How to cut a thread on the lathe

Single-point screwcutting on a manual lathe, from setting up to checking the fit. Use it alongside the thread calculator and the change-gear calculator.

Cutting a leadscrew? See the Acme thread table (1/4" to 3", standard and alternative TPIs).

Feeding the tool in with the compound slide set at about half the thread angle means it cuts mainly on one flank.

Before you start

Look up the thread: pitch or TPI, major diameter and depth. The thread calculator gives all of these.
Turn the work to the major diameter (slightly under for an external thread). Chamfer the start and cut an undercut or groove at the end if you can.

Grind or fit the tool. Use a 60° tool for metric and unified threads, or 55° for BSW/BSP. Check it with a thread gauge (fishtail gauge). Set it exactly at centre height and square to the work.
Set the pitch. On a gearbox lathe, set the levers from the chart on the headstock. On a change-gear lathe, fit the train from the change-gear calculator. Pick your machine there and it fills in the leadscrew for you.
Set the compound slide to 29–30° for 60° threads (27–27.5° for 55°), so the tool cuts on its leading flank.
Slow down. Use a quarter or less of your normal turning speed, slow enough to disengage in time. Backgear is common on hobby lathes.
Take a scratch pass. Touch on, zero the cross-slide and compound dials, and engage the half-nuts on a thread dial number. Take a very light pass, then check the pitch with a gauge or rule.
Feed in with the compound. At the end of each pass, withdraw the cross-slide, return the carriage, reset the cross-slide to zero and put on the next cut with the compound. Take smaller cuts as you go deeper.
Re-engage correctly. For metric threads on an imperial leadscrew (or the other way round), you can’t use the thread dial. Leave the half-nuts closed and reverse the spindle back.
Check the fit as you near full depth: use the mating nut, a thread ring gauge, or measure over three wires. Finish with a spring pass (a repeat pass with no extra cut).

Cutting a multi-start thread

A multi-start thread has two or more separate grooves running side by side. Each turn of the screw moves a nut by the lead, which is the number of starts times the pitch, so it moves fast without a deep, weak thread. The thread calculator gives the lead, helix angle and indexing for 1 to 4 starts.

Set the machine for the lead, not the pitch. A twin-start 5 TPI Acme is set up as if cutting 2½ TPI. The depth, diameters and tool width still come from the pitch.
Grind for the helix. The helix angle is much steeper than on a single-start thread, so give the leading side of the tool extra clearance or it will rub.
Cut the first start to full depth, but don’t move the cross-slide setting afterwards: every start must finish at the same depth.
Index to the next start in one of two ways. Either move the topslide (set parallel to the axis) along by exactly one pitch, taking up backlash the same way each time, or turn the work relative to the spindle by 360° ÷ starts, using driver-plate slots, a dividing chuck or a marked change gear, without moving the carriage.
Cut each start in turn, then check the fit with the nut. Light finishing passes over all starts help to even them out.

Pipe threads: NPT vs BSPT vs BSPP

NPT (US) is a 60° thread on a 1 in 16 taper. It seals on the thread with PTFE tape or compound. NPTF (dryseal) has the same sizes but is made to seal without sealant.
BSPT (R external, Rc taper internal, Rp parallel internal) is a 55° Whitworth-form thread, also 1 in 16. It seals with tape or compound.
BSPP / G is parallel and does not seal on the thread. It seals on a washer, O-ring or bonded seal.
They are not interchangeable. The angles differ, and so does the TPI at some sizes (1/2" is 14 TPI on both, but 1" is 11.5 TPI NPT and 11 TPI BSPT).
Cutting a taper thread: set the topslide or taper attachment to 1°47′ (half of the 3°34′ included angle). 0°47′ is a common mistake. Keep the threading tool square to the lathe axis, not to the taper. Stop at the hand-tight length and test with a gauge or the fitting.

The thread calculator and tap-drill page give the TPI, gauge diameters, engagement lengths and tap drills for NPT, NPTF, NPSM, BSPT, Rp and G.

Pro tips

Total compound infeed ≈ thread depth ÷ cos(29.5°). The thread calculator shows the infeed schedule in Detailed mode.
Cutting towards the chuck? Make a mark on the work, or fit a carriage stop, to show where to disengage.
Use cutting oil on steel. For stainless and tough alloys, use sharp carbide or HSS and positive cuts. Don’t let the tool rub.
For internal threads, use a boring-bar threading tool and work out the minor diameter first. Our tap-drill page gives the bore size.

Safety: keep hands away from the chuck and leadscrew. Never use emery on work held between centres while the half-nuts are engaged. Always remove the chuck key. Wear eye protection, and no gloves near rotating work.