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Fixing "modifications"

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Posted by Ian McKernan on August 9, 2002 03:07:35 UTC

Seems that about the only way that you can completely stuff up a mirror blank is to drop it, or, as in one artcle i read, try and heat it with a blowtorch while making the pitch lap, and shatter it by thermal stress! (warming things up in hot water is much gentler and works well).

You are right when you say that you interchange tool on top (TOT) and mirror an top (MOT) to get the right radius of curvature. I cant think of any reason you would want to grind it flat again. With my mirror i started with 19mm thick and was shooting for sagitta if 2.6mm ( about f6 ). The EDGE of my mirror is now about 15.8mm thick (ive hacked off a lot of glass!!) and sagitta is about 2.8mm (around f 5.4).

I overshot a LONG way during initial grinding, as i was trying to use a steel covered tool with a pre-cast curve which was to deep. It ground plate glass really fast though!! Maybe i should have interchanged MOT/TOT more often during initial hogging out?


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