U.S. patent application number 15/126335 was filed with the patent office on 2017-03-23 for percussive drill bit with multiple sets of front cutting inserts.
The applicant listed for this patent is SANDVIK INTELECTUAL PROPERTY AB. Invention is credited to Rasmus HEMPH, Anders NORDBERG, Anna NORDSTRAND.
Application Number | 20170081920 15/126335 |
Document ID | / |
Family ID | 50289505 |
Filed Date | 2017-03-23 |
United States Patent
Application |
20170081920 |
Kind Code |
A1 |
HEMPH; Rasmus ; et
al. |
March 23, 2017 |
PERCUSSIVE DRILL BIT WITH MULTIPLE SETS OF FRONT CUTTING
INSERTS
Abstract
A percussive rock drill bit includes a head provided with an
elongate shank. A plurality of front cutting inserts are
distributed at a front face of the head and are grouped into a
plurality of sets, with each set being positioned at a different
radial distance from a central axis. Each set of inserts includes
the same number of inserts to maximize drilling rate and service
lifetime of the drill bit.
Inventors: |
HEMPH; Rasmus; (Gavle,
SE) ; NORDSTRAND; Anna; (Gavle, SE) ;
NORDBERG; Anders; (Sandviken, SE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SANDVIK INTELECTUAL PROPERTY AB |
Sandviken |
|
SE |
|
|
Family ID: |
50289505 |
Appl. No.: |
15/126335 |
Filed: |
March 9, 2015 |
PCT Filed: |
March 9, 2015 |
PCT NO: |
PCT/EP2015/054846 |
371 Date: |
September 15, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
E21B 10/46 20130101;
E21B 10/36 20130101; E21B 10/40 20130101 |
International
Class: |
E21B 10/40 20060101
E21B010/40 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 18, 2014 |
EP |
14160500.6 |
Claims
1. A percussive rock drill bit comprising: a head provided at one
end of an elongate shank, the head having a forward facing front
face defined by a perimeter edge; and a plurality of front cutting
inserts distributed at the front face at different radial positions
between a central axis extending through the drill bit and the
perimeter edge of the front face, wherein the front cutting inserts
are grouped into a plurality of sets, each set including an equal
number of inserts positioned at a different radial distance from
the axis, each insert of each respective set of inserts being
positioned at substantially the same radial distance and positioned
circumferentially between a pair of inserts of a neighbouring
radially outer and/or radially inner set inserts, such that the
positions of the inserts of neighbouring sets are staggered
circumferentially.
2. The drill bit as claimed in claim 1, wherein each set includes
from three to six inserts (104).
3. The drill bit as claimed in claim 2, wherein each set includes
four inserts.
4. The drill bit as claimed in claim 1, further comprising from two
to eight sets of front cutting inserts.
5. The drill bit as claimed in claim 1, wherein a diameter of the
inserts within at least some of the sets increases in a direction
radially outward from the axis.
6. The drill bit as claimed in claim 1, wherein a central region of
the front face is positioned axially forward of the perimeter
edge.
7. The drill bit as claimed in claim 1, wherein the front face is
generally convex such that a central region of the front face is
positioned axially forward of the perimeter edge.
8. The drill bit as claimed in claim 1, wherein the inserts of at
least two sets overlap radially.
9. The drill bit as claimed in claim 1, wherein one of the sets
represents a radially innermost set of the front cutting inserts,
the innermost set comprising having the same number of inserts as
the radially outer sets.
10. The drill bit as claimed in claim 1, further comprising a
radially innermost group of front inserts that include a number of
inserts being less than the number of inserts within each of the
sets that are positioned radially outside the sets.
11. The drill bit as claimed in claim 1, further comprising: a
first inner set of inserts positioned radially innermost at or
towards the axis relative to other sets; and a first intermediate
set of inserts positioned radially between the first inner set and
a gauge group of cutting inserts positioned at or towards the
perimeter edge.
12. The drill bit as claimed in claim 11, further comprising a
second intermediate set of inserts positioned radially intermediate
the first intermediate set and the gauge group of inserts.
13. The drill bit as claimed in claim 12, further comprising a
second inner set of inserts positioned radially intermediate the
first inner set and the first intermediate set of inserts.
14. The drill bit as claimed in claim 13, wherein the inner and
intermediate sets are positioned at different axial positions with
the first inner set (205) of inserts positioned axially forwardmost
and the second intermediate set of inserts positioned axially
rearwardmost, the gauge group of inserts being positioned axially
rearward of the second intermediate set.
15. Percussive drilling apparatus comprising: a drill bit, the
drill bit including a head provided at one end of an elongate
shank, the head having a forward facing front face defined by a
perimeter edge, and a plurality of front cutting inserts
distributed at the front face at different radial positions between
a central axis extending through the drill bit and the perimeter
edge of the front face, wherein the front cutting inserts are
grouped into a plurality of sets, each set including an equal
number of inserts positioned at a different radial distance from
the axis, each insert of each respective set of inserts being
positioned at substantially the same radial distance and positioned
circumferentially between a pair of inserts of a neighbouring
radially outer and/or radially inner set inserts, such that the
positions of the inserts of neighbouring sets are staggered
circumferentially
Description
FIELD OF INVENTION
[0001] The present invention relates to a percussive rock drill bit
and in particular, although not exclusively, to a drill bit formed
with a cutting head mounting a plurality of front cutting inserts
arranged into sets with each set distributed at different radial
positions at a front face of the cutting head.
BACKGROUND ART
[0002] Percussion drill bits are widely used both for drilling
relatively shallow bores in hard rock and for creating deep
boreholes. For the latter application, drill strings are typically
used in which a plurality of rods are interconnected to advance the
drill bit and increase the depth of the hole. In `top hammer
drilling` a terrestrial machine is operative to transfer a combined
impact and rotary drive motion to an upper end of the drill string
whilst a drill bit positioned at the lower end is operative to
crush the rock and form the boreholes. In `down the hole` drilling
the impact is delivered not through the upper end of the string,
but by a hammer directly connected to the drill bit within the
hole.
[0003] The drill bit typically comprises a drill head that mounts a
plurality of hard cutting inserts, commonly referred to as buttons.
Such buttons comprise a carbide based material to enhance the
lifetime of the drill bit. Conventionally, the drill bit comprises
a plurality of gauge buttons distributed circumferentially at an
outer perimeter of the head that are configured to engage material
to be crushed and to determine the diameter of the borehole. The
head also mounts a plurality of front buttons provided at a front
face of the head for engaging material to be crushed at the axial
forwardmost region of drilling. Example percussive drill bits are
disclosed in U.S. Pat. No. 3,357,507; U.S. Pat. No. 3,388,756, U.S.
Pat. No. 3,955,635; US 2008/0087473; US 2008/0078584; WO
2012/17460; WO 2006/033606; WO 2009/067073; U.S. Pat. No.
7,527,110; U.S. Pat. No. 7,392,863; EP 2592216; WO 2012/038428 and
EP 2383420.
[0004] Commonly, the cutting inserts are distributed over the front
face in sets or seemingly randomly at different locations not on a
common circumferential path. Typically, the multiplicity of inserts
increases progressively from the radially inner to radially outer
region of the front face. In order for the bit to advance into the
rock during drilling, each insert must crush the rock in its
circumferential path. Where a particular region of the front face
includes a single insert, a full 360.degree. rotation is needed to
advance the bit axially forward. Accordingly, the inserts at those
regions of the bit with the fewer neighbouring inserts have a
higher wear rate and also limit the penetration rate of the bit.
Accordingly, what is required is a drill bit that addresses these
problems.
SUMMARY OF THE INVENTION
[0005] It is a primary objective of the present invention to
provide a percussive drill bit that is configured to maximise a
penetration rate into rock during drilling and to maximise the
service lifetime of the bit as far as possible.
[0006] It is a further specific objective to provide a distribution
of hardened cutting inserts at a drill head part of the bit that is
optimised to withstand frictional wear consistently at varying
radial positions from the axial centre of the bit. It is a further
specific objective to configure the front buttons to maximise the
drilling rate via their organisation and distribution at the front
face relative to the bit axis and a perimeter edge of the drill bit
head.
[0007] According to a first aspect of the present invention there
is provided a percussive rock drill bit comprising: a head provided
at one end of an elongate shank, the head having a forward facing
front face defined by a perimeter edge; a plurality of front
cutting inserts distributed at the front face at different radial
positions between a central axis extending through the drill bit
and the perimeter edge of the front face; wherein the front cutting
inserts are grouped into a plurality of sets with each set
comprising the same number of inserts and positioned at a different
radial distance from the axis and each insert of each respective
set is positioned at substantially the same radial distance
characterised in that: each insert of each set is positioned
circumferentially between a pair of inserts of a neighbouring
radially outer and/or radially inner set such that the positions of
the inserts of neighbouring sets are staggered
circumferentially.
[0008] Reference within this specification to a `cutting insert`
encompasses alternative and equivalent terms such as cutting
buttons and the like being specifically adapted to comprise a
hardness that is greater than the main body of the bit.
[0009] Reference within this specification to `each insert of each
respective set positioned at substantially the same radial
distance` encompasses the axial centres of each of the inserts
within each set positioned on the same circumferential path and at
the same radial separation distance from the axis. The term also
encompasses the axial centres of at least some of the insert within
each respective set being positioned radially outside or inside
other inserts within the same set. Such a variation includes the
positioning of the axial centre of one or more inserts within a set
so as to be eccentric (off-set) relative to the other inserts
within the same set by up to a diameter (D), a radius, a third
diameter (D/3) or a quarter diameter (D/4) of the inserts within
the set.
[0010] Reference within this specification to `angular distance`
encompasses the angle between the inserts and in particular the
angle between neighbouring inserts in the circumferential
direction.
[0011] Circumferentially staggering the positions of the
neighbouring sets of inserts is advantageous for a number of
reasons. Firstly, by positioning the inserts of one set
circumferentially between the inserts of a neighbouring radially
outer and/or radially inner set, the diameters of the inserts of
the neighbouring sets may overlap radially to provide a more
compact design. Such an arrangement also maximises the number of
inserts that may be embedded at the front face. Additionally,
positioning the inserts of the different sets so as to be
circumferentially off-set relative to one another provides a more
efficient and effective cutting action at the drill hole bottom. In
particular, such an arrangement has been found to be more effective
to break the rock as the cutting grooves created in the rock
overlap which acts to chip or fragment the rock to be propelled
radially outward over the cutting face.
[0012] Optionally, each set comprises from three to six cutting
inserts. Preferably, each set comprises four inserts. Such an
arrangement represents an optimised compromise between minimising
wear of the inserts and also minimising an overall weight of the
cutting bit that naturally increases with a larger number of
inserts.
[0013] Preferably, each set comprises from two to eight sets of
front cutting inserts. Optionally, the bit comprises four sets of
inserts. Optionally, an inner set of the inserts represents a
radially innermost set of front buttons. Optionally, the drill bit
comprises an additional set of innermost front buttons that
comprises a number of cutting inserts being less than the number of
inserts within the radially outer sets (with each of these radially
outer sets comprising the same number of cutting inserts in
accordance with the subject invention). An additional set of
innermost inserts having a lower number of inserts relative to the
radially outer sets may be advantageous for use with smaller
diameter drill bit heads.
[0014] Optionally, a diameter of the inserts of each set increases
in a direction radially outward from the axis. Alternatively, a
diameter of the inserts within all or some of the sets may be
substantially equal. Such configurations are beneficial to maximise
the service lifetime of the drill bit having consideration of the
different angular velocities of the inserts within the different
sets. That is, the radially outer sets may comprise larger diameter
inserts relative to the inner sets to compensate for the increased
angular velocity during drilling so as to provide a uniform insert
wear rate across all sets.
[0015] Optionally, a central region of the front face is positioned
axially forward of the perimeter edge. Optionally, the front face
is generally convex such that a central region of the front face is
positioned axially forward of the perimeter edge. The front face
may be dome-shaped that is tilted axially rearward towards the
shank such that a radially innermost part of the front face is
positioned axially forward relative to a perimeter edge of the
head. According to further embodiments, the axially forward region
of the head may be substantially planar or flat or comprise a
cavity so as to be generally concave having a peripheral collar
that projects axially forward of a radially innermost region within
which is mounted the plurality of sets of inserts. In some
embodiments, the annular collar of the head may comprise one or a
plurality of sets of the front buttons in addition to gauge
buttons.
[0016] Advantageously, the head and shank are formed integrally.
Such an arrangement is advantageous to provide a robust and
reliable bit configured to withstand the significant torque and
axial loading forces transmitted during percussive drilling.
Additionally, the bit head is formed as a single piece component
integrally with the elongate shaft to optimise the strength of the
bit to provide a rigid bit that is not susceptible to independent
movement of separate components which is otherwise disadvantageous
with multi-component bits.
[0017] Preferably, the front face is formed as a single and common
support substrate for the inserts such that all the inserts are
embedded in a single common face. Such an arrangement is
advantageous to distribute the loading forces uniformly at the
front face to maximise the service lifetime of the bit and to
minimise wear. The inserts are embedded directly in the bit head,
via the front face, and are not mounted via an intermediate and
detachable body. Such an arrangement is advantageous to provide a
structurally robust configuration such that the inserts are
resistant to the torque and axial forces encountered during
use.
[0018] Preferably, an angle between neighbouring inserts within
each respective set is substantially equal. That is, the separation
distance between neighbouring inserts in a circumferential
direction around the central axis within each set is substantially
equal. Preferably the angle between neighbouring inserts (within
each set) is substantially equal for all sets at the front face.
Preferably, a distance in the circumferential direction between
neighbouring inserts within each set increases between the sets in
a direction radially outward from the central axis. As will be
appreciated, the increase in circumferential separation distance
between inserts within the sets in a direction progressively
outward from the axis is a consequence of the uniform angular
separation between neighbouring inserts within all sets.
[0019] Optionally, the positions of the inserts of at least two
sets overlap radially. Optionally the positions of the inserts of
three, four, five, six or all sets overlap radially. Optionally the
radially inner, radially outer and/or mid-radial sets comprise
inserts that are positioned to overlap radially. Such a
configuration optimises the cutting efficiency of the bit head and
maximises the penetration rate as the density of the distribution
of the cutting grooves in the rock is enhanced as at least some of
the as-formed grooves overlap radially.
[0020] Optionally, the drill bit may comprise a first inner set
positioned radially innermost at or towards the axis relative to
other sets; and a first intermediate set positioned radially
between the first inner set and a gauge group of cutting inserts
positioned at or towards the perimeter edge. Optionally, the drill
bit may further comprise a second intermediate set positioned
radially intermediate the first intermediate set and the gauge
group. Optionally, the drill bit may further comprise a second
inner set positioned radially intermediate the first inner set and
the first intermediate set. Such an arrangement is beneficial to
maximise the drilling rate and service lifetime of the drill bit
configured with a domed or convex front face. As will be
appreciated, a number of different sets of inserts may vary
according to the size, geometry and shape profile of the bit head
including in particular flat and concave front face regions.
[0021] According to a second aspect of the present invention there
is provided percussive drilling apparatus comprising a drill bit as
claimed herein.
BRIEF DESCRIPTION OF DRAWINGS
[0022] A specific implementation of the present invention will now
be described, by way of example only, and with reference to the
accompanying drawings in which:
[0023] FIG. 1 is an external perspective view of a drill bit
comprising a plurality of front cutting inserts distributed and
arranged into groups or sets at a head part of the bit according to
a specific implementation of the present invention;
[0024] FIG. 2 is a further perspective view of the head part of the
drill bit of FIG. 1;
[0025] FIG. 3 is a plan view of the front face of the head of FIG.
2;
[0026] FIG. 4 is a side elevation view of the head of FIG. 2.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT OF THE INVENTION
[0027] Referring to FIGS. 1 to 4 a drill bit 100 comprises a drill
head 101 formed at one end of a generally elongate shaft 102 with
both head 101 and shaft 102 centred on a longitudinal axis 108.
Shaft 102 comprises splines 109 to engage with corresponding
splines of a drive tool (not shown) that surrounds the shaft 102
during use. Head 101 comprises an axially forward section 111 and
an axially rearward skirt section 110 that interfaces with shaft
102. Section 111 comprises a larger diameter than skirt 110 and is
defined, in part, by an annual outermost perimeter edge 106. A
forward facing front face indicated generally by reference 105
projects axially forward from edge 106 such that the forward facing
part of section 111 is generally convex and comprises a dome-shaped
profile when viewed from the side or in cross section. Forward head
section 111 tapers rearwardly from edge 106 via a short
longitudinally extending trailing surface 107 that mates with an
axially forward region of skirt 110.
[0028] Front face 105 may be considered to be segmented radially
into a plurality of annular regions extending between axis 108 and
perimeter edge 106. In particular, face 105 comprises an outer
peripheral region 200 that is defined at its perimeter by edge 106.
Region 200 is chamfered to taper rearwardly relative to axis 108 so
as to be declined relative to a radially innermost region 203 that
is aligned substantially perpendicular to axis 108. Region 203 is
axially raised to form a platform 204 that is upstanding from a
first intermediate region 202. Region 202 is also aligned
substantially perpendicular to axis 108. The first intermediate
region 202 is surrounded by a second intermediate region 201 that
is chamfered to taper rearwardly relative to axis 108. Second
intermediate region 201 is positioned radially between the first
intermediate region 202 and the perimeter region 200. The declined
angle by which region 201 extends from axis 108 is less than the
corresponding angle by which peripheral region 200 extends relative
to axis 108. Accordingly, and as indicated in FIG. 4, regions 200
to 203 collectively define a generally convex front face 105 that
projects axially forward from perimeter edge 106.
[0029] Head 101 mounts a plurality of hardened cutting inserts
indicated generally by references 103 and 104 formed from a carbide
based material such as cemented carbide or tungsten carbide for
example. According to the specific implementation, inserts 103, 104
are generally dome-shaped. However, according to further specific
embodiments, inserts 103, 104 may have axially forward cutting tips
that are rounded, conical, ballistic, semi-spherical, flat or
pointed according to conventional cutting insert configurations.
The inserts 103, 104 are embedded in head section 111 so as to
stand axially proud of front face 105. As will be appreciated,
gauge inserts 103 are positioned at or towards perimeter edge 106
to determine and maintain a predetermined diameter of the borehole
during formation. Gauge inserts 103 are tilted radially outward so
as to be generally inclined and outward facing from axis 108
consistent with peripheral region 200.
[0030] Referring to FIGS. 2 and 3, inserts 104 that are positioned
radially within gauge inserts 103 are grouped into a plurality of
sets with each set positioned at a different radial distance from
axis 108. Additionally, each insert 104 of each set is positioned
at the same radial separation from axis 108 such that the inserts
104 of each set are positioned at a common circumferential path
around axis 108.
[0031] According to the specific embodiment, inserts 104 are
divided into a first inner set 205 positioned radially innermost at
or towards axis 108 and mounted within inner region 203. A second
inner set 206 surrounds first set 205 and is mounted at region 202.
A first intermediate set 207 surrounds second inner set 206 and is
mounted at region 201. A second intermediate set 208 (also mounted
at region 201) is positioned radially outside first intermediate
set 207 and radially inside the group of gauge inserts 103.
Advantageously, to maximise the service lifetime and to maximise
the penetration rate of bit 100, each set 205 to 208 comprises the
same number of inserts 104 being four inserts per set according to
the present embodiment. Accordingly, during rotation of bit 100
about axis 108, each insert 104 of each set is configured to
collectively define an annular cutting path when bit 100 is rotated
through 90.degree. during drilling. This is to be contrasted with
conventional arrangements where for example the innermost set 205
comprises a single insert 104 that is required to be rotated
through 360.degree. to complete a single annular cutting path
within the rock. As will be appreciated, the penetration rate of
the present bit 100 is appreciably greater than conventional
arrangements and the wear of the inserts is lower and more evenly
distributed.
[0032] First and second intermediate sets 207, 208 comprise inserts
104 having axes that are inclined to taper radially outward from
axis 108 consistent with the angled orientation of region 201
(relative to axis 108) within which sets 207, 208 are mounted. In
contrast, inserts 104 within first and second inner sets 205, 206
have axes that are aligned substantially parallel with axis 108. As
illustrated in FIG. 4, the angle by which the inserts 104 of first
and second intermediate sets 207, 208 extend radially outward from
axis 108 is less than a corresponding angle by which gauge inserts
103 tilt radially outward from axis 108.
[0033] Referring of FIG. 3, each insert 104 of first inner set 205
is separated from axis 108 by the same radial separation distance
R1. Each insert 104 of the second inner set 206 is separated from
axis 108 by the same radial separation distance R2. Each insert 104
of the first intermediate set 207 is separated from axis 108 by the
same radial separation distance R3. Each insert 104 of the second
intermediate set 208 is separated from axis 108 by the same radial
separation distance R4. According to the present embodiment,
R1<R2<R3<R4. However, the magnitude of the sequential
increase from R1 to R4 (via R2 and R3) is-non uniform according to
the present embodiment.
[0034] According to the present embodiment, an angle by which
adjacent inserts 104 (neighbouring in a circumferential direction)
are separated is 90.degree. within all sets 205 to 208. That is,
the angle between adjacent inserts 104 of each set is uniform and
equal over all sets 205 to 208. Such a configuration is
advantageous in that the inserts wear consistently at varying
radial positions from the axial centre of the bit and the bit
penetration rate is maximised.
[0035] Referring to FIG. 3, the position of each set of inserts 205
to 208 is circumferentially staggered at front face 105. That is,
each insert 104 of each set 205 to 208 is positioned
circumferentially between two inserts 104 of a neighbouring
radially inner or radially outer set 205 to 208. In particular,
each insert 104 of set 205 is positioned circumferentially between
the inserts 104 of set 206; the inserts 104 of set 206 are
positioned circumferentially between the inserts 104 of set 205 and
207; the inserts of set 207 are positioned circumferentially
between the inserts 104 of sets 206 and 208 and the inserts 104 of
set 208 are positioned circumferentially between the inserts 104 of
set 207. According to the specific implementation, the inserts 104
of set 208 are also positioned circumferentially between the gauge
inserts 103. Additionally, and when viewed in plan as illustrated
in FIG. 3, the respective diameters of the inserts 104 of at least
some of the sets overlap radially with this being possible due to
the circumferential staggering of the inserts of neighbouring sets.
In particular, the diameters of the inserts 104 of sets 207 and 208
overlap radially. Also, as will be noted from FIG. 3, each insert
104 of each set is positioned approximately mid-way
circumferentially between the pair of inserts 104 of the
neighbouring set. With this configuration, the inserts of all sets
are spaced apart so as to be positioned as far apart as possible at
front face 105. Such an arrangement is advantageous to maximise the
cutting efficiency of each insert and provide a structurally strong
bit. The circumferential staggering of the inserts and the radial
overlap between inserts 104 of sets 207, 208 is further
advantageous to provide a radially compact head 101 and to maximise
the number of sets 205 to 208 embedded at front face 105. The
present configuration functions to aggressively break the rock at
the drill hole bottom, due in part, to partial radial overlap of
sets 207, 208 and the grouping of the inserts 104 into the sets
(206, 208) positioned at different radial distances from axis
108.
[0036] The mounting of the first inner set 205 at platform 204 is
advantageous to provide collaring as the inserts of set 205 meet
the rock initially during drilling. Such an arrangement is also
considered to be beneficial for enhancing penetration speed due to
the provision of a local `lower` level within the rock into which
cracks may form during drilling. Additionally, the raised or convex
front face 105 having raised innermost region 203 is beneficial to
minimise the risk of cracks within head 101 originating from
flushing holes 112 extending within head 101.
[0037] According to the present embodiment, a diameter of inserts
104 of the first inner set 205 is less than a corresponding
diameter of second inner set 206. Similarly, a diameter of the
inserts 104 of the first and second inner sets 205, 206 is less
than a corresponding diameter of inserts 104 of the first and
second intermediate sets 207, 208. As shown, and according to the
present embodiment, a diameter of the gauge inserts 103 is greater
than each one of the inner and intermediate inserts 104 of sets 205
to 208. However, according to further embodiments, a diameter of
inserts 104 over all sets 205 to 208 may be substantially equal or
may be smaller or larger than the diameter of gauge inserts
103.
[0038] According to further embodiments, the axially forward facing
region of head section 111 may be substantially planar (flat) or
comprise a concave region defined by an axially forward extending
perimeter collar. Additionally, head 101 may comprise peripheral
axially extending flushing channels associated with each flushing
hole 112 to facilitate axially rearward transport of the flushing
fluid and the rock fragments and fines.
[0039] Additionally, according to further embodiments, head 101 may
comprise an additional innermost set of inserts 104 positioned
radially within set 205 comprising the same number or less inserts
(at the same and common radial position) relative to the radially
outer sets 205 to 208. That is, an innermost set of inserts 104 may
comprise one, two or three inserts 104 whilst the radially outer
sets 205 to 208 may comprise four or more inserts with the inserts
104 of each set positioned at the same radial separation distance
R1 to R4 relative to axis 108.
* * * * *