When one screw is used the jaws will hold the work so that the centre of rotation will be midway between the points of contact of the jaws of the chuck and the work, hence work cannot be set eccentrically, unless pieces of iron are inserted between it and one of the jaws. When two screws are used the jaws may be operated separately, and one jaw may be set to such distance from the centre of rotation as the necessities of the work may require; but in this case more adjustment is required to set either square or cylindrical work to rotate on its axis than when the jaws operate simultaneously as with a right and left-hand screw. It is obvious that the axial line of the screw or screws must stand parallel with the plane of the face f. It will be observed that the back of each jaw is cut away at b: this serves two purposes, first it permits of a piece of work having a small flange, head or projection being held in the Vs of the jaws; and secondly, it equalizes the wear on the jaws of the chuck, because in jaw chucks generally there is more wear at the outer than at the inner end of the jaws, because work shorter than the length of the jaws, or requiring to be held as far out from the jaws as possible, does not have contact at the back end of the work holding jaw faces, hence the jaws are apt to wear, in course of time, taper. By cutting away the jaws at the back, the tendency to unequal wear is greatly reduced, hence this plan is adopted to a more or less degree in the dogs or jaws of all chucks, being in many cases merely a small recess from 1⁄16 to 1⁄8 inch deep only.
When the jaws have a V-groove as in the cut, the face f of the chuck does not form a guide in setting the work, the truth of the V-grooves being solely relied upon for that purpose.
Fig. 830.
The form of two-jawed chuck shown in [Fig. 830] is intended for square or rectangular work, and is mainly used by wood workers. It may be operated by a right and left-hand screw, but is generally preferred with independent screws. The face f of the chuck may be employed to serve as a guide in setting the work as shown in the cut, in which w represents a piece of work held between the jaws a, a, and resting against the face f, which therefore serves as a guide against which to set the work to insure that its axial line shall stand parallel with the face f, or in other words at a right angle to the line of centres of the lathe.
Fig. 831.