PRINCIPLES OF INTERCHANGEABLE MANUFACTURING
PRINCIPLES OF
INTERCHANGEABLE
MANUFACTURING
A TREATISE ON THE BASIC PRINCIPLES INVOLVED IN SUCCESSFUL INTERCHANGE- ABLE MANUFACTURING PRACTICE COVERING DESIGN, TOLERANCES, DRAW- INGS, MANUFACTURING EQUIPMENT, GAGING AND INSPECTION
BY EARLE BUCKINGHAM, A.S.M.E., S.A.E.
ENGINEER, PRATT & WHITNEY Co.
FIRST EDITION FIRST PRINTING
NEW YORK THE INDUSTRIAL PRESS
LONDON: THE MACHINERY PUBLISHING CO.. LTD
COPYRIGHT, 1921,
BY
THE INDUSTRIAL PRESS NEW YORK
COMPOSITION AND ELECTROTYPING BY THE PLIMPTON PRESS, NORWOOD, MASS., U.S.A.
PREFACE
WHILE many articles dealing with various phases of inter- changeable manufacturing have appeared from time to time in the technical press, no complete and comprehensive treatise dealing with this subject as a whole has heretofore been avail- able to those interested in interchangeable manufacturing in the machine building and metal working fields.
The development of interchangeable manufacturing is closely interwoven with many distinctly American manufacturing methods and processes. Every large American industry has contributed its share to the progress made in interchangeable manufacturing. Different plants working along independent lines have often achieved the same results by widely different methods. The author has attempted to define and emphasize the underlying basic principles, using specific methods only when necessary to illustrate the application of these principles in actual manufacturing processes. He has gathered the in- formation upon which this treatise is based from many manu- facturing plants, both large and small, in this country and in Canada. He has seen every method discussed in successful operation, some in one plant, some in another — but not all in any one.
For more than ten years the author has been in constant touch with many of the detailed manufacturing problems that arise in the production of interchangeable mechanisms in large quantities. During the World War his work took him for four years into many manufacturing plants in connection with ord- nance work, first for private corporations and later for the Ordnance Department. When engaged in this work it became apparent to him that the absence of common methods of inter- pretation of drawings, tolerances, and specifications, the lack of uniform gaging methods and misunderstanding of many of
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VI PREFACE
the factors of interchangeable manufacturing, presented an urgent need for a complete treatise on this subject.
In arranging the material available on the subject of inter- changeable manufacturing, the author has first taken up the general principles involved in the industrial application of this method of production, and has then devoted a separate chapter to the definition of the terms used, so that there will be no mis- understanding as to the meaning of the terms used later in the book. The influence of interchangeable manufacturing processes on machine design and the purposes of models are then dealt with, followed by a complete and minute discussion on the dimensioning of drawings intended for use in interchangeable manufacturing. This is followed by a discussion of the principal elements that govern mechanical production, the equipment required for interchangeable manufacturing (including machines, jigs and fixtures); the gaging equipment necessary; and the principles of inspection and testing. Special chapters are also devoted to the manufacture for selective assembly, and methods used in small quantity production on an interchangeable basis. An entire chapter deals with the service factor in interchangeable manufacturing, because in the final analysis no manufactured machine or device is ever purchased for itself alone, but is acquired for the service which it is supposed to render.
The Pratt & Whitney Co., Hartford, Conn., with whose cooperation this treatise is written, submits it to the public as a part of the company's contribution to the art of inter- changeable manufacturing with the hope that it will assist manufacturers and mechanics to employ effectively the prin- ciples of interchangeable manufacturing and to reap the benefits that a rational application of these principles make possible. The author also wishes to acknowledge at this time the assist- ance that has been given him by many other manufacturing plants that he has visited. To name them all would mean a long list of prominent plants manufacturing machine tools, auto- mobiles, tractors, ordnance, typewriters, watches, phonographs, instruments, etc.
EARLE BUCKINGHAM
CONTENTS
CHAPTER I
PRINCIPLES OF INTERCHANGEABLE MANUFACTURING
PAGES
Economy — Extent of Interchangeability — Clearances — Tolerances — Component Drawings — Specifications — Gages for Checking Results — Manufacturing Equipment — Pro- duction Problems — Inspection of Product 1-17
CHAPTER II
TERMS USED IN INTERCHANGEABLE MANUFACTURING
Interchangeability - - Selective Assembly - - Function — Limit — Tolerance — Basic and Model Size — Maximum and Minimum Metal Size — Maximum and Minimum Clearance — Interference — Operating, Functional, and Clearance Sur- faces — Elementary and Composite Surfaces — Compound Tolerances — Register Points — Unit Assembly — Component and Operation Drawings 18-28
CHAPTER III
MACHINE DESIGN IN INTERCHANGEABLE MANUFACTURING
Classes of Design — Simplifying Design — Choice of Ma- terials — Clearances and Tolerances — Application of Inter- changeable Principle — Advantages of Unit Assembly - Designing for Assembling and Service 29-39
CHAPTER IV PURPOSE OF MODELS
Manufacturing Model to Test Functioning — Experimental Model — Testing Tolerances — Model for Standard of Pre- cision 40-45
vii
viii CONTENTS
CHAPTER V
PRINCIPLES IN MAKING COMPONENT DRAWINGS
PAGES
Functional Drawings — Manufacturing Drawings — Laws of Dimensioning — Inspection Gage Requirements — Composite Surfaces — Compound Tolerances — Force Fits — Profile Sur- faces — Dimensioning Holes — Location of Holes — Con- centricity and Alignment — Gears 46-76
CHAPTER VI PRACTICE IN MAKING COMPONENT DRAWINGS
Maintaining Functional Requirements of a Mechanism — Basic Dimensioning — Maintaining a Common Locating Point — Length Dimensions from Common Locating Point — Draw- ing of Separate Parts — Compound Tolerances 77-104
CHAPTER VII ECONOMICAL PRODUCTION
Specifications — Functions and Requirements of Product — Manufacturing Data — Factory Cost — Direct Labor Cost — Machine Hour Rate — Product Overhead — Clerical and Ac- counting Work — Specific and General Information 105-120
CHAPTER VIII
EQUIPMENT FOR INTERCHANGEABLE MANUFACTURING
Selection of Machine Tools — Designing Jigs and Fixtures — Cutting Tools — Locating Points — Chip Clearances — Check- ing and Testing Jigs and Fixtures — Maintaining Tolerances — Special Equipment for Machining Automobile Transmission Cases — Drilling Holes Simultaneously — Pneumatic Clamp- ing Devices — Multiple-Tool Facing Bar — Milling and Drill- ing Fixtures 121-176
CHAPTER IX GAGES IN INTERCHANGEABLE MANUFACTURING
Classification According to Use — Accuracy — Working and Inspection Gages — Interchangeability between Parts made in
CONTENTS IX
PAGES
Different Shops — Snap, Ring, and Plug Gages — Contour or Profile Gages — Receiving Gages — Flush-pin Gages — Sliding Bar Gages — Depth and Length Gages — Hole Gages - Gaging Threads — Tolerances on Threaded Parts — Wing and Indicator Gages — Functional Gages — Gaging Gears - Master and Reference Gages 177-216
CHAPTER X INSPECTION AND TESTING
Discrepancy between Part and Drawing — Incomplete Drawings and Specifications — Shop Inspection — Final In- spection — Inspecting Gages and Material — Testing As- sembled Mechanisms 217-223
CHAPTER XI MANUFACTURING FOR SELECTIVE ASSEMBLY
Clearances and Tolerances — Dimensions and Tolerances on Drawings — Laws of Dimensioning — Similarity of Specifica- tions, Equipment, Gages, and Inspection Methods 224-225
CHAPTER XII SMALL-QUANTITY PRODUCTION METHODS
Standardization of Nominal Sizes — Clearances and Toler- ances — Economy of Standardization — Standardizing Unit Assemblies to Suit Several Machines — Component Drawings - Manufacturing Equipment — Gages and Methods of In- spection 230-240
CHAPTER XIII
SERVICE FACTOR IN INTERCHANGEABLE MANUFACTURING
Functional and Manufacturing Designs — Keeping Specifica- tions up to Date — Planning Production to Obtain Required Service. . 241-245
PRINCIPLES
OF INTERCHANGEABLE MANUFACTURING
CHAPTER I PRINCIPLES OF INTERCHANGEABLE MANUFACTURING
INTERCHANGEABLE manufacturing consists of machining the component parts of a given mechanism in the manufacturing departments within such limits that they may be assembled in the assembling department without fitting or further machining. > Component parts may also be replaced or transferred from one mechanism to another without detriment to the function- ing and without machining. The advantages of such a method of manufacture are self-evident, and need not be dwelt upon further. It is obvious that with proper equipment and control, the component parts of a mechanism can thus be manufactured in large quantities at a low direct labor cost.
Economy of Interchangeable Manufacturing. In all private industrial enterprises ultimate economy is the controlling factor of any method of procedure. This does not necessarily mean that the methods adopted always are actually the most economi- cal. Methods which will promote this economy are, however, the ideals toward which manufacturers are constantly striving. Now, a careful analysis will show that interchangeability does not always result in ultimate economy. In such cases the at- tempt to maintain it is a fault, not a virtue.
To make this point clear, consider the matter first from the standpoint of production alone. The equipment and prepa- ration necessary to produce interchangeable parts are expensive. In making only a small number of special mechanisms, it would be gross extravagance to maintain any high degree of inter- changeability. Viewed simply as a question of production, the
2 ; .; :\ .INTERCHANGEABLE MANUFACTURING
problem of interchangeable parts is solved by establishing a balance between manufacturing and assembling costs, whether the quantity of production be great or small, whether the mecha- nism involved be a standard or a special product.
Ultimate economy, however, must include the factor of service. Suppose automobiles, typewriters, sewing machines, or sporting rifles are sold. Parts will wear out or be broken by accident. The maintenance of service stations, where extra parts are quickly available, tends to keep customers satisfied. Service stations will be least expensive if the product is truly inter- changeable and the agent can replace a part with the aid of a screwdriver or wrench — or, still better, if the customer can replace it himself. Since the advent of the automobile, people have been much more interested in things mechanical than before, and have taken pride in making their own repairs. The more nearly interchangeable mechanisms are made, the more this desirable trait is fostered and the less will service stations cost. Ultimate economy, then, requires that service costs be balanced against total productive costs.
Degree of Interchangeability Desirable. It should not be as- sumed from this that entire interchangeability or none at all must be had. In almost every mechanism certain parts are begun as separate units in order to simplify the manufacture, but are later permanently assembled into a single unit and machined to completion as such. In many such cases, the expense of attaining interchangeability would be too great to justify the attempt, because of the many mechanical difficulties to be overcome. It would be more economical, in case of break- age, to discard and replace the entire assembled unit. In other cases, the functional requirements may be so severe that a system of selective assembly will prove to be the proper course, although this entails carrying a double or triple number of spare parts in service stations, or involves some fitting when replacing unserviceable parts.
In general, however, interchangeability is a desirable goal, and is readily attained in the majority of cases if the proper attention is given to the basic principles governing it, including
INTERCHANGEABLE MANUFACTURING 3
the design of the mechanism and the process of manufacture; yet it is limited in several directions by the inadequacy of many present manufacturing conditions. With improved facilities, it may be that in future years a much greater degree of inter- changeability will be possible than at present.
The following paragraphs, which are based on manufacturing conditions as they now exist, trace the progress of a commodity through all stages of its manufacture, from its inception as a mechanical project to the final testing that determines its suc- cessful completion. An attempt has been made to single out for special comment those factors which make possible, or promote, the interchangeable manufacture of its parts.
Design as it Affects Success. The development of any new mechanism starts with a mental conception of some function to be performed. This conception then takes detailed form, first mentally, then on paper, and finally in metal. The experi- mental model — if such be constructed — is usually made by the cut-and-try method. Little attention is paid in the beginning to future manufacturing requirements. The main object is to construct a mechanism that will function properly regardless of the exact design. When this end is reached, what may be called the inventive or functional design has demonstrated its success.
Before manufacturing is begun, however, a manufacturing design must be perfected which will modify the inventive design so as to allow its economical production on a large scale. Several manufacturers recognize this twofold nature of design- ing, and maintain a separate department for each type. In- dispensable as is the original invention, it is the manufactur- ing design which largely determines the success or failure of a given project. This manufacturing designing necessarily con- tinues throughout the whole course of production because of the almost infinite number of petty detailed questions involved, only a few of which can be foreseen and answered in advance. One of the important functions of an engineering department is to keep in close touch with the progress of the work in the shops, deduce general principles therefrom, and apply these
4 INTERCHANGEABLE MANUFACTURING
principles not only to the work in hand, but also to all new work that may be developed.
The Manufacturing Model. Assume that the functional re- quirements of the mechanism are established and that the manufacturing design has been adopted. The first concern is to test this design as far as possible. The most certain method of accomplishing this is to develop a physical model. Such a model must not be confused with the experimental model, as its purpose is quite different. The experimental model shows that the mechanism will perform certain functions. The manufacturing or physical model, if properly developed, proves that the mechanism, as modified and developed to facili- tate manufacture, still retains the functional advantages of the experimental model.,, The manufacturing model is naturally an expensive piecei of equipment, but if a large output of a new commodity is under consideration, it is money well invested. In the case of a small total output, a "pilot" mechanism is often built for this purpose, which is not set aside for future reference but incorporated in the product itself.
There are many other services which a manufacturing model is capable of rendering. It may serve as a physical standard of dimensions for the future product. In this case, it must be made with much greater care than if it were to be used merely to test the functioning of the manufacturing design. Such a model will be of great value as a reference at all times during pro- duction: In itself, it comprises an effective functional gage to test any completed part. It should be used but rarely, however, for that purpose. In addition, the component parts of the model are of great assistance in checking the manufacturing equipment in the early stages of the work.
Clearances. Clearances are vital factors in interchangeable manufacturing. Fits can be secured without interchangeability, but the latter cannot be maintained without proper clearances. It is self-evident that a certain space must be left between operating parts. The minimum clearances should be as small as the assembling of the parts and their proper operation under service conditions will allow. The maximum clearances should
INTERCHANGEABLE MANUFACTURING 5
be as great as the functioning of the mechanism permits. The variation between a maximum and a minimum clearance de- termines the manufacturing tolerance. It is clear, then, that determining at the outset the permissible clearances establishes also the extent of the tolerances which control the final inspection.
Clearances should be one of the principal considerations in developing the manufacturing design. This design should aim to allow the greatest possible amount of clearance between companion parts. The more the design lends itself to this end, the greater the economy of manufacture and the greater the degree of interchangeability obtainable. In determining which parts of a mechanism can be made interchangeable, this matter of permissible clearances plays the largest part. A mechanism which is so designed that it cannot permit fairly liberal clear- ances is not a suitable one to be manufactured on a strictly interchangeable basis with the standard equipment now avail- able. Every operating part of a mechanism must be located with- in reasonably close clearances in each plane. After such require- ments of location are met, all other surfaces should have liberal clearances, unless the factor of strength is the controlling one.
Manufacturing Tolerances. The general tendency in the past has been to establish manufacturing tolerances by trying to hold the product as closely as possible to a fixed size. The natural result of this policy is that the tolerances established on paper are often exceeded; yet the actual working variations remain unrecorded, because it is argued that under certain conditions the original requirements might be met and, therefore, the tolerances noted are the proper ones, even though they are not maintained. Every effort to make the recorded tolerances represent the actual working tolerances is opposed on the ground that such a procedure would lower the shop standards. As a matter of fact, it is hard to understand how anything could lower the standards of the shop more than the absolute disregard of the rules it is supposed to be obeying.
There is a further argument for the acceptance of liberal tolerances. Too often in manufacturing concerns, and especially in the case of interchangeable manufacturing, one finds details
6 INTERCHANGEABLE MANUFACTURING
being made ends in themselves rather than means to a larger end. In producing a component part, the main object should not be to demonstrate how closely a fixed size can be approached; the aim should be to construct, as economically as possible, a mechanism that will satisfactorily perform certain functions. The knowledge of how accurately a machining operation can be performed is indeed invaluable in making the manufacturing design; but when that design has once been completed, interest should shift to the proper functioning of the completed mecha- nism. Finally, it may be said that in most cases the tolerances originally fixed are increased during the process of manufactur- ing without detriment to the mechanism. It is rarely that a tolerance has to be reduced.
The proper minimum clearances can be determined quite readily and definitely for most cases in the early stages of the work — the manufacturing model is of great value in this respect — but the maximum clearances become established only after extended experience with the particular mechanism. In many cases the extreme maximum is never found, because long before that point is reached, the tolerances have become so liberal that there is no need, from the standpoint of economical production, to increase them further.
Component Drawings. Component drawings have two main functions to perform. The first is to give such information about the design and the tolerances that the manufacture of the product can begin. This does not seem like a very difficult task, but the notation of the tolerances on component drawings has created new problems of interpretation that have not, as yet, been fully solved. At the present time, the language of drawings is not altogether clear and exact.
The first tendency in introducing tolerances on drawings seems to have been to attempt to express a permissible variation on every dimension given. The results obtained in the shop depend, then, upon the particular combination of dimensions used. Different organizations using different combinations could obtain radically different results; and of the possible number of different combinations there is no end.
INTERCHANGEABLE MANUFACTURING 7
The existence of a tolerance on a drawing is an acknowledg- ment that variations are inevitable in the physical dimensions of the product. Any dimension given on such a drawing without a tolerance should not be construed to denote an absolute size without error, but rather to indicate either that the permissible variation for that point or surface is controlled by tolerances given on other co-related dimensions, or that the dimension is so relatively unimportant that no attempt had been made to determine its permissible variation.
In making component drawings, the effort should be made to so give the dimensions and necessary tolerances that it would be possible to lay out one, and only one, representation of the " maximum metal" condition and one, and only one, of the " minimum metal" condition. If such lay-outs were super- imposed, the difference between them would represent the permissible variation on every surface. Any condition of the product which fell within the zone thus established could be considered as meeting the requirements of the drawing. If one will make a few such lay-outs, it will soon be clear to him that there are always a number of dimensions that should be given without tolerances if drawings are to be kept consistent and intelligible.
Information on Component Drawings. It must be realized at the start that it is impossible in every case to give on one com- ponent drawing all the dimensions that are needed to construct the patterns, tools, gages, and other manufacturing equipment, without introducing many inconsistencies. Certain dimensions could be correct if one set of holding points and one series of operations were to be used, but would be incorrect under differ- ent conditions. If the component drawings are made so that they represent the proper completed conditions — call them inspection gage requirements if you will — the end in view is attained. Any figures that the shop desires to use are correct if they insure this result.
It is impossible to amplify this point without entering into a prolonged discussion of the effect of using different holding or registering points in the manufacturing processes. Yet it
8 INTERCHANGEABLE MANUFACTURING
may be of interest to know that several manufacturing plants solve this problem by adding operation drawings, which give only the specific dimensions required at a particular operation. Some of the dimensions are duplicates of those on the component drawing, while others are computed to serve their restricted purpose. This proves an effective means of recording additional information required in the manufacturing departments, which cannot be put on component drawings without danger of misuse.
After production is well under way, the component drawings have served their first purpose. In the meantime, the actual manufacturing operations have made available a store of new information regarding the proper conditions to be maintained. It should be the second function of the component drawings to record as much of this information as possible. Conflicting information or misinformation should be eliminated at the same time; in short, the drawings should be revised to agree with actual conditions and requirements. It has been a great fault in the past to neglect this second function almost entirely. It is a difficult task to make the component drawings represent from the first conditions that must be maintained. In time, the shop will discover many of them, often after bitter experience, even though they have been omitted from the component draw- ings. Frequently, however, it happens that this information does not make its way back to the office, but is retained by the shop men among themselves. Often this is the fault of the office, which is prone to consider such information as criticism, so that the shop, after a few rebuffs, makes no further attempt to pass it along. It is most essential, however, that such in- formation be recorded in permanent form, not only because of its value to the work in hand, but also because of its helpful application to new work in the future.
Dimensioning of Component Drawings. The problem of the proper dimensioning of component drawings is strictly a mathe- matical one. There are a few basic principles in regard to it as fixed and simple as Newton's three laws of motion, but even more difficult at times to apply correctly. When either of the two following principles is violated, trouble will inevitably follow:
INTERCHANGEABLE MANUFACTURING 9
1. In interchangeable manufacturing, there is but one dimen- sion (or group of dimensions) in the same straight line that can be controlled within fixed tolerances. That is the distance between the cutting surface of the tool and the locating or registering surface of the part being machined. Hence, it is incorrect to locate any point or surface with tolerances from more than one point in the same straight line.
2. Dimensions should be given between those points which it is essential to hold in a specific relation to each other. The majority of dimensions, however, are relatively unimportant in this respect. It is good practice to establish locating points in each plane, and to give, as far as possible, all such dimensions from these common locating points.
There are also a few other general principles which it is good practice to follow. Although violations of them are not errors in themselves, they lead to many unnecessary errors. In all of this work it must be realized that it is impossible to create anything that is altogether fool-proof; the best that can be expected is to make conditions such that little or no excuse remains for making a mistake. The three following principles are of this order:
1. The basic dimensions given on component drawings for / interchangeable parts should be the maximum metal sizes, except for force fits and other unusual conditions. The direct 1 comparison of the basic sizes should check the " danger zone" I or the minimum clearance conditions in most cases. It is evident ! that these sizes are the most important ones, as they control the interchangeability. They should be the first determined and, once established, they should remain fixed if the mechanism functions properly and the design is unchanged. The direction of the tolerances, then, would be such as would increase this clearance. For force fits, such as taper keys, etc., the basic dimensions should be those which determine the minimum inter- ference (which is the " danger zone" in this case) and the direc- tion of the tolerances for this class of work should be such as would increase this interference.
2. Dimensions should not be duplicated between the same
10 INTERCHANGEABLE MANUFACTURING
points. The duplication of dimensions causes much needless trouble, due to changes being made in one place and not in the others. It causes less trouble to search a drawing to find dimen- sions than it does to have them duplicated and, though more readily found, inconsistent.
3. As far as possible, the dimensions on companion parts should be given from the same relative locations. This pro- cedure assists in detecting interferences and other improper conditions.
If careful thought is given to these component drawings, much time and effort will be saved later in the shop. If they are neglected, all the future work will suffer. A large percentage of. the mistakes made in the manufacturing departments may be traced back to improper component drawings.
Specifications for Interchangeable Manufacturing. The in- formation that can be included on component drawings, except in the case of a very simple or familiar mechanism, is seldom sufficient in itself to enable the manufacturer to proceed in- telligently with a new product. It is desirable that he know the particular purpose for which the mechanism is to be made. The better he is informed on this subject, the greater service he can render in promoting its economical manufacture and future development. Specifications are supposed to supplement the drawings by giving all the needed additional information which has no place on the drawings. I say "supposed" because it is only in rare cases that the specifications commonly en- countered give all the desirable information. They usually deal with only the most exacting requirements and make no mention of the others, thus establishing a severe precedent for the solution of all questions in regard to the requirements, important and unimportant. They seldom indicate the essential object in view, namely, the economical production of mechanisms which will function satisfactorily.
Specifications should state the end to be accomplished, and should give all possible information to assist in the attaining of that end. Any unusual conditions should be explained in detail. All exacting requirements should be specified with the reasons
INTERCHANGEABLE MANUFACTURING II
for the same, including requirements of functioning and of materials to be used. But they should not stop here. The less exacting conditions should be noted also. If a certain material is specified, and the chief consideration is economy, it should be so stated, with the substitution allowed. The material that might be most economical under one set of conditions might be otherwise under different circumstances. Parts which are detailed on the drawings but for which commercial articles can be substituted should be so designated. The specifications should list those parts which must be made interchangeable and those which need not be. A description of the tests for materials, for physical dimensions, and for functioning should be included. In fact, any information that will assist in the manufacture of the product should be given. Some of it will specify the results to be obtained; more of it should be information to assist the manufacturer, not hard and fast rules which he must follow regardless of consequences.
Such specifications would undoubtedly be far from com- plete at first. Provision should be made to keep them abreast of the actual progress of the work. The shop should use them as a place to record as much of the experience gained as possible. If certain methods have been found unsatisfactory, here is an ideal place to record the fact, and perhaps save a duplication of the mistake in the future. If other methods have proved satisfactory, they, too, should by all means be recorded. In fact, specifications of this kind, although they would in time become voluminous, would be a history of a mechanism and furnish valuable data to assist in developing new mechanisms.
Written specifications are held in low esteem by the majority of manufacturers. They do without written specifications for their own products, and when obliged to meet them for contract work, find them an additional annoyance instead of a help. This is due, in large measure, to the fact that this subject, as also the matter of tolerances, has been regarded as an end in itself instead of as a means to a larger end. Manufacturers do have specifications, although they are seldom called by that name and are seldom written or grouped together for ready
12 INTERCHANGEABLE MANUFACTURING
reference. Some of them may be found in the cost and pro- duction records, some in the shop correspondence, but most of them are carried in the memories of the foremen and older employes who maintain the traditions of the shop.
Gages for Checking Results. Thus far those elements which form the groundwork for actual manufacturing operations have been discussed. The manufacturing design has been developed; it has been tested with the manufacturing model; the first guess as to the proper manufacturing tolerances has been made; all suitable and available information has been recorded on the component drawings; and the specifications to supplement these drawings have been partially developed by recording there all further information available that will assist in accomplishing the main purpose, namely, to produce satisfactory mechanisms as economically as possible. The means of carrying on the work of actual production and the facilities that should be provided for checking the results, must now be considered.
There are two important reasons for inspecting the product during manufacture: First, spoiled parts must be eliminated as soon as possible to save the expenditure of useless effort on unserviceable pieces. Second, the completed components must be checked before assembly to eliminate the unserviceable parts and thus insure the proper functioning of the mechanism. For these purposes, gages are extensively employed.
A gage should be provided whenever its use is more economical than the use of standard measuring instruments. For example, if the total production of a certain mechanism amounts to about a dozen units, it is extravagance to provide special gages. On the other hand, if this production amounts to several thousand units, a complete set of gages is both desirable and necessary. The extent to which gages are necessary, therefore, depends in great measure upon the amount of the total production. Further- more, gages should be provided to check only those conditions which it is essential to maintain. The nature and extent of the gages required depend upon the manufacturing conditions. In many cases, a check on one or two points is sufficient to detect
INTERCHANGEABLE MANUFACTURING 13
any unsatisfactory results. Under varying manufacturing con- ditions different faults must be guarded against. Gages are a preventive and not a cure. The point to be emphasized is that they should be provided whenever their addition will result in the production of more or better components with a total ex- penditure of the same or less effort.
Main Classes of Gages. There are two kinds of gages to con- sider, which, for want of better terms, will be called limit gages and functional gages. A limit gage is one that checks a specified dimension to specified tolerances. A functional gage is one that checks the relationship of several dimensions to insure the proper functioning of the assembled mechanism. As with other manufacturing equipment, the exact design of a gage is unim- portant, if it fulfils its purpose simply and efficiently.
The degree of accuracy or precision required depends upon the extent of the tolerances. In all cases, on limit gages, the variation must be inside the established limits of the component. The dimensions given on component drawings are limit gage sizes. For example, the limits given for the diameter of a stud should be interpreted to mean that such diameter must be made to satisfy ring or snap gages of the sizes specified.
As yet master gages, or reference gages, as they are variously called, have not been touched upon. A master is a physical standard of size or form used for reference purposes. It is needed only where the degree of precision required is so exacting that the errors inherent in direct measurements with standard measur- ing instruments will be great enough to prevent the proper functioning of the product. If a manufacturing model is care- fully developed, few, if any, masters will be required. For simple dimensions of length, it is usually sufficient to establish reference pieces of, say, tenth-inch units. For important functional contours, masters are essential.
Test pieces for individual gages are necessary only when the amount of gage checking is so great that too much time is con- sumed by using standard measuring instruments, or when no skilled labor is available for this checking. Test pieces are therefore desirable for checking complicated profile and fixture
14 INTERCHANGEABLE MANUFACTURING
gages that receive hard usage, but they are seldom necessary for plain plug, ring, and snap gages.
Manufacturing Equipment. Suitable tools and equipment with which to manufacture a product must also be provided. The first logical step to this end is to make operation lists, planning in detail the successive operations, and specifying the type of machine, fixture, tool, and gage required. These operation lists are an integral part of the specifications, subject, of course, to such modifications as are found necessary. Of the machines themselves but little mention need be made at this time. Stand- ard machine tools are now on the market for making almost every variety of machining cut. Special machines are required only for very unusual operations or for extremely large pro- ductions where many automatic operations are performed.
The design of the fixture and the tool depends to a great extent upon the design of the piece to be machined. Great care should be taken to maintain the same locating or register- ing points in the fixtures as are used for the gages. The ideal condition is to have the registering points for both fixtures and gages identical with the points on the component drawings from which the surfaces in question are dimensioned. After the equipment is complete, the component drawings should be checked and revised T^here necessary to obtain this result.
Another factor which must be considered in the design of the equipment is the required rate of production. In the case of a small output, the cost of the equipment amounts to a large percentage of the total cost of production. As the output in- creases, the proportionate cost of the equipment decreases, thus making it desirable to refine this equipment, if by so doing the production can be increased with the expenditure of less productive effort. Here, as elsewhere, it is a question of balanc- ing the cost of one item against that of another and of selecting the most economical combination.
In most cases, except with some automatic machines or on very large work, the operator spends more time in handling the work than the machine takes to perform the machining operation. Therefore, whenever the rate of production is high enough to make
INTERCHANGEABLE MANUFACTURING 15
it economical, the fixtures should be made for rapid operation, even though this greatly increases the initial cost of equiqment.
Production Problems. The actual production consists of taking the raw material and passing it through the equipment until it emerges as a finished component. The production problems are many and varied. Any part of the preceding work which has been slighted or left undone must be completed here in addition to the many tasks which are involved in the production itself. The greatest problem involved in production is that most uncertain factor — human nature. The present tendency is to provide equipment that can be operated by semi-skilled labor. Equipment, however, cannot be made altogether fool- proof. As noted before, the best that can be done is to arrange matters so that little or no excuse remains for making mistakes.
People thoughtlessly speak of unskilled labor. The more this problem is studied, the more it is realized that there is no place in interchangeable manufacturing for such assistance. That is, there is no task so elementary but that better and more economical results can be obtained by a certain degree of train- ing or skill in the operator. An attempt is made to subdivide productive operations into the most elementary tasks so that labor can be readily trained to perform them satisfactorily. Each manufacturer is forced to train the majority of his own operators. Naturally, then, the shorter the time required for this training, the sooner the results will show in the production. On the other hand, the less skill required of the operator, the more elaborate and complete the equipment must be. The amount of supervision required for both operators and equip- ment is also greatly increased, in both quantity and quality.
In any case, the better the training that these operators receive, the higher is the quality of the work produced. And the matter of honest, serviceable quality as distinguished from mere appearance is more appreciated than formerly. The operator should be taught to maintain the established tolerances. If the specified tolerances prove too severe in practice for eco- nomical production, they should be corrected, provided the functional requirements of the mechanism will permit. If they
1 6 INTERCHANGEABLE MANUFACTURING
are not too severe, there is no excuse for violating them. The practice of adhering to the specified tolerances will do much to promote a high quality of product.
Shop Inspection of Product. The inspection and acceptance or rejection of the components falls logically into two divisions. The first is the shop inspection which is made while the material is in process of manufacture. The object is to cull out defective work as soon as possible and also to detect any defects in the equipment that would result in faulty work. If the percentage of rejections is normal, it is evident that the requirements speci- fied and the manufacturing facilities provided are satisfactdry. If the percentage is high, it is evidence of improper conditions somewhere which should be investigated, and the trouble should be corrected at its source. Sometimes an error occurs, with the result that the requirements are exceeded on a large number of parts. Such matters should be investigated and settled according to their merits. If the pieces will be serviceable and can be completed without undue cost, the factor of economy will play a large part in the decision. In such cases, the require- ments specified should not be changed unless it is evident that such a change will result in an economic benefit in the future. As in all other cases, ultimate economy is the goal.
Final Inspection. The second division of the inspection is the final examination of the completed parts. The object of this inspection is to see that all components which will function properly are accepted and that all unserviceable parts are re- jected. This inspection is largely governed by the requirements of the component drawings — often represented by gages — and by the specifications. It is therefore most important that the drawings and specifications give as nearly as possible the limits of parts which will function properly. Yet, as has been already noted, these drawings and specifications are incomplete at the beginning, and probably will always be so, to a certain extent. Therefore, a rigid adherence to the letter but not to the spirit of the drawings and specifications is unwise, as it will not aid in the acceptance of all serviceable material, nor in the ultimate economy of manufacture. In addition to the written require-
INTERCHANGEABLE MANUFACTURING iy
ments, inspectors must have a certain amount of education and experience with the mechanisms involved, or with similar mechanisms; otherwise the inspection will always prove a hindrance to the main purpose.
The characteristic needed for a successful inspector is a judicial mind. Since the requirements are laws, the inspection should equitably enforce them. The spirit of the requirements should be enforced in those cases where their exact expression is incomplete. If the essentials are always specified definitely and completely, it will be a fair assumption that incompletely specified conditions are relatively unimportant. Wherever possible, the requirements should be revised to make the letter and the spirit agree, but the attempt to cover every minute and unimportant detail will prove impossible in practice.
The functional requirements should be maintained in the final inspection strictly according to the specified conditions. The non-functional requirements should be handled in a more judicial manner, each case being decided on its merits. As a matter of fact, this final inspection should be in the nature of a functional inspection only. Little attention should be given here to the non-essentials other than, perhaps, a visual inspection for general quality, and some supervision of the shop inspection to see that proper precautions are taken during production to insure a good product. In all cases, the main effort throughout the work should be to establish, define, and maintain the essential conditions first, letting the non-essentials develop in practice. No secret, however, should be made of the fact that these non- essentials are left to work out their own salvation.
Test of Success. The final and complete evidence as to whether the aim has been accomplished is furnished after the mechanisms have been assembled and tested. If the total costs have been reasonable and the completed mechanisms assemble properly and perform satisfactorily all the required functions, it is con- clusive evidence that all essentials have been mastered. On the other hand, if the costs are excessive or if the mechanism fails to assemble or to operate properly after being assembled, it is equally conclusive evidence of failure.
CHAPTER II TERMS USED IN INTERCHANGEABLE MANUFACTURING
IN order to describe concisely characteristics peculiar to interchangeable manufacturing, it is necessary to use many words and phrases in an arbitrary sense. Therefore, to avoid misunderstanding, space is taken here to define several of the important terms. The interpretation of these terms is limited to the ideas they express in this treatise.
Interchangeability. The term interchangeability, as used here, refers to absolute interchangeability. In this sense, inter- changeable parts are parts that are so made that they can be assembled or interchanged after final inspection without machin- ing or fitting, and any possible combination of these parts will assemble, interchange, and function properly. To insure this end, the most extreme limits permitted must be constantly checked against each other.
Selective Assembly. Selective assembly refers to a method of manufacturing similar in many of its details to interchange- able manufacturing, in which component parts are sorted and mated according to size and assembled or interchanged with little or no machining. Companion parts made to the extreme limits are not supposed to interchange. For instance, a maximum male component will not assemble with a minimum female part. However, the maximum male and female, or the minimum male and female must interchange. A good example of this method of assembling is found in the production of ball bearings. The balls are sorted into groups, according to their size, to facili- tate the assembly of any bearing with balls of uniform size. As a matter of fact, nearly every so-called interchangeable article represents a combination of the two methods of quantity production — interchangeable and selective.
18
DEFINITIONS OF TERMS 1 9
Function. The term function is used extensively and with various shades of meaning. The word itself has many meanings. The dictionary gives one as "fulfillment or discharge of a set duty or requirement"; and another as "that mode of action or operation which is proper to any structure," etc. As applied to component parts, the word has been used to express both these meanings. This includes all requirements of interchange- ability and service which the part must render throughout the normal life of the mechanism of which it forms a part. The same meaning is intended when it is applied to the assembled mechanism. The functional design refers specifically to the combination of mechanical movements required to make the completed mechanism perform its specified duties. Functional gages are those which test the functional operation of components without strict adherence to their exact physical dimensions.
Limit. In every interchangeable mechanism there are certain maximum and minimum sizes for each part, between which the parts will function properly in conjunction with each other and outside of which they will not. These sizes are the absolute limits of the parts. The established limits are the maximum and minimum dimensions specified on the component drawings. The established limits should approach as closely to the absolute limits as normal manufacturing conditions require. Limits established without regard to the absolute limits result either in excessive cost of manufacture or faulty mechanisms or both. If the established limits are much more severe than the absolute limits, needless expense is incurred in manufacturing. On the other hand, if the established limits are more liberal than the absolute limits, unsatisfactory mechanisms will be produced.
Tolerance. Tolerance is the amount of variation permitted on dimensions or surfaces. The tolerance is equal to the differ- ence between the maximum and minimum limits of any specified dimension. For example, if the maximum limit for the diameter of a shaft was 2.000 inches and its minimum limit was 1.990 inches, the tolerance for this diameter would be o.oio inch. By determining the maximum and minimum clearances required on operating surfaces, the extent of these tolerances is established.
2O
INTERCHANGEABLE MANUFACTURING
The application of the tolerances to the basic dimensions fixes the limits.
Basic and Model Size. Obviously, the absolute limits of the various dimensions and surfaces indicate danger points, inas- much as parts made beyond these limits are unserviceable. A careful analysis of a mechanism shows that one of these danger points is more sharply defined than the other. For example, a certain stud must always assemble into a certain hole. If the stud is made beyond its maximum limit, it will soon be tqo large to assemble. If it is made beyond its minimum limit, it will be too loose or too weak to function. The absolute maximum
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Fig. 1. Graphic Illustration of the Meaning of the Terms Limit and Tolerance
limit in this case can be defined within a range of o.ooi inch, whereas the absolute minimum limit cannot be defined within a range of at least 0.004 mcn- In this case the maximum limit is the more sharply defined.
The basic size expressed on the component drawing is that limit which defines the more vital of the two danger points, while the tolerance defines the other. In general, the basic dimension of a male surface is the maximum limit which re- quires a minus tolerance. Similarly, the basic dimension of a female surface is the minimum limit requiring a plus tolerance,
DEFINITIONS OF TERMS 21
as shown in Fig. i. There are, however, dimensions which define neither a male nor a female surface. Such are dimensions for the location of holes. In a few cases of this kind, a variation in one direction is less dangerous than a variation in the other. Under these conditions, the basic dimension represents the dan- ger point, and the tolerance permits a variation only in the less dangerous direction. At other times, the conditions are such that any variation from a fixed point in either direction is equally dangerous. In such a case, the basic size represents this fixed point. Tolerances, when given on the component drawing, extend equally in both directions.
If a model is developed as a standard of precision, the model parts become the physical representations of the basic sizes. In other words, for all practical purposes, the model size and the basic size are identical.
Maximum Metal Size. Maximum metal size is that limit at which the part contains the maximum amount of metal. This would be the maximum male limit and the minimum female limit. In many cases, a careful analysis is necessary to determine which limit represents the maximum metal conditions, as many dimensions are neither male nor female. In other cases, such as locations of holes, there are neither maximum nor minimum metal conditions. With few exceptions, however, the maximum metal sizes are also the basic sizes.
Minimum Metal Size. Similarly, the minimum metal size is that limit at which the part contains the minimum amount of metal. This is the minimum male limit and the maximum female limit, when the dimensions can be so classified.
Minimum Clearance. It is evident that there must be a definite amount of clearance between male and female components which operate together. The minimum clearance should be as small as will permit the ready assembly and operation of the parts, while the maximum clearance should be as great as the functioning of the mechanism will allow. The difference between the maximum and minimum clearances defines the extent of the tolerances. On companion elementary surfaces, the difference between the maximum male limit and the minimum female
22
INTERCHANGEABLE MANUFACTURING
limit determines the minimum clearance, as shown in Fig. 2. On composite surfaces, careful study is required to determine which limit should be used. In fact, it is impossible in certain cases to have the minimum clearance conditions at all points at the same time. In general, however, the comparison of the basic sizes of companion parts gives the minimum clearance conditions. The minimum clearance is quite commonly known as the " allowance."
Maximum Clearance. On elementary surfaces, the difference between the minimum male limits and the maximum female limits establishes the maximum clearances. In general, the
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Graphic Illustration of the Meaning of the Terms Maximum and Minimum Clearance
terms maximum or minimum clearance refer only to the clear- ance between surfaces which operate together or within close proximity to each other. When surfaces stand well clear of each other, and there is little or no danger of interference, as between unfinished forged or cast surfaces, the matter of maximum and minimum clearance plays little part in determining the toler- ances.
Interference. If a male member is larger than a female mem- ber, it is obvious that there will be interference when these parts are assembled together. Such interference is required where
DEFINITIONS OF TERMS
force fits are specified. If interchangeable parts are to be forced together, this interference performs a similar function to that of clearance on operating surfaces. In this case, the minimum interference establishes the danger point. This means that for force fits the basic male dimension is the minimum limit requiring a plus tolerance, while the basic female dimension is the maximum limit requiring a minus tolerance. (See Fig. 3.) When the component drawings permit an interference where
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a clearance is required, they are wrong. The term interference is often used to express such conditions of error.
Operating Surfaces. The term operating surface is used to distinguish the working surfaces of the mechanism from the others. It is clear that the working surfaces are the essential ones; all others are present only because of the necessity of holding the mechanism together. Generally speaking, the oper- ating surfaces are the machined surfaces, while the others often retain their original forged or cast finish. The operating surfaces are divided into two classes, which are designated functional and non-functional, or clearance, surfaces.
Functional Surfaces. The functional surfaces are those oper- ating surfaces which control the functioning of the mechanism,
INTERCHANGEABLE MANUFACTURING
as shown in Fig. 4. These must naturally be held to the closest limits. Every operating part of a mechanism must be controlled in operation within reasonably close limits in each plane. After these functional requirements of location are met, all other surfaces should have as large clearances as possible, unless the factor of strength is the controlling one. Those surfaces that affect the relative location of the operating parts in operation are the functional surfaces. For example, the surface of a pad on which a bracket that carries operating parts is fastened is a
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Fig. 4. Illustration showing the Meaning of the Terms Functional and Clearance Surfaces
functional surface; whereas, the surface of a pad that supports a bracket for holding wrenches or oil-cans is not.
Clearance Surfaces. Clearance surfaces are those operating surfaces which are not functional surfaces. In this class are surfaces which do not control the location of operating members while functioning, but which either prevent them from being disassembled or locate them approximately in their inactive position, or both.
Atmospheric Fits. Atmospheric fits, as the name implies, refers to those surfaces which, under all conditions, stand entirely clear of any other operating or functional members of the mech- anism. Such is the outside of a machine frame. Many surfaces
DEFINITIONS OF TERMS
on operating parts are themselves also atmospheric fits. With few exceptions, the majority of the surfaces of all mechanisms are atmospheric fits.
Elementary Surfaces. An elementary surface is one which is defined with a single dimension, such as a cylinder, a plane, or a sphere. For example, a reamed hole of a speicfied depth rep- resents two elementary surfaces. The diameter defines one and the depth the other. Obviously, most surfaces which are not elementary in themselves are a combination of elementary surfaces. In so far as such surfaces are machined and measured according to their elements, they are considered elementary surfaces. When the combination as a whole is measured or
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Fig. 5.
Illustration showing the Meaning of the Terms Elementary and Composite Surfaces
machined, and a variation on one surface affects the dimension of another, they are not elementary but composite surfaces.
Composite Surfaces. Composite surfaces are those surfaces which are required to maintain a co-relation which cannot be expressed by a single dimension. For example, Fig. 5 shows a yoke end. The over-all dimension (2.500 inches) controls elementary surfaces. The dimension of the slot (0.750 inch) and that of its location (0.875 inch) also define elementary surfaces when used independently. If, however, surfaces marked A, B, and C are required to be checked concurrently, these elementary
26 INTERCHANGEABLE MANUFACTURING
surfaces become composite. Irregular profiles, the co-relation of several holes to each other, tapered surfaces, thread sizes and forms, the contour and location of gear teeth, etc., are examples of more complicated composite surfaces than the example shown in Fig. 5.
Compound Tolerances. A compound tolerance refers to those conditions where the established tolerances on more than one dimension determine the required limits. These exist in con- junction with the dimensioning of composite surfaces. For example, a compound tolerance exists in establishing the location of surface C in Fig. 5 from surface A. This condition of com- pound tolerances will be covered in greater detail in a subsequent chapter.
Working or Register Points. The working or register points are those surfaces that are employed for locating the parts in the jigs and fixtures during the process of manufacture. Sometimes important functional surfaces are used for this purpose. In other cases, for parts of irregular form, special lugs are provided to serve this end. These are removed after the machining operations are complete. Register points become functional surfaces when they are employed to machine other functional surfaces. As few locating points as possible should be estab- lished; this practice simplifies the design of the gages and other equipment.
Unit Assembly. Many mechanisms are a combination of several semi-independent mechanisms which may be assembled and tested individually before they are assembled together. This is known as unit assembly; it is of particular value for a plant which manufactures a varied product where such unit assemblies are interchangeable. An example would be a feed- box that could be used on several types of machines. This practice enables a plant to obtain the benefits of quantity pro- duction on these unit assemblies although the quantity of pro- duction on any one type of machine is small.
Precision. There are two characteristics pertaining to the physical dimensions of the parts manufactured. For purposes of discussion, they will be called precision and accuracy. The
DEFINITIONS OF TERMS 27
two are often considered identical, and if ideal conditions could be maintained, they would be identical. In ordinary manu- facturing practice, however, precision alone is usually obtained, and precision is all that is necessary in most cases. It is when several factories attempt independently to produce a common interchangeable product that accuracy is required. The degree of precision is measured by the amount of variation that exists between duplicate parts. For example, a reamed hole is dimen- sioned as 0.500 inch in diameter. In manufacture, a product is obtained in which the difference between the largest and smallest holes produced does not exceed 0.005 inch. The degree of pre- cision in this case would be 0.005 mcn> even though the absolute size of the largest hole was 0.508 inch.
Accuracy. The accuracy of any determination is measured by its limits of error from a fixed standard. For example, a length of one inch is to be measured. We will assume, for the sake of argument, that the measuring instruments are absolute. This length measured with an ordinary steel scale gives a result correct within a limit of error of about o.oi inch. If measured with a micrometer, the result is correct within a limit of error of about 0.005 incn- If measured on a sensitive measuring machine, the result is correct within a limit of error of about o.ooooi inch; while if measured by optical methods, advantage being taken of the principle of interference of light waves, the result is correct within a limit of error of o.oooooi inch or less. It may be of interest to note here that standards of length have been defined by the Bureau of Standards in terms of light waves. By this means, an absolute standard is established, since the lengths of light waves are absolute. The term accuracy implies a comparison with a fixed standard. In the example given to illustrate precision, the limit of precision is 0.005 inch, while the limit of accuracy is 0.008 inch. It is obvious from this that it is more difficult to maintain a limit of accuracy of o.ooi inch than it is to maintain a limit of precision of the same amount.
Component Drawings. Component drawings are detailed drawings of the component parts of a mechanism. For inter- changeable manufacturing, these drawings show the completed
28 INTERCHANGEABLE MANUFACTURING
dimensions required (or inspection gage requirements) of the parts. These include all tolerances and any sub-assemblies that may be necessary to assist in their proper interpretation.
Operation Drawings. An operation drawing is a detailed drawing or sketch which gives the dimensions required on an individual machining operation. These drawings are used to record much supplementary information that might be confusing or misleading on the component drawings, such as allowances for finishing cuts and grinding operations, etc. •
CHAPTER III
MACHINE DESIGN IN INTERCHANGEABLE MANUFACTURING
THE improvement in manufacturing methods and facilities during the past forty or fifty years has been very rapid. Quantity production is now the order of the day. New problems have arisen and old ones must be constantly re-solved to meet the situations thus created. Manufacturing on an interchangeable basis has been a direct development of this advance. The pur- pose of the present chapter is to discuss the effects of this develop- ment on the design of a machine or device, and to emphasize those practices which will promote economical manufacture on an interchangeable basis.
In early days, the design of a new mechanism existed only in the mind of the mechanic engaged in its construction. It was made piece by piece, each detail taking definite shape as it was constructed. The original mechanism was completed, tested, and corrected or rebuilt before the design was finished. Dupli- cate mechanisms that might be constructed were patterned after the original, and modified or improved as suited the ideas of the mechanics who performed the actual work. Needless to say, interchangeability and quantity production were non- existent factors.
Sketches and drawings were next employed to express the ideas of the inventor, but little attempt was made to indicate more than the general idea and construction. The details of the design and the dimensions of the individual parts were matters for the workmen to decide. A competent mechanic was required to determine these for himself. It was part of his training. Details and dimensions, more or less complete and consistent, made their appearance on the drawings later. Errors and omissions were of little moment, as the mechanic who worked to the drawings expected to select and use the proper infor-
29
30 INTERCHANGEABLE MANUFACTURING
mation given and to ignore the incorrect, supplying all omissions from his own store of mechanical knowledge and experience. The quantity of production was small. Little or no importance was placed on interchangeability. A few workmen thoroughly acquainted with the requirements of the mechanism or with the intentions of the designer performed all the actual work of con- struction. Under these conditions, functional drawings, which make no pretense of giving more than the general construction or combination of mechanical movements and- the general out- line of the detailed parts, are sufficient.
Function of Design. Under present manufacturing conditions, with productive operations subdivided into elementary tasks, with productive labor trained along specialized lines, with pro- ductive equipment specialized and more nearly complete, with the rate of production greatly increased, with larger organiza- tions in which but few individuals are thoroughly conversant with all the detailed requirements of the mechanism, the design must cover a wider field and be much more comprehensive and accurate. In addition to expressing the ideas of the inventor, it must supply most of the knowledge and experience formerly brought to this work by the mechanic.
We now have, therefore, two types of designing to consider, which we will call the functional design and the manufacturing design. The manufacturing design is a detailed development of the functional design. It corrects and modifies the functional design where necessary, to facilitate the economical production of the mechanism, giving as much as possible of the information previously supplied by the workman. It is evident that the manufacturing design will always be incomplete to a certain extent. Suitable provision for its modification must be made to obtain the advantage of the new and improved methods of manu- facture which are constantly developed. Changes, however, in proved manufacturing designs should be avoided when pos- sible. As much or greater care should be taken in adopting changes as is exercised in establishing the original manufacturing design. After equipment has been completed, changes are very costly. A change which might be justified in the early stages of
MACHINE DESIGN 31
work often costs more than it is worth in the later stages. This makes it of the utmost importance that great care be exercised in the development of the original manufacturing design of a new commodity which is to be manufactured in large quantities on an interchangeable basis.
Classes of Design. For the construction of a small number of special machines, or tools and fixtures, which are built in a general machine shop or tool-room, the functional design is all that is required. The number of men engaged in the produc- tion is small, their training is general, and the requirements of the mechanisms can be explained to them personally by the designer as questions arise; therefore, the additional expense of a manufacturing design is not justified. However, in the manufacture of a large quantity of any article, particularly if interchangeability is sought, a complete manufacturing design is necessary. True, this design will work itself out in practice in the course of time, but this is a very slow and expensive method. It means that experimental work on a large scale is carried on, whereas it can be done on a smaller scale with better and speedier results. Furthermore, this method results in con- tinual alterations in the equipment and a loss of interchange- ability. However, this chapter is not concerned with designs of special mechanisms, tools, fixtures, etc.; attention will here be given to the requirements of designing as applied to the manu- facture of a product in large quantities.
In both types of designing, the end in view is the same as far as the functioning of the mechanism is concerned. This is to develop a product capable of performing certain results which will fill or create a public demand for itself. The means of at- taining this are governed by various considerations. For the functional design, any solution is satisfactory. As regards the manufacturing design, the methods adopted must result in ultimate economy. Also, the manufacturing design must re- semble the functional to such an extent that all patents will be retained, while those of competitors must not be infringed. This is one of the commercial difficulties that, at times, prevents the true economic development of a commodity.
32 INTERCHANGEABLE MANUFACTURING
It is plain that the manufacturing designer must take into consideration every circumstance involved in the production of the commodity. To be successful, he must work in close cooperation with all who will be engaged in the development and operation of the manufacturing equipment. This will include the tool designers, and the superintendents and foremen of the various manufacturing and assembling departments. In general, there is too much detail involved for any one person to carry it alone to a successful completion.
Simplifying Design. When considering the manufacture of a new product, one of two conditions usually obtains. Either it is to be produced in an established plant with an existing variety of manufacturing equipment, or a new plant must be created. In the first case, the designer must be familiar with the available equipment and must modify the functional design so as to utilize these facilities to the best advantage. In the second case, he is not restricted to the use of any specified equip- ment. In either case, unless the volume of production is to be extremely large with many automatic operations, every effort must be made to reduce the machined surfaces of the various com- ponents to simple, elementary surfaces which can be readily machined on standard machine tools with simple, rugged, and inexpensive tools, jigs, and fixtures. If, in the manufacturing design, the component parts are thus simplified, a further advantage is gained. The productive operations on these parts are resolved into simple, elementary tasks, and this simplifies the problem of securing and training the necessary productive labor. Simplicity is a primary source of economy. The number of machining operations is reduced and the direct labor cost thereby lowered. The amount of time that raw material is tied up in process of manufacture is reduced and quicker re- turns are secured on the money invested in direct labor and materials. The many other economies resulting from simplicity in design, such as lower equipment and maintenance costs are obvious.
Factors Governing Choice of Materials. Those responsible for the manufacturing design must pay close attention to the
MACHINE DESIGN 33
character of the materials they specify for the individual com- ponents. Ultimate economy is the desired end. This is affected by many different and sometimes opposing factors.
Cost. The first cost of the material is one of these. When several thousand duplicate mechanisms are manufactured, the slightest saving in the cost of direct materials is multiplied over and over again in the course of time. As many parts as possible should be made of the same size and kind of material. This permits purchasing in larger quantities and reduces the gross amount of raw material carried in the store-room. As far as possible, this material should be of standard sizes and forms that can be purchased in the open market at the lowest prices.
Source of Supply. Due consideration must be given to the possible sources of supply for the materials specified. It is a serious matter when production is held up because of lack of material which has a limited or uncertain source of supply. Every effort must be put forth, in making the manufacturing design, to specify materials which are readily secured.
Machining Qualities. The actual economy of low-priced mate- rial is governed by the ease with which it can be machined. If a part requires many machining operations, a low initial cost for material is often overbalanced by the greater cost of manu- facture. Therefore, if a more expensive material can be machined at a lower cost, ultimate economy dictates its purchase. For this reason, the use of extruded or rolled bars of special form is often adopted in the manufacture of small parts for adding machines, typewriters, counters, and other similar mechanisms.
An illustration of this point occurred in a large plant which makes small duplicate parts. Several of these parts were made of brass castings because of the lower cost of machining, but the price of copper began to rise and was soon about double its normal price. It was decided to substitute cast iron for brass because the difference in the cost of machining was less than the difference in the market price of the materials. Luckily, an investigation was made before the change went into effect. This plant had its own brass foundry but no iron foundry. It was discovered that the foundry had purchased no copper
34 INTERCHANGEABLE MANUFACTURING
for several years. In fact, a large stock of pig copper had been stored in a shed and was never touched. Another department of this plant was engaged in making copper matrices by a plat- ing process, and the trimmings from these supplied all the pure copper which the foundry required. This, with scrap brass stock from other departments, made it unnecessary to purchase any metal for the brass foundry in the open market. Needless to say, no change was made in the material of the castings. This incident indicates in some degree the many factors that must be considered to secure genuine economy. It is not a matter of mere addition and subtraction; every existing con- dition must be taken into account.
Weight of Finished Product. Whenever the weight of the finished product is an important consideration, as with auto- mobiles, etc., the materials used in making it must be of a better grade than when the weight is less important. In every case, the materials specified must be sufficiently strong and rigid to hold their form throughout the normal life of the mechanism. Thus, the detailed design of the various components is governed to a great extent by the nature of the materials which are used in their manufacture. For example, if forged steel is substituted for cast iron, the component will be of much lighter design.
Service Required. The composition of the materials used is governed by the nature of the service which the part must render. One that is subjected to excessive wear must be made of material hard or tough enough to withstand it. Material for parts liable to corrosion or other chemical action must be of the proper composition to counteract it. Material for parts under constant vibration must not crystallize readily. In every event, the materials must be selected to withstand both the use and abuse which they will eventually meet.
It is of interest to note, as an indication of the importance of materials in relation to the total cost of production, that census statistics show that the cost of materials — direct and indirect — is from 30 to 60 per cent of the selling price of the majority of mechanical products which are manufactured in this country.
MACHINE DESIGN 35
Clearances and Tolerances. The establishment of suitable clearances and tolerances is a vital, if not the most vital, factor in the manufacturing design. Tolerances are, in many respects, like laws. There are two classes of laws. One is so severe and exacting in its nature that it cannot be enforced, and soon falls into disrepute and is disregarded, even though it remains on the statute books. The other is drawn up with a full understanding of existing conditions, and its justice to all concerned is so evident that it is readily and consistently enforced.
Similarly, tolerances fall into two classes. Those which represent the extreme conditions of accuracy obtainable from the equipment under ideal conditions can be specified without regard to the functional requirements of the product. In such cases they, too, soon fall into disrepute and are disregarded, even though they still remain on the drawings. On the other hand, tolerances are readily and consistently maintained when they represent the widest variations that the functioning of the mechanism will safely permit.
Liberal tolerances and clearances result in easier manu- facturing conditions of every sort and thus promote economy; they make quantity production possible. The serviceable life of tools depends directly on the extent of the tolerances. Every exacting tolerance is a direct check on the economical and rapid production of the mechanism. On the other hand, if the functional requirements do not permit wide tolerances, the functional requirements must prevail.
It is evident, then, that the construction must be carefully studied so that the manufacturing design will permit the widest possible tolerances. It is only in exceptional cases that a mechan- ism cannot be modified so as to retain all functional advantages and yet allow liberal tolerances on the majority of its dimen- sions. Very often, when there is a severe functional requirement to maintain, the introduction of simple means of adjustment promotes easier manufacturing conditions. In other cases, a system of selective assembly is more desirable.
Applying Interchangeable Principle. The designer must de- termine which parts will be interchangeable. Interchangeability
36 INTERCHANGEABLE MANUFACTURING
can be carried too far and thus allowed to defeat its own purpose as noted in a previous chapter. Interchangeability and liberal maximum clearances are closely connected. Whenever reason- able clearances are out of the question on certain components, these parts are not suitable ones to be manufactured on an inter- changeable basis. In this matter, the relative accuracy of the available equipment plays a large part. For example, if the surfaces are elementary and can be finished by a simple grinding operation, much closer tolerances can be economically main- tained than if they are composite and require milling or turning operations. The variations on work finished by grinding are about one-third those resulting from milling and one-half those from turning; and the effort expended is no greater. On the other hand, grinding is not always suitable nor possible. There- fore, in determining whether or not certain required conditions permit reasonable tolerances, the designer must consider pos- sible methods of manufacture and must be well informed regarding the normal variations which result from them in actual practice.
This knowledge is the outcome of experience in checking and analyzing results previously secured. This is a matter to which little attention has been paid in the past. For example, in a large and long-established plant, where many milling operations are performed, it had been assumed that these operations were maintained within a tolerance of o.ooi inch. Actual measure- ments brought out the fact that the normal variation was over three times as great as that, and always had been. A similar misconception of actual conditions was apparent in the majority of shops engaged in government work during the recent war. When their product was actually checked by limit gages and held to the specified tolerances, a variation of 0.002 or 0.003 incQ was found to be an extremely small manufacturing tolerance. It is, therefore, one of the duties of the maker of the manu- facturing design to specify the parts which are to be made inter- changeable, those to be selectively assembled, and those to be fitted to each other. Careful attention to this detail saves much wasted effort in the shops subsequently.
MACHINE DESIGN 37
Advantages of Unit Assembly Construction. Almost every mechanism can be subdivided into smaller units which are dis- tinct in their purpose. For example, an automobile contains an engine, transmission, axle drive, carburetor, magneto, etc., which are assembled and tested as units and later assembled into the completed car. In like manner a typewriter is sub- divided into the carriage, the escapement, the type-bar and the segment assembly, etc. The assembly is greatly facilitated if the design of the mechanism permits such unit assembly con- struction; and efforts should be made to obtain this result whenever practicable.
There are many other advantages of this unit assembly con- struction. Not only the various manufacturing departments of one factory but also entire plants are specializing more and more. The automobile has hastened this trend more than any other one thing. Where such unit assemblies are of equal value on several articles, separate plants spring up to produce them as a specialty. This gives the benefits of quantity production where otherwise they would not exist. Therefore, as a direct result of unit assembly construction, there are separate plants specializing in engines, rear axles, carburetors, magnetos, etc., for automobiles; ball and roller bearings for all types of machin- ery; and many other similar specialized products.
Standardization of Parts. Another practice which allows the benefits of quantity production to be obtained in the production of smaller numbers of complete mechanisms is the standard- ization of many of the individual components. For example, most manufacturing concerns have standardized their screws, nuts, studs, rivets, and others mall parts. The majority of machine tool builders also standardize their handwheels, mi- crometer thimbles, gears, tool-holders, work-arbors, etc. A good illustration of the economy of this practice is found in the ex- perience of one plant which originally manufactured over one hundred and fifty special screws and studs for its particular product. Little effort was required to reduce this number to less than half, thus increasing the rate of production of these parts and also reducing the stock of spare parts. This practice
38 INTERCHANGEABLE MANUFACTURING
is extending to larger and more important components. Not only are similar parts produced by individual plants being standardized, but parts used in common by several manufactur- ers are also standardized and often manufactured as specialties by other concerns.
Designing for Assembling and Service. The design must per- mit the ready assembly of the product. Parts which require attention in service must be accessible. .Attention to these details reduces assembling and service costs, and these must be considered to insure ultimate economy.
The service requirements are the most difficult to determine. Time alone brings the desired information. Experiments and endurance tests in the factory are insufficient to give it. After a mechanism is on the market, it receives use and abuse that the makers never dreamed of. Yet if the product fails under these unforeseen conditions, the manufacturing plant is blamed. Naturally the nature of the commodity determines what sort of service it must render. The service requirements of an automobile are distinct from those of a typewriter; those of a precision machine tool — which is supposedly used by skilled mechanics only — differ from those of a lawn-mower; etc.
The service requirements include the protection of the working parts from dirt and other foreign matter, the provision of proper lubricating facilities, and the protection of the operator from moving parts. The question of the best preservative finishes, such as japanning, plating, painting, etc., must also be answered to meet the service requirements, both of use and appearance. For these and many other similar problems a solution is sought that will result in the maximum amount of service at a minimum expense.
It should be clearly understood that the manufacturing design is not undertaken with the idea of wilfully altering the functional design, but is*made to facilitate manufacture and to furnish as much as possible of that vast amount of detailed information previously brought to the productive work by the mechanic who carried out the inventor's ideas. The alterations made in the functional design by the manufacturing design should not
MACHINE DESIGN 39
be looked on as any criticism of the original lay-out. Each has its distinct purpose to perform. Many large plants recognize clearly the difference between the two types of designing and maintain separate departments for each. The original research and inventive work is carried on independently of the factory operations. New or improved designs are turned over to the factory organization where they are redesigned to meet the manufacturing and service needs.
CHAPTER IV PURPOSE OF MODELS
A MODEL mechanism, constructed personally by the inventor, or by the workmen under his immediate direction, was the original form of making and recording a new design. The introduction and development of mechanical drawings superseded many of the functions previously performed by the model. At the present time, therefore, the practice of developing models has been relegated to a comparatively insignificant place in most lines of manufacturing. They are still employed to a limited degree, however, by several manufacturers for a variety of purposes.
The primary purpose of any model at the present time is to prove — not to originate — a new or improved design that has been developed only on paper. It may be either to prove the possibility of the functional design or to check the manu- facturing design. This may be done by a single mechanism in some cases, or several duplicate mechanisms may be required to prove its operation under various service conditions.
Manufacturing models may be used for one or more of the following purposes: First, to check the operation of the manu- facturing design against the experimental model; second, to prove the manufacturing design in regard to the service require- ments; third, to test the* manufacturing tolerances which may be contemplated; and fourth, to create a physical standard of precision for future manufacturing.
Manufacturing Model to Test Functioning. A manufacturing model used to test the functioning of the manufacturing design is merely a sample mechanism constructed in the tool-room or machine shop to detect as many faults as possible in the design or to discover possible errors in the component drawings. It is essentially a precautionary measure. It is more economical to
40
PURPOSE OF MODELS 41
detect and correct a fault on one sample than it is to salvage a large number of parts after production is started, with the addi- tional expense of correcting the manufacturing equipment. After such a model has demonstrated the success of the manu- facturing design and the correctness of the component drawings, its purpose has been achieved. Its future disposition is a matter of little moment.
Such a model is seldom necessary on simple mechanisms that are merely new combinations of old and proved mechanical movements, or on minor variations of proved designs, such as standard motors, dynamos, various types of engines, and many machine tools. The actions of such mechanisms under many conditions have been so well established that practically all of the necessary experimental work can be accomplished on paper. On many other mechanisms, however, such as typewriters, add- ing machines, small arms, watches, etc., the mechanical move- ments of which are delicate and intricate and not so positive, manufacturing models are a vital necessity. In general, such a model will be constructed when the insurance against the possi- ble errors in the design is worth the expense entailed. For this reason it is often customary to build a pilot machine before putting through a lot of new or special machines.
If a new commodity is designed, particularly if it is to fill a new demand, it is advisable to determine its action in actual service before extensive productive operations are far advanced. The only sure method of obtaining this information is to have one or more mechanisms built and operated under the condi- tions with which they are expected to contend. The manu- facturing model, which is built to test the manufacturing design, is often used for this purpose. As a matter of fact, the test for functioning should also include the tests for service require- ments, inasmuch as this factor of functioning should include the measure of service which the mechanism must render throughout its normal life.
For example, a large plant in the Middle West goes thor- oughly into this preliminary work on all new models. Three successive designs are developed and tested. First, the func-
42 INTERCHANGEABLE MANUFACTURING
tional or inventive design is made and tested. Second, the manufacturing design is carefully developed and tested by means of a manufacturing model. When this last design seems satisfactory, it is turned over to the tool designing department which goes over it a third time solely to simplify the tooling and mechanical productive operations. The changes made at this time, however, affect minor details only. From twenty-five to fifty mechanisms are built to this design and sent into the field for actual service. These last models must give satisfactory service for from one to two years before further preparations for manufacture are considered. It is of interest to note that the manufacturing equipment provided by this factory is com- plete; also that changes here in the process of manufacture or in the product under production are very rare.
Many of the so-called improvements in new commodities which result in frequent modifications of the product under manufacture are only steps taken to correct mistakes, omis- sions, and other faults due in large measure to neglect of the manufacturing design (both neglect to make and neglect to test it) because of haste to rush into actual production. This has been forcibly brought out by the conditions which developed in the manufacture of many devices during the war.
Models to Test Tolerances. It is desirable to know at the earliest possible moment whether or not the specified tolerances define the limits of parts that will function properly. The sooner this information is obtained, the sooner can efforts be concentrated on problems of production alone. Until this matter is settled within a reasonable degree of certainty, each problem in production is complicated by many considerations relating to the design and tolerances. This causes innumer- able revisions on the component drawings with the attendant changes in tools, fixtures, and gages, resulting in delays in production and additional expense.
Some concerns try to solve this problem by carefully build- ing several models which represent as closely as practicable the extreme conditions permitted by the component drawings. These model parts are assembled and reassembled, and tested
PURPOSE OF MODELS 43
for operation after each assembly. Necessary alterations of the drawings are made before manufacturing operations are under way. This practice is, naturally, expensive, as each of these models costs much more to construct than any of the preceding ones. However, if insurance against future changes is worth the expense, the practice is well worth while.
One typewriter manufacturer makes a practice of building from six to ten sets of model parts before any new or revised machine is manufactured. When only one unit assembly is affected, model parts for that mechanism only are made. These parts are then sent to the assembling department for trial. Except on purely experimental models, the men who make the parts are not allowed to assemble them. No effort is made to do anything more than to duplicate the kind of work normally produced in the manufacturing departments. The parts are not cornered or burred unless that operation is required in manufacture. In other words, the attempt is made to determine how little effort will be required to manufacture the mechanism satisfactorily. The sizes of these parts usually cover the entire range between the specified limits, but no distinct effort is made to have them meet either limit exactly. Any combination of these parts must assemble and operate properly before changes or new models are adopted. This practice has saved the com- pany several times from making unnecessary and improper changes.
Model for Standard of Precision. The component drawings give many dimensions. Strictly speaking, the expressed di- mensions represent absolute sizes. Dimensions of elementary surfaces can be produced and reproduced within relatively small limits of error. Often, however, these dimensions define de- veloped and complicated profiles, locations, and other com- posite surfaces which cannot be reproduced as readily. Yet they must be reproduced many times over in the course of manu- facture to within relatively small limits of error.
A choice must be made between accuracy and precision at the outset. In the case of elementary surfaces, accuracy is usually the better choice; but for many composite surfaces,
44 INTERCHANGEABLE MANUFACTURING
precision will often give the quicker, more economical, and more practical results. On the other hand, it must be clearly understood that when precision is chosen, it becomes prac- tically impossible for another plant, working in entire indepen- dence of the first, to produce a common interchangeable product. If more than one plant is engaged on the production, they must maintain close relations in almost every detail of the work. In order to maintain this precision within reasonably close limits, physical standards of some sort must be provided at the very beginning. By developing a model for this purpose, two results can be accomplished at the same time. Such a model will test the manufacturing design for functioning, and will also provide the desired physical standards.
This model must be made with the greatest care and should represent the "danger zone" — that is, in most cases, the maximum metal or minimum clearance conditions. It is evi- dent that these sizes are the most important. They control the interchangeability. No cuts, other than slight cornering and similar burring operations, should be made with a hand tool, such as a file. Whenever the contour of the surface in question is important, templets should be made to check the special tools used. These templets are an integral part of the model. All important locations of holes should be established from master plates. Templets, master plates, etc., as well as the model parts, are invaluable when the equipment is built; properly utilized, they will insure a high degree of uniformity at relatively small expense. Such a model must be used with the greatest care and becomes the court of last appeal in many of the perplexing questions which inevitably arise in the manu- facturing departments of a plant engaged in producing an inter- changeable product in large quantities.
This practice in regard to models is extensively followed in the manufacture of small arms. As stated before, it has a certain disadvantage when more than one factory is involved. As it is impossible to duplicate the model exactly, one of two courses is open. Either one model is standard for all plants — which entails much lost time in referring many detailed ques-
PURPOSE OF MODELS 45
tions back to the central plant — or additional models may be made, which results in different basic standards at the vari- ous plants. If the second course is followed, and all parts of all models are mutually interchangeable, the product of the various factories will be interchangeable. However, this results in reducing the amount of the absolute tolerances available for manufacturing variations, as the variations in the different models consume a certain amount. In cases where the func- tional conditions are exacting, this method is often found im- practicable. On the other hand, if the design of the mechanism is such that the absolute tolerances are liberal, the second method gives an economical solution.
All the foregoing model work, regardless of its purpose, is essentially a preliminary measure in the manufacture of a new or revised product. Properly conducted, it will stabilize the manufacturing design at a minimum expense. It takes con- siderable time, however, and that is one reason why models are not more extensively employed. For any commodity that is already under manufacture and the design of which is already standardized, models are of doubtful value.
CHAPTER V PRINCIPLES IN MAKING COMPONENT DRAWINGS
THE art of expressing mechanical information by means of drawings is still in the process of evolution. Many details have become conventionalized, yet these comprise little more than the alphabet of the language of drawings and relate principally to conventional meanings of the lines, figures, and relative locations of the several projections which go to complete the drawings. Such, for example, are the full lines which represent the visible outlines of the part; the dotted lines which represent the hidden outlines; the light dot-and-dash lines which indicate center lines; the light dimension lines — and all the other con- ventional lines and characters which are employed. The third- angle projection is also fairly well established in mechanical drawing. This branch of drawing is fully covered in text-books, so no further mention of it will be made here. The subject of dimensioning, however, is so incompletely covered that this chapter will be devoted to a detailed discussion of this subject. The addition of tolerances on component drawings has created new problems which have not, as yet, been fully solved, and which, therefore, require considerable and thoughtful study.
The matter of dimensioning, as given in books and taught in various schools, receives only minor attention. Little more than the a b c of the subject is taught. In actual practice — particularly where tolerances are involved — so many different conditions are to be met, so many different shades of meaning must be clearly expressed, and so many different types of work- men must be informed by these drawings that this alphabet must be fully understood and carefully used to enable it to serve its purpose. It is necessary, in order to consider intelligently this subject of dimensioning with tolerances, to discard all school training in the application of dimension lines, etc.
46
COMPONENT DRAWINGS 47
The main purpose of a mechanical drawing is to express or record information. This information is of many kinds and is used for many purposes. The drawing, to be correct, must clearly and consistently express the particular information re- quired to serve its specific purpose. For example, a type of drawing that may be correct for the use of a toolmaker in build- ing a jig may be incorrect for the use of a machine operator in the manufacturing department engaged in quantity production. Inasmuch as the drawings are the written or pictured expres- sion of the design, they may be roughly classified into func- tional drawings and manufacturing or component drawings.
Functional Drawings. The functional drawing, like the func- tional design, primarily expresses the functional conditions to be maintained. The detailed information relating to many of the manufacturing problems that are involved which does not appear on these drawings is supplied by the mechanic who uses them. Thus, in those cases where only a few special mechan- isms, or jigs, fixtures, tools, etc., are to be made in a general machine shop or tool-room, where the type of workman is such that this detailed information is unnecessary, functional draw- ings only are required. Such drawings need not express toler- ances, clearances, and other minor details so essential on the manufacturing drawings. For example, a notation such as "drive fit" or " sliding fit" is sufficient to indicate and obtain the desired results. Yet, even here, if the drawings are to serve their purpose efficiently, the information given must be so expressed that it may be used directly. In order to attain this end, every line drawn and every dimension expressed must be made with a full understanding of the final results required and of the means to be employed to obtain them.
Manufacturing Drawings. The manufacturing drawings, to be complete, must express all suitable information that is avail- able. For the purposes of the present discussion, we will con- fine ourselves to component drawings of an interchangeable product. As stated in a preceding chapter, the proper dimen- sioning of component draiwngs with tolerances is a mathe- matical problem. Five laws are given, which, if carefully
48 INTERCHANGEABLE MANUFACTURING
observed, will simplify many of the equipment and production problems.
Laws of Dimensioning, i. In interchangeable manufactur- ing there is only one dimension (or group of dimensions) in the same straight line which can be controlled within fixed toler- ances. This is the distance between the cutting surface of the tool and the locating or registering surface of the part being machined. Therefore, it is incorrect to locate any point or surface with tolerances from more than one point in the same straight line.
2. Dimensions should be given between those points which it is essential to hold in a specific relation to each other. The majority of dimensions, however, are relatively unimportant in this respect. It is good practice to establish common location points in each plane and to give, as far as possible, all such dimensions from these points.
3. The basic dimensions given on component drawings for interchangeable parts should be, except for force fits and other unusual conditions, the maximum metal sizes. The direct com- parison of the basic sizes should check the danger zone, which is the minimum clearance condition in the majority of cases. It is evident that these sizes are the most important ones, as they control the interchangeability, and they should be the first determined. Once established, they should remain fixed if the mechanism functions properly and the design is unchanged. The direction of the tolerances, then, would be such as to recede from the danger zone. In the majority of cases, this means that the direction of the tolerances is such as will increase the clear- ance. For force fits, such as taper keys, etc., the basic dimen- sions determine the minimum interference, while the tolerances limit the maximum interference.
4. Dimensions must not be duplicated between the same points. The duplication of dimensions causes much needless trouble, due to changes being made in one place and not in the others. It causes less trouble to search a drawing to find a dimension than it does to have them duplicated and more readily found but inconsistent.
COMPONENT DRAWINGS
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tions, it is evident from the foregoing that a different product would be received from each plant. The example given is the simplest one possible. As the parts become more complex, and the number of dimensions increase, the number of different com- binations possible and the extent of the variations in size that will develop also increase.
Fig. 4 shows the correct way to dimension this part if the length of the body and the length of the stem are the essential dimensions. Fig. -5 is the correct way if the length of the body and the length over all are the most important. Fig. 6 is correct if the length of the stem and the length over all are the most important.
If the part is dimen- sioned in accordance with either Fig. 4, Fig. 5, or Fig. 6, the product from any number of factories should be alike. There is now no excuse for them to misinterpret the meaning of the drawing. The point may be raised that the manufacturer should study the drawing to deter- mine what his sequence of operations should be in order to main- tain all dimensions and tolerances given. On such a simple part as was given for the first example, this would not be difficult. On a more complicated piece, however, it would be almost im- possible. Such conditions occur when the draftsman makes little or no effort to reduce as many surfaces as possible to elementary ones. Furthermore, when the manufacturer or workman sees such dimensions on a component drawing, he is justified in as- suming that the designer or draftsman who made them had little or no idea as to the essential conditions to be maintained. In such cases, the sequence of operations and the register points for machining will be established to facilitate production, or to suit the ideas of individuals as to the most essential conditions.
Machinery
Fig. 7. A Third Interpretation of Dimen- sioning in Fig. 1
INTERCHANGEABLE MANUFACTURING
Often, this will result in some of the operations on a component being arranged to suit one idea, while the remainder are com- pleted in accordance with an almost diametrically opposed con- ception. It cannot be too strongly impressed upon the drafts- man that when a drawing leaves his hands it must not be open to more than one interpretation. This, in turn, demands that a uniform method of interpretation be adopted and published by each plant for the guidance of all concerned. It is self-evident that a universal method of interpretation of drawings with tolerances would be of great benefit to all manufacturing plants.
ON EACH SIDE
WIN. CLEARANCE 0.005 MAX. CLEARANCE 0. 025*'
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(MIN. CLEARANCE 0.002" IMAX.CLEARANCE 0.0121'
Machinery
Fig. 8. Sketch showing Functional Requirements of Slide
This is a field where the various engineering societies, working in close cooperation, could render valuable service.
Violation of the Second Law. Let us take as the second example the slide shown in the sub-assembly, Fig. 8. This sketch gives the functional conditions which must be main- tained. It is well to note that it is a very desirable practice to add to a set of component drawings a series of sub-assemblies of this kind. These would show graphically the functional requirements of the most important operating members of the mechanism, when the detail drawings are insufficient, in them- selves, to express them clearly. Such a practice will prove of great assistance in limiting the interpretation of the component drawings.
Fig. 9 illustrates a common method of dimensioning such details. This is wrong, as it violates the second law previously stated. These parts are dimensioned in Fig. 10 in accordance
COMPONENT DRAWINGS
53
with the foregoing laws. It will be noted that all dimensions for height are given from the bearing surface A, which is the most important in this case. If the slide should be designed to bear at B instead of at A , surface B would become the most important, and the various dimensions of height would be given from there instead of from A. The same functional conditions (see Fig. 8) are maintained in Figs. 9 and 10. Attention is
Marhincry
Fig. 9. Incorrect Dimensioning of Slide shown in Fig. 8
Fig. 10. Proper Dimensioning of Slide shown in Fig. 8
called to the fact that in Fig. 10 it is possible to allow a toler- ance of o.o 10 inch on the dimension to the top surface B, whereas in Fig. 9 only 0.005 mch can be allowed as a manufacturing tolerance when making this cut.
Increasing Possibility of Draftsman's Errors. Thus, the im- proper and careless dimensioning of component drawings results directly in reducing the manufacturing tolerances, in addition to creating uncertainty by not indicating the essential surfaces. Furthermore, the possibility of draftsman's errors is greatly
54
INTERCHANGEABLE MANUFACTURING
increased by dimensioning as shown in Fig. 9 because the drafts- man, in this case, niust make several additions and subtractions of basic figures and tolerances in order to check the maximum and minimum clearances. In Fig. 10, on the other hand, the direct comparison of the basic dimensions checks the minimum clearance. The maximum clearance is readily checked by adding the sum of the tolerances to this minimum clearance. In general, the direct comparison of the basic dimensions should establish the minimum clearances between elementary surfaces on com- panion parts.
No mention has been made of the dimensions of width in the previous example. Strictly speaking, dimensions so given
Machinery
Fig. 11. Graphical Illustration of Application of Tolerance
are central with the center line. Half of the tolerances for width may be utilized on either side of the center line. This does not mean that the surfaces must be absolutely central; one side can be made to the maximum dimension and the other side to the minimum. In general, the tolerances should be understood to establish a parallel zone of acceptable work, all parts falling within this zone being acceptable. Fig. n illus- trates how the dimensions and tolerances in Fig. 10 establish such a zone. The full lines show the basic or maximum metal conditions, while the dotted lines show the minimum metal conditions.
Inspection Gage Requirements. In the previous example, each surface has been considered as an independent elementary surface, and the meaning of the drawing interpreted accordingly. But there is also a certain condition of alignment which these
COMPONENT DRAWINGS 55
various surfaces must maintain in relation to each other. When considering this phase of the subject, the surfaces become com- posite. Whenever composite surfaces are involved, the func- tional requirements of these surfaces must be taken into con- sideration. The only satisfactory method of solving such problems is in terms of the inspection gage requirements. If the succeeding solutions are accepted, the accompanying inter- pretations, expressed in terms of functional gages, must also be accepted.
To a certain extent, the amount of tolerance required to machine a given surface depends on the methods employed to check the results obtained. For example, the maximum thick- ness of the tongue of the slide shown in Fig. 10 is 0.623 inch. If this thickness is checked with an ordinary snap gage, prac- tically the entire tolerance is available for variations in thick- ness. If, however, the width of this snap gage were equal to or greater than the length of the tongue, any deviations in the surfaces checked from true parallel planes would tend to pre- vent the part from entering the gage. In this case, part of the tolerance would be consumed by the errors in alignment of the two surfaces, leaving the remainder for variations in the dis- tance between them.
One of the principal reasons for providing clearances in the design is to discount this condition of misalignment. In develop- ing functional gages to check these conditions, therefore, we are justified in utilizing a fair percentage of the minimum clearance. In order to insure strict interchangeability, the functional gage for the male component should never be larger than the func- tional gage for its companion female component. In general, if the functional gages never invade this minimum clearance more than fifty per cent, we shall remain on the safe side. Con- ditions sometimes arise, of course, where it is desirable to utilize a greater percentage on one component and a correspondingly lesser percentage on the other. For the purposes of this dis- cussion, however, we shall assume that the conditions are such that a maximum of fifty per cent for each component represents a fair distribution.
50 INTERCHANGEABLE MANUFACTURING
The dimensions for functional gages to check the parts shown in Fig. 10 are given in Fig. 12. The various dimensions of the parts should first be checked as elementary surfaces with limit gages representing the specified limits. This functional gage would then be employed to test the relative alignment of these surfaces necessary to insure interchangeability.
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Jl ( MIN. |CLEARANCE^0.002"^ON EACH
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/ALIGNING OR FUNCTIONAL ff SURFACE
HNH
N. CLEARANCE O.OIo'J AX. CLEARANCE O.OSo"
MAXJCLEARANCE 0.008" J SIDE
MIN. CLHARANCE 0.025||l E°gH MAX. CLEARANCE 0.045 J SIDE
!! (MIN. CLEARANCE 0.025" » ~nP>AX. CLEARANCE 0. 045 f
ON EACH SIDE
Machinery
Fig. 13. Sketch showing Functional Requirements of Dovetail Slide
Dimensioning Composite Surfaces. Thus far we have been considering parts whose surfaces are susceptible of individual checking as elementary surfaces. We must also consider parts whose surfaces cannot be resolved into elementary ones and checked as such. Take, for example, a dovetail slide, such as shown in Fig. 13, which introduces an angular surface. Such angular surfaces are almost always composite ones. Great care must be exercised in such cases to avoid compound tolerances.
COMPONENT DRAWINGS
57
A compound tolerance exists when the application of a toler- ance on one dimension develops a variation in another dimen- sion which also has a tolerance specified. Such a condition immediately raises the question as to whether the resultant variation of both tolerances is permissible or whether the toler- ances specified are final and complete for their respective di- mensions. In either event, confusion and misunderstanding will result. Here, as with the introduction of more than one dimen- sion in the same straight line (see first law of dimensioning) to locate a given surface, the final results will depend on the se- quence of operations adopted, with all the attendant differences.
Machinery
Fig. 14. Correct Dimensioning of Dovetail Slide shown in Fig. 13
As stated in the chapter " Principles of Interchangeable Manu- facturing," in making component drawings, the effort should be made to so give the dimensions and necessary tolerances that it would be possible to lay out one — and only one — • repre- sentation of the maximum metal condition and one — and only one — minimum metal condition. If such lay-outs were super- imposed, the difference between them would represent the per- missible variation on every surface. If a few such lay-outs are made, it will soon be evident that there are always a number of dimensions that should be given without tolerances.
A compound tolerance is an error — often a serious one. It can and should always be eliminated. Fig. 14 illustrates a method of dimensioning the dovetail slide shown in Fig. 13 which avoids compound tolerances. The dimensions that con- trol the position of the angular flanks are given to the sharp
INTERCHANGEABLE MANUFACTURING
corners at the top of the dovetail. The tolerances on these dimensions limit the permissible variation of these angular flanks. The angle is given as a flat dimension. As is evident from the functional drawing, Fig. 13, the bearing surface A and these angular flanks are the essential functional surfaces of this dovetail. All other surfaces are clearance surfaces, as should be apparent from the extent of the tolerances, Fig. 14, even though the functional drawing were not available. Fig. 15 shows graphically the applications of the tolerances given in Fig. 14. The full lines represent the maximum metal con-
Machlncry
Fig. 15. Graphical Illustration of Application of Tolerances
ditions, while the dotted lines indicate the minimum metal conditions.
Compound Tolerances. The dimensioning of tapered plugs and holes introduces a somewhat similar problem which will result in a condition of compound tolerances if great care is not exercised. Fig. 16 shows such a tapered hole as it is usually dimensioned. This method of dimensioning is wrong, as it creates a condition of compound tolerances. With these di- mensions, it is impossible to determine what final result is re- quired, since there are so many possible combinations. It is evident that as the diameter of either the large or the small hole varies, the taper will change. This makes an uncertainty about the reamers, as these tools have a fixed taper. If we assume that the taper is constant, questions will be raised as to which combination of limits to employ to establish the taper. If we further assume that the basic dimensions are to be used
COMPONENT DRAWINGS
59
for this purpose, the next question will be whether this taper, considered as a constant, is required to remain in the position indicated by the dimension i.ooo inch -o'S^> under all con- ditions, or whether it can also vary in addition by the amount resulting from the variations in diameters. Also a tolerance is given on the length of the taper. This is entirely meaningless. It cannot be measured readily even with an elaborate laboratory equipment and there is no use for this tolerance in the course of manufacture. With a fixed taper, the variation in this length is controlled absolutely by the relative size of the holes. All in all, as the drawing stands, it is a puzzle without any solution.
r
rnA'-f- 0.010., 500 — 0.000 "
Machinery
Fig. 16. Incorrect Method of dimensioning Tapered Hole
We will assume that the intent of Fig. 16 is to indicate a constant taper with a tolerance of -fo.oio inch in regard to its position. Fig. 17 shows the correct method of dimensioning such a surface to maintain such a condition. An arbitrary point is taken on the taper and a fixed dimension given for its diameter at that point. The location of this fixed diameter is dimensioned with the tolerance. Three methods of dimensioning this taper are shown. Either of the first two, A or B, is preferable to the third, C, because any reference figures desired can be readily computed from them without recourse to trigonometry or any tables or handbooks.
Fig. 17 gives the manufacturer definite information which he can use and which he can use in only one way. The tolerances
6o
INTERCHANGEABLE MANUFACTURING
given on each dimension apply only to the specific surface in question. No tolerance can be given on the diameter of the taper nor on the angle without introducing compound tolerances again, with resultant confusion. The permissible variations on this tapered surface are fully established by
TAPER 0.400 PER INCH
B
THREE METHODS OF DIMENSIONING ANGLE
Machinery
Fig. 17. Correct Method of dimensioning Tapered Hole
Machinery
Fig. 18. Graphical Illustration of Application of Tolerance
the tolerance given on its location. Fig. 18 shows graphically the maximum and minimum metal conditions established by the dimensions and tolerances given in Fig. 17. It will be noted that a parallel zone for the permissible variations has been established on every surface. When this has been accomplished, no further tolerances should be given.
COMPONENT DRAWINGS
6l
The method of dimensioning a taper shown in Fig. 17 usually meets with more or less opposition from the shop men. The objection is raised that more dimensions are necessary in order to make up the proper reamers, etc. Although the needed di- mensions can be readily computed, it is desirable to reduce the amount of such computations in the shop as much as possi- ble. This objection can be eliminated in several ways. First, if a drawing is made for the reamers, all the additional checking dimensions can appear on these drawings. Second, if opera- tion drawings are provided, these dimensions would appear
BEARING OR FUNCTIONAL SURFACES
CLEARANCE
MIN. INTERFERENCE FOR KEY 0.002" MAX. INTERFERENCE FOR KEY O.OOS"
Machinery
Fig. 19. Functional Requirements for a Taper Key
there. Third, if neither of the two foregoing practices is adopted, the required dimensions may appear on the component drawing in parentheses, and may be marked " Basic" or "Reference." It should be clearly understood, however, that such dimensions are supplementary and apply only in connection with the other basic dimensions given. No tolerances should under any cir- cumstances be given on such reference figures. As far as possi- ble they should be eliminated from the drawing.
Dimensioning Force Fits. The dimensioning of a taper key and its seat offers a very instructive example. In this case, we have a drive fit so that instead of clearances we must concern ourselves with the establishment of the proper interferences. Fig. 19 illustrates such a key and its seat, The functional con-
62
INTERCHANGEABLE MANUFACTURING
ditions to be maintained demand that we have always an inter- ference of at least 0.002 inch and never have a greater interference than 0.008 inch. The illustration shows clearance at those points at which no bearing is required. Often, however, we find draw- ings for such functional conditions specifying fits on all surfaces. Such conditions add nothing to the strength or effectiveness of the construction but entail unnecessary refinement in the manu- facture of the detailed parts with a correspondingly increased
« II
=>=> 00
5
f-H
2
4-0.000.
-o.oio"
TAPER 0.625 INCH
—PER FOOT 2>5001
TAPER 0.625 INCH PER FOOT
_JL
Machinery
Fig. 20. Incorrect Dimensioning and Design of Details shown in Fig. 19
cost. Fig. 20 illustrates the details of such a condition dimen- sioned in a very common manner. This method of dimensioning is wrong. The key in this sketch violates the first, second, third, and fifth laws of dimensioning; the slide violates the second, third and fifth laws; while the dimensioning on the seat vio- lates the first, second, third, and fifth laws. With the dimen- sions given as they are, it is impossible to specify tolerances that will insure the required functional conditions unless we reduce each tolerance to a fraction of a thousandth. The dimensions and the tolerances as they stand permit, in some cases, the key
COMPONENT DRAWINGS
to be tight in the slide and loose in the seat of the slide. In other cases, the reverse is true. Parts made to the basic di- mensions will have a fit on all surfaces.
Fig. 21 shows these parts dimensioned in accordance with the laws of dimensioning. It will be noted that with parts made to the basic dimensions, the key will be driven home to its head, with an interference on the bearing surfaces specified of 0.002 inch. The direction of the tolerances on every dimen-
TAPER 0.625 INCH PER FOOT (SIDES OF SLOT MAY BE PARALLEL)
do
TAPER 0.625 INCHL<4o-> PER FOOT
Machinery
Fig. 21. Correct Dimensioning and Design of Details shown in Fig. 19
sion affecting these bearing surfaces is such that this interference is increased as the sizes of the parts vary from the basic dimen- sions. Under maximum metal conditions, the bottom of the key will be flush with the bottom of the slide with an inter- ference of 0.008 inch on the functional surfaces. It should be noted that by giving the dimensions in this manner, the required conditions are always maintained, while the manu- facturing tolerances are greatly increased. Both slots are made with parallel sides to facilitate machining. Fig. 21 offers a good example of the application of the fifth law of dimen-
INTERCHANGEABLE MANUFACTURING
sioning. This illustration should be carefully studied and com- pared with Fig. 20. Note, in particular, the ease of checking the functional conditions in Fig. 21 as contrasted with the diffi- culty and confusion which arises if we attempt to determine the possible combinations permitted in Fig. 20. Note also how the relative extent of the tolerances specified in Fig. 21 calls atten tion to the essential functional surfaces. These same relative conditions exist between any drawings that are dimensioned without careful study as compared with those which are ra- tionally and logically dimensioned. No attempt has been made
0.560 RAD.
Machinery
Fig. 22.
Sketch showing Satisfactory Method of specifying Tolerance on Contours
in this example to express any dimensions other than those which affect the taper key and its seat. The example given in Fig. 14 shows the proper dimensioning of the dovetail slide.
Dimensioning of Profile Surfaces. The dimensioning of con- tours with tolerances introduces still another problem. To give tolerances on the various dimensions which establish the basic contour inevitably introduces compound tolerances. On the other hand, it is often impossible to resolve such composite sur- faces into elementary ones for the purposes of dimensioning and checking, because their dimensions and relative locations are inseparable. Fig. 22 illustrates one satisfactory solution of this problem. The basic dimensions of the profile are given without tolerances. A dotted line is drawn parallel to the basic
COMPONENT DRAWINGS
contour which indicates the direction of the tolerance. A dimension is given between the full (or basic) outline and this dotted line which specifies the extent of the tolerances. This method of dimensioning gives definite information which can be used directly in the manufacturing departments.
Dimensioning of Holes. The dimensioning of the location of holes with tolerances is a most difficult problem. These dimen- sions are usually given to the centers of the holes and define neither male nor female surfaces. They must be used in con- junction with the diameters of the holes, thus establishing a composite surface condition. The introduction of tolerances on
Machinery
Fig. 23. Diagram illustrating Conditions met with in measuring Holes
these dimensions of location immediately will produce com- pound tolerances.
We might dimension them as shown in Fig. 23 by giving one dimension to the inside edges of the holes (which is a male dimension), another to the outside edges of the holes (a female dimension), and eliminate the dimension for diameter. This would give us a better opportunity of applying the five laws of dimensioning in a similar manner to that employed for elemen- tary surfaces. However, this would prove unsatisfactory in practice because it does not give directly the information which is of most value in the shop — namely, the diameters of the holes and the center distances.
66
INTERCHANGEABLE MANUFACTURING
No rules can safely be given for dimensioning the location of holes in which the permissible variations are distinctly expressed, unless the required functional conditions are duly considered. The following examples give possible solutions of a few of these problems. If these solutions are accepted, the corresponding interpretations, expressed in terms of inspection gage require- ments, must also be accepted. For the first example, we will take the base for a bracket and its pad on a frame illustrated in Fig. 24. We will assume that the position of this bracket on the frame is important and must be held as closely as manu- facturing conditions will permit. We will assume also that the
DIAMETER OF STUD 0.746+°'°°°
Machinery
Fig. 24. Methods of dimensioning the Location of Holes
jigs from which these holes are drilled locate the parts on the finished surfaces from which the dimensions are given.
Causes of Variation in Manufacture. Variations of locations in manufacture develop from three main causes: First, from a fixed error in the jig; second, from a difference in size between the drill and its bushing in the jig; and third, from improper location of the parts in the jigs. Variations occurring because of the first cause will affect the locations of the holes both in relation to each other and to their locating surfaces. Varia- tions because of the second cause will have similar effects to those developing from the first cause. Variations because of the third cause will affect only the location of all holes as a unit from the locating surfaces. Thus, with these problems, there are always composite variations with which to contend. The
COMPONENT DRAWINGS
surfaces involved are always composite, and a condition of compound tolerances is always present.
If precision, rather than absolute accuracy, is the main con- sideration, and if these locations in the two jigs check with each other, the variations due to the first cause may be disregarded, provided that the gages which check these locations are made to agree with the jigs.
The variations due to the second cause may be reduced to comparatively small amounts by closely maintaining the rela- tive diameters of the drills and their bushings. This naturally involves a somewhat increased maintenance cost of the equip- ment. The extent of the variations due to the third cause
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Machinery |
Fig. 25. Functional Gage for Part shown in Fig. 24
depends upon the design of the jigs and the care exercised by the operator. In general, the third cause is responsible for the largest amount of variation.
The locations in Fig. 24 are given without tolerances, yet the drawing should not be interpreted to mean that no variations are permissible. The minimum clearance between the studs and the holes is 0.004 mcn- This clearance is provided to allow for the variations in their locations. Therefore, this clearance should be considered in testing the locations of these holes.
Gages for Checking Location of Holes. The inspection gage for testing these locations would be a functional gage which invades this minimum clearance. There are two conditions to be considered here which affect the amount of the minimum
68
INTERCHANGEABLE MANUFACTURING
clearance that may be used on the gage. If the studs used are loose, individual pieces which pass through both parts and are bolted or riveted at assembly, the functional gage may utilize the entire minimum clearance. On the other hand, if the studs are first driven or riveted into one member, these functional gages could invade the minimum clearance not over fifty per cent. In either case, it is possible to make a single gage which will check both parts.
We will first consider the functional gage to check the first of the above conditions. This gage would consist of a plate with four pins, as shown in Fig. 25. It checks both the loca- tions of the four holes relative to each other and the location of
\< 5.000" —
Machinery
Fig. 26. Another Type of Functional Gage for Part shown in Fig. 24
the group from the edges of the part. The locations of the pins are identical with the corresponding basic dimensions given on the component drawings. The diameter of the pins is 0.746 inch (basic diameter of hole minus minimum clearance). The gage must always enter all four holes on the part. When the gage is held against the upper edge of the holes, the lower edge of the gage must not project below the lower edge of the com- ponent. When held against the lower edge of the holes, the lower edge of the gage must not be above the lower edge of the part. This checks the vertical position of the holes. The hori- zontal locations are checked in a similar manner. The diameters of the holes are checked as elementary surfaces with limit plug gages made to the specified limits.
COMPONENT DRAWINGS
69
Thus, although both the drawings and the gages are made to flat dimensions, a tolerance on all positions of the holes has been established. If their relative locations were perfect, under maximum metal conditions of the various holes, a variation of 0.002 inch either way would be permitted on their location from the edges of the parts. If the various holes were made to the maximum limits, this variation could be 0.005 mcn either way. On the other hand, if the position of these holes as a unit were perfect in regard to the edges of the components, a variation of 0.004 inch would be permitted on their relative locations under maximum metal conditions, with a correspondingly increasing
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Fig. 27. Method of dimensioning Location of Holes without Tolerances
tolerance as the parts approach minimum metal conditions. This would amount to o.oio inch at the extreme minimum metal condition. Inasmuch as variations will develop in both types of locations, all that is not consumed by one is available for the other.
We will now consider the functional gage to check the con- dition where the studs are rigidly fastened to one of the parts. A gage for this purpose would be similar to the one shown in Fig. 25 except that it would contain four holes instead of four pins. Such a gage is shown in Fig. 26. Four plugs would be used with this gage for testing the locations of the holes in one piece, while the holes in the gage would go over the studs fas- tened to the companion part. The diameter of the holes in this
INTERCHANGEABLE MANUFACTURING
gage and also of the plugs is 0.748 inch (basic diameter of hole minus one-half minimum clearance or basic diameter of stud plus one-half minimum clearance). This gage would be used in exactly the same manner as the first. The permissible varia- tions under maximum metal conditions of the holes and studs would be but one-half that permitted in the first case. As these holes and studs approach minimum metal conditions, the per- missible variations in location would increase in the same man- ner and extent as in the first case.
In Fig. 24, the locating dimensions are given from common locating surfaces in each direction. They could be given as
ONE METHOD OF DIMENSIONING
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DIAMETER OF STUD= 0.746 Ij^w SECOND METHOD OF DIMENSIONING
Machinery
Fig. 28. Two Methods of Dimensioning with Tolerance which maintain Identical Conditions
shown in Fig. 27. As long as no tolerances are expressed, the method most convenient for the shop is best.
Expressed Tolerances on Location of Holes. If expressed tolerances for locations of holes are insisted upon, it is impossi- ble to avoid compound tolerances. Then an arbitrary method of interpretations must be promulgated to prevent continual argument and misunderstanding. Fig. 28 illustrates two methods of indicating such tolerances. We assume that the functional conditions are identical with those previously dis- cussed in the first case of Fig. 24. This is one example where the mean size is the proper basic dimension, and tolerances apply equally plus and minus.
In order to establish the sizes of inspection gages we must consider the tolerances, instead of minimum clearances. Re-
COMPONENT DRAWINGS 71
ferring to Fig. 23, an inspection gage to check the relative locations of the holes must be made to the maximum dimension A and the minimum dimension B. In the horizontal direction, the maximum limit of A is equal to the maximum center distance (4.004 inches) minus the minimum diameter of the hole (0.750 inch) which amounts to 3.254 inches. The minimum limit of B in the same direction is equal to the minimum center distance (3.996 inches) plus the minimum diameter of the hole (0.750 inch) which is equal to 4.746 inches. The difference between A maximum and B minimum gives double the diameter of the pins on the inspection gage, which amounts to 1.492 inches. The diameter of these pins is therefore 0.746 inch, and the inspection gage for the relative location of the holes is identical with the one shown in Fig. 25.
In like manner, we must consider the location of these holes from the edge of the components. For simplicity in notation, call the dimension from the lower edge of the piece (in Fig. 28) to the upper edge of the circumference of the lower left-hand hole, C. Call the distance from the lower edge of the piece to the bottom edge of the hole, D. On the gage, evidently, C must be minimum, while D must be maximum. The minimum di- mension of C is equal to 0.998 inch plus half the minimum diameter of the hole (0.375 mcn) which amounts to 1.373 inches. The maximum dimension D is equal to 1.002 inches minus half the minimum diameter of the hole, which equals 0.627 mcn- The diameter of the pins in the gage is equal to the difference between C and D, which equals 0.746 inch. Therefore, the gage shown in Fig. 25 applies to both Fig. 24 and Fig. 28. Or, to put it in other words, Fig. 24 and Fig. 28 express the same information.
Therefore, in those cases where no tolerances are given for center distance (and this applies equally to locations of holes or grooves or slots, etc.), the minimum clearance must be analyzed, and utilized accordingly to determine the inspection gage re- quirements, and a suitable minimum clearance must be pro- vided to allow for the inevitable variation in these dimensions. On the other hand, where tolerances are specified on such di-
INTERCHANGEABLE MANUFACTURING
mensions, these must be analyzed and applied accordingly, to establish the inspection gage dimensions, and the minimum clearance between the holes and studs must be sufficient to prevent interferences. In either case, the basic dimensions should be identical, and the inspection gages would also be identical. For conditions such as those described, experience will teach that the safest plan is to eliminate the tolerances from the component drawings.
The above interpretations of drawings are arbitrary to a cer- tain extent. It would be possible to demonstrate that under certain combinations of conditions, the exact letter of the com- ponent drawings would be violated. This is inevitable in this
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— H r — 4<00°'' — H
1 1.000"
±0.040" DIAMETER OF STUDS 0.746"+°-°OOjJ
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Fig. 29. Another Method of Dimensioning with Tolerance Part shown in Fig. 24
connection. As stated before, if these solutions are accepted, the corresponding interpretations must also be accepted.
Tolerances for " Group " Locations. The same parts, but with different functional conditions, will now be considered. Naturally, the holes in the companion parts must line up suffi- ciently to enable the studs to pass through. Therefore, the importance of the location of these holes in relation to each other is constant. We will assume, however, that in this case the position of the bracket on the frame is unimportant. The only method of indicating this condition that will be consistent with the general practice of dimensioning discussed heretofore will be to express a tolerance. This is done in Fig. 29. No toler- ances are shown in this sketch on the dimensions controlling the relative locations of the holes to each other.
COMPONENT DRAWINGS
73
The interpretation of this drawing is that a variation of 0.040 inch, plus and minus, over and above that allowed by the mini- mum clearance between studs and holes is permitted on the location of the holes as a group. The inspection gage for test- ing this is shown in Fig. 30. This gage differs from that shown in Fig. 25 only in the addition of the steps on the edges which check the additional tolerance. It is used as follows: When the gage is held against the upper edges of the holes, the mini- mum lower step of the gage must not extend below the lower edge of the part. When the gage is held against the lower edge of the holes, the lower maximum step must not be above the lower edge of the part. The horizontal locations are checked
Machinery
Fig. 30. Functional Gage for the Part shown in Fig. 29
in a similar manner. If the functional conditions permit a liberal variation in one direction (say, horizontal) but not in the other direction (vertical), a combination of the methods of dimen- sioning and checking meets the situation.
Conditions arise where the locations of holes must be estab- lished and checked from other points than a flat surface. This often requires quite elaborate fixture gages. A full understand- ing of the preceding principles and a careful study of the par- ticular conditions will point the way to a consistent solution of the problem. The present space is not sufficient to go into the subject in greater detail. Simple examples have been purposely selected to indicate and illustrate the general principles involved.
74
INTERCHANGEABLE MANUFACTURING
The preceding examples involve maintaining the relative position of several holes with each other in addition to the loca- tion of a group as a whole. In those cases where only a single hole is involved which must maintain its position in relation to elementary surfaces, the problem is simple. In most cases it can be solved by the application of methods previously discussed for elementary surfaces. In other cases, the functional require- ments may be such as to demand a functional gage similar to those shown in Figs. 25 and 30.
Concentricity and Alignment. The expression of permissible variations in concentricity and alignment introduces another difficult problem. The succeeding examples offer one solution.
U' " I 3.500 ±S:«£' — *j h-2.0l6'i°;^(;->|
h-
Machinery
Fig. 31. Methods of dimensioning Holes and Studs to fit
As in the case of the locations of holes, if these solutions are accepted, the corresponding interpretations, expressed in terms of inspection gage requirements, must also be accepted.
We will assume that the stud shown in Fig. 31 must always assemble into the hole shown in the same sketch. In the process of manufacture, a certain amount of eccentricity will develop. If we attempt to give on the component drawings the permis- sible eccentricity in every case, the drawing will become more and more complicated. The more complicated the drawings become, the greater the possibility of undetected errors. With the following interpretations of the drawings, the conditions of eccentricity are almost automatically covered.
COMPONENT DRAWINGS
75
There is a minimum clearance on diameters of 0.004 inch between the parts shown in Fig. 31. Inspection gages to test the concentricity of these parts are functional gages and invade this minimum clearance to a fair amount. In general, this should not be over fifty per cent. The lengths of the studs and depths of the holes do not enter into this discussion. They are elementary surfaces which should be readily maintained and checked. The gages for testing the concentricity of these parts
Machinery
Fig. 32. Functional Gages for Part shown in Fig. 31
are shown in Fig. 32. It will be noted that the diameters invade the minimum clearance by an amount equal to fifty per cent. A somewhat similar condition which involves the alignment of slides, or profile grooves and tongues, has been previously dis- cussed in connection with Fig. 10. Functional gages for such conditions are shown in Fig. 12.
Occasionally, the situation arises where a sub-assembly as a whole must meet such conditions as are described above. This
76 INTERCHANGEABLE MANUFACTURING
may entail individual tests for concentricity or alignment on individual component parts of the sub-assembly. The mini- mum clearances must, therefore, be subdivided proportionately. In such cases, it is good practice to include on the component drawing an outline with the dimensions of the functional gage required to test the conditions of concentricity or alignment. This practice will eliminate many arguments in the course of future production.
Gears. Gear teeth offer another problem of composite sur- faces. In general the tolerances on the tooth forms can best be given by specifying the permissible amount of backlash between the pair. No tolerances should ever be given on pitch diameters of gears. Specifying a limit on the backlash makes it possible to eliminate all compound tolerances. Furthermore, the most effective inspection of the gears is obtained by measur- ing this backlash with the gears at a fixed center distance. All the foregoing examples are comparatively simple ones. They should, however, be sufficient to indicate the manner in which a component drawing with tolerances should be dimensioned.
As stated previously, it is seldom possible at the very start to collect and record on the component drawings all of the de- tailed information which belongs there. The development of tools, gages, and other equipment and the final solution of many of the manufacturing problems will make apparent omissions and errors. Therefore, the component drawings should not be considered as complete until the product is actually being pro- duced in strict accordance with them. This requires that the designer, responsible for the accuracy of the. drawings, keep in close touch with both the designers of the manufacturing equip- ment and the various manufacturing departments in order to keep these component drawings up to date.
CHAPTER VI PRACTICE IN MAKING COMPONENT DRAWINGS
As a practical and specific illustration of the principles gov- erning the dimensioning of component drawings set forth in the preceding chapter, a small unit assembly showing the per- cussion firing mechanism for a large cannon is taken as an example. This particular mechanism is chosen because it is composed of a small number of parts; also because it contains several examples of comparatively unusual conditions. In studying the various component or detail drawings to be re- ferred to, the relation between the methods of dimensioning, the tolerances and clearances specified, and the functional re- quirements of each part should be carefully considered.
Drawing of Firing Mechanism Assembled. The assembly of this mechanism is shown in Fig. i. The operation is as follows: The firing mechanism container assembled must be withdrawn before the breech of the cannon can be opened, and cannot be replaced until the breech is closed. (The safety mechanism con- trolling this is not shown on this drawing.) While this assem- bled container is being withdrawn, a primer A is inserted in the primer extractor. This primer is held in place by the pressure of the firing pin guide spring acting against the firing pin guide B. After the breech has been closed again, the container C with the primer is inserted into the housing D and screwed home by hand. The primer must seat tightly on the sharp taper in the spindle plug E. A lanyard is attached to the striker F with a connection that slips off when the end of the striker is withdrawn beyond the end of the container cover G, thus allowing the striker to move forward at the proper moment under the im- pulse of the firing spring H. The firing pin transmits the blow of the striker to the primer, thus detonating it and igniting the charge in the cannon.
77
78 INTERCHANGEABLE MANUFACTURING
COMPONENT DRAWINGS 79
Functional Requirements of the Mechanism. The following functional conditions must be maintained: The primer must be seated in the spindle plug in such a manner that no gases can escape when the gun is fired. Any leakage of these gases, which are at a very high temperature and under high pressure, will quickly erode or burn out the parts of the mechanism, thus destroying its effectiveness. This requires that the surfaces of the seat for the primer be smooth and that its dimensions be maintained within close limits. The blow imparted by the striker must be sufficient to insure that the primer will always be detonated, since the sole object of the mechanism is to de- tonate the primer. In order to insure this result, the firing pin must always protrude, in operation, a certain minimum distance (determined by experiments), while, in order not to pierce the primer cup, it must never protrude beyond a certain maximum distance (also determined by extensive experiments). The vari- ous unit assemblies of the mechanism must be interchangeable in order to allow quick replacements in service — a vital re- quirement. As far as proves economical, the various component parts of the unit assemblies should be interchangeable to permit ready repairs in service. Unless noted otherwise, the parts of this mechanism must be interchangeable. These are the most important of the functional requirements. Others will be dis- cussed as they arise in connection with the details.
Drawing of Firing Pin Guide. A detail drawing of the firing pin guide is shown in Fig. 2. The outside diameter is 0.782 inch plus o.ooo, minus 0.003. This guide must be an easy slide fit in the container. It has a minimum clearance of 0.002 inch, as will be seen by a comparison with that part of the container (see Fig. 7) which receives the guide. With a tolerance of 0.003 inch on each part, it has a maximum clearance of 0.008 inch. With a reasonably smooth finish, such as that obtained by a finishing cut on the guide and a finish-reaming operation on the hole in the container, these clearances will maintain the condi- tions required.
The firing pin must be an easy slide fit in the guide. The diameter of the firing pin hole is 0.118 inch plus 0.003, minus
8o
INTERCHANGEABLE MANUFACTURING
o.ooo. The diameter of the firing pin is 0.117 inch plus o.ooo, minus 0.003; hence the minimum clearance is o.ooi and the maximum clearance (with tolerance of 0.003 on each part), 0.007 inch. With a reamed finish in the hole and a finishing cut on the pin, these clearances will maintain the proper conditions.
The diameter of the large counterbore in the rear of the guide is 0.584 inch plus 0.003, minus o.ooo. The flange of the firing pin must be an easy slide fit in this counterbore. The diameter of the flange is 0.582 inch plus o.ooo, minus 0.003; therefore the minimum clearance is 0.002 and the maximum clearance, 0.008
0.118+8:8$
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Machinery
Fig. 2. Firing Pin Guide
inch. With a reamed finish in the counterbore and a finishing cut on the flange of the firing pin, these clearances will maintain the proper conditions.
The diameter of the smaller counterbore is 0.484 plus o.oi, minus o.oo. This counterbore contains the firing pin guide spring. The minimum clearance is 0.020 and the maximum clearance 0.040 inch. It is apparent that this surface is of minor importance. The only limit to the increase in diameter of this surface is controlled by the width of shoulder at its mouth which is needed to act as a stop for the firing pin. The basic width of this shoulder is 0.05 inch; the tolerance on the counterbore diameter is o.oi, thus reducing the effective width of the shoulder by 0.005 inch. The tolerance specified should be sufficient to enable this counterbore to be finished in one cut. No finishing
COMPONENT DRAWINGS 8 1
operations are necessary on this surface either for smoothness or accuracy.
Exception to General Rule for Basic Dimensions. The length of the guide (Fig. 2) is 0.525 inch plus 0.003, minus o.ooo. This dimension is an exception to the general rule of making the basic dimension represent the maximum metal conditions, be- cause of the functional conditions which must be maintained in this case. When the firing pin and the guide are seated solidly in the container, the face of the guide and the end of the firing pin should be as nearly flush as possible. Under no conditions must the firing pin project, because any such projection makes possible a premature explosion of the primer. The basic dimen- sions on the firing pin and guide are identical for this point, thus making these surfaces flush under basic conditions. The direc- tion of the tolerances in each case is such that the pin can never project. This method of dimensioning, therefore, adheres to the principle of making the basic dimensions represent the danger point, while the direction of the tolerance is such as to move away from this danger point.
On the other hand, there is another danger point in the other direction, although not as serious a one as the first. In such cases, the basic dimension should always represent the more dangerous point, while the tolerances should limit the extent of the other. In this case, the second danger point is that the end of the firing pin should be held as nearly flush with the face of the guide as possible so as not to form a pocket into which the primer cup might be forced under firing conditions. If this happens, it is very difficult to remove the exploded primer. This may retard the rate of fire and possibly put the gun out of action. The tolerances on these dimensions limit the depth of this pocket to 0.006 inch which is as great as is considered safe. The front face of the guide must be smooth; a polished surface is desir- able, as this facilitates the insertion and removal of the primers.
The dimension from the bottom of the large counterbore to the front face is 0.395 inch plus o.ooo, minus 0.004. This di- mension controls the protrusion of the firing pin and is, there- fore, the dimension to be maintained. Experiments show that
82 INTERCHANGEABLE MANUFACTURING
the firing pin should protrude at least 0.026 inch in order to insure detonation, while it should not protrude over 0.034 inch or there will be danger of piercing the primer cup; therefore, the corresponding length of the firing pin is made 0.421 inch .which gives the minimum protrusion of 0.026 inch while the tolerance of 0.004 inch applied to each part limits the maximum protrusion to 0.034 inch.
Maintaining a Common Locating Point. Inasmuch as the im- portant functional dimensions of length are given from the front face, the bottom of the small counterbore is also located from that surface so as to maintain one common locating point. This surface is unimportant; a dimension of 0.136 inch plus o.oo, minus o.oi is specified, which should give wide enough limits to enable it to be machined in a single cut.
The bevel at the front of the guide is provided to assist in the insertion of the primer. The diameter of the intersection of this bevel with the front face is given as 0.551 inch plus o.oo, minus 0.02. These limits should be wide enough to meet any normal manufacturing conditions. The surface of the bevel must be smooth; a polished surface would be desirable. The angle of the bevel is given as 25 degrees. No tolerance is specified, as the permissible variation is controlled by the tolerance given for the face. The angle of the corresponding surface of the primer extractor is 14 degrees. The angle on the guide is made greater to insure that the forward corner of the guide will not project above the bottom of the primer slot in the extractor. Any such projection would interfere with the ready insertion of primers.
No tolerances are given for the radii of the corners of the guide. In the first place, a reasonable variation is already estab- lished for them by the tolerances given on other dimensions. In the second place, their exact contour is of no importance, their purpose being to remove the sharp corners. A straight bevel of the same dimensions would be as effective.
Drawing of Firing Mechanism Container Cover. The thread of the container cover screws into the container and must be set up as tightly as possible. The outside or major diameter
COMPONENT DRAWINGS
of the thread is 1.125 mcn plus o.ooo, minus 0.008 (see Fig. 3). The pitch diameter is 1.0979 inch plus o.ooo, minus 0.004. The minimum clearance is o.ooo and the maximum clearance on the pitch diameter, 0.008 inch, as will be seen by comparing Figs. 3 and 7. This tolerance should be kept as small as normal manufacturing methods will permit.
The diameter of the flange is 1.25 inch plus o.oo, minus o.oi. This surface is an atmospheric fit and of little importance, as regards either smoothness or accuracy; hence it should be com- pleted in a single machining operation. The diameter of the stem is i.io inch plus o.oo, minus o.oi. This surface is an
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Machinery
Fig. 3. Firing Mechanism Container Cover
atmospheric fit and should be machined in a single operation. The distance across the flats on the stem is 0.945 inch plus o.oo, minus 0.02. This surface is for the wrench used in assembling and is of little importance. It should be machined in a single operation by a straddle-milling tool.
The diameter of the hole and the width of slot is 0.520 inch plus 0.006, minus o.ooo. These surfaces are for the striker and lanyard connection. They should be as smooth as careful ream- ing and finish-milling operations will leave them. The hole and the slot must be matched so that no shoulder will be left at their intersection, which would retard the striker in its action. This
INTERCHANGEABLE MANUFACTURING
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COMPONENT DRAWINGS 85
ing point. As the front surface is the logical working point, this has been chosen.
The length of the thread is 0.394 inch plus o.oo, minus o.oi. The width of the recess is 0.08 inch plus o.oi, minus o.oo. The requirements of these dimensions are that there shall be suffi- cient threads to hold the cover in position and that the length of this stem shall be less than the depth of the recess in the con- tainer, as the cover must always seat on its flange. The depth of the recess into which the thread projects may extend about o.oi inch below the bottom of the threads to give a suitable clearance for threading. Also, the end of the stem and edge of the recess may be beveled about thirty degrees to facilitate this operation. The depth of the counterbore is identical with the length of the threaded stem. This depth is of relatively small importance.
The location of the rear face of the flange is 0.494 plus o.oo, minus o.oi. This surface is an atmospheric fit and should be finished in a single operation. The bottom of the wrench cuts is 0.51 plus o.oo, minus o.oi. This is a clearance surface of little importance. It is left above the rear surface of the flange to eliminate all matching operations. The bottom of the slot is located by these same dimensions, and its surface is of equal unimportance.
The length over all -is 1.544 inches plus o.oo, minus 0.02. The rear surface of the cover is an atmospheric fit of minor im- portance. No tolerances have been specified for any of the radii, as their exact contour is of no importance. Their purpose is to remove the sharp corners. Furthermore, sufficient varia- tions have been established for these radii by the tolerances on other dimensions.
Drawing of Striker. The diameter of the front end of the striker (see Fig. 4) is 0.591 inch plus o.oo, minus o.oi. This surface must clear the counterbore in the container (see Fig. 7). The minimum clearance is 0.019 inch and the maximum clear- ance 0.039 inch. The radius of the air grooves is 0.07 inch plus o.oi, minus o.oo. No finishing cuts are required on these sur- faces. The stem which has a diameter of 0.38 inch plus o.ooo,
86
INTERCHANGEABLE MANUFACTURING
minus 0.005, must be a free sliding fit in the spring seat washers. The minimum clearances are 0.002 inch and the maximum 0.012 inch. This stem must have a smooth surface, such as will be secured by a finishing tool.
The rear shoulder which has a diameter of 0.517 inch plus o.ooo, minus 0.005, must be a free sliding fit in the cover. The minimum clearance is 0.003 mcn an<^ tne maximum clearance 0.014 inch. This surface must be as smooth as a careful finish- ing cut will leave it. The length of the front end (0.829 inch plus o.ooo, minus 0.005) should be held within reasonably close
SOLID HEIGHT, NOT MORE THAN 0.621"
ASSEMBLED HEIGHT. 1.4&"
LOAD AT ASSEMBLED HEIdHT.NOT LESS THAN
3. 3 LBS.
SPRING TO BE COMPRESSED TO A HEIGHT 0.77l"
FOR 48 HOURS, AFTER WHICH IT SHALL SUPPORT
A LOAD OF 27.7 LBS. ATA HEIGHT OF 0.77l'±0.03
FREE HEIGHT "'' V_ 0.092"
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Fig. 5. Spring Seat Washer
Fig. 6. Firing Spring
limits in 'order to insure a uniform blow on the firing pin. Varia- tions in this dimension will affect the force of this blow. The front face of the striker must be as smooth as a finishing cut will leave it. The length of the stem (1.70 inches plus o.oi, minus o.oo) should also be held within reasonably close limits, as it controls, to a certain extent, the force of the blow of the striker. This stem could be machined with a form tool, the roughing tool being made to form the neck for assembling the washers. The neck is, therefore, located from the front end of the stem, the dimension being 0.23 inch plus o.oi, minus o.oi. The width is 0.145 mc^ P^us °-OI> minus o.oo. These limits should be adequate to permit this groove to be finished with a roughing tool without any unnecessary refinements.
COMPONENT DRAWINGS 87
The location of the rear shoulder from the front end (2.73 inches plus o.oo, minus 0.02) and the width of the bottom of the groove (0.40 inch plus o.oi, minus o.oo) are relatively unim- portant. The surfaces, however, must be reasonably smooth ones such as are obtained with a finishing tool. These limits should be sufficient for all manufacturing purposes. The length over all (3.30 inches plus o.oo, minus 0.02) is also relatively unimportant. A sufficiently smooth surface for the rear end will be obtained with a cutting-off tool.
No tolerances are given for the 45-degree bevel at the rear end, nor for the radii, because none are required. A reasonable variation is already established by tolerances given on other dimensions. Sufficiently accurate radii will be obtained by touching with a file the various corners which are not broken by form tools, to remove the sharp edges.
Drawing of Spring Seat Washer. If a large number of spring seat washers (see Fig. 5) were to be manufactured, they might be made in a punch and die. The surface obtained in a well made sub-press die would be sufficiently smooth, but the surface obtained on the usual punch press in an open die would prob- ably require some polishing. For a small number of parts, bar stock could be used. The surface obtained with a finishing tool would be satisfactory.
The hole must be a free sliding fit on the stem of the striker. The minimum clearance is 0.002 inch and the maximum clear- ance o.oi 2 inch. A surface equal to that obtained with a reamer should be secured. The width of the assembling slot is unim- portant. The faces of the washer are of but minor importance. The original surface of flat stock, if that is used, or the surface obtained with a cutting-off tool, if bar stock is used, will be satisfactory.
Tolerances are not needed for the radius of the corners, but enough of the corner must be removed to permit the washer to seat properly in the counterbore in the cover. This corner may be removed on a polishing wheel or with a file in the lathe.
Drawing of Firing Spring. No tolerances are given on the dimensions of the firing spring (see Fig. 6), because the functional
88
INTERCHANGEABLE MANUFACTURING
COMPONENT DRAWINGS 89
requirements are covered by the weight specifications, and the manufacturer is allowed reasonable latitude in these dimensions as long as the weight requirements are maintained. The di- mensions given are nominal. A variation of 0.005 mcn m the diameter of the wire, or of 0.030 inch in the diameter of the coils or free length of the spring will be of no moment. If these toler- ances were expressed on the drawing, some manufacturer would complain that the weight specifications would not allow him to take full advantage of them, and would seek to have the weight requirements altered or removed. These weight con- ditions are the essential ones, as they control the force of the blow on the primer. A minimum load of 3.3 pounds is required at the assembled height of 1.45 inches. A load of 27.7 pounds is required at a height of 0.771 inch plus 0.03, minus 0.03. By thus specifying loads at two heights, the strength of the spring is very closely controlled.
Drawing of Firing Mechanism Container. The container is shown in Fig. 7. The housing thread (which has an outside diameter of 1.417 inches plus o.ooo, minus 0.006) is a special thread and will undoubtedly be milled. It must be a very free fit in the housing, as the container is inserted and removed every time the gun is fired. It must assemble readily, even if a certain amount of dirt and grit is present. The minimum clear- ance is 0.008 inch and the maximum clearance 0.020 inch. The surfaces must be smooth. A finish- turning or milling cut will be satisfactory. It will be necessary to match the turning and milling cuts where the bottom of this thread matches the cylin- drical portion of the container with a file after the part is machined.
The thread for the primer extractor (outside or major diameter 1.024 inches plus o.ooo, minus 0.008) must be left hand to pre- vent the primer extractor from unscrewing as the mechanism is removed from the housing. The minimum clearance is o.ooo and the maximum clearance on the pitch or effective diameter, 0.008 inch. The primer extractor must be screwed home as firmly as possible, and the variations on these threads should be kept as small as normal manufacturing methods will permit.
go INTERCHANGEABLE MANUFACTURING
The small counterbore in the rear end (diameter 0.6 10 inch plus o.oi, minus o.oo) is for clearance and is unimportant. It should be finished at a single operation of a counterbore. The large counterbore in the rear end (diameter 0.950 inch plus 0.005, minus o.ooo) must be a free sliding fit for the washer and should be reamed. The minimum clearance between the hole having a diameter of 0.35 inch plus o.oi, minus o.oo, and the firing pin is 0.05 inch, while the maximum clearance is 0.07 inch. This surface is unimportant and should be machined by a single drilling operation. The diameter of the counterbore in the front end is 0.784 inch plus 0.003, minus o.ooo. This surface must be an easy sliding fit for the guide and requires a careful finish- reaming operation. The length over all (3.271 inches plus o.oo, minus 0.02) is relatively unimportant, yet both the front and the rear faces must be smooth, as they form the seats for the cover and primer extractor. The majority of the length di- mensions are located from the front face. A few are given from the rear end because of manufacturing considerations. The remainder are given from intermediate points because of the functional requirements of the mechanism.
The length of the thread for the extractor is 0.209 mch plus o.oi, minus o.oo, and the width of the under-cut, 0.06 inch plus 0.005, minus o.ooo. The requirements of these dimensions are that there shall be sufficient threads to hold the extractor prop- erly and that the length of the stem be always long enough to permit the extractor to seat on the front face of the container.
The bottom of the large counterbore, which is i.n inches plus o.ooo, minus 0.005, from the firing pin seat, is an impor- tant functional surface and should be finished by a special opera- tion, locating from the firing pin seat. This dimension controls, to a great extent, the force of the blow of the striker. The location of the housing thread is also controlled from the firing pin seat. This dimension (0.291 inch plus o.ooo, minus 0.005) controls the angular position of the mechanism when it is screwed home in the housing. A plus variation on this dimen- sion might prevent the mechanism from locking. The width of the housing thread is 0.405 inch plus o.oo, minus o.oi. The
COMPONENT DRAWINGS
rear surface or flank of this thread is a bearing surface and must be smooth. A corner of this rear flank is beveled with a file to facilitate entering the housing (see view showing development of thread). The length of bevel is 0.158 inch plus 0.02, minus o.oo, and the width of bevel 0.04 inch plus 0.02, minus o.oo. This should give sufficiently liberal tolerances for all manufacturing purposes. Double this tolerance could be given, however, if necessary.
The latch-pin and spring holes are both located from the front face of the container, as this is the logical locating point for the drill jig. The distance from the latch-pin hole to the
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Machinery
Fig. 8. Primer Extractor
center of the container is 0.625 inch plus 0.005, minus o.ooo. A minus variation on this last dimension would develop inter- ference between the bottom of the slot and the latch. The latch-pin thread is a standard No. 5-44 A.S.M.E. thread. Taps should be avalaible from stock from any reliable tap manu- facturer. This is a fine thread and should be held as close to size as normal manufacturing conditions will permit.
The distance to the cut on the left side of the flange is 0.30 inch plus o.oo, minus o.oi, and the height from the bottom of the cut is 0.56 inch plus o.oo, minus o.oi. The requirements of this cut are that the cutter shall not gouge the knurled handle
§2 INTERCHANGEABLE MANUFACTURING
by cutting too deeply and that the tap shall not gouge the bottom of this cut. The limits specified give the greatest per- missible variations under maximum metal conditions and should be great enough to allow this cut to be machined in a single operation. The width of the latch slot is 0.300 inch, plus 0.006, minus o.ooo. This surface must be reasonably smooth so as to maintain an easy action of the latch. The minimum clearance is 0.003 inch and the maximum clearance, 0.013 inch. With the proper surfaces, these clearances will maintain the desired conditions.
The location of the bottom of the slot from the center of the container is 0.45 inch plus o.oo, minus 0.02. A plus variation on this dimension would develop interference with the latch. The surface of the bottom and ends of this slot is not important and requires no finishing cuts. The cuts on the side of the slot must be matched to provide a smooth bearing for the latch.
The ends of the housing thread are 0.02 inch plus 0.02, minus o.oo, from the center of the container. These surfaces require no finishing cuts. The length of the knurling is 1.60 inches plus o.oo, minus 0.05. A scale measurement will be sufficient to check this dimension. No tolerances are given on the vari- ous radii or angles, as none are required. Tolerances on other dimensions establish liberal variations for these surfaces.
Drawing of Primer Extractor. The outside diameter of the primer extractor (see Fig. 8) which is 1.102 inches, plus o.ooo, minus 0.006, should approximately match the corresponding diameter on the container shown in Fig. 7. The surface should be reasonably smooth. The diameter of the recess for the firing pin guide (0.791 inch, plus o.oi, minus o.oo) is clearance and is unimportant. The depth (0.264 inch, plus 0.005, minus o.ooo) is more important, as it controls the amount of surface which engages the head of the primer. A finish cut will be required on this surface.
The width of the extractor slot (0.472 inch, plus 0.004, minus o.ooo) is an important dimension and the surface must be smooth. A finish cut will be required. The bevel at the end of this slot is at an angle of 30 degrees and is located from the
COMPONENT DRAWINGS 93
center of the extractor at a distance of 0.236 inch, plus o.oi, minus o.oi. The exact dimensions of the bevel are unimpor- tant, as it is provided merely to facilitate assembling the primer. The surfaces must be smooth, however, even if an extra filing operation is needed to match the cuts.
The distance across the flats (0.945 inch, plus o.oo, minus o.oi) is for the wrench used in assembling and is unimportant. No finish cuts are required. The length of the extractor is 0.394 inch, plus o.oo, minus o.oi. The front face must clear the rear face of the spindle plug when the primer is seated; there- fore, no plus variation is permissible. Any great minus varia- tion will weaken the extractor. The tolerance given should be liberal enough for all normal manufacturing purposes. Both the front and rear surfaces should be reasonably smooth. This will require finishing cuts.
The depth of the tapped hole is 0.209 inch, plus o.oo, minus o.oi, and the width of the thread under-cut, 0.06 inch, plus 0.005, minus o.ooo. Enough threads must be secured to hold the extractor firmly in position, yet the depth must be shallow enough to permit the extractor to seat on the front face of the container. It is permissible to make the depth of the thread under-cut not over 0.005 inch below the bottom of the threads to provide clearance for the tap. A greater diameter of under- cut than 1.042 inch (maximum outside or major diameter of thread or 1.032 inch plus o.oi in diameter allowed for tap clearance) would weaken the extractor to such an extent that it would not be safe to use it in service.
The location of the bottom of the primer slot from the rear face is 0.248 inch, plus o.ooo, minus 0.005. This surface should never come below the corner on the firing pin guide, for if it did, it would be difficult to insert the primer. The surface should be smooth and all corners about this slot must be care- fully broken.
Dimensioning to Prevent Compound Tolerances. The coun- tersink which merges into the beveled surface on the under side of the primer head is located from a theoretical point, where its angle of 35 degrees intersects the center line of the extractor.
94
INTERCHANGEABLE MANUFACTURING
The distance from this intersecting point to the front face is 0.306 inch, plus 0.005, minus o.ooo. Such a method of dimen- sioning is necessary to prevent compound tolerances. It will be noted that no dimensions are given for the intersections of this angle with the primer slot or bottom of the recess. Such dimensions are unnecessary and could not be measured directly in any event. The dimensions given locate this surface defi- nitely and completely. It will be necessary for the manufacturer to compute the diameters on this countersink to suit his own
ENDS GROUND SQUARE
6 COILS NO. 10 MUSIC WIRE
No.5 44-U.S.Form-R.H. Pitch Dia. 0.1102 +£$&'' Core Dia. 0.0955 +0.000;;
Machinery
Fig. 9. Locking Latch, Spring and Pin
particular needs. This surface must be smooth and will require a careful finishing operation.
No tolerances are given on any of the angles or radii because none are needed. Sufficient variation on these surfaces is per- mitted by the tolerances given on other dimensions.
Drawing of Locking Latch, Spring, and Pin. The surfaces of the locking latch (Fig. 9) must be smooth, as they bear on the sides of the slot in the container. This part has a tolerance of minus o.oi inch on the entire contour. This means that the
COMPONENT DRAWINGS
95
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+ 1 +1+1 +1
96 INTERCHANGEABLE MANUFACTURING
piece may vary o.oi inch normal to the profile at any point in the direction that will make the piece smaller, or, in other words, any variation from the normal dimensions must remove more metal. The diameter of the pin hole (0.092 inch, plus 0.004, minus o.ooo) corresponds with the pin hole in the container. The diameter of the spring hole (0.175 inch, plus o.oi, minus o.oo) also corresponds with the spring hole in the container.
The locking latch spring (Fig. 9) is a part of minor importance. It is made of No. 10 music wire, and no tolerance is specified for its diameter. This means that commercial music wire bought in the open market will be satisfactory. No difficulty should be experienced in maintaining the limits given.
The thread of the locking latch pin is a No. 5-44 U. S. form. This is a standard A.S.M.E. thread, and dies should be avail- able in stock at any reliable die manufacturer's. After this pin is assembled into the container, the end thread of the tapped hole in the container should be upset slightly with a punch to prevent this pin from falling out. It should be understood that it is permissible to bevel at both ends of the thread to facilitate the threading. The surface of the stem forms a bearing for the latch and should receive a finish cut. This part should be com- pleted in a single operation on a screw machine.
Drawing of the Housing. The outside diameter of the hous- ing (Fig. 10) is 2.638 inches, plus o.oo, minus o.oi. This is an atmospheric fit and requires no finishing cut, which also applies to the shoulder, the diameter of which is 1.925 inches plus o.oo, minus o.oi. The Whitworth thread, the full or major diameter of which is 2.100 inches, plus 0.012, minus o.ooo, must assemble on the hinged collar (Fig. n). The Whitworth form of thread is used to suit other types of firing mechanisms now in service; otherwise the U. S. form of thread would be preferable.
The counterbore in the front end (diameter 1.502 inches, plus 0.02, minus o.oo) must clear the spindle plug. No finish- ing cut is required. The counterbore in the rear end of the same size must clear the flange on the container. The minimum clearance is 0.007 incri and the maximum clearance, 0.047 inch. No finish cut is required. The full or major diameter of the con-
COMPONENT DRAWINGS
97
tamer thread is 1.425 inches, plus 0.006, minus o.ooo. A sector of this thread is removed to permit the assembly of the con- tainer. All of these surfaces require finishing cuts. The rear flank is the bearing flank and the most essential. The re- quirements were previously given in connection with the firing mechanism container. The front face of the housing should be reasonably smooth, as it seats against the hinged collar and is the most important working point for other machining operations. The depth of the Whitworth thread is 0.80 inch, plus 0.02, minus o.oo. The hole must be deep enough for the hinged collar,
0.160
Machinery
Fig. 11. Hinged Collar assembled
and enough threads must be maintained to hold the firing mechanism in position during the firing of the gun. This thread is subjected to a considerable strain at this time. The depth of the counterbore in the front end is 1.08 inches, plus 0.02, minus o.oo. This surface must clear the end of the spindle plug. No finish cut is required.
The location of the thread for the container is 1.987 inches, plus 0.005, minus o.ooo from the front end. This is an impor- tant functional dimension and must be carefully watched, as it controls the angular position of the firing mechanism when
98 INTERCHANGEABLE MANUFACTURING
it is screwed home. A minus variation on this dimension would prevent the mechanism from locking. A slot is shown in the lower right-hand side of the housing for the safety mechanism. The safety bar should be a very free fit in this slot. The slot is 1.59 inches, plus o.oi, minus o.oi from the front end; the length, 0.75 inch, plus 0.02, minus o.oo; and the width, 0.375 inch, plus 0.02, minus o.oo. The surface in the slot should be reasonably smooth. A tapped hole, having a full or major diameter of 0.50 inch, plus o.oi, minus o.oo, is shown in the bottom of the housing for the firing mechanism pin. It is per- missible to run the tap drill below the thread, provided that this drill does not break through into the hole in the center of the housing.
A recess is milled in the rear face of the housing to engage the locking latch. This recess allows for a possible variation in the locked position of the container of 90 degrees. The toler- ances given on the various controlling dimensions will permit a variation of approximately 30 degrees. The variations on the primer plug and the primer will permit approximately 30 degrees more. This leaves the remaining 30 degrees to allow for wear. The depth of the recess is o.io inch, plus 0.02, minus o.oo and the ends of the recess are located 0.15 inch, plus o.oi, minus o.oo from the center lines through the rear face, 90 degrees apart.
The hole and counterbores for the collar-catch (shown in Fig. 12) are located from the center of the housing because these holes will be drilled in a jig which should locate the housing centrally. The counterbores are 1.088 inches, plus o.ooo, minus 0.005, and the hole, 1.183 inches, plus 0.005, minus o.ooo from the center line (see end view Fig. 10). The diameter of the hole is o.i 6 inch, plus 0.006, minus o.ooo. This hole should be a free sliding fit for the stem of the collar-catch. The mini- mum clearance is 0.002, and the maximum clearance, 0.014 inch. The full or major diameter of the tapped hole is 0.375 inch, plus 0.008, minus o.ooo, and the pitch diameter, 0.3417 inch, plus 0.004, minus o.ooo. No core or minor diameter is given, as the small counterbore, which is a few thousandths inch larger
COMPONENT DRAWINGS
99
then the theoretical core diameter, limits the height of the threads. The V-form of thread is used to obtain as great an area of contact as possible. After assembly of the collar-catch screw, the metal should be upset slightly with a cold chisel into the slot of the screw to prevent disassembling. The bottom of the counterbore is 0.125 inch, plus 0.02, minus o.oo from the rear face of the body. This counterbore receives the head of the collar-catch screw and also forms a groove in the stem which provides clearance for drilling and tapping.
The width of the thread for the container is 0.41 inch, plus o.oi, minus o.oo. A 45-degree bevel (0.03 inch, plus 0.02, minus o.oo wide) is required on the corner of this thread to facilitate
TOLERANCE 'n/ AS SHOWN —
Machinery
Fig. 12. Collar-catch
the insertion of the container. This bevel may be made with a rile; the tolerance should be great enough to cover this method of manufacture. The ends of the thread sector are located from the 45-degree center line of the housing (see end view) at 0.02 inch, plus 0.02, minus o.oo. This sector is at an angle so as to always insure a minimum contact on this thread of 135 degrees. No tolerances are given on the various angles and radii, as none is required.
Drawing of Hinged Collar. The hinged collars and housings are not interchangeable and must be furnished in pairs. To make these parts interchangeable and insure that the housing would be screwed tightly against the shoulder on the hinged collar when the locking holes in each part were in correct align-
IOO INTERCHANGEABLE MANUFACTURING
ment, would require very expensive manufacturing methods. In such a case, the position of the start of the Whitworth thread in the housing would have to be held very closely in relation to the position of the locking hole. The same would be true on the hinged collar. Some variation must of necessity be allowed. This would introduce a further variation longitudinally of the position of the firing mechanism. The effect of such a variation would be an additional angular variation in the locked position of the mechanism. If the original pairs of housings and hinged collars become separated, an additional locking hole will have to be drilled in the flange of the collar, Fig. n, transferring it from the housing which is to be used. The diameter of the lock- ing hole is 0.160 inch, plus 0.006, minus o.ooo. It should be drilled in one operation by using its companion housing as a jig.
Drawing of Collar-catch. For convenience of manufacture, the collar-catch (see Fig. 12) is made in two parts which are permanently assembled. The stem and the finger piece may be made interchangeable, or a system of selective assembly may be employed. This is matter to be determined by the manu- facturer to suit his own convenience. Therefore no tolerances will be given on the dimensions of the riveted end. The stem must be a snug fit in the finger piece, and the two parts must be solid after riveting. Any parts made within o.oi inch of the nominal dimensions and which meet the above conditions will be acceptable. The rear face of the finger piece will be finished after riveting.
As the diameter of the stem is 0.158 inch, plus o.ooo, minus 0.006, it should be possible to secure drill rod well within these limits; no further machining will be required on this surface. The length of the stem is of minor importance. The surface left by the cutting-off tool will be satisfactory. The length of the finger piece (0.525 inch) is an atmospheric fit; no tolerances are given, because the note in regard to the profile gives a tolerance of minus 0.04 inch.
The thickness of the flange (0.125 inch plus o.oo, minus o.oi) is of minor importance; hence the flange can be completed in a single operation after the stem is riveted. The width of the
COMPONENT DRAWINGS
101
flange (0.245 inch, plus o.oo, minus o.oi) must be free in the slot in the housing. The minimum clearance is 0.005 mcn and the maximum clearance, 0.035 inch.
The dimensions of the profile of the finger piece are given without tolerances, but a note is added: "Tolerance plus o.oo, minus 0.02, as shown by dotted line." The entire upper part of the finger piece is an atmospheric fit. It must be reasonably smooth because the finger operates this part. The note permits a minus variation of 0.02 on this profile where no other toler- ances are given. This variation is measured as normal to the profile at any point. If a clean drop-forging is secured, this
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.frig. 13. Firing Pin Guide Spring and Firing Pin
surface may be finished by removing all rough scale, flash, and other rough spots with a file or on a polishing wheel. The con- tour of this surface is not important enough to require expen- sive form-milling cuts.
Drawing of Firing-pin Guide Spring and Firing Pin. The diameter of the wire for the firing-pin guide spring (Fig. 13) is 0.04 inch, plus o.ooo, minus 0.002 ; the outside diameter of the coils, 0.464 inch, plus o.oo, minus o.oi; and the free height, 0.69 inch, plus 0.02, minus o.oo. These limits should be readily maintained under normal manufacturing conditions.
The firing-pin flange is 0.582 inch in diameter, plus o.ooo, minus 0.003. This surface must be a free sliding fit in the
102
INTERCHANGEABLE MANUFACTURING
firing-pin guide. The surface will require a careful finishing cut. The diameter of the front end (0.117 inch, plus o.ooo, minus 0.003) must be a free sliding fit in the firing pin guide. This surface requires a careful finishing cut.
The surface of the rear end clears the hole in the container by 0.050 inch and requires no finishing cut. The diameter of the large end of the taper is an unimportant clearance surface. The taper is provided to strengthen the end of the firing pin. No finishing cut is required on this tapered surface. The length over all is an important functional dimension and, in part,
26 V-Thds. per in. R.H. Pitch Dia. 0.3417'±g;9$>,'/' Core Dia. 0.3804'+ J}-{gjj« Dia.of Undercut 0.3804' t°-9?o:
3 COILS NO. 15 MUSIC WIRE
MacMnerv
Fig. 14. Collar-catch Screw and Spring
controls the force of the blow on the primer. The front face of the pin must be as smooth as possible, and a polished surface is desirable. The rear face should be as smooth as a careful finish- ing cut will leave it.
The location of the rear face of the flange (0.525 inch, plus o.ooo, minus 0.003) *s important, as this dimension controls the location of the end of the firing pin. The surface should receive a finishing cut. The distance to the front face of the flange is an important functional dimension which controls the protrusion of the firing pin, as noted in connection with the firing-pin guide. A finishing cut is required on this surface. The
COMPONENT DRAWINGS 103
location of the beginning of the taper is relatively unimportant. This dimension maintains clearance with the bottom of the counterbore in the firing-pin guide.
No tolerances are given for the radii because none are needed. Attention is called, however, to the radius of 0.04 inch at the front end. This must not be exceeded. The purpose of this radius is to remove the sharp corner, but care must be taken to remove as little material as possible.
Drawing of Collar-catch Screw and Spring. The collar- catch screw is shown in Fig. 14. The diameter of the head must enter the counterbore in the housing. The thread, which is a sharp V-form, must assemble into the tapped hole in the hous- ing. Sufficient threads must be secured to hold the screw in position. It is permissible to bevel both under-cut and end to facilitate threading. This screw should be completed in a single operation on a screw machine.
No. 15 music wire is specified for the collar-catch spring. Commercial wire of this number will be satisfactory. The func- tion of this spring is to hold the collar catch in its locked posi- tion. The limits given should be maintained readily under normal manufacturing conditions.
All dimensions and tolerances given on these drawings repre- sent limit gage sizes. If a hole is given as 1.25 inches, plus o.oi, minus o.oo, this means that the hole must be made so that a plug gage 1.25 inches in diameter will always enter, while a plug gage 1.26 inches in diameter will not. In general, the extent of the tolerances allowed on any surface is a good index of the char- acter of the finish required. All burrs, fins, etc., and unneces- sary sharp corners must be removed. All cuts and surfaces, whether rough or finished, must show no evidence of careless- ness. All cuts must be made with clean and sharp tools. Gouges, tears, and unnecessary scratches produced by dull or improper tools and careless workmanship or carelsss handling should be sufficient cause for rejection.
Unless noted otherwise, common manufacturing practices, such as under-cutting and beveling for threads, extending the tap drill a reasonable amount below the threads in tapped holes,
104 INTERCHANGEABLE MANUFACTURING
countersinking to guide the tap, providing reasonable grinding clearances where necessary, burr-beveling corners on screw machine parts, etc., are permissible. Whenever any differences exist between the dimensions and tolerances expressed on the drawing and the above specifications, the figures on the drawing should be used. The dimensions and tolerances given on the component drawings should be strictly maintained. If modi- fications are possible which will relieve the situation, they should be made. No deviations from the specified requirements are permissible, however, until definite modifications are authorized.
CHAPTER VII
ECONOMICAL PRODUCTION
WHEN certain manufacturing methods are to be decided upon, the decision made in this connection should be recorded, to- gether with the reasons for it. This practice tends to eliminate many expensive, unnecessary refinements which are often arbi- trarily specified, because, instead of baldly specifying the vari- ous requirements, the necessity for adding sufficient reasons therefor demands a careful analysis of the mechanism and its purpose; and a careful analysis of almost any mechanism will soon make it apparent that only a small proportion of the di- mensions and other requirements are exacting. This and many other subjects bearing upon the attainment of economical prac- tice in interchangeable manufacturing are dealt with in the present chapter.
Principal Elements in Economical Production. There are three principal elements in the economical and successful pro- duction of a commodity. Stated briefly, they are as follows: (i) A thorough knowledge of the object (function) of the article and of all the conditions essential in attaining it. (2) The de- velopment of manufacturing methods and facilities that will most economically produce a satisfactory product. (3) The development of testing methods and apparatus to determine in an economical manner, at any stage, whether or not the desired results are being achieved.
Duplication of work never results in economy. Therefore, a record should be made of any solution reached in regard to these questions. Almost every problem has more than one satisfactory method of solution. The multiplication of solu- tions, however, particularly in manufacturing, is a hindrance to team work. For example, if the foreman of one department uses one solution of a problem, while the foreman of another department who performs succeeding operations on the same or
105
106 INTERCHANGEABLE MANUFACTURING
companion parts arrives independently at another solution and uses it, the final results may be chaos; whereas, if each solution is recorded, whenever or wherever made, an opportunity is created to check these solutions against each other, thus making possible the elimination of inconsistencies at an early stage of the work. This practice will aid greatly in promoting team- work, and thereby eliminate many misunderstandings.
Specifications. Specifications, in their broadest sense, in- clude the solutions of all the three problems mentioned. This information may be compiled and recorded in one place or it may be scattered throughout the plant. In general, if the entire control of the design and manufacture of a commodity is held in one plant, the compiling and assembling of much of this information may be of doubtful value. As long as it is on record somewhere and available when needed, that' is sufficient. On the other hand, if the control of the design rests with one organi- zation, the control of the production with another, while the control of the final inspection is distinct from either of the two foregoing establishments, reasonably complete specifications are imperative if economical and expeditious production is to be obtained.
Specifications thus defined include component drawings. For purposes of discussion, however, component drawings and specifications will be considered as distinct. In this case the specifications are supplementary to the component drawings and include all information which is not given on these drawings.
Function and Essential Requirements of Product. The com- ponent drawings consist of pictures of the parts, statements of the physical dimensions required, and usually specifications of the material to be employed. By themselves they only partially solve the first of the major problems noted previously. They tell little or nothing of the object of the commodity. They state requirements, but give no reasons therefor. Thus, the first function of the specifications is to state briefly the purpose of the mechanism and its functional requirements. The preceding chapter, " Practice in Making Component Drawings," indicates the lines which specifications should follow.
ECONOMICAL PRODUCTION 107
A second function of the specifications is to indicate the quality of workmanship desired. The extent of the tolerances given on the drawings indicates, to a certain degree, the proper character of the finished surfaces. The specifications should supplement this information by stating not only desired results, but also reasons therefor. The preceding chapter previously referred to illustrates this practice. To a certain extent, per- haps, many of the conditions discussed there are so obvious as to need no mention, yet no harm is done by being explicit.
Another subject to be included in the specifications is the matter of the materials to be employed, and their nature, com- position, and ultimate use. When standardized material is used, this can be called for directly on the component drawing, together with the proper heat-treatment. It is of interest to note that the Society of Automotive Engineers has done much valuable work in establishing standard specifications and methods of heat- treatment for nearly every kind of material used in automobile construction. The adoption of such stand- ards greatly simplifies the provision of proper component draw- ings and specifications. In those cases where standardized material cannot be used, the specifications should give all perti- nent information to enable the proper material to be secured. For preservative finishes the drawings or specifications should give complete information as to nature, need, and use.
When the component drawings have been completed and the specifications have reached this stage, the first important ele- ments of the work are established. Until this is accomplished, those responsible for the manufacturing design have not done all in their power to secure the economical production of dupli- cate mechanisms in large quantities. Undoubtedly, many minor revisions will be required before the proper solutions of the succeeding problems will be found. But without the fore- going information, such revisions cannot be made intelligently. Furthermore, this information, in most cases, will point the way to a simple and direct solution of the succeeding problems. "Well begun is half done" was never nearer the truth than in connection with interchangeable manufacturing.
108 INTERCHANGEABLE MANUFACTURING
Specific Manufacturing Data. We will now consider the solution of the second major problem — the development of suitable manufacturing methods and facilities. The first step to be taken is to make up the operation lists for every compo- nent, noting in detail the type of machine, fixture, and tool required. It is of great assistance in many cases to develop concurrently the operation drawings, indicating on them the work to be performed at each operation. In addition, it is a good plan to include in this part of the work an estimate of the production time on each operation. This information is neces- sary to establish the amount of equipment required and also to make a comparison, when desired, of the economy of several methods.
This information should be revised and kept up-to-date after production is under way. This furnishes valuable data for estimating on new commodities and facilitates comparison be- tween the costs of different methods of manufacturing. Such information is invaluable when it becomes necessary to call on outside plants to assist in obtaining greater production. It need not be bound together with other parts of the specifica- tions, but it should be in such shape that it can be quickly found and readily applied. The foregoing information serves as the basis for designing the special manufacturing equipment necessary as well as for arranging the manufacturing depart- ments so that the component parts can be produced rapidly and efficiently.
This second problem is seldom or never fully solved. Im- proved methods are being devised constantly, and these intro- duce new factors into old problems. Even greater care must be exercised in adopting a new method on work already in process of production than is required in adopting the original methods, because in these cases, ultimate economy requires that such changes result in a saving which will pay for the dis- carded equipment as well as for the new. The effect of a possible interruption in production must also be carefully considered.
The production records of the manufacturing equipment should be so kept that it will be always possible to trace back
ECONOMICAL PRODUCTION 1 09
through every change in equipment and make direct compari- sons between the results obtained by each method. At the same time, these records should be so simple as not to entail unnecessary clerical expense. Whenever a change is made, the reason for making it should be on record. All these data furnish information which cannot be secured in any other way. As a matter of fact, many plants keep a complete record of changes, but do not provide this class of information when the produc- tion of an entirely new mechanism is undertaken.
General Manufacturing Data. In addition to the specific in- formation required for each individual part and each assembled mechanism, there is a vast amount of general data which must be had before decisions as to the economy of different methods of manufacture can be made with certainty. Much of this information should be available from the cost department records, but, in most cases, these records are kept merely for accounting purposes and their use as engineering data often gives incorrect results.
Factory Cost of Production. It is not the purpose to outline here a new system of cost-accounting. A discussion of some of the factors entering into the factory costs of production, how- ever, is necessary to indicate the character of the information needed to promote economical production. For the purpose of simpler accounting, it is often customary to prorate the entire amount of indirect or overhead charges against the total output of the plant, distributing them according to the direct labor costs. From the accountant's viewpoint, this method is correct. If the product of the plant consists of one simple specialized article, such a method of accounting undoubtedly gives suffi- cient data for general engineering purposes. On the other hand, if the products are varied, or if the productive operations are subdivided into elementary operations, performed in vari- ous departments, the data so collected are incomplete and mis- leading for engineering purposes, because the direct labor cost alone will be the determining factor in selecting the apparent economical methods of production. As a matter of fact, this direct labor cost is but a small percentage of the total cost of
IIO INTERCHANGEABLE MANUFACTURING
production. It seldom amounts to 25 per cent. Furthermore, as the volume of business increases, the percentage of direct labor charges decreases. Thus, as the quantity of production increases, the data so obtained become more and more unreliable.
Another method of distributing the indirect expense consists in establishing overhead rates for each department, prorating these charges in proportion to the direct labor cost as before. If the departments are arranged to contain only one type of equipment, and to perform similar operations, the data so ob- tained are valuable, but such an arrangement of machines and operations is seldom possible or desirable. Different types of work creep into a department. When this condition exists, the information obtained from the use of a departmental overhead will again lead to false conclusions. Such a condition will cause manufacturing methods that are not economical to be accepted.
Certain types of equipment are always duplicated to some extent in several departments. All other things being equal, the cost of duplicate operations on duplicate equipment is identical regardless of the physical location in the plant. But with the use of departmental overhead charges, the book costs will show otherwise. For example, in one plant a sheet-metal part required a foot-press operation between two power-press operations. Foot presses were available in two departments: the power-press department with an overhead charge of 150 per cent, and a sub-assembly department in a distant part of the factory with an overhead charge of only 50 per cent. The original operation list assigned all three operations to the power- press department to eliminate unnecessary trucking and transfer. This was changed so that the second operation would be per- formed in the other department because of the lower overhead there. Actually, this last method cost more than the first because of the trucking and transfers back and forth, but be- cause the book records showed a higher cost for the first method, it was disapproved despite all arguments. This is not an ex- treme case. Similar conditions exist in the majority of manu- facturing plants.
ECONOMICAL PRODUCTION III
It is realized that book costs and actual costs are not identical. To obtain such accuracy would entail a system so complex and elaborate that its cost alone would overbalance all other ex- penses. Yet some simple way must be found to give more nearly true costs of production in order to promote true economy of manufacture. The direct labor and direct material costs are readily obtained. Most accounting methods apply these charges directly against the individual parts, which is the proper distribution. But this, in most cases, disposes of less than half of the total cost of production. Indirect expenses are not only the most difficult to distribute equally, but also involve the larger amount of the costs. The total amount of these charges can be easily determined. This is purely a matter of bookkeeping. Their equitable distribution, however, is more an engineering than an accounting problem.
Distribution of Indirect Factory Expenses. There are three main factors to which most of these indirect expenses can be logically applied: First, the direct labor; second, the general productive equipment; third, the component parts themselves. There are also a number of other indirect expenses which must be charged to the general factory expense. As these are rela- tively few, they can be arbitrarily distributed over the entire product without affecting the value of the data sought. Even- tually, of course, the product must carry them all. The great problem is to distribute them simply and properly.
If the attempt is made to apply all indirect charges to any one of the above factors, many economic errors will result. Attempts have been made to carry them all on the direct labor factor with far from satisfactory results. Neither can they all be applied to the equipment factor with any better results. Each factor must bear only its own indirect expenses. In order to determine where each indirect expense belongs, a process of elimination should be adopted. Without such a factor, would this expense exist? If the expense remains after direct labor, general productive equipment, and individual components are eliminated, it belongs to the general factory expenses or factory management.
112 INTERCHANGEABLE MANUFACTURING
Expenses Due to Direct Labor. Let us first consider the indirect expenses due to direct labor. Hereafter, for the sake of brevity, direct labor will be referred to as labor.
Cost of Supervision. One charge against labor is the cost of supervision as represented by the salary of the foremen and their assistants. The number of these depends chiefly on the number of men to be controlled. This charge could be prorated against the number of men employed. Such a method might, in extreme cases, be erroneous because the higher-priced men should require less supervision. Although, in such cases, it might be more logical to proportion this charge in an inverse ratio to the wages of the men, the clerical work necessary to accomplish this would cost more than the information would be worth.
Making up Payroll. The cost of time-keeping and making up the payroll logically belongs to the labor factor. Eliminate labor and there is no payroll to make up. This should also be distributed on the basis of the number of men employed, as it costs as much to make up the pay account of a man getting fifteen dollars a week as it does to make up the pay account of a man getting thirty dollars. Here again, the indirect expense of high-priced labor is proportionately less than that of lower- priced labor.
Employment Department. The cost of the employment de- partment also belongs to the labor factor. Eliminate labor and there is no further need of an employment bureau. These charges should also be distributed on the basis of the number of men employed, for it costs as much to hire one man as another. In many cases the higher-priced men are the most reliable — not always, of course, as so many other conditions enter into this — and thus it is possible that a closer result would be obtained by distributing the cost in an inverse proportion to the wages of the men.
Educational Department. Wherever personnel or educational departments are established or other similar departments the objects of which are to promote cooperation between the em- ployer and employe to their mutual advantage, all expense
ECONOMICAL PRODUCTION 113
incurred should be charged against labor. It is extremely diffi- cult to analyze such charges accurately — so much depends upon the nature of the activities of such departments and upon the character of the persons with whom they deal. Therefore it might be best to distribute these charges also on the basis of the number of men employed so as to simplify the accounting by prorating all labor charges in a uniform manner.
Maintaining Health and Safety of Employes. All charges for installing safety devices, fire escapes, improved sanitary equip- ment, for heating and lighting, and other similar expenses neces- sary for maintaining the health and safety of the employes as required by law or promoted by the dictates of humanity, belong to the labor factor. However, some of these expenses might be included in other general items — fire escapes may be included in the cost of the buildings, for example — and it may not be possible or feasible to isolate them. Those few cases where it is not practicable to apply expenses directly where they logi- cally belong will not affect the values of the final data much.
It should offer no great accounting difficulties to isolate the majority of the expenses enumerated and all other kindred items. This is all that the accountant would necessarily do. The total amount so determined could be prorated on the basis of the average number of men engaged in actual production, and thus give the labor overhead. This would be used as a constant for a predetermined period in a similar manner to the usual overhead charges, and would be close enough for all prac- tical purposes. Of course, it