US2976927A - Rotary drill bits and cutters - Google Patents

Description

March 28, 1961 A. w. KAMMERER, JR., ET AL 2,976,927

ROTARY DRILL BITS AND CUTTERS 2 Sheets-Sheet 1 Filed May 8, 1958 r O Zm Alec/me March 1961 A. w. KAMMERER, JR., ET AL 2,976,927

ROTARY DRILL BITS AND CUTTERS Filed May 8, 1958 2 Sheets-Sheet 2 Alec/nae 11 flaw/505w, zk,

04140 E LAMP/44565 INVENTORS.

BY i.-

Arm/mars:

ROTARY DRILL BITS AND CUTTERS Archer W. Kammerer, Jr., Fullerton, and David E. Lamphere, Orange, Calif., assignors, by direct and mesne assignments, to Rotary Oil Tool Company, Buena Park, Calif., a corporation of California Filed May 8, 1958, Ser. No. 733,995

6 Claims. (Cl. 166-553) The present invention relates to rotary drill bits, and more particularly to bits capable of milling out a portion of easing, or similar well conduit, disposed in a well bore.

Rotary drill bits of the expansible type have been used for milling away desired lengths of casing disposed in a well bore. Most effective operation of such bits requires that they be appropriately centered in the casing during their use, which has been difficult to achieve, particularly since the inside diameter of a given size of casing is different for different weights of such size casing. For example, a 7" O.D., 34 pound casing will have a much smaller diameter than a 7' OD, 17 pound casing. Bits have been used with pilots to secure their centering in the casing, but the pilot size must be different for different weights of easing of the same outside diameter. Despite the use of a proper size pilot, the drill bit still tends to wobble in the well bore, resulting in its uneven operation I and non-uniform wear on the bit cutters, shortening the life of the bit, inasmuch as one cutter will wear out before the other bit cutters. The unevenness in the operation of the bit also imposes additional wear on the-drill bit as a whole. Moreover, it tends to initiate splits in the casing.

It is an object of the present invention to provide a rotary drill bit or milling tool'forcutting well casing in a well bore, in which a single tool will be properly centered with respect to casing of the same outside diameter but of different weights.

Another object of this invention is to provide a rotary drill bit or milling tool for cutting away casing in a well bore which centers itself with respect to the outer surface of the casing rather than its inner surface, thereby enabling a single tool to operate elfectively on different casings of the same outside diameter but of different inside diameters.

A further object of the invention is to provide a rotary drill bit or milling tool for milling away casing in. a well bore in which the cutters of the tool effect its centering I with respect to the casing.

An additional object of the invention is to provide a rotary drill bit or milling tool which is less susceptible to wobbling while milling away casing in a well bore, resulting in a practically uniform wearing away of the tool cutters, less wear on the entire tool, and a longer useful life of the cutters and the tool as a whole.

Yet another object of the invention is to provide a rotary drilling tool for cutting away casing in a well bore which has a substantially reduced tendency to initiate splits in the casing.

Still another object of the invention is to provide a milling tool having cutters thatdo not chatter while in operation on well casing in a well bore, which is of strong and sturdy construction, and which possesses a comparatively long milling life.

This invention possesses many other advantages, and has other objects which may be made more clearly apparent from a consideration of a form in'which it may be embodied. This form is shown in the drawings accompany- 2,976,927 7 Patented Mar. 28, 1 961 ice now be described in detail, for the purpose of illustrating the general principles of the invention; but it is to be understood that such detailed description is not to be taken in a limiting sense, since the scope of the invention is best defined by the appended claims.

. Referring to the drawings: v

Figure 1 is a longitudinal section through a milling tool disposed within a well casing, with the cutters and other parts occupying an initial retracted position;

Fig. 2 is a longitudinal section similar to Fig. 1, dis closing the cutters locked in their fully expanded position;

Fig. 3 is an enlarged side elevation of a cutter structure;

Fig. 4 is an outer side elevational view of the cutter structure;

Fig. 5 is a bottom plan view'taken along the lines 5-5 on Fig. 3;

Fig. 6 is a longitudinal section taken along line 6--6 on Fig. 5.

'As shown in the drawings a rotary expansible drill bit or milling tool A is secured to the lower end of a string of drill pipe B that extends to the top of the well bore C, and by means of which the drill bit is lowered through a string of well casing D to a location therein where a casing milling operation is to commence.

The upper portion of the rotary drill bit consists of a mandrel 10 having an upper pin 11 threadedly connected to the lower end of the string of drill pipe B. This man drel includes an upper kelly or drill stem member 12 slidably splined to the main body 13 of the drill bit. The exterior of the lower portion 14 of the kelly is non-circular in shape, being telescopically received in a companion non-circular socket 15 formed in the main bit body 13. Specifically, the kelly exterior and the socket 15 may be of hexagonal shape, to enable the kelly 12 to be moved longitudinally with respect to the body 13, while still being capable of transmitting rotary motion to the body.

The mandrel 10 has a limited range of longitudinal movement within the body, its downward movement being determined by engagement of the lower end 16 of the kelly with an inwardly directed body shoulder 17, and its upward movement being limited by engagement of an external shroulder or piston portion 18 of the kelly with a cylinder head 19 secured to the body. The upper end of the head has a flange 20 engaging a body shoulder 21, the flange being prevented from moving upwardly of the body by split snap retainer rings 22 fitting in a body groove 23 and overlying the flange 20. An annular guide 24 is releasably secured to the body 13 by a split snap ring 25 above the retainer rings.

. The body 13 has a plurality of expansible parts mounted on it. These include cutter supporting members 26 pivotally mounted in body slots 27 on hinge pins 28 and v to sever the casing and mill it away by operating upon the upper end B of the casing therebelow. To accomplish the expansion, each cutter supporting member 26 has an inclined expander surface 30 on its inner portion below the hinge'pin 28 which tapersin a downward and inward direction. Each expander surface terminates in a lock surface 31 formed on a lock portion 32of the cutter supporting member. The outward"expansion is accomplished by producingrelative longitudinal movement between the mandrel 10 and the bit body 13, which will produce relative longitudinal movement between the cutter supporting members 26 and the tubular member 33 of the mandrel ill This tubular member includes a lower portion 34 slidable within a guide bushing 35 mounted in a bridge 36 secured to the body and extending across the body slots 27'. This guide bushing 35 is disposed below the look portions 32 of the cutter supporting members 26.

Located initially substantially above the guide bushing 35 and below the hinge pins 28, and in cutter member recesses 37, is a mandrel lock and expander 38 which has outer surfaces 3-9 adapted to engage the expander surfaces 3t) and the lock surfaces 31-. The lock and expander 33 may be formed integral with the tubular member 33, the upper end of the latter being piloted within a socket 49 formed in the lower portion of the kelly 112. An enlarged boss 41 on the tubular member 35 engages a downwardly facing shoulder 42 of the kelly, the tubular member being held against this shoulder by a suitable split retainer or lock ring snapped into an internal groove idencompassing the kelly socket and engaging the lower end of the tubular member boss 41.

Drilling mud or other fluid can pass down through the central passage 45 in the kelly or drill stem 12 and into the central passage 46 extending completely through the tubular member 33. Leakage of fluid around the exterior of the tubular member 33 is prevented by a suitable side seal ring 47, such as a rubber O ring, in a peripheral groove 4? in the kelly, which engages the'interior of the boss 51.

Assuming that tr e body 13 of the tool is elevated relatively along the tubular mandrel it, the inclined expander surfaces 3%? of the cutter supporting members 26 will be shifted upwardly along the lock and expander portion 33 of the tubular member 33. During such upward shifting, the cutter supporting members 2s, and the cutter structures 29 carried thereby, will be pivoted about the hinge pins 28 and urged in an outward direction. The upward movement of the body 13 with respect to the tubular mandrel Hi can continue until the cutter structures 2? have been shifted outwardly to their fullest extent, as determined by engagement of stop shoulders on the cutter supporting members 26 with cornpanion shoulders it? formed in the body on opposite sides of the body slots 2'7. When such engagement occurs, the lower end in of the kelly portion 3.2 of the tubular mandrel will engage the body shoulder 17, and the lock and expander on the tubular member 33 will be disposed behind and in engagement with the lock portions 32 on the cutter supporting members as (Fig. 2).

It is to be noted that the surfaces 31 of the lock portions 32. of the cutter supporting members 26 and the companion surfaces 39 on the lock and expander portion 38 of the tubular member are substantially parallel to the axis of the drill bit, to prevent the reactive forces on the cutter structures 29 from moving the latter inwardly. As a practical matter, it is preferred that the coengaging lock surfaces 39, 31 be inclined slightly in a downward direction toward the axis of the tool, to insure release of the 'lock and the expander portion 38 from the cutter supporting members 26 when the latter and the cutter structures 29 are to be shifted to retracted position.

The relative longitudinal movement between the tubular mandrel and the body 13 of the tool is accomplished hydraulically in the specific form of apparatus disclosed in the drawings. Thus, the piston or enlarged portion 18 on the drill stem 12 is received within a counterbore 51 formed on the upper portion of the body of the tool. This upper portion actually constitutes a cylinder 52 having a cylindrical wall 53 extending from a lower shoulder 54, defining the bottom of the counterbore 51, to the cylinder head 19.

A confined cylinder space 55 is formed between the piston portion 18 of the kelly, the periphery of the kelly 4 above the piston, and the cylinder 52. A suitable packing or side seal ring as may be disposed in a suitable piston ring groove 5'7 formed in the piston 18, which is adapted to slidably seal against the cylindrical wall Q2 the cylinder E52. Fluid is thereby prevented from lg a downward direction between the piston and the cylinder. Similarly, iluid is prevented from passing in an upward direction out of the annular cylinder space by an inner side seal ring 53 carried in an internal ro ve in the cylinder head and slidably and sealthe periphery of .ne kelly 12 above the piston to, and also by an outer side seal ring 6% disposed in an external groove tilt in the head 19 an sealingly engaging the cylinder wall under pressure in the string or" drill pipe B and the tubular mandrel passage 2-5 can be fed into the cylinder space 55 through one or more side ports 62 e""blishing communication between theccntral passage through the kelly and the cylinder space. Such fluid under pressure is developed, in the-form of invention disclosed in the drawings, by virtue of the fact that the pass ,e as through the tubular member 33 of the drel is of a restricted diameter as comparco to the passage through the kelly portion of the mandrel. As a result, the pumping of drilling mud, or other fluid, at an adequate rate through the apparatus will build up a back pressure of fluid in the passage 55, which pressure will be imposed on the fiuid in the cylinder space 55, acting upon the cylinder head 19 to urge the body of the tool in an upward direction with respect to the tubular mandrel lb, to secure the outward expansion of the cutter supporting member 26 and cutter structures 29 to their fullest extent, as above described.

As apparently disclosed in the drawings a set of dia metrically opposed supporting members as and cutter structures are supplied d po ed substantially lSO degrees apart from each other. Actually, it is preferred to supply three sets of supporting members and cutter structures dispiaced substantially 26 degrees from each other to secure a smoothly running rotary expansible bit. The two sets of supporting member strucures are shown the interest of simplicity of the drawings. Each cuter structure 29 is secured to the lower poi n of a cutter supporting member 255 by use of welding material 763, which fun ionally integrates the cut r structure 29 to the depending supporting member The lowplurality of depending legs 71, 72, one another by substantial spaces feet, constitute cutter tee- T e leg which depend to a substan-v tial extent from the upper body portion 7d of the cutter structure. The leading faces of each cutter tooth are preferably provided with hardfacing material 75, which may be of any suitable type. By way of example, it is found that the brazing of tungsten carbide particles on the leading faces of the teeth 71, 72, 73 is very ef fective in milling away casing s ctions. The hardfacing material extends from the lower end of these teeth substantially to the upper body portion 7 of the cutter structure 2?. i

in addition to hardfacing the leading faces of each tooth or cutter, the outer surface or face of one of the teeth 72 is provided with hardfacing material 76. Such outer surface will extend radially from the axis of the drill bit to a greater extent than the outer surfaces '7 of the other teeth 71, 73 which need not be hardfacec The provision of hardfacing material on only one too h 72, or on a lesser number of teeth ti an are contained in each cutter structure, facilitates expansion of each cutter structure 29 and its milling away of the casing at point where the operation is to commence. The hydraulic pressure exerted in the cylinder 52, to elevate the body 13 relative to the mandrel 1t and effect outward expansion of the cutters, is applied on only a single tooth 72 of each cutter structure 29, so that the unit force of amete ,5 v such cutter structure against the casing D during the severing action is increased, insuring efiective milling away or severing of the casing and outward expansion of the cutters to the fullest extent, which is the condition illustrated in Fig. 2.

The cutter structures are so designed that they will etfect'their own appropriate centering on the well casing D. after the latter has been severed by the cutters, reliance not being placed upon a pilot structure 80 that is secured to the lower portion'of' the body 13 of the drill bit and which has the purpose of effecting substantial centermg of the entire milling apparatus in the well casing. After the casing D has been severed by the cutters 29, as described hereinbelow, andthe latter have been expanded outwardly to the full extent, the cutter structures are lowered to engage the severed end E of the casing thereinbelow. When this occurs, the upper end E of the lowered casing portion will be received within a centering groove 81 extending upwardly from the lower ends 82 ofthe cutter structure teeth 71, 72, 73. The centering grooves 81 lie collectively on an arc corresponding to the curvature of the casing D with the outer side 83 of each groove 81 extending from the axis of the bit a distance substantially equalling the outside radius of the casing D. Accordingly, following expansion of the cutter structures 29 and the disposition of the lower grooves 81 over the severed casing section E below, each outer side 83 of each centering groove 81 will ride along the periphery 84 of the casing D. In view of the use of a plurality of centering structures (preferably three in a set, spaced 120 degrees apart) the cutter blades 71, 72, 73 will center themselves upon'the casing D and elfect an appropriate centering of the entire rotary drill bit A relative to the casing. Accordingly, reliance need not be had upon the centering effectiveness of the pilot portion 80 of the apparatus, which need only place the cutters in an approximate centered position in which the grooves 81 of the cutter structure will fit over the upper severed end B of the casing structure. The width of each centering groove 81 is substantially greater than the thickness of the casing section D, so that casings of the same outside diameter but of different inside diameters willlfit appropriately within the centering grooves, the outer sides 83 of the grooves still performing the centering action in the well casing for all of the cutter structures 29, and of the entire drill bit.

The depending legs or teeth 71, 72, 73 of each cutter structure 29 are secured together by an inner web 85, forming the inner portion of the cutter structure and preferably made integral withthe legs. The web may have longitudinally spaced holes 86 therethrough between adjacent depending blades to facilitate hardfacing of the leading faces of the teeth. Such hardfacing is facilitated since the gases of the welding torch can escape through the openings 86 and insure the proper deposit of the hardfacing material 75 upon the tooth surfaces. Further reinforce ment between the blades 71, 72, 73 of each cutter structure is provided by welding struts 87 to adjacent teeth extending across and between said teeth. -The struts 87, coupled with the inner web 85 integral with the blades, provide cutter blades or teeth that are rigid, and which will not chatter norvibrate during the milling operation. Neither the web 85 nor the struts 87 interferes with the cutting action of the individual blades on the casing D, since the struts aredisposed outwardly of the outside diameter of the casing D and the webs 85 are disposed substantially inwardly ofthe inside diameter of the casing It is to be noted that the Webthickness is at a maximum adjacent the upper body portion 74 of each body structure 29, and that it gradually decreases toward the lower ends of the blades. Due to this arrangement, the blades or teeth 71, 72, 73 cannot deflect, since the point subject to the greatest deflection is adjacent to the upper body portion 74 and, at such portion, the web has its maximum thickness.

, 6 In the operation of the apparatus A, it is run in the" well casing D with the cutter supporting members 26 and the cutter structures 29' in their initially retracted posi tions, such as disclosed in Fig. '1. When the apparatus has been lowered to the desired point in the well casing at which the latter is to be severed, the pumps (not shown) at the top of the well bore are started to pump fluidat a suflicient rate through the drill pipe B and into the mandrel passages, building up a back pressure in the kelly passage 45 and, of the fluid in the ports 62 and cylinder space 55, which pressure acts upon the cylinder head 19 to urge the body 13, the cutter supporting members 26 and the cutter structures 29 in an upper direction with respect to the mandrel. During such upward movement, the expander surfaces 30 are brought to bear against the lock and expander portion 38 of the mandrel, the cutter structures 29 being'urged in an outward direction against the wall of the well casing D. The drill pipe B and the rotary drill bit A are rotated at a proper speed, while fluid is being pumped through the apparatus, the outer casing severing or reaming edges 76 of the cutter teeth .72 acting upon the wall of the casing and gradually milling it away. During this operation,the apparatus is retained in substantially the same longitudinal position in the well casing. As the casing is cut away, the hydraulic force acting upon the body 13 raises it and the cutter supporting members 26 and cutter structures 29 to a further extent, until the reamer edges 76 of the cutter teeth 72 have completely severed the casing. Such severing action will occur before the outer-faces 77 of the other cutter teeth 71, 73 can engage the inner wall of the casing. Thereafter, rotation of the drill pipe B and the rotary drill bit A continues, so that the reamer edges 76; continue to cut away the casing and dig into the formation, until the cutters 29 have been fully expanded outwardly to the maximum extent, as determined by engagement of the stop shoulders 49, 50 and the lower end 16 of'the kelly portion 12 of the mandrel with the body shoulder 17. With the parts in this position, the lock portion 32 of each cutter supporting member 26 will bear against the lock and expander portion 38 of the tubular member 33 to preclude inadvertent partial retraction of the cutter structures 29 from their fully expanded position. i

Downweight in the proper amount is now imposed on a string of drill pipeB, this downweight being transmitted through the'kelly 12 to the body shoulder 17 and from the body 13 to the stop shoulders 50, 49 directly to the cutter supporting members 26 and the cutter structures 29, urging the three blades 71, 72, 73 of each cutter structure against the upwardly facing severed end B of the well casing. Inasmuch as the outer sides 83 of the grooves 81 have substantially the same radial distance from the axis of thedrill bit as the outer surface 84 of the casing, the cutter teeth will slide over the outside 84 of the casing, the latter being received within the centering grooves 81 provided in the lower portion of the cutter teeth. As noted above, the proper reception of casings of different weights, but of the same outside diameter, within the grooves 81 will occur, the cutters being held in their outer expanded positions and appropriately centered by the outer surface 84 of the casing itself. h The imposition of the proper downweight or drilling weight on the apparatus and its rotation at the proper speed will cause the hardfacing material75 to effect a cuttingaction upon the severed end E of the casing and mill it .away. During such milling action, drilling'fluid is being pumped down through the drill pipe B and the apparatus A, discharging from the latter and carryingthe cuttings upwardly around the tool A and the drill pipe B to the top of the hole.

As the cutters. perform their drilling action, the teeth 71, 72, 73 will be 'worn away progressively, the casing wearing anever-increasing, upward-extending groove in each tooth. The outer side 83a of such groove will.

conform to theoutside diameter of the casing D and will more effectively center the blades 71, 72, 73 and the entire drill bit structure with respect to the casing, insuring smooth operation of the apparatus. Such smooth operation will occur until the casing has worn the groove 9?; upwardly through the entire length of the cutter teeth inasmuch as each cutter structure 29 and the teeth of h cutter structure are maintained by the casing equal ices from the casing axis, each tooth and each cutter st acture will be subject to uniform wear, the groove 90 eing worn upwardly along the teeth to the same uniform extent. The teeth will not be shifted off center, since the outer surface 84 of the casing holds them centered with respect thereto, such that one tooth or cutter structure will not be subject to more wear than another tooth or cutter structure. In addition, the fact that the blades center themselves on the outer surface 84 of the casing will insure that the tool will operate concentrically of the axis of the casing and will not be subject to awobbling or eccentric motion, which would produce wearing out of one of the cutters before another cutter wears out, and which may also have a tendency to initiate splits in the pipe D. Moreover, the uniformity in the distance of each cutter tooth 71, '72, 73 from the axis of the casing D, and of the cutter structures 29 from the axis of the casing insures smooth operation of the entire bit A, its tendency to vibrate and to chater being grealy minimized, if not eliminated entirely. Such chattering or vibration, and even flexing of the cutters, are also not present with the construction disclosed in view of the reinforcement provided by the inner webs 85 of the cutter structures 29 and by the struts 87 extending between the outer portions of the cutter teeth.

After the desired length of casing has been milled away, or the cutters have been completely worn out, the apparatus can be removed from the well casing. All that need be done is to discontinue the pumping of the drilling fluid through the drill pipe B and the apparatus, to relieve the pressure in the cylinder 52, and then elevate the drill pipe. Such elevating movement will elevate the tubular mandrel it} with respect to the body 13 and the cutter supporting members 26, raising the lock and expanded portion 33 above the expander surfaces 30, whereupon the cutter structures 26, 29 will drop back to their retracted position, such as disclosed in Fig. 1. In the event the cutters 29 are reluctant to move to such position, the outer surfaces of the cutter supporting members 26 will engage the casing above and adjacent to the location of the severing cut, upon elevation of the apparatus in the well bore, which will force the cutter supporting members 26 inwardly, enabling the apparatus to be Withdrawn through the well casing D to the top of the well bore.

The inventors claim:

l. in a rotary'drill bit to be lowered in a well conduit disposed in a well bore to mill away a portion of the well conduit: a body; a plurality of circumferentially spaced supporting members mounted on said body for expansion laterally outward of said body, each supporting member having a drag cutter thereon; means for expanding said cutters and members laterally outward to a predetermined max mum for operatior upon the weli con 1.; cutters having preformed grooves extending upwardly from their lower ends to receive the conduit and center said cutters relative to the axis of the well conduit; said :eing equidistant from the common axis of the E, and body when said cutters occupy their maximum expanded positions; and means preventing outward expansion of said cutters beyond said predetermined maxi- .n wherein said cutters rest upon the upper end of the cond It with the conduit received within said grooves; the of each groove providing a cutting face.

2. in a rotary drill bit to be lowered in a well conduit disposed in a well bore to mill away a portion of the well conduit: a body; a plurality of circumferentially spaced supporting members mounted on said body for expansion laterallyoutward of said body; a drag cutter structure secured to each of said supporting members; means for expanding said members and cutter structure laterally outward of said body to a predetermined'maximum for operation upon the well conduit; each of said drag cutter structures having a plurality of depending teeth spaced from each other arcuately with respect to the bit axis; said teeth having preformed grooves extending upwardly from their lower ends to receive the conduit and center said teeth relative to the axis of the well conduit; said grooves being equidistant from the common axis of the conduit and body when said cutter structures and teeth occupy their maximum expanded positions; and means preventing outward expansion of said cutter structures and their teeth beyond said predetermined maximum wherein said teeth upon the upper end of the conduit with the conduit received within said grooves; the base of each groove providing a cutting face.

3. in a rotary drill bit to be lowered in a well conduit disposed in a well bore to mill away a portion of the well conduit: a body; a plurality of circumferentially spaced supporting members mounted on said body for expansion latcraliy outward of said body, each supporting member raving a drag cutter thereon; means for expanding said cutters and members laterally outward to a predetermined maximum for operation upon the well conduit; said cutters having preformed grooves extending upwardly from their lower ends to receive the conduit and center said cutters relative to the axis of the well conduit; the radial distance of the outer sides of said grooves from the common axis of the conduit and body being substantially equal to the radius of the outer surface of the conduit when said cutters occupy their maximum expanded positions; and means preventing outward expansion of said cutters beyond said predetermined maximum wherein said cutters rest upon the upper end of the conduit with the conduit received within said grooves; the base of each groove providing a cutting face.

4. In a rotary drill bit tobe lowered in a well conduit disposed in a well bore to mill away a portion of the well conduit: a body; a plurality of circumferentially spaced supporting members mounted on said body for expansion laterally outward of said body; a drag cutter structure secured to each of said supporting members; means for expanding said members and cutter structure laterally outward of said body to a predetermined maximum for operation upon the well conduit; each of said drag cutter structures having a plurality of depending teeth spaced from each other arcuately with respect to the bit axis; said teeth having preformed grooves extending upwardly from their lower ends to receive the conduit and center said teeth relative to the axis of the well conduit; said grooves being so arranged in said teeth that the outer sides of said grooves engage the outer surface of the conduit when the cutter structures occupy their maximum expanded positions with the conduit in said grooves; and means preventing outward expansion of said cutter structures and said teeth beyond said predetermined maximum wherein said teeth rest upon the upper end of the conduit with the conduit received Within said grooves; the base of each groove providing a cutting face.

5. In a rotary drill bit to be lowered in a well bore to mill away a portion of a well conduit in the well bore: a body; a plurality of circumferentially spaced drag outter means on said body; each of said cutter means having a preformed groove extending upwardly from its lower end to receive the conduit and center said cutter means relative to the axis of the well conduit; and means positio ng said cutter means on said body wherein said ooves are equidistant from the common axis of the conand body and wherein said cutter means rest upon the u er end of the conduit with the conduit received within said grooves; the base of each groove providing a cutting face.

6. In a rotary drill bit to be lowered in a well bore to mill away a portion of a well conduit in the well bore: a body; a plurality of circumferentially spaced drag cutter means on said body; each of said cutter means having a preformed groove extending upwardly from its lower end to receive the conduit and center said cutter means relative to the axis of the well conduit; means positioning said cutter means on said body wherein said cutter means rest upon the upper end of the conduit with the conduit received within said grooves; the base of each groove providing a cutting face the radial distance of the outer side References Cited in the file of this patent UNITED STATES PATENTS 1,667,155 Higdon Apr. 24, 1928 2,290,595 Kinnear July 21, 194.2 2,823,901

Kammerer Feb. 18, 1958

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