Note: Descriptions are shown in the official language in which they were submitted.
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TOOL FOR CUTTING SOLID MATERIAL
BACKGROUND OF THE INVENTION
The present invention relates to a tool for cutting
solid material, said tool comprising a tool body and a cutting
insert of cemented carbide, said cutting insert being secured to
the tool body by brazing. The invention also relates to a
cutting insert per se.
When a tool according to the present invention is
cutting a relatively hard, solid material, e.g., sandstone, the
cutting insert will be subjected to very high forces, said forces
creating a turning moment that gives rise to tensile stresses in
certain portions of the surface of the cutting tip. Also the
turning moment will eventually be transformed to the brazed
joint.
Cutting inserts of cemented carbide that are subjected
to high bending stresses must have a high toughness, i.e., lower
hardness compared to cutting inserts that are subjected basically
to compressive stresses. In mineral and asphalt cutting, lateral
forces are present to a relatively high degree. Therefore,
cutting inserts of the type having a relatively low hardness and
high Co-content are chosen for mineral and asphalt cutting. A
high Co-content is also favorable in reducing brazing stresses.
The wear resistance of a cutting insert as described
above consequently is low and in no way optimal as regards length
of life. It is therefore common to choose big cutting inserts
having a big volume of cemented carbide for mineral and asphalt
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cutting. By way of such an arrangement, one can ~an`dle2the
bending stresses and the tool also gets an acceptable length of
life.
In conventional tools for mineral and asphalt cutting,
the big volume cutting inserts are properly embedded in the tool
blank made out of steel. Such an arrangement makes sure that
the cutting insert is not subjected to too high stresses.
However, such a design means that the steel of the
blank surrounding the cutting insert quite soon gets in contact
with the mineral or asphalt that is worked. Especially when
minerals are worked, the contact between minerals and steel will
initiate sparking that can be very dangerous, e.g., in mines
having inflammable gases. Contact between a cutting insert of
cemented carbide and minerals will normally not initiate
sparking.
Since the cemented carbide cutting insert for cutting
mineral and asphalt has a relatively big volume, the tool itself
is also voluminous. This means that very powerful machines are
needed to carry the tools.
As mentioned above, the turning moment acting upon the
cutting insert will be transferred to the brazed joint. A
conventional brazed joint between the cutting insert and the tool
body has normally a substantially constant thickness. This means
that only a peripheral part of the brazed joint will be active in
absorbing the turning moment.
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Especially in mineral cutting one speaks of technically
cuttable material and economically cuttable material. The
technically cuttable material is the hardest material that can be
worked by a cutting action. The economically cuttable material
is the hardest material that can be worked by cutting action in
economic superiority to other methods.
OBJECTS AND SU~ARY OF THE I~lv ~:Nl lON
The aim of the present invention is to present a tool
and a cutting insert for the cutting of mineral or asphalt, said
tool/cutting insert demanding a relatively low energy to perform
cutting and has a high wear resistance. A preferred embodiment
of the tool has a brazed joint that to a greater degree is active
in absorbing the turning moment acting upon the cutting insert.
Consequently, harder material can thereby be considered
economically cuttable. The tool according to the invention also
to a high degree avoids sparking when working. The aim of the
present invention is realized by a tool/cutting insert that has
been given the characteristics of the appending claims.
In one aspect of the inspection there is provided a
tool for cutting solid material, said tool including a tool body
having a supporting surface, and a cutting insert having a
generally conical tip portion and a shoulder portion that is
intended to rest against the supporting surface, said cutting
insert being secured to the tool body wherein an intermediate
portion of the cutting insert, seen in axial direction of the
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cutting insert, includes a concave portion extending
circumferentially around the cutting insert, the cutting insert
comprising a core of cemented carbide, an intermediate layer of
cemented carbide surrounding said core and a surface layer of
cemented carbide, the surface layer, the intermediate layer and
the core all containing WC (alpha-phase) with a binder phase
(beta-phase) based upon at least one of cobalt, nickel or iron,
the core further containing eta-phase, the intermediate layer and
the surface layer being free of eta-phase, the content of binder
phase in the surface layer being lower than the nominal content
of binder phase for the cutting insert, and the content of binder
phase in the intermediate layer being higher than the nominal
content of binder phase for the cutting insert.
In another aspect of the invention there is provided a
cutting insert of cemented carbide adapted to be fastened to a
supporting surface of a tool body, said cutting insert having a
generally conical tip portion and a shoulder portion that is
intended to rest against the supporting surface, wherein an
intermediate portion of the cutting insert, seen in axial
direction of the cutting insert, includes a concave portion
exten~ing circumferentially around the cutting insert, the
cutting insert comprising a core of cemented carbide, an
intermediate layer of cemented carbide surrounding said core and
a surface layer of cemented carbide, the surface layer, the
intermediate layer and the core containing WC (alpha-phase) with
a binder phase (beta-phase) based upon at least one of cobalt,
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nickel or iron, the core further containing eta-phase, the
intermediate layer and the surface layer being free of eta-phase,
the content of binder phase in the surface layer being lower than
the nominal content of binder phase for the cutting insert, and
the content of binder phase in the intermediate layer being
higher than the nominal content of binder phase for the cutting
insert.
BRIEF DESCRIPTION OF THE DRAWINGS
Below an embodiment of the tool according to the
invention will be described with reference to the accompanying
drawings, where Fig. 1 discloses a cutting drum of an excavating
machine; Fig. 2 discloses a detail in enlarged scale of a part of
a tool carried by the drum; Fig. 3 shows a sectional view of a
cutting insert according to the invention; Fig. 4 shows a diagram
of how the compressive stresses in the surface layer vary by
varying cobalt content; Fig. 5 shows a diagram of how the
hardness varies in relation to the distance from the surface of
two cutting inserts; Fig. 6 shows a diagram of how the wear is
related to the cutting length for a number of cutting inserts;
Fig. 7 shows the head of a cutting insert of type B; Fig. 8 shows
a tool according to the invention having a preferred design of
the brazed joint; and Fig. 9 shows a detail in enlarged scale of
Fig. 8.
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DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The cutting drum 10 (only partly shown) in Fig. 1
carries a number of holders 11 that each support a tool 12
for cutting solid material. The cutting drum 10 is rotated
in direction of the arrow 13. When a tool 12 is in
engagement with the material to be worked, the cutting
insert 14 of the tool 12 is subjected to a normal force FN
and a force parallel to chord FT.
If very hard material is worked then the normal
force FN is considerably bigger than the force parallel to
chord FT. The force FN can be up to four times the force FT
and in such a case it is at once realized that a portion of
the surface of the cutting inset 14 will be subjected to
high tensile stresses.
In order to handle these high tensile stresses it
is necessary to use a special type of cemented carbide
disclosed in U.S. Patent Nos. 4,743,515 and 4,820,482.
The cutting insert 14 in Fig. 3 has a core 15 of
cemented carbide containing eta-phase carbide, that is, the
low carbon phases of the W-C-Co system such as the M6C- and
M~2C- carbides and kappa phase which is approximately M4C.
The core 15 is surrounded by an intermediate layer 16 of
cemented carbide free of eta-phase and having a high content
of cobalt relative to the nominal content of cobalt in the
entire insert. The surface layer 17 consists of cemented
carbide free from eta-phase and
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having a low content of cobalt relative to the nominal content of
cobalt in the entire insert. An intermediate part of the
cutting insert 14 includes a concave portion 18 extending
circumferentially around the cutting insert 14.
The thickness of the surface layer 17 is 0.8 - 4,
preferably 1 - 3, of the thickness of the intermediate layer 16.
The core 15 and the intermediate, cobalt rich layer 16
have high thermal expansivity compared to the surface layer 17.
This means that the surface layer 17 will be subjected to high
compressive stresses. The bigger the difference in thermal
expansivity, i.e., the bigger the difference in cobalt content
between the surface layer 17 and the rest of the cutting insert
14, the higher the compressive stresses in the layer 17. The
content of binder phase in the surface layer 17 is 0.1 - 0.9,
preferably 0.2 - 0.7, of the nominal content of binder phase for
the cutting insert 14. The content of binder phase in the
intermediate layer 16 is 1.2 - 3, preferably 1.4 - 2.5, of the
nominal content of binder phase for the cutting insert 14.
From what is said above, it can be realized that a
higher nominal cobalt content of the cutting insert gives higher
compressive stresses in the surface layer. This is shown by the
diagram of Fig. 4.
It should be pointed out that the core 15 of cemented
carbide containing eta-phase is stiff, hard and wear resistant.
Said core 15 in combination with an intermediate layer 16 free of
eta-phase and having a high content of cobalt and a surface layer
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17 free of eta-phase and subjected to high compressive stresses
presents a cutting insert 14 that fulfills the requirements
discussed above for cutting of mineral and asphalt, i.e., a
cutting insert demanding relatively low cutting forces and having
a relatively high wear resistance.
In Fig. 5, a diagram is disclosed showing the hardness
distribution of a cutting insert according to the present
invention and a cutting insert of standard cemented carbide, both
inserts having a nominal content of cobalt of 15% by weight. The
measurements are carried out from the surface up to the center of
the cutting inserts. By studying Fig. 5 it is at once noticed
that the surface layer 17 of a cutting insert according to the
invention has a relatively seen very high hardness up to about
1.5 mm from the surface, said layer 17 having a low content of
cobalt. The layer 16 having a high content of cobalt has a
relatively low hardness. The core 15 again has a relatively high
hardness.
The cutting insert of standard cemented carbide has a
constant hardness, as can be seen in Fig. 5.
Tests have been made of the parameter wear relative to
the parameter cutting length for three difference cutting
inserts. Said tests are shown in a diagram in Fig. 6.
The cutting insert of type A has a geometrical design
in accordance with Fig. 3. However, the material in said cutting
insert is cemented carbide of standard type. The cutting insert
of type B is of conventional geometrical design for cutting
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mineral, see Fig. 7, and the cutting insert of type C is a
cutting insert 14 according to the present invention, i.e., in
accordance with Fig. 3.
As can be seen from Fig. 6, the cutting insert of type
A is worn out to 100% after a cutting length of about l90 m. The
cutting insert of type B is worn out to about 80% after a cutting
length of about 375 m. The cutting insert of type C is worn out
to about 50% after a cutting length of about 940 m. In this
connection it should also be pointed out that the cutting inserts
of type A and C have a weight of 80 g while the cutting insert of
type B has a weight of 150 g, i.e., the volume of the cutting
insert of type B is almost twice the volume of the cutting
inserts of type A and C.
For a man skilled in the art, the results presented in
Fig. 6 are very surprising. Compared to conventional cutting
inserts for cutting mineral or asphalt, the cutting insert
according to the present invention has a relatively large axial
projection, see, e.g., Fig. 2. The composition of the cutting
insert 14 according to Fig. 3 makes it possible to handle the
relatively large tensile stresses and bending moments that act
upon the cutting insert 14 due to its relatively large axial
projection.
A further advantage with a tool according to the
present invention compared to conventional tools is that less
dust is produced when cutting is effected, i.e., the grain-size
distribution of the cut material is displaced towards bigger
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grain-sizes for the cutting insert of the present invention than
for a cutting insert of type B, see Fig. 7. The reason for that
is the geometry in combination with the high wear resistance of
the cutting insert according to the invention.
In Fig. 8 and 9 a preferred embodiment of a brazed
joint 19 is disclosed. The brazed joint 19 is located between
the tool body 12 and the cutting insert 14. The tool body
includes a recess 20 adapted to receive the cutting insert 14.
In the described embodiment the recess 20 has a flat
bottom portion 21 located in a plane perpendicular to the
longitudinal center axis 22 of the tool. The recess also
includes a conical surface portion 23 extending from the bottom
portion 21 towards the periphery of the tool body 10. The
conical portion 23 is symmetrical in respect of the longitudinal
center axis 22.
The recess 20 also includes an annular surface portion
24 having an extension in the longitudinal direction of the tool.
In the conical surface portion 23 an annular groove 25
is provided, said groove 25 being used for fixation of the
cutting insert 14 in the recess.
The cutting insert 14 according to the described
emhoAiment has a flat bottom surface 26 adapted to be located
above the bottom surface 21 of the recess in mounted position of
the cutting insert 14.
The cutting insert 14 further includes a conical
surface portion 27 extending from the bottom surface 26 up to a
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cylindrical periphery surface 28 of the cutting insert 14, said
surface 28 defining the biggest diameter of said cutting insert
14.
The conical surface portion 27 of the cutting insert is
provided with a number of spacing buttons 29 cooperating with the
groove 25 in mounted position of the cutting insert 14. The
buttons 29 and the groove 25 make sure that the cutting insert is
in correct position before brazing takes place.
As is indicated in Fig. 8 the conical surface portion
23 of the recess 20 and the conical surface portion 27 of the
cutting insert between them include an angle ~ that preferably
has a value of 2-4. The surface portions 23 and 27 resp.,
diverge in direction towards the periphery of the tool.
From Fig. 9 it can be learnt that the bottom surfaces
29 and 26 resp., are at a small distance from each other in the
disclosed embodiment.
When brazing is about to take place the tool body 10
and the cutting insert 14 are oriented relative to each other as
is shown in Figs. 8 and 9, i.e., they have a common longitudinal
center axis 22.
Brazing is then effected and preferably a copper based
brazing alloy is used. It is also preferred to use vacuum
brazing. The upper surface of the brazed joint 19 is marked by
30 in Fig. 9.
Due to the included angle ~ between the conical surface
portions 23 and 27 resp., the brazed joint 19 has generally
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wedge-like cross-sections in an axial plane through the tool
according to the invention. The thickness of the brazed joint 19
is increasing towards the periphery of the cutting insert 14.
This described design of the brazed joint 19 is very
S effective in that almost the entire portion of the brazed joint
19 located between the conical surface portions 23 and 27 resp.,
is active in absorbing the turning moment acting upon the cutting
insert 14. At one side the brazed joint 19 will be subjected to
tension forces while the diametrically opposed side will be
subjected to compression forces. The most difficult forces to
handle are of course the tension forces.
In order to describe the function of the brazed joint
according to the present invention it could be looked upon as a
number of elastical springs 31, 32, and 33. In such a case the
in radial direction outer portion of the brazed joint will be
more extended/compressed than the inner portions. Although the
springs 31-33 are extended/compressed to a different degree they
exert substantially the same force due to their different
lengths. This is illustrated by the diagram in Fig. 9. The
vertical axis indicates the force F and the horizontal axis
indicates the extension E. The disclosed brazed joint of Fig. 9
is subjected to a turning moment M and it is realized at once
that the springs 31-33 are subjected to tension forces that in a
conventional way are negative in the diagram. The tension force
in each spring 31-33 is the same while the extensions are
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different. Of course this theory will not be fulfilled
completely in practice but the principle is important.
A preferred but non-limiting dimensional example of the
brazed joint can be given. In the area of spring 31 the brazed
joint can have a thickness of 0.7 mm and in the area of spring 33
the thickness is 0.3 mm. The diameter of the cutting insert 14
is 24 mm measured at the cylindrical periphery surface 28.
In this connection it should be pointed out that the
brazed joint described above is not limited to be used with a
cutting insert 14 according to the present invention. Also the
rest of the invention is of course not restricted to the
described embodiments but can be varied freely within the scope
of the appending claims.
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