William Noble wrote:
ok, some setup - I have a mill (Abene), I just bought a chinese coaxial

And, once all this is solved, how can I hold the indicator so I am sure it's
doing it's job and not finding me a hole that is offset from the center?
A centering indicator will work best in a collet. An end mill
holder may force it off-center a bit. An Abene has a pretty
fancy articulated head, you need to tram it to make sure it is
perfectly normal to the X-Y plane of table motion. I have a CNC
routine I use, but it can be done on a manual machine. Probably
the easiest way is to take a small end mill (3/8 or 1/2" max)
and plane off the top of a piece of scrap aluminum. Don't use a
fly cutter, you want it to make a sawtooth if the head is out of
tram. Now, put a dial test indicator in the spindle and with
the spindle centered over the scrap, sweep around in a circle,
and see what is high and low. Retram the head to get as
parallel in both front-back and left-right as possible, then
repeat. This time there should be very little sawtooth in the
surface, but recheck with the indicator. You want to get down
into the tenths of a thousandth of an inch over the largest
scrap you have, I use a 7" block.

(The other method is to level a block in the vise by moving the
table and reading the 4 corners with an indicator, then sweep
the indicator in a circle and read and tram as above.)

The idea here is that if the head is not in very good tram, then
any length change in the spindle puts the tool off-center from
where you set up. Only if the tip of the drill bit was at the
same length as the tip of the coax indicator will you be
spot-on. Also, if the head is out of tram and you drill by
raising the knee, you will bend the drill and can't get a
straight hole.

Jon