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Caden Kraft’s open-source quasi-direct-drive actuator is a 3D-printed motor-and-gearbox project, not a $40 finished product for sale. Kraft estimated about $40 in actuator parts, or about $70 including its motor controller, in a 2025 project write-up. He ordered the parts before US tariffs took effect, so those figures are historical estimates—not verified current prices.
What is Kraft’s actuator?
The design combines a radial brushless DC motor with a 7:1 cycloidal-profile planetary gearbox integrated into the assembly. Kraft describes this arrangement as quasi-direct drive: the reduction increases torque while keeping the actuator compact in height.
Its rotor uses a Halbach magnet arrangement to direct more magnetic flux toward the stator without a ferrous backing. Kraft’s design description says the chosen cycloidal tooth profile maintains constant contact and has no backlash; that is a design claim, not an independent durability assessment.
What the magnetic simulations show
Kraft compared four rotor arrangements in FEMM simulations: iron-backed and ironless versions of both Halbach and conventional magnet arrays. In his fixed-angle comparison, the ironless Halbach configuration produced 91.381% of the simulated average torque of the iron-backed Halbach version, while the ironless conventional array reached 72.455%. Kraft cautions that these figures are for comparing the simulated configurations; they are not measurements of the finished actuator’s torque.
#1 Best Overall
- Material: The integrated joint motor module is constructed from durable materials to ensure long-lasting performance in harsh environments
- Design: The brushless DC motor module features a lightweight, compact design that saves space
- Operation: The robotic actuator is plug-and-play, making it easy to use.
- Performance: The high-torque robotic joint actuator provides powerful drive support for humanoid robot joints
- Applications: The integrated joint module is widely applicable in various scenarios, including servo motors, exoskeletons, robotic joints, and quadruped robot arms
How much does the actuator cost?
Kraft’s July 2025 estimate was about $40 for the actuator bill of materials and about $70 with the controller. He explicitly noted that he placed the orders before US tariffs took effect and that pricing and availability may vary. Treat both figures as the creator’s historical estimates, not current retail quotes.
Hackster.io also reported the approximate $40 parts estimate and $70 controller-inclusive estimate, and described the 29.4 Nm result as a holding-torque test limited by the controller’s 50 A setting. Neither figure establishes what it would cost to source the parts now.
Rank #2
- Material: The integrated joint motor module is constructed from durable materials to ensure long-lasting performance in harsh environments
- Design: The brushless DC motor module features a lightweight, compact design that saves space
- Operation: The robotic actuator is plug-and-play, making it easy to use.
- Performance: The high-torque robotic joint actuator provides powerful drive support for humanoid robot joints
- Applications: The integrated joint module is widely applicable in various scenarios, including servo motors, exoskeletons, robotic joints, and quadruped robot arms
What does the build require?
The project uses a 10010, 36-slot stator, wound with six turns per slot—five full turns and one partial turn—using six parallel strands of 0.4 mm wire. Its specified rotor has 42 main-pole N52 magnets measuring 12 × 5 × 3 mm, paired with 42 smaller N50 magnets. Kraft reports a 0.7 mm air gap between rotor and stator.
The controller is an MKS X Drive Mini, which Kraft describes as based on the older ODrive 3.6 and equipped with an integrated Hall-effect encoder. The design therefore still depends on purchased motor, magnet, wire, and controller components, even though the custom mechanical parts are printed.
Rank #3
- Material: The integrated joint motor module is constructed from durable materials to ensure long-lasting performance in harsh environments
- Design: The brushless DC motor module features a lightweight, compact design that saves space
- Operation: The robotic actuator is plug-and-play, making it easy to use.
- Performance: The high-torque robotic joint actuator provides powerful drive support for humanoid robot joints
- Applications: The integrated joint module is widely applicable in various scenarios, including servo motors, exoskeletons, robotic joints, and quadruped robot arms
Printed parts and assembly notes
Kraft reports printing the custom mechanical parts in PA6-GF. In a retrospective comment, he suggests ordinary nylon might be preferable for the gears because glass additive could cause abrasion over time. That is the designer’s guidance, not a general material test result. A fixture was used to position and glue the rotor magnets.
The first-party project article links CAD, FEMM simulations, a bill of materials, ODrive configuration, and printable/CAD files hosted on MakerWorld. Kraft asks people who use the files to credit him. Check the source files for current reuse and licensing terms: Caden Kraft’s Ironless Rotor Cycloidal Planetary Actuator.
Rank #4
- Material: The integrated joint motor module is constructed from durable materials to ensure long-lasting performance in harsh environments
- Design: The brushless DC motor module features a lightweight, compact design that saves space
- Operation: The robotic actuator is plug-and-play, making it easy to use.
- Performance: The high-torque robotic joint actuator provides powerful drive support for humanoid robot joints
- Applications: The integrated joint module is widely applicable in various scenarios, including servo motors, exoskeletons, robotic joints, and quadruped robot arms
How much torque did Kraft measure?
Kraft reports two peak holding-torque tests, under different electrical limits. In the first, the actuator reached 14 Nm while powered by a 24 V, 25 A supply; that supply limited the test. In a later test using a different power source, it reached 29.4 Nm with the controller at the 50 A current limit set for the test. The wire surface reached 78°C. Kraft stopped the test because he felt the windings and plastic gearbox were approaching their limits.
The 29.4 Nm result is a peak holding-torque measurement in that test, not a continuous torque rating or evidence of long-term operation at that load. The reported finished actuator weight is 728 g. Kraft also calculates a cost-to-torque ratio of $2.47/Nm from the project’s figures; that is his calculation, not a current market comparison.
Best Value
- Material: The integrated joint motor module is constructed from durable materials to ensure long-lasting performance in harsh environments
- Design: The brushless DC motor module features a lightweight, compact design that saves space
- Operation: The robotic actuator is plug-and-play, making it easy to use.
- Performance: The high-torque robotic joint actuator provides powerful drive support for humanoid robot joints
- Applications: The integrated joint module is widely applicable in various scenarios, including servo motors, exoskeletons, robotic joints, and quadruped robot arms
What the $40 figure does—and does not—tell you
The estimate is useful as a project-specific parts-cost reference, but it does not establish a current price, a commercial product price, or a standardized performance rating. To compare this build fairly with another actuator, match the scope and test conditions:
- Compare actuator-only cost with actuator-only cost, or include the controller on both sides.
- Distinguish peak holding torque from continuous or dynamic torque.
- Account for the supply voltage and current limits used in each test.
- Compare mass and dimensions, along with the effort and equipment needed for printing and winding.
- Include the purchased components that are not printed.
For the original components, files, measurements, and project context, see Kraft’s project article. Hackster.io’s independent coverage is available at Hackster.io.
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