About this project
Parametric external Geneva drive ("Maltese cross"), the classic intermittent-motion mechanism that turns continuous crank rotation into stepped indexing. Three separately printed parts: a base with two axles, the driver (crank disc, drive pin, relieved locking disc and knob) and an N-slot wheel.
Geometry
Everything is derived from the slot count n and the centre distance C: crank radius a = C sin(π/n), wheel radius from b = C cos(π/n), slot depth reaching C - a, stop-arc radius a - 1.5p, locking disc one clearance inside it, and a locking-disc relief sized so the wheel tips clear it through the whole index. Validators block combinations that cannot work (locking disc too small, no room for the wheel pedestal or knob). At the default 50 mm centre distance that allows 4 to 8 slots; 9 and 10 need a larger one.
Turning it
Crank angle 0° is the pin pointing away from the wheel: the middle of the dwell, wheel locked. 180° is the pin at the bottom of a slot, mid-index. Move the Crank angle slider or drag the handle above the pin; the wheel angle is solved from the crank angle, so you can watch it index and then lock. Readouts give the wheel step, dwell share and peak wheel speed (4 slots: 90° step, 75 % dwell, 2.41× crank speed).
Verified
Interference and clearance checks through the whole crank cycle for 4 to 10 slots, and at the ends of the size and clearance ranges: no overlap, half the print clearance at the pin (0.2 mm by default) and the full clearance at the lock (0.4 mm).
Printing
All three parts print as modelled with no supports at 0.2 mm layers: base flat, driver crank disc down, wheel flat. The only bridge is the ceiling of the driver's blind axle bore (about 8 mm). Default clearance 0.4 mm (0.2 mm per side in the slots); raise it if your printer runs tight. The axles and drive pin are short, thick pegs with light loads, so vertical layer lines are fine here. Assembly: drop the driver on the left axle and the wheel on the pedestal axle; gravity holds them.