High speed end milling of hardened AISI D2 tool steel (∼58 HRC)

Tool steels conforming to AISI group D are used widely in the manufacture of blanking and cold-forming dies, on account of their excellent wear resistance and deep hardening characteristics. The present paper gives details of machining experiments with hardened AISI D2 cold work tool steel (∼58 HRC) using indexable insert ball nose end mills employing carbide and cermet tools, and solid carbide ball nose end mills. The aim was to identify tool wear mechanisms and appropriate cutting parameters. Tool life in terms of length cut and workpiece surface roughness (Ra) data are presented, together with comparative results for hardened AISI H13 (∼52 HRC) hot work tool steel. Preliminary results on the performance of polycrystalline cubic boron nitride (PCBN) tools when face milling are also detailed. Little difference was found in terms of tool life between the cemented carbide products tested, despite the differences in tool format, coating and geometry. An analysis of the flank wear patterns indicated that chipping, adhesion and attrition wear, were the principal mechanisms responsible for tool wear. Cut lengths were between 10 and 35 m for a tool life criterion of 0.3 mm maximum flank wear, at cutting speeds 25–200 m/min. This compares with 50–250 m for hardened H13 when using cutting speeds between 150 and 350 m/min. Cermet performance on D2 was poor, with lengths cut typically only 10–50% of that obtained with carbide tools. Direct comparison of results with PCBN tools is not possible due to differences in the mode of operation and the step-over distance employed, however, the volume of material removed during the life of the tool was ∼40 cm3.