Fig. 1709.
In [Fig. 1708] we have a piece containing three holes, which are to be drilled in a certain position with regard to each other, and with regard to the face a. This brings us to the consideration that in all cases the work must be chucked or held true by the faces to which it is necessary that the holes must be true, and as in this case it is the face a, the jig must be made to hold the piece true by a, the construction being as in [Fig. 1709], which represents a top view, and a sectional side view. The upper plate d carries three hardened steel bushes, a, b, and c, to receive the drilling tools, and thus determine that the holes shall be drilled at their proper positions with relation to each other, and is provided with a face n, against which the face (a, [Fig. 1708]) may be secured by the screw h, and thus determine the positions of the holes with, regard to that face. At e, f, and g are eye-bolts for clamping the work between the cap and the base plate, which is made large so that it may lie steadily on the table of the drilling machine. When the nuts e, f, and g and the screw h are loosened the cap d may be lifted off and the work removed.
If the holes are required to be made very exact in their positions with relation to one edge, as well as to the face a of the work, two screws k would be required, one binding the cap against the lug m of the base, and the other binding the edge of the work against the same lug.
The usefulness of jigs, or fixtures, is mainly confined to small work in which a great many duplicate pieces are to be made, and their designing calls for a great deal of close study and ingenuity. They can obviously be applied to all kinds of small work, and as a general principle the holes and pins of the work are taken as the prime points from which the work is to be held.
Drilling fixtures may, however, be applied with great advantage to work of considerable size in cases where a number of duplicate parts are to be made, an example of this kind being given in the fixtures for drilling the bolt holes, &c., in locomotive cylinders.
Fig. 1710.
