Before machiningafter

Fig. 5 Schematic of deburring and radiusing process

ECD systems have current ratings from 100 to 2000 A at a dc voltage of 7 to 25 V. Typical outputs are 100 to 1000 A and 0 to 30 V. The high power requirements result in heating of the equipment and thus it requires cooling. The units are normally air- or water-cooled. Depending on the thickness of the material and the current density, there are various methods of supplying the electric current. With thick materials, the current can be supplied directly to the workpiece by means of sliding contacts or pressure contacts. With thin material, the current can be conducted through a conductor beneath it, such as platinum. If higher current densities are required, a system can be used in which the workpiece floats with respect to the electrodes and the current is supplied through the electrolyte.

In an ECD system, the electrolyte pressure ranges from 0.15 to 0.5 MPa (15 to 70 psi), at a flow rate of 3 to 15 L/min (0.8 to 4 gal/min) for each 100 A. The electrolyte used is usually a neutral pH salt solution, such as sodium nitrate or sodium chloride.

The gap between the tool and the burr ranges from 0.2 to 1.2 mm (0.008 to 0.05 in.). ECD systems can handle workpieces manually as well as on completely automated transfer lines. ECD tools are usually made from brass, copper, or stainless steel and are insulated on all surfaces except the surface adjacent to the burr. The insulations range from acrylic and polyvinyl chloride to Delrin and epoxy coatings.

Applications and Limitations. Examples of tooling for ECD are shown in Fig. 6 and 7. ECD is used in industries ranging from consumer appliances and automobiles to biomedical and aerospace products. ECD is used to deburr a variety of parts including gears, gear plates, and fuel injector nozzles.

Fig. 6 Schematic of electrochemical machining: smoothing, deburring, and radiusing of piston pin. Machining parameters: U (in Fig. 2) = 17 V; electrolyte pressure, 0.3 MPa; electrolyte, 15% NaCl; time of machining, 75 s; maximum current per piece, 180 A
Fig. 7 Tooling for electrochemical machining deburring. (a) Valve casing. (b) A fragmentary schematic of the production jig. Machining parameters: 15% water solution of NaNO3; U (in Fig. 2) = 15 V; machining time, 8 s; electrolyte pressure, 1 MPa; maximum current per piece, 20 A

Limitations. One of the major limitations of the ECM and ECD processes is their inability to machine electrically nonconductive materials. Usually, sharp corners or clear cuts cannot be obtained by ECM. The complexity of the shape to be machined, the workpiece material, and the electrolyte put a limit on the dimensional accuracy and the surface finish that can be achieved by ECM. Etching of the constituents, grain-boundary attack, and pitting due to electrochemical action may have drastic effects on the mechanical properties of the material, particularly the fatigue strength.

Recent Advances in ECM as a Surface Finishing Process. As mentioned earlier, different anodic reactions take place at high current densities during ECM, depending on the metal-electrolyte combination and operating conditions. Electropolishing is another finishing process (discussed in the next section) that involves anodic reactions. The rate of these reactions depends to a great extent on the ability of the system to remove the reaction products as soon as they are formed. All of these factors influence the machining performance (i.e., the dissolution rate, shape control, and the surface finish of the workpiece). An understanding of the kinetics and stoichiometry of anodic reactions and their dependence on mass transport conditions is therefore essential in order to optimize relevant ECM and electropolishing parameters. The high rate of anodic dissolution of metals that is applicable to ECM and electropolishing has been reviewed recently (Ref

6). Electrochemical and hydrodynamic parameters influence the nature of the anodic reactions and their rates. These in turn influence the performance of metal shaping and finishing operations. Surface finish during high-rate anodic dissolution depends on the mass transport conditions at the anode. At potentials below the limiting current range, rough surfaces are obtained as a result of crystallographic etching and grain boundary attack. The limiting current, which appears at higher potentials, is mass transport controlled. This corresponds to the formation of a salt layer at the anode that suppresses the influence of the metallurgical phenomena on the dissolution process (Ref 6). Dissolution at or above the limiting current therefore yields smooth surfaces (0.12 to 0.2 ^m Ra). Pulsed dissolution is considered most suitable for electrochemical micromachining of thin films and foils when low dissolution rates are desirable for better control over the machining process (Ref 7).

In another recent development, "maskless" and "throughmask" electrochemical micromachining (EMM) techniques have been developed for the processing of thin films and foils of materials that are difficult to machine by other methods (Ref

7). In these processes at higher potentials, a layer of salt forms at the anode. The presence of this salt film also influences the current dissolution, hence the uniformity of metal dissolution and the shape profiles during throughmask EMM.

A new finishing method has been developed using an electrochemical finishing machine (Ref 8). This method enables the operator to set the timing of a switchover to pulses having a high current density, to provide an optimum working condition and thereby improve working efficiency. This method permits the removal of an oxide layer generated on a three-dimensional surface of an object, so that a highly accurate surface can be obtained in a short period of time.

Recently, ECM with an ac source was used for machining of some special alloys such as tungsten carbide. The pulse train used for the ac was asymmetric. This type of ECM is also known as alternate polarity ECM. Alternate polarity ECM has been found to be good for obtaining a uniform dissolution of tungsten carbide and for suppressing the dissolution of the tool electrode (Ref 9). A smooth surface (3.5 to 8 ^m Rmax) without any heat-affected layer or cracks was obtained by this method, where Rmax is the maximum peak-to-valley roughness height.

The quality of surfaces produced by ECM has been investigated recently, based on how changing process variables affects the resulting plasticity index, which is an indication of surface capacity to bear plastic deformation during service of electrochemically machined surfaces (Ref 10).

Another variation of the ECM process is the use of pulsed current during ECM. This is known as pulse electrochemical machining (PECM). The electrochemical principles of PECM are identical to those of ECM with continuous current. In the PECM process, discrete machining pulses are applied, and the system is allowed to relax between these pulses. It is this alternating pulse on-time and off-time that fundamentally distinguishes PECM from continuous current ECM. Recent studies have shown that PECM results in higher anodic dissolution localization, smaller gap sizes, higher peak current densities and better surface finish (0.16 to 0.63 ^m Ra) (Ref 11). Experimental results with pulse EMM indicate that good surface finish can be obtained even at low average current densities. In PECM, shorter pulse on-times (0.2 to 4 ms) are preferable for achieving better surface finishes.

References cited in this section

4. K.P. Rajurkar, C.W. Walton, and T.A. Kottwitz, "State of the Art Assessment of the Electrochemical Machining Process," Final Report, National Center for Manufacturing Sciences, 1989

5. K.P. Rajurkar and C.L. Schnacker, Some Aspects of ECM Performance and Control, Ann. CIRP, Vol 37/1, 1988,p 183-186

6. M. Datta, Anodic Dissolution of Metals at High Rates, J. Res. Dev., Vol 37 (No. 2), 1993, p 207-226

7. M. Datta, Electrochemical Microfabrication, M. Datta, K. Sheppard, and D. Snyder, Ed., The Electrochemical Society, Pennington, NJ, 1992, p 61

8. Y. Kuwabara, T. Asaoka, S. Yoshioka, and H. Sugiyama, "Finishing Method Employing Electro-Chemical Machining, and an Electro-Chemical Finishing Machine," U.S. patent 4,956,060, 1990

9. T. Masuzawa and M. Kimura, Electrochemical Surface Finishing of Tungsten-Carbide Alloy, CIRP Ann., Vol 40 (No. 1), 1991, p 199-202

10. M. Abdel-Rahman and H.M. Osman, Surface Quality of Electrochemically Machined Surfaces, Proc. 10th Int. Symp. Electromachining, Magdeburg Technical University, Germany, 1992, p 543-552

11. K.P. Rajurkar, J. Kozak, and B. Wei, Study of Pulse Electrochemical Machining Characteristics, Ann. CIRP, Vol 42 (No. 1), 1993, p 231-234

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