TECHNICAL FIELD
[0001] The present invention relates to a so-called double pipe type drilling tool that
performs drilling using a ring bit disposed on a tip side of a casing pipe, and a
pilot bit inserted into the casing pipe.
BACKGROUND ART
[0003] As such a double pipe type drilling tool, PTL 1 suggests a drilling tool in which
a ring bit is rotatably inserted into a tip part of a casing pipe with their inner
and outer periphery surfaces being made to face each other; an inner bit is attached
to a tip of a transmission member inserted into the casing pipe; striking force and
impelling force are transmitted to the casing pipe and rotating force in addition
thereto are transmitted to the ring bit via this inner bit so as to perform drilling;
and after a borehole having a predetermined depth is formed, the ring bit be removed
from the casing pipe and left in the borehole.
[0004] PTL 2 describes a so-called under-reaming bit in which an extendable bit is attached
to an outer periphery of a tip part of a shank device rotated around an axis; a borehole
having a predetermined internal diameter is formed while the extendable bit is positioned
in a extending state and protrudes to a tip of a casing pipe during drilling; and
after the end of the drilling, the extendable bit is shrunk and then is recovered
with the shank device through the inside of the casing pipe.
CITATION LIST
PATENT LITERATURE
[0005]
[PTL 1] Japanese Patent No. 4887857
[PTL 2] Japanese Patent No. 4501407
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0006] In recent years, in specific drilling works using such drilling tools, there have
been increasing numbers of cases in which a borehole is formed with a greater internal
diameter than an internal diameter sufficient enough to insert the casing pipe itself.
For example, in a case where a steel-pipe diaphragm wall is buried, a casing pipe
in which a joint is provided at an outer peripheral part of a steel pipe is inserted
into a borehole. Further, in a case where the casing pipe is connected by a coupling
having a greater external diameter than the external diameter of the steel pipe, this
coupling is inserted into the borehole, and therefore, a borehole with an internal
diameter for which the external diameter to the joint or the coupling is taken into
consideration must be formed. Moreover, also in a case where the periphery of the
steel pipe is cemented at a water well or the like for water stoppage, and a borehole
with a greater internal diameter than the external diameter of the steel pipe is required.
[0007] In a case where such a borehole with a greater internal diameter is formed by the
double pipe type drilling tool described in PTL 1, the external diameter of the ring
bit is increased. However, as described above, the ring bit is rotatably inserted
with the outer peripheral surface of the posterior end part thereof being made to
face the inner peripheral surface of the casing pipe. Therefore, the internal diameter
of the ring bit does not change and the size in the radial direction of the ring bit
becomes larger. Since this ring bit is finally left in the borehole without being
recovered, an increase in construction cost is caused. Further, since the rotating
force is transmitted to the ring bit via the inner bit inserted into the ring bit
through the inside of the casing pipe, sufficient rotating force cannot be transmitted
if the external diameter of the ring bit becomes larger, and also there is a concern
that drilling performance may degrade.
[0008] In a case where a borehole with a greater internal diameter is formed using the under-reaming
bit described in PTL 2, the radius of the extendable bit from the axis in the extending
state is increased. However, there is also a limitation to enlarge the extendable
bit in order to shrink the extendable bit and recover it with the shank device after
the end of the drilling. Moreover, in forming a borehole having a greater internal
diameter, load becomes larger. As a result, there is also a concern that damage may
occur on a shaft that rotatably supports the extendable bit. Further, there is also
a limitation to the number of drilling tips disposed on the extendable bit, and drilling
performance degrades as the diameter of a borehole becomes larger.
[0009] The present invention has been made in view of such a background, and the objective
thereof is to provide a borehole that can prevent degradation of drilling performance,
occurrence of damage, and an increase in construction cost, in a case where a borehole
with a greater internal diameter than the external diameter of a casing pipe is drilled.
SOLUTION TO PROBLEM
[0010] In order to solve the above problems and achieve the objective, the present invention
provides a drilling tool including: a cylindrical casing pipe centered on an axis;
an annular ring bit that is coaxially disposed on a tip side of the casing pipe and
has a greater external diameter than the casing pipe; and a pilot bit that is inserted
into an inner peripheral part of the ring bit through the inside of the casing pipe.
The pilot bit is rotatable around the axis and a bit head is provided at an outer
peripheral part of a tip of the pilot bit. The bit head is rotatable around a centerline
eccentric from the axis, and is configured that when the pilot bit is rotated in a
tool rotation direction during drilling, the bit head is extended such that a radius
of the bit head from the axis is enlarged and thereby is supported by the pilot bit.
The ring bit is provided with: a part to be engaged that is configured to be engaged
with the extended bit head in the tool rotation direction during drilling; and a first
abutting part capable of abutting against the tip side of the extended bit head in
a direction of the axis.
[0011] In this drilling tool, the bit head provided at the outer peripheral part of the
tip of the pilot bit is extended during drilling, and the first abutting part provided
at the ring bit abuts against the tip side of the extended bit head in the direction
of the axis. Thus, the ring bit can be prevented from coming off to the tip side thereof.
Also, since the part to be engaged of this ring bit is engaged with the extended bit
head in the tool rotation direction during drilling, rotating force can be transmitted
from the pilot bit via the bit head to the ring bit.
[0012] Therefore, even if the external diameter of the ring bit is made to be greater than
the external diameter of the casing pipe, via the extended bit head of which the radius
from the axis is enlarged, sufficient rotating force can be transmitted to the ring
bit, and drilling performance can be guaranteed. Additionally, when the part to be
engaged of the ring bit is engaged with the extended bit head in the tool rotation
direction during drilling, in this way, the need for making an outer peripheral surface
of a posterior end part of the ring bit face an inner peripheral surface of the casing
pipe to allow the posterior end part to be rotatably inserted into the casing pipe
is also eliminated. Therefore, the internal diameter of the ring bit can be increased,
that is, a required material can be reduced by making the volume of the ring bit small.
Thus, even in a case where after the end of drilling, the bit head is rotated in a
direction opposite to the direction during drilling and the ring bit is left in a
borehole, construction cost can be prevented from increasing.
[0013] In contrast, in the pilot bit, even if the radius of the extended bit head from the
axis is not made as large as the radius of the borehole, a borehole with a large internal
diameter can be formed by the ring bit, and damage or the like to the bit head can
be prevented without exerting an excessive load. Additionally, the number of the drilling
tip disposed on the annular ring bit that drills the outer peripheral side of the
borehole can be relatively freely set, and it is also possible to prevent degradation
of drilling performance resulting from shortage of chips.
[0014] Here, when a recessed portion that is recessed to an outer peripheral side is formed
at the inner peripheral part of the ring bit, and the recessed portion serves as the
part to be engaged, the volume of the ring bit can be further reduced, and construction
cost can be further reduced. In addition, in this case, a tip surface of the ring
bit adjacent to the recessed portion may serve as the first abutting part, and a bottom
surface that faces the tip side in the direction of the axis may be formed in the
recessed portion so as to serve as the first abutting part.
[0015] In addition, in building the casing pipe in a borehole with the striking force and
the impelling force to the tip side in the direction of the axis to be applied to
the pilot bit, as in the drilling tools described in PTLs 1 and 2, a smaller-diameter
part of which an internal diameter is one step smaller may be formed at an inner peripheral
part of a tip of the casing pipe, and a second abutting part like a stepped part capable
of abutting against the smaller-diameter part from a posterior end side in the direction
of the axis may be formed at an outer peripheral part of a posterior end of the pilot
bit so as to transmit the striking force and the impelling force. In this case, by
making the internal diameter of the ring bit equal to or greater than the internal
diameter of the smaller-diameter part, the volume of the ring bit can be made small
as described above, and construction cost can be reliably reduced.
[0016] In transmitting the striking force and the impelling force to the tip side in the
direction of the axis to the ring bit, the striking force and the impelling force
may be directly transmitted to the ring bit from the pilot bit as in the drilling
tool described in PTL 1. However, in that case, when the smaller-diameter part is
formed at the inner peripheral part of the tip of the casing pipe and is made to be
capable of abutting against the stepped part of the pilot bit as described above,
a smaller-diameter part of which the internal diameter is further smaller than the
smaller-diameter part of the casing pipe must be formed at the ring bit to abut against
the pilot bit, and consequently, there is a concern that it becomes difficult to make
the internal diameter of the ring bit small as described above to reduce construction
cost.
[0017] Thus, particularly in such a case, by providing the bit head with a third abutting
part which is capable of abutting against a surface of the ring bit that faces a posterior
end side in the direction of the axis in a state where the bit head is extended, it
becomes unnecessary to form a smaller-diameter part having a smaller internal diameter
than the smaller-diameter part of the casing pipe in the ring bit, construction cost
can be much more reliably reduced, and it is possible to reliably transmit the striking
force and the impelling force from the pilot bit to the tip side in the direction
of the axis to the ring bit via the third abutting part of the bit head.
[0018] Moreover, in this case, by making the greatest radius of the third abutting part
of the extended bit head from the axis greater than a radius of an outer peripheral
part of the tip of the casing pipe from the axis, the striking force and the impelling
force can be transmitted to the outer peripheral side of the ring bit where drilling
is performed. In a case where a borehole with a greater internal diameter than the
external diameter of the casing pipe is formed, it is possible to perform even more
efficient drilling, the thickness of the ring bit in the direction of the axis can
also be made small, and much more construction cost reduction can be achieved.
ADVANTAGEOUS EFFECTS OF INVENTION
[0019] As described above, according to the present invention, even in a case where a borehole
with a greater internal diameter than the external diameter of the casing pipe is
formed, it is possible to transmit sufficient rotating force to the ring bit and to
perform efficient drilling, without causing degradation of drilling performance, an
increase in construction cost, or damage to the tool.
BRIEF DESCRIPTION OF DRAWINGS
[0020]
FIG. 1 is a cross-sectional view showing a state where a bit head is extended in one
embodiment of the present invention.
FIG. 2 is an enlarged front view when the embodiment shown in FIG. 1 in a state where
the bit head is retracted is seen from a tip side in a direction of an axis (illustration
of a casing pipe and a casing top is omitted).
FIG. 3 is an enlarged front view when the embodiment shown in FIG. 1 in a state where
the bit head is extended is seen from the tip side in the direction of the axis (illustration
of the casing pipe and the casing top is omitted).
FIG. 4 is an enlarged front view when the ring bit of the embodiment shown in FIG.
1 is seen from the tip side in the direction of the axis.
FIG. 5 is Z-Z sectional view in FIG. 4.
DESCRIPTION OF EMBODIMENTS
[0021] FIGS. 1 to 5 show one embodiment of a drilling tool of the present invention. In
the present embodiment, a casing pipe 1 is formed in a cylindrical shape centered
on an axis O using metallic materials, such as a steel material, and a casing top
1A formed in a multi-stage cylindrical shape using metallic materials, such as a steel
material, is attached to a tip part (left side in FIG. 1) of the casing pipe 1.
[0022] In the casing top 1A, the internal diameter thereof is a constant internal diameter
that is one step smaller than the internal diameter of the casing pipe 1, the external
diameter of a tip part is the same diameter as the casing pipe 1, and the external
diameter of a posterior end part is of a size such that the posterior end part can
be inserted and fitted into the casing pipe 1. The posterior end part of the casing
top 1A is inserted and fitted into the casing pipe 1 from a tip side of the casing
pipe 1 and then jointed by welding or the like, whereby the casing top 1A is coaxially
integrated with the casing pipe 1.
[0023] By attaching the casing top 1A in this way, a smaller-diameter part 1B of which the
internal diameter is one step smaller is formed at an inner peripheral part of the
tip of the casing pipe 1. In addition, a posterior end surface of the smaller-diameter
part 1B is formed in the shape of a concave conical surface which is centered on the
axis O and tilts toward an inner peripheral side so as to become slightly closer to
the tip side.
[0024] A pilot bit 2 is inserted into the casing pipe 1 from a posterior end side thereof
(right side in FIG. 1). The pilot bit 2 is made of metallic materials such as a steel
material and has an outer shape that is also formed in a multi-stage columnar shape,
and a posterior end part thereof is made into a smaller-diameter shank part 2A. Striking
force directed to the tip side in the direction of the axis O is transmitted to the
pilot bit 2 from a down-the-hole hammer H attached to the shank part 2A.
[0025] Additionally, a drill rod (not shown) is added if necessary and is coupled to the
posterior end side of the down-the-hole hammer H, and a drill rod disposed at the
most posterior end is attached to a drilling device. The impelling force directed
to the tip side in the direction of the axis O and the rotating force directed in
a tool rotation direction T during drilling are transmitted to the pilot bit 2 from
the drilling device via the drill rod and the down-the-hole hammer H. In addition,
the casing pipe 1 is also added to the posterior end side if necessary and is inserted
into a borehole.
[0026] A stepped part of which the external diameter becomes a maximum is formed at an outer
periphery of the pilot bit 2 at a position closer to the tip side than the shank part
2A, and serves as a second abutting part 2B of the present embodiment. The external
diameter of the second abutting part 2B is slightly smaller than the internal diameter
of the casing pipe 1 and greater than the internal diameter of the smaller-diameter
part 1B formed by the casing top 1A. Moreover, a tip surface of the second abutting
part 2B is formed in the shape of a convex conical surface which tilts toward the
inner peripheral side so as to become slightly closer to the tip side, and the tilt
angle thereof is equal to the tilt angle of the posterior end surface of the smaller-diameter
part 1B that forms the concave conical surface shape.
[0027] Therefore, the pilot bit 2 is coaxial with the casing pipe 1 and the casing top 1
A movable integrally with the casing pipe 1 and the casing top 1A to the tip side
in the direction of the axis O, and rotatable around the axis O relative to the casing
pipe 1 and the casing top 1A, in a place where the pilot bit 2 is inserted into the
casing pipe 1 from the posterior end side thereof and the second abutting part 2B
abuts against the smaller-diameter part 1B. Additionally, the external diameter of
the pilot bit 2 at a position closer to the tip side than the second abutting part
2B is a constant external diameter slightly smaller than the internal diameter of
the smaller-diameter part 1B formed by the casing top 1A, and thus a tip part of the
pilot bit 2 is formed so as to protrude greatly from the tip of the casing top 1A
in a state where the second abutting part 2B abuts against the smaller-diameter part
1B.
[0028] A housing recess 3 is formed at an outer periphery of the tip part of the pilot bit
2 protruding from the tip of the casing top 1A so as to be located closer to the tip
side than the casing top 1A. The housing recess 3 includes: a bottom surface 3A that
is located closer to the tip side than the casing top 1 A, faces the tip side, and
is perpendicular to the axis O; and a wall surface 3B that extends to tip side in
parallel with the axis O from an inner peripheral edge of the bottom surface 3A and
reaches a tip surface of the pilot bit 2. The housing recess 3 is formed so as to
be open to an outer peripheral surface and a tip surface of the tip part of the pilot
bit 2. In the present embodiment, a plurality of (three) such housing recesses 3 having
the same form and the same size are formed at equal intervals in a circumferential
direction.
[0029] A wall surface 3B of each housing recess 3 includes: a first wall part 3a that is
a plane facing an outer peripheral side of the pilot bit 2; a second wall part 3b
that is a plane located on a side opposite to the first wall part 3a in the tool rotation
direction T and facing the tool rotation direction T; and a third wall part 3c that
is a plane located on the same side as the first wall part 3a in the tool rotation
direction T and facing the side opposite to the tool rotation direction T. The second
and third wall parts 3b and 3c are formed such that a gap therebetween in the circumferential
direction becomes greater toward the outer peripheral side, and the second wall part
3b out of these wall parts extends toward the outer peripheral side so as to tilt
to the tool rotation direction T.
[0030] Additionally, a fourth wall part 3d and a fifth wall part 3e are formed in the shape
of a concave cylindrical surface centered on a straight line parallel to the axis
O at a boundary part between the first and second wall parts 3a and 3b and a boundary
part between the first and third wall parts 3a and 3c, respectively. The fourth wall
part 3d connects with the first and second wall parts 3a and 3b, and the fifth wall
part 3e connects with the first and third wall parts 3a and 3c. The radius of the
concave cylindrical surface as the fourth wall part 3d formed at the boundary part
between the first and second wall part 3a and 3b is greater than the radius of the
concave cylindrical surface as the fifth wall part 3e formed at the boundary part
between the first and third wall parts 3a and 3c.
[0031] Moreover, a discharge groove 2C for cuttings is formed which extends toward the posterior
end side in parallel with the axis O from the tool rotation direction T side of the
bottom surface 3A of each housing recess 3 and reaches an outer peripheral side of
the shank part 2A beyond the second abutting part 2B. Each discharge groove 2C forms
a substantially rectangular shape in a cross-section perpendicular to the axis O and
is open to an outer peripheral surface of the tip part of the pilot bit 2. A bottom
surface of each discharge groove 2C which faces the outer peripheral side of the pilot
bit 2 forms a recessed curved shape and is slightly swept toward the outer peripheral
side in a place where the bottom surface reaches a posterior end of the second abutting
part 2B. A portion where this bottom surface and the bottom surface 3A of the housing
recess 3 intersect each other is chamfered by an inclined surface 2D that intersects
with both the bottom surfaces at an obtuse angle.
[0032] On the other hand, a fitting hole 3C that has a centerline C parallel to the axis
O and has a circular cross-sectional shape is formed on the side opposite to the bottom
surface 3A of each housing recess 3 in the tool rotation direction T. The centerline
C of the fitting hole 3C coincides with the centerline of the concave cylindrical
surface as the fourth wall part 3d formed at the boundary part between the first and
second wall parts 3a and 3b, and is eccentric to the outer peripheral side of the
axis O. Additionally, the internal diameter (radius) of the fitting hole 3C is approximately
equal to or slightly smaller than the radius of the concave cylindrical surface as
the fourth wall part 3d.
[0033] A bit head 4 is attached to each of the housing recesses 3 of the pilot bit 2. In
the bit head 4, a columnar shaft part 4A to be inserted and slidably fitted into the
fitting hole 3C and a head main body 4B provided on the tip side of the shaft part
4A are integrally formed using metallic materials, such as a steel material. The bit
head 4 is attached so as to be rotatable around the centerline C. The bit head 4 is
positioned in a state where as shown in FIG. 2, the head main body 4B abuts against
the first wall part 3a and is housed within the housing recess 3 and the radius thereof
from the axis O is reduced, or is positioned in a state where as shown in FIG. 3,
the head main body 4B abuts against the second wall part 3b and the radius thereof
from the axis O is enlarged. A posterior end surface of the head main body 4B is a
plane perpendicular to the centerline C.
[0034] A cutout 4C is formed at an outer periphery of the shaft part 4A such that the cutout
4C forms a semi-oval shape as shown in FIG. 1 in a cross-section along the centerline
C and extends to form a substantial L-shape as shown in FIGS. 2 and 3 in a section
perpendicular to the centerline C. A pin 5 is driven into the tip part of the pilot
bit 2 in a tangential direction of the fitting hole 3C in the cross-section orthogonal
to the axis O, at a position that faces the cutout 4C in the direction of the axis
O in a state where the shaft part 4A is inserted into the fitting hole 3C, a posterior
end surface of the shaft part 4A is made to abut against the bottom surface of the
fitting hole 3C, and a posterior end surface of the head main body 4B is made to abut
against the bottom surface 3A of the housing recessed portion 3. A peripheral surface
of each pin 5 is exposed inside the fitting hole 3C and is engaged with the cutout
4C, and thereby the bit head 4 is prevented from coming off to the tip side while
being made to be rotatable around the centerline C.
[0035] Additionally, a first side surface 4a, which is located on an extension of an outer
peripheral surface of the shaft part 4A among side surfaces of the head main body
4B, is formed in the shape of a convex cylindrical surface centered on the centerline
C which flushes with this outer peripheral surface of the shaft part 4A or has an
external diameter slightly greater than that of the outer peripheral surface of the
shaft part 4A. The first side surface 4a is made to be slidable on the fourth wall
part 3d of the wall surface 3B of the housing recess 3. Moreover, second and third
side surfaces 4b and 4c that sandwich the first side surface 4a therebetween are formed
in a planar shape. As shown in FIG. 2, in a state where the bit head 4 is retracted,
the second side surface 4b out of these side surfaces is made to abut against the
first wall part 3a of the wall surface 3B of the housing recess 3 while the third
side surface 4c faces the outer peripheral side of the pilot bit 2. In a state where
the bit head 4 is extended, the third side surface 4c is made to abut against the
second wall part 3b while the second side surface 4b is directed to the tool rotation
direction T.
[0036] Moreover, a fourth side surface 4d, which is located between the second and third
side surfaces 4b and 4c on a side opposite to the first side surface 4a, is formed
so as to protrude to the outer periphery of the pilot bit 2 and be located on a cylindrical
surface centered on the axis O, as shown in FIG. 3 in a state where the bit head 4
is extended. In addition, an intersecting ridgeline part between the fourth side surface
4d and the third side surface 4c is formed so as to be chamfered by a cylindrical
surface that has a diameter slightly smaller than the external diameter of the tip
part of the pilot bit 2 and is centered on the axis O in a state where the bit head
4 is retracted as shown in FIG. 2. Accordingly, the head main body 4B retracted and
housed in the housing recess 3 is located inside a cylindrical surface of the outer
peripheral surface of the tip part of the pilot bit 2.
[0037] Additionally, the fourth side surface 4d is formed in a multi-stage (three-stage
in the present embodiment) shape that becomes concave and convex to the inner and
outer peripheral sides with respect to the axis O toward the direction of the centerline
C as shown in FIG. 1. A stage at the most posterior end among these stages is made
to protrude to the outermost peripheral side from the axis O, and a portion in which
the stage at the most posterior end is formed serves as a third abutting part 4D in
the present embodiment. The third abutting part 4D has a surface thereof facing the
tip side which is a flat surface perpendicular to the centerline C, and a greatest
radius R of the third abutting part 4D from the axis O is greater than a radius r
of the outer peripheral parts of the tips of the casing pipe 1 and the casing top
1A from the axis O in a state where the bit head 4 is extended as shown in FIG. 1.
[0038] In addition, an intersecting ridgeline part between the fourth side surface 4d and
the second side surface 4b in which the third abutting part 4D is formed is chamfered
in the shape of a convex cylindrical surface with a radius approximately equal to
a concave cylindrical surface as the fifth wall part 3e of the housing recess 3, and
as shown in FIG. 2, is made to abut against the fifth wall part 3e in a state where
the bit head 4 is retracted. Additionally, a stage at a foremost end of the fourth
side surface 4d slightly tilts toward the posterior end side so as to become closer
to the inner peripheral side in a stage where the bit head 4 is extended.
[0039] Moreover, at a middle stage located between the stage at the foremost end and a stage
at a most posterior end in which the third abutting part 4D is formed, the fourth
side surface 4d is formed so as to extend in parallel with the axis O. Furthermore,
an engaging part 4E is formed at a corner part where that the fourth side surface
4d the second side surface 4b intersect with each other in this middle stage, such
that the engaging part 4E cuts out the corner part in a substantial L-shape in a section
orthogonal to the centerline C.
[0040] The engaging part 4E has a first wall surface 4e facing the outer peripheral side
and a second wall surface 4f facing the tool rotation direction T, in a state where
the bit head 4 is extended, a bottom surface 4g that is made to be flush with a surface
of the third abutting part 4D which faces the tip side, and a ceiling surface 4h that
faces the bottom surface 4g in parallel therewith and faces the posterior end side.
[0041] Similarly, in a state where the bit head 4 is extended, the first wall surface 4e
is located on a cylindrical surface that has a slightly greater external diameter
than the second abutting part 2B of the pilot bit 2 and is centered on the axis O,
and the second wall surface 4f is formed such that the second wall surface 4f extends
toward the outer peripheral side so as to slightly tilt to the tool rotation direction
T.
[0042] Moreover, an intersecting ridgeline part between a stage at a foremost end of the
fourth side surface 4d and a tip surface of the head main body 4B is formed as an
inclined surface that extends toward the centerline C so as to become closer the tip
side in such a manner the surface forms a truncated conical surface shape centered
on the axis O in a state where the bit head 4 is extended. Additionally, an intersecting
ridgeline part between the tip surface and the outer peripheral surface in the pilot
bit 2 is also formed as an inclined surface that similarly forms a truncated conical
surface shape centered on the axis O and tilts toward the inner peripheral side so
as to become closer the tip side, except for the portion cutout by the housing recess
3.
[0043] Furthermore, the tip surface of the pilot bit 2 and the tip surface of the head main
body 4B of the bit head 4 except the portions made to have these inclined surfaces
are respectively flat surfaces perpendicular to the axis O and the centerline C.
[0044] Additionally, the length of the head main body 4B in the direction of the centerline
C is equal to the depth from the bottom surface 3A of the housing recess 3 to the
tip surface of the pilot bit 2. Therefore, the tip surfaces of the pilot bit 2 and
the head main body 4B become flush with each other in a state where the bit head 4
is housed in the housing recess 3.
[0045] The tip surfaces and the respective inclined surfaces of the pilot bit 2 and the
head main body 4B of the bit head 4 are provided with a plurality (large number) of
drilling tips 6 made of cemented carbide or the like which is harder than a steel
material or the like that forms the pilot bit 2 and the bit head 4. Each of the drilling
tips 6 is one in which, for example, hemispherical head part protruding from the tip
surfaces and the inclined surfaces and columnar trunk part which are integrally formed,
and is fixed by press-fitting, hot-shrink fitting, cold-shrink fitting, or brazing
the trunk parts into each of circular holes formed perpendicularly to the tip surfaces
and the inclined surfaces.
[0046] Moreover, an annular ring bit 7 is disposed coaxially with the axis O on the tip
side of the casing pipe 1. The ring bit 7 is also formed in an annular plate shape
using metallic materials, such as a steel material, and a tip surface and a posterior
end surface thereof that face the direction of the axis O are perpendicular to the
axis O. However, the intersecting ridgeline part between the tip surface and the outer
peripheral surface is made to be a truncated cone-shaped inclined surface centered
on the axis O. The drilling tips 6 made of hard materials, such as cemented carbide,
are also provided on the inclined surface and the outer peripheral part of the tip
surface so as to protrude perpendicularly thereto, similar to the pilot bit 2 and
the bit head 4.
[0047] Additionally, the external diameter of the ring bit 7 is greater than the external
diameter of the casing pipe 1 and the casing top 1A and greater than the external
diameter of the extended bit head 4. Further, the internal diameter of the ring bit
7 is slightly greater than the external diameter of the second abutting part 2B of
the pilot bit 2 and therefore greater than the internal diameter of the smaller-diameter
part 1 B formed within the casing pipe 1 by the casing top 1A. The internal diameter
of the ring bit 7 is smaller than the external diameter of the extended bit head 4
and is of such a size that the first wall surface 4e of the engaging part 4E is fittable.
Additionally, the thickness of the ring bit 7 in the direction of the axis O is smaller
than the width between the external and internal diameters of the ring bit 7, and
is slightly smaller than a gap between the bottom surface 4g and the ceiling surface
4h of the engaging part 4E.
[0048] Moreover, three recessed portions that are recessed to the outer peripheral side
and are of the same number as that of the bit heads 4 are formed at equal intervals
in the circumferential direction at the inner peripheral part of the ring bit 7, and
as shown in FIG. 3, each of the recessed portions serves as a part 7A to be engaged
that is engaged with the engaging part 4E of each bit head 4 in the tool rotation
direction T during drilling.
[0049] The part 7A to be engaged includes a first wall surface 7a that recedes by one step
from the inner peripheral part of the ring bit 7 to the outer peripheral side and
then faces the inner peripheral side, a second wall surface 7b facing the side opposite
to the tool rotation direction T and a third wall surface 7c facing the tool rotation
direction T which extend from the first wall surface 7a to the inner peripheral part.
In the present embodiment, the part 7A to be engaged is formed so as to pass through
the ring bit 7 in the direction of the axis O.
[0050] The first wall surface 7a among these surfaces is located on the cylindrical surface
centered on the axis O. The radius of the first wall surface 7a from the axis O is
slightly greater than the radius of the stage at the foremost end and the middle stage
from the axis O in the fourth side surface 4d of the extended bit head 4 which faces
the outer peripheral side and is smaller than the radius R of the third abutting part
4D. Further, the circumferential length of the first wall surface 7a is slightly greater
than a length except the engaging part 4E in the circumferential length of the middle
stage of the fourth side surface 4d.
[0051] Additionally, the second and third wall surfaces 7b and 7c extend toward the outer
peripheral side so as to tilt to the tool rotation direction T, and an angle that
the second wall surface 7b out of these wall surfaces makes with respect to a radial
direction with respect to the axis O is equal to an angle that the second wall surface
4f in the engaging part 4E of the extended bit head 4 makes with the radial direction
with respect to the axis O. Further, as shown in FIG. 3, the third wall surface 7c
is formed in a concave cylindrical surface shape centered on the centerline C of the
fitting hole 3C in the housing recess 3 of the pilot bit 2, in a state where the part
7A to be engaged is engaged with the engaging part 4E.
[0052] In addition, in the pilot bit 2, a bottomed supply hole 8 is drilled from a posterior
end of the shank part 2A along the axis O to the vicinity of a central part of the
housing recess 3 in the direction of the axis O so as to be capable of supplying compressed
air therethrough from the down-the-hole hammer H side. First to third blow holes 8A
to 8C, which are three for each and have a smaller diameter than the supply hole 8,
branch obliquely from the supply hole 8 and extend toward the outer peripheral side
so as to tilt to the tip side.
[0053] Each first blow hole 8A is open to the tip side of the second abutting part 2B in
the outer peripheral surface of the tip part of the pilot bit 2. A fourth blow hole
8D with a much smaller diameter branches from each first blow hole 8A in parallel
with the axis O and is open to the center of the bottom surface of the fitting hole
3C. Further, each second blow hole 8B branches from the supply hole 8 at a position
closer to the tip side than each first blow hole 8A, and is open substantially perpendicularly
to the inclined surface 2D between the bottom surface of each discharge groove 2C
for cuttings and the bottom surface 3A of each housing recess 3. Moreover, each third
blow hole 8C has a greater diameter than the first and second blow holes 8A and 8B,
branches at the tip of the supply hole 8, and is open to the fifth wall part 3e side
of the first wall part 3a of the housing recess 3.
[0054] In this drilling tool, the pilot bit 2 is inserted from the posterior end side of
the casing pipe 1 in a state where the bit head 4 is retracted and the head main body
4B is housed in the housing recess 3, and is positioned in the direction of the axis
O in a place where the second abutting part 2B abuts against the posterior end surface
of the casing top 1A. Next, with the head main body 4B being housed, as shown in FIG.
2, a circumferential position of the part 7A to be engaged is aligned with the housing
recess 3, and the ring bit 7 is inserted into the tip part of the pilot bit 2 from
the tip side and is disposed at the position of the engaging part 4E of the head main
body 4B in the direction of the axis O.
[0055] When, from this state, the ring bit 7 is relatively rotated to the side opposite
to the tool rotation direction T during drilling while the bit head 4 is being extended,
as shown in FIG. 3, the second wall surface 4f in the engaging part 4E of the extended
bit head 4 comes in close contact with and abuts against the second wall surface 7b
in the part 7A to engaged of the ring bit 7 and thereby is engaged with the part 7A
to be engaged, and the third side surface 4c in the head main body 4B abuts against
the second wall part 3b of the housing recess 3 and is supported by the housing recess
3. Thereby, the ring bit 7 becomes integrally rotatable with respect to the pilot
bit 2 and the bit head 4 in the tool rotation direction T.
[0056] Additionally, in the direction of the axis O, as shown in FIGS. 1 and 3, when the
portion of the part 7A to be engaged on the tool rotation direction T side in the
tip surface of the ring bit 7 faces the ceiling surface 4h of the engaging part 4E
with a slight gap therefrom and is abuttable against the ceiling surface 4h, the ring
bit 7 is prevented from coming off to the tip side. That is, in the present embodiment,
the portion of the part 7A to be engaged on the tool rotation direction T side in
the tip surface of the ring bit 7 serves as a first abutting part 7B that is abuttable
against the extended bit head 4, on the tip side in the direction of the axis O. Moreover,
the bottom surface 4g of the engaging part 4E and the surface of the third abutting
part 4D flush with the bottom surface 4g which faces the tip side abut against the
posterior end surface of the ring bit 7, and support the ring bit 7 on the tip side
thereof. Thereby, the casing pipe 1 and the ring bit 7 are made to be movable to the
tip side in the direction of the axis O integrally with the pilot bit 2 and the bit
head 4.
[0057] Therefore, when, from this state, the striking force directed to the tip side in
the direction of the axis O is transmitted to the pilot bit 2 and the bit head 4 and
to the ring bit 7 via the third abutting part 4D by the down-the-hole hammer H and
the impelling force, and the rotating force directed in the tool rotation direction
T are transmitted from the drilling device, drilling work is performed by the drilling
tips 6 provided on the pilot bit 2, the bit head 4, and the tip surface of the ring
bit 7, and the casing pipe 1 is inserted into the formed borehole. In addition, compressed
air is blown off from the supply hole 8 via the first to fourth blow holes 8A to 8D
during drilling. Thereby, cuttings generated by the drilling tip 6 are discharged
through the inside of the casing pipe 1 from the discharge groove 2C, and biting of
the cuttings into the fitting hole 3C or the smaller-diameter part 1B is prevented.
[0058] After the borehole is formed up to a predetermined depth in this way, in the drilling
tool of the above configuration, the pilot bit 2 is rotated to the side opposite to
the tool rotation direction T during drilling by the drilling device. Then, the head
main body 4B of the bit head 4 is guided by friction with the borehole and by the
third wall surface 7c of the part 7A to be engaged, and thereby the bit head 4 is
retracted as shown in FIG. 2. Thus, the pilot bit 2 and the bit heads 4 can be recovered
with the ring bit 7 being left in the borehole by pulling out the pilot bit 2 together
with the down-the-hole hammer H as it is from the casing pipe 1.
[0059] In this way, according to the drilling tool of the above configuration, the rotating
force in the tool rotation direction T is transmitted from the head main body 4B of
the extended bit head 4 to the part 7A to be engaged of the ring bit 7. Thus, the
rotating force can be efficiently transmitted at a position farther from the axis
O which becomes the rotation center of the pilot bit 2 and the bit head 4. Accordingly,
even in a case where a borehole with a greater internal diameter than the external
diameter of the casing pipe 1 is formed, sufficient rotating force can be transmitted
to the ring bit 7, and drilling performance can be guaranteed.
[0060] Moreover, in the present embodiment, the pilot bit 2 and the bit head 4 protrude
by one step to the tip side of the ring bit 7 as shown in FIG. 1. Therefore, the drilling
tips 6 on the ring bit 7 drill an outer peripheral part of the borehole of which an
inner peripheral part is drilled by the drilling tips 6 on the pilot bit 2 and the
bit head 4, and becomes apt to be crushed. For this reason, the load to the ring bit
7 can be reduced, and more efficient drilling can be performed. Here the tip surfaces
of the pilot bit 2 and the bit head 4 may be made to be flush with the tip surface
of a ring bit 7, and the tip surface of the ring bit 7 may protrude from the tip surfaces
of the pilot bit 2 and the bit head 4.
[0061] Additionally, since the outer peripheral side of the borehole is drilled by the ring
bit 7, in the pilot bit 2 and the bit head 4, it is not necessary to make the radius
of the extended head main body 4B from the axis O as large as the internal diameter
of the borehole. For this reason, a burden to the shaft part 4A of the bit head 4
or the like can be reduced, and damage can be prevented. Moreover, since the ring
bit 7 forms an annular shape, the number and positions of the drilling tips 6 can
be relatively freely set, for example, like the drilling tips 6 being disposed in
ranges other than a range in the circumferential direction where the extended bit
head 4 as shown in FIG. 3 is located. Therefore, degradation of the drilling performance
resulting from partial insufficiency of the drilling tips 6 can also be prevented.
[0062] In the ring bit 7, the part 7A to be engaged is engaged with the extended bit head
4 as described above, while the ring bit 7 is supported by the bit head 4 so as to
be rotatable integrally therewith in the tool rotation direction T and the rotating
force is transmitted to the ring bit 7. Thus, it becomes unnecessary to support the
ring bit 7 with the casing pipe 1, and the internal diameter of the ring bit 7 can
be increased. For this reason, required materials such as a steel material can be
reduced by making the volume of the ring bit 7 small, and even in a case where the
ring bit 7 is left in a borehole after the end of drilling, an increase in construction
cost can be suppressed.
[0063] Additionally, as described above, the inner peripheral part of the borehole is drilled
by the drilling tips 6 on the pilot bit 2 and the bit head 4. Thus, in the present
embodiment, it is not necessary to provide the ring bit 7 with the drilling tip 6,
as shown in FIGS. 1 and 3, in a range where the drilling tips are provided on the
tip surface of the head main body 4B of the extended bit head 4 in the radial direction
from the axis O. For this reason, it is possible to avoid providing more drilling
tips 6 made of expensive cemented carbide than needed in the ring bit 7 left in the
borehole, and cost reduction can be achieved.
[0064] Moreover, in the present embodiment, a recessed portion that is recessed toward the
outer peripheral side is formed in the inner peripheral part of the ring bit 7, and
serves as the part 7A to be engaged. In this regard, for example, it is also possible
to form a protrusion on the tip surface of the ring bit 7 as a part to be engaged
to engage the head main body 4B of the extended bit head 4 with this protrusion in
the tool rotation direction T. However, in that case, there is a concern that the
load resulting from the rotating force may be concentrated on the protrusion to cause
damage, and the volume of the ring bit 7 may also become as great as the protrusion
and material cost may increase. In contrast, in the present embodiment, it is possible
to receive the rotating force with a main body itself of the annular ring bit 7, and
the volume and cost of the ring bit 7 can be further reduced.
[0065] In addition, in the present embodiment, the portion adjacent to the part 7A to be
engaged at the tool rotation direction T side thereof which is formed as the recessed
portion recessed from the inner peripheral part toward the outer peripheral side in
this way in the tip surface of the ring bit 7 faces the ceiling surface 4h of the
engaging part 4E, and serves as the first abutting part 7B abuttable against the ceiling
surface 4h of the engaging part 4E on the tip side in the direction of the axis O.
For example, even if the ring bit 7 collides against the ceiling surface 4h with the
striking force transmitted via the bit head 4 from the pilot bit 2, a shock can be
received over the overall thickness of the ring bit 7, and occurrence of damage or
the like can be prevented. However, the bottom surface that faces the tip side may
be formed in this recessed portion so as to face the ceiling surface 4h, and may be
used as the first abutting part 7B.
[0066] Also in the present embodiment, in inserting the casing pipe 1 into a borehole with
the striking force and the impelling force to the tip side to be applied to the pilot
bit 2, similar to the drilling tools described in PTLs 1 and 2, the casing top 1A
is attached to the tip part of the casing pipe 1 to form the smaller-diameter part
1 B, and the second abutting part 2B of the pilot bit 2 is made to abut against the
smaller-diameter part 1B so as to transmit the striking force and the impelling force.
However, in the present embodiment, the internal diameter of the ring bit 7 is increased
with respect to the internal diameter of this smaller-diameter part 1B. Thus, as described
above, construction cost can be reliably reduced compared to the drilling tool described
in PTL 1 in which the internal diameter of the ring bit has to be made smaller. In
addition, the internal diameter of the ring bit 7 may be equal to the smaller-diameter
part 1B.
[0067] Furthermore, in the present embodiment, the striking force and the impelling force
are transmitted by providing the casing pipe 1 with the smaller-diameter part 1 B
in this way. In contrast, in transmitting the striking force and the impelling force
from the pilot bit 2 to the ring bit 7, the striking force and the impelling force
are not directly transmitted from the pilot bit 2 unlike the drilling tool described
in PTL 1, but the bit head 4 is provided with the third abutting part 4D that is abuttable
against the posterior end surface of the ring bit 7 in an extended state so as to
transmit the striking force and the impelling force from the third abutting part 4D.
For this reason, in a case where the smaller-diameter part 1B is provided as described
above, it is unnecessary to make the internal diameter of the ring bit 7 still smaller,
and it is possible to reduce construction cost even more reliably.
[0068] Moreover, in the present embodiment, in a case where the striking force and the impelling
force are transmitted from the third abutting part 4D provided in the bit head 4 in
this way to the ring bit 7, the greatest radius R of the third abutting part 4D of
the extended bit head 4 from the axis O is greater than the radius r of the outer
peripheral part of the tip of the casing pipe 1 from the axis O, that is, the radius
of the casing top 1A. For this reason, the striking force and the impelling force
can be more reliably transmitted to the ring bit 7 on the outer peripheral side where
drilling is performed, and even in a case where a borehole with a greater internal
diameter than the external diameter of the casing pipe 1 as in the present embodiment
is formed, it is possible to perform drilling much more efficiently.
[0069] Then, in a case where the striking force and impelling force are transmitted from
the third abutting part 4D that is enlarged and has the larger radius R than the radius
r of the outer peripheral part of the tip of the casing pipe 1 to the ring bit 7 in
this way, it is possible to reliably form a borehole with a large internal diameter
without impairing the strength or rigidity of the ring bit 7 even if the thickness
of the ring bit 7 in the direction of the axis O is smaller than the width between
the external and internal diameters of the ring bit 7 like, for example, the present
embodiment. Therefore, according to the present embodiment, the volume of the ring
bit 7 can be further reduced, and a much greater reduction of construction cost can
be achieved.
INDUSTRIAL APPLICABILITY
[0070] As described above, according to the drilling tool of the present invention, even
in a case where a borehole with a greater internal diameter than the external diameter
of the casing pipe is formed, it is possible to transmit sufficient rotating force,
striking force, and impelling force to the ring bit and to perform efficient drilling,
without causing degradation of drilling performance, an increase in construction cost,
or damage to the tool. Therefore, the present invention can be industrially applied.
REFERENCE SIGNS LIST
[0071]
1: CASING PIPE
1A: CASING TOP
1B: SMALLER-DIAMETER PART
2: PILOT BIT
2B: SECOND ABUTTING PART
2C: DISCHARGE GROOVE
3: HOUSING RECESS
3C: FITTING HOLE
4: BIT HEAD
4A: SHAFT PART
4B: HEAD MAIN BODY
4D: THIRD ABUTTING PART
4E: ENGAGING PART
5: PIN
6: DRILLING TIP
7: RING BIT
7A: PART TO BE ENGAGED
7B: FIRST ABUTTING PART
8: SUPPLY HOLE
O: AXIS OF CASING PIPE 1
T: TOOL ROTATION DIRECTION DURING DRILLING
C: CENTERLINE OF FITTING HOLE 3C
H: DOWN-THE-HOLE HAMMER
R: GREATEST RADIUS OF THIRD ABUTTING PART 4D OF EXTENDED BIT HEAD 4 FROM AXIS O
r: RADIUS OF OUTER PERIPHERAL PART OF TIP OF CASING PIPE 1 FROM AXIS O