Tri-Gear Fidget Spinner — Print-in-Place Herringbone Gears, No Bearing, No Supports, No Assembly

Four gears, one print, zero assembly. Flick any one of them and all four turn.

What makes it work

No bearing, no hardware. Every axle, every gear and every hex cap is printed in place, in one go. Nothing to buy, nothing to press-fit, nothing to glue. It comes off the plate as a finished fidget.

Herringbone teeth. Not just for looks: the V-shaped teeth cage each gear against its neighbours axially, so the train stays aligned and the mesh feels smooth and continuous instead of clicking.

Hex caps that actually do something. The cap on each axle is the axial retention — it overhangs the gear hub so nothing can lift off. Its chamfer is hexagonal all the way down, so it reads as one clean cap rather than a hexagon stuck on a cylinder.

Verified, not hoped

The gear profiles are proper involutes (25° pressure angle) generated by the included Python scripts, which also simulate the full rotation and measure the minimum gap between tooth flanks at every step — not just in the assembly position.

• Minimum backlash, any position: 0.36 mm

• Gears: sun 10 : planets 14, module 1.5 (55 mm) / 2.2 (81 mm)

• First layer of every gear: 100 % supported on the frame boss below it

• Air gap under each gear: exactly one layer

That last pair of rows is the whole trick, and it is where this kind of model usually fails. Read on if you want to know why.

Print settings — please read, it is short

• Layer height 0.2 mm. Not 0.15, not 0.3. The gap between each gear and the boss under it is one layer. Print thicker and the gears fuse to the frame; print thinner and you get two layers of air with nothing to sit on.

• No supports. No brim.

• 3 walls, 10-15 % infill. PLA is perfect.

• 55 mm version: ~14 g, ~35 min. 81 mm version: ~40 g, ~1 h 20.

Do not scale this model. Scaling shrinks the clearances too: at 75 % the one-layer gap becomes 0.15 mm and the 0.36 mm backlash becomes 0.27 mm, and the whole thing comes out welded solid. That is why two sizes are included — and if you want a third, regenerate it with the scripts (below), which keep the clearances absolute while changing the tooth size.

First spin: hold the frame and turn one gear firmly. Each gear rests on about 40 mm² of one-layer contact, which breaks free with a twist. After a few turns it runs smooth.

Three colours, no assembly

Insert a filament change in your slicer (right-click the layer in preview):

• Frame done → gears start: z = 2.6 mm (55 mm version) / z = 3.0 mm (81 mm version)

• Gears done → hex caps start: z = 15.5 mm (55 mm version) / z = 20.6 mm (81 mm version)

Frame in one colour, gears in another, caps in a third.

Remix it

generator.py writes the OpenSCAD file and gear_profiles.py computes the involutes. Change the tooth counts or the module, run it, and it verifies the mesh through a full rotation before writing anything — it will tell you if your variant binds, before you waste filament. The clearances (backlash, axle fit, the one-layer Z gap) are absolute values in mm, deliberately independent of the tooth size.

No non-printed parts. Nothing at all.