Summary

Gearbox for FTC Tetrix / AndyMark motors. Slows motor speed down by 6:1 (actually (22/9)^2).
Requires four 6-32 x 1" screws, a Tetrix axle cut to about 23-24mm, a Tetrix motor hub, small screws to mount the part to your motor of choice, and a few standard Tetrix screws and nuts.
https://www.youtube.com/watch?v=56S-BnZ7S0I

Instructions

Print the individual parts and gather the additional materials described in the description. Don't print the STL file that says "don't print": it's there to show what the final assembly looks like.
Screw the bottom mount on the motor. Use 3 and 9 o'clock screw holes for Tetrix motor, 1, 5, 7, and 11 o'clock screw holes for AndyMark. Tetrix option uses same screws as used to add an encoder. AndyMark sheet says it uses M3 screws.
Put the 23 mm cut axle in the axle hole on the bottom mount.
Put the double gear on the axle.
Put the small motor shaft gear with the D-shaft on the motor shaft, with the gear part down toward the motor. This is intended to be tight and may require a few taps with a hammer.
Add a standard small Tetrix screw to the small motor shaft gear as a set screw. Screw it in gently but tightly on the flat part of the motor shaft.
Screw the motor hub onto the remaining gear. You can do this with 2 screws to leave 2 screw holes on the hub free for gears, sprockets, etc. Or you can drill out the threads in the holes in the hub and pre-assemble the printedgear-drilledhub-gearorsprocket-secondhub sandwich with the top motor mount in place.
Assemble the remaining parts and use four 6-32 x 1" screws and nuts to hold the top and bottom mounts together.
If you haven't already, add what you want to the motor hub now: gear, sprocket, etc.

FTC Tetrix / AndyMark Motor 6:1 Gearbox - V7