A solid tap made to cut to exact size, having no leeway for wear, soon becomes too small. This fault is overcome by making a tap that may be adjusted from time to time. Another advantage of adjustable taps is that the holes may be tapped to fit hardened screws, which vary in size because of the hardening.
Probably the most common form of adjustable tap is the one shown in Fig. 112. This tap is made in one piece, and then split. It has some means of adjustment whereby the tap can be expanded or contracted through a limited range. This can be accomplished by using a taper-bodied screw. The hole to receive the screw should be drilled, tapped, and taper-reamed before the tap is turned to size. The thread should then be cut, and the taper thread cut on the end at A. There is less tendency to spring, when the tap is hardened, if the projection shown in Fig. 113 is provided; this may be ground off after the tap is hardened and tempered. When the flutes have been cut, the tap should be split in the milling machine by using a metal slitting saw, the tap being held between centers. It is split on two opposite sides, as shown at B, Fig. 112. The splitting should not go to the end of the projection.
Fig. 112. Section of Common Form of Adjustable Tap.
For hardening taps, pack hardening is best. If, however, this method cannot be used, the tap should be heated very carefully in a muffle furnace, or in a tube, the hole for the adjusting screw having previously been plugged with fire clay mixed with water to the consistency of dough. When heated to the proper degree, the tap should be dipped into a bath of lukewarm brine, and worked up and down rapidly. After hardening, it should be ground in the flutes, and the temper drawn to a full straw color. The projection on the end may be ground off, the taper screw inserted, and the locking nut B, Fig. 112, screwed to place. This nut has a taper thread cut inside to correspond with the thread on the tap at A. It will be found necessary to cut the taper thread on the tap and in the nut, by means of the taper attachment.
Fig. 113. Split Tap.