Re: ATM Glass removal with bench grinder wheel

[The Silent Observer <silent1@ix.netcom.com>]

Adam Perkins wrote:
> 
> Hey,
> 
> Has anyone ever tried using an abrasive wheel from a bench grinder as a
> hogging out tool?  Not spinning in the machine, but used as one might
> use a glass or tile tool to rough grind.
> 
> Just an idea...

I haven't tried it, but I see (at least potentially) a large flaw in the
idea.

The problem is this: the grinding action during hogging is primarily due
to the grit rolling between blank and tool, and the sharp corners on the
grit grains chipping out tiny flakes of glass under high local pressure
as they roll.  This action won't occur with the locked grains in a
grinding wheel -- instead, you'll be scratching the glass with every
stroke, setting up the kind of stresses that let a glass cutter do its
job.  You'll be (if I've understood all this stuff correctly) setting
yourself up to crack, split, or shatter your blank instead of grinding
it to the desired curve.

Even if the blank doesn't simply break (and it might not, given fine
annealed glass to start from), it will take a great deal longer to
remove the require amount of glass than it would with loose grit, and
leave deep cracks under the scratches that you'll then have to grind out
with the 120 grit after your curve is generated.

As far as I'm concerned, the proper way to use a grinding wheel for
curve generating is to spin it in the appropriate grinder, keep it very
wet, and use it to make the curve you want in a few minutes to a half
hour or so (depending on your setup, blank size, and what depth of curve
you're after -- might be much longer for a big mirror) with either a
rotating mirror and tilted wheel setup, or a radius rod setup, and a
slow feed of mirror into wheel or vice versa in either case.  You'll
still get cracks you have to grind out, but you'll get the curve fast
enough that you'll still save time over conventional, strong-arm
hogging.

For myself, I found the hogging on my 8" Pyrex blank enjoyable -- a
fairly mindless way to get the feel of pushing the glass before it
really mattered what technique I was using, and it only took about three
hours, total, with 60-90 lapidary grit and tile tool to get the 8" to
f/7.

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Opinions expressed are my own -- take them for what they're worth
and don't expect them to be perfect.