METHOD FOR ELECTRICAL DISCHARGE MACHINING

Abstract

A method for electrical discharge machining (EDM) a workpiece by means of a train of machining pulses. During the machining time the machining pulses are applied to the working gap between workpiece and electrode.

An open voltage is first applied, the ignition delay time t.sub.d is measured, then, at the beginning of the discharge, its fall time t.sub.f is measured, and certain shape features (e.g. the pedestal and ramp) of the pulse are adapted in real time for the very same discharge, as a function of said ignition delay time and/or fall time. Moreover, instead of shaping the very same discharge, one or more subsequent discharges can be shaped as a function of t.sub.d and/or t.sub.f of a single discharge, or of an average of t.sub.d and/or t.sub.f over several discharges.

Claims

1. A method for electrical discharge machining of a workpiece by a tool electrode, wherein a plurality of discrete electrical discharge machining pulses are applied to a gap between the work piece and the tool electrode, comprising wherein, an open voltage U.sub.o is applied between the electrode and the work piece to induce a discharge; a gap voltage U.sub.Gap is measured; at least one time parameter related to the gap voltage is computed, a shape characteristics of a current pulse is determined based on the determined at least one time parameter, and the current pulse is generated according to the determined shape characteristics and applied to the tool electrode.

2. The method for the electrical discharge machining according to claim 1, whereas the time parameter is an ignition delay time t.sub.d and/or a fall time t.sub.f.

3. The method for the electrical discharge machining according to claim 1, wherein the determined time parameter is normalized, in particular with reference to the pulse duration t.sub.i, or with reference to a reference timeframe T.sub.ff, preferably the normalized value of the time parameter is used as a pointer to a memory location that contains the shape characteristics of the current pulse.

4. The method for the electrical discharge machining according to claim 3, wherein a combination of normalized values of two time parameters, in particular the combination of the normalized ignition delay time t.sub.d % and the normalized fall time t.sub.f100 is used as a pointer to a memory location that contains the shape characteristics of the current pulse.

5. The method for the electrical discharge machining according to claim 1, wherein a defined auxiliary current I.sub.o is issued at least until the current pulse is switched on, preferably during the whole pulse.

6. The method for the electrical discharge machining according to claim 1, wherein an average value of the time parameter of a plurality of consecutive voltage pulses is computed and used to determine the shape characteristics of the current pulse and the determined shape characteristics is applied for a plurality of subsequent machining discharge pulses.

7. The method for the electrical discharge machining according to claim 1, wherein the memory includes a set of look-up tables, in which the shape characteristics of the current pulse is stored and the look-up table is selected according to the given priority of the technological results to be achieved, in particular low wear or high material removal rate.

8. The method for the electrical discharge machining according to claim 1, wherein the shape characteristics of the current pulse includes a first shape feature and a second shape feature, whereas the first shape feature is a pedestal of the current pulse and the second shape feature is a ramp of the current pulse.

9. The method for the electrical discharge machining according to claim 8, and the first shape feature is configured to maintain the discharge and the second shape feature is configured to optimise the material removal rate and tool wear.

10. The method for the electrical discharge machining according to claim 1, wherein the shape features included in the look-up tables is optimized in advance by iterative erosion tests.

11. The method for the electrical discharge machining according to claim 1, wherein a discharge voltage U.sub.e of each electrical discharge pulses is acquired and stored, whereby front discharges are determined out of a plurality of successive discharges based on the voltage U.sub.e_front of said front discharges.

12. The method for the electrical discharge machining according to claim 11, wherein a generator power supply voltage is adapted, as a function of the determined front discharge voltage U.sub.e_front in order to be just slightly higher than the erosion voltage of the front discharges U.sub.e_front but lower than the erosion voltage of the side discharges U.sub.e_side.

13. The method for the electrical discharge machining according to claim 1, wherein the machining pulses having a delay time which is longer than a set reference ignition delay t.sub.d_side are cut off.

14. The method for the electrical discharge machining according to claim 1, wherein a dedicated current pulse (exemplarily less energetic) for discharges longer than a set reference ignition delay t.sub.d_side is issued.

15. The method for the electrical discharge machining according to claim 1, wherein the discharge machining is a die-sinking electrical discharge machine, a wire electrical discharge machine or a fast-wire electrical discharge machine.

16. A machine tool for electrical discharge machining of a workpiece by a tool electrode including a power generator and a control unit, wherein a plurality of discrete electrical discharge machining pulses are applied to a gap between the work piece and the tool electrode, comprising wherein, an open voltage U.sub.o is generated by the power generator and applied between the electrode and the work piece to induce a discharge; a gap voltage U.sub.Gap is measured; at least one time parameter related to the gap voltage is computed by the control unit, a shape characteristics of a current pulse is determined based on the determined at least one time parameter by the control unit, and the current pulse is generated according to the determined shape characteristics and applied to the tool electrode.

Description

DRAWINGS

[0053] Preferred embodiments of the invention will now be detailed with reference to the attached drawings, in which

[0054] FIG. 1a is a schematic plot of a typical gap voltage, machining current pulse and oscillator clock pulse;

[0055] FIG. 1b is a schematic plot of a typical gap voltage, machining current pulse and oscillator clock pulse; and

[0056] FIG. 1c is a schematic plot of a typical gap voltage, machining current pulse and oscillator clock pulse;

[0057] FIG. 2 is a graph illustrating MRR, relative tool wear, and gap width as a function of the reference ignition delay t.sub.dref in percent of the total pulse duration t.sub.i, as known in the art;

[0058] FIG. 3 is a graph illustrating the dependency of the tool electrode wear on the fall time t.sub.f, as known in the art;

[0059] FIG. 4 is a graph illustrating the dependency of the relative tool electrode wear on the reference ignition delay t.sub.dref, as known in the art;

[0060] FIG. 5a is a schematic plot of a voltage pulse with a small tool electrode wear;

[0061] FIG. 5b is a schematic plot of a voltage pulse with a big tool electrode wear;

[0062] FIG. 6 is a schematic plot of the gap voltage, the corresponding current pulse and a t.sub.d %-look-up table according to a first embodiment of the invention;

[0063] FIG. 7 is a schematic plot of the gap voltage, the corresponding current pulse and a t.sub.f-look-up table according to a second embodiment of the invention;

[0064] FIG. 8 is a schematic plot of the gap voltage, the corresponding current pulse and a t.sub.d+t.sub.f-look-up table according to a third embodiment of the invention;

[0065] FIG. 9a is a schematic plot of the gap voltage with potentially low tool erosion wear and the corresponding current pulse according to an embodiment of the invention;

[0066] FIG. 9b is a schematic plot of the gap voltage with potentially high tool erosion wear and the corresponding current pulse according to an embodiment of the invention;

[0067] FIG. 10 is a schematic plot of a current pulse with characteristic shape features;

[0068] FIG. 11 is a schematic plot of a sequence of current pulses with varying pulse shape;

[0069] FIG. 12a is a schematic plot of exemplary erosion current pulses with various wear limiting shapes;

[0070] FIG. 12b is a schematic plot of exemplary erosion current pulses with various wear limiting shapes;

[0071] FIG. 12c is a schematic plot of exemplary erosion current pulses with various wear limiting shapes;

[0072] FIG. 12d is a schematic plot of exemplary erosion current pulses with various wear limiting shapes;

[0073] FIG. 12e is a schematic plot of exemplary erosion current pulses with various wear limiting shapes; and

[0074] FIG. 12f is a schematic plot of exemplary erosion current pulses with various wear limiting shapes;

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0075] First, referring to FIGS. 1a and 1b there is illustrated the gap voltage and current profile of an ideal machining pulse as a function of time in a known die-sinking electrical discharge machine, with an anodic-poled machining electrode. FIG. 1c depicts the oscillator clock signal OCP controlling the pulse duration t.sub.i of the machining pulses and the pulse pause to between two pulses.

[0076] To initiate the discharge channel, an open voltage is applied to the gap between the tool electrode and the workpiece. The ignition occurs after an ignition delay time t.sub.d. With the ignition, the gap voltage falls from the value of the open voltage U.sub.o with a variable slope, during a fall time t.sub.f, to a discharge voltage U.sub.e. It is possible that a small current I.sub.o flows also during the fall time t.sub.f, which is illustrated in the FIG. 1b showing the current flowing across the gap.

[0077] The material removal occurs during the discharge time t.sub.e, whereas the tool electrode wear occurs at the beginning of this time interval. During the ignition delay time t.sub.d and in the pause time t.sub.o no machining current flows and there is practically no material removal.

[0078] The main aspect of the present invention is to reduce the tool electrode wear to the maximum to ensure a high copy precision and at the same time to achieve a high MRR.

[0079] FIG. 2 is taken from Fritz Klocke and Wilfried Konig, Abtragen, Generieren, Laser-materialbearbeitung, Springer Verlag, 4th edition, 1997, FIG. 2.20. This figure illustrates the gap width and technological parameters as a function of the machining voltage, and is complemented with a scale of the reference ignition delay t.sub.dref[%] and with the labels P1 and P2, as discussed here below. The curves illustrate the tool wear and the material removal rate in t.sub.dref relation to t.sub.dref The reference ignition delay presents the percentage of the desired ignition delay time t.sub.dref to pulse duration t.sub.i. For this machining regime, the point P1 with the maximum MRR is found with a t.sub.dref of about 15%, whereas the tool electrode wear increases sharply for values of t.sub.dref smaller than 30%. The reference ignition delay t.sub.dref is the imposed setpoint value for ignition delay t.sub.d, i.e. the actual t.sub.d is kept equal to t.sub.dref by the servo control loop.

[0080] One sees that either the machining is carried out around the point P1, thus maximizing the MRR, or around the point P2, thus obtaining a small tool wear, or in a work point in between, representing a trade-off between MRR and tool wear.

[0081] FIG. 3 illustrates the dependency of the tool electrode wear on the fall time t.sub.f, as known in the art. This FIG. 3 is published by Dirk Dauw, dissertation KU Leuven, On line identification and Optimization of electro-discharge machining, 1985, as FIG. 3-28.

[0082] FIG. 4 is published as FIG. 1-10 in the same dissertation, and illustrates the dependency of the relative tool electrode wear on the reference ignition delay t.sub.dref

[0083] FIGS. 5a and 5b illustrate the two main wear indicators of an EDM machining pulse: [0084] the fall time t.sub.f1 and t.sub.f2; and [0085] the ignition delay time t.sub.d1 and t.sub.d2, respectively for a high wear (5a) and a low wear (5b) ideal machining pulse.

[0086] In this example the fall time t.sub.f defines the time interval when the gap voltage falls from a first threshold voltage L.sub.1 to a second threshold voltage L.sub.2, and the ignition delay time t.sub.d defines the time interval between the crossing of a first threshold voltage L.sub.1 by the rising flank of the gap voltage and falling below the threshold voltage L.sub.1 by the falling flank of the gap voltage.

[0087] A small or missing ignition delay means that the gap is still ionized, therefore the next discharge is going to occur at the same spot, or very near to the preceding discharge, causing in this way a localized high tool electrode wear.

[0088] As said, the tool electrode wear occurs at the beginning of the discharge, by the electronic current.

[0089] Another discharge pulse parameter that influences the electrode wear is the fall time t.sub.f, since it correlates with the electron current density. The shorter the fall time is, the higher the current density is. Consequently, the tool electrode wear is higher with a shorter fall time than a longer fall time.

[0090] The method of the present invention provides the possibility to use one or both of these two indicators to predict the electrode wear caused by the present discharge, and to adapt the current pulse shape to optimize the tool wear and the material removal rate.

[0091] The current pulse shape is adapted in real time as a function of the time parameter t.sub.d and/or t.sub.f featured by the gap voltage, whereas the pulse shape features are retrieved from a fast memory to generate the present current pulse accordingly. These pulse shape features are illustrated exemplarily in FIG. 11. The auxiliary current I.sub.o is omitted for clarity. One feature is I.sub.ped which defines the pedestal current to be set at the beginning of the discharge current pulse. The second pulse shape feature is the current slope which is set after I.sub.ped based on dI.sub.ramp. Additional current pulse shape features may be specified and stored to get particular current pulse shapes, featuring e.g. additional slopes, steps, peaks, etc.

[0092] The slope of the current pulse can be defined as an increment dI.sub.ramp of current per time unit dt, with a suitable predefined time unit dt. Alternatively the slope can be indicated as the current rising time, that is, the time between the reaching of the pedestal current and the reaching of the machining current I.sub.e.

[0093] FIG. 9a shows a voltage pulse with a comparably long ignition delay time, and comparably long fall time. A suitable current pulse shape is quasi rectangular, providing high MRR. In contrast, FIG. 9b shows a voltage pulse with a short ignition delay time and steep falling flank. Here, a suitable current pulse comprises a pedestal current followed by current slope. FIG. 10 shows a sequence of current pulses, where the pulse shape is adapted in real time, pulsewise in consideration of the determined time parameters.

[0094] In some embodiments disclosed hereafter, during the ignition time and right thereafter, the generator issues a small auxiliary current I.sub.o. This current is very small during the ignition delay and is illustrated in FIGS. 9a and 9b, during the fall time, before the switching of the machining current. For simplicity the current I.sub.o is represented as a small current step. This current I.sub.o is preferably applied to overcome the small gap conductivity, and to give time to the generator to prepare the main current pulse. The inventive method requires a t.sub.d and/or t.sub.f computation, preferably for each machining pulse.

[0095] These computations can be done in a computation time t.sub.c of a few hundreds of ns, e.g. in a fast programmable gate array (FPGA). Nevertheless one must avoid that the discharge turns off and the plasma channel collapses, else a phase of instability would be triggered, where high frequency voltage and current spikes would enter an oscillation, causing a high wear. So, a minimal current is preferred to be sustained over the whole pulse.

[0096] FIG. 6 shows a first embodiment of the present invention. The method includes the following steps: [0097] for at least one, preferably every single erosion pulse the ignition delay time t.sub.d is determined; [0098] the percent value of the ignition delay time t.sub.d referred to the pulse duration t.sub.i is computed, yielding the ignition delay t.sub.d % (t.sub.d %1 to t.sub.d %40); [0099] the resulting value is used as a pointer in a look-up table, which is schematically illustrated in FIG. 6; [0100] the pulse shape features, e.g. a pedestal and ramp value for the current pulse are taken from a look-up table; and [0101] the generator issues the current pulse accordingly.

[0102] As shown in the FIG. 6, the current pulse applied to the electrode features the auxiliary current I.sub.o, a pedestal current I.sub.ped4 and a current ramp dI.sub.ramp4 at the beginning of the pulse. The auxiliary current serves to maintain the discharge channel after breakdown, until the machining current I.sub.e is initiated. The pedestal current is to get a stable discharge condition as quick as possible. Following the current pedestal, the ramp serves to modulate the current density at the beginning of the current pulse, to keep the material removal rate high and the tool wear low, as the wear is especially caused at the beginning of the discharge by high current density.

[0103] FIG. 6 shows an example of 40 different values of shape characteristics of the current pulses stored in the look-up table, but the number of the values of shape characteristics is not limited. If the ignition delay has a normalized value of t.sub.d %1, the values of I.sub.ped1 and dI.sub.ramp1 are used to generate the current pulses. If the ignition delay has a normalized value of t.sub.d %37, the values of I.sub.ped37 and dI.sub.ramp37 are used to generate the current pulse for this pulse.

[0104] For example, using the ignition delay time, the current shape is formed and applied as follows: the open voltage is applied to the gap and the ignition delay time is determined and normalized with reference to the duration of the set oscillator clock pulse which corresponds to the pulse duration t.sub.i. The normalized ignition delay time is used to determine the current pulse features from a look-up table as the one shown in FIG. 6. I.sub.ped4 defines the pedestal current of the machining pulse, and dI.sub.ramp4 defines the steepness of the following current ramp. By way of example, dI.sub.ramp is defined as the increment of current per time unit dt, e.g. dt=20 ns.

[0105] In an exemplary embodiment, dI.sub.ramp represents the multiplier of 20 ns units to increment the ramp current by 100 mA. In other words, to have a ramp of 0.1A/?s, or 0.1A/1000 ns, the multiplier is dI.sub.ramp=1000 ns/20 ns=50, meaning one increment of 0.1A every 20 ns*50=1 ?s. The auxiliary current I.sub.o is applied immediately at the detection of the breakdown. The pedestal current I.sub.ped is applied as soon as the current pulse features are determined based on the look-up table. The current rises sharply from the auxiliary current I.sub.o to the value of the pedestal current I.sub.ped4. Then, the current rises further from I.sub.ped4, with a steepness dI.sub.ramp4/dt, to the desired machining current I.sub.e which is maintained up to the end of the pulse.

[0106] Preferably, the time parameters of the present voltage pulse are used to determine the shape characteristics for the present current pulse only. Alternatively, the time parameters t.sub.d and/or t.sub.f of a plurality of machining impulses are averaged and/or these values are used to determine the shape characteristics for a present and a plurality of successive the current pulses.

[0107] FIG. 7 illustrates a second embodiment of the present invention, which includes the following steps: [0108] for every single erosion pulse the fall time t.sub.f is measured; [0109] the measured value is normalized, e.g. by rounding to 100 ns units, yielding t.sub.f100 (t.sub.f101 to t.sub.f140); [0110] the resulting value is used as a pointer in a look-up table, which is schematically illustrated in FIG. 7; [0111] the pulse shape features, e.g. a pedestal and ramp value for the current pulse are taken from the look-up table; [0112] the generator issues the current pulse accordingly.

[0113] As shown in the FIG. 7, the current pulse applied to the electrode features an auxiliary current I.sub.o, a pedestal value I.sub.ped104 and a current ramp dI.sub.ramp104.

[0114] For example, using the fall time t.sub.f of the gap voltage, the current shape is formed and applied as follows: the open voltage is applied to the gap and the fall time is determined and rounded. The rounded fall time, e.g. t.sub.f104 is used to determine the current pulse features from a look-up table which comprises for instance 40 different values of shape characteristics of the current pulses, as the one shown in FIG. 7. I.sub.ped104 defines the pedestal current to be applied, and dI.sub.ramp104 defines the steepness of the following current ramp. The current is applied in the same way as discussed with reference to FIG. 6.

[0115] FIG. 8 illustrates a third embodiment, in which a combination of both time parameters of the gap voltage are used to determine the current pulse shape characteristics, where following steps are carried out: [0116] For every single erosion pulse the ignition delay time t.sub.d and the fall time t.sub.f are measured and normalized, yielding the ignition delay time t.sub.d % and the fall time t.sub.f100; [0117] A wear evaluation function is created, exemplarily similar to:

WE.sub.n=k1*(100?t.sub.d %)+k2*t.sub.f100

Where k1 and k2 are fitting coefficients found by application tests; [0118] WE.sub.n is an index to a look-up table that yields the pulse shape characteristics, e.g. a pedestal and ramp value in consideration of the normalized delay time t.sub.d % and fall time t.sub.f100; [0119] The pulse generator outputs a current shape accordingly.

[0120] For example, using the ignition delay time t.sub.d and the fall time t.sub.f of the gap voltage, the current shape is formed and applied as follows: the open voltage is applied to the gap and the ignition delay time t.sub.d is determined and normalized e.g. as described with reference to the first embodiment. Moreover, the fall time t.sub.f is determined and rounded, e.g. as described with reference to the second embodiment. The wear evaluation function including the normalized ignition delay time t.sub.d % and the fall time t.sub.f100 of the gap voltage is computed. The output of the wear evaluation function, e.g. WE.sub.3 is used as a pointer to determine the current pulse features from the look-up table of FIG. 8. The current features are applied in the same way as for the case of the first and second embodiment.

[0121] These steps can be implemented using fast electronics in order to react within few hundreds of nanoseconds.

[0122] The pulse features are advantageously pre-computed and stored in a fast memory, e.g. used as a look-up table.

[0123] During this computation time, the current I.sub.o is applied to avoid the discharge extinction.

[0124] The current pulse shape is not limited to the variants outlined above. FIGS. 12a-12f shows several examples of current pulses with other shapes, which can be applied to the tool electrode. The number of the pulse shape features is not limited to one or two; the current pulse shape may be defined by an arbitrary number of features, however this number should be limited to limit the complexity. In particular, the current pulses features a pedestal at the beginning of the current pulse and a current ramp. FIG. 12a shows a current pulse having the auxiliary current followed by the pedestal and current ramp rising with the same steepness. Such pulse is very effective in terms of MRR, but could be undesirable in terms of electrode wear. FIG. 12b shows a current pulse with a pedestal and a ramp having a moderate slope. FIG. 12c shows a variant that the current pulse has no pedestal, but an initial current followed by a rising current profile and a phase in which the machining current is maintained till the switch off. FIG. 12d shows the example of applying a rising flank followed by a falling flank. FIG. 12e shows a current pulse in which the slope is obtained by stepwise increase the current. FIG. 12f shows a current pulse featuring a pedestal current followed by a rising current profile.

[0125] The invention is supported by certain exemplary embodiments. It goes without saying that the person skilled in the art may consider alternative embodiments by which the current pulse shape is adapted to the relevant voltage pulse characteristic, in real time. For instance, an ignition delay time may be rounded down to full microseconds, and directly used as a pointer to address the suitable prestored current shape.

[0126] The embodiments discussed herein feature an auxiliary current I.sub.o which may have a different shape and duration than shown, or be omitted entirely.

[0127] Preferably, the discharge time t.sub.e is a fixed time (so called isoenergetic pulses, when rectangular shaped impulses are used), but this is not imperative. For instance, the discharge time t.sub.e can be controlled as a function of the pulse shape, such as to still get isoenergetic machining pulses, even when using non-rectangular shaped pulses.

[0128] On the other hand, while keeping the discharge time t.sub.e constant, the current pulse shape characteristics can be adapted such as to still get isoenergetic machining pulses, or to at least to at least partially reduce the difference of the energy of pulses. For instance, for current pulse shapes displaying a small initial slope, the current is highly increased in the course of the discharge time t.sub.e to a level above of the current of a corresponding rectangular pulse.

[0129] Moreover, the number of the prestored current pulse shapes and the number of features of the pre-stored current pulse shapes can be set as needed. For instance, the invention was presented using exemplarily 40 current pulse shapes. However, the advantages of the invention are already appreciable with as few as two different pulse shapes, that is, a first pulse shape with a moderate slope and a second pulse shape with a rather steep slope. In further one current pulse shape may be used by default, and one or more other may replace the default current pulse shape depending on the value of the time parameter featured by the gap voltage.