Wire electrode for electro-discharge machining and method of manufacturing same

A process of manufacturing spark erosion electrode particularly pertaining to electrode in wire form, for use in electrical discharge machining, the core of the electrode being of comparatively low zinc alpha brass with top layer of highly rich zinc beta and gama brass to facilitate better flashability of the electro-erosion process, and also to achieve comparatively higher tensile strength of the core material of the electrode, whereby relatively higher accurate vertical cut of the workpiece is possible, due to reduction of amplitude of high tension vibration of the core while cutting workpiece by electro discharge machining process (hereinafter referred to as EDM process).

BACKGROUND OF THE INVENTION 
Electro discharge machining process is a metal-removal process (also called 
electric-spark machining) in which materials that conduct electricity can 
be removed by an electric spark. The spark is a transient electric 
discharge through the space between the tool (cathode)-and the workpiece 
(anode). It is used to form holes of varied shape in materials of poor 
machinability which takes long time to complete with the help of 
conventional machine tool. 
1. Field of the Invention 
The present invention relates to a process of manufacturing spark erosion 
electrodes and more particularly to electrode in the wire form. 
2. Prior Art Disclosure 
When the EDM process was invented and subsequently commercial application 
started from mid-sixties, initial electrode material used was copper. 
Copper had its own drawbacks like low tensile strength and poor 
flashability. Both the reasons caused inaccurate cutting of the workpiece 
and low cutting speed. So copper was replaced by plain brass wire, whereby 
the conductivity of the core was sacrificed, but considerable strength 
thereof over copper, and comparatively better flashability over copper, 
was gained. 
As ordinary brass was not in a position to provide the ideal requirements 
of an EDM electrode wire, various other materials have so far been adopted 
and used, some of which are mentioned herebelow, in brief. 
U.S. Pat. No. 4,287,404 discloses an electrode for machining a workpiece by 
electrical discharges, the electrode having an active surface comprising 
at least 50% by weight of a metal or alloy selected from the group 
consisting of zinc, cadmium, tin, lead, antimony and bismuth. The 
mechanical strength of the wire and the intensity of the current flow 
through the wire has been proposed to be greatly increased by providing a 
wire having a steel core surrounded by a layer of copper or silver 
provided in turn by the protective thermal coating consisting of, for 
example, zinc, cadmium, tin, lead, antimony or bismuth, or alloys thereof. 
U.S. Pat. No. 4,424,432 discloses an electrode material for travelling-wire 
type electrical discharge machining, said electrode material being drawn 
from a composition of 0.1 to 3% by weight zirconium, 0.3 to 10% zinc and 
the balance copper. 
U.S. Pat. No. 4,631,237 discloses wire electrode for spark-eroding systems, 
said electrode having a core of a current-conducting material and a wire 
coating of a material with a lower evaporation temperature, for example 
zinc. The core consisting of one of the following alloys 
a) Cu Mg 0.4; 
b) Cu Fe 2P; 
c) Cu Cr Zr; 
d) Cu Zr. 
U.S. Pat. No. 4,717,804 discloses an electrical discharge machining 
electrode which comprises a composite member having an electrically 
conductive entirely metal wire length of ferrous alloy metal for its core 
and with said core being clad with a layer of copper whose outer surface 
is oxidized and coated with graphite. 
U.S. Pat. No. 4,935,594 discloses a method for the manufacture of an 
eroding wire electrode for use in the spark-erosive cutting of 
electrically conductive materials, said electrode comprising a core of one 
of a metal and a metal alloy and at least one coating of one of a metal 
having a low volatilization temperature and an alloy of said 
last-mentioned metal, the improvement wherein for producing an altered 
coating, which can be cold-shaped only to a limited degree and is active 
during spark-erosive cutting, out of said first-mentioned coating, said 
wire electrode is annealed substantially at said low volatilization 
temperature until said altered coating is created, said altered coating 
including an alloy which extends from an outer surface of said wire 
electrode toward said core with a corresponding decreasing content of said 
metal having said low volatilization temperature, and wherein said wire 
electrode is subsequently cooled in a controlled manner to fix the 
diffusion states. 
U.S. Pat. No. 4,968,867 discloses a wire electrode for wire cut electric 
discharge machining having a core wire of high thermal conductivity (made 
of copper, silver, aluminium or their alloys), an intermediate layer 
formed by a low-boiling point material(zinc), and an outermost layer of 
brass having high mechanical strength. 
U.S. Pat. No. 4,977,303 discloses a method for forming an EDM wire 
electrode which comprises coating a copper wire core with zinc, and then 
heating the coated wire in an oxidizing atmosphere to simultaneously 
provide a copper-zinc alloy layer over the copper core and a zinc oxide 
surface on the alloy layer. The oxide and alloy-coated wire is then 
reduced in diameter to reduce the thickness of the alloy layer by about 
one-half of its initial thickness. The resulting electrodes wire permits a 
greater current density and a greater tractional force to be employed, 
yielding a significantly greater machining speed in the EDM process. 
U.S. Pat. No. 5,196,665 discloses a method for manufacturing a multilayer 
electrode wire comprising the steps of: 
superimposing a plurality of alternate fine layers of a first metal with 
high electrical conductivity and a second metal with a low melting and 
vaporization point onto a core made of electrically conductive material; 
finishing said superimposed alternate fine layers with a said layer of 
said-second metal; and 
cold drawing said superimposed fine layers and said finishing layer to 
cold-form said electrode wire without causing any of said metals to 
diffuse into an adjacent said layer. 
U.S. Pat. No. 4,341,939 discloses a metallic wire, for cutting a workpiece 
by electrical discharge machining, coated with at least one layer of a 
metal having a low temperature of vaporization and film of a metal oxide 
on the metal layer. The metallic coating is preferably made of zinc and is 
subjected to an oxidizing thermal or electrolytic treatment such as to 
form on the surface of the metallic layer a thin film of zinc oxide. 
U.S. Pat. No. 4,952,768 discloses an electrode for precision electric 
discharge machining is formed by silicon or other materials, a base of a 
low melting point metal or an alloy of such a low melting point metal is 
covered with a layer composed of a mixture of the metal land a high 
resistivity inorganic compound. 
U.S. Pat. No. 5,206,480 discloses a wire electrode for electro-discharge 
machining made of a Cu alloy containing 38 to 50 percent by weight of Zn. 
Cu alloy may also contain 0.01 to 1.0 percent by weight of Zr, 0.001 to 
0.05 percent by weight in total of at least one element selected from a 
group of Ce, Ti, Mg, Bi and Mn, and/or 0.01 to 2.0 percent by weight of at 
least one element selected from a group of Al, Si, Fe, Ca and La. A method 
of manufacturing a wire electrode comprising the steps of preparing a Cu 
alloy material containing 38 to 50 percent by weight of Zn and thinning 
the Cu alloy material employing roll working and/or warm working at least 
in a part of working process. 
U.S. Pat. No. 4,673,790 discloses a wire electrode for use in 
electro-discharge machining formed essentially of Cu, including Zn of 54 
through 38% by weight and Si of 0.1 through 0.5% by weight so as to 
suppress sputtering of the electrode material to the workpiece and to 
improve workability The wire electrode may include Zn of 30 through 40% by 
weight, Si of 0.1 through 1.2% by weight and Al of 0.01 through 0.2% by 
weight in addition to Cu. 
U.S. Pat. No. 4,806,271 discloses production of base-type conductive 
polymers, particularly from the family of conductive polyaniline, by 
reacting a base-type non-conductive polymer containing carbon-nitrogen 
linkages, e.g. polyaniline, with a cation donor compound, such as R SO 
R'SO Cl or R" SiC1, where R, R' and R" are alkyl or aryl, such as dimethyl 
sulfate or tosyl chloride, and forming an electrically conductive polymer 
in which the R groups of R SO, the R'SO groups of R'SO Cl, or the R" Si 
groups of R" SiCl are covalently linked to the nitrogen atoms of the 
polymer. 
U.S. Pat. No. 4,740,666 discloses an improved electrical discharge 
machining electrode permitting improved cutting action to be achieved at 
the same level of electrical power being applied to the wire electrode. 
U.S. Pat. No. 4,988,552 discloses a wire electrode for a traveling wire EDM 
apparatus that comprises a copper clad steel core that is clad with a 
homogenous, outer brass layer using a classical bonding process. The 
copper clad steel core has a conductivity in the range of 50%-70% IACS 
(International Annealed Copper Standard) and the brass layer comprises 
35%-50% by volume of the electrode. The brass layer comprises 65% copper 
and 35% zinc in an optimum construction. 
The spark erosion electrode for "EDM" process, and the technology for the 
production thereof, as invented and developed by the inventor is cheaper, 
totally novel and facilitates faster cutting of workpiece. 
The processes and the products according to the aforesaid prior art are 
relatively expensive. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide a comparatively less 
expensive process for manufacturing wire electrode for use in EDM process, 
which affords comparatively better physical and chemical characteristics 
of the wire to facilitate better EDM erosion for accurate cutting of 
workpiece. 
It is known that tensile strength of Brass increases with the increase of 
Zinc content. It rises very rapidly with the appearance of Beta-Phase. At 
32% zinc content the tensile strength of brass becomes maximum when alpha 
and Beta phases are present in approximately equal proportion. 
Keeping the above aspect in view, the present invention contemplates to 
produce spark erosion electrode, the core whereof is of comparatively low 
zinc alpha brass with top layer of highly rich zinc beta and gama brass to 
facilitate better flashability of the electro-erosion process, and also to 
achieve comparatively higher tensile strength of the core material of the 
electrode. 
In use of the spark erosion electrode, so produced, in "EDM" process, the 
following material removal rate, compared to what is achieved from normal 
brass electrode, has been noticed 
______________________________________ 
Material Removal 
Rate 
Present 
Normal Invention 
Brass Wire 
Material Machine Type MM/Min. MM/Min. 
% increase 
______________________________________ 
Hardened ELECTRONICS 16.4 18.8 14.6% 
Steel 
Aluminium 
ELECTRONICS 13.0 18.0 38.5% 
Hardened Stl. 
MAKINO 36.0 50.4 40% 
Hardened Stl. 
SODIK 77.0 101.2 31.4% 
Hardened Stl. 
AGIE 200-D 78.0 105.0 34.6% 
______________________________________ 
DETAILED DESCRIPTION OF THE INVENTION 
Accordingly, the present invention provides a process of manufacturing 
spark erosion electrode, which comprises plating with zinc, by 
galvanising, a core wire made of brass, and of diameter more than the 
desired diameter of the final wire; putting coil(s) of the zinc plated 
wire in a bath pot; heating the pot in double vacuum furnace at a 
temperature below the melting temperature of zinc for a prolonged period 
of more than 24 hours, the temperature of the furnace being increased 
gradually from a starting temperature of 60.degree.-70.degree. C., with 
the final temperature, depending on the required thickness of zinc to be 
absorbed within the brass, and the diameter of the wire, being held for 10 
to 20 hours, gradually cooling the pot upto the ambient temperature over a 
prolonged period of about 24 hours; drawing the wire to reduce its 
diameter to an intermediate stage almost upto the desired final diameter 
thereof; subjecting coil(s) of the wire to gradual heating in double 
vacuum atmosphere for prolonged period, as done earlier, and carrying out 
final drawing of the wire upto its desired diameter, optionally, followed 
by resistance annealing thereof, in the manner such as herein described. 
Preferably, the core wire is produced from a square wire, spirally cut from 
a disc/plate made of the starting composition of brass, by centrifugal 
casting, said square wire being gradually shaped to the said core wire of 
desired diameter, by drawing, followed by annealing and pickling, as 
required. 
In a particular embodiment, centrifugal casting is done with a composition 
of 61.5% copper and 38.5% zinc with plus minus 0.2%. To get precise 
composition both outer portion as well as the central portion of the disc 
are discarded. At high speed centrifugal process, heavier particles go to 
the top portion of the plate and the lighter particles come at the central 
portion. The middle portion is more or less of uniform composition. 
However, the above composition of the wire is extremely difficult to draw 
and frequent inter annealing is required to get a diameter of around 3.00 
mm. 
The prolonged heating of the coil(s) of wire, below the zinc melting 
temperature (i.e. 419.degree. C.) results into: 
(i) absorption of zinc inside the brass to a great extent forming highly 
enriched zinc composition brass known as Kirkendal effect, and 
(ii) to get a granular structure which provides better conductivity as well 
as better splashability due to lower vapor pressure.

PREFERRED EMBODIMENT 
In a preferred embodiment, the square wire is drawn through round dies to 
obtain round wire, the latter being welded together, if necessary, to get 
a big coil thereof followed by mechanical polishing of the same, with the 
help of grooved grinding wheel. As stated earlier, the outer and central 
portion of the disc/plate are discarded, while making the square wire from 
the disc/plate, by spiral cutting, with a view to get uniform composition 
of brass. 
Preferably, prior to plating with zinc, the coil(s) of wire is(are) 
annealed and then pickled. 
The coil(s) of wire is(are) subjected to double vacuum atmosphere by 
creating vacuum in the bath pot and also in the furnace. Preferably, the 
vacuum is created in the bath pot and in the furnace by usual sucking 
followed by introduction of nitrogen, and pumping out the same, and 
repeating the said procedure, if necessary, with a view to make the inside 
of the bath pot and that of the furnace totally free from oxygen. 
The coil(s) of wire, duly cooled inside the pot by gradual cooling of the 
latter for a prolonged period, is(are) pickled e.g. by sulphuric acid, 
prior to further drawing thereof. 
The resistance annealing of this finally drawn wire may be done by 
resistance heating arrangement, provided in-built with the drawing 
machine, as known in the art, where high current of electromotive force is 
caused to be passed through the wire at low voltage so as to heat the wire 
almost immediately due to the resistance of the wire. 
In a particular embodiment, the centrifugal casting is done at 600 RPM to 
get a disc/plate of 600 mm outer diameter and thickness of 16 mm. 
Preferably, the cast disc/plate surface is subjected to grinding/scalping 
for surface cleaning thereof. Prior to spiral cutting, rolling of the 
plate/disc is done to reduce the thickness thereof upto 6.5 mm. 
Thereafter, square wire of 6.5 mm.times.6.5 mm is made from the disc/plate 
by spiral cutting. Then, the square wire is drawn through round dies to 
obtain round wire of 5.60 mm diameter. In the next stage, the diameter of 
the wire is reduced to 3.0 mm by drawing, preceded by inline annealing and 
pickling, and said wire is plated with zinc by galvanising. Preferably, 
the inline annealing of the wire is done in double vacuum furnace at 
620.degree. C. for a period of 5 hours. The pickling of wire may be done 
with sulphuric acid. Preferably, 10 to 40 microns of zinc is provided on 
the surface of the wire by zinc plating. Thereafter, the coil(s) of wire 
drawn upto 3.0 mm diameter and kept inside the bath pot, is(are) gradually 
heated in the furnace, both the said pot and the furnace being kept under 
vacuum and in total absence of oxygen, as aforesaid, the temperature of 
the furnace being gradually raised upto 370.degree. C. to 395.degree.C. 
and the duration of the heating at the said temperature is maintained for 
24-26 hours. After creation of the vacuum in the pot and in the furnace, 
the temperature is gradually increased from 60.degree.-70.degree.C. in 
steps of 50.degree.C. at an interval of 2 hours upto 300.degree.C., and 
then at 25.degree.C. upto 350.degree.C., and at 10.degree.C. upto 
380.degree.C., and thereafter at 5.degree.C. at the intervals of half an 
hour, and holding the final temperature, so raised, for a period of 18 to 
20 hours. The coil(s) of wire, as obtained finally, is(are) spooled. 
In the process according to this invention highest possible overall zinc 
content can be achieved but at the same time it can be drawn to fine sizes 
with maximum reduction of upto 96% area reduction without inter annealing. 
This gives huge process cost reduction and finally the wire is stress 
released at a temperature of about 37.degree.C. and ultimately a tensile 
strength of more than 95 kgs/cm can be achieved. This tensile strength 
will be most ideal for modern auto feed machines and also for high tension 
cutting to set most accurate vertical cut. 
It would be appreciated that in the hither-to-known process/systems the 
wire stays inside the pipe type furnace around 8 to 10 seconds. This is 
equivalent to treatment of about 40 mm wire per minute. For a standard 
size of 0.25 mm wire at a speed of 40 mm per minute, a production of 24 
kg/24 hours may be expected. For a standard furnace of 31 pipe at 75% 
efficiency approximately 540 kgs of production is expected against the 
production of 1000 kgs per 24 hours according to the process of this 
invention. So, the production rate is about double in the type of 
production according to this invention. 
Again, in the case of Pipe type furnace production, the cost is higher 
because of continuous supply of reducing atmosphere i.e. combination of 
Nitrogen and Hydrogen, whereas in the process of this invention, this is 
not required. 
The invention will be illustrated hereinafter with reference to the 
following example: A disc was made, by centrifugal casting, from molten 
brass having composition of 61.5. copper and 38.5% zinc .+-.0.2%. 
The centrifugal casting was done at a speed of 600 RPM with cast disc of 
outer diameter 600 mm and thickness of 16 mm. The disc was properly 
scalped and rolled to reduce the thickness thereof upto 6.5 mm, and to 
increase the diameter of the same. 
Spiral cutting of the disc was done to get rod coil, and a square wire of 
6.5 mm.times.6.5 mm was cut. Prior to spiral cutting, the outer and the 
central portions of the disc were discarded to get uniform composition, 
remaining at the middle portion thereof. 
The square wire was then passed through round die and ultimately at 5.60 mm 
diameter round shaped wires in coil form, of 100 kgs weight each were 
obtained. Then these coils were annealed in double vacuum furnace at 
620.degree. C. for a period of 5 hours, and then pickled with sulphuric 
acid. The wires were welded to make the coils. 
Thereafter the wires, in coil form, were drawn to 3.00 m dia, followed by 
annealing and pickling thereof. The wires, so treated, were then plated 
with zinc by age-old galvanic process and then put under the furnace of 
double vacuum. For such purpose, the coils of brass wire, coated with zinc 
were kept inside a bath pot, and the pot was inserted in a furnace. The 
bath pot as well as the furnace were processed to create double vacuum. 
The vacuum was created in the bath pot and in the furnace by usual sucking 
followed by introduction of nitrogen, and pumping out the same, and 
repeating the said procedure, if necessary, with a view to make the inside 
of the bath pot and that of the furnace totally free from oxygen. 
After the vacuum was created the temperature was gradually increased in 
steps of 50.degree. C. at an interval of two hours upto 300.degree. C. 
then 25.degree. C. upto 35.degree. C., and 10.degree. C. upto 380.degree. 
C. and thereafter 5.degree. C. at the interval of half an hour. The time 
of holding the final temperature was 18 to 20 hours, and the total time of 
keeping the coils inside the furnace was between 24 to 26 hours. 
The pot was then taken to cooling vat and cooled very gradually upto room 
temperature for about 24 hours. 
Then the coils of wire were taken out from the annealing pot and were 
subjected to both inline mechanical polishing of the wire surface by means 
of grooved grinder wheel as well as for chemical surface cleaning as 
preparatory steps for the lubricant carrier facility of the wire to help 
maximum possible drawing of the coils of wire in a single shape at a 
reduction of more than 94.3% in area to get the diameter of the wire upto 
an intermediate shape between 1 to 1.25 mm. 
Then the wires were again charged inside the double vacuum furnace and same 
procedure of prolonged heating was done. With this second prolonged 
heating of the wires, top layer zinc further penetrated towards the core 
and there was a gradually higher zinc content towards the surface of the 
wire. 
The purpose of the gradual change of zinc content was to provide better 
splashibility of the EDM wire so that it would perform far better than the 
conventional EDM wire. 
Thereafter, final drawing of the wire upto 0.15 to 0.30 mm was done in a 
wet type drawing machine and in the same mechanical composition these were 
put to resistance annealing. This was done by resistance heating 
arrangement, provided in-built with the drawing machine, as known in the 
art, where high current of electromotive force was caused to be passed 
through the wire at low voltage so as to heat the wire almost immediately 
due to the resistance of the wire. The said resistance annealing was done 
to release the in built stress and to get the desired straightness of the 
finished wire, and the wire was heated when the wire was in tension. 
Finally the wire was spooled and said spools were put in a chamber, wherein 
water was sprayed to form steam with a view to protect the wire surface. 
It is to be understood that various modifications of the process according 
to this invention are possible within the scope of what has been described 
hereinbefore and will be claimed hereinafter. 
BRIEF DESCRIPTION OF THE DRAWINGS 
Various changes which occur in the composition and metallurgical structures 
of the electrode wire, during its production, following the process 
according to this invention, are illustrated by way of photomicrographs of 
the sections of the wire, at different stages, as shown in the 
accompanying drawings, wherein, 
FIG. 1: shows, in section, one embodiment of a core wire made of brass 
(indicated by 1) in annealed condition, but prior to plating with zinc by 
galvanizing. 
FIG. 2: shows, in section, the same core wire, as shown in FIG. 1, which 
has been plated with zinc, where the .alpha.-brass core is indicated by 1, 
and the zinc-coating therearound is indicated by 2. 
FIG. 3: shows, in section, the same wire, as shown in FIG. 2, which has 
been heated in vacuum at temperature below the melting temperature of zinc 
for 24 to 26 hours, with the result, due to Kirkendal effect, that the 
core remains .alpha.-brass, as indicated by 1, while the zinc coating 
(indicated by 2 in FIG. 2)--transforms to .beta.-and .alpha.-brass, (as 
indicated by 3. 
FIG. 4: shows, in section, the finally drawn wire from the treated wire, 
shown in FIG. 3, after reduction in cross sectional area of about 95%, 
where the core remains .alpha.-brass, as indicated by 1, and the 
surrounding layer is .beta.-and .gamma.-brass, as indicated by 3. 
FIG. 5: shows, in, section the same embodiment of the finally drawn wire, 
as shown in FIG. 4, which has been stress-released after final resistance 
annealing, as described hereinbefore. As seen, the composition and 
structure of the core 1 and the outer layer 3 remains unchanged.