I`d like to use longer ballscrews in my next CNC project:

Max 120 RPM under load or jogging,
Pitch 0.5",
Lenght- possibly 96", maybe less.

Which diameter should I use to avoid whipping? 0.75"?

Thanks

It depends how it's mounted. You might want to look here. http://www.nookind.com/ball/BallGlossary.cfm#ScrewCritical

Has anyone tried to resolve the ballscrew whipping by fixing the ballscrew and rotating only the ball nut? How does one design something like this?