Cutting insert and indexable face mill

A pair of sub-flank surfaces is formed at a vertex angle portion of an insert body having a substantially polygonal, plate-like shape, and between the sub flank surfaces, a vertex flank surface at a vertical angle portion is formed substantially in a protrusive, curved shape, and is smoothly connected to the sub-flank surfaces in a tangential direction. The vertex flank surface at the vertex angle portion is extended across the entire thickness of the insert body, and boundary lines, between the vertex flank surface at the vertex angle portion and a pair of minor cutting edges, are parallel to each other. A second curved ridge line, which is formed between (i) the flank surface and (ii) the rake surface, and a first curved ridge line, which is formed between (i) the flank surface and (ii) the seat surface, have approximately same shape.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a cutting insert and an indexable face mill on which such cutting inserts are mounted.

2. Description of the Related Art

Cutting inserts have been proposed that are available both for a case wherein, when the cutting inserts are mounted on a face mill, they are to be fed to the right of the inserts, as viewed from the front in the rotational direction of the tool (hereinafter, this is referred to as a right hand cut), and for a case wherein they are to be fed to the left of the inserts, as viewed from the front in the rotational direction of the tool (hereinafter, this is referred to as a left hand cut).

A cutting insert disclosed in Japanese Patent Laid-Open No. 2006-224278 has a nearly polygonal shape, whereon paired minor cutting edges are provided for the individual corners, and are extended along the side ridges, while being inclined inwardly from the side ridges, and whereon clearance parts are formed between the individual minor cutting edge pairs, at a location inward from the insert relative to intersecting points of lines extended from the paired minor cutting edges. The clearance parts each include a flat portion and a V-groove or a round groove, formed between the minor cutting edge pair.

According to the Japanese Patent Laid-Open No. 2006-224278, the clearance parts are positioned between the minor cutting edge pairs, at the interior of the cutting insert, relative to the intersecting points of lines extended from the paired minor cutting edges. Therefore, in a case wherein this insert is employed for both a right hand cut and a left hand cut, when one of the paired minor cutting edges has been worn out by cutting, an increase in the wear can be prevented by the presence of the clearance part, and wear of the other minor cutting edge can be avoided.

However, since the cutting insert described in Japanese Patent Laid-Open No. 2006-224278 includes the clearance parts positioned between the minor cutting edge pairs, that are formed at the interior of the insert, relative to the intersecting points of the lines extended from the paired minor cutting edges, the strength of the portions of the individual minor cutting edges adjacent to the clearance parts is reduced. Therefore, chipping or fracturing of the minor cutting edges may occur, and the finished surface roughness may be lowered. Furthermore, during the face milling process, when the cutting insert is positioned to the front of the center of the face mill in the feeding direction F, the cutting of a workpiece is to be performed, while when the cutting insert is positioned to the rear of the center in the feeding direction F, the cutting insert does not have an apparent contact with the workpiece. However, in actuality, since the inclination of the face mill body itself caused by cutting resistance or the main spindle of a machining tool, etc., are present a phenomenon has occurred that cutting inserts101, which are located to the rear of the center of the face mill100in the feeding direction F, contact the work surface of the workpiece and perform micro-cutting on the work surface (seeFIG. 8). During this micro-cutting, the portions where the minor cutting edges and the clearance parts are adjacent to each other contact the workpiece first, and cut the work surface. Since as described above the strength of the adjacent portions is lowered, the load due to micro-cutting is easily imposed on these portions, and chipping tends to occur. Therefore, there is a probability that the chipped minor cutting edges and the clearance parts will contact the work surface and degrade the surface roughness of the work surface.

SUMMARY OF INVENTION

To resolve this problem, one objective of the present invention is to provide a cutting insert that can be employed both for a left hand cut and a right hand cut, and for which the chipping resistances of minor cutting edges and adjacent portions are improved, in order to obtain a better surface roughness for a work surface, and an indexable face mill.

In order to achieve the above objective, the present invention provides a cutting insert of an almost polygonal shaped plate that is to be mounted detachably on a tool body10of an indexable face mill, comprising:a rake surface2that is provided for the top face of the polygonal shape;a seat surface3that is provided for the bottom face, opposite the top face;main flank surfaces4athat are substantially flat surfaces having positive relief angles, provided on side faces extended from side ridges of the top face;pairs of sub-flank surfaces4bthat are provided on side faces extended from vertex angle portions of the polygonal shaped faces; andvertex flank surfaces4cthat are provided at the vertex angle portions between the pairs of sub-flank surfaces4b, and are formed in a substantially projected outward curved shape,wherein the vertex flank surfaces4care smoothly connected with the sub-flank surfaces4bin a tangential direction, and are extended along a thickness of the cutting insert in a direction along a vertex angle bisector line B;wherein boundary lines6a, formed by the vertex flank surfaces4cand the pairs of sub-flank surfaces4b, are parallel to each other; and

wherein as viewed from a direction opposite the rake surface2, almost the same shape is provided for upper, second curved ridges5cformed along an intersecting ridge line portion of the vertex flank surfaces4cand the rake surface2and for lower, first curved ridges that are formed along an intersecting ridge line portion of the vertex flank surfaces4cat the vertex angle portion and the seat surface3.

According to the present invention, the vertex flank surface4cat the second vertex angle portion, provided between the pair of sub-flank surfaces4b, separates the sub-flank surfaces4b. Further, the second curved ridge5c, formed between the pair of minor cutting edges5b, separates these minor cutting edges5b. As a result, when one of the pair of minor cutting edges5bis damaged, the affect of such damage on the other minor cutting edge5bcan be prevented.

The boundary portion where the vertex flank surface4cat the second vertex angle portion intersects the pair of sub-flank surfaces4b, and the boundary portion where the second curved ridge5cintersects the pairs of the minor cutting edges5bare smoothly connected, in a tangential direction, to the pair of sub-flank surfaces4band the pair of minor cutting edges5b, respectively, and the reduction in the strength in the vicinity of the boundary portion can be prevented. Furthermore, the vertex flank surface4cat the second vertex angle portion is extended across the thickness of the insert body1, and the boundary lines between the vertex flank surface4cat the second vertex angle portion and the pair of sub-flank surfaces4bare extended, parallel to each other, and with almost the same width, in a direction perpendicular to the direction of the thickness of the insert body1. In addition, the intersecting ridge line portion of the vertex flank surface4cat the second vertex angle portion and the seat surface3is substantially curved in consonance with the shape of the vertex flank surface4cat the second vertex angle portion. As a result, an appropriate strength can be obtained for the entire vertex flank surface4cat the second vertex angle portion to cope with a force (a cutting resistance) that is exerted on the second curved ridge5c. With this arrangement, since the chipping resistance is increased for the vertex flank surfaces4cat the second vertex portion and for the boundary portion between these surfaces and the pair of sub-flank surfaces4b, the service life of the cutting insert can be extended, and degrading of the surface roughness of the work surface can be prevented. Especially, when micro-cutting for the work surface of a workpiece is performed by the cutting insert that is positioned to the rear of the center of the face mill in the feeding direction, the occurrence of chipping can be avoided for the portion where the minor cutting edge5b, which contacts the workpiece first, and the second curved ridge5care adjacent to each other, and degrading of the surface roughness on the work surface can be prevented.

Further, the present invention provides the cutting insert, wherein an angle E, formed by the pair of minor cutting edges5b, is in a range of 100° to 178°. Thus, since the angle formed by the pair of minor cutting edges5bis set to 178° or smaller, one of the paired minor cutting edges5bcan be employed for cutting, while the other minor cutting edge5bcan be maintained not in use, without being in contact with the workpiece. However, when the angle formed by the minor cutting edges5bis set smaller than 100°, the intersecting portion of the minor cutting edges5bbecomes sharp, and the strength might be reduced.

Furthermore, the present invention provides the cutting insert, wherein a radius of curvature Rc of the second curved ridge5cis in a range of 0.4 mm to 2.5 mm. When a radius of curvature Rc for the second curved ridge5cis set to 0.4 mm or greater, the strength for the second curved ridge5ccan be obtained. However, when the radius of curvature Rc is too great, the range of damage, caused when the second curved ridge5ccontacts the workpiece, will be increased, and accordingly, the cutting resistance is increased. Therefore, it is preferable that the radius of curvature Rc be 2.5 mm or smaller.

Moreover, the present invention provides the cutting insert, wherein a length Lb for the pair of minor cutting edges5bis in a range of 0.5 mm to 3 mm. Since the length Lb for the minor cutting edges5bis 0.5 mm or greater, efficient face milling processing can be performed at a higher feed rate, without the surface roughness on a working surface being degraded. However, when the length Lb for the minor cutting edges5bis too great, the range where the minor cutting edges5bcontact the work piece is increased, and the cutting resistance is increased to cause chattering marks, or to degrade the surface roughness on the work surface. Therefore, it is preferable that the length Lb for the pair of minor cutting edges5bbe 3 mm or smaller.

Further, the present invention provides an indexable face mill comprising:a tool body10, which is to be rotated at a center line O, and on which one of the above described cutting inserts is to be mounted detachably,wherein, when the cutting insert is mounted on the tool body10, an axial rake angle AR is positive and a radial rake angle RR is negative, and of the pairs of the minor cutting edges5b, the minor cutting edges5blocated at an outer circumference are positioned substantially at a right angle relative to the center line O.

According to the present invention, when one of the above described cutting inserts is mounted, the chipping resistance is increased at the portion where the minor cutting edges5band the second curved ridge5care adjacent to each other, and the surface roughness of the work surface can be improved.

The cutting insert is mounted on the tool body10to provide a so-called negative-positive edge type, for which the axial rake angle AR is positive and the radial rake angle RR is negative. With this arrangement, the cutting resistance of the face mill can be reduced. Especially, a thrust force exerted in a direction counter to a direction in which the workpiece is pressed can be reduced. This is effective for improving the chipping resistance of the portion where the second curved ridge5cand the minor cutting edges5bare adjacent to each other.

According to the cutting insert and the face mill of the present invention, since the second curved ridge5cis formed to separate one minor cutting edge5bfrom the other, when one of the minor cutting edges5bin a pair is damaged, the other minor cutting edge5bcan be protected from being affected by such damage.

The vertex flank surface4cat the second vertex angle portion and the second curved ridge5cintersect, respectively, the pair of sub-flank surfaces4band the minor cutting edges5bsmoothly in a tangential direction, so that the reduction in the strength in the vicinity of the intersecting portion can be prevented. As a result, an appropriate strength can be obtained for the entire vertex flank surface4cat the second vertex angle portion to cope with a force (a cutting resistance) that is exerted on the second curved ridge5c. Furthermore, the vertex flank surface4cat the second vertex angle portion is extended across the thickness of the insert body1, and the boundary lines between the vertex flank surface4cat the second vertex angle portion and the pair of sub-flank surfaces4bare extended, parallel to each other, and with almost the same width, in a direction perpendicular to the direction of the thickness of the insert body1. In addition, the intersecting ridge line portion of the vertex flank surface4cat the second vertex angle portion and the seat surface3is substantially curved in consonance with the shape of the vertex flank surface4cat the second vertex angle portion. As a result, an appropriate strength can be obtained for the entire vertex flank surface4cat the second vertex angle portion to cope with a force (a cutting resistance) that is exerted on the second curved ridge5c. With this arrangement, since the chipping resistance is increased for the portion where the pair of sub-flank surfaces4bare adjacent to the vertex flank surfaces4cat the second vertex portion, and for the portion where the minor cutting edges5bare adjacent to the second curved ridge5c, and degrading of the surface roughness of the work surface can be prevented.

The cutting insert is mounted on the tool body10to provide a so-called negative-positive edge type, for which the axial rake angle AR is positive and the radial rake angle RR is negative. With this arrangement, the cutting resistance of the face mill can be reduced. Especially, a thrust force exerted in a direction counter to a direction in which the workpiece is pressed can be reduced. This is effective for improving the chipping resistance of the portion where the second curved ridge5cand the minor cutting edges5bare adjacent to each other.

DESCRIPTION OF THE EMBODIMENTS

A cutting insert according to one embodiment of the present invention is an exemplary cutting insert to be mounted on a face mill having an approach angle of 48°. As shown inFIGS. 1 to 3, an insert body1is a plate having an almost square shape. The almost square upper face of the insert body1serves as a rake surface2, the lower face opposite the upper face serves as a substantially flat seat surface3, and the side faces extended between the upper and lower faces serve as flank surfaces. The inscribed circle diameter D of the almost square upper face is 12.7 mm. The intersecting ridge portions of the rake surface2and the flank surfaces serve as cutting edges. The side surfaces employed as the flank surfaces are gradually inclined to the interior of the insert body1in a direction from the intersecting portion of the upper face to the intersecting portion of the lower face, and a positive flank angle is formed. The rake surface2of the insert body1is provided as a flat upper face; however, a chipbreaker, such as a concave groove, may be formed adjacent to part of, or all of the cutting edges. The cutting edges are made of at least a hard material, such as cemented carbide, coated cemented carbide, cermets, ceramics, polycrystalline diamond or cubic boron nitride. In addition to an almost square shape, an almost polygonal shape, such as a rectangular, a parallelogram, a rhombic, a triangular, a pentagonal or a hexagonal shape, can be employed for the upper face of the insert body1. The size for the polygonal shape can also be appropriately changed.

The cutting insert1includes at least main flank surfaces4a, sub-flank surfaces4band vertex flank surfaces4cat second vertex angle portions. As is apparent fromFIGS. 3 to 5, the main flank surfaces4aare formed on four almost flat side faces that are extended from the side ridges of the upper face in a square shape. The sub-flank surfaces4bare formed on a pair of almost flat side faces near the corner C of the insert body1. The pair of sub-flank surfaces4bare bent toward the interior of the insert body1, relative to the adjacent main flank surface4a, i.e., are inclined toward the adjacent main flank surface4aat an angle smaller than 180°. The vertex flank surface4cat the second vertex angle portion is formed on the substantially curved side face between the pair of sub-flank surfaces4b.

As is apparent, especially fromFIGS. 2 to 4, the pair of sub-flank surfaces4band the vertex flank surface4cat the second vertex angle portion are almost symmetric along a bisector line B for the vertex angle. Further, an angle E formed by the pair of sub-flank surfaces4branges from 100° to 178°, and is 174° in this embodiment. A rounded flank surface4d, which is a curved surface, is formed between the sub-flank surface4band the adjacent main flank surface4a, and smoothly connects these surfaces4band4a. The rounded flank surface4dmay be changed to an almost flat chamfered flank surface that obliquely intersects the sub flank surface4band the main flank surface4a. The rounded flank surface4dmay not be formed, and the sub-flank surfaces4bmay be directly connected to the main flank surface4a. Further, the paired sub-flank surfaces4bmay be curved surfaces that are projected outward of the insert body1. In this case, the angle E formed by the pair of sub-flank surfaces4bis regarded as an angle formed by virtual flat planes that pass both ends of the protruded curved face. In this case, the angle E also ranges from 100° to 178° C. The four side faces that serve as the main flank surfaces4aare also employed as constrained surfaces that selectively contact a tool body10of a face mill when the cutting insert is positioned in the tool body10.

The following relief angles are employed for the individual flank surfaces. The relief angle of the main flank surface4ais set for a range of 7° to 25°, the relief angle of the sub-flank surface4bis set for a range of 10° to 35°, and the relief angle of the vertex flank surface4cat the second vertex angle portion in the direction of a vertex angle bisector line B, i.e., in the direction of the maximum inclination, is set for a range of 10° to 35°. Further, the relief angle of the sub-flank surface4bis set equal to or greater than the relief angle of the main flank surface4a. In this embodiment, the relief angle for the main flank surface4ais set as 20°, and the relief angle for the sub flank surface4bis set as 30°.

The cutting insert1includes at least a major cutting edge5a, the pair of minor cutting edges5band the second curved ridge5c. The major cutting edge5ais linearly formed along the intersecting ridge portion of the rake surface2and the main flank surface4a. The minor cutting edges5bare formed almost linearly at the intersecting ridge portions of the rake surface2and the sub-flank surfaces4b, and are provided as a pair at the corner C of the insert body1. The second curved ridge5cis formed almost curvilinearly at the intersecting ridge portion of the rake surface2and the vertex flank surface4cat the second vertex angle portion. The pair of minor cutting edges5bare substantially symmetric along the angle bisector line B of the corner C. The second curved ridge5cis formed between the minor cutting edges5b, and is smoothly connected to the individual minor cutting edges5bin a tangential direction. A rounded corner5dis formed in a curved shape between the minor cutting edges5band the adjacent major cutting edges5a, and smoothly connects the minor cutting edges5bto the major cutting edges5a. The rounded corner5dmay be changed to a chamfered corner having a linear shape that obliquely intersects the minor cutting edges5band the major cutting edge5a. Instead of forming the rounded corner5dor a chamfered corner, the minor cutting edges5band the major cutting edge5amay be directly connected.

A length Lb for the individual minor cutting edges5bis set for a range of about 0.5 mm to 3 mm, and is set to 1.6 mm in this embodiment. When the pair of sub-flank surfaces4bare substantially curved surfaces, the minor cutting edges5bare also curved, in consonance with the shape of the sub-flank surfaces4b. In this case, it is preferable that the radius of curvature for the minor cutting edges5bbe set for a range of 50 mm to 600 mm, while taking the finished surface roughness into account.

The angle E formed by the pair of the minor cutting edges5bis set within a range of 100° to 178°, viewed in a direction opposite the rake surface2of the insert body1, and is set to 174° in this embodiment. Furthermore, a radius of curvature Rc for the second curved ridge5cis set for a range of 0.4 mm to 2.5 mm, and is set to 1 mm for this embodiment. A radius of curvature Rd for the rounded corner5dis set for a range of 0.4 mm to 2.5 mm, and is set to 1.3 mm in this embodiment. When the corner is formed almost straight, the length of the cutting edge is set for a range of 0.4 mm to 2.5 mm.

As shown inFIG. 5, when the cutting insert is viewed along the vertex angle bisector line B, two boundary lines6aare present at borders between the pair of sub-flank surfaces4band the vertex flank surface4cat the second vertex angle portion. The two boundary lines6aare extended in a direction parallel to the perpendicular insert axis (not shown) passing through the rake surface2(or the seat surface3), from the intersecting point for the rake surface2to the intersecting point for the seat surface3, while being at a distance from each other in a direction perpendicular to the direction of thickness of the insert body1. That is, the vertex flank surface4cis extended from the rake surface2to the seat surface3, with the constant radius of curvature being maintained. Further, the radius of curvature for the vertex flank surface4cis set to an arbitrary value at which abnormal damage, such as chipping, will not occur. It should be noted that although the boundary lines6ado not actually appear, these lines are indicated by solid lines inFIGS. 3 to 5in order to make this embodiment easy to understand. Further, for the same reason, the boundary line between the rounded flank surface4dand the main flank surface4aand the boundary line between the rounded flank surface4dand the sub-flank surface4care also indicated by solid lines inFIGS. 3 to 5.

The relief angles are not constant in the individual cross sections taken along normal lines of the second curved ridge5c. The relief angle in the cross section along the bisector line B is a maximum value. The relief angles in the cross sections taken along the other normal lines are gradually reduced in a direction leading from the intersecting point of the second curved ridge5cand the bisector line B to either of the two ends of the second curved ridge5c.

The above described cutting insert is employed for a face mill100shown inFIGS. 6 and 7. This face mill includes the tool body10, which is to be rotated at the center line O, and for which the cutting inserts are to be detachably mounted on the peripheral face of the front end, at almost the same intervals in the circumferential direction.

A boss, formed at the rear end of the tool body10, has an end face that abuts upon the end face of the main spindle of a cutting machine, or the end face of a milling head arbor. Further, the tool body10also has a through hole extended along the center line O. At the front face of the tool body10, a plurality (twelve in this embodiment) of chip pockets11are recessed in the peripheral wall in the circumferential direction and at the same intervals. Insert mounting grooves12are formed at the rear, adjacent to the chip pockets11in the tool rotational direction R. Wedge insertion grooves13are formed at the front, adjacent to the insert mounting grooves12in the tool rotational direction R.

Holding members (locators), each of which includes an insert mounting seat20ain which a cutting insert is to be seated, are securely fixed, by seat screws, to the insert mounting grooves12. Wedge members21are inserted into the wedge insertion grooves13and are to be driven forward or backward along the depth of the wedge insertion grooves13.

When the cutting insert is seated, the seat surface3contacts the bottom face of the insert mounting seat20aof the holding member20; and of the side faces that serve as the main flank surfaces4a, a pair of side faces directed toward the rear end and not concerned with cutting serve as constrained faces, and abut on a pair of walls rising from the bottom face. The wedge21inserted in the insertion groove13in the direction of depth presses the top face of the cutting insert, i.e., the rake surface2, toward the seat surface3to securely hold the cutting insert.

Since the relief angle of the sub-flank surface4bof the cutting insert is equal to or greater than the relief angle of the main flank surface4a, the cutting insert is mounted, on the tool main body10of the face mill, so that the axial rake angle AR has a positive value, and the radial rake angle RR has a negative value. In this embodiment, the cutting insert is mounted on a so-called negative-positive edge type face mill, for which the axial rake angle AR is set as about 24° and the radial rake angle RR is about −6°. Furthermore, the cutting insert is mounted on the tool body10, so that, of pairs of the minor cutting edges5bat the front end of the tool body10, the minor cutting edges5bpositioned outside, in the direction of diameter of the tool body10, are located perpendicular to the center line O of the tool body10. When the above described angles are employed to mount the cutting insert on the tool body10, the approach angle of the face mill is determined. The approach angle for the face mill in this embodiment is about 48°.

The second curved ridge5c, adjacent to one of the minor cutting edges5bof each pair, is not projected forward from the minor cutting edge5balong the center line O. The other minor cutting edge5b, located inward, in the direction of the diameter of the tool body10, is gradually inclined to the rear along the center line O, relative to the plane perpendicular to the center line O, and to the interior in the direction of the diameter.

According to the cutting insert of the present invention, the vertex flank surface4cat the second vertex angle portion, provided between the pair of sub-flank surfaces4b, separates the sub-flank surfaces4b, and the second curved ridge5c, formed between the pair of minor cutting edges5b, separates these minor cutting edges5b. As a result, when one of the pair of minor cutting edges5bis damaged, or worn out, the effect of such damage on the other minor cutting edge5bcan be prevented. Therefore, since the individual minor cutting edges5bof a pair can be employed either for a right hand cut or a left hand cut, the number of nose corners available for the cutting insert can be doubled, and economic efficiency is improved.

The boundary portion6awhere the vertex flank surface4cat the second vertex angle portion intersects the pair of sub-flank surfaces4b, and the boundary portion6awhere the second curved ridge5cintersects the pairs of the minor cutting edges5bare smoothly connected, in a tangential direction, to the pair of sub-flank surfaces4band the pair of minor cutting edges5b, respectively, and the reduction in the strength in the vicinity of the boundary portion can be prevented. Furthermore, the vertex flank surface4cat the second vertex angle portion is extended across the thickness of the insert body1, and the boundary lines between the vertex flank surface4cat the second vertex angle portion and the pair of sub-flank surfaces4bare extended, parallel to each other, and with almost the same width, in a direction perpendicular to the direction of the thickness of the insert body1. In addition, the intersecting ridge line portion of the vertex flank surface4cat the second vertex angle portion and the seat surface3is substantially curved in consonance with the shape of the vertex flank surface4cat the second vertex angle portion. As a result, an appropriate strength can be obtained for the entire vertex flank surface4cat the second vertex angle portion to cope with a force (a cutting resistance) that is exerted on the second curved ridge5c. With this arrangement, since the chipping resistance is increased for the vertex flank surfaces4cat the second vertex portion and for the boundary portion between these surfaces and the pair of sub-flank surfaces4b, the service life of the cutting insert can be extended, and degrading of the surface roughness of the work surface can be prevented. Especially, when micro-cutting for the work surface of a workpiece is performed by the cutting insert that is positioned to the rear of the center of the face mill in the feeding direction, the occurrence of chipping can be avoided for the portion where the minor cutting edge5b, which contacts the workpiece first, and the second curved ridge5care adjacent to each other, and a problem on degrading of the surface roughness on the work surface can be resolved.

For each cross section taken along the normal line of the second curved ridge5c, the relief angle of the vertex flank surface4cat the second vertex angle portion reaches the maximum in the cross section along the bisector line B of the second curved ridge5c, and becomes smaller in a direction leading from the intersecting point of the second curved ridge5cand the bisector line B to the two ends of the second curved ridge5c. With this structure, the edge strength of the second curved ridge5cis increased near the boundary portions between the second curved ridge5cand the adjacent minor cutting edges5b. As a result, since the chipping resistance is increased in the vicinity of the boundaries between the second curved ridge5cand the pair of minor cutting edges5b, the service life of the cutting insert is extended, and the deterioration of the surface roughness of the finished surface can be prevented.

The relief angle of the vertex flank surface4cat the second vertex angle portion, in the cross section along the normal line of the second curved ridge5c, is increased in a direction leading from the two ends of the second curved ridge5cto the intersecting point of the second curved ridge5cand the bisector line B. Therefore, when damage has occurred in the vicinity of the boundary between one of the minor cutting edges5bof the pair and the second curved ridge5c, any effect of the damage on the other minor cutting edge5bcan be prevented. Further, in the micro-cutting process for the working surface of a workpiece employing the cutting insert positioned to the rear from the center of the face mill in the feeding direction, even when damage has occurred and is rapidly increasing in the vicinity of the boundary between one of the minor cutting edges5b, which first contacted the workpiece, and the second curved ridge5c, the spread of the damage to the other minor cutting edge5bcan be appropriately prevented. As a result, the other minor cutting edge5bcan be used properly, and the degrading of the surface roughness of the finished surface can be appropriately prevented.

Since the range of the angle formed by the adjacent minor cutting edge5bacross the second curbed ridge5cis from 100° to 178°, one of the minor cutting edges5bof a pair and the second curved ridge5care not projected from a line extended from the other minor cutting edge5b, and are gradually separated from this extension line in a direction leading toward the intersecting portion of the main cutting edge5a. Therefore, the minor cutting edge5band the vicinity of the boundary, between this minor cutting edge5band the second curved ridge5c, can be appropriately prevented from contacting the workpiece. As a result, the minor cutting edge5band the vicinity of the boundary can be properly protected from being affected by damage to the other cutting edge5b.

Since the radius of curvature Rc for the second curved ridge5cis equal to or greater than 0.4 mm, the strength of the second curved ridge5ccan be obtained. However, when the radius of curvature Rc is too large, the range of damage due to the second curved ridge5ccontacting the workpiece would be increased, and the cutting resistance would be much greater. Therefore, it is preferable that the radius of curvature Rc for the second curved ridge5cbe 2.5 mm or smaller.

Since the length Lb for the pair of minor cutting edges5bis set to 0.5 mm or greater, efficient face milling can be performed at a high feed rate, without degrading the surface roughness of the finished surface. However, when the length Lb of the minor cutting edges5bis too great, the range wherein the minor cutting edge5bcontacts the workpiece would be increased, and the cutting resistance would be greater, so that chattering may occur, or the surface roughness of the finished surface may be degraded. Therefore, it is preferable that the length Lb for the pair of minor cutting edges5bbe equal to or smaller than 3 mm.

As described above, according to the face mill on which the cutting inserts of this embodiment are mounted, the chipping resistance can be increased at the portion where the minor cutting edge5band the second curved ridge5care located adjacent to each other, and the surface roughness of the finished surface can also be improved. Since the cutting insert is mounted on the tool body10to provide a so-called negative-positive edge structure, wherein the axial rake angle AR has a positive value and the radial rake angle RR has a negative value, the cutting resistance, especially of a thrust force exerted in a direction counter to the direction in which a workpiece is pressed, can be reduced. This is effective for improving chipping resistance at the portion where the second curved ridge5cand the minor cutting edges5bare adjacent to each other. Further, since the inclination of the face mill is suppressed due to the thrust force, it is possible to reduce the occurrence of a phenomenon that the cutting insert, located at the rear of the center of the face mill in the feeding direction, performs micro-cutting for the finished surface of a workpiece. As a result, the occurrence of chipping can be prevented at the portion where the minor cutting edge5b, which first contacts the workpiece in the micro-cutting process, and the second curved ridge5care adjacent to each other, and the degrading of the surface roughness of the finished surface can be prevented.

There is a tendency for a general negative-positive edge type face mill that, when the boundary portion, between the minor cutting edges, which is formed on the front end of the face mill and is located forward in the tool rotational direction, and the adjacent second curved ridge, cuts into the workpiece, the boundary portion is damaged at an early time by receiving too high a load. On the other hand, according to the face mill in this embodiment, since the second curved ridge5cexhibits a superior chipping resistance, degrading of the surface roughness of the finished surface can be prevented, and the service life of the cutting insert can be extended.

The present invention is not limited to the above described embodiment, and naturally, modifying, adding to, and partial deleting of the structure for the present invention are available without departing from the scope of the present invention.