First I'd like to give due credits to Kees Koese who started with the idea of the magnetic ball joint and made the original design of the effector plate. Give his website a visit to have a look at his multispindle drilling machines. Technicians will like it :)
This post is a how and why about the geometry of the ball seat in the effector plate. Since the original effector plate was made with SLS printing we wanted to be able to print this with FDM printing (both being entirely different processes).
During the first test prints there was still manual rework needed. I wanted to end up having to do nothing at all regarding removing filaments, grinding etc. I think making geometry only with the printer without having to rework it is why 3D printing can be such a fantastic way to manufacture.
Before going into details I was amazed how rewarding and exciting it is to draw your thought/improvement in your favourite CAD program (mine is SolidWorks) and having it on your desk in 30 minutes... Every engineers dream.
The picture below shows the original geometry Kees drew a few (almost 6) months ago. It depicts the ball seat with the area (cylinder) where the magnet needs to fit. Because of the original being SLS printed there were virtually no limits regarding overhang or printing resolution. Notice the second (bigger and lower) sphere that ensures the ball will fit in the topmost seat.
When initially printing this test piece to check if the resulting dimensions were good enough to fit the cylindrical magnet (this situation needs a ∅10.5 mm diameter to result in a snug fit of the ∅10 mm magnet) I had a lot of problems printing because of 2 non-printing areas coming together in mid-air. Also the points where the 2 areas merged were sharp so the filament kept sticking to the nozzle on it's way back. (a lot of words for saying the result was horrible...)
What to do... Make sure there were no pointy encounters so that meant closing the gap between the seat and the cylinder volume. See below:
This was not the way to go. The wall thickness was so thin (wall thin-ness) that it could not be closed (and the ugly thin-ness needed to be drilled away manually... bah...). Because the overhang is at a 15 degree angle there is not enough previous layer to adhere to. Improving (but not really) solving) this could be done by adding wall thickness. But alas... Every solution frequently introduces it's own problem... The distance between magnet and ball was getting so big the magnet lost it's force-field on the ball... Thanks for nothing Master Yoda!
There had to be a way to ensure a minimal gap between magnet and ball without the overhang problem and without wall-thinness situation... If you have something you don't want: (re)move it... See below:
All that's needed is a defined sphere and a axial constraint preventing the magnet being pulled onto the face of the ball (creating friction when the ball is turning). If you add a partial ceiling coming from the side (instead of bottom or top) of the cylindrical hole then during printing you will see a very gentle alteration of the contour of the non-printing area. Then there is enough support from the previous layer to create the overhang.
Final measurement and test were done with calliper (measuring the distance over Ball and Magnet, subtracting the ball diameter and length of the magnet) and multimeter.
I measured a gap of 0.1 - 0.15 mm and when doing the beeeep testing for I got no beeeep. (measuring short circuit between ball and magnet).
This evening Master Simon helped me taking a picture of the actual force of the ball joint connection. Result: One connection can pull 1690 gram. Take that Master Yoda?
Below some pictures of the test, some prints and last but certainly not least the geometry of the connection.
Pulling water in Sigg bottles. I am holding the blue printed part.
Resulting weight being pulled:
Seat of ball with magnet underneath (view from top):
Fit of magnet in area under steel ball (view from bottom):
Misprints being mentioned in this post, next to the result:
Finally after having bored you to death the resulting geometry. First the cut-revolve of seat and magnet volume:
Second the cut-extrude at ∅10.5 with a width of 8 mm. Seen from above. This prevents the magnet from getting in contact with the ball. I also added a radius of R1 at the inside of the cylinder making the sliced contour even more printer friendly.