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

PRINCIPLES OF DRILL JIGS
At times jigs are made of such size and design that it seems wise
to core the bushing holes. In such cases it is necessary, in order that
we may lay out the location of the centers of desired holes, to press
a piece of sheet steel or sheet brass into the cored hole, as shown in
Fig. 27, and locate the center on this piece. When the holes are properly
located for machining, the sheet metal may be removed and the
holes finished to the desired size. If an error of 0.001 or 0.002 inch is
not permissible, the method described above should not be employed.
Method Employed for Highest Degree of Accuracy
Where extreme accuracy is essential we must locate round pieces of
steel on the face of our work. These pieces of steel are called buttons
and are of exact size and perfectly round. To do away with any possibility
of their becoming bruised in any way, they are hardened and
carefully ground to size. The buttons are attached to the work by
means of machine screws, as shown in Fig. 28, the holes in the but-
Fig. 26.
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Taking Vertical Measurements by Means of Height Gage
tons being larger than the screws used; this difference in size allows
us to move the button until it is accurately located. The diameter of
the buttons should be some standard size, exactly divisible by two,
because, in making our computations we only consider the distance
from the center of the button to its circumference, that is, the radius.
When we start to lay out the centers for the bushing holes we first
determine our working surface, then lay out on the face of the jig, by
means of a surface gage, as described in a previous operation, the
centers of the holes to be produced. We then drill and tap screw holes
to receive the screws to be used in holding the buttons to the jig.
When we have prick punched the surface, and before drilling the holes,
we scribe by means of dividers a circle the size of the button on
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