Tool engineering - jigs and fixtures

The aim and purpose of this book is to furnish information with respect to the science of tool engineering. Nothing has previously been published on the subject except in short articles dealing with specific examples of jigs and fixtures. Information of value regarding principles of design in connection with production tools is sadly lacking and mechanical literature contains only spasmodic efforts to remedy the deficiency.

In order to cover the subject properly three volumes were planned, each of these being complete in itself. This volume, which is the first, deals with the design of jigs and fixtures. It covers the important points connected with the design, shows the reasons why certain methods are better than others, takes up principles and their application to design and gives many graphic examples which illustrate the use of the principles involved. An (endeavor has been made to simplify the subject matter as far as possible and to treat it in a practical common sense manner which can be easily understood by the designer. A careful study of the illustrations and descriptive matter will enable a progressive man to understand both the theory and practice necessary for this line of work.

The second volume takes up turret lathe and vertical boring mill tooling together with grinding fixtures. The third volume deals with punches, dies and gages.

For a number of years the machines and tools used for production have been undergoing a process of evolution and although the development work has progressed rapidly, much still remains to be done. Present manufacturing methods are of the highest order and tooling for high production is of interest to all the mechanical fraternity. There are however, comparatively few men in this country who really know the science in all its fundamentals and for this reason the tooling in many factories is probably not over 50% efficient.

A great many of those responsible for tooling are not well informed as to the fundamentals of design. Tools are worked out more or less by using ideas in vogue in the factory where the work is being done and the design is usually influenced by previous practice for work of the same character. Progressive tool engineering requires first of all, a thorough knowledge of principles and the ability to specify the machining operations necessary on a given piece of work. With this as a basis, mechanical problems can be analyzed and the solution obtained by the application of known principles. For this reason our books take up the subject fundamentally and deal largely with principles although many examples of interesting fixtures are illustrated. Mechanical principles are fixed and do not change from year to year as designs often do; hence, the man whose knowledge of tools is firmly grounded on sound mechanical principles is independent, original and progressive, so that his designs are practical, economical and productive.

The superintendent, factory manager, foreman and tool engineer will find theory and practice combined in such a way that the principles on which the science is based will be readily understood. The reasons why one design is better than another are graphically shown in numerous examples, dealing with actual cases observed during the writers' long experience in handling production problems both in shop and drafting room. Problems are analyzed; causes of trouble shown; correct and incorrect methods illustrated; and much valuable data are given regarding designs and proportions of jigs, fixtures, turret lathe tools, punches, dies and gages.

It is our belief that the work will be appreciated by mechanical men throughout the country. We hope that the many practical examples will provide food for thought and eventually bring about a general revision and radical improvement in tooling methods.

The science of tool engineering as it is now practiced dates back comparatively few years and very high production tooling is of even more recent date. A few years ago when production was small the majority of jigs were made as cheaply as possible, no great attention being paid to upkeep, because production was not sufficiently high to warrant it, except in the case of products which had been to some extent standardized, such as military rifles, army pistols, sewing machines, and similar work. The usual practice in the old days was to make a rough list of operations which was to be followed and then give a few free-hand sketches to the toolmaker to show him approximately how the tools were to be made, leaving many of the details to the man himself. When this man needed a pattern he went to the pattern maker and told him what he wanted, leaving the proportioning of the pattern to him. Then after the casting had been made the toolmaker “whittled” it out until a makeshift jig was evolved, which served its purpose in the production of so-called interchangeable parts.

The tool engineer of the present day must be up to date in the manufacturing field; must have a broad knowledge of machine tools; must understand the theory and practice of cutting tools, speeds, feeds and kindred subjects and should have practical shop training of such a nature that he knows from his own experience just how a given machine is handled and what the requirements are for tools to be used on it. It he does not have this knowledge he will not be able to do the work required of him in the most efficient manner.

Listing of Operations - In up-to-date tool engineering the first step in the process is the listing of the various operations necessary to machine each component part of the mechanism which is to be manufactured. In this listing of operations each step in the manufacture is considered carefully from various viewpoints, such as: Economy in handling; machine tools most suitable; tooling equipment available; production required; accuracy required; jigs and fixtures necessary; gages necessary, etc. In many cases, also, work may require heat treatment, local hardening, grinding after hardening, welding, riveting to some other unit, polishing, bluing or nickel plating. All of these matters must be considered in the listing of the operations. Hence it is evident that the tool engineer must not only be familiar with the processes of machining, hardening, gaging and grinding, but he must also understand the construction of the mechanism as a whole in order that he may decide on the necessity for machining this or that surface so that it will bear a distinct relation to some other hole or surface in order that the entire unit will function properly.

Points To Be Considered - Let us assume that the blueprint of a certain part is turned over to the tool engineer, with the statement that the part drawing has been approved and is ready for tooling. The following points must then be taken into consideration in listing the operations:

1. Production Required. - This is an important consideration, which affects the method of handling to a considerable extent. If a comparatively small number of pieces is to be manufactured, the tools must be simple and cheap in order to keep the tool cost as low as possible. If a large number of pieces is to be manufactured, multiple fixtures and rapid clamping devices would be called for, in order to produce the work as rapidly as possible. In the latter case the cost of tools would be distributed over such a great number of pieces that the unit cost for took would not be excessive.

2. Material of Which the Work Is Made. - It may be a casting, a forging or stamping, or it may be made from bar or flat stock. If a casting, it may need to be pickled, sandblasted and snagged on a rough grinding wheel before machining. If it is a thin or irregular casting it should first be inspected both for quality and to see whether it has warped out of shape so that it cannot be machined to proper dimensions. If a forging it may require heat treatment before or during machining or it may be hardened and afterward ground so that necessary allowances must be made during the machining to provide sufficient stock for grinding. If made from round stock it may be found best to machine it from the bar on a screw machine, or perhaps its length and general shape may make handling it more profitable on a manufacturing lathe after cutting it into lengths on a cold saw or a cutting-off machine. It is evident from the foregoing that the material of which the part is made is an important factor in the machining.

3. Surfaces To Be Machined - In considering the various holes to be drilled, bored or reamed and the various surfaces to be machined, it is important first to decide whether the various holes can be drilled in one jig, or several jigs will be required; next, whether several milled surfaces can be machined in one setting or it will be more economical to make several operations. It is also necessary to decide whether any other operations that may be necessary can be handled to best advantage in combination, or by several operations. It is not good practice to drill small holes and large ones in the same jig, unless drilling machines can be so arranged as to obtain correct spindle speeds for the different sizes of drills required. In special cases it may be found profitable to do something of this kind in order to avoid a resetting of the work and the cost of an extra jig.

4. Accuracy Required - In any mechanism there are certain fundamental principles affecting the successful operation of the device. In order that it may function properly as a unit the various components which make it up as a whole must fit each other within certain limits of accuracy. These limits are usually specified on the drawings of each part and the tool engineer must keep them in mind when listing the operations as well as when designing the limit gages used in the production of the parts.

The accuracy with which various machine tools will work must be taken into consideration and if their accuracy is not sufficient to produce the results required, a final fitting or grinding operation may be necessary. So it is apparent that the accuracy required is a factor of importance in listing operations.

5. Selection of Working Points - In order to obtain the best results in production it is advisable to select working points which can be used for location in all of the operations on the work. It is difficult to give a hard and fast rule for determining which points are the best to work from, due to the fact that different cases require different treatment and various pieces of work are of such widely different design that no fixed rule can be given to apply to all instances. A very good thought in connection with the establishment of locating points is first to obtain a flat surface and next machine two or more holes perpendicular thereto if the nature of the piece will permit it. In a case of this kind it is possible to work from the finished surface for all the subsequent operations, locating by means of pins in the drilled or reamed holes, and in this manner making certain that correct relations are kept for all the operations with the points established as working points. Sometimes it may be necessary to vary this procedure on account of the shape of the work, but the matter of establishing the working points must always be considered very early in the listing of operations.

6. Provision for Chucking, - In the handling of work on the turret lathe it is frequently necessary to provide means for clamping or holding the work during the first operation. There are many cases where the shape of the work is such that it can be held in a chuck without difficulty, but in other instances it may be found necessary to provide the work with lugs in order to hold it properly. A case of this kind will be noted in the hub, illustrated in Pig. 2. In this case it was decided to machine the surfaces marked / in the same setting, and obviously it would be difficult to hold by means of the tapered portion A, By the addition of three lugs B the work can be readily held by the chuck jaws C, as indicated in the illustration. When lugs of this kind are added to a casting they may be removed by a subsequent operation or they may be left as they are, provided they do not interfere with the appearance or utility of the finished product.

7. Concentricity of Cylindrical Surfaces - In the listing of operations the importance of concentricity of the cylindrical surfaces which must be in alignment should be carefully considered, as any variation from the truth will cause the mechanism when completed to cramp and not run smoothly. It is advisable wherever possible to machine concentric cylindrical surfaces in the same setting, but as this is not always practical, particular attention must be paid to the method of holding, when several operations are used, in order that the work may be true when completed. A very good example of a piece of work of this character is shown in Fig. 3. In this case the bearing seats A and B must be concentric to each other, and yet it is apparent that the two surfaces cannot be machined in the same setting of the work. For this reason the greatest care must be exercised in designing the tool equipment so that the first bearing seat B will be used as a location from which to produce the second bearing seat A, Many other examples could be given of work of this character, but the instance given is a representative one which will serve to illustrate the points involved.

8. Machines Required and Available - In the selection of machines for the work in process it is necessary that the tool engineer should be familiar with the various types of machine tools most suited to the work. In listing operations for an old plant having a considerable assortment of machine tools from which to choose the tool engineer must have a list of these machines together with necessary data on their capacities and their working ranges. It must alwics be borne in mind, however, that the selection of a machine for high production should not be dependent entirely upon the machine tools which are in stock, and it may be more profitable to purchase new equipment rather than to use old equipment which is out of date and does not give maximum efficiency.


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