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Old 15-06-2012, 05:41 PM
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Satchmo
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Join Date: Dec 2005
Location: Sydney
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Steven

NZ amateur optician Graeme Loftus told me about 20 years ago that he never had any success with any plate glass mirrors where the ratio of the saggitta to edge thickness was less than 5/8...or 0.5625. Your 25" comes in at 0.44 which does not pass what he called ` the 5/8 rule'. What he was referring to was figure bending effect of having more thermal inertia at the edge than the center. The 36" X 1" thick mirror he made gave apparently fair images for about 5 minutes each night as it passed from under to overcorrected at sunset.

The issues you speak of of the mirror sagging under the weight of the tool sound quite plausable. I'm not sure why you want to push so big with this thin glass. The thinnest I would ever conceive of trying would be about 14 inch at 19mm thickness



Quote:
Originally Posted by sopticals View Post
Hi grinders,

Really having some trouble making progress with this 25" f4.7 (19mm thick) float glass mirror. Its polished up really nicely, but just no joy with the figuring. What I think is happening is that because of its extreme thinness, its flexing too much as I work the laps (TOT). I may have pushed beyond the "thinness limit" on this one. Was ok with 22"x 19mm with one usable mirror so far, and will get a second 22" to an acceptable stage before too long. What I am intending with the 25" is to make it a "sandwich" by marrying a second 19mm float disc to it. In between will be approximately 100 25mmx25mmx7mm glass tiles centers around 2 1/2" arranged in concentric pattern. The overall thickness of the composite will go up from 19mm(3/4") to about 45mm(1.77"). As stiffness increases with the cube of thickness the substate (in theory) will have an increase in stiffness of 13.285. This should enable me to finish figuring without a flexture problem.
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