Electric heating – Metal heating – Cutting or disintegrating
Patent
1993-09-07
1995-12-12
Evans, Geoffrey S.
Electric heating
Metal heating
Cutting or disintegrating
219 6913, 219 6918, B23H 102, B23H 704
Patent
active
054751950
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to the machining of metallic workpieces by electrical discharge machining, in particular with a wire cut machine, and concerns a device to protect the workpiece against secondary corrosion and parasitic phenomena due to the electrical-discharge machining process itself.
During the electrical-discharge machining process, the machining area between the workpiece-electrode and the tool-electrode, in which the erosive sparks appear, is submerged or sprayed with a dielectric liquid which is most frequently water.
Water (or a liquid of comparable electrical conductivity), even when deionized, is always somewhat conductive (on the order of several microsiemens/cm to several dozens of microsiemens/cm; one siemen=1/ohm). Thus, when the machining liquid is water (or a comparable liquid) and the tool-electrode has negative polarity (cathode) and the workpiece is positive (anode), an eddy or parasitic current, called a "leakage (or vanishing or drain) current", can be observed which circulates from the workpiece to the tool-electrode and is caused by the potential difference between these two electrodes. This phenomenon is especially noticeable during the so-called off time, the period prior to the spark discharge, when the voltage between the electrodes can be between 80 and 200 volts, depending on the machining settings and the generator employed. This creates a displacement of material and, therefore, corrosion of the workpiece, a phenomenon referred to as anodic dissolution. Moreover, this change in the workpiece geometry is accompanied by a change in its surface quality and a weakening of its mechanical properties.
This phenomenon is illustrated in FIG. 1, where the tool-electrode is a wire-electrode 2 connected to the negative pole of an electric pulses generator 3. The workpiece-electrode 1 is connected to the positive pole of the generator 3 and functions therefore as an anode. FIGS. 2 and 3 illustrate general principles of electrical discharge machining. Here the variation over time of the potential difference V set up in the gap between the electrodes, and the electric current I, circulating through this gap are represented. With every pulse emitted by the generator 3, the voltage V rises to Va, forming a plateau during the waiting delay time, and decreases thereafter to an average value Ve, which fluctuates slightly. This drop corresponds to the discharge of a spark 4 between the tool-electrode and the workpiece. The voltage V then decreases to a nil value, Vr, during an off time.
The current Ie corresponds to the discharge phase, i.e. to the passage of the spark 4 between the tool-electrode and the workpiece, and current Ir to the off time. Owing to the low conductivity of the dielectric fluid steeping the space between the electrodes, an eddy current Ia, called "leakage current", circulates between the electrodes. It is this leakage current Ia which is liable for the anodic dissolution. At the area near the surface of the workpiece which are the most affected by the eddy current Ia, metal atoms are ionized, resulting in the release of positive ions and of free electrons, as represented by the following formula:
These positive ions, along with the inversely freed electrons, maintain the eddy current Ia in the water or comparable dielectric fluid circulating in the machining area. Because machining time often requires several hours, or even days, and workpieces are most often steel, this corrosion quickly becomes unacceptable, for it destroys the accuracy of previously machined areas before they are even attached to the electrical discharge machine (reference surfaces, ground faces, etc.). In addition to this change in the geometry of the machined workpiece, its surface is altered and its mechanical strength weakened. The surface finish of the corroded workpiece is often incompatible with the desired finish quality. This parasitic (leakage) current dissolves not only the workpiece but all unprotected and electrically conductive machine parts subject to the action of the dielect
REFERENCES:
patent: 4678884 (1987-07-01), Obara et al.
patent: 4975557 (1990-12-01), Yamada et al.
patent: 5171956 (1992-12-01), Haruki et al.
Charmilles Technologies SA
Evans Geoffrey S.
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