Electric discharge machining apparatus

The present invention relates to an electric discharge machining device. The electric discharge machining device includes: a first switch provided between the positive pole of a power supply and a work piece; a second switch provided between the negative pole of the power supply and the work piece; a third switch provided between the negative pole of the power supply and a tool electrode; a fourth switch provided between the positive pole of the power supply and the tool electrode; and a pulse generating device. In order to supply current pulses with a straight polarity, the pulse generating device repeatedly switches on and off either the first or third switch while the other switch is on. In order to supply current pulses with a reverse polarity, the pulse generating device repeatedly switches on and off either the second or fourth switch while the other switch is on.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a 371 application of an international PCT application serial no. PCT/JP2013/071350, filed on Aug. 7, 2013, which claims the priority benefit of Japan application no. 2012-175874, filed on Aug. 8, 2012. The entirety of each of the above-mentioned patent applications is hereby incorporated by reference herein and made a part of this specification.

BACKGROUND OF THE INVENTION

Field of the Invention

The invention relates to an electric discharge machining apparatus that supplies a current pulse to a machining gap formed between a work piece and a tool electrode to machine the work piece. In particular, the invention relates to an electric discharge machining apparatus that is capable of supplying a current pulse with a first polarity, in which the work piece is a positive potential and the tool electrode is a negative potential, and supplying a current pulse with a second polarity, in which the work piece is the negative potential and the tool electrode is the positive potential.

Description of Related Art

A polarity, in which the work piece is the positive potential and the tool electrode is the negative potential, is called “straight polarity”. A polarity, in which the work piece is the negative potential and the tool electrode is the positive potential, is called “reverse polarity”. The polarity of a power pulse is an important condition for electric discharge machining. Generally, in order to prevent electric corrosion of the work piece, a wire electric discharge machine maintains the average of voltages in the machining gap (“gap voltage”) at 0V as much as possible by switching the polarity.

The wire electric discharge machine that applies a high frequency AC voltage to the machining gap to machine the work piece is widely known. With such a wire electric discharge machine, the current pulse having a short ON time improves the surface roughness. However, as the size of the machining gap and the machining area change, the impedance in the machining gap changes. If the impedance changes significantly, the no-load voltage does not go up high enough and the power supplied to the machining gap would be lower than expected.

Patent Literature 1 discloses an electric discharge machining apparatus, in which an impedance matching circuit is provided between an AC power supply and the machining gap. The AC power supply is connected to the machining gap through a proper power cable. The impedance matching circuit suppresses undesirable influence of the electrostatic capacity that exists in the power cable.

PRIOR ART LITERATURE

Patent Literature

SUMMARY OF THE INVENTION

Problem to be Solved

However, the impedance matching circuit would increase the cost of the apparatus that supplies power to the machining gap. An object of the invention is to provide an electric discharge machining apparatus that is capable of supplying a current pulse with an intended waveform to the machining gap without disposing the impedance matching circuit.

Solution to the Problem

The invention relates to an electric discharge machining apparatus for machining a work piece (4) by supplying a current pulse to a machining gap (9) formed between the work piece and a tool electrode (2) while alternately switching between a straight polarity, in which the work piece is a positive potential and the tool electrode is a negative potential, and a reverse polarity, in which the work piece is the negative potential and the tool electrode is the positive potential. According to an embodiment of the invention, the electric discharge machining apparatus includes: a power supply (30) having a positive pole and a negative pole; a first switch (41) disposed between the positive pole of the power supply and the work piece; a second switch (42) disposed between the work piece and the negative pole of the power supply; a third switch (43) disposed between the tool electrode and the negative pole of the power supply; a fourth switch (44) disposed between the positive pole of the power supply and the tool electrode; and a pulse generating device (10) controlling the first switch, the second switch, the third switch, and the fourth switch. While one of the first switch and the third switch is on, the pulse generating device repeats an on/off switching operation of the other switch of the first switch and the third switch so as to supply a series of current pulses with the straight polarity to the machining gap. In addition, while one of the second switch and the fourth switch is on, the pulse generating device repeats an on/off switching operation of the other switch of the second switch and the fourth switch so as to supply a series of current pulses with the reverse polarity to the machining gap.

Preferably, a bridge circuit is formed, in which a first node (51) is disposed between the first switch and the second switch, a second node (52) is disposed between the second switch and the third switch, a third node (53) is disposed between the third switch and the fourth switch, and a fourth node (54) is disposed between the fourth switch and the first switch. The first node is connected to the work piece, the second node is connected to the negative pole of the power supply, the third node is connected to the tool electrode, and the fourth node is connected to the positive pole of the power supply.

The electric discharge machining apparatus further includes: a first transistor disposed in a circuit where a current flows through the first switch, the machining gap, and the third switch with the straight polarity; a first resistor having a terminal connected to a base of the first transistor and the other terminal connected to a collector of the first transistor; a second transistor disposed in a circuit where a current flows through the fourth switch, the machining gap, and the second switch with the reverse polarity; and a second resistor having a terminal connected to a base of the second transistor and the other terminal connected to a collector of the second transistor. The first transistor and the second transistor may be bipolar transistors.

The pulse generating device may switch off at least one of the second switch and the fourth switch while one of the first switch and the third switch is on so as to supply the current pulse with the straight polarity to the machining gap. The pulse generating device may switch off at least one of the first switch and the third switch while one of the second switch and the fourth switch is on so as to supply the current pulse with the reverse polarity to the machining gap.

The pulse generating device may switch off both the second switch and the fourth switch while one of the first switch and the third switch is on so as to supply the current pulse with the straight polarity to the machining gap. The pulse generating device may switch off both the first switch and the third switch while one of the second switch and the fourth switch is on so as to supply the current pulse with the reverse polarity to the machining gap.

According to another embodiment of the invention, an electric discharge machining apparatus includes: a first power supply (31) having a positive pole and a negative pole; a second power supply (32) having a positive pole and a negative pole; a first switch (41) disposed between the positive pole of the first power supply and the work piece (4); a second switch (42) disposed between the work piece and the negative pole of the second power supply; a third switch (43) disposed between the tool electrode (2) and the negative pole of the first power supply; a fourth switch (44) disposed between the positive pole of the second power supply and the tool electrode; and a pulse generating device (10) controlling the first switch, the second switch, the third switch, and the fourth switch. While one of the first switch and the third switch is on, the pulse generating device repeats an on/off switching operation of the other switch of the first switch and the third switch so as to supply a series of current pulses with the straight polarity to the machining gap. While one of the second switch and the fourth switch is on, the pulse generating device repeats an on/off switching operation of the other switch of the second switch and the fourth switch so as to supply a series of current pulses with the reverse polarity to the machining gap.

Effects of the Invention

The electric discharge machining apparatus of the invention is capable of supplying a current pulse to the machining gap at high frequency, and since the polarity switching cycle is long, the impedance matching circuit can be omitted.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, an electric discharge machining apparatus of the invention is described in detail with reference to the figures.FIG. 1illustrates an embodiment of the electric discharge machining apparatus of the invention. A tool electrode in the electric discharge machining apparatus is a wire electrode2that travels vertically. A machining gap9is formed between the wire electrode2and a work piece4. The work piece4is fixed in a machining tank (not shown in the figure). A machining fluid is supplied into the machining tank, and the machining gap9is filled with the machining fluid. A major component of the machining fluid is deionized water or oil.

A bridge circuit is disposed between a DC power supply30and the machining gap9. The bridge circuit includes a first switch41, a second switch42, a third switch43, and a fourth switch44. The switches41,42,43, and44form four nodes51,52,53, and54between the adjacent switches and are connected in series. The first switch41, the second switch42, the third switch43, and the fourth switch44may be MOSFET.

The first node51is disposed between the first switch41and the second switch42and connected to the work piece4through a proper power cable. The second node52is disposed between the second switch42and the third switch43and connected to a negative pole (−) of the DC power supply30. The third node53is disposed between the third switch43and the fourth switch44and connected to the wire electrode2through a proper power cable. The fourth node54is disposed between the fourth switch44and the first switch41and connected to a positive pole (+) of the DC power supply30. The aforementioned proper power cable is a coaxial cable, for example.

A first transistor45is disposed between the node53and the third switch43, and a second transistor46is disposed between the node51and the second switch42. The first transistor45and the second transistor46are bipolar transistors. An emitter E of the first transistor45is connected to the third switch43. A base B of the first transistor45is connected to a terminal of a first resistor65. A collector C of the first transistor45is connected to the other terminal of the first resistor65and the wire electrode2. An emitter E of the second transistor46is connected to the second switch42. A base B of the second transistor46is connected to a terminal of a second resistor66. A collector C of the second transistor46is connected to the other terminal of the second resistor66and the work piece4.

The first resistor65determines a base current of the first transistor45. The first transistor45limits the current flowing with the straight polarity and protects the first switch41and the third switch43. Moreover, the second resistor66determines a base current of the second transistor46. The second transistor46limits the current flowing with the reverse polarity and protects the second switch42and the fourth switch44.

On/off switching of the first switch41, the second switch42, the third switch43, and the fourth switch44is controlled by a pulse generating device10. The pulse generating device10supplies a first gate signal G1, a second gate signal G2, a third gate signal G3, and a fourth gate signal G4respectively to the first switch41, the second switch42, the third switch43, and the fourth switch44. When the gate signals G1, G2, G3, and G4are on, the first switch41, the second switch42, the third switch43, and the fourth switch44are on respectively.

The pulse generating device10is described in detail with reference toFIG. 2andFIG. 3. As shown inFIG. 2, in the pulse generating device10, a high frequency clock signal CLK is generated. The clock signal CLK determines the frequency of a current pulse supplied to the machining gap9. A cycle time c1of the current pulse is 100 ns, for example. In addition, a signal PS and a signal DLY are generated. The signal PS determines a cycle time c2of the polarity. The cycle time c2of the polarity is 20 μs, for example. The cycle time c1of the current pulse is much shorter than the cycle time c2of the polarity. The signal DLY is on when the signal PS is off, and the signal DLY is off when a delay time td passes.

The clock signal CLK is supplied to AND gates23and22. The signal PS is supplied to AND gates23and21. As shown inFIG. 3, the AND gate23allows the clock signal CLK to pass only when the signal PS is on so as to generate the third gate signal G3. The signal PS inverted by an inverter26is supplied to AND gates22and24. The AND gate22allows the clock signal CLK to pass only when the signal PS is off so as to generate the second gate signal G2. An inverted signal of the signal DLY is supplied to the AND gates21and24. The AND gate21allows the signal PS to pass only when the signal DLY is off so as to generate the first gate signal G1. The AND gate24allows the inverted signal of the signal PS to pass only when the signal DLY is off so as to generate the fourth gate signal G4.

As shown inFIG. 3, at a time t1, when the first gate signal G1and the third gate signal G3are on, the first switch41and the third switch43are on. Meanwhile, the second gate signal G2and the fourth gate signal G4are off, and the second switch42and the fourth switch44are off Thus, the voltage of the DC power supply30is applied to the machining gap9with the straight polarity, in which the work piece4is a positive potential and the wire electrode2is a negative potential. As a result, a voltage Vgap of the machining gap9rises rapidly and electric discharge occurs. Due to the electric discharge, a current Igap flows through the machining gap9. The one-dot dashed line inFIG. 1indicates the flow of the current in the straight polarity. At a time t2, when the third gate signal G3is off and the third switch43is off, the current Igap drops rapidly to 0. Because the circuit has no current limiting resistor, the current pulse has a steep rising edge.

As a time t3when the cycle time c1passes after the time t1, the third gate signal G3is on and the third switch43is on, and the current pulse is generated again. While the first switch41is on, the on/off switching of the third switch43is repeated at a high frequency, and a series of current pulses is supplied to the machining gap9with the straight polarity. The pulse generating device10may switch off at least one of the second switch42and the fourth switch44while the first switch41is on. In the embodiment ofFIG. 3, the pulse generating device10switches both the second switch42and the fourth switch44off during the period from the time t1to a time t4. As a result, the current pulse that rises steeply and drops steeply is generated.

At the time t4when half of the cycle time c2passes after the time t1, when the first gate signal G1is off and the first switch41is off, the voltage Vgap decreases rapidly. At a time t5when the delay time td passes after the time t4, when the fourth gate signal G4and the second gate signal G2are on, the fourth switch44and the second switch42are on. Meanwhile, the first gate signal G1and the third gate signal G3are off, and the first switch41and the third switch43are off Thus, the voltage of the DC power supply30is applied to the machining gap9with the reverse polarity, in which the wire electrode2is the positive potential and the work piece4is the negative potential. As a result, the voltage Vgap of the machining gap9rises rapidly and electric discharge occurs. Due to the electric discharge, the current Igap flows through the machining gap9, and the dashed line inFIG. 1indicates the flow of the current in the reverse polarity.

In a period when the fourth gate signal G4is on and the fourth switch44is on, the on/off switching of the second switch42is repeated and a series of current pulses is supplied to the machining gap9with the reverse polarity. While the fourth switch44is on (t5-t6), the pulse generating device10may switch off at least one of the first switch41and the third switch43. In the embodiment ofFIG. 3, the pulse generating device10switches off both the first switch41and the third switch43during the period from the time t5to the time t6. The series of current pulses with the straight polarity and the series of current pulses with the reverse polarity are supplied to the machining gap9alternately. Since the cycle time c2for switching the polarity is relatively long, the voltage Vgap can go up to a sufficiently high value. In addition, because the small current pulses are generated at high frequency, the roughness of the machining surface is improved efficiently.

An operation of another pulse generating device is described with reference toFIG. 4. The pulse generating device10switches off the fourth gate signal G4while the first gate signal G1is on. Moreover, the pulse generating device10switches off the first gate signal G1while the fourth gate signal G4is on. In the embodiment ofFIG. 4, another pulse generating device repeats the on/off switching of the second switch42while the first switch41is on. As a result, while the first switch41is on, the current flows with the straight polarity, as indicated by the one-dot dashed line inFIG. 1, and the current flows from the node51to the node52through the second switch42.

Further, the aforesaid another pulse generating device repeats the on/off switching of the third switch43while the fourth switch44is on. As a result, while the fourth switch44is on, the current flows with the reverse polarity, as indicated by the dashed line inFIG. 1, and the current flows from the node53to the node52through the third switch43. As described above, because a part of the current bypasses the machining gap9, the current Igap is reduced and the roughness of the machining surface is improved.

The second gate signal G2and the third gate signal G3have the same on time and cycle time c1. While the first gate signal G1is on, the second gate signal G2temporally deviates from the third gate signal G3and rises.FIG. 4illustrates three types of pulses A, B, and C of the second gate signal G2. The pulse A indicates the second gate signal G2that rises when the third gate signal G3drops. The pulse B indicates the second gate signal G2that rises slightly earlier than the rising of the third gate signal G3. The pulse C indicates the second gate signal G2that rises slightly later than the rising of the third gate signal G3. While the fourth gate signal G4is on, the third gate signal G3temporally deviates from the second gate signal G2and rises.FIG. 4illustrates three types of pulses D, E, and F of the third gate signal G3.

The pulse A or D can form a current pulse that drops steeply. The pulse B or E can form a current pulse that rises gradually. The pulse C or F can form a current pulse that rises gradually at the peak. The aforesaid another pulse generating device can generate a current pulse sequence, which has a different shape from the current pulse ofFIG. 3, in the machining gap9.

Next, another embodiment of the wire electric discharge machining apparatus of the invention is described with reference toFIG. 5. Elements the same as those ofFIG. 1are assigned with the same reference numerals, and detailed descriptions thereof are omitted hereinafter. The wire electric discharge machining apparatus includes a first power supply31that supplies a current pulse with the straight polarity to the machining gap9and a second power supply32that supplies a current pulse with the reverse polarity to the machining gap9. The current with the straight polarity flows through the first switch41, the work piece4, the wire electrode2, and the third switch43from the first power supply31. Moreover, the current with the reverse polarity flows through the fourth switch44, the wire electrode2, the work piece4, and the second switch42from the second power supply32.

The descriptions are not intended to limit the electric discharge machining apparatus of the invention to the form disclosed above. Various improvements and modifications may be made with reference to the above descriptions. For example, in order to supply the current with the straight polarity to the machining gap9, the first gate signal G1may be supplied to the third switch43and the third gate signal G3may be supplied to the first switch41. In order to supply the current with the reverse polarity to the machining gap9, the second gate signal G2may be supplied to the fourth switch44and the fourth gate signal G4may be supplied to the second switch42.

Furthermore, the positions of the first transistor45and the second transistor46are not limited to the disclosure ofFIG. 1. The first transistor45may be disposed in a circuit where the current flows through the first switch41, the machining gap9, and the third switch43with the straight polarity. The second transistor46may be disposed in a circuit where the current flows through the second switch42, the machining gap9, and the fourth switch44with the reverse polarity.

DESCRIPTIONS OF REFERENCE NUMERALS