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SO-100 (SO-ARM100)

Original Standard Open Arm 100: a 3D-printable 6-DOF low-cost arm that served as the first reference hardware for LeRobot; superseded by the SO-101.

discontinuedDIYL2 Sim-verifiednot buildable

Difficulty index

3.4/13
3.4
Index breakdown
Classarm+2
Cost$244 BOM+0.4
DOF6 DOF+0.6
Parts26 parts+0.4
How the index works

Specimen photographs

6 items
  • SO-100 (SO-ARM100) studio render, three-quarter front view
    Studio render of the published MJCF, keyframe pose from the model. Geometry only; colours are the model's own materials.Rendered by Hyperspawn from mujoco_menagerie/trs_so_arm100 (Apache-2.0) · Hyperspawn render · source
  • SO-100 (SO-ARM100) studio render, side view
    Side view of the same MJCF render, keyframe pose from the model.Rendered by Hyperspawn from mujoco_menagerie/trs_so_arm100 (Apache-2.0) · Hyperspawn render · source
  • 4 s turntable of the MJCF studio render.Rendered by Hyperspawn from mujoco_menagerie/trs_so_arm100 (Apache-2.0) · Hyperspawn render · source
  • SO-100 leader and follower arms, orange and yellow 3D-printed parts
    From the SO-ARM100 repo media folder.The Robot Studio (SO-ARM100) · Apache-2.0 · source
  • Build guide for Standard Open ARM 100 5DOF - Low cost DIY 3dprinted robot arm Le Robot Hugging Facetherobotstudio · official video, embedded · source
  • Assemble and Calibrate SO-100: LeRobot Tutorial #7 by Jess MossHugging Face · official video, embedded · source

Spec card

4/13 known
Degrees of freedom
6
BOM total
$244 · 6/8 lines priced
Part count
26
Actuators
Feetech STS3215 serial bus servos

Not in the registry: height, mass, payload, speed, runtime, battery, price, compute, sensors. Left blank rather than guessed.

What you learn building it

  • +Forward and inverse kinematics
  • +Joint calibration and homing
  • +Trajectory planning
  • +Teleoperation and collecting demonstrations for learning
  • +FDM 3D printing: 5 parts
  • +Tools used: FDM printer, 220x220 mm bed or larger, PLA filament, Putty knife, Phillips #0, Phillips #1, Hugging Face account, CUDA or Apple-silicon machine, or HF Jobs
  • +13 assembly steps across 8 subassemblies
  • +Flashing firmware for Feetech STS3215 servo (EEPROM: ID 1-6 and baudrate via lerobot-setup-motors), Waveshare serial bus servo driver (USB bridge, no user firmware)
  • +Testing in simulation (URDF/MJCF) before touching hardware
  • +Running published learned policies

Openness

100/100
  • publicCAD
  • publicBOM
  • publicFirmware
  • publicSim model
  • publicSoftware
  • publicPolicies

Score 100/100 computed from what is public: CAD 25, BOM 20, sim 20, firmware 15, software 10, policies 10. Hardware license: Apache-2.0. Software license: Apache-2.0.

Repos & models

URDF ↗MJCF ↗MuJoCo Menagerie ↗
SiteDocs

Build it

Discontinued.
BOM total
$244
Lines priced
6/8
Parts
26
Steps
13
Version
0.1.0

2 lines have no price yet, so the real total is higher.

Largest BOM lines

Largest bill of materials lines
PartQtyUnit
Feetech STS3215 servo, 7.4 V (6 follower + 6 leader; gears are removed from the 6 leader units)STS321512$15
Motor control board: Waveshare serial bus servo driver (USB-C), one per arm2$11
DC power supply, 5 V, 5.5x2.1 mm barrel (one per arm)2$10
Table clamp, 4 pcs1$9
USB-C cable, 2-pack1$7
Precision screwdriver set (Phillips)1$6
Wrist camera (optional): 32x32 mm USB UVC camera module, min 720p / 30 fps1?
Overhead / scene camera (optional): USB webcam1?

Fabrication

  • FDM 3D printing5
Firmware repo
  • Superseded. The SO-ARM100 README marks the SO-100 docs as deprecated and points new builds to the SO-101 (improved wiring, no gear removal, purpose-geared leader servos). The files remain for existing arms and DIY.
  • SO-100 vs SO-101: SO-100 leader servos are standard 7.4 V STS3215 with the gears removed by hand, while the SO-101 leader uses intact 1/191 and 1/147 geared servos. SO-100 motor IDs must be set before assembly because the connectors are hard to reach afterwards. The SO-100 gripper motor takes 3 screws per side, the SO-101 2. The two use different STL sets (SO_ARM100_08k vs *_SO101), and some SO-101 add-ons, such as the hex-nut wrist camera mount, are stated not to fit the SO-100.
  • USD prices are the SO100.md figures (two-arm total $232, 244€, ￥1343.16) and exclude printing and cameras. The two-arm servo line links a 6-pack listing while the unit price is per servo.
  • The table-clamp row in SO100.md is misaligned: a ￥9.2 TaoBao entry sits in the EU columns and there is no EU link.
  • SO100.md footnotes 1-3 (servo, power supply, screwdriver) are referenced but not present in the file. Its printing links point to /stl_files_for_3dprinting/..., which no longer exists; the files are now under STL/SO100/.
  • The upstream BOM has no fastener line. Screws are not itemised and the SO-100 assembly text gives no screw sizes (the SO-101 guide uses M2x6 and M3x6).
  • Cameras are not in the core BOM; the camera lines come from the optional mount guides and are marked optional.

Simulate

Load it in the browser simulator and train a policy before building.

Open on humanoidrobots.training ↗

Next rung

Aero Hand Open →Chestnut Robotics (formerly TetherIA) · hand · index 5.5

Easiest buildable hand, the next class up from arm. Adds tendon and linkage transmissions.

Note'discontinued' = superseded by SO-101 in the same repo (SO100.md kept); files remain available for DIY.

Sources

  1. [1]GitHub repochecked 2026-09-24
  2. [2]Hackster: SO-101 as next-gen of SO-100checked 2026-09-24
  3. [3]LeRobot SO-100 docschecked 2026-09-25