The effect of forced torsional vibrations on the stability of metal cutting processes using the example of cold circular sawing

• Applicants: Dr.-Ing. Martin Kolouch, Ph.D.; Professor Dr. Günter Radons (†)
• Subject Area: Metal-Cutting and Abrasive Manufacturing Engineering
• Term: from 2016 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 321138034

Increasing quality requirements and decreasing product life cycles demand flexible metal cutting processes at the performance limit of modern machine tools. Large vibrations during machining, which lead to bad surface finish, increased tool wear and noise, are the main reason, why only a fraction of the full potential of machine tools can be used in production. For the optimization of the cutting processes and the construction of machine tools, typically, mechanical models are used for the prediction of machine tool vibrations and for getting an understanding of the so-called machine tool chatter. The main reason for chatter is the regenerative effect, where a wavy surface is left on the workpiece due to small vibrations of the structure and the repetitive cutting of the same wavy surface leads to larger vibrations of the structure. An essential parameter for the description of the regenerative effect is the time delay between two subsequent cuts at the same workpiece surface. Forced torsional vibrations due to a periodic variation of the cutting torque, for example, in face milling, in deep-hole drilling or in sawing result in state-dependent variations of the time delay. Whereas the classical regenerative effect with a constant time delay is well-understood and frequently studied in the literature, the effect of state-dependent delays due to forced torsional vibrations is nearly unexplored.The main objective of this project is the theoretical investigation of these effects with dynamic process models and experiments at a test rig and a real machine tool. In particular, the effects due to the dynamic displacements of the workpiece entry and exit angles of the cutting teeth and the effect of state-dependent delays on the dynamics and stability of metal cutting processes are studied using the example of cold circular sawing of thin-walled workpieces. It is known that in cold circular sawing instabilities due to torsional vibrations can appear and, in addition, cutting of thin-walled workpieces results in large forced vibrations. Previous studies at the Fraunhofer IWU have shown that there are two additional effects, which has not been studied until now. These are, on the one hand, transient vibrations during the whole process due to continuously varying entry and exit angles of the cutting teeth including a continuous variation of the location of the excitation. On the other hand, it is possible that temporarily no cutting tooth is engaged in the workpiece, which leads to a vanishing preload of the structure and a significant nonlinear structural behavior for torsional vibrations. These two effects, which can appear in milling or deep hole drilling as well, are also investigated in this project. At the end a clear identification of the relationship between cause and effect should be possible, such that a fundamental understanding of the effect of forced torsional vibrations on the dynamics and stability of metal cutting processes can be obtained.

• DFG Programme: Research Grants