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I have to concur with everything that Bill has said.

My experience of gluing mirrors to backing plates produced the
results that Bill predicted - as the glue cured, it shrank and bent
the mirror. Too, because one side of the metal backing plate is
effectively insulated (by the mirror) temperature variations are
going to have a dramatic affect on the image quality.

When wishing to grind a flat surface, it is usual to use three
substrates and grind each against the other two in rotation to
produce three flat surfaces. That is the only way to do it...

I am not sure what is meant here by a 10 second level as the distance
between the divisions has not been defined - that I have seen anyway.
But never-the-less I am also not sure that practically, a 10 second
level is a good idea. Levelling a ten foot lathe with such a
sensitive level is one thing. But levelling a 6 inch mirror to that
precision with such a sensitive level will be a real pain. And I
would suspect that even the weight of the level will become a problem.

Geoffrey Kolbe

At 23:12 10/01/2012, you wrote:

>Randy,
>
>I don't think it would be wise to glue the mirror to the levelling
>plate. As the glue cures, the mirror may take on the contour of the
>plate. A simple way of retaining the mirror would be to use three
>pan-head screws, tapped into the plate just outside the periphery of
>the mirror, and tightened so the heads are just short of touching
>the mirror. This will prevent you carelessly decanting the mirror on
>to the floor, but will not introduce any strains into the glass.
>
>Grinding two plates together may well give you a surface that is
>smooth, but it is very unlikely to be flat to the standard required.
>A 10 second level with feet 100 mm apart will comfortably detect a
>difference in level of .005 mm over this distance. If you grind two
>circular plates together, more likely than not one will be concave
>and the other convex to match. This is quite desirable when mounting
>drums of marble on top of each other, but not when you want a truly
>flat surface. I am inclined to doubt, however, that the technology
>of the times of the Parthenon, was able to produce steel plates of
>the required size or even at all.
>
>I suggest you have a friend face the plate in his lathe. A properly
>adjusted lathe will face very slightly concave, so the mirror will
>be well supported at its periphery, which is where the feet of your
>level will rest when setting level. I don't think it's a good idea
>to divorce the level from the surface that you wish to level, and
>suggest you stick to resting the level on the mirror face.
>
>Bill Morris
>Pukenui
>New Zealand
>
>
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