NEMA17 Cycloidal Joint Actuator

This is a compact cycloidal reducer that bolts straight onto a standard NEMA 17 stepper. The result is a 96 mm square rotary joint module with a 15:1 reduction, very little backlash and far more torque than the bare motor. A printed eccentric cam on the 5 mm motor shaft wobbles a 15-lobe cycloidal disc around 16 rolling 5 mm steel ring pins. Six steel output pins pass through the disc's oversize holes and turn that motion into smooth, concentric rotation of the output carrier. The carrier's top face has four M3 heat-set inserts, so you can bolt on the included demo arm, a gripper, or the next joint in your own arm. It's for hobby roboticists who want a real geared arm joint without buying a harmonic drive.

The stack is simple. A base plate takes the motor and the housing bolts. The ring-pin housing and the cover clamp down onto it with four long M3 screws. The output carrier is trapped under the cover lip and runs in a plain Ø50 bearing. If the motor shaft is long enough, its tip also runs in a 625 bearing in the carrier. The output turns the opposite way to the motor, 1 turn per 15 motor turns. With 1/16 microstepping that's 48,000 steps per output revolution.

Assembly order:

1. Press the M3 heat-set inserts into the base plate (4) and the output carrier (4).
2. Put the motor's pilot boss into the recess under the base plate and fix it with 4x M3x8 screws from above; the heads sit below the top face in their counterbores.
3. Drop the M3 nut into the cam's slot, slide the cam onto the shaft with its collar down, turn it so the set screw lines up with the shaft's D-flat, then tighten the M3x10 set screw through the side hole.
4. Set the housing ring on the base, line up its 4 bolt holes and drop the 16 ring pins (5x18 mm) into their holes. They should turn freely.
5. Grease the cam lobe and the ring pins, then lower the cycloidal disc over the lobe so it meshes with the pins.
6. Press the 6 output pins (5x14 mm) and the 625 bearing into the underside of the output carrier. Lower it so the pins enter the disc holes and the 625 goes over the shaft tip.
7. Fit the cover over the carrier boss and clamp the whole stack with 4x M3x30 screws into the base inserts.
8. Bolt the output arm (or your own payload) to the carrier face with 4x M3x10 screws.

Tuning notes: print the disc, cam and carrier in PETG or PLA+ and use plenty of PTFE or lithium grease on the lobe bore and pins. The ring pins sit in 5.3 mm slip holes so they can roll, which cuts friction a lot. There's only one disc, so it isn't balanced. Keep input speeds under about 600 rpm to keep vibration down. For a stiffer output, use a motor whose shaft sticks out at least 22 mm so the 625 bearing carries the carrier. With a shorter shaft, leave the 625 out and the carrier runs on the cover's plain bearing alone. Mount the module to 2020 extrusion through the four M5 corner holes.