Thread milling cuts a thread using a rotating tool on a helical (circular ramping) path, rather than a tool the same size as the thread. One tool diameter can produce several different thread diameters at the same pitch, and the same tool cuts both internal and external threads.
Thread milling vs tapping. A thread mill produces a complete thread profile from the first pass, where a tap only reaches full profile after several threads of engagement. Cutting forces are lower and chip evacuation is more reliable, which matters in blind holes and in materials that are hard to tap cleanly. Thread mills can also cut hardened steels up to around HRC 65, titanium and nickel-based superalloys where taps struggle or break.
Solid carbide thread mills (3 to 6 flute, with or without through-coolant) suit smaller diameters and general-purpose work in steel, stainless and aluminium, including combination drill/chamfer/thread mill tools for single-operation blind or through holes.
Indexable thread mills use a toolholder with replaceable inserts — one holder can produce a range of diameters at a given pitch, both left and right hand threads, which reduces tooling costs on larger-diameter or higher-volume work.
Coatings and grades. Uncoated or bright finishes suit aluminium and free-machining materials. TiN and TiCN coatings cover general steel and stainless work. AlCrN and AlTiN coatings hold up at the higher temperatures generated in stainless, titanium and nickel alloys. Dedicated hardened-material grades are used for steels above 45 HRC and up to 62 HRC.
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