Lathes · Explorer · Simulator

Lathe change gear explorer

Search simple and compound change-gear trains for a manual lathe, compare actual thread pitch against the target, and see when metric-imperial conversion needs a proper transposing gear instead of wishful thinking.

Machine and thread target

Target search intent: lathe change gear calculator, metric thread on imperial lathe, imperial thread on metric lathe, and compound change gear setup.

Leadscrew system

Leadscrew threads per inch (TPI)

Target thread system

Target thread pitch (mm)

Search mode

Error check length

Available gears

Enter tooth counts separated by spaces or commas. Results assume one copy of each gear unless you list a tooth count more than once in real life.

Why this is worth using

It searches your actual gear pile instead of assuming a specific factory chart.
It shows both exact and near matches, with drift over length so approximations are easier to judge.
It covers simple trains and compound trains, which is where many threading jobs become possible.
It makes the metric-imperial translation problem explicit instead of quietly burying it.

Scope note: the page models change-gear ratios only. It does not check banjo fit, stud spacing, tumbler direction, or gearbox settings on specific machines.

Read the launch note for this lathe tool.

Required ratio

Best simple trains

Best compound trains

Translation and setup notes

How the ratio works

The carriage advances by the leadscrew pitch times the leadscrew revs per spindle rev.
For a simple train, that rev ratio is spindle-driver teeth divided by leadscrew-driven teeth.
For a compound train, multiply both stages: (A / B) × (C / D).
Idlers reverse direction and help with spacing, but they do not change the ratio.

Common lathe realities

Crossing between metric and imperial often needs a 127-tooth transposing gear for an exact answer.
A close approximation can still be unusable if thread error accumulates over a long engagement.
Gear ratio is only part of the job. You still need the train to fit physically on the banjo.
If your machine has a quick-change gearbox, this tool is still useful for oddball pitches and add-on gear trains.