GB2127463A

GB2127463A - Two-piece drill bit

Info

Publication number: GB2127463A
Application number: GB08322798A
Authority: GB
Inventors: Robert Wesley Hughes
Original assignee: Ingersoll Rand Co
Current assignee: Ingersoll Rand Co
Priority date: 1982-08-30
Filing date: 1983-08-24
Publication date: 1984-04-11
Also published as: SE8304638L; GB8322798D0; DE3330025A1; BR8304683A; JPS5976394A; SE8304638D0

Abstract

A two-piece drill bit 10 for drilling a hole in earth or rock has means for isolating mating screw threads 24, 44 from forces due to bending moments occurring between the shank 12 and the bit head 14. The bit head 14 has a smooth bore 46 of a diameter less than the minor diameter of the screw threads on one side of the threaded bore and a counter bore 42 of a diameter greater than the major diameter of the screw threads on the other. The shank has a locating diameter 22 at one end and a flange 25 with an outer diameter 26 on the other side of the screw threads. The diameters 22 and 26 are such as to locate and engage the bores 46 and 42, respectively, when the screw threads are in threading engagement. <IMAGE>

Description

SPECIFICATION Two-piece drill bit This invention relates to drill bits for drilling holes in earth or rock for example attached by screw threads to the down-hole end of a drill or drill steel string.

It is well known in the art to connect a bit head to a shank, in a two-piece drill bit arrangement through the use of a threaded connection. This connection usually provides a solid contact between the head and the shank either when the shank threads into and bottoms against the bit head, or when a shoulder on the shank pressingly engages an annular end of the bit head. When such two-piece threaded bits are used in an earth drilling machine the threads between the shank and the bit head progressively tighten, thereby increasing the tensile stress in the regions adjacent the interface. This can result in transverse fatigue fracture developing in the bit head skirt in the plane of the shank bottom and localised thread galling which in turn encourages the development of fatigue fractures in the threaded shank.A solution to the above described problem is disclosed in copending application No.

8313205 (Publication No Typically, two-piece threaded bits known in the art utilise relatively long threaded portions so that when assembled, the parts are reasonably rigid and can withstand bending moments encountered in the course of drilling. These bending moments necessarily cause the mating threads to flex and deform somewhat thereby aggravating the thread galling and fracture problems described above.

Moreover, concentricity of the bit and shank must be controlled solely by the mating threaded surfaces, often resulting in misalignment of the bit with respect to the center of rotation of the drill string.

It is, therefore, an object of this invention to provide a two-piece threaded bit of simple construction which substantially isolates the threaded interface from bending moments.

It is another object of this invention to provide a two-piece threaded bit that maintains a relative high degree of concentricity between the bit and shank independent of the mating threaded surface.

According to the present invention, there is provided a two-piece drill bit for drilling holes in earth and rock, the drill bit including a shank portion and a bit portion. The shank portion has one end adapted for attaching to a drill means, the other end being of cylindrical shape and having screw threads formed on its outer surface. The bit portion is suitably arranged for drilling rock and has a threaded bore formed therein for threading engagement with the screw threads of the shank portion. Means are provided for nullifying bending moments between the screw threads and the threaded bore and for maintaining concentricity therebetween.

Other objects, features, and advantages will become apparent upon reading the following detailed description in conjunction with the accompanying drawings in which: FIG. 1 is an axial cross-section view of portions of a two-piece drill bit showing the novel features of this invention; FIG. 2 is an axial cross-section view of portions of a two-piece drill bit showing a typical construction found in the prior art; and FIG. 3 is a view similar to that of FIG. 1 showing an alternative embodiment.

Referring to FIG. 1 there is shown a two-piece bit 10 having a shank portion 12 and a bit portion or bit head 14. The shank portion 12 has at its one end 1 6 a suitable interface arrangement 20 for coupling to a drill or drill steel string, not shown.

Formed on the outer end 1 8 of the shank portion is a first cylindrical surface 22, screw threads 24 having a length E, and a second cylindrical surface 26 formed mutually concentric. The first cylindrical surface 22 has an outside diameter that is smaller than the minor diameter of the screw threads 24 and the second cylindrical surface 26 has an outside diameter that is larger than the major diameter of the screw threads 24 and may comprise the outer diameter of a flange 25. A shoulder 28 is formed between the first cylindrical surface 22 and the screw threads 24.

The bit portion 14 incorporates the standard tungsten carbide inserts 30 (only three shown) and hole cleaning exhaust conduits 32 (only one shown) which mate with similar conduits 34, and a passageway 36 formed in the shank portion 1 2.

The bit portion 14 incorporates a cavity 40 bounded by a cylindrical bore or counter bore 42, a threaded bore having screw threads 44, a cylindrical bore 46, and a recessed bottom 48. A shoulder 50 is formed in the cavity 40 intermediate the screw threads 44 and the bore 46.

The surfaces 42 and 46, the screw threads 44, and the shoulder 28 are arranged so that when the bit portion is assembled to the shank portion, the screw threads 24 and 44 are threadingly engaged and tightened so that the shoulder 28 is in pressing engagement with the shoulder 50, and the cylindrical surfaces 22 and 26 slidingly engage the bores 42 and 46 respectively. The screw threads 24 are thereby maintained coaxial to the threads 44. Clearance between the cylindrical surface 26 and the bore 42 and between the cylindrical surface 22 and the bore 46 is made relatively small so that bending moments are transferred through these surfaces and not through the mating screw threads 24, 44. The cylindrical surfaces 22 and 26 are thereby in locating engagement with the bores 46 and 42 respectively.

Referring now to FIG. 2 there is shown a typical prior art two-piece bit 100 having a shank portion 112 and a bit portion or bit head 114. The shank portion 112 has at its one end 116 a suitable interface arrangement 1 20 for coupling to a drill or drill steel string or, alternatively, the shank portion 112 may simply be the lower end of a section of drill steel as is commonly known in the art. Formed on the outer end 118 of the shank portion is a length LL of screw threads 124.

Similar to the bit portion 14, the bit portion 114 incorporates the standard tungsten carbide inserts 30 and hole cleaning exhaust conduits 32 which mate with similar conduits 34 and the passageway 36 formed in the shank portion 112.

The bit portion 114 incorporates a cavity 140 having screw threads 144 formed therein and a bottom surface 148. When the bit portion 114 is assembled to the shank portion 11 2, the screw threads 124 and 144 are threadingly engaged and tightened so that the end 149 of the shank portion 112 is in pressing engagement with the bottom surface 148. As can be appreciated by those skilled in the art, all bending moments that occur between the bit portion 114 and the shank portion 112 must pass through the mating screw threads 124 and 144. The length LL of the screw threads 124 and 144 must be sufficient to enable the screw threads to withstand, without serious deformation, substantial bending moments that occur in the course of drilling in the demanding rock drilling environment.

Referring to FIG. 3, there is shown a two-piece bit 200 having a shank portion 212 and a bit portion or bit head 214 that are similar to the twopiece bit 10 except as follows. Formed on the end 218 of the bit portion 214 is a first cylindrical surface 222, screw threads 224 having a length E, and a second cylindrical surface 226 formed mutually concentric. A shoulder 250 is formed adjacent the surface 226 and extends to the outer periphery of the bit portion 214. The first cylindrical surface 222 has an outside diameter that is smaller than the minor diameter of the screw threads 224 and the second cylindrical surface 226 has an outside diameter that is larger than the major diameter of the screw threads 224.

The shank portion 212 incorporates a cavity 240 bounded by a recessed bottom 248, a cylindrical bore 246, screw threads 244, and a cylindrical bore or counter bore 242. The shank portion terminates in an abutting end 228. The surfaces 242 and 246, the screw threads 244, and the abutting end 228 are arranged so that when the bit portion is assembled to the shank portion, the screw threads 224 and 244 are threadingly engaged so that the abutting end 228 is in pressing engagement with a shoulder 250 and the cylindrical surfaces 222 and 226 slidingly engage the bores 246 and 242 respectively and are thereby in locating engagement therewith. In all respects the two-piece bit 200 is similar to the two-piece bit 10 shown in Fig. 1.

The length L shown in Figs. 1 and 3 of the two-piece bits 10 and 200 respectively should approximate the length LL shown in Fig. 2 for comparable results, however, the length E of the mating screw threads 24-44 and 224-244 may be considerably less and need only be sufficiently long to accommodate a portion of a single thread. While the principles of this invention may be practised with a two-piece drill bit having mating screw threads comprising at least a portion of a single screw thread, two and one half turns of screw thread are preferable.

Accordingly, there has been disclosed an arrangement for obviating the effect of bending moments on the mating screw threads in a two-piece drill bit. It is understood that the above described embodiment is merely illustrative of the application of the principles of this invention. Upon reviewing the present disclosure numerous other embodiments may be devised by those skilled in the art without departing from the scope of this invention, as defined by the appended claims.

Claims

1. A two-piece drill bit for drilling a hole in earth or rock, the bit having a shank portion and a bit portion, wherein one of the shank and bit portions is of generally cylindrical shape for a selected portion and has at least one screw thread portion formed on the outer surface thereof, and the other one of the shank and bit portions has a threaded bore formed therein, adapted for threading engagement with the screw thread portion, the shank portion having an end adapted for attaching to a rock drill, the bit portion being suitably arranged for drilling earth or rock, and means for nullifying bending moments between the screw thread portion and the threaded bore.

2. A bit according to Claim 1, in which the one of the shank and bit portions has screw threads formed on the outer surface thereof, adapted for threading engagement with the threaded bore.

3. A bit according to Claim 1 or 2, wherein the shank portion is the one of the shank and bit portions.

4. A bit according to Claim 1 or 2, wherein the bit portion is the one of the shank and bit portions.

5. A bit according to Claim 1, 2, 3 or 4, wherein the means for nullifying bending moments comprises a smooth bore formed in the portion with the threaded bore and coaxial therewith, and a locating diameter disposed on the other portion and adapted for locating engagement with the smooth bore, when the threaded bore is in threading engagement with the screw thread portion of screw threads.

6. A bit according to Claim 5, wherein the smooth bore is of smaller diameter than the minor diameter of the screw thread portion or screw threads.

7. A bit according to any preceding claim, wherein the means for nullifying bending moments includes a counter bore formed adjacent the threaded bore in the portion with the threaded bore, and a flange formed on the other portion, the flange having an outer diameter adapted for locating engagement with the counter bore when the threaded bore is in threading engagement with the screw thread portion or screw threads.

8. A bit according to Claim 7, wherein the counter bore is of larger diameter than the major diameter of the screw thread portions or screw threads.

9. A bit according to Claim 7 or 8, as appendant to Claim 5 or 6, wherein the smooth bore and counter bore are on opposite sides of the threaded bore.

10. A bit according to Claim 2 or any claim appendant to Claim 2, wherein there are substantially two and one half turns of screw thread upon the portion.

11. A two-piece drill bit substantially as hereinbefore particularly described and illustrated in Fig. 1 of the accompanying drawings.

12. A two-piece drill bit substantially as hereinbefore particularly described and illustrated in Fig. 3 of the accompanying drawings.