İhsan Gökhan Serin - 503111309 - 6th Week's Unanswered Term

Electroerosion Dissolution Machining (Nontraditional Machining Process)

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Novel methods of machining hard metals, which are difficult to cut by conventional methods, continue to attract attention. Electrochemical machining and electro discharge machining have proven to be very useful. However, drawbacks such as the expense of tooling for machin­ing large cavities, the high cost of machining systems, low rates of metal removal, and the presence of a recast layer, which often has to be removed in EDM, have hindered wider acceptance of these techniques.

EEDM (also called ECDM or ECAM) is a new development, which combines features of both ECD and EDE. It utilizes electrical dis­charges in electrolytes for material removal. Such a combination allows high metal removal rates to be achieved. EEDM has found a wide range of applications in the field of wire cutting, hole drilling, and fin­ishing of dies and molds. Further applications regarding machining of composites using NaOH electrolyte have been reported.




The EEDM process is a further development of pulsed electrochemi­cal machining (PECM) where, according to Saushkin, et al. (1982), at high input power, phenomena that limit further dissolution may arise. Under such circumstances, the machining medium changes to a gas­vapor mixture that interferes with the ion transfer in the electric field.

  If the field strength is high enough to cause gap breakdown, the nature of charge transfer is altered causing the combined action of EEDM. Such a machining process is associated with a glow appearing in the intereleetrode gap. The glow and the subsequent breakdown of the inter­electrode gaps were localized at points where the gas content and tem­perature of the working medium are likely to be highest.

  The machining system for EEDM wire cutting is shown in Fig. 7.2.  It adopts pulsed voltage and liquid electrolytes as the machining medium that ensures the occurrence of ECD along with the discharge phase. A further arrangement used in hole drilling is shown in Fig. 7.3.  A full wave rectified voltage is applied during the vibration of the workpiece or the tool at 100 Hz and an amplitude and phase shift with respect to the voltage as shown in Fig. 7.4. The tool feed, vibration amplitude, and phase angle determine the instantaneous machining gap width and hence the intensity and duration of each phase.

  EEDM is affected by many variables that control its performance, accuracy, and surface quality. Among these are the electrical parame­ters such as pulse time, relaxation interval, and pulse current. Workpiece characteristics such as melting point and specific heat have been dealt with by El-Hofy (1992, 1996a). During EEDM, the machin­ing medium is subjected to varying contamination conditions. This is caused by many interfering phenomena such as gas generation and varying dissolution intensity. The presence of different types of pulses that are responsible for the erosion phase in the form of metal resolid­ified particles, and the change of electrolyte flow rate, are possible causes of gap contamination. The breakdown characteristics change with time, and hence the initially required dimensional accuracy cannot be reached anymore. The gas-liquid wedge, formed during electrolysis, is the determining factor in this particular hybrid machining process. In this regard, it has been found that the superposition of low-voltage pulse components (Fig. 7.5) stabilizes the machining process and makes it possible to reduce the breakdown voltage, enhance the machining productivity, and reduce the surface roughness. 

   Figure 7.6 presents the main machining phases and process compo­nents of EEDM. According to Fig. 7.7, spark discharges occur at random locations across the machining gap while electrolysis is believed to be localized in the proximity of the pits of the formed craters which are soon made smooth, probably as a result of the high temperature of the metal and electrolyte. The EEDM material removal rate is enhanced by the sparking action and not by the arcing one because the latter usually results in a low and localized material removal rate and yields more irregular machined surfaces. 

(Hassan Abdel-Gawad El-Hofy, Advanced Machining Processes, pg. 204-208)