I reached into the way back machine to modify this design. While my wife was putting together her PolyBot, she cut her finger on the sharp corners of this thing: http://www.thingiverse.com/thing:5126As much as I love that design, it was innevitable that sharp corners would cut someone. My wife's comment was; "If you're going to have kids assemble this, there can't be any sharp corners".This thing takes a different direction than the socket based joint. Instead of using 'sockets', it uses protrusions that can grab onto the dimples of another thing.In this particular case, it will match well with this other thing: http://www.thingiverse.com/thing:8272Combined, they form a vertex/joint that can bring together three rods of any size. A nifty thing for building space frames.You can change parameters in the OpenScad to adjust the size of the protrusions, or the width of the slot, or whatever.1) Print a few of these 2) Insert dimpled things for the protrusions to grab 3) Rejoice!This design is based on what I would call 'progressive friction holding'. That is, there are no fasteners. The hub is somewhat flexible, so the first thing you put in place will go in easily, due to that flexing. The second one will go in harder, because there's less flex available, and thus more friction. The last one should be downright challenging to get in, but once in, all three should be fit in very snugly, with nothing more than the friction holding them in place. If you find that you've got the angles you need, you can pour a bit of epoxy or whatever ABS melting/binding/hardening thing you have, over the joints, and things can be solidified.