[ATM] Yet another ultra thin project

[vorblesnak <vorblesnak@peak.org>]

Something there is that loves a fool and a mirror grinder.

I had slumped a piece of 3/16ths glass over a bad form.  What I got was 
an outer ring with a nasty case of ripply droop.  I ground on it a bit 
and realized I was going to grind through before I got the ripple and 
the droop out.  So, having never tried to cut glass before, and having 
this piece of now worthless glass, I decided to cut the outer 1.25 
inches off.

I will post pictures later, but I made a beam compass out of a scrap 
piece of fir stock.  I did this by nailing a wheel cutter to the end of 
the scrap and the running a screw through it at the turn point.  The 
screw acted as my point for the compass.  I put a couple of layers of 
masking tape near the middle of the glass and then using a ruler I 
marked the middle of the circle on the masking tape.  I etched/scribed a 
circle on the back of the curve and then using the ball on the end of 
the cutter I tapped from the inside of the curve following the etch line 
through the glass.  It worked!  I cannot believe I got away with that.  
So now it is a 15.5 inch instead of an 18 inch.

The specs on this one:

15.5 inch diameter, 3/16th inches thick
3/16 inch of sagitta
80 inch +/- focal length based on r squared / sagitta
f5 more or less

Should be a fun project.

BTB: The original is on its way to the coater.  Tested out pretty well 
at OSP.

David Davis
Toledo, Oregon 97391
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