Unless you already work with steel on a regular basis, you probably do not have a connection with a steel yard or supplier. Finding a good one is a very good idea if you're serious about building a kinetic sculpture. Only a steel supplier can get you that 20' long stick of 3"x4" .120 wall tube in 1016 alloy.

But if you're just getting started, aren't that hardcore, or just need something that's easy to get and easy to work with, conduit is your friend. "Conduit" is a generic term for Electrical Metal Tubing or EMT. It usually comes in two varieties, rigid and thinwall. Rigid conduit is akin to galvanized pipe. Its heavy and threaded on the ends. It may have a few applications in kinetics, but not many. Thinwall on the other hand is light and fairly strong. It bends easily into a variety of shapes, can be flattened at the ends with a hammer, and can be welded with caution (see below). And best of all, you can get it at any bog-box store in the Electrical department.

Thinwall EMT conforms to standards set for all steel tubing in its class. Its made by rolling a flat strip of steel into a cylinder and then welding the seam via electrical resistance. Which means that its similar to the kind of tubing used to build cheap dept. store bicycle frames. EMT is always galvanized, which creates two problems. First, oil-based paint won't permanently stick to it. Second, the zinc galvanizing creates fumes during welding that can produce flu-like symptoms and puking. But, there is a way to remove it!

EMT can be immersed in a 10% solution of water and muriatic acid. Muriatic acid is used to clean swimming pools and is available at box-box stores in the summer. The acid-water produces a fizzing chemical reaction that releases hydrogen in large quantities. Its flammable, might be explosive, and its acidic. So no smoking, wear super-duper rubber gloves, and do it outside in the yard. Oh yea, it'll also eat the concrete, so stay out of the driveway. Be sure to rinse your tubing with lots of water after etching!

Assuming you've now etched your EMT, you can use it like any other steel tubing and weld it safely. Here is a link to a list of EMT's vital statistics:http://www.westerntube.com/electrical_metallic_tubing.htm

Now get out there and start building something. Bend it, weld it, flatten the ends and bolt it. Its all good. Just remember to be safe and have fun.