GB2146276A - A cutting tool and an indexable insert therefor - Google Patents

Abstract

A cutting tool preferably embodied as a ball nosed end mill (100) has a cylindrical shank (101) which terminates in a spherically shaped cutting end (110) with recesses shaped for receipt of at least two indexable cutting inserts (10, 11, 12). Each insert is mounted in lay-down fashion and provides a portion of an effective overall arcuate cutting edge, the cutting zones of each insert overlapping each other. Each insert (5) has the form of an equilateral polygon bounded by planar flanks (54), each flank intersecting a convex portion of a major face of an insert to form an arcuate cutting edge (51). The convex major face portion forms the clearance face, and the planar flank portion (54) forms the rake face for each arcuate cutting edge (51). <IMAGE>

Description

SPECIFICATION A cutting tool and an indexable insert therefor The present invention relates to a cutting tool and to an indexable cutting insert for such a tool.

For example, the invention relates generaliy to chip forming cutting tools utilizing indexable cutting bits or inserts, such as a spherical or ball nosed end mill having a cylindrical shank and mechanically held, multiple edge cutting inserts on its spherical cutting end with arcuate cutting edges for forming a radiused cut.

Ball nosed end mills are known which can perform omni-directional cutting and contouring of irreguiar shapes on workpieces. One class of such prior art tools features cutting edges integrally formed with the cutter body or cemented carbide cutting elements brazed onto the cutter body. Such designs require considerable manufacturing effort and their cutting edges can be reground or sharpened only with considerable difficulty and with the required use of suitable grinding machines.

After regrinding, a loss of diametrical size occurs, necessitating careful machine adjustments when the reground tool is returned to service. Those tools with integrally formed cutting blades are of a single material and therefore are limited to use on certain narrow categories of workpiece materials. One such known brazed tool is disclosed in US Patent No. 4,132,493.

A second class of known ball nosed mills features indexable cutting elements or inserts.

Such tools are disclosed, for example, in US Patents Nos. 4,175,896 and 4,252,400. US Patent Specification No. 4,175,896 describes an insert with a conical rake or chip engaging surface, whilst US Patent Specification No.

4,252,400 describes an insert mounted in a so-called "stand-up" position with each insert's minor dimension lying behind the cutting edge and receiving the cutting forces generated. This latter specification also shows specifically non-overlapping of the cutting zones of each insert employed in the cut.

Milling cutters with inserts mounted in standup position require more mounting room thereby leading to lowered rigidity of the cutter body. Additionally, inserts in the standup mounting arrangement are subjected to maximum cutting forces through a minimum dimension of the insert body material. Cutters employing inserts with non-planar rake faces produce narrower, stringy chips of non-uniform width which are more prone to chip clogging conditions in the cut.

According to a first aspect of the present invention there is provided a cutting tool comprising: a cylindrical shank terminating in a spherical cutting end, the cutting end including a first and second mounting cavities shaped for receipt, in lay-down fashion, of respective first and second indexable cutting iserts each having an identical arcuate cutting edge along each side of a first major polygonal face thereof, the first cavity oriented such that an active arcuate cutting edge of a first insert placed in lay-down fashion in the first cavity will extend radially outwardly from a corner arc of the insert passing through a longitudinal axis of the shank, and the second cavity positioned on the cutting end with respect to the first cavity such that a second insert placed in lay-down fashion therein presents an arcuate cutting edge whose cutting zone partially overlaps that of the first insert's cutting edge and extends the arcuate cutting action of the first insert substantially in a common plane passing through the shank axis, thereby enabling the cutting tool to generate an arcuate cut of substantially uniform radius.

The invention also extends to an indexable cutting insert comprising: first and second equilateral polygonal major faces, a plurality of identical planar flank surfaces forming the sides of the polygon with two of the flank surfaces interseeting at each corner thereof, the first major face including convex portions intersecting each flank surface in an acute angle to form identical arcuate cutting edges along each side of the polygon, and the second major face comprising a planar surface intersecting each flank surface in an obtuse angle.

In an embodiment, a cutting tool for generating arcuate profiles comprises a ball nosed end mill having a cylindrical shank terminating in a spherically shaped cutting end. The cutting end includes at least two cavities shaped for lay-down mounting receipt of first and second cutting inserts, each insert presenting an arcuate cutting edge with a radius of curvature equal to the radius of the cut to be taken. The arcuate cutting edge of the first insert extends radially outwardly from the longitudinal axis of the shank. The cutting zone of the second insert overlaps that of the first insert. Each insert has first and second major faces in the form of equilateral polygons connected by planar flank surfaces. The first major face includes convex portions intersecting each flank surface in an acute angle to form identical arcuate cutting edges along each side of the polygon.The second major face is planar and intersects each flank surface in an obtuse angle. Mounting the fully indexable inserts in the so-called "lay-down" or "on edge" configuration results in placing the major dimension of the insert body behind the active cutting edge to enable the insert to better withstand the cutting forces imposed on it. Additionally, lay-down insert mounting results in shallower mounting cavities in the tool body, thereby providing added strength and rigitidy thereto.

Embodiments of the present invention will hereinafter be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a ball nosed end mill; Figure 2 is a plan view of the end mill of Fig. 1 taken normal to the longitudinal axis of the tool shank; Figure 3 is a plan view of the end mill of Fig. 1 taken parallel to the longitudinal axis of the tool shank from the cutting end; Figure 4 is a sectional view taken along line 4-4 of Fig. 2; Figure 5 is a top plan view of an indexable cutting insert; Figure 6 is a front view of the insert of Fig.

5; Figure 7 is a side view of the insert of Fig.

5; Figure 7A is a perspective view of the insert of Fig. 5; and Figures 8 and 9 are respectively top and front views of an alternative embodiment of a cutting insert.

With reference to Figs. 1 to 4, a ball nosed end mill 100 has a substantially cylindrical shank 101 terminating in a substantially spherical cutting end 110 and a mounting end (not shown) which can be fashioned in any of a number of conventional shapes for retention by a machine spindle or tool mounting device.

In the spherical cutting end 110 are positioned mounting cavities for idexable cutting inserts mounted in so-called "lay-down" or "on-edge" configuration i.e. with a major dimension of the insert lying rearwardly of the active cutting edge whereby a major face of the insert acts as a clearance face, while the minor flank faces serve as rake or chip engaging surfaces for their respective cutting edges.

In the embodiment illustrated, the ball nosed end mill 100 has three cavities for mating laydown receipt of three inserts 10, 11 and 1 2.

This invention contemplates the use of at least two cutting inserts. The number of inserts depends upon the size of the inserts used along with the desired radius of the arcuate profile to be machined in a workpiece by the tool. Additionally, peripheral inserts (not shown) could be mounted along a portion of the cylindrical shank to extend the depth of cut taken by the disclosed cutting tool.

Chip gullet 102 is provided adjacent arcuate cutting edge 1 2c of insert 12, while chip gullet 103 lies adjacent arcuate cutting edges 10c and 11 c of inserts 10 and 11, respectively. Edges 10c, 11 c and 1 2c ali lie in substantially the same axial plane. The cutting zone of insert 1 2 overlaps both those of insert 10 and insert 11.This is best seen with reference to Fig. 2 where a projection of insert 1 2 is shown in dashed phantom lines and is designated 1 2p. In this manner, inserts 1 0, 11 and 1 2 cut a continuous arcuate profile having a substantially uniform radius--i.e. a substantially hemispherical profile with no gaps due to insert spacing.

The radially innermost corner 1 0a of insert 10 intersects or extends just beyond longitudinal axis 44 of the cutter body shank. This provides strength and shear cutting action at the normally vulnerable centre cutting portion of the tool.

The radially innermost end of cutting edge 1 2c lies, as seen from Fig. 2, at an angle a1 from axis 44, and angle a1 is less than angle a, the angle of the radially outermost end of cutting edge 10c with respect to axis 44.

Hence,the cutting zone of insert 23 overlaps that of insert 10 by the angle b-i.e.

a = a, + b. The purpose of this overlap is twofold. First. it prevents leaving of cusps on the work surface generated and second, tool life is enhanced. In the area of overlap (angle b), inserts 10 and 1 2 are subjected to only one half the feed rate encountered where no overlap occurs (angle a1). With no overlap, i.e.

with angle a, equal to angle a, a minute amount of insert corner wear would cause the cutter body to impact the work surface.

Overlap b is equal to the angle created by lines extending from the centre point 20 of the hemispherical cutting end 110 over the respective corner radii of insert 10 and of insert 1 2 projected 180 out of its mounting position as insert 12p. The magnitude of angle b is dependent upon three factors: insert inscribed circle diameter, insert corner radius, the cutter body diameter.

The side surfaces of inserts 10, 11 and 12, for example surface 1 OR of insert 10 (Fig. 2), are commonly referred to as flanks. However, when the inserts are mounted in lay-down fashion as shown, flank surface 1 OR becomes the chip engaging or rake face. The rake faces of the inserts are flat planar surfaces. In operation, chips are generated having substantially uniform thickness and flow freely across the unrestricted planar rake faces and are explelled via chip gullets 102 and 103.

Arcuate rake faces, on the other hand, tend to result in thinner, stringy chips more prone to clogging.

In Fig. 4 it will be apparent that by positioning insert 10 angularly inwardly towards the tool axis, cutting clearance 43 is obtained.

The insert's supporting cavity surface 41 is at a relatively shallow depth, approximately equaling the insert's thickness, thereby leaving substantial mass in body 110 surrounding the insert mounting cavities. This results in strong insert support and cutter body stiffness. The difference between the cutting radius CR and tool body radius BR equals cutter body clearance 45.

Also, as seen from Fig. 4, each insert has a countersunk cavity for receipt of an appropriately headed mounting screw member 42 which threadingly engages a tapped hole in body portion 110.

A preferred insert is set forth in Figs. 5, 6, 7 and 7A. Insert 5 presents three identical arcuate cutting edges 51 and has the form of an equilateral triangle. However, the insert may alternatively be of any equilateral polygonal shape.

The equilateral triangular shape of insert 5 is formed by three planar flanks 54, a planar bottom mounting surface 55 and a major top face including three arcuate cylindrical surfaces A, B and C. Surfaces A, B and C have common boundaries at lines 50, 56 and 58.

The intersection of surfaces A, B and C with a corresponding adjacent flank surface 54 provides three identical arcuate cutting edges 51 each having cutting radius R (Fig. 7A).

Surfaces A, B and C have respective axes of generation generally transverse to their associated cutting edges and extending at an acute angle 70 to a plane parallel to the plane of surface 55. A preferred range of angle 70 runs from about 5 degrees to about 20 degrees and the value of angle 70 is governed by the cutter body diameter. The larger the cutter body diameter, the smaller is angle 70.

Faces 54 are sloped inwardly from cutting edges 51 to bottom face 55 such that each rake face 54 intersects its associated surface A, B or C in an acute angle and such that each rake face intersects planar surface 55 in an obtuse angle.

A central hole 53 having countersunk portion 52 is provided for receipt of a suitable insert mounting screw.

Planar surfaces 54 function as the rake faces while cylindrical surfaces A, B and C function as clearance or relief surfaces for their respective cutting edges when insert 5 is mounted to body portion 110 of the ball nosed end mill 100 of Figs. 1 to 4.

An alternative insert is shown in Figs. 8 and 9. Insert 8 has an equilateral triangular shape bounded by three planar flank surfaces 86 sloping inwardly from major top surface 80 to bottom planar mounting surface 83. In this alternative embodiment, top surface 80 is spherical and intersects each flank surface 82 to form three identical arcuate cutting edges 81. A central aperture 85 with countersunk portion 84 is provided for receipt of a suitable mounting screw. When used with the cutting tool of Figs. 1 to 4, flat surfaces 86 serve as rake faces for their respective cutting edges, and spherical surface 80 serves as a clearance or relief surface.

Claims (11)

1. A cutting tool comprising: a cylindrical shank terminating in a spherical cutting end, the cutting end including a first and second mounting cavities shaped for receipt, in lay-down fashion, of respective first and second indexable cutting inserts each having an identical arcuate cutting edge along each side of a first major polygonal face thereof, the first cavity oriented such that an active arcuate cutting edge of a first insert placed in lay-down fashion in the first cavity will extend radially outwardly from a corner arc of the insert passing through a longitudinal axis of the shank, and the second cavity positioned on the cutting end with respect to the first cavity such that a second insert placed in lay-down fashion therein presents an arcuate cutting edge whose cutting zone partially overlaps that of the first insert's cutting edge and extends the arcuate cutting action of the first insert substantially in a common plane passing through the shank axis, thereby enabling the cutting tool to generate an arcuate cut of substantially uniform radius.

2. A cutting tool as claimed in Claim 1, wherein each arcuate cutting edges is formed by the intersection of a convex surface portion of each insert's first major face with a planar flank surface.

3. A cutting tool as claimed in Claim 2, further comprising first and second chip gullets respectively intersecting the first and second cavities such that chips formed by the first and second active arcuate cutting edges are directed by the planar flank surfaces into the first and second chip gullets.

4. A cutting tool as claimed in Claim 2 or Claim 3, wherein the convex surface portion is spherical.

5. A cutting tool as claimed in Claim 2 or Claim 3, wherein the convex surface portion is cylindrical.

6. A cutting tool as claimed in any of Claims 2 to 5, wherein a second major face of each insert is planar, intersects each planar flank surface in an obtuse angle, and rests upon a correspondingly planar mounting floor of its respective mounting cavity.

7. A cutting tool as claimed in Claim 6, wherein each planar monting floor is intersected by a threaded cavity in the spherical cutting end for receipt of an insert mounting screw.

8. An indexable cutting insert comprising: first and second equilateral polygonal major faces, a plurality of identical planar flank surfaces forming the sides of the polygon with two of the flank surfaces intersecting at each corner thereof, the first major face including convex portions intersecting each flank surface in an acute angle to form identical arcuate cutting edges along each side of the polygon, and the second major face comprising a planar surface intersecting each flank surface in an obtuse angle.

9. An insert as claimed in Claim 8, wherein each convex portion is spherical.

10. An insert as claimed in Claim 8, wherein each convex portion is cylindrical.

11. An insert as claimed in Claim 10, wherein the axis of each cylindrical portion extends transversally to the cutting edge formed by the intersection of the cylindrical portion with an adjacent flank surface.

1 2. An insert as claimed in Claim 10 or Claim 11, wherein the axis of each cylindrical portion extends at an acute angle to a plane parallel to the plane of the second major face.

1 3. An insert as claimed in Claim 12, wherein the acute angle lies in the range from 5 degrees to 20 degrees.

1 4. A cutting tool substantially as hereinbefore described with reference to the accompanying drawings.

1 5. An indexable cutting insert substantially as hereinbefore described with reference to the accompanying drawings.