US2894726A - Drilling bit - Google Patents

Description

July 14, 1959 E. M. WEAVER ET AL 2,894,726

DRILLING BIT Filed April 10, 1956 5 Sheets-Sheet 2 FIG. 3.

IN VENTORfl EARL M. WEAVER F1624. BURLE CHAWTHOR/VE JESSE LSTOCKARD ATTORNEYS July'l4, 1959 E. M. WEAVER ET AL I 2,894,726

' Y DRILLING BIT Filed April 10, 1956 5 Sheet s-Sheet 3 FIG 5 H616 INVENTORS EARL M. WEAVER BURLE c. HAWTHORNE JESSELSTOCKARD I ATTORNEYS July 14, 1959 E. M. WEAVER ET AL 2,894,726

DRILLING BIT Filed April 10, 1956 5 Sheets-Sheet 4 FIGJZ INVENTORS EARL M. WEAVER BU/PLE 6. HAWTHORNE JESSE L.STOCKARD ATTORNEYS July 14, 1959 WEAVER E AL 2,894,726

DRILLING BIT Filed April 10, 1956 -5 Sheets-Sheet 5 24A v- 248 2 8 A 2eA 44 42 26B INVENTORS JESSE L. STOCK/1RD United States Patent DRILLING BIT Earl M. Weaver, Burle C. Hawthorne, and Jesse L. Stockard, Houston, Tex.

Application April 10, 1956, Serial No. 577,233

13 Claims. (Cl. 255-61) This invention relates to earth-boring drag blade bits and more particularly to a blade bit incorporating inserts in the cutting edges thereof.

This application constitutes a continuation-in-part of our copending application Serial No. 511,452, filed May 27, 1955, now abandoned.

An object of the present invention is to provide an insert blade bit having a novel cutting edge constructed to obtain highly efiicient cutting. It has been found that several factors contribute to the efiicient cutting of a blade bit, for example, the weight of the drilling loads imposed upon the drill bit, the rate of advance and rotation of the drill bit and the condition of the material through which the blade bit is cutting. In order to more effectively take care of these conditions so that the greatest possible efiiciency in cutting may be obtained, the cutting edge of the blade bits should be constructed so that the pilot cutting edge is of the proper size, the stepping of the cutting edge is of proper inclination and dimension to stabilize the side movement of the blade bit and eliminate all unnecessary drag or friction.

Accordingly, it is another object of the present invention to provide a blade bit having inserts incorporated in the cutting edges thereof, which inserts are disposed so as to provide a pilot cutting edge of improved construction operable to enable the blade bit to operate efficiently for an extended period of use without failure.

A still further object of the present invention is the provision of a blade bit having inserts incorporated within a cutting edge thereof, which inserts are positioned so as to provide proper stepping in the cutting edge thereby providing proper stabilizing effect upon the side movement of the blade bit.

A still further object of the present invention is the provision of a blade bit having inserts incorporated in the cutting edges thereof, the blade bit being constructed so as to minimize all unnecessary drag and friction which may occur during the operation thereof.

A still further object of the present invention is the provision of a blade bit having improved inserts incorporated therein, which inserts operate during the use of the blade bit to maintain the maximum step condition of the cutting edge during use.

These and other objects of the present invention will become more apparent during the course of the following detailed description and appended claims.

The invention may best be understood with reference to the accompanying drawings wherein illustrative embodiments are shown.

In the drawings:

Figure 1 is a front elevational view of a blade bit assembly embodying the principles of the present invention;

Figure 2 is a bottom view of the assembly shown in Figure 1;

Figure 3 is an elevational view of the assembly shown in Figure 1 rotated 90 about its vertical axis;

Figure 4 is a bottom view of one of the blade bits;

2,894,726 Patented July 14, 1959 Figure 5 is an elevational view of the blade bit illustrating the cutting edge thereof before use;

Figure 6 is a side elevational view of the blade bit shown in Figure 5;

Figure 7 is a bottom view of the blade bit shown in Figure 5;

Figure 8 is a view similar to Figure 5 showing the cutting edge of the bit in a first stage of wear;

Figure 9 is a side elevational view of the bit shown in Figure 8;

Figure 10 is a bottom view of the bit shown in Figure 8;

Figure 11 is a view similar to Figure 5 showing the bit in a second stage of wear;

Figure 12 is a side elevational view of the bit shown in Figure 11;

Figure 13 is a bottom view of the bit shown in Figure 11;

Figure 14 is a view similar to Figure 5 showing the bit in a final stage of wear;

Figure 15 is a side elevational view of the bit shown in Figure 14;

Figure 16 is a bottom view of the bit shown in Figure 14;

Figure 17 is a view similar to Figure 11 illustrating the bit of modified construction;

Figure 18 is a side elevational view of the bit shown in Figure 17;

Figure 19 is a bottom view of the bit shown in Figure 17;

Figure 20 is a view similar to Figure 14 illustrating the final stage of wear of the bit shown in Figure 17 Figure 21 is a side elevational view of the bit shown in Figure 20;

Figure 22 is a bottom view of the bit shown in Figure 20;

Figures 23-26 are views similar to Figure 5 illustrating various arrangements of the inserts along the cutting edge of the bit;

Figure 27 is an enlarged fragmentary elevational view of a blade having a modified pilot cutting edge; and

Figure 28 is a view similar to Figure 27 of a blade having a pilot edge of a further modified form.

Referring now more particularly to the drawings, there is shown in Figures 1-3, a blade bit assembly, generally indicated at 10, which is of the type disclosed in Hawthorne et al. Patent No. 2,695,758 and Hawthorne Patent No. 2,666,622. Assemblies of this type include a bit body (not shown) having a threaded shank and a lower tapered portion. The tapered portion of the body is provided with an annular groove on the outer periphery thereof and a clamping collar 12, provided with an internal taper, cooperates with the body to secure a plurality of cutters or blades 14 on the body. In order to prevent slipping of the clamping collar 12, a drill collar 16 is provided with internal threads arranged to engage the threaded portion of the body. When the bit is assembled, the drill collar 16 will engage the end of the collar 12 to clamp the blades 14 in position on the body. As shown in Figure 1, the assembly is centrally hollow, as indicated at 17, to receive a drilling fluid. This general arrangement of a bit assembly is conventional and forms no part of the present invention.

It will be understood that the present invention is concerned more particularly with the cutting action of the blades and the construction that achieves such action. Accordingly, the above manner of securing the blades to the drilling shaft or bit body is merely exemplarly and other means may be provided, as for example, welding or the likev As indicated above, three identically shaped blades 14 are utilized in the blade bit assembly to accomplish the actual earth boring and since each of the.

blades is identical, description of one should suffice to describe all.

The blade 14 comprises in general, an upper shank end or portion 18 to which the bit body and collars are attached and a lower cutting end or portion 20. The blade may be drop-forged in a single piece so that the cutting end presents a substantially planar leading surface 21 having its lower edge recessed, as at 22, for the reception of a plurality of inserts 24, of a hard material such as tungsten carbide or the like. The lower edge of the cutting end 20 of the blade includes two aligned inserts 24A and 24B, the lower edges of which are disposed in a horizontal plane so as to provide a continuous pilot cutting edge 26. The inner corner 26A of the cutting edge 26 is spaced radially from the axis of rotation of the blade and the outer corner 26B terminates at a point intermediate the inner corner 26 A and the outermost position of the cutting end 20 of the blade. Disposed above and outwardly of the pilot cutting edge 26 are suitable inserts 24C and 24D which define a plurality of steps 28. These steps are related to the pilot cutting edge in such a way that a line drawn through the lower outer corner 23A of each step and the lower outer corner 26B of the pilot edge is inclined with respect to the axis of rotation of the bit assembly an angle which is preferably between 45 and 50, as shown in Figure 5. Disposed above the outermost insert 24D of the steps 28 is a plurality of vertically aligned inserts 24E and and 24F which together with the outermost step insert define a peripheral cutting edge 30 of the blade.

Referring now more particularly to Figures 3 and 4, the thickness of the under or bottom cutting edge surfaces including the inserts and the adjacent portion of the cutting end 20 of the blade is substantially equal to only twice the thickness of the inserts. Stated differently, the thickness of the back up material provided by the cutting end of the blade at the bottom cutting edge surfaces is substantially equal to the thickness of the adjacent inserts themselves. From these relatively thin bottom edge surfaces the blade tapers upwardly or increases in thickness toward the upper end thereof. Preferably, the trailing face of the blade includes a plurality of strengthening ribs 32 which extend upwardly at each step of the blade and at the pilot edge.

As was previously mentioned above, the characteristics and construction of the cutting edge of the blade is related to the drilling load, speed of rotation and feed of the bit and the material within which the bit is to be utilized. Consequently, the construction of the cutting edge of the blades constitutes the essential feature of the present invention.

It will be understood that in drilling a hole through soft material, there will quite frequently be strata or sections of harder formation encountered. Upon entering such sections, the pilot cutting edges will first contact the hard formation. Thus, at the exact moment that such contact is made, the entire load on the bit is placed on these leading pilot cutting edges and therefore, it is important that they be of proper size to accommodate and support the weight of the drilling load. Moreover in normal operation, the blade bit will be drilling at a certain rate of speed so that if a hard section is encountered, the operator will have insufficient time to regulate the drill ing rig with regard to speed and rate of penetration before excessive stresses are placed upon the blade bits. Under these conditions, the drill may be running at a rate of speed and effecting a rapid penetration in a manner that is totally inconsistent with the drilling rates and speeds which should be used in drilling hard formations. As a result, a bit that is designed for one particular type of formation may not stand up in another due to the fact that it is not run at the proper speed. Heretofore, the pilot edges have been relatively small so that if the load required to perform ordinary drilling is excessive, the leading pilot edges may fail. With the pilot cutting edge of the present invention, the size of the same is related to the size of the rig and drilling loads to be imposed, but yet is sufficient to take care of excessive conditions.

Referring now to Figure 1, it will be seen that the pilot cutting edge 26 of the blade has a distance equal to the radius B less the radius A and that during operation the same will describe a pilot hole having a radius B. The relationship of this pilot hole to the total size of the hole being cut will be the proportion between radius B and radius D. It will be understood that the number of steps provided may vary in accordance with the size bits utilized as well as other considerations. Since the size of the bit assemblies vary, it is impossible to give a single physical dimension of the pilot cutting edge which would be applicable to all. However, in order to exemplify this condition, a radius B size of approximately 1 /4 of an inch has been found to be advantageous where a 4% inch hole is being drilled under a load of approximately 8,000 pounds. Where holes of 4 inches or 4% inches are being drilled under loads of 6,000 to 7,000 pounds maximum, the radius B would be approximately /a of an inch or 1 inch. Where drilling loads on the order of 3,000 to 5,000 pounds are used in drilling holes of 3 inch size, radius B would be approximately /2 inch to /4 inch maximum. In general, therefore, it is preferable to provide the pilot cutting edge with a radius dimension or radial extent B which is approximately onehalf the radius dimension or radial extent D. A further preferred form of the bit embodies a blade profile having outward and upward steps, with the pilot step being approximately twice the radius dimension or radial extent of each of the remaining steps. That is, in Figure 1, B -A would be twice E, where E is a typical radial extent or radius dimension of each of the remaining steps.

Another advantage obtained by the provision of a pilot cutting edge constructed as indicated above, is that the clearance angle of the outer vertical surface 26C of the step formed thereby need not be as large as is necessary with the relatively short pilot cutting edges heretofore provided. Referring to Figure 4, it will be seen that the leading vertical edge extending upwardly from the outer corner 26B of the pilot cutting edge 26-, during rotation, will described a circle of radius B and in order to provide proper clearance, all points on the adjacent vertical face 26C must be disposed at most a distance B away from the axis of rotation. The angle that this face makes with a tangential plane touching a circle drawn from the axis of rotation through the leading vertical edge may be termed a clearance angle. Hence, the clearance angle or curvature of such a vertical face will become greater the closer it is to the axis of rotation. Thus, when the radius B is made relatively short, as heretofore proposed, as for example, as indicated at B in Figure 4, the greater clearance angle or curvature of the outer vertical face would substantially reduce the amount of material provided to back up the pilot cutting edge. This condition necessitates a thicker wall at the cutting edge in order to prevent failure thereof and with such thicker edge wall, excessive drag results. This is of particular significance where inserts are employed since it is essential that adequate backup material 'be provided therefor so as to prevent failure.

Also of significance in connection with the pilot cutting edge 26 is the length of the radius A. It is well known in the tungsten carbide art that such material must have a positive surface speed in operation in order to properly perform its function. Since the center of the bit has no surface speed, problems are encountered at this point when using tungsten carbide inserts or the like. In practice, a bit drilling a hole tends to walk around or have some side play so that the entire actual bottom of the hole within the radius A is contacted by the insert, although it is not actually considered to be cut by the inserts in the ordinary sense. Thus with the bit walking around on the bottom of the hole, the inside corner 26A of the innermost insert may slide across to the center of the hole and rub across its face. While this condition is most usually the case in practice, in some hard formations the bit will drill exactly the same size hole as the size of the bit. In this case, there will actually be a small core of material of a radius A extending up between the blade bit to some extent.

The proper size of the radius A is, therefore, controlled by two factors. First, with regard to the non-drilling in the center of the hole, the radius A must be large enough that a slight speed of drilling is obtained at the inside corner 26A so as to do some cutting. Second, the radius A must be large enough so that with flexing of the blades, the inside corner 26A and inside cutting edge thereof does not come to the center of rotation or contact one another, as the pressures may cause the inserts to chip. Furthermore, with respect to the walking around of the blade bit mentioned above, the radius A must be sufficiently large to definitely prevent the inside corner 26A from moving across the center of rotation. With this condition, it will be understood that a small part of the insert would then extend across the center and be traveling actually in a reverse direction with respect to the remainder of the blade. Thus, a reverse pressure would be applied opposite from the cutting direction which would tend to chip the insert and break the bond with the blade itself. The radius A, of course, cannot be too large since the center of the hole must be drilled. In general, this radius has been found to be between A3 of an inch to A of an inch depending upon the size of the bit assembly itself. Thus, a 4% inch bit in some formations may actually drill a 5 inch hole which means that the hole would be /4 of an inch oversize and hence, the radius A would be /s of an inch. Stated in general terms, the size of radius A should be such as to give the innermost insert an opportunity to cut through the bottom of the hole with a positive surface speed.

As was indicated above, the steps 28 are preferably disposed so that their corners 28A fall within a line extending at an angle of 45 to 50 with respect to the axis of rotation. The height of the steps should be such that they have a tendency to resist the side play or walking around of the bit mentioned above. It is to be noted that when a bit enters a hard strata of rock, the pilot cutting edges 26 must do the initial drilling followed up by the full diameter of the bit. Consequently, each step functions as a consecutive pilot as the bit enters into the hard section. Stated differently, if one of the stepped cutting edges entering a hard section acts as a pilot, it can maintain the bit in circular axial alignment. It has been found that the amount of side play in the bit assembly may cause considerable damage to the cutting edges so that by properly dimensioning the steps to reduce this Side play, the wearing characteristics of the inserts may be substantially improved.

Another feature of the present invention is the provision of minimizing all unnecessary drag or friction caused by the blades during operation. Here again, the concept that the bit tends to walk around on the bottom of the hole so as to drill a hole larger than the size of the bit determines to some extent the various clearances of the blade during operation. Ordinary clearances are those which will just barely keep the cutting end of the blades from touching the wall of the hole of each stepped radius as the cutting edge moves into operative position. With the present invention, these ordinary clearances are increased to accommodate the walking around of the bit within the hole.

Referring now to Figure 4, it will be seen that the outer corner 26B of the pilot cutting edge describes a radius B, the inside step corner 28A a radius C and the peripheral cutting edge 30 a radius D. Obviously, the angle which the vertical surfaces corresponding to these radii make relative to the vertical planar face 21 will be less, the greater they are disposed outwardly of the axis of rota- 6 tion. The portion of the cutting end of the blade which backs the inserts is given an initial relatively slight angle of relief which provides the proper amount of support for the inserts and at the same time still provides clearance in the hole. The angle of relief may be defined as the increased angular inclination of the vertical surfaces with respect to the angle of clearance noted above, as indicated in Figure 4. The trailing portion of each vertical surface is given a still greater angle of relief insuring that the trailing edge will not contact the wall of the hole even when walking occurs. The relief of the cutting edges as shown in Figure 4 is such that there is no rubbing of the outer vertical surfaces of the bit against the wall of the hole for each stepped radius. In this manner, undue side bearing or drag is greatly reduced or substantially eliminated. Another factor in this regard is the generation of extreme heat due to friction which may cause failure of the bond of the inserts.

Another factor in the eliminating of unnecessary drag is occasioned by the sharpness of the cutting edge of the blade bit. Quite frequently during operation, a bit assembly will drill five feet per minute and it is not unusual to have periods of operation where the bit will drill ten feet per minute. At a rate of penetration of five feet per minute with a blade rotation of rpm, 60 inches of hole will have been drilled with 120 rotations. This means that the bit has advanced /2 an inch with each rotation. Assuming that each blade cuts an identical amount of formation, it then follows that each cutting edge has cut /6 of an inch during one rotation, since there are three blades.

Referring now to Figure 6, it will be seen that the area of the under or bottom surfaces of the blades, which are in contact with the bottom of the hole, should be relatively small in order to prevent undue drag since the same are penetrating a considerable extent during each rotation. The greater the area of contact of the blade, the greater will be the drag thereof. Thus, at any given drilling load, there is a certain area of engagement of the end of the blade into the formation against which pressure must be applied. By providing a relatively sharp or thin bottom cutting edge surface, this area of contact is minimized thus enabling the present bit to operate with less drag.

Heretofore, it has been generally regarded that the use of inserts required the provision of a relatively thick blade portion to properly back up the inserts so that they would not fail too readily. With the present invention, the blade adjacent the insert is upwardly tapered sufficiently to provide adequate back up metal, but yet to enable the bottom surfaces of the cutting edge to be relatively sharp. As previously mentioned, the thickness or width of these bottom surfaces of the cutting edges is approximately twice the thickness of the inserts. As best shown in Figure 6, these bottom cutting edge surfaces are initially provided with a ground relief angle of approximately 7. That is, the bottom cutting edge surfaces extend upwardly from the leading cutting edges an angle approximately 7 with respect to the horizontal. Beyond these substantially horizontal cutting edge surfaces, the trailing surfaces of the blade taper upwardly at an angle of the order of 35 to 43 with respect to the vertical.

Of particular significance, is that the contact area of the blade does not substantially increase during wear. Figures 8-10 illustrate the blade after a first stage of wear and it will be noted that only a slight amount of increased area will be in contact with the bottom of the wall. This wearing away of the blade in a self-sharpening manner so that a minimum load is required during use, renders the present bit highly efficient in operation. It will be understood that where a greater area of contact is made, greater loads must be utilized for a given rate of penetration or conversely for a given load, the greater the area of contact, the less the rate of penetration will be. Figures 11-16 illustrate the manner in which the cutting edges of the present blades are worn successively during normal operation.

Note particularly that the width and/or area of the bottom surfaces are not substantially greater in the final stages of wear shown in Figures and 16 than the width and/or area originally provided.

As set forth in the patents mentioned above, the fracturability of a formation in drilling depends upon the dimension of the horizontal edge or face of the various steps of the drill bit so that the force of penetration will cause fracturing to occur. When the drilling load is imposed upon the bit with a substantial width of horizontal face, more fracturing will occur. Thus, in accordance with the present invention, the outer ends of the inserts forming the steps may be provided with harder particles 33 which will cause the cutting edge to wear as indicated in Figures 1719. The actual particles pressed in the insert may be somewhat larger and harder tungsten carbide particles, diamond-like particles, actual diamond particles themselves or the like. It has been found that the optimum condition to obtain this wearing effect is when the width of the outer end of the inserts, which contain the harder particles, is approximately one-fourth the total width of the insert. With this arrangement, the blade has a long efiicient life and the effects of wear on the cutting edge are minimized until the final stage of wear as shown in Figures 2022.

It will be understood that the inserts themselves may take many forms and may be disposed within the recess of the blade in different positions. Briefiy, tungsten carbide inserts are manufactured by the use of processes of powdered metallurgy wherein tungsten carbide is sifted through screen mes. s of approximately 100 lines per inch. With the addition of alloy elements such as cobalt for toughness and possible eutectic material, the powdered particles are pressed into a mold and then placed in a furnace for sintering. This powdered metallurgical process results in a very dense material which is extremely hard and particularly suited to promote more efiicient drilling. Briefly, the inserts are usually rectangular in shape and are applied within the recess 22 of the blade after first placing a shim of soldering material in place on the blade within the recess. Flux is then sprinkled over the blade and it is then placed in a furnace for preheating. In the furnace, the temperature of the blade is raised approximately to the soldering temperature, after which the blade is withdrawn from the furance and worked by hand in order to insure that the imerts are placed in their proper locations. A final grinding may be performed if desired.

The inserts illustrated in connection with Figures 122 are rectangular in shape and placed so that the inserts forming the pilot cutting edge are disposed with their long sides horizontal. The inserts forming the steps 28 are likewise placed with their long sides horizontal and the inserts of the outer cutting edge are placed so that their long sides are disposed vertically.

In Figures 2326, various other arrangements and shapes of inserts are illustrated. Figure 23 shows rectangular inserts similar to the arrangement previously described except that the steps are spaced apart by two inserts 34 disposed at right angles with respect to other step inserts. in Figure 24, steps are formed by two per pendicularly disposed rectangular inserts 3s and the pilot cutting edge is formed by a similar pair of inserts disposed perpendicularly with respect to each other. In Figure 25, the inner end of the inserts 33 forming the steps is angular or is cut off so that these inserts are substantially triangular in shape. Figure 26 illustrates an arrangement similar to Figure wherein the corresponding inserts have their inner surfaces notched so that their combined effect is of a serration of greater step effect. It will be appreciated that numerous other arrangements may be utilized within the scope of the present invention.

While the blades illustrated in Figures l26, which have horizontal pilot cutting edges, have proven satisfactory under most conditions, it has been found that in drilling through very hard formations, a considerable amount of walking occurs in operation. In examining the bottom profile of a plug cut from the bottom of a hole being dug by the use of the blades described above, it can be noted that the profile in plan assumes the shape of a substantially square figure with its sides bowed in slightly and corners rounded off. The significance of the configuration is that a three-bladed bit is forming a four-sided hole of a size greater than the diameter of the bit. This phenomena graphically illustrates the walking action of the bit within the bottom of the hole. It can be concluded from an examination of the profile of the hole bottom that, during the rotation of the bit two blades are moving about an axis through the third and that the two that are moving are progressively changing. Where such a substantially square hole is cut, it is evident that there is considerable walking of the bit with the attendant probability that the inner corner of the pilot edge will pass inwardly to and beyond the axis of rotation so as to have a reverse direction of travel. it is this reverse direction of travel which causes the chipping off of the inserts themselves. In other words, under extreme conditions, the amount of ofiset of the inner corner of the pilot cutting edge as described above, at times, is not sufiicient to prohibit the inner corner from crossing the axis of rotation. Rather than make the offset greater, it has been found that walking can be substantially reduced even under the most difiicult conditions by grinding off the corners of the pilot cutting edge as shown in Figures 27 and 28. Since these embodiments operate effectively under the most extreme conditions, as well as under ordinary conditions, they constitute the preferred embodiments of the invention.

When dealing with small size bits, it is necessary only to grind off the outer corner of the pilot edge, as at 4'2, and this cut is preferably made along a line disposed at an angle of approximately 20 with respect to the horizontal. Otherwise, the blade is substantially identical to those described above, and it will be understood that the pilot edge construction of the preferred embodiments may be utilized in any of the embodiments previously described.

With a completely horizontal pilot edge as disclosed in Figures 1-26, the outer corner of the pilot cutting edge functions to fracture the material in the hole as it rotates in contact therewith. When this corner is cut on, the ability of the edge to fracture the formation is substantially reduced and the same tends to cut through the formation in a relatively smooth manner. Accordingly, since the formation does not fracture so readily adjacent the inclined outer pilot cutting edge, the same offers more lateral stability or resistance and, hence, the provision of the inclined cut on the pilot edge greatly reduces walking of the bit within the hole. Under such conditions the resulting forces on the inside corner 26A serves to fracture the center core.

Where larger size bits are utilized and operated under relatively heavy loads, it is preferred to also grind off the inner corner of the pilot edge, as at 44, also along a line disposed at an angle of approximately 20 with respect to the horizontal. Thus, in this embodiment the profile of the pilot cutting edge presents a wide spread V. The provision of the additional inclined surface adds still more lateral stability in operation so that Walkmg is reduced to a minimum. At any rate, walking is reduced sufficiently that the inner corner of the pilot edge will never pass inwardly beyond the axis of rotatron so as to have a reverse direction of movement. This favorable reduction in walking permits a decrease in the amount of offset heretofore necessary in regard to the inner corner of the pilot cutting edge. Whereas, with the straight pilot cutting edge the embodiments disclosed in Figures 126, a /8 to A offset was necessary, it has been found that an offset of the order of to A2" is all that is required with the improved inclined pilot cutting edges of Figures 27 and 28. This, of course, slightly increases the radius dimension of the pilot cutting edge, but the relationships noted in connection with the previous embodiments are substantially retained. Moreover, it has been found that with the use of the inclined pilot cutting edges as shown in Figures 27 and 28, a round hole results, even in the hardest foundations.

It is to be understood that the forms of the invention herewith shown and described are to be taken as the preferred embodiment of the same and that various changes may be made without departing from the spirit of the invention or the scope of the appended claims.

We claim:

1. A blade for a rotary drilling bit assembly rotatable about a longitudinal axis, said blade comprising a cutting section in the lower end thereof of a hard material having generally radially extending leading surface means, the radially inwardly disposed portion of the cutting section having downwardly facing bottom engaging surface means intersecting with the associated leading surface means to define a pilot cutting edge extending, between an inward terminal position spaced radially outwardly from the axis of rotation and an outward terminal position disposed radially substantially onehalf of the entire radial extent of the cutting portion of said blade, such that no point on said pilot cutting edge disposed radially inwardly from a given point thereon substantially midway of the radial extent of said pilot cutting edge to said inward terminal position is disposed below said given point and all points on said pilot cutting edge disposed radially outwardly from said given point are disposed progressively above said given point radially outwardly to said outward terminal position, the outermost portion of said pilot cutting edge defining portion of said cutting section including a stabilizing surface extending upwardly from said outward terminal position, the remaining portions of the blade cutting section including bottom engaging surface means and stabilizing surface means extending upwardly from the outer ends of said bottom surface means, said bottom engaging surface means intersecting with said leading surface means to define a plurality of stepped cutting edges extending upwardly and outwardly with respect to said pilot cutting edge.

2. A blade as defined in claim 1 wherein said pilot cutting edge extends from said given point to said outward terminal position along a straight line inclined at an angle of approximately 20 with respect to a horizontal plane.

3. A blade as defined in claim 2 wherein said pilot cutting edge extends inwardly and upwardly from said given point to said inward terminal position along a straight line inclined at an angle of approximately 20 with respect to a horizontal plane.

4. A blade as defined in claim 1 wherein said pilot cutting edge extends inwardly and upwardly from. said given point to said inward terminal position.

5. A blade for a rotary drilling bit assembly rotatable about a longitudinal axis, said blade comprising a cutting portion having recess means formed in the lower end thereof, and inserts of a hard material mounted in said recess means each having a leading surface disposed in a generally radial plane with respect to said axis, the radially inwardly disposed inserts having downwardly facing bottom engaging surfaces intersecting with the associated leading surfaces to define a pilot cutting edge extending, between an inward terminal position spaced radially outwardly from the axis of rotation and an outward terminal position substantially one half of the entire radial extent of the cutting portion of said blade, such that no point on said pilot cutting edge disposed radially inwardly from a given point thereon substantially midway of the radial extent of said pilot cutting edge to said inward terminal position is disposed below said given point and all points on said pilot cutting edge disposed radially outwardly from said given point are disposed progressively above said given point radially outwardly to said outward terminal position, the outermost insert of said pilot cutting edge defining inserts including a stabilizing surface extending upwardly from said outward terminal position, the remaining inserts including bottom engaging surface means and stabilizing surface means extending upwardly from the outer end thereof, said bottom engaging surface means intersecting with the leading surface of the inserts to define at least one stepped cutting edge disposed upwardly and outwardly with respect to said pilot cutting edge.

6. A blade as defined in claim 5 wherein said pilot cutting edge extends from said given point to said outward terminal position along a straight line inclined at an angle of approximately 20 with respect to a horizontal plane.

7. A blade as defined in claim 6 wherein said pilot cutting edge extends inwardly and upwardly from said given point to said inward terminal position along a straight line inclined at an angle of approximately 20 with respect to a horizontal plane.

8. A blade as defined in claim 5 wherein said pilot cutting edge extends inwardly and upwardly from said given point to said inward terminal position.

9. A blade as defined in claim 5 wherein the inward terminal position of said pilot cutting edge is spaced from the axis of rotation a distance between $6 to A.

10. A blade as defined in claim 5 wherein the bottom engaging surface means and the leading surfaces of the remaining inserts intersect to define two stepped cutting edges, each having a radial extent substantially equal to one-half the entire radial extent of said pilot cutting edge.

11. A blade as defined in claim 10 wherein a line passing through the outer corners of said two stepped cutting edges is inclined an angle of between 45 and 50 with respect to the axis of rotation.

12. A blade for a rotary drilling bit assembly rotatable about a longitudinal axis, said blade comprising a cutting portion having a continuous recess formed in the lower end thereof, and inserts of a hard material mounted in said recess in substantially abutting relation, each of said inserts having a leading surface disposed in a common generally radial plane with respect to said axis, the radially inwardly disposed inserts having downwardly facing bottom engaging surfaces intersecting with the associated leading surfaces to define a pilot cutting edge extending, between an inward terminal position spaced radially outwardly from the axis of rotation and an outward terminal position substantially one half of the entire radial extent of the cutting portion of said blade, such that no point on said pilot cutting edge disposed radially inwardly from a given point thereon substantially midway of the radial extent of said pilot cutting edge to said inward terminal position is disposed below said given point and all points on said pilot cutting edges disposed radially outwardly from said given point are disposed progressively above said given point radially outwardly to said outward terminal position, the outermost insert of said pilot cutting edge defining inserts including a stabilizing surface extending upwardly from said outward terminal position, the remaining inserts including bottom engaging surfaces and stabilizing surfaces extending upwardly from the outer ends thereof, the bottom engaging surfaces of said remaining inserts intersecting with the associated leading surfaces to define a pair of stepped cutting edges extending upwardly and outwardly with respect to said pilot cutting edge such that a line passing through the outer corners thereof is disposed at an angle of between 45 to 50 with respect to the axis of rotation, the cutting portion of said blade including bottom engaging surfaces of a thickness substantially equal to the thickness of the bottom engaging surfaces of said inserts and trailing surfaces extending upwardly from the trailing edges of said bottom engaging surfaces at an angle of between 35 and 43 with respect to the radial plane of the leading surfaces of said inserts, the bottom engaging surfaces of said cutting portion and the associated bottom engaging surfaces of said inserts being disposed in common planes extending upwardly from the associated cutting edge an angle of approximately 7 with respect to a horizontal plane.

13. A blade as defined in claim 12 wherein said pilot cutting edge extends from said given point to said outward terminal position along a straight line inclined at an angle of approximately 20 with respect to a horizontal plane and from said given point to said inward terminal position upwardly and inwardly along a straight line inclined at an opposite angle of approximately 20 with respect to the horizontal plane, said inward terminal position being spaced from the axis of rotation a distance between to A".

References Cited in the file of this patent UNITED STATES PATENTS

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