Hi there. I am building my first 3D printer (Reprap Prusa i3) and I typically would test each axis using the LCD controller connected to RAMPS (running on Arduino Mega with Marlin). When I would move each axis, everything would work fine except for the two Z-axis motors. They would make this awful grinding noise. I tried adjusting the potentiometer and also tried putting the motors in series as oby benlabscdk - RAMPS Electronics