Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02331003 2001-02-05
CUTTING INSERT
The present invention relates to a cutting
insert for chipbreaking machining tools, in
particular for milling tools such as facemilling
tools, and to a process for making the insert.
Such an insert is typically manufactured by
form-pressing and sintering of an insert-forming
powder material to form a body which comprises a top
chip surface, a suitably planar bottom surface that
can rest upon a seat surface of the machining tool,
and at least one edge surface extending between the
top and bottom surfaces. The edge surface, which can
be placed in abutment with at least one cooperating
side abutment surface of the tool, is generally
inclined at an acute angle with respect to the chip
surface and at an obtuse angle with respect to the
bottom surface, whereby a cutting edge is formed
along the intersection of the chip surface and the
edge surface, adjacent to which cutting edge there
are one or several relief or clearance faces.
For the manufacturing of such cutting inserts,
in particular indexable cutting inserts, of hard
metal, a direct-pressing method is frequently used,
in which a hard metal-forming powder first is formed
to the intended shape in a suitable pressing die and
then given the final strength and size by sintering
in an oven at a temperature above 1000°C. The
pressing operation as such has been further developed
over the years and is today so advanced that it
enables the formation of the cutting edges and
adjacent chipforming faces and possible reinforcing
faces with great dimensional precision. However,
during the sintering operation a shrinkage takes
place (usually amounting to about 18% of the original
length in each dimension) and due to this, the
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cutting insert loses some of its original precision.
For some types of machining, e.g., some sorts of
facemilling, the requirements of form and dimensional
precision have become more rigorous over the last
years. Particularly insert geometries with a
positive cutting edge require a very high degree of
dimensional accuracy to guarantee a satisfactory
function at small tooth feeds. These precision
requirements have up to now been met by so-called
to contour grinding, which consists of aftergrinding the
surfaces) adjacent to the individual cutting edge in
one step after sintering. However, a serious
disadvantage of such contour grinding is that it
causes modifications in the micro-geometry of the
insert, i.e., in the surface structure of the
insert's edge-shaping parts after a surface treatment
such as blasting, face grinding or deposition of a
hardness-improving surface layer, which is usually
done as soon as possible after the sintering has been
2o finished. Thus, the width of existing negative
reinforcing surfaces is altered, as well as the
distance from the cutting edge to the chipforming
surfaces. In practice, this means that the
chipforming ability and the cutting performance of
the cutting insert are diminished and that its
strength and life are reduced.
SUMMARY OF THE INVENTION
An object of the present invention is to remove
the above mentioned disadvantages by eliminating
3o every form of after-grinding in the immediate
proximity of the cutting edge(s). Thus, a primary
object of the invention is to produce a cutting
insert whose working dimensions can be established
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with high accuracy without necessitating any after-
grinding of the cutting edges in question. A further
object of the invention is to enable a simple and
rational production of such inserts.
In a process aspect of the invention, a powder
is pressed to form an insert body having a top chip
surface, a bottom surface, and at least one edge
surface interconnecting the top and bottom surfaces.
The edge surface includes a relief surface portion
to intersecting the top surface at an acute angle to
form therewith a cutting edge. That press-formed
body is then sintered. Finally, the edge surface is
ground, but only along a lower portion thereof
disposed below the relief surface portion.
That lower portion of the edge surface is,
during a cutting operation, placed against a locating
surface of a machine tool. By subjecting only that
lower surface portion to a grinding operation, the
insert dimensions assume a much greater degree of
precision while avoiding the aforementioned problems
resulting when the entire edge surface and/or top
surface are subjected to grinding.
BRIEF DESCRIPTION OF THE DRAWINGS
A preferred embodiment of the invention will be
described below in detail with reference to the
accompanying drawings in which:
FIG. 1 is a simplified top perspective view of a
cutting insert according to the present invention,
FIG. 2 is a transparent top perspective view of
the same cutting insert,
FIG. 3 is a side view of the cutting insert in
connection with a schematically shown grinding tool,
FIG. 4 is an end view of a facemilling tool
equipped with cutting inserts according to the
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invention and illustrating the positioning of the
inserts on the facemill, and
FIG. 5 is a side view of the facemill of Fig. 4.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
OF THE INVENTION
In Figs. 1-3 a cutting insert 1 of square basic
shape is shown. The cutting insert comprises a top
chip surface 2 and a suitably planar bottom surface 3
which is substantially parallel with the plane PZ of
the chip surface 2. In this square embodiment four
identical edge surfaces 4A, 4B, 4C and 4D extend
between the chip surface 2 and the bottom surface 3.
The cutting insert has a positive geometry, meaning
that the insert's edge surfaces 4 lie in planes that
on the one hand form an obtuse angle with the plane
of the bottom surface 3 and on the other hand form an
acute angle with the plane of the chip surface P2.
Disposed between adjacent edge surfaces are
corners 5, 5', 5" and 5 "'. In the area between the
chip surface 2 and one of the edge surfaces 4A are
disposed two cutting edges, viz. a main cutting edge
6 and a secondary cutting edge 7. In an analogous
manner, between chip surface 2 and another edge
surface 4B are disposed a main cutting edge 6' and a
secondary cutting edge 7', similar pairs of cutting
edges 6" , 7" and 6 "' , 7 "' being formed in the
transitions between the chip surface and each one of
the edge surfaces 4C and 4D. Each main cutting edge
6 forms a certain angle with the secondary cutting
edge 7 as the surface 2 is viewed in plan. In
practice, this angle should lie within the range of
0.5 - 4°.
Along a certain part of each edge surface, the
cutting insert 1 has a planar relief surface 8, which
extends along a substantial extent of the respective
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main cutting edge 6 and whose width or height
increases toward a corner located adjacent that main
cutting edge. For example, at the edge surface 4A,
the width of relief surface 8 gradually decreases in
the direction from the corner 5"' towards the corner
5. This relief surface 8 is oriented in a plane
substantially perpendicular to the plane PZ, which is
clearly shown in Fig. 3. Along a dividing or
junction line 9 this first relief surface 8 joins a
second relief surface 10, whose width decreases in
the direction from the corner 5 towards the corner
5"'. This second relief surface 10 is inclined at an
acute angle ~ in relation to the plane Pz of the chip
surface, for instance an angle within the range of 65
- 75°, suitably around 70°, as shown in Fig. 3. The
top surface 2 includes an upper reinforcing land 11
joining the cutting edges 6, 7 and having a
substantially constant width, at least along the main
cutting edge 6. This land preferably lies in the
plane PZ and joins an intermediate surface portion 11A
which, in turn, joins a chipbreaking portion 2A of
the top surface 2. The latter can include many
different chip breakers.
In the center of the cutting insert there is a
hole for the application of a suitable fastening
means.
The cutting insert described so far has been
previously described in SE 9003827-4 (& EP-A-
489 702).
Besides the two relief surfaces 8, 10, each
edge surface 4A-4D according to the present invention
also comprises a third surface portion 12 which is
formed by grinding of the cutting insert. In Fig. 3
it is schematically illustrated how this third
surface 12 can be produced by applying a grinding
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tool 13 against the edge surface in question. In
Fig. 3 the grinding tool 13 is in contact with
surface 4A. The cutting edge 6" on the diametrically
opposed side 4C functions as an abutment for
positioning the insert 13 during the grinding
operation. More specifically, the grinding tool is
brought to such a depth that a precise dimension A is
attained between the cutting edge 6" and the
diametrically opposed surface 12. This is important,
since the ground surface 12 will abut a locating
surface of a machine tool in order to position the
diagonally opposite cutting edge during a cutting
operation.
The inclination angle 8 of the ground surface 12
in relation to the plane P2 of the chip surface is in
practice somewhat smaller than the previously
mentioned inclination angle y of the other relief
surface 10. In practice, the difference between
those angles should lie within the range 1 - 6° and
preferably about 4°. Thus, if the angle ~ is 70°,
then the angle B should amount to about 66°. A
junction edge 14 is thus formed between the two
surfaces 10 and 12. In practice, this edge 14 should
extend parallel to the bottom surface 3, giving the
surface 12 a constant width W along its whole length.
In practice, the width of this surface 12 is about
half of the insert's thickness T, although it could
alternatively be less or greater. However, the width
W of the ground surface 12 should always amount to at
least 40% of the insert s thickness T.
Naturally, the grinding operation is resource-
demanding in that it requires time and energy. In
order to reduce time and energy consumption to a
minimum, the cutting insert has been provided with a
preferably centrally placed recess 15 on each edge
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surface 4A-4D during the form-pressing step. This
recess divides the ground surface on each side of the
cutting insert into two surface portions 121 and 122.
In practice, the length of the recess 15 can be as
much as 25 - 35% of the whole length L of the ground
surface 12, so that the total surface area of the
surface portions 121 and 122 is about 75 - 65% of the
area that the ground surface would have had if it had
not been interrupted by the recess 15.
While the surfaces 8 and 10 next to the cutting
edges are_kept in the same shape as formed by form-
pressing and sintering steps, the surfaces 12 are
produced by grinding. This enables a very high
degree of dimensional accuracy to be attained in that
the tolerance of the aforedescribed dimension A
between the individual grinding surface and a
diametrically opposed cutting edge (i.e., the
variance of that dimension A from a desired value)
will lie within a very small range, i.e., the range 1
- 20 Vim, preferably 1 - 10 Vim. The individual ground
surface 12 is not a real relief surface under any of
the cutting edges; it merely serves as an abutment
surface in connection with a side abutment surface of
the machining tool for positioning the insert during
cutting.
Figures 4 and 5 illustrate a facemiller 15
equipped with a number of cutting inserts 1 according
to the invention (although the cutting inserts in
Fig. 5 are shown with a differently shaped chip
surface than the cutting insert shown in Fig. 1).
The cutting inserts are placed into recesses of the
milling cutter 15 in such a way that the milled angle
in the working piece will be 90°. In each individual
recess of the milling cutter there are three separate
abutment surfaces, viz. a bottom abutment or seat
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surface 16 against which the bottom surface 3 of the
cutting insert is placed, a first side abutment
surface 17 and a second abutment surface 18 against
which two of the edge surfaces 12 of the cutting
insert are placed, while the cutting edges of the
other two edge surfaces are in a working position.
Generally, the geometry is such that the radial rake
angle a is negative and the axial rake angle ~ is
positive. Decisive for the machining accuracy is on
the one hand the radial dimension B between the
rotation_axis of the milling cutter and a
peripherally situated main cutting edge 6 on the
individual cutting insert, and on the other hand the
axial dimension C between the upper, planar surface
19 of the milling cutter and the secondary cutting
edges 7 of the individual cutting insert. These
dimensions B and C are of course dependent upon the
dimension A between the individual cutting edge and
the diametrically opposed abutment surface 12 which
abuts either of the surfaces 17 and 18. By making
this abutment surface 12 in the form of a ground
surface, not only the benefit of high dimensional
precision is realized, but also the basic advantage
that the micro-geometry of the cutting insert can be
maintained unchanged after sintering and a possible
surface treatment. In this way, inter alia the
reinforcing faces 11 which were produced during the
form-pressing and sintering steps, can keep their
original width, and the chip breakers in the chip
surface 2 can keep their positions in relation with
the cutting edges.
The invention is naturally not restricted to
what has been described above or to the embodiment
illustrated in the drawings. Thus, it is also
feasible to use the inventive concept for cutting
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inserts with another polygonal basic form than the
square shape, e.g., a triangular shape. It is even
feasible to produce circular cutting inserts with the
lower portion of the sole circumferential edge
surface being ground. Further, the cutting insert
according to the invention can also be applied to
other chipbreaking machining tools than just
facemillers.
Moreover, it is pointed out that the cutting
l0 insert according to the invention need not
necessarily have two cutting edges 6, 7 and two
distinct relief surfaces 8, 10 separated by an
inclined juncture edge 9. Rather, it is also
possible to produce the cutting insert with only one
relief surface and one single cutting edge along each
side (in the case of a polygonal cutting insert),
whereby that relief surface joins a ground surface
having a smaller angle in relation with the chip
surface than the relief surface.
Furthermore, the design of the chip breakers on
the top surface of the cutting insert can vary quite
considerably, as well as the form and the dimensions
of all possible reinforcing faces.
The inventive concept is also applicable on
cutting inserts made of other materials than hard
metal, as long as a powder is form-pressed and
sintered.
