The influence of spatially nonuniform temperature fields on the vibration and stability characteristics of EDM wires

This is an investigation into the effect of spatially nonuniform temperatur e fields on the free vibration and stability characteristics of the wire el ectrode in the wire electro-discharge machining (EDM) process. In particula r, a nonlinear multi-mode structural model is developed for the translating wire along with a thermal model which accounts for heat input (from sparki ng), axial conduction, radial convection, and energy storage. The two model s are then coupled through the internal thermal stress terms of the structu ral model. Using these combined models, an equilibrium and eigenvalue analy sis is performed to examine the behavior of the small amplitude natural fre quencies as a function of the transport speed under normal cutting conditio ns. It is shown that both divergence and Hopf bifurcations may occur in thi s system. Specifically, at low transport speeds the straight wire configura tion is unstable due to a thermal buckling instability. At intermediate spe eds, the straight configuration is restabilized by the reduced residency ti me of the wire in the kerf. At high speeds the straight configuration is ag ain unstable; this is due to a series of transport instabilities, The influ ence of the input discharge energy and the applied axial wire tension on th e size of the restabilized zone is determined and the implications for cutt ing accuracy are discussed. (C) 2000 Elsevier Science Ltd. All rights reser ved.