These are rod-ends for Rostock's printed u-joints parametric so you can use tubes of any size. Unlike the stock Rostock ones, these are designed with a plug to fit into a tube rather than as a socket for tubes to fit into. The current settings are for 3/8" hollow Aluminium rods. This was inspired by http://www.thingiverse.com/thing:39662, but thingiverse seems wonky and I can't mark this piece as a remix of that so I'm just linking it for now. Print without support, then chop off the square and epoxy it into your tubes.