A cyber-physical system that can automatically monitor the chatter of a machine tool and adjust the machining parameters to suppress the chatter is in need in the industry. For the CNC machine tools without network connection capability, to implement such a cyber-physical system is difficult. To overcome the problem, we developed a device that can be mounted on the panel of the controller of a CNC machine tool to monitor the chatter and to adjust the feed rate when the chatter occurs. A microphone was used as the sensor for the chatter detection. Preliminary results demonstrated the feasibility of the device. The device can be used on CNC machines with various controllers and can upgrade an unconnected CNC machine to become a CNC machine with cyber-physical capability.