Turn the center knob and watch three different motions happen at once: a fast input, a wobbling nine-lobe cycloidal disc, and a slow output rotating in the opposite direction.

This compact mechanical fidget transforms the mathematics of a roller-offset hypotrochoid into a functional 9:1 speed reducer. Ten stationary pins engage with nine cycloidal lobes, so nine complete turns of the input knob produce one full rotation of the output carrier in the opposite direction.

Features:
• Functional 9:1 cycloidal reduction
• Visible wobbling cycloidal motion
• Seven separately printed parts
• No screws, bearings or additional hardware
• No supports required
• Stable 68 mm geometric frame
• Press-fit assembly
• Designed for a 0.4 mm nozzle
• Educational STEM model and mechanical desk fidget

Recommended settings:
• 0.20 mm layer height
• 4 walls
• 15–20% infill
• PLA or PETG
• Print all parts in the supplied orientation
• Supports: No

Before fully pressing the retaining ring and center cap into place, perform a dry fit and make sure all rotating components move freely. Slightly clean up any elephant foot around the holes if necessary.

This model makes an abstract mathematical curve tangible: rotate the input, observe the cycloidal disc orbit and wobble, and watch the reduced reverse output.

If you print it, I would love to see your make! Feedback and ideas for future mathematical mechanisms are always welcome.

AI disclosure: The thumbnail was designed with AI assistance. The other pictures are original real life photos.