TECHNICAL TIP #42; THREADING; THREAD MILLS:
Thread milling has become quite popular in recent years as an alternative to tapping or other forms of threading.
Thread mills look similar to taps, but function entirely different.  Taps feed into the part at the rate of the lead of the thread utilizing the chamfer and first full thread beyond the chamfer to cut and enlarge the thread to finished size.  On the other hand, a thread mill has no chamfer.  The mill is inserted into the hole along the axis of the spindle, deep enough to produce the full thread depth required.  The controller moves the thread mill out to the hole diameter until the threads cut into the sidewalls of the hole.  The thread mill then moves in a 360° circular motion until it is back to it starting position.  During this circular motion the mill must be lifted toward the top of the hole or moved along the “Z” axis of the machine one thread pitch or lead to produce a thread.  This lifting movement in conjunction with the circular motion is called “helical interpolation”.  All machines must have a helical interpolation program capability to utilize a thread mill.  After the 360° rotation, the tool returns to the center of the hole and extracted from the part.

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.
Thanks and a gift to Don Doggett, Vickie Finch, Martha Houdesheldt and Eddie Collins for suggesting this topic!

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