Cutting insert having an improved chip breaker

Disclosed is a cutting insert capable of enhancing the surface roughness of worked materials and lengthening the durability of a cutting tool which has utilized in conjunction with the cutting insert, capable of efficiently removing cut chips from the cutting region by making a chip breaker into a complete free curved surface to discharge the cut chips in the most natural direction and form, which are formed when performing a desired cutting operation of ferrous or nonferrous metals such as aluminum, copper, stainless, etc., to also minimize the resistance to chip flow and the occurrence of the melted-sticking phenomenon. The cutting insert has a specific chip breaker formed on an upper surface thereof with the aid of a pressure molding operation using a mold and in a sintering operation. A lower surface of the cutting insert is formed as a plane. The lower surface is firmly fixed on a holder for cutting tool and supports the cutting insert when mounting the cutting insert in the holder for cutting tool. It is also possible to provide the chip breaker on the lower surface. At least one cutting corner is formed at corners of the upper surface. The cutting insert includes main cutting edge portions slanted to the cutting corner at a certain angle. The main cutting edge portions includes main cutting edges and main cutting edge land surfaces extend from the main cutting edges toward the upper surface. A circle opening is formed through the center of the cutting insert. A boss is mounted in the circle opening.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a cutting insert, and more particularly to
 a cutting insert capable of enhancing the surface finish of worked
 materials and lengthening the durability of a cutting tool which has
 utilized in conjunction with the cutting insert, capable of efficiently
 removing cut chips from the cutting region by making a chip breaker into a
 complete non uniform curved surface to flow the cut chips in the most
 natural direction and curling, which are formed when performing a desired
 cutting operation, to also minimize the resistance to chip flow and the
 occurrence of the adhesion phenomenon.
 2. Description of the Related Art
 The machine industry such as the automotive industry etc. has been well
 developed heretofore, and thereby a variety of manufacturing processes
 have been automatized and performed at high speed. As a result, there
 exists a need for enhancing the performance of the cutting tool and for
 lengthening the durability of the cutting tool which has been well
 utilized in the machine industry.
 The cutting tool is mainly used to cut ferrous or nonferrous metals under
 the state that it is mounted in a machine tool. The processes of cutting
 the metals using the cutting tool can be classified into two ways.
 According to the first way, the cutting tool cuts a metal workpiece under
 the state that an edge of cutting tool is brought into contact with a
 surface of the metal workpiece when rotating the workpiece, According to
 the second way, a cutting insert having a cutting edge cuts a metal
 workpiece, which is fixed at a predetermined position on a work station,
 when rotating the cutting insert under the state that the cutting insert
 is mounted in a machine tool using a holder for cutting tool.
 The cutting insert is used to cut the workpiece under the state that it is
 mounted to a portion of the cutting tool directly contacting with the
 workpiece. The cutting insert is the most important factor in a metal
 cutting process. Further, the quality and the shape of the cutting insert
 is the most important factor for determining the durability of the cutting
 tool and the quality of the worked materials.
 However, in the metal cutting process using the cutting insert, it is the
 most important things that metal chips generated from the workpiece, in
 the form of chip, when cutting unessential portions of the workpiece are
 securely and effectively flowed from the cutting region. Accordingly, a
 variety of endeavors for developing a cutting insert capable of securely
 flowing the metal chips from the cutting region without interfering with
 the continuing cutting process and without endangering the operating
 personnel have been proposed.
 As a part of the endeavors, a method of changing the shape of the cutting
 insert for effectively cutting the workpiece has been proposed. Generally,
 the cutting insert has a rake surface and a flank surface. The rake
 surface allows the cut chips for flowing. An angle and a shape of the rake
 surface play an important role in determining the durability of the
 cutting insert, the surface finish of worked materials, a chip breaking,
 cut chip flowing and a cutting resistance, etc. The specific shape of the
 sloping surface and the surplus surface is called as "chip breaker". Chip
 breakers having a variety of shapes have been developed heretofore in the
 field of the art with respect to manufacturing the cutting insert. A
 standard of developing the chip breaker in relation to the shape thereof
 is the quality of the workpiece, the shape of the workpiece, a size of the
 cut portion of the workpiece, a precision of the cutting process, the
 quality of the cutting tool, etc.
 However, according to the prior art, since a predetermined angle is present
 between a rake surface and a certain plane, a unnatural cutting
 phenomenon, which is not associated with a natural cutting mechanism
 proceed in the metal cutting process, can be generated. Due to this
 phenomenon, the cutting resistance is increased when cutting the
 workpiece. Further, the workpiece can be melted and stuck to the cutting
 edge of the cutting tool at the time that the workpiece begins to be melt.
 In addition, there are many problems such that the badness of the chip
 breaking, the excess wear of the cutting tool, the chipping of the cutting
 edge, the breakdown of the cutting tool, etc.
 FIG. 11A is a top view of a cutting insert according to the prior art, and
 FIG. 11B is a sectional view taken along the line XI--XI of FIG. 11A.
 Referring to FIGS. 11A and 11B, a cutting insert 10 according to the prior
 art includes a main body 12 having substantially parallel an upper surface
 11 and a bottom surface 13. A circle opening 18 is formed through the
 center of the main body 12. Circle opening 18 provides a means whereby a
 holder for cutting tool (not shown) can be fitted into circle opening 18
 to secure cutting insert 10 to the holder for cutting tool.
 Cutting insert 10 includes four cutting corners 15. Two cutting edges 16
 and two chip grooves 17 adjacent to the cutting edges 16 extend from
 cutting corners 15. Cutting edges 16 extend from cutting corners 15 to the
 middle portion of cutting edges 16 at a predetermined angle. Cutting edges
 16 comprise a straight sloping portion 16a and a horizontal center portion
 16b, respectively.
 Straight sloping portions 16a of cutting edges 16 vary in width, widening
 from cutting corners 15 to the middle portion of cutting edges 16.
 Chip grooves 17 continuously extend across sections of cutting corners 15
 and sections adjacent to straight sloping portions 16a. Chip grooves 17
 are provided with sloping side surfaces 17a,17b. Sloping side surfaces
 17a,17b uniformly maintain in width at the center portions 16b of cutting
 edges 16. Sloping side surfaces 17a,17b and upper surface 11 of cutting
 insert 10 form an angle greater than 130.degree..
 Upper surface 11 includes a flat horizontal section 11a and flat triangular
 sections 11b. Flat triangular sections 11b form a predetermined angle with
 flat horizontal section 11a. Flat horizontal section 11a meets with
 sloping side surfaces 17a,17b of chip grooves 17 in an adjoining
 relationship and accordingly form an angle therebetween. The altitude of
 flat horizontal section 11a is lower than that of cutting edge 16.
 However, in cutting insert 10 as described above, sloping side surfaces
 17a,17b of the chip breaker allowing the cut chips to flow there through
 are plane in figure. Further, the common junctures between flat horizontal
 section 11a and flat triangular sections 11b, sloping side surface 17a and
 flat triangular section 11b, sloping side surface 17b and flat horizontal
 section 11a, etc. form an angle therebetween. Accordingly, the cut scraps
 produced during the metal cutting process undergo compulsive bending
 stresses while passing through the planes such as sloping side surfaces
 17a,17b and the common junctures. As a result, the irregular and excess
 stresses can be applied to cutting edge 16 playing an important role in
 the cutting process, and thereby the cut chips are melted and stuck to
 cutting edge 16. Further, the cut chips cannot smoothly and securely
 flowed from the cutting region, and the surface finish of worked materials
 is not good. In addition, the chipping of the cutting edge can be
 generated. Consequently, the cutting tool has a short life.
 U.S. Pat. No. 5,772,366 issued to Jorgen Wiman et al on Jun. 30, 1998
 discloses a cutting insert having an integral chip control surface. In
 this patent, the cutting insert comprises a compound body consisting of a
 sintered cemented carbide substrate with a surface coating which is a
 diamond coating with a thickness of 1.about.20 .mu.m deposited directly
 from a gas phase in a reactor by CVD or PVD technique. Therefore, it is
 necessary to perform any post-processing for a plurality of curved
 projections and an integral chipformer provided on an upper surface of the
 cutting insert.
 However, in the Wiman al.'s invention, since a central portion of an upper
 surface of the cutting insert is perpendicular to acute-angled corners of
 the cutting insert, there is no chip breakers in a direction parallel with
 a corner cutting edge. Accordingly, the resistance to chip flow is
 relatively high. Since an acute-angled angle is in existence at the
 juncture between the central portion and a sloping flank on the upper
 surface of the cutting insert, the cutting resistance to the cutting edges
 is increased and therefore the built-up edge phenomenon frequently
 generates at the cutting edges. If a workpiece to be cut has a
 sharp-shaped outer surface, the separation of the thin film of the
 workpiece can be generated on the outer surface and thereby the surface
 finish of the workpiece is not good. Consequently, it is impossible to
 maintain pointed cutting edges during the workpiece of aluminum cutting
 process.
 SUMMARY OF THE INVENTION
 The present invention is contrived to solve the foregoing problems. It is
 an object of the present invention to provide a cutting insert capable of
 enhancing the surface finish of worked materials and lengthening the
 durability of a cutting tool life which has utilized in conjunction with
 the cutting insert, capable of efficiently removing cut chips from the
 cutting region by making a chip breaker into a non uniform curved surface
 to flow the cut chips in the most natural direction and form, which are
 formed when performing a desired cutting operation of ferrous or
 nonferrous metals such as aluminum, copper, stainless, etc., to also
 minimize the resistance to chip flow and the occurrence of the
 melted-sticking phenomenon.
 In order to achieve the above object, the present invention provides a
 cutting insert, comprising:
 an upper surface having a chip breaker formed in a pressure molding
 operation using a mold and in a sintering operation, the upper surface
 including at least one ridge and valley constituting a non uniform curved
 surface;
 a lower surface formed as a plane, the lower surface being firmly fixed on
 a holder for cutting tool and supporting the cutting insert when mounting
 the cutting insert in the holder for cutting tool;
 at least one corner cutting edge portion formed at corners of the cutting
 insert, the corner cutting edge portion extending from a first side face
 of the cutting insert, a curved surface of the corner cutting edge portion
 comprising a corner cutting edge, a curved corner cutting edge land
 surface, a curved corner chip groove and a curved surface extending from a
 first valley closed by a curved surface of the corner chip groove to an
 apex of a first ridge neighboring to the first valley, in which a center
 of the curved surface of the corner cutting edge portion is projected on a
 plane along a bisecting line going from a center of the corner cutting
 edge toward a center of an opening formed through a center portion of the
 cutting insert, and a radius of curvature of a curved surface of the
 corner cutting edge land surface is within the range of from 1 to 30 mm;
 and
 main cutting edge portions including sloping main cutting edge land
 surfaces extending downwards from main cutting edges of the main cutting
 edge portions toward the bisecting line, the main cutting edges meeting
 with the corner cutting edge in an adjoining relationship and forming a
 corner included angle of between 35.degree. and 180.degree. therebetween,
 in which the main cutting edge land surfaces comprise at least one curved
 surface extending from a second side face of the cutting insert and
 connecting the ridge and the valley with each other, a radius of curvature
 of a curved surface comprising the main cutting edge land surfaces and the
 ridge or comprising the main cutting edge land surfaces and the valley is
 within the range of from 5 to 15 mm, the main cutting edges comprises at
 least one curved line and at least one straight line which smoothly meet
 with each other in an adjoining relationship, the curved line extends
 downwards from the corner cutting edge toward the center portion of the
 cutting insert, and a radius of curvature of the curved line is within the
 range of from 3 to 20 mm.
 Further, the present invention provides a cutting insert, comprising:
 an upper surface having a first chip breaker formed in a pressure molding
 operation using a mold and in a sintering operation, the upper surface
 including at least one ridge and valley constituting a non uniform curved
 surface;
 a lower surface having a second chip breaker formed in a pressure molding
 operation using a mold and in a sintering operation, the second chip
 breaker being mirror symmetrical with respect to the first chip breaker
 along a horizontal bisector of the cutting insert, the lower surface
 including at least one ridge and valley constituting a non uniform curved
 surface;
 at least one corner cutting edge portion formed at corners of the cutting
 insert, the corner cutting edge portion extending from a first side face
 of the cutting insert, a curved surface of the corner cutting edge portion
 comprising a corner cutting edge, a curved corner cutting edge land
 surface, a curved corner chip groove and a curved surface extending from a
 first valley closed by a curved surface of the corner chip groove to an
 apex of a first ridge neighboring to the first valley, in which a center
 of the curved surface of the corner cutting edge portion is projected on a
 plane along a bisecting line going from a center of the corner cutting
 edge toward a center of an opening formed through a center portion of the
 cutting insert, and a radius of curvature of a curved surface of the
 corner cutting edge land surface is within the range of from 1 to 30 mm;
 and
 main cutting edge portions including sloping main cutting edge land
 surfaces extending downwards from main cutting edges of the main cutting
 edge portions toward the bisecting line, the main cutting edges meeting
 with the corner cutting edge in an adjoining relationship and forming a
 corner included angle of between 35.degree. and 180.degree. therebetween,
 in which the main cutting edge land surfaces comprise at least one curved
 surface extending from a second side face of the cutting insert and
 connecting the ridge and the valley with each other, a radius of curvature
 of a curved surface comprising the main cutting edge land surfaces and the
 ridge or comprising the main cutting edge land surfaces and the valley is
 within the range of from 5 to 15 mm, the main cutting edges comprises at
 least one curved line and at least one straight line which smoothly meet
 with each other in an adjoining relationship, the curved line extends
 downwards from the corner cutting edge toward the center portion of the
 cutting insert, and a radius of curvature of the curved line is within the
 range of from 3 to 20 mm.

DETAILED DESCRIPTION OF THE INVENTION
 Generally, the important factors of determining the performance of the
 cutting insert are the shapes of a main cutting edge, a cutting corner and
 an auxiliary cutting edge. More particularly, the ends, the land surfaces
 or the chip grooves of the cutting edge are very important in the cutting
 process with regard to the work materials of aluminum, copper, stainless
 steel, etc. Since the cutting process of soft materials such as aluminum
 is performed at a high speed and the cut chips are flowed along sloping
 surfaces, the chip grooves, an upper surface of the cutting insert, then
 the shape of the upper surface, which is positioned at the downstream in
 the direction of chip flowing, also plays an important role in the cutting
 process. Therefore, this invention is contrived to improve the shape of
 the chip breaker which is an important factor in the cutting mechanism.
 The cutting insert according to the present invention is formed by
 depositing a hard coating layer onto the surface of substrate of the
 cutting tool or the wear resistance tool. For example, in order to produce
 the cutting insert, the powder materials such a cemented carbide of
 tungsten carbides, a various kinds of cement alloys of titanium carbides,
 a ceramic, a steel such as a high speed steel, etc., undergo the molding
 and the sintering process. As a result, a sintered body is produced. The
 sintered body passes through the grinding, polishing, cleaning process.
 Thereafter, in order to give the wear resistance and the heat shock
 resistance to the sintered body, at least one layer of materials selected
 from the group consisting of carbides, nitrides, carbo-nitrides of
 titanium(Ti), zirconium(Zr) or hafnium(Hf) which are the IV-A group
 metals, and aluminum oxides are deposited on the surface of the sintered
 body by using PVD or CVD. As a result, the cutting insert having improved
 hot hardness and oxidation resistance can be produced.
 Hereinafter, preferred embodiments of the present invention will be
 explained in more detail with reference to the accompanying drawings.
 FIG. 1 is a top view of a cutting insert according to a preferred first
 embodiment of the present invention, FIGS. 1A and 1B are three-dimensional
 perspective views of the cutting insert shown in FIG. 1. FIG. 2A is a
 sectional view taken along the line A--A of FIG. 1, and FIG. 2B is an
 enlarged view of "A" section illustrated in FIG. 2A.
 Referring to FIGS. 1 to 2B, cutting insert 100 cuts ferrous or nonferrous
 metals by only using an upper surface 110 thereof. Accordingly, a chip
 breaker is only provided on upper surface 110. A lower surface 111 is a
 plane in figure and thereby it can be firmly fitted into a holder for
 cutting tool (not shown) during the installation of cutting insert 100
 into the holder for cutting tool. Lower surface 111 firmly supports
 cutting insert 100 within the holder for cutting tool. The chip breaker of
 upper surface 110 is created in a pressing molding operation using a mold
 and in a sintering operation. Accordingly, there is no necessity for
 performing any post-processing to the chip breaker portion.
 The V-type of cutting insert 100 includes at least one corner cutting edge
 portion formed at corners of cutting insert 100. As best seen in FIG. 2B,
 a curved surface of the corner cutting edge portion comprises a corner
 cutting edge 101a, a curved corner cutting edge land surface 102a, a
 curved corner chip groove 122a and a curved surface extending from a first
 valley 114a.sub.1, closed by a curved surface 122a to an apex of a first
 ridge 112a.sub.1 neighboring to first valley 114a.sub.1. At this time, a
 radius of curvature (R.sub.1) of corner cutting edge land surface 102a is
 within the range of from 1 to 30 mm. Further, a radius of
 curvature(R.sub.4) of ridge 112a.sub.1 and a radius of curvature(R.sub.5)
 of valley 114a.sub.1 are within the range of from 0.1 to 2.5 mm.
 Meanwhile, a center of the curved surface of corner cutting edge portion is
 projected on a plane along bisecting line 150 going from a center of
 corner cutting edge 101 toward a center(CP) of an opening 109 which is
 formed through a center portion of cutting insert 100. At this time,
 bisecting line 150 divides upper surface 110 into a first region
 110H.sub.1 and a second region 110H.sub.2 so that first region 110H.sub.1
 is mirror symmetrical with respect to second region 110H.sub.2 along
 bisecting line 150.
 As best seen in FIG. 2B, bisecting line 150 is parallel with lower surface
 111. A tangential line 155 connecting ridges 112a.sub.1, 112a with each
 other determines a curling radius of the cut chips produced during the
 ferrous or nonferrous metal cutting process. Further, tangential line 155
 determines a radius of curvature of at least one chip groove 124 and a
 flowing direction of the cut chips. Tangential line 155 and bisecting line
 150 form a predetermined angle(.alpha.), preferably -10.degree. to
 +10.degree. therebetween. The angle(.alpha.) depends on an included
 angle(.gamma.) of inclination of corner cutting edge 101.
 Corner cutting edge 101 extends from a first side face 107 of cutting
 insert 100. At this time, first side face 107 extends upwardly and slants
 outwardly from lower surface 111 to corner cutting edge 101 so that the
 interference between corner cutting edge 101 and the workpiece is
 obviated.
 Referring to now to FIG. 1, cutting insert 100 includes main cutting edge
 portions comprising main cutting edges 103,105 and sloping main cutting
 edge land surfaces 104,106. The main cutting edge land surfaces 104,106
 extend downwards from main cutting edges 103,105 toward bisecting line
 (150).
 Main cutting edges 103 and 105 merge with each other and constitute a
 natural curved surface. Although radius of curvature (R.sub.2) of main
 cutting edges 103,105 depends on the size of cutting insert 100, it is
 within the range of from 3 to 20 mm.
 Main cutting edges 103 and 105 smoothly meet with corner cutting edge 101
 in an adjoining relationship. A corner included angle(.gamma.) of
 35.degree. is formed between main cutting edges 103,105 and corner cutting
 edge 101.
 Alternatively, in case of the R-type cutting insert, the corner included
 angle(.gamma.) of 180.degree. is formed therebetween. In case of the
 C-type cutting insert, the corner included angle (.gamma.) is 80.degree..
 In case of the T-type cutting insert, the corner included angle (.gamma.)
 is 60.degree.. In case of the D-type cutting insert, the corner included
 angle (.gamma.) is 55.degree.. Finally, in case of the S-type cutting
 insert, the corner included angle (.gamma.) is 90.degree..
 Main cutting edge land surfaces 104,106 comprise at least one curved
 surface extending from a second side surface 108(referred to FIG. 3). Cut
 chips produced during the ferrous or nonferrous metal cutting process meet
 with main cutting edge land surfaces 104,106 at first. Accordingly, main
 cutting edge land surfaces 104,106 are formed as smooth non uniform curved
 surfaces in order to reduce resistance to chip flow. As a result, the wear
 resistance and the surface finish of the worked materials are enhanced.
 Upper surface 110 of cutting insert 100 not undergoes in the grinding
 process. Alternatively, upper surface 110 undergoes in the pre-honing
 process, buffing process or surface hardening process. The altitude of
 upper surface 110 is lower than those of corner cutting edge 101 and main
 cutting edges 103,105. In the same manner as that of main cutting land
 surfaces 104,106, upper surface 110 is formed as a non uniform curved
 surface in order to reduce the resistance to chip flow. For this purpose,
 at least one ridge 112 and valley 114 are formed on upper surface 110, and
 thereby at least one corner chip groove 122 and a plurality of central
 grooves 124 are formed on upper surface 110. At this time, radius of
 curvature of ridge 112 and valley 114 are within the range of from 0.1 to
 2.5 mm.
 Referring to FIGS. 1 and 3, a curved surface, which is consisted of main
 cutting edge land surfaces 104,106 and ridge 112 or consisted of main
 cutting edge land surfaces 104,106 and valley 114, has a radius of
 curvature (R.sub.3) within the range of from 1 to 50 mm, preferably from 5
 to 15 mm. The recessed chip grooves 122,124 extend from main cutting edge
 land surfaces 104,106 toward bisecting line 150. Chip grooves 122,124 are
 non uniform curved surfaces in figure so that the capability of removing
 the cut chips produced during the ferrous or nonferrous metal cutting
 process is enhanced.
 A circle opening 109 is formed through the center of cutting insert 100 and
 a boss 140 is installed in circle opening 109. Boss 140 provides a means
 whereby a holder for cutting tool (not shown) can be fitted into boss 140
 to secure cutting insert 100 to the holder for cutting tool. The altitude
 of boss 140 is higher or lower than that of upper surface 110 of cutting
 insert 100. When cutting insert 100 is fixed into the holder for cutting
 tool, then a clamping screw or a clamping lever can be inserted into boss
 140. It is also possible to use a clamp having a specific shape to hold
 cutting insert 100 in the holder for cutting tool without forming circle
 opening 109 at the center of cutting insert 100.
 FIG. 3 is a sectional view taken along the line B--B of FIG. 1. Referring
 to FIG. 3, a main cutting land surface 106b extends from second side
 surface 108 and comprises a smooth curved surface. At this time, in order
 to avoid the interference between corner cutting edge 101 and the
 workpiece such as ferrous or nonferrous metal, second side surface 108
 extends upwardly and slants outwardly from lower surface 111 to main
 cutting edges 103,105. A central chip groove 124b having the shape of
 valley extends from main cutting edge land surface 106b toward bisecting
 line 150(referred to FIG. 1). Upper surface 110b comprises ridges 112b and
 valleys 114b which continuously meet with each other. The altitude of
 upper surface 110b is lower than those of a corner cutting edge 110b and a
 main cutting edge 105b.
 FIG. 4A is a sectional view taken along the line C--C of FIG. 1, and FIG.
 4B is an enlarged view of "C1" section illustrated in FIG. 4A.
 Referring to FIGS. 1, 4A and 4B, ridge 112 and valley 114 extend from a
 sloping main cutting edge land surface 106c adjacent to main cutting edge
 105 toward bisecting line 150(referred to FIG. 1). At this time, as best
 seen in FIG. 1, ridge 112 and valley 114 form a predetermined
 angle(.theta.), that is +10.degree. to -20.degree., preferably -15.degree.
 to -5.degree. with a line 154 perpendicular to main cutting edge 105.
 Further, ridge 112 and valley 114 of first region (110H.sub.1) are mirror
 symmetrical with respect to ridge 112 and valley 114 of second region
 (110H.sub.2) along bisecting line (150).
 As shown in FIGS. 4A and 4B, main cutting edge land surface 106c comprises
 a smooth curved surface meeting with a chip groove 124c. At this time, a
 radius of curvature (R.sub.3) of the curved surface is within the range of
 from 5 to 15 mm.
 FIG. 5A is a sectional view taken along the line D--D of FIG. 1, and FIG.
 5B is an enlarged view of "D1" section illustrated in FIG. 5A.
 Referring to FIGS. 5A and 5B, upper surface 110 comprises ridges 112d and
 valleys 114d. The uneven surface comprising ridges 112d and the valleys,
 that are central chip grooves 114d, has a radius of curvature of between
 0.1 and 2.5 mm.
 FIG. 6A is a sectional view taken along the line E--E of FIG. 1, and FIG.
 6B is an enlarged view of "E1" section illustrated in FIG. 6A.
 Referring to FIGS. 6A and 6B, upper surface 110 is consisted of ridges 112e
 and valleys, that are central chip grooves 114e. The uneven surface
 comprising ridges 112e and central chip grooves 114e extends toward
 bisecting line 150(referred to FIG. 1). At this time, radius of curvature
 of ridges 112e and central chip grooves 114e is within the range of from
 0.1 to 2.5 mm.
 FIG. 7 is a front view taken in the direction of the arrow "Y" of FIG. 1.
 Referring to FIG. 7, when a person see cutting insert 100 in the direction
 of the arrow "Y", then the main cutting edge seems to be a generally
 curved line. The main cutting edge is consisted of at least one curved
 line 103y and at least one straight line 105y meeting with each other in
 an adjoining relationship. Preferably, curved line 103y is a part of
 circle or ellipse. A radius of curvature (R.sub.2) of curved line 103y is
 within the range of from 1 to 50 mm, preferably 3 to 20 mm.
 FIG. 8 is a front view taken in the direction of the arrow "Z" of FIG. 1.
 Referring to FIG. 8, when a person see cutting insert 100 in the direction
 of the arrow "Z", then the main cutting edge portion seems to be a curved
 line. The main cutting edge portion is consisted of at least one main
 cutting edge 103z, which is a curved line, and at least one main cutting
 edge 105z, which is a straight one, main cutting edge 103z and 105z
 meeting with each other in an adjoining relationship.
 FIG. 9 is a longitudinal sectional view of a cutting insert having chip
 breakers provided on both surfaces thereof according to a preferred second
 embodiment of the present invention.
 A cutting insert 100A according to the preferred second embodiment of the
 present invention is formed by improving cutting insert 100 according to
 the preferred first embodiment of the present invention considering the
 economical efficiency. That is, cutting insert 100A has the same
 constitution as cutting insert 100 according to the preferred first
 embodiment of the present invention, except that it has same chip breakers
 on an upper surface 110A and a lower surface 111A thereof in order to cut
 ferrous or nonferrous metals selectively using upper surface 110A and
 lower surface 111A. Accordingly, descriptions of constitutional elements
 which are identical to the constitutional elements of cutting insert 100
 according to the preferred first embodiment of the present invention will
 be omitted.
 In the same manner as that of the first embodiment of the present
 invention, on upper surface 110A and lower surface 111A, cutting insert
 100A includes main cutting edge portions comprising main cutting edges and
 sloping main cutting edge land surfaces extending downwards from the main
 cutting edges toward bisecting line 150. At this time, the altitudes of
 upper surface 110A and lower surface 111A are lower than those of the
 corner cutting edges and the main cutting edges.
 Further, at least one ridge and valley are formed on upper surface 110A and
 lower surface 111A of cutting insert 100A, and thereby at least one corner
 chip groove and a plurality of central chip groove are provided thereon.
 In addition, a circle opening 109A is formed through the center of cutting
 insert 100A. A boss 140A having an upper portion and a lower portion is
 installed in circle opening 109A.
 The chip breaker on upper surface 110A is mirror symmetrical with respect
 to the chip breaker on lower surface 111A along a horizontal bisector
 (158) of cutting insert (110). Boss 140A provides a means whereby a holder
 for cutting tool (not shown) can be fitted into boss 140A to secure
 cutting insert 100A to the holder for cutting tool.
 FIG. 10 is a longitudinal sectional view of a shim which has used with the
 cutting insert as shown in FIG. 9.
 Referring to FIG. 10, a shim 200 according to the present invention
 provides a means whereby cutting insert 100A is firmly supported by shim
 200 and secured to a holder for cutting tool(not shown) during the use of
 cutting insert 100A.
 For this purpose, an upper surface 210 of shim 200 has a shape
 corresponding to that of a lower surface 111A of cutting insert 100A. An
 opening 209 is formed through the center of shim 200. A boss 240 is
 installed in opening 209A. Boss 240 provides a means whereby a holder for
 cutting tool (not shown) can be fitted into boss 240 to secure cutting
 insert 100A to the holder for cutting tool. Since a lower surface 211 of
 shim 200 is a plane in figure, it firmly supports shim 200 during the
 fixation of shim 200 into the holder for cutting tool.
 Therefore, when cutting insert 100A according to the present invention is
 positioned onto upper surface 210 of shim 200 fixed in the holder for
 cutting tool, then lower surface 110A having the chip breaker is exactly
 fitted with upper surface 210 of shim 200. Under this state, cutting
 insert 100A is firmly clamped by means of a clamping screw or an
 additional clamp for pressing and fixing upper surface 210 of cutting
 insert 100A.
 As described above, cutting inserts 100, 100A according to the present
 invention have the specific chip breakers, the chip breakers being given
 by corner cutting edge 101 and main cutting edges 103,105, which are
 smoothly curved at a certain radius of curvature, corner cutting edge land
 surface 102 and main cutting edge land surfaces 104,106, upper surfaces
 110,110A, which have uneven shapes and are curved at a certain radius of
 curvature in order to meet with corner cutting edge 101, main cutting
 edges 103,105, corner cutting edge land surface 102 and main cutting edge
 land surfaces 104,106.
 Accordingly, if cutting inserts 100,100A according to the present invention
 are used to cut ferrous or nonferrous metals, the resistance to chip flow
 is minimized due to the shape of the uneven surface extending among
 cutting land surfaces 102,104,106, chip grooves 122,124 and upper surfaces
 110,110A. Further, since the radius of curvature of the cut chips is
 optimized, the cutting resistance being applied to cutting edges
 101,103,105 is minimized. In addition, since the cutting force is equally
 distributed to individual cutting edges, the strength of the cutting edge
 is enhanced.
 Further, since the chip breakers of cutting insert 100,100A according to
 the present invention are produced in a pressing molding operation using a
 mold and in a sintering operation, there is no necessity for performing
 any post-processing to the chip breakers. As a result, it is possible to
 provide a uniform product of which a quality is maintained within a lot or
 between lots. In addition, since the chip breakers of cutting insert
 100,100A undergo in the honing process, buffing process or surface
 hardening process, the lubricity of the chip breakers is increased and
 thereby the cutting resistance is minimized. Accordingly, cutting insert
 100, 100A can have wear resistance and wear toughness better than those of
 cutting insert 10 according to the prior art. Further, the surface
 roughness of worked materials is highly enhanced due to the remarkable
 decrease of the adhesion phenomenon, etc.
 Finally, due to the characteristic between main cutting edge land surfaces
 104,106 and the surface of the cut chip, the radius of curvature of the
 cut chip is optimized to be corresponded with a cutting condition. As a
 result, cutting insert 100,100A can smoothly remove the cut chip as
 compared with cutting insert 10 according to the prior art. Accordingly,
 the wear resistance and the wear toughness of cutting insert 100,100A are
 enhanced. The surface finish of worked materials is also enhanced.
 While the present invention has been particularly shown and described with
 reference to particular embodiments thereof, it will be understood by
 those skilled in the art that various changes in form and details may be
 effected therein without departing from the spirit and scope of the
 invention as defined by the appended claims.