JPH0453646B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention is an improvement in an electrical discharge machining method that supplies machining pulses to the machining gap between a machining electrode and a workpiece by on/off switching control of a switch element. In particular, the present invention relates to a control method for quickly extinguishing abnormal discharges such as sustained arc discharges and short circuits, and increasing normal discharges.

[Background of the invention]

As is well known, electrical discharge machining is performed by repeated electrical discharge from machining pulses supplied to the machining gap, but the discharge state changes from moment to moment due to changes in the concentration of machining debris in the machining gap. Harmful or ineffective sustained arc discharge, abnormal discharge such as short circuit, etc. occur, resulting in a decrease in machining speed and deterioration of machining accuracy. For this reason, it is necessary to reduce abnormal discharge and perform machining stably, and many methods and devices related to optimal control of machining pulses have been proposed.

For example, as described in Japanese Unexamined Patent Publication No. 53-44994, a machining pulse is applied when the gap resistance is at a constant value during off-time. However, when the gap is in an abnormal state, the gap resistance does not reach a certain value, and its recovery can be done by using the electrode feed servo system, machining fluid jet device, etc.
In general, since the processing is dependent on components with slow response speeds, recovery is delayed, and therefore the frequency of normal discharge is low, making it impossible to expect an increase in machining speed. Furthermore, as described in Japanese Patent Publication No. 45-36200, an abnormality in the machining state is detected for each machining pulse immediately after its application, and control is performed to immediately extinguish the machining pulse when an abnormality is determined. However, in practice, it is not necessary to control each machining pulse one by one to prevent damage to the machined surface due to abnormal electrical discharge.
In addition, since the machining pulse is extinguished in the event of an abnormality, recovery from the abnormal state is delayed as in the above example, and an increase in the machining speed cannot be expected. Furthermore, JP-A-59-69226
As described in the publication, the discharge delay time after applying the machining pulse is detected, and if it is too short, it is determined to be a sustained arc discharge, and only current pulses with a shorter pulse width and lower peak current value than usual are used. On the other hand, the presence or absence of a short circuit is detected during the off-time, and if a short circuit is detected, a normal current pulse is generated only once to remove a pseudo short circuit caused by inclusions such as processing debris. If the short circuit still cannot be removed, it is determined that it is a mechanical short circuit, and the machining gap is enlarged by the electrode feeding system, and current pulses with the same short-time and low current value as sustained arc discharge are applied in between, until the short circuit is removed. There is an example in which control is performed in which the voltage is continued to be applied after a certain off time of the same width as normal.

However, active removal of sustained arc discharge through pulse control is not considered, nor is consideration given to the current pulse width for removing pseudo short circuits and the early occurrence of the next normal discharge pulse due to short circuits during its off time. It had not been done. Furthermore, since both the peak current value and pulse width of the current pulse are controlled when an abnormal discharge occurs, there is a drawback that the control device becomes complicated.

[Purpose of the invention]

The present invention was made in view of the above problems, and its purpose is to quickly extinguish both abnormal discharges such as sustained arc discharges and short circuits using a simple pulse control device, and thereby improve the frequency of occurrence of normal discharges. The object of the present invention is to provide an electric discharge machining method that has a high machining speed and is relatively inexpensive.

[Summary of the invention]

That is, the present invention uses a check pulse to determine and check the state of the machining gap after applying a machining pulse, and if it is normal, machining is performed using a method in which the machining pulse frequency is constant or a discharge duration is constant;
In abnormal cases, it is divided into sustained arc discharge and short circuit, and in the case of the former, the machining pulse is stopped immediately, and in the latter case, the machining pulse is stopped after a period of time shorter than the normal discharge duration, and then the processing pulse is turned off for a shorter time than usual. The pulse is applied again after a certain period of time, and the on/off control of the machining pulse and the determination check of the machining gap state are repeated until these abnormal conditions are recovered.

[Embodiments of the invention]

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

In FIG. 1, 1 is a machining electrode, 2 is a workpiece facing the machining electrode 1 and forms a machining gap, and 3
is a machining power source, which is connected to the machining gap through a transistor switch 4 and a current limiting resistor 5 in series, and supplies machining pulses to the gap by on/off switching control of the switch 4 by a control circuit 6. 7 is a check pulse generation circuit, and control circuit 6
The time from when the on-start signal 15 of the switch 4 is detected and the on-start signal is output until the switch 4 is turned on and the voltage between electrodes reaches a voltage sufficient to discharge (for example, 50 V or more) After delaying the check pulse 16, a check pulse 16 is generated.

The width of this check pulse is very small (for example, 0.5 μsec or less) compared to the first pulse-on time tp or the discharge duration te set as the machining conditions.
shall be. The first pulse on time tp is the time during which the switch element 4 is turned on by the switching control circuit 6 and a machining pulse is applied between the machining electrode 1 and the workpiece 2. Further, the time period during which the switch element 4 is periodically turned off and the machining pulse is paused is referred to as the first pulse off time to.
Reference numeral 8 denotes a gap state discrimination circuit, in which a check pulse 16 outputted from the check pulse generation circuit 7 is used to discriminate whether there is a sustained arc discharge or a short circuit.
During the generation period, the detected voltage of the gap is compared and determined with the reference voltage, and when it is determined that it is a sustained arc discharge, the sustained arc discharge determination signal 17 is sent to the control circuit, and when it is determined that it is a short circuit, the short circuit determination signal 18 is sent to the control circuit. Output to 6.

Discrimination between normal discharge, sustained arc discharge, and short circuit is performed as follows. In the case of normal discharge, the voltage of the applied machining pulse is equal to the no-load voltage Eo (generally
100V, but it may be higher than that) and then discharge (the voltage during which the discharge continues)
(Ve is about 18 to 30 V), whereas in the case of sustained arc discharge, an arc-shaped discharge occurs without rising to the above no-load voltage. In addition, in the case of a short circuit, the voltage decreases to the short circuit voltage Vs.

Therefore, the first reference voltage level e ₁ is set between a voltage Vc (for example, 50 V or more) that is sufficient to discharge the no-load voltage Eo (Eo > e ₁ > Vc), and at the same time another reference level e ₂ is set. is set between the discharge voltage Ve and the short-circuit voltage Vs (Ve > e ₂ > Vs), and is compared and determined by the gap state determination circuit.

If the level E of the interelectrode voltage detected during the generation period of the check pulse is less than _e1 , it is an abnormal discharge (if it is more than _e1 , it is a normal discharge).

When this detection level E is e ₁ > E > e ₂ , it is a sustained arc discharge, and when E > e ₂ , it can be determined that it is a short circuit, and thus a sustained arc discharge discrimination signal or a short circuit discrimination signal can be output, respectively. become.

Although the discharge voltage in the gap has been shown here as a signal for detecting and discriminating the gap state, it goes without saying that the electric discharge machining current or impedance can also be used as a signal for detecting and discriminating.

For example, in a normal discharge, the discharge current flows after a discharge delay time (occurrence of no-load voltage) after the machining pulse is applied, whereas in the case of an abnormal discharge, the discharge current flows immediately, and in the case of a short circuit, the short-circuit current Therefore, each can be distinguished. Also, regarding impedance, since an impedance corresponding to the voltage between electrodes is generated, the gap state can be determined.

9 and 10 are circuits for setting the on/off switching time of switch 4 when abnormal discharge occurs due to sustained arc discharge or short circuit, and 9 sets the pulse on time to be shorter than the normal discharge duration te. It is a second pulse on time setting circuit, and also has a first pulse on time setting circuit.
0 is a second pulse off time setting circuit that sets a time shorter than the normal off time, that is, the first pulse off time to.

Figure 2 shows the waveforms of the gap voltage a, machining current b, and check pulse c, where A, B, F, and K are normal discharges, C, D, and E are sustained arc discharges, and G, H shows the case of short circuit.

When the control circuit 6 that controls on/off of the switching element 4 receives an abnormal state signal of sustained arc discharge or short circuit from the discrimination circuit 8 that detects and discriminates whether the gap state is good or bad, the control circuit 6 controls whether the signal is a sustained arc discharge discrimination signal or not. For example, turn switch 4 off immediately, and turn it on after the second pulse off time, which is set shorter than the normal off time, has elapsed.
The gap state is determined by the determination circuit 8, and this process is repeated until the sustained arc discharge determination signal is no longer generated. In this case, even if the switch 4 is turned off immediately after the sustained arc discharge discrimination signal is generated, the current pulse (a small pulse energy with the same peak current value as during normal discharge and only the pulse width reduced) for longer than the generation period of the check pulse. occurs and serves to remove sustained arc discharge without damaging the machined surface.

On the other hand, when the abnormal state signal is a short circuit determination signal (mostly pseudo short circuits due to inclusions such as machining debris), for example, switch element 4 is activated for a normal discharge duration.
By turning off after the second pulse on time, which is set shorter than te, pseudo short circuits caused by machining debris are removed, and the second pulse off time, which is set shorter than the normal off time to, is turned off. After the time has elapsed, the switch element 4 is turned on again, the gap state is determined by the gap state determination circuit, and this process is repeated until the short circuit determination signal is no longer generated. In addition,
If the short circuit is not a pseudo short circuit but a mechanical short circuit, it can be dealt with by enlarging the machining gap using an electrode feeding system (not shown). Furthermore, damage to the switch element due to short circuit current is prevented by inserting a current limiting resistor.

Therefore, for each machining pulse applied to the machining gap, the gap condition is detected by a check pulse at the initial stage, and if an abnormal discharge such as a sustained arc discharge or a short circuit occurs, it can be immediately detected. By removing the metal and then generating normal discharge in a short period of time, the frequency of normal discharge increases overall, and the machining speed can be increased. Also,
Since only the pulse width is controlled, the device can be made inexpensive with only a small addition to conventional circuits or devices.

As described above, according to the embodiment of the present invention, machining can be stabilized especially in the finishing machining area where electric discharge tends to be unstable due to the small pulse energy and narrow machining gap, and the machining speed is faster than that of the conventional method. A significant increase in can be achieved. Furthermore, even in the rough machining region where the pulse energy is high, the machining speed is improved without deteriorating the machined surface.

[Effects of the Invention] As described above, according to the present invention, abnormal discharges such as sustained arc discharges and short circuits can be extinguished at an early stage, and the frequency of occurrence of normal discharges increases. Machining speed can be increased over the entire area without deteriorating the machined surface.

[Brief explanation of the drawing]

Fig. 1 is a circuit configuration diagram of an embodiment of the present invention;
The figure is an explanatory diagram of the discharge waveform, where a is the interelectrode voltage waveform, and b
is a machining current waveform, and c is a check pulse waveform. DESCRIPTION OF SYMBOLS 1... Electrode for processing, 2... Workpiece, 3... Power source for processing, 4... Switching element, 5... Current limiting resistor, 6... Switching control circuit, 7... Check pulse generation circuit, 8... ...Gap state discrimination circuit, 9...Second pulse on time setting circuit, 1
0...Second pulse off time setting circuit, 15...
...On start signal, 16...Check pulse, 17
...Sustained arc discharge determination signal, 18...Short circuit determination signal.

Claims (1)

[Claims] 1. An electric discharge machining method in which machining is performed while supplying a machining pulse by on/off switching control of a switch element to a machining gap formed by facing a machining electrode and a workpiece, comprising: The voltage level of the machining voltage waveform during electrical discharge machining is detected by a gap state discriminating circuit, and the time point at which the machining pulse is applied is detected by a check pulse generating circuit to generate a check pulse with a minute width. The voltage level E of the inter-electrode voltage waveform detected by the check pulse generating circuit is compared with the reference voltage levels e ₁ and e ₂ during the generation period of the check pulse output from the check pulse generating circuit, and the detected voltage level E is determined as e ₁ >E>e ₂ and E<e ₂ , the gap condition is determined to be abnormal, and a sustained arc discharge and short circuit discrimination signal is generated, respectively, and when the discrimination signal is a sustained arc discharge discrimination signal, the switch is activated. The element is immediately turned off, and when the signal is a short-circuit determination signal, the switch element is turned off after a time shorter than the normal discharge duration te has elapsed to pause the machining pulse, and then the switch element is turned off again after a time shorter than the normal off time to has elapsed. is turned on and the machining pulse is reapplied, and the on/off switching control of the switch element and the check to determine whether the gap condition is good or bad are performed until generation of the sustained arc discharge determination signal and the short circuit determination signal is avoided. An electric discharge machining method characterized in that the following steps are repeated.