Work piece dynamics influence on stability in machining

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This project was carried out at Sandvik Coromant in Sandviken. The objective was to investigate how the machining dynamics was influences by the workpiece and different positions of the fixtures. The project consisted of two parts: investigation of one simple beam model to gain knowledge and understanding of the problem and secondly: study of alternative clamping positions for a jet engine turbine exhaust case. For both cases natural frequencies, mode shapes, frequency response functions and machining stability was investigated. For the beam model the natural frequency changes resulting from machining were found to have specific patterns which were dependent on the boundary conditions. The changing natural frequencies were reflected on to the stability lobes. In order to maximize the production rate the spindle speed should be altered to natural frequencies changes. The natural frequencies can change considerably when the beam cross section is machined down. The mode shapes of the beam is however fairly unchanged. This makes it possible to choose a good position of an additional support based on knowledge of the original beam. A position of an additional support at a node point for a particular mode maximizes the natural frequency for the mode one number lower. A position of the support near the middle of the beam minimizes the highest flexibility that occurs and also maximizes the first natural frequency for a prismatic beam. For the jet engine casing six lower and six upper clamping sections with a 30 degree phase shift was found to have both the highest first natural frequencies and the highest machining stability limits of the investigates cases. Locking th...