WEEK 6
Arms (Both Working, 6DOF Each)
Two working arms mounted to torso. Each arm has 3-DOF shoulder/elbow/forearm + 3-DOF wrist/grip.
Est. Hours
35h
Est. Spend
$0
Tools Required
angle grinderband sawsoldering ironRaptorX scannerArduino MegaPCA9685
CAD FILES
arm_assembly_left.sldasmarm_assembly_right.sldasmfinger_mechanism.sldasm
PRINT FILES
bearing_housings_cf.stlarm_covers_left.stlarm_covers_right.stlfinger_segments_cf.stl
Goal
Two working arms mounted to torso. Each arm has 3-DOF shoulder/elbow/forearm + 3-DOF wrist/grip.
Arms are the second-hardest mechanical subsystem. You’ve now done precision servo work on the head — same techniques apply, but at larger scale with higher-torque servos. The ASME-04B servos are powerful enough to break things (including fingers) — respect them.
Day-by-Day Tasks
| Day | Task | Method | Notes |
|---|---|---|---|
| 1-2 | Build arm structure (both arms) | 6061 aluminum square tube (1”x1”) + CF-PETG bearing housings | Cut tube to length (angle grinder or band saw). Print bearing housings. Dry-fit before drilling. |
| 2-3 | Install ASME-04B servos (6 total) | 3 per arm: shoulder pan, shoulder lift, elbow | Mount to laser-cut shoulder plates. These servos are 380KG.cm — secure them properly. Lock-tight on every bolt. |
| 3-4 | Install MG996R servos (6 total) | 3 per arm: forearm rotation, wrist, grip | Smaller servos, simpler mounting. Wire to PCA9685 board #2. |
| 4-5 | Build articulated fingers (both hands) | 3D-printed segments (CF-PETG), fishing line tendons, springs | Print fingers. Thread fishing line through channels. SG90 servos pull tendons (2 per hand = 4 total). Springs for return force. |
| 5 | Mount cosmetic arm covers | 3D-printed shells (PLA), clips/screws | Cover the mechanism. Test full range of motion — covers must NOT interfere with servo movement. |
| 5-6 | Mount arms to torso | Bolts through shoulder plates | Arms are heavy with 3 ASME servos each. Have someone hold while you bolt. |
| 6 | Wire all arm servos to PCA9685 board(s) | Soldering, labeled JST | 12 servo connections + 4 finger servos = 16 wires to route down each arm and through torso. |
| 6-7 | Test all arm servos | Arduino sweep test | Each joint: min/center/max. Check for binding, clearance, torque. KEEP HANDS CLEAR during ASME-04B tests. |
| 7 | Test finger mechanism | Grip test with soft objects | Tennis ball, water bottle, cardboard box. Adjust tendon tension and spring force. |
| 7 | RaptorX: Scan complete arm assemblies | RaptorX | Check range of motion in scan. Verify no interference at extreme positions. |
Skills Practiced
High-torque servo mounting, mechanical linkages, tendon-driven fingers, cable routing through articulated joints.
Print Queue
Misc remaining parts, RaptorX quick-release mount