The specifications for the Tentacle System are quite simple. The ball and cup have the same dimensions: 15mm. There is no clearance between them because they are supposed to have significant friction to resist moving.
Any sphere that is 15mm in diameter can work with the Tentacle system as a base so long as more than half of the sphere is unobstructed. The cup covers slightly more than half the sphere, by 2.6mm. Thus, a perfect hemisphere could not work.
The cups have a split in the side so they can flex over the sphere. The split is 1mm wide and 7mm long. The edges are chamfered by 0.4mm.
.step files for a the ball and cup are included in the main tentacle system download archive.
The cups can be printed facing straight up or straight down. Avoid printing them at an angle because it can lead to an uneven interior surface and the ball won’t fit well.
If they are printed facing downward they should have a relief cone inside. Without this the top of the cup may not print correctly and it can interfere with the fit of the ball in the cup. The diameter of the recess is 6mm and the depth is at least 3mm so that the bottom edge of the relief can be chamfered at 3mm. I use 8mm deep relief cylinder measured from the center of the sphere inside the cup. The cup is slightly more than a 1/2 sphere, so the excess isn’t included in the depth of the relief cylinder.
If you design a part where a cup would have to be other than cup straight up or down, you should separate the model into objects so the cup can be printed vertically. When it is separated it should have a pin on it. Cups have pins and balls have sockets. The connection kit and all tentacle system components that have separate parts follow this convention (see “Pins and Sockets” below).
The balls must be printed pointing straight up. If they are not, the ball will not be well-formed and it won’t fit well within the cups. If your model cannot be printed with the ball facing up, you should split the model into objects so that the ball can be printed separately. You can just use the ball from the connector kit or you can model your own and add a socket as described in Pins and Sockets below.
Pins and Sockets
Some components are printed in pieces that are connected together with pins and sockets. These pins are always 5mm in diameter and the corresponding sockets are 5.10mm in diameter. The pins are 5mm long and the sockets are 5.5mm deep.
The leading edge of each is chamfered at 0.4mm. The sockets must not be printed horizontally – only vertically or at an angle less than or equal to 45º from the build plate.
Pins can be printed at any angle. If a pin is printed in a horizontal orientation or less than 45º from the build plate, leave about 1mm of the bottom clipped so it has a flat lower face. This allows the pins to fit in the sockets regardless of any print artifacts from supports.
You will need to superglue these parts to make useful combinations that do not simply fall apart. While sometimes these press-fit with a satisfying and useful amount of friction, they often do not. It can vary from filament to filament and printer-to-printer.
A few Tentacle System components included neodymium magnets. They are all 6mm in diameter and 2mm thick: the same size used for Gridfinity components. There are various qualities of these magnets with the cheapest variety having the weakest field. I buy good quality magnets like these. I bought some described as “refrigerator magnets” that were the same size and they were very weak, even for use on a refrigerator.
How the Tentacle System Fails
By far the weakest part of the Tentacle System is the cup. If the pieces are not pulled straight apart, one side of the cup or the other can crack. Once it is cracked you might be able to superglue it, but it is important not to deform the interior of the cup or a ball might not fit anymore.
The next weakest part of the system is the friction between the balls and cups. Because they are printed vertically, the layer lines line up when the components are straight. Friction is highest there. As the parts move off axis the friction reduces slightly. Over time the parts may wear and the friction may reduce even more.
More components can mean more chances to have a looser junction. Using fewer components helps up to a point, but the system can only handle just so much weight before friction will always be overcome with any number of components. The parts are inexpensive to print so useful configurations of connectors can be superglued together, which bypasses the above weaknesses.
In general it is better to put the baseplate on an elevated surface than to build a “tall” tentacle. And, it is better to move the baseplate closer to the work area than to use a long tentacle. That is part of why there are such a wide variety of baseplates: to make it easier to bring the tentacle closer to the work area.
I printed my parts in PLA and they worked great, but of course if you want to use them in a car or in any hot environment you might need to print them in PETg. It isn’t a weakness of the Tentacle System, per se, but it is a failure mode if the wrong filament is chosen.
The pins and sockets rarely fit with enough friction to not need glue. If they do happen to have enough friction, it is likely to disappear over time if the parts ever move. My printer (Prusa MKS3S+ is only warranted to printing within 0.3mm of where the slicer tells it to print, so a difference in sizes of 0.1mm is beyond its accuracy. But, the way printers work, both the pins and sockets are likely to both be too big or both too small, so they will generally still fit.
Finally, the system fails when there is no useful base for the work area in use, or when there is no gripper / holder that can do the job of holding something lightweight that needs to be held. However, this kind of failure can be addressed. If you run into this problem, please send me an email at hoco at timefold dot see oh em and I will see what I can do to provide a tentacle model for you and the public domain.