Change gear calculator

Enter your leadscrew, the thread you want to cut and the gears you actually own. The calculator searches every simple (driver → driven) and compound (A/B × C/D) train and ranks them by how close they come.

How it works

Your lathe’s change gears link the spindle to the leadscrew. Pick the right ones and the carriage moves exactly one thread pitch per turn. The top row of the results is the best train to fit. New to this? Read the thread-cutting guide.

The gear train must turn the leadscrew so the carriage moves one thread pitch per spindle revolution. Ratio = target pitch ÷ (leadscrew pitch × fixed ratio). A simple train is driver/driven. A compound train is (A/B) × (C/D), where A drives B, and C is keyed to B and drives D. Idlers only change direction, so they are ignored.

Worked example: to cut 1.25 mm on an 8 TPI leadscrew (3.175 mm), the ratio is 1.25 / 3.175 = 0.3937 = 50/127. The 127-tooth gear is exact because 127 × 0.2 = 25.4. A good train is 20/40 × 100/127 (= 0.3937, error 0%).

The Myford preset is the standard ML7/Super 7 set as listed on lathes.co.uk (2×20, 25, 30, 35, 38, 40, 45, 50, 55, 60, 65, 70, 75T) plus the two 21T wheels used for metric pitches. Always enter what is actually in your drawer, and check that the train physically fits your banjo.

Safety: isolate the machine before changing gears. Fit the gear guard before you start the machine.

Spotted a mistake? Report an error.