How Many Thread Processing Processes Do You Know?
Seven thread processing processes cover nearly every internal and external thread we cut, roll, or grind. This guide explains how each one works, the parameters that matter, and when to pick it. Written for engineers and buyers who need to specify threads correctly the first time.
In this article
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- 2
- 3
- 4
- 5
- 6
- 7
Key takeaways
What thread processing processes actually do
A thread is a helix with a defined pitch, flank angle, and major diameter. Every thread processing processes removes or displaces metal to reach that geometry. The choice changes tool life, thread class, surface finish, and whether the part can be disassembled without galling.
Cutting processes, like tapping and milling, shear material away and leave a visible root radius. Forming processes, like rolling, push material into shape and leave a smoother flank with a cold-worked surface. Grinding sits in between: it removes very little material but holds the tightest tolerance.
The decision usually starts with three questions: is the thread internal or external, what material and hardness, and what thread class does the drawing call out. A 6H internal thread in 6061 aluminium and a 4H thread in 4140 steel at 40 HRC will not use the same process.
We run all seven on the same floor, so the process is picked per feature, not per part. That is why a single housing can carry tapped M4 mounting holes, a milled NPT port, and a rolled external stud thread.
- 1Internal threadsTapping, thread milling, thread whirling, and single-point boring.
- 2External threadsDie threading, thread rolling, single-point turning, and grinding.
- 3Hardened partsThread grinding after heat treatment, usually above 45 HRC.
Tapping and thread milling: the two workhorses
Tapping uses a fluted or form tap that enters a drilled hole and cuts or displaces the internal thread in one pass. For M3 in aluminium, a cutting speed of 15–25 m/min and a feed equal to the pitch works well. In 304 stainless, drop to 8–12 m/min and use a forming tap if the hole is not too deep.
The pilot hole diameter matters more than most operators admit. For a 6H M6 × 1.0 thread, a 5.0 mm drill gives about 75% thread engagement. Going undersize by 0.1 mm raises torque sharply and snaps taps. Going oversize weakens the thread and may fail a go/no-go gage.
Thread milling interpolates a helical path with a single-point or multi-tooth cutter. It needs a machine with helical interpolation and enough rigidity to hold the helix. The advantage is one tool for many diameters: an M6 thread mill can cut M6 × 1.0 or M6 × 0.75 by changing the pitch in the program.
Milling also handles interrupted threads, thin walls, and blind holes with no chip packing. It is slower per hole, so we use it when tapping fails or when the part cost justifies the extra cycle time.
- 1Tapping tipUse spiral-flute taps for blind holes and spiral-point for through holes.
- 2Milling tipLeave 0.05–0.1 mm radial stock for a clean finish pass.
Thread rolling and die threading on external features
Thread rolling displaces material between two or three dies. The blank diameter is not the major diameter; it is the pitch diameter, roughly the nominal minus 0.3 × pitch. For a 10 mm rolled thread, the blank is usually 9.6–9.7 mm. Roll too large and the dies overload; roll too small and the crests come out flat.
The payoff is fatigue strength. Rolled threads have continuous grain flow and compressive residual stress at the root, so they survive vibration better than cut threads. Automotive and aerospace drawings often specify rolled threads for exactly this reason.
Die threading is the manual or turret-lathe cousin of rolling. A die cuts the external thread in one or more passes. It is fine for repair work and small batches, but it tears the surface on stainless and is hard to keep concentric on long parts.
We run rolled threads on a dedicated machine when the batch justifies it. For one-off prototypes, single-point turning is faster to set up and easier to inspect.
- 1Rolling blank sizeNominal pitch diameter, typically 0.1–0.3 mm under major diameter.
- 2Die threading limitBest for Ø6 mm and under, or for chasing damaged threads.
Grinding and whirling for tight classes and hard material
Thread grinding uses a form-dressed wheel to cut the thread after heat treatment. It holds lead accuracy and flank angle better than any cutting process, which is why gage makers and aerospace shops use it for class 3A and 3B threads. Wheel dressing is the critical step; a worn profile shows up as a drift in pitch diameter.
Grinding removes very little material per pass, so it is slow and generates heat. Flood coolant and a sharp wheel keep the surface from burning. If the part is not hardened, grinding is usually overkill and adds cost without benefit.
Thread whirling is a CNC lathe process where a rotating cutter head surrounds the workpiece and the tool feeds along the axis. It cuts long, slender threads, bone screws, and deep lead screws in one pass. The counter-rotation balances cutting forces, so the part does not bend.
Whirling is the right answer for medical bone screws and long threaded shafts where a die or a single-point tool deflects. It is not a general-purpose process, and the tooling is specific to the thread form.
- 1Grinding thresholdUse above 45 HRC or when the drawing calls out class 3 fits.
- 2Whirling advantageOne-pass threading on long, slender parts without deflection.
Step by step: selecting the right process
- 1Read the thread calloutNote diameter, pitch, class, and whether it is internal or external. A 1/4-20 UNC 2B internal thread points straight to tapping or milling.
- 2Check material and hardnessAluminium and brass tap easily at 20–30 m/min. Stainless and titanium need 8–12 m/min and more coolant. Above 45 HRC, plan for grinding after heat treatment.
- 3Measure the hole depthFor blind holes, thread depth should be at least 1.5 × diameter. Below that, use thread milling to avoid tap breakage.
- 4Pick the pilot or blank sizeInternal: drill 0.1–0.2 mm under nominal minor diameter. External rolling: turn the blank to pitch diameter, about 0.1–0.3 mm under major diameter.
- 5Set cutting parametersTapping: speed 8–25 m/min, feed = pitch. Milling: 0.05–0.1 mm radial stock, climb cut. Turning: 0.1–0.2 mm depth per pass.
- 6Inspect the first partUse go/no-go gages for internal threads and a thread micrometer for external. Check pitch diameter, not just the major diameter.
- 7Adjust for chip evacuationBlind holes need through-coolant or peck cycles. If chips pack, switch to thread milling or add a retract dwell.
Thread processing processes compared
Pick the process by feature type, material, and thread class.
| Process | Best for | Typical speed | Watch out for |
|---|---|---|---|
| Tapping | Internal M1–M12, rigid setup | 8–25 m/min | Chip packing, tap breakage |
| Thread milling | Large or thin-wall internal threads | 100–200 m/min | Needs helical interpolation |
| Single-point turning | External threads, prototypes | 80–150 m/min | Tool deflection on long parts |
| Thread rolling | High-fatigue external threads | 20–60 m/min | Blank diameter must be exact |
| Die threading | Small external, repair work | 5–15 m/min | Torn flanks on stainless |
| Thread grinding | Hardened steel, class 3 fits | 25–35 m/s wheel | Burn if coolant is weak |
| Thread whirling | Long slender threads, bone screws | 200–400 m/min | Dedicated tooling per form |
The short version
Match the process to the feature: tap small internal threads, mill large or thin-wall ones, roll external threads that see fatigue, and grind only when the part is hard or the class is tight.
Frequently asked questions
What is the strongest thread processing process?
Rolled threads have the best fatigue life because the grain flow follows the thread profile and the root carries compressive stress. For static strength, a cut thread with full engagement is close, but rolling wins under vibration or cycling loads.
Grinding does not improve strength; it improves accuracy and surface finish after hardening.
When should I use thread milling instead of tapping?
Use milling when the hole is larger than M12, when the wall is thin, when the material is hard or gummy, or when the part cannot risk a broken tap. One mill cuts a range of diameters in the same pitch family.
Milling is slower per hole. For a hundred M4 holes in aluminium, tapping is still the right call.
Why did my tapped thread fail the go/no-go gage?
The most common cause is pilot hole diameter. A hole 0.1 mm undersize raises torque and can tear the thread. A hole oversize reduces engagement and the go gage enters too far.
Check the drill size against the thread chart, then verify spindle speed and coolant. Stainless often needs a forming tap and a slightly larger pilot.
Can you cut threads on a 5-axis machine?
Yes. Thread milling and single-point turning both run on our 5-axis centers, which lets us cut threads on angled or compound features in one setup. Tapping also runs on 3-axis and 4-axis machines with rigid tapping enabled.
For rolled and ground threads, we use dedicated equipment to hold the class and finish.
What pitch diameter tolerance can you hold?
We hold ±0.005 mm on turned and ground diameters, which covers class 2 and class 3 fits for most inch and metric threads. Rolled threads typically hold a slightly wider band because the dies wear over the run.
Specify the thread class on the drawing so we can pick the process and the gage.
Do you inspect every thread?
We inspect 100% of parts before shipment. Internal threads are checked with go/no-go gages, external with thread micrometers or ring gages. Reports are available on request.
For critical threads, we log pitch diameter and class on the inspection sheet.
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