Influence of top-surface condition and process parameters on cutting forces and surface integrity in CFRP milling

The machinability of unidirectional carbon-fibre reinforced polymer (UD-CFRP) composites is strongly dependent on fibre architecture, cutting parameters and the initial surface condition; however, their combined effects remain insufficiently quantified. This study systematically evaluates the influence of fibre orientation (0°, 45°, 90°, 135°), feed (0.05 and 0.15 mm), cutting depth (1 and 2 mm) and top-surface quality (pre-machined by machining vs. abrasive water jet machining (AWJM)) on cutting forces and surface integrity during dry face milling. UD-CFRP composites were up-milled using a Ø12 mm Alcrona-coated two-flute carbide endmill. Three-component cutting force was recorded with a Kistler 9257B dynamometer and processed using a Butterworth low-pass filter. Surface roughness was measured in accordance with ISO 4288:1996, and surface morphology was assessed by optical and scanning electron microscopy. ANOVA results identify cutting depth and feed as the primary drivers of resultant cutting force, while the initial top-surface condition was also found to be significant. Fibre orientation did not significantly affect force in the pooled dataset, but became significant for pre-machined specimens, indicating that consistent allowance is required to reveal orientation effects. Surface roughness was strongly dependent on fibre orientation — optimal finishes were observed at 0° and 90°, whereas 135° produced the poorest results. SEM analysis revealed extensive matrix smearing that can obscure subsurface defects (fibre pull-out, microcracks), potentially biasing profilometric measurements. The results support the need for pre-machining of AWJ-cut CFRP specimens to improve repeatability and consistency, and recommend that fibre orientation and subsurface inspection be considered when specifying machining strategies for CFRPs.