This table uses an old road sign for the top, but you could use a lot of things for the surface -- laminated wood, plywood, an old piece of countertop, butcher block, or whatever else you can scavenge that's about 30" square. The advantage of the road sign is its relative light weight and high strength, making this table truly transient. All the wood was scavenged pine, cut-offs from tongue-and-groove flooring, along with two pieces of cedar.
I built it in about a day. It cost somewhere around thirty dollars to make, as I had to buy hinges, bolts, nuts, some copper fittings, and super-strong neodymium magnets to hold the legs in the folded position.
It is currently for sale at the Grace Aberdean Gallery in Tuscaloosa, AL: http://graceaberdean.wordpress.com/
Thanks to Ramell Ross for the first five pictures. http://www.ramellross.com
You will need these materials:
Approx. 20 running feet of wood ripped down to 3/4" or 1" square
Approx. 6 running feet of wood ripped down to 1-1/2" x 2"
Suitable table-top material 30"-36" square (old sign, laminated wood, 3/4" plywood, etc.)
4 small triangle strap hinges
16 3/8" dia. neodymium magnets
8 1/4" x 2-1/2" machine bolts
24 1/4" cut washers
8 1/4" nuts
a handful of pan-head screws
You will need these tools:
Impact driver (optional)
Step 1: X-Bracing
Once you have two equal pieces cut, mark the center on one and cut the width of its matching piece out of it, so you have three pieces to compose your "X". Drill a 1/4" hole about four inches from the center of the "X" on each piece, making four holes to hinge the legs to. Sand liberally, seal if desired. Attach the legs to the underside of your table top with superglue or clamps to hold them in place temporarily, then pre-drill and screw them in from above. If your table top is not metal, and sufficiently thick to take screw from the underside, you can screw in from below to keep the surface smooth.
Step 2: Legs!
Drill holes in the top of each leg; first a counter-bore for your nut and washer, then a 1/4" hole for the bolt. Do the same a ways down each leg for the copper fitting where the leg brace will eventually lock, about 4" above the foot in this case. The copper fitting has to be past where the "X" brace ends so the legs will fold flat. Sand liberally, seal if desired.
Use your ratchet to bolt the legs to the "X" brace where you drilled holes before. The bolt should be tight but not so tight the legs don't rotate fairly easily. Down towards the feet, bolt the legs together as well, but put a piece of 1/2" x 1-1/2" copper pipe around the bolt to hold the legs apart.
Step 3: Leg Braces
Unfold one of your newly attached legs and measure from the copper fitting to the end of the "X" brace. Cut four pieces of leftover leg wood to that measurement. Drill a hole with a spade bit as wide as the wood is at one end before you cut it to length, making the cut bisect the hole, so you end up with a "C"-shaped notch in one end. Sand liberally, seal if desired.
Attach the brace to the tapered end of the "X" brace with the hinge. Test that they lock together ok. Getting the braces to the right length may take some messing around with the 'ol guess-and-check method.
Step 4: Magnets!
Pop a shallow hole in the "X" brace the same diameter of your magnet. Slop some superglue in here, and hammer your magnet in. Let the glue dry and sand it down. Put a dab of ink on the magnet and close the brace down on top, which will mark where the magnet should go in the leg brace. Do the same drill-glue shenanigan on the leg brace. To get the magnet really in there, put a magnet on the jaw of the clamp and work it into the hole so it sits perfectly flush.
For the legs, I ran into a problem because the aluminum top is not magnetic. I bolted on a little steel piece for the magnets to interact with; you could do that part any number of ways depending on what your table top is made out of. Your legs should now stay retraced when you retract them.
Congrats! Bust open a couple of beers and check out your sweet new low folding table . . .