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Challenges: Achieving independent rotation in a low profile, low mass package; maintaining concentricity and a tuned gap within 3D printed tolerances; implementing magnetomechanical on/off that held on thin steel without false triggers while balancing magnet strength and back iron for secure yet releasable attachment; routing a compact wiring harness through moving parts without drag; ensuring reliable motor startup, smooth rotation, and low vibration; managing LED and motor power from a small lithium ion pack with safe charging; preserving screen accurate aesthetics without sacrificing assembly or serviceability.
Why: The project aimed to convert a screen-accurate arc reactor concept into a reliable wearable electromechanical system. It targeted magnetomechanical activation on ferromagnetic surfaces, independent motion of nested assemblies, and low-profile, low-mass packaging using CAD-driven design, 3D printing, and accessible components.
Learned: Established an end-to-end electromechanical workflow that included parametric CAD, design for additive manufacturing, and print-informed tolerance control. Practiced rapid iteration and failure analysis across fifteen revisions to reduce mass and stack height and to increase reliability; balanced aesthetic fidelity with functional requirements and validated performance through magnetic holding and relative motion tests.
Microwave Magnetron
In a domestic microwave oven, the magnetron converts high-voltage DC into microwave radiation. A heated filamentary cathode emits electrons that experience a strong radial electric field from an anode held near ground and a cathode pulsed at roughly 2–4 kV negative. Two permanent ring magnets provide an axial magnetic flux, producing crossed electric and magnetic fields. Under these conditions, the electrons follow cycloidal paths and bunch into azimuthal spokes. As the spokes sweep past slots leading into the anode’s resonant cavities, they excite and phase-lock RF currents; the oscillation frequency is defined by the cavity dimensions and is standardized at 2.45 GHz (λ≈12.24 cm) for household units. Energy is coupled out through a probe into a waveguide and then into the cooking chamber, where a mode stirrer or turntable improves field uniformity.
To me, the magnetron visually resembles the arc reactor from the "Iron Man" movie, which was the reason behind pursuing the project.
Highschool attempt
The prototype was completed in three days from available components. An optical cavity filled with glycerol provided a transparent medium, illuminated by UV LEDs and secondarily backlit by a white LED behind a blue filter. Copper windings were salvaged from old transformers and power supplies. Electrical power originated from a salvaged lithium-ion phone battery. Mechanical attachment used a neodymium magnet recovered from a hard drive, engaging a steel plate affixed to the chest.
Napkin design
The arc-reactor was revisited as a learning platform with intentionally expanded requirements. Illumination and magnetic chest mounting were preserved, but added objectives included screen-accurate geometry achieved through CAD-driven design and additive manufacturing, as well as autonomous rotation of the central core independent of the outer shell. To enable these functions, the mechanism adopted a three-section layout (outer shell, intermediate carrier, core rotor) with mechanical decoupling for relative motion. Packaging constraints prioritized low profile and low mass to maintain comfort and operational stability. A magnetomechanical on/off mechanism was conceptualized, enabling automatic power-up of lighting and rotation on contact with a ferromagnetic surface and fail-safe off otherwise.
Early Versions
Initial attempts encompassed all the major design features necessary for the desired functionality. The design lacked tolerance fine tuning, goodness of fit of components and concentricity. Concentricity primarily suffered due to the print orientation of a threaded component where part strength was sacrificed for dimensional accuracy. The thread size of the component was small, and limited by the ID of the bearing used. Additionally, reliable assembly of all the components had to be ensured, implementing nestability and disassemblable design.
Magnetic Actuation
The base contained three recesses, each comprising one zinc-plated steel washer serving as a back iron and two neodymium ring magnets. A central depression captured molten plastic to lock the magnets in place. A metal rod linked the actuator pin to the actuator foot, which functioned as the stand surface when the magnets were disengaged.
Bearings
Initially, ceramic hybrid bearings were chosen for a lower friction compared to conventional bearings, while keeping the dimensional accuracy and load-bearing capacity. In early iterations, hybrid ceramic bearings with the lowest profile and largest ID on the market were used. Being only 5mm tall, it was sufficiently short, but the ID was too small, making the threaded components fit in that volume too unreliably, both by strength and dimensional accuracy. Later, low-profile concentric bearings with the largest ID were sourced, sacrificing some friction for better concentricity and reliability. The new bearing height was only 4mm, which allowed for sufficient room for a low-profile spacer, ensuring proper spacing between the inner core and the outer cage.
Design Evolution
Across fifteen iterations, the design evolved substantially. Hybrid-ceramic bearings were replaced with conventional bearings, whose larger inner diameter permitted a larger thread size. The magnetron assembly was shortened to reduce mass and stack height. A shaft collar was incorporated to improve concentricity and coupling stiffness. Hard stops were added to the outer cage and inner core housing to define a tuned, precise gap. A spacer was inserted between the bearings to enhance reliability and simplify assembly.
Coil Winding
The copper windings served as aesthetic features required for a movie-accurate appearance. A dedicated winding jig compatible with drill-driven rotation was designed to produce uniform turns. Coils were placed at equal angular pitch around a polymer ring and bonded to the upper member of the outer cage, completing the subassembly.
Rechargeable
A 1000mAh, 3.7V (3.7Wh) Li-ion battery was used for powering the LED array and the geared DC motor. A USB Type-C charge-discharge module was used to ensure proper operation and rechargeability of the battery.
Final Design
The final configuration integrated the most recent revisions of each component and subassembly. Verification was performed via multiple additive builds and full mechanical assemblies to confirm fit, proper nesting, function, and tolerance stack-up.
Components
Magnetic actuator foot that served as the primary support when not actuated.
Base that connected the inner core and outer cage via bearings and housed the actuator foot and magnets.
Compartment within the base that housed zinc-plated steel washers and two neodymium magnets.
Outer cage that connected to the copper-winding subassembly and to the base through a bearing.
Inner core housing that connected to the base through a bearing and to the electronics housing.
Bearings that provided structural support, ensured concentricity, and enabled free rotation of components.
Bearing spacer that separated the bearing faces and isolated the inner core housing from the outer cage.
Threaded interface that coupled the motor shaft to the base to permit relative motion.
Cavity that housed a compression spring to bias the device’s weight and enable magnetic actuation.
Action pin that interfaced with a levered, wheel-actuated microswitch to control electronic power.
Cavity that housed the motor shaft collar, maintaining concentricity and structural integrity.
Cavity that housed the geared DC motor, providing high starting torque and smooth operation.
Wheel-actuated microswitch that provided on/off control of the electronics.
Bay that housed the USB-C Li-ion charge/discharge module.
Bay that housed the Li-ion battery.
Light diffuser to which an LED array attached.
Raceway that housed wiring between all electronic components.
Copper windings spaced on a ring to reproduce the screen-accurate appearance.
Copper-winding subassembly, separated from the outer cage to enable reliable assembly of nested components.
Magnetron housing that also supported decorative elements.
Microwave magnetron.
Decorative ring imitating the movie’s palladium core.
Assembly
Assembly used drilling, adhesive joints, and press fits. The base held three magnet bays, each with one zinc plated steel washer and two neodymium ring magnets locked by a central recess filled with molten plastic. Bearings and a spacer set concentric low low-friction rotation. The motor shaft assembly was threaded into the base. A compression spring and an action pin linked to the actuator foot operated a levered microswitch for on/off control. The outer cage and inner core housing rode on bearings; the copper winding ring bonded to the cage top; the shortened magnetron and decorative parts were installed. Electronics comprised a geared DC motor, an LED array, a lithium-ion cell, and a USB-C charge and discharge module wired to a common harness with the microswitch in series. Final checks verified free relative motion and stable magnetic engagement.
Testing
Functionality was validated on a thin ferromagnetic substrate, such as a metal can, verifying consistent magnetic engagement and relative concentricity. When mounted on a 90° surface, the gravitational moment did not overcome the magnetic holding force, and the unit remained attached. The recording showed low-friction, mechanically decoupled rotation of the outer cage relative to the core/base with illumination and inner core spinning.
Tony Stark built his reactor in a cave with a box of scraps. I significantly improved the design and functionality of my previous build, made with a box of scraps.
Future Work
A steel chest mounting plate remains to be developed to demonstrate screen-accurate magnetic docking and automatic actuation.
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