Re: [ATM] Polishing with vacuum

["Bob May" <rmay@nethere.com>]

I'll note that Bob Goff used air pressure in his HexTek mirror
grinding and polishing to keep from getting printthrough.  Then
again, it was easier for him as the cells all had a hole in the
back just right for a plug with an air hose in it to keep the
inside of the mirror at the right pressure.
Bob May

rmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net

----- Original Message -----
From: Ted & Joan Foster <tjfoster@kos.net>
To: <atm@atmlist.net>
Sent: Wednesday, May 28, 2008 7:55 PM
Subject: [ATM] Polishing with vacuum


>     Perhaps the time is right for me to jump in here with great
> intrepidation and describe my  experiment with " large, thin".
First a
> little back ground.  A while back I was attempting to machine
grind and
> figure a 16" X 3/4" porthole mirror but couldn't get rid of
stig.  This
> was the incentive to try a 18" sandwich mirror with plate glass
front,
> back,  and webs, or spokes, radiating out from the center to
separate
> front and back.  By keeping the ratio of the distance between
webs to
> the thickness of the front glass at any given radius near 6
to1, I had
> hoped to avoid print through. No such luck.  As polishing out
was
> finishing it became obvious that there was a valley in the
surface over
> each spoke, no matter how lightly I tried to use the lap.  At
this
> point  I decided to try something different.  Using dabs of
pitch to
> block off the channels in the lap I  made a continuous circular
dam
> about half way out to the edge of the lap, drilled a hole
through the
> center of the lap, and arranged a connection to a refrigerator
> compressor used as a vacuum pump.   After a normal  pressing I
found
> that the area inside the dam, between lap and mirror, would
hold a
> vacuum  and I was able to do a half hour of polishing manually
lifting
> up on the lap, (could even lift the mirror off the turn table),
and
> still have good adhesion between lap and mirror.  The vacuum
did suck
> some of the  slurry off the mirror, so a small jar was used in
the line
> prior to the pump as a sump.  So did it work?  Well I can't say
with
> certainty, but at the tester I thought that the shadows were
actually
> reversed and the valleys were now ridges!  And there the
experiment sits
> for more than a year now (got side tracked  into
interferometery this
> winter).  At any rate, I'm convinced that polishing can be done
with
> little or no  weight between mirror and lap. Can it be
controlled to
> make a fine surface remains to be seen.  When I get back at it
I will
> arrange to draw the vacuum via the poker pin, insert a vacuum
gauge and
> bleed valve in the line, and probably use a bungee cord to a
hook in the
> ceiling to exactly balance the weight of the lap and over arm.
Also
> need to make a new lap with the dam designed into the channel
pattern.
>   Now  someone will probably point out that the Rev Ellison
tried this
> back in the eighteen hundreds and it didn't work.  Okay, but
I'm sure he
> didn't have a scrap frig compressor to draw the vac, : -).
>             Hope this is of interest,
>                       Ted Foster  ....   (now there are two
Teds on here
> to take the heat,   ....   be careful what you say and to
whom!)
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
>

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