WO1998058154A1

WO1998058154A1 - A drill bit

Info

Publication number: WO1998058154A1
Application number: PCT/AU1998/000462
Authority: WO
Inventors: Malcolm Bicknell Mcinnes
Original assignee: Sds Digger Tools Pty. Ltd.
Priority date: 1997-06-16
Filing date: 1998-06-16
Publication date: 1998-12-23
Also published as: ZA985220B; AUPO737497A0; ZA985216B; WO1998058153A1

Abstract

The invention relates to a drill bit for use with a percussive hammer. The drill bit (14) has a cutting face (31) at the lower end of the drill bit (14) and a body extending from or adjacent the cutting face (31) to the upper end of the drill bit (14). The upper end of the drill bit (14) has a thrust face and an impact face (17) and there are a plurality of drive (23) and retainer surfaces (21) on, and radially spaced around, the external surface of the body. The drill bit (14) is retained with respect to the hammer so that in use the upper end of the drill bit (14) abuts against the lower end of the hammer. This results in a drill bit (14) having a significantly reduced length which can also have the advantage of a body having a constant diameter along its length.

Description

A DRILL BIT

BACKGROUND OF THE INVENTION

This invention relates to a drill bit and in particular to improvements with drill bits that are used with down hole percussive hammers.

Conventional percussive drill bits for use with down hole hammers are well-known. They comprise a drill bit cutting face with hardened inserts, a drill bit head or body behind the face and a shank extending from the drill bit body. The shank is normally inserted into the lower end of the hammer and in particular into a component known as a drive sub-assembly. The shank has a splined portion which engages with a corresponding spline portion in the drive sub-assembly. The end of the shank has an impact surface which is struck by the piston of the hammer.

The attachment of the drill bit to the hammer enables some longitudinal movement of the bit with respect to the hammer body. The drill bit is normally retained within the hammer by way of a split ring which locates in a recess portion at the end of the shank and abuts against a shoulder on the drive sub-assembly. This enables the drill bit to hang in a reaching position. When the drill bit is pushed into the hammer, the hammer piston is able to strike the impact end of the drill bit, and surfaces of the hammer can abut against the drill bit to provide a downward thrust loading.

There are several difficulties with this particular design which result in limitations to either the operation loads that can be applied to the drill bit, and also effect the durability of the drill bit.

For example, the spline drive means is prone to damages through excessive wear of the spline surfaces, or through fracturing as a result of the base of each spline providing stress raisers and crack initiation sites. It is very difficult to ensure that all of the splines on the shank are engaged by the corresponding drive splines of the drive sub-assembly. Normally, there is only point or line contact between only a few of the splines. This leads to excessive wear and surface spalling. The load applied through the splines is quite high which is due to the relatively small diameter of the spline and the large torque which is being transferred. The small diameter of the spline results from the spline surface having to fit within the drive sub-assembly which itself normally fits within the hammer barrel. The spline also usually has a diameter that is smaller than the diameter of the thrust face on the drill bit. In addition the thrust face on the drill bit is forward of the splines.

Further, the conventional drive sub-assembly needs to be removed before the drill bit can be released from the hammer. The split ring bit retainer is held in place by an elastomeric component that is easily damaged by high temperatures. The retention system uses small parts which are easily dropped and may be lost down the hole

Existing bits are normally manufactured from forgings due to the great variation in diameter along their lengths. The shank portion of the drill bit is often half the diameter of the body of the drill bit. Forgings are costly, particularly where a great number of drill bits of differing dimensions are used by one manufacturer. The forgings also have some difficulty in maintaining consistent material strength throughout the length of the drill bit. It would be desirable to produce drill bits from bar stock material but this would not be cost effective due to the amount of machining required and resultant waste of material.

Another difficulty with existing drill bits is the rear retention means. Apart from the number of components required for the assembly, which of itself is a disadvantage, fracturing of the drill bit in certain positions forward of the rear retention means will result in the whole of the fractured portion of the drill bit being lost down hole. The fractured portion would simply drop out of the hammer.

Accordingly, it is an objection of the invention as described in this specification to overcome the abovementioned problems and to provide means of simplifying the construction of the drill bit. Further aspects of the invention include improving the retaining means and providing an improved drive means for the hammer and drill bit assembly.

SUMMARY OF THE INVENTION

In its broadest form, the invention is a drill bit for use with a percussive hammer comprising: a cutting face at the lower end of said drill bit, a body extending from or adjacent to said cutting face to the upper end of said drill bit, a thrust face and impact face on the upper end of said drill bit, and a plurality of drive and retainer surfaces on, and radially spaced around, the external surface of said body, said drill bit retained with respect to said hammer so that in use the upper end of said drill bit abuts against the lower end of said hammer.

Preferably, the drill bit body has a substantially constant diameter along its length. For example, from the shoulder of the drill bit, which has approximately the same diameter as the gauge diameter of the drill bit, the body may have a smaller diameter that remains constant from the shoulder to the upper end of the drill bit. This means that apart from the shoulder adjacent the cutting face, the body does not have any significant steps or reductions in diameter between the cutting face and the upper end of the drill bit.

The drive and retainer surfaces are preferably formed by machining so that they are on the external surface of the drill bit body between the cutting and impact face. Such a configuration of a drill bit would enable economic production from cylindrical bar stock which is not economically feasible for conventional drill bits which have substantial diameter changes along its length.

The hammer subject of this aspect of the invention is designed so that the impact end does not need to engage substantially within the hammer. In this case, a drill bit retainer and drive means will extend beyond the end of the hammer, where the end of the hammer is defined by the maximum reach of the piston when it is operating normally to provide impact to the end of the drill bit. Preferably, the thrust face of the drill bit may also be on the same surface as the impact face, or may be close to the impact face.

Apart from the drill bit having a more constant diameter along its length, this aspect of the invention means that the length of the drill bit between the impact and cutting face can be significantly reduced by comparison to current drill bit designs. This has some assistance in transferring impact load from the piston to the cutting face, and also improves the resistance to bending of the drill bit about its longitudinal axis.

Preferably, the drill bit retainer and drive means (referred to hereafter as drive means) attaches to the end of the hammer, and may comprise a threaded coupling on the external surface of the end of the hammer. In the case of a reverse circulation hammer, the diameter of the drive means may be sufficient for it to act as a seal due to the external surface of the drive means engaging with the wall of the hole that is being drilled to prevent air flow to the surface between the hole and the drill rod. The forward end of the drive means may also extend towards the drill bit cutting face to ensure the sealing effect is as close to the cutting face as possible, and may also act to direct exhaust air flow towards the drill bit face.

Preferably, the drive surfaces comprise a plurality of channels that are radially spaced around the body of the drill bit. The channels are substantially parallel to the longitudinal axis of the drill bit and have side walls that comprise the drive surfaces. The channels are engaged by drive lugs that are preferably on the drive means. The meshing of the drive lugs with the channels causes the drill bit to rotate with the hammer.

Preferably, the channels are formed between two adjacent splines which are formed on the external surface of the body of the drill bit. In addition to a drive lug locating within the channel between the splines, further adjacent drive lugs may be positioned outside of the channel so that they engage the external surface of the splines to also provide drive engagement.

Preferably, the drive means is substantially tubular, and the drive lugs are formed on the internal surface of the drive means towards its lower end.

The width of the splines that form the channels and the drive lugs can be significantly larger that the teeth that comprise the conventional spline drive arrangement. This is due to the drive surfaces being positioned on a surface of much greater diameter than is normally the case, and also the fact that as few as two or three sets of drive surfaces may be positioned around the periphery of the drill bit body.

Preferably, the retainer surfaces comprise walls that extend across the end of the channels. These close off the channels, preferably toward the impact end of the drill bit which in turn means that the drill bit will be retained in the drive means when the drive lugs engage the retaining surfaces. This will occur when the drill bit hangs within the drive means as the hammer is lifted off the bottom of the hole being drilled.

Alternatively, the channels may be completely closed off at both the upper and lower ends, and the drive means may be split in two halves in order to assembly the drive means around the drill bit. In this case, some form of means (similar to that shown in International Patent Application No PCT/ AU97/ 00482) is required to hold the two halves of the drive means together and to hold the assembled drive means to the end of the hammer.

Preferably though, a bayonet style connection may be used. In this example, pairs of splines on the drill bit are associated with a drive lug or a set of drive lugs. The lower end of each channel has an opening through which a drive lug may pass as the drill bit is rotated. The drill bit is inserted into the drive means, with the drive means only partially threaded on to the hammer, so that the lugs locate adjacent the openings.

The drill bit is then rotated so that each drive lug aligns with and engages in the channel. The drive means is then fully threaded on to the hammer which prevents the drill bit from moving forward enough to allow the lugs to algin with the openings. The ends of the channels are closed off by the retaining surfaces which enable the drill bit to be held in the drive means.

The location of the drive lugs and channels may be reversed. In other words, the spline-like drive lugs may be located on the drill bit body with the corresponding channels being located on the internal surface of the drive means.

Preferably, the engagement of the drive means is at a diameter which is substantially the same as the diameter of the hammer barrel. This has a significant advantage in that the rotation force transferred to the drill bit as the hammer is rotated is much lower at the larger diameter by comparison to common drill bits which use smaller diameter splines that locate within the hammer. A significant advantage of the design referred to above is that drive means is able to operate a much larger diameter by comparison to conventional spline drives.

In addition, the outer radial surfaces of the splines act as bearing surfaces on the inner surface of the drive sub-assembly so that the drill bit is held centrally with respect to the hammer.

In respect of all of the aspects of the invention, drill bits used for reverse circulation hammers may be provided with a plurality of exhaust conduits that extend from the rear of the drill bit in the vicinity of the sample tube aperture through to the cutting face or to a region adjacent the cutting face or shoulder of the drill bit. The entry of the exhaust port adjacent the sample tube aperture ensures that exhaust flow from the hammer has a clear passage beyond any of the forward bit retainers or drive means and the thrust face. BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention, preferred embodiments will now be described. These preferred embodiments are a combination of the inventive aspects described above, but it should be realised that the invention is not limited or confined to precise details of each of the embodiments or the manner in which the aspects of the invention are combined.

The embodiments are illustrated in the attached drawings in which:

Fig 1 shows a cross-sectional view of an end portion of the hammer showing the drill bit held in relation to the end of the hammer,

Fig 2 shows a perspective view of a drill bit in accordance with a first embodiment of the invention,

Fig 3 shows a perspective view of a drive means used in conjunction with the drill bit shown in Fig 2,

Fig 4 shows a cross-sectional view of the end of a hammer showing a first embodiment of a thrust ring,

Fig 5 shows a cross-sectional view of an end of a hammer with a second embodiment of a thrust ring,

Fig 6 shows a perspective view of a drill bit according to a second embodiment,

Fig 7 shows a perspective view of a drive means used with the drill bit shown in Fig

6,

Fig 8 shows a drill bit according to a third embodiment of this invention, and

Fig 9 shows a perspective view of a drive means used with the drill bit shown in Fig

8.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Fig 1 shows the forward end of an assembled percussive hammer 10 which comprises a hammer barrel 11 and a piston 12. The percussive hammer 10 illustrated is a reverse circulation hammer, so it incorporates a sample tube 13. The impact position for the piston 12 is approximately level with the end of the barrel 11 as shown in Fig 1. The drill bit 14 comprises a body 36, a shoulder 20, a cutting face 31 and an impact face 17. The impact face is on a raised portion which is surrounded by a thrust face 26. In its operative position, the upper end of the drill bit 14 abuts against the end of the hammer barrel 11. The drive means 15 is a generally tubular shaped component which threadably engages to the end of the hammer barrel 11.

Threads 16 are provided on the internal surface of the drive means 15 and the end of the hammer barrel 11.

As can be seen in Fig 1 and Fig 2, the drill bit 14 does not have any portion such as a shank which needs to locate within the hammer 10. Accordingly, the body 36 of the drill bit 14 has a substantially constant cross-section throughout its length, particularly behind the shoulder 20 of the drill bit 14. This enables the drill bit 14 to be manufactured from bar stock material rather than having to use a forging as would be the case with a normal drill bit.

The drill bit 14 is provided with three pairs of channels 23 formed between pairs of splines 18 and 19. The region between adjacent pairs of splines 18 and 19 form a channel 22 that extends between the impact face 17 and the cutting face 31. A retainer surface comprising a wall 21 is positioned at the end of channel 23 so that it does not open onto the impact face 17. Spline 19 is shorter than spline 18 in each set and therefore forms an opening 24 between the end of spline 19 and the shoulder 20.

The drive means 15 has three drive lugs 25 at its forward end. These drive lugs are radially spaced so that they locate in the channels 22 thereby enabling the drill bit to be inserted into the lower end of the drive means 15. With the drive means 15 partially attached to the hammer barrel 11, each drive lug 25 can simultaneously be aligned with a respective channel 22. The drill bit 14 can then be pushed into the drive means 15 with the drive lugs 25 sliding along the channels 22. With the drive means 15 only being partly threaded onto the end of the barrel 11, the drill bit 14 can be pushed all the way into the drive means 15 so that the openings 24 align with the drive lugs 25. In this position, the drill bit 14 can be rotated so that each of the drive lugs 25 travel through the openings 24 into the channels 23. The end wall 21 of the channels 23 abut against the drive lugs 25 thereby preventing the drill bit from falling out of the end of the drive means 15. In this position, the drive means 15 can then be fully threaded onto the end of the hammer barrel 11. When in this position, the drill bit 14 cannot be pushed back far enough for the drive lugs 25 to again align with the openings 24. Therefore, the drive lugs 25 are held captive within the channels 23.

This allows sufficient movement of the drill bit 14 with respect to the end of the hammer barrel 11 the normal operation, and enables the impact face 17 to abut against the end of the hammer barrel 11, and therefore be in position for impact by the piston 12.

Accordingly, rotation of the percussive hammer 10 will result in the drive lugs 25 bearing against the side wall of the splines 18 or 19 and therefore rotating the drill bit 14. When the percussive hammer 10 is lifted, the drill bit will be retained by the drive lugs 25 abutting against the walls 21. The outer radial surfaces of the splines act as bearing surfaces that engage against the inner surface of the drive means 15. This holds the drill bit 14 centrally with respect to the hammer 10.

The walls 21 are provided with a groove 33 and recesses 34 at the top of each channel 23. Both the groove 33 and recesses 34 allow airflow into and around the channel 23 to help clear any debris that may accumulate in the end of the channel 23.

The drill bit 14 is provided with an exhaust conduit 27. The impact face 17 of the drill bit 14 has an aperture 28 into which the sample tube 13 locates. The end of the aperture 28 is enlarged to a large diameter portion 29 into which the exhaust conduit 27 opens. The other end of the exhaust conduit 27 exits the side of the drill bit 14 adjacent the shoulder 20 of the drill bit 14. A channel 30 is cut into the shoulder 20 of the drill bit 14 adjacent the exit of the exhaust conduit 27. There are three exhaust conduits 27 in the embodiment illustrated in Figs 1 to 3. The exhaust air from the exhaust conduit 27 travels through the channel 30 and across the cutting face 31 of the drill bit. The exhaust air and entrained cuttings travel through return conduit 32 which connects with the sample tube 13 for return of air and cuttings to the surface.

At least one return conduit 32 is used on each drill bit 14, but preferably, the number of return conduits 32 equal the number of exhaust conduits 27.

Although this embodiment describes a reverse circulation hammer, it would equally be suited to a normal circulation hammer. In such a hammer, there is no sample tube and the exhaust supply exits through the centre of the drill bit and out through the face, the air and cuttings then return to the surface between the hole and the drill rod.

The drive means 15 is provided with wear indicators 35 which comprise two circumferential grooves of varying depth. The external surface of the drive means 15 abuts against the wall of the hole being drilled and is therefore subject to significant wear. The wear indicator grooves 35 provide a clear indication as to the extent of wear and assist in determining when to replace the drive means 15.

As a result of the drive means 15 abutting against the wall of the hole being drilled, it provides a sealing effect to restrain air from travelling to the surface between the hole and drill rod. In addition, the forward end of the bit drive means 15 extends towards the face of the hole being cut, and therefore provides a sealing effect at a position that is very close to the cutting face 31 of the drill bit. It therefore fills a volume that otherwise might result in a reduced velocity of airflow exiting from the exhaust conduits 27. This ensures that the velocity of the exhaust air that moves around the cutting face 31 is as high as possible. It also ensures that the exhaust air is directed towards the cutting face. Figs 4 and 5 show the use of a hardened thrust ring. In Fig 4, the thrust ring 37 transfers thrust force from the end of the hammer barrel 11 to the end of the drill bit 14. This preserves the hammer barrel from wear. It also forms part of the lower piston chamber and piston seal/bearing surface of the hammer barrel 11.

In the embodiment shown in Fig 5, the thrust ring 38 is similar to that shown if Fig 4, other than that the thrust ring forms the lower piston chamber by itself rather than in combination with the end of the hammer barrel 11.

The embodiments shown if Figs 6 and 7 and Figs 8 and 9 show variations in the drive lugs 25 at the end of the drive means and bit retainer 15. In Figs 6 and 7, a pair of drive lugs 40 are used. The drill bit 14 still has a channel 22 and an opening 24 but has a pair of channels 41 formed by splines 46, 47 and 48, into which the drive lugs 40 each locate. This version spreads the load applied to the sides of the second channels 41 over twice the area by comparison to the embodiment shown in Figs 2 and 3.

In Figs 8 and 9, a pair of drive lugs 43 and 44 are used where drive lug 44 is longer than or the same length but non-aligned with drive lug 43. In the drill bit 14 shown in Fig 8 the channel 22 is wide enough for both drive lugs 43 and 44, but the opening 24 only allows drive lug 43 to locate into the channel 45. Channel 45 is formed between splines 18 and 19. Therefore, only drive lug 43 performs any retaining function in terms of holding drill bit 14 to the drive means 15, but both drive lugs 43 and 44 act against the walls of the splines 18 and 19 to cause rotation of the drill bit 14.

As can be seen from the above descriptions, the drive means 15 provides a unique way of holding a drill bit 14 into a percussive hammer 10. It not only provides a retention means, but also provides a drive means for rotating the drill bit 14 with respect to the percussive hammer 10. The fact that the drive means 15 is forward of the end of the hammer barrel 11 means that the effective diameter at which the drive lugs 25 engage the drill bit 14 is much larger by comparison to the splined portion used on a shank that has to be located within the hammer body. Accordingly, the loads applied to the drill bit 14 that cause it to rotate are greatly reduced by comparison to the loads that will be applied through the splines on a shanked drill bit.

As the drill bit 14 does not have any components which locate within the hammer 10, it can have a substantially constant cross-sectional diameter from the impact face 17 to the cutting face 31. The only major variation in the diameter would be any drill bit shoulder 20 that may be used. However, in the majority of cases this is only a minor diameter variation and it would still enable the drill bit 14 to be manufactured in a cost effective manner from simple bar stock and would therefore be an alternative to the use of forged components.

The overall diameter of the drill bit 14 is constant and therefore avoids weakness that would result in fractures through changes in diameter or to smaller diameter components such as a shank. This greatly reduces the potential for fracture of the drill bit 14 through normal use.

However, a shorter drill bit in accordance with most aspects of the invention, but with some diameter change on the body, will still have significant advantages and be within the scope of the invention.

Another important aspect of the invention is the ability with the drill bit 14 to have the thrust face of the hammer barrel and the impact plane of the piston 12 in substantial alignment or close to each other. This has an effect of minimising bit length, enhancing bit alignment with the piston to minimise stress and to maximise the energy that is effectively transferred to the cutting face 31.

The process by which the piston 14 connects to the drive means and bit retainer 15 involves a very simple operation that can be easily performed in the field, and reduces the number of components for assembly and therefore improves the ease of attaching the drill bit 14 to the end of the hammer 10. The drill bit can be removed from the hammer with great ease. The drive means 15 is partly unscrewed, the bit 14 pushed in and rotated and then simply pulled out. There are no small components that could be easily lost down the hole or any elastomeric parts that can be damaged through high temperatures.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A drill bit for use with a percussive hammer comprising: a cutting face at the lower end of said drill bit, a body extending from or adjacent to said cutting face to the upper end of said drill bit, a thrust face and impact face on the upper end of said drill bit, and a plurality of drive and retainer surfaces on, and radially spaced around, the external surface of said body, said drill bit retained with respect to said hammer so that in use the upper end of said drill bit abuts against the lower end of said hammer.

2. A drill bit according to claim 1 wherein said body has a substantially constant diameter along its length.

3. A drill bit according to either claim 1 or 2 wherein said thrust and impact faces are on the same surface.

4. A drill bit according to either claim 1 or 2 wherein said impact face comprises an annular projection that is surrounded by said thrust face.

5. A drill bit according to any one of the preceding claims further comprising a plurality of channels radially spaced around the body of said drill bit that are substantially parallel to the longitudinal axis of said drill bit, the side walls of said channels comprising said drive surfaces.

6. A drill bit according to claim 5 wherein said retainer surfaces comprise a wall that extends between the side walls of a said channel.

7. A drill bit according to either claim 5 or 6 wherein said channels are formed between a pair of spaced parallel splines.

8. A drill bit according to either claim 6 or 7 wherein said channels extend to the upper end of said drill bit, said retainer surfaces adjacent the upper end of said channels.

9. A drill bit according to either claim 7 or 8 further comprising a shoulder extending around the lower end of said drill bit.

10. A drill bit according to claim 9 wherein one of the splines of said pair extend from the upper end of said drill bit to said shoulder, the other extending from the upper end of said drill bit to a position intermediate of the upper end of said drill bit and said shoulder.

11. A drill bit according to any one of claims 7 to 10 wherein channels formed between adjacent pairs of said splines form air passages that direct air flow to the cutting face in respect of a drill bit used with a reverse circulation hammer.

12. A drill bit for use with a reverse circulation hammer according to any one of the preceding claims further comprising sample return conduit extending through said body and exhaust passages that extend from within said sample return conduit to the external surface of said body, said exhaust passages positioned between adjacent pairs of said splines.

13. A drill bit and combined retainer and drive means for said drill bit for use with a percussion hammer comprising: a drill bit according to claim 1, and said retainer and drive means comprising: an upper portion secured to the lower end of said hammer, a lower portion extending beyond the lower end of said hammer, a recess in said lower portion within which said drill bit locates such that- the upper end of said drill bit abuts against the lower end of said hammer, and a plurality of drive projections radially spaced on the inner surface of said recess that engage said drive surfaces so that said drill bit rotates with said hammer, and that engage said retainer surfaces to retain said drill bit in said lower portion.

14. A drill bit and combined retainer and drive means for said drill bit for use with a percussion hammer comprising: a drill bit according to any one of claims 5 to 12, and said retainer and drive means comprising: an upper portion secured to the lower end of said hammer, a lower portion extending beyond the lower end of said hammer, a recess in said lower portion within which said drill bit locates such that the upper end of said drill bit abuts against the lower end of said hammer, and a plurality of drive projections radially spaced on the inner surface of said recess that locate within said channels and engage said drive surfaces so that said drill bit rotates with said hammer, and that engage said retainer surfaces to retain said drill bit in said lower portion.

15. A drill bit as hereinbefore described with reference to and as illustrated in the accompanying drawings.

16. A drill bit and combined retainer and drive means as hereinbefore described with reference to and as illustrated in the accompanying drawings.