Abstract:
A fixture for etching PCD drill inserts is provided. The fixture design allows the fixture to be injection molded, significantly reducing costs and allowing the fixture to be disposed of after a single use. The fixture allows for faster use and more accurate etching of the PCD insert.

Description:
PRIORITY 
       [0001]    The present application is a continuation of U.S. application Ser. No. 13/030,776, filed Feb. 18, 2011, which is herein incorporated by reference in its entirety, and which claims the benefit of U.S. Provisional Application Ser. No. 61/306,347, filed Feb. 19, 2010, which is herein incorporated by reference in its entirety. 
     
    
     THE FIELD OF THE INVENTION 
       [0002]    The present invention relates to acid etching of polycrystalline diamond compacts inserts. More specifically, the present invention relates to a support fixture for the acid etching of polycrystalline diamond (PCD) inserts used in drill bits and industrial cutters. 
       BACKGROUND 
       [0003]    PCD inserts are used to form the cutting tips on underground drill bits, such as those used to drill oil and gas wells. Such inserts are cylindrical in nature, having a substrate which is typically sintered carbide and a layer of sintered polycrystalline diamond on an end of the cylinder. Multiple of such inserts are attached to drill bits as the PCD forms a durable cutting edge. 
         [0004]    One limitation in the use of PCD cutting tips is the solvent metal which occupies the interstitial spaces between the diamond crystals. The diamond accounts for about 85 to 95 percent of the PCD, and the remaining material is a metal which acts as a solvent for carbon and a catalyst for diamond formation while sintering the PCD. The fraction of solvent metal is sufficient to cause problems in using the resulting PCD cutting insert. One problem is that the solvent metal expands more with temperature than diamond, and can cause cracking of the PCD layer as the cutting insert is used. Another limitation is that the solvent metal, being a solvent for carbon during the formation of diamond crystals, also acts as a carbon solvent for the degradation of the diamond at elevated temperatures. As such, the solvent metal remaining in the PCD causes the diamond to convert into carbon dioxide, carbon monoxide, or graphite at temperatures near 700 degrees Celsius. 
         [0005]    As such, it is desirable to remove the solvent metal from the PCD cutting inserts before use. The solvent metal may be etched from the PCD using a mixture of strong acids, such as hydrofluoric and nitric acids (HF and HNO 3 ). U.S. Patent Publication 2007/0284152 discusses the use of PCD cutting inserts, the problems associated with the solvent metal remaining in the PCD, and the etching of the PCD in acid to remove the solvent metal. In removing the solvent metal from the sintered diamond with acid, it is necessary to protect the substrate from the acid, as it is not desirable to etch or erode the substrate. 
         [0006]    U.S. 2007/0284152 shows a fixture in FIG. 12 which is used to hold the PCD insert during etching and to protect the substrate from the acid. For discussion, the fixture is reproduced as Prior Art  FIG. 2 .  FIG. 1  shows a typical PCD cutter insert  10 . The insert  10  includes a substrate  14  and a PCD layer  18 . As discussed, the substrate  14  is typically sintered carbide, which is comprised of metal carbides sintered together by metals. The PCD layer  18  typically includes about 85 to 95 percent diamond crystals and the remainder an appropriate solvent catalyst metal. The insert  10  is typically about 0.5 inches in diameter and about 0.75 inches in length. To increase the useful life of the insert  10 , it is desirable to remove the solvent metal from between the diamond crystals. 
         [0007]      FIG. 2  shows a cross-sectional view of a prior art fixture  22  used to hold the insert  10  in order to acid etch the PCD layer  18  to remove the solvent metal from between the diamond crystals. The fixture  22  has a center bore  26  which receives in insert  10 , a hole  42  connecting the center bore through the back side of the fixture, and a groove  34  formed adjacent the front of the center bore. In use, the insert  10  is placed into the center bore  26  of the fixture  22 . Afterwards, an elastomeric o-ring  30  is placed into the O-ring groove  34  formed in the front part of the bore  26 . The insert  10  is then slid out of the bore  26  into the position shown, causing the o-ring  30  to seat on the diamond layer  18 . A rubber stopper  38  is then placed into the hole  42  formed in the back of the fixture  22 . The insert  10  is thus sealed into the fixture  22 , having only a portion of the diamond table  18  exposed for etching. Etching is accomplished by placing the fixture  22 , with the diamond table  18  facing downwardly, into a shallow bath of concentrated acid. The acid bath is kept at a desired temperature for a desired time period. Typically, the inserts  10  are etched for a period of 5 to 10 days in order to remove the solvent metal to a sufficient depth. 
         [0008]    There are several problems associated with the fixtures  22  of  FIG. 2 . One significant problem is the expense of the fixture  22 . The o-ring groove  34  must be machined into the fixture  22 , making the cost of the fixture about $4.00 each. Since the fixtures typically may be used only a few times, the cost per insert etched is high. Another problem with the fixtures  22  is the time required to load the insert  10  into the fixture. Multiple steps are required to load the insert  10 , install the o-ring, and set the insert at the proper depth. This increases the time required for assembly prior to etching, raising the cost of etching the insert  10 . 
         [0009]    Additionally, the O-ring  30  itself also presents a weakness in the design. Since the O-ring is elastomeric, it can be nicked or damaged while pushing the diamond table  18  through the o-ring during installation. Damage to the o-ring often results in a failed seal and thus an insert which is damaged during etching. Additionally, the O-ring  30  itself adds significant cost to the procedure, since the O-ring costs about $0.50, and is replaced after each use. Even using an O-ring  30  properly selected for the acids, such as a Viton® o-ring, the o-ring periodically fails while etching, resulting in a damaged part. Even if the o-ring  30  does not fail, it is typically softened by the acid and must be laboriously removed from the PCD insert  10  after etching. 
         [0010]    A final limitation of the fixture  22  is the inability to precisely delineate the etched and non-etched portions of the diamond layer  18 .  FIG. 3  illustrates an etched PCD insert  10   a.  The o-ring  30  and fixture  22  produce an irregular border between the non-etched diamond layer  18  and the etched portion of the diamond layer  18   a.  The irregular boundary between the etched and non-etched portions of the diamond layer  18  require conservative placement of the insert  10  in the fixture  22  so as to prevent etching of the substrate  14 . Additionally, an irregular boundary between etched and non-etched diamond layer  18  may result in damage to or failure of the insert  10  at the portions of the diamond layer  18  which still have solvent metal therein. 
         [0011]    There is thus a need for an improved fixture for etching PCD drilling inserts. There is a need for an etching fixture which is easier to use, more reliable, and less expensive than prior art fixtures. 
       SUMMARY OF THE INVENTION 
       [0012]    It is an object of the present invention to provide an improved fixture for etching PCD drilling inserts. 
         [0013]    According to one aspect of the invention, a fixture is provided which does not require the use of an o-ring seal. The fixture thus eliminates the various modes of o-ring failure which may occur, and eliminates the expense of the O-rings. The fixture also provides a sharp delineation between etched and non-etched diamond, allowing the diamond to be etched more consistently and allowing the diamond layer to be etched to a level closer to the substrate. 
         [0014]    According to another aspect of the invention, a fixture design is provided which may be injection molded rather than machined, significantly reducing the cost of the fixture. By reducing the cost of the fixture, the fixture may simply be discarded after use rather than cleaning the fixture for reuse. 
         [0015]    According to another aspect of the invention, a fixture is provided which creates a positive pressure therein when loaded. The positive pressure helps keep the acid from leaking into the fixture and provides an additional measure of safety in etching the PCD inserts. 
         [0016]    These and other aspects of the present invention are realized in a fixture for acid etching PCD drilling inserts as shown and described in the following figures and related description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    Various embodiments of the present invention are shown and described in reference to the numbered drawings wherein: 
           [0018]      FIG. 1  shows a perspective view of a known PCD drilling insert; 
           [0019]      FIG. 2  shows a partial cross-sectional view of a prior art etching fixture; 
           [0020]      FIG. 3  shows a side view of a PCD insert etched with the prior art fixture of  FIG. 2 ; 
           [0021]      FIG. 4  shows a perspective view of an etching fixture of the present invention; 
           [0022]      FIG. 5  shows cross-sectional view of the fixture of  FIG. 4 ; 
           [0023]      FIG. 6A  shows a detailed view of the indicated section of the fixture of  FIG. 5 ; 
           [0024]      FIG. 6B  shows another detailed view of the indicated section of the fixture of  FIG. 5 ; 
           [0025]      FIG. 7  shows a side view of the fixture of  FIG. 4 ; 
           [0026]      FIG. 8  shows a bottom view of the fixture of  FIG. 4 ; and 
           [0027]      FIG. 9  shows a cross-sectional view of the fixture of  FIG. 4 . 
       
    
    
       [0028]    It will be appreciated that the drawings are illustrative and not limiting of the scope of the invention which is defined by the appended claims. The embodiments shown accomplish various aspects and objects of the invention. It is appreciated that it is not possible to clearly show each element and aspect of the invention in a single figure, and as such, multiple figures are presented to separately illustrate the various details of the invention in greater clarity. Similarly, not every embodiment need accomplish all advantages of the present invention. 
       DETAILED DESCRIPTION 
       [0029]    The invention and accompanying drawings will now be discussed in reference to the numerals provided therein so as to enable one skilled in the art to practice the present invention. The drawings and descriptions are exemplary of various aspects of the invention and are not intended to narrow the scope of the appended claims. 
         [0030]    Turning now to  FIG. 4 , a perspective view of a fixture  46  of the present invention is shown. The fixture has a body  50  which is generally cylindrical, and has a bore  54  therethrough and a base  58  formed at the bottom thereof. The base  58  extends radially outwardly from the bottom of the body  50 . The bore  54  is sized to receive a PCD insert  10 . As there are different diameters of PCD inserts, different diameters of fixtures  46  are made. A plurality of feet  62  extend downwardly from the base  58 . The feet  62  elevate the base  58  and the face of the insert  10  which is being etched to raise these off of the bottom of the etching tank and allow for better circulation of the acid around the PCD insert. This improves the etching of the insert. 
         [0031]    Currently, the PCD inserts  10  are commonly 13, 16 or 19 millimeters in diameter. This application primarily discusses the 13 mm diameter insert as an example. The larger sizes of inserts  10  would use a correspondingly larger fixture  46 , with similar clearance or interference in the fit. The 13 millimeter insert may be casually referred to herein as a one half inch insert, since 13 mm is 0.512 inches in diameter. 
         [0032]      FIG. 5  shows a cross-sectional view of the fixture body  50 . As shown, the bore  54  may be made with two sections of different diameter. As shown, the top portion  54   a  of the bore (approximately the top half) has a diameter of 0.533 inches. The lower portion  54   b  of the bore (approximately the lower half) has a diameter of 0.525 inches. These diameters allow an insert  10  having a diameter of 0.512 inches to easily be placed within the fixture body  50  while keeping the insert aligned within the body. A small rib  66  is formed at the bottom of the bore  54 . The rib  66  seals against an insert  10  which is pressed through the top of the bore  54 , through the lower end of the bore  54  and past the rib  66  by a desired amount. 
         [0033]      FIG. 6A  and  FIG. 6B  show detailed views of the rib  66 . The rib  66  extends approximately 0.03 inches into the bore  54 , making the diameter of the bore  54  at the rib  66  approximately 0.47 inches. The rib thus forms an interference fit with a 0.512 inch diameter PCD drill insert. It is currently preferred to have a rib  66  which is between about 0.01 inches and 0.04 inches smaller in diameter than the insert. When an insert  10  is pressed into the body  50 , the rib  66  seals against the insert. As shown in  FIG. 6A , the rib  66  may have a radiused upper portion  66   a  which transitions into a lower sealing portion  66   b.  The upper portion and lower portion may both be between about 0.01 and 0.03 inches in height, and have a protrusion into the bore  54  as discussed. 
         [0034]    As shown in  FIG. 6B , the rib  66  may have an upper portion  66   c  which transitions from the bore  54  to a lower sealing portion  66   d.  The sealing portion  66   d  protrudes into the bore  54  as discussed above to create an interference fit between about 0.01 and 0.03 inches with the insert. The upper transition portion  66   c  and the lower sealing portion  66   d  are both between about 0.01 and 0.03 inches in height. The rib  66  may also have a smaller secondary rib  66   e  extending outwardly from the lower portion  66   d  and further into the bore  54 . The secondary rib  66   e  is typically between about 0.001 and 0.01 inches in both height and width (protrusion into the bore  54 ), and preferably may be about 0.003 inches in height and protrusion into the bore. 
         [0035]    The upper transition region  66   a,    66   c  helps the insert move smoothly past the rib  66  without causing damage. The lower sealing region  66   b,    66   d  presses against the insert to seal thereto. The secondary rib  66   e,  if used, provides a more easily deformable section of material to the sealing rib  66  and can improve the effectiveness and reliability of the sealing rib  66 . 
         [0036]    Different etching conditions such as time or temperature may affect the inner size of the rib  66 , requiring the rib to be larger or smaller in size. Thus, the interior diameter defined by the rib  66  may be a few hundredths of an inch larger or smaller. Typically, the same amount of interference is used between the rib  66  and a larger insert  10 , such as a 16 or 19 millimeter insert. That is to say that the difference in size between the inner diameter of the rib  66  and the outer diameter of the insert  10  would be approximately the same. Advantageously, the fixture  46  may be adapted to receive 16 or 19 millimeter diameter inserts by changing the diameter of the body  50  while leaving the diameter of the base  58  and location of the feet  62  the same. This allows the use of the same loading and processing equipment for different insert sizes. 
         [0037]      FIG. 7  shows a side view of the fixture body  50  with an insert  10  loaded therein. The insert  10  is placed into the top of the bore  54  and pressed downwardly past the rib  66 . A simple pressing jig can be made which contacts the bottom of the base  58  and which allows the insert  10  to move downwardly past the base  58  a predetermined distance before stopping the insert. This allows the insert  10  to be easily and repeatably loaded into the fixture body  50 . The prior art fixture  22  requires more time to load, requiring the insert  10  to be placed into the fixture, then the o-ring  30  to be placed into the groove  34 , and finally requiring the insert to be pressed past the O-ring into position. Thus, the fixture  46  achieves a significant time savings in loading the insert  10  as well as providing a much more accurate and repeatable loading and etching process. The improved accuracy and repeatability of loading and etching allows the diamond layer  18  to be etched closer to the substrate  14 . 
         [0038]      FIG. 8  shows a bottom view of the fixture body  50 , showing the placement of the feet  62 .  FIGS. 7 and 8  illustrate how the fixture body  50  keeps the diamond layer  18  off of the bottom of the etching reservoir, and allows better circulation of acid around the etched face of the diamond layer  18 . This allows for more consistent etching of the diamond layer  18 . 
         [0039]      FIG. 9  shows a cross-sectional view of the fixture  46  ready for etching. The fixture  46  has a PCD insert  10  loaded into the body  50 . After pressing the insert  10  into place, a cap  70  is pressed into the top of the bore  54 . The cap  70  extends downwardly into the bore approximately 0.2 inches. The cap  70  has a slight interference fit with the bore  54 , sealing against the bore  54  as it is pushed into place. As such, inserting the cap compresses the air in the bore  54  and causes a positive pressure to be formed inside of the bore  54 . This positive pressure helps to keep the etching acid out of the bore  54  while etching the insert  10 , further reducing the risk of leakage. 
         [0040]    The cap  70  extends outwardly beyond the body  50  and forms a lifting flange which makes it easier to move the fixtures  46  into and out of the acid reservoir. The fixture body  50  and cap  70  are preferably made from a plastic such as polypropylene, polyethylene, polyvinylidene fluoride, polytetraflouroethylene, and mixtures thereof. Other plastics that may also work could be Liquid Crystal Polymer (LCP) or PolyEtherKetone (PEK). A currently preferred material is C3350 TR polypropylene co-polymer. 
         [0041]    One significant advantage of the fixture  46  is that the boundary between etched and non-etched portions of the diamond layer  18  can be precisely controlled. The rib  66  forms a sharp delineation between etched and non-etched diamond compact. The precise control of the etching boundary allows the insert  10  to be mounted into the fixture  46  with a greater amount of the diamond layer  18  exposed, improving the temperature stability and useful life of the etched insert. 
         [0042]    Another significant advantage of the fixture  46  is the reduction of leaks during etching. The prior art fixtures  22  had a failure rate of between 2 and 5 percent. The present fixture  46  has a failure rate of less than one percent. The reduction of the failure rate is significant because of the cost associated with producing the inserts  10  and the time and cost of etching the inserts. 
         [0043]    Another significant advantage of the fixture  46  is the ease with which it is used. The fixture  46  may be loaded in much less time than the prior art fixture  22 . The fixture  46  may also be quickly unloaded and disposed of where the relatively expensive prior art fixture needed to be cleaned for reuse. Cleaning of the prior art fixture  22  and the produced insert  10  took significant time because the o-ring was damaged by the acid and became sticky and difficult to remove from the insert  10  and fixture  22 . 
         [0044]    Another advantage of the fixture  46  is that the design of the cap  70  and body  50  allow the fixture to be more easily moved into and out of the acid reservoir for etching, and also allow a closer spacing between adjacent fixtures in the etching reservoir. This allows more inserts  10  to be etched in a batch. This is advantageous as the batch time is quite long (typically between 5 and 10 days) and the etching acid is not reused. 
         [0045]    There is thus disclosed an improved etching fixture for PCD drill inserts. It will be appreciated that numerous changes may be made to the present invention without departing from the scope of the claims.