| Summary: | Electrochemical machining has found numerous applications in recent years. The process offers significant economic advantages over conventional machining methods where complex shapes are required in high performance metals. However, a general lack of information on the design of a machining operation has resulted in an unacceptable amount of trial and error development work in many instances. This thesis is concerned with the selection of operating conditions and the design of tooling for electrochemical machining. An introduction to this aspect of the process is followed by a review of current literature on the subject. A model is adopted for the prediction of the one-dimensional equilibrium gap. The model utilises two parameters which require experimental determination. The model also predicts an upper bound to the electrolyte velocity in the electrode gap which corresponds to a limiting value of the tool feedrate. Various other apparent limitations on the metal removal rate are discussed. Preliminary experimental work was carried out to test the applicability of the theoretical model with particular reference to the pressure drop across the electrode gap. The results verify the model but the model parameters show dependence on the operating conditions of machining. The experimental work was extended, using a production electrochemical machine, to cover operating conditions used in practice. Sodium chloride and sodium nitrate solutions were the electrolytes in the machining tests. The anode workpieces were manufactured from mild steel and were initially plane and parallel to the cathode. The machining characteristics of the two electrolytes with mild steel are discussed. Anode profiles are presented at various operating conditions with the model parameters required to match the experimental test data. Phenomena other than the electrolyte conductivity appear to influence the anode profile in tests with sodium nitrate electrolyte. The theoretical model is not verified with respect to the upper bound to the electrolyte velocity. Tool feedrates and electrolyte flow velocities are used which exceed those predicted theoretically. The upper bound to the electrolyte velocity is the performance limit of the electrolyte pump. A separate investigation was required to determine the cause of machining failure. Miniature bead thermistors were inserted through the cathode into the electrode gap to measure the temperature of the electrolyte close to the cathode. Temperatures in this region are found to be greater than those measured at outlet. Machining failure is shown to be the result of boiling of the electrolyte around the gas bubbles evolved at the cathode. The conductivity of the electrolyte becomes less due to the formation of steam bubbles and the anode converges towards outlet so that sparking or arcing occurs across the gap. A correlation is introduced to allow prediction of this condition. The operating characteristics of electrochemical machining are discussed in relation to the correlation, and other limitations. An optimum outlet pressure exists which allows an increase in feedrate particularly at low machining voltages. A procedure for the selection of operating conditions is proposed.
|
|---|
