The limited strength of the joint seems pretty advantageous for this application - it means that when any of the various software, hardware, or electronics failures occur that could result in the actuators fighting each other or the kinematic constraints of the mechanism, the outcome will be magnets letting go instead of the robot destroying itself.

If the magnetic coupling isn't strong enough, you could add an extra magnet to the pin that screws into the metal ball

Kuel, 😎if you put some machine polish in the ball socket and drive the ball with a drill until it contacts the magnet, you may end up with 1)best x,y fit in the socket 2)best z mag strength 3)best z ball support 4)best x,y precision. That ball won't be getting any closer to the mag, just be sure to get all the polish out, and lube with carbon powder or something.

Cool video! Really interesting to see you walk through your design process and show the progression on each iteration of joint.

The max retaining force would be achieved if the magnet had a concave surface grind just a few thousandths larger radius as the ball. But even then, very fast reversal of direction could possibly cause the ball to lift out of the socket = crash. But so far so good...

Hey everyone, I hope you enjoyed this video on upgrading Jugglebot's Stewart platform joints! I'd love to know your thoughts on the new design and its benefits. Also, if you have any suggestions for future improvements or topics you'd like to see covered on the channel, please let me know in the comments below! 🤖🔧

me, watching the first part of the video: "I wonder if something like PTFE sheet might help reduce friction and avoid wear on the magnet". you, a few minutes later: "I've bought some PTFE sheet". 👍

Nice! If it doesnt slide well enough dry lubricants might be an option. I'm thinking graphite and the like

provided the sheet doesnt dampen the magnetic force too much you shoould definetly holepunch a few pieces of it, the coating on most commerical magnets is designed to resist corrosion, not friction based wear.

Man, I keep finding your videos and they're so similar to things I've worked on here (different, but similar). I went on quite a ball-joint (and, separately, magnets) kick for a while. Great stuff, love the design. One thing I'd suggest, regarding friction of the printed cup - print that part in nylon. Nylon is exceptionally smooth, and also very very strong. If you're able to print in HDPE, that'd be even better, but man that is tricky stuff to print with. Another thing that came to mind, not sure this would really be necessary but you might want to make mental note of it for later if it comes up: switchable magnets are a thing and they're pretty cool. Basically, by either wedging a piece of metal between some magnets, or not having that metal there, you can completely enable/disable a magnet. This is all done without active power. This video "How to build a switchable magnet" by Andrew Klein explains it well along with some designs. So a modified version could perhaps be used in your cup assembly, if you want a way to use overly strong magnets but still be able to easily disassemble it later. Toggle the magnet into off position, falls apart. Toggle it back to on position, probably far stronger than you'd ever need it. But as you say, you may already be at that 'stronger than you need it' level. Anyways, best of luck. Really enjoying these videos.

I did something very similar a few years back for a delta bot I was working on. Never did get the full machine working, but the joints seemed to work great!

So essentially your joint is the ball sliding inside the 3d printed cup, with the magnetism just providing a preload to your ball joint. This is very similar to existing ball joint designs often found in delta robots. A cup (usually coated with PTFE) with a ball preloaded (generally by a spring between two legs). You might be able to just buy such a PTFE coated socket + ball off the shelve... Regarding the question about joint 'strength'. Important to recognize is that stiffness of your mechanism is likely not dominated by the stiffness of the rods, but by the stiffness of your bearings. Because the bearings are preloaded, the stiffness can be found as the contact between the ball and the 3D printed part, and hence the 3D printed part is likely a huge bottleneck in the design. The joints themselves present a similar stiffness in both tension and compression, with the exception in tension if the force exceeds the preload force (leading to the magnet detaching). So again, if you make the cup somehow more stiff, it would lead to a higher stiffness joint in general and improve also hysteresis, which is my following point: What would additionally concern me, is the large and unpredictable amount of friction from the ball sliding against the 3D part. This can (of course) cause a lot of wear at best, but the large friction combined with limited stiffness can cause hysteresis, which will limit the accuracy of your mechanism. Additionally, if the static friction is large enough, it can (combined with the stiffness) create stick-slip type problems which are particularly difficult to deal with in your application. As for the shape of a ball and cup joint, this is a whole science in itself. As you can imagine, getting the exact curvature the same between ball and cup is nearly impossible, so that can lead to undefined points of contact and lots of variation. Some suggestion I saw in another comment was to have the magnet directly contact the ball. I would highly advise against this, as the plating on magnets is quite brittle and I think it would wear out pretty quickly.

I think you will want more strength. There are magnets that are cylinders with counter sunk holes in them. So you can secure them with a screw (gently, they like to shatter) but that counter sunk hole makes the ball have closer contact with the magnet. So if you don’t have enough strength, get a bigger magnet with a countersunk hole in the centre so it can hug the ball. Or a ring magnet?

Use ruby bearings for the ball cups.

These videos are awesome. I'm trying to look for a project to start of my own. I've been a blank slate for a while and when I say this series I'm like damn that's such a sick idea. Let me know if you have any other ideas or projects in mind 😂😂.

You might gain strength by having three smaller magnets in a cup configuration - though as you speculate you probably won't need it.

If you have problems with it not being smooth enough, you can add little steel roller balls that'll limit the points of contact

I'd recommend some sort of lubricant, like silicon oil or graphite rather than trying to add more stuff between the ball and the magnet

I would suggest to allow the steel balls to make full contact with the magnet, and use that as your main bearing surface, and use the concentric plastic part only to constrain the slipping motion of the ball. i would probably use a ball end mill to bring the spherical cutout of the 3d print to a tiny and smooth clearance.

Excellent

What, no cats!?

I wonder how a joint based on a flexure would look like.

I mean the robot is cool but the calculus is way off 117 parts is not nuts, nuts is clearly 18. :D

3x fewer!! So before when it was the same number of parts of itself and it had zero fewer parts, now it has three times that many fewer parts!! So that's . . . zero. Hey, maybe using the phrase "three times fewer" when you mean "one-third as many" is dumb.

Excellent