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Re: Rough Grinding Leaves Smooth Edge

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Posted by Robert May on September 6, 1999 18:03:41 UTC

"...held to more rigidly for max stroke" means. Are you saying that I need to be more precise with the 1/3 W stroke. Can you clarify? What I mean is that you don't want to overhang by more than that. lesser strokes are ok but you don't want to dig out the center more. Look at how to parabolize to see what I mean. The process is the same - large overhangs shorten the radius and make nonspherical surfaces. Putting the tool on top will both open the radius of your mirror and "shine up" the edge first but don't do that just to shine it up as you will be keeping the figure of the previous grit shape which won't be correct for the next grit. When you get to the point of polishing, your mirror should have an almost spherical shape and you will find that the polishing will start from the center and go to the edges. It's ok to do one grit with the tool on top and the next with the tool on the bottom to try to keep the curvature you want, just don't do both with the same grit. My recomendation is to go to 80% of depth on the first grit and work most of the rest with the next grit and finish with the rest of the grits. For the finer grits, reflect a light bulb off of the surface and get the color of the light to orange and you will be able to see the finish of the center vs. the edge. This test works well starting at about 320 grit.

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