A browser-based tool that calculates gear-train combinations for screwcutting on a manual lathe.
The calculator searches through all permutations of a user-supplied gear set and finds combinations that produce a specified thread pitch (given in threads-per-inch or millimetres). Results are ranked by closeness to the target pitch and displayed alongside a schematic diagram showing how the gears are arranged on the lathe.
Open javascript/changegears.html in a modern web browser (no server required).
- Gears – Enter the tooth counts of all gears available to you as a comma-separated list.
- Leadscrew – Enter the leadscrew pitch and select whether the value is in TPI or mm.
- Target – Enter the thread pitch you want to cut and select TPI or mm.
- Validation – Choose how strictly impossible gear combinations are filtered out.
- Click Search for combinations. The dropdown lists every valid gear train found, sorted by accuracy. Selecting an entry shows the exact pitch produced, the error, and a diagram of the gear layout.
At the top of javascript/changegears.js there are two variables you can adjust to match your lathe:
| Variable | Default | Description |
|---|---|---|
ILX |
38 | Horizontal distance (mm) between input and leadscrew shafts |
ILY |
70 | Vertical distance (mm) between input and leadscrew shafts |
These values are used both to draw the on-screen gear diagram and to filter out combinations that physically cannot fit on your lathe.
GPLv3 – see LICENSE.
Originally created by Carl Williams (2011) at https://lathenovice.wordpress.com/. Forked and updated by Jeff Pedlow from 2020 onwards.