Re: ATM: newbie polishing questions

[Bruce Swayze <swayze@europa.com>]

At 04:59 PM 9/8/97 -0400, you wrote:
>OK, I am about 7-8 hours into polishing my 6" pyrex with cerium oxide.
>I've finally learned how to get really good contact, I think. Still
>some gray. Here are my questions:
>
>1. It looks like I'm making negative progress in some areas (i.e. the center
>   looked polished out and now looks gray).  I think there may be some CeO
>   being deposited on the glass overnight between polishing sessions (I keep
>   the tool on top of the mirror, with CeO on it, in a plstic bag).  The
>   reason I think this is that under a 50x microscope the gray areas look
>   more like dried drops (i.e. roughly circular, with a line around the edge
>   and less gray in the middle).

First of all, why are you keeping the mirror and pitch lap together during
off hours?  This is not wise for several reasons:  1. They can become stuck
together very firmly, and it's very hard to get them apart.  High
frustration levels associated with trying to separate them have often led
to damage to either the mirror or the lap, or both.  2. With that much
"cold pressing" going on, you will prematurely deform (flatten out) your
pitch facets requiring much more trimming and thinning of your lap.  btw,
if your facets are NOT flattening out and squeezing together under this
kind of treatment, your pitch is way too hard.    It may need to be
remelted and softened a bit with a small bit of turpentine.  Pitch that is
too hard leads to scratches, zones, and it will be very difficult to
parabolize your mirror properly if the lap won't deform.  

Far better to keep the lap and mirror separate.  It's best to keep the lap
covered from dust.  Wash the mirror under the tap briefly to remove any
dust before the next polishing session, and cold press for 10-15 minutes to
bring good contact back before polishing again.  

>   I have been rinsing the mirror under my (very soft) tap water, rinsing
>   briefly with distilled water, then patting dry with a clean damp
>   synthetic sponge.  Is there anything I should do to better clean off the
>   glass?

You are being much too careful.  No need to treat a piece of glass like you
would treat an aluminized surface.  No need for distilled water.  Don't be
afraid to take a soft cloth and scrub the surface with some non-abrasive
liquid soap.  I suspect the greyness you see in the middle may only be oils
or residue from long hours of contact with pitch, mixed with CeO.  See if
your fingernail will remove any residue from the glass.  You may even try
some turpentine or paint thinner or glass cleaner to clean the glass once.

>2. How does one secure the mirror?  I hogged out on a board with 3 cleats
>   and a shim in the basement, but moved to a cleaner area (the kitchen) for
>   later work, where I simply put the mirror on some wet paper on a wooden
>   counter.  Now, the drag in polishing is such that the mirror slides
>   around.  I tried moving back to the board in the basement (seeing the
>   picture in Texereau where the mirror is loosely held by the 3 cleats, but
>   immediately (after 1/2 hour) got distortions in the mirror's figure (as
>   measured on my foucault tester) that I'm still polishing out.

First of all, you definitely need the tool secured by cleats.  Polishing
creates much more lateral drag than grinding does, and you need a secure
lap.  We spend most of the time with the mirror on top.  It sounds like you
are polishing with the tool on top.  There are differences of opinion on
this, but try the mirror on top, letting it cold press with just it's own
weight for 10 minutes after every 15 to 30 minutes of polishing.  

I don't know what you mean by the word "distortions", but its common to
have zones, raised hills and depressed holes, and all the possible defects
that you will see during the course of figuring a mirror to it's final
ideal shape.  By keeping the mirror on top and using a nice 1/3 stroke you
should end up with something reasonably close to a sphere by the time you
are totally polished out.  

>3. How much drag is correct?  I find at times I am hard pressed (no pun
>   intended) to push the glass with 5 lbs. of weight on it.  By the way, how
>   much weight should I have on the glass?

You shouldn't have any weight on it at this point.  And you are
experiencing the correct amount of drag.  This is where the polishing
action is the most effective.  

>4. Given the above, should I go back to fine grinding and try again?  Would
>   5 micron be a good place to try, or go back to 16 micron?  I know I had
>   good contact throughout fine grinding, but it was hard for me to tell at
>   16 and 5 microns when I was "done".

I don't think you need to go back to grinding.  You will experience this
all over again.  Try the above suggestions and you should be okay.
>
>To describe my setup, my original grinding stand was a 3/4" piece of plywood
>with 3 cleats screwed to it, and a shim to hole the glass.  I moved to the
>kitchen for fine grinding, where I put wet paper down on a butcher block
>dishwasher top.  I've been testing the mirror since it was polished enough
>to test, in order to learn how to use the tester.  I've watched the figure
>go from ellipse through sphere, and right now it's sort of parabolic (with
>no attempt on my part to make it so, I'm ignoring the data until the
>mirror's finished polishing out, and doing 1/3 COC strokes) with some divots
>at 120 degrees (corresponding to the 3 wets in the old grinding stand).
>
>So, advice from all you great mirror makers?

I don't know what these "divots" are that you mention, but I would advise
against the dishwasher for several reasons.  If it's on wheels, I don't
know how you keep it from rolling around.  All the vigorous polishing
action will rock the thing around and cause a lot of wear and tear on the
dishwasher's frame and/or wheels (much to the chagrin of the wife).  All
that wetness can't help the surface wood last any longer, not to mention
having CeO around the food preparation area (or foodstuffs getting into the
CeO).  How about a 55 gal. barrel in the basement?  

Hope this helps.  You're well on your way!


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Bruce Swayze
swayze@europa.com
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