MACHINE-SHOP PRACTICE 427 



shop 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 speci- 

 fied by their amount of "open" and the size of their jaws. 

 A No. 18 vise has a 2^g in. jaw and opens 3}/2 in., while a No. 

 20 has a 4 J/2 m - 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." 

 A piece of pipe should never be used to obtain more leverage 

 on the vise handle. 



493. Screw Machine. The screw machine is practically 

 the same as the turret lathe, but is used for different purposes; 

 in fact, the screw machine when first designed had a turret, 

 the tools of which were advanced in the proper order by 

 means of levers and guide straps fastened to a guide drum. 

 The screw machine is now built along the general lines of a 

 lathe, but without a carriage. The tail-stock is replaced by 

 a turret arranged to hold six or more tools, so that each one 

 may be brought into play in the order in which it is needed, 

 and each operation may be thus performed in its progressive 

 order. 



