Module 0.8, explained without the engineering degree

What module means

Module is the size of a gear tooth. For a straight-toothed gear it links three things:

• Pitch diameter = module × number of teeth. A 100-tooth gear at module 0.8 has an 80 mm pitch diameter.
• Outside diameter (tooth tips) = module × (teeth + 2). The same gear measures 81.6 mm across the tips.
• Tooth spacing around the circle is module × π, about 2.51 mm at module 0.8.

Two gears mesh when they share the same module and tooth shape. The tooth count can differ as much as you like. That is why a tiny motor pinion drives a big lens gear.

Why your outside diameter jumps in steps

A gear needs a whole number of teeth. You cannot have 100.4 of them. So when you ask for a particular outside diameter, the generator rounds to the nearest whole tooth, and the real diameter moves in steps of 0.8 mm.

That is not a flaw, it is geometry. The generator shows you the diameter it actually built and engraves it inside the ring.

One outside diameter for all your lenses

Here is a trick worth knowing: if all your lenses get gears with the same outside diameter, your follow focus or motor sits in the same position on every lens. No sliding the motor bracket between takes.

In the generator, choose Outside Ø instead of Auto and type the same number for every lens. The opening changes per lens; the teeth line up the same way.

Imperial "32 pitch"

Some older follow focus gear is specified in imperial "32 pitch". That works out to about 0.794 module: close to 0.8, and usually close enough to mesh, but not identical. If you mix systems, check that the teeth run smoothly over a full turn.

Why tooth count matters for hard stops

For a plain focus gear, any module 0.8 motor works. For hard stops, the motor's gear becomes part of the geometry: the stops have to meet the motor's teeth at exactly the ends of your focus travel. A 30-tooth pinion and a 40-tooth pinion are different sizes, so the stops sit in different places.

That is why the hard-stop generator asks for your motor model, or its tooth count and gear width for a custom motor. In the generator you find:

• Reference tooth counts: 26, 30, 33, 39 and 40
• Named motors: ARRI cforce mini, DJI Focus Pro and RS focus motors, PD Movie, Tilta Nucleus

If you are not sure about your motor's tooth count, count it. It takes a minute and saves a print.

Build a clamp ring or hard stops for your motor · Start from a lens in the lens library.