Technical Field of the Invention

In a first aspect, this invention relates to a drilling tool intended for percussive rock drilling and of the type that comprises two drill bits provided with crushing members, viz. a central pilot bit and a ring bit surrounding the same, which individually have a rotationally symmetrical basic shape in relation to a centre axis and include front and rear ends, and which are detachably connectable with each other by means of a bayonet coupling comprising a number of pockets recessed in one of the bits and in which drivers included in the other bit are insertable to transmit driving rotary motions from the pilot bit to the ring bit, the drilling tool including a flush duct mouthing in the front end of the pilot bit for the supply of flushing medium, as well as a number of internal evacuation channels for the removal of drill cuttings together with the flushing medium.

In further aspects, the invention also relates to an expendable kit, a ring bit and a casing shoe for drilling tools of the type in question.

Background of the Invention

Rock drilling tools of the type generally mentioned above are well suitable for the drilling of holes in which casing tubes are to be left permanently. This is especially important in connection with the driving of tunnels in rock that is sometimes solid, sometimes loose. When unconsolidated rock, i.e., rock that is not self-supporting, is encountered, the same has to be consolidated, more precisely by in the same introducing casing tubes in which an injection mass, e.g., concrete, can be pressed in and be spread out in the unconsolidated rock structure, more precisely via radial holes in the tube, to strengthen the rock after hardening. Such injection tubes are distributed in the form of an arch-shaped umbrella over the part of the rock that after strengthening can be excavated and/or blasted out to form a part of the tunnel. By those skilled in the art, this strengthened or consolidated structure is denominated "protection umbrella". Examples of other situations, which require that a casing tube is left in a hole drilled in the rock or the ground, are when liquids or other fluids, e.g., water, oil, gas or the like, are to be raised to the ground or surface level.

Irrespective of the object of driving in casing tubes into rock/ground by means of a drilling tool of the type in question, it is often a desire that the tube should not rotate while abutting against the surrounding hole wall during the driving-in, because otherwise the energy consumption would become unacceptably great, in particular when the holes are deep and the casing tubes long. This requires that the flushing medium and the accompanying drill cuttings are evacuated internally through the drilling tool and the casing tube.

Common to such previously known rock drilling tools, which allow driving-in of casing tubes having the same diameter as the drill hole without they rotating, and which therefore require moderate energy consumption, is that the ring bit of the tool can rotate in relation to the casing tube, as well as compulsorily pull the casing tube along axially into the drill hole as this deepens. For this reason, the ring bits of the drilling tools in question are connected to the appurtenant casing tubes (and possible casing shoes of the same) via coupling means, which makes retraction of the ring bit impossible when the casing tube is to be left in the drill hole. However, when this takes place, the central pilot bit should, by detachment from the ring bit, be removable from the hole so as to allow reuse This means that the ring bit and a possible casing shoe thereof forms an expendable unit, which is finally expended for each one of the different drill holes, while the pilot bit is reusable. Therefore, it is important that the last-mentioned one is removed from the drill hole in the best possible state. For this reason, the pilot bit and the ring drill bit are interconnected via a coupling of the bayonet type, which allows the pilot bit to be removed from the ring bit remaining in the drill hole, more precisely by the simple measure of turning the pilot bit a distance in the direction opposite to the direction of rotation. In such a way, drivers included in the bayonet coupling are made free to be pulled out axially through the ring bit.

Although the type of drilling tools in question advantageously can be used for percussive rock drilling of holes in which casing tubes are left, the same may, if required, also be used for the drilling of holes from which the tubes are removed.

Because the ring bit and a possible casing shoe remain in the drill hole together with the casing tube, the same are in practice expendable units, in spite of the fact that they form integrated parts of the drilling tool during the proper drilling operation. For this reason, said units are delivered in the form of separate expendable kits, which can co-operate with one and the same pilot bit as long as the same is capable of functioning. A desire among users is that the cost of such kits is kept low and that the pilot bit and the ring bit should be possible to be coupled together and be detached from each other, respectively, in a simple and reliable way, i.e., without obstacle of, for instance, sticking drill cuttings, complicated coupling details or other faults. It is naturally of interest also that the pilot bit has a long service life; all with the purpose of creating the best possible economy.

Prior Art

Drilling tools for percussive rock drilling are in general widely described in the patent literature. See for instance: US 5957224, US 3227230, EP 1837481 , WO94/12760, WO98/13575 and SE 519312. Among these documents, the first- mentioned one discloses a rock drilling tool that works with internal evacuation of drill cuttings. In such a way, the casing tube does not need to rotate in the drill hole, whereby considerable amounts of energy are saved. More precisely, this drilling tool is in the form of a down-the-hole tool, i.e., a tool the percussion mechanism of which is directly acting on the pilot bit, contrary to such tools that are intended for top hammer drilling. The pilot bit is connected to the ring bit via a bayonet coupling, the rotation-transmitting drivers of which act against shoulder surfaces in pockets recessed in the envelope surface of the pilot bit, the drivers being in the form of projections on the inside of the ring bit. These projections can be brought axially through chutes, which are countersunk in the envelope surface of the pilot bit and extend between the front and rear ends thereof. Impact impulses from a percussion mechanism of sinker drill type connected to the pilot bit are transmitted from the pilot bit to the ring drill bit via the bayonet coupling, more precisely by axially rear bordering surfaces in the pockets acting against the projections on the inside of the ring bit.

Although the drilling tool known by US 5957224 affords the energetic advantage that the casing tube does not rotate in relation to the drill hole, the same is associated with a plurality of disadvantages. One such a disadvantage is that the fairly small drivers will constantly strike on one and the same impact surface (of a limited size) on the ring bit. This means that the stresses on the drivers of the pilot bit become great and shorten the service life of the pilot bit. Another disadvantage is that the casing tube is directly connected to the ring bit. Therefore, in order for the casing tube not to be set in rotation, the joint between the same and the ring bit has to be made so that it guarantees free rotation of the ring bit; something that requires delicate technical solutions for the severe environment in which the tool will work. An additional disadvantage is that the axial chutes in the pilot bit, which are required to bring the projections of the bayonet coupling into and out of engagement with the pockets, simultaneously are utilized as evacuation channels for drill cuttings.

Therefore, a mixture of flush water and drill cuttings will constantly pass through the chutes during drilling, wherein drill cuttings easily may penetrate into the gaps between the drivers and pockets of the bayonet coupling. In such a way, it is risked that the drill cuttings get stuck in the pockets and make the detachment and retraction of the pilot bit more difficult. A manufacturing disadvantage is due to the fact that the projections are formed on the ring bit and the pockets in the pilot bit. This means that the costly material (steel) required to form the drivers or the projections will be included in the expendable unit that should be inexpensive, viz. the ring bit, while the pockets requiring no material are included in the reusable pilot bit.

Objects and Features of the Invention

The present invention aims at obviating the above-mentioned disadvantages of the drilling tool known by US 5957224, and at providing an improved drilling tool. Therefore, a primary object of the invention is to give the pilot bit a long service life at the same time as the ring bit obtains optimum strength within the margin of the manufacturing cost, more precisely by distributing the impact stresses on the respective bit along as large impact surfaces as possible. An additional object of the invention is to provide a drilling tool, the bayonet coupling members, in the form of pockets and drivers, of which can be brought into and out of engagement with each other in a smooth and reliable way, at the same time as the tool allows internal evacuation of drill cuttings. Another object is to provide a drilling tool in which the risk of the casing tube unintentionally being set in rotation as a consequence of the rotation of the ring bit is efficiently counteracted. Yet an object of the invention is to provide a drilling tool the expendable ring bit of which is inexpensive to manufacture not only as a consequence of low material costs, but also as a consequence of low machining costs. An additional object is to provide a drilling tool that affords the possibility of placing the crushing members of the two bits in a crushing-wise optimal way also when the diameter of the tool is small.

According to the invention, at least the primary object is attained by means of the features defined in the characterizing clause of claim 1. Preferred embodiments of the drilling tool according to the invention are further defined in the dependent claims 2-13.

In a second aspect, the invention also aims at providing an expendable kit intended for percussive rock drilling tools and that includes not only a ring bit but also a casing shoe that may be left together with the casing tube in the drill hole. In this aspect, the object of the invention is to provide possibilities of transmitting impact impulses from the pilot bit to the ring bit via the casing shoe. In addition, both these components should be possible to be manufactured at a low cost, at the same time as the possibility for the ring bit to rotate freely in relation to the casing shoe should be reliable also under severe external conditions.

The features of the expendable kit according to the invention are seen in the independent claim 14. Preferred embodiments of the same are further defined in the dependent claims 15-21.

In a third aspect, the invention aims furthermore at providing an improved ring drill bit as such. The vital features of this improved ring drill bit are seen in the independent claim 22, while preferred embodiments of the same are defined in claims 23-29. In a fourth aspect, the invention also aims at providing an improved casing shoe suitable for the purpose. The fundamental features of this casing shoe are seen in the independent claim 30, besides which claims 31-33 define preferred embodiments of the casing shoe.

Further Elucidation of Prior Art

By EP 1837481 , a drilling tool intended for percussive drilling is previously known, the ring bit of which is connected to a casing shoe, which in turn may be welded onto the front end of a casing tube. However, in this case, the outer diameter of the ring bit is greater than the outer diameter of the casing tube in order to create a slit between the casing tube and the hole wall, through which slit the flushing medium and the drill cuttings can be evacuated. This slit interrupts all frictional contact between the outside of the tube and the hole wall, and makes rotational securing of the tube in the hole impossible.

For the sake of completeness, it should be pointed out that the drilling tool according to the invention should be possible to be formed for top hammer drilling as well as sinker drilling.

Brief Description of the Appended Drawings

In the drawings:

Fig. 1 is a perspective view of an assembled drilling tool according to the invention,

Fig. 2 is a partly sectioned perspective view showing the tool connected to the front end of a casing tube, besides which a drill rod intended for top hammer drilling being shown coupled to the rear end of the tool,

Fig. 3 is a perspective exploded view showing the components included in the tool individually, viz. a ring drill bit, a clamp ring, a casing shoe, and a pilot bit,

Fig. 4 is a perspective view corresponding to Fig. 2 showing how the drivers of the bayonet coupling assume a locking position in relation to the ring bit, Fig. 5 is a perspective view showing the same drivers turned into an open position, in which the pilot bit can be retracted axially out of the ring bit,

Fig. 6 is a perspective view showing the pilot bit during retraction out of the ring bit as well as of the casing tube,

Fig. 7 is a planar view showing the drivers in the same position as in Fig. 4,

Fig. 8 is a planar view showing the drivers in the position according to Fig. 5,

Fig. 9 is a side view of only the pilot bit,

Fig. 10 is an end view from above of the pilot bit according to Fig. 9,

Fig. 11 is a longitudinal section Xl-Xl in Fig. 10,

Fig. 12 is a side view of the ring bit,

Fig. 13 is an end view of the same ring bit,

Fig. 14 is a section XIV-XIV in Fig. 13,

Fig. 15 is an enlarged longitudinal section through the above-mentioned casing shoe,

Fig. 16 is a partially cut side view of the assembled drilling tool,

Fig. 17 is an enlarged detailed view showing how the casing shoe is connected to the ring bit, and

Fig. 18 is a perspective view of a pilot bit according to the invention intended for sinker drilling.

Detailed Description of Preferred Embodiments of the Invention

The drilling tool shown in Figs. 1-3 is in its assembled state according to Fig. 1 generally designated 1. In this tool, four separate components are included that are shown in Fig. 3, viz. a pilot bit 2, a ring bit 3, a casing shoe 4, as well as a clamp or coupling ring 5 having the purpose of connecting the casing shoe to the ring bit. In Fig. 2, the drilling tool is shown connected to a casing tube 6, more precisely by the fact that the front end of the casing tube has been welded onto the rear end of the casing shoe 4. In order to apply combined impact and rotary motions to the two bits, a drill rod 7 is connected to the rear end of the pilot bit, which rod is dhvable by a ground-based top hammer unit (not shown).

The designation C used for the assembled tool according to Figs. 1-2 for the centre axis of the tool will henceforth be used also for the individual centre axes of the respective components 2, 3, 4.

As is seen in Figs. 9-11 , the pilot bit 2 has a rotationally symmetrical basic shape in that a cylindrical envelope surface 8 in this is case concentric with the centre axis C, and extends between front and rear ends 9, 10. The front end 9 includes a central, flat end surface 11 , as well as a conical end surface 12 surrounding the same. At a certain distance from the front end 9, a ring-shaped bulge or girth 13 is formed, which is delimited by front and rear, ring-shaped end surfaces 14, 15, and includes two sections 16, 17, the front one 16 of which has a diameter that is smaller than the diameter of the rear section 17. Between cylindrical envelope surfaces 18, 19 along the sections 16, 17, a conical surface 20 extends, which forms an impact surface for transmitting impact impulses from the pilot bit to the casing shoe 4.

During drilling, the pilot bit 2 is intended to rotate in the direction of the arrow R.

As is seen in Fig. 10 the ring bulge 13 is broken through by a number of peripherally spaced-apart passages 21. In this case, the number of passages is three, the passages being uniformly placed at a pitch of 120°. Each individual passage 21 is delimited by a bottom surface 22 and two opposite side surfaces 23.

In Figs. 9 and 10, it is furthermore seen that the pilot bit includes three drivers 24, which in the preferred embodiment are placed in the immediate proximity of the front end 9 of the pilot bit, and which are in the form of projections having an essentially parallelepipedic basic shape. Each such driver 24 includes an axially rear end surface 25, two side surfaces 26, 27, an outside 28, as well as a front end surface, which in this case is a part of the conical end surface 12. The arc length, with which a driver 24 is peripherally displaced in relation to a rotationally trailing passage 21 in the ring bulge 13 (see Fig. 10), is designated by A.

In the flat end surface 11 as well as the conical end surface 12, there are mounted members 29, 30 having the purpose of crushing rock. In practice, said crushing members may consist of buttons of cemented carbide or another wear- resistant material, which are anchored in holes that are bored in the end surfaces. In this connection, it should be pointed out that the proper pilot bit in a conventional way is manufactured from steel or another equivalent material that is softer than the material in the crushing members 29, 30.

As is seen in Fig. 11 , in the rear end of the pilot bit 10, a hole 31 mouths, which is a part of a flushing medium duct, which furthermore includes two duct sections 32, 33 having successively decreasing diameters, the most slender 33 of which mouths in the end surface 11. In the hole 31 , a female thread 34 is formed for receipt of a male thread (not shown) on the front end of the drill rod 7. In this connection, it should be pointed out that the drill rod 7 also includes an internal duct (not shown) for the supply of flushing medium, in particular water, to the drill head with the purpose of evacuating the broken-away drill cuttings. It should also be noted that radial holes 35 extend between the duct section 32 and the envelope surface 8 of the pilot bit. More precisely, the individual hole 35 mouths at a point situated at a distance from the front end 9 of the pilot drill bit, as well as between the side surfaces 26, 27 of two adjacent drivers 24. As seen in the direction of rotation R, the side surface 27 forms a rear surface on a first driver and the surface 26 a front, driving side surface or drive surface on a trailing driver. In this connection, the hole 35 is situated between each rear side surface 27 and a (rotationally) trailing passage 21 through the ring bulge 13.

With continued reference to Figs. 9-11 , it should in conclusion be pointed out that three grooves 36 countersunk in the end surfaces 11 , 12 jointly meet each other at a point where the flush duct section 33 mouths. Supplied flush water will therefore be distributed out to the outer ends of the grooves 36. The same are situated axially flush with the passages 21 through the ring bulge 13. Reference is now made to Figs. 12-14, which illustrate the ring bit 3. Like the pilot bit, the ring bit 3 has a rotationally symmetrical basic shape by including an envelope surface 37 that is concentric with the centre axis C and in this case is slightly conical, as well as two opposite, ring-shaped surfaces 38, 39, which form the front and rear ends of the ring bit. An inner surface designated by 40 (see also Fig. 3) is cylindrical. Outside the flat, ring-shaped front end surface 38, there is also in this case a conical end surface 41. Crushing members 29a, 30a in the form of cemented- carbide buttons or the like are mounted in the flat end surface 38 as well as the conical end surface 41.

A front material section 42 surrounded by the envelope surface 37 has a greater diameter than a rear material section 43, the envelope surface of which is designated 44. In said envelope surface 44, a circumferential groove 45 opens. Internally in the ring bit, a number of cavities are formed in the inner surface 40. More precisely, three first chutes 46 are countersunk at a pitch of 120° and that extend axially between the front and rear ends of the ring bit. In front, said chutes 46 transform into a pocket 47 each, which extends laterally from the appurtenant chute and is delimited by a bottom surface 48, which extends perpendicularly to the centre axis C, as well as by an axially running shoulder surface 49. The chutes 46 and the pockets 47 form, together with the drivers 24 of the pilot bit 2, a bayonet coupling, the function of which will be described in more detail below, reference being made to Figs. 4-8.

With continued reference to Figs. 12-14, it should be noted that in the area between adjacent first chutes 46, second chutes 50 are formed, which, like the first ones, are placed at a pitch of 120° and extend axially between the front and rear ends 38, 39 of the ring bit. Each such second chute 50 are spaced-apart from an adjacent first chute 46 by means of a crest or partition wall 51 , the inside 40a of which is a part of the inner surface 40 in its entirety. In other words, the inner surface 40 has the same diameter in the area of the partition walls 51 as in the area of the pockets 47. Furthermore, it should be pointed out that the rear, flat end surface 39 of the ring bit is surrounded by a conical surface 52, which forms an impact surface on the ring bit. Now reference is made simultaneously to Figs. 2, 3 and 15, which illustrate the essential features of the casing shoe 4. Like the other components, the casing shoe has a rotationally symmetrical basic shape by including an envelope surface 53, which is cylindrical and concentric with the centre axis C, as well as extends between front and rear ends in the form of ring-shaped end surfaces 54, 55. It should be particularly noted that the rear end surface 55 is conical in order to, together with a corresponding conical surface on the casing tube 6, form a fillet for welding together of the casing shoe and the casing tube. On the generally cylindrical inside 56 of the casing shoe, there is formed, in accordance with the invention, a ring-shaped ridge 57 having a reduced inner diameter. Said ridge 57 is delimited between ring-shaped end surfaces 58, 59, both of which are conical and form impact surfaces, the rear one 59 of which is arranged to co-operate with the conical impact surface 20 on the pilot bit, and the front one 58 with the likewise conical impact surface 52 on the ring bit (see Fig. 17). In the area between the front end 54 and the ridge 57, there is arranged an internal, ring-shaped groove 60, the width of which is greater than the previously mentioned groove 45 in the ring bit.

In Figs. 16 and 17, parts of the drilling tool in the assembled state thereof are illustrated. Among other things, it is shown how the coupling ring 5 co-operates with the ring bit 3 and the casing shoe 4 with the purpose of interconnecting the same in such a way that they, on one hand, follow each other axially during drilling, but on the other hand allow free rotation of the ring bit 3 in relation to the casing shoe 4. For this purpose, the ring 5 consists of a Seeger ring, which can be compressed to be threadable into the front end portion of the casing shoe and then expand to its initial position by a full diameter. In the last-mentioned, expanded state, shown in Figs. 16 and 17, the inner part of the ring 5 engages the external groove 45 in the ring bit 3, while the outer part is accommodated in the groove 60 on the inside of the casing shoe. The groove 45 has essentially the same width as the ring 5, while the groove 60 is considerably wider than the ring. In such a way, the ring will be fixed axially in relation to the ring bit 3 at the same time as the same can move axially in relation to the casing shoe 4. More precisely, the width of the groove 60 should be sufficiently large to allow the ring to move axially in relation to the casing shoe a distance that is at least somewhat greater than the amplitude of the impacts. Since the clamp ring 45 only interconnects the ring bit and the casing shoe axially, but not peripherally, the ring bit 3 can rotate freely in relation to the casing shoe.

Function and Advantages of the Drilling Tool According to the Invention

When a hole is to be drilled and lined, e.g., with the purpose of, in unconsolidated rock, providing a consolidating protection umbrella of the kind mentioned by way of introduction, first the present casing tube 6 (see Fig. 2) is united with the casing shoe 4, preferably by welding. In the next step, the ring bit 3 is interconnected with the casing shoe 4, more precisely by means of the coupling ring 5 in the way described above. In a concluding step, the ring bit 3 is interconnected with the pilot bit 2. This is effected by the fact that the drivers of the pilot bit 24 are inserted axially through the chutes 46 until they are located on a level with the pockets 47 in the front end of the ring bit. Next, the pilot bit is turned in the direction of rotation R of the tool so that the drive surfaces 26 on the drivers 24 contact the shoulder surfaces 49 included in the pockets. In this state, shown in Figs. 4 and 7, the tool is ready for drilling. The drilling is effected by a combination of impact and rotary motions by means of which the rock is crushed by the crushing members 29, 29a, 30, 30a at the same time as the same constantly changes the peripheral positions thereof in relation to the rock. More precisely, the impacts are transmitted directly to the own crushing members 29, 30 of the pilot bit, as well as to the crushing members 29a, 30a of the ring bit, viz. via the casing shoe 4.

The last-mentioned transmission is effected by the fact that the impact surface 20 of the pilot bit acts against the rear impact surface 59 of the casing shoe and sets the casing shoe in intermittent, axial impact motions, which in turn are transmitted to the ring bit 3 via the front impact surface 58 on the internal ridge 57 in the casing shoe and the rear impact surface 52 on the ring bit.

The transmission of said impact motions occurs entirely without hindrance from the ring 5, which is movable in the wide grooves 60 of the casing shoe. The rotation of the ring bit in relation to the casing shoe, which is required for the same to accompany the pilot bit with the purpose of intermittently altering also the positions of the crushing members 29a, 30a included in the ring bit in relation to the rock, is effected by means of the drivers 24. More precisely, the drive surfaces 26 of the drivers 24 (at least intermittently) is held pressed against the shoulder surfaces 49 included in the pockets 47 of the ring bit. During the drilling, when the drivers 24 engage the pockets 47, the evacuation of the flush water and accompanying drill cuttings occurs via the channels that are delimited by, on one hand, the second chutes 50 in the inside of the ring bit, and on the other hand the envelope surface 8 of the pilot bit. In this state, the chutes 50 are situated axially aligned with a trailing passage 21 through the ring bulge 13 of the pilot bit. This means that the flush water flows through the drilling tool occur via ducts in the form of the second chutes 50, which are spaced-apart from the first chutes 46 that are required for the application of the drivers 24 of the bayonet coupling in a locked, driving position. In other words, the individual soil water flow is directed linearly through the chute 50 and the axially trailing passage 21 in the ring bulge without tendency to penetrate into the groove 46 and the pocket 47. When the drilling tool is in its active state according to Figs. 4 and 7, the radial hole 35 is located opposite the chute 46 so that the fed out water should rinse said chute and the connected pocket 47.

When the pilot bit 2 is to be detached from the ring bit 3 and be pulled out of the drill hole (see Figs. 5 and 8), e.g., when the drill hole is finished or upon overhaul of some type, the pilot bit is turned an arc length B in Fig. 8 in the opposite direction to the direction of rotation R during driving. In such a way, the drivers 24 are located flush with the chutes 46 and can be pulled out rearward through the same, as is shown in Fig. 6.

A substantial advantage of the invention is that the impact forces from the pilot bit are not transmitted to the ring bit via the drivers of the bayonet coupling, but via impact surfaces on the casing shoe, which all are circumferential or endless and thereby ample. Also the impact surface on the pilot bit is comparatively large because the same is interrupted only by the axial passages of the ring bulge that have a moderate width. The stresses on the impact surfaces are furthermore reduced as a consequence of the fact that the impact surfaces of the pilot bit as well as of the ring bit constantly rotate in relation to the impact surfaces of the casing shoe and vary the mutual points of impact thereof. In addition to this, the impact surfaces are conical, and therefore the components included in the drilling tool are always centered in relation to each other in an effective way. Another essential advantage is that the evacuation of drill cuttings is effected in other ways than via those chutes that are required in order for the drivers of the bayonet coupling to be insertable into the appurtenant pockets. In such a way, the risk is minimized that the drill cuttings penetrate into and stick in the pockets, and thereby making the bringing of the drivers into and out of engagement with the pockets more difficult. Also the rinse described above of the first chutes and the pockets via the radial flushing holes contributes significantly to this effect. An additional advantage of the invention is based on the fact that the pockets for the drivers of the bayonet coupling are situated in the front part of the ring bit and open in the forward direction. In such a way, the axial extension or thickness of the ring bit can be reduced to a minimum while reducing the material consumption in the ring bit. Furthermore, not only the pilot bit but also the ring bit can be manufactured by means of simple and fast machining operations, among other things as a consequence of the fact that the internal surfaces to be machined are freely exposed and easily accessible. Another advantage is afforded by the unique locking or the coupling between the ring bit and the casing shoe. Thus, by means of a simple Seeger ring, a very reliable coupling is obtained between said components, more precisely what relates to the freedom of the ring bit to rotate in relation to the casing shoe as well as to the axial union of the same. A further advantage of the invention is that the drivers of the pilot bit afford room for mounting of buttons or crushing members situated near the periphery of the tool. This possibility is particularly important when the diameter of the tool is small and the ring bit thin, because the requisite crushing members in the vicinity of the periphery can be mounted on the pilot bit instead of on the thin ring bit.

In Fig. 12, t designates the thickness of the ring bit 3, such this is defined by the axial distance between the front and rear ends 38, 39 of the ring bit, while the outer diameter of the ring bit is designated OD. In a concrete embodiment of a ring bit according to the invention, the same has the diameter OD = 120 mm, and a thickness t = 40 mm. In other words, the thickness t amounts to only 1/3 or 33 % of the diameter OD. The possibility of reducing the thickness of the ring bit - and thereby the volume - to this minimum is based on the fact that the pockets for the drivers of the pilot bit are diminutive and situated in the immediate vicinity of the front end of the ring bit. The result of this becomes a reduced consumption of material and lowered manufacturing costs.

In this connection, it should be pointed out that the absolute thickness t of the ring bit does not necessarily need to be increased if the diameter OD is increased.

In Fig. 18, a pilot bit made in accordance with the invention suitable for sinker drilling is shown. For this purpose, the rear part of the pilot bit, which is present behind the ring bulge 13, is formed with spline bars for the connection of the pilot bit with a sinker drill assembly of traditional type (not shown). However, the front part of the pilot bit, which is present in front of the ring bulge 13, is identical to the front part of the previously described pilot bit, and therefore the same can be interconnected with ring bits of the kind in question.

The invention is not limited only to the embodiments described above and shown in the drawings. Thus, it is feasible to form one or both pairs of co- operating impact surfaces in the form of flat ring surfaces, which extend perpendicularly to the centre axis of the tool. However, the exemplified, conical surfaces are preferred. Furthermore, it is possible to form the tool with another number of co-operating drivers and pockets than three. Because the forces that are required to drive the ring bit in the rotary motions of the pilot bit are very moderate in comparison with the forces that act between the impact surfaces, it is even feasible to form the tool with only one driver. On the other hand, it is possible to use several more drivers than three, in particular when the tool has a large diameter. However, at all events, the different drivers, when they are numerous, should be equidistantly spaced-apart along the periphery. Within the scope of the subsequent claims, it is furthermore possible to form the pocket for the driver without the same opening in the front end of the ring bit. In other words, a fairly short first chute, which only opens in the rear end of the ring bit, may be combined with a laterally projecting pocket. With the purpose of reducing the material consumption, the shown embodiment is, however, preferred with forwardly open pockets. Furthermore, the two impact surfaces, which are included in the casing shoe, may be formed on two separate ridges instead of on a common ridge. The disclosures in Swedish patent application No. 0802615-5, from which this application claims priority, are incorporated herein by reference.