To: "Tom Stokes"
,
From: Dennis Rech 
Cc: "atm" 
Reply-To: Dennis Rech 


At 09:48 AM 5/3/03 -0700, Tom Stokes wrote:

>First, I need to explain how I got here.
>
>I pitched an 8"x3/4" plate glass disk to a 12"x3/4"
ply disk. I then
>bolted the ply disk to my MOM 20 turntable which is also 3/4" ply.
>I used 3 bolts spaced 120 degrees apart and used a washer on
>each bolt to keep the ply disks separated. This 3 point support
>slowly failed as the washers became recessed into the wood and
>the wood itself became slightly warped. Every time I tighten or
>loosen the bolts I'm putting a bending stress into the glass. Now
>I have astigmatism. Using a knife edge near the focus, I can see
>a Ying Yang pattern. Inside or outside of focus, the knife edge
>is strongly tilted and not straight.
>
>The fix is simply to use more (or thicker) washers to get back to the
>3 point support system. The pitch supporting the glass should take
>care of the rest. Will the astigmatism ever polish out or do I go
>back to fine grinding?
>
>Tom Stokes
>http://home.pacbell.net/tstokes shows my MOM 20.

HI Tom,
This is a repeat of the reply from the m-o-m e-group.

I think that you have put your self three strikes behind with your method
of polishing.
1. By mounting your mirror on a piece of wood, you leave yourself open to
changes in humidity and temperature. The bonded glass and wood will move at
different rates with each change and it will warp the mirror.
2. By pitching the glass to the plywood, you have now created three
insulated spots that are of different temperature in the glass. Those spots
will swell or shrink at a different rate than the rest of the mirror with
temperature changes.
3. You just cannot polish or figure a mirror properly unless it is properly
supported. A three point support is valuable when suspending a finished
mirror in a telescope, but will cause big problems when used for figuring
or polishing. The three point cell is designed for a uniform load coming
from the weight of the glass only. When you add a point load from a
pitchlap, the glass deflects dozens of waves from the weight of the tool.
Mirrors should always be placed on some sort of fairly stiff backing when
being worked on in order to give constant support. Most people use short
napped carpet or woven rubberized shelf liner under their mirrors.
You may want to go back to the conventional method of securing a mirror to
the turntable. The four slotted blocks shown in the m-o-m plans work well.
Hope this helps.
Dennis

--- BBBS/NT v4.00 MP