When holes are to be drilled from opposite sides of a piece of work, as shown in Fig. 285, a jig may be constructed having legs on both upper and lower sides, but both sets of legs should be solid with the base, as shown in Fig. 286.
If the two end holes in Fig. 285 are of the same size, and it is necessary to use a drill press having but two spindles, the legs on each side must be of a length that will make it possible to set the stops so that the drill will cut the required depth on each side. If a drill press having three or more spindles is to be used, the jig legs may be of a convenient length, as two drills of the same diameter can be used in two different spindles, each one to drill the required depth when the stop is set.
Drill jig legs are generally made of tool steel and are screwed into the base of the jig. The thread on the legs should be a good fit in the base. After having been screwed into place, the ends of the legs should be machined to length by milling or planing; the legs can then be removed, and the ends that come in contact with the drill-press table hardened. The legs should now be polished, if that is allowed, and screwed into place. The ends are then ground to such a length that the surface where the work is seated will be of the correct height above the drill-press table.
Fig. 287. Flat Lap Block for Lapping Jig Legs.
Grinding the ends of the legs can best be done in a surface grinder, or some form of universal grinder designed for surface grinding. After grinding, the ends of the legs should be lapped to remove any irregularity that may result from grinding. A very good lap may be made from a flat plate or block of cast iron. The surface to be used should be planed flat and smooth, then a series of grooves cut to form squares, as shown in Fig. 287. These grooves should be cut with a V-shaped tool and should be \ inch apart, and 1/54 inch to 1/32 inch deep. The grooves catch the emery and feed it to the work being lapped. If the pressure is not equal, one leg may be cut shorter than the other, or may be lapped out of true, causing the jig to rock.