Okay now you guys made me get up off my bed leave my coffee go out to the garage get an old busted rocker shaft and pull out a flat file and use the edge of it. And it was kind of like I thought. Because of the roundness of the shaft using a straight edge it would get deeper in the center naturally. I think using something round would make it even deeper in the center? Also it's before 7 in the morning here on the West Coast and I'm not firing up my grinder with my wife still sleeping LOL to see what the round wheel would do...
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