Cutting insert and cutter for milling

Abstract: An insert (2) for milling applications in which a radial rake face (38,40) includes a concave conical portion (38,40) defined to achieve constant positive radial rake throughout the length of a radial cutting edge (18,22) at the angle of inclination (110) thereof as the insert is intended to be mounted in a cutter body. The insert includes an axial rake face (72,74) having a sloping portion defined to achieve positive axial rake as the insert is intended to be mounted in a cutter body. The upper surface (6) of the insert includes planar regions (60,66) and bend surfaces (72,74) merging smoothly with the radial rake faces (18,22) and axial rake faces (26,30) to assist in chip expulsion during cutting.

Description: The invention relates to a milling insert with the features cited in the preamble of claim 1. Inserts as such are particularly adapted for being detachably mounted on a tool holder for use in milling operation. Cutting inserts are generally made of cemented metal carbide, such as tungsten carbide, and are formed by pressing techniques after which the pressed articles are sintered. A cutting insert of the above-mentioned type disclosed in WO 94/16847 comprises a chip surface extending along a cutting edge, said chip surfaces being helically twisted in a counter-clockwise direction, starting off from an operative cutting insert corner. The rake angle of the chip surface increases with increasing distance from the insert corner. DE-A-2626335 discloses a cutting insert for a facing head of a milling machine. The cutting insert has a cutting groove extending from a corner of the insert and along a cutting edge. The cutting groove becomes wider in direction associated with increasing depth of cut along the cutting edge. The rake angle varies along the cutting edge so that the groove is adapted to the respective cutting depth and the thickness of a chip. Figs. 7 and 8 illustrate a milling tool of another known type with an insert 104 mounted therein. While only a single insert is illustrated in Figs. 7 and 8, tools of this type may include plural flutes, such as flute 120, each providing a recess for mounting of an insert. In use, tool body 100 is rotated about axis 102, and surfaces of revolution generated by the cutting edge(s) of the insert(s) are cylindrical. Fig. 9 illustrates the circle circumscribed by a point on a radial cutting edge 108 of insert 104 by rotation of tool body 100. An axial cutting edge 106 cuts a face at the base of the cylinder cut by radial cutting edge 108. So called positive inserts are intended for applications in which a cutting edge leads the insert rake face during chip formation. Such applications rely on relatively greater shear in formation of chips than applications in which the cutting edge lags the insert rake face in chip formation. To provide for cutting edge lead and allow clearance between the insert flank and the work during chip formation, an included angle between the insert rake face and insert flank surface is made acute. However, cutting edge strength tends to be directly proportional to the magnitude of the included angle, i.e., the greater the included angle, the greater the cutting edge strength. Insert design strikes a balance between these competing demands according to the intended application, tradeoffs being dictated by the nature of the material to be cut and the type of cutting. For example, for cutting materials such as aluminum which are relatively highly machinable, relatively narrow included angles can be used. Conversely, for materials such as steel which are relatively less machinable, edge strength must be greater and hence relatively wide included angles are preferred. Further, applications involving rough cutting involving relatively thicker chip formation, require relatively greater edge strength than do finishing operations involving relatively thinner chip formation. In the tool of Figs. 7 and 8 a so called positive insert is an insert providing both positive radial rake and positive axial rake, i.e. the cutting edges 108 and 106 are intended to lead the respective radial and axial rake faces of the insert during cutting. It is known to provide inserts for such applications in which features in the surface containing the rake faces are intended to increase radial and axial rake in accordance with intended orientation of the insert within a cutter body. However, with such known inserts, at increasing depths of cut design tradeoffs result in weakness of the cutting edge which are manifest in edge breakdown away from the lead corner. In addition, such known inserts are provided with increased material around the periphery of the mounting aperture which can result in jamming of chips, inhibiting their expulsion during cutting. A further drawback of known inserts arises from orientation of the insert within the recess of the tool body. In Fig. 8 an angle 112 (shown greatly exaggerated) is measured between a longitudinal centerline of the insert and a line parallel to the axis of rotation of the tool. From Fig. 8 it will be understood that an insert having a straight cutting edge 106 will make point contact with the work when the angle 112 is other than zero. The point contact will result in grooving of the work as the tool is translated relative thereto. Known inserts include crowned axial cutting edges to effectively overcome such point contact. It is the object of the present invention to provide a milling insert of the above-mentioned type which has an improved cutting action and an enhanced chip expulsion. This object is achieved with a milling insert having the features of claim 1. Advantageous features and embodiments are cited in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS This invention may be clearly understood by reference to the attached drawings in which like parts are designated by like reference numerals and in which: Fig. 1 is a perspective view of a milling insert of the present invention; Fig. 2 is a plan view of the milling insert of Figure 1; Fig. 3 is an elevational view of Figure 2; Fig. 4 is a cross-sectional view of Figure 1 taken along lines 4-4; Fig. 5 is a cross-sectional view similar to Figure 4 but of an alternative embodiment of the milling insert of the present invention; and Fig. 6 is a cross-sectional view similar to Figure 4 but of another alternative embodiment of the milling insert of the present invention. Fig. 7 is a side view of a milling tool of known type. Fig. 8 is a second side view of the milling tool of Fig. 7. Fig. 9 illustrates the circle circumscribed by rotation of the tool of Figs. 7 and 8. DESCRIPTION OF THE PREFERRED EMBODIMENT The embodiment which is depicted in the drawings is one which is particularly suited for achieving the objects of the invention. Figures 1 and 2 depict a milling insert 2 which comprises a lower surface 4 and an upper surface 6. In the embodiment of Figure 1, lower surface 4 is planar. A plurality of side surfaces are provided which extend from lower surface 4 to upper surface 6, the intersection of each side surface with upper surface 6 forming a respective cutting edge. For example, in the preferred embodiment depicted in the drawings, a plurality of peripheral side surfaces 8, 10, 12, 14 are provided which extend upwardly and outwardly from lower surface 4 to upper surface 6. Side surface 8 extends upwardly and outwardly to a radial cutting edge 18 and side surface 10 extends upwardly and outwardly to radial cutting edge 22. Similarly, side surface 12 extends upwardly and outwardly to an axial cutting edge 26, and side surface 14 extends upwardly and outwardly to axial cutting edge 30. In the preferred embodiment of Figure 1, the side surfaces 8 and 10 are each planar to provide planar flank areas which extend upwardly and outwardly from lower surface 4 to upper surface 6. In one alternative embodiment side surfaces 8 and 10 may each comprise more than one portion, including planar and curved portions to provide a compound flank surface at each such side surface. A mounting aperture 32 extends through the milling insert 2 from lower surface 4 to upper surface 6. A mounting screw 122 such as shown in Fig. 8 typically extends through aperture 32 to mount the milling insert 2 in the usual manner. As intended to be mounted in a milling cutter, cutting edges 18, 22, 26, and 30 of insert 2 are active, in, at most, pairs, i.e., edges 18 and 26 will be engaged in cutting while cutting edges 22 and 30 are located interiorly of the cutter body periphery. Upon indexing of the insert, cutting edges 22 and 30 are made the active cutting edges. In the illustration of Fig. 9 the angle 114 between a cutter body radius 116 passing through a point on the radial cutting edge 108 and a tangent 118 to the insert rake surface passing through the same point on cutting edge 108 is known as the radial rake angle, also referred to as radial rake. Positive radial rake is achieved when the radius 116 leads the tangent 118 in the direction of advance of cutting edge 108. As intended to be mounted in cutter body 100, radial cutting edge 108 is inclined at an angle 110 relative to a line parallel to tool axis 102. As a result of angle 110 of Fig. 7, the orientation of the insert in cutter body 100 changes the relationship of tool body radii, such as radius 116 of Fig. 9, to the rake face along the length of radial cutting edge 108. Applicants have determined that to produce constant radial rake throughout the length of cutting edge 108 it is advantageous to vary the direction of the tangent to the rake surface, such as tangent 118 of Fig. 9. Referring to Figs. 1 and 2, upper surface 6 of milling insert 2 includes a first conical portion 38 and a second conical portion 40 each of which is concave relative to the upper surface. Conical portion 38 has an associated cone axis 58 and narrows in the direction of arrow 42. Conical portion 40 has an associated cone axis 56 and narrows in the opposite direction as designated by arrow 48. In the embodiment of Figure 1, conical portions 38 and 40 are each defined by cones having circular cross-section relative to respective cone axes 56 and 58. In one alternative embodiment, conical portions 38 and 40 may be defined by cones have an elliptical cross-section relative to respective cone axes 56 and 58. In accordance with the definition of radial rake, and considering the effects on radial rake of angle 110 of Fig. 7, conical portions 38 and 40 are designed to produce substantially constant radial rake along the co-extensive length of the active radial cutting edge at a particular value of angle of inclination 110 of Fig. 7. By virtue of the concave conical surface, the effective volume of material of the insert body directly behind the radial cutting edge increases in the direction of increasing depth of cut of the milling cutter. Hence, increasing edge strength is realized at locations along the radial cutting edge associated with increasing depth of cut. Further, constant radial rake along the length of the radial cutting edge results in uniformity of cutting action, i.e., the shear angle along the length is constant and hence shear is constant along the cutting edge. In Fig. 7, the angle 113 between a vertical 109 passing through a point on axial cutting edge 106 and a tangent 111 to the rake surface at the same point on the cutting edge 106 is the axial rake angle, also known as axial rake. Positive axial rake is achieved when the vertical 109 leads the tangent 111 in the direction of advance of cutting edge 106. Upper surface 6 includes sloping portions 72 and 74 each of which extend or slope downwardly towards lower surface 4 as depicted in Figure 1. Sloping portion 72 extends from second or axial cutting edge 26 towards first common corner 44, narrowing in the direction away from common corner 44. An opposite sloping portion 74 extends from fourth or axial cutting edge 30 towards second common corner 50, narrowing away from common corner 50. Surfaces 72 and 74 serve to reduce axial cutting forces by increasing the effective axial rake at the active one of cutting edges 26 and 30 as the insert is mounted in a milling cutter such as that shown in Figs. 7 and 8. Narrowing of sloping portions 72 and 74 in the direction away from the respective common corners 44 and 50 increases the volume of the insert body adjacent the narrow ends of conical portions 38 and 40 serving to provide additional support to strengthen cutting edges 18 and 22 at the ends associated with greatest depth of cut. Upper surface 6 of milling insert 2 also includes a first planar region 60 which extends between and merges with first conical portion 38, second concave conical portion 40 and sloping portion 72. In a like manner, a second planar region 66 at the opposite end of the milling insert 2 extends between and merges with second concave conical portion 40, first concave conical portion 38 and sloping portion 74. By allowing segments 60 and 66 to merge or come together with adjacent concave conical portions, without any abrupt change in surface contour, segments 60 and 66 serve to enhance chip expulsion during the milling operation. Upper surface 6 includes a first blend surface 76 which merges the sloping portion 72 with concave conical portion 38 at common corner 44 and also merges concave conical portion 40 with corner 78 of upper surface 6 as depicted in Figures 1 and 2. A second blend surface 80 is provided which merges sloping portion 74 with concave conical portion 40 at common corner 50 and also merges the concave conical portion 38 with corner 82 of upper surface 6. Blend surfaces 76 and 80 smoothly and gradually merge all surfaces adjacent to such blend surfaces to facilitate chip expulsion. In one embodiment, relatively narrow land areas are provided adjacent to at least two cutting edges. For example, in the embodiment depicted in Figures 1 to 4 all of the upper cutting edges 18, 22, 26, 30 comprise a land area 84. Although not necessary, such land area may be parallel to lower surface 4 as depicted in Figure 4. In an alternative embodiment, as depicted in Figure 5 a land area 86 may be provided which slopes downwardly relative to lower surface 4 and in this configuration is referred to as a negative-T land area. An angle of inclination 88 of negative-T land area 86 is conveniently defined relative to a plane transverse to the axis of mounting aperture 32 and has a value in the range of 5° to 20°. By providing a land area around the periphery of the milling insert, the upper cutting edge will be strengthened. In another embodiment, the narrow land area may be eliminated and the cutting edge may be in the configuration of a sharp edge 89 as depicted in Figure 6. In order to provide a smooth finish upon the workpiece, axial cutting edges 26, 30 are advantageously slightly curved outwardly relative to the center of upper surface 6 in the direction of respective arrows 90, 92. The curve of axial cutting edges 26, 30 is effective, within a range of values of angle 112 of Fig. 8, to wipe or sweep an area of workpiece material during cutting to eliminate point contact grooving and hence enhance the finish of the workpiece. The embodiments which have been described herein are but some of several which utilize this invention and are set forth here by way of illustration but not of limitation. It is apparent that many other embodiments which will be readily apparent to those skilled in the art may be made without departing from the of the claims.

Claims: A milling insert comprising: a polygonal body having an upper surface (6), a lower surface (4) and a plurality of side surfaces (8, 10, 12, 14) extending between said upper surface and said lower surface, the side surfaces sloping outwardly from the lower surface (4) to intersection with the upper surface (6); a first radial cutting edge (18) formed at the intersection of a side surface (8) with the upper surface (6), the first radial cutting edge (18) extending between corners (44, 58) of said upper surface (6), characterised in that the upper surface (6) comprises a first concave conical portion (38) adjacent said first cutting edge (18), the first concave conical portion (38) extending between corner portions (44, 58) of the insert and narrowing in a direction (42) associated with increasing depth of cut along the first radial cutting edge (18), and being defined to achieve constant positive radial rake throughout the length of the first radial cutting edge (18) at the angle of inclination thereof as the insert is to be mounted in a cutter body with the first radial cutting edge (18) positioned as an active radial cutting edge. The milling insert of claim 1 characterised in that a narrow land (84) is interposed between the radial cutting edge (18) and the concave conical portion (38). The milling insert of claim 1 or 2 characterised in that the upper and lower surfaces (4, 6) are quadrilateral. The milling insert of one of the claims 1 to 3 characterised in that it comprises a first axial cutting edge (26) extending substantially perpendicularly from a corner (44) common with the first radial cutting edge (18). The milling insert of claim 4 characterised in that the upper surface (6) further comprises a first sloping portion (72) adjacent said first axial cutting edge (26), said first sloping portion (72) sloping downwardly from said first axial cutting edge (26), extending between corner portions (44, 78) of the insert, and narrowing in the direction away from the first common corner (44), the sloping portion (72) defining a positive axial rake as the insert is intended to be mounted in a cutter body with the first axial cutting edge (26) positioned as an active axial cutting edge. The milling insert of one of the claims 1 to 5, characterised in that it further comprises a mounting aperture (32) extending through said milling insert from said lower surface (4) to said upper surface (6). The milling insert of claim 6 characterised in that it further comprises a first planar region (60) adjacent the first concave conical surface (38) and interiorly of the first sloping portion (72) and extending to the mounting aperture (32), the first planar region (60) sloping downwardly from the narrowest end of the concave conical portion (38) in the direction transverse thereto and sloping downwardly along the concave conical portion (38) in the opposite direction of its narrowing. The milling insert of claim 5, 6 or 7 characterised in that it further comprises a first blend surface (76) merging smoothly with the planar region (60), the first concave conical portion (38), the first sloping portion (72) and corners of the upper surface (6) adjacent the first sloping portion (72). The milling insert of one of the claims 5 to 8 characterised in that it comprises a second radial cutting edge (22) and a second axial cutting edge (30), the second radial cutting edge (22) and the second axial cutting edge (30) extending from a second common corner (50) diagonally opposite the first common corner (44) and being substantially perpendicular to each other, wherein the upper surface (6) further comprises: a second concave conical portion (40) adjacent the second radial cutting edge (22), the second concave conical portion (40) extending between corner portions (50, 78) of the insert and narrowing in the direction (48) away from the second common corner (50), and being defined to achieve constant positive radial rake throughout the length of the second radial cutting edge (22) at the angle of inclination thereof as the insert is intended to be mounted in a cutter body with the second radial cutting edge (22) as an active radial cutting edge; a second sloping portion (74) adjacent the second axial cutting edge (30), the second sloping portion (74) sloping downwardly from the second axial cutting edge (30) and narrowing in the direction away from the second common corner (50) and defined to achieve positive axial rake as the insert is mounted in a cutter body with the second axial cutting edge (30) positioned as an active axial cutting edge; and a second planar region (66) adjacent the second concave conical portion (40) and interiorly of the second sloping portion (74) and extending to the mounting aperture (32), the second planar region (66) sloping downwardly from the narrowest end of the second sloping portion (74) toward the second common corner (50) and upwardly from the second common corner (50) in the direction of narrowing of the second concave conical portion (40). The milling milling insert of claim 9 characterised in that a second blend surface (80) merges smoothly with the second concave conical portion (40), the second sloping portion (74), the second planar region (66) and corners of the insert adjacent the second sloping portion (74). The milling insert of claim 9 or 10, characterised in that the first and second cutting edges (18, 20) curve outwardly relative to the upper surface (6). The milling insert of claim 9, 10 or 11 characterised in that the cones defining the first and second concave conical portions (38, 40) are circular. The milling insert of claim 9, 10 or 11 characterised in that the cones defining the first and second concave conical portions (38, 40) are elliptical. A milling cutter comprising a cutter body and a cutting insert according to one of the preceding claims.

Inventors: DOXEN PAUL R, LARK II JAMES D, PLUTSCHUCK LAWRENCE, REITERMAN LEE, DOXEN, PAUL R., LARK II, JAMES D., PLUTSCHUCK, LAWRENCE, REITERMAN, LEE
Applicants: VALENITE INC, VALENITE INC.
Application year: 1996
Main classification: B23C5
IPC codes: B23C5