46 Laboratory Arts 



as possible in the shape of a long truncated cone, by which 

 means a large proportion of the cut threads will be removed. 

 The total reduction of diameter at the commencing end should 

 not exceed the depth of one thread. 



Grooves are then filed with a small round file into the 

 flattened surfaces, deep at the beginning and more shallow at 

 the shank (see Fig. 37) ; the tap is cut off from the rod about 

 i" above the last thread, and is now ready for hardening. This 

 is done by making it red hot, quenching for 

 the full threaded distance and allowing it to 

 become hot gradually through conduction until 

 of a pale straw colour. Then it is quenched 

 fully, and the shank ground to a square 

 FIG. 37. End section. 



^^cSknce^ Jt ma y be more convenient to file up the 



grooves. shank of the tap to a square before hardening, 



in which case the hardening is done by placing 



the tap between two pieces of iron and heating them all to 



redness, quenching as before. 



The making of small taps is rather a delicate matter (for 

 very small ones, the grooves may be omitted), but not of such 

 great delicacy as to dismay teachers of science, and in addition 

 to its being extremely useful, the work of making them is very 

 interesting. 



Whenever possible a taper tap should be used first, and 

 followed by a plug tap, as a properly made taper tap has only 

 one or two full threads, consequently though it enters easily, 

 and indicates the threads, it does not fully cut them. 



By making taps from the screw plate, one is assured that 

 nut and bolt will fit correctly, as the small amount of " play " 

 that usually comes into being in following out the above method 

 ensures an easy fit. 



Taps are driven by " tap wrenches " plates having square 

 holes which just fit over the shanks of the taps. 



Just as the steel to be made into a tap is tapered to enable 

 the screw plate to take up the cut evenly, so should all rods be 

 tapered before cutting screw threads upon them. The taper 

 tap serves a similar purpose upon holes. 



