The servo motor and the stepper motor are both DC electric motors. Their significant torque during blocking (or starting) makes them commonly used motors for robots and special machines. These motors are also used to control the axes of computer numerical control milling machines.
The Stepper Motor
The stepper motor is better suited for small machining operations. Its simple composition makes it both easy to produce and reliable.
However, when the forces applied to the rotor are greater than the motor’s maximum permissible torque (a value in N.m provided by the manufacturer), a phenomenon of step loss occurs: the motor accidentally shifts, and small “cracks” are heard inside the motor.
If you are milling a part in a somewhat hard material and the feed rates of your milling machine are poorly adjusted, the stepper motors driving your axes are subject to step loss. You will experience the same phenomenon if the speeds and accelerations of your axis movements are too high.
The problem is that during a long machining process, the accumulation of successive step losses will result in significant discrepancies on your milling machine’s axes. The cutter will not have passed through the initially intended location, and your final part will no longer be suitable.
Stepper motors are sufficient for milling small parts and short processes in soft materials. However, for manufacturing large parts, they are no longer suitable.
The Servo Motor
The servo motor is constructed like a stepper motor, with the difference that it also has an encoder. This allows it to continuously know the rotational speed of the rotor. Thus, the controller can determine if the rotation command has been fully executed. In the event of step loss, it will know if the motor rotated less than expected and will immediately adjust the movement. This prevents discrepancies during the machining process.
Therefore, the servo motor is not sensitive to the effect of step loss and is thus better suited for large-scale milling. A milling machine equipped with servo motors will withstand high accelerations and high travel speeds on its axes while maintaining its positioning accuracy. They are therefore recommended for manufacturing large parts that require high precision.