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3D PRINT A ROBOT ARM THAT COPIES YOUR EVERY MOVE

A 3D-printed robotic arm that mirrors your hand in real time — move the leader, the follower copies you.

by NormaCore

FULL CAD BOM FIRMWARE DOCS

RoboticsOpen-hardware

Built with3D printing

difficulty
●●●●○
time
a weekend-plus
license
MIT
repo
repo ACTIVE644 stars

WHAT YOU’LL NEED

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COMPAREE VERDICT

ElRobot is a leader-follower robotic arm built for imitation learning: you move the leader arm by hand and the follower copies it, while cameras record the demonstration for AI training. Both arms are 3D-printed PLA driven by Feetech ST3215 serial bus servos — eight per arm — connected to a computer over USB through a small bus servo driver board. 7+1 degrees of freedom, 430 mm reach, about 800 g per arm, roughly 29 hours of printing for the pair and about 220 dollars in parts per arm. The repository gives you STEP and STL files, ready-made Bambu/Orca print profiles, a BOM with supplier links, PDF assembly manuals and a URDF model. The hard part is not the printing — it is sourcing the right servo variants (the leader uses low-ratio servos for back-drivability) and getting the software stack running. Licence is Apache-2.0 for the hardware and MIT for the software, so commercial use is allowed.

GOOD TO KNOW

  • —STEP and STL files for both leader and follower arms are in the repository.
  • —BOM lists Feetech ST3215 serial bus servos (eight 12 V/30 kg for the follower, eight low-ratio 7.4 V for the leader), a bus servo driver board per arm, power supplies, cameras and a powered USB hub.
  • —Control software is NormaCore Station, a single binary with a web UI for teleoperation, data collection and inference; the servos need no custom firmware.
  • —Assembly guide and URDF model are included.
  • —The ElRobot hardware files are Apache-2.0 and the software is MIT — both allow commercial use.
  • —No custom PCB; the servos daisy-chain to an off-the-shelf serial bus servo driver board on each arm.

Parts to buy

7 items

From our check of the build. Exact quantities and part numbers are in the creator’s BOM.

  • 16 Feetech ST3215 serial bus servosFind
  • 2 serial bus servo driver boardsFind
  • 12 V/4 A and 5 V/3 A power suppliesFind
  • Powered USB hubFind
  • Data USB cablesFind
  • Arm camera and a table cameraFind
  • Optional clampsFind

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Can I build this?

PrintLeader arm (~320 g PLA, ~13.5 h) and follower arm (~360 g PLA, ~16 h), plus a camera mount and optional driver enclosure. Ready-made .3mf profiles for Bambu/Orca; strong_parts need 6 walls and supports. 0.4 mm nozzle, 0.2 mm layers.
Buy16 Feetech ST3215 serial bus servos (8× 12 V/30 kg for the follower, 8× 7.4 V ST-3215-C044 or C046 for the leader), 2 serial bus servo driver boards, 12 V/4 A and 5 V/3 A power supplies, a powered USB hub, data USB cables, an arm camera and a table camera, optional clamps.
ToolsWell-calibrated FDM 3D printer with a 0.4 mm nozzle, PH1 and PH2 screwdrivers, a computer with USB ports and a powered USB hub.
SkillsIntermediate 3D printing (supports, 6-wall strong parts), careful mechanical assembly from the PDF manuals, servo wiring, and installing and running the NormaCore Station software. No PCB soldering.
TimeAbout 29 hours of printing for both arms, then assembly from the PDF manuals and software setup in NormaCore Station.
Cost$$ — about 220 dollars per arm according to the project, dominated by the eight ST3215 servos at roughly 20–22 dollars each; PLA for both arms is about 700 g.
SafetyNone beyond ordinary electronics care. Low-voltage DC servos, no mains, no sharp edges, no hazardous materials. Keep fingers clear of moving joints during calibration.

Build at your own risk. Projects involve tools, electronics and sometimes mains voltage — follow the creator’s safety notes.

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Start here

Navigation into the creator’s own docs — we don’t rewrite the guide, we route you to the source.

  1. 1.Read the ElRobot README and the follower and leader PDF assembly manuals in hardware/elrobot to understand the build order. (Start with the BOM and print order — you need both arms and all brackets before assembly.)
  2. 2.Print the leader (~13.5 h) and follower (~16 h) from the ready-made .3mf profiles or the combined STL files.(Use PLA with a 0.4 mm nozzle and 0.2 mm layers. If you slice the combined STLs yourself, print panels.stl without supports, regular_parts.stl with tree supports and strong_parts.stl with 6 walls, supports and a brim. The .3mf files include three camera mounts and two optional driver enclosures: keep only the ones you need.)
  3. 3.Order the servos, driver boards, power supplies, USB hub and cameras from the BOM. Test each servo before installation.(The follower uses 12 V/30 kg ST3215 servos, the leader the 7.4 V low-ratio C044 (1:191) or C046 (1:147). On AliExpress they come in 6-packs, so eight servos means buying two packs.)
  4. 4.Connect both servo driver boards to your computer over USB and run NormaCore Station.(Use data-rated USB cables — charge-only cables stop the computer from seeing the driver boards.)
  5. 5.Assemble the follower arm from its PDF manual, then the leader arm (its manual shows only the different steps), and wire the servos.(Work through the manual in order; the leader and follower share most parts.)

KNOWN ISSUES

  • Buy the right servo variants. The follower needs 12 V/30 kg ST3215s; the leader needs the 7.4 V low-ratio versions (C044 or C046) so you can move it by hand. On AliExpress they come in 6-packs, so eight servos means buying twelve.
  • Cameras need a powered USB hub. High-frame-rate video can overload a laptop's or Raspberry Pi's own ports, and use data-rated USB cables or the driver boards will not show up.
  • Print quality matters for the joints: calibrate flow and bed levelling first, print strong_parts.stl with 6 walls and supports, and follow the follower manual step by step (the leader manual only shows the differences).
  • The arm itself weighs about 800 g and the project does not publish a lifting capacity. Keep loads light until you have tested it.
  • Check every servo and its ID before closing up the arm, so you are not taking a joint apart later. Run both arms in NormaCore Station before you start recording demonstrations.

Can I use different servos?

The design is built around Feetech ST3215 bus servos, and the STEP files let you modify mounts, but stick to the BOM servos for the first build: the leader specifically needs the low-ratio C044 or C046 variants to be back-drivable.

What controls the arms?

No ESP32 at all. Each arm has its own small serial bus servo driver board connected to your computer over USB. The leader's ST3215 servos report their positions and the software sends them to the follower.

Can I control it with a VR headset or joystick instead of the leader arm?

The documented setup uses the physical leader arm in NormaCore Station. Other input devices would mean changing the software yourself; the project does not document it.

How much does the print weigh?

About 680 g of PLA for both arms (roughly 320 g leader, 360 g follower), so one 1 kg spool covers it.

What is the repeat accuracy?

Not documented. The project does not publish repeatability or payload figures, so test it with your own task before relying on it.

Community builds

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Discussion1

FROM THE COMPAREE TEAM

About 29 hours of printing, 16 serial bus servos, and you get a follower arm that mirrors the leader arm in real time. What would you make it grab first?

CompareeTEAM2mo agoedited

Practical notes from our verification: ElRobot lives in the norma-core repository, with the hardware README, BOM, STL and 3MF files and PDF manuals for the leader and follower arms in hardware/elrobot. The arms use Feetech ST3215 serial bus servos, eight per arm (12 V on the follower, 7.4 V on the leader), each arm with its own serial bus servo driver connected over USB to a computer running the NormaCore Station software; there is no ESP32 and no WiFi. The project estimates about 220 dollars per arm, and printing takes roughly 13.5 hours for the leader and 16 hours for the follower. The servos are often sold in packs of six, so buying eight can mean buying twelve. The README links an official NormaCore video. Licensing is permissive: the ElRobot hardware is Apache-2.0 and the rest of the repo is MIT, so commercial use is allowed. Correction (4 October 2026): we re-checked this page line by line against the project's own repository, documentation and videos, and fixed errors in earlier versions.

NormaCore

NormaCore builds an open toolkit for developing and operating physical systems, from robots to sensor networks. ElRobot is their fully 3D-printed 7+1 DoF arm for imitation learning, published in 2026 alongside their Station control software, with a community on Discord and YouTube.

GitHub

Star the project on GitHub

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