The last Y-axis moving test was not good as following link.
http://www.thingiverse.com/thing:106254

It was too slow, noisy, and non straightness of lead screw.
In addition, the wiring of motor was wrong.

I fixed the straightness of lead screw using by dial gauge.
I re-designed the nut of lead screw, and it replaced instead of the backlash adjusting nuts as following link.
http://www.thingiverse.com/thing:98894

Marlin firmware and Cura used for moving test.
I've setted the default steps per unit value to 200.0 on configuration.h of Marlin for 8 mm lead screw and 1/8 micro stepping condition.

I used the Cura's jog buttons manually for moving the Y-axis.
The testing speed was 100 mm/sec.
The moving test was not bad as following video.
http://youtu.be/AMEa2v2D23w

The uploaded nut has a big tolerance.
Because my Rep2 can't make exact circle, so the nut fitted little bit tight to the lead screw.
The length of the nut was reduced from 25 mm to 4 mm in order to minimize the friction and I used only one nut. because I couldn't feel the backlash with one nut.
And I used teflon oil for lubricant. It shows more smoother feeling than the WD-40.

If you want more small tolerance model, please let me know.

PLA, 0.1 mm Layer, 50% infill