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Cast Tool Grinding

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Posted by Ian McKernan on March 27, 2002 15:01:50 UTC

Hiyah! Following up a posting i did last year about shaped, cast tools. After much faffing about getting glass, ( i wound up with 2 pieces of 254 * 19mm plate) i finally started grinding mirror tonight. I'm doing it on a plaster tool that i cast with (hopefully) something like the a spherical curve of approx the right radius on it. This was then covered in the "dumps" from the centres of 2mm mild steel washers.
In about 1 - 1.5 hrs grinding with #80 i've got a hole about 3/4 of the way to the edge and still a fair ways to go. Since the tool has a curve on it, im grinding centre over centre, and the steel dumps in the centre of the tool are wearing down slowly. There are however a few spots where the surface of the dumps is obviously low and isn't wearing at all. If i keep turning the tool and shuffling round the drum i think it should come out to a sphere and it will certainly get the gross hogging out done. I'm a little dubious about what will happen as i go down the grit sizes though, and wonder if these low spots will give me a problem with either the shape of the mirror or scratches? Any opinions / advice out there? I'm thinking that if the surface of the tool doesn't look uniform when ive hogged out the first bit of glass i might do the second bit as well to wear the tool a bit more before i go on the finer grits. Is this overkill?

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