Are you way-scraping? A couple of years ago I got three chunks of enormously thick glass (not thick enough to stop a car, it turned out, which is why I got them) and cross-ground them all. It was kind of embarrassing, though, since when I stopped it was visually obvious that the grinding pressure had warped the glass more than when I'd started with it: it was perceptibly clearer around the outside than at the center, on each piece.
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A couple of years ago I got three chunks of enormously thick glass (not thick enough to stop a car, it turned out, which is why I got them) and cross-ground them all. It was kind of embarrassing, though, since when I stopped it was visually obvious that the grinding pressure had warped the glass more than when I'd started with it: it was perceptibly clearer around the outside than at the center, on each piece.