OK I’ve got about an hour for lunch.
That’s plenty of time to do some truly questionable shenanigans.
You see, I’ve got a mechanism which requires a really solid connection between a magnet and a ball bearing. Problem is, I can’t find magnets with spherical inset shapes. Plenty of countersunk shapes but no spherical.
After a whole lot of searching and reading so. many. posts. stating that shaping magnets is a fools errand…I gave up and said screw it I’ll do it myself.
The prototype was made with the first of my blown up nomad 3 spindles. (We’ve seen this little monster a few times before)
Basically just a platform to turn things in a relatively precise way along an axis.
High torque dc motor geared down.
12mm copper ball bearing.
400 grit diamond paste.
Time.
Smother your copper sphere in paste and place it against the magnet, letting it spin freely every few seconds to avoid wearing the copper bearing unevenly. It doesn’t take long for the very hard magnet to charge the copper with the diamond paste which is when some real material removal happens.
I don’t have a force gauge (yet) to measure the increase in force from even this little bit of grinding but I assure you it’s a material improvement.
Once I get the general shape correct, I’ll start bumping to finer grits. The force of a magnet on a lapped surface is incredible, so I figure why not lap the magnet
Improvements are in the works to make this less effort…CNC spherical grinding? Maybe. Just be sure to cover those rails
A couple of questions.
What diameter ball bearing do you need to grab?
How strong does the ball bearing need to be held?
It seems that a more effective way to approach this problem is 3D print a magnetic Kinematic mount. Basically, arrange three edge magnetized bar magnets in a cone arrangement (along the inside edge of a cone, so when the ball bearing is in contact, the three bars positively positions it.)
When I first was thinking about this, I thought three points (this is what we usually did for kinematic optic mounts)
Another thought, does it have to be a permanent magnet? You should be able to fab up an electromagnet that might meet your requirements.
This is a prototype so I don’t have a whole lot of hard numbers, mostly just vibes and how it feels.
It’s a ~12mm ball that will have a short dowel sticking out of it for mounting base plates and mini figures. It’s essentially an assistive painting tool for miniatures.
Locating the ball in space isn’t much of a concern, the real goal is to get a really good hold on the bearing while maintaining a very smooth motion and a relatively rigid stable state.
I want the ball to be tiltable when wanted but applying light pressure via a marker it brush should not so much as wiggle that figure.
The challenge here is getting enough friction on the ball without some sort of mechanical advantage. (I thought about a non-magnetic screw cap to apply pressure and tried o-rings as well to decent effect. Nothing as smooth as I’d like though so we’re going for this Hail Mary)
I did end up grinding more with the ball and it’s been awesome so far. I think I can still increase force quite a bit as well if it seems necessary, plenty more material to remove and lapping with higher grits to do.
I’m looking for a decent, reasonably priced force gauge to characterize the forces involved before and after grinding the seat. That should be more interesting that me saying it’s working or not
I am a touch worried about the loss of stiction that I might get from nickel plating the magnet again though.
I might give it a thick layer of copper coating to provide a wear surface that isn’t quite so hard.
Does the magnet itself need to be ground? What about a mild steel pole piece on top of it that itself has the spherical contour easily(ish) machined into it? I like John’s electromagnet concept. Except I’d make a little assembly with a permanent magnet with a pole piece on top of it, and a concentric electromagnet with a momentary switch to nullify the magnetic force for repositioning.
I’ve played with a few spherical magnets to achieve this goal in an easier way but something about the field results in a much weaker attraction compared to a ring magnet. (Again all anecdotal until the force gauge arrives)
I’ll be trying out the various combinations to see what works in the coming week or so. This custom grind has been by far the best but if I can get away with not custom grinding each magnet I wouldn’t be mad about it
I am trying to achieve a relatively light weight head that’s dead simple and reliable. So any electronics are out for the time being.
I’ll certainly be playing with the electro approach at some point on another project though.
I actually don’t know why this problem is fascinating to me, but I found some COTS set ups for they magnetic ball problem. May be wrong size and a bit too pricey but its some leads. The keywords that seemed to hit the concept was “magnetic ball joint”
Just a note: I’ve used them quite a bit over the years and always had a great experience. The quality of their products and reliability in getting them to me on time has been excellent. Strongly recommend them for all your magnet needs
And if you like weird magnet stuff in general, I’d recommend looking at this Magic:
I imagine that each half is magnetized across the thickness, so the magnetic force is concentrated along the gap of the assembly. The tube they’re mounted in completes the magnetic circuit. Think of a transmogrified loudspeaker magnet assembly…
