Wear on the tool's flank, measured as the width of the worn land, and the standard tool life criterion.
Flank wear is gradual and predictable, unlike crater wear or chipping. That makes it the criterion most tool life standards use: the tool is spent when the wear land reaches a specified width, regardless of whether it still cuts.
It is caused by abrasion from the machined surface, so it accelerates in abrasive materials and in hard inclusions. Coated tooling exists largely to slow it down.
Judging tool life by surface finish alone. Flank wear may be well past the criterion while the finish still looks acceptable — until dimensions drift out of tolerance.
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