Method of and apparatus for electroerosively wire-cutting a conductive workpiece

An electroerosive contouring wire-cutting apparatus using a distilled or deionized water machining medium of a specific resistivity controlled to be in a range between 10.sup.2 and 10.sup.5 ohm-cm, and having abrasive particles of TiC, TiN, B.sub.4 C, BN, SiC, Al.sub.2 O.sub.3 and/or SiO.sub.2 of for example a particle size in the order of microns for delivery by the medium to the machining gap. A high-frequency vibration of a frequency of 1 kHz to 1 MHz is imparted to the wire electrode traveling through the workpiece in a direction transverse to the wire axis to enhance the combined electroerosive and abrasive actions at the machining gap. The machining medium is delivered by a nozzle, retrieved at a collector, and separated into its liquid and solid components in a separator. After deionizing the liquid and extracting the machining products, the abrasive particles and deionized water are mixed and recirculated through the nozzle.

FIELD OF THE INVENTION 
The present invention relates to an apparatus for electroerosively 
wire-cutting an electrically conductive workpiece to form a desired 
contour therein wherein a water medium is continuously supplied into a 
machining gap defined between the workpiece and a wire electrode bridged 
under tension across supply and takeup sides and axially displaced 
continuously to travel between a pair of machining guide members while 
traversing the workpiece. A succession of electrical pulses are applied 
between the traveling wire electrode and the workpiece across the 
machining gap flushing with the water medium to produce time-spaced 
electrical discharges through the water medium, thereby electroerosively 
removing material from the workpiece. As material removal proceeds, the 
workpiece is displaced relative to the traveling wire electrode 
transversely to the axis thereof along a prescribed path to form the 
desired contour in the workpiece. The invention particularly relates to an 
improvement in the electroerosive wire-cutting apparatus of the type 
described. 
BACKGROUND OF THE INVENTION 
In the art of electroerosive wire-cutting defined above, a thin continuous 
wire or flamentary electrode is commonly employed which has a thickness as 
small as 0.005 to 0.5 mm. In addition, an extremely small gap must be 
formed between the workpiece and the traveling wire electrode. This 
condition unavoidably imposes a restriction on the desired smooth and 
sufficient passage of the water medium through the machining site. Thus, 
only a small fraction of the water medium supplied to the region of the 
workpiece juxtaposed with the traveling wire electrode is actually allowed 
to enter and flow through the machining gap at a limited flow rate. 
Difficulty therefore arises for machining chips and gases produced at 
discharge sites to be carried away smoothly. As a result, arcing and 
short-circuiting tend to develop between the workpiece and the wire 
electrode to disturb the progress of cutting and often causes breakage of 
the wire electrode and impairs the cutting stability. 
OBJECT OF THE INVENTION 
It is accordingly an important object of the invention to provide an 
improved apparatus for electroerosively wire-cutting an electrically 
conductive workpiece in the manner described, which permits the workpiece 
to be machined with increased stability and efficiency, and with less 
tendency towards the wire breakage. 
Other objects will become apparent as the description which follows 
proceeds. 
SUMMARY OF THE INVENTION 
The invention is directed to the electroerosively wire-cutting of an 
electrically conductive workpiece to form a desired contour therein, in 
which method a water medium is continuously supplied to a machining gap 
defined between the workpiece and a wire electrode bridged under tension 
across supply and takeup sides and axially displaced continuously to 
travel between a pair of machining guide members while traversing the 
workpiece, a succession of electrical pulses are applied between the 
traveling wire electrode and the workpiece across the machining gap 
flushed with the water medium to produce time-spaced electrical discharges 
through the water medium, thereby electroerosively removing material from 
the workpiece, and the workpiece is displaced relative to the traveling 
wire electrode transversely to the axis thereof along a prescribed path to 
form the desired contour in the workpiece. In this method there are 
included the steps of controlling the specific resistivity of the water 
medium to be in a range between 10.sup.2 and 10.sup.5 ohm-cm and 
introducing abrasive particles into the water medium supplied to the 
machining gap. 
In accordance with a further important feature of the present invention, a 
vibration of a frequency in the range between 100 Hz and 1 MHz, preferably 
not less then 1 kHz and with an amplitude between 1 and 50 microns, 
preferably not greater than 10 microns, is imparted intermediate the said 
machining guide members to the traveling wire electrode in a direction 
transverse to the axis thereof so that the traveling wire electrode 
acquires an undulating oscillatory motion along the axis with more than 
two nodes and antinodes or loops with the amplitude at most smaller than 
the size of the machining gap in the said direction. 
The vibration may be imparted to the traveling wire electrode by disposing 
an electrochemical transducer energized by a high-frequency power supply 
and disposed in a contacting relationship with a stretch of the wire 
electrode supported between the guide members. A pair of such transducers 
may be disposed one on one side of the workpiece and the other on the 
other side of the workpiece so that the two vibrations, preferably with 
different frequencies or modes, are superimposed upon each other as 
applied to the traveling wire electrode. The electromechanical transducer 
or each of the electromechanical transducers may be in the form of a disk 
and the wire electrode may be passed through the disk in the region of a 
center thereof in the direction of its thickness. 
Thus the apparatus for electroerosively wire-cutting an electrically 
conductive workpiece to form a desired contour therein, comprises means 
for continuously supplying a water medium into a machining gap defined 
between the workpiece and a wire electrode supported under tension across 
supply and takeup sides and axially displaced continuously to travel 
between a pair of machining guide members while traversing the workpiece, 
power supply means for applying a succession of electrical pulses across 
the machining gap flushed with the water medium between the workpiece and 
the wire electrode to produce time-spaced electrical discharges through 
the water medium, thereby electroerosively removing material from the 
workpiece and contouring feed means for displacing the workpiece relative 
to the traveling wire electrode transverse to the axis thereof along a 
predetermined path to form the desired contour in the workpiece. The 
apparatus also includes ion-exchange means for controlling the specific 
resistivity of the water medium for delivery to the machining gap by the 
said supply means to be in a range between 10.sup.2 and 10.sup.5 ohm-cm 
and means for introducing abrasive particles into the 
resistivity-controlled water medium for delivery to the machining gap. 
In accordance with a further feature of the apparatus aspect of the 
invention, means is provided for imparting a vibration of a frequency in 
the range between 100 Hz and 1 MHz, preferably not less than 1 kHz an with 
an amplitude between 1 and 50 microns, preferably not greater than 10 
microns intermediate the machining guide members to the traveling wire 
electrode in a direction transverse to the axis thereof so that the 
traveling wire electrode acquires an undulating oscillatory motion along 
the axis with more than two nodes and antinodes or loops with the 
amplitude at most smaller than the size of the machining gap in the said 
direction. 
The vibration means may include an electromechanical transducer energized 
by a high-frequency power supply and disposed in a contacting relationship 
with a stretch of the wire electrode supported between the guide members. 
A pair of such transducers may be disposed one on one side of the 
workpiece and the other on the other side of the workpiece so that the two 
vibrations, preferably with different frequencies or modes, are 
superimposed upon each other as applied to the traveling wire electrode. 
The two vibrations effectively create in the traveling wire a beat or the 
periodic vibrations in amplitude of a wave that is the superimposition of 
the corresponding two simple harmonic waves of the different frequencies. 
The electromechanical transducer or each of the two electromechanical 
transducers may be in the form of a disk and the wire electrode may be 
passed through the disk in the region of a center thereof in the direction 
of its thickness. 
BRIEF DESCRIPTION OF THE DRAWING 
These and other objects, features and advantages of the present invention 
will become more readily apparent from the following description of 
certain embodiments thereof as taken with reference to the accompanying 
drawing in which:

SPECIFIC DESCRIPTION 
Referring to FIG. 1, an electroerosive wire-cutting arrangement according 
to the invention includes a wire electrode 1 composed of, e.g., copper or 
brass and having a diameter of 0.005 to 0.5 mm, preferably not greater 
than 0.1 mm. The wire electrode 1 is axially advanced from its supply side 
shown in the form of a supply reel 2 to its takeup side shown in the form 
of a takeup reel 3 continuously through a cutting zone defined between a 
pair of machining guide members 4 and 5. A workpiece 6 is disposed in the 
cutting zone and traversed by a linear stretch of the wire electrode 1 
tightly bridged across and continuouslly traveling between the machining 
guide members 4 and 5. Further guide means 7 and 8 are provided in the 
path of wire travel to change the direction of advance of the wire 
electrode 1 from the supply side 2 to the cutting zone and from the latter 
to the takeup side 3, respectively. The axial displacement of the wire 
electrode 1 at an appropriate rate and under an appropriate tension may be 
effected by drive means disposed between the guide 8 and the takeup reel 3 
and brake means disposed between the supply reel 2 and the guide 7. 
In the cutting zone, a mixture of a water machining medium 16 and abrasive 
particles 17 is continuously supplied from a nozzle 9 into a machining gap 
G formed between the workpiece 6 and the traveling wire electrode 1. An 
EDM (electrical discharge machining) power supply 10 is electrically 
connected on one hand to the workpiece 6 and on the other hand to the wire 
electrode 1 via a brush 11 to apply a succession of EDM pulses across the 
machining gap G through the water medium to electroerosively remove 
material from the workpiece 6. The water medium 16 should, for the 
purposes of the invention, be of a specific resistivity in the range 
between 10.sup.2 and 10.sup.5 ohm-cm. 
The workpiece 6 is securely mounted on a worktable 12 and a contour-feed 
drive system for displacing the workpiece 6 relative to the wire electrode 
1 transversely to the axis thereof or in an X-Y plane includes a first 
motor 13 for feeding the worktable 12 along the X-axis and a second motor 
14 for feeding the worktable 12 along the Y-axis. A numerical controller 
15 is provided having data for a prescribed contour-feed path 
preprogrammed therein. The data are reproduced and the corresponding drive 
signals are furnished from the numerical controller 15 to the motors 13 
and 14 to displace the workpiece 6 relative to the wire electrode 1 along 
the prescribed path so that a desired contour is machined in the 
workpiece. 
The abrasive particles 17 should preferably be composed of TiC, TiN, TiB, 
TiB.sub.2, (TiB.sub.2)C, (TiB.sub.2)N, HfC, HfB.sub.2, TiCN, TiHfC, 
B.sub.4 C, BN, SiC, Al.sub.2 O.sub.3 and/or SiO.sub.2 and of a particle 
size in the order of microns, preferably between 1 and 100 .mu.m.phi. and 
still favorably not greater than 50 .mu.m.phi.. These particles supplied 
in suspension with the water medium 16 are forced to enter into the gap 
spacing G and there act to abrade the workpiece contour surface being 
eroded by electrical discharges as they are carried by the traveling wire 
electrode 1. Thus, the mechanical abrading action is added to the 
electroerosive action, giving rise to a marked increase in material 
removal from the workpiece 6. The abrasive particles dynamically moving 
through the machining gap G also act to mechanically carry away the 
machining chips and other gap products formed by the electroerosive action 
and at the same time serve as a spacer between the traveling wire 
electrode 1 and the workpiece 6 to prevent them from direct contact or 
short-circuiting The results are a marked increase in the cutting 
efficiency and performance, and an increased operational stability 
practically without arcing and with less intensity towards wire breakage. 
In addition or optionally, a vibrator unit 18 comprising an 
electromechanical transducer assembly 19 energized by a high-frequency 
power supply 20 is provided. The assembly 19 is shown having an amplifying 
horn portion whose tip is disposed in a contacting relationship with the 
wire electrode 1 traveling through the cutting zone defined between the 
guide members 4 and 5. The transducer 19 is energized by the power supply 
20 having a frequency not less than 100 Hz, preferably not less than 1 kHz 
and up to 1 MHz to impart to the traveling wire 1 stretch between the 
guide means 4 and 5, a vibration of an amplitude preferably between 1 and 
50 microns and still more preferably between 1 and 10 microns. The 
vibration is imparted to the wire electrode in a direction transverse to 
the axis thereof so that an undulating oscillatory motion with more than 
two nodes and antinodes or loops is provided in the wire 1 traveling 
between the guide members 4 and 5 positioned at opposite sides with 
respect to the workpiece 6. This vibration creates a highly faborable 
pumping action for the mixture of the water medium 16 and the abrasive 
particles 17 which can thus be entrained on the traveling wire electrode 1 
into the machining site G at an increased volume rate of flow. Since the 
water medium 16 and the abrasive particles 17 are thus carried into the 
machining site G at an increased smoothness, a still further increase in 
the material-removal rate is achieved. The pumping action also serves to 
carry away the machining and other gap products at an increased smoothness 
from the gap site G to assure continuation of the steady machining 
operation. 
The mixture of water medium and abrasive particles leaving from the 
workpiece 6 with the traveling wire 1 is directed with a stream of clean 
water medium supplied from a nozzle 21 disposed below the workpiece 6 onto 
a trough 22. The mixture with the clean water medium is introduced in a 
centrifugal separator 23 where it is separated into a liquid (water 
medium) and solid particles (abrasive particles and machining chips). The 
liquid is led into a vessel 24 for temporary storage therein and then 
forced by a pump 25 to flow into a conduit 26 leading to the supply and 
dispensing nozzle 9. The solid particles from the centrifugal separator 23 
are led on a moving endless belt 27 which has a magnetic separator 28 
disposed along its path to magnetically collect the machining chips. The 
abrasive particles 17 separated from the machining chips continue to be 
carried on the moving belt 27 and are collected into a funnel 29 and then 
are led to a mixing chamber 30 provided at a portion of the conduit 26. At 
the mixing chamber 30 the clean abrasive particles 17 and thus 
homogeneously mixed with the clean water medium 16 pumped from the vessel 
24, the mixture being led via the conduit 26 to the nozzle 9 for delivery 
to the machining gap G. In the vessel 24 or at a portion of the conduit 26 
upstream of the mixing chamber 30 there is provided an ion-exchange unit 
(not shown) for controlling the electrical specific resistivity of the 
water medium 16 to be in a range between 10.sup.2 amd 10.sup.5 ohm-cm. 
EXAMPLE 
A copper wire electrode of 0.02 mm.phi. is mounted in an arrangement 
generally of the type shown in FIG. 1 and displaced axially at a rate of 
travel of 2 m/min for electroerosively wire-cutting a workpiece composed 
of S55C material having a thickness of 11 mm. The water medium is tap 
water treated by an ion-exchanger to have a specific resistivity of 
5.times.10.sup.4 ohm-cm. The electroerosive machining pulses have an 
on-time .tau.on of 10 microseconds, an off-time .tau.off of 15 
microseconds and a peak current of 56 amperes. Abrasive particles as 
suspended in the water medium are composed of SiC and have particle sizes 
of 600 meshes. They are mixed at a proportion of 15% by volume with the 
water medium. A vibration of 35 kHz is applied to the traveling wire 
electrode. Removal rates obtained for cases, viz. (A) without the abrasive 
particles and without wire vibration, (B) with abrasive particles and (C) 
with the abrasive particles and wire vibration are shown in the following 
table: 
TABLE 1. 
______________________________________ 
Case Removal Rate 
______________________________________ 
A 0.9 mm/min 
B 1.6 
C 2.1 
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It is seen that an increase in removal rate amounting to or even more than 
two times is obtained according to the present invention. It should also 
be noted that with C, the contouring feed rate can be increased up to 30% 
compared with B. It is therefore apparent that the total machining time 
with the present invention is reduced to about or less than one half that 
with the prior art. 
Instead of a single vibrator unit 19 as shown, two such vibrators may be 
provided, one on one side of the workpiece 6 and the other on the other 
side of the workpiece 6; they are preferably energized with different 
frequencies so that two resulting vibrations are superimposed upon one 
another to create a composite undulating oscillatory motion with more than 
two nodes and antinodes or loops in the wire 1. 
FIG. 2 shows a modified vibration system in an electroerosive wire-cutting 
arrangement in which some of the parts or elements the same as those in 
FIG. 1 are referred to by the same reference numerals and some are omitted 
to avoid duplication. The vibration system shown in FIG. 2 makes use of 
one or two disk-shaped electromechanical transducers 31 and 32, instead of 
a horn structure as shown in FIG. 1, the transducers 31 and 32 being 
energized by a common high-frequency power supply 33. The transducers 31 
and 32 each comprise a disk formed with a central opening 31a (32a) or 
with a slit 31b (32b) exterding between the center and the periphery 
thereof as shown in FIGS. 3A and 3B, respectively, which serves as a 
passageway for the traveling wire electrode 1. When each of the disk 
transducers 31 and 32 is energized by the high-frequency power supply 33, 
a high-frequency mechanical vibration is generated therein and imparted to 
the traveling wire electrode 1. Since each teransducer 31, 32 is disposed 
in a contacting relationship with the wire stretch 1a between the 
machining guide members 4 and 5, an undulatory oscillatory motion develops 
in the wire stretch 1a which thus acquires an external mechanical 
vibration in a direction transverse to the axis thereof in the manner 
previously described. 
Each of the transducers 31 and 32 may, as in the assembly 19 of FIG. 1, be 
composed of quarz, lithium tantanate, barium titanate, lead 
zircon-titanate or the like known transducer material and is energized to 
produce a high-frequency mechanical vibration in the direction of its 
radius. The disk form of vibrator is particularly advantageous in that it 
can be mounted in close proximity to the workpiece 6 and hence the 
machining site. 
FIG. 4 is a graph showing experimental results wherein the increase in 
cutting efficiency is plotted along the ordinate and the distance of the 
location of an electromechanical transducer away from the workpiece 6 is 
plotted along the abscissa. The graph shows that a vibration frequency in 
excess of 100 kHz is preferred and is advantageous to achieve an increase 
in the cutting efficiency with the greater distance but a shorter distance 
is preferred generally to attain a greater increase in the cutting 
efficiency. 
In a further embodiment of the invention shown in FIG. 5, each of the disk 
vibrators 31 and 32 traversed by the traveling wire electrode 1 is 
received in a plenum chamber 34, 35 supplied with a mixture of the water 
medium and abrasive particles already described. Each of the chamber 34 
and 35 has an inlet 34a (35a) leading to the conduit 26 shown in FIG. 1 
and an outlet 34b (35b) disposed in close proximity to the working region 
for delivering the water/abrasive mixture into the machining gap. The 
water medium in the mixture here also effectively serves to cool the 
vibrator body 31, 32 thereby assuring the operational stability of these 
units. 
There is thus provided an improved method of and apparatus for 
electroerosively wire-cutting an electrically conductive workpiece whereby 
a marked enhancement in the machining performance, efficiency and 
operational stability is achieved.