Hey Dave,


If you think about what is actually happening, it's easy to work with.
Mind you it's far easier with a DRO than without, but recall that
edgefinders have been around a LOT longer that DRO's.

I have found that when you start the spindle up to find an edge the
tip of the edgefinder should be "flicked" with a finger so as to have
a quite definite "wobble". I know you can't "see it" as so, but this
eccentric wobble will tend to act like a fixed lobe on a cam, in that
it will tend to remain in one place on the edgefinder relative to the
edgefinder body, so to speak. As you approach and the tip of the
edgefinder contacts the edge of the work, the tip will get pushed
over to become less and less eccentric and more and more concentric,
until it's impossible to visually see any eccentricity. But the
eccentricity is still there, ever so slightly. Your moving the table
is pushing the tip closer and closer to being centred, and it does so
because it "can" get less and less eccentric. But as soon as the
edgefinder does reach concentricity, that is it becomes centred, then
any motion past that point will cause eccentricity to occur again.
But this "new" eccentricity cannot remain in one place relative to
the edgefinder body as it did on the approach. The over-centering
causes the eccentricity to occur in one place relative to the work
face instead, so the edgefinder seems to "kick" over immediately.
That is the point you are looking for. That ever so slight movement
of the handwheel causing the kick. Accurate to at least a thou, and
it is also in a dynamic mode, so accounts even for a non-concentric
spindle, or collet, or holder, or drill-chuck, too. That's why it is
best to chuck the edgefinder in whatever holder or drill chuck that
you intend to use for cutting.

As to finding the centre of a hole, it can be tougher to see, but
really all you have to do is get a rough location, move the table so
you get kick on one direction, then traverse only that axis in the
opposite direction until you get kick again, note the difference in
the two points and halve it. Move the table to this "halved" point,
then do the other axis the same way. Now the hole should be fairly
close to centre. Then do them both again, then once again, and this
last time the halving will now give you the centre. This is only
easily possible with a DRO. With a bit of math, you can even
determine how close you are by the motion distance required to see the
kicks, as it should be hole diameter minus edgefinder diameter divided
by 2. For example, presuming you know that the hole diameter is say
..375, and the edgefinder tip is .200, then when starting correctly
centred on either axis the kick point should occur at [ (0.375" -
0.200") / 2 = 0.0875"] of motion in either direction along either
axis. You can even reverse this theory, and find the centre then note
the motions and that will give you a really good indication as to hole
diameter, and even as to concentricity, in that all movements from
centre to kick point should be equal.

Take care.

Brian Lawson,
Bothwell, Ontario.
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On Wed, 12 Nov 2003 03:11:43 GMT, "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