Wet vs dry CFRP drilling : comparison of cutting fluid delivery methods

This work studied the impact of flood and through-tool cutting fluid delivery methods during the drilling of carbon fibre reinforced polymer (CFRP) composite panels against a dry baseline condition. The cutting fluid considered was a water-based, composite-specific fluid, and the cutting tool was a Ø6.35 mm diamond-coated drill bit. After an initial design of experiments (DOE) approach enabled the establishment of appropriate cutting parameters to minimise delamination, both delivery methods and dry baseline drilling were compared in terms of tool wear and hole quality. The increased heat removal efficiency of through-tool delivery method is theorized to have prevented the localised softening of the resin and consequently maintained high abrasiveness of the composite workpiece, ultimately developing the highest tool wear for the given parameters. After drilling 600 holes, dry drilling exhibited the best results and lowest variation of hole quality metrics, i.e. the most stable process. However, after drilling 300 holes, flood delivery produced a better performance for cylindricity and perpendicularity and similar exit delamination factor (Fd) than dry drilling. Interestingly, through-tool delivery exhibited a slightly better performance than dry drilling when the analysis and comparison is conducted after the first 100 holes drilled only. These results indicate that hole diameter is highly dependent on tool wear and demonstrates that heat evacuation helps to produce holes with better cylindricity and perpendicularity, since hole distortion is highly affected by heat up and cool down cycles and the different coefficient of thermal expansion (CTE) across the laminate.