JPS6210769B2 - - Google Patents

Description

Detailed Description of the Invention The present invention is an electric discharge machining method in which machining is performed by intermittently passing a pulse current through a machining gap formed between an electrode and a workpiece. This invention relates to improvement of electric discharge machining control method for performing machining under optimal machining conditions.

In general, in electric discharge machining, the condition of the machining gap changes very easily, and sometimes the condition of the machining gap deteriorates significantly, causing an abnormal discharge state and causing unexpected damage to the electrode and the workpiece. For example, in rough machining, if an excessive machining current is supplied relative to the machining area, or if the machining depth becomes deeper and the ejection of the workpiece worsens, the insulation state of the machining gap deteriorates, resulting in a so-called steady arc. This will cause damage to the electrode and workpiece. In addition, in the case of finishing machining, the machining gap is extremely narrow, so it is particularly difficult to discharge the machining powder, and when the machining area is large, the machining powder stays locally, causing electrical discharge to concentrate there. This creates a small dent. In this state, the damage to the workpiece is small compared to the steady arc state during rough machining, but since the finished surface is finely rough as required by the finishing process itself, the workpiece is damaged as a result. This can damage the

Therefore, in the past, the operator constantly monitored the machining status, judged the machining gap status based on the movement of the machining depth indicator, changes in machining sound, or the way smoke was generated during machining, and averaged the machining process. By changing the current or changing the conditions of the electrode fixed time pulling device,
In other words, machining is performed by changing the period at which the electrode is pulled up, but it is difficult to appropriately select such conditions, and not only does the operator require considerable skill, but the operator must also Since it is on the side of machines, it is inevitable that it will be at a considerable disadvantage from the point of view of labor saving.

In addition, in order to improve this point and automatically adjust the average machining current, the state of the machining gap is electrically detected, and the detection signal is used to determine the pulse width, rest time, or current peak value of the electric discharge machining pulse. There are also some that change the . This is configured, for example, as shown in Fig. 1, and its basic principle is that a current limited by a resistor 2 from a DC power source 1 is switched on and off by a switching transistor group 3, and an electrode 4 and a workpiece are connected to each other. Electric discharge machining is performed by flowing the material into the machining gap formed by 5. On the other hand, the state of the machining gap is electrically detected by a detection device 6, further evaluated and determined by an evaluation device 7 based on a certain evaluation method, and the output signal is sent to a pulse generator 8.
send to

When an abnormality is detected in the state of the machining gap, the average current flowing in the machining gap is reduced in order to reduce the amount of the machining layer staying in the machining gap, that is, the current pulse current flowing in the machining gap is A signal is generated for controlling in the direction of shortening the pulse width, extending the pause time, and decreasing the current peak value, which are the main conditions, and drives the control device 9 using this signal to control the switching transistor group 3.
Here, the machining gap detection device 6 divides the voltage of the machining gap by resistors 10 and 11, smoothes the divided voltage by a capacitor 12, and obtains the voltage V ₂
A comparator 15 compares the voltage V ₁ determined by the DC power supply 13 and the variable resistor 14, and outputs a signal when V ₂ ≧V ₁ . Here, the voltage across the machining gap in the case of an abnormal state in electrical discharge machining is generally as shown in FIG. In the figure, a is the state when the so-called steady arc state is approaching, and the no-load voltage application time becomes extremely short, and b is the state when the iron is processed with an iron electrode or the gain of the servo system is increased too much. This is a condition that occurs when the machining gap is repeatedly opened or short-circuited. Further, c is a case where electric discharge is concentrated due to accumulation of machining debris, which often occurs during finishing machining, and there is almost no change in the voltage waveform. When such an abnormal condition occurs, if the detection method using the detection device 6 shown in FIG. 1 attempts to detect the steady arc condition shown in FIG.
It also detects conditions such as repeated opening. This is because when processing iron with a graphite electrode, which is generally well-processed, or when processing iron with a copper electrode, if the operator sets the servo system conditions and current pulse conditions appropriately, the second Since the repeated short-circuit and open-circuit state shown in FIG. 2B does not occur very often, only the state that is becoming the steady arc state shown in FIG. 2A can be detected as a result. However, when processing iron with iron electrodes, where short circuits and open circuits often occur even under normal conditions, instability may be detected too much.
For example, the average machining current is reduced too much, resulting in extremely inefficient machining. In addition, conditions such as concentration of electrical discharge due to accumulation of machining debris as shown in Fig. 2c can hardly be detected, and for example, in low-consumption finishing machining, the operator may set the electrode lifting cycle to be shorter before machining. In addition, machining was carried out under conditions that suppressed the machining current as much as possible to prevent the accumulation of machining debris, resulting in extremely inefficient machining.

In general, in electric discharge machines, the condition of the machining gap formed by the electrode 4 and the workpiece 5 changes from time to time, and in response to these changes, the average machining gap voltage is adjusted to a predetermined value as described later. Since the position of the electrode 4 is changed from time to time in order to approach the value, the electrode 4 also moves toward the workpiece 5 while vibrating minutely (with an amplitude). For example, the magazine "Mitsubishi Electric Technical Report" (Vol.45.No.10) published on October 25, 1970, No.
It is introduced on pages 1248-1249. Here the third
The figure shows how the electrode position changes in various discharge states, where a is a normal state when machining iron with a copper electrode, and the amplitude 16 is about 0.01 mm. On the other hand, b is a case where iron is machined with an iron electrode, and even when the machining is normal, there are many short circuits and opens in the machining gap, so the amplitude 16 is generally large, 0.02
~0.1mm. While the machining is being performed normally, as shown in FIGS. 3a and 3b, the position 17 that moves from when the electrode 4 descends to when it rises (hereinafter referred to as the lowest point) is where the machining is progressing. However, if an abnormal condition occurs in the machining gap, for example if the insulation condition of the machining gap deteriorates, electrical discharge will occur even if the machining gap length is long, so as shown in Figure 3c. As shown, the lowest point 17 tends to rise. In other words, since the position of the electrode 4 is controlled by detecting the average machining gap voltage so that it approaches a predetermined standard, if the convection of the machining fluid in the machining gap is poor and the insulation degree of the machining fluid decreases, the appearance of the machining gap will change. resistance decreases, and the average machining gap voltage decreases. Therefore, in order to bring the average machining gap voltage close to a predetermined reference value, the machining gap is widened (in other words, the apparent resistance of the machining gap is increased), and the machining gap is controlled (the machining gap voltage is servo controlled) to reach the lowest point. 17 will gradually rise. Furthermore, even if processed powder is locally accumulated on the workpiece 5, the machining gap length is expanded by the amount of accumulated processed powder.
As shown in , the lowest point 17 rises. In other words, electric discharge occurs between the accumulated machining powder and the electrode 4, but as mentioned above, the feed of the electrode 4 is controlled so that the average machining voltage is constant, so the accumulated machining powder As the powder grows, the electrode 4 moves away from the workpiece 5 until it reaches the lowest point 17.
The position of will rise. (This phenomenon is explained, for example, in Japanese Utility Model Publication No. 52-31519.) In abnormal conditions, this rise in the lowest point occurs when the amplitude of electrode vibration is small, such as when machining iron with a copper electrode. This occurs even when the amplitude of electrode vibration is large, such as when processing iron.

The present invention focuses on this point, and by detecting the movement of the electrode, especially the movement of its lowest point, the machining gap, which is the cause of the abnormal state, can be detected before the machining gap shifts to an abnormal state or at an early stage. In order to reduce the amount of machining debris that accumulates, the electrical conditions of the current pulse flowing in the machining gap are shortened, the pulse width is extended, the pause time is extended, the current peak value is reduced, the electrode lifting period is shortened, and the electrode lifting width is increased. In order to improve the shortcomings of the method of electrically detecting the state of the machining gap and to perform optimal machining by operating at least one of the following: increasing the pressure of the machining fluid supplied to the machining gap, It is something to do.

FIG. 4 is an example of an apparatus for carrying out the method of the present invention. The operation of this device is to connect a resistor 2 from a DC power source 1.
Electrical discharge machining is performed by causing the current limited by the above to be turned on and off by the switching transistor group 3 and flowing through the machining gap formed by the electrode 4 and the workpiece 5. On the other hand, the distance of the machining gap is controlled by the servo control device 18 and the servo motor 19 so that the average voltage of the machining gap is constant. In addition to the servo control device 18 and servo motor 19, an electrode forcible lifting motor 20 and an electrode lifting control device 21 are installed in order to discharge machining debris from the machining gap. In addition, in order to eject machining fluid from the center of the electrode 4 and discharge machining debris in the machining gap, a machining fluid supply pump 22 and a machining fluid pressure control valve 23 for controlling the pressure of the machining fluid.
is installed. Here, in order to detect the movement of the electrode 4, a digital pulse scale 24 that generates a number of pulses according to the movement of the electrode 4 is added, and the pulses generated by this digital pulse scale 24 are used to detect the lowest point. The lowest point of the feed is detected by the device 25. The value of the lowest point detected here is evaluated and determined by the evaluation device 26 based on a certain evaluation method, and as a result of the judgment, when the movement of the lowest point is increasing, the state of the machining gap is determined to be abnormal. A signal is generated, and the pulse generator 8, electrode lifting control device 21, and machining fluid pressure control valve 23 are activated by the signal.
By controlling one or a combination of two or more of the following, the amount of machining debris generated during machining is suppressed or the amount of machining debris is discharged in order to reduce the amount of machining debris that remains in the machining gap. It operates in such a way as to increase. Specifically, this operation includes reducing the pulse width of the current pulse, increasing the pause time, and decreasing the current peak value for the pulse generator 8, and shortening the electrode lifting period and increasing the electrode lifting width for the electrode lifting control device 21. For the machining fluid control valve 23, the machining fluid pressure is also increased.

In the above embodiment, the digital pulse scale 24 is used to detect the movement of the electrode 4.
However, if a pulse motor is used as the servo motor 19, the digital pulse scale 24 is not required, and it is natural that the pulse motor driving pulse can be used instead, and the other electrodes 4
Of course, it is also possible to use a device that converts the movement of the object into an electrical signal.

Here, FIG. 5 shows details of essential parts of an example of the lowest point detection device 25 and the evaluation device 26 that implement the method of the present invention.
As for the output pulses, the pulse (up pulse) that occurs in the upward direction is connected to the addition terminal 2511 of the reversible counter 251, the downward pulse that occurs in the downward direction is connected to the subtraction terminal 2512, and the upward pulse is connected to the flip-flop. (F/F)252
The lower pulse is applied to the set terminal of AND gate 25.
Connected to 3. The output of the AND gate 253 is input to the reset terminal of the flip-flop 252 via the delay element 254, and is also input to the latch 255.
It is configured to hold the output contents of 1.
Further, the borrow output of the reversible counter 255 is connected to a reset terminal, so that when the content of the reversible counter 255 becomes negative, it always becomes 0. The output of the latch 255 is connected to a D/A converter 256, converted to an analog voltage, and input to a comparator 261 in the evaluation device 26.

FIG. 6 shows a timing chart of signals between each element constituting the lowest point inspection device 25 and evaluation device 26 shown in FIG. The operation of 26 will be explained below.

When the electrode moves as shown in FIG. 6a, for example, the digital pulse scale 24 outputs up pulses and down pulses as shown in FIGS. 6b and 6c. These pulses are processed by a reversible counter 25
1, and the counted contents are as shown in FIG. 6b. Further, the lowering pulse signal sets the flip-flop 252 to the set state, and the Q output of the flip-flop 252 is set to the sixth
AND is set to 1 as shown in Figure d.
Open gate 253. Next, when the above-mentioned rising pulse signal is output, it is input to the latch 255 via the AND gate 253 at the timing shown in FIG. 6e, and the contents of the reversible counter 251 at that time (FIG. 6b) are held. works. FIG. 6d shows the contents of latch 255 when held by latch 255, which is input to comparator 261 as an analog voltage signal via D/A converter 256. The output content of the latch 255 at this time is the content immediately before the first rising pulse signal after the falling pulse signal [FIG. 6c] which is the output of the digital pulse scale 24, in other words, This shows the state of the lowest point. Above AND gate 253
The rising pulse signal passed through the delay element 254 returns the flip-flop 252 to the reset state after a predetermined delay time. Here D/
The output of the A converter 256 is compared with a predetermined reference voltage by a comparator 261, and when it is higher than the reference voltage, it is output as an abnormal signal.

As described in detail above, according to the method of the present invention, the lowest point that changes from when the electrode descends to when it rises is detected, and the electrical conditions of the current pulse supplied to the machining gap, the conditions for raising the electrode, the pressure of the machining fluid, etc. Since these are controlled singly or in combination, highly efficient and optimal electrical discharge machining can be performed, and the implementation effect is extremely large.

[Brief explanation of the drawing]

Fig. 1 is a diagram for explaining the conventional method, Fig. 2 is an explanatory diagram of the machining gap voltage in the case of an abnormal state in electric discharge machining, Fig. 3 is an explanatory diagram of the electrode position in various discharge conditions, and Fig. 4 is a diagram for explaining the present invention. A diagram for explaining the method of the invention, FIG. 5 is a configuration diagram of a lowest point detection device and an evaluation device showing an example of implementing the method of the invention, and FIG. 6 shows a diagram of each signal of the device of FIG. It is a timing chart diagram. In addition, the same reference numerals in the figures indicate the same or equivalent parts, and in the figure, 4 is the electrode, 5 is the workpiece, and 8
is a pulse generator, 21 is an electrode pulling device, 23
24 is a processing fluid pressure control valve, 24 is a digital pulse scale, 25 is a lowest point detection device, and 26 is an evaluation device.

Claims (1)

[Claims] 1 Current pulses are intermittently supplied to the machining gap formed between the electrode and the workpiece, and the size of the machining gap is adjusted so that the average machining voltage of the machining gap approaches a predetermined standard. In the method of processing the workpiece by controlling
An electrical discharge machining control method characterized by simultaneously performing one or more of the operations (f). (a) Narrow the pulse width of the current pulse. (b) Increase the rest time of the current pulse. (c) Lower the current peak value of the current pulse. (d) Shorten the period of electrode lifting. (e) Increase the amount of electrode lifting. (f) Increase the pressure of the machining fluid.