The effect of resin on tool wear in the drilling of CFRP composites

Tool wear developed in the drilling of carbon fibre reinforced polymer (CFRP) and other FRP composites has been typically attributed in literature to the abrasive nature of the fibres. Unexpectedly, early research conducted by Sandvik Coromant found that the type of resin had an important influence on the tool wear developed in the drilling of CFRP composites with similar carbon fibre fabrics and different resins using coated and uncoated WC-Co tools. This thesis studied the influence of resin on tool wear in the drilling of CFRP composites by investigating the influence of workpiece constituents on a number of aspects relevant to the drilling operation.

The resin showed a significant impact on the forces developed in the drilling of CFRP composites. Resins with a high degree of cross-linking developed greater thrust force (Fz) and torque (Mz) than low cross-linked matrices. On the other hand, torque showed to be more sensitive to changes in cutting speed (Vc) in drilling composites with high modulus (HM) CF fabrics than in composites with high strength (HS) CF fabrics. This behaviour was found to be related to the lower temperatures developed in the machining of HM CF-reinforced composites compared to HS CF-reinforced systems. The higher temperatures developed at lower cutting speeds compared to higher cutting speeds increased the ability of the composite to form longer and more twisted chips, thus requiring a higher torque to produce these.

Considering the influence of the workpiece constituents on the aspects commented above, tool wear mechanisms in the drilling of CFRP composites were further investigated. No evidence of chemical wear was found, rendering the tool wear process fundamentally mechanical. Two types of mechanical wear regimes occurred; mild wear, involving the removal of the cobalt (binder) and the subsequent removal of carbide grains and severe wear, involving the hard polishing of the carbide phase and high tool material removal rates.

Pin-on-disc experiments and further tool wear tests revealed that the abrasiveness of the composite and the tool-workpiece coefficient of friction (μ) are higher at the beginning of drilling than at later stages, i.e. when the workpiece temperature is below Tg. Hence, the influence of resin on tool wear relies on the ability of the resin to hold the reinforcement firmly during the drilling operation. At temperatures above Tg, the resin cannot longer hold the fibres tight against the cutting edge and tool wear-related phenomena decrease (fibre-flank face brushing and fibre spring back).