Traveling-wire EDM method and apparatus with a cooled machining fluid

A traveling-wire EDM method and apparatus in which a machining fluid in liquid phase is sufficiently cooled in its supply conduit to less than a predetermined critical temperature, e.g. 4.degree. C. To this end, the wire electrode prior to introduction into the fluid supply nozzle may be cooled by passage through a refrigerant or by thermoelements. Preferably, the wire electrode is passed between a pair of ice-formed guide members disposed across the nozzle units. The eventual machining fluid may be gas or liquid in which fine ice particles or fragments are suspended.

FIELD OF THE INVENTION 
The present invention relates to traveling-wire EDM and, more particularly, 
to a new and improved method of and apparatus for electroerosively 
machining a workpiece with a traveling wire electrode in the presence of a 
machining fluid. By the term "wire electrode" is meant herein a thin, 
elongate electrode in the form of a wire, filament, ribbon or the like. 
BACKGROUND OF THE INVENTION 
As is well known, the traveling-wire EDM process makes use of a thin 
metallic wire composed, say, of copper or brass and having a diameter, 
say, of 0.1 to 0.5 mm. The wire may be continuously unwound from a supply 
reel and passed through a workpiece and taken up onto a takeup roller. In 
the path of wire travel, a pair of machining positioning guide members are 
arranged to support and guide the traveling wire in machining relationship 
with the workpiece. A machining fluid, typically distilled water, is 
supplied into the cutting zone from nozzle means which preferably 
comprises two nozzles disposed on the opposite sides of the workpiece, 
respectively. Preferably, the machining fluid is injected into the cutting 
zone from a nozzle coaxially with the traveling wire electrode. An EDM 
power supply is electrically connected to the wire electrode and the 
workpiece to apply a machining current, commonly in the form of a 
succession of electrical pulses, therebetween. Time-spaced, discrete 
electrical discharges are thereby created across the machining gap defined 
between the traveling wire electrode and the workpiece to electroerosively 
remove material from the workpiece. As electroerosive material removal 
proceeds, the workpiece is displaced relative to the axis of the traveling 
wire electrode transversely thereto along a predetermined path which 
determines a contour of cut eventually imparted to the workpiece. 
During the traveling-wire EDM process, the machining liquid tends to be 
heated up by successive electrical discharges. It has now been found that 
a rise in temperature of the machining liquid, especially when constituted 
by a distilled water liquid, is a source of reduction in the cutting 
accuracy, insufficiency of the removal rate and wire breakage. As the 
water liquid is heated up, its specific resistivity lowers and deviates 
from the desired setting, resulting in an enlargement of the machining gap 
size and the consequent deviation of the overcut. Furthermore, the cooling 
capacity of the water liquid when heated is reduced and the consequent 
increase in liability of the wire to break requires that the machining 
current be limited to an unsatisfactory level. 
OBJECTS OF THE INVENTION 
The present invention accordingly seeks to provide an improved 
traveling-wire EDM method which enables the traveling wire electrode to be 
less liable to break, the machining accuracy to be increased, the 
machining stability to be improved and the EDM machining efficiency to be 
markedly increased, and further to provide an apparatus for carrying out 
the improved method. 
SUMMARY OF THE INVENTION 
In accordance with the present invention there is provided a method for 
machining a workpiece by electroerosion with a wire electrode wherein the 
wire electrode is axially displaced to traverse the workpiece between a 
pair of wire supply supporting members while defining a machining gap in 
an opening being developed in the workpiece, a machining fluid is injected 
into the workpiece opening from nozzle means adjacent to the workpiece and 
machining current is passed between the traveling wire electrode and the 
workpiece to create electrical discharges across the machining gap, 
thereby electroerosively removing material from the workpiece, which 
method comprises: providing the machining fluid in liquid phase in supply 
conduit means connected to the nozzle means; cooling the liquid in the 
supply conduit means; and pumping the fluid in the cooled liquid phase to 
feed it without a substantial rise in temperature via the nozzle means 
into the workpiece opening. 
The machining fluid is, preferably, distilled water having a predetermined 
specific resistance given in the supply conduit means and cooled to less 
than a predetermined temperature, viz. generally less than 20.degree. C., 
preferably less than 10.degree. C. and more preferably less than 4.degree. 
C. The predetermined specific resistance should preferably range between 
5.times.10.sup.3 and 5.times.10.sup.5 ohm-cm. 
Specifically, the machining fluid in liquid state is cooled by cooling the 
wire electrode in contact therewith, independently of cooling by the 
machining fluid. Alternatively, the wire electrode may be cooled by 
contact with the cooled machining fluid passing through the nozzle means 
disposed upstream of the workpiece opening. Thus, the wire electrode may 
be cooled within or upstream of the nozzle means disposed ahead of the 
workpiece. 
In a further embodiment of the invention, the wire electrode is cooled at a 
temperature below the freezing point of the water and thereafter passed 
through the nozzle means fed with the distilled water liquid to allow a 
layer of ice to build up on the surface of the wire electrode and the 
latter to be cladded therewith prior to entry into the workpiece opening. 
The wire electrode may be cooled by bringing a refrigerant into contact 
therewith. Alternatively, the wire electrode may be cooled by passing the 
wire electrode through a wire guide member composed of ice and disposed 
ahead of the nozzle means. 
In a still further embodiment of the present invention, the maching fluid 
is admixed with particles or fragments of ice prior to passage into the 
workpiece opening and preferably prior to passage out of said nozzle 
means. 
The present invention also provides, in a second aspect thereof, an 
apparatus for machining a workpiece by electroerosion with a wire 
electrode wherein the wire electrode is axially displaced to traverse the 
workpiece between a pair of wire supporting members while defining a 
machining gap in an opening being developed in the workpiece, a machining 
fluid is injected into the workpiece opening from nozzle means adjacent to 
the workpiece and machining current is passed between the traveling wire 
electrode and the workpiece to create electrical discharges across the 
machining gap, thereby electroerosively removing material from the 
workpiece, which apparatus comprises: supply conduit means for providing 
the machining fluid in liquid phase; the said nozzle means connected to 
the supply conduit means for injecting the machining fluid into the 
workpiece opening; means in the supply conduit means for cooling the 
machining fluid in liquid phase; and means for pumping the fluid in the 
cooled liquid phase to feed it without a substantial temperature rise to 
the said nozzle means.

SPECIFIC DESCRIPTION 
Referring now to the drawing, there is shown a conventional traveling-wire 
EDM 1 arrangement incorporating the principles of the present invention 
embodied in various fashions. 
The conventional traveling-wire EDM arrangement makes use of a continuous 
wire electrode E which is stored on, say, a supply reel not shown and 
dispensed therefrom, typically at a continuous rate, for cutting a 
workpiece W by electroerosion. The wire electrode E typically is composed 
of a copper or brass and has a diameter or thickness ranging between 0.05 
and 1 mm. 
As shown in FIG. 1, the wire electrode E is linearly bridged between a pair 
of guide members 2, here each in the form of a roller, and axially 
transported through the workpiece W disposed therebetween. The wire 
electrode E is shown traveling vertically from up to down through the 
workpiece W and in the direction indicated although it may be transported 
in the opposite direction. A pair of conductor rollers 3 are shown, one of 
which is held in contact with the traveling wire electrode E between the 
upper guide member 2 and the upper surface of the workpiece W and the 
other of which is held in contact with the travelinfg wire electrode E 
between the lower surface of the workpiece W and the lower guide member 2. 
The conductive rollers 3 are electrically connected to one terminal of an 
EDM power supply 4 whose other terminal is electrically connected to the 
workpiece W. 
The wire electrode E leaving the lower guide member 2 is fed to a takeup 
reel or the like collection means (not shown). The wire electrode E is 
driven by a capstan and pinchroller unit (not shown) which is disposed 
between the collection means and the lower guide member 2 to establish a 
desired rate of travel of and a desired tension on the wire electrode E 
traveling through the workpiece W in conjunction with a braking unit (not 
shown) disposed between the supply reel and the upper guide member 2. The 
functions of the guide members 2 are to change the direction of wire 
travel at a right angle or so from the supply side to the machining zone 
and from the latter to the collection means, respectively, and to 
establish a linear traveling stretch of the wire electrode E across the 
workpiece W and through an opening or cut groove H therein. 
Disposed immediately adjacent to the workpiece W are a pair of nozzles 5 
and 6 fed with a machining fluid such as a water liquid to supply it into 
the opening H. The upper nozzle 5 is designed to create a downwardly 
directed stream of the machining fluid which is substantially coaxial with 
the traveling wire electrode E so as to be led into the opening H through 
the upper side of the workpiece W. Likewise the lower nozzle 6 is designed 
to create an upwardly directed stream of the machining fluid which is 
substantially coaxial with the traveling wire electrode E so as to be led 
into the opening H through the lower side of the workpiece W. 
A machining gap G is formed in the opening H between the traveling wire 
electrode E and the workpiece W. With the wire electrode E and the 
workpiece W energized by the EDM power supply 4, a succession of 
time-spaced electrical discharges are produced through the machining gap G 
between the traveling wire electrode E and the workpiece W to 
electroerosively remove material from the workpiece W. As material removal 
proceeds, a worktable (not shown) on which the workpiece W is securely 
mounted is driven by a drive control unit (not shown) to displace the 
workpiece W in a horizontal X-Y plane transversely to the traveling wire 
electrode E along a preprogrammed cutting path which determines a desired 
contour of cut to be imparted to the workpiece W. 
The nozzles 5 and 6 are securely supported in position by holders 7 and 8 
so that they are held adjacent to the upper and lower surfaces of the 
workpiece W, respectively and so as to be coaxial with the traveling wire 
electrode E. It will be apparent that the holders 7 and 8 may be secured 
to upper and lower arms (not shown) to which the upper and lower guide 
rollers 2 are secured in position, respectively. The nozzles 5 and 6 are 
fed with the machining fluid by inlet conduits 9 and 10, which are carried 
by the holders 7 and 8 and connected via valves 11 and 12, respectively, 
to a supply conduit 13 leading from a pump 14. In accordance with a 
feature of the present invention, a provision is incorporated as will be 
described to assure that the temperature of the machining fluid for 
delivery into the opening H or the machining gap G is not in excess of 
20.degree. C., preferably of 10.degree. C. or, more preferably, of 
4.degree. C., independently of the temperature of the environment in which 
the foregoing arrangement or the traveling-wire EDM machine is placed. 
The spent machining fluid away from the workpiece W is allowed to fall by 
gravity and collected by a pan 15. The spent machining liquid which 
contains machining products, i.e. sludges, chips and other impurities. is 
then led to a liquid-treatment system 16 which includes two reservoirs 17 
and 18. The first reservoir 17 is a sedimentation tank for receiving the 
spent machining liquid from the pan 15 to allow sludges and chips therein 
to be sedimented generally by gravity towards the bottom thereof. A 
clearer upper layer of the machining liquid in the first reservoir 17 is 
drawn by a pump 19 and passed through a filter 20 for reception in the 
second reservoir 18. 
The second reservoir 18 is designed to treat especially the machining 
liquid when constituted by a water liquid. The water liquid in the 
reservoir 18 is recycled by a pump 21 through an ion-exchange cartridge 22 
to control its specific conductivity or resistivity. The reservoir 18 is 
also equipped with a temperature-control or cooling unit 23 for 
sufficiently cooling the conductivity-adjusted water liquid therein. 
The conductivity-adjusted water liquid is drawn from the tank 12b by a pump 
24 and is thereby fed through an ultrafine filter 25 into a further 
reservoir or receptable 26. A conductivity (resistivity) detecting sensor 
27 is provided between the filter 25 and the receptacle 26 and is 
electrically connected to a control circuit 28 which is designed to 
control the operation of the pump 21 in response to a deviation of the 
conductivity from a predetermined value, thereby maintaining constant the 
conductivity or resistivity of the water liquid furnished to the 
receptacle 26. When a deviation of the conductivity or resistivity from 
such a predetermined value is detected by the sensor 27, the control 
circuit 28 is operated to actuate the pump 21 or modify the rate of drive 
of a motor for the pump to circulate the water liquid or to control the 
rate of circulation of the water liquid in the reservoir 18 through the 
ion-exchange cartridge 22 until the predetermined conductivity or 
resistivity of the water liquid is restored. 
The receptacle 26 is provided to temporarily store the purified and 
conductivity-adjusted water liquid therein and has a cooling coil 29 in 
contact with the stored water liquid. The cooling coil 29 is constituted 
by a conventional coiled heat-exchanger tubing having an outer wall in 
contact with the stored water-liquid and an inner passage traversed by a 
cooling medium such as ammonia or Freon. The cooling medium which is 
cooled by a refrigerator 30 is driven by a pump 31 to flow through the 
tubular passage of the coil 29 and is allowed to boil there to cool the 
water liquid in heat-exchanging relationship therewith. The receptacle 26 
has also a temperature sensor 32 immersed in the stored water liquid to 
provide an electrical output signal representing the temperature thereof. 
The output signal of the sensor 32 is furnished to a control circuit 33, 
which has a predetermined threshold value preset therein and is connected 
to act on one or both of the refrigerator 30 or the pump 31. Thus, when 
the temperature of the water liquid is detected by the sensor 32 to exceed 
a maximum temperature corresponding to the preset threshold value, thus 
generally 20.degree. C. and, in a preferred embodiment, 10.degree. C. or 
4.degree. C., the rate of flow of the cooling medium through the coil 29 
is controlled so as to hold the temperature of the stored water liquid not 
to exceed the preset temperature. The water liquid sufficiently cooled in 
this manner is drawn by the pump 14 and thereby fed into the workpiece 
opening H in the manner previously described. 
EXAMPLE I 
A steel workpiece composed of S55C JIS (Japanese Industrial Standard) and 
having a thickness of 25 mm is machined using a brass wire electrode of a 
diameter of 0.2 mm and a water machining liquid of a specific resistance 
in the range of 10.sup.4 ohm-cm while varying the temperature of the 
machining liquid supplied into the region of the workpiece and the 
traveling wire electrode. It has been found that the removal rate varies 
in inverse proportion to the temperature of the machining liquid as 
depicted in the graph of FIG. 4 in which the abscissa represents the 
temperature and the ordinate represents the removal rate. Thus, the 
removal rate which is 2 mm/min when the machining liquid has a temperature 
of 25.degree. C. is increased to 2.6 mm/min when the temperature is 
reduced to 10.degree. C. The removal rate is further increased to 3.1 
mm/min when the temperature is further reduced to 4.degree. C. immediately 
above the temperature at which the machining liquid is frozen. 
In the embodiment of FIG. 2 in which the same references as in FIG. 1 are 
used to designate the same parts, the machining liquid is subjected to 
cooling immediately prior to entry into the machining region. In this 
embodiment, each inlet conduit 9, 10 leading from the pump 14 is coupled 
with a cylinder or cylindrical collar 34, 35 constituting cooling means 
for the traveling wire electrode E. Thus, the cylinders 34 and 35 have 
their respective cylindrical inner passages 34a, 35a which are coaxial 
with each other and through which the wire electrode E is passed to 
traverse the workpiece W linearly between the wire guidance and support 
members 2. The machining liquid pumped through each inlet conduit 9, 10 is 
thus forced to flow and to be injected into the workpiece opening H in a 
stream coaxial with the traveling wire electrode E. The machining liquid 
when sufficiently cooled at the source side as described with reference to 
FIG. 1 therefore effectively cools the traveling wire E in the cutting 
zone. 
Furthermore, the embodiment of FIG. 2 is designed to cool the 
wire-electrode E via the coaxially flowing envelope of machining liquid by 
externally cooling the cylinders 34 and 35. Each cylinder 34, 35 thus has 
a plurality of thermoelements 36 attached thereto, each of which 
constitutes an electric cooling system utilizing the Peltier effect. When 
contacted dissimilar metals are traversed by electric current, there 
develop at the junctions generation and absorption of heat which are 
reversible, depending on the directions of the electric current. By 
arranging the heat absorbing portion in contact with the outer wall of the 
cylinder 34, 35 to absorb the heat of the machining fluid passing through 
the internal passage 34a, 35a, the machining fluid is cooled to cool the 
traveling wire electrode E as well. 
EXAMPLE II 
A steel workpiece composed of SK JIS (Japanese Industrial Standard) and 
having a thickness of 50 mm is electroerosively machined with a brass wire 
electrode having a diameter of 0.2 mm and axially traveling at a rate of 3 
m/min. A water machining liquid is supplied into the cutting zone at a 
volume flow rate of 5 liters/min and has a temperature at its source 
controlled to 10.degree. C., yielding a removal rate of 0.8 mm/min. The 
removal rate is increased to 3 mm/min in an arrangement generally as shown 
in FIG. 2 when the cylindrical collars 34, 35 are cooled at a temperature 
of 1.degree. C. 
FIG. 3 shows another embodiment of the invention in which an electric 
cooling system as described in connection with FIG. 2 is used to cool the 
wire electrode E independently of the cooled machining fluid (refer to 
FIG. 1), thereby sufficiently holding the temperature of the machining 
liquid lowered below a threshold point at a supply site, viz. the 
receptacle 26, as it is fed coaxially with the traveling wire electrode E 
into the cutting zone H. Thus, the wire electrode E passing over the 
upstream guide member 2a and the upstream electricity-conducting roller 3a 
is passed proximal to or in contact with the heat-absorbing portion 37A of 
thermoelement 37 as described, prior to entry into the upstream nozzle 5 
through which the cooled machining liquid from the source, viz. the 
receptacle 26, is injected into the opening H in the workpiece W. The heat 
of the traveling wire electrode E is sufficiently absorbed by the 
thermoelement 37 to sufficiently cool the wire electrode E led into the 
machining liquid nozzle 5. 
EXAMPLE III 
Example I is followed except that the wire electrode E is cooled by a 
thermoelement 37 at a portion of its travel path between the upstream 
guide member 2a and conducting roller 3a. It is found that the removal 
rate is increased to 2.9 mm/min and 33 mm/min when the machining liquid is 
reduced in temperature at the source (viz. the system 16 and or the 
receptacle 26) to 10.degree. C. and 4.degree. C., respectively. 
In addition, it should be noted that a water liquid may advantageously be 
supplied over or into a cavity in, the heat-absorption portion 37A of the 
thermoelement 37 so as to be partially frozen there to enhance the cooling 
of the wire electrode E. 
It will be apparent that the present invention enables the traveling-wire 
EDM removal rate to be increased by nearly or even more than 50% over the 
conventional system. By limiting the machining liquid, especially water 
liquid, in temperature to a lower value, it is found that its specific 
resistivity can be substantially held to a fixed level favorable for EDM 
electroerosion. Since this allows the essential machining gap spacing to 
be effectively fixed at a constant value, the machining accuracy can be 
largely improved as well. 
In FIG. 5 there is shown a further embodiment of the invention 
incorporating an improved machining fluid flushing and cooling 
arrangement. The arrangement includes a pair of nozzle assemblies 38 and 
39, each of which comprises respectively a wire passage 38a, 39a coaxial 
with the wire electrode E, a fluid inlet 38b, 39b connected to a source of 
a water liquid (see FIG. 1), a nozzle chamber 38c, 39c open to the opening 
H in the workpiece W and an end flange 38d, 39 d extending radially about 
the end opening 38c', 39c' of the nozzle chamber 38c, 39c. The nozzle 
assemblies 38 and 39 may be secured to upper and lower arms (not shown) 
extending in parallel with one another from a vertical column (not shown) 
of a conventional traveling-wire EDM machine. Each fluid inlet 38b, 39b is 
fed with the water liquid to inject it through the nozzle chamber 38c, 39c 
into the workpiece opening H under an elevated pressure of 1 kg/cm.sup.2 
or more. Each flange 38d, 39d has an area several times greater than the 
cross-sectional area of the nozzle opening 38c', 39c' and closely adjacent 
to the workpiece W with a small spacing, say 2 mm or preferably 1 mm or 
less. By virtue of the provision of the flange portion 38c, 39c mentioned 
which substantially restains the supplied water liquid from escaping 
radially over the workpiece W, the improved nozzle assembly 38, 39 ensures 
a highly smoothed, effective and efficiency-enhanced delivery and renewal 
of the workpiece opening H with the supplied water machining liquid while 
maintaining the desirable injection pressure thereof. 
The wire passage 38a, 39a in each nozzle assembly 38, 39 should be 
sufficiently elongated and narrow just to allow the wire electrode to be 
smoothly passed and to limit the machining liquid against leaking 
therethrough. Optionally, a seal member adapted to slidably accept the 
wire electrode therethrough may be plugged in each wire passage 38a, 39a. 
Each flange member 38d, 39d may, as shown, be formed with grooves 38e, 39e 
thereon proximal to the workpiece W, which grooves are preferably spiral 
or labyrinthine to provide turbulence in the flow of the machining liquid 
tending to escape radially outwardly through the narrow spacing between 
the flange 38d, 39d and the workpiece W. By virtue of the formation of 
such, the machining liquid tending to more radially outwards is markedly 
limited. Each nozzle assembly 38, 39 may be composed of an electrically 
nonconductive material such as a synthetic resin. The flange 38d, 39d 
formed with the grooves 38e, 39e may be composed of a rubber. In this 
manner, each assembly 38, 39 can be held sufficiently close to the 
workpiece W to achieve the machining liquid delivery with an enhanced 
effectiveness, smoothness and efficiency. For example, a workpiece having 
thickness of 300 mm can be machined with a wire electrode having a 
diameter of 0.2 mm. It has been found that where the machining arrangement 
is, as is conventional, devoid of flanges 38d, 39d, the wire electrode is 
broken when the average machining current exceeds 9.2 amperes. When, 
however, the nozzle assemblies 38, 39 are each provided with a flange 38d, 
39d as shown and having a diameter of 40 mm and formed with spiral or 
labyrinthine grooves 38e, 39f, there occurs no breakage of the wire 
electrode when the machining current is increased to as high as 12 
amperes. The machining liquid is effectively injected into the workpiece 
opening H under a pressure of 2 kg/cm.sup.2. 
It is desirable that cooling means be provided to cool the machining liquid 
immediately prior to entry into the workpiece opening H and further to 
cool the wire electrode E at a site immediately upstream thereof for the 
reasons previously noted with reference to FIGS. 2 and 3. To this end, 
each nozzle assembly 38, 39, possibly except the flange 38d, 39d and the 
inlet conduit 38b, 39b is composed of a metal and has the tubular passages 
38a, 39a and the nozzle chamber 38c, 39c provided with the respective 
heat-absorbing portions 40a, 41a, 42a, 43a of thermoelements 40, 41; and 
42, 43, respectively. In addition, these thermoelements have their 
respective heat-emitting ends 40b, 41b, 42b and 43b, respectively, which 
are cooled by a coolant passing through a cooling conduit 44. 
Each nozzle assembly 38, 39 is also shown provided with a further fluid 
inlet 38f, 39f which is narrower in cross section than and coaxial with 
the first fluid inlet 38b, 39b and the nozzle chamber 38c, 39c. Each 
second fluid inlet 38f, 39f is arranged to terminate and to be open 
immediately ahead of the workpiece opening H in the nozzle chamber 38c, 
39c. It is desirable that the second fluid inlet 38f, 39f be supplied with 
a cooled hydrocarbon machining liquid F2 such as kerosene and the first 
fluid inlet 38b, 39b be supplied with the water liquid F1. The two liquids 
F1 and F2 are simultaneously supplied to the machining system in this 
manner whereby the second liquid F2 is at least predominantly injected 
through the second inlet conduit 38f into the workpiece opening H and the 
first liquid F1 is supplied as an auxiliary machining liquid for admixture 
with the primary, hydrocarbon machining liquid F2 or as an auxiliary 
working fluid exclusively functioned to cool and curtain the workpiece W 
and the wire electrode E. For the latter purpose, the water liquid F1 acts 
in the region between the open end of the second conduit 38f, 39f and the 
workpiece opening H as an envelope fluid to enclose the hydrocarbon liquid 
and is thereby forced to flow into regions other than the machining gap in 
the workpiece opening H and elsewhere outside the workpiece W. 
In a further embodiment of the invention illustrated in FIG. 6, a wire 
electrode E unwound from a supply reel 45 is passed through a refrigerant 
46 such as liquefied nitrogen retained in a container 47. The wire 
electrode E fed from the supply reel 45 enters into the refrigerant 46 in 
the container 47 and is guided over a pair of guide rollers 48 and 49 
located therein. The wire electrode E is thereby cooled to a temperature 
lower than the freezing point of a water liquid. The wire electride E 
leaving the refrigerant 46 is then passed over the lower machining guide 
member 2a and a lower electricity-conducting brush 3a and fed through a 
lower fluid supply nozzle 50, a workpiece opening H, an upper 
electricity-conducting brush 3b and an upper machining guide member 2b, 
and eventually taken up into collection means (not shown). The lower 
nozzle 50 has a nozzle chamber and opening shown to be coaxial with the 
traveling wire electrode E as in the FIG. 1 arrangement. An upper nozzle 
51 is shown to be a nozzle unit disposed by the traveling wire electrode E 
and trained towards the workpiece opening H but may be of the same type as 
the lower nozzle unit 50. Both nozzle units 50 and 51 are fed with a water 
liquid sufficiently cooled at its source to be a temperature below a 
predetermined level as described previously. 
Since the wire electrode E is cooled below the freezing point of the water 
liquid prior to entry into the lower nozzle unit 50, it follows that an 
ice layer of the water develops on the surface of the wire electrode E 
passing out of the nozzle opening of the lower nozzle unit 50, that is a 
layer of water completely frozen or partially iced, e.g. in the form of 
sleet or snow, depending upon the particular rate of travel of the wire 
electrode and the particular reduced temperature of the supplied water 
liquid contacting the cooled wire electrode in the nozzle chamber. Since 
the wire electrode E fed into the cutting zone H is cooled enough, its 
heat capacity is markedly enhanced to absorb the machining heat there. In 
addition, the ice layer provides a temporary and/or localized protection 
against any possible damage from electrical discharges and mechanical 
damage, hence giving rise to an increased machining stability and removal 
rate. The discharge repetition or average machining current case be 
increased. The wire-electrode durability against breakage can be improved. 
It will be apparent that various modifications of the arrangement shown in 
FIG. 6 are possible. For example, the cooling means for the wire may not 
be limited to the use of a refrigerant such as nitrogen liquid. The wire 
reel itself may be cooled to around 0.degree. C. or less. The wire 
electrode may, immediately prior to entry into the upstream nozzle unit 
50, be passed through liquified carbon dioxide retained in a casing. 
EXAMPLE IV 
A steel workpiece composed of S55C JIS (Japanese Industrial Standard) and 
having a thickness of 25 mm is machined in an arrangement as generally 
shown in FIG. 6, using a water liquid having a specific resistivity of 
5.times.10.sup.4 ohm-cm and using a wire-electrode composed of brass and a 
diameter of 0.2 mm. The wire electrode is fed to axially travel at a rate 
of 3 m/min. The water liquid cooled to 5.degree. C. at its source is 
allowed to be injected into the workpiece opening from the upper nozzle 
under a pressure of 0.3 kg/cm.sup.2 and from the lower nozzle under a 
pressure of 1 kg/cm.sup.2. The upper nozzle has its nozzle opening spaced 
by a distance of 3 mm from the upper surface of the workpiece while the 
lower nozzle has its nozzle opening spaced by a distance of 2 mm from the 
lower surface of the workpiece. With the wire electrode of a room 
temperature passed through the arrangement described, the removal rate is 
observed to be at maximum 2.0 mm/min. When, however, the wire electrode 
is passed through liquid nitrogen immediately prior to its passage into 
the lower nozzle, it is observed that an ice layer of water having a 
thickness of 0.045 mm develops on the surface of the wire electrode 
leaving the same nozzle and the removal rate is found to be increased to 
2.5 mm/min. 
In a modification of the arrangement of FIG. 6 shown in FIG. 7, the wire 
electrode E is guided through an ice guide member 52 provided immediately 
upstream of the upstream nozzle unit 50 and optionally also through an 
additional ice guide member 53 provided immediately downstream of the 
downstream nozzle unit 51. The ice guide members 52 and 53 are held by ice 
forming and retention members 54 and 55, respectively, which are connected 
to the cooling poles of thermoelements 56 and 57, respectively, which have 
their respective radiating fins 58 and 59 disposed in a conduit 60 in 
which a coolant 61 flows. Each thermoelement 56, 57 in which the 
heat-radiating fin 58, 59 is cooled by the coolant 61 is the arrangement 
that as shown in FIG. 8 the radiating fin 58, 59 and the cooling pole 
plate 62, 63 are connected by a semiconductor 64 (N-type), 65 (P-type). 
With an electric current passed through the latter, a heat-absorbing and 
cooling action is created at the plate 62, 63 and hence at the retention 
member 54, 55. In this manner, highly effective and efficient pre-cooling 
of the wire electrode E is achieved by the ice wire-supporting and 
guidance members of reduced temperature to enhance the EDM removal rate 
and to improve the EDM cutting performance while minimizing the possiblity 
of wire breakage. The water liquid containing a large number of ice 
particles or fragments and having a temperature slightly more or less than 
0.degree. C. is injected into the workpiece opening H from the nozzles 50 
and 51. Designated at 66 and 67 (FIG. 7) are nozzles for replenishing a 
water liquid into the retention members 54 and 55, respectively, where the 
supplied water liquid is being frozen. 
In FIG. 9, the wire electrode E is shown as linearly bridging between the 
upstream and downstream machining guide members 2a and 2b and as traveling 
vertically through the workpiece opening H from down to up as in FIGS. 6 
and 7. It should be noted, however, that the opposite direction of travel 
of the wire electrode may be employed and is often preferable especially 
with an arrangement which will now be described. In this embodiment as 
well, a nozzle chamber 70 has a nozzle opening 71 coaxial with the 
traveling wire electrode and trained into the working opening H. The 
nozzle chamber 70 has an inlet conduit 72 adapted to be fed with a fluid, 
e.g. water liquid 73 which has been sufficiently cooled at its source as 
described hereinbefore. Alternatively the fluid 73 may be a refrigerant 
gas. In this embodiment, however, there is further provided means for 
dispensing fine particles of ice 74 into the fluid 73. This means 
comprises an ice forming chamber 76 fed with a water liquid 75 through an 
inlet conduit 77 and opening into the water-liquid conduit 72. The chamber 
76 is cooled by a thermoelement including a coupling 78 to freeze 
completely or partially the water liquid 75 supplied through the inlet 
conduit 77. The frozen water or ice 74 is ground by a grinding member 79 
in the form of a roller having a multiplicity of grinding edges 89 and 
driven by a motor 89 via a drive shaft 83. Particulate ice is thus 
produced by the grinding edges and is mixed into the water liquid from its 
source to form a sleet thereof or a mixture 84. 
EXAMPLE V 
A water liquid having a specific resistivity of 10.sup.5 ohm-cm is ground 
by a grinder in an arrangement as generally shown in FIG. 9 to form ice 
particles, which are mixed at a proportion of 1/1 in and entrained on a 
flow of frozen nitrogen gas to form a mixture of a gas with the gas 
containing these particles. Thus gas/ice mixture fluid is injected into 
the workpiece opening H coaxially with the traveling wire electrode E. 
This has been found to increase the average machining current which is 
limited to 5 to 6 amperes simply with such a water machining liquid to 
about 10 amperes. 
EXAMPLE VI 
A machining fluid consisting of a distilled water liquid having a specific 
resistivity of 10.sup.5 ohm-cm containing sorbtol is cooled below a 
freezing point thereof ahead of an upstream nozzle as in the embodiment of 
FIG. 9 and injected therethrough into the cutting zone coaxially with a 
traveling wire electrode composed of 65% Cu and 35% Zn having a diameter 
of 0.2 mm. It is found that no breakage of the wire electrode occurs with 
the average machining current increased to 13 amperes. 
EXAMPLE VII 
A water machining liquid having a specific resistivity of 10.sup.5 ohm-cm 
is, after and without cooling below its freezing point, supplied onto the 
brass wire electrode of 0.25 mm diameter traveling into a cutting zone 
defined thereby with a S55C (Japanese Industrial Standard) workpiece of 
100 mm thickness at a rate of travel of 4 meters/min. In each case, the 
minimum volume flow rate of the machining fluid required to avoid wire 
breakage and the resultant EDM removal rate are measured, yielding the 
following table: 
______________________________________ 
Minimium 
Volume 
Machining Fluid 
Flow Rate Removal Rate 
______________________________________ 
Distilled Water 
10 l/min 92 mm.sup.2 /min 
Not Cooled 
Distilled Water 
50 cc/min 140 mm.sup.2 /min 
Cooled to Form 
Sleet-Like 
Fluid 
______________________________________ 
It will be apparent that according to the embodiment of the invention, only 
a considerably reduced amount of the water liquid is required, yet to 
yield a largely increased removal rate. In addition, adverse wire 
vibration is substantially reduced, thus improving the cutting accuracy in 
the finish range.