Tool geometry optimization for drilling CFRP/Al-Li stacks with a lightning strike protection

Abstract

One-shot drilling of Carbon Fiber-Reinforced Polymer materials with a Lightning Strike Protection (LSP)/metal stacks is a challenging task due to the inherent difference physical and mechanical properties and processing mechanisms of each component. The objective of the present work is to optimize tool geometry width in drilling of CFRP/Al-Li with a LSP. Firstly, a set of conventional uncoated carbide drills which are commercially available for the drilling of aeronautic composites was used to study the effect of tool geometry on drilled-hole quality. The set encompasses a twist drill bit, a step drill bit and a point spur drill bit. Based on references and cutting conditions recommended by drill manufacturers, the drilling tests performed are based on full-factorial experimental design using three cutting speeds and two feed rates. Results showed that, on the one hand, spur drill gave the best results causing small damage extension in the hole perimeter but we noticed a rapid tool wear at the spur which increases with feed. On the other hand, step drill presented higher LSP delamination located at the hole entrance but reduces the level of thrust force. The choice of tool geometry process should be a compromise in drilling aluminium as well as drilling carbon fiber with LSP. In the second phase of the current work, three different new uncoated carbide geometries were developed: a Spur Step Drill, a Three Steps Drill and a Square Step Drill. Same cutting conditions were used for the three drills. Results showed a rapid tool wear for the Spur Step Drill at the spur. In terms of LSP delamination, burr and drill wear, the drill adapted to drilling CFRP/Al-Li with LSP stacks is the three steps drill.