Analysis of exit delamination mechanism and critical axial force in ball helical milling of CFRP

To determine the critical axial force of exit delamination in the ball helical milling (BHM) of CFRP, based on the classical laminar theory, the critical delamination is judged by the theory of virtual work displacement and the linear elastic fracture mechanics. The model that describes the crucial axial force in the BHM process is set up, given the anisotropy of CFRP and the specific shape of the ball end mill. The external force is a novel set as a quadratically distributed load dependent on the milling parameters in the model, and the relationship between the quantity of uncut remaining layers and the critical axial force is indicated. When the tool rotation speeds were 4000–8000 rpm, with 0.1–0.2 mm/rev axial feed and 0.02–0.04 mm/tooth tangential feed, helical milling experiments were carried out. The correlation among the milling parameters and axial force and exit delamination factor was investigated, taking into account the effect of tool wear. Comparing with the experimental data, combined with the exit delamination, the proposed model can well predict the critical axial force under different cutting conditions, and the milling parameters have a great impact on the axial forces and delamination. The hole-making delamination of the ball end mill is smaller compared to the end mill, and exit delamination of CFRP can be reduced in BHM technique.