Method and apparatus for preventing wire breakage during electrospark erosion machining

Abstract

A device for machining a part by electrical discharge machining using an electrode wire (4), comprising: - means (20, 30, 60) for keeping the electrode wire taut and driving the electrode wire to translate longitudinally in a spark initiation zone (5) near the part to be machined; - means (50) for causing a stream of dielectric liquid to flow through the spark initiation zone between the electrode wire and the part to be machined; - a power supply (10) for generating an electric pulse for initiating a spark between the electrode wire and the part to be machined in the spark initiation zone; - a control device (40) for controlling the device according to machining parameters; - a measuring device (70) for evaluating the amount of bubbles present in the spark initiation zone between the electrode wire and the part to be machined and for transmitting a signal (73) representing the amount of bubbles to the control device; - an adaptation device (41) in the control device, which is used to modify the machining parameters so that the value of the signal is maintained within a suitable range of signal values.

Description

Technical Field

[0001] The present invention relates to a wire cutting EDM (Electro Discharge Machining, Electrospark Machining) method (process, procedure), wherein, in an EDM machine, a conductive material is cut using an electrode wire by using spark erosion in a dielectric (dielectric) between the electrode wire (electrode wire, welding wire) and the material. Prior art

[0002] The well-known EDM method allows material to be removed from a part made of conductive material by generating sparks in a spark initiation zone between the part to be machined and a conductive wire electrode. The wire electrode is continuously routed in the vicinity of the part in the length direction of the wire held by a guide and is gradually moved in the transverse direction towards the part by a transverse translation of the wire guide or of the part.

[0003] In an EDM machine, the spark initiation zone between the electrode wire and the part to be machined is immersed in a suitable dielectric fluid. The power supply is connected between the part to be machined and the electrode wire via electrical contacts located away from the spark initiation zone. The power supply allows sparks to be generated via the transmission of appropriate electrical pulses, which gradually erode the part and the electrode wire. The particles separated from the electrode wire and the part by the spark are dispersed in the dielectric fluid, and they are removed from the dielectric fluid. The longitudinal routing of the electrode wire makes it possible to permanently maintain a wire diameter that is generally sufficient to withstand the mechanical tension applied to it in the spark initiation zone without breaking. The relative movement of the wire and the part in the transverse direction allows the part to be cut or, where appropriate, its surface to be processed.

[0004] In an EDM machine, an electronic control device controls the components of the machine in order to establish or change the machining parameters selected by the user according to the nature of the electrode wire, the nature and size of the part to be machined and the particularity sought after machining. The machining parameters generally include the frequency of the electric pulses, the duration of the pause between two consecutive electric pulses, the starting voltage, the energy of the spark current, the peak amplitude of the spark current, the duration of the spark current, the characteristics of the dielectric fluid flow, the speed of the longitudinal routing of the electrode wire and the speed of the relative movement of the part to be machined relative to the electrode wire.

[0005] In order to obtain precise machining, in particular to produce cuts with a small radius, it is necessary to use a wire of small diameter having a high ultimate mechanical strength, allowing it to remain taut in the spark initiation zone and to limit the amplitude of vibrations.

[0006] Most modern EDM machines are designed to use a typical diameter of 0.25 mm and an ultimate strength between 400 and 1000 N / mm. 2 The metal wires in between.

[0007] Since EDM is a relatively slow process, there is a simultaneous need to maximize machining speed, particularly roughing speed.

[0008] It is now generally accepted that the machining speed depends directly on the spark initiation energy released in the spark initiation zone between the electrode wire and the part to be machined, and therefore on the electrical energy that the wire can conduct to the spark initiation zone. However, the erosive discharge in the spark initiation zone and the Joule heating produced by the current flowing through the wire tend to heat the wire, while reducing its ultimate mechanical strength.

[0009] Thus, one limitation of wire used for EDM is that it will break under the combined influence of heat and mechanical tension. This requires the user to limit the processing power of their EDM machine, which in turn limits the processing speed.

[0010] Therefore, a problem that often occurs in the wire cutting EDM method is the premature breakage of the electrode wire. Since this effect is obviously impossible to completely eliminate, devices have been provided in EDM machines to allow the electrode wire to be automatically rethreaded after it breaks. However, after the electrode wire breaks, the processing process is necessarily interrupted during the time required to rethread the electrode wire, which greatly reduces the overall speed of processing the part.

[0011] Various approaches have been pursued in an attempt to reduce the frequency of premature wire breakage during EDM processes.

[0012] For example, the mechanical strength of the wire electrode has been increased by appropriately selecting a constituent metal having high mechanical strength, such as steel.

[0013] According to another example, it has been proposed to use an EDM wire comprising a metal core and a continuous zinc coating, the function of the coating being to limit the heating of the metal core by means of the thermal energy consumed by the zinc during its evaporation. Attempts have been made to reduce the effects of the inevitable heating of the electrode wire caused by machining sparks and Joule heating, hoping to thereby prevent such heating from leading to fractures by reducing the mechanical strength of the electrode wire.

[0014] According to another example, document CH 633 739 A5 describes an EDM wire and a method for manufacturing the same, the wire having a core that can be made of copper or a copper-zinc alloy and having a coating made of a different copper-zinc alloy and covered with a film made of zinc oxide. The document mentions that the coating made of the copper-zinc alloy has a porous structure and that the wire has a rough surface, which increases the contact area with the water used as a dielectric machining liquid, thereby increasing the cooling of the wire and allowing a higher current to pass.

[0015] Document EP 0 930 131 B1 also describes an EDM wire and a method for manufacturing the same, the wire comprising a core made of a first copper-containing metal, an alloy layer formed on the core, and a surface layer formed on the alloy layer and made of a second metal having a lower evaporation temperature than the first metal. The document teaches that, in order to better cool the wire, it is advantageous to obtain a porous layer comprising open pores on the surface of the wire, which increases the contact area between the wire and the machining dielectric liquid, so as to better cool the wire.

[0016] According to another example, a high conductivity electrode wire is used to reduce heating of the wire caused by the current flowing therethrough, thereby hoping to prevent such inevitable heating leading to fracture by reducing the mechanical strength of the electrode wire.

[0017] According to another example, the average current flowing through the electrode wire is limited, thereby limiting the inevitable heating of the electrode wire by the current flowing through the electrode wire.

[0018] According to another example, attempts have been made to reduce the frequency of rapid spark triggering or short circuits that inevitably occur randomly in the spark initiation zone between the electrode wire and the part to be machined by promoting the removal of machining debris that easily leads to the triggering of dielectric breakdown, hoping that the less frequent short circuits will reduce the risk of electrode wire breakage. For example, in document US 5,945,010, it is proposed to anneal galvanized α-phase brass to produce a peripheral layer made of γ-phase brass and then stretch the intermediate product thus obtained to obtain the final diameter. This stretching produces a broken γ-phase brass surface layer. The resulting EDM wire surface is irregular, which the document indicates helps to remove machining debris.

[0019] Document FR 2 527 960 A1 teaches that one of the causes of wire breakage may be at least partially due to the presence of air bubbles transported by the wire itself into the spark initiation zone when the wire penetrates the processing liquid. The solution described is to reduce the amount of air bubbles entrained by the wire by mechanically cleaning the wire before it enters the spark initiation zone.

[0020] Unfortunately, despite the many attempts that have been developed over the years, none of these known methods has yet achieved satisfactory results, since random breakage of the electrode wire is still often observed during the EDM process. Furthermore, the limitation of the average current flowing through the electrode wire or the use of steel cored wire leads to an inevitable reduction in the EDM speed, which in practice significantly reduces the advantages of these methods. Summary of the invention

[0021] Therefore, it remains of great interest to reduce the frequency of random wire breakage in EDM processes without - if possible - reducing the processing speed.

[0022] Therefore, one problem solved by the present invention is to effectively prevent electrode wire breakage during EDM process without causing a significant decrease in the overall processing speed.

[0023] The present invention arises from the following observations.

[0024] The inventors have observed that during the EDM process, bubbles are formed inside the dielectric liquid in the spark initiation zone itself. These bubbles may accumulate above the spark initiation zone. In most cases, under conventional machining conditions, these bubbles are gradually removed from the spark initiation zone, in particular by escaping upwards or in the flow of dielectric liquid that removes machining debris.

[0025] Cleaning the wire before it is penetrated into the dielectric liquid - tensioned as in document FR 2 527 960 A1 - does not prevent the formation of bubbles in the spark initiation zone itself and this is probably why the solution proposed in this document does not sufficiently reduce the risk of wire breakage.

[0026] Based on these observations, the idea behind the present invention is that random breakage of the electrode wire is essentially due to the formation of bubbles in the spark initiation zone when, under certain processing conditions, bubbles are formed extremely excessively and agglomerated on the surface of the electrode wire around at least a section of its useful length in the spark initiation zone: when the bubbles on the surface of the electrode wire agglomerate excessively over a certain length of the electrode wire, the bubbles on the surface of the electrode wire tend to cause strong local heating of the wire and its breakage.

[0027] One explanation seems to be that the agglomerated bubbles insulate the electrode wire from the dielectric liquid, thus preventing it from being locally cooled. Another explanation seems to be that the agglomerated bubbles promote the initiation of sparks in the area occupied by the bubbles, thereby increasing the local heating of the electrode wire. These two effects may be combined.

[0028] Therefore, in order to avoid wire electrode breakage during EDM without significantly reducing the overall processing speed, the present invention proposes a method for machining a part by electrical discharge machining using a wire electrode, wherein:

[0029] - tensioning the wire electrode and driving it to translate longitudinally in the vicinity of the part to be machined in a spark initiation zone through which a stream of dielectric liquid moves,

[0030] - using a power source connected between the part to be machined and the wire electrode to generate an electrical pulse, said electrical pulse initiating a spark in a spark initiation zone between the wire electrode and the part to be machined,

[0031] - assessing (estimating) the amount of gas bubbles present in the spark initiation zone between the wire electrode and the part to be machined,

[0032] - modifying at least one processing parameter according to the estimated amount of gas bubbles so as to maintain or return the estimated amount of gas bubbles to a suitable value range.

[0033] The processing parameters mentioned in the description of the above solution are selected from conventional processing parameters of EDM machines, such as those listed in the opening part of this description with respect to the prior art.

[0034] It has been observed that the fact that the amount of gas bubbles is kept below the tolerance threshold leads to a significant reduction in the risk of wire breakage during the EDM process.

[0035] According to a first embodiment, the processing parameter acted upon to reduce the risk of wire breakage is the peak amplitude (peak value) of the spark current. In this case, when the estimated amount of bubbles increases, the peak amplitude of the spark current is reduced, and after a preset dwell time or preferably when the estimated amount of bubbles decreases, the peak amplitude of the spark current is increased.

[0036] In this first embodiment, the peak amplitude of the spark current may advantageously decrease when the estimated amount of bubbles increases above an upper threshold, and preferably may increase again when the estimated amount of bubbles decreases below a lower threshold.

[0037] Indeed, in a simpler variation of this first embodiment:

[0038] - comparing the estimated amount of said bubbles with at least one preset threshold amount,

[0039] - when the estimated amount of the gas bubbles reaches or exceeds the at least one preset threshold amount, reducing the peak amplitude of the spark current within a preset dwell time.

[0040] According to a second embodiment, the processing parameter acted upon to reduce the risk of wire breakage is the pause time between successive electrical pulses. In this case, the pause time between successive electrical pulses is increased when the estimated amount of bubbles increases, and is decreased after a preset dwell time or, preferably, when the estimated amount of bubbles decreases.

[0041] In this second embodiment, when the evaluated amount of bubbles has increased to exceed the upper threshold, the interval time between successive electric pulses can advantageously be increased, and preferably, when the evaluated amount of bubbles decreases to below the lower threshold, the interval time between successive electric pulses can be reduced again.

[0042] Indeed, in the simpler second embodiment:

[0043] - comparing the estimated amount of said bubbles with at least one preset threshold amount,

[0044] - when the estimated amount of gas bubbles reaches or exceeds the at least one preset threshold amount, increasing the pause time between consecutive electric pulses for a preset dwell time.

[0045] In any embodiment, by measuring the electrical impedance between the wire electrode and the part to be machined during the time interval between two consecutive pulses of the spark current, the amount of gas bubbles present in the spark initiation zone between the wire electrode and the part to be machined can be advantageously estimated indirectly.

[0046] In practice, the value of the capacitive element of the electrical impedance between the wire electrode and the part to be machined is measured during the time interval between two consecutive pulses of the spark current. Thus, disturbances of the erosive spark are avoided, thus maintaining machining efficiency.

[0047] According to a first possibility, the value of the capacitive component of the electrical impedance between the wire electrode and the part to be machined is derived by measuring the rate of rise of the voltage between the wire electrode and the part to be machined during the application of the starting voltage pulse and before the starting voltage pulse initiates a spark.

[0048] According to a second possibility, the amount of bubbles present in the spark initiation zone between the wire electrode and the part to be machined is indirectly evaluated by injecting a high-frequency current between the wire electrode and the part to be machined during the pause time between two consecutive electric pulses and by measuring the impedance of the circuit at this high frequency in order to deduce therefrom the value of the capacitive component of the electrical impedance between the wire electrode and the part to be machined.

[0049] For the same purpose, according to another aspect, the present invention provides an apparatus for machining a part by electric discharge machining using an electrode wire, comprising:

[0050] - means for keeping the wire electrode taut and for driving it in longitudinal translation in the spark initiation zone in the vicinity of the part to be machined,

[0051] - means for passing a stream of dielectric liquid through the spark initiation zone between the wire electrode and the part to be machined,

[0052] a power source connected between the workpiece to be machined and the wire electrode and capable of generating electrical pulses capable of initiating sparks in a spark initiation zone between the wire electrode and the workpiece to be machined,

[0053] - control means which control said power source, means for causing the flow of dielectric liquid to flow, and other means for establishing or changing process parameters by setting the characteristics of said electrical pulses, the characteristics of the flow of dielectric liquid and other process parameters,

[0054] a measuring device for evaluating the amount of gas bubbles present in the spark initiation zone between the wire electrode and the part to be machined and for transmitting a signal representative of the estimated amount of gas bubbles to a control device,

[0055] - adaptation means in the control device for modifying at least one machining parameter as a function of said signal representative of the estimated amount of gas bubbles present in the spark initiation zone between the wire electrode and the part to be machined, so as to maintain or return the value of said signal to a suitable range of signal values.

[0056] According to a first embodiment, the adapting device may include:

[0057] - a comparator for comparing said signal representative of the amount of gas bubbles present in the spark initiation zone between the wire electrode and the part to be machined with at least one preset signal threshold;

[0058] - a recorded control program for controlling the power supply so as to reduce the peak amplitude of the spark current when said signal representative of the amount of gas bubbles present in the spark initiation zone reaches or exceeds said at least one signal threshold.

[0059] According to a second embodiment, the adapting device may include:

[0060] - a comparator for comparing said signal representative of the amount of gas bubbles present in the spark initiation zone between the wire electrode and the part to be machined with at least one preset signal threshold,

[0061] - a recorded control program for controlling the power supply so as to increase the pause time between successive electrical pulses when said signal representative of the amount of gas bubbles present in the spark initiation zone reaches or exceeds said at least one signal threshold.

[0062] The means for assessing the amount of gas bubbles present in the spark initiation zone between the wire electrode and the part to be machined may advantageously comprise means for measuring the capacitive component of the impedance of the circuit formed by the spark initiation zone between the wire electrode and the part to be machined.

[0063] According to a first possibility, the means for measuring the capacitive impedance component measures the rate of rise of the initial voltage between the wire electrode and the part to be machined.

[0064] According to a second possibility, the means for measuring the capacitive impedance component comprises:

[0065] -High frequency generator, which can inject high frequency current between the electrode wire and the part to be processed,

[0066] - means for measuring the complex impedance of the circuit into which said high-frequency generator injects said high-frequency current,

[0067] - calculation means for extracting from the measured complex impedance the value of the capacitive impedance component of the circuit formed by the spark initiation zone between the wire electrode and the part to be machined. BRIEF DESCRIPTION OF THE DRAWINGS

[0068] Other objects, features and advantages of the present invention will become apparent from the following description of specific embodiments, which is given with reference to the accompanying drawings, in which:

[0069] Figure 1 is a schematic diagram of a wire electrode EDM machine incorporating an apparatus according to the present invention;

[0070] Figure 2 is a schematic perspective view showing, on a larger scale, a part to be machined during machining with a wire electrode;

[0071] Figure 3 yes Figure 2 a schematic front view of the whole;

[0072] Figure 4 yes Figure 2 A schematic top view of the whole;

[0073] Figure 5 is a schematic front elevational view in cross-section showing the spark initiation zone when no bubbles are present;

[0074] Figure 6 is a schematic front elevational view in cross-section showing the spark initiation zone in the presence of a bubble;

[0075] Figure 7 is a schematic diagram showing a waveform of a voltage between a wire electrode and a part to be machined during an erosive spark initiation step; and

[0076] Figure 8 is a schematic diagram showing the waveform of the current flowing through the electrode wire, the dielectric and the part to be machined during the erosive spark initiation step. DETAILED DESCRIPTION

[0077] First consider Figures 1 to 6 , these figures illustrate a method and a device according to the invention which allow machining by means of spark erosion of a wire electrode 4 while reducing the risk of premature breakage of the wire electrode 4 and without significantly reducing the speed of electrical discharge machining of the part 8 to be machined.

[0078] EDM machinery—e.g. Figure 1The process shown in the figure mainly comprises a processing chamber 1 containing a dielectric flow, such as deionized water, means such as pulleys 2 and 3 and wire guides 20 and 30 for holding an electrode wire 4 and keeping it taut in a spark initiation zone 5 inside the chamber 1, a part holder 6, and means 7 for moving the part holder 6 relative to the electrode wire 4 in the spark initiation zone 5. The part 8 to be processed held by the part holder 6 is placed in the spark initiation zone 5. The wire guides 20, 30 are located on both sides of the part 8 to be processed and accurately guide the electrode wire 4. To this end, these wire guides are positioned close to the part 8 to be processed, and their diameter is slightly larger than the diameter of the electrode wire 4, for example, a diameter of 254 μm for an electrode wire 4 of 250 μm. The electrode wire 4 is longitudinally routed (runned, fed, runoff) in the spark initiation zone 5 as shown by arrow 9 facing the part 8 to be processed. The power source 10 is electrically connected to the wire electrode 4 between the pulley 2 and the wire guide 20 via a line 18 and at least one contact 18 a, which contacts the wire electrode 4 during its passage through the dielectric of the chamber 1, and to the part to be machined 8 via a line 19, on the other hand, and generates electrical energy in the spark initiation zone 5 suitable for forming a spark or an arc between the part to be machined 8 and the wire electrode 4.

[0079] Now consider Figure 7 and Figure 8 , these figures show the waveforms of voltage U and current I respectively as a function of time t during an EDM electrical pulse. The electrical pulse first comprises a starting voltage pulse UA between times t0 and t2 and then comprises a spark current pulse IE between times t2 and t3.

[0080] exist Figure 7 In the embodiment, the power source 10 generates a starting voltage pulse UA, the rising front edge FM of which extends from the initial time t0 to the end time t1 of the establishment. The establishment of the maximum starting voltage U0 between the electrode wire 4 and the part to be machined 8 in the spark initiation zone 5 is not instantaneous, but occurs via an exponentially rising front edge FM, because it involves the problem of establishing a voltage between the terminals of the circuit formed by the spark initiation zone 5, so that the terminals essentially have the characteristics of a capacitor: in the absence of sparks, the dielectric liquid present in the spark initiation zone 5 insulates the metal elements, namely the electrode wire 4 and the part to be machined 8, and they together form a capacitive element.

[0081] After time t1 and until time t2 , power supply 10 maintains voltage U0 while waiting for dielectric breakdown.

[0082] At time t2, the dielectric breaks down, and the voltage U between the wire electrode 4 and the workpiece 8 to be machined drops sharply and remains low until time t3.

[0083] exist Figure 8 , the waveform of the current I supplied by the power supply 10 is illustrated, the current I remains low between time t0 and t1, and remains almost zero between time t1 and t2. After time t2 when the breakdown occurs, the power supply 10 generates a spark current IE in the form of a current pulse having a peak value or amplitude I0 and ends at time t3. It is this current pulse or spark current IE that feeds or sustains the EDM spark.

[0084] After time t3, the power supply 10 waits for the rest time Tp, and the cycle restarts with a new voltage pulse.

[0085] It will be appreciated that each pulse of the spark current IE will generate an erosive spark. Figure 8 The time interval Te between two consecutive sparks, ie between two consecutive pulses of the spark current pulse IE, is shown.

[0086] The power source 10 can act on the machining power, in particular by modulating the peak value or amplitude I0 of the spark current IE, the pulse duration of the spark current IE between times t2 and t3 and the pause time Tp between two consecutive electrical pulses UA-IE.

[0087] Consider Again Figure 1 The EDM machine comprises control means 40 for controlling the various units of the EDM machine according to suitable machining parameters. The user can select certain machining parameters depending in particular on the nature and shape of the part 8 to be machined, the composition of the electrode wire 4 and the type of machining to be performed (roughing, finishing).

[0088] Therefore, the control device 40 controls the power supply 10 in particular by adjusting the value, waveform and other parameters of the electrical energy generated by the power supply 10 in the spark initiation zone 5.

[0089] The control device 40 also controls other units of the EDM machine, in particular means 50 for causing a flow of dielectric liquid to flow through the spark initiation zone 5, drive means 60, such as an electric motor, for driving the electrode wire 4 to translate in the longitudinal direction as indicated by the arrow 9 and for adjusting its running speed in the spark initiation zone 5, and movement means 7 for ensuring that the part 8 to be machined is moved relative to the electrode wire 4 according to the desired machining step.

[0090] In the illustrated embodiment, the EDM machine further comprises a measuring device 70, the input-output lines 71 of which are electrically connected to the workpiece 8 to be machined and the wire electrode 4, respectively, and which can evaluate the amount of bubbles present in the spark initiation zone 5 between the wire electrode 4 and the workpiece 8 to be machined. The measuring device 70 transmits a signal 73 representing the estimated amount of bubbles to the control device 40 via a transmission line 72.

[0091] According to a first embodiment, the measuring device 70 may include a high-frequency generator 70a and a measuring device 70b, wherein the high-frequency generator 70a can inject a high-frequency current between the electrode wire 4 and the part to be processed 8 via an input-output line 71, and the measuring device 70b is used to measure the capacitive element of the complex electrical impedance of the circuit between the terminals of the input-output line 71 at the high frequency, and to derive therefrom a signal 73 representing the evaluated bubble amount.

[0092] In practice, any circuit forming the following capacitance meter can be used, which can quickly measure the capacitance of about 5pF to 100pF in a measurement time shorter than the conventional pause time Tp (about 0.5ms) between two consecutive electrical pulses. During this pause time Tp, the output impedance of the power supply 10 must be very high, an open circuit type impedance, so as not to interfere with the capacitance measurement.

[0093] Alternatively, according to a second embodiment, the measuring device 70 can receive a waveform of the voltage existing between the electrode wire 4 and the part to be processed 8 during the initial voltage pulse UA via the input-output line 71, and can measure the rising speed of the starting voltage U between the electrode wire 4 and the part to be processed 8 during the application of the starting voltage pulse UA by the power supply 10 so as to derive the value of the capacitive element of the electrical impedance existing between the electrode wire 4 and the part to be processed 8 from the waveform shown, thereby deriving a signal 73 characterizing the evaluated bubble amount.

[0094] In the control device 40, the adaptation device 41 is programmed to modify the processing parameters as a function of the signal 73 representing said estimated amount of gas bubbles in order to maintain the value of said signal 73 or to return the value of said signal 73 to a suitable range of signal values.

[0095] The limits of the suitable range of signal values ​​depend in particular on the nature and shape of the part 8 to be machined, the composition of the electrode wire 4 and the type of machining to be performed. These limits must therefore be determined by the user via routine machining trials during which satisfactory machining speeds and a significant reduction in the frequency of potential electrode-wire fractures will be observed.

[0096] In practice, a signal value range limited only by an upper value may be chosen, the adaptation means 41 being programmed to reduce the spark energy during a preset waiting time, allowing sufficient removal of the bubbles, and to allow the spark energy to return to its previous level afterwards at the end of the preset waiting time.

[0097] However, a signal value range defined by an upper limit value and a lower limit value will enable the EDM speed to be optimized.

[0098] The control means 40 and the adaptation means 41 may take the form of a microprocessor or a microcontroller associated with a suitable program. In fact, according to the method of the invention, a conventional control unit of the EDM machine may itself be programmed to perform the above functions and to control the constituent units of the EDM machine accordingly.

[0099] from Figure 2 As can be seen in the figure, by moving the part to be machined in the transverse direction - as indicated by arrow 11 - the spark erosion gradually causes the electrode wire 4 to penetrate into the conductive body of the part to be machined 8 and produce a groove 12. In the example shown, the cutting portion is a straight groove 12, which occupies the entire height H of the part to be machined 8. The cutting portion can be non-linear via the non-linear movement 11 of the part to be machined 8.

[0100] The movement of the workpiece 8 to be processed must follow the erosion caused by the electric spark and cannot exceed it. Too slow a speed will reduce the processing speed. Too fast a speed will cause the electrode wire 4 to contact the workpiece 8 to be processed, and the machine will stop working due to this short circuit.

[0101] Figure 3 and Figure 4 The spark initiation zone 5 in the slot 12 is shown in more detail. By adjusting the moving speed of the workpiece 8 to be machined, the spacing G between the outer surface of the wire electrode 4 and the inner surface of the slot 12 remains substantially constant. Therefore, the spark initiation zone 5 is located between the semi-cylindrical back of the slot 12 and the semi-cylinder formed by the half surface of the wire electrode 4 facing the semi-cylindrical back of the slot 12. The width L of the slot 12 is equal to the sum of the diameter D of the wire electrode and twice the spacing G.

[0102] Figure 5 and Figure 6 A cross-sectional view of the spark initiation zone 5 between the electrode wire 4 and the workpiece 8 to be machined is shown along the middle plane of the groove 12. Figure 5 In FIG. 5 , the spark initiation zone 5 is shown in a “normal” state, where no bubbles accumulate around the electrode wire 4. In contrast, in Figure 6 , the spark initiation zone 5 is shown as being in a "breakage risk" state, where gas bubbles 100 have accumulated around the electrode wire 4. It should be understood that in the length section occupied by the gas bubbles 100, the electrode wire 4 is thermally insulated from the dielectric liquid filling the rest of the spark initiation zone 5. As a result, the temperature of the length section of the electrode wire rises rapidly, resulting in an increased risk of the electrode wire 4 breaking in the length section occupied by the gas bubbles 100.

[0103] The present invention prevents the occurrence of such bubbles 100 having a large volume and accumulating on the electrode wire 4 by detecting the presence of an abnormally high amount of bubbles in the spark initiation zone 5 and by adjusting the processing parameters accordingly and instantaneously until the amount of bubbles returns to an acceptable level. As a result, the risk of breakage of the electrode wire 4 is significantly reduced.

[0104] Simulations show that when the diameter D of the electrode wire 4 is 250 μm, the height H of the part to be machined is 0.1 m and the conventional spacing G is 40 μm, when the dielectric liquid is deionized water, the value of the capacitive component of the electrical impedance present in the spark initiation zone 5 is approximately 800 pF, because the electrical permittivity of liquid water is approximately 81.

[0105] In contrast, the same simulations show that, assuming that the gas bubble 100 occupied all of the spark initiation zone 5, the value of the capacitive component of the electrical impedance present in the spark initiation zone 5 would be approximately 10 pF, since the permittivity of the gas bubble is approximately 1.

[0106] It will be appreciated that the presence of bubbles occupying only a portion of the spark initiation zone 5 will result in a capacitive element having intermediate values, which will be between 800 pF and 10 pF when the dielectric liquid used is deionized water.

[0107] Therefore, it can be seen that by measuring the value of the capacitive electrical impedance element present in the spark initiation zone 5, the amount of bubbles present in the spark initiation zone 5 can be evaluated. When the value decreases, this indicates that the presence of bubbles increases, and therefore, Figure 6 The illustrated bubble 100 has an increased risk of insulating bubbles being present.

[0108] It should be noted that when using this method to assess bubble volume, it is advantageous to use deionized water rather than an oil-based dielectric because an oil-based dielectric will have a permittivity closer to 2.2, i.e. closer to the permittivity of the gas that forms the bubble, which reduces the sensitivity of detection by measuring capacitance.

[0109] Preferably, the capacitive component of the complex electrical impedance present between the wire electrode 4 and the part to be machined 8 must be measured without interfering with the EDM method. In fact, in a first embodiment in which high frequencies are injected by means of the measuring device, the measuring step must be performed during the pause time Tp. In a second embodiment in which the speed of rise of the starting voltage is measured, the measuring step will be performed in the time interval between times t0 and t1 - that is, during the rising front FM of the starting voltage pulse - without changing the value and waveform of this pulse.

[0110] The invention is not limited to the embodiments that have been explicitly described, but comprises variants and generalizations thereof that are covered by the scope of the appended claims.

Claims

1. A method for machining a part (8) by electrical discharge machining using an electrode wire (4), wherein: - tensioning the electrode wire (4) and driving it in longitudinal translation (9) in the vicinity of the part (8) to be machined in a spark initiation zone (5) through which a stream of dielectric liquid moves, - using a power source (10) connected between the part to be machined (8) and the wire electrode (4) to generate an electric pulse (UA-IE), said electric pulse initiating a spark in said spark initiation zone (5) between said wire electrode (4) and said part to be machined (8), - indirectly assessing the amount of gas bubbles present in the spark initiation zone (5) between the wire electrode (4) and the part (8) to be machined by measuring the value of the capacitive component of the electrical impedance between the wire electrode (4) and the part (8) to be machined during the time interval (Te) between two consecutive pulses of the spark current (IE), - modifying at least one processing parameter according to the estimated amount of gas bubbles so that the estimated amount of gas bubbles remains within or returns to a suitable value range.

2. The processing method according to claim 1, characterized in that: When the estimated amount of the bubbles increases, the peak amplitude (I0) of the spark current (IE) is reduced, and when the estimated amount of the bubbles decreases, the peak amplitude (I0) of the spark current (IE) is increased.

3. The processing method according to claim 1 or 2, characterized in that: - comparing the estimated amount of said bubbles with at least one preset threshold amount, - when the estimated amount of the bubbles reaches or exceeds the at least one preset threshold amount, reducing the peak amplitude (I0) of the spark current for a preset dwell time.

4. The processing method according to claim 1, characterized in that: When the estimated amount of the air bubbles increases, the interval time (Tp) between the consecutive electric pulses (UA-IE) ​​is increased, and when the estimated amount of the air bubbles decreases, the interval time (Tp) between the consecutive electric pulses (UA-IE) ​​is decreased.

5. The processing method according to claim 1 or 2, characterized in that: - comparing the estimated amount of said bubbles with at least one preset threshold amount, - when said estimated amount of gas bubbles reaches or exceeds said at least one preset threshold amount, increasing the pause time (Tp) between consecutive electrical pulses (UA-IE) ​​for a preset dwell time.

6. The processing method according to claim 1, characterized in that: The value of the capacitive component of the electrical impedance between the wire electrode (4) and the part (8) to be machined is derived from the rate of rise of the voltage (U) between the wire electrode (4) and the part (8) to be machined measured during the application of a starting voltage pulse (UA) and before it initiates a spark.

7. The processing method according to claim 1, characterized in that: By injecting a high-frequency current between the wire electrode (4) and the part (8) to be machined during the interval time (Tp) between two consecutive electrical pulses (UA-IE), and by measuring the impedance of the circuit at this high frequency in order to deduce therefrom the value of the capacitive component of the electrical impedance between the wire electrode (4) and the part (8) to be machined, the amount of bubbles present in the spark initiation zone (5) between the wire electrode (4) and the part (8) to be machined is indirectly evaluated.

8. An apparatus for machining parts by electrical discharge machining using an electrode wire, comprising: - means (2, 3, 20, 30, 60) for keeping the electrode wire (4) taut and driving it in longitudinal translation (9) in the spark initiation zone (5) in the vicinity of the part (8) to be machined, - means for causing a flow of dielectric liquid to flow through the spark initiation zone (5) between the wire electrode (4) and the part to be machined (8), - an electric power source (10) which is electrically connected between the workpiece (8) to be machined and the wire electrode (4) and is capable of generating electric pulses (UA, IE) which are capable of initiating sparks in the spark initiation zone (5) between the wire electrode (4) and the workpiece (8) to be machined, - control means (40) which control said power source (10), said means (50) for causing the flow of dielectric liquid to flow, and other means for establishing or changing process parameters by setting the characteristics of said electrical pulses (UA, IE), the characteristics of said dielectric liquid flow and other process parameters, - a measuring device (70) for evaluating the amount of gas bubbles present in the spark initiation zone (5) between the wire electrode (4) and the part (8) to be machined, and for transmitting a signal (73) representative of the estimated amount of gas bubbles to the control device (40); - the means (70) for evaluating the amount of gas bubbles present in the spark initiation zone (5) between the wire electrode (4) and the part (8) to be machined comprises means for measuring the capacitive component of the impedance of the circuit formed by the spark initiation zone (5) between the wire electrode (4) and the part (8) to be machined; - an adaptation device (41) in the control device (40), which is used to modify at least one of the processing parameters based on the signal (73) representing the estimated amount of the bubbles present in the spark initiation zone (5) between the electrode wire (4) and the part to be processed (8) so that the value of the signal (73) is maintained or returned to a suitable range of signal values.

9. The processing equipment according to claim 8, characterized in that The adapting device (41) comprises: a comparator for comparing the signal (73) representative of the amount of gas bubbles present in the spark initiation zone (5) between the wire electrode (4) and the part (8) to be machined with at least one preset signal threshold, - a recorded control program for controlling the power supply (10) so as to reduce the peak amplitude (I0) of the spark current (IE) when the signal (73) representative of the amount of the gas bubbles present in the spark initiation zone (5) reaches or exceeds the at least one signal threshold.

10. The processing equipment according to claim 8, characterized in that The adapting device (41) comprises: a comparator for comparing the signal (73) representative of the amount of gas bubbles present in the spark initiation zone (5) between the wire electrode (4) and the part (8) to be machined with at least one preset signal threshold, - a recorded control program for controlling the power supply (10) so as to increase the pause time (Tp) between consecutive electrical pulses when the signal (73) representative of the amount of gas bubbles present in the spark initiation zone (5) reaches or exceeds the at least one signal threshold.

11. The processing equipment according to claim 8, characterized in that The device for measuring the capacitive impedance component measures the rising speed of the initial voltage (U) between the electrode wire (4) and the part to be processed (8).

12. The processing equipment according to claim 8, characterized in that The device for measuring the capacitive impedance component comprises: - a high frequency generator capable of injecting a high frequency current between the electrode wire (4) and the part to be processed (8), - means for measuring the complex impedance of the circuit into which said high-frequency generator injects said high-frequency current, - calculation means for extracting from the measured complex impedance the value of the capacitive impedance component of the circuit formed by the spark initiation zone (5) between the wire electrode (4) and the part to be machined (8).

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