Machining zone

Peripheral pitting and etching associated with the low current densities arising outside the main machining zone occur when higher current densities of 45-75 A/cm are appHed. This is a recurrent difficulty when high alloy, particularly those containing about 6% molybdenum, titanium alloys are electrochemicaHy machined. 

Process variables also play a significant part in determination of surface finish. For example, the higher the current density, generally the smoother the finish on the workpiece surface. Tests using nickel machined in HCl solution show that the surface finish improves from an etched to a poHshed appearance when the current density is increased from ca 8 to 19 A/cm and the flow velocity is held constant. A similar effect is achieved when the electrolyte velocity is increased. Bright smooth finishes are obtained over the main machining zone using both NaCl and NaNO electrolyte solutions and current densities of 45-75 A/cm. ... 

In the local, one-dimensional approximation [9], the determination of the shape of one electrode is reduced to the determination of the width of the I EG in the machining zone. For instance, if the TE surface is given by the vector function... 

The majority of the known methods of solving the direct and inverse problems with moving boundaries in ECM were elaborated within the framework of the so-called model of ideal processes, ignoring the variation of the electrolyte properties in the machining zone owing to heat and gas generation and also the peculiarities of mass transfer in the diffusion boundary layer ([9] and references cited therein, [34-42], etc.). In this case, the distribution of current density over the WP surface is determined solely by the distribution of electric potential over the machining zone. 

For solving Eq. (15), appropriate boundary conditions must be prescribed. Normally, the boundary of the machining zone consists of several sections at which the boundary conditions of different types are prescribed. The type of the boundary conditions depends on the character of the boundary section TE, WP, insulator, or the line (the plane of symmetry), and also on the operating conditions of the power supply the conditions of stabilization of the applied voltage, the conditions of current stabilization, and the natural current-voltage characteristics. In the general case, the boundary conditions that account for the kinetics of the electrode reactions and the transfer processes in the near-electrode diffusion layers can be written as follows ... 

The machining zone is formed. The boundary of this zone consists of the TE (3), the WP (4), the insulator surfaces, and the planes of symmetry (5). The surface of this zone is meshed by triangle or quadrilateral boundary elements (Fig. 10b). In each element, the potential and its gradient are taken to be constant. 

For the given scheme of partition of the machining zone boundary on the elements, the discretization of the boundary integral equation and the boundary conditions is performed. The set of nonlinear equations, which is obtained by discretization, is solved by Newton s method. As a result, the distribution of the current density over the WP surface is obtained (Fig. 10c). 

First, instead of determining the shape of one electrode, it is sufficient to find the distribution of the IEG over the machining zone. This appears to be much simpler than determining, immediately, the electrode s shape, because the IEG depends only on three parameters (u,v,t), whereas the electrode surface is determined by four parameters (x,y, z,t). 

For the chosen machining conditions, the distribution of gap width over the machining zone is calculated using Eq. (24). 

In ECM sinking, the accuracy and surface finish depend considerably on the proper choice of TE design, a scheme of electrolyte feed through the slots and holes into the machining zone, the scheme, and parameters of machining. ECM sinking is... 

From Fig. 5.8d one can observe that the temperature of the workpiece at the bottom of the tool-electrode is lower than that at the edges, near the gas film (the difference is more than 100°C). Figure 5.8a-c gives a clear picture of the machining zone. The depth of the machining zone is higher near the edge of the tool-electrode than under the electrode. 

Optimal surface quality is achieved by maximising the chemical etching and at the same time minimising the local heating in order to avoid the formation of heat affected zones. This implies in particular an optimal supply of electrolyte to the machining zone. 

Increasing the electrolyte supply to the machining zone can be achieved by using appropriate tool-electrode shapes or by promoting the flow of the electrolyte by tool-electrode motions. These strategies are discussed below. Note that until now no attempt has been made to study the potential benefit of using hollow electrodes, which would allow injecting the electrolyte from inside the tool. 

The heat released by the electrochemical discharges is only partially transferred to the workpiece. Part of it is evacuated through the electrolyte and the tool-electrode. The greater the heat removed from the machining zone, the better is the machining in terms of quality. As expected, the major drawback is the decreased material removal rate. In general, a trade-off has to be reached between quality and fabrication time. 

The heat conductivity of the tool-electrode affects the machining in two ways. First, it controls the heat removed from the machining zone. The heat conductivity of the tool-electrodes is generally significantly higher than that of the electrolyte and the workpiece. It follows that a major part of the heat supplied by the electrochemical discharges is removed through it. 

Depending on the operating conditions and metal-electrolyte combinations, different anodic reactions take place when sufficient pulse power is applied. Rate of anodic reactions is influenced by the supply of fresh electrolyte, which enables the removal of reaction products as soon as they generates into the machining zone. The electrolyte flow velocity is negligible in case of EMM. So there is not sufficient transfer of mass from one electrode to the other. This gives rise to the formation of diffusion layer at the electrode-electrolyte interface at anode. Machining performance e.g. MRR, accuracy and surface finish of the workpiece is affected by the factors as discussed already. 

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