MG996R 12-DOF Quadruped Robot Dog

This is a four-legged walking robot dog in the Spot-Micro style. It uses 12 MG996R standard servos, three per leg for hip roll, shoulder pitch and knee pitch. A Raspberry Pi 4 and a PCA9685 servo driver run it, and it can walk, trot, sit and lean. The body spine is two 200 mm lengths of 2020 extrusion with a printed hip frame at each end. Builders often complain that the shoulder and knee joints on printed dogs hang off the servo spline alone and that the bearing bores are too tight. Here both of those joints sit in a clevis: the servo drives one side through its horn, and the other side turns on a 625 bearing on an M5 bolt. The published designs don't give MG996R tab-hole spacing or output-shaft position, so the parts don't assume them. The servo tab holes are slots, and every bearing bolt passes through a slot, so you line up each joint axis by hand when you assemble it.

Every part works on either side of the robot. Left and right legs use the same prints, just flipped over. Each leg is built from a shoulder bracket, an inner and an outer upper-leg plate joined by four heat-set posts, and a lower leg with a built-in foot. The hip servos sit in the hip frames and drive the shoulder brackets directly. Each shoulder bracket holds a shoulder servo that drives the upper leg, and the upper leg holds the knee servo that drives the lower leg.

Assembly order:

1. Center all 12 servos at 90° with a tester before fitting any horns. Screw a round 25T horn onto each spline.
2. Bolt both hip frames to the two 2020 rails with M5 T-nuts under the ledges, then bolt the electronics deck across the rail tops.
3. Fit the two hip servos into each hip frame, tabs on the front face, using M3x16 screws, washers and nuts through the slots.
4. Press a 625 bearing into each lower leg and each upper-leg outer plate. Bolt the knee servo into the upper-leg inner plate with its output shaft toward the foot end.
5. Bolt the lower-leg horn face to the knee horn. Close the upper leg with the outer plate: four M3x10 screws go into the heat-set posts, and an M5x20 bolt goes through the knee bearing and slot. Slide the bolt in its slot until the joint turns freely, then tighten it.
6. Bolt the shoulder servo into the shoulder bracket and fix the upper-leg horn face to it. Put the M5 bolt through the bracket's slot cheek into the upper-leg bearing, then align and tighten it as in step 5.
7. Bolt each shoulder bracket's horn plate to its hip servo horn. Wire the servos to the PCA9685 and set a trim offset for each joint in software.

Tuning notes: print the load-bearing parts in PETG or PLA+ with 5 walls and 40% infill. Take up the space between each bearing and its slot cheek with M5 washers until there's no side play. Power the servos from a separate 6 V UBEC rated for high current (10 A or more), not from the Pi.