Re: ATM Beginning my 16"

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

Actually, the stroke is not all that important other than to insure that you
don't grind the edges (you just take material away without getting a curve
into the glass and the final mirror just ends up being that much thinner).
Rotating the mirror as you go does help get the hole spherical but the
stroke that I usually have the guys use is just a small to medium stroke
around rather than linear back and for on the glass.  This seems to be
easier for most and the rotational symmetry is easier to do on a benchtop
(what I have to work with for a support surface) with occasional rotation of
the glass.  The problem here is that the grinder does need to take a bit
more care to keep the curve constant across the glass and making a cheap
spherometer (super cheap?  3 ball bearings on a wood circle and a screw in
the middle with or without a dial on it!) will help in keeping the curve
fairly constant.  Remember that you are mostly digging out most of the hole
with something that is cheap and doesn't need to be repaired part way
through.  After you get the hole dug, a few wets with the tile tool will
take off an amazing amount of glass in the zones that are the problem.  You
will also see the effects of a poor shape in the tool that you make and that
will smooth out in 3-4 wets.  At that point, it's time to start going down
through the grits.
Bob May
http://nav.to/bobmay
bobmay@nethere.com