Lathe Change Gear Calculator

Inputs

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How the target thread is specified.
Target thread pitch. Used when the thread is metric.
mm
Target TPI (11.5 and similar fractional pipe pitches are fine). Used when the thread is imperial.
How the lathe leadscrew is specified.
Leadscrew pitch. Used when the leadscrew is metric.
mm
Leadscrew threads per inch. Used when the leadscrew is imperial. 8 TPI is the classic engine-lathe leadscrew.
Generic sets for the search — real lathes ship different gears; check the required ratio against yours.

Results

Default result
Edit inputs
Required ratio(i)
0.472441
Pass

60 driver → 127 driven (idler between, any size)

Spindle → leadscrew: thread advance ÷ leadscrew pitch. The universal answer for any gear set.

Also computed

Driver 1Pass60

On the spindle/stud side.

Driven 1Pass127

Driver 21

Simple train — second pair not needed.

1 means a simple train — one pair plus an idler.

Driven 21

Achieved pitch1.5mm

Pitch error0

Exact train — the thread tracks the spec perfectly.

Parts per million; 0 when the train is exact.

Method notes 4 notes
  • Ratio = thread advance per spindle rev ÷ leadscrew pitch; the train multiplies as driver₁·driver₂ / (driven₁·driven₂). Idlers fill center distance and set rotation direction without changing the ratio.
  • Cross-system jobs carry the exact factor 1 in = 25.4 mm = 127/5 — a 127-tooth gear makes them exact; without one the solver returns the closest train and its ppm error.
  • The searched set is generic (every 5 teeth, one of each). Real lathes ship different sets — hold the required ratio against your own gears, and check banjo clearance and thread HAND (idler count flips direction) on the machine.
  • Quick-change gearboxes do this internally; this screen is for change-gear lathes, gearbox gaps and special pitches.

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