Choosing a Stepper Motor for your CNC Machine


Why stepper motors? What about other types of motors?

Stepper motors are by far the most common types of motors used for CNC Machines. Stepper motors have a unique property that makes them particularly well suited for CNC motion. Each time a stepper motor receives a pulse, it only moves a certain amount. For a full step, this is usually 1.8 degrees and for a half-step this is usually 0.9 degrees. The reason this is important is because the computer running the motor doesn’t need to sense the actual position of the motor because the motor’s position can be calculated solely by how many pulses have been sent out. Because no position sensor is required, this is called an open loop control system, and is extremely common for hobby CNC projects. If you are considering building a CNC machine for the first time, you’d be well advised to use stepper motors to achieve movement for all of your axis.

Another possibility is to use PWM motors. These motors produce different torque depending on the duty cycle of a square wave signal it receives. However, a more complicated controller and an expensive quadrature encoder must be used to determine the current location of the motor at all times. This is a closed loop control system, where the system reacts based on position data which is detected through a sensor. This is more expensive and complicated than using stepper motors, but could theoretically gain better accuracy and would be smoother than stepper motors, since there are no actual ‘steps’ with PWM motors. Still, achieving great accuracy with stepper motors is possible by using threaded or ACME rods with a large number of threads per inch. That being said, PWM motors could possibly achieve greater speed and accuracy when compared to stepper motors, but would have greater cost, a more complicated control system, and less holding torque.

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