50V supply on 50V drive is bad idea.! . . . your in danger of back emf frying them on de-acceleration. On a lathe it probably won't be a massive issue because it wont be de-accelerating from high feeds but @50V your on the edge.!!

First I would double, triple check all your wiring, esp the motors phase connections.

Check your amps are not set too high on drive, then like john says try other settings.

What model motors are they.?
How have you got the motors wired.? Parallel or series or unipolar.