Improving hole exit quality in rotary ultrasonic machining of ceramic matrix composites using a compound step-taper drill

Mechanical-machining-induced tearing defects at hole exits restrict the application of C/SiC composites. Rotary ultrasonic machining (RUM) is suitable for hole manufacture in brittle composites, providing reduced tearing size as compared with conventional grinding. Even so, substantial tearing defects at the hole exit remain with RUM. In this study, a novel compound step-taper diamond core drill for RUM of C/SiC was developed to further improve the hole exit quality. Contrastive machining tests were conducted to evaluate the effectiveness of the new type drill. Experimental results show that the compound drill can help reduce the tearing size by 30% on average. Results of variance analysis indicate that there is little dependency of tearing size on processing variables with the compound drill, whereas the common drill shows substantial dependence. Detailed observation of the thrust force reveals that the tearing size reduction using the compound drill is due to the reprocessing effects of its taper face. In the reprocessing process of the taper face, the thrust force gradually decreases at the hole exit. Increasing the ultrasonic amplitude can help further improve the hole exit quality when using our compound drill.