Spur Gear Generator
Involute spur gears from module and tooth count — download SVG or DXF.
Tooth size. Two gears mesh only if their modules match.
Tooth thickness is reduced by half this on each flank.
0 to skip. Used for centre distance and ratio.
Geometry for the pitch circle and the flanks. It does not check bending or contact stress; for a loaded drive, size the teeth against a standard such as AGMA 2001 or ISO 6336.
How it works
Gear teeth are involute curves, not arcs. An involute is the path traced by the end of a string unwound from a circle — the base circle — and the reason it is used is that two involute flanks transmit motion at a constant velocity ratio even if the centre distance is slightly wrong. No other simple curve does that, which is why every mechanical gear you have ever handled has this profile.
The geometry here is standard ISO: pitch diameter is module x teeth, the base circle is the pitch circle times the cosine of the pressure angle, addendum is one module and dedendum 1.25, giving the root clearance that stops the tip of one tooth bottoming out in the root of the other.
Below about 17 teeth at 20 degrees, the cutter removes material from the flank near the base circle — undercutting — which weakens the tooth and shortens the contact path. The generator warns when you cross that line.
Common questions
What is module?
Pitch diameter divided by the number of teeth — the size of one tooth. It is the metric standard; the imperial equivalent is diametral pitch, which is teeth per inch of pitch diameter and runs the other way round. Two gears mesh only if their module and pressure angle both match.
How far apart do I mount them?
Half the sum of the two pitch diameters. For a 24-tooth and a 12-tooth gear at module 2, that is (48 + 24) / 2 = 36 mm. The generator shows this when you enter the partner's tooth count.
Why does it warn below 17 teeth?
Undercutting. At 20 degrees pressure angle, a generating cutter starts eating into the flank below the base circle at 17 teeth, thinning the root. It still works — small gears are made every day — but expect reduced strength, and profile shift is the proper fix.
SVG or DXF?
SVG for laser cutting and for dropping into a drawing. DXF for CAD and CNC — it is written as a polyline in millimetres at 1:1 scale, so it imports at the right size without scaling.