Fig. 2. Plan View of Lathe Headstock showing Back-gears
As some metals are much harder than others, and as the diameters of parts to be turned also vary considerably, speed changes are necessary, because if the speed is excessive, the turning tool will become dull in too short a time. These speed changes (with a belt-driven lathe) are obtained by placing the driving belt on different steps of cone-pulley P, and also by the use of back-gears. The cone-pulley can be connected directly with the spindle or be disengaged from it by means of bolt m. When the pulley and spindle are connected, five speeds (with this particular lathe) are obtained by simply shifting the driving belt to different steps of the cone. When a slower speed is required than can be obtained with the belt on the largest step of the cone, the latter is disconnected from the spindle, and the back-gears G and G1 (shown in the plan view [Fig. 2]) are moved forward into mesh by turning handle O; the drive is then from cone-pulley P and gear L to gear G, and from gear G1 to the large gear J on the spindle. When driving through the back-gears, five more speed changes are obtained by shifting the position of the driving belt, as before. The fastest speed with the back-gears in mesh is somewhat slower than the slowest speed when driving direct or with the back-gears out of mesh; hence, with this particular lathe, a series of ten gradually increasing speeds is obtained. Changes of feed for the turning tool are also required, and these are obtained by shifting the belt operating on pulleys p and p1 to different-sized steps. On some lathes these feed changes are obtained through gears which can be shifted to give different ratios. Many lathes also have gears in the headstock for changing the speeds.
Fig. 3. Feed Mechanism of Lathe Apron
Front and rear views of the carriage apron, which contains the feeding mechanism, are shown in [Figs. 3] and [4], to indicate how the feeds are engaged and reversed. The feed-rod R ([Fig. 1]) drives the small bevel gears A and A1 ([Figs. 3] and [4]), which are mounted on a slide S that can be moved by lever r to bring either bevel gear into mesh with gear B. Gear B is attached to pinion b (see [Fig. 3]) meshing with gear C, which, when knob k ([Fig. 1]) is tightened, is locked by a friction clutch to pinion c. The latter pinion drives gear D which rotates shaft E. A pinion cut on the end of shaft E engages rack K ([Fig. 1]) attached to the bed, so that the rotation of E (which is controlled by knob k) moves the carriage along the bed. To reverse the direction of the movement, it is only necessary to throw gear A into mesh and gear A1 out, or vice versa, by operating lever r. When the carriage is traversed by hand, shaft E and gear D are rotated by pinion d1 connected with handle d ([Fig. 1]).
