JPH1177303A - Arc welding equipment - Google Patents

Abstract

PROBLEM TO BE SOLVED: To control oscillation of a welding arc so as to stabilize the arc position, and also to suppress a large weld ripple on the surface of a weld bead, spatter and the width fluctuation of a back bead. SOLUTION: In this equipment, a hollow cylindrical member 100, 200 for absorbing magnetic flux is attached to the nozzle of a welding torch. The flux absorbing member 100 is a cylindrical block of a magnetic substance laminated with silicon steel plates, absorbing the magnetic flux, which is caused by a welding wire current and which deflects a weld arc, and reducing the magnetic field around the arc. The flux absorbing member 200 is provided with a pair of magnetic legs 202, 203 which extends downward from the magnetic cylindrical block and which is capable of entering into the groove. Since the legs 202, 203 are close to the arc, they bring a strong effect for reducing a magnetic field around the arc.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an arc welding apparatus, and more particularly to, but not limited to, an arc welding apparatus for welding a relatively wide groove with a relatively large current arc. It relates to a welding device.

[0002]

2. Description of the Related Art For example, in the construction of a hull, the construction of a bridge, the construction of a large tank, or the construction or construction of another steel structure in shipbuilding, when welding a groove between two butted steel plates, Run the welding bogie along the welding line (the direction in which the groove or corner extends) while performing welding,
The welding line is automatically welded by the welding torch mounted on the bogie. In order to make the penetration of the filler material uniform, the filler material is a granular cut wire or iron powder obtained by finely cutting a small-diameter steel wire,
This is sprayed into a groove before welding (for example, Japanese Patent Publication No. 50-7543).

[0003] Since the steel plate to be welded is thick and the groove cross section is large, the welding torch is swung in a direction perpendicular to the direction in which the groove extends, and in order to increase the efficiency of welding work, In some cases, two or more welding torches are mounted on a trolley, and two layers are welded in one run of the trolley. The present applicant has already developed and put to practical use such a welding apparatus (for example, Japanese Patent Application Nos. 8-64705 and 8-2).
98439).

By mounting two or more welding torches on a truck,
Even when performing two or more layers of welding in one run of the bogie,
Since the steel plate to be welded is thick, the welding arc current of each torch is large, and accordingly, the arc of the preceding welding torch swings in the direction opposite to the welding progress direction by magnetic blowing development. Repeat. Therefore, there are problems such as a large irregularity of the weld bead surface due to the preceding welding torch, a large amount of spatter, and a random change if the width of the back bead is wide / narrow.

[0005] Regarding investigation into causes of instability of welding arc and countermeasures, Japanese Patent Laid-Open Nos.
No. 8677, JP-A-59-107772 and JP-A-8-155463. JP-A-56-11185 discloses that after welding a high nickel-based welding member to one side of a 9% nickel steel base material that is easily magnetized,
This relates to the case where TIG welding is performed on the other side.
TIG welding is difficult due to the occurrence of bubbles. Therefore, the groove to be welded is heated before the demagnetizing TIG torch is advanced to the welding TIG torch, and TIG welding is performed after reducing the residual magnetism.

Japanese Unexamined Patent Publication No. 57-28677 discloses that a base material on both sides of a groove is magnetized in opposite polarities and a magnetic field appears in a direction crossing the groove, whereby a welding arc extends in a direction in which the groove extends. Is considered to be the cause of arc instability. Therefore, an electric coil is wound around the welding torch, and a perpendicular magnetic field is applied to the arc through a direct current or an alternating current to reduce or cancel the magnetism in the groove.
It proposes a stabilization method.

Japanese Patent Application Laid-Open No. Sho 59-107772 discloses an
A magnetic detector is placed on the edge of the steel material to be welded to detect the direction and strength of the magnetic field in the groove, and a magnetic field is applied to the steel material by an electromagnet attached to the steel material end surface in a direction to cancel it The present invention proposes a method for preventing magnetic arc blow of a welding arc.

Japanese Patent Application Laid-Open No. 8-155,643 discloses a method in which a contact for a base material earth is provided on a traveling vehicle, and the contact is moved together with a torch to secure the base material earth in the immediate vicinity of the welded portion. A magnetic blowing prevention type welding device is presented.

The inventor of the present invention disclosed in Japanese Patent Application No. 9-55038, a block of a thin magnetic plate straddling a groove was applied to a material to be welded forward in a welding traveling direction, and this block was connected to a torch carriage. Thus, there has been proposed a magnetic-blowing-suppression welding apparatus in which the torch carriage also travels in the welding traveling direction when traveling.

[0010]

The above-mentioned JP-A-56-11
The method disclosed in Japanese Patent Publication No. 185 is limited to the demagnetization of a high nickel-based material having a large residual magnetism. JP-A-57-2
In the arc stabilization method disclosed in Japanese Patent No. 8677,
A large electric coil is provided around the torch, so that the torch is heavy and handling of the torch is difficult. Since the electric coil and a power supply for supplying power to the coil are required, the equipment becomes expensive. There are problems such as an increase in the work of removing spatters attached to the switch and the electric coil and low workability.

In the method of preventing magnetic arc blow of welding arc disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 59-107772, the effect is obtained only at the end of the base material. Since a controller for processing and controlling the energization of the electromagnet is equipped, the equipment is expensive, it is difficult to handle, and it is virtually impossible to carry out welding work on site where the base material is located. Can be

In the magnetic blowing prevention type welding apparatus disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 8-155,643, since a thick base metal cable is pulled by a traveling cart, handling of the base metal cable is difficult. Even if the magnetic blowing caused by the current flowing through the base material is reduced, the effect of the magnetic blowing due to the magnetic imbalance before and after the torch is low. Dragging of the thick base metal cable by the traveling cart is impossible. Therefore, it is conceivable that the welding is limited to welding of low current small overlay.

[0013] The inventor of the present invention proposed in Japanese Patent Application No. 9-55038 that a superposed block of a thin magnetic plate straddling a groove mechanically comes into contact with a material to be welded and magnetically intervenes with the material to be welded. Since a suction force acts, a large driving force is required to cause the traveling in the welding traveling direction, and the traveling load of the torch carriage is increased. In particular, when the groove edge is rough, the running load fluctuates and becomes excessive.

SUMMARY OF THE INVENTION It is a first object of the present invention to suppress the run-out of the arc of a welding torch and to stabilize the arc position. A second object is to suppress fluctuations in the width of the wave, and a third object is to avoid an increase in the running load of the torch carriage due to a magnetic member added thereto.

[0015]

Means for Solving the Problems The present inventor has described the above-mentioned 2
In welding thick steel plates in which two or more welding torches are mounted on a trolley and two or more layers are welded in one run of the trolley, the cause of the instability of the welding arc of the preceding torch is considered. investigated.
First, when two welding torches are used, the above-mentioned problems appear with respect to the welding of the preceding torch, and the above-mentioned problems do not substantially appear in the welding of the following torch. Focusing on this, the difference in the welding environment between the preceding torch and the following torch can be seen. In the preceding torch, the front of the torch moving direction (welding direction) is a groove and the rear is a welding bead. -While the following
In this case, the welding bead is formed at both the front and rear sides, and the front welding bead (formed by the preceding torch) is also at a temperature below the Curie point immediately before the following torch (magnetic material). It is.

Accordingly, regarding the preceding torch, the arc
As shown by the two-dot chain line in FIG. 6, the magnetic flux flows through the steel plates W1 and W2 to be welded, but the front is the groove, and the back is the non-magnetic material (the welding wire above the Curie point). -) And the far back is a magnetic material (weld bead below the Curie point), so that the welding wire wr of the preceding torch is
Magnetic flux goes to a long distance behind the welding wire wr, so that the magnetic flux density (magnetic field) is rough (weak) immediately behind the welding wire wr.
However, in front of the welding wire wr, it is considered that the magnetic flux density (magnetic field) is close (strong) immediately in front of the welding wire wr without the magnetic flux going far because the front is a groove (gap). In some cases, a cut wire is sprayed as a filler material in the groove, but the effect of cutting off the magnetic flux by the cut wire is low. As described above, if the magnetic flux density is high in front of the welding wire wr of the preceding torch and the magnetic flux density is low behind, the welding flux is applied to the welding arc by the force P in accordance with Fleming's left-hand rule. As a result, the welding arc is swung in the direction opposite to the torch advancing direction as shown in FIG.

(1) Therefore, the arc welding apparatus of the present invention
An arc welding torch (Tr) for welding two opposed steel plates (W1, W2) to be welded in the space extending in the x-direction and leaving a space (α) in the y-direction; -Magnetic absorption of the magnetic material attached to the arc welding torch (Tr) in order to absorb the magnetism generated by the current flowing through the welding wire (Wr) of the electrode passing through the arc welding torch (Tr) Member (100);

In order to facilitate understanding, the symbols of the corresponding elements in the embodiments shown in the drawings and described later are added in parentheses for reference.

According to this arc welding apparatus, the magnetic absorbing member (1)
00) absorbs the magnetic flux (magnetic field) around and below the nozzle (Trn) of the torch (Tr) generated by the arc current flowing through the welding wire (Wr). Accordingly, the magnetic flux (magnetic field) immediately below the torch (Tr) is reduced (weakened), thereby weakening the electromagnetic force that tends to swing the welding arc (Ar) to the back of the torch (Tr). The corrugation of the weld bead surface is low, the generation of spatter is drastically reduced, and a backwash with small width fluctuation is obtained.

FIG. 5A is a cross-sectional view schematically showing a state of a magnetic flux generated in an arc welding torch (Tr) by a conventional arc welding apparatus. A voltage is applied from the welding power source (Ppr) between the welding wire (Wr) penetrating the welding torch (Tr) and the welding steel plates (W1, W2), and a current (I) flows through the arc (Ar). . The current (I) changes due to arc discharge, and the welding torque
This changes because the switch (Tr) swings in the y direction. Welding wire (W
A magnetic flux φ is generated around r) by the current (I) according to the right-hand screw rule. A part of the magnetic flux φ is deflected and flows to the steel plate (W1, W2) and the weld bead (Wb) having higher magnetic permeability than the space.
The nozzle (Trn) of the welding torch (Tr) is a non-magnetic material made of copper or a copper alloy and has no relation to the deflection of magnetic flux.

In the groove α portion having a V-shaped cross section in front of the welding point (left side in the figure), more magnetic flux is concentrated on the groove bottom where the steel plates (W1, W2) are approaching. The magnetic flux is dispersed over a wide range because the closest is a non-magnetic material (welding bead above the Curie point) and the farther back is a magnetic material (weld bead below the Curie point). That is, since a difference occurs in the magnetic flux density before and after the arc (Ar), the arc (Ar) is deflected in the direction of arrow P. That is, magnetic blowing occurs.

FIG. 5B schematically shows the state of the magnetic flux when the magnetic absorbing member (100) of the present invention is mounted on the torch (Tr). Current flowing through welding wire (Wr)
(I) generates a magnetic flux φ around the welding wire (Wr) as in (a) above, but since the magnetically absorbing member (100) is attached to the torch (Tr), the torch (Tr) Most of the magnetic flux φ present in the lower space as well as the outer circumference flows to the magnetic absorbing member (100), the magnetic flux absorbed by the welded steel plates (W1, W2) decreases, and the magnetic flux density in the space at the lower end of the torch decreases. descend. That is, the magnetic flux φ
Absorbed by (100). As a result, the magnetic field acting on the arc (Ar) is weakened, and magnetic blowing is greatly reduced.

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION

(2) The magnetic absorbing member (100) is formed by laminating magnetic thin plates (103a to 103m) having the same flat ring shape to form a hollow cylinder block, and a welding wire (Wr) penetrates the inner space thereof. (FIG. 3). Electrode welding wire (Wr)
The magnetic flux circulating around the welding wire (Wr) flows due to the current (I) flowing through, and the circulating magnetic flux fluctuates due to the fluctuation of the welding current (I). A current flows through the body thin plate in the thickness direction, and the magnetic absorbing member (100) generates heat due to the Joule heat of the current.
Since the thin plates are laminated, the electric resistance between the thin plates is high, and the current is suppressed. In the embodiment described later, the magnetic absorption member (100)
Is a stack of silicon steel sheets having an oxide layer on the surface,
Low Joule heat generation.

(3) The magnetic absorption member (100) is a hollow cylindrical block (201) made of a magnetic material, and is continuous with the block, opposed to each other in the x direction in which the space extends, and protrudes toward the space. Hollow cylinder block including a pair of magnetic legs (202, 203)
An electrode welding wire (Wr) penetrates the inner space of (201) (FIG. 4).

According to this, the magnetic legs (202, 203) are grooved.
(α), the magnetic flux concentrates there, and the arc in the groove
Since the magnetic field in the vicinity of the magnetic field is weakened, the effect of preventing magnetic blowing is further enhanced. Other objects and features of the present invention will become apparent from the following description of embodiments with reference to the drawings.

[0026]

【Example】

-First Embodiment- FIG. 1 shows a first embodiment of the present invention. The arc welding apparatus 1 shown in FIG. 1 includes a preceding welding torch (arc welding torch) Tr.
And two subsequent welding torches Tt, that is, C
O ₂ Gasushi - is intended to perform field sided welding, a groove (space) alpha, which is welded in a direction x extending.

Workpieces W1 and W2, which are steel sheets arranged horizontally (steel sheets to be welded), have their opposing end faces abutted against each other and are opened between both steel sheets (hereinafter sometimes referred to as “workpieces”). The tip α is formed. The cart 11 of the welding device 1
In this embodiment, it is on the workpiece W1 and moves in the x direction along the groove α. Reference numeral 50 denotes a start tab for preventing the molten metal from burning off. Dolly (torch carriage)
Reference numeral 11 supports the control panel 20 below via the support frame 12. The support frame 12 further supports, via a support member 13, a base of a support arm 13a extending in the y-direction perpendicular to the bogie traveling direction x. It projects at right angles to it.

The tip of the support arm 13a is fixed to a slide rod 14 extending in parallel to the traveling direction x of the carriage. That is, the cart 11 is moved through the aforementioned 12, 13, 13a,
The slide bar 14 is supported in parallel with the traveling direction. The slide bar 14 supports the vertical mechanism 15r so as to be slidable in the x direction. The up-down mechanism 15r is fixed to the slide bar 14 by tightening a set screw (not shown).

The up-down mechanism 15r supports a leading torch swing mechanism 16r via an internal elevating mechanism (not shown). When the operator rotates the knob connected to the elevating mechanism of the up-and-down mechanism 15r forward or backward, the preceding torch swinging mechanism 16r moves in the upward direction z or the downward direction z with the rotation of the knob. The leading torch swing mechanism 16r supports the leading torch Tr via the leading torch support arm 17r.
The swing controller in the control panel 20 swings and drives the preceding torch Tr with a swing width and a swing frequency / min according to the swing width signal and the swing speed signal from the indicating board 30. The driving of the electric motor of the preceding torch swing mechanism 16r is controlled.
As a result, the preceding torch Tr moves in the bogie traveling direction x (groove α).
Swinging (reciprocating) in a direction y perpendicular to the direction in which

The slide bar 14 supports the up-down mechanism 15t so as to be slidable in the x-direction at a position that is more rearward (to be referred to as a trailing side) than the up-down mechanism 15r in the bogie traveling direction. The up-down mechanism 15t is fixed to the slide bar 14 by tightening a set screw (not shown).

The up-down mechanism 15t supports a trailing torch swing mechanism 16t via an internal elevating mechanism (not shown). When the operator rotates the knob connected to the elevating mechanism of the up-and-down mechanism 15t in the forward or reverse direction, the trailing torch swinging mechanism 16t moves upward or downward z as the knob rotates. The trailing torch swing mechanism 16t supports the trailing torch Tt via the trailing torch support arm 17t.
The swing controller in the control panel 20 swings the trailing torch Tt with a swing width and a swing frequency / min according to the swing width signal and the swing speed signal from the indicating board 30. The driving of the electric motor of the trailing torch swing mechanism 16t is controlled.
This causes the trailing torch Tt to swing in a direction y perpendicular to the bogie traveling direction x.

A wire reel 18t for the trailing torch Tt is mounted on the trailing side surface of the support frame 12, and a welding wire wt for the trailing torch Tt is wound therearound. The welding wire wt is supplied to the trailing torch Tt when the wire supply motor 19t provided on the trailing side of the support frame 12 is driven. Further, a wire reel 18r for the preceding torch Tr is provided on the side of the support frame 12 in the front direction (referred to as the leading side) in the bogie traveling direction x, and the welding wire wr for the leading torch Tr is wound. I have. The welding wire wr is supplied to the preceding torch Tr when a wire supply motor (not shown) provided on the leading side of the support frame 12 is driven. The magnetic absorbing member 100 of the present invention is fixed to the nozzle of the preceding torch Tr.

FIG. 2 shows an enlarged view of the nozzle Trn of the preceding torch Tr and the magnetic absorption member 100, and FIG.
0 shows a cross section. Please refer to these drawings. A male screw 105 is cut on the outer periphery of the nozzle Trn of the preceding torch Tr, and a flange 104 is formed on the lower part. Outside the nozzle Trn, a magnetic absorbing member (100) of a hollow cylindrical block having the flange 104 as a bottom or a thickness of 0.5 m
m, a plurality of ring-shaped silicon steel plates 103a to 103m, which are integrally laminated and fixed by a tightening nut 102 and a lock nut 101 screwed to a male screw 105. Since the ring-shaped silicon steel plates 103a to 103m each having a thickness of 0.5 mm have an oxide layer (insulating layer) on the surface, an induced current in the z direction does not substantially flow through the magnetic absorbing member 100. Few.

The magnetic flux caused by the current flowing through the welding wire Wr is absorbed by the magnetically absorbing member 100, whereby the electromagnetic force for swinging the welding arc of the preceding welding torch Tr to the rear of the torch becomes weak. In addition, the wavy irregularities on the surface of the weld bead are low, the generation of spatter is drastically reduced, and a backwash with a small width variation can be obtained.

Since the magnetic absorption member 100 is not in contact with the workpieces W1 and W2, even if the magnetic absorption member 100 is attracted to the workpieces W1 and W2, the magnetic absorption member 100 (welding torch Tr) can be moved in the welding direction. There is substantially no load increase that would hinder movement.

Second Embodiment FIG. 4A shows a main part of a second embodiment. The magnetic absorption member 200 is formed by integrally integrating a hollow cylindrical block 201 made of a magnetic material and a pair of magnetic material legs 202 and 203. FIG. 4B shows a cross section of the magnetic absorption member 200 (a cross section taken along line BB of FIG. 4C), and FIG.
(A view as seen from the line CC in FIG. 4B).
The magnetic absorbing member 200 is formed by cutting a mild steel pipe to form a pair of magnetic legs 202 and 203 at the lower end of the hollow cylinder. A magnetic material such as pure iron, silicon steel or amorphous metal may be used.

The hollow cylinder block 201 of the magnetic absorption member 200 is provided with a female screw hole 204r that penetrates the cylinder wall. A set screw 204s is screwed into this hole, and the tip of the screw 204s presses the nozzle of the torch Tr. As a result, the magnetic absorption member 20
0 is attached to the torch Tr. At this time, the posture of the magnetic absorption member 200 is set so that the magnetic legs 202 and 203 are aligned in the direction in which the groove α extends and the lower ends thereof are at a height above or within the groove α. And adjust the position.

At the lower ends of the magnetic legs 202 and 203, there are wide claws in the x direction, and the magnetic flux at the lower end of the torch concentrates on these claws, so that the magnetic field acting on the arc is weakened. Magnetic leg 20
Nos. 2 and 203 have a wedge shape with an angle of θ1 (FIG. 4C) so as to reduce the wire-facing surface on the center side of the cylinder in order to reduce the deposition of spatter (FIG. 4C), and the claw has a wedge shape with an angle of θ2. ((B) of FIG. 4). In this embodiment, the angle θ1 is 35
The degree and the angle θ2 are 45 degrees.

Since the magnetic absorption member 200 moves up and down in the z direction together with the torch Tr, the magnetic legs 20 are not used during arc welding.
So that the claw at the lower end of 2,203 enters the groove α.
By adjusting the mounting height of the welding bead, the magnetic flux near the welding arc is concentrated on them, the magnetic field acting on the welding arc is weakened, and the wavy irregularities on the welding bead surface are low. The occurrence of spatter is drastically reduced, and a backwash with small width fluctuation is obtained.

The magnetic path member 200 of the second embodiment has a simple structure, and has a set screw 204 with respect to the torch Tr.
Since it is fixed at s, it can be easily replaced when the magnetic legs 202 and 203 are melted or a large amount of spatter is deposited.

The welding apparatus of the present invention can be applied to fillet welding and lap fillet welding. Further, the present invention can be applied to a welding torch of a non-consumable electrode.

[Brief description of the drawings]

FIG. 1 is a perspective view showing an appearance of a first embodiment of the present invention.

FIG. 2 is an enlarged perspective view showing a tip of a welding torch which is a main part of the embodiment shown in FIG.

FIG. 3 is an enlarged vertical sectional view of the magnetic absorption member 100 shown in FIG.

FIGS. 4A and 4B are views showing a main part of a second embodiment of the present invention, in which FIG. 4A is a perspective view of the tip of the torch, and FIG. 4B is a longitudinal sectional view of the magnetic absorption member 200 shown in FIG. , (C) shows the magnetic absorption member 20
0 is a bottom view.

FIG. 5 is a cross-sectional view schematically showing an estimated magnetic flux distribution around a welding torch. FIG. 5A shows a conventional example in which arc welding is performed by a torch Tr having no magnetic absorbing member 100; (B) shows a case where the magnetic absorption member 100 is provided.

FIG. 6 is a plan view schematically showing the estimated magnetic flux distribution of the works W1 and W2 at the time of arc welding of the conventional example shown in FIG.

FIG. 7 is a side view of the works W1 and W2 shown in FIG. 6, and a part thereof shows a longitudinal section.

[Explanation of symbols]

 1: Welding device 11: Dolly 12: Support frame 13: Support member 13a: Support arm 14: Slide bar 15r: Vertical mechanism 15t: Vertical mechanism 16r: Leading torch rocking mechanism 16t: Trailing torch rocking mechanism 17r: Leading torch Support arm 17t: Trailing torch support arm 18r: Wire reel 18t: Wire reel 19t: Wire supply motor 20: Control panel 20a: Signal receiving side connector 21: Operation board 30: Welding condition designation panel 50: Start tab 100 : Magnetic absorption member 101: Lock nut 102: Tightening nut 103 a to 103 m: Ring-shaped silicon steel plate 104: Flange 105: Male screw 200: Magnetic absorption member 201: Hollow cylinder block 202, 203: Magnetic leg 204 r: Female screw hole 204 s : Set screw Ar: Welding arc cw: Cut wire M: Base Drive motor Ppr, Ppt: welding power source Tr: prior welding torch Trn: nozzle Tt: trailing welding torch W1, W2: Wa - click (welded steel) Wb: Welding bi - de wt, wr: Welding wire alpha: groove

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Osamu Maruyama 7-6-1, Higashi Narashino, Narashino City, Chiba Prefecture Nippon Steel Welding Industry Co., Ltd. Equipment Division (72) Inventor Tsukasa Yoshimura Tsukiji, Chuo-ku, Tokyo Nippon Steel Welding Industry Co., Ltd. (72) Inventor Tetsuo Miyazaki 1 Ariake, Oaza, Nagasu-cho, Tamana-gun, Kumamoto Prefecture Inside the Ariake Plant of Hitachi Zosen Corporation (72) Inventor Yoshio Nakajima Kumamoto Prefecture 1 Ariake, Nagasu-cho, Tamana-gun, Hitachi, Ltd.Ariake Plant (72) Inventor Hiroaki Nakahara 1 Ariake, Nagasu-cho, Tamana-gun, Kumamoto Prefecture, Ariake Plant, Hitachi Zosen Corporation

Claims (3)

[Claims] 1. An arc welding to weld two opposite steel plates to be welded extending in the x-direction and spaced in the y-direction.
And a magnetically absorptive member attached to the arc welding torch for absorbing magnetism generated by an electric current flowing through the electrode passing through the arc welding torch. Welding equipment. 2. The magnetic absorption member is formed by laminating magnetic thin plates having the same flat ring shape into a hollow cylinder block.
The arc welding apparatus according to claim 1, wherein the electrode penetrates the inner space. 3. The magnetic absorption member includes a hollow cylindrical block made of a magnetic material, and an x extending continuously from the block and extending the space.
The arc welding apparatus according to claim 1, further comprising a pair of magnetic legs which are opposed to each other in the direction and project toward the space, and wherein the electrode penetrates the inner space of the hollow cylinder block.