Who said we couldn't print useful things? Here's a flat-pack modular rotary hydroponic unit to be produced by most CNC tools.
My Blog Post about this here: http://engineeringourfreedom.blogspot.com/2011/05/do-cartwheels-feed-people.html
For working examples of such a system, see OmegaGarden.com RotoGro.com or H2ODynamic.com
Call it the ModRotoHydro, RapFarm, FeederBot, [email protected], or whatever else you like, I don't care, just help me build them and feed the world. ;D
Some bits of the assembly are made from or based on standard mendel parts, such as the frame vertex and Prusa's Y-bar clamps. The current gear-tooth profile is based on http://www.thingiverse.com/thing:8077; I might try a version with an easier-to-print zigzag, though it may be less efficient.
The cylinder has an outer diameter of 1 metre, ID of 80cm, and the trays are designed to hold 75mm/3" rockwool cubes, though it would be very easy to design alternate holders for smaller sizes.
Currently I have designed the system to use a gear ratio of 11/1056, i.e. 44 teeth on each of 24 segments, using a 3-3.6rpm synchronous motor pulled from the base of a broken halogen heater. If you want me to release alternative parts, e.g. with different numbers of teeth on the segment, just say so and I'll get on it for you.
I have been designing the base-frame to try and use 1m lengths of studding with as little random waste as possible, but you could construct this at different sizes with odds and ends if you so wish. It doesn't make a difference so long as a tray and reservoir can fit under the cylinder.
The tray support designed is now up. It uses 8mm studding to span the gap in the design, but I think you could possibly use some 6mm dowel, but don't take my word for it until you or I have done a stress calculation. I'm thinking of having it so you simply hang a polyethylene sheet across the gap in the base to make a sump for the nutrients. There needs to be a hole in the bottom to drain excess away into a reservoir though, and some kind of hose line up from the reservoir to pump it in.
The weight-saving gaps have mostly been placed conservatively using intuition and best practice of avoiding sharp corners, but I have yet to do any FEA on the components to see if this can be improved for even more strength and less weight. However, I expect design 1.0 to work as it is with lightweight infill settings.
Any DXF files that end with a measurement are giving the reccomended thickness of sheet to cut the part out of, in order to fit within this design, however you may be able to change these around a bit and file holes down to fit.
Let me know if you want another file format. I added a DXF of the geared segment face on request, for anyone able to cut that profile halfway through a 10mm board. I promise I'll get round to drawing up a less-botched-together motor mounting part that follows some kind of one-size-fits-all geometric rule (eventually).