An extendable gear play set, with more gears than you can shake a stick at. Suitable for education and play. The latest version with any minor updates (and now also an extendable linear rack) can be found here. Do note that there is now also a version for a younger audience.

The playing field is a perforated plate with holes spaced on a 15mm rectangular grid. By printing multiple boards, and linking them from below with the ‘Joiner’ pieces, the joy can be extended without limit, in two dimensions. The gears themselves have sturdy M6 threaded axles, which turn freely in the holes of the board. However, each gear also has a threaded hole on top, which facilitates stacking to create compound gears, and hence the attainment of ridiculous gear ratios

The third dimension is therefore also conquered, though not without limit as in the case of the other two. You can stack as high as you like, but you are unlikely to like it very high, because the play on the bottom gear limits the practical height to maybe three levels. Which is still plenty - rarely will you need to go above two layers.

Most gears mesh with each other. There are also three ring gears: 36/48, 48/60, and 56/80, with the first number denoting the internal tooth count, and the second the external. These enable planetary gears to be constructed.

And then there is the delightful 6-slot Geneva wheel, which requires both parts (GenevaA6 and GenevaB6); they of course only speak to each other. Unlike the other, more conventional gears, these cannot be back-driven; part B (with the pin) turns part A (with the slots), and never the other way round. The resultant punctuated movement is used in mechanical movie projectors to advance film frames, and the mechanism also found application in watches and clocks, hence the name, with its Swiss flavour.

Printed in PLA, at 0.2mm and 15% infill with a 0.4mm nozzle on my trusty Prusa Mk4. All without supports, except GenevaB6 - sorry about that. You do not need supports in the threaded holes - the roof of the hole is tapered/slanted.

Because a standard M6 x 1mm thread is used, you can clean the printed threads using a metal M6 bolt and nut, although I have had uniform success by simply screwing the gears into each other as they come off the printer bed. You want a snug fit - just be careful not to cross-thread when screwing them in the first time. If it really is too tight (or loose) you may consider playing with the XY-size compensation to the tune of a few tens of microns at a time. Print a few of the spacers first, and adjust the fit if needed, until you are comfortable, then use the same settings for all the gears. I did not need to do this.

Print lots of the smaller gears (12, 16, 20 teeth), and maybe a handful of the spacers. The larger gears take up a lot of space (and time, and filament), so maybe you only want one or two of each.

I also include a crank and handle (which click into the crank), though I would not bother printing these if I were you - you can turn the gears by hand, or screw a spacer into the top of one or more gears and spin them that way.