[ATM] help me get back to sphere -- A report

["Rich Ball" <ShooterBall@hotmail.com>]

My thanks to all who contributed to my (interim) success.

My 8" f-6 project was stalled out for a few years because I had lost hope of
getting control over it.  It's figure was too deep and it had a TDE.  I
picked it up again a few months ago with a determination to get it done.
But all I did made it worse.

With all my gratitude, some experienced ATMers on this list answered my call
for help.  I determined what was wrong with my technique and am happy to
report that I now have a mirror ready to figure.

This is my report.  I hope this might be helpful to other neophytes who
struggle as I did.  (Note -- I still have to figure the mirror.
Nevertheless, getting to where I am is a milestone based upon where I've
been so far.)

For a long time everything I did to the mirror exacerbated the problem.  I
had allowed the channels to close on my lap and I did not re-cut them. I hot
pressed with netting in order to it taken up by the pitch.  And I cold
pressed too.  After this I used Bob May's "little half stroke"  but the
situation continued to deteriorate and the hyperbola deepened.  No
improvement on the TDE.  I had great confidence in that stroke.  It made
sense to me. But I think I was doing so many other things wrong it could not
be made to work.

I believe that I was using too much CeO relative to the amount of water and
no detergent. Often the mirror would be stuck to the tool after hot
pressing.  I had to use a hand screw to urge them apart.  I'd repress after
this jury rigged solution.  But I suspect I always distorted the lap and
never got it back to perfect contact.  And I was using too much pressure
while polishing.  At the worst my Foucault numbers showed a disparity of
over an inch between center and edge!  A dark moment.

At this point I communicated a great deal off-list with Mike Lockwood.  I
had so many questions I did not want to clutter up this web site.  Mike
talked me down.  I ate some humble pie, learned a few things and changed my
approach.

I would say for the sake of other neophytes that it is very difficult and
very important to understand the difference between grinding and polishing.
When we grind we are cutting glass by the abrasive action of grit.  I
continued to use that "model" as I began work on the pitch lap.  I thought
that I needed to bear down at least somewhat, use enough "grit" (in this
case micron-sized CeO) and grind away ever smaller bits of glass.  I had no
clear idea of the almost mystical effect the pitch has when the glass is
moved across it and how little polishing agent is really useful. I also had
no idea how rapidly the figure, as seen on the tester, would change for
better or worse when things were going well or badly.  One time, 10 minutes
of over-zealous polishing produced a frightening ripple as if my mirror had
the look of pealing skin that comes from bad sunburn.  Light pressure, short
strokes do not seem convincing to a newbie.  What effect could this possibly
have on hard glass?

This has been the hardest lesson to learn.  Truth is, my intuition tells me
that light pressure and short strokes could not do anything at all. But they
are magic.  Who understands this?

When I finally established my routine I monitored my work-room temperature
and never pushed glass over 72*.  I kept my channels open with a copper
stripped looped to the end of my Weller Junior soldering tool.  I trimmed
the lap and chamfered the mirror.  I did not use the netting anymore,
although there is still some pattern from the old net in the lap. I cold
pressed at least 15 minutes for every polishing session.  I mixed up slurry
of 1/4 teaspoon CeO, 1 oz. water and a drop of detergent. The mirror and lap
never stuck together again. Most sessions were under 20 minutes and then I
would cold press another 15 minutes. I did not take the mirror off the lap
after pressing.  I simply removed the weight (25 pounds) and began pushing
so as not to disturb the contact. I worked MOT about 2/3rds of the time.
Contact was consistently excellent.  I used a narrow "W" stroke with an
average length of 3" (1 1/2" each way), but with a lot of variety.  No
pressure beside the weight of my hands. Like a miracle the center zones
became spherical and then at last the TDE vanished.  Under the knife edge
the mirror darkens all at once.  And this is the first time I've ever seen
that happen.  It's neat.

One other thing:  Several months ago I took my mirror to Jim Seaver's shop,
urged on by a contact from this list.  Jim spent less than a New York minute
looking at the mirror with his Foucault tester and pronounced it "too deep,
with a TDE."  He said "get it back to sphere and start over."   I don't
think he's on this list.  But he is a gentleman first class and I thank him
for being available and for being encouraging.

I also thank all those who helped on line.  But I am not out of the woods
yet.  I'll need more route-finding as I start to figure.  So be patient.
I'll be back with more questions.


--Rich Ball
Oak Park, Illinois




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