Lenz’s Law is one of those physics tricks that look like magic if you don’t understand what’s happening. [Seth Robinson] was inspired by the way eddy currents cause a cylindrical neodymium magnet to levitate inside a rotating copper tube, so he cast a spherical copper cage to levitate a magnetic sphere.
Metal casting is an art form that might seem simple at first, but is very easy to screw up. Fortunately [Seth] has significant experience in the field, especially lost-PLA metal casting. While the act of casting is quick, the vast majority of the work is in the preparation process. Video after the break.
[Seth] started by designing and 3D printing a truncated icosahedron (basically a low-poly sphere) in two interlocking halves and adding large sprues to each halve. Over a week, the PLA forms were repeatedly coated in layers of ceramic slurry and silica sand, creating a thick shell around them. The ceramic forms were then heated to melt and pour out the PLA and fired at 870°C/1600°F to achieve full hardness.
With the molds prepared, the molten copper is poured into them and allowed to cool. To avoid damaging the soft copper parts when breaking away the mold, [Seth] uses a sandblaster to cut it away sections. The quality of the cast parts is so good that 3D-printed layer lines are visible in the copper, but hours of cleanup and polishing are still required to turn them into shiny parts. Even without the physics trick, it’s a work of art. A 3d printed plug with a brass shaft was added on each side, allowing the assembly to spin on a 3D-printed stand.
[Seth] placed a 2″ N52 neodymium spherical magnet inside, and when spun at the right speed, the magnet levitated without touching the sides. Unfortunately, this effect doesn’t come across super clearly on video, but we have no doubt it would make for a fascinating display piece and conversation starter.
Using and abusing eddy currents makes for some very interesting projects, including hoverboards and magnetic torque transfer on a bicycle.
I see it drop at the end of the spin, so it really levitates nicely. Despite the video being 60fps it’s difficult to look past the motion blur. Whoever has a high speed camera should contact him so he can make a follow up ultra slow motion movie. Great work!
Looks like a truncated icosahedron, not a sphere.
Looks like you didn’t watch the video.
This is a beautiful structure. It looks like the kind of thing Bathsheba Grossman makes:
https://www.bathsheba.com/
though her metal structures are 3D printed directly. One of the oldest methods of casting is the lost-wax method, and this is technically a lost-PLA method!
its nice to see someone do a shell casting with proper meltout and burnout instead of doing a sandcasting and relying on the melt pour to vaporize captive pla
That’s a really interesting and well executed project. Chapter 2 would be something like having an Arduino (or whatever controller, even a PC) control the speed of the motor to rotate the piece and have it register sound as well, to capture the click-click sounds of the magnet hitting the “sphere”. That way, at a certain Hz, you get pure silence which means the magnet is floating perfectly. You could then test harmonics of that speed to see if it still floats. Awesome mini scientific article it would give!
Its just me or those two halves look terribly close to the two halves of a nuke core? More precisely, the explosives that are going to compress the radioactive core.
If this was during WW2, you will quietly come along with me and…
A soccer ball was once a “secret”!
Everything worth anything was once a secret. Smetling metals, making swords, growing tulips, blowing glass, gun powder, nukes, enigma machine, semiconductors, credit card pins, email passwords, computer passwords and more. Yeah, knowledege is power.
Yet both of us enjoy slow forward time travel (with the speed of 1 year per 365 days) and those secrets are out.
I hope you know that during the war the 3 letter agencies from US allowed the publication of a SF novel detailing the construction of a nuclear bomb (after cheking how the author find out about Los Alamos business, and found no link) because not allowing the publication could rise suspicions/alarms about the matter to the enemy.
Unfortunately for them, the russians already had spies there.
I’m not exactly sure what they would look like, only what popular fiction would have us believe what they look like.
Regardless though, if popular fiction is a close enough approximation to reality, and based upon the public information that rapid compression of core fissile material can cause criticality, the resemblance should be no surprise. A polyhedron is an n-faced approximation of a sphere. If you wanted to use discrete lumps of material to direct explosive energy into a sphere, you would arrange them in a polyhedron.
He got so close to a beautiful levitating ball video until the end. Me yelling at the screen: “come on!” “Sync-strobe your camera or lights with the spinny copper shape so we can see the ball float!”.
Try applying an electric field at right angles to the magnetic field, and see what happens.
if you get the frequency tuned properly it will open inter dimensional wormholes
You have remarkable skills, an abundance of good tools, and have executed the project masterfully. ⭐️⭐️⭐️⭐️⭐️
Would a sphere formed from copper mesh be easier? The skin depth at 1 kHz is about 2mm, so if spun fast enough it should create a repelling field, without the need for casting, it seems to me. Perhaps just a cylindrical copper mesh cage would be sufficient, brazed together.
It should work even better with an aluminum frame.
Maybe a stroboscopic lighting (synchronized with the rotation period) would allow seeing the shell immobile, giving a better view of the ball?
Really didn’t work as well as hoped/expected, but that may have been due to the holes and amount a icosahedron varies from a sphere. A sheet metal hammered / pressure formed copper sphere with drilled holes might have been less work. The lost-PLA cast shape was neat looking.
I think the lack of continuous levitation is due to magnetic slip. Once the ball magnet is spinning the same speed as the copper cage, there’s no induced current and it stops levitating. Just like an electric motor. So if you want to keep it levitating, you need to maintain some slip (difference in rotational vector) between the ball and cage. I’m just disappointed he didn’t rotate in multiple axis to keep the slip vector changing.
IR camera to visualize heating effects while object is rotating.
I once managed to demonstrate this effect to my daughter when I dropped a reasonably strong magnet (that I have no idea of it’s origin) through the middle of a brand new roll of kitchen foil. Nowhere near as slow as the magnet through the tube at the start of this, but it was readily apparent it was falling through slower than expected.
Is there a wheight difference of this device when rotating or not ?