Thread mills cut internal or external threads by milling—rotating while helically interpolating—ideal for large diameters, difficult-to-cut or hardened materials, thin walls, or where a broken tap is unacceptable. Versus taps, one thread mill makes multiple diameters of the same pitch, breaks chips well, won't seize in the hole if it fails, and can thread near the bottom. Mostly carbide with coatings. Confirm pitch, thread standard (metric/imperial) and diameter range; helical interpolation is CNC-controlled.