How to 3D print a focus gear that actually fits

Start with the right number

Every good print starts with a good measurement. Measure the band of the focus ring where the gear will sit, not the filter thread on the front. If you have not done that yet, read how to measure a lens for a focus gear first. A perfect print of the wrong diameter is still the wrong gear.

If your lens is in the lens library, start from those numbers and check them with a tape anyway. Lenses change between versions, and sometimes between copies.

Material: it depends on the clamps

For a slip-on ring or a lens cap, PLA or PETG is all you need. Both print easily and hold their shape on a lens.

Clamp rings are different. The three clamps only hold if the ring is stiff, so I print them in ASA. It is much stiffer than PETG, which gives a little when you tighten the set screws. PLA works for clamp rings too.

Whatever you use: print the ring in the material you will actually shoot with. Every material shrinks differently.

My settings

• 0.4 mm nozzle
• 0.20 mm layers
• Slip-on rings and caps: your usual walls
• Clamp rings: 6–7 walls (perimeters), so the ring and its clamps are solid plastic, not infill
• Print flat, with the ring's axis pointing up: no supports, round teeth, no seam through the tooth flanks if you can place it on the inside

The generator already puts the ring in that orientation. You only need to slice it.

Shrinkage belongs in your slicer, not in the model

Every material shrinks as it cools, ASA more than most. With my printer and my ASA, I compensate roughly 3 percent in the filament profile of the slicer. Your number will be different: it depends on the material, the brand, the colour, the printer and the room.

Two rules keep this sane:

1. Keep the model at its true size. The STL from Focus Rings is always nominal, in millimetres. Do not scale it in the slicer to fix the fit.
2. Put shrink compensation in one place only, the filament profile, so you never apply it twice.

Print a test ring first

Before you print a full gear with clamps or hard stops, print the free slip-on ring at your diameter, or a short test ring. It costs a few minutes and a few grams of filament, and it answers the only question that matters: does it slide on with a little resistance and stay put?

• Too tight? Increase the opening by 0.1–0.2 mm and print again.
• Too loose? Decrease it by the same amount, or choose a clamp ring, which takes up the difference with three set screws.

Change the diameter in the generator, not the scale in the slicer. The teeth stay correct that way.

Check the dimensions you printed

Every ring from the generator has its opening, outside diameter and width engraved inside. When you have three test rings on the desk, you will be glad you can tell them apart.

Measure the printed opening with calipers. If it is consistently smaller than the number engraved, that is your printer's real offset, and it will be the same next time.

Then mount it and turn it end to end

Put the ring on the lens and turn focus from close focus to infinity with the motor or follow focus attached. Listen for skipping teeth, watch for the gear riding up on the lens, and check that nothing rubs on the aperture ring or the lens hood.

If it all turns smoothly, you are done. If it does not, it is almost always one of three things: the diameter, the width, or a motor that is not centred on the gear.

Make a free focus ring for your lens · No account, one STL, dimensions engraved inside.