Applicability of Specific Acoustic Emission for the Investigation of the Transient Phase of Cutting

In the metal chip formation zone, during the large-scale and high-speed shaping of the raw material, high-frequency but low-energy pressure waves are generated within the solid material. The entire set of these pressure waves can be detected in machining research through acoustic emission measurement. The magnitude and features of the acoustic emission mainly depend on the material of the workpiece and tool, the technological parameters, and tool wear. Exploiting this property, it has become widespread in the field of process monitoring. By relating the acoustic emission to the theoretical chip cross-section, we obtain a characteristic indicator proportional to the chip formation process. Examining the specific acoustic emission as a function of the theoretical chip cross-section allows us to clearly observe and measure the transient phase of the cutting process, i.e., when the tool edge enters and exits the cutting zone. The range of small chip thickness is considered to be the range of theoretical chip thicknesses that are equal to or smaller than the tool edge radius. In this range, the statistical behavior of the specific acoustic emission differs from that observed at larger theoretical chip thicknesses, indicating a change in chip formation quality—referred to as process transition or instability. Cutting in this unstable range is undesirable and can deteriorate the finish of the machined surface. This study presents the measurement technique for the specific acoustic emission, the development of related process indicators, and their application to monitor the transient stage of cutting.