On Fri, 12 Nov 2010 07:44:48 -0800 (PST), Jim Wilkins
wrote:

On Nov 12, 1:16*am, "Michael Koblic" wrote:
....
1) I have the optical punch. I think I mentioned it passing in my
conclusion. I certainly prefer it, more so now after I
tried the hard way :-)
...
Michael Koblic,
Campbell River, BC

I use a fine-tipped prick punch (home-made from a broken tap) to make
a very small dimple at the line crossing, followed by a center punch
ground somewhat sharper than the angle of a drill bit. Usually the
drilled hole position is within 0.005".

I have grown partial to an automatic punch if I can see the marks well
enough. That also needs a follow-up punch with something bigger. But
for the tricky bits I have found nothing that beats the optical punch.

The punches are a kind of side issue to the original problem, i.e..
the center drill not agreeing with the twist drill. The obvious
question was "which one hits the true center". That is when I
discovered that defining the true center is not as simple as I thought
it would be.

If I define the drilling point by edge-finding and measurement from
edges on the mill will the center drill hit that point accurately?
Will the twist drill? I know you cannot use the latter to start the
hole, but if the center drill is off, the twist drill will be off
also.

Michael Koblic,
Campbell River, BC