Abstract:
A modified prior art twist drill embodies structural features that require less energy to drill a hole during a normal plunge operation and allow for lateral cutting movement during extraction of the twist drill from a drilled work piece.

Description:
RELATED APPLICATIONS 
   This application claims benefit of U.S. Provisional Patent Application No. 60/314,625, filed Aug. 23, 2001, and is a continuation-in-part of Application No. 10/007,005, filed Nov. 9, 2001, abandoned, which is a division of Application No. 09/174,887, filed Oct. 19, 1998, now U.S. Pat. No. 6,315,505, which is a division of Application No. 08/698,722, filed Aug. 15, 1996, now U.S. Pat. No. 5,823,720. 

   TECHNICAL FIELD 
   This invention provides in a conventional twist drill or “jobber bit” structural features that require less energy to drill a hole during a normal plunge operation and allow for lateral cutting movement during extraction from a drilled work piece. 
   BACKGROUND OF THE INVENTION 
     FIGS. 1A ,  1 B, and  1 C are three side elevation views and  FIG. 1D  is a work end view of a conventional twist drill  10 . The three side elevation views of  FIGS. 1A ,  1 B, and  1 C show twist drill  10  in different angular orientations of and about its longitudinal axis  12  to clearly present certain structural features. Specifically, twist drill  10  has a body that includes an elongated shaft  14  extending between a tip  16  and a shank  18 . Each of a pair of flutes  20  spirals along the length of shaft  14  and has a trailing edge  22  and a sharpened leading cutting edge  24 . Two top relief portions  26  provide at a work end downwardly sloping surfaces that form a taper from tip  16  to the full diameter of shaft  14 . 
     FIGS. 2A ,  2 B, and  2 C show twist drill  10  in, respectively, a reference (substantially 0 degrees) angular orientation about longitudinal axis  12 , a 90 degree angularly displaced orientation about longitudinal axis  12  relative to that of  FIG. 2A , and the orientation of  FIG. 2B  with an outwardly directed tilt from the plane of the figure.  FIG. 2D  is an enlarged fragmentary view of the work end of twist drill  10  in the angular orientation shown in  FIG. 2A . Increasing amounts of the lengths of trailing edge  22  and leading cutting edge  24  of shaft  14  contact the wall of a hole cut by twist drill  10  as it plunges into a work piece (not shown). The contact of shaft  14  against the hole wall creates friction between twist drill  10  and the work piece. This friction generates heat stored in twist drill  10  and necessitates energy to overcome drag slowing the rate of plunge of twist drill bit  10  into the work piece. Moreover, twist drill  10  is not configured to accommodate lateral cutting movement in a drilled hole to facilitate correction of axial misalignment of holes in two work pieces positioned to be joined by a fastener inserted in them. 
   What is needed, therefore, is a twist drill bit configured to reduce the amount of friction produced and thereby reduce the amounts of heat generated and energy consumed during cutting a hole into a work piece. Reducing energy consumption would be especially beneficial in the operation of a twist drill held in a cordless (i.e., battery operated) drill power head. 
   SUMMARY OF THE INVENTION 
   The present invention provides in a prior art twist drill structural features that require less energy to drill a hole during a normal plunge operation and allow for lateral cutting movement during extraction of the twist drill from a drilled work piece. In a first embodiment, reduced friction is accomplished by provision of a depthwise recess along the length of the shaft between the tapered end and the shank of the twist drill. The recess establishes a transition portion between the tapered end and the shank and thereby determines at the tapered end the lengths of cutting edges angularly inclined from the tip in directions away from the longitudinal axis. The terminations of the inclined cutting edges at the transition portion defines the wall diameter of the hole cut by the twist drill. In a second embodiment, a leading edge tooth formed in the transition portion enables lateral cutting movement during extraction of the twist drill from the work piece. 
   Additional objects and advantages of this invention will be made apparent from the following detailed description of preferred embodiments thereof, which proceeds with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A ,  1 B, and  1 C are side elevation views and  FIG. 1D  is a plan view of the work end of a conventional twist drill. 
       FIG. 2A and 2B  are side elevation views in different angular orientations of the prior art twist drill of  FIGS. 1A–1D ;  FIG. 2C  is a view of the twist drill of  FIG. 2B  with an outwardly directed tilt from the plane of the figure; and  FIG. 2D  is an enlarged, fragmentary side elevation view of the work end of the twist drill of  FIG. 2A . 
       FIGS. 3A ,  3 B, and  3 C correspond to, respectively, the views of  FIGS. 2A ,  2 B, and  2 C and show a first embodiment of the invention with a depthwise recess formed in and along the length of the shaft of the twist drill; and  FIG. 3D  is an enlarged, fragmentary view of the work end of the twist drill of  FIG. 3B . 
       FIGS. 4A ,  4 B, and  4 C correspond to, respectively, the views of  FIGS. 2A ,  2 B, and  2 C and show a second embodiment of the invention formed with the depthwise recess in the shaft, and a leading edge tooth to enable lateral cutting upon extraction, of the twist drill; and  FIG. 4D  is an enlarged, fragmentary view of the work end of the twist drill of  FIG. 4A . 
       FIGS. 5A ,  5 B, and  5 C correspond to, respectively, the views of  FIGS. 4A ,  4 B, and  4 C and show the second embodiment of the invention formed with an optional top relief region that creates a clearance path for material removed by the twist drill operating in its reverse direction;  FIG. 5D  is an enlarged, fragmentary view of the work end of the twist drill of  FIG. 5B ; and  FIG. 5E  is a plan view of the work end of  FIG. 5A . 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIGS. 3A ,  3 B, and  3 C;  FIGS. 4A ,  4 B, and  4 C; and  FIGS. 5A ,  5 B, and  5 C show corresponding angular orientations of twist drill  10  undergoing in three process steps modifications that form the relieved reversible twist drill of the invention. The drawing figures are identified by alphanumeric figure numbers. The numerals  3 ,  4 , and  5  indicate different modification process steps, and common letter suffices of the numerals  2 ,  3 ,  4 , and  5  indicate the same angular orientation about longitudinal axis  12  of twist drill  10  undergoing modification to form the present invention. 
     FIGS. 3A ,  3 B, and  3 C show that a first modification process step entails imparting, such as by grinding, to shaft  14  a cylindrical relief  28  of 0.1D in depth along the length of shaft  14  to within 0.4D of the cutting edge, where “D” represents the diameter of the hole (see  FIG. 2 ) cut by the twist drill.  FIG. 3D , which is a 5:1 enlarged view of the work end shown in  FIG. 3B , shows the dimensions 0.1D and 0.4D that pertain to this first modification process step.  FIG. 3A  shows the length, L R , of the recess. Imparting cylindrical relief  28  to shaft  14  forms a recessed shaft  14   R  and establishes a transition portion  30  between a tapered work end  32  and recessed shaft  14   R . Tapered work end  32  includes tip  16  and is of the same dimensions as those of the corresponding portion of the work end of prior art twist drill  10 . The flute edges extending along the length, L R , are, of course, not sharpened because of the relief from the hole wall. 
   With particular reference to  FIGS. 3B and 3D , the unmodified portion of leading cutting edge  24  of each flute  20  is in the form of an angularly inclined cutting edge  34  and a transition portion cutting edge  36 . Inclined cutting edge  34  extends from tip  16  to a termination point  38  at transition portion  30 , and transition portion cutting edge  36  follows the spiral direction of flute  20  and is substantially concentric with longitudinal axis  12 . Inclined cutting edge  34  is the standard leading sharpened flute edge for conventional plunge operation.  FIG. 3D  shows cylindrical relief  28  forming a 90 degree relief angle relative to transition portion cutting edge  36 ; however, other relief angles would be suitable. The radial distance between termination point  38  and longitudinal axis  12  sets the twist drill diameter.  FIGS. 3A ,  3 B,  3 C, and  3 D show a first embodiment of the invention in which the twist drill requires less energy to drill a hole. The 0.1D recess depth and 0.4D transition portion length dimensions preferably apply to a twist drill diameter of ⅜ inch (953 millimeter) or smaller. A twist drill diameter of greater than ⅜ inch (953 millimeter) can be modified with the dimensions established for a ⅜ inch (953 millimeter) diameter drill. 
     FIGS. 4A ,  4 B, and  4 C show that a second modification process step entails providing back relief  48  of preferably about 60 degrees following the cylindrical contour of the shaft as shown.  FIG. 4D , which is a 5:1 enlarged view of the work end shown in  FIG. 4A , together with  FIG. 5D , shows the back relief angle of 60 degrees and features it creates to enable lateral cutting movement during twist drill extraction from a work piece. In particular, back relief  48  forms a leading edge tooth  50  that enables cutting during extraction of the twist drill rotating in either a clockwise or counter-clockwise direction. Back relief  48  also provides the space required for removal of work piece material cut as the drill bit rotates in a counter-clockwise (reverse) direction during extraction.  FIGS. 4A ,  4 B,  4 C, and  4 D show a second embodiment of the invention in which the twist drill enables lateral cutting during extraction from a drilled hole in a work piece. 
   Leading edge tooth  50  is positioned at the bottom of transition portion cutting edge  36 . A shallow back relief area  52  of triangular shape beginning at a relief line  54  extends rearwardly from transition portion cutting edge  36 . 
     FIGS. 5A ,  5 B, and  5 C show that a third, optional modification process step entails providing, such as by grinding, top relief at a compound angle of preferably about 25 degrees in z-axis, 30 degrees in y-axis, and 15 degrees in x-axis, as measured from the coordinate axes shown in  FIG. 5C .  FIG. 5D , which is a 5:1 enlarged view of the work end shown in  FIG. 5B , shows a top relief region  58  of crescent shape that provides relief for transition portion cutting edge  36  that determines the hole diameter.  FIG. 5E  shows a plan view of the work end.  FIGS. 5A ,  5 B,  5 C,  5 D, and  5 E show in final form the twist drill implemented with both embodiments of the present invention. 
   The twist drill resulting from implementation of the three modification process steps described above provides the following advantages. Cylindrical relief  28  results in less drill shank contact with the work piece, and thereby reduces heat generated during cutting and provides a consequent longer life of the sharpened cutting edge. There is also an increase in battery life for cordless drill power heads. Back relief  48  following the cylindrical contour of the shaft allows for removal of debris in a freshly cut hole and lateral movement with cutting in either rotational direction of the twist drill during its extraction from the work piece. 
   Skilled persons will appreciate that the work end need not be restricted to a point but can alternatively be, for example, a spade bit end, a three-point end, or any other conventional drill bit work end. 
   The construction of the relieved reversible twist drill has been described with reference to the practice of a three-step modification process on a conventional twist drill. Skilled persons will appreciate that the drill bit of the present invention can be manufactured in accordance with a more extensive process starting with metal stock that does not embody features of a conventional twist drill. 
   It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments of this invention without departing from the underlying principles thereof. The scope of the invention should, therefore, be determined only by the following claims.