The screw thread finish jobs not smooth, new CNC lathe machine, new cutting tools, but the others like screw pitch are within the tolerance.
1- Is the spindle vibration too big?
2- Are the tools bad quality?
3- Check if the frequency inverter output smooth or not?
4- Any problem of the X or the Z axis, are they flexible?
After checked above then found the spindle vibration was about 0.2mm, which is too big and caused the result, change the spindle bearing, the machining size is good.