Both the servo motor and the stepper motor are DC electric motors. Their high stall (or start-up) torque makes them among the most commonly used motors for robots and special-purpose machines. These motors are also used to drive the axes of CNC milling machines.
The stepper motor
The stepper motor is better suited to small machining operations. Its simple design makes it easy to manufacture and reliable.
However, if the forces applied to the rotor are greater than the motor’s maximum permissible torque (N·m value specified by the manufacturer), a step-loss phenomenon occurs: the motor moves unintentionally and you hear small “cracks” inside the motor.
If you are milling a workpiece made of a hard material and the feed rates of your milling machine are poorly set, the stepper motors driving your axes will be subject to step loss. You will observe the same phenomenon if the speeds and accelerations of your axis movements are too high.
The issue is that during a long machining process, the accumulation of successive step losses leads to large offsets on your milling machine’s axes. The cutter is no longer guided to the originally intended position, and your finished part is no longer acceptable.
For milling small parts and short processes in soft materials, stepper motors are sufficient. However, for producing large parts, they are no longer suitable.
Servo motor
The servo motor is built like a stepper motor, with the difference that it also has an encoder. This allows it to continuously measure the rotor’s rotational speed. The controller can therefore determine whether the rotation command has been fully executed. In the event of step loss, it detects that the motor has rotated less than planned and immediately adjusts the movement. This avoids delays during the machining process.
As a result, the servo motor is not susceptible to step loss and is therefore better suited to milling large dimensions. A milling machine equipped with servo motors can withstand high accelerations and high axis traverse speeds while maintaining positioning accuracy. They are therefore recommended for producing large parts that require high precision.