An ongoing question among CNC machine users is whether or not to get a machine using a stepper motor or a servo motor. Well, the thing is they are both capable of being quality products and getting the job done. They both have good repeatability and accuracy. The main differences are that a Servo runs on a closed loop circuit whereas the stepper runs on an open loop circuit. This means that the servo is always in communication with the computer, this means that it is able to consistently send and receive commands. Whereas an open circuit system, steppers, take a command, do the task, and then report to computer. This means that if there is an error or slight change, it will not be able to make up the difference during the task, something that a servo motor is capable of.
Both motors are good at low speeds, steppers are considered slightly more powerful when operating at low speeds but servos are also okay at low speeds. However, servos maintain rated torque even at 90% of its max RPM, whereas a stepper loses up to 80% of its torque when it reaches 90% of its max RPM. The servo also has an encoder which is constantly monitoring the RPM and its position, the stepper does not have an encoder. This allows more motion control.
As for long lasting, the steppers only components that are wearing parts are its bearings. This is an advantage over the servo as the brushes on the servo will need to be replaced after 2000 hours of operation.
When it comes to cost, the stepper is also a cheaper option, however all of the larger machines, talking 4’x8’ machines and greater, typically only run on servo motors and this relates back to the torque at high speeds.
In conclusion, if you are buying a larger CNC router or are looking for torque at high speeds, get a servo. If you are getting a smaller machine that you can take a bit more time to make cuts and want low maintenance, then get a stepper motor.