WO1996025265A1

WO1996025265A1 - Plasma cutting method

Info

Publication number: WO1996025265A1
Application number: PCT/JP1996/000304
Authority: WO
Inventors: Katsuo Saio; Yoshihiro Yamaguchi
Original assignee: Komatsu Ltd.; Komatsu Industries Corporation
Priority date: 1995-02-13
Filing date: 1996-02-13
Publication date: 1996-08-22

Abstract

A plasma cutting method using a plasma cutting apparatus that comprises a nozzle having an orifice for thinning a plasma arc and secondary gas jet means for supplying a secondary gas in such a manner as to encompass the end of the nozzle. While a nonoxidizing gas is used as starting plasma gas, a nonoxidizing gas is supplied as a secondary starting gas so as to establish a nonoxidizing gas atmosphere in the proximity of the nozzle.

Description

Description Plasma cutting method Technical field

The present invention relates to a plasma cutting method using a plasma cutting machine, and more particularly, to a plasma cutting method capable of preventing oxidation damage of an orifice portion of a nozzle at a cutting start. ^

As shown in FIG. 1, a plasma torch used in a conventional plasma cutting machine has an electrode 1 at a central portion and a cooling chamber 8 formed therein. Further, a plasma gas passage 2 is formed outside the electrode 1, and a nozzle 3 surrounding the electrode 1 via the plasma gas passage 2 is arranged. A cooling chamber 9 and a secondary gas passage 4 are formed outside the tip of the nozzle 3, and a shield cap 5 is disposed so as to connect the cooling chamber 9 and the secondary gas passage 4. I have.

Cutting with a plasma torch having such a configuration is performed by a plasma arc which is a main arc between the electrode 1 and the workpiece 6 while flowing the plasma gas 20 through the plasma gas passage 2. 7 and the plasma arc 7 is narrowed down by the orifice 3 a of the nozzle 3, and is sprayed at a high temperature and a high speed onto the material 6 to be cut, thereby causing the material 6 to be cut. Cutting is performed by melting and removing a part.

At this time, cooling water is circulated through cooling chambers 8 and 9 provided inside the electrode 1 and outside the nozzle 3, respectively, so that the electrode 1 and the nozzle 3 are cooled. I have. At this time, the secondary gas 21 is injected from the secondary gas passage 4 provided inside the shielded cap 5, and the plasma arc 7 is surrounded by the secondary gas 21.

The procedure for generating the plasma arc 7 is as follows. First, a high-frequency voltage is applied between the electrode 1 and the nozzle 3, and a spark arc discharges to generate a pilot arc. Then, along with the flow of the plasma gas 20, the discharge point on the electrode 1 side of this pilot arc shifts to the center of the tip of the electrode 1 and the discharge point on the nozzle 3 side Passes through the orifice 3 a of the nozzle 3, moves to the periphery of the outlet, and finally reaches the workpiece 6 to form a plasma arc 7.

At the same time, the power supply between electrode 1 and nozzle 3 is stopped. At this time, the plasma arc 7 is narrowed down by the orifice 3a of the nozzle 3, and is in a state of high-temperature and high-speed jet jet. As a result, a narrow cutting groove is formed in the workpiece 6, and the workpiece 6 is cut ii. ₀

At this time, both the electrode 1 and the nozzle 3 are exposed to a high temperature by the plasma arc 7, and these are cooled by the cooling water or the air as described above. In addition, a high melting point material is used for the electrode 1, which is heated to several thousand degrees by thermionic emission to reduce its consumption. As the material, hafnium is used when a plasma gas containing oxygen is used, and tungsten is used when the plasma gas 20 is a non-oxidizing gas containing no oxygen. Use

Further, in the conventional plasma cutting, a brush is required depending on the material of the material 6 to be cut. The use of different types of Zuma Gas 20 is being carried out.Oxygen is used as the plasma gas 20 when cutting mild steel, and oxygen is used as the plasma gas 20 when cutting stainless steel or aluminum. Non-oxidizing gas containing no is used. As the non-oxidizing gas, use a single component gas such as nitrogen, argon, or hydrogen, or a mixed gas thereof.

By the way, as described above, in the plasma cutting, a high-temperature and high-speed plasma arc 7 is blown out from the nozzle 3, thereby locally melting the material 6 to be cut and blowing away the molten metal in that portion. The cutting groove is formed, and the cutting of the workpiece 6 proceeds.

Therefore, the cutting quality in the plasma cutting largely depends on the shape of the nozzle 3 for narrowly squeezing and ejecting the plasma arc 7, and the nozzle 3 wears out and changes its shape. When the diameter of the screw 3a becomes large, the cutting quality deteriorates.

In particular, the outlet of the orifice 3a of the nozzle 3 greatly affects the direction and spread of the plasma arc 7 ejected therefrom. The cutting surface of the cutting material 6 is inclined, the molten metal cannot be completely blown off, and molten metal scum called dross remains in the cutting groove, giving a large bad shadow to the cutting quality.

Further, as described above, in the conventional plasma cutting machine, a pilot arc is generated between the electrode 1 and the nozzle 3 at the time of arc start, and this pilot arc is used as a pilot flame, When the electrical continuity between 1 and workpiece 6 is ensured, plasma arc 7 is formed between electrode 1 and workpiece 6, which is the main arc, and is replaced with plasma arc 7. Instead, the power supply to nozzle 3 is stopped, and the pilot arc is stopped. Thereafter, the cutting proceeds by this main arc. Therefore, in a plasma cutting machine, when performing a cutting operation by generating a main arc, a pilot arc is generated every time the arc is started.

Since this pilot arc is generated between the electrode 1 and the nozzle 3 as shown in FIG. 2, the point (arcing point) P at which this pilot arc 17 is generated Is exposed to high temperature arcs. In addition, an entrainment flow 18 of air is generated near the tip of the nozzle 3 at this time, and the air flows into the orifice 3 a of the nozzle 3. Therefore, even if a non-oxidizing gas is used as the plasma gas 20, damage 19 due to oxidation occurs at the orifice 3a of the nozzle 3. For this reason, it is inevitable that the wear of the nozzle 3 progresses every time the cutting is performed, due to the pilot arc 17 generated at the time of the arc start.

In the pilot arc 17, a high-frequency high voltage is applied between the electrode 1 and the nozzle 3 at the beginning of the arc start, and a spark discharge is generated. At this time, the pilot arc 17 occurs at the shortest distance between the electrode 1 and the nozzle 3. Then, riding on the flow of plasma gas 20, the firing point on the electrode 1 side is flowed to the center of the tip of the electrode 1, and the firing point P on the nozzle side moves the orifice 3 a of the nozzle 3. After passing through, it flows around the exit of the nozzle wall 3a and stops around the exit until a main arc is generated.

Therefore, as shown in Fig. 2, the consumption of the nozzle 3 at the time of the generation of the pilot arc is intensively progressed at the outlet of the orifice 3a. And

As described above, in the conventional plasma cutting machine, each time cutting is performed, the pilot arc at the time of the arc start causes the nozzle 3 orifice 3a, which has a particularly large effect on the cutting quality. As the wear at the outlet of the pipe increases, it is inevitable that the cutting quality will deteriorate. Therefore, the nozzle 3 had to be changed frequently to maintain good cutting quality.

In cutting mild steel, oxygen or a gas containing oxygen is generally used as the plasma gas 20.In this case, a non-oxidizing gas is used as the plasma gas 20. As compared with the conventional method, the consumption of the nozzle 3 by the pilot arc becomes more severe, and the nozzle 3 needs to be replaced only by cutting work for several hours or tens of hours, thereby improving the durability of the nozzle 3. It was a big challenge.

In other words, such frequent nozzle replacement not only raises the running cost due to the cost of the nozzle 3 to be replaced, but also reduces the cutting time due to the loss time required for nozzle replacement, thereby reducing productivity. It will cause it. In addition, there is a need for personnel to constantly monitor the deterioration of the cutting quality due to the deterioration of the nozzle 3, determine the time to replace the nozzle 3, and carry out the replacement, resulting in severe consumption of the nozzle 3. Has been a major obstacle to unmanned plasma cutting machines. The present invention has been made in view of the above circumstances, and can significantly improve the durability of a nozzle, maintain high-quality cutting quality for a long time, reduce running costs, and improve productivity. The purpose is to provide a realized plasma cutting method. Disclosure of the invention

To achieve the above object, according to the present invention,

Plasma cutting using a plasma cutting device having a nozzle having an orifice for narrowing down a plasma arc and a secondary gas blowing means for supplying a secondary gas so as to surround a tip of the nozzle. In the method,

A non-oxidizing gas is flowed as a plasma gas for the arc start, and a non-oxidizing gas is flowed as a secondary gas for the arc start, so that the vicinity of the nozzle outlet is a non-oxidizing gas atmosphere. A plasma cutting method characterized by this is provided.

According to the above configuration,

An oxygen-free, non-oxidizing gas is used as the plasma gas for the arc start for plasma cutting, and the arc start is used to surround the nozzle orifice outside the nozzle. The secondary gas is blown out to prevent the air from being drawn into the orifice, and the secondary gas also flows a non-oxidizing gas that does not contain oxygen, like the plasma gas. Since oxygen is not present in the orifice, the consumption of the nozzle orifice can be greatly reduced. In the above configuration,

At substantially the same time as the transition from the pilot arc to the main arc, the plasma gas may be switched from a non-oxidizing gas to oxygen or a gas containing oxygen.

In the above case, it is desirable that the plasma gas be switched when a pilot arc or a main arc occurs.

In the above configuration, Substantially simultaneously with the transition from the pilot arc to the main arc, the secondary gas may be switched from the non-oxidizing gas to oxygen or a gas containing oxygen.

In the above case, it is desirable that the switching between the plasma gas and the secondary gas be performed when a pilot arc or a main arc occurs.

Further, in the above configuration,

The non-oxidizing plasma gas and the secondary gas flowing at the time of arc start are nitrogen, and the plasma gas flowing at the time of cutting substantially simultaneously after the transition from the pilot arc to the main arc is oxygen and the secondary gas is air. Alternatively, a mixed gas of oxygen and nitrogen may be used.

In the above configuration,

The plasma gas flowing at the time of cutting substantially at the same time as the transition from the pilot arc to the main arc may be a non-oxidizing gas. In the above configuration,

The non-oxidizing gas may be used as the secondary gas flowing substantially simultaneously after the transition from the pilot arc to the main arc.

Further, in the above configuration,

It is desirable that the plasma gas and the secondary gas be nitrogen. BRIEF DESCRIPTION OF THE FIGURES

The invention will be better understood from the following detailed description and the accompanying drawings illustrating an embodiment of the invention. The embodiments shown in the accompanying drawings are not intended to specify the invention, but merely to facilitate explanation and understanding. In the figure,

FIG. 1 is a sectional view showing an example of a plasma torch used in a conventional plasma cutting method.

FIG. 2 is a cross-sectional view showing a state in which a nozzle is consumed by a pilot arc at the time of an arc start in a conventional plasma cutting method. FIG. 3 is a sectional view showing an example of a plasma torch used in the plasma cutting method according to the present invention.

FIG. 4 is a sectional view showing another example of the plasma torch used in the plasma cutting method according to the present invention.

FIG. 5 is a sectional view showing still another example of the plasma torch used in the plasma cutting method according to the present invention.

FIG. 6 is a gas supply circuit diagram when only plasma gas is switched in the method of the present invention.

FIG. 7 is a gas supply circuit diagram when switching between the plasma gas and the secondary gas in the method of the present invention.

FIG. 8 is a timing chart showing an example in which only the plasma gas is switched in the method of the present invention.

FIG. 9 is a timing chart of another example in which only the plasma gas is switched in the method of the present invention.

FIG. 10 is a timing diagram showing an example of the case where the plasma gas and the secondary gas are switched in the method of the present invention.

FIG. 11 is a timing chart of another example in the case of switching between the plasma gas and the secondary gas in the method of the present invention. Preferred state for carrying out the invention! _ Hereinafter, a plasma cutting method according to a preferred embodiment of the present invention will be described with reference to the accompanying drawings.

An embodiment of the plasma cutting method according to the present invention will be described below. The method of the present invention is performed using a plasma torch having a general configuration shown in FIG.

According to the method of the present invention, in order to generate a pilot arc at the start of plasma cutting, a non-oxidizing gas containing no oxygen is supplied as the plasma gas 30 and the outside of the nozzle 3 is also discharged. The secondary gas 31 is released so as to surround the orifice 3a to prevent the air from being drawn in, and the secondary gas 31 also contains the same oxygen as the plasma gas 30. Pour no non-oxidizing gas. Therefore, by setting the state in which oxygen does not exist near the orifice 3a of the nozzle 3, the consumption of the orifice 3a of the nozzle 3 can be greatly reduced.

Experiments conducted by the inventors and the results thereof are shown below to prove the above effects.

The experiment used a plasma torch with secondary gas supply means to supply secondary gas so as to surround the tip of the nozzle, and repeatedly ignited the pilot arc to reduce the nozzle The consumption of the orifice is advanced, and the weight of the copper nozzle with an orifice diameter of 2.8 mm before and after the experiment is measured, and the reduced weight is used as the orifice of the nozzle. And the amount of wear on the wheels.

The types of plasma gas and secondary gas were used in the following combinations.

(1) Plasma gas: oxygen Secondary gas: air

(2) Plasma gas: nitrogen Secondary gas: air (3) Plasma gas: nitrogen Secondary gas: nitrogen

The operating conditions were a plasma gas pressure of 2.0 kg / cm ² , a secondary gas pressure of 3.5 kg / cm ² , a current value of 50 A, an arc firing frequency of 50 times, and an arc firing time of 3 seconds.

Table 1 below shows the measured values of nozzle orifice wear obtained as experimental results.

table 1

From the results in Table 1 above, it can be seen that if oxygen is contained in the plasma gas, the orifice of the nozzle due to the pilot arc is extremely intense. In addition, when the plasma gas is changed to a gas containing no oxygen (nitrogen), the consumption of the nozzle orifice is considerably reduced, but progresses to a degree that affects the cutting quality. Furthermore, if not only the plasma gas but also the secondary gas is a gas (nitrogen) that does not contain oxygen, the consumption of the nozzle orifice hardly progresses.

From the above experimental results, it was found that oxygen was greatly involved in the consumption of the nozzle orifice.

That is, if oxygen is present near the nozzle's orifice, the At the ignition point of the nozzle of the torque, not only the melting due to the high temperature state, but also the oxidation due to the high temperature progresses, and the consumption of the nozzle orifice by this oxidation causes a large proportion. It turned out that it was occupied.

In addition, the oxygen that causes oxidation of the nozzle orifice naturally becomes a supply source if oxygen is contained in the plasma gas at the time of the pilot arc, but not only that. If the nozzle orifice is exposed to the atmosphere (air), the plasma arc flow blown from the nozzle orifice at a high speed causes the atmosphere to be changed to the nozzle orifice. It was found that oxygen drawn into the atmosphere along with the oxygen also caused the oxidation of the nozzle orifice, which in turn accelerated the depletion of the nozzle orifice.

Accordingly, as in the method of the present invention, a non-oxidizing gas containing no oxygen is supplied as a plasma gas for the arc start of the plasma cutting, and the nozzle orifice is provided outside the nozzle. A secondary gas for the arc start is blown out to surround the gas to prevent air from being drawn into the orifice, and the secondary gas contains oxygen as well as the plasma gas By flowing no non-oxidizing gas and leaving oxygen free in the nozzle orifice, the consumption of the nozzle orifice can be significantly reduced. .

In the above method, the secondary gas 31 has a function of narrowing down the plasma gas 7 that contributes to the cutting, that is, the plasma arc 7, and its accuracy has a decisive influence on the cutting quality. It functions to shut off the exit of nozzle 3 from the atmosphere.

In this case, the shape of the tip end of the shielded cap 5 constituting the secondary gas passage 4 becomes narrower toward the tip of the torch as shown in FIG. The shape of the nozzle allows the tip of the nozzle to be shielded more efficiently with a small amount of the secondary gas 31.In this case, the plasma ejected from the nozzle 3 by the secondary gas Since it is necessary to prevent the arc 7 from being disturbed, the diameter of the opening 5a of the shielded cap 5 constituting the secondary gas passage 4 is the orifice 3a of the nozzle 3. It must be larger than the diameter of

As another example of the means for ejecting the secondary gas 31, as shown in FIG. 4, a shielded cap 5 having a cylindrical shape, or as shown in FIG. A secondary gas blowout nozzle 16 is arranged on the side of the nozzle 3 outlet, and a secondary gas 31 is blown from the side to the outlet of the nozzle 3 to shut off the outlet from the atmosphere. It is also possible to do so.

Next, the supply circuit of the plasma gas 30 and the secondary gas 31 for carrying out the method of the present invention is as shown in FIG. 6 or FIG.

In FIGS. 6 and 7, 10 is a non-oxidizing gas supply circuit, and 11 is an oxidizing gas supply circuit.

FIG. 6 shows a case where only the plasma gas 30 is switched. Before the occurrence of the plasma arc 7, in order to completely replace the gas in the supply circuit, open the plasma gas on / off valve 12 for the arc start at a predetermined time before the generation of the pilot arc, and A non-oxidizing gas is passed through the plasma gas passage 2 as the plasma gas 30, and the secondary gas on-off valve 13 is opened, and the non-oxidizing gas is also passed through the secondary gas passage 4 as the secondary gas 31. Gas. As a result, there is no oxygen around the outlet of the nozzle 3. In this state, a pilot arc is generated and the arc is started. After the completion of the pilot arc, the Closing the plasma gas on-off valve for starter 12 and opening the plasma gas on-off valve 14 for cutting at the same time, switching the plasma gas 30 from non-oxidizing gas to oxygen or a gas containing oxygen. Sink and perform cutting work.

The switching timing at this time is as shown in FIG. 8 or FIG. In FIG. 8, switching between the valves 12 and 14 is performed when a pilot arc occurs. In FIG. 9, switching between the valves 12 and 14 is performed when a main arc occurs.

When switching between both the plasma gas 30 and the secondary gas 31, as shown in Fig. 7, the arc gas must be completely replaced before the plasma arc is generated. A predetermined time before the start, the plasma gas on / off valve 12 for the arc start is opened, a non-oxidizing gas as the plasma gas 30 flows through the plasma gas passage 2, and the secondary gas on / off valve for the arc start is opened. 13 Open the same non-oxidizing gas as the secondary gas 3 1 in the secondary gas passage 4 as well as the secondary gas passage 4 so that there is no oxygen around the outlet of the nozzle 3. Generates a pilot arc and starts the arc. After the generation of the pilot arc is completed, the plasma gas on-off valve 14 for arc start is closed and the plasma gas on-off valve 14 for cutting is opened at the same time, and the plasma gas 30 is converted from non-oxidizing gas to oxygen or oxygen. Switch to a gas containing oxygen, and close the arc start secondary gas on-off valve 13 and open the cutting secondary gas on-off valve 15 at the same time to deoxidize the secondary gas 31 Switch from the neutral gas to oxygen or a gas containing oxygen, and then perform the cutting operation.

The timing for switching between the two gases 30 and 31 at this time is shown in Fig. 10 Or as shown in Figure 11. In Fig. 10, the valves 12, 13, 13, 14, 15 are switched when a pilot arc occurs, but in Fig. 11, the valves 12, 13, 33, 1 are switched. Switching between 4 and 15 is performed when a main arc occurs.

The timing of the above gas switching is when the generation of the pilot arc or the generation of the main arc is detected and the signal is received.

In addition, the gas switching timing should be set in consideration of the timing of replacement of the non-oxidizing gas for the start and the oxidizing gas for the cutting in the nozzle 3 orifice. Is good. Desirably, if the replacement is completed at the nozzle 3 orifice at the same time as the occurrence of the main mark, there is no effect on the disconnection. However, in practice, the length of the gas piping will also affect the time required for replacement. Therefore, when the gas is switched, the gas piping distance is short, and if the gas that has passed through the on-off valve quickly reaches the orifice of nozzle 3, the main arc is detected. When it is necessary to receive a signal and the gas piping distance is long, if it takes a long time to replace the gas at the orifice, it is better to do so before the main arc occurs. As for the timing of gas switching, in the order of the time required for gas replacement, in the order of the shortest time required for gas replacement, one of the pilot arc generation detection signal, high frequency generation detection signal, and start signal is detected to switch the open / close valve By doing so, it is possible to minimize the influence of gas replacement on cutting.

In the method of the present invention, the gas is switched between when a pilot arc occurs and during cutting. However, it is more desirable that the gas be switched between when the pilot arc occurs and after the cutting is completed. Similarly, non-oxidizing gases If the non-oxidizing gas flows for a predetermined time, the gas pipe will be filled with the non-oxidizing gas, and when the non-oxidizing gas flows to replace the gas in the gas pipe at the next arc start Since cutting can be shortened, cutting starts earlier and cutting work can be made more efficient.

In the method of the present invention, the oxidizing gas is a gas containing oxygen such as oxygen or air or a mixed gas of oxygen and nitrogen, and the non-oxidizing gas is a gas such as nitrogen, argon, helium, and hydrogen. So-called inert gas is used alone or in combination.

When cutting mild steel by plasma cutting, it is common to use oxygen as the plasma gas 30. In this case, nitrogen is used as the plasma gas 30 and the secondary gas 31 during the arc start, and oxygen is used as the plasma gas 30 during the cutting after the completion of the pilot arc, and air or air is used. A gas containing oxygen is used as the secondary gas 31.

Here, the reason why oxygen is used as the plasma gas 30 for cutting is that cutting is promoted by the reaction heat of the oxidation reaction between mild steel and oxygen plasma. In this case, the secondary gas 31 is preferably a gas containing oxygen. This is because the use of a non-oxidizing gas reduces the oxygen purity of the plasma gas 30 and adversely affects cutting. The reason for using nitrogen as a non-oxidizing gas when generating a pilot arc is that the characteristics when converted into plasma are almost the same as oxygen, and arc instability due to switching is unlikely to occur. It is.

In cutting stainless steel pots and aluminum, non-oxidizing gas containing no oxygen is used as plasma gas 30. Non-oxidizing As the gas, nitrogen, argon, hydrogen, or the like is used alone, or a mixed gas thereof is used. Even in this case, as described above, the consumption of the nozzle by the pilot arc is smaller than that of the oxygen plasma, but proceeds. Therefore, even in such a plasma cutting machine using a non-oxidizing gas, the non-oxidizing gas flows as a secondary gas 31 when a pilot arc is generated, so that the nozzle 3 Durability can be improved.

According to the present invention, the following effects can be obtained.

(1) The nozzle 3 orifice can be shielded from the atmosphere during the arc start and its oxidation damage can be prevented.

(2) The non-oxidizing gas flows at the time of arc start, but it switches to the oxidizing gas at the time of cutting, so there is no loss of cutting quality when cutting mild steel.

(3) As a result of preventing oxidation damage, good cutting quality over a long period of time can be maintained. That is, the durability of the nozzle 3 can be improved.

(4) The durability of the nozzle 3 is improved, so that the number of times the nozzle 3 needs to be replaced is reduced, and the labor of the operator is reduced.

(5) The reduction in the number of replacements of the nozzles 3, that is, the extension of the replacement cycle of the nozzles 3, greatly contributes to the realization of unmanned operation of the plasma cutting machine.

(6) Loss time required for replacement of the nozzle 3 is eliminated, and cutting work efficiency is improved.

(7) Since the cost of purchasing the nozzle 3 is reduced, the running cost can be reduced.

It should be noted that the present invention has been described with reference to an exemplary embodiment, but has been disclosed. It will be apparent to those skilled in the art that various modifications, omissions, and additions can be made to the embodiments without departing from the spirit and scope of the present invention. Therefore, the present invention should not be limited to the above embodiments, but should be understood to include the scope defined by the elements recited in the claims and their equivalents.

Claims

The scope of the claims

1. A nozzle having an orifice for narrowing down a plasma arc, and a plasma cutting device having a secondary gas ejecting means for supplying a secondary gas so as to surround the tip of the nozzle. In the plasma cutting method used,

A non-oxidizing gas is allowed to flow as the plasma gas for the arc start, and a non-oxidizing gas is allowed to flow as the secondary gas for the arc start so that the vicinity of the nozzle outlet has a non-oxidizing gas atmosphere. And a plasma cutting method.

2. The method according to claim 1, wherein the plasma gas is switched from the non-oxidizing gas to oxygen or a gas containing oxygen substantially simultaneously with the transition from the pilot arc to the main arc. Plasma cutting method.

3. The plasma cutting method according to claim 2, wherein switching of the plasma gas is performed when a pilot arc is generated.

4. The plasma cutting method according to claim 2, wherein switching of the plasma gas is performed when a main arc is generated.

5. The method according to claim 2, wherein the secondary gas is switched from the non-oxidizing gas to oxygen or a gas containing oxygen at substantially the same time as the transition from the pilot arc to the main arc. Plasma cutting method.

6. The plasma cutting method according to claim 5, wherein switching between the plasma gas and the secondary gas is performed when a pilot arc occurs.

7. The plasma cutting method according to claim 5, wherein switching between the plasma gas and the secondary gas is performed when a main arc occurs.

8. The non-oxidizing plasma gas and secondary gas flowing at the time of arc start are nitrogen, and the plasma gas flowing at the time of cutting substantially simultaneously with the transition from the pilot arc to the main arc is oxygen. 6. The plasma cutting method according to claim 5, wherein the secondary gas is air or a mixed gas of oxygen and nitrogen.

9. The plasma cutting method according to claim 1, wherein the plasma gas flowing at the time of cutting substantially simultaneously with the transition from the pilot arc to the main arc is a non-oxidizing gas.

10. The plasma cutting method according to claim 9, wherein the secondary gas flowing at substantially the same time as the transition from the pilot arc to the main arc is a non-oxidizing gas.

11. The plasma cutting method according to claim 10, wherein the plasma gas and the secondary gas are nitrogen.