A Comparative Analysis of Vibration and Acoustic Signals in Machining Condition Monitoring of a Universal Lathe

Lathe is a basic machining equipment and widely used in industrial production. It is of great significance to monitor the running state of the lathe for production efficiency and quality control. Commonly, the method of monitoring the running state of the lathe is to arrange vibration sensors on the equipment or use non-contact acoustic sensors to collect vibration signals or acoustic signals generated during the machining process and to analyze the signals for monitoring and diagnosing the running state of the lathe. Compared with acoustic sensor, vibration sensors have the characteristics of strong anti-interference and high sensitivity, but they need to contact the equipment and need more sensors placed in adequate positions. The acoustic sensors can perceive different sound sources with fewer sensor locations and conveniently placed for remote monitoring, and, but the sensitivity is low, and easy to be disturbed by environmental noise. Aiming at developing cost-effective monitoring system in machining workshops, this study evaluates vibration signal and acoustic signal for monitoring the turning processes of a universal lathe with different depths of cut (DOC) and spindle speeds. Common Fourier spectrum and Hilbert envelope spectrum analysis are used to extract monitoring information in low and high frequency ranges respectively. It has found that both vibration and acoustics allow the changes in characteristic frequency values of motor and spindle to be resolved for representing the progression of DOC. Particularly the frequency exhibit a gradual decrease due to the gradual increase of overall cutting load with DOCs and can be an effective monitoring feature. However, the amplitudes at these frequencies and others such as gear meshes change in nonlinear ways due to dynamic effects in structure resonances, cutting forces and background noises. In collusion, the frequency change is a reliable feature for both vibration and acoustics for monitoring the machining of lathe.