Effect of workpiece stiffness on thrust force and delamination in drilling thin composite laminates

During drilling thin composite laminates, workpiece stiffness affects drilling process through bending and release movement. A new mechanical model for thrust force prediction was developed to quantitatively describe influence of workpiece stiffness. Its effect on delamination was also investigated. Derivative of workpiece deflection caused by thrust force was calculated to obtain dynamic feed rate. Then, thrust force was predicted as a function of drill geometry, specific energy and dynamic feed rate. Experiments of drilling woven CFRP plates had been performed with different feed rate and unsupported length. Results showed that the presented model could fit measurement thrust force nicely except exit stage, in which local deformation and crack of uncut material brought forward the force drop point. Delamination damage was observed at low stiffness even though thrust force did not exceed the critical value. Analysis results indicated that dynamic feed rate was a more suitable critical parameter for delamination-free drilling.