Wire electrode for electro-discharge machining

Details Wire electrode for electro-discharge machining Wire electrode for electro-discharge machining

2000-08-15

2001-09-18

Elve, M. Alexandra (Department: 1725)

Electric heating

Metal heating

Cutting or disintegrating

C219S069110, C219S069150

Reexamination Certificate

active

06291790

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an improvement of a wire electrode used for electro-discharge machining.
2. Description of the Background Art
Wire electro-discharge machining is a method of melting and cutting a workpiece into a desired configuration by causing intermittent discharge between a workpiece and a linear working electrode referred to as a wire electrode for electro-discharge machining via a working liquid such as water or oil and by moving the workpiece relative to the wire electrode for electro-discharge machining. This method is utilized for manufacturing various types of metal molds and the like. Such wire electro-discharge machining requires processing characteristics such as excellent finish of a workpiece, high process accuracy, good finished surface condition of the workpiece, no wire electrode material adhesion to the workpiece, and a short electro-discharge machining time. A brass wire, having good drawability and high strength required for a wire electrode, is generally used as a wire electrode for such wire electro-discharge machining.
In recent years, as improvements and advancements are made on working power sources, a need exists for a wire electrode that can improve processing speed and accuracy. In particular, a wire electrode that allows improved processing speed and accuracy is desired for applications employing a power source of a wire electro-discharge machining apparatus that repeatedly applies a high and short-time pulse voltage.
Conventional wire electrodes for electro-discharge machining prepared from a brass wire, however, are incapable of attaining a sufficiently high processing speed. Further, brass wires are problematic in that large amount of electrode material adheres to the workpiece, that the cut surface of the workpiece is roughened, and that the wire electrode is easily broken when electro-discharge machining is attempted at a high processing speed.
Moreover, some applications use a wire electrode for wire electro-discharge machining prepared by covering a core formed of Cu or a Cu alloy with Zn, or that, after covering with Zn, heat treated to produce a Cu—Zn alloy at a surface layer by diffusion and thus having an oxide film on the outermost surface. The former Zn-coated Cu or Cu alloy wire electrode results in improved cut surface of the workpiece; however, it cannot provide a sufficiently high processing speed. On the other hand, the latter diffusion alloy-coated wire electrode improves the processing speed to some extent, but the property of the cut surface of the workpiece is not sufficiently improved. It is also difficult to position the latter wire electrode prior to the start of electro-discharge machining. As compared with the brass wire, although the conventional wire electrode for wire electro-discharge machining having a core and a coating layer formed on an outer periphery of the core affords improved electro-discharge machining characteristics, the wire electrode itself, a wire contact (a roller or die for feeding electricity), a guide die and the like suffer wear and undergo considerable damage so that their lifetimes are shortened, which significantly increases the cost of electro-discharge machining.
SUMMARY OF THE INVENTION
Hence, an object of the present invention is to provide at a low cost a wire electrode for wire electro-discharge machining that reduces the electro-discharge machining time, suppresses electrode material adhesion to the workpiece, and provides a smoothly cut workpiece surface. Another object of the present invention is to provide a wire electrode for wire electro-discharge machining that can be easily positioned, does not reduce the lifetime of a wire contact and a guide die, and thus reduces the overall cost of electro-discharge machining.
According to the present invention, the wire electrode for wire electro-discharge machining includes a core and a plurality of coating layers covering the outer peripheral surface of the core, wherein the core and the plurality of coating layers are formed of different materials, and each of coating layers is made of a metal or a metal alloy including a metal selected from the group consisting of Cu, Sn, Ag, Al, Zn, Cs, Se, Te, Mg, Bi, Ti, P, In, Cr, and Fe. The wire electrode for wire electro-discharge machining including a plurality of coating layers made of such specific materials has excellent electro-discharge machining characteristics (in other words, has improved electro-discharge machining speed, reduced material adhesion to the cut surface of the workpiece, and thus a smoothly cut workpiece surface), and particularly, shows such excellent characteristics in electro-discharge machining where a high and short-time pulse voltage is repeatedly applied.
In addition, a first coating layer directly in contact with the outer peripheral surface of the core is preferably made of a Cu—Zn alloy. Moreover, the outermost layer of the plurality of coating layers is preferably made of Zn. Further, from the viewpoint of conductivity and workability of the wire electrode, it is preferred that at least the surface layer of the core is made of copper or a copper alloy.
According to the present invention, in addition to the plurality of coating layers made of different metals or alloys in the above-described wire electrode, at least one coating layer other than the outermost layer may additionally include a discontinuous coating layer including a compound of a nitride, an oxide, or a carbide. Inclusion of such additional discontinuous compound coating layer facilitates suppression of a localized concentrated discharge and allows a fine and highly frequent discharge to be maintained with ease. Moreover, since the outermost layer of the plurality of coating layers is formed of a metal or an alloy, the compound coating layer generally having a high degree of hardness does not aggravate wear and damage of the wire contact, the guide die and the like to shorten their lifetimes, and the positioning of the wire electrode prior to the start of electro-discharge machining can be facilitated. Furthermore, when the outermost layer of the plurality of coating layers is formed of a metal or an alloy, the outermost layer, at the time of discharge, can easily be vaporized, acting to inhibit the rise in the temperature of the wire electrode so that a high-speed electro-discharge machining becomes possible without causing the wire electrode to break even under severe electro-discharge machining conditions.
Preferably, the wire electrode is subjected to plastic working with area reduction rate (reduction rate in the cross sectional area) in the range of about 40% to about 99% after the plurality of coating layers are formed or after the final heat treatment after the formation of the plurality of coating layers. Here, the possible techniques of plastic working include roll working, wire drawing, or the like. Plastic working with the area reduction rate of less than 40% does not afford sufficient improvement in the tensile strength of the wire electrode, while plastic working with the area reduction rate exceeding 99% is not preferable in that the high-temperature strength of the wire electrode is lowered, resulting in frequent breaking of the wire electrode during electro-discharge machining. The area reduction rate in the plastic working is expressed by the following equation:
Area reduction rate (%)
=
d
1
2
-
d
2
2
d
1
2
×
100
where
d
1
is the wire diameter before processing; and
d
2
is the wire diameter after processing.
The wire electrode preferably has a tensile strength of 70 kg/mm
2
or greater and has a conductivity satisfying the relation of (tensile strength) ×(conductivity)
3
/10
5
≧1.2, since a wire electrode having the tensile strength of less than 70 kg/mm
2
frequently breaks during electro-discharge machining, while the sufficient electro-discharge machining speed cannot be attained with a wire electrode having a low conductivity.
The wire electrode preferably has a wire diameter and an average

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