Three-dimensional volumetric characterization of CFRP surface integrity in milling: comparison with conventional roughness parameters

Abstract Machined surface quality in carbon fibre reinforced polymer (CFRP) is commonly assessed using roughness parameters, which may not fully represent heterogeneous three-dimensional topography. This study investigates whether volumetric descriptors complement profile and areal roughness in milled CFRP surfaces. Dry up-milling experiments were conducted using a two-flute polycrystalline diamond end mill under a full factorial design with cutting speeds of 200 and 300 m/min and feed per tooth values of 0.017, 0.033 and 0.050 mm/tooth. Surface quality was characterized by focus-variation optical metrology using Ra, Sa, Above Surface Volume and Below Surface Volume. Cutting force, final cutting-edge radius and selected µCT observations supported interpretation of the machining response. Two-way ANOVA showed that feed per tooth accounted for 84.1% of Ra variation, while cutting speed and its interaction with feed were not significant. In contrast, Sa and both volumetric descriptors were significantly affected by cutting speed, feed and their interaction. At 0.050 mm/tooth, increasing cutting speed from 200 to 300 m/min changed Ra only from 1.527 to 1.426 µm, whereas the response shifted from above-surface dominated (V above = 0.975 mm³, V below = 0.033 mm³) to below-surface dominated (V above = 0.106 mm³, V below = 0.231 mm³). The 300 m/min condition exhibited more numerous and extensive µCT-visible discontinuities than the 200 m/min condition. Analysis over cumulative cutting length showed no universal monotonic relationship between cutting force and surface quality. Overall, the separated volumetric descriptors complement Ra and Sa by retaining the direction of three-dimensional topographic deviation, providing additional information for CFRP machining-quality assessment.