Stable Rotary Drill Bit

Abstract

A drill bit comprises a bit body having a plurality of upstanding main blades formed thereon. The main blades have a plurality cutting elements mounted thereon, and they are shaped such that a cutting profile of the bit includes a nose portion, an inner cone region located radially inwards of the nose portion and an outer region located radially outwards of the nose portion. The bit further has at least one additional blade. The additional blade carrying cutting elements located radially inwards of the outer region of the cutting profile, but being substantially free of cutting elements located in the outer region of the cutting profile.

1 . A drill bit comprising a bit body having a plurality of upstanding main blades formed thereon, the main blades having a plurality cutting elements mounted thereon, the main blades being shaped such that a cutting profile of the bit includes a nose portion, an inner cone region located radially inwards of the nose portion and an outer region located radially outwards of the nose portion, the bit further comprising at least one additional blade, the additional blade carrying cutting elements located radially inwards of the outer region of the cutting profile, but being substantially free of cutting elements located in the outer region of the cutting profile, wherein the cutting elements provided on the main blades and on the additional blades are located such that the lateral forces generated by the cutting elements, in use, are substantially balanced.  
   
   
       2 . A drill bit according to  claim 1 , wherein a part of the additional blade located in the outer region of the cutting profile is of increased blade height as compared to parts of the main blades located in the outer region of the cutting profile.  
   
   
       3 . A drill bit according to  claim 1 , wherein the bit has a diameter falling within the range of about 12¼″ to 26″.  
   
   
       4 . A drill bit according to  claim 1 , wherein the cutting elements comprise polycrystalline diamond compact cutters.  
   
   
       5 . A drill bit according to  claim 1 , wherein the cutting elements comprise diamond crystals mounted upon or embedded in the main and additional blades.  
   
   
       6 . A drill bit according to  claim 1 , wherein the cutting elements comprise polycrystalline diamond crystals mounted upon or embedded in the main and additional blades.  
   
   
       7 . A drill bit according to  claim 1 , wherein an outer surface of a part of the additional blade located in the outer region of the cutting profile is rendered of improved resistance to wear.  
   
   
       8 . A drill bit according to  claim 7 , wherein the said part of the additional blade located in the outer region is rendered of improved resistance to wear by having suitably shaped and positioned, non-cutting, polycrystalline diamond elements mounted thereon.  
   
   
       9 . A drill bit according to  claim 7 , wherein the said part of the additional blade located in the outer region is rendered of improved resistance to wear by having a hard facing material coating applied thereto.  
   
   
       10 . A drill bit according to  claim 1 , wherein the plurality of main blades comprise a first plurality of blades which extend inwardly to a centerline of the bit body, and a second plurality of blades which stop short of the centerline of the bit body.  
   
   
       11 . A drill bit according to  claim 1 , wherein a plurality of the additional blades are provided, the additional blades being substantially equally spaced around the bit body.  
   
   
       12 . A drill bit according to  claim 1 , wherein the main blades and the additional blades each have a gauge region, the gauge regions being of substantially uniform length and being aligned with one another.