Fig. 1509.
For example, suppose we have a thin piece of work, as in [Fig. 1509], and the amount of surface bearing against the fixed jaw is so small in comparison to its width between the jaws that e would form no practical guide in setting the work. If then the edges of such a piece of work were shaped first the face or faces may or may not be made at a right angle to them, or square as it is termed. But if the faces were shaped first, then when the work was held by them to have the edges shaped there would be so broad an area of work surface bedding against the jaw surface, that the edges would naturally be shaped square with the faces.
In cases, therefore, where the area of bedding surface of the work against the faces of the jaws is too small to form an accurate guide and the work is not thick enough to rest upon the surface d, [Fig. 1507], it is set true to that surface by a parallel piece.
If the work is wide or long enough to require it, two parallel pieces must be used, both being of the same thickness, so that they will keep the work true with the surface d.
Fig. 1510.
Pieces such as p, [Fig. 1510], are also used to set work not requiring to be parallel. Thus in figure are a number of keys placed side by side and set to have their edges shaped, and piece p is inserted not only to lift the narrow ends of the keys up, but also to maintain their lower edges fair one with the other, and thus insure that the keys shall all be made of equal width.
They are also serviceable to interpose between the work and the vice jaws when the work has a projection that would receive damage from the jaw pressure.