WO2012038342A1

WO2012038342A1 - A rock drill bit and a drilling assembly for percussive rock drilling

Info

Publication number: WO2012038342A1
Application number: PCT/EP2011/066114
Authority: WO
Inventors: Mauri Esko; Markku Keskiniva; Juha Piispanen
Original assignee: Sandvik Intellectual Property AB
Current assignee: Sandvik Intellectual Property AB
Priority date: 2010-09-22
Filing date: 2011-09-16
Publication date: 2012-03-29
Anticipated expiration: 2013-03-22
Also published as: AU2011304461B2; RU2571783C2; BR112013006623A2; RU2013118269A; AU2011304382B2; BR112013006621A2; EP2619396A1; EP2434086B1; CN103261561A; CN103339339A; AU2011304382A1; WO2012038428A1; US20130180785A1; KR20140009149A; AU2011304461A1; AU2015238819A1; RU2013118265A; US20130180784A1; EP2434086A1; CA2811674A1

Abstract

A rock drill bit (30) for percussive drilling comprises a bit head configured to be attached at an end of a drill element (33) of a drilling assembly and having a diameter larger than that of said drill element. The bit head has at a front end (38) as seen in the intended drilling direction a plurality of buttons (41) distributed over said bit head and configured to engage material to be crushed. At least one of said buttons is allowed to rotate about its own symmetry axis.

Description

A ROCK DRILL BIT AND A DRILLING ASSEMBLY FOR PERCUSSIVE ROCK DRILLING

TECHNICAL FIELD OF THE INVENTION AND BACKGROUND ART

The present invention relates to a rock drill bit for percussive drilling comprising a bit head configured to be attached at an end of a drill element of a drilling assembly and having a diameter larger than that of said drill element, said bit head having at a front end as seen in the intended drilling direction a plurality of buttons distributed over said bit head and configured to engage material to be crushed, as well as a drilling assembly for percussive rock drilling according to the preamble of the independent claim therefor.

The invention is not restricted to any type of drilling assembly for use of a said rock drill bit, but the former may be a down-the- hole hammer drill just as well as a top hammer drill, although the rock drill bit shown is especially intended for the latter type.

Furthermore, said rock drill bit may have any conceivable size and has normally a diameter of 30 mm - 300 mm. The same ab- sence of limitations applies to the intended percussion frequency and rotational speed of the rock drill bit in operation, although it may be mentioned that these are typically within the ranges 20 Hz - 100 Hz and 20 - 500 revolutions per minute, respectively, but the invention does not exclude the use of the rock drill bit in high frequency assemblies operating at a frequency above 250 Hz and which may reach more than 1 kHz.

A known so-called standard rock drill bit 1 of the type defined in the introduction will now be described while referring to both Fig 1 and Fig 2. The drill bit has a bit head 2 configured to be attached at an end of a drill element, for example in the form of a drill tube or drill rod, of a drilli ng assembly and having a diameter larger than that of a said drill element. This drill element is not shown in these figures but may be intended to be received in a so-called ski rt 3 integ ral with a bit head and having a smaller diameter than the bit head. Other ways of connecting the dril l bit to the dril l element are conceivable and known withi n the art. The bit head has at a front end 4 as seen i n the intended dril ling direction a plu rality of gauge buttons 5 distributed along the circumference of the bit button head 2. The gauge buttons are configured to engage material to be crushed and to determine the diameter of a hole 6 (see Fig 1 ) to be drilled by the rock drill bit. These gauge buttons are made of hard material , such as cemented carbide or tu ngsten carbide. Front buttons 7 also of hard material are arranged on a front surface 8 for engaging material to be crushed. It is also indicated how a flush channel opens at the front by a flush ing hole 9 in the front surface.

In operation the gauge buttons 5 will engage and break rock close to the walls of a hole 6 in which the rock drill bit with said rod is located and the front buttons 7 will break rock closer to the centre of such a hole by impacts carried out by the rock drill bit in the direction of the arrow A. The drill bit wi ll rotate somewhat, typically about 5 °, between each such i mpact. The operation efficiency of a rock dril l bit of th is type is of course an important featu re and th is may be expressed as the penetration rate of the rock drill bit defined as the length of a hole drilled per time unit (meter/m inute) . The penetration rate of known rock dril l bits of this type is dependent upon the wear of said buttons, especially the gauge buttons. It is indicated in Fig 2 that during the operation of such a rock drill bit material is abraded at the periphery of the gauge buttons resulting in a flat surface 1 0 there, which makes them less sharp and reduces the penetration rate. These flat surfaces 1 0 wil l du ring the operation of the rock dril l bit grow and finally result in a diameter of a hole drilled determi ned by said gauge buttons being so m uch reduced that the rock drill bit has to be replaced. It is of cou rse an ongoing attempt to increase the penetration rate and prolong the life time of a rock drill bit of the type defined in the introduction . SUMMARY OF TH E INVENTION

The object of the present invention is to provide a rock drill bit of the type defined in the introduction being improved in at least some aspect with respect to such rock dril l bits al ready known .

This object is according to the invention obtained by providing such a rock dri ll bit in which at least one of said buttons is configured or allowed to rotate about its own symmetry axis. By rotatably fitting at least one said button in the bit head this button wil l whi le drilling be i nfluenced by the impacts thereof and rotation of the rock drill bit to rotate about its own symmetry axis, so that the parts of said button engaging rock will vary and the button will be evenly worn and by that self-sharpened. This means that this button will thanks to the self-sharpening effect maintain its contribution to the penetration rate of the rock drill bit longer than would it be fixed in the bit head.

According to an embodiment of the invention said pl urality of buttons includes gauge buttons distributed along the circumfer- ence of said bit head and configu red to determi ne the diameter of a hole to be dril led by the rock drill bit, and at least one of said gauge buttons is config ured or allowed to rotate about its own symmetry axis. This means that th is gauge button will thanks to said self-sharpening effect maintain its contribution to the penetration rate of the rock drill bit until th is gauge button has been worn so much that the diameter of a said hole drilled determined thereby has reached a m inimu m acceptable level . However, this will take longer than would said at least one gauge button be fixed with respect to the bit head, so that the lifeti me of the rock drill bit may also be prolonged. According to another embodi ment of the invention the rock dri ll bit comprises a first member havi ng a substantially ci rcumferential ri ng surface defining a substantial ly frusto-conical shape as seen in the intended drilling direction , and said at least one gauge button has a base portion resting on said ring su rface configured to transfer impact forces to the gauge button and to allow said base portion to move thereon when rotati ng . Transfer of impact forces to the gauge button by such a surface acting on said base portion of said at least one gauge button may in- crease the efficiency of the i mpacts and reduce stresses on said gauge button .

According to another embodi ment of the invention the rock dri ll bit comprises a second member defining said front end of said bit head and having said plu rality of gauge buttons extending out thereof, and said at least one gauge button allowed to rotate is arranged in a hole in said second member and allowed to move with respect to walls defini ng said hole when rotating . According to another embodi ment of the invention constituting a further development of the embodiment last mentioned said at least one gauge button allowed to rotate has a base portion with a larger cross-section than the rest of the gauge button , and at least a part of said hole in said second member has a cross- sectional area bei ng smaller than that of said base portion so as to maintai n the gauge button received in said second member. By arrangi ng such a base portion in form of a shoulder said at least one gauge button may reliably be maintai ned in said second member of the rock dri ll bit while bei ng allowed to rotate.

According to another embodi ment of the invention said hole is a through-hole, and said gauge button is arranged with the base portion thereof on a rear side of said hole as seen i n the intended dri lling direction . According to another embodi ment of the invention said fi rst member has means config ured to secure this member to a said drill element. This means that the rock dril l bit may be secured to a drill element throug h said first member providing said ring surface for transfer of i mpact forces to said at least one gauge button .

According to another embodi ment of the invention the rock dri ll bit has both a said first and second member and comprises means configured to secure said second member to said first member, which means that said ring surface will be located inside the bit head behind said second member. Said means configured to secu re said second member to said first member is according to another embodi ment of the invention config ured to releasably secure these members to each other, which means that a second member with gauge buttons may be removed and replaced when the diameter determined by the gauge buttons has reached the m inimu m limit whi le keepi ng said fi rst member, so that not the enti re rock dri ll bit has to be replaced resulting in saving of costs.

According to another embodi ment of the invention said fi rst member is a ring config ured to be supported on and/or secured to an end of said drill element. Such a so-called pusher ring is of particular interest and results in a simple construction of the rock drill bit if the rock drill bit also comprises means for secu ring said second member and by that said rock dri ll bit to a said dril l element. I n such a case the first member has neither to be secured to a drill element nor to said second member, but it wil l be kept in place by said second member.

Accordi ng to another embodi ment of the invention a majority of said gauge buttons are allowed to rotate about thei r own symmetry axis, and accordi ng to a fu rther embodiment of the inven- tion all said gauge buttons of the rock dril l bit are allowed to rotate about thei r own sym metry axis. The advantages of arrang- ing gauge buttons so as to be allowed to rotate will be the greater the more of the gauge buttons that are so arranged resulting in a said prolonged life time of the rock drill bit and a high penetration rate maintained over a longer time.

According to another embodiment of the invention said bit head has a front surface provided with a plurality of front buttons configured to engage material to be crushed. These front buttons may be fixed with respect to said front surface, but ac- cording to another embodiment of the invention at least one of said front buttons is allowed to rotate about its own symmetry axis, which will result in an even wear of said front button and by that a self-sharpening action thereof, so that it will carry out its task more efficiently and over a longer time than would it be fixed with respect to said front surface of the bit head.

According to another embodiment of the invention said at least one front button has a design similar to that of said at least one gauge button, and said second member is configured to receive said at least one front button similarly to said at least one gauge button.

According to another embodiment of the invention a majority of said front buttons or all said front buttons are allowed to rotate about their own symmetry axes.

According to another embodiment of the invention the rock drill bit comprises a flush channel extending through said bit head and having at least one flushing hole opening at said front end and passing the circumference of at least one said button allowed to rotate about its own symmetry axis. By letting the flushing medium, such as water, flowing over the circumference of such a button allowed to rotate surfaces and spaces of said bit head and said button allowing said rotation will be kept clean while eliminating any wear problems while the button is rotating inside a hole in which it is received in said bit head. According to another embodi ment of the invention said at least one button allowed to rotate is arranged in a hole in said bit head and allowed to move with respect to walls defi ni ng said hole when rotating , said at least one button is provided with a circumferential an nular groove, said walls defi ning said hole have a female member, and the rock drill bit comprises means configured to engage said groove and said female member so as to lock said button in said hole whi le allowing it to rotate. This is a preferable and simple way of locki ng said button with respect to said bit head while allowing it to freely rotate.

According to another embodi ment of the invention said female member is an annular groove i n said hole walls, and said engaging means comprises a ri ng of elastic material . This allows easy mounting of said button in said hole by pushing it thereinto while compressi ng said ri ng , which will when arriving to said annular groove in said hole walls expand and lock the button in place while allowing it to freely rotate. According to another embodi ment of the invention said engagi ng means comprises a pin config ured to be pushed i nto said groove and said female member for being fixed with respect to said bit head and lock said button in said hole while allowing it to rotate. Such a lock pi n wil l also by simple means provide a reliable locking of said button in said hole of the bit head wh ile allowing it to freely rotate.

The i nvention also relates to a drilling assembly for percussive rock drilli ng according to the appended independent claim for such an assembly. Such a drilling assembly makes it possible to carry out rock drilli ng with a high penetration rate over a longer time and while replacing the rock drill bit thereof more seldom with respect to such dril ling assemblies uti lizing known rock drill bits. The i nvention also relates to a use of a rock dril l bit according to the invention for percussive rock drilling into earth material , such as rock. Further advantages as well as advantageous features of the invention wi ll appear from the following description .

BRI EF DESCRI PTION OF TH E DRAWINGS With reference to the appended drawings, below fol lows a specific description of embodiments of the invention cited as examples.

In the drawings:

Fig 1 is a very simplified view of a rock drill bit according to prior art in operation ,

Fig 2 is a perspective view of a rock drill bit according to prior art after some time of operation ,

Fig 3 is a perspective view illustrati ng the pri nciple of a rock drill bit according to the present invention ,

Fig 4 shows a long itudi nal section th roug h a part of a rock drill bit according to a first embodiment of the invention in operation ,

Fig 5 is an exploded view of the rock dril l bit according to

Fig 4,

Fig 6 is a view correspondi ng to Fig 4 of a rock drill bit according to a second embodiment of the invention , Fig 7 is an exploded view of the rock dril l bit according to

Fig 6, is a simplified view correspondi ng to Fig 4 of a rock dril l bit according to a third embodiment of the invention , is a simplified view of a button allowed to rotate i n a bit head of a rock dril l bit according to a fourth embodiment of the i nvention , and is a very simplified view of a drilling assembly for percussive rock drill ing according to an embodiment of the present invention in operation .

DETAI LED DESCRI PTION OF EMBODIM ENTS OF TH E INVENTION

Fig 3 shows very schematical ly the principle of a rock drill bit according to the present invention , in which all gauge buttons 20 and all front buttons 21 are config ured or allowed to rotate about their own symmetry axis by bei ng received in holes 22 in a substantially circu mferential ring surface 23 defining a

substantially frusto-conical shape as seen in the intended drilli ng di rection and in a front surface 24, respectively. The buttons are preferably of tu ngsten carbide.

A rock dril l bit 30 according to a first embodiment of the present invention will now be described while maki ng reference to Figs. 4 and 5. The rock drill bit comprises a first member 31 having a substantially circu mferential ring surface 32 defining a substan- tially frusto-conical shape as seen in the i ntended drilling direction . This first member 31 is provided with means configured to secure this member to a dril l element 33, i n which th is securi ng means is formed by a sleeve-like portion 34 of the first member 31 provided with engagement means in the form of an internal thread 35 configured to engage engagement means i n the form of an external thread 36 on the drill element. The rock drill bit fu rther comprises a second member 37 defini ng a front end 38 of a bit head 39 of the rock dri ll bit. This second member is provided with a plurality of throug h holes 40 receiving the gauge buttons 41 and front buttons 42 while allowing these to rotate about their own sym metry axis. The throug h-holes 40 each have a diameter sl ightly exceedi ng (suitably by a diameter difference in the order of 30-80 m) the diameter of the respective button received therein for allowi ng the button to move with respect to wal ls 43 in the second member 37 defining said hole when rotating . However, this difference in diameter has been exaggerated in this fig ure and also in the embodiment shown in Fig 6 and described below for better i llustrating this featu re. The gauge buttons as well as the front buttons are pro- vided with a base portion 44 with larger cross-section than the rest of the button and also than the respective hole 40 so as to maintain the button received in the second member.

A gauge button 41 rests by the base portion 44 thereof on said ring surface 32 configu red to transfer impact forces to the gauge button and allow the base portion to move thereon when rotating . This means that impact forces are transferred to the gauge buttons from a surface 32 located inside the dri ll bit. The first member has also surfaces 45 directed in an intended drilling di- rection for supporti ng base portions of front buttons and

transferring impact forces thereto whi le allowing these base portions to move on these su rfaces 45 when rotating . Furthermore, the bit head 39 wil l through a shoulder 47 on the first member 31 provide a clearance C with respect to this member 31 , so that the button 41 may rotate freely without jamm ing . Particular measures are taken for flushing the surfaces and spaces surrounding the button , which will be explained more in detai l below. The rock drill bit comprises means 46 config ured to secure the second member 37 to the first member 31 . The securi ng means is preferably configured to releasably secure these members to each other, for instance by m utual ly securi ng them by engagement of th reads. This would then mean that it would be possible to remove said second member with buttons for replacement while keeping the first member after the buttons have been that much worn that they have to be replaced. Welding or press fitting are other possible alternatives of said securing means 46 easier to accomplish . When carrying out percussive dril ling with the rock drill bit shown in Fig 4 and 5 as illustrated in Fig 4 the buttons thereof will be allowed to rotate about their own axes, which means that the gauge buttons 41 wil l be worn evenly and maintain their sharpness, so that a high penetration rate may be maintained over a long period of time and the diameter of the hole defined by the gauge buttons wil l be reduced more slowly than would the gauge buttons be fixedly arranged in the bit head.

Figs. 6 and 7 i llustrate a rock dril l bit 50 according to a second embodiment of the invention . This rock dri ll bit has a fi rst member 51 in the form of a ring configured to be supported on and/or secured to an end 52 of a drill element 53 and havi ng a ring surface 54 form ing a support for a base portion 55 of each gauge button 56 in the same way as the correspondi ng surface 32 in the embodiment shown in Figs. 4 and 5. Thus, impact forces will through this surface be transferred to the gauge buttons while the base portions thereof are allowed to move thereon when rotating . A second member 57 of the rock dri ll bit has through holes 58 receiving said gauge buttons and allowing them to move with respect to walls of these holes when rotati ng . The front buttons 59 are, as an example, in this embodiment fixedly secured to a front end 60 of the second member 57. The second member 57 is in this embodiment provided with means for secu ring this member to a drill element 53 by having a sleeve-l ike portion 61 designed to receive a dril l element and having engagement means in the form of an internal th read 62 for engagi ng with engagement means i n the form of an external thread 63 on the drill element for releasably securing said second member to the drill element and by that also keeping said ring 51 , a so-called pusher ring , in place. The first member 51 is provided with a col lar 64, so that the first 51 and second 57 members are fixed with respect to each other while leaving a clearance 66 therebetween for the button to freely rotate. Proper flushi ng of a button allowed to rotate is also important. It is indicated in Fig 6 that the rock drill bit has a conventional flush channel 67 extendi ng th roug h the bit head. The flush channel has also at least one fl ushing hole 68 (see the arrows F

indicating the flow of flushing medium) opening at the first end 60 and passing the clearance 66 and the circumference of the button 56 allowed to rotate. This will keep said clearance 66 clear and elimi nates wear problems whi le the button rotates inside the hole 58. The function of this embodiment of the invention in operation appears clearly from the above discussion of inter al ia the first embodiment of the present invention .

A part of a rock dri ll bit according to a third embodi ment of the invention is very schematical ly shown i n Fig 8. This rock drill bit is provided with alternative means to lock a button 80 to a dri ll bit head 81 wh ile allowing the button to rotate. A bli nd hole 82 in the bit head desig ned to receive the button 80 is provided with an an nular groove 83, and the button 80 is provided with a cor- responding ann ular groove 84 receivi ng an elastic lock ring 85, for example a ring , such as a C-ri ng , made from steel . When the button 80 is pushed into the hole 82 the lock ring will first be compressed until reachi ng the groove 83 in the bit head. It will then expand outwards into that g roove and lock the button to the bit head 81 wh ile allowing the button to rotate. Fig 9 illustrates an alternative way of locki ng a button 90 to a bit head not shown in a rock dril l bit according to a fou rth embodiment of the invention wh ile allowi ng the button to rotate. This is achieved by providing the button 90 with an an nular groove 91 as in the embodiment shown in Fig 8. However, a lock pin 92 is used instead of a lock ri ng , and this lock pin is after pushing the button 90 into a corresponding hole i n the bit head pushed i nto the g roove 91 while locking the button in place and still allowing it to rotate about its own symmetry axis.

Fig 1 0 illustrates very schematical ly a drilling assembly for percussive rock drilling according to the present invention having a rock drill bit 70 accordi ng to an embodiment of the invention provided with gauge buttons 71 . This drilli ng assembly is a so- called top ham mer drill acting upon the rock drill bit from a location above the ground and has power means 72, such as diesel engine and hydrau lic pu mp, configured to drive the rock dril l 76, which in turn makes said dril l element 73 and the rock drill bit to rotate and carry out percussions and by that crush the rock. A desig n of the drilli ng assembly as a down-the-hole hammer equipment is also within the scope of the present invention .

The drilling assembly has also means 74, such as a compressed air generator, configured to flush cuttings resulted from en- gagement of the gauge buttons and the front buttons of the drill bit away from the region occupied by the drill bit. The drilling assembly has a control arrangement 75 configured to control the operation of the power means 72 so as to adapt the frequency of impacts and the rotational speed of the drill bit. It has turned out that drill bits according to the present invention with buttons allowed to rotate about their own symmetry axis are particu larly well suited to be used i n dril ling assemblies controlled to have frequencies above 250 Hz, preferably above 350 Hz and most preferred in the range of 350 Hz - 1 000 Hz. Drilli ng with a drilli ng assembly according to Fig 8 with a rock drill bit according to the present i nvention will be more efficient than with rock drill bits already known , since the penetration rate may be kept at a h igh level longer and the stops needed for re- placi ng the rock drill bit or parts thereof will be less frequently occurring .

It is preferable that the base portion of each rotatable button rests agai nst or contacts the bottom of the hole to transfer impact forces to the button and while allowing the base portion to move thereon when rotating .

The i nvention is of course not in any way restricted to the embodiments described above, but many possibilities to modifica- tions thereof would be apparent to a person with skill i n the art without departi ng from the scope of the invention as defined in the appended clai ms.

The n umber and positions of the buttons of the rock drill bit may differ a lot with respect to the embodi ments shown in the figures.

"Substantially" used in the expressions "substantial ly a frusto- conical shape" and "substantially circumferential ring" also cover the case when cutting recesses or grooves and/or gauge buttons intersect the ri ng , as shown i n the fig ures.

The disclosures in EP Patent Appl ication No. 1 01 78387.6, from which this appl ication claims priority, are i ncorporated herei n by reference.

Claims

1. A rock drill bit (30, 50) for percussive drilling comprising a bit head (39) configured to be attached at an end of a drill ele- ment (33, 53) of a drilling assembly and having a diameter larger than that of said drill element, said bit head having at a front end (38, 60) as seen in the intended drilling direction a plurality of buttons (41, 42, 56, 59, 80, 90) distributed over said bit head and configured to engage material to be crushed, characterized in that at least one of said buttons (41 , 42, 56, 80, 90) is allowed to rotate about its own symmetry axis.

2. A rock drill bit according to claim 1 , characterized in that said plurality of buttons includes gauge buttons (41, 56, 80,

90) distributed along the circumference of said bit head (39, 81) and configured to determine the diameter of a hole to be drilled by the rock drill bit, and that at least one of said gauge buttons (41, 56, 80, 90) is allowed to rotate about its own symmetry axis.

3. A rock drill bit according to claim 2, characterized in that it comprises a first member (31, 51) having a substantially circumferential ring surface (32, 54) defining a substantially frusto-conical shape as seen in the intended drilling direction, and that said at least one gauge button (41, 56) has a base portion (44, 55) resting on said ring surface configured to transfer impact forces to the gauge button and to allow said base portion to move thereon when rotating.

4. A rock drill bit according to claim 2 or 3, characterized in that it comprises a second member (37, 57) defining said front end (38, 60) of said bit head and having said plurality of gauge buttons (41, 56) extending out thereof, and that said at least one gauge button allowed to rotate is arranged in a hole (40, 58) in said second member and allowed to move with respect to walls defin ing said hole when rotating .

5. A rock drill bit according to claim 4, characterized in that said at least one gauge button (41 , 56) allowed to rotate has a base portion (44, 55) with a larger cross-section than the rest of the gauge button , and that at least a part of said hole (40 , 58) in said second member (37, 57) has a cross- sectional area being smaller than that of said base portion so as to maintai n the gauge button received in said second member.

6. A rock drill bit according to claim 5, characterized in that said hole (40, 58) is a through hole, and that said gauge button (41 , 56) is arranged with the base portion (44, 55) thereof on a rear side of said hole as seen in the intended dril ling direction .

A rock drill bit according to claim 3 or any of clai ms 4-6 dependent upon claim 3 , characterized in that said first member (31 ) has means (34, 35) config ured to secure th is member to a said dril l element (33) .

8. A rock drill bit according to claim 4 and 7, characterized i n that it comprises means (46) configured to secu re said second member (37) to said first member (31 ) .

9. A rock drill bit according to claim 8, characterized in that said means (46) config ured to secure said second member (37) to said first member (31 ) is configured to releasably secure these members to each other.

1 0. A rock drill bit according to claim 3 or any of clai ms 4-6 dependent upon claim 3 , characterized in that said first mem- ber is a ring (51 ) configured to be supported on and/or secured to an end (52) of a said drill element (53) .

1 1 . A rock drill bit according to any of claims 4-6 and clai m 1 0 , characterized in that it comprises means (61 , 62) for secu ring said second member (57) and by that said rock drill bit to a said drill element (53) .

1 2. A rock drill bit according to any of claims 2- 1 1 , characterized in that a majority of said gauge buttons (41 , 56, 80, 90) are al lowed to rotate about their own sym metry axes.

1 3. A rock drill bit according to any of the claims 2- 1 2, characterized in that all said gauge buttons (41 , 56, 80, 90) of the rock dril l bit are allowed to rotate about thei r own symmetry axes.

1 4. A rock drill bit according to any of the preceding claims,

characterized in that said bit head has a front su rface provided with a plurality of front buttons (42, 59) configured to engage material to be crushed.

1 5. A rock drill bit according to claim 1 4, characterized in that at least one (42) of said front buttons is allowed to rotate about its own symmetry axis.

1 6. A rock drill bit according to any of claims 4-6 and clai m 1 5, characterized in that said at least one front button (42) has a desig n sim ilar to that of said at least one gauge button (41 ) , and that said second member (37) is configured to receive said at least one front button similarly to said at least one gauge button .

1 7. A rock drill bit according to claim 1 5 or 1 6, characterized in that a majority of said front buttons (42) or all said front buttons are allowed to rotate about their own symmetry axis.

1 8. A rock drill bit according to any of the preceding claims,

characterized in that it comprises a flush channel (67) extending through said bit head (39) and having at least one flushing hole (68) opening at said front end and passi ng the circumference of at least one said button (56) allowed to rotate about its own symmetry axis.

1 9. A rock drill bit according to any of claims 1 , 2, 1 2- 1 5, 1 7 and 1 8, characterized in that said at least one button allowed to rotate is arranged in a hole (82) in said bit head (81 ) and allowed to move with respect to walls defin ing said hole when rotating , that said at least one button is provided with a circumferential annular groove (84, 91 ) , that said walls defini ng said hole have a female member (83) and that the rock dril l bit comprises means (85, 92) configu red to engage said groove (84) and said female member (83) so as to lock said button i n said hole wh ile allowi ng it to rotate.

20. A rock drill bit according to claim 1 9, characterized in that said female member is an annular groove (84) in said hole wal ls, and that said engag ing means comprises a ri ng (85) of elastic material .

21 . A rock drill bit according to claim 1 9, characterized in that said engaging means comprise a pin (92) configured to be pushed into said groove (91 ) and said female member for bei ng fixed with respect to said bit head and lock said button (90) in said hole while allowing it to rotate.

22. A drilling assembly for percussive rock drilling comprising a rock dri ll bit (70) , power means (72) configured to act upon said rock dri ll bit so as to make the latter rotate and carry out impacts for engag ing material to be crushed and a control arrangement (75) configured to control the operation of said power means, characterized in that said rock drill bit is a rock dri ll bit according to any of claims 1 -21 .

23. A drilling assembly according to claim 22, characterized in that the control arrangement (75) is configured to control said power means (72) to obtain a frequency of said impacts exceeding 250 Hz, exceeding 350 Hz or being in the range of 350 Hz - 1000 Hz.

24. Use of a rock drill bit (30, 50) according to any of claims 1-21 for percussive drilling into earth, such as rock.