Patent Publication Number: US-2017355103-A1

Title: Drill Bit

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 U.S.C. §119 of Great Britain Patent Application Nos. 1610155.2, filed Jun. 10, 2016, and 1613753.1, filed Aug. 10, 2016, the disclosures of which are hereby incorporated herein by reference in their entireties. 
     FIELD OF THE INVENTION 
     The present invention relates to drill bits, and in particular to drill bits having an annular cutting portion. 
     BACKGROUND TO THE INVENTION 
     With drill bits used for cutting holes from tiles and the like, an annular cutting surface is provided which on rotation of the drill bit removes a circular disc from the tile or other object being drilled. Such drill bits are often known as hole saws and to prevent excessive heating of the abrasive cutting surface, a wax plug can be incorporated within the hollow annular body of the hole saw, the wax melting to provide a fluid that can cool the cutting surface by evaporation. Unfortunately, as drilling takes place, loose waste material is generated which combines with the melted wax, eventually forming a slug located within the drill bit which blocks the flow of wax to the cutting surface. This prevents cooling taking place and heat damage occurs, shortening the useable lifetime of the drill bit. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided a drill bit for cutting tiles comprising a hollow body portion, typically cylindrical and so tubular, bearing an abrasive annular cutting surface at one end and a wax plug located within the hollow body portion, wherein an internal wall of the hollow body portion has at least one region of reduced wall thickness, thereby to provide a channel within the wall along which liquid wax can flow. This ensures that the wax can reach the region where cutting is taking place faster, which reduces heating occurring at the cutting surface, so improving the lifetime of the drill bit. 
     The region of reduced wall thickness preferably extends along the body portion at least as far as the wax plug. 
     The hollow body portion may be formed by a passage offset from the cross-sectional centre of the hollow body portion. As the drill bit is rotated by a drill or angle grinder, the rotation of the body portion with its region of reduced thickness provides a channel extending along the body portion and along which melted wax can flow to reach the cutting surface. 
     The passage is preferably offset from the cross-sectional centre of the hollow body portion by 50% or less of the width of the hollow body portion. Thus, the passage may be offset from the cross-sectional centre of the hollow body portion so that the region of reduced wall thickness has a minimum wall thickness of 0.5 mm. 
     The region of reduced wall thickness may be provided by at least one channel formed within the internal wall of the hollow body portion. 
     The hollow body preferably has an external diameter of between 3 to 16 mm, and more preferably between 3 to 7 mm with the wax cooling particularly effective for an external diameter of 7 mm or less. 
     If desired, a plurality of channels may be provided, preferably as circumferentially equispaced grooves around the internal wall of the hollow body portion. 
     The at least one channel may be a helical groove, and may be a reverse spiral thread such that the handedness of the helical groove is opposite to the sense of rotation of the drill bit during use. One or more helical grooves may be provided, for example as two, three or four threads. 
     The drill bit as aforesaid is particularly advantageous when used with a drilling tool, operating between 500 to 30,000 rpm such as corded and cordless drills, angle grinders and high speed rotary tools and of particular advantage for angle grinders operating between 8000 to 14000 rpm. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described by way of example with reference to the accompanying drawings in which: 
         FIG. 1  is a perspective view of a first embodiment of a drill bit in accordance with the invention; 
         FIG. 2  is an illustrative diagram showing an internal rotational path of the first embodiment; 
         FIG. 3  is a perspective view of a second embodiment; 
         FIGS. 4( a ) and ( b )  are end and horizontal cross-sectional views respectively of the second embodiment; 
         FIG. 5  is a perspective view of a third embodiment; and 
         FIGS. 6( a ) and ( b )  are end and horizontal cross-sectional views respectively of the third embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows drill bit, or hole saw,  10  comprising a shank  12  for attachment to a manually held drill or angle grinder and an elongate thin-walled cylindrical tube  14  having an annular cutting surface  16  for engaging with a workpiece such as a tile or work surface. Typically, the external diameter of tube  14  will be in the range 3 mm to 16 mm. Cutting surface  16  has abrasive cutting material  18  attached to it, typically in the form of adhered particles of granular diamond or tungsten carbide. Rotation of drill bit  10  by a drill or angle grinder connected to shank  12  rotates abrasive cutting surface  16  so removing circular discs of material from a workpiece. 
     Internal passage  20  through tube  14  is circular in cross-section and is offset by at most 50% from a centre axis  22  about which drill bit  10  rotates. Passage  20  is thus typically displaced between 0.1 to 1 mm from centre axis  22  so that the wall thickness of tube  14  varies, with a minimum wall thickness of around 0.5 mm at the narrowest region  24 . Wax plug  26  is located within passage  20  close to cutting surface  16 . Wax plug  26  gradually melts from heat generated by friction as cutting takes place, with liquid wax acting as a coolant on cutting surface  16  and passing between the granular cutting material to achieve cooling. 
     Rotation of drill bit  10  results in an elliptical rotation path  28  of offset passage  20  as shown in  FIG. 2 . Where a slug  30  forms due to waste material mixing with liquid wax, slug  30  will be circular in cross-section and of reduced diameter compared to passage  20 , leaving gap  32  unblocked and forming an internal channel within tube  14  that reaches wax plug  26 . Thus, during rotation there exists an internal channel extending within tube  14  along which wax can flow to reach cutting surface  16 . 
     The creation of a channel that cannot be blocked by slug formation maintains wax cooling at abrasive cutting surface  16 , ensuring cutting material  18  is not damaged through excess heat. This increases the working lifetime of drill bit  10  as can be seen in Table 1 below where the number of holes drilled through 8 mm thick porcelain tiles at 1100 rpm was compared for prior art hole saws with wax plugs and hole saws according to the invention with an offset internal passage and wax plug. Drill bits of external diameters 5 mm and 6.5 mm were compared, with the offset passage or offset hole version increasing the number of holes drilled by over 130%. This is a significant improvement on existing hole saws. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Comparison of holes drilled 
               
            
           
           
               
               
            
               
                   
                 Diameter 
               
            
           
           
               
               
               
            
               
                   
                  3/16″ or 5.0 mm 
                 ¼″ or 6.5 mm 
               
            
           
           
               
               
               
               
               
            
               
                   
                 without 
                 with 
                 without 
                 with 
               
               
                   
                 offset 
                 offset  
                 offset 
                 offset  
               
               
                 Modification 
                 hole 
                 hole 
                 hole 
                 hole 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Test 1 
                 18 
                 100 
                 26 
                 65 
               
               
                 Test 2 
                 27 
                 48 
                 20 
                 93 
               
               
                 Test 3 
                 23 
                 46 
                 19 
                 69 
               
               
                 Test 4 
                 31 
                 44 
                 19 
                 56 
               
               
                 Test 5 
                   
                 57 
                 17 
                 56 
               
               
                 Test 6 
                   
                   
                 32 
                   
               
               
                 Test 7 
                   
                   
                 28 
                   
               
               
                 Average Life 
                 24.8 
                 59.0 
                 23.0 
                 67.8 
               
               
                 Difference 
                 ref. 
                 138% 
                 ref. 
                 195% 
               
               
                   
               
            
           
         
       
     
     A second embodiment shown in  FIGS. 3 and 4  comprises hollow tube  34  having an internal passage  36  centred on rotational axis  22  and which is formed with a plurality of internal semi-circular channels  40  extending along tube  34  towards shank  12 , with the four channels  40  being circumferentially equispaced and providing multiple pathways for melted wax from plug  26  to reach a workpiece. Typically channels  40  extend into wall  42  to produce a minimum wall thickness  24  of 0.5mm, with the wall thickness in regions without a channel being around 1 mm. Any slug formed from waste material combined with melted wax will sit within passage  36  and will not obstruct flow of liquid wax along channels  40 . 
     In a third embodiment shown in  FIGS. 5 and 6 , one or more internal helical channels  50  are provided within tube  52 , such that the channels extend along and around the internal wall. The helical channel can be provided as a single thread, or as two, three or four multiple threads so as to provide multiple exit points adjacent cutting surface  16 . The threads can be provided as reverse spiral threads, such that the handedness of the helical groove left-handed and opposite to the sense of rotation of the drill bit during use. 
     Increasing the number of helical threads or helical channels provides increased flow of coolant/lubricant, delivering more cooling. 
     While the present invention has been illustrated by description of various embodiments and while those embodiments have been described in considerable detail, it is not the intention of Applicants to restrict or in any way limit the scope of the appended claims to such details. Additional advantages and modifications will readily appear to those skilled in the art. The present invention in its broader aspects is therefore not limited to the specific details and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of Applicants&#39; invention.