Patent Publication Number: US-2022228445-A1

Title: Auger bit

Description:
TECHNICAL FIELD 
     The present invention relates to an auger bit for an auger head. 
     BACKGROUND ART 
     In an existing pile working method using a pile driver, for example, an auger head is connected to a drive mechanism, a foundation hole for a building, a construction hole for a continuous underground wall, or the like is excavated in the ground, and a cutting part for excavation is provided to an auger bit provided at a leading end of the auger head. This is substantially same for a steel pipe inner excavation working method and the like. 
     For example, Patent Literature 1 discloses a removable earth auger bit including a bit body, a bit holder, and a coupling member, in which the bit body includes an excavation head including an excavation blade made of a superhard alloy, and a shank part provided continuously with the excavation head, the shank part includes a middle constriction part and circular parts and has a cross-section of a substantially 8-figure shape, a semi-opened retaining recess is formed at a substantially middle site in a longitudinal direction, the bit holder has a receiving hole that is drilled through an end face of the bit holder and into which the shank part is inserted, and also has an attachment hole drilled through each of upper and lower surfaces of the bit holder, intersecting the receiving hole, and having a hole shape complemented by the retaining recess of the shank part, and a middle part of the coupling member is positioned in the retaining recess of the shank part when the coupling member is inserted into the attachment hole of the bit holder. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: Japanese Patent Laid-open No. 2002-339680 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     However, Patent Literature 1 merely discloses a removable earth auger bit and an earth auger that have an improved removable structure. 
     Conventionally, there have been no auger-head auger bit that has a tapered shape and an R shape curved at a predetermined curvature to achieve improved excavation efficiency, nor auger-head auger bit that has a larger bit width to avoid loss and improve abrasion resistance, improves the excavation efficiency while ensuring an excavation diameter, and further improves the excavation efficiency by preventing interference with a guide by employing an R shape. 
     The present invention is achieved in view of such a problem and intended to provide an auger-head auger bit having improved excavation efficiency. 
     Solution to Problem 
     To solve the above-described problem, an auger bit according to a first aspect of the present invention is an auger bit mounted on an auger head, the auger bit including: a base part having a leading end protruding in an axial direction of the auger head; and two leg parts continuously extending from the base part and facing each other at a predetermined interval, the base part includes a plane part parallel to and continuous with planes of the leg parts, a first tilted part that is tilted from the plane part toward a side closer to the leading end in the axial direction of the auger head, a second tilted part and a third tilted part that are continuous with the first tilted part, a first side surface part continuous with the second tilted part, a second side surface part continuous with the third tilted part, and a fourth tilted part that is tilted from the first to third tilted parts toward a side closer to the leg parts in the axial direction of the auger head, a superhard chip is disposed across parts of the first side surface part and the fourth tilted part at a lower end part where the second tilted part is continuous with the first side surface part and the fourth tilted part, a superhard chip is disposed across a part of the fourth tilted part at a lower substantially central part of the second tilted part, a superhard chip is disposed across the first to third tilted parts, the second side surface part, and the fourth tilted part at a lower end part where the first to third tilted parts are continuous with each other, the second side surface part, and the fourth tilted part, and the plane part and the leg parts each have a shape curved at a predetermined curvature relative to the axial direction of the auger head. 
     An auger bit according to a second aspect of the present invention is the auger bit according to the first aspect in which the base part has a wide shape of a width of 85 mm to 105 mm in a width direction of the plane part of the auger head on the side closer to the leg parts. 
     An auger bit according to a third aspect of the present invention is an auger bit mounted on an auger head, the auger bit including: a base part having a leading end protruding in an axial direction of the auger head; and two leg parts continuously extending from the base part and facing each other at a predetermined interval, the base part includes a plane part continuous with planes of the leg parts, a first tilted part that is tilted from the plane part toward a side closer to the leading end in the axial direction of the auger head, a side surface part continuous with the first tilted part, and a second tilted part tilted from the first tilted part toward the side closer to the leg parts in the axial direction of the auger head, the plane part of the base part has a tapered elongated shape of a width that gradually decreases toward the side closer to the leading end so that a length of a side along which the plane part is continuous with the first tilted part and the second tilted part on the side closer to the leading end is smaller than a length of a side along which the plane part is continuous with the one leg part on the side closer to the leg parts, and the plane part and the leg parts each have a shape curved at a predetermined curvature relative to the axial direction of the auger head. 
     Advantageous Effects of Invention 
     The present invention can provide an auger bit having improved excavation efficiency for a casing auger. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view of an auger bit according to a first embodiment of the present invention. 
         FIG. 2  is a configuration diagram of the auger bit according to the first embodiment of the present invention. 
         FIG. 3  is a perspective view of an auger bit according to a second embodiment of the present invention. 
         FIG. 4  is a configuration diagram of the auger bit according to the second embodiment of the present invention. 
         FIG. 5  is a perspective view of an auger bit according to a third embodiment of the present invention. 
         FIG. 6  is a configuration diagram of the auger bit according to the third embodiment of the present invention. 
         FIG. 7  is a perspective view of an auger bit according to a fourth embodiment of the present invention. 
         FIG. 8  is a configuration diagram of the auger bit according to the fourth embodiment of the present invention. 
         FIG. 9  is a perspective view of an auger bit according to a fifth embodiment of the present invention. 
         FIG. 10  is a configuration diagram of the auger bit according to the fifth embodiment of the present invention. 
         FIG. 11  is a perspective view of an auger bit according to a sixth embodiment of the present invention. 
         FIG. 12  is a configuration diagram of the auger bit according to the sixth embodiment of the present invention. 
         FIG. 13  is a diagram illustrating a status in which an auger bit is attached to an auger head. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Configurations and effects of auger bits for a casing auger according to first to sixth embodiments of the present invention will be described below in detail with reference to the accompanying drawings. 
     First Embodiment 
     The configuration of an auger bit according to a first embodiment of the present invention will be described below with reference to  FIGS. 1, 2 ( a ), and  2 ( b ). More specifically,  FIG. 1  is a perspective view of the auger bit,  FIG. 2( a )  is a plan view thereof, and  FIG. 2( b )  is a side view thereof. 
     As illustrated in these diagrams, an auger bit  1  includes a base part  2 , and two leg parts  3  and  4  extending from the base part  2 . In the first embodiment, a side closer to the base part  2  is also referred to as a leading end of the auger bit  1 , and a side closer to the leg parts  3  and  4  is also referred to as an auger-head mounting side or a rear end of the auger bit  1 . 
     The base part  2  has what is called a chevron shape having a top part protruding toward the leading end side of the auger bit  1  in an axial direction of an auger head. More specifically, a plane part  2   a  of the base part  2  of the auger bit  1  is parallel to planes of the leg parts  3  and  4  and continuous with three tilted parts  2   b ,  2   c , and  2   d  that are tilted toward the leading end side in the axial direction of the auger head. 
     In this example, a left end of the tilted part  2   b  when viewed from the leading end side in the axial direction of the auger head is continuous with a side surface part  2   e , and a right end of the tilted part  2   d  when viewed from the leading end side in the axial direction of the auger head is continuous with a side surface part  2   f.    
     The tilted parts  2   b ,  2   c , and  2   d  of the base part  2  are continuous in the stated order in a lateral direction when viewed from the leading end side in the axial direction of the auger head, in other words, in a circumferential direction of the auger head. In this example, the tilted part  2   b  is tilted relative to the tilted part  2   c  in the circumferential direction of the auger head, and the tilted part  2   d  is tilted relative to the tilted part  2   c  in the circumferential direction of the auger head (direction opposite to the direction in which the tilted part  2   c  is tilted) of the tilted part  2   b.    
     Lower ends of the tilted parts  2   b ,  2   c , and  2   d  of the base part  2  are continuous with a tilted part  2   g . The tilted part  2   g  is tilted toward the rear end side in the axial direction of the auger head, in other words, the auger-head mounting side of the auger bit  1  at 15° relative to a plane parallel to the plane part  2   a . Note that, in this example, a hard building-up 6 is disposed at the base part  2 . 
     The leg parts  3  and  4  extend with a predetermined interval therebetween in a U shape from the base part  2  toward the rear end side in the axial direction of the auger head. The leg parts  3  and  4  are provided with four holes  3   a  and four holes  4   a , respectively, for mounting the auger bit  1  on the auger head with bolts or the like. 
     Superhard chips  5   a ,  5   b , and  5   c  are disposed at the tilted parts  2   b ,  2   c , and  2   d  of the base part  2  of the auger bit  1 . More specifically, the superhard chip  5   a  is disposed also across a part of the side surface part  2   e  and the tilted part  2   g  at a lower end part where the tilted part  2   b  is continuous with the side surface part  2   e  and the tilted part  2   g . The superhard chip  5   b  is disposed across parts of the tilted parts  2   b ,  2   c ,  2   d , and  2   g  at a lower end part where the tilted parts  2   b ,  2   c , and  2   d  are continuous with each other. The superhard chip  5   c  is disposed also across parts of the side surface part  2   f  and the tilted part  2   g  at a lower end part where the tilted part  2   d  is continuous with the side surface part  2   f  and the tilted part  2   g.    
     With the superhard chips  5   a  to  5   c , abrasion outside a contact surface can be prevented. Moreover, the materials and hardness of a plurality of superhard chips can be freely combined as appropriate in accordance with soil or an obstacle as an excavation target. For example, in the auger bit  1  according to the first embodiment, the superhard chips can be optionally selected from among superhard chips of five materials at maximum and used in combination so that loss and the like at excavation of a wide variety of excavation targets can be effectively prevented. 
     The base part  2  of the auger bit  1  has a shape bilaterally symmetric with respect to a target axis that is a line segment passing through the center of the plane part  2   a  in the axial direction of the auger head. In this manner, protection performance is increased with the bilaterally symmetric shape as well as a configuration in which respective surfaces of the tilted parts  2   b  to  2   d  and  2   g  are tilted at angles and superhard bits are arrayed at predetermined intervals as described above. 
     As for materials, parent materials of the leg parts  3  and  4  may be SCM440 (chromium/molybdenum steel) or the like, and the materials of the superhard chips  5   a  to  5   c  may be E3 (material name: MG30), E4 (material name: MG40), E5 (material name: MG50), E6 (material name: MG60), or the like among the JIS usage classification symbols or may be G4 (CIS material symbol: VC-40), G5 (CIS material symbol: VC-50), or the like in the CIS standard. 
     As described above, according to the first embodiment of the present invention, since the three divided superhard chips are provided, cutting edges of the auger bit are less likely to suffer loss and abrasion, thereby achieving improved excavation efficiency. Moreover, the auger bit needs to be less frequently replaced because not all superhard chips become unusable due to loss at one place unlike a configuration in which the superhard chips are integrated. 
     Second Embodiment 
     The configuration of an auger bit according to a second embodiment of the present invention will be described below with reference to  FIGS. 3, 4 ( a ), and  4 ( b ). More specifically,  FIG. 3  is a perspective view of the auger bit,  FIG. 4( a )  is a plan view thereof, and  FIG. 4( b )  is a side view thereof. 
     As illustrated in these diagrams, an auger bit  11  includes a base part  12 , and two leg parts  13  and  14  extending from the base part  12 . In the second embodiment, a side closer to the base part  12  is also referred to as a leading end of the auger bit  11 , and a side closer to the leg parts  13  and  14  is also referred to as an auger-head mounting side or a rear end of the auger bit  11 . 
     The base part  12  has what is called a chevron shape having a top part protruding toward the leading end side of the auger bit  11  in an axial direction of the auger head. More specifically, a plane part  12   a  of the base part  12  of the auger bit  11  is parallel to planes of the leg parts  13  and  14  and continuous with three tilted parts  12   b ,  12   c , and  12   d  that are tilted toward the leading end side in the axial direction of the auger head. 
     In this example, a left end of the tilted part  12   b  when viewed from the leading end side in the axial direction of the auger head is continuous with a side surface part  12   e , and a right end of the tilted part  12   d  when viewed from the leading end side in the axial direction of the auger head is continuous with a side surface part  12   f.    
     The tilted parts  12   b ,  12   c , and  12   d  of the base part  12  are continuous in the stated order in a lateral direction when viewed from the leading end side in the axial direction of the auger head, in other words, in a circumferential direction of the auger head. In this example, the tilted part  12   b  is tilted relative to the tilted part  12   c  in the circumferential direction of the auger head, and the tilted part  12   d  is tilted relative to the tilted part  12   c  in the circumferential direction of the auger head (direction opposite to a direction in which the tilted part  12   b  is tilted) of the tilted part  12   b.    
     Lower ends of the tilted parts  12   b ,  12   c , and  12   d  of the base part  12  are continuous with a tilted part  12   g . The tilted part  12   g  is tilted toward the rear end side in the axial direction of the auger head, in other words, the auger-head mounting side of the auger bit  11  at 15° relative to a plane parallel to the plane part  12   a . Note that, in this example, a hard building-up 16 is disposed at the base part  12 . 
     The leg parts  13  and  14  extend with a predetermined interval therebetween in a U shape from the base part  12  toward the rear end side in the axial direction of the auger head. The leg parts  13  and  14  are provided with four holes  13   a  and four holes  14   a , respectively, for mounting the auger bit  11  on the auger head with bolts or the like. 
     One superhard chip  15  is disposed at lower parts of the tilted parts  12   b ,  12   c , and  12   d  of the base part  12  of the auger bit  11 . With the superhard chip  15 , abrasion outside a contact surface can be prevented. Moreover, the material and hardness of the superhard chip can be freely combined as appropriate in accordance with soil or an obstacle as an excavation target. 
     As for the material of each component, parent materials of the leg parts  13  and  14  may be SCM440 (chromium/molybdenum steel) or the like. The material of the superhard chip  15  may be E3 (material name: MG30), E4 (material name: MG40), E5 (material name: MG50), E6 (material name: MG60), or the like among the JIS usage classification symbols or may be G4 (CIS material symbol: VC-40), G5 (CIS material symbol: VC-50), or the like in the CIS standard. 
     The plane part  12   a  of the base part  12  and the leg parts  13  and  14  of the auger bit  11  each have an R shape curved relative to the axial direction of the auger head at a predetermined curvature in accordance with the diameter of the auger head. In this example, it is designed that leading ends of cutting edges are positioned on a circumference of ϕ850 mm, and the R shape has a curvature of R500 (inner periphery R395 (wide R shape)). With such a configuration, efficiency of excavation at the outer periphery of the auger head can be improved when a holder dedicated for the auger head is used together. 
     As described above, according to the second embodiment of the present invention, since the plane part and the like each have an R shape curved at a predetermined curvature in accordance with the diameter of the auger head, it is possible to prevent interference with a guide casing and thus reduce loss of cutting edges due to collision with the guide casing. In addition, with the R shape, the cutting edges are positioned not at middle positions but on the outer side so that excavation is performed further on the outer side than when the cutting edges are positioned at the middle positions, which enables not only securement of an excavation diameter but also size increase thereof. Moreover, with the R shape, it is possible to reduce a load at excavation, thereby achieving improved excavation efficiency. 
     Third Embodiment 
     The configuration of an auger bit according to a third embodiment of the present invention will be described below with reference to  FIGS. 5, 6 ( a ), and  6 ( b ). More specifically,  FIG. 5  is a perspective view of the auger bit,  FIG. 6( a )  is a side view thereof, and  FIG. 6( b )  is a bottom view thereof. 
     As illustrated in these diagrams, an auger bit  21  includes a base part  22 , and two leg parts  23  and  24  extending from the base part  22 . In the second embodiment, a side closer to the base part  22  is also referred to as a leading end of the auger bit  21 , and a side closer to the leg parts  23  and  24  is also referred to as an auger-head mounting side or a rear end of the auger bit  21 . 
     The base part  22  has what is called a chevron shape having a top part protruding toward the leading end side of the auger bit  21  in an axial direction of the auger head. More specifically, a plane part  22   a  of the base part  22  of the auger bit  21  is parallel to planes of the leg parts  23  and  24  and continuous with three tilted parts  22   b ,  22   c , and  22   d  that are tilted toward the leading end side in the axial direction of the auger head. 
     In this example, a left end of the tilted part  22   b  when viewed from the leading end side in the axial direction of the auger head is continuous with a side surface part  22   e , and a right end of the tilted part  22   d  when viewed from the leading end side in the axial direction of the auger head is continuous with a side surface part  22   f.    
     The tilted parts  22   b ,  22   c , and  22   d  of the base part  22  are continuous in the stated order in a lateral direction when viewed from the leading end side in the axial direction of the auger head, in other words, in a circumferential direction of the auger head. In this example, the tilted part  22   b  is tilted relative to the tilted part  22   c  in the circumferential direction of the auger head, and the tilted part  22   d  is tilted relative to the tilted part  22   c  in the circumferential direction of the auger head (direction opposite to a direction in which the tilted part  22   b  is tilted) of the tilted part  22   b.    
     Lower ends of the tilted parts  22   b ,  22   c , and  22   d  of the base part  22  are continuous with a tilted part  22   g . The tilted part  22   g  is tilted toward the rear end side in the axial direction of the auger head, in other words, the auger-head mounting side of the auger bit  21  at 15° relative to a plane parallel to the plane part  22   a . Note that, in this example, a hard building-up 26 is disposed at the base part  22 . 
     The leg parts  23  and  24  extend with a predetermined interval therebetween in a U shape from the base part  22  toward the rear end side in the axial direction of the auger head. The leg parts  23  and  24  are provided with four holes  23   a  and four holes  24   a , respectively, for mounting the auger bit  21  on the auger head with bolts or the like. 
     One superhard chip  25  is disposed at lower parts of the tilted parts  22   b ,  22   c , and  22   d  of the base part  22  of the auger bit  21 . With the superhard chip  25 , abrasion outside a contact surface can be prevented. Moreover, the materials and hardness of a plurality of superhard chips can be freely combined as appropriate in accordance with soil or an obstacle as an excavation target. 
     As for the material of each component, parent materials of the leg parts  23  and  24  may be SCM440 (chromium/molybdenum steel) or the like. The material of the superhard chip  25  may be E3 (material name: MG30), E4 (material name: MG40), E5 (material name: MG50), E6 (material name: MG60), or the like among the JIS usage classification symbols or may be G4 (CIS material symbol: VC-40), G5 (CIS material symbol: VC-50), or the like in the CIS standard. 
     The plane part  22   a  of the base part  22  and the leg parts  23  and  24  of the auger bit  21  have cutting edges wider than those of a typical auger bit. Specifically, a width of the plane part  22   a  in a direction orthogonal to the axial direction of the auger head, in other words, in the circumferential direction of the auger head is larger than that of a typical auger bit. 
     More specifically, the width of the plane part  22   a  in the direction orthogonal to the axial direction of the auger head is larger than that of a typical auger bit. For example, it is typical that, in a typical auger bit for a casing auger, a cutting edge width is equal to a holder width like the cutting edge width is 60 mm for a holder of 60 mm width and is 80 mm for a holder of 80 mm width, but the auger bit according to the present embodiment has such a wide shape that the cutting edge width is 85 mm for a holder of 60 mm width and is 105 mm for a holder of 80 mm width, and thus an already mounted holder can be used for the auger bit. 
     Conventionally, to increase the width of the auger head, it has been needed to increase the sizes of holders or weld auger bits for an auger of 20 to 25 mm width, but with the above-described configuration, auger bits can be used only by mounting the auger bits on existing holders. 
     As described above, according to the third embodiment of the present invention, loss and abrasion of auger bits can be prevented with the wide shape. Moreover, auger bits can be mounted by utilizing existing holders of the auger head. 
     Fourth Embodiment 
     The configuration of an auger bit according to a fourth embodiment of the present invention will be described below with reference to  FIGS. 7, 8 ( a ), and  8 ( b ). More specifically,  FIG. 7  is a perspective view of the auger bit,  FIG. 8( a )  is a plan view thereof, and  FIG. 8( b )  is a side view thereof. 
     As illustrated in these diagrams, an auger bit  31  includes a base part  32 , and two leg parts  33  and  34  extending from the base part  32 . In the fourth embodiment, a side closer to the base part  32  is also referred to as a leading end of the auger bit  31 , and a side closer to the leg parts  33  and  34  is also referred to as an auger-head mounting side or a rear end of the auger bit  31 . 
     The base part  32  has what is called a chevron shape having a top part protruding toward the leading end side of the auger bit  31  in an axial direction of the auger head. More specifically, a plane part  32   a  of the base part  32  of the auger bit  31  is parallel to planes of the leg parts  33  and  34  and continuous with three tilted parts  32   b ,  32   c , and  32   d  that are tilted toward the leading end side in the axial direction of the auger head. 
     In this example, a left end of the tilted part  32   b  when viewed from the leading end side in the axial direction of the auger head is continuous with a side surface part  32   e , and a right end of the tilted part  32   d  when viewed from the leading end side in the axial direction of the auger head is continuous with a side surface part  32   f.    
     The tilted parts  32   b ,  32   c , and  32   d  of the base part  32  are continuous in the stated order in a lateral direction when viewed from the leading end side in the axial direction of the auger head, in other words, in a circumferential direction of the auger head. In this example, the tilted part  32   b  is tilted relative to the tilted part  32   c  in the circumferential direction of the auger head, and the tilted part  32   d  is tilted relative to the tilted part  32   c  in the circumferential direction of the auger head (direction opposite to a direction in which the tilted part  32   b  is tilted) of the tilted part  32   b.    
     Lower ends of the tilted parts  32   b ,  32   c , and  32   d  of the base part  32  are continuous with a tilted part  32   g . The tilted part  32   g  is tilted toward the rear end side in the axial direction of the auger head, in other words, the auger-head mounting side of the auger bit  31  at 15° relative to a plane parallel to the plane part  32   a . Note that, in this example, a hard building-up 36 is disposed at the base part  32 . 
     The leg parts  33  and  34  extend with a predetermined interval therebetween in a U shape from the base part  32  toward the rear end side in the axial direction of the auger head. The leg parts  33  and  34  are provided with four holes  33   a  and four holes  34   a , respectively, for mounting the auger bit  31  to the auger head with bolts or the like. 
     Superhard chips  35   a ,  35   b , and  35   c  are disposed at the tilted parts  32   b ,  32   c , and  32   d  of the base part  32  of the auger bit  31 . More specifically, the superhard chip  35   a  is disposed also across parts of the side surface part  32   e  and the tilted part  32   g  at a lower end part where the tilted part  32   b  is continuous with the side surface part  32   e  and the tilted part  32   g . The superhard chip  35   b  is disposed also across a part of the tilted part  32   g  at a lower substantially central part of the tilted part  2   b . The superhard chip  35   c  is disposed across the tilted parts  32   b ,  32   c , and  32   d , the side surface part  32   f , and the tilted part  32   g  at a lower end part where the tilted parts  32   b ,  32   c , and  32   d , the side surface part  32   f , and the tilted part  32   g  are continuous with each other. 
     With the superhard chips  35   a  to  35   c , abrasion outside a contact surface can be prevented. Moreover, the materials and hardness of a plurality of superhard chips can be freely combined as appropriate in accordance with soil or an obstacle as an excavation target. For example, in the auger bit  31  according to the present embodiment, the superhard chips can be optionally selected from among superhard chips of five materials at maximum and used in combination so that loss and the like at excavation of a wide variety of excavation targets can be effectively prevented. 
     As for materials, parent materials of the leg parts  33  and  34  may be SCM440 (chromium/molybdenum steel) or the like. The materials of the superhard chips  35   a  to  35   c  may be E3 (material name: MG30), E4 (material name: MG40), E5 (material name: MG50), E6 (material name: MG60), or the like among the JIS usage classification symbols or may be G4 (CIS material symbol: VC-40), G5 (CIS material symbol: VC-50), or the like in the CIS standard. 
     The plane part  32   a  of the base part  32  and the leg parts  33  and  34  of the auger bit  31  each have an R shape curved relative to the axial direction of the auger head at a predetermined curvature in accordance with the diameter of the auger head. In the present example, it is designed that leading ends of cutting edges are positioned on a circumference of ϕ850 mm, and the R shape has a curvature of R475. With such a configuration, efficiency of excavation at the outer periphery of the auger head can be improved when a holder dedicated for the auger head is used together. 
     The plane part  32   a  of the base part  32  and the leg parts  33  and  34  of the auger bit  31  have cutting edges wider than those of a typical auger bit. Specifically, a width of the plane part  32   a  in a direction orthogonal to the axial direction of the auger head, in other words, in the circumferential direction of the auger head is larger than that of a typical auger bit. 
     Specifically, the width of the plane part  32   a  in the direction orthogonal to the axial direction of the auger head is larger than that of a typical auger bit. 
     For example, it is typical that, in a typical auger bit for a casing auger, a cutting edge width is equal to a holder width like the cutting edge width is 60 mm for a holder of 60 mm width and is 80 mm for a holder of 80 mm width, but the auger bit according to the fourth embodiment has such a wide shape that the cutting edge width is 85 mm for a holder of 60 mm width and is 105 mm for a holder of 80 mm width, and thus an already mounted holder can be used for the auger bit. 
     As described above, according to the fourth embodiment of the present invention, since the three divided superhard chips are provided, cutting edges of the auger bit are less likely to suffer loss and abrasion, thereby achieving improved excavation efficiency. Moreover, the auger bit needs to be less frequently replaced because not all superhard chips become unusable due to loss at one place unlike a configuration in which the superhard chips are integrated. 
     In addition, according to the fourth embodiment, since the plane part of the base part has an R shape, it is possible to prevent interference with a guide casing and thus reduce loss of cutting edges due to collision with the guide casing. The cutting edges are positioned not at middle positions but on the outer side so that excavation is performed further on the outer side than when the cutting edges are positioned at the middle positions, which enables securement of an excavation diameter and size increase thereof. Moreover, since the R shape has a curvature in accordance with the diameter of the auger head, it is possible to reduce a load at excavation, thereby achieving improved excavation efficiency. 
     In addition, according to the fourth embodiment, loss and abrasion of auger bits can be prevented with the wide shape. Moreover, auger bits can be mounted by utilizing existing holders of the auger head. 
     Fifth Embodiment 
     The configuration of an auger bit according to a fifth embodiment of the present invention will be described below with reference to  FIGS. 9, 10 ( a ), and  10 ( b ). More specifically,  FIG. 9  is a perspective view of the auger bit,  FIG. 10( a )  is a plan view thereof, and  FIG. 10( b )  is a side view thereof. 
     As illustrated in these diagrams, an auger bit  41  includes a base part  42 , and two leg parts  43  and  44  extending from the base part  42 . In the fifth embodiment, a side closer to the base part  42  is also referred to as a leading end of the auger bit  41 , and a side closer to the leg parts  43  and  44  is also referred to as an auger-head mounting side or a rear end of the auger bit  41 . 
     The base part  42  has what is called a chevron shape having a top part protruding toward the leading end side of the auger bit  41  in an axial direction of the auger head. More specifically, a plane part  42   a  of the base part  42  of the auger bit  41  is tilted downward at 15° toward the leading end side of the auger head relative to planes of the leg parts  43  and  44  and is continuous with three tilted parts  42   b ,  42   c , and  42   d  that are further tilted toward the leading end side in the axial direction of the auger head. 
     In this example, a left end of the tilted part  42   b  when viewed from the leading end side in the axial direction of the auger head is continuous with a side surface part  42   e , and a right end of the tilted part  42   d  when viewed from the leading end side in the axial direction of the auger head is continuous with a side surface part  42   f.    
     The tilted parts  42   b ,  42   c , and  42   d  of the base part  42  are continuous in the stated order in a lateral direction when viewed from the leading end side in the axial direction of the auger head, in other words, in a circumferential direction of the auger head. In this example, the tilted part  42   b  is tilted relative to the tilted part  42   c  in the circumferential direction of the auger head, and the tilted part  42   d  is tilted relative to the tilted part  42   c  in the circumferential direction of the auger head (direction opposite to a direction in which the tilted part  42   b  is tilted) of the tilted part  42   b.    
     Lower ends of the tilted parts  42   b ,  42   c , and  42   d  of the base part  42  are continuous with a tilted part  42   g . The tilted part  42   g  is tilted toward the rear end side in the axial direction of the auger head, in other words, the auger-head mounting side of the auger bit  41  at 15° relative to a plane parallel to the plane part  42   a . Note that, in this example, a hard building-up 46 is disposed at the base part  42 . 
     The leg parts  43  and  44  extend with a predetermined interval therebetween in a U shape from the base part  42  toward the rear end side in the axial direction of the auger head. The leg parts  43  and  44  are provided with four holes  43   a  and four holes  44   a , respectively, for mounting the auger bit  41  on the auger head with bolts or the like. 
     One superhard chip  45  is disposed at lower parts of the tilted parts  42   b ,  42   c , and  42   d  of the base part  42  of the auger bit  41 . With the superhard chip  45 , abrasion outside a contact surface can be prevented. Moreover, the material and hardness of the superhard chip can be freely selected as appropriate in accordance with soil or an obstacle as an excavation target. 
     As for the material of each component, parent materials of the leg parts  43  and  44  may be SCM440 (chromium/molybdenum steel) or the like. The material of the superhard chip  45  may be E3 (material name: MG30), E4 (material name: MG40), E5 (material name: MG50), E6 (material name: MG60), or the like among the JIS usage classification symbols or may be G4 (CIS material symbol: VC-40), G5 (CIS material symbol: VC-50), or the like in the CIS standard. 
     The base part  42  of the auger bit  41  has a tapered elongated shape as compared to that of a typical auger bit and thus can provide a desirable bite into an excavation target, which leads to improved excavation efficiency. 
     More specifically, the cutting edge width is 60 mm for a holder of 60 mm width and is 80 mm for a holder of 80 mm width in a case of a typical auger bit, but the cutting edge width is 40 mm for a holder of 60 mm width and is 50 mm for a holder of 80 mm width in a case of the auger bit  41  according to the fifth embodiment. In the illustrated example, the plane part  42   a  of the base part  42  has such a tapered shape that the plane thereof has a maximum width of 80 mm and a minimum width of 50 mm, and the width gradually decreases from 80 mm to 50 mm as the position becomes closer to the leading end side. 
     Moreover, the cutting edge of a typical auger bit has a width of 60 mm and a length of 185 mm for a holder of 60 mm width and has a width of 80 mm and a length of 215 mm for a holder of 80 mm width, but the cutting edge of the auger bit  41  according to the fifth embodiment has a width of 40 mm and a length of 250 mm for a holder of 60 mm width, and has a width of 50 mm and a length of 280 mm or 310 mm for a holder of 80 mm width. 
     The auger bit according to the fifth embodiment may be used together with an existing auger bit or may be used alone. Specifically, for example, three kinds of auger bits, namely, normal auger bits of lengths 280 mm and 310 mm and the auger bit according to the present embodiment may be used together in a case in which the cutting edge has a width of 50 mm and a length of 280 mm or 310 mm for a holder of 80 mm width. 
     As described above, according to the fifth embodiment of the present invention, the base part of the auger bit has a tapered elongated shape and thus can provide a desirable bite into an excavation target, which leads to improved excavation efficiency. 
     Sixth Embodiment 
     The configuration of an auger bit according to a sixth embodiment of the present invention will be described below with reference to  FIGS. 11, 12 ( a ), and  12 ( b ). More specifically,  FIG. 11  is a perspective view of the auger bit,  FIG. 12( a )  is a plan view thereof, and  FIG. 12( b )  is a side view thereof. 
     As illustrated in these diagrams, an auger bit  51  includes a base part  52 , and two leg parts  53  and  54  extending from the base part  52 . In the sixth embodiment, a side closer to the base part  52  is also referred to as a leading end of the auger bit  51 , a side closer to the leg parts  53  and  54  is also referred to as an auger-head mounting side or a rear end of the auger bit  51 . 
     The base part  52  has what is called a chevron shape having a top part protruding toward the leading end side of the auger bit  51  in an axial direction of the auger head. More specifically, a plane part  52   a  of the base part  52  of the auger bit  51  is tilted downward at 15° toward the leading end side of the auger head relative to planes of the leg parts  53  and  54  and is continuous with three tilted parts  52   b ,  52   c , and  52   d  that are further tilted toward the leading end side in the axial direction of the auger head. 
     In this example, a left end of the tilted part  52   b  when viewed from the leading end side in the axial direction of the auger head is continuous with a side surface part  52   e , and a right end of the tilted part  52   d  when viewed from the leading end side in the axial direction of the auger head is continuous with a side surface part  52   f.    
     The tilted parts  52   b ,  52   c , and  52   d  of the base part  52  are continuous in the stated order in a lateral direction when viewed from the leading end side in the axial direction of the auger head, in other words, in a circumferential direction of the auger head. In this example, the tilted part  52   b  is tilted relative to the tilted part  52   c  in the circumferential direction of the auger head, and the tilted part  52   d  is tilted relative to the tilted part  52   c  in the circumferential direction of the auger head (direction opposite to a direction in which the tilted part  52   b  is tilted) of the tilted part  52   b.    
     Lower ends of the tilted parts  52   b ,  52   c , and  52   d  of the base part  52  are continuous with a tilted part  52   g . The tilted part  52   g  is tilted toward the rear end side in the axial direction of the auger head, in other words, the auger-head mounting side of the auger bit  51  at 15° relative to a plane parallel to the plane part  52   a . Note that, in this example, a hard building-up 56 is disposed at the base part  52 . 
     The leg parts  53  and  54  extend with a predetermined interval therebetween in a U shape from the base part  52  toward the rear end side in the axial direction of the auger head. The leg parts  53  and  54  are provided with four holes  53   a  and four holes  54   a , respectively, for mounting the auger bit  51  to the auger head with bolts or the like. 
     One superhard chip  55  is disposed at lower parts of the tilted parts  52   b ,  52   c , and  52   d  of the base part  52  of the auger bit  51 . With the superhard chip  55 , abrasion outside a contact surface can be prevented. Moreover, the material and hardness of the superhard chip can be freely selected as appropriate in accordance with soil or an obstacle as an excavation target. 
     As for the material of each component, parent materials of the leg parts  53  and  54  may be SCM440 (chromium/molybdenum steel) or the like. The material of the superhard chip  55  may be E3 (material name: MG30), E4 (material name: MG40), E5 (material name: MG50), E6 (material name: MG60), or the like among the JIS usage classification symbols or may be G4 (CIS material symbol: VC-40), G5 (CIS material symbol: VC-50), or the like in the CIS standard. 
     The base part  52  of the auger bit  51  has a tapered elongated shape as compared to that of a typical auger bit and thus can provide a desirable bite into an excavation target, which leads to improved excavation efficiency. 
     More specifically, the cutting edge width is 60 mm for a holder of 60 mm width and is 80 mm for a holder of 80 mm width in a case of typical auger bit, but the cutting edge width is 40 mm for a holder of 60 mm width and is 50 mm for a holder of 80 mm width in a case of the auger bit  51  according to the present embodiment. In the illustrated example, the plane part  52   a  of the base part  52  has such a tapered shape that the plane thereof has a maximum width of 80 mm and a minimum width of 50 mm, and the width gradually decreases from 80 mm to 50 mm as the position becomes closer to the leading end side. 
     Moreover, the cutting edge of a typical auger bit has a width of 60 mm and a length of 185 mm for a holder of 60 mm width and has a width of 80 mm and a length of 215 mm for a holder of 80 mm width, but the cutting edge of the auger bit  51  according to the sixth embodiment has a width of 40 mm and a length of 250 mm for a holder of 60 mm width, and has a width of 50 mm and a length of 280 mm or 310 mm for a holder of 80 mm width. 
     The auger bit according to the present embodiment may be used together with an existing auger bit or may be used alone. Specifically, for example, three kinds of auger bits, namely, normal auger bits of lengths 280 mm and 310 mm and the auger bit according to the sixth embodiment may be used together in a case in which the cutting edge has a width of 50 mm and a length of 280 mm or 310 mm for a holder of 80 mm width. 
     The plane part  52   a  of the base part  52  and the leg parts  53  and  54  of the auger bit  51  each have an R shape curved relative to the axial direction of the auger head at a predetermined curvature in accordance with the diameter of the auger head. In the present embodiment, it is designed that leading ends of cutting edges are positioned on a circumference of ϕ850 mm, and the R shape has a curvature of outer periphery R475 and inner periphery R395. With such a configuration, efficiency of excavation at the outer periphery of the auger head can be improved when a holder dedicated for the auger head is used together. 
     As described above, according to the sixth embodiment of the present invention, the base part of the auger bit has a tapered elongated R shape and thus can provide a desirable bite into an excavation target, which leads to improved excavation efficiency. 
     Moreover, since the plane part of the base part has an R shape, it is possible to prevent interference with a guide casing and thus reduce loss of cutting edges due to collision with the guide casing. In addition, the cutting edge of the base part is positioned not at a middle position but on the outer side so that excavation is performed further on the outer side than when the cutting edge is positioned at the middle position, which enables size increase of the excavation diameter. Furthermore, since the R shape is curved at a predetermined curvature in accordance with the diameter of the auger head, it is possible to reduce a load at excavation, thereby achieving improved excavation efficiency. 
     Lastly, a status in which the auger bits according to the fourth embodiment and the sixth embodiment are attached to the auger head will be described with reference to  FIG. 13 . More specifically,  FIG. 13( a )  is a side view illustrating the attachment status, and  FIG. 13( b )  is a front view illustrating the attachment status. 
     As illustrated in these diagrams, an auger head  100  has what is called a screw shape, and at one of two bifurcated leading ends, an auger bit  101  according to the fourth embodiment and auger bits  102  and  103  according to the sixth embodiment are mounted in parallel in the stated order from outside toward a central part. At the other leading end, an auger bit  104  according to the fourth embodiment and an auger bit  105  according to the sixth embodiment are mounted in parallel in the stated order from outside toward the central part. In this manner, the auger bits  101  and  104  according to the fourth embodiment are disposed on the outermost side and have R shapes approximate to the R shape of the auger head  100  and thus achieve smooth rotation and excavation, and moreover, the auger bits  101  and  104  have wide shapes and thus have high resistance against damage due to abrasion or the like. In addition, the auger bits  102 ,  103 , and  105  according to the sixth embodiment are disposed toward the central part and have tapered elongated shapes, and can achieve desirable excavation irrespective of the kind of an excavation target. 
     The first to sixth embodiments of the present invention are described above, but the present invention is not limited thereto and may be modified and changed in various kinds of manners without departing from the scope of the present invention. 
     For example, an auger bit for a casing auger according to each embodiment can be employed in, for example, work using an existing pile working method, a steel pipe inner excavation working method, or the like, and accordingly, crack, dropping, and abrasion of a superhard chip are less likely to occur during the work as compared to conventional cases. 
     REFERENCE SIGNS LIST 
     
         
           1  auger bit 
           2  base part 
           2   a  plane part 
           2   b ,  2   c ,  2   d  tilted part 
           2   e ,  2   f  side surface part 
           2   g  tilted part 
           3  leg part 
           3   a  hole 
           4  leg part 
           4   a  hole 
           5   a ,  5   b ,  5   c  superhard chip 
           6  hard building-up 
           11  auger bit 
           12  base part 
           12   a  plane part 
           12   b ,  12   c ,  12   d  tilted part 
           12   e ,  12   f  side surface part 
           12   g  tilted part 
           13  leg part 
           13   a  hole 
           14  leg part 
           14   a  hole 
           15  superhard chip 
           16  hard building-up 
           21  auger bit 
           22  base part 
           22   a  plane part 
           22   b ,  22   c ,  22   d  tilted part 
           22   e ,  22   f  side surface part 
           22   g  tilted part 
           23  leg part 
           23   a  hole 
           24  leg part 
           24   a  hole 
           25  superhard chip 
           26  hard building-up 
           31  auger bit 
           32  base part 
           32   a  plane part 
           32   b ,  32   c ,  32   d  tilted part 
           32   e ,  32   f  side surface part 
           32   g  tilted part 
           33  leg part 
           33   a  hole 
           34  leg part 
           34   a  hole 
           35   a ,  35   b ,  35   c  superhard chip 
           36  hard building-up 
           41  auger bit 
           42  base part 
           42   a  plane part 
           42   b ,  42   c ,  42   d  tilted part 
           42   e ,  42   f  side surface part 
           42   g  tilted part 
           43  leg part 
           43   a  hole 
           44  leg part 
           44   a  hole 
           45  superhard chip 
           46  hard building-up 
           51  auger bit 
           52  base part 
           52   a  plane part 
           52   b ,  52   c ,  52   d  tilted part 
           52   e ,  52   f  side surface part 
           52   g  tilted part 
           53  leg part 
           53   a  hole 
           54  leg part 
           54   a  hole 
           55  superhard chip 
           56  hard building-up