JP5869348B2 - Auger head - Google Patents

Description

  The present invention relates to an auger head.

  Conventionally, an auger head is attached to the tip of an auger screw used for ground excavation. There are types of auger heads that are compatible with various grounds such as viscous ground, general and hard gravel, cobblestone ground, and rock, so that auger heads of various types can be attached to auger screws for excavation. It has become. For example, Patent Documents 1 and 2 disclose a double auger head for viscous ground, general gravel / cobblestone ground, and Patent Document 3 discloses a single auger head for rock.

JP 2002-129863 A JP 2004-124695 A JP 2006-022594 A

  By the way, in general, there are few cases where each board is single, and most of them are mutually layered. For example, if the viscous ground and the gravel and cobblestone ground are alternating layers and the underlying layer is rock, for example, if the auger head for rock shown in Patent Document 3 is used, the viscous soil sticks to the auger head and cannot be excavated. There is. In this case, the entire auger screw is pulled out from the ground, and the fixed earth and sand are removed from the auger head and then re-excavated. This is a factor in increasing the excavation time and cost.

  On the other hand, for example, if the viscous ground shown in Patent Documents 1 and 2 and the two-row auger head for general gravel / cobblestone are used for similar ground, the excavation performance itself is inferior for hard gravel / cobblestone or rock. End up. In addition, centering cannot be performed, and the chip of the bit may be lost due to swinging from side to side. In that case, the auger screw must be pulled out of the ground and the bit replaced or replaced with an auger head for hard gravel or cobblestone or rock, which in any case will increase the drilling time and cost. I was supposed to invite.

  For this reason, the subject of this invention is providing the auger head which can be excavated efficiently even if it is the kind of ground.

An auger head according to the invention of claim 1
A connecting shaft attached to the tip of the auger screw;
A head shaft that is disposed on the distal end side of the connection shaft and has the rotation axis of the connection shaft as a rotation center;
Holding a bit, a plurality of holders arranged in a spiral form of two or three stripes gradually spreading outward from the tip end to the base end of the head shaft ;
A blade portion formed along each spiral and provided with the holder on an outer end surface ;
Among the plurality of holders arranged in each of the two or three spirals , the innermost holder is a central holder, the outermost holder is an outermost outer holder, and the central part When a holder other than the holder and the outermost peripheral holder is a middle holder,
The central holder disposed in each of the two or three spirals is disposed on a first circle around the rotation axis,
The outermost circumferential holders arranged in each of the two or three spirals are arranged on a second circle around the rotation axis,
Any two toes arbitrarily selected from the toes of the bit held by the middle holder arranged in the two or three spirals are arranged offset from each other in the radial direction around the rotation axis. And
At least one of the center of gravity and the excavation center is arranged inside the first circle.

The invention according to claim 2 is the auger head according to claim 1,
The bits held by the holder arranged in at least one of the two or three spirals are adjacent to each other and partially overlap in the circumferential direction around the rotation axis. It is said.

The invention according to claim 3 is the auger head according to claim 1 or 2,
Each of the two Articles or Sanjo helix before Kiho Ruda is being equal arranged, n-th holder between from the tip side spaced a substantially equal angle with respect to the rotation axis, yet The leading end of the bit held by the nth holder is positioned near the base end of the bit held by the n-1 th holder or near the base end of the n-1 th holder. It is characterized by being arranged in.

The invention according to claim 4 is the auger head according to any one of claims 1 to 3,
Drilling width of the bit is better positioned on the distal end side of the head shaft, it is characterized in Tei Rukoto are wider than those located on the proximal side of the head shaft.
Invention of Claim 5 is an auger head as described in any one of Claims 1-4,
A trajectory of rotation of the toe of each of the bits held by the plurality of holders around the rotation axis is projected on the same plane parallel to the rotation axis, and the toe of each of the bits on the same plane is projected. The angle between the first approximate line from the tip side of the approximate straight line connecting the outer ends of the two and the horizontal line orthogonal to the rotation axis is α2, the second approximate line from the tip side and the horizontal line orthogonal to the rotation axis When the angle is α1, 35 ° ≦ α1 ≦ 75 ° and α2 <α1 are set.
Invention of Claim 6 is an auger head as described in any one of Claims 1-4,
A trajectory of rotation of the toe of each of the bits held by the plurality of holders around the rotation axis is projected on the same plane parallel to the rotation axis, and the toe of each of the bits on the same plane is projected. The line connecting the outer ends of the two lines is straight, and when the angle between the line and the horizontal line orthogonal to the rotation axis is α1, 35 ° ≦ α1 ≦ 75 ° is set.

The invention according to claim 7 is the auger head according to any one of claims 1 to 6 ,
The mounting angle β1 of the bit held by the middle holder and the outermost holder with respect to a horizontal line perpendicular to the rotation axis is 45 ° ≦ β1 ≦ 75 °,
In addition, the attachment angle β1 is characterized by being larger than an angle β2 with respect to the horizontal line at a portion of the blade portion where the middle stage holder and the outermost peripheral holder are provided.

The invention according to claim 8 is the auger head according to any one of claims 1 to 7 ,
Among the middle stage holder and the outermost peripheral holder, holders other than the innermost holder hold the bit so that the tip end portion of the bit is arranged outside the base end portion. It is characterized by that.

  According to the present invention, it is possible to provide an auger head that can be excavated efficiently regardless of the type of ground.

It is a front view showing a schematic structure of an auger head concerning this embodiment. It is a bottom view which shows schematic structure of the auger head which concerns on this embodiment. It is the projection figure seen from the upper part which shows the occupation range of the auger head with respect to the excavation range of the auger head which concerns on this embodiment. It is the projection figure seen from the upper part which shows the occupation range of the auger head with respect to the excavation range of the auger head (excavation head 20) shown by patent document 3. FIG. It is a front view which shows the 1st modification of the auger head which concerns on this embodiment. It is a front view which shows the 2nd modification of the auger head which concerns on this embodiment. It is a bottom view which shows schematic structure of the auger head of FIG. It is the projection view seen from the upper side which shows the occupation range of the auger head with respect to the excavation range of the auger head of FIG.

  The best mode for carrying out the present invention will be described below with reference to the drawings. However, although various technically preferable limitations for implementing the present invention are given to the embodiments described below, the scope of the invention is not limited to the following embodiments and illustrated examples.

  FIG. 1 is a front view showing a schematic configuration of an auger head according to the present embodiment. As shown in FIG. 1, the auger head 1 has a connection shaft 2 attached to a tip portion of an auger screw (not shown) and a distal end side of the connection shaft 2, and the rotation axis S <b> 1 of the connection shaft 2 is the center of rotation. A head shaft 3 is provided.

At the center of the connecting shaft 2 and the head shaft 3, a through hole 4 for supplying air or water to the front of the head shaft 3 is formed over the entire length so as to follow the rotation axis S1.
The head shaft 3 is provided with two blade portions 5 formed in a spiral shape on the outer peripheral surface of the head shaft 3 so as to gradually spread outward from the tip end to the base end portion of the head shaft 3. The outer end surface of each blade portion 5 is a continuous curved surface, and the same number of holders 7 for holding the bits 6 are provided on the outer end surface at predetermined intervals.

  The bit 6 is detachable from the holder 7. Specifically, the holder 7 has a male shape, and the bit 6 has a female shape, and both are fixed by fitting the male shape of the holder 7 into the female shape of the bit 6.

Further, when the sword claw having a vertex near the center as shown in FIG. 6 or the sword claw having a vertex inside is used as the bit 6, the biting into the ground is improved and the excavation ability is improved. In addition, in the gravel and cobblestone ground, the action of pushing out to the outside during excavation becomes higher, the amount of earth and sand discharged is reduced, and the earth and sand can be easily treated.
On the other hand, when the bit 6 is a flat type as shown in FIG. 1 or a sword tip claw having an apex on the outside, the crushing action is increased during excavation and the excavation efficiency in the rock can be increased.
These bits 6 are appropriately selected according to the ground conditions and mounted on the holder 7.

Here, among the plurality of holders 7 arranged in each of the spirals , the innermost holder 7 is a central holder 7a, the outermost holder 7 is an outermost holder 7b, The holders 7 other than the central holder 7a and the outermost peripheral holder 7b are referred to as middle stage holders 7c.

FIG. 2 is a bottom view showing a schematic configuration of the auger head 1. In FIG. 2, concentric circles about the rotation axis S <b> 1 are shown as virtual lines. As shown in FIG. 2, the center holder 7a disposed in each of the spirals is disposed on a first circle C1 centered on the rotation axis S1. Moreover, the outermost periphery holder 7b arrange | positioned at each of the said helix is arrange | positioned on the 2nd circle C2 centering on rotating shaft S1. Then, any two toes arbitrarily selected from the toes of the bit 6 held by the middle holder 7c arranged in the spiral are arranged so as to be offset from each other in the radial direction around the rotation axis S1. Yes. Then, at least one of the center of gravity of the auger head 1 itself and the excavation center is disposed inside the first circle C1. Here, the first circle C1 is the outer diameter of the excavation of the bit 6 held in the center holder 7a, but the circle C1 ′ (the bit 6 held in the center holder 7a) inside the first circle C1. It is preferable that at least one of the center of gravity of the auger head 1 itself and the center of excavation is arranged inside the excavation inner diameter of the auger head 1.

  In addition, among the middle holder 7c and the outermost peripheral holder 7b, the holders 7c and 7b other than the innermost holder 7c1 are arranged so that the tip of the bit 6 is arranged outside the base end. Holding. Thereby, since the front-end | tip part of the bit 6 is hold | maintained so that it may be offset ahead of a rotation direction, the upper clearance angle and outer periphery clearance angle of the bit 6 are securable.

In addition, among the plurality of holders 7 disposed in each of the spirals, the n-th holders 7 from the front end side have a substantially equiangular interval with respect to the rotation axis S1 (about 180 ° in the case of two strips). It is arranged in the space. Furthermore, the tip end portion of the bit 6 held by the nth holder 7 is near the base end portion of the bit 6 held by the (n-1) th holder 7 or near the base end portion of the (n-1) th holder 7. The n-th holder 7 from the front end side is disposed so as to be positioned.

A step-like virtual line L1 shown in FIG. 1 is obtained by projecting a locus accompanying rotation about the rotation axis S1 of the toe of each bit 6 on the same plane parallel to the rotation axis S1. The horizontal portion of the staircase-like virtual line L1 is the excavation width of each bit 6. As is clear from this imaginary line L1, the excavation width of the bit 6 is set wider at the tip side of the head shaft 3 than at the base end side of the head shaft 3.

Also, a trajectory associated with the rotation about the bit 6 each toe axis of rotation S1, held by a plurality of holders 7, projected on the same plane parallel to the rotation axis S1, toe of the bit, respectively, in the same plane Of the approximate straight line L2 connecting the outer ends of the locus of the head axis 3, the angle between the first approximate line from the front end side of the head shaft 3 and the horizontal line orthogonal to the rotation axis S1 is α2, and the angle between the front end side of the head shaft 3 is two. When the angle between the first approximate straight line and the horizontal line orthogonal to the rotation axis S1 is α1, 35 ° ≦ α1 ≦ 75 ° and α2 ≦ α1 are set .
When a compressive load is applied to the ground, a break line corresponding to the ground strength (internal friction angle) is generated in the ground. When α1 is set to an angle that approximates the angle formed by the break line and the horizontal line (break angle αS), it is preferable to make α1 larger than the break angle αS in order to induce the generation of a break line. Furthermore, if the ground compressive strength is high, the breaking angle αS also increases, so α1 must also be increased. Considering the breaking angle αS and practicality, α1 is preferably set to 35 ° ≦ α1 ≦ 75 ° as described above. In this case, since the main component is not the cutting main body but the crushing main body, the efficiency is high and the wear of the bit 6 can be reduced.

Further, the mounting angle β1 of the bit 6 held by the middle holder 7c and the outermost holder 7b with respect to the horizontal line is set to 45 ° ≦ β1 ≦ 75 °. The attachment angle β1 is set to a value larger than the angle β2 with respect to the horizontal line of the portion of the blade portion 5 where the middle stage holder 7c and the outermost peripheral holder 7b are provided. Since the attachment angle β1 is set to such a value, a space is formed behind the bit 6 and the holder 7 in the rotation direction. During excavation, since earth and sand flows into this space, it is possible to increase biting into the ground.
The rake angle β3 of the bit 6 is preferably set to 0 ° ≦ β3 ≦ 30 °, and the clearance angle β4 is preferably set to 10 ° ≦ β4 ≦ 30 °.

As described above, according to the present embodiment, the pair of center holders 7a are arranged on the first circle C1 with the rotation axis S1 as the center. Since the bit 6 held by the center holder 7a has a role of centering at the beginning of excavation, the center holder 7a is disposed on the first circle C1, and the auger head 1 is located inside the first circle C1. Since at least one of the center of gravity and the center of excavation is arranged, centering can be performed smoothly.
In addition, since any two toes arbitrarily selected from the toes of the bit 6 held by the middle holder 7c are arranged offset from each other in the radial direction around the rotation axis S1, during excavation, Since all the bits 6 held by the middle holder 7c always exhibit the excavation capability, the excavation efficiency can be improved. Further, the burden on one bit 6 can be reduced, and damage to the bit 6 and the middle stage holder 7c can be suppressed.

  Further, the bit 6 held by the outermost peripheral holder 7b and the bit 6 held by the middle stage holder 7c inside one of the outermost peripheral holders 7b are arranged on different circles. Although the rotation axis S1 may be displaced with respect to the excavation center, since the outermost peripheral holders 7b are arranged on the second circle C2, the bit 6 held by the outermost peripheral holder 7b is removed from the auger head 1 during excavation. As a result, the above-described deviation can be prevented.

As described above, in the auger head 1 of the present embodiment, the excavation efficiency is greatly increased as compared with the conventional one, and the centering performance is also improved, so that any kind of ground can be excavated efficiently. Is possible.
In hard gravel and cobblestone ground including viscous ground, the excavation capacity is significantly improved by 1.5 times compared to the conventional auger head, while the pressure input is reduced to 2/3 or less and the torque to 2/3. Therefore, it is possible to perform high-precision construction even on the ground with high top edge construction and ground with low ground reaction force. Further, even if an auger having a low pressure input or an auger apparatus having a low auger rotational torque is used, the ground excavation one rank higher is possible if the auger head 1 of this embodiment is used. In addition, the environmental load factor can be reduced to 2/3 or less of the conventional one, and an energy saving effect is also expected.
In rock excavation, the pressure input and torque are the same while drastically improving the excavation capability by about three times compared to the conventional three-head. Even if excavation is possible with the conventional auger head, it is profitable. Even in the ground that has adopted down-the-hole, if the auger head 1 with greatly improved excavation capability is used, profitable construction is possible. In addition, noise and vibration are lower than down-the-hole, and because it is used in combination with a press-fitting and pulling machine that uses a pile that has already been press-fitted as a reaction force, it can also prevent falls.

FIG. 3 is a projection view seen from above showing the occupation range of the auger head 1 with respect to the excavation range of the auger head 1. In FIG. 3, a circle Q <b> 1 indicates the excavation range of the auger head 1, and a hatched portion Q <b> 2 indicates the occupation range of the auger head 1. Moreover, the blank part in the circle Q1 becomes a passage space for earth and sand. The flow of earth and sand when the auger head 1 is pulled out is indicated by an arrow Y1.
On the other hand, FIG. 4 is a projected view showing the occupation range of the auger head with respect to the excavation range of the auger head (excavation head 20) shown in Patent Document 3. In FIG. 4, a circle P1 indicates the excavation range of the auger head, and a hatched portion P2 indicates the occupation range of the auger head. Further, the blank portion in the circle P1 is a passage space for earth and sand, and the flow of earth and sand when the auger head is pulled out is indicated by an arrow Y2.

As shown in FIG. 4, in the case of the auger head of Patent Document 3, the occupation ratio of the auger head is approximately 68%. When the auger head is pulled out of the ground, there is only about 32% of the space for the earth and sand to pass down, and the earth and sand flow is in one direction. For this reason, the resistance acting on the auger head at the time of pulling is increased, which causes an increase in pulling time and deformation of the blade portion.
However, in the auger head 1 of the present embodiment, the occupation ratio of the auger head 1 is approximately 61%. When the auger head is pulled out from the ground, the space through which the earth and sand are pulled out is about 39%, which is 1.2 times or more compared with the case of FIG. In addition, since the earth and sand flow is in two directions, the earth and sand flow also becomes faster, and as a result, the pulling time of the auger head 1 can be shortened.
In FIG. 4, the rotation axis and the center of gravity of the auger head are greatly deviated (see Z <b> 1 in FIG. 4). However, in FIG. 3, since the centering performance of the auger head 1 is enhanced, the rotation axis S <b> 1. It can be seen that the deviation from the center of gravity is greatly reduced (see Z2 in FIG. 3). When the deviation is reduced in this way, hole bending is reduced, and smooth high-efficiency excavation with reduced vibration noise is possible.

Further, when the trajectory accompanying the rotation about the rotation axis S1 of the toe of each bit 6 is projected on the same plane parallel to the rotation axis S1, it is arranged in a step shape as shown in FIG. Also, the centering performance can be improved.
Further, since the excavation width of the bit 6 is set wider at the tip end side of the head shaft 3 than that at the base end side of the head shaft 3, the excavation width at a short excavation peripheral length is set. The excavation width is wide and the excavation circumference is long. Therefore, the burden at the time of cutting becomes uniform as a whole, and not only the excavation efficiency is increased, but also the wear of the bit 6 can be suppressed.

Note that the present invention is not limited to the above embodiment, and can be modified as appropriate. In the following description, the same parts as those in the above embodiment are denoted by the same reference numerals and the description thereof is omitted.
For example, in the above embodiment, the case where the outer end surface of the blade portion 5 is a continuous curved surface has been described as an example. However, the outer end surface of the blade portion 5a is formed stepwise as in the auger head 1A shown in FIG. May be. In this case, each of the bits 6 held by the holder 7 on the lower stage and the angle α3 between the approximate straight line L3 of the line connecting the outer ends of the locus of the tip of each of the bits 6 held by the upper holder 7 and the horizontal line. The angle α4 between the approximate straight line L4 of the line connecting the outer ends of the locus of the tip of the toe and the horizontal line may be the same or different.
In the case of FIG. 5, the outer end surface of the blade portion 5 a is formed in a stepwise manner in two upper and lower stages, but the outer end surface can also be formed in three or more steps. It is.

Further, as the auger head 1B shown in FIGS. 6 and 7, among the spiral, the center bit 6 held by the holder 7 arranged in a spiral of at least Ichijo, adjacent, the rotation axis S1 You may arrange | position so that a part may overlap in the performed circumferential direction . Specifically, with respect to the bit 6 held by the holder 7 on the distal end side of the head shaft 3, the distal end portion of the bit 6 held by the holder 7 adjacent on the proximal end side is a periphery around the rotation axis S1. The holder 7 of the single blade portion 5c is arranged so as to overlap in the direction . When the adjacent bits 6 are overlapped in the circumferential direction in this way, the gravel and cobblestone pushed out by the lower bit 6 is pushed further outward by the upper bit 6, so that the gravel and cobblestone in the ground Will move smoothly. Thereby, the uptake | capture of the earth and sand in the auger head 1 is restrict | limited, and since it compacts outside, the circumference of the already press-fitted pile can be compacted and the reaction force at the time of press-fitting can be increased. Therefore, a large press-fit load can be applied, and the excavation speed can be improved. Moreover, since the gravel and cobblestone are consolidated, the amount of soil discharged can be reduced, and the soil removal processing can be simplified.
When the adjacent bits 6 are arranged so as to overlap each other in the circumferential direction, the auger head 1 itself can have a compact overall length. If the auger head 1 is made compact, the auger head 1 can be replaced while the entire auger apparatus is held by the press-fitting and drawing machine. In addition, since the auger head 1 itself is also light, it is possible to improve the workability of replacement.

  Further, as shown in FIG. 7, a straight line L6 that is 5 ° on the front side in the rotation direction around the rotation axis S1 with respect to the straight line L5 that connects the toes of the bit 6 held by the pair of center portion holders 7a, A straight line L7 of 85 ° on the rear side is set. Within the range formed by the straight line L6 and the straight line L7, the toe position of the bit 6 held by the middle stage holder 7c adjacent to the center holder 7a is arranged. With this arrangement, the flow of earth and sand at the toe of the bit 6 is improved, and earth and sand consolidation can be suppressed.

FIG. 8 is a projection view seen from above showing the occupation range of the auger head 1B with respect to the excavation range of the auger head 1B. In FIG. 8, a circle Q3 indicates the excavation range of the auger head 1B, and a hatched portion Q4 indicates the occupation range of the auger head 1B. Moreover, the blank part in the circle Q3 becomes a passage space for earth and sand. The flow of earth and sand when pulling out the auger head 1B is indicated by an arrow Y3.
In the case of the auger head 1B, the occupation ratio of the auger head 1B is approximately 45%. When the auger head 1B is pulled out from the ground, the space through which the earth and sand pass is approximately 39%, which is 1.7 times or more compared to the case of FIG. In addition, since the earth and sand flow is in two directions, the earth and sand flow also becomes faster, and the drawing time of the auger head 1B can be greatly shortened.
Further, in FIG. 8, since the centering performance of the auger head 1B is enhanced, it can be seen that the deviation between the rotation axis S1 and the center of gravity is significantly reduced as compared with the case of FIG. 4 (in FIG. 8). , Z3).

Moreover, although the said embodiment demonstrated and demonstrated the case where the said spiral was provided with two strips , the said spiral may be provided with three strips . Here, among the plurality of holders 7 disposed in each of the three spirals, the n-th holders 7 from the front end side are spaced at substantially equal angles with respect to the rotation axis S1 (about 120 ° in the case of the three stripes). ).

DESCRIPTION OF SYMBOLS 1 Auger head 2 Connection axis | shaft 3 Head axis | shaft 4 Through-hole 5 Blade | wing part 6 Bit 7 Holder 7a Center part holder 7b Outermost periphery holder 7c Middle stage holder C1 1st circle C2 2nd circle S1 Rotating shaft

Claims (8)

A connecting shaft attached to the tip of the auger screw;
A head shaft that is disposed on the distal end side of the connection shaft and has the rotation axis of the connection shaft as a rotation center;
Holding a bit, a plurality of holders arranged in a spiral form of two or three stripes gradually spreading outward from the tip end to the base end of the head shaft ;
A blade portion formed along each spiral and provided with the holder on an outer end surface ;
Among the plurality of holders arranged in each of the two or three spirals , the innermost holder is a central holder, the outermost holder is an outermost outer holder, and the central part When a holder other than the holder and the outermost peripheral holder is a middle holder,
The central holder disposed in each of the two or three spirals is disposed on a first circle around the rotation axis,
The outermost circumferential holders arranged in each of the two or three spirals are arranged on a second circle around the rotation axis,
Any two toes arbitrarily selected from the toes of the bit held by the middle holder arranged in the two or three spirals are arranged offset from each other in the radial direction around the rotation axis. And
An auger head, wherein at least one of the center of gravity and the excavation center is disposed inside the first circle. The bits held by the holder arranged in at least one of the two or three spirals are adjacent to each other and partially overlap in the circumferential direction around the rotation axis. The auger head according to claim 1. Each of the two Articles or Sanjo helix before Kiho Ruda is being equal arranged, n-th holder between from the tip side spaced a substantially equal angle with respect to the rotation axis, yet The leading end of the bit held by the nth holder is positioned near the base end of the bit held by the n-1 th holder or near the base end of the n-1 th holder. The auger head according to claim 1, wherein the auger head is disposed on the auger head. Drilling width of the bit is better positioned on the distal end side of the head shaft, any one of the preceding claims, characterized in Tei Rukoto are wider than those located on the proximal side of the head shaft The auger head according to the item. Wherein the plurality of the trajectory due to rotation about the axis of rotation of the toe of each of the bits held by a holder, is projected on the same plane parallel to the rotation axis, of the toe of the bit, respectively, in the same plane trajectory Of the approximate straight line connecting the outer ends of the head axis, the angle between the first approximate straight line from the front end side of the head axis and the horizontal line orthogonal to the rotation axis is α2 , and the second approximate straight line from the front end side of the head axis wherein when an angle between a horizontal line perpendicular to the rotation axis and α1, 35 ° ≦ α1 ≦ 75 °, and, [alpha] 2 <any one of claims 1 to 4, characterized in that it is set to [alpha] 1 and Auger head as described in. Wherein the plurality of the trajectory due to rotation about the axis of rotation of the toe of each of the bits held by a holder, is projected on the same plane parallel to the rotation axis, of the toe of the bit, respectively, in the same plane trajectory A line connecting the outer ends of each of the lines is straight, and when an angle between the line and a horizontal line orthogonal to the rotation axis is α1, 35 ° ≦ α1 ≦ 75 ° is set. Item 5. The auger head according to any one of Items 1 to 4 . The mounting angle β1 of the bit held by the middle holder and the outermost holder with respect to a horizontal line perpendicular to the rotation axis is 45 ° ≦ β1 ≦ 75 °,
Yet the mounting angle β1 is claimed in any one of claims 1 to 6, wherein the greater than an angle β2 with respect to the horizontal line of the in the wing portions middle holder and the outermost periphery holder is provided moiety Auger head. Among the middle stage holder and the outermost peripheral holder, holders other than the innermost holder hold the bit so that the tip end portion of the bit is arranged outside the base end portion. The auger head according to any one of claims 1 to 7 , wherein

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