On Fri, 12 Nov 2010 16:46:53 -0800, 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.

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

http://www.miketreth.mistral.co.uk/Hardware.htm

This is what I use...

http://www.penntoolco.com/catalog/pr...ategoryID=5873


Gunner
--
"Confiscating wealth from those who have earned it, inherited it,
or got lucky is never going to help 'the poor.' Poverty isn't
caused by some people having more money than others, just as obesity
isn't caused by McDonald's serving super-sized orders of French fries
Poverty, like obesity, is caused by the life choices that dictate
results." - John Tucci,