How to choose motor size for lathe machine?

Q: We want to retrofit two 6280 universal lathe machine into CNC, the ball screw diameter of Z axis is 60mm, what sizes of the X and Z axis motors should I choose?

A: Is the max. machining diameter of this 6280 lathe 800mm? If so we suggest 15Nm for X and 18Nm for Z, if the machine is old, then 18Nm+22Nm could be better.