Problem with Z axis (4 replies)

Hi all.
I'm new to 3d printing and I'm building a Prusa Mendel.
I've all the structure built, all the electronics tested and working.
My problem is the Z axis won't move if connected to the threaded rods just vibrate and make noise, but if I disconnect from the threaded rods they work smoothly
Rechecked the structure measurements, Z motors connections, the motors coils and all is OK
Even tried to level up Imax a little bit (my motors are 2.5A rated) but with no success
I'm getting really mad with this as I don't know what's next step to try...
Any help will be welcome.
Thanks and sorry for my poor english

CoNiCiO