Most will agree that 5/16" galvanized threaded rod is not the best leadscrew for a printer's z-axis yet Printrbots still ship with them. This thing is a bit of a hack to adapt the printer to an M6 threaded rod leadscrew with a pair of captive nuts and a spring. The printed adapter is designed to fit the space occupied by the 5/16" hex nut and require no changes to the printer. This upgrade increases the steps/mm to a nice even 3200 (as opposed to 2267.7183), reduces any sort of backlash, and makes the z axis run smoothly.
Loosely based on some work I was doing with the x-axis ends for the Wallace.