Re: [ATM] cutting a big piece of plate glass into mirror or tool blanks

[Dave Smith <smithersscope@yahoo.com>]

Hey Guy,
I've used my grinding table to trepan a couple of 13"
blanks from 3/4" plate glass. (I think it was 3/4"
anyway. Haven't looked at it in a year or more.)

Took about 2.5 hours each. The first attempt was a
bust as a helpful neighbor said "put more weight on
there" as he leaned into it and broke the
soon-to-be-disk in half.

I've tried a couple of different "tools", the most
productive being thin tin flashing screwed to the
circumference of a plywood disk. Cut notches around
the perimeter to facilitate grit accessing the groove
and you're golden. Starting the groove was the biggest
challenge for me. I found that a 1/8" hardboard disk
(routed to the target mirror diameter) lightly RTV'd
to the glass was an excellent and easily removable
guide to get things started. My blanks all have a
larger diameter on the bottom than on top. The tool
seems to splay or cut outward as the cut proceeds. I
suppose the deeper the groove, the more limited the
play in the tool, the more precise the cut. YMMV.

I have not tried the other method mentioned
(square/octagon/nipping method).

Good luck!
Dave

--- Guy Brandenburg <gfbrandenburg@yahoo.com> wrote:

> How many folks have actually tried and succeeded in
> cutting reasonably sized mirror or tool blanks from
> a large piece of plate glass about 1 inch thick? Did
> you find the metal tabs on a wooden spindle worked
> well? Or was it easier to use a window-cutter and
> just cut numerous sides, then try to shape that into
> a rough circle?
> 
> 
> 
> Guy Brandenburg, Washington, DC
> My home page on astronomy, mathematics, education:
>
http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
> or else 
> http://tinyurl.com/r6fh2
> 
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