The performance of coated tungsten carbide drills when machining carbon fibre-reinforced epoxy composite materials

This paper is concerned with the effect of coatings on the performance of tungsten carbide (WC) drills in the drilling of carbon fibre-reinforced epoxy. Although composites are becoming increasing popular, there is a deficit in the existing knowledge of drilling composites, and in particular carbon fibre-reinforced epoxy resins. Two coated drills, namely titanium nitride (TiN)-coated and diamond-like carbon (DLC)-coated drills, were investigated and for comparative purposes an uncoated drill. The testing involved drilling a series of consecutive holes. During these tests the thrust forces and torques were monitored, following which the tool was inspected for flank wear and the workpiece inspected for damage in terms of hole tolerance, delamination and spalling. For all three tool types (uncoated, TiN coated and DLC coated), only a small number of drilled holes were found to satisfy an H8 tolerance criterion. An investigation of the hole diameter through the thickness of the composite revealed that it was the outermost plies that caused the hole to fail this tolerance criterion. The effect of tool wear caused the measured thrust forces and torques to increase over the life of the tool. While the degree of measured tool wear was small by comparison with that associated with drilling conventional materials, the effects were found to result in unacceptable damage to the composite. The damage was apparent in the form of spalling, chip-out and matrix cracking. The coatings were not found to reduce either tool wear or damage to the composite.