The Design of Jigs, Tools & Fixtures

This article was originally published in Practical Engineering 1940 Vol1 No1. Information within this article is therefore correct as of 1940. The publication of this material aims to provide historical insight on the subject rather than modern specifics. Enjoy.

Toolmaking is a term that may loosely be applied to a wide range of activities embracing the manufacture of all classes of tools, including machine tools.

The particular phase of tool making with which these articles are concerned is that covered by the products of the engineering works department known as the Tool Room. Such a department forms an essential part of every engineering concern engaged on work of a repetition character. The actual nature of the work which the tool room is called upon to perform is dictated by the class of job to be produced within the works, but it may consist of the manufacture of any or all of the following devices: — jigs, fixtures, gauges, special cutting tools, press, tools, moulds and dies.

A tool room is completely equipped with machine tools, capable of producing accurate work, many of them being of a special character or specially equipped to cope with the machining problems peculiar to this particular class of work. Additional to this, there is a fitting department, and usually provision is made for effectively dealing with necessary heat treatment processes within the tool room. Accurate measuring instruments and standards are available for the purpose of comparison and checking work during the progress of manufacture. Thus it will be seen that the tool room is in reality a completely equipped and self-contained works within a works.

Its purpose is to supply the manufacturing department with special tools and devices designed to assist production and made to such limits of accuracy as to ensure interchangeability for the products resulting there from. The repair and maintenance of such devices and small tools is also taken care of by this department.

How Cost Affects Design

The basis of tool designing is a thorough knowledge of machine shop practice, plus some practical toolmaking experience. In large establishments the designs are produced by a tool-drawing office and the various stages of the work executed by specialists, whereas in smaller ones the matter of design may be left to the skilled toolmaker entrusted with the work. Where such a practice prevails the toolmaker must be capable of carrying the work right through from start to finish and is usually required to do this.

Jigs and Fixtures

The question of tool design is governed to a considerable extent by the number of pieces to be, or likely to be, handled by the device under consideration. Thus it might well be that a proposed scheme, while admirable for the purpose intended, would be unwarranted on account of excessive cost against the total number of pieces required. Therefore, it is apparent that an alternative arrangement, while being inferior from the viewpoint of operating time, is superior to the more ambitious scheme when the ultimate total cost per piece is considered.

A jig is a device provided with one or more guides (for cutting tools) in which the work is located and held in correct disposition in relationship to the guides. Jigs are commonly associated with drilling and like, operations and may consist in some cases, where the holes to be drilled are in one plane, of little else than a flat plate in which are housed the tool guides. This arrangement presents the jig in its simplest form. Other circumstances such as where the holes lie in more than one plane, or where the points of location are inconveniently situated, dictate the conformation of the jig, which in many cases consists of a box-like structure.

A fixture is a work-holding device pure and simple, and is intended to provide a means of mounting work on a machine for the purpose of performing an operation in correct relationship to a previously machined surface, or surfaces, without the necessity of setting-up the work. In order that the fixture itself may be located correctly on the working surface or table of the machine tool with a minimum of trouble, keys are provided in die base to engage with a table slot, or where the fixture is intended for use on a lathe or similar tool a short spigot is provided 'on the back to correspond with a register turned in the face plate. Apart from machining, fixtures are also widely used in connection with assembling operations. Examples of a jig and fixture both of simple design are illustrated in Fig. 1.

The Elements of Jig and Fixture Design

For a jig to prove successful in use, the following points must have been studied in the course of design. 1.— Correct choice of locating points. 2.— Making adequate provision for removing swarf from locating surfaces easily. 3.—The means of holding or clamping the part must be such that it will not cause distortion. 4.—Making provision to support the part where necessary to guard against cutting pressure causing distortion. 5.—Ease of loading and removing the work after completing the operation. 6.—Arranging the tool guides as near to the work as possible and making them of ample length. 7.—The elimination as far as possible of the likelihood of swarf preventing the jig from standing level on the machine table by raising the base on feet. 8.— Arranging the feef in such a manner that the cutting pressure cannot cause the jig to tilt and lastly that the jig is easy to keep clean during use. The same points arise where a fixture is concerned with the exception of those stressed in numbers 6, 7 and 8.

In order to amplify the foregoing points it will be as well perhaps to give examples of the results likely to accrue from having failed to observe any one of them. The locating points chosen, i.e., the surfaces of the work which abut against or rest on stops placed in the jig to determine its position relative to the tool guides, should be those which have to be maintained at a constant dimension and from which other main dimensions depend. As for instance, it will be seen by referring to Fig. 1, that the jig is located in the centre hole of the work. Where the jig is located on the outside diameter, the drilled holes, while right in themselves as regards spacing, could lie on a pitch circle eccentric to the bore by as much as the tolerance allowable on the comparatively unimportant external diameter. This, of course, is an elementary example, but it plainly shows the trouble that is likely to ensue from incorrect location in a more complicated piece of work or where perhaps more than one jigging is necessary.

It is obvious that if the stops provided in the jig as location points are not kept free from metallic particles, the work produced cannot be uniform. This liability is best frustrated by having the surfaces of the means of location visible to the operator and by the avoidance of sharp corners in which swarf may collect.

Clamping arrangements which cause distortion may permanently deform the work or allow the work to resume its normal shape upon removal from the jig, when an inaccuracy in the drilling becomes apparent.

The lack of proper provision, where necessary, against the work springing or distorting during machining, will produce similar results. Where the design of a jig is such that the loading operation is made difficult, the loss of machining time may seriously affect production. While the loading may be an easy matter the removal of the work can be rendered difficult by the failure to provide clearance for machining burrs. Such a happening will necessarily show up the work and most likely cause damage to the work by having to apply force to assist in its removal.

Unless the tool is guided almost to the point where it makes contact with the work, inaccuracies are liable to creep in on account of " drill run." A guide that is too short in comparison with its internal diameter can also produce similar results. Jigs that are made to stand on a flat base can cause constant' trouble by reason of drilling or metal chips becoming embedded therein, thus leading to inaccuracy and tool breakage. An improper arrangement of the feet provided to counteract this objection will have the same effect.

The nature of the service that tool guides have to perform demands that they be made from material not easily damaged by the cutting edges which come into contact with them. These guides are made in the form of bushes, cast steel being employed for making the smaller sizes. Larger ones are usually made from case hardening mild steel. After suitable heat treatment the bores are finished by lapping or grinding to suit the " land" sizes of the cutting tools for which they are intended. The external diameters are then ground to size to ensure that the bushes are truly concentric.

Use of Bushes

Various types of bushes are used according to the nature of the work to be done. It may be that in certain instances the use of a headed bush is precluded. In such cases a flush type of bush is employed. Where a hole requires reaming after drilling a slip bush is used. Here the hole in which the bush or bushes fit is provided with a liner to guard against the jig body becoming worn by constant insertion and removal of the slip bushes. The question of whether a separate bush is required for guiding the reamer will depend upon the positional accuracy demanded.

A slip bush is also necessary where a counter boring or spot facing operation follows the drilling. Where this occurs the internal diameter of the finer requires to be such that it will guide the body of a pilotless cutter or if otherwise to clear it. Provision has to be made to prevent the slip bushes from lifting out or turning in the liner when in use. This is accomplished in a variety of ways, but most usually by the provision of a bayonet catch arrangement.

Image Credit: Robert S. Donovan