JPS6246286B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electric discharge machining apparatus, and particularly to an improvement of a discharge circuit for obtaining a discharge current waveform with short rise and fall times.

FIG. 1 is a diagram showing an example of a circuit of a conventional electric discharge machining device, in which an electrode 1 and a workpiece 2 are opposed to each other with a machining gap 3 interposed therebetween, and a DC power source 4 and a current limiting resistor are connected to the machining gap 3. 5 and a control switching element 6 such as a transistor are connected in series via processed pulse transmission lines 10 and 11 to form a closed circuit, and the control switching element 6 is repeatedly turned on and off by a signal from the control section 7. A machining pulse is applied to the machining gap 3, and the workpiece 2 is machined by the pulsed discharge generated in the machining gap.

In such electrical discharge machining equipment, in order to efficiently machine cemented carbide etc. as a workpiece, it is necessary to increase the energy of the machining pulse applied to the machining gap, and it is also necessary to increase the energy of the machining pulse applied to the machining gap. In order to improve the surface roughness, it is effective to shorten the pulse width of the discharge current.

That is, whether or not a pulse discharge with a short current pulse width and a high current peak value can be generated is a factor that determines the performance of an electric discharge machining apparatus for machining cemented carbide or the like.

However, in the conventional discharge circuit as described above, when the current pulse width is shortened, only a current with a low current peak value can flow, making it difficult to obtain a desired current peak value.

FIG. 2 is a diagram showing the waveform of the voltage E _G appearing in the machining gap of the conventional discharge circuit described above and the waveform of the current I _L flowing through the machining pulse transmission lines 10 and 11. a, b are voltages when voltage application time is long;
In the current waveform diagram, when the voltage application time T ₀ transitions to the discharge start time T ₁ , the current I _L starts flowing, rises with a delay due to the equivalent impedance 12 and 13 of the line, and reaches the predetermined current piece value I _P1 . ing. When the control switching element 6 is turned off at time T ₂ , the voltage E _G drops to a value that cannot sustain discharge, but the current due to the electromagnetic energy accumulated in the equivalent impedances 12 and 13 continues to flow until time T ₃ .

If the voltage application time T ₀ −T ₂ is long in this way, the scheduled energization time due to the on/off control of the control switching element reaches the predetermined current peak value I _P1 .
The current pulse width becomes longer as T ₁ −T ₃ compared to T ₁ −T ₂ .

Figures 2c and d are voltage and current waveform diagrams when the voltage application time is short; discharge starts at time _T1 , and the control switching element 6 is turned off at time _T4 ;
Because this time T ₁ -T ₄ is short, the peak value of the current I _L reaches only I _P2 , which is much lower than I _P1 .
Note that the current I _G flowing through the machining gap 3 is as shown in Fig. 2b,
Both d are equal to _IL .

One of the reasons for this is that the control switching element 6
The high-speed switching characteristics of the device are controlled by the charge accumulation time of the element, and other causes include the high frequency impedance of the DC power source 4 and the machining pulse from the DC power source 4 and the current limiting resistor 5 to the machining gap 3. Examples include the high frequency impedance of the transmission lines 10 and 11, and the high frequency impedance of the current limiting resistor 5. These high frequency impedances are shown in Figure 1 as equivalent impedances of 12,
13.

Among the above causes, the high frequency impedance of the DC power supply 4 is affected as shown in Figure 1.
By adding a high-frequency capacitor 9 of polymer film type in parallel with the high-frequency capacitor 9, the high-frequency impedance can often be sufficiently reduced.

To improve the high-speed switching characteristics of control switching elements, select appropriate elements and
In order to increase the number of elements connected in parallel to lower the current per element, and to shorten the turn-on time during switching, which is dominated by the charge accumulation time, for example, in the case of a transistor, the speed not shown in the base drive circuit is increased. Methods are known, such as adding a boost capacitor or limiting the base current to shorten the turn-off time to drive the transistor at the very edge of its saturation region. Further, the high frequency impedance of the current limiting resistor 5 can be improved by using a metal film resistor.

Regarding the high frequency impedance of the processed pulse transmission lines 10 and 11, which cannot be solved even with these improvements, conventionally, as shown in FIG.
A capacitor 8 is placed near the machining gap 3, and energy is stored in the capacitor 8 by the charging circuit. A solution was sought by adopting a so-called capacitor-added discharge circuit, in which a discharge current is passed through the circuit.

This capacitor-added discharge circuit can keep the high-frequency impedance between the capacitor and the machining gap extremely low, so it is easy to obtain a discharge current with a minute pulse width and a high current peak value as desired. However, on the other hand, since the discharge starting voltage is affected by the physical conditions of the machining gap (such as the unevenness of the machined surface and the distribution of inclusions in the machining fluid), the discharge energy per shot, that is, the capacitor This method has the drawback that large variations occur in the stored energy 1/2CV ² and the desired machined surface roughness is not necessarily obtained.

Another method for improving the high frequency impedance of the processed pulse transmission lines 10 and 11 is to replace the transmission lines themselves with high frequency low impedance lines such as coaxial cables. When using a coaxial cable, the purpose of improvement can be achieved sufficiently if the equivalent impedance is several ohms, but normally electric discharge machining equipment can handle both rough machining areas with large current values and finishing machining areas with small current values and minute pulse widths. It is extremely difficult or often impossible to manufacture a low impedance high frequency cable as described above that has a heat capacity that can withstand use with large currents.

The present invention has been made in view of the above points, and can be used in both rough machining and finishing machining areas, and can obtain a desired current peak value even when the discharge current pulse width is extremely short, such as in finishing machining. The purpose is to provide processing equipment.

In order to achieve the above object, in the present invention, in addition to the normal machining pulse transmission line between the power supply section and the machining gap, electric discharge is caused by the high frequency impedance of the machining pulse transmission line and other parts (current limiting resistor, etc.). A series circuit of a capacitor and a low impedance line for high frequency is provided so that only the current that compensates for the delay in the current waveform flows, and the rise and fall times of the discharge current are shortened by the compensation current flowing through this series circuit. This is what I did.

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 3 is a circuit diagram showing an embodiment of the present invention, and the same reference numerals as in FIG. 1 indicate corresponding parts. This embodiment is different from FIG. 1 in that in order to improve the delay in the rise and fall of the discharge current waveform due to the high frequency impedance of the processed pulse transmission lines 10 and 11, the processed pulse transmission line 10 and the processed pulse transmission line are This is because it is installed in parallel with 11. 14 is a coaxial cable shown as an example of a low impedance line for high frequency, one end of the internal conductor 14-1 of the coaxial cable 14 is connected to the current limiting resistor 5, and the other end is connected to the workpiece 2 via the capacitor 15. One end of the outer conductor 14-2 of the coaxial cable 14 is connected to the electrode 1, and the other end is connected to the DC power source 4 via the shunt resistor 16.

In this way, the coaxial cable 14 connects the machining gap 3, DC power source 4, and current limiting resistor 5 to the capacitor 15 in parallel with the machining pulse transmission lines 10 and 11.
The charging/discharging current of the capacitor 15 flowing through this coaxial cable 14 will be referred to as I _C .

When the control switching element 6 is turned on by a signal from the control section 7, a voltage is applied to the machining gap 3 and electric discharge is started. At this time, the sum of the current I _L flowing through the machining pulse transmission lines 10 and 11 and the current I _C flowing through the coaxial cable 14 becomes the current I _G flowing through the machining gap 3, but the rising speed of the current I _C is determined by the machining pulse It is not affected by the equivalent impedances 12 and 13 of the transmission lines 10 and 11, and is determined only by the high frequency characteristics of the coaxial cable 14, capacitor 15, and shunt resistor 16. Furthermore, the magnitude of the current I _C is determined only by the amount of charge charged in the capacitor 15, and it is only necessary to compensate for the delay in the rise of the current I _L.
The effective value of _C is extremely small compared to _IL .

The purpose of providing the shunt resistor 16 is that when the current I _L rises to a current value determined by the applied voltage and the current limiting resistor 5, a part of this current I _L is shunted to the coaxial cable 14. This is to suppress the flow of electricity and allow it to flow mainly through the lines 10 and 11, but generally the DC resistance value of the coaxial cable 14, which is not intended for power supply, is larger than the DC resistance value of the lines 10 and 11. Therefore, providing the shunt resistor 16 is not an essential requirement. When providing the shunt resistor 16, one with good high frequency characteristics, such as a metal film resistor, is used.

The coaxial cable 14 used as a high frequency low impedance line may be a coaxial cable for broadcasting equipment.

Furthermore, it is sufficient that the high frequency characteristics of the capacitor 15 are comparable to those of a discharge capacitor that has conventionally been used by being connected in parallel to the machining gap.

This capacitor 15 is useful for improving high frequency impedance as will be described later, but at the same time it also serves to prevent excessive continuous current from flowing into the coaxial cable 14.

The operation of the compensation circuit constructed from such readily available parts will be explained below with reference to FIGS. 4 and 5.

FIG. 4a is a waveform diagram of the voltage E _G applied to the machining gap, which is the same as the E _G waveform of FIG. 2a. Fourth
Figure b is a waveform diagram of the current I _L flowing through the processed pulse transmission lines 10 and 11, which is the same as the I _L waveform in Figure 2 b, and regardless of the presence or absence of a compensation circuit, the waveforms of E _G and I _L are basic. There is no difference.

FIG. 4c is a waveform diagram of the capacitor charging/discharging current I _C , which starts to rise at the same time as I _L at the start time of discharge in the machining gap T ₁ . Unlike the current I _L , the current I _C is not affected by the equivalent impedances 12 and 13 of the processed pulse transmission line, and is not affected by the current limiting resistor 5 and the shunt resistor 1.
6, etc., and the capacitance C of the capacitor 15.
It shows a rise characteristic proportional to the time constant determined by the inductance L included in the line and the inductance L included in the line. Here, the shunt resistor 16 is either absent or set to a very small value, for example, 1 ohm or less, preferably 0.5 ohm or less, while the resistance value of the current limiting resistor 5 is set as shown in FIGS. 1 and 3. As shown in , it is a composite value of multiple parallel resistances, and generally has a value of several ohms to several tens of ohms, so the DC resistance value of the line through which the current I _C flows is mainly determined by the value of resistor 5. . Under such circuit constants, the current I _C exhibits a steep rise characteristic as shown in FIG. 4c. Ideally, the slope of this rise would be infinite with respect to time, but in reality, a slight delay occurs due to the existing inductance component of the line, resulting in a waveform as shown in FIG. 4c.

The waveform of the current I _C that started rising from time _T1 is the time
After reaching the peak value at T ₁ ′, it gradually decreases. This means that at the initial discharge start time T ₁ , the terminal voltage maintains the value of electromagnetic energy, which was zero, due to the electromagnetic energy conservation characteristic of the equivalent impedances 12 and 13, and the equivalent impedance becomes 1.
2 and 13, and the capacitor 15 is charged by this terminal voltage, but the capacitor terminal voltage becomes high at time T ₁ ', and the charging current begins to attenuate thereafter.

In the process in which the current I _L increases while storing electromagnetic energy in the inductance components of the equivalent impedances 12 and 13, the amount of accumulated electromagnetic energy and the amount of electrostatic energy accumulated in the capacitor 15 are equal to each other, and at a certain time. If the slope of the fall of the I _C waveform after T ₁ ' is approximately equal to the slope of the rise of the I _L waveform, the current I _C +I
_L , that is, the rise of the current I _G flowing through the machining gap is the steepest, and the current peak value is approximately equal to I _P1 .

Even after the voltage applied to the machining gap is cut off at time T ₂ , the current I _L continues to flow until time T ₃ as in FIG. 2b. This delay in the fall of the current waveform is due to the release of electromagnetic energy stored in the equivalent impedances 12 and 13 when the current rises.

When the amounts of electrostatic energy and electromagnetic energy stored are equal as described above, after the applied voltage is cut off, only the stored energy of the former two becomes a current source, and the charge stored in the capacitor 15 flows backwards. , the electromagnetic energy of the equivalent impedance 13 is consumed as a current I _L that stores it, and the energy stored in equal amounts cancels each other out, and as a result, the current on the outside line, that is, the line passing through the machining gap 3, is The current waveform is rapidly cut off and has a steep fall after time _T2 , as shown in FIG. 4d.

In summary, when the current rises, the delay in the rise of the discharge current I _G due to the equivalent impedances 12 and 13 is compensated by the charging current of the capacitor 15, resulting in a steep rising waveform, while when the current falls, it is stored in the equivalent impedances 12 and 13. The generated electromagnetic energy can be rapidly eliminated by the discharge current from the capacitor 15, resulting in a steep falling waveform.

This is true even when the voltage application time is short, and FIGS. 5 a to d are similar to FIGS.
It is shown that the rise and fall of the waveform of the current I _C +I _L is improved under the short condition of voltage application time T ₀ -T ₄ as in d.

FIG. 6 is a diagram showing another embodiment of the present invention, in which a capacitor 15 is provided on the power supply side of the coaxial cable 14, that is, on the side opposite to the processing gap 3. In this way, the capacitor 15 needs to be placed in series with the coaxial cable 14 and in parallel with the processed pulse transmission line 11, and it does not matter which side of the coaxial cable 14 it is placed. In this embodiment, series resistors 17-1 to 17-3 are connected between the capacitor 15 and the control switching element 6 of the power supply section. As a result, these resistors 17-1 to 17-3 are provided in parallel with the current limiting resistor 5, making it easy to set the capacitor charging/discharging current.

FIG. 7 shows still another embodiment of the present invention, in which the positions of resistors 17-1 to 17-3 and capacitor 15 in FIG. 6 are replaced. That is, the resistors 17 are arranged in a concentrated manner, and the capacitors 1
5-1 to 15-3 are distributed in the same number as the control switching elements 6, and when the number of elements that perform on/off control among the control switching elements 6 is changed, the capacitor capacity also changes accordingly. The charge/discharge current is increased or decreased in proportion to the number of control switching elements 6 that perform on/off control.

As explained above, the present invention improves the delay in the rise and fall of the discharge current waveform due to the high frequency impedance of the processed pulse transmission line by adding a series circuit of another high frequency low impedance line and a capacitor. By implementing the present invention, it is possible to obtain a desired current peak value even when the discharge current pulse width is extremely short, thereby providing a high-performance electrical discharge machining device that can efficiently perform machining with good surface roughness. can.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram showing an example of a conventional electrical discharge machining device, Figure 2 is a waveform diagram of the voltage and discharge current applied to the machining gap when impedance compensation of the machining pulse transmission line is not performed, and Figure 3 is a diagram of the present invention. FIGS. 4 and 5 are circuit diagrams showing one embodiment of the invention, and FIGS.
7 and 7 are circuit diagrams showing other embodiments of the present invention. 1... Electrode, 2... Workpiece, 3... Machining gap, 4... DC power supply, 5... Current limiting resistor,
6... Control switching element, 10, 11... Processed pulse transmission line, 12, 13... Equivalent impedance, 14... Coaxial cable which is a low impedance line for high frequency, 15... Capacitor.

Claims (1)

[Claims] 1. The electrode and the workpiece are opposed to each other with a machining gap interposed therebetween, and a machining pulse is applied to the machining gap by repeated on/off operations of a control switching element connected between the machining gap and a DC power source to generate electrical discharge. In the electrical discharge machining device that generates electric discharge, the power supply unit and the machining gap are connected via a capacitor in parallel with a machining pulse transmission line that connects the machining gap to a power supply unit including the DC power source and control switching element. An electrical discharge machining device characterized by being provided with a low impedance line for high frequency.