Abstract:
In a power source for a wire-cut, electric-discharge machine, a plurality of capacitors of different capacitances are provided as the discharge capacitors in a transistor-capacitor circuit and the capacitors are selectively used in accordance with the machined surface of the workpiece. Each capacitor has connected thereto an inductor, the inductance value of which increases as the capacitance value of the capacitor decreases, and this action of the inductor ensures stable discharge.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a wire-cut, electric-discharge machining power source, and more particularly to a wire-cut, electric-discharge machining power source with which it is possible to perform electric-discharge machining operations at as high a speed as possible from roughing to finishing through utilization of a plurality of capacitors of different capacitances. 
     2. Description of the Prior Art 
     Wire-cut, electric-discharge machines are capable of highly accurate machining of a metal molds or the like having complicated configurations through the use of wire electrodes for electric-discharge machining. An example of a power source which has hitherto been employed with such discharge machine is shown in FIG. 1, in which the power source is formed by a transistor-capacitor discharge circuit comprising a power source V 1 , a charging resistor R, a transistor Tr for control use and a capacitor C 0  connected between a wire electrode WIR and a workpiece WK. With such a power source, high-speed cutting of the workpiece WK can be performed by making the capacitance of the capacitor C 0  large, the inductance of the discharge circuit small and the voltage of the power source V 1  low. In this case, however, the roughness of the machined surface of the workpiece WK is increased by the increased capacitance of the capacitor C 0 . To avoid this, it is general practice in the prior art to imporve the surface roughness by decreasing the capacitance of the capacitor C 0  in the case where a high degree of accuracy is required of the machined surface. But the conventional circuit arrangement of FIG. 1 has the defect that decreasing the capacitance of the capacitor C 0  may sometimes result in unstable discharge and extremely lowered cutting speed. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a power source suitable for use with a wire-cut, electric-discharge machine. 
     Another object of the present invention is to provide a wire-cut, electric-discharge machining power source which, even in the case of using a small-capacitance discharge capacitor for reducing machined surface roughness, ensures a stable discharge to raise the cutting speed as high as possible. 
     Yet another object of the present invention is to provide a wire-cut, electric-discharge machining power source which employs a plurality of discharge capacitors of different capacitances so that machining operations from roughing to finishing can be performed at as high a speed as possible. 
     Briefly stated, the wire-cut, electric-discharge machining power source of the present invention employs a plurality of capacitors of different capacitances as discharge capacitors of a transistor-capacitor discharge circuit. The capacitances of the capacitors are each set to a value suitable for a particular machining operation, for example, rough, medium and finish machining operations. The capacitors each have connected thereto an inductor the value of which increases with a decrease in the capacitance of the capacitor, and this action of the inductance ensures a stable discharge. By operating switches for selectively changing over the discharge capacitors in accordance with the machining operations, machining from roughing to finishing can be achieved at high speed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an electric circuit diagram illustrating a conventional wire-cut, electric-discharge machining power source; and 
     FIG. 2 is an electric circuit diagram illustrating an embodiment of the wire-cut, electric-discharge machining power source of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is based on the confirmation by the present inventor&#39;s experiments that in the case of electric-discharge machining using a capacitor of a small capacitance, a relatively large inductance of the discharge circuit and a high power source voltage suppress the possibility of short-circuiting to ensure stable discharge, permitting an increase in the cutting speed without degrading the machined surface roughness, and on the idea that machining operations from roughing to finishing can be performed at the highest possible speed by changing the inductance of the discharge circuit depending on the capacitance of the capacitor in view of the fact that high-speed machining can be achieved by increasing the capacitance of the capacitor and using a low-inductance discharge circuit as mentioned previously. 
     FIG. 2 is an electric circuit diagram illustrating an embodiment of the present invention, in which the parts corresponding to those in FIG. 1 are identified by the same reference characters. In FIG. 2, reference characters C 1  to C 3  indicate capacitors; L 1  and L 2  designate inductors; S 1  to S 3  identify switches; and V 2  denotes a power source. 
     In FIG. 2, the capacitors C 1  to C 3  are each connected in parallel with the power source V 2 . The inductor L 1  and the switch S 1  are connected in series with the capacitor C 1 , and the inductor L 2  and the switch S 2  are connected in series with the capacitor C 2 . No particular inductor is connected to the capacitor C 3  since a lead wire or the like has a stray inductance, and only the switch S 3  is connected in series with the capacitor C 3 . The capacitor C 3  is provided for rough machining and has the largest capacitance among the capacitors. The capacitor C 1  is for finishing and has the smallest capacitance. The capacitor C 2  is for medium machining and has a capacitance substantially intermediate between those of the capacitors C 1  and C 2 . The inductors L 1  and L 2  respectively connected to the capacitors C 1  and C 2  are provided for increasing the cutting speed, and their inductance values are set so that they may increase as the capacitance values of the capacitors decrease. In this case, the inductance value of the inductor L 1  is set to be larger than that of the inductor L 2 . 
     As referred to above, it has been confirmed experimentally that in the case of using a capacitor of a small capacitance, a certain increase in the inductance of the discharge circuit suppresses the occurrence of short-circuiting to stabilize the discharge, thereby permitting an increase in the cutting speed. It is assumed that the increase in the inductance causes a decrease in the pulse height of one shot of discharge current to reduce the discharge pressure, imparting no unnecessary vibration to the wire. Furthermore, it has been confirmed experimentally that an excessive increase in the inductance decreases the pulse height of the discharge current to lower the machining performance. Accordingly, there are optimum inductance values of the inductors L 1  and L 2 . For example, in the case where a stray inductance of about 0.3 μH exists, the capacitor C 3  may have a capacitance value of 2 to 3 μF, and in the case of the capacitor C 2  having a capacitance value of 0.1 to 0.5 μF, it is preferred that the inductance value of the inductor L 2  is in the range of 0.4 to 0.7 μH. Further, it has been confirmed experimentally that in the case of setting the capacitance value of the capacitor C 1  in the range of 0.01 to 0.1 μF, it is preferred that the inductance value of the inductor L 1  is 1 to 1.2 μH or so. Incidentally, when machining was conducted using the capacitor C 1  having a capacitance value of 0.08 μF without the inductor L 1 , the cutting speed was 60 to 70% lower than that in the case of employing the inductor L 1 . 
     With the arrangement described above, when only the switch S 3  is closed, the large-capacitance capacitor C 3  with no inductor connected thereto is connected in parallel to the power source V 2 , permitting high-speed rough machining. When closing the switch S 1  alone, the series circuit of the capacitor C 1  and the inductor L 1  is connected in parallel to the power source V 2  and finish machining takes place. In this case, since the inductor L 1  is connected to the capacitor of small capacitance, the discharge is made stable and a sufficient cutting speed can be maintained. Likewise, when closing the switch S 2  alone, the series circuit of the capacitor C 2  and the inductor L 2  is connected to the power source V 2  and medium machining is carried out. In this way, by changing over each capacitor by one of the switches S 1  to S 3 , the inductance of the discharge circuit can be changed depending on the capacitance value of the capacitor, and as a consequence, a substantially sufficient cutting speed can be maintained from rough to finish machining operations. 
     While in the foregoing embodiment use is made of three capacitors C 1  to C 3  which are selectively changed over by the switches S 1  to S 3 , the number of capacitors used need not always be limited specifically thereto but may also be selected to be two or more as desired. Also, the switches S 1  to S 3  can be substituted with one rotary switch. 
     As will be appreciated from the foregoing description, the present invention is to change the inductance of the discharge circuit depending on the capacitance value of each of the capacitors used, by which a sufficient cutting speed can be maintained over the entire machining operation from rough to finish machining. 
     It will be apparent that many modifications and variations may be effected without departing from the scope of the novel concepts of this invention.