Impact of ultrasonic assisted cutting of steel on surface integrity

Abstract

Diamond tools are typically used to fabricate optical molds. However, the excessive tool wear of diamond tools during machining steel still prevents their application for optical mold manufacturing. As steel molds potentially offer a higher lifetime than non-ferrous molds, the machining of steel to optical molds with diamond tools is desirable. This implies a great challenge for the production of tools for injection molded parts with complex geometry and high surface quality. As the most suitable method to prevent tool wear, ultrasonic assisted diamond cutting has been established. Since it is not yet known how the ultrasonic vibration during machining affects the surface integrity of manufactured components, an experimental study was carried out for this purpose. The conventional machining mode and the ultrasonic assisted elliptical vibration mode were compared. In addition to diamond tools, tools made of cubic boron nitride (CBN) were also included in the investigations to check whether ultrasonic assisted cutting with these significantly cheaper tools can achieve a similar supreme surface finish. For the evaluation of the surface integrity, the surface topography was measured post-process with 3D white light interferometry and the generated subsurface zone was characterized by metallographic analysis of the processed samples. Additionally, the forces were measured in-process to determine the local mechanical load on the material. The results show that in ultrasonic assisted cutting, compared to conventional machining, an improvement in surface quality is achieved with diamond tools as well as CBN tools. In terms of plastic deformation, it was also found that the influence on the surface layer is significantly different compared to conventional cutting with both diamond tools and CBN tools. The differences are attributed to the ultrasonic impact. Regarding the process forces, an elliptical vibration lowers the process forces with a factor supposedly depending on the cutting edge radius and the material hardness.