Geeetech

I finally assembled my prusa i3 but I have come to a problem, the z axis won't turn and it's farly hard to screw in unlike the first time I did it. Now I've unscrewed them both and hooked them up to a drill and put them through the brass piece a couple hundred times which seemed to help (easier to screw in) but the motors still won't turn, they work fine without the rod connected but when it's connected it just vibrates and makes noise. Does someone know how to fix this?

Have you checked the stepper driver voltage settings yet? They may be set too low, I have mine set to. 8v

How do I change the input voltage of those? Through repitier?

It is not able to change the input voltage by repetier unless you screw the rotary knob in A4988,the motor drive chip in the board.

I changed it to the highest it could go and it slightly turns for a second then makes a horrible grinding noise

The speed is too fast

Ddman114, I was having the same issue and it has been due to binding between linear Z-rod and the Z-screws. As a temporary fix while I try to iron out the other issue I am having, I was able to loosen the 4 screw holding each of the nuts to allow the nut to slightly wobble and not bind it up. There are some youtube videos on some work arounds people have done.

Hodman is right. And I think you should loose the Z yellow nut a little.