Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B24—GRINDING; POLISHING
  • B24B—MACHINES, DEVICES, OR PROCESSES FOR GRINDING OR POLISHING; DRESSING OR CONDITIONING OF ABRADING SURFACES; FEEDING OF GRINDING, POLISHING, OR LAPPING AGENTS
  • B24B1/00—Processes of grinding or polishing; Use of auxiliary equipment in connection with such processes
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B24—GRINDING; POLISHING
  • B24B—MACHINES, DEVICES, OR PROCESSES FOR GRINDING OR POLISHING; DRESSING OR CONDITIONING OF ABRADING SURFACES; FEEDING OF GRINDING, POLISHING, OR LAPPING AGENTS
  • B24B39/00—Burnishing machines or devices, i.e. requiring pressure members for compacting the surface zone; Accessories therefor
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B24—GRINDING; POLISHING
  • B24B—MACHINES, DEVICES, OR PROCESSES FOR GRINDING OR POLISHING; DRESSING OR CONDITIONING OF ABRADING SURFACES; FEEDING OF GRINDING, POLISHING, OR LAPPING AGENTS
  • B24B9/00—Machines or devices designed for grinding edges or bevels on work or for removing burrs; Accessories therefor
  • B24B9/02—Machines or devices designed for grinding edges or bevels on work or for removing burrs; Accessories therefor characterised by a special design with respect to properties of materials specific to articles to be ground
  • B24B9/06—Machines or devices designed for grinding edges or bevels on work or for removing burrs; Accessories therefor characterised by a special design with respect to properties of materials specific to articles to be ground of non-metallic inorganic material, e.g. stone, ceramics, porcelain
  • B24B9/16—Machines or devices designed for grinding edges or bevels on work or for removing burrs; Accessories therefor characterised by a special design with respect to properties of materials specific to articles to be ground of non-metallic inorganic material, e.g. stone, ceramics, porcelain of diamonds; of jewels or the like; Diamond grinders' dops; Dop holders or tongs

Definitions

• This invention relates to a method for rapidly polishing diamond which achieves a scratch-free surface. It is particularly useful for finishing polycrystalline diamond bearing surfaces.
• diamond is the hardest substance known and is therefore difficult to polish. Other materials can be polished with harder substances, but diamond is polished only with diamond in the form of j diamond powder or "grit". Polishing is necessarily slow “ " and a large amount of diamond abrasive powder is consumed. Polishing is desirable not only on natural gem stones but also on synthetic and polycrystalline diamond used for industrial purposes. For example, bearings have recently been devised with both bearing faces being made of polycrystalline diamond. The diamond faces are complemen- tary and are polished smooth to take advantage of the low coefficient of friction of diamond-on-diamond (in the order of 0.02).
• Polishing of a pair of complementary conical bearing surfaces can be considered as an example of the shortcomings of prior polishing techniques and an advantage of this invention.
• Each of the bearing surfaces comprised areas of polycrystalline diamond that collectively formed a conical surface, one external cone and one internal cone.
• the bearings were ground and polished by relative rotation with the bearing surfaces engaged, using diamond grit as an abrasive between the bearing surfaces. As much as 0.2 millimeters (0.005 inch) of material was removed from some areas of each diamond surface.
• Such polishing by conventional techniques took as long as three weeks for each bearing pair and about 25 carats of diamond grit was consumed for each 6.5 square centimeter (one square inch) of polished surface.
• Polycrystalline diamond is usually, but not necessarily, made from synthetic diamonds rather than natural diamonds.
• Synthetic diamonds are typically made by subjecting graphite to high temperature at a sufficiently high pressure that diamond is the thermodynamically stable crystal structure for carbon. Conversion of graphite to carbon is preferably conducted in the presence of a catalytic metal such as cobalt.
• Diamond can be synthesized at a pressure of 65 kilobars and 1500° Kelvin. Various other pressures and temperatures can be used as is well known to those skilled in the art. Depending on the operating parameters during synthesis of diamond various crystal sizes of diamond can be produced, Much of the diamond is in the form of individual crystals, although twins and other polycrystalline forms are not uncommon.
• Polycrystalline diamond for industrial purposes such as bearings can be made by subjecting a mass of diamond crystals to high temperatures and pressures for sintering the diamond and producing diamond-to-diamond bonds between crystals. Such sintering can be without catalyst as described by H. Tracy Hall in Science, Volume 169, August 28, 1970, pages 868. and 869.
• diamond can be sintered in the presence of a catalytic metal such as cobalt as described in U.S. Patent No. 3,141,746.
• a catalytic metal such as cobalt as described in U.S. Patent No. 3,141,746.
• Other techniques for forming polycrystalline diamond can also be practiced.
• a layer of polycrystalline diamond can be formed on a cemented carbide backing. Such product is desirable for bearings and ' for a variety of other industrial appli ⁇ cations where the strength and stiffness of the cemented carbide helps support the diamond layer.
• conventional polishing commences with relatively coarse hard grit which continually scratches the surface- of the material being polished until all of the scratches remaining on the surface are as small as can be made with that size grit. The next step is to polish with a smaller size grit until all of the larger scratches are removed and the only remaining scratches are the smallest that can be produced with this second size grit. This continues with successively smaller grit sizes until the desired degree of polishing is obtained.
• the largest scratches remaining in the surface are about half the size of the grit being used.
• the smallest practical grit size is about one micron.
• a typical well-polished diamond surface has one-half micron scratches.
• Grit refers to discrete particles of abrasive in a chosen size range used for polishing. For most purposes, the grit is harder than the material being polished. When diamond is being polished, the grit is diamond powder. Polishing grit is continually reduced in size during polishing and must be replenished as it is consumed. For most polishing the grit is "loose", that is, it is not attached to either surface and can tumble or slide in the polishing interface. During lapping the grit is pressed into the surface of the lap so as to be more or less held in place and caused to slide across the surface being polished. The particles become blunted as edges break or wear away and the polished surface has rounded scratches.
• Waviness is a periodic or aperiodic wave ⁇ like variation from a perfect surface which is generally much larger and wider than the roughness in the form of scratches left by grinding or polishing. Depending on the application of the product, waviness may be undesirable while minute scratches can be tolerated. For example in -. gauge blocks, the polished steel surface has little waviness but on a microscopic scale is scratched.
• Polished metallographic specimens may have moderate waviness but are commonly etched to provide a scratch- free surface.
• Diamond can be roughly cut by a technique known as bruting. This involves cutting a diamond by rubbing it with another diamond or diamond chip. There is considerable chipping of the diamond surface and a very rough surface is obtained, which is then polished by conventional grit polishing.
• a technique where the surface is polished by rubbing with a hard smooth object can be used for polishing some materials such as metals. This is often referred to as burnishing. For example, silver or leather can be burnished with a steel tool. In this form of burnishing there is plastic flow of the material being polished. Material is not ordinarily removed from the burnished surface, the surface is merely "rearranged". When done with clean tools, the surface may retain appreciable waviness but be largely scratch-free. Burnishing diamond has not been previously considered since there is nothing harder than diamond or with greater compressive strength, nor is diamond subject to plastic flow at anything approaching practical pressures.
• diamond can be burnished with a complementary diamond surface by rubbing the two surfaces together with a sufficient pressure and velocity to heat the surface being finished above the spontaneous thermal degradation temperature of diamond. Since no grit is present, a scratch-free polished diamond surface can be obtained.
• Frictional energy is applied to a surface being burnished sufficiently rapidly to heat the surface to a temperature where diamond is converted to non-diamond.
• the diamond is cooled and/or the interval of heating is sufficiently short to avoid degradation of any more than a minute surface depth.
• the surface can be intermittently cooled with water. In such an embodiment the most rapid burnishing is obtained when the diamond surface is heated just to the stage where nucleate boiling of the water occurs.
• FIG. 1 illustrates in longitudinal cross section an exemplary device for burnishing a set of conical diamond bearing members
• FIG. 2 illustrates in perspective such a conical diamond member
• FIG. 3 illustrates in perspective one member of a thrust bearing used in the device of FIG. 1;
• FIG. 4 illustrates semi-schematically in longitudinal cross section apparatus for burnishing a pair of conical bearing sets
• FIG. 5 is a fragmentary, semi-schematic illustration of apparatus for burnishing a flat diamond surface
• FIG. 6 is a plan view of the face of a burnishing wheel used in the apparatus of " FIG. 5;
• FIG. 7 is a fragmentary view of the face of an exemplary wheel for burnishing small diamonds
• FIG. 8 is a fragmentary cross section of the wheel of FIG. 7;
• FIG. 9 is an isometric semi-schematic view of an exemplary cylindrical diamond bearing which can be polished in practice of this invention.
• FIG. 10 illustrates schematically a technique for burnishing a cylindrical diamond bearing as illustrated in FIG. 7;
• FIG. 11 illustrates schematically a technique for burnishing the inside of a cylindrical diamond bearing
• FIG. 12 illustrates in longitudinal cross section another device for burnishing a set of conical diamond bearing members.
• Diamond is a somewhat unique material in that it has a crystal structure that is thermodynamically metastable at ordinary temperature and pressure. If diamond is heated to about 1400°K in inert gas it spontaneously recrystallizes or graphitizes. It may form amorphous carbon instead of the graphite crystal structure. In the presence of oxidizing substance such as air, diamond may thermally degrade at a lower temperature. In a polycrystalline diamond the presence of a catalytic metal in interstices of the diamond matrix may cause thermal degradation at a lower.temperature.
• Polycrystalline diamond in some embodiments such as may be produced in accordance with U.S. Patent No. 3,141,746 or by other processes, comprises a network of diamond crystals bonded to each other with some interstices containing a catalytic metal, ordinarily cobalt.
• the diamond constitutes 85 to 90% by volume and the cobalt phase the other 10 to 15%.
• Such a material may be subject to thermal degradation due to differential thermal expansion between the cobalt and diamond. Upon sufficient expansion the interdiamond bonding may be ruptured and cracks and chips may occur.
• This mechanism of thermal" degradation is postulated since it is known that such a material thermally degrades at a temperature lower than the thermal degradation of an other ⁇ wise identical material from which most of the cobalt phase has been leached away.
• the lower temperature thermal degradation may be in part a chemical interaction between the cobalt or carbon, instead of or in addition to degradation due to differential thermal expansion.
• cubic boron nitride Another material with properties analogous to those of diamond is cubic boron nitride.
• This material has a diamond-like crystal structure that is metastable at ambient temperature and pressure. This material is formed at high temperature and pressure from hexagonal boron nitride in a process analogous to formation of synthetic diamond. Cubic boron nitride thermally degrades upon heating at a temperature somewhat above the thermal degradation temperature of diamond. Cubic boron"nitride is extremely hard and in some applications can be substituted for diamond. Wurtzitic boron nitride is another material produced at high temperature and pressure that has a metastable crystal structure and high hardness. It too will thermally degrade upon heating at ambient pressures.
• Friction bearings where one material rubs against a similar or dissimilar material, with or without lubrication, have frictional heating at the bearing interface. Such heating is dependent upon the pressure at the bearing interface, relative velocity between the surfaces and coefficient of friction. Practical bearings have a limit on the pressure, which may be determined by the compressive strength of one of the materials making up the bearing interface. Practical bearings ordinarily have a velocity limit as well.
• a dry carbon-graphite bearing may have a limiting Pv of 15,000 foot pounds per square inch minute or 52.5 watts per square centimeter.
• Friction bearing with lubrication can have much higher Pv values depending on the service required.
• a well lubricated friction bearings of hard materials will run indefinitely with a Pv of 40,000.
• a sintered bronze bearing impregnated with lubricant may have.a limiting Pv of 50,000 for a reasonable useful life.
• the friction journal bearing of a cutter cone mounted on a rock bit may have a conventional high strength hard facing alloy as one bearing material and aluminum bronze as another bearing material.
• Such a bearing has sealed-in lubricant and under service conditions has a Pv of 200,000.
• an exemplary embodiment for burnishing diamond in practice of this invention employs a final Pv in excess of 1,000,000 or 3.5 kilowatts per square centimeter. Burnishing is also dependent on adequate cooling for avoiding subsurface thermal degradation, but too much cooling can completely prevent burnishing.
• the Pv actually used may be more or less, depending on the effectie rate of cooling. In an exemplary embodiment Pv may be as much as 10,000,00 or 35 kilowatts per square centimeter.
• burnishing of diamond be conducted at a Pv in the range of 2,000,000 to 10,000,000 foot pounds per square inch minute or 7 to 35 kilowatts per square centimeter.
• Pv is less than 7 kilowatts per square centimeter the rate of burnishing is low and to maintain a high rate, cooling water flow must be carefully controlled to avoid excessive thermal degrada ⁇ tion. It is easier to maintain a high rate of burnishing with a higher Pv.
• a Pv of 7 kilowatts per square centimeter is at least twice the Pv contemplated for bearing applications.
• the area of the surface layer being degraded depends on how well the complementary surfaces actually fit together.
• the complemen ⁇ tary surfaces may have slightly higher and lower areas due to manufacturing variations. The high areas will rub together during initial burnishing and a surface layer is degraded in such areas. As burnishing progresses these areas of contact become larger. With a given load or force between the surfaces, the pressure at the beginning of burnishing would be higher than at the end because the area of contact is smaller. Thus, the force applied is preferably increased as burnishing progresses to maintain a high rate of material removal.
• the area of the surace layer is very much larger than the crystals of diamond.
• crystal size in polycrystalline diamond can be provided over a broad range, one example may have diamonds about 60 microns across commingled with smaller crystals. Even near the beginning of j burnishing the areas being thermally degraded extend " for millimeters instead of microns.
• non-steady state or intermittent frictional heating may be most desirable for burnishing diamond or other superhard material.
• frictional heating may be applied at the surface being burnished with heat being extracted from the opposite face of the diamond.
• a temperature gradient is established through the diamond which is more or less linear, deviating from linearity to the extent that the coefficient of thermal conductivity varies as a function of temperature.
• non-steady state frictional heating which is preferred for burnishing diamond in practice of this invention, the temperature gradient adjacent to the surface being heated can be extremely high when the rate of energy input at the surface is high and an extremely thin layer at the surface thermally degrades. It is desirable to employ non-steady state frictional heating for ease of* assuring that thermal degradation beneath the surface does not occur.
• the discontinuous diamond surface can be provided by a plurality of separate areas of diamond spaced apart from each other, or grooves can be provided in the surface of the diamond being used for burnishing.
• a cooling medium such as water is circulated through the discontinuities of the diamond surface for extracting heat from the diamond being burnished.
• the rate of cooling influences the rate of burnishing. If there is too much cooling, there is essentially no burnishing. If there is inadequate cooling, excessive thermal degradation occurs. The most rapid burnishing occurs when the cooling rate is slightly more than the medium required to prevent massive thermal degradation.
• nucleate boiling When operating burnishing apparatus manually, such nucleate boiling is audible as the water vapor bubbles collapse. Conversely, if there is steaming of the cooling water, film boiling may occur with consequent damage to the diamond.
• Suitable manual control can be provided by . maintaining a given Pv and gradually reducing the rate of cooling water flow until nucleate boiling becomes audible. Flow is then increased slightly to provide a margin to prevent inadvertent damage to the diamond.
• Nucleate bo.i ⁇ ng can be detected as a "sizzle" in the apparatus. Thus, it is preferable to burnish diamond at or just below a sizzle.
• the pressure of the cooling water is another variable to be aware of and possibly use for control. Pressurizing the cooling water inhibits boiling and can be used to prevent film boiling which would significantly lower cooling rate and cause excessive thermal degradation. For example, a given Pv and water flow rate at atmospheric pressure may result in film boiling and damage to a rock bit bearing made of diamond. The same Pv and water flow may be adequate when the same bearing is in use for drilling a well since the hydrostatic head in the well prevents film boiling and assures adequate cooling to prevent thermal degradation. Thus, one could hold Pv and flow rate constant and control pressure of the cooling water to maintain nucleate boiing for rapid burnishing. It is believed that this is more difficult than controlling bearing pressure, velocity or coolant flow rate.
• polycrystalline diamond is preferred, namely diamond with a continuous network of diamond to diamond bonding.
• Such a material has the strength to resist dislodging of significant diamond from the surface. This is to be contrasted with a surface containing diamond made by infiltration techniques in which individual diamond crystals are set in a metal or metal carbide matrix. The bonding achieved is not sufficient to secure all of the diamonds in place under the high Pv conditions of burnishing.
• Polycrystalline diamond commonly includes other material in interstices of the diamond network.
• Cobalt or other catalyst, silicon or silicon carbide, and cemented tungsten carbide are examples of such materials.
• Such interstitial materials do not interfere with burnishing since much weaker and softer than the diamond.
• Exemplary polycrystalline diamond has from 70 to 95 percent by volume diamond and from 5 to 30 percent by volume non-diamond material in the interstices.
• Diamond sintered at high pressure and high temperature (in the diamond forming range) with less than 70 percent by volume diamond has extensive diamond-to-diamond bonding, and is hence considered polycrystalline. There may not, however, be a continuous network of intercrystalline bonding throughout such a body. It is preferred to employ polycrystalline diamond with a continuous network of diamond bonding for burnishing in practice of this invention.
• FIG. 1 illustrates in longitudinal cross section a device for manually burnishing a pair of conical diamond bearings such as may be used as a combined radial and thrust bearing for supporting a cutter cone on a rock bit.
• An exemplary concial bearing is illustrated in perspective in FIG. 2. Unless indicated otherwise, the parts of the burnishing device illustrated in FIG. 1 are round in transverse cross section.
• the two principal parts of the diamond burnishing device are a conical steel socket 10 and a conical steel plug 11.
• the socket When used, the socket is securely chucked in the head stock of a lathe for rotation.
• a bar (not shown) is inserted through a transverse hole 12 in the plug to prevent rotation of the plug.
• a conical bearing set comprising an inner sleeve 13 and an outer sleeve " 14 are fitted between the socket and plug for burnishing.
• a representative inner sleeve comprises a conical steel member supporting a plurality of circular bearing pads 16.
• Each bearing pad comprises a substrate of cemented tungsten carbide with a layer of polycrystalline diamond (not separately shown) on its exterior face.
• Each pad is curved to have a conical surface at both the interior and exterior of the sleeve.
• Each of the bearing pads is brazed into the sleeve with a silver solder having a melting point below the thermal degradation temperature of diamond.
• the cemented tungsten carbide faces are a continuation of the conical interior face of the cone; that is, on the interior of the sleeve, there is a smooth continuous conical surface.
• this interior conical surface seats on the journal of the leg of a rock bit. If necessary to obtain a good fit, the interior of the sleeve can be ground after brazing the bearing pads in place.
• each bearing pad extends slightly beyond the face of the steel sleeve.
• the face may protrude from the outer surface of the sleeve 0.4 to 0.8 millimeters.
• the resulting space between the inner and outer sleeves around the bearing pads provides a path for cooling fluid during service of the bearing and also for the cooling fluid during burnishing.
• the height of the pads above the steel surface is selected for assuring, adequate cooling during service of the bearing,
• the diamond layer on the surface of each bearing pad comprises polycrystalline diamond sintered in a high temperature, high pressure process.
• the surface layer comprises 90% by volume diamond and 10% by volume cobalt. Crystal size is in the range of 1 to 60 microns, primarily in the range of 40 to 60 microns.
• the diamond layer may be 0.75 millimeters thick before burnishing. As much as 0.1 to 0.15 milli-
• meters of diamond may be removed during burnishing.
• the outer sleeve 14 is similar in construction to the inner sleeve.
• the diamond bearing pads are, however, reversed, with the cemented carbide substrate being at the exterior of the cone and the diamond layer being inside.
• the exterior of the sleeve and inserts is ground to a smooth continuous conical surface.
• the interior face of each of the bearing pads in the outer sleeve extends above the inside face of the outer sleeve a small distance, e.g., 0.4 to 0.8 millimeters.
• the diamond faces of the outer bearing pads form a conical surface complementary to the conical surface formed by the outside faces of the bearing pads in the inner sleeve.
• the smaller diameter of the sleeves at the bearing interface between the inner and outer sleeves is 35.5 millimeters.
• the larger diameter of the conical bearing interface is 71 millimeters.
• the included half angle of the cone is 20°.
• Each of the bearing pads has a diameter of 12.7 millimeters.
• the pads are arranged in two circumferentially extending rows in each sleeve.
• a different number of bearing pads are employed in the corresponding inner and outer rows so that a substantial area of bearing interface always remains in contact in each row.
• the two rows in the inner sleeve may have six and nine inserts, respectively, while the complementary rows in the outer sleeve have eight and eleven bearing pads, respectively.
• Sleeves with other included angles, three rows of inserts, different numbers of inserts and different sizes are also usable.
• the inner sleeve 13 has a plurality of notches 17 around its larger end. Each notch is located midway between a pair of bearing pads in the larger row.
• An j equal number of pins 18 protrude from the plug 11 so that when fitted into the notches in the sleeve, the sleeve is prevented from rotating.
• the outer sleeve 14 has a plurality of notches (not shown) at its smaller end which engage a like number of pins 19 in the socket for preventing rotating of the outer sleeve. Similar pin arrangements are used on the leg journal and cone of a rock bit to hold the sleeves stationary.
• the plug 11 is connected to the socket 10 by a high strength bolt 21.
• the bolt extends through the axis of the plug and is threaded into the socket with a selected torque as hereinafter described.
• a lock nut 22 is threaded tight against the end of the socket to lock the bolt tightly in position, thereby preventing further tightening or loosening of the bolt.
• a number of heavy duty Belleville washers 23 are provided between the bolt head and the body of the plug to act as a spring.
• a thrust bearing 24 is mounted between the Belleville washers and the body of the plug.
• the thrust bearing comprises a pair of substantially similar rings, one of which is illustrated in FIG. 3.
• Each ring comprises a rigid steel washer 26 supporting a plurality of diamond bearing sectors 27.
• Each bearing sector comprises a substrate of cemented tungsten carbide and a surface layer of polished or burnished polycrystalline diamond. The tungsten carbide substrate is brazed to the steel washer so that the diamond faces of the sector are co- planar.
• the two bearing rings of a set differ only in the number of sect'ors in each ring.
• the ring illustrated in FIG. 3 has, for example, eight sectors.
• the ring used with that one to form a thrust bearing has nine sectors.
• the thrust bearing rings are fitted between the plug 11 and Belleville washers 23 with the diamond faces of the sector 27 forming a bearing interface.
• the spaces between adjacent sectors of diamond on the rings provide radial cooling fluid passage through the thrust bearing. -21- A plurality of cooling fluid passages 28 axrs drilled through the plug. In an exemplary embodiment, three or more passages may be used as required to obtain adequate flow of water.
• the passages convey water to an annular space 31 between the end of the plug and the base of the socket adjacent to the small end of the bearing sleeves.
• the water flows through the conical bearing interface to escape between the plug and socket.
• Water is also supplied into the annular clearance between the bolt and plug. This delivers water to the thrust bearing 24 for self- pumped flow through the spaces between the sectors 27 of the rings.
• the outer end of the plug can be closed and water forced through the annular clearance between the bolt and plug for delivery to both bearing interfaces.
• the device When it is desired to burnish a set of conical bearing sleeves, the device is assembled as illustrated in FIG. 1. Initially the bolt is tightened with a torque of 0.7 kilogram meters. Because of the pitch of the threads in the socket and the angle of the cone, this applies a normal force on the conical bearings of about 135 kilograms. The lock nut is tightened to prevent loosening. The socket end of the assembly is then chucked
• a bar is inserted through one of the holes 12 in the plug 11 to prevent the plug from rotating.
• the bar is connnected to a spring balance or the like so that the friction force or torque across the bearing interfaces can be determined.
• the lathe is started at a relatively low speed such as 100 RPM. Since there are surface asperities and other irregularities to be burnished away, the actual area of contact between the bearing pads is less than the total bearing area. Thus, even at the relatively low - loads and rotational speeds used at the beginning of burnishing, there are areas where the Pv is sufficiently high to thermally degrade some of the surface. It is observed that initially the bearing torque is high. After several seconds of running, the diamond surfaces are burnished to the extent that the local Pv is insuffi ⁇ cient for further thermal degradation. It is observed that the torque drops to a low value, for example, about one-third of the initial torque and remains steady indefinitely at that Pv. The initial noisy operation of the bearing also drops off rapidly.
• the speed of the lathe is increased to increase the Pv on the bearing.
• a similar cycle of increased torque followed by a decrease to a steady value is observed. This cycle is repeated until the desired maximum speed of the lathe is reached.
• the entire assembly is then removed from the lathe and the bolt is tightened another 0.7 kilogram meters.
• the assembly is put back in the lathe and the cycle of gradually increasing speeds repeated. This procedure is repeated until a desired maximum Pv on the bearing interface is achieved.
• the bolt may be tightened to 1 kilogram meters and the speed increased to 1000 RPM.
• the power required to rotate a set of conical bearing sleeves under such conditions is in the order of 6 kilowatts.
• Such a bearing is burnished at conditions at least twice as severe as expected in service. This assures that the bearing will last a long time in service with no noticeable degradation.
• the severity of service can be considered on the basis of three parameters, pressure P, velocity v, and cooling rate dQ/dt.
• the severity of service can be considered on the basis of three parameters, pressure P, velocity v, and cooling rate dQ/dt.
• Pv bearing is burnished with AQ / A *' . at least twice what is to be expected during normal service of the bearing. Cooling rate is a sensitive control parameter.
• Controlled cooling of the surface being burnished is important "for proper burnishing. There should be sufficient cooling that only a thin, uniform layer at the surface is thermally degraded. If there is inadequate cooling, thermal degradation below the surface actually roughens rather than smooths the surface. If there is too much cooling, the surface does not reach a sufficiently high temperature to thermally degrade and no burnishing occurs.
• Cooling is maintained in the range sufficient to elevate the temperature of a surface layer above the temperature where diamond spontaneously decomposes to non-diamond, and insufficient to heat the diamond below the surface layer to the thermal degradation temperature.
• such cooling is applied intermittently at the surface being polished. This occurs in the conical bearings because of the discontinuities in the complementary surfaces; that is, the recessed areas between the bearing pads. Water flows through these recesses and the bearing pads are alternately heated by friction on an opposing pad and quenched by water between the pads. This intermittent cooling inhibits thermal degradation of the diamond in both complementary surfaces.
• the burnishing can be accomplished by changing any of three variables separately or collectively. These are pressure P on the area being burnished, velocity v between the two surfaces, and cooling rate dQ/dt. One can, with a given pressure and cooling rate gradually increase velocity in a control-led manner. Each cycle of the burnishing technique in the device illustrated in FIG. 1 is exemplary. Alternatively, with constant cooling rate and velocity pressure can be gradually increased. Alternatively, although it is less precise, Pv can be held constant and cooling rate gradually decreased to effect burnishing.
• a burnished diamond is free of surface scratches. There are no discrete diamond particles or polishing grit in the interface being burnished, so there is nothing to scratch the surface.
• surface waviness may remain as an artifact of the original surface irregularities on the diamond pads. Such waviness is in circumferentially extending streaks since the same diamond areas of the inner and outer sleeves are continually rubbed together. Such waviness is acceptable in a conical bearing. In other applications it is desirable to minimize waviness, as well as eliminate scratches and by varying the area of the diamond in contact during burnishing, waviness can be reduced or largely eliminated.
• Burnishing is described herein as free from polishing grit since the grit is not needed, is expensive, and may actually reduce the rate of burnishing. Further, addition of grit in the final stages of burnishing could leave surface scratches or rounded grooves. Any diamond grit introduced would be rapidly thermally decomposed and comminuted. Any particles of material soften than diamond would be rapidly destroyed. Thus, burnishing of diamond is provided on a complementary diamond surface and temporary presence of grit is not material.
• FIG. 4 illustrates schematically apparatus for burnishing diamond where velocity is maintained constant and pressure is gradually increased. If desired, velocity can also be increased preferably in steps.
• conical bearing sleeves of the type illustrated in FIG. 2 are burnished two sets at a time.
• the apparatus has a rigid frame 36, the bed of which supports a lower fixed socket 37.
• An outer bearing sleeve 114 is fitted into the lower socket 37 and secured against rotation by pins (not shown) as described above with reference to FIG. 1.
• An inner bearing sleeve 113 is secured on the lower half of a double conical plug 38 by pins (not shown) as hereinabove described.
• the upper cone of the plug 38 is in an upper socket 39 and another bearing set comprising an inner sleeve 213 and outer sleeve 214 is mounted between the plug and upper socket.
• the bearing sleeves are also indicated schematically and include diamond bearing pads such as those illustrated in FIG - 2 -
• the double conical plug 38 is connected to an electric motor 41 by a shaft 42.
• the motor is mounted on a platen 43 connected to the frame by bearings 44 which permit the platen to move longitudinally on the frame.
• the platen is connected to the piston 46 of a hydraulic actuator 47 which is securely connected to the frame 36.
• the electric motor rotates the double conical plug and inner sleeves 113 and 213.
• the diamond pads on these sleeves rub against the diamond pads on the outer sleeves 114 and 214 respectively.
• the hydraulic actuator applies an axial load on the bearings for burnishing the diamonds.
• the torque required to rotate the two bearing sets is measured, preferably by simply measuring the motor power.
• the principal component of the torque is the friction force between the. bearings or Fv where F is the normal force on the bearings and v is the apparent coefficient of friction.
• the pressure applied by the hydraulic actuator is continually adjusted to maintain a torque somewhat above the equilibrium torque of the diamond bearings when fully burnished.
• the torque is measured and a conventional feedback circuit (hot shown) increases pressure as torque begins to drop. This maintains a high .
• This burnishing apparatus can be operated with constant motor speed and gradually increasing pressure or can be run with motor speed incrementally increasing with varying pressures to obtain a desired Pv. Cooling water is delivered to the lower bearing set through water passages (not shown) in the lower bed of the frame. Similarly water is delivered to the upper bearing set through passages in the moveable platen.
• Burnishing rate can also be controlled by varying cooling water flow to the two sets of bearings. This can be done separately or coordinated with changes in Pv.
• burnished conical diamond bearings have surfaces that are substantially free of scratches. Appreciable waviness may remain when viewed in an axial plane. Little if any, non-circumferential waviness remains.
• the diamond bearings form fully complementary surfaces of rotation with the waviness on the inner sleeve being complementary to waviness in the outer sleeve.
• the two sleeves of a bearing set are matched and are used together in service.
• bearings that are polished by conventional grit abrasive techniques. It appears that the axial waviness can be minimized by varying both pressure and rotational speed so that the elasticity of the assemblies permits different paths of the inner and outer sleeves to rub together at different stages of the burnishing.
• Waviness can also be minimized by partly burnishing one set of inner and outer sleeves then swapping one of the sleeves for additional burnishing. By doing this repeatedly the waviness of several sleeves is "averaged” and it is believed that universally usable inner and outer sleeves can be prepared without need for matched pairs.
• FIGS. 5 and 6 illustrate schematically apparatus for j burnishing a flat diamond face.
• the diamond to be burnished comprises a layer 51 of polycrystalline diamond bonded to a cemented tungsten carbide slug 52.
• the slug and diamond layer may have a diameter of about 12 millimeters and the diamond layer may have a thickness of about 0.5 to 1.0 millimeters.
• the slug 52 is secured in one arm 53 of a three or four legged spider 54. Similar slugs are mounted in other arms (not shown) of the spider so that the diamond layers are co-planer.
• the spider is secured to the piston 56 of a hydraulic ram (not shown).
• the burnishing medium is provided by a plurality of similar cylindrical carbide slugs 57 mounted in a rigid wheel 58 so that the diamond faces 59 on the slugs are co-planer.
• These diamond layers can be the same diameter as the diamond being polished, or preferably are of a different size, such as 9.5 millimeters.
• the diamond layers are placed as close together as possible on the face of the wheel to maximize the diamond area while still leaving a narrow flow path between the slugs for cooling water flow. Cooling water is readily provided by merely flooding the wheel inboard of the spiders so that the centrifical action of the rotating wheel carries the cooling water radially outward.
• the carbide slugs 57 are brazed into cylindrical sockets in the wheel.
• a vent hole 61 through the wheel can be used to admit a probe that assures that each diamond layer is in tight engagement with the flat surface.
• the hole also serves as a vent during furnace brazing. Irregularities in the resultant diamond surface can be removed by conventional diamond grinding and lapping or by burnishing as provided in practice of this invention.
• the diamond faces on the inserts in the wheel are essentially continually cooled as the wheel . rotates, except when in contact with the diamond being burnished. They therefore have an equilibrium temperature near that of the cooling water.
• the diamond layers in the spider are intermittently cooled by water in the passages between the diamond faces on the wheel.
• the temperature at the surface of the diamond being burnished can be substantially higher than the temperature of the diamond layers on the wheel. This results in thermal degradation of the diamond being burnished with little, if any, degradation o ' f the diamonds on the wheel.
• FIGS. 5 and 6 An arrangement as illustrated in FIGS. 5 and 6 can be used to minimize waviness in the diamond being burnished as well as substantially eliminating scratches. This is accomplished by moving the diamond being burnished radially relative to the wheel so that different paths on the wheel are traversed by the diamond. This is readily • accomplished by mounting the spider eccentrically relative to the wheel and gradually rotating or oscillating the spider. Alternatively, radial movement is readily accomplished in a four-arm spider by translating the spider parallel to one side of a square array of diamonds 1 mounted in the spider. This has the advantage o ' f making a small change in the angle that diamonds on the wheel rub across the diamond being burnished as well as shifting the path on the wheel traversed. This is, in effect, a
• the wheel 66 has
• each of which is in the form of a truncated radial sector of a circle.
• These diamond sectors are assembled in almost abutting relation to fill in annulus on the face of the wheel.
• the sectors each have a cemented
• the diamond layer has a plurality of radiating grooves 71 that may be about 0.3 to 0.4 millimeters wide.
• the lands 72 between the grooves may be in the order of 1 millimeter or more in circumferential width and collectively the lands form a plane surface.
• the small space between the adjacent sectors is filled with brazing alloy 73.
• This may also be grooved at the surface to provide a cooling channel analogous to the cooling grooves 71.
• Such a wheel is appropriate for burnishing diamonds with relatively small areas. A very stiff dop is used for pressing the diamond against the rapidly rotating wheel with enough pressure to burnish the diamond.
• FIG. 9 illustrates an exemplary cylindrical diamond bearing for carrying radial loads.
• a bearing has a steel shaft or sleeve 76 with curved diamond pads 77 on a cemented carbide substrate embedded therein to collectively form an external cylindrical surface. It is not ordinarily feasible to polish or. burnish such a surface in direct engagement with a mating bearing sleeve since wear of the two parts can result in too loose a fit.
• a suitable arrangement for polishing such a cylindrical bearing is illustrated schematically in FIG. 10. i
• the bearing sleeve 76 is rotated in one direction with its external face in engagement with a drum 78 of larger diameter.
• the external face of the drum has diamond pads (not shown) embedded therein much the same as the sleeve 76.
• the drum rotates in the opposite direction from the sleeve for achieving a high relative velocity at the engaging surface.
• Cooling water is introduced into the nip between the rotating sleeve and drum, to be carried through the interface where burnishing occurs.
• the drum is rotated at a higher peripheral speed than the sleeve and water is added at the top of the nip. Water is thereby swept between the sleeve and drum in the spaces between the diamond pads. It is desirable to burnish the diamond faces 77 on the sleeve with minimal wear of the diamond faces on the drum 78.
• One way to accomplish this is to heat the sleeve 76 internally while at the same time cooling the drum 78 internally.
• the higher equilibrium temperature of the diamond on the sleeve promotes greater thermal degradation of the faces being burnished than of the diamond faces on the burnishing drum.
• waviness of the surface being burnished can be minimized by shifting the drum axially relative to the sleeve so that burnishing of a given area on the sleeve is not continually by the same area on the drum.
• the drum can have an axial extent different from the length of the sleeve and can b.e moved axially along the sleeve for burnishing, much as one would use a grinding wheel to grind a long shaft.
• a pair of drums can be used on opposite sides of the sleeve for doubling the rate of burnishing and providing a backup to prevent bending of the sleeve under high transverse loads.
• FIG. 11 illustrates schematically a suitable arrangement for burnishing the diamond pads (not shown) inside a cylindrical bearing sleeve 81.
• a burnishing drum 82 can be provided inside the sleeve with diamond faced inserts for polishing the diamond pads inside the sleeve.
• the drum is mounted eccentrically with respect to the sleeve and is rotated in the opposite direction from the sleeve to obtain a high surface velocity. Cooling water is fed into the nip between the sleeve and drum at the advancing side of the rotating member having a higher speed and the retreating side of the member having the lower speed. For example, if the surface speed of the sleeve 81 is twice the surface speed of the drum 82, cooling water would be introduced at the top to be swept through the interface between the two members.
• burnishing of the diamonds in the sleeve is promoted by heating the sleeve and cooling the drum, thereby assuring that the degradation temperature is achieved at the surface of the diamonds in the sleeve without appreciable degradation of the diamonds on the drum.
• the drum and sleeve are preferably moved axially relative to each other for minimizing waviness. It can. also be desirable to employ a drum having a substantially shorter length than the sleeve so that a higher local Pv can be obtained with a given force between the sleeve and drum.
• FIG. 12 illustrates another embodiment of apparatus '-for burnishing a set of conical diamond beawrings as hereinabove described and illustrated.
• the apparatus is generally similar to the one described and illustrated in FIG. 1. Since many of the parts of the device are similar, the same reference -numerals are used ion FIG. 12 to refer to the same parts as in FIG. 1, except that each reference numeral has been increased by 100. Thus, for example, diamond bearing pads identified as 16 in FIG. 1 are identified as 116 in FIG. 12. Since most of the parts are substantially identical, only those parts that are different are specifically described.
• FIG. 12 emnploys a hydraulic actuator which enhances the speed and accuracy of application of varying load on the bearings.
• the bolt 121 is somewhat longer than in the embodiment illustrated in FIG. 1 and instead of having a head it is threaded at its outer end for receiving a nut 186.
• a hollow hydraulic actuator 187 is positioned between the nut 186 and the plug 111 which supports the inner bearing sleeve 113. The head of the hollow piston 188 of the actuator bears against the nut so that the bolt is placed in tension, thereby pulling the plug 111 and socket 110 towards each other and loading the bearings being burnished.
• a conventional hydraulic actuator of the type employed in this device includes a built in linear variable differential transformer to measure the displacement of the piston. This displacement measure is usefully employed in practice of this invention.
• the socket is chucked in a lathe and the plug held stationary against rotation as hereinabove described.
• the lathe is run at a selected speed setting such as, for example, 1200 RPM. Cooling water is applied to the bearings using a positive displacement pump that can deliver water at a carefully controlled and adjustable rate.
• the current on the lathe motor is measured. which provides a measure of the torque resisting rotation of the bearings.
• Pressure on the hydraulic cylinder can be varied for applying a controlled load on the bearings. Displacement of the piston of the hydraulic cylinder can be measured for determining the rate of burnishing of the bearing.
• the coefficient of friction is higher than whjjfen there is diamond-on- diamond contact.
• the resisting torque is increased to a level that provides rapid burnishing but is less than would cause excessive thermal degradation to cause seizure of the bearings.
• An acceptable rate of displacement can be determined experimentally to be sufficient for rapid burnishing without seizing.
• the lathe In one mode of operation the lathe is set at a given speed and a selected load is applied on the bearings by way of the hydraulic actuator.
• the flow rate of water is controlled electronically in response to current variations to maintain a torque level known to provide rapid burnishing.
• the burnishing can also be monitored by way of the displacement occurring as material is removed from the diamond interfaces. When the total displacement is sufficient to indicate that burnishing is completed to the desired extent, the apparatus is shut down. Burnishing can be discontinued by decreasing paressure, decreasing speed, or by increasing water flow.
• lathe speed is constant and water flow is set at a sufficiently low rate to permit burnishing. Motor current is monitored and the pressure on the hydraulic actuator is varied in response to variations in current to maintain a desired rate of burnishing.
• speed can be kept constant and both water flow and pressure varied to maintain a desired high rate of burnishing. These can be continually varied or can be varied in predetermined increments. The variations can be simultaneous, independent, or alternating. It should be recognized that rotational speed can also be varied, but pressure and water flow are the preferable control variables.
• the conical bearing pads 116 and thrust bearings 124 may be burnished at the same time and the resultant set of bearings can then be used in one embodiment of rock bit. burnished, some of the burnishing occurs as the conical bearings are burnished. Adequate burnishing of the thrust bearing may not be achieved, however, with simul ⁇ taneous burnishing since the loads and speeds are different on the two sets of bearings. Differential burnishing of the two types of bearings can readily be provided by isolating cooling water flow to the two bearing sets and introducing different water flow rates for preferential burnishing.
• a relatively high flow rate of water can be applied to the conical bearings while at the same time a low flow rate is applied to the thrust bearings.
• the high flow of water prevents further burnishing of the conical bearing while thermal degradation is promoted in the thrust bearing.
• a diamond-on- diamond bearing can be burnished in the apparatus in which the bearing is to be used in service.
• the bearings of a rock bit can be burnished by assembling the rock bit and rotating the cone or
• Diamond can be used to.burnish cubic boron nitride (CON) when the diamond is kept below its thermal degradation temperature. Diamond has a lower thermal degradation temperature than CBN and a temperature differential can be aintatined to assure that the CBN decomposes preferentially. More '-. surprisingly, it appears diamond may be "burnished" by the softer CBN when an appropriate temperature differential is maintained. Care should be taken in such embodiments to avoid bearing pressures that would unduly deform CBN which has a lower modulus of elasticity than diamond. Burnishing CBN with a complementary CBN surface is similar to burnishing diamond on diamond.
• thermal degradation believed to predominate is recrystallization of diamond to non-diamond carbon. It is known, however, that oxidizing substances promote thermal degradation at a lower temperature and it may be that some oxidation is also occurriong during burnishing. Thus, inclusion of materials with a higher oxidation potential than air may promote more rapid burnishing. Examples would include peroxide, permangate and perchlorate solutions, high oxygen or ozone concen ⁇ trations in the burnishing environment, or the like.

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• 1985
  • 1985-06-24 EP EP19850903543 patent/EP0227651A1/de not_active Withdrawn
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