NEW VERSION AVAILABLE: http://www.thingiverse.com/thing:602481
This is a rigid self centering axis coupler.
It's ideal for printers like Prusa i3, because the z-axis threaded rods normally have no dedicated bearings or bushings to ensure coaxiality between the steppers and the rod itself.
This Thing adds some rigidity to this system and keeps the axis coaxial to each other.
Make sure your rods are as straight as possible.
Bent rods can lead to Z-axis wobble. In this case you should install an anti wobble system.
14-07-16: Added a 5/8mm version
14-07-17: Added a 5/6mm version - use nut_5mm for both sides
Print this as fine as possible to get nice threads.
This one is printed with 0.16mm layer height, 2 perimeter, 100% infill.