Re: [ATM] mirror grinding

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

If you started with a parallel faced blank, you're having
problems doing the right grinding procedure.  You need to rotate
around the tool as well as turn the mirror as you grind.  This is
the only way that you can end up getting all of the surface
ground evenly as the repetitive rotation of each of the pieces
tends to even out the errors from grinding pressure.
With a mirror, it really doesn't matter if you have some wedge in
the mirorr for all prectible purposes as the glass just carries
the mirror surface and only if the mirrror is free to rotate will
cause alignment problems.  The oldtimers put the tool on a post
or barrel and walkes around that barrel as they did their
grinding.  In addition, they rotated the mirror every few strokes
in their hands to insure that all angles were attained in the
movement of the glass on the tool.  There is a reason for
rotating the mirror and the tool and that is to obtain an evenly
ground sourface that is a spherical surface.
You can just grind in one area but that will just be a bit of
fussing about that really isn't necessary.
I'll note two other things here, the initial grinding should be
done with a metal tool (a pipe end cap is an excellent piece of
metal for dong the initial grinding) to get the majority of the
glass out of the way before starting with a tile tool.  As it is,
you'll find that you're going to have to make a new tile tool
when you are done getting the depth right.
Second off, 60 grit is really quite coarse for such a small
mirror.  The depth that the cracks go with that coarse of a
material make the reat of the grinding take a lot longer to do.
80 grit is coarse enough to do a decent job of glass removal and
digging a hole for a F6 should take about 3-4 hours, depending
upon how much you futz around.
Bob May
bobmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net
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