A NEMA17 behind a printed cycloidal reducer: twenty-two steel dowel pins on a 72 mm circle, a 21-lobe disc running 1.0 mm off centre, a second disc 180 degrees round to cancel the wobble, six output dowels through oversized holes. Twenty-one to one in a single stage, and several lobes carry the load at once, which is why a printed disc can take what printed spur teeth would strip. Three identical joints, each on a 42 N cm NEMA17 with no balancer spring, are the yaw, shoulder and elbow of an arm with 150 mm links; two servos do wrist pitch and roll, and three two-joint fingers close on a 66 mm can. A laptop turns webcam pose into joint angles and an ESP32-S3 steps the motors. The question the build answers is not whether it turns but two numbers: backlash at the output and holding torque, measured against a prediction written down before the test.
Fusion 360, Blender, ngspice, C++ on ESP32-S3, Python, MediaPipe, PETG on a Bambu P1S
- Joint
- 22 steel dowel pins on a 72 mm circle, a 21-lobe disc thrown 1.0 mm, two discs 180 degrees apart, six output dowels through 7.2 mm holes: 21 to 1 in one stage
- Why cycloidal
- Several lobes share the load, so PETG carries what printed spur teeth would strip. The price is backlash from the pin clearances, and that is what the test measures.
- Predicted
- 4.28 N m stall per joint, from 0.34 N m x 21 x 0.60; the 0.60 is a guess the test replaces. Backlash 0.76 degrees from the drawn clearances, against a 0.9 degree gate.
- Arm
- Yaw, shoulder and elbow on three identical 21:1 joints, one 42 N cm NEMA17 each and no balancer spring; 150 mm links; wrist pitch and roll on hobby servos; three two-joint fingers rated 150 g on a 66 mm can
- Control
- Webcam pose through MediaPipe to joint angles on a laptop; step pulses from an ESP32-S3; no encoders, homed by hand
- Checks
- 143 geometric checks on the joint and 144 on the arm, a collision scan over 82,485 poses, every print plate mesh-checked before slicing. Fits are estimates until the first joint is read.
- Test
- Backlash under 1 degree at the output, the arm holding 1 kg at 150 mm, and measured stall within 20 percent of the locked prediction, on film, by 30 November 2026