Planetary gear fidget

A pocket fidget with a real planetary gear train: a ring gear you hold, a sun gear you spin with a fingertip, and three planets rolling between them under a press-fit cap. Pick the tooth size and counts and the ring follows; the readouts give the ratio and the outside size. The gears print in place on the ring's floor and the cap presses on afterwards, and the backlash slider is the number to tune so it spins freely.