The Tetrahedron Machines were conceived and sketched in 1992, while I was active in SRL. The grand scheme was to make a completely impractical ten-foot tall machine which took up a lot of space, and was capable of intimidating an audience while roaming freely. Though he was dubious, Mark Pauline generously provided a four-way valve bank after I found three sets of six 12"x5/8" Bimba cylinders at the now defunct A-Z surplus in Campbell for what was to become Prime Tetrahedron (11"-19" tall). The parts sat in a box while I moved several times and moped about how to implement the compound corner joints, since I didn't have a machine shop. The joints had several constraints:
rotate through 3 simultaneous angles from five to 180 degrees,
protect cylinder casings and shafts from impact,
preserve contracted/extended length ratio as much as possible,
stretch in case of mechanical interference, to minimize side stress to the cylinder shafts,
be simple and cheap to repair in the field.
These requirements seemed to rule out ball-and-socket joints and compound hinges.
To break the impasse, Chico MacMurtrie enthusiastically assisted me in assembling the Prime Tetrahedron in 1994 using duct tape for the corner connections. Wrapped correctly, tape met all requirements, and lasted one full performance, about 20 minutes before some repair was needed. MTTR was also 20 minutes.
The Skinny Tetrahedron (12"-21"tall ) and first Big Tetrahedron (31"-54" tall) were constructed in May 2000, and all three machines received new reliable corner joints based on the original design sketches from 1992. Corners were realized in machined threaded aluminum blocks by SMP Machining, brass and stainless steel hose barbs, and very flexible polyethylene tubing. These joints have very long life, and are very quickly repaired in the field.
An all-metal compound joint mechanism is feasible, but would require many custom parts, rather than the current single custom-machined part per corner.