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CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority, under 35 U.S.C. §119, to European Patent Application No. 12169829.4, filed May 29, 2012, entitled “Drill Bits,” which is incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to drill bits and cutting heads for drill bits, in particular for drilling concrete and rock. 
       BACKGROUND 
       [0003]    Drill bits may comprise a steel fluted section with a hard material cutting head, for example a tungsten carbide head, attached at one end. Located at the other end of the shaft is a shank to releasably connect to a tool holder or a drill or the like, for example, a rotary drill, a rotary percussion drill or a rotary hammer. In order to minimise the amount of power required to drive the drill, the cutting head conventionally comprises two opposing main cutting arms extending from a central point. Conventionally, the drilling of holes with a cross-section closely approaching a geometric circle is assisted by provision of two auxiliary hard material head parts axially set back from the main cutter and radially set back from the outer diameter swept by the end of the main cutting arms, which are either connected to the main cutting head as in U.S. Pat. No. 7,861,807, or are spaced apart from the main cutting plate on the head of the steel fluted section as in European Patent No. EP 1 506 830. 
       SUMMARY 
       [0004]    The present disclosure seeks to provide improved drill bits and improved cutting heads for drill bits. 
         [0005]    A first aspect of the disclosure provides a cutting head for a drill bit, the cutting head comprising four substantially identically shaped cutting arms extending radially from a common central axial point, each cutting arm comprising a cutting edge extending outwardly and axially backwards from the common central axial point. The cutting edges are equiangularly spaced apart from each other at all points about the central axis of the cutting head. Each cutting arm further comprises a side chamfer extending between its radially outer face and its rotationally trailing side face. 
         [0006]    Each cutting edge may extend in a straight unbroken line from the central axial point to the outer radius of the cutting head. Each cutting edge may comprise three or more sections, for example four sections, wherein the point angle between corresponding sections of opposing cutting edges is different to the point angle of any neighbouring sections. Each section may transition to its neighbouring sections at a transition having a large curve radius. 
         [0007]    Each cutting arm may comprise two side faces falling axially away from each cutting edge, wherein each side face comprises a number of side face sections and transitions, corresponding to the sections and transitions of the cutting edge. The two side faces of each cutting arm may be angled symmetrically about the longitudinal central plane of the cutting arm. 
         [0008]    A further aspect of the disclosure provides a drill bit including a cutting head according to the first aspect of the disclosure. 
         [0009]    Advantages of the disclosed inventions may include one or more of the following. An advantage of the four identically shaped cutting arms of the cutting head of the present invention is that the beat force or hammering force is evenly distributed to all four cutting edges, rather than being distributed unequally between a main cutting edge and auxiliary cutting edges. This even distribution of the beat force over a larger number of cutting edges than conventional heads leads to reduced breakage of the cutting edges. 
         [0010]    In addition, it has been found that the cutting head according to the invention does not require significantly more power to drive than conventional heads, even though it has four cutting arms extending to the same outer radius. The cutting head according to the invention improves the durability, speed and overall life of a drill bit. 
         [0011]    Further areas of applicability will become apparent from the description provided herein. The description and specific examples herein disclosed are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
           [0013]      FIG. 1  is a side elevation view of a drill bit in accordance with the present disclosure. 
           [0014]      FIG. 2  is a perspective view of a cutting head for a drill bit in accordance with the present disclosure. 
           [0015]      FIG. 3  is a side elevation view of the cutting head of  FIG. 2 . 
           [0016]      FIG. 4  is a top plan view of the cutting head of  FIG. 2   
           [0017]      FIG. 5  is a perspective view of an alternative cutting head for a drill bit in accordance with the present disclosure. 
           [0018]      FIG. 6  is a side elevation view of the cutting head of  FIG. 5 . 
           [0019]      FIG. 7  is a top plan view of the cutting head of  FIG. 5 . 
           [0020]      FIG. 8  is a perspective view of the cutting head of  FIG. 2 , set into the top part of the fluted section of a drill bit. 
           [0021]      FIG. 9  is a side elevation view of the cutting head of  FIG. 8 . 
           [0022]      FIG. 10  is a top plan view of the cutting head of  FIG. 8 . 
           [0023]      FIG. 11  is a cross sectional view of a fluted section of a drill bit in accordance with the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    Referring to  FIG. 1 , a drill bit  10  comprises a fluted section  20  and a cutting head  30 . The fluted section  20  comprises four helical discharge grooves or flutes. The cutting head  30  may be joined to the fluted section  20  by any known method. For example, appropriately sized head accepting areas, which may be in the form of a roughened surface, slots, holes, or any other suitable platform area is formed at the top end  21  of the fluted section, for example by milling. The cutting head, in one piece or in component parts which together comprise the cutting head once fitted, may then be fitted and brazed into place. 
         [0025]    The fluted section may be formed using any known method. Preferably, the fluted section for use with the cutting head of the present invention has four flutes. As shown in  FIG. 11 , the fluted section may have four lands  22  joined by a web in the conventional way. The cut-away curve joining two neighbouring lands may be a smooth curve. Alternatively, the fluted section may have cut-away curves which are not smooth, to result in a cog-like cross section with protrusions  23 . Such protrusions can reduce bending of the fluted section during drilling, and therefore help to prevent drill bit failure due to fatigue fracture. 
         [0026]    Turning to  FIG. 2 , a cutting head  30  of the present invention comprises four cutting arms  300 , each of the same shape, and at right angles to both neighbouring cutting arms. Furthermore, the cutting edge  301  of each cutting arm  300  extends from a common central axial point  40  to the outermost radius of the cutting edge, along a straight line. Each cutting arm  300  has an axis of reflectional symmetry along the cutting edge  301 , until the arm approaches the outermost radius. 
         [0027]    Each cutting arm may be symmetric about the longitudinal central plane of the cutting arm (defined by the central axis and the cutting edge of the cutting arm), except for the formation of a side chamfer  350  extending between a radially outer face  302  of the cutting arm and a rotationally trailing side face  351  of the cutting arm. As shown in  FIG. 4 , the side chamfer  350  may for example have an angle (Y) of approximately 45° to a longitudinal plane (L) of the cutting arm, although it may be at a greater or smaller angle. The side chamfer  350  may be in a plane which extends parallel to the axial direction (Z) of the cutting head. The rotationally forward edge  351  of the side chamfer preferably approaches but does not intersect with the longitudinal central plane (L) of the respective cutting arm. Such a side chamfer reduces the area of the radially outer face  302  of the cutting arm which decreases the force required for drilling using the cutting head. The radially outer face  302  of each cutting arm may also include a check groove  303 . 
         [0028]    The cutting head  30  may be formed with base chamfers  360  at the base of the sides of the arms, as shown in  FIG. 3 . Such base chamfers  360  assist in fixing the cutting head securely into head accepting areas in the form of rectangular slots  368  in the top end  21  of the fluted section of a drill bit, as shown in  FIGS. 8-10  (continuation of fluted section not shown). Such slots may have a slightly curved joining line between the walls and the bottom of the slot due to the manufacturing process, which slightly protrudes into the slot. The base chamfers  360  ensure that any such protrusions do not interfere with the proper insertion of the cutting head. Base face  365  of the cutting head  30  can closely approach a bottom surface  366  of the slot  368  regardless of any protrusions at the line joining the walls and bottom of the slot, which improves the assembly tolerances and strength. 
         [0029]    The cutting head  30  may also be formed with end chamfers  370  at the base of the outer end of each arm  300 . If the cutting head  30  is attached to the top end  21  of the drill by insertion of the arms  300  into the slots  368 , end chamfers  370  may protrude from the ends of the slots  368  after assembly, as shown in  FIG. 8 . End chamfers  370  may be at any suitable angle, such as for example 45° to the base  365  and radially outer faces  302  of the cutting head  30 . End chamfers  370  assist in drilling, for example, when drilling in rebar or similar material, such chamfers allow easy retraction of the drill bit after a drilling operation. 
         [0030]    As shown in  FIGS. 2 and 3 , the four cutting arms  300  meet at a central axial point  40  which is the axially highest point of the cutting head  30 . The cutting head may comprise a small rounded protruding tip of which the central axial point is the apex. Such an arrangement permits excellent performance in both centering and in speed while drilling. The central axial point  40  is joined to the most central first section  310  of the cutting edge of each cutting arm via a transition  304  having a large curve radius. 
         [0031]    The cutting edge  301  of each identical arm is divided into four sections,  310 ,  320 ,  330 ,  340 , of different radial lengths and angles. Each cutting edge section transitions into the next cutting edge section via a transition  314 ,  324 ,  334 , having a large curve radius. 
         [0032]    As shown in  FIG. 4 , a similar large curve radius transition  344  may separate each of the four cutting arms axially. Each curved transition avoids sharp transitions between adjacent non-contiguous planes, and therefore avoid points of high stress which can cause failure of the head. 
         [0033]    A point angle (A) between opposing first sections  310  may be between 140° and 150°, for example 145°. A point angle (B) between opposing second sections  320  may be between 105° and 115°, for example 112°. A point angle (C) between opposing third sections  330  may be between 135° and 145°, for example between 140° and 142°, for example 142°. A point angle (D) between opposing fourth sections  340  may be between 100° and 110°, for example 105°. 
         [0034]      FIGS. 5-7  show an alternative embodiment of a cutting head according to the present disclosure. Cutting head  50  has parts corresponding to the parts of cutting head  30 , numbered correspondingly. Hence, cutting head  50  has cutting arms  500 , each with a cutting edge  501 , a radially outer face  502  with a check groove  503 , and a side chamfer  550  with a leading edge  551 . As shown on  FIG. 6 , cutting head  50  has a base  565  with base chamfers  560 , and end chamfers  570 . 
         [0035]    In the alternative cutting head  50 , as shown in  FIGS. 5 to 7 , the point angle (A′) between opposing first sections  510  may be between 95° and 120°, for example between 95° and 105°,for example 100°. The point angle (B′) between opposing second sections  520  may be between 110° and 120°, for example 115°. The point angle (C′) between opposing third sections  530  may be between 135° and 145°, for example between 140° and 142°, for example 142°. The point angle (D′) between opposing fourth sections  540  may be between 100° and 110°, for example  105 ° 
         [0036]    The point angles (D, D′) between opposing fourth sections  340 ,  540 , in the embodiments of  FIGS. 2-4  and  FIGS. 5-7 , respectively, is relatively small. This improves durability of the cutting head by reducing the likelihood of breakage of the ends of the cutting arms due to drilling, and also allows the speed while drilling to be high. The point angles (C, C′) between opposing third sections  330 ,  530 , in the embodiments of  FIGS. 2-4  and  FIGS. 5-7 , respectively, allows the speed while drilling to be high in comparison to axially flatter cutting heads. 
         [0037]    For cutting head  30 , the first section  310  of the cutting edge may comprise approximately 17% to 23%, for example 20% of the length of the entire cutting edge  301  of the cutting arm. The second section  320  may comprise approximately 8% to 13%, for example 10% of the length of the entire cutting edge. The third section  330  may comprise approximately 22% to 50%, for example 35% to 45%, for example 40% of the length of the entire cutting edge. The fourth section  340  may comprise approximately 8% to 13%, for example 10% of the length of the entire cutting edge. The transitions  304 ,  314 ,  324 ,  334  between the central axial point and the four sections each comprise approximately 2% to 6%, for example 5% of the length of the entire cutting edge. Such an arrangement provides a compromise between an axially protruding central area for higher drilling speed and good centering performance, with low overall power requirements to drive the cutting head while drilling. The corresponding sections  510 ,  520 ,  530 ,  540  and transitions  504 ,  514 ,  524 ,  534  of cutting head  50  may have corresponding proportions. 
         [0038]    Two side faces  308   a,    308   b  fall axially away from each cutting edge  301 , symmetrically. The side faces  308   a  facing in the drilling direction (ω) will act as rake faces and relief faces  308   b  facing the other direction will act as relief faces. As shown in  FIG. 3 , each pair of side faces  308   a,    308   b  has an internal angle (X), of approximately 80° to 90°, for example 85°, forming a relatively steep roof shape arm. The symmetry of the cutting edge causes the forces produced when drilling, in particular the beat forces, to be transmitted evenly through the cutting head and into the bottom of the slot, which reduces stresses on the connection between the cutting head and the end of the fluted section, and on the end of the fluted section itself. Cutting head  50  has corresponding side faces  508   a,    508   b.    
         [0039]    The cutting head may be of a material conventionally used for cutting heads, for example, tungsten carbide. The cutting head may be made using any known method for forming parts from such material, for example by pressing or grinding. 
         [0040]    The cutting head may be made in one piece. It is also possible to make the cutting head in more than one piece and bring the pieces together when the cutting head is attached to the end of the fluted section. For example, the cutting head could comprise three separate plates, one central plate forming two opposite arms, and two identical side plates one of which could be positioned extending from either side of the central plate, to form the other two opposing arms. 
         [0041]    It should be understood that although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the scope of the claims.

Summary:
The invention provides a cutting head for a drill bit, the cutting head comprising four substantially identically shaped cutting arms extending radially from a common central axial point, each cutting arm comprising a cutting edge extending outwardly and axially backwards from the common central axial point. The cutting edges are equiangularly spaced apart from each other about the central axis of the cutting head. Each cutting arm further comprises a side chamfer extending between its radially outer face and its rotationally trailing side face.