x and y axis work, but z axis makes the motors buzz and stick

I have a RAMPS 1.4 board that I installed the newest Marlin firmware on. I have the printer (Prusa i3) mostly assembled, except I disconnected z-threaded rods and need end stops.

Controlling with the smart LCD controller, I can move X, Y without problems. However when I move the z axis the motors buzz/hum and try to rotate, but can't. This seems to be to be a problem with the amps so I dialed the potmeter but there is not 'stop' so I don't know when it is turned up or down. As even at full amps the motors should burn, I don't think the stepper board is the problem.

So I was wondering if this could somehow be the firmware preventing the z from spinning? Or if it could be broken electronics.

Thanks in advance.

- Nic

is it a high pitch hum, and what feedrate are you trying to move it at?

my guess it that z axis speed is set higher than it should be or your speed in mm/s and mm/m are off. try 180mm/m or 3mm/s on z axis. you can also change the max feedrate in firmware.

I'm having the same issue.

I'm also not sure that + and - are going where they're supposed to. I can reverse the direction in the Marlin firmware, correct?

try slightly backing out the screw on the z axis stepper driver. the problem might be that theres not enough current going to the z motors. I had the same problem.

I often see this problem on IRC, and I tell people to adjust their Z feedrate, then it will works. In your configuration.h (Marlin), set the Z HOMING_FEEDRATE and Z DEFAULT_MAX_FEEDRATE to 2 or 3.

If it doesn't help, maybe it is a wiring error?