Abstract:
A spade drill bit for use in association with a drill having a direction of rotation includes an elongate shank, a spade portion and a center spur. The elongate shank portion has a central longitudinal axis and one end adapted to engage the drill. The spade portion extends longitudinally from the other end of the elongate shank. The spade portion has opposed spaced apart planar faces and each planar face has a leading shoulder edge and a trailing shoulder edge. Each planar face has a leading face portion and a trailing face portion proximate to the respective leading shoulder edge and trailing shoulder edge that are twisted in the direction of rotation. The center spur extends outwardly from the spade portion along the central longitudinal axis. Preferably the center spur has an elongate flute that has a volume that increases toward the spade portion. Preferably a dimple is formed proximate to a corner leading edge which is between the leading shoulder edge and the leading longitudinal edge.

Description:
FIELD OF THE INVENTION 
     This invention relates to drill bits for forming bores in wood or other soft material and in particular this invention relates to spade type drill bits that range in size from about ¼″ to 1½″ and that are commonly used with a small portable electric or cordless drill or with a drill press. 
     BACKGROUND OF THE INVENTION 
     Drill bits are a very common tool used in the construction and maintenance industries. There are a wide variety of drill bits that have been developed. Spade type drill bits, which have a spade-like blade or cutter, are generally used in association with small portable electric or cordless drills but they may also be used in association with a dill press. 
     Spade drill bits are generally inexpensive as compared to auger or twist type drill bits. One of the reasons that spade drill bits are affordable is their simple design and manufacture. A spade drill bit is made by flattening a portion of a cylindrical bar by forging and the formation of cutting edges on the flattened section by grinding. The remainder of the unflattened bar is used to engage the drill by way of a chuck. In addition the spade drill bit may be modified through secondary forging steps thereby modifying the flattened section to include performance improving impressions, for example folds, grooves and bends. 
     Other attributes of spade drills over auger or twist type drills include convenience of storage and ease of resharpening the bits in the field. 
     In general, spade type drills have a shank region of cylindrical cross section. The end of the shank section may have either ground or forged flats to assist engagement of the drill in the chuck of the portable electric or cordless drill or drill press. The flattened end of the spade drill has two parallel opposing planar face regions, each having a leading face portion and a trailing face portion and an axially extending centre spur that points away from the shank end of the spade. The centre spur extends between the opposing planar face regions of the spade bit. Each opposing face of the spade is bounded by leading and trailing longitudinal edges, by leading and trailing shoulder edges and by leading and trailing shank edges. 
     The longitudinal sides between the longitudinal edges are slightly tapered toward each other toward the shank end of the bit, further, the longitudinal sides occur at an acute angle from the face region of the spade drill along the leading longitudinal edge. 
     The shoulder sides are generally radially located between the centre spur and the longitudinal sides and are sloped so as to occur at an acute angle from the face region of the spade drill along the leading shoulder edge. 
     As the leading and trailing shank edges of the spade are removed from all cutting activity, the side located between these edges is left unfinished beyond the as forged condition. 
     The centre spur has centre spur faces that are bounded by the face region of the spade drill and by leading and trailing centre spur edges, between the leading and trailing edges are the centre spur sides, the sides occurring at an acute angle from the centre spur faces along the centre spur leading edges. 
     The centre spur sides, longitudinal sides and shoulder sides occur at acute angles from their respective faces to provide relief for the centre spur leading edge, longitudinal leading edge and shoulder leading edge respectively during operation of the drill. 
     Additionally, side spurs are often provided. These spurs generally are extensions of the longitudinal sides extending beyond the shoulder sides and have leading and trailing faces that are continuations of the leading face portion and trailing face portions respectively of the face region of the spade. Further the inward side of the side spur Is a non-coplanar extension of the shoulder side of the spade drill. 
     In operation, with the spade drill bit installed in an electric or cordless drill or drill press the centre spur is the first part of the bit to engage the work whereupon the centre spur leading edges cut out a conical impression in the work initiating cutting and providing stability for the spade drill. Further advancement of the drill allows the side spurs, if present, to cut a circular “v” shaped groove in the work whereupon further advancement causes the shoulder side leading edges to engage the work and remove material between the centre spur and side spur. This action continues until the centre spur exits out the other side of the workpiece and the side spurs cut a circular exit hole. During cutting action where the longitudinal sides are engaged with the workpiece and particularly upon break through of the centre spur and side spurs from the workpiece, the longitudinal sides provide stability of the bit in the formed bore. 
     A review of the prior art reveals that considerable effort has been taken to provide increased cutting efficiency of the spade drill at all of the cutting edges. 
     For example U.S. Pat. No. 2,782,824 issued to Robinson on Feb. 26, 1957, shows a groove in the centre spur face along a side of the centre spur leading edge. However, the inside edge of the centre spur flute is generally parallel to the centre spur leading edge and there is not an increase in volume of the flute in the longitudinal direction. Alternatively, U.S. Pat. No. 3,997,279 issued to Porter on Dec. 14, 1976 shows a full centre spur flute that has concave sides proximate to the centre spur leading edge and the centre spur trailing edge. The concave sides extend from the tip to the face of the spade drill bit. However, there is a considerably reduced amount of material in this centre spur which leads to an increased likelihood of failure. 
     An alternate prior art spade drill bit disclosed in U.S. Pat. No. 4,682,917 issued to Williams on Jul. 28, 1987, shows a groove in the face of the spade along a side of the shoulder leading edge with side spurs extending in the direction of the centre spur. In addition, the leading face of the side spur is sloping in the direction of rotation. 
     Despite these and many other improvements to spade drills there still remain deficiencies and it is the ambition of this invention to overcome these. In particular it would be advantageous to have a spade drill bit that has good cutting characteristics, that can be easily sharpened by the end user and is relatively easy to manufacture. 
     SUMMARY OF THE INVENTION 
     A spade drill bit for use in association with a drill having a direction of rotation includes an elongate shank, a spade portion and a centre spur. The elongate shank portion has a central longitudinal axis and one end adapted to engage the drill. The spade portion extends longitudinally from the other end of the elongate shank. The spade portion has opposed spaced apart planar faces and each planar face has a leading shoulder edge and a trailing shoulder edge. Each planar face has a leading face portion and a trailing face portion proximate to the respective leading shoulder edge and trailing shoulder edge that are twisted in the direction of rotation. The centre spur extends outwardly from the spade portion along the central longitudinal axis. 
     In another aspect of the invention, a spade drill bit includes an elongate shank, a spade portion and a centre spur. The elongate shank portion has a central longitudinal axis and one end adapted to engage the drill. The spade portion extends longitudinally from the other end of the elongate shank. The spade portion has opposed spaced apart planar faces. Each planar face has a leading face portion, a trailing face portion, a leading shoulder edge, a trailing shoulder edge, a leading longitudinal edge and a trailing longitudinal edge. There is a corner leading edge between the leading shoulder edge and the leading longitudinal edge. A dimple is formed in each leading face portion proximate to each corner leading edge such that a cutting edge is formed at each corner leading edge. A centre spur extends outwardly from the spade portion along the central longitudinal axis. 
     In a further aspect of the invention, a spade drill bit includes an elongate shank, a spade portion and a centre spur. The elongate shank portion has a central longitudinal axis and one end adapted to engage the drill. The spade portion extends longitudinally from the other end of the elongate shank. The centre spur extends outwardly from the spade portion along the central longitudinal axis. The centre spur has a pair of opposing centre spur faces. Each centre spur face has a centre spur leading edge and a centre spur trailing edge which meet at a point. A centre spur elongate flute is formed in each centre spur face proximate to the centre spur leading edge. The centre spur elongate flute has an inside boundary that is generally parallel to a central longitudinal axis such that the volume of the centre spur elongate flute increases as it approaches the spade portion. 
     Further features of the invention will be described or become apparent in the course of the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described by way of example only, with reference to the accompanying drawings, in which: 
     FIG. 1 is a front view of the spade drill bit constructed in accordance with the present invention; 
     FIG. 2 is a side view of the spade drill bit of FIG. 1; 
     FIG. 3 is an enlarged partial perspective view of the spade portion of the spade drill bit of FIG. 1; 
     FIG. 4 is a cross section taken on line  4 — 4  of FIG. 1; 
     FIG. 5 is an end view of the spade drill bit of FIG. 1; 
     FIG. 6 is a cross section taken on line  6 — 6  of FIG. 5; 
     FIG. 7 is an enlarged cross section of a rounded corner and dimple of the present invention shown engaging a workpiece, also shown in cross section; 
     FIG. 8 is an enlarged partial perspective view of a rounded corner and dimple of the present invention; 
     FIG. 9 Is an enlarged partial perspective view of a prior art side spur; 
     FIG. 10 is an enlarged front view of a conical dimple; 
     FIG. 11 is an enlarged cross section of the conical dimple of FIG. 10; 
     FIG. 12 is an enlarged front view of a frustoconical dimple; 
     FIG. 13 is an enlarged cross section of the frustoconical dimple of FIG. 12; 
     FIG. 14 is an enlarged front view of a pyramidal dimple; 
     FIG. 15 is an enlarged cross section of the pyramidal dimple of FIG. 14; 
     FIG. 16 is an enlarged front view of a frustopyramidal dimple; 
     FIG. 17 is an enlarged cross section of the frustopyramidal dimple of FIG. 16; 
     FIG. 18 is an enlarged front view of an elliptical dimple; 
     FIG. 19 Is an enlarged cross section of the elliptical dimple of FIG. 18; 
     FIG. 20 Is an enlarged cross section of a hemispheric dimple showing a thin leading edge; 
     FIG. 21 is an enlarged cross section of a hemispheric dimple showing a thicker leading edge as compared to the leading edge shown in FIG. 20; 
     FIG. 22 is an enlarged partial cross section of a centre spur of the present invention; 
     FIG. 23 is an enlarged partial cross section of a prior art centre spur; and 
     FIG. 24 is an enlarged partial cross section of another prior art centre spur. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The spade drill bit of the present invention is a one piece forged steel spade drill bit for forming holes in wood or other similar material when used with an electric or cordless drill or drill press. Referring to FIGS. 1 and 2 the spade drill bit of the present invention is shown generally at  10 . Spade drill bit  10  has an elongate shank portion  12 , a spade portion  14  and a centre spur  16 . 
     The driving end  18  of the elongate shank portion  12  has a plurality of flat sides  20 . The flat sides  20  form a generally hexagonal shape in cross section. The driving end  18  facilitates engagement of the spade drill bit  10  in a chuck of an electric or cordless drill or drill press (not shown). 
     Spade portion  14  is wider than the elongate shank portion  12  due to the forging and flattening of the cylindrical bar. Spade portion  14  has two opposing generally planar faces  24  with each face  24  having a leading face portion  26  and a trailing face portion  28 . Each face  24  is bounded by a leading shoulder edge  30 , a trailing shoulder edge  32 , a leading longitudinal edge  34 , a trailing longitudinal edge  36 , a leading shank edge  38 , a trailing shank edge  40  and the centre spur  16 . Longitudinal sides  42  join the leading longitudinal edge  34  of one face  24  with the trailing longitudinal edge  36  of the other face  24 . Similarly, shoulder sides  44  join the leading shoulder edge  30  of one face  24  with the trailing shoulder edge  32  of the other face  24 . The distance between faces  24  is relatively small as compared to the width of each face  24 . 
     As shown in FIG. 3 the centre spur  16  has two oppositely facing generally planar centre spur faces  46  that are extensions of the faces  24 . Each centre spur face  46  is bounded by the face  24  of the spade drill bit  10 , by the centre spur leading edge  48  and the centre spur trailing edge  50 . Centre spur sides  52  join the centre spur leading edge  48  of one centre spur face  46  with the centre spur trailing edge  50  of the other centre spur face  46 . 
     Each leading shoulder edge  30 , leading longitudinal edge  34  and centre spur leading edge  48  defines an acute angle between their respective sides  44 .  42  and  52  and faces  26  and  46 . Conversely, each trailing shoulder edge  32 , trailing longitudinal edge  36  and centre spur trailing edge  50  defines an obtuse angle between their respective sides  44 ,  42  and  52  and faces  28  and  46 . 
     As shown in FIGS. 3,  4 ,  5  and  6 , the portion of the planar face  24  adjacent to the leading shoulder edge  30  and trailing shoulder edge  32  has a right handed twist  54 , in a smooth continuous curve in the direction of rotation. Spaced from the leading shoulder edge  30  and trailing shoulder edge  32 , planar faces  24  are generally straight. Preferably twist  54  is arranged such that a straight line  55  perpendicular to the longitudinal axis  56  of the spade drill bit  10  and extending from a leading longitudinal edge  34  to the opposed trailing longitudinal edge  36  will be in continuous contact with the face  24 . This line may be placed anywhere along longitudinal axis  56  along a planar face  24 , as shown in FIGS. 3,  5  and  6 . 
     Referring to FIG. 6, drill bit  10  has two relevant angles at the leading shoulder edge  30 , namely a positive rake angle  63  and a relief angle  65 . The positive rake angle  63  is the angle that the leading face portion  26  of the face  24  is swept backwardly from the vertical along the shoulder leading edge  30  and is formed by the twist  54  of the drill bit  10 . The relief angle  65  is the angle that the shoulder side  44  is swept downwardly from the horizontal along the shoulder leading edge  30 . The relief angle is formed by grinding the shoulder side  44  to the preselected angle. 
     Twist  54  provides each leading shoulder edge  30  with a positive rake  63  to allow a chip or “curl” of wood to form in the workpiece  60  (shown in FIG. 7) at the leading shoulder edge  30  during the operation of the spade drill bit  10 . The “curl” or chip is suggested by the curved arrow  62  shown at the leading shoulder edge  30  in FIG.  6 . The maximum amount of rake  63  for leading shoulder edge  30  provided by the twist  54  varies from one size of drill bit to the next size of drill bit, but ranges between 4° to 15°. The greater the rake  63  the greater the cutting efficiency but the greater the instability. For most applications a rake of 6° balances the efficiency with stability. 
     The relief angle  65  will also affect the stability of the drill bit  10 . Similarly, the larger the relief angle  65  the larger the pull into the workpiece and the greater the instability. Thus to further balance the instability associated with the positive rake  63 , the relief angle  65  can be selected to limit the maximum depth of cut of the drill bit  10  and so limit the maximum thickness of the resulting chip. By lowering the relief angle  65 , the depth of cut and so the thickness of the chip is reduced and so the net cutting rate is maintained at a level that taxes neither the operator and the electric drill nor the strength of the drill bit  10  as a whole, while continuing to provide an acceptable cutting rate. Typically this angle will be between 5° and 10° and preferably relief angle is 6°. 
     Referring to FIG. 3, there is a gradual transition of the rounded corner  64  between shoulder side  44  and longitudinal side  42 . The rounded corner leading edge  66  of the rounded corner  64  is a smooth continuous link between the leading shoulder edge  30  and leading longitudinal edge  34 . Similarly, the rounded corner trailing edge  68  of the rounded corner  64  is a smooth continuous link between the trailing shoulder edge  32  and trailing longitudinal edge  36 . The rounded corner trailing edge  68  has a smaller radius of curvature than the rounded corner leading edge  66  because of the acute and obtuse angles relative to longitudinal side  42  and shoulder side  44 . 
     Referring to FIG. 3, a dimple  70  is positioned on the leading face portion  26  proximate to the rounded corner  64  such that rounded corner leading edge  66  forms a cutting edge. Dimple  70  is generally hemispherical in shape. A portion of dimple  70  is bounded by the rounded corner leading edge  66 . Preferably dimple  70  has the same radius as the radius of the rounded corner  64 . A cross section of the spade drill bit  10  through the dimple  70  and across the face  24  is shown in FIG.  7 . As can be seen in FIG. 7, dimple  70  provides a positive rake to rounded corner leading edge  66 . The rounded corner leading edge  66  and dimple  70  engages the work piece  60  and severs wood fibres therein. As shown in FIGS. 7 and 8, the cutting edge provided by the sharp rounded corner leading edge  66  between the rounded corner  64  and dimple  70  is an improvement over the cutting edge provided with a side spur  72  of prior art drill bit  74 , shown in FIG.  9 . 
     As best seen in FIG. 3, an elongate flute  76  is formed in centre spur face  46  proximate to the centre spur leading edge  48 . Elongate flute  76  is narrower and shallower at the tip of the centre spur  16  than toward and into the face  24 . An inside boundary  78  of the elongate flute  76  spaced from the centre spur leading edge  48  is generally parallel to longitudinal axis  56  of the spade drill bit  10 . Elongate flute  76  in conjunction with centre spur leading edge  48  provides a positive rake angle to the centre spur leading edge  48 . The centre spur  16  has an elongate flute  76  in each centre spur face  46 . Accordingly the depth of each elongate flute  76  should not interfere with the other elongate flute  76  nor compromise the strength of the centre spur  16  as a whole. 
     Centre spur leading edge  48 , leading shoulder edge  30 , leading longitudinal edge  34  and rounded corner leading edge  66  are all sharpened edges. The centre spur leading edge  48  is field resharpened by use of a grinding stone or flat file, the stone or file being applied across the centre spur side  52 , the new centre spur side  52  forming a new centre spur leading edge  48  in cooperation with the centre spur elongate flute  76 . Similarly, the leading shoulder edge  30  is field resharpened by use of the same grinding stone or flat file, the stone or file being now applied across the shoulder side  44 , the new shoulder side  44  forming a new leading shoulder edge  30  in cooperation with the twist  64  at the leading face portion  26 . In addition, in continuation with the leading shoulder edge resharpening, the stone or file can easily be applied tangentially around the rounded corner  64 , to form a new rounded corner leading edge  66  in cooperation with dimple  70 . As the rounded corner leading edge  66  does not extend axially upwardly from the leading shoulder edge  30 , a particularly narrow stone or file is not required to sharpen the leading shoulder edge  30  as is the case with prior art spade drill bits  74  that include side spurs  72  which limit the width of stone or file that can be used. Further, complex side spur geometry does not have to be preserved, beyond the rounded corner aspect, during resharpening the spade drill bit  10  of the present invention. 
     Referring to FIGS. 10 through 19, the dimple could have a number of alternate shapes. The hemispheric dimple  70  described above has a spherical shape. Resharpening of the hemispheric dimple  70  will result in an erosion of the rounded corner leading edge  66  and a different rake of the rounded corner leading edge  66 . FIGS. 10 and 11 show a conical dimple  88 . The conical dimple  88  has a constant rake even after repeated sharpening. The frustoconical dimple  90  shown in FIGS. 12 and 13 is similar to the conical dimple  88  but limits the depth thereof. Similarly the frustoconical dimple  90  has a constant rake even after repeated sharpening. A pyramidal or diamond dimple  92  shown in FIGS. 14 and 15 has a straight corner leading edge  94 . The pyramidal dimple  92  has a constant rake. Similarly frustopyramidal dimple  98  shown in FIGS. 16 and 17 has a straight corner leading edge with a limiting depth. The frustopyramidal dimple  98  has a constant rake. The elliptical dimple  96  shown in FIGS. 18 and 19 is similar to the hemispheric dimple  70  but has a longer rounded corner leading edge. 
     The shape and the rake of the dimple can be chosen by the manufacturer. However, it should be noted that if the shape and rake of the dimple is such that the leading edge is very sharp it is also very thin and very week and is subject to breakage and chipping when in use. An example of a dimple  70  with a sharp edge is shown in FIG.  20 . Alternatively a dimple that balances the sharp edge with a relatively thick corner edge is shown in FIG.  21 . The dimple of FIG. 21 is preferable over the dimple of FIG.  20 . 
     Spade drill bit  10  is shaped using the forging process and thereafter grinding and sharpening. The spade portion  14  is forged by flattening a cylindrical bar of steel. The elongate shank portion  12  is the unmodified cylindrical bar. The flat sides  20  of the driving end  18  are ground or forged. Spade portion  14  is wider than the elongate shank portion  12  due to the forging and flattening of the cylindrical bar. The twist  54  is forged into the cylindrical bar with the spade portion  14  formed therein. A hole  58  is provided in the centre of planar faces  24 . Hole  58  allows the end user to hang drill bit  10  for storage. Further hole  58  is used during the grinding and sharpening process to position the drill bit properly for each successive step of the manufacturing process. 
     There are a number of advantages of spade drill bit  10  over the prior art. For example, while the provision of a centre spur elongate flute  80  of prior art drill bit  82  with both edges generally parallel to the centre spur leading edge has been shown (FIG. 23) the elongate centre spur flute  76  of the present invention provides increased efficiency by providing an increased volume as it approaches and enters the face  24 , thereby providing the path for increased chip flow along the flute  76 . Further, the elongate centre spur flute  76  of the present invention is an improvement over other centre spur flutes  84  of prior art drill bit  86  such as those shown in FIG. 24 because it has improved strength. A comparison of FIGS. 22,  23  and  24  suggests that the centre spur  16  and centre spur elongate flute  76  of the present invention have improved chip capacity and improved strength over the prior art while retaining a positive rake angle. 
     Further, although rounded corners have been shown in the prior art the provision of dimple  70  provides improved cutting characteristics. Overall, the spade drill bit  10  of the present invention has shown, through testing, improvements in the drilling rate and a reduction in the amount of wood splintering at the entrance and exit regions of the drill hole as compared to prior art drill bits. 
     It will be appreciated that the above description relates to the invention by way of example only. Many variations on the invention will be obvious to those skilled in the art and such obvious variations are within the scope of the invention as described herein whether or not expressly described.