Patent Publication Number: US-2022234121-A1

Title: Drill and method of producing drilled product

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2021-012526, filed on Jan. 28, 2021; the entire contents of which are incorporated herein by reference. 
     FIELD 
     Implementations described herein relate generally to a drill and a method of producing a drilled product. 
     BACKGROUND 
     Conventionally, a drill having cutting edges of which point angle and relief angle change continuously toward the maximum diameter position of the cutting edges, and also having a relief angle at the maximum diameter position of the cutting edges, is known as a drill which can drill not only a metal but also an FRP (Fiber Reinforced Plastic), such as GFRP (Glass Fiber Reinforced Plastic) and CFRP (Carbon Fiber Reinforced Plastic), which is also called a composite material, with high quality (for example, refer to Japanese Patent Application Publication JP2008-36759 A, Japanese Patent Application Publication JP2012-135873 A and Japanese Patent Application Publication JP2010-214478 A). Since this drill can machine a hole with remarkably higher quality and have a remarkably extended tool life by improving wear resistance, the drill a become famous with a common name of R-drill (registered trademark). 
     Moreover, since a projected shape in a radial direction of cutting edges whose point angle continuously decreases becomes a curved one, such as a parabola or a circle, a multi-point-angle drill whose point angle intermittently decreases has also been proposed from a viewpoint of simplifying production (for example, refer to Japanese Patent Application Publication JP2015-221467 A). 
     When drilling is performed by attaching the above-mentioned R-drill (registered trademark) to a machine tool, such as a drill press, a milling machine or a machining center, of which spindle has high rigidity, a hole can be machined with high quality and exchange frequency of the drill caused by wear of the drill can be remarkably reduced. 
     However, when drilling is performed by attaching the R-drill (registered trademark) to a handheld tool rotating device, such as a hand to or a drilling device, such as a simple drill press, of which spindle has low rigidity or accuracy, the quality of a hole sometimes deteriorate. As a concrete example, it has been confirmed that there was a case where vibration of the R-drill (registered trademark) became large during drilling, and thereby the shape of a cross section of a hole did not become circular but polygonal when the number of the cutting edges is not less than three, as a result of drilling tests performed by holding the R-drills (registered trademark) with a handheld tool rotating device. Such deterioration in quality has been confirmed whether a material of a workpiece is a composite material or a metal. 
     Accordingly, an object of the present invention is to allow drilling a workpiece, made of a composite material, a metal, or the like, with high accuracy, not only in a case where a spindle of a drilling machine has sufficient rigidity but also in a case where drilling is performed by a handheld tool rotating device, and a spindle of a drilling machine has low rigidity or accuracy. 
     SUMMARY OF THE INVENTION 
     In general, according to one implementation, a drill includes at least one first cutting edge, at least one second cutting edge and a deflection reducer. The at least one first cutting edge drills a prepared hole to a workpiece. The at least one first cutting edge is formed in a tip side of the drill. The first point angle of the at least one first cutting edge at a tip position is more than 0 degrees and less than 180 degrees. The first point angle continuously or intermittently decreases from the tip side toward a rear end side of the drill. The first relief angle of the at least one first cutting edge continuously or intermittently decreases from the tip side toward the rear end side. The at least one second cutting edge finishes the prepared hole. The at least one second cutting edge is formed at a position away in the rear end side from the at least one first cutting edge. The at least one second cutting edge has a second relief angle at a maximum diameter position of the at least one second cutting edge. The deflection reducer reduces deflection of the at least one second cutting edge by being inserted into the prepared hole. The deflection reducer is formed between the at least one first cutting edge and the at least one second cutting edge. 
     Further, according to one implementation, a method of producing a drilled product includes drilling the workpiece with the above-mentioned drill in order to produce the drilled product. 
    
    
     
       BRIEF DESCRIPT 1 ON OF THE DRAWINGS 
       In the accompanying drawings 
         FIG. 1  is a front view, of which arrow direction is partially changed, of a drill according to the first implementation of the present invention; 
         FIG. 2  is an enlarged left side view of the drill shown in  FIG. 1 ; 
         FIG. 3  is an enlarged partial top view of the tip part of the drill shown in  FIG. 1 ; 
         FIG. 4  is an enlarged cross-sectional view at the position A-A of the drill shown in  FIG. 1 ; 
         FIG. 5  is an enlarged projection view of the cutting edge part on a projection plane parallel to the tool axis for explaining the shape of each cutting edge part shown in  FIG. 1 ; 
         FIG. 6  is a partial front view showing an example of the point angles and the relief angles of the tip cutting edge shown in  FIG. 1 ; 
         FIG. 7  is a projection view showing an example of the margins formed only within a partial range of the hold part shown in  FIG. 1 ; 
         FIG. 8  is a view for explaining an example of a method of drilling a workpiece by attaching the drill to a handheld tool rotating device; 
         FIG. 9  is a partial front view of a drill according to the second implementation of the present invention; 
         FIG. 10  is a partial front view of a drill according the third implementation of the present invention; 
         FIG. 11  is an enlarged left side view of the tip part of the drill shown in  FIG. 10 ; 
         FIG. 12  is an enlarged partial top view of the tip part of the drill shown in  FIG. 10 ; 
         FIG. 13  is an enlarged cross-sectional view at the position B-B of the drill shown in  FIG. 10 ; 
         FIG. 14  is an enlarged projection view of the tip part of the drill shown in  FIG. 10 ; 
         FIG. 15  is a partial front view of a drill according to the fourth implementation of the present invention; 
         FIG. 16  is an enlarged left side view of the tip part of the drill shown in  FIG. 15 ; 
         FIG. 17  is an enlarged partial top view of the tip part of the drill shown in  FIG. 15 ; 
         FIG. 18  is an enlarged cross-sectional view at the position C-C of the drill shown in  FIG. 15 ; 
         FIG. 19  is an enlarged partial front view showing the first example of a shape of a cutting edge part formed in a drill according to the fifth implementation of the present invention; 
         FIG. 20  is an enlarged partial front view showing the second example of a shape of a cutting edge part formed in a drill according to the fifth implementation of the present invention; 
         FIG. 21  is an enlarged partial projection view of reamer edges formed in a drill according to the sixth implementation of the present invention; 
         FIG. 22  is an enlarged partial projection view showing an example of the reamer edge whose point angle remains 0 degrees without decreasing; 
         FIG. 23  is a projection view showing the first example of a shape of a tip cutting edge formed in a drill according to the seventh implementation of the present invention; 
         FIG. 24  is a projection view showing the second example of a shape of a tip cutting edge formed in a drill according to the seventh implementation of the present invention; 
         FIG. 25  is a projection view showing the third example of a shape of a tip cutting edge formed in a drill according to the seventh implementation of the present invention; and 
         FIG. 26  is a projection view showing the fourth example of a shape of a tip cutting edge formed in a drill according to the seventh implementation of the present invention. 
     
    
    
     DETAILED DESCRIPT 1 ON 
     A drill and a method of producing a drilled product according to implementations of the present invention will be described with reference to the accompanying drawings. 
     (First Implementation) 
     (Structure and Function of Drill) 
       FIG. 1  is a front view, of which arrow direction is partially changed, of a drill according to the first implementation of the present invention.  FIG. 2  is an enlarged left side view of the drill shown in  FIG. 1 .  FIG. 3  is an enlarged partial top view of the tip part of the drill shown in  FIG. 1 .  FIG. 4  is an enlarged cross-sectional view at the position A-A of the drill shown in  FIG. 1 . Note that, the lower side than the tool axis AX in  FIG. 1  shows the outline of the outer shape of a cutting edge of the drill when the rake face of the cutting edge is developed so that the rake face may be parallel to the figure, and illustration of the detailed outline of the inner shape of the cutting edge has been omitted since the cutting edges formed in the drill are symmetric with each other around the tool axis AX, i.e., the shapes of the cutting edges formed in the drill are the same. 
     A drill  1  is a drilling tool which can drill a workpiece with high quality whether the workpiece is an FRP or a metal. That is, the drill  1  has a shape which can drill not only a metal, such as aluminum and titanium, hut also an FRP, such as CFRP and GFRP, and a material obtained by overlapping an FRP with a metal, with high accuracy. 
     The drill  1  has a body  2 , in the tip side in the tool axis AX direction of the drill  1 , and a shank  3  in the rear end side in the tool axis AX direction of the drill  1 . Each of the body  2  and the shank  3  can be made of a typical material, such as a cemented carbide, a tool steel or a PCD (Polycrystalline diamond). When the body  2  and the shank  3  are made of a tool steel, it is practical to use a high-speed steel. Moreover, a desired coating, such as diamond coating or DLC (Diamond Like Carbon) coating, may be applied in order to improve wear resistance, thermal shock resistance, resistance to melting, or the like, regardless of a material of the body  2  and the shank  3 . 
     The shank  3  is a portion for holding the drill  1  with a holder of a handheld tool rotating device, a machine tool or the like. Although the shank  3  has a special shape which can be easily attached to and detached from a holder in an example shown in  FIG. 1 , the shank  3  may have a desired shape, such as a straight shank or a tapered shank. When the shank  3  has a simple shape like a straight shank and a tapered shank, the body  2  and the shank  3  may consist of an integral material, i.e., the drill  1  may be a solid drill. On the contrary, when the shape of the shank  3  is complicated, the drill  1  may be a tipped drill of which the body  2  is inserted into and bonded with the shank  3  as exemplified in  FIG. 1 . 
     A cutting edge part  4  for drilling a workpiece is formed at the tip of the body  2 . The cutting edge part  4  has tip cutting edges  5 , which are the first cutting edges, reamer cutting edges  6 , which are the second cutting edges, and a holding part  7 . The tip cutting edges  5  are formed in the tip side of the cutting edge part  4  in the tool axis AX direction. Meanwhile, the reamer cutting edges  6  are formed in the rear end side of the cutting edge part  4  in the tool axis AX direction. The reamer cutting edges  6  are formed at positions away, in the rear end side, from the tip cutting edges  5 . The holding part  7  is formed between the tip cutting edges  5  and the reamer cutting edges  6 . 
     Each of the tip cutting edges  5  has a rake face  5 A and a flank  5 B, and a ridgeline formed as a line of intersection of the rake face  5 A and the flank  5 B is a tip of the tip cutting edge  5 . Similarly, each of the reamer cutting edges  6  also has a rake face  6 A and a flank  6 B, and a ridgeline formed as a line of intersection of the rake face  6 A and the flank  6 B is a tip of the reamer cutting edge  6 . Note that, a flank is also called a second face, and a relief angle, which is a clearance angle between a flank and a machined face, is also called a second angle. 
     Although the three tip cutting edges  5  and reamer cutting edges  6  have been formed in the cutting edge part  4  in an example shown in  FIG. 1  to  FIG. 4 , the number of the tip cutting edges  5  and reamer cutting edges  6  is flexible. Specifically, although the drill  1  exemplified in  FIG. 1  to  FIG. 4  has three cutting edges consisting of the tip cutting edges  5  and the reamer cutting edges  6 , formed on three lands  9  formed between three grooves  8  respectively, adjacent planar relief faces  9 A, the drill  1  may have two cutting edges, four cutting edges, or five cutting edges. Since the tip cutting edges  5  and the reamer cutting edges  6  are formed in the lands  9  respectively, the number of the tip cutting edges  5  is the same as the number of the reamer cutting edges  6 . That is, the number of the tip cutting edges  5  and the number of the reamer cutting edges  6  are equal to the number of the lands  9 . 
     Moreover, although the drill  1  exemplified in  FIG. 1  to  FIG. 4  is a straight fluted drill having the untwisted grooves  8 , the drill  1  may be a twisted drill having the twisted grooves  8 . When the groove  8  is a V-groove, i.e., the drill  1  is a straight fluted drill, the rigidity of the drill  1  can be increased. On the contrary, when the groove  8  is a twisted groove, cutting resistance can be decreased. 
       FIG. 5  is an enlarged projection view of the cutting edge part  4  on a projection plane parallel to the tool axis AX for explaining the shape of each cutting edge part  4  shown in  FIG. 1 .  FIG. 6  is a partial front view showing an example of the point angles α and the relief angles γ 1  of the tip cutting edge  5  shown in  FIG. 1 . 
       FIG. 5  shows an example of a shape obtained by projecting a passing area of the cutting edge part  4 , on a projection plane parallel to the tool axis AX, in case of rotating the cutting edge part  4  around the tool axis AX. That is,  FIG. 5  shows an example of a shape obtained by projecting the cutting edge part  4 , rotated around the tool axis AX, in a direction perpendicular to the tool axis AX. 
     The point angle α of the tip cutting edges  5  is more than 0° and less than 180° at the tip. Specifically, the point angle α of the tip cutting edges  5  at the tip of the drill  1  in the tool axis AX direction has a relation of 0°&lt;α&lt;180°. In other words, the drill  1  is not a flat drill or a candle shaped drill (a candle sharpened drill) of which point angle is 180°. 
     A point angle of a typical drill is 118° or 120°, and the point angle α at the tool center position at the tip of the drill  1  can be also determined as an appropriate angle according to cutting conditions, such as a material of a workpiece and the tool diameter D. When a workpiece is made of a metal or an FRP, it is desirable to set the point angle α at the tool center position at the tip of the tip cutting edges  5  to 90°&lt;α&lt;150° from a viewpoint of making cutting conditions, such as cutting resistance, preferable. 
     Moreover, each tip cutting edge  5  has such a shape that the point angle α continuously decreases from the tip side of the tool axis AX direction and the drill  1  toward the rear end side. Therefore, the point angle α at the tip of the tip cutting edges  5  becomes the maximum value of the point angle α. Moreover, the diameter of the tip cutting edges  5  continuously increases from the tip side toward the rear end side. Meanwhile, the relief angle γ 1  of each tip cutting edge  5  is also formed so as to continuously decrease from the tip side of the tool axis AX direction and the drill  1  toward the rear end side. Therefore, the relief angle γ 1  of each tip cutting edge  5  also becomes the maximum value at the tip. 
       FIG. 6  shows an example of the point angle α and the relief angle γ 1  at the position P 1 , the position P 2  and the position P 3  of the tip cutting edge  5 . In an example shown in  FIG. 6 , the point angle α of the tip cutting edge  5  gradually and continuously decreases so as to be 60°, 30° and 20° between the tip side and the rear end side while the relief angle γ 1  of the tip cutting edge  5  gradually and continuously decreases so as to be 37°, 33° and 31° between the tip side and the rear end side. 
     It is desirable to determine the relief angle γ 1  at the tip of the tip cutting edge  5  so that a relation of γ 1 &gt;(180°−α)/2 may be established between the relief angle γ 1  at the tip and the point angle α at the tip, from a viewpoint of improving cutting conditions. Therefore, when the point angle α at the tip of the tip cutting edge  5  is 120°, the relief angle γ 1  at the tip of the tip cutting edge  5  can be determined to about 45°, for example. 
     When the point angle α of the tip cutting edges  5  is decreased continuously, an effect that the risk of chipping can be reduced can be attained since no corner which may cause chipping is generated on the ridgeline of each tip cutting edge  5 . As a result, the tool life of the drill  1  can be increased. In addition, when the point angle α of the tip cutting edges  5  is decreased gradually, an effect that generation of delamination can be reduced in case of drilling an FRP can be attained. This is because delamination temporarily generated due to drilling an FRP by the tip cutting edges  5 , having the relatively large point angle α, in the tip side of the drill  1  is removed by drilling by the tip cutting edges  5 , having the relatively small point angle α, continuing to the rear side because of small cutting resistance. 
     Moreover, the more a shape of the ridgelines of the tip cutting edges  5  on a projection plane parallel to the tool axis AX is approximated to a parabola, the more the volume of a workpiece cut by the ridgelines per unit length in the tool axis AX direction becomes uniform. Therefore, an effect that cutting resistance can be made uniform over the whole tip cutting edges  5  can be attained. When cutting resistance can be made uniform over the whole tip cutting edges  5 , it becomes possible to reduce not only the generation of delamination and burrs at the time of drilling an FRP but also wear of the tip cutting edges  5 . 
     Meanwhile, when a shape of the ridgelines of the tip cutting edges  5  on a projection plane parallel to the tool axis AX is approximated to an ellipse, the volume of a workpiece cut by the ridgelines per unit length in the tool axis AX direction can be decreased at a constant rate as the ridgelines are located in the rearer side of the tip cutting edges  5 . Therefore, the situation that the wear in posterior portions of the tip cutting edges  5 , of which a cutting speed is relatively fast due to different radii of rotation, progresses earlier than the wear in anterior portions of the tip cutting edges  5 , of which a cutting speed is relatively slow, can be avoided, and thereby an effect that the degree of progress of the wear can be made uniform over the whole tip cutting edges  5  can be attained. When the degree of progress of the wear can be made uniform over the whole tip cutting edges  5 , the life of the tip cutting edges  5  can be improved. 
     Therefore, the shape of the ridgelines of the tip cutting edges  5  on a projection plane can be approximated to a parabola or an ellipse depending on an effect which is thought as important. As a matter of course, the shape of the ridgelines of the tip cutting edges  5  on a projection plane may he determined so that a part may have a shape close to an ellipse while another part may have a shape close to a parabola. 
     Note that, if the shape of the ridgelines of the tip cutting edges  5  on a projection plane parallel to the tool axis AX is entirely made to a parabola or an ellipse, the point angle α at the tip of the tip cutting edges  5  becomes 180°. Therefore, it is necessary to determine the shape of the ridgeline of the tip cutting edge  5  so that the point angle α at the tip of the tip cutting edges  5  may become an angle desirable for drilling a workpiece. 
     Accordingly, the shape of the ridgeline of the tip cutting edge  5  can be determined so that only a portion near the tip of the ridgelines of the tip cutting edges  5 , e.g., the tip side from positions PT 0  shown in  FIG. 5 , may be straight lines on a projection plane as exemplified by  FIG. 5 ,  FIG. 6  and the like, for example. In this case, the shape of the ridgeline of the tip cutting edge  5  can be determined so that the shape on a projection plane of the ridgelines, in the rear side from the positions PT 0 , following the linear ridgelines at the tip of the tip cutting edges  5  may be a quadratic curve, such as a parabola or an ellipse. 
     Alternatively, the shape of the ridgeline of the tip cutting edge  5  may be determined so that an intersection point of two line-symmetric parabolas derived by offsetting their symmetrical axes in the directions opposite to each other by a same distance from a projected line of the tool axis AX on a projection plane or an intersection point of two line-symmetric ellipses derived by offsetting their long axes in the directions opposite to each other by a same distance from a projected line of the tool axis AX on a projection plane may be a projected point of the tip of the tip cutting edges  5  and a part of the two symmetric parabolas or ellipses in the side close to the projected line of the tool axis AX may be a projected line of the ridgelines of the tip cutting edges  5 . 
     Since producing the tip cutting edges  5  of which outline on a projection plane becomes an ellipse, two ellipses, a parabola or two parabolas becomes complicated, the ellipse, the two ellipses, the parabola or the two parabolas may be simulated by connecting circles to each other along the ellipse, the two ellipses, the parabola or the two parabolas serving as a reference line RL or reference lines RL. In case of an example shown in  FIG. 5 , a position PT 0  may be connected with a position PT 1  by a single circle having a same radius, the position PT 1  may be connected with a position PT 2  by another single circle having a larger radius, and the position PT 2  may be connected with a position PT 3  by another single circle having an even larger radius, on a projection plane, for example. That is, determining the shape of the ridgeline of the tip cutting edge  5  so that a projected line of the ridgelines of the tip cutting edges  5  becomes a smooth curve without corners obtained by connecting lines and circles with each other can attain a merit of approximating the projected line of the ridgelines to an ellipse, two ellipses, a parabola or two parabolas while reducing manufacturing costs. 
     When the relief angle γ 1  of each tip cutting edge  5  is gradually decreased as mentioned above as well as the point angle α of the tip cutting edges  5  is gradually decreased, an effect that wear resistance of the tip cutting edges  5  can be improved is attained. This is because corners easily worn out are not generated on the flank  5 B of each tip cutting edge  5  when the relief angle γ 1  of each tip cutting edge  5  is continuously and smoothly decreased. 
     A rake angle of each tip cutting edge  5  can be determined according to a material of a workpiece. When the rake angle of each tip cutting edge  5  is made 0°, i.e., the rake face  5 A of each tip cutting edge  5  is made perpendicular to the cutting face of a workpiece, an effect that delamination is hardly generated at the time of drilling an FRP is attained. This is because the tip cutting edges  5  without a rake angle can finely cut an FRP and reduce open seams of fibers included in the FRP. Therefore, when improving quality of a hole machined on an FRP is considered as important, the rake angle of each tip cutting edge  5  can be made zero. 
     On the contrary, when each tip cutting edge  5  has the rake angle which is not zero, drilling conditions for drilling a metal can be made preferable. Therefore, when improving quality of a hole machined on a metal is considered as important, each tip cutting edge  5  may has the rake angle which is not zero. In an illustrated example, the rake angle of each tip cutting edge  5  is 0°. That is, the rake angle is not formed on each tip cutting edge  5 . 
     On the other hand, each reamer cutting edge  6  may also have such a shape that a point angle β continuously decreases down to 0° and a relief angle γ 2  continuously decreases from the tip side of the tool axis AX direction and the drill  1  toward the rear end side. Therefore, the diameter of the reamer cutting edges  6  also continuously increases from the tip side toward the rear end side. As a result, the point angle β becomes 0° at the maximum diameter position of the reamer cutting edges  6 . That is, each reamer cutting edge  6  has the non-zero relief angle γ 2  at the maximum diameter position at which the point angle β becomes 0°. 
     The maximum diameter of the reamer cutting edges  6  in the rear end side is larger than the maximum diameter D 1  of the tip cutting edges  5 . Therefore, the maximum diameter of the reamer cutting edges  6  is the tool diameter D of the drill  1 . The maximum diameter of the holding part  7  is made not more than the maximum diameter D 1  of the tip cutting edges  5 . Therefore, when a workpiece is machined by the tip cutting edges  5  of which maximum diameter D 1  is smaller than the tool diameter D, a prepared hole having a diameter smaller than that of a finished hole to be machined is machined on the workpiece. Accordingly, the tip cutting edges  5  undertake a role of drilling a prepared hole on a workpiece. 
     When the drill  1  is fed out in the tool axis AX direction while drilling a workpiece, the holding part  7  is inserted into the prepared hole machined by the tip cutting edges  5  since the maximum diameter of the holding part  7  following the tip cutting edges  5  is not more than the maximum diameter D 1  of the tip cutting edges  5 . Accordingly, the outer surface having the maximum diameter of the holding part  7  contacts the inner surface of the prepared hole, and thereby the holding part  7  is guided by the prepared hole. 
     As a result, deflection of the holding part  7  and the reamer cutting edges  6  following the holding part  7  can be reduced during drilling. That is, the holding part  7  functions as a deflection stopping part which reduces deflection of the reamer cutting edges  6  by being inserted into a prepared hole machined by the tip cutting edges  5  during drilling a workpiece. 
     It is practical to form the holding part  7  by margins  7 A each having an appropriate width, and not to form any cutting edge in the holding part  7 , from a viewpoint of letting the holding part  7  fulfill a function as a deflection stopper of the reamer cutting edges  6  while avoiding an excessive rise in temperature due to friction between a workpiece and the holding part  7 . When the holding part  7  is formed by the margins  7 A having no cutting edges, the margins  7 A can be formed so that the tip cutting edges  5  are connected with the reamer cutting edges  6  by the margins  7 A respectively. 
     The width of each margin  7 A included in the holding part  7  can be determined to an appropriate width according to the maximum diameter D 1  of the tip cutting edges  5  equivalent to the diameter of a prepared hole. As a concrete example, when the maximum diameter D 1  of the tip cutting edges  5  is about 3 mm to 10 mm, the width of each margin  7 A can be determined within a range of 0.1 mm to 1.5 mm, e.g., to about 0.3 mm. 
     When the drill  1  is further fed out in the tool axis AX direction while drilling the workpiece, the prepared hole is reamed by the reamer cutting edges  6  formed at positions away, in the rear end side, from the tip cutting edges  5 . Specifically, hole-finish processing for extending the inside diameter of the prepared hole, machined on the workpiece, up to the tool diameter D equivalent to the maximum diameter of the reamer cutting edges  6  can be performed. Thereby, a hole having a diameter corresponding to the tool diameter D can be machined on the workpiece. 
     Note that, although “a reamer cutting edge” may mean a cutting edge, having a specific shape for finishing a hole, formed on the lateral face of a tool, in a narrow sense, “a reamer cutting edge” is used in a broad sense here, and a cutting edge for finishing a hole is written as “a reamer cutting edge” regardless of the shape of the cutting edge. Therefore, “reaming” is also used in a broad sense here, and finishing a hole is written as “reaming” regardless of the shape of a cutting edge for finishing of a hole. 
     As exemplified by  FIG. 5 , when the ridgelines of the reamer cutting edges  6  are determined so that the ridgelines of the reamer cutting edges  6  and the ridgelines of the tip cutting edges  5  on a projection plane parallel to the tool axis AX may lie on the reference line RL consisting of a common quadratic curve or common quadratic curves, such as an ellipse, ellipse, a parabola or parabolas, the drill  1  can be produced by a production method by which a semi-product of the drill  1  whose ridgelines of the reamer cutting edges  6  have been connected with the ridgelines of the tip cutting edges  5  respectively is produced, and subsequently the holding part  7  is formed by cutting or polishing process. 
     In this case, steps are generated in the tip side of the reamer cutting edges  6  just by forming the holding part  7  on a semi-product of the drill  1  having cutting edges along the reference line RL consisting of at least one quadratic curve. That is, the tool diameter of the drill  1  increases stepwise at the tip of the reamer cutting edges  6 . As a result of tests, it was confirmed that steps in the tip side of the reamer cutting edges  6  allowed reaming an FRP under a satisfactory condition while the steps in the tip side of the reamer cutting edges  6  made it difficult to ream a metal under a satisfactory condition. 
     Accordingly, when a hole is made machinable with satisfactory quality not only on an FRP but also on a metal, it is appropriate to form cutting edges, having the largest and constant point angle β, at the tip of the reamer cutting edges  6 , and connect the cutting edges, of which point angle β and each relief angle γ 2  decrease continuously, with the rear side of the cutting edges having the constant point angle β as illustrated. As a result of tests, it was confirmed that it is preferable to make the largest point angle β in the tip side of the reamer cutting edges  6  not less than 80° and not more than 100° when a hole is made machinable with satisfactory quality not only on an FRP but also on a metal. 
     Note that, when the drill  1  is dedicated for drilling an FRP, the cutting edges having the constant point angle β may not be formed in the tip side of the reamer cutting edges  6  so that steps may be formed since the point angle β of not less than 80° and not more than 100° is not necessarily required for reaming an FRP. In that case, the largest point angle β in the tip side of the reamer cutting edges  6  is the maximum of the point angle β which decreases continuously. 
     Regardless of whether the cutting edges having the constant point angle β are formed in the tip side of the reamer cutting edges  6 , the point angle β in the rear end side of the reamer cutting edges  6  is made 0° as mentioned above. Since any parabola has no tangent line parallel to the symmetrical axis, it is convenient to determine a shape of the ridgelines of the reamer cutting edges  6  so that a projected line of the ridgeline of the whole reamer cutting edge  6  or a projected line of at least a part of the ridgeline of the reamer cutting edge  6  in the rear end side may become a portion of an ellipse which crosses a short axis, in order to make the point angle β in the rear end side of the reamer cutting edges  6  be 0°. 
     Therefore, when the respective shapes of the ridgelines of the tip cutting edges  5  and the ridgelines of the reamer cutting edges  6  are determined so that the ridgelines of portions other than the tip, of the tip cutting edges  5  and the ridgelines of the reamer cutting edges  6  on a projection plane parallel to the tool axis AX may lie on circles which simulates the reference line RL consisting of a common ellipse or common ellipses, it becomes possible to reduce labor and costs for manufacturing the drill  1  whose point angle β in the rear end side of the reamer cutting edges  6  is 0°, by adopting a production method by which a semi-product of the drill  1  whose ridgelines of the tip cutting edges  5  have been connected with the ridgelines of the reamer cutting edges  6  respectively is produced, and subsequently the holding part  7  is formed by cutting work or polishing processing. 
     When the non-zero relief angle γ 2  is formed at the maximum diameter position at which the point angle β of the reamer cutting edges  6  becomes 0°, reaming can be performed without generating burrs since the reamer cutting edges  6  at the maximum diameter position contact in parallel with the inner surface of the prepared hole machined by the tip cutting edges  5 . As a result, a finished hole can be machined on a workpiece with satisfactory quality. 
     Note that, it is desirable to make the relief angle γ 2  of each reamer cutting edge  6  at the maximum diameter position of the reamer cutting edges  6  be less than 15° from a viewpoint of preventing chipping and wear of the reamer cutting edges  6 . Therefore, the maximum relief angle of each of the cutting edges having the constant point angle β in the tip side of the reamer cutting edges  6  can be made about 11° to 12° while the minimum relief angle γ 2  of each reamer cutting edge  6  at the maximum diameter position of the reamer cutting edges  6  can be made about 10°, for example. Meanwhile, a rake angle of each reamer cutting edge  6  can be formed at 0° or an appropriate angle, similarly to the tip cutting edge  5 . 
     A margin  10  can be formed in the rear end side of each reamer cutting edge  6  so that the margin  10  continues from the reamer cutting edge  6  as exemplified by  FIG. 1 ,  FIG. 3  and  FIG. 4 . Thereby, the body  2  of the drill  1  can be guided by a hole, which has been finished by the reamer cutting edges  6 , so that deflection of the drill  1  can be reduced. The width of each margin  10  can be determined as an appropriate width according to the tool diameter D. As a concrete example, when the tool diameter D is about 3 mm to 10 mm, the width of each margin  10  can be determined within a range of 0.1 mm to 1.5 mm, e.g., to about 0.3 mm. 
     When the margins  7 A are formed in the holding part  7  while the margins  10  are also formed behind the cutting edge part  4 , the margins  7 A in the holding part  7  can be guided by a prepared hole while the margins  10  behind the reamer cutting edges  6  can be guided by a finished hole. That is, deflection of the reamer cutting edges  6  can be suppressed at two positions in the tool axis AX direction which sandwich the reamer cutting edges  6 . As a result, off-centering of the drill  1  in the portion of the reamer cutting edges  6  can be prevented, and thereby a finished hole can be machined with satisfactory quality. 
     The margins  7 A formed in the holding part  7  may have a constant diameter or may have a back taper. When the diameter of the holding part  7  is made constant with no back taper, the effect of reducing deflection of the holding part  7  can be maximized since the whole margins  7 A of the holding part  7  are contacted with the inner surface of a prepared hole. 
     When the whole margins  7 A of the holding part  7  are contacted with the inner surface of a prepared hole, the possibility that temperature of a workpiece may rise due to frictional heat between the margins  7 A and the workpiece becomes high. Accordingly, when a workpiece is made of a material, such as aluminum, of which melting point is low, the workpiece may be welded. Thus, when a workpiece made of a material, such as aluminum, having a low melting point is drilled, welding of the workpiece can be prevented by forming a back taper on the margins  7 A of the holding part  7 . 
     On the contrary, when a hard-to-cut material, such as titanium or an FRP, having low thermal conductivity is drilled, an effect of reduction in deflection of the holding part  7  may be improved by omitting a back taper or forming a small back taper on the margins  7 A of the holding part  7  since cutting resistance becomes large and the hard-to-cut material is hard to be welded. 
     As a result of tests, it became clear that it was appropriate to form a back taper of not less than 0.02/100 and not more than 0.06/100 on the holding part  7  in order to achieve both preventing welding of a workpiece and preventing deflection of the holding part  7  as long as the back taper was formed. In view of the same reason, it is also appropriate to omit a back taper or form a back taper of not less than 0.02/100 and not more than 0.06/100 on the margins  10  formed behind the cutting edge part  4 . 
     When the maximum diameter of the holding part  7  is made too small, the maximum diameter D 1  of the tip cutting edges  5  also becomes small, by which the rigidity of the tip cutting edges  5  and the holding part  7  deteriorates, and the amount of depth of cut in finish processing by the reamer cutting edges  6  may be excessive. In addition, the smaller the maximum diameter D 1  of the tip cutting edges  5  is, the more the risk of chip clogging increases since the grooves  8  become shallower. Accordingly, it is important to secure the rigidity and sizes of the tip cutting edges  5  and the holding part  7  while making the amount of depth of cut in finish processing by the reamer cutting edges  6  appropriate in order to stably machine a hole with high quality. As a result of tests, it became clear that it was desirable to make the maximum diameter of the holding part  7  be not less than 0.5 times and not more than 0.9 times of the tool diameter D equivalent to the maximum diameter of the reamer cutting edges  6 . 
     The length Lh of the holding part  7  in the tool axis AX direction can be determined based on the cutting edge length L of the whole drill  1  and the appropriate length L 2  of the reamer cutting edges  6 . The cutting edge length L of the whole drill  1  is expressed as the sum of the length L 1  of the tip cutting edges  5  in the tool axis AX direction, the length Lh of the holding part  7  in the tool axis AX direction, and the length L 2  from the minimum diameter position of the reamer cutting edges  6  to the position, at which the diameter of the reamer cutting edges  6  firstly becomes the maximum, in the tool axis AX direction. 
     Specifically, a condition for determining realistic shapes of the ridgelines of the tip cutting edges  5  and the reamer cutting edges  6  along a preferable quadratic curve or preferable quadratic curves while determining the point angle α of the tip cutting edges  5  as an appropriate angle is that the cutting edge length L expressed as the sum of the length L 1  of the tip cutting edges  5  in the tool axis AX direction, the length Lh of the holding part  7  in the tool axis AX direction and the length L 2  from the minimum diameter position of the reamer cutting edges  6  to the position, at which the diameter of the reamer cutting edges  6  firstly becomes the maximum, in the tool axis AX direction is made not less than one time and not more than twice of the maximum diameter of the reamer cutting edges  6  equivalent to the tool diameter D (i.e., 1D≤L≤2D). 
     This is because a too short cutting edge length L compared with the tool diameter D geometrically necessitates to enlarge the point angle α or to approximate the projected lines of the tip cutting edges  5  and the reamer cutting edges  6  to an arc, an ellipse whose short axis is parallel to the tool axis AX, or the like, which is unfavorable. 
     Meanwhile, in case of drilling a workpiece, such as a plate, to form a through hole, a too long cutting edge length L compared with the tool diameter D of the drill  1  causes a problem that drilling may become impossible due to interference with a jig, another portion of the workpiece, or the like since the cutting edge part  4  having the cutting edge length L projects behind the workpiece. 
     In addition, when the cutting edge length L of the drill  1  is too long compared with the tool diameter D of the drill  1 , only a deep hole can be finished with favorable accuracy inconveniently. This is because the tip cutting edges  5  has penetrated a workpiece before the reamer cutting edges  6  at the maximum diameter position starts to finish a prepared hole, and therefore the effect that the deflection of the reamer cutting edges  6  is suppressed by both the holding part  7  and the margins  10  is spoiled, except for a case of machining a through hole whose diameter is equivalent to the tool diameter D, on the workpiece thicker than the cutting edge length L of the drill  1 . That is, the longer the cutting edge length L of the drill  1  is, the longer machining a through hole is required for certainly preventing the accuracy of the finished hole from deteriorating. 
     Under the above-mentioned restriction, the cutting edge length L of the whole drill  1  is determined depending on the tool diameter D. Accordingly, when the maximum diameter D 1  of the tip cutting edges  5  and the maximum diameter of the holding part  7  are determined depending on the tool diameter D, the length L 1  of the tip cutting edges  5  is inevitably determined since the radius of rotation of the tip cutting edges  5  is determined on the basis of the reference line RL which consists of a quadratic curve or quadratic curves, such as a parabola, parabolas, an ellipse or ellipses. Therefore, the length derived by subtracting the length L 1  of the tip cutting edges  5  from the cutting edge length L of the whole drill  1  becomes the sum of the length Lh of the holding part  7  and the length L 2  of the reamer cutting edges  6 . Accordingly, the length derived by subtracting the length L 2  appropriate as the reamer cutting edges  6  can be determined as the length Lh of the holding part  7 . 
     As a concrete example, the length L 2  of the reamer cutting edges  6  can be set to 0.1 times the tool diameter D (i.e., L 2 =0.1D) while the sum of the length L 1  of the tip cutting edges  5  and the length Lh of the holding part  7  can be set to 1.5 times the tool diameter D (i.e., L 1 +Lh=1.5D). 
     Note that, the length of the margins  7 A required to let the holding part  7  fulfill the function to stop deflection may become shorter than the length Lh of the holding part  7 . Moreover, when a contact area between the margins  7 A and a prepared hole machined by the tip cutting edges  5  is too large, temperature may increase excessively. 
     For that reason, a back taper may be formed partially instead of forming a back taper over the whole holding part  7 , or a portion having a constant diameter may be formed partially. Experientially, it is considered that making the diameter or the back taper constant in a range whose length is at least ⅕ of the length Lh of the holding part  7  in the tool axis AX direction (≥Lh/5) makes it possible to obtain the effect of stopping deflection of the reamer cutting edges  6  sufficiently. That is, it is considered that the effect of reduction in deflection of the reamer cutting edges  6  can be attained so long as the margins  7 A having the constant maximum diameter or the constant back taper are formed in the range having at least ⅕ length in the tip cutting edge  5  side of the holding part  7 . 
       FIG. 7  is a projection view showing an example of the margins  7 A formed only within a partial range R of the hold part  7  shown in  FIG. 1 . 
     As shown by the projection view in  FIG. 7 , it is a preferred condition to form the margins  7 A in the range R whose length is not less than 20% and not more than 100% of the length Lh of the holding part  7 . In other words, the margins  7 A may be formed only in the partial range R of the holding part  7  while the radius of rotation may be decreased in the remaining range. Therefore, when the margins  7 A are formed by polishing, only a certain range R may be polished with high accuracy to form the margins  7 A while a back clearance may be formed in the other range by rough polish processing. 
     (Method of Producing Drilled Product) 
       FIG. 8  is a view for explaining an example of a method of drilling a workpiece W by attaching the drill  1  to a handheld tool rotating device  20 . 
     A drilled product can be produced as a product or a semi-product by drilling the workpiece W using the drill  1  held by the handheld tool rotating device  20  as shown in  FIG. 8 . When the workpiece W is drilled by the handheld tool rotating device  20 , the drill  1  can be positioned to the workpiece W by attaching a nosepiece  22  having a bushing chip  21  to the tool rotating device  20  while placing a drill plate  23  on the workpiece W. 
     Specifically, the position and direction of the drill  1  can be positioned by fixing the bushing chip  21  to the drill plate  23 . Then, not only the workpiece W made of a single material, such as an FRP or a metal, but also the workpiece W made of materials, such as overlapped materials of an FRP and a metal, can be drilled using the drill  1 , for example. 
     Although the tool rotating device  20  in an example shown in  FIG. 8  has an automatic feeding function of the drill  1 , the workpiece W may be drilled by the simple tool rotating device  20  having only a rotation function of the drill  1 . As a matter of course, a drilled product may be produced by attaching the drill  1  to a machine tool, such as a drill press or a machining center, or a drilling machine which is not handheld. 
     (Effects) 
     As described above, the drill  1  has the tip cutting edges  5  each having the continuously decreasing point angle α and relief angle γ 1 , the reamer cutting edges  6  each having the continuously decreasing point angle β and relief angle γ 2  which does not becomes zero at the maximum diameter position at which the point angle β becomes 0°, and the hold part  7  consisting of the margins  7 A and the like, for reducing deflection of the reamer cutting edges  6 , between the tip cutting edges  5  and the reamer cutting edges  6 . Meanwhile, a method of producing a drilled product is to produce the drilled product using the above-mentioned drill  1 . 
     Accordingly, the drill  1  and the method of producing a drilled product allow reaming of a prepared hole drilled by the tip cutting edges  5 , by the reamer cutting edges  6  while being guided with the holding part  7  by utilizing the prepared hole. That is, vibration of the reamer cutting edges  6  can be suppressed by the holding part  7 . Therefore, a hole having circular cross section can be machined with high quality. 
     In particular, there was a problem that a hole drilled by a drilling device whose spindle had low rigidity or accuracy unfavorably became a polygon in a case where the number of cutting edges of the conventional R-drill (registered trademark), of which point angle and relief angle of each cutting edge continuously changed toward the maximum diameter position of the cutting edges and the relief angle at the maximum diameter position of the cutting edges was nonzero, was not less than three. It is considered that this conventional problem is because the coaxiality of the tool axis of the R-drill (registered trademark) cannot be secured, and thereby the R-drill (registered trademark) is deflected when the rigidity or accuracy of a spindle of a drilling device holding the R-drill (registered trademark) is low. Specifically, it is considered that a hole unfavorably becomes a polygon since the cutting edges in the lateral side of the R-drill (registered trademark) contact with the inner surface of the hole while vibrating. 
     On the other hand, the above-mentioned drill  1  derived by improving the R-drill (registered trademark) is used, a finished hole having a circular cross section can be machined by finish processing of a prepared hole by the reamer cutting edges  6  of which deflection has been suppressed by the holding part  7  even when the shape of the cross section of the prepared hole drilled by the tip cutting edges  5  becomes a polygon due to the deflection of the tip cutting edges  5 . Accordingly, a hole can be machined with satisfactory quality even when the number of the tip cutting edges  5  is made not less than three. 
     That is, even under drilling conditions under which the shape of the cross section of a hole unfavorably becomes a polygonal using the conventional R-drill (registered trademark) having at least three cutting edges, the above-mentioned drill  1  can machine a hole having a circular cross section. As a concrete example, even when the workpiece W is drilled by the drill  1  having not less than three cutting edges, which is held by the handheld tool rotating device  20  as exemplified in  FIG. 8 , a hole of which cross section has a circular shape can be machined. That is, drilling can be manually performed by the drill  1  having not less than three cutting edges. 
     When the number of the tip cutting edges  5  and the reamer cutting edges  6  of the drill  1  is increased, the cutting amount per one cutting edge of the tip cutting edges  5  and the reamer cutting edges  6  is decreased. Therefore, when the number of the tip cutting edges  5  and the reamer cutting edges  6  of the drill  1  is made not less than three, a tool life, i.e., the number of holes which can be drilled with securing quality can be increased compared with the conventional R-drill (registered trademark) having two cutting edges. 
     Moreover, also in a case where the number of the tip cutting edges  5  and the reamer cutting edges  6  is two, the quality of a hole can be improved since the vibration in the tool diameter D direction can be suppressed compared with the conventional R-drill (registered trademark) having two cutting edges. In particular, since metal chips are finely cut as a result of restricting the drill  1  to vibrate only in the tool axis AX direction, an effect that clogging with metal chips can be prevented is also obtained. That is, the vibration energy in the tool diameter D direction of the drill  1  can be transformed into the vibration energy in the thrust direction by the holding part  7 , and thereby the energy can be utilized for preventing clogging with metal chips. 
     (Second Implementation) 
       FIG. 9  is a partial front view of a drill according to the second implementation of the present invention. Note that, the lower side than the tool axis AX in  FIG. 9  shows the outline of the outer shape of a cutting edge of the drill when the rake face of the cutting edge is developed so that the rake face may be parallel to the figure, and illustration of the detailed outline of the inner shape of the cutting edge has been omitted since the cutting edges formed in the drill are symmetric with each other around the tool axis AX, i.e., the shapes of the cutting edges formed in the drill are the same. 
     A drill  1 A in the second implementation shown in  FIG. 9  is different from the drill  1  in the first implementation in a point that a portion  6 C having the constant maximum diameter is extended in the rear end side of the reamer cutting edges  6 . Other structures and actions of the drill  1 A in the second implementation are not substantially different from those of the drill  1  in the first implementation. Therefore, only a partial front view of the drill  1 A is shown, and the same signs are attached to the same elements or the corresponding elements while explanation thereof is omitted. 
     As explained in the first implementation, deflection of the reamer cutting edges  6  is suppressed by the margins  7 A of the holding part  7 . Accordingly, the margins  10  in the rear side of the reamer cutting edges  6  may be omitted while the portion  6 C which has the constant maximum diameter can be formed instead. Alternatively, the portion  6 C which has the constant maximum diameter may be formed, and the margins  10  may he formed in the rear side of the portion  6 C. Anyway, the portion  6 C, of which point angle β is 0°, having non-zero relief angle γ 2  can be lengthened. 
     In this case, even when the reamer cutting edges  6  are worn out and thereby a radius of rotation is decreased, it becomes possible to perform finish processing of a hole with the reamer cutting edges  6  at the new maximum diameter position since the maximum diameter position merely moves toward the rear side of the reamer cutting edges  6 . As a result, a tool life of the drill  1 A can be lengthened. Moreover, a deep hole can also be machined by lengthening the portion  6 C which has the constant maximum diameter. In particular, an effect of improving the quality of a deep hole can be attained when the deep hole is machined on a workpiece which has the thickness not less than 2.6 times the tool diameter D. 
     (Third Implementation) 
       FIG. 10  is a partial front view of a drill according to the third implementation of the present invention.  FIG. 11  is an enlarged left side view of the tip part of the drill shown in  FIG. 10 .  FIG. 12  is an enlarged partial top view of the tip part of the drill shown in  FIG. 10 .  FIG. 13  is an enlarged cross-sectional view at the position B-B of the drill shown in  FIG. 10 .  FIG. 14  is an enlarged projection view of the tip part of the drill shown in  FIG. 10 . 
     A drill  1 B in the third implementation shown in  FIG. 10  to  FIG. 14  is different from each of the drill  1  in the first implementation and the drill  1 A in the second implementation in a point that the point angle of the reamer cutting edges  6  continuously changes so that any step may not be generated in the tip side of the reamer cutting edges  6 . Other structures and actions of the drill  1 B in the third implementation are not substantially different from those of each of the drill  1  in the first implementation and the drill  1 A in the second implementation. Therefore, the same signs are attached to the same elements or the corresponding elements while explanation thereof is omitted. 
     As shown in  FIG. 10  to  FIG. 14 , the point angle β of the reamer cutting edges  6  can be smoothly and continuously decreased down to 0° from the tip side of the drill  1  and the tool axis AX direction toward the rear side so that any portion at which the point angle β of the reamer cutting edges  6  changes intermittently toward the tool axis AX direction may not be generated. In addition, the non-zero relief angle γ 2  can be formed at the maximum diameter position of the reamer cutting edges  6  where the point angle β of the reamer cutting edges  6  becomes 0°, as exemplified by  FIG. 14 , similarly to the first implementation. In this case, it is also practical to smoothly and continuously decrease the relief angle γ 2  of the reamer cutting edges  6  from the tip side of the drill  1  and the tool axis AX direction toward the rear side so that any portion at which the relief angle γ 2  of the reamer cutting edges  6  changes intermittently may not generated, similarly to the point angle β. 
     The portion  6 C having the constant maximum diameter may be extended in the rear end side of the maximum diameter position of the reamer cutting edges  6  where the point angle β has decreased down to 0°, as described in the second implementation. The margin  10  can be extended in the rear end side of the reamer cutting edge  6  itself, as described in the first implementation, regardless of whether the portion  6 C having the constant maximum diameter is extended. 
     The number of the tip cutting edges  5  and the reamer cutting edges  6  is flexible and not limited to a case of three cutting edges as exemplified by  FIG. 11  and  FIG. 13 . When the number of the tip cutting edges  5  and the reamer cutting edges  6  is increased, the effect that the tool life of the drill  1 B and the number of holes, which can be drilled, can be increased can be attained since the cutting amount per one cutting edge of the tip cutting edges  5  and the reamer cutting edges  6  decreases, as described in the first implementation. Moreover, the drill  1 B is not limited to a straight fluted drill having the untwisted grooves  8 , and may be a twist drill having the twisted grooves  8 , similarly to the first implementation. 
     When the point angle β of the reamer cutting edges  6  is decreased smoothly so that two point angles β may not be generated at any position of the ridgelines of the reamer cutting edges  6  as shown in  FIG. 10  and  FIG. 14 , the effect similar to the effect attained in the case where the point angle α of the tip cutting edges  5  is decreased smoothly and continuously can be attained in the case of reaming a prepared hole by the reamer cutting edges  6 . 
     More specifically, when the point angle β of the reamer cutting edges  6  is decreased continuously, an effect that the risk of chipping can be reduced can be attained since no corner which may cause chipping is generated on the ridgeline of each reamer cutting edge  6 . As a result, the tool life of the drill  1 B can be increased. In addition, when the point angle β of the reamer cutting edges  6  is decreased gradually, an effect that generation of delamination can be reduced in case of drilling an FRP can be attained. This is because delamination temporarily generated due to reaming an FRP by the reamer cutting edges  6 , having the relatively large point angle β, in the tip side of the drill  1 B is removed by reaming by the reamer cutting edges  6 , having h relatively small point angle β, continuing to the rear side because of small cutting resistance. 
     Moreover, the more a shape of the ridgelines of the reamer cutting edges  6  on a projection plane parallel to the tool axis AX is approximated to a parabola, the more the volume of a workpiece cut by the ridgelines per unit length in the tool axis AX direction becomes uniform. Therefore, an effect that cutting resistance can be made uniform over the whole reamer cutting edges  6  can be attained. When cutting resistance can be made uniform over the whole reamer cutting edges  6 , it becomes possible to reduce not only the generation of delamination and burrs at the time of reaming an FRP but also wear of the reamer cutting edges  6 . 
     Meanwhile, when a shape of the ridgelines of the reamer cutting edges  6  on a projection plane parallel to the tool axis AX is approximated to an ellipse, the volume of a workpiece cut by the ridgelines per unit length in the tool axis AX direction can be decreased at a constant rate as the ridgelines are located in the rearer side of the reamer cutting edges  6 . Therefore, the situation that the wear in posterior portions of the reamer cutting edges  6 , of which a cutting speed is relatively fast due to different radii of rotation, progresses earlier than the wear in anterior portions of the reamer cutting edges  6 , of which a cutting speed is relatively slow, can be avoided, and thereby an effect that the degree of progress of the wear can be made uniform over the whole reamer cutting edges  6  can be attained. When the degree of progress of the wear can be made uniform over the whole reamer cutting edges  6 , the life of the reamer cutting edges  6  can be improved. 
     Therefore, the shape of the ridgelines of the reamer cutting edges  6  on a projection plane can be approximated to a parabola or an ellipse depending on an effect which is thought as important. As a matter of course, the shape of the ridgelines of the reamer cutting edges  6  on a projection plane may be determined so that a part may have a shape close to an ellipse while another part may have a shape close to a parabola. Production of the reamer cutting edges  6  of which at least a part of the outline on a projection plane resides on an ellipse or a parabola is complicated. Therefore, an ellipse or a parabola may be simulated by smoothly connecting line segments and/or circular arcs to each other so that no corner may be generated. 
     When the point angle β of the reamer cutting edges  6  is decreased continuously so that no corner may be generated while the shape of the reamer cutting edges  6  on a projection plane parallel to the tool axis AX is determined to a shape simulating an ellipse or a parabola with line segments and/or arcs as mentioned above, the ridgelines of the tip cutting edges  5  and the reamer cutting edges  6  respectively become ridgelines simulating reference lines RL 1  and RL 2  different from each other, as exemplified by  FIG. 14 . 
     More specifically, the shape of the projected line  5 C derived by projecting a passing area of the tip cutting edges  5 , rotated around the tool axis AX, on a projection plain parallel to the tool axis AX becomes a shape, derived by connecting the first circular arcs and/or line segments with each other, which is line symmetric around a straight line derived by projecting the tool axis AX on the projection plane. As a concrete example, the first circular arcs and line segments can be determined so that at least six points PT 10 , PT 11 , and PT 12  on the projected line  5 C derived by connecting the first circular arcs and line segments with each other may be on the first reference line RL 1  consisting of the first quadratic curve, as shown in  FIG. 14 . 
     The first quadratic curve used as the first reference line RL 1  can be determined as any one of a single parabola, two parabolas, a single ellipse or two ellipses, line symmetric around a straight line derived by projecting the tool axis AX on the projection plane, as described in the first implementation.  FIG. 14  shows an example in which the first reference line RL 1  is a single ellipse. 
     Meanwhile, the shape of the projected lines  6 D derived by projecting a passing area of the reamer cutting edges  6 , rotated around the tool axis AX, on a projection plain parallel to the tool axis AX becomes a shape, derived by connecting the second circular arcs and/or line segments with each other, which is line symmetric around a straight line derived by projecting the tool axis AX on the projection plane. As a concrete example, the second circular arcs and line segments can be determined so that at least six points PT 20 , PT 21 , and PT 22  on the two projected lines  6 D derived by connecting the second circular arcs and line segments with each other may be on the second reference line RL 2  consisting of the second quadratic curve, as shown in  FIG. 14 . 
     The second quadratic curve used as the second reference line RL 2  can be determined as any one of a single parabola, two parabolas, a single ellipse or two ellipses, line symmetric around a straight line derived by projecting the tool axis AX on the projection plane and different form the first quadratic curve used as the first reference line RL 1 .  FIG. 14  shows an example in which the second reference line RL 2  is a single ellipse. 
     As a matter of course, the shape of at least a part of at least one of the tip cutting edges  5  and the reamer cutting edges  5  on a projection plane may be a quadratic curve itself or quadratic curves themselves consisting of a single ellipse, two ellipses, a single parabola or two parabolas, serving as the reference, instead of simulating a single quadratic curve or two quadratic curves. That is, the shape of at least a part of the tip cutting edges  5  and the reamer cutting edges  6  may be overlapped with the reference line RL 1  and/or the reference line RL 2 . 
     Note that, when the shape of the projected line  5 C of the tip cutting edges  5  is determined by using a single ellipse or parabola as the first reference line RL 1 , the projected line  5 C of the tip cutting edges  5  near the tip of which point angle α becomes the maximum cannot be overlapped with the single ellipse or parabola since the largest point angle α at the tip must not be 180°. For that reason, it is practical to make the shape of the projected line  5 C of the tip cutting edges  5  near the tip of which point angle a becomes the maximum be two line segments connected so as to become line symmetric with respect to the tool axis AX, from a viewpoint of simplifying production. Also in case of simulating the first reference line RL 1  by connecting arcs as exemplified by  FIG. 14 , or in case of using two quadratic curves, line symmetric with respect to the tool axis AX, as the first reference line RL 1 , it is practical to make the shape of the projected line  5 C of the tip cutting edges  5  near the tip of which point angle α becomes the maximum be two line segments connected so as to become line symmetric with respect to the tool axis AX, from a viewpoint of simplifying production. 
     On the other hand, the reamer cutting edges  6  are away from each other at the position where the point angle β becomes the maximum, unlike the tip cutting edges  5 . Therefore, each of the shapes of the portions of the projected lines  6 D of which point angle β decreases, including the portions of the projected lines  6 D at which point angle β becomes the maximum, can be simulated with circular arcs or overlapped with the second reference lines RL  2 , regardless of whether the second reference line RL 2  consists of a single quadratic curve or two quadratic curves line symmetric with respect to the tool axis AX. When the portions  6 C of which point angle β is constantly 0° are formed, the shapes of the projected lines  6 D of the portions  6 C of which point angle β is 0° each becomes a line segment, as exemplified by  FIG. 14 . 
     The portion whose length is L 2 , at which the point angle β of the reamer cutting edges  6  changes, from the position, at which the diameter of the reamer cutting edges  6  becomes the minimum, to the position, at which the diameter of the reamer cutting edges  6  firstly becomes the maximum, which corresponds to the projected lines  6 D of the portion of the reamer cutting edges  6  from the position, at which the point angle β of the reamer cutting edges  6  becomes the maximum, to the position, at which the point angle β of the reamer cutting edges  6  firstly becomes 0°, had better not be simulated with line segments from the viewpoint of increasing the degree of coincidence with the second reference line RL 2  consisting of a single quadratic curve or two quadratic curves. Accordingly, when the second reference line RL 2  is simulated, it is preferable to simulate the second reference line RL 2  by connecting only circular arcs without line segments, as exemplified by  FIG. 14 . 
     Therefore, a preferable condition for improving drilling quality and the tool life is to snake the shape of each of the projected lines  6 D derived by projecting a passing area of the portion of the reamer cutting edges  6  having the length L 2 , at which the point angle β of the reamer cutting edges  6  changes, rotated around the tool axis AX, on a projection plain parallel to the tool axis AX be a part of a quadratic curve or quadratic curves, regardless of whether the shape of each of the projected lines  6 D is made to be circular arcs simulating the second quadratic curve or a part of the second quadratic curve itself, since a circular arc is a part of a circle which is also a quadratic curve. 
     Note that, in the third implementation in which the point angle β of the reamer cutting edges  6  does not change intermittently, there are no common points between the projected line  5 C derived by projecting the passing area of the tip cutting edges  5 , rotated around the tool axis AX, on a projection plain, and a quadratic curve or quadratic curves of which a part is the projected lines  6 D of the reamer cutting edges  6  (i.e., circles for simulating the second reference line RL 2 , circles for simulating the two second reference lines RL 2 , the second reference line RL 2  itself or the two second reference lines RL 2  themselves) even if the quadratic curve or the quadratic curves forming the projected lines  6 D are extended infinitely in the length direction, as exemplified by  FIG. 14 , whereas a quadratic curve or two quadratic curves used as the reference line RL for the projected lines of the reamer cutting edges  6  in the first implementation in which the point angle β of the reamer cutting edges  6  may change intermittently may be common to a quadratic curve or two quadratic curves used as the reference line RL for the projected line of the tip cutting edges  5 , as exemplified by  FIG. 5 . 
     By the way, each reamer cutting edge  6  is adjacent to the back end side of the margin  7 A of the holding part  7 , in the tip side at which the point angle β becomes the maximum. Therefore, it may be difficult to process and produce the tip of each reamer cutting edge  6  or the tolerance near the tip of each reamer cutting edge  6  may have to be roughened because of the interference with the back end side of the margin  7 A. For that reason, a clearance  30  may be formed between each reamer cutting edge  6  and the holding part  7 , and thereby the minimum diameter of the reamer cutting edges  6  can be made not more than or less than the maximum diameter of the holding part  7 , as illustrated. 
     Consequently, the reamer cutting edges  6  of which point angle β decreases smoothly can be easily produced. In addition, the portions of the reamer cutting edges  6  of which point angle β decreases smoothly can be certainly contacted with an edge of a prepared hole which has been processed by the tip cutting edges  5  and held by the holding part  7 . 
     In the above-mentioned third implementation, the shape of the reamer cutting edges  6  is further idealized from a viewpoint of improving the tool life while securing drilling quality of an FRP. Therefore, according to the third implementation, it becomes possible to secure the drilling quality and increase the tool life of the drill  1 B even when the number of the tip cutting edges  5  and the reamer cutting edges  6  is increased, especially in case of drilling an FRP. 
     (Fourth Implementation) 
       FIG. 15  is a partial front view of a drill according to the fourth implementation of the present invention.  FIG. 16  is an enlarged left side view of the tip part of the drill shown in  FIG. 15 .  FIG. 17  is an enlarged partial top view of the tip part of the drill shown in  FIG. 15 .  FIG. 18  is an enlarged cross-sectional view at the position C-C of the drill shown in  FIG. 15 . 
     A drill  1 C in the fourth implementation shown in  FIG. 15  is different from the drill  1 B in the third implementation in a point that the number of the tip cutting edges  5  is different from the number of the reamer cutting edges  6 . Other structures and actions of the drill  1 C in the fourth implementation are not substantially different from those of the drill  1 B in the third implementation. Therefore, the same signs are attached to the same elements or the corresponding elements while explanation thereof is omitted. 
     When the number of the tip cutting edges  5  and the reamer cutting edges  6  is increased, the web thickness of the cutting edge part  4  decreases due to increase in the number of the grooves  8  whereas the tool life is improved since the cutting amount per one cutting edge decreases. When the web thickness of the cutting edge part  4  decreases, the rigidity of the cutting edge part  4  deteriorates, which leads to degradation in the drilling quality. For that reason, the number of the tip cutting edges  5 , having the relatively small radius of rotation, of which the degree of consumption due to wear and chipping is relatively small, can be made lower than the number of the reamer cutting edges  6 , having the relatively large radius of rotation, of which the degree of consumption is relatively large. 
     In this case, the web thickness and rigidity of the tip cutting edges  5  to which cutting resistance is relatively large whereas the degree of consumption is relatively small can be secured by restricting the number of the tip cutting edges  5 . Meanwhile, the progress in consumption of the reamer cutting edges  6  of which degree of consumption is relatively large can be delayed by increasing the number of the reamer cutting edges  6 . As a result, the tool life of the entire drill  1 C can be increased. 
     As a result of cutting tests, it became clear that it was preferable to make the number of the tip cutting edges  5  two or three, from a viewpoint of securing the web thickness and rigidity. Meanwhile, it is preferable to make the number of the reamer cutting edges  6  not less than three, from a viewpoint of delaying the progress of consumption. Therefore, when the number of the tip cutting edges  5  is made identical to the number of the reamer cutting edges  6  in consideration of simplifying production, it is preferable to make the number of the tip cutting edges  5  and the reamer cutting edges  6  three, as exemplified in the first to third implementations. Meanwhile, when the number of the reamer cutting edges  6  is made more than the number of the tip cutting edges  5 , it is preferable to make the number of the tip cutting edges  5  two and make the number of the reamer cutting edges  6  not less than four. 
     Although the number of the reamer cutting edges  6  in the example shown in  FIG. 18  is four in consideration of simplifying production, the number of the reamer cutting edges  6  may be not less than five so that wear resistance can be improved. As for the number of the margins  7 A of the holding part  7 , it is realistic to make the number of the margins  7 A identical to the number of the tip cutting edges  5 , and extend the margins  7 A from the rear end sides of the tip cutting edges  5  respectively since the margins  7 A are inserted into a prepared hole processed by the tip cutting edges  5 . 
     Thus, the number of the tip cutting edges  5  and the number of the reamer cutting edges  6  can be determined independently from each other. Therefore, the number of the reamer cutting edges  6  in the first or second implementation may be more than the number of the tip cutting edges  5 . Nevertheless, when the number of the tip cutting edges  5  is made identical to the number of the reamer cutting edges  6  in the first or second implementation, the drill  1  or the drill  1 A can be produced by a simple production method by which a semi-product of the drill  1  or the drill  1 A whose ridgelines of the reamer cutting edges  6  are connected with the ridgelines of the tip cutting edges  5  respectively is produced, and subsequently the holding part  7  is made by cutting work or polishing processing, as described in the first implementation. Therefore, whether the number of the tip cutting edges  5  is made identical to or different from the number of the reamer cutting edges  6  can be determined according to a production method of the drill  1  or the drill  1 A, and the shape of the reamer cutting edges  6 . 
     Note that, regardless of whether the number of the reamer cutting edges  6  is the same as the number of the tip cutting edges  5 , a projection view of the outline of the cutting edge part  4  on a projection plane parallel to the tool axis AX becomes similar since the tip cutting edges  5  and the reamer cutting edges  6  rotate around the tool axis AX. 
     In the above-mentioned fourth implementation, the rigidity of the tip cutting edges  5  is secured by restricting the number of the tip cutting edges  5  while the life tool of the while drill  1 C is intended to be increased by increasing the number of the reamer cutting edges  6 . 
     Therefore, according to the fourth implementation, drilling quality can be secured while a tool life can be improved whether an FRP or a metal is drilled. That is, many holes can be drilled with high quality by the single drill  1 C. 
     Actually, the drill  1 C having the two tip cutting edges  5  and the four reamer cutting edges  6  has been produced, and a cutting test in which a CFRP having a thickness of 10 mm was drilled has been performed. As a result, it has been confirmed that about twice as many as holes could be drilled and the drilling speed was improved up to about 1.2 times, compared with the conventional R-drill (registered trademark) having two cutting edges. 
     (Fifth Implementation) 
       FIG. 19  is an enlarged partial front view showing the first example of a shape of a cutting edge part formed in a drill according to the fifth implementation of the present invention while  FIG. 20  is an enlarged partial front view showing the second example of a shape of a cutting edge part formed in a drill according to the fifth implementation of the present invention. 
     A drill  1 D in the fifth implementation shown in  FIG. 19  or  FIG. 20  is different from each of the drill  1 ,  1 A,  1 B and  1 C in the first to fourth implementations in a point that a size or sizes of an element or elements included in the cutting edge part  4 , such as the length L 1  of the tip cutting edges  5 , the length L 2  of the reamer cutting edges  6  and/or the maximum diameter D 1  of the tip cutting edges  5 , are made suitable for a method of using the drill  1 D. Other structures and actions of the drill  1 D in the fifth implementation are not substantially different from those of each of the drill  1 ,  1 A,  1 B and  1 C in the first to fourth implementations. Therefore, only enlarged partial front views showing examples of a shape of the cutting edge part  4  are shown, and the same signs are attached to the same elements or the corresponding elements while explanation thereof is omitted. 
     In each of  FIG. 19  and  FIG. 20 , the upper part above the tool axis AX shows the outline of the cutting edge part  4  while the lower part below the tool axis AX shows a shape of a projected line obtained by projecting a passing area of the cutting edge part  4 , rotated around the tool axis AX, on a projection plain parallel to the tool axis AX. 
     As explained in the first implementation, it is desirable to make the cutting edge length L of the cutting edge part  4  not less than one time and not more than twice the tool diameter D (i.e., 1D≤L≤2D from a viewpoint of improving the quality of a finished hole without restriction on a shape, such as thickness of a workpiece, as much as possible. Meanwhile, the larger the maximum diameter D 1  of the tip cutting edges  5  is made, the more the rigidity of the tip cutting edges  5  can be raised and the more satisfactorily chips can be discharged since the grooves  8  become deeper. 
     When the maximum diameter D 1  of the tip cutting edges  5  is enlarged, the length L 2  of the reamer cutting edges  6  becomes short since the length L 1  of the tip cutting edges  5  becomes long. Also in the drills  1 ,  1 A,  1 B, and  1 C exemplified in the first to fourth implementations, the length L 2  of the reamer cutting edges  6  has been made short in order to enlarge the maximum diameter D 1  of the tip cutting edges  5 . 
     The shorter the length L 2  of the reamer cutting edges  6  is made while keeping the cutting edge length L of the cutting edge part  4  constant, the more the cutting amount by the tip cutting edges  5  increases since the maximum diameter D 1  and the length L 1  of the tip cutting edges  5  become large and long while the cutting amount by the reamer cutting edges  6  decreases. When the difference between the cutting amount by the tip cutting edges  5  and the cutting amount by the reamer cutting edges  6  becomes large, the difference in cutting resistance between machining a prepared hole by the tip cutting edges  5  and finishing the prepared hole by the reamer cutting edges  6  also becomes large. That is, after a prepared hole has been formed by the tip cutting edges  5 , the prepared hole is finished by the reamer cutting edges  6  with extremely decreased cutting resistance. 
     When any of the drills  1 ,  1 A,  1 B, and  1 C in the first to fourth implementations is hold and used by not only a machine tool, such as a machining center or a drill press, whose feeding speed of spindle is automatically controlled, but the handheld tool rotating device  20  as exemplified by  FIG. 8  having an automatic feeding function allowing controlling the feeding speed in the tool axis AX direction, the drill  1 ,  1 A,  1 B, or  1 C can be fed out in the tool axis AX direction with an appropriate feeding speed during both machining a prepared hole and finishing the prepared hole even when the prepared hole is finished by the reamer cutting edges  6  with cutting resistance extremely decreased after the machining of the prepared hole by the tip cutting edges  5 . That is, so long as a drilling machine which allows controlling the feeding speed of the drill  1 ,  1 A,  1 B, or  1 C in the tool axis AX direction is used, the feeding speed of the drill  1 ,  1 A,  1 B, or  1 C can be made constant or an appropriate one regardless of change of cutting resistance. 
     Conversely, when an operator conducts drilling by attaching any of the drills  1 ,  1 A,  1 B, and  1 C in the first to fourth implementations to a handheld tool rotating device having no automatic feeding function, a speed with which the operator pushes out the handheld tool rotating device toward a workpiece is the feeding speed of the drill  1 ,  1 A,  1 B, or  1 C. Therefore, it is required to adjust the force to push out the handheld tool rotating device according to decrease in cutting resistance so that the feeding speed of the handheld tool rotating device holding the drill  1 ,  1 A,  1 B, or  1 C may remain an appropriate one at finishing of a prepared hole by the reamer cutting edges  6  even when the cutting resistance extremely decreases after the machining of the prepared hole by the tip cutting edges  5 . 
     As a result, there arises a problem that an unskilled operator may maintain the force for pushing out a tool rotating device even with extremely decreased cutting resistance after machining of a prepared hole, and thereby the feeding speed of the drill  1 ,  1 A,  1 B, or  1 C may become excess during finishing of the prepared hole by the reamer cutting edges  6  although a skilled operator can adjust the force for pushing out the tool rotating device so that the drill  1 ,  1 A,  1 B, or  1 C may be fed out with an appropriate feeding speed during both of the machining of the prepared hole and the finishing of the prepared hole. If the feeding speed of the drill  1 ,  1 A,  1 B, or  1 C becomes excess during finishing of a prepared hole by the reamer cutting edges  6 , delamination may arise in case of drilling an FRP. That is, if cutting resistance in finishing of a hole decreases extremely compared with that in machining of a prepared hole, delamination may arise in drilling of an FRP unless an operator is skilled. 
     For that reason, when a handheld tool rotating device having no automatic feeding function is to be used, cutting resistance in finishing of a prepared hole by the reamer cutting edges  6  may be increased intentionally so that an operator can machine a finished hole with high accuracy without requiring advanced skill. What is necessary in order to increase cutting resistance in finishing of a prepared hole is to lengthen the length L 2  of the reamer cutting edges  6  to a certain extent, and enlarge the difference between the maximum diameter D 1  of the tip cutting edges  5  and the tool diameter D, which is the maximum diameter of the reamer cutting edges  6 , to a certain extent. 
     The drill  1 D in the first example of the fifth implementation exemplified by  FIG. 19  has the reamer cutting edges  6  whose length L 2  is longer than that in each of the first to fourth implementations so that cutting resistance may not decrease extremely at finishing of a prepared hole. In addition, the maximum diameter D 1  of the tip cutting edges  5  is smaller than that in each of the first to fourth implementations so that the cutting amount by the reamer cutting edges  6  may be secured. 
     Note that, the drill  1 D in the example shown in  FIG. 19  has the cutting edge length L which is 1.6 times the tool diameter D (L=1.6D), the length L 1  of the tip cutting edges  5  which is 0.3 times the tool diameter D (L 1 =0.3D), the maximum diameter D 1  of the tip cutting edges  5  which is 0.6 times the tool diameter D (D 1 =0.6D), and the length L 2  of the reamer cutting edges  6  which is 0.96 times the tool diameter D (L 2 =0.96D) wherein the cutting edge length L of the drill  1 D is defined as the length in the tool axis AX direction from the tip point of the tip cutting edges  5  to the position at which the diameter of the reamer cutting edges  6  first becomes the maximum diameter of the reamer cutting edges  6 , i.e., the tool diameter D while the length L 2  of the reamer cutting edges  6  is defined as the length in the tool axis AX direction from the forward position, at which the diameter of the reamer cutting edges  6  becomes the maximum diameter D 1  of the tip cutting edges  5 , to the rear position, at which the diameter of the reamer cutting edges  6  first becomes the maximum diameter of the reamer cutting edges  6 . 
     As a result of repeated drilling examination, is has been confirmed that it was suitable to make the length L 2  of the reamer cutting edges  6  not less than 0.7 times the tool diameter D and not more than 1.2 times the tool diameter D (i.e., 0.7D≤L 2 ≤1.2D) while making the maximum diameter D 1  of the tip cutting edges  5  not less than 0.5 times the tool diameter D and not more than 0.7 times the tool diameter D (i.e., 0.5D≤D 1 ≤0.7D) from a viewpoint of preventing cutting resistance from decreasing extremely at finishing of a prepared hole just after machining the prepared hole, and thereby allowing feeding out a handheld tool rotating device together with the drill  1 D with an appropriate feeding speed easily without requiring advanced skill even when the handheld tool rotating device had no automatic feeding function. 
     When the length L 2  of the reamer cutting edges  6  is lengthened without changing the cutting edge length L of the cutting edge part  4  and the length Lh of the holding part  7 , the maximum diameter D 1  of the tip cutting edges  5  necessarily becomes small. For this reason, the drill  1 D having the reamer cutting edges  6  whose length L 2  has been lengthened has decreased discharging performance of chips, as mentioned above. 
     Nevertheless, when a handheld tool rotating device having no automatic feeding function is used, it is easy to decrease the feeding speed of the tool rotating device and the drill  1 D against possible large cutting resistance as well as to perform step machining in which drilling is performed in small steps with repeating alternate forward movement and backward movement of the tool rotating device and the drill  1 D. Accordingly, step machining or decreasing the feeding speed can be performed through machining of a prepared hole with large cutting resistance, and thereby trouble that the drill  1 D is clogged with chips can be avoided. 
     On the other hand, also when a drilling machine having an automatic feeding function is used, configuring the drilling machine to allow step machining makes it possible to avoid chip clogging even in case of using the drill  1 D whose maximum diameter D 1  of the tip cutting edges  5  is small. As a concrete example, in case of using a machine tool, the machine tool can be operated for step machining by numerical control program. Alternatively, in case of using a handheld tool rotating device having an automatic feeding function, mounting an air signal circuit or the like, for controlling the tool rotating device so as to repeat forward movement and backward movement of the drill  1 D, like one disclosed in Japanese Patent Application Publication JP2014-50943 A or the like, with the tool rotating device allows step machining. 
     As another method, both of the length L 2  of the reamer cutting edges  6 , and the length L 1  and the maximum diameter D 1  of the tip cutting edges  5  may be secured so that drilling can be performed without step machining whether a handheld tool rotating device having no automatic feeding function or a drilling machine having an automatic feeding function is used. That is, the ratio between the length L 1  of the tip cutting edges  5  and the length L 2  of the reamer cutting edges  6  may be made suitable. 
     In case of the third or fourth implementation in which the shapes of the ridgelines of the tip cutting edges  5  and the reamer cutting edges  6  are respectively determined along the two reference lines RL 1  and RL 2  different from each other, the length Lh of the holding part  7  can be made a voluntary length, and therefore examination was performed. As a result, it has became clear that it was sufficient to make the length Lh of the holding part  7  not less than 0.25 times the tool diameter D (i.e., Lh≥0.25D) as long as the entire holding part  7  consisted of the margins  7 A. Therefore, the length derived by subtracting the length Lh required for the holding part  7  from the cutting edge length L of the drill  1 D can be distributed to the length L 1  of the tip cutting edges  5  and the length L 2  of the reamer cutting edges  6 . 
     The drill  1 D in the second example of the fifth implementation exemplified by  FIG. 20  has the maximum diameter D 1  of the tip cutting edges  5  and the length L 2  of the reamer cutting edges  6  each determined from a viewpoint of securing chip discharging performance while reducing drastic change in cutting resistance between machining of a prepared hole and finishing of the prepared hole. That is, the drill  1 D shown in  FIG. 20  has the maximum diameter D 1  of the tip cutting edges  5 , which allows avoiding chip clogging even without step machining, and the length L 2  of the reamer cutting edges  6 , which allows reducing a decrease amount in cutting resistance between machining of a prepared hole and finishing of the prepared hole. 
     Note that, the drill  1 D in the example shown in  FIG. 20  has the cutting edge length L which is 1.6 times the tool diameter D (L=1.6D), the length L 1  of the tip cutting edges  5  which is 0.5 times the tool diameter D (L 1 =0.5D), the maximum diameter D 1  of the tip cutting edges  5  which is 0.675 times the tool diameter D (D 1 =0.675D), and the length L 2  of the reamer cutting edges  6  which is 0.74 times the tool diameter D (L 2 =0.74D). 
     As a result of repeated drilling examination, is has been confirmed that making the maximum diameter D 1  of the tip cutting edges  5  not less than 0.6 times the tool diameter D and not more than 0.7 times the tool diameter D (i.e., 0.6D≤D 1 ≤0.7D) could secure chip discharging performance even without special machining, such as step machining. 
     On the contrary, when it is not intended to use a handheld tool rotating device having no automatic feeding function, it is preferred to shorten the length L 2  of the reamer cutting edges  6  and enlarge the maximum diameter D 1  of the tip cutting edges  5  as illustrated in each of the first to fourth implementations from a viewpoint of raising both the chip discharging performance and the rigidity of the tip cutting edges  5 . Nevertheless, in case of the reamer cutting edges  6  whose point angle changes, the longer the length L 2  of the reamer cutting edges  6  is, the more the progress of wear of the reamer cutting edges  6  can be advantageously delayed since the length of the portions of the reamer cutting edges  6  contacted with a workpiece becomes longer. Therefore, when it is important to delay the progress of wear of the reamer cutting edges  6 , the length L 2  of the reamer cutting edges  6  may be lengthened within a range where the maximum diameter D 1  of the tip cutting edges  5  is not less than 0.6 times the tool diameter D and not more than 1.0 times the tool diameter D (i.e., 0.6D≤D 1 ≤1.0D) in order to secure the chip discharging performance, the length Lh of the holding part  7  consisting of the margins  7 A is not less than 0.25 times the tool diameter D (i.e., Lh≥0.25D) in order to secure the function to stop deflection of the holding part  7 , and the cutting edge length L of the drill  1 D is not less than the tool diameter D and not more than twice the tool diameter D (i.e., 1D≤L≤2D). 
     Note that, each of the drill  1 B and the drill  1 C in the examples shown in  FIG. 10  and  FIG. 15  respectively has the cutting edge length L which is 1.6 times the tool diameter D (L=1.6D), the length L 1  of the tip cutting edges  5  which is 0.6 times the tool diameter D (L 1 =0.6D), the maximum diameter D 1  of the tip cutting edges  5  which is 0.75 times the tool diameter D (D 1 =0.75D), and the length L 2  of the reamer cutting edges  6  which is 0.56 times the tool diameter D (L 2 =0.56D). 
     The number of the tip cutting edges  5  formed on the drill  1 D in the fifth implementation may be same as that of the reamer cutting edges  6 , and both the numbers of the tip cutting edges  5  and the reamer cutting edges  6  may be three, for example, as described in the first to third implementations. On the contrary, the number of the tip cutting edges  5  of the drill  1 D may be different from that of the reamer cutting edges  6 , and the number of the tip cutting edges  5  may be two while the number of the reamer cutting edges  6  may be four, for example, as described in the fourth implementation. As for the point angle β of the reamer cutting edges  6 , the point angle β at the tip of the reamer cutting edges  6  may be constant as described in the first and second implementations, or conversely, the point angle β of the reamer cutting edges  6  may be changed smoothly as described in the third and fourth implementations. 
     According to the above-mentioned fifth implementation, a finished hole can be machined with high accuracy without requiring advanced skill of an operator also in case of using a handheld tool rotating device having no automatic feeding function. Specifically, in case of drilling an FRP, the feeding speed at finishing of a prepared hole can be prevented from becoming excess, and thereby the risk that delamination arises can be reduced. 
     (Sixth Implementation) 
       FIG. 21  is an enlarged partial projection view of reamer edges formed in a drill according to the sixth implementation of the present invention. 
     A drill  1 E in the sixth implementation shown in  FIG. 21  is different from each of the drill  1 ,  1 A,  1 B,  1 C and  1 D in the first to fifth implementations in a point that the projected line of the ridgeline of each reamer cutting edge  6  is not a curved line but a straight line. Other structures and actions of the drill  1 E in the sixth implementation are not substantially different from those of each of the drill  1 ,  1 A,  1 B,  1 C and  1 D in the first to fifth implementations. Therefore, only an enlarged partial projection view of the reamer cutting edges  6  is shown, and the same signs are attached to the same elements or the corresponding elements while explanation thereof is omitted. 
     As exemplified by  FIG. 21 , a shape of the ridgeline of each reamer cutting edge  6  may be determined so that the shape of the ridgeline of the reamer cutting edge  6  on a projection plane parallel to the tool axis AX may become line segments connected with each other. In other words, each reamer cutting edge  6  can have a shape that the point angle β intermittently decreases down to 0° from the tip side toward the rear end side while the relief angle γ 2  becomes nonzero at the maximum diameter position where the point angle β is 0°. 
     In this case, labor and costs for manufacturing the drill  1 E can be reduced since a shape of the reamer cutting edges  6  can be simplified compared with a case where the point angle β of the reamer cutting edges  6  is decreased continuously like the first to fifth implementations. 
       FIG. 22  is an enlarged partial projection view showing an example of the reamer cutting edge  6  whose point angle β remains 0 degrees without decreasing. 
     When an FRP is a main target to be drilled, the point angle β of the reamer cutting edges  6  may not be decreased intermittently but be limited to 0°. Specifically, so long as the nonzero relief angle γ 2  is formed at the maximum diameter position of the reamer cutting edges  6  at which point angle β is 0°, other shapes may be omitted or changed. Thereby, a shape of the reamer cutting edge  6  can be simplified further, and labor and costs for manufacturing the drill  1 E can be reduced. 
     Note that, in the sixth implementation, the margins  10  may be formed behind the reamer cutting edges  6 , or the portion  6 C of which diameter is constant and maximum may be formed like the second implementation. Moreover, a shape of the reamer cutting edge  6  can be simplified by making the relief angle γ 2  of the reamer cutting edge  6  decrease intermittently according to the change of the point angle β, or constant. As a matter of course, the number of the reamer cutting edges  6  may be made more than that of the tip cutting edges  5  like the fourth implementation. In addition, in the sixth implementation, a size or sizes, such as the length of the tip cutting edges  5 , the length L 2  of the reamer cutting edges  6  and/or the maximum diameter D 1  of the tip cutting edges  5 , may be made preferable for a method of using the drill  1 D, similarly to the fifth implementation. 
     (Seventh Implementation) 
       FIG. 23  is a projection view showing the first example of a shape of a tip cutting edge formed in a drill according to the seventh implementation of the present invention.  FIG. 24  is a projection view showing the second example of a shape of a tip cutting edge formed in a drill according to the seventh implementation of the present invention.  FIG. 25  is a projection view showing the third example of a shape of a tip cutting edge formed in a drill according to the seventh implementation of the present invention.  FIG. 26  is a projection view showing the fourth example of a shape of a tip cutting edge formed in a drill according to the seventh implementation of the present invention. 
     A drill  1 F in the seventh implementation shown in each of  FIG. 23  to  FIG. 26  is different from each of the drills  1 ,  1 A,  1 B,  1 C,  1 D and  1 E in the other implementations in a point that projected lines of the ridgelines of the tip cutting edges  5  and the reamer cutting edges  6  are not curved lines but line segments connected with each other. Other structures and actions of the drill  1 F in the seventh implementation are not substantially different from those of each of the drills  1 ,  1 A,  1 B,  1 C,  1 D and  1 E in the other implementations. Therefore, only a projected view of the cutting edge parts  4  is shown, and the same signs are attached to the same elements or the corresponding elements while explanation thereof is omitted. 
     Although an example in which a projected view of the ridgeline of the reamer cutting edge  6  was made at least one line segment has been explained in the sixth implementation, a projected view of the ridgeline of the tip cutting edge  5  may also be made connected line segments. Specifically, the shapes of the respective ridgelines of the tip cutting edges  5  and the reamer cutting edges  6  can be determined so that a projected line of each ridgeline of the tip cutting edges  5  and the reamer cutting edges  6  may be a polygonal line which simulates a quadratic curve, such as a parabola and an ellipse. In this case, the drill  1 F can also be classified into a multi-point-angle drill which has the tip cutting edges  5 , having the point angles α, and the reamer cutting edges  6 , having the single point angle β of 0° or the point angles β. 
     As a concrete example, the shape of the tip cutting edge  5  can be determined so that a projected line of the ridgelines of the tip cutting edges  5  on a projection plane parallel to the tool axis AX may be a polygonal line, symmetric with respect to a projected line of the tool axis AX as the center line, derived by connecting, with each other, at least six line segments of which respective both ends are on the reference line RL consisting of a single parabola symmetric with respect to the projected line of the tool axis AX as the center line, as exemplified by  FIG. 23 . In this case, each tip cutting edge  5  is an at least three multi-stage cutting edge having at least three point angles α 1 , α 2  and α 3 . Note that, the reference line RL may be replaced from a parabola to an ellipse. 
     Alternatively, the shape of the tip cutting edge  5  can be determined so that a projected line of the ridgelines of the tip cutting edges  5  on a projection plane parallel to the tool axis AX may be a polygonal line, symmetric with respect to a projected line of the tool axis AX as the center line, derived by connecting, with each other, at least eight line segments which are each tangent on the reference line RL consisting of a single parabola symmetric with respect to the projected line of the tool axis AX as the center line, as exemplified by  FIG. 24 . In this case, each tip cutting edge  5  is an at least four multi-stage cutting edge having at least four point angles α 1 , α 2 , α 3  and α 4 . Note that, the reference line RL may be replaced from a parabola to an ellipse. 
     As another example, the shape of the tip cutting edge  5  can be determined so that a projected line of the ridgelines of the tip cutting edges  5  on a projection plane parallel to the tool axis AX may be a polygonal line, symmetric with respect to a projected line of the tool axis AX as the center line, derived by connecting, with each other, at least six line segments of which respective both ends are on the reference line RL consisting of two ellipses, in the side closer to the projected line of the tool axis AX, symmetric with respect to the projected line of the tool axis AX as the center line, as exemplified by  FIG. 25 . In this case, each tip cutting edge  5  is an at least three multi-stage cutting edge having at least three point angles α 1 , α 2  and α 3 . In an example shown in  FIG. 25 , four point angles α 1 , α 2 , α 3  and α 4  have been formed. Note that, the reference line RL may be replaced from two ellipses to two parabolas. 
     As yet another example, some of line segments composing a polygonal line which is a projected line of the ridgelines of the tip cutting edges  5  on a projection plane parallel to the tool axis AX may be each tangent on the reference line RL, such as a parabola, parabolas, an ellipse, or ellipses while the others may be line segments of which respective both ends are on the reference line RL, as exemplified by  FIG. 26 . In an example shown in  FIG. 26 , the projected line of the ridgelines forming the maximum point angle α 1  of the tip cutting edges  5  consists of two symmetric line segments which are each tangent on the reference line RL consisting of a single parabola or ellipse symmetric with respect to the projected line of the tool axis AX as the center line while the projected lines of the ridgelines forming the other point angles α 2 , α 3  and α 4  consist of symmetric line segments of which respective both ends are on the reference line RL. 
     Note that, some of line segments composing a projected line of the ridgelines of the tip cutting edges  5  on a projection plane parallel to the tool axis AX may be replaced with curved lines, such as arcs. For example, when line segments are connected to each other by an arc disposed therebetween, the tip cutting edge  5  can have a smooth shape without corners. In that case, a risk of causing chipping on the tip cutting edge  5  can be reduced, and thereby a tool life can be lengthened. Alternatively, as explained in the first implementation with reference to  FIG. 5  and  FIG. 6 , the shapes of the ridgelines of the tip cutting edges  5  may be determined so that only the projected line of the ridgelines of the tip cutting edges  5  which form the maximum point angle α at the tip point may consist of line segments while the projected lines of the other ridgelines may consist of curved lines each derived by smoothly connecting arcs, having mutually different radii, with each other. 
     When the point angle α of the tip cutting edge  5  is decreased intermittently, decreasing the relief angle γ 1  intermittently according to the decrease of the point angle α leads to the simplification of the shape of the tip cutting edge  5 . Therefore, both of the point angle α and the relief angle γ 1  of the tip cutting edge  5  may be decreased continuously or intermittently from the tip side toward the rear end side. In addition, a shape obtained by projecting a passing area of the tip cutting edges  5 , rotating around the tool axis AX, on a projection plane parallel to the tool axis AX can be made a shape, derived by connecting line segments and/or curved lines, such as circular arcs with each other, line symmetric with regard to a projected line of the tool axis AX on the projection plane as the center line, so that the shape of the tip cutting edge  5  can be simplified. 
     As a concrete example, the shape of the tip cutting edge  5  can be determined so that positions of at least seven points on curved lines and/or line segments composing a projected line of the ridgelines of the tip cutting edges  5  on a projection plane may be on a quadratic curve or quadratic curves, consisting of a single parabola, two parabolas, a single ellipse or two ellipses, line symmetric with regard to the projected line of the tool axis AX on the projection plane as the center line. Thereby, an effect that cutting resistance to the tip cutting edges  5  and the degree of wear progress of the tip cutting edges  5  can be uniformed as much as possible in the length Lh direction of the tip cutting edges  5  can be obtained as explained in the first implementation. 
     Note that, when the point angle β of the reamer cutting edges  6  is also decreased continuously or intermittently, the reamer cutting edge  6  can have a shape derived by connecting line segments and/or curved lines, such as arcs, with each other on a projection plane, as described with reference to  FIG. 14  and the like in the third implementation. Specifically, the shape of the reamer cutting edge  6  can be determined so that a shape obtained by projecting a passing area of the reamer cutting edges  6 , rotated around the tool axis AX, on a projection plane parallel to the tool axis AX may become a shape obtained by connecting line segments and/or curved lines, such as arcs, with each other, and line symmetric with regard to the projected line of the tool axis AX on the projection plane as the center line. 
     As for the shape of the reamer cutting edge  6 , the shape of the projected line on a projection plane can be also determined using the reference line consisting of a quadratic curve or quadratic curves same as those for determining the shape of the tip cutting edge  5 , as described with reference to  FIG. 5  and the like in the first implementation. In this case, a merit that a production method of forming the holding part  7  on a semi-product of which ridgelines of the tip cutting edges  5  have been connected with those of the reamer cutting edges  6  respectively can be adopted can be obtained. Alternatively, the shape of the projected line of the reamer cutting edge  6  may be determined based on a quadratic curve different from a quadratic curve for determining the shape of the projected line of the tip cutting edge  5  so that the change amount of the point angle β of the reamer cutting edges  6  may be reduced, as described with reference to  FIG. 14  and the like in the third implementation. 
     According to the above-mentioned seventh implementation, a shape of each ridgeline of the tip cutting edges  5  and the reamer cutting edges  6  can be simplified further, and thereby labor and costs for producing the drill  1 E can be reduced further. 
     Note that, the shape of the ridgeline of the reamer cutting edge  6  may be a curved one in which the point angle β continuously and smoothly decreases as described in the third implementation while only the point angle α of the tip cutting edges  5  may be decreased intermittently for simplification as exemplified in  FIG. 23  to  FIG. 26 . In that case, as for the tip cutting edge  5  in which wear progress is relatively slow because of the small radius of rotation, production costs can be reduced by simplifying the shape. Meanwhile, as for the reamer cutting edge  6  in which wear progress is relatively fast because of the large radius of rotation, wear progress can be effectively delayed by making the shape preferable. 
     (Other Implementations) 
     While certain implementations have been described, these implementations have been presented by way of example only, and are not intended to limit the scope of the invention. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the invention. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the invention.