Re: [ATM] Blank ratios for different materials.

[Dominic-Luc Webb <dlwebb@canit.se>]

On Tue, 19 Jun 2007, Anthony Anconetani wrote:

> Hi Guys,
>
> >From my basic understanding of mirror grinding, it sounds like there is
> a typical ratio of blank diameter to thickness that minimizes complexity
> in polishing, figuring and mounting.
>
> I think the ratio for a Pyrex blank is something like 6:1. Is that
> correct?
>
> And, what would be a good ratio for materials like plate glass, BVC and
> Quartz?
>
> Thanks,
>
> -Anthony
> Round Rock TX

The 6:1 is an old empirical rule of thumb, and I have many times
thought to hunt for a mathematical/physical proof. For typical
mirrors, like my first 8", it certainly worked, but so too did
the plate glass tool I got in the kit, and that was more like
3:1. I am not remembering the proper engineering terms for these
properties of glass, but I know that sodium and borosilicate
capillaries have very different properties. I have made micro-
pipettes and electrodes from each and at a microscopic level
the amount you can bend them before they break was remarkably
different. That led me to believe that the amount they would
sag under their own weight would also be very different. In
this case, I would expect things can very as much or even more
for more diverse materials like quartz and BVC. I have not seen
anything specifying if the ease of bending a capillary is
actually related to these thickness ratios like 6:1, but certainly
longer pieces of stock capillaries (1-2 meters) of same dimensions
will noticably sag differently. It was never clear to me how the
relative sag between different glass types would be affected as a
function of thickness. The "Handbook of Optics" by the American
Optical Society gives some physical properties of different types
of glass, including mechanical properties, but I do not know how
to apply these to the present question.

Dominic

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