ATM stained mirror edge and water delivery system

[Michael Conron <mconron@yahoo.com>]

Someone mentioned recently a problem of stains on
their mirror while polishing.  

I ran into the same issue with a 30 inch miror in
process and was a bit bafffled by it.  The mirror is
being produced on a Hindle machine, face up and
polished with an 18 inch tool.  The polishing in the
central 90% of the mirror was proceeding normally but
the edge developed what appeared to be some kind of
mineral deposit that had a reflectance of its own
simpilar to a metalic sheen!  

Anyway, I threw the mirror on the test stand and
observe an oblate sphere with a turned down edge.  I
never had that situation before using a subdiameter
tool on top with this machine! Normally a parabolic
like figure happens with this configuration.  

In talking with my fellow local scope makers, we
determined that maybe support was an issue.  I had
mentioned earlier about using 1/4 bubble pack as a
support and now I'm suspicious about it. Our theory is
that a "dynamic sag" was occurring as the weight of
tool plus weights on the tool back glided from edge to
edge in the process flexing the central area down and
perhaps tipping the edge up slightly into the work. 
We replaced the bubble pack with carpet.  

That still did not effect a change in the staining at
the edge. 

Another theory emerged.  We noticed a complete
evaporation of water on the mirror's edge during its 2
minute rotation on the turntable.  The reason for that
was the fact that the 18 inch tool was in contact with
the edge for about 30% of the 2 minute rotation time
of the mirror.  Not only was evaporation occuring but
a large amount of dried CeO lay on the edge only to be
crashed into by the tool as the mirror rotated into
the tool action.  I suspect this was causing the
turned edge. 

We found that in keeping the edge wet, the staining
disappaered!  The turned edge is diminishing too! 

We also sped up the machine by a factor of 2 so the
mirror rotation is about 1 minute now and stroke
action about 35 strokes per minute. 
 
The solution to water application at the edge was
simple and cheap.  A garden sprayer with the nozzle
set to a drip made all the difference!  

Picture number 5 shows the setup 

http://hometown.aol.com/mikepic/HindleMachine/index.htm


Mike Conron
 

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