"Pete Snell" wrote in message
...
Harold and Susan Vordos wrote:


If you have drill blanks, it's easy to install them (snug fit and
vertical)
and then mic across them. Subtract half the diameter of the blanks
for
your true bolt circle.


Don't you mean subtract the diameter of the blanks Harold?

No, half the diameter of each blank should be subtracted in order to
determine the bolt circle. My rational is that each hole may vary enough
that he could end up with two different diameters of drill blanks. That's
not at all uncommon, even though each hole should be identical. Half the
diameter of the two blanks would be true center, yes?

If you have
a digital caliper or mike this is easy. Measure the diameter of the
pin/screw/screwhead, zero the caliper or mike, insert pin/screw into
opposite holes, and measure accross the pins/screws/screwheads. The
measurement shown on the caliper is the bolt hole circle.

But would include the amount of error that was half the difference between
two different diameters. That's why I suggested subtracting half the
diameter of the two drill blanks.

I don't own anything electronic in the way of measuring tools (not even a
DRO, which I refuse to use), so I was not familiar with the feature you
spoke of. It's certainly a nice convenience, assuming you're working with
the same diameter blanks or other devices.

Harold