I have in the recent past been gripped with the problem of choosing a better joint for delta arms on my 3d printer. Following is my criteria for choosing what is best. Importance decreases as we go in the list
1) Accurate and Error Free, no unwanted play in joint.
2) Reasonable good freedom of movement (30 degrees in any direction minimum)
3) Cheaply available or can be manufactured with access to semi skilled labour. Cost comparisons should include the entire arm assembly.
4) Ease of maintenance.
My first candidate was to use a POS6 ball joint. Here is a photo of this joint.
It required some thought process on how to use this in the best possible manner. The joint has a 26 degrees movement in one of the axis. The problem that I faced was with accuracy. The joint has some play, and the carriage itself would be slightly shaky, enough to ruin delicate parts. It is cheapely available( 1 USD), and the entire arm assembly can be laser cut in acrylic. I used two of these joints for a single arm, and two acrylic pieces of 6 inches X 10mm with holes on either sides to put a M6 bolt through them.
I then read about using a magnetic joint using a spherical metal ball and Neodymium Magnets. After doing some of my own research, I think I have been able to design a better joint. Here is what is required
1) A M6 Allen bolt
2) A M8 allen bolt
3) 1/2 inch Steel or MS sphere/ball.
4) A neodymium magnet ring magnet. I used a magnet with an ID of 6mm, OD of 20mm and 6mm thick.
To construct the joint, push the M6 bolt through the magnet, enough that it touched the head of the bolt. Place the steel ball on the side of the bolt where you place the allen wrench. Place the second M8 bolt on the steel ball. This joint, for me satisfies all the above criterias, but is marginally expensive than the POS6.
Here is a quick photo of how the joint I made looks like.
I did a few measurements to measure the holding strength of the joint. I tied a plastic bag around the M8 bolt, and poured water inside the bag, until the join starts to give away. 250ml of water was measured, and it roughly equals 250gms.