Abstract:
A cutting insert incorporates a pre-formed discrete cutting tip which wraps around a curved cutting corner of the insert. The cutting tip includes a top layer and a bottom layer made of a relatively harder material than the top layer. In one embodiment, the bottom layer is made of polycrystalline diamond (PCD) or a polycrystalline cubic boron nitride (CBN) material and the top layer is made of cemented carbide. The cutting insert may further incorporate a chip control structure formed in the cutting tip including a plurality of facets formed on each side of a centerline for providing chip control during a cutting operation. The chip control structure can be formed in a two-step process to expose a portion of the bottom layer. The discrete cutting tip can be brazed to the cutting insert prior to forming the chip control structure.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    It is a continuing goal in the metal cutting industry to provide metal cutting inserts with sharp cutting edges that stay sharp longer even when machining non-ferrous and abrasive materials. To this end, it has been known to incorporate polycrystalline diamond (PCD) materials and polycrystalline cubic boron nitride (CBN) materials on a carbide substrate to provide an improved cutting edge. However, a significant portion of the cutting tip is embedded within the slot and this embedded portion acts only as structural material to retain the tip within the slot and does not contact the workpiece. 
         [0002]    The cost of PCD or CBN materials used as a substrate is very high as compared to the cost of cemented carbide. One attempt to reduce cost is to provide a cutting insert that incorporates a discrete cutting tip that wraps around the nose of a cutting insert and having a top layer made of PCD or CBN material that is supported by a bottom layer of cemented carbide substrate. It has been found, however, that although the top layer of PCD or CBN material has sufficient thickness for allowing a chip breaker to be ground therein, the thickness of the top layer of PCD or CBN material is insufficient for machining operations, thereby reducing the machine life of the cutting insert. 
       SUMMARY OF THE INVENTION 
       [0003]    The problem of reduced machine life of a cutting insert with a discrete tip made of a top layer of relatively harder material and a bottom layer of relatively softer material is solved by reversing these materials and providing a cutting insert with a discrete tip having a top layer made of a relatively softer material and a bottom layer made of a relatively harder material, and then removing material from the top layer to expose a portion of the relatively harder bottom layer to form the cutting edge. 
         [0004]    In one aspect, a cutting insert comprises a body having upper face, a base face, at least two side walls joining the upper face and the lower face, a curved cutting corner at the intersection of the at least two side walls, and a cutting edge formed at the intersections of each side wall and the upper face; a recess formed in the curved cutting corner; and a cutting tip mounted in the recess, the cutting tip including of a top layer made of a first material, and a bottom layer disposed adjacent a floor of the recess and made of a second material having a greater hardness than the first material. 
         [0005]    In another aspect, a method of fabricating a cutting insert having a cutting tip with a chip control structure defined by a plurality of facets, the cutting tip including of a top layer made of a first material, and a bottom layer disposed adjacent a floor of the recess and made of a second material having a greater hardness than the first material, the method comprising:
       removing material from the top layer at a first angle, A 1 , to expose the bottom layer; and   removing material from the top layer at a second angle, A 2 , to further expose the bottom layer.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    While various embodiments of the invention are illustrated, the particular embodiments shown should not be construed to limit the claims. It is anticipated that various changes and modifications may be made without departing from the scope of this invention. 
           [0009]      FIG. 1  is an exploded an isometric view of a cutting insert in accordance with an aspect of the invention; 
           [0010]      FIG. 2  is an enlarged, partial view of the cutting insert shown in  FIG. 1  in accordance with an aspect of the invention; 
           [0011]      FIG. 3  is a top view of  FIG. 2 ; and 
           [0012]      FIG. 4  is a cross-sectional view taken along line  4 - 4  of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]    Referring now to  FIGS. 1-4 , a cutting insert is shown generally at  10  according to an embodiment of the invention. In general, the cutting insert  10  has a polygonal body  12  with a central axis A extending therethrough. The body  12  has an upper face  14 , a substantially planar base face  16 , and a plurality of side walls  18  perpendicular to and joining the faces  14  and  16 . For the specific embodiment shown in the figures, a central pin receiving hole  17  is provided through the insert  10  for retention of the insert within a tool holder (not shown). In another embodiment, the cutting insert  10  does not include a hole therein for securing the cutting insert to a tool. Rather, the cutting insert  10  is retained in the tool by a clamping mechanism, which securely retains the cutting insert within the tool. Curved cutting corners  20  are respectively at the intersections of the side walls  18 , while cutting edges  22  are respectively formed at the intersections of each side wall  18  and a T-land  34 . In the event the cutting insert  10  does not have a T-land, the cutting edges  22  are formed at the intersection of each side wall  22  and a recessed land  38 . 
         [0014]    The body  12  may be made of a suitable material, such as cemented carbide, and the like, and may be fabricated by using any number of conventional processes known in the art, such as molding, and the like, such that a recess, shown generally at  24 , is molded into the insert body  12 . In the alternative, the recess  24  may be generated after the insert body  12  is fabricated by any number of commercial techniques for removing material from a cemented carbide substrate. The recess  24  is formed in each curved cutting corner  20  and extends between two adjacent side walls  18 . As illustrated in  FIG. 1 , the recess  24  has a floor  24   a , a back wall  24   b , and end walls  24   c ,  24   d.    
         [0015]    One aspect of the invention is that the cutting insert  10  includes a pre-formed discrete cutting tip, shown generally at  26 . The cutting tip  26  may be comprised of a bottom layer  28  made of a suitable material, such as polycrystalline diamond (PCD) or polycrystalline cubic boron nitride (PCBN) and a top layer  30  made of a suitable material, such as cemented carbide (line  39  is the separation line between the bottom layer  28  and the top layer  30 ). PCD is a common material used for non-ferrous applications, whereas PCBN is a common material used for hardened steel applications. It is known by those skilled in the art that PCD and PCBN has a greater hardness than cemented carbide. The bottom layer  28  defines a bottom surface  28   a  of the cutting tip  26  disposed adjacent the floor  24   a  of the recess  24 , and the top layer  30  defines a top surface  30   a  of the cutting tip  26  (and also a portion of the top surface  14  of the cutting insert  10 ). The bottom surface  28   a  substantially corresponds in shape to the floor  24   a  of the recess  24  such that the cutting tip  26  mates with the recess  24 . 
         [0016]    The cutting tip  26  can be fabricated by using any number of conventional processes known in the art, such as molding, and the like. In one embodiment, the top layer  30  made of cemented carbide material is molded and the bottom layer  28  made of PCD or CBN material is deposited on the top layer  30 , such as by vapor deposition, and the like. The tip  26  may be secured within the recess  24  of the cutting insert  10  by brazing, and the like. The cutting tip  26  forms a portion of the curved cutting corner  20  when secured within the recess  24 . 
         [0017]    In another aspect of the invention, the cutting insert  10  includes a chip control structure, shown generally at  32 , formed in the upper face  14  and extends continuously along the cutting edges  22 . In one aspect of the invention, the chip control structure is formed at least substantially in or on the cutting tip  26 . In the illustrated embodiment, the cutting insert  10  is symmetrical about a centerline or bisector line X-X of the curved cutting corners  20  of the cutting insert  10 . In one embodiment, the chip control structure  32  includes a plurality of facets  32   a ,  32   b ,  32   c  and  32   d  that are formed on opposing sides of a centerline X-X of the curved cutting corners  20  of the cutting insert  10 . More specifically, the facets  32   a  and  32   b  are formed on one side of the centerline X-X, while facets  32   c  and  32   d  are formed on the opposite side of the centerline A-A. Alternatively, the chip control structure  32  may be formed as a continuously curved surface, rather than the faceted surface of the illustrated embodiment. 
         [0018]    Although the form of the facets  32   a ,  32   b ,  32   c  and  32   d  may be any of a variety of forms known in the art to promote the formation of chips,  FIG. 4  illustrates an embodiment in which the facets  32   a ,  32   b ,  32   c  and  32   d  are formed at a facet angle  36  with respect to the top surface  14  of the insert body  12 . In particular, the facet angle  36  may have a range between about five (5) degrees and about forty-five (45) degrees. In the illustrated embodiment, the facets  32   a ,  32   b ,  32   c  and  32   d  are formed in a T-land  34  that extends at an angle  35  between the upper face  14  and the side wall  18 . In one embodiment, the angle  35  may be in a range between about five (5) degrees and about forty-five (45) degrees. For example, the angle  35  may be about twenty-five (25) degrees. The facets  32   a ,  32   b ,  32   c  and  32   d  terminate in the recessed land  38  formed by exposing the bottom layer  28  during a two-step process explained below. 
         [0019]    As shown in  FIG. 3 , a line, L 1 , extends along the intersection between the facets  32   a ,  32   d  and the top surface  14  of the cutting insert  10 . Similarly, a line, L 2 , extends along the intersection between the facets  32   b ,  32   c  and the top surface  14  of the cutting insert  10 . A angle, A 1 , is defined between the centerline X-X and the line, L 1 , and a angle, A 2 , is defined between the centerline X-X and the line, L 2 . As seen in  FIG. 4 , the angle, A 2 , is greater than the angle, A 1 . However, both angles, A 1  and A 2 , are less than ninety degrees. For example, angle, A 1 , may be about five (5) degrees, and angle, A 2 , may be about twenty-two (22) degrees. 
         [0020]    In another aspect of the invention, the facets  32   a ,  32   b ,  32   c ,  32   d  may begin below the upper face  14  and slope downwardly toward a recessed land  38  in which the bottom layer  28  made of PCD or CBN material is exposed to form the cutting edge  22 . It will be appreciated, that additional facets, i.e. more than the illustrated two facets on each side of the centerline X-X of the curved cutting corner  20  may be provided in accordance with aspects of the invention. Additionally, while a single cutting tip  26  has been discussed with respect to a single curved cutting corner  20 , it should be appreciated that other cutting tips  26  may be associated with the other curved cutting corners  20 , respectively, as shown in  FIG. 1 . 
         [0021]    In addition, a C-shaped cutting insert  10  is shown in  FIGS. 1-4 . However, it will be appreciated that the invention is not limited by a particular shape of the cutting insert. For example, the principles of the invention can be practiced with a D-shaped cutting insert, a V-shaped cutting insert, and the like. Further, the cutting insert  10  of the invention should not be limited by a cutting insert having four side walls. For example, the cutting insert of the invention can have a triangular shape with three side walls, a pentagonal shape with five side walls, a hexagonal shape with six side walls, an octagon with eight side walls, and the like. 
         [0022]      FIGS. 1-4  illustrate the facets  32   a ,  32   b ,  32   c  and  32   d  are formed at an angle  36  with respect to the top surface  14  of the cutting insert  10 . In the illustrated embodiment, the facets  32   a  and  32   d  are formed at the same angle  36 , while the facets  32   b  and  32   c  are formed at a different angle  36  than the facets  32   a  and  32   d . However, it will be appreciated that the invention is not limited by the angle  36  of the facets  32   a ,  32   b ,  32   c  and  32   d , and that the invention can be practiced with the facets  32   a ,  32   b ,  32   c  and  32   d  formed at the same angle  36 . For example, the facets  32   a ,  32   b ,  32   c  and  32   d  can have an undulating profile such that the angle  36  defined in  FIG. 4 , varies along the recess  24 . In this embodiment, the angle  36  may have a value ranging between about 5 degrees to about 45 degrees. Other angles known to those skilled in the art are within the scope of the invention. 
         [0023]    What has so far been described are cutting inserts having two sides which converge to define the radiused or curved cutting corners  20 . However, it is entirely possible for the two sides to be parallel and act as a bridge between these two sides, thereby providing a recess to accept the cutting tip  26 . 
         [0024]    A method of fabricating the cutting insert  10  of the invention will now be described. First, a molding process is performed to form the polygonal body  12  made of cemented carbide with the recesses  24  at each curved cutting corner  20  of the cutting insert  10  in the mold. Each recess  24  has a depth that may be, for example, less than one half of the distance of a straight line connecting the end walls  24   c ,  24   d  of the recess  24 . Alternatively, the polygonal body  12  may be molded, and then material can be removed in a separate step to form the recess  24 . 
         [0025]    Next, the discrete cutting tip  26  is formed by molding the cemented carbide top layer  30 , and then depositing the PCD or CBN material onto the top layer  30 . Then, the cutting tip  26  is positioned within the recess  24 . Next, the cutting tip  26  is secured within the recess  24  by brazing, and the like. 
         [0026]    In accordance with an aspect of the invention, the facets  32   a ,  32   b ,  32   c  and  32   d  of the chip control structure  32  are formed in the cutting insert  10  by a two-step process. In the first step, the facets  32   a  and  32   d  are formed by removing material from the top layer  30  of cemented carbide material at the angle, A 1 , with respect to the the centerline, X-X, to form the facets  32   a  and  32   d  at the angle  36 . The material can be removed by any suitable means, such as grinding, laser, and the like. During the forming of the facets  32   a  and  32   d , a portion of the top layer  30  is removed in such a way that a portion of the bottom layer  28  of PCD or CBN material is exposed to form a first portion of the cutting edge  22 . In the second step, the facets  32   b  and  32   c  are formed by removing material from the top layer  30  of cemented carbide material at the angle, A 2 , with respect to the centerline, X-X, to form the facets  32   b  and  32   c  at the angle  36 , which may or may not be different than the angle  36  of the facets  32   a  and  32   d . During the forming of the facets  32   b  and  32   c , a portion of the top layer  30  is removed in such a way that a second portion of the recessed land  38  is exposed to form a second portion of the cutting edge  22 . 
         [0027]    Throughout this discussion, inserts having a variety of shapes and configurations for holding such inserts have been presented. It should be appreciated any shape of insert may be secured within a toolholder, using any of a number of different configurations, whether it be a hold down screw extending into a bore through the insert body and threaded into the toolholder pocket, or a clamp holding the insert within the pocket of a toolholder. Therefore, the manner of holding inserts, and the modifications to an insert body necessary for holding such inserts discussed herein should not be interpreted as a limitation upon the scope of the invention. 
         [0028]    The patents and publications referred to herein are hereby incorporated by reference. 
         [0029]    Having described presently preferred embodiments the invention may be otherwise embodied within the scope of the appended claims.