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Counterbore Tool Issue

petertha

H-M Supporter - Premium Content
H-M Supporter-Premium
#1
I bought some mixed brands of HSS capscrew counter bores & its becoming obvious some are not cutting well. First I thought it was a 3-flute vs 2-flute thing. But upon closer inspection, the cutting edge is not consistent. Viewing from the side, some have what would form a flat bottom hole. Others have a slope meaning they would cut deeper out near the edge & shallower in where the pilot stub is. Not like an outer pip either, its a linear taper. The better quality (happen to be 3-flute euro) are all flat bottom. What makes me suspicious is the angle &straight are from the same set - almost like they messed up the grinding relief. Correct me if I'm wrong but isn't the whole objective of a capscrew counterbore to be flat across the bottom, not tapered?