Throw-away tipped drill bit
This application is a continuation of application Ser. No.: 07/658,210, filed on: Feb. 19, 1991, now abandoned. 
    
    
     CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is related to copending application U.S. Ser. No.: 07/649,615, filed on Feb. 1, 1991, now abandoned. 
     FIELD OF THE INVENTION 
     The present invention relates to the structure of a drill bit which is mainly adapted to drilling steel, cast iron or super alloy, and more particularly, it relates to the structure of a high-quality throw-away tipped drill bit, which is excellent in wear resistance and toughness. 
     BACKGROUND INFORMATION 
     A drill bit is a rotating cutting tool for drilling steel products and the like. FIG. 1 shows an exemplary structure of a conventional twist drill bit. The twist drill bit is formed by a cutting portion 1 which is applied to drilling, and a shank 2 which is adapted to discharge chips and is held in a chuck etc. of a cutting machine such as a drilling machine. 
     In general, materials for drill bits are prepared from high-speed steel and cemented carbide. The high-speed steel, which is excellent in toughness but inferior in wear resistance, is improper for high-speed cutting. On the other hand, cemented carbide, which is excellent in wear resistance and tool accuracy characteristics but brittle, may cause breakage when the same is applied in a machine tool having a low rigidity, for example. 
     In order to improve such materials, there has been proposed a structure attained by coating a cutting portion of high-speed steel with hard TiN, a structure attained by brazing a cutting portion of cemented carbide, and the like. 
     In recent years, there has further been proposed a structure attained by brazing different cemented carbide materials (P30 and D30) for improving the wear resistance and toughness as disclosed in Japanese Utility Model Laying-Open No. 58-143115 (1983), or by metallurgically integrating/connecting such different materials as disclosed in Japanese Utility Model Publication No. 62-46489 (1987). A drill bit having a double structure with central and outer peripheral portions has been made of different cemented carbide materials in consideration of the differences between the characteristics required for such drill bit portions, as disclosed in Japanese Patent Laying-Open No. 62-218010 (1987). A method of attaining such a double structure by injection molding has been disclosed in Japanese Patent Laying-Open No. 63-38501 or 38502 (1988). There has also been proposed a structure attained by preparing a material for a drill bit from cermet Ti-base cemented carbide in order to improve the adhesion resistance, as disclosed in Japanese Patent Laying-Open No. 62-292307 (1987). 
     A cutting portion and a shank of a drill bit are exposed to different load conditions during drilling. Therefore, different material characteristics are required for the respective parts or portions of such a drill bit. For example, wear resistance and adhesion resistance are required for a tip of the cutting portion, while toughness for maintaining strength of the tool is required for the shank. As to the tip of the cutting portion, different characteristics are required for central and outer peripheral portions thereof, since these portions are driven at different cutting speeds. 
     When a drill bit with a coated cutting portion for meeting the complicated requirements for the material characteristics, is resharpened for general use, the coating layer is inevitably separated at least from a front flank side, and hence most of the coating effect is lost. 
     On the other hand, a drill bit which is formed by brazing cemented carbide to its cutting portion, cannot be used for deep hole drilling of a hard to cut material. If the shank is made of steel, a significant difference between the thermal expansion coefficients of the steel and of the cemented carbide forming the cutting portion, exists whereby splitting or cracking may easily be caused during brazing. 
     In recent years, a cemented carbide material for the shank of a drill bit, has been brought into a coarse grain or high binder phase state, in order to improve the toughness of the shank. However, the strength of the material is reduced by distortion of the elastic limit. Hence, the shank has a tendency to break during perforating, or due to vibration of a workpiece, an unstable rotation of a cutting machine or the like. 
     While a drill bit formed by inseparably and integrally connecting the cutting portion and the shank, can be repeatedly used by resharpening the cutting portion after a prescribed operating time, the number of such resharpenings is limited, and production costs are increased. Further, sharpness and the tool life of such bits are non-uniform depending on conditions under which the resharpening is performed. In addition, it is necessary to successively and correctly ascertain the length of the resharpened drill bit if the bit is used in a numerically controlled and automated cutting machine tool. Thus, the length of the drill bit must be measured by a complicated operation every time the same has been resharpened. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to solve the aforementioned problems, by providing a drill bit with a throw-away insert forming a drill bit which has an excellent wear resistance, an adhesion resistance, and a shank which has sufficient toughness against breakage, which does not require resharpening for continuous use during the life of the drill bit. 
     In order to attain the aforementioned object, the present drill bit with a throw-away tip comprises a tip insert for cutting a workpiece and a shank to be mounted in a prescribed position of a cutting machine, wherein the insert is disengageably mechanically connected with the shank. At least a cutting edge of the insert is formed of a diamond sintered body, and the shank is made of steel. 
     The diamond sintered body forming at least the cutting edge of the insert preferably contains diamond in a range of at least 70 volume percent and not more than 99 volume percent. 
     The entire insert body need not be formed of a diamond sintered body, rather, a diamond sintered body forming the cutting edge may be bonded onto a base of cemented carbide. Further, only the cutting edge of the insert may be formed of a diamond sintered body or a material prepared by bonding a diamond sintered body onto a base of cemented carbide, and the base is then connected by brazing with the remaining portion of the insert, which is made of cemented carbide. In other words, at least the cutting edge of the insert must be formed of a diamond sintered body. 
     Characteristics required for a drill bit are roughly classified into wear and adhesion resistance for an insert, and breakage resistance, which is provided by the toughness of a shank. According to the present invention at least the cutting edge of the insert is formed of a diamond sintered body, whereby the insert is improved in wear resistance, adhesion resistance and toughness, because diamond is extremely hard and has an excellent high thermal conductivity as compared with WC and Al 2  O 3 , which are generally employed as main components for tool materials. A diamond sintered body is also excellent in toughness. Thus, the present drill bit is suitable for cutting materials such as cemented carbides, an Al alloy containing a large amount of Si, ceramics, a copper alloy, carbon, and the like. 
     At least the cutting edge of the diamond sintered body forming the insert, preferably contains diamond in a range of at least 70 volume percent and not more than 99 volume percent since it is impossible to sufficiently improve the wear resistance if the diamond content is less than 70 volume percent, while the degree of sintering deteriorates if the content exceeds 99 volume percent. 
     The shank, which is made of steel, has an excellent toughness and breakage resistance. Further, the material costs have been reduced. 
     In the drill bit according to the present invention, the insert and the shank are disengageably mechanically connected with each other, whereby it is possible to easily disengage and throw away the insert, when it has been worn out. 
     In another aspect of the present invention, the drill bit comprises an insert of which at least the cutting edge is formed of a cubic boron nitride sintered body, and the shank is made of steel. 
     The cubic boron nitride sintered body forming at least the cutting edge of the insert preferably contains cubic boron nitride in a range of at least 40 volume percent and not more than 80 volume percent. 
     The insert need not be formed entirely of a cubic boron nitride sintered body, but at least the cutting edge must be made of cubic boron nitride, whereby a cubic boron nitride sintered body forming the cutting edge or a portion of the insert including the cutting edge is bonded onto a base of cemented carbide. 
     The remaining portion of the insert, which is made of cemented carbide, is connected to the cutting edge or cutting edge portion by brazing. In other words, at least the cutting edge of the insert may be formed of a cubic boron nitride sintered body. 
     The insert and the shank of the present drill bit are connected with each other by a self-grip connection to be described below. 
     By forming at least the cutting edge of the insert as a cubic boron nitride sintered body, the wear resistance, the adhesion resistance, and the toughness are improved, because cubic boron nitride is extremely hard and has an excellent thermal conductivity as compared with WC and Al 2  O 3 , which are generally employed as main components for tool materials. A cubic boron nitride sintered body is also excellent in toughness. Further, such an insert formed as cubic boron nitride sintered body can be used for drilling ferrous materials, particularly very hard steel such as hardened steel, since the insert does not contain any carbon. In addition, the cubic boron nitride sintered body is also capable of cutting an Ni-group or Co-group heat resistant alloy, which is regarded as a hard to cut material, and a ferrous sintered component. 
     The cubic boron nitride sintered body which is employed as a material for the insert, preferably contains cubic boron nitride in a range of at least 40 volume percent and not more than 80 volume percent since it is impossible to sufficiently improve the wear resistance if the content is less than 40 volume percent. The continuity between particles of cubic boron nitride structurally deteriorates if the CBN content exceeds 80 volume percent, whereby the toughness is reduced. 
     The present inserts have an excellent wear resistance, adhesion resistance, and thermal cracking resistance or chipping resistance. The shank has an excellent toughness. The costs have been reduced. Thus, it is possible to provide a drill bit with a throw-away tip having a high reliability, a long life, and a high quality. Chipping of the insert and abrupt breakage of the shank are avoided at a low cost. The insert exchange is easy and resharpening is not required for a continuous use during the life of the insert, whereby the drill bit length remains constant and sharpness is maintained during a prolonged bit life. Repeated length measurements are avoided. 
     The blank for the insert can be formed by injection molding, whereby it is possible to easily form a chip breaker or the like, to further reduce the working cost. 
     These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a structural diagram showing a conventional twist drill bit; 
     FIG. 2A is a front elevational view showing a two-flute screwed throw-away tipped drill bit of the present invention; 
     FIG. 2B is a right side elevational view of the drill bit shown in FIG. 2A; 
     FIG. 2C is a perspective view on an enlarged scale showing an insert for use in FIGS. 2A and 2B; 
     FIG. 3 is an exploded perspective view showing a throwaway tipped drill bit of the invention with a two-flute screwed system; 
     FIG. 4 shows, on an enlarged scale an exploded perspective view showing a connection part and insert of a self-grip type drill bit of the invention; 
     FIG. 5A is a front elevational view of the assembled drill bit shown disassembled in FIG. 4; 
     FIG. 5B is a right side elevational view of the assembled bit; 
     FIG. 6A is a sectional view taken along the line 6 - 6 in FIG. 5A, for illustrating an initial press fitting of an insert into a shank; 
     FIG. 6B is a sectional view similar to that of FIG. 6A but illustrating the completion of the press fitting; 
     FIG. 7 is a sectional view illustrating another example embodiment of an insert connection part provided with slits on both sides of the holding portions of the connection part. 
     FIG. 8A is a perspective view showing an exemplary insert for drill bits of the invention, only the cutting edge of which is formed of a diamond or cubic boron nitride sintered body; 
     FIGS. 8B and 8C are sectional views illustrating two examples of a portion around the cutting edge shown in FIG. 8A; and 
     FIG. 9 is a perspective view showing an exemplary insert for drill bits of the invention having a surface entirely covered with a diamond film. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Examples of the present invention are now described. 
     An insert and a shank of the throw-away tipped drill bit according to the invention are connected with each other without screws as shown in FIGS. 4, 5A, 5B, 6A, 6B and 7. FIGS. 2A to 2C show a two-flute throw-away tipped drill bit, wherein inserts 11a and 11b are fixed by screwing to outer and inner peripheral portions of a forward end of a shank 12 respectively. FIG. 3 shows a typical example of a throw-away tipped drill bit of a two-flute system, wherein an insert 21 is fitted in a shank 22 as shown by an arrow. The insert 21 is fixed by a screw 23 in a screw hole 24. A coolant supply hole 25 directly supplies a coolant to the tip of the insert 21. Further, a chip breaker 26 is provided on the cutting edge of the insert 21 for separating chips. 
     In a drill bit shown in FIGS. 4, 5A and 5B, an insert 31 is fitted in a shank 32 as indicated by an arrow shown in FIG. 4, so that the insert 31 and the shank 32 are connected with each other without any screw or the like, in a so-called self-grip connection shown in FIGS. 5A and 5B. In this self-grip connection side walls of a held portion 31a of the insert 31 are brought into contact with inner end surfaces of holding portions 33a and 33b of the shank 32 with a frictional force fit, whereby the insert 31 is fixed to the shank 32. 
     At least one of the holding portions 33a and 33b is able to elastically yield radially outwardly relative to a central longitudinal axis of the shank 32 by reason of a slit 34 extending axially between the holding portion 33a and a rigid axially A corresponding slit not shown may be provided between the holding portion 33b and a rigid axially extending shank section 32b. 
     With reference to FIGS. 6A and 6B, the state of such connection of the insert 31 with the shank 32 caused by engagement therebetween is explained as follows: Before the insert 31 is engaged with the shank 32, an angle θ 1  formed by the left and right side walls of the held portion 31a is slightly greater than an angle θ 2   formed by the opposite inner end surfaces of the holding portions 33a and 33b. When the insert 31 is press-fitted in the shank 32, the angle θ 2  is gradually expanded by wedge action due to tapering of the left and right side walls of the held portion 31a and due to the slit 34 provided on the holding portion 33a as shown in FIG. 4. When θ 1  &gt;θ 2 , the held portion 31a is in contact with the holding portion 33b only at an upper end of the inner end surface of the holding portion 33a. When θ 2  reaches θ 1 , the contact areas between both side walls of the held portion 31a and the inner end surface of the holding portion 33a, are maximized as shown in FIG. 6B. The press fitting operation is stopped in this state, and the holding portion 33a is elastically deformed to cause a pressing force in the contact surfaces of the holding portions and the held portion 31a , so that the insert 31 is connected with or fixed to the shank 32 by frictional force between the contact surfaces. 
     At least one coolant supply hole 35 shown in FIG. 4 directly supplies a coolant to the cutting edge of the insert 31. Further, a chip breaker 36 is provided on the cutting edge of the insert 31, in order to part chips also as shown in FIG. 3 at 25. 
     FIG. 7 shows another example of a shank 32, which is provided with two slits 34, as shown in FIG. 4, not only in a holding portion 33a, but also in another holding portion 33b. In this case, an insert 31 is press-fitted in the shank 32 so that both holding portions 33a and 33b are simultaneously expanded, thereby holding a held portion 31a by an elastic force. FIG. 7 also shows a second cut-back groove 33d mirror-symmetrical relative to the groove 33c and relative to a central longitudinal axis of the shank 32, whereby a symmetrical elastic expansion of the holding portions 33a, 33b is assured. 
     The just described throw-away tipped drill bit according to a first Example of the present invention, is formed by making the insert 31 of a material obtained by bonding a diamond sintered body onto a base of cemented carbide which is disengageably mechanically connected with a shank 32 made of steel as shown in FIG. 4. 
     Table 1 shows composition values of various powder materials for diamond sintered bodies forming insert samples A, B, C, and D of the invention and comparative samples E and F. Table 1 also shows steel materials for shanks of the inventive and comparative samples. The samples A, B, C, and D and the comparative samples E and F shown in Table 1, have the inserts connected to the shanks as shown in FIG. 4. 
     Sample D of the present samples A to D is substantially within the scope of the present invention, but the content (volume percent), shown by marks **, of diamond is considerably below a preferable range. The material for the insert of the comparative sample E is out of the scope of the present invention since the cutting edge of its insert is formed of coated cemented carbide. In the comparative sample F, the material of the cutting edge of the insert is made of Si 3  N 4 , which is out of the scope of the present invention. Symbol * shows materials which are out of the scope of the present invention. 
     
                       TABLE 1                                                     
______________________________________                                    
Sample   Composition of Insert (vol. %)                                   
                                Shank                                     
______________________________________                                    
Example of                                                                
the Invention                                                             
A        Diamond Powder (92), WC(7), Co (1)                               
                                SCM435                                    
B        Diamond Powder (88), WC (containing                              
                                SKS2                                      
         15 wt. % of Co)                                                  
C        Diamond Powder (93), TiC (5), Co (2)                             
                                SKH51                                     
D        **Diamond Powder (65), WC (30),                                  
                                SCM435                                    
         Co (5)                                                           
Comparative                                                               
Example                                                                   
E        Cutting Edge: Coated Cemented                                    
                                SCM435                                    
         Carbide (P30-Grade Cemented                                      
         Carbide Coated with TiN Film)                                    
F        *Cutting Edge: Si.sub.3 N.sub.4 Ceramics                         
                                SCM435                                    
______________________________________                                    
 SCM435 Chromium Molybdenum Steels According to Japanese Industrial       
 Standard (JIS G 41051979)                                                
 SKS2 Alloy Tool Steels (JIS G 4404  1983)                                
 SKH 51 High Speed Tool Steels (JIS G44031983)                            
 
    
     Performance evaluation tests for the aforementioned samples were made on drill bits having a diameter of 8 mm, under the following conditions: 
     Workpiece: AC4C 
     Cutting Speed: 120 m/min., dry type (water-soluble cutting oil) 
     Feed Rate: 0.10 mm/rev. 
     Depth of Cut: 25 mm 
     Criterion: Tip conditions etc. were observed after working up to end of life. 
     Life: Generally regarded as ended when the outer peripheral flank was worn by at least 0.2 mm. 
     Table 2 shows the results of the aforementioned 15 performance evaluation tests. Excellent results were obtained for samples A to C of the invention as compared with comparative samples E and F. The present sample D, the number of drilled holes that could be drilled is lower than for samples A, B, and C. The depth of adhesive wear of the inner peripheral face was increased for sample D with respect to the same amount of outer peripheral front flank abrasion, since the amount of diamond contained in the diamond sintered body forming the insert was less than the preferable range of 70 to 99 volume percent. 
     
                       TABLE 2                                                     
______________________________________                                    
                                  Depth of                                
                                  Adhesive                                
                       Abrasion   Wear of                                 
              Number   of Outer   Inner                                   
              of       Peripheral Peripheral                              
              Worked   Front Flank                                        
                                  Face                                    
Sample        Holes    (mm)       (mm)                                    
______________________________________                                    
Sample of the Invention                                                   
A             3500     0.2        0.01                                    
B             4300     0.2        0.01                                    
C             4700     0.2        0.01                                    
D             1520     0.2        0.10                                    
Comparative Sample                                                        
E              840     0.2        0.15                                    
                       (significantly                                     
                       chipped)                                           
F             1210     test stopped due to                                
                       cutting edge chipping                              
______________________________________                                    
 
    
     A second Example of the present invention will now be described. 
     The samples of throw-away tipped drill bits of the second example of the invention were formed by inserts and shanks of the same materials as those for the sample A of the first Example, to compare cutting characteristics of three types of connection systems. The self-grip drill bit shown in FIGS. 4 to 7 corresponds to sample G. The two-flute screwed drill bit shown in FIGS. 2A to 2C corresponds to sample H. The two-flute drill bit shown in FIG. 3 corresponds to sample I. A comparative sample of a brazed drill bit having an insert of cemented carbide, which was out of the scope of the present invention corresponds to Sample J. The cutting conditions were as follows: 
     Workpiece: AC4C 
     Cutting Speed V: 50 m/min. and 150 m/min. (water-soluble cutting oil) 
     Feed Rate: 0.1 mm/rev. 
     Depth of Cut: 40 mm 
     Diameter of Worked Hole: 20 mm 
     Table 3 shows the results of the evaluated characteristics. 
     Cutting characteristics, represented by stability, can be regarded excellent since horizontal load components, thrust and torque load values of a cutting balance acting on the drill bits by the cutting resistance are reduced and the values depending on speed are also reduced. 
     From the results obtained for the samples of the invention, it is understood that the self-grip system of the sample G attained the most excellent cutting characteristics among the connection systems for the present throw-away tipped drill bits. 
     
                                           TABLE 3                                 
__________________________________________________________________________
                     Compared Characteristics                             
                     Cutting Balance     Hole                             
                     (Horizontal         Diameter                         
                     Component)                                           
                             Thrust  Torque                               
                                         Over Size                        
                     (N)     (N)     (J) (μm)                          
                     Cutting Speed V (m/min.)                             
Sample               50 150  50 150  50  150    Remarks                   
__________________________________________________________________________
Example of                                                                
the Invention                                                             
G      Self-Grip Type                                                     
                     100                                                  
                         90  1200                                         
                                1200 1500                                 
                                         30     Throw-Away Type           
                                                Shown in FIG. 4.          
H      Two-Flute Screwed Type                                             
                     800                                                  
                        320  1600                                         
                                1600 1750                                 
                                         200    Throw-Away Type           
       with two inserts                         Shown in FIG. 2A to 2C    
I      Two-Flute Screwed Type                                             
                     240                                                  
                        Unmea-                                            
                             5200                                         
                                Unmea-                                    
                                     2050                                 
                                         35     Throw-Away Type           
       with One Insert  surable surable         Shown in FIG. 3.          
                                                Insert Departed at V =    
                                                150                       
Comparative                                                               
Example                                                                   
J      Brazed Type with Cutting                                           
                      80                                                  
                        Unmea-                                            
                             2000                                         
                                Unmea-                                    
                                     2900                                 
                                         30     Helical Flute, Cross      
       Edge of Cemented Carbide                                           
                        surable surable         Thinning, Resharpening    
                                                Available 5 Times.        
                                                Incapable of Parting      
                                                Chips and Broken at V =   
                                                150                       
__________________________________________________________________________
 (Note) Materials for inserts and shanks of present samples G, H and I are
 identical to those of sample A in Table 1.                               
 
    
     The self-grip type throw-away tipped drill bit shown in FIGS. 4 to 7 was embodied in sample G of the invention, whereby the insert is fixed to the shank through elastic force exerted by the steel shank, which is provided with at least one slit 34 in the shank as described above. Thus, this drill bit requires no separate fastener means such as screw means. Therefore, it is possible to manufacture for the first time a drill bit with a throw-away tip having a diameter exceeding 10 mm, which has not been implemented by conventional screw mountins due to an inferior fastening strength, causing breaking of the mounting screws and the like. Further, the use of the present drill bits is improved since no screwing is required for connecting the insert to the shank in the work place. Rather, the drill bit can be assembled by simply press-fitting the insert into the shank. 
     Example 3 of the present invention will no be described. 
     A drill bit with a throw-away insert according to the third Example of the present invention is formed by preparing an insert from a material obtained by bonding a cubic boron nitride sintered body onto a base of cemented carbide and disengageably mechanically connecting the insert with a shank of steel as shown in FIGS. 4 to 7. 
     Table 4 shows composition values of various powder materials for cubic boron nitride sintered bodies forming inserts of the invention and comparative samples. Table 4 also shows steel materials for shanks of the invention and comparative samples. In all the samples shown in Table 4, the inserts and shanks are connected with each other as shown in FIG. 4. Samples N and 0 of the samples K to 0 are substantially within the scope of the present invention but the contents (volume percent), shown by marks **, of cubic boron nitride power materials are out of the preferred range of 40 to 80 volume percent. The comparative sample P is out of the scope of the invention because its shank is made of K30-grade cemented carbide, althouth its insert is formed of a cubic boron nitride sintered body. 
     
                       TABLE 4                                                     
______________________________________                                    
Sample   Composition of Insert (vol. %)                                   
                               Shank                                      
______________________________________                                    
Sample of                                                                 
the Invention                                                             
K        Cubic Boron Nitride Powder (60)k                                 
                               SCM435                                     
         TiC (40)                                                         
L        Cubic Boron Nitride Powder (75),                                 
                               SKS2                                       
         TiN (25)                                                         
M        Cubic Boron Nitride Powder (55),                                 
                               SK2                                        
         WC (45)                                                          
N        **Cubic Boron Nitride Powder (90),                               
                               SCM435                                     
         TiN (10)                                                         
O        **Cubic Boron Nitride Powder (35),                               
                               SK2                                        
         TiN (40), WC (25)                                                
Comparative                                                               
Sample                                                                    
P        Cubic Boron Nitride Powder (60),                                 
                               K30-Grade                                  
         TiC (40)              Cemented                                   
                               Carbide                                    
______________________________________                                    
 SCM435 Chromium Molybdenum Steels According to Japanese Industrial       
 Standard (JIS G41051979)                                                 
 SKS2 Alloy Tool Steels (JIS G 44041983)                                  
 SK2 Alloy Tool Steels (JIS G 44041983)                                   
 
    
     Performance evaluation tests for the aforementioned samples were made with drill bits of 12 mm in diameter, under the following conditions: 
     Workpiece: SNCM420 (H RC  =50) 
     Cutting Speed: 100 m/min., dry type (water-soluble cutting oil) 
     Feed Rate: 0.15 mm/rev. 
     Depth of Cut: 20 mm 
     Criterion: Tip conditions etc. were observed after working up to end of the tip life. 
     Life: Generally regarded as ended when the outer peripheral flank was worn by at least 0.2 mm. 
     Table 5 shows the results of the aforementioned 15 performance evaluation tests. Excellent results were obtained for the samples K to M of the invention. The present samples N and 0 are somewhat inferior in wear resistance and chipping resistance since the contents of the cubic boron nitride in the sintered bodies forming the inserts are out of the preferred range of 40 to 80 volume percent with respect to bonding phases of cemented carbide. For the purpose of reference, similar experiments were made for four types of conventional drill bits as shown in the lower part of Table 5. It is understood from the results that the samples K to M of the invention are particularly superior compared to the reference samples. 
     
                       TABLE 5                                                     
______________________________________                                    
                                   Depth                                  
                                   of Wear                                
                        Abrasion   of Inner                               
               Number   of Outer   Peripheral                             
               of       Peripheral Cutting                                
               Worked   Front Flank                                       
                                   Face                                   
Sample         Holes    (mm)       (mm)                                   
______________________________________                                    
Sample of the Invention                                                   
K              2680     0.2        0.02                                   
L              2410     0.2        0.03                                   
M              2840     0.2        0.01                                   
N              1200     0.2        0.02                                   
                        (significantly                                    
                        chipped)                                          
Comparative Samples                                                       
O               840     0.2        0.05                                   
P               670     test stopped due to                               
                        shank breakage                                    
Reference Example                                                         
Cutting Edge: Coated                                                      
                140     0.2        0.11                                   
Cemented Carbide                                                          
Cutting Edge: Al.sub.20 3                                                 
                460     test stopped due to                               
                        significant cutting                               
                        edge chipping                                     
Integrated Coated High-                                                   
                 0      cutting edge                                      
Speed Steel             extremely worn                                    
Integrated Coated                                                         
                66      test stopped due to                               
Cemented Carbide        shank breakage                                    
______________________________________                                    
 
    
     A fourth Example of the present invention will now be described. 
     In this Example, the present samples were formed by inserts and shanks of the same materials as those for the sample K of the third Example, to compare the cutting characteristics with regard to three types of connection for the self-grip drill bit shown in FIG. 4, sample Q. The two-flute drill bit shown in FIGS. 2A to 2C represents sample R. The two-flute drill bit shown in FIG. 3 represents sample S. A comparative sample of a brazed drill bit, having an insert of cemented carbide, which was out of the scope of the present invention is represented by sample T. The cutting conditions were as follows: 
     Workpiece: S50C, H B  =220 
     Cutting Speed V: 50 m/min. and 150 m/min. (water-soluble cutting oil) 
     Feed Rate: 0.2 mm/rev. 
     Depth of Cut: 40 mm 
     Diameter of Worked Hole: 20 mm 
     Table 6 shows the results of the evaluated characteristics. 
     Cutting characteristics, represented by stability, can be regarded excellent since horizontal load components, thrusts and torque load values of a cutting balance acting on the drill bits by the cutting resistance are reduced and the values depending on speed are also reduced. 
     From the results obtained for the samples of the invention, it is understood that the self-grip system of the sample Q attained the most excellent cutting characteristics among the connection systems for the present throw-away tipped drill. 
     
                                           TABLE 6                                 
__________________________________________________________________________
                     Compared Characteristics                             
                     Cutting Balance      Hole                            
                     (Horizontal          Diameter                        
                     Component)                                           
                              Thrust  Torque                              
                                          Over Size                       
                     (N)      (N)     (J) (μm)                         
                     Cutting Speed V (m/min.)                             
Sample               50  150  50 150  50  150   Remarks                   
__________________________________________________________________________
Samples of                                                                
the Invention                                                             
Q      Self-Grip Type                                                     
                     190 180   800                                        
                                  185 3300                                
                                          20    Throw-Away Type           
                                                Shown in FIG. 4.          
R      Two-Flute Screwed Type                                             
                     1610                                                 
                         590  1980                                        
                                 2050 3800                                
                                          200   Throw-Away Type           
       With Two Inserts                         Shown in FIG. 2A to 2C    
S      Two-Flute Screwed Type                                             
                     400 Unmea-                                           
                              7200                                        
                                 Unmea-                                   
                                      4200                                
                                          30    Throw-Away Type           
       With One Insert   surable surable        Shown in FIG. 3.          
                                                Insert Departed at V =    
                                                150                       
Comparative                                                               
Sample                                                                    
T      Brazed Type with Cutting                                           
                     150 Unmea-                                           
                              2700                                        
                                 Unmea-                                   
                                      6200                                
                                          20    Helical Flute, Cross      
       Edge of Cemented Carbide                                           
                         surable surable        Thinning, Resharpening    
                                                Available 5 Times.        
                                                Incapable of Parting      
                                                Chips and Broken at V =   
                                                150                       
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 (Note) Materials for inserts and shanks of present samples G, H and I are
 identical to those of sample A in Table 1.                               
 
    
     In first to fourth Examples, at least the cutting edge of each insert is made of a diamond or cubic boron nitride sintered body, specifically as shown in FIG. 8A. For example, an insert cutting edge 43 of a diamond or cubic boron nitride sintered body is attached to an insert base 42, which is made of cemented carbide or steel to form the insert 31. In order to attain such a structure, a cutting edge piece forming an insert tip 43 of a diamond or cubic boron nitride sintered body is connected to a tip base 44 of cemented carbide by brazing. The tip base 44 in turn is secured in a groove provided in the insert base 42 also by brazing as shown in FIGS. 8B and 8C. 
     With the present structure it is possible to use a hard diamond sintered body or cubic boron nitride sintered body as a material for making only the cutting edge of the insert for which high wear resistance and hardness are required and to use such a strong material as cemented carbide or steel as a material for the main body of the insert. These features prolong the life of the present drill bits. 
     It is also possible to attain a similar function or effect by making an insert 31 entirely of cemented carbide or ceramics and coating the overall surface thereof with a diamond film of less than 20 μm in thickness by vapor phase synthesis. 
     The method of manufacturing inserts wherein a diamond coating is applied is more preferable, because such inserts can be produced at a low cost by injection molding, whereby complicated figures can be easily formed. 
     As a result of a performance evaluation test under the same cutting conditions as the first embodiment, after drilling 3200 holes, the abrasion of the outer peripheral front flank was 0.2 mm with a slight chipping and the depth of wear of the inner peripheral cutting face was less than 0.01 mm, which was nearly equal with the samples A to C. 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.