NEMA17 Twin-Motor Strandbeest Walker

This is a desk-sized Strandbeest: a Theo Jansen linkage walker scaled to exactly 2x the classic "holy numbers". It stands about 220 mm tall and 310 mm long. Each side has its own NEMA17 stepper turning a 30 mm crank. The crank drives a mirrored front and rear leg, so the two sides can run at different speeds and the walker turns like a tank. It is for builders who want a motor-driven Strandbeest with published hole-centre distances and generous M4 pivot clearances, instead of STLs that only fit one printer's tolerances.

The frame is two side plates tied together by a centre deck tray. The deck bolts into heat-set inserts in the plates. Each motor bolts to the inside of its plate, and the crank passes through the plate to the outside. The legs are flat printed parts on M4 socket-head pins with nylock nuts. There are four link types (upper rocker triangle, foot triangle, 100 mm upper link, 123.8 mm lower link) plus a 78.7 mm tie link used twice per leg. Every joint follows one layer stack (interface I3): each part has 6 mm hubs (8 mm for layer L1), so a link printed on its own lands in the right plane. A rear leg is just a flipped front leg, and the same holds for left and right sides.

Assembly order:

1. Press the 16 M3 heat-set inserts into the side plates and one M3 insert radially into each crank.
2. Bolt a NEMA17 to the inside of each side plate with 4x M3x8. The screw heads sit in the counterbores on the outside face.
3. Bolt the deck tray between the two plates with 16x M3x10, and strap on the battery and electronics.
4. Push each crank over its motor shaft, with the boss going into the plate's 16 mm hole. Leave 1 mm of air under the crank arm and tighten the M3 set screw onto the shaft flat.
5. On each side, pin the upper triangles and tie links (c) to the frame pivots B and B' with M4x30 pins.
6. Add the foot triangles, the tie links used as f, and the 100 mm and 123.8 mm links, one layer at a time as listed in I3. Put an M4 nylon spacer at J' (layer L2) and K' (layer L4).
7. Fit the crank pin last: an M4x35 with its head in the crank's counterbore, going up through j, j', k and k'.
8. Snug every nylock nut, then back it off until the joint swings freely under its own weight.

Tuning: set the left crank 180° out of phase with the right one, so a foot is always down on each side at opposite ends. Run the steppers at 30–60 rpm, 1/8 microstepping, 12 V (3S). The linkage has a high load ratio near the bottom of the stride, so slow and steady works better than fast. To steer, slow one side down or reverse it. If the pins bind, check that no link has elephant's-foot flare on its hub faces. The 0.5 mm hub stand-off on each face is the only running clearance between layers.