Optimization of tool structure, diamond coating thickness, and cutting parameters for low-damage milling of T800/epoxy CFRP composites

This study addresses machining challenges of T800-grade unidirectional CFRP laminates by optimizing the tool structure, the CVD diamond coating thickness, and the cutting parameters. An asymmetric four-tooth milling cutter with varying helix angles (5°–45°) and segmented edge lengths was proposed, and six tooth arrangements were evaluated by finite element simulation. The optimal sequence—alternating long/short left-handed segments with varied helix angles—achieved the best thermal-mechanical balance, with only 26.5% high-stress zones and a peak temperature of 277°C, whereas unbalanced designs caused stress concentration or overheating above 350°C. Cutting tests were then performed with the optimized tool under a full factorial design covering coating thicknesses of 5–20 μm, spindle speeds of 8000–12,000 rpm, and feed rates of 300–500 mm/min. Regarding the machining parameters, the 10–20 μm coatings maintained stable cutting forces over the spindle speed range, whereas the 5 μm coating exhibited a marked force escalation at high speeds due to thermal softening. In addition, the axial force of the 5 μm-coated tool decreased with increasing feed rate, which is attributed to a transition from fiber ploughing to direct shearing. Concerning the coating thickness, wear resistance was non-monotonic, peaking at 15 μm (2696 μm ² worn area) and offering the best balance between protection and adhesion, while the 20 μm coating suffered from brittleness and chip adhesion that degraded the surface quality. Overall, the proposed tool combined with a 10–15 μm CVD diamond coating enables low-damage, high-quality milling of T800-grade CFRP laminates.