Sherline CNC retrofit

Hello:

I am in the process of upgrading my Sherline 4400 lathe with mill
column and manual rotary table to CNC control. I am a clockmaker and
need to cut clock gears. My lathe still operates smoothly with
minimal backlash. I noticed that the Sherline CNC upgrade
includes "ball screws". I am not familiar with this term. Can
anyone explain to me why I should upgrade to ball screws? Why can't
I use the current lead screw and cross slide screw?? Do I also need
different lead screws for the rotary table and mill column??? Any
help on this would be much appreciated.

Thanks,

Michael