What is gear module, and how does it relate to diametral pitch?

Module is metric tooth size in millimetres: module = pitch diameter ÷ number of teeth. Diametral pitch (DP) is the imperial equivalent. Convert with module = 25.4 ÷ diametral pitch — so a 12 DP gear is roughly module 2.12.

Which gears will mesh together?

Two gears mesh when they share the same module and the same pressure angle. Tooth counts can differ freely — that difference is the gear ratio. A module 2 20-tooth gear meshes with a module 2 40-tooth gear for a 2:1 reduction.

What is the minimum number of teeth for a spur gear?

Around 12 teeth for a standard 20° pressure angle. Below that the teeth undercut at the root, which weakens them and degrades the involute contact.

What file formats can I download the gear in?

STEP for CAD and CNC, STL for 3D printing, or 3MF. STEP keeps exact involute curves, which matters if you plan to machine the gear or edit it in other CAD software.

Gear Generator

Generate precise involute gears — spur, helical, herringbone, bevel, internal ring, and rack — by describing what you need in plain English. Set the module, tooth count, face width, bore, and pressure angle, then download a real STEP, STL, or 3MF file. No CAD experience needed.

Make a gear now: Generate a 20-tooth spur gear — opens the studio with that description already filled in. Change any number before you send it, then inspect and export the solid model.

Gear types you can generate

Spur

Straight teeth parallel to the axis. The default choice — simple, strong, easy to print or machine.

Helical

Teeth cut at an angle so contact is gradual. Quieter and smoother than spur, at the cost of axial thrust.

Herringbone

Two opposing helices in a V. Keeps the smoothness of helical while cancelling the axial thrust.

Bevel

Conical gears for shafts that intersect — typically turning drive through 90°.

Internal ring

Teeth on the inside of the rim. The outer element of a planetary gearset.

Rack

A flat, linear gear. Pair it with a spur pinion to turn rotation into linear travel.

The parameters that define a gear

Parameter	What it means	Typical values
Module	Tooth size in mm. Sets how big each tooth is, and which gears will mesh.	0.5, 1.0, 1.5, 2.0, 2.5, 3.0
Teeth	Tooth count. With module, this fixes the gear’s diameter, and between two gears it sets the ratio.	12–80 (min ~12 for spur)
Face width	How thick the gear is along the shaft. Wider spreads the load over more tooth.	3×–5× the module
Bore	The centre hole for the shaft.	Match your shaft (3, 5, 6, 8 mm…)
Pressure angle	The angle of tooth contact. Must match between meshing gears.	20° (standard), 14.5° (legacy)

The three formulas worth knowing

Pitch diameter

module × teeth. Module 2 with 20 teeth → a 40 mm pitch diameter. This is the circle the gears actually roll on.

Outside diameter

≈ module × (teeth + 2). The same gear measures about 44 mm across the tips — useful for checking clearance.

Imperial → metric

module = 25.4 ÷ diametral pitch. A 12 DP gear is module 2.12. Working from an imperial drawing? Convert first.

Centre distance for a pair: module × (teeth₁ + teeth₂) ÷ 2. A module 2 pair of 20 and 40 teeth sits 60 mm apart, shaft to shaft.

Worked example: a 2:1 gear pair

Say you need a 2:1 reduction on 8 mm shafts. Keep the module identical — that is the rule that makes gears mesh — and change only the tooth count:

	Pinion (driver)	Gear (driven)
Module	2	2 (must match)
Teeth	20	40
Pitch diameter	40 mm	80 mm
Bore	8 mm	8 mm

Ratio 40 ÷ 20 = 2:1. Centre distance = 2 × (20 + 40) ÷ 2 = 60 mm. Ask for them one at a time:

Generate the 20-tooth pinion
Generate the 40-tooth gear

Tips for 3D-printed gears

Do not go too fine. Below about module 1, FDM printers struggle to resolve the tooth profile and the gear binds. Module 1.5–2 prints reliably on most machines.
Add clearance. Printed gears come out slightly oversized. If a pair binds, reprint the pinion a touch smaller or open the centre distance by a few tenths — see 3D printed part tolerances.
Print flat. Gears printed with the face on the bed get layer lines running around the tooth instead of across it, which is much stronger.
Helical needs support thought. Angled teeth overhang; herringbone doubly so. Spur is the safe printed default.
Export STEP for machining. STL approximates the involute curve with flat triangles. STEP keeps it exact — see STL vs STEP.

Common questions

What is module, and how does it relate to diametral pitch?

Module is metric tooth size in millimetres: pitch diameter divided by tooth count. Diametral pitch is the imperial equivalent, and the two convert with module = 25.4 ÷ DP. A 12 DP gear is about module 2.12.

Which gears will mesh?

Two gears mesh when they share the same module and the same pressure angle. Tooth counts are free to differ — that difference is the ratio.

What is the minimum tooth count?

About 12 for a standard 20° pressure angle. Fewer than that and the teeth undercut at the root, weakening them and spoiling the involute contact.

Which file format should I download?

STEP for CAD or CNC (exact curves), STL or 3MF for 3D printing. More on exporting files.

Generate your gear

Tell PartWork.ai the gear you need — type, module, teeth, width, bore — and it builds real involute geometry you can inspect, adjust in follow-up messages, and export. If you are unsure of a number, say what the gear is for and start from there.

Start from a ready-made description and edit the numbers: spur · helical · bevel · rack · internal ring · herringbone — or open a blank studio. More on creating parts and exporting files.