How to Design a Gear in CAD (Spur Gear Basics)

Designing a gear sounds like advanced engineering, but a basic spur gear is defined by just a handful of numbers. Get those right and the gear will mesh and turn. Here is what each number means, how to make two gears work together, what to watch for when 3D printing, and how to generate a gear without drawing every tooth by hand.

The few numbers that define a gear

A spur gear — a flat gear with straight teeth, the most common kind — is described by three related values. Pick two and the third is fixed:

Module (or DP)

The tooth size. Module is used in metric (mm per tooth of pitch diameter); diametral pitch (DP) is the imperial equivalent. Two gears must share the same module to mesh.

Number of teeth

How many teeth around the gear. This sets the gear ratio between two meshing gears (for example, 20 teeth driving 40 teeth = 2:1 reduction).

Pitch diameter

The effective diameter where the teeth engage. For a metric gear, pitch diameter = module × number of teeth. A module-2 gear with 20 teeth has a 40 mm pitch diameter.

Pressure angle

The angle of the tooth face, almost always 20° for standard gears. Leave it at 20° unless you have a specific reason not to — both gears must match.

The one rule for meshing

Two spur gears mesh if they share the same module and the same pressure angle. The number of teeth can differ — that is how you change the ratio. The distance between their centres is the sum of the two pitch radii (half of each pitch diameter).

A worked example: a 2:1 gear pair

Say you want to halve a motor's speed and double its torque. You need a 2:1 ratio. Choose a module and the tooth counts:

Module: 1.5 (a reasonable size for a desktop 3D printer).
Driving (pinion) gear: 20 teeth → pitch diameter 30 mm.
Driven gear: 40 teeth → pitch diameter 60 mm.
Centre distance: (30 + 60) / 2 = 45 mm between the two shafts.

Add a bore for the shaft (and a flat or set-screw boss if you need to lock it), pick a gear thickness (face width) of roughly 3–5× the module, and you have a working pair.

Watch the tooth count on small gears

Below about 17 teeth at a 20° pressure angle, standard gear teeth begin to undercut — the base of the tooth gets thinned and weakened. For a small pinion, keep the tooth count at 17 or above when you can, or accept that a very small gear will be less robust.

3D printing gears: practical tips

Add clearance. Printed teeth are never perfect. A little backlash (loosening the fit slightly) keeps gears from binding. If they jam, scale one gear down a hair or increase the centre distance by a few tenths of a millimetre.
Print flat. Lay the gear on the bed so the teeth are in the strong XY plane, not built up layer-by-layer along the tooth.
Mind the bore fit. Holes print undersized. See 3D printed part tolerances for the clearance values that make a shaft fit the first time.
Use enough perimeters. Gear teeth take load — more wall perimeters and higher infill make them last.

The tooth profile itself is an involute curve — mathematically precise and tedious to draw by hand. That is exactly the kind of thing worth letting software generate.

Generate a gear in your browser

Instead of plotting an involute curve, describe the gear you need to PartWork.ai and get editable solid geometry back. For example: “a module 1.5 spur gear with 20 teeth, a 5 mm bore, and 6 mm thick.” Then iterate — “make it 40 teeth” for the matching driven gear.

Start with 2 free AI generations — no card required

Open the studio and describe your gear. Export STL for printing or STEP for machining. More credits: 100 for $4.99 (~5¢ each).