[ATM] The Ultrathin project, update

[vorblesnak@peak.org]

I have been polishing on the 15.5 and have about three hours into it.  I
have been using the Hindle machine.  I am polishing with a sub-diameter
lap, (8 inches in diameter), made of 3/4 inch plywood.  The stroke is
around five inches and the side swing is two inches.  I set the stroke for
about a 1.5 inch over hang at the edge.  For a bit of weight I center a
2.5 pound bar bell weight in the middle of the lap.  Total weight of the
lap is about 3 pounds.  I would say I am about 1/4 of the way done, maybe.

Having the glass glued to the wooden disk warped it and make it look
astigmatised.  I also think that is where the zones I am seeing came from.
 I cut it off of the wood, cleaned up the back and then set it on a piece
of styrofoam with a ring of supports near the edge.  The action of the lap
was too much and it would push the glass off of the styrofoam.  So,
thinking the styrofoam would not expand much or distort the glass, I glued
the mirror to the foam using silicone RTV in a couple of spots and then
polished on it a bit.  Four wets of 15 minutes each.

I checked for basic shape using a 50 line Ronchi tester.  The mirror
showed some astigmatism but I let it cool for about an hour.  I noticed
while doing pie plate mirrors that when they come off of the lap they have
a nasty astigmatism to them that disappears as the glass cools.  An hour
later the mirror still showed astigmatism.  I cut it off of the styrofoam,
cleaned the back up and tested it again.  It is zoney but no astigmatism.

The issue is I must control the glass somehow while I polish.  I need a
way to hold the glass steady on the turntable during the stroke.  Or, I
could use a full size lap and move the glass.  I thought about a handle in
the center of the back of the mirror, working mirror on top.  Should it
warp the middle somehow that zone would be hidden by the secondary.  A
full size lap would correct the zones a bit better.

Maybe a ring on the turntable with that rubber shelf liner on it to resist
the movement of the glass and cushion it.

What I have found is, if the glass hangs freely from the edge the
polishing goes better.  If the glass is supported zones will form where
the supports are.  With glass on top, no weight can be added to the glass
to speed polishing, the weight of the glass is what you got to work with. 
The handle must be very light.  I think 3/8ths of an inch is about the
minimum hand worked thickness.

Any ideas from the list?

David Davis
Toledo, OR 97391



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