How to sharpin a point post hole digger

I recently purchased a used Husky 3 point post hole digger. It works fine in regular soil but I need to dig some holes in shale. I sharpened the point and that works very well. The cutting teeth just ride in top of the shale. I tried turning the cutting points upside down but that didn't help. so my question is how should I sharpen the the removable cutters. Should I sharpen at a certain angle? Should I sharpen the top side or the bottom side. Should I grind a relief behind the cutting...

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How to sharpin a point post hole digger