Workpiece-fixture frictional contact and its influence on machining performance

The significance of the fixture lies in the fact that the fixture has a strong impact on the machining performance. The workpiece-fixture interaction affects the dynamic behavior of a machining process via the configuration of the fixture layout and the frictional contact at the workpiece-fixture interfaces. The fixture serves as a bridge between the workpiece and machine structure and friction forces developed at the contact interface serve as a damper to dissipate the input energy from the cutting process. This research has sought to understand and model the effects of the frictional contact on the machining performance.; After an extensive review of the frictional contact problem, the variational inequality theory was employed in this research because of its mathematical elegance and ability to handle multibody frictional contact. Static and dynamic frictional models have been developed. The static model aims at predicting the friction forces at the contact interface, thus helping the fixture designer to exposing more features to the machining process by relying on the friction forces. The model is also capable of predicting the static displacement due to the clamping forces, which is an important factor for part quality. Towards developing a comprehensive dynamic model, the evolution of the workpiece-fixture dynamics with varying interface conditions has been studied through both modeling and experimentation. It was discovered that the increased clamping force tends to force the fixture element to move with the workpiece, thus “locking” the interface. A comprehensive dynamic model has been developed to capture the interface “locking”. The model considers both workpiece and fixture element vibrations. An automatic 3D mesh refinement scheme has been employed by the dynamic model for better solution accuracy. It has been shown that the developed dynamic model is capable of predicting the workpiece-fixture dynamics.; With the comprehensive dynamic model, it is possible to predict machining process stability by taking the workpiece-fixture interaction into account. Results from machining process stability showed that the machining stability is strongly influenced by the frictional contact and the incorporation of its influence allows for optimal fixture design.