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Hardware · Robot arm

UJ201

A desktop arm with a two-axis universal-joint shoulder, reworked from the open-source SO-ARM100. Scroll to take it apart, then see how it was built.

Parts
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Bodies
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Joints
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Mass
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How it works ↓

01 · Lineage

Not an SO-100 any more

UJ201 started as TheRobotStudio's open-source SO-ARM100. It has since been heavily reworked.

The highlighted parts are new: a universal-joint shoulder and U-bridge, a redesigned forearm, wrist and end effector, and a base binder strip, cradle and roof.

02 · Exploded view

parts, moving bodies

Each group that slides apart is one rigid body: everything bolted together between two motors. Inside a group, the servos and brackets lift off their main part.

The U-joint yoke separates into two halves. That split is deliberate, and section 04 explains why.

03 · Mass

of PLA and servos

The printed parts' masses come from their geometry at PLA density (1240 kg/m³). Fusion had assumed steel for every body, which made the arm about six times too heavy.

The servos are surface bodies in the CAD, so Fusion gives them zero mass. Their ~60 g each comes from the datasheet.

04 · The shoulder

Two axes, one joint

The shoulder is a universal joint. A yoke pivots on one axis, and the housing carrying the rest of the arm pivots on a second, perpendicular axis.

In the CAD, the joint's parent and child were swapped, so it read as the handle hangs off the bridge. In fact the bridge hangs off the handle. Reversing that one joint turned two loose pivots into a proper 2-DOF shoulder.

The yoke was one body carrying both axes, which a URDF tree can't express. It was split in CAD at its real parting line, 43/57 by mass, and rejoined with a fixed joint.

05 · Kinematics

Eight joints, one chain

From the base yaw to the thumb, every motion is a revolute joint driving everything after it. The highlighted part is what the current joint moves.

    06 · From CAD to URDF

    Read the assembly as it is

    The usual Fusion exporter keys links off component names. That means flattening the design, breaking links and renaming copies before it will export.

    UJ201's exporter keys off occurrences, the instances, instead. Nested and shared components just work, and the CAD is left untouched. Every link frame shown here comes straight from the assembly.

    07 · Your turn

    Drive it

    Set each joint with the sliders. With a mouse, drag the drawing to orbit.

    08 · Build log

    Building UJ201

    A year of printing, breaking and reprinting an arm, from an SO-100 kit to a pair of arms with universal-joint shoulders.

    Intro: your words. Why build an arm at all? Why start from SO-100, and why go after the shoulder? Two or three sentences to set up the story.

    Starting from a kit

    Your words. Where did the SO-100 parts come from (printed yourself? bought?), and what did you want to change before you'd even assembled it?
    SO-100 parts, fresh off the printer A full set of red printed parts laid out on a cutting mat next to the print bed: gripper jaws, the honeycomb base plates and the servo housings, with bagged screws and drivers alongside.

    Redrawing the shoulder

    Your words. What was wrong with the stock shoulder? Why a universal joint? This is a good place to link the teardown's shoulder section.
    The yoke A U-shaped bracket in Fusion 360, with a bolt pattern on each ear: the shape that becomes the U-joint yoke.
    Servo stand-in A simplified servo body sitting in a housing, modelled as blocks so the parts around it can be sized.
    Yoke meets housing The yoke, a servo housing and a base plate brought together in one assembly, still floating apart.
    Dimensioning The same parts across several views, with sketch dimensions on the yoke (one reads 24.00).
    Questions. Were these the first sketches or a later revision? What was the 24 mm dimension constrained by?

    First prints

    Blue prototypes on the bench A tall housing with a servo inside and a bracket bolted to its horn, two U-brackets, a dark-blue cup, and a loose servo for scale. A Leatherman for company.
    Your words. Printed the same day as the CAD above. What did the first fit-up teach you? Is the black servo the one that ended up in the arm?

    Gearing the shoulder

    A gear train on top An orange housing on a blue base, with two spur gears on its face. A servo sits sideways inside the blue yoke underneath.
    From the side The servo driving the gear pair, mounted in the orange housing above the honeycomb base.
    Your words. Why gear it: torque, range, or getting the servo out of the joint? What ratio? Did it survive into the later arms? (The December arms have a large gear on the shoulder.)

    A whole arm, then two

    First full arm A complete blue arm with an orange shoulder standing on the desk, wired up, with a screwdriver beside it.
    Blue and red The blue arm reaching up, with a second, red arm folded down beside it.
    Side by side The blue arm upright behind a red arm with a long gripper, both on the desk.
    Two orange shoulders Close up, both arms now share the same orange shoulder assembly; the red one has gained it too.
    Questions. Is the red arm a stock SO-100 that got converted? Why two arms (leader and follower for teleoperation?), and why the colour coding?

    It moves

    First motion The red arm with an orange shoulder at the edge of the desk, sweeping through its range and reaching out with the gripper.
    Your words. How was it driven: hand-written poses, a leader arm, a script? What broke or surprised you once it moved?

    Baseplates and second revisions

    Red, with a big gear The red arm mounted on a grey printed baseplate, with a large orange gear on the side of the shoulder and a small controller board nearby.
    Blue, same treatment The blue arm on a matching baseplate with the same orange geared shoulder.
    Yellow lobes The blue arm with a yellow upper arm, and a yellow lobed part at the shoulder in place of the gear.
    Red, refined The red arm on its baseplate from another angle, upper arm raised.
    Questions. What is the yellow lobed part: a cam, a counterbalance, a cable guide? What did the baseplates fix? What changed between 3 and 12 December?

    Picking things up

    Your words. There's a nine-month gap here (December → September). What happened in between? This is where the URDF, the simulation and the teardown come in.
    Pick and place The red arm picks a blue cube up off a box, lifts it clear and carries it across.
    The pair today Both arms (red, and blue with yellow) on their grey baseplates on a workbench, each with its own small controller board.
    Questions. In the pick-and-place video, is the arm being teleoperated or running a learned policy? Is the second arm the leader?

    Unplaced

    Decide. These two were in the folder but don't show the arm: a clay figure study, dated 6 September 2025. Do they belong as a prologue (anatomy, how a shoulder moves), or should they go? The base has a name written on it, so check that's OK to publish.
    Clay figure, front A crouching figure sculpted in clay, one arm braced on the board.
    Clay figure, back The same figure from behind, showing the shoulder blades and back.