Model the pair
Dial in tooth counts, circular pitch, pressure angle, compound shafts, ring gears, racks, or a dividing plate. Rotate or animate the pair to catch a bad mesh early, and undo or redo your edits.
Field note 01 / workshop build
ExactCog
A gear generator for people who would rather make the part than shop for it.
One HTML file. No install. No account. No internet once it is downloaded.
Why it exists
The useful bit is not the gear. It is the confidence before the cut.
Most gear calculators stop at a pretty circle. This one lets you sanity-check the mesh, fit it to the way you cut, print a measured template, and take the design to the shop as a file you own.
What it does now
Start from a working set
Seven examples open ready to cut, from a 3:1 pair to a clock train. The planetary builder adds a sun, planets, and ring in one click, and warns first if the planets cannot be spaced evenly. Three short guided tours walk you through a paper template, a laser or CNC cut, and a 3D print.
Millimetres or inches
Switch to inches and circular pitch becomes diametral pitch, the way US catalogues size teeth. Common bore sizes are named, and cut sheets add a 4 in calibration bar.
Fit it to the way you cut
Fit clearance presets for paper or scroll saw, laser, FDM, and resin set the backlash. Pick a standard bore size and make it round, D-flat for a motor shaft, or with a keyway.
Measure your printer first
"STL: fit test" exports one small print: a gear pair and a strip of test holes. Turn the pair by hand, find the hole that fits, and set hole compensation to match.
Print, save, export
Calibrate against the 100 mm bar and print at 100%; big gears tile across sheets with crop marks. Save a .cog4 project, or export SVG, DXF, PNG, or STL.
A clear boundary
Free means useful, not watered down.
Everything in it today is free and stays free. A paid add-on is planned for new features that belong in a deeper workshop tool, such as sprockets, ratchets, and timing pulleys. Nothing free today moves behind it. No fake launch date, no bait-and-switch.
Start with the tool
Make one before you bookmark it.
Download the self-contained build, open it in a modern browser, and keep the file wherever your projects live. Or install the live app from Edge or Chrome: it runs in its own window, works offline, opens .cog4 files on double-click, and waits for you to say when to update.
Open the live app
Download the standalone HTML
View ExactCog on GitHub →
Coming to the Microsoft Store.