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, Farm@Home, 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).

Use the supplied eDrawings executable assembly file to take relevant measurements.

• Acquire studding, nuts, bolts, washers, bearings, plastic or mdf, lights, a low-speed motor, some grow medium and hydroponic fertiliser. (Optional: water pump and timer) (Alternative - build the base out of MDF or any scrap wood/fibreboard pulled from a skip, with a few holes to mount rollers/bearings on)

• Print or laser-cut all the flat structural parts.

• Cut and file studding to size.

• Build a base appropriate to the number of rings you want in a cylinder.

• Fit your lights, motor and feeding tray.

• Build as many rings as you can fit on the base.

• Test that the motor turns the cylinder through one full rotation in about 30-40 minutes.

• Soak rockwool cubes and balance the pH of the surrounding solution to about 6.

• Germinate seeds or propagate cuttings in 1-inch rockwool cubes.

• Plug the starter cubes with well-rooted seedlings into holes in the 3" cubes.

• Quickly load the cubes into the trays on an assembled ring, and secure them with the grow-medium retainer pins.

• Rest your planted ring on the base (Optional: adjust the base width if you already have one or more on there, and bolt them together)

• Hit the lights and start the motor.

• Set your timer to pump nutrient solution into the sump for 1 hour a day, every day, or DIY with a jug.

• Eat tasty fresh food! ^_^ (Especially things out of season that would be wasteful to import)

*...Oh wait, there's a step in there somewhere about replacing old media, but who cares... :P