In the same figure, at B, is shown a portion of a milling machine vise, the work being held between steel jaws. If the work were of a character that made it possible to use jaws extending but little above the top of the vise, it would not be necessary to use heavy ones; but in the illustration, the jaws extend considerably above the top of the vise, and even heavy ones would spring, or would draw away from the vise at the bottom, thus throwing the work out of true. To prevent this, they are made of the form shown, and bear on the top of the vise.
Fig. 251. Forms of Milling Machine Holdfasts.
When pieces are milled, a burr is thrown out, as shown at C. At times, this burr will bear against the bearing surfaces of the fixtures, and throw the work out of true; and it will also be pressed into the work, thus mutilating and spoiling it. Frequently these burrs are removed by filing or grinding. At times this seems an unwarrantable expense, as subsequent operations would cut them away at no expense; under such conditions, it is possible to cut into the bearing surface and remove enough stock to provide a place for the burr, as shown at D.
When it seems advisable to hold work in the vise, and the opening is not sufficient to take in the piece, the jaws may be made as shown at E.