From: "Russell Jocoy" 
To: kpatel{at}hotpop.com, atm{at}shore.net
Reply-To: "Russell Jocoy" 


>From: Kartik Patel 
>Reply-To: Kartik Patel 
>To: atm{at}shore.net
>Subject: ATM Flat 10 incher polishing lap
>Date: Sun, 15 Jun 2003 16:28:52 +0530
>
>
>Hello, all
>
>This is an update on the 10" (well, 9.5", actually) mirror I am in the
>process of making, and a query.
>
>The grinding process is over. The last stage (1200 mesh SiC) was
>completed some time ago. The focal length is 57", corresponding to
>f/6, which was the aim. The tool was made of  ordinary cement with
>glass tiles; the channels between the tiles were filled with candle
>wax. The wax was remelted and refilled between stages, and completely
>replaced before starting the 600 grit stage.
>
>There are no visible scratches or pits on the blank and so, hopefully,
>it can proceed to the polishing stage.
>
>Having not made a base for the polishing lap, I am toying with the
>idea of using a circular piece of plywood, cut to the size of the
>blank, as a base instead of cement or plaster. Since the central
>portion won't be elevated, the plywood being plane all over, I am
>thinking of placing a two millimetre thick circular piece in the
>centre, having a radius of two inches or so. The pitch will be poured
>over this and pressed with the blank in the usual way.
>
>I remember reading (perhaps on the list?) that this procedure can be
>used to make a base for the polising lap. Or should I go the usual way
>and cast a plaster or cement base?
>
>Sincerely,
>Kartik Patel,
>http://www.geocities.com/letapk/index.html
>kpatel{at}hotpop.com
>
>
>
>
>



("atm{at}shore.net")

   Kartik Patel,

        I did both, I poured a thin cast (about 1") on the mirror and glued
it to two 5/8" plywood
discs the same diameter. I used "hydraulic cement" which sets up
in about 30 minutes. The
cast follows the curve of the mirror.  The plywood discs matched the
diameter and were wood glued
together. I sanded and painted the wood with exterior latex. I let the
cement cast dry real good
in a dry hot area for about a week. Next I glued the cement disc to the
plywood disc with Polyurethane
based adhesive ( this comes in a caulk tube)  This stuff is better than
liquid Nails And is waterproof.
This adhesive doesn't set up brittle but you can't budge it when it dries.
I bought the adhesive at
Lowes or the Depot I can't remember. The stuff is made by P L products.
   I also coated the entire cement disc with the adhesive with a throw away
brush to seal the concrete.
  I did have to come back and roll down bubbles on the surface. Once the
adhesive dried it leaves a
protective "skin" on the front suface and sides against water
getting in or chips getting out.
       I then poured the pitch directly onto the dried adhesive skin.

                                                    Russ Jocoy
("atm{at}shore.net")

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