94 SCREW-CUTTING. 



' worm,' or thread, has been removed by filing either flat surfa 

 or concave grooves along its length, the angles left by this operatic 

 forming a series of cutters. The head of the tap is squared to fit it 

 into handles or a vice, by means of which it may be turned ; the 

 next portion is cylindrical, and the tap itself is filed towards the end 

 in such a manner as to leave there only a trace of the worm. The 

 taps are generally made in sets of three ; the first (shown in the 

 figure), called the entering or taper-tap, is regularly tapering through- 

 out its length ; the second or middle tap is generally cylindrical 

 throughout with just a few threads at the end tapered off ; the third 

 tap, which is also called the plug or finishing tap, is always cylinclri- 

 cal, except at the two or three first threads, which are slightly re- 

 duced. In cutting a hollow screw right through a piece of flat 

 metal, the first kind of tap is turned with its whole length through 

 the metal ; but for tapping shallow holes the second kind must be 

 used, and it is necessary to have a succession of three or four of 

 such taps, each a little larger than the preceding. 



Of the two corresponding parts of the screw the nut is cut first. 

 A hole is drilled in the piece of metal (see further on), and the tap 

 worked into the hole with gentle pressure. The tap may be either 

 fixed into the middle of a long handle, by which it can be turned 

 with considerable purchase, or driven round by a tail- vice, whilst 

 the work is fixed in the vice. Tap- wrenches, or levers with central 

 holes to fit the square ends of the tap, are however better. In tap- 

 ping iron or steel, which latter must be softened in the manner 

 explained further on, the tap must be oiled with common olive-oil ; 

 for brass .it must be slightly greased with tallow. Copper is badly 

 adapted for cutting screws ; when used, the tap should be moistened 

 with soap-water. The metal piece in which the screw is to be cut 

 is protected from injury by being placed in the vice between two 

 flat pieces of lead (3 to 5 mm thick); moderately thick (O mm '5) sheet- 

 copper may also be used ; the pieces should be twice as broad, and 

 about as long, as the cheeks of the vice. They are first placed in the 

 vice, their lower edges in a line with the lower edges of the cheeks ; 

 the projecting rims are then bent apart and hammered flat with a 

 wooden mallet, until they have nearly the shape of the cheeks. 



The cylinder for the screw is made, when practicable, of a piece 

 of wire which is bought of the required thickness, namely that of the 

 corresponding screw- taps. The sliding gauge, fig. 68, is well adapted 

 for measuring the thickness of wire and many other objects. It con- 

 sists of a hollow square measuring rule of brass, in which a second 

 rule of iron moves with some little friction. Each rule carries at 

 one end, at right angles, a projecting arm or beak ; both beaks 

 touch each other closely when one rule is completely within the 



