Thread mills are most commonly found in solid carbide, with either straight or helical flutes. They are also available in indexable style with carbide inserts, or premium high-speed steel (powdered metal) with helical flutes. Ideally all thread mills are coated with TiN, TiCN, or TiAlN depending upon the application. Indexables are typically for sizes ¾” or larger, and accommodate a variety of threads per inch by replacing the insert. Solid carbide is generally for production threading and for materials up to 62 Rc. Powdered metal HSS thread mills are recommended for materials softer than 30/32 Rc, less rigid setups, interrupted bores and machines that have a limited speed capability.
When should a thread mill be selected over taps? In general, for
production threading up to 3/8”, taps are more efficient. However, if
producing a wide variety of parts, threads, and materials on the same machine,
threads mill are far more versatile. They will produce right or left hand,
internal or external threads, single or multiple lead from #2-56 with the same
mill. Materials range from soft, non-ferrous alloys to heat-treated
steels, or tough alloys such inconel and titanium, where tap breakage often
occurs. Pipe threads are easily produced without leaving the normal
“stop lines” and creating the troublesome stringy chips normally produced by
taps. In addition, thread mills can produce full threads to within one
pitch of the bottom of the drilled hole.
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