Abstract:
A graphite core mold having its own cavity is pressed into a cavity of a graphite shell mold. The core cavity is filled with tungsten carbide, mixed with a nickel alloy, heated and then cooled to thereby provide a carbide matrix body. After milling the carbide matrix body to thereby provide a carbide bit body, the intact shell mold is removed and then used again.

Description:
BACKGROUND OF INVENTION 
       [0001]    This invention relates generally to the manufacture of fixed cutter drill bits used to drill oil and gas wells. 
         [0002]    Various types of drill bits have been developed and found useful in different drilling environments. Bits typically used for drilling boreholes in the oil and gas industry include roller cone bits and fixed cutter bits. Cutting structures on bits vary depending on the type of bit and the type of formation being cut. Roller cone cutting structures typically include milled steel teeth, tungsten carbide inserts, or diamond enhanced inserts. Cutting structures for fixed cutter bits typically include polycrystalline diamond compacts, commonly referred to as “PDC” cutters. The selection of a bit type and cutting structure for a given drilling application depends on many factors including the formation type to be drilled, rig equipment capabilities, and the time and cost associated with drilling. 
         [0003]    It is common in the art of manufacturing PDC, fixed cutter drill bits, to manufacture the carbide matrix body of the drill bit in a graphite mold. Such a mold known in the prior art is typically made with a single mold manufactured from graphite. 
         [0004]    Fixed cutter drill bits, also referred to as fixed head bits or drag bits, are generally more expensive than mill tooth roller cone drill bits and are considered to offer less aggressive cutting structures than roller cone drill bits. However, in several applications, fixed cutter bits can be used to drill longer well segments in a single run and can be rebuilt and reused multiple times to provide an overall economic benefit that outweighs the higher cost. Fixed cutter bits which include PDC cutters are typically referred to as PDC bits. 
         [0005]      FIG. 1  of the drawings illustrates a conventional single piece graphite mold which is known in the prior art for manufacturing the carbide matrix body for a PDC drill bit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a pictorial, isometric view of a graphite mold known in the prior art used for manufacturing a carbide matrix body for PDC drill bits; 
           [0007]      FIGS. 2A ,  2 B and  2 C respectively illustrate an exploded view of the two piece mold in accordance with the present invention; 
           [0008]      FIG. 3A  illustrates another view of the elements illustrated in  FIGS. 2A ,  2 B and  2 C, as assembled, after the drill bit has been at least partially milled in accordance with invention; 
           [0009]      FIG. 3B  is a schematic view, partially in cross section, illustrating the assembled two piece mold and the gauge ring which is used as a funnel, illustrated in  FIG. 2C , resting on top of the two piece mold illustrated in  FIGS. 2A and 2B ; 
           [0010]      FIG. 4A  is an elevated, schematic view, in cross section, illustrating the mold core illustrated in  FIG. 2  mounted within the mold shell illustrated in  FIG. 2A  and having the gauge ring illustrated in  FIG. 2  sitting on top of the mold part shown in  FIG. 2B ; 
           [0011]      FIG. 4B  is a top plan, schematic view of the embodiment illustrated in  FIG. 4A , but without its top portion illustrated; 
           [0012]      FIG. 5  is a side view, in cross section of the mold shell illustrated in  FIG. 2A  in accordance with the invention; 
           [0013]      FIG. 6  is a side view, in cross section, illustrating the mold core illustrated in  FIG. 2B , which in operation resides within the mold shell illustrated in  FIG. 5 ; 
           [0014]      FIG. 7  is a pictorial, isometric view of the assembled two piece mold illustrated in  FIGS. 2A and 2B , and having a drill bit residing within the interior of the mold core in  FIG. 2B  in accordance with the invention; and 
           [0015]      FIG. 8  illustrates the equipment used in milling the drill bit within the mold core illustrated in  FIG. 2B  while resting on a fixture to support the mold core illustrated in  FIG. 2B . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0016]    Referring now to the drawings in more detail,  FIG. 1  is a pictorial, isometric view of the one piece mold which is commonly used in the PDC drill bit industry to manufacture PDC bits. The mold  10  has a lower portion  12  and an upper ring  14  which contains a cavity which is used as the mold for manufacturing the PDC bit body. In the process of using the prior art mold illustrated in  FIG. 1 , the mold is first partially filled with a tungsten carbide powder to which is then applied a metal alloy, such as a nickel alloy. Upon heating, the nickel alloy will mix with the molten tungsten carbide, which upon cooling results in a carbide matrix which forms the bit body in the prior art. A conventional milling process such as is illustrated in  FIG. 8  having milling device  16  is then used to mill out the various components of the matrix body to result in a finished PDC matrix body. 
         [0017]    In using the single piece mold of the prior art illustrated in  FIG. 1 , as soon as the process has been finished as illustrated in  FIG. 1 , the mold  10  is removed from the molded graphite  18 , typically by breaking the mold  10  with a hammer, a crowbar or the like. This results in a lot of wasted material because the mold  10  cannot be reused because of its broken condition. 
         [0018]    Referring now to  FIGS. 2A ,  2 B and  2 C, there is illustrated a mold shell  30  and a mold core  32  which is illustrated in  FIG. 2B . The mold core  32  is sized to closely fit within the cavity  34  illustrated in  FIG. 2A  which allows the mold core  32  to be easily removed from the shell  30  after the molding process has been completed. The mold shell  30  is not broken and can be reused several times in the molding process. As illustrated in  FIG. 2B , the milling process has been used to at least partially complete the milling of the matrix body to result in a matrix drill bit body. Gauge ring  36  which sits on top of the mold shell  30 , surrounds the cavity in the mold core  32 , all is illustrated in  FIG. 4A . 
         [0019]    The funnel  38  through which the nickel alloy and the tungsten carbide powder is filled, allows the tungsten carbide powder and the nickel alloy to go into the cavity within the mold core  32  as illustrated in  FIG. 2B . Sitting atop the funnel  38  is a top end piece  42 . The end cap  42  can be threaded onto the top of the funnel  38  or can merely sit on top of the funnel as desired. In  FIG. 4A , the drill bit parts, are illustrated schematically, such as the nozzles  44  and the other components such as the cutter pockets  46  which will eventually house the PDC cutters once all the molding process has been completed. All of these parts of the conventional PDC bit are well known in the art and need no further description herein. 
         [0020]      FIG. 4B  illustrates a top plan view of the mold shell, mold core and gauge ring, as well as carbide matrix which has been milled to accept the various components of the finished drill bit in accordance with the invention. 
         [0021]      FIG. 7  is a pictorial, isometric view of the two piece mold according to the invention, is much like the embodiment of  FIG. 3A , but having the milling process further completed for forming the various components of the PDC drill bit. Assuming that the milling process is completed, the core  32  is easily removed from the mold shell  30 , without any need to break the mold shell  30 , all as contemplated by the present invention. The matrix body for the drill bit can then be removed from the mold core, with breaking of the mold core as may be necessary. It allows a mold shell to be used several times without a need to break the shell as done in the prior art single piece molds. The use of the two piece mold enables the manufacture of several sized PDC bit bodies from a single size mold shell which cannot be done in the prior art because the one piece mold is broken with a single use and does not allow the manufacture of different sized drill bit bodies to be made from a one piece mold, or even a repetition of the process for the same sized bits using the same shell.