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How to Cut Threads on a Lathe

Guide · Updated October 28, 2026

Single-point threading cuts a thread with one form tool, taking many light passes along the same helix. The lathe's leadscrew and half-nuts keep each pass in step, so the setup before the first pass decides whether the thread comes out right.

Check the thread when you are done

Measure pitch diameter with three wires or a thread micrometer, and use the free Gage Block Calculator to set up a stack for checking your mic or wires.

Step by step

Set the gearbox: Set the quick-change gearbox to the threads per inch or pitch you need, and engage the leadscrew, not the feed rod.
Grind or pick the tool: Use a 60° threading insert or a ground tool checked with a center gage (fishtail). Set it on center height.
Square the tool: Set the compound to 29.5° (for a right-hand thread) and square the tool to the part with a center gage.
Turn the major diameter: Turn the blank to major diameter, usually a few thousandths under nominal, and cut a relief groove or chamfer at the end.
Touch off: Touch the tool to the work, zero the cross slide and compound dials, and back out the cross slide.
Take passes: Engage the half-nuts on the threading dial mark, feed in with the compound, cut, then withdraw the cross slide and return. Repeat with smaller cuts as you go deeper.
Check and finish: Check with a nut, thread gage or three wires. Take a final light spring pass at the same setting before calling it done.

Worked example: 1/2-13 UNC

Pitch is 1/13 = 0.0769 in. Basic thread depth for a sharp V is about 0.866 × P, and the infeed along a 29.5° compound for a UN external thread is roughly 0.64 × P = about 0.049 in. Start with 0.008–0.010 in passes, then drop to 0.003–0.002 in near full depth, and check fit with a nut or ring gage before the last passes.

Threading dial rules

For even threads per inch, any line on the dial works. For odd TPI, use any numbered line. For half-threads (e.g. 11-1/2), use the same number every pass. For metric on an inch leadscrew, leave the half-nuts engaged and reverse the spindle to return.

Why 29.5° on the compound

Feeding at just under half the 60° thread angle makes the leading edge do most of the cutting while the trailing flank takes a light shave. That gives a better finish and less chatter than plunging straight in.

Tips

Use slow spindle speed until you are comfortable stopping at the shoulder; see turning speeds and feeds.
Check pitch with a thread pitch gage after the first scratch pass.
Measure pitch diameter with three wires or a thread micrometer.
Final fit check with thread ring gages.

FAQ

What angle should the compound be set to for threading?

For 60° UN and metric threads, set the compound to 29 to 29.5° so the tool feeds mostly along one flank.

How deep do I cut a thread on a lathe?

A rough compound infeed for external UN threads is about 0.64 times the pitch; finish by fit with a nut, ring gage or three-wire measurement.

Do I need a threading dial for metric threads?

On an inch leadscrew lathe, keep the half-nuts engaged and reverse the spindle to return instead of using the dial.

Why are my threads torn or rough?

Common causes are a dull or off-center tool, too deep a pass, too much speed, or no cutting oil.

Can I cut an internal thread the same way?

Yes, with a boring-bar style threading tool; set the compound to the opposite side and watch clearance in the bore.

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