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

1G No. 3 DRILL JIGS
blow holes in castings, a reamer is liable to alter its course and so
change the location of the hole. While for many purposes this slight
alteration of location might be of no account, yet for work where accu-
racy is essential, it is out of the question.
After drilling and boring the first hole, the jig may be moved on
the face-plate, and the other holes produced. It is abvious that in
order to produce holes that will be at right angles to the base of the
jig, the face-plate of the lathe must run true, and should be tested
each time it is used for any work where accuracy must be observed.
Method of Locating- Holes "When Accuracy is not Essential
Where there is no model, and it is not considered advisable to make
working models of the various parts, the location of the bushing holes
may be obtained by laying out the various points on the jigs. In such
cases a drawing is usually furnished, and the dimensions on same are
transferred to the face of the jig. If it is not necessary to have the
holes exact as to measurements, the laying out may be done with a
surface gage, the point of the needle being set to a scale. The scale
Machinery, N. Y.
Fig. 25. Angle Iron -with Groove for Scale
may be clamped against an angle iron, as shown in Fig. 24, or an angle
iron may have a groove of the width of the scale cut across its face
at right angles to the base, as shown in Fig. 25. The scale should be
a good fit in the groove, so fitted that it will stay securely at any
point from frictional contact with the sides of the slot, or a spring may
be so arranged as to insure the proper tension.
Method Assuring- a Fair Degree of Accuracy
Where greater accuracy is essential, the working points should be
obtained by means of a height gage, as shown in Fig. 26. By means
of such a tool the measurements may be fairly accurate, as the vernier
scale allows of readings to one-thousandth inch. When the lines have
been scribed at the proper locations they are prick punched. In order
to prick punch exactly at the intersection of lines the operator must
wear a powerful eye-glass, and use a carefully pointed punch, ground
to an angle of 60 degrees. If the punch marks are made very light at
first, the exact location may be observed nicely. The punch marks
should not be deep, as there is a liability of alteration of location if
the punch is struck with heavy blows. After the various points have
been located and punched, the jig may be clamped to the face-plate
of the lathe, and the bushing holes carefully drilled and bored to size.