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

14 No. 3 DRILL JIGS
as few shops carry such steel in stock, crucible tool steel is generally
used. The ends of the legs should be ground true with the seating
surface that is, where the work rests of the jig. To accomplish this
a surface grinder hould be used. As the operation of grinding leaves
a number of projections on the surface ground, and as these ridges
or projections would wear away as the legs were moved back and forth
t
PIECE OF WORK I
Fig. 22. Jig with Pivoted Leaf
Machinery, If. T.
on the drill press table, it is advisable to remove them by lapping on a
flat lap, thus producing a perfectly smooth, true surface. In this way
we reduce the wear to a minimum.
For certain classes of jigs the legs may be short, not more than
% inch long; but for jigs of the style shown in Fig. 22, where the
tool is held in the hand, it is necessary to make the legs longer to
Machinery, If. Y.
Pig. 23. Part of Jig -with Pivoted Leaf, Showing Method of Holding Round Work
freep the fingers from coming in contact with the chips on the drill
press table. The legs should be located so as to do away with any
tendency of the jig to tip up when the work is being drilled.
Relation Between Accuracy of Jigs and Accuracy of Machines on
which They are Used
While it is necessary to observe extreme care in designing drill jigs
to prevent any tendency of the jig to tip, and to have the legs ground
and lapped on a true plane, it is just as necessary that the drill
press table should be perfectly at right angles to the spindle, and that
it should be true and flat. Otherwise, the holes will not be at the
desired angle with the working surface of the work.
In shops where interchangeable work is produced, or where the
work must in all respects be machined correctly, the condition of the
various machines is closely watched, and especially such parts of the
machines as affect the accuracy of the finished product. Drill press
tables are planed over when out of true, or are lined up to insure their
being at right angles to the spindles of the drill press. This may be
done by placing a bent wire in the drill chuck, the wire being bent
so that it will describe as large a circle as possible, and yet be free
to swing. The end of the wire is bent so that the point will come in