GB2135612A - Ballnose end mill and cutting insert therefor - Google Patents

Abstract

A ballnose end mill having two identical indexable on-edge convex inserts 10, 10' with part-circular cutting edges 28, 28' is capable of drilling and milling, and apportioning the major milling load between the inserts. The mill also has a third insert 36 with a straight cutting edge. <IMAGE>

Description

SPECIFICATION Ballnose end mill and insert therefor This invention relates to ballnose end mills of the kind having replaceable cutting inserts.

The invention in particular relates to a ballnose end mill having so-called "on-edge" inserts, i.e.

inserts in which the cutting force is transmitted 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.

The present invention therefore seeks to provide a ballnose end mill which employs inserts of cutting material which are configured for onedge 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.

The invention also seeks 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 cutting principles.

According to this invention we provide an indexable cutting insert for milling cutters and the like comprising a block of cutting material having a first major face which is convex, an opposite major face adapted for seating the block in a recess in a tool holder, and a plurality of minor boundary faces surrounding said convex face, at least one of said boundary faces being a cutting face which intersects said convex face at an acute included angle to form a cutting edge which is substantially circular, and which intersects at least one of its adjacent boundary faces in a second acute included angle.

Furthermore we provide a ballnose end mill comprising a tool holder in the form of a generally cylindrical shank having at its cutting end at least two chip gullets each followed in the direction of rotation by an on-edge cutting insert seated and secured in a recess formed in the shank, said inserts each having curved cutting edges which together sweep a cutting path which is a substantially circular quadrant in projection to a radial plane through the axis of the shank and is continuous from the axis of the shank to the maximum radius of the path measured from said axis, the cutting edge of one of said inserts being positioned substantially radially of the shank, terminating at one end of the axis of the shank, and extending along a major portion of said quadrant, the cutting edge of the other of said inserts being positioned with a substantial combined negative radial rake and positive axial rake and extending along a major portion of said quadrant from said maximum radius toward said point, the cutting edges of said inserts being disposed on opposite sides of said axis, and the cutting paths of said cutting edges individually overlapping by a major portion of the length of each at the mid portion of said quadrant to apportion the heavier cutting load between the inserts.

The invention is explained by way of example in the following detailed specification taken in conjunction with the accompanying drawings, in which: Figure 1 is an end view of the ballnose end mill of our invention seen from the cutting end; Figure 2 is a side view of the same similarity showing the two inserts which provide the hemispherical sweep and a third and optional insert which cuts an essentially cylindrical path tangent to the hemispherical cutting path swept by the two curved-edge cutters; Figure 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 Figures 4a-d inclusive are enlarged end, back, top, and front views respectively of the curvededge, indexable, on-edge 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 Figure 3.

Referring first to the several view of Figure 4 for the details of the cutter itself, the on-edge insert 10 is a block of suitable cutting material, and is generally in the form of a flat, six-sided block having two major faces, viz., a flat, planar seating face 12 and an opposite face 14 which is 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 1 8 (Figures 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 preferred form, two of these are cutting faces 20 which are formed as though cut by converging planes which make equal angles ct with a bisecting plane perpendicular to the axis 22 of the cylindrical face 14 of the insert (Figure 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 1 2 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 (Figure 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 intended 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 Figures 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 1 8 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 (Figure 3).

The point insert 10 is positioned with one of its cutting surfaces 20 essentially radially of the tool holder 1 8 and with the very end of its cutting edge 28 at the acute angle corner of the insert positioned at the rotational axis (compare Figures 1 and 2). From Figure 1 the reason for the acute angle corner will be apparent, namely it provides a centering 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 taken, the seating surface of the 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 make 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 rotational 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 10' 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 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.S. Patent No.

2,186,417, issued in 1940.

It will again be apparent from Figure 1 that the same rhomboidal shape which provides the cutting point of insert 10 is compensated by the positive axial rake of the upper insert 1 0' with the result that the cutting edge 28' of the upper insert 10' is fully backed throughout its entire length in opposition to the cutting forces while the insert 10' 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 axtend the cutting path cylindrically tangent to the circular-quadrant projection 32 of the overlapped cutting paths of the two novel inserts 10 and 10' of this invention.

The latter is preferably also of rhomboidal shape for the solid support 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 1 8 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 from the foregoing descriptions of the individual cutting inserts 10 and 1 0' 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 from 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 where a more nearly plane surface must be left upon the work.

Figure 3, as earlier noted, illustrates diagrammatically the projection of the several aforedescribed cutting paths upon a radial plane through the centre 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, 1 0' 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 portion 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 only by the upper cutter 10', peripheral chip-thinning due to the transverse feed 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 10 and upper insert 10', those inserts share the heavy load, each taking a new cut which is only half the thickness that would 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 position, 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 feature of the invention believed new and patentable are set forth in the following claims.

Claims (11)

1. An indexable cutting insert for milling cutters and the like comprising a block of cutting material having a first major face which is convex, an opposite major face adapted for seating the block in a recess in a tool holder, and a plurality of minor boundary faces surrounding said convex face, at least one of said boundary faces being a cutting face which intersects said convex face at an acute included angle to form a cutting edge which is substantially circular, and which intersects at least one of its adjacent boundary faces in a second acute included angle.

2. The cutting insert of claim 1 in which two of said boundary faces are cutting faces each as defined by claim 1.

3. The cutting insert of claim 2 in which the block is generally rhomboidal, the convex major face is cylindrical, the two cutting faces are formed by converging planes which intersect in a line in a plane perpendicular to the axis of cylindrical face and at the same angle with said perpendicular plane, and the other boundary faces are at least two in number each intersecting one of said cutting faces in said second acute included angle.

4. The cutting insert of claim 3 in which the opposite major face is flat and parallel to the axis 'of the cylindrical surface, and said other two boundary faces are parallel to each other and perpendicular to said opposite major face.

5. A ballnose end mill comprising a tool holder in the form of a generally cylindrical shank having at its cutting end at least two chip gullets each followed in the direction of rotation by an on-edge cutting insert seated and secured in a recess formed in the shank, said inserts each having curved cutting edges which together sweep a cutting path which is a substantially circular quadrant in projection to a radial plane through the axis of the shank and is continuous from the axis of the shank to the maximum radius of the path measured from said axis, the cutting edge of one of said inserts being positioned substantially radially of the shank, terminating at one end of the axis of the shank, and extending along a major portion of said quadrant, the cutting edge of the other of said inserts being positioned with a substantial combined negative radial rake and positive axial rake and extending along a major portion of said quadrant from said maximum radius toward said point, the cutting edges of said inserts being disposed on opposite sides of said axis, and the cutting paths of said cutting edges individually overlapping by a major portion of the length of each at the mid portion of said quadrant to apportion the heavier cutting load between the inserts.

6. The end mill of claim 5 in which the inserts are interchangeable and comprise a block of cutting material having a first major face which is convex, an opposite major face adapted for seating the block in a recess in a tool holder, and a plurality of minor boundary faces surrounding said convex face, at least one of said boundary faces being a cutting face which intersects said convex face at an acute included angle to form a cutting edge which is substantially circular, and which intersects at least one of its adjacent boundary faces in a second acute included angle.

7. The end mill of claim 5 or 6 in which the inserts are interchangeable and indexable, and comprise a block of cutting material having a first major face which is convex, an opposite major face seated in the tool holder recess, and a plurality of minor boundary faces surrounding said convex face, wherein at least two of said boundary faces are cutting faces which intersect said convex face at an acute included angle to form a cutting edge which is substantially circular, and each cutting face intersects at least one of its adjacent boundary faces in a second acute included angle.

8. The end mill of claim 5, 6 or 7 in which the inserts are interchangeable and indexable, and comprise a block of cutting material having a first major face which is cylindrical, an opposite major face seated in the recess in the tool holder, and four minor boundary faces surrounding said cylindrical face in rhomboidal outline, two of said boundary faces being cutting faces formed by converging planes which intersect in a line in a plane perpendicular to the axis of said cylindrical face and make the same angle with said perpendicular plane, and the other boundary faces each intersect one of said cutting faces in an acute included angle.

9. The end mill of any one of claims 5 to 8 having at least one additional insert on the shank disposed to sweep a substantially cylindrical cutting path about the axis of the shank and tangent to the cutting path swept by the curvededge inserts.

10. An indexable cutting insert substantially as shown in the accompanying drawings and described herein with reference thereto.

11. A ballnose end mill substantially as shown in Figures 1 and 2 of the accompanying drawings and described herein with reference thereto.