When producing work by milling operations, it is necessary to use good cutters; it is equally necessary to employ suitable means of holding the work. It is a waste of money to make costly cutters and to purchase a strong, heavy machine, and then to use a weak, poorly designed holding device. When unsuitable holding fixtures are used, accurate work cannot be produced unless extremely slow feeds are employed, and even then it is many times impossible. In fact, the designing of fixtures to hold work in the milling machine calls for as great a display of ingenuity as the designing of any class of tools used in the shop.
Years ago almost all pieces produced by the milling machine were purposely left large in order that they might be brought to exact size by filing. Today most pieces are milled to finish size, thus doing away with the costly operation of filing. But in order to produce work of the desired accuracy, cutters must be used that are of the right shape; machines must be provided that are strong, rigid, and easily operated; holdfasts must be employed which will hold the work and insure its being presented to the cutter in such a manner that all pieces will be alike, so that perfect inter-changeability of parts will be secured. This is impossible with light fixtures, as they will spring, and not only will the sizes vary, but the surface of the work will be covered with chatter marks.
The fixture must be rigid and strong, and the holding devices must be of a design that allows rapid handling of the work. All bearing and locating surfaces and points must be accessible, for the sake of ease in cleaning, as dirt, or a collection of chips, at times the presence of even one chip, might change the location of the piece to an extent that would prove fatal to the work.
Most fixtures of this kind are made from cast iron, and as this is a cheap metal that is easily shaped, it is possible to supply a sufficient amount to insure necessary rigidity. For fixtures that are to be used over and over, it is advisable, generally speaking, to supply seating and binding surfaces of hardened steel.
When designing fixtures, plan to have the strain incident to binding and cutting the work come, if possible, against the strongest part. Fig. 251 shows several holding devices. A is made in the form of an angle iron. The binding and the cutter strain both come against the rigid part. If the fixture were reversed, the cutter strain would come against the strap, and it would be found impossible to produce work to gage if heavy cuts or moderately coarse feeds were employed.