Electrical discharge machine

An electrode for electrical discharge machining. This spark-machining electrode improves the accuracy at which the workpiece is machined by electrical discharge machining. The electrode can dispense with a mechanism which scans the spark-machining electrode or the workpiece. A plurality of needlelike electrodes are formed on the surface of the spark-machining electrode. The needlelike electrodes are so arranged that they are present in craters created by their respective adjacent needlelike electrodes. The plural electrodes form a group. The shape of the surface of this group is formed according to the desired shape to be formed in the workpiece. Art electric discharge occurs mainly at the tips of the needlelike electrodes and so the capacitance is smaller than the capacitance of the prior art flat-plate electrode. Also, the energy of a single electric discharge can be reduced. Furthermore, electric discharge at the side surfaces of the needlelike electrodes can be suppressed because of the effect of concentration of electric field.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to the structure of an electrode used for 
electrical discharge machining which is employed for an electrical 
discharge machine that machines a workpiece by the action of electric 
discharge and, more particularly, to an electrode capable of machining 
parts having intricate shapes by electrical discharge machining. 
2. Description of the Related Arts 
Conventional methods of intricately spark-machining workpieces include a 
method utilizing photolithography adapted to machine semiconductors, a 
method using an LIGA process, and a method of machining a workpiece into a 
desired shape by electrical discharge machining. In the method using the 
LIGA process, a photosensitive material is molded, using synchrotron 
radiation. Then, the material is plated with a metal. Thus, the material 
is machined into a desired shape. 
The electrical discharge machining which is also known as electron 
discharge machining, electric spark machining, electroerosive machining, 
and electrospark machining is described now. The following three kinds of 
electrodes can be employed for electrical discharge machining. 
A first kind of electrode is an electrode 20 shown in FIG. 11. This 
electrode 20 takes the form of a flat plate and is provided with a 
plurality of holes 20a. A workpiece 21 is mounted opposite to the 
electrode 20. A power supply 22 for electrical discharge machining applies 
a voltage, which produces an electric discharge from the flat-plate 
electrode 20 to thereby machine the workpiece 21. 
A second kind of electrode is a single needlelike electrode 23 shown in 
FIG. 13. A voltage is applied between the electrode 23 and a workpiece 21 
to create an electric discharge. Utilizing this discharge, holes are 
formed in the workpiece 21. Since the area of the front end of the 
electrode 23 is much smaller than the area of the flat-plate electrode 20, 
the energy of a single electric discharge is small. Hence, the machining 
accuracy can be enhanced. 
A third kind of electrode is a wire electrode 24 shown in FIG. 14. A 
voltage is applied between the wire electrode 24 and a workpiece 21. At 
the same time, the workpiece 21 is rotated. In this manner, the workpiece 
is machined into a needlelike shape. During the machining, the wire 
electrode 24 is fed in to suppress the wear of the electrode due to the 
electric discharge. Also, the machining accuracy can be improved. 
However, where a workpiece 21 is machined so as to leave minute regions, or 
minute cylindrical portions 21a, as shown in FIG. 12 by the prior art 
machining techniques, various difficulties arise. 
In the method using photolithographical techniques principally relying on 
etching used to machine semiconductors, if the height of the cylindrical 
portions 21a shown in FIG. 12 reaches tens of microns, lateral etching 
also occurs, thus deteriorating the dimensional accuracy. 
In the LIGA process, the cost is very high because of the use of 
synchrotron radiation. 
Furthermore, in the prior art method making use of electrical discharge 
machining, the machining accuracy and the machining time present problems. 
The problems with the above-described electrode structures are described 
next. 
Where the flat-plate electrode 20 shown in FIG. 11 is used, the area of the 
electrode is large and so the energy produced by a single electric 
discharge is large. Therefore, the outer surfaces of the cylindrical 
portions 21a formed by the holes 20a tend to be uneven. Consequently, it 
cannot be anticipated that high machining accuracy is obtained. 
Where the single needlelike electrode 23 shown in FIG. 13 is used, it may 
be possible to enhance the accuracy at which the cylindrical portions 21a 
shown in FIG. 12 are machined, by scanning the electrode 23 or the 
workpiece 21. However, the machining time is increased. Furthermore, a 
mechanism for scanning the electrode 23 or the workpiece 21 is needed. 
Also, measures must be taken against the wear of the electrode 23. 
Where the wire electrode 24 shown in FIG. 14 is employed, the wear of the 
electrode can be suppressed but the machining time is prolonged. Also, a 
mechanism for scanning the electrode or the workpiece is needed. In 
addition, the machining accuracy is lower than the machining accuracy 
obtained where the needlelike electrode 23 is used, because the 
electric-discharge area at the front end of the wire electrode is wider 
than the electric-discharge area of the needlelike electrode 23. 
SUMMARY OF THE INVENTION 
It is a first object of the present invention to provide an electrode 
(hereinafter often referred to as the spark-machining electrode) which is 
used for electrical discharge machining and which is comparable in 
machining accuracy to the prior art single needlelike electrode and does 
not need any complex mechanism for scanning the electrode or the 
workpiece. 
It is a second object of the invention to provide a spark-machining 
electrode which accomplishes the above-described first object and is 
capable of accurately forming convex portions of desired form on a 
workpiece. 
It is a third object of the invention to provide a spark-machining 
electrode which accomplishes the above-described first object and is 
capable of accurately forming recesses of desired shape in a workpiece. 
It is a, fourth object of the invention to provide a spark-machining 
electrode which accomplishes the above-described first object and is 
capable of accurately forming desired shapes having varying heights on a 
workpiece. 
The first object is achieved by a first structure in which groups of 
needlelike electrodes are formed on the surface of the spark-machining 
electrode and the needlelike electrodes are arranged in craters created by 
their respective adjacent needlelike electrodes. 
The second object is achieved by a structure which has the features of the 
first structure and in which the base plate of the spark-machining 
electrode and the groups of the needlelike electrodes have holes larger 
than the craters. 
The third object is achieved by a structure which has the features of the 
first structure and in which the groups of the needlelike electrodes are 
spaced apart from each other such that the craters created by the adjacent 
groups of the needlelike electrodes are spaced apart from each other. 
The fourth object is achieved by a structure which has the features of the 
first structure and in which the groups of the needlelike electrodes have 
varying heights. 
Where the novel electrode is used for electrical discharge machining, an 
electric discharge starts at the tips of the needlelike electrodes which 
could produce electric discharge most readily immediately prior to the 
machining. As the electric discharge progresses, the distance between each 
needlelike electrode and the workpiece increases. Then, the needlelike 
electrodes which could produce electric discharge less easily create an 
electric discharge. In this way, successive electric discharges occur, 
whereby the workpiece is machined. In the machined workpiece, the craters 
formed in the adjacent needlelike electrodes overlap with each other and 
hence do not reflect the shape of the needlelike electrodes but reflect 
the shapes of the groups of the needlelike electrodes. 
Also, the area of the tip of each needlelike electrode is small. Therefore, 
the energy of the electric discharge which is proportional to the 
capacitance between the tip of each needlelike electrode and the workpiece 
is also small. In consequence, the accuracy at which the workpiece is 
machined is enhanced. Furthermore, the total area of the surfaces of the 
needlelike electrodes is larger than the area of the surface of a 
flat-plate electrode. During electric discharge, a dielectric oil is 
circulated through the gaps between the needlelike electrodes and so heat 
is dissipated effectively. This also improves the machining accuracy. 
Successive electric discharges take place on each individual needlelike 
electrode and trigger electric discharges on all the groups of needlelike 
electrodes. Hence, a mechanism which scans the spark-machining electrode 
or the workpiece is made unnecessary. 
The second structure has the features described above. In addition, convex 
portions conforming in shape to the holes formed inside the groups of the 
needlelike electrodes can be accurately machined in the workpiece 
simultaneously. 
The third structure has the features described above. In addition, concave 
portions conforming in shape to the separate shapes of the groups of the 
needlelike electrodes can be accurately machined in the workpiece 
simultaneously. 
The fourth structure has the features described above. In addition, shapes 
conforming to the shapes of the groups of the needlelike electrodes having 
varying heights can be machined accurately simultaneously. 
Other objects and features of the invention will appear in the course of 
the description thereof which follows.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Electrical discharge machining using a spark-machining electrode according 
to the invention is next described by referring to FIGS. 1-6. As shown in 
FIG. 3, a pole 2 extends upright from a bed 1. A support arm 3 is held 
horizontally to the upper end of the pole 2. A support plate 4 is rigidly 
mounted to the lower surface of the arm 3 and depends from it. A 
spark-machining electrode 5 forming a first embodiment of the invention is 
removably mounted to the lower end of the support plate 4. 
A support base 6 is mounted on the upper surface of the bed 1. A machining 
vessel 8 is mounted on the support base 6 in such a way that the vessel 
can be moved up and down by an elevating motor 7. A horizontal support 
plate 9 and a dielectric oil O are contained within the vessel 8. An 
electric discharge circuit 10 having a power supply for electrical 
discharge machining is connected between the support plates 9 and 4. A 
workpiece 11 is placed on the upper horizontal support plate 9. A given 
voltage is applied between the support plates 4 and 9 from the electric 
discharge circuit 10. The workpiece 11 is moved upwardly together with the 
machining vessel 8 toward the spark-machining electrode 5, whereby the 
workpiece 11 is machined by electrical discharge machining. 
FIG. 4 is a cross-sectional view of the above-described spark-machining 
electrode 5. FIG. 5 shows the structure of the surface of the 
spark-machining electrode 5. The electrode 5 has a base plate 5i a on 
which a number of needlelike electrodes 12 are arranged closely at a given 
density so as to form needlelike electrode groups 13. In this embodiment, 
a desired shaped is formed on one electrode group 13 to machine the 
workpiece 11 into the desired form by electrical discharge machining. 
The spark-machining electrode 5 shown in FIGS. 4 and 5 is manufactured in 
the manner described now. First, the needlelike electrode groups 13 are 
formed on the electric discharge surface of the base plate 5a of the 
spark-machining electrode by photolithography techniques used in 
semiconductor fabrication processes in such a way that each needlelike 
electrode 12 has a depth H of 5 microns and a diameter D of 2 microns and 
that the spacing g between adjacent electrodes 12 is 1 micron, as shown in 
FIG. 1. Then, circular holes 14 having a diameter L of several millimeters 
are formed on the rear surface of the base plate 5a of the electrode by 
electrical discharge machining. 
The method of fabricating the spark-machining electrode 5 is now described 
in detail. The base plate 5a of this electrode 5 is made of tungsten (W). 
A photosensitive resin such as a photoresist is applied to the surface of 
the base plate 5a forming an electric discharge surface. The resin is then 
exposed, using a photomask having desired shapes. The resin is developed. 
Thus, the shapes of the photomask used to fabricate the needlelike 
electrode groups 13 are transferred to the photosensitive resin. 
Thereafter, the base plate 5a of the spark-machining electrode is etched, 
using the photosensitive resin as a masking material. In this way, the 
needlelike electrode groups 13 are formed on the whole electric discharge 
surface of the base plate 5a. Extraction holes 14 of a desired shape is 
machined in the rear surface of the electrode base plate 5a by electrical 
discharge machining. The etching described above and etching described 
later may be either a wet chemical etching process using a liquid or a dry 
etching process using a plasma gas. 
The obtained spark-machining electrode 5 is mounted on the electrical 
discharge machine shown in FIG. 3 and disposed at a given distance from 
the workpiece 11 to be machined. A voltage is applied from the electric 
discharge circuit 10. The machining vessel 8 is elevated by the elevating 
motor 7. In this manner, the machining operation illustrated in FIG. 2 is 
performed. Then, the workpiece 11 is machined into the forms shown in FIG. 
6. As shown in FIG. 2, under this condition, the front ends of the 
needlelike electrodes 12 create concentration of electric field and can 
easily produce an electric discharge. Therefore, an electric discharge is 
produced at the front ends of the electrodes 12. An electrical discharge 
machining process which leaves the cylindrical portions 11a on the 
workpiece 11 is carried out through the holes 14 in one operation without 
using a scanning mechanism. Since the needlelike electrodes 12 exist in 
the craters formed by the electric discharges produced by their respective 
adjacent electrodes 12, the shapes of the electrodes 12 are not 
transferred to the workpiece 11. In other words, the workpiece does not 
enter between the adjacent needlelike electrodes 12. 
The energy (in joules) of a single electric discharge from each needlelike 
electrode 12 is given by 
EQU E=(1/2).times.C.times.V.sup.2 (1) 
where C is the capacitance, and V is the applied voltage. The capacitance C 
is given by 
EQU C=.epsilon..times..epsilon..sub.0 .times.(S/d) (2) 
where .epsilon. is the dielectric constant of the dielectric oil O, 
.epsilon..sub.0 is the dielectric constant of vacuum, S is the area of the 
front end surface of each needlelike electrode 12, and d is the distance 
between the electrode 12 and the workpiece 11. Therefore, the energy E of 
the electric discharge can be reduced by reducing the area of the front 
end surface of each needlelike electrode 12. Also, the accuracy at which 
the workpiece 11 is machined can be enhanced. 
In this first embodiment, the region of the spark-machining electrode 5 in 
which an electric discharge should be produced is formed into the 
needlelike electrode groups 13. This facilitates generation of an electric 
discharge between the front end of each needlelike electrode 12 and the 
workpiece 11. In the regions where no electric discharge should be 
produced, i.e., on the side surfaces of the outermost needlelike 
electrodes 12 of the electrode groups 13, electric discharge with the 
workpiece can be prevented. This also improves the machining accuracy. 
Furthermore, the area of the surface of the spark-machining electrode is 
increased, because the surface of the spark-machining electrode 5 is 
shaped into the needlelike electrode groups 13. Since the dielectric oil O 
enters the gaps g between the adjacent electrodes 12, the heat created 
during the electric discharge can be effectively dissipated. This also 
improves the machining accuracy. Additionally, the gaps g in the electrode 
group 13 permit the sludge created during the electric discharge to be 
expelled more effectively. 
A second embodiment of the invention is next described by referring to 
FIGS. 7 and 8. FIG. 7 is a cross-sectional view of main portions of an 
electrode 5 for electrical discharge machining. The electrode 5 forms the 
second embodiment of the invention and has needlelike electrode groups 13. 
To fabricate this electrode 5, cylindrical portions 5b having a depth H1 
of 100 microns are formed on the surface of the base plate 5a of the 
spark-machining electrode 5 by photolithographical techniques. Then, the 
needlelike electrode groups 13 consisting of the needlelike electrodes 12 
are formed on the base plate 5a and on the front end surfaces of the 
cylindrical portions 5b. In the same way as in the electrode groups 13 of 
the first embodiment, the electrode groups 13 are so formed that the depth 
H of the electrodes 12 is 5 microns, the diameter D of the electrodes 12 
is 2 microns, and the spacing g between the adjacent electrodes 12 is 1 
micron. 
The method of fabricating this spark-machining electrode is now described 
in detail. The base plate 5a of the spark-machining electrode 5 is made of 
tungsten. A photosensitive resin such as a photoresist is applied to the 
surface of the base plate 5a forming an electric discharge surface. The 
resin is then exposed, using a photomask having the desired shapes, i.e., 
the shapes of the cylindrical portions 5b. The resin is developed. Thus, 
the shapes of the photomask is transferred to the photosensitive resin. 
Thereafter, the tungsten electrode plate 5a is etched, using the 
photosensitive resin as a masking material. In this way, the cylindrical 
portions 5b are formed on the plate 5a. Again, a photosensitive resin such 
as a photoresist is applied to the base plate 5a and to the electric 
discharge surfaces of the cylindrical portions 5b. The resin is exposed, 
using a photomask having the shapes for fabricating the needlelike 
electrode groups 13. The resin is then developed to transfer the shapes of 
the photomask to the resin, the photomask being used for formation of the 
needlelike electrode groups 13. Using the photosensitive resin as a 
masking material, the tungsten base plate 5a and the front end surfaces of 
the cylindrical portions 5b are etched to form the desired needlelike 
electrode groups 13. 
As shown in FIG. 8, circular holes 11b can be accurately spark-machined in 
the workpiece 11 by means of the electrical discharge machine shown in 
FIG. 3, using the obtained spark-machining electrode 5. 
A third embodiment of the invention is next described by referring to FIGS. 
9 and 10. FIG. 9 is a cross-sectional view of main portions of an 
electrode 5 used for electrical discharge machining, the electrode 5 
having needlelike electrode groups 13. The surface of the spark-machining 
electrode 5 is entirely covered with the needlelike electrodes 12 forming 
one group 13. The needlelike electrodes 12 of this group 13 have mildly 
varying heights. 
The spark-machining electrode 5 shown in FIG. 9 is fabricated in the manner 
described now. The needlelike electrodes are scanned and spark-machined to 
form intricate three-dimensional shapes on the surface of the 
spark-machining electrode 5. Then, the needlelike electrode group 13 is 
fabricated photolithographically in such a way that the depth H of each 
electrode 12 is 5 micron, the diameter D of each electrode 12 is 2 
microns, and the spacing g between the adjacent electrodes 12 is 1 micron. 
More specifically, mild slopes 5c are formed at plural locations on the 
electric discharge surface of the tungsten base plate 5a through the use 
of the electrical discharge machine shown in FIG. 13. A photosensitive 
resin such as a photoresist is applied to the base plate 5a and to the 
electric discharge surfaces of the slopes 5c. Subsequently, the resin is 
exposed, using a photomask having shapes for fabricating the electrode 
group 13. The resin is developed, whereby the shapes of the photomask are 
transferred to the resin. 
Finally, the tungsten base plate is etched, using the photosensitive resin 
as a masking material. Thus, the desired electrode group 13 is formed on 
the electric discharge surfaces of the slopes 5c and on the tungsten base 
plate 5a. 
The workpiece 11 is spark-machined, using the obtained spark-machining 
electrode 5. As a result, as shown in FIG. 10, the plural mild slopes 11c 
can be formed accurately on the surface of the workpiece 11 at a time. 
In the above embodiments, the diameter D of the needlelike electrodes 12 
are set to 2 microns. The needlelike electrodes 12 can be made thinner by 
adopting a modified method. If these thinner electrodes are used, the 
machining accuracy can be enhanced further. 
As described in detail thus far, by using any of the novel spark-machining 
electrodes, the capacitance can be reduced compared with the case in which 
the prior art flat-plate electrode is used. Consequently, the energy of a 
single electric discharge can be reduced. By the effect of the 
concentration of electric field, the electric discharge occurs principally 
at the front end surfaces of the needlelike electrodes. This suppresses 
electric discharge at the side surfaces. For these reasons, the accuracy 
of electrical discharge machining can be made comparable to the accuracy 
obtained by the use of the prior art single needlelike electrode. 
Furthermore, a complicated mechanism for scanning the spark-machining 
electrode or the workpiece can be dispensed with. This allows the 
electrical discharge machine to be made smaller and lighter in weight. 
Further, the machining time can be shortened, since the workpiece can be 
machined in one electrical discharge machining process. This results in an 
improvement in the productivity. 
In addition, the area of the electric discharge surface is increased. This 
enhances the cooling and cleaning effects on the spark-machining 
electrode, thus improving the durability of the electrode. Also, the 
accuracy at which the workpiece is machined is improved. 
Moreover, the workpiece can be accurately machined into the desired form by 
appropriately arranging the needlelike electrodes, i.e., by adequately 
contriving the whole shape of the spark-machining electrode. For example, 
convex portions conforming to the holes formed inside the needlelike 
electrode groups can be formed simultaneously in the workpiece accurately. 
Also, concave portions conforming to the separate shapes of needlelike 
electrode groups can be machined in the workpiece simultaneously. Further, 
desired convex or concave shapes of needlelike electrode groups can be 
accurately formed in the workpiece simultaneously.