U.S. patent application number 13/825533 was filed with the patent office on 2013-07-18 for rock drill bit for percussive drilling and a rock drill bit button.
This patent application is currently assigned to SANDVIK INTELLECTUAL PROPERTY AB. The applicant listed for this patent is Mauri Esko, Markku Keskiniva, Pauli Lemmetty, Juha Piispanen. Invention is credited to Mauri Esko, Markku Keskiniva, Pauli Lemmetty, Juha Piispanen.
Application Number | 20130180785 13/825533 |
Document ID | / |
Family ID | 43500325 |
Filed Date | 2013-07-18 |
United States Patent
Application |
20130180785 |
Kind Code |
A1 |
Esko; Mauri ; et
al. |
July 18, 2013 |
ROCK DRILL BIT FOR PERCUSSIVE DRILLING AND A ROCK DRILL BIT
BUTTON
Abstract
A button and a rock drill bit for percussive drilling including
a bit head attached at an end of a drill element of a drilling
assembly. The bit head has at a front end, as seen in the intended
drilling direction, a plurality of buttons distributed over the bit
head to engage material to be crushed. At least one of the buttons
has a shank portion made of a substrate material of particles of a
first material embedded in a binder phase. The first material is
harder than the binder phase. The shank portion at least partially
forms a bearing portion, the material of which is harder than the
binder phase. At least one of the buttons is allowed to rotate
about its own respective symmetry axis.
Inventors: |
Esko; Mauri; (Ikaalinen,
FI) ; Keskiniva; Markku; (Tampere, FI) ;
Piispanen; Juha; (Tampere, FI) ; Lemmetty; Pauli;
(Lempaala, FI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Esko; Mauri
Keskiniva; Markku
Piispanen; Juha
Lemmetty; Pauli |
Ikaalinen
Tampere
Tampere
Lempaala |
|
FI
FI
FI
FI |
|
|
Assignee: |
SANDVIK INTELLECTUAL PROPERTY
AB
Sandviken
SE
|
Family ID: |
43500325 |
Appl. No.: |
13/825533 |
Filed: |
September 20, 2011 |
PCT Filed: |
September 20, 2011 |
PCT NO: |
PCT/EP2011/066323 |
371 Date: |
March 21, 2013 |
Current U.S.
Class: |
175/415 ;
175/428 |
Current CPC
Class: |
E21B 10/36 20130101 |
Class at
Publication: |
175/415 ;
175/428 |
International
Class: |
E21B 10/36 20060101
E21B010/36 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 22, 2010 |
EP |
10178387.6 |
Claims
1. A rock drill bit for percussive drilling comprising a bit head
attached at an end of a drill element of a drilling assembly, said
bit head having at a front end and as seen in an intended drilling
direction a plurality of buttons distributed over said bit head to
engage material to be crushed, at least one of said plurality of
buttons having a shank portion of a substrate material including
particles of a first material embedded in a binder phase, said
first material being harder than the binder phase, wherein the
shank portion at least partially forms a bearing portion, the
bearing portion being made of a material that is harder than the
binder phase, and wherein at least one of said plurality of buttons
is allowed to rotate about its respective symmetry axis.
2. A rock drill bit according to claim 1, wherein the material of
the bearing portion is substantially homogenous.
3. A rock drill bit according to claim 1, wherein the bearing
portion comprises a material generally free from particles that are
harder than the surrounding material.
4. A rock drill bit according to claim 1, wherein the bearing
portion at is at least partially coated with a barrier coating.
5. A rock drill bit according to claim 1, wherein the bearing
portion has a friction coefficient against steel which is less than
0.5, preferably in the range of 0.1-0.49, and/or most preferably in
the range of 0.2-0.4.
6. A rock drill bit according to claim 1, wherein the bearing
portion has a microhardness (HV 0.05) of at least 3000, preferably
in the range of 3000-3500, and/or most preferably in the range of
3100-3400.
7. A rock drill bit according to anyone of claims 1 to 6, claim 1,
wherein the material of the bearing portion is selected from the
group of titanium-aluminium nitride (TiAlN), aluminum-chromium
nitride (AlCrN), titanium carbide (TiC), titanium nitride (TiN),
chromium nitride (CrN), zirconium nitride (ZrN) and/or diamond
coatings, or mixtures of such.
8. A rock drill bit according to claim 1, wherein each of the
plurality of buttons includes a button retaining means.
9. A rock drill bit according to claim 1, wherein a base portion
(44, 55) of at least one of the plurality of buttons rests against
or contacts a bottom of a button hole to transfer impact forces to
the button while allowing the base portion to move thereon when
rotating.
10. A rotatable rock drill bit button, comprising: a shank portion
of a substrate material; particles of a first material embedded in
a binder phase, said first material being harder than the binder
phase, wherein the shank portion at least partially forms a bearing
portion, the bearing portion being made of a material that is
harder than the binder phase.
11. A rock drill bit button according to claim 10, wherein the
material of the bearing portion is substantially homogenous.
12. A rock drill bit button according to claim 10, wherein the
bearing portion comprises a material generally free from particles
that are harder than the surrounding material.
13. A rock drill bit button according to claim 10, wherein the
bearing portion has a friction coefficient against steel which is
less than 0.5, preferably in the range of 0.1-0.5, and/or most
preferably in the range of 0.2-0.4.
14. A rock drill bit button according to claim 10, wherein the
bearing portion has a microhardness (HV 0.05) of at least 3000,
preferably in the range of 3000-3500, and/or most preferably in the
range of 3100-3400.
15. A rock drill bit button according to claim 10, wherein the
bearing portion is selected from the group of titanium-aluminium
nitride (TiAlN), aluminum-chromium nitride (AlCrN), titanium
carbide (TiC), titanium nitride (TiN), chromium nitride (CrN),
zirconium nitride (ZrN) and/or diamond coatings, or mixtures of
such.
16. A rock drill bit button according to claim 10, wherein each of
the plurality of buttons includes a button retaining means.
17. (canceled)
Description
TECHNICAL FIELD OF THE INVENTION AND BACKGROUND ART
[0001] The present invention relates to a rock drill bit for
percussive drilling and a rock drill bit button according to the
preambles of the independent claims.
[0002] The invention is not restricted to any type of drilling
assembly for use of a said rock drill bit, but the former may be a
down-the-hole hammer drill just as well as a top hammer drill,
although the rock drill bit shown is especially intended for the
latter type.
[0003] Furthermore, said rock drill bit may have any conceivable
size and has normally a diameter of 30 mm-300 mm. The same absence
of limitations applies to the intended percussion frequency and
rotational speed of the rock drill bit in operation, although it
may be mentioned that these are typically within the ranges 20
Hz-100 Hz and 20-500 revolutions per minute, respectively, but the
invention does not exclude the use of the rock drill bit in high
frequency assemblies operating at a frequency above 250 Hz and
which may reach more than 1 kHz.
[0004] A known so-called standard rock drill bit 1 of the type
defined in the introduction will now be described while referring
to both FIG. 1 and FIG. 2. The drill bit has a bit head 2
configured to be attached at an end of a drill element, for example
in the form of a drill tube or drill rod, of a drilling assembly
and having a diameter larger than that of a said drill element.
This drill element is not shown in these figures but may be
intended to be received in a so-called skirt 3 integral with a bit
head and having a smaller diameter than the bit head. Other ways of
connecting the drill bit to the drill element are conceivable and
known within the art. The bit head has at a front end 4 as seen in
the intended drilling direction a plurality of pressed-in gauge
buttons 5 distributed along the circumference of the bit button
head 2. The gauge buttons are configured to engage material to be
crushed and to determine the diameter of a hole 6 (see FIG. 1) to
be drilled by the rock drill bit. These gauge buttons are made of
hard material, such as cemented carbide or tungsten carbide. Front
buttons 7 also of hard material are pressed into a front surface 8
for engaging material to be crushed. It is also indicated how a
flush channel opens at the front by a flushing hole 9 in the front
surface.
[0005] In operation the gauge buttons 5 will engage and break rock
close to the walls of a hole 6 in which the rock drill bit with
said rod is located and the front buttons 7 will break rock closer
to the centre of such a hole by impacts carried out by the rock
drill bit in the direction of the arrow A. The drill bit will
rotate somewhat, typically about 5.degree., between each such
impact.
[0006] The operation efficiency of a rock drill bit of this type is
of course an important feature and this may be expressed as the
penetration speed of the rock drill bit defined as the length of a
hole drilled per time unit (meter/minute). The penetration speed of
known rock drill bits of this type is dependent upon the wear of
said buttons, especially the gauge buttons. It is indicated in FIG.
2 that during the operation of such a rock drill bit material is
abraded at the periphery of the gauge buttons resulting in a flat
surface 10 there, which makes them less sharp and reduces the
penetration speed. These flat surfaces 10 will during the operation
of the rock drill bit grow and finally result in a diameter of a
hole drilled determined by said gauge buttons being so much reduced
that the rock drill bit has to be replaced. It is of course an
on-going attempt to increase the penetration speed and prolong the
life time of a rock drill bit of the type defined in the
introduction.
SUMMARY OF THE INVENTION
[0007] The object of the present invention is to provide a rock
drill bit of the type defined in the introduction being improved in
at least some aspect with respect to such rock drill bits already
known.
[0008] This object is according to the invention obtained by
providing such a rock drill bit in which at least one of said
buttons having a shank portion at least partially comprising a
bearing portion, the material of which is harder than the binder
phase, and allowing the button to rotate about its own symmetry
axis. By rotatably fitting at least one said button in the bit head
this button will while drilling be influenced by the impacts
thereof and rotation of the rock drill bit to rotate about its own
symmetry axis, so that the parts of said button engaging rock will
vary and the button will be evenly worn and by that self-sharpened.
This means that this button will thanks to the self-sharpening
effect maintain its contribution to the penetration speed of the
rock drill bit longer than would it be fixed in the bit head. The
provision of a bearing portion on the button will substantially
avoid any grinding action on the hole wall.
[0009] According to an embodiment of the invention the material of
the bearing portion is substantially homogenous or stated another
way it comprises a material generally free from particles that are
harder than the surrounding material so as to avoid exposure of
abrasive particles towards the hole wall.
[0010] According to another embodiment of the invention the bearing
portion is at least partially coated with a barrier coating, which
substantially stops dissolution of binder phase.
[0011] According to another embodiment of the invention the bearing
portion can have a friction coefficient against steel which is less
than 0.5 that will substantially avoid wear on the hole wall.
[0012] According to another embodiment of the invention the bearing
portion may have a microhardness (HV 0.05) of at least 3000 to make
the bearing portion endure abrasion.
[0013] According to another embodiment of the invention the bearing
portion comprises anyone of or several of titanium-aluminium
nitride (TiAlN), aluminum-chromium nitride (AlCrN), titanium
carbide (TiC), titanium nitride (TiN), chromium nitride (CrN),
zirconium nitride (ZrN) and/or diamond coatings to achieve
non-abrasive effect on the hole.
[0014] According to another embodiment of the invention the button
comprises button retaining means such that the button may be
reliably held in the rock drill bit while being allowed to
rotate.
[0015] According to another embodiment of the invention a base
portion of at least one button rests against or contacts a bottom
of a button hole to transfer impact forces to the button while
allowing the base portion to move thereon when rotating.
[0016] The invention also relates to a rock drill bit button
according to the invention for percussive rock drilling into earth
material, such as rock.
[0017] The invention also relates to a use of a rock drill bit
according to the invention for percussive rock drilling into earth
material, such as rock.
[0018] Further advantages as well as advantageous features of the
invention will appear from the following description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] With reference to the appended drawings, below follows a
specific description of embodiments of the invention cited as
examples.
[0020] In the drawings:
[0021] FIG. 1 is a very simplified view of a rock drill bit
according to prior art in operation,
[0022] FIG. 2 is a perspective view of a rock drill bit according
to prior art after some time of operation,
[0023] FIG. 3 is a perspective view illustrating the principle of a
rock drill bit according to the present invention,
[0024] FIG. 4 shows a longitudinal section through a part of a rock
drill bit according to a first embodiment of the invention in
operation,
[0025] FIG. 5 is an exploded view of the rock drill bit according
to FIG. 4,
[0026] FIG. 6 is a view corresponding to FIG. 4 of a rock drill bit
according to a second embodiment of the invention,
[0027] FIG. 7 is an exploded view of the rock drill bit according
to FIG. 6,
[0028] FIG. 8 is a simplified view corresponding to FIG. 4 of a
rock drill bit according to a third embodiment of the
invention,
[0029] FIG. 9 is a simplified view of a button allowed to rotate in
a bit head of a rock drill bit according to a fourth embodiment of
the invention, and
[0030] FIG. 10 is a very simplified view of a drilling assembly for
percussive rock drilling according to an embodiment of the present
invention in operation.
[0031] FIG. 11 is a graph showing drilled meters versus drill
penetration speed, wherein drill bits B and C represent the present
invention.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0032] FIG. 3 shows very schematically the principle of a rock
drill bit according to the present invention, in which all gauge
buttons 20 and all front buttons 21 are allowed to rotate about
their own symmetry axis by being received in holes 22 in a drill
bit body in a substantially circumferential ring surface 23
defining a substantially frusto-conical shape as seen in the
intended drilling direction and in a front surface 24,
respectively.
[0033] Each button may be manufactured from pressed and sintered
cemented carbide. By the term "cemented carbide" is here meant WC,
TiC, TaC, NbC, etc., in sintered combination with a binder metal
such as, for instance, Co or Ni. The button is preferably at least
partially coated with a barrier coating which will be discussed
more in detail. In certain cases, it may be justified that at least
the exposed part of the button comprises superhard materials such
as polycrystalline diamond or cubic boron nitride.
[0034] A rock drill bit 30 according to a first embodiment of the
present invention will now be described while making reference to
FIGS. 4 and 5. The rock drill bit comprises a first member 31
having a substantially circumferential ring surface 32 defining a
substantially frusto-conical shape as seen in the intended drilling
direction. This first member 31 is provided with means configured
to secure this member to a drill element 33, in which this securing
means is formed by a sleeve-like portion 34 of the first member 31
provided with engagement means in the form of an internal thread 35
configured to engage engagement means in the form of an external
thread 36 on the drill element.
[0035] The rock drill bit further comprises a second member 37
defining a front end 38 of a bit head 39 of the rock drill bit.
This second member is provided with a plurality of through holes 40
receiving the gauge buttons 41 and front buttons 42 while allowing
these to rotate about their own symmetry axis. Each gauge button 41
comprises a shank portion 41' preferably integral with a tip
portion. Preferably, the shank portion 41' defines a larger
diameter than any chosen diameter of the tip portion. The
through-holes 40 each have a diameter slightly exceeding (suitably
by a diameter difference in the order of 30-80 .mu.m) the diameter
of the respective shank portion received therein for allowing the
button to move with respect to walls 43 in the second member 37
defining said hole when rotating. However, this difference in
diameter has been exaggerated in this figure and also in the
embodiment shown in FIG. 6 and described below for better
illustrating this feature. The gauge buttons as well as the front
buttons are provided with a base portion 44 with larger
cross-section than the rest of the button and also than the
respective hole 40 so as to maintain the button received in the
second member.
[0036] A gauge button 41 rests by the base portion 44 thereof on
said ring surface 32 configured to transfer impact forces to the
gauge button and allow the base portion to move thereon when
rotating. This means that impact forces are transferred to the
gauge buttons from a surface 32 located inside the drill bit. The
first member has also surfaces 45 directed in an intended drilling
direction for supporting base portions of front buttons and
transferring impact forces thereto while allowing these base
portions to move on these surfaces 45 when rotating. Furthermore,
the bit head 39 will through a shoulder 47 on the first member 31
provide a clearance C with respect to this member 31, so that the
button 41 may rotate freely without jamming. Particular measures
are taken for flushing the surfaces and spaces surrounding the
button, which will be explained more in detail below.
[0037] The rock drill bit comprises means 46 configured to secure
the second member 37 to the first member 31. The securing means is
preferably configured to releasably secure these members to each
other, for instance by mutually securing them by engagement of
threads. This would then mean that it would be possible to remove
said second member with buttons for replacement while keeping the
first member after the buttons have been that much worn that they
have to be replaced. Welding or press fitting are other possible
alternatives of said securing means 46 easier to accomplish.
[0038] When carrying out percussive drilling with the rock drill
bit shown in FIGS. 4 and 5 as illustrated in FIG. 4 the buttons
thereof will be allowed to rotate about their own axes, which means
that the gauge buttons 41 will be worn evenly and maintain their
sharpness, so that a high penetration speed may be maintained over
a long period of time and the diameter of the hole defined by the
gauge buttons will be reduced more slowly than would the gauge
buttons be fixedly arranged in the bit head.
[0039] FIGS. 6 and 7 illustrate a rock drill bit 50 according to a
second embodiment of the invention. This rock drill bit has a first
member 51 in the form of a ring configured to be supported on
and/or secured to an end 52 of a drill element 53 and having a ring
surface 54 forming a support for a base portion 55 of each gauge
button 56 in the same way as the corresponding surface 32 in the
embodiment shown in FIGS. 4 and 5. Each gauge button 56 comprises a
shank portion 56' preferably integral with a tip portion.
Preferably, the shank portion 56' defines a larger diameter than
any chosen diameter of the tip portion. Impact forces will be
transferred by the ring surface 54 to the gauge buttons while the
base portions thereof are allowed to move thereon when
rotating.
[0040] A second member 57 of the rock drill bit has through holes
58 receiving said gauge buttons and allowing them to move with
respect to walls of these holes when rotating. The front buttons 59
are, as an example, in this embodiment fixedly secured to a front
end 60 of the second member 57.
[0041] The second member 57 is in this embodiment provided with
means for securing this member to a drill element 53 by having a
sleeve-like portion 61 designed to receive a drill element and
having engagement means in the form of an internal thread 62 for
engaging with engagement means in the form of an external thread 63
on the drill element for releasably securing said second member to
the drill element and by that also keeping said ring 51, a
so-called pusher ring, in place. The first member 51 is provided
with a collar 64, so that the first 51 and second 57 members are
fixed with respect to each other while leaving a clearance 66
therebetween for the button to freely rotate. Proper flushing of a
button allowed to rotate is also important. It is indicated in FIG.
6 that the rock drill bit has a conventional flush channel 67
extending through the bit head. The flush channel has also at least
one flushing hole 68 (see the arrows F indicating the flow of
flushing medium) opening at the first end 60 and passing the
clearance 66 and the circumference of the button 56 allowed to
rotate. This will keep said clearance 66 clear and eliminates wear
problems while the button rotates inside the hole 58. The function
of this embodiment of the invention in operation appears clearly
from the above discussion of inter alia the first embodiment of the
present invention.
[0042] A part of a rock drill bit according to a third embodiment
of the invention is very schematically shown in FIG. 8. This rock
drill bit is provided with alternative means to lock a button 80 to
a drill bit head 81 while allowing the button to rotate. Each gauge
button 80 comprises a shank portion 80' preferably integral with a
tip portion. Preferably, the shank portion 80' defines the largest
diameter of the button. A blind hole 82 in the bit head designed to
receive the button 80 is provided with an annular groove 83, and
the shank portion 80' is provided with a corresponding annular
groove 84 receiving an elastic lock ring 85, for example a ring,
such as a C-ring, made from steel. When the button 80 is pushed
into the hole 82 the lock ring will first be compressed until
reaching the groove 83 in the bit head. It will then expand
outwards into that groove and lock the button to the bit head 81
while allowing the button to rotate.
[0043] FIG. 9 illustrates an alternative way of locking a button 90
to a bit head not shown in a rock drill bit according to a fourth
embodiment of the invention while allowing the button to rotate.
This is achieved by providing a shank portion 90' with an annular
groove 91 as in the embodiment shown in FIG. 8. However, a lock pin
92 is used instead of a lock ring, and this lock pin is after
pushing the button 90 into a corresponding hole in the bit head
pushed into the groove 91 while locking the button in place and
still allowing it to rotate about its own symmetry axis.
[0044] The base portions 44, 55 and the annular groove 91 are all
examples of button retaining means and each said portion may define
a largest diameter of the button.
[0045] FIG. 10 illustrates very schematically a drilling assembly
for percussive rock drilling according to the present invention
having a rock drill bit 70 according to an embodiment of the
invention provided with gauge buttons 71. This drilling assembly is
a so-called top hammer drill acting upon the rock drill bit from a
location above the ground and has power means 72, such as diesel
engine and hydraulic pump, configured to drive the rock drill 76,
which in turn makes said drill element 73 and the rock drill bit to
rotate and carry out percussions and by that crush the rock. A
design of the drilling assembly as a down-the-hole hammer equipment
is also within the scope of the present invention.
[0046] The drilling assembly has also means 74, such as a
compressed air generator, configured to flush cuttings resulted
from engagement of the gauge buttons and the front buttons of the
drill bit away from the region occupied by the drill bit. The
drilling assembly has a control arrangement 75 configured to
control the operation of the power means 72 so as to adapt the
frequency of impacts and the rotational speed of the drill bit. It
has turned out that drill bits according to the present invention
with buttons allowed to rotate about their own symmetry axis are
particularly well suited to be used in drilling assemblies
controlled to have frequencies above 250 Hz, preferably above 350
Hz and most preferred in the range of 350 Hz-1000 Hz.
[0047] Drilling with a drilling assembly according to FIG. 8 with a
rock drill bit according to the present invention will be more
efficient than with rock drill bits already known, since the
penetration speed may be kept at a high level longer and the stops
needed for replacing the rock drill bit or parts thereof will be
less frequently occurring.
[0048] The inventors of the present invention found during tests
that button hole wear is of major importance. Numerous experiments
were made to avoid hole wear including hardening of the steel bit
body, different flushing solutions for avoiding cuttings to enter
into the holes, polishing of the buttons, etc. The results of the
tests regarding button hole wear showed that surface hardness of
the drill bit body and entrance of rock cuttings into the hole
clearance have no significant effect on wear rate. The inventors
surprisingly found that tungsten carbide grains are responsible for
the steel wear in the button holes. Surface quality of the button
has tremendous effect on wear rate but the wear rate increases
rapidly after a certain time of use of polished buttons. It is
believed that after a certain period of drilling time the cobalt
binder of the cemented carbide dissolves from the button surface
thereby exposing abrasive wolfram carbide grains and the button
surface quality is lost so that the wear rate in the hole increases
rapidly.
[0049] The aim of the further tests was to maintain the integrity
of the envelope surface of the button.
[0050] One way of achieving that aim is to coat at least the shank
portion 41', 56', 80', 90' of the button with a barrier such as a
barrier coating to substantially eliminate cobalt dissolution. The
button will then substantially maintain the surface quality and the
button hole wear is substantially eliminated. It is preferable that
also the button retaining means and/or the exposed portions of the
rotatable buttons are coated.
[0051] Two coating materials were used in tests, i.e. one material
comprising TiAlN and one material comprising AlCrN.
[0052] FIG. 11 is a graph showing drilled meters versus drill
penetration speed of A--a drill bit with fixed, uncoated gauge
buttons, B--a drill bit with coated, rotatable gauge buttons, the
coating being BALINIT.RTM. FUTURA NANO, i.e. titanium-aluminium
nitride (TiAlN), and C--a drill bit with coated, rotatable gauge
buttons, the coating being BALINIT.RTM. ALCRONA PRO, i.e.
aluminum-chromium nitride (AlCrN). The coating thickness was about
3 micrometers in both cases. All bits had fixed, i.e. pressed-in,
front buttons during the drilling tests. Each drilled hole was
about 4.1 m deep. The drill bits all had conical button tips and
were made for hole diameter of 48 mm. They all had five 10 mm
buttons on gauge and three uncoated 9 mm buttons fixed to the front
surface.
[0053] Both drill bits B and C with coated gauge buttons
outperformed the drill bits A with uncoated gauge buttons. While
drill bit A only could drill about 40 m, drill bit B managed about
80 m and drill bit C about 170 m. Thus with a suitable barrier
against binder phase dissolution the life of a drill bit can be
extended up to at least 400%. After drilling about 143 m with drill
bit C the feed was increased since by then the buttons became blunt
and a further 20 m could be drilled. The latter action is depicted
in FIG. 11 as "Higher power".
[0054] Properties for a suitable coating can be that the bearing
portion has a friction coefficient against steel which is less than
0.5, preferably in the range of 0.1-0.5, most preferably in the
range of 0.2-0.4. The bearing portion may have a microhardness (HV
0.05) of at least 3000, preferably in the range of 3000-3500, most
preferably in the range of 3100-3400. The coating thickness at the
bearing portion can be thin such as 1-5 micrometers, preferably 2-4
micrometers, most preferably about 3 micrometers.
[0055] The coatings form diffusion barriers which prevent the
interaction between the hole wall and the button substrate
material. Other coatings that can be used are titanium carbide
(TiC), titanium nitride (TiN), chromium nitride (CrN), zirconium
nitride (ZrN) and diamond coatings.
[0056] A material generally free from particles that are harder
than the surrounding material is here called substantially
homogenous.
[0057] It is preferable that the base portion of each rotatable
button rests against or contacts the bottom of the hole to transfer
impact forces to the button and while allowing the base portion to
move thereon when rotating.
[0058] The invention is of course not in any way restricted to the
embodiments described above, but many possibilities to
modifications thereof would be apparent to a person with skill in
the art without departing from the scope of the invention as
defined in the appended claims. For example, the rotatable button
can be provided with bearing portion in the shape of a sleeve
secured to its shank portion instead of a coating such that the
substrate does not reach the hole wall in the drill bit.
[0059] The number and positions of the buttons of the rock drill
bit may differ a lot with respect to the embodiments shown in the
figures.
[0060] "Substantially" used in the expressions "substantially a
frustoconical shape" and "substantially circumferential ring" also
cover the case when cutting recesses or grooves and/or gauge
buttons intersect the ring, as shown in the figures.
[0061] The disclosures in EP Patent Application No. 10178387.6,
from which this application claims priority, are incorporated
herein by reference.
* * * * *