Modelling of thrust force for worn drill bits characterised by cutting edge radius in drilling carbon fibre–reinforced plastic/Ti-6Al-4V alloy stacks

Wear rates are rapid when drilling carbon fibre–reinforced plastics/Ti-6Al-4V alloy stacks because of their distinct mechanical properties. Tool wear leads to a high thrust force, thereby reducing the quality of the drilled holes. This article develops a novel mechanistic model for carbon fibre–reinforced plastics/Ti-6Al-4V stacks, which is characterised by the cutting edge radius, to predict the variation of the thrust force when drilling with worn drill bits. Drilling experiments with varying feed rates were performed using carbide twist drill bits. The thrust force and drill edge profile were measured to calibrate and validate the presented model. The edge radius increases with both the cutting distance and number of drilled holes at varying feed rates. It was found that the growth rate of the edge radius increased with the feed rate with identical cutting distances, whereas it decreased slightly with the feed rate when the number of drilled holes was identical. Tool wear reduces the equivalent rake angle of the drill edge, resulting in higher thrust force. The maximum thrust force increases almost linearly with the edge radius of worn drills for both materials. The predicted thrust force curves are in very good agreement with the measured curves during the entire process. Average absolute errors of the maximum thrust force for carbon fibre–reinforced plastics and Ti-6Al-4V alloy are 3.24% and 1.88%, respectively.