Re: [ATM] Hoping to Grind a Mirror

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

Ah, doing a mirror where there is little support from the area is
always fun!
Silicon Carbide, while the best cutting agent for tlass, isn't
the only substance that works well.  The finer grits used are
usually Aluminum Oxide which gives a nicer cut to the glass.
Even sand itsellf can be used if it is the right sand.  If you
can scratch the glass with the sand, you can use it to grind a
mirror.
Youi may be able to find the grit at a sandblasting shop and you
may even come across some Silicon Carbide (it's black in color)
and you can even use the coarse grit only if you sort the grit
out after doing some grinding with it (the grit breaks into
pieces which will be fine for the finer grits needed) and thus a
half a kilogram of grit may be enough to fully do a mirror from
start to finish.
The 12mm glass will be fine for a 6" mirror but any larger it can
become a bit more difficiult to do as the glass starts to fflex
while grinding and polishing.  A 10L1 thickness ratio is
acceptable without going to special techniques (basically less
pressure and more control of turning things) to keep astigmatism
(differential focus depending upon the direction of the image
across the mirror) to a minimum.  Be sure to turn the mirror on
the tool, the tool on the stand as well as walking around the
stand to keep the nasty astigmatism at bay.  Long strokes
decrease the ROC of the surface while short strokes tend to keep
it the same.
Trepanning (cutting a round piece of glass out of a bigger piece)
is easy to do if you have a slow speed turntable of some
strength.  You merely need to make a round piece of wood if you
don't have a can of the right diameter and attach a nice vertical
piece of steel sheet to it to make a cutting tool.  Put some V
slots in the sheetmetal to drive the grits down to the bottom and
make sure that it nicely goes around in a circle and you have
your tool.  Apply grit and water to the glass and the cutter
(either turn the cutter or the glass as you desire, either works
the szme) and watch the disk get cut by the cutter.  There is
nothing fancy about the process, just have the cutter grind over
the same path on the glass and it will get cut.  FWIW, I recently
cut a 14" diameter mirror out of a 3/4" piece of glass and it
took about 3 hours using 80 grit SiC and about 20lb. on the tool
to press it down.  The weight was all I could apply without the
machine I was using stalling out from the high forces involved.
Bob May
bobmay at nethere.com
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http: slash /bobmay dot astronomy.net
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Thank you for your understanding!

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