Metal cutting insert and a milling tool

Abstract

An indexable cutting insert for a milling tool includes an upper side defining an upper extension plane and a lower side defining a lower extension plane parallel to the upper extension plane. A side surface connects the upper side and the lower side, the side surface including a plurality of upper main clearance surfaces and secondary clearance surfaces. At least six identical and alternately usable upper cutting edges extend around the upper side. Each cutting edge has a chip removing main cutting edge portion and at least one secondary cutting edge portion. The main cutting edge portion is formed in a transition between the upper side and one of the upper main clearance surfaces. The secondary cutting edge portion is formed in a transition between the upper side and one of the secondary clearance surfaces between two main cutting edge portions. The upper main clearance surfaces are formed at an obtuse inner angle with respect to the upper extension plane as seen in side elevation view.

Claims

1. An indexable cutting insert for a milling tool, the cutting insert comprising: an upper side defining an upper extension plane; a lower side defining a lower extension plane parallel to the upper extension plane, wherein a center axis extends perpendicularly through the upper extension plane and the lower extension plane; a side surface connecting the upper side and the lower side, the side surface including a plurality of upper main clearance surfaces and secondary clearance surfaces; and at least six identical and alternately usable upper cutting edges extending around the upper side, wherein each cutting edge includes a chip removing main cutting edge portion and at least one secondary cutting edge portion, wherein the main cutting edge portion is formed in a transition between the upper side and one of said upper main clearance surfaces, and the secondary cutting edge portion is formed in a transition between the upper side and one of said secondary clearance surfaces in a region between two main cutting edge portions, wherein each of said upper main clearance surfaces is formed at an obtuse inner angle with respect to the upper extension plane as seen in side elevation view, wherein the inner angle between the upper extension plane and each of said upper main clearance surfaces is within the range 100118.

2. An indexable cutting insert for a milling tool, the cutting insert comprising: an upper side defining an upper extension plane, wherein the upper side includes a recessed upper base surface extending in parallel with the upper extension plane, and an upper chip surface extending between the upper cutting edges and the upper base surface; a lower side defining a lower extension plane parallel to the upper extension plane, wherein a center axis extends perpendicularly through the upper extension plane and the lower extension plane; a side surface connecting the upper side and the lower side, the side surface including a plurality of upper main clearance surfaces and secondary clearance surfaces; and at least six identical and alternately usable upper cutting edges extending around the upper side, wherein each cutting edge includes a chip removing main cutting edge portion and at least one secondary cutting edge portion, wherein the main cutting edge portion is formed in a transition between the upper side and one of said upper main clearance surfaces, and the secondary cutting edge portion is formed in a transition between the upper side and one of said secondary clearance surfaces in a region between two main cutting edge portions, wherein each of said upper main clearance surfaces is formed at an obtuse inner angle with respect to the upper extension plane as seen in side elevation view, wherein the inner angle between the upper extension plane and each of said upper main clearance surfaces is within the range 100118.

3. The cutting insert according to claim 2, wherein the upper side includes at least one upper reinforcement land connecting the upper cutting edges with the upper chip surface.

4. The cutting insert according to claim 1, wherein each of said secondary clearance surfaces is formed at an inner angle with respect to the upper extension plane as seen in side elevation view, wherein <.

5. The cutting insert according to claim 4, wherein the inner angle between the upper extension plane and the secondary clearance surface below at least a part of the upper secondary cutting edge is within the range 85100.

6. The cutting insert according to claim 1, comprising at least seven identical and alternately usable upper cutting edges.

7. The cutting insert according to claim 1, wherein the cutting insert is double-sided with the lower side being identical to the upper side.

8. The cutting insert according to claim 1, wherein the side surface includes a plurality of recessed support surfaces.

9. The cutting insert according to claim 1, wherein the main cutting edge portion is rectilinear or essentially rectilinear.

10. The cutting insert according to claim 1, wherein the secondary cutting edge portion is in the form of a curved edge portion extending between two adjacent main cutting edge portions and having at least one radius of curvature.

11. The cutting insert according to claim 1, wherein the at least one secondary cutting edge portion is in the form of a surface-wiping secondary edge.

12. The cutting insert according to claim 11, wherein each upper cutting edge has a first and a second surface-wiping secondary edge formed at an angle with respect to each other as seen in plan view.

13. A face milling tool configured for chip-removing machining comprising: a tool body including a front end and a rear end, between which a central rotation axis extends around which the tool is rotatable in a direction of rotation; at least one insert seat formed in a transition between the front end and an envelope surface extending between the front end and the rear end of the tool body, the at least one insert seat including a bottom support surface, wherein a chip pocket is provided in front of the at least one insert seat in the direction of rotation of the tool; and at least one cutting insert securely and detachably mounted in the at least one insert seat, the at least one insert including an upper side defining an upper extension plane, a lower side defining a lower extension plane parallel to the upper extension plane, wherein a center axis extends perpendicularly through the upper extension plane and the lower extension plane, a side surface connecting the upper side and the lower side, the side surface including a plurality of upper main clearance surfaces and secondary clearance surfaces, and at least six identical and alternately usable upper cutting edges extending around the upper side, wherein each cutting edge includes a chip removing main cutting edge portion and at least one secondary cutting edge portion, the main cutting edge portion being formed in a transition between the upper side and one of said upper main clearance surfaces, and the secondary cutting edge portion being formed in a transition between the upper side and one of said secondary clearance surfaces in a region between two main cutting edge portions, wherein each of said upper main clearance surfaces is formed at an obtuse inner angle with respect to the upper extension plane as seen in side elevation view, wherein the inner angle between the upper extension plane and each of said upper main clearance surfaces is within the range 100118.

14. The face milling tool according to claim 13, wherein the tool is configured so that a main cutting edge portion is at an entering angle smaller than 80, and so that the upper extension plane of the cutting insert is on one hand radially tipped in at a radial tipping-in angle f within the range 60f25 and on the other hand axially tipped in at an axial tipping-in angle m within the range 20m0.

15. The cutting insert according to claim 1, wherein the inner angle between the upper extension plane and each of said upper main clearance surfaces is within the range 100114.

16. The cutting insert according to claim 1, wherein the upper side includes a recessed upper base surface extending in parallel with the upper extension plane and an upper chip surface extending between the upper cutting edges and the upper base surface, wherein a chip surface angle (1) is within the range 35155 with respect to the upper extension plane and along the main cutting edge portion.

17. The cutting insert according to claim 16, wherein chip surface angle (1) is within the range of 40155.

18. The cutting insert according to claim 1, wherein an end portion of the main cutting edge portion forms a recess so that the end portion of the main cutting edge portion is located below a successive secondary cutting edge portion with respect to the upper extension plane.

19. The cutting insert according to claim 1, wherein an end portion of the main clearance surface at an end portion of the main cutting edge portion has a smaller inner angle than the obtuse inner angle () of the remaining main clearance surface.

20. The cutting insert according to claim 1, wherein the main cutting edge portion is inclined as seen in side elevation view of the cutting insert so that the main cutting edge portion is declining with respect to the upper extension plane in a direction toward an end of the main cutting edge portion, wherein an end portion of the main cutting edge portion is located below a successive secondary cutting edge portion with respect to the upper extension plane.

21. The cutting insert according to claim 19, wherein the end portion of the main cutting edge portion includes an ascending transition edge connected to the successive secondary cutting edge portion.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a perspective view of a cutting insert according to a first embodiment of the disclosure.

(2) FIG. 2 is a side view of the cutting insert of FIG. 1.

(3) FIG. 3 is a partial top view of the cutting insert of FIG. 1.

(4) FIGS. 4a-c are partial cross-sections taken along lines IVa, IVb, and IVc of FIG. 3, respectively.

(5) FIG. 5 is a perspective view of a cutting insert according to a second embodiment of the disclosure.

(6) FIG. 6 is a side view of the cutting insert of FIG. 5.

(7) FIG. 7 is a perspective view of a cutting insert according to a third embodiment of the disclosure.

(8) FIG. 8 is a partial top view of the cutting insert of FIG. 7.

(9) FIGS. 9a-c are partial side views and a cross-section taken along lines IXa-IXa, IXb-IXb and along line IXc-IXc in FIG. 8, respectively.

(10) FIG. 10 is a perspective view of a cutting insert according to a fourth embodiment of the disclosure.

(11) FIG. 11 is a perspective view of a cutting insert according to a fifth embodiment of the disclosure.

(12) FIG. 12 is a top view of the cutting insert of FIG. 11.

(13) FIG. 13 is a cross-section taken along the line in FIG. 12.

(14) FIG. 14 is a perspective view of a cutting insert according to a sixth embodiment of the disclosure.

(15) FIG. 15 is a perspective view of a milling tool according to the invention.

(16) FIG. 16 is a side view of the milling tool of FIG. 15;

(17) FIG. 17 shows the axial tipping-in angle in a partial side view of the milling tool of FIG. 15.

(18) FIG. 18 shows the radial tipping-in angle in a partial planar view of the milling tool of FIG. 15.

(19) FIG. 19 shows the entering angle in a partial side view of the milling tool of FIG. 15.

(20) FIG. 20 shows the angle of inclination in a partial perspective view of the milling tool of FIG. 15.

(21) FIGS. 21a-b are a perspective view and a side view of a seventh embodiment of the cutting insert;

(22) FIG. 22a-b are a perspective view and a side view of an eighth embodiment of the cutting insert;

(23) FIG. 23a-b are a perspective view and a side view of a ninth embodiment of the cutting insert; and

(24) FIG. 24a-b are a tenth and eleventh embodiment respectively of a transition edge in the ninth embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

(25) The cutting insert according to a first embodiment of the disclosure is shown in FIGS. 1-4. The cutting insert 1 is double-sided with a polygonal basic shape and includes an upper side 2 defining an upper extension plane P.sub.U and an identical lower side 3 defining a lower extension plane P.sub.L, which is parallel to the upper extension plane P.sub.U. A center axis C2 extends perpendicularly through the upper extension plane P.sub.U and the lower extension plane P.sub.L.

(26) The upper side 2 and the lower side 3 are connected by a side surface 4, which includes several main clearance surfaces 5, 15 and secondary clearance surfaces 6a, 6b, 16a, 16b. Around the upper side 2, seven identical and alternately usable cutting edges 7 extend. Each cutting edge includes an essentially rectilinear chip removing main cutting edge portion 8 and a first and a second secondary cutting edge portion 9, 10, formed as surface-wiping edges. The main cutting edge portion 8 is formed in a transition between the upper side 2 and one of the upper main clearance surfaces 5. The first secondary cutting edge portion 9 is formed in a transition between the upper side 2 and a first upper secondary clearance surface 6a in a region between two main cutting edge portions 8, that is, in a corner region of the cutting insert 1. The second secondary cutting edge portion 10 is formed in a transition between the upper side 2 and a second upper secondary clearance surface 6b. The first secondary cutting edge portion 9 is here configured to act as a surface-wiping secondary edge when the cutting insert 1 is mounted in a milling tool with an entering angle of approximately 25.

(27) Alternatively, if the cutting insert 1 is mounted in a milling tool with an entering angle of approximately 42, the first secondary cutting edge portion 9 acts as a corner edge, while the second secondary cutting edge portion 10 at this entering angle is configured to act as a surface-wiping secondary edge. Thus, the milling insert 1 according to this embodiment can be used for two different entering angles. The edge portions between the main cutting edge portion 8, the first secondary cutting edge portion 9, the second secondary cutting edge portion 10 and the next main cutting edge portion 8 are formed as radial transitions.

(28) The cutting insert 1 further includes a recessed upper base surface 11 extending in parallel with the upper extension plane P.sub.U. An upper chip surface 12 extends in the region between the upper cutting edges 7 and the upper base surface 11. Furthermore, between the cutting edges 7 and the base surface 11, a reinforcement land 13 extends. The cutting insert 1 in this first embodiment also includes, in its side surface 4, several recessed support surfaces 14 forming a waist around the cutting insert. As can be seen in FIGS. 4a and 4b, the radial distance measured from the center axis C2 to the recessed support surface 14 below one of the main cutting edge portions 8 equals the radial distance from the centre axis C2 to the main cutting edge portion 8. However, in the corner region, the corresponding distance between the recessed support surface 14 and the centre axis C2 is smaller than the distance from the center axis C2 to the secondary cutting edge portions 9, 10. Transition surfaces are formed between the recessed support surface 14 and the clearance surfaces 5, 6a, 6b.

(29) As can be seen in FIG. 2, the main clearance surface 5 is formed at an obtuse inner angle with respect to the upper extension plane P.sub.U as seen in side elevation view. In FIG. 4a, a partial cross-section taken across the main clearance surface shows the obtuse angle . In this embodiment, the inner angle is 107. The secondary clearance surfaces 6a, 6b are formed at inner angles .sub.1, .sub.2 with respect to the upper extension plane P.sub.U as seen in side elevation view in FIG. 4b, shown in cross section for the upper second secondary clearance surface 6b, formed at an angle .sub.2=97, and in FIG. 4c shown in cross section for the upper first secondary clearance surface 6a, formed at an angle .sub.1=90.5.

(30) The cutting insert 1 is indexable to different index positions. In one index position, one of the upper cutting edges 7 is cutting, wherein the upper side 2 partially forms a rake surface and the lower side 3 forms a support surface resting on a bottom support surface of an insert seat of a milling tool. In another index position, one of a number of lower cutting edges 17 extending around the lower side 3 is cutting, wherein the lower side 3 partially forms a rake surface, and the upper side 2 forms a support surface resting on the bottom support surface of the insert seat.

(31) FIGS. 15-18 show the cutting insert 1 according to a variety of the first embodiment of the disclosure, mounted in a milling tool 101 according to the disclosure. The milling tool 101 includes a tool body 102 and several cutting inserts 1. The tool body 102 includes a front end 104 and a rear end 105, between which a central rotation axis C1 extends. The tool is rotatable in a direction of rotation R around the central rotation axis C1 and an envelope surface 106 is concentric with the axis C1. Several insert seats 107 are formed in a transition between the front end 104 and the envelope surface 106.

(32) Each insert seat 107 has a bottom support surface against which the lower side 3 of the cutting insert 1 rests, a side support having two side support surfaces against which two of the recessed support surfaces 14 rest, and a chip pocket 110 provided in front of the insert seat 107 in the direction of rotation R of the tool. The cutting inserts 1 are securely and detachably mounted in the insert seats 107 by means of a screw 111.

(33) The tool shown in FIGS. 15-19 is configured such that the chip-removing main cutting edge portion 8 is at an entering angle of about 42, so that the first secondary cutting edge portion 9 acts as a corner edge, while the second secondary cutting edge portion 10 acts as a surface-wiping secondary edge. The entering angle is the angle that the main cutting edge portion 8 makes with the direction of feed of the milling tool as seen in side elevation view, as shown in FIG. 19. The entering angle is more specifically defined as the angle between a plane P.sub.tan and a plane P.sub.f measured in a reference plane P.sub.ref2, which planes P.sub.tan, P.sub.f and P.sub.ref2 will be defined below. The entering angle varies along the edge, even though the edge is straight.

(34) The cutting insert 1 is tipped in so that the upper extension plane P.sub.U is at a negative radial tipping-in angle .sub.f of 35. The radial tipping-in angle .sub.f, shown in FIG. 18, is the angle between the upper extension plane P.sub.U and a line along the radial vector r of the tool as seen in planar view. More specifically, the radial tipping-in angle .sub.f is obtained by taking a plane P.sub.f normal to the central rotation axis C1 and passing through a point p.sub.k, and in the plane P.sub.f measure the angle between a reference plane P.sub.ref and the upper extension plane P.sub.U as shown in FIG. 18, which is a view in the plane P.sub.f. The reference plane P.sub.ref is a plane spanned by the central rotation axis C1 and a radial vector r perpendicular to the central rotation axis C1 and passing through the point p.sub.k.

(35) The radius r of the tool is measured between the central rotation axis C1 and the point p.sub.k, which for this cutting insert 1 is located in the transition between the main cutting edge portion 8 and the adjacent second secondary cutting edge portion 10, in this embodiment a surface-wiping secondary edge. With a negative radial tipping-in angle .sub.f, the upper extension plane P.sub.U is directed outwards with regard to the central rotation axis C1 of the tool.

(36) The cutting insert 1 is further tipped in so that the upper extension plane P.sub.U is at a negative axial tipping-in angle .sub.m of 10. The axial tipping-in angle .sub.m, shown in FIG. 17, is the angle between the upper extension plane P.sub.U and the central rotation axis C1 of the tool. More specifically, the axial tipping-in angle .sub.m is obtained by measuring the angle between the upper extension plane P.sub.U and the reference plane P.sub.ref in a plane P.sub.m (not shown), which plane P.sub.m is perpendicular to the upper extension plane P.sub.U, parallel to the central rotation axis C1 and passes through the point p.sub.k. With a negative axial tipping-in angle .sub.m, the upper extension plane P.sub.U is inclined towards the front end 104 of the milling tool. With an entering angle of approximately 42, a radial tipping-in angle .sub.f of 35 and an axial tipping-in angle .sub.m of 10, the main cutting edge portion 8 is at an angle of inclination of approximately 20. The angle of inclination , shown in FIG. 20, is the angle that the main cutting edge portion 8 in a point p.sub.a, or a tangent t to the main cutting edge portion 8 in that point, makes with a second reference plane P.sub.ref2. The second reference plane P.sub.ref2 is parallel with and includes the central rotation axis C1 and includes the point p.sub.a on the main cutting edge portion 8. The angle of inclination is measured in a tangential plane P.sub.tan. The tangential plane P.sub.tan is tangential to the main cutting edge portion 8 in the point p.sub.a and is perpendicular to the second reference plane P.sub.ref2.

(37) In FIG. 20, the angle of inclination is shown by looking at the main cutting edge portion 8 from below the front end 104 of the tool 101, along a line which is normal to the tangential plane P.sub.tan. For the cutting insert 1 according to the first embodiment, the angle of inclination is approximately constant along the main cutting edge portion 8, since the main cutting edge portion 8 is essentially rectilinear. For a curved main cutting edge portion, the angle of inclination will vary along the edge.

(38) With the cutting insert 1 according to the first embodiment mounted in the milling tool 101 as described above, the clearance behind the main cutting edge portion 8 in the direction of rotation R of the tool is optimised with regards to the obtuse inner angle so that the cutting insert 1 has high strength, while still providing sufficient clearance. The clearance behind the surface-wiping secondary cutting edge 10 is sufficient thanks to the negative axial tipping-in angle .sub.m. With the chosen values for the inner angles , .sub.1 and .sub.2, the clearance behind the main cutting edge portion 8 and the secondary cutting edges 9, 10 is in a suitable range. The recessed upper base surface 11 ensures that a positive rake angle is achieved despite the large negative radial tipping-in angle .sub.f. For this purpose, the base surface 11 is in this embodiment formed at a distance of 1.2 mm from the main cutting edge portion 8. The chip surface 12 is at the main portion of the main cutting edge 8 inclined at an angle .sub.1 between 40 and 55, here approximately 44, with respect to the upper extension plane P.sub.U. The reinforcement land 13 is at an angle .sub.2 between 25 and 45, as shown in FIG. 4a. The recessed support surfaces 14 formed in the side surface 4 of the cutting insert 1 provide a large support area resting on the side support surfaces of the milling tool 101. This prevents rotation of the cutting insert 1 within the insert seat 107 of the milling tool 101.

(39) The milling tool in which the cutting insert 1 according to the first embodiment is mounted may instead be configured for an entering angle of approximately 25, in which case the first secondary cutting edge portion 9 acts as a surface-wiping secondary edge. The second secondary cutting edge portion 10 is for moderate cutting depths not active as a cutting edge. However, the second secondary cutting edge portion 10 adjacent the active main cutting edge portion 8 may be used as a prolongation of the main cutting edge portion 8 if the cutting depth is large. For an entering angle of approximately 25, the axial tipping-in angle .sub.m may be set to 17 and the radial tipping-in angle .sub.f to 45, in which case the angle of inclination is approximately 33. It is preferable to adjust the radial and the axial tipping-in angles so that the angle of inclination is within the range 1550.

(40) Further embodiments of the cutting insert 1 will now be described. It is to be noted that the same reference sign designates the same or a similar element in all embodiments disclosed.

(41) A second embodiment of the cutting insert according to the disclosure is shown in FIGS. 5-6. The cutting insert 1 according to this embodiment only differs from the cutting insert of the first embodiment in that it lacks recessed support surfaces. Instead, the side surface 4 extends without recesses from the upper cutting edges 7 to the lower cutting edges 17, including upper clearance surfaces 5, 6a, 6b and lower clearance surfaces 15, 16a, 16b. The side surface 4 also includes non-recessed support surfaces 14 extending between the main clearance surfaces 5, 15.

(42) A third embodiment of the cutting insert according to the disclosure is shown in FIGS. 7-9. The cutting insert 1 according to this embodiment is also double-sided and indexable and differs from the cutting insert according to the first embodiment in that it comprises upper cutting edges 7, each including a main cutting edge portion 8 and one secondary cutting edge portion 9 in the form of a surface-wiping secondary edge. Between the secondary cutting edge portion 9 and the subsequent main cutting edge portion 8 is a radial transition. Since the cutting insert 1 is double-sided, the lower side 3 is identical to the upper side 2, with lower cutting edges 17 extending around the lower side 3.

(43) The cutting insert 1 according to this embodiment further differs from the first embodiment in that it lacks a reinforcement land. Instead, the upper side 2 is formed with a chip surface 12 extending between the upper cutting edges 7 and a recessed upper base surface 11. The cutting insert 1 also differs in the design of the side surface 4. Here, the side surface 4 includes upper and lower main clearance surfaces 5, 15 and a secondary clearance surface 6 that extends all the way between the upper secondary cutting edge portion 9 and a corresponding lower secondary cutting edge portion 19. The recessed support surfaces 14 are rounded and are only formed below the upper main cutting edge portions 8.

(44) As can be seen in FIGS. 9a-c, the upper main clearance surface is formed at an obtuse inner angle of 107 with respect to the upper extension plane P.sub.U while the secondary clearance surface 6 is formed at a nearly right inner angle with respect to the upper extension plane P.sub.U. With these angles, the cutting insert is optimised such that at slightly negative axial tipping-in angles and strongly negative radial tipping-in angles, the clearances behind the main cutting edge portion 8 and the secondary cutting edge portion 9 are within a suitable range.

(45) A fourth embodiment of the cutting insert according to the disclosure is shown in FIG. 10. The cutting insert 1 according to this embodiment only differs from the cutting insert according to the third embodiment in that it lacks recessed support surfaces. Instead, the side surface 4 extends without recesses from the upper cutting edges 7 to the lower cutting edges 17, including upper main clearance surfaces 5, lower main clearance surfaces 15, non-recessed support surfaces 14 extending between the main clearance surfaces 5, 15, and upper and lower secondary clearance surfaces 6, 16.

(46) A fifth embodiment of the cutting insert according to the disclosure is shown in FIGS. 11-13. The cutting insert 1 according to this embodiment differs from the third embodiment in that instead of a surface-wiping secondary cutting edge, the secondary cutting edge portion 9 is formed as a curved cutting edge 9 with a corner radius defining the radius of curvature. The curved cutting edge portion 9 extends between two adjacent main cutting edge portions 8. The cutting insert further differs from the third embodiment in that it comprises a reinforcement land 13 extending between the upper cutting edges 7 and the upper chip surface 12.

(47) As in the third embodiment, the side surface 4 is formed with rounded recessed support surfaces 14 below the upper main clearance surfaces 5. The secondary clearance surface 6 is formed as a curved surface, with a gradual transition between the main clearance surface 5 and the secondary clearance surface 6. Since the cutting insert 1 according to the fifth embodiment is formed with curved cutting edges 9 with a corner radius, the cutting insert according to this embodiment has mirror symmetry with respect to the line shown in FIG. 12, i.e. a bisector cutting the curved cutting edge 9 in two equal parts. As can be seen in FIG. 13, the secondary clearance surface 6, below the bisector, is formed at a right angle with respect to the upper extension plane P.sub.U, while the main clearance surface 5 is formed at an obtuse inner angle of around 107.

(48) With a cutting insert 1 according to this embodiment mounted in a milling tool with a negative radial tipping-in angle .sub.f of 35 and a negative axial tipping-in angle .sub.m of 10, the functional clearance behind both the main cutting edge portion 8 and behind the secondary cutting edge portion 9 is approximately 10.

(49) A sixth embodiment of the cutting insert according to the disclosure is shown in FIG. 14. The cutting insert 1 according to this embodiment differs from the fifth embodiment only in that it lacks recessed support surfaces, but instead has non-recessed support surfaces 14 below the upper main clearance surfaces 5.

(50) A seventh embodiment of the cutting insert is shown in FIGS. 21a and 21b. The cutting insert according to this embodiment only differs from the cutting insert of the first embodiment in that an end portion 8a of the main cutting edge portion 8 forms a recess 8b as seen in a side elevation view of the insert so that the end portion 8a of the main cutting edge portion 8 is located below a successive secondary cutting edge portion 9, 10 with respect to the upper extension plane P.sub.U. A reliable clearance between the workpiece and an inactive main cutting edge portion 8 adjacent an active secondary cutting edge portion 9, 10 is hereby achieved. Hence, the recess 8b in the end portion 8a of the inactive main cutting edge 8 situated radially inside an active secondary cutting edge portion 9, 10 during milling (see FIG. 19) provides the clearance to the machined surface Pf. This embodiment is also double-sided with the lower side 3 identical to the upper side 2 so that the cutting insert is indexable in seven different index positions on the upper side 2 and seven different index positions on the lower side 3.

(51) An eighth embodiment of the cutting insert is shown in FIGS. 22a and 22b. The cutting insert according to this embodiment only differs from the cutting insert of the first embodiment in that an end portion 5a of the main clearance surface 5 at an end portion 8a of the main cutting edge portion has a smaller inner angle than the obtuse inner angle of the remaining main clearance surface. The end portion 5a of the main clearance surface 5 may be provided with an inner angle at around 90, or in the same range as the inner angle provided on the secondary clearance surface. This embodiment provides another way of achieving a reliable clearance between the workpiece and an inactive main cutting edge portion 8 adjacent an active secondary cutting edge portion 9, 10. The end portion 5a of the main clearance surface 5 at the end portion 8a of the inactive main cutting edge 8 is situated radially inside an active secondary cutting edge portion 9, 10 during milling (see FIG. 19) and clears the machined surface Pf. This embodiment is also double-sided with the lower side 3 identical to the upper side 2 so that the cutting insert is indexable in seven different index positions on the upper side 2 and seven different index positions on the lower side 3.

(52) A ninth embodiment of the cutting insert is shown in FIGS. 23a and 23b. The cutting insert according to this embodiment only differs from the cutting insert of the first embodiment in that the main cutting edge portion 8 is inclined as seen in side elevation view of the cutting insert so that the main cutting edge portion 8 is declining with respect to the upper extension plane P.sub.U in a direction toward an end of the main cutting edge portion 8, wherein an end portion 8a of the main cutting edge portion 8 is located below a successive secondary cutting edge portion 9, 10 with respect to the upper extension plane P.sub.U. This embodiment also achieves a reliable clearance between the workpiece P.sub.f (see FIG. 19) and an inactive main cutting edge portion situated radially inside an adjacent and active secondary cutting edge portion 9, 10 during milling. There may be a risk that at least the end portion 8a of the inactive main cutting edge portion, and in particular its main clearance surface 5, adjacent the active secondary cutting edge portion 9, 10, will collide with the workpiece (surface P.sub.f) during milling. Clearance is hereby achieved by inclining the (inactive) main cutting edge 8 portion so that at least its end portion 8a is situated below the successive (active) secondary cutting edge portion 9, 10 with respect to the upper extension plane P.sub.U. FIGS. 23a and 23b show a main cutting edge portion being formed as a straight edge 8 having a constant inclination along the entire length of the cutting edge 8. It may however be partly inclined or curved in a side elevation view of the cutting insert as long as the end portion 8a is located below the secondary cutting edge portion 9, 10.

(53) FIGS. 24a and 24b show a tenth and eleventh embodiment respectively of a transition between the inclined main cutting edge portion 8 and the secondary cutting edge portion 10 of the ninth embodiment. The end portion 8a of the main cutting edge portion 8 is hereby connected to or includes an ascending transition edge 8c connected to the successive secondary cutting edge portion 10. The ascending transition edge 8c is relatively short and is used to connect the end cutting edge portion 8a to the successive secondary cutting edge portion in a smooth manner and thereby avoid abrupt/sharp corners. This ca increase the strength of the cutting edge line in the transition between cutting edge portions. In the tenth embodiment shown in FIG. 24a, the end portion 8a of the inclined main cutting edge portion 8 partly extends into and thereby recesses (removes) a minor part of the secondary cutting edge portion 10, wherein an ascending transition edge 8c is formed and connected to the secondary cutting edge portion 10. In the eleventh embodiment shown in FIG. 24b, the end portion 8a of the inclined main cutting edge portion 8 includes an ascending transition edge 8c connected to the successive secondary cutting edge portion 10.

(54) The disclosure is not limited to the embodiments disclosed but may be varied and modified within the scope of the following claims. For instance, the cutting edges may include curved main cutting edge portions, the cutting insert may be single-sided with cutting edges extending only around the upper side, a cutting insert with curved secondary cutting edge portion with a corner radius may be formed with planar recessed side support surfaces instead of rounded, the insert geometry could be with or without reinforcement land or lands, the reinforcement land and/or the chip surface may be curved surfaces, or the cutting insert may be formed with a larger number of cutting edges, such as eight cutting edges or more. The cutting insert may be designed for left hand rotation of the tool as well as for right hand rotation of the tool. The cutting insert may also, instead of being screw mounted, be secured by for example clamps.

(55) Although the present embodiment(s) has been described in relation to particular aspects thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred therefore, that the present embodiment(s) be limited not by the specific disclosure herein, but only by the appended claims.