Thread cutting on the lathe is one of the few operations where the insert geometry must match the specification exactly — a 60° ISO thread will not accept a 55° Whitworth insert, and an internal thread needs a different profile than the external one on the same part. This guide walks through how thread turning inserts are named and how to pick the right one.
ER vs IR: external vs internal
- ER (external): the insert profiles the outside of the workpiece — bolts, shafts, stud threads. The profile faces up; chip flow is outward.
- IR (internal): the insert cuts threads inside a hole — nuts, tapped holes. The profile faces down into the hole; chip evacuation is the harder problem, which is why IR inserts often carry a more aggressive chip-breaker geometry.
The size prefix is the nominal insert diameter class: 08IR, 11IR, 16ER/16IR, 22ER/22IR, 27VE… A 16ER covers roughly M8–M14 external threads; a 22ER/22IR covers M16–M24. Match the size class to the thread diameter first — running a 16ER on M20 pushes the insert past its profile and weakens the corner.
The three standard profiles
| Profile | Included angle | Where you see it | Insert families |
|---|---|---|---|
| ISO metric | 60° | General metric threads (M, MA, BSW-adjacent) | 16ER/16IR, 22ER/22IR (ISO versions) |
| Whitworth | 55° | BSW/BSF, pipe threads, legacy imperial | 55° grades of 16ER/16IR, 22ER |
| Trapezoid / ACME | 29°–30° | Lead screws, power transmission | TR/ACME versions (22ER4.0TR type) |
The model code tells you which: 16IR1.0ISO = 16-series internal, 1.0 mm pitch, ISO 60°; 22ER4.0TR = 22-series external, 4.0 mm pitch, trapezoid. When you send us a thread spec, we match the insert to the exact profile and pitch — most thread inserts are pitch-specific.
Pitch: the second number that must be exact
Unlike general turning (where one insert covers a range), a thread insert’s cutting profile is milled to a specific pitch. Common ranges:
- 0.5 – 1.0 mm pitch: fine threads (M6, sensor housings, instrument parts)
- 1.0 – 2.0 mm pitch: the bulk of general metric work
- 2.0 – 3.0 mm pitch: coarse threads, fasteners, large diameters
- 3.0 mm and above (including 4.0 TR): power threads, lead screws
Cutting a 1.5 mm pitch with a 1.0 mm pitch insert gives you the wrong flank angle and a thread that won’t pass a gauge. Always confirm pitch before ordering.
Cutting strategy that protects the insert
- Multiple passes, full-width only at the end: cut to diameter-minus-one-pitch at full width, then take the final profile pass at ~0.2–0.3 mm depth. This is where most thread breakage happens — loading a full-width insert past the depth it was designed for.
- Feed = pitch, exactly. Thread turning feed per revolution must equal the thread pitch; anything else smears the flanks.
- Back off, don’t lift: retract axially between passes where the machine allows; radial lift loads the profile edge.
- Stainless and titanium: drop speed 30–50% versus steel and use the M-grade (stainless-optimized) inserts — chip welding is the main killer in thread grooves.
Internal threads: the chip problem
In IR work, chips must escape a narrow hole behind the insert. Two practical rules: (1) use the IR-specific chip-breaker geometry (the IR models in our turning insert range), not an ER insert flipped over; (2) flood coolant in deep holes — in shallow holes, compressed-air chip blasters between passes keep the flanks clean and the gauge happy.
Quick pick for common jobs
- M6–M12 external, general: 16ER, ISO 60°, pitch 1.0 or 1.5
- M14–M20 external: 22ER, ISO 60°
- M8–M20 internal: 16IR / 22IR matching pitch
- Lead screw 29°: 22ER TR version at the actual lead
- 55° Whitworth: 16ER/22ER 55° grades
Send us the thread callout
Every AMJtool thread insert page lists the profile, pitch and size class, and the material compatibility chart. If you have the thread specification (or just a part drawing), contact our sales team and we will identify the exact insert — including OEM pitch variants for special threads.