A method for processing a workpiece on a numerically controlled machine tool by a tool includes the steps of: controlling a relative movement of the tool relative to the workpiece for processing the workpiece, producing an ultrasonic vibration of the tool by an ultrasonic generator, detecting at least one sensor signal outputted from the ultrasonic generator and identifying a change in the material at the workpiece while controlling the relative movement of the tool relative to the workpiece on the basis of the at least one sensor signal outputted from the ultrasonic generator.