WO2000005024A1

WO2000005024A1 - Wirecut electric discharge machining method and apparatus

Info

Publication number: WO2000005024A1
Application number: PCT/JP1998/003306
Authority: WO
Inventors: Kiyoshi Inoue
Original assignee: Inoue K
Priority date: 1998-07-24
Filing date: 1998-07-24
Publication date: 2000-02-03
Also published as: AU8357498A

Abstract

A high speed machining process and equipment is disclosed. Machining is carried out mainly by evaporation and pulse duration is controlled to below one micro-sec. Various techniques of reducing system impedance and reducing wire electrode vibrations are disclosed including changes in the pulse generator circuit construction, the lead wire, wire guides and current pick-ups for wire electrode and work piece. This reduces debris size and permits good surface finish of work piece. A novel process information controlled power supply circuit is also disclosed in which the energy requirement, at any instant, of load is estimated and only the estimated energy is supplied, the balance and residual energy being stored in energy storing means. This makes the process very efficient. A shield of novel construction is also provided which absorbs the electro-magnetic disturbance waves.

Description

00/05024 DESCRIPTION PCT/JP98/03306

IRECUT ELECTRIC DISCHARGE MACHINING METHOD AND APPARATUS

FIELD OF INVENTION :

This invention relates to the field of Electric discharge machining, (EDM) particularly travelling wire EDM (W-EDM) machining process particularly where a good surface finish of work piece, high quality of machined surface and safe, high speed machining over uninterrupted extended periods is required with efficient use of power supplied to the machining process.

Further this invention also relates to current pick-up elements and wire guides for W-EDMε, power supply circuits for machining equipment using electrical energy directly with fast response, low loss power supply, such as W-EDM, ram type EDM, means to prevent electromagnetic disturbances (EMD) emanating from EDM equipment, particularly, W-EDM and control means for power supply and servo feed.

BACKGROUND OF THE INVENTION : PRIOR ART : In W-EDM of the prior art, typically, for machining of an iron work piece, maximum machining speed achieved is approximately 300 mm²/min which corresponds to a maximum peak value of current pulse of 500 A. The maximum cutting speed of a W-EDM depends upon the maximum peak value of the current pulse. This peak value can be increased only by increasing the pulse duration because the current pulse wave form is triangular due to inductance of lead wire which is greater than 1 micro-H/metre. Prior art pulse generators are able to achieve a maximum peak value of current pulse even upto 950 A max which is above 500 A by increasing the current pulse or discharge duration beyond 1 micro-sec, typically in the range of 1.2 to 2 micro-sees. The limitation of prior art is that for achieving high machining speeds the pulse interval is required to be controlled to be as small as possible so that arc discharge is not generated during discharge interval or quiescent time.

In case of repeated discharges, machining can be considered as the aggregate of a series of machinings by each of the discharges. In this case, even by increasing the discharge initiation voltage, maximum discharge current of 700 to 1000 A is not possible with the discharge duration below 1 micro-sec. Hitherto, machining speed is increased by increasing the discharge repetition frequency by shortening the discharge quiescent time.

Since higher machining peak current is achieved by increasing pulse duration, there is low discharge power density resulting in the occurrence of machining particle size distribution of larger than 1 to 2 micro-m diameter, which further results in short circuit occurring many a time and gradually the machining process shifts to thermal machining, and high speed becomes a difficult task. Again, for high speed machining, it is necessary to pass high current, but wire electrode tends to break when high current is applied, thus discharge of high current can only be used for an extremely short duration. Especially, in the case of large size work pieces, inductance also becomes very large and therefore high peak current cannot be applied.

In high speed wire EDM of the prior art, which provides machined surface roughness in the range of 3 to 5 micron-Ra minimum, the wire electrode generally used has a diameter ranging from 0.3 to 0.35 mm. It is essential that during the machining process this wire should not fuse by discharge. It is therefore necessary to generate a discharge of high power density for machining mainly by evaporation. It is also necessary to stabilize the discharge gap by :

(i) providing sufficient quiescent time between any two consecutive discharges ;

(ii) providing a pulse generating circuit with sufficiently low inductance;

(iii) improving the current pick-up and the discharge machining fluid supply system.

Stabilization of the discharge gap will result in reducing the generation of large diameter debris, and keeping the machining area effectively and electrically clean.

Further, in the prior art, WC material - which is a good conductor and has high hardness is mainly used in sintered form as a material for the manufacture of current pick-ups. There are variations according to conditions but the current pick-up so manufactured cannot be used continuously for longer durations such as say, 50 hours, without generating on the current pick-up, a groove of the order of 0.1mm depth caused by the travelling wire. Again, sapphire or diamond is used as a guide for positioning of the travelling wire electrode and the current pick-up element and the wire guides are distinct elements used separately for energising and positioning.

The problem areas which this invention particularly seeks to resolve include:

(i) there should be less wear between the travelling wire electrode and the current pick-up element;

(ii) even though some wear occurs it should be as minimum as possible; and

(iii) wear resistant property of the current pick-up should be improved with better energising to achieve longer life of the element and maintain optimum performance of the machining process.

Further in the prior art, the power supplies used for aforesaid equipment, supply energy intermittently to the load in micro-sec periods. The load impedance may fall suddenly as a result of a short circuit. To overcome this load variation a series resistance is introduced to protect semiconductor elements of the power supply circuit. However, the use of the series resistance results in high power losses. To overcome this limitation, PWM (Pulse Width Modulation) control has been proposed but in this case also the power loss is still high and response is slow.

Still, further in the prior art, means to prevent Electro Magnetic Disturbances during the machining process include covering the total area of disturbance wave by a metallic mesh, but as wave generation area is open, the internal area has strong EM field intensity (5 to 20V reading taken at 2m position using single loop antenna of lm) .

The intensity of the field and the level of the noise is high inside the equipment and these waves also radiate outside the equipment. The computer and other electronic systems are prone to malfunction in the presence of EMD waves .

The problem attempted to be resolved by this invention is to prevent noise radiation wave diffusion by making the EMD wave small and weak.

Still further, in the prior art, machining condition is judged by detecting the mechanical or sound vibrations based on the voltage applied across the machining gap between a travelling wire electrode and a work piece. Gap short circuit or open state is detected by level of changes, and servo feed is given which controls the discharge repetition frequency and the machining gap. Force is applied on the wire electrode by the wire feed mechanism or discharge, as a result of which wire electrode starts vibrating which results in bad machining or wire electrode breakage. Also, in the voltage applying circuit, a switching element and a series resistance is incorporated in the pulse generating circuit, resulting in the reduction of the efficiency of pulse equipment and increase in heat generation.

Problems which this invention is trying to resolve are to find the distance between the work piece and the electrode for machining, and to generate the control action with respect to vibrations. Further, the number of parts are reduced to get higher efficiency.

SUMMARY OF THE INVENTION

According to this invention there is provided a high speed

W-EDM machining process, the W-EDM including a work piece upon which machining is to be performed and a guided travelling wire electrode spaced apart from said work piece defining therebetween a machining gap; a power supply means, including pulse generating means, supplying, through lead wires and a plurality of current pick-ups and contact elements, a train of intermittent current pulses to the travelling wire electrode and work piece respectively; and means to supply machining fluid in the machining region surrounding the machining gap, via nozzles; in which the machining process is carried out mainly by evaporation using a series of repeated high discharge current density discharges by reducing the total impedance in the power supply means and the repeated intermittent current pulses are applied in such a manner that each discharge in the series is completed in such a short time before plasma builds up in the discharge machining gap.

Typically the process is characterised in that the pulse current rise time characteristic of said pulses is set at least at 500A/micro-sec and includes further steps of maintaining the quiescent time between each of the said pulses such that debris particles are eliminated from the machining region by taking sufficient quiescent time before repeating a discharge and maintaining the quiescent time between each of the said pulses such that at the point of time of initiating every repeated discharge , an insulated state of time of initiating every repeated discharge, an insulated state (cold condition of the discharge particle) in the machining gap is established by sufficiently increasing the breakdown voltage characteristic of the machining gap, the quiescent time is maintained at least at 10 micro-sec and a delay time of at or below 0.1 micro-sec is set between the application of voltage pulse and the initiation of discharge and the discharge initiation voltage of each repeated discharge is maintained above 50V and preferably above 100V.

Preferably, the discharge debris size has fine size distribution. In accordance with a preferred embodiment of the invention, the high speed W-EDM machining process includes the steps of estimating the load condition of the process from the current time characteristic of load and /or machining position change time characteristics and using the estimation to control the pulse energy of the power supply means.

Advantageously, the process includes the step of twisting the lead wires to keep the self inductance during the process at or below 0.3 micro-H max.

The lead wires are made of Litz wires and includes the step of coating the lead wires with an insulation coating thickness of at the most 10 micro-m Typically, the coating is done by a CVD process using a thermopolymer such as paraxylene and forming a coating preferably of 2.5 micro-m thickness or a coating of polysilazane of 3.5 micro-m thickness.

The invention also discloses a method of supplying intermittent power pulses to a low impedance load, comprising the steps of providing a DC power supply; supplying the power to a pulse generating means; generating intermittent current pulses in a controlled manner; applying the pulses to the load via lead wires in such a manner that high current density, short duration pulses of preferably less than one micro-sec are generated at the load.

Typically , in accordance with one embodiment of the invention the high current density pulses are alternating current pulses and are applied to the load in groups of trains of pulses each pulse in the train having pulse width smaller than one micro-sec, the pulse train width of each group being at least larger than one micro-sec, the time interval between each group being fixed.

The method of supplying intermittent power pulses may further include the step of detecting the energy consumption status of overall load or timewise variation of load characteristics, typically, the load impedance, to obtain a detected signal and using the detected signal to control the pulse generating means and the step of storing only the energy required by the load at any given instant into a main energy storing means and applying the pulses to the load by discharge of the energy storing means and the step of storing residual energy, during the process of storing energy in the main energy storing means.

Typically, method includes the step of storing residual energy, during the process of application, in an auxiliary storing means and returning the energy stored in the auxiliary means to the main energy storing means.

It is envisaged that in the method of supplying intermittent power pulses while supplying pulses to the load the main energy storing means is made to discharge in a time closer to resonance time of capacity of the main energy storing means and inductance during application of pulses to the load.

In accordance with another aspect of this invention there is provided a W-EDM equipment for carrying out a machining process, said equipment including a work piece upon which machining is to be performed and a guided travelling wire electrode spaced apart from said work piece defining therebetween a machining gap; a power supply means, including pulse generating means, supplying, through lead wires and a plurality of current pick-ups and contact elements, a train of intermittent current pulses to the travelling wire electrode and work piece respectively; and means to supply machining fluid in the machining region surrounding the machining gap, via nozzles; characterised in that the operative total impedance of the power supply means is relatively very low to obtain high density current discharges, and the power supply means includes control means for applying the repeated intermittent current pulses in such a manner that each discharge in the series is completed in such a short time before plasma builds up in the discharge machining gap.

Typically, the machining equipment includes a group of at least two synchronously movable make and break contact elements provided to energise the work piece as close as possible to the machining gap, said contact elements being fitted to the nozzles and being cooperatively displaceable with the movement of the work piece in such manner that at least one of the contact elements makes continuously energising contact with the work piece even during the movement of the work piece when contact between other contact elements and work piece is broken. The make and break frequency of the oveable contact elements corresponds ideally to movement speed of said work piece. Each of the contact elements may be perforated thereby increasing their surface area to reduce losses due to skin effect of discharge current.

In accordance with another embodiment of the invention, spring loaded movable contact elements are provided to abut the work piece from the operative top and bottom surface of said work piece under spring bias .

Typically, the contact elements are adapted to provide machining fluid in the contact area between the contact elements and work piece to flush away debris particles which may lodge between the contact elements and the work piece thereby preventing the desired contact between said contact elements and work piece.

Advantageously, said contact elements are used also to support the work piece core during the machining process. Preferably, twisted Litz wires are provided to supply power to each contact element.

It is conceived in this invention that the current pick-up also functions as a wire guide and thereby inductance between the current pick-up and work piece is reduced.

In the W-EDM equipment the power supply means can include an isolation transformer or choke fitted between the pulse generating means and the load. Typically, the windings of said isolation transformer or choke are made of Litz wires. Further, said transformer or choke includes an auxiliary winding the output signal derived from which is used for controlling the machining process.

It is also envisaged that the W-EDM equipment includes a shield of a composite EM wave absorbing material which is made by laminating graphite with conductive material mesh and magnetic material mesh and includes a synthetic polymeric material covering which encloses the conductive material mesh, the magnetic material mesh and graphite in the form of at least one fibre mesh, is used to cover the electromagnetic disturbance wave generating members and the electromagnetic disturbance wave sensitive members.

Typically, the shield is used to cover the exposed parts (antenna ) of the travelling wire electrode or the shield is used in sheet form to cover the machining tank area to absorb the EMD waves, reduce reflection, and combine the action of a splash guard and prevention of EM wave diffusion.

In accordance with another aspect of this invention is proposed a power supply circuit for supplying intermittent power pulses to a low impedance load, typically for carrying out a W-EDM process, which includes

(i) main energy storing means (MESM) ;

(ii) charging means to charge the MESM in a controlled manner;

(iii) discharging means to discharge MESM in a controlled manner to supply power pulses to the load;

(iv) control means to control charging and discharging means to store and to discharge at any given time only the expected energy required by the load; and

(v) detection means cooperating with the control means adapted to detect the expected energy requirement of the load at any given instant of time.

In the power supply circuit the charging and discharging means operate in such a manner that they do not close simultaneously and discharging means includes a transformer or choke. Typically, the charging means includes PWM means to prevent excess current flow at the time of charging. In accordance with a preferred embodiment of the invention, the power supply circuit includes an auxiliary energy storing means for storing the residual energy in the power supply circuit and path means for supplying the energy stored in the auxiliary energy storing means to the main energy storing means and the charging and the discharging means include switching elements whose drive circuit impedance is made as small as possible to control the charging and discharging of MESM very efficiently.

The detection means consists of a detection coil fitted on the transformer or choke and the controlling means includes a computer. Twist wires are used to supply signals from the detection coil to the pulse generating circuit.

In accordance with another aspect of this invention there is provided a machining equipment, which includes power supply means having control means cooperating with means to estimate the load condition of the operative equipment derived from the current time characteristic of load and /or machining position change time characteristics .

In accordance with another aspect of this invention there is provided a machining equipment, which includes a shield of a composite EM wave absorbing material made by laminating graphite with conductive material mesh and magnetic material mesh for covering at least the operational portion of the equipment generating the EM disturbance (EMD ) waves and/or sensitive to EMD waves.

Also proposed is a discharge machining method, typically the high speed W-EDM process, consisting of performing machining by producing discharges in a machining gap between an electrode and a work piece comprising : passing a pulse discharge current through the machining gap, said current produced by a power supply means including a pulse generating circuit, ; detecting the discharge current waveform, that is the discharge power consumption to obtain a signal; and using said signal to control the pulse generating circuit so that the discharge current value is 60 to 80 % of the value of the current in a short circuit condition of the machining gap.

Typically the discharge machining method, consists of performing machining by producing discharges in a machining gap between an electrode and a work piece comprising : passing a pulse discharge current through the machining gap, said current produced by a power supply means including a pulse generating circuit, ; detecting the discharge current waveform, that is the discharge power consumption to obtain a signal; and using said signal to control the pulse generating circuit in such a way that discharge power consumption at the machining gap becomes maximum.

Still further, the discharge machining method , consists of performing machining by producing discharges in a machining gap between an electrode and a work piece comprising : passing a pulse discharge current through the machining gap, said current produced by a power supply means including a pulse generating circuit, ; detecting the discharge current waveform, that is the discharge power consumption to obtain a signal ; and using moving type wire electrode and damping the vibrations of the wire electrode either by directly detecting the vibrations of wire electrode or contacting a damping body with the wire electrode for absorbing the wire vibrations corresponding to the pulse discharge.

Further the discharge machining method, consists of performing machining by producing discharges in a machining gap between an electrode and a work piece comprising : passing a pulse discharge current through the machining gap, said current produced by a power supply means including a pulse generating circuit, ; detecting the discharge current waveform, that is the discharge power consumption to obtain a signal ; and using moving type wire electrode and damping the vibrations of the wire electrode due to tension and feed motors for wire electrode by using anti-phase type servo feed. The invention also proposes an equipment for discharge machining, including a work piece upon which machining is to be performed and an electrode spaced apart from said work piece defining therebetween a machining gap; a power supply means supplying, through lead wires, intermittent pulsed current to said machining gap; characterised in that the power supply means includes a pulse generating circuit having a pulse output isolation transformer adapted to deliver a high power efficient discharge without inserting series resistance for short circuit prevention.

Further the invention proposes an equipment for discharge machining including a work piece upon which machining is to be performed and an electrode spaced apart from said work piece defining therebetween a machining gap; a power supply means supplying, through lead wires, intermittent pulsed current to said machining gap; characterised in that said power supply means supplying pulse current to said machining gap includes an isolation transformer having a primary coil receiving pulse generation supply and a secondary coil supplying pulse current to the machining gap, both coils formed by winding twist Litz wires, said transformer fitted with a detection coil for detecting the discharge power consumption signal, which signal is used to control the pulse generating circuit.

Still further the equipment for discharge machining, includes a work piece upon which machining is to be performed and an electrode spaced apart from said work piece defining therebetween a machining gap; a power supply means supplying, through lead wires, intermittent pulsed current to said machining gap; characterised in that said power supply means supplying pulse current to said machining gap includes a choke in parallel with the machining gap formed by winding twist Litz wires, said choke fitted with a detection coil for detecting the discharge power consumption signal , which signal is used to control the pulse generating circuit.

Advantageously, the power supply means includes a plurality stacks, each stack comprising a plurality of pulse generation switching elements alternately mounted on a plurality of heat sink plates, the stack tightened with fasteners to form close contact between alternate members of the stack, the stack being further cooled typically by air or water, the output leads of the switching elements being connected directly to output bus bars by crimped fasteners thereby providing low inductance output terminals for the switching elements.

In accordance with another aspect of this invention there is provided a low resistance energising current pick-up cum wire guide for travelling wire of W-EDM equipment, having a body or contact surface of very hard material , in which the surface roughness of the contact surface used for energising travelling wire electrode is between 2 micro-Rmax to 15 micro-Rmax with minimum wear as a result of deposition of wire material on the relatively rough contact surface area and its adhesion thereof during the operative configuration of the device.

Typically, the contact surface is formed by dispersing a very hard material on hard electrically conductive body and the dispersion material SiC, A1203, Tie, Diamond, CBN, B4C used alone or in combination.

In one embodiment, said body is made of a very high hardness electrically high conductive material having expansion coefficient equivalent to diamond, and diamonds are deposited or bonded on its contact surface and the contact surface is formed by adhering by deposition or otherwise travelling wire electrode material or similar material, before hand, on the energising contact region to make further adhesion of travelling wire material during machining easier.

In another embodiment, said body is formed by stacking a plurality of thin layers of pure WC and wire electrode material alternately and said body has at least one groove at an angle to the movement direction of said travelling wire electrode.

In another embodiment, a low resistance energising current pick-up for travelling wire of W-EDM equipment is envisaged having an elastic support for its contact body in which, the vibration characteristics of the elastic material of the elastic support is predetermined to control the vibrations of the travelling wire electrode. Also envisaged is a shield of a composite EM wave absorbing material which is made by laminating graphite with conductive material mesh and magnetic material mesh and includes a synthetic polymeric material covering which encloses the conductive material mesh, the magnetic material mesh and graphite in the form of at least one fibre mesh used to cover the electromagnetic disturbance wave generating members and the electromagnetic disturbance wave sensitive members of an equipment.

A splash guard, for EDM equipment has also been conceived , said splash guard being of a composite EM wave absorbing material made by laminating graphite with conductive material mesh and magnetic material mesh enclosed in a covering of synthetic polymeric material and the operative inner walls in the machining area are made rough to achieve low reflection of EM waves impinging thereon.

BRIEF DESCRIPTION OF THE DRAWINGS :

The invention will now be described with reference to the accompanying drawings in which is illustrated Electric discharge machining, (EDM) particularly a travelling wire EDM (W-EDM) machining process particularly where a good surface finish of work piece, high quality of machined surface and safe, high speed machining over uninterrupted extended periods is required with efficient use of power supplied to the machining process. Also illustrated in the drawings, are current pick-up elements and wire guides for W-EDMs, power supply circuits for machining equipment using electrical energy directly with fast response, low loss power supply, such as W-EDM, ram type EDM, means to prevent electromagnetic disturbances (EMD) emanating from EDM equipment, particularly, W-EDM and control means for power supply and servo feed.

Figure 1 is a typical graphical representation showing the relationship of firing potential, with volumetric ratio expressed as percentage debris volume to the total volume of debris and machining fluid, in a discharge gap of 1 mm, for different types of machining fluid and different particle sizes of debris;

Figure 2 is a typical schematic drawing illustrating discharge characteristics showing the repeated sequence of events occurring in a machining gap during a single discharge in an EDM process;

Figure 3 is a graphical representation showing the relationship between the machining speed and machining roughness in accordance with the machining process of this invention as compared to that of the prior art;

Figure 3A is a table showing tabulation of the characteristics of W-EDM machine relating to the prior art and in accordance with the present invention. Figure 4A is a schematic view (not according to scale) of the machining region of a W-EDM equipment in accordance with this invention showing side view details of a nozzle showing the contact elements fitted thereto and Figure 4B is a sectional cut away view of contact element of fig 4A.

Figure 5 is a plan view of the nozzle of fig 4A;

Figure 6 is a sectional side view of a current pick-up cum wire guide body for W-EDM in accordance with this invention, showing the travelling wire passing therethrough;

Figure 7 shows deposition adhesion area on body of fig 6;

Figure 8 shows a sectional view of a current pick-up construction;

Figures 9A & 9B show front elevation and plan view of an alternative embodiment of the body of fig 6;

Figure 10 shows the wear characteristics of the current pick-up body of fig 9A & 9B.

Figure 11 shows a power supply circuit in accordance with this invention;

Figure 12 is a detailed view of a driver for high speed switching -of elements for the circuit of fig 11. Figure 13 shows a schematic drawing for a shield for the machining process of this invention;

Figure 14 shows a graphical representation, in a discharge machining process generally, of the relationship between discharge electrical power, discharge current peak value, discharge voltage and the machining gap when pulse voltage applied in micro-sec;

Figure 15 shows a scheme for carrying out the method according to this invention using an isolation transformer;

Figure 16 shows a scheme for carrying out the method according to this invention using a choke;

Figure 17 shows electrode damping control block diagram for motor control, typically for W-EDM;

Figure 18 shows a schematic drawing of vibration damping device for the invention;

Figure 19 shows a switching element assembly of this invention; and

Figure 20 shows a front view of switching element assembly cross section, section being viewed along line X-Xl in fig 19. DETAILED DESCRIPTION OF THE INVENTION : From the view point of energy efficiency, machining by melting is preferable because in the case of wire EDM, only a limited average machining current suitable to wire diameter can be passed. But although machining by evaporation has apparently low energy efficiency, the resulting suppression of temperature rise when the power density is increased is the crux of the present invention. This principle is diametrically opposite to conventional thinking. Further, the speed at which power is supplied to the machining region should be maintained as high as possible to prevent the diffusion of energy from the work piece to other areas as far as possible. As a result, conversion efficiency of electrical energy to mechanical energy also increases. To generate discharge when power density is high, the number of charged particles should be reduced to sufficiently increase the resistance value of the discharge gap and pulse voltage needs to be reapplied only when the gap breakdown voltage characteristic is more than 30 V and preferably more than 50 V and more preferably above 100 V.

Normally, atomic volume of a substance in solid state is around 20 to 50cm^Λ3/mol. As Avogadro constant is N=6.02 x 10^A23 / mol, apparent volume of one atom of a substance is 3 to 8 x 10^A-23. The surface barrier energy per atom is 6 to 25 eV ( 1 eV := 1.6 x 10^Λ-19 J ). Therefore, on the basis of calculations the energy required for machining is of the order of 10 to 40 x 10 '-19 J/atom. Here, if we consider mainly metallic materials, volume removal energy density (α) becomes 10^A5 to 10^Λ6 J /cm3. Large number of research results are published in this connection. Therefore, if rate of reacting atoms in reacting mass of unit mass, in other words, reaction speed ( reacting atoms/unit time -unit mass ) is taken as r, then r can be expressed as :

Formula 1 :

O — F r = A exp( ) (1) kT where Q = reaction activation energy - J/atom

E = free energy of reacting atoms - J/atom k = Boltzmann's constant - J/ K- atoms

T = temperature of atoms of reacting material -°K

A = reaction constant (related to profile or density)

Reaction response is fast corresponding to higher activation energy density and higher temperature.

For machining, it is necessary that energy is applied to the atoms and molecules of the machining area to shift them from their lattice structure and separate these atoms and molecules away from the surface. For example, in the case of iron (Fe) , the atom bonding energy is 10^Λ-18 Joule/atom and surface barrier energy is 10^Λ-18 Joule/atom minimum, therefore, even on a macro level, an energy density of at least 10^A5 to 10^Λ6 Joule/cm3 minimum is required to enable the discharge crater region to be machined by evaporation and maintain a small debris size.

In case of a discharge point generated on the surface of a work piece, the temperature gradient (psi) of the discharge point is as follows : Formula 2 :

(psi ) = KQh/ (lambda) (2) where K = constant

Qh = heat input power density in Joule/cm2 • sec and (lambda) = heat transfer rate in Joule/cm- sec. -K Thus it is clear that, higher the value of Qh, higher will be the concentration of power at the discharge point.

Therefore it is seen that, larger the value of Qh and smaller is the energy supply time, the temperature gradient becomes high resulting in higher concentration of power. Power concentration will reduce as (lambda) becomes large. Eventually, when machining is to be performed mainly by evaporation, requirement of energy density is 1.1 * 10^Λ6 Joule/cm2 ■ sec. minimum. At the time of discharge, release of electrons from metal becomes the main action. Particularly, in the case of repeated discharge, the subsequent discharge occurs after the previous discharge after a certain interval of time. Considering Fermi-Divac energy distribution, if electron density is taken as Formula 3 :

J = AT²- exp(-eV/kT) (3) where J = current density T = temperature V = work constant e = charge of electron k = Boltzmann's constant A = constant Power density, therefore, can be expressed by JVE/ cm3

The start of discharge occurs when charged particles are formed as discharge takes place. Model for this is shown in

Fig 2 of the accompanying drawings . As seen in Fig 2 , injected energy A eventually gets converted into energy B as a result of discharge as machining progresses. Energy of region A becomes small if temperature T of the medium, which includes the electrode, is high. Therefore, energy density of B becomes low. From the point of view of machining, following aspects are quite important: (i) how fast the previous discharge is extinguished and (ii) cooling i.e. to what extent the temperature of electrode including that of the medium can be kept low. If, time taken for shifting from point B to A as shown in Fig 2, that is to say the discharge cut-off time is called ts, then it can be represented as follows :

Formula 4 :

-1 i -h TS = V • d • (In ) ( 4 ) i - il where rs = discharge stop delay time

V = average speed of charged particles i = charged particle current = ( ιτ/2 ) ^ * SeVn (n=concentration of charged particles, S=electrode area and e = amount of charge) il = current at the time of discharge stop d = discharge gap length

It can be considered that during fast response the charged particles which are displaced are not ions but electrons. Therefore, the time (which may vary according to material) when almost no ions are formed in the discharge gap is approximately less than 1 micro-sec, and discharge plasma voltage can be around 30 V. Generally, conductivity of discharge column, in case of short time is decided in more than 90 to 99 % cases by electrons since transfer constant is proportionate to the square of electron concentration.

In this case it can be considered that normally the strength of the electric field rapidly varies in the discharge column. There is hardly any direct relationship between discharge excitation time and stopping time.

While machining, there is a repetition of starting and stopping of discharge as well as heating and cooling as machining takes place. If we consider, based on the theory of thermal conduction, the thermal input power density as q

(discharge power density) , q can be expressed as

Formula 5 :

Q T q(Xltl) = -F ( • wt ) (5) π - a² T

T where F ( • wt ) = Fourier series sq. wave period

T function, w = 2iι / T

Q = round section discharge crater power input a = radius of crater

T = discharge power supply time T = repetition period

This differs from material to material. If thermal conductivity is lambda (Joules/cm- S k) , P is specific gravity (kg/m3), c is specific heat (Joule/g-k), a(cm) is radius of discharge column and Ts is repetition period, then thermal response characteristic is as follows:

Formula 6 :

Ts- (lambda) -2 = 1.5 x 10 (6)

P c - π -a² and from the relation with discharge time τ(s), r/Ts is = 5 x 10 -2, and it is understood that temperature gradient becomes maximum between the discharge crater region and base metal.

In this case, the objective is achieved when the power density is made 7.6 x 10 6 W /cm².

Discharge initiation is the basic requirement for generating a completely new discharge. In case of W-EDM, as discharge is repeated one after another, considering delay time in completing the previous discharge and delay time in generating the subsequent discharge, power consumption becomes maximum when the subsequent discharge is to be initiated. Thus, considering the conditions of discharge initiation, it is seen that, effective machining characteristics can be achieved by supplying pulsed power.

Referring to Fig 1 and 2 of the accompanying drawings, Fig 1 is a typical graphical representation showing the relationship of firing potential, with volumetric ratio expressed as percentage debris volume to the total volume of debris and machining fluid, in a discharge gap of 1 mm, for different types of machining fluid and different particle sizes of debris and Fig 2 is a typical schematic drawing illustrating discharge characteristics showing the repeated sequence of events occurring in a machining gap during a single discharge in an EDM process.

Fig 2 shows the phenomenon from starting of discharge (block ES) till the end of discharge (block CI) i.e. from point A to B. From the relation of firing potential, size of debris and mixture percentage shown in Fig 1 , it is clear that the discharge is initiated at a relatively low voltage and therefore it is not possible to have high power density discharge. Even though large number of debris particles exist, due to their small size, it is envisaged that residual ions in the machining zone decrease rapidly. Referring to Fig 2, discharge energy is supplied at A (block ES) in the neutral state of the machining gap, charged particles are generated (block FL) during rd delay and radiate (block RA) , radiation is absorbed (block LA) . Thereafter, discharge occurs generating the heating (block HG) , formation of electron cloud (block EC) and cation formation (block CI), thereafter there is a lag time TS (block LT) followed by discharge cut-off quiescent time rq (block QT) . As seen in the figure, evaporation starts during block HG and continues within block EC and is overlapped by the process of melting and subsequent scattering which occurs after block EC. It is to be noted that there is no clear cut demarcation between the processes of evaporation, melting and scattering. At the time of evaporation, the impact force causes a crater formation. At B, the discharge is complete. At the discharge point, heat energy is transformed into mechanical energy and is diffused even by noise (vibrations) , and after a quiescent time again new discharge is generated to continue the repetition. Reference numeral r is the time interval between commencement of block FL and end of block EC which is the preferred time interval in which, in accordance with this invention, that the discharge should take place for material removal mainly by evaporation.

This invention can be considered to be effective when minimum τ/T = 1/20 is retained at minimum current rise of 500 A /micro-sec, with a pulse width of 1 micro-sec max (Iron). As per the calculations, when evaporation is to be prominent (80% min), power density should be about 1*10^A7 W/cm² and the power supply time duration t for generating a discharge should be between lmicro-sec and 0.1 micro-sec. Power density reduces by ft of power supply time t. If power supply time duration is greater than one micro-sec, machining shifts progressively from evaporation machining to machining by melting, in which case, the sides of wire electrode tend to fuse easily to the work piece.

When one gm of material is to be removed by 100% evaporation from iron work piece, calculations show that energy )£ approximately 7400 Joules is necessary per unit mass. Thus, removal of 0.135 mg is possible by one Joule of energy. As per the past data, 1000 Joules were necessary to machine 7.8 mg in a high precision grinding process. That is 1 Joule of energy was necessary to machine 0.0078 mg.

Further, according to published data, if evaporation is to be formed by releasing the grid connection energy, 0.065 mg of material can be removed by one Joule (0.48 * 10 -18 Joule /atom). However, in actual machining, it is necessary to carry out machining at a higher level of energy ( 10 -17 Joule/atom) for carrying out evaporation in machining area of about 10^A-6 to 10 -4 mm diameter.

In this case, temperature gradient becomes large as per the large Q/lambda i.e. supply of energy at smaller thermal conductivity and temperature gradient at the time of evaporation is of the order of 1.4 * 10 6 °K/cm. Mass machined per Joule is about 0.1 mg to 0.01 mg max and in case of melting, the mass melted per Joule is 0.3 mg to 0.03 mg.

Especially, in the case of large work pieces, considering the movement of lead wires, it is necessary to equip the machine with twisted cables.

If coefficient of self inductance per lead wire of the pulse power supply lead wires is taken as L then Formula 7 :

L = 47T • mu • n ² • s • 1 ( 7 ) where mu : magnetic permeability n : number of turns

1 : length of coil ( length of magnetic circuit) s : area of coil. In the case of twisted cables, as a result of the twist, the layers of insulation between the cables is formed in such a way that s tends to be equal to zero and inductance of total cable can therefore be retained low. This formation can be achieved, in accordance with the invention, by applying a thermopolymer such as paraxylene by CVD coating or forming layer using polysilazane on the cables. A cable coated with 2.5 micro-m paraxylene or 3.5 micro-m polysilazane (average molecular weight 1200) can withstand 1060 to 1100 V. The self inductance L of the such lead wires, when twisted, is 0.03 micro-H/metre which 1/10 times the inductance of conventional twisted cables.

Further, L can be retained to a small value when thickness of insulation layer is kept at around 400 .. In this case, when electrical resistance value is taken as R, then Formula 8 :

R = σ-l/Sl (8) where σ = specific resistance 1 = length SI = cross sectional area as the cross sectional area tends to decrease, due to electromagnetic effect, decrease of SI is prevented by means of split and Litz formation, thus resistance value as a whole can be retained at lower level.

Referring to the accompanying drawings, Fig 4A is a schematic view (not according to scale) of the machining region of a W-EDM equipment in accordance with this invention showing side view details of a nozzle showing the contact elements fitted thereto, while Fig 4B and 5 are respectively the sectional cut away view of contact element and the plan view of the nozzle.

In the machining region of a W-EDM, shown in Fig 4A, the work piece is represented by numeral 1. The work piece 1 is mounted on a table 9 whose movement is numerically controlled. Nozzles 4 supplying machining fluid to the machining region are disposed on top and bottom sides of the work piece 1 (bottom nozzle not shown) . The W-EDM includes the travelling wire electrode 3 positioned in the machining region by wire guide 13. The travelling wire electrode 3 is energised by the current pick-up 5 also fitted to nozzle 4. Movable contact elements 2a, 2b .. 2n are fitted to the nozzle 4. Supply cable 6 provides pulse energy both to the contact elements 2a, 2b .. 2n and the current pick-up 5. Machining fluid is supplied to the nozzle 4 via inlet 7 and is supplied to the machining region via outlet 8 8n.

Contact elements 2a , 2b .. 2n are spring loaded via springs

11A, 11B and are movable on nozzle 4 by means of contact element retraction moving coil 14A .... 14N and timed switching relay means 10 comprising contact maker 15, coil for contact maker 16, contact element selector coil 12, energised by auxiliary power supply 17.

Each of the contact elements 2a, 2b..,.2n is perforated by means of holes 2A, 2B .... 2N.

In Fig 4 of the accompanying drawings, contact elements 2a .. 2n are seen to cooperate with current pick-up 5 and wire guide 13 and power lead cable 6 of 3 metre length. The inductance is 0.2 micro-H when current pick-up 5 and wire guide 13 are used separately and the inductance is reduced to 0.18 micro-H when wire guide 13 is used to function as a current pick-up. This means that the total inductance of the pulse supply circuit can further be reduced, when the current pick-up 5 and the wire guide 13 are used as a common body.

Generally, when discharge current wave peak value is taken as Im then Formula 9 :

Im := -__..•» yycc//LL ( 9 )

R

E delta

Im = — exp ( - θ ) JL/C w

2L

R := • In A 4Tm

-1 where θ = <Ξot (delta / w) delta = R/2L TT

Tm = current rise time : = JLC

2

A = current attenuation ratio

R = total circuit resistance

E = Discharge initiation voltage

C = capacitance of circuit

L = inductance of circuit The discharge current rise time, till the discharge current reaches its peak value Im, is in proportion to the square root of the product of circuit capacitance and circuit inductance. For increasing the power density, it is necessary to retain inductance (formula 7) and resistance (formula 8) of the discharge circuit at smaller values. According to formula 8, suppose 24 enamelled wires of 0.7 mm diameter each are twisted together, the total resistance is 1.5 mΩ/metre. Even after considering the skin effect, it is 1.96 mΩ/metre. On the other hand, inductance of such lead wires of 2 meter length when connected between the work piece and current pick-up is 0.1 micro-H, and the inductance in the region between the work piece and the current pick-up is 0.2 micro-H when the work piece is of a large size of the order of 2 length, and thus, the overall inductance value will be of the order of 0.3 micro-H or less.

If a capacitor of 0.22 micro-F is used to supply power via a lead wire of 3 m length, the discharge wave peak current value becomes nearly 418 A. In accordance with a preferred embodiment, a moving type contact element alternately retracting and extending at frequencies ranging from 0.1 to 10 Hz can be mounted to the nozzle region of a W-EDM. If a capacitor of 0.15 micro-F is used (inductance of 0.25 micro-H) , it is possible to calculate wave peak value according to formula 9. When the switching element of a pulse generator is brought near the machining tank (thereby eliminating lead wires) with an inductance of 0.2 micro-H and capacitance of 0.5 micro-F, a pulse current of 600 A peak value and 0.65 micro-sec width is obtained with a discharge voltage of 100V. Preferably, the contact element should be 30% WCu and the contact pressure should be 2 kg/cm2.

Machining fluid must be kept constantly flowing from the central region of the contact elements to avoid accumulation of debris along the contacts . Particularly when two or more contact elements are used, at least one of the contact elements must be kept in constant contact with the work piece, and to ensure continuous stable contact during movement, maximum debris particles should be eliminated. This can be achieved by drilling a large number of holes in the contact elements and spraying machining fluid gently through these holes. When 5 such holes of 2 mm diameter, i.e. holes 2A' .. 2N' , as shown in Fig 4B of the accompanying drawings, were tried, it is possible to increase the peak current value by about 10A. The holes also reduce losses due to the skin effect and retain the resistance value to a low level . By use of the arrangement suggested according to this invention, it is possible to pass stable high peak value current even in the case of large size work pieces and effectively, current with significantly smaller pulse width can be supplied resulting in high speed machining with good surface finish irrespective of work piece size. In typical cases, the work piece core can be supported with the help of selected core side and work piece side contact elements by NC control.

Table 1 in Fig 3A shows the tabulation of data relating to the prior art and in accordance with the present invention. In Table 1, work piece thickness (CI 1) in mm, pulse discharge time (CI 2) in micro-sec,discharge repetition frequency (CI 3) in kHz, discharge to quiescent time ratio (CI 4), discharge machining peak current rise time (CI 5) in A/micro-sec, discharge machining average current (Cl 6) in A, machining speed (Cl 7) in mm²/min, machined surface roughness (Cl 8) in micron-Rraax, machining efficiency in terms of machining speed/ machined surface roughness ratio (Cl 9) in mm²/mim/micron-Rmax are expressed in relation to electrode diameter (Cl 10) in mm. The results in accordance with this invention are represented by item b in Table 1 which are further graphically represented by line b in Fig 3 of

drawings .

The data represented in item a in Table 1 are characteristics of wire EDM in the prior art graphically represented by line a in Fig 3. The data represents machining carried out under the following conditions - machining fluid pressure 15 kgf/cm² , machining fluid resistivity 5 x 10^Λ5 Ω cm, wire feed speed 15 m/min, wire tension one kgf . A brass wire electrode was used for machining.

When iron (Fe) material is machined using a travelling wire electrode, the material removal per Joule of energy is 0.0175 mg/Joule and this is within the values 0.1 to 0.01 mg/J stated above. Further, machining surface roughness is proportional to the electrode roughness after machining and electrode wear after machining should be retained at 1/10 of electrode diameter. This reduces the possibility of wire breakage .

The relationship of machined surface roughness and machining speed is shown in Fig 3. Line a represents machined surface roughness of 30 to 32 micron-Rmax achieved by 300 mm² /min machining speed by the process of the prior art. Line b represents the 14 to 15 micron-Rmax at 300 mm²/min machining speed. The graph shows the difference in the results of the process in accordance with this invention in comparison with the results of the prior art process. Therefore, when machining at 500 mm²/min, 20 to 25 micron-Rmax is possible.

A slightly alkaline machining fluid of 8 * 10^A3 Ω-cm was used and repeated machining at a frequency of 60 kHz was carried out using similar condition and a wire electrode of 0.33 mm diameter at a tension of 1.8 kgf . It was observed that the wear rate of brass wire became almost 1/2 to 1/3 when machining fluid of high specific resistance 5 * 10^A5

Ω.cm was used with a wire tension of 1.1 kgf. Thus, when a machining fluid of lower resistivity was used similar machining was possible at higher tension.

In this case when machining was done using AC power supply, electrode wear was reduced by 6 to 37 % as compared with a case when a DC pulse current was passed. The possibility of wire breakage was also further reduced.

The examples described hitherto relate to machining of simple flat work pieces. But it is possible to extend the use of the arrangement of this invention including the novel contact elements for rod / column shaped work pieces or work pieces of different materials using an NC drive.

Fig 4B shows cut away side elevation of the nozzle for ease of understanding and Fig 5 shows the plan view of the nozzle of Fig 4. Referring to Fig 4A, machining is carried out by relatively moving the wire electrode 3 towards NC driven work piece 1 , and selecting the movable contact elements 2a .. 2n on the machining nozzle region to contact the work piece 1 from top and/or bottom side. The work piece 1 moves along with table 9 relative to the nozzles 4. Machining fluid is also sprayed gently through the holes 2A, 2B ...2N which are provided in the central region of contact elements 2a, 2b .. 2n, thereby debris particles do not obstruct contact. Further, the contact elements 2a ..

2n are made to alternately retract and extend at a frequency between 0.1 Hz and 10 Hz with the help of timed switching relay means 10. The contact elements are normally in contact with the work piece by springs 11A, 11B ... , and the contact can be broken if desired during movement of the table 9. Various systems such as electrically driven screws or pneumatic, hydraulic arrangements and the like can be used for maintaining the contact pressure.

Power supply of relay means 10 can be adapted to vary the frequency of movable contact elements 2a .. 2n corresponding to discharge frequency when voltage is applied to coil 12. Contact maker 15 and control coil 16 function simultaneously at the time of jog feed, and pull contacts 2a .. 2n away from work piece to open their contact with the work piece 1. With suitable modifications, wire guide 13 can be adapted to function as a current pick-up in accordance with a second aspect of this invention.

In use, nozzle is moved in Z-axis direction and the nozzle 4 tip position is aligned with work piece 1. Machining fluid having 5 * 10^A5 Ω.cra resistance value and 15 kgf/cm2 pressure is supplied via machining fluid inlet 7. Simultaneously, the machining fluid is spray at 2kg/cm2 pressure through holes 2A .. 2N of approx. 1mm diameter in the contact elements 2a .. 2n and the contact elements are made to alternately extend and retract at a frequency in the range of 0.1 to lOHz corresponding to the movement of the work piece 1. When movement of the work piece becomes faster, the power supply feeding the coil 12 of relay means 10 is controlled to increase the frequency of movement of the contact elements. For example, while contact element 2a is touching the work piece 1, contact element 2b is made to touch and then contact element 2a is pulled away so as to separate it from the work piece 1. This ensures that at any given instant, at least one contact element of the set supplies energy to the work piece even while the work piece is in motion.

It is possible to obtain peak current value of 700A in the case of a work piece measuring 2 metre in length and 1 metre in width of material S55C, where actual impedance was 0.4 Ω and pulse width of 0.65 micro-sec at 280 V. In this case, machining speed of 1.74 g/min could be achieved. A work piece of S45C material and of 100 mm thickness was machined at a similar speed using 0.35 mm diameter brass wire and travelling at 12 M/min. Machining was done by spraying machining fluid at 15 kgf/cm2 pressure.

Figs 6 to 10 illustrate current pick-ups and current pick-up cum wire guide for use with the process and in the W-EDM equipment envisaged in accordance with this invention, in which Fig 6 is a sectional side view of a current pick-up cum wire guide body for W-EDM in accordance with this invention, showing the travelling wire passing therethrough; Fig 7 shows deposition adhesion area on the body of Fig 6; Fig 8 shows a sectional view of a current pick-up construction; Figs 9A & 9B show front elevation and plan view of an alternative embodiment of the body of Fig 6; Fig 10 shows the wear characteristics of the current pick-up body of Fig 9A & 9B.

Referring to the drawings, Fig 6 is a sectional side view of the current pick-up cum wire guide body 202 of pure WC material for W-EDM in accordance with this invention, showing the travelling wire 205 passing therethrough. A diamond deposition layer 201 is provided on one surface as will be hereinafter described.

Fig 7 shows the relatively rough deposition adhesion contact surface 210 having crests and troughs in which the layer 209 of wire material is coated on WC+Co body 202. Fig 9A & 9B show front elevation and plan view of an alternative embodiment of the body 202 having grooves 202a, 202b, 202c ... 202n with 15° inclination to the direction of movement of the travelling wire 205. Fig 10 shows the wear characteristics in the form of surface profile of surface 210 of the current pick-up body 202 of Fig 9A & 9B showing wear condition 211 of normal machining transposed on the height 212 and width 213 of coating layer formed on the contact surface 210 by wire 205.

Fig 8 shows a sectional view of a current pick-up of an alternative construction in which a contact body 215 of pure WC material is damped in use by means of composite rubber element (206, 207). Preferably, the deposition adhesion contact surface 216 of the contact body 215 is made relatively rough and is coated with the layer 209 of wire material. The contact body 215 is connected to terminal 208 of the twisted Litz lead wire and the other terminal 208' is connected to the work piece (not shown).

The surface region of the current pick-up in contact with the wire electrode should be made rough. The required surface roughness on the current pick-up material can be achieved by various processes such as grinding, SSD, laser, mechanical alloying, EDM and the like. If we take contact area as A, relation with contact pressure can be expressed as follows: Formula 10 :

A = K-Pⁿ (10) where K = a constant relating to surface roughness, P = contact pressure, n = a constant related to elastic deformation.

Further, if energising width is taken as d, then Formula 11: d l/3 = Kl-r-F-P -l (11) where F = frictional force; r = wire electrode radius; P = contact pressure; Kl = constant.

Considering, the wire electrode movement velocity as v, if temperature of contact point is T, then Formula 12 :

T = K2 - H -3 - mu - P^A-l/3 - V ( 12 ) where K2 = constant related to thermal conductivity;

H = peak value of surface roughness; mu = coefficient of friction; v = moving velocity of contact region. Important aspects especially in formula 12 are: (i) the entity corresponding to surface roughness may differ from material to material and becomes H^A-4 for range of surface roughness below 2 to 3 micron-Rmax; and (ii) characteristics can be used for range 2 to 3 micron-Rmax and above.

(iii) In combination of pure WC and brass wire, 6 micron-Rmax can be recommended as against 2.5 micron-Rmax. (iv)due to temperature T stated in formula 12 soft material of the wire electrode gets easily coated on the hard material surface of the current pick-up. (v) there is a range for machining surface roughness depending on the wire electrode material which varies according to the conditions.

Due to the temperature rise in the width d and the surrounding region, a constant thickness of soft material of wire electrode is coated and retained on the hard material surface of the current pick-up. The thickness of coating layer is about 3 to 5 micro-m. As a result of wire vibrations due to discharges in the machining region, the wire electrode material gets coated more effectively. In general, hile machining, effectively, coating layer of soft material of wire electrode gets formed on the hard material of current pick-up (refer Fig 7). Contact surface of current pick-up becomes less rough when wire electrode material grazes to the current pick-up surface and by pressure, electrode material is coated on the current pick-up surface easily. In this process, the soft material fills the troughs on the relatively rough hard surface. Under constant pressure due to wire tension, there is perfect contact between the similar materials of the wire electrode and the coated layer.

Fig 6 of the accompanying drawings shows the construction of a current pick-up cum wire guide in accordance with this invention. A deposition layer 201 is applied on the surface of the body 202 by electro-deposition process in which, the body 202 of pure WC material is made the cathode, diamond #80 is fitted as anode with 3 mm gap and 380 V DC is applied in an environment containing a mixture of 0.1 % alcohol and hydrogen gas at the rate of 0.5 litres / min and 5 * 10^A3 Pa pressure. A diamond coating of approximately 5.2 micro-m thickness is deposited in 15 hours. . As the coefficient of expansion of pure WC which is 3.5 * lθ -6 ^°C resembles that of diamond which is 3.2 * 10 -6 ^°C, bonding strength is improved by about 10 times when compared with WC 6% Co material for current pick-ups in the prior art.

Fig 7 shows an alternative multilayered embodiment in which a pure WC layer 214 is deposited on the surface of WC-Co base material of body 202. In this surface discharge coating process (SSD) , an anode of pure WC material is selected, the base material being treated as cathode and coating takes place in an air environment by passing a peak discharge of 30 to 40 A current in 5 to 10 micro-sec to create a pure WC layer 214, lOmicro-m thick. This layer 214 is tipped with a diamond layer 201. Energising of the wire electrode is achieved with the help of layer 214 or inner surface 210 of body 202, whereas positioning is achieved with the help of diamond tipped region 201. As wire 205 travels through the annular region defined by surface 210 of body 202, wire material deposition takes place over the surface 210 forming a layer 209 over this surface.

Brass wire electrode material of 60 % Cu and 40 % Zn having 0.25 mm diameter is moved at 12 M/min speed, current pick-up of 99.8 % density WC material is used, pulled by 1.1 kgf tension contact pressure with respect to current pick-up, contact is made by moving in such a way that angle along the wire electrode is 1° , average current 25A is passed under the conditions - pressure approx. 0.3 kg, pulse current 350A, pulse time 0.8 micro-sec, under water cooling, roughness of contact surface (Ra micro-m) in machining time of one hour is shown in Table 2. Wear occurring on the relatively smooth machining surface in the prior art is represented by D, E of Table 2. A, B, C represent the results of this invention. Table 2 •

Ra micro-m Thickness of brass layer on WC

A 20.0 + 3.00 micro-m

B 4 . 0 + 0.75 micro-m

C 2 . 5 + 0.50 micro-m

D 1 . 5 Wear - 6.00 micro-m

E 0.05 Wear - 11.00 micro-m Cu wire electrode shows similar results under similar conditions. When 15° inclined groove of 0.035 mm width and 5 mm depth is formed on body 202 and machining fluid envelops the complete surface of the wire, it is possible to have stable energising of travelling wire. Table 3 :

Thickness of Cu layer on WC A + 11 micro-m B + 6 micro-m C + 5 micro-m D - 5 micro-m E - 10 micro-m In another embodiment, brass layer is applied on WC material. Brass layer thickness is taken as 0.3 mm. Twenty layers of WC material, each having thickness of 0.5 mm, and 19 layers of brass material, each having thickness of 0.3 mm, are used by stacking on each other. Table 4 :

Thickness of brass layer on WC A + 6 micro-m B + 4 micro-m C + 3 micro-m D - 2 micro-m E - 5 micro-m

Fig 8 shows a current pick-up, particularly for W-EDM. The pick-up includes brush 215 mounted on double layer rubber, one of polyurethane 206 and one of silicon 207. The brush 215 receives power supply via twisted litz wires 208. By use of the double layer of different hardness, the hardness of the rubber composite can be modified selectively for the purpose of damping to suit the wire electrode resonance frequency, say 0.9 kHz. As wire (not shown in Fig 8) travels across the surface 210 of body 215, wire material deposition takes place over the surface 210 forming a layer 209 over this surface.

Wire damping is improved when resonance frequency of brush 215 of said current pick-up is brought closer to the wire electrode resonance frequency. Table 5 :

Resonance frequency of brush Resonance frequency cycle

/damping material time of wire electrode

A Normal brush with ceramic 600 micro-sec insulation B 0.9kHz rubber 300 micro-sec

(polyurethane) C 1.8 kHz rubber 160 micro-sec

(polyurethane , polycarbonate) D 2.7 kHz resin 68 micro-sec

(epoxy lamination) Fig 9A & 9B show a current pick-up having a body 202 of pure WC material. Using the current pick-up, machining for one hour is done at Ip 230A Ton 1.1 micro-sec at a contact pressure of 280 g. Fig 10 illustrates the resultant surface profile of surface 210 where the surface roughness is 15 micron-Rmax and the deposition layer height 212 is 50 micro-m with a width 213 of 90 micro-m. Transposing the state of a conventional current pick-up and normal machining, wear is 75 micro-m represented by numeral 211 in Fig 10.

In the current pick-up of Fig 9A & 9B, as shown, grooves 202a, 202b, 202c ... 202n of 0.03 mm width at an inclination of 15° with the wire are formed on the body 202 of pure WC material and these grooves are filled with a mixture of lubricating materials BaCr04 10 %, W2S 1%, graphite 6 % and balance In02. As a result, coefficient of friction became 0.28, wear resistance and charging ability improved.

Also disclosed in this invention is a new and improved power supply circuit which is illustrated in Fig 11 & 12, Fig 11 showing the power supply circuit in accordance with this invention and Fig 12 is a detailed view of a driver for high speed switching of elements for the circuit of Fig 11.

Fig 11 shows a power supply circuit for supplying intermittent power pulses to a low impedance load such as a W-EDM. The circuit includes a main energy storing means in the form of accumulator 302, charging means including a DC power supply 301, power supply accumulator 319, inductor element 317, MOSFET switch 303, to charge the accumulator 302 in a controlled manner, discharging means including MOSFET switch 304 to discharge the accumulator 302 in a controlled manner to supply power pulses to the load terminals 309 or 310 or 310'. The circuit further includes control means in the form of computer 307 to control charging and discharging means via MOSFET drivers 305, 306 or 305', 306' as the case may be to store and to discharge at any given time only the expected energy required by the load. Still further, the circuit includes detection means cooperating with the control means adapted to detect the expected energy requirement of the load at any given instant of time. The detection means may be in the form of a load characteristics detection coil 318 through its terminals 311 fitted to an output transformer or choke 308 or a Logoski coil (link core) detection coil 311' or a photo transistor detector 311". The power supply circuit further includes an auxiliary energy storing means in the form of residual energy (charge) absorption accumulator 312 in series with residual energy rectifier 313 for storing the residual energy in the discharge means and first path means including residual energy (charge) pumping back transformer 314, residual energy rectifier 315 for supplying the energy stored in accumulator 312 to accumulator 302 and second path means to return the- energy stored in the charging circuit to the accumulator 302 in the form of residual energy rectifier 316.

The theoretical aspects of this portion of the invention can be dealt with as under :

When electrical energy is to be supplied to a load, in the case when an accumulator is used as an energy storing device, if the current rise time is taken as τ , then

Formula 13: π

T := (L.C) (13)

2 Here , maximum current Im can be expressed as :

Formula 14:

lm:= E. (C/L) (14)

Further, usable energy W where

Formula 15

W = C-V² / 2 (15) stored in the accumulator is controlled by voltage V.

Capacity C of the accumulator can be changed as required.

Therefore when the inductance of the energy supplying circuit containing one ore more electrostatic capacitors is determined, the value of electrostatic capacitance C of the accumulator can be calculated based on Formula 13 and Formula 14, and energy finally required can be calculated by Formula 15. It is possible to supply energy evenly for longer duration by repeating time r . The energy pulse control time is in the range of nsec to msec.

Time energy characteristics for energy supply is selected and its repetition can be controlled over a wide range. The principle of this invention, namely, high speed response

(nsec) and controlled energy supply speed can be precisely realised over a wide range of energy requirements ( nJoule to kJoule) . This energy can be directly or indirectly used for machining by heat (fusing, evaporation, temperature rise ) or by mechanical pressure (driving force, explosion energy ), or energy of motion and the like. Secondly, the energy can also be used as a reaction generating phenomenon by receiving the energy from diffusion, excitation or intermolecular energy.

The detailed working of the power supply circuit of fig 11 of the accompanying drawings, in accordance with this invention, for supplying power to a low impedance load, typically a W-EDM, will now be described. Reference numeral 301 represents DC power source which charges the accumulator 319. When this accumulator 319 is charged, the control switches 303 and 304 are driven by drivers 305, 306 to supply pulsed energy. The switches 303 and 304 can be of the MOSFET type and operate in such a manner that they do not close simultaneously. The control for driver 305 is pulse width modulated.

Typical driver coupling capacitors 305', 306 ' for the MOSFET switches 303 and 304 are shown in Fig 12. The capacitors 305', 306' have sufficiently large capacity and are driven by low impedance drivers, thereby the loss in the MOSFET switches 303, 304 is reduced to 1/10. The accumulator 302 which stores the energy, is controlled in such a way that the charging voltage of accumulator becomes 200 to 500 V in about 5 to 10 micro-sec by PWM control, when the signal is received from the computer 307 and the transformer 308. Charging energy of 0.001 to 0.1 Joule (max) gets stored in accumulator 302 and this energy is supplied to the load via switch 304 and transformer 308 output terminals 309 or choke terminals 310 of transformer 308. After the required energy is supplied, switch 304 opens and PWM control for switch 303 is enabled. When the switch 304 is opened after a time period less than the time required for the cycle width of LC, the residual energy is absorbed by a series circuit comprising accumulator 312 and rectifier 313. The accumulator 312 is connected to the primary winding of transformer 314 which steps up the voltage to the desired high level and charges the energy accumulator 302 via rectifier 315 within 5 to 10 micro-sec as stated above. Repeated discharge quiescent time is in the range of 5 to 10 micro-sec, and main energy pulse can be maintained between 0.1 and 1 micro-sec, which is typically used for W-EDM.

The circuit is such that, the rectifier 316 also provides a path for the residual energy generated at the time of charging of accumulator 302 and the residual energy stored in the inductance element 317 in the charging circuit. Further, a detection coil 318 is wound around choke or transformer 308 for detection of load condition. The charging condition of main accumulator 302 is controlled by output 311 of coil 318 via computer 307. The signal detected at output 311 can also be used for servo feed control in the equipment process such as for controlling the machining gap in an EDM. Similarly, residual energy in the discharge circuit of load connected to terminals 309 or 310 of transformer or choke 308 is stored in the accumulator 312 via series rectifier 313 and this energy is returned to the main accumulator 302 via step up transformer 314 and rectifier 315 and this improves the efficiency of the power supply circuit.

In case of ram type EDM, circuit configuration similar to that in W-EDM machining equipment can be used, except that, in this case, it is preferable if polarity exists at the load connected at terminals 310' (optionally by removing the transformer or choke 308). Current is detected by detection coil as link core 311" and Logoski coil 311' and their output signal can be used for detection of impedance time variation of gap by condition of gap load via computer 307 which in turn controls the charging-discharging of main accumulator 302 and control the feed servo of the machining gap.

Further, in this case, pulses are applied to the load in groups of trains of about 10 to 500 pulses, each pulse in the train having relatively smaller pulse width of less than one micro-sec with pulse interval larger than at least one micro-sec, and the quiescent time interval between each group being equivalent to that of 10 to 30 pulses. Stable machining with low electrode wear is possible by injecting machining fluid e.g. water and surface activator. This machining is achieved by detecting various phenomenon occurring in the machining gap and current time characteristics with the help of detection coil 311'. Electrode wear characteristics can further be controlled by controlling saturation of transformer or choke 308.

In the case of surface coating equipment, charging energy is 0.01 Joule to 100 Joule. The process is carried out in air environment, however, depending on the circumstances, gas as a medium to increase diffusion action or various types of oil to prevent oxidation are used. Electrode polarity similar to that of ram type EDM is given. Pulses having width of about 50 to 500 micro-sec and quiescent time of about 10 to 500 micro-sec are applied between coating material as anode and the body of the surface to be coated as cathode thereby generating discharge. Current rise time characteristic is detected by detector 311' and accordingly the electrode feed is controlled to move the discharge position. It is possible to carry out localised i st treatment, of the order of 0.05 mm to 0.1 mm depth, by using graphite electrode. The electrode should be selected which will not show bad effects due to alloying. Diffusion is controlled by controlling the saturation of transformer or choke 308.

If ionized water is mainly used as machining fluid, under the above conditions, it is possible, using the above configuration, to obtain electrolytic discharge and control its level at the time of occurrence of ion action.

In the same way, as in the case of an electrolytic liquid, particularly when the liquid is one of the electrodes, discharge is generated between electrode and liquid. Using the same level of energy range as that of surface coating system, in this case, it is possible to carry out activation treatment or diffusion treatment or welding at a pulse width of one to 100 micro-sec. For example, with 5 micro-sec discharge time, 20 micro-sec quiescent time, by using terminals 310' as load terminals and by carrying out discharge in saturated solution of phosphoric acid with electrode temperature from 20 °C to 1750°C, temperature of cathode can be controlled in a wide range. Depending on the electrode area, the quiescent time is controlled in inverse proportion relation. Again, it is possible to use for anodic process, cathodic electro-deposition process by electrolytic action.

In the case of discharge between solid bodies, applicable energy of 0.1 kJoule to 100 kJoule is used by sufficiently increasing the winding ratio of transformer 308, because the load impedance is low. Detector 311 output is used to control the pressure characteristics between particles and this response is used to detect the relationship between a compacting die and punch, and thereby, to, for example, easily sinter pure WC or B4C material. Using this process, it is possible to get sintered WC of specific gravity 15.5 and B4C of specific gravity 2.45 within 3 to 5 min.

Further, when sintering starts, pressure characteristics get reversed at specific point as contraction starts. It is necessary to stop charging just before this reversing starts. By controlling the current time characteristics sufficiently at constant value, it is possible to perform binderless stable sintering. Similarly, it is possible to sinter materials used for switches, such as WCu, WAu, WAg, or spring materials such as FeCu and the like.

The power supply, in accordance with this invention, can be used for high frequency applications such as induction heating, high frequency welding, high frequency melting. In this case, energy is supplied via transformer 308, load impedance is detected by output 311 which corresponds to the physical characteristics of the heating body and the detected signal is fed to computer 307. Switching speed of switch 303, 304 is controlled to respond to the load factor. This configuration makes it possible to supply high energy with stable load factor. Particularly in the case of magnetic material, as the magnetic permeability reduces according to temperature, pulse repetition frequency is controlled according to the variation of coefficient of magnetic coupling with load, and it is possible to get high efficiency process at constant magnetic coupling.

The power supply, in accordance with this invention, can be used for laser equipment in which energy requirement is between 1 and 10 kJoule and load characteristics is controlled by detector 311. Depending upon the detection requirement, by replacing the detection coil 311" by a photo transistor which detects the laser irradiation light and/or reflected light from direct optical fibre and/or temperature. A group of pulses is used in this application.

Further, by computer control, precise control is achieved by micro-sec order pulses and it is possible to achieve precise control without resetting as compared to prior art.

As illustrated above, just by modifying energy and control time it is possible to use the pulses of this high frequency power supply, which can be controlled very efficiently with high speed response, in different ways. It is possible to use the power supply and techniques of this invention to totally different types of processes just by modifying the computer software. This invention envisages a new type of process information controlled power supply system which can be used for a variety of processing equipment.

Fig 13 shows a schematic drawing for a shield for the machining process of this invention.

Characteristics of electromagnetic (EM) waves are based on characteristics of electric dipole and magnetic dipole. Therefore, surge impedance, i.e. when distance is sufficiently longer than wave length, impedance Z occurring at propagation characteristics of EM wave in air is Formula 16 :

Z = JmuO/eO := 377Ω (16) where muO = magnetic permeability of air eO = dielectric constant of air

Diffusion prevention and absorption of disturbance wave is carried out by wrapping origin of EM wave generation by retaining impedance value of prevention shield at a small value.

Conduction current as a necessary characteristics of EM wave absorber is Formula 17 :

J = delta -E (17) where delta : electric conductivity and considering phase displacement current has following characteristics - Formula 18 : dD = j-w-e-E (18) dt where e = dielectric constant

D = dielectric flux density eE

Here item delta + j w e of displacement current of conduction current and phase forms the basis for evaluation of EM shield. Again, when conductor is to be kept within the propagation of EM wave, when field reflection is taken as Re, reflection coefficient is

Formula 19 :

Re = (Z2 - ZO) / ( Z2 + ZO) (19) where ZO, Z2 : are impedance for atmosphere and absorber respectively (refer Fig 13).

Electric field transmission coefficient Te is

Formula 20 :

Te = 2 Z2 / (Z2 + Z0 (20)

Magnetic field transmission coefficient Th is Formula 21 :

Th = 2-ZO / (Z2 + Z0) (21)

Further, nonreflection condition is Formula 22 :

Z0 = Z2-tan h r 1 (22) where r = propagation constant + j β 1 = length h = thickness Further, damping constant is Formula 23 :

7 1 + (delta/ w.e)² - 1 a = w-./e-mu-( ) % ....(23)

2 and phase constant β is

Formula 24 :

J 1 + (delta/ w.e)² + 1 β = w-7e-mu-( ^ (24)

2 As a result, if practical shield effect is taken as S dB

Formula 25 :

S := 10 log(delta- f)^A-l x 1.7h ( f / delta ) i> ..(25) where delta = volumetric natural resistance (conductivity) f - frequency Hz h = thickness

As per formula 25, it is possible to achieve shield effect of 30-40 dB at h=0.2cm, within the range of 10 to 1000 MHz. Formulae 16 to 25 express the general condition, by which the effective means of this invention include inducing a current into a short circuit coil using mesh. Further, a strong magnetic mesh having high magnetic permeability (mu) is used. The disturbance EM wave is converted in to heat by using graphite as a resistive material. In this manner, the EM waves can be absorbed very effectively by a relatively thin walled shield.

The energy possessed by EMD wave is

Formula 26 :

1

W = ( E² + H² ) (26)

8-7T where E = strength of electric field

H = strength of magnetic field

However, by locating electrically conductive mesh and magnetic mesh at electric field and magnetic field region at each point, EM wave energy is short circuited and consumed. According to H. Poynting, absorption energy per unit area of a spherical surface around EM wave generation point is

Formula 27 :

where C = EM wave flux Graphite (carbon) absorbs the energy as a resistor.

Alternative to formula 22, self impedance of composite shield with respect to EM wave can be expressed as Formula 28 : mu ^ 2 π

Z = ( ) -tan(j d-(mu-e) ) (28) e lambda where mu = complex ratio magnetic permeability e = complex ratio electric permeability d = thickness of layer lambda= wave length

Here, complete absorption occurs by Z = 1.

Therefore, if actual value of complex ratio magnetic permeability is 1/3 of actual value of complex ratio electric permeability, by way of an example, it is seen that absorption is possible (mu = e/3). Therefore, composite carbon fibres are suitable material for making the shield according to this invention.

Example :

First layer is of copper mesh of #60 of 0.3 mm, the layer of graphite fibres of 50 micro-m diameter, unit of 10 cm length, P A N type resin is heat treated from 240°C for 3 hours (3°C/min) to 350°C under the flow of pure nitrogen gas. Pulsed voltage of 7 kV peak value is applied on this graphite material and peak current of 1.2A is passed per piece 30 times. At this time, specific resistance changed from 1000 Ω to one Ω.

This carbon fibre is woven into a mesh and 5 layers of this mesh are taken in the centre, this is stacked from outside by 100 micro-m woven mesh of stainless and on the other side copper mesh similar to stainless mesh, after that sandwich the outer side by copper mesh, and total body is made of 20 mm thickness in which graphite is held in the centre. Further, total body surface is put in a synthetic resin bag. This shield is able to absorb EMD waves generated by discharge machining ranging from 1 to 100 MHz, reflection waves in the range of -15 to -18 dB, transmission waves in the range of -25 to -30 dB. It is possible to use the shield in accordance with this invention around a machining tank of any type of EDM or around a computer for reducing the EM noise level .

Finally, Figs 14 to 20 illustrate a discharge machining process in accordance with this invention using a new and improved power supply circuit, in which Fig 14 shows a graphical representation, in a discharge machining process generally, of the relationship between discharge electrical power, discharge current peak value, discharge voltage and the machining gap when pulse voltage applied in micro-sec, Fig 15 shows a scheme for carrying out the method according to this invention using an isolation transformer, Fig 16 shows a scheme for carrying out the method according to this invention using a choke, Fig 17 shows electrode damping control block diagram for motor control, typically for W-EDM, Fig 18 shows a schematic drawing of vibration correcting device for the invention, Fig 19 shows a cooling method for the switching element of this invention and Fig 20 shows a front view of switching element cooling part cross section, section being viewed along line X-Xl in Fig 19. When machining between electrode and work piece is in progress, pulse of the power supply is controlled by a signal derived from the discharge current peak value or discharge power which has relation with gap length and the electrode feed servo is operated according to gap voltage at that instant. To avoid the machining accuracy defects caused by wire breaking or burning, occurring due to wire electrode vibrations resulting in short circuit between the wire electrode and the work piece, wire electrode vibrations are controlled by using a current pick-up having damping characteristics. Higher efficiency is achieved by directly rectifying the power supply, modulating the rectified power supply pulse and supplying the rectified pulse modulated power supply to the machining region via a transformer.

Referring to the drawings :

Fig 14 shows a graphical representation, in a discharge machining process generally, of the relationship between discharge electrical power, discharge current peak value, discharge voltage and the machining gap when pulse voltage applied in micro-sec. In the drawing, I represents machining current, V the machining gap voltage, W the power at the time of machining. In Fig 14, A' represents the machining gap length region in which the energy density is maximum at the time of machining. Line a5 - a' 5 represents machining average current value increase discharge gap length characteristic at the time of machining. Line b'5-b5 represents arc discharge current gap length characteristic. Line c5 - c'5 represents discharge gap length voltage characteristic at the time of machining. Line d'5 - d5 represents arc discharge voltage gap length characteristic. Line e5 - e'5 represents power gap length characteristic at the time of machining starting. Line f'5 - f5 represents electrical power machining gap length characteristic at the time of arc discharge.

Figs 15 and 16 show schemes for carrying out the method according to this invention using an isolation transformer and a choke respectively. In Figs 15 and 16, the following reference numerals represent the parts mentioned along side and their working will be described hereinbelow : Reference numeral 511 : switching semiconductor element; 512 : isolation transformer (either with core or air core); 512a : primary Litz wire with parallel twist; 512b : secondary Litz wire with parallel twist; 513 : machining gap; 514 : signal terminal; 515 : DC power source; 515a, 515b : twist wire; 516 : choke coil; 516a : Litz coil; 517 : switching power source; 518 : signal output terminal and 519 : machining gap.

Figs 17 and 18 show the electrode damping control block diagram for motor control, typically for W-EDM and a schematic drawing of vibration correcting device for the invention. In Figs 17 and 18, the following reference numerals represent the parts mentioned along side and their working will be described hereinbelow : Reference numeral 520 : wire electrode; 521, 522 : wire electrode guide; 523 : work piece; 524 : vibration detector coil drive type which detects vibration signals in wire 520 and feed signal, via preamplifier 525 and computer 526 to servo control tension and feed motors 527 and 528 for damping by anti-phase servo feed; 529 : magnetostrictive material; 530 : detection and vibration operator; 531 : Amplifier for magnetic distortion vibration detection combined drive equipment; and Z5 : input-output terminals of computer.

Fig 19 shows a cooling method for the switching element of this invention; and Fig 20 shows a front view of switching element cooling part cross section, section being viewed along line X-Xl in Fig 19.

In Fig 19 and 20, reference numerals 501, ... 501n represent switching element MOSFET; 502, ... 502n-l, 502n : cooling fin; 503 : source bus bar; 532 : drain bus bar; 533 : gate bus bar; 503a : gate bus bar coupling fastener; 503b : drain bus bar coupling fastener; 503N, 503nl : source bus bar coupling fastener; 504 : fastening bar of cooling plate and semiconductor element; 505 : fastening screw of cooling plate and semiconductor element; 506, 507 : dielectric ceramics; 508, 509 : cooling fan blades; and 510 : motor for cooling fan.

Discharge current peak value is generally found out by Formula 29 : di 1 L—(t) + Ri(t) = f i(t)-dt (29) dt C where L = Inductance of the discharge circuit

(time function),

R = Resistance of the discharge circuit

(time function) ,

C = Static charge capacity, i = Current (time function).

In formula 29, under the condition of R² < 4 L/C, then

Formula 30 :

V -at i(t) = e sin wt (30) wL where a = Ratio of resistance and inductance R/2L

w = Angular velocity Jvι0² - ² t = Time,

V = Accumulator terminal voltage

From formula 30, if peak current value is taken as Im, then Formula 31 :

If t

n as tm, then

Formula 32 :

tm := (π/2) . J (32)

Therefore, inductance is

Formula 33 :

L := 4 - tm² / 7τ ² - c ( 33 ) and resistance is Formula 34 :

R : = ( 2L/T ) ln A ( 34 )

Here, if period T is taken as 4 tm, then, A indicates the damping ratio of current. Power consumed at the discharge gap is equal to the energy consumed by equivalent resistance value R(t), thus Formula 35 : t f R-I²-dt = W (35)

0 where 1= Effective current -> Im

R= This is an addition of discharge gap resistance r and discharge circuit resistance (time function) shown in formula 6.

Further, if maximum temperature is taken as θm from formula

35, then

(However, ignore both the temperature coefficients rho, k)

Formula 36 : rho • I ² θm = ( 36 )

8 -7T² -k-rO² where rho = Conductivity k = Thermal conductivity rO = Radius of discharge crater

Thermionic current increases further, as a result of the temperature rise of the discharge point region and radiation. At θm of about 2800 °C to 3000^°C, evaporation takes place at the discharge point and generates discharge pressure. Finally, cylindrical wave pressure is generated between the wire electrode and the work piece at the discharge point. If radius of high pressure region is taken as R , it increases by Formula 37 :

R Jt ( 37 ) and also it varies according to the machining condition. As pressure to the level of g to lOg is generated, at fixed combination, the frequency of oscillations of wire electrode, if taken as f, then Formula 38 : f a JP (38)

When discharge is generated, wire electrode vibrates causing repetition of open-short circuits in the machining gap.

From the above characteristics, the range used for constant discharge machining is as follows -

Discharge current I = 5 to 1000 A

Discharge start voltage V = 50 to 500 V

Pulse width r = 10 nsec to 100 micro-sec

Discharge energy W = 10^A-7 to 1 Joule

Repeat frequency = 300 kHz to 10 k Hz

A typical representation of above ranges is illustrated in

Fig 1.

In other words, if the positive and the negative current carrying wires are twisted and wound around each other to form Litz cable, it become a cable of extremely low inductance. The primary winding and secondary winding are formed by coil - using Litz twist made from twisted Litz wire. Discharge peak current value Im can be increased, as shown by formula 31, and efficiency increases with decrease in phase shift of voltage and current.

According to Fig 14, if voltage gap characteristic is represented in a graphical form, (a' 5) indicates the case of machining gap short circuit for zero discharge voltage condition when resistance R is at lowest value in formula 29, voltage reduces to (c'5) and the current peak value (a'5) becomes max current Ip at short circuit condition. When arc discharge continues, V in formula 31 reduces, Voltage (d'5) becomes almost equal to the addition of voltage (c'5) and arc discharge voltage. As a certain arc voltage exists, arc current (b'5) is less than short circuit current (a' 5). As discharge resistance exists at the time of discharge, machining peak current Im is 60 to 80 % of short circuit Ip value.

During arc discharge the discharge gap voltage and current do not vary much with respect to the discharge gap. Moreover, arc voltage is normally 12 to 16 V, and when the discharge starts, it usually switches over to arc discharge within a fixed period of micro-sec order after starting. Discharge starting voltage is higher than arc discharge voltage. Accordingly, the power consumed in the machining gap (e5) -> reaches the peak value in the discharge machining region (A') -> becomes arc discharge by momentary short circuit at (e'5) and moves to arc condition power consumption characteristic (f'5). Lowering of discharge starting voltage upto arc voltage normally occurs within less than one micro-sec. A discharge within one micro-sec can take place effectively under high power density condition. Discharge machining uses a range in which energy consumption at the time of machining is maximum. Using pulse discharge is more effective than using arc discharge in machining. For this purpose, machining region A' of the discharge process becomes proper machining gap distance. Therefore, it is difficult to detect machining gap distance from gap voltage directly.

(a5) is the characteristic of current peak value when gap is taken as a parameter, similarly, (c'5) is the characteristic when gap of discharge start voltage is taken as a parameter. But, if the machining progress is in the direction towards short circuiting in which gap becomes narrower, detection of machining gap distance is possible. Therefore, until now, the gap voltage is used as a signal for detection. Frequently, process of machining with repeated short-open circuit occurs. In this case, current peak value changes to (a'5) and voltage to (c'5), at the next instance, current becomes (b'5) and voltage becomes (d'5), current stabilizes as (b'5) -> (b5), and voltage as (d'5) -> (d5).

In the machining zone of A', situation is uncertain. Voltage changes as (c5) -> (c'5) -> (d'5) -> (d5), discrimination of normal or abnormal signal is extremely inaccurate. From a utility point of view, it is not possible to control the discharge current peak value by controlling the discharge gap length as it requires gap control response time of the order of micro-sec, whereas in reality the mechanical time constant of the machining gap control system is of the order of msec. However, it is possible to control the frequency of pulse discharge by equivalent current peak value which respond in the order of

2 micro-sec to 100 micro-sec Therefore, it is possible to control the discharge current peak value by the current peak value signal.

If gap control servo feed signal is detected from the machining gap and used while controlling pulse generation in such a way that power consumption is maximum in the gap, then servo electrode gap feed and pulse discharge supply control do not hunt and stable machining is possible. Eventually, during machining feed, when short circuit occurs for the first time and continues for more than one msec, the electrode feed control to get a constant feed, which does not exceed, so that no short circuit occurs at the gap between wire electrode and work piece, is the main subject.

In the case of W-EDM, the wire electrode vibrates in the machining gap at a relatively high frequency at the time of discharge causing repeated short & open circuits in short time intervals of msec order. Even in the case of ram type EDM with solid electrode, low frequency vibrations occur causing short and open circuits in the machining gap. By using a computer in the control circuit, pulse power and max current control according to this invention can be achieved easily.

Example 1 :

A 50 mm thick S45C material is machined using machining fluid of 5 * 10 4 Ω-cm resistivity, 15 kgf/cm² pressure, and using brass wire electrode of 0.25 mm diameter, travelling at 12 m/min under 1.2 kgf tension. A discharge start voltage 320 V, Ipeak 310 A, pulse width of 800 nsec is used with a) a non-electrolytic machining transformer coupling is used in combination with rectifier integrating circuit having response time of 3 to 15 micro-sec for voltage servo control of discharge repetition frequency aiming at the peak value; or b) a capacitive coupling output having response time of 3 to 15 micro-sec for voltage servo control of discharge repetition frequency; or c) control of same range carried out by detecting current peak value of above mentioned a) or d) control of same range carried out by detecting current peak value of above mentioned b) or e) control of same range carried out by detecting power at above mentioned conditions a) or f) control of same range carried out by detecting power at above mentioned conditions b) . Same machining is executed 3 times and average data is as follows. a) Machining speed is 179 mm²/min .... 17.5A average machining current b) Machining speed is 214 mm²/min .... 19. OA average machining current c) Machining speed is 228 mm²/min d) Machining speed is 234 mm²/min e) Machining speed is 253 mm²/min f) Machining speed is 273 mm²/min

Therefore, detecting current or power is extremely advantageous in improving machining speeds.

In b) of above mentioned example 1, the machining power is detected by sampling over a period of 200 micro-sec, by sensing peak power value above a certain value, say 5 kW, the wire electrode feed motor is controlled at a response of 5 kHz to 2 kHz . Based on the machining power the motor speed is equivalently servo controlled in the range of 88 to 122 RPM, as against the conventional motor having fixed 100 RPM. Machining speed of 268 mm²/min is obtained when the servo motor is controlled at response time of 3 msec.

Example 2 :

In fig 18, a wire guide of 2 mm diameter is fitted in a damping body of 5 mm length and having vanes at 120° direction to each other made of TbO .3 Dy0.7 Fel.92 type material is used in the conditions similar as b) above, machining speed becomes 258 mm²/min when oscillations of 10 micro-m were imparted to the wire electrode for 50 micro-sec with a delay of 20 micro-sec. Accordingly, it is clear that electrode vibration prevention is extremely advantageous for improving machining speed and machined surface roughness .

Example 3 :

When AC voltage is rectified directly and is fed to a resonant circuit formed by a capacitor and transformer primary via a chopper of 5 micro-sec to resonate at 280 V. The secondary voltage at 100 kHz is fed to a machining gap where the pulse width is varied from 0.12 micro-sec to 1 micro-sec. The entire efficiency became 82 %. Efficiency of the power supply of prior art is 45 to 50 %.

Example 4 :

When pulse output transformer is used with 1:1 turns ratio, and when twist type Litz wire winding is made together with 60 wires of 0.16 mm diameter, output / input ratio (efficiency) becomes 86 %. Very precise voltage time characteristic of gap is obtained at the tertiary winding. When wire length is more than 10 cm, normal twist wire is used.

Example 5 :

For both transformer tertiary winding output and Logoski coil output, about 2 m twist wiring is used and electrical power output is obtained by computer. By this method, equipment can be controlled efficiently. Example 6 :

When twist Litz wire is used as an element with 0.2 mm diameter wires and 0.7 mm diameter wires having equal cross section area, discharge peak current value for 0.2 mm diameter wire is 420A whereas for 0.7 mm diameter wire, it is 380A. (Formal wire used). Therefore, using twist Litz wire is advantageous while detecting current for control. Example 7 :

Fig 19 and 20 show a novel heat sink configuration for the equipment of this invention. The heat sink includes power MOSFET switching elements 501, ... 501n shown in Fig 19, Fig 20 and cooling plates 502, ... 502n alternately stacked and tightly contacted by means of coupling devices 504 & 505. Bus bars 503, 503N1, 503n, 532 are arranged with minimum inductance as shown in the front view drawing, Fig 20. The entire construction is easy to dismantle. When ten MOSFETs are used, it is possible to get 750A peak pulse current. By using twenty five MOSFETs, the inductance is 0.006 micro-H. With this configuration, it is possible to generate current pulses of 50 nsec to 1 micro-sec width. For further cooling, water and air cooling are used optionally. All parts are joined by threaded fasteners.

Various alterations and modifications are envisaged from the basic aspects suggested in this invention without departing from its nature and its scope. The examples teach only the best method for performing the invention and are in no way limiting the scope of the invention and the ambit of the claims.

Claims

1. A high speed W-EDM machining process , the W-EDM including a work piece upon which machining is to be performed and a guided travelling wire electrode spaced apart from said work piece defining therebetween a machining gap; a power supply means, including pulse generating means, supplying, through lead wires and a plurality of current pick-ups and contact elements, a train of intermittent current pulses to the travelling wire electrode and work piece respectively; and means to supply machining fluid in the machining region surrounding the machining gap, via nozzles; in which the machining process is carried out mainly by evaporation using a series of repeated high discharge current density discharges by reducing the total impedance in the power supply means and the repeated intermittent current pulses are applied in such a manner that each discharge in the series is completed in such a short time before plasma builds up in the discharge machining gap.

2. A high speed W-EDM machining process as claimed in claim 1, characterised in that the pulse current rise time characteristic of said pulses is set at least at 500A/micro-sec.

3. A high speed W-EDM machining process as claimed in claims 1 or 2, which includes a further step of maintaining the quiescent time between each of the said pulses such that debris particles are eliminated from the machining region by taking sufficient quiescent time before repeating a discharge.

4. A high speed W-EDM machining process as claimed in any one of the preceding claims, which includes a further step of maintaining the quiescent time between each of the said pulses such that at the point of time of initiating every repeated discharge, an insulated state (cold condition of the discharge particle) in the machining gap is established by sufficiently increasing the breakdown voltage characteristic of the machining gap.

5. A high speed W-EDM machining process as claimed in any one of the preceding claims, in which the quiescent time is maintained at least at 10 micro-sec.

6. A high speed W-EDM machining process in as claimed in any one of the preceding claims, in which a delay time of at or below 0.1 micro-sec is set between the application of voltage pulse and the initiation of discharge.

7. A high speed W-EDM machining process as claimed in any one of the preceding claims, in which the discharge debris size has fine size distribution.

8. A high speed W-EDM machining process as claimed in claim 4, in which discharge initiation voltage of each repeated discharge is maintained above 50 V and preferably above 100 V.

9. A high speed W-EDM machining process as claimed in any one of the preceding claims, which includes the steps of estimating the load condition of the process from the current time characteristic of load and /or machining position change time characteristics and using the estimation to control the pulse energy of the power supply means.

10. A high speed W-EDM machining process as claimed in any one of the preceding claims, in which the process includes the step of twisting the lead wires to keep the self inductance during the process at or below 0.3 micro-H max.

11. A high speed W-EDM machining process as claimed in claim 10, in which the lead wires are made of Litz wires .

12. A high speed W-EDM machining process as claimed in claims 10 or 11, which includes the step of coating the lead wires with an insulation coating thickness of at the most 10 micro-m.

13. A high speed W-EDM machining process as claimed in claim 12, in which the coating is done typically by a CVD process using a thermopolymer such as paraxylene and forming a coating preferably of 2.5 micro-m thickness or a coating of polysilazane of 3.5 micro-m thickness .

14. A method of supplying intermittent power pulses to a low impedance load, comprising the steps of providing a DC power supply; supplying the power to a pulse generating means; generating intermittent current pulses in a controlled manner; applying the pulses to the load via lead wires in such a manner that high current density, short duration pulses of preferably less than one micro-sec are generated at the load.

15. A method of supplying intermittent power pulses to a low impedance load as claimed in claim 14, in which the high current density pulses are alternating current pulses.

16. A method of supplying intermittent power pulses to a low impedance load as claimed in claims 14 or 15, in which the pulses are applied to the load in groups of trains of pulses each pulse in the train having pulse width smaller than one micro-sec, the pulse train width of each group being at least larger than one micro-sec, the time interval between each group being fixed.

17. A method of supplying intermittent power pulses to a low impedance load as claimed in any one of claims 14 to 16, which includes the step of detecting the energy consumption status of overall load or timewise variation of load characteristics, typically, the load impedance, to obtain a detected signal and using the detected signal to control the pulse generating means.

18. A method of supplying intermittent power pulses to a low impedance load as claimed in any one of claims 14 to 17, which includes the step of storing only the energy required by the load at any given instant into a main energy storing means and applying the the pulses to the load by discharge of the energy storing means.

19. A method of supplying intermittent power pulses to a low impedance load as claimed in claim 18, which includes the step of storing residual energy, during the process of storing in the main energy storing means .

20. A method of supplying intermittent power pulses to a low impedance load as claimed in claims 18 or 19, which includes the step of storing residual energy, during the process of application, in an auxiliary storing means and returning the energy stored in the auxiliary means to the main energy storing means.

21. A method of supplying intermittent power pulses to a low impedance load as claimed in any one of claims 18 to 20, in which while supplying pulses to the load the main energy storing means is made to discharge in a time closer to resonance time of capacity of main energy storing means and inductance during application of pulses to the load.

23. A W-EDM equipment for carrying out a machining process in accordance with any one of claims 1 to 23, said equipment including a work piece upon which machining is to be performed and a guided travelling wire electrode spaced apart from said work piece defining therebetween a machining gap; a power supply means, including pulse generating means, supplying, through lead wires and a plurality of current pick-ups and contact elements, a train of intermittent current pulses to the travelling wire electrode and work piece respectively; and means to supply machining fluid in the machining region surrounding the machining gap, via nozzles; characterised in that the operative total impedance of the power supply means is relatively very low to obtain high density current discharges, and the power supply means includes control means for applying the repeated intermittent current pulses in such a manner that each discharge in the series is completed in such a short time before plasma builds up in the discharge machining gap.

24. A machining equipment as claimed in claim 23, which includes a group of at least two synchronously movable make and break contact elements provided to energise the work piece as close as possible to the machining gap, said contact elements being fitted to the nozzles and being cooperatively displaceable with the movement of the work piece in such manner that at least one of the contact elements makes continuously energising contact with the work piece even during the movement of the work piece when contact between other contact elements and work piece is broken.

25. A W-EDM equipment as claimed in claim 24 which includes control means to control the make and break frequency of the movable contact elements correspond to the movement speed of said work piece.

26. A W-EDM equipment as claimed in claims 24 or 25, in which each of the contact elements are perforated thereby increasing their surface area to reduce losses due to skin effect of discharge current.

27. A W-EDM equipment as claimed in any one of claims 23 to 26, in which spring loaded movable contact elements are provided to abut the work piece from the operative top and bottom surface of said work piece under spring bias .

28. A W-EDM equipment as claimed in any one of claims 23 to

27, in which the contact elements are adapted to provide machining fluid in the contact area between the contact elements and work piece to flush away debris particles which may lodge between the contact elements and the work piece thereby preventing the desired contact between said contact elements and work piece.

29. A W-EDM equipment as claimed in any one of claims 23 to

28, in which said contact elements are used also to support the work piece core during the machining process.

30. A W-EDM equipment as claimed in any one of claims 23 to

29, in which twisted Litz wires are provided to supply power to each contact element.

31. A W-EDM equipment as claimed in any one of claims 23 to

30, in which current pick-up also functions as a wire guide and thereby inductance between the current pick-up and work piece is reduced.

32. A W-EDM equipment as claimed in any one of claims 23 to

31, in which the power supply means includes an isolation transformer or choke fitted between the pulse generating means and the load.

33. A W-EDM equipment as claimed in any one of claims 23 to

32, in which the windings of said isolation transformer or choke are made of Litz wires.

34. A W-EDM equipment as claimed in any one of claims 23 to

33, in which said transformer or choke includes an auxiliary winding the output signal derived from which is used for controlling the machining process.

35. A W-EDM equipment as claimed in any one of claims 23 to

34, in which a shield of a composite EM wave absorbing material which is made by laminating graphite with conductive material mesh and magnetic material mesh and includes a synthetic polymeric material covering which encloses the conductive material mesh, the magnetic material mesh and graphite in the form of at least one fibre mesh, is used to cover the electromagnetic disturbance wave generating members and the electromagnetic disturbance wave sensitive members.

36. W-EDM equipment as claimed in claim 35, in which the shield is used to cover the exposed parts (antenna ) of the travelling wire electrode.

37. A W-EDM equipment as claimed in claims 35 or 36 in which the shield is used in sheet form to cover the machining tank area to absorb the EMD waves, reduce reflection, and combine the action of a splash guard and prevention of EM wave diffusion.

38. A power supply circuit for supplying intermittent power pulses to a low impedance load, typically for carrying out a W-EDM process as claimed in claim 1, which includes

(i) main energy storing means (MESM);

(ii) charging means to charge the MESM in a controlled manner;

(iii) discharging means to discharge MESM in controlled manner to supply power pulses to the load;

39. A power supply circuit as claimed in claim 38, in which the charging and discharging means operate in such a manner that they do not close simultaneously.

40. A power supply circuit as claimed in claims 38 or 39, in which discharging means includes a transformer or choke.

41. A power supply circuit as claimed in any one of claims 38 to 40, in which the charging means includes PWM means to prevent excess current flow at the time of charging.

42. A power supply circuit as claimed in any one of claims 38 to 41, which includes an auxiliary energy storing means for storing the residual energy in the power supply circuit and path means for supplying the energy stored in the auxiliary energy storing means to the main energy storing means.

43. A power supply circuit as claimed in any one of claims 38 to 42, in which the charging and the discharging means include switching elements whose drive circuit impedance is made as small as possible to control the charging and discharging of MESM very efficiently.

44. A power supply circuit as claimed in claim 40, in which the detection means consists of a detection coil fitted on the transformer or choke.

45. A power supply circuit as claimed in any one of claims 38 to 44, in which the controlling means includes a computer.

46. A machining equipment, which includes power supply means having control means cooperating with means to estimate the load condition of the operative equipment derived from the current time characteristic of load and /or machining position change time characteristics.

47. A machining equipment, which includes a shield of a composite EM wave absorbing material made by laminating graphite with conductive material mesh and magnetic material mesh for covering at least the operational portion of the equipment generating the EM disturbance (EMD ) waves and/or sensitive to EMD waves.

48. A discharge machining method, typically the high speed W-EDM process of Claim 1, consisting of performing machining by producing discharges in a machining gap between an electrode and a work piece comprising : passing a pulse discharge current through the machining gap, said current produced by a power supply means including a pulse generating circuit;detecting the discharge current waveform, that is the discharge power consumption to obtain a signal; and using said signal to control the pulse generating circuit so that the discharge current value is 60 to 80 % of the value of the current in a short circuit condition of the machining gap.

49. A discharge machining method, typically the high speed W-EDM process of Claim 1, consisting of performing machining by producing discharges in a machining gap between an electrode and a work piece comprising : passing a pulse discharge current through the machining gap, said current produced by a power supply means including a pulse generating circuit, ; detecting the discharge current waveform, that is the discharge power consumption to obtain a signal; and using said signal to control the pulse generating circuit in such a way that discharge power consumption at the machining gap becomes maximum.

50. A high speed W-EDM machining process as claimed in any one of Claims 1 to 13, in which the vibrations of the wire electrode are damped by contacting a damping body with the wire electrode for absorbing the wire vibrations corresponding to the pulse discharge.

51. A high speed W-EDM machining process as claimed in any one of Claims 1 to 13, in which the vibrations of the wire electrode are damped by detecting the vibrations of wire electrode to generate a vibration signal and using the said signal to provide anti-phase servo feed to a tension and a feed motor between which said wire electrode is travelling.

52. An equipment for discharge machining,typically the high speed machining equipment of Claim 23, for carrying out the process as claimed in any one of claims 48 to claim 51, including a work piece upon which machining is to be performed and an electrode spaced apart from said work piece defining therebetween a machining gap; a power supply means supplying, through lead wires, intermittent pulsed current to said machining gap; characterised in that the power supply means includes a pulse generating circuit having a pulse output isolation transformer adapted to deliver a high power efficient discharge without inserting series resistance for short circuit prevention.

53. An equipment for discharge machining for carrying out the process as claimed in claims 48 to 51, including a work piece upon which machining is to be performed and an electrode spaced apart from said work piece defining therebetween a machining gap; a power supply means supplying, through lead wires, intermittent pulsed current to said machining gap; characterised in that said power supply means supplying pulse current to said machining gap includes an isolation transformer having a primary coil receiving pulse generation supply and a secondary coil supplying pulse current to the machining gap, both coils formed by winding twist Litz wires, said transformer fitted with a detection coil for detecting the discharge power consumption signal, which signal is used to control the pulse generating circuit.

54. An equipment for discharge machining, typically the high speed machining equipment of Claim 23, for carrying out the process as claimed in claim 48 to claim 51, including a work piece upon which machining is to be performed and an electrode spaced apart from said work piece defining therebetween a machining gap; a power supply means supplying, through lead wires, intermittent pulsed current to said machining gap; characterised in that said power supply means supplying pulse current to said machining gap includes a choke in parallel with the machining gap formed by winding twist Litz wires, said choke fitted with a detection coil for detecting the discharge power consumption signal, which signal is used to control the pulse generating circuit.

55. An equipment for discharge machining as claimed in any one of claims 52 to 54, in which the power supply means includes a plurality of stacks, each stack comprising a plurality of pulse generation switching elements alternately mounted on a plurality of heat sink plates, the stack tightened with fasteners to form close contact between alternate members of the stack, the stack being further cooled typically by air or water, the output leads of the switching elements being connected directly to output bus bars by crimped fasteners thereby providing low inductance output terminals for the switching elements.

56. An equipment for discharge machining as claimed in any one of claims 52 to 55, in which twist wires are used to supply signals such as pulse discharge power and current signals, from the machining gap or the detection coil to the pulse generating circuit, which signals are used to control the machining gap length or the pulse power supplying circuit.

57. A low resistance energising current pick-up cum wire guide for travelling wire of W-EDM equipment, having a body or contact surface of very hard material, in which the surface roughness of the contact surface used for energising travelling wire electrode is between 2 micro-Rmax to 15 micro-Rmax with minimum wear as a result of deposition of wire material on the relatively rough contact surface area and its adhesion thereof during the operative configuration of the device.

58. A low resistance energising current pick-up cum wire guide for travelling wire of W-EDM equipment as claimed in claim 57, in which the contact surface is formed by dispersing a very hard material on hard electrically conductive body.

59. A low resistance energising current pick-up cum wire guide for travelling wire of W-EDM as claimed in claim 58, in which said dispersion material Sic, A1203, Tic, Diamond, CBN, B4C used alone or in combination.

60. A low resistance energising current pick-up cum wire guide for travelling wire of W-EDM as claimed in any one of claims 57 to 59, in which said body is made of a very high hardness electrically conductive material having expansion coefficient equivalent to diamond, and diamonds are deposited or bonded on its contact surface.

61. A low resistance energising current pick-up cum wire guide for travelling wire of W-EDM equipment as claimed in any one of the preceding claims 57 to 60, in which the contact surface is formed by adhering by deposition or otherwise travelling wire electrode material or similar material, before hand, on the energising contact region to make further adhesion of travelling wire material during machining easier.

62. A low resistance energising current pick-up cum wire guide for travelling wire of W-EDM equipment as claimed in claim 57, in which said body is formed by stacking a plurality of thin layers of pure WC and wire electrode material alternately.

63. A low resistance energising current pick-up cum wire guide for travelling wire of W-EDM equipment as claimed in any one of Claims 56 to 62, in which said body has at least one groove at an angle to the movement direction of said travelling wire electrode.

64. A low resistance energising current pick-up for travelling wire of W-EDM equipment as claimed in any one of claims 23 to 37, having an elastic support for its contact body in which, the vibration characteristics of the elastic material of the elastic support is predetermined to control the vibrations of the travelling wire electrode.

65. A low resistance energising current pick-up for travelling wire of W-EDM equipment as claimed in any one of claims 23 to 37, in which the contact surface roughness of the contact surface used for energising travelling wire electrode is between 2 micro-Rmax to 15 micro-Rmax with minimum wear as a result of deposition of wire material on the relatively rough contact surface area and its adhesion thereof during the operative configuration of the device.

66. A shield of a composite EM wave absorbing material which is made by laminating graphite with conductive material mesh and magnetic material mesh.

67. A shield as claimed in claim 66, in which graphite is in the form of at least one layer of fibre mesh.

68. A shield as claimed in claims 66 and 67, in which the composite EMD wave preventing material is enveloped by synthetic polymeric material covering.

9. A splash guard, for EDM equipment, said splash guard being of a composite EM wave absorbing material made by laminating graphite with conductive material mesh and magnetic material mesh enclosed in a covering of synthetic polymeric material and the operative inner walls in the machining area are made rough to achieve low reflection of EM waves impinging thereon.