ATM Behavior of glass upon trepanning

[ToobWiz@aol.com]

May I impose upon the experience of the list again?

I have two matching spherical mirrors that are to be the auxiliaries for my 
SPT. I am just about finished figuring the spherical surfaces, and would now 
like to trepan the mirrors to size, 45mm and 60mm dia.

The mirrors are made on 70 X 10 mm opthalmic crown pressed blanks. The backs 
are semi-ground and flash polished; the edges are untouched. Can the behavior 
of the glass be predicted? If it is a simple matter like "the edges always 
turn up", then I can trepan pretty close to my finished sizes. I'd leave 
aobut 2mm for the bevel and another 2mm for tool runout/cut width (that's how 
I arrived at the diameters stated above.

OTH, if there is likely to be a large and unpredictable edge anomaly, I'd 
likely not trepan them and adjust the OTA design to accomodate the larger 
sizes. I'd liefer have them small, though.

Thanks in advance/Don Carron