Thread Mills
Thread mills are the milling cutters that make threads by interpolation instead of by tapping, and they have changed how the modern machining centre threads its holes. A tap, as the tapping entry describes, is a tool of one thread: it must be the exact size and pitch of the thread it cuts, and it cuts that thread only in a hole. A thread mill is different in kind. It is a small carbide cutter, rather like a tiny end mill, whose teeth are ground in the profile of a thread form; the machine drives it round the hole in a helix — it turns and climbs one pitch as it makes one circuit — so that the cutter’s teeth mill the thread form into the wall as it goes. Because the thread’s diameter is set by the machine’s path rather than by a tool of that size, one thread mill cuts many thread sizes; a multi-tooth cutter is made for one pitch, while a single-profile tool can cut a range of pitches. This entry sets out what a thread mill is, how thread milling works and where it beats the tap.
What a thread mill is
A thread mill is a milling cutter made to cut a thread form, and its two forms serve two ends. The single-profile thread mill carries one row or a short run of thread-form teeth, like a single-point threading tool wrapped round a shank; it makes one small arc of thread per pass and must climb the helix in several circuits to cut a full thread, each pass taking a deeper bite. The multi-tooth thread mill — the full-form cutter — carries a full length of thread-form teeth, several pitches of them, so that it cuts the whole thread in a single helical circuit, turning round the hole once while climbing one pitch; it is the faster and the commoner tool, made in sizes to suit the thread pitches it cuts. Both are solid carbide, coated for the material, small and rigid, and both are held in the spindle like any end mill — which is the first of their advantages over the tap, because they need no tapping attachment and no special holder, only the machine’s ordinary milling motion.
Cutting the thread by interpolation
Thread milling works by the machine moving the cutter in a helix, and the motion is what gives the method its reach. The machining centre interpolates a circular path of the diameter of the thread while the cutter feeds axially by one pitch for each full circuit; the cutter’s teeth, sweeping that helix, mill the thread form into the hole’s wall. For an internal thread the cutter is first plunged or ramped into the hole to its full depth, then run round the helix to cut the thread; for an external thread — a threaded boss or spigot — the same helical path is run round the outside. Because the pitch is set by the machine’s motion and not by the tool’s grinding, the same thread mill cuts a thread of any diameter at that pitch, and the machine can cut right-hand or left-hand threads, full or partial, simply by the direction and the length of its helix. The CNC control that this wiki treats elsewhere does the interpolation, and the cutter follows it truly.
Where the thread mill wins
The thread mill earns its place where the tap struggles, and the cases are the ones that make a machinist reach for it. Its first win is one tool for every size: a tap is one thread and one hole, but one thread mill and the right program cut a family of thread sizes at its pitch, so the shop that threads many sizes keeps one cutter where it would keep a rack of taps — the saving that matters for the expensive, seldom-used sizes. Its second win is the fine and the special thread: fine pitches, multi-start threads, and threads in materials that defeat a tap — the hard and the work-hardening alloys where a tap breaks and a thread mill, cutting with the light, interrupted bite of a milling cutter, survives. Its third win is the blind and the partial thread: the thread mill can cut a thread to the very bottom of a blind hole and can cut a partial thread or a thread right up to a shoulder, places where a tap cannot follow or must stop short. And its fourth is quality: the thread it mills is cut, not formed or chased, so it is accurate and its start is clean.
Where the tap still wins
For all its reach, the thread mill does not replace the tap everywhere, and the comparison is the one this wiki’s tapping and tap entries frame. The tap is still the faster tool for the ordinary small hole: one pass, one operation, and the thread is cut, where a thread mill must interpolate its helical path — so for the production run of small, common threads in easy material, the tap machines the hole in a fraction of the time. The tap also needs only a simple spindle and a rigid tapping cycle, where the thread mill needs the machine’s full circular-interpolation motion, which the older or simpler machines lack. The thread mill is thus the specialist tool of the machining centre that has the interpolation and the work to justify it: the shop taps its small, standard, high-volume holes and thread-mills its large holes, its fine pitches, its hard materials and its specials — turning the threads that a tap cannot cut with one small carbide cutter.