Abstract:
A throw-away tipped drill has an insert removably connected to a shank for cutting a workpiece. The insert (31) and the shank (32) have mutually engaging surfaces respectively, and a slit (34, 39) is formed either in the insert (31) or in the shank (32) for mounting the insert to the shank, whereby the insert (31) is fixed to the shank (32) by an elastic force which is caused by an elastic deformation upon mutual movement of opposite surfaces of this slit (34, 39) in engagement of the insert (31) and the shank (32). Therefore, the insert (31) and the shank (32) are coupled by simple press fitting without any requirement for a screw or other connection, whereby the assembly is improved. The cutting edge of the insert (31) has an excellent wear resistance, adhesion resistance and thermal cracking resistance and the shank (32) has an excellent toughness while the drill is relatively low in costs, by employing surface-coated cemented carbide as the material for the insert while employing steel as the material for the shank.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a drill which is mainly applied to piercing steel, and more particularly, it relates to the structure of and materials for a throw-away tipped drill whose insert portion is separably mechanically connected with a shank portion. 
     BACKGROUND INFORMATION 
     A drill is a cutting tool which can be used for piercing steel products and the like. For example, the structure of a twist drill is shown in FIG. 1. The twist drill is formed by a cutting portion 1 which is applied to piercing, and a shank portion 2 which is not concerned in cutting but is mainly used for discharging chips and to be clamped in a chuck on the shaft of a drilling machine. 
     In general, materials for drills are high-speed steel (rapid tool steel) and cemented carbide. The high-speed steel, which has an excellent toughness but is inferior in its wear resistance, is improper for high-speed cutting. On the other hand, the cemented carbide, which has an excellent wear resistance and accuracy characteristics as a tool but is quite brittle and hence may break when used in a machine tool having low rigidity, for example. 
     In order to improve drills there has been considered a drill structure with a coating of hard TiN on the cutting portion made of high-speed steel, or a drill structure with a cutting portion made of cemented carbide and brazing the same. There have further been devised different connecting structures e.g. by brazing different materials of cemented carbide (P30 and D30) for improving the wear resistance and toughness, (Japanese Utility Model Laying-Open Gazette No. 58-143115) or a structure of metallurgically attaining an integration or connection (Japanese Utility Model Laying-Open Gazette No. 62-46489), or a structure that takes into account the difference between the characteristics required for a central portion and an outer peripheral portion of a drill and preparing the central portion and the outer peripheral portion from different materials of cemented carbide in a double structure (Japanese Patent Laying-Open Gazette No. 62-218010), or a method of forming such a double structure by injection molding (Japanese Patent Laying-Open Gazette No. 63-38501 or 38502). Further, there is a structure of forming the material for a drill as a cermet, in order to improve adhesion resistance of the drill (Japanese Patent Laying-Open Gazette No. 62-292307). 
     A cutting portion and a shank portion of a drill are exposed to different loaded conditions in a drilling operation. Therefore, characteristics required for the respective portions of the drill are different from each other. For example, wear resistance, adhesion resistance and the like are required for a cutting edge of the cutting portion, while toughness for maintaining the strength of the tool is required for the shank portion. Further, different cutting edge characteristics are required for different portions along the cutting edge since cutting speeds are different at the center and at the outer peripheral portions of a drill cutting edge. 
     In a drill whose cutting edge is coated in order to satisfy these different requirements at least a coating layer of a front flank side is inevitably removed when resharpening the drill as is performed in general, whereby most of the coating effect is lost. A drill having such a structure that cemented carbide is brazed to its cutting portion cannot be used for deep hole drilling of a hard to cut material since brazing itself is a method which is essentially inferior in thermal strength and mechanical strength. Further, when cemented carbide is brought into a coarse grain state or a high binder phase state in order to improve the toughness of the shank portion of a drill, the strength of the material is reduced or the elastic limit of distortion is reduced, and hence the shank portion is undesirably broken during drilling, due to vibration of a workpiece, unstable rotation of a drive shaft and the like. Also, the cost of a drill having a large diameter and a long length for drilling deep holes, depends on the drill structure. Drills entirely made of cemented carbide are problematically high-priced both in alloy costs and processing costs. 
     In order to solve the aforementioned problems, there has recently been devised the so-called throw-away tipped drill, whose cutting portion and shank portion are separately manufactured and mechanically connected to be separable from each other, thereby enabling the use of materials having the characteristics required for the respective drill portions. 
     Conventional throw-away tipped drills include those shown in FIGS. 2 to 5, for example. FIGS. 2 to 4 show a two-flute throw-away tipped drill, wherein the tips 11a and 11b are fixed to the outer peripheral portion and the inner peripheral portion of a forward end of a shank portion 12 by respective screw connections. On the other hand, FIG. 5 shows a typical example of a one-flute throw-away tipped drill, wherein a cutting portion 21 is fitted in a shank portion 22 as shown by an arrow, and fixed to a screw hole 24 by a screw 23. A coolant is directly supplied from a coolant supply hole 25 to the cutting edge of the cutting portion 21. A chip breaker 26 for chip parting control is formed on the cutting edge of the cutting portion 21. 
     As another type of a one-flute throw-away tipped drill, there is a spade drill which is defined in JIS B0171-1609. The spade drill is common to each of the aforementioned throw-away tipped drills in that a plate-type cutting portion is fixed to the forward end of a shank portion by screwing. 
     The aforementioned conventional throw-away tipped drills, whose cutting portions and shank portions are formed of different members, can solve the various problems of the integral drill whose cutting portion is integrated with and made of the same material as its shank portion and of the brazed drill formed by brazing cemented carbide to its cutting portion. 
     However, each of these throw-away tipped drills, whose cutting portions and shank portions are connected with each other by screwing, causes the following problems: 
     (1) When the screw hole is increased in size, the tip strength is reduced. 
     (2) In order to reduce the size of the screw hole, it is necessary to use a small screw, and hence an allowable load for the screw is reduced to inevitably lower the strength of the connection between the cutting portion and the shank portion. 
     (3) Further, a small screw is inferior in its workability in assembling. In the conventional throw-away tipped drill, consequently, the cutting portion and the shank portion have been easily broken during drilling operations due to insufficiency in strength of the cutting portion and insufficiency in the connection strength between the cutting portion and the shank portion in the case of a small-diameter drill of not more than 20 mm in diameter. 
     OBJECTS OF THE INVENTION 
     In consideration of the aforementioned problems of the prior art, a first object of the present invention is to provide a throw-away tipped drill whose insert portion and shank portion can be fastened to each other without any screwing connection. 
     A second object of the present invention is to provide a throw-away tipped drill which has an excellent wear resistance and adhesion resistance in an insert portion of the drill with a shank portion having sufficient toughness and breakage resistance. Drills of the invention shall not require any resharpening for continuous use. 
     SUMMARY OF THE INVENTION 
     The following first four embodiments of the invention comprise features for attaining the aforementioned first object, while the next three embodiments comprise features for attaining the aforementioned second object. 
     The first embodiment of a throw-away tipped drill according to the invention comprises an insert portion for cutting a workpiece and a shank portion to be connected to a drive shaft of a cutting machine, and the insert portion is separably mechanically connected with the shank portion. Further, the insert portion and the shank portion have mutually engaging portions forming the connection between the insert and the shank, and a slit formed in at least in either the insert portion or the shank portion so that opposite surfaces of such a slit mutually move for an elastic deformation when surfaces of the engaging portions engage each other, whereby the insert portion is fixed to the shank portion by an elastic force. 
     According to the invention, the insert portion and the shank portion are connected with each other using an elastic force which is caused by an elastic deformation upon mutual movement of the opposite surfaces of the slit in an engaging state of both portions, whereby the insert portion can be connected with or fixed to the shank portion by simply press-fitting the insert portion into the shank portion without employing any fastening component such as a screw. 
     The second embodiment of a throw-away tipped drill according to the invention comprises an insert portion for cutting a workpiece and a shank portion to be connected to a drive shaft of a cutting machine, and the insert portion is separably mechanically connected with the shank portion. Further, the insert portion and the shank portion have mutually engaging portions for forming a connection between insert and shank. The mutually engaging portions are formed by a held portion of the insert, and holding portions of the shank for holding the held surfaces which are provided on left and right sides of the held portion. The feature of this embodiment of the invention resides in the fact that both or either one of the held surfaces of the insert and opposite holding surfaces of the shank are tapered and the angle of such tapering is so formed that the holding surfaces and the held surfaces come into surface contact with each other and enter a press-fitted state that holds the insert in the shank. 
     According to this invention, an elastic force caused by an elastic deformation of the holding portions or the held portion acts on the overall engaging surfaces in the engaging state. As a result, a frictional force is caused by a wedging action between the engaging surfaces, so that the insert is fixed to the shank. Therefore, it is possible to assemble the drill by simply pressing the held portion of the insert into the holding portions of the shank with no requirement for other fastening means such as screws. 
     The third embodiment of the invention relates to a shank for a throw-away tipped drill. The present shank for a throw-away tipped drill is separably mechanically connected with an insert for cutting a workpiece. The shank is connectable to a drive shaft of a cutting machine, and has holding portions for mutually engaging and holding the insert. The feature of this third embodiment of the invention resides in the fact that slits are provided in the holding portions to connect or fix the insert to the shank by holding a held portion of the insert with an elastic force which is caused by an elastic deformation upon mutual movement of opposite surfaces of these slits when the insert engages the shank, whereby the insert can be connected with or fixed to the shank by simply press-fitting the insert into the shank without employing any fastening component such as a screw. 
     The fourth embodiment of the invention relates to an insert tip for a throw-away tipped drill, wherein the insert tip is separably mechanically connected with a shank portion to be connected to a drive shaft of a cutting machine, for cutting a workpiece. The feature of this insert tip resides in that it has a held portion which mutually engages with the shank portion, and a slit is formed in this held portion so that the tip is connected with or fixed to the shank portion by an elastic force which is caused by an elastic deformation upon mutual movement of opposite surfaces of this slit when the insert engages the shank, whereby the insert tip can be connected with or fixed to the shank portion by simply press-fitting the insert tip into the shank portion without using any fastening component such as a screw. 
     The fifth embodiment of the invention relates to a throw-away tipped drill with an insert for cutting a workpiece and a shank portion to be connected to a drive shaft of a cutting machine, wherein the insert portion is separably mechanically connected with the shank portion. The feature of this invention resides in that its insert portion is made of surface-coated cemented carbide or cemented carbide, and the shank portion is formed of steel. 
     When surface-coated cemented carbide is employed as the material for making the insert, the surface coating material is preferably made of one or more materials selected from carbides, nitrides, borides, carbo-nitrides, carbo-oxides, carbo-oxy-nitrides and boronitrides of metals belonging to the groups IVa, Va and VIa of the periodic table such as Al 2  O 3 , TiC, TiN, TiCN, TiAlN and the like. This surface coat may be formed not only in a single layer but also in a plurality of layers. Alternatively, the surface coat may be provided in mixed multiple layers obtained by forming a plurality of coating layers in which at least two of the aforementioned materials are mixed. 
     It is possible to use any surface treatment method of the so-called physical vapor deposition and chemical vapor deposition as a method of surface-coating the cemented carbide. 
     The cemented carbide serving as the base material for the insert portion is preferably formed by a hard phase which is made of a B1 type solid solution consisting of WC and one or more types of carbides, nitrides and carbo-nitrides of metals belonging to the groups IVa, Va and VIa of the periodic table, and a binder phase which is made of an iron family metal mainly composed of 5 to 30 percent by volume of Co. 
     A drill type shown in FIGS. 2 to 4 and a drill type shown in FIG. 5, and the so-called self-grip type throw-away tipped drill having the structure shown in the aforementioned first or second embodiment of the invention can be used as a system of connecting the insert portion and the shank portion of the throw-away tipped drill according to this invention. 
     Characteristics required for a drill are roughly classified into wear resistance and adhesion resistance of an insert portion and breakage resistance, represented by toughness, of a shank portion. In this invention, the heat resistance and hardness are improved by employing cemented carbide as the material for the insert portion. Thus, high-speed cutting is enabled, and wear resistance and adhesion resistance are improved as compared with a drill made of high-speed steel, so-called rapid tool steel. In addition, the wear resistance and adhesion resistance are further improved by coating the surface of the insert portion which is made of cemented carbide. 
     Further, since steel is employed as the material for the shank portion, the present drills have an excellent toughness and a good breakage resistance. Yet, it is possible to reduce the material cost. 
     The sixth embodiment of the throw-away tipped drill according to the invention also comprises an insert portion and a shank portion, and the insert portion is separably mechanically connected with the shank portion. The feature of this invention resides in the fact that its insert is made of a silicon nitride sintered body, and the shank portion is formed of steel. 
     As to the silicon nitride sintered body for the insert portion, a crystal phase of Si 3  N 4  or SIALON (Si 6  to  z  Al z  N 8-z  wherein z=0 to 4.2) preferably occupies at least 80 percent by weight of the sintered body. More preferably, the silicon nitride sintered body contains 5 to 20 percent of Al 2  O 3 , Y 2  O 3 , MgO and an oxide of a rare earth metal, by weight. These concentrations are desirable because strength is reduced due to reduction of density if the oxide content is less than 5 percent by weight. On the other, the heat resistance, thermal cracking resistance and wear resistance are reduced if the oxide content is in excess of 15 percent by weight. 
     Further, the surface of the insert portion may be subjected to coating of TiC, TiN, TiCN, TiAlN, Al 2  O 3  or the like in a single layer or multiple layers. 
     Similarly to the aforementioned third invention, the system shown in FIGS. 2 to 4, that shown in FIG. 5, and the self-grip system may be used as a connecting device for securing the insert to the shank of the throw-away tipped drill according to this invention. 
     According to this invention, the heat resistance and wear resistance are substantially increased by utilizing a silicon nitride sintered body as the material for the insert portion, instead of high-speed steel (rapid tool steel) or cemented carbide. Thus, adhesion resistance is also improved. 
     Further, since steel is used as the material for the shank, the drill has an excellent toughness, a good breakage resistance, and it is possible to reduce the material cost of the drill, similar to the aforementioned fifth invention. 
     The seventh embodiment of a throw-away tipped drill according to the invention, comprises an insert and a shank. The insert of the throw-away tipped drill according to this invention is made of cermet. This cermet is made of a hard dispersed phase and a metallic binder phase. The hard dispersed phase is mainly composed of titanium and any one of a carbide, a nitride and a double carbo-nitride of at least one of metals belonging to the groups IVa, Va and VIa of the periodic table, excluding titanium. The metallic binder phase is mainly composed of nickel and cobalt. The composition of the hard dispersed phase is expressed as (TiaMb) (CcNd). Symbol M represents at least one of metals belonging to the groups IVa, Va and VIa of the periodic table, excluding Ti. Symbols a, b, c and d show molar fractions. Symbols a, b, c and d are defined in the following relational expressions: 
     
         a+b=1 
    
     
         c+d=1 
    
     
         0.5≦a≦0.95 
    
     
         0.1≦d≦0.7 
    
     The hard dispersed phase comprises at least a fine grain group and a coarse grain group. The mean particle diameter of the fine grain group is at least 0.2 μm but not more than 0.6 μm. The mean particle diameter of the coarse grain group is at least 1 μm but not more than 3 μm. The volume ratio of the fine grain group to the aforementioned coarse grain group is at least 0.3 but not more than 3. The ratio of the metallic binder phase contained in the cermet is at least 5 percent by weight but not more than 30 percent by weight. 
     The shank of the throw-away tipped drill according to this embodiment of the invention is separably mechanically connected with the insert. The shank is made of alloy steel. As to the coupling system for the insert and the shank of the throw-away tipped drill according to this invention, a system which is similar to that of the aforementioned fifth embodiment or sixth embodiment of the invention is applicable. 
     Reasons for restriction of compositions, particle diameters, etc. in the throw-away tipped drill according to this invention are now described. 
     When wear of a cutting edge advances due to the adhesion of a workpiece and the drill material, the life of the drill comes to a premature end due to adhesion of chips. According to this invention, the composition and the particle diameter of the hard dispersed phase are restricted in order to improve wear resistance and adhesion resistance. The amount of titanium in a metal atom group which is contained in the hard dispersed phase is restricted within a range of 0.5 to 0.95 in molar fraction If the amount is less than 0.5, wear resistance and adhesion resistance are reduced. If the amount exceeds 0.95, on the other hand, the sintering property of cermet itself deteriorates. Since affinity with iron is reduced by such a high content of titanium, the machined surface of a workpiece containing iron has an improved quality. 
     The rate of nitrogen in a nonmetal atom group which is contained in the hard dispersed phase is restricted within a range of 0.1 to 0.7 in molar fraction. If the rate is less than 0.1, nitrogen shows no effect of suppressing grain growth of the hard dispersed phase in the sintering of cermet. If the rate exceeds 0.7, on the other hand, the sintering property of cermet deteriorates. 
     The hard dispersed phase is made of a mixture of a fine grain group having particle diameters of 0.2 to 0.6 μm, and a coarse grain group having particle diameters of 1 to 3 μm. The volume ratio of the fine grain group to the coarse grain group is restricted within a range of 0.3 to 3. If this volume ratio is less than 0.3, the toughness of cermet deteriorates so that chipping is caused in the cutting edge portion of the drill. If this volume ratio exceeds 3.0, on the other hand, thermal cracking is caused in the cutting edge of the drill. 
     The amount of the metallic binder phase is restricted to a range of 5 to 30 percent by weight. If the amount is less than 5 percent by weight, toughness of cermet is so insufficient that chipping is caused in the cutting edge of the drill. If the amount exceeds 30 percent by weight, wear resistance is insufficient, resulting in extreme wear being caused in a flank or a margin of the cutting edge. 
     Alloy steel is employed for the shank of the throw-away tipped drill according to this embodiment of the invention. The alloy steel exhibits excellent toughness against slight impact since its elastic deformation range is wide, and therefore the shank is hard to break. While steel materials are roughly classified into carbon steel, alloy steel and high alloy steel, alloy steel (nickel chrome molybdenum steel, chrome molybdenum steel, tool steel, etc.) is employed in this invention in consideration of both wear resistance and workability. In the case of an insert type one-flute throw-away tipped drill, it is necessary to also consider elasticity material selection and spring steel or the like may also be employed. Further, alloy steel which is generally easier to manufacture is employed for the shank, and sintered hard alloy which requires complicated manufacturing steps is employed only for the insert, thereby reducing the manufacturing cost, and this invention is also cost efficient. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing a conventional twist drill; 
     FIG. 2 is a front elevational view showing an example of a conventional two-flute throw-away tipped drill; 
     FIG. 3 is a right side elevational view of FIG. 2; 
     FIG. 4 is a perspective view showing on an enlarged scale, the tip of the drill of FIG. 2; 
     FIG. 5 is an exploded perspective view of a conventional one-flute screwed type throw-away tipped drill; 
     FIG. 6 is an exploded perspective view showing an insert 31 separated from a shank 32 of a throw-away tipped drill with the insert holding portions in the shank and insert, in accordance with an embodiment of the present invention; 
     FIG. 7 is a front elevational view of the embodiment; 
     FIG. 8 is a right side elevational view thereof; 
     FIG. 9 is a sectional view taken along the line A--A in FIG. 7, showing a state at a point of time for starting press fitting of the insert 31 into the shank in the embodiment shown in FIG. 6; 
     FIG. 10 is the same sectional view showing such a state that the insert 31 is connected with or fixed to the shank 32 as shown in FIG. 9; 
     FIG. 11 is a sectional view showing the embodiment of the present invention, in which slits 34 are provided on both sides of holding portions 33a and 33b; 
     FIG. 12 is a sectional view showing a state at a point of time for starting press fitting of an insert 31 into a shank 32 in another embodiment of the present invention; 
     FIG. 13 is a sectional view showing a connected or fixed state as shown in FIG. 12; 
     FIG. 14 is a sectional view taken along the line B--B in FIG. 10 or FIG. 13; 
     FIG. 15 is a sectional view taken along the line B--B in FIG. 11; 
     FIG. 16 is a sectional view of the section taken along the line B--B in FIG. 10 or FIG. 13; 
     FIG. 17 is a partially enlarged sectional view of line B--B in FIGS. 10 or 13; 
     FIG. 18 is a diagram showing an embodiment of an insert comprising displacement preventive means for joints of an insert 31 and a shank 32; 
     FIG. 19 is a sectional view taken along the line D--D in FIG. 18, showing such a state that the insert 31 of the embodiment engages with the shank 32; 
     FIG. 20 is a perspective view showing the insert 31 shown in FIG. 18; 
     FIG. 21 is a sectional view showing a state at a point of time for starting press fitting of an insert 31 into a shank 32 in still another embodiment of the present invention: 
     FIG. 22 is a sectional view showing means by which the insert 31 and the shank 32 are connected with or fixed to each other; and 
     FIG. 23 is a distribution chart showing a particle size distribution of a hard dispersed phase of a cermet alloy of which inserts of the invention are made. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of the first invention is now described on the basis of FIGS. 6 to 10. Referring to FIG. 6, the throw-away tipped drill of this embodiment employs the so-called self-grip system, in which an insert 31 is fitted into a shank 32 along the direction shown by an arrow in FIG. 6, so that the insert 31 and the shank 32 are connected without using a screw etc. In this embodiment, a state in which the insert 31 and the shank 32 are connected is as shown in FIGS. 7 and 8. In this connected state, the insert 31 is fixed to the shank 32 by frictional force which is caused by the side portions of a held portion 31a of the insert 31 coming in contact with inner end surfaces of holding portions 33a and 33b of the shank 32. With reference to FIGS. 9 and 10, the state of connection attained by engagement of the insert 31 and the shank 32 in this embodiment is explained as follows: In a state before engagement of the insert 31 and the shank 32, an angle θ 1  formed between left and right side portions of the held portion 31a is slightly larger than an angle θ 2  formed between opposite inner end surfaces of the holding portions 33a and 33b, as shown in FIG. 9. When the insert 31 is press-fit into the shank 32, the angle θ 2  is gradually expanded by a wedge action due to tapering of the left and right side portions of the held portion 31a and formation of a slit 34 on the side of the holding portion 33a. While the angles are in a relation θ 1  &gt;θ 2 , the held portion 31a and the holding portion 33a are in contact with each other only on an upper end of the inner side surface of the holding portion 33a. When θ 2  coincides with θ 1 , the contact areas between both side portions of the held portion 31a and the inner end surface of the holding portion 33a are maximized as shown in FIG. 10. The press fitting is stopped in this state, so that pressing force is caused in the contact surface with the held portion 31a by elastic force resulting from elastic deformation of the holding portion 33a. The insert 31 is connected with or fixed to the shank 32 by frictional force between the contact surfaces. 
     A coolant is directly supplied to the cutting edge of the insert 31 from a coolant supply hole 35. A chip breaker 36 for parting chips is formed on the cutting edge of the insert 31. 
     The angle θ 1  formed by the left and right side portions of the held portion 31a is set at an angle of 4° to 20°, preferably 8° to 15°. The reason for such setting of the range of the value of the θ 1  is as follows: First, if θ 1  is reduced, positioning accuracy of the insert 31 in the connected state shown in FIG. 10 is substantially influenced by the dimensional accuracy of 1, the held portion 31a and the holding portions 33a and 33b. If θ 1  &lt;4°, it is rather very difficult to implement dimensional accuracy of the held portion 31a and the holding portions 33a and 33b for attaining desired positioning accuracy of the insert 31. If θ 1  is increased, on the other hand, a component of pressing force, which is applied to the held portion 31a from the holding portions 33a and 33b, along the axial direction of the shank 32 is increased. Since this force acts in such a direction that the insert 31 is disengaged against frictional force, the force for fixing the insert 31 is weakened as θ 1   is increased. It has been confirmed that desired fixing force for the insert 31 with respect to the shank 32 cannot be obtained when θ 1  exceeds 20°. 
     As another embodiment of the shank 32, slits 34 may be formed not only on the side of the held portion 33a but also on the side of the holding portion 33b, as shown in FIG. 11. In this case, both of the holding portions 33a and 33b are flared as the insert 31 is press-fitted into the shank 32, so that the held portion 31a is held by the elastic force. Also in this case, the value of θ 1  is set at 4° to 20°, similar to the aforementioned value of θ 1 . 
     Still another embodiment as to the configurations of the held portion 31a and the holding portions 33a and 33b is shown in FIGS. 12 and 13. In this embodiment, a side portion of the held portion 31a of the insert 31 is formed by a tapered portion 31c which forms an angle 83 with a parallel portion 33b, as shown in FIG. 12. Further, the inner end surfaces of the holding portions 33a and 33b of the shank 32 have tapering (angle θ 4 ) which is inverse to the tapering (angle θ 1 ) of the held portion shown in FIG. 9 etc. As the insert 31 is press-fit along the arrow direction shown in FIG. 12, opposite surfaces of slits 34 relatively move and the held portion 33a is flared by a wedge action of the tapered portion 31c, and when the press fitting is further performed, the insert is fixed in the state shown in FIG. 13, i.e., such that the upper end surface of the holding portions 33a and 33b are in contact with left and right lower end surfaces of the insert 31. θ 3  and θ 4  are so selected that the inner end surfaces of the holding portions 33a and 33b are parallel to each other and contact areas with the side portions of the held portion 31a are maximized in this state. Also since the inner end surfaces of the holding portions 33a and 33b of the shank 32 are parallel to each other, it is possible to attain sufficient holding power although the contact areas are reduced. Also in this embodiment, the held portion 31a is held by elastic force following elastic deformation of the holding portion 33a having the slit 34 similar to the embodiment of FIG. 9, and the insert 31 is connected with or fixed to the shank 32 by frictional force. According to this embodiment, the upper end surfaces of the holding portions 33a and 33b come into contact with the insert 31 in the connected state of the insert 31 and the shank 32, whereby the insert 31 is further reliably positioned with respect to the shank 32 as compared with the aforementioned respective embodiments. 
     Each B--B section of the embodiments shown in FIGS. 10 and 13 is as shown in FIG. 14. In this embodiment, the cross-section of the held portion 31a of the insert 31 is in the form of a parallelogram, and both of the inner end surfaces of the holding portions 33a and 33b are inclined. The inclination angle (θ 6  shown in FIG. 14) of the inner end surfaces of the holding portions 33a and 33b is larger than the inclination angle (θ 5  shown in FIG. 14) of the side portions of the held portion 31a. In the state shown in FIG. 3B or FIG. 5B, therefore, force F in a direction shown in FIG. 14 acts substantially in line contact with the side portion of the held portion 31a in the vicinity of one edge of the inner surface of each of the holding portions 33a and 33b. This force F acts in a direction for pressing the end surface of the held portion 31a against the inner wall surface of the shank 32, whereby the insert 31 and the shank 32 are still further reliable connected with or fixed to each other. Further, since the drill rotates in a arrow C direction shown in FIG. 14, torque which acts on the insert 31 during perforating by the drill also acts in the direction for pressing the end portion of the held portion 31a against the inner wall surface of the shank 32. 
     The B--B section of the embodiment shown in FIG. 11 is as shown in FIG. 15, and relations between inclination angles θ 5  and θ 6 , force F, and a rotational direction C are similar to those shown in FIG. 14, except that the slit 34 is also formed on the side of the held portion 33b. 
     Alternatively, means for generating the force F may be implemented by making both side portions of the held portion and the inner end surfaces of the holding portions in configurations of cylindrical surfaces and changing the radii of curvature thereof while making the central positions thereof eccentric, in place of making the cross-section of the held portion 31a in the form of a parallelogram as in the aforementioned embodiment. Namely, as shown in FIG. 17 in an enlarged manner, the radius R 1  of curvature of the holding surfaces and the radius R 2  of curvature of the held surfaces are so set that R 1  &lt;R 2  and the centers thereof are slightly displaced, whereby holding portions 33a and 33b and held portions 31a and 31b are brought into line contact states and force F acts similarly to the embodiments shown in FIGS. 14 and 15, so that a similar function or effect can be attained. 
     FIGS. 18 to 20 show still another embodiment of this invention. In this embodiment, depressed portions 31d are formed on front and back end surfaces of a held portion 31a of an insert 31, and projections 37 are provided at substantially central portions thereof. These projections 37 engage with recessed portions 38 which are provided on inner walls of a shank 32 in such a manner that the insert 31 is connected with the shank 32. Thus, the insert 31 is stopped in the shank 32 in the position of the connected state, and prevented from disengagement. 
     Although the slits 34 are provided on the holding portion sides of the shank 32 as means for generating elastic force for connecting or fixing the insert 31 and the shank 32 with or to each other in each of the aforementioned embodiments, a substantially similar function or effect can be attained also by providing a slit 39 on an insert 31 side, as shown in FIGS. 21 and 22. In this case, a lower tapered portion 31e and an upper tapered portion 31f of a side portion of a held portion 31a form angles θ 7  and θ 8  as shown in FIG. 21 respectively. When the insert 31 is press-fit into a shank 32 along the arrow direction, the held portion 31a is first held by holding portions 33a and 33b by inclination of the lower tapered portion 31e, and fixed in the state shown in FIG. 22. At this time, a clearance of the slit 39 is so narrowed that the upper tapered portion 31b is substantially parallel to maximize contact areas with the inner end surfaces of the holding portions 33a and 33b, whereby the insert 31 and the shank 32 are connected with or fixed to each other by pressing force and frictional force following elastic deformation of the held portion 31a. 
     In any case of connection or fixing in the aforementioned respective embodiments, it is possible to replace the insert by easily removing the insert 31 from the shank 32 using a jig. 
     While both the insert 31 and the shank 32 of each of the aforementioned embodiments are somewhat complicated in configuration, substantially final required configurations are relatively easily manufactured in a molding stage when an injection molding technique is employed. 
     EMBODIMENTS OF SECOND INVENTION 
     All embodiments of the second invention are common to the aforementioned embodiments of the first invention, and are explained similarly to the aforementioned embodiments of the first invention, on the basis of FIGS. 6 to 22. 
     EMBODIMENTS OF THIRD INVENTION 
     As to the third invention, the shank 32 in FIGS. 6 to 17, which have been explained as the embodiments of the aforementioned first invention, correspond to embodiments thereof. Therefore, the embodiments of this invention are explained similarly to the aforementioned embodiments of the first invention, using these figures. 
     EMBODIMENTS OF FOURTH INVENTION 
     As to the fourth invention, the insert 31 shown in FIGS. 21 and 22, which has been explained as the embodiment of the aforementioned first invention, corresponds to its embodiment. Therefore, the embodiment of this invention is explained similarly to the aforementioned embodiment of the first invention, using these figures. 
     EMBODIMENTS OF FIFTH INVENTION 
     Embodiments of the fifth invention are now described. 
     First Embodiment 
     A throw-away tipped drill in a first embodiment of this invention is formed by employing surface-coated cemented carbide for an insert portion while employing steel for a shank portion, and mechanically connecting both portions to be separable in the system shown in FIG. 6. 
     Surface-coated cemented carbide materials forming the insert portions in this embodiment are prepared by mixing various types of powder materials so that compositions of hard phases and binder phases reach numerical values shown in Table 1 after sintering. As to steel materials forming the shank portions, those shown in Table 1 are employed. Inventive samples and comparative samples of this embodiment, shown in Table 1, are those whose insert portions and shank portions are connected in the system shown in FIG. 6. Among the inventive samples A to D, the sample D, which is in the scope of this invention for the time being, is an example of such a case that the volume percentage of the binder phase, shown with **, is considerably less than a preferably value. As to the comparative sample E, the insert portion is made of cemented carbide but not surface-coated. As to the comparative sample F, K30-grade cemented carbide, which is out of the scope of this invention, is employed as the material for the shank portion. Numerals which are out of the scope of this invention are shown with marks * respectively. 
     
                                           TABLE 1__________________________________________________________________________  Insert Portion  Composition of Binder              Binder Phase                     Surface CoatingSample Phase (mol ratio)              (vol. %)                     Material                             Shank Portion__________________________________________________________________________InventiveSampleA      WC:(TiTa)(CN) = 7:3              13     TiN     SCM435B      WC:(TiTa)(CN) = 7:3              13     Al.sub.2 O.sub.3                             SKS2C      WC:(TiW)C = 5:5              18     TiCN    SNCM420D      WC:(TiTa)(CN) = 7:3              **3    TiN     SCM435ComparativeSampleE      WC:(TiTa)(CN) = 7:3              13     *None   SCM435F      WC:(TiTa)(CN) = 7:3              13     TiC + Al.sub.2 O.sub.3                             *K30-Grade Cemented Carbide                             (WC-11 wt. % Co)__________________________________________________________________________ 
    
     Performance evaluation tests of drills were carried out using drills of 18 mm diameter under the following conditions: 
     Workpiece: S50C (H B  =230) 
     Cutting Speed: 100 m/min., wet process (water-soluble cutting oil) 
     Feed: 0.15 mm/rev. 
     Depth: 35 mm 
     Criterion: Conditions of cutting edges etc. are observed after working up to end of life. 
     Life: Generally regarded as ended when the outer peripheral flank is worn in excess of 0.2 mm. 
     Results of the aforementioned drill performance evaluation tests are shown in Table 2. An analysis yields excellent results attained from the inventive samples A to C. Sample D of the invention was prone to cutting edge chipping because the binder phase of the cemented carbide forming the insert portion was less than the preferable values of 5 to 30 percent by volume, as shown with ** in Table 1. 
     Similar experiments were performed on four conventional types of drills for the purpose of reference and are recorded in the lower area of Table 2. Comparing these results to those of the inventive samples A to C reveals the superiority of the inventive samples. 
     
                       TABLE 2______________________________________                Amount       Number   of Outer   Depth of       of       Peripheral Inner Peripheral       Worked   Front Flank                           Face AdhesiveSample      Holes    Wear (mm)  Wear (mm)______________________________________InventiveSampleA           3500     0.2        0.01B           4100     0.2        0.02C           3700     0.2        0.01D            117     Test stopped by cutting edge chipComparativeSampleE           1200     0.2        0.12F           1550     Test stopped by breakage of                shank portionReferenceCutting Edge:        20      0.2 (chipping:                           0.10Coated High-Speed    2 portions)SteelCutting Edge:       1800     Test stopped by cutting edge chipAluminaIntegral Coated        40      Test stopped by cutting edgeHigh-Speed Steel     chipping.Integral Coated       1990     Test stopped by breakage ofCemented Carbide     shank portion______________________________________ 
    
     Second Embodiment 
     A second embodiment of this invention is now described. In this embodiment, cutting characteristics were compared in relation to inventive samples of throw-away tipped drills having insert portions and shank portions of the same materials as those of the sample A in the aforementioned first embodiment respectively, as to three types of connection systems, i.e., the self-grip system shown in FIG. 6 (sample G), the two-flute screwed drill shown in FIGS. 2 to 4 (sample H) and the one-flute screwed drill shown in FIG. 5 (sample I), and a brazing-connected drill having a cutting edge of cemented carbide (sample J), which was out of the scope of the present invention. Cutting conditions thereof are as follows: 
     Workpiece: S50C (H B  =220) 
     Cutting Speed V: 50 m/min., 150 m/min. (water-soluble cutting oil) 
     Feed: 0.2 mm/rev. 
     Depth: 40 mm 
     Diameter of Drilled Hole: 20 mm 
     Table 3 shows the results of evaluated characteristics in this embodiment. The cutting characteristics, represented by stability, can be regarded as displaying excellent cutting characteristics as horizontal components and thrusts of cutting balance acting on the drills by cutting resistance shown in Table 3 are reduced and values of dependency are reduced with respect to the speeds. From the results of this embodiment, it is understood that the self-grip system of the sample G exhibits the most excellent cutting characteristics as compared with other systems, as the connection system for the throw-away tipped drill to which this invention is applied. 
     
                                           TABLE 3__________________________________________________________________________Compared Characteristics                                     Hole            Cutting Balance          Diameter            (Horizontal          Torque                                     Over            Component) Thrust (N)                                 (J) Size (μm)            Cutting Speed V            (m/min.)Sample           50  150    50 150    50  150   Remarks__________________________________________________________________________InventionG      Self-Grip 190 180    1800                          1850   3300                                     20    Throw-Away Type  System                                   shown in FIG. 1H      Two-Flute 1610                590    1980                          2050   3800                                     200   Throw-Away Type  Screwed                                  of FIGS. 5A to C  SystemI      One-Flute 400 unmeasurable                       7200                          unmeasurable                                 4200                                     30    Throw-Away Type  Screwed                                  shown in FIG. 6  System                                   Cutting Edge                                           Departure at                                           V = 150.Comparative  Brazing   150 unmeasurable                       2700                          unmeasurable                                 6200                                     20    Helical Flute,Sample Connection                               Cross Thinning,J      System with                              Number of  Cutting Edge of                          Resharpenable  Cemented Carbide                         Times: 5.                                           Could not part                                           chips and broken                                           at V = 150__________________________________________________________________________ Note) Materials for insert portions and shank portions of the inventive samples G, H and I were prepared from those identical to the sample A in Table 1. 
    
     Third Embodiment 
     The same cemented carbide base material as the inventive sample B of the aforementioned first embodiment, i.e., cemented carbide having a hard phase of WC:(TiTa)(CN)=7:3 and 13 percent by volume of a binder phase was worked into a configuration for a drill of 10 mm in diameter, and coated with TiCN of 2 μm and Alzk2 0  3 of 2 μm, as well as TiN of 1 μm for the uppermost layer, as surface coating materials. 
     This drill was subjected to a performance evaluation test under the following conditions: 
     Workpiece: S50C (H B  =220) 
     Cutting Speed: 80 m/min., wet process (water-soluble cutting oil) 
     Feed: 0.18 mm/rev. 
     Depth: 20 mm 
     As the result, it was possible to work 2000 holes (each length of cut: 40 mm), and the worn state was normal. 
     EMBODIMENT OF SIXTH INVENTION 
     An embodiment of the sixth invention is now described. Throw-away tipped drills in the embodiment of this invention are formed by employing silicon nitride sintered bodies for insert portions while employing steel for shank portions, and separably mechanically connecting both portions with each other in the system shown in FIG. 6. 
     The silicon nitride sintered bodies forming the insert portions in this embodiment are prepared by mixing various types of powder materials so that compositions reach numerals shown in Table 4 after sintering. As to the steel forming the shank portions, those of materials shown in Table 4 are employed. As to inventive samples and comparative samples in this embodiment, shown in Table 4, all of the insert portions and shank portions are connected with each other in the system shown in FIG. 6. Among the inventive samples A to C, the sample C, which is in the scope of this invention for the time being, is an example of such a case that the weight percentage of Si 3  N 4  shown with ** is considerably less than a preferable value. The comparative sample D, whose insert portion is of a silicon nitride sintered body, is out of the inventive scope due to the fact that the material for the shank portion, shown with *, is not steel but P30-grade cemented carbide. 
     
                                           TABLE 4__________________________________________________________________________  Insert PortionSample Starting Composition (wt. %)                           Shank Portion__________________________________________________________________________InventiveSampleA      Si.sub.3 N.sub.4 (93.0), MgO (1.5), Y.sub.2 O.sub.3 (4.5),  Al.sub.2 O.sub.3 (1.0)   SKH51B      SIALON (94.0), MgO (2.0), Y.sub.2 O.sub.3 (2.0), Al.sub.2 O.sub.3  (2.0)                    SCM435C      **Si.sub.3 N.sub.4 (78.0), MgO (10.0), Y.sub.2 O.sub.3                           SCM435Comparative  β-Si.sub.3 N.sub.4 (93.0), MgO (2.5), Y.sub.2 O.sub.3 (4.5),  Al.sub.2 O.sub.3 (5.0)   *P30-Grade CementedSample                          Carbide__________________________________________________________________________ Note 1) Starting composition indicates distribution ratio of powder in mixing. Note 2) *shows that out of scope of this invention, and **shows deviation from preferable range of this invention. 
    
     Performance evaluation tests of drills were carried out using drills of 18 mm in diameter under the following conditions: 
     Workpiece: S50C (H B  =230) 
     Cutting Speed: 150 m/min., wet process (water-soluble cutting oil) 
     Feed: 0.10 mm/rev. 
     Depth: 15 mm 
     Criterion: Conditions of cutting edges etc. are observed after working up to end of life. 
     Life: Generally regarded as ended when the outer peripheral flank is worn in excess of 0.2 mm. 
     Results of the aforementioned drill performance evaluations tests are shown in Table 5. From these results, excellent results have been obtained regarding inventive samples A and B. As to the inventive sample C, the number of worked holes with respect to the same amount of wear is smaller than that compared to A and B, due to the fact that the weight percentage of Si 3  N 4  in the silicon nitride sintered body forming the insert portion does not satisfy the condition of at least 80%, which is a preferable value, as shown in Table 4 with **. For the purpose of reference, similar experiments were also made using conventional drills having cutting edges of coated cemented carbide or coated high-speed steel (lower column of Table 5). Again, the results yielded by the present samples A to C are superior. 
     
                       TABLE 5______________________________________                  Amount of Outer       Number of  Peripheral Front FlankSample      Worked Hole                  Wear (mm)______________________________________InventiveSampleA           3810       0.2B           3250       0.2C           1890       0.2Comparative 2240       Shank portion brokenSampleReferenceE            840       0.2F             0        Cutting edge remarkably                  worn______________________________________ Note) E: Cutting edge of coated cemented carbide F: Cutting edge of coated highspeed steel 
    
     EMBODIMENT OF SEVENTH INVENTION 
     An embodiment of the seventh invention is now described. 
     Throw-away tipped drills in this invention are formed by employing cermet alloys for insert portions while employing alloy steel materials for shank portions, and separably mechanically connecting the both portions with each other. The cermet alloys forming the insert portions were prepared by mixing various powder materials so that compositions of hard dispersed phases, compositions of binder phases and grain size abundance ratios of the hard dispersed phases reached the numerical values shown in Table 6 after sintering. The alloy steel materials forming the shank portions were prepared from those of the materials shown in Table 6. 
     FIG. 23 shows grain size distribution of a hard dispersed phase in a cermet alloy forming the insert portion of a drill. Referring to this figure, symbol A represents abundance degree distribution of fine grains, and symbol B represents abundance degree distribution of coarse grains. The volume ratio of a fine grain group to a coarse grain group is shown in Table 6 as A/B. 
     Inventive samples and comparative samples shown in the following embodiment were prepared using drills of 16 mm diameter, utilizing the connection systems shown in FIGS. 6 to 8. 
     Table 6 shows compositions and grain size distributions etc. of hard alloys forming the inventive drills and drills prepared for the purpose of comparison. Referring to Table 6, the comparative sample F was prepared so that a nonmetal atom ratio and a grain size abundance ratio of the hard dispersed phase of the insert portion were out of the scope of this invention, as shown with *. The comparative sample G was prepared so that the amount of a metallic binder phase of the insert portion was out of the scope of this invention. As to the comparative sample H, K30-grade cemented carbide, which is out of the scope of this invention, was employed as the shank portion. 
     Numerals which are out of the scope of this invention are shown with * respectively. 
     
                                           TABLE 6__________________________________________________________________________      Insert Portion      Hard Dispersed Phase                       Amount of                             Grain Size                 Nonmetal                       Binder                             Abundance      Component Ratio of                 Atomic                       Phase Ratio of      Metal Atom Ratio (wt. %)                             Hard Phase                                   Shank PortionClassification  Sample      Ti        Ta          W Mo Nb                 N/C + N                       Ni Co A/B (*1)                                   (under JIS Standard)__________________________________________________________________________Inventive  A   80        5 10            1  4 0.41  10 10 2.0   SKD61Sample B   88        5  7            -- --                 0.38   7 15 1.0   SCM435  C   78        9  8            2  3 0.32  10 8  1.0   SNCM439  D   82        8 10            -- --                 0.43   8 8  1.8   SNC415  E   75        5 15            2  3 0.48  11 8  2.5   SUP7Comparative  F   80        5 10            1  4  0.05*                       10 10  0.2* SKD61Sample G   80        5 10            1  4 0.43   15*                          20*                             1.2   SCM435  H   88        5  7            -- --                 0.47  11 8  1.8   K30-Grade Cemented                                   Carbide (WC-10 wt. %__________________________________________________________________________                                   Co) (*1) Grain size abundance ratio of hard phase: A/B (volume ratio), (see FIG. 23) 
    
     Performance evaluation tests of drills were carried out under the following conditions: 
     Workpiece: S50C (H B  =220) 
     Cutting Speed: 120 m/min., wet process (water-soluble cutting oil) 
     Feed Rate: 0.15 mm/rev. 
     Depth: 35 mm 
     Criterion: Conditions of cutting edges etc. are observed after working up to end of life. 
     Life: Generally regarded as ended when the outer peripheral flank is worn in excess of 0.2 mm. 
     Results of the aforementioned drill performance evaluation tests are shown in Table 7. Referring to Table 7, first, the toughness of cermet forming the insert portion of comparative sample F was reduced compared with present samples A and B and the cutting edge portion of sample F chipped. 
     Next, it was recognized that the inner peripheral face adhesive wear depth was deep in the comparative sample G. Comparative sample G was also inferior in adhesion resistance in comparison to present samples A and B. 
     In comparing the inventive samples A and B with the comparative sample H, the comparative sample H was inferior in shank strength, and the shank portion was broken. 
     Further, the samples C, D and E of the invention also had excellent performances substantially similar to those of the samples A and B of the invention, as shown in the tables. 
     The performance tests were made also with currently used coated high-speed steel or coated cemented carbide drills for the purpose of reference. In comparison of these drills with the drills of the invention samples A, B, C, D and E, it is obvious that performances of the drills of the inventive samples are superior. 
     
                                           TABLE 7__________________________________________________________________________                   Amount of Outer                             Depth of Inner             Number of                   Peripheral Front                             Peripheral FaceClassification  Sample     Drilling                   Flank Wear                             Adhesive Wear__________________________________________________________________________Inventive  A          2100 holes                   0.2       0.01Sample B          1989 holes                   0.2       0.02  C          1970 holes                   0.2       0.01  D          2050 holes                   0.2       0.02  E          1959 holes                   0.2       0.02Comparative  F           710 holes                   Test stopped by cutting edge chipSample G           385 holes                   0.2       0.12  H           625 holes                   Test stopped by breakage of shank portionReference  Coated High-Speed              124 holes                   Test stopped by cutting edge chip  Steel  Coated Cemented             1005 holes                   Test stopped by breakage of shank portion  Carbide  (Single Material)  Coated Cemented              989 holes                   Test stopped by cutting edge chipping  Carbide  (Cutting Edge Alone  Brazed)__________________________________________________________________________ 
    
     INDUSTRIAL APPLICABILITY 
     According to the first invention to the fourth invention, as hereinabove described, the insert portion and the shank portion are connected with or fixed to each other without separate fastening means such as screwing. Therefore, it is possible to manufacture a small-diameter throw-away tipped drill of not more than 10 mm in diameter, which has not been implemented by the conventional system of fixing an insert portion to a shank portion by screwing or the like. Workability is improved in a step of connecting the insert portion and the shank portion to each other, by simply performing press fitting, without requiring an operation such as screwing. 
     According to the fifth, sixth and seventh invention, a material which is excellent in wear resistance, adhesion resistance or thermal cracking resistance (chipping resistance) is employed as a material for the insert portion of the throw-away tipped drill while a material which is excellent in toughness and is of relatively low cost is employed for the shank portion and both portions are mechanically connected with each other, thereby forming a drill. Thus, a throw-away tipped drill having high reliability, high life and high quality, which displays no abrupt breakage or the like, is provided at a relatively low cost.