GlossaryKind of term: Manufacturing termThread milling
What is thread milling?
Thread milling cuts a thread with a cutter that follows a helix, advancing one pitch along the axis per orbit, instead of using a tap.
In thread milling a thread mill orbits the centre line of a pre-drilled hole, or of a shaft for an external thread, and advances one lead axially per orbit, one pitch on a single-start thread: helical interpolation. A multi-form cutter with several profiles at one pitch makes different diameters at that pitch; a single-form cutter can also make different pitches with the same flank angle, within the limits of its tooth form. Right-hand or left-hand thread follows from the orbit direction and the axial direction of travel. You set the size with the radius offset.
What does thread milling ask of machine and tool?
Whether thread milling fits depends on the control (helical interpolation), the ratio of cutter diameter to hole, the thread length relative to the cutter diameter, the material and the stiffness. A long thread milled with a slender cutter can come out tapered through deflection; extra passes or an adjusted offset can compensate. Because the cutter is smaller than the hole, a broken cutter can usually be removed, whereas a broken tap in a blind hole can stay stuck. The thread can run closer to the bottom of a blind hole than with a tap, because a thread mill has no chamfered lead.
How does thread milling work in practice?
For a small thread in free-machining material, tapping is usually quicker per hole. Thread milling costs more cycle time per hole, but can be the better choice in tough or hard material, on large diameters, in expensive parts and where many sizes share one pitch. The choice trades minutes per hole against magazine pockets and scrap risk. Checking is usually done with a go/no-go thread gauge to the tolerance class on the drawing, for example 6H.
What is thread milling often confused with?
Thread milling is not tapping: a tap cuts or forms the thread in one move in and out, and has one size. Thread turning is different again: on a lathe a tool cuts the thread in several passes while the workpiece rotates.
How do you see thread milling in Blake?
A thread such as M6-6H sits in Blake as a proposal from the drawing, ready to confirm or correct. Drilling and tapping have their own line with the machining time as an estimate. If you mill the thread and the time differs, you record a correction on the line with its reason. More on estimating.
Frequently asked questions
When should you mill a thread instead of tapping it?
In tough or hard material, on large threads, in blind holes where the thread must run close to the bottom, and in expensive parts where a broken tap can mean scrap. For small threads in a batch, tapping is usually quicker.
Is thread milling slower than tapping?
Per hole, usually yes, because the cutter has to travel a helix. Across an order it can come out better: fewer tools across many sizes, no stuck taps, and a size you adjust with an offset.