Abstract:
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.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of U.S. Ser. No. 09/203,122 filed Dec. 1, 1998, now abandoned. 
    
    
     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.  11 A. 
     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  16   a  and a horizontal center portion  16   b , respectively. 
     Straight sloping portions  16   a  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  16   a . Chip grooves  17  are provided with sloping side surfaces  17   a , 17   b . Sloping side surfaces  17   a , 17   b  uniformly maintain in width at the center portions  16   b  of cutting edges  16 . Sloping side surfaces  17   a , 17   b  and upper surface  11  of cutting insert  10  form an angle greater than 130°. 
     Upper surface  11  includes a flat horizontal section  11   a  and flat triangular sections  11   b . Flat triangular sections  11   b  form a predetermined angle with flat horizontal section  11   a . Flat horizontal section  11   a  meets with sloping side surfaces  17   a , 17   b  of chip grooves  17  in an adjoining relationship and accordingly form an angle therebetween. The altitude of flat horizontal section  11   a  is lower than that of cutting edge  16 . 
     However, in cutting insert  10  as described above, sloping side surfaces  17   a , 17   b  of the chip breaker allowing the cut chips to flow there through are plane in figure. Further, the common junctures between flat horizontal section  11   a  and flat triangular sections  11   b , sloping side surface  17   a  and flat triangular section  11   b , sloping side surface  17   b  and flat horizontal section  11   a , 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  17   a , 17   b  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˜20 μm 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.&#39;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° and 180° 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° and 180° 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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above object and other characteristics and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings, in which: 
     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; 
     FIG. 2B is an enlarged view of “A” section illustrated in FIG. 2A; 
     FIG. 3 is a sectional view taken along the line B—B of FIG. 1; 
     FIG. 4A is a sectional view taken along the line C—C of FIG. 1; 
     FIG. 4B is an enlarged view of “C1” section illustrated in FIG. 4A; 
     FIG. 5A is a sectional view taken along the line D—D of FIG. 1; 
     FIG. 5B is an enlarged view of “D1” section illustrated in FIG. 5A; 
     FIG. 6A is a sectional view taken along the line E—E of FIG. 1; 
     FIG. 6B is an enlarged view of “E1” section illustrated in FIG. 6A; 
     FIG. 7 is a front view taken in the direction of the arrow “Y” of FIG. 1; 
     FIG. 8 is a front view taken in the direction of the arrow “Z” of FIG. 1; 
     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; 
     FIG. 10 is a longitudinal sectional view of a shim which has used with the cutting insert as shown in FIG. 9; 
     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.  11 A. 
    
    
     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.  2 A. 
     Referring to FIGS. 1 to  2 B, 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  101   a , a curved corner cutting edge land surface  102   a , a curved corner chip groove  122   a  and a curved surface extending from a first valley  114   a   1 , closed by a curved surface  122   a  to an apex of a first ridge  112   a   1  neighboring to first valley  114   a   1 . At this time, a radius of curvature (R 1 ) of corner cutting edge land surface  102   a  is within the range of from 1 to 30 mm. Further, a radius of curvature(R 4 ) of ridge  112   a   1  and a radius of curvature(R 5 ) of valley  114   a   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  110 H 1  and a second region  110 H 2  so that first region  110 H 1  is mirror symmetrical with respect to second region  110 H 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  112   a   1 ,  112   a  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(α), preferably −10° to +10° therebetween. The angle(α) depends on an included angle(γ) 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 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(γ) of 35° 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(γ) of 180° is formed therebetween. In case of the C-type cutting insert, the corner included angle (γ) is 80°. In case of the T-type cutting insert, the corner included angle (γ) is 60°. In case of the D-type cutting insert, the corner included angle (γ) is 55°. Finally, in case of the S-type cutting insert, the corner included angle (γ) is 90°. 
     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 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  106   b  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  124   b  having the shape of valley extends from main cutting edge land surface  106   b  toward bisecting line  150 (referred to FIG.  1 ). Upper surface  110   b  comprises ridges  112   b  and valleys  114   b  which continuously meet with each other. The altitude of upper surface  110   b  is lower than those of a corner cutting edge  110   b  and a main cutting edge  105   b.    
     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.  4 A. 
     Referring to FIGS. 1,  4 A and  4 B, ridge  112  and valley  114  extend from a sloping main cutting edge land surface  106   c  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(θ), that is +10° to −20°, preferably −15° to −5° with a line  154  perpendicular to main cutting edge  105 . Further, ridge  112  and valley  114  of first region ( 110 H 1 ) are mirror symmetrical with respect to ridge  112  and valley  114  of second region ( 110 H 2 ) along bisecting line ( 150 ). 
     As shown in FIGS. 4A and 4B, main cutting edge land surface  106   c  comprises a smooth curved surface meeting with a chip groove  124   c . At this time, a radius of curvature (R 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.  5 A. 
     Referring to FIGS. 5A and 5B, upper surface  110  comprises ridges  112   d  and valleys  114   d . The uneven surface comprising ridges  112   d  and the valleys, that are central chip grooves  114   d , 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.  6 A. 
     Referring to FIGS. 6A and 6B, upper surface  110  is consisted of ridges  112   e  and valleys, that are central chip grooves  114   e . The uneven surface comprising ridges  112   e  and central chip grooves  114   e  extends toward bisecting line  150 (referred to FIG.  1 ). At this time, radius of curvature of ridges  112   e  and central chip grooves  114   e  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  103   y  and at least one straight line  105   y  meeting with each other in an adjoining relationship. Preferably, curved line  103   y  is a part of circle or ellipse. A radius of curvature (R 2 ) of curved line  103   y  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  103   z , which is a curved line, and at least one main cutting edge  105   z , which is a straight one, main cutting edge  103   z  and  105   z  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  100 A 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  100 A 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  110 A and a lower surface  111 A thereof in order to cut ferrous or nonferrous metals selectively using upper surface  110 A and lower surface  111 A. 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  110 A and lower surface  111 A, cutting insert  100 A 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  110 A and lower surface  111 A 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  110 A and lower surface  111 A of cutting insert  100 A, and thereby at least one corner chip groove and a plurality of central chip groove are provided thereon. In addition, a circle opening  109 A is formed through the center of cutting insert  100 A. A boss  140 A having an upper portion and a lower portion is installed in circle opening  109 A. 
     The chip breaker on upper surface  110 A is mirror symmetrical with respect to the chip breaker on lower surface  111 A along a horizontal bisector ( 158 ) of cutting insert ( 110 ). Boss  140 A provides a means whereby a holder for cutting tool (not shown) can be fitted into boss  140 A to secure cutting insert  100 A 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  100 A is firmly supported by shim  200  and secured to a holder for cutting tool(not shown) during the use of cutting insert  100 A. 
     For this purpose, an upper surface  210  of shim  200  has a shape corresponding to that of a lower surface  111 A of cutting insert  100 A. An opening  209  is formed through the center of shim  200 . A boss  240  is installed in opening  209 A. Boss  240  provides a means whereby a holder for cutting tool (not shown) can be fitted into boss  240  to secure cutting insert  100 A 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  100 A according to the present invention is positioned onto upper surface  210  of shim  200  fixed in the holder for cutting tool, then lower surface  110 A having the chip breaker is exactly fitted with upper surface  210  of shim  200 . Under this state, cutting insert  100 A is firmly clamped by means of a clamping screw or an additional clamp for pressing and fixing upper surface  210  of cutting insert  100 A. 
     As described above, cutting inserts  100 ,  100 A 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 , 110 A, 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 , 100 A 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 , 110 A. 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 , 100 A 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 , 100 A 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 ,  100 A 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 , 100 A 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 , 100 A 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.