Fig. 556.
Fig. 557.
[Fig. 555] represents another design of cone bearing, in which the spindle is threaded to receive the nuts a which draw it within the front bearing and thus adjust the fit, and at the same time prevent end motion. The back bearing is provided with a bush parallel outside, and furnished with a nut at b to adjust the fit of the end bearing. To prevent the end pressure of the dead centre from forcing the spindle cones too tightly within their bearings a cross piece p is employed (being supported by two studs provided in the head), and through p passes an adjusting screw d, having nuts n and c, one on each side of p. Between the end of d and of the lathe spindle a washer of leather or of raw hide is placed to prevent the end faces from abrading. A similar device for taking up the end thrust is often provided to lathes in which the journals are both parallel, fitting in ordinary boxes, a top view of the device being illustrated in [Fig. 556], in which b is the back bearing box, s s two studs supporting cross-piece p, and n and c are adjusting nuts. g is the gear for driving the change wheels for screw cutting or for ordinary feeding as the case may be. In this design the gear wheel g remains fixed and the combinations of gears necessary to cut various pitches of thread must be made on the lead screw and on the swing frame, which must be long enough to permit the change gear stud to pass up to permit the smallest change wheel to gear with wheel g, and which is provided with two grooves e and f, [Fig. 557], for two studs to carry two compounded pairs of change wheels. This compounding in two places on the swing frame enables gear g to be comparatively large, and thus saves the teeth from rapid wear, while it facilitates the cutting of left-hand threads, because it affords more convenience for putting in a gear to change the direction of feed screw revolution.
Fig. 558.
In many lathes of American design the journals are made parallel, and the end play is taken up at the back bearing, an example being given in [Fig. 558], in which the back bearing boxes are made in two halves a and b, the latter having a set screw (with check nut) threaded through it and bearing against a washer that meets the end of the spindle.