POWER SOURCE FOR DISCHARGE MACHINING

Abstract: A power source for discharge machining of the type in which the charging current of a discharging capacitor (C) is controlled by switching elements (Tr1, Tr2). A large-current charging circuit (A) which will be turned on and off with a pulse width comparable to the pulse width of discharge current of the discharging capacitor (C) is arranged in parallel with a small-current circuit (B) which will be turned on and off maintaining a much longer period. The large-current charging circuit (A) participates in the charging operation only when the small-current circuit is being turned on and the charging voltage of the discharging capacitor (C) is smaller than a predetermined value.

Description: This invention relates to an electrical discharge machining power supply. Fig. 3 is a view showing the arrangement of a known wire-cut electrical discharge machine, in which numeral 1 denotes an electrical discharge machining power supply, 2 a upper wire guide, 3 a lower wire guide, 4 an XY table, 5 a workpiece, 6 a wire electrode, 7 an X-axis servomotor, 8 a Y-axis servomotor, 9 a dielectric treating tank, 10 a CNC, and 11 a machining command tape. The electrical discharge machining power supply 1, which produces a spark discharge across the workpiece 5 and wire electrode 6, is adapted to generate a pulse-shaped voltage impressed across the wire electrode 6 and workpiece 5. The specific construction of the electrical discharge machining power supply will now be described in detail. Fig. 4 shows a transistor-controlled capacitor discharge circuit used in the above electrical discharge machining power supply 1. The technique used here is described in, for example, "Wire-cut Electrical Discharge Machining Techniques", p. 25, edited by Nagao Saito, published by Nikkan Kogyo Shimbun. The circuit includes transistorized switching elements inserted in the charging circuit section of the discharge circuit of a capacitor C. With this circuit, the peak Ip of a discharge current can be changed by switching transistors Tr1, Tr2 through use of a control circuit. Such a circuit is disclosed for example in JP-A 56-107833. This circuit enables the power supply side and the discharge electrodes to be cut off from each other by the switching elements, so that the power supply can be opened when a discharge current is flowing between the discharge electrodes. Accordingly, in-flow of an electric current in a state where insulation recovery has not been attained can be prevented. Further, with the circuit shown in Fig. 4, the discharge current has a high peak Ip and the discharge current pulse obtained has a narrow pulse width in comparison with a transistor discharge circuit. For these reasons, the circuit is widely employed as a power supply for wire electrical discharge machines, machines for forming small holes and special-purpose machines for carbide alloys. Thus, with a transistor-controlled capacitor discharge circuit, electrical discharge machining is possible at a high machining speed by virtue of a discharge current having a high peak value Ip and a small pulse width. Where finishing machining is concerned, however, discharge current cut-off is poor, thus leaving a trail, and the charging current of the capacitor is caused to fluctuate by leakage current across the discharge electrodes, so that stable discharge cannot be achieved. An object of the present invention is to provide an electrical discharge machining power supply for controlling the charging current of a discharge capacitor by switching elements, which power supply enables finishing machining for a good surface smoothness, and in which a stable discharge is obtained by suppressing fluctuation of charging voltage. According to the present invention there is provided an electrical discharge machining power supply for controlling charging current of a discharge capacitor by switching elements, comprising: a first switching element and a first resistor connected in series with the discharge capacitor; a first oscillator which oscillates at a period having a pulse width larger than the width of a discharge pulse of the discharge capacitor for controlling switching of the first switching element; a series circuit consisting of a second switching element and a second resistor connected in parallel with the series circuit consisting of said first switching element and said first resistor; said second resistor having a resistance value smaller than a resistance value of the first resistor; a second oscillator which oscillates at a period having a pulse width substantially equal to the width of a discharge current pulse of said discharge capacitor for controlling switching of the second switching element; and control means for applying an output from the second oscillator to the second switching element only when the first switching element is in an on state and a charging voltage of the discharge capacitor is less than a predetermined value. According to the present invention, cut-off of the capacitor discharge circuit can be improved and discharge can be maintained in a stable state. This makes it possible to raise the machined precision of a surface machined by electrical discharge and to form a machined surface having a surface smoothness which is uniformly good. Particularly, in finishing electrical discharge machining, how the electrical discharge machining energy is supplied to the machined surface is a factor which determines the quality of the machined product. The present electrical discharge machining power supply is well-suited for such finishing electrical discharge machining and contributes greatly to an improvement in the quality of the machined product. Figs. 1(a), (b) are circuit diagrams illustrating an electrical discharge machining power supply embodying the present invention, Figs. 2(a) through (e) are waveform diagrams of signals associated with various portions of the circuit of Fig. 1, Fig. 3 is an overall view of an electrical discharge machining system, Fig. 4 shows a conventional transistor-controlled capacitor discharge circuit, and Fig. 5 is a waveform diagram of voltage (V) and discharge current (Ip) across discharge electrodes in Fig. 4. An embodiment of the present invention will now be described in detail with reference to the drawings. Figs. 1(a), (b) are circuit diagrams of an electrical discharge machining power supply according to the present invention. In these Figures, Tr1, Tr2 denote transistors serving as switching elements, and R1, R2 represent resistors for limiting current during charging. To make the circuit including the transistor Tr1 function as a large current charging circuit A and the circuit including the transistor Tr2 function as a small current charging circuit B, the resistance value of resistor R1 is made smaller than that of resistor R2; in other words, the resistance values are set in such a manner that R1<R2 will hold. R3, R4 denote voltage dividing circuits for measuring voltage during a discharge interval, C denotes a capacitor, numeral 21 designates a DC power supply, and numeral 22 designates an electrical discharge machining section including a workpiece 5 and an electrode 6. Further, numeral 23 denotes a first oscillator for generating pulses of a comparatively low frequency at a duty of approximately 50%, 24 a second oscillator for generating pulses of a higher frequency and smaller pulse width in comparison with the first oscillator, 25 and 27 AND circuits, 26 a comparator, and 28, 29 preamplifiers. Output terminals G1, G2 in Fig. 1(b) naturally are connected to gates G1, G2 of the transistors in Fig. 1(a). Figs. 2(a) through (d) are waveform diagrams associated with various portions of the circuits shown in Figs. 1(a) and (b). The operation of the electrical discharge machining power supply will now be described on the basis of these drawings. First, as shown in Fig. 2(a), the first oscillator 23 generates a signal having a large pulse width (low frequency). The signal is amplified by the preamplifier 28, so that a signal of the kind shown in Fig. 2(a) is applied to the gate G2 of the transistor Tr2. Meanwhile, as shown in Fig. 2(b), the second oscillator 24 generates a signal having a higher frequency and smaller pulse width in comparison with the first oscillator 23. The AND circuit 25 takes the AND of this signal and the abovementioned signal of large pulse width obtained from the first oscillator 23 and applies the resulting signal to one input terminal of the AND circuit 27. The voltage Vc of the capacitor C, namely the voltage across discharge electrodes, is divided by the voltage dividing resistors R3, R4 and is thus detected as a voltage Eg. The detected voltage Eg is compared with a reference voltage Vr by the comparator 26. When the voltage Vc of capacitor C surpasses the reference voltage Vr, as shown in Fig. 2(c), the comparator 26 outputs a signal. Thus, overcharging of the capacitor C is monitored. When the comparator 26 generates the output signal, control is effected in such a manner that a signal applied to the gate G1 of transistor Tr1 is turned off, as shown in Fig. 2(e). In other words, charging of the large current circuit is performed intermittently with regularity at the short pulse width under a condition where the capacitor C has not attained the overcharged state. However, when the capacitor C becomes overcharged, supply of the pulses to the gate G1 of transistor Tr1 in the large current circuit is terminated and the charging current is suppressed. In other words, overcharging of the capacitor C can be prevented automatically. Note that the preamplifier 29 is provided between the gate circuit 27 and the gate G1 of transistor Tr1. When the capacitor C is charged, as depicted in Fig. 2(c), a discharge current Ic from the capacitor C flows between the electrodes, as illustrated in Fig. 2(d), so that electrical discharge machining is carried out. In the circuit arrangement of this electrical discharge machining power supply, the small current charging circuit B comprising the transistor Tr2 and resistor R2 is connected in parallel with the large current charging circuit A comprising the transistor Tr1 and resistor R1 and passing a charging current greater than that of the above circuit. In addition, the small current charging circuit B limits the charging current to a small current. The result is excellent charging current cut-off. In other words, the charging current does not continue after the discharge of the capacitor C. Further, since the control pulse width of the small current charging circuit B is large, discharge can be stabilized and so maintained. Meanwhile, the large current charging circuit A is turned on and off at a very small pulse width. This pulse width is made the same as the discharge current width of capacitor C. By adopting such an arrangement, in-flow of the discharge current of capacitor C can be prevented. The reason for adopting the large current is that the charging voltage of capacitor C would fluctuate due to current leakage across the electrode gap if only a small current were used; the large current prevents this fluctuation of the charging voltage. In the present embodiment, it has been described that transistors are used to switch the charging circuits, though it goes without saying that switching elements such as thyristors can be used instead. The present invention is not limited to the illustrated embodiment but can be modified in various ways, such modifications being within the scope of the invention as defined by the following claim. The present invention is applicable to wire-cut electrical discharge machining and can also be used in power supplies for engraving-type electrical discharge machining.

Claims: An electrical discharge machining power supply for controlling charging current of a discharge capacitor by switching elements, comprising: a first switching element (Tr2) and a first resistor (R2) connected in series with the discharge capacitor (C); a first oscillator (23) which oscillates at a period having a pulse width larger than the width of a discharge pulse of the discharge capacitor (C) for controlling switching of the first switching element (Tr2); a series circuit consisting of a second switching element (Tr1) and a second resistor (R1) connected in parallel with the series circuit consisting of said first switching element (Tr2) and said first resistor (R2); said second resistor (R1) having a resistance value smaller than a resistance value of the first resistor (R2); a second oscillator (24) which oscillates at a period having a pulse width substantially equal to the width of a discharge current pulse of said discharge capacitor (C) for controlling switching of the second switching element (Tr1); and control means (25,26,27) for applying an output from the second oscillator (24) to the second switching element (Tr1) only when the first switching element (Tr2) is in an on state and a charging voltage of the discharge capacitor (C) is less than a predetermined value.

Inventors: OBARA, HARUKI
Applicants: FANUC LTD.
Application year: 1985
Main classification: B23H1
IPC codes: B23H1