Designing a 3R Robotic Arm

Brief

A 3R (three-revolute) robotic arm (one joint for base yaw, two more articulating within a single plane) built as a foundation exercise in sequential assembly, actuation, and moving frames. The arm has three degrees of freedom, one per joint.

Problem

A 3R arm is straightforward to model in simulation, but physical assembly has to solve real ordering and tolerance problems simulation skips over: which part gets fixed to the world first, how each joint transmits torque without slipping or breaking under repeated actuation, and how to keep the whole assembly rigid without over-constraining it.

Approach

• Fixed the base to a stable surface using three anchor points, with an extra wing added to distribute stress rather than loading only two anchors during lifting.
• Used ST3215 servo motors with a stainless-steel flange shaft for the turntable drive: 3D-printed PLA was ruled out for this shaft because torsion breaks along the print layers.
• Assembled the turntable base, motor housings, and two link pairs in sequence: base → turntable → motor housing → first link pair → second link pair to the end effector, using Allen screws as rigid inter-link supports (again avoiding 3D-printed structural supports, which are layer-weak).
• Generated the URDF from the CAD model using the onshape-to-robot extension and visualized/actuated it in RViz2.

Results

A hardware-assembled 3R arm with a stable, wing-reinforced base and a URDF that actuates correctly in RViz2.

Status / Next

Completed as a foundations exercise. Actuation and full ros2_control integration are queued for the next development phase.