CA1231091A - Chip relief for rock bits - Google Patents

Abstract

CHIP RELIEF FOR ROCK BITS

ABSTRACT OF THE DISCLOSURE

This invention relates to roller cone, air circulation type rock bits. Means are provided on the rock bit body as well as on the shirttail portion of each of the legs extending from the body to provide a relief to pass rock chips from the borehole bottom and up the drill string as the air circulation roller cone bit works in a formation.

Description

~Z3~()91 CHIP RELIEF FOR ROCK BITS

BACKGROUND OF THE INVENTION
1. Field of the Invention This invention relates to air circulation roller cone rock bits.
More particularly, this invention relates to modern ate to high velocity and volume air circulation roller cone rock bits and a means formed in the rock bit to enhance rock chip removal from a Barlow bottom as the bit works in the earth formation.

2. Description of the Prior art It is well known in the rock bit art -to provide well fortified rock bit legs in multi-cone rock bits to assure that the rock b t maintains "gage" of a bore-hole while working in a formation. The leading edges of the shirttail portions of most of these hits are I hard faced to resist erosion of the bit shirttail since the shirttail portion is almost the same diameter as the cutting end of the rock bit. Additionally, the back of the leg is often studded with flush-type lung-stun carbide inserts to resist erosion wear caused by the legs coming in con-tact with the Barlow wall.
In petroleum drilling where -the clearance around the rock bit is minimal, the likelihood or drilling "mud"
circulating fluid pumped into the drill string is suffix ciently viscous to suspend the cut~illgs within itself -I and carry them out of the Barlow at a relatively low -1- I.

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Nate ox flow. With the introduction of air drilling, basic bit geometry did not change and the generally large detritus material in tune Barlow bottom could not be carried out of the hole by the less dense and less vise cows air until the rock particles were reduced in size by regrinding by the bit. Regrinding the detritus slow-Ed down the formation penetration rate of the bit and shortened the life of the bit. The reground rock chips tend to dull the cutters and wear away the shirttail portion of the bit. In addition, the finely ground particles get into the bearing surfaces formed between the roller cones and the journals supported by the bit, further limiting bit life. It is imperative then that the Barlow cuttings be immediately removed from the Barlow bottom so that the bit cutting surface is con-tunnel exposed to uncut rock as it penetrates the formation.
The relative rock cuttings transport capabilities of liquid and gas drilling fluids are defined in the follow-in analysis. Table 1 lists properties of rock cuttings transport capabilities of the fluids.
Table 1 of Rock Drilling Fluids ABSOLUTE
FLEDGED TEMPERATURE PRESSURE DENSITY VISCOSITY
Fahrenheit pounds pounds pounds per square per cubic per foot-inch feet second . _ Air 64 14.70.076 12.3 X 10 6 Air 165 54.70.236 14.1 X 10 6 Air 165 314.71.359 14.1 X 10 6 water 68 62.4 6.73 X 10 4 Mud I - 75.0 336 X 10 Mold 68 - 135.0 504 X 10 I

A small spherical particle falling under the action ox gravity through a viscous medium ultimately acquires a constant velocity expressed by Stokes' Law.

guy (dl-d2) v -- _ 9z where: v = velocity (feet per second) g = gravitational acceleration feet per second per second) a = radius of the sphere (feet) do = density of the sphere (pounds per cubic foot) do = density of the medium (pounds per cubic foot) z = viscosity (pounds per foot second) Using nominal particle size of one-eighth inch radius, particle density of 158 pounds per cubic foot, drilling mud density of 75 pounds per cubic foot, drilling mud viscosity of 0.0336 pounds per foot-20 second, and standard gravitational acceleration, we have:

2(32~2) (0.010~)2(158-75) v = 9 feet per second 9(0.0336) v = 115 feet per minute In theory, the velocity of the drilling mud up the annular area between the drilled hole wall and the out-side diameter of the drill pipe must exceed this velocity to transport the assumed spherical rock cutting particle out of the drilled hole.

~23~91 In practice, most drilled rock cuttings tend to be flat or lens-shaped and Pigtail suggests that the probable velocity will be about 40 percent of that calculated by the above equation. This gives good agreement with Naomi-net drilling mud velocities encountered in practice and Allen where this velocity (called slip velocity) does not exceed 50 percent of the drilling mud annular velocity:
slip = 115 feet per minute x 40~ = 46 feet per minute Mud annular velocity = slip 46 feet per minute x 2 = 92 feet per minute Stokes' law is applicable to viscous fluids only and cannot be applied to gaseous fluids. Even for high density air (314.7 pounds per square inch absolute pressure) the velocity becomes:
2(32.2) (0.0104)2(158-1.359) v = = 8598 feet 9(0.0000141) per second v = 515880 feet per minute which is obviously absurd.
Where air is the cooling, lubricating, and flushing medium Gray developed the following equation for rock cutting particle velocity (slip velocity):
.9456(2a)T(dl) v p Where:
T = Bottom hole temperature (degrees Ranking) P = Bottom hole pressure (pounds per square inch absolute) -ISSUE

Using the same rock particle data and air at 54.7 pounds per square inch absolute pressure, 160 Foreign-hell (625~ Ranking) temperature, and assuming bottom hole pressure equal to delivered pressure:
.9456 (.0208) 625 (158) v = = 35.5 feet per second 54.7 v = 2130 feet per minute Pro slip velocity at 50 percent of annular velocity we have:
Air Annular Velocity = V(slip)2130 X 2 = 4260 feet per minute Annular fluid volume flow from:
Q = VA
Where:
Q = annular fluid volume flow (cubic feet per minute) V = annular fluid velocity (feet per minute) A = annular area (square feet) For an 8 1/2 inch diameter rock bit Wyeth 5 inch outside diameter drill pipe, the annular area is 0.258 square feet and the annular fluid volume flow will be:
Mud = 92(0.258) = 23.7 cubic feet per minute Pair = 4260(0.258) = 1099 cubic feet per moonlit These fluid velocities and volumes are typical for mud and air drilling conditions.
In this analysis, the mud and air drilling annular areas are equal for transporting the same size of purity ale. It should be noted, however, that the selected rock particle size is most closely related to the relatively low drilling penetration rites associated with mud :123~09~

drilling. It should also be noted that the selected rock particle density is most closely related to that of the shales, limestones, and sandstones associated with petrol Lomb deposits where mud drilling is practiced. In mud S drilling, the annular area and rock bit to hole wall clearance around the bit body are more than adequate. The flow of the incompressible mud is governed by bit nozzle diameters of less cross-sectional area than either the rock bit body clearance or the drilled hole annular area.
Mud flow velocity through the nozzles, and therefore mud volume, is restricted by nozzle wear, cavitation effects, ,urhulence, pressure differentials, and available hydra-fig horsepower.
Generally, air drilling produces large rock particle sizes and high drilling penetration rates, particularly or blast-hole drilling in surface mining where 50 foot maximum hole depths are typical. The compressible air flows contracting and expanding down the drill pipe, through rock bit nozzles and open air passages through the rock bit bearings, around the bit cutting structures and body, and up the drill pipe annular area. The annum far area is usually adequate, but the rock bit to hole wall clearance around the bit body is often inadequate if designed to mud drilling standards. Additional bit body clearance is required for many air drilling apply-cations to permit passage of large rock particles and the greater volume of air required to transport the fang-or particles. Drilling penetration rates and related rock particle sizes commonly encountered in mud and ax driliiny are compared in Table 2.

~123~()931 Table 2 - Penetration Rates and Common Rock Particle Sizes Do I Do MUD DRILLING AIR DRILLING
Slow Drill Jo Rate -Penetration rates (feet per hour) < 3 < 30 Rock particle large dimensions (inches) < 1/4 < 1/4 Moderate Drilling Rate -Penetration rates feet per hour) 3-20 30-100 Rock particle large dimensions winches) 1/4 1/4-1/2 High Drilling Rate Penetration rates (feet per hour) > 20 100 Rock particle large dimensions (inches) > 1/4 > 1/2 The volume of rock cuttings passed over the bit body and up the drilled hole annular area is not significant for mud or air drilling. Table 3 shows the volume of rock particles removed from an 8 1/2 inch diameter hole (0.394 square feet cross-sectional area) at various penetration rates.
Table 3 - V fume Penetration Rate Penetration Rate Volume of Pock Removed (feet per hour) (feet per minute) (cubic feet per minute)

3 0.05 O.Olg 0.17 0.065 0.50 0.197 I 60 1.00 0.39~
loo 1 . 67 0.652 3~.)9~

Four a penetration rate of 160 feet per hour and using slip velocities equal to 50 percent of the fluid velocities previously calculated (92 feet per minute for mud drilling and 4260 feet per minute for air drilling), the areas no-squired to transport the rock cuttings will be:
Q
A =
V
1.051 (cubic feet per minute) A = = 0.023 square feet mud 92 X .5 (feet per minute) which is less than 10 percent of the annular area (0.258 square feet) 1.051 (cubic feet per minute) A . = = 0.0005 square feet at r ---- - -4260 X .5 (feet per minute) which is less than 0.2 percent of the annular area.
Using Gray's equation, the larger rock particle sizes for moderate (3/8 inch rock particle large dimensions) to high (1/2 inch rock particle large dimensions) air drill-in rates will produce a corresponding increase of one and one-half to two times the air velocity (6390 to 8520 feet per minute) and resulting air volume (1649 to 2198 cubic feet per minute) flowing in the drilled hole annum far area.
Although the relatively high penetration rate air drilling practices of surface mining are possible in petroleum drilling, the constraints of directional control, maintaining hole diameter for emplacing casing, and avoid-in bit damage to preclude premature removal of a length I

drill string from a deep hole dictate deliberately slow drilling. In contrast, surface mining blast hole air drilling permits rough directional control, rough hole diameter control, since casing it not emplaced, and is virtually insensitive to bit damage and bits are drilled to destruction. Consequently, higher penetration rates and large chips, with a corresponding requirement for greater clearance between the mining bit body and the drilled hole wall, are normal for virtually all surface mining air drilling relative to petroleum drilling.
As a practical matter, the clearance between a bit body and the drilled hole wall cannot be greater than the clearance between the shoulder of the threaded con-section at the threaded pin end of the bit. This clear- -ante is further restricted by the requirement for bit shirttail structural integrity, including allowances for lubricating and cooling passages. Using the bit cross-sectional clearance area through the threaded jet nozzles relative to the drilled hole annular area we have the lot-lowing typical ratios:
Petroleum bit ratio = .28- Mining bit ratio = .37 Mining air drilling bit clearance areas should be at least 37 percent of the available area and should be I about 30 percent more than that of a comparable petroleum mu drilling bit.
Experience has shown that in state of the art mining bits, the penetration rate is slow, wear rate is rapid and a heightened erosion rate of the shirttail leg port lion of each of the bits is evident. Therefore, the present invention overcomes these major problems in the mining industry. This is accomplished through careful removal of material from the shirttail portion of the rock bit, thus providing greater clearance so the rock chips or detritus may more easily pass from the Barlow bottom up the drill string and out OX the formation.

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- SUMMARY OF THE IN NOTION
It is an object of this invention to provide a mint in bit with superior means to pass detritus from a bore hole bottom to the surface of a formation.
More particularly, it is an object of this invention to provide an air circulation mining bit that has select-Ed portions of the shirttail of each of the legs of the rock bit removed to enhance chip removal from the bore-hole bottom.
This invention relates to an air circulation, air lug bracketed rock bit commonly used in the mining industry.
The bit consists of a rock bit body having a first cut-tying end and a second pin end, the body forming a champ bier therein. The chamber communicates with circulation air through an opening formed in the second pin end of the bit, the pin end of course being connected to a drill string. At least a pair of legs extend from the rock bit body (there are normally three legs in a three cone rock bit), each leg forming a shirttail portion and journal bearing, each journal bearing serving to support a roller cutter cone at a first cutting end of the bit. Cutting elements, such as tungsten carbide rock bit inserts, are positioned adjacent the largest diameter of each of the roller cones. These inserts serve to form tune means to I cut the gage (major diameter of the Barlow) of a bore-hole in a formation.
There is at least one nozzle formed in the dome area of the bit body, the nozzle being in communication with the chamber within the bit. The nozzle directs air past I each roller cone into the Barlow to lift detritus or rock chip material out of the bottom of the Barlow.
Relief means are formed in each of the legs, the relief means serve to pass the rock chips or detritus material from the Barlow past the rock bit body and out of the Barlow.
An annular space is provided between an outer surface of the bit body including the leg portions, and walls formed by the Barlow. The annular space, in a plane perpendicular to an axis of the bit, about adjacent an exit end of the nozzle. That is, the cross-sectional area of the resulting bit body clearance, measured through the jet nozzles (the jet nozzles are typically threaded), exceeds thirty-five percent of the cross-sectional annular area defined by the shoulder of the threaded pin end or connection and the drilled Barlow wall and increases as the bit cross section approaches the shouldered connection.
Additionally, each of the legs extending from the rock bit include channel-type grooves on the leading and trailing edge of each of the legs -to further enhance rock chip removal from the Barlow by relieving further the material of each leg of the rock bit body.
An advantage then over state of the art rock bits is the removal of material from the body of the bit to provide greater space for the removal of rock chips from a Barlow bottom.
Yet another advantage over the state of the art air circulation rock bits is the elimination of the need to hard face a portion of the leg, namely the leading edge ~Z3~(.)91 of the shirttail, to prevent erosion of the leg as it comes in contact with a Barlow wall.
Still another advantage is the elimination of the need to further protect the shirttail portion of the leg of a rock bit by embedding flush-type tungsten carbide inserts into the surface of the shirttail to further prevent Eros soon of this portion of the rock bit as it works ion a Barlow.
The above noted objects and advantages of the present invention will be more fully understood upon a study of the following description in conjunction with the detailed drawings.

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ERIE I)ESCP~IPT.-[ON OF THE DRAWINGS
FIG. 1 is a perspective view of a typical air air--culation mining bit illustrating the relieved portions of the bit that enhance rock chip removal from the bore-hole bottom;
FIG. 2 is an illustration of one leg of a typical three cone rock bit partially in cross section/ thus-trating.the relieved portions of the leg along the shirttail surface to enhance removal of rock chips;.
FIG. 3 is a side view of one leg of a rock bit, illustrating the relieved portions of each leg to en-hence chip removal; and FIG. 4 is a view taken through 4-4 of FIG 1, illustrating the annular hole wall clearance between the Barlow wall and the bit body.

DESCRIPTION OF THE P REFERRED EMBODIMENTS ED
Beat MODE phyla< CA TRYING OUT THE INVENTION
With reference now to FIG. 1, the rock bit, goner-ally designated as 10, is comprised of a bit body 12 having a cutting end 14, shown in phantom. The cutting end I forms a Barlow, generally designated as 32, in an earth formation. At the opposite end of bit body 12 is pin end 16, adapted to be connected to a drill string 25 (shown in phantom) of a drilling rig. Within the bit body 12 is formed a chamber 13 trot shown), the chamber directing fluid, such as air, through the pin end 16 into chamber 13 and out of nozzle 30 inserted through dome 19 of the rock bit body 12. Three legs, generally designated as 18, extend from bit body 12. Each leg 18 forms a shirttail portion 20. Shirttail portion 20 is relieved above the cutter cones 15 in area 28 by remove in material therefrom. The shirttail then is stepped down from the cones toward the pin end 16 of rock bit body 12. In addition to relieving material from the leg in the area shown as 28, the leg is further reduced in size by providing a scalloped or concave groove 21, formed in both the leading edge 22 and the trailing edge 24 of the legs 18. Normally, the shirttail port lion of a standard rock bit leg is much more massive than is shown in FIG. 1. Since the leg shirttail port lion is nearly as large as the gage of the rock bit in standard bits, the shirttail needs to be protected as heretofore described. The instant invention circus-vents the need for protection of the shirttail by simply 3~J removing material from the shirttail to both prevent -15~

3~)91 erosion of the leg of the rock bit as well as enhance rock chip removal, the latter being the more important of the two.
An annular space 36 is shown between the rock bit body 12 and the Barlow wall 33. The annular space or cross-sectional area 36 through a plane 37 perpendicular to an axis of the bit approximately through an exit end of the jet nozzles 30 its at least thirty-five percent of the annular cross-sectional area defined by the shoulder of the threaded pin end or connection and the wall of the Barlow 32.
Turning now to FIG. 2, the leg portion is shown in a Barlow 32. Cone 15 is illustrated in contact with the bottom of the Barlow and, as the roller cone rotates in the Barlow bottom, the cutting elements (tungsten carbide inserts 23) scrape, gouge, and crush the formation, thus creating detritus or rock chips 34 which must be removed from the Barlow bottom. In mining bits, air is used as both a bit lubricant and a means to remove detritus from the Barlow bottom. Air is directed though the nozzle 30 (FIG.
1) toward the Barlow bottom and the rock chips 34 are blown out of the Barlow bottom past the bit body and up the Barlow 32. The rock bit leg then is relieved by removing material from the shirttail 20 in the area indicated as 28 and by providing a concave groove 21 in the leading and trailing edges 22 and 24 of the leg 18 of the rock bit body 12. Detritus 34 then easily passes by the cutter cones 15, past the bit body 12 and up the Barlow, being enhanced by the relieved portions in both the shirttail surface and the leading and trailing edges of the leg 18 of the bit 10.

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With reference now to Fig. 3, the bit body 12, being turned 90 from FIG. 2, further illustrates the areas of the leg 18 which are removed, namely the stepped area 28 of the shirttail portion 20 and the scalloped grooves 21 along the leading edge 22 and trailing edge 24.
Turning now to FIG. 4, the annular space 36 through plane 37 defined a cross-sectional area at least thirty-five to thirty-seven percent of the annular cross-sectional area defined by the shoulder of the threaded pin end or connect lion and the walls 33 of the Barlow 32. The cross hatched portion of the illustration represents each of the legs that support cutter cones 15. As heretofore mentioned, jet air nozzles 30 direct compressed gaseous fluid toward the bore-hole bottom to lift variously sized detritus out of the Barlow.
This relatively simple procedure produced a dramatic increase in Barlow penetration in the mining field. For example, recent tests have revealed a standard 6 3/4 inch mining bit, without chip relief, would normally cut 2500 feet of earth formation. A 6 3/4 inch bit with chip removal features as taught in this invention, in the very same formation, cut 4500 feet of earth formation, resulting in a 77% increase in rock bit performance. Several bits were run to confirm this phenomenal increase in rock bit penetration with an average increase in performance of about 75%
overall. This indeed is a new and unusual result from a rock bit modification, especially in air circulation mining bits. Field reports have shown that chip grooves, such as the scalloped grooves 21 in leading edges 22 and trailing edges 24 of the rock bit, adds significantly I

to chop flow with increased bit life and performance. It was also confirmed that the chip relief is equally effect live for milled tooth and tungsten carbide insert bits, the latter being illustrated in the instant invention.
Field engineers have observed that when large rock bit stabilizers are attached to the rock bits, the diameter of the stabilizer being near the diameter of the bore-hale, rock chip removal is again inhibited, even with a bit with chip relief. This observation confirmed that rock chips or detritus is reground over and over again to enable them to finally pass by the large diameter stabilizer. Where stabilizers are used in conjunction with air circulation bits with chip relief, the dram-ever of the stabilizer must be reduced accordingly to complement the modified bit and its greater capacity to pass detritus material thereby. Where this practice is followed, a 75~ increase in bit performance can be ox-pealed. Air flow through an air circulation bit must have a clear path of escape once it passes through the nozzles 30 of the rock bit. Free flow of air is needed if remitting or recutting of the chips it to be pro-vented. Engineering tests confirm that mining bits, as modified by the teachings of this invention, do indeed exhibit increased rock bit penetration rates. The life of the cutting end of the bit is prolonged with a more efficient means to remove more and larger detritus from the Barlow bottom thus contributing to the phenomenal increase in rock bit efficiency and performance.
Chip relief for sealed bearing rock bits used in the oil field will enhance their performance as well. detritus I

material washed out of the bottom of a Barlow by drill-in mud will more easily pass by the bit with chip relief.
It will of course be realized that various modific2-lions can be made in the design and operation of the pros-end invention without departing from the spirit thereof.
Thus, while the principal preferred construction and mode of operation of the invention have been explained in what is now considered to represent its best e~odiments, which have been illustrated and described, it should be under-stood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated and described.

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References Pig got, R.J.S., "Mud Flow in Drilling", Drilling and Production Practices, APT ~1941), pp. 91-103 Allen, James H., "How to Relate Bit Weight and Rotary Speed to Bit Hydraulic Horsepower", Drilling DOW, May 1975 Gray, K. E., "The Cutting Carrying Capacity of Air at Pressure Above Atmospheric", MY Thesis, University of Tulsa, Oklahoma (1957)

Claims (8)

1. An air circulation, air lubricated roller cone rock bit comprising:
a rock bit body having a first cutting end and a second threaded pin end for connection to a drill string a shoulder on the bit body forming a thread termination base for the second threaded pin end of the rock bit;
an interior chamber in the body communicating with circulation air from the drill string through an opening formed in the second pin end of the bit body;
at least a pair of legs extending downwardly from the bit body, each leg including a shirttail portion at the outside of the bit body and adjacent to the wall of a bore hole drilled by such a rock bit, each leg also comprising a bearing supporting a roller cutter cone at the first cutting end of the bit, such a roller cone including cutting elements adjacent to the largest diameter of the roller cone for forming the gage of a bore hole drilled by such a roller bit;
at least one air nozzle in the bit body in communication between the chamber and the exterior of the bit body for directing air past each roller cone into a bore hole to lift detritus material out of the bottom of the bore hole; and an annular space between the outer surface of the bit body and walls formed by such a bore hole drilled by the rock bit for passing the detritus material from the bottom of the bore hole past the rock bit body and out of the bore hole;
said rock bit being characterized by the cross-sectional area of the annular space between the outside surface of the rock bit body and the walls of the bore hole in a plane at the exit end of such a nozzle being at least 35% of the annular cross-sectional area defined by the shoulder of the threaded pin end and the walls of the bore hole; and wherein the cross-sectional area of the annular space progressively increases in successive planes perpendicular to the axis of the rock bit from the lower end of the shirttail to the shoulder.

2. A rock bit as recited in Claim 1 further comprising a longitudinally extending channel groove in each of the leading edge and trailing edge of the shirttail portion of each leg.

3. A rock bit as recited in Claim 2 wherein each of the channel grooves progressively increases in size from the lower end of the shirttail portion toward the pin end of the rock bit.

4. An air circulation, air lubricated, roller cone rock bit comprising:
a rock bit body having a first cutting end and a second threaded pin end and a shoulder at the base of the threads for connection to a drill string, the body having a chamber therein, communicating with circulation air from a drill string through an opening formed in said second pin end, at least a pair of legs extending from said bit body, each leg forming a shirttail portion on the exterior of the bit body and a bearing, each bearing serving to support a roller cutter cone at said first cutting end of said bit with cutting elements adjacent the largest diameter of each roller cone for forming a gage of a bore hole in a formation, at least one nozzle formed in said bit body in communication with said chamber for directing air past each roller cone into said bore hole to lift detritus material out of the bottom of the bore hole, and relief means formed by said legs for passing said detritus material from the bottom of said bore hole past the rock bit body and out of said bore hole by providing an annular space between an outer surface of said legs and walls formed by said bore hole, characterized by the annular space, in a plane perpendicular to an axis of said bit adjacent to an exit end of such a nozzle, being thirty-five percent or more of the annular area defined by the shoulder of the threaded pin end and the wall of said bore hole through said plane, said annular space progressively enlarging in perpendicular planes between said exit end of such nozzle and a shoulder formed on said bit body, each shoulder forming a thread termination base end for said second threaded pin end of said rock bit.

5. The invention as set forth in Claim 4 wherein the relief means formed by the legs further includes channel grooves formed in the sides of each leg in a leading edge and a trailing edge of the shirttail portion of such a leg.

6. The invention as set forth in Claim 5 wherein the relief means formed by the legs further includes a space formed between the shirttail portion of each of the legs and the wall of the bore hole, wherein the shirttail space is formed by relieving the surface of the shirttail adjacent to the bore hole wall, the leading and trailing edge grooves and the relieved portion of the shirttail serving to enhance the removal of relatively large detritus material from the bottom of the bore hole.

7. An air circulation, air lubricated, roller cone rock bit comprising:
a rock bit body having a first cutting end and a second threaded pin end, said body forming a chamber therein, said chamber communicates with said circulation air through an opening formed in said second pin end, said pin end being connected to a drill string, at least a pair of legs extending from said bit body, each leg forming a shirttail portion and a bearing, each bearing serves to support a roller cutter cone at said first cutting end of said bit, cutting elements adjacent the largest diameter of each roller cone form a gage of a borehole in a formation, at least one nozzle formed in said bit body in communication with said chamber, said nozzle directs air past each roller cone into said borehole to lift detritus material out of the bottom of the borehole, and relief means formed by said legs, said relief means serves to pass said detritus material from the bottom of said borehole by the rock bit body and out of said borehole by providing an annular space between an outer surface of said legs and walls formed by said borehole, said annular space, in a plane perpendicular to an axis of said bit, about adjacent an exit end of said at least one nozzle, is thirty-five percent or more of the area formed by said borehole through said plane, said annular space progressively enlarges in perpendicular planes between said exit end of said at least one nozzle and a shoulder formed in said bit body, said shoulder forms a thread termination base end for said second threaded pin end of said rock bit.

8. An air circulation, air lubricated, roller cone rock bit comprising:
a rock bit body having a first cutting end and a second threaded pin end, said body forming a chamber therein, said chamber communicates with said circulation air through an opening formed in said second pin end, said pin end being connected to a drill string, at least a pair of legs extending from said bit body, each leg forming a shirttail portion and a bearing, each bearing serves to support a roller cutter cone at said first cutting end of said bit, cutting elements adjacent the largest diameter of each roller cone form a gage of a borehole in a formation, at least one nozzle formed in said bit body in communication with said chamber, said nozzle directs air past each roller cone into said borehole to lift detritus material out of the bottom of the borehole, and relief means formed by said legs, said relief means serves to pass said detritus material from the bottom of said borehole by the rock bit body and out of said borehole by providing an annular space between an outer surface of said legs and walls formed by said borehole, said annular space progressively enlarges in perpendicular planes between said exit end of said at least one nozzle and a shoulder formed in said bit body, said shoulder forms a thread termi-nation base end for said second threaded pin end of said rock bit, said relief means formed by said legs include channel grooves formed in the sides of the leg in a leading edge and a trailing edge of said shirttail portion of said leg, said relief means formed by said legs further include .

a space formed between said shirttail portion of each of said legs and said wall of said borehole, said space is formed by relieving the surface of said shirttail substan-tially paralleling said borehole wall, said leading and trailing edge grooves and said relieved portion of said shirttail portion paralleling said borehole wall serve to enhance the removal of relatively large detritus material from the bottom of said borehole, a cross-sectional area of said annular space measured in a plane through said one or more nozzle exceeds thirty-five percent of the cross-sectional area of the borehole and increases as the bit cross-section approaches said shoulder of said second pin end of said bit.