Knob caps for an Etch A Sketch: LEGO gear and band versions

3D-printed caps that slide over the 28 mm knobs of a classic Etch A Sketch, so a LEGO motor can turn them. I made them for a LEGO Mindstorms EV3 drawing machine. The 40-tooth gear cap is the one that worked; the lipped cap is for anyone who wants to try a rubber-band drive. Not affiliated with or endorsed by the makers of Etch A Sketch or LEGO. Files - knob-cap-gear40T.stl: 40-tooth, module 1, 20° pressure angle, so it meshes with standard LEGO Technic gears. This is the one that worked in my build. - knob-cap-lipped-band.stl: a cap with raised lips to hold a rubber band. Bands kept slipping in my tests, so treat this one as a challenge (see below).

Fit - Knob: 28 mm diameter, 14 mm tall (measure yours first). - Bore: 28.3 mm (0.3 mm clearance), which was a snug push fit on the first print. The cap sits on the face of the Etch A Sketch, not on top of the knob.

Print settings (what worked for me) - Gear cap: print gear side down, no supports. Elephant's foot compensation of about 0.15 mm keeps the teeth clean. - Lipped cap: print with the closed top on the bed.

Gear spacing (axle to axle, from the cap to a LEGO gear) - 8-tooth: 24 mm (3 studs) - 24-tooth: 32 mm (4 studs) - 40-tooth: 40 mm (5 studs)

A gear pushes the motor away from the knob as it turns, so brace the motor well. My frame flexed until I reinforced it. The challenge I couldn't stop rubber bands from slipping, but if you can, show me. The two-tick slip test code is on GitHub: https://github.com/bricksandbrains/ev3-etch-a-sketch-plotter. Run it on your build, and if your ticks line up, post it in the video's comments: https://youtu.be/OWJuXen_HOY.