JP2003080419A - Aerial discharge working method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve a problem of conventional aerial discharge work that the work is unstable and working speed cannot be increased because a working clearance is narrow and because of re-attachment of working chips to an electrode and a working body, etc., even though working and removing quantity for one discharge is roughly the same as that of a conventional submerged discharge work in rough work more than medium work. SOLUTION: The aerial discharge work is made to continue by supplying a discharge pulse a product of pulse duration and a discharge electric current amplitude value of which is larger to the clearance under a down time condition to be a larger duty factor and maintaining a transition state of sub-continuous arc discharge to increase as the working and removing quantity of the working body immediately before gas to chemically react with the working body in supplied gas and the working body continuously cause chemical reaction suddenly changes and increases.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a discharge in which a machining voltage is repeatedly applied in a pulsed manner while a fluid machining medium is circulated in a discharge gap formed between a machining electrode and a workpiece. The present invention relates to an improvement of an electric discharge machining method for generating an arc and machining a workpiece by using an electric discharge machining method in which a gas containing a gas that chemically reacts with a workpiece material is used as the machining medium.

[0002]

2. Description of the Related Art Such an air-electric discharge machining method has been disclosed by the present inventors, for example, (1) 1998 Journal of Precision Engineering, VOL. 64, No.
12, PP1735-1738, "Air EDM",
(2) Japanese Patent Application No. 8-71,071 (Japanese Patent Application Laid-Open No. 9-239,
(See Japanese Patent No. 622), "Electrical discharge machining method" and the like. Further, according to such an air-electric discharge machining method, the wear of the tool electrode is very small. The processing characteristics can be improved depending on the type of assist gas. There is little damage on the machined surface. Since the processing reaction force is small, it is advantageous for micro processing. Since no processing liquid is used, no processing equipment or processing tank is required, and environmental pollution is small. It has been attracting attention because of its features such as. That is, in the conventional submerged electric discharge machining method, attention has been paid to the fact that the electrode is low or non-consumable and has a high machining speed in the machining region of finish machining where the electrode is largely consumed and the machining speed is slow.

The machining performance such as the machining speed (machining removal rate) in the finishing machining area below the above-mentioned mid-finishing machining in the air-electric discharge machining is considerably dependent on the reaction between the workpiece material and the gas such as oxygen which chemically reacts. It seems that it has contributed, but above 2
According to the data such as machining conditions in the literature, the electric energy supplied per unit area during machining (average machining current density A / cm ² ) is the maximum average machining current in the case of conventional submerged electric discharge machining. It is considered that the large contribution to the density (25 to 30 A / cm ² ) contributes greatly.

[0004]

However, in rough machining more than medium machining in air electric discharge machining, the amount of machining removed per electric discharge pulse is almost the same as that in liquid electric discharge machining. Despite this, the discharge gap is small due to the retention and reattachment of machining waste, and the ratio of short circuit or short circuit discharge becomes extremely high, the machining state is not stable, and high voltage machining is required to increase the gap of the electric discharge gap. Although a power supply was used or a means for maintaining control of the gap with a high response was taken, the amount of machining removed depends on the increase in electric energy to the discharge gap and the action due to the chemical reaction with the supply gas of the workpiece material. It was not in a situation where it was successfully used to improve processing performance.

However, the inventors of the present invention depend on the material combination of the material to be processed and the gas that chemically reacts with each other, but under certain conditions, the electric energy is supplied to the discharge gap by the voltage pulse as the discharge pulse. Then, when the normal voltage pulse pause time (τOFF) is increased by decreasing it, the chemical reaction between the workpiece material and the supplied chemical reaction gas is continuous under a certain processing condition, and the discharge is continuous arc. The uncontrolled runaway state of the discharge state and the processing of the quasi-runaway immediately before the runaway state and the quasi-continuous arc discharge state by controlling the supply condition of the electric energy and / or the chemical reaction condition. We have found that there is a transition region where the amount of body material removed by machining increases rapidly, and that by maintaining this transition region, it is possible to achieve ultra-high-speed machining with controllable machining. The present invention is proposed.

[0006]

The above-mentioned objects of the present invention are as follows.
(1) The machining electrode and the work piece are opposed to each other with a minute gap therebetween, and a gas containing a gas that chemically reacts with the work piece material is forced to flow through the gap, and the two are interposed therebetween. With a pause time τOFF, intermittent voltage pulses are applied to repeatedly generate a discharge pulse, and relative machining feed in the facing direction and / or a plane direction perpendicular to the facing direction is applied between them. In the air electric discharge machining method for machining a workpiece under machining conditions of high speed rough machining, a discharge pulse in which the product of the pulse width τON (μs) of the discharge pulse and the discharge current amplitude value Ip (A) is larger , Larger duty factor (τON / τON + τOF
F) and supply it under the OFF time τ OFF condition,
In a quasi-continuous arc discharge state immediately before a continuous arc discharge state in which a chemical reaction between the supply gas and the work material in the discharge gap occurs continuously, the amount of work removal of the work material is This is achieved by an air-electric discharge machining method in which the above-mentioned air-electric discharge machining is continued while maintaining the machining conditions in the transition region where the abrupt increase changes.

The above-mentioned object of the present invention is: (2) The discharge pulse is a discharge pulse having a ratio of the discharge current amplitude value to the pulse width (Ip / τON shock value) of more than 0.5, and the duty pulse This is achieved by the air-electric discharge machining method described in (1) above, in which the factor is supplied under a pause time condition of greater than 0.80.

The above-mentioned object of the present invention is: (3) The transition region of the discharge state is controlled by changing the content ratio of the gas which chemically reacts with the workpiece material in the gas which is circulated in the gap. This is achieved by using the air discharge machining method described in (1) or (2) above.

The above-mentioned object of the present invention is (4) cooling control of the temperature of the workpiece so that the discharge state before the reaction between the supply gas and the workpiece material is continuous is in the transition region. This can be achieved by the air discharge machining method described in (1), (2) or (3) above.

The above-mentioned object of the present invention is: (5) When the gas that chemically reacts with the workpiece is iron, it is oxygen, and when it is titanium, both oxygen and nitrogen, or This can be achieved by the method for air discharge machining according to any one of (1), (2), (3) or (4).

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is an explanatory view of the overall configuration of an embodiment for carrying out the air discharge machining method of the present invention, in which 1 is a cylindrical machining electrode, 2 is an electrode chuck, and 3 is a spindle main shaft. Rotating device for rotating the electrode 1 around a central axis or a desired deviation axis, 4 is a machining head, 5 is a feed screw, 6 is a DC or AC servomotor for machining feed and positioning of the machining head, and 7 is the servomotor 6, a feed position (current position) of the tip of the electrode 1 is detected, and a detection signal is supplied to a CNC controller described later. A rotation speed detecting device such as a finger speed generator or an encoder for detecting and supplying a detection signal to the CNC control device, 9 is a workpiece, 10 is a processing table also serving as a temperature control plate for cooling or the like for mounting the workpiece 9, 1 xy the processing table is placed in
Cross table, 12 and 13 are cross tables 11
The machining feed and positioning servomotors for the xy axes, not shown, of the position detection and rotation speed detection device for machining feed and positioning feed of the machining table 10 in the xy axis directions via the servomotors 6, 14 are the servomotors 6, 12,
Drive devices 13 and 13, 15 is a CNC for electric discharge machining, and a control device, 15A is a computer, 15B is an input device such as a keyboard, 15C is a paper tape, a magnetic tape,
External storage device for electrical discharge machining data or programs such as floppy disk or compact disk, 15
D is a numerical controller.

Further, 16 is a pulse power source for electric discharge machining for supplying an intermittent voltage pulse between the machining electrode 1 and the workpiece 9, 17 is a DC voltage source, 18 is an ON / OFF electronic switch element such as FET, Reference numeral 19 is connected in series with the switch element 18 and the voltage source 17 and is connected to the current amplitude Ip of the discharge pulse.
(A) is a current limiting resistor that is set to be switched together with the number of FETs connected in parallel, and 20 is a signal from the CNC controller 15 that selects the on time τON (μs) and the rest time τOFF (μs) of the electronic switch element 18. Voltage pulse condition setting device for switching setting, 21 is a reverse voltage prevention diode,
Reference numeral 22 is an electric discharge machining state detection device for a machining gap between the electrode 1 and the workpiece 9.

Reference numeral 23 is an air compressor or a compressed or liquefied gas cylinder or a combination thereof, and a compressed gas supply device containing a gas that chemically reacts with the material to be processed, and 24 is a vapor removal device provided as necessary. Air dryer, 25 is a decompression adjusting valve for compressed gas supplied. Further, 23A, 24A, and 25A are another set of compressed gas supply devices provided as necessary, and control and set the concentration of the gas which chemically reacts through the mixing adjusting means 26 as desired. Reference numeral 27 denotes one or more temperature sensors that detect the temperature of the workpiece 9 and supply a detection signal to the CNC control device 15, and 28 denotes a temperature control device that supplies a temperature control medium or the like to the temperature control plate 10. The CNC control device 15 executes a predetermined temperature control according to the detection signal.

Next, the phenomenon of aerial electrical discharge machining, which is the basis of the aerial electrical discharge machining method of the present invention, will be described. Carbon steel is used as the work piece, and the gas that chemically reacts with the work piece material is 1.0% oxygen gas as the working medium gas.
Used at MPa. The carbon steel workpiece has an outer diameter of φ7.
A pipe having a thickness of 0 mm and a wall thickness of 0.5 mm was used, the pipe was rotated at 180 rpm, the machining electrode was swung as a copper block, and the intermittent voltage pulse power supply 16 was positively connected. To increase the energy supplied per unit time to the discharge gap by the voltage pulse power supply 16, the product of the pulse width τON (μs) and the discharge current amplitude Ip (A) (Ip × τON) as the discharge pulse of the voltage pulse. Of a certain value or more, and it is necessary to supply this discharge pulse at a high time density so as to switch the inter-voltage pulse pause time τOFF (μs) to a small setting. Further, in this case, it is desirable that the discharge pulse has a certain ratio of the discharge current amplitude value to the pulse width (Ip / τON shock value). The size, shape, and form of the processing electrode and the work piece in the above-described experiment are set in the opposite manner to the normal one so that the work piece is in a pipe state. This is for avoiding the influence on the processing due to the change of the processing mode such as the state of the gas jetting from the outer circumference of the pipe and the discharge of the processing waste due to the formation of some cavities as the processing progresses.

FIG. 2 shows the width of the discharge pulse τON (μ
s) is 40 μs, the discharge current amplitude Ip of the discharge pulse
(A) shows 24A: □ mark, 32A: ▲ mark, and 40A:
For each discharge pulse set to ●, switch the discharge pulse pause time τOFF (μs) to a smaller value (horizontal axis)
Each volume processing speed (cm ³ / min, vertical axis)
And the state of electric discharge or machining. In normal electrical discharge machining, the voltage pulse width τON (time)
And the width τD of the discharge pulse, from the time of applying the voltage pulse,
Since there is a delay time τW (μs) until the start of discharge, they do not match (τON = τW + τD), but in the present invention,
The amplitude value Ip (A) of the discharge current is the pulse width τ of the discharge pulse.
It is set to a value that is relatively large with respect to the value of D (μs), and the discharge pulse pause time τOFF (μs)
However, the delay time τW is usually sufficiently small, or almost τ.
W≈0, therefore, the width τD of the discharge pulse is almost the same as the width τON of the voltage pulse (τD≈τON), so τON
Only one will be described. For example, namely, the duty factor is properly τD / τW + τD + τO
Although it is FF, it is as described by describing as τON / τON + τOFF.

As is apparent from the figure, the impact value (I
When the discharge pulse has a large p / τ ON), the rest time τ
While OFF is still larger (duty factor is smaller), the discharge state of the discharge gap is mainly due to oxygen (O 2) which chemically reacts with iron of the workpiece.
In contrast to 2), the above-mentioned chemical reaction is continuously or continuously generated, and becomes a runaway continuous arc discharge state that is almost uncontrollable.
When the above-mentioned impact value (Ip / τON) is the smallest discharge pulse (marked with □), even if the pause time is less than 3 μs (duty factor about 0.93), At the density of the discharge energy, it is considered that the runaway state does not occur because the temperatures of the working electrode and the work piece have not risen by the time the chemical reaction continues.

The runaway state of the electric discharge gap in the above-mentioned air-electric discharge machining is because the temperature becomes higher than the temperature of the work piece and becomes an excessive oxidation reaction state. Therefore, in order to prevent or control this. , The temperature of the discharge point of the work piece and the vicinity thereof is cooled so as not to reach the above-mentioned high temperature, or the oxygen concentration (content ratio) in the high-speed gas flowing through the discharge gap is appropriately reduced, or, The electrical conditions of the discharge pulse for the electric discharge machining indicated by the ▲ and ● marks, and the impact value (Ip) of the discharge pulse
/ ΤON) and duty that is the supply condition of the discharge pulse
It is considered that the oxidation reaction may be controlled by reducing the factor (τON / τON + τOFF) or both.

FIG. 3 shows the condition of the discharge pulse of the air-electric discharge machining indicated by ▲ in FIG. 2 in the case of machining in the same pure oxygen gas as in FIG. It is shown by comparing each processing state when air of about 21% in volume percentage is used. That is, the conditions of the air-electric discharge machining in this case are that the pulse width τON of the discharge pulse is 40 μs,
Since the discharge current amplitude Ip is 32 A, the energy τON
× Ip = 1280 (A · μs), impact value Ip / τON
= 0.8 (A / µs) is supplied to perform electric discharge machining in the air, the gas is pure oxygen gas (mark ●)
As described above, the supply condition of the discharge pulse for changing the discharge state of the discharge gap to the runaway state is the rest time τOFF = 5 μs.
(The duty factor is about 0.89). However, in contrast, the oxygen gas content is about 2
When 1% air was used (marked with ▲), the machining removal amount started to suddenly change and increase with a pause time of 5 μs (duty factor of about 0.89), which was a runaway state in the case of pure oxygen. Rest time 4 μs (duty factor approx.
In 91), the machining speed increased to about 4 times that before the pause time of 5 μs, and the discharge state was not yet in the runaway state.

That is, in the air electric discharge machining, it depends on the combination of the material to be machined and the kind of the gas containing the gas which chemically reacts with the material to be machined and the gas concentration. , When the supply of electric energy by the discharge pulse to the discharge gap is increased per unit time, the discharge state of the discharge gap changes depending on the progress state of the chemical reaction between the workpiece material and the gas. It was found that in the critical phenomenon in which the state of medium electric discharge machining shifts to the runaway state, there is a transition region in which the machining removal amount sharply increases and changes in the condition region immediately before the shift to the runaway state. Therefore, this transition region is called a quasi-runaway state or a quasi-continuous arc discharge state. In this case, according to the observation of the discharge gap, the arc column is not spread over the entire surface of the workpiece and is formed at each discharge pulse. The arc column is localized, and therefore, it is recognized that the machined surface is formed by the accumulation of the discharge marks generated by the localized arc column. Is different from. However, the amount of machining removal in the above-mentioned quasi-runaway state is so large that it cannot be explained by using the discharge frequency or the rate of increase in the density of electric energy as an index when the discharge pause time is shortened. However, the reason is the activation in which the chemical reaction at the discharge point becomes apparent.

Next, an example of shape processing in the above-mentioned quasi-runaway state will be described. A Cu pipe electrode having an outer diameter of 4 mm and a wall thickness of 1 mm was used to form a slit having a width of 6 mm and a depth of 2 mm over a length of 60 mm on the surface of a carbon steel block to be processed. The rotation speed of the electrode is 180 rpm, the swing width is 20% of the electrode thickness, the gas used is compressed dry air supply pressure of 1 MPa, and the voltage pulse conditions are the voltage pulse of the electric discharge machining indicated by ● in FIG. τON = 40 μs, discharge current amplitude Ip = 40 A (impact value 1.0), and a positive polarity, and voltage pulse supply conditions were rest time τOFF = 5 μs (duty factor about 0.89).

FIG. 4 shows the resulting processing speed (cm ³ / m
in) and the electrode wear rate (E / W%) are shown in comparison with the results of conventional in-oil processing. There is still much room for improvement in the machined surface roughness and shape accuracy, but the electrode consumption rate is about 1
/ 3, and the machining speed is about 15.7 times ultra-high per the same electric energy, though there is a contribution of chemical processing by the reaction with oxygen gas, and it can be used as electrical discharge rough machining. Has sex.

A few experiments were conducted to see if it is possible to use a metal other than iron-based material to achieve high speed using the quasi-runaway state. First of all, although it is copper, since copper has a high thermal conductivity and a lower surface temperature at the discharge location than iron, it was expected that copper would not easily run into a runaway state. Therefore, similar to the first experiment described above, a copper block electrode is used as a cathode, and a copper pipe work piece (φ 9 mm, wall thickness 0.
5 mm) was used for processing. At this time, the processing speed when the down time is gradually shortened is shown in FIG. In addition,
The conditions such as the electric voltage pulse are the same as those shown in FIG. 4 (the same applies hereinafter). In pure oxygen processing of copper, the thermal conductivity was high and the temperature of the work piece did not rise so much, so the runaway state did not occur as expected, but the rest time τOFF = 6
It was found that the processing speed drastically increased from μs. By observation, the emission color of the discharge changes from blue to green. It is considered that the flame reaction of copper appeared due to the oxidation reaction. As a result, there was a quasi-runaway state in the oxygen processing of copper, and it was speculated that the processing speed could be increased.

Next, regarding the processing of titanium and titanium alloy materials, titanium is considered to be a difficult material to be processed regardless of cutting or electric discharge machining. A problem with electric discharge machining is that the machining speed is slow. The reason for this is that titanium has a sticky property, and short-circuiting occurs frequently due to the buildup of discharge marks, leading to a reduction in processing speed. In addition, titanium is an element having a very high affinity with nitrogen, and it is considered that a quasi-runaway state can be obtained by violently causing a chemical reaction with nitrogen instead of oxygen. If quasi-runaway machining with nitrogen is possible, it is less dangerous and advantageous than using oxygen. Therefore, the copper block electrode was used as a cathode and the anode was processed using a titanium pipe work piece (φ 9 mm, wall thickness 0.5 mm). Here, the processing conditions were discharge current Ip = 40 A and discharge pulse width τON = 40 μs. The processing speed when the supply gas is nitrogen and air is shown in FIG. As a result, as expected, an improvement in the processing speed peculiar to the quasi-runaway state can be seen even in the nitrogen processing. Further, the air processing has a much higher processing speed in the quasi-runaway state. By the way, pure oxygen processing was also performed, but even with a very small value of the discharge current value amplitude Ip = 4, the oxidation reaction occurred violently, leading to a runaway state, suggesting the necessity of another control.

As described above, the air electric discharge machining method of the present invention is still in the research and development stage, but it is capable of significantly increasing the machining speed in rough machining of medium machining or more, and is of a conventional type. Practical use of aerial electrical discharge machining, which has characteristics such as almost no electrode consumption even under finishing conditions and high machining speed, in comparison with electrical discharge methods that use petroleum-based machining fluids, over all machining conditions from finishing to rough machining. It is a useful invention that has the potential to make it possible.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of an overall configuration of an apparatus for carrying out an air-electric discharge machining method of the present invention.

FIG. 2 shows different discharge pulse conditions (discharge current amplitude value Ip (A) with respect to voltage pulse width τON (μs)) in air-discharge processing, and a pause time τOFF between the voltage pulses.
(Μs) Explanatory drawing of a machining speed and an electric discharge state when it is made small (when the duty factor is made large).

FIG. 3 is a comparative explanatory diagram of the machining speed and the discharge state when the content ratio of the gas that chemically reacts with the workpiece material in the gas used as the machining medium in FIG. 2 is different.

FIG. 4 is a diagram showing the acceleration and the electrode wear rate when the machining conditions of the air electric discharge machining of the present invention are applied to the shape machining of a workpiece, in comparison with those of the conventional submerged machining.

FIG. 5 is a machining speed characteristic diagram for explaining the possibility of applying the air electric discharge machining method of the present invention to copper other than iron material.

FIG. 6 is a machining speed characteristic diagram for explaining the possibility when applied to another material titanium material, similar to FIG.

[Explanation of symbols]

1: Processing electrode 2: Chuck 3: Rotating device 4: Processing bed 5: Feed screw 6: Servo motor 7: Position detection device 8: Rotational speed detector 9: Workpiece 10: Temperature control plate and processing table 11: Cross table 12, 13: x, y axis servo motors 14: Servo motor drive device 15: CNC control device 16 ： Pulse power supply for electrical discharge machining 17: DC power supply 18: Switch element 19: Current limiting source 20: Voltage pulse condition setting device 21: Diode 22: EDM state detection device 23: Compressed gas supply device 24: Air dryer 25: Pressure reducing adjustment valve 26: Mixing adjusting means 27: Temperature sensor 28: Temperature control device

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Claims (5)

[Claims] 1. A machining electrode and a work piece are opposed to each other with a minute gap therebetween, and a gas containing a gas that chemically reacts with the work piece material is forced to flow through the gap. Intermittent voltage pulses are applied with a pause time τOFF between them to repeatedly generate discharge pulses, and a relative machining feed in the opposing direction and / or a plane direction perpendicular to the opposing direction is applied between them. In the air electric discharge machining method for machining a workpiece under machining conditions of high speed rough machining, the product of the pulse width τON (μs) of the discharge pulse and the discharge current amplitude value Ip (A) is larger Pulse to a larger duty factor (τON / τON + τO
FF), a quasi-continuous arc immediately before a continuous arc discharge state occurs by supplying and generating under a rest time τ OFF condition, and a chemical reaction between the supply gas and the workpiece material continuously occurs in the discharge gap. An aerial electrical discharge machining method, characterized in that the aforesaid electrical discharge machining is continued in a discharge state while maintaining the processing conditions in a transition region where the amount of machining removal of the workpiece material changes rapidly. 2. The discharge pulse has a ratio of discharge current amplitude value to pulse width (Ip / τON impact value) of 0.
The electric discharge machining method according to claim 1, wherein the electric discharge pulse is larger than 5 and is supplied under a pause time condition in which the duty factor is larger than 0.80. 3. The transition region of the discharge state is controlled by changing the content ratio of the gas that chemically reacts with the workpiece material in the gas that is circulated in the gap. Or the electric discharge machining method described in 2. 4. The cooling control of the temperature of the object to be processed is performed so that the discharge state before the reaction between the supply gas and the material to be processed is continuous is in the transition region. 2. The electric discharge machining method according to 2, or 3. 5. The gas that chemically reacts with the workpiece material,
5. The air discharge according to claim 1, wherein the material to be processed is oxygen when it is an iron material, and oxygen and / or nitrogen when it is a titanium material. Processing method.