Drill bit assembly for fluid-operated percussion drilling tool

Abstract

The present invention relates to a drill bit assembly for a fluid operated percussion drilling tool. The drill bit assembly includes a percussion drill bit having a head portion with an axially extending shank, a plurality of first axially extending splines on the shank being slidably engageable with a plurality of first complementary splines formed in an interior of a drive chuck, such that rotational drive from the chuck can be transmitted to the shank. The assembly further includes an engagement member on the chuck for connecting the chuck with a drive member of the fluid operated percussion drilling tool. During drilling, the drill bit and the chuck are held in a first relative orientation in which the drill bit is retained in the chuck and the drill bit is axially movable relative to the chuck to a drill bit removal position in which the drill bit is rotatable relative to the chuck to a second relative orientation in which the drill bit is removable from the chuck. The drill bit is movable to the drill bit removal position when the chuck is fully engaged with the drive member.

Description

TECHNICAL FIELD

[0001] The present invention relates to a drill bit assembly for use in a fluid operated percussion drilling tool. In particular, the present invention relates to a drill bit assembly for use with a down-the-hole hammer. BACKGROUND

[0002] Conventional down-the-hole hammers and fluid operated percussion drilling tools generally comprise an outer cylinder or outer wear sleeve within which an inner cylinder is mounted, which in turn is engaged with a back head assembly. A sliding reciprocating piston cooperates with the inner cylinder and back head assembly so that, when air pressure is supplied through the back head assembly, the piston imparts an impact to a drill bit held within a chuck which is threadably attached to the bottom of the outer wear sleeve. The drill bit shank is externally formed with a plurality of axially extending splines which are spaced around the circumference of the drill bit shank. The splines on the drill bit are slidably engaged with complementary splines formed in the inner wall of the chuck for transmitting rotational drive from the chuck to the drill bit.

[0003] In conventional constructions, in order to retain the drill bit in the chuck, a drill bit retaining ring is provided which sits above the chuck and cooperates with an annular shoulder on the drill bit. In alternative constructions, the part of the drill bit retaining ring which is used to engage with the retaining shoulder to retain the drill bit in the assembly is arranged within the chuck. Typically, the drill bit retaining ring is provided in two halves which are held together by an O-ring.

[0004] In each of these existing constructions, removal of the drill bit from the assembly requires unscrewing the chuck from the outer wear sleeve in order to enable the drill bit to be removed from the chuck. When the chuck is unscrewed from the outer wear sleeve, the drill bit retaining ring is removed and the drill bit falls out. A new drill bit is then arranged within the chuck, the drill bit retaining ring is arranged over the end of the chuck so that it engages with the shoulder or tail splines on the drill bit shank, and the O-ring is arranged over the drill bit retaining ring in order to hold it together. The assembly is then reattached to the hammer by screwing the chuck into the wear sleeve. A disadvantage of this construction is that unscrewing the chuck from the hammer can be time consuming. Down-the-hole hammers are typically designed so that the threaded connection between the chuck and the wear sleeve will be tightened during drilling. As the torque used to tighten the thread before drilling and during drilling is very high, loosening the thread in order to change the drill bit can be difficult and can require specialist loosening equipment. In poor ground conditions, it can be necessary to remove the drill bit several times during the life of the hammer for regrinding or replacement. Furthermore, drillers are typically paid per metre drilled and the time taken to change the drill bit reduces the amount of time which can be spent drilling.

[0005] It is therefore desirable to provide a drill assembly from which the drill bit can be removed without the need to unscrew the chuck from the wear sleeve. One such arrangement is proposed in Irish Patent No. S87041 in which a retaining sleeve is externally mounted to the chuck. The retaining sleeve is movable between a drilling position in which the drill bit and chuck are held in a first relative orientation and a drill bit removal position in which the drill bit can be rotated relative to the chuck to a second relative orientation in which the drill bit can be removed from the chuck. However, problems can arise in relation to the flushing of the retaining sleeve and it is therefore desirable to provide a simpler and more robust arrangement. SUMMARY

[0006] The present application relates to a drill assembly for a fluid operated percussion drilling tool comprising:

[0007] a percussion drill bit having a head portion, the percussion drill bit being formed with an axially extending shank, a plurality of first axially extending splines on the shank being slidably engageable with a plurality of first complementary splines formed in an interior of the drive chuck so that rotational drive from the chuck can be transmitted to the shank; and

[0008] engagement means on the chuck for connecting the chuck to drive means of the fluid operated percussion drilling tool;

[0009] wherein during drilling the drill bit and chuck are held in a first relative orientation in which the drill bit is retained in the chuck and the drill bit is axially movable relative to the chuck to a drill bit removal position in which the drill bit is rotatable relative to the chuck to a second relative orientation in which the drill bit can be removed from the chuck, the drill bit being movable to the drill bit removal position when the chuck is fully engaged with the drive means, that is when the chuck is in its operating position.

[0010] The drill bit removal position of the drill bit can be an axial position or range of axial positions in which the drill bit is rotatable relative to the chuck.

[0011] The advantage of this arrangement is that the drill bit can be removed from the drill assembly without the need to unscrew the chuck from the wear sleeve because the drill bit is axially movable to the drill bit removal position when the chuck is in its operating position, that is, without the need to disconnect the engagement means on the chuck from the percussion drilling tool. This improves the speed of changing the drill bit, thereby maximising drilling time. The removal or replacement of the drill bit also does not require specialist unthreading equipment. Problems associated with flushing the retaining sleeve are avoided because a separate retaining sleeve is not required to retain the drill bit and chuck in the first relative orientation.

[0012] The drill bit assembly can further include an alignment member on the drill bit engageable with a complementary alignment member on the chuck to prevent rotation of the drill bit relative to the chuck to maintain the drill bit and chuck in a first relative orientation in which the drill bit is retained in the chuck, the alignment member disengaging from the complementary engagement member in the drill bit removal position to allow the drill bit to rotate relative to the chuck to a second relative orientation.

[0013] The complementary alignment member can include a set of axially extending alignment splines formed inside the chuck at a forward end of the chuck, the alignment member can include one or more sets of axially extending alignment splines formed at a forward end of the drill bit shank, such that the alignment splines on the chuck are engageable with at least one of the sets of alignment splines on the drill bit shank to maintain the drill bit and chuck in the first relative orientation. In the drill bit removal position, the alignment splines on the chuck disengage from the one or more sets of alignment splines on the drill bit shank to allow the drill bit to rotate relative to the chuck to the second relative orientation.

[0014] A circumferential portion of the drill bit shank corresponding in axial length to at least one length of at least one of the sets of alignment splines on the chuck and located adjacent to the one or more sets of alignment splines can be un-splined, and in the drill bit removal position, the alignment splines on the chuck can register with the un-splined portion of the drill bit shank to allow the drill bit to rotate relative to the chuck. Thus, in the design of the drill bit assembly, the drill bit removal position is determined by the appropriate placement of the un-splined portion of the drill bit shank. An advantage of this arrangement is that the drill bit removal position can be selected so that the drill bit is less likely to be inadvertently moved into the drill bit removal position while drilling.

[0015] Suitably, a drill bit retention member on the drill bit engages a complementary drill bit retention member on the chuck when the drill bit and chuck are in the first relative orientation to retain the drill bit in the chuck, the drill bit retention member disengaging from the complementary drill bit retention member when the drill bit and chuck are in the second relative orientation to allow the drill bit to be removed from the chuck.

[0016] The drill bit assembly can include at least one retention spline at a rear end of the drill bit shank, wherein the retention spline or splines are arranged to engage a corresponding one of a first plurality of complementary splines formed inside the chuck when the drill bit and chuck are in the first relative orientation to retain the drill bit in the chuck. The at least one retention spline can thus constitute the drill bit retention member.

[0017] The at least one retention spline can be offset from the complementary splines formed in the chuck when the drill bit and chuck are in the second relative orientation to allow the drill bit to be removed from the chuck. At least one edge of the at least one retention spline can be radially offset from a corresponding edge of a corresponding one of the first plurality of splines on the drill bit shank.

[0018] The drill bit assembly can also include an aligner bushing disposed behind the drive chuck, such that when the drill bit is in the drill bit removal position, the aligner bushing is configured to engage with the at least one retaining spline to prevent rotation of the drill bit relative to the aligner bushing. The aligner bushing can include at least one spline formed on its interior that is configured to engage with the at least one retaining spline on the drill bit shank to prevent rotation of the drill bit relative to the aligner bushing when the drill bit is in the drill bit removal position. The aligner bushing can be sized such that, during normal operation of the hammer, frictional forces associated with the fit between the outer surface of the aligner bushing and the inner surface of the wear sleeve of the fluid-operated percussive drilling tool prevent rotation of the aligner bushing relative to the wear sleeve, thereby preventing rotation of the drill bit relative to the chuck. For example, an interference fit (e.g., a location interference fit) can be provided between the aligner bushing and the wear sleeve. However, when a rotational force is applied between the drill bit and the wear sleeve that is sufficient to overcome the frictional forces associated with the fit between the aligner bushing and the wear sleeve of the fluid-operated percussive drilling tool, the drill bit can rotate relative to the chuck to a second relative orientation.

[0019] The rotational force can be applied by the fluid-operated percussive drilling tool or by hand (using a suitable hand tool). For example, where the drill bit removal position requires it to be fully pushed into the hammer, the force can be applied by pushing the hammer into the ground, leaving the drill bit stationary and driving the hammer in the opposite direction to normal operation, to cause reverse rotation between the aligner bushing and the wear sleeve (and hence between the drill bit and the chuck). Where the drill bit removal position is intermediate between the drill bit being fully extended and the drill bit being fully pushed into the hammer, the rotational force can be applied by applying a slight reverse rotational force to the drill bit as it is pulled out of the hammer. A greater force can be applied to the drill bit as the alignment splines on the chuck disengage from the alignment splines on the drill bit shank, to cause the drill bit and aligner bushing to rotate relative to the wear sleeve and chuck, thereby causing the drill bit and chuck to move to a second relative orientation.

[0020] Thus, when the drill bit is moved axially to the drill bit removal position during normal drilling operation, the drill bit is prevented from rotating relative to the aligner bushing. Since the aligner bushing is a tight fit in the wear sleeve, it is rotationally fixed relative to the wear sleeve and chuck under normal operating conditions, meaning that the drill bit is also prevented from rotating relative to the chuck to a second relative orientation. By applying a rotational force sufficient to overcome the frictional forces between the aligner bushing and the wear sleeve, the drill bit (and aligner bushing) can only rotate relative to the wear sleeve (and hence the chuck). Thus, accidental relative rotation of the drill bit and chuck to a second relative orientation (and hence accidental release of the drill bit from the chuck) during drilling is avoided.

[0021] According to another aspect of the application, there is provided a hammer drill comprising: an outer cylindrical outer wear sleeve; a sliding piston mounted for reciprocating movement within the outer wear sleeve to impact a striking bit of a drill bit assembly located at a forward end of the outer wear sleeve, wherein the drill bit assembly is as described above. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 is an exploded view of a drill bit assembly according to a first embodiment of the application;

[0023] Figure 2 is Figure 1 a longitudinal sectional view of the drill bit assembly.

[0024] Figure 3 is Figure 1 a longitudinal sectional view of the drill bit assembly of

[0025] Figure 4 is Figure 1 a longitudinal sectional view of the drill bit assembly of

[0026] Figure 5 is Figure 3 a transverse sectional view of the drill bit assembly of

[0027] Figure 6 is Figure 4 a transverse sectional view of the drill bit assembly of

[0028] Figure 7 is Figure 4 a transverse sectional view of the drill bit assembly of

[0029] Figure 8 is an elevational view of a drill bit assembly according to a second embodiment of the application.

[0030] Figure 9 is an exploded view of a drill bit assembly according to a third embodiment of the application.

[0031] Figure 10 is Figure 9a longitudinal section view of the drill bit assembly of

[0032] Figure 11 is Figure 9 a longitudinal section view of the drill bit assembly of

[0033] Figure 12 is Figure 9 a longitudinal section view of the drill bit assembly of

[0034] Figure 13 is a longitudinal section view of a down-the-hole hammer including the drill bit assembly as shown in Figure 10 ; and

[0035] Figure 14 is Figure 13 a partial longitudinal section view of the down-the-hole hammer of DETAILED DESCRIPTION

[0036] A first embodiment of a drill bit assembly 1 for a fluid operated percussive drilling tool according to the present invention is shown in Figures 1 to 7 .

[0037] The assembly includes a percussive drill bit 2 having a head portion 3 formed with an axially extending shank 4. A plurality of first axially extending splines 5 spaced around the circumference of the drill bit shank 4 are slidably engaged with a plurality of first complementary splines 6 formed inside a drive chuck 7, so that rotational drive from the chuck can be transmitted to the shank. Threads 8 are provided on the chuck 7 for connecting the chuck 7 with drive components of the fluid operated percussive drilling tool.

[0038] A plurality of second splines 14, referred to as retaining splines, are provided at a rear end 15 of the drill bit shank. As shown most clearly in Figure 1 , each retaining spline 14 is wider than a corresponding one of the plurality of first splines 5, so that one edge of each spline 14 is offset from an edge of the corresponding spline 5. The spacing between the splines of the plurality of first internal splines 6 on the drive chuck is wide enough to accommodate one of the plurality of second splines 14 on the drill bit shank. As shown most clearly in Figure 1 and 2 , a plurality of third splines 13, referred to as alignment splines, are formed adjacent the drill bit head 3 around the outer circumference of an outwardly projecting shoulder 17 at a forward end 18 of the drill bit shank.

[0039] As shown most clearly in Figure 2 , a plurality of second splines 12, referred to as alignment splines, are formed inside the drive chuck at a forward end 16 of the drive chuck. As shown most clearly in Figure 3As shown, during drilling, the alignment spline 12 on the chuck is arranged to engage with the alignment spline 13 on the drill shank, thereby preventing the drill bit from rotating relative to the chuck. Therefore, the drill bit 2 and the chuck 7 remain in a position... Figure 5 The first relative orientation is shown, wherein each tail spline 14 is arranged to engage with the rear end 20 of a corresponding spline 6 formed inside the chuck 7, so as to retain the drill bit in the chuck. Thus, during drilling operations of a fluid-operated impact drill tool, the drill bit is held in the chuck.

[0040] For example Figure 1 and 2 As shown, the circumferential portion 19 of the drill bit shank is spline-free. The axial length of this circumferential portion 19 corresponds to the length of the aligned splines 12 on the chuck, has a small clearance, and is positioned adjacent to one or more sets of aligned splines 13 on the drill bit. When the drill bit moves axially relative to the chuck to... Figure 4 When the drill bit is removed from the position shown (in this embodiment, this requires the drill bit to be fully pushed into the hammer), the alignment splines 12 on the chuck align with the unsplined portion 19 of the drill shank so that they disengage from the alignment splines 13 on the drill shank. This allows the drill bit to rotate relative to the chuck to... Figure 6 The second relative orientation is shown. In this orientation, the splines 14 are kept offset from the complementary splines 6 formed inside the chuck so that they no longer engage with the rear end 20 of the complementary splines 6. The spacing between the plurality of second splines 14 and the internal splines 6 on the chuck is sized such that when the drill bit and the chuck are in the second relative orientation, the drill bit can slide out of the assembly without needing to loosen the chuck from the drilling tool. Since the drill bit is restricted only in its axial movement in the rearward direction by the interaction between the drill bit head 3 and the chuck 7, the drill bit can be fully pushed into the hammer when the chuck is fully engaged with the wear sleeve. It is not necessary to loosen the chuck from the wear sleeve to move the drill bit to the drill bit removal position.

[0041] like Figures 1 to 7 As shown, the assembly also includes an aligner bushing 21 disposed at the rear end 22 of the drive chuck. The aligner bushing includes a plurality of splines 23 formed internally therein, which engage with a retaining spline 14 on the drill shank to prevent rotation of the drill bit relative to the aligner bushing when the drill bit is in the drill bit removal position. In this embodiment, the aligner bushing is sized to provide a positional transition fit between the outer surface of the aligner bushing and the inner surface of the wear sleeve of the fluid-operated impact drill tool. This can be achieved by dimensionalizing the aligner bushing such that its diameter is the same as or slightly larger than the inner diameter of the wear sleeve. In other embodiments, a suitable fit can be achieved by, for example... Figure 8 This is achieved by the circumferential groove 24 in the outer surface of the aligner bushing, into which a hydraulic seal or O-ring can be fitted.

[0042] When the drill bit is in Figure 4 In the drill bit removal position shown, the drill bit can be rotated to a second relative orientation when a rotational force is applied (sufficient to overcome the friction associated with the fit between the aligner bushing 21 and the worn sleeve of the fluid-operated impact drill tool). For example, the hammer can be pressed into the ground to position the drill bit in the drill bit removal position and prevent rotation, while the hammer rotates counterclockwise (i.e., in the opposite direction to the normal operation of the hammer). In this way, the torque from the drill string can overcome the friction associated with the fit between the aligner bushing and the worn sleeve, thereby allowing the worn sleeve and chuck of the hammer to rotate relative to the drill bit and aligner to the second relative orientation. In this orientation, the drill bit can slide out of the hammer for replacement or maintenance without having to loosen the chuck from the worn sleeve. When the drill bit has a relatively small diameter, a pipe wrench can be used instead to rotate the drill bit and aligner bushing relative to the worn sleeve (and therefore the chuck). Because in this embodiment, the drill bit can only rotate relative to the chuck when it is fully pushed into the hammer, thereby disengaging the second spline, the possibility of the drill bit bouncing out of position during operation is very small.

[0043] Another embodiment of the present invention is in Figures 9 to 14 This is represented in the text. This embodiment is similar to... Figures 1 to 7 The embodiment shown is illustrated. However, in this embodiment, two sets of alignment splines 13 are located at the front end 18 of the drill shank. A spline-free portion 19 is located between the two sets of alignment splines. Furthermore, in this embodiment, the desired fit between the alignment bushing 21 and the wear sleeve 25 of the impact drill tool is achieved through a circumferential groove 24 in the outer surface of the alignment bushing, in which a hydraulic seal or O-ring 26 is fitted.

[0044] like Figure 10 As shown, when the drill bit is fully pushed into the hammer, the alignment splines 12 on the chuck engage with the first of two sets of alignment splines 13 on the drill shank. Figure 11 As shown, when the drill bit is fully extended, the alignment spline 12 on the chuck engages with the second of two sets of alignment splines 13 on the drill shank. Thus, in each of these axial positions of the drill bit, the drill bit 2 and the chuck 7 are held in a first relative orientation, wherein each tail spline 14 is arranged to engage with the rear end 20 of a corresponding spline 6 formed inside the chuck 7 in order to hold the drill bit in the chuck.

[0045] The drill bit removal position in this embodiment is as follows: Figure 12 and 14The chuck is then rotated to the fully extended position, as shown in FIG. 1 1, and the drill bit is removed from the assembly. The drill bit is then reinserted into the assembly and the chuck is rotated to the fully retracted position, as shown in FIG. 12. The drill bit is then removed from the assembly. The drill bit is then reinserted into the assembly and the chuck is rotated to the fully extended position, as shown in FIG. 1 1, and the drill bit is removed from the assembly. This process is repeated until the drill bit is removed from the assembly in the intermediate drill bit removal position, as shown in FIG. 13. As shown in these figures, when the drill bit is in this position, the alignment spline 12 on the chuck is aligned with the un-splined portion 19 on the shank of the drill bit, thereby disengaging them from both sets of alignment splines 13 on the shank of the drill bit. Thus, in this embodiment, the drill bit removal position is a position intermediate the fully extended position of the drill bit (as shown in FIG. 1 1 ) and the fully retracted position of the drill bit within the hammer (as shown in FIG. 12). In operation, to locate this intermediate drill bit removal position, a slight rotational pressure can be applied to the drill bit while it is being pulled from the chuck. Once a "click" is heard or felt, the splines will disengage and the drill bit will be in the drill bit removal position. Since the drill bit spends virtually no time in this position during operation, the likelihood of the drill bit rotating to this second relative orientation position during operation is very small. Figure 11 Figure 10 13 In operation, to locate this intermediate drill bit removal position, a slight rotational pressure can be applied to the drill bit while it is being pulled from the chuck. Once a "click" is heard or felt, the splines will disengage and the drill bit will be in the drill bit removal position. Since the drill bit spends virtually no time in this position during operation, the likelihood of the drill bit rotating to this second relative orientation position during operation is very small.

[0046] Once the drill bit is in the drill bit removal position, it can be rotated relative to the chuck to a second relative orientation by applying a rotational force sufficient to overcome the frictional forces associated with the mating between the aligner bushing 21 and the wear sleeve 25, in which the retaining splines 14 are offset from the complementary splines 6 formed in the interior of the chuck such that they are no longer engaged with the rear end 20 of the complementary splines 6, thus enabling the drill bit to be slipped out of the assembly without having to unscrew the chuck from the drilling tool. As in the previous embodiments and as shown in FIG. 14, the chuck does not need to be unscrewed from the wear sleeve in order to move the drill bit to the drill bit removal position. Figure 14

[0047] As used herein in reference to the present application, the words "comprise / comprising" and the words "have / having" are used in their open-ended, non-limiting sense to specify the presence of stated features, integers, steps, or components but do not preclude the presence or addition of one or more other features, integers, steps, components, or groups thereof.

[0048] It will be appreciated that, for clarity, purposes, certain features of the application that are, for brevity, described in solely in combination with other features; in other embodiments, various features of the application will also be provided alone or in any suitable sub-combination.​​​

Claims

1. A bit assembly for a fluid operated percussion drilling tool, the bit assembly comprising: a percussion bit having a head portion, the percussion bit formed with an axially extending bit shank, a plurality of first axially extending splines on the bit shank being slidably engageable with a plurality of first complementary splines formed in an interior of a drive chuck, whereby rotational drive from the drive chuck can be transmitted to the bit shank; and an engagement member on the drive chuck for connecting the drive chuck with a drive member of the fluid operated percussion drilling tool; wherein, during drilling, the percussion bit and the drive chuck are held in a first relative orientation in which the percussion bit is held in the drive chuck, wherein the percussion bit is axially movable relative to the drive chuck to a bit removal position in which the percussion bit is rotatable relative to the drive chuck to a second relative orientation in which the percussion bit is removable from the drive chuck, the percussion bit being movable to the bit removal position when the drive chuck is fully engaged with the drive member such that the percussion bit is removable from the bit assembly when the drive chuck is fully engaged with the drive member; the bit assembly further comprising an alignment member on the percussion bit engageable with a complementary alignment member on the drive chuck to prevent rotation of the percussion bit relative to the drive chuck such that the percussion bit and the drive chuck are held in the first relative orientation in which the percussion bit is held in the drive chuck, wherein, in the bit removal position, the alignment member is disengaged from the complementary alignment member such that the percussion bit is rotatable relative to the drive chuck to the second relative orientation.

2. The drill bit assembly of claim 1, wherein: the complementary alignment member comprising a set of axially extending alignment splines formed in the interior of the drive chuck at a forward end of the drive chuck, the alignment member comprising one or more sets of alignment splines formed at a forward end of the bit shank such that the alignment splines on the drive chuck are engageable with at least one of the sets of alignment splines on the bit shank to hold the percussion bit and the drive chuck in the first relative orientation, the alignment splines on the drive chuck being disengaged from the one or more sets of alignment splines on the bit shank in the bit removal position to allow the percussion bit to rotate relative to the drive chuck to the second relative orientation.

3. The drill bit assembly of claim 2, wherein: a circumferential portion of the bit shank, at least along an axial length corresponding to a length of the alignment splines on the drive chuck and located proximate to the one or more sets of alignment splines on the percussion bit, is splines-free, the alignment splines on the drive chuck being aligned with the splines-free portion of the bit shank in the bit removal position to allow the percussion bit to rotate relative to the drive chuck.

4. The drill bit assembly of any of claims 1-3, further comprising: at least one retaining spline at a rearward end of the bit shank, wherein, when the percussion bit and the drive chuck are in the first relative orientation, the or each retaining spline is arranged to engage a rearward end of a corresponding one of the complementary splines formed in the drive chuck to hold the percussion bit in the drive chuck.

5. The drill bit assembly of claim 4, wherein: the at least one retaining spline being offset from the complementary splines formed in the drive chuck when the percussion bit and the drive chuck are in the second relative orientation to allow the percussion bit to be removed from the drive chuck.

6. The drill bit assembly of claim 4, wherein: The at least one edge of the at least one retention spline is radially offset from a corresponding edge of a corresponding one of the plurality of first retention splines on the bit shank.

7. The drill bit assembly of claim 4, further comprising: An aligner bushing is arranged at the rear end of the drive chuck, whereby the aligner bushing is for engagement with the at least one retention spline when the impact bit is in the bit removal position so as to prevent rotation of the impact bit relative to the aligner bushing.

8. The drill bit assembly of claim 7, wherein: The aligner bushing comprises at least one spline, the spline of the aligner bushing being formed in the interior of the aligner bushing and being engageable with the at least one retention spline on the bit shank so as to prevent rotation of the impact bit relative to the aligner bushing when the impact bit is in the bit removal position.

9. The drill bit assembly of claim 7, wherein: The aligner bushing is sized such that, during normal operation of the hammer, frictional forces associated with the fit between the exterior surface of the aligner bushing and the interior surface of the wear sleeve of the fluid operated impact drill tool prevent rotation of the aligner bushing relative to the wear sleeve.

10. The drill bit assembly of claim 9, wherein: The impact bit is rotatable to a second relative orientation when the applied rotational force is sufficient to overcome the frictional forces associated with the fit between the aligner bushing and the wear sleeve of the fluid operated impact drill tool.

11. The drill bit assembly of claim 9, wherein: The fit between the aligner bushing and the wear sleeve is a transition fit.

12. The drill bit assembly of claim 11, wherein: The fit between the aligner bushing and the wear sleeve is a location transition fit.

13. A hammer drill, the hammer drill comprising: An outer cylindrical exterior wear sleeve; Sliding A piston is mounted for reciprocal movement within the exterior wear sleeve so as to strike the impact bit of the bit assembly located at the forward end of the exterior wear sleeve, wherein the bit assembly is as claimed in any one of claims 1-12.

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