ELECTRIC SPARK DISCHARGE MACHINE

Abstract: An electric discharge machine is provided with first and second power supply sources which are connected in parallel during a duty cycle and diodes are arranged in series for the output of the first supply to prevent reverse current in the second supply when the loaded output voltage of the first supply is higher than that of the second supply.

Description: ELECTRIC SPARK DISCHARGE MACHINE This invention relates to electric spark discharge machines. Electric spark discharge machines exist in various forms all having the common characteristic that a spark is struck across a spark gap between a workpiece and an electrode to erode, and thus shape, the workpiece. The conditions in which a spark is struck vary dependant on the intended work rate, finish, materials and so on, but to ensure the spark strikes reliably it is often desired to strike at a much higher voltage across the gap than that needed to maintain the spark. Conventionally series resistance (including internal resistance), is used to regulate the output of the generator providing the discharge current, so that before the spark strikes the (open circuit) output voltage across the spark gap is high, and so that as the spark discharge draws a rapidly increasing current, the voltage across the spark gap falls to that required to maintain the discharge current at a desired level. It is observed that this arrangement wastes large amounts of power in the regulating resistance. For example, if the striking voltage is 100 volts, the discharge current is 100 amps and the gap voltage required to maintain the discharge is 27 volts (all typical examples), then once the spark has been struck, 7300 watts is dissipated as wasted heat during the remainder of the duty cycle. In addition to the waste of energy, this leads to the need for high voltage generators capable of delivering a high output current, which generators are expensive. GB-A-1196644 discloses an electric discharge machine having means to produce a high voltage high impedance low current followed by a low voltage high current. Power switching means are provided to connect the power supplies to the workgap. The switching means is complex, and there is a transition period between switching out of the high voltage supply and the switching in of the low voltage supply during which the spark will extinguish. This results in poor machining. GB-A-947363 discloses a machine in which the strike current and the low voltage supply are drawn from the main output transformer or the high voltage supply is drawn from the transformer and a transistorised switched low voltage high current supply is used to generate the low current. It is difficult to ensure the timing of the strike current with respect to the low voltage high current supply in such a system and as a result the correct spark gap will not always be present. It is one object of this invention to provide a simple and relative inexpensive system for providing the power for an electric discharge machine in such a way that there is little or no waste of power. In accordance with one aspect of the invention there is provided an electric discharge machine for generating a spark across a spark gap between an electrode and a workpiece to erode the workpiece, the machine including: a first power supply having a relatively low short circuit output current and a relatively high open circuit output voltage for striking a spark discharge between the workpiece and the electrode; and a second power supply having a relatively low open circuit output voltage and a relatively high short circuit output current for maintaining spark discharge between the electrode and the workpiece, characterised in that the first and second supplies are connected in parallel during a duty cycle and in that means are present to prevent reverse current in the second supply when the loaded higher output voltage of the first supply is higher than that of the second supply. In use, the spark is struck by the first power supply which, since it has a low short circuit current is, firstly, not expensive and, secondly, when the spark strikes and the voltage across the gap falls to, say, 27 volts, the wasted power is low. For example, if the striking voltage is the same 100 volts and the short circuit current of the first supply is 0.5 amps, the waste power is only 36.5 watts. Further, since the second supply which, which supplies the bulk of the discharge current has a low open circuit voltage, its cost is far less and the power wasted is again reduced. For example, if the open circuit output voltage of the second supply is 45 volts, at 100 amps discharge current, the wasted power is only 1800 watts. The arrangement can be quite simple in principle. For example, a preferred embodiment is so arranged that the first and second power supplies are connected in parallel during a duty cycle, one or more diodes being arranged in series with the output of the second supply to prevent reverse current in the second supply when the loaded output voltage of the first supply is higher than that of the second supply. There is thus no need for switch control gear to time the operation of the second supply relative to the application of the striking voltage to the spark gap. The term "prevent" in this context is intended to include the practical reality of there being small so-called "leakage" currents. The duty cycle of the machine preferably is controlled by first switch means in parallel with the spark gap, second switch means in series with the output of the second supply, and switch control means for opening the first switch means and closing the second switch means in a duty cycle and for closing the first switch means and opening the second switch means in a non-duty cycle. So that different operating conditions may be set, the first supply may preferably be set to provide one of a plurality of different striking voltages. Because the second supply is not subject to high voltages, transistors may be used to control its output current. Thus the machine preferably includes transistor means having a controlled path in series with the output of the second supply, and means providing a current control signal to a control gate of the transistor means, to determine the current in the controlled path. This has the advantage that since the control signal can be varied automatically between one duty cycle and the next, the discharge current can be varied automatically. Experiments show that if suitable variations are chosen, a high work rate can be achieved at the same time as a fine finish. For example, the current control means may be arranged to control the transistor means to provide a periodic current profile, e.g. the ratio of the currents in consecutive cycles is 1:2:4 or 1:4:16 in a repeating sequence. In an alternative, the current control means includes a pseudo-random sequence generator arranged to determine the control signal so as to produce a pseudo-random sequence of output currents from the second supply. In another aspect the invention provides a method of treating a workpiece by an electric spark discharge, the method comprising striking a spark discharge between the workpiece and an electrode from a first power supply having a relatively high open circuit output voltage and providing a relatively low short circuit output current, and maintaining the spark discharge between the workpiece and the electrode by means of a second power supply having a relatively low open circuit current until the treatment is finished, characterised by connecting the first and second supplies in parallel during a duty cycle and by preventing reverse current in the second supply when the loaded higher output voltage of the first supply exceeds that of the second supply. One embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which Figure 1 is a block diagram of the spark generator of an electric discharge machine embodying the invention. Figure 2 is a schematic detail of high voltage and high current generators of Figure 1, and Figure 3 shows the switching wave forms. Referring to the drawings, a spark gap 2 is schematically illustrated between a workpiece 4 and an electrode 6. In one example, the electrode is an inverse replica of the shape to which it is desired to erode the workpiece and, in use, is advanced towards the workpiece at a rate controlled by measured spark gap parameters. A first, high voltage, generator 8 is connected directly across the gap 2. The circuit of the high voltage generator 8 is illustrated schematically in Figure 2. Here a mains transformer T1 has a primary winding 10 connected to a mains electricity supply, not shown. A secondary winding 12 of the transformer T1 , has taps giving +45 V, -55 V, -155 V and -155 V relative to a 0 V tap. As shown, the + 45 V tap is connected directly to one side of an input to a strike supply rectification and smoothing network 14. The other side of the input to the rectification and smoothing network 14 is connected to relay means, illustrated schematically at 16, which enables one of the 0 V, -55 V, -155 V or -255 V taps of the winding 12 to be selected so that the output of the rectifying and smoothing network 14, connected across the gap 2 is selectable between 45 V, 100 V, 200 V or 300 V, as determined by a voltage select input from a DIP switch, not illustrated. The winding 12 is not intended to carry a heavy current and the internal resistance of the winding 12 and rectifying and smoothing circuit 14 combined is such that the short circuit current output of the generator 8 is limited to, for example 0.5 amps at 300 V and 1 amp at 100 V. Also connected across the gap 2 is a series circuit containing a second, high current generator 16 and a current control circuit 18 illustrated schematically in more detail in Figure 2. The transformer T1 has a further secondary winding 20 which is capable of carrying heavy currents and, accordingly, has a low internal resistance. The winding has 45 V and 0 V outputs connected to the input of a high current rectifying and smoothing circuit 22 to produce a rectified output having an open circuit voltage of 45 V and a short circuit current of 120 amps, for example. The current control circuit 18 comprises a bank of parallel transistors, only one is illustrated in Figure 2. The current control system utilises the transistor characteristic that a constant base-emitter voltage applied to a transistor produces a constant current in the base-collector path for a known temperature. The variables are controlled by adding emitter resistors to each transistor, using a temperature reference diode and thermally balancing the transistor heat sinks (not illustrated in the drawings). In order to control the duty cycle of the machine a spark timebase generator 24 is provided by which the duty cycle and repetition rate of a spark timebase on lead 26 can be varied. The spark timebase is input to a parallel FET drive 28 and a series FET drive 30, which respectively control switch means in the form of FETs 32 connected in parallel across the gap 2, and FETs 24 connected in series with the high current generator 16 and current control circuit 18, so that the FETs 32 go open circuit and the FETs 34 conduct in a duty cycle, whereas the FETs 32 conduct and the FETs 34 go open circuit in a non-duty cycle. In a non-duty cycle, the gap is thus a) short circuited by the FETs 32, and b) isolated from the high voltage end of the supply 16 by the FETs 34. In a duty cycle, initially the selected high voltage supply open circuit voltage appears across the gap 2, eg 300 V. In order to prevent reverse currents in the high current supply at the beginning of a duty cycle, schottky diodes 36 are connected in series therewith. The high voltage rapidly establishes sparking between the workpiece and the electrode, leading to a rapidly increasing current which, due to the regulation characteristics of the high voltage generator 8 rapidly pulls the gap voltage down towards 45 V. As soon as the schottky diodes 36 become forward biassed, discharge current is also supplied by the high current generator 16 which supplies the bulk of the gap current, as determined by the current control means 18, up to 120 amps. The voltage across the gap 2 is sensed by a servo control signal circuit 38 and the resulting control signal is used to control the position of the electrode relative to the workpiece so that the voltage across the gap is regulated to 27 V. If the voltage across the gap is higher, the electrode is advanced towards the workpiece. If the voltage across the gap is lower than 27 V advances is stopped or the electrode is backed off the workpiece. The servo control signal is also supplied to a voltmeter via a drive 40 and to a speaker via a drive 42 to provide visual and audio indications of conditions at the gap. The gap current is selected from a DIP switch (not shown) the setting of which is decoded by a digital to analog convertor 44 which, in turn, provides an input to a circuit 46 which drives the bases of the transistor bank in the current control circuit 18. Since the gap current is constant at a given setting, and since that determines a constant voltage drop in the generator 16, too low a gap voltage implies a higher voltage drop across, and thus heat dissipation in, the bank of transistors in the current control circuit 18. Although a low gap voltage will result in the servo control backing the electrode off the workpiece (or at least stopping its advance), the reaction is too slow to protect the current control transistors. To provide protection the gap voltage (essentially) is sensed at the node between the FETs 34 and the current control circuit by an arc suppression circuit 48 and compared with a variable reference signal on lead 50. If the arc suppression circuit sets the output of the convertor 44 to zero, thus suppressing the arc until the gap voltage has recovered. The wave forms of the switching are shown in Figure 3. Other digital inputs (not shown) to the digital to analog convertor may be automatically cycled by an input from the spark time base. Thus currents in proportions 1:2:4 or 1:4:16, for example may be provided, or a pseudo-random digital sequence generator may be indexed by the time base so that the input to the digital to analog convertor is a pseudo-random sequence. Experiments have shown that such arrangements enable a fine finish, eg corresponding to a current of 2 amps in a conventional machine, may be attained at a work rate corresponding to a much higher current, eg 30 amps, in a conventional machine. The invention is not limited to the embodiment shown. For example the secondary winding 12 of the taps of the transformer may be 0.60 V (ac), 155 V (ac), 220 V (ac); and that of the secondary winding 20 may be 18 V (ac) N; 18 V (ac) N; 18 V (ac) N.

Claims: CLAIMS 1. An electric discharge machine for generating sparks across a spark gap between an electrode and a workpiece to erode the workpiece, the machine including: a first power supply having a relatively low short circuit output current and a relatively high open circuit output voltage for striking a spark discharge between the workpiece and the electrode; and a second power supply having a relatively low open circuit output voltage and a relatively high short circuit output current for maintaining spark discharge between the electrode and the workpiece, characterised in that the first and second supplies are connected in parallel during a duty cycle and in that means are present to prevent reverse current in the second supply when the loaded higher output voltage of the first supply is higher than that of the second supply. 2. A machine as claimed in Claim 1 , characterised in that the means to prevent reverse current in the second supply when the loaded output voltage of the first supply is higher than that of the second supply comprises one or more diodes arranged in series with the output of the second supply. 3. A machine as claimed in Claim 1 or 2, characterised by first switch means in parallel with the spark gap, second switch means in series with the output of the second supply, and switch control means for opening the first switch means and closing the second switch means in a duty cycle, and for closing the first switch means and opening the second switch means in a non-duty cycle. 4. A machine as claimed in any preceding Claim, characterised by means whereby the first supply may be set to provide one of a plurality of different striking voltages. 5. A machine as claimed in any preceding Claim, characterised by transistor means having a controlled path in series with the output of the second supply, and means providing a current control signal to a control gate of the transistor means to determine the current in the controlled path. 6. A machine as claimed in Claim 5, characterised by the current control means for automatically changing the current control signal between one duty cycle and the next. 7. A machine as claimed in Claim 6, characterised in that the current control means is arranged to control the transistor means to provide a periodic current profile. 8. A machine as claimed in Claim 6, characterised in that the current control means includes a pseudo-random sequence generator arranged to determine the control signal so as to produce a pseudo-random sequence of an output currents from the second supply. 9. A method of treating a workpiece by an electric spar discharge, the method comprising striking a spark discharge between the workpiece and an electrode from a first power supply having a relatively high open circuit output voltage and providing a relatively low short circuit output current, and maintaining the spark discharge between the workpiece and the electrode, by means of a second power supply having a relatively low open circuit output voltage and providing a relatively high short circuit output current until the treatment is finished, characterised by connecting the first and second supplies in parallel during a duty cycle and by preventing reversed current in the second supply when the loaded higher output voltage of the first supply exceeds that of the second supply. 10. A method as claimed in Claim 9, characterised by arranging one or more diodes in series with the output of the second supply to prevent reverse current in the second supply when the loaded output voltage of the first supply is higher than that of the second supply.

Inventors: DOSSAJI TAIBALI, POCOCK GORDON JEFFREY, RANDALL MARTIN, DOSSAJI, TAIBALI, POCOCK, GORDON, JEFFREY, RANDALL, MARTIN
Applicants: HURCO HOLDINGS LTD, HURCO HOLDINGS LIMITED
Application year: 1992
Main classification: B23H1
IPC codes: B23H1