The Design of Jigs, Tools and Fixtures

Posted By Richard Barker on 13 May 2014

Posted in The Vintage Machinery Almanac

This article first appeared in Practical Engineering 1940 Vol1 No3. The information that appears within this article is accurate as of 1940. The article provides details of small tools utilised in workshops at the time.

Jig Bodies

As stated previously the bodies of box jigs are mainly made from iron castings. It is necessary for this form of construction to have a pattern prepared from which to produce the casting. A typical casting for an open-sided box jig represents but one form of this class of jig, and for its completion it needs surface machining and boring correctly to receive the guiding bushes, and must be provided with suitable work-holding arrangements. Other forms of work may need a complete box-shape, when the lid of the box may be hinged to permit the loading and unloading of the jig. Large jigs of this description may require to be built up from cast sections, or the same principle may have to be adopted where the internal machining of the jig would otherwise be rendered difficult or impossible.

It will, of course, be understood that the jig casting requires to be designed in accordance with the shape of the job to be handled, when bosses for bushes, pads for supporting the work, and seatings for locating pieces can be correctly disposed on the pattern for incorporation in the casting.

It is intended at a later stage to give suggestions for “jigging" several concrete examples of work with a view to covering briefly the problems likely to be encountered. This particular type of jig can then be more fully dealt with in its completed form.

Methods of Clamping

While it is true that certain work may be held in a jig without being clamped in the ordinary sense—as, for instance, where a large, bored cross-hole laying parallel with a machined surface requiring to be drilled can be held in position by means of a cross-pin or mandrel passing through the jig and the work, in the majority of cases the holding has to be accomplished by some form of clamp or clamps.

The point of application of each clamp needs to be considered from the viewpoint of its liability to cause distortion of the work upon being tightened, and at the same time the clamping arrangement must provide adequate support where the machining is being performed against it.

Generally speaking, the operation of any jig is retarded where it is necessary to remove nuts from studs before the jig can be loaded and unloaded. In fact, it is better wherever possible to avoid loose pieces of any description. This is not, however, always possible without the addition of unnecessary complications.

A particularly simple form of clamping that is effective for work having a central hole that has been machined at a previous operation is one that has a central stud tapped into the bushed surface of the jig and is provided with a substantial slotted washer, of sufficiently large diameter to encompass the hole in the work, which is tightened down by means of a flanged nut. It will be apparent that by slackening the nut slightly the washer may be slid out and the work removed over the stud without taking the nut off the stud.

Where circumstances permit, the nut shown may be modified to dispense with the necessity of employing a separate spanner or key for tightening and loosening. This form of clamping is satisfactory for work of substantial proportions, or where the holes to be drilled are disposed reasonably close to the point of support, otherwise for similar work of a lighter nature the clamping arrangement consists of a spider, having legs of suitable length to engage the surface of the work, the central hole being large enough to clear the clamping nut.

Where the surface against which the legs bear is irregular in relation to the machined surface of the work opposing the locating face of the jig;—as, for instance, a rough cast or stamped surface—the ends of the legs should be domed and the surface about the centre hole in the spider machined concave. The under surface of the washer will then need to be machined convex to correspond, thus forming a compensating clamp which will take care of any unevenness in the surface of the work.

Plate Clamps

A simple form of plate clamp may consist of a plain slotted plate, when the heel piece of suitable height to correspond with the thickness of the work is attached to the jig. The clamping stud should be arranged in such a position as to permit the front of the clamp being slid clear of the work without entirely removing it from the stud. Where this type of clamp is used, the front of the clamp is retained at its normal height when the work is removed by means of a light compression spring passed over the stud.

Where it becomes necessary to remove the clamp entirely to facilitate loading and unloading, the slot may be run out to the front of the clamp or the front end of the slot may terminate in a hole large enough in diameter to clear the clamping nut. A certain degree of compensation for taking care of variations in thickness of the work may be obtained by providing a lip on the underside of the clamp bearing against the work. Where this is done the surface clamping nut requires to be convexed and the top of the clamp machined concave to suit in the appropriate position. The surface of the heel will also need to be made convex, these precautions being necessary to avoid bending. the stud when tightening.

Swinging Clamps

While the clamps just described serve for holding on the edges of work, an alternative form is necessary in certain instances. Use can then be made of a swing type of clamp which is mounted on pillars, but the pivoting stud and anchor could be incorporated on the edges of a box jig. The stud is made with two diameters so that the nut and washer may be tightened down, leaving the plate or strap free to swing on the larger diameter. The anchor pillar is necked in to receive the slotted plate freely, and the centre of the slot is cut on a radius equal to that of the pillar centres. The distance between the pillars is made to suit requirements and one or more clamping screws may be provided.

A similar form of clamp for use under different conditions is one in which the plate swings about a pin set in the bossed end of the plate. At the opposite end the plate shuts down on to the edge of the jig. With such a clamp it would be necessary to remove the knurled nut before the plate could be raised if a fixed stud were employed. To obviate the necessity it is usual to fit a swing stud. Here the stud is bossed at its lower end and drilled to swing about a pin set in a fork or slot cut in the jig body.

With this type of clamp it is possible to dispense with the stud and nut, and fit a refinement in the shape of a quick-acting latch device. This, naturally, involves more work in the making, but where the job warrants adopting this alternative arrangement the trouble will be amply repaid on the saving in operating time.

Sliding Bars

A similar effect to that just described may be obtained in a simple manner with a loose square or rectangular bar sliding in suitably shaped holes cut in both sides of the jig body. To render the removal of the clamping screws unnecessary, one of the holes is slotted through to permit the screws to pass. Owing  to  the   severe  work  usually imposed on the clamping details of jigs during their use, nail studs should be made from high tensile steel and all plates, nuts and the ends of clamping screws hardened.

Pneumatically-Operated Clamps

Where an air line is available it is often possible to incorporate pneumatically-operated clamping devices. For larger classes of jigs it is possible to obtain air-operated units made for this express purpose. Needless to say, where it is possible to adapt such power for this purpose the operator is relieved of much tedious work and its benefit is greatly reflected in output on work of a heavy nature. Apart from this the pressure exerted on the work to hold in the position is always constant.