I have a strange issue with my Z axis. When I set it to move at 2200mm / minute (or faster), it moves very well. When I try and make it move at lower speeds, it stutters or won't move at all, as if the load is too great for the motor.

Most of what I read indicates that this is the reverse of what should happen. I.e. The suggested fix for a struggling motor is to gradually reduce speed and acceleration until it stops struggling.

My Z axis is moving a 35lb spindle and I installed a 35lb gas spring from little machine shop to counter weight it. I have 20mm pitch ball screws on all axis. I thought that maybe the spring is not enough but it struggles to move in both directions (at low speed).

Does anyone know what sort of problem makes a stepper motor struggle at 800mm / minute but move easily at 2200mm / minute? I keep reading that steppers produce the most torque at lower speeds...

The motor is a Nema 34 3.5a 465 oz / in.