Precision Plasma LLC sells DIY CNC plasma kits. Here is a model of a simple cnc rotary tubing cutter.
This design uses a 6" 4 jaw chuck in which the tube moves under the torch. The complexity of the tubing cutter depends on the application. For instance, if you just wanted to cut tubes for fabricating roll cages, you would not require torch height control or even a motorized Z axis. If you want to cut square tubing, you would need both THC and a motorized Z axis, along with a UHMW tubing support, but with a round cut-out to match the diameter of the square tubing.