Comparative study on cutting performance of conventional and ultrasonic-assisted bi-directional helical milling of CFRP

Compared with the conventional drilling, the helical milling has obvious advantages in making holes of carbon fiber-reinforced plastic (CFRP). Nevertheless, the rapid wear of cutting edges readily causes some defects in the outlet holes, such as burrs and tearing. In order to improve the hole-making quality of CFRP, a comparative experimental study on conventional and ultrasonic-assisted bi-directional helical milling of CFRP was carried out. The wear mechanism of the forward and reverse cutting edges was analyzed in the two types of machining, and the change laws of cutting forces and hole wall quality were obtained by different machining means. The experimental results indicated that the flank face of forward and reverse cutting edges was dominated by the abrasive wear mechanism in the ultrasonic-assisted milling. With aggravation of the tool wear, no obvious coating peeled off the forward cutting edge, the reverse cutting edge remained relatively intact, and the wear form of neither cutting edge changed. Furthermore, in the ultrasonic-assisted reverse milling, the axial force and hole diameter deviation were restrained better than in the conventional milling, and especially when the tool wear occurred, the cutting force fluctuation varied slowly. In the ultrasonic-assisted milling, the shear fracture predominated over bending fracture. Meanwhile, the time variation of effective rake angles improved the chip breaking and removing performance of cutters, and thus, the machining quality of hole wall was enhanced obviously.