JPS6126456B2 - - Google Patents

Description

[Detailed description of the invention]

This invention relates to electric discharge machining in which an electrode and a workpiece are opposed to each other via a non-insulating liquid and a spark discharge is generated in the gap between the electrodes. In addition, in a plane perpendicular to this direction, a predetermined electrical discharge machining gap is maintained between the electrode and the workpiece while changing the relative positions thereof, and electrical erosion in non-opposing gaps is actively suppressed. The present invention relates to a method and apparatus for controlling. As is well known in the art, electrical discharge machining methods that use water as a machining fluid have many characteristics. That is, (1) There is no risk of fire. (2) Do not generate toxic or flammable hydrocarbon gases. (3) Since there is no carbon, tar and carbon will not be generated even if an abnormal arc occurs, and arc marks will not grow. etc. On the other hand, the disadvantages are: (1) It has an electrolytic effect. (2) Rust occurs on the workpiece. (3) Accuracy is poor due to poor insulation and wide side clearance (due to the ability to discharge over a considerable distance). etc. By the way, water is used as machining fluid in wire cut electric discharge machining, but this is because the electrode is 0.2 mm
It is a thin wire of approximately φ diameter, and the part facing the workpiece changes one after another, and does not face the workpiece for a long time in close proximity at one point, so it can be used over long distances due to the aforementioned electrolytic action and low withstand voltage. Discharge (several 10 to several
Up to 100μm. ) This is thought to be due to the fact that the gap between the poles does not expand due to this phenomenon. Therefore, even in general electric discharge machining that performs machining that corresponds to the shape of the electrode, as in the wire-cut electric discharge machining described above, as long as there is a sufficient distance between the electrode and the workpiece except for the opposing gap where the discharge occurs,
It can be understood that unnecessary erosion is not caused to the workpiece. Here, let us consider the discharge erosion effect and the electrolytic erosion effect when a machining fluid that is not highly insulating, such as water, is introduced into the gap between the electrodes. Figure 1 shows the relationship between the electrode and the workpiece in electrical discharge die carving when water is used as the machining fluid. Electrode 1 is machining workpiece 2 in the main axis (Z-axis) direction. is illustrated. Gap G ₁ between electrode 1 and workpiece 2 on the bottom surface
can be narrowed to the same extent as in an insulating machining fluid. This is true to the basic principle of electrical machining, where the gap between the poles and the average machining voltage Vg have an almost linear relationship when the gap length is narrow, so it is theoretically possible to control the gap to the point where it is on the verge of shorting. This is due to this. Note that the above-mentioned average machining voltage Vg is obtained by smoothing the machining voltage of the machining power source 3. As the power supply 3 for normal processing,
A high-frequency pulse power source consisting of a DC power source 4, a switching element 5, and a current limiting resistor 6 is used, and machining is performed while randomly selecting four states: no-load state, arc discharge state, rest state, and short-circuit state, depending on the gap between the poles. The average voltage Vg is the reference setting voltage Vr
Comparator 7 compares whether the voltage is higher or lower, and this error voltage ε is amplified by amplifier 8, and Z-axis feed control is performed by Z-axis direction drive motor 9, and average voltage Vg≒reference setting voltage Vr. The gap between the poles is such that In FIG. 1, 10 is a resistor, 11 is a capacitor, 12 is a machining fluid, and 13 is a machining fluid tank. Next, focusing on the side gaps G ₂ , G ₃ , G ₄ , and G ₅ , these gaps are not artificially controlled in any way, so as a function of the facing time with the electrode 1, G ₂ → It increases in the order of G ₃ → G ₄ . The reason for this increase is that the probability that a discharge will occur after the pulse voltage is applied by the machining power source 3 is higher in the narrower the gap and lower in the wider gap, but the number of repetitions of the discharge is 10 to 100,000 times per second. Due to the large number of discharges, over time, even if only a few percent of them are left on the side, a considerable number of discharges will occur. In particular, when water is used as a machining fluid, the probability of electrical discharge is high even over a considerable distance due to its low insulating properties, and the gap G ₂ to _{G 4} increases significantly as a function of time compared to insulating machining fluids. It is clear that By the way, the probability of discharge is inversely proportional to the gap length in a narrow range, k=A/G …………(1)

The inventors have confirmed through experiments that the relationship shown in [Table] exists. Also, the expansion amount ΔG of the gap G during the time Δt is expressed as ΔG=kΔt ......(2), so by substituting equation (1) into equation (2), ΔG=A/GΔt... ......(3), and solving this gives t=1/A∫GdG=1/2AG ² ......(4) From this, finding G as a function of t, G=√2+G1... ......(5) (When the initial value t=0, G=G1) It can be seen that the gap length increases in proportion to the 1/2 power of time. However, in reality, as the gap length increases, A in equation (1) above begins to decrease significantly, so after a certain time or a certain gap, the increase in the gap becomes negligible ( _G4 ≒ _G5 ). However, when water is used as the machining fluid, this gap length is
It reaches 0.1~0.3mm, and the difference with the electrode size is too large for precise die engraving (gap length of about 0.005~0.02mm)
It could not be used for processing. Furthermore, when processing in water, there is a problem that electrolytic action occurs and the anode portion is ionized and eluted. In particular, in order to eliminate electrode wear due to spark discharge, the electrode is made a positive electrode and the energization pulse time is set to 50μsec or more during processing, so the electrode is worn out by electrolytic action more than the wear caused by spark discharge. As expected, the drawback was that the engraving accuracy deteriorated. This invention was made in order to eliminate the above-mentioned drawbacks of the conventional ones.Firstly, regarding the increase in the discharge gap due to the low-insulating machining fluid, it is necessary to solve the problem in the plane perpendicular to the main machining direction of the electrode and the workpiece. By applying a displacement that exceeds the critical discharge gap (gap length at which the gap does not increase further) to the relative positions of the electrode and workpiece, a controlled minute gap is maintained in the opposing side gap, and a controlled minute gap is maintained in the non-opposing side gap. In addition to widening the gap beyond the critical discharge gap where no discharge occurs in the gap,
To prevent electrolysis from occurring, the voltage applied to the gap between the electrodes is alternating current, and the current pulse during machining is positive,
The present invention provides a method for accurately machining a workpiece into a desired shape by setting the polarity to a desired polarity, and an apparatus for implementing the method. The principle of the method of this invention will be explained below with reference to FIG. An AC power source 20 and a DC pulse power source 21 are connected to the electrode 1 and the workpiece 2, and the switching element 22 of the DC pulse power source 21 is in an off state until discharge occurs, and the AC power source 20 is in an off state. An alternating current voltage is applied via a current limiting impedance element 23. Therefore, no electrolytic action occurs. Next, when a discharge occurs, an alternating current flows through the current detection resistor 24, so this signal S is transmitted to the pulse control oscillation circuit 25 of the direct current pulse power supply 21, and the switching element 22 is turned on.
The insulation of the gap between the electrodes has already been applied to the AC power supply 2 at this time.
Since the electrode is broken down due to the discharge current caused by zero, a DC pulse current flows into the gap between the electrodes. The pulse current is determined by the voltage of the DC pulse power supply 21, the resistance value of the current limiting resistor 26, and the arc voltage (usually 20 to 30 V), and the width of the current pulse is controlled and set by the oscillation circuit 25. After the time has elapsed, the switching element 22 is turned off again and only alternating current is applied to the gap between the electrodes. The alternating current is a small current necessary for dielectric breakdown, and is 0.5
~1.5A is enough, and there is no transition to AC arc. In this way, the power source is controlled so that there is no electrolytic action, and the electrode 1 is displaced so as to gradually spread from the center in the X-Y plane perpendicular to the Z-axis. Some part of the surface of the electrode 1 is controlled to maintain a narrow inter-electrode gap at all times, and the gaps other than the opposing electrode gaps are extremely wide, so that no extra discharge occurs. Next, an embodiment of this method will be explained with reference to FIG. 3, which is an embodiment of the apparatus of the present invention. That is, FIG. 3 shows an embodiment of the apparatus for carrying out this invention, in which an electrode 1 and a workpiece 2 are connected.
and are arranged opposite to each other in a machining tank 13 that uses water as a machining fluid 12. This electrode 1 is attached to a Z-axis drive head 30 driven by a Z-axis drive motor 9, and the Z-axis is driven by a combination of a pole gap servo and a position servo in electrical discharge machining. It is done. This method is disclosed in Japanese Patent Publication No. 53-32112, and the Z-axis servo is selected by the discrimination selection circuit 32 for the lower of the output ε ₁ of the position detector 31 and the interpolation servo signal ε ₂ . The above Z-axis drive head 3
0 can be stopped at a predetermined position. At this time, the X-Y drive table 33 on which the workpiece 2 is placed
is driven by the numerical control device 34 while maintaining a predetermined amount of gap between the poles. That is, the above-mentioned displacement trajectory is programmed in advance in a storage medium 35 such as a paper tape, and the X and Y vector distribution circuits 36 and 37 are operated in accordance with this command value. This X, Y
The vector distribution circuits 36 and 37 include a latch circuit 38 that temporarily stores and holds the X and Y command values of the numerical control device 34, and a pulse multiplier circuit 39 (generally
Bynaly rate multiplyer (BRM) and examples of use such as Texas Instruments' SN7497 are well known) and positive/negative direction discrimination gate 4
0, 41, and X- and Y-axis drive amplifiers 42 and 43, by which an X-axis drive motor 44 and a Y-axis drive motor 45 can be moved. By the way, this X-axis and Y-axis drive motor 44,
45 by the above-mentioned displacement amount and control to keep the gap between the poles constant is as follows. That is, the state of the gap between the poles is determined by detecting the average machining voltage Vg generated by the envelope detection circuit 46 of the signals at the output terminals of the machining power supplies 20 and 21 connected to the electrode 1 and the workpiece 2. It is determined whether this average machining voltage Vg is higher or lower than the reference setting voltage Vr, and whether the gap between the electrode 1 and the workpiece 2 in the X-Y plane is wide or narrow, or if a short circuit accident has occurred. You can find out if there are any. Next, the pole-to-pole servo signal ε ₂ is rectified by a diode 47 and a resistor 48, and only when this signal ε ₂ is positive, it is converted to a frequency proportional to the pole-to-pole servo signal ε ₂ via a frequency converter 49. When input as a pulse to the pulse multiplier circuit 39, the output pulse from the pulse multiplier circuit 39 is modulated by the voltage between the poles, and the speed of the X-Y displacement vector is controlled according to the gap between the poles. . Also, X
- Each predetermined amount of the Y displacement vector is determined by a frequency converter 49.
Since the multiplicand is confirmed by the counter 50 that counts the output pulses from the counter 50, there is no possibility of overfeeding or underfeeding. As described above, this device can perform a desired displacement movement while keeping the gap between the poles constant, so that the processing method according to the present invention can be easily and reliably implemented. In addition, in the apparatus explained above, the table on which the workpiece is placed is driven, but there is no problem at all if an X-Y crosshead is provided between the electrodes and this electrode is operated in the same way. Machining efficiency can be improved by controlling the drive of the X-Y drive motor when ₂ is positive, and by controlling the drive in the opposite direction both when it is negative and when it is positive. In addition, the machining conditions can be programmed in advance on a paper tape or the like that is input into the numerical control device so that the machining conditions can be switched in response to the progress of machining, such as rough machining, semi-machining, and finishing machining, and the machining conditions can be automatically switched. In contrast, it is clear that machining efficiency is significantly improved.

[Brief explanation of the drawing]

Figure 1 is a diagram explaining the state of the gap between the electrode and the workpiece in underwater electrical discharge machining, Figure 2 is a diagram explaining the principle of the method according to the present invention, and Figure 3 is a diagram for implementing the method of the present invention. FIG. 1 is a diagram showing an embodiment of the device of the present invention. In the figure, 1 is a power source, 2 is a workpiece, 3 is a machining power source, 9 is a Z-axis drive motor, 12 is a machining fluid,
20 is an AC power supply, 21 is a DC pulse power supply, 31 is a position detector, 33 is an X-Y drive table, 34 is a numerical controller, 36 and 37 are vector distribution circuits,
44 is an X-axis drive motor, and 45 is a Y-axis drive motor. Note that the same reference numerals in the figures indicate the same or equivalent parts.

Claims (1)

[Claims] 1. In electrical machining in which electricity is applied to the gap between the electrode and the workpiece through water or a low-insulating machining fluid, the gap between the electrode and the workpiece is set in the main machining direction. and a direction substantially perpendicular to the main machining direction, maintaining a controlled gap between the poles for machining in the main machining direction, and generating electric discharge in the direction substantially perpendicular to the main machining direction. In the direction, the gap length between the electrode and the workpiece is controlled to be small, and in the direction substantially perpendicular to the main machining direction, the gap length between the electrode and the workpiece is controlled to be large. In addition to controlling
A light current voltage is applied until a discharge occurs between the electrode and the workpiece, and once a discharge occurs between the electrode and the workpiece, a desired unipolar current is applied to process the workpiece. Electrical discharge machining method to be performed. 2 In electrical discharge machining equipment that supplies electricity to the machining gap between the electrode and the workpiece through water or a low-insulating machining fluid, there is a machine in the main machining direction that determines whether the machining process is acceptable or not and maintains the machining gap in an appropriate state. a first drive device that moves the electrode relative to the workpiece so as to A command signal for regulating the amount of relative movement of the electrodes by the second drive device and a signal based on the state of the gap between the electrodes are input, and when the state of the gap between the electrodes is in a proper state, the second drive is performed based on the movement amount regulation signal. An AC power supply that applies an AC voltage to the gap between the electrodes, and a DC pulse that is operated by the AC current from the AC power supply when a discharge occurs and supplies a DC pulse current to the gap between the electrodes. Electrical discharge machining equipment equipped with a power source.