Is process damping effective in the stability of robotic milling?

Abstract

Chatter stability is a major constraint in milling, where low and high cutting speeds are used. At low cutting speed regime, process damping leads to increased stability, whereas at high cutting speeds lobing effect is beneficial. Excitation frequency depends on spindle speed and the number of cutting flutes on the milling tool. Hence the vibration mode governing chatter stability varies for multi-mode milling systems. In CNC milling, low frequency structural modes are stiffer than cutting spindle-holder-tool (SHT) assembly. However, robotic milling demonstrates a distinct behavior as low frequency modes are significantly more flexible. This study investigates the effect of robot structure induced low frequency vibration modes on stability limits at low cutting speeds, where process damping is expected to increase stability limits. Time domain simulations are used to explain the variation of the dominant mode from high frequency to low frequency with the decreasing spindle speed. Simulated stability diagram for the multi-mode robotic milling system is verified by experiments. It was shown that especially the vibration modes in the range of 15 to 20 Hz do not generate enough process damping force due to long vibration waves, i.e. cutting speed – to – chatter frequency ratio, when low frequency modes govern chatter stability. Simulation of stability diagrams showed that there is a spindle speed region where the stability lobes governed by the robot structure crosscut the stability lobes governed by the THS assembly. Due to the inherent effect of tool diameter (D) and number of cutting flutes (Z) on cutting speed and excitation frequency, this region shifts according to the D/Z ratio. It was shown through simulations that D/Z ratio is a critical metric to benefit from process damping without the interference of the low frequency excitation of the robotic structure. The simulation results are used to provide suggestions for milling tool selection in robotic milling, where the main conclusion is to use lower D/Z ratio, which means that using high number of cutting flutes.