Update: created new 16 tooth GT2 gear head (6 to 7mm wide)
A 4 to 1 planetary gear box for common small stepper motors. Takes your typical 7.5 degree per step motor and turns it into a 1.8 degree per step. I have a ton of small stepper motors I have ripped out of printers but the resolution sucked... so I made this. Plan on making more gear heads later to fit different belts/pulleys.
Print the Planetary and Sun gear at about 95% the normal size. This lets them mesh better. Print all other parts normal. Take a razor and trim a few things if you have to.
I acetone treated all my parts which allowed me to smoosh the sun gear (about 5 minutes after acetone treatment) onto the 2mm stepper shaft that had a flat grounded on it. I used a dremel to grind a flat. Another way to do this is use JB Weld and a tooth pic which will fill in the flat part creating a snug fit.
*acetone treatment also cause the teeth to fade on the gear head... so I used Sugru and filled the open slot, lubed a typical printer belt, and rotated the gearhead on it form form teeth. I let it sit for 1 day to dry then razor off any excess.
USE GREASE! On all gears, sliding parts, and the bearing which holds the gear head. I used PTFE infused synthetic grease which I got from Enco.com for about 3 bucks on sale. That stuff is magic.
Hooked the prototype up to a stepper motor tester (Arduino Uno and a A4988 driver) and ran it at 100 rpm for 1 day with no load. Still looked good.
-4 to 1 gear ratio
-M0.7 module on the case, planetary, and sun gear
-M0.3 for the gear head (fits common printer belts)
-Case 39 teeth
-Sun and Planetary are 13 teeth
Stepper motor I used was a:
7.5 degree per step