On Fri, 12 Nov 2010 18:39:13 -0500, Joseph Gwinn
wrote:

[...]

What I do is to use an optical centering scope that mounts in the 3-jaw
or collet chuck of the spindle, followed by a large spotting drill.

http://www.use-enco.com/CGI/INSRIT?PMAKA=240-0404

I have found that with this scope, it's more accurate to not
center-punch the workpiece. I just center on the scribed lines and use
the spotting drill to make the initial dimple. A center-punch dimple
tends to pull the spotting drill sideways, reducing accuracy.

The other thing the scope is good for is locating zero-reference
features on a workpiece, subsequently using the DRO to move to places
where holes are desired. This is fast and accurate. I often scribe all
the hole centers, to catch errors, but again I don't center punch the
hole locations.

The only problem with the scope is that it was a bit expensive.


You are not kidding about the price, but wow, what a cool toy! I can
see that it would solve all sorts of problems.

Then of course you are relying on the center drill being on the mark
which is something the OP questions :-)

Another tool that comes to mind and which nobody mentioned is this:
http://littlemachineshop.com/product...y=131031042 9

I have heard conflicting opinions on it.

Michael Koblic,
Campbell River, BC