POWER SUPPLY SYSTEM FOR ELECTRIC DISCHARGE MACHINE AND ELECTRIC DISCHARGE MACHINING METHOD

Abstract: A power supply circuit (8) incorporates a DC power supply (8A), a switching element (8B), and a diode (8C). A transformer (12) incorporates a ring core (13), primary windings (13B), and secondary windings (13C). The switching element (8B) goes ON and OFF on the basis of a signal pulse supplied from a pulse emitting device (5), so that a DC current pulse is supplied to the primary side of the transformer (12). An inverting device (15) connected across the power supply circuit (8) and transformer (12) incorporates switches (151), (152), (153), and (154), and is controlled by a signal from a relay circuit. When the switches (151) and (152) are closed, the switches (153) and (154) are open. The first half-wave of the voltage pulse of the current induced on the secondary side of the transformer (12) is supplied to a machining gap formed between a wire (1) and work piece (3) in the direction of the work piece (3). The subsequent half-wave is then supplied to the machining gap in the direction of the wire (1). At this time, the voltage waveform of the current pulse is biased to the positive side in terms of the potential of the work piece with respect to the wire (1). Thus, undesirable coating of the material of the wire (1) onto the surface of the work piece (3) is prevented. When the switches (151) and (152) are opened and the switches (153) and (154) are closed, the AC current pulse induced on the secondary side of the transformer (12) is biased to the negative side.

Description: Technological Field The present invention concerns a power supply system for electrical discharge machining (EDM) units that supplies an AC pulse to a machining gap formed between a working electrode and a work piece to machine the work piece. In particular, the present invention concerns an ideal power supply system for electrical discharge machining units that converts the DC pulse from a DC power supply to an AC pulse, supplies this AC pulse to the machining gap, and emits an electrical discharge to machine the work piece. In addition, the present invention concerns an electrical discharge machining method that makes it possible to prevent the undesirable coating of electrode material onto the surface of the work piece surface that occurs when a work piece is being electrically discharge machined with the use of this type of power supply system. Technological Background Electrical discharge machining units are divided into a mechanical device featuring a member that supports the work piece and a mechanical member that moves the working electrode and the work piece relative to one another, and a power supply device that is invariably located at a physical distance from the mechanical part and emits a power pulse. The power supply device generally possesses an electrical circuit that emits a power pulse, a control device that control the amount of energy in the power pulse and its ON and OFF times, and a control device that controls the relative movement of the working electrode and work piece with respect to one another. The power supply device is connected by a suitable conductor to the mechanical device bearing the work piece and working electrode. The members comprising the electrical discharge circuit that is provided in the mechanical device, eg the lead wires and the conductive members connecting the lead wires and electrode, can also be regarded as part of the power supply device. Such electrical discharge machining units feature a known power supply system of a type that supplies AC voltage to the machining gap, and supplies the work piece. Other EDM systems, such as those disclosed in European Patent No 0 545 156 and British Patent Application GB-A-2 097 708, employ a DC power supply, but the DC pulse is not converted to an AC signal, rather it is supplied directly to the work piece gap. Particularly in machining process where a water-base machining fluid is used, electrical discharge machining devices that employ an AC power pulse to machine the work piece enable the average voltage at the machining gap to be brought to substantially zero, making it possible to prevent electrolysis that corrodes the machined surface. Power supply systems that supply AC voltage include systems that operate by converting a high-frequency DC pulse into an AC pulse, and supplying the high-frequency AC pulse to the machining gap. In concrete terms, these systems are provided with at least a DC power supply and a switching element, and turn the switching element ON and OFF at a specified signal from a pulse emitting device so as to emit a continuous high-frequency DC pulse having a specified ON timing and OFF timing. They convert the high-frequency DC voltage into an AC pulse with a transformer, and supply the high frequency AC pulse to the machining gap. Although this type of device will deliver comparatively small amounts of energy in the individual power pulses when used to supply several (s (microseconds)-long power pulses of AC current per cycle, it can still deliver sufficiently large amounts of energy in a given unit time. This principle allows machining to be carried out quickly without compromising the desired surface roughness. Moreover, wire-cut electrical discharge machining units that use a wire as the working electrode to cut the work piece can furnish a large voltage at the discharging wire electrode regardless of the small amounts of energy per cycle, enabling a reduction in the amplitude of wire electrode oscillation. As a result, such devices can be used to machine work pieces to a better configurational accuracy and a lower roughness on the machined surface. Despite such advantages, however, electrical discharge machining devices that supply an AC pulse to machine the work piece have the drawback of frequently allowing the electrode material (depending on the electrode and work piece material) to stick to the work piece and coat its surface. This coating is particularly marked when the work piece material is tungsten carbide containing cobalt (WC-Co) or copper (Cu), and the electrode material is copper (Cu) or brass (Cu-Zn). In order to obtain the work piece as a finished product, the surface must be ground or subjected to other treatment to remove this coating. A thick coating of electrode material on the surface of the work piece can sometimes prevent the desired surface roughness from being obtained. In the past, the AC voltage pulse waveforms provided at the machining gap by power supply systems that convert a DC pulse into an AC pulse were regarded as being substantially sinusoidal. However, under careful study, the waveform of this AC voltage pulse was revealed to be formed such that the potential of the work piece with respect to the electrode was biased to the negative side. To simplify the explanation, the voltage when the work piece is positive and the electrode negative will be designated as being on the positive side, and as being on the negative side in the opposition situation. Even if a sinusoidal voltage pulse is supplied, this undesirable coating frequently occurs. Accordingly, even when a sinusoidal AC voltage pulse waveform is obtained in this type of electrical discharge machining device by adjusting the ON and OFF timing of the switching element by a frequency corresponding to a constant of the power supply circuit so as to shape the voltage waveform of the AC pulse, the problem of undesirable coating remains difficult to solve. Moreover, simply performing such an operation with this type of device is difficult in and of itself. For a further discussion of such conventional devices, the Reader is directed to the Journal of the Japan Society of Electrical-Machining Engineers, Vol 24/No 47 and the article written by Kiyonori Masui and Takumi Sone contained therein. In order to solve the above-mentioned problem, when the inventors supplied an AC voltage pulse having a waveform biased to the positive side to carry out repeated machining of numerous workpieces, they found that this undesirable coating was virtually eliminated. On the other hand, when the work piece material was eg steel (Fe), the absence of coating was marked. When a high-frequency AC voltage pulse biased to the positive side was then supplied and a steel workpiece was machined, there was an even greater effect on the roughness of the machined surface. The present invention has been conceived in the light of the above situation, and has the purpose of providing a power supply system that is constituted so as to supply an AC pulse to the machining gap, that is particularly ideal for devices that convert a DC pulse into an AC pulse and supply the AC pulse to the machining gap, and that is capable of preventing the undesirable coating of electrode material that can stick to the surface of the workpiece. Moreover, the purpose of the present invention is to provide a power supply system for electrical discharge machining units with which the waveform of the high-frequency AC pulse supplied to the machining gap can be selectively biased in response to machining conditions that include differences in workpiece materials to the positive side or the negative side as seen in terms of the potential of the work piece with respect to the electrode to machine the workpiece. The other purpose of the present invention will be partially discussed in the following explanation, and will be partially revealed by means of embodiments to those skilled in the art. Disclosure of the invention In order to achieve the aforementioned objectives, the power supply system of the present invention is a power supply system for electrical discharge machining devices that supplies a power pulse to a machining gap formed by a tool electrode and a workpiece so as to machine the work piece by electrical discharge, which includes a power supply system for electrical discharge machining units that incorporates a serial circuit that incorporates a DC power supply and at least one switching element; a pulse emitting means that supplies a specified gate pulse having a specified ON timing and OFF timing for turning the aforementioned switching element ON and OFF; a conversion means that is provided between the aforementioned machining gap and the aforementioned serial circuit and converts the DC pulse of the aforementioned serial circuit to an AC pulse; and an inverting means that is provided between the aforementioned machining gap and the aforementioned serial circuit and inverts the direction in which the AC pulse is supplied to the aforementioned machining gap. Preferably, the aforementioned conversion means contains a ring core, at least one primary winding that is wound upon this ring core and connected to the aforementioned serial circuit, and at least one secondary winding that is wound upon the aforementioned ring core and connected to the aforementioned machining gap. More preferably, the aforementioned inverting means incorporates a plurality of switches that switch the connection between the aforementioned DC power supply and the aforementioned machining gap, and it is yet more preferable that these switches be relay switches. Moreover, the power supply system of the present invention is a power supply system for electrical discharge machining devices that supplies a power pulse to a machining gap formed by a tool electrode and a workpiece so as to machine the work piece by electrical discharge, wherein the power supply system for electrical discharge machining units incorporates a serial circuit that incorporates a DC power supply and at least one switching element; a pulse emitting means that supplies a specified gate pulse having a specified ON timing and OFF timing for turning the aforementioned switching element ON and OFF; a conversion means that is provided between the aforementioned machining gap and the aforementioned serial circuit and converts the DC pulse of the aforementioned serial circuit to an AC pulse; and a bias power supply device that incorporates a DC bias power supply connected to two terminals on the circuit path connecting the aforementioned conversion means and the aforementioned machining gap, switches that are respectively provided between the ends of the aforementioned DC bias power supply and the aforementioned two terminals, and switches that are provided between the aforementioned two terminals on the aforementioned circuit path. The electrical discharge machining method of the present invention is an electrical discharge machining method for supplying a power pulse to a machining gap formed by a tool electrode and a workpiece so as to machine the work piece by electrical discharge, which incorporates a step in which at least one switching element is controlled ON and OFF to emit a DC pulse having a specified ON timing and OFF timing; a step in which aforementioned DC pulse is converted to an AC pulse; a step in which the aforementioned AC pulse whose average voltage is biased to either the positive or negative side in terms of the potential of the work piece with respect to the working electrode is biased to the other side; and a step in which the aforementioned AC pulse biased to the other side is supplied to the machining gap to machine the work piece. Moreover, the electrical discharge machining method of the present invention is an electrical discharge machining method for supplying a power pulse to a machining gap formed by a working electrode and a workpiece so as to machine the work piece by electrical discharge, wherein at least one switching element is controlled ON and OFF to cause a DC pulse having a specified ON timing and OFF timing to be emitted, this DC pulse is converted into an AC pulse, the aforementioned AC pulse is biased so that its average voltage becomes negative in terms of the potential of the work piece with respect to the working electrode, and the aforementioned negatively biased AC pulse is supplied to the machining gap, machining the work piece and preventing undesirable coating of the electrode material onto the surface of the workpiece. Brief description of the figures Figure 1 illustrates a power supply system for a wire-cut electrical discharge machining unit pertaining to an embodiment of the present invention. Figure 2 illustrates a power supply system for a wire-cut electrical discharge machining unit pertaining to another embodiment of the present invention. Figure 3 illustrates a power supply system for a wire-cut electrical discharge machining unit pertaining to yet another embodiment of the present invention. Figure 4 illustrates waveforms for current or voltage in the present invention, where Figure 4A illustrates the signal pulse from the pulse emitting means, Figure 4B the waveform of the current pulse supplied to the primary side of the transformer, Figure 4C the waveform of the AC voltage pulse output from the transformer as seen in terms of potential of the work piece with respect to the electrode, and Figure 4D the various waveforms of the current flowing at the gap. Figure 5 illustrates the current rise characteristics when a MOS-FET constituting one switching element is used to emit pulse current. Figure 6 illustrates a power supply system for a wire-cut electrical discharge machining unit pertaining to yet another embodiment of the present invention. Preferred embodiments of the present invention Figure 1 illustrates a power supply system for a wire-cut electrical discharge machining unit pertaining to an embodiment of the present invention. Unless otherwise noted, the following explanation concerns wire-cut electrical discharge machining in which a water-base machining fluid is used. As the working tool, a wire electrode (1) composed of brass is used. This wire (1) is hung under a specified tension, and is guided through a specified course by a pair of wire guides (2A) and (2B). In the present embodiment, the wire guides (2A) and (2B) can also be used as members supplying current to the wire. (3) is a work piece that is provided at a specified gap from the wire (1) and held by a work stand (4). The electrical discharge circuit incorporates a power supply circuit (8), an inverting device (15), a cable (11), a transformer (12) for converting DC into AC, the wire (1), and the work piece (3). The power supply circuit (8) incorporates a variable DC power supply (8A), a field-effect transistor (MOS-FET) (8B) constituting a switching element, and a diode (8C) that prevents reverse flow of current, and said power supply circuit (8) is connected in series. If a plurality of MOS-FETs (8B) are connected together in parallel between the DC power supply (8A) and the transformer (12), it will be possible to increase the current value of the current pulse supplied to the primary side of the transformer (12). Since this power supply circuit (8) has no limiting resistance, one way of obtaining a current pulse waveform having the sharpest possible rises and falls is to turn the MOS-FET (8B) OFF before a current value that would damage the MOS-FET (8B) is reached. Also, the ability of the power supply circuit (8) to obtain a current pulse having sharp rises and falls allows it so supply high-frequency pulses of 1 MHz (megahertz) or above. A signal pulse having a pre-set ON timing and OFF timing is supplied from the pulse emitting device (5) to the gate of the MOS-FET (8B) on the basis of a control signal from a control device (6), and the MOS-FET (8B) goes ON and OFF on the basis of this signal pulse. The transformer (12) incorporates a primary winding (13B) wound once upon a ring core (13A), and a secondary winding (13C) wound twice upon the ring core (13A), and is used as a device for converting DC into AC. The ring core (13A) is preferably composed of ferrite, and is designed so as to withstand becoming magnetically saturated when a current pulse is supplied to the primary winding (13B) from the power supply device (10). This ring core (13) is more effective when it consists of two ring cores (13A). The transformer (12) is preferably provided in close proximity to the machining part. The secondary side of the transformer (12) is essentially connected to the wire ( I ) and work piece (3). The wire (I) and work piece (3) are provided at a suitable distance from one another, forming the machining gap. The inverting device (15) incorporates synchronously operating switches (151). (152), (153), and (154). These switches are relay switches, and are controlled on the basis of a relay signal from a relay circuit not illustrated in the figures to ensure that current can be supplied to the primary winding without shorting the electrical discharge circuit. This inverting device (15)is provided between the power supply device (10) and the transformer (12). The power supply device and the mechanical device are connected with power lines (11a) and (1B) such as coaxial cables or shielded cables. In the embodiment illustrated in Fig. 1, the inverting device (15) is provided on the power supply device (10) side. On the basis of a command from the control device (6), a signal pulse supplied from the pulse emitting device (5) and having e.g. an ON time T(ON) of 100 ns (nanoseconds) and an OFF time T(OFF) of 1 (s such as illustrated in Fig. 4A is supplied to the gate of the MOS-FET (8B). At this time, the switches (151) and (152) of the inverting device (15) illustrated in Fig. 1 are closed, and the switches (153) and (154) are open. At this time, as illustrated in Fig. 5, the MOS-FETs constituting the switching element are ON, depending on the MOS-FETs' characteristics, so that it takes longer for the current value to drop to an essentially blocked condition after the MOS-FETs have been turned OFF than it does for the current to reach its required peak value after the MOS-FETs have been turned ON. While this period of time is extremely belief, the difference between the current's rise time and fall time grows greater as higher peak values are required for the current. For this reason, a current pulse like that illustrated in Figure 4B having a waveform whose fall time is longer than its fall [sic; "rise"] time is supplied to transformer's primary side. The voltage (V) induced on the transformer's secondary side with respect to current (I) is expressed by V ( dI/dt (where t is time), and is proportional to the change in current (I). Accordingly, as illustrated in Fig. 4C, observation with a synchroscope shows that the seemingly sinusoidal waveform for the voltage pulse of the current induced on the secondary side of the transformer is, in fact, not sinusoidal. The second half of the waveform of this voltage pulse for one cycle of current voltage is smaller than its first half-wave as seen in terms of the potential of the work piece with respect to the wire, and is biased to the positive side. The characteristics of the switching elements are not the sole cause of the bias in the current pulse's voltage waveform, but are one of the major factors at work. (IS) in Fig. 5 is expressed by the relation IS = E/R when the voltage and resistance of the power supply circuit (8) incorporating the DC voltage supply and the switching elements are designated (E) and (R), respectively, and represents the maximum current value that can be supplied with this circuit. Since the power supply circuit (8) has no current-limiting resistance under this scenario, the resistance (R) consists principally of the resistance value present in the circuit path. (IP) illustrated in Fig. 5 is the current value supplied to the primary side of the transformer (12). For example, when (IP) illustrated in the figure is 180A (amperes), it is expressed by the sum value obtained when a plurality of MOS-FETs connected to one another in parallel are used. Although MOS-FETs were cited in this example, the same also holds for switching transistors or other switching elements as well as for MOS-FETs. As illustrated in Fig. 4C, when the work piece is made positive in the machining gap and the first half-wave of the current voltage pulse cycle is supplied to the positive side, an electrical discharge is emitted at the machining gap, and a specified current flows from the work piece to the wire as illustrated in Figure 4D. When the work piece is then made negative and the next half-wave of the cycle is supplied to the negative side, an electrical discharge is emitted at the machining gap, and current flows from the wire to the work piece as illustrated in Figure 4D. When the work piece is machined with the average value of the AC voltage being biased to the positive side as seen in terms of the work piece's potential with respect to the wire in this way, no copper or zinc from the material of the wire are coated onto the surface of cobalt-containing tungsten carbide work pieces. On the other hand, when switches (151) and (152) of the inverting device (15) are opened and switches (153) and (154) are closed, the voltage waveform of the current supplied to the gap appears substantially inverted, as shown in the latter half of Fig. 5C. For this reason, the average value of the AC voltage is biased to the negative side as seen in terms of the potential of the work piece with respect to the wire. If the average value of the AC voltage is biased to the negative side in this way, an ideal surface roughness will be obtained on steel workpieces. The decision of whether to bias the voltage to the positive side or the negative side depends on the workpiece material. Naturally, the direction in which the voltage pulse is supplied can be inverted from the beginning to the end of the machining process. For example, the effect of the present embodiment can still be obtained when the direction in inverted during any several of the machining steps performed in the latter half of the machining process. In the present embodiment, when brass is used as the wire material and cobalt-containing tungsten carbide as the work piece material, the AC voltage pulse is biased to the positive side as seen in terms of the potential of the workpiece, and the initial machining process in which the surface of the work piece is cut is followed by three "second cut" machining processes in which the wire electrode is moved relative to the cut surface to reduce its roughness, i.e., the work piece is machined in four machining processes. As a result, the roughness of the obtained machined surface is approximately 1.1-2.4 (Rmax, which is an ideal outcome. Trace analysis by electron probe (EPMA analysis) shows the value of copper or zinc from the wire coated onto the machined surface to be approximately 540 cps (Zn-K( value), a result that can be regarded as indicating substantially no coating. A work piece composed of copper was machined with the waveform of the AC the voltage pulse biased to the positive side as seen in terms of the work piece's potential, with the result that the roughness of the machined surface obtained in four machining processes was approximately 1.4-2.4 (Rmax. The value of brass from the wire coated onto the machined surface was substantially the same as when the work piece consisted of cobalt-containing tungsten carbide, and could be regarded as indicating that substantially no coating had occurred. When the wire was brass, the work piece was steel, and four machining process were carried out with the waveform of the AC voltage pulse biased to the negative side as seen in terms of the work piece, the roughness of the machined surface obtained was approximately 1.8-2.5 (Rmax. However, when the waveform was biased to the positive side and the work piece was machined in the same way, the roughness of the machined surface obtained was on the order of 1.5 (Rmax. Given that the same roughness was anticipated for the machined surface in the machining operation where the waveform was biased to the positive side in this way, an effect of increased machining speed was obtained. When the work piece is steel, it is preferable that it be machined with the waveform biased to the positive side. Given the above results, a more desirable way of machining the work piece is realized by using an inverter to selectively switch the direction in which high-frequency AC voltage is supplied in accordance with the materials of which the wire electrode and work piece are composed. Fig. 2 illustrates another embodiment of the present invention. Elements identical to those illustrated in Fig. 1 are given identical symbols, and will not be described in detail. The present embodiment differs from the embodiment in Fig. 1 in that the inverting device (15) is provided between the transformer (12) and the machining gap. The four switches of the inverting means (15) are used to invert the direction in which the voltage pulse of the AC current induced on the secondary side of the transformer (12) is supplied. Under this scenario, the operations performed in machining and the objectives obtained are generally the same as those in the first embodiment. Notwithstanding that the transformer (12) and inverting device (15) are provided on the power supply device (10) side in the present embodiment, when the transformer (12) is placed in close proximity to the machining part, the inverting device (15) is also placed in close proximity to the machining part together with the transformer (12). Figure 3 shows yet another embodiment of the present invention. Elements identical to those illustrated in Fig. 1 are given identical symbols, and will not be described in detail. In the present embodiment, the inverting device (15) is dived into two units. The first of these is provided between the transformer (12) and power supply circuit (8), and the other is provided between the transformer (12) and the machining gap. The unit (15A) of the inverting device (15) incorporates switches (151) and (153) that operate in opposition to one another, and the unit (15A) incorporates switches (152) and (154). These switch groups are controlled on the basis of signals from a relay circuit not illustrated in the figures. The transformer (12) incorporates a ring core (13), and primary windings (13B-1) and (13B-2) and secondary windings (13C-1) and (13C-2) wound upon the ring core (13). The respective ends of the two primary windings (13B-1) and (13B-2) connected to the negative side of the variable DC power supply (8A) are coupled. In addition, the other end of the primary winding (13B-1) is connected to the switch (151), and the other end of the primary winding (13B-2) is connected to the switch (153). Accordingly, when the switch group of unit (15A) is operated, one of the primary windings conducts, and the other primary winding is blocked. Also, the respective ends of the two secondary windings (13C-1) and (13C-2) connected to the wire (1) are coupled. The other respective ends of the secondary windings (13C-1) and (13C-2) connected to the work piece (3) are respectively connected to the two switches (152) and (154) of unit (15B). Accordingly, when the group of switches of the unit (15B) is operated, one of the secondary winding conducts, and the other secondary winding is blocked. The two primary windings (13B-1) and (13B-2) are wound upon the ring core (13) so as to produce oppositely oriented magnetic flux induced by the current pulse in the ring core (13), and the two secondary windings (13C-1) and (13C-2) are wound in the same way. For this reason, when the primary winding (13B-1) and the secondary winding (13C-1) conduct or the primary winding (13B-2) and the secondary winding (13C-2) conduct to form an electrical discharge circuit, the first half-wave of the AC voltage pulse cycle is supplied to the positive side with the work piece made positive. Accordingly, as explained earlier, the average value of the continuously supplied AC voltage pulse is biased to the positive side. On the other hand, when the primary winding (13B-2) and secondary winding (13C-1) conduct, or when the primary winding (13B-1) and secondary winding (13C-2) conduct to form an electrical discharge circuit, the first half-wave of the AC voltage pulse cycle is supplied to the negative side as seen in terms of the work piece's potential. As a result, the average voltage is biased to the negative side. In this manner, the waveform of the AC voltage pulse supplied to the machining gap can be biased to the positive side or the negative side by selectively connecting the primary and secondary windings. Therefore, the present embodiment consequently possesses the same effect as the embodiment illustrated in Fig. 1. Figure 6 illustrates yet another application example of the present invention. Elements identical to those that have already been explain are given identical symbols, and will not be explained in detail. When a continuous high-frequency DC current pulse is supplied to the primary side of the transformer (12) from the power supply circuit (8) as the switching element (8B) goes ON and OFF, a continuous high-frequency AC voltage pulse is supplied to the machining gap with its first half-wave making the work piece positive and its subsequent half-wave making the work piece negative. The DC bias power supply device (16) incorporates a serial circuit comprising a DC power supply (16A), variable resistor (16B), diode (16C), and reactor (16D) that prevents high-frequency oscillation. A non-inductive resistor (16E) with a low resistance value is connected to this serial circuit in parallel. The two terminals of the bias power supply device (16) are respectively connected to two terminals (71) and (72) on the circuit path linking the transformer (12) and work piece (3). A switch (141) is provided between one terminal of the DC power supply (16A) and the terminal (71), and a switch (142) is provided between the other terminal of the DC power supply (16A) and the terminal (72). These switches (14) synchronously go ON and OFF, and permit the bias power supply device (16) to be connected to and disconnected from the electrical discharge circuit. Moreover, a switch (9) is provided between the terminals (71) and (72) on this circuit path. When this switch (9) is closed, the switches (14) are open, disconnecting the bias power supply device (16) from the electrical discharge circuit. Thus, a high-frequency AC voltage pulse is supplied to the machining gap via these switches (17) [sic; "(14)"]. Alternatively, when the switch (9) is opened, the switches (14) are closed, and DC voltage is supplied from the DC power supply (16A) of the bias power supply device (16). At this time, the DC voltage of the bias power supply device (16) is supplied to the machining gap via the circuit of the non-inductive resistor (16B) [sic] so as to cancel out the output of the first half-wave of the high-frequency AC voltage pulse cycle supplied from the transformer (12) with the work piece made positive. Alternatively, the DC voltage of the bias power supply device (16) is serially supplied to the output of the subsequent half-wave of the high-frequency AC voltage pulse cycle supplied with the work piece made negative. As a result, the waveform of the high-frequency AC voltage pulse supplied to the machining gap is biased to the positive side as seen in terms of the work piece's potential. The extent of this bias varies with the voltage value of the DC power supply (16A), and can be adjusted by altering the resistance value of the resistor (16B) or the non-inductive resistor (16C) [sic; "(16E)"]. As a result, the present embodiment enables undesirable coating to be prevented in the same way as in the previous group of embodiments. The group of illustrated embodiments explained above were selected for the purpose of explaining the essence of the present invention and its practical applications. The present invention is not intended to be limited to exactly the same forms that have been disclosed herein, but can be improved and varied in a variety of ways. For example, the inverting device can be provided in any location that corresponds favorably with its design and operation, and can also be combined with the bias power supply device. Moreover. the invention is not limited to using only the power supply circuit (8), e.g., it is also possible for a power supply circuit provided with a resistor in series with a DC power supply, or a power supply circuit incorporating a DC power supply having a voltage higher than that of the DC power supply (8A), to be connected in parallel to the power supply circuit (8). Naturally, the concrete configurations of the inverting device, conversion device, or bias power supply device need not be the configurations cited in the embodiments. The scope of the present invention will now be defined by means of the appended claims.

Claims: A power supply system for an electrical discharge machining device for supplying power pulses to a machining gap formed between a tool electrode (1) and a workpiece (3) to machine the workpiece by electrical discharges, comprising: a serial circuit (8) comprising a DC power supply (8A) and at least one switching element (8B); a pulse generator (5) for supplying a gate pulse having a specified ON time and OFF time to said switching element for turning the switching element ON and OFF; and a DC to AC converter (12) connected between the machining gap and the serial circuit for converting DC pulses from said switching element to AC pulses; said power supply system being characterized by an inverter (15) connected between the machining gap and the serial circuit for inverting the polarity of the AC pulses as applied across the machining gap. The power supply system according to Claim 1, wherein the DC to AC converter comprises a ring core (13A), at least one primary winding (13B) wound on the ring core and connected to the serial circuit, and at least one secondary winding (13C) wound on the ring core and connected to the machining gap. The power supply system according to Claim 1, wherein the inverter is connected between the serial circuit and the DC to AC converter. The power supply system according to Claim 1, wherein the inverter is connected between the DC to AC converter and the machining gap. The power supply system according to Claim 1, wherein the inverter comprises a first switching unit (15A) connected between the DC power supply and the DC to AC converter, and a second switching unit (15B) connected between the DC to AC converter and the machining gap. The power supply system according to Claim 5, wherein the DC to AC converter comprises a ring core (13), a plurality of primary windings (13B-1, 13B-2) wound on the ring core and connected to one of the first switching unit and a plurality of secondary windings (13C-1, 13C-2) wound on the ring core and connected to the second switching unit. The power supply system according to Claim 1, wherein the inverter comprises a bridge circuit having four switches (151, 152, 153, 154) provided on each side. The power supply system according to Claim 7, wherein the four switches are relay switches. A power supply system for an electrical discharge machining device for supply power pulses to a machining gap formed between a tool electrode (1) and a workpiece (3) to machine the workpiece by electrical discharges, comprising: a serial circuit (8) comprising a DC power supply (8A) and at least one switching element (8B); a pulse generator (5) for supplying a gate pulse having a specified ON time and OFF time to said switching element for turning the switching element ON and OFF; and a DC to AC converter (12) connected between the machining gap and the serial circuit for converting DC pulses from said switching element to AC pulses; said power supply system being characterized by a switch (9) positioned on a circuit path connecting the DC to AC converter and one of the tool electrode and the workpiece; a bias power supply (16A) connected in parallel to the switch (9) between two nodes (71, 72) on the circuit path for adding a DC bias to the AC voltage as applied across the machining gap; and a pair of switches (141, 142) connected between the bias power supply and two nodes, respectively. A method for supplying a power pulse to a machining gap formed between a tool electrode and a workpiece so as to machine the workpiece by electrical discharge machining, comprising the steps of: controlling at least one switching element (8B) to be ON or OFF to generate DC pulses having a specified ON time and OFF time; converting the DC pulses to AC pulses, said AC pulses having an average voltage which is biased to one side on which the workpiece is negative and the tool electrode is positive; and applying the AC pulses across the machining gap to machine the workpiece; said method being characterized by the steps of: biasing an average voltage of the AC pulse as applied across the machining gap to an opposite side on which the workpiece is positive and the tool electrode is negative. The method according to Claim 10, wherein the step of biasing comprises the step of inverting the polarity of the AC pulses as applied across the machining gap. The method according to Claim 11, wherein the step of biasing further comprises the step of adding a DC bias to the AC pulses as applied across the machining gap.

Inventors: KANEKO YUJI, TOYONAGA TATSUO, KANEKO, YUJI, TOYONAGA, TATSUO, KANEKO, Yuji, Sodick Co., Ltd., TOYONAGA, Tatsuo, Sodick Co., Ltd.
Applicants: SODICK CO LTD, SODICK CO., LTD.
Application year: 1995
Main classification: B23H1
IPC codes: B23H1, B23H7