U.S. patent application number 11/987214 was filed with the patent office on 2008-06-12 for button bit.
This patent application is currently assigned to SANDVIK INTELLECTUAL PROPERTY AB. Invention is credited to Christer Lundberg.
Application Number | 20080135308 11/987214 |
Document ID | / |
Family ID | 39468149 |
Filed Date | 2008-06-12 |
United States Patent
Application |
20080135308 |
Kind Code |
A1 |
Lundberg; Christer |
June 12, 2008 |
Button bit
Abstract
A button bit is disclosed of the type that is rotatable in a
predetermined direction of rotation (R) around a centre axis (C)
and includes a number of spaced-apart peripheral buttons, which
protrude forward from a front end surface to which an envelope
surface connects having a rotationally symmetrical shape, the
individual peripheral button being tilted partly in relation to the
centre axis (C) of the drill bit, partly in the forward direction
of rotation (R) in relation to the surface from which the button
protrudes. The hole for the individual peripheral button mouths in
a countersunk entering surface which is planar, ring-shaped and
extends perpendicularly to the centre axis (C1) of the hole. By
tipping the button in the forward direction of rotation, the same
will operate aggressively and efficiently. Also, the peripheral
button may be mounted in accurate drilled apertures which are
simple to provide, despite the complex space geometry
predicament.
Inventors: |
Lundberg; Christer;
(Sandviken, SE) |
Correspondence
Address: |
DRINKER BIDDLE & REATH (DC)
1500 K STREET, N.W., SUITE 1100
WASHINGTON
DC
20005-1209
US
|
Assignee: |
SANDVIK INTELLECTUAL PROPERTY
AB
|
Family ID: |
39468149 |
Appl. No.: |
11/987214 |
Filed: |
November 28, 2007 |
Current U.S.
Class: |
175/420.1 |
Current CPC
Class: |
E21B 10/56 20130101;
E21B 10/46 20130101 |
Class at
Publication: |
175/420.1 |
International
Class: |
E21B 10/52 20060101
E21B010/52 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 29, 2006 |
SE |
0602559-7 |
Claims
1. A button bit of the type that is rotatable in a predetermined
direction of rotation around a centre axis, the button bit
comprising: a body; and a number of spaced-apart peripheral buttons
each positioned in a hole, wherein the buttons protrude forward
from a front end surface to which an envelope surface connects,
wherein the body has a rotationally symmetrical shape, wherein at
least one individual peripheral button is tilted partly in relation
to a centre axis of the bit, and partly in a forward direction of
rotation in relation to a surface from which the button protrudes,
wherein a mouth of the hole for the individual peripheral button
includes a countersunk entering surface which is planar and
ring-shaped, and wherein the countersunk entering surface extends
perpendicularly to a centre axis of the hole.
2. The button bit according to claim 1, wherein the individual
peripheral button has a centre axis tilted at an acute angle in
relation to the centre axis of the bit and in that the countersunk
entering surface is delimited by a cylinder surface, the depth of
which decreases in a direction from a rotationally leading end
toward a trailing end.
3. The button bit according to claim 2, wherein the individual
peripheral button, in addition to a tapering tip, comprises a
cylindrical base part that is fixed in a cylindrical hole, which
mouths in a surface via a mouth opening having an oval shape.
4. The button bit according to claim 1, wherein the centre axis of
the individual peripheral button is tilted not only at a primary
tip angle (.alpha.) in relation to the centre axis of the bit, but
also at a secondary tip angle (.epsilon.) in relation to a centre
plane (MP), wherein the centre plane (MP) is oriented perpendicular
to the surface from which the button protrudes and intersects the
mouth of the hole halfway between the rotationally leading and
trailing end points along a borderline of the hole.
5. The button bit according to claim 4, wherein the secondary tip
angle (.epsilon.) is at least 5.degree..
6. The button bit according to claim 5, wherein the secondary tip
angle (.epsilon.) is at most 20.degree..
7. The button bit according to claim 4, wherein all peripheral
buttons are tilted at said primary tip angle (.alpha.) as well as
at said secondary tip angle (.epsilon.).
8. The button bit according to claim 7, wherein the secondary tip
angle (.epsilon.) is at least 5.degree..
9. The button bit according to claim 8, wherein the secondary tip
angle (.epsilon.) is at most 20.degree..
10. The button bit according to claim 7, wherein the peripheral
buttons are arranged along a ring-shaped, peripheral cone surface,
the cone angle (.beta.) of which is obtuse, and which is located
between the envelope surface and a central front surface from which
a number of centre buttons protrude forward, a part of the
individual peripheral button protruding peripherally in relation to
the envelope surface.
11. The button bit according to claim 10, wherein the individual
peripheral button, in addition to a tapering tip, comprises a
cylindrical base part that is fixed in a cylindrical hole, which
mouths in a surface via a mouth opening having an oval shape.
12. The button bit according to claim 11, wherein the oval shape
has a major axis extending tangentially along the cone surface and
a minor axis extending radially along the cone surface.
13. The button bit according to claim 4, wherein the peripheral
buttons are arranged along a ring-shaped, peripheral cone surface,
the cone angle (.beta.) of which is obtuse, and which is located
between the envelope surface and a central front surface from which
a number of centre buttons protrude forward, a part of the
individual peripheral button protruding peripherally in relation to
the envelope surface.
14. The button bit according to claim 13, wherein the individual
peripheral button, in addition to a tapering tip, comprises a
cylindrical base part that is fixed in a cylindrical hole, which
mouths in a surface via a mouth opening having an oval shape.
15. The button bit according to claim 14, wherein the oval shape
has a major axis extending tangentially along the cone surface and
a minor axis extending radially along the cone surface.
16. The button bit according to claim 3, wherein the individual
peripheral button, in addition to a tapering tip, comprises a
cylindrical base part that is fixed in a cylindrical hole, which
mouths in a surface via a mouth opening having an oval shape.
17. A button bit of the type that is rotatable in a predetermined
direction of rotation around a centre axis, the button bit
comprising: a body having a front end surface and an envelope
surface rearward of the front end surface; and a number of
spaced-apart peripheral buttons each positioned in a hole in the
front end surface, wherein the buttons protrude forward from the
front end surface, and wherein at least one individual peripheral
button is tilted partly in relation to a centre axis of the bit,
and partly in a forward direction of rotation in relation to a
surface from which the button protrudes.
18. The button bit according to claim 17, wherein a centre plane is
defined by a plane orientated perpendicularly to the surface from
which the button protrudes and intersects a mouth of the hole for
the button at a point halfway between a rotationally leading end
point along the hole edge and a rotationally trailing end point
along the hole edge, and wherein a mass of material of the button
situated rotationally in front of the centre plane is greater than
a mass of material of the button situated rotationally behind the
centre plane.
19. The button bit according to claim 18, wherein a centre axis of
the individual peripheral button is tilted at a primary tip angle
(.alpha.) in relation to the centre axis of the bit, and wherein
the centre axis of the individual peripheral button is tilted at a
secondary tip angle (.epsilon.) in relation to the centre
plane.
20. A button bit of the type that is rotatable in a predetermined
direction of rotation around a centre axis, the button bit
comprising: a body having a front end surface and an envelope
surface rearward of the front end surface; and a number of
spaced-apart peripheral buttons each positioned in a hole in the
front end surface, wherein the buttons protrude forward from the
front end surface, wherein at least one individual peripheral
button is tilted partly in relation to a centre axis of the bit,
and partly in a forward direction of rotation in relation to a
surface from which the button protrudes, wherein the buttons have
an apex at a first end, the first end exposed when mounted in the
hole, and wherein a first generatrix from the apex to a
rotationally leading end point along the hole edge has a first
length and a second generatrix from the apex to a rotationally
trailing end point along the hole edge has a second length, and the
first length is less than the second length.
21. The button bit according to claim 20, wherein a mouth of the
hole for the individual peripheral button includes a countersunk
entering surface, and wherein the countersunk entering surface
extends perpendicularly to a centre axis of the hole.
Description
RELATED APPLICATION DATA
[0001] This application claims priority under 35 U.S.C. .sctn.119
and/or .sctn.365 to Swedish patent application No. 0602559-7, filed
Nov. 29, 2006, the entire contents of which are incorporated herein
by reference.
FIELD
[0002] The present disclosure relates to a button bit intended for
percussive rock drilling and of the type that is rotatable in a
predetermined direction of rotation around a centre axis, and
comprises a number of spaced-apart peripheral buttons, which
protrude forward from a front end surface to which an envelope
surface connects having a rotationally symmetrical basic shape, the
individual peripheral button is tilted partly in relation to the
centre axis C, partly in the forward direction of the rotational
direction R in relation to the surface from which the button
protrudes.
BACKGROUND
[0003] In the discussion of the background that follows, reference
is made to certain structures and/or methods. However, the
following references should not be construed as an admission that
these structures and/or methods constitute prior art. Applicant
expressly reserves the right to demonstrate that such structures
and/or methods do not qualify as prior art.
[0004] Button bits can be realized in most varying embodiments
depending on the specifics of the field of application. Thus, the
different embodiments differ not only as a consequence of varying
diameters of the holes to be drilled, but also depending on the
nature of the rock or the soil type to be machined, e.g., in
respect of the hardness thereof. However, common to previously
known button bits is that they comprise a generally rotationally
symmetrical basic body that is manufactured from steel or the like
and equipped with a number of buttons of a material, e.g., cemented
carbide, that has greater hardness and wear resistance than the
material of the basic body. Said buttons are placed in different
configurations on a front end surface, which via a circular
borderline transforms into an envelope surface having a
rotationally symmetrical basic shape, which partly tapers in the
backward direction. More precisely, the front end surface is
included in a head having a greater diameter than a shank behind,
which may be in the form of a sleeve or skirt into which a drill
rod can be inserted and interconnected with the drill bit. The head
is strongly dimensioned in order to withstand above all
considerable impulsive forces. The individual button is usually
formed with a cylindrical base part and a tapering top or tip,
which effects the proper machining of the rock. The button is
permanently united to the drill bit by a shrink-fit joint, which is
provided by inserting the base part of the button into a drilled,
cylindrical hole in a heated drill bit, which is allowed to cool
down in order to clamp the button in the hole. The button may also
be applied by pressing into a cold steel body. The buttons are
grouped centrally as well as peripherally along the front end
surface of the drill bit. More precisely, a certain number of
centre buttons are placed in different locations along a circular,
central front surface, while a usually greater number of peripheral
buttons are placed tangentially spaced apart along a peripheral,
ring-shaped surface between the central end surface and the
envelope surface. This peripheral ring surface is conical, having
cone angles within the range of 90-140.degree., which means that
the centre axes of the different peripheral buttons commonly meet
the centre axis of the drill bit at equal angles of
20-45.degree..
[0005] Examples of known button bits of the kind in question are
disclosed in SE 9001081-0, SE 8305048-4, SE 607972, FR 1514998 and
FR 2646875.
[0006] Previously known rock-drilling equipment for top-hammer
drilling allows machining of the rock at a relatively moderate
rotational speed and moderate percussion frequency. Thus, the
rotational speed has previously been limited to about 200 r/min
(slightly more than 3 revolutions per second), and the percussion
frequency to about 100 Hz (i.e., 100 percussions per second). Under
these conditions, the individual button performs repeated forwardly
directed impact motions, which results in rock being crushed, at
the same time as the same slowly rotates around the centre axis of
the drill bit, the entire drill bit fairly slowly being fed axially
into the rock. Therefore, because the speed of rotation is moderate
in comparison with the percussion frequency, the rock is in all
essentials machined by crushing.
[0007] However, rock-drilling equipment has recently been developed
that allows rotation of the drill bit at a considerably higher
rotational speed and also at a higher percussion frequency. During
drilling by means of such equipments, it has turned out that the
rock is machined not only by being crushed as a consequence of the
axial percussion motions of the button, but also by being cut into
pieces as a consequence of the rotation of the button. In other
words, the rock is disintegrated by a combination of percussive
crushing and rotating, cutting machining.
[0008] FR 1514998 and FR 2646875 disclose drill bits. In practice
such constructions are troublesome to produce, because the drilling
of the individual hole must be performed in a geometrically
complicated position in relation to a cone surface. Thus, the drill
tip may easily glide along the cone surface during the initial
drilling and thereby deviate from the determined entering
point.
SUMMARY
[0009] The presently disclosed button bits aim at eliminating the
above mentioned problems and at further developing previously known
button bits so far that they are particularly suitable for drilling
at high rotational speed and high percussion frequency. Therefore,
one object is to provide a button bit having peripheral buttons
that may be mounted in accurately drilled apertures which are
simple to provide, despite the complex space geometry predicament.
Another object is, by the utilization of the understanding that
high-speed rotating buttons fragmentize the rock through a
combination of percussive crushing and rotary cutting into pieces,
to provide an efficiently and aggressively operating button bit, by
means of which the times for the requisite drilling operation per
hole meter are reduced. An additional object is to provide a
high-speed rotating drill bit having long service life, in
particular such as this is determined by the capacity of the
peripheral buttons to resist wear.
[0010] The above mentioned objects are at least partly attained by
a mouth of the hole for the individual peripheral button including
a countersunk entering surface which is planar and ring-shaped and
extends perpendicularly to a centre axis of the hole.
[0011] For example, an exemplary embodiment of a button bit that is
rotatable in a predetermined direction of rotation around a centre
axis comprises a body, and a number of spaced-apart peripheral
buttons each positioned in a hole, wherein the buttons protrude
forward from a front end surface to which an envelope surface
connects, wherein the body has a rotationally symmetrical shape,
wherein at least one individual peripheral button is tilted partly
in relation to a centre axis of the bit, and partly in a forward
direction of rotation in relation to a surface from which the
button protrudes, wherein a mouth of the hole for the individual
peripheral button includes a countersunk entering surface which is
planar and ring-shaped, and wherein the countersunk entering
surface extends perpendicularly to a centre axis of the hole.
[0012] Another exemplary embodiment of a button bit of the type
that is rotatable in a predetermined direction of rotation around a
centre axis comprises a body having a front end surface and an
envelope surface rearward of the front end surface, and a number of
spaced-apart peripheral buttons each positioned in a hole in the
front end surface, wherein the buttons protrude forward from the
front end surface, and wherein at least one individual peripheral
button is tilted partly in relation to a centre axis of the bit,
and partly in a forward direction of rotation in relation to a
surface from which the button protrudes.
[0013] A further exemplary embodiment of a button bit of the type
that is rotatable in a predetermined direction of rotation around a
centre axis comprises a body having a front end surface and an
envelope surface rearward of the front end surface, and a number of
spaced-apart peripheral buttons each positioned in a hole in the
front end surface, wherein the buttons protrude forward from the
front end surface, wherein at least one individual peripheral
button is tilted partly in relation to a centre axis of the bit,
and partly in a forward direction of rotation in relation to a
surface from which the button protrudes, wherein the buttons have
an apex at a first end, the first end exposed when mounted in the
hole, and wherein a first generatrix from the apex to a
rotationally leading end point along the hole edge has a first
length and a second generatrix from the apex to a rotationally
trailing end point along the hole edge has a second length, and the
first length is less than second length.
[0014] It is to be understood that both the foregoing general
description and the following detailed description are exemplary
and explanatory and are intended to provide further explanation of
the invention as claimed.
BRIEF DESCRIPTION OF THE DRAWING
[0015] The following detailed description can be read in connection
with the accompanying drawings in which like numerals designate
like elements and in which:
[0016] FIG. 1 is a perspective exploded view showing an exemplary
embodiment of a button bit, an individual peripheral button being
shown spaced-apart from the basic body of the drill bit.
[0017] FIG. 2 is a central longitudinal section through the drill
bit of FIG. 1.
[0018] FIG. 3 is an enlarged end view from the front of the drill
bit according to FIG. 1.
[0019] FIG. 4 is a schematic section through only the front head of
a previously known drill bit (PRIOR ART).
[0020] FIGS. 5-8 are a series of schematic illustrations, which in
two dimensions explain the difference between an exemplary
embodiment of a disclosed button bit (FIGS. 7-8) and the previously
known technique (FIGS. 5-6).
[0021] FIGS. 9-10 are detailed sections showing how a hole for an
individual peripheral button can be created.
DETAILED DESCRIPTION
[0022] In FIG. 1, an exemplary embodiment of a button bit is shown,
which comprises a basic body 1 as well as a number of buttons 2, 3
that, in this case, are in the form of separately manufactured
bodies, which are permanently connected to the basic body 1. The
buttons consist of peripheral buttons 2 as well as centre buttons
3. In practice, the button bodies are manufactured from cemented
carbide or another material that is harder and more wear-resistant
than the material of the basic body, which usually consists of
steel.
[0023] The basic body 1 comprises a front head 4 and a rear shank 5
that is thinner than the head and, in the example, is
sleeve-shaped. The shape of the basic body is generally
rotationally symmetrical so far that the envelope surface thereof
comprises a cylindrical surface 7, which forms the outside of the
shank 5, and a slightly conical clearance surface 8 on the head 4.
Said cone surface converges in the backward direction and is
spaced-apart from the cylinder surface 7 via a ring-shaped neck 9.
A circular borderline 10 forms a transition between the clearance
surface 8 and the front end surface of the head 4. Said front end
surface comprises also two part surfaces, viz. a central front
surface 12 and a peripheral cone surface 13. The clearance surface
8 is interrupted by a number of chipways 14. Furthermore, at least
one flushing-liquid channel 15 terminates in the clearance surface
8. Other flushing-liquid channels 16 can terminate in the front
surface 12.
[0024] The drill bit is rotatable in a predetermined direction of
rotation R, around a centre axis designated C.
[0025] From the enlarged end view in FIG. 3, it is seen that in the
example, the number of peripheral buttons 2 amounts to six, and the
number of chipways 14 to three, the peripheral buttons being placed
in pairs between adjacent chipways. In the example, the number of
centre buttons 3 amounts to four. Of these four, three are situated
peripherally along the central front surface 12, in the immediate
vicinity of a circular borderline 11 between the surfaces 12 and
13, while a fourth one (designated 3a) is situated closer to the
centre axis C of the drill bit.
[0026] The three outermost centre buttons 3 are equidistantly
spaced-apart by a partition of 120.degree.. The button 3a is placed
in such a way inside a pair of outer centre buttons, that the areas
swept by the buttons cover each other. Radially inside two pairs of
outer buttons, there terminate two channels 16 for flushing liquid
that can be flushed from the interior of the drill bit and has,
among other things, the purpose of evacuating the debris broken off
from the drill hole.
[0027] The number and arrangement of buttons, chipways and channels
are non-limiting examples; other suitable numbers and arrangements
of these features are within the scope of the disclosure.
[0028] As is seen in FIG. 3, the individual button comprises a
cylindrical base part 18 and a tapering tip or top 19 that, in the
example according to FIGS. 1 and 2, has a so-called ballistic shape
so far that the same is delimited by one or more convex surfaces.
The base part 18 is applied in a hole 20 in the head of the drill
bit and is permanently united to the same in a suitable way, e.g.,
via a shrinkage fit, which is provided by applying the button in
the hole when this is widened by heating of the drill bit, and then
the drill bit is allowed to cool down so that the button is clamped
in the hole.
[0029] Reference is now made to FIG. 4, which illustrates the basic
geometry of previously known button bits of the kind in question.
In the example, the angle .alpha. between the peripheral cone
surface 13 and an imaginary plane P in the extension of the front
surface 12 amounts to 30.degree., i.e., the cone angle .beta.
amounts to 120.degree. (2.times.60.degree.). The centre axis C1 of
the individual hole 20 and the button 2, respectively, extends
perpendicularly to the cone surface 13, such as this is represented
by the line L in FIG. 4, the centre line C1 of all buttons meeting
each other in a common point "O" along the centre axis C of the
drill bit. In other words, the individual centre axis C1 and the
line L may be said to be situated in a common plane that is
traversed by the centre axis C of the drill bit. A consequence of
this geometry is that the borderline 21, which forms a transition
between the cone surface 13 and the cylindrical inside of the hole
20, obtains a circular shape as viewed along the axis C1 (even if
the same is deflected in a plane as a consequence of the conicity
of the surface 13).
[0030] In FIG. 2, it is shown how the rear shank part 5 of the
drill bit in this case delimits a hollow space 22 in which the
front end of a drill rod or drill string can be inserted and
interconnected with the drill bit, e.g., via a threaded joint. From
this hollow space 22, flushing liquid can be fed out via the
channels 15, 16.
[0031] As far as the shown drill bit has been described hitherto,
the same is in all essentials previously known.
[0032] Reference is now made to FIGS. 5-9. Because the general idea
of the invention is difficult to clearly perceive in the
three-dimensional illustrations according to FIGS. 1-3, FIGS. 5-9
have been made in two dimensions, so far that the in reality
conical surface 13 of the head of the drill bit here is shown
extended in a plane.
[0033] In FIGS. 5 and 6, the placement of the peripheral buttons 2
according to prior art is shown, i.e., with the centre axes C1 of
the buttons orientated perpendicularly to the surface 13 (even if
the axes--thanks to the conicity of the surface 13--in reality
extends at the angle .alpha. to the centre axis C of the drill
bit). If the button is regarded from outside in the direction of
the axis C1, such as in FIG. 6, it is realized that the leading
half 19a of the protruding tip 19 of the button, which first
impinges against the rock during rotation, is identical to the
trailing half 19b facing rearward. In relation to the front head of
the drill bit, hence the individual button is tilted only at a
primary tip angle, viz. the angle .alpha. according to FIG. 4.
[0034] In FIGS. 7 and 8, the placement of the peripheral buttons
according to exemplary embodiments of the presently disclosed
button bits are illustrated. In this case, the individual button is
tilted not only at the primary tip angle .alpha. (not shown here),
but also at a secondary tip angle .epsilon.. This is effected by
locating the individual button hole 20, which in practice is
manufactured by chip removing machining, such as drilling or
possibly milling, with the centre axis C1 thereof directed in the
desired angle, without because of this needing to modify the design
of the button. Instead of directing, for instance, a drill linearly
inward toward the centre axis C of the drill bit (only at the angle
.alpha.), the same is directed toward a point situated beside the
centre axis C. The cone angle for the cone surface 13 amounts to
120.degree. (2.times.60.degree.). The individual peripheral button
2 is tilted partly in relation to the centre axis C (30.degree. in
the example), partly in the forward direction in relation to the
cone surface 13 seen in the rotational direction R.
[0035] In such a way, a double inclination may be said to arise,
which also may be described such that the imaginary plane (see FIG.
4), in which the line L and the centre axis C1 are mutually
situated, is turned in the backward/upward direction around the
line L. Thus, one example measure to realize the invention during
practical manufacture of the drill bit is to alter the entering
angle of the drill to the desired one, wherein buttons of standard
design, e.g., conical or ballistic, can still be used.
[0036] In order for the aggressiveness and efficiency of the
buttons to increase in comparison with conventionally mounted
buttons, the angle .epsilon. has of course to be greater than 0
(zero). In order to improve the effect of cutting into pieces upon
rotation considerably, the angle .epsilon. should, however, amount
to at least 5.degree.. Upward from this limit value, the angle
.epsilon. may vary most considerably. However, the same should not
be more than 25.degree., because if a larger inclination would be
chosen, risk arises that the button cracks or is damaged in the
percussive phase of the operation. Suitably, the angle .epsilon.
should be within the range of 10-20.degree.. Thus, the same amounts
to 15.degree. in the embodiment shown in FIGS. 1-3, and to
20.degree. in the schematic embodiment according to FIG. 7.
[0037] If the button hole 20 terminates directly in the cone
surface 13 (see FIG. 3 as well as FIGS. 7 and 8), the endless
borderline 21, which forms a transition between the surface 13 and
the cylindrical inside of the hole, will obtain an oval, more
precisely elliptical shape, the major axis of the ellipse extending
tangentially and the minor axis radially along the cone surface.
However, in practice, it is cumbersome to enter a drill in the
desired, secondary tip angle .epsilon. to the cone surface 13. For
this reason, such as is shown in FIGS. 9 and 10, a particular
entering surface 23 may be milled or formed in another way in the
cone surface 13 before the drilling of the hole takes place. Said
entering surface 23 is planar and located perpendicularly to the
centre axis C1 of the hole to be made. In the example according to
FIGS. 9 and 10, the entering surface is ring-shaped and delimited
by a shallow cylinder surface 24, the depth of which decreases in
the direction from a rotationally leading end toward a trailing
end. The entering of the conical surface 13 at the desired angle by
means of, for instance, a shank-end mill, presents no practical
difficulties. As soon as the planar entering surface has been
provided, a gimlet can be pressed against the same and with high
accuracy drill the hole at the desired angle.
[0038] In addition to the individual peripheral button, as a
consequence of the forwardly tipped position thereof, operating
aggressively and efficiently during the rotating, cutting-machining
phase, an increased service life of the same is also gained. In
FIGS. 7 and 8, MP designates a centre plane that is orientated
perpendicularly to the surface 13 and intersects the hole mouth at
a point halfway between the (rotationally) leading and trailing end
points 25, 26 along the hole edge or borderline 21. Such as is
clearly shown to the right in FIG. 7, the quantity or mass of
material (cemented carbide) in the buttons situated to the left,
i.e., rotationally in front, of the plane MP is considerably
greater than the mass of material situated behind the same. This
means that the part of the wear of the button caused by the rotary
cutting into pieces of the rock (isolated from the crushing) takes
long time in comparison with the rotation wear of a button mounted
in a conventional way, the two material masses of which on opposite
sides of the centre plane are the same.
[0039] In FIG. 7, a and b designate two imaginary, generatrix-like
lines extending from a common apex point on the top of the button
to the two end points 25, 26 of the hole mouth. As a consequence of
the forwardly tipped position of the button, the generatrix line a
becomes always shorter than the line b. More precisely, the ratio
a:b decreases with increasing tilt angle .epsilon..
[0040] The invention is not limited only to the embodiment
described above and shown in the drawings. Thus, it is feasible to
tilt only some of the peripheral buttons in the forward direction
of rotation, instead of all, such as is shown in the preferred
embodiment example. For instance, every second peripheral button
could be tipped forward, but not the other ones. It is also
feasible to tip forward different peripheral buttons at different
angles. Within the scope of the invention, it is also feasible to
tip forward one or more of the centre buttons. Neither is the
invention limited to such bits that have the peripheral buttons
mounted in an outer cone surface of the head. Thus, within the
scope of the invention, it is feasible to mount the peripheral
buttons in question in a substantially planar, ring-shaped surface,
which extends perpendicularly to the centre axis of the drill bit.
Neither is the invention limited to such bits that are composed of
a basic body of steel and a number of separately manufactured
buttons of another material. Thus, the invention envisages the
possibility of integrating the buttons with at least the front head
of the drill bit. This could be realized by forming button-like
projections integrally with the rest of the head in accordance with
the known MIM-technique (Metal Injection Moulding). Although such
projections would come to lack geometrical centre axes in a proper
sense, the same may, however, be tilted forward in the sense that a
leading generatrix line a (see FIG. 7) becomes shorter than a
corresponding trailing generatrix line b.
[0041] Although described in connection with preferred embodiments
thereof, it will be appreciated by those skilled in the art that
additions, deletions, modifications, and substitutions not
specifically described may be made without department from the
spirit and scope of the invention as defined in the appended
claims.
* * * * *