The threads upon bolts are usually produced either by a head containing chasers or by a solid die such as shown at a in [Fig. 264], b representing a bolt being threaded. The bore of a is threaded and fluted to provide cutting teeth, and the threads are chamfered off at the mouth to assist the cutting by spreading it over several teeth, which enables the bolt to enter the die more easily.
We may now consider the effect of continued use and its consequent wear upon the threads or teeth of a tap and die or chaser.
Fi.g 265.
The wear of the corners at the tops of the thread (as at a b in [Fig. 265]) of a tap is greater than the wear at the bottom corners at e f, because the tops perform more cutting duty.
First, the top has a larger circle of rotation than has the bottom, and, therefore, its cutting speed is greater, to an amount equal to the difference between the circumferences of the thread at the top and at the bottom. Secondly, the tops of the teeth of tap perform nearly all the cutting duty, because the thread in the nut is formed by the tops and sides of the tap, which on entering cut a groove which they gradually deepen, until a full thread is formed, while the bottoms of the teeth (supposing the tapping hole to be of proper diameter and not too small) simply meet the bore of the tapping hole as the thread is finished. If, as in the case of hot punched nuts, the nut bore contains scale, this scale is about removed by the time the bottoms of the top teeth come into action, hence the teeth bottoms are less affected by the hardness of the scale.
Fig. 266.