Fig. 257.
Fig. 258.
Fig. 259.
Referring to the second requirement, screw threads or the tools that produce them are originated in the lathe, and the difficulty with making a round top and bottom thread lies in shaping the corner to cut the top of the thread. This is shown in [Fig. 257], where a Whitworth thread and a single-toothed thread-cutting tool are represented. The rounded point a of the tool will not be difficult to produce, but the hollow at b would require special tools to cut it. This is, in fact, the plan pursued under the Whitworth system, in which a hob or chaser-cutting tool is used to produce all the thread-cutting tools. A chaser is simply a toothed tool such as is shown in [Fig. 258]. Now, it would manifestly be impracticable to produce a chaser having all the curves, a and b, at the top and at the bottom of the teeth alike, by the grinding operations usually employed in the workshop, and hence the employment of the hob. [Fig. 259] represents a hob, which is a threaded piece of steel with a number of grooves such as shown at a, a, a, which divide the thread into teeth, the edges of which will cut a chaser, of a form corresponding to that of the thread upon the hob. The chaser is employed to produce taps and secondary hobs to be used for cutting the threads in dies, &c., so that the original hob is the source from which all the thread-cutting tools are derived.