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I never heard of a protective layer of nickel over the abrasive diamond particles. My impression is that they are embedded in a layer of nickel on steel. :shrug: I was wrong once before though. :)
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Like FloorPizza I took the shaft on a 12" Stanley screwdriver and after wetting the plate I did some round trips on the flat part and on the edges. I used a fair amount of pressure. It doesn't round...