The Design of Jigs, Tools and Fixtures - Methods of Work Location

Posted By Richard Barker on 14 May 2014

Posted in The Vintage Machinery Almanac

This article first appeared in Practical Engineering 1940 Vol1 No4. The article’s contents are accurate as of 1940. The article describes new methods employed by the Engineering industry at the time.

Background

As mentioned earlier in this series, the work is positioned in the jig in such a manner that the holes, when machined through the guide bushes, are correctly located in every respect. In order that this object may be achieved it is necessary to provide in the jig stops against which previously machined surfaces may abut or fit. In many cases the choice of locating surfaces is automatic, and where two or more separate jigs are required for one job the original datum faces are maintained throughout the operation.

It will be realised that it is not possible to deal exhaustively with this particular subject as so much will depend upon the actual shape of the part to be handled, but a few methods used singly or in combination will briefly be described. These should cover the classes of work most commonly encountered.

Locating With Vee Blocks

Vee blocks provide the means of locating work from a radiused end. The depth of the block is made to suit the job, but provision should be made for swarf clearance. For the location of plane surfaces, the face is cut away at the bottom to prevent swarf building up in the corner. Where it is not convenient to use a block form of location one or more pads may be employed. These may have plain ground shanks for pressing into a drilled or reamed hole, or screwed to fit into a tapped hole. In either case the underside of the head is undercut to permit grinding and the face of the head ground to give a predetermined thickness after hardening.

A female coned pad is suitable for the location of spherical-ended work. Where the work is slotted, a bar is arranged in front of the pad but clear of it and clear of the bottom of the slot. Here again such a pad may have a parallel ground shank pressed into a reamed hole or a thread provided in addition and the pad secured in position with a nut.

Bossed Levers

A bossed lever is located from the large bored hole by means of a plug. The smaller boss is positioned centrally by a sliding Vee block working in guides and backed up by a cam or an adjusting screw. As will be seen in this arrangement the location so far as the centre distance of the holes is concerned is governed by a plug fitting into the bored hole, the Vee block positioning the smaller boss in a lateral direction. A great deal of machined work can be located from bored holes or registers for drilling.

In certain circumstances such as where a long bored hole passes through the work from a machined face, the opposing sides of the jig body are bored to receive a hardened and ground locating pin. This pin is chamfered at the front end to assist in leading it into the work. The head may be knurled or provided with a cross pin as illustrated to facilitate its removal.

When locating from a male or female register the jig body is usually appropriately bored or provided with machined projections to suit the work. The question of swarf clearance has to be studied carefully, and for this reason the mouth of the hole or recess in the jig used as a register requires to be chamfered, or where a male plug or insert provides the location the base of the plug should be undercut to avoid loose metal chips, likely to prevent a proper seating, from being trapped in a corner.

Seldom is it necessary to jig parts entirely from surfaces as cast or forged. While it is true that machine-moulded castings and drop-forgings generally are produced approximately uniform in size, some form of adjustment or compensation should be allowed for in the locating points of the jig in which such a part is to be handled. Flat surfaces should be supported where dimensions depend from such at three points on raised pads.

So far, the parts comprising an average jig have been dealt with briefly so as to impart a general idea of requirements covering different classes of work, but it will already be realised that many of the details such as bushes, clamps, latches, studs and pins can be standardised.

In jig designing each problem presents its own difficulties, but there are recurrences of familiar types of work, and where a scheme has proved satisfactory on a similar job it may be re-adopted with perhaps small improvements or modifications.

Examples of Drilled Work

It is now proposed to suggest methods of dealing with a few examples of drilled work, and wherever possible the solution suggested will have a wide range of application.

A common type of drilling job is one in which the requirements are that a drilled hole passes through the centre of the work in both directions square with the axis. The simple jig is one that will produce the results desired and is also capable of rapid manipulation. A casting forms the jig body which is machined on the top and bottom faces. The inverted 90 degrees Vee, with the root terminating in a shallow slot, at the top of the jig, is machined parallel with the base. A hole for the drill bush is bored central with the Vee and a hole drilled and tapped in the centre lug in line with it to receive the pressure screw.

Securing the Jig to the Table

Holes in the base lugs are drilled for the purpose of securing the jig to the drilling-machine table. The headed pressure screw is drilled through its centre to clear the size of drill used for the work. A short tommy bar secured into the head of the screw provides the means of securing and releasing the work-piece. A steel bracket screwed and dowelled to the side of the jig, and provided with an adjustable stop in the form of a screw tapped into the bracket and secured with a locking nut, affords the means of end location. Where more than one operation is necessary the jig is equipped with a liner and slip bushes, in which case, where the jig is to be operated on a two- or three-spindle machine, the base may be modified and a handle provided.

Where a series of jobs of a like nature that are to be handled, and, differing slightly, perhaps, in diameter, length and hole size, one jig may be utilised to deal with them all. When this is intended the end stop will require a range of adjustment sufficient to cover the longest and shortest of the parts. Slip bushes will be substituted for the fixed bush, and a series of tommy holes drilled and reamed in the bush head to permit the lever being moved to a convenient position when a smaller or larger diameter of work is being handled.

For Long Service

Should a lot of work have to be drilled, or when the jig will see service over a long period, its durability will be enhanced if the Vee block portion is made from steel which is subsequently finished by hardening and grinding. This would be attached below the bushed portion with hollow cap screws and dowels. It will be apparent also that this jig lends itself to a built-up form of construction from steel. Naturally this will be the course to adopt where the work is of small proportions, say below 1in. in diameter.

By a slight modification this design lends itself admirably to the drilling of pin holes; for the majority of split-pin holes in bolts or clevis pins the pressure screw would need to be at a point behind the bush. The form of end location shown could be dispensed with as hole centres for this class of work usually given are from under the head.

The edge of the Vee block could be used as a locating face, or where length variation has to be coped with an adjustable stop sliding through and clamped in a hole in the back of the jig can be provided. Reverting to the original purpose of the jig where two holes are required side by side, one solid pressure screw between the holes will the method of clamping.