How Flat Glass?

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On Tue, 4 Oct 2005 09:44:37 -0700, with neither quill nor qualm,

As I said, I thoroughly cleaned the thing, then inspected it carefully. Sure enough, it was a scratch. Of that I have absolutely no doubt. It caught my fingernail and had a gritty sound when it did. But it was gone at a later date. I'm convinced that some formulations of glass do heal. Oh, I also looked for some sort of coating on the glass and found none. It "ticked" when I tapped it with my fingernail. I'm still tickled that it fixed itself. BTW, I've been sober for 20 years straight now, so I wasn't liquidly mistaken. ;)
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wrote in message

From my original message ... "The underlying surface is flat".
Thanks for the vote of confidence. Is this where I say, "you'd think they read the original post." and you reply, "Ass-me. No, I'd not expect that" :)
Just to clarify, I measured it on my table saw top, my 8" jointer, and my workbench ... all of which have been checked for flatness. But the real indicator is that if the piece is flipped over, the bow likewise switches from facing top to facing bottom.
-- Mark
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wrote in message

there you go again, bringing in facts to ruin a good argument.
could it be tempered? tempered glass almost always has a very slight bow in it.
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wrote:

any chance of gluing it flat to something stiffer and flatter than it is?
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[...]

The rule of thumb for mirrors in telescopes is to make them only six times wider than they are thick to prevent them from sagging under their own weight.
For scary sharpening or waterstone flatening I use a ground and polished concrete fake granite plate of the kind used in garden paths, I cannot detect any deviation from flat with a straightedge and it was cheap, about 3EUR for a 40cm times 40cm size.
--
Dr. Juergen Hannappel http://lisa2.physik.uni-bonn.de/~hannappe
mailto: snipped-for-privacy@physik.uni-bonn.de Phone: +49 228 73 2447 FAX ... 7869
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Modern float glass isn't designed with a "flatness" tolerance per se, the flatness is really just a result of the way it is manufactured. For window glass the .02 discrepancy would certainly be acceptable. :)
If you're on good terms with the glass shop maybe the will let you swap it for another piece. Bring you straight edge with you this time. Or, you could buy a granite surface plate, the ARE manufactured to a specified tolerance.
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