Influence of cooled and heated compressed air on the surface finish in face milling of CF30-reinforced PEEK

Carbon fiber-reinforced poly (ether-ether-ketone) (CF/PEEK) is a widely used composite material in aerospace, automotive, and biomedical applications. However, achieving a high-quality surface finish during machining remains challenging due to its low thermal conductivity and high abrasiveness. This study investigates the thermal effects of using cooled compressed air (CCA at −40°C) and heated compressed air (HCA at 60°C) on surface finish during face milling of PEEK reinforced with 30% short carbon fiber (CF30), compared to dry machining (DRY). A Box-Behnken experimental design was applied with three levels of cutting speed ( v c ), feed per tooth ( f z ), and axial depth of cut ( a p ). Surface profiles and roughness parameters ( R a and R z ) were statistically evaluated using ANOVA, Abbott–Firestone curves (AFC), and probability density functions (PDF). Scanning electron microscopy (SEM) and 3D interferometry were employed to analyze the optimized and best-performing samples. Results indicated that the lowest roughness values were achieved under CCA, while HCA yielded the highest. In both cases, f z was the most significant factor. AFC and PDF analyses confirmed that surfaces machined with CCA exhibit a more homogeneous 2D roughness profile, with less pronounced peaks and valleys. The regression model accurately predicted roughness values under CCA, with an error of approximately 4%, aligning well with experimental results ( R a = 0.307 μm, R z = 2.210 μm) and maintaining a suitable material removal rate (MRR = 1723 mm 3 /min). SEM images revealed that CCA enhances surface finish by increasing composite stiffness, facilitating cutting. 3D roughness profiles confirmed isotropic characteristics for all conditions. After optimization, CCA exhibited the lowest S a (0.371 µm), DRY the lowest S z (12.42 µm), and HCA the roughest surface profile, confirming the trends observed in 2D analysis.