Residual stress fluctuates periodically via the workpiece rotation phase during low frequency vibration cutting

Abstract

The turning process is a standard machining process employed in diverse sectors. However, it produces long continuous chips that can affect the efficiency of the process, accelerate the tooling's wear, or damage the machined surface. As a solution, low frequency vibration cutting synchronizing with the spindle rotation has recently been developed as a new machining method in turning operation. It applies vibrations in the tool feed direction and can synchronously control the applied vibrations and the spindle rotation. It can also effectively divide the long continuous chips generated by the turning process and has the potential to reduce thermo-mechanical load on the tool by periodically enabling the tool to leave the workpiece due to the vibrations. Low frequency vibration cutting, the cutting characteristics of which differ from those of conventional turning, induces residual stresses in the machined surface; however, the properties of these stresses have not yet been studied. Residual stress can have both beneficial and negative effects on the fatigue life of products. While compressive residual stress increases the fatigue life of the product, tensile residual stress facilitates the growth of fine cracks on the product's surface and reduces fatigue life. Therefore, it is important to understand the characteristics of the residual stress developed on a machined surface. In this study, an annealed 0.45% C steel bar was machined via straight turning of the vibration cutting process synchronizing with the spindle rotation, and the residual stress on the finished surface was measured. Particular focus was placed on analyzing the effects of the spindle phase on the characteristics of the residual stress inside the machined surface, especially the effects of the number of vibrations per spindle rotation, D, which is a unique parameter defining the vibration condition. Our results revealed that the residual stress varied depending on the position of the finished surface owing to the change in feed with the spindle phase during the process. Furthermore, D was observed to heavily influence the distribution of the residual stress on the finished surface. By means of adjusting its value, the residual stress value could either fluctuate periodically according to the phase of the workpiece or not fluctuate.