Fig. 1507.
The bottom surface a a, [Fig. 1507], of a planer vice is parallel with the surfaces d, d′ and as surface a is secured to the upper face of the slider table shown in figure, and this face is parallel to the line of motion of the slide a, and also parallel with the cross slide in that figure, it follows that the face d is also parallel both with the line of motion of slide a and with the surface of the slider table. Parallel work to be held in the vice may therefore be set down upon the surface d (between the jaws), which surface will then form a guide to set the work by. The work-gripping surfaces b and e, [Fig. 1507], of the jaws are at a right angle to surface a, and therefore also to d, therefore the upper surface of work that beds fair upon d, or beds fair against b, will be held parallel to the line of motion x of the tool and the line z of the feed traverse. Similarly the upper surfaces a, b of the gripping jaws are parallel to a a, hence they may be used to set the work true with the line of feed traverse. The sliding jaw, however, must be a sufficiently easy fit to the slideways that guide it to enable it to be moved by the screw that operates it, and as a result it has a tendency to lift upon its guideways so that its face e will not stand parallel to b or at a right angle to d. In [Fig. 1508], for example, is a side view of a vice holding a piece of work w, the face f of the work being at an angle. As a consequence there is a tendency to lift in the direction of c. If the jaw does lift or spring in this direction it will move the work, so that instead of its lower face bedding down upon face d, [Fig. 1507], it will lie in the direction of h, [Fig. 1508], while its face parallel to f, instead of bedding fair against the face of jaw j, will lie as denoted by the line g, and as a result the work will not be held fair with either of those faces and the value of faces b, d and e in [Fig. 1507] is impaired.
Fig. 1508.
This lifting of the movable or sliding jaw is prevented in some forms of chuck, to be hereafter described, by bolts passing through which hold it down, but the tendency is nevertheless present, and it is necessary to recognise it in treating of chucking or holding work in such vices.
The work gripping face b, [Fig. 1507], of the fixed jaw, however, is not subject to spring, hence it and the surface d are those by which the work may be set. The work, however, is held by the force of the screw operating the sliding jaw, hence the strain is in the direction of the arrow p in [Fig. 1508], which forces it against the face of the fixed jaw. All the pressure that can be exerted to hold work down upon the surface d, [Fig. 1507], is that due to the weight of the work added to whatever effort in that direction there may be induced by driving the work down by blows upon surface d after the jaws are tightened upon the work. This, however, is not to be relied upon whenever there is any tendency for the work not to bed down fair. It follows, then, that surface b of the work-gripping jaw is that to be most depended upon in setting the work, and that the surface that is to act as a guide at each chucking should be placed against this surface unless there are other considerations that require to be taken into account.