Abstract:
Significantly improved resistance to breaking while drilling in metal can be realized in twist drill bits having a web thickness taper which is about twice the web thickness taper specified in Standards and in common use in the industry. This creates much shallower flutes and flies in the face of conventional wisdom in the art which holds that the web taper rate should be low to maximize the amount of flute volume available to convey chips and sawdust out of the hole. A flute length of about 1/2″ less than the flute lengths specified by the Standards makes it possible to have such a large taper rate but still control variations in tolerances for the flute length. It also provides a marking zone for placing size indicia on the drill bit shank outside the portion which is gripped by the chuck jaws of a power drill. The use of the new web taper rate, in conjunction with modifications to the tip geometry of a self-centering drill bit with pilot tip, produces a drill bit optimized both for metal and wood drilling which is strong enough to withstand the forces exerted on the bit while drilling in metal using a hand-held power drill, yet which need not be retracted from a 2×4 to clear sawdust prior to drilling a hole completely through the thickness of the wood.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation of U.S. patent application Ser. No. 09/087,157, filed May 29, 1998, now U.S. Pat. No. 6,050,754 which claims priority to U.S. Provisional Patent Application Ser. No. 60/048,016 filed May 29, 1997, and entitled “Improved Self-Centering Drill Bit with Pilot Tip”, the specification and drawings of which are herein expressly incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to twist drill bits having flutes that extend from a tip of the drill bit to a termination point on the shank. The drill bit flutes define a web thickness which increases along the length of the flute from the tip to the termination point. The invention uses a much steeper web taper rate than is used in the industry. It has been discovered that bits employing the present invention experience significantly enhanced toughness and resistance to breakage in metal, yet still perform well in wood. This enhanced effect is particularly pronounced when the drill bit is driven by a portable drill. 
     BACKGROUND OF THE INVENTION 
     Conventional wisdom in the art holds that it is advisable to use a relatively shallow web taper rate in twist drills so that the flute depth along the length of the flute is as great as practicable. This should provide the maximum amount of volume to convey chips, swarf or sawdust back from the tip and out of the hole being drilled. This convention is embodied in two Standards: The American Society of Mechanical Engineers (ASME B94.11M-1993), and The National Aerospace Standard of the Aerospace Industries Association of America, Inc., (NAS 907) (hereinafter collectively referred to as “Standards”), both of which Standards are specifically incorporated in their entireties by reference herein. The Assignee of the present invention has been selling conventional Jobber-length, straight-shank drill bits which have parameters that track the Standards, and which have conventional web thickness taper rates between 0.024 inches to 0.030 inches. Also, it has been selling a self-centering drill bit with pilot tip under the BULLET® trademark having a web taper rate of about 0.027 inches per inch of flute length. This self-centering drill bit was designed with the end user who uses a portable drill in mind, particularly one who desires a bit which is optimized to drill in both metal and wood. The first versions ofthese bits are disclosed in U.S. Pat. No. 4,968,193, issued Nov. 6, 1990, to Chaconas et al., and U.S. Pat. No. 5,288,183, issued Feb. 22, 1994, to Chaconas et al., each of which is assigned to the same Assignee of the present invention. The entire disclosures of each of these two patents are also specifically incorporated by reference herein. 
     One of the needs of an operator who uses the bit in a hand-held power drill is increased toughness, or resistance to breaking (typically in the flute portion of the drill bit). The increased robustness is needed because in metal drilling the portable power drill user puts considerable side stresses on the drill bit, not having the stability that a drill press provides. However, the BULLET® drill bit, which was optimized to drill well both in metal and in wood, not only needs strength while drilling metal, it also must drill holes through wood with a minimum of “woodpeckering”, that is, repetitively retracting and reinserting the drill bit to clear the flutes of sawdust. There is also a need to enhance the BULLET® drill bit&#39;s longevity if it should be subjected to an overspeed condition in metal, as well as the strength in its tip portion. Finally, there is a need to reduce its tendency to produce a disc in laminated materials. 
     SUMMARY OF THE INVENTION 
     It has been discovered that by defying convention, and instead, using a radically-increased web taper rate, a much more robust drill bit will be created. In addition, it has been discovered that a particular range of increased web taper rates will not only provide sufficiently enhanced robustness or strength when drilling in metal, it simultaneously provides a bit which minimizes the amount of woodpeckering required when drilling wood. Accordingly, it is an object of the present invention to manufacture a twist drill bit having a web thickness taper rate over the length of the flute which is about twice as great as the web taper rate of conventional drill bits, thereby providing significantly increased resistance to breakage while drilling in metal, yet, for drill bits having nominal diameters of 0.250 inch or greater, enabling the user to drill holes in inch and one-half thick wood in one pass. 
     It is a particular object of the preferred embodiments of the invention to provide a twist drill bit in which the web thickness increases from the tip to the shank portion at a uniform taper rate along the length of the flute in the range from about 0.050 inch to about 0.071 inch per inch of flute length. It is yet another particular object of the preferred embodiments of the invention to provide a bit marking zone on the shank portion of the drill bit which is outside of the area of the shank typically gripped by the jaws of a chuck in which the drill bit is inserted. 
     A feature by which the above objects may be attained is by forming the flutes with the web taper rate of the present invention along a flute length which is less than the flute length specified for a predetermined bit nominal diameter by the ASME B94. 11-M-1993 Standard, and specifically by reducing the flute length by about 0.3 inch to about 0.7 inch. Optimally, the flute length is reduced by about one-half inch, for all nominal diameters of the drill bits. A preferred feature is to form indicia such as the nominal size ofthe bit, in the marking zone, so that in the engagement by the chuck of the bit shank does not erase the indicia. 
     Another feature by which the above objects can be attained is in the manufacturing process: causing relative axial movement between a grinding wheel and drill bit stock while rotating the drill bit stock about its axis, grinding the flute having the desired flute length with the web thickness taper rate of the present invention, then retracting the grinding wheel from the drill bit stock upon reaching a point which is a predetermined distance before the point at which the flute would terminate at the shank if the taper rate were to be maintained. 
     It is yet another object of the present invention to enhance the robustness of self-centering drill bits with pilot tip of the type sold under the trademark BULLET®. One feature by which the above object can be attained is by optionally providing the drill bit tip portion with a fishtail angle of less than 180°. Yet another preferred feature is to form a chamfer adjacent the fishtail such that a cutting edge on the chamfer connects a cutting lip on the fishtail with the outer diameter of the drill bit. Still another preferred feature is to provide the pilot portion with a back taper in the axial direction. Another preferred feature is to provide a back taper which is at least 1° negative. And yet another feature is to size the width or diameter of the pilot portion to be about one-half the nominal bit diameter. 
     Other objects, features and advantages of the present invention will become more fully apparent from the following detailed description of the preferred embodiments, the appended claims and the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings, in which like reference characters in the same or different Figures indicate like part: 
     FIG. 1 is a side elevational schematic view of a conventional drill bit to which a force F is applied to such an extent that the bit breaks, usually in the flute portion; 
     FIG. 2 is a side elevational schematic view of a twist drill bit according to the present invention in which a much higher force F must be applied to break the drill bit; 
     FIG. 3 is a side elevational schematic view of a twist drill bit according to another embodiment of the present invention to which a force F is applied to cause the bit to break outside the flute portion; 
     FIG. 4 is a schematic detail view of the drill bits of FIGS. 2 and 3 superimposed upon the drill bit of FIG. 1, employing the web taper rates of the present invention; 
     FIG. 5 is similar to FIG. 4, but illustrating solely a web taper rate of the present invention, in combination with a pilot tip of the present invention; 
     FIG. 6 is similar to FIG. 5, but illustrating the web taper rate of the present invention in a conventional twist drill bit; 
     FIG. 7 is a view similar to the view of FIG. 5, illustrating an embodiment of the present invention in which the flute length is the same as the conventional flute length; 
     FIG. 8 is an elevational schematic view of the process for forming the web taper rate of the present invention; 
     FIG. 9 is an elevational detail schematic view of the tip portion of a self-centering drill bit having a pilot tip; 
     FIG. 10 is one embodiment of an improved tip portion according to the present invention; 
     FIG. 11 is another embodiment of the tip portion of the present invention; 
     FIG. 12 is yet another embodiment of the tip portion according to the present invention; 
     FIG. 13 is still another embodiment of the tip portion of the present invention; 
     FIG. 14 is partial enlarged elevational detail view of the bit of FIG. 9 rotated to illustrate a secondary lip relief angle (“F2”); and 
     FIG. 15 is an enlarged schematic detail view of the process for forming a portion of the tip portion of FIG.  9 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring first to FIG. 1, a conventional twist drill bit is shown generally as  10 , and includes a shank portion  12 , a flute portion  14  containing two flutes  16  and a tip portion  18 . When drilling in metal with a portable power drill, the drill bit  10  encounters greater than normal forces F transverse to the axis of the drill bit, as shown schematically in FIG.  1 . If the force F is great enough, the drill bit will break or fail as shown at  15 . Tests run on a quarter inch diameter conventional twist drill bit  10  found that the failure  15  occurred primarily in the fluted portion  14 , when the bit was subjected to a peak force at failure in the range from 150 to 175 lbs. 
     A twist drill using the taper rate of the present invention is shown generally as  20  in FIG. 2, and includes homogenous shank portion  22 , flute portion  24  having two flutes  26 , and a tip portion  28 . In tests of quarter inch nominal diameter twist drill bits incorporating the taper rate of the present invention, it took a peak force at failure in the range of 250 to 300 lbs. to break the bit. In most cases the break occurred not in the fluted portion  24 , but in the strongest part of the bit, namely the shank portion  22 . This result was attained using the same hardness of the bit steel as conventional bits. The first three quarters of the fluted portion is maintained at full hard, decreasing to a lesser hardness from that point to the end of the shank. 
     FIG. 3 shows an alternate embodiment of a twist drill incorporating the web thickness taper rate of the present invention, and illustrates its ability to perform well in wood while still exhibiting its enhanced strength while drilling in metal. It has been found that a twist drill bit made according to the invention shown in FIG.  3  and having a nominal bit diameter of ¼″ can be used to drill a hole through a 1½″ thick piece of wood  30 , such as 2×4, without withdrawing the bit to clear the sawdust from the flutes and then reinserting the bit to finish the hole. Thus the present invention embodied in the twist drill bit shown in FIG. 3 reduces the amount of “woodpeckering”, so that the operator can use a ¼″ metal-drilling drill bit to drill a hole through the board  30  in one pass, as indicated by arrow  32 . The web taper rates of the present invention are shown schematically in various drill bits in FIGS. 4 through 7. 
     FIG. 4 schematically illustrates the web taper of the present invention (shown in short dashes) superimposed upon the web taper of a conventional bit (shown as long and short dashes). The profile of the conventional web taper is referred to as  34 , and the web profile of the present invention is referred to as  36 . 
     The most common drill bit used by the person operating a hand-held power drill is known as a “Jobber&#39;s-length, straight-shank drill bit”, specifications for which are common in the industry and are set out in the Standards. These Standards collectively set forth commonly-accepted ranges of drill bit parameters for various nominal bit diameters&#39;, and further reflect conventional wisdom in the art of drill bit technology. As previously noted, these teachings hold that the more chips or sawdust a flute can remove from thd hole being drilled, the better. To that end, conventional wisdom in the art dictates that the web thickness should taper outwardly from the tip towards the shank at a very gradual rate. For example, the NAS 907 Standard specifies that the web taper rate for Types “A”, “B” and “J” straight shank, Jobber&#39;s length drill bits be uniform tapers of 0.017″, 0.024″ and 0.006″, respectively, with respective tolerances of plus or minus 0.003″, plus or minus 0.003″, and plus or minus 0.002″, respectively. Indeed the taper rates of conventional bits sold by the Assignee of the present invention follow this teaching. For example, certain drill bits sold under the DeWALT® trademark have a web taper rate of 0.024″ inches per inch of flute length for drill bit nominal diameters up to and including {fraction (3/16)}″, and a web taper rate of 0.030 inches per inch of flute length for drill bits having nominal diameters greater than {fraction (3/16)} inch. As previously noted, self-centering drill bits having pilot tips sold under the BULLET® trademark have a web taper rate of 0.027 inches per inch of flute length. 
     Referring once again to FIG. 4, bits following the teaching of the Standards have flutes along the flute portion  24  which exit at a termination point  38  with the outer diameter of the shank portion  22 , the shank portion being defined as the portion of the drill bit outside of the portion that contains the flute and which is inserted in the chuck of a power drill. Here the web thickness of the conventional web profile  34  increased from a thickness T to a thickness designated by  40  at termination, leaving a relatively large flute depth  42  for receiving the chips or sawdust. In marked contrast, the web taper profile  36  of the present invention exits the bit  20  at a flute termination point  44 . This provides a much thicker web  46  at the flute termination point  44  and yields a very small flute depth  48  at that point. Note that the flute grinding wheel has left a relatively large radius portion  50  at the conventional flute termination point  38 , but a relatively small radius  52  at the flute termination point  44  of the present invention. 
     In one embodiment of the present invention a uniform web taper rate of 0.068 inch per inch of flute length was found to provide durability that far exceeded expectations. The peak transverse force F at failure applied to a rotating cantilever-mounted drill bit exceeded 250 lbs. (for a ¼ inch nominal diameter drill bit), with very acceptable chip removal characteristics when drilling in metal. In another embodiment of the present invention with the same nominal diameter, the web thickness taper rate was set at 0.054 inches per inch of flute length (plus or minus 0.003 inches) for all sizes of drill bits from ⅛ inch nominal diameter ½ inch nominal diameter. Not only did the drill bit exhibit the same strength characteristics, but it reduced the amount of woodpeckering when drilling wood. It was able to drill a hole in 1½ inch thick wood (such as a 2×4) in one pass. The web thickness T at the tip or point in the preferred embodiment generally follows the NAS 907 Standard, and for that matter, the other parameters of bits according to the preferred embodiments generally follow one or more of the Chaconas et al. teachings or the Standards, unless otherwise specified herein. 
     The other drill bit parameters pertaining to the drill bit shown in FIG.  4  and to the other embodiments are as follows: “D” stands for the nominal diameter of the drill bit; “BL” stands for the bit length (note that it does not extend to the tip but instead to the axially forwardmost point of the bit where the flute cuts the outer diameter of the bit); “FL1” stands for the flute length of a flute having a conventional web thickness taper rate and is the flute length referred to in the previously-identified Standards; and “FL2” is the flute length of one of the preferred embodiments of the present invention, created by diminishing FL1 by a predetermined amount MZ. As will be described shortly, selecting FL2 in this fashion confers a double benefit upon the drill bit of the present invention. 
     Still referring to FIG. 4, for the most part, the parameters just described, and those in the Standards, pertain to a double-flute, straight shank, Jobber&#39;s length drill bit. However, it is believed that the concept of the present invention will also have utility in the environment of a drill bit with a single flute. In that case, instead of measuring the taper rate of a web thickness T, the taper rate of the shortest radial distance between the flute and the axis  54  of the drill bit (T/2) would be specified. In either event, the preferred web thickness taper rate ranges from about 0.050 inch to about 0.071 inch per inch of flute length, and the flute length FL lies in the range from about 0.3 inch to about 0.7 inch less than the flute length specified by the ASME Standard. 
     Ideally, MZ is about ½ inch for all nominal bit diameters in the range of from ⅛ inch through ½ inch. Although it is anticipated that the web taper rate of the present invention will do well in a drill bit having a conventional chisel point or web-thinned point (such as a split point) as schematically shown in FIG. 6, it is particularly useful in the unitary metal/wood-drilling, self-centering drill bit with pilot tip developed by the Assignee of the present invention, and as further described in the Chaconas et al. patents cited above. These embodiments of the drill bits incorporated the web-taper rate of the present invention are shown in FIGS. 4 and 5, which bits can be equipped with any of the tip portions  28  shown in FIGS. 9 through 13. 
     The nomenclature of the parameters for these embodiments are noted in FIGS. 9 and 11. The tip portion  28  includes a fishtail portion  56  and a pilot portion  58  extending axially outwardly from the fishtail portion. The fishtail portion  56  includes fishtail cutting lips  62  arranged at a fishtail angle  60 . In this embodiment, the cutting lips extend from the pilot portion to the outer diameter of the drill bit. The pilot portion  58  extends a distance “L” from the fishtail portion  56 , and has a diameter “d” smaller than the nominal diameter “D” of the drill bit. The pilot portion  58  defines an annular pilot wall  64  which is both radially and axially relieved as taught in the above-identified Chaconas et al. patents. The pilot portion  58  is formed with pilot cutting lips arranged at a point angle  68  (for example 135°) and, in the preferred embodiment, defines a split point  66 . The pilot portion  58  is joined to the fishtail portion  56  with a radially-relieved fillet  70  having a radius “R”. The drill bit has the following parameters: the fishtail angle  60  is greater than 180°, namely about 190°, and the pilot wall  64  defines an axial back taper  65  of about 5°. For the purposes of this description, the back taper on the pilot  58  which is directed radially inwardly from the point end  66  of the bit  20  toward the fishtail portion  56  will be considered a “positive” angle, whereas a back taper which is directed radially outwardly, as identified as  67  in FIG. 13, will be considered a “negative” back taper angle. A negligible back taper angle is shown as  69  in the embodiment shown in FIG.  12 . For the purposes of this description, “negligible” means in the range from greater than minus 1° to less than positive 1°. Other parameters noted in FIG. 9 are also used in the embodiments shown in FIGS. 10 through 13 and include the pilot length “L”, pilot diameter “d” and point angle  68 . 
     Referring now to the embodiment shown in FIG. 10, it was discovered that the fishtail portion  56  will exhibit increased longevity if the fishtail angle  60  is reduced to lie in the range from about 168° to about 182°, and preferably about 170°. It has also been discovered that the pilot portion  58  will exhibit increased longevity if its diameter d is approximately one-half the nominal bit diameter D. 
     Yet another discovery was made that adding a chamfer portion  72 , as shown in FIG. 11, significantly improves the life of the bit  20  if it were to be run at an overspeed condition. The chamfer portion  72  includes at least one chamfer  74 , and in the preferred embodiment includes two chamfers. The chamfers  74  are oriented at a chamfer angle  76  which is in the range of from about 75° to about 140°, and is preferably about 90°. In the single-chamfer embodiment, the chamfer angle  78  is about 45°, or one-half the two-chamfer included angle  76 . The length of the chamfer  80  is in the range of from about 5% to about 15% of the nominal diameter and is preferably about 10%. Another advantage of the chamfer is the significant reduction in creation of “discs” in composite materials. Furthermore, in the embodiment shown in FIG. 11, the included angle  68  remains at 135°, but the preferred back taper angle is in the range from 0° to 5.5°, with the preferred angle being 5°. Other tip parameters for the embodiment shown in FIG. 11 are set forth in TABLE 1. 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 TABLE OF TIP PARAMETERS 
               
             
          
           
               
                 Nom. 
                   
                   
                   
                   
               
               
                 Bit Diam. 
                 Pilot Diam. 
                 Pilot Length 
                 Secondary Lip 
                 Fillet Radius 
               
               
                 (In.) 
                 (In.) 
                 (In.) 
                 Relief Angle 
                 (In.) 
               
               
                 D 
                 d 
                 L 
                 f2 
                 R 
               
               
                   
               
               
                 1/8 
                 .067 
                 .05  
                 14° 
                 .020 
               
               
                  9/64 
                 .072 
                 .052 
                 14° 
                 .020 
               
               
                  5/32 
                 .080 
                 .052 
                 14° 
                 .020 
               
               
                 11/64 
                 .087 
                 .052 
                 14° 
                 .020 
               
               
                  3/16 
                 .094 
                 .052. 
                 14° 
                 .020 
               
               
                 13/64 
                 .102 
                 .052 
                 14° 
                 .023 
               
               
                  7/32 
                 .109 
                 .056 
                 14° 
                 .023 
               
               
                 15/64 
                 .117 
                 .061 
                 14° 
                 .023 
               
               
                 1/4 
                 .125 
                 .064 
                 14° 
                 .023 
               
               
                 17/64 
                 .133 
                 .068 
                 14° 
                 .026 
               
               
                  9/32 
                 .141 
                 .073 
                 14° 
                 .026 
               
               
                 19/64 
                 .148 
                 .077 
                 14° 
                 .026 
               
               
                 5/16 
                 .156 
                 .08  
                 14° 
                 .026 
               
               
                 21/64 
                 .164 
                 .085 
                 14° 
                 .030 
               
               
                 11/32 
                 .172 
                 .089 
                 14° 
                 .030 
               
               
                 23/64 
                 .180 
                 .093 
                 14° 
                 .030 
               
               
                 3/8 
                 .188 
                 .097 
                 14° 
                 .030 
               
               
                 25/64 
                 .195 
                 .101 
                 12° 
                 .035 
               
               
                 13/32 
                 .203 
                 .105 
                 12° 
                 .035 
               
               
                 27/64 
                 .211 
                 .109 
                 12° 
                 .035 
               
               
                  7/16 
                 .219 
                 .113 
                 12° 
                 .035 
               
               
                 29/64 
                 .227 
                 .117 
                 12° 
                 .035 
               
               
                 15/32 
                 .234 
                 .121 
                 12° 
                 .035 
               
               
                 31/64 
                 .242 
                 .125 
                 12° 
                 .035 
               
               
                 1/2 
                 .250 
                 .129 
                 12° 
                 .035 
               
               
                   
               
             
          
         
       
     
     The embodiment shown in FIG. 12 is very similar to that of FIG. 11, except the back taper angle  69  is negligible. Similarly, the embodiment shown in FIG. 13 uses a negative back taper angle greater than minus 1°. 
     Referring now to FIG. 15, the process for generating the pilot portion  58 , the fishtail portion  56  and the radially-relieved fillet  70  is shown. The area of the pilot diameter relief is shown at  82 ; the fillet relief area is shown at  84 , and the area of fishtail cutting lips relief is shown as  86 . It will be appreciated that in the drill bit of FIG. 15, a continuous cutting edge is created by the fishtail cutting lip  62 , fillet edge  71  and pilot wall cutting edge  65 . In order to enable these edges to cut satisfactory, they all must be provided with relief surfaces as shown in FIG. 15, and as discussed in Chaconas et al, cited above. To form such surfaces, a grinding wheel  88  having a grinding wheel face  90  is oriented with the bit axis  54 , such that the fishtail angle  60  divided by two is in the desired range. The pilot back taper angle  65  (positive or negative) is achieved by providing the grinding wheel  88  with a predetermined form, which also defines the shape of the fillet  70 . To generate the desired edges and surfaces, the bit is positioned in the desired angular relationship to the grinding wheel  88  and is rotated about its axis  54  in the direction shown by arrow  92  and at the same time is moved into the wheel in the direction shown by arrow  94 , thereby achieving the reliefs for the cutting edge  65 ,  71  and  62 . The process is repeated on the opposite side of the bit to generate both sets of cutting edges and reliefs. 
     Returning now to the web thickness taper rate of the present invention, it was discovered that a modification to the Standard flute lengths resulted in benefits both in reducing variations in flute length tolerances as well as in creating a special marking zone “MZ” where such indicia  96  as bit size may be formed, to thereby stay clear of the jaws of the drill chuck to which the bit is inserted (see FIG.  4 ). This means that the drill chuck will not eradicate the size markings from the shank, as often happens in conventional drill bits. 
     Again referring to FIG. 8, a grinding wheel  98  is positioned at a predetermined angle relative to the axis  54  of drill bit  20  and is brought radially inwardly into contact with the bit at the tip portion  28 , or starting point of the flute, as shown by arrow  100 . The bit stock  20  and the grinding wheel  98  are then caused to have relative movement towards one another along  30  an axial direction  102 . In the preferred embodiment, the grinding wheel  98  is held for movement only in the radially inward and outward directions as shown by arrows  100 , and the bit stock  20  is moved axially into the grinding wheel. A cam then causes the grinding wheel  98  to move radially outwardly while the bit stock moves into it, thereby creating a desired web taper rate. If the grinding wheel  98  producing the taper rate of the present invention were to be retracted at the termination point  38 , creating a flute length FLI corresponding to the flute length recommended by the Standards, the variation in manufacturing tolerances of the grinding wheel and bit, in combination with the taper angle, would create a flute length FLI of unacceptable breadth of variability as shown at  106 . Inasmuch as the flute length is a very important parameter in the manufacture of twist drill bits, it is preferable to maintain such variations to be within a plus or minus ⅛″, namely ¼″, in breadth. To reduce the variation in flute length tolerances, it was discovered that if the grinding wheel  98  were retracted radially outwardly at termination point  44 , rather than at  38 , the variation in flute lengths could be significantly reduced. The point of retraction is determined by the flute length FLI diminished by amount MZ, where MZ is in the range of from 0.3 inch to about 0.7 inch, and preferably about ½ inch. 
     The process is completed when a second flute is formed diametrically opposite the flute just described using the same process. In the preferred embodiments, the process forms a modified parabolic flute. 
     Although the web thickness taper rate of drill bits of the present invention is believed to significantly improve the strength of drill bits having conventional point geometry, as schematically shown in FIG. 6, it was particularly effective in enhancing the durability of drill bits having tip configurations  28  shown in FIGS. 9 through 13. The coaction of the web taper rate, pilot width, back taper angle, and chamfer of the embodiment shown in FIG. 11, will produce a twist drill bit of significantly improved strength overall while keeping the versatility that has help make it a commercial success. 
     The above-described embodiments, of course, are not to be construed as limiting the breath of the present invention. Modifications, and other alternative constructions, will be apparent which are within the spirit and scope of the invention as defined in the appended claims.