Patent Publication Number: US-2012039681-A1

Title: Cutting insert, cutting tool and cutting method using the same

Description:
RELATED APPLICATIONS 
     This application is a continuation application of U.S. patent application Ser. No. 12/994,627, entitled “CUTTING INSERT, CUTTING TOOL AND CUTTING METHOD USING THE SAME”, which claims priority rights of Japanese Patent Application No. 2008-251198, filed on Sep. 29, 2008 and incorporates herein the entire disclosure of the application for reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a cutting tool used for working a metal or the like. 
     BACKGROUND ART 
     Conventionally, there has been used a throw away type cutting tool structured such that a cutting insert having a cutting edge is installed to a holder. 
     As the cutting insert used in the cutting tool mentioned above, for example, in patent document 1, there is disclosed a cutting insert provided with a cutting edge which is positioned at an intersection of an upper surface and a side surface, and a groove portion which is provided on the side surface in such a manner as to divide the cutting edge. Divided cutting edges divided by the groove portion contribute to a reduction of a cutting resistance. 
     On the other hand, in the case of using the cutting insert having the divided cutting edges as mentioned above, since an uncut part is generated in a portion corresponding to the groove portion, in the workpiece, it is necessary to install a cutting insert having a different arrangement of the groove portion into the holder. Specifically, two kinds of cutting inserts are attached to the holder in such a manner that located at a position on a rotational locus of the groove portion of one cutting insert installed to the holder is a cutting edge of the other cutting insert. 
     However, in the cutting tool to which these cutting inserts are attached, a great load is applied to the cutting edge portion of the other cutting insert corresponding to the groove portion of one cutting insert. Therefore, the cutting edge portion tends to be chipped. 
     PRIOR ART PUBLICATION 
     
         
         Patent Publication 1: Japanese Unexamined Patent Publication No. 07-225908 
       
    
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a cutting tool and a cutting method which can suppress a chip of a cutting edge. 
     Means for Solving the Problem 
     A cutting tool in accordance with the present invention includes a first cutting insert, a second cutting insert, and a holder to which the first cutting insert and the second cutting insert are installed. The first cutting insert has a first side surface connecting a first upper surface to a first lower surface, a first cutting edge positioned at an intersection of the first upper surface and the first side surface, and a first groove portion positioned at the first side surface and reaching the first upper surface so as to divide the first cutting edge. On the other hand, the second cutting insert has a second side surface connecting a second upper surface to a second lower surface, a second cutting edge positioned at an intersection of the second upper surface and the second side surface, and a cutting edge reinforcement portion positioned at an end portion close to the second cutting edge on the second upper surface. Further, the first cutting insert and the second cutting insert are installed to the holder in such a manner that the first cutting edge and the second cutting edge are positioned at an outer peripheral side of the holder, the first lower surface and the second upper surface come close to each other, and a rotational locus of the cutting edge reinforcement portion and a rotational locus of the first groove portion overlap partly. 
     A cutting insert in accordance with the present invention is a reinforced type cutting insert for a cutting tool, which is installed to a holder together with a groove portion type cutting insert. The groove portion type cutting insert has a first side surface connecting a first upper surface to a first lower surface, a first cutting edge positioned at an intersection of the first upper surface and the first side surface, and a first groove portion positioned at the first side surface and reaching the first upper surface so as to divide the first cutting edge. On the other hand, the reinforced type cutting insert has a second side surface connecting a second upper surface to a second lower surface, a second cutting edge positioned at an intersection of the second upper surface and the second side surface, and a cutting edge reinforcement portion positioned at an end portion close to the second cutting edge in the second upper surface. Further, the groove portion type cutting insert and the reinforced type cutting insert are installed to the holder in such a manner that the first cutting edge and the second cutting edge are positioned on an outer peripheral side of the holder, the first lower surface and the second upper surface come close to each other, and the cutting edge reinforcing portion and the first groove portion are arranged at a corresponding position of a rotational locus of the holder. 
     A cutting method of a workpiece in accordance with the present invention includes a step of rotating the cutting tool, a step of bringing the cutting tool into contact with the workpiece, and a step of separating the workpiece from the cutting tool. 
     Effect of the Invention 
     In accordance with the cutting tool of the present invention, it is possible to suppress a chip of the cutting edge portion existing at the position corresponding to the groove portion of the first cutting insert, in the cutting edge of the second cutting insert. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a general perspective view of a cutting tool  1  in accordance with a first embodiment of the present invention; 
         FIG. 2  is a general perspective view of a cutting insert  2 A installed to the cutting tool in  FIG. 1 ; 
         FIG. 3(   a ) is a plan view, and  FIG. 3(   b ) is a side elevational view of the insert  2 A in  FIG. 2 ; 
         FIG. 4  is a general perspective view of a cutting insert  2 B installed to the cutting tool in  FIG. 1 ; 
         FIG. 5(   a ) is a plan view, and  FIG. 5(   b ) is a side elevational view of the insert  2 B in  FIG. 3 ; 
         FIG. 6(   a ) is a side elevational view of the cutting tool  1  as seen from a direction X in  FIG. 1 , and  FIG. 6(   b ) is a side elevational view of the cutting tool  1  as seen from a direction Y in  FIG. 1 ; 
         FIG. 7  is a schematic view illustrating a cutting state using the cutting tool  1 , and respectively shows enlarged views of substantial parts of  FIGS. 6(   a ) and  6 ( b ) in parallel; 
         FIG. 8(   a ) is a plan view of a cutting insert  2 B′ installed to a cutting tool  1 ′ in accordance with a second embodiment of the present invention,  FIG. 8(   b ) is a cross sectional view along a line B-B, and  FIG. 8(   c ) is a cross sectional view along a line C-C in  FIG. 8(   a ); 
         FIG. 9(   a ) is a plan view of a cutting insert  2 B″ installed to a cutting tool  1 ″ in accordance with a third embodiment of the present invention,  FIG. 9(   b ) is a cross sectional view along a line B-B, and  FIG. 9(   c ) is a cross sectional view along a line C-C in  FIG. 9(   a ); 
         FIG. 10(   a ) is a plan view of a cutting insert  2 B″ installed to a cutting tool  1 ′″ in accordance with a fourth embodiment of the present invention,  FIG. 10(   b ) is a cross sectional view along a line B-B, and  FIG. 10(   c ) is a cross sectional view along a line C-C in  FIG. 10(   a ); and 
         FIG. 11  ( a ),  FIG. 11(   b ),  FIG. 11  ( c ), and  FIG. 11  ( d ) is a process chart illustrating a cutting method of a workpiece in accordance with an embodiment of the present invention. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     A description will be in detail given below of an embodiment in accordance with the present invention with reference to the accompanying drawings. 
     &lt;Cutting Tool&gt; 
     First Embodiment 
     A cutting tool  1  in accordance with the present embodiment is provided with a holder  3 , and a cutting insert  2  (hereinafter, abbreviated as insert  2 ) installed to the holder  3 , as shown in  FIG. 1 . 
     The holder  3  is formed as an approximately rotating body shape around an axis S. Further, four insert pockets  17  are provided in a front end side of the holder  3 . The insert pocket  17  is a portion to which the insert  2  is installed, and an outer peripheral surface and a front end surface of the holder  3  are open thereto. 
     At least two kinds of inserts (inserts  2 A and  2 B) having different shapes are installed to the holder  3 . Specifically, two inserts  2 A and two inserts  2 B are installed to the holder  3 . The inserts  2 A and the inserts  2 B are alternately arranged in a peripheral direction of the holder  3 . 
     In the present embodiment, the insert  2  is installed to the insert pocket  17  by a fixing screw  16 . That is, the insert  2  is installed to the holder  3  by inserting the fixing screw  16  to a through hole  15  mentioned below of the insert  2 , and engaging a front end of the fixing screw  16  with a thread hole formed in the insert pocket  17 . 
     The insert  2  is installed to the holder  3  further in such a manner as to have a positive axial rake. In accordance with the structure mentioned above, it is possible to achieve a reduction of a cutting resistance applied at a time of cutting. 
     The insert  2 A is provided with an upper surface  11 A, a lower surface  12 A and a side surface  13 A connecting the upper surface  11 A to the lower surface  12 A, as shown in  FIG. 2 . Further, the insert  2 A is provided with a cutting edge  4 A which is provided on an intersection of the upper surface  11 A and the side surface  13 A. The upper surface  11 A is formed as an approximately square shape having four corners  10 A, as shown in  FIG. 3(   a ), and the cutting edge  4 A is arranged in each of sides of the upper surface  11 A. In other words, the insert  2 A is a so-called four-corner type insert having four cutting edges  4 A. In this case, the insert  2 A has a through hole  15 A penetrating from the upper surface  11 A to the lower surface  12 A. Further, four cutting edges  4 A are arranged in such a manner as to be rotationally symmetrical at 90 degree with respect to a center line of the through hole  15 A. 
     Further, the insert  2 A has a groove portion  5 A provided on the side surface  13 A. The groove portion  5 A reaches the upper surface  11 A, and is provided in such a manner as to divide the cutting edge  4 A into a plurality of small cutting edges  41 A, as shown in  FIGS. 2 and 3(   b ). Further, the groove portion  5 A is formed in such a manner as to form an outward open concave shape in the top view, as shown in  FIG. 3(   a ). 
     In the present embodiment, the cutting edge  4 A is divided into five small cutting edges  41 A by the groove portion  5 A. 
     In the present embodiment, further, two groove portions  5 I positioned at the center side among four groove portions  5 A are provided from an end portion close to the upper surface  11 A side to an end portion close to the lower surface  12 A side on the side surface  13 A. On the other hand, the remaining two groove portions  5 II positioned close to the corner  10 A side do not reach the end portion close to the lower surface  12 A side on the side surface  13 A, as shown in  FIG. 3(   b ). Since the groove portions  5 II mentioned above are provided at the corner side, it is possible to inhibit a strength from being lowered in the vicinity of the corner  10 A of the insert  2 A. 
     Further, a positive clearance angle is applied to the insert  2 A in accordance with the present embodiment. The clearance angle in this case means an angle of gradient θ of the side surface  13  with respect to an auxiliary line which is approximately vertical to the lower surface  12 A, in the side view, as shown in  FIG. 3(   b ). In the present embodiment, the clearance angle θ is fixed in an area extending along the cutting edge  4 . 
     Next, a description will be given of the insert  2 B installed to the cutting tool  1  in accordance with the present embodiment with reference to  FIGS. 4 and 5 . 
     The insert  2 B is provided with an upper surface  11 B, a lower surface  12 B, and a side surface  13 B connecting the upper surface  11 B to the lower surface  12 B. Further, the insert  2 B is provided with a cutting edge  4 B which is provided at an intersection of the upper surface  11 B and the side surface  13 B, and a through hole  15 B penetrating from the center of the upper surface  11 B to the center of the lower surface  12 B. As shown in  FIG. 5(   a ), in the insert  2 B, the upper surface  11 B is formed as an approximately rectangular shape, and the cutting edge  4 B is arranged on each of sides of the upper surface  11 B. These four cutting edges  4 B are arranged so as to be rotationally symmetrical at 90 degree with respect to a center line of the through hole  15 B, in the same manner as the insert  2 A. 
     Here, the insert  2 B has a cutting edge reinforcing means  6  in the vicinity of the cutting edge  4 B. The cutting edge reinforcing means  6  is provided at a position corresponding to the groove portion  5 B of the insert  2 A. 
     The cutting edge reinforcing means  6  is arranged for the purpose of suppressing a chip of the cutting edge  4 B. Specifically, the cutting edge reinforcing means  6  is a protruding portion which is formed on the upper surface  11 B and extends in an approximately vertical direction to the cutting edge  4 B. Since the cutting edge reinforcing means  6  is arranged in the vicinity of the cutting edge  4 B, a thickness in the vicinity of the cutting edge is increased, and it is possible to suppress the chip of the cutting edge  4 B. In terms of a further suppression of the chip of the cutting edge  4 B, it is preferable that the cutting edge reinforcing means  6  is formed continuously in the cutting edge  4 B. The cutting edge reinforcing means  6  is provided, for example, on a rake face formed on the upper surface  11 B or on a land. 
     It is preferable that the cutting edge reinforcing means  6  is arranged in the vicinity of the small cutting edge  41   a  positioned at a front end side of the holder  3  among four small cutting edges  41  divided by the groove portion  5 B, as shown in  FIG. 6(   b ). That is, the cutting edge reinforcing portion  6  is positioned closer to the front end side of the holder  3  than one or more groove portions  5 B. In accordance with the structure mentioned above, it is possible to keep a strength of the small cutting edge  41   a  coming into contact with the workpiece at the beginning. 
     Further, in the present embodiment, the insert  2 B has a land  7  which is provided along the cutting edge  4 . In the case that the land  7  is arranged, the cutting edge reinforcing means  6  may be arranged continuously to the land  7 . As shown in  FIG. 7 , in the case of setting a dimension of the cutting edge reinforcing means  6  in the approximately vertical direction to the cutting edge  4  to D 6 , and setting a dimension of the land  7  in the approximately vertical direction to the cutting edge  4  to D 7 , D 6  and D 7  have a relationship D 6 &gt;D 7 . As mentioned above, in the present embodiment, the dimension D 6  of the cutting edge reinforcing means  6  in which a substantial feeding amount is larger becomes larger than the dimension D 7  of the land  7 . Note that D 6  means a maximum value of the dimension of the cutting edge reinforcing means  6  in the vertical direction to the cutting edge  4 , as shown in  FIG. 7 . In the present embodiment, since the dimension becomes maximized in the center in an approximately parallel direction to the cutting edge  4 B (a width direction), a value in the center is set to D 6 . D 7  also means a maximum value of the dimension of the land  7  in the same manner as mentioned above. 
     In the present embodiment, further, as shown in  FIG. 7 , a width of the groove portion  5 A close to the cutting edge  4 A side is set to W 5 , in the insert  2 A. Further, a width of the cutting edge reinforcing means  6  close to the cutting edge  4 B side is set to W 6 , in the insert  2 B. At this time, W 5  and W 6  are approximately identical. In this case, in the present specification, the width means a dimension in a direction extending along the cutting edge. 
     In accordance with the structure mentioned above, it is possible to improve a cutting edge strength while achieving a reduction of a cutting resistance. 
     Further, as shown in  FIG. 7 , the width W of the cutting edge reinforcing means  6  becomes smaller as it heads for an inner side. Specifically, in the present embodiment, the cutting edge reinforcing means  6  is formed as an approximately semicircular shape, in the top view. In accordance with the structure mentioned above, it is possible to achieve an improvement of a strength of the cutting edge reinforcing means  6  itself. 
     Further, in the present embodiment, as shown in  FIG. 5(   b ), the cutting edge reinforcing means  6  and the land  7  are formed in such a manner that an angle of gradient with respect to the lower surface  12 B becomes constant. In other words, in the present embodiment, the cutting edge reinforcing means  6  and the land  7  are constructed on the same plane. In accordance with the structure mentioned above, it is possible to achieve an improvement of a strength of the land  7  as a whole. 
     In this case, in the present embodiment, there is exemplified the embodiment in which the angles of gradient of the cutting edge reinforcing means  6  and the land  7  become constant, but the structure is not limited to this. The angles of gradient of the cutting edge reinforcing means  6  and the land  7  may be different from each other (not shown). For example, even in the case that the angle of gradient of the cutting edge reinforcing means  6  is smaller than the angle of gradient of the land  7 , the effect mentioned above can be achieved. 
     Further, the cutting edge reinforcing means  6  may be structured so as to be protuberate upward as it heads for the inner side of the upper surface  11 B. In other words, the cutting edge reinforcing means  6  may be structured so as to be inclined in such a manner as to be away from the lower surface  12 B as it heads for the inner side of the upper surface  11 B. Note that, in the case of the protuberate structure as mentioned above, as the cutting edge reinforcing means  6 , for example, there can be listed up an approximately semicircular shaped structure protruding upward with respect to the land  7 . 
     The insert  2 B has the groove portion  5 B dividing the cutting edge  4 B, in the same manner as the insert  2 A. The cutting edge  4 B is divided into four small cutting edges  41 B by three groove portions  5 B. As mentioned above, since the insert  2 B also has the groove portion  5 B, it is possible to achieve a reduction of the cutting resistance. Here, in the present embodiment, they are arranged in such a manner that a rotational locus of the groove portion of the insert  2 A does not lap over a rotational locus of the groove portion of the insert  2 B, in the side view. 
     Further, as shown in  FIGS. 2 and 4 , both the insert  2 A and the insert  2 B have a projection portion  16  which is provided on the upper surface  11  in correspondence to the small cutting edge  41 . Accordingly, a chip generated by the small cutting edge  41  is curled by the projection portion  16 , and the chip is smoothly discharged. In this case, in the present embodiment, each of the projection portions  16  corresponding to a plurality of small cutting edges  41  is formed as approximately in the same shape. As a structure of the projection portion  16 , for example, the structure of the projection portion  16  corresponding to the small cutting edge  41  in which a first land  71  forming the cutting edge reinforcing means  6  is provided may be different from the projection portion  16  corresponding to the other small cutting edge. 
     The cutting tool  1  is structured, as mentioned above, such that the inserts  2 A and  2 B are installed to the holder  3 .  FIG. 6(   a ) is a side elevational view of the cutting tool  1  as seen from a direction X in  FIG. 1 , and  FIG. 6(   b ) is a side elevational view of the cutting tool  1  as seen from a direction Y in  FIG. 1 . The direction X here is a direction which is approximately vertical to an axis S of the holder  3 , and a direction which heads for the lower surface  12 A side from the upper surface  11 A side of the insert  2 A. Further, the direction Y here is a direction which is approximately vertical to the axis S of the holder  3 , and a direction which heads for the lower surface  12 B side from the upper surface  11 B side of the insert  2 B. 
     As shown in  FIGS. 6(   a ) and  6 ( b ), both the insert  2 A and the insert  2 B are arranged in the holder  3  in such a manner that the cutting edge  4 A and the cutting edge  4 B protrude from the outer peripheral surface of the holder  3 . At this time, the cutting edge  4 A and the cutting edge  4 B are arranged along a direction of the axis S of the holder  3 . 
     Further, the insert  2 A and the insert  2 B are arranged in such a manner that the lower surface  13 A of the insert  2 A and the upper surface  11 B of the insert  2 B come close to each other. In accordance with the arrangement mentioned above, a smooth work surface can be obtained by cutting an uncut portion of the work surface of the workpiece which is generated by the groove portion  5 A provided on the insert  2 A, by means of the cutting edge  4 B of the insert  2 B. 
     Specifically, since the insert  2 A has the groove portion  5 A, the portion corresponding to the groove portion  5 A is left without being cut, on the work surface of the workpiece which is cut by the insert  2 A. As a result, a band-like protuberant portion corresponding to the groove portion  5 A exists on the work surface of the workpiece. The insert  2 B cuts the work surface having the protuberant portion mentioned above. 
     Further, the insert  2 A and the insert  2 B are arranged in such a manner that a rotational locus of the groove portion  5 A provided on the insert  2 A partly laps over a rotational locus of the cutting edge reinforcing means  6  provided on the insert  2 B. In accordance with the structure mentioned above, the protuberant portion of the work surface generated by the groove portion  5 A can suppress the chip of the cutting edge  4 B which may be generated at a time of coming into collision with the insert  2 B. The rotational locus of the groove portion  5 A and the rotational locus of the cutting edge reinforcing means  6  have a width in an axial direction of the holder, in the side view. Specifically, in the side view, these rotation loci may overlap partly or may be coincident. 
       FIG. 7  shows a cutting edge portion at a position corresponding to the groove portion  5 A of the insert  2 A in the cutting edge  4 B of the insert  2 B indicated by reference symbol A. The cutting edge portion A is a cutting edge portion which cuts the band-like protuberant portion corresponding to the groove portion  5 A of the insert  2 A on the work surface of the workpiece. A substantial feeding amount of the cutting edge portion A is larger than a substantial feeding amount of the other cutting edge portion. Specifically, the feeding amount of the cutting edge A cutting the protuberant portion corresponds to a total amount of the substantial feeding amount of the other cutting edge portion and a dimension at which the protuberant portion is protuberate upward. Accordingly, a greater load than the other cutting edge portion is applied to the cutting edge portion A cutting the protuberant portion. 
     Accordingly, in the present embodiment, as shown in  FIG. 7 , the cutting edge reinforcing means  6  is provided in the vicinity of the cutting edge portion A to which the greater load is applied. Specifically, the cutting edge reinforcing means  6  is provided at a position corresponding to the groove portion  5 A of the insert  2 A (that is, on the rotational locus of the groove portion of the installed insert  2 A), in the vicinity of the cutting edge  4 B. In accordance with the structure mentioned above, it is possible to improve a strength of the cutting edge portion of the insert  2 B. As a result, it is possible to inhibit the cutting edge portion A from being chipped. 
     Second Embodiment 
     An insert  2 B′ shown in  FIG. 8  is another embodiment of the insert  2 B installed to the cutting tool  1  in accordance with the first embodiment mentioned above. 
     In this case, the same reference numerals are attached to the same structures, as those of the insert  2 B in the first embodiment and a description thereof will be omitted. 
     In the insert  2 B′ in accordance with the second embodiment, a structure of the cutting edge reinforcing means  6  is different from that of the insert  2 B in accordance with the first embodiment. Specifically, the insert  2 B′ is structured such that a honing process is applied to the cutting edge  4 B. The honing process means a process for improving the strength of the cutting edge  4 B by grinding the cutting edge  4 B with a grindstone or the like. 
     The cutting edge  4 B of the insert  2 B′ has a first partial cutting edge  4   i  corresponding to the cutting edge reinforcing means  6 , and second partial cutting edges  4   ii  positioned on both sides of the first partial cutting edge  4   i . The first partial cutting edge  4   i  is positioned at a position corresponding to the groove portion  5 A of the one insert  2 A, in the direction of the axis S of the holder  3 , in the cutting edge  4 B. 
     In the present embodiment, as shown in  FIGS. 8(   b ) and  8 ( c ), a honing amount of the first partial cutting edge  4   i  is larger than a honing amount of the second partial cutting edge  4   ii . In other words, the honing process is applied more to the first partial cutting edge  4   i  than to the second partial cutting edge  4   ii.    
     In accordance with the structure mentioned above, it is possible to enhance a strength of the first partial cutting edge  4   i  corresponding to the cutting edge portion of the insert  2 B′ to which the great load is applied at a time of cutting as mentioned above. Accordingly, it is possible to inhibit the cutting edge portion from being chipped. 
     Here, a honing amount A 1  of the first partial cutting edge  4   i  means a distance which is approximately in parallel to the lower surface  12 B, of a distance from an outer end portion of the upper surface  11 B to an upper end of the side surface  13 B, in the cross sectional view, as shown in  FIG. 8(   b ). A honing amount A 2  of the second cutting edge  4   ii  can be calculated in the same manner as the honing amount A 1 , as shown in  FIG. 8(   c ). 
     Third Embodiment 
     An insert  2 B″ shown in  FIG. 9  is another embodiment of the insert  2 B installed to the cutting tool  1  in accordance with the first embodiment mentioned above. 
     In this case, the same reference numerals are attached to the same structures, as those of the insert  2 B in the first embodiment and a description thereof will be omitted. 
     In the insert  2 B″ in accordance with the third embodiment, a structure of the cutting edge reinforcing means  6  is different from that of the insert  2 B in accordance with the first embodiment. 
     Specifically, an upper surface  11 B of the insert  2 B″ has a rake face  8  extending from the cutting edge  4 B toward the center of the upper surface  11 B. Here, in the present embodiment, the cutting edge reinforcing means  6  can be used as the rake face. 
     In the present embodiment, as shown in  FIGS. 9(   b ) and  9 ( c ), a rake angle α 6  of the cutting edge reinforcing means  6  is smaller than a rake angle α 8  of the rake surface  8 . 
     In accordance with the structure mentioned above, it is possible to enhance a strength of the cutting edge portion to which the great load is applied at a time of cutting as mentioned above, in the cutting edge  4  of the insert  2 B″. Accordingly, it is possible to inhibit the cutting edge portion from being chipped. 
     In this case, the rake angle α 6  of the cutting edge reinforcing means  6  is an angle of gradient of the cutting edge reinforcing means  6  with respect to the lower surface  12 , as shown in  FIG. 9(   b ). Further, the rake angle α 8  of the rake surface  8  can be calculated in the same manner as the rake angle α 6 , as shown in  FIG. 9(   c ). 
     In this case, in the present embodiment, as shown in  FIGS. 9(   b ) and  9 ( c ), a land  7 ′ is provided along a whole periphery of the cutting edge  4 . 
     Fourth Embodiment 
     An insert  2 B″ shown in  FIG. 10  is another embodiment of the insert  2 B installed to the cutting tool  1  in accordance with the first embodiment mentioned above. 
     In this case, the same reference numerals are attached to the same structures, as those of the insert  2 B in the first embodiment and a description thereof will be omitted. 
     In the insert  2 B″ in accordance with the fourth embodiment, a structure of the cutting edge reinforcing means  6  is different from that of the insert  2 B in accordance with the first embodiment. 
     Specifically, a side surface  13 B of the insert  2 B″ has a clearance  9  extending from the cutting edge  4 B toward the lower surface  12 B. 
     Further, the clearance  9  of the insert  2 B″ has a first clearance  91  corresponding to the cutting edge reinforcing means  6 , and second clearances  92  positioned on both sides of the first clearance  91 . 
     In the present embodiment, as shown in  FIGS. 10(   b ) and  10 ( c ), a clearance angle β 91  of the first clearance  91  is smaller than a clearance angle β 92  of the second clearance  92 . 
     In accordance with the structure mentioned above, it is possible to enhance a strength of the cutting edge portion to which the great load is applied at a time of cutting as mentioned above, in the cutting edge  4 B of the insert  2 B′. Accordingly, it is possible to inhibit the cutting edge portion from being chipped. 
     Here, the clearance angle β 91  of the first clearance  91  is an angle of gradient of the first clearance  91  with respect to the lower surface  12 B, as shown in  FIG. 10(   b ). Further, the clearance angle β 92  of the second clearance  92  can be calculated in the same manner as the clearance angle β 91 , as shown in  FIG. 10(   c ). 
     In this case, in the present embodiment, a land  7 ′ is provided along a whole periphery of the cutting edge  4  as shown in  FIGS. 10(   b ) and  10 ( c ), in the same manner as the insert  2 B″ according to the third embodiment. 
     As mentioned above, in the insert installed to the cutting tool in accordance with four embodiments of the present invention, there is exemplified the insert in which the upper surface  11  is formed as the approximately rectangular shape as mentioned above; however, the structure is not limited to this, but may be made such that the upper surface may be formed as the other shapes such as a rhomboid shape, a triangular shape and the like. Note that, since it is possible to achieve a reduction of a working cost by having a plurality of cutting edges such as the present embodiment, the structure is desirable. 
     Further, in the inserts  2 A and  2 B in accordance with the embodiments mentioned above, there are exemplified the embodiments in which three or four groove portions  5  dividing the cutting edge  4  are formed, but the number of the groove portions  5  dividing the cutting edge  4  may be set to one or more. 
     Further, in the cutting tools in accordance with the embodiments mentioned above, there are exemplified the embodiments in which two kinds of inserts  2 A and  2 B are installed to the holder  3 , but the structure is not limited to this. The kinds of the inserts  2  installed to the holder  3  may be two or more, and three or four kinds of inserts  2  may be installed to the holder  3 . 
     In this case, in the embodiment mentioned above, the inserts  2 A and  2 B are installed directly to the insert pocket  17  of the holder  3 , though the structure may be made such that the inserts  2 A and  2 B may be installed to the insert pocket  17  of the holder  3  via a seat member. In accordance with the structure mentioned above, in the case that the chip of the cutting edge  4  of the insert  2  or the like is generated, it is possible to reduce the chip of the holder  3  portion positioned at the chip portion of the cutting edge  4 . Therefore, it is possible to elongate a service life of the holder  3 . 
     In this case, in the embodiment mentioned above, there is exemplified the embodiment in which the insert  2  is installed to the holder  3  by the fixing screw  16 ; however, the structure is not limited to this, but may be made such that the insert  2  is installed to the holder  3  by another clamp mechanism. 
     Further, in the cutting tool in accordance with the embodiment mentioned above, the face mill cutter is exemplified; however, the present invention is not limited to this, but may be applied to various rolling tools such as an end mill, a side cutter and the like. 
     &lt;Cutting Method&gt; 
     Finally, a description will be given of a cutting method of a workpiece in accordance with an embodiment of the present invention with reference to  FIGS. 11(   a ) to  11 ( d ) by exemplifying the case that the rolling tool (the cutting tool  1 ) mentioned above is used. 
     A cutting method of the workpiece in accordance with the present embodiment includes the following steps (a) to (d). 
     (a) a step of rotating the cutting tool  1  in a direction of an arrow A around the axis S of the holder  3 , as shown in  FIG. 11(   a ). 
     (b) a moving closer step of moving the cutting tool  1  close to a workpiece  100  by moving the cutting tool  1  in a direction of an arrow B, as shown in  FIG. 11(   b ). 
     (c) a step of cutting a surface of the workpiece  100  by bringing the cutting edge  4  of the insert  2  into contact with the surface of the workpiece  100 , and moving the cutting tool  1  in a direction of an arrow C, as shown in  FIG. 11(   c ). 
     (d) a step of keeping the cutting tool  1  away from the workpiece  100  by moving the cutting tool  1  in a direction of an arrow D, as shown in  FIG. 11(   d ). 
     Thus, since the excellent cutting edge strength is provided and the chip discharging property is provided, and the workpiece  100  is worked by using the cutting tool  1  having a long tool service life, as mentioned above, it is possible to achieve an improvement of a working efficiency and a finished surface precision. That is, it is possible to suppress the chip of the cutting tool  4 . As a result, it is possible to stably carry out the cutting work having a high working precision for a long term. 
     In this case, in the step (a) mentioned above, at least one of the cutting tool  1  and the workpiece  100  may be rotated. Further, in the step (b) mentioned above, it is sufficient to relatively move the cutting tool  1  close to the workpiece  100 , for example, the workpiece  100  may be moved close to the cutting tool  1 . Likewise, in the step (d) mentioned above, it is sufficient to relatively keep the workpiece  100  away from the cutting tool  1  for example, the workpiece  100  may be kept away from the cutting tool  1 . In the case of carrying over the cutting work, it is sufficient to repeat the step of bringing the cutting edge  4  of the insert  1  into contact with the different positions of the workpiece  100  while holding a state in which the cutting tool  1  and/or the workpiece  100  is rotated. When the cutting edge in use wears, an unused cutting edge is used by rotating the insert  2  at 90 degree around a center axis of the through hole  15 . 
     The embodiments in accordance with the present invention are exemplified above; however, it goes without saying that the present invention is not limited to the embodiments, but may be optionally structured without departing from the purpose of the invention.