On-line chatter detection in milling using drive motor current commands extracted from CNC

This paper presents an on-line chatter detection method in milling using drive motor current commands supplied by the CNC system in real-time. The methodology is described by using the spindle drive motor current command although it has been applied to the feed drives as well which is demonstrated in the results section. The transfer function of spindle velocity controller is constructed by reading the control law parameters and measuring the Frequency Response Function (FRF) of the system automatically using an external computer communicating with the CNC in real time. By subtracting the rigid body based FRF from the measured FRF of the velocity controller that includes the flexibilities, the structural modes of the spindle drive are identified. The closed loop transfer function between the cutting torque at the tool and corresponding noise free digital current commanded by the CNC is formed. The effects of structural dynamic modes of the spindle are compensated via a proposed observer. The bandwidth of the compensated FRF of the current command over cutting torque disturbance has been increased to 2.5 kHz with 10 kHz communication speed limit of the CNC with external PC. After removing the forced vibration components, the frequency and presence of chatter are detected from the Fourier Spectrum of the current commands supplied by CNC in real time. The proposed system is experimentally validated in milling tests.