JIG AND FIXTURE DESIGN
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JIG AND FIXTURE DESIGN
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JIG AND FIXTURE DESIGN
A TREATISE COVERING THE PRINCIPLES OF JIG AND FIXTURE DESIGN, THE IMPORTANT CONSTRUCTIONAL DETAILS, AND MANY DIF- FERENT TYPES OF WORK-HOLDING DEVICES USED IN INTERCHANGEABLE MANUFACTURE
EDITED BY
FRANKLIN Df JONES
ASSOCIATE EDITOR OF MACHINERY
AUTHOR OF "TURNING AND BORING," "PLANING AND MILLING,
" MECHANISMS AND MECHANICAL MOVEMENTS,"
"THREAD-CUTTING METHODS," ETC.
FIRST EDITION
NEW YORK THE INDUSTRIAL PRESS
LONDON: THE MACHINERY PUBLISHING CO., LTD. I Q2O
COPYRIGHT, 1020
BY
THE INDUSTRIAL PRESS NEW YORK
COMPOSITION AND ELECTROTYPING BY THE PLIMPTON PRESS, NORWOOD, MASS., U. S. A.
PREFACE
The development of machine tools has been accompanied by a corresponding development of auxiliary equipment for in- creasing the quantity and improving the quality of the products of these machines. Whenever duplicate parts require some operation such as drilling, planing, or milling, the selection of a suitable type of machine is often followed by the design of whatever special tools or attachments are needed to adapt the machine to the operation required. The tool-guiding and work-holding jigs and fixtures which are now used in prac- tically all machine shops represent the most important class of special equipment, and this book deals exclusively with their design and construction.
As most jigs are used for drilling operations, a book was previously published entitled "Drilling Practice and Jig De- sign," covering different types of drilling machines and their use, the design of drill jigs, and, to some extent, the design of fixtures such, for example, as are used on milling machines. While the subjects of drilling and jig design are closely allied, it is no longer possible to cover them both in a single volume, owing to the extensive changes in drilling practice and the increasing use of jigs and fixtures of various types on different classes of machine tools. Therefore, the book referred to has been replaced by two volumes, of which this is one. The other book, "Modern Drilling Practice," is already well known to many designers, shop foremen, and machinists interested in the latest types of drilling machines and their use.
This new book, "Jig and Fixture Design," contains that part of the volume on "Drilling Practice and Jig Design" which dealt with jigs and fixtures. This material was used because it is a treatise on the principles of jig and fixture design which contains information that is indispensable in a book of this
VI PREFACE
kind. These original chapters which explain the general pro- cedure in designing jigs and fixtures and how work should be located, clamped, etc.,' have been supplemented by a large amount of new matter, thus making the present book unusually complete. A great variety of jig and fixture designs have been described and illustrated in order to show just how the principles and important details referred to in the forepart of the book are applied under many different conditions and to jigs and fixtures used on various types of machine tools.
Most of the designs illustrated in this book have been sent to MACHINERY from men in the machine-building field, be- cause the designs were considered unusual and worth placing on record. While it would not be possible to give credit to each individual contributor, we are indebted to all who have assisted indirectly in preparing this treatise, and especially to Einar Morin and Albert A. Dowd, recognized tool experts and production engineers, who have supplied valuable material for several of the chapters on jig design.
THE EDITOR.
New York, October, 1920.
CONTENTS CHAPTER I
PAGES
PRINCIPLES OF JIG DESIGN
Objects of Jigs and Fixtures — Difference between Jigs and Fixtures — Fundamental Principles of Jig Design — Locating Points — Clamping Devices — Weight of Jigs — Jigs provided with Feet — Materials for Jigs — General Remarks on Jig Design — Summary of Principles of Jig Design — Types of Jigs — Open Jigs — Box Jigs — De- tails of Jig Design 1-20
CHAPTER II DESIGN OF OPEN DRILL JIGS
Jig Drawings — Designing Open Jigs — Improving the Simple Form of Jig by Adding Locating Screws — Pro- viding Clamps and Feet for the Jig — Examples of Open Drill Jigs 21-44
CHAPTER III DESIGN OF CLOSED OR BOX JIG
General Procedure in the Design of Closed or Box Jigs - Jigs for Rapid Production — Special Features of Box Jigs — Examples of Closed or Box Jigs 45-6?
CHAPTER IV JIG BUSHINGS
Removable Bushings — Material for Jig Bushings - Dimensions of Stationary Jig Bushings — Miscellaneous Types of Jig Bushings — Means for Preventing Loose Bushings from Turning — Dimensions of Removable Bush- ings — Screw Bushings — Special Designs of Guide Bush- ings — Methods of making Jig Bushings — Hardening Jig Bushings — Grinding and Lapping. .... 68-91
vii
Viii CONTENTS
CHAPTER V
LOCATING POINTS AND ADJUST- ABLE STOPS PAGES
Pins and Stops used as Locating Means — Locating by Means of V-blocks — Cup and Cone Locating Points — Screw Bushings and Sliding Bushings used as Locating Means — Adjustable Locating Points — Special Types of Adjustable Stops — Locating from Finished Holes — Lo- cating by Keyways in the Work — Common Defects in Jig Design 92-109
CHAPTER VI JIG CLAMPING DEVICES
Types of Clamps — Hook-bolts — Screw-tightening De- vices — Swinging Leaves — Wedge or Taper Gibs — Ec- centric Clamping Arrangements — Applications to Jig Design 110-150
CHAPTER VII EXAMPLES OF DRILL JIG DESIGN
Different Types of Indexing Jigs — Jig for Deep-hole Drilling — Jig of Simple Design for drilling Straight and Angular Holes — Drill Jig equipped with Milling Attach- ment—Jig for Cross-drilling Pistons and Facing Wrist- pin Bosses — Universal Jigs — Machine Vises with Drill Jig Attachments — Miscellaneous Designs 151-194
CHAPTER VIII BORING JIGS
Boring Jig of Simple Design — Adjustable Boring Jigs - Boring Jig supported on Work — Jigs designed for Sup- porting Bar on One Side of Hole Only — Jigs for Multiple Boring — Combination Drill and Boring Jig 195-210
CONTENTS ix
CHAPTER IX
MILLING AND PLANING FIXTURES PAGES
Fixture for milling to a Given Length — Duplex Fixture -Adjustable Fixture for Angular Work — Fixture ar- ranged for Lateral and Angular Adjustment — Indexing Milling Fixtures — Various Designs of Radial Milling Fixtures — Examples of Planer Fixture Design 211-241
CHAPTER X
ADJUSTABLE FIXTURES FOR TURRET LATHES AND VERTICAL BORING MILLS
Important Points in the Design — Adjustable Fixture for Holding Castings of Different Diameters — Adjustable Fixture for Special Bevel Gear Blanks — Provision for maintaining Accuracy in Adjustable Fixture — Various Designs of Adjustable Fixtures for Vertical Boring Mills. . . 242-256
CHAPTER XI
THE FLOATING PRINCIPLE AS APPLIED TO FIXTURE WORK
Important Points in the Application of Floating Principle — Piston Drill Jig with Floating Clamps — Drill Jig for Rough Collar — Drill Jig with Floating Bushings and Locating Vees — Milling Fixture with Floating Clamps and Locator — Various other Designs of Locating Devices illustrating the Application of the Floating Principle 257-275
CHAPTER XII
APPLICATION OF THE THREE-POINT PRINCI- PLE IN FIXTURES
Three-point Locating and Clamping Devices — Three- point Support for Flywheel Fixture — Three-point Fixture for Pot Casting — Two Methods of Obtaining a Three-point Support on a Hub Casting — Fixture having Three Clamp- ing Jaws and Three Locating Pads — Double Three-point Locating Device 276-287
X CONTENTS
, CHAPTER XIII
SPECIAL JIG AND FIXTURE MECHANISMS PAGES
Equalizing the Pressure of Clamping Devices — Clamps that draw the Work down Firmly on the Locating Means — Multiple-clamping Devices — Clamping Devices for Fixtures that do not interfere with the Tools used — Three-point Clamping Devices 288-305
CHAPTER XIV
PROVIDING FOR UPKEEP IN DESIGNING JIGS AND FIXTURES
Points Pertaining to Upkeep — Drill Jig for a Receiver Forging — Drilling and Reaming Jig — Indexing Fixture for a Clutch Gear — Fixture with Inserted Jaws — Bevel Gear Fixture with Adjustable Features — Fixture for a Hub Casting 306-315
JIG AND FIXTURE DESIGN
CHAPTER I PRINCIPLES OF JIG DESIGN
Jigs and fixtures may be defined as devices used in the manu- facture of duplicate parts of machines and intended to make possible interchangeable work at a reduced cost, as compared with the cost of producing each machine detail individually. Jigs and fixtures serve the purpose of holding and properly locat- ing a piece of work while machined, and are provided with neces- sary, appliances for guiding, supporting, setting, and gaging the tools in such a manner that all the work produced in the same jig or fixture will be alike in all respects, even with the employ- ment of unskilled labor. When using the expression "alike," it implies, of course, simply that the pieces will be near enough alike for the purposes for which the work being machined is intended. Thus, for certain classes of work, wider limits of variation will be permissible without affecting the proper use of the piece machined, while in other cases the limits of varia- tion will be so small as to make the expression "perfectly alike" literally true.
Objects of Jigs and Fixtures. — The main object of using jigs and fixtures is the reduction of the cost of machines or machine details made in great numbers. This reduction of cost is ob- tained in consequence of the increased rapidity with which the machines may be built and the employment of cheaper labor, which is possible when using tools for interchangeable manu- facturing. Another object, not less important, is the accuracy with which the work can be produced, making it possible to assemble the pieces produced in jigs without any great amount of fitting in the assembling department, thus also effecting a great saving in this respect. The use of jigs and fixtures practically does away with the fitting, as this expression was understood in the old-time shop; it eliminates cut-and-try methods, and does
2 JIG DESIGN
away with so-called "patch- work" in the production of machin- ery. It makes it possible to have all the machines built in the shop according to the drawings, a thing which is rather difficult to do if each individual machine in a large lot is built without reference to the other machines in the same lot.
The interchangeability obtained by the use of jigs and fixtures makes it also an easy matter to quickly replace broken or worn- out parts without great additional cost and trouble. When machines are built on the individual plan, it is necessary to fit the part replacing the broken or worn-out piece, in place, involv- ing considerable extra expense, not to mention the delay and the difficulties occasioned thereby.
As mentioned, jigs and fixtures permit the employment of practically unskilled labor. There are many operations in the building of a machine, which, if each machine were built indi- vidually, without the use of special tools, would require the work of expert machinists and toolmakers. Special tools, in the form of jigs and fixtures, permit equally good, or, in some cases, even better results to be obtained by a much cheaper class of labor, provided the jigs and fixtures are properly designed and cor- rectly made. Another possibility for saving, particularly in the case of drill and boring jigs provided with guide bushings in the same plane, is met with in the fact that such jigs are adapted to be used in multiple-spindle drills, thereby still more increasing the rapidity with which the work may be produced. In shops where a great many duplicate parts are made, containing a number of drilled holes, multiple-spindle drills of complicated design, which may be rather expensive as regards first cost, are really cheaper, by far, than ordinary simple drill presses.
Another advantage which has been gained by the use of jigs and fixtures, and which should not be lost sight of in the enu- meration of the points in favor of building machinery by the use of special tools, is that the details of a machine that has been provided with a complete equipment of accurate and durable jigs and fixtures can all be finished simultaneously in different departments of a large factory, without inconvenience, thus mak- ing it possible to assemble the machine at once after receiving
JIG DESIGN 3
the parts from the different departments; and there is no need of waiting for the completion of one part into which another is required to fit, before making this latter part. This gain in time means a great deal in manufacturing, and was entirely impossible under the old-time system of machine building, when each part had to be made in the order in which it went to the finished machine, and each consecutive part had to be lined up with each one of the previously made and assembled details. Brackets, bearings, etc., had to be drilled in place, often with ratchet drills, which is a slow and always inconvenient operation.
Difference between Jigs and Fixtures. — To exactly define the word "jig, " as considered apart from the word "fixture," is difficult, as the difference between a jig and a fixture is often- times not very easy to decide. The word jig is frequently, al- though incorrectly, applied to any kind of a work-holding appli- ance used in the building of machinery, the same as, in some shops, the word fixture is applied to all kinds of special tools. As a general rule, however, a jig is a special tool, which, while it holds the work, or is held onto the work, also contains guides for the respective tools to be used; whereas a fixture is only holding the work while the cutting tools are performing the oper- ation on the piece, without containing any special arrangements for guiding these tools. The fixture, therefore, must, itself, be securely held or fixed to the machine on which the operation is performed; hence the name. A fixture, however, may sometimes be provided with a number of gages and stops, although it does not contain any special devices for the guiding of the tools.
The definition given, in a general way, would therefore clas- sify jigs as special tools used particularly in drilling and boring operations, while fixtures, in particular, would be those special tools used on milling machines, and, in some cases, on planers, shapers, and slotting machines. Special tools used on the lathe may be either of the nature of jigs or fixtures, and sometimes the special tool is actually a combination of both, in which case the term drilling fixture, boring fixture, etc., is suitable.
Fundamental Principles of Jig Design. — Before entering upon a discussion of the minor details of the design of jigs and
4 JIG DESIGN
fixtures, the fundamental principles of jig and fixture design will be briefly outlined. Whenever a jig is made for a compo- nent part of a machine, it is almost always required that a corre- sponding jig be made up for the place on the machine, or other part, where the first-mentioned detail is to be attached. It is, of course, absolutely necessary that these two jigs be perfectly alike as to the location of guides and gage points. In order 'to have the holes and guides in the two jigs in alignment, it is advis- able, and almost always cheaper and quicker, to transfer the holes or the gage points from the first jig made to the other. In many instances, it is possible to use the same jig for both parts. Cases where the one or the other of these principles is applicable will be shown in the following chapters in the detailed descrip- tions of drill and boring jigs.
There are some cases where it is not advisable to make two jigs, one for each of the two parts which are to fit together. It may be impossible to properly locate the jig on one of the parts to be drilled, or, if the jig were made, it may be so complicated that it would not be economical. Under such conditions the component part itself may be used as a jig, and the respective holes in this part used as guides for the tools when machining the machine details into which it fits. Guide bushings for the drills and boring bars may then be placed in the holes in the component part itself. In many cases, drilling and boring opera- tions are also done, to great advantage, by using the brackets and bearings already assembled and fastened to the machine body as guides.
One of the most important questions to be decided before mak- ing a jig is the amount of money which can be expended on a special tool for the operation required. In many cases, it is possible to get a highly efficient tool by making it more compli- cated and more expensive, whereas a less efficient tool may be produced at very small expense. To decide which of these two types of jigs and fixtures should be designed in each individual case depends entirely upon the circumstances. There should be a careful comparison of the present cost of carrying out a certain operation, the expected cost of carrying out the same operation
JIG DESIGN 5
with an efficient tool, and the cost of building that tool itself. Unless this is done, it is likely that the shop is burdened with a great number of special tools and fixtures which, while they may be very useful for the production of the parts for which they are intended, actually involve a loss. It is readily seen how uneconomical it would be to make an expensive jig and fixture for a machine or a part of a machine that would only have to be duplicated a few times. In some cases, of course, there may be a gain in using special devices in order to get extremely good and accurate results.
Locating Points. — The most important requirements in the design of jigs are that good facilities be provided for locating the work, and that the piece to be machined may be easily inserted and quickly taken out of the jig, so that no time is wasted in placing the work in position on the machine performing the work. In some cases, a longer time is required for locating and clamp- ing the piece to be worked upon than is required for the actual machine operation itself. In all such cases the machine per- forming the work is actually idle the greater part of the time, and, added to the loss of the operator's time, is the increased expense for machine cost incurred by such a condition. For this reason, the locating and clamping of the work in place quickly and accurately should be carefully studied by the designer before any attempt is made to design the tool. In choosing the locat- ing surface or points of the piece or part, consideration must be given to the facilities for locating the corresponding part of the machine in a similar manner. It is highly important that this be done, as otherwise, although the jigs may be alike, as far as their guiding appliances are concerned, there may be no facility for locating the corresponding part in the same manner as the one already drilled, and while the holes drilled may coincide, other surfaces, also required to coincide, may be considerably out of line. One of the main principles of location, therefore, is that two component parts of the machine should be located from corresponding points and surfaces.
If possible, special arrangements should be made in the design of the jig so that it is impossible to insert the piece in any but
6 JIG DESIGN
the correct way. Mistakes are often made on this account in shops where a great deal of cheap help is used, pieces being placed in jigs upside down, or in some way other than the cor- rect one, and work that has been previously machined at the expenditure of a great deal of time is entirely spoiled. There- fore, whenever possible, a jig should be made " fool-proof ."
When the work to be machined varies in shape and size, as, for instance, in the case of rough castings, it is necessary to have at least some of the locating points adjustable and placed so that they can be easily reached for adjustment, but, at the same time, so fastened that they are, to a certain extent, positive. In the following chapters different kinds of adjustable locating points will be described in detail.
Clamping Devices. — The strapping or clamping arrangements should be as simple as possible, without sacrificing effectiveness, and the strength of the clamps should be such as to not only hold the piece firmly in place, but also to take the strain of the cutting tools without springing or " giving." When designing the jig, the direction in which the strain of the tool or cutters acts upon the work should always be considered, and the clamps so placed that they will have the highest degree of strength to resist the pressure of the cut.
The main principles in the application of clamps to a jig or fixture are tha£ they should be convenient for the operator, quickly operated, and, when detached from the work, still con- nected with the jig or fixture itself, so as to prevent the oper- ator from losing them. Many a time, looking for lost straps, clamps, screws, etc., causes more delay in shops than the extra cost incurred in designing a jig or fixture somewhat more com- plicated, in order to make the binding arrangement an integral part of the fixture itself. Great complication in the clamping arrangements, however, is not advisable. Usually clamping arrangements of this kind work well when the fixture is new, but, as the various parts become worn, complicated arrangements are more likely to get out of order, and the extra cost incurred in repairing often outweighs the temporary gain in quickness of operation.
JIG DESIGN 7
The judgment of the designer is, in every case, the most im- portant point in the design of jigs and fixtures. Definite rules for all cases cannot be given. General principles can be studied, but the efficiency of the individual tool will depend entirely upon the judgment of the tool designer in applying the general prin- ciples of tool design to the case in hand.
When designing the jig or fixture, the locating and bearing points for the work and the location of the clamps must also be so selected that there is as little liability as possible of springing the piece or jig, or both, out of shape, when applying the clamps. The springing of either the one or the other part will cause in- correct results, as the work surfaces will be out of alignment with the holes drilled or the faces milled. The clamps or straps should therefore, as far as possible, be so placed that they are exactly opposite some bearing point or surface on the work.
Weight of Jigs. — The designer must use his judgment in re- gard to the amount of metal put into the jig or fixture. It is desirable to make these tools as light as possible, in order that they may be easily handled, be of smaller size, and cost less in regard to the amount of material used for their making, but, at the same time, it is poor economy to sacrifice any of the rigidity and stiffness of the tool, as this is one of the main considerations in obtaining efficient results. On large-sized jigs and fixtures, it is possible to core out the metal in a number of places, without decreasing, in the least, the strength of the jig itself. The corners of jigs and fixtures should always be well rounded, and all burrs and sharp edges filed off, so as to make them convenient and pleasant for handling. Smaller jigs should also be made with handles in proper places, so that they may be held in posi- tion while working, as in the case of drilling jigs, and also for convenience in moving the jig about.
Jigs Provided with Feet. — Ordinary drill jigs should always be provided with feet or legs on all sides which are opposite the holes for the bushings, so that the jig can be placed level on the table of the machine. These feet also greatly facilitate the making of the jig, making it easier to lay out and plane the differ- ent finished surfaces. On the sides of the jig where no feet are
u
8 JIG DESIGN
required, if the body is made from a casting, it is of advantage to have small projecting lugs for bearing surfaces when laying out and planing. While jigs are most commonly provided with four feet on each side, in some cases it is sufficient to provide the tool with only three feet, but care should be taken in either case that all bushings and places where pressure will be applied to the tool are placed inside of the geometrical figure obtained by con- necting, by lines, the points of location for the feet.
While it may seem that three feet are preferable to use, because the jig will then always obtain a bearing on all the three feet, which it would not with four feet, if the table of the machine were not absolutely plane, it is not quite safe to use the smaller number of supports, because a chip or some other object is liable to come under one foot and throw the jig and the piece out of line, without this being noticed by the operator. If the same thing happens to a jig with four feet, it will rock and invariably cause the operator to notice the defect. If the table is out of true, this defect, too, will be noticed for the same reason.
Jig feet are generally cast solid with the jig frame. When the jig frame is made from machine steel, and sometimes in the case of cast-iron jigs, detachable feet are used.
Materials for Jigs. — Opinions differ as to the relative merits of cast iron and steel as materials from which to construct the jig and fixture bodies. The decision on this point should depend to a great extent upon the usage to which the fixture is to be put and the character of the work which it is to handle. For small and medium sized work, such as typewriter, sewing machine, gun, adding machine, cash register, phonograph, and similar parts, the steel jig offers decided advantages, but for larger work, such as that encountered in automobile, engine, and machine tool fixtures, the cast-iron jig is undoubtedly the cheaper and more advisable to use. The steel jig should be left soft in order that at any future time additional holes may be added, or the existing bushings changed as required. With a cast-iron jig this adding of bushings is a difficult matter, as the frame is usually bossed and "spot finished" at the point where the bushings are located, and it is very difficult to build up on the jig frame in order to
JIG DESIGN 9
locate or change the bushings. When designing the jig, these points should be remembered and provision made for them, where possible.
General Remarks on Jig Design. — One mistake, quite fre- quently made, is that of giving too little clearance between the piece to be machined and the walls or sides of the jig used for it. Plenty of clearance should always be allowed, particularly when rough castings are being drilled or machined in the jigs; besides, those surfaces in the jig which do not actually bear upon the work do not always come exactly to the dimensions indicated on the drawing, particularly in a cast-iron jig, and allowance ought to be made for such differences.
In regard to the locating points, it ought to be remarked that, in all instances, these should be visible to the operator when placing the work in position, so that he may be enabled to see that the work really is in its right place. At times the construc- tion of the piece to be worked upon may prevent a full view of the locating points. In such a case a cored or drilled hole in the jig, near the locating seat, will enable a view of same, so that the operator may either see that the work rests upon the locating point, or so that he can place a feeler or thickness gage between the work and the locating surface, to make sure that he has the work in its correct position. Another point that should not be overlooked is that jigs and fixtures should be designed with a view of making them easily cleaned from the chips, and provision should also be made so that the chips, as far as possible, may fall out of the jig and not accumulate on or about the locating points, where they are liable to throw the work out of its correct position and consequently spoil the piece.
The principles so far referred to have all been in relation to the holding of the work in the jig, and the general design of the jig for producing accurate work. Provisions, however, should also be made for clamping the jig or fixture to the table of the machine, in cases where it is necessary to have the tool fixed while in operation. Small drilling jigs are not clamped to the table, but boring jigs and milling and planing fixtures invariably must be firmly secured to the machine on which they are used.
10 JIG DESIGN
Plain lugs, projecting out in the same plane as the bottom of the jig, or lugs with a slot in them to fit the body of T-bolts, are the common means for clamping fixtures to the table. For boring jigs, it is unnecessary to provide more than three such clamping points, as a greater number is likely to cause some springing action in the fixture. A slight springing effect is almost unavoidable, no matter how strong and heavy the jig is, but, by properly applying the clamps, it is possible to confine this spring- ing within commercial limits.
Jigs should always be tested before they are used, so as to make sure that the guiding provisions are placed in the right relation to the locating points and in proper relation to each other.
Summary of Principles of Jig Design. — Summarizing the principles referred to, the following rules may be given as the main points to be considered in the designing of jigs and fixtures:
1. Before planning the design of a tool, compare the cost of production of the work with present tools with the expected cost of production, using the tool to be made, and see that the cost of building is not in excess of expected gain.
2. Before laying out the jig or fixture, decide upon the locat- ing points and outline a clamping arrangement.
3. Make all clamping and binding devices as quick-acting as possible.
4. In selecting locating points, see that two component parts of a machine can be located from corresponding points and sur- faces.
5. Make the jig " fool-proof "; that is, arrange it so that the work cannot be inserted except in the correct way.
6. For rough castings, make some of the locating points adjustable.
7. Locate clamps so that they will be in the best position to resist the pressure of the cutting tool when at work.
8. Make, if possible, all clamps integral parts of the jig or fixture.
9. Avoid complicated clamping arrangements, which are liable to wear or get out of order.
JIG DESIGN II
10. Place all clamps as nearly as possible opposite some bearing point of the work, to avoid springing.
11. Core out all unnecessary metal, making the tools as light as possible, consistent with rigidity and stiffness.
12. Round all corners.
13. Provide handles wherever these will make the handling of the jig more convenient.
14. Provide feet, preferably four, opposite all surfaces con- taining guide bushings in drilling and boring jigs.
15. Place all bushings inside of the geometrical figure formed by connecting the points of location of the feet.
1 6. Provide abundant clearance, particularly for rough castings.
17. Make, if possible, all locating points visible to the operator when placing the work in position.
18. Provide holes or escapes for the chips.
19. Provide clamping lugs, located so as to prevent spring- ing of the fixture, on all tools which must be held to the table of the machine while in use, and tongues for the slots in the tables in all milling and planing fixtures.
20. Before using in the shop, for commercial purposes, test all jigs as soon as made.
Types of Jigs. — The two principal classes of jigs are drill jigs and boring jigs. Fixtures may be grouped as milling, planing, and splining fixtures, although there are a number of special fixtures which could not be classified under any special head.
Drill jigs are intended exclusively for drilling, reaming, tap- ping, and facing. Whenever these four operations are required on a piece of work, it is, as a rule, possible to provide the neces- sary arrangements for performing all these operations in one and the same jig. Sometimes separate jigs are made for each one of these operations, but it is doubtless more convenient and cheaper to have one jig do for all, as the design of the jig will not be much more complicated. Although it may be pos- sible to make a distinction between a number of different types of drill jigs, it is almost impossible to define and to get proper
12 JIG DESIGN
names for the various classes, owing to the great variety of shapes of the work to be drilled. There are, however, two general types that are most commonly used, the difference between them being very marked. These types may be classified as open jigs and closed jigs, or box jigs. Sometimes the open jigs are called clamping jigs. The open jigs usually have all the drill bushings in the same plane, parallel with one another, and are not provided with loose or removable walls or leaves, thereby making it possible to insert the piece to be drilled without any manipulation of the parts of the jig. These jigs are often of such a construction that they are applied to the work to be drilled, the jig being placed on the work, rather than the work being placed in the jig. The jig may be held to the work by straps, bolts, or clamps, but in many cases the jig fits into or over some finished part of the work and in this way the jig is located and held in position.
The closed drill jigs, or box jigs, frequently resemble some form of a box and are intended for pieces where the holes are to be drilled at various angles to one another. As a rule, the piece to be drilled can be inserted in the jig only after one or more leaves or covers have been swung out of the way. Some- times it is necessary to remove a loose wall, which is held by bolts and dowel pins, in order to locate the piece in the jig. The work in the closed drill jig may be held in place by set- screws, screw bushings, straps, or hook-bolts.
The combination drilling and boring jig is another type of closed jig designed to serve both for drilling and boring opera- tions. Before designing a combination drill and boring jig, the relation between, and number of, the drilled and bored holes must be taken into consideration, and also the size of the piece to be machined. In case there is a great number of holes, it may be of advantage to have two or even more jigs for the same piece, because it makes it easier to design and make the jig, and very likely will give a better result. The holes drilled or bored in the first jig may be used as a means for locating the piece in the jigs used later on. Combination drill and boring jigs are not very well adapted for pieces of large size.
JIG DESIGN
Open Jigs. — Open jigs of the simpler forms are simply plates provided with bushed holes which are located to cor- respond with the required locations for the drilled holes. While holes are sometimes drilled by first laying out the holes directly upon the work, it is quite evident that this method of drilling would not be efficient if a large number of duplicate parts had to be drilled accurately, as there is likely to be more or less variation in the location of the holes, and considerable loss of time. In the first place, a certain amount of time is required for laying out these holes preparatory to drilling. The operator,
Fig. 1. Jig for Cylinder Flange and Head, and its Application
when starting the drill, must also be careful to make it cut concentric with the scribed circle, which requires extra time, and there will necessarily be more or less variation. To over- come these objections, jigs are almost universally used for hold- ing the work and guiding the drill, when drilling duplicate parts, especially when quite a large number of duplicate pieces must be drilled.
The ring-shaped jig shown at A in Fig. i is used for drilling the stud bolt holes in a cylinder flange and also for drilling the cylinder head, which is bolted to the cylinder. The position of
JIG DESIGN
the jig when the cylinder flange is being drilled is shown at B. An annular projection on the jig fits closely in the cylinder counterbore, as the illustration shows, to locate the jig concentric with the bore. As the holes in the cylinder are to be tapped or threaded for studs, a "tap drill," which is smaller in diameter than the bolt body, is used and the drill is guided by a remov- able bushing b of the proper size. Jigs of this type are often held in position by inserting an accurately fitting plug through the jig and into the first hole drilled, which prevents the jig from turning with relation to the cylinder, when drilling the other holes. When the jig is used for drilling the head, the
opposite side is placed next to the work, as shown at C. This side has a circular recess or counterbore, which fits the projection on the head to properly locate the jig. As the holes in the head must be slightly larger in diameter than the studs, another sized drill and a guide bushing of corresponding size are used. The cylinder is, of course, bored and the head turned before the drilling is done.
Jigs of the open class, as well as those of other types, are made in a great variety of shapes, and, when in use, they are either applied to the work or the latter is placed in the jig. When the work is quite large, the jig is frequently placed on it, whereas small parts are more often held in the jig, which is so designed that the work can be clamped in the proper position. The form of any jig depends, to a great extent, on the shape of the work for which it is intended and also on the location of the holes to be drilled. As the number of differently shaped pieces which go to make up even a single machine is often very
Fig. 2. Drill Jig of the Box Type
JIG DESIGN 15
great, and as most parts require more or less drilling, jigs are made in an almost endless variety of sizes and forms. When all the holes to be drilled in a certain part are parallel, and es- pecially if they are all in the same plane, a very simple form of jig can ordinarily be used.
Box Jigs. — A great many machine parts must be drilled on different sides and frequently castings or forgings are very irregular in shape, so that a jig which is made somewhat in
J L
Fig. 3. Box Jig for Drilling Ball shown enlarged at A
the form of a box, and encloses the work, is very essential, as it enables the guide bushings to be placed on all sides and also makes it comparatively easy to locate and securely clamp the part in the proper position for drilling. This type of jig, which, because of its form, is known as a closed or "box jig," is used very extensively.
A box jig of simple design is shown in Fig. 2. This particu- lar jig is used for drilling four small holes in a part (not shown) which is located with reference to the guide bushings B by a central pin A attached to the jig body. This pin enters a hole in the work, which is finished in another machine in connection
i6
JIG DESIGN
with a previous operation. After the work is inserted in the jig, it is clamped by closing the cover C, which is hinged at one end and has a cam-shaped clamping latch D at the other, that engages a pin E in the jig body. The four holes are drilled by passing the drill through the guide bushings B in the cover.
Another jig of the same kind, but designed for drilling a hole having two diameters through the center of a steel ball,
Fig. 4. Box Jigs for Drilling Parts shown by Heavy Dot-and-dash Lines
is shown in Fig. 3. The work, which is shown enlarged at A, is inserted while the cover is thrown back as indicated by the dotted lines. The cover is then closed and tightened by the cam-latch Z), and the large part of the hole is drilled with the jig in the position shown. The jig is then turned over and a smaller drill of the correct size is fed through guide bushing B on the opposite side. The depth of the large hole could be gaged for each ball drilled, by feeding the drill spindle down to a certain position as shown by graduation or other marks, but
JIG DESIGN
if the spindle has an adjustable stop, this should be used. The work is located in line with the two guide bushings by spherical seats formed in the jig body and in the upper bushing, as shown. As the work can be inserted and removed quickly, a large num- ber of balls, which, practically speaking, are duplicates, can be drilled in a comparatively short time by using a jig of this type.
A box jig that differs somewhat in construction from the design just referred to is illustrated at A in Fig. 4, which shows
Fig. 5. Jig shown at A, Fig. 4, in Two Different Drilling Positions
a side and top view. The work, in this case, is a small casting the form of which is indicated by the heavy dot-and-dash lines. This casting is drilled at a, b, and c, and the two larger holes a and b are finished by reaming. The hinged cover of this jig is opened for inserting the work by unscrewing the T-shaped clamping screw s one-quarter of a turn, which brings the head in line with a slot in the cover. The casting is clamped by tighten- ing this screw, which forces an adjustable screw bushing g down against the work. By having this bushing adjustable, it can be set to give the right pressure, and, if the height of the cast-
1 8 JIG DESIGN
ings should vary, the position of the clamping bushing could easily be changed.
The work is properly located by the inner ends of the three guide bushings ai, bi, and ci, and also by the locating screws I against which the casting is held by knurled thumb-screws m and n. When the holes a and b are being drilled, the jig is placed with the cover side down, as shown at A in Fig. 5, and the drill is guided by removable bushings, one of which is shown at r. When the drilling is completed, the drill bushings are replaced by reamer bushings and each hole is finished by ream- ing. The small hole c, Fig. 4, is drilled in the end of the cast- ing by simply placing the jig on end as shown at B, Fig. 5. Box jigs which have to be placed in more than one position for drilling the different holes are usually provided with feet or extensions, as shown, which are accurately finished to align the guide bushings properly with the drill. These feet extend beyond any clamping screws, bolts, or bushings which may protrude from the sides of the jigs, and provide a solid support. When inserting work in a jig, care should be taken to remove all chips which might have fallen upon those surfaces against which the work is clamped and which determine its location.
Still another jig of the box type, which is quite similar to the one shown at A, Fig. 4, but is arranged differently, owing to the shape of the work and location of the holes, is shown at B in the same illustration. The work has three holes in the base h, and a hole at i which is at an angle of 5 degrees with the base. The three holes are drilled with the jig stand- ing on the opposite end y, and the angular hole is drilled while the jig rests on the four feet k, the ends of which are at such an angle with the jig body that the guide bushing for hole i is prop- erly aligned with the drill. The casting is located in this jig by the inner ends of the two guide bushings w and the bushing o and also by two locating screws p and a side locating screw q. Adjustable screws t and t\ in the cover hold the casting down, and it is held laterally by the two knurled thumb-screws u and v. If an attempt were made to drill this particular part without a jig (as would be done if only a few castings were
JIG DESIGN 19
needed) it would have to be set with considerable care, provided the angle between hole i and those in the base had to be at all accurate, and it would be rather difficult to drill a number of these castings and have them all duplicates. By the use of a jig, however, designed for drilling this particular casting, the relative positions of the holes in any number of parts are practically the same and the work can be done much more quickly than would be possible if it were held to the drill-press table by ordinary clamping appliances. Various designs of jigs will be described in Chapter VII.
Details of Jig Design. — The general principles of the design and use of jigs have been explained. The details of jig design will now be considered. Generally speaking, the most im- portant parts of a jig are the guide bushings for the drills and other tools, the clamping devices, and the locating points, against which the work is placed to insure an accurate posi- tion in the jig. The guides for the cutting tools in a drill jig take the form of concentric steel bushings, which are placed in the jig body in proper positions.
The drill bushings are generally made of tool steel, hardened and lapped, and, where convenient, should be ground inside and out. They should also be long enough to support the drill on each side regardless of the fluting, and they should be so located that the lower end of the bushings will stop about the same distance above the work as the diameter of the drill, so that chips will clear the bushings readily. Where holes are drilled on the side of a convex or a concave surface, the end of the bushing must be cut on a bevel and come closer to the part being drilled, to insure the drill having adequate support while starting into the work. The bushings should have heads of sufficient diameter. Long bushings should be relieved by in- creasing the hole diameter at the upper end. The lower end of the bushing should have its edges rounded, in order to permit some of the chips being shed from the drill easily, instead of all of them being forced up through the bushing. It is also good practice to cut a groove under the head for clearance for the wheel when grinding the bushing on the outside. A com-
20 JIG DESIGN
plete treatise covering dimensions and design is given in the chapter on "Jig Bushings."
In order to hold the work rigidly in the jig, so that it may be held against the locating points while the cutting tools operate upon the work, jigs and fixtures are provided with clamping devices. Sometimes a clamping device serves the purpose of holding the jig to the work, in a case where the work is a very large piece and the jig is attached to the work in some suitable way. The purpose of the clamping device, however, remains the same, namely, that of preventing any shifting of the guiding bushings while the operation on the work is performed. The clamping device should always be an integral part of the jig body in order to prevent its getting lost. Different types of clamping devices are shown and described in the chapter on "Jig Clamping Devices. "
The locating points may consist of screws, pins, finished pads, bosses, ends of bushings, seats, or lugs cast solid with the jig body, etc. The various types used are described in detail in the chapter on "Locating Points and Adjustable Stops."
CHAPTER II DESIGN OF OPEN DRILL JIGS
To give any rational rules or methods for the design of drill jigs would be almost impossible, as almost every jig must be designed in a somewhat different way from every other jig, to suit and conform to the requirements of the work. All that can be done is to lay down the principles. The main principles for jigs as well as fixtures were treated at length in Chapter I. It is proposed in the present chapter to dwell more in detail on the carrying out of the actual work of designing jigs.
Jig Drawings. — Before making any attempt to put the lay- out of the jig on paper, the designer should carefully consider what the jig will be required to do, the limits of accuracy, etc., and to form, in his imagination, a certain idea of the kind of a jig that would be suitable for the purpose. In doing so, if a model or sample of the work to be made is at hand, it will be found to be a great help to study the actual model. If the draw- ing, as is most often the case, is the only thing that is at hand, then the outline of the work should be drawn in red (or other colored) ink on the drawing paper, on which the jig is subse- quently to be laid out, and the jig built up, so to speak, around this outline. The designing of the jig will be greatly simplified by doing this, as the relation between the work and the jig will always be plainly before the designer, and it will be more easily decided where the locating points and clamping arrangements may be properly placed. When drawing and projecting the different views of the jig on the paper, the red outline of the work will not in any way interfere, and when the jig is made from the drawing, the red lines are simply ignored, except to the extent to which the outline of the pieces may help the toolmaker to understand the drawing and the purpose of certain locating points and clamping devices.
21
22 JIG DESIGN
If possible, the jig should be drawn full size, as it is a great deal easier to obtain the correct proportions when so doing. Of course, in many cases, it will be impossible to draw the jigs full size. In such cases the only thing to do is to draw them to the largest possible regular scale. Every jig draftsman should be supplied with a set of blueprints containing dimensions of standard screws, bolts, nuts, thumb-screws, washers, wing-nuts, sliding points, drills, counterbores, reamers, bushings, etc.; in short, with blueprints giving dimensions of all parts that are used in the construction of jigs, and which are, or can be, standardized. It should be required of every designer and draftsman that he use these standards to the largest possible extent, so as to bring the cost of jigs down to as low a figure as possible.
It is highly desirable, for the obtaining of best results, that, before starting on the drawing, the draftsman who is to lay out the jig should consult the foreman who is actually going to use the jig. Oftentimes this man will be able to supply the best idea for the making of the jig or tool. The combined experience of the draftsman and the foreman will generally produce a much better tool than could either of them alone.
As a jig drawing, in most cases, is only used once, or at most only a very few times, it is not advisable to make a tracing or blueprint from the drawing, but, as a rule, the pencil drawing itself may be used to advantage. If, however, it is given out in the shop directly as it comes from the drawing-board, it is likely to become soiled, so that, after a while, it would be impossible to make out the meaning of the views shown on it. For this reason jig drawings should be made on heavy paper, preferably of brown color, which is not as quickly soiled as white paper; and in order to prevent the drawing from being torn, it should be mounted on strawboard, and held down along the edges by thin wooden strips, nailed to the board. It is also desirable to cover the drawings with a thin coat of shellac before they are sent out into the shop. When this is done, dirt and black spots may be washed off directly; and the shellac itself may be washed off by wood alcohol, when the drawing is returned to the draft- ing-room. The drawing, after having been cleaned, is then
OPEN DRILL JIGS 23
detached from the strawboard, which may be used over and over again. The drawing is, of course, filed away according to the drafting-room system. The most advantageous sizes for jig drawings for from medium to heavy work are about as follows:
1. Full-size sheet, 40 X 27^ inches.
2. Half-size sheet, 27^ X 20 inches.
3. Quarter-size sheet, 20 X 13! inches.
4. Eighth-size sheet, i3f X 10 inches.
Of course, these sizes will vary in different shops, and in many cases, particularly when the tool-designing department and the regular drafting-room are combined as one drafting department, the jig drawings should be of the same regular sizes as the ordi- nary machine drawings.
It is common practice in a great many shops to make no de- tailed drawings of jigs, but simply to draw a sufficient number of views and sections, and to dimension the different parts directly on the assembly drawings. In cases where the jig drawings are complicated, and where they are covered with a large number of dimensions which make it hard to read the drawing and to see the outlines of the jig body itself, it has proved a great help to trace the outlines of the jig body, and of such portions as are made of cast iron, on tracing paper, omitting all loose parts, and simply putting on the necessary dimensions for making the pat- terns. A blueprint is then made from this paper tracing, and is sent to the patternmaker, who will find the drawing less of a puzzle, and who will need to spend far less time to understand how the pattern actually looks. It is, however, good policy to detail jig drawings completely, the same as other machine de- tails.
When jigs are made for pieces of work which require a great many operations to be carried out with the same jig, and where a great number of different bushings, different sizes of drills, reamers, counterbores, etc., are used, a special operation sheet should be provided, which should be delivered to the man using the jig, together with the jig itself. This enables him to use the jig to best advantage. On this sheet should be marked the order in which the various operations are to be performed and the
aj
JIG DESIGN
o
L-L1L.
a
qp
L-.-.-1
t
OPEN DRILL JIGS 25
tools and bushings which are to be used. The bushings should be numbered or marked in some way so as to facilitate the selec- tion of the correct bushing for the particular tool with which it is used. If this system is put in force and used for simpler classes of jigs also, the operator will need few or no instructions from the foreman, outside of this operation sheet.
Designing Open Jigs. — The present chapter will be de- voted to explaining and illustrating the application of the prin- ciples previously outlined, to the simplest and most common design of drill jig — the open jig. Assume that the drill jig is to be designed for a piece of work, as shown in Fig. i. Con- sideration must first be given to the size of the piece, to the finish given to the piece previous to the drilling operation, the accu- racy required as regards the relation of one hole to the other, and in regard to the surfaces of the piece itself. The number of duplicate pieces to be drilled must also be considered, and, in some cases, the material.
The simplest kind of drill jig that could be used for the case taken as an example would be the one illustrated in Fig. 2, which simply consists of a flat plate of uniform thickness of the same outline as the piece to be drilled, and provided with holes for guiding the drill. Such a jig would be termed a jig-plate. For small pieces, the jig-plate would be made of machine steel and casehardened, or from tool steel and hardened. For larger work, a machine-steel plate can also be used, but in order to avoid the difficulties which naturally would arise from harden- ing a large plate, the holes are simply bored larger than the required size of drill, and are provided with lining bushings to guide the drill, as shown in Fig. 3. It would not be necessary, however, to have the jig-plate made from steel, for large work, as a cast-iron plate provided with tool steel or machine-steel guiding bushings would answer the purpose just as well, and be much cheaper. The thickness of the jig-plate varies accord- ing to the size of the holes to be drilled and the size of the plate itself.
The holes in the jig in Fig. 2 and in the bushings in the jig in Fig. 3 are made the same size as the hole to be drilled in the work,
26 JIG DESIGN
with proper clearance for the cutting tools. If the size and loca- tion of the holes to be drilled are not very important as regards accuracy, it is sufficient to simply drill through the work with a full-sized drill guided by the jig-plate, but when a nice, smooth, standard-sized hole is required, the holes in the work must be reamed. The hole is first spotted by a spotting drill, which is of exactly the same size as the reamer used for finishing, and which nicely fits the hole in the jig-plate or bushing. Then a so-called reamer drill, which is o.oio inch, or less, smaller in diameter than the reamer, is put through, leaving only a slight amount of stock for the reamer to remove, thereby obtaining a very satisfactory hole. Sometimes a separate loose bushing is used for each one of these operations, but this is expensive and also unnecessary, as the method described gives equally good results.
By using the rose reaming method very good results will also be obtained. In this case two loose bushings besides the lining bushing will be used. These bushings are described and tabu- lated in a following chapter. The drill preceding the lose chucking reamer is TV inch smaller than the size of the hole. This drill is first put through the work, a loose drill bushing made of steel being used for guiding the drill. Then the rose chucking reamer is employed, using, if the hole in the jig be large, a loose bushing made of cast iron.
When dimensioning the jig on the drawing, dimensions should always be given from two finished surfaces of the jig to the center of the holes, or at least to the more important ones. In regard to the holes, it is not sufficient to give only the right angle dimensions, a, 5, c, and d, etc., Fig. 2, but the radii between the various holes must also be given. If there are more than two holes, the radii should always be given between the nearest holes and also between the holes that bear a certain relation to one another, as, for instance, between centers of shafts carry- ing meshing gears, sprockets, etc. This will prove a great help to the toolmaker. In the case under consideration, the dimen- sions ought to be given from two finished sides of the work to the centers of the holes, and also the dimension between the centers of the holes to be drilled.
OPEN DRILL JIGS
When using a simple jig, made as outlined in Figs. 2 and 3, this jig is simply laid down flat on the work and held against it by a C-clamp, a wooden clamp, or, if convenient, held right on the drill-press table by means of a strap or clamp, as shown in Fig. 4. Here two pieces of the work are shown beneath the jig- plate, both being drilled at one time.
Improving the Simple Form of Jig. — The first improvement that could be made on the jig shown in Fig. 3 would be the plac- ing of locating points in the jig-plate in the form of pins, as shown in Fig. 5, in which the dotted lines represent the outline of the work. The plate need not necessarily have the shape shown in
n
rr
I
Fig. 7. Simple Jig with Locating Screws Holding the Work in Place
Fig. 5, but may have the appearance shown in Fig. 6, according to the conditions.
The adding of the locating points will, of course, increase the cost of the jig, but the amount of time saved in using the jig will undoubtedly make up for the added expense of the jig, provided a fair number of pieces is to be drilled; besides a great advantage is gained in that the holes will always be located in the same relation to the two sides resting against the locating pins on all the pieces drilled. The locating pins are flattened off to a depth of TV inch from the outside circumference, and dimensions should be given from the flat to the center of the pin
28 JIG DESIGN
holes and to the center of the nearest or the most important of the holes to be drilled in the jig. The same strapping or clamp- ing arrangements for the jig and work, as mentioned for the simpler form of jig, may be employed.
Improving the Jig by Adding Locating Screws. — The next step toward improving the jig under consideration would be to provide the jig with locating screws, as shown in Fig. 7. By the addition of these, the locating arrangements of the jig be- come complete, and the piece of work will be prevented from shifting or moving sideways. These locating screws are placed so that the clamping points come as nearly opposite to some bearing points on the work as possible. In order to provide for locating set-screws in our present jig, three lugs or projections A are added which hold the set-screws. If possible the set-screw lugs should not reach above the surface of the work, which should rest on the drill-press table when drilling the holes.
The present case illustrates the difficulty of giving exact rules for jig design. Two set-screws are used on the long side of the work, but in a case like this, where the piece is comparatively short and stiff, one lug and set-screw, as indicated by the dotted lines at B in Fig. 7, would be fully sufficient. The strain of the set-screw placed right between the two locating pins will not be great enough to spring the piece out of shape. When the work is long and narrow, two set-screws are required on the long side, but, in the case illustrated, two lugs would be considered a waste- ful design.
Providing Clamps and Feet for the Jig. — The means by which the work has been clamped or strapped to the jig when drilling in the drill press (see Fig. 4) have not been integral parts of the jig in the simple types shown. If clamping arrangements that are integral parts of the jig are to be added, the next improve- ment would be to add four legs in order to raise the jig-plate enough above the surface of the drill-press table to get the re- quired space for such clamping arrangements. The completed jig of the best design for rapid manipulation and duplicate work would then have the appearance shown in Fig. 8. The jig here is provided with a handle cast integral with the jig body, and
OPEN DRILL JIGS
with a clamping strap which can be pulled back for removing and inserting the work. Instead of having the legs solid with the jig, as shown in Fig. 8, loose legs, screwed in place, are some- times used, as shown in Fig. 9.
These legs are round and provided with a shoulder A, prevent- ing them from screwing into the jig-plate. A headless screw or pin through the edge of the circumference of the threads at the top prevents the studs from becoming loose. These loose legs are usually made of machine steel or tool steel, the bottom end
Standard Jig Feet
Me
Me
M
9ia
Me
H
5/i2
Me
'Me %
N
H
Me
Me
Screws for Jig Feet
o. 160 o. 191 0.213 0.233 0.256
%4
Me
O.IIO
0.123
0.137
0.150 0.164
Ma
Me
0.299
0-343 0.386 0.426
H
9xi2
Me
0.192 0.219 0.246 0.273
Me
being hardened and then ground and lapped, so that all four legs are of the same length. It is the practice of many tool- makers not to thread the legs into the jig body, but simply to provide a plain surface on the end of the leg, which enters into the jig-plate, and is driven into place. This is much easier, and there is no reason why, for almost all kinds of work, jigs provided with legs attached in this manner should not be equally durable.
Jig feet are also made of the form shown in the accompanying table, where a separate screw is used for holding the jig feet to the jig body.
When jigs are made of machine or tool steel, and feet are
3o'
JIG DESIGN
OPEN DRILL JIGS 31
required, the only way to provide them is to insert loose feet. In the case of cast-iron jigs, however, solid legs cast in place are preferable. The solid legs cast in place generally have the appear- ance shown in the upper right-hand corner of Fig. 8. The two webs of the leg form a right angle, which, for all practical pur- poses, makes the leg fully as strong as if it were solid. The leg is tapered 15 degrees, as a rule, as shown in the engraving, but this may be varied according to conditions. The thickness of the leg varies according to the size of the jig, the weight of the work, and the pressure of the cutting tools, and depends also upon the length of the leg. The length b on top is generally made one and one-half times a. As an indication of the size of the legs required, it may be said that for smaller jigs, up to jigs with a face area of 6 square inches, the dimension a may be made from -£$ to f inch; for medium-sized jigs, J to f inch; for larger-sized jigs, f to i| inch; but, of course, these dimensions are simply indications of the required dimensions. As to the length of the legs, the governing condition, evidently, is that they must be long enough to reach below the lowest part of the work and the clamping arrangement, as clearly indicated in the design in Fig. 8.
If a jig is to be used in a multiple-spindle drill, it should be designed a great deal stronger than it is ordinarily designed when used for drilling one hole at a time. This is especially true if there is a large number of holes to drill simultaneously. It is evident that the pressure upon the jig in a multiple-spindle drill is as many times greater than the pressure in a common drill press as the number of drills in operation at once.
Referring again to Fig. 8, attention should be called to the small lugs A on the sides of the jig body which are cast in place for laying out and planing purposes. The handle should be made about 4 inches long, which permits a fairly good grip by the hand. The design of the jig shown is simple, and fills all requirements necessary for producing work quickly and accu- rately; at the same time, it is strongly and rigidly designed. Locating points of a different kind from those shown can, of course, be used; and the requirements may be such that adjust-
JIG DESIGN
able locating points, as described in a following chapter, may be required. A more quick-acting, but, at the same time, a far more complicated clamping arrangement might be used, but the question is whether the added increase in the rapidity of manipu- lation offsets the expense thus incurred.
A question which the designer should always ask himself is: Can more than one piece be drilled at one time? In the present case, the locating pins can be made longer, or, if there is a locat- ing wall, it can be made higher, the legs of the jig can be made longer, and the screw holding the clamp can also be increased in length. If the pieces of work are thick enough, set-screws for
e-
Fig. 9. Legs Screwed into Jig Body
holding the work against the locating pins can be placed in a vertical line, or if the pieces are narrow, they can be placed diagonally, so as to gain space. If the pieces are very thin, the locating might be a more difficult proposition. If they are made of a uniform width, they may simply be put in the slot in the bottom of the jig, as shown in the lower right-hand corner of Fig. 8, or if a jig on the principles of the one shown to the left is used, they might be located sideways by a wedge, as shown in Fig. 10. A couple of lugs A would then be added to hold the wedge in place and take the thrust. In both cases the pieces must be pushed up in place endways by hand. If the pieces are not of exactly uniform size and it is desired to drill a number
OPEN DRILL JIGS
33
at a time, they must be pushed up against the locating pins by hand from two sides, and the clamping strap must be depended upon to clamp them down against the pressure of the cut, and at the same time prevent them from moving side or endways. If the accuracy of the location of the holes is important, but one piece at a time should be drilled.
Examples of Open Drill Jigs. — A typical example of an open drill jig, very similar to the one just developed and explained, is shown in Fig. n. The work is located against the three locat- ing pins A, and held in place against these pins by the three set-screws B. The three straps C hold the work securely against
Fig. 10. Jig with Wedge for Holding the Work
the finished pad, in the bottom of the jig. These clamps are so placed that when the work has been drilled and the clamp screws loosened, the clamps will swing around a quarter of a turn, allow- ing the work to be lifted directly from the jig and a new piece of work inserted; then the clamps are again turned around into the clamping position, and the screws tightened. These straps are integral parts of the jig; at the same time, they are quickly and easily manipulated, and do not interfere with the rapid removal and insertion of the work. The strength and rigidity of the feet in proportion to the jig should be noted, this strength being ob- tained by giving proper shape to the feet, without using an un- necessary quantity of metal.
The jig in Fig. 1 1 is also designed to accommodate the compo- nent part of the work when it is to be drilled. When this is done, the work is held on the back side of the jig, shown in Fig. 12.
34 JIG DESIGN
This side is also provided with feet, and has a finished pad against which the work is held. The locating pins extend clear through the central portion of the jig body, and, consequently, will locate the component part of the work in exactly the same position as the piece of work drilled on the front side of the jig. The same clamping straps are used, the screws being simply put in from the opposite side into the same tapped holes as are used when clamping on the front side of the jig. The four holes D are guide holes for drilling the screw holes in the work, these being drilled the body size of the bolt in one part, and the tap drill size
Fig. ii. Example of Open Drill Jig. View showing Front Side
in the component part. The lining bushing in the holes D serves as a drill bushing for drilling the body size holes. The loose bushing E, Fig. n, is used when drilling the tap holes in the component part, the inside diameter of this bushing being the tap drill size, and the outside diameter a good fit in the lining bushing. The two holes F, Fig. 12, are provided with drill bushings and serve as guides when drilling the dowel pin holes, which are drilled below size, leaving about o.oio inch, and are reamed out after the two component parts of the work are put together. The two holes shown in the middle of the jig in Fig. n, which are provided with lining bushings, and also with loose bushings, as shown inserted in Fig. 12, may be used for
OPEN DRILL JIGS 35
drilling and reaming the bearing holes for the shafts passing through the work. In this particular case, however, they are used only for rough-drilling the holes, to allow the boring-bars to pass through when finishing the work by boring in a special boring jig, after the two parts of the work have been screwed together.
The large bushings shown beside the jig in Fig. n are the loose bushings shown in place in Fig. 12. It will be noted that the bushings are provided with dogs for easy removal, as ex- plained in a following chapter. As the central portion of the
Fig. 12. Rear View of Drill Jig shown in Fig. n
jig body is rather thin, it will be seen from Fig. 12 that the bosses for the central holes project outside of the jig body in order to give a long enough bearing to the bushings. This, of course, can be done only when such a projection does not interfere with the work. The bosses, in this particular case, also serve another purpose. They make the jig " fool-proof ," because the pieces drilled on the side of the jig shown in Fig. n cannot be put on the side shown in Fig. 12, the bosses preventing the piece from being placed in position in the jig.
Attention should be called to the simplicity of the design of this jig. It simply consists of a cast-iron plate, with finished seats, and feet projecting far enough to reach below the work
JIG DESIGN
when drilling, three dowel pins, set-screws for bringing the work up against the dowel pins, three clamps, and the necessary bushings. The heads of all the set-screws and bolts should, if possible, be made the same size, so that the same wrench may be used for tightening and unscrewing all of them. It can also be plainly seen from the halftones that there are no unnecessarily finished surfaces on the jig, a matter which is highly important in economical production of tools.
Another example of an open drill jig, similar in design to the one just described, is shown in Fig. 13. The work to be drilled
*
Fig. 13. Drill Jig Used for Drilling Work shown to the Right
in this jig is shown at A and B at the right-hand side of the jig. In this case, the work is located from the half-circular ends. The pieces A and B are component parts and, when finished, are screwed together. The piece A is located against three dowel pins, and pushed against them by set-screws C, and held in posi- tion by three clamping straps, as shown in Fig. 14. In this case, the straps are provided with oblong slots as indicated, and when the clamp screws are loosened the clamps are simply pulled backward, permitting the insertion and removal of the work without interference. It would improve this clamping arrange- ment to place a stiff helical spring around the screws under each strap, so that the straps would be prevented from falling down to
OPEN DRILL JIGS 37
the bottom of the jig when the work was removed. At the same time this would prevent the straps from swiveling around the screws when not clamped.
In Fig. 15, the part B in Fig. 13 is shown clamped in position for drilling, the opposite side of the jig being used for this pur- pose. In jig design of this kind it is necessary to provide some means so that the parts A and B will be placed each on the correct side of the jig, or, as mentioned, the jig should be made " fool-proof. " In the present case, the parts cannot be exchanged and placed on the wrong side, because the cover or guard B can- not be held by the three straps shown in Fig. 14, as the screws
Fig. 14. Drill Jig shown in Fig. 13 with Work in Place
for the straps are not long enough. On the other hand, the piece A could not be placed on the side shown in Fig. 15, because the long bolt and strap used for clamping on this side would interfere with the work.
It may appear to be a fault in design that three straps are used to fasten the piece A in place, and only one is employed for hold- ing piece B. This difference in clamping arrangement, however, is due to the different number and the different sizes of holes to be drilled in the different pieces. The holes in the piece A are larger and the number of holes is greater, and a heavier clamping arrangement is, therefore, required, inasmuch as the thrust on
3 8 JIG DESIGN
the former is correspondingly greater, the multiple-spindle drill being used for drilling the holes. If each hole were drilled and reamed individually, the design of the jig could have been com- paratively lighter.
In the design shown, the locating of each piece individually in any but the right way is also taken care of. The piece A, which is shown in place in the jig, Fig. 14, could not be swung around into another position, because the strap and screw at E would interfere. For the same reason, the cover or guard B could not be located except in the right way. As shown in Fig.
Fig. 15. Rear View of Drill Jig shown in Fig. 13, with Cover to be Drilled in Place
15, the strap and screw would have to be detached from the jig in order to get the cover in place, if it were turned around. The locating pins for the work pass clear through the body of the jig, and are used for locating both pieces. The pieces are located diagonally in the jig, because, by doing so, it is possible to make the outside dimensions of the jig smaller. In this particular case the parts are located on the machine to which they belong, in a diagonal direction, so that the additional advantage is gained of being able to use the same dimensions for locating the jig holes as are used on the drawing for the machine details them- selves. This also tends to eliminate mistakes in making the jigs. Sometimes, when more or less complicated mechanisms are
OPEN DRILL JIGS 39
composed of several parts fitted together and working in relation to each other, as, for instance, friction clutches, one jig may be made to serve for drilling all the individual parts, by the addition of a few extra parts applied to the jig when different details of the work are being drilled. In Figs. 16, 17, and 18, such a case is illustrated. The pieces A, B, and C, in Fig. 16, are component parts of a friction clutch, and the jig in which these parts are being drilled is shown in the same figure, to the left. Suppose now that the friction expansion ring A is to be drilled. The jig is bored out to fit the ring before it is split and when it is only
Fig. 1 6. Drill Jig for Parts of Friction Clutches shown at the Right
rough-turned, leaving a certain number of thousandths of an inch for finishing. The piece is located, as shown in Fig. 17, against the steel block D entering into the groove in the ring, and is then held by three hook-bolts, which simply are swung around when the ring is inserted or removed. The hook-bolts are tightened by nuts on the back side of the jig. Three holes marked E in Fig. 17 are drilled simultaneously in the multiple- spindle drill, and the fourth hole F (see Fig. 16) is drilled by turning the jig on the side. The steel block D, Fig. 17, is hard- ened, and has a hole to guide the drill when passing through into the other side of the slot in the ring. The block is held in place by two screws and two dowel pins.
3J
40 JIG DESIGN
When drilling the holes in the lugs in the friction sleeve B, Fig. 1 6, the block D and the hook-bolts are removed. It may be mentioned here, although it is a small matter, that these parts should be tied together when removed, and there should be a specified place where all the parts belonging to a particular jig should be kept when not in use. The friction sleeve B fits over the collar G} Fig. 180 This collar is an extra piece, belong- ing to the jig, and used only when drilling the friction sleeve; it should be marked with instructions for what purpose it is used. The collar G fits over the projecting finished part H in
Fig. 17. Drill Jig shown in Fig. 16, with One of the Pieces in Place
the center of the jig, and is located in its right position by the key ways shown. The keyway in the friction sleeve B} which must be cut and placed in the right relation to the projecting lugs before the piece can be drilled, locates the sleeve on the collar G} which is provided with a corresponding keyway. A flange on the collar G} as shown more plainly at L in Fig. 18, locates the friction sleeve at the right distance from the bottom of the jig, so that the holes will have a proper location sideways. Two collars, G and L, are used for the same piece B, this being necessary because the holes M and M in the projecting lugs shown in Fig. 16 are not placed in the same relation to the sides of the friction sleeve. The collars are marked to avoid mis- takes, and corresponding marks on the jig provided so as to
OPEN DRILL JIGS
assure proper location. The friction sleeve is clamped in place by a strap which, in this case, does not form an integral part of the jig. This arrangement, however, is cheaper than it would have been to carry up two small projections on two sides of the jig and employ a swinging leaf and an eye-bolt, or some arrange- ment of this kind. Besides, the strap is rather large, and could not easily get lost. The jig necessarily has a number of loose parts, on account of being designed to accommodate different details of the friction clutch.
The friction disks C, in Fig. 16, when drilled, fit directly over the projecting finished part H of the jig, and are located on this
Fig. 18. Drill Jig shown in Fig. 16 used for Drilling Friction Sleeve
projection by a square key. The work is brought up against the bottom of the jig and held in this position by the strap shown in Fig. 1 8 for holding the friction sleeve. The bushings of different sizes shown in Fig. 18 are used for drilling the different sized holes in the different parts.
In all the various types of drill jigs described, the thrust of the cutting tools is taken by the clamping arrangement. In many cases, however, no actual clamping arrangements are used, but the work itself takes the thrust of the cutting tools, and the locating means are depended upon to hold the piece or jig in the right position when performing the drilling operation.
42 JIG DESIGN
It may be well to add that loose bushings ought to be marked with the size and kind of cutting tool for which they are intended; and the corresponding place in the jig body where they are to be used should be marked so that the right bushing can easily be placed in the right position.
A few more examples of open drill jig designs of various types may prove instructive. In Fig. 19 are shown two views of a jig for drilling two holes through the rim of a handwheel. To the left is shown the jig itself and to the right the jig with the hand-
Fig. 19. Drill Jig for Holes in Rim of Handwheel
wheel mounted in place, ready for drilling. As shown, the hand- wheel is located on a stud through its bore, and clamped to the jig by passing a bolt through the stud, this bolt being provided with a split washer on the end. The split washer permits the easy removal of the handwheel when drilled, and the putting in place of another handwheel without loss of time. The hand- wheel is located by two set-screws B passing through two lugs projecting on each side of a spoke in the handwheel, the set- screws B holding the handwheel in position, while being drilled, by clamping against the sides of the spoke. The jig is fastened on the edge of the drill-press table, in a manner similar to that indicated in the illustration, so that the table does not interfere
OPEN DRILL JIGS
43
with the wheel. The vertical hole, with the drill guided by bushing G, is now drilled in all the handwheels, this hole being drilled into a lug in the spoke held by the two set-screws B. When this hole is drilled, the jig is moved over to a horizontal drilling machine, and the hole D is drilled in all the handwheels, the jig being clamped to the table of this machine in a manner similar to that on the drill press.
Fig. 20. Miscellaneous Examples of Open Drill Jigs
In Fig. 20, at A, an open drill jig of a type similar to those shown in Figs, n and 13, is shown. This jig, however, is pro- vided with a V-block locating arrangement. An objectionable feature of this jig is that the one clamping strap is placed in the center of the piece to be drilled. Should this piece be slender, it may cause it to bend, as there is no bearing surface under the
44 JIG DESIGN
work, at the place where the clamp is located, for taking the thrust of the clamping pressure.
At B and C in the same illustration are shown the front and back views of a drill jig, where the front side B is used for drill- ing a small piece located and held in the jig as usual; and the back side C, which is not provided with feet, is located and applied directly on the work itself in the place where the loose piece is to be fastened, the work in this case being so large that it sup- ports the jig, instead of the jig supporting the work.
At D in the same illustration is shown a jig for locating work by means of a tongue E. This tongue fits into a corresponding slot in the work. This means for locating the work was referred to more completely in connection with locating devices. Finally, at F, is shown a jig where the work is located by a slot G in the jig body, into which a corresponding tongue in the work fits.
CHAPTER HI DESIGN OF CLOSED OR BOX JIGS
In the preceding chapter, the subject of the design of open drill jigs has been dealt with. In the present chapter it is pro- posed to outline the development of the design of closed or box
jigs-
Assume that the holes in a piece of work, as shown in Fig. i, are to be drilled. Holes A are drilled straight through the work, while holes B and C are so-called " blind holes," drilled into the work from the opposite sides. As these holes must not be drilled through, it is evident that the work must be drilled from two sides, and the guiding bushings for the two blind holes must be put in opposite sides of the jig. The simplest form of jig for this work is shown in Fig. 2. The piece of work D is located between the two plates E, which form the jig, and which, if the jig is small, are made of machine steel and casehardened. If the jig is large these plates are made of cast iron. The work D is simply located by the outlines of the plates, which are made to the same dimensions, as regards width, as the work itself. The plates are held in position in relation to each other by the guiding dowel pins F. These pins are driven into the lower plate and have a sliding fit in the upper one. In some cases, blocks or lugs on one plate would be used to fit into a slot in the other plate instead of pins. These minor changes, of course, depend upon the nature of the work, the principle involved being that some means must be provided to prevent the two plates from shifting in relation to each other while drilling. The whole device is finally held together by clamps of suitable form. The holes A may be drilled from either side of the jig, as they pass clear through the work, and the guides for the drills for these holes may, therefore, be placed in either plate. Opposite the bushings in either plate a hole is drilled in the other plate
45
JIG DESIGN
for clearance for the drill when passing through, and for the escape of the chips.
The two plates should be marked with necessary general in- formation regarding the tools to be used, the position of the plates, etc., to prevent mistakes by the operator. It is also an advan- tage, not to say a necessity, to use some kind of connection be- tween the plates in order to avoid such mistakes as, for instance, the placing of the upper plate in a reversed position, the wrong pins entering into the dowel pin holes. This, of course, would locate the holes in a faulty position. Besides, if the upper plate be entirely loose from the lower, it is likely to drop off when the jig is stored, and get lost. Some means of holding the two parts
tfTt
Fig. i. Work to be Drilled
together, even when not in use, or when not clamped down on the work, should therefore be provided. Such a means is em- ployed in Fig. 2, where the screw G enters into the guiding dowel pin at the left and holds the upper plate in place. A pin H, fitting into an elongated slot in the dowel pin, as shown at the left, could also be used instead of the screw. The design shown presents the very simplest form of box jig, consisting, as it does, of only two plates for holding the necessary guiding arrange- ments, and two pins or other means for locating the plates in relation to each other.
In manufacturing, where a great number of duplicate parts would be encountered, a jig designed in the simple manner shown in Fig. 2 would, however, be wholly inadequate. The simplest
BOX JIGS
47
form of a jig that would be used in such a case would be one in which some kind of locating means is employed, as indicated in Fig. 3, where three pins are provided, two along the side of the work and one for the end of the work, against which the work
w
Fig. 2. Simple Form of Closed Jig for Drilling Work shown in Fig. i
may be pushed prior to the clamping together of the two jig- plates. In this illustration, the jig bushings are not shown in the elevation and end view, in order to avoid confusion of lines. The next improvement to which this jig would be subjected would
Fig. 3. Locating Pins added to Jig
be the adding of walls at the end of the jig and the screwing together of the upper and lower plate, the result being a jig as shown in Fig. 4. This design presents a more advanced style of closed jig — a type which could be recommended for manufac- turing purposes. While the same fundamental principles are
JIG DESIGN
still in evidence, this jig embodies most of the requirements necessary for rapid work. This design provides for integral clamping means within the jig itself, provided, in this case, by the screws /. The upper plate K is fastened to the walls of the lower plate L by four or more screws M and two dowel pins N. The cover K could also be put on, as shown in Fig. 5, by making the two parts a good fit at 0, one piece being tongued into the other. This gives greater rigidity to the jig. In this jig, also, solid locating lugs F are used instead of pins.
Referring again to Fig. 4, by providing a swinging arm P with a set-screw Q, the work can be taken out and can be inserted
Fig. 4. Jig Suitable for Manufacturing Purposes
from the side of the jig, which will save making any provisions for taking off or putting on the top cover for every piece being drilled. If there is enough clearance between the top cover and the piece being drilled, the screw Q could, of course, be mounted in a solid lug, but it would not be advantageous to have so large a space between the top plate and the work, as the drill would have to extend unguided for some distance before it would reach the work. The set-screws Q and U hold the work against the locating points, and the set-screws / on the top of the jig, pre- viously referred to, hold the work down on the finished pad R on the bottom plate. These screws also take the thrust when the hole C is drilled from the bottom side. It is immaterial on which side the bushings for guiding the drills for the two holes A are placed, but by placing them in the cover rather than in
BOX JIGS
49
the bottom plate, three out of the four bushings will be located in the top part, and when using a multiple-spindle drill, the face R will take the greater thrust, which is better than to place the thrust on the binding screws /. In the designs in Figs. 4 and 5 the whole top and bottom face of the jig must be finished, or a strip marked/ in Fig. 6, at both ends of the top and bottom sur- faces, must be provided, so that it can be finished, and the jig placed on parallels D as illustrated.
While the jig itself, developed so far, possesses most of the necessary points for rapid production and accurate work, the
J L
-
00
Fig. 5. Alternative Design of Jig shown in Fig. 4
use of parallels, as indicated in Fig. 6, for supporting the jig when turned over so that the screw-heads of the clamping screws point downward, is unsatisfactory. Therefore, by adding feet to the jig, as shown in Fig. 7, the handling of the jig will be a great deal more convenient. The adding of the protruding handle 5 will still further increase the convenience of using the jig. The design in Fig. 7 also presents an improvement over that in Fig. 4, in that, besides the adding of feet and handle, the leaf or strap E is used for holding screw Q instead of the arm P. This latter is more apt to bend if not very heavy, and would then bring the set-screw in an angle upwards, which would have a tendency to tilt the work. The strap can be more safely relied upon to clamp the work squarely. Two set-screws / are shown for holding the work in place. The number of these set-screws,
.50
JIG DESIGN
of course, depends entirely upon the size of the work and the size of the holes to be drilled. Sometimes one set-screw is quite sufficient, which, in this case, would be placed in the center, as indicated by the dotted lines in Fig. 4.
The type of jig shown in Fig. 7 now possesses all the features
W' "."in
Fig. 6. Jig in Fig. 4 used in Combination with Two Parallels
generally required for a good jig, and presents a type which is largely used in manufacturing plants, particularly for medium and heavy work. The jig shown in Fig. 8, however, represents another type, somewhat different from the jig in Fig. 7. The
Fig. 7. Jig improved by Adding Feet opposite Faces containing Drill Bushings
jig in Fig. 7 is composed of two large separate pieces, which, for large jigs, means two separate castings, involving some extra expense in the pattern-shop and foundry. The reason for mak- ing the jig in two parts, instead of casting it in one, is because it makes it more convenient when machining the jig. The locat-
BOX JIGS
ing points, however, are somewhat hidden from view when the piece is inserted. The jig shown in Fig. 8 consists of only one casting L, provided with feet, and resembles an open drill jig. The work is located in a manner similar to that already described, and the leaf D, wide enough to take in all ,the bushings except the one for the hole that must be drilled from the opposite side, is fitted across the jig and given a good bearing between the lugs in the jig wall. It swings around the pin E and is held down by the eye-bolt F with a nut and washer. Sometimes a wing- nut is handier than a hexagon nut. Care should be taken that
Fig. 8. Alternative Design of Jig in Fig. 7
the feet reach below the top of the nut and screw. The set- screw G holds the work down, and takes the thrust when the hole from the bottom side is drilled. The three holes A, A and B are drilled from the top so that the thrust of the drilling of these three holes will be taken by the bottom of the jig body L. If one set-screw G is not sufficient for holding the work in place, the leaf may be made wider so as to accommodate more binding screws.
It is, however, an objectionable feature to place the clamping screws in the bushing plate. If the leaf has not a perfect fit in its seats and on the swiveling pin, the screws will tilt the leaf
JIG DESIGN
one way or another, and thus cause the bushings to stand at an angle with the work, producing faulty results. In order to avoid this objectionable feature, a further improvement on the jig, indicated in Fig. 9, is proposed. In the jig body, the locating points and the set-screws which hold the work against the locat- ing pins are placed so that they will not interfere with two straps Gy which are provided with elongated slots, and hold the work securely in place, also sustaining the thrust from the cutting tools. These straps should be heavily designed, in order to be able to take the thrust of the multiple-spindle drill, because in this case all the bushings, except the one for hole B, are placed
Fig. 9.
Jig in which Thrust of Drilling Operations is taken by Clamps
in the bottom of the jig body. The leaf is made narrower and is not as heavy as the one shown in Fig. 8, because it does not, in this case, take any thrust when drilling, and simply serves the purpose of holding the bushing for hole B. The leaves and loose bushing plates for jigs of this kind are generally made of machine steel, but for larger sized jigs they may be made of cast iron. The leaf in Fig. 9 is simply held down by the thumb-screw H.
If the hole B comes near to one wall of the jig, it may not be necessary to have a leaf, but the jig casting may be made with a projecting lug D, as shown in Fig. 10, the jig otherwise being of the same type as the one illustrated in Fig. 9. The projecting
BOX JIGS
53
part D, Fig. 10, is strengthened, when necessary, by a rib E, as indicated. Care must be taken that there is sufficient clearance for the piece to be inserted and removed. Once in a while it happens, even with fairly good jig designers, that an otherwise well-designed jig with good locating, clamping, and guiding arrangements, is rendered useless, for the simple reason that there is not enough clearance to allow the insertion of the work.
Fig. ii shows the same jig as before, but with the additional feature of permitting a hole in the work to be drilled from the end and side as indicated, the bushings E and F being added
Fig. 10. Modification of Jig Shown in Fig. 9
for this purpose. The bushings, in this case, extend through the jig wall for some distance, in order to guide the drill closely to the work. Bosses may also be cast on the jig body, as indicated by the dotted lines, to give a longer bearing for the bushings.
Feet or lugs are cast and finished on the sides of the jig opposite the bushings, so that the jig can be placed conveniently on the drill-press table for drilling in any direction. When drilling the holes from the bushings E and F, the thrust is taken by the stationary locating pins. It is objectionable to use set-screws to take the thrust, although in some cases it is necessary to do so. When designing a jig of this type, care must be taken that strapping arrangements and locating points are placed so that
54
JIG DESIGN
they, in no way, will interfere with the cutting tools or guiding means. In this case the strap H is moved over to one side in order to give room for the bushings F and the set-screw K. Strap G should then be moved also, because moving the two straps in opposite directions still gives them a balanced clamping action on the work. If the strap G had been left in place, with the strap H moved sideways, there would have been some ten- dency to tilt the work.
Sometimes one hole in the work comes at an angle with the faces of the work. In such a case the jig must be made along
1 ilk-p.^-^-4 ;t~~~
rrjrrir"""""" ir ~~]~
Fig. ii. Jig for Drilling Holes from Two Directions
the lines indicated in Fig. 12, the feet on the sides opposite to where the drill bushings are placed being planed so that their faces will be perpendicular to the axis through the hole A . This will, in no way, interfere with the drilling of holes which are perpendicular to the faces of the work, as these can be drilled from the opposite side of the work, the jig then resting on the feet B. Should it, however, be necessary to drill one hole at an angle and other holes perpendicular to the face of the work from the same side, an arrangement as shown in Fig. 13 would be used. The jig here is made in the same manner as the jig shown in Fig. 1 1 , with the difference that a bushing A is placed at the required angle. It will be seen, however, that as the
BOX JIGS
55
other holes drilled from the same side must be drilled perpendicu- larly to the faces of the work, it would not be of advantage to plane the feet so that the hole A could be drilled in the manner previously shown in Fig. 12. Therefore the feet are left to suit the perpendicular holes, and the separate base bracket B} Fig. 13, is used to hold the jig in the desired inclined position when the hole A is drilled.
Stand B in Fig. 13 is very suitable for this special work. It is made up as light as possible, being cored at the center, so as to remove superfluous metal. These stands are sometimes pro-
Fig. 12. Jig for Drilling Holes at an Angle
vided with a clamping device for holding the jig to the stand. Special stands are not only used for drilling holes at angles with the remaining holes to be drilled, but sometimes such stands are made to suit the jig in cases where it would be inconvenient to provide the jig with feet, finished bosses, or lugs, for resting directly on the drill-press table.
When a jig of large dimensions is to be turned over, either for the insertion or removal of the work, or for drilling holes from opposite sides, it is, in cases where the use of a crane or hoist can be obtained, very satisfactory to have a special device at- tached to the jig for turning it over. Fig. 14 shows such an arrangement. In this illustration, A represents the jig which is
4;
JIG DESIGN
to be turned over. The two studs B are driven into the jig in convenient places, as nearly as possible in line with a gravity axis. These studs then rest in the yoke C, which is lifted by the crane hook placed at D. The jig, when lifted off the table, can then easily be swung around. The yoke is made simply of round machine steel.
i
Fig- 13* Jig and Stand for Drilling Holes at an Angle
Fig. 14. Device for Turning over and Handling Heavy Jigs
Examples of Closed or Box Jigs. — The development of a closed or box jig has now been treated. In the following pages a number of examples of closed jig designs will be shown and described. There is, however, no distinct division line between open and closed drill jigs, so that in many cases it is rather in- consistent to attempt to make any such distinction.
BOX JIGS
57
In Fig. 15, for instance, is shown a box jig which looks like a typical open jig. The jig body A is made in one solid piece, cored out as shown in order to make it lighter. The piece to be drilled, B, shown inserted in the jig, has all its holes drilled in this jig, the holes being the screw holes C, the dowel pin holes D, and the large bearing hole E. The bosses of the three screw holes C are also faced on the top, and the bearing is faced on both sides while the work is held in the jig. The work is located against two dowel pins driven into the holes F, and against two lugs at G, not visible in the illustration, located on either side of the
Fig. 15. Box Jig which Resembles the Open Type
work. In these lugs are placed set-screws or adjustable sliding points. It may seem incorrect not to locate the bracket in regard to the hole E for the bearing, so as to be sure to bring the hole concentric with the outside of the boss. This ordinarily is a good rule to follow, but in this particular case it is essential that the screw holes be placed in a certain relation to the outline of the bracket in order to permit this to match up with the pad on the machine on which the bracket is used. Brackets of this shape may be cast very uniformly, so that locating them in the manner described will not seriously interfere with drilling the hole E approximately in the center of its boss. The work is firmly held in the jig by the three straps H, care being taken in designing the jig that these straps are placed so they will not inter- fere with the facing tools.
JIG DESIGN
The swinging strap /, which really is the only thing that makes this jig a closed jig, serves the sole purpose of taking the thrust of the heavy cutting tools when drilling the hole E and of steady- ing the work when facing off the two ends of the hub. The two collar-head screws K hold the strap to the jig body and the set-
Fig. 16. Plan and Elevation of Jig Shown in Fig. 15
screw L bears against the work. This strap is easily swung out of the way by loosening one of the collar-head screws, a slot being milled at one end of the strap to permit this. Stationary bush- ings are used for the screw hole and dowel holes, but for the bear- ing hole E three loose bushings and a lining bushing are employed.
BOX JIGS 59
The hole E is first opened up by a small twist drill, which makes the work considerably easier for the so-called rose-bit drill. The latter drill leaves TV inch of stock for the rose reamer to remove, which produces a very smooth, straight, and concentric hole. The last operation is the facing of the holes. The holes just drilled are now used to guide the pilots of the facing tools, and, as the operation is performed while the work is held in the jig, it is important that the locating or strapping arrangements should not be in the way.
In connection with the opening up of a hole with a smaller drill, it may be mentioned that it is not only for large holes that this method of procedure will save time, but the method is often a time-saving one also for smaller holes, down to | inch in diameter, when drilled in steel.
The use of lubrication in jigs is a very important item, the most common lubricant being oil or vaseline, but soap solution is also used. The objection to the latter is that unless the machine and tools are carefully cleaned it is likely to cause rusting. Using a lubricant freely will save the guiding arrangements, such as the drill bushings, the pilots on counterbores, etc., to a great extent.
The jig in Fig. 15 is shown in Fig. 16 and a clear idea of the design of the jig will be had by studying this line engraving. The bracket B, in Fig. 15, could have been drilled in a different way than described, which would sometimes be advantageous. It could be held in a chuck, and the hole E reamed and faced in a lathe, which would insure that the hole would be perfectly central with the outside of the boss. Then a jig could be designed, locating the work by a stud entering in hole E, as indicated in Fig. 17, additional dowel pins and set-screws being used for locating the piece sidewise. The whole arrangement could be held down to the table by a strap and bolt, a jack-screw support- ing it at the overhanging end.
Fig. 1 8 shows another jig of the closed type, with the work inserted. The piece A is a casing, and the holes to be drilled vary greatly in size. The casing rests on the flat, finished bottom surface of the jig and is brought up squarely against a finished pad at B. It further locates against the finished lug C, in order
6o
JIG DESIGN
to insure getting the proper amount of metal around the hole D. At the bottom it is located against the sliding point E, the latter being adjustable, because the location of the work is determined by the other locating points and surfaces. The work is held against the locating points by the long set-screw shown to the left. This clamping arrangement, however, is not to be recom- mended, because this screw must be screwed back a considerable distance in order to permit insertion and removal of the work.
|
,*^^ SCREW |
jGjj |
|||
|
1 |
^^-•^""^ |
II ~\\ |
||
|
! |
(,*-*""^ |
M |
||
|
i |
^^* |
|||
|
i i i |
^^' |
DRILL-PRESS TABLE
Fig. 17.
Simple Plate Jig for Drilling Bracket shown in Fig. 15, after Hole £ has been Bored in the Lathe
An eye-bolt used in the manner described in a preceding chapter would have given better service. The three straps G hold the work against the bottom surface, and the two straps H hold it against the finished surface at B. There is not a long finished hole through the casting, as would be assumed from its appear- ance, but simply a short bearing at each end, the remaining part of the hole being cored out. For this reason, the hole is drilled and reamed instead of being bored out, as the latter operation
BOX JIGS
61
would be a slower one. Although the two short bearings are somewhat far apart, the guiding bushings come so close to these bearings that the alignment can be made very good. The screw holes and dowel pin holes at the bottom of the casing are not shown in the illustration, as the inserted casing is not yet drilled. The hole drilled from bushing / is a rather important hole, and the bushing requires a long bearing in order to guide the drills straight when drilling. When this jig was made, the projecting lug which was provided solid with the jig body, to give a bearing
Fig. 18. Box Jig for Casing drilled from Five Directions
to the jig bushing, came so much out of the way in the rough casting for the jig that half of the lining bushing would have been exposed. It was therefore planed off and a bushing of the type shown in Fig. 5, in the chapter on "Jig Bushings," inserted instead, in order to provide for a long bearing.
Leaf K, which carries the bushings for drilling the hole D, fits into a slot planed out in the jig body and is held down by the eye-bolt L. Two lugs M are provided on the main casting for holding the pin on which the leaf swivels. Around the hole
62
JIG DESIGN
D there are three small tap holes 0 which are drilled by the guiding afforded by the bushing P, which is made of cast iron and provided with small steel bushings placed inside as illustrated in Fig. 14, in the chapter on " Jig Bushings. " In the bushing P is another hole Q which fits over a pin located in the top of the leaf and which insures that the three screw holes will come in the right position. It should be noted that large portions of the
Fig. 19. Box Jig for Drilling Work shown by Dash-dotted Lines
jig body are cored out at top and bottom in order to make it light and easy to handle. Of course some metal is also saved by the construction of jigs in this manner, but comparing the price of cast iron with the total price of a finished jig of this type, the saving in this respect is so insignificant that it is not worth while mentioning. The leaf K is also made of cast iron, being of
BOX JIGS 63
particularly large size, and it is planed at the places where it has a bearing on the jig body.
Fig. 20 shows a closed jig about which there can be no doubt but that it should be classified as a box jig. The piece of work drilled, the foot trip A, has two holes B and C which are drilled in this jig. The cylindrical hub of the work is located against V-blocks and held in place by a swinging strap D. The work is further located against a stop-pin placed opposite the set-screw E. The trip is located sidewise by being brought against another
Fig. 20. Jig shown in Detail in Fig. 19
stop by the set-screw F. One-quarter of a turn of the collar-head screw on the top of the jig releases the swinging strap which is then turned out of the way; this permits the trip to be removed and another to be inserted. Half a turn or less of the set-screws is enough to release and clamp the work against the stops men- tioned. A line engraving of this jig is shown in Fig. 19 which gives a better idea of some of the details of the construction.
In Figs. 21 and 22 are shown two views of another type of closed drill jig. The work A, to be drilled, is shown at the left
JIG DESIGN
in both illustrations, and consists of a special lathe apron with large bearing holes, screw holes, and dowel pin holes to be drilled. The apron is located in the jig body in the same manner as it is located on the lathe carriage, in this case by a tongue which may be seen at B in Fig. 22. This tongue fits into the slot C in the jig,
Fig. 21. Jig of Typical Design, and Work for which it is Used
Fig. 22. Another View of the Jig in Fig. 21
care being taken in the construction of the jig that the slot is made deep enough to prevent the tongue from bearing in the bottom of the slot. A good solid bearing should be provided, however, for the finished surface on both sides of the tongue. The surface D should also have a solid bearing on the surface E in the jig, the difference in height between the two bearing sur- faces in the jig being exactly the same as between the two bear- ing surfaces on the lathe carriage where the lathe apron is to be fitted. The work is brought up against, and further located by,
BOX JIGS 65
a dowel pin at the further end of the slot, by the set-screw in the block F, Fig. 21. As it is rather difficult to get the tongues on all the pieces exactly the correct width for a good fit in the slot, the latter is sometimes planed a little wider and the tongue is brought up against one side of the slot by set-screws. In the case in hand, a few thousandths inch clearance is provided in the slot, and the set-screw G in Fig. 22 is used for bringing the work against the further edge, which stands in correct relation to the holes to be drilled. The apron is held down against the bottom surface of the jig by four heavy set-screws H.
It will be noticed that the jig is open right through the sides in order to facilitate the finishing of the pads at the ends of the
Fig. 23. Jigs in which the Work is Located by Means of Beveled Surfaces
work, and a swinging leaf, like the one previously described, reaches across one side for holding the lining and loose bushings for the hole K which is drilled and rose-reamed in the usual way. The large hole V, Fig. 21, is bored out with a special boring tool If, as there are no standard drills obtainable for this large size of hole. This special boring tool is guided by a cast-iron bush- ing which fits into the lining bushing; it is provided with two cutters, one for roughing and one for finishing. The small screw holes O around the large hole V are drilled from the bush- ing P. For drilling the rest of the holes, except the hole Q, stationary bushings are used. The screw holes ought to be drilled simultaneously in a multiple-spindle drill. The jig is provided with feet and cored out in convenient places in order
66
JIG DESIGN
to make it as light as possible to handle. Lugs project wherever necessary to give ample bearings to the lining bushings and, in turn, to the loose guiding bushings.
Fig. 23 shows two closed jigs made up of two main parts which are planed and assembled by screws and dowels as indicated, the reason for making the jigs in this way being the ease of planing the bottom section. The work drilled in these jigs, some special slides, is located by the dovetail and held up against one dove- tail side by set-screws A} as shown in the illustration. In the jig
Fig. 24. Jig for Drilling Holes at other than go-degree Angles
to the left, the work is located endwise against a dowel pin and is held up against this stop by a set-screw through the block shown to the left. This block must be taken out when the slide is in- serted, this being the reason why a lug cast directly in place, through which the set-screw could pass, is not used. The top plate D is held down on the main body by six fillister-head screws Ej and two dowel pins F prevent it from shifting. No clamping arrangements, except the set-screws A, are necessary. The holes being drilled from the top, the main body of the jig takes the thrust. These jigs are also used in multiple-spindle drills.
One objectionable feature of the jig to the right in Fig. 23 is that set-screws A are difficult of access. There are, therefore,
BOX JIGS
67
holes piercing the heads of the set-screws in two directions in order to allow a pin to be used when tightening the screws. A better idea, however, is to have the screw-heads extend out through the wall and, if this is solid, to have cored or drilled holes through which the heads of the screws may pass.
In Fig. 24 is another closed drill jig in which the work is located against the finished seats and held down by the set-screws A in the straps B. All the holes, except those marked C, are drilled
Fig. 25. Jig in Fig. 24 in Position for Drilling Holes at an Oblique Angle with Jig Base
in the usual manner, the jig standing on its own feet, but when drilling the holes C, which come on an angle, the special stand D is employed, which brings the holes in the right position for drilling, as illustrated in Fig. 25. If only the holes C were to be drilled, the feet on the side opposite the guiding bushings for these holes could have been planed off, so that they would have been in a plane perpendicular to the axis of the holes. This last jig has a peculiar appearance, on account of the end walls coming up square, as shown in the illustrations, but this design was adopted only to simplify matters for the patternmaker, it being easier to make the pattern this way.
CHAPTER IV JIG BUSHINGS
The drills, counterbores, reamers, etc., used in connection with drill jigs are guided by steel bushings, which are hardened and ground, and placed in the jig body in their proper location. These bushings may be of two kinds: stationary and removable, the latter usually being known as ''loose" bushings. The most common and the preferable form for the stationary bushing is shown in Fig. i. This bushing is straight both on the inside and on the outside, except that the upper corners A on the in- side are given a liberal radius, so as to allow the drill to enter the hole easily, while the corners B at the lower end of the out- side are slightly rounded for the purpose of making it easier to drive the bushing into the hole, when making the jig, and also to prevent the sharp corner on the bushing from cutting the metal in the hole into which the bushing is driven.
Removable Bushings. — When removable bushings are used, they should never be placed directly in the jig body, except if the jig be used only a few times, but the hole should always be provided with a lining bushing. This lining bushing is always made of the form shown in Fig. i. If the hole bored in the jig body receives the loose or removable bushing directly, the in- serting and removing of the bushing, if the jig is frequently used, would soon wear the walls of the hole in the jig body, and after a while the jig would have to be replaced, or at least the hole would have to be bored out, and a new removable bushing made to fit the larger-sized hole. In order to overcome this, the hole in the jig body is bored out large enough to receive the lining bushing referred to, which is driven in place. This lining bushing then, in turn, receives the loose bushing, the outside diameter of which closely fits the inside diameter of the lining bushing, as shown in Fig. 2, in which A is the jig body, B the
68
BUSHINGS
69
lining bushing, and C the loose bushing. Both of these bush- ings are hardened and ground so that they will stand constant use and wear for some length of time. When no removable bushings are required, the lining bushing itself becomes the drill bushing or reamer bushing, and the inside diameter of the lining bushing will then fit the cutting tool used. The bushing shown in Fig. i is cheaper to make, and will work fully as well, when driven in place in the hole receiving it, as do bushings having a shoulder at the upper end, such as the loose bushing shown in Fig. 2. It was the practice some years ago to make all bushings with a shoulder, but this is unnecessary, and simply increases the cost of making the bushing.
Material for Jig Bushings. — Bushings are generally made of a good grade of tool steel to insure hardening at a fairly low temperature and to lessen the danger of fire cracking. They
Fig. i.
Fig. 2.
Fig. 3-
can also be made from machine steel, which will answer all practical purposes, provided the bushings are properly case- hardened to a depth of about TV inch. Sometimes bushings for guiding tools may be made of cast iron, but only when the cut- ting tool is of such a design that no cutting edges come within the bushing itself. For example, bushings used simply to sup- port the smooth surface of a boring-bar or the shank of a reamer might, in some instances, be made of cast iron, but hardened steel bushings should always be used for guiding drills, reamers, taps, etc., when the cutting edges come in direct contact with the guiding surfaces. If the outside diameter of the bushing is very large, as compared with the diameter of the cutting tool, the cost of the bushing can sometimes be reduced by using an outer cast-iron body and inserting a hardened tool steel bush-
yo JIG DESIGN
ing. Occasionally a bushing having a large outside diameter is required as, for example, when a large counterbore must be used in a small hole, which makes it necessary to have a large opening in the jig body.
Dimensions of Stationary Jig Bushings. — Standard dimen- sions for jig bushings, applicable under all circumstances, can- not be given, as these depend, in most cases, on the different conditions of the various classes of jigs in which the bushings are inserted. The common practice is to make the length of the bushing twice the inside diameter of the hole in the bushing for stationary drill bushings. On very small bushings, however, say J inch diameter hole and less, the length of the bushing will have to be made longer than twice the diameter, while on very large bushings the length may be made somewhat less than twice the diameter. Table I gives proportions of stationary drill bushings. The dimensions, as here given, will be found suitable in all cases where no special conditions demand devia- tion from ordinary practice. If the jig wall is thin, the bushing may project out as shown in Fig. 3, so as to give the cutting tool the proper guiding and support as close to the work as possible. In Table II are given dimensions for lining bushings, not in- tended to directly guide the drill, but to hold removable bush- ings, which, in turn, guide the cutting tools. The dimensions given in Tables I and II are for bushings made from either tool steel or machine steel.
While it is difficult, in some cases, to draw a distinct line be- tween stationary drill bushings and lining bushings, it may be said, in general, that the bushings in Table I are used for guid- ing the drills when drilling holes directly, either with a full- sized drill, when the hole is not required to be very smooth or accurate, or, if greater accuracy is required, for guiding a spot- ting drill which fits the bushings exactly, after which the hole is drilled out with a so-called " reamer-drill," which is o.oio inch or less under the size of the finished hole, and finally reamed out with a reamer exactly fitting the hole in the bushing. These bushings are thus, in general, used when no tapping or counter- boring would be required. The lining bushing in Table II,
BUSHINGS
again, may guide one of the tools for the holes to be finished directly, and then removable bushings are inserted to guide the other tools used.
Miscellaneous Types of Drill Bushings. — As mentioned, it was, some years ago, general practice to provide even station- ary bushings with a shoulder or head, as shown in bushing C, Fig. 2. This will prevent the bushing from being pushed through the jig by the cutting tool, but this seldom happens if the bushings are made to fit the tool correctly. Sometimes the shoulder is used to take the thrust of a stop-collar, which is
Table I. Dimensions of Stationary Drill Bushings
|
r |
|||||||||||
|
* — ~r |
|||||||||||
|
t |
i |
||||||||||
|
<- * |
i |
||||||||||
|
v L ^ |
|||||||||||
|
JfacAfaen/>.F. |
|||||||||||
|
A |
B |
L |
A |
£ |
L |
A |
- |
L |
A |
JB |
L |
|
He |
Me |
% |
Me |
'Me |
iH |
I He |
IMe |
2 |
I Me |
I 'Me |
294 |
|
H |
M |
/i |
M |
J$ |
i'/4 |
1/6 |
iH |
2% |
!5/i |
2 |
2/6 |
|
Me |
Me |
M |
'He |
'Me |
i^i |
iMe |
I Me |
2l/i |
I 'He |
2tt |
27/6 |
|
M |
N |
96 |
N |
I |
i/i |
iH |
1% |
2\i |
1 94 |
2H |
3 |
|
Me |
4 |
$6 |
'Me |
i'/6 |
i^ |
I Me |
I 'He |
2M |
I'9ie |
2Me |
3H |
|
H |
Me |
N |
ji |
I Me |
i^i |
1 96 |
1 94 |
29^ |
iH |
296 |
3M |
|
Me |
% |
£6 |
'Me |
iH |
iM |
I Me |
I 'Me |
2H |
i 'Mo |
2Me |
396 |
|
4 |
'He |
i |
I |
iH |
ZH |
iH |
IH |
2H |
2 |
2% |
3H |
clamped on the drill, to allow it to go down to a certain depth, as shown in Fig. 4, in which C is the stop-collar, D the wall of the jig, and E the stationary bushing; F is the work. In such a case, a shoulder on the bushing should be provided.
If the work to be drilled is located against a finished seat or boss on the wall of the jig, and the wall is not thick enough to take a bushing of standard length, then it is common practice to make a bushing having a long head, as shown in Fig. 5. The length A of the head can be extended as far as necessary to get the proper bearing. As the bushing is driven in place
5J
JIG DESIGN
and the shoulder of the head bears against the finished surface of a boss on the jig, it will give the cutting tool almost as rigid a bearing as if the jig metal surrounded the bushing all the way
up- Removable bushings are frequently used for work which must
be drilled, reamed, and tapped, there then being one bushing for each of the cutting tools. They are also used when different parts of the same hole are to be drilled out to different diam- eters, or when the upper portion of the hole is counterbored,
Table II. Dimensions of Lining Bushings
|
T"" ft |
||||||||
|
i |
||||||||
|
r* |
||||||||
|
J |
||||||||
|
Machinery, N.T. |
||||||||
|
A |
B |
L |
A |
B |
L |
A |
B |
L |
|
Me |
H |
H |
iU |
l!-'2 |
1 1/2 |
2H |
2% |
2% |
|
% |
Me |
H |
iM |
iM |
iW |
2Me |
21 He |
2>4 |
|
Me |
M |
% |
I Me |
I Hie |
1% |
214 |
2% |
2% |
|
Me |
13/16 |
% |
iN |
1% |
I7xi |
2Me |
2?4 |
25* |
|
% |
n |
?4 |
I Me |
113,1s |
2 |
2H |
3 |
3 |
|
JHe |
JMe |
H |
I Me |
liMe |
2H |
2Me |
3Me |
3H |
|
% |
I |
i |
iW |
2 |
2W |
2% |
3H |
3H |
|
iMe |
IK |
iH |
IN |
2M |
2i/, |
2i He |
3Me |
33/^ |
|
»*i« |
iM |
i!4 |
HM« |
2Me |
2H |
2% |
33-^ |
3H |
|
i |
iH |
i% |
1% |
2^ |
2^ |
|||
|
I He |
I Me |
iJ4 |
I1 Me |
2^6 |
2^ |
|||
or when a lug has to be faced o.L In this case, each tool, of course, has its own guide bushing. The common design of removable bushings is shown in Fig. 6. The outside is made to fit the inside of the lining bushing with a nice sliding fit, so that it can be gently pressed into the lining bushing by the hand. The distance A under the head of the bushing should be the same length as, or longer than, the guide bushing. The thickness B of the head varies, of course, according to the •size of the bushing. The diameter C of the head should be
BUSHINGS
73
from J to J inch larger than the diameter D of the bushing. A groove E, f to J inch wide, is cut immediately under the head, so that the emery wneel can pass clear over the part being ground.
Means for Preventing Loose Bushings from Turning. — In order to prevent the bushings from turning, in some shops a
Fig. 4-
Fig. 5-
Fig. 6.
collar, with a projecting tail, as shown in Fig. 7, is forced over the head of the bushing. This arrangement also makes it easy to remove the bushing. The dog, as it is commonly called, is usually bent at the end of the tail, as shown in the illustration, one end resting against some part of the jig, the proportions of which the dog must suit. Sometimes the bent end is left straight,
Fig. 7.
Fig. 8. Fig. 9. Fig. 10.
if there is a possibility for the tail to strike against some lug in the same plane. The making of such dogs involves some extra expense, but it is very effective in avoiding troubles with bushings turning and working their way out of the holes. In some cases simply a hole is drilled in the shoulder of the bushing at the edge, and a corresponding pin is driven into the jig body. This serves the same purpose as the dog. It is probably cheaper,
74
TIG DESIGN
but it does not add the convenient means for removing the bushing as does the dog. To make such a bushing more easily removable, the arrangement shown in Fig. 8 is probably the most common. A step A is turned down on the head, which, in this case, will have to be a trifle larger in diameter. This step permits some kind of a tool — a screw driver, for instance, to be put underneath, and with a jerk the bushing may be lifted enough to get a good hold on it. The half-round slot at B is milled or filed in the periphery of the head, and fits over a pin or screw which is fastened in the jig body, as mentioned before.
Machinery
Fig. n. Methods used for Preventing Jig Bushings from Turning
In Fig. ii are shown three methods of holding bushings to prevent them from turning, the methods all being on the prin- ciple described: A shows a bushing having a pin inserted which slips in a slot cut in the lining bushing; B shows a bushing hav- ing a slot milled through the collar, a pin being located in the jig to engage this slot; and C illustrates a more elaborate device that is sometimes used. The stop button which is fastened to the jig prevents the bushing from being drawn out of the liner while withdrawing drills or reamers, as well as preventing it from turning.
The following method for holding slip jig bushings in place
BUSHINGS
75
has been found to be a very good one: Drill and tap a J- or f-inch hole in the side of the jig bushing, as indicated in Fig. 12. After the bushing is hardened and ground, screw in a pin and cut it off so that it projects about T\ inch outside of the bushing, as at B. Chip out a slot in each hole in the jig as indicated at A, the hole being chipped in the direction of a spiral. By engaging the projecting pin in this slot, the bushing is prevented from turning and from rising out of the hole. At the same time it can easily be removed when required, and there is no projection on the jig of any kind that can be broken off while handling. It is not always necessary to tap a hole for the pin in the jig bushing. A plain drilled hole is sufficient when the bushing is at least f inch thick. If the wall of the bushing is thinner than this, the pin cannot be driven in tightly enough to stay in place securely.
Machinery
Fig. 12. Another Method for Preventing Drill Bushings from Turning
Dimensions of Removable Bushings. — In Table III are given dimensions for removable bushings of the type shown in Fig. 8. Table IV gives dimensions for bushings for holes which are reamed with a rose chucking reamer, after having first been drilled with a drill TV inch smaller than the diameter of the reamer with which the hole is finally reamed out. The bushing to the extreme right, over the table, is the lining bushing, which is made of machine steel, casehardened and ground. The bushing to the extreme left is the bushing for the rose chucking reamer. It is made of cast iron and ground. The bushing in the center is the drill bushing which is made from tool steel, hardened and ground, or, in cases where it does not seem war- ranted to make the bushing of tool steel, of machine steel, case- hardened and ground.
The tapered removable bushing shown in Fig. 9 is objection-
76 JIG DESIGN
Table III. Dimensions of Removable Drill Bushings
|
T*fl |
q |
||||||||
|
. |
|||||||||
|
I |
cb |
f |
|||||||
|
I |
•p II |
1 |
|||||||
|
I5 |
1 |
||||||||
|
* * |
|||||||||
|
j |
. _~L |
"L |
|||||||
|
j± |
1 |
f- |
>| |
||||||
|
"*1 |
|||||||||
|
3focA/nerj/,A[.r. |
|||||||||
|
A |
» |
C |
D |
• |
* |
H |
/* |
K* |
|
|
H |
Me |
M |
M |
H |
H |
M |
M |
Ma |
|
|
Me |
H |
M |
^ |
H |
H |
M |
M |
^2 |
|
|
M |
Me |
H |
M |
N |
54 |
M |
H |
J-^2 |
|
|
Me |
Me |
H |
M |
N |
% |
M |
H |
]/^2 |
|
|
H |
H |
M |
M |
n |
n |
M |
M |
H2 |
|
|
Me |
n/le |
Jj |
M |
i |
i |
M |
% |
J-^2 |
|
|
M |
N |
i |
M |
zM |
i |
M |
% |
Ma |
|
|
Me |
JMe |
l^i |
H |
zM |
zM |
M |
i |
Ma |
|
|
H |
iMe |
1 1/4 |
M |
6 |
I Me |
iH |
N |
zM |
Ma |
|
iMe |
I |
zH |
6 |
IMe |
zM |
M |
zM |
He |
|
|
N |
I He |
zM |
Me |
Z'Ma |
ZH |
M |
I H |
He |
|
|
JMe |
zM |
I V3 |
M |
6 |
In/l6 |
i% |
M |
iH |
He |
|
M |
zM |
XH |
Me |
i me |
zM |
M |
i?i |
He |
|
|
JMe |
I Me |
i?4 |
Me |
I me |
zH |
M |
zM |
He |
|
|
I |
l3^ |
zj$ |
M |
2M |
iM |
Me |
!•}£ |
Hi |
|
|
I He |
I Me |
2 |
N |
2H |
zji |
Me |
iM |
Ha |
|
|
zW |
iMe |
2j4 |
H |
2H |
2 |
Me |
I JMe |
Ha |
|
|
I Me |
zH |
2}<6 |
H |
2% |
2 |
Me |
IiMe |
Ha |
|
|
IM |
i% |
2H |
Me |
2Me |
2M |
Me |
I iMe |
M |
|
|
iMe |
iiMfi |
2H |
Me |
2Me |
2H |
Me |
2He |
M |
|
|
1% |
i7-6 |
2% |
Me |
2i He |
2H |
Me, |
2He |
M |
|
|
I Me |
I1 Me |
2M |
Me |
2iMe |
23/6 |
Me |
2Mo |
M |
|
|
zH |
2H |
•2^6 |
% |
3 |
21/2 |
Me |
2Me |
M |
|
|
I Me |
2Me |
234 |
% |
3M |
2^i |
Me |
2Me |
M |
|
|
zH |
2]4 |
2H |
% |
3M |
2H |
Me |
2Me |
M |
|
|
JiHe |
2Me |
2H |
% |
3H |
2% |
Me |
?Me |
M |
|
|
iM |
2% |
3 |
H |
33/^ |
2% |
Me |
2Me |
M |
|
|
I iMe |
2Me |
3M |
1A |
3H |
3 |
H |
2lMe |
Ha |
|
|
Zjtj |
254 |
3H |
M |
33/4 |
3Vi |
N |
215/16 |
5^2 |
|
|
I iMe |
21 He |
33^ |
M |
3Ji |
3M |
H |
215/16 |
5^2 |
|
|
2 |
23/4 |
SH |
M |
4 |
3M |
M |
3 Me |
Ha |
* When using dogs as illustrated in Pig. 7, the dimensions in these columns are omitted.
able on account of being expensive to make, and also on account of being likely to be thrown out of true by chips, etc., forced in between the outside of the bushing and the hole.
BUSHINGS
77
Screw Bushings. — Sometimes removable bushings are threaded on the outside and made to fit a tapped hole in the jig, as shown in Fig. 10. The lower part of the bushing is usu- ally turned straight, and ground, in order to center the bushing perfectly in the hole in the jig. The head of the bushing is either knurled or milled hexagon for a wrench. When these bushings are used, they are, as a rule, not used for the single purpose of guiding the cutting tool, but they combine with this the purposes of locating and clamping the work. For such
Table IV. Bushings for Holes Reamed with Rose Chucking Reamers
7f TT
it H
-± i. *
1^ * K ^1
ii=.
|
— |
— |
- |
|
|
:-!< |
F —
l<— I
I
I He IH
I Me iH iMe
1 Me I1 He I 'Me
I1 Me 2
2 He 2^
2H 2%
He
I I He
IMe
IMe IMe
xH
I Me I 'He
I 'Me 2 2 He
2Me 2Me 2Me
2'H«
IMe I Me
1 Me
I 'He I 'Me I 'Me 2
2 He
2Me 2% 2Me
2Me 2H 2i He
3 3H
i»H
I 'Me
I 'Me 2
2Me
2^ 2Me 2*i 2i He 3
3W
3%
35/*6 3'Me
4M
4W
iM
2H 2H 2%
23/4
3 3
3W
3H
4 4W
I Me
1 Me
IM
2 21/6
2?6
2 ^
2% 2% 2%
3 3«
3M
3W
Me
Me
Ha
•Mr.
!4
M
M
M
Me He
Me
Me
Ha
Me Me Me Me
M
W Hi
M
Me He
Me Me
I M
I 'Me
2
2Me
2Me
2Me
2Me
2i He
3
3H
3H
3'He
4 4
4M
4W
4H
I Me
I1 Me
2
2He 2H
2M
3 Me
3M
4M
4W 4W 43/i 4W 4H
W Mo Me Me Me Me Me Me Me Me Me Me Me Me
M
J/4 M H
2
2H
2M
2H 2% 2H 2% 2H 2 'Me
3H 3H
3«
W M
Me Me Me Me Me Ha Me Me Me Me Me Me
78
JIG DESIGN
purposes they are quite frequently used. These bushings are not commonly used as removable bushings, as it would take considerable time to unscrew, and to again insert, a bushing of this type into the jig body.
Special Designs of Guide Bushings. — When the guide bush- ings are very long, and consequently would cause unnecessary friction in their contact with the cutting tools, they may be recessed, as shown in Fig. 13. The distance A of the hole in the bushing is recessed enough wider than the diameter of the tool so as not to bear on it. The length B, being about twice the diameter of the hole, gives sufficiently long guiding sur- faces for the cutting tool, to prevent its running out. If the outside diameter of the bushing is very large compared with
t
:,
~
f
4-
Fig. 13. Fig. 14. Fig. 15.
the diameter of the cutting tool, as indicated in Fig. 14, the expense of making the bushings may be reduced by making the outside bushing of cast iron, inserting into this a hardened tool-steel bushing, driven in place. The steel bushing is then given dimensions according to Table I for stationary bushings. The reason why there may be a necessity of a bushing having so large an outside diameter and so small a hole may be that the bushing is required to be removed for counterboring part of the small hole being drilled by a counterbore of large diam- eter, in which case the hole in the jig body has to be large enough to accommodate the large counterbore.
If a loose or removable bushing is longer than the lining bush- ing, as illustrated in Fig. 15, it will prove advantageous to have the diameter of the projecting portion of the bushing about ¥V inch smaller in diameter than the part of the loose bushing which
BUSHINGS
79
fits the lining bushing. This lessens the amount of surface which has to be ground, and, at the same time, makes it easier to insert the bushing, giving it, so to say, a point, which will first enter the lining bushing, and it interferes in no way with the proper qualities of the bushing as a guide for the cutting tool.
In some cases, the holes in the piece to be drilled are so close to one another that it is impossible to find space for lining bushings in the jig. If this happens, it is necessary to make a leaf, or a loose wall, or the whole jig, of machine steel or tool steel, hardening a portion or the whole jig thus made.
Table V. Allowances for Grinding and Lapping Bushings
|
Operation |
Diameter of Bushings in Inches |
|||||
|
H |
• |
iH |
2 |
2\i |
3 |
|
|
A B C D |
0.008 0.0005 0.008 0.0003 |
O.OIO 0.0005 O.OIO 0.0005 |
0.013 0.0007 0.013 0.0007 |
0.016 o . 0008 0.016 0.0008 |
0.020 0 . OOOQ 0.020 o . 0009 |
O.O25 O.OOI 0.025 O.OOI |
A — Grind outside; B — Lap outside after grinding; C — Grind inside; D — Lap inside after grinding.
Methods of Making Jig Bushings. — There are several methods followed in turning jig bushings. Some toolmakers prefer to " chuck out" the hole to the desired size and then finish the outside of the bushing by placing it on an arbor; others prefer to turn up the bushings two at a time, end to end, cut them apart, and then bore as the final operation. This is an excellent method to follow when making large bushings. The most rapid method, however, is to chuck out the hole and finish the outside at one setting, using bar stock held in the chuck of a rigid en- gine lathe. This method is not always practicable on large bushings.
In making allowances for grinding and lapping, many tool- makers use too small limits, which is the cause of many bush- ings having to be made over again on account of not " finishing out." On the other hand, many toolmakers leave too liberal an allowance for finishing, thereby causing unnecessary trouble
80 JIG DESIGN
and labor. The allowances given in Table V can be safely used when the bushings are made somewhere near the propor- tions indicated in Tables I to IV, but for extra long bushings more liberal allowances should be made.
Before hardening, the bushings should be plainly stamped with the size and purpose for which they are intended, "Jf drill," "f ream," etc. They should be stamped with a set of plain sharp figures, reserved solely for this purpose. It is poor practice to try to stamp the words " drill," "ream," etc., in a straight line, as this is difficult to do. If, however, the words are laid out on a slight curve the results are more satisfactory, as slight irregularities of alignment are not then so noticeable. Sharp clean figures and letters, neatly laid out, not only improve the appearance of the toolmaker's work, but also save the drilling operator's time, as sharp clean-cut figures can be read at a glance.
Hardening Jig Bushings. — When hardening bushings made of tool steel they should be brought to an even red heat in a clean fire; the heating should never be hurried. When bush- ings are heated quickly, they are apt to heat unevenly, which results in warping or distortion that makes it impossible to finish them to the required size. Gas furnaces are excellent for heat- ing, but a clean charcoal fire will answer the purpose. As soon as the bushing has been brought to an even red heat, it should be dipped in water just warm enough to take off the chill. The bushing should then be heated to a "sizzling" heat, after which it is left in the air to cool. Some toolmakers draw bushings to a medium straw color. This is a mistake as it only tends to shorten their life.
Grinding and Lapping. — There are four methods in common use for finishing holes in jig bushings: i. Lapping with a lead lap. 2. Lapping with a lead lap followed by a cast-iron or copper lap. 3. Internal grinding. 4. Internal grinding fol- lowed by a cast-iron or copper lap for removing the last 0.0005 inch. The first method is erroneous, as it invariably results in bell-mouthed holes, especially when the toolmaker charges the lap while in use, which is an unsatisfactory but very common
BUSHINGS
8l
D
method. The second method is correct for holes too small to be ground conveniently. The third method is inadvisable, as the grinding wheel, no matter how fine, leaves innumerable very fine scores and high spots. These high spots soon wear away leaving the hole oversize. The last method is correct and should be used whenever possible.
In Fig. 1 6 is shown a lead lap with a steel tapered spindle, and a convenient mold for casting the laps. This mold is pro- vided with a base having a hole to receive the spindle that the lap is cast on. A number of laps can be cast in this mold at one heating of the metal, and the laps are afterwards turned to the size required. Fig. 17 represents a familiar form of cast-iron lap. This lap is split in three places and provided with a taper-end screw for expanding it to compensate for wear.
Laps should be charged before using — not while they are in use. A good way to charge a lap is to lay it on a cast-iron plate on which some of the abrasive mate- rial has been sprinkled. A cast-iron plate small enough to be conveniently handled is then held on the lap and moved back and forth with a regular motion. The lap being rolled between the two surfaces picks up a certain amount of the abrasive material. A lead lap can be charged in this manner very rapidly, as the grains of abrasive material readily imbed them- selves in the soft metal. A cast-iron lap, being of a harder material, requires more time to properly charge.
Until the last few years emery was the abrasive generally used for lapping. At the present time, however, artificial abra-
Machinery
Fig. 1 6. Lead Lap and Mold used for Casting it
82 JIG DESIGN
sives, products of the electric furnace, are displacing emery, as they cut faster, producing excellent results in a comparatively short time as compared to emery. Nos. 90 to 150 are used in connection with lead laps for roughing operations. For the final finishing with cast-iron laps, flour abrasive is used. When not in use, any abrasive used for lapping should be kept in a covered box to protect it from dirt and other foreign substances. A small chip or piece of grit will often cut a deep score in a piece of work.
Laps should always be run at a fairly low speed. Fifteen to twenty feet surface speed for a lead lap used for roughing and
Machinery
Fig. 17. Usual Form of Cast-iron Lap
twenty to twenty-five feet surface speed for a cast-iron lap used for finishing are about right. A high surface speed causes the lap to wear out without cutting as rapidly as it should. Many toolmakers make the mistake of running laps too fast, often causing unsatisfactory work. For light lapping, the work can be held by hand, but for a heavy roughing cut it is best to hold the work with an ordinary lathe dog, care being taken to see that the dog is not clamped so tightly as to spring the work out of shape. Lead laps should be split to compensate for wear, and the spindles should have a groove cut along their entire length to prevent the lap from turning.
Before testing with a size plug, the work should be washed with benzine or gasoline to remove all traces of the abrasive material, a few grains of which will wear the size plug below standard size in a surprisingly short time.
Many toolmakers look on the finishing of jig bushings by internal grinding as a rather uncertain method, whereas it is a comparatively simple process when the following important factors are carefully considered. First, proper selection of grind- ing wheels; second, correct wheel speeds or at least as nearly
BUSHINGS 83
correct as the design of the machine will permit; third, correct alignment of the headstock in regard to the travel of the platen; and fourth, proper truing of wheels.
Wheels for internal grinding should be of a medium grit, soft grade and open bond. As a rule the grit should never be finer than 60 grit; in fact, a coarser grit can often be used to advantage. Wheels with fine grit cut slowly, and fill up readily, glazing and invariably heating the work, and causing chattering and other troubles. In fact, the only argument in favor of a fine grit wheel is that it leaves a smooth surface. However, no matter how smooth the surface appears, even under a powerful glass, it must be lapped to remove the wheel marks.
For the internal grinding of jig bushings, aloxite wheels, if inch in diameter, f -inch face, 60 grit, P grade, 0-495 bond, may be used with good results, the wheel speed being 12,000 R.P.M. For bushings averaging 2\ inches long, if -inch hole, the holes rough-bored, 0.015 inch being left for grinding, the grinding time per bushing, including chucking and truing up, would be about twelve minutes each, and the finish left good, 0.0005 inch being sufficient to lap out the wheel marks. Reference is made to the holes being rough-bored; this is good practice, as the rather rough surface tends to wear the wheel just a little while removing the fire scale, thus preventing the wheel from glazing. Once the scale is removed from the hole, the wheel should not glaze readily, provided it is of the proper grit and grade.
Wheels for internal grinding should be run at a surface speed of 5000 feet per minute. This, however, is a general rule open to exceptions. A safe practical rule to follow is to speed up the wheel if it wears away too readily, and to reduce the speed where the wheel shows a tendency to glaze. Attention to this rule will often save much trouble. The toolmaker should bear in mind the fact that it is easier to adjust the speed to suit the wheel than it is to try to keep on hand a large variety of wheels to suit all speed conditions.
Assuming that the work in question is to be done on an ordi- nary universal grinder, the headstock must be set parallel with
84 JIG DESIGN
the travel of the platen to produce straight holes. A practical way to determine parallelism is to clamp a piece of round stock in the headstock chuck, letting it project from the jaws a little farther than the length of the holes to be ground. This piece should have a groove turned in it for the wheel to dwell in during reversal. This test piece is then ground in the regular way with the wheel used for cylindrical work, the headstock being adjusted by means of its swivel base until the test piece is ground parallel. Before calipering, the wheel should be al- lowed to grind until very few sparks are visible. When once this test piece has been ground straight the setting can be de- pended upon to produce straight holes, provided, of course, that the swivel adjustment of the headstock and the angular adjustment of the platen are not disturbed. To try to align the headstock by calipering the work while the internal grinding is in process is, at best, difficult, and the operator is never sure of accurate results.
It is common practice to true wheels for internal grinding with a diamond fed by hand, using the eye as a guide. This is poor practice, as the wheel is seldom turned parallel, one edge being left to do all the cutting, which glazes it readily. A more practical way to true these comparatively soft wheels is to feed them past the end of a carborundum rub, in 20 grit, H grade. The rub can be held in a suitable holder strapped to the platen of the grinder or held firmly by hand against the end of the work. A carborundum rub shows high efficiency when used for this purpose.
In holding work in the chuck for internal grinding, it is well to exercise due care to see that the work is not clamped hard enough to spring it out of shape. As a rule it does not require much pressure to hold work of this nature, as the grinding cut is comparatively light. As it is general practice to grind internal work dry, a certain amount of expansion from frictional heat is always present. For this reason considerable care has to be used in calipering the work with the sizing plug. As the plug is many degrees cooler than the work, it is liable, on being inserted, to contract the bushing suddenly, causing bushing and plug to
BUSHINGS 85
"freeze" together firmly. This can be avoided by cooling the work with a plug that is known to be undersize before caliper- ing with a plug of the desired size.
When a wheel of 60 grit is used, a hole one inch or under in diameter should be left approximately 0.0005 mcn undersize. This amount is sufficient to lap out the wheel marks and leave a "dead smooth" mirror finish to the hole. This is a general rule based on the fact that a certain amount (in this case 0.00025 inch) is enough allowance to lap out the marks left on a surface by a grinding wheel, and that should suffice for all holes regard- less of size. With comparatively large holes, one and one-half inch diameter or over, it is better, however, to make allowance for finishing, owing to the fact that the area of contact of wheel
|
ftZ^^^^ |
||||
|
\ |
1 |
|||
|
l^^^^ |
||||
|
Machinery |
Fig. 18. Arbor for Holding Bushings
and work is generally not so great and the ground surface is not quite so smooth.
In regard to the external grinding of bushings, there are two important points that should be given consideration: the selec- tion of wheels and the method of holding the work. The wheel should be fast cutting and at the same time it should hold its shape and leave a good finish. For this work good results may be obtained with an aloxite wheel of 1 2 inches diameter, |- inch face, 5-inch hole, 405 grit, N grade, D-497 bond, the wheel being run at a speed of 1800 R.P.M.
When a number of bushings arc to be ground one after another it is best to mount them on arbors of the same length, when practicable to do so, thus saving considerable time generally spent in re-setting the platen, which has to be done whenever the tailstock is moved to accommodate arbors of different lengths. An arbor for holding bushings should be made as shown in Fig. 18. The straight part should be a good fit in the
86 JIG DESIGN
bushing, a slight taper on the remainder of the arbor being sufficient to prevent the bushing from turning on the arbor. When bushings are held on an ordinary arbor or mandrel the operator is never quite sure that the hole and the outside of the bushing are concentric, as one end of the arbor, owing to its taper, does not quite fill the hole. This is illustrated in Fig. 19. Both Figs. 1 8 and 19 are somewhat exaggerated to illustrate the principle.
In grinding lining and solid bushings, due allowance must be made for a driving fit in the body of the jig. There are three methods in common use for making driving fits on this class of work: First, grinding the bushing until the lower end just enters the hole, the bushing being slightly tapered to bring it to a snug fit when pressed into place; second, grinding the
|
v//////////////////^^^ |
|||
|
\ \ |
|||
|
V//////////////////^^^ |
Fig. 19. Improper Fit of Bushing on Ordinary Arbor
bushing straight for its entire length, leaving it just enough oversize to make a good driving fit; and third, grinding the bushing for nearly its entire length just enough oversize to make a good driving fit, and grinding about one-eighth its length just enough undersize to enter the hole.
The first method is not considered very good practice, as the bushing contracts more at the top than elsewhere, owing to the taper, which leaves the hole in the bushing tapered. The sec- ond method is very poor practice, as the bushing is liable to cramp while being forced in place, which results in an unsatis- factory job, as the hole in the jig is generally sheared by the sharp end of the bushing. The third method is correct, as the part that is ground to fit the hole acts as a pilot, thus insuring the proper starting of the bushing, and the body, being straight, insures even contraction.
In making allowances for driving fits, o.ooi inch for each
BUSHINGS
inch diameter of the bushing is considered practical where the holes are one inch or over, and where the holes in the jig are bored smooth. If the holes are rough-bored, a more liberal allowance is required. After the lining bushings are driven in place, they require re-lapping, as they always contract a little.
The outside of the removable bushings should be finished by lapping to a "dead smooth" finish, as otherwise they will soon wear loose. This should never, under any circumstances, be done with emery cloth, but with a cast-iron lap as illustrated in Fig. 20. The abrasive used in this case should be of flour grit with lard oil as a lubricant, the abrasive and oil being applied through a hole in the top of the lap. The work should be lapped with a regular even motion to insure its being
Machinery
Fig. 20. Lap for Finishing Outside of Slip Bushings
straight, and should be brought to the temperature of the room by being cooled in benzine or gasoline before testing for a fit. The lapping should be carried to a point where the bushing is a wringing fit in its liner, but not tight enough to stick when left for a moment.
After the grinding and lapping of the removable bushings, their tops can be finished by lapping on a carborundum stone, in medium grit, wet with gasoline. A regular motion should be used across the face of the stone without turning or altering the relative position of the bushing. This lapping gives the bushings a good appearance, and, as the dimensions stamped are left black from the action of the fire in hardening, they can be read at a glance.
Driving Fit Allowances for Jig Bushings. — Standard dimen- sions for driving fit allowances for jig bushings, arranged ac- cording to the outside diameter of the bushing, are given in
6J
88
JIG DESIGN
Table VI. Oftentimes difficulty is experienced in assembling the bushings on account of not having allowed the proper amount of stock for fitting.
Plate Bushing Holders for Multiple Drilling. — When a number of holes are to be drilled and reamed on a multiple- spindle machine, the most simple method is to place the piece in a suitable jig and use individual slip bushings, so that after the holes are drilled the bushings can be replaced with reamer- Table VI. Allowances for Driving Fit for Drill Bushings
|
Outside Diameter, Inches |
Allowance for Drive Fit, Inch |
Outside Diameter, Inches |
Allowance for Drive Fit, Inch |
Outside Diameter, Inches |
Allowance for Drive Fit, Inch |
|
3A6 |
O.OOI |
7/8 |
0.0015 |
i 5/8 |
0.0025 |
|
S/i6 |
O.OOI |
0.0015 |
13/4 |
0.0025 |
|
|
7/16 |
O.OOI |
1/16 |
O.002 |
i 15/16 |
0.0025 |
|
1/2 |
O.OO-I |
1/8 |
O.OO2 |
2 1/8 |
0.003 |
|
9/i6 |
O.OOI5 |
3M |
O.OO2 |
21/4 |
0.003 |
|
11/16 |
0.0015 |
5/i6 |
O.O02 |
2 7/16 |
0.003 |
|
3/4 |
0.0015 |
3/8 |
O.OO2 |
25/8 |
0.0035 |
|
13/16 |
0.0015 |
7/16 |
0.002 |
2 3/4 |
0.0035 |
size bushings, the jig moved under the reamers, and the holes machined. The loss of time in handling these slip bushings is so great that the production costs increase very rapidly, especially when the operator has to stop to pry up bushings with a screwdriver or some other tool, as is often the case. This style of bushing will frequently catch the drilling or ream- ing tool and turn with it, thus wearing the bushing plate. To prevent its turning, the groove-cut bushing is sometimes used. This consists of an ordinary slip bushing in which a slot is cut spirally around one-quarter of the outer periphery. This slot engages a pin in the bushing plate, so that, when the bushing starts to slip, the pin prevents its making a full turn. A modi- fication of this method was described in connection with Fig. 12. One source of trouble from individual slip bushings is the accumulation of chips, which must be carefully removed before the bushings are changed; another is the possibility of inter-
BUSHINGS
89
changing the drilling and the reaming bushings (even though they are carefully marked) and thus spoiling the tools or the work. An improvement over the individual slip bushings is
\ '•' /' / TO FIT
BUSHING BORE IN JIG
rTh
] L
«_ DIAMETER BUSHINQ BORE IN JIG
Marlilnrry
Fig. 21. Drill with Guide Bushing attached
Fig. 22. Stationary Guide for Multiple Drilling and Reaming Tools
the plate bushing holder, which is especially useful on such work as crankcases, cylinders, etc., and in practically all work where six or more holes are to be drilled. The work is placed in a box jig or frame in which there are either two dowel-pins
90 JIG DESIGN
or two slots. The removable bushing plates used with this frame have holes or hinged binders to correspond with these pins or slots and so are correctly located.
Guide Bushings attached to Drills. — When several small holes necessitating two or more operations are to be machined, the following plan works well from a production standpoint. Guide bushings of the same diameter are fastened to the drills, reamers and other tools to locate them in the bushings in the plate, which are uniform in diameter. Thus, when drilling or reaming, the tools will be guided from the bushing A, Fig. 21. This method is not recommended for holes over one inch deep, as there is a tendency for the drills to spring out of alignment, especially if the drilling is done against a rough surface, since the end of the drilling tool will be some distance from the aux- iliary bushing guiding it. This arrangement is effective for drilling steel, as the space between the jig plate and the work allows room for the curled chips. The diameter of the guide bushing, however, must be kept as small as possible, since this piece has a tendency to heat and stick owing to the peripheral speed. This sticking and the wear on the bushing plate may be avoided by using a stationary pilot similar to that shown in Fig. 22. A Z-shaped casting with a bore equal to the tool size and a nose equal to the jig bushing diameter is secured to the arm of the multiple-spindle drilling machine by a bolt that extends through the slot in the arm, as shown in the illustration.
General Notes on Bushings. — When accurate work is neces- sary, the bushings should support the cutting tool to within one diameter of the tool from the work. If a A-inch drill is used, the end of the bushing should not be more than -fe inch from the work, and it may be carried to within J inch of the work. Bushings should not be located close to the work with the object of carrying the chips up through the bushing. It is much better to provide other means in the jig for the removal of the chips.
The shape of the work frequently requires bushings of con- siderable length in order to carry the cutting tool close to the work. When the length exceeds four diameters of the tool to be guided, the bushing presents considerable friction surface.
BUSHINGS gi
A length equal to two diameters of the cutting tool is usually sufficient for a bearing surface in the bushing. The remainder of the length of the hole in the bushing may be counterbored or relieved. The end that should be relieved is, of course, that which is farthest from the work into which the tool is to be guided.
Screw bushings are generally avoided when accurate work is required. There must be a certain amount of clearance in the ordinary tapped hole, and a threaded bushing is likely to be out of true on that account. Sometimes, however, it happens that no other type of bushing can be used for the work in hand.
The headed or flanged bushing is preferred by many tool designers as a lining bushing, whenever it is possible to utilize it. If it is desired to have the head of the bushing flush with the surface of the jig, the jig is counterbored to receive the head.
As previously mentioned, slip bushings are employed when several operations are to be performed through the same lining bushing. For example, when it is desired to drill and ream a hole and to finish a boss or spot around the hole while the work is still in the jig, a lining bushing is selected that will guide a counterbore iV inch larger than the boss to be finished. A slip bushing is then made to guide the drill, the body of which is a sliding fit in the lining bushing. Another slip bushing is made for the reamer which is also a sliding fit in the lining bushing. The slip bushing walls may have any thickness, providing they are not too thin. Should the conditions require bushings with too thin walls, the counterboring operation in the jig must be abandoned and some different method of procedure adopted.
CHAPTER V LOCATING POINTS AND ADJUSTABLE STOPS
The locating points in a jig usually consist of finished pads, bosses, seats, or lugs, cast solid with the jig, as illustrated in Fig. i . In this engraving the surfaces marked / are the locat- ing points, which bring the piece to be machined in correct re- lation to the bushings guiding the drills, or to the gages to which other cutting tools may be set. This method of locating the work is satisfactory when the work done is finished in a uniform way and where there is very little variation in the parts inserted in the jig.
Pins and Studs used as Locating Means. — Another com- monly used method for locating the work in jigs is by means of dowel pins, as shown at A and B in Fig. 2. The sides of the dowel pins which rest against the work are usually flattened, as indicated, so as to give more bearing than a mere line con- tact with the pins could give, and, at the same time, prevent too rapid wear on the locating pins, as would be the case if the work bear against the pins along a line only.
Sometimes pins or studs are inserted in jigs to act as locating points, instead of having lugs cast directly on the jig as shown in Fig. i. A case where a pin is used for this purpose is shown in Fig. 3, where B is the body of the jig, A the pin inserted to act as a locating and resting point, and C the work located against this point. Locating pins of this character should always be provided with a shoulder or collar, so that they will firmly resist the pressure of the work they support, without possibility of moving in the hole in which they are inserted.
Locating by Means of V-blocks. — A common method of locating cylindrical pieces or surfaces is that of placing the cylindrical surface in a V-block, as shown in Fig. 4. This V-block, as a rule, is stationary, and is held in place by screws
Q2
LOCATING POINTS
93
and dowel pins, as indicated in the engraving, but sometimes this V-block may also be made adjustable, in order to take up the variations of the pieces placed in it, and also in order to act as a clamp. A V-block of this character is shown in Fig. 5. In this, A is the adjustable V-block, having an oblong
\-fJ
C
n
Fig. i. Locating Pads in Jigs
Fig. 2. Pins used for Locating Work
hole B to allow for the adjustment. The block is held down in place by a collar-head screw C, which passes through the elongated hole. The under side of the block is provided with a tongue Z), which enters into a slot in the jig body itself, the V-block being thereby prevented from turning sideways. The
Fig. 3. Inserted Pin used for Locating and Support- ing Work
Fig. 4. V-block for Locating Round Work or Cylindrical Surfaces
screw E passes through the wall of the jig, or through some lug, and prevents the V-block from sliding back when the work is inserted into the jig. It is also used for adjusting the V-block and, in some cases, for clamping the work. The V-blocks are usually made of machine steel, but when larger sizes are needed they may be made of cast iron. Little is gained, however,
94
JIG DESIGN
in making these blocks of cast iron, as most of the surfaces have to be machined, and the difference in the cost of material on such a comparatively small piece is very slight.
Cup and Cone Locating Points. — When it is essential that a cylindrical part of the work be located centrally either with the outside of a cylindrical surface or with the center of a hole
Fig. 5. Adjustable V-block used for Locating Purposes
passing through the work, good locating means are provided by the designs shown in Figs. 6 and 7. In Fig. 6, the stud A is countersunk conically to receive the work. The stud A is made of machine or tool steel, and may, in many cases, serve as a bushing for guiding the tool. In Fig. 7, the stud is turned conically in order to enter into a hole in the work. These two
WORK
Fig. 6. Recessed Stud used Fig. 7. Conical Stud used for Locating Round Work for Locating Work in Re- in a Jig with Relation to lation to the Center of the Center a Hole
locating appliances are always made stationary, and are only used for locating the work, never for binding or clamping.
Screw Bushings and Sliding Bushings used as Locating Means. — Screw bushings may be used for locating and clamp- ing purposes by making them long enough to project through the walls of the jig and by turning a conical point on them, as
LOCATING POINTS
95
shown in Fig. 8, or by countersinking them, as shown in Fig. 9. In all cases where long guide bushings are used, the hole in the bushing ought to be counterbored or recessed for a certain dis- tance of its length.
Another type of bushing which serves the same purpose as a screw bushing is illustrated in Fig. 10. This bushing, together with the forked lever D and clamping bolt and wing-nut shown, will serve not only to locate but also to clamp the work in place. This sliding bushing gives very good results and is preferable to the screw bushing in cases where accurate work is required; but, as a rule, where extreme accuracy would be required, this kind of locating means is not used.
In Fig. 10 the sliding bushing A has a close sliding fit in the lining bushing B. In the head of the bushing A there are two
Figs. 8 and 9. Screw Bushings
screws with hardened heads, which fit into elongated slots in the forked lever or yoke D, which, in turn, swivels around pin E. The eye-bolt F fits into a slot G in the yoke, and the wing- nut tightens down the bushing against the work as clearly indi- cated in the engraving. A comparatively long bearing for the bushing is required in order to produce good results. On work that varies considerably in size, this arrangement works some- what quicker than does a screw bushing, but it is clearly evident that it is a rather expensive appliance and that the construction of the jig does not always permit of its application.
In some instances it is necessary to have the screw bushing movable sideways, for instance, when the piece of work to be made is located by some finished surfaces, and a cylindrical part is to be provided with a hole drilled exactly in the center
JIG DESIGN
of a lug or projection, the relation of this hole to the finished surfaces used for locating being immaterial. The piece of work, being a casting, would naturally be liable to variations between the finished surfaces and the center of the lug, particularly if there are other surfaces and lugs to which the already finished surfaces must correspond, and in such a case, the fixed bushing for drilling a hole that ought to come in the center of the lug, might not always suit the casting. In such a case, so-called " floating" bushings, as shown in Fig. n, are used. The screw
WORK
Fig. 10. Sliding Bushing for Locating and Clamping Work
bushing A is conically recessed and locates from the projection on the casting. It is fitted into another cylindrical piece B, provided with a flange on one side. The piece B, again, sets into the hole C in the jig body Z), this hole being large enough to permit the necessary adjustment of the jig bushing.
When the bushing has been located concentric with the lug E on the work, the nut F, having a washer G under it, is tightened. The flange on piece B and the washer G must be large enough to cover the hole C even if B is brought over against the side
LOCATING POINTS
97
of the hole. It is not often necessary, however, to use this floating bushing, because it is seldom that a drilled hole in a piece of work can be put in without having any direct relation to other holes or surfaces.
Adjustable Locating Points. — The most common form of ad- justable locating points is the set-screw provided with a check-
Fig, ii. Floating Drill Bushing
Fig. 12. Adjustable Locat- ing Point
nut, as shown in Fig. 12. The screw A is a standard square- head set-screw, or, in some cases, a headless screw — with a slot for a screw driver; this screw passes through a lug on the jig, or the jig wall itself, and is held stationary by a check-nut C
Fig. 13. Adjustable Locating Point consisting of a Flatted Stud held in Place by a Set-screw
tightened up against the wall of the jig. Either end of this screw may be used as a locating point, and the check-nut may be placed on either side. By using a square-head screw, adjust- ment is very easily accomplished, but unless the operator is familiar with the intentions of the designer of the jig, locating
JIG DESIGN
points of this kind are often mistaken for binding or clamping devices, and the set-screws are tightened up and loosened to hold and release the work, when the intention is that these screws should be fixed when once adjusted. It is not even possible to depend upon the check-nut stopping the operator from using the screw as a binding screw. A headless screw, therefore, is preferable, as it is less apt to be tampered with.
The sliding point, as illustrated in Figs. 13 and 14, is another adjustable locating point which is used to a great extent in jig work. A flat piece of work or a plate which is not perfectly level will always rock if put down on four stationary locating
Fig. 14. Sliding Point used for Locating Work
points, but the difficulty thus encountered is very easily over- come by making one of the locating points adjustable, and, as a rule, the sliding point is used for this purpose.
One design is shown in Fig. 13, where A represents the work to be located, B the sliding point itself, and C the set-screw, binding it in place when adjusted. The sliding point B fits a hole in the jig wall and is provided with a milled flat slightly tapered, as shown, to prevent its sliding back under the pressure of the work or the tool operating upon the work. This design of sliding point is frequently used, but it is not as efficient as the one illustrated in Fig. 14. In this design the sliding point A consists of a split cylindrical piece, with a hole drilled through it, as illustrated in the engraving, and a wedge or shoe B tapered on the end to fit the sides of the groove or split in the sliding point itself. This wedge B is forced in by a set-screw C, for the
LOCATING POINTS
99
purpose of binding the sliding point in place. Evidently, when the screw and wedge are forced in, the sliding point is expanded, and the friction against the jig wall D is so great that it can withstand a very heavy pressure without moving. Pin E pre- vents the sliding point from slipping through the hole and into the jig, when loosened, and also makes it more convenient to get hold of. In the accompanying table are given the dimen- sions most commonly used for sliding points and binding shoes and wedges.
Special Types of Adjustable Stops. — Adjustable stops are used to a greater extent in milling fixtures than in drill jigs, but
Dimensions of Sliding Points and Shoes or Binders
i
Screws Me
H 21A to 3
2J4 to 3 H
2J4 to 3
5A6
U 2}4 to 3
M M
Ms
9*2
Ha
the principles employed are the same. The examples shown in connection with the following description of adjustable stops have been applied to milling fixtures, and, in some cases, to drill jigs. In Fig. 15 is shown the simplest type of adjustable stop, provided with a helical spring beneath the plunger, to press it against the work. The objection to this type of stop is that the plunger A will slip back under the pressure of the clamps or cutting tools upon the work. There is also danger of the milled flat on the plunger clogging with dirt, so that the stop will not work properly. Considerable time is, therefore, lost in using jigs or fixtures with this type of stop. The method of clamping the plunger is also slow, as it is necessary to use a wrench in tightening or loosening the set-screw B. In Fig. 16 is shown an adjustable stop which is an improvement over that shown in Fig. 15. The flat on the side of plunger A is milled at a slight
100
JIG DESIGN
angle instead of parallel with the center-line, as in Fig. 15. This prevents the plunger from slipping after being clamped. A piece of hardened drill rod B, which is kept from turning by a small pin C, engaging a flat milled in piece B, is used between the plunger A and the clamp. A wing-nut D is fastened to the
| WORK
Machinery
Fig. 15. Simple Type of Adjustable Stop
Fig. 1 6. Improvement on Stop shown in Fig. 15
end of the screw as shown, in order to eliminate the use of a wrench.
In Fig. 17 is shown another adjustable stop which presents a further improvement over that shown in Fig. 16. A bronze bushing B is driven into the base of the jig and allowed to pro-
Machinery
Fig. 17. A Further Improvement upon the Adjustable Stops shown in Figs. 15 and 16
ject above the base, as indicated. Plunger A is a sliding fit in the bushing. A cap C is driven onto the end of the plunger and extends down over the outside of the bushing, as indicated, making the stop dirt-proof. This stop, however, as well as that shown in Fig. 16, is not entirely satisfactory, because it will
LOCATING POINTS
101
shift at the time it is tightened, although when once tightened it will remain in position.
In Fig. 1 8 a different arrangement is shown. Here the thumb-screw and spring plunger used in the preceding device is abandoned, and the sliding wedge A is used to obtain the pressure upon plunger C. The wedge is provided with a handle B attached so that it can easily be operated, and is held in place by two shoulder screws that are inserted through two elongated slots milled in the wedge. These screws are tightened after the stop has been brought up to position. The difficulty met with in using this stop is that the wedge is liable to slip back, owing to
Machinery
Fig. 1 8. Simple Form of Adjustable Wedge Stop
the vibration of the machine while in operation, so that plunger C drops down.
In Fig. 19 is shown a further development of the method indicated in Fig. 18. In this case, means are provided for pre- venting wedge A from slipping back. A stud is riveted into the wedge A, this stud extending up through an elongated slot cut in the base of the fixture. The end of the stud is threaded for the knurled nut B, which also acts as a handle for shifting the wedge. When this nut is tightened, it clamps the wedge A and the shoe C against the base. The friction between shoe C and the base prevents the slipping of wedge A. Shoe C also acts as a covering for the slot cut in the base, and thus acts as a dirt and chip shield. It is prevented from turning, when the nut B is tightened or loosened, by a stud D, driven into it and sliding in a slot cut in the base. The difficulty with this design is that wedge A rests upon the table of the machine, and, if there is slight unevenness in the table, the plunger is liable to spring down slightly under the pressure of the cut.
102
JIG DESIGN
In order to overcome this difficulty, an adjustable stop, as shown in Fig. 20, has been designed. The flat style of wedge is abandoned, and the wedge A is made of drill rod and slides in a hole drilled in the base of the fixture. The stud at the back end of the wedge is screwed into it instead of being riveted, as in the previous example. Bushing C is provided with a shoulder and a headless set-screw D is added to prevent plunger E from dropping out when the fixture is not in use. The wedge A is subjected to considerable friction and the fixture is, therefore, not so sensitive to the touch of the operator as would be desir-
x Machinery
Fig. 19. Improvement upon the Adjustable Wedge Stop shown in Fig. 18
able. It is difficult for the operator to feel when the stop is against the work, when tightening the wedge in position.
Fig. 21 shows a modification of the design shown in Fig. 20, the only change made being in bushing A, which has been lengthened so that it will act as a support for the end of wedge B. The bushing is made of cold-rolled steel and casehardened. The bottom part of the base is cut away in order to reduce the friction between the base and the wedge. This design is better than that shown in Fig. 20.
In Fig. 22 is shown a somewhat complicated and expensive adjustable stop which, however, has the advantages of almost perfect operating conditions. Bushing A is lengthened and has
LOCATING POINTS
I03
a much larger shoulder in order to take the thrust to which it will be subjected when the device is operated. A small pin B replaces the headless set-screw used in the designs in Figs. 20 and 21. The arrangements for clamping the wedge have been considerably changed, and bronze casting C is added. A hole is cut in the base into which the casting is inserted, clearance
Machinery
Fig. 20.
A Further Improvement upon the Adjustable Wedge Stops shown in Figs. 18 and 19
being permitted all around so that the casting can be aligned easily with the wedge. The casting is held in place by two fillister-head screws and two dowels; a hole is drilled through the lower part of it which acts as a support for the back end of the wedge, as indicated. The front end is supported in the bushing A in such a manner that the friction is reduced to a
Machinery
Fig. 21.
A more Satisfactory Form of Adjustable Wedge Stop than that shown in Fig. 20
minimum. Casting C also supports the shoe D and raises it from the base of the fixture. A tongue is cut on the lower side of shoe D which fits into a groove in casting C, thereby prevent- ing the shoe from turning when the nut is tightened or loosened. Stud E is screwed into the side of the knurled nut and a small pin F is driven into the shoe. This pin acts as a stop for the
7J
104
JIG DESIGN
stud, preventing the operator from turning the nut more than is necessary in tightening or loosening.
The adjustable stop shown in Fig. 22 meets practically all requirements placed on an adjustable stop. It will not slip back under the pressure of the stop; it will not slip in tighten- ing; it is dirt-proof; all the parts form integral parts of the jig; and it will not become loose, due to vibration of the machine, or spring down under the pressure of the cut, due to unevenness of the tables of the machines on which the fixture is used. It can be rapidly operated and is so sensitive that the operator feels instantly when plunger G is in contact with the work.
X- — JJUIM-
\| WORK j \
' *^^:r — h"
SECTION A-A
Machinery
Fig. 22. Principle of the Final Improvement in the Adjustable Wedge Stop
The only objection to this design is that so much of the metal of the base has been cut away that it is seriously weakened, and the design shown in Fig. 23 is superior in this respect. In the making of the fixture, difficulties were also encountered in aligning the holes in bushing A with the holes in casting C, Fig. 22. This was remedied by making the bushing an easy fit and adding a small pin D and the round-head screw C, Fig. 23, to keep the bushing from turning or working loose. The wedge was also jointed and made in two parts, as indicated, in order to take care of the variations that might occur in drilling
LOCATING POINTS
105
the holes in the bushing A and casting C, Fig. 22, in which the wedge slides. This practically makes the wedge self-aligning.
Locating from Finished Holes. — If the work to be finished in the jig has some holes already finished, it is sometimes most satisfactory to locate the work by these holes, which may be done by means of studs or plugs similar to the one shown in Fig. 3, which then enter the holes; preferably, these studs should be ground and hardened to the standard size of the hole. If the finished hole should be of a character that varies somewhat in size, expansion studs with bushings may be used. These studs
Machinery
Fig. 23. The Adopted Form of Adjustable Wedge Stop
may be of a great many different designs and styles, but, as a rule, they always work on the same principle as the one shown in Fig. 24. In this, A is the bushing, fitting the finished hole in the work. This bushing is split in several different ways, either by having one slot cut entirely through it, and two more slots cut to within a short distance of the outside periphery, or by having several slots cut from the top and from the bottom, alternating, but not cut entirely through the full length of the bushing. The method of splitting, however, in every case, accomplishes the same object, that of making the bushing capable
io6
JIG DESIGN
of expansion, so that when the stud J3, which is turned to fit the tapered hole in the bushing, is screwed down, the bushing is expanded.
Locating by Keyways in the Work. — Sometimes the work to be finished in the jig is provided with a keyway or a slot, or with some other kind of a seat, by means of which it is located on its component part on the machine for which it is ultimately in-
Fig. 24. Fig. 25. Fig. 26.
tended, and it is always essential that the work be located in the same way in the jig as it is to be located on the machine on which it is to go; thus, if the work has a keyway suitable for locating, a corresponding keyway ought to be put into the jig, and the work located by means of a key, as shown in Figs.
Fig. 27. Work which is Milled as Indicated at E
25 and 26. Instead of a loose key, a tongue may be planed or milled solid with the jig, but, as a rule, it is more satisfactory to have the loose key, as, if it should happen to wear, it is pos- sible to replace it; and if the width of the keyway should vary in different lots of the parts made, it is possible, with little ex- pense, to make a new key to fit the variation, whereas if the key is made solid with the jig, and found to be either too large or too small, the trouble of fixing this would be considerably greater.
LOCATING POINTS
107
Common Defects in Jig Design. — The first consideration of the jig designer should be to determine what degree of accuracy is essential in the part that is to be produced, and also whether absolute interchangeability is necessary. This information will be a guide for the economical production of the jig. The de- signer must also consider any operations which are to be per- formed on the work prior to the one for which the jig under consideration is intended; for while this preliminary machining may not need to be accurately done, inaccuracy or uniformity may result in improperly locating the work in the next jig,
-GH
Fig. 28.
Defective Design of Fixture for Holding Piece shown in Fig. 27
which should be so designed as to locate the part with the re- quired accuracy.
The locating points of any jig should be such as to allow as wide a range of inaccuracy on any preceding operation as is compatible in the part. For example, if the part has to be turned to, say, a limit of o.ooi inch, it will require more skill and time than if a limit of 0.005 inch is allowable. Again, as far as practicable, the portion of the work that requires to be the most accurate should be used in locating it in the jig for the succeeding operation. Often a surface is selected to locate from, which, in consequence, must be machined to an accurate limit, when accuracy otherwise would be unnecessary. This, of
io8
JIG DESIGN
course, only adds to the cost of production. After considering the points mentioned, the best method of arranging the details of the jig, so that it has as few dimensions as possible requiring absolute accuracy, should also receive attention; that is, the jig should be as simple as possible, and still be so designed as to accurately locate the parts to be machined.
In Figs. 28 and 29 are shown two jig designs which will serve to illustrate these points. The part for which a jig is required is shown in Fig. 27. In the preliminary machining operation the work is turned to diameters A and B and to lengths C and D. The limit of accuracy required on end A is — -^, or any diameter from i £ f inch as a minimum to i f inch. For end B a
Fig. 29. Fixture which will hold a Number of Pieces, Fig. 27, properly, even when Diameters of Locating Parts vary
finer limit of —0.002 is necessary, so that this end should be used as the locating part for the next operation; viz., the milling out of the slot E which must be central with the part B. A design such as shown in Fig. 28 is not uncommon for this operation, and with it fairly accurate results will be secured; but if the locating diameter on the work is slightly small, say 0.002 inch, then the forcing of the piece over to one side by the locking screw A will result in an inaccuracy in the milling operation. The locating holes B must be the exact size of the locating part of the work, and unless every piece is a push fit (which is un-
LOCATING POINTS
IOQ
necessary accuracy in the part) the location is not accurate, as the work is clamped against a small area on one side of the hole and the point of the set-screw on the other. This can be avoided by locating the part against V-blocks, as shown in Fig. 29, which locate each shank central, irrespective of the variations in their diameters. The construction of this jig illustrates the points which have been referred to. The V-blocks provide four lines of contact, and the part is secured very rigidly in a central position irrespective of the variations in the diameter of the locating part. This jig, though more expensive than the one shown in Fig. 28, is quite simple in its construction. A central slot is machined to a width which need not be to any particular dimension as the steel V-blocks will be accurately fitted to this slot. Steel plates are secured to the ends of the
Fig. 30. The Way the V-blocks for the Jig, Fig. 29, are planed
jig after machining the slot as shown. By closing these ends after the slot is machined, the tool has a clear passage through, which, of course, would be impossible were the ends cast on. The V-blocks are planed in one piece, as shown in Fig. 30. The only important dimension is the width of the block. The exact position of the V in relation to the sides is immaterial provided that after the blocks have been sawed off they are inserted in the slot in the jig with the long or short sides to- gether. To avoid trouble from this source, one side of the slot and a corresponding side on the blocks should be marked to insure the correct insertion of the latter. In the event of a design requiring the V's to be strictly central with the sides, the cost would, of course, be increased, as much more care would be required in machining. The jig shown in Fig. 29 is for holding three of the pieces shown in Fig. 27 at one time; this number could be increased as desired.
CHAPTER VI JIG CLAMPING DEVICES
The clamping devices used in connection with jigs and fix- tures may either clamp the work to the jig or the jig to the work, but very frequently the clamps simply hold in place a loose or movable part in the jig, which can be swung out of the way to facilitate removing the work from, and inserting it in, the jig. The work itself is in turn clamped by a set-screw or other means passing through the loose part, commonly called the leaf.
Types of Clamps. — The simplest form of clamping device is the so-called clamp, of which a number of different forms are commonly used. Perhaps the most common of all clamps is the one shown in Fig. i. This kind of clamp is also commonly termed a strap. It is simple, cheap to make, and, for most purposes, it gives satisfactory service. The clamp shown in Fig. 2 is made on practically the same principle as the one shown in Fig. i, but several improvements have been intro- duced. The clamp is recessed at the bottom for a distance 6, to a depth equal to a, so as to give a bearing only on the two extreme ends of the clamp. Even if the strap