I couldn't find a printable U-joint for Deltabot rods that I liked, so I made one. The bearings give the joint high load capacity with low friction and zero wear.
Yes, it could be much smaller with special bearings (like the Kossel Pro U-joints) but 623zz is a common and cheap size that fits M3 hardware.
The rod pocket is sized for TriDPrinting.com 5.8mm rods, but feel free to modify the size to fit whatever you want.
I will be posting up-rev'd files for overall size and range of motion as I iterate the design. No promises that different revs work together.
Print two of each part. I recommend high infill, 0.5mm extrusion width, 0.2mm layer height, with ABS, but use whatever you want. Bed leveling is very important for the "inner" pieces to get a snug and square fit.
Add four 623 bearings to the bearing pockets. They should be a snug fit on a well-calibrated printer.
Hold the two inner pieces bottom to bottom, and press the 3mm shaft into the bearings on the outer halves. Press together the two outer halves. Again, they should be a snug fit on a well-calibrated printer.
Fasten the two outer halves with a 30mm long M3 screw. I recommend using washers and a lock-nut, but do what you want.
Epoxy the rod into the U-joint socket using two of the included jig screwed to a piece of 2020 to ensure all rods are the same length.