Overview
A six degree of freedom desk arm built from 3D-printed structure and serial-bus servos, with a parallel-jaw gripper and a custom controller board. The design covers actuator sizing, bearing support, assembly access, cable routing and servo power distribution.
Status, October 2026: The arm is assembled. The controller board in the base drives all eight servos, and the wrist camera is working. The elbow balance springs are fitted. Initial picks are complete. Current work: pick-and-place applications, camera calibration, and motion smoothness, accuracy and robustness.
Joint Architecture
The shoulder and elbow carry the largest gravity loads, so they set the actuator layout. Joint sizing came from a gravity-torque model using the CAD mass properties of every printed part, servo and bearing, checked against each servo's continuous rated torque rather than its stall torque. The sizing case is the arm held straight out with a 200 g payload.
| Joint | Load, arm straight out, 200 g | Servo | Rated | Stall |
|---|---|---|---|---|
| Shoulder | 41.8 kg·cm | 2 × STS3250 | 32 kg·cm | 100 kg·cm |
| Elbow | 20.2 kg·cm | STS3250 | 16 kg·cm | 50 kg·cm |
| Wrist pitch | 6.5 kg·cm | STS3215 | 10 kg·cm | 30 kg·cm |
Two STS3250 servos share the shoulder and a third drives the elbow. The base yaw, forearm roll, wrist and tool roll joints carry little gravity load and use STS3215 servos.
Held straight out, the arm reaches about 650 mm. Picking straight down from the desk, it reaches from 240 to 440 mm.
The fully extended 200 g case exceeds the continuous rating of the shoulder and elbow, so it is excluded from the sustained working envelope. Picking straight down from the desk, 200 g stays within rated torque out to 400 mm. For holds of up to 30 seconds, with every servo limited to half its stall torque, the arm lifts 550 g at 440 mm, the edge of its top-down reach, and 300 g held straight out.
The base yaw joint turns on a thin-section bearing that carries the arm's overturning moment, so the servo carries only torque. The forearm roll joint has its own bearing.
Elbow Balance Springs
During joint testing after assembly, the elbow servo overheated and melted under continuous load, deforming its printed mount. The elbow was redesigned with spring assistance to reduce the servo's continuous torque demand.
- Two extension springs (McMaster-Carr 9654K322), one on each side of the elbow, run from grooved pegs on the upper arm's side faces to an M3 screw across a slot in the lower end of each elbow side plate.
- The springs are always preloaded: about 11 N each with the arm straight, which leaves the elbow servo 0.2 N·m to hold with the arm upright.
- As the elbow bends, the side plate's tip swings back and the springs carry about two thirds of the forearm's weight in working poses, at most 23 N each against the spring's 38.6 N maximum load.
- The pegs are closest with the elbow straight, so the springs pull toward straight from both sides and assist against gravity in either bend direction. The elbow keeps its full travel of −75° to 110°.
Mechanical Design
The Fusion 360 assembly carries the joint limits, the packaging geometry and the assembly checks. Servo pockets and screw holes were designed against the manufacturer's STEP models, and the cable windows follow each servo's connector position. Cables run outside the links with service loops, held by printed hooks.
Every screw is reachable with a straight driver at the step where it goes in. The forearm roll joint uses a separate hub so the servo horn screws can be driven straight down before the forearm is bolted on.
Gripper
Two meshed spur gears drive parallelogram linkages, so both fingers move together from one servo and stay parallel through about 62 mm of opening. The servo sits behind the palm, which leaves room for a wrist camera.
Controls and Electronics
All eight servos share one half-duplex serial bus and report their position. The base holds a two-layer controller board that combines the USB-C interface to the bus with fused, reverse-polarity protected 12 V servo distribution, in place of an off-the-shelf bus adapter and separate power wiring. The board is installed in the base and drives all eight servos; details are on the Robotic Arm Controller PCB page.
The Fusion model exports a URDF. ROS 2 and MoveIt integration for motion planning is planned.
