Re: ATM : Hole in the middle

["Jeff Baldwin" <bald@gotnet.net>]

I've done a few perforated mirrors, but I really like keeping that plug in.
I have an idea.
You said that it is a 20" mirror with a 3.5" perforation.  That means that
from the edge of the perforation to the edge of the mirror  is about
(20-3.5)/2 = 8.25"  You can make an 8" mold of the mirror with plaster
between the perforation and the edge.  When it is cured, set the mirror onto
it so the perforation is over the center of the mold.  This will allow you
to drop the plug into the perforation onto the mold, keeping it close to
being cospherical with the remaining mirror.  Once it is there replaster a
little on the seam on the back side so the plug doesn't move.  Then you can
make a pitch lap incorporating the plug.  There will be a ring at the
interface seam, but it can be shaved off easily.  Then remove the plug again
and polish the mirror.  If you leave the plug in while polishing, it
probably won't be perfectly cospherical with the remaining mirror.  That
would have to be accounted for when testing, but at least would act as a
support while polishing.
JB

Jeff Baldwin,  Stockton  CA
bald@gotnet.net
http://members.gotnet.net/bald
http://members.gotnet.net/bald/atm.htm
http://www.jps.net/pleman/sas
-----Original Message-----
From: robert allen <rallen@iland.net>
To: atm <atm@shore.net>
Date: Monday, July 26, 1999 3:56 PM
Subject: ATM : Hole in the middle


>
>
> We got a problem. The A.S.K.C. has ground and
> smoothed a 20" blank with a 3.5" hole in the "middle". The
> work was done with a "plug" held in with dental plaster and
> the grinding went well with a 16" tile tool.
> BUT, at the first attempt to polish we found severe
> scratches and sleeks around the hole from debris picked up
> from the depressed ring around the "plug". The "plug" was
> removed and the smoothing was redone.
> We are now ready to polish but cannot decide how
> to build a lap that can be pressed properly.
> Hope some one out there has beat this problem before.
> We are wide open for advise or sugestions.
>
> HELP !!!     Bob Allen
>