Basically, the stepper motor in milling machine utilises the well-known principle that unlike poles attract; the cnc milling  machine, cnc machining center, cnc gantry machine, cnc machine tools, if one assumes that the rotor and stator were made to appear like toothed wheels,  with the general convention being for the external teeth on the rotor and the internal teeth on the stator. 

With this approach, if we consider that the alternative teeth are magnetised as either a "north" or "south" pole, this means that the corresponding pole on the rotor will be attracted to the opposite pole in the stator. Assuming now that we turn the stator, this would also turn the rotor, but naturally the stator is not physically turned as it is the fixed part of the motor, although it is possible to turn it electrically. 

Normally a number of sub-stators are used to make up the total stator, with each sub-stator being displaced from the next one by a "step" of 50-100. If the sub-stator is now sequentially energised - that is one after the other - the rotor is attracted to the next pole and so on. So as the poles are displaced a definite distance, the motor is said as "step". Nowadays, it  is not so common in cnc milling machine, not to say vmc and many other machining centers. 

Datan provides a series of CNC milling machine that can be with step motor  that is really welcomed at home and abroad:

http://www.cncdatan.com/milling/mini-cnc-milling-machine-ZX25s.html