1. Field of the Invention
The present invention generally relates to a power supply device for electric discharge machining, and in particular to a power supply device for finish machining.
2. Description of the Related Art
In electric discharge machining, arc discharges are generated by applying voltages between an electrode and a workpiece in a machining fluid. The heat generated by these electric discharges melts the workpiece while rapidly heating the machining fluid, causing its vaporization and explosion that blows off the molten portion of the workpiece. Machining progresses by repeating this process at a high frequency. Since the machined surface is formed by a collection of small craters produced by electric discharges, the size of each crater determines the surface roughness.
In wire electric discharge machining, which is a kind of electric discharge machining, it is generally known that electric discharge of short duration is repeated at a high frequency by applying a high-frequency AC voltage 102, which is a power supply output voltage 100, across a machining gap between a wire electrode and a workpiece to achieve micromachining, as shown in FIGS. 10A and 10B. FIG. 10A shows an exemplary high-frequency AC voltage 100 that is generated by the power supply in a wire electric discharge machine. Rectangular wave voltages generated by the power supply are transmitted through a coaxial cable or the like from the power supply to the machining gap and applied in the sinusoidal form of blunt waveforms as shown in FIG. 10B across the machining gap.
Japanese Patent Application Laid-Open No. 61-260915, for example, discloses that a machined surface with a surface roughness of 1 μm Rmax or lower can be obtained by applying high-frequency AC voltages in the range of 1-5 MHz. Japanese Patent Application Laid-Open No. 2010-194693 discloses a machining technique, in which positive and negative voltages are applied, as shown in FIG. 12A, across the machining gap between a wire electrode and a workpiece with a quiescent period equal to or longer than individual voltage application periods provided between individual voltage applications so as to obtain trapezoidal wave voltages.
The technique disclosed in Japanese Patent Application Laid-Open No. 61-260915, however, apparently has the following problems:
(1) Degradation in Straightness Accuracy
If high-frequency AC voltages are used in wire electric discharge machining, the wire electrode bends due to electrostatic attraction between the wire electrode and the workpiece, because voltages are continuously applied across the machining gap, as shown in FIG. 10B. This degrades the straightness accuracy of machining, because the amount of machining increases in the central part of the sheet thickness of the workpiece, resulting in a barrel-like form.
(2) Degradation in Roughness of the Machined Surface
When machining is performed using an AC voltage, electric discharges are theoretically interrupted at every half cycle of the voltage, i.e., at zero crossing points 104 of the discharge current, as shown in FIG. 10B, because the voltages are reversed from positive to negative or from negative to positive over time. At higher AC frequencies, individual electric discharge arcs are not sufficiently extinguished. If electric discharges occur immediately after voltage applications, the electric discharges tend to occur repeatedly at the same place. If electric discharges continue at a high frequency, the resultant surface roughness would become worse than that obtained by AC half-wave electric discharges. Since the surface roughness tends to vary with the density of electric discharges, streaks may be produced on the machined surface.
(3) Difficulty in Determining the Machining State
In electric discharge machining, the machining state is typically determined by measuring an average voltage applied across the machining gap in order to control the electrode feeding speed and to change the machining conditions. With high-frequency AC voltages at several MHz or higher, however, measurement errors increase because a rectifier circuit for obtaining the average voltage does not respond. At high frequencies, resonance phenomena often occur between the machining power supply and the machining gap. If the electric discharge gap length, sheet thickness, the machining fluid flowing state, or the like changes, electric constants of the machining gap change, which inevitably causes the machining voltage to vary. This makes it more difficult to determine the machining state from the average voltage. This is a bottleneck in improving the machining accuracy, because it is difficult to perform feedback control responsive to the machining state, so that the electrode is fed at a constant speed in the finishing region, for example.
As a solution to these problems, the above-mentioned Japanese Patent Application Laid-Open No. 2010-194693 discloses a machining technique using a trapezoidal wave voltage 118 as shown in FIG. 12C. The trapezoidal wave voltage 118 is formed by applying a power supply output voltage 110 across the machining gap between an electrode and a workpiece with a quiescent period 112 equal to or longer than individual voltage application periods provided between applications of positive voltage 114 and negative voltage 116, as shown in FIG. 12A. FIG. 12B shows a power supply output current which corresponds to the power supply output voltage shown in FIG. 12A, wherein positive currents 111 and negative currents 113 are alternately applied.
In electric discharge machining, multiple machining is typically performed by gradually weakening the intensity of machining pulses in the order of rough machining, intermediate machining, and finish machining until the desired accuracy and surface roughness are achieved. In recent years, in order to reduce the machining period, attempts have been made to reduce the number of machining times by performing part of the intermediate machining using a power supply for finish machining. More specifically, in the prior art, rough machining and intermediate machining are performed until a surface roughness of 3-5 μm Rz is achieved and then machining is performed using a power supply for finish machining several times up to a surface roughness of about 1 μm Rz. In recent years, instead, when a surface roughness of about 10 μm Rz is achieved, machining is performed using a power supply for finish machining to reduce the number of machining times and the machining period.
In this case, the amount of machining per unit time of the power supply for finish machining increases than ever and accordingly the output voltage from the machining power supply increases, which overheats switching elements and other components in the conventional power supply exceeding their rated values and makes them unserviceable. There is the need, therefore, to provide a new power supply that can output a higher current.
FIG. 11 is a schematic block diagram of the bipolar voltage application circuit 10 that is typically used as in Japanese Patent Application Laid-Open Nos. 61-260915 and 2010-194693 mentioned above.
Reference numerals 11 and 12 represent DC power supplies; reference numerals 13 and 14 represent switching elements. Reference numerals 15, 16, and 17 represent a damping resistance, inductance, and a resistance, respectively. Reference numerals 18 and 19 represent a line-to-line capacitance and an electrode, respectively. Reference numerals 20, 21, and 22 represent a workpiece, machining gap stray capacitance, and a leak resistance, respectively. The inductance 16, resistance 17 and line-to-line capacitance 18 represent equivalent components included in the wiring route represented by the feeding cable 24 between the power supply and the machining gap. Reference character Vbb represents a voltage applied across the machining gap between the electrode 19 and the workpiece 20. The switching elements 13, 14 are turned on and off by a control circuit (not shown) and output power supply output voltages shown in FIGS. 10A and 12A, for example.
The inductance 16, resistance 17, line-to-line capacitance 18 of the feeding cable 24 exist in the bipolar voltage application circuit 10 and the machining gap stray capacitance 21 and leak resistance 22 exist in the machining gap between the opposite surfaces of the electrode 19 and the workpiece 20. In rough machining and intermediate machining, a machining current having sharply rising peaks is favorable, so the circuit is configured such that the impedance in the entire circuit becomes as small as possible, which reduces the inductance L and resistance R and increases the line-to-line capacitance C. If the output energy from the power supply is reduced to improve the surface roughness as in the finish machining, the stray capacitance cannot be charged rapidly and the frequency of the high-frequency AC voltages applicable across the machining gap including the line-to-line capacitance is limited to about 200-300 kHz.