The influence of tool-surface contact on tool life and surface roughness when milling free-form geometries in hardened steel

Abstract

Machining process planning for milling hard materials with free-form surfaces using a ball nose end mill still requires empirical determination of the machining parameters, which can limit the efficiency of the process, reduce tool life, and adversely affect workpiece surface quality. Milling free-form surfaces differs from ordinary milling because the tool-surface contact changes constantly, causing the cutting speed to vary from the programmed value (in regions where the tool touches the surface with its nominal diameter and the tool axis is parallel to the surface) to zero (in regions where the center of the tool tip cuts the material and the tool axis is almost perpendicular to the surface at the point of contact). This paper focuses on this issue and investigates the influence of effective cutting speed and tool-surface contact on tool wear and surface roughness. High-speed milling experiments were carried out in which convex circular surfaces of hardened D6 steel were machined with a ball nose end mill keeping the effective tool diameter along the tool’s circular trajectory constant in each experiment. The input variables were the lead angle (and consequently the effective tool diameter, which was kept constant in each experiment but varied from one experiment to another) and feed direction (ascendant and descendant). As the effective tool diameter increased from one experiment to another, the feed rate decreased. The results show, in contrast to other findings in the literature, that contact between the center of the tool tip and the workpiece can increase tool life and reduce roughness when milling free-form surfaces in hardened steels. Furthermore, machining time is reduced as the smaller effective tool diameter leads to a higher feed rate. A relationship was also observed between the axial machining force and process stability when the tool tip is involved in the cutting process.