Note: Descriptions are shown in the official language in which they were submitted.
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BALLNOSE END MILL AND INS~RT THEREFOR
This invention relates to ballnose end mills of
the kind having replaceable cutting inserts.
The invention in particular :is a ballnose end
mill having so-called "on-edge" inserts, i.e., inserts
in which the cutting force is tranqmitted through one of
the major dimensions of the insert, rather than through
its short dimension.
On-edge inserts are demonstrably more resistant
to breakage from shock loads, and while their
superiority in this respect is well-known, they have not
heretofore been applied successfully to ballnose end
mills or other rotating tools intended for fillet
cutting, such as are used, for example, in the
manufacture of dies for press work, or simply for the
machining of machine parts, or structural components of
aircraft, for example, where filleting is essential for
the prevention of undue stress concentration.
It is accordingly the objective of the present
invention to provide a ballnose end mill which employs
inserts of cutting material which are configured for
on-edge orientation and which, in preferred form, are
not only replaceable, but indexable, i.e., which have
multiple cutting edges which can be used in succession
as each cutting edge becomes dull or eroded.
; It is a further objective to provide an
indexable on-edge insert for ballnose end mills and
comparable fillet-cutting usage which is so configured
that two such identical inserts properly mounted in a
tool holder sweep the desired hemispherical path, are
capable equally of axial plunge cutting, i.e., drilling,
to initiate a cut, and share the major cutting load
between them in a substantial overlap of their
respective cutting paths, with at least one of the
cutters having its cutting edge oriented for maximum
bevel-cutting effectiveness, i.e., with negative radial
rake and positive axial rake, in accordance with known
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cutting princlples. ~ 3~
Spècifically, the invention relates to a multiple-
use indexable cutting insert for on-edge cutting in a ballnose
end mill or the like comprising a block o:E cutting rnaterial
having a first major face which is cylindrically convex,
an opposite major face which is plane, rhomboidal in outline,
with its opposite major edges in chordal relation to the
cylindrical major face, and upon which the block is adap-ted
to be seated in a recess in a tool holder, a pair of opposed,
plane, minor boundary surfaces which intersect the plane
rhomboidal face on the two major edges and diverge therefrom
at equal angles to intersect the convex major face at an
acute included an~le to form therewith two substantially
circular cutting edges on the boundary faces, a second pair
of opposed, plane, minor boundary surfaces which intersect
the plane rhomboidal face perpendicularly on its two minor
edges, and a central bore extending between the major faces
to receive a securing fastener, wherein the pair of diverging
minor boundary surfaces constitute alternate rake faces and
wherein the second pair of minor boundary surfaces, by their
acute-angle intersection with the pair of diverging minor
boundary surfaces, form edges which intersect the substantially
circular cutting edges in alternate centering points when
the block is positioned at the nose of an end mill and provide
clearance behind the rake faces respectively when the block
is positioned in an end mill for bevel cutting with positive
axial rake.
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The invention is explained in the following
detailed specification takerl in conjunction with the
accompanying drawings in which:
FIGURE 1 is an end view of the ballnose end
mill of my invention seen from the c~tting end;
FIGURE 2 is a side view of the same similarly
showing the two insarts which provide the hemispherical
sweep and a third and optional insert which cuts a~
essentially cylindrical path tangent to the
hemispherical cutting path swept by the two curved~edge
cutters;
FIGUR~ 3 is a diagram of the profile of the
cutting path taken on a radial plane through the axis of
the tool holder to show the relationship of each insert
to the composite resulting cutting path; and
FIGUR~5 4a-d inclusive are enlarged end, back,
top, and front views respectively of the curved-edge,
indexable, on-edye insert which is a key element of the
invention and which, when appropriately applied to a
rotatable shank, provides the cutting path of
circular-quadrant outline illustrated diagrammatically
in FXGUP~ 3.
Referring first to the several views of FIGURE
4 for the details of the cutter itself, the on-edge
- insert 1 is a block of suitable cutting material, and
is generally in the form of a flat, si-x-sided block
having two major faces, viz., a flat, planar seating
face 12 and an opposite face 14 which i5 convex. A hole
16, cored through the minor dimension of the block and
countersunk at the convex face 14, receives a fastener,
not shown, which anchors the insert 10 in place in a
recess provided for it in the tool-holder 18 ~Figs. 1
and 2), as later explained.
The convex major face 14 of the insert is
cylindrical in the illustrated case, and is limited by
four minor boundary surfaces. In the prefarred formJ
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two of these are cutting faces 20 which are formed as
though cut by converging planes which make equal
angles with a bisecting plane perpendicular to the
axis 22 of the cylindrical face 14 of the insert ~Fig.
4a) and intersect the convex face 14 at an acute angle
to form the curved cutting edge 28.
The other two minor boundary surfaces 24 are
parallel planes perpendicular to the flat seating face
12 of the insert, but askew to the parallel edges 26 of
the flat seating face 12 formed by the converging
cutting faces 20. The cutting insert is therefore
generally rhomboidal (FigO 4c), its diagonally opposite
corners being respectively acute and obtuse at the
junctures of the cutting faces 20 with their respective
adjacent end surfaces 24.
The radius of the cylindrical surface 14 in the
illustrated case is the same as the radius of the
intencled hemispherical cutting path of the end mill, and
while the cutting edge 28 of each of the two identical
cutting faces of the insert is, in fact, elliptical in
the plane of that face, the profile of the cutting path
to be generated by the use of such inserts depends upon
the orientation of the cutting edge, and, more
particularly, upon the projection of the convex surface
of the seated insert, along the rotational path, to a
radial plane through the axis of rotation of the cutter.
That geometry is shown in FIGVRES 1 and 2 which
illustrate the composite cutting tool of the invention.
The hemispherical cutting path of the end mill
of this invention is generated by two inserts 10 and 10'
of the kind illustrated in FIGURE 4 which are positioned
at the nose of the tool holder 18 so as to sweep
overlapping cutting paths which, projected to a radial
plane through the axis 30 of the tool holder, extend as
a circular quadrant 32 from the axis of rotation to the
point of maximum radius measured from that axis (Fig. 3).
The point insert 10 is positioned with one of
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its cutting surfaces 20 essentially radially o~ the tool
holder 18 and with the very end of its cutting edge 28
at the acute an~le corner of the insert positioned at
the rotational axis (compare Figs. 1 and 2~. From
5 FIGURE l the reason for the acute angle corner will be
appaxent, namely it provides a centerin~ point to
facilitate an axially plunging or drilling cut into the
work, and at the same time provides clearance behind the
cutting point. By the same token, the seating surface
of tne recess which receives the point insert 10 is
pitched to drop the opposite and trailing edge of the
insert, i.e., the reserve cutting edge, below the
generally spherical outline traced by the active cutting
edge during rotation.
The need for trailing clearance to prevent the
insert from "heeling" on the freshly cut surface of the
work, and the desirability of maintaining the direction
of the cutting forces upon the tool holder as nearly
tangential as possible, limit the dimension of the
insert in the axial direction of its cylindrical
surface. In end mills of very small diameter, space
limitations may ~ake it necessary to forego indexability
in favor of solid support for a narrower single-edge
insert.
The second or upper insert 10', identical in
form and indeed interchangeable with the point insert
10, is positioned to sweep the spherical path from its
major radius measured from the xotational axis of the
cutter downwardly along the circular quadrant 32 of the
cutting path (Fig. 3), but terminating well short of the
point. Again referring to FIGURE 1, the upper insert
lO' is positioned with its cutting edge 28'
approximately diametrically opposite the cutting edge of
the point insert 10~ but with a substantially different
orientation. The seat of the recess for the upper
insert 10' is pitched and configured to position the
upper insert with negative radial rake and positive
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axial rake to provide the optimum orientation for the
bevel cut which constitutes the major part of the
cutting load of the upper insert. The benefits of such
orientation for bevel cutting are well-known, having
been disclosed in U.5. Patent No. 2,186,417, issued in
1940 to the assignee of this invention.
It will again be apparent from FIGURE 1 that
the same rhomboidal shape which provides the cutting
point of insert lO is compensated by the positive axial
rake of the upper insert lO' with the result that the
cutting edge 28' of the upper insert lO' is fully backed
throughout its entire length in opposition to the
cutting forces while the insert lO' is adequately
pitched to prevent the trailing, reserve cutting edge
from heeling on the freshly cut work.
A third cutting insert 36 having a straight
cutting edge is employed to extend the cutting path
cylindrically tangent to the circular-quadrant
projection 32 of the overlapped cutting paths of the two
novel inserts lO and lO' of this invention. The latter
is preferably also of rhomboidal shape for the solid
~upport of its cutting edge, although positioned with
positive axial rake. The third insert is similarly
pitched for assurance that its trailing reserve cutting
edge is clear of the cut, and all three inserts are
positioned with their active cutting edges trailing
irregularly shaped and positioned chip gullets 38, 40
and 42 which are gashed in the body of the tool holder
18 at an angle to the axis of rotation to allow the
chips to flow up and out when drilling, or indeed when
feeding transversely of the cutter axis.
It may be appreciated fro~ the foregoing
descriptions of the individual cutting inserts lO and
lOI and of their orientations upon the tool holder 18
that the cutting edges 28 and 28' do not sweep a path
which is precisely circular in the radial plane, but
rather one whose radial projection deviates slightly
elliptically rom circular with a somewhat tighter
radius of curvature. The deviation from circular,
however, is very minor and essentially of no consequence
in the contour machining involved, for example, in die
work, nor in the machining of fillets for their own
sake. Similarly, it will be appreciated that the
axially raked straight cutting edge of the "cylindrical"
insert 36 sweeps a path which is not precisely
cylindrical, but rather hyperbolic. Again, however,
this deviation from a truly straight surface tangent to
the fillet generated by the ball nose is of no
consequence in most applications and may be overcome by
a mildly convex or helical cutting edge on the third
insert for cutting applications ~here a more nearly
plane surface must be left upon the work.
FIGURE 3, as earlier notecl, illustrates
diagrammatically the projection of the several
aforedescribed cutting paths upon a radial plane through
the center line of the cutter, i.e., through the axis of
rotation. Although FIGURE 3 is diagrammatic rather than
a precise orthographic projection, the cutter inserts
10, 10' and 36 are indicated generally in outline form
projected upon a radial plane of the tool body. The
matter to be observed is the degree to which the
respective cutters 10 and 10' occupy the circular
quadrant 32 of their combined cutting paths and, in
particular, the overlap of those paths which permits the
two cutters to share the major cutting load. For
convenience, the poxtion cut only by the point insert 10
is labelled "A", the overlap portion "B", and the
portion cut only by the upper insert 10' is labelled "C".
It is understood by those skilled in the art
that the cutting load in zone "A", during ordinary
transverse milling as distinguished from axial plunge
cutting, i.e., drilling, is a very light load because of
the chip~thinning effect resulting from the bevel of the
cutting edge. On the other hand, in zone "C", swept
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only by the upper cutter 10', peripheral chip-thinning
due to th~ transverse ~eed is drastic, but thinning
attributable to bevel is negligible. Quantitatively,
the greater metal removal rate occurs in the overlap
zone "B" where, with the approximate diametrical
placement of the cutting edges of the point insert lO
and upper insert lO', those inserts share the heavy
load, each taking a new cut which is only half the
thickness that ~ould be encountered by a single cutter
covering the same sector at the same feed rate~
The novel ballnose end mill of the invention is
a heavy duty tool capable of withstanding heavy cutting
loads because its inserts cut "on edge", i.e., they
receive and transmit the cutting load essentially
through a major dimension of the insert. Moreover, they
are arranged to sweep paths which greatly overlap,
making for greater tool life on both counts.
The individual novel insert is usable
interchangeably in either nose pGsition, i.e., as the
point cutter 10 or as the upper cutter 10', and both are
reversibly indexable to double their service in either
role.
The features of the invention believed new and
patentable are set forth in the following claims.
