On Aug 15, 1:25 pm, F. George McDuffee gmcduf...@mcduffee-
associates.us wrote:
On Fri, 15 Aug 2008 03:11:49 -0700 (PDT), Jim Wilkins



wrote:
On Aug 15, 1:15 am, Don Foreman wrote:
On Thu, 14 Aug 2008 19:43:47 -0700 (PDT),
wrote:...
Here is a question: Has somebody here actually TESTED an edgefinder
to determine accuracy and repeatability?
...
Wolfgang
...I trust my best edgefinder to half a thou, though I very rarely need
that kind of accuracy.

I checked how repeatable mine was the other day because of this
thread. It repeated to about half a thousandth turning at 600 RPM on a
filed surface and I could get it within a thousandth by feeling for a
step when it was stationary, which is how I use the center finder.
When I want accuracy I surface-grind the block before milling and
drilling. A surface-ground edge seems to behave differently with an
edge finder, maybe it's the friction or lack of it?

I haven't made anything recently that could be used to check the
accuracy of the edge finder. Maybe a carefully bored hole near an edge
would do.

=========
Anyone have accuracy numbers for the cheapscrew method of using a
dowel pin and feeler gage or slip of paper?

Unka' George [George McDuffee]
-------------------------------------------
He that will not apply new remedies,
must expect new evils:
for Time is the greatest innovator: and
if Time, of course, alter things to the worse,
and wisdom and counsel shall not alter them to the better,
what shall be the end?

Francis Bacon (1561-1626), English philosopher, essayist, statesman.
Essays, "Of Innovations" (1597-1625).


Unka George, (we're probably a similar age?)

I think that by using a dowel pin in a collet the accuracy of picking
up an edge depends very much on the run-out(TIR)of the collet/pin
combination.

The kick-off type edge locator is much less affected by run-out.

As others have mentioned the surface finish used to locate off is
important; The better the surface finish the smaller the locating
error, up to the point where the inherent process error governs.

The point about lapping the two sliding surfaces on the locator is
also a good idea. If one were really anal one would check the
flatness of these surfaces with an optical flat and a monochromatic
light :-)).

As to whether the kick-off method or 'dead-nuts' concentricity gives
better locating accuracy off an edge... I don't know for sure; but
here is an observation based on physical principles:

1) Two objects cannot occupy the same space at the same time.
2) A 1" diameter pin will not fit into a 1" diameter hole without
force.

Based on this I would venture an opinion that the kick-off method is
more accurate; it is certainly more precise ie. it has greater
repeatability.

..0005" accuracy is, in all likelihood sufficient for most work. I run
at 1000 RPM or so when edge finding, and also place a drop of oil on
the pick-up edge to "improve" things :-)).

Wolfgang