Workholding with Difficult Shapes

Problems do arise from time to time regarding the methods of holding work in the milling machine. In full scale engineering these problems are not nearly so acute as components are more solid and clamps can be applied without crushing the pieces. Often with model parts it is difficult to get a hold sufficiently firmly without more or less mutilating the piece. One method I often use both for castings and bar material is to arrange for an accurate chucking piece to be left on the component until all operations are complete and then to remove this. Generally this piece is made to suit one of the Myford collets, from $ in. diameter downwards, since the collets do hold the part with great accuracy, and after turning operations it can be transferred to the miller with the collet placed in a dividing head; even if no indexing has to be done, the head acts as a very effective vice.

Sometimes thin components present problems in holding on the milling machine. If one side is already flat one can use double-sided sticky tape, available from drawing office supply shops and some stationers. If two or three strips can be used, an astonishingly firm grip can be obtained, which will stand up to shear forces induced by milling. I have also in an industrial plant stuck down metal which had to be tooled all the way across the piece, on a false base with woodworker's glue and a sheet of newspaper. After the operations are completed a fine chisel is knocked in between the parts and the paper tears within its thickness, so the pieces come apart with some paper sticking to each. This can then be washed off with hot water. There is nothing very original about this, of course, it is an age-old patternmakers' method of producing a pattern which has eventually to be in halves, but it is a sound method not nearly so well known as it should be. Fig. 53 shows a light alloy casting being faced right across with a flycutter, the casting being stuck to the table with sticky tape and nothing else.

However, the various examples given throughout the book should serve to show how to undertake a really wide range of jobs. At the risk of seeming repetitive I would again stress that it is often worth while to make a jig for holding or locating the work, just to make sure it can be held firmly enough without damage in the right attitude. The kind of jigs and fixtures needed in modelling seldom involve more than a few minutes or perhaps an hour to make, and if this safeguards the compo-

Fig. 53 Tape-held workpiece being flycut

Fig. 54 Three-face ang/eplate used to align vee-biocks

Fig. 55 Three-face angleplates used as main packing nent, as well as the tools and machine, it is time well spent. If a duplicate component is ever needed that will be produced expeditiously without risk too.

Angleplates

A new type of angleplate has been introduced by Hemingway. This has three faces machined at 90 degrees to one another. I have found over many years that cast-in slots in angleplates never seem to be in the right place for any job. and it seems better to just drill a hole where it happens to be needed. These angleplates, which are sent out unmachined in light alloy, are in three sizes very convenient for use in home workshops and can easily be faced up on the Myford lathe boring table or faceplate. Fig. 54 shows one of them set across a miller table to locate the two vee-blocks in which the work is resting. Fig. 55 shows another pair used as main packings with Picador stepped blocks on top to give the last bit of height adjustment for the clamp plates. So many sizes are available by selecting different attitudes of these blocks that they are very useful indeed.

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