Fig. 145.
Figs. 143, 146, 148.
This roller, which may be grooved or plain, may be replaced by a second small pulley, which is capable of being slid along the round bar which forms its axle and turns with it between the centre screws. In this case the bar is made with a groove or channel along its length, [Fig. 147], and a pin projecting from the central hole in the pulley enters this groove. Thus the two will turn together, and at the same time the position of the pulley is adjustable at pleasure. The tension of the springs B, B, is increased or lessened by a turn or two of the nuts C, C, just in the same manner as the spring of the safety valve is adjusted on the boiler of a locomotive.[12] The application of the overhead motion is plainly shown in Figs.[143] and [144]. The work is fixed in the lathe, and the mandrel kept stationary. The cord passes to the overhead directly from the fly wheel, and thence to a pulley on the screw of the slide rest, as in [143], to a drill, as in 144, or to any other apparatus at pleasure. To fit the work in the lathe in such a manner as to enable any point in the face or side to be operated upon, a division plate and index are required. The first is a round plate of brass or gun metal, 1/16th of an inch thick, drilled with holes in concentric circles. The index is a steel spring with a projecting point, which, entering any one of the holes, retains the plate, and with it the pulley, on the face of which it is fixed immovable. There are generally four circles of holes, the number in each selected with reference to its divisibility by the greatest possible number of divisions—thus 360, which is usually on the outside or largest circle, can be divided without a remainder by 2, 3, 4, 5, 6, 8, or 9—144 is a good number for the second circle, being divisible by 2, 3, 4, 6, 8, 9. The other two may be 112 and 96. The uses of the division plate are many. Eccentric cutting and drilling could not be done without its aid, and wheel cutting for clocks, or for making cycloidal and other chucks, is entirely dependent on this contrivance.
[12] The frontispiece shows another superior form of overhead, with balance-weights hanging close to the ground behind the lathe.
Fig. 147.
The division plate and index are shown in [Figs. 147, A, B, C]. The part B is a knob with a round hole through it, to take the tail of the spring C. This is shown again at C on a larger scale, to bring to view a slot which allows of some slight adjustment of the point, to suit the different-sized circles. The milled-headed screw clamps the point at any desired part of this slot. When not in use, the spring is drawn back so as to release it from the division plate, and turned down in the position shown by the dotted line.