Fig. 839.
In [Fig. 839], for example, let a a and b b represent grooves between the scroll threads, and if the thread on the jaws be made to the curve and width of a a, it would not pass in that of b b, and vice-versâ, and it would take but five revolutions of the thread to pass a nut thread from a to b. To overcome this difficulty the jaw threads are not made correct to either curvature but so formed as to fit at points c, d, e, when in the groove a and at points f, g, h, when in groove b. This obviously reduces their bearing area and therefore their durability. To avoid this defect the jaws of many universal chucks are operated by screws in the same way as independent jaw chucks, but provision is made whereby the operation of any one of the jaw screws will simultaneously operate all the others, so that all the jaws are moved by the operation of one screw.
Thus in the following figures is shown the Sweetland chuck.
Fig. 840.
[Fig. 840] represents the chuck partly cut away to show the mechanism, which consists of a pinion on each jaw screw, and a circular rack beneath. The rack is shown in gear with a pinion at o, and out of gear with a pinion at c, which is effected as follows:—
The rack is stepped, being thicker at its outer diameter, and the thin part forms a recess and the shoulder between the thick and thin part forms a bevel or cone. Between this circular rack and the face of the plate at the back of the chuck is placed, beneath each jaw, a cam block bevelled to correspond with the bevelled edge of the recess in the ring. The cam block stem passes through radial slots in the face of the chuck, so that it can be moved to and from the centre of the chuck. When it is moved in, its cam head passes into the recess in the ring rack, which then falls out of gear with the jaw screw pinion; but when it is moved outward the cam head slides (on account of the bevelled edges) under the ring rack and puts it in gear with the jaw screw pinion. Thus, to change the chuck from an independent one to a universal one all that is necessary is to push out the bolt heads on the cam block stems, the said heads being outside the chuck. The washers beneath these heads are dished to give them elasticity and enable them to steady the cams without undue friction.
