JP3468864B2 - Plasma arc interruption control method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a control method for cutting off a plasma arc formed at the tip of a plasma torch at the end of cutting of a plasma cutting apparatus. 2. Description of the Related Art A conventional plasma arc cutoff control method in a plasma cutting apparatus will be described with reference to FIG. FIG.
(A) shows the time change of the plasma arc current, and FIG.
(B) shows the time change of the voltage applied to the electrode. After the plasma arc is ignited between the electrode provided at the tip of the plasma torch and the workpiece by the constant current power supply, the plasma arc is applied to a predetermined condition corresponding to the cutting processing conditions as shown in FIG. Cutting is performed with the constant current value set to I1. At this time, if the voltage applied to the electrode is in a steady cutting state, the voltage is stabilized while substantially maintaining V1 as shown in FIG. 4B. Next, at the end of cutting, as shown in FIG. 4 (a), at an arbitrary time t1 at which the cutting is completed, the arc current is reduced to zero, the plasma arc is cut off, the operation of the plasma torch is stopped, and cutting is performed. To end. At this time, at the moment when the arc current is reduced to zero, a no-load voltage is generated at the output terminal of the constant current power supply, so that a shocking no-load voltage V4 is applied to the electrode at time t1 as shown in FIG. Is done. Thereafter, the no-load voltage V4 applied to the electrode gradually drops to zero by a predetermined time constant. [0004] For example, if the arc current I1 during the cutting process is set to 260A, the voltage V
1 is about 130V, and the no-load voltage V4 when the plasma arc is interrupted rises to 350V to 450V. That is, when the plasma arc is cut off, the no-load voltage V4, which is about three times the voltage V1 when the cutting is performed, is applied to the electrode in a shocking manner. [0005] In the above-described conventional plasma arc cutoff control method, the plasma arc current is reduced to zero at the same time at the end of cutting to cut off the plasma arc. The no-load voltage V4 is applied to the electrode immediately after the operation. At this time, the electrode material immediately after the cutting process is in a molten state because of the high temperature, and in this state, when the no-load voltage V4 is applied to the electrode, the molten layer on the surface of the electrode exhibits an abnormal state. Further, there is a problem that the melt is scattered and the consumption of the electrode is increased. SUMMARY OF THE INVENTION An object of the present invention is to provide a plasma arc shutoff control method capable of preventing the electrode from being consumed by a no-load voltage applied to the electrode when the plasma arc is interrupted and extending the life of the electrode. [0007] The present inventors have repeated various experiments to elucidate the process leading to the destruction of the electrode. As a result, in the process of performing the steady cutting of the workpiece, the plasma arc voltage becomes a substantially constant value, and the formation state and the thickness of the molten layer on the electrode surface also become the steady state. That is, it was found that the amount of melting of the electrode material was maintained at a substantially constant state. The molten state of the electrodes at this time will be described with reference to the drawings. FIG. 5A is a cross-sectional view of the electrode 2 and the holder 3, and FIG. 5B is a surface view when the electrode 2 and the holder 3 are cooled. Generally, the electrode 2 and the holder 3 are forcibly cooled by cooling water. When a plasma arc 10 is formed between the electrode 2 and a workpiece (not shown), the surface of the electrode 2 is melted by the heat of the arc 10 and a molten layer 2a is formed.
Is formed. The molten state of the electrode 2 becomes a steady state when the plasma arc 10 is stabilized and the heat input and the cooling reach an equilibrium state, and the same molten state is maintained thereafter. In this state, the molten layer 2a has a state in which the molten electrode material is distributed thicker at the center and thinner toward the outer periphery. Accordingly, the electrode material is uniformly evaporated and oxidized from the entire surface of the molten layer 2a, and the electrode 2 is consumed.
When the molten layer 2a is cooled while maintaining a steady molten state, the surface of the electrode 2 has a mirror-like shape as shown in FIG. 5B. That is, the molten electrode material solidifies as it is, and retains the function as the electrode material. However, as described above, the no-load voltage applied to the electrode when the plasma arc is cut off at the end of cutting suddenly rises to about three times the plasma arc voltage at the time of steady cutting. In the case of an electrode in a molten state that has not yet been cooled sufficiently immediately after the cutting process has been performed, an abnormality occurs in the state of the molten layer on the surface of the electrode, and a large amount of electrode material is instantaneously melted and scattered. That is, it was found that when the voltage applied to the electrode in the molten state when the plasma arc was cut off suddenly increased to the no-load voltage, the electrode was rapidly consumed and the life was shortened. The molten state of the electrode 2 at this time will be described with reference to FIG. When the voltage applied to the electrode 2 in the molten state is not sufficiently cooled immediately after cutting when the plasma arc is cut off and the voltage applied to the electrode 2 in the molten state suddenly rises to the no-load voltage, some force acts on the molten layer 2a, as shown in FIG. The molten electrode material is forcibly moved from the central portion to the peripheral portion. Although the cause of this force has not yet been elucidated, it is supposed that the electric field generated between the electrode 2 and the workpiece may fluctuate instantaneously and the molten layer 2a of the electrode material may move. Seem. The effect that occurs here is similar to the fact that the surface of the water moves when you blow on the top of the water just filled with the glass. Further, since the voltage applied to the electrode 2 fluctuates in an impact manner, the molten electrode material scatters as droplets,
The electrode 2 is consumed at a stretch. In particular, when the plasma arc is interrupted in a state where the molten electrode material is thickly distributed in the peripheral portion, the molten electrode material may be removed from the electrode material or the holder 3.
6B, and cannot be maintained, and scatters as fine droplets as shown in FIG. 6B. The action that occurs here is similar to the above-mentioned cup water, in which the water that has just been blown cannot keep the adhesive force to the water in the cup or the cup and becomes water droplets and scatters outside the cup. I have. This is evident from the fact that when the electrode 2 is cooled from the state shown in FIG. 6A, the electrode material solidified in a patchy manner is scattered on the surface of the electrode 2 as shown in FIG. 6C. is there. [0014] The present inventors based on the results of the above findings,
In order to secure the cooling time of the electrode before completely shutting off the plasma arc current, a current lower by a predetermined value than the plasma arc current value during cutting is applied for a predetermined time, and then the plasma arc is cut. An experiment was performed in which the life of the electrode when the current was interrupted was compared with the number of usable times (usable time). As a result, the arc current during the cutting process is reduced.
In the case of 260A, the arc current is 65A after cutting is completed.
Under the condition that the arc current was cut off after the current was passed for 80 msec and the arc current was cut off, the life of the electrode could be about three times that of the case where the arc current was cut off simultaneously with the end of the conventional cutting. found. Therefore, in order to solve the above-mentioned problems, a plasma arc shutoff control method according to the present invention is directed to a plasma arc shutoff method for shutting off an ignited plasma arc. The plasma arc current at the time of performing the cutting is reduced to a constant current value lower than the plasma arc current value at the time of performing the cutting processing and a constant value capable of maintaining the plasma arc, and the current value is maintained for a predetermined time. After flowing the electrode surface to cool it and solidify it in a steady state, the plasma arc current is cut off. According to the method for controlling the interruption of the plasma arc,
The electrodes can be protected from being consumed by no-load voltage when the plasma arc is cut off. That is, when the plasma arc is cut off at the end of cutting, a current that is lower by a predetermined value than the plasma arc current value during the cutting process is passed for a predetermined time, thereby generating a no-load voltage applied to the electrode. Can be delayed for the predetermined time. By maintaining the plasma arc having a low current value during the delay time, the cooling of the electrode is promoted, the temperature of the electrode is reduced, and the molten electrode material can be solidified in a steady state. After the surface of the electrode is solidified, the plasma arc current can be cut off to stop the operation of the plasma torch. At this time, even if a no-load voltage is applied to the electrode, the surface of the electrode is solidified, and the no-load voltage may cause the melted layer on the surface of the electrode to exhibit an abnormal state and be scattered. There is no. Therefore, the use of the plasma arc interruption control method as in the present invention can extend the life of the electrode. Next, an embodiment of a method for controlling interruption of a plasma arc according to the present invention will be described with reference to the drawings. FIG. 1 is a configuration diagram of a plasma cutting device for carrying out the present invention, FIG. 2 is an explanatory diagram for carrying out a plasma arc cutoff control method according to the present invention, and FIG. 3 is a graph showing the relationship between delay time T and electrode life. It is a graph which shows a relationship. According to the present invention, when a workpiece is cut using a plasma torch, and when the plasma arc is cut off and the operation of the plasma torch is stopped, the arc current value when cutting is performed for a certain period of time. A current of a lower value is passed, and by delaying the generation of the no-load voltage applied to the electrode for a predetermined time, the cooling of the electrode is promoted to solidify the surface of the electrode, and then the arc current is cut off. It is configured to cut off the plasma arc. By performing the method of the present invention, it is possible to protect the electrodes from being consumed by no-load voltage, and as a result, the life of the electrodes can be significantly extended. First, a schematic configuration of a plasma cutting apparatus for carrying out a plasma arc cutoff control method of the present invention will be described with reference to FIG. In the figure, a holder 3 having an electrode 2 is detachably attached to a plasma torch 1, and a cap 5 having a nozzle 4 is detachably attached to the tip. The working gas filled in the cylinder 6 is supplied from a passage 7 formed between the holder 3 and the cap 5 and is ejected from the nozzle 4 to the outside. The holder 3 and the cap 5 are each cooled by cooling water. The holder 3 (electrode 2) is connected to a negative electrode of a plasma power source 8 controlled by a constant current, and the cap 5 and the workpiece W are connected to a positive electrode of the plasma power source 8. The power supply control device 9 is a device for controlling the plasma power supply 8 and controls the current of the plasma arc 10 formed between the electrode 2 and the workpiece W and the voltage between the electrode 2 and the workpiece W. The current and the voltage can be controlled while measuring. In addition, the time for delaying the interruption of the plasma arc can be measured, and the delay time can be controlled. In the cutting apparatus configured as described above,
When cutting the workpiece W, first, a voltage is applied between the electrode 2 and the cap 5 to form a pilot arc, and then a main arc for cutting is provided between the electrode 2 and the workpiece W. A plasma arc 10, which is an arc, is formed.
By moving the plasma torch 1 at a predetermined speed, the workpiece W can be melted by the heat of the plasma arc 10 and the cutting can be performed while excluding the molten material. In the plasma arc interruption control method of the present invention, first, a value of a current (hereinafter referred to as "interruption current") lower than an arc current value at the time of cutting by a predetermined value and an energizing time of the interruption current (hereinafter referred to as "interruption current"). "Delay time" is set. The value of the breaking current is set to a current value lower than the arc current value at the time of cutting, which corresponds to various cutting conditions, by a predetermined value. In setting the value of the cutoff current, when the arc current is reduced at time t1 as shown in FIGS. 2A and 2B (detailed description of FIG. 2 will be described later), corresponding to this. The voltage applied to the electrode 2 increases. Therefore, if the cutoff current is reduced at once, the voltage applied to the electrode 2 during the delay time sharply increases, and undesirably increases to near the no-load voltage V3. Therefore, the value of the cutoff current is preferably set so that the voltage value applied to the electrode 2 during the delay time takes a value close to the voltage value applied to the electrode 2 at the time of cutting. It is preferable that the value of the cutoff current is set to a minimum value which satisfies the above conditions and can continuously maintain the plasma arc. With this configuration, the timing at which the no-load voltage V3 is applied to the electrode 2 can be delayed, and the temperature of the electrode 2 due to an arc current can be minimized to promote cooling of the electrode 2. . For example, the workpiece W is made of a material SS41 and a thickness of 12
mm, the cutting speed is 3200 mm / min, the nozzle diameter of the plasma torch 1 is 2.3 mm, and the arc current is 260 A.
It is preferable to set to about. In this embodiment, the value of the interruption current is set to 65 A, and this data is input to the power supply control device 9 in advance and stored. The power supply control device 9 is provided with a timer capable of continuously changing the interruption time T for supplying the interruption current in the range of 0 msec to 600 msec, and the data of the preset interruption time T is stored in the power supply control device. 9 and stored. The interruption time T is set to a time required for the surface of the electrode 2 that has cooled and melted to solidify. Here, if the cut-off time T is short, the electrode 2 is not sufficiently cooled, and before the molten electrode surface solidifies,
Is applied, a molten layer 2a formed on the surface of the electrode 2 scatters in an abnormal state, and the electrode 2 is consumed. If the cut-off time T is longer than necessary,
This is counterproductive because the wear of the tire continuously accumulates and the degree of wear increases. Next, a description will be given of an embodiment of a method for controlling the interruption of a plasma arc with reference to FIG. FIG. 2A shows the time change of the plasma arc current, and FIG. 2B shows the time change of the voltage applied to the electrode. In the plasma cutting apparatus configured as described above, the plasma power source 8 is controlled by the power source control device 9 during the cutting process in the steady state, so that a constant plasma arc current I1 is constantly generated between the electrode 2 and the workpiece W. Power is supplied, and the cutting process is continuously performed. At this time, in this embodiment, the plasma arc current I1 is set to 260 A, and the voltage V1 applied to the electrode 2 during the cutting process is approximately 130V. On the surface of the electrode 2, a stationary molten layer 2a is formed as shown in FIG. Next, when a stop switch (not shown) is operated at an arbitrary time t1, a stop signal is transmitted to the power supply controller 9. Then, the plasma power supply 8 is controlled by the control device 9, and as shown in FIG.
Set to 2 The delay current I2 at this time is approximately 65
A. At this time, the voltage applied to the electrode 2 increases to V2 (approximately 150 V) in response to the decrease in the arc current. This V
The value of 2 is sufficiently smaller than the no-load voltage V3 (350 V to 450 V), and is set so that the electrode 2 is not consumed by the impact voltage as described above. Next, when the arc current I2 is supplied for T time, and at time t2, the power supply controller 9 controls the plasma power supply 8 to reduce the arc current to zero as shown in FIG. To cut off the plasma arc 10. At this time, electrode 2
Is applied with the no-load voltage V3. This voltage V3 reaches 350 V to 450 V as described above. However, the electrode 2 is forcibly cooled by a cooling means (not shown) while the low value arc current I2 is supplied for T time, and the surface of the electrode 2 solidifies in a steady state. Therefore, even if the no-load voltage V3 is applied to the electrode 2 after coagulation, the electrode 2 can be protected without the above-mentioned consumption of the electrode 2 occurring. Then, the no-load voltage V3 gradually drops to 0V by a predetermined time constant, and the plasma arc can be cut off to stop the operation of the plasma torch 1. FIG. 3 shows that a plurality of electrodes manufactured according to the same specifications are used to intermittently repeat a steady cutting state and a stop operation in a cycle in which the arc time is 1 minute and the pause time is 30 seconds. Is plotted as the number of times the electrode can be used, and the relationship between the delay time T and the life of the electrode is plotted by changing the set time of the delay time T in the range of 0 msec to 600 msec. The number of times the electrode can be used corresponds directly to the usable time (unit is minutes). The horizontal axis in FIG. 3 shows the delay time T, and the vertical axis shows the number of usable electrodes. The amount of consumption of the electrode 2 can be measured by measuring the depth of a dent generated at the tip of the electrode 2. When the electrode 2 is broken, the tip of the electrode 2
A depression of 2.0 mm to 2.5 mm is formed. Therefore, by appropriately setting the depth of the depression before reaching the depth of the depression when the electrode 2 is destroyed (for example, 80% to 90% of the depth of the depression at the time of destruction), the life of the electrode 2 is reached. Can be set as a wear limit value for recognizing. As shown in FIG. 3, when the delay time T was set to zero, that is, when the cutting current was being cut as in the conventional plasma arc cutoff control method, the arc current was cut off at once. Electrode 2 can be used 1 time
20 times. Next, when the delay time T is set in the range of 5 msec to 500 msec, the number of usable times of the electrode 2 becomes 200 or more, as shown in FIG. 3, and the life of the electrode can be extended by about 60%. . When the delay time T was set to 5 msec or less, the effect of reducing the consumption of the electrode 2 hardly appeared. When the delay time T is set to 500 msec or more, the wear rate of the electrode 2 is extremely increased.
It has been reduced to less than 00 times, which has the opposite effect. Therefore,
It has been found that it is preferable to set the delay time T in the range of 5 msec to 500 msec. When the delay time T is set in the range of 40 msec to 120 msec, the number of usable times of the electrode 2 becomes 300 times or more as shown in FIG. 3, and the life of the electrode 2 can be increased by about 2.5 times. Was. Therefore, it has been found that setting the delay time T within this range is more preferable. In this embodiment, when the delay time T is set to 80 msec, the number of usable times of the electrode 2 reaches the peak of 360 times, and a very favorable experimental result that the life of the electrode 2 can be approximately tripled is obtained. I was able to do it. Since the plasma arc cutoff control method according to the present invention has the above-described configuration and operation, when cutting off the plasma arc, the cutting process is performed before the arc current is cut off. By supplying an arc current having a value lower than the current arc current by a predetermined value for a predetermined time, cooling of the electrode can be promoted before no-load voltage is applied to the electrode, and the surface of the electrode can be solidified. Therefore, even if the arc current is cut off after the surface of the electrode is solidified and a no-load voltage is applied to the electrode, no wear occurs due to the application of the no-load voltage to the electrode. Therefore, the life of the electrode can be significantly extended, which is economical. In addition, since the frequency of electrode replacement can be reduced, the working efficiency is improved.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a configuration diagram of a plasma cutting device for carrying out the present invention. FIG. 2 is an explanatory diagram for carrying out a plasma arc cutoff control method according to the present invention. FIG. 3 is a graph showing a relationship between a delay time T and an electrode life. FIG. 4 is a diagram illustrating a conventional plasma arc cutoff control method. 5A and 5B are explanatory diagrams when the electrode is stably and constantly melted, and FIG. 5A is a cross-sectional view of the electrode and the holder,
(B) is a surface view when the electrode and the holder are cooled. FIGS. 6A and 6B are explanatory diagrams when the molten state of the electrode is in an abnormal state, and FIG. 6A is a cross-sectional view of the electrode and the holder showing a state in which the molten electrode material is forcibly moved from a central portion to a peripheral portion; FIG. 2B is a cross-sectional view of the electrode and the holder showing how the molten electrode material is scattered as fine droplets,
(C) is a surface view showing a state in which the electrode material solidified in the form of patches is scattered on the surface of the electrode 2 when the electrode and the holder are cooled. [Description of Signs] 1 ... plasma torch, 2 ... electrode, 2a ... molten layer, 3 ... holder, 4 ... nozzle, 5 ... cap, 6 ... cylinder, 7 ...
Passageway, 8: Plasma power supply, 9: Power supply control device, 10: Plasma arc, W: Workpiece

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int. Cl. ⁷ , DB name) B23K 10/00

Claims (1)

(1) In a plasma arc shut-off method for shutting off an ignited plasma arc, the plasma arc current at the time of cutting is reduced when the plasma arc is shut off. When the cutting process is carried out, the current is reduced to a constant current value lower than the plasma arc current value and the plasma arc can be maintained, and the current value is maintained for a predetermined time to cool the electrode surface to a steady state. A plasma arc interruption control method, which comprises interrupting the plasma arc current after solidification.