How the two processes differ
A tap cuts or forms a thread in one continuous pass, with every tooth of the tool engaged along the thread length. A thread mill cuts the same thread with a single small insert or solid cutter, orbiting helically through the hole one thread at a time.
That difference in mechanics explains every practical consequence, including the ones that have nothing to do with cost.
| Characteristic | Tapping | Thread milling |
|---|---|---|
| Tool cost per thread size | One tap per size and pitch | One cutter covers a diameter range at one pitch |
| Cycle time per hole | Fast | Slower, typically several times longer |
| Torque on the spindle | High, and it rises with thread size | Low, because the cutter is small |
| Failure mode | Tap breaks in the hole, often scrapping the part | Cutter breaks clear of the hole |
| Thread size limit | Practical limit around M36 | Any diameter, including oversized repair threads |
| Material suitability | Good in free-cutting material | Good in hard, gummy and work-hardening material |
| Blind hole capability | Depends on tap geometry and chip clearance | Full thread to the bottom of a blind hole |
Where tapping wins
Tapping is the correct choice more often than not, and pretending otherwise does not help anyone plan a process.
- High-volume small threads. For thousands of M4 holes in free-cutting steel, tapping is several times faster and thread milling cannot compete on cycle time.
- Cheap parts where a broken tap is recoverable. If the part can be scrapped or the hole re-tapped oversize without consequence, the risk that makes thread milling attractive does not apply.
- Machines without helical interpolation. Thread milling requires the control to synchronise three axes. Where that is not available, tapping is the only option.
- Through holes in ductile material. A spiral point tap pushes the chip ahead of it and clears the hole with no retraction, which is about as efficient as threading gets.
Where thread milling wins
Thread milling becomes the better answer in situations where the cost of a failure, or the difficulty of the cut, exceeds the cycle-time penalty.
- Expensive parts. A tap broken at the bottom of a hole in a part that already carries hours of machining is the most expensive tool failure in a machine shop. A thread mill breaks above the hole and can be removed.
- Hard or work-hardening materials. Tapping torque in tough stainless or a nickel alloy rises until the tap fails. Thread milling imposes a fraction of that load, so the operation becomes feasible rather than marginal.
- Large threads. Above roughly M20 the torque needed to drive a tap becomes impractical on many machines, while a thread mill of the same pitch handles it comfortably.
- Blind holes needing full thread depth. A thread mill can cut to the very bottom of a blind hole, because it engages one thread at a time rather than needing the full thread length of engagement.
- Repair and salvage. An existing thread that is damaged or needs to be opened to a repair size can be recut by interpolation, which no tap can do.
- Low-volume special threads. Where only a handful of parts need an unusual pitch, one thread mill replaces a tap that would sit in the crib for years.
The blind hole question
Blind holes are where the decision most often gets made wrongly, because the constraint is not the tool but the chip.
A spiral flute tap pulls the chip back out of the hole, but it has limited flute volume, so a deep blind hole fills and the tap breaks. A forming tap produces no chip at all, which solves blind holes entirely, but it needs a larger tapping drill and can distort a thin wall.
A thread mill has no chip problem at all: the chip is a fine particle washed out by coolant, and the tool can cut a complete thread to within a fraction of a millimetre of the hole bottom. Where the thread must run to the bottom of a blind hole in a difficult material, thread milling is usually the answer that works first time.
What to check before deciding
The decision usually comes down to four facts about the job, and they are worth establishing before any tooling is ordered.
- The thread size and pitch, because above about M20 the tapping torque becomes the deciding factor
- The workpiece material, and whether it work-hardens or is abrasive
- The value of the part at the threading operation, since that sets the cost of a broken tool
- Whether the machine control supports helical interpolation and rigid tapping
Frequently asked questions
Is thread milling slower than tapping?
Yes, typically several times slower per hole, because the thread is produced one helical pass at a time rather than in a single continuous engagement. Whether that matters depends on the part: the cycle-time penalty is frequently smaller than the cost of one broken tap in a nearly finished component.
What happens when a thread mill breaks?
It breaks above the hole rather than inside it, because the cutter is small and the shank is the weakest section. The fragment can normally be removed and the operation repeated, which is the main practical advantage over tapping in an expensive part.
Can one thread mill cut several thread sizes?
Yes, at a single pitch it covers a range of diameters, including a repair size larger than the original thread. That is why thread milling suits low-volume special threads, where a dedicated tap for each size would never be used enough to justify itself.
Does thread milling need a special machine?
It needs a control that can synchronise three axes for helical interpolation. Most modern machining centres can do it. Older controls without that capability leave tapping as the only option regardless of which process would suit the part better.
Which is better for blind holes?
Thread milling, in most difficult cases. It cuts to the bottom of a blind hole because it engages one thread at a time, and its chip is a fine particle rather than a curl that has to be evacuated through the tap flutes. Forming taps also solve blind holes by producing no chip, but they need a larger hole and can distort thin walls.