Asymmetric three-cone rock bit

Aug. 20, 1968

D. R RE1CHMUTH

3,397,751

ASYMMETRIC THREE-CONE ROCK BIT

Sheets-Sheet 1

Filed March 2, 1966

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BY

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Aug. 20, 1968

3,397,751

D. R REICHMUTH

ASYMMETRIC THREE-CONE ROCK BIT

Filed March 2, 1966

Sheets-Sheet 2

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INVENTOR.

OpMALO &amp; /Pe-iCHMUEH

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Aug. 20, 1968

D. R. REICHMUTH

3,397,751

ASYMMETRIC THREE-CONE ROCK BIT

Filed March 2, 1966 3 Sheets-Sheet 3

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<img file="US3397751A_D0007.tif" />

BY

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3,397,751

Patented Aug. 20, 1968

United States Patent Office

3,397,751

ASYMMETRIC THREE-CONE ROCK BIT

Donald R. Reichmuth, Austin, Tex., assignor to Continental Oil Company, Ponca City, Okla., a corporation of Delaware

Filed Mar. 2,1966, Ser. No. 531,301

Claims. (Cl. 175—341)

ABSTRACT OF THE DISCLOSURE

A three-cone rock drill bit having one roller cone of smaller diameter than the other two roller cones to provide asymmetric positioning of the roller cones around the vertical axis, the axis of rotation of each cone being inclined to form the same angle with the vertical axis and pass through a common point on the vertical axis, the teeth of the smaller cone interfitting with each of the two larger cones, and a large return port for drilling fluid between the two larger roller cones.

Brief summary

This invention involves a three-cone rock bit wherein one small cone and two larger cones are asymmetrically positioned around the vertical axis to decrease vibrational tendencies characteristic of symmetrical bits. The cones are inclined at a common angle with the vertical axis so that the attack angle of the cone teeth on the bottom is optimized. The teeth of the smaller cone interfit with the teeth of each of the larger cones to improve cleaning of the teeth. A large return port for drilling fluids is provided between the two larger cones to minimize plugging of the drill bit by solids being carried out of the hole in the drilling fluid.

This invention relates to roller cone type drilling bits utilized for drilling oil and gas wells. More particularly, the present invention relates to a roller cone rock bit having three roller cones positioned asymmetrically from each other around the vertical axis of the drill string.

In the course of the drilling of oil and gas wells, a drill string is frequently rotated at a rate such that vibrations of the string tend to be reinforced by periodic contact of the drill bit with obstructions or irregularities on the bottom of the hole being drilled so that the vibrations of the string build up and the drilling apparatus becomes damaged as a result of excessive vibration and mechanical shock.

Another problem which is encountered in drilling operations using roller cone type rock bits is the propensity of the nozzles and ports provided in the drill head for circulating the drilling fluid therethrough to become plugged with cuttings or foreign objects carried or picked up by the drilling fluid. Frequently also, the ports provided for injecting the drilling fluid from the drill string into the bottom of the hole around the drill bits extend at converging angles with respect to each other so that the drilling fluid discharged therefrom tends to flow through mutually interfering paths and the efficiency of the circulation of the drilling fluid is impaired.

The present invention comprises an improved roller cone type rock bit in which the three cones of the bit are set asymmetrically with respect to each other around the vertical axis of the drill string. Stated differently, each of the roller cones is mounted on three downwardly projecting legs of a bit head with the cones mounted for rotation about their axes of symmetry, and with the rotational and symmetrical axes of the cones extending through a common point on the vertical axis of the drill stem. The rotational axes are further oriented with respect to each other so that the distance between two of the rotational axes as measured in a plane exending at a right angle to the vertical axis of the drill string is greater than the distance between these two rotational axes and the rotational axis of the third cone as measured in the same plane. This arrangement prevents the development during drilling, as a result of the bit encountering a fixed obstruction or vibration-creating protuberance in the bottom of the drill hole, of certain harmonics of the fundamental frequency at which the drill string will vibrate, so that vibrations in the drill string are not built up by resonance to an undesirable extent, and damage of the equipment as a result of excessive vibration is avoided.

The asymmetric arrangement of the three cones on the bit head also permits a more efficient construction of the drilling fluid nozzles and return ports in the bit head so that cuttings may be more efficiently removed from the bottom of the hole, and plugging or obstruction of the nozzles and cuttings return ports occurs less frequently.

To more specifically describe one exemplary embodiment of the invention, the bit is utilized in combination with a vertically extending drill string having a vertical axis of rotation, and comprises a bit head located at one end of the drill string and formed about the vertical axis of the drill string, three legs projecting downwardly from the bit head, and a generally conically shaped roller cone journaled on each of the legs for rotation about the axis of symmetry of the cone, with the three roller cones being positioned asymmetrically about the vertical axis of the bit head and the drill string. The cones have their apices converging toward the vertical axis, and their rotational axes pass at equal angles through a common point on the vertical axis of the bit head and drill string, and also pass through three points, at least two of which are unequally spaced from the third point, which three points lie in a common plane extending normal to the vertical axis.

Each of the roller cones carries cutter teeth which project from the periphery of the roller cone and are aligned in a series of planes extending normal to the rotational axis of the cone. These planes containing the cutter teeth are spaced axially from each other along the cone, and each of the cutter teeth have a cutting edge or tip lying in one of the series of planes which extends normal to the rotational axis of the cone. All of the cutting edges of the cutter teeth also lie in an imaginary conical surface extending parallel to the surface of the roller cone on which the respective cutter teeth are mounted. To describe more specifically this latter aspect of the embodiment of the invention under discussion, each of the cutter teeth have a wedged-shaped tip defined by two converging sides, and each mounted on the cone so that the lines of intersection of the two converging sides of all of the teeth on the respective cone lie in a common plane during rotation of the roller cone about its axis of rotation.

The asymmetric positioning of the roller cones about the vertical axis of the bit head and drill string permits a portion of the bit head between the two cones which are most widely spaced with respect to each other to be cut away to provide a large return port for permitting drilling fluid and the cuttings removed thereby to be circulated up the hole after the hole bottom has been swept out by the drilling fluid. A pair of jet nozzles or injection ports are extended through the bit head at points disposed between the third roller cone and the two cones which include between them the return port so that, in the operation of the drill bit, drilling fluid injected through the two nozzles at one side of the bit head passes across the bottom of the hole and returns upwardly on the other side of the hole to the return port located on the opposite side of the bit head from the injection nozzles. In this way,

3,397,751 the drilling fluid is permitted to more efficiently sweep the bottom of the hole, and a smoother flow pattern for the fluid is provided than has heretofore been accomplished with conventional three-cone rock bit designs.

From the foregoing description of the invention, it will have become apparent that an important object of the invention is to provide an improved roller cone type rock bit which effectively reduces the vibrational stresses set up in the drill string during normal drilling operation, and which permits a greater variety of rotational speeds to be imparted to the drill bit without concern for inducing unacceptably high vibrational stresses in the drill string.

Another object of the present invention is to provide a three-cone rock bit which includes a novel arrangement of drilling fluid injection nozzles and fluid return ports which permits a more effective sweeping of the hole bottom with drilling fluid to be accomplished.

An additional object of the present invention is to provide a three-cone rock bit having a longer tooth life than conventional bits.

A further object of the invention is to provide a threecone rock bit which permits the gauge teeth of the roller cones employed to be especially effective in their cutting action.

Another object of the invention is to provide a threecone rock bit which is characterized in having interfitting cutter teeth which clean each other during the operation of the bit.

An additional object of the invention is to provide a three-cone rock bit in which most vibrational frequencies which are resonant harmonics of the fundamental frequency of vibration of the drill string, and which are caused by contact of the roller cones with obstructions or protuberances in the hole bottom, have been eliminated by the novel bit design.

In addition to the foregoing described objects and advantages, other objects will become apparent as the following detailed description is read in conjunction with the accompanying drawings which illustrate the invention.

In the drawings:

FIGURE 1 is a partially schematic horizontal sectional view taken through the bit head upon which the three cones of the bit of the present invention are mounted, and showing the asymmetric disposition of the several cones relative to each other on the bit head.

FIGURE 2 is a sectional view taken along line 2—2 of FIGURE 1 with the section taken, for purposes of illustration, to show the two rollers X and Y as having their axes parallel to the plane of the paper, rather than in their true angular relation, as illustrated in FIGURE 1.

FIGURE 3 is a folded-out sectional view of the pyramid A, B, C, D in FIGURE 1 illustrating the manner in which the cutter teeth of the several roller cones overlap or interfit during the operation of the bit.

FIGURE 4 is a partially schematic horizontal sectional view similar to FIGURE 1 illustrating a modified embodiment of the three-cone rock bit of the invention.

FIGURE 5 is a sectional view similar to that shown in FIGURE 2, but illustrating the appearance of the modified embodiment depicted in FIGURE 4.

FIGURE 6 is a sectional view similar to that shown in FIGURE 3, but portraying the folded-out view of the pyramid defined by lines E, F, G and H in FIGURE 4.

Referring initially to FIGURES 1 and 2 of the drawings, the illustrated embodiment of the three-cone rock bit of the invention includes a bit head indicated generally by reference character 10 which includes a hollow, externally threaded shank portion 12 at the upper end thereof and three horizontally spaced, downwardly depending leg portions 14. The shank portion 12 is adapted to be threadedly engaged with the lower end of a drill string 16 in accordance with conventional drill bit construction, and the legs 14 provide supporting elements for rotatably supporting on suitable bearing structures (not shown), a plurality of conically shaped roller cones designated generally by reference character 18. The earth traversed by the bit in the drilling operation is indicated by reference character 20.

Each of the conically shaped roller cones 18 is journaled on its respective supporting leg 14 for rotation about the axis of symmetry of the conical body. The axis of rotation and symmetry of each roller cone 18 is inclined with respect to the vertical so as to form an angle of from about 25° to about 50° therewith or, stated differently, an angle of from about 40° to about 65° with a horizontal plane passed through the vertical axis of rotation of the drill string 16 and bit head 10 as indicated by the angle a depicted in FIGURE 2. In the preferred embodiment of the invention, the angle a is about 53°. The rotational axes of the roller cones 18 which are shown as passing through points A, B and C in the respective cones, also pass through a common point D in the vertical axis of rotation of the drill string 16 and bit head 10, as depicted in FIGURES 1 and 2. Stated in another way, the roller cones 18 are contained within imaginary conical envelopes having their apices at point D.

For purposes of clarity in referring to the three roller cones 18 utilized in the bit of the present invention, the three cones have been assigned letter designations X, Y and Z in FIGURES 1-3. The roller cones X, Y and Z are asymmetrically positioned with respect to each other on the bit head 14, this asymmetric relationship being most perceptible in FIGURE 1. Thus, it will be noted that, as measured in a plane extending substantially normal with respect to the vertical rotational axis of the drill string 16 (and corresponding to the plane of the drawing paper in FIGURE 1), the angular distance separating the rotational axes of the roller cones Y and Z from each other is 132° whereas the angular distance separating the axis of roller cone X from the respective axes of roller cones Y and Z as measured in the same plane is only 114°.

The large angular spacing of the roller cones Y and Z from each other on the bit head 10 permits a large fluid return port 22 to be formed in the side of the bit head between these two roller cones so as to accommodate a large volume flow of drilling fluid and cuttings from the bottom of the hole upwardly past the bit head 10 en route to the surface. For the purpose of injecting drilling fluid into the bottom of the hole being drilled with the bit, two drilling fluid injection nozzles 24 and 26 are provided on the opposite side of the bit head 10 from the return port 22, and are disposed at positions between roller cone X and the respective roller cones Y and Z. The drilling fluid injection nozzles 24 and 26 communicate with the hollow interior of the drill string 16 through the shank 12 of the drill head 10 in accordance with conventional construction in the drill bit art.

Each of the roller cones X, Y and Z is provided with a plurality of cutter teeth 28 which extend circumferentially around the main body of the roller cones in several axially spaced rows, as best illustrated in FIGURE 2. In the illustrated embodiment of the invention, each cutter tooth 28 extends in continuous or uninterrupted fashion around the entire periphery of its respective roller cone and carries a wedge-shaped tip 30 which is defined by two converging sides 30α and 306. The wedge-shaped tips 30 of each of the cutter teeth 28 are, in the embodiment of the invention illustrated in FIGURES 1-3, aligned in a conical surface which forms an angle β of from about 5° to about 40° with the horizontal (or more accurately stated, with a plane passed normal to the vertical axis of the drill string), as illustrated in FIGURE 2. In this embodiment of the invention, the conical surface in which each of the tips 30 of the cutter teeth 28 is disposed is illustrated as inclined at an angle β of 20° with respect to a plane extending normal to the vertical rotational axis of the drill string 16.

In a preferred construction of the drill bit of the invention, the two converging sides 30α and 306 of each of the teeth 28 define an angle of at least 40° with a plane containing the line 31 as measured from either of the con3,397,751 .⁵ verging sides directly to said plane. A more specific description of the continuous circumferential cutter teeth 28 and the manner in which they function is provided in U.S. application Ser. No. 329,130 filed Dec. 9, 1963, and assigned to the assignee of the present application. The specific configuration of the cutter teeth employed in the drill bit of this invention constitutes no part of the invention except as used in combination with the several asymmetrically oriented roller cones utilized in the bit of the invention.

Roller cones Y and Z are each provided with a full size circumferential gauge tooth 32 which terminates in a wedge-shaped tip 34 occupying the same conical surface as the wedge-shaped tips 30 of the cutter teeth 28. The roller cone X, however, carries only a partial gauge tooth 36, as best illustrated in FIGURES 1 and 2, so that this gauge tooth does not extend the entire distance to the bottom hole surface upon which the cutter teeth roll during the operation of the bit. The described arrangement in which two of the roller cones Y and Z are provided with full size gauge teeth extending from the cone body into the same plane as that occupied by the cutting edges or tips 30 of the cutter teeth 28, and in which the third roller cone X is provided with only a partial gauge tooth 36 permits the three roller cones to be mounted in interfitting relation, as best illustrated in FIGURE 3, so that the cutter teeth overlap each other and provide a cleaning action. It will be noted in the referring to FIGURE 3 that the roller cones Y and Z each have their respective cutter teeth 28 interfitting with the cutter teeth 28 mounted on the roller cone X, but do not have mutually interfering or overlapping teeth. Thus, the roller cones Y and Z can be provided with the full gauge teeth 32 without interfering with the rolling action of each other, and yet each of these cones is cleaned, and dirt prevented from becoming impacted between the cutter teeth 28 and the gauge tooth 36 by reason of the interfit of the several teeth with the teeth provided on the roller cone X.

For the purpose of accomplishing the lead cutting at the center of the bit and thereby assure that the hole will be propagated along a straight vertical axis, a generally cylindrical cutting point 38 is mounted in the roller cone Y at the apex thereof. The cutting point 38 is constructed of a hardened metal or alloy, such as tungsten carbide, and in the illustrated embodiment, takes the form of a cylindrical plug which is pressed into an aperture formed in the small end of the roller cone.

Operation

In the operation of the drill bit of the invention, the drill string 16 is rotated about a vertical axis and carries with it in rotation, the bit head 14 and the three roller cones 18 which are mounted thereon, and which are designated for discussion purposes as cones X, Y and Z. The three roller cones X, Y and Z, by virtue of their individual symmetrical conical configuration, and because of the fact that their rotational axes all pass through a common point on the vertical axis of rotation of the drill string 16, have a true rolling action on the bottom of the hole drilled by the bit. They thus function to continuously bring the circumferential cutter teeth 28, as well as the gauge teeth 32 carried by the cones Y and Z, into cutting contact with the bottom of the bore hole. The hard metal cylindrical point 38 provided on the lower end or at the vertex of the roller cone Y continually cuts away a small pilot hole toward the center of the bore hole, and aids in maintaining a true vertical alignment of the bore hole. In other words, the lead cutting by the bit is done at the center of the bit which tends to maintain the straightness of the hole being drilled.

The asymmetric arrangement of the roller cones X, Y and Z around the vertical axis of rotation of the drill string 16 provides several marked advantages in drilling operations accomplished with the bit of the invention. One of the most important of these advantages is the damping out or suppression of drill string vibrations resulting from contact of the roller cones wtih protuberances or obstructions on the bottom of the bore hole. These vibrations, in being harmonics of the fundamental frequency of vibration of the drill string, tend to reinforce or build up vibration in the drill string to a point where damage may occur. To understand the manner in which the asymmetrically positioned cones accomplish this result, the recognized relationship between the length of the drill string and the fundamental frequency at which it will vibrate over its length must be considered. This relationship is expressed by the mathematical formula

237,000 L where f is the fundamental frequency of vibration (in vibrations per minute) of the drill string, and L is the length of the string in feet. For example, a drill string which is 2,000 feet in length will have a fundamental vibration frequency of about 120 vibrations per minute, a string 4,000 feet in length will have a fundamental frequency of about 60 vibrations per minute, and a string 8,000 feet in length will have a fundamental frequency of about 30 vibrations per minute. Where obstructions, protuberances or depressions are encountered in the bottom of the bore hole by the teeth of the roller cones, the rate at which the drill string is being rotated may be such that the impact of the cutter teeth with the obstruction or depression occurs at exactly the same frequency as either the fundamental frequency of vibration of the drill string, or at some harmonic or subharmonic of the fundamental frequency. Thus, assuming that the fundamental frequency of vibration of the drill string is 30 vibrations per minute (about 8,000· feet of string), if the drill string were rotated at 30 r.p.m., and the roller cones were oriented symmetrically thereon, they would each strike a protuberance, depression or the like in the hole bottom at the rate of 30 impacts per minute and the vibrations resulting from the impact of the cones with the protuberance or depression would establish a resonant condition in the drill string and cause the magnitude of drill string vibration to build up to an unacceptable level. The same condition is true of the asymmetrically oriented cones, if the condition exists that the rate of rotation of the drill string, in r.p.m., is numerically equivalent to the fundamental frequency, in vibrations per minute.

Let the conditions of operation be changed, however, so that the drill string is rotated at 90 r.p.m. and the length of the drill string is such that its fundamental frequency is 30 r.p.m. In this case, where there are three roller cones disposed symmetrically on the bit head, the teeth of each cone strikes the protuberance or depression at the rate of 90 times per minute, which is, if considered in terms of vibrations per minute, an odd harmonic of the fundamental frequency of the drill string. If all three roller cones are considered conjunctively, the frequency of impact of vibration transmitted from the three roller cones to the drill string is 270 vibrations per minute, the ninth harmonic of the fundamental frequency. Again an undesirable resonant condition results. With the asymmetric cone construction, however, the build-up of drill string vibration is not experienced. This is because vibration waves from the asymmetrically positioned roller cones are out of phase with each other and tend to cancel or damp each other out. Stated differently, the impact of at least one of the cones with the vibration causing obstruction in the hole bottom will occur at some period following the impact of another cone which is more or less than any given multiple or submultiple of the period of the fundamental frequency of vibration of the drill string. Thus, except when the drill string is rotating at a rate in r.p.m. which is numerically equivalent to the fundamental frequency of the drill string in vibrations per minute, the vibrations transmitted from the roller cone to the drill string will be interfering in nature rather than reinforcing.

3,397,751

Another important advantage of the asymmetric disposition of the three roller cones is the ability to orient the drilling fluid injection nozzles and the fluid return port to provide for more efficient and trouble-free sweeping of the hole bottom. Thus, with the cones Y and Z having their axes BD and CD, respectively, spaced widely from each other as portrayed in FIGURE 1, a large fluid return port 22 can be disposed on the opposite side of the bit head 10 from two drilling fluid injection nozzles 24 and 26. Although three drilling fluid injection nozzles have usually been used in conventional bit construction, the two nozzles 24 and 26 provided in the bit of the present invention are constructed to have a port or opening area which, in the case of each nozzle, is about fifty percent larger than the standard or conventional nozzle so that the two nozzles which are provided deliver the same amount of drilling fluid to the hole bottom, and provide substantially the same pressure drop as is characteristic of bits employing the conventional three-nozzle arrangement. The nozzles, in being enlarged, are made less susceptible to plugging by foreign objects entrained in the drilling fluid, and the jets of drilling fluid can reach the hole bottom with less loss in effectiveness because the ratio of the jet area to its circumference is increased. Moreover, as contrasted with the geometric arrangement of the fluid injection nozzles on bits heretofore in use, the nozzles 24 and 26 of the bit of this invention discharge their jets of drilling fluid in directions which do not allow the jets to intermingle and interfere with each other prior to reaching the hole bottom, thus resulting in less non-useful dissipation of jet energy. By having the fluid injection ports 24 and 26 disposed on one side of the bit head 10 and the fluid return port 22 on the opposite side thereof, the drilling fluid is permitted to sweep the bottom of the hole in a smooth non-interfering flow pattern as it passes from one side of the hole across the bottom and up the other side thereof to the return port.

A second embodiment of the invention has been illustrated in FIGURES 4, 5 and 6 of the drawings. The embodiment here illustrated is generally similar to the embodiment illustrated in FIGURES 1-3 except that the configuration of the roller cones and the cutter teeth carried thereby which is required to give a true rolling action on the hole bottom has been sacrificed in order to gain additional cone volume and interlocking of cutter teeth. In other words, instead of the roller cones depicted in the embodiment under discussion having only a simple rolling action on the bottom of the bore hole, the cones also skew or scrape to some extent as a result of their particular structural configuration as hereinafter described.

Referring initially to FIGURES 4 and 5 of the drawings, the modified bit there shown includes a bit head 50 having a hollow generally cylindrical externally threaded shank portion 52 and three downwardly extending, horizontally spaced legs 54. The shank portion 52 of the bit head 50 is threaded into the lower end of a drill string 56 in substantially identical fashion to that which has been described in referring to the embodiment illustrated in FIGURES 1-3. Three roller cones 58 which, in order to clarify the following discussion, have been designated by the letters R, S and T, are supported on the legs 54 of the bit head 50 by suitable bearing structures (not shown). The earth traversed by the bit in the drilling operation is indicated by reference character 60.

Each of the roller cones R, S and T is journaled on its respective supporting leg 54 for rotation about its axis of symmetry. The axis of rotation and symmetry of each of the roller cones 58 is inclined with respect to the vertical so as to form an angle of from about 25° to about 55° therewith, or, stated differently, an angle of from about 35° to about 65° with a horizontal plane passed through the vertical axis of rotation of the drill string 56 and bit head 50 as indicated by the angle a depicted in FIGURE 5. In the embodiment of the invention illustrated in FIGURE 5, the angle a is preferably about 48°. The rotational axes of the several roller cones R, S, and T which are shown as passing through points E, F and G in the respective cones, also pass through a common point H in the vertical axis of rotation of the drill string 56 and bit head 50, as depicted in FIGURES 4 and 5. In the foregoing respects, therefore, the mounting of the several roller cones R, S and T is similar to the mounting of the roller cones X, Y and Z in the embodiment appearing in FIGURES 1-3.

Also similarly to the roller cones X, Y and Z depicted in FIGURE 1, the roller cones R, S and T are asymmetrically positioned with respect to each other on the bit head 50 in accordance with one of the salient, fundamental features of the present invention. It will therefore be noted, in referring to FIGURE 4, that the angular distance separating the rotational axes of the roller cones S and T from each other i.e. 13,2°, as such distance is measured in a plane extended at a right angle to the vertical rotational axis of the drill string 56. The angular distance separating the axes of the roller cone R from the respective axes of the roller cones S and T as measured in the same plane is, however, only 114°.

A fluid return port 60 is provided in the side of the bit head 50 which extends between the roller cones S and T. Drilling fluid injection nozzles 62 and 64 are formed in the bit head 50 on the opposite side thereof from the return port 60 and are disposed at positions between the roller cone R and each of the respective roller cones S and T. The size and function of the drilling fluid injection ports 62 and 64 and the fluid return port 60 utilized in the embodiment of the invention depicted in FIGURES 4-7 is substantially identical to that which has been hereinbefore described in referring to the embodiment shown in FIGURES 1-3.

The major difference which exists between the embodiment of the invention under discussion and the embodiment which has previously been described resides in the configuration and arrangement of the cutter teeth on the several roller cones. Thus, in referring to FIGURE 5, the roller cones R and S which appear therein each carry a plurality of circumferential cutter teeth which extend outwardly from the periphery of the conical roller cone body and bear against the bottom of the bore hole. The several circumferential cutter teeth are spaced axially from each other along the body of the cone, and have been designated by reference characters 68α, 686, and 68c. The reference numeral 68 is the general designation for the several circumferentially extending cutter teeth, and the postscript alphabetical letter refers to the position on the cone body occupied by the respective cutter tooth. Thus, cutter tooth 68α refers to the cutter tooth which is positioned on the roller cone body in closest proximity to the vertical walls of the bore hole (with the exception of the gauge teeth which are hereinafter described), cutter tooth 686 is positioned intermediate the cone body and farther removed from the vertical wall of the bore hole than the cutter tooth 68α, and cutter tooth 68c is the cutter tooth which is located nearest the center of the bore hole on each of the cones which carry as many as three of the cutter teeth.

Each of the cutter teeth 68a-68c carries a wedgeshaped tip 70 which is defined by two converging sides 72 and 74. As contrasted with the cutter teeth 28 provided in the embodiment of the invention illustrated in FIGURES 1-3, the wedge-shaped tips 70 of the cutter teeth 68a-68c do not terminate in a common imaginary conical surface. Instead, the several cutter teeth 68a-68c each have their wedge-shaped tips 70 positioned varying distances from the conical sides of the respective roller cone bodies on which the cutter teeth are mounted. Thus, each of the wedge-shaped tips 70 on the several cutter teeth 68a-68c pass through lines (or planes) extending at different angles with respect to a generally horizontal plane extended normal to the vertical axis of the drill

3,397, 9 string 56 when the wedge-shaped tips 70 are in their position of closest approach to such generally horizontal plane. These angles are designated β in FIGURE 5.

The relationship of the several wedge-shaped tips 70 of the cutter teeth is schematically illustrated in FIGURE 5 5 of the drawings where the generally horizontal reference plane is designated by reference character P. In referring to this figure, it will be noted that the wedgeshaped tip 70 of the cutter tooth 68έ on roller cone R, in its closest approach to the plane P, passes through a ₁₀line which extends at an angle of about 5° with respect to the plane P, whereas the wedge-shaped tip 70 of the cutter tooth 68α on this same roller cone passes through a line extending at an angle of about 10° with respect to the plane P which extends normal to the vertical axis 15 of the drill string. Stated in different terms, the wedgeshaped tips 70 of each of the cutter teeth 68a-68c on each of the roller cones extends a different distance toward a horizontal plane passed through the point H in the vertical axis of the drill string 56 so that the 20 roller cones do not undergo a true rolling action on the bottom of the hole, but must develop a skewing or slicing movement along with the quasi-rolling movement during their operation. The orientation of the wedge-shaped tips 70 of the several cutter teeth may also be described 25 with reference to the generally horizontal plane containing the points E, F and G as illustrated in FIGURE 4, which plane extends normal to the vertical axis of the drill string 56. In this context, each of the tips 70, at the lowest point in its rotational movement on the conically 30 shaped roller cone body, passes through a line extending at an actute angle with respect to this plane (plane EFG), the several lines thus passed through by the tips extending at different acute angles with respect to this plane, with the lines passed through by the tips of teeth mounted 35 relatively nearer the base of the conically shaped body extending at larger acute angles to this plane than the lines which are passed through by the tips of teeth mounted relatively further from the base of the roller cone and closer to the tip or apex thereof. 40

As in the case of the roller cones X, Y and Z illustrated in the FIGURES 1-3 embodiment, the gauge teeth carried by the roller cones R, S and T of the embodiment of the invention under discussion are constructed so that the cones S and T carry a full gauge tooth 82, whereas ₄the cone R is not provided with any gauge tooth. The gauge teeth 82 are wedge-shaped and terminate in cutting edges or tips which pass through a line which preferably extends at an angle of from about 15° to about 40° to the plane P. 50

The roller cone 76 carries at its apex, a generally cylindrical cutting point 86 which is constructed of a hardened metal or alloy. The cutting point 86 functions, as previously described, to perform the lead cutting toward the center of the bore hole, and thus aids in main- _5gtaining the vertical alignment or straightness of the hole which is drilled.

The manner in which the cutter teeth 68a-68c provided in the embodiment of the invention depicted in FIGURES 4, 5 and 6 are constructed permits the cones to be θθ constructed of relatively larger volume so that more room is provided within the cone bodies for the accommodation of bearing structures to mount the cones on the respective legs 54 of the bit head. Moreover, the configuration of the teeth permits the teeth of the three cones to _6gbe provided in a more completely interlocking or interfitting relationship as depicted in FIGURE 6 of the drawings. Thus, whereas in the embodiment of the invention illustrated in FIGURES 1-3, the teeth carried by the cones Y and Z do not interfit with each other, the teeth carried by roller cones S and T in the FIGURE 6 embodiment do interfit with each other, as well as with the cutter teeth provided on the roller cone R.

The operation of the embodiment of the invention depicted in FIGURES 4, 5 and 6 is substantially the same 75

751 as that shown in FIGURES 1, 2 and 3. The efficient sweeping action of drilling fluid on the bottom of the bore hole resulting from the positioning of the drilling fluid injection ports and the fluid return port on the bit head is retained, and the asymmetrical orientation of the three roller cones on the bit head provides a damping or suppressing action on vibrational stresses which would otherwise tend to build up in the drill string. A better cleaning action resulting from the interfit of roller cone teeth is accomplished with the FIGURES 4-6 embodiment than with the FIGURES 1-3 embodiment due to the ability to construct the former embodiment with the teeth of all three of the cones mutually interfitting to provide a double cleaning action on each cone.

From the foregoing description of the invention, it will have become apparent that the invention provides a novel three-cone rock bit in which the roller cones which are provided are asymmetrically positioned on the bit head so that vibrations resulting from the impact of the roller cone teeth against obstructions in the bottom of the bore hole, and which are harmonics or subharmonics of the fundamental frequency of the drill string, can be caused to interfere with each other, rather than being cumulative or additive in their effect. The bit can be economically constructed, and is characterized by a long and troublefree service life. Drilling operations can be carried out utilizing the bit with relatively little danger of the drilling fluid injection ports becoming plugged or obstructed, and with a more efficient continuous sweeping of the hole bottom being effected as a result of the novel injection nozzle and return port design.

Although certain preferred embodiments of the invention have been herein described in order to provide an example enabling one skilled in the art to practice the invention, certain structural modifications and changes in the relationship of various elements of the bit construction to each other can be effected without departure from the basic principles which underlie the invention. For example, instead of continuous, uninterrupted, circumferential teeth being provided on the several roller cones, a plurality of teeth extending from the periphery of the roller cone body and occupying the same plane extending normal to the axis of rotation of the cone may be provided to good advantage for some types of drilling operations. Cones carrying interrupted teeth of this type are described in U.S. application Ser. No. 329,130 filed Dec. 9, 1963, and assigned to the assignee of the present invention. All such innovations and changes which are made in the structure of the asymmetric three-cone rock bit, but which continue to utilize the basic principles of the invention as hereinbefore enunciated, are deemed to be circumscribed by the spirit and scope of the invention except as the same may be necessarily limited by the appended claims or reasonable equivalents thereof.