John,
What you are trying to do is, for all practical purposes, an impossible
task. The cumulative error you build in when going from hole to hole is
killing you. For an example, look at the 96 hole index disk. That's 360
deg. / 96 holes or 3.75 degrees per hole. But, if you are in error by only
0.05 degrees when you drill the first 2 holes, the cumulative error over 96
holes is approximately 1 degree - or about 1/4 the distance between two
holes. .05 degrees is a very small error when drilling holes with even very
high quality machine tools.
Jim Seelye

"John Weeks" wrote in message
...
I've been trying to construct and indexing disk for my lathe. It has
24 hole indexing built in, but I want 48 and 96 as well. I'm trying to
come up with the best way to lay out the holes and dril them on the
drill press. My first thought was to use a pin guide, something like
cutting box joints on a table saw, to avoid having to position the
drill bit by eye.

I carefully drilled the first two 3/16th inch holes the correct
distance apart, carefully positioning the disk with a 1/16th inch
drill in the chuck, clamping thedisk down, then switching to a 3/16th
inch bit and drilling the hole. The center of the disk is revolving on
a pin at the radius I wanted. Then, with the bit in the 2nd hole, I
pressed a pin through a guide block into the first hole and clamped
the guide block in place.

I should be able to remove the pin, rotate the second hole under under
the guide, position it with the pin, drill the third hole, etc.

It's not working. When I get around the diameter I have lost or gained
a third to a half the distance between the holes. The drill press
table is locked down tight, the pin is a good tight fit. But there is
slop creeping in someplace.

This time I'm going to print a compass template with 48 radii and
position the bit by eye. The individual holes may be off slightly but
they should each be within a certain degree of error from the right
point.

Anybody have any better way of doing this?

I thought about using the index on the lathe, drilling the 24 holes,
then slipping the disk half the distance and going around again. But
I'd have to build a positioning jig for lathe drilling first.

JW.