Abstract:
A modified Forstner type drill bit includes a shank and a drill head. The drill head includes a center brad, first and second radial cutters, first and second following rims and first and second chip channels. The center brad defines a longitudinal direction. The first and second of radial cutters extend radially from the center brad. The first and second following rims extend circumferentially from and following from in the direction of rotation the distal end of the first and second radial cutters respectively. Each following rim is divided into a plurality of segments, each have a top portion. The top portion of the segments of each following rim are generally in the same plane. The first and second chip channels are positioned adjacent to and before in the direction of rotation the first and second radial cutters respectively. The first and second chip channels are between the one of the first and second radial cutter and the second and first following rims. The shaft extend longitudinally from the radial cutters in the longitudinal direction and from the side opposed from the center brad. In one embodiment a plurality of depressions divide each following rim into segments. In another embodiment a plurality of elongate grooves divide each following rim into segments.

Description:
FIELD OF THE INVENTION 
     This invention relates to drill bits for use in woodworking and in particular to Forstner type drill bits. 
     BACKGROUND OF THE INVENTION 
     Forstner type drill bits are shaft mounted drills that have a cylindrical cutting head with a pair of radial cutters that are perpendicular to the axis of the drill bit. These radial cutters allow for the drilling of flat bottomed holes. Located between the radial cutters, on the axis of the drill, is a center brad that acts to provide stability to the drill when engaged in drilling a hole. As the center brad extends longitudinally beyond the radial cutters only a short distance, perhaps only 10 percent of the diameter of the bit, and has a similarly small base, again perhaps only 10 percent of the diameter of the bit, the impression of the center brad left at the bottom of a blind hole is only small and tends not to diminish the flat bottomed aspect of the hole. 
     Directly leading and extending longitudinally behind each radial cutter is a chip channel that allows waste wood from the drilling operation to pass from the radial cutters to the space behind the cylindrical cutting head and then out of the so bored hole. Each chip channel has a radial width equal to the width of each radial cutter and is bounded by a trailing surface that is an extension of the radial cutter. Further the trailing surface is angled such that the radial cutter leads all other parts of the trailing surface. 
     Trailing behind and spaced slightly apart from each radial cutter are a spur and a following rim. Since the spur and following rim are positioned at the outer most diameter of the drill bit, the spur works to sever wood fibers that are at the diameter of the drilled hole. Each spur and following rim are set slightly outwardly longitudinally from the radial cutters such that each spur severs wood fibers and so defines a clean boundary for the drilled hole before the radial cutters uplift the wood that occurs between the spur and the center brad. The spur acts to sever wood fibers and the following rim acts as a stabilizer in the sense that it follows closely the circular groove cut in the work piece by the spur. This close following of the circular groove acts to keep the drill bit from wandering. As the inner surface of the following rim is generally conical and as the circular groove cut in the work piece is also made generally of a conical surface there is considerable tendency for the two conical surfaces to stay engaged and so there exists little tendency for the bit to wander. This engagement between the conical surfaces of the bit and work piece is sufficient to allow the bit to be used to drill a hole through the edge of a work piece wherein the center brad of the bit entirely overhangs the edge of the work piece being drilled. During this operation the sole source of stability of the bit, beyond the stability offered by the tool that is driving the bit, lies between the close engagement of the following rim surfaces and the circular groove in the work piece. 
     The outer surface of the following rim extends toward the shank of the bit and so forms a surface that offers additional stability to the drill bit in the drilled hole by being in close engagement with the bore of the so formed hole. 
     As there is a considerable amount of surface to surface contact between the drill bit and the work piece, particularly on larger sized holes, only a limited longitudinal displacement of the bit into the work piece is achieved per unit of longitudinal force applied to the bit. That is to say, the actual cutting rate into the work piece can be quite low. Further, due to the surface to surface contact, considerable heat is generated and time must be allowed between drilling operations for the drill bit to cool off as overheating of the drill bit will tend to alter the hardness and consequently shorten the life of the bit. 
     In an effort to improve the cutting rate of Forstner bits in general, a variation has been developed that has saw type teeth formed into the following rim. The saw teeth effectively replaces the conical inner surface of the following rim with a plurality of triangular saw tooth inner surfaces. The formation of saw teeth in the following rim of a Forstner drill can mean the reduction of more than 90 percent of the area of the conical inner surface that contacts the work piece during operation. By reducing this surface area, a greater displacement of the drill into the work piece is achieved per unit of longitudinal force applied to the drill thereby resulting in increased drilling rate and reduced friction and heat buildup experienced by the drill. However this loss of conical inner surface area can result in a considerable loss of aligning tendency that normally exists between the drill and the work piece thereby resulting in a reduced ability for the drill to form generally cylindrical holes in the work piece and reduced ability to form holes where the center brad of the drill overhangs an edge or is over a void in the work piece. 
     Additionally, the formation of saw teeth in the following rim of the Forstner drill bit also reduces the area of the outer surface of the following rim, but only by as little as 8 percent and as the outer surface of the drill is tapered slightly, having a smaller diameter as it progresses away from the following rim, a loss of area here does not greatly reduce the aligning tendency between the drill and the work piece. 
     In an additional effort to further improve the drilling performance and reduce both the resulting friction and the power requirements of saw tooth Forstner drills, flutes have been introduced. The flutes extend across the outer surface of the cylindrical cutting head from the following rim and generally downwardly away from the following rim. The flutes are angled slightly from the longitudinal direction in a forward direction such that the portion of the flute proximate to the following rim radially leads the remaining portion of the flute, further the width of the flute is approximately one half the pitch of the saw teeth of the drill. Two embodiments exist for the flutes, the first including flutes of generally parabolic shape having greatest width and depth at the following rim and extending to least width and depth at the shank edge of the cylindrical cutting head. The second embodiment includes flutes with generally parallel sides such that the flutes have constant width and depth over their entire extent. In either the parabolic or parallel sided flute case, the intersection of the flutes with the saw teeth of the drill are generally the same. 
     The advantage of the application of flutes to a Forstner drill that already includes saw teeth is that there is reduced friction between the drill and the formed bore by reducing the area of the outer surface of the drill that contacts the formed bore in the work piece. As saw tooth Forstner drills already have limited contact between the following rim of the drill and the work piece due to the portion of the following rim that were removed to form the saw teeth, the additional application of flutes to the outer surface of the drill does little to further reduce the area of contact between the inner conical surface of the following rim and the work piece. The disadvantage of the application of flutes to a saw tooth Forstner drill bit is that there is an increased tendency to wander. That is the drill is more likely to move sideways and thus create a hole that is not straight through the work piece. 
     It would be advantageous to provide a Forstner type drill bit that increases the cutting rate over a conventional Forstner drill bit but reduces the tendency to wander over the fluted saw tooth Forstner drill bit. 
     SUMMARY OF THE INVENTION 
     It has been determined that advantages can be realized by applying spaced apart depressions to the following rim of a prior art Forstner drill, where the pitch between depressions is greater than the extent of each depression. It has been further realized that such depressions can be achieved either by having the depressions extend generally perpendicularly to both the following rim and the axis of the bit and or by having the depressions extend generally perpendicularly to the following rim and parallel to the axis of the bit. 
     The present invention is a modified Forstner type drill bit which includes a shank and a drill head. The drill head includes a center brad, first and second radial cutters, first and second following rims and first and second chip channels. The center brad defines a longitudinal direction. The first and second of radial cutters extend radially from the center brad. The first and second following rims extend circumferentially from and following from in the direction of rotation the distal end of the first and second radial cutters respectively. Each following rim is divided into a plurality of segments, each have a top portion. The top portion of the segments of each following rim are generally in the same plane. The first and second chip channels are positioned adjacent to and before in the direction of rotation the first and second radial cutters respectively. The first and second chip channels are between the one of the first and second radial cutter and the second and first following rims. The shaft extends longitudinally from the radial cutters in the longitudinal direction and from the side opposed from the center brad. In one embodiment a plurality of depressions divide each following rim into segments. In another embodiment a plurality of elongate grooves divide each following rim into segments. 
     Further features of the invention will be described or will 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 perspective view of a drill of the present invention showing arcuate depressions; 
     FIG. 2 is a top view of the drill of FIG. 1; 
     FIG. 3 is a partial side view of the drill of FIG. 1; 
     FIG. 4 is a partial perspective view of an alternate embodiment of the drill of the present invention showing “V” shaped depressions; 
     FIG. 5 is a partial perspective view of an alternate embodiment of the drill of the present invention showing alternate elongate depressions; 
     FIG. 6 is a top view of the drill of FIG. 5; 
     FIG. 7 is a partial side view of the drill of FIG. 5; 
     FIG. 8 is a partial perspective view of an alternate embodiment of the drill of the present invention showing progressive elongate depressions; 
     FIG. 9 is a partial perspective view of a prior art Forstner drill; 
     FIG. 10 is a partial perspective view of a prior art saw tooth drill; 
     FIG. 11 is a partial perspective view of another prior art saw tooth drill with parabolic flutes and 
     FIG. 12 is a partial perspective view of another prior art saw tooth drill with parallel sided flutes. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1,  2  and  3 , the drill bit of the present invention is shown generally at  10  where the bit  10  has a shank  12  that extends along the axis of the drill  10  and a cutting head  14  attached to the shank  12  that is designed to drill a cylindrical bore upon rotation. 
     At the center of the cutting head  14  is a center brad  16  that is generally a pyramid shaped point that extends longitudinally beyond the cutting head  14 . The point of the center brad  16  has the greatest longitudinal extension of all parts of the bit  10  and so, upon operation, provides stability for the bit  10  as it provides a fixed point in the work piece for the bit  10  to rotate around. 
     Extending radially in opposite directions from the base of the center brad  16  is a pair of radial cutters  18 . The radial cutters  18  act as chisels to uplift wood on either side of the center brad  16 . Extending downwardly and backwardly from the radial cutters  18  are cutter planes  20 . 
     A chip channel  22  is positioned in front of each radial cutter  18 . Each cutter plane  20  forms one side of the adjacent chip channel  22 . The chip channels  22  are designed to allow for the flow and removal of dust and chips that have been cut by the radial cutters  18 . 
     A following rim  24  extends behind each radial cutter  18  on the circumference of the drill  10 . The following rim  24  has an inner conical surface  26  and an outer surface  28 . The outer surface  28  extends generally longitudinally from the following rim  24 . 
     Formed in each following rim  24  is a plurality of arcuate depressions  30  spaced apart from each other such that the pitch or distance between arcuate depressions  30  is greater than the extent or length of each arcuate depression  30 . (Compare this to the saw tooth Forstner drill bit wherein the pitch and the extent are equal). Thereby there are a plurality of spaced apart segments  32 , which are a portion of the following rim  24 , between adjacent arcuate depressions  30 . The arcuate depressions  30  are arranged on the following rim  24  so as to reduce the area of inner conical surface  26  that actually contacts the work piece during operation. 
     Each arcuate depression  30  extends from the inner conical surface  26  through the following rim  24  to the outer surface  28 . Thus the continuity of the following rim  24  is divided into a series of segments  32  separated from each other by an arcuate depression  30 . The intersection of a following portion of a arcuate depression  30 , the outer surface  28 , the inner conical surface  26 , and a leading portion of a segmented edge  32  creates a supplementary spur  34 . The supplementary spur  34  provides additional wood fiber severing capacity to the drill during operation. Generally the length of the following rim of a Forstner drill bit is approximately 57% of the circumference. The sum of the lengths of the plurality of segments  32  is in a range of 30-55% of the Forstner following rim or 15-40% of the circumference. Preferably the sum of the lengths of the plurality of segments  32  is 25% of the circumference. 
     As the arcuate depressions  30  extend into the inner conical surface  26  of the following rim  24 , the inner conical surface  26  exhibits a series of notches that reduce the overall area of inner conical surface  26 . This is particularly evident proximate to the following rim  24 . The longitudinal position of each following rim  24  is between the point of the center brad  16  and radial cutter  18 , as best illustrated in FIG.  3 . The longitudinal position of the following rim  24  is considerably closer to radial cutter  18  than to the point of center brad  16 . Generally only a small portion of the area of conical inner surface  26  actually contacts the work piece during operation. In practice it is not uncommon for the following rims  24  to be only a few hundredths of an inch longitudinally beyond the radial cutters  18 . Accordingly each arcuate depression  30  need extend into the area of inner conical surface  26  only slightly to have a relatively large impact on the area of inner conical surface  26  that actually contacts the work piece during operation. 
     Hereinafter alternative embodiments of the invention will be described and where the element is the same as described above the same reference numeral will be used. Only those features which are different from the embodiment described above will be described in detail. 
     An alternate embodiment is shown in FIG. 4 wherein the depression is a generally “V” shaped depression  36 . The spacing and depth of “V” shaped depression  36  is similar to those of arcuate depression  30 . 
     FIGS. 5,  6  and  7  show an alternate embodiment of the drill bit of the present invention shown generally at  40  wherein elongate grooves  42  are formed in both the following rim  24  and the outer surface  28 . The elongate grooves  42  are generally in the form of flutes which extend from the following rim  24  to the opposite edge of the outer surface  28 . The elongate grooves are angled in the direction of rotation as best illustrated in FIG.  7 . Thus, the portion of each groove  42  proximate to the following rim  24  leads all other portions of the elongate groove  42 . 
     The ends of elongate grooves  42  proximate to the following rim  24  divide the following rim  24  into a plurality of segments  44  similar to those segments  32  described above with regard to bit  10 . The elongate grooves  42  intersect the inner conical surface  26  thus decreasing the area of the conical surface  26 . As discussed above in regard to drill bit  10 , similarly in regard to drill bit  40  the longitudinal position of each following rim  24  is between the point of the center brad  16  and radial cutter  18 , as best illustrated in FIG.  7 . The longitudinal position of the following rim  24  is considerably closer to radial cutter  18  than to the point of center brad  16 . Generally only a small portion of the area of conical inner surface  26  actually contacts the work piece during operation. In practice it is not uncommon for the following rims  24  to be only a few hundredths of an inch longitudinally beyond the radial cutters  18 . Accordingly each elongate groove  42  need extend into the area of inner conical surface  26  only slightly to have a relatively large impact on the area of inner conical surface  26  that actually contacts the work piece during operation. 
     Similarly the formation of elongate grooves  42  in outer surface  28  and the inner conical surface  26  creates supplementary spurs  34  at the leading end of each segment of following rim  24 . Each supplementary spur  34  provides additional wood fiber severing capacity to the drill during operation. 
     Each elongate groove  42  is adjacent to a supplementary spur  34 . Each elongate groove  42  provides a path for the removal of wood dust and chips cut by the supplementary spurs  34 . Thus the elongate grooves minimize the accumulation of wood dust and chips thereby allowing the supplementary spur  34  to have the opportunity to cut further without being clogged with dust or chips. 
     Referring to FIG. 8 an alternate embodiment of the drill bit of the present invention is shown generally at  50  wherein the shape of the progressive elongate groove  52  is varied. The progressive elongate groove  52  has a width  54  that increases as the distance from the following rim  24  increases and a depth  56  that increases as the distance from the following rim  24  increases. The progressive elongate grooves  52  have shape and extent proximate to the following rim  24  similar to the shape and extent of arcuate depressions  30  and elongate grooves  42  of drill bits  10  and  40  respectively. 
     Prior art Forstner drill bit  60 , as shown in FIG. 9, has a pair of spur cutters  62  and following rims  64 . The purpose of the spur cutters  62  is to sever wood fibers that occur at the perimeter of the drilled hole and the purpose of following rims  64  is to provide aligning stability between the groove cut in the work piece by the spur cutter  62  and the bit  60 . 
     As the inner surfaces  66  of following rims  64  are generally conical and the outer surfaces  68  are generally cylindrical, there develops considerable aligning tendency between the following rim  64  and the groove cut in the work piece during operation of the drill  60 . However, as there exists considerable surface to surface contact between inner surface  66 , outer surface  68  and the circular groove cut in the work piece, considerable heat is generated due to friction between these surfaces. Further, only limited displacement is achieved by the drill per unit of longitudinal force applied to the drill, so it remains an object of the drill of the present invention to reduce the surface to surface contact between the drill and the work piece, to reduce friction and increase the cutting rate over conventional Forstner drills. 
     Alternatively, referring to FIG. 10, a prior art saw tooth Forstner drill  70  is shown with saw teeth  72  formed into its following rims  74 . The object of saw teeth  72  is to provide additional cutters, as spur cutters  62  did for prior art bit  60  and to reduce the amount of friction between the following rim  74  and the work piece by having reduced the area of the following rim  74 . 
     In greater detail, the reductions to the area of following rims  74  also reduced the areas of both the outer surface  76  and the inner conical surface  78 . As outer surface  76  is tapered slightly such that the drill  70  has a decreasing diameter as it progresses away from the following rim  74 , the application of saw teeth  72  has only a limited effect on reducing the area of outer surface  76  that actually bears on the inner surface of the drilled hole. The application of saw teeth  72  does not greatly reduce drag between the outer surface  76  of the drill bit  70  and the drilled hole during operation. The application of saw teeth  72  to the following rim  74  does however have a greater influence on the effective area of inner conical surface  78 . As with Forstner drills  60 , the following rims  74  are at a longitudinal position that leads the radial cutters often by only a few hundredths of an inch (that is, less than a millimeter). A small margin of inner conical surface  78  actually bears on the circular groove cut in the work piece during operation and thus by forming saw teeth  72  in following rim  74  a considerable portion of that margin is removed. By reducing the area of inner conical surface  78  that bears on the work piece during operation an increased drilling rate is achieved, but at a cost of greatly reducing the engaging and aligning tendencies that exist between the inner conical surface  78  and the circular groove in the work piece, as in the case of a Forstner drill. 
     Further embodiments of the saw tooth drill are illustrated in FIGS. 11 and 12 wherein parabolic flutes  80  or parallel sided flutes  82  are formed into the outer surfaces  76  of the drill bits  84  and  86 . The object of both parabolic flutes  80  and parallel sided flutes  82  are to further reduce the area of outer surface  76  of the drill bits  84  and  86 , beyond the reductions achieved by the provision of saw teeth  72 . The surface area reductions due to the employment of flutes  80  and  82  are achieved solely on the outer surfaces  76  of the drills  84  and  86 . As the surface area of the inner conical surfaces  78  of the drill bits  84  and  86  have already been reduced, in the areas proximate to the flutes  80  and  82 , by the formation of saw teeth  72 , no further reduction of this inner conical surface  78  area is achieved by the additional application of the flutes  80  and  82 . 
     As described above there are a number of limitations in the prior art drill bits, examples of which are shown in FIGS. 10,  11  and  12 . The modified Frostner drill bit of the present invention achieves an increased work rate without greatly increasing the wandering of the drill bit during use. 
     Application of spaced apart depressions to the following rim of a Forstner drill bit as described herein with regard to the present invention yields a segmented following rim with reduced following rim outer surface area, reduced following rim inner conical surface area and creates supplementary spur edges. As the following rim is modified only at the areas where a depression exists, the remaining portion of the following rim is unmodified and continues to act in its intended work piece engaging manner wherein the remaining outer and inner surfaces of the following rim closely engage the work piece, thus offering reduced wandering of the drill over saw tooth and fluted saw tooth drills during operation. As the modified and segmented following rim of the drill of the present invention has the reduced surface areas and supplementary spur edges, an improved rate of drilling is realized over the prior art Forstner drills. As the depressions are applied to an otherwise prior art Forstner drill, the geometry of the drill of the present invention is achieved without the effort and expense incurred to machine or otherwise form the actual saw teeth into the drill. 
     As a result of having applied depressions to the following rim of an otherwise prior art Forstner drill bit, a hybrid Forstner drill bit is created that exhibits improved cutting speed over Forstner drills and improved wander resistance and reduced cost over saw tooth and fluted saw tooth Forstner drill bits. 
     It will be appreciated that the above description related 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.