Re: [ATM] Grit for 10" Quartz Blank - 3 questions

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

FWIW, I watched a guy at Stellafane one year do an 8" blank from
a pregenerated curve to a blank ready for polishing in one houir!
What he had was a big cast iron disk of the right curvature on
the face and it was spun at a couple of rpm.  He threw some grit
on the wheel and put the blank on the surface and put an arm over
the glass and pushed down.  Since the wheel was maybe three feet
in diameter, he had a lot of room on which to move the blank
(and you have to to keep the iron tool to the right radius!) and
he used three grits in doing his work - 120 grit, 320 grit and 9
micron - to do the grinding. Much of his time was spent cleaning
the wheel of the old grit so that he didn't get scratches on the
blank from leftover grit.
In other words, using every size grit from the coarse 80 grit to
the finest 5 micron grit is really a bit of a waste of time.  I
find that after hogging out with the 800 grit, a jump to 220 is
good followed by a bit of time with 25 micron and then, just for
insurance, 12 micron where you cqn start polishing if desired
otherwise the 5 micron which will decrease the polishing time a
bit.
I'll also note that 25 micron is where I make sure that the
surface is indee a decent spherical shape and there are no deep
pits in the glass.
Bob May
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