JP2001198679A - Plasma torch and method for setting initial height of plasma torch - Google Patents

Abstract

PROBLEM TO BE SOLVED: To set an initial height in a relatively short time without being influenced by fumes and sputter scattered by plasma arc cutting, also even if coating of electric insulation is applied to the surface of a workpiece and further even in a torch having a shield cap. SOLUTION: The shield cap 7 and an electrically connected signal line 25 are incorporated in the plasma torch 1. The signal line 25 is connected to a detecting circuit 29. Voltage is applied between the shield cap 7 and a workpiece 101 by the detecting circuit 29, the difference of voltage changed when those are mutually brought into contact and are electrically conducted is detected. An annular projection 33 is provided on the surface coming into contact with the workpiece 101 at the tip end of the torch 1. In setting of the initial height, the torch 1 is lowered at a relatively high speed to detect the rough position of the workpiece 101, thereafter the torch 1 is lifted by the prescribed distance to separate the torch 1 from the workpiece 101, and this time the torch 1 is lowered at a relatively low speed to detect the position of the workpiece 101.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a plasma torch for plasma arc cutting or plasma arc welding,
In particular, it relates to a technique for setting the height of a plasma torch from a workpiece.

[0002]

2. Description of the Related Art In plasma arc machining, for example, plasm arc cutting, the height of a tip (nozzle) of a plasma torch (nozzle) from the surface of a work piece (member to be cut or cut) ("stand-off").
) Is important for obtaining good cutting quality. As a control method for maintaining a constant standoff, a control method using an arc voltage that changes as the arc length increases or decreases according to the height of the standoff (AVC (Arc Voltage Control or Automated Control))
atic Voltage Control).

In the process of plasma arc cutting in AVC, if an arc is generated, a stand-off can be detected based on the arc voltage. However, the arc voltage cannot be used before the arc is generated. It is necessary to set the standoff (hereinafter, the initial height). The standoff in the cutting process changes depending on the thickness of the workpiece, and even if the thickness is the same, it changes depending on the inclination or warpage of the workpiece. It must be set each time a new disconnection is started. Setting the initial height is important in setting the standoff in the cutting process.

In setting the initial height, a sensor for detecting the position of the work piece ("initial height sensor") is used.
) Is used. The initial height sensor is generally provided near the torch, and a relative position with respect to the torch is predetermined. In the setting of the initial height using the sensor, the initial height is set based on the position of the workpiece detected when the sensor contacts the workpiece. Specifically, JP-A-59-16668, 4-12
No. 0315 or Japanese Patent Application Laid-Open No. H11-226741.

(A) In JP-A-59-16668, a contact-type sensor which can be moved up and down is mounted on the side of the torch as an initial height sensor, and the sensor is detected by lowering the sensor and bringing it into contact with a workpiece. A method is disclosed in which an initial height is set using a standoff.

(B) In Japanese Utility Model Laid-Open No. 4-120315, a contact-type sensor is mounted on an arm that can rotate not only vertically but also vertically about its movement line, so that it can move from the torch to the workpiece directly below it. To detect the height of the object, and set the initial height using the detected height.
Is disclosed.

(C) In Japanese Patent Application Laid-Open No. H11-226741, a torch and a holder for holding the torch are connected by a spring so as to be relatively movable, and when the torch is brought into contact with a workpiece, the torch and the holder are connected to each other. A method is disclosed in which an initial height is set by detecting relative movement of an object.

(D) In Japanese Patent Application Laid-Open No. H11-226741, as a prior art, a voltage of several volts is applied between a nozzle of a torch and a workpiece, and when the nozzle and the workpiece come into contact with each other, There is also disclosed a method of setting the initial height by using the detection of the flow of current (or the decrease in voltage) between the workpiece and the workpiece.

[0009]

The conventional methods for setting the initial height (a) to (d) have the following problems.

That is, in the above methods (a) and (b) for setting the initial height, it is necessary to provide an initial height sensor near the torch, and even if the sensor is avoided above the torch during cutting, Fumes and spatters scattered during cutting may reach the sensor, which degrades the sensor. Also, since the condition of the workpiece surface is not clean, when the sensor contacts the workpiece,
The sensor becomes dirty and malfunctions easily occur at the next initial height setting. Further, every time the initial height is set, the sensor must be moved up and down, and the operation time is added to the substantial required time as the required time of the cutting step, which is necessary for the actual cutting step. The time is lengthened, which reduces productivity.

In the initial height setting method (c), since the sensor is not in the vicinity of the torch but is in the holder holding the torch, the fume (particle size is several (A fine powder of iron oxide of up to several tens of μm, which penetrates even in a small gap) may deteriorate the durability and reliability of the sensor.

As described above, in any of the initial height setting methods (a) to (c), the provision of the sensor near the plasma torch or inside the holder is performed while the cutting is performed near the torch or inside the holder. Considering that fumes are scattered, it can be said that it is not desirable from the viewpoint of reliability or durability of the initial height sensor.

In the method of setting the initial height in the above (d), as described in JP-A-11-226741 as a problem, the initial height is set in a state where the workpiece surface is electrically insulated by coating. Setting is not possible. In the initial height setting method (d), the nozzle of the plasma torch is used as the initial height sensor. However, when a shield cap for protecting the nozzle is provided on the torch, the shield cap is placed before the nozzle. Contact the workpiece surface. The shield cap is electrically insulated from the nozzle to protect the nozzle, so that the shield cap hindered the flow of current (or reduced voltage) between the nozzle and the workpiece. Cannot be detected. Therefore, when the shield cap is provided on the plasma torch, the method for setting the initial height (d) cannot be used.

Further, in any of the above methods (a) to (d) for setting the initial height, a sensor or a torch (nozzle)
Is lowered to contact the workpiece, but if the descending speed is too fast, between detecting the contact with the workpiece and stopping the descending of the sensor or torch,
An accurate workpiece position cannot be detected because the sensor or torch pushes down the workpiece. Therefore, in order to be able to accurately detect the workpiece position, it is necessary to reduce the descending speed of the sensor or the torch. For this reason, the time required for setting the initial height becomes longer, and the time required for the actual cutting step becomes longer as a whole, so that the productivity is reduced.

Accordingly, an object of the present invention is to make it possible to set the initial height without being affected by fumes and spatter scattered by plasma arc cutting.

Another object of the present invention is to enable the initial height to be set even if the surface of the workpiece is coated with an electrically insulating material.

Still another object of the present invention is to make it possible to set an initial height even with a torch having a shield cap.

Still another object of the present invention is to enable the initial height setting to be performed in a relatively short time.

[0019]

A plasma torch according to a first aspect of the present invention is a plasma torch for cutting or welding a workpiece by ejecting a plasma arc from a nozzle, wherein the nozzle is detachably mounted. , A torch body that is movably arranged relative to the workpiece when incorporated in the plasma processing machine, and a workpiece that is attached to the torch body outside the nozzle, and when the plasma torch is closest to the workpiece And a signal line connected to the cap for transmitting an electric signal for detecting whether or not the cap has contacted the workpiece.

According to the present invention, the electrical conduction between the workpiece and the cap, which is generated when the workpiece comes into contact with the cap of the plasma torch, can be detected, so that the plasma torch has a cap (for example, a shield cap). Even if it does, the initial height of the plasma torch can be set.

In a preferred embodiment, the signal line is built in the torch body.

A cap according to a second aspect of the present invention includes a nozzle mounted on a torch body that is movably disposed relative to a workpiece when incorporated into a plasma processing machine, and a cap mounted outside the nozzle. Used in a plasma torch having a cap attached to a torch body and a signal line connected to the cap and transmitting an electric signal for detecting whether the cap has contacted a workpiece. The cap has a projection for contacting the workpiece when the plasma torch is closest to the workpiece.

[0023] In a preferred embodiment, the projection has a pointed shape toward the tip that contacts the workpiece.

According to a third aspect of the present invention, there is provided a method for setting an initial height of a plasma torch, comprising: lowering a plasma torch toward a workpiece at a relatively high speed; Detecting the electrical conduction between the plasma torch and the workpiece; stopping the lowering of the plasma torch when detecting the electrical continuity; raising the stopped plasma torch from the workpiece; The step of separating, the step of lowering the plasma torch from the separated position toward the workpiece at a relatively low speed, and the step of lowering makes the tip of the plasma torch come into contact with the workpiece, and the plasma torch and the workpiece are electrically connected. As soon as the electrical conduction is detected, stop the plasma torch descent. When, and a step of raising the plasma torch by a predetermined distance on the basis of the stop position.

According to a fourth aspect of the present invention, there is provided a method for setting an initial height of a plasma torch, comprising: lowering a plasma torch at a relatively high speed toward a workpiece; Detecting that the plasma torch and the workpiece are electrically connected; stopping the lowering of the plasma torch when detecting the electrical connection; raising the stopped plasma torch at a relatively low speed; A step of measuring the position of the plasma torch immediately after detecting that the electrical continuity is lost due to the rise, and a step of further raising the plasma torch by a predetermined distance based on the measured position of the plasma torch. Have.

[0026]

FIG. 1 is a perspective view of a plasma processing machine according to one embodiment of the present invention.

The work piece 101 is set on the work stage 103.
The steel bar is placed horizontally. The Y carriage 105 is installed so as to be able to move horizontally in the Y direction with respect to the work stage 103. A guide rail 107 extends from the Y carriage 105 in the X direction, and the X carriage 109 can move horizontally on the guide rail 107 in the X direction. The X carriage 109 has a Z
A Z carriage 111 that moves in the (vertical) direction is mounted, and a plasma torch 1 described later is fixed to the Z carriage 111 so as to face directly below. During processing of the workpiece 101, the plasma torch 1 has a certain standoff (from the upper surface of the workpiece 101 to the tip of the torch 1) with respect to the workpiece 101 by the XYZ moving system including the carriages 105, 109, and 111. While keeping the distance), is horizontally moved in the X and Y directions along a processing line that matches the product shape.

FIG. 2 shows the overall structure of the plasma torch 1, and FIG. 3 is an enlarged view of the tip of the plasma torch 1.

The plasma torch 1 has a substantially cylindrical shape as a whole, forms a base end portion thereof, and has a torch main body 2 fixed to the Z carriage 111 shown in FIG. And a plurality of parts detachably attached to the part. The torch main body 2 includes a water pipe 16, an inner sleeve 17, an annular insulator 97, and an outer sleeve 1 in order from the torch axis to the outside.
5. Nozzle pedestal 95, fixing ring 99, torch outer cylinder 10
0 and so on. The components detachably attached to the torch main body 2 include an electrode 3, an insulating guide 13, a nozzle 5, an insulating ring 19, a shield cap 7, a retainer cap 13, and a rotating ring 11 in order from the torch axis to the outside. is there. The details will be described below.

A water pipe 16 having a circular cross section for introducing cooling water into the electrode 3 is arranged at the center axis position of the torch main body 2. A cylindrical metal inner sleeve 17 is fitted around the outer periphery of the water guide tube 16 in a coaxial positional relationship with the water guide tube 16, and the cylindrical electrode 3 is fitted inside the distal end of the inner sleeve 17. I have. A heat-resistant insert 4 made of haunium or the like is attached to a portion of the electrode 3 where the arc is generated.

An annular insulator 97 for electrically insulating the electrode 3 from the nozzle pedestal 95 is fitted. A cylindrical outer sleeve 15 which is an insulator is fitted on the outer periphery of the base end portion of the inner sleeve 17, and a metal cylindrical fixing ring 99 is fitted on the outer periphery of the base end portion of the outer sleeve 15. Have been. The fixing ring 99 is fixed to a tip of a torch outer cylinder 100 that forms an outer shell of the torch main body 2. Inner sleeve 17 and fixing ring 9
9 is electrically insulated by an outer sleeve 15 of an insulator.

A metal short cylindrical nozzle pedestal 95 is fixed to the distal end of the outer sleeve 15, and a substantially cylindrical nozzle 5 tapered toward the distal end is attached to the distal end of the nozzle pedestal 95. ing. The nozzle 5 covers the outside of the electrode 3 in a coaxial positional relationship with the electrode 3, and narrows the plasma arc to the center axis position of the tip of the nozzle, which is the front of the heat-resistant insert 4, to eject the plasma arc forward. It has a nozzle orifice 5A. A cylindrical insulating guide 13 for electrically insulating the nozzle 7 and the electrode 3 is fitted between the nozzle 7 and the electrode 3.

At the tip of the nozzle 5, a workpiece 101
And a metal-made short cylindrical shield cap 11 for protecting the nozzle 5 from molten metal or the like which blows up from the workpiece 101. An insulating ring 19 is fitted between the nozzle 5 and the shield cap 7 to electrically insulate them. Outside of this structure,
A cylindrical metal retainer cap 9 tapered toward the tip is covered.

A space 18 surrounded by the retainer cap 9 on the outer periphery of the nozzle 5 is a cooling water passage for forming a cooling water jacket for cooling the nozzle 5. In the cooling water passage 18, the cooling water supplied from the water guide pipe 16 into the electrode 3 flows through a flow path (not shown). A drain pipe 20 extends through the outer sleeve 15 to discharge the cooling water flowing through the cooling water passage 18 to the outside of the torch. This drain pipe 20 is connected to the nozzle base 95,
It communicates with the above-mentioned cooling water passage 18 through a hole (not shown) in the nozzle seat 95.

A cylindrical rotating ring 11 made of metal is fitted to the outside of the base end of the retainer cap 9. An annular claw 11B formed to be bent inward at the distal end of the rotating ring 11 is engaged with a flange 9B formed at the base end of the retainer cap 9, whereby the rotating ring 1 is rotated.
1 raises the retainer cap 9. Further, as described above, the cylindrical fixing ring 99 is fitted around the outer periphery of the base end of the outer sleeve 15. A female screw (not shown) formed on the inner circumference of the rotating ring 11 is connected to a male screw (not shown) formed on the outer circumference of the fixing ring 99. By rotating the rotating ring 11 about the torch center axis, the screwing between the rotating ring 11 and the fixed ring 99 can be tightened or loosened. The retainer cap 9 is attracted and fixed to the torch 1 by the rotating ring 11 in a state where the rotating ring 11 is most tightly fastened to the fixing ring 99.

The annular claw 9A at the distal end of the retainer cap 9 is connected to the flange 8 at the proximal end of the shield cap 7.
, So that the retainer cap 9 is
A set of detachable parts such as the shield cap 7, the insulating ring 19, the nozzle 5, the insulating guide 13, and the electrode 3 is attracted to the torch body 2 and fixed.

A plasma power source 23 for generating a plasma arc is connected to the plasma torch 1. The negative pole of the plasma power source 23 is connected to a point 93 at the base end of the inner sleeve that can be electrically connected to the electrode 3,
The positive pole is connected to the workpiece 101, and the pilot arc terminal P is connected to the
0 is connected to the point 91. With this configuration, a pilot arc is generated between the electrode 3 and the nozzle 5 by the high-frequency voltage of the plasma power supply 23, and a main arc is generated between the electrode 3 and the workpiece 101.

Further, a detection circuit 29 for detecting contact with the workpiece 101 is connected to the plasma torch 1. A signal line 25 for connecting to the detection circuit 29 is accommodated inside the torch outer cylinder 100,
A rotating ring 11 for fixing the retainer cap 9 is connected to a base end portion of a fixing ring 99 to which the rotating ring 11 is attached.

In setting the initial height of the plasma torch 1 in the present embodiment, the plasma torch 1 is lowered, and the tip of the plasma torch 1 and the workpiece 10 are described in detail later.
The workpiece 101 is detected by making contact with
The initial height is set based on the detection. In setting the initial height by this method, the plasma torch 1 is devised as follows.

In the plasma torch 1, the signal line 25 is housed inside the outer cylinder 100 of the torch as described above. The signal line 25 includes a fixed ring 99, a rotating ring 11,
The shield cap 7 can be energized via the retainer cap 9. The signal line 25 is connected via a switch S to a detection circuit 29 that detects that the shield cap 7 and the work piece 101 have come into contact with each other. The detection circuit 29 includes a resistor 33 and a DC power supply (for example, DC24V).
And a voltage change detecting device 27 are connected in parallel. The plus pole of the DC power supply 31 is connected to the plus terminal of the voltage change detection device 27 and the signal line 25, and the minus pole of the DC power supply 31 is connected to the minus terminal of the voltage change detection device 27 and the workpiece 101. I have.
With this configuration, the shield cap 7 and the workpiece 1
When the shield cap 7 and the work piece 101 come into contact with each other, the current flows through the work piece 101, the shield cap 7, the retainer cap 9, the rotating ring 11, the fixing ring 99, and the signal line 25. Flows, the voltage difference between the shield cap 7 and the workpiece 101 changes to zero (or DC2
4V), and the voltage change detecting device 27
Can be detected by This eliminates the need to provide a special sensor for detecting the workpiece 101 in the vicinity of the plasma torch 1, which deteriorates due to fumes, spatters, and the like scattered by plasma arc cutting as in the related art. And the need to worry about durability.

As shown in FIG. 3, the shield cap 7 which is located at the tip of the plasma torch 1 and covers the tip of the nozzle 5 is provided with the shield cap 7 on the tip face facing the workpiece 101. An annular protrusion 33 is provided so as to surround the distal end opening 35. The annular projection 33 is formed in a shape that is pointed toward its tip (that is, tapered toward its tip). Thus, even if there is a coating such as an electrically insulating paint or a protective seal on the surface of the work piece 101, if the torch 1 is pressed against the work piece 101 with a small force, the annular protrusion on the tip surface of the torch 1 is formed. The film can be broken by 33, and as a result, the shield cap 7 and the work piece 101 are electrically connected, and the work piece 101 can be detected. According to a test conducted by the inventor of the present invention, an annular protrusion 33 having a height of about 0.5 mm is provided on the tip end surface of the torch 1 and the torch 1 is placed on the workpiece 101 by about 1 kg (with such a force). If there is an electrical insulation paint film on the surface of the work piece 101,
It has been confirmed that the contact between the torch 1 and the workpiece 101 can be detected by breaking the film.

The annular projection 33 has the following effects. That is, since the annular projection 33 is present on the front surface of the shield cap 7, the shield cap 7
Is pressed against the workpiece 101, the surface having the shield cap opening 35 does not directly contact the workpiece 101. Therefore, it is possible to avoid a problem that the shield cap opening 35 is damaged by the foreign matter attached to the surface of the work piece 101 or the foreign matter is attached to the shield cap opening 35.

As already described, this torch 1 includes:
Signal line 2 electrically connected to shield cap 7
5 is built-in. That is, the signal line 25 is accommodated inside the torch outer cylinder 1A to which the rotating ring 11 for fixing the retainer cap 9 is attached. Therefore, when replacing the consumables (mainly the electrode 3 and the nozzle 5) of the torch 1, the rotating ring 11 is loosened to remove the retainer cap 9, or the retainer cap 9 is attached again to tighten the rotating ring 11. , The signal line 25 does not interfere.

The procedure for setting the initial height of the torch 1 in the present embodiment using the plasma torch 1 having the above configuration will be described with reference to FIGS.

First, as shown in FIG.
To the carriages 107, 10 of the XY movement system shown in FIG.
9 moves the torch 1 so that the center axis (arc generation point) is located directly above the piercing point (the position where a hole is first drilled to start cutting) 40 of the workpiece 101.

Next, as shown in FIG.
Is moved by the Z carriage 111 shown in FIG. 1 to a height stored in advance, for example, to a piercing height position in the previous cutting step (that is, a standoff in the cutting step) (for example, at a speed of 10 m / min). Lower it.

Next, as shown in FIG.
After applying 24 V DC between the tip (shield cap 7) and the work piece 101, the torch 1 is intermittently lowered (skip feed) at a relatively high speed (for example, 1 m / min).
Let me do it. Then, the tip of the torch 1 and the workpiece 10
As soon as it is detected that the voltage has changed (becomes 0 V) due to contact with the torch 1, the lowering of the torch 1 is stopped immediately.

However, even if the descent of the torch 1 is stopped immediately, the torch 1 goes too far below the position of the workpiece 101 as shown in FIG. Would. Therefore,
As shown in FIG. 4D, the torch 1 is moved from the stopped position by a predetermined distance (for example, 3 m at a speed of 0.2 m / min).
m).

Next, as shown in FIG.
The torch 1 is moved at a relatively low speed (for example, 0.2 m /
Min) intermittently (skip feed). Then, when it is detected that the tip of the torch 1 comes into contact with the workpiece 101 and the voltage value is changed, the torch 1 is immediately turned on.
And stop applying the voltage.

Next, as shown in FIG.
From the position where is stopped, the torch 1 is raised by a predetermined distance H. Thereby, the tip of the torch 1 and the workpiece 1
The distance H to 01 is the initial height (piercing height) H in this cutting step.

Next, as shown in FIG. 5 (g), a plasma power source is activated to generate a plasma arc 41 between the torch 1 and the workpiece 101, and a hole is made in the pierce point 40.

After the piercing step of FIG. 5 (g) is completed, the torch 1 is lowered to the stand-off (cutting height) at the time of cutting as shown in FIG. The torch 1 is moved relative to the workpiece 101 along a previously programmed XY trajectory to cut the workpiece 101 by NC (numerical control).

In each step of plasma arc cutting, 1
As one cycle, the steps of FIGS. 4A to 5H are performed.

In this embodiment, as described above, the torch 1 is lowered at a relatively high speed (for example, 1 m / min) to detect the approximate position of the work piece 101, and then the torch 1 is moved for a predetermined distance (for example, 3 mm). Then, the torch 1 is moved away from the workpiece 101, and then the torch 1 is lowered at a relatively low speed (for example, 0.2 m / min) to detect an accurate position of the workpiece 101. Thereby, the initial sensor is lowered at a low speed from the beginning in order to accurately detect the position of the workpiece, and after detecting the workpiece position, the sensor is moved above the tip of the torch 1 and then the torch 1 is lowered. As compared with the conventional method, the work piece position can be accurately detected in a short time.

As a modification of this embodiment, FIG.
After the torch 1 is lowered at a relatively high speed in the steps (a) to (c) to detect and stop the approximate position of the workpiece 101, in the step in FIG. 4D, the torch 1 is stopped. When the changed voltage value (0 V) returns to the original value (24 V
), The position of the torch 1 is measured immediately (that is, the position of the torch 1 at the moment when the torch 1 is separated from the workpiece 101 and the electrical conduction is lost), and based on the position, The initial height may be set by raising the torch 1 by a predetermined initial height H. According to this method, the torch 1 needs to be moved at a low speed only at the time of lifting in FIG.
The initial height of the torch 1 can be set in a short time.

Although the preferred embodiment of the present invention has been described above, this is an exemplification for explaining the present invention, and is not intended to limit the scope of the present invention only to this embodiment. The present invention can be implemented in other various forms.
Instead of the annular projection 33 of the shield cap 11, for example, a number of pointed projections may be arranged around the opening 35.

[Brief description of the drawings]

FIG. 1 is a perspective view of a plasma processing machine according to an embodiment of the present invention.

FIG. 2 is a diagram showing an overall configuration of the plasma torch 1.

FIG. 3 is an enlarged view of a tip portion of the plasma torch 1.

FIG. 4 is a diagram showing a procedure for setting an initial height of the torch 1 according to one embodiment of the present invention.

FIG. 5 is a diagram showing a procedure for setting an initial height of the torch 1 according to one embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Plasma torch 3 Electrode 5 Nozzle 7 Shield cap 9 Cap 13 Insulation guide 15 Insulation cylinder 19 Insulation ring 29 Detection circuit 33 Annular protrusion 35 Shield cap opening

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Kazuhiro Kuraoka 5 Yokaichi-cho, Komatsu-shi, Ishikawa Komatsu Industries Co., Ltd. F-term (reference) 4E001 AA01 BA04 DA01 DF09 DG05 EA10 ME04 ME11

Claims (6)

[Claims] 1. A plasma arc is ejected from a nozzle,
In a plasma torch for cutting or welding a workpiece, a torch body to which the nozzle is detachably attached and which is arranged to be movable with respect to the workpiece when incorporated in a plasma processing machine; A cap attached to the outside of the nozzle and coming into contact with the workpiece when the plasma torch comes closest to the workpiece; and the cap connected to the cap, wherein the cap has contacted the workpiece. A signal line for transmitting an electric signal for detecting whether or not the plasma torch is detected. 2. The plasma torch according to claim 1, wherein said signal line is built in said torch main body. 3. A torch main body which is movably arranged with respect to a workpiece when incorporated in a plasma processing machine, a nozzle attached to the torch main body, and a torch main body outside the nozzle. The cap used in a plasma torch, comprising: a cap to be attached; and a signal line connected to the cap, for transmitting an electric signal for detecting whether the cap has contacted the workpiece. A cap having a projection for contacting the workpiece when the plasma torch is closest to the workpiece. 4. The cap according to claim 3, wherein the projection has a shape that is pointed toward a tip that contacts the workpiece. 5. A method for setting an initial height of a plasma torch for injecting a plasma arc from a nozzle to cut or weld a workpiece, comprising: lowering the plasma torch toward the workpiece at a relatively high speed. A step of detecting that the tip of the plasma torch and the work piece are in contact with each other and that the plasma torch and the work piece are electrically connected to each other; Stopping, raising the stopped plasma torch and separating it from the workpiece, lowering the plasma torch from the released position toward the workpiece at a relatively low speed, The tip of the plasma torch and the workpiece come into contact with each other, A step of stopping the lowering of the plasma torch as soon as it is detected that the plasma torch is electrically connected to the workpiece, and a step of raising the plasma torch by a predetermined distance based on the stopped position. How to set the initial height of the torch. 6. A method for setting an initial height of a plasma torch for injecting a plasma arc from a nozzle to cut or weld a workpiece, comprising: lowering the plasma torch toward the workpiece at a relatively high speed. A step of detecting that the tip of the plasma torch and the work piece are in contact with each other and that the plasma torch and the work piece are electrically connected to each other; Stopping, raising the stopped plasma torch at a relatively low speed, measuring the position of the plasma torch immediately after detecting that the electrical continuity is lost due to the raising, The plasma torch is updated by a predetermined distance based on the position of the plasma torch. Raising the plasma torch to an initial height.