How to make 3D-printed gears that actually mesh

Module: the size of the tooth

The module (m) is the pitch diameter divided by the number of teeth. In practice it is tooth size: module 1 means small teeth, module 3 big ones. Two gears mesh only if they share the same module.

For FDM printing, below module 1 the teeth are too small for a 0.4 mm nozzle. Module 1.5–3 is the safe zone for most parts.

The formulas

• Pitch diameter: d = m × z (z = number of teeth).
• Outside diameter: m × (z + 2).
• Centre distance: a = m × (z₁ + z₂) / 2.
• Gear ratio: i = z₂ / z₁. The small gear turns i times faster than the big one.

A full example

You want to reduce speed by 3. Choose module 2, a 12-tooth pinion and a 36-tooth gear.

• d₁ = 2 × 12 = 24 mm, d₂ = 2 × 36 = 72 mm.
• Centre distance: 2 × (12 + 36) / 2 = 48 mm.
• Ratio: 36 / 12 = 3.
• For printing backlash, move the shafts 0.1–0.2 mm further apart: 48.15 mm.

What jams a printed gear train

• Shafts too close — with no backlash the plastic rubs and melts. Add 0.1–0.2 mm to the centre distance.
• Elephant's foot on the first layer: chamfer the bottom edge of the teeth by 0.3–0.5 mm.
• Shaft hole too tight: see tolerances — for free rotation, 0.4–0.5 mm clearance per side.
• Too thin: a gear roughly 5–10 times thicker than its module will not flex under load.

In GrafitGo

The gear in the mechanical editor takes teeth, module, thickness and bore as parameters. Place two gears with the same module at the distance from the formula; the “Gear train” template shows one ready-made.

The tooth profile is simplified (trapezoidal), not involute. It is fine for slow mechanisms, demonstrations, toys and light reducers. For heavily loaded or quiet transmissions, use an involute profile generator.