Generator 35 is gears
29 August 2026
The site has a gear generator now. Type a module, a tooth count, a pressure angle and a helix angle, and it hands you an STL of a gear that actually meshes, plus an optional pinion cut to run against it. It is generator number 35, and it is the one people have been asking for the longest.
I put it off for a reason. A box is walls and a floor. A gear tooth is a specific curve. Cheat on it and the print looks like a gear but grinds like a coffee mill.
The curve is not optional
The working face of a tooth is an involute, the path a string end traces while unwinding from a cylinder. That cylinder is the base circle, and its size falls out of two numbers you pick, the pitch diameter and the pressure angle. The magic property is that two involutes rolling on each other transmit rotation at a constant ratio through the whole contact. Teeth drawn as triangles or sine bumps do not have that property. They speed up and slow down a little on every tooth, which you hear as noise and feel as wear.
So the generator samples the true involute. Fourteen points per flank, denser near the tip where the curvature moves fastest, then the mirror, the tip arc, the root arc, and around the wheel it goes. A 24 tooth gear comes out as an outline of a few hundred points, and every one of them sits on the curve it should sit on.
Helix, and the tooth that is two teeth
Straight-cut gears are the default. Set a helix angle and the teeth wind around the wheel instead, which makes them engage gradually, front edge first. Quieter, smoother, and printable, because to a printer a helical gear is just a stack of layers that each rotate a fraction of a degree.
A single helical gear pushes itself sideways under load, so the generator also does herringbone. The tooth climbs one way for half the width and mirrors back for the other half, and the side loads cancel. Chevron teeth also happen to print beautifully with no supports. The matching pinion always gets the opposite hand, because two gears of the same hand will not mesh, a fact I re-checked three times against a printed pair before trusting my own signs.
What the numbers guard
The generator refuses configurations that cannot work, and it tells you the way out. Ask for a 40 mm bore in a 52 mm gear and it answers that only 1.5 mm of rim would remain under the teeth, and that 38.2 mm is the ceiling for this wheel. Below 17 teeth at a 20 degree pressure angle it warns about undercut and suggests 25 degrees. There is backlash control, 0.10 mm by default, a keyway, and a hub.
One honest limitation surfaced during testing. The bore twists together with the teeth, so on a helical gear the keyway comes out slightly helical through the width. For a 10 mm wide gear it is a fraction of a degree and a printed key does not care, but the generator says it out loud and points you at spur or herringbone if you want the slot dead straight.
The launch that 404ed
I ran the mesh checks and every part came back watertight on the first try. Felt great for about a minute. Then I shipped it and the public route answered 404 while every internal check kept insisting the backend was healthy. The router reads its route table at startup, and adding a 35th route to the config file does not make a running router re-read anything. One reload later the gear generator existed for the rest of the world too, and the deploy script now reloads the router every time so route number 36 will not repeat this.
If you print a pair, the README in the ZIP has the center distance to drill your mounting holes at, down to the millimeter.
Next on the wishlist are racks and a planetary set. If you need one of those sooner, the report button on the generator page reaches me directly.