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How to Cut Multiple-Start Threads on a Lathe

Guide · Updated October 28, 2026

A multiple-start thread has two or more thread helixes interleaved, so the nut advances faster per turn without a deeper, weaker thread. The key idea is lead versus pitch: lead is how far the nut moves per turn, pitch is the distance between adjacent threads. Lead = pitch × number of starts.

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Step by step

Work out lead and pitch: Lead = pitch × starts. Thread depth is based on pitch, not lead.
Set the gearbox to the lead: The lathe must cut the lead.
Grind the tool for the helix: The helix angle uses the lead, so it is steeper than a single-start thread of the same pitch.
Cut the first start to full depth: Use the compound as normal, recording the dial readings.
Index to the next start: Either rotate the work 360° ÷ starts (slotted driver plate or indexing the chuck) or move the tool one pitch along the axis with the compound set parallel to the ways.
Cut the second start: Use the same depth settings.
Check: Measure pitch with a gage and fit the nut.

Worked example

A 1 in diameter, 2-start thread with 0.100 in pitch: lead = 0.100 × 2 = 0.200 in, so set the gearbox to 5 TPI (1 ÷ 0.200). Thread depth for a 60° form ≈ 0.6134 × 0.100 = 0.0613 in, set from the pitch. Helix at the 0.94 in pitch diameter: tan⁻¹(0.200 ÷ (π × 0.94)) = 3.9°, versus 1.9° for a single start of the same pitch. To index by moving the tool, set the compound parallel to the axis and advance it exactly 0.100 in; to index the work, rotate it 180°.

Indexing accuracy

An error in indexing makes the two starts unequal, so the nut binds. Moving the compound 0.100 in on a 0.001 in dial is easy to get right; rotating work in a driver plate depends on the slot accuracy.

Common problems

A nut that binds means unequal starts. Using pitch for the gearbox instead of lead gives a single-start thread. Rubbing flanks mean the tool helix clearance was ground for the pitch, not the lead.

Tips

Basics: cut threads on a lathe.
Acme: Acme threads are often multi-start.
Pitch: thread pitch gages.

FAQ

What is the difference between lead and pitch?

Pitch is the distance between adjacent threads; lead is how far the nut moves per turn. Lead = pitch × starts.

Which do I set on the gearbox?

The lead.

How deep do I cut?

Based on the pitch, the same as a single-start thread of that pitch.

How do I index between starts?

Rotate the work 360° ÷ starts or move the tool one pitch with the compound parallel to the axis.

Why use multiple starts?

Faster travel per turn without a deeper thread.

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