[z_beeblebrox]

1/4" baltic birch plywood is most of our metal, whether for structures or gussets. It's exceptionally strong, lightweight, easy-to-machine (even without a laser cutter) and inexpensive. We also use some 1/8" plywood for small or non-structural stuff.

In terms of actual metal, we mostly use 1x1x1/16" square 6061 tube. Its profile gives good resistance to bending and twisting and its thin wall keeps weight low. Also, lots of Vex 1/2" tube axle (with the ends tapped for 5/16-18 bolts) for both axles and standoffs.

We also have some small parts that take high loads (axle mounts, winch ratchet and pulleys) laser cut from .125" 5052 aluminum by a sheet metal shop that sponsors us.

We'll also make small parts out of HDPE or Delrin on our CNC router or from thin delrin sheet on our laser cutter. Lexan sheets are also occasionally used.

We occasionally use 1x1x1/8" or 2x2x1/8" 6061 angle and 1/8" or 1/16" 5052 sheet if it better suits the application.

For fasteners we use #8 Torx screws for wood-wood joints, 3/16" rivets where possible in metal (lighter, easier to work with than bolts), 10-32 and 1/4-20 bolts where rivets won't fit or aren't strong enough and 5/16-18 bolts for attaching tube axle. Mostly socket-head cap screws from McMaster-Carr with nylock nuts.

Edit: glad to see a good mechanical design discussion thread going. I've been missing these.