Abstract:
An apparatus and method for manufacturing a downhole tool that reduces failures occurring along a bondline between a cemented matrix coupled around a blank. The cemented matrix material is formed from a tungsten carbide powder, a shoulder powder, and a binder material, wherein at least one of the tungsten carbide powder or the shoulder powder is absent of any free tungsten. The blank, which optionally may be coated, is substantially cylindrically shaped and defines a channel extending from a top portion and through a bottom portion of the blank. The absence of free tungsten from at least one of the tungsten carbide powder or the shoulder powder reduces the reaction with iron from the blank, thereby allowing the control and reduction of intermetallic compounds thickness within the bondline.

Description:
RELATED APPLICATIONS 
     The present application is a continuation-in-part of U.S. patent application Ser. No. 13/476,662, entitled “Heavy Duty Matrix Bit,” and filed on May 21, 2012, which claims priority to U.S. Provisional Patent Application No. 61/489,056, entitled “Heavy Matrix Drill Bit” and filed on May 23, 2011, the disclosures of which are incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to downhole tools and methods for manufacturing such items. More particularly, this invention relates to infiltrated matrix drilling products including, but not limited to, fixed cutter bits, polycrystalline diamond compact (“PDC”) drill bits, natural diamond drill bits, thermally stable polycrystalline (“TSP”) drill bits, bi-center bits, core bits, and matrix bodied reamers and stabilizers, and the methods of manufacturing such items. 
     Full hole tungsten carbide matrix drill bits for oilfield applications have been manufactured and used in drilling since at least as early as the 1940&#39;s.  FIG. 1  shows a cross-sectional view of a downhole tool casting assembly  100  in accordance with the prior art. The downhole tool casting assembly  100  consists of a thick-walled mold  110 , a stalk  120 , one or more nozzle displacements  122 , a blank  124 , a funnel  140 , and a binder pot  150 . The downhole tool casting assembly  100  is used to fabricate a casting (not shown) of a downhole tool. 
     According to a typical downhole tool casting assembly  100 , as shown in  FIG. 1 , and a method for using the downhole tool casting assembly  100 , the thick-walled mold  110  is fabricated with a precisely machined interior surface  112 , and forms a mold volume  114  located within the interior of the thick-walled mold  110 . The thick-walled mold  110  is made from sand, hard carbon graphite, ceramic, or other known suitable materials. The precisely machined interior surface  112  has a shape that is a negative of what will become the facial features of the eventual bit face. The precisely machined interior surface  112  is milled and dressed to form the proper contours of the finished bit. Various types of cutters (not shown), known to persons having ordinary skill in the art, can be placed along the locations of the cutting edges of the bit and can also be optionally placed along the gage area of the bit. These cutters can be placed during the bit fabrication process or after the bit has been fabricated via brazing or other methods known to persons having ordinary skill in the art. 
     Once the thick-walled mold  110  is fabricated, displacements are placed at least partially within the mold volume  114  of the thick-walled mold  110 . The displacements are typically fabricated from clay, sand, graphite, ceramic, or other known suitable materials. These displacements consist of the center stalk  120  and the at least one nozzle displacement  122 . The center stalk  120  is positioned substantially within the center of the thick-walled mold  110  and suspended a desired distance from the bottom of the mold&#39;s interior surface  112 . The nozzle displacements  122  are positioned within the thick-walled mold  110  and extend from the center stalk  120  to the bottom of the mold&#39;s interior surface  112 . The center stalk  120  and the nozzle displacements  122  are later removed from the eventual drill bit casting so that drilling fluid (not shown) can flow though the center of the finished bit during the drill bit&#39;s operation. 
     The blank  124  is a cylindrical steel casting mandrel that is centrally suspended at least partially within the thick-walled mold  110  and around the center stalk  120 . The blank  124  is positioned a predetermined distance down in the thick-walled mold  110 . According to the prior art, the distance between the outer surface of the blank  124  and the interior surface  112  of the thick-walled mold  110  is typically twelve millimeters (“mm”) or more so that potential cracking of the thick-walled mold  110  is reduced during the casting process. 
     Once the displacements  120 ,  122  and the blank  124  have been positioned within the thick-walled mold  110 , tungsten carbide powder  130 , which includes free tungsten, is loaded into the thick-walled mold  110  so that it fills a portion of the mold volume  114  that is around the lower portion of the blank  124 , between the inner surfaces of the blank  124  and the outer surfaces of the center stalk  120 , and between the nozzle displacements  122 . Shoulder powder  134  is loaded on top of the tungsten carbide powder  130  in an area located at both the area outside of the blank  124  and the area between the blank  124  and the center stalk  120 . The shoulder powder  134  is made of tungsten powder. This shoulder powder  134  acts to blend the casting to the steel blank  124  and is machinable. Once the tungsten carbide powder  130  and the shoulder powder  134  are loaded into the thick-walled mold  110 , the thick-walled mold  110  is typically vibrated to improve the compaction of the tungsten carbide powder  130  and the shoulder powder  134 . Although the thick-walled mold  110  is vibrated after the tungsten carbide powder  130  and the shoulder powder  134  are loaded into the thick-walled mold  110 , the vibration of the thick-walled mold  110  can be done as an intermediate step before, during, and/or after the shoulder powder  134  is loaded on top of the tungsten carbide powder  130 . 
     The funnel  140  is a graphite cylinder that forms a funnel volume  144  therein. The funnel  140  is coupled to the top portion of the thick-walled mold  110 . A recess  142  is formed at the interior edge of the funnel  140 , which facilitates the funnel  140  coupling to the upper portion of the thick-walled mold  110 . Typically, the inside diameter of the thick-walled mold  110  is similar to the inside diameter of the funnel  140  once the funnel  140  and the thick-walled mold  110  are coupled together. 
     The binder pot  150  is a cylinder having a base  156  with an opening  158  located at the base  156 , which extends through the base  156 . The binder pot  150  also forms a binder pot volume  154  therein for holding a binder material  160 . The binder pot  150  is coupled to the top portion of the funnel  140  via a recess  152  that is formed at the exterior edge of the binder pot  150 . This recess  152  facilitates the binder pot  150  coupling to the upper portion of the funnel  140 . Once the downhole tool casting assembly  100  has been assembled, a predetermined amount of binder material  160  is loaded into the binder pot volume  154 . The typical binder material  160  is a copper alloy or other suitable known material. Although one example has been provided for setting up the downhole tool casting assembly  100 , other examples can be used to form the downhole tool casting assembly  100 . 
     The downhole tool casting assembly  100  is placed within a furnace (not shown) or other heating structure. The binder material  160  melts and flows into the tungsten carbide powder  130  through the opening  158  of the binder pot  150 . In the furnace, the molten binder material  160  infiltrates the tungsten carbide powder  130  and the shoulder powder  134  to fill the interparticle spaces formed between adjacent particles of tungsten carbide powder  130  and between adjacent particles of shoulder powder  134 . During this process, a substantial amount of binder material  160  is used so that it fills at least a substantial portion of the funnel volume  144 . This excess binder material  160  in the funnel volume  144  supplies a downward force on the tungsten carbide powder  130  and the shoulder powder  134 . Once the binder material  160  completely infiltrates the tungsten carbide powder  130  and the shoulder powder  134 , the downhole tool casting assembly  100  is pulled from the furnace and is controllably cooled. Upon cooling, the binder material  160  solidifies and cements the particles of tungsten carbide powder  130  and the shoulder powder  134  together into a coherent integral mass  310  ( FIG. 3 ). The binder material  160  also bonds this coherent integral mass  310  ( FIG. 3 ) to the steel blank  124  thereby forming a bonding zone  190 , which is formed along at least a chamfered zone area  198  of the steel blank  124  and a central zone area  199  of the steel blank  124 . The coherent integral mass  310  ( FIG. 3 ) and the blank  124  collectively form the matrix body bit  200  ( FIG. 2 ), a portion of which is shown in  FIGS. 2 and 3 . Once cooled, the thick-walled mold  110  is broken away from the casting. The casting then undergoes finishing steps which are known to persons having ordinary skill in the art, including the addition of a threaded connection (not shown) coupled to the top portion of the blank  124 . Although the matrix body bit  200  ( FIG. 2 ) has been described to be formed using the process and equipment described above, the process and/or the equipment can be varied to still form the matrix body bit  200  ( FIG. 2 ). 
       FIG. 2  shows a magnified cross-sectional view of the bonding zone  190  located at the chamfered zone area  198  ( FIG. 1 ) within the matrix body bit  200  in accordance with the prior art.  FIG. 3  shows a magnified cross-sectional view of the bonding zone  190  located at the central zone area  199  ( FIG. 1 ) within the matrix body bit  200  in accordance with the prior art. Referring to  FIGS. 2 and 3 , the coherent integral mass  310  is bonded to the steel blank  124  via the bonding zone  190  that is formed along and/or adjacent the surface of the steel blank  124 . The binder material  160  causes a portion of the iron from the steel blank  124  to diffuse into the binder material  160  and react with the free tungsten within the shoulder powder  134  and the tungsten carbide powder  130 , thereby forming this bonding zone  190 . The bonding zone  190  includes intermetallic compounds  290 . These intermetallic compounds  290  have an average hardness level of about 250 HV, which corresponds to about twice the hardness of the binder and steel matrix. According to  FIG. 2 , the bonding zone  190  is formed having a thickness  215  ranging from about sixty-five micrometers (μm) to about eighty μm in the chamfered zone area  198  ( FIG. 1 ). According to  FIG. 3 , the bonding zone  190  is formed having a thickness  315  ranging from about ten μm to about twenty μm in the central zone area  199  ( FIG. 1 ). The thicknesses  215 ,  315  and/or volumes of the bonding zone  190  are dependent upon the exposure time and the exposure temperature. Exposure temperature is related to the type of binder material  160  that is used to cement the tungsten carbide particles to one another. Manufacturers typically use the same binder material  160  over long periods of time, such as ten year or more, because of the knowledge gained with respect to the binder material  160  used. Thus, the exposure temperature is substantially the same from one casting to another. Exposure time is not always the same, but instead, is related to the bit diameter that is to be manufactured. When the bit diameter to be manufactured is relatively large, there is a larger volume of tungsten carbide particles that are to be cemented to one another. Hence, the exposure time also is relatively longer, thereby providing more time for cementing the larger volume of tungsten carbide particles. Thus, since the exposure temperature is the same from one casting to another, and the exposure time is the same for casting similar bit diameters, it follows that the thicknesses  215 ,  315  of intermetallic compounds  290  formed within the bit is consistent from one casting to another for a same bit diameter. 
     Initially, natural diamond bits were used in oilfield applications. These natural diamond bits performed by grinding the rock within the wellbore, and not by shearing the rock. Thus, these natural diamond bits experienced little to no torque, and hence very little stress was experienced at the bonding zone  190  of the natural diamond bits. With the advent of PDC drill bits, the bits sheared the rock within the wellbore and began experiencing more torque. However, these initial PDC drill bits were fabricated relatively small, about six inch diameters to about 12¼ inch diameters, and the prior art fabrication method described above continued to perform well. Later, PDC drill bits were fabricated having larger diameters and failures began occurring along the bonding zone  190 . Specifically, decohesion began occurring between the blank  124  and the coherent integral mass  310 , or matrix, at the bonding zone  190 . These intermetallic compounds  290  are a source for causing mechanical stresses to occur along the bonding zone  190  during drilling applications because there is a contraction of volume occurring when the intermetallic compounds  290  are formed. These intermetallic compounds are very brittle and some cracks in the intermetallic compounds could occur during the drilling process. These cracks could weaken the bit and lead to catastrophic failure. Now that cutter technology has improved, the demand placed upon the bits have also increased. Bits are being drilled for more hours. Bits also are being used with much more energy, which includes energy produced from increasing the weight on bit and/or from increasing the rotational speed of the bit. This increased demand on the bits is causing the decohesion failure to become a recurring problem in the industry. As the thickness or volume of the intermetallic compounds  290  increases, the risk of decohesion also increases. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other features and aspects of the invention will be best understood with reference to the following description of certain exemplary embodiments of the invention, when read in conjunction with the accompanying drawings, wherein: 
         FIG. 1  shows a cross-sectional view of a downhole tool casting assembly in accordance with the prior art; 
         FIG. 2  shows a magnified cross-sectional view of a bonding zone located at a chamfered zone area within the matrix body bit in accordance with the prior art; 
         FIG. 3  shows a magnified cross-sectional view of a bonding zone located at a central zone area within the matrix body bit in accordance with the prior art; 
         FIG. 4  shows a cross-sectional view of a blank in accordance with an exemplary embodiment; 
         FIG. 5  shows a cross-sectional view of a downhole tool casting assembly using the blank of  FIG. 4  in accordance with the exemplary embodiment; 
         FIG. 6  shows a magnified cross-sectional view of a bonding zone located at a chamfered zone area within the downhole tool in accordance with the exemplary embodiment; 
         FIG. 7  shows a magnified cross-sectional view of a bonding zone located at a central zone area within the downhole tool in accordance with the exemplary embodiment; 
         FIG. 8  shows a magnified cross-sectional view of a bonding zone located at a chamfered zone area within the downhole tool in accordance with another exemplary embodiment; 
         FIG. 9  shows a magnified cross-sectional view of a bonding zone located at a central zone area within the downhole tool in accordance with another exemplary embodiment; 
         FIG. 10  shows a cross-sectional view of a downhole tool casting assembly in accordance with another exemplary embodiment; 
         FIG. 11  shows a partial cross-sectional view of a downhole tool casting formed using the downhole tool casting assembly of  FIG. 10  in accordance with the exemplary embodiment; 
         FIG. 12  shows a cross-sectional view of a downhole tool casting assembly in accordance with yet another exemplary embodiment; 
         FIG. 13  shows a partial cross-sectional view of a downhole tool casting formed using the downhole tool casting assembly of  FIG. 12  in accordance with the exemplary embodiment; 
         FIG. 14  shows a cross-sectional view of a downhole tool casting assembly in accordance with yet another exemplary embodiment; and 
         FIG. 15  shows a partial cross-sectional view of a downhole tool casting formed using the downhole tool casting assembly of  FIG. 14  in accordance with the exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     This invention relates generally to downhole tools and methods for manufacturing such items. More particularly, this invention relates to infiltrated matrix drilling products including, but not limited to, fixed cutter bits, polycrystalline diamond compact (“PDC”) drill bits, natural diamond drill bits, thermally stable polycrystalline (“TSP”) drill bits, bi-center bits, core bits, and matrix bodied reamers and stabilizers, and the methods of manufacturing such items. Although the description provided below is related to a drill bit, embodiments of the present invention relate to any infiltrated matrix drilling product. 
       FIG. 4  shows a cross-sectional view of a blank  400  in accordance with an exemplary embodiment. The blank  400  includes an internal blank component  410  and a metal coating  420  coupled around at least a portion of the surface of the internal blank component  410 . The internal blank component  410  is similar to the blank  124  ( FIG. 1 ) above. The internal blank component  410  is a cylindrically, hollow-shaped component and includes a cavity  412  extending through the entire length of the internal blank component  410 . According to some exemplary embodiments the internal blank component  410  also includes a top portion  414  and a bottom portion  416 . The top portion  414  has a smaller outer circumference than the bottom portion  416 . According to some exemplary embodiments, the internal blank component  410  is fabricated from steel; however, any other suitable material known to people having ordinary skill in the art is used in other exemplary embodiments. 
     The metal coating  420  is applied onto at least a portion of the surface of the internal blank component  410 . In some exemplary embodiments, the metal coating  420  is applied onto the surface of the entire internal blank component  410 . In other exemplary embodiments, the metal coating  420  is applied onto a portion of the surface of the internal blank component  410 . For example, the metal coating  420  is applied onto the surface of the bottom portion  416 , which is the portion that bonds to the matrix material, or a coherent integral mass  710  ( FIG. 7 ), which is described below. The metal coating  420  is applied onto the internal blank component  410  using electroplating techniques. Alternatively, other techniques, such as plasma spray, ion bombardment, electro-chemical depositing, laser cladding, cold spray, or other known coating techniques, are used to apply the metal coating  420  onto the internal blank component  410  in other exemplary embodiments. The metal coating  420  is fabricated using a material that reduces the formation of intermetallic compounds  690  ( FIG. 6 ) along and/or adjacent the surface of the blank  400  ( FIG. 4 ). Specifically, the metal coating  420  reduces the migration of iron from the internal blank component  410  into the binder material  560  ( FIG. 5 ) for reacting with the free tungsten at the temperature and exposure time during the fabrication process. The metal coating  420  is fabricated from nickel according to some exemplary embodiments. Alternatively, the metal coating  420  is fabricated using at least one of brass, bronze, copper, aluminum, zinc, cobalt, titanium, gold, refractory transitional materials such as molybdenum and tantalum, carbide, boride, oxide, metal matrix composites, a metal alloy of any previously mentioned metals, or any other suitable material that is capable of reducing the migration of iron from the internal blank component  410  into the binder material  560  ( FIG. 5 ) for reacting with the free tungsten. Alternatively, a different type of coating, such as a polymer coating, is used in lieu of the metal coating. 
     The metal coating  420  is applied onto the internal blank component  410  and has a thickness  422  ranging from about five μm to about 200 μm. In another exemplary embodiment, the metal coating  420  has a thickness  422  ranging from about five μm to about 150 μm. In yet another exemplary embodiment, the metal coating  420  has a thickness  422  ranging from about five μm to about eighty μm. In a further exemplary embodiment, the metal coating  420  has a thickness  422  ranging less than or greater than the previously mentioned ranges. In certain exemplary embodiments, the thickness  422  is substantially uniform, while in other exemplary embodiments, the thickness  422  is non-uniform. For example, the thickness  422  is greater along the surface of the internal blank component  410  that would typically form a greater thickness of the intermetallic compound during the fabrication process, such as the chamfered zone area  598  ( FIG. 5 ). 
       FIG. 5  shows a cross-sectional view of a downhole tool casting assembly  500  using the blank  400  in accordance with the exemplary embodiment. Referring to  FIG. 5 , the downhole tool casting assembly  500  includes a mold  510 , a stalk  520 , one or more nozzle displacements  522 , the blank  400 , a funnel  540 , and a binder pot  550 . The downhole tool casting assembly  500  is used to fabricate a casting (not shown) of a downhole tool, such as a fixed cutter bit, a PDC drill bit, a natural diamond drill bit, and a TSP drill bit. However, the downhole tool casting assembly  500  is modified in other exemplary embodiments to fabricate other downhole tools, such as a bi-center bit, a core bit, and a matrix bodied reamer and stabilizer. 
     The mold  510  is fabricated with a precisely machined interior surface  512 , and forms a mold volume  514  located within the interior of the mold  510 . The mold  510  is made from sand, hard carbon graphite, ceramic, or other known suitable materials. The precisely machined interior surface  512  has a shape that is a negative of what will become the facial features of the eventual bit face. The precisely machined interior surface  512  is milled and dressed to form the proper contours of the finished bit. Various types of cutters (not shown), known to persons having ordinary skill in the art, are placed along the locations of the cutting edges of the bit and are optionally placed along the gage area of the bit. These cutters are placed during the bit fabrication process or after the bit has been fabricated via brazing or other methods known to persons having ordinary skill in the art. 
     Once the mold  510  is fabricated, displacements are placed at least partially within the mold volume  514 . The displacements are fabricated from clay, sand, graphite, ceramic, or other known suitable materials. These displacements include the center stalk  520  and the at least one nozzle displacement  522 . The center stalk  520  is positioned substantially within the center of the mold  510  and suspended a desired distance from the bottom of the mold&#39;s interior surface  512 . The nozzle displacements  522  are positioned within the mold  110  and extend from the center stalk  520  to the bottom of the mold&#39;s interior surface  512 . The center stalk  520  and the nozzle displacements  522  are later removed from the eventual drill bit casting so that drilling fluid (not shown) flows though the center of the finished bit during the drill bit&#39;s operation. 
     The blank  400 , which has been previously described above, is centrally suspended at least partially within the mold  510  and around the center stalk  520 . The blank  400  is positioned a predetermined distance down in the mold  510 . The distance between the outer surface of the blank  400  and the interior surface  512  of the mold  510  is about twelve millimeters or more so that potential cracking of the mold  510  is reduced during the casting process. However, this distance is varied in other exemplary embodiments depending upon the strength of the mold  510  or the method and/or equipment used in fabricating the casting. 
     Once the displacements  520 ,  522  and the blank  400  have been positioned within the mold  510 , tungsten carbide powder  530  is loaded into the mold  110  so that it fills a portion of the mold volume  514  that is around the bottom portion  416  of the blank  400 , between the inner surfaces of the blank  400  and the outer surfaces of the center stalk  520 , and between the nozzle displacements  522 . Shoulder powder  534  is loaded on top of the tungsten carbide powder  530  in an area located at both the area outside of the blank  400  and the area between the blank  400  and the center stalk  520 . The shoulder powder  534  is made of tungsten powder or other known suitable material. This shoulder powder  534  acts to blend the casting to the blank  400  and is machinable. Once the tungsten carbide powder  530  and the shoulder powder  534  are loaded into the mold  510 , the mold  510  is vibrated, in some exemplary embodiments, to improve the compaction of the tungsten carbide powder  530  and the shoulder powder  534 . Although the mold  510  is vibrated after the tungsten carbide powder  530  and the shoulder powder  534  are loaded into the mold  510 , the vibration of the mold  510  is done as an intermediate step before, during, and/or after the shoulder powder  534  is loaded on top of the tungsten carbide powder  530 . Although tungsten carbide material  530  is used in certain exemplary embodiments, other suitable materials known to persons having ordinary skill in the art is used in alternative exemplary embodiments. 
     The funnel  540  is a graphite cylinder that forms a funnel volume  544  therein. The funnel  540  is coupled to the top portion of the mold  510 . A recess  542  is formed at the interior edge of the funnel  540 , which facilitates the funnel  540  coupling to the upper portion of the mold  510 . In some exemplary embodiments, the inside diameter of the mold  510  is similar to the inside diameter of the funnel  540  once the funnel  540  and the mold  510  are coupled together. 
     The binder pot  550  is a cylinder having a base  556  with an opening  558  located at the base  556 , which extends through the base  556 . The binder pot  550  also forms a binder pot volume  554  therein for holding a binder material  560 . The binder pot  550  is coupled to the top portion of the funnel  540  via a recess  152  that is formed at the exterior edge of the binder pot  550 . This recess  552  facilitates the binder pot  550  coupling to the upper portion of the funnel  540 . Once the downhole tool casting assembly  500  has been assembled, a predetermined amount of binder material  560  is loaded into the binder pot volume  554 . The typical binder material  560  is a copper alloy or other suitable known material. Although one example has been provided for setting up the downhole tool casting assembly  500 , other examples having greater, fewer, or different components are used to form the downhole tool casting assembly  500 . For instance, the mold  510  and the funnel  540  are combined into a single component in some exemplary embodiments. 
     The downhole tool casting assembly  500  is placed within a furnace (not shown) or other heating structure. The binder material  560  melts and flows into the tungsten carbide powder  530  through the opening  558  of the binder pot  550 . In the furnace, the molten binder material  560  infiltrates the tungsten carbide powder  530  to fill the interparticle space formed between adjacent particles of tungsten carbide powder  530 . During this process, a substantial amount of binder material  560  is used so that it fills at least a substantial portion of the funnel volume  544 . This excess binder material  560  in the funnel volume  544  supplies a downward force on the tungsten carbide powder  530  and the shoulder powder  534 . Once the binder material  560  completely infiltrates the tungsten carbide powder  530 , the downhole tool casting assembly  500  is pulled from the furnace and is controllably cooled. Upon cooling, the binder material  560  solidifies and cements the particles of tungsten carbide powder  530  together into a coherent integral mass  710  ( FIG. 7 ). The binder material  560  also bonds this coherent integral mass  710  ( FIG. 7 ) to the blank  400  thereby forming a bonding zone  590 , which is formed at least at a chamfered zone area  598  of the blank  400  and a central zone area  599  of the blank  400 , according to certain exemplary embodiments. The coherent integral mass  710  ( FIG. 7 ) and the blank  400  collectively form the matrix body bit  600  ( FIG. 6 ), a portion of which is shown in  FIGS. 6 and 7 . Once cooled, the mold  510  is broken away from the casting. The casting then undergoes finishing steps which are known to persons of ordinary skill in the art, including the addition of a threaded connection (not shown) coupled to the top portion  414  of the blank  400 . Although the matrix body bit  600  ( FIG. 6 ) has been described to be formed using the process and equipment described above, the process and/or the equipment can be varied to still form the matrix body bit  600  ( FIG. 6 ). 
       FIG. 6  shows a magnified cross-sectional view of the bonding zone  590  located at the chamfered zone area  598  ( FIG. 5 ) within the downhole tool in accordance with the exemplary embodiment.  FIG. 7  shows a magnified cross-sectional view of the bonding zone  590  located at the central zone area  599  ( FIG. 5 ) within the downhole tool in accordance with the exemplary embodiment. Referring to  FIGS. 6 and 7 , the blank  400  includes the internal blank component  410  and the metal coating  420 , which is applied onto the surface of the internal blank component  410 . The coherent integral mass  710  is bonded to the blank  400  via the bonding zone  590  that is formed along and/or adjacent the surface of the blank  400 . According to some exemplary embodiments, the metal coating  420  is thinly applied onto the internal blank component  410  so that a portion of the iron from the blank  400  to diffuses into the binder material  560  and reacts with the free tungsten within the shoulder powder  534  and the tungsten carbide powder  530 , thereby forming this bonding zone  590 . The bonding zone  590  includes intermetallic compounds  690 , which are similar to the intermetallic compounds  290  ( FIG. 2 ). According to  FIG. 6 , the bonding zone  590  is formed having a thickness  615  ranging from about five μm to less than sixty-five μm in the chamfered zone area  598  ( FIG. 5 ). In another exemplary embodiment, the bonding zone  590  is formed having a thickness  615  ranging from about five μm to less than fifty μm in the chamfered zone area  598  ( FIG. 5 ). In yet another exemplary embodiment, the bonding zone  590  is formed having a thickness  615  ranging from about five μm to less than thirty μm in the chamfered zone area  598  ( FIG. 5 ). According to  FIG. 7 , the bonding zone  590  is formed having a thickness  715  ranging from about two μm to less than about ten μm in the central zone area  599  ( FIG. 5 ). In another exemplary embodiment, the bonding zone  590  is formed having a thickness  715  ranging from about two μm to less than eight μm in the central zone area  599  ( FIG. 5 ). In yet another exemplary embodiment, the bonding zone  590  is formed having a thickness  715  ranging from about two μm to less than six μm in the central zone area  599  ( FIG. 5 ). The thicknesses  615 ,  715  and/or volumes of the bonding zone  590  are dependent upon the exposure time, the temperature, and the thickness of the metal coating  420  that is applied onto the internal blank component  410 . As previously mentioned, the metal coating  420  reduces the migration of iron from the blank  400  into the binder material  560 , thereby decreasing the reaction with the free tungsten within the shoulder powder  534  and the tungsten carbide powder  530  during the fabrication process. 
       FIG. 8  shows a magnified cross-sectional view of the bonding zone  590  located at the chamfered zone area  598  ( FIG. 5 ) within the downhole tool in accordance with another exemplary embodiment.  FIG. 9  shows a magnified cross-sectional view of the bonding zone  590  located at the central zone area  599  ( FIG. 5 ) within the downhole tool in accordance with another exemplary embodiment. Referring to  FIGS. 8 and 9 , the blank  400  includes the internal blank component  410  and the metal coating  420 , which is applied onto the surface of the internal blank component  410 . The coherent integral mass  710  is bonded to the blank  400  via the bonding zone  590  that is formed along and/or adjacent the surface of the blank  400 . According to some exemplary embodiments, the metal coating  420  is applied onto the internal blank component  410  such that a smaller portion of the iron from the blank  400  diffuses into the binder material  560 . The diffused iron reacts with the free tungsten within the tungsten carbide powder  530  and the tungsten powder  534  to form this bonding zone  590 . The bonding zone  590  includes intermetallic compounds  690 , which are similar to the intermetallic compounds  290  ( FIG. 2 ). According to  FIG. 8 , the bonding zone  590  is formed having a thickness  815  ranging from about five μm to less than sixty-five μm in the chamfered zone area  598  ( FIG. 5 ). In another exemplary embodiment, the bonding zone  590  is formed having a thickness  815  ranging from about five μm to less than fifty μm in the chamfered zone area  598  ( FIG. 5 ). In yet another exemplary embodiment, the bonding zone  590  is formed having a thickness  815  ranging from about five μm to less than thirty μm in the chamfered zone area  598  ( FIG. 5 ). According to  FIG. 9 , the bonding zone  590  is formed having a thickness  915  ranging from about two μm to less than about ten μm in the central zone area  599  ( FIG. 5 ). In another exemplary embodiment, the bonding zone  590  is formed having a thickness  915  ranging from about two μm to less than eight μm in the central zone area  599  ( FIG. 5 ). In yet another exemplary embodiment, the bonding zone  590  is formed having a thickness  915  ranging from about two μm to less than six μm in the central zone area  599  ( FIG. 5 ). The thicknesses  815 ,  915  and/or volumes of the bonding zone  590  are dependent upon the exposure time, the temperature, and the thickness of the metal coating  420  that is applied onto the internal blank component  410 . As previously mentioned, the metal coating  420  reduces the migration of iron from the blank  400  into the binder material  560 , thereby decreasing the reaction with the free tungsten within the shoulder powder  534  and the tungsten carbide powder  530  during the fabrication process. 
       FIG. 10  shows a cross-sectional view of a downhole tool casting assembly  1000  in accordance with another exemplary embodiment. Referring to  FIG. 10 , the downhole tool casting assembly  1000  includes a mold  1010 , a stalk  1020 , one or more nozzle displacements  1022 , a blank  1024 , a funnel  1040 , and a binder pot  1050 . The downhole tool casting assembly  1000  is used to fabricate a casting  1100  ( FIG. 11 ) of a downhole tool, such as a fixed cutter bit, a PDC drill bit, a natural diamond drill bit, and a TSP drill bit. However, the downhole tool casting assembly  1000  is modified in other exemplary embodiments to fabricate other downhole tools, such as a bi-center bit, a core bit, and a matrix bodied reamer and stabilizer. 
     The mold  1010  is similar to mold  510  and forms a mold volume  1014 , which is similar to mold volume  514 . Since mold  510  has been previously described above, the details of mold  1010  are not repeated again herein for the sake of brevity. The center stalk  1020  and the one or more nozzle displacements  1022  are similar to the center stalk  520  and the nozzle displacements  522 , respectively, and therefore the descriptions of each also are not repeated herein for the sake of brevity. Further, the blank  1024  used within the downhole tool casting assembly  1000  is similar to either the blank  124  ( FIG. 1 ) or the blank  400  ( FIG. 4 ) and therefore also is not repeated herein for the sake of brevity. 
     Once the displacements  1020 ,  1022  and the blank  1024  have been positioned within the mold  1010 , tungsten carbide powder  1030 , similar to tungsten carbide powder  530 , is loaded into the mold  1010  so that it fills a portion of the mold volume  1014  that is around the bottom portion  1026  of the blank  1024 , between the inner surfaces of the blank  1024  and the outer surfaces of the center stalk  1020 , and between the nozzle displacements  1022 . According to the exemplary embodiment shown in  FIG. 10 , this tungsten carbide powder  1030  is the same as tungsten carbide powder  530  described above and includes at least W 2 C and some free tungsten. The process of fabricating W 2 C generally involves the inclusion of free tungsten. However, in other exemplary embodiments as shown in  FIG. 12  for instance, this tungsten carbide powder  1030  is absent any free tungsten. Thus, the tungsten carbide powder  1030 , which is absent any free tungsten, includes only WC in some exemplary embodiments. Alternatively, the tungsten carbide powder  1030 , which is absent any free tungsten, includes W 2 C, WC, or a combination of both, while excluding any free tungsten. Thus, any free tungsten is removed either during or after the fabricating process before placing the tungsten carbide powder  1030  within the mold  1010 . 
     Shoulder powder  1034  is loaded on top of the tungsten carbide powder  1030  in an area located at both the area outside of the blank  1024  and the area between the blank  1024  and the center stalk  1020 . The shoulder powder  1034  is made of stainless steel powder or other known suitable material that is absent any free tungsten. Some examples of other suitable materials that is usable for the shoulder powder  1034  include other steel powders, nickel powder, cobalt powder, refractory transitional materials such as molybdenum powder and tantalum powder, and/or other metals that have a higher melting temperature than the binder alloy material  1060  but are soft enough to be machined. This shoulder powder  1034  acts to blend the casting to the blank  1024  and is machinable. Once the tungsten carbide powder  1030  and the shoulder powder  1034  are loaded into the mold  1010 , the mold  1010  is vibrated, in some exemplary embodiments, to improve the compaction of the tungsten carbide powder  1030  and the shoulder powder  1034 . Although the mold  1010  is vibrated after the tungsten carbide powder  1030  and the shoulder powder  1034  are loaded into the mold  1010 , the vibration of the mold  1010  is done as an intermediate step before, during, and/or after the shoulder powder  1034  is loaded on top of the tungsten carbide powder  1030 . Although tungsten carbide material  1030  is used in certain exemplary embodiments, other suitable materials known to persons having ordinary skill in the art are used in alternative exemplary embodiments. 
     The funnel  1040  is similar to funnel  540  and forms a funnel volume  1044  therein, which is similar to funnel volume  544 . Since funnel  540  has been previously described above, the details of funnel  1040  are not repeated again herein for the sake of brevity. Further, the binder pot  1050  is similar to binder pot  550  and forms a binder pot volume  1054  therein, which is similar to binder pot volume  554 , for holding a binder material  1060 , which is similar to binder material  560 . Since binder pot  550  and binder material  560  have been previously described above, the details of binder pot  1050  and binder material  1060  are not repeated again herein for the sake of brevity. Although one example has been provided for setting up the downhole tool casting assembly  1000 , other examples having greater, fewer, or different components are used to form the downhole tool casting assembly  1000 . For instance, the mold  1010  and the funnel  1040  are combined into a single component in some exemplary embodiments. 
     The downhole tool casting assembly  1000  is placed within a furnace (not shown) or other heating structure. The binder material  1060  melts and flows into the shoulder powder  1034  and the tungsten carbide powder  1030  through an opening  1058  of the binder pot  1050 . In the furnace, the molten binder material  1060  infiltrates the shoulder powder  1034  and the tungsten carbide powder  1030  to fill the interparticle space formed between adjacent particles of the shoulder powder  1034  and the tungsten carbide powder  1030 . During this process, a substantial amount of binder material  1060  is used so that it fills at least a substantial portion of the funnel volume  1044 . This excess binder material  1060  in the funnel volume  1044  supplies a downward force on the tungsten carbide powder  1030  and the shoulder powder  1034 . Once the binder material  1060  completely infiltrates the shoulder powder  1034  and the tungsten carbide powder  1030 , the downhole tool casting assembly  1000  is pulled from the furnace and is controllably cooled. Upon cooling, the binder material  1060  solidifies and cements the particles of shoulder powder  1034  and tungsten carbide powder  1030  together into a coherent integral mass  1110  ( FIG. 11 ). The binder material  1060  also bonds this coherent integral mass  1110  ( FIG. 11 ) to the blank  1024  thereby forming a bonding zone  1190  ( FIG. 11 ) therebetween. The coherent integral mass  1110  ( FIG. 11 ) and the blank  1024  collectively form the casting  1100  ( FIG. 11 ) or the matrix body bit  1100  ( FIG. 11 ), a portion of which is shown in  FIG. 11 . Once cooled, the mold  1010  is broken away from the casting  1100  ( FIG. 11 ). The casting  1100  ( FIG. 11 ) then undergoes finishing steps which are known to persons of ordinary skill in the art, including the addition of a threaded connection (not shown) to the casting  1100  ( FIG. 11 ). Although the casting  1100  ( FIG. 11 ), or the matrix body bit  1100  ( FIG. 11 ), has been described to be formed using the process and equipment described above, the process and/or the equipment can be varied to still form the matrix body bit  1100  ( FIG. 11 ). 
       FIG. 11  shows a partial cross-sectional view of a downhole tool casting  1100  formed using the downhole tool casting assembly  1000  of  FIG. 10  in accordance with the exemplary embodiment. Referring to  FIG. 11 , the downhole tool casting  1100  includes the coherent integral mass  1110 , the blank  1024 , and the passageways  1120  formed from the removal of the displacements  1020 ,  1022 . As mentioned above with respect to  FIG. 10 , the coherent integral mass  1110  is formed using the tungsten carbide material  1030 , as described above, and the shoulder powder  1034 , also as described above. According to the exemplary embodiment illustrated in  FIGS. 10 and 11 , the shoulder powder  1034  is absent of free tungsten material and the tungsten carbide material  1030  is the same as tungsten carbide powder  530  described above and includes at least W 2 C and some free tungsten. However, in other exemplary embodiments as shown in  FIG. 12  for instance, this tungsten carbide powder  1030  is absent any free tungsten. Thus, the tungsten carbide powder  1030 , which is absent any free tungsten, includes only WC in some exemplary embodiments. Alternatively, the tungsten carbide powder  1030 , which is absent any free tungsten, includes W 2 C, WC, or a combination of both, while excluding any free tungsten. 
     The intermetallic compounds are formed when iron reacts with free tungsten. According to one of the present exemplary embodiments, the typical shoulder powder  134  having free tungsten is replaced with shoulder powder  1034 , thereby reducing and/or eliminating the formation of these intermetallic compounds, which is very brittle. The shoulder powder  1034  occupies the area adjacent a chamfered portion  1198  of the blank  1024 , similar to chamfered portion  598  ( FIG. 5 ), which experiences high stresses. Thus, by reducing and/or eliminating these intermetallic compounds from that region, the casting or bit  1100  is more durable and has a greater longevity. According to alternative exemplary embodiments, a type of tungsten carbide powder  1030  which also is tungsten free may be used in place of the typical tungsten carbide powder  130 , which includes free tungsten. The tungsten carbide powder  1030  occupies the area adjacent a central zone area  1199  of the blank  1024 , similar to central zone area  599  ( FIG. 5 ), which also experiences high stresses. Thus, by reducing and/or eliminating these intermetallic compounds from that region, the casting or bit  1100  is more durable and has a greater longevity. According to the exemplary embodiments, either or both shoulder powder  1034  and tungsten carbide powder  1030  (which are tungsten free) may be used in lieu of the typical shoulder powder  134  and typical tungsten carbide powder  130 . 
       FIG. 12  shows a cross-sectional view of a downhole tool casting assembly  1200  in accordance with yet another exemplary embodiment. Referring to  FIG. 12 , the downhole tool casting assembly  1200  includes a mold  1210 , a stalk  1220 , one or more nozzle displacements  1222 , a blank  1224 , a funnel  1240 , and a binder pot  1250 . The downhole tool casting assembly  1200  is used to fabricate a casting  1300  ( FIG. 13 ) of a downhole tool, such as a fixed cutter bit, a PDC drill bit, a natural diamond drill bit, and a TSP drill bit. However, the downhole tool casting assembly  1200  is modified in other exemplary embodiments to fabricate other downhole tools, such as a bi-center bit, a core bit, and a matrix bodied reamer and stabilizer. 
     The mold  1210  is similar to mold  510  and forms a mold volume  1214 , which is similar to mold volume  514 . Since mold  510  has been previously described above, the details of mold  1210  are not repeated again herein for the sake of brevity. The center stalk  1220  and the one or more nozzle displacements  1222  are similar to the center stalk  520  and the nozzle displacements  522 , respectively, and therefore the descriptions of each also are not repeated herein for the sake of brevity. Further, the blank  1224  used within the downhole tool casting assembly  1200  is similar to either the blank  124  ( FIG. 1 ) or the blank  400  ( FIG. 4 ) and therefore also is not repeated herein for the sake of brevity. 
     Once the displacements  1220 ,  1222  and the blank  1224  have been positioned within the mold  1210 , tungsten carbide powder  1230  is loaded into the mold  1210  so that it fills a portion of the mold volume  1214  that is around the bottom portion  1226  of the blank  1224 , between the inner surfaces of the blank  1224  and the outer surfaces of the center stalk  1220 , and between the nozzle displacements  1222 . According to the exemplary embodiment shown in  FIG. 12 , this tungsten carbide powder  1230  is absent any free tungsten, and includes W 2 C, WC, or a combination of both, while excluding any free tungsten. In certain exemplary embodiments, the tungsten carbide powder  1230 , which is absent any free tungsten, includes only WC. 
     Shoulder powder  1234  is loaded on top of the tungsten carbide powder  1230  in an area located at both the area outside of the blank  1224  and the area between the blank  1224  and the center stalk  1220 . The shoulder powder  1234  is tungsten powder according to some exemplary embodiments; however, in other exemplary embodiments the shoulder powder  1234  is made of stainless steel powder or other known suitable material that is absent any free tungsten. Some examples of other suitable materials that is usable for the shoulder powder  1234  include other steel powders, nickel powder, cobalt powder, and/or other metals that have a higher melting temperature than the binder alloy material  1260  but are soft enough to be machined. This shoulder powder  1234  acts to blend the casting to the blank  1224  and is machinable. Once the tungsten carbide powder  1230  and the shoulder powder  1234  are loaded into the mold  1210 , the mold  1210  is vibrated, in some exemplary embodiments, to improve the compaction of the tungsten carbide powder  1230  and the shoulder powder  1234 . Although the mold  1210  is vibrated after the tungsten carbide powder  1230  and the shoulder powder  1234  are loaded into the mold  1210 , the vibration of the mold  1210  is done as an intermediate step before, during, and/or after the shoulder powder  1234  is loaded on top of the tungsten carbide powder  1230 . Although tungsten carbide material  1230  is used in certain exemplary embodiments, other suitable materials known to persons having ordinary skill in the art are used in alternative exemplary embodiments. 
     The funnel  1240  is similar to funnel  540  and forms a funnel volume  1244  therein, which is similar to funnel volume  544 . Since funnel  540  has been previously described above, the details of funnel  1240  are not repeated again herein for the sake of brevity. Further, the binder pot  1250  is similar to binder pot  550  and forms a binder pot volume  1254  therein, which is similar to binder pot volume  554 , for holding a binder material  1260 , which is similar to binder material  560 . Since binder pot  550  and binder material  560  have been previously described above, the details of binder pot  1250  and binder material  1260  are not repeated again herein for the sake of brevity. Although one example has been provided for setting up the downhole tool casting assembly  1200 , other examples having greater, fewer, or different components are used to form the downhole tool casting assembly  1200 . For instance, the mold  1210  and the funnel  1240  are combined into a single component in some exemplary embodiments. 
     The downhole tool casting assembly  1200  is placed within a furnace (not shown) or other heating structure. The binder material  1260  melts and flows into the shoulder powder  1234  and the tungsten carbide powder  1230  through an opening  1258  of the binder pot  1250 . In the furnace, the molten binder material  1260  infiltrates the shoulder powder  1234  and the tungsten carbide powder  1230  to fill the interparticle space formed between adjacent particles of the shoulder powder  1234  and the tungsten carbide powder  1230 . During this process, a substantial amount of binder material  1260  is used so that it fills at least a substantial portion of the funnel volume  1244 . This excess binder material  1260  in the funnel volume  1244  supplies a downward force on the tungsten carbide powder  1230  and the shoulder powder  1234 . Once the binder material  1260  completely infiltrates the shoulder powder  1234  and the tungsten carbide powder  1230 , the downhole tool casting assembly  1200  is pulled from the furnace and is controllably cooled. Upon cooling, the binder material  1260  solidifies and cements the particles of shoulder powder  1234  and tungsten carbide powder  1230  together into a coherent integral mass  1310  ( FIG. 13 ). The binder material  1260  also bonds this coherent integral mass  1310  ( FIG. 13 ) to the blank  1224  thereby forming a bonding zone  1390  ( FIG. 13 ) therebetween. The coherent integral mass  1310  ( FIG. 13 ) and the blank  1224  collectively form the casting  1300  ( FIG. 13 ) or the matrix body bit  1300  ( FIG. 13 ), a portion of which is shown in  FIG. 13 . Once cooled, the mold  1210  is broken away from the casting  1300  ( FIG. 13 ). The casting  1300  ( FIG. 13 ) then undergoes finishing steps which are known to persons of ordinary skill in the art, including the addition of a threaded connection (not shown) to the casting  1300  ( FIG. 13 ). Although the casting  1300  ( FIG. 13 ), or the matrix body bit  1300  ( FIG. 13 ), has been described to be formed using the process and equipment described above, the process and/or the equipment can be varied to still form the matrix body bit  1300  ( FIG. 13 ). 
       FIG. 13  shows a partial cross-sectional view of a downhole tool casting  1300  formed using the downhole tool casting assembly  1200  of  FIG. 12  in accordance with the exemplary embodiment. Referring to  FIG. 13 , the downhole tool casting  1300  includes the coherent integral mass  1310 , the blank  1224 , and the passageways  1320  formed from the removal of the displacements  1220 ,  1222 . As mentioned above with respect to  FIG. 12 , the coherent integral mass  1310  is formed using the tungsten carbide material  1230 , as described above, and the shoulder powder  1234 , also as described above. According to the exemplary embodiment illustrated in  FIGS. 12 and 13 , the shoulder powder  1234  includes tungsten powder and the tungsten carbide material  1030  is absent free tungsten and includes either WC, W 2 C, or a combination of both. However, in other exemplary embodiments as shown in  FIG. 12  for instance, this shoulder powder  1234  is absent any free tungsten. Thus, the shoulder powder  1234 , which is absent any free tungsten, includes stainless steel powder or any other suitable material described above. 
     The intermetallic compounds are formed when iron reacts with free tungsten. According to one of the present exemplary embodiments, the typical tungsten carbide powder  130  having free tungsten is replaced with tungsten carbide powder  1230  which is absent of free tungsten, thereby reducing and/or eliminating the formation of these intermetallic compounds, which is very brittle. The tungsten carbide powder  1230  occupies the area adjacent a central zone area  1399  of the blank  1024 , similar to central zone area  599  ( FIG. 5 ), which experiences high stresses. Thus, by reducing and/or eliminating these intermetallic compounds from that region, the casting or bit  1300  is more durable and has a greater longevity. According to alternative exemplary embodiments, the shoulder powder  1234  which is tungsten free, according to some exemplary embodiments, may be used in place of the typical shoulder powder  134 , which includes free tungsten. The shoulder powder  1234  occupies the area adjacent a chamfered portion  1398  of the blank  1224 , similar to chamfered portion  598  ( FIG. 5 ), which also experiences high stresses. Thus, by reducing and/or eliminating these intermetallic compounds from that region, the casting or bit  1300  is more durable and has a greater longevity. According to the exemplary embodiments, either or both shoulder powder  1234  and tungsten carbide powder  1230  (which are tungsten free) may be used in lieu of the typical shoulder powder  134  and typical tungsten carbide powder  130 . 
       FIG. 14  shows a cross-sectional view of a downhole tool casting assembly  1400  in accordance with yet another exemplary embodiment. The downhole casting assembly  1400  is similar to downhole casting assembly  1000  ( FIG. 10 ) and/or downhole casting assembly  1200  ( FIG. 12 ) except an intermediate layer  1438  is disposed between the shoulder powder  1434  and the tungsten carbide powder  1430 . The intermediate layer  1438  is meant to minimize stresses caused by thermal expansion according to some exemplary embodiments. The shoulder powder  1434  is similar to shoulder powder  1034 ,  1234  ( FIGS. 10 and 12 , respectively) and the tungsten carbide powder  1430  is similar to tungsten carbide powder  1030 ,  1230  ( FIGS. 10 and 12 , respectively). At least one of the shoulder powder  1434  and the tungsten carbide powder  1430  is absent of free tungsten. The intermediate layer  1438  is formed by including an amount of tungsten carbide powder  1430  that is used to the shoulder powder  1434  that is used thereby transitioning from the tungsten carbide powder  1430  to the shoulder powder  1434 . The amount of tungsten carbide powder  1430  that is included with the shoulder powder  1434  in the intermediate layer  1438  is about twenty percent to thirty percent by volume with respect to the shoulder powder  1434 . According to some other exemplary embodiments, the amount of tungsten carbide powder  1430  that is included in the intermediate layer  1438  is between ten percent and less than fifty percent by volume. According to certain exemplary embodiments, the composition of the intermediate layer  1438  gradually varies from the bottom of the intermediate layer  1438  to the top of the intermediate layer  1438 , where the composition at the bottom of the intermediate layer  1438  is close to the composition of the tungsten carbide powder  1430  and the composition at the top of the intermediate layer  1438  is close to the composition of the shoulder powder  1434 . This intermediate layer  1438  is harder than the areas where the shoulder powder  1434  is, but is still machinable according to certain exemplary embodiments. 
       FIG. 15  shows a partial cross-sectional view of a downhole tool casting  1500  formed using the downhole tool casting assembly  1400  of  FIG. 14  in accordance with the exemplary embodiment. The downhole tool casting  1500  is similar to downhole tool casting  1100  ( FIG. 11 ) and/or downhole tool casting  1300  ( FIG. 13 ) except an intermediate layer  1438  is disposed between the shoulder powder  1434  and the tungsten carbide powder  1430 , as described above. 
     Although the invention has been described with reference to specific embodiments, these descriptions are not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternative embodiments of the invention will become apparent to persons skilled in the art upon reference to the description of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. It is therefore, contemplated that the claims will cover any such modifications or embodiments that fall within the scope of the invention.