For locating holes at school we mount a dial test indicator in the
spindle, then move the table around until the dial reads 0 all the way
around the hole when the spindle is rotated.

For finding edges we have a beeping edge finder - when the ball on the
edge-finder makes contact with the workpiece it completes a circuit or
whatever and makes it beep. I've used it a few times, but mostly I've
been using a 20 thou feeler gauge and bringing the cutter in (with the
power off!) until the feeler gauge is just barely snug between the
cutter and the workpiece. Add 20 thou plus half the diameter of the
cutter and the middle of the spindle is over the edge of the workpiece.
I'm not sure how accurate this method really is, but I haven't had any
problems so far.


chem

David Anderson wrote:
Thanks to all for good comments - the link to the article by Mike Rehmis
entitled "Edge Finder" was right on target. Also, his comments about a
cheap tool being useless (which is what I was using) made perfect sense. So,
I immediately bought a Starrett #827A which is much better. But, all these
comments evidently assume that we are locating a straight surface from the
spindle centerline. What about locating a curved, internal edge of a hole?
Maybe that was my problem. I was attempting to locate the edge of a 0.402
inch diameter hole with the 0.200 tit on the finder. The "kick" now depends
on the direction of rotation of the tool and whether the curved surface
being contacted is coming nearer or further away during the "kick". Thanks
once again. Dave




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