A small, obviously scalable, U-Joint. printed in ABS with 3mm walls and 10% infill on my Ultimaker 2. Instead of using the normal Cura support I used my own which can simply be combined with the file (you can find it here: http://www.thingiverse.com/thing:991392)
The picture shows two u-joints glued together with an axle inside for strength.
-The axle size for the unscaled .stl is ~5/16" (~8mm) in diameter
-The tiny whole going through the axle is for a cotter pin <1/16" (2mm) in diameter (rebar tie wire is perfect)
-The bolts used to hold the joint together are 3/16" (~4mm)
Just because of imperfections and expansion of plastic when printing it may be necessary to use a drill bit to make the wholes the right size. I used the same ABS juice that I use on my print bed to glue the nuts in just to make them harder to loose.