On Fri, 9 Oct 2009 17:36:34 +0100, AdeV
wrote:

David Billington may or may not have intoned:
AdeV wrote:
Recently, I bought a 24" digital scale from Chronos, which I fitted to
the Z axis of my machine. It looks very similar to this one, only
longer:

http://www.chronos.ltd.uk/acatalog/info_SGU___.html


I'm somewhat annoyed to discover it has a 0.004" error in every 0.100"
of travel! Is this normal for this kind of scale, or have I got a duff
one?



What machine is it you've fitted it too and how have you determined the
scale is at fault rather than the machine.

It's fitted to a Bridgeport series 1. I spotted the error as soon as I
finished the mounting adaptor, as it read 0.096" for one complete turn
of the Z-axis handwheel (for some bizzare reason, my Z-axis is in
inches, wheras X & Y are both in mm). A second turn yielded 0.184", and
so on.

To prove it was the Chronos unit & not the machine; I faced a block of
aluminium with a 1/2" end mill, then cut a 0.100" deep slot based on the
handwheel reading. I then returned the mill to the start point, measured
0.100" on the Chronos, and cut a second slot, adjacent to the first. So
I ended up with two "U" channels in the top of my ali. Used a depth mic
to establish the depth of the channel; the handwheel was pretty well
spot on 100 thou, the Chronos one was almost exactly 0.104"

I only did the test once, mind, and I can't remember now if I made sure
I accounted for backlash. The Chronos unit seems to have about 0.010" of
"backlash", compared to only 0.002"-0.003" of actual Z-axis backlash.



The scale is bolted to the main body of the machine. I made some
aluminium pieces to hold rubber swarf covers (which replace the z-axis
gib cover/felt pad scraper); into this is screwed an L-shaped piece
which attaches firmly to the scale (it is sized to fit the base of the
scale slider exactly AND it screws into place). Although it's aluminium,
it's pretty rigid; and whilst I can believe that maybe the "backlash"
comes from flex in the fittings, it can't create a cumulative error -
and certainly not such a repeatable error.


While I cannot speak for Chronos..most reader heads have internal dip
switches that allow you to correct for scaling issues..which are quite
common.

Check your manual

Gunner

GUNNER'S PRAYER:
"God grant me the serenity to accept the people
that don't need to get shot, the courage to shoot
the people that need shooting and the wisdom to know the difference.
And if need be, the skill to get it done before I have to reload."


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