As a rough check, find a piece of bar, end of a can, large washer, etc., etc. or
turn a disk, perhaps even cut a disk out of card, having the same diameters as those
chosen, and offer these up against one of the chuck's normal jaws.
Examination of the chuck's normal jaws, Photograph 1, gives the impression that the
face is machined at an angle to the jaw. However, closer examination will show that
the crest of the curve is not central to the width of the jaw hence the reason why
one side is lower than the other, see Sk. 7. and is therefore just a case of offsetting
the jaw when machining it, but by how much is the difficult question. For simplicity,
earlier sketches did not show this feature.
As precision is not required, determining the offset by taking a rule measurement
from the crest to the edge of the jaw is a possibility. However, determining just
where the crest is, is not that easy, particularly in the case of the flatter inner
curve. Measuring the angle, Photograph 2, between the tips of the jaw tooth, doing
this for both curves as they are different, and using these to calculate the two
offset values as shown in Sk. 8, is probably a better option. Do note that the offset
may not be the same for both sides of the teeth.
Measure the width of the existing jaws, the width of the groove along each side and
the position of the grove relative to the base of the jaw's teeth. These are critical
dimensions but the fourth dimension, that of the depth of the slot along each side,
is less important as it is just clearance for the rail on which the jaw slides.
The width of the jaw has to be a close sliding fit in the chuck body and the groove
a close sliding fit along the rails either side of the chuck jaw aperture. Cut three
pieces of steel and machine the width to be a tight fit between the sides of the
aperture in which they are to fit. Then with some emery paper (say 180 grit) on a
flat surface lap the sides until a close sliding fit results. The lucky readers will
have access to a surface grinder. You may like to consider starting with four pieces
so that you have one that you can use for setting up purposes, or seven pieces, being
two sets of jaws plus one for setting up.
A fence is required
Rather than milling the groves in the side, now produce the teeth as follows. However,
before starting actual work on the jaw, a fence for use on the rotary table, must
be made and with locating holes at the same pitch as the teeth on the jaws, see Sk.
9, and Photograph 3.