Sound mapping for identification of stability lobe diagrams in milling processes

Abstract

This paper deals with milling sound information. In milling operations, cutting edge impacts excite vibrations due to the interaction between the cutter and the workpiece, and because of the system's lack of dynamic stiffness. It is possible to distinguish between free, forced and self-excited vibrations. During a milling operation these three different types of mechanical vibrations propagate through air and generate a sound that intrinsically contains information about the process. A sound map is a graphical sound-level representation of a certain zone or region that is divided into points by means of a mesh. Sound maps have typically been used with social considerations in mind: to determine, for instance, noise levels in cities. The goal of this paper is to determine the stability lobe diagram (SLD) of a milling process by applying sound mapping methodology. Stability lobe diagrams show the stability frontier as combinations of spindle speeds (i.e. the abscissas axis) and radial depths of cut (i.e. the ordinate axis). In this investigation the SLD was obtained from a mesh of 30 spindle speeds per 20 axial depths of cut, resulting in a total of 600 experiments. A data acquisition platform was developed to collect the milling process sound through a microphone placed inside the machine-tool enclosure. Data were analysed off-line in order to recognise chatter frequencies. A 3D sound map was built by plotting, on each corresponding point of the mesh described above, the sound amplitude at frequencies around chatter frequency. The difference between stable and unstable zones is shown. This is the stability lobe diagram. The extensive experimentation detailed in this work reasserts and confirms the current state of knowledge of the chatter phenomenon