Abstract:
A cutting plate ( 4 ) has a working end ( 20 ) to engage a workpiece and an axis ( 8 ) of rotation. A pair of oppositely arranged first faces ( 34 ) are inclined axially rearward from the working end ( 20 ) in an axial plane intersecting said first faces ( 34 ). A pair of second faces ( 30 ) are alternately arranged between the first faces ( 34 ). The second faces ( 30 ) are inclined axially rearward from the working end ( 20 ) in an axial plane ( 37 ) intersecting the second faces ( 30 ), and a pair of axis parallel front faces ( 24 ). Each of the front faces ( 24 ) is bound on three sides by a respective first face ( 34 ) and the pair of second faces ( 30 ). Each second face ( 30 ) is inclined radially inward from a transitional edge with a respective rotationally leading first face ( 34 ) in a plane normal to the axis ( 8 ) intersecting the transitional edge with a respective rotationally leading first face ( 34 ). Each first face ( 34 ) is inclined radially inward from a transitional edge ( 36 ) with a respective rotationally leading second face ( 30 ) in a plane normal to the axis ( 8 ) intersecting the transitional edge ( 36 ) with a respective rotationally leading second face ( 30 ), as the cutting plate ( 4 ) rotates.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority to Great Britain Patent Application No. 0308287.2 filed Apr. 10, 2003, which application is herein expressly incorporated by reference. 
   FIELD OF THE INVENTION 
   The present invention relates to a cutting plate for a drill bit and relates particularly, but not exclusively, to a cutting plate for a drill bit for rotary cutting of ceramic tiles. The invention also relates to a drill bit incorporating such a cutting plate. 
   BACKGROUND OF THE INVENTION 
   Rotary cutting of a ceramic tile presents various difficulties because of the hardness and smoothness of both the glazed protective layer of the tile and the tile substrate material itself. It is known to cut ceramic tiles by initially using a small diameter drill bit to cut a small hole in the protective layer to minimise the possibility of cracking the protective layer and the tile substrate. Next drill bits of successively larger diameters are used to enlarge the hole to the required size once the tile has been pierced with a small hole and the risk of damage to the tile is reduced. However, this known method suffers from the drawback that several drill bits of different fixed diameters are required in order to cut a hole in the tile to a desired diameter. 
   One known drill bit which seeks to overcome the above mentioned problem is disclosed by International patent publication no. WO 03/061927A1. This drill bit has a cutting plate seated in one end of a cylindrical shank with a rotational axis. Viewed in side elevation, the cutting plate has a rectangular main body portion seated in the shank and a triangular portion extending from the main body portion. The triangular portion is terminated by a small pyramidal portion with a small chisel edge at its foremost end. The chisel edge is defined by a pair of inclined front faces and a pair of inclined side flanks. The intersection between the pair of inclined side flanks forms the chisel edge. The inclined front faces define the length of the chisel edge. Each inclined side flank is inclined axially rearward away from the chisel edge until each meets a respective inclined side face. Each inclined side face is also inclined axially rearward away, but at a steeper angle than that of an adjacent inclined side flank. Each inclined front face is inclined axially rearward from a respective end of the chisel edge until each meets a respective axis parallel front face. In use, the hole in the ceramic tile is steadily increased from a very small diameter to the full working diameter of the drill bit, in one operation. 
   A primary cutting edge is formed at the intersection between the inclined side flanks and a respective rotationally leading inclined front face. A second cutting edge is formed at the intersection between the inclined side faces and a respective rotationally leading front face. The inclined side flanks and side faces are also inclined radially inward from their respective rotationally leading primary and secondary cutting edges. However, the inclined front faces are inclined radially inward from their respective rotationally trailing primary cutting edges, whilst the parallel front faces are not radially inclined, one way or the other. For the sake of clarity, a face or edge that is ‘inclined radially inward’ from a given point is one that departs from said point in a direction tending, or inclined, towards the axis of rotation. 
   Thus, a cross section through a plane normal to the rotational axis and including the primary cutting edges would reveal that the inclined side flanks and front faces circumscribe a first parallelogram shape. In this first parallelogram the primary cutting edges are located in the corners furthest apart and it is the inclined side flanks that provide relief to the primary cutting edges. 
   Likewise, another cross section through any plane normal to the rotational axis, including the secondary cutting edges, would reveal that the inclined side faces and front faces circumscribe a second parallelogram shape. In this second parallelogram the secondary cutting edges are located in the corners furthest apart and the inclined side faces provide relief to the secondary cutting edges. 
   In both parallelograms, opposing faces are parallel. In the case of the second parallelogram, the included angle at the secondary cutting edges is 90° minus the angle of inward radial inclination of the inclined side faces. This is because the parallel front faces are not radially inclined. In the case of the second parallelogram, each inclined side flank and front face is inclined radially inward from a respective intersecting primary cutting edge. Therefore, the included angle at the primary cutting edges would be 90° minus the sum of the angles of inward radial inclination of the inclined side flanks and side faces. Accordingly, the first parallelogram is more collapsed and has sharper primary cutting edges than the second parallelogram which is fuller and has duller, or less sharp, secondary cutting edges. For the sake of clarity, a more ‘collapsed’ parallelogram is intended to mean one that circumscribes a smaller area than a fuller, or less ‘collapsed’, parallelogram having equivalent sides. Of course, a rectangle circumscribes the fullest possible area of a parallelogram of a given length sides. 
   Whilst the sharp primary cutting edges of the drill bit disclosed by WO 03/061927 A1 have an initial advantage of cutting a ceramic tile more quickly this initial advantage is short lived and these sharp primary cutting edges soon wear and become blunt when used upon relatively hard and abrasive materials such as ceramic tiles. Such sharp cutting edges are also brittle and more prone to chipping. This is especially problematic at the foremost working end of the drill bit in the region of the chisel edge where the cutting plate&#39;s geometry is small and there is not enough spare material to re-sharpen chipped cutting edges more than a few times, if at all. This reduces the life span of the drill bit. Further, if the inclined side faces are steeply inclined axially rearward with an included angle falling within the range of 30° to 60° then the cutting plate&#39;s geometry in the region of the chisel edge becomes especially small. In this case breakage of the cutting edges could result complete removal of the foremost working end of the cutting plate making its re-sharpening impossible. 
   SUMMARY OF THE INVENTION 
   The present invention seeks to overcome these disadvantages, or at least mitigate them. A cutting plate comprises a working end to engage a workpiece and axis of rotation. A pair of oppositely arranged first faces are inclined axially rearward from the working end in an axial plane intersecting the first faces. A pair of second faces are alternately arranged between the first faces. The second faces are inclined axially rearward from the working end in an axial plane intersecting the second faces. A pair of axis parallel front faces are included on the cutting plate. Each of the front faces is bound on three sides by a respective first face and the pair of second faces. Each second face is inclined radially inward from a transitional edge with a respective rotationally leading first face in a plane normal to the axis intersecting the transitional edge with a respective rotationally leading first face. Each first face is inclined radially inward from a transitional edge with a respective rotationally leading second face in a plane normal to the axis intersecting the transitional edge with a respective rotationally leading second face, as the cutting plate rotates. The pair of second faces can be a pair of planar faces or a pair of faces comprising a plurality of individual planar surfaces. 
   Thus, the parallelogram circumscribed by the first and second faces can be any parallelogram shape ranging from a partially collapsed parallelogram, with sharp cutting edges, up to and including, a rectangle with four right-angled edges. The closer the parallelogram resembles a rectangle the greater the enclosed area therein and the stronger the cutting plate at its foremost working end. This makes the cutting plate more resistant to breakage caused by a chipped cutting edge. Thus the cutting edges can be orientated with respect to the workpiece in a more aggressive manner to increase speed of cutting while limiting the chance of chipping the cutting edges to an acceptable level. 
   Preferably a chisel edge is defined by the intersection of one of the pair of first or second faces at the foremost of the working end. The chisel edge provides an edge to penetrate the outer layer of a workpiece to begin the hole cutting process. A chisel edge at the foremost end of the cutting plate is less brittle than a single cutting point and is therefore less prone to chipping. The length of the chisel edge is defined by the other of the pair of first or second faces. If the chisel edge is chipped then it can be re-sharpened by grinding either of both of the pair of first or second faces. 
   Preferably, an angle, subtended by the one of the pair of first or second faces forming the chisel edge, falls within a range of 90° to 150°. Thus, the chisel edge is sharp enough to cut the outer layer of the ceramic tile without being easily chipped. More preferably, the angle subtended by the one of the pair of first or second faces forming the chisel edge is 100°. 
   Preferably, the transitional edge between each first face and a respective rotationally leading second face is a primary cutting edge. Also, a transitional edge between each second face and a respective rotationally leading front face is a secondary cutting edge. The primary and secondary cutting edges can be continuous, but need not be for satisfactory cutting of a workpiece. The second and first faces provide relief to the primary and secondary cutting edges, respectively. 
   Additionally, the cutting plate further comprises a pair of axis parallel side faces each bound on three sides by a respective second face and the pair of front faces. A side cutting edge is defined at the intersection of each side face with a respective rotationally leading front face. Each side face is inclined radially inward from a respective side cutting edge in a plane normal to the axis intersecting the side cutting edge. 
   Preferably, the angle, subtended by the second faces in an axial plane orthogonal to the second faces, falls within a range of 30° to 60°. More preferably, the angle subtended by the second faces in an axial plane orthogonal to the second faces is 43°. The cutting plate, when viewed from the side, has an increasingly sharp arrow head shape as the angle subtended by the first faces is decreased. A relatively sharp included angle between the second faces helps to cut a cleaner hole in a workpiece for the reasons discussed above. 
   Preferably, the cutting plate further comprises a mechanism for precise attachment of the cutting plate to a shank of a drill bit. Such an attachment mechanism is useful for easily and reliably attaching the cutting plate to a shank of a drill bit. Thus, the axes of rotation of the cutting plate and shank are concentric. 
   The cutting plate is best suited for cutting holes in ceramic material. The cutting plate is particularly well suited to cut holes in ceramic tiles. 
   Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
       FIG. 1  is a perspective view of a drill bit in accordance with the present invention; 
       FIG. 2  is a perspective view of a cutting plate of the present invention; 
       FIG. 3  is an enlarged front elevation view of the cutting plate of  FIG. 2 ; 
       FIG. 4  is an enlarged plan view of the cutting plate of  FIG. 2 ; 
       FIG. 5  is an enlarged side elevation of the cutting plate of  FIG. 2 ; 
       FIG. 6  is a cross-sectional view along the line A—A of  FIG. 3 ; 
       FIG. 7  is a cross-sectional view along the line B—B of  FIG. 4 ; 
       FIG. 8  is a cross-sectional view along the line C—C of  FIG. 3 ; 
       FIG. 9  is a cross-sectional view along the line E—E of  FIG. 3 ; and 
       FIG. 10  is a cross-sectional view along the line F—F of  FIG. 3 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
   Referring to  FIG. 1 , a drill bit  2  is shown for rotary cutting of ceramic material. The drill bit  2  has a cutting plate  4  and an elongate cylindrical shank  6  defining a central longitudinal axis  8  of rotation. One end of the shank  6  engages a chuck of a rotary power drill (not shown). A groove  12  is at the opposite end of the shank  6 . The groove  12  seats the cutting plate  4 . The groove  12  is flanked by a pair of opposing jaws  14 . The cutting plate  4  is removably clamped by the jaws  14  to secure it to the shank  6 . The cutting plate  4  is formed from a suitably hard material like, for example, hardened steel or tungsten carbide. In use, the drill bit  2  rotates in the direction of the circular arrow R, as is shown in  FIG. 4 . 
   When viewed in front elevation, as is shown in  FIG. 3 , the cutting plate  4  is generally shaped as a flattened arrow-head. At one end the cutting plate  4  has a planar base  16 . The base  16  faces a trough  18  of the groove  12  when the cutting plate  4  is seated in the groove  12 . At the opposite end the cutting plate  4  has a sharp tip  20  to cut a workpiece. The tip  20  has a straight chisel edge  22  which forms the foremost end of the bit  2 . The base  16  and the chisel edge  22  are normal to the axis  8 . The central axis of the cutting plate  4  is concentric with the axis  8  of the shank  6 . 
   When viewed in plan, as is shown in  FIG. 4 , the cutting plate  4  is generally parallelogram shaped with a pair of axis parallel opposing front faces  24  and a pair of axis parallel opposing narrow side faces  26  arranged alternately between the front faces  24 . 
   Referring to  FIGS. 3 and 5 , each side face  26  extends from the base  16  towards the tip  20 , to a respective straight side shoulder edge  28 . Each side shoulder edge  28  forms a transition between one side face  26  and a respective narrow inclined side face  30 . Like the side faces  26 , the inclined side faces  30  are also arranged alternately between the front faces  24 . The inclined side faces  30  are convergent such that each inclined side face  30  extends from a respective side shoulder edge  28  towards the tip  20  where they abut at each end of the chisel edge  22 . The length of the chisel edge  22  is defined by the inclined side faces  30 . The converging inclined side faces  30  subtend an angle α, as measured in an axial plane orthogonal to the inclined side faces  30 . The cutting plate  4  can be manufactured with an angle α in the range between 30° to 60°. Angle α is 43° in  FIG. 3 . The smaller angle α, the steeper the axially rearward inclination of the inclined side faces  30 . Accordingly, the smaller angle α, the more gradual the enlargement of the diameter of the hole cut in the workpiece. This results in a cleaner hole with fewer, or no, chips around its circumference. 
   Referring to  FIG. 3 , each front face  24  extends from near the base  16  towards the tip  20  to a respective straight front shoulder edge  32 . Each front shoulder edge  32  forms a transition between a front face  24  and a respective inclined front face  34 . The inclined front faces  34  are convergent such that each inclined front face  34  extends from a respective front shoulder edge  32  towards the tip  20  where they intersect to form the chisel edge  22 . The converging inclined front faces  34  subtend an angle β, which is measured in an axial plane perpendicular to the chisel edge  22 . The inclined front faces  34  are arranged alternately between the inclined side faces  30 . The cutting plate  4  can be manufactured with an angle β in a range between 90° to 150°. An angle β falling in the range of 100° to 110° provides a chisel edge  22 . The best compromise between strength and speed of cutting is when angle β is 100° as in  FIG. 7 . 
   As is shown in  FIGS. 4 and 5 , a primary cutting edge  36  is formed at the intersection between each inclined front face  34  and a respective rotationally leading inclined side face  30 . Each inclined front face  34  is inclined radially inward from a respective rotationally leading primary cutting edge  36  in a plane normal to the axis  8  intersecting the primary cutting edge  36 . The inclined front faces  34  create relief behind the primary cutting edges  36 . An angle θ of inward radial inclination of each inclined front face  34  is measured from an axial plane  37  bisecting the front faces  24 . The cutting plate can be manufactured with an angle θ in the range of 1° to 20°. Angle θ is 9° in  FIG. 8 . The working diameter D 36  of the primary cutting edges  36  steadily increases as the cutting plate  4  penetrates the workpiece W. 
   A secondary cutting edge  38  is formed at the intersection between each inclined side face  30  and a respective rotationally leading front face  24 . Each inclined side face  30  is inclined radially inwardly from a respective rotationally leading secondary cutting edge  38  in a plane normal to the axis  8  intersecting the secondary cutting edge  38 . The inclined side faces  30  create relief behind the secondary cutting edges  38 . An angle λ, of inward radial inclination of each inclined side face  30 , is measured from a plane  39  orthogonal to the axial plane  37  bisecting the front faces  24 . The cutting plate  4  can be manufactured with an angle λ in the range of 30 to 15°. Angle λ is 5° in  FIGS. 8 and 9 . The working diameter D38 of the secondary cutting edges  38  steadily increases as the cutting plate  4  penetrates the workpiece W. 
   A side cutting edge  40  is formed at the intersection between each side face  26  and a respective rotationally leading front face  24 . Each side face  26  is inclined radially inwardly from a respective rotationally leading side cutting edge  40  in a plane normal to the axis  8  intersecting the side cutting edge  40 . The side faces  26  create a relief behind the side cutting edges  40 . An angle γ of inward radial inclination of each side face  30  is measured from the plane  39  orthogonal to the axial plane  37  bisecting the front faces  24 . The cutting plate can be manufactured with an angle γ in the range of 3° to 15°. Angle γ is 5° in  FIG. 10 . The side cutting edges  40  define the maximum working diameter D 40  of the cutting plate  4 . 
   The chisel edge  22  is approximately 5% to 20% of the working diameter D 40  of the cutting plate  4 . The chisel edge  22  is continuous with the primary cutting edges  36  located on either side. Each secondary cutting edge  38  is continuous with a respective side cutting edge  40 . However, there exists a break between the primary cutting edges  36  and the secondary cutting edges  38 . As is most clearly shown in  FIG. 6 , the chisel edge  22  is rotationally advanced of the trailing primary cutting edges  36 . The primary cutting edges  36  are rotationally advanced of respective secondary  38  and side  40  cutting edges. The chisel edge  22  is rotationally advanced of the trailing secondary  38  and side  40  cutting edges by an angle of rotation of 180°—angle δ, as is shown in  FIG. 4 . Angle δ is between 13° to 23° and 18° as shown in  FIG. 4 . 
   Referring to  FIGS. 3 and 4 , the base  16  has a pair of chamfers  42  extending between the side faces  26 . Each chamfer  42  joins the base  16  with a respective front face  24 . The chamfers  42  result in partial tapering of the cutting plate  4  to ease its insertion into the groove  12  of the shank  6  during assembly. The cutting plate  4  also has a shallow rectangular recess  44  in each front face  24 . The recess  44  extends from near the base  16  to approximately the midpoint of the cutting plate  4 . The recesses  44  are symmetrical about the axis  8 . Each recess  44  has a planar recess face  46  parallel to the plane  37  beisecting the front faces  24 . The recesses  44  are open at one end facing the base  16 . Thus, the recess faces  46  each encroach halfway into a respective chamfer  42 . Each recess  44  has a pair of axially parallel recess side walls  48 . The sidewalls  48  extend from the open end  50  of the recess  44  to a recess end wall  52  located at the opposite end of the recess  44  and arranged perpendicular to the axis  8 . The recesses  44  provide a location on each side of the cutting plate  4  to accommodate the opposing jaws  14  when the cutting plate  4  is seated in the groove  12  and clamped to the shank  6 . The side walls  48  and end wall  52  of the recesses  44  register precisely with a respective jaw  14  when the cutting plate  4  is seated in the groove  12 . This ensure, that the central axis  8  of the cutting plate  4  is concentric with the axis  8  of the shank  6 . 
   Returning to  FIG. 1 , the groove  12  is deeper than the length of the recess  44  in the cutting plate  4 . At a location between the trough  18  of the groove  12  and the base  16  of the cutting plate  4  (when the cutting plate is clamped to the shank) a transverse cylindrical hole  54  passes through both jaws  14  and the axis  8 . The hole  54  in one jaw  14  is unthreaded and has a cylindrical recess  56  facing the outside of the shank  6 . The hole&#39;s recess  56  is sized to accommodate the head  58  of a small bolt  60 . The hole  54  of the other jaw  14  is threaded to mesh with the threaded shank of the bolt  60 . In the present embodiment the bolt  60  is a cylindrical head bolt with a hexagonal recess  62  to receive a hexagonal key (not shown) to turn the bolt  60  while its head  58  is accommodated by the hole&#39;s recess  56 . The bolt  60  could be a standard hexagonal head bolt, however the hole&#39;s recess  56  would need to be enlarged to facilitate engagement of a socket spanner with the hexagonal head. 
   The action of turning the bolt  60  to fasten the cutting plate  4  pulls the jaws  14  closer together to clamp the cutting plate  4  in place. Conversely, the action of turning the bolt  60  to loosen the cutting plate  4  allows the jaws  14  to move apart so that the cutting plate  4  can be released from the groove  12  for replacement or repair. 
   Alternatively, the cutting plate  4  could be brazed, or glued with a strong adhesive, into the groove  12 . Accordingly, this would eliminate the transverse hole  54  and bolt  60  arrangement. The cutting plate  4  may even be integral with the shaft  6 . 
   In use, the drill bit  2  rotates in the direction of the circular arrow R. The chisel edge  22  begins the cutting operation by engaging a workpiece W, for example, a ceramic tile. The chisel edge  22  is relatively short and forms a point to initially pierce the hard outer layer of the ceramic tile. Once the chisel edge  22  has pierced the workpiece W, the hole formed therein is steadily enlarged as the primary cutting edges  36  and then the secondary cutting edges  38  cut the workpiece W. The final cutting operation is performed by the side cutting edges  40  which help cut a clean hole in the workpiece W. The side cutting edges  40  also direct the drill bit  2  in a straight line and promote removal of debris away from the tip of the cutting plate  4 . 
   As is shown in  FIGS. 8 ,  9  and  10 , the cutting plate  4  is parallelogram shaped in cross-sections taken at planes normal to the axis  8 . The figures show the primary  36 , secondary  38  and side  40  cutting edges, respectively. These cutting edges  36 ,  38 ,  40  are always located in opposite corners furthest apart. Angle λ is fixed by the inclination of the inclined side faces  30  thus angle θ controls the shape of the parallelogram circumscribed by the inclined side  30  and front  34  faces. If angle θ is equal to angle λ then the parallelogram circumscribed by the inclined side  30  and front  34  faces is rectangular and all its edges become right angled. If angle θ is greater than the angle λ, then the primary cutting edge  36  jumps approximately a quarter revolution ahead of the secondary  38  and side  40  cutting edges. This is shown by comparing the position of the primary cutting edge  36  shown in  FIG. 8  with that of the secondary and side cutting edges  38 ,  40  shown in  FIGS. 9 and 10 . As a result there is a small discontinuity between each primary cutting edge  36  and a respective secondary cutting edge  38 . 
   If the cutting plate  4  is made of tungsten carbide type KCR05 then preferably the primary cutting edge  36  has an included angle μ of 86°. Primary cutting edges  36 , with an included angle μ of 86°, provide a good compromise between being sharp and hard enough to cut ceramic tiles without being excessively brittle. 
   The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.