Development of a practical model for selection of stable tooling system configurations in internal turning

When machining high precision mechanical parts, self excited chatter vibrations must be absolutely avoided since they cause unacceptable surface finish and dimensional errors. This is of particular relevance in internal turning, where the boring bar is usually the most flexible element of the machining system. In this application, the critical ratio can be defined as the maximum admissible ratio of boring bar overhang L to boring bar external diameter D assuring a stable cutting process. In general, it is not easy to predict (L/D)cr since data available in literature are unreliable, especially for modern boring bars. In this paper, an experimental study on the influence of boring bar material (conventional alloy steel and high-damping carbide), boring bar external diameter D, workpiece material cutting pressure ks and tool nose radius rε on the critical ratio is illustrated. Statistical analysis of experimental results was carried out, and a semi-empirical model of the critical ratio (L/D)cr as a function of boring bar material and geometry, cutting edge geometry and workpiece material was obtained. The model is able to predict the critical ratio with good accuracy (±10%) and it can be practically applied for selection of stable tooling system configurations in internal turning.