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TECHNICAL FIELD 
       [0001]    The present disclosure relates to tips in rock or mining bits for use in a rock excavation apparatus. 
         [0002]    The disclosure particularly relates to tips with hollow bases. 
       BACKGROUND 
       [0003]    There are several different methods used for drilling in earth formations. Some use rotary movement and some use a combination of rotary and percussive movement. One common aspect of these methods is that a drill bit rotates at an end of a drill string. 
         [0004]    Rotary drilling is conducted by rotating a rigid string of tubular rods to which a rock-cutting bit is attached. The rotary drill imparts two basic actions through the drill rod and bit into the rock, i.e. axial thrust and rotational torque. Percussive drills break rock predominantly by crushing and chipping rock with the repeated application of high-frequency, high-energy blows through a drill bit. The impact energy is developed by a piston that strikes the bit (down-the-hole drill) or drill steel (surface-mounted drill). 
         [0005]    To improve the wear resistance and increase the lifetime of such drill bits, cutting elements in the form of tips or inserts are attached to the drill bit body. These tips are often made of cemented carbide, most commonly tungsten carbide, due to its excellent combination of high hardness and high toughness. The tips can also be made of polycrystalline diamond (PCD). The purpose of the tips is mainly to apply pressure to and fracture rock. Sometimes tips are also positioned on the drill bit body as protection for the surrounding steel. The tips must therefore withstand high compressive and transverse loads. 
         [0006]    The tungsten carbide tips are commonly mounted in cylindrical recesses in the outer surface of the drill bit body. The tips can be made a few hundredths of a mm larger than the recess and are pressed in to have a tight interference fit to prevent loosening during usage. 
         [0007]    Tungsten carbide tips are also used for soft cutting conditions such as excavation of coal. The tips are, in this application, often named caps and are often adhesively bonded to a pick body by, for example, brazing or welding. 
         [0008]    Several standard shapes are used for tungsten carbide tips, such as a part-spherical, conical, a double cone, a ballistic and a chisel crest. Common for these different shapes is that the base, also called mounting portion, of the tip is generally cylindrical. 
         [0009]    In many applications it is advantageous to use tips having a diameter of about 2 cm or larger. The advantages being that fewer tips need to be used and also that the protrusion of the tip from the surface of the bit body can be greater while maintaining adequate strength to avoid transverse failure during the excavation operation. Having large tips concentrates the load to fewer tips and greater rock penetration can be obtained without engagement of the steel surface, resulting in improved excavation rate. Having large tips that extend a greater distance from the bit also increases the lifetime of the bit as the large tips can accommodate appreciable wear before they are worn out. 
         [0010]    One problem with large diameter tips is that they are expensive. This is due to the high cost of the material required to manufacture a tip. The material quantity required increases with the square of the diameter of the tip. When using tungsten carbide tips it is more costly to use few tips with a diameter of about 2 cm than using a larger amount of small tips. 
         [0011]    This problem is addressed in U.S. Pat. No. 4,150,728. Here tungsten carbide tips with hollow bases are shown. Such a tip has a cavity opening to the inner end of the tip with a volume in the range of from about 15 to 30% of the volume of the base portion of the tip. 
         [0012]    The present inventor has surprisingly found that several prominent problems occur at tips with hollow bases. One is that the base of the tip tends to get oval in shape. Another problem is that the form of the tip tends to get conical, i.e. the diameter of the mounting portion decreases when approaching the very bottom of the tip. Ovality and conicity in the mounting portion decreases the force for pulling the tip from the drill bit body and therefore loosening of the tip is an unfavorable consequence. 
         [0013]    Another problem is that the mounting portion may crack when a tip is press fitted into a recess in the drill bit body. 
         [0014]    These and other aspects of, and advantages with the present invention will be apparent from the detailed description and the accompanying drawings. 
       SUMMARY 
       [0015]    One object of the present invention is to provide a hollow tip concept, which has better strength than known hollow tips. Another object is to provide a hollow tip concept which can be produced at a lower cost compared to solid tips, thus solving the above mentioned problems. 
         [0016]    According to a first aspect, there is provided a rock bit tip comprising: a mounting portion, an end portion converging from a top end of the mounting portion to form a work surface and at least one recess in a bottom of the mounting portion, the recess extending towards said end portion, wherein there are more than one recess. 
         [0017]    In a second aspect, there is provided a rock bit comprising a tip with more than one recess. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    In the detailed description of the present invention reference will be made to the accompanying drawings, wherein, 
           [0019]      FIG. 1  schematically illustrates a conventional tip or button, in a side view, suitable for excavation of rock, 
           [0020]      FIG. 2  schematically illustrates a tool for rotary drilling, in a side view, 
           [0021]      FIG. 3  schematically illustrates a tool for percussive drilling, in a perspective view from above, 
           [0022]      FIG. 4  schematically illustrates a mining pick in a side view, 
           [0023]      FIG. 5   a  schematically illustrates a first embodiment of a tip according to the invention in a perspective view from below, 
           [0024]      FIG. 5   b  schematically illustrates a first embodiment of the tip according to the invention in an axial cross-sectional view through the tip centre axis A, 
           [0025]      FIG. 6  schematically illustrates a second embodiment of a tip according to the invention in a perspective view from below, 
           [0026]      FIG. 7  schematically illustrates a third embodiment of a tip according to the invention in a perspective view from below, 
           [0027]      FIG. 8  schematically illustrates a fourth embodiment of a tip according to the invention in a perspective view from below, 
           [0028]      FIG. 9   a  schematically illustrates a fifth embodiment of a tip according to the invention in a perspective view from below, 
           [0029]      FIG. 9   b  schematically illustrates the tip according to  FIG. 9   a  in a side view, 
           [0030]      FIG. 9   c  schematically illustrates the tip according to the line IX C-IX C in  FIG. 9   b , and 
           [0031]      FIG. 9   d  schematically illustrates the tip according to the line IX D-IX D in  FIG. 9   b.    
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0032]    The features and advantages of the present invention are well understood by reading the following detailed description in conjunction with the drawings in which like numerals indicate similar elements and in which: 
         [0033]      FIG. 1  illustrates a tip  10  suitable for rock excavation. The tip comprises an end portion  11  having a top  13 . The end portion is intended to project from a front surface of a bit to form a work surface that is in contact with the rock. The tip also comprises a mounting portion  12  extending from the end portion  11  towards a bottom end  14 . The mounting portion of the tip is intended to be positioned in a recess in a drill bit body and assures that the tip is securely fixed to the bit body. If the tip  10  is comprised in a mining pick, the bottom end  14  may be secured to the bit, for example, by means of brazing or welding. 
         [0034]      FIG. 2  illustrates a rock bit for rotary drilling  20 . The bit comprises a thread  24  whereby the rotary bit  20  is to be connected to a drill string. The bit further comprises legs  25  with roller cones  21  attached at one end. Tips, of the type herein described, can be used as protective tips  23  on the legs  25  and/or as active tips  22  on the roller cones  21 . The term “protective” means that the main purpose of the tip  23  is to protect the steel in the drill bit from being too heavily worn. The term “active” means that the main purpose of the tip  22  is to apply pressure to and fracture rock. The technology is more closely described in U.S. Pat. No. 6,446,739. 
         [0035]      FIG. 3  illustrates a percussive rock drill bit  30  that comprises a drill bit head  31  and a shank  32 . The drill bit head  31  may comprise tips, of the type described below, such as peripheral tips  33  and/or front tips  34 . The technology is more closely described in U.S. Pat. No. 7,296,641. 
         [0036]      FIG. 4  illustrates a mining pick  40  used in operations such as cutting soft minerals such as, for example, coal. The mining pick  40  includes a body  41  having a head  42  and a shank  43 . The tip  44  is made according to the invention and is the part that actively cuts minerals. It is made of a hard material such as cemented carbide, diamond, SiC-D or combinations thereof. The technology is more closely described in U.S. Pat. No. 8,210,618. 
         [0037]      FIGS. 5   a  and  5   b  illustrate a first embodiment of the invention. The tip  110  has a mounting portion  112 . Four identical open recesses  115  extend axially from the bottom end  114  of the mounting portion  112 . The open recesses  115  have cross sections at the bottom end  114  that can be substantially like circle sectors with rounded corners for avoiding stress concentrations. The recesses  115  are spaced apart by a support structure  116 . At least one open recess  115  has its geometric centre axis B separate from the longitudinal tip centre axis A. 
         [0038]      FIG. 6  illustrates a second embodiment of the invention. The tip  120  has a mounting portion  122 . Three identical open recesses  125  extend axially from the bottom end  124  of the mounting portion  122 . The open recesses  125  may have cross sections at the bottom end  124  that are substantially like truncated circle rings. The recesses  125  are spaced apart by a support structure  126 . At least one open recess  125  has its geometric centre axis B separate from the longitudinal tip centre axis A. 
         [0039]      FIG. 7  illustrates a third embodiment of the invention. The tip  130  has a mounting portion  132 . Seven identical open recesses  135  extend axially from the bottom end  134  of the mounting portion  132 . The open recesses  135  have cross sections at the bottom end  134  that are substantially hexagonal. The recesses  135  are spaced apart by a support structure  136 . At least one open recess  135  has its geometric centre axis B separate from the longitudinal tip centre axis A. A central open recess  135  has its geometric centre axis B substantially coinciding with the longitudinal tip centre axis A. 
         [0040]      FIG. 8  illustrates a fourth embodiment of the invention. The tip  140  has a mounting portion  142 . Six open recesses  145  extend axially from the bottom end  144  of the mounting portion  142 . The open recesses  145  have cross sections at the bottom end  134  that are substantially circular. The recesses  145  are spaced apart by a support structure  146 . At least one open recess  145  has its geometric centre axis B separate from the longitudinal tip centre axis A. 
         [0041]      FIGS. 9   a - 9   d  illustrate a fifth embodiment of the invention. The tip  150  has a mounting portion  152 . One open recess  155  extends axially from the bottom end  154  of the mounting portion  152 . Between the bottom end  154  and the end portion, the open recess  155  is split into three identical separate recesses  157 . The recesses  157  are spaced apart by a support structure  156 . At least one of the recesses  157  has at least one cross section (IX D-IX D), parallel to the bottom of the mounting portion, between the bottom of the mounting portion and the end portion, which has its geometric centre axis B separate from the longitudinal tip centre axis A,  FIG. 9   d.    
         [0042]    The number of recesses, and their sizes, may be altered compared to the examples described in the above-captioned embodiments. It is also possible to combine recesses with different sizes and shapes into a new embodiment. The depth of the recesses can also be varied, but the recesses are always of the blind hole type. 
         [0043]    The present invention is not limited to the above described embodiments. Different alternatives, modifications and equivalents might be used. The above mentioned embodiments should therefore, not be considered limiting to the scope of the invention, which is defined by the patent claims.

Summary:
A rock bit tip includes a mounting portion, an end portion converging from a top end of the mounting portion to form a work surface and at least one recess in a bottom of the mounting portion, the recess extending towards the end portion. Particularly, there are more than one recess. The disclosure further relates to a rock bit.