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 edge? I seems line a waste to buy new points if I have a grinder, I just need to know where to grind. If anyone has a picture of new cutters I certainly would appreciate seeing them.