Fig. 568.
Fig. 569.
Fig. 570.
But if lever l be lowered as in [fig. 568], then wheel e will gear with and receive motion from a, which it will convey to b, and c will revolve in the opposite direction to that in which the lathe spindle runs. To secure lever l in position, a pin f passes through it and into holes as i, j, provided in the lathe head. Lever l is sometimes placed inside the head, and sometimes outside as in [Fig. 569], and it will be obvious that it may be used to cut left-hand threads without the use of an extra intermediate change gear, which is necessary in the construction shown in [Fig. 570], in order to reverse the direction of lead screw revolution.
Sometimes the pin f is operated by a small spring lever attached to l, so that the hand grasps the end of l and the spring lever simultaneously, removing f from the hole in h, and therefore freeing l, so as to permit its operation. By relaxing the pressure on the small spring lever pin f finds its own way into the necessary hole in h, when opposite to it, without requiring any hand manipulation.
In larger lathes the lever l is generally attached to its stud outside the end bearing of the head h.