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Werner,
Please don't go any further with this idea because it is practically impossible to make a useful flat with the diameter to thickness ratio that you describe.
I understand that you intend to make an optical flat with a D/T=18 - way beyond the realm of amateur telescope makers.
You can epoxy two of the discs together and use it as a grinding tool for making a 14" mirror (but not using the third disc - you would need a thicker blank for that).
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