Re: ATM: Help - Mistake fine grinding mirror

["Bill T." <twentiethwave@hotmail.com>]

>I realized that I've been using the wrong stroke for fine
>grinding!

Michael,

Actually, you're probably ok as is.  It's pretty hard to grind two pieces of 
glass together for a long time and not come up with something like a sphere. 
  I'd just keep on with that W stroke, adjusting the stroke to keep the 
amount of overhang consistent on all sides -- overhang maybe 25% of 
diameter.  Give it about 7 vectors to cross the mirror, but try not to be 
too consistent -- some variation is good!  25 micron is probably still rough 
enough to automatically correct whatever minor problems your curve may have. 
  Personally, assuming that the glass is now "pit-free" even at the edge, 
I'd give it another hour with 25 micron and then go on 9 micron and then 
polishing.  Oh, and be sure to rotate the mirror and tool in opposite 
directions, often!

You can use the famous Sharpie test to evaluate your present grind.  Use a 
black Magic Marker to cover the mirror with a grid pattern and start 
grinding.  If everything is about right, the pattern will grind off very 
evenly starting at the center and moving rapidly towards the edge.  This is 
easiest to monitor with mirror on top, with the back of the glass wet.  If 
you consistently see isolated islands or circumferential zones with very 
fast or very slow removal, then you may want to back up to 120 grit.

Bill Tondreau



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