Quality monitoring and control for drilling of CFRP laminates

Widespread uptake of Carbon Fiber Reinforced Plastics (CFRP) in the aerospace and automotive industries over the past two decades has highlighted the importance of machining composites in large production environments. The abrasive nature of carbon fibers and inherently heterogeneous structure of CFRP laminates create tool wear conditions that are unlike those typical of metal machining. The present work quantifies thrust force, tool flank wear and push-out delamination (POD) for drilling of 8 mm thick CFRP laminates with 8 mm high-speed steel drill bits. Thrust force, directly responsible for delamination effects, is found to increase with feed rate and cutting speed, while flank wear increases at lower feed rate and higher cutting speed. POD is found to increase with feed rate and tool wear, with excessive values of the latter found to change the material removal mechanism from cutting to tearing of fibers resulting in increases in local workpiece temperature and matrix degradation. A quality monitoring and control strategy for drilling of CFRP laminates is then proposed based on these observations by exploiting a fuzzy logic algorithm to predict POD based on thrust force and flank wear measurements. Feed rate, in turn, is controlled based on the POD to ensure acceptable machining quality over the entire lifespan of the tool. The developed approach offers an effective and simple way of maximizing throughput and tool duration while guaranteeing part quality.