Machining performance in longitudinal-torsional ultrasonic-assisted core drilling of CFRP

Ultrasonic vibration-assisted core drilling has achieved some beneficial results in carbon fiber-reinforced plastics (CFRP) hole machining. Among them, longitudinal-torsional ultrasonic-assisted core drilling (LTUACD) shows more significant advantages in decreasing cutting force and reducing delamination defects and other aspects compared to longitudinal ultrasonic-assisted core drilling (LUACD). However, the reduction mechanism of cutting force and delamination defects in LTUACD is still not clear enough. Therefore, this paper researched the machining performance of LTUACD of CFRP. Specifically, the cutting force in LTUACD of CFRP is analyzed based on hertz contact theory and indentation depth theory. Then, a series of experiments were conducted to verify the analysis. And delamination suppression mechanism was discussed from the aspects of surface morphology of hole wall, exit morphology of hole, and bottom surface morphology of blind hole. The results show that the cutting force is significantly reduced in LTUACD compared to conventional core drilling (CCD) and LUACD due to the additional torsional vibration of tool, which changes the contact state between abrasive grain and material. Therefore, the exit delamination is further suppressed. In addition, the adhesion phenomenon of chips on the tool is also reduced, which enhances the tool cutting ability to obtain even fiber fracture surfaces and greatly improves hole quality.