US2773673A - Pilot bit - Google Patents

Description

Dec. 11, 1956 G. A. M. PETERSEN PILOT BIT Filed March 4, 1954 2 Sheets-Sheet l INVENTOR. 6.60MB 4. MPH'EFS/V 1956 G. A. M. PETERSEN 2, 73,673

PILOT BIT Filed March 4, 1954 2 Sheets-Sheet 2 Fl E'Qll INVENTOR.

1 15.13 Q- W/M United States Patent PILOT BIT Gerald Alger Monroe Petersen, Santa Clara, Calif. Application March 4, 1954, Serial No. 414,007 11 Claims. (Cl. 255-61) This invention relates to earth boring tools or angers, and more particularly to that portion of such tools which encounters the earth first, commonly referred to as a pilot bit, for breaking the way for larger portions of the auger or drill shaft therefor.

The present invention contemplates certain novel improvements in pilot hits such as the one described and claimed in my U. S. Patent No. 2,639,122 dated May 19, 1953. While the pilot bit of this prior application is satisfactory for the purpose intended, I have discovered that certain advantages thereover are attained by structural deviation therefrom. It is therefore one object of this invention to provide a pilot bit which is basically an inverted cone, the base of which conforms substantially with the mean diameter of the drill shaft to which it is adapted to be attached. Along these lines it is another object to provide what I term a fish tail leader in advance of and beyond the apex of the cone body.

It is another object of this invention to provide in combination with the cone body of my new pilot bit, spirally developed blades or moldboards extending upwardly from the fish tail leader and cooperating with the side wall of the cone body for diverting earth spirally upward and outward to a zone beyond the mean diameter of the drill shaft or the base of the cone body. In conjunction with the foregoing, it is a further object to provide moldboards as aforementioned with a radial shape terminating in marginal spiral edges which are self-sharpening as they wear during use.

One of the maior problems confronting manufacturers of pilot bits relates to the answer of how to make these tools hard enough to withstand wear and yet strong enough to avoid breakage. It is well known that a hardened tool is more brittle and likely to snap or break when too much bending or lateral strain is exerted against its blades. With this in mind it is another object of this invention to so construct and design a pilot bit with integral spirally developed blades the marginal spiral edges of which lead all other parts of such blades so as to be put under compression rather than a bending moment relative to earth. In this manner the material from which the blades are cast can be hardened to a much greater extent and will not normally break due to the inward compression created. This object of the invention also applies to the manner of construction of the fish tail leader in which a pair of fins constituting this leader each present chisel-like portions having their cutting edges offset equidistant in advance of a radial line SH'HCK from the axis about which the bit rotates. In this manner each of these chisel-like portions, although extended beyond the apex of the cone body proper of the bit, slicingly engage the earth instead of entering the earth in a direction perpendicular to their width. In other words, by this arrangement the strain placed upon these chisel-like fins is-of a compressive nature diagonally through the wide dimension thereof rather than of a bending nature which would occur in the case of a direct perpendicular thrust against such fins. Therefore the shape, lead and 2,773,673 Patented Dec. 11, 1956 position of the blades and fins of the instant invention can be hardened to much higher degree with assurance against breakage.

The conical main body of my new pilot bit is conducive to a further advantage over prior tools of this general character in that it affords the formation of a socket or recess Within the pilot bit for the reception of a male type coupling. When it is considered that pilot bits must be replaced frequently due to wear and tear it will be appreciated that there is a great waste of material when the male type coupling is formed integrally with the pilot bit body as has heretofore been the practice. It is therefore another object of this invention to provide a pilot bit so formed as to reduce the amount of material required without sacrificing strength but rather creating a more sturdy structure embodying fewer weak or critical stress points.

Other objects and advantages of the present invention will become apparent in the following description in the light of the drawings in which:

Fig. 1 is a top plan view of a present invention.

' Fig. 2 is a side view of the pilot bit shown in Fig. 1. v Fig. 3 is a bottom plan view of the pilot bit shown in Figs. 1 and 2.

Fig. 4 is a transverse section through one fin of the fish tail leader shown in Fig. 2, and taken substantially along line 44 thereof.

Fig. 5 is an elevation, from line 55 therein.

- Fig. 6 is a longitudinal section through one fin of the fish tail leader shown in Fig. 2, and taken substantially along line 6-6 in Fig. 3.

Fig. 7 is a vertical section through the cone body of the pilot bit shown in Fig. 1 and taken along line 77 thereof. a

Fig. 8 is an enlarged side view of a coupling at the end ofa drill shaft adapted for use with the pilot bit of the present invention.

Fig. 9 is a side elevation of an anger and drill shaft adapted for use with the pilot bit of the present invention.

Fig. 10 is a side elevation of a modified form of the pilot bit shown in Figs. 1 through 7.

Fig. 11 is a top plan view of a modified form of pilot bit shown in Fig. 10.

Fig. 12 is a bottom in Figs. 10 and 11.

Fig. 13 is a fragmentary vertical section through the adapter socket zone of the cone body portion of the pilot bit shown in Figs. 10 through 12 taken along line 1313 of Fig. 11.

Fig. 14 is an enlarged side view of a modification of the coupling shown in Fig. 8 and adapted for use with the pilot bit construction shown in Figs. 10 through 12.

Fig. 15 is a section through a further modification of the couplings shown in Figs. 8 and 14.

The pilot bit of the present invention is generally designated 10 in the drawing. However, in order to distinguish between the modified form of Figs. 10, 11 and 12 from the preferred embodiment of Figs. 1, 2 and 3, prime numerals will hereinafter be used to designate the same parts in the modified form. Accordingly the pilot bit shown in Figs. 11), 11 and 12 is generally designated 10'.

Briefly, the pilot bit 10 or 10' embodying the present invention comprises a main body 11 having a plurality of radially extending blades, preferably two, 12 and 13 formed integrally therewith. The main body 11 is of substantially conical shape having its base 14 adapted for connection to a drill shaft 15. The base 14 of the coneshaped .body is ,disposed coaxialwith the axis A- of the' drill shaft 15 and this axis A is coincident with the axis A of the cone-shaped body 11. It will thus be seen that pilot bit embodying the partly in section, of Fig. 2 taken plan view of the pilot bit shown the apex a ofthe cone is disposed to enter the earth first. The present invention resides in the particular contour of the radial blades 12 and 13 relative to and in combination' with the cone-shaped body 11 at will now be explained in specific detail.

The blades 12 and 13 as best seen in Figs. 1 and 11 are diametrically opposed to one another relative to the axis A of the cone shaped body 11. The base 14 of the cone is of a mean diameter conforming substantially to the diameter of the drill shaft 15. At the plane of the base 14 of the cone body the blades 12- and 13 each extend equidistant beyond the mean diameter of the cone body. This is for cutting earth to a desired diameter sufficien't to allow passage of fragments of earth upwardly around the code base and the drill shaft 15. The blades 12 and 13 extend in spiral fashion from the side wall of the cone body, the spiral advancing is screw fashion in the general direction of rotation of the pilot bit during cutting of the earth thereby.

In addition to the foregoing, it will be noted that each blade 12 and 13' is of concave-convex contour, the concave face 16 leading and the convex face 17 trailing relative to the direction of rotation of the pilot bit. It will therefore be seen that each blade has its outer spiral edge 18 disposed slightly in advance of the zone of connection of the blade with the cone body. This affords a cutting action which in effect is a shaving of the earth at the spiral edge 18 as well as a tendency for the earth cut thereby to curve rearwardly into the concave face 16 at the leading side of the blade.

The disposition of the spiral edge 13 of each blade diminishes in dimension radially from the axis A gradually in each increment of descent from the plane of the base 14 of the cone shaped body 11. At the plane of the apex a of the cone body 11, the spiral edge 18 of each blade is still a slightly greater distance radially from the axis A" than the perimeter of the base 14 of the cone body 11. Each blade continues spirally beyond the plane in which the apex a lies to provide a pair of fins each designated 19 which together form what I prefer to call a fish tail leader 20 for the pilot bit. The spiral edge 18 of the fins 19 at the terminal end of the fish tail leader 20'are spaced from each other a distance substantially corresponding to the mean diameter of the base 14 of the cone body.

From the foregoing it will be seen that the two fins 19 of'the fish tail leader 20 in advance of the apex at of the cone shaped body 11 constitute a pair of chisel-like cutters. Each of these chisel-like cutters has a leading cutting edge 21 which is disposed in a plane substantially parallel to the base 14 of the cone body, i. e., perpendicular'to the axis A about which the body 11 rotates.

It should here be noted that while each blade 1213 is of concave-convex contour in a radial direction throughout its length from the base 14 down to the apex a of the cone body 11, the chisel-like extension 19 of each blade is substantially flat on its leading and trailing faces as best illustrated in Figs. 4 and 5. However, it should be noted that although the extension 19 is substantially fiat, the convexity of the trailing face 17 thereof prevails adjacent the outer spiral edge 18 whereas this trailing face 17 is provided with a concave cove 22 adjacent the inner edge 23 of the extension. Moreover, the leading face 16 of the extension 19 is provided with a convex surface 24 adjacent its inner edge 23.

I As best seen in Figs. 2 and 3 as well as Figs. 10 and 12, the leading cutting edge 21 of each extension 19 is disposed at right angles relative to the inner edge 23 thereof. Also note that the inner edges 23 of both extensions 19 are aligned with each other and radially disposed relative to the ax is' A of the pilot bit. Thus it will -be'seen that the leadingcutting edges 210i the two-extensions 19 are offset relative to an imaginary radial line I (Figs. 3 and I2)" traversing the aligned inner edges 23 at the axis A of the bit. Moreover, each leading cutting edge 21 is in advance of such imaginary radial line I relative to the direction of rotation of the bit about the axis A.

From the foregoing it will be noted that the cutting performed by the cutting edges 21 is a slicing action due to the angular disposition of these cutting edges relative to the direction of their movement. In other words, as indicated by the arrow S in Figs. 3 and 12, the inner corner of each cutting edge 21 is in advance of the outer corner thereof. in this manner resistance of the earth to the cutting action is minimized.

The leading face of each forward extension 19 is preferably tapered to provide a sharp edge at the leading cutting edge 21. Consequently, both leading cutting edges 21 engage the earth at a common plane in chisellike fashion for shaving the fragments of earth ofi while diverting such fragments of earth upwardly along the concave leading face 16 of the respective blade 12 or 13 as the case may be. Since the fish tail leader 20, as explained above, has substantially the same spread as the mean diameter of the base 14 of the cone shaped body 11 the initial cut made by the two chisel-like extensions 19 will provide a hole in the earth sufiicient to admit the cone body 11 as well as the drill shaft 15. This is, of course, the ideal result desired. It should be noted that there will be Wear at the cutting edge 21. However, since the fore face of the extension 19 is tapered the leading edge 21 is in effect self-sharpening until worn right up to the plane in which the apex a of the cone body 11 is disposed. It will thus be appreciated that during use and despite Wear, the fish tail leader 20 will continue to bore a hole comparable to the mean diameter of the cone base and drill shaft.

In this connection, it should also be noted that the inner edge 23 of each fin extension 19 presents its convex upper corner 24 to the earth for diverting fragments of earth upwardly onto the leading face of the fin 19. Simultaneous with the foregoing it will be noted that the scraping of the earth by the down-turned inner edge 23 leaves a void below the forward extension 19 of each blade so that the downward thrust of the pilot bit is accomplished with the least possible resistance. At the same time, note that the spiral outer edge 18 of each blade 1213 at the extension 19 effects a spiral cutting action which is downward and forward in a screw-like but slicing fashion. Consequently, a cutting action takes place along the spiral outer edge 18 of each blade diverting the spoil of earth fragments onto the concave leading face 16 thereof. This action takes place at each strata upwardly along the spiral outer edge 18 of each blade.

Since the first tail leader 20 initially cuts a hole into the earth sufiicient to admit the mean diameter of the cone 11 and the fragments of eanth thus cut are immediately diverted up the concave leading face 16 of each blade 12-13, it will be apparent that compression of such earth fragments or spoil is minimized. This is an important feature of the present invention because it assures against plugging up of the pilot bit particularly when heavy clays and such pliable soils with homogeneous tendencies are encountered. In other words, each of the spiral blades 12 and 13 serves as a moldboard for shaping the spoil, confining it within the radial extent of the spiral edges 18 while conveying such spoil in serpentine fashion upwardly around the cone shaped body 11. Thus the spoil is promptly displaced and removed as the pilot bit advances into the earth and packing of or resistance by the spoil eliminated.

As hereinbefore explained, the blades 12-13 extend equidistant radially beyond the face of the cone 11 sufficiently to allow passage of the eanth upwardly past the base 14 of the cone. In other words, the distance between the outer; spiral edge 18 of each blade 12-13 gradually increases from the mean diameter at the strata of the apex a of the cone up to the base 14 of the cone. Consequently, it will be realized that the cutting action along the spiral edge 18 gradually increases the diameter of thehole being cut'into earth. That is to say,- a minimum amount of earth is cut at each strata of the spiral edges 18 from the apex to the base of the pilot bit. Moreover,- this cutting action is a gradual slicing and corresponding lifting of the spoil so that a minimum of strain is put upon the radially extending blades 12-13. The strength of these blades 12-13 is funther enhanced by reason of their relative shortness or stubbedness relative to the face of the cone body 11 and of course the breadth of each blade at its juncture with the cone body 11.

From the foregoing it will be appreciated that the tendency of the earth to resist the advancement of the blades is directed in a compressive direction due to the forward curvature of each blade and the disposition of their spiral cutting edges 18 ahead of other portions of the blade. Therefore the metal from which the pilot bit is made can be appreciably hardened to withstand wear with assurance that it will not break under normal stress. This is also true with respect to the forwardly projecting chisellike fins 19 on the fish tail leader in which the resistance of earth to advancement thereof is in an arcuate direction through the width of these fins rather than in direct perpendicular relation thereto. Consequently these chisellike fins 19 although appreciably hardened will be confronted with a compressive strain across their widest dimension and the bending moment within them minimized to withstand normal wear while assuring against breakage of them.

Concerning the wearing of the cutting edges 21 and I 18, it has already been pointed out that the extensions 19 of the fish tail leader 20 are self sharpening until-they are worn away clear up to the apex a of the cone body 11. By reason of the concave-convex contour of each blade 12-13, a similar self-sharpening effect takes place along the spiral edges 18 as the latter wear during use. This is created by reason of the fact that both faces 1617 of each blade recede angularly backward from the spiral edge 18 in relation to .the radial disposition of the latter relative to the axis A about which the pilot bit rotates. Moreover, since the angular disposition of the leading and trailing faces 16-17 of each blade is relatively small, and since the trail-ing face 17 at an acute angle relative to the circular path of the spiral edge 18 as designated by the arrows in Fig. 1, any appreciable wear at the spiral edge 18 presents a minimum of arcuate worn face due to such wear. Therefore, even though some friction might be created at such 'arcuate worn faces adjacent the spiral edges 18 this friction is kept to'a minimum.

The relationship of the cone body 11 to the spiral blades .or mcldlboards 12 13 has "an advantage with respect .to the adaptation of the pilot bit of the'present invention for connection to the drill shaft 15. Since these pilot bits must be replaced after being worn to an inefficient shape, it will be apparent that there'is nothing left but'scrap metal. With prior type angers or pilot bits the adaptation thereof to a drill shatfit required that an extension or male fitting be formed integrally'with the same. Consequently such male fitting entailed additional metal which had to be scrapped along with the worn out pilot bit.

With the cone type structure embodied in the present invention, a female adapter 30 is possible as illustrated in Figs..l and 7 as well as Figs. 11 and 13; As is shown,- this female adapter 38 is accomplished by forming a recess or socket 31 in the base end of the cone shaped body 11 which reduces the amount of metal without weakening .the bit or otherwise reducing its efficiency.

The socket 311 in the present disclosure is relatively narrow .in one direction and elongated in the other direction so as to span diametrically of the'con'e base 14 between the point of juncture of the blades 12'-13 with the cone body '11.

In addition to the foregoing, the socket 31 has lateral recesses 32 which are formed in the cone body 11-transversely'of the larger or elongated portion of thesocket. These lateral recesses traverse the larger portion 'ofthe socket 31 at the axis A of the cone body and extend only about half way to the face of the cone. Moreover, these lateral recesses 32 extend downwardly but partially the depth of the main or larger portion of the recess 31.

The counterpart of the female adapter 30 is a male adapter '33 which is illustrated in Figs. 8 and 9 of the drawing. This male adapter 33 is a key-like fitting 34 conforming in shape to the socket 31 so as to fit snugly into the same for turning the pilot bit. The key-like fitting 34 has a disc formation 35 on its upper face formed integrally with the lower key portion 34. This disc formation 35 conforms to the mean diameter of the cone base 14 as well as the drill shaft 15 and is secured to the latter in any suitable manner, but preferably by welding as at 35 as shown in Figs. 8 and 9.

As shown in Figs. 11, 13 and 14 the socket 31' as well as the key-like fitting 34' may be provided with a twist in the direction of rotation of the pilot bit. This twist is provided to follow substantially the radial disposition of the blades 1213' and to avoid any undue projections on the side walls of the cone-shaped body which might interfere with the normal passage of spoil upwardly around the same.

As an alternative of the above forms of connection to the drill shaft 15 a threaded shank 37 may be employed. Such threaded shank 37, as shown in Fig. 15, includes a head 38 which is substantially the same diameter as the drill shaft 15 for welding thereto as at 36. The disk 35 or 35 is provided with a sleeve 39 having an inside thread to receive the shank 37. Of course it should be obvious that the screw shank37 can be formed integrally with the disk 35 or 35 of either male adapter for threaded connection into a threaded socket (not shown) formed in the lower end of the drill shaft 15 or 15'.

In either manner of connecting the male fitting 34 to a drill shaft, the pilot bit 10 of the present invention is effectively connected to the drill shaft for turning movement therewith in cutting direction or to afford reverse turning of the pilot bit. In order to enable the withdrawal of the pilot bit from earth, the lower key portion 34 of the male fitting 33 has a hole 40 formed therethrongh for the reception of a bolt 41' (Fig. 15). The cone body 11 likewise has aligned holes 42 43 therethrough transversely of the enlarged portion of the female socket 31 and in axial alignment with the hole 40 when the male adapter 33 is reposed in the socket 31. Thus the bolt 41' extends through the aligned holes 40, 42 2nd 43 to enable withdrawal of the pilot bit from eart Having thus described my new pilot bit in specific detail, it will be apparent to those skilled in the art that the same is susceptible to modifications, variations, and/or alterations without departing from the spirit of the invention. I therefore desire to avail myself of all modifications, variations, and/or alterations as fairly come within the purview of the appended claims.

What I claim as new and desire to protect by Letters Patent is:

1. A pilot bit adapted for connection to a drill shaft for turning in a predetermined direction about an axis, comprising a cone-shaped body having its base end of a mean diameter comparable to the diameter of said drill shaft, a pair of blades formed integrally with said coneshaped body and extending radially therefrom, each said blade having a concave-convex cross section radially from said axis to provide a concave leading face and a convex trailing face relative to the direction of turning of said pilot bit, each said blade descending from the base end of said cone-shaped body in spiral fashion and terminating in a fin extending beyond the apex of said cone-shaped body whereby the extended fins of each of said blades provide a pair of chisel-like blades in advance of said cone-shaped body within an orbit coinciding substantially with said mean diameter of the base end of said coneshaped body.

2. A pilot bit adapted for connection to a drill shaft for turning movement in a predetermined direction about an axis, comprising a cone-shaped body having a base of a mean diameter comparable to the diameter of said drill shaft for aligned connection therewith, a fish tail leader on the apex end of said cone-shaped body including a pair of chisel-like fins each having a cutting edge disposed perpendicular to the axis of said cone-shaped body and beyond the apex end thereof, each said cutting edge being disposed at right angles to a line bisecting said fish tail leader and traversing the axis of said cone-shaped body, the cutting edge of each said chisel-like fin being in advance of an imaginary line transverse to said line bisecting said fish tail leader for cutting into earth in a slicing fashion upon turning of said pilot bit, said chisel-like fins having an orbital spread coinciding substantially with the mean diameter of the base of said cone-shaped body for initially cutting into earth a hole of a diameter to receive said cone-shaped body and for diverting fragments of earth upwardly adjacent the face of said coneshaped body, and a pair of radially extending helical blades formed integrally with said cone-shaped body and said pair of chisel-like fins each having a substantially fiat leading face and an outer spiral cutting edge for diverting fragments of earth cut thereby back toward the face of said cone-shaped body While conveying such fragments of earth in serpentine fashion upwardly around said coneshaped body.

3. A pilot bit comprising a cone-shaped body adapted for turning movement about its base to apex axis, a fish tail leader on the apex end of said cone-shaped body ineluding a pair of chisel-like fins each having a cutting edge disposed perpendicular to the axis of said cone-shaped body and beyond the apex end thereof, each said cutting edge disposed at right angles to a line bisecting said fishtail leader and traversing the axis of said cone-shaped body, the cutting edge of each said chisel-like fin being in advance of an imaginary line transverse to said line bisecting said fish tail leader for cutting into earth in a slicing fashion upon turning of said pilot bit, said chisel-like fins having a mean diameter coinciding substantially with the diameter of the base of said cone-shaped body for initially cutting a hole into earth for the reception of said cone-shaped body, each said chisel-like fin having its leading end tapered from its cutting edge up to the plane of the apex of said cone-shaped body to facilitate a selfsharpening effect thereto upon wearing of said chisel-like fins clear up to the apex of said cone-shaped body, and a pair of radially extending helical blades formed integrally with said cone-shaped body and said pair of chisellike fins each having an outer spiral cutting edge which gradually increases in radial dimension from the axis of said cone-shaped body as said outer spiral cutting edge ascends toward the upper base end of said cone-shaped body for gradually cutting additional earth beyond the mean diameter of said fish tail leader.

4. A pilot bit comprising a cone-shaped body adapted for turning movement about its base to apex axis, a fish tail leader on the apex end of said cone-shaped body including a pair of chisel-like fins each having a cutting edge disposed perpendicular to the axis of said cone-shaped body and beyond the apex end thereof, each said cutting edge disposed at right angles to a line bisecting said fish tail leader and traversing the axis of said cone-shaped body, the cutting edge of each said chisel-like fin being in advance of an imaginary line transverse to said line bisecting said fish tail leader for cutting into earth in a slicing fashion uponturning of said pilot bit, said chisellilse fins having a mean diameter coinciding substantially with the diameterof the base of "saidcone-shaped body for ini tially cutting a hole into earth for the reception of said cone-shaped body, each said chisel-like fins having its leading end tapered from its cutting edge up to the plane of the apex of said cone-shaped body to facilitate a selfsharpen'ing effect thereto upon wearing of said chisel-like fins clear up to the apex of said cone-shaped body, and a pair of radially extending helical blades formed integrally with said cone-shaped body and said pair of chisel-like fins each having an outer spiral cutting edge which gradu* ally increases in radial dimension from the axis of said cone-shaped body as said outer spiral cutting edge ascends toward the upper base end of said cone-shaped body for gradually cutting additional earth beyond the mean diameter of said fish tail leader, each said blade having a concave leading and a" convex trailing face receding an gularly rearwardly from said outer spiral cutting edge to facilitate a self-sharpening effect to the latter during wearing of said outer spiral cutting edge as it slicingly engages the earth and diverts fragments of earth onto the concave leading face of said blade.

5, A pilot bit comprising a cone-shaped body adapted to turn about its base to apex axis, a fish tail leader on the apex end of said cone-shaped body including a pair of chisel-like fins each having a cutting edge disposed perpendicular to the axis of said cone-shaped body and beyond the apex end thereof, each said cutting edge being disposed at right angles to a line bisecting said fish tail leader and traversing the axes of said cone-shaped body, the cutting edge of each said chisel-like fin being in advance of an imaginary line transverse to said line bisecting said fish tail leader for cutting into earth in a slicing fashion upon turning of said pilot bit, said pair of chisellike fins being of a mean diameter coinciding substantially with the diameter of the base of said cone-shaped body for initially cutting into earth a hole of a diameter to receive said cone-shaped body, the inner edge of each said chisel-like fin along said line bisecting said fish tail leader having a convex upper corner for diverting fragmentsof earth up onto said fin and a downturned lower edge for scraping earth at a lower level than the lower'face of said fin to form a void in the earth below the same to minimize resistance against normal advancement of said pilot bit into earth, and a pair of radially extending spirally ascending helical blades formed integrally with said pair of chisel-like fins and said cone-shaped body for molding fragments of earth while conveying such fragments of earth in serpentine fashion upwardly around said cone-shaped body.

6. A pilot bit comprising a cone-shaped body adapted to turn about its base to apex axis, a fish tail leader on the apex end of said cone-shaped body including a pair of chisel-like fins each having a cutting edge disposed perpendicular to the axis of said cone-shaped body and beyond the apex end thereof, each said cutting edge being disposed at right angles to a line bisecting said fish tail leader and traversing the axis of said cone-shaped body, the cutting edge of each said chisel-like fin being in advance of an imaginary line transverse to said line bisecting said fish tail leader for cutting into earth in a slicing fashion upon turning of said pilot bit, said pair of chisellike fins being of a mean diameter coinciding substantially with the diameter of the base of said cone-shaped body for initially cutting into earth a hole of a diameter to receive said cone-shaped body, the inner edge of each said chisel-like fin along said line bisecting said fish tail leader having a convex upper'corner for diverting fragments of earth up onto said tin and a downturned lower edge for scraping earth at a lower level than the lower face of said fin to form a void in the earth below the same to minimize resistance against normal advancement of said pilot bit into earth, and a pair of radially extending spirally ascending helical blades formed integrally with said pair of chisel-like tins and said cone-shaped body, each said blade having a concave leading face and a convex trailing face to provide a spiral outer cutting edge on said blade in advance of said concave leading face thereof and at gradu ally increasing radial dimension from the axis of said cone-shaped body for slicingly engaging earth exteriorly of the hole initially cut by said fish tail leader and for diverting fragments of earth back onto said concave leading face for conveyance thereby upwardly in serpentine fashion adjacent the face of said cone-shaped body.

7. A pilot bit adapted for connection to a drill shaft for turning movement in a predetermined direction about an axis, comprising a cone-shaped body having a base of a mean diameter comparable to the diameter of said drill shaft for aligned connection therewith, a pair of radially extending blades formed integrally with said cone-shaped body and each having a concave-convex contour radially with the concave face leading relative to the direction of turning of said pilot bit and disposed at an acute angle with respect to said convex trailing face to provide an outer spiral cutting edge in advance of said concave leading face of said blade, each said blade descending from the base of said cone-shaped body in spiral fashion in the direction of turning thereof and diminishing in radial dimension from the axis of said cone-shaped body and terminating in a pair of chisel-like fins beyond the apex of said cone-shaped body to provide a fish tail leader in advance thereof, said cone shaped body having a diametrically extending socket formed in its base end, and a keylike extension secured to the end of said drill shaft conforming in shape to said socket for reception therein for coupling said pilot bit to said drill shaft for turning therewith.

8. A pilot bit adapted for connection to a drill shaft for turning in a predetermined direction about an axis, comprising a cone-shaped body having its base end of a mean diameter comparable to the diameter of said drill shaft, a pair of blades each formed integrally with said cone-shaped body and extending radially therefrom and descending from the base end of said cone-shaped body in spiral fashion and terminating in a fin extending beyond the apex of said cone-shaped body within an orbit defined by the mean diameter of the base end of said coneshaped body, each said blade having a concave-convex cross section radially with its concave face leading at an acute angle relative to its convex trailing face to present a cutting edge at the point of convergence of said concaveconvex faces for advance engagement with earth so that the resistance of earth to cutting thereby effects a compression stress radially inward of said blades.

9. A pilot bit adapted for connection to a drill shaft for turning movement in a predetermined direction about an axis, comprising a cone-shaped body having a base of a mean diameter comparable to the diameter of said drill shaft for aligned connection therewith, a fish tail leader on the apex end of said cone-shaped body including a pair of chisel-like fins each having a cutting edge within the orbit of said mean diameter and disposed perpendicular to the axis of said cone-shaped body and beyond the apex end thereof, each said cutting edge being disposed at right angles to a line bisecting said fish tail leader and in advance of an imaginary line traversing said line bisecting said fish tail leader at the axis of rotation of said pilot bit whereby the inner corners of said chisel-like fins engage earth in advance of the remainder of the same and exert a strain crosswise said fin.

10. A pilot bit adapted for connection to a drill shaft for turning in a predetermined direction about an axis, comprising a cone-shaped body having its base end of a mean diameter comparable to the diameter of said drill shaft, a pair of blades formed integrally with said coneshaped body and extending radially therefrom and descending from the base end of said cone-shaped body in spiral fashion and terminating in a fin extending beyond the apex of said cone-shaped body to provide a fish tail leader thereon, each said blade having a concave-convex cross section radially with its concave face leading at an acute angle relative to its convex trailing face to present a cutting edge at the point of convergence of said concave-convex faces for advance engagement with earth so that the resistance of earth to cutting thereby efiects a compression stress radially inward of said blades, and each of said fins having a cutting edge within the orbit of said mean diameter and disposed perpendicular to the axis of said cone-shaped body and beyond the apex end thereof, each said cutting edge being at right angles to a line bisecting said fish tail leader and in advance an imaginary line traversing said line bisecting said fish tail leader along the axis of rotation of said cone-shaped body to present the inner corner edges of said fins to earth in advance of other portions of said fins during turning of said pilot bit so that the resistance of the earth to being cut thereby exerts a compression strain crosswise of said fins.

11. A pilot bit adapted for connection to a drill shaft for turning in a predetermined direction about an axis, comprising a cone-shaped body having its base end of a mean diameter comparable to the diameter of said drill shaft for aligned connection thereto, a pair of radially extending blades formed integrally with said cone-shaped body, each said blade having a concave-convex cross section radially with the concave face thereof leading relative to the direction of turning of said pilot bit, said blades each descending from the base end of said cone-shaped body in spiral fashion in thedirection of turning of said body and terminating in a chisel-like fin beyond the apex of said cone-shaped body to provide a fish-tail leader in advance of the apex of said cone-shaped body, each said blade diminishing in radial dimension relative to the axis of said cone-shaped body for confining said fish tail leader to an orbit comparable to the mean diameter of the base end of said coneshaped body, said chisel-like fins being divided by a line bisecting the apex of said cone-shaped body so that aligned inner edges are presented by each of said fins, and the leading edges of each said chisel-like fins being parallel to and offset in advance of an imaginary line traversing said line bisecting the apex of said cone-shaped body.

References Cited in the file of this patent UNITED STATES PATENTS

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