Diamond cutter insert with a convex cutting surface

Abstract: A diamond drag bit has cylindrical polycrystalline diamond faced inserts with a convex cutting surface, the insert being imbedded in the cutting face of a drag bit. The inserts are of the type in which a polycrystalline diamond layer is adhered to a cemented tungsten carbide substrate to form a composite, and the composite is bonded to a support stud or cylinder. The diamond surface has a convex curvature radius optimized to the extent that the best shear action on the earthen formation is achieved. The convex curvature has a radius in the range of from five to six times the radius of the cylindrical substrate. The resultant positive and negative side rake angles assure that each insert remains free of detritus, presenting a clean cutting edge to the formation.

Description: Background The present invention relates to diamond drag bits having cylindrical polycrystalline diamond faced inserts imbedded in the cutting face of a drag bit. More particularly, the present invention relates to the optimization of the geometry of the cutting face of cutting elements, particularly of the type in which a diamond layer or other superhard material is adhered to a cemented carbide substrate to form a composite, and the composite is bonded to or integral with a support stud or cylinder. One type of cutting element used in rotary drilling operations in subterranean earth formations comprises an abrasive composite or compact mounted on a support cylinder or stud. The composite typically comprises a diamond layer adhered to a cemented carbide substrate, e.g., cemented tungsten carbide, containing a metal binder such as cobalt, and the substrate is brazed to the support cylinder or stud. Alternatively, the support cemented tungsten carbide cylinder may be integrally formed as part of the polycrystalline diamond substrate backing. Mounting of these cutting elements in a drilling bit is achieved by press fitting, brazing or otherwise securing the stud or cylinder into pre-drilled holes in the drill bit head. Fabrication of the composite is typically achieved by placing a cemented carbide cylinder into the working volume of a press. A mixture of diamond grains and a catalyst binder is placed atop the substrate and is compressed under ultra-high pressure and temperature conditions. In so doing, the metal binder migrates from the substrate and "sweeps" through the diamond grains to promote a sintering of the diamond grains. As a result, the diamond grains become bonded to each other to form a diamond layer and also bonded to the substrate along a planar interface. Metal binder (e.g. cobalt) remains disposed within the pores defined between the diamond grains. A composite formed in this manner may be subject to a number of shortcomings. For example, the coefficient of thermal expansion of the cemented tungsten carbide and diamond are somewhat close, but not exactly the same. Thus, the heating and cooling of the composite in the manufacturing process or during the work cycles the cutter undergoes in the drilling process, create significantly high cyclic tensile stresses at the boundary of the diamond layer and the tungsten carbide substrate. The magnitude of these stresses is a function of the disparity of the thermal expansion coefficients. These stresses are quite often of such magnitude to cause delamination of the diamond layer. This limitation has been greatly minimized by adding a transition layer of mixed diamond particles and pre-sintered tungsten carbide between the full diamond layer and the carbide substrate, as taught by U.S. Patent Numbers 4,525,178 and 4,604,106. Another shortcoming of state of the art diamond composite compact technology described above is the difficulty of producing a composite compact with any shape other than a flat planar diamond cutting layer that has low enough residual tensile stresses at the diamond/carbide interface that will permit its use as a drilling tool. Using the technology of the above described U.S. patents, it is relatively simple to produce diamond composite compacts with concave, convex or other non flat cutting surfaces. This allows much greater freedom of design of drag type diamond compact drilling bits that are fitted with diamond cutters having significantly greater impact strength and wear resistance. This technology is taught in U.S. Patent Number 4,858,707. Brief Summary of the Invention One object of the present invention is to modify the curvature geometry of the diamond cutting surface to significantly increase the drilling rate of the bit compared to the prior art. This curvature radius is optimized to the extent that, for a given range of rock strengths and types, the curvature gives the optimum back rake angle (negative rake angle) range to provide the best shear action on the rock considering the internal friction factor for that range of geological formations. A preferred diamond rock bit has one or more diamond inserts secured within a first end presenting a cutting face formed on a rock bit body. The body has a second end which is an open threaded pin end, a fluid chamber and one or more nozzle passages through the cutting face. Such a diamond insert comprises a diamond cutter end, an intermediate cylindrical body and a base end. The cutter end has a convex surface with a radius from five to six times the radius of the cylindrical body. The curved surface provides positive and negative side rake angles to deflect detritus from the curved diamond surface and to help cool and clean the diamond cutters while drilling an earthen formation. The curvature radius is optimized to the extent that, for a given range of rock strengths and types, the curvature gives the optimum back rake angle range to provide the best shear action on the rock formation. The idealized curvature of the diamond cutting surface provides both positive and negative side rake to promote removal of drilled cuttings or other detritus from the cutting face, thereby presenting a clean cutting edge to the formation. The curved side rake surfaces are constantly wiped clean providing for constant drilling fluid flushing the diamond cutting edge. This greatly aids in cooling the cutters below their thermal degradation limit. This permits less wear on the cutter and greater drilling life. Brief Description of the Drawings The above noted objects and advantages of the present invention will be more fully understood upon a study of the following description in conjunction with the detailed drawings wherein: FIGURE 1 is a perspective view of a diamond drag bit of the present invention; FIGURE 2 is a top view of the cutting head of the drag bit; FIGURES 3a and 3b depict a side view of a prior art diamond dome insert and a prior art diamond flat disc type insert, respectively; FIGURE 4 is a side view of a diamond insert of the present invention having a slightly convex diamond cutter disc with a disc cutter radius about five times the radius of the supporting stud body; FIGURE 5 is a top view of one of the cylindrical diamond inserts secured in a matrix forming the face of the drag bit; FIGURE 6 is a partial cross-section of a cylindrical diamond cutter illustrating the varying negative rake angle of the convex diamond face as the insert penetrates an earthen formation; FIGURE 7 is a chart indicating torque response of a dome vs. flat diamond cutter; FIGURE 8 is a chart comparing weight response of a flat vs. first and second generation diamond dome cutters; FIGURE 9 is a chart comparing RPM response of a flat vs. first and second generation diamond dome cutters; and FIGURE 10 is a cutter life chart comparing a flat vs. first and second generation diamond dome cutters. Description of the Preferred Embodiments FIGURE 1 illustrates a diamond drag rock bit generally designated as 10. The drag bit comprises a bit body 12, threaded pin end 14 and cutting end generally designated as 16. A pair of tool groove slots 13 on opposite sides of the bit body 12 provide a means to remove the bit from a drill string (not shown). At the cutting end 16 is formed a cutting face 18 that contains a multiplicity of diamond faced cylindrical studs generally designated as 20 extending therefrom. Such a diamond stud 20, for example, comprises a diamond disc 22, a cylindrical backing support segment 24 and a cylindrical stud body 26. The disc 22 is fabricated from a cemented tungsten carbide substrate with a polycrystalline diamond layer sintered to the face of the substrate. The diamond layer, for example, is formed with a convex surface having a radius between five and six times the radius of the stud body 26. The convex surface preferably forms a portion of a sphere with a radius about five times the radius of the stud body 26. FIGURE 2 illustrates the cutting end of the bit with the inserts 20 imbedded in, for example, a matrix of cemented tungsten carbide making up the head of the bit. Each of the inserts is strategically positioned in the face 18 of the bit. Formed in the cutting face of the bit is one or more fluid passages generally designated as 30. Each fluid passage communicates with a plenum chamber (not shown) formed within the bit body. A nozzle 34 is, for example, threaded into a nozzle opening 33 at the end of the fluid passage 30. Drilling fluid or "mud" is directed out of the nozzles 34 toward a borehole bottom 35 (Fig. 6) to clear detritus 37 from the bottom and to cool and clean each of the diamond inserts 20. The cutting face 18 additionally has raised ridges 40 that support insert protrusions 42. Each insert protrusion 42 partially encapsulates the base 26 of an insert. Each insert is positioned with the convex diamond disc 22 at a negative rake angle "A" with respect to the bottom of the borehole 35 (Fig. 6), that is, a negative rake angle relative to a plane transverse to the bit axis. Obviously, with a convex or spherically shaped disc 22, the deeper the diamond cutter penetrates the formation 35, the negative rake angle will change accordingly. The rake angle "A" will be less negative the deeper the penetration of the disc 22. Moreover, with reference to FIGURE 5, since the disc 22 is convex, detritus 37 is deflected away (angle "B") from the diamond cutting surfaces 23, hence flushing and cooling fluid is more readily able to maintain the integrity of the diamond during operation of the bit in a borehole. The prior art depicted in FIGURE 3a illustrates a typical diamond domed insert 50 with a cylindrical base 51 having a 12.7 mm (0.500 inch) diameter with a dome radius of 12.7 mm (0.500 inch). While the foregoing domed insert 50 has many attributes of the present invention, it does not have the penetration rate of the insert 20. The slightly convex surface of disc 22 more closely approximates the fast penetration rate of a flat diamond insert 54 illustrated in the prior art of FIGURE 3b. Referring now to the prior art shown in FIGURE 3b, the insert 54 has a cylindrical body 56 with a flat diamond disc 58 sintered to a tungsten carbide substrate cylinder 60 that is typically brazed to the body 56. The flat diamond insert 54 has been demonstrated to have an excellent penetration rate however, detritus build up in front of each disc 58 during bit operation in a borehole results in heat generation and ineffective cleaning and cooling that unfortunately equates to short bit life and early destruction of the diamond cutters 54. The diamond inserts 20 with a relatively large convex radius to the diamond cutting face 22 has the advantage of a fast penetration rate such as that demonstrated by the flat diamond cutter while retaining the detritus deflecting capabilities of the foregoing prior art dome cutter 50. The slightly domed insert 20 thus incorporates the best features of the prior art cutters 50 and 54 with none of the undesirable characteristics of either. Referring now to FIGURES 5 and 6, FIGURE 5 illustrates an insert 20 mounted in a raised protrusion 42 extending above a ridge 40. The cutting end 16 affixed to bit body 12 is preferably fabricated from a matrix of tungsten carbide 19 molded in a female die. The die, for example, forms insert pockets, raised protrusions 42, ridges 40, fluid passages 33, face 18, etc. Each insert 20 is partially encapsulated in the matrix 19 and is angled such that the end diamond disc 22 is at a negative rake angle "A" (Fig. 6). This angle "A" is between ten and twenty degrees with respect to a borehole bottom 35. The preferred rake angle is 20 degrees. The top of such an insert 20 (Fig. 5) with the slightly curved surface 23 provides both positive and negative side rake angles relative to a radial line and deflects debris away from an apex of the disc 22. This characteristic is indicated by angle "B". As heretofore described, detritus does not build up against the curved face 23 hence, the cutting face stays free of obstruction. The drilling rig mud or fluid easily cleans and cools each of the multiple diamond inserts affixed within the face of the cutting head. The side rake angle is relative to a radial line from the center of the bit. If one has a flat cutter face, a positive side rake angle is presented when the cutter face is skewed with the edge nearer the center of the bit trailing the edge nearer the periphery of the bit. Conversely, a negative rake angle is when the edge of a flat cutter face is skewed so that the edge of the cutter face nearer the periphery of the bit trails the edge of the cutter face nearer the center of the bit. The slightly convex curvature of the present insert means that there is positive rake toward the center of the bit, while at the same time there is negative rake toward the periphery. This permits detritus to move laterally in both directions. The double cleaning action is obtained without sacrificing the desirable negative rake in the longitudinal direction on the bit (Fig. 6) because of the small curvature. An advantage of the present invention over the prior art is that the rearwardly curved surfaces of the inserts perform as small individual bit stabilizers, reducing the tendency of the drag bit to drill off-center, gyrate or whirl. This substantially reduces the injurious vibrations common to prior art flat face cutter bits. Minimizing vibrations greatly reduces impact damage to the diamond cutter edges and faces, thereby measurably increasing the life expectancy of the bit. Moreover, the use of curved diamond surfaces shows a marked reduction in damaging torque variations when drilling broken or laminated formations. An advantage of importance in the present invention is maintaining or increasing the physical strength and wear resistance of the diamond cutters. This is provided by having optimum diamond surface curvature to provide high drilling rates, but concurrently putting the diamond layer in a high compressive stress which minimizes delamination, chipping or fracturing of the diamond layer. Referring now to FIGURE 7, the chart illustrates a reduction in torque when a domed insert (20 and 50) is utilized. The flat diamond inserts 54 tend to easily torque up and as a result, vibrate badly in a formation. With the dome insert 50 of the prior art, while it has less of a tendency to torque up and vibrate, bit penetration rate is far less than the flat faced prior art insert 54. This phenomenon is clearly shown in the weight response chart of FIGURE 8 and the RPM response chart of FIGURE 9. In FIGURE 8, the ROP (rate of penetration) is increased for the second generation domed insert 20 of the present invention over both the prior art dome insert 50 and the flat insert 54. As the WOB (weight on bit) increases, the bit penetration "tails off" for both the prior art dome and flat insert type bits. The chart of FIGURE 9 indicates as the RPM (revolutions per minute) increases, the ROP is better for the second generation insert 20 than the prior art flat insert 54 and much better than the first generation dome insert 50. Finally, the FIGURE 10 chart reveals the extended life of the insert 20 of the present invention over both the flat and dome inserts of the prior art. It will of course be realized that various modifications can be made in the design and operation of the present invention without departing from the spirit thereof. Thus, while the principal preferred construction and mode of operation of the invention have been explained in what is now considered to represent its best embodiments, which have been illustrated and described, it should be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated and described.

Claims: A diamond rock bit (10) having on or more diamond inserts (20) secured within a first end (16) presenting a cutting face (18) formed on a rock bit body (12), the body (12) further having a second end which is an open threaded pin end (14), a fluid chamber and one or more nozzle passages (30,33,34) through the cutting face (18), such a diamond insert (20) comprising a diamond cutter end, an intermediate cylindrical body (24,26) and a base end, and characterized by the cutter end having a convex surface (23) with a radius of curvature between five and six times the radius of the cylindrical body (24,26), the convex diamond cutter end providing optimum rock shearing ability, and positive and negative side rake angles (B) to deflect detritus (37) from the convex diamond face (23) and to help cool and clean the diamond cutters while drilling an earthen formation. A diamond rock bit as set forth in claim 1 wherein the convex surface (23) is a portion of a sphere atop a cylindrical substrate (22), the substrate being secured to the cylindrical body (24,26). A diamond rock bit as set forth in either of claims 1 or 2 wherein the cylindrical substrate (22) and the cylindrical body (24,26) are tungsten carbide. A diamond rock bit as set forth in any of the preceding claims wherein the diamond cutter end comprises polycrystalline diamond sintered to the substrate (22). A diamond rock bit as set forth in any of the preceding claims wherein the diamond insert (20) is secured within the cutting face (18) of the bit body (12) with a negative rake angle (A) with respect to a plane transverse to the bit axis. A diamond rock bit as set forth in claim 5 wherein the negative rake angle (A) with respect to a transverse plane is between ten and twenty degrees. A diamond rock bit as set forth in claim 5 wherein the negative rake angle (A) with respect to a transverse plane is 20 degrees.

Inventors: KNOWLTON HELEN R, KNOWLTON, HELEN R.
Applicants: SMITH INTERNATIONAL, SMITH INTERNATIONAL, INC.
Application year: 1992
Main classification: E21B10
IPC codes: E21B10