Experimental study on drilling load and hole quality during rotary ultrasonic helical machining of small-diameter CFRP holes

Small-diameter CFRP holes have a number of applications such as aerospace structures, circuit boards and rehabilitation medical equipments. High quality hole-making technology for the small-diameter CFRP holes is urgently needed due to various mechanical damages in twist drilling and chip removal clog in core drilling. In order to improve the hole quality, rotary ultrasonic helical machining (RUHM) was developed for machining small-diameter CFRP holes in this paper. The trajectory of cutting edge in RUHM was modelled and the intermittent cutting mode in RUHM was analyzed. Afterwards, the comparison experiments were conducted between RUHM and conventional grinding (CG). The results shows that compared to CG, both thrust force and transverse force were remarkably reduced in RUHM with a maximum decrement of 71.3% and 61.5%, respectively. Meanwhile, both the hole edge quality and surface integrity were significantly improved in RUHM. Compared to CG, delamination factor at hole exit was reduced by 12.8-25.7% and surface roughness for hole inner surface was reduced by 51.9%-53.2% for Ra value in RUHM. Moreover, the mechanisms of cutting force reduction, delamination formation and suppression, and surface roughness improvement in RUHM were also analyzed. The results suggest that RUHM is a promising processing strategy for machining small-diameter CFRP holes.