Throw-away tipped drill bit
This application is a continuation of U.S. patent application Ser. No. 07/649,615, filed on Feb. 1, 1991, now abandoned. 
    
    
     CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is related to copending application U.S. Ser. No. 07/658,210, filed on Feb. 19, 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 highspeed 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 Nos. 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 hold 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 present invention to provide a throwaway tipped drill bit comprising an insert having an excellent wear resistance, an adhesion resistance, and a shank having a sufficient toughness against breakage, which requires no resharpening for prolonged, uninterrupted use. 
     In order to attain the aforementioned object, the present throwaway tipped drill bit comprises an insert for cutting a workpiece, and a shank to be mounted in a prescribed position of a cutting machine, wherein the insert is disengageably and mechanically connected with the shank. The insert is formed of a silicon carbide whisker reinforced alumina sintered body, and the shank is made of steel. 
     Preferably, the silicon carbide whisker reinforced alumina sintered body forming the insert, is prepared from a sintered body of alumina containing 3.0 to 40 volume percent of silicon carbide whiskers, whereby the aspect ratio of the silicon carbide whiskers is within the range of 3 to 200, whereby this ratio is a length to width, e.g. diameter ratio. 
     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, a material for the insert is prepared from a silicon carbide whisker reinforced alumina sintered body for the following reason: The silicon carbide whisker reinforced alumina sintered body is prepared by introducing silicon carbide (SiC) whiskers into an alumina (Al 2  O 3 ) sintered body. Compared to other ceramic materials, such a sintered body has an extremely high heat resistance and an excellent wear resistance as a tool, thereby providing an improved toughness and an increased impact resistance. This material has a small coefficient of friction with respect to iron, and is superior in wear resistance to a sintered body of silicon nitride (Si 3  N 4 ). Further, an insert made of such a silicon carbide whisker reinforced alumina sintered body is suitable for working of not only workpieces of steel and casting but also a normally uncuttable or hard to cut material such as a heat-resistant Ni alloy. 
     The content of the silicon carbide whiskers is preferably 3.0 to 40 volume percent since the effect of improving the toughness is reduced if the content is less than 3.0 volume percent, while strength is reduced if the content exceeds 40 volume percent. 
     The shank, which is made of steel, is excellent in 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 mechanically disengageable from each other, whereby it is possible to easily disengage and throw away the insert, when it has been worn-out. 
     The present insert has an excellent wear resistance, adhesion resistance, and thermal cracking or chipping resistance. The shank has an excellent toughness. The manufacturing costs have been reduced compared to conventional bits. Thus, it is possible to provide a throw-away tipped drill bit having a high reliability, a long life and a high quality. Chipping of the insert and an 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 sharpeness 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 throw-away tipped drill bit of the invention with a two-flute screwed system; 
     FIG. 4 shows, on an enlarged scale, an explosed 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; and 
     FIG. 7 is a sectional view illustrating another example embodiment of an insert connection part provided with slits on both sides of holding portions of the connection part. 
    
    
     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 extending shank section 32a and due to a cut-back groove 33c. 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 silicon carbide whisker reinforced alumina sintered body 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 silicon carbide whisker reinforced alumina sintered bodies forming inserts of the invention and of comparative samples. Table 1 also shows steel materials for shanks of the invention and for comparative samples. The present shanks and inserts and the comparative shanks and inserts shown in Table 1, are connected with each other respectively, in the manner shown in FIG. 4. Samples A to D are prepared according to the invention, although the sample D has a content (volume percent) of SiC whiskers in excess of a preferably range. In the comparative sample E, the material for the insert is out of the scope of the present invention since sample E does not contain SiC whiskers. In the comparative sample F, the material for the shank is made of a K30-grade cemented carbide material, which is out of the scope of the present invention. 
     
                                           TABLE 1                                 
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         Insert                                                           
Sample   Starting Composition (vol. %)                                    
                                      Shank                               
__________________________________________________________________________
Samples of                                                                
       A SiC Whisker (20), TiC powder (5), Al.sub.2 O.sub.3 powder        
         (Rest)                       SCM435                              
the invention                                                             
       B SiC Whisker (15), MgO (0.5), Y.sub.2 O.sub.3 (3), Al.sub.2       
         O.sub.3 Powder (Rest)        SKS2                                
       C SiC Whisker (30), ZrO.sub.2 (5.0), Al.sub.2 O.sub.3 powder       
         (Rest)                       SCM415                              
       D **SiC Whisker (50), TiN powder (5), Al.sub.2 O.sub.3 powder      
         (Rest)                       SCM435                              
Comparative                                                               
       E *TiC (10), Y.sub.2 O.sub.3 (3), Al.sub.2 O.sub.3 powder          
                                      SCM435                              
Sample F SiC Whisker (20), TiN powder (5), Al.sub.2 O.sub.3 powder        
                                      K30-Grade                           
                                      Cemented                            
                                      Carbide                             
__________________________________________________________________________
 *out of claimed range                                                    
 **out of preferable range                                                
 ##STR1##                                                                 
 SKS2  Alloy Tool Steels (JIS G44041983)                                  
 
    
     Performance evaluation tests for the aforementioned samples were made on drill bits having a diameter of 8 mm, under the following conditions: 
     Workpiece: Inconel 718 
     Cutting Speed: 100 m/min., dry type (water-soluble cutting oil) 
     Feed Rate: 0.12 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 according to the invention. The cutting by the sample D of the invention was stopped after cutting 620 holes, due to heavy damage of the insert, which was caused by the content of Si whiskers in the insert being out of the preferable range of 3.0 to 40 volume percent, as shown in Table 1 by marks **. For the purpose of reference, similar experiments were made as to four types of conventional drill bits as shown in the lower part of Table 2. It is understood from the results that the present samples A to C are superior also to the conventional drill bits. 
     It has been experimentally confirmed that the silicon carbide whisker reinforced alumina sintered body of the present inserts can effectively contain a carbonitride, a carbonized oxide or an oxide of titanium, instead of TiC or TiN. 
     
                                           TABLE 2                                 
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                                Abrasion of Outer                         
                                            Depth of Adhesive Wear        
                         Number of                                        
                                Peripheral Front Flank                    
                                            of Inner Peripheral Face      
Sample                   Drilled Holes                                    
                                (mm)        (mm)                          
__________________________________________________________________________
Samples of                                                                
       A                 1620   0.2         0.02                          
the Invention                                                             
       B                 1550   0.2         0.04                          
       C                 1740   0.2         0.05                          
       D                 620    0.2         0.13                          
Comparative                                                               
       E                 840    test stopped due to insert chipping       
Example                                                                   
       F                 115    test stopped due to shank breakage        
Reference                                                                 
       Tip: Si.sub.3 N.sub.4 Sintered Body                                
                         700    0.2         0.10                          
Example                                                                   
       Tip: Coated Cemented Carbide                                       
                         240    test stopped due to insert chipping       
       Tip: Coated High-Speed Steel                                       
                         150    0.2         0.15                          
       Integrated Coated Cemented Carbide                                 
                          45    test stopped due to shank                 
__________________________________________________________________________
                                breakage                                  
 
    
     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 sample A of the first Example, to compare the cutting characteristics of three types of connection systems. Sample G was a self-grip drill bit shown in FIG. 4. Sample H relates to a two-flute screwed drill bit shown in FIGS. 2A to 2C. Sample I relates to a two-flute screwed drill bit shown in FIG. 3. Sample J is a comparative sample of a brazed drill bit, having an insert of cemented carbide, which was out of the scope of the present invention. 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 3 shows the results of the evaluated characteristics. 
     Cutting characteristics, represented by stability, can be regarded excellent since horizontal load components and thrusts of a cutting balance acting on the drills by the cutting resistance are reduced and values of speed dependency 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                                 
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                   Compared Characteristics                               
                   Cutting Balance       Hole Diameter                    
                   (Horizontal       Torque                               
                                         Over Size                        
                   Component) (N)                                         
                            Thrust (N)                                    
                                     (J) (μm)                          
                   Cutting Speed V (m/min.)                               
Sample             50  150  50  150  50  150     Remarks                  
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Sample                                                                    
     G Self-Grip Type                                                     
                   190 180  1800                                          
                                1850 3300                                 
                                         20      Throw-Away Type          
of the                                           shown in FIG. 4          
Invention                                                                 
     H Two-Flute Screwed                                                  
                   1610                                                   
                       590  1980                                          
                                2050 3800                                 
                                         200     Throw-Away Type          
       Type With Two Inserts                     Shown in FIG. 2A to 2C   
     I Two-Flute Screwed                                                  
                   400 Unmeas-                                            
                            7200                                          
                                Unmeas-                                   
                                     4200                                 
                                          3      Throw-Away Type          
       Type With One Insert                                               
                       urable   urable           Shown in FIG. 3.         
                                                 Insert Departed at V =   
                                                 150                      
Compar-                                                                   
     J Brazed Type with                                                   
                   150 Unmeas-                                            
                            2700                                          
                                Unmeas-                                   
                                     6200                                 
                                         20      Helical Flute, Cross     
ative  Cutting Edge of urable   urable           Thinning, Resharpening   
Sample Cemented Carbide                          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.                               
 
    
     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.