Hi. Here is a report on a substitute surface plate made from a discarded granite countertop offcut. Somebody who doesn't like to measure things gave me a 6 x 6" square of discarded that he probably picked up somewhere in his neighborhood. He told me that it's good, but wouldn't say how good. When he saw my expression, he said I could throw it away if I wanted since it wasn't costing me anything. I just threw it in a junk box and didn't think about it until today. Somebody else dropped my desoldering iron and broke the glass filter cartridge. I epoxied it together, but the glue was proud and needed to seal against a silicone ring. A good chance to use the ersatz surface plate, but how good was it? I ran a DTI with a base over it and noticed two things. First, the surface is very smooth. No needle jiggling was observed. Second, there was about 0.004" of variation over the area. Not so good, huh? This is a good lesson for those who don't like to measure. Despite the obvious error, I used it to sand the sealing surface. The error will be much less than the 0.004" I measured. It will probably be as good as a newly calibrated Starrett. It should seal just fine. This is better than using a block of wood or sanding the sealing surface freehand. And it keeps grit away from the real surface plate.
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