Surface Finish Analysis After Face Milling of PEEK under Different Cutting and Lubri‑cooling Conditions

Poly(ether-ether-ketone) (PEEK) is a high-performance thermoplastic widely used in orthopedic and traumatological implants due to its biocompatibility. In such applications, complex geometries and low surface roughness require optimized machining strategies. This study evaluated the influence of cutting parameters and lubri-cooling conditions on the surface finish of face milled PEEK using a TiAlN-coated solid carbide end mill. The tested conditions were cooled compressed air at − 40 °C (CCA), dry machining (DRY), and nanofluid minimum quantity lubrication (NMQL). A three-factor, three-level Box–Behnken design was applied for experimental planning and statistical analysis. Results showed that DRY machining provided the best surface finish, with average roughness values 32.45 and 28.45% lower than CCA and NMQL, respectively. The DRY condition also produced a more homogeneous peak-to-valley profile, confirmed by the Abbott–Firestone curve and probability density function. Analysis of variance indicated that feed per tooth and depth of cut were the most significant factors affecting roughness under DRY machining. SEM images revealed a more uniform surface under DRY conditions, while CCA promoted material pull-out, likely due to increased brittleness at low temperature. Under NMQL, feed marks appeared influenced by nanoparticle drag during tool–workpiece interaction.