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

18 No. 3 DRILL JIGS
the face of the jig with the punch mark as center. This enables us to
approximately locate the button. If the hole to be produced has its
center 2 inches from the base a and 4 inches from vertical side &,
Fig. 29, we would locate the button provided it was '% inch diameter
1% inches from a, and 3% inches from &. This can be done accurately
by the use of a vernier caliper, or we can lay the jig on the side &, and
by means of a length gage, or a piece of wire filed to the right length,
accurately determine the distance from 6 to the button. The jig is
then placed on the base a and the other dimension obtained in the
same manner. The buttons may be located more easily by the use of
a vernier height gage, if one is at hand.
If there are to be several bushings on the face of a jig, a button
may be accurately located where each hole is to be. The jig may be
clamped to the face-plate of the lathe so that one button is located to
run exactly true. This is done by means of a lathe indicator. When
BRASS PIECES-
Machinery, A. Y.
Fig. 27. Cored Holes with Inserted Brass Pieces for Centers
the jig has been so located that the button runs perfectly true, the
button may be removed and the hole enlarged by means of a drill, so
that a boring tool can be used to bore it to the proper diameter.
1
Locating the Holes on the Milling- Machine
In some shops it is not considered advisable to locate a button at
the desired position of each bushing hole. One button is located and
the jig is fastened to the table of a milling machine having a corrected
screw for each adjustment. Then, after one hole is accurately located
and bored, it is a comparatively easy matter, by means of the graduated
dials, to obtain the other locations; however, this method should never
be used unless the machine has all its movements governed by "cor-
rected" screws, as the screws ordinarily sent out on milling machine t s
are not correct as to pitch, and if used, serious defects in measurements
will result. Many tool-makers, therefore, prefer using a vernier scale
and vernier attached to the knee and table of the milling machine,
for accurate work, as they are then independent of the inaccuracies
that may be present in the feed-screw.