6-DOF Robotic Arm

Background

The goal of the project was a desk-scale robotic arm built from 3D-printed parts and hobby-grade actuators. I previously attempted Version 1, a four degree of freedom arm on MG90S hobby servos, and did not finish it. Those servos do not report position, so the controller commands a joint angle without confirming the joint reached it. Version 2 is a six degree of freedom design on serial-bus servos that report position at every joint, complete in CAD and not yet built.

Design Requirements

• Six joints and a gripperBase yaw, shoulder, elbow, forearm roll, wrist pitch, tool roll.
• Position feedbackEncoder position feedback at every joint on a single serial bus.
• Payload200 g payload held within rated servo torque across the working reach.
• MountingPrinted structure that clamps to a desk edge.
• AssemblyEvery fastener reachable with a straight driver at its assembly step.

Structure and Actuation

• ServosFeetech STS3250 and STS3215 serial-bus servos with magnetic encoders, daisy-chained on one bus. Two STS3250 servos drive the shoulder together and an STS3250 drives the elbow.
• SizingServos sized from a gravity-torque model: a 200 g payload is held within every servo's rated torque out to 400 mm.
• BearingsA 6810 thin-section bearing carries the base yaw joint and a 6806 bearing the forearm roll joint.
• Forearm hubThe forearm mounts on a separate hub, so the forearm roll servo's horn screws are driven straight down before the forearm is bolted on.
• Desk clampThe base clamps to a desk edge with a printed Tr16x4 trapezoidal clamp screw.

Gripper

• Parallel linkageTwo meshed spur gears (module 1.25, 24 teeth), each driving a crank and a passive link that form a parallelogram, so the fingers stay parallel through a 62 mm opening.
• Grip facesStaircase grooves across both grip faces.
• Servo housingThe gripper servo sits in an enclosed housing and slides in from the back, behind a cover that carries its mounting screws and its wire exit. Minimum wall thickness is 3 mm.
• PalmPin bolts thread 8 mm into bosses behind the palm; side cheeks tie the palm to the front plate.
• CameraA pod on the back of the palm, on the servo side, holds the wrist camera.

Verification in CAD

• Parametric modelModelled parametrically in Python (build123d) and rebuilt as a parametric Fusion 360 assembly with revolute joints and joint limits. A URDF is generated from the model.
• Fit checksEvery servo pocket and screw hole checked against the manufacturer STEP models: no overlaps, and every hole coaxial with the servo's hole.
• Cable routingWire windows sized from the servo's connector position, and printed hooks on the arm links.
• Assembly sequenceGenerated from the CAD: eight subassemblies built on the bench, then joined into the arm in 44 steps. Every insertion path is checked for intersection with the placed parts, and every screw head for a straight driver path, using exact geometry in Fusion 360.

Result

A complete six degree of freedom arm design in CAD, with servo sizing, fit checks against manufacturer models and a 44-step assembly sequence whose every insertion path and screw access is checked against the model.

Next Steps

I am building the arm now, working from the checked assembly sequence.

• Print and assemblePrinting the structural parts and building the eight subassemblies on the bench, then joining them into the arm in the 44 steps generated from the CAD.
• Bring-upWiring the servos onto the single serial bus, setting each joint's zero and limits against the CAD values, and driving the joints from the generated URDF.
• TrainingTeaching the arm tasks once it moves, with the wrist camera in the loop, and comparing what the built arm does against the model it was designed from.