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.