Process and device for preventing breakage of electrode wire during machining by spark erosion

Abstract

A device for machining a part by electrical discharge machining using an electrode wire. The device includes equipment for holding the electrode wire taut and driving the wire to translate longitudinally, in proximity to the part to be machined, in a sparking zone. The device further includes equipment for making a stream of dielectric liquid flow through the sparking zone between the electrode wire and the part to be machined. An electrical power source generates electrical pulses that cause sparks in the sparking zone between the electrode wire and the part to be machined. The quantity of gas bubbles present in the sparking zone is measured, and a signal is produced, representative of the quantity of bubbles, the signal being delivered to a controller. The controller modifies machining parameters so as to maintain the value of the signal within a suitable range.

Claims

1. An adaptive process for machining a part by electrical discharge using an electrode wire, the process comprising the steps of: tightening and driving said electrode wire to translate longitudinally in proximity to said part being machined, in a sparking zone through which a stream of dielectric liquid is moved, using an electrical power source connected between the part being machined, and the electrode wire, to generate electrical pulses which cause sparks in the sparking zone between the electrode wire and the part being machined, measuring a quantity of gas bubbles present in the sparking zone between the electrode wire and the part, the measuring step being performed while the machining process is being conducted, modifying at least one machining parameter, in response to measurement of said quantity of gas bubbles, so as to maintain or return the measured quantity of bubbles to a suitable range of values, the modifying step being performed adaptively while the machining process is being conducted, wherein the measuring step is performed indirectly by measuring an electrical impedance between the electrode wire and the part being machined during a time interval between two successive pulses of spark current.

2. The process of claim 1, wherein a peak amplitude of a spark current is decreased when said quantity of gas bubbles increases, and the peak amplitude of the spark current is increased when said quantity of gas bubbles decreases.

3. The process of claim 1, further comprising: comparing said quantity of gas bubbles with at least one preset threshold quantity, decreasing a peak amplitude of spark current for a preset time, when the quantity of gas bubbles reaches or exceeds said preset threshold quantity.

4. The process of claim 1, further comprising: increasing a pause time between successive electrical pulses when said quantity of gas bubbles increases, and decreasing the pause time between said successive electrical pulses when said quantity of gas bubbles decreases.

5. The process of claim 1, further comprising: comparing said quantity of gas bubbles with at least one preset threshold quantity, and increasing a pause time between successive electrical pulses, for a preset time, when said quantity of gas bubbles reaches or exceeds said preset threshold quantity.

6. The process of claim 1, wherein the measuring step includes measuring a value of a capacitive element of the electrical impedance between the electrode wire and the part being machined, during the time interval between two successive pulses of spark current.

7. The process of claim 6, wherein the value of the capacitive element of the electrical impedance between the electrode wire and the part being machined is deduced by measuring a rise speed of the voltage between the electrode wire and the part being machined, during an application of an initiation electrical-voltage pulse before the latter causes a spark.

8. The process of claim 1, wherein the measuring step is performed indirectly, by injecting, between the electrode wire and the part being machined, a high-frequency electrical current during a pause time between two successive electrical pulses, and by measuring an impedance, at this high frequency, of an electrical circuit formed by the sparking zone between the electrode wire and the part being machined, in order to deduce therefrom a value of a capacitive element of the electrical impedance between the electrode wire and the part being machined.

9. A device for machining a part by electrical discharge machining using an electrode wire, the device comprising: means for holding the electrode wire taut and for driving the electrode wire to translate longitudinally in proximity to said part being machined in a sparking zone, a stream of dielectric liquid flowing through the sparking zone between the electrode wire and the part being machined, an electrical power source electrically connected between the part being machined and the electrode wire, the power source being capable of generating electrical pulses sufficient to cause sparks in the sparking zone between the electrode wire and the part being machined, measuring means for evaluating a quantity of gas bubbles present in the sparking zone between the electrode wire and the part, while the part is being machined, and for delivering a signal to a controller, the signal being representative of said evaluated quantity of gas bubbles, wherein the controller includes adapting means for modifying at least one machining parameter, depending on a value of said signal, so as to maintain or return the value of said signal to a suitable range of signal values while the part is being machined, wherein the measuring means comprises means for measuring an electrical impedance between the electrode wire and the part being machined, during a time interval between two successive pulses of spark current.

10. The machining device of claim 9, wherein said adapting means comprise: a comparator for comparing, to at least one preset signal threshold, said signal representative of the quantity of gas bubbles present in the sparking zone between the electrode wire and the part being machined, a recorded control program for controlling the electrical power source so as to decrease a peak amplitude of a spark current when said signal representative of said quantity of gas bubbles present in the sparking zone reaches or exceeds said at least one signal threshold.

11. The machining device of claim 9, wherein said adapting means comprises: a comparator for comparing, to at least one preset signal threshold, said signal representative of said quantity of gas bubbles present in the sparking zone between the electrode wire and the part being machined, a recorded control program for controlling the electrical power source so as to increase a pause time between successive electrical pulses when said signal representative of said quantity of gas bubbles present in the sparking zone reaches or exceeds said at least one signal threshold.

12. The machining device of claim 9, wherein the measuring means comprises means for measuring a capacitive component of an impedance of an electrical circuit formed by the sparking zone between the electrode wire and the part being machined.

13. The machining device of claim 9, wherein said means for measuring a capacitive component takes a measurement of a rise speed of an initiation voltage between the electrode wire and the part being machined.

14. The machining device of claim 12, wherein said means for measuring a capacitive component comprises: a high-frequency generator able to inject, between the electrode wire and the part being machined, a high-frequency electrical current, means for measuring a complex impedance of an electrical circuit into which said high-frequency generator injects said high-frequency electrical current, a microprocessor or a microcontroller associated with a program for extracting, from said measured complex impedance, a value of a capacitive impedance component of an electrical circuit formed by the sparking zone between the electrode wire and the part being machined.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other objects, features and advantages of the present invention will become apparent from the following description of particular embodiments, which description is given with reference to the appended figures, in which:

(2) FIG. 1 is a schematic view of a wire-electrode EDM machine incorporating a device according to the present invention;

(3) FIG. 2 is a schematic perspective view illustrating, at larger scale, a part to be machined during machining with the electrode wire;

(4) FIG. 3 is a schematic face-on view of the ensemble of FIG. 2;

(5) FIG. 4 is a schematic top view of the ensemble of FIG. 2;

(6) FIG. 5 is a schematic face-on view in cross section illustrating the sparking zone without the presence of gas bubbles;

(7) FIG. 6 is a schematic face-on view in cross section illustrating the sparking zone in the presence of a gas bubble;

(8) FIG. 7 is a schematic view illustrating the waveform of the voltage between the electrode wire and the part to be machined during an erosive sparking step; and

(9) FIG. 8 is a schematic view illustrating the waveform of the electrical current flowing through the electrode wire, the dielectric and the part to be machined during an erosive sparking step.

DESCRIPTION OF PREFERRED EMBODIMENTS

(10) FIGS. 1 to 6 will firstly be considered, these figures illustrating the process and device according to the present invention, which allow machining to be carried out by spark erosion by means of an electrode wire 4, while decreasing the risk of untimely breakage of the electrode wire 4 without substantially decreasing the speed at which a part to be machined 8 is electrically discharge machined.

(11) The EDM machine, such as illustrated in FIG. 1, essentially comprises a machining chamber 1 containing a stream of a dielectric 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 sparking zone 5 in the interior of the chamber 1, a part holder 6 and means 7 for moving the part holder 6 with respect to the electrode wire 4 in the sparking zone 5. The part 8 to be machined, held by the part holder 6, is placed in the sparking zone 5. The wire guides 20, 30 are located on either side of the part 8 to be machined, and guide the electrode wire 4 with precision. To do so, they are positioned close to the part 8 to be machined, and their diameter is slightly larger than that of the electrode wire 4, for example a diameter of 254 μm for an electrode wire 4 of 250 μm. The electrode wire 4 runs off longitudinally as indicated by the arrow 9 in the sparking zone 5 facing the part 8 to be machined. An electrical power source 10, electrically connected on the one hand to the electrode wire 4 via a line 18 and at least one contact 18a that touches the electrode wire 4 during its passage through the dielectric of the chamber 1 between the pulley 2 and the wire guide 20, and on the other hand to the part to be machined 8 via a line 19, generates in the sparking zone 5 an electrical energy that is suitable for making sparks or electric arcs appear between the part 8 to be machined and the electrode wire 4.

(12) FIGS. 7 and 8 will now be considered, these figures respectively illustrating, as a function of time t, the waveform of the voltage U and the waveform of the current I during an EDM electrical pulse. The electrical pulse firstly comprises, between the times t.sub.0 and t.sub.2, an initiation voltage pulse UA, then, between the times t.sub.2 and t.sub.3, a spark current pulse IE.

(13) In FIG. 7, the electrical power source 10 generates an initiation voltage pulse UA the rising front FM of which extends from the initial time t.sub.0 to the establishment end time t.sub.1. The establishment of the maximum initiation voltage U.sub.0 between the electrode wire 4 and the part 8 to be machined in the sparking zone 5 is not instantaneous, but occurs via an exponential rising front FM because it is a question of the establishment of a voltage across the terminals of an electrical circuit formed by the sparking zone 5 essentially having the properties of an electrical capacitor: in the absence of sparks, the dielectric liquid present in the sparking zone 5 insulates the metal elements that are the electrode wire 4 and the part 8 to be machined, and they together form a capacitive element.

(14) After the time t.sub.1 and up to a time t.sub.2 the electrical power source 10 maintains the voltage U.sub.0 while waiting for the dielectric to break down.

(15) At the time t.sub.2 the breakdown of the dielectric occurs, and the voltage U between the electrode wire 4 and the part 8 to be machined drops abruptly, and remains low up to the time t.sub.3.

(16) In FIG. 8, which illustrates the waveform of the electrical current I supplied by the electrical power source 10, the electrical current I remains low between the times t.sub.0 and t.sub.1, and it remains almost zero between the times t.sub.1 and t.sub.2. After the time t.sub.2 at which breakdown occurs, the electrical power source 10 generates the spark current IE in the form of a current pulse having a peak magnitude or amplitude I.sub.0 and that ends at the time t.sub.3. It is this current pulse, or spark current IE, that feeds and sustains the EDM spark.

(17) After the time t.sub.3, the electrical power source 10 waits a pause time Tp, and the cycle restarts with a new voltage pulse.

(18) It will be understood that an erosive spark is produced on each pulse of spark current IE. FIG. 8 illustrates the time interval Te between two successive sparks, i.e. between two successive pulses of spark current IE.

(19) The electrical power source 10 may act on the machining power, in particular by modulating the peak magnitude or amplitude I.sub.0 of the spark current IE, the pulse duration of the spark current IE between the times t.sub.2 and t.sub.3, and the pause time Tp between two successive electrical pulses UA-IE.

(20) Consider once again FIG. 1. The EDM machine comprises controlling means 40 for controlling the various units of the EDM machine depending on suitable machining parameters. The user may choose certain machining parameters in particular depending on the nature and shape of the part to be machined 8, the makeup of the electrode wire 4, and on the type of machining (rough machining, finishing) to be carried out.

(21) Thus, the controlling means 40 control the electrical power source 10 in particular by adapting the value, the waveform and the other parameters of the electrical energy generated in the sparking zone 5 by the electrical power source 10.

(22) The controlling means 40 also control the other units of the EDM machine, in particular the means 50 for making the stream of dielectric liquid flow through the sparking zone 5, the driving means 60 such as an electric motor for driving the electrode wire 4 to translate longitudinally, as illustrated by the arrow 9, and for adapting its run-off speed in the sparking zone 5, and moving means 7 that ensure the movement of the part 8 to be machined with respect to the electrode wire 4 depending on the desired machining steps.

(23) In the illustrated embodiment, the EDM machine furthermore comprises measuring means 70, the input-output lines 71 of which are electrically connected to the part 8 to be machined and to the electrode wire 4, respectively, and that are able to evaluate the quantity of gas bubbles present in the sparking zone 5 between the electrode wire 4 and the part 8 to be machined. The measuring means 70 deliver, to the controlling means 40, via the transmission line 72, a signal 73 representative of said evaluated quantity of gas bubbles.

(24) According to a first embodiment, the measuring means 70 may comprise a high-frequency generator 70a able to inject via the input-output lines 71 a high-frequency electrical current between the electrode wire 4 and the part 8 to be machined, and measuring means 70b for measuring the capacitive element of the complex electrical impedance of the electrical circuit across the terminals of the input-output lines 71 at this high frequency and for deducing therefrom the signal 73 representative of the evaluated quantity of gas bubbles.

(25) In practice, any circuit forming a capacitance meter capable of rapidly measuring capacitances of about 5 to 100 pF in a measurement time shorter than the conventional pause time Tp (about 0.5 ms) between two successive electrical pulses will possibly be used. During this pause time Tp, the output impedance of the electrical power source 10 must be very high, of open-circuit type, in order not to perturb the capacitance measurement.

(26) Alternatively, according to a second embodiment, the measuring means 70 may receive, via the input-output lines 71, the waveform of the voltage present between the electrode wire 4 and the part 8 to be machined during the initiation voltage pulse UA, and may deduce therefrom, by measuring the rise speed of the initiation voltage U between the electrode wire 4 and the part 8 to be machined during the application of an initiation voltage pulse UA by the electrical power source 10, the value of the capacitive element of the electrical impedance present between the electrode wire 4 and the part 8 to be machined, in order to deduce therefrom the signal 73 representative of the evaluated quantity of gas bubbles.

(27) In the controlling means 40, adapting means 41 are programmed to modify the machining parameters depending on the signal 73 representative of said evaluated quantity of gas bubbles, so as to maintain or return the value of said signal 73 to a suitable range of signal values.

(28) The limits of the suitable range of signal values in particular depend on the nature and shape of the part 8 to be machined, on the makeup of the electrode wire 4, and on the type of machining to be carried out. These limits must therefore be determined by the user, via routine machining trials during which a satisfactory machining speed and a substantial decrease in the frequency of potential electrode-wire breakages will be observed.

(29) In practice, a range of signal values defined solely by an upper limit will possibly be chosen, the adapting means 41 then being programmed to decrease the sparking energy during a preset waiting time allowing a sufficient removal of the gas bubbles, and to then return the sparking energy to its prior level at the end of the preset waiting time.

(30) However, a range of signal values defined by an upper limit and a lower limit will allow the EDM speed to be optimized.

(31) The controlling means 40 and the adapting means 41 may take the form of a microprocessor or a microcontroller associated with a suitable program. In practice, the conventional control units of EDM machines may themselves be programmed to perform the functions described above and to accordingly control the constituent units of the EDM machine according to the process of the present invention.

(32) As may be seen in FIG. 2, by moving the part to be machined in a transverse direction, as indicated by the arrow 11, spark erosion gradually causes the electrode wire 4 to penetrate into the bulk of the part 8 to be machined, which is electrically conductive, and produces a slot 12. In the illustrated example, the cut is a rectilinear slot 12, which occupies the entire height H of the part 8 to be machined. Via a non-linear movement 11 of the part 8 to be machined, the cut may be non-linear.

(33) The movement of the part 8 to be machined must follow the erosion produced by the sparks, without excess. Too slow a speed decreases the machining speed. Too high a speed leads to contact of the electrode wire 4 and the part 8 to be machined, and stoppage of the machine as a result of this short-circuit.

(34) FIGS. 3 and 4 illustrate in more detail the sparking zone 5 in the slot 12. By adapting the speed of movement of the part 8 to be machined, the spacing G between the exterior surface of the electrode wire 4 and the interior surface of the slot 12 is kept substantially constant. Thus, the sparking zone 5 is located between the semi-cylindrical back of the slot 12 and the semi-cylinder formed by the half-surface of the electrode wire 4 that is oriented toward the semi-cylindrical back of the slot 12. The width L of the slot 12 is equal to the diameter D of the electrode wire increased by two times the spacing G.

(35) FIGS. 5 and 6 illustrate, in cross section along the median plane of the slot 12, the sparking zone 5 between the electrode wire 4 and the part 8 to be machined. In FIG. 5, the sparking zone 5 is illustrated in a “normal” state in which no gas bubble has amassed about the electrode wire 4. In contrast, in FIG. 6, the sparking zone 5 is illustrated in a “risk of breakage” state in which a gas bubble 100 has amassed around the electrode wire 4. It will be understood that, in the segment of length occupied by the gas bubble 100, the electrode wire 4 is thermally insulated from the dielectric liquid filling the rest of the sparking zone 5. As a result, the temperature of said segment of length of electrode wire rapidly increases and causes an increase in the risk of breakage of the electrode wire 4 in said segment of length occupied by the gas bubble 100.

(36) The present invention, by detecting the presence of an abnormally high quantity of gas bubbles in the sparking zone 5, and by adapting accordingly and momentarily the machining parameters until this quantity of gas bubbles returns to an acceptable level, aims to prevent the appearance of such a gas bubble 100 of large volume and amassed on the electrode wire 4. A very noticeable decrease in the risk of breakage of the electrode wire 4 results.

(37) A simulation has shown that, in the case of an electrode wire 4 the diameter D of which is 250 μm, of a part to be machined the height H of which is 0.1 m, and of a conventional spacing G of 40 μm, the value of the capacitive element of the electrical impedance present in the sparking zone 5 is about 800 pF when the dielectric liquid is deionized water, since the electrical permittivity of liquid water is about 81.

(38) In contrast, the same simulation shows, assuming that a gas bubble 100 occupies all the sparking zone 5, that the value of the capacitive element of the electrical impedance present in the sparking zone 5 would be about 10 pF, since the electrical permittivity of the bubble gases is about 1.

(39) It will be understood that the presence of gas bubbles occupying only some of the sparking zone 5 leads to an intermediate value of the capacitive element, which value will be comprised between 800 pF and 10 pF when the dielectric liquid used is deionized water.

(40) It may therefore be seen that it is possible to evaluate the quantity of gas bubbles present in the sparking zone 5 by measuring the value of the capacitive electrical-impedance element present in the sparking zone 5. When this value decreases, this indicates an increase in the presence of gas bubbles, and therefore an increase in the risk of presence of insulating gas bubbles such as the gas bubble 100 illustrated in FIG. 6.

(41) It will be noted that, when using this method for evaluating the quantity of gas bubbles, it is more advantageous to use deionized water than an oil-based dielectric, because an oil-based dielectric will have a permittivity close to 2.2, i.e. closer to that of the gases forming the gas bubbles, this decreasing the sensitivity of the detection by measurement of capacitance.

(42) The capacitive element of the complex electrical impedance present between the electrode wire 4 and the part 8 to be machined must preferably be measured without perturbing the EDM process. In practice, in the first embodiment in which a high frequency is injected by the measuring means, the measuring step will have to be carried out during the pause time Tp. In the second embodiment in which the rise speed of the initiation voltage is measured, the measuring step will be carried out during the time interval between the times t.sub.0 and t.sub.1, i.e. during the rising front FM of the initiation voltage pulse, without modifying the value and waveform of this pulse.

(43) The present invention is not limited to the embodiments that have been explicitly described, but includes the various variants and generalizations thereof encompassed by the scope of the following claims.