Parallel turning technology is considerably important in future multi-tasking machine tool because it has the potential to enhance the stability limits, compared to turning operations using a single tool. Although stability prediction models for parallel turning have been developed recently, the technique of in-process monitoring of chatter is almost out of focus. In this study, the monitoring of chatter based on the sensorless cutting force/torque technique was evaluated in the parallel turning and cutting of the same surface of an elongated workpiece. Two cutting force/torque estimation methods were evaluated: a conventional disturbance observer (DOB) using internal information from a servomotor and a multi-encoder-based disturbance observer (MEDOB) using load-side position/ angular information as well. In the DOB-based monitoring, chatter frequency components were observable regardless of the guideway type and drive system. However, chatter monitoring may be difficult when the angle of the servomotor is changed slightly because of the damping properties of the sliding guideway. In the MEDOB-based monitoring, the waveform of the estimated cutting force reflected the vibrational state at the cutting point well, and the extraction of chatter frequency components became easier regardless of the guideway type.
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