COMMON FASTENING AGENTS 



259 



(a) Taper Tap 



Plug lap 



The word " standard" means that the number of threads 

 to the inch is United States standard, and all taps made to 

 this standard are exactly alike. 



302. Teeth of Taps. The teeth or cutting edges of taps 



are radial. The cutting edge of a tap penetrates the metal 



very much like a wedge. 



For this reason taps for 



taking very heavy cuts 



are backed off much 



more than finishing taps 



which take light cuts. 



Too much backing off 



makes the tap wobble in 



the hole and weakens 



the cutting edges of the 



teeth. 



Taps made for cutting 



the threads in solid wood 



and split dies for screw cutting are called hobs. They differ 



from ordinary taps chiefly in having from six to eight more 



flutes. A large number of flutes makes the tap stiffer and less 



likely to wobble. As a result, the thread cut will be more 



perfect than if made with 

 an ordinary tap. The 

 term hob is also applied 

 to the milling cutter used 

 for cutting the teeth of 

 worm wheels. 



(c) Bottoming Tap 

 Fia. 119. Taps. 



Copyrighted by Miller Falls Co. 



FIG. 120. 



303. How to Determine the Size of a Tap Drill. A simple 

 method of finding a tap drill for a V thread, or a United States 

 standard thread tap, is provided by the following formulas (see 



