The machinist's vise is one of the simplest as well as one of the most important parts of the machineshop outfit. Its mechanical principle is based on that of the screw. Much of the success of the work on lathe and planer often depends on the accuracy with which the work at the vise has been done. Such preparatory machine operations as center punching, the scribing of outlines, and the laying off of holes and machine cuts require experience in handling a vise. Machinists' vises are made in various shapes and sizes to meet different requirements and in two styles of base: (1) stationary and (2) swivel. Some have patent self-adjusting jaws, while others have solid jaws. Different sizes are specified by their amount of "open" and the size of their jaws. A No. 18 vise has a 2 5/8 in. jaw and opens 3 1/2 in., while a No. 20 has a 4 1/2 in. jaw and opens 6 in. Rough pieces to be filed or chipped may be clamped between the jaws, but smooth finished pieces are held between copper or lead strips, fitted over the vise jaw so as not to bear too hard on the stock. No undue strain should be put on a vise to make it grip firmly as already noted in Chapter XXIII (Common Hand-Tools. 306. Kinds Of Hammers), "Common Hand-Tools." A piece of pipe should never be used to obtain more leverage on the vise handle.