Electric heating – Metal heating – Cutting or disintegrating
Reexamination Certificate
2003-04-28
2004-12-21
Evans, Geoffrey S. (Department: 1725)
Electric heating
Metal heating
Cutting or disintegrating
Reexamination Certificate
active
06833523
ABSTRACT:
TECHNICAL FIELD
The present invention relates to improvements of a method of and an apparatus for wire electrical discharge machining for machining a workpiece on the basis of electric discharge energy by supplying working electric power across a gap between a wire electrode and the workpiece.
BACKGROUND ART
Electrical discharge machining has established a solid position as a machining technology for such as dies and molds, and has been extensively used in the fields of the automobile industry, the household electrical appliance industry, the semiconductor industry, and the like.
FIG. 7
is an explanatory diagram of the mechanism of electrical discharge machining. In the drawing, reference numeral
1
denotes an electrode;
2
, a workpiece;
3
, an arc column;
4
, a working liquid; and
5
, machining debris produced in electrical discharge machining. Removal machining based on the electric discharge in the workpiece
2
progresses while repeating the cycle (corresponding to steps (a) to (e) in
FIG. 7
) of the following steps (a) to (e): Namely, these steps are (a) the formation of the arc column
3
due to the generation of electrical discharge, (b) the local fusion and the vaporization of the working liquid
4
due to the thermal energy of electric discharge, (c) the generation of an explosive force of vaporization of the working liquid
4
, (d) the scattering of a fused portion (machining debris
5
), and (e) cooling, solidification, and recovery of insulation between the electrodes due to the working liquid.
This invention concerns wire electrical discharge machining which is used in boring, cutting, and the like in electrical discharge machining. In particular, there has been a growing demand for higher precision in the wire electrical discharge machining, and high machining accuracy on the order of 1 to 2 &mgr;m or thereabouts has come to be required in the machining of high-precision dies and molds used in the semiconductor industry and the like.
FIG. 8
is an explanatory diagram illustrating an example of the machining process of wire electrical discharge machining. In the drawing, reference numeral
1
a
denotes a wire electrode;
2
, the workpiece;
4
a
, a working liquid which is, for example, water; and
6
, an initial hole. The part (a) of
FIG. 8
shows the state of a first cut which is rough machining, the part (b) of
FIG. 8
shows the state of a second cut which is semi-finish machining after rough finishing, and the part (c) of
FIG. 8
shows the state of a third cut which is final finish machining.
The example of the machining of the first cut in the part (a) of
FIG. 8
shows machining in which the wire electrode
1
a
is passed through the initial hole
6
, and the workpiece
2
is bored. In the case of such a first cut, since the surface roughness and accuracy are finished in subsequent machining, very strict surface roughness and accuracy are not required so much, and it is important to increase the machining speed, in particular, so as to improve productivity. In wire electrical discharge machining, in order to increase the machining speed, the injection of the working liquid
4
a
to the gap between the wire electrode
1
a
and the workpiece
2
is practiced to efficiently discharge the machining debris from the gap. In addition, in order to eliminate the unevenness of the application of the working liquid
4
a
to the gap and prevent the disconnection of the wire electrode
1
a
, a method is adopted in which the working liquid
4
a
stored in an unillusirated working tank and the workpiece
2
is immersed in it. Thus a working-liquid supplying section is used for supplying the working liquid to the gap between the wire electrode
1
a
and the workpiece
2
.
In the above-described conventional wire electrical discharge machining, the machining after the first cut (the part (a) in FIG.
8
), such as the second cut (the part (b) in
FIG. 8
) and the third cut (the part (c) in FIG.
8
), is also performed in the working liquid
4
a.
When a voltage is applied across the gap between the wire electrode
1
a
and the workpiece
2
, a force acts in which the positive polarity and the negative polarity are attracted toward each other, so that the wire electrode
1
a
having small rigidity is pulled toward the workpiece
2
side by this electrostatic force. This causes the vibration of the wire electrode
1
a
, so that there has been a problem in that high-accuracy machining is made difficult due to such vibration.
In addition, in a state in which the explosive force of vaporization of the working liquid has been generated due to the discharge energy (e.g., the part (c) in FIG.
7
), a large force acts on the wire electrode
1
a
in a direction opposite to that of the workpiece
2
due to the explosive force of vaporization of the working liquid, so that vibrations occur. There has been a problem in that irregularities occur in the shape of the workpiece
2
due to such vibrations, which leads to the deterioration of the accuracy.
Further, in the semiconductor industry and the like, which are the fields of application of wire electrical discharge machining, in the machining of such as a die for IC leadframes, applications are increasing in which extremely high accuracy and very smooth surface roughness are required for a workpiece whose form accuracy is 1 &mgr;m and whose surface roughness is 1 &mgr;m Rmax or less. In such uses, in particular, the above-described problem ascribable to the vibration and the like of the wire electrode has been noticeable.
As a measure for overcoming such problems of wire electrical discharge machining in a liquid, a technique concerning aerial wire electrical discharge machining has been disclosed in which wire electrical discharge machining is performed in the atmosphere without a working liquid interposed in the gap between the wire electrode and the workpiece (Adachi, Tokyo University of Agriculture and Technology, et al.: “Attaining High Precision in Second Cuts by Aerial EDM,”
Die & Mold Technology
, Vol. 14, No. 7, 1999, p. 154, The Nikkan Kogyo Shimbun, Ltd.). In this technique, it is disclosed that accuracy in the straightness of cut surfaces of workpieces can be improved by wire electrical discharge machining in the atmosphere. However, although its significance from the perspective of the attainment of high precision is large as compared with the ordinary wire electrical discharge machining in a working liquid, it is impossible to make use of the cooling effect by means of the working liquid (e.g., the part (e) of
FIG. 7
) in a case where wire electrical discharge machining is performed in such an atmosphere or wire electrical discharge machining is performed in a mist. Therefore, it is impossible to sufficiently cool the gap between the wire electrode and the workpiece, and machining progresses in the state in which thermal strains have occurred in the workpiece due to the heat generated by electric discharge. Hence there has been a problem in that this technique cannot cope with applications in which extremely high accuracy and very smooth surface roughness are required for workpieces such as those mentioned above.
DISCLOSURE OF THE INVENTION
This invention has been devised to overcome the above-described problems, and its object is to provide a method of and an apparatus for wire electrical discharge machining which are suitable for attainment of high accuracy and high quality in wire electrical discharge machining.
The method of wire electrical discharge machining in accordance with this invention is a method of wire electrical discharge machining for machining a workpiece by generating electric discharge in a gap between a wire electrode and the workpiece, wherein machining is performed by combining at least two kinds of machining among three kinds of machining including machining in a working liquid, machining in a mist, and machining in a gas, and in a process of machining in the mist or the gas, machining is performed while cooling the workpiece.
The wire electrical discharge machining apparatus in accordanc
Goto Akihiro
Kimura Munenori
Evans Geoffrey S.
Mitsubishi Denki & Kabushiki Kaisha
Sughrue & Mion, PLLC
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