The press fit as bad as it may look does not concern me as much as the steppers holding ballscrew in place. The reason being is I can measure the shaft of the motor moving along the motors axis as the load on the ballscrew changes. Inside the motor is a spring washer, if you pickup a stepper and try pulling and pushing the shaft you will see it takes very little force to move it. Maybe your lathe has different types of steppers with thrust bearings built in, or maybe its designed to work within the range of the spring washer inside?