Assembling TARS: Arms, Hands, and Magnets

In the full-arms build, TARS gets hands that gesture and grip, and they are the most delicate part of the whole project. If you built the legs-only version, TARS is already complete and this lesson is optional. If you are doing the full build, the same principle applies as before: the TARS-AI guide gives you the exact assembly steps, and this lesson gives you the handful of ideas that make those steps make sense and keep you from breaking a finger you spent hours printing.

Legs-only or full arms

TARS comes in two forms, and both are legitimate finishes to the program. The legs-only version stands and moves without hands and is a complete robot. The full-arms version adds articulated arms and fingers driven by the smaller servos, and while it is more work and more fragile, it adds a lot of expressiveness. Which one you build is your call, and the TARS-AI documentation covers both; just know which one you are doing before you start, because it changes which parts you assemble here.

Small servos, same home-position rule

The hands use the smaller servos because fingers move light loads in a tight space. Each finger servo installs into the printed forearm or hand and drives a finger through a linkage, and, exactly as with the legs, you set each one to its home position before attaching its horn so the fingers rest in a natural open or closed pose and can travel their full range. The rule that mattered for the legs matters even more here, because there is less room for a mis-set joint to hide.

The magnetic fingertips, and why orientation matters

The fingertips include small magnets, which let the fingers hold onto compatible objects and give the hands a real, functional grip. You set these magnets into their printed pockets, and there is one thing you must get right: a magnet has a polarity, a north and a south face, and it has to be oriented so the fingers attract rather than repel. Install a fingertip magnet backwards and the finger will push objects away instead of holding them, which is both useless and maddening to debug after the fact. Check the orientation as you go, and if a fingertip repels when it should grip, flip the magnet.

Fit the extension cables now, not later

The arm servos sit far from the PCA9685 in the torso, so their wires need extension cables to reach the driver. The strong advice, which the TARS-AI guide echoes, is to install those extensions during the build rather than after, because retrofitting them means taking the arm apart again, and you will not want to. Route them cleanly back toward the driver as you assemble, and add each arm servo to your channel map so every finger and arm joint has a known channel for when you program motion.

Everything about the hands rewards patience: snug the small fasteners gently so you do not crack a printed finger, and test each finger's range and each magnet's grip before you move on. Delicate as they are, working hands are what make TARS feel truly alive.