I'm in the process of building/ purchasing an Avid 4x8 pro machine. CNC Depot has an ATC spindle package for the Avid machine. They have two options that I'm looking at; the S30 (18,000rpm) and the S30C (24,000rpm ceramic bearings) the S30C is about 500 bucks more. I don't usually run my bits much over 18,000 rpm. Should I be? Is there any reason to spend more for the higher rpm? Is that for cutting aluminum? 500 is not much more, and I'm willing to spend the extra, if I need it. Thanks for the advice