Jay Cee,
It is not actually an inability of the motor to react quickly. In actuality, both types of motors will begin to react within a microsecond or so 'with no load'. However, this is not reality with any machine tool, because you have the inertial load of the driven stage to contend with. You'll notice that I said "physical reaction time" in my previous post. I will attempt to analogize this with a very basic concept. Two identical cars with the same amount of horsepower apply throttle at the same moment. The "linear car" is in 3rd gear, while the "ballscrew car" is in 1st gear. The gearing analogy relates to the result of any given force being applied to the ballscrew as opposed to that same force being applied directly to the driven stage. Obviously, the mechanical advantage of the ballscrew produces a much higher linear force. While the linear will eventually build up enough speed to overtake the ballscrew, the ballscrew overcomes the inertial load much more quickly, hence reacting in far less time. In essence, the ballscrew car has moved a foot or so before the linear car has moved an inch. Reduced to units applicable to EDM, this first critical reaction might be a mere few microns, but directly comparable in scale. In the case of EDM, this "hole-shot" (as it would relate to drag-racing, not adult film) is far more important than the top speed. I also stated that this relates to a "properly applied" ballscrew. This would apply to systems in which the driven stage is coupled to a rigidly anchored ballscrew which is directly driven by the servo through a rigid (ie: bellows type) coupling, and not by a Gilmer pulley and timing belt type of system attached to a floating ballscrew as seen in nearly all models of a very popular brand of EDM. The reaction time of such a system is considerably slower than either of the other two types discussed here, and for obvious reasons. This is not intended as an attack on commonly (and popularly) utilized construction techniques, this is simple physics.