njc-articulated-hinge: sim-verified articulated 3D objects

Fifteen household objects rebuilt as physics-valid articulated assets for robot learning. Each object has one revolute joint (a lid, door, or clamshell hinge) and ships as URDF and native MuJoCo MJCF with textured visual meshes, convex collision decomposition, computed mass and inertia, a tuned joint, and its own actuator. Every object was actuated through its full range in MuJoCo and had to pass an automated 26-check physics gate before inclusion. Executability: 15/15.

Built by notjustchairs.ai.

Why this dataset

Most articulated-object assets are kinematics-only: joints with a type, axis, and limits, but no validated dynamics. Loaded into a physics simulator they interpenetrate, jam, fly apart, or fall open under gravity. This set inverts the priority: the simulator is the gatekeeper. An asset only ships if, loaded cold from the published files with contacts fully active, it:

• rests closed on the floor without interpenetration (worst-case contact depth budgeted at 5 mm),
• actuates through at least 90% of its authored joint range,
• holds position when released with the actuator disabled (no gravity flop),
• survives a full-force slam without going unstable,
• does not topple from its own articulation (instability is recorded when present),
• and passes structural checks on meshes, textures, inertia tensors, and file formats.

The 15 objects

Class	Objects
Lid (8)	wooden box, jewelry box, toolbox, bread box, cooler, toy chest, trash bin, toilet
Door (6)	kitchen cabinet, medicine cabinet, minifridge, microwave, safe, locker
Clamshell (1)	laptop

What's in each asset

Each asset is a folder under assets/<id>/:

File	Description
object.urdf	Free-standing URDF: two links, one revolute joint with limits, effort/velocity bounds, damping and friction, explicit inertials.
object.xml	Native MuJoCo MJCF: free-floating base, hinge joint with full dynamics, position actuator, visual and collision geoms.
scene.xml	MuJoCo scene wrapper (object + floor + light), Menagerie-style; load this to simulate immediately.
base.glb, lid_closed.glb	Textured visual meshes (movable part baked at the closed pose).
base_final.obj, lid_final.obj (+ .mtl, .png)	Visual meshes referenced by URDF/MJCF.
collision/*.obj	Convex decomposition (CoACD), separate from visuals.
web_joint.json	Hinge point, axis, and limit for browser viewers.
meta.json	Scale provenance, material, masses, inertia method, joint schema, gravity-torque analysis, holding mechanism, and the full QC gate record.
qc.json	The 26-check validation report for this object.
sim.gif	The MuJoCo verification sweep, rendered.
source.png	The concept image the object was reconstructed from.

A top-level metadata.jsonl holds one row per asset; stats.json has aggregate numbers.

Physics fields worth knowing

• Real-world scale: every object is metrically scaled from a user-provided real height (scale_source: "user"), not normalized units.
• Mass and inertia: computed from the actual mesh shell (voxel-shell method for non-watertight parts, recorded per part in inertia_method), optionally rescaled to user-provided masses. A hollow box is not a solid block here.
• Joint dynamics: each joint records its maximum gravity torque and a holding_mechanism. Gravity-opening joints receive dry friction or a counterbalance spring (in the MJCF; URDF has no native spring element, so URDF consumers should read meta.json and inject equivalents at load time, the same pattern ManiSkill uses for PartNet-Mobility).
• Collision honesty: convex decomposition cannot represent every nesting shape perfectly; where a simplification was applied it is recorded in meta.json (collision_note), never silent.

Intended uses

• Robot-learning environments: drop scene.xml into MuJoCo and train open/close manipulation immediately; URDF for other simulators.
• Articulated-object understanding: paired source image, textured mesh, parts, and joint parameters per object.
• Scene composition: retrieval-ready articulated assets for simulation scene generators.

Limitations

• 15 objects, one revolute joint each. No prismatic joints, no multi-joint objects in this release.
• Collision is convex decomposition: shapes that nest (a flip-lid skirt over a rim) carry a documented clearance floor in meta.json.
• Visual meshes are single-image 3D reconstructions: stylized, occasional small surface artifacts, not CAD or scans of specific real products.
• Masses and materials are assigned per object (recorded with provenance), not measured from physical references.
• Validated in MuJoCo. URDF loads cleanly elsewhere, but other simulators' defaults differ; PhysX/Isaac validation is planned, not included.

License & attribution

Released under CC-BY-4.0:

"njc-articulated-hinge: sim-verified articulated 3D objects" by notjustchairs.ai, CC-BY-4.0.

Reconstruction and processing use open tools and models with permissive licenses, including TRELLIS (MIT outputs), BiRefNet, CoACD, and MuJoCo. See THIRD-PARTY-LICENSES.md.

Citation

@misc{notjustchairs_articulatedhinge_2026,
  title  = {njc-articulated-hinge: sim-verified articulated 3D objects},
  author = {notjustchairs.ai},
  year   = {2026},
  howpublished = {Hugging Face Datasets},
  note   = {CC-BY-4.0}
}

Dataset statistics

• 15 objects: 8 lids, 6 doors, 1 clamshell
• 390 automated QC checks passed (26 per object, zero failures)
• Executability: 15/15 (full-range actuation in MuJoCo from the shipped files, contacts active)
• 1296 convex collision hulls across the set
• All objects metrically scaled with user provenance; masses assigned per object

Object	Class	Mass (base+lid kg)	Open range	Hulls	QC
laptop	clamshell	3.0 + 1.5	105 deg	53	26/26
kitchen cabinet	door	6.0 + 1.6	105 deg	124	26/26
locker	door	25.0 + 4.0	127 deg	49	26/26
medicine cabinet	door	8.0 + 2.0	80 deg	71	26/26
microwave	door	12.0 + 1.5	83 deg	95	26/26
minifridge	door	18.0 + 3.0	113 deg	59	26/26
safe	door	35.0 + 7.0	89 deg	82	26/26
bread box	lid	1.5 + 0.5	113 deg	75	26/26
cooler	lid	4.0 + 1.5	129 deg	121	26/26
jewelry box	lid	0.5 + 0.1	142 deg	53	26/26
toilet	lid	25.0 + 2.0	94 deg	124	26/26
toolbox	lid	3.0 + 0.8	130 deg	60	26/26
toy chest	lid	7.0 + 2.0	171 deg	130	26/26
trash bin	lid	2.5 + 0.4	120 deg	110	26/26
wooden box	lid	3.2 + 1.3	139 deg	90	26/26

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Questions, custom articulated collections, or verified data for your pipeline? Get in touch.