Study on tool wear and workpiece surface integrity following drilling of CFRP laminates with variable feed rate strategy

Drilling carbon fibre reinforced plastic (CFRP) composites is often accompanied by workpiece delamination particularly at hole exit, which is dependent to a large extent on the applied feed rate/thrust force. In this paper, a variable feed rate strategy was investigated where drill feed was decreased from a maximum of 0.3 mm/rev to 0.01 mm/rev for the final 2 mm of hole depth, in order to reduce thrust force and minimise hole exit damage but without severely impacting process productivity. A full factorial experiment involving two variable factors (CFRP lay-up configuration at two levels and primary feed rate at three levels) was performed using diamond coated carbide tools. All tests achieved the 384 drilled hole criterion with tool flank wear ranging from 114 to 193 μm, chipping/delamination of the coating and abrasion were the principal tool wear modes. Delamination factor values at hole entry generally increased with primary feed rate and number of holes drilled by up to ~23%, with limited difference in exit delamination factor between the first and last holes drilled (maximum of 0.04). The reduced feed rate regime immediately prior to hole exit led to lower surface roughness by up to ~6.46 μm Ra compared with the entry location together with reduced fibre pull out and cavities compared to the rest of the hole when drilling with a new tool. Surface quality was however less apparent with increased numbers of holes drilled and tool wear.