Segment Engine. Fig. 287
In very many cases of ornamentation it is required that the mandrel, instead of making an entire revolution, should stop at a given point in both directions, so that, for instance, the turner should be able to move it 60, 80, or 100 divisions to and fro, with the certainty of its not advancing beyond that distance. This is effected by the racked and divided brass wheel B, fixed on the mandrel against the small end of the pulley. This wheel is sufficiently thick to allow of racking part of its edge to be acted on when necessary by the tangent screw, and leaving the other part for divisions, which are generally seventy-two in number, and marked in figures at every sixth division. On the other side of the plate are a number of holes drilled through its whole thickness to receive stop pins, Fig. 289, P, which are sawn through as shown, that they may spring, and fit the holes tightly. There are seventy-two holes corresponding with the divisions. These pins are about 3/16ths of an inch diameter, generally with flattened heads, and a hole through them to receive a pin to aid in removing them. The holes are sometimes made in the edge, instead of the side of the segment plate, but the latter is the best position. At Fig. 288, T, is seen the interior part of the poppet, with a piece of brass let in, and fixed securely, in which are inserted two screws, against which the segment stops abut, and prevents further rotation of the pulley. Side by side with this latter piece is placed the frame which carries the tangent screw. It is shown at Fig. 289. This frame is not fixed to the base of the poppet, but pivotted at e, between two short standards screwed into the poppet for that purpose. When not in use, the whole frame, therefore, drops down towards the front, but it can be raised by the small cam, K, Fig. 289, so as to gear with the worm wheel. In many cases the latter is not used, but the pulley turned by hand. The screw, however, gives a steadier and more easily regulated movement, essential in delicate operations, and sometimes convenient, even when the stops do not require to be inserted. The use of the cam, acting on the frame which carries the tangent screw, is now generally followed in the eccentric and oval chucks, and also in the dome chuck. It enables the workman, by throwing out of gear this part, to turn the worm wheel with the fingers, to set it at the required number on the division plate, a slow process when effected by the screw.