A small planetary gears system with a ratio of 4:1 per stage, as many stages as wanted, small, open enclosure to see the gears, and hardware free.
The gears are designed with a circular pitch of 2.5mm and they are also slightly beveled at the tips of the teeth to avoid the need of cleaning the excess of material from the firsts layers.
The OpenSCAD is not parametric, there's only parameters for clearance (set to 0.2), backslash (set to 0.1), pressure angle (set to 22.5), tolerances between nuts and bolts (set to 0.4) and length of the bolts, sometimes referred as rods. Any other change will not work.
To play with the OpenSCAD you will need the following libraries:
Public Domain Involute Parameterized Gears by LeemonBaird
Poor man's openscad screw library
Knurled surface finishing library
The libraries have been included in the ZIP file together with STL files for every single part, build plates for complete sets from 1 to 4 stages, just the crank and upgrade kits from 1 stage to 2, 3 or 4.
1 minute video: http://youtu.be/MUH0MGP7AGc
A shorter video (59") featuring a 10 stages set: http://www.youtube.com/watch?v=FyVZ31tLGCU&feature=plcp
My setup is a Makerbot Replicator with 0.4mm nozzle, ReplicatorG - 0034 and Skeinforge 47. The gears where not printing fine with the standard profile and printomatic, also, the threads like to print slowly and a little bit fat.
It is important to set Layer Height at 0.25mm (or the planets can find it hard to spin). Infill should not be set bellow 60% or the bolts will break too easyly!
(The profile I used is included in the ZIP file)
Use multiply to print 4 planets and see if you need to adjust the profile. Then print the stage to be sure that the gears are usable. If you don't clean the sides of the teeth it will be a tight fit perfectly usable. If you clean them they may be too loose.
Be very careful with the bolts, do not fasten the nuts too much (just a very little bit) and be extremely careful when using the crank
Update: Scaling this thing
I'm always afraid about the result of scaling multipart things, specially when the design is full of threads and gears.
Check out the awesome set printed by mread ( thingiverse.com/derivative:29726 )
He scaled it up by 25% and it looks terrific!
I don't know what are the specs of the printer he used but it could very well well be the case that scaling this thing up by 25% is the right thing to do when the extruder is equiped with a 0.5mm nozzle.
Let's check it out! bot1334 has a 0.6mm nozzle so I scaled it up by 50% (0.6 / 0.4), sliced at 0.4mm layer height and... I had a lot of trouble with the threads taped in the plates.
Amazingly, the issues disappeared when I lowered down the layer height to 0.38mm. Is it a coincidence that 0.38 is close to 0.25 scaled up a 50%?
In this bigger (huge) version everything fits but quite tight, the motion is not as smooth and I did had to clean every single teeth. After assembling it and a couple of minutes of operation with a powertool it works quite fine.
Difficult to say if the gears not meshing as nice as they do in the original is due to bot1334's limitations, poor profile, manual cleaning or side effect of the scaling, anyway, I'm very pleased with the result.