Novel chip-breaking structure of step drill for drilling damage reduction on CFRP/Al stack

Abstract

Abstract CFRP/Al stacks are popular for aerospace structures which require one-shot drilling thousands of stack holes within strict damage tolerance. However, CFRP is susceptible to cutting damages; the evacuation of Al chips through the upper CFRP may also cause unexpected damages even with a preferable step drill bit, of which second step faces the challenge of breaking chips due to its space and cutting angle limitations. This article proposes a novel structure on the second step of a step drill bit to effectively break the Al chips and reduce the chip-induced damage in one-shot drilling of CFRP/Al stack. The cutting chip generation/motion of the second step is analyzed, and it is found that adding a block surface beside the rake surface to restrain the side-curl of the chips could increase the surface strain of the chip and possibly break it. More importantly, the block surface is an additional structure beside the secondary cutting edge, which will neither affect its cutting angle nor be limited by space. Then a novel chip breaking structure based on the block surface is invented at the inner side on the rake surface of the secondary cutting edge. Comparative experiments are carried out for one-shot drilling of CFRP/Al, and the interactions between chips and CFRP/Al are revealed. With the novel structure, effective Al chip breaking and significant improvement of CFRP drilling quality are achieved.