The torso is the visual centerpiece of the B9 robot. It contains the chest light panel (the array of colored lights that pulse and flash during the robot’s dialogue), the control surface details, and the rotation mechanism that allows the torso to spin independently of the tread and legs. Building the torso is the most complex single phase of the B9 construction project.

Torso Dimensions

The B9 torso is roughly barrel-shaped — wider than the leg section, narrower than the tread base. Approximate dimensions from reference photography:

  • Maximum diameter: approximately 22–24 inches at the widest point
  • Height: approximately 18–20 inches from leg collar to upper collar
  • Front face: features the chest light panel, various control details, and the main speaker grille

The torso tapers slightly from waist to shoulder area. The exact profile varies between Season 1 and Seasons 2–3 of the show — the original prop was modified between productions.

Shell Construction Options

Fiberglass Over a Buck

The traditional approach: build a full-scale foam or plaster positive form (the “buck”), surface it to shape, then lay up fiberglass over it.

  1. Build the torso buck from foam, plaster, or cardboard over a wooden armature — the buck just needs to hold its shape during fiberglassing
  2. Smooth the buck surface with body filler and sand to a final shape
  3. Wax thoroughly (6+ coats of paste wax) for release
  4. Lay up 3–4 layers of fiberglass cloth with polyester or epoxy resin
  5. After cure, cut the shell off the buck along a vertical centerline (creating front and back halves)
  6. Trim and bond the halves with an internal flange

Split Mold and Cast Urethane

A more repeatable approach: build the buck, make a silicone or fiberglass production mold from it, then cast the torso halves in urethane resin. This is more work upfront but produces a smoother surface and allows casting multiple copies.

Two-part molds for the torso are large — you’re looking at a mold that weighs 15–25 lbs and requires 2–4 gallons of silicone. Some builders use fiberglass “mother molds” (rigid outer shells) with a thin silicone inner face to reduce silicone cost.

Machined Aluminum or Steel

The highest-end approach for museum-quality replicas: have the torso shell panels CNC-machined from aluminum. This is expensive but produces a robot with the weight and feel of the original.

Chest Light Panel

The chest light panel is the most recognizable detail on the B9’s torso front face. The original features a grid of colored lights in red, blue, amber, and green that flash in patterns during the robot’s scenes.

Panel Construction

  1. Build a sub-panel from sheet aluminum or cut acrylic — the same dimensions as the panel opening in the torso
  2. Drill or route the light openings in a grid pattern matching the original
  3. Install colored acrylic or polycarbonate lenses into each opening (snap-fit or glued)
  4. Wire LED modules behind each lens

Lighting Control

Modern replicas typically use an Arduino or Raspberry Pi to control the chest light patterns. A simple Arduino sketch can pulse, chase, and flash the lights in patterns that match the show’s look. Several open-source sketches exist in the B9 builder community.

For the wiring, star-topology wiring (each LED group returns to the controller) is easier to troubleshoot than daisy-chain. Use 22-gauge wire for individual LEDs and 18-gauge for power runs.

Torso Rotation Mechanism

The original B9’s torso rotated continuously during scenes where the robot was expressing emotion. The rotation mechanism sits between the leg tops and the torso bottom.

Turntable Bearing

The simplest and most reliable modern approach uses a lazy Susan turntable bearing — a heavy-duty ring bearing rated for vertical loads. These are available in 12-inch diameter ratings up to 2,000 lbs and cost $20–$60 depending on size.

Mount the bearing between the leg mounting plate and the torso floor plate. The torso bolts to the upper ring, the leg frame mounts to the lower ring.

Motorized Rotation

To motorize the rotation:

  • Mount a DC gear motor (12V, 10–30 RPM) to the static frame
  • Drive a rubber wheel or gear against the inner edge of the lower turntable ring
  • Control motor speed and direction with a motor driver board (L298N or similar) controlled by Arduino

Rotation speed on the original was approximately one revolution per 4–6 seconds. Slow enough to look deliberate but fast enough to be clearly moving.

Internal Armature

The torso needs an internal frame to:

  • Support the chest panel and its electronics
  • Provide mounting points for the collar (top) and the rotation bearing (bottom)
  • Create structure for any internal operator (on walk-in builds)

Steel square tube welded into a simple rectangular frame gives you mounting surfaces on all faces. Drill and tap the frame for bolted connections — avoid relying on welds for frequently disassembled joints.

Surface Details

The torso surface carries a variety of control details: knobs, switches, dial faces, pipe fittings, and assorted hardware items that were sourced from industrial surplus catalogs in the 1960s. Identifying the exact original parts is an ongoing community research project.

For replication, cast the details in urethane resin using the techniques in the plastic casting guides, then bond to the torso shell with two-part epoxy.

See the B9 construction overview for how the torso connects to the leg section below and the collar and upper body above.