Note: Descriptions are shown in the official language in which they were submitted.
BALL END MILL AN~ THROW AWAY
INSERT FOR SUCH EN~ MI~L
Background of the Invention
The present invention relates to a ball end mill
for the chipforming machining of metal workpieces. The
end mill comprises a tool body that is rotatable about its
longitudinal axis. The forward end of the tool body is
hemispherically shaped and has several separate pockets
for the receipt of indexable inserts. One of th~ inserts
is substantially triangular in shape and is mounted
tangentially in the mantle surface of the tool body close
to its center such that the main cutting edge of the
insert is oriented generally radially and has one of its
corner portions located at the center of the tool body.
The invention furthermore relates to a polygonal throw
away insert specifically designed for use with such an
end mill.
Such ball nose end mills with indexable inserts
are previously known, for instance as disclosed in U.S.
Pateht No. 4,252,480. That ball nose end mill includes a
central insert having a cutting edge of convex shape along
a portion of the periphery of one of its side surfaces for
the purpose of reducing the thrust forces. A basically
similar ball end mill with throw away inserts is known
from U.S. Patent 4,618,296 in which all inserts have a
generally triangular shape and are tangentially oriented
in the tool body. The convex formation is achieved by
making the outermost surface of the insert partially
convex in shape.
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It has been discovered, however, that the cutting
forces acting on each insert of the above described known
ball end mills become high and are unfavorably distributed
over the tool body. Further, each insert must be active
during a long cutting engagement in the workpiece.
It would, therefore, be desirable to provide a ball end
mill with several inserts that are mounted in an optimal
fashion over the mantle surface of the end mill so that a
more limited portion of the cutting insert corner comes
into engagement with the workpiece at different
engagements.
With this background in mind, it is an object of
the present invention to provide a ball nose end mill in
which a more optimal and even distribution of the cutting
forces acting on the inserts is achieved while providing
for good chip removal at each insert.
Summary of the Invention
These and other objects have been achieved by
a ball nose end mill comprising a rotary body which is
rotatable about a longitudinal axis. A forward end
surface of the body is of generally hemispherical shape
and includes a plurality of cavities. Indexable cutting
inserts are mounted in respective ones of the cavities.
A central one of the inserts is of substantially
triangular shape and is mounted closely adjacent the
longitudinal axis. The front insert includes a forwardly
facing, substantially triangular top face, and a plurality
of edge faces intersecting one another to form three
corner regions and intersecting the top face to form
therewith a plurality of main cutting edges. Each of the
main cutting edges is disposed between two of the corner
regions and is convexly curved when viewed in a direction
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toward its respective edge face. The central insert is
mounted such that one of the main cutting edges thereof
constitutes an active cutting edge positioned in a cutting
orientation. One of the corner reyions of the central
insert is situated at an end of the active cutting edge
and is located immediately adjacent the axis. The cutting
inserts include additional inserts of substantially
rectangular shape. Each of the additional cutting inserts
includes a circumferentially facing, generally rectangular
top face and a plurality of edge faces interconnecting
such top face to form therewith a plurality of cutting
edges. Each of the additional inserts is arranged such
that one cutting edge thereof constitutes an active
cutting edge positioned in a cutting orientation. The
plurality of additional inserts includes first and second
additional inserts whose cutting edges are convexly curved
when viewed in a direction toward their respective top
faces. The first and second additional inserts have their
active cutting edges disposed on opposite sides of the
axis and lying generally in a first plane which contains
the axis.
The present invention also relates to an
indexable cutting insert which is adapted for use in a
ball nose end mill. That insert comprises a substantially
flat bottom face from which the insert is to be supported,
and a generally convexly top face disposed opposite the
bottom face. A plurality of edge faces interconnect the
top and bottom faces and intersect the top face at an
acute angle. The intersection of the end faces with the
top face forms a plurality of main cutting edges and a
plurality of secondary cutting edges. The secondary
cutting edges are located at respective corner regions of
the insert and are convexly curved when the top face is
viewed in plan. Each o~ the main cutting edges is located
between respective pairs of the corner regions and is
generally convexly curved when viewed in a direction
toward its respective edge face. ~ach of the main cutting
edges is straight when the top face is viewed in plan.
Each of the secondary cutting edges intersects first and
second ones of the main cutting edges at first and second
points of intersection, respectively. A first line
tangent to the first point of intersection coincides with
the first main cutting edge as the top face is viewed in
plan. A second line tangent to the second point of
intersection forms an angle with the second main cutting
edge as the top face is viewed in plan.
Brief Description of the Drawings
The objects and advantages of the invention will
become apparent from the following detailed description of
preferred embodiments thereof in connection with the
accompanying drawings, in which like numerals designate
like elements, and in which:
FIG. 1 is an end view of a ball end mill having a
plurality of indexable inserts, according to the present
invention;
FIG. lA is a side view of the center insert as
seen in direction D-D in FIG. 1;
FIG. 2 is a side elevational view of the ball end
mill as seen in direction A-A in FIG. l;
FIG. 3 is a side elevational view as seen in
direction C-C in FIG. l;
FIG. 4 is a side elevational view as seen in
direction B-B in FIG. l;
FIG. 5 is a side elevational view as seen in
direction D-D in FIG. 1;
FIG. 6 is a plan view of the central insert;
FIG. 7 is a sectional view taken along the line
VII-VII in FIG. 6;
FIG. 8 is a sectional view taken along the line
VIII-VIII in FIG. 6;
FIG. 9 is a side elevational view of an
alternative embodiment of a ball end mill according to the
invention;
FIG. 10 is a plan view of the alternative
embodiment of the central insert of FIG. 9; and
FIG. 11 is a side view of the alternative insert
as seen from the right hand side in FIG. lo.
Detailed Description of Preferred
Embodiments of the Invention
With reference to FIGS. 1 and 2-5, a ball nose
end mill has a cylindrical shank 10 rotatable in a
direction af cutting CD around a central longitudinal
axis 11. The shank terminates in a substantially
hemispheriaal cutting face 12 in which there are provided
cavities. The surface 12 is generated by a radius R' (see
FIG. 2). The cavities are shaped to receive indexable
cutting inserts 13-19 made preferably of cemented carbide.
A central insert 13, i.e., the insert located close to the
longitudinal axis 11, includes a bottom face 20 and a top
face 21 (see FIG. lA). The insert 13 is of substantially
triangular shape when viewed in plan, i.e., in a direction
toward its top face (see FIG. 1). The top face 21 is
convexly shaped, more specifically is in the general shape
of a portion of a sphere so as to generally conform to
the profile of the hemispherical face 12 (see FIG. 3).
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All of the additional inserts 14-19 of this end mill are
substantially rectangular in shape and have planar,
parallel bottom and top faces.
The top faces of all of the inserts 13-19 are
centrally apertured for the receipt of central clamp
screws for securing the inserts to the end mill body.
That aperture extends completely through the top and
bottom faces of the respective insert. Each insert
includes edge faces (e.g., see the edge faces 22 of the
insert 13) which intersect the top face to form cutting
edges therewith.
The central insert 13 has three identical cutting
edges 23, 24, 25. Bevels 33, 32 and 31 are situated
immediately inwardly of the cutting edges 23, 24, 25,
respectively.
The cutting inserts 13-19 are of positive shape,
i.e., each of the edge faces thereof intersects the top
face at an acute angle. The degree of inclination of the
edge faces relative to the top face varies around the
insert. More specificallyj the inclination of such edge
face is greater at the corner regions of the insert than
at the intermediate regions of the edge face. By
"intermediate" region is meant a region of the edge face
situated between the corner regions of the insert. When
the inserts 13-19 are mounted in the end mill body, the
cutting edges present an effective chip angle that is
negative with respect to the radial direction RD (see
FIG. 1) and positive with respect to the axial direction
11 (see FIG. 2), which contributes to a smooth and
efficient working behavior of the tool.
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-- 7 --
First and second ones of the additional inserts
16 and 19 have their active cutting edges located on
opposite sides o~ the axis 11 and substantially in a
common first radial plane P1 which contains the axis 11
(see FIG. l). A third one of the additional inserts 15
has a convexly curved active cutting edge arranged
substantially in a second plane P2 which is different from
the first plane and which contains the axis 11. Fourth
and fifth ones of the inserts 14, 18 have straight main
cutting edges arranged substantially in a common plane,
e.g., the second plane P2, which is oriented substantially
perpendicular to the first plane Pl and which contains the
axis 11. Thus, the inserts 14, 15 and 18 have their
active cutting edges arranged substantially in the second
plane P2. A sixth one of the additional inserts 17 has a
convexly curved active cutting edge arranged substantially
in a third plane P3 which contains the axis 11 and which
is different from both of the first and second planes P1,
P2. Alternatively, the sixth additional insert 17 could
be positioned such that its cutting edge is arranged
substantially within the second plane P2.
The inserts 15 and 17 are located axially
rearwardly relative to the central insert 13 and axially
forwardly relative to the inserts 16 and 19. The inserts
16 and 19 are located axially forwardly relative to the
inserts 14 and 18. The inserts 14 and 18 have straight
active cutting edges arranged substantially diametrically
opposite cne another and extending substantially parallel
to the axis 11.
. ~s
As shown in FIG. 5, the inserts 14 and 18, which
are located axially rearwardly of all other inserts, are
provided with straight elongated main cutting edges 14a
and 18a. The more forwardly located inserts 15, 16, 17
are provided with cutting edges 1sa, 16a, 17a which are
convexly curved when viewed in a direction toward the
respective top face (e.g., see FIG. 4). The insert 19,
which is visible in FIG. 1, has a design corresponding to
that of the inserts 15, 16, 17. Inserts 14 and 18 could
alternatively be square in shape. In another embodiment,
insert 14 could have a design similar to insert 15.
The cutting edges of the insert 13 comprise a
plurality of main cutting edges 23, 24, 25 each extending
between the insert corners, and a plurality of secondary
cutting edges 26 extending around the insert corners.
Each of the secondary cutting edges 26 is generated by a
radius R (see FIG. 1). ~ach main cutting edge is convexly
curved when viewed in a direction toward the respective
edge face (see FIG. lA). When the insert 13 is viewed in
plan (FIG. 1), it can be seen that each of the secondary
cutting edges 26 is convexly curved. As a result, the
chips produced will be curved and small and thus can be
easily transported away from the center of the tool.
Each of the secondary cutting edges 26 intersects
two of the main cutting edges. For example, the secondary
cutting edge 26 shown at the lower right in FIG. 1
intersects the main cutting edge 25 at point 27', and
intersects the main cutting edge 23 at point 27A. As
shown in FIG. 6, a line drawn tangent to point 27A
coincides with the main cutting edge 23. However, a line
drawn tangent to the point 27' forms an obtuse angle~ with
the main cutting edge 25. Hence, each of the secondary
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cutting edges is not symmetrical about a plane bisecting
the corner. The edges 24 and 25 intersect the secondary
edges at points 27" and 27''', respectively.
As can be seen in FIG. 1, the insert 13 is
positioned such that the main cutting edge 23 constitutes
the "active" main cutting edge (the other main cutting
edges 24, 25 being inactive). Also, the secondary cutting
edge which constitutes an active secondary cutting edge is
the one located adjacent the center axis 11 of the mill.
It is preferable that the active secondary cutting edge
intersect or at least nearly intersect that center axis
when viewed in FIG. 1. During a cutting operation, there
occurs a relative rotary movement between the chip and the
secondary cutting edge which reduces a tendency for edge
build-up to occur. Such edge build-up can occur as the
result of the adhesion of undesirable material to the
cutting edge (the undesirable material coming from the
workpiece), and could lead to premature failure of the
cutting edge.
The top face 21 of the insert 13 is shaped so
that each of the three corner regions 28 is located at a
different elevation relative to other portions of the top
face 21. For example, when the insert is viewed along a
section line VII-VII which bisects both a corner region
and the opposite main cutting edge 24, a central portion
36 of the top face which surrounds the aperture 28 is
situated at an elevation higher than the corner region 28,
as shown in FIG. 7. Moreover, an intermediate portion 30
of the top face situated between the portion 36 and the
corner region 28 is at a lower elevation than adjacent
beveled portions 31, as is apparent from FIG. 8. Thus,
the central portion 36 of the top face is elevated in
comparison with each corner portion, whereas the
-- 10 --
intermediate portion 30 is at a lower level than the
bevels 31, 32, 33 located next to cutting edges 25, 24
and 23, respectively. In accordance with one preferred
embodiment, the insert 13 is so constructed and arranged
that, as shown in FIG. 2, two lines drawn from a center of
the radius R' (FIG. 2) to the points 27', 27",
respectively, form an acute angle~ of about 30 degrees.
The insert 13 includes a plurality of interjacent
surface portions extending from the central portion 36 to
the mid-region of a respective main cutting edge. As
shown in FIG. 7, each interjacent surface portion includes
(i) a beveled face 32 (or 31 or 33) having a width a,
(ii) a flat part 34 (having a width k) which is parallel
to the bottom face 20, and (iii) an obliquely inclined
part 35 (having a width c) which connects the flat part 34
with the central portion 36. The flat part 34 lies at a
higher elevation than the central portion 36.
The width a of each of the beveled faces 31,
32, 33 is such as to provide a substantially constant
clearance angle. The width b is larger than the width a
of the beveled face. The width a should preferably be
approximately one-half the width b.
Due to such an arrangement of surface portions
32, 33, it has been found possible to use this end mill as
a drilling end mill. For instance, it has been found
possible to drill a hole to a depth of one-half of the
diameter of the end mill in a normally workable metal
workpiece. It is also possible to use this tool for
drilling obliquely in ~elation to the longitudinal axis,
i.e., so-called ramping. The invention makes such metal
working possibIe within an angular range from the angle
0 to 90 degrees.
An alternative embodiment of the central insert
is shown in FIGS. 9-11 in which the central insert 13~ is
provided with recesses 37 of concave shape, each of which
intersects and extends through the respective main cutting
edge for the purpose of obtaining increased clearance of
the central portion of the tool during a working
operation. This contributes to avoiding small pieces
breaking off from the edge which could cause tool failure.
This recess 37 starts at about the point 27' (or 27"
or 27''') and extends along a limited portion af the
adjacent main cutting edge. At the same time, the
recess 37 extends obliquely inwards in relation to
the main cutting edge, whereby the extension inwards
substantially corresponds to the double the width a
of the beveled face 33.
Further, the insert 13A is provided with a
strengthening face 38 on its chip surface which extPnds
adjacent to the beveled face 33. Preferably, the
strengthening face 38 intersects with the beveled face at
an angle of 75 to 85 degrees. The clearance face 22
extends downwards and intersects at an obtuse angle with
an edge surface 39 which is obliquely oriented in relation
to the bottom face 20.
Although the present invention has been described
in connection with preferred embodiments thereof, it will
be appreciated by those skilled in the art that additions,
modifications, substitutions, and deletions not
specifically described may be made without departing from
the spirit and scope of the invention as defined in the
appended claims.
