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How to Cut Left-Hand Threads on a Lathe

Guide · Updated October 28, 2026

Left-hand threads tighten counterclockwise and show up on spindles, pedals and turnbuckles. On the lathe you reverse the leadscrew so the carriage feeds toward the tailstock, start from a relief groove and swing the compound the other way.

Check the major diameter

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Step by step

Reverse the leadscrew: Use the tumbler or reverse lever.
Cut a relief groove: Groove at the chuck end so the tool can start there.
Set the compound: Swing it to 29.5° on the opposite side from right-hand.
Set tool height: On center, squared with a thread gage.
Cut toward the tailstock: Start in the groove and feed out.
Passes: Decreasing depth.
Check: Thread gage or nut.

Worked example

A 1/2-20 LH thread: P = 0.050 in, thread depth ≈ 0.6134 × 0.050 = 0.0307 in. With the compound at 29.5°, compound infeed ≈ 0.0307 ÷ cos 29.5° = 0.0353 in. Make the relief groove about 0.035 in deep and 0.100 in wide (two pitches) so the tool can start cleanly.

Using the thread dial

Works the same as for right-hand threads.

Common problems

Wrong-hand thread: leadscrew not reversed. Torn start: no relief groove.

Tips

Worked example: setting up a 1/2-13 left-hand thread

For a 1/2-13 UNC LH thread, set the gearbox for 13 TPI and reverse the leadscrew direction with the tumbler or reverse lever so the carriage moves toward the tailstock. Set the compound to 29.5° on the opposite side from right-hand threading. Single thread depth for a 13 TPI external thread is about 0.6134 × pitch = 0.6134 × 0.07692 = 0.0472 in; with the compound at 29.5° the compound dial travel is a little more, about 0.054 in. Confirm size with three wires or a thread mic at the end.

Start each pass from a relief groove at the headstock end, which gives the tool room to engage the half-nut. Make the groove at least as deep as the thread and about 1.5 pitches wide, roughly 0.12 in here. Check pitch diameter against a 1/2-13 UNC-2A LH ring gage or with wires: the 2A pitch diameter limits for 1/2-13 are 0.4485 to 0.4435 in. Because the thread is left-hand, keep the half-nut engaged and reverse the spindle between passes if the dial is not usable.

Left-hand threads are used on parts where rotation would loosen a right-hand thread, such as the left side of bench grinder arbors and some pedals and turnbuckles. Mark LH parts clearly so nobody forces a right-hand nut on.

FAQ

How do I cut LH threads?

Reverse the leadscrew and cut toward the tailstock.

Compound angle?

29.5° to the opposite side.

Need a relief groove?

Yes, at the start.

Thread dial?

Same as RH.

Check?

LH gage or nut.

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