Influences of the workpiece material and the tool-surface engagement (TSE) on surface finishing when ball-end milling

This paper explains and quantifies the effects of the tool-surface engagement (TSE) on the workpiece finishing when using a ball-end tool in free-form milling of metallic mold materials with distinct elastoplastic responses. Electrolytic copper and AISI H13 steels were machined under different TSE and monitored for Sz roughness and cutting energy. As a result, two different cutting mechanisms named Tip-On and Tip-Out were identified. In Tip-On, the tooltip directly contributes to the part final surface formation, while at Tip-Out the tooltip only takes part of the chip removal without participating in the part surface generation. Accumulated cutting energy during every chip formation was also calculated based on machining force measured experimentally. With accumulated cutting energy 10 times lower than down-milling, the up-milling mode improved the part roughness by 43% at the Tip-Out mechanism. Contrary, down-milling produced four main surface damages due to the higher accumulated cutting energy, i.e., plastic deformation and side flow in ductile parts and cavities and scratches in harder workpieces. Real Sz roughness was 7 to 19 times higher than the theoretical one with major influences of the TSE and workpiece material. In conclusion, hand finishing can be reduced by applying up-milling and Tip-Out strategies when milling free-forms of harder workpieces.