A small planetary gears system with a ratio of 4:1 per stage, as many stages as wanted, small, open enclosure to see the gears, and hardware free.

The gears are designed with a circular pitch of 2.5mm and they are also slightly beveled at the tips of the teeth to avoid the need of cleaning the excess of material from the firsts layers.

The OpenSCAD is not parametric, there's only parameters for clearance (set to 0.2), backslash (set to 0.1), pressure angle (set to 22.5), tolerances between nuts and bolts (set to 0.4) and length of the bolts, sometimes referred as rods. Any other change will not work.