Modeling dynamic stability in high-speed micromilling of Ti–6Al–4V via velocity and chip load dependent cutting coefficients

Abstract

An increase in the demand for miniaturized components has resulted in the development of mechanical micromachining processes, such as micromilling. However, scaling down the process for micromilling operations require micro-tools, whose stiffness values are orders of magnitude lower than the conventional tools. The limited stiffness of the micro-end mills is a big impediment in machining difficult-to-cut materials, such as hardened steels and Ti-alloys. To address this issue, the cutting forces and hence the chip loads need to be reduced by using very high spindle rotational speeds. However, at lower chip loads ploughing may occur instead of cutting resulting in cutting force variation and high spindle speeds can excite higher order modes. Consequently, high spindle speeds and low chip loads in a tool with limited stiffness can lead to chatter induced dynamic instability which deteriorates the part quality, surface finish and tool life. Hence, identification of stable cutting parameters is necessary to avoid the chatter in high speed micromilling. Since the dynamic stability depends on the speed and the chip load (feed/flute), mechanistic force model with a constant cutting coefficient will yield inaccurate results. In this paper, the mechanistic force model based on velocity and chip load dependent cutting coefficient has been incorporated into the analytical stability model to predict the cutting forces and the stability lobe diagrams for high-speed micromilling of Ti6Al4V. The force predictions from the mechanistic model using velocity and chip load dependent cutting coefficient are in better agreement with experimentally measured forces as compared to constant cutting coefficients. Up to a spindle speed of 70,000rpm, the maximum prediction errors in stability boundary for a 500µm diameter end-mill using constant and velocity–chip load dependent cutting coefficients are ~33% and ~11%, respectively.