Drill Jigs Principles of Design Machinery ... - Evenfall Studios

EXAMPLES OF DRILL JIGS 41
from the foundry. It is not very difficult to design a jig when there is
some part of the casting finished to size, but when there is practically
nothing to start from, it becomes quite a different matter. If we are to
judge from the number of discarded jigs in the shops, it seems that
quite a few tool designers have "fallen down" on this problem.
One principal feature of these jigs is the screw bushings, two of
which are shown enlarged in Fig. 58. By screwing down on the bUSh-
Fig. 56. Work to be Drilled in Jig, Fig. 57
ing the casting is clamped between the screw bushing and a plain
bushing in the bottom of the jigs. Thus it will be seen that these
bushings perform the double function of locating the hole and also
holding the casting securely in its proper position in the jig. When
only one end of the boss is accessible, the plain bushing cannot be used,
and other means must be devised to back up the thrust of the screw
bushing. Being movable, screw bushings will take care of any reasonable
variation in the size of the casting^ and also insure that the hole
shall be drilled in the center of the boss, the bushing being recessed
Fig. 57. Jig for Drilling Work Shown in Fig. 56
in the portion binding against the boss in order to center it. This
latter condition is very desirable in work of this kind, for the sake of
appearance and strength. In this form, screw bushings are rendered
applicable to all forms of castings having any kind of a circular projection
or boss over which the bushings may be fitted, as shown in the
cuts, Figs. 57 and 60.
When headless bushings are necessary (as on both ends of the jig,
Fig. 57), they are tightened down with a spanner, whereas a plain
drill rod pin is sufficient for the other. When both ends of the boss
are held by bushings, the holes to receive these bushings must be in
[;iii