Fig. 847.
[Fig. 846] represents a front, and [Fig. 847] a sectional view, of the Westcott combination chuck. f is the main body of the chuck screwing on to the lathe spindle. f carries the annular ring d, which has a thread on its face, as shown. d is kept in place by the ring e, which screws in an annular recess provided in the back of the chuck. c is a box fitting in the radial slots of the chuck. The back of the box c meshes into the radial thread on d, hence, when d is revolved, the boxes c move radially in the slots. Now the boxes c afford journal bearing to, and carry the worm or screws b as well as the chuck jaws a, hence revolving d operates the jaws simultaneously and concentrically as in a scroll or universal chuck. By means of the screws b, the jaws may be operated individually (the boxes c and ring d remaining stationary) as in an independent jaw chuck.
Suppose, now, the jaws to have been used independently, and that they require to be set to work simultaneously and concentric to the centre of the chuck, then the screws b may be operated until the jaws at their outer edge are even with the circumference of the chuck (or, if the jaws are nearer the centre of the chuck, they may be set true with a pointer), and the ring d may be operated. In like manner, if a number of pieces of work are eccentric, the screws b may be used to chuck the work to the required eccentricity, and when the next piece is to be chucked the ring d may be operated, and the chuck will be used as a universal one, although the shape of the work be irregular, all that is necessary being to place the same part of the work to the same jaw on each occasion.
Fig. 848.
The faces of the jaws of jaw chucks when they are true with the face of the chuck (or what is the same thing, run true, and are at a right angle to the axial line of the lathe centres), form guides wherefrom to set the work true, but this will only be the case when they remain true, notwithstanding the pressure of the jaws upon the work. Their truth, however, is often impaired by their wear in the chuck slots which gives them play and permits them to cant over. Thus in [Fig. 848] is shown a chuck gripping a piece of work w, and it is obvious that to whatever extent the jaws may spring, or have lost motion in the ways or slots in the chucks, the jaws will move in the direction of the dotted lines a a, the face of the jaw then standing in the direction of dotted lines b b, instead of being parallel to the chuck face. If the spring or wear of the mechanism were equal for each jaw, the work would be held true, notwithstanding that the jaws be out of line, but such is not found to be the case, and as a result the work cannot be set quite true.