Vibration surveillance system with variable stiffness holder for milling flexible details

Abstract

Efficient milling of the flexible details (i.e. rotor blades, thin-walled elements) using slender ball-end tools is a difficult task due to possibility of vibration occurrence. Because of the existence of certain conditions (small depths of cutting, regeneration phenomena), cutting process may lose stability and selfexcited chatter vibration may appear. Frequency of the chatter vibration is close to dominant natural frequency of the workpiece or the tool. One of the methods of chatter vibration avoidance is matching the spindle speed to the optimal phase shift between subsequent cutting edge passes (i.e. the Liao–Young condition). In previous works the authors successfully implemented the idea of optimal speeds map where optimal speed was calculated for every point of the machined surface based on the dominant natural frequencies for local areas. During milling, spindle speed was set according to the map. However, changing spindle speed during tool pass may reduce surface quality in speed change point and is difficult to perform it in some milling centres. The article presents the idea of a new workpiece holder with adjustable stiffness. Milling process will be performed with constant spindle and feed speed. In order to avoid vibration, stiffness of the specially designed workpiece holder will be modified off-line. Stiffness changes modify natural frequencies of the workpiece and thus, it is possible to modify dynamic properties of the workpiece in such a way that arbitrary chosen, constant spindle speed will be optimal, due to the Liao–Young condition performance. This will need calculation of the optimal stiffness map (referred to different spindle speeds), which will be performed before milling based on the workpiece’s modal identification results and Finite Element Model simulations.