Wire electrode for wire-cut electrical discharge machining

A wire electrode for use in wire-cut electrical discharge machining which can be used under a high machining tension and which produces no appreciable deposit on the workpiece it is used to machine. The wire of the invention may be made of an amorphous metal or amorphous alloy composed primarily of iron, cobalt, or copper, and coated on its surface with a layer of a metal such as zinc, magnesium, tin, lead, cadmium or alloys thereof. Alternatively, the wire electrode may be a wire of highly fine crystalline particles fabricated by super-quenching from a pure metal or an alloy of a pure metal in a molten state at a cooling rate of 10.sup.3 to 10.sup.5 .degree.C./sec in a super-quenching process such as a spinning process which forms a wire in a rotating liquid layer.

BACKGROUND OF THE INVENTION 
The present invention relates to a wire electrode for use in a wire-cut 
electrical discharge machining process. 
Wire electrodes for wire-cut electrical discharge machining are generally 
in the form of a wire of copper, brass, tungsten or the like and having a 
diameter in a range of from 0.05 to 0.3 mm. FIG. 1 of the accompanying 
drawings is illustrative of the manner in which electrical discharge 
machining is carried out with such a wire electrode. The wire electrode, 
designated at 1, is tensioned and fed at a constant speed in the direction 
of the arrow A while being held in a confronting relation to a workpiece 
2. Then, a machining solution 3 is applied in a direction coaxial with the 
wire electrode 1 while a pulsed voltage is impressed between the wire 
electrode 1 and the workpiece 2. An electrical discharge is now repeatedly 
produced through the medium of the machining solution 3 across a small gap 
between the wire electrode 1 and the workpiece 2 to melt and scatter away 
a desired amount of material of the workpiece 2 by heat energy generated 
upon the electrical discharge. An XY crosstable (not shown) coupled to the 
workpiece 2 is numerically controlled to achieve desired relative movement 
between the wire electrode 1 and the workpiece 2 while keeping the 
electrode-to-workpiece gap constant at all times and ensuring continuous 
electrical discharge. 
By repeating the electrical discharge and controlling the XY crosstable in 
the above manner, a groove 4 can be continuously cut in the workpiece 2 to 
machine the workpiece 2 to a desired contour. Such wire-cut electrical 
discharge machining has been widely used in blanking and cutting general 
dies, for example. 
The speed of wire-cut machining is dependent on the degree of tension 
applied to the wire electrode 1, as shown in FIG. 2 where the abscissa 
indicates the tension T (g) and the ordinate the cutting speed F 
(mm/minute). FIG. 2 shows a characteristic curve which progressively rises 
as it goes to the right, the indication being that the cutting speed is 
higher as the tension is larger. It has been confirmed that as the tension 
is made larger, the wire electrode 1 is subjected to smaller vibrations 
and the electrode-to-workpiece gap can be controlled more uniformly for 
stabler electrical discharge repetitions, resulting in a higher cutting 
speed. 
One conventional electrode is disclosed in U.S. Pat. No. 4,287,404, 
entitled "Electrode for Electrical Discharge Machining", in which a wire 
electrode for electrical discharge machining is made of a material of high 
tensile strength and a metal of good machinability. 
Wire electrodes of copper, brass or steel, for example, having conventional 
crystalline structures suffer a limitation on the tensile strength 
thereof, and it is not possible to achieve a higher cutting speed through 
an increase in tensile strength. 
When a conventional wire electrode 1 of copper, brass or steel is fed 
upwardly or downwardly with respect to a workpiece during machining as 
shown in FIG. 3, portions of the wire electrode 1 are often scattered and 
deposited on an upper or lower end of a groove 4 cut in the workpiece 2. 
The deposited material 5 is mainly composed of copper or steel, and it has 
been observed that the material is deposited behind the wire electrode 1 
as it cuts into the workpiece 2 as illustrated in FIGS. 3A, 3B, 4A and 4B. 
The deposit 5 on the machined surface tends to impair the dimensional 
accuracy of the cut groove 4. Such a deposited layer 5 has a thickness in 
the range of about 10 to 100 microns in areas where large machining energy 
is applied. As the machining energy is increased, the cut groove 4 is 
sometimes filled with the deposited material as shown in FIG. 4. This 
undesirable phenomenon results in various shortcomings. The workpiece 
having been machined cannot be removed from the wire electrode. During 
machining, the machining solution 3 ejected coaxially with the wire 
electrode 1 does not enter the electrode-to-workpiece gap, causing a 
gaseous electrical discharge to lower the cutting speed and resulting in 
the danger of breaking the wire electrode 1. The deposit 5 mainly of 
copper, iron or the like, can only be removed with a dangerous chemical 
such as fuming nitric acid, a procedure which is tedious, timeconsuming, 
and unsafe. 
Therefore, the conventional wire electrodes have suffered from many 
difficulties and have proven unsatisfactory. 
SUMMARY OF THE INVENTION 
The present invention has been made in view of the foregoing conventional 
drawbacks. 
It is an object of the present invention to provide a wire electrode which 
has a high tensile strength, will deposit no appreciable amount of its 
material on a workpiece, and can machine a workpiece at an increased speed 
and a high accuracy. 
The wire electrode of the invention comprises a wire made of an amorphous 
metal or an amorphous alloy which may be coated on its surface with a 
layer of a metal such as zinc, magnesium, tin, lead, aluminum, cadmium or 
alloys thereof. 
Alternatively, the wire electrode may be a wire of highly fine crystalline 
particles which is fabricated by being super-quenched from a pure metal or 
an alloy in its molten state at a cooling rate of 10.sup.3 to 10.sup.5 
.degree. C./sec in a super-quenching process such as a spinning process 
which forms a wire in a rotating liquid layer. Alternatively, the wire 
thus fabricated is further drawn into a thin wire for use as a wire 
electrode. 
The inventor has found that the wire electrode of the invention has a 
tensile strength much higher than that of conventional wire electrodes. 
The above and other objects, features and advantages of the present 
invention will become more apparent from the following description when 
taken in conjunction with the accompanying drawings in which preferred 
embodiments of the present invention are shown by way of illustrative 
example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The wire electrodes heretofore used in wire-cut electrical discharge 
machining have had crystalline structures which fail to provide a high 
tensile strength. 
EXAMPLE 1 
According to this example, a pure metal or an alloy in a molten state is 
quenched by a super-quenching process such as a spinning process for 
producing a thin amorphous wire in a rotating liquid layer. The rate of 
quenching the metal or alloy is in the range of 10.sup.5 to 10.sup.6 
.degree. C./sec. The thus-produced thin amorphous wire may be used 
directly as a wire electrode, or it may further be drawn into a wire 
electrode. The resultant wire electrode has a tensile strength much higher 
than that of the prior wire electrodes. 
FIGS. 6A and 6B schematically show a spinning device for forming an 
amorphous wire in a rotating liquid layer, the spinning device being 
generally composed of a heating furnace, an ejector for ejecting molten 
metal, and a cooling unit for cooling a rotating drum. The rotating drum 
has an inside diameter of 600 mm and, when rotated, forms a uniform 
rotating layer of water on an inner periphery. The molten metal is ejected 
in the same manner as an amorphous ribbon is formed, but is ejected 
through a nozzle having a circular cross section. The spinning process 
continuously forms an amorphous wire which is accumulated in cooling water 
in the drum, the amorphous wire thus fabricated having a diameter ranging 
from 100 to 200 microns for an iron-base metal. 
Other super-quenching processes include a spinning process for forming an 
amorphous wire in a water stream and a spinning process for forming an 
amorphous wire coated with glass, as shown in FIGS. 7A and 7B, 
respectively. 
Amorphous-metal wires generally have a tensile strength which is 1.5 to 3 
times that of crystalline-metal wires. As an example, FIG. 5 illustrates 
stress vs. strain curves of an amorphous-metal wire and a conventional 
piano wire. 
Wire electrodes are made of copper or a copperbase alloy. Where an 
amorphous wire electrode is made of copper-base metal, part of the wire 
electrode will be scattered and deposited on a machined surface of a 
workpiece due to the electrical discharge. 
To prevent the electrode material from being scattered and deposited, an 
amorphous wire electrode 6 (FIG. 8) is coated on its surface with a layer 
7 of a material having a low melting point and which is capable of being 
easily evaporated, such as zinc, magnesium, tin, lead, aluminum, cadmium 
or alloys thereof. The wire electrode thus constructed will produce 
substantially no deposit on the workpiece. 
The amorphous-metal wire electrode thus fabricated by super-quenching 
molten metal has a tensile strength which is 1.5 to 3 times that of 
conventional crystalline-metal wire electrodes. Since the tension applied 
to the amorphous-metal wire electrode can be increased in actual machining 
operation, the machining speed can be increased and the machining accuracy 
can also be improved. 
Table 1 shows a comparison of various characteristics of a conventional 
wire electrode of brass plated with a coating layer of zinc about 10 
microns thick, a wire electrode of brass, and a wire electrode of copper, 
the characteristics being obtained when machining a steel workpiece and 
indicated by way of percentage with brass characteristics used as a 
reference. Table 1 clearly indicates that the zinc coating layer is highly 
effective in reducing the unwanted wire material deposit and increasing 
the machining speed. It is apparent that the advantage of the coating 
layer remains the same when the electrode core is an amorphous-metal wire. 
TABLE 1 
______________________________________ 
Wire Coating 
electrode 
Diameter thickness Tensile 
Machining 
material 
(mm) (microns) 
Deposit 
strength 
speed 
______________________________________ 
Brass 0.2 -- 100 100 100 
Copper 0.2 -- 700 50 80 
Zinc- 0.2 10 5-8 75 180 
coated 
brass 
______________________________________ 
The amorphous wire electrode of the present invention may be made of any 
metal that can be in an amorphous state. Since the wire electrode should 
be of good conductivity, the amorphous wire electrode may be coated with 
electrically conductive material. FIG. 9 shows such a multilayer amorphous 
wire electrode comprising an amorphous wire electrode 6, a layer 8 of 
electrically conductive material coated on the amorphous wire electrode 6, 
and a layer 7 of metal or alloy coated on the layer 8 for preventing 
electrode material from being scattered and deposited on a workpiece. The 
multilayer-coated amorphous wire electrode can machine workpieces at an 
increased machining accuracy and speed. 
Metals and alloys that can be used to prepare amorphous wires of the 
invention include: 
1. Pure metal or alloys of pure metal; 
2. Iron, aluminum, magnesium, copper, cobalt, niobium and alloys thereof; 
3. Iron-base alloy (alloy composed mainly of iron), copper-base alloy 
(alloy composed mainly of copper), and cobalt-base alloy (alloy composed 
mainly of cobalt); 
4. Fe - Si - B alloy, 
Fe - P - C alloy (Fe - P - C - Cr alloy), and 
Fe - Co - Si - B alloy; 
Cu - Zr alloy, 
Cu - Sn - P alloy, and 
Cu - Zn - Ag alloy; 
Co - Nb - B alloy, and 
Co - Fe - Si - B alloy; 
5. Fe=70-75%, Si=10%, B=15-20%, 
Fe=72-77.5%, P=12.5%, C=10% (Cr=0-5.5%), and 
Fe=71%, Co=4%, Si=10%, B=15%; 
Cu=60%, Zr=40%, 
Cu=65-70%, Zn=20-25%, Ag=5-15%, and 
Cu=70-80%, Sn=10-20%, P=0-10%; 
Co=67.5%, Fe=5%, Si=12.5%, B=15% 
Other advantages of the amorphous wire electrode than the higher tensile 
strength are as follows: 
By adding a passive film element such as Cr, corrosion resistance 
capability is greatly increased. Where a wire electrode is made of an 
amorphous metal alloy only with no surface coating, the fabricated wire 
electrode can be packaged in a simple process. Under current practice, 
wire electrodes are made of brass and packaged by vacuum packaging. 
However, the amorphous wire electrodes of the invention can be packaged 
more easily. 
In general, amorphous wire electrodes with no passive film formed thereon 
have a surface which is highly chemically active. By coating such an 
electrode surface with Zn or the like, a strong and stable bond will be 
formed between the electrode surface and the coating. 
Advantages in the manufacturing process are as follows: Wire electrodes can 
be completed simply by ejecting a molten metal material for 
super-quenching. It is not necessary therefore to repeat conventional wire 
drawing processes to form a thin wire. Accordingly, the manufacturing 
process can be simplified and the time required for manufacturing a wire 
electrode can be shortened. 
EXAMPLE 2 
A molten metal is super-quenched at a cooling rate of 10.sup.3 .degree. 
C./sec or higher to fabricate a highly fine crystalline wire. The highly 
fine crystalline wire has a tensile strength that is 1.5 to 2 times that 
of a conventional wire electrode as indicated in Table 2. The highly fine 
crystalline wire electrode, if made primarily of copper, will have its 
material deposited on a workpiece during machining operation. This is true 
of a highly fine crystalline wire electrode if made primarily of steel. To 
avoid this shortcoming, the wire electrode is coated on its surface with a 
layer of a material having a low melting point and capable of being easily 
evaporated, such as zinc, magnesium, tin, lead, aluminum, cadmium or 
alloys thereof, as illustrated in FIG. 8. The wire electrode thus 
constructed will produce substantially no deposit on the workpiece and 
will increase the machining accuracy and speed. 
TABLE 2 
______________________________________ 
Cooling Crystal Tensile Machining 
Wire rate particle strength 
speed 
type (.degree.C./sec) 
dia. (mm) (kg/mm.sup.2) 
(mm.sup.2 /sec) 
______________________________________ 
1 up to 10 0.025-0.03 50 80 
2 .sup.- 10.sup.3 or 
0.001-0.005 
90 120 
higher 
______________________________________ 
1: Conventional copper wire 
2: Highly fine crystalline copper wire 
Table 3 shows a comparison of various characteristics of a conventional 
wire electrode of brass plated with a coating layer of zinc about 10 
microns thick, a wire electrode of brass, and a wire electrode of copper, 
the characteristics being obtained when machining a steel workpiece and 
indicated by way of percentage with brass characteristics used as a 
reference. Table 3 clearly indicates that the zinc coating layer is highly 
effective in reducing the unwanted wire material deposit and increasing 
the machining speed. It is apparent that the advantage of the coating 
layer remains the same when the electrode core is a highly fine 
crystalline wire. 
TABLE 3 
______________________________________ 
Wire Coating 
electrode 
Diameter thickness Tensile 
Machining 
material 
(mm) (microns) 
Deposit 
strength 
speed 
______________________________________ 
Brass 0.2 -- 100 100 100 
Copper 0.2 -- 700 50 80 
Zinc- 0.2 10 5-8 75 180 
coated 
brass 
______________________________________ 
The highly fine crystalline wire electrode of the present invention may be 
made of any metal that can have finely divided crystalline particles when 
super-quenched from its molten state. Since the wire electrode should be 
of good conductivity, the wire electrode may be coated with a electrically 
conductive material. More specifically, as illustrated in FIG. 9, a highly 
fine crystalline wire electrode 6 is coated with a layer 8 of electrically 
conductive material, which in turn is coated with a layer 7 of zinc, 
magnesium, tin, lead, aluminum, cadmium or alloys thereof for preventing 
electrode material from being scattered and deposited on a workpiece. The 
multilayer-coated highly-fine crystalline wire electrode can machine 
workpieces at an increased machining accuracy and speed. 
As described above, the wire electrode for use in wire-cut electrical 
discharge machining according to the present invention has many practical 
advantages. 
Although certain preferred embodiments have been shown and described, it 
should be understood that many changes and modifications may be made 
therein without departing from the scope of the appended claims.