JPH1018775A - Cutter head - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a cutter head of a tunnel excavator from lowering its excavating efficiency even though a cutter pin is worn, and enhance the safety and the working efficiency. SOLUTION: A cutter spoke 32 of a cutter head 31 is rotatably attached thereto with a plurality of cutter bit holders 34, and a first cutter bit 37 and a second cutter bit 38 are arranged adjacent to and fixed to the cutter bit holders 34, Normally, the first cutter bit 37 is held at a position where it is projected forward. If the first cutter bits 37 is worn, the cutter bit holders 34 are rotated by a rotary jack 40 so as to the first cutter bit 27 is moved to a retracted position while the second cutter bit 38 is moved to the position where the second cutter bit is projected.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cutter head having a plurality of cutter bits mounted on a front portion of an excavator body and excavating a ground, a tunnel excavator equipped with the cutter head, and a cutter head. The present invention relates to a tunnel excavation method using the method.

[0002]

2. Description of the Related Art FIG. 6 is a schematic cross-sectional view of a general shield excavator, FIG. 7 is a front view of the shield excavator, and FIG. 8 is a cross-sectional view of a main part of a conventional cutter head showing a mounted state of a cutter bit (FIG. VIII-VIII section), and FIG. 9 schematically shows the wear state of the cutter bit due to long-term use.

As shown in FIGS. 6 and 7, in a general shield excavator, a cutter head 12 is rotatably mounted at the front of a cylindrical skin plate 11 serving as an excavator body. Spokes 13 are fixed radially to the front surface of the cutter head 12, and a number of cutter bits 14 are fixed to the spokes 13.
A ring gear 15 is fixed to the rear of the cutter head 12, while the hydraulic motor 1 is mounted on the skin plate 11.
The drive gear 17 of the hydraulic motor 16 is engaged with the ring gear 15. Accordingly, when the hydraulic motor 16 is driven to rotate the drive gear 17, the cutter head 12 can be rotated via the ring gear 15.

[0004] A bulkhead 18 is attached to the skin plate 11 behind the cutter head 12, and a chamber 19 is formed between the cutter head 12 and the bulkhead 18. In the chamber 19, one end of a mud feeding pipe 20 and the other end of a mud discharging pipe 21 that are extended outside the shield excavator are open,
An agitator 22 that stirs and mixes excavated earth and sand with muddy water is installed near the opening of the drainage pipe 21.

[0005] Further, a plurality of shield jacks 23 are arranged in the circumferential direction around the rear outer periphery of the skin plate 11, and the shield jacks 23 extend rearward in the excavation direction of the shield excavator and excavate in a tunnel. By pressing against an existing segment S (not shown) constructed on the peripheral surface, the skin plate 11, that is, the entire shield excavator can be advanced by the reaction force. On the other hand, a segment elector 24 is provided at the rear center of the skin plate 11, and the segment elector 24 is provided in a space between the skin plate 11 advanced by the shield jack 23 and the existing segment S.
Can be used to build a tunnel.

By the way, the above-mentioned cutter bit 14
As shown in FIG. 8, the spokes 13 are fixed to both sides by welding or bolt-up. The cutter bit 14 is composed of a cutter bit chip 14a for crushing the ground and a cutter bit shank 14b holding the cutter bit chip 14a and forming a cutter bit body. And this cutter bit chip 14a is WC
While the cutter bit shank 14b is not required to be as hard as the cutter bit tip 14a, the cutter bit shank 14b is required to have a hardness that can withstand the flow wear of the earth and sand, while being made of a high-hardness material such as a series cemented carbide having excellent wear resistance to earth and sand. In addition, a material having good workability is required, and tool steel or the like is used.

Therefore, as shown in FIG. 6, in order to excavate and form a tunnel, a plurality of shield jacks 23 are first extended, and the skin plate 11 is advanced by a reaction force against the existing segment S, while the hydraulic pressure is increased. Motor 16
, The cutter head 12 is rotated, and the ground in front is excavated by the multiple cutter bits 14. Then, the earth and sand excavated by the cutter bit 14 is taken into the chamber chamber 19 and supplied with water from the mud feed pipe 20 to be supplied to the agitator 2.
2 excavated earth and sand and water supply are stirred and mixed.
1 to the outside. Next, shield jack 2
3 is operated in the contracting direction to form a space between the existing segment S and a new segment S is mounted in the space by the segment elector 24. By repeating this operation, a tunnel of a predetermined length is excavated and formed.

[0008]

In recent years, tunnels formed by excavation have tended to be long-hauled, and a cutter bit 14 mounted on a cutter head 12 has been worn out during excavation work by a shield excavator, and the excavation resistance of earth and sand has been reduced. And the excavation efficiency decreases, or excavation becomes impossible. That is, when the cutter head 12 makes a forward rotation in the direction of the arrow shown in FIG. 8, the tip 14a of the cutter bit 14 at the front (right side in FIG. 8) excavates the ground by a predetermined cutting amount with respect to the ground. The soil moves to the rear of the excavator along the front part of the cutter bit 14, and the excavated soil causes the cutter bit 1 to move.
Wear is likely to occur on the front part of No. 4. At this time, the excavated soil moves along the rear surface of the cutter bit 14 at the rear (left side in FIG. 8), and the excavated sediment easily causes wear on the rear surface of the cutter bit 14. When the cutter head 12 rotates in the reverse direction, the operation is the reverse of that described above. As shown in FIG. 9, the cutter bit 14 becomes worn at the tip end of the tip 14 a and becomes less sharp as a result of long-term use.
_1, the rear part 14R wear P ₂ are generated respectively, there is a problem that hindered the discharge of drilling and soil of the ground.

Therefore, when the cutter bit 14 is worn, the excavation operation must be stopped and the worn cutter bit must be replaced. Conventionally, the excavation work by the shield excavator was stopped, and first, the ground in front of the shield excavator for which the cutter bit 14 was replaced was solidified and improved by injection of a chemical solution, freezing, and the like, and the water was completely stopped. Drill a tunnel excavator to the ground improvement point. here,
The shield excavator is stopped to discharge all the excavated earth and sand in the chamber 16, and the worker enters the chamber 19,
In addition, the cutter bit 14 is replaced in the front of the cutter head 12, that is, the worn cutter bit 14 is removed and a new cutter bit is mounted.

However, the work of replacing the cutter bit in this shield excavator is costly and uneconomical in order to improve the ground and requires a chemical solution, and requires a long working time, resulting in good workability. Absent. In addition, the operator can work in the narrow space in the chamber 19 or the cutter head 12.
Therefore, there is a problem that the work is performed in the space in front of the operator, which is inconvenient and dangerous, and the burden on the operator increases.

The present invention has been made to solve such a problem, and a cutter head and a tunnel excavator which prevent a decrease in excavation efficiency even when a cutter bit is worn, and improve safety and work efficiency. Machine and tunnel excavation method.

[0012]

According to the present invention, there is provided a cutter head having a plurality of cutter bits mounted on a front portion of an excavator body and excavating the ground. A plurality of cutter bit holders are rotatably mounted on the front part, a first cutter bit is fixed to each of the cutter bit holders, and a second cutter is provided on each of the cutter bit holders adjacent to the first cutter bit. A bit is fixed, and the cutter bit holder is moved to a storage position in which either the first cutter bit or the second cutter bit is stored by rotating the cutter bit holder, while the other protrudes forward. It is characterized in that cutter bit exchanging means movable to a position is provided.

Therefore, at the beginning of the excavation operation, the first cutter bit has a sharp tip, and the second cutter bit is at the accommodation position accommodated inside the cutter head. In this state, when the cutter head operates as the excavator body advances, the plurality of first cutter bits attached to the cutter head excavate the ground to form a tunnel. And
When excavation work is performed for a long period of time, the tip of the first cutter bit is worn, and the excavation resistance of earth and sand is increased and the excavation efficiency is reduced. By moving the cutter bit, the second cutter bit at the accommodation position inside the cutter head is moved to a projecting position projecting from the front surface of the cutter head, and the excavation work of the ground is continuously performed by the new second cutter bit.

Further, in the cutter head of the present invention, the first cutter bit and the second cutter bit are continuously formed in the respective cutter bit holders so as to form an arc over a predetermined angle range. Is what you do.

Therefore, when the tip of the first cutter bit is worn out due to long-term use, the cutter bit holder is rotated by the cutter bit exchange means so that the projecting portion from the front surface of the cutter head is moved from the first cutter bit to the second cutter bit. The operation is shifted to the cutter bit, and the excavation work of the ground is continuously performed by the new second cutter bit.

Further, the tunnel excavator of the present invention comprises: a tubular excavator body; a propulsion means for moving the excavator body forward;
A cutter head rotatably mounted on a front portion of the excavator body, cutter head driving means for driving and rotating the cutter head, radial spokes fixed to a front portion of the cutter head, and the spokes; A plurality of cutter bit holders rotatably mounted on each of the cutter bit holders, a first cutter bit fixed to each of the cutter bit holders, and a second cutter bit fixed to each of the cutter bit holders adjacent to the first cutter bit. The cutter bit is connected to the cutter bit holder and the cutter bit holder is rotated to rotate the cutter bit holder.
Cutter bit exchanging means for moving either the cutter bit or the second cutter bit to a storage position in which the other bit is stored therein, while being able to move the other to a protruding position in which the other protrudes forward. Things.

Therefore, at the beginning of the excavation work, the first cutter bit has a sharp tip, and the second cutter bit is at the accommodation position accommodated inside the cutter head. In this state, while the cutter head driving means drives and rotates the cutter head, the excavator body is advanced by the propulsion means, so that the first cutter bit fixed to the front part of the cutter head crushes the ground in front. Excavate the tunnel.
Then, the tip of the first cutter bit is worn by long-term use, the excavation resistance of earth and sand is increased, and the excavation efficiency is reduced. In this case, the cutter bit holder is rotated by the cutter bit exchange means. Then, the second cutter bit at the accommodation position inside the cutter head is moved to a projecting position projecting from the front surface of the cutter head, and the excavation work of the ground is continuously performed by the new second cutter bit.

Further, in the tunnel excavation method of the present invention, the cutter head mounted on the front portion of the excavator body is driven to rotate while the excavator body is advanced, so that the cutter head is rotatable on the front portion of the cutter head. When the first cutter bit of the cutter bit holder mounted on the first cutter bit excavates the ground in front and forms a tunnel, and when the first cutter bit wears and the excavating ability decreases, the cutter bit holder is rotated. A second cutter bit adjacent to the first cutter bit is projected forward from the storage position to the front of the cutter head, and the ground is excavated by the second cutter bit to form a tunnel. is there.

Therefore, when the wear of the first cutter bit progresses during excavation of the tunnel, the second cutter bit housed in the cutter head is projected forward of the cutter head, and the excavation work is continuously performed by the second cutter bit. And excavation efficiency does not decrease.

[0020]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a sectional view of a main part of a cutter head mounted on a shield excavator as a tunnel excavator according to a first embodiment of the present invention, FIG. 2 is a sectional view taken along line II-II of FIG. 1, and FIG.
2 shows a cross section of a main part of a cutter head showing a state where a cutter bit projects.

In the cutter head mounted on the shield excavator according to the present embodiment, as shown in FIGS. 1 and 2, a large number of cutter spokes 32 of the cutter head 31 extend in the longitudinal direction (the forward and backward direction of the excavation direction). (In this embodiment, 2
) Cutter bit housing 33 is fixed,
A disk-shaped cutter bit holder 34 is located in the cutter bit housing 33.
A rotating shaft 35 integral with the support 4 is rotatably supported via a bearing 36. A first cutter bit 37 and a second cutter bit 38 are fixed to the outer periphery of the cutter bit holder 34 so as to be adjacent to each other in the circumferential direction. This cutter bit 3
7, 38 are shank 37a, 38a and tip 37b, 3
8b. A seal member 39 for preventing intrusion of earth and sand is interposed between the cutter bit housing 33 and the cutter bit holder 34 and the rotating shaft 35.

A rotary jack 40 as cutter bit changing means is mounted inside the cut-spoke 32 along its longitudinal direction. The rotary jack 40 has a drive rod 41 which can be extended and contracted. One end of the connecting rod 42 is connected to the distal end of the driving rod 41 by a connecting shaft 43, and the other end of the connecting rod 42 is fixed to the rotating shaft 35. Normally, rotating jack 40
Is in a contracted state, and the first cutter bit 37 is located at a projecting position projecting from the front surface of the cutter head 31. On the other hand, the second cutter bit 38 is located at a storage position where the second cutter bit 38 is stored inside the cutter head 31 (the cut spoke 32). Then, if necessary, the drive rod 41 of the rotary jack 40 is extended, and the connecting rod 42 is extended.
By rotating the rotary shaft 35 and the cutter bit holder 34 through a predetermined angle via the, the second cutter bit 38 located at the storage position can be moved to a projecting position projecting from the front surface of the cutter head 31. In the state where the second cutter bit 38 is housed in the cut-spoke 32, a filler of a soft material such as clay (not shown) is attached to a peripheral gap portion to prevent intrusion of earth and sand. When the second cutter bit 38 is projected by the rotary jack 40, the filler is broken and can be projected.

Here, the operation of the cutter head 31 mounted on the shield excavator of this embodiment described above will be described.

When a tunnel is formed by excavation, usually, a plurality of shield jacks are extended to advance the skin plate, while the cutter head 31 is rotated by a hydraulic motor, and a plurality of cutter bits 37 (37) are used to cut the ground in front. Drill and do. In the cutter head 31 of the present embodiment, as shown in FIG.
Is in a contracted state, a large number of first cutter bits 37 fixed to the cutter bit holder 34 are exposed to the outside, and the sharp tips 37b of the first cutter bits 37 excavate the ground in front. I do. On the other hand, the second cutter bit 38 is located at a housing position where the second cutter bit 38 is housed inside the cutter head 31.

The earth and sand excavated by the first cutter bit 37 is discharged to the outside by a not-shown discharging device. On the other hand, one of the shield jacks is operated in the contraction direction to form a space between the existing segment and a new segment is attached to this space by the segment erector, and a tunnel of a predetermined length is formed by excavation. I will do it.

When the tunnel is excavated and formed for a long time by the operation of the shield excavator, the tip 37b of the first cutter bit 37 in the cutter head 31 is formed.
Will wear out. In this case, in this embodiment,
This is achieved by exposing the second cutter bit 38 instead of the worn first cutter bit 37.

That is, when the rotary jack 40 is driven from the state shown in FIG. 1 and the drive rod 41 is extended as shown in FIG. 3, the cutter bit holder 34 rotates through a predetermined angle via the connecting rod 42. Then, the second cutter bit 38 located at the accommodation position moves to the projecting position projecting from the front surface of the cutter head 31 and is exposed to the outside. Therefore,
The second cutter bit 38 projects in front of the cutter head 31 in place of the worn first cutter bit 37, and the new second cutter bit 38 enables continuous excavation of the ground in front.

When the first cutter bit 37 is worn due to long-term use, the excavation work is interrupted by projecting a new sharpened second cutter bit 38 in place of the worn first cutter bit 37. Instead, the excavator can excavate the ground as before the first cutter bit 37 is worn, and can continuously form a tunnel without lowering the excavation efficiency.

FIG. 3 is a sectional view of a main part of a cutter head mounted on a shield excavator as a tunnel excavator according to a second embodiment of the present invention, and FIG. 4 is a main part of the cutter head showing a projected state of a second cutter bit. 2 shows a partial cross section.

In the cutter head mounted on the shield excavator of the present embodiment, as shown in FIG. 4, a large number (two in the present embodiment) of the cutter spoke 52 of the cutter head 51 is arranged along its longitudinal direction. Cutter bit housing 53
Is fixed, and a disk-shaped cutter bit holder 54 is located in the cutter bit housing 53, and a rotary shaft 55 integrated with the cutter bit holder 54 is rotatably supported. On the outer peripheral portion of the cutter bit holder 54, a fan-shaped cutter bit 56 in which the first cutter bit 37 and the second cutter bit 38 of the first embodiment are integrally formed is formed over a predetermined angle range. ing. The cutter bit 56 includes a shank 56a and a tip 56b.

A rotary jack 57 as cutter bit changing means is mounted inside the cut spoke 52 along its longitudinal direction. The rotary jack 57 has a drive rod 58 which can be extended and contracted. One end of the connecting rod 59 is connected to the distal end of the driving rod 58 by a connecting shaft 60, and the other end of the connecting rod 59 is fixed to the rotating shaft 55. Usually, rotating jack 57
Of the cutter bit 5 is in a contracted state.
Reference numeral 6 denotes a clockwise rotation limit. The drive rod 58 of the rotary jack 57 is extended as necessary, and the rotary shaft 55 and the cutter bit holder 54 are rotated by a predetermined angle via the connecting rod 59. By doing so, the cutter bit 56 can be moved to the rotation limit in the counterclockwise direction.

Here, the operation of the cutter head 31 mounted on the shield excavator of the above-described embodiment will be described.

As shown in FIG. 4, at the beginning of the excavation, the driving rod 58 of the rotary jack 57 is in a contracted state, and the cutter bit 56 is located at the clockwise rotation limit together with the cutter bit holder 54. A part of the sharp tip 56b of the cutter bit 56 projecting outward from the front part of the cutter head 31 excavates the ground in front. When the tunnel is excavated and formed for a long time by the operation of the shield excavator, a part of the tip 56b of the cutter bit 56 protruding outward in the cutter head 51 is worn. In this case, when the rotary jack 57 is driven to extend the drive rod 58 as shown in FIG.
9, the cutter bit holder 54 rotates a predetermined angle in the counterclockwise direction. Then, the wear range A of the tip 56b of the cutter bit 56 at the projecting position moves to the storage position, while the sharp range B of the tip 56b of the cutter bit 56 at the storage position projects from the front surface of the cutter head 51. To the projected position, and is exposed to the outside. Therefore,
Sharp area B instead of wear area A of cutter bit 56
Projecting forward of the cutter head 51, and the new portion of the cutter bit 56 enables continuous excavation work of the ground in front.

As described above, since the cutter bit 56 is formed in a fan shape over the predetermined angle range of the cutter bit holder 54, the cutter bit 56 can be used for a long time.
Even if a part of the excavator is worn, the worn part of the cutter bit 56 can be replaced with a sharp part by rotating the cutter bit holder 54 little by little. Excavates the ground in the same manner as before wear, and can continuously form a tunnel without reducing the excavation efficiency. And. In this embodiment, the life of the cutter bit 56 can be further extended.

[0036]

As described in detail in the above embodiment, according to the cutter head of the present invention, a plurality of cutter bit holders are rotatably mounted on the front portion, and the first cutter bit is mounted on the cutter bit holder. And a second cutter bit is fixed adjacent to the first cutter bit,
By rotating the cutter bit holder by the cutter bit changing means, one of the first cutter bit and the second cutter bit is moved to the accommodation position accommodating therein, and the other is moved to the protruding position where it protrudes forward. When the first cutter bit is worn out due to long-term use, the sharp second cutter bit is protruded in place of the worn first cutter bit, so that the operation can be performed without interrupting the operation by the two cutter bits. Excavation work is performed continuously, so that a decrease in excavation efficiency can be prevented.

Further, according to the cutter head of the present invention, since the first cutter bit and the second cutter bit are continuously formed in the cutter bit holder so as to form an arc over a predetermined angle range, the cutter bit is exposed after long-term use. When the cutter bit is worn, excavating work can be performed continuously without interrupting the work by exposing the sharpened part instead of the worn part, and the cutter bit is rotated by a predetermined angle and exposed. By doing so, the life of the cutter bit can be extended.

According to the tunnel excavator of the present invention,
The tubular excavator body can be advanced by the propulsion means, the cutter head mounted on the front part of the excavator body can be driven and rotated by the cutter head driving means, and a plurality of cutter bit holders are turned around the spokes of the cutter head. It is mounted movably, the first cutter bit is fixed to the cutter bit holder, the second cutter bit is fixed adjacent to the first cutter bit, and the cutter bit holder is rotated by the cutter bit exchange means. One of the first cutter bit and the second cutter bit is moved to the accommodation position in which the first cutter bit is accommodated therein, and the other is movable to a protruding position in which the first cutter bit protrudes forward. Then, the excavation work is interrupted by projecting the sharp second cutter bit in place of the worn first cutter bit. In fact, the excavator can excavate the ground as before the first cutter bit wear, continuously excavate the tunnel without lowering the excavation efficiency, and replace the cutter bit in a narrow space as in the past. This eliminates the need and improves safety and work efficiency.

According to the tunnel excavation method of the present invention, the excavator body is advanced while driving and rotating the cutter head mounted on the front part of the excavator body, whereby the cutter head is rotated to the front part. The first cutter bit of the freely mounted cutter bit holder excavates the ground in front to form a tunnel, and when the wear of the first cutter bit is reduced and the excavating ability is reduced, the cutter bit holder is rotated. Then, the second cutter bit adjacent to the first cutter bit is projected forward from the accommodation position to the cutter head, and the second cutter bit excavates the ground to form a tunnel. It is possible to excavate the ground continuously without causing a decrease in excavation efficiency, thereby improving safety and work efficiency.

[Brief description of the drawings]

FIG. 1 is a sectional view of a main part of a cutter head mounted on a shield excavator as a tunnel excavator according to a first embodiment of the present invention.

FIG. 2 is a sectional view taken along line II-II of FIG.

FIG. 3 is a sectional view of a main part of a cutter head showing a state where a second cutter bit projects.

FIG. 4 is a sectional view of a main part of a cutter head mounted on a shield excavator as a tunnel excavator according to a second embodiment of the present invention.

FIG. 5 is a cross-sectional view of a main part of the cutter head showing a state where a second cutter bit projects.

FIG. 6 is a schematic sectional view of a general shield excavator.

FIG. 7 is a front view of a general shield excavator.

8 is a cross-sectional view (a cross section taken along line VIII-VIII of FIG. 7) of a main part of a conventional cutter head showing a mounted state of a cutter bit.

FIG. 9 is a schematic view showing a worn state of a cutter bit due to long-term use.

[Explanation of symbols]

 31, 61 Cutter head 32, 62 Cutter spoke 33, 63 First cutter bit 34, 64 First cutter bit chip 41 Opening 43, 73 Second cutter bit 45, 75 Second cutter bit chip 46, 76 Jack guide 47, 77 rotary jack (cutter bit driving means)

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Ken Murakami 1-1-1 Wadazakicho, Hyogo-ku, Hyogo-ku, Kobe City, Mitsubishi Heavy Industries, Ltd. 1-1-1 Tazakicho Mitsubishi Heavy Industries, Ltd., Kobe Shipyard

Claims (4)

[Claims] 1. A cutter head having a plurality of cutter bits mounted on a front portion of an excavator body and excavating a ground, wherein a plurality of cutter bit holders are rotatably mounted on the front portion, and each of the cutter bits is mounted on the cutter head. A first cutter bit is fixed to the bit holder, and a second cutter bit is fixed to each of the cutter bit holders adjacent to the first cutter bit, and the first bit is rotated by rotating the cutter bit holder.
A cutter bit exchanging means is provided, which is capable of moving either the cutter bit or the second cutter bit to a housing position in which the other is housed, while being capable of moving the other to a protruding position where the other protrudes forward. Cutter head. 2. The cutter head according to claim 1, wherein
The cutter head according to claim 1, wherein the first cutter bit and the second cutter bit are continuously formed in the respective cutter bit holders so as to form an arc over a predetermined angle range. 3. A cylindrical excavator body, propulsion means for advancing the excavator body, a cutter head rotatably mounted on a front portion of the excavator body, and driving and rotating the cutter head. Cutter head driving means, radial spokes fixed to the front surface of the cutter head, a plurality of cutter bit holders rotatably mounted on the spokes, and a first fixed to each of the cutter bit holders. A cutter bit; a second cutter bit fixed to each of the cutter bit holders adjacent to the first cutter bit; and a first cutter connected to the cutter bit holder and rotating the cutter bit holder. A cutter bit that moves either the bit or the second cutter bit to a storage position where the bit or the second cutter bit is stored therein, while the other bit is movable to a protruding position that protrudes forward. A tunnel excavator, comprising: 4. A cutter bit holder rotatably mounted on a front portion of the cutter head by advancing the excavator body while driving and rotating a cutter head mounted on a front portion of the excavator body. When the first cutter bit excavates the ground in front to form a tunnel, and when the wear of the first cutter bit progresses and the excavating ability is reduced, the cutter bit holder is rotated and the first cutter bit is rotated. Adjacent second
A tunnel excavation method, comprising: projecting a cutter bit from a storage position forward of the cutter head, and excavating the ground with the second cutter bit to form a tunnel.