Inspired by the elegant, beautifully realized work of Shigeru Ban (en.wikipedia.org/wiki/Shigeru_Ban), I put this table together out of cardboard tubes and corrugated plastic. The frame is triangulated out of the tubes that come at the center of rolls of paper for architectural plotters. Each tube is an inch and a half in diameter and three feet long. They are incredibly strong, if loaded linearly, straight down their length -- each can support the weight of an adult with no problem. However, if laid flat between two supports and loaded in the middle, the tubes will buckle and fail.
The trick, then, is to find a structure that takes advantage of that strength while retaining some visual delicacy. Mr. Ban has experimented with several ways of joining his tubes, and, following his research, I chose to bolt through with quarter-inch bolts, which is simple, straightforward, and strong. The top is made from PolyGal, a corrugated industrial plastic used in architectural applications, such as greenhouses.
I salvaged all the tubes for free, as well as the plastic for the top; I spent maybe fifteen dollars on hardware, nuts, washers, etc. The table was built with simple hand tools -- drill, hacksaw, wrench, and screwdriver. It took about ten to fifteen hours to make. It is strong enough to stand on, yet light enough to lift with one hand, and one hundred percent recyclable at the end of its life.
Step 1: The "X"
Start with the two tubes that will make the "X" directly underneath the table top. Mark a centerline on each, then push them together and scribe a circle from a third tube. Using a hacksaw to start the cut, carve out two saddle notches with a sharp box cutter. The advantage of using a hacksaw is the fine teeth on the blade don't catch and tear the cardboard like a handsaw made for wood. Clamp together at the middle and drill a hole slightly bigger than the bolts you are using through the exact center of each notch. 1/4" bolts are sufficient, maybe even too big.
I also reinforced the notches by gluing a scrap of tube on the inside of each notch. This isn't strictly necessary, but be forewarned that once you take a big chunk out of the middle of the tube, they are likely to bend and crease when handled if you aren't careful. Treat them gently until bolted together.
Lay one of the "X" tubes with two full tubes as legs coming off from it as shown in the last picture. Cut two short, 18" braces for the legs, each with a 3" slot in both ends. Move the tubes around on the floor until the angles look ok; the goal is to have the end of the legs come slightly past the end of the "X" piece. This provides a wide enough stance for stability while maximizing knee and leg space for the user.
Step 2: Legs!
Cut the same 3" slot in one end of four full-length tubes. These will be your legs. Bend the "tabs" created by your cuts to fit around the "X" tube, tight against the center saddle notch. Clamp in place to drill a hole. Unclamp, apply glue to the inside of the tabs, replace, and then bolt through. Don't over-tighten the bolts, as you can crush the tube. Use a clamp to flatten the tabs against the surface of the tube they are joining to until the glue dries. I rounded off the tabs with a box cutter and hit them with a little sandpaper to clean up the cuts.
Attach the braces the same way; start flush at the end of the "X" tube, pin, then pivot down until you hit the leg. Clamp, drill, glue, and bolt. You may want to scribe the tab cuts on the braces to fit more tightly against the legs. All the compound curves and conic sections involved in cutting cylinders can be tricky, so just do your best with a pencil, a box cutter, and your eye.
Repeat until all four legs are attached and you have two identical frames with two legs, two braces, and one cross-piece each. Lay the saddle joints across one another and bolt through. Hand-tighten the "X" bolt, as it will need to be removed later in the process.
Step 3: Finishes
Cut off all the tails and ends of bolts with a hacksaw or a Dremel. Sand any rough or ragged edges. I finished the tubes with a paste wax, which is a low-VOC, non-toxic treatment; however, the tubes take polyurethane quite well. Be careful not to let polyurethane or varnish to puddle on the surface of the tubes, as it will leave a stain after the finish dries where the varnish soaked in deeper than elsewhere.
Step 4: Top
The top is made from a heavy-duty corrugated plastic called PolyGal. Plywood would probably work equally well, but not look as good. The PolyGal can be cut with a sharp box cutter and straight edge, or on a band saw or table saw.
The top works as a brace for the whole structure, locking the legs together. It has to be stiff to achieve this; in that vein, I made the cuts so the corrugations would lie diagonally across the the "X" structure for maximum stiffness. An even better approach would be to layer two pieces of PolyGal together, with the corrugations running perpendicular to one another.
Measure and make bolt holes in the top in an X shape. Scribe and make corresponding holes in the tubes. Take out the "king" bolt at the center of the legs. Attach one set of legs. I used regular and neoprene washers to make sure the top was tight to the structure, yet protected from crushing. Again, don't over-tighten the bolts, as it will dimple the plastic and crush the tubes.
The second set of legs will actually be a little bit lower than the first, because of the saddle notching. Make up the difference in height between the top of the tubes and the underside of the PolyGal with some washers and a nut as a shim.
Step 5: Braces
The last bracing is minimal but important: two 3/16" dia. threaded rods tie the opposing legs together, so that when loaded, the legs don't kick outwards. A second set of braces is made from scraps of tubes in a square ring near the top of the legs; this prevents the tendency for the legs to roll left or right. The last set of braces can be seen in the pictures at the introduction, which are made from PolyGal scraps glued together and brought out from the center of the table, radially, in between the arms of the "X" bracing. All the braces are held with glue and drywall screws.
To trim the feet, flip the table up on its legs. Shim under the feet that are high. Use a piece of cardboard or wood to transfer the line of the floor onto the tubes, several inches up. Cut. If the feet still don't lie flat, there is a considerable amount of adjustment in the threaded rods; by tightening and loosening those, you can pull the legs in or out until the table sits flat.