In article ,
wrote:

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.

All in all, I don't regret the purchase, despite the whining.


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

The centering scope followed by a big (at least 3/8" diameter) spotting
drill and then by a stub drill is the best way I have found to get the
hole at the scribed location. If the location is off, oh well.

What is even more accurate is to use the DRO to locate a collection of
holes with respect to one another, but this process usually starts with
using the centering scope to set the DRO to a reference point on the
workpiece.


Another tool that comes to mind and which nobody mentioned is this:
http://littlemachineshop.com/product...=2604&category
=1310310429

I have heard conflicting opinions on it.

I have never used one of these. I didn't buy one because I could not
see how one could achieve 0.001" accuracy. Even if the laser beam focus
spot is that small on the workpiece, my eyes are not that good to tell
if the beam is exactly on the scribe marks. And I bet that the focal
spot is dazzlingly bright and scintillates.

The catalog pictures all show a laser-pointer beam, which would be
perhaps a millimeter in diameter (0.040" diameter), but the pictures may
be artistic versus technical.

My guess that in my hands, accuracy would be more like 0.005" to 0.010".
This is certainly good enough for woodworking.

With the centering scope, the limitations of my old eyes have little
effect on achievable accuracy, and I easily achieve 0.001", probably
less. This scope is how I discovered that the Millrite table gibs were
loose, when I saw the table cocking slightly when drive direction
reversed.

Joe Gwinn