DRILL BIT MANUFACTURE

Abstract: There is described a process for the manufacture of a drill bit. The process comprises the steps of forming a cast metal blank having a substantially predetermined form and thereafter coating predetermined external portions of the previously cast blank with a hard-facing material.

Description: "Drill Bit Manufacture" This invention relates to a process for the manufacture of drill bits, and relates more particularly, but not exclusively, to a process for the manufacture of drill bits of the type used in the drilling of oil and gas wells . To date there have been three methods of forming bodies for such bits; the first method involves machining a steel body directly from a casting or forging, the second method involves forming a tungsten carbide matrix body around a pre-machined steel blank and the third method utilises a casting process to produce a bit with all of the geometric features peculiar to these types of products in an almost finished state. Each of the above methods results in a bit which has some but not all of the attributes desirable in an oilfield bit body. Some acknowledged advantages and shortcomings of each of the processes are as follows: Body Advantages Shortcomings Steel Low cost, Erosion resistance, (first Good structural Lack of design method) integrity. freedom. Matrix Good erosion resistance. Poor structural (second integrity, method) High cost. Cast Good structural Poor erosion (third integrity, resistance. method) Design freedom. In many cases the blank used in matrix bits takes an uncomplicated form such as a turned shaped in order to ease manufacturing difficulties and to reduce cost, but in such cases design freedom is restricted (eg holes must have a straight axis). In recent years, developments have taken place whereby complicated shaped blanks are machined (in similar fashion to the steel bodied product) prior to forming the matrix body. This combination is the preferred manufacturing process in some instances, and provides good structural integrity and good erosion resistance as advantages but lack of design freedom and high cost as disadvantages. The design freedom advantage of the cast method of manufacture arises from the ability to form heavily sculptured surfaces and complicated internal flow paths in order that the drilling mud may be used in as efficient a way as possible. This advantage is not gained by the previously described combination. According to the present invention there is provided a process for the manufacture of a drill bit, the process comprising the steps of forming a cast metal blank having a substantially predetermined form and thereafter coating predetermined external portions of the previously cast blank with a hard-facing material. The cast blank is preferably formed of a suitable steel. The blank is preferably cast by an investment casting process, with the consequent advantage of allowing external surfaces and internal conduits and ports to be almost any shape (including shapes which would be uneconomic or impossible to machine) with little or no subsequent machinery being necessary. For example, the blank can be cast with pre-formed nozzle ports and cutter blades . The hard-facing material is preferably tungsten carbide. Subsequent coating of the cast blank with tungsten carbide (or any other suitable hard-facing material) can be restricted to those parts of the exterior of the cast blank where hard-facing is required (eg the leading faces and the outer edges of the blades), while avoiding the coating of tungsten carbide on parts of the bit where hard-facing is not required (eg the rear (trailing) faces of the blades). Alternatively, the entire exterior surface of the bit may be coated with tungsten carbide, but preferably with a non-uniform thickness such that the thickest carbide coating goes on surface portions most requiring hard-facing and the thinnest carbide coating goes on surface portions requiring little or no hard-facing. Where the exterior of the pre-cast blank is selectively coated with tungsten carbide and/or any other suitable t* hard-facing material(s) by a procedure in which the blank is placed within a surrounding mould which is over-sized with respect to the uncoated cast blank by the intended thickness of tungsten carbide, followed by the incanting of a mixture of granular tungsten carbide and a suitable matrix material (eg brass), the mixture being previously or subsequently liquified by heat such as to flow into gaps between the blank and the mould where it is then solidified by cooling from liquefaction temperature towards ambient temperature, those parts of the blank which substantially touch the surrounding mould by reason of these parts being intended to remain substantially free of tungsten carbide (ie those parts intended to receive a zero thickness of carbide) are preferably provided with expansion compensation means interposed between these parts and the adjacent parts of the mould to obviate or mitigate excessive stresses induced by thermal expansion of the blank within the mould during the application of tungsten carbide to the blank. The expansion compensation means may be sheet paper. Embodiments of the invention will now be described by way of example with reference to the accompanying drawing, the sole figure of which is a diametral vertical section of a cast blank in a mould for the selective coating of the blank with tungsten carbide as part of the process for forming a drill bit. The first stage of manufacture of the drill bit involves the production of the cast blank 10 by the process of investment casting. The blank 10 is cast with desirable features built-in, eg nozzle ports and the general form of the bit body and the blades. However, it is preferred to avoid the incorporation of features which are more suitably produced by the subsequent matrix infiltration cycle, for instance the cutter pockets where bit body erosion is common or might be expected. As shown in the drawing, the investment-cast blank 10 is formed with a hollow interior 12 from which a conventional nozzle port 14 leads to the exterior of the blank 10, immediately in front of a cutter blade 16 (one of several cutter blades in the eventual drill bit) . The blank 10 is also formed with a lateral nozzle 18 which leads from the hollow interior 12 to the exterior of the blank 10 at the inboard end of the blade 16. Whereas the conventional nozzle port 14 has a straight axis such that in this and other aspects of its shape, the nozzle port 14 could (in theory, though not necessarily in this invention) be machined, the nozzle port 18 has a highly curved axis and could not be machined by any conventional boring tool. However, the fact that the nozzle port 18 is formed by investment casting (as is the whole of the blank 10) allows substantially complete design freedom and the ability to select the nozzle shape from considerations of functional effect rather than machinability. As shown in the drawing, the downstream (outer) end of the nozzle port 18 is directed to blow mud radially outwards along the leading face of the blade 16, which increases the ef ectiveness of debris clearance and reduces erosion in comparison to a conventional nozzle port (eg such as the nozzle port 14). The second stage of manufacture of the drill bit would involve the utilisation of the cast blank 10 in a matrix infiltration cycle. This process comprises the steps of forming a mould from graphite (or another suitable mould material), placing the blank in the mould, filling the empty spaces between the blank and the mould with tungsten carbide powder, arranging above the tungsten carbide powder material suitable as a binder such as brass for instance, and heating the assembly in order that the binder melts and flows into the interstices between the particles of tungsten carbide followed by a cooling stage solidifying the binder bonding the tungsten carbide particles together and to the blank. As shown in the drawing, the mould comprises a generally annular graphite base section 20 having a central cavity 22. The bottom of the cavity 22 is closed by a ceramic plate 24. The base section 20 is surmounted by a graphite ring 26 which acts as a funnel for the initially particulate and unconsolidated hard-facing material (if initially unmelted) or as a funnel for the hot liquid hard-facing material (if initially molten) . After the hard-facing material is placed in the mould, the mould is closed by a ceramic lid 28. The lower half of the cavity 22 contains a sand mould 30 having an upper surface whose shape, in conjunction with the shape of the inner surface of the remainder of the mould cavity 22 (ie the cavity surface not covered by the sand mould 30), is a three-dimensional negative of the intended final exterior surface of the eventual drill bit. The sand mould 30 also accommodates parts of an array of PDC cutter bits 32 such as to support the bits 32 until they are integrated with the drill bit by the matrix metal during the second stage of the drill bit manufacturing process. An array of gauge bits 34 is temporarily attached to the inner surface of the mould cavity 22 for incorporation into the finished drill bit as conventional gauge protectors . During the second stage of the drill bit manufacturing process when the hard-facing is being applied around the blank 10, the nozzle ports 14 and 18 are temporarily blocked to prevent the unwanted ingress of molten material. The nozzle port 14 is blocked by a sand core (not shown) which fills the nozzle port 14 and extends beyond it to contact the facing surface of the sand mould 30. The nozzle port 18 is blocked by a ceramic disc (not shown) which is plugged into the outer end of the nozzle port 18. (The ceramic of this disc will be selected to be unwetted by molten hard-facing material such that solidified hard-facing material does not bridge the outlet of the nozzle port 18). The mould would contain those features required by the overall design but not incorporated into the casting as mentioned above. The resulting bit body would then have the desirable elements of both processes as listed below without compromising any essential design features related to the performance of the product. Further, the judicious use of the expensive tungsten carbide powder by placing the erosion resistant material only where it is needed would not result in high cost as a shortcoming as indeed it is in traditional matrix products. Where the blank 10 touches the inner surface of the mould cavity 22 at locations not intended to be coated with tungsten carbide, excessive stressing of the mould section 20 due to thermal expansion of the blank 10 is avoided by placing expansion compensations between the blank 10 and the inner surface of the mould cavity 22 at these locations. The expansion compensations can be pieces of paper. The benefits of the invention are summarised below in the same analytical format as was used in the introduction to discuss the prior art. Body Advantages Shortcomings Cast/ Structural integrity Matrix. Design freedom, Erosion resistance. The method of the invention results in an erosion resistant drill bit body which incorporates advantages not available in previously proposed methods. The invention is not restricted to the preferred embodiment described above, and modifications of the described drill bit manufacturing process can be adopted without departing from the scope of the invention as defined in the appended claims.

Claims: CLAIMS 1. A process for the manufacture of a drill bit, the process comprising the steps of forming a cast metal blank having a substantially predetermined form and thereafter coating predetermined external portions of the previously cast blank with a hard-facing material. 2. A process as claimed in Claim 1, wherein the cast blank is formed of steel. 3. A process as claimed in Claim 1 or Claim 2, wherein the hard-facing material comprises tungsten carbide. 4. A process as claimed in any preceding claim, wherein the blank is cast by an investment casting process. 5. A process as claimed in Claim 4, wherein the blank as cast comprises at least one pre-formed nozzle port having a longitudinal axis which is curved. 6. A process as claimed in any preceding claim, wherein the predetermined external portions which are hard-faced are selected to substantially only those portions requiring hard-facing. 7. A process as claimed in Claim 6, wherein the exterior of the pre-cast blank is selectively coated with hard-facing material by a procedure in which the blank is placed within a surrounding mould which is over-sized with respect to the uncoated cast blank by the intended thickness of tungsten carbide, followed by the incanting of a mixture of granular hard-facing material and a suitable matrix material, the mixture being previously or subsequently liquified by heat such as to flow into gaps between the blank and the mould where it is then solidified by cooling from liquefaction temperature towards ambient temperature, those parts of the blank which substantially touch the surrounding mould by reason of these parts being intended to remain substantially free of tungsten carbide are provided with expansion compensation means interposed between these parts and the adjacent parts of the mould to obviate or mitigate excessive stresses induced by thermal expansion of the blank within the mould during the application of tungsten carbide to the blank. 8. A process as claimed in Claim 7, wherein the expansion compensation means comprises sheet paper. 9. A drill bit when manufactured by the process as claimed in any preceding claim.

Inventors: WARDLEY MICHAEL THOMAS, WARDLEY, MICHAEL, THOMAS
Applicants: BBL, WARDLEY MICHAEL THOMAS, BBL BRIT BIT LIMITED, WARDLEY, MICHAEL, THOMAS
Application year: 1996
Main classification: B22D19
IPC codes: B22D19, C23C30