How to Use LatheGearPro

Introduction

LatheGearPro calculates change gear arrangements for cutting threads on lathes. It finds all valid combinations of gears that produce the ratio needed to cut your desired thread pitch.

Understanding the Inputs

Leadscrew TPI / Pitch

The threads per inch (or pitch in mm) of your lathe's leadscrew. This is typically stamped on the lathe or found in its manual. Common values are 8 TPI or 4 TPI. Use the inch/metric toggle to switch between TPI and millimetre pitch.

Desired Thread TPI / Pitch

The threads per inch (or pitch in mm) you want to cut on your workpiece. For example, 20 TPI for a 1/4-20 bolt, or 1.5 mm for an M10x1.5 thread.

Available Gears

Enter the tooth counts of all change gears you have available, separated by commas. Include duplicates if you have multiple gears with the same tooth count. For example: 22, 22, 38, 42, 48, 48, 127.

They can be entered in any order, but storing them from smallest to biggest makes it easiest to keep track and confirm your list is correct.

Error Tolerance

The maximum acceptable percentage deviation from the exact ratio. Use 0 for precision work. Small values (0.1–1%) are acceptable for most applications. If you just want to try the app, use a higher percentage such as 5.

Understanding the Results

Each result shows a valid gear combination:

• Drivers: Gears that connect to the spindle side (driving gears)
• Drivens: Gears that connect to the leadscrew side (driven gears)

The overall ratio is (product of drivers) ÷ (product of drivens). Install drivers on the spindle side and drivens on the leadscrew side, with intermediate gears (idlers) as needed.

Results are colour-coded by accuracy:

• Green — Exact match (zero error)
• Orange — Very close (less than 0.2% error)
• Red — Within your tolerance but with noticeable error

Metric and Imperial

To cut metric threads on an imperial lathe (or vice versa), you'll normally need a 127-tooth gear or approximation gears (like 63T or paired 21T gears). The app handles the conversion automatically when you toggle between TPI and mm pitch values.

Lathe Profiles

You can save multiple lathe profiles, each with its own leadscrew specification, gear list, and default settings. This is useful if you have access to more than one lathe, or if you want to keep separate gear sets for different jobs.

Use the profile selector on the home screen to switch between saved profiles.

Settings

The settings screen lets you configure default values that are pre-filled when you open the app:

• Default Leadscrew: Your most common leadscrew TPI or pitch
• Default Gears: Your standard set of available change gears
• Default Error Tolerance: Your preferred error threshold
• Calculation Depth: Maximum number of gear pairs (1–4). Higher values find more combinations but take longer to compute