Figure 7.—Thread of modern form recently cut, using the old screw and nut but with a new tool. The material threaded is carbon-steel drill rod. (Smithsonian photo 49276A.)
[Figure 12], the headstock of another and more advanced lathe than shown in figures [9] and [11] but of the same type, shows “keys” (D), each of which is a partial nut of different pitch to engage with a thread of mating pitch. The dotted lines in [figure 13] show the engaged and disengaged positions of one of these keys, and [figure 14] shows the spindle with the various leads, C. At D is a grooved collar to be engaged by the narrow key shown in operating position at the left in [figure 12] for the purpose of controlling the endwise movement of the spindle when used for ordinary turning instead of thread-cutting. In return for greater convenience and freedom from the expense of the many separate spindles, as typified by the Wetschgi machine, a sacrifice has been made in the length of the thread which can be cut without interruption.
Figure 8.—Binding screw seen at A in [figure 3], showing the long smooth fadeout of the thread below the shoulder. (Smithsonian photo 49276.)
