Re: [ATM] processing techniques for soft optical glass?

["Bob May" <rmay@nethere.com>]

The making of two tools for any particular radius is very
sensical.  It is always easier to measure a concave radius then a
convex radius and get it very close to what it needs to be as you
can alwqys do a foucault test on the concave and then use that as
a reference surface to go against the convex glass surface of the
final lens.  It is relatively very difficult to get a spherical
shape measured  than it is for a concave surface and doing gringe
testing is the way to go.
Water is quite good enough to do grinding and polishing with even
the most sensitive of the glasses.  Water really doesn't stain
glass that fast and just making sure that the glass doesn't sit
for a while in the water is sufficient to keep it from staining
the glass.
One thing tho is that when you get the final shapes for the
glass, you want ot get any outside surfaces anti-reflection
coated as a protection against staining, etc.
You will also find that the finer grits will fill up with glass
faster than with other glasses of normal hardness so make your
wets shorter.  I'd also rather spend the time with a finer grit
than any ral hogging out grit as the cracks of the grinding
proceess will propagate further.
And, yes, the glass is softer than standard plate glass but not
so much as to be worn away in minutes by the grinding process.
One of the spec items is how hard the glass is and you should
take note of that when doing the grinding and polishing.
Good luck!
Bob May
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