Thor is an open source, 3D printed, 6 degrees of freedom robotic arm. Designed to be affordable and accessible, Thor is ideal for educational settings, makers, and robotics enthusiasts. With a height of 625mm it has the capacity to lift up to 750g. Its yaw-roll-roll-yaw-roll-yaw configuration is common in industrial manipulators.

✨ Key Features

• 🧩 Born Open Source: Designed using open source software such as FreeCAD, KiCAD, GRBL, RRF and ROS.
• 🕊️ Released Open Source: All source files published under CC-BY-SA-4.0 license.
• 💰 Low Cost: Hardware cost below 350€.
• 📚 Suitable for education: Perfect for robotics courses in schools and universities, there are already success cases!
• 🧰 G-code Controlled: Uses the same language as 3d printers and CNCs to move.
• 🐳 ROS2 Integration: Available implementation using Docker for flexibility.

📦 Repository Structure

• freecad-src – Source files of 3D models created with FreeCAD
• mods – Modified models and improvements to standard models
• stl – Printable STL files
• step – STEP files

🔗 Related Repositories

• Thor-ROS: ROS2 & Moveit2 configuration files and packages to work with Thor.
• ThorControlPCB: Source & manufacture files of Arduino Mega shield designed to control Thor.
• Asgard: Control software for thor motors with a simple interface.

🚀 Getting Started

1. Read the Documentation: Comprehensive guides on printing, assembly, electronics, and firmware are available at thor.angel-lm.com/documentation.
2. Print the Parts: Use the STL files in this repository.
3. Get the Hardware: Take a look to the Bill of Materials.
4. Assemble the Robot: See here for assembly videos and interactive instructions.
5. Assemble & Setup the Electronics: The Wiring & Setup guide will help you prepare your electronic board and make the hardware connections.
6. Install the Firmware: Depending on which electronics you have chosen, you will have to perform a different firmware configuration. This page explains the steps to follow.
7. Control the Robot: Use Asgard to move the robot. Here is how to do it.

🧠 Technical Specifications

• Degrees of Freedom: 6 (yaw-roll-roll-yaw-roll-yaw).
• Height Stretched: 625mm (without end effector).
• Payload Capacity: 750g max (including end effector weight).
• Motors: Stepper motors.
• Electronics: DIY PCB or commercial boards.
• Transmission: 3D printed gears, GT2 pulleys and belts.
• Software: FreeCAD, KiCAD, GRBL, RRF, ROS2.

📚 Additional Resources

• 🌐 Official Website: thor.angel-lm.com
• ❓ Frequently Asked Questions: FAQ
• 💬 Thor Community Forums: Thor Forums
• 💬 Thor Discord Server: Thor Robot Community

🤝 Contributing

Contributions are welcome! If you'd like to improve Thor — whether it's design, documentation, or code — feel free to open an Issue or submit a Pull Request.

🤖 Simulation

This repository also carries a solid-node simulation layer under simulation/: the whole machine assembled from its own published parts, six joints and a gripper a maker can drive in a browser, and contracts that fail when an interface stops holding. Nothing under stl/, step/, freecad-src/, mods/ or doc/ is edited by it — the printed parts are read from step/*.step as they are, and every placement comes from the FreeCAD Assembly4 documents' own solved values.

Running it

With solid-node installed:

solid build                       # build once and publish the model
solid develop                     # a live viewer that rebuilds on save
solid test --exact simulation/thor.py     # the contracts
solid snapshot -o thor.png --autocenter --viewall

The contracts run on the exact kernel and only on it. Seven of Thor's printed parts tessellate to a mesh that is not closed at the default deflection — their solids are sound — and the faceted kernel refuses those meshes rather than answering, so --faceted cannot run this machine.

What the sliders mean

driver	what it measures	range
art1	the base's yaw about Z	±180°
art2	the shoulder's roll about Y, 202.0 mm up	±90°
art3	the forearm's roll in the machine frame	±135°
art4	the forearm's yaw about its own axis	±180°
art5	the wrist's roll about Y, 556.0 mm up	±105°
art6	the tool's roll about the wrist output axis	±180°
grip	the clear opening between the two jaws	0–69.94 mm

art3 is absolute, not relative to the upper arm, because the machine is: the belt that sets the elbow runs from a pulley carried by the shoulder housing rather than by the arm. Swing art2 alone and the forearm keeps pointing the same way while no elbow motor turns. The published joint limits are not in this repository, so the ranges above are the mechanism's own travel where the geometry bounds it and the full circle otherwise; they are presentation only and clamp nothing.

The instructions are Home, Ready, Reach, Pick, Place and Park. A scenario contract triggers all six in order, at the rate each joint's own mechanism allows, and asks the machine on the way whether it has driven any part through another.

Looking at it in pieces

Each sub-assembly builds on its own, in the order the robot stands up:

solid build simulation/base.py:Base       # the plinth and the electronics box
solid build simulation/art1.py:Art1       # the shoulder housing and up
solid build simulation/art2.py:Art2       # the upper arm and up
solid build simulation/art3.py:Art3       # the elbow's output link and up
solid build simulation/art4.py:Art4       # the forearm and the wrist
solid build simulation/art56.py:Art56     # the wrist differential and gripper
solid build simulation/gripper.py:Gripper # the end effector alone

What the model found in the design

Assembling Thor in software is the first time anything has asked its files every question at once, and some of the answers are worth having. All of these are measured; docs/measurements.md carries the numbers and how they were read, and none of them is fixed upstream.

• The elbow-to-wrist span is 194.0 mm, not the 195.00 the axis drawing states. The drawing's other two spans, 202.00 and 160.00, match the assembly exactly.
• No gear pair in the assembly is phased to mesh. All five printed pairs interpenetrate at the pose the assembly records, by 0.2 to 46.2 mm³. A builder puts a pinion on at an angle that drops into mesh; the model measures that angle for each pair.
• The two shoulder pinions need different angles — 31.5° and 19.5° — although the design places them symmetrically. They are mirrored instances of one part.
• The base pinion has no interference-free angle at all. It stands 2 mm taller than the ring gear it drives, and its top rim cuts the rim above the teeth by 0.20 mm³ whatever angle it is put on.
• The forearm pinion's lower flange fouls the column's gear by 24.30 mm³ at every angle: the flange is 1.2 mm larger in radius than the teeth, and the ring at the foot of the column's gear is not relieved for it.
• The arm's two sprung tensioners do not reach its belt. The taut run between the shoulder and elbow pulleys passes 5.41 mm clear of their rims, so they are drawn retracted and the belt is a plain two-pulley loop of 462.98 mm — 231.5 teeth, which is what the tensioners are there to take up.
• The arm's two belt pulleys share only 3.65 mm of land for a 6 mm belt. Their running surfaces overlap between 18.000 and 21.651 along the shoulder axis and nowhere else. The forearm's pulleys share their whole 7.5 mm land, which is how you know this one is the arm's.
• The forearm belts are named for 208 mm and their pulleys need 223.5 mm. The design's own belt solid measures about 220, so it is the name that is wrong — but a builder buying by the name gets a belt that will not close.
• All three belt solids are placed where their pulleys are not. Both forearm belts lie flat in a plane perpendicular to their pulleys' axes, and the arm's belt clears both of its own pulleys.
• Art1Top is one solid that tessellates into five bodies — the part, three zero-volume patches and a detached lug — which is why every question here is asked of the solids.
• The wrist's belt pulleys are 4 mm-bore pulleys on a 5 mm shaft. The design's own GT2x40PulleyM4 is bored 2.000 in radius and its own Shaft_5x102mm is 2.500, so half a millimetre of bore wall is solid shaft over the pulley's whole 17 mm.

The interference contract that finds these is red on purpose: 272 pairs of solids share space at the home pose and thirteen are recorded, so the model is still telling the truth about how much it has explained. docs/measurements.md breaks the rest down.

• GripperBot is sunk 3248.9 mm³ into Art56GearPlate. The gripper base's mounting boss occupies the same space as the plate it bolts to; neither part rebates for the other.
• Art4Optodisk passes 24.1 mm³ through Art3Body's wall rather than running in a slot cut for it.
• Art2MotorGear and Art4BodyBot are exported inside out, measuring −8435.0 and −113674.9 mm³.
• GripperActiveArm and GripperPassiveArm each hold two solids: the arm, and a loose Ø3.4 × 5.0 pin standing in its outer pivot hole. A builder slicing those files gets a plug printed inside the hole.
• GripperFinger is used mirrored but published only one way, so a builder printing from step/ or stl/ gets two identical jaws rather than the handed pair the assembly uses.
• Art1OptoFix, Art4BearingPlug and Art56Interface are drawn and exported but placed in no assembly.
• The design models no fastener. Every screw and nut in the model is derived from the parts' own Ø3.4 holes, Ø5.9 counterbores and 5.80 across-flats pockets: 181 fasteners, 77 of them with nuts, and 85 further hole stacks the geometry cannot classify.

📜 License

Thor is licensed under the Creative Commons Attribution-ShareAlike 4.0 International License. You’re free to use, modify, and distribute this project under its terms.

🎥 Videos