Note: Descriptions are shown in the official language in which they were submitted.
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SYMMETRICAL BIT FOR DIRECTIONAL DRILLING TOOL
Field of the Invention
The present invention relates to horizontal directional drilling and, in
particular, to a
drill bit for a directional drilling tool.
Background to the Invention
It is known to use directional drilling apparatus to form substantially
horizontal drill
holes in the ground for installing electrical or telephone cables, gas or
water pipes, or
HI the like. The directional drilling equipment usually comprises a
percussion drill bit
operable through a drill string and includes a steering device so that the
drill bit can be
steered in a desired substantially horizontal direction below and along the
route of roads
and streets, and under river beds, roads and the like.
In a typical directional drilling system, a drill string incorporating a
percussion hammer,
to apply axial impact forces to the drill bit, is utilised to drill an initial
pilot hole of a
smaller diameter (for example 133 mm) than the diameter of the ultimate
passage
desired. The ultimate drill passage may have a diameter of anything from 200
mm to
760 mm depending upon the size of the cables, pipes or conduits to be inserted
in the
drilled passageway. With the known horizontal drilling method, when the
percussion
drill bit and hammer breaks through the surface after having drilled the pilot
hole, the
percussion hammer system is removed and a reamer bit is fixed to the
protruding drill
rod. Using the drill string, the reamer bit is then pulled back against the
face of the
drilled pilot hole. The present invention is concerned with directional
drilling tools for
drilling the initial pilot hole.
Steering of such horizontal directional drill tools is typically carried out
by providing a
percussion drill bit having an asymmetric chisel-shaped or slant-faced bit
head. US
Patent No. 4.867,255 discloses a technique for steering a downhole hammer. The
hammer has a forward most cutting bit, which is asymmetrically configured with
respect
to the elongation axis of the hammer. In order to cause the hammer to move
along a
straight path, the asymmetrical cutting bit is rotated continuously either
clockwise or
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counter clockwise at a constant speed. To cause the hammer to turn, rotation
of the
cutting bit is modulated in a particular way, depending upon how the hammer is
to turn.
Other directional drilling apparatuses having slant-faced bit heads are
described in US
Patent No. 6,397,956, US Patent No. 6,454,025, US Patent No. 6,533,052 and US
Patent
No. 6,705,415.
One example of a known directional drilling apparatus using a slant-faced bit
head will
now be described with reference to Figures 1, 2, 5a, 5b and 6 of the
accompanying
drawings. As shown in Figure 6, the percussion drill bit has a head portion
formed with
an axially extending shank. The head portion of the bit is similar to that of
a normal
percussion drill bit, but with a portion machined off one side of the bit to
provide a slant
face for steering purposes. In the example shown, the slant face is provided
with flat-
wear carbides, although these are not essential. Figures 1, 2, 5a and 5b show
the slant-
faced bit in assembly with the hammer and a locating device known as a sonde.
The
sonde is located in a locator housing in a fixed axial and angular location.
The sonde
sends out a signal which provides data at the surface which indicates depth,
angle or
pitch and angular position of the drill tool. The locator housing is
threadably connected
to a hammer with the slant-faced bit located at the end of the hammer. The
connection
between the locator housing and the hammer can be made using either a bent or
a
straight sub. A bent sub allows for more aggressive steering of the tool. The
bend is
typically of the order of 1 to 3 degrees. The bit is assembled so that the
slant face is
located at the six o'clock position of the sonde.
In order to steer the hammer in an upward direction, the sonde locator is
checked so that
its angular position reads six o'clock, indicating that the slant face of the
bit is at the six
o'clock position. The assembly is rocked about this point by about 60 degrees
(or two
"hours") either side of the initial point as the hammer drills forward,
causing the
hammer to steer upwards. When the required direction has been achieved, normal
rotation is applied and the system drills straight.
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There are a number of disadvantages of this type of system. In particular, all
of the
steering forces act on a single area of the bit which can cause extreme
bending stresses
on the bit shank and these stresses can lead to failure of the bit.
Summary of the Invention
According to an aspect of the present invention, there is provided a
percussion drill bit
for directional drilling tools comprising:
a head portion formed with an axially extending shank;
engagement means on the shank engageable with complementary engagement
means formed on a drive chuck whereby rotational drive from the chuck may be
transmitted to the shank;
characterised in that the head portion of the drill bit is formed with a
plurality of
slant faces arranged symmetrically about the head portion.
The term "slant faces" is used herein to indicate that the faces are angled
with respect to
an elongate axis of the bit.
The term "arranged symmetrically" is used herein to indicate that the slant
faces are
arranged radially around the head portion of the bit in such a way that the
positioning of
the slant faces is symmetrical about at least one axis through the head
portion of the bit.
Preferably, the slant faces are spaced at regular intervals around the
circumference of
the head portion of the bit.
In one embodiment, each of the slant faces comprises a substantially planar
surface
angled with respect to an elongate axis of the bit. In another embodiment,
each of the
slant faces may comprise a plurality of contiguous planar surfaces, wherein
each of the
planar surfaces is angled with respect to the elongate axis of the bit. Each
of the planar
surfaces may be continuous or non-continuous.
The angle at which the slant faces are arranged with respect to the elongate
axis of the
bit may be selected depending on how aggressively the bit is required to
steer. The less
acute the angle of the slant faces, the more aggressively the bit may be
steered.
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An advantage of the present invention is that, because a plurality of slant
faces are
provided, the forces on the bit due to steering can be spread over the
circumference of
the bit by indexing the bit in the chuck. In this way, each of the slant faces
may be used
in turn to steer the bit, so that no one area of the bit shank is subjected to
extreme
bending stresses, thereby increasing the lifetime of the bit.
The head portion of the drill bit is preferably formed with an odd number of
slant faces.
This allows the slant faces to be arranged around the circumference of the bit
such that
carbides may be provided diametrically opposite to each slant face to allow
cutting of
material in the steering direction, while maintaining symmetry of the bit. For
example,
if the tool is to be steered upwards, the bit may be positioned such that one
of the slant
faces is at the six o'clock position in the drill assembly, that is, at the
bottom of the
assembly. In order to drill upwards, it is preferable that carbides are
provided at the top
of the drill bit, that is, at the 12 o'clock position, to allow the drill to
move upwards.
Ideally, the head portion of the drill bit is provided with three slant faces.
The slant faces may be provided with a hard-wearing material to improve the
wear
resistance of the slant faces. In one embodiment, the hard-wearing material is
provided
in the form of flat wear carbides. In another embodiment, a layer of a hard-
wearing
material may be applied to the slant faces. In other embodiments, strips of
the material
may be applied to the slant faces.
In a preferred embodiment, the engagement means comprises a plurality of
axially
extending splines on the shank slideably engageable with complementary splines
formed on the drive chuck. Ideally, the number of splines is an integer
multiple of the
number of slant faces provided on the bit.
Brief Description of the Drawings
Figure 1 is a side elevation of a known horizontal directional drilling tool;
Figure 2 is a side elevation of an alternate embodiment of a known horizontal
directional drilling tool;
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Figure 3 is a side elevation of a horizontal directional drilling tool in
accordance with an
embodiment of the invention;
Figure 4 is a side elevation of a horizontal directional drilling tool in
accordance with an
alternate embodiment of the invention;
5 Figure 5a is an isometric view of the drilling tool of Figure 1;
Figure 5b is an isometric view of the drilling tool of Figure 2;
Figure Sc is an isometric view of the drilling tool of Figure 3;
Figure 5d is an isometric view of the drilling tool of Figure 4;
Figure 6 is an isometric view of the percussion drilling bit of the drilling
tool of Figures
1 and 2;
Figure 7 is an isometric view of a percussion drilling bit according to an
embodiment of
the invention;
Figure 8 is an isometric view of a percussion drilling bit according to an
alternate
embodiment of the invention;
Figure 9 is a plan view of the drilling bit of Figure 7; and
Figure 10 is a plan view of the drilling bit of Figure 8.
Detailed Description of the Drawings
Referring first to Figures 7 and 9, there is illustrated a percussion drill
bit 1 for a
directional drilling tool 10. The drill bit 1 comprises a head portion 2
formed with an
axially extending shank 3. The head portion has a front cutting face 6, which
is
provided with a plurality of carbide buttons or inserts 7. Gauge row carbides
8 are
provided at the periphery of the bit. The drill bit further comprises twelve
axially
extending splines 4 on the stub shank 3 slideably engageable with
complementary
splines formed on a drive chuck. In accordance with the invention, the head
portion 2
of the drill bit 1 is formed with three slant faces 5 arranged symmetrically
about the
head portion 2. The three slant faces 5 are equally spaced around the
circumference of
the head portion 2 of the bit. Each of the slant faces 5 is provided with a
plurality of flat
wear carbides 9.
As can be seen from Figure 7, the slant faces 5 are inclined at an angle with
respect to
the elongate axis X-X of the bit. Each of the slant faces 5 comprises two
contiguous
planar surfaces 5a and 5b, and each of the planar surfaces 5a, 5b is angled
with respect
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to the elongate axis of the bit. In the illustrated embodiment, each of the
planar surfaces
is non-continuous, in the sense that each of the surfaces is interrupted by a
channel 15 in
the bit head 2.
As can be seen from Figure 9, the slant faces 5 are arranged radially around
the head
portion 2 of the bit 1 at regular intervals in such a way that the positioning
of the slant
faces is symmetrical about axis Y-Y.
Because the head portion 2 of the drill bit 1 is formed with an odd number of
slant faces,
the slant faces can be arranged around the circumference of the bit such that
gauge row
carbides 8 are provided diametrically opposite to each slanted face.
An alternate embodiment of the percussion drill bit of the invention is shown
in Figures
8 and 10. In this embodiment, a fewer number of larger carbides 8 are provided
on the
gauge. This allows larger slant faces 5 to be machined off the bit head 2, so
that each of
the slant faces has a larger surface area.
Figures 3, 4, Sc and 5d show the percussion drill bit 1 according to the
present invention
in assembly with a hammer and a locating device known as a sonde. A
directional
drilling tool 10 comprises a locator housing 11, a sub 12 and a hammer 13. The
drill bit
1 is disposed at a forward end 14 of the hammer 13. The sonde is located in
the locator
housing 11 in a fixed axial and angular location, as described above in
relation to
Figures 1 and 2. The sonde sends out a signal which provides data at the
surface which
indicates depth, angle or pitch and angular position of the drill tool. The
locator housing
11 is threadably connected to a hammer 13 with the symmetrical slant-faced bit
located
at the end 14 of the hammer. The connection between the locator housing and
the
hammer can be made using either a bent or a straight sub 12. A bent sub allows
for
more aggressive steering of the tool. As shown in Figure 3, the bend is
typically of the
order of 1 to 3 degrees. The bit is assembled so that one of the slant faces 5
is located at
the six o'clock position of the sonde.
In order to steer the hammer 13 in an upward direction, the sonde locator is
checked so
that its angular position reads six o'clock, indicating that a slant-face is
at the six
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o'clock position. The assembly is rocked by about 60 degrees (or 2 "hours")
either side
of this initial position as the hammer drills forward, causing the hammer to
steer
upwards. When the required direction has been achieved, normal rotation is
applied and
the system drills straight. In order to distribute the forces on the bit 1 due
to steering,
the bit may be indexed in the chuck. In this way, each of the slant faces 5
may be used
in turn to steer the bit, so that no one area of the bit shank is subjected to
extreme
bending stresses, thereby increasing the lifetime of the bit.
The words "comprises/comprising" and the words "having/including" when used
herein
with reference to the present invention are used to specify the presence of
stated
features, integers, steps or components but does not preclude the presence or
addition of
one or more other features, integers, steps, components or groups thereof
It is appreciated that certain features of the invention, which are, for
clarity, described in
the context of separate embodiments, may also be provided in combination in a
single
embodiment. Conversely, various features of the invention which are, for
brevity,
described in the context of a single embodiment, may also be provided
separately or in
any suitable sub-combination.
