This page descibes technical aspects of the CNC machine in more detail.
The torque of a stepper driver is not linear over the number of revolutions per second. It is nearly linear in the beginning but drops down at a specific point for each motor until it reaches zero. In our current setup we use Sanyo Denki 103h7126-0740 Motors, which are linear until about 1000 to 2000 pulses/second. We are using 8 Microsteps, so the normal 200 steps per revolution (1.8 Degree step size) must be multiplied by 8, which gives 1600 steps per revolution. Now we can calculate the maximum speed for a axis by taking the maximum pulses/second and divide it by the steps needed per mm (266,6 on our machine): We need the factor 8 here because of the microstepping.
This aspect of a stepper motor means also: lower microstep setting allows higher speed and vice versa.