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

42 No. 3 DRILL JIGS
line, and when they are so aligned, it is impossible for the hole to
come out of center on either end of the boss. The simplest and safest
way to align these holes is to run a single-pointed boring bar through
the screw bushing into the bottom of the jig, after the screw bushing
has been fitted to the jig, the shank of the boring bar, of course, being
a good fit in the hole of the screw bushing, which has been previously
lapped to size. On the larger sizes of bushings, it has been found
advantageous to use a good quality of machine steel, case-hardened and
having a smaller tool steel bushing inserted in the center. When made
Fig. 58. Screw Bushings Fig. 59. Work to be Drilled in Jig, Pig. 6O
entirely from tool steel, the distortion in hardening is too great to
allow a good fit, which is essential on the threaded portion. The bodies
of the jig should be. made of cast iron, cradle-shaped, and cut out where
possible, to facilitate cleaning. The covers which hold the screw bushings
should be of machine steel, held in place by means of screws and
dowels.
Two examples of jigs of this class are shown. The larger jig, Fig.
57, was designed for drilling the breast drill frame shown in Fig. 56.
Fig. 6O. Jig for Drilling "Work Shown in Fig. 59
The casting is clamped by the large bushing first, and then the smaller
bushings on the ends are brought up just tight enough not to cause
any spring in the casting. There are two holes in this frame which
must be reamed square with each other. After trying unsuccessfully
to ream the holes by hand after drilling in the jig, the holes were
reamed in the jig as follows: The hole in the bushing was made the
exact size of the hole to be reamed in the casting. A drill of this
size was used to spot the hole, following with a reamer drill, and lastly
with a rose reamer, making in every respect a satisfactory job.