I've always wondered this, I've ran several different CNC lathes in my life and all of them will cross thread if you make a thread and then decide you need to change the speed in the middle of the thread to reduce chatter, or if your running a roughing tool and a finish tool the speed has to be the same for both, you can't run say 500 RPM on the rougher and 800 RPM on the finisher. It seems to me the machine should know where it is at all times and speed wouldn't matter.