Multi-margin drill structure for improving hole quality and dimensional consistency in drilling Ti/CFRP stacks

Low damage and accurate dimension are crucial requirements in the drilling of aerospace structures, especially for bearing components of Ti/CFRP stacks. However, tremendous differences in material properties between Ti and CFRP have brought significant challenges to the drilling quality/accuracy control. This paper presents a novel cutting depth control drill structure for improving drilling quality/accuracy of Ti/CFRP stack. Considering the different elastic modulus of two materials, a two-dimensional cutting model was applied to analysis the cutting process of Ti, CFRP, Ti/CFRP interface and CFRP exit. Then, the effects of the cutting depth on the spring-back of the machined surface and the delamination of the outlet are clarified, and the small cutting depth is suggested to be adopted. A multi-margin structure is proposed on second-step of the step drill bit in order to perform the cutting depth control strategy, and the geometrical analyses reveal that the multi-margin structure could effectively reduce the cutting depth in drilling Ti/CFRP stack. Finally, comparative experimental verifications are carried out, and the multi-margin structure is proved to be effective in reducing drill-exit damages of CFRP and controlling hole diameter deviations for both Ti and CFRP within the tolerance.