I have studied a lot of both the home built and comercial CNC plasma cutters, and as far as I can tell, none are using ballscrews (except, as stated above, perhaps on the z axis). There are several problems with using ballscrews on a plasma cutter. The reasons are somewhat intertwined. In order to get smooth corners, especialy in thin metal, you need fast rapids and acceleration. One thing you don't need is exceptional accuracy. Since the kerf that the cutter takes is wide, and somewhat vairiable, trying for more than .010-.015 inch accuracy is not neccicary. For the same reason, as small amount of backlash (the primary reson for using ballscrews to begin with) is not detrimental. Another major concern is the dust from the plasma process, which would quicly ruin a ballscrew. Combine the additional expense, short life expectancy and a lack of need for thier primary strenghts and you don't see many on pure plasma tables (a combination plasma/router table would be a different story, but those added requirements would drive a huge amount of cost into the build). Most that I have seen use belt, chain or rack and pinion, or a combination. I had planned on building a combination plasma/router that would use ballscrews, but the size was fairly small (2' x 4' capacity) and most of the design decisions were driven by the router capability, not the plasma. A small table in the 2' x 2' possibly up to the 4' x 4' range could probably be built with acme screws, but thier protection from grit and dust would drive more complexity into the system than is neccecary. Do some googling on home built CNC plasma tables, there quite a few out there, with nearly as many different designs.