Correlating tool life and workpiece surface roughness with tool stiffness in the milling of Ti-6Al-4V alloy with toroidal tool

Abstract

Titanium and its alloys are known as difficult to machine materials because of their unique combination of properties, such as high strength at elevated temperatures, low thermal conductivity, high chemical reactivity with almost all tool materials, and low Young’s modulus (which favors vibration). Because of the high tendency of vibration titanium machining (specially milling process) has, it is necessary to keep vibration under control in order to achieve high productivity. This work focused on the study of tool life, cutting forces, and workpiece roughness in the end milling of Ti-6Al-4V alloy. The main objective was to find a kind of tool holder to provide reduction of vibration in the milling process, increasing tool life, and decreasing surface roughness. Besides the conventional tool holders made of steel and of cemented carbide, two others were tried, both made of steel: the first was hollow (the steel tool holder was drilled in its axis direction) and in the second tool holder, the hole was filled with polyurethane (PU). These modified tool holders exhibited higher natural frequency than the conventional holders but, due to its lower stiffness, tool lives in the milling experiments were shorter when they were used. The most rigid tool holders (the conventional ones) were able to resist to the high-frequency vibration caused by both the fluctuation of chip thickness and the segmented chips and, consequently, presented longer tool lives. Adhesion was the main wear mechanism in all tests.