JPH0592272A - Method and device for protecting plasma torch electrode - Google Patents

Abstract

(57) [Summary] [Purpose] To protect electrodes from melting damage that may occur when cutting a workpiece using a plasma torch. [Configuration] A protection voltage for protecting the electrode from melting damage is set, and a plasma arc voltage is measured and compared with the protection voltage in the process of performing plasma cutting. When the measured voltage becomes equal to the protection voltage, the plasma power supply is shut off or the plasma arc current is gradually reduced to stop the plasma power supply. [Effect] The molten layer formed on the surface of the electrode is maintained in a stable steady state, and the consumption of the electrode due to the scattering of the molten electrode material is prevented to protect the electrode from unexpected melting damage and extend the life of the electrode. It can be done.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrode protection method and apparatus for protecting an electrode of a plasma torch used for plasma cutting from unexpected melting damage.

[0002]

2. Description of the Related Art Conventionally, a work material having conductivity such as a steel plate, a stainless steel plate, and an aluminum plate has been cut by a plasma torch. This processing is
A constant current is applied between the plasma torch and the work material to form a plasma arc, and the plasma arc is cooled by the nozzle part of the plasma torch and finely squeezed and jetted to melt the work material. The melt is removed from the base material and cut. When performing the cutting process, an operating gas such as oxygen gas or air is jetted from the plasma torch. In the above processing, the temperature of the plasma arc whose extreme point is the surface of the electrode attached to the plasma torch is extremely high and reaches about 30,000 ° C. Therefore, the surface of the electrode is melted and evaporated to be consumed, resulting in destruction of the electrode. When the electrode of the plasma torch is broken, the plasma torch is melted and the cutting of the material to be processed cannot be continued, resulting in defective products. Therefore, various techniques have been proposed in order to prevent melting damage of the plasma torch due to the destruction of the electrodes or to prevent product defects. For example, a technique has been proposed in which the destruction of the electrode is detected to take measures such as shutting off the plasma power supply and stopping the operation of the cutting device. In addition, a technique in which an operating time or a cutting length or the number of operations of the electrode is set in advance and used as a reference, and when the actually used electrode reaches the reference, the electrode is exchanged regardless of the consumption of the electrode. Is being done.

[0003]

Each of the above techniques is a technique for detecting the breakage of the electrode or a technique for predicting the breakage of the electrode. Based on the recognition that the electrode of the plasma torch will eventually break down, This is a technology that seeks to solve the problem of how to detect or predict the destruction of an electrode. That is, at present, there is no technology that protects the electrode to prolong the life of the electrode, or even if it exists, there is no technology that can substantially exert the effect.

An object of the present invention is to provide an electrode protection method and an apparatus for protecting the electrode from unexpected melting damage during the cutting process.

[0005]

The present inventors have conducted various experiments in order to elucidate the process leading to the destruction of electrodes. As a result, during the steady cutting of the work material, the plasma arc voltage has a substantially constant value, and the formation and thickness of the molten layer on the electrode surface are also in a steady state. That is, it was found that the melting amount of the electrode material maintained a substantially constant state. The melting state of the electrode at this time will be described with reference to the drawings. 4A is a sectional view of the electrode 2 and the holder 3, and FIG. 4B is a surface view of the electrode 2 and the holder 3 when they 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 to form a molten layer 2a. 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 the equilibrium state, and thereafter, the molten state is substantially the same. In the molten layer 2a in this state, the molten electrode material is thick in the central portion and thinly distributed in the outer peripheral direction. Therefore, the electrode material is uniformly evaporated and oxidized from the entire surface of the molten layer 2a, and the consumption of the electrode 2 progresses. When the molten layer 2a is cooled while maintaining a steady molten state, the surface of the electrode 2 has a mirror surface shape as shown in FIG. That is, the melted electrode material is solidified as it is and maintains the function as the electrode material.

However, when the plasma arc voltage suddenly and steadily cuts to a voltage more than about double the voltage due to some reason, the state of the molten layer on the electrode surface is abnormal. Occurs, and a large amount of electrode material melts and scatters instantaneously. That is, it was found that when the plasma arc voltage suddenly increased more than twice during the cutting process, the consumption of the electrode rapidly progressed (melted). The molten state of the electrode 2 at this time will be described with reference to FIG. When the plasma arc voltage rises sharply, some force acts on the molten layer 2a and the molten electrode material is forcibly moved from the central portion to the peripheral portion as shown in FIG. It has not been clarified as to what causes the force). For this reason, the surface area of the molten layer 2a becomes large, and evaporation and oxidation of the molten electrode material proceed more quickly than in the case where a steady molten state is maintained. Further, the melted electrode material is dispersed as droplets, and the consumption of the electrode 2 progresses at once. In particular, when the plasma arc is stopped in a state where the molten electrode material is thickly distributed in the peripheral portion, the molten electrode material cannot retain the adhesive force to the electrode 2 or the holder 3, and as shown in FIG. It becomes fine droplets and scatters. This means that the electrode 2 is shown in FIG.
It is also clear from the fact that when cooled from the above state, the electrode material solidified in spots is scattered on the surface of the electrode 2 as shown in FIG. From the above experimental results, it is presumed that when an unexpected plasma arc voltage suddenly rises during cutting of the work material, the time until the destruction occurs even if the electrodes have the same specifications.

Considering the cause of the rapid increase in the plasma arc voltage during cutting of the material to be processed, for example, in the method of cutting and cutting the material to be processed, a certain distance after the plasma torch leaves the end surface of the material to be processed. After proceeding, the plasma power supply is shut off, but in such a case, the plasma arc voltage rises sharply. Further, in the method of performing the closed loop cutting on the material to be processed, it is preferable that the desired cutting is completed and the plasma power supply is cut off during the run-up section of the plasma torch. However, when the plasma power supply is shut off after the plasma torch is stopped, the plasma arc voltage rapidly rises at the same time when the plasma torch is stopped. An ordinary plasma cutting device has a height detection sensor and an elevating device that operates according to a signal from this sensor in order to keep the height of the plasma torch with respect to the workpiece constant. However, in many numerically controlled plasma cutting devices, in order to avoid the plasma torch from colliding with other members, the plasma torch is raised when an obstacle is detected. Therefore, the plasma arc voltage may increase as the plasma torch increases.

In the past, the cause of variations in the life of electrodes having the same specifications has been found in the machining tolerance. However, as a result of the above experiment, the machining tolerance alone does not always cause the variation in the life of the electrode, but it is also caused by the sudden increase in the plasma arc voltage due to some cause or the method of stopping the plasma arc at the end of cutting. Thought to be a thing.

Based on the results of the above findings, the inventors of the present invention cut and cut the work piece for the purpose of sharply increasing the plasma arc voltage during cutting, and at the same time, keep the plasma torch constant from the end surface of the work piece. An experiment was performed to compare the life when the plasma power supply was cut off after a distance, and the life when the plasma power supply was forcibly cut off when the plasma arc voltage sharply increased during cutting. As a result, the service life of the electrode when the plasma power supply was forcibly cut off was increased by about 45% compared to the service life when the electrode was disconnected and cut. Furthermore, if the plasma arc voltage rises sharply during cutting, gradually reduce the plasma arc current,
An experiment was conducted to control the so-called slope down, and as a result, the life of the electrode when the plasma arc current was sloped down was increased by about 55% as compared with the life when the electrode was cut and cut.

Incidentally, at present, whether the abnormal state of the molten layer on the electrode surface caused when the plasma arc voltage sharply rises is due to the increase of Joule heat or the vibration due to the abrupt fluctuation of the plasma arc voltage It has not been clarified whether it is the cause.

Therefore, in order to solve the above-mentioned problems, a first plasma torch electrode protection method according to the present invention is an electrode protection method for protecting the electrodes of the plasma torch from unexpected melting damage. The voltage is set, and the plasma power supply is shut off when the plasma arc voltage reaches the protection voltage during the cutting process on the workpiece using the plasma torch.

The second electrode protection method for the plasma torch is an electrode protection method for protecting the electrodes of the plasma torch from unexpected melting damage, in which the protection voltage of the electrodes is set in advance, and the workpiece is processed using the plasma torch. It is characterized in that the plasma arc current is controlled so as to be gradually reduced when the plasma arc voltage reaches the protection voltage during cutting of the material.

Further, the first protection device includes a storage means for storing a preset protection voltage, a measuring means for measuring a plasma arc voltage in the course of performing cutting work on a workpiece, and the measuring means. And a control means for comparing the plasma arc voltage measured by the method with the protection voltage stored in the storage means and for shutting off the plasma power supply when the measured plasma arc voltage becomes equal to the protection voltage. Is done.

The second protection device is a storage means for storing a preset protection voltage, a measuring means for measuring a plasma arc voltage in the course of performing a cutting process on a workpiece, and the measuring means. And a control means for comparing the plasma arc voltage measured by the protective voltage stored in the storage means and controlling the plasma arc current to gradually decrease when the measured plasma arc voltage becomes equal to the protective voltage. It is configured to have.

[0015]

According to the first electrode protection method for the plasma torch (hereinafter referred to as "first protection method"), the electrode attached to the plasma torch can be protected from unexpected melting damage. That is, the voltage higher than the plasma arc voltage required to perform cutting on the work material and before the electrode reaches melting damage is set as the protection voltage, and the plasma arc voltage rises to the protection voltage during cutting. At this time, by cutting off the plasma power supply, the operation of the plasma torch can be stopped before the molten layer on the surface of the electrode becomes abnormal. Therefore, by cooling the molten layer on the electrode surface without exhibiting an abnormal state, it is possible to prevent the molten electrode material from scattering and protect it from melting damage.

According to the second electrode protecting method for the plasma torch (hereinafter referred to as "second protecting method"), the electrode attached to the plasma torch is protected from unexpected melting damage and the electrode is more consumed. Can be reduced.
That is, when the plasma arc voltage rises to the protection voltage during cutting, the plasma arc current is gradually reduced to prevent a steady thermal change in the molten layer on the surface of the electrode and to maintain a steady melting state. Can be maintained and cooled, and by preventing the molten electrode material from scattering and forming a stable cooling surface, the electrode can be protected from melting damage and the life of the electrode can be extended.

According to the first protection device, the first protection method can be reasonably implemented, and according to the second protection device, the second protection method can be reasonably implemented. Can be done.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Each protection method and protection device will be described below with reference to the drawings. FIG. 1 is a schematic explanatory view of a protection device according to the present invention, FIG. 2 is an explanatory view for carrying out a first protection method according to the present invention, and FIG. 3 is a second protection method according to the present invention. It is explanatory drawing for doing. According to the present invention, when the plasma arc voltage rises during the process of cutting the work piece using the plasma torch, the rise of the plasma arc voltage is detected to shut off the plasma power supply, or the plasma arc current is gradually increased. The electrode is protected by stopping the plasma arc before the molten layer formed on the surface of the electrode exhibits an abnormal state by lowering the temperature. By carrying out the method of the present invention, it is possible to protect the electrode from unexpected melting damage, and as a result, it is possible to extend the life of the electrode.

First, the schematic structure of the protection device 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 a tip thereof. 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 cooled by cooling water. The holder 3 (electrode 2) is connected to the-terminal of the plasma power supply device 8 that is controlled by a constant current, and the cap 5 and the workpiece A are connected to the + terminal of the plasma power supply device 8. The power supply control device 9 is a device for controlling the plasma power supply 8 and is input with preset protection voltage data and measures the voltage of the plasma arc 10 formed between the electrode 2 and the workpiece A. However, when the voltage rises and becomes equal to the protection voltage, the plasma power supply 8 is shut off when the first protection method is performed, and the plasma power supply 8 is turned on when the second protection method is performed. In the cutting device configured as described above, which controls to gradually reduce the plasma arc current without interruption,
When performing the cutting process on the work material A, a current is first applied between the electrode 2 and the cap 5 to form a pilot arc, and then the main cutting work is performed between the electrode 2 and the work material A. A plasma arc 9 which is an arc is formed.
Then, the plasma torch 1 can be moved at a predetermined speed to melt the material A to be processed by the heat of the plasma arc 9, and at the same time, the melted material can be removed to perform cutting.

In the first and second protection methods according to the present invention, first, the protection voltage for protecting the electrode 2 is set. The value of this protection voltage is set as low as possible according to the performance of the plasma power source 8, the specifications of the electrode 2 for cutting the workpiece A, and the like. For example, if the work material A is the material S
When cutting with S41, plate thickness 12 mm, cutting speed 3200 mm / min, plasma torch 1 nozzle diameter 2.3 mm and applied current 250 A, the plasma arc voltage is 160 V.
However, considering the variation range of the plasma arc voltage due to the variation of the height of the plasma torch 1 during cutting and the variation range of the plasma arc voltage due to the variation of cutting speed, the variation range of the plasma arc voltage is set to 140V to 180V. It is preferable to keep it. Therefore, in this embodiment, the protection voltage is set to 200 V, and this data is input to the control device 9 and stored. The value of the protection voltage is not more than twice the value because the minimum value of the fluctuation range of the plasma arc voltage is set to 140 V, which is sufficient to protect the electrode 2.

Next, the implementation of the first protection method will be described with reference to FIG. In the figure, the solid line shows the change in the plasma arc voltage when the first protection method is carried out, and the two-dot chain line shows the change in the plasma arc voltage in the conventional method. In the plasma cutting apparatus configured as described above, a momentary no-load voltage is generated at the same time that the plasma arc forming signal is generated at the start of cutting. This voltage is
Since the voltage reaches 380 V, the control device 9 delays the measurement start time of the plasma arc voltage by a predetermined time from the time when the plasma arc formation signal is generated. The controller 9 constantly measures the plasma arc voltage between the electrode 2 and the work material A, and compares the measured voltage with a preset protection voltage. Even if the measured voltage of the plasma arc fluctuates, if this maximum value is below the protection voltage,
The control device 9 continues the disconnection without performing any special operation. At this time, a steady molten layer 2a is formed on the surface of the electrode 2 as shown in FIG. When the control device 9 detects that the measured voltage has risen to the protection voltage, the control device 9 transmits an operation stop signal to the plasma power source 8 to forcibly interrupt the plasma arc current. Therefore, the molten layer 2a in the electrode 2 is cooled while maintaining the steady state of FIG. 4 without shifting to the state shown in FIG. Therefore, it is possible to protect the electrode 2 without melting it.

With the plasma cutting apparatus to which the first protection method is applied, the material A to be processed is SS41, the plate thickness is 12 mm, the cutting speed is 3200 mm / min, and the nozzle diameter of the plasma torch 1 is
2.3mm, the applied current is 250A.
The wear depth of the electrode 2 in the second cutting (arc operation time 1 minute) was measured, and the time until the electrode 2 was broken was measured. As a result of the above experiment, the average value of the first wear depth was 0.038 mm, and the average life was 90.8 minutes. Further, a cutting experiment was conducted under the same cutting conditions as above using a conventional plasma cutting apparatus, and the first wear depth and the time until the electrode 2 was broken were measured, and the average of the first wear depth was measured. The value was 0.736 mm, and although the consumption depth after the second time tended to gradually decrease, the life was shortened due to the influence of the initial large amount of wear, and the average life was 62.3 minutes.
From this, it is apparent that the electrode 2 can be protected by using the first protection device, and the life of the electrode 2 can be extended by about 45%.

Next, the implementation of the second protection method according to the present invention will be described with reference to FIG. In the figure, the solid line shows the change in the plasma arc voltage when the second protection method is performed, and the dotted line shows the change in the plasma arc current with the change in the plasma arc voltage. Further, the two-dot chain line shows the change in the plasma arc voltage in the conventional method. In the second protection method, when the control device 9 detects that the measured voltage has risen to the protection voltage, the control device 9 controls the plasma power supply 8 to gradually reduce the plasma arc current and then stop the operation. To do. With this method, the plasma arc 10 is continuously formed even when the measured voltage becomes equal to the protection voltage. Therefore, the molten electrode material in the molten layer 2a is gradually cooled while maintaining a steady molten state, and the cooled surface of the electrode 2 can be maintained in a good state.

With the plasma cutting apparatus to which the second protection method is applied, the material A to be processed is SS41, the plate thickness is 12 mm, the cutting speed is 3200 mm / min, and the nozzle diameter of the plasma torch 1 is
2.3mm, cutting the applied current 250A condition, measure the wear depth of the electrode 2 in the first cutting (arc operation time 1 minute), and measure the time until the electrode 2 breaks. did. As a result of the above experiment, the average value of the first wear depth is
It was 0.035 mm, and the average life was 96.3 minutes. Therefore,
It is clear that it is possible to protect the electrode 2 and to extend the life of the electrode 2 by about 55% by using the second protection device.

[0025]

As described above in detail, according to the first protection method and the second protection method according to the present invention, the preset protection voltage and the plasma arc voltage measured during the plasma cutting are performed. When the measured voltage is equal to the protection voltage, the plasma power supply is shut off, or the plasma current is gradually decreased to stop the discharge, so that the electrode can be protected from unexpected melting damage. As a result, the life of the electrode can be extended.

The first protection device according to the present invention can reasonably implement the first protection method, and the second protection device reasonably implements the second protection method. Can be implemented.

[Brief description of drawings]

FIG. 1 is a schematic explanatory view of a protection device according to the present invention.

FIG. 2 is an explanatory diagram for carrying out a first protection method according to the present invention.

FIG. 3 is an explanatory diagram for carrying out a second protection method according to the present invention.

FIG. 4 is an explanatory diagram of a case where an electrode is stably and constantly melted.

FIG. 5 is an explanatory diagram when the molten state of the electrode is in an abnormal state.

[Explanation of symbols]

A is a work material, 1 is a plasma torch, 2 is an electrode, 2a is a molten layer, 3 is a holder, 4 is a nozzle, 5 is a cap, 6
Is a cylinder, 7 is a gas passage, 8 is a plasma power source, 9 is a control device, and 10 is a plasma arc.

Claims (4)

[Claims] 1. An electrode protection method for protecting an electrode of a plasma torch from unexpected melting damage, in which a protection voltage of the electrode is set in advance, and a plasma arc is performed during cutting of a workpiece by using the plasma torch. A method of protecting an electrode of a plasma torch, characterized in that the plasma power supply is shut off when the voltage reaches the protection voltage. 2. An electrode protection method for protecting an electrode of a plasma torch from unexpected melting damage, wherein a protection voltage of the electrode is set in advance, and a plasma arc is performed during cutting of a workpiece by using the plasma torch. A method of protecting an electrode of a plasma torch, comprising controlling the plasma arc current so as to gradually decrease when the voltage reaches the protection voltage. 3. A storage means for storing a preset protection voltage, a measuring means for measuring a plasma arc voltage during a process of cutting a workpiece, and a plasma arc measured by the measuring means. And a control means for comparing the voltage with the protection voltage stored in the storage means and for shutting off the plasma power supply when the measured plasma arc voltage becomes equal to the protection voltage. Electrode protection device. 4. A storage means for storing a preset protection voltage, a measuring means for measuring a plasma arc voltage in the course of performing a cutting process on a workpiece, and a plasma arc measured by the measuring means. Control means for comparing the voltage and the protection voltage stored in the storage means and controlling the plasma arc current to gradually decrease when the measured plasma arc voltage becomes equal to the protection voltage. Electrode protection device for plasma torch.