I was very impressed by the "Tiny Planetary Gears Set" by aubenc but had trouble with the long screws so have redesigned the connecting system.
My design uses D section rods which are printed lying down, and spacers to position the ring gears correctly. More parts to print, but easier to print and easier to assemble.
The ring gear has been modified to accept the D section rods.
I've also modified the design of the crank - I've used a left hand thread so the handle tends to self tighten when the crank is turned clockwise (with the crank nearest you and the gear set pointing away from you).
Note that this design requires the sun and planetary gears, and the planet holder from "Tiny Planetary Gears Set" http://www.thingiverse.com/thing:23030
A couple of weeks ago I posted http://www.thingiverse.com/thing:204809 which also used parts from "Tiny Planetary Gears Set" but at 2X scale. The 1x scale parts are close to the limits of what is sensible on my equipment and required quite a bit of post print trimming etc.
I've been trying to think of a way to show the gearbox working, that is to see movement at the output end. At the moment I'm rather at a loss. I calculate that if I wanted to observe the output moving one tooth pitch (1/9th of a revolution), and drove the input at 350 RPM (slow speed on my cordless drill) it would take over 5 hours! Suspect that the drill would lose charge and the gear box would overheat and seize up.
The D section rods should be printed lying down, and the spacers standing vertically.
I recommend making one set of sun, planetary and ring gears together with one planetary holder and one rod to check fits and clearance before starting 'mass production'.
The D section rods should be a close sliding fit through the ring gears (once the parts have had excess material removed after printing).
This helps to keep the frame pretty rigid. The spacers should be a freely sliding fit.
The planetary gears sun gear from "Tiny Planetary Gears Set" I cleaned with a 3.2mm drill.
When assembling the gear set I suggest pre-assembling a ring gear/sun gear/planetary-gears-and-holder sub-assembly and sliding the sub-assembly onto the D section rods as one unit.
Because of the large number of components you may find it necessary to make some custom 'z' scaled spacers to get the end plate close to the tapered pins.