Analysis of the effects of different milling strategies on helical milling of unidirectional CFRP under minimum quantity lubrication

During the helical milling of carbon fiber-reinforced polymer (CFRP), issues such as low surface quality of workpieces and severe tool wear commonly occur. In this study, helical milling experiments were conducted on a unidirectional CFRP under minimum quantity lubrication (MQL) condition. It comparatively analyzed the effects of down/up milling, as well as the fiber cutting angle, on the cutting force, vibration, temperature, tool wear, and surface quality. It then proposed and validated a hybrid (up - down - up) milling strategy. The results indicate that, for both milling strategies, when cutting against the fiber (θ ∈ [90, 180°]), the cutting force is larger than along the fiber (θ ∈ [0, 90°]). Cutting force and vibration follow axial force F_z > radial force F_n > tangential force F_t, and axial vibration a_y > radial vibration a_x. Compared with down milling, the temperature decreased by 22.37 % and 9.30 % in the early stages (I and II, respectively) and increased by 20.00 %, 23.74 %, and 8.47 % in the middle and later stages (III, IV, and V, respectively) for the up milling. Lower cutting forces, vibrations, and tooltip area led to a significant decrease in surface defects at the orifice. However, more defects, including fiber residues and fibers pulling out, were evident on the hole wall surface during up milling. Severe pitting and scratching were observed on the rear surface of the tool, which were absent in down milling. Compared with down/up milling, the proposed hybrid milling strategy effectively enhanced the surface quality and tool life. These findings can contribute to improvements in the helical milling process of CFRP and optimize overall machining performance.