13096WO_1 2011-08-31 1 A ROCK DRILL BIT AND A DRILLING ASSEMBLY FOR PER CUSSIVE ROCK DRILLING 5 TECHNICAL FIELD AND BACKGROUND ART The present disclosure 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 di 10 ameter larger than that of said drill element, said bit head hav ing at a front end as seen in the intended drilling direction a plu rality 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 inde 15 pendent 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, 20 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 25 sence of limitations applies to the intended percussion fre quency 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 30 rock drill bit in high frequency assemblies operating at a fre quency 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 35 1 and Fig 2. The drill bit has a bit head 2 configured to be at tached at an end of a drill element, for example in the form of a 6771586 1 (GHMatters) P93026.AU WILSONC 13096WO_1 2011-08-31 2 drill tube or drill rod, of a drilling assembly and having a diame ter 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 skirt 3 integral with a bit head and having a 5 smaller diameter than the bit head. Other ways of connecting the drill bit to the drill element are conceivable and known within the art. The bit head has at a front end 4 as seen in the intended drilling direction a plurality of gauge buttons 5 distributed along the circumference of the bit button head 2. The 10 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 tungsten carbide. Front buttons 7 also of hard material are arranged on a front 15 surface 8 for engaging material to be crushed. It is also indicated how a flush channel opens at the front by a flushing hole 9 in the front surface. In operation the gauge buttons 5 will engage and break rock 20 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 will rotate some what, typically about 50, between each such impact. 25 The operation efficiency of a rock drill bit of this type is of course an important feature and this may be expressed as the penetration rate of the rock drill bit defined as the length of a hole drilled per time unit (meter/minute). The penetration rate of 30 known rock drill 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 10 there, which makes them less sharp and reduces the 35 penetration rate. These flat surfaces 10 will during the operation of the rock drill bit grow and finally result in a diameter of a hole 6771586 1 (GHMatters) P93026.AU WILSONC 13096WO_1 2011-08-31 3 drilled determined by said gauge buttons being so much reduced that the rock drill bit has to be replaced. It is of course an on-going attempt to increase the penetration rate and prolong the life time of a rock drill bit of the type defined in the 5 introduction. SUMMARY OF THE DISCLOSURE 10 The disclosure provides a rock drill 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 will while drilling be influenced by the impacts thereof and rotation of the rock drill bit to rotate about 15 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 penetra tion rate of the rock drill bit longer than would it be fixed in the 20 bit head. According to an embodiment of the invention said plurality of buttons includes gauge buttons distributed along the circumfer ence of said bit head and configured to determine the diameter 25 of a hole to be drilled by the rock drill bit, and at least one of said gauge buttons is configured or allowed to rotate about its own symmetry axis. This means that this gauge button will thanks to said self-sharpening effect maintain its contribution to the penetration rate of the rock drill bit until this gauge button 30 has been worn so much that the diameter of a said hole drilled determined thereby has reached a minimum 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 lifetime of the rock drill bit may also be prolonged. 35 6771586 1 (GHMatters) P93026.AU WILSONC 13096WO_1 2011-08-31 4 According to another embodiment of the invention the rock drill bit comprises a first member having a substantially circumferen tial ring surface defining a substantially frusto-conical shape as seen in the intended drilling direction, and said at least one 5 gauge button has a base portion 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. 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 10 crease the efficiency of the impacts and reduce stresses on said gauge button. According to another embodiment of the invention the rock drill bit comprises a second member defining said front end of said 15 bit head and having said plurality 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 defining said hole when rotating. 20 According to another embodiment 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 25 sectional area being smaller than that of said base portion so as to maintain the gauge button received in said second member. By arranging such a base portion in form of a shoulder said at least one gauge button may reliably be maintained in said second member of the rock drill bit while being allowed to 30 rotate. According to another embodiment 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 in the in 35 tended drilling direction. 6771586 1 (GHMatters) P93026.AU WILSONC 13096WO_1 2011-08-31 5 According to another embodiment of the invention said first member has means configured to secure this member to a said drill element. This means that the rock drill bit may be secured to a drill element through said first member providing said ring 5 surface for transfer of impact forces to said at least one gauge button. According to another embodiment of the invention the rock drill bit has both a said first and second member and comprises 10 means configured to secure said second member to said first member, which means that said ring surface will be located in side the bit head behind said second member. Said means con figured to secure said second member to said first member is according to another embodiment of the invention configured to 15 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 minimum limit while keeping said first member, so that not the entire rock drill bit has to be replaced resulting in 20 saving of costs. According to another embodiment of the invention said first member is a ring configured to be supported on and/or secured to an end of said drill element. Such a so-called pusher ring is 25 of particular interest and results in a simple construction of the rock drill bit if the rock drill bit also comprises means for secur ing said second member and by that said rock drill bit to a said drill element. In such a case the first member has neither to be secured to a drill element nor to said second member, but it will 30 be kept in place by said second member. According to another embodiment of the invention a majority of said gauge buttons are allowed to rotate about their own sym metry axis, and according to a further embodiment of the inven 35 tion all said gauge buttons of the rock drill bit are allowed to ro tate about their own symmetry axis. The advantages of arrang 6771586 1 (GHMatters) P93026.AU WILSONC 13096WO_1 2011-08-31 6 ing gauge buttons so as to be allowed to rotate will be the greater the more of the gauge buttons that are so arranged re sulting in a said prolonged life time of the rock drill bit and a high penetration rate maintained over a longer time. 5 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 but tons may be fixed with respect to said front surface, but ac 10 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 15 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 20 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 25 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 30 and passing the circumference of at least one said button al lowed 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 35 while eliminating any wear problems while the button is rotating inside a hole in which it is received in said bit head. 6771586 1 (GHMatters) P93026.AU WILSONC 13096WO_1 2011-08-31 7 According to another embodiment 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 defining said hole when rotating, said at least one button is provided with a 5 circumferential annular groove, said walls defining 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 while allowing it to rotate. This is a preferable and simple way of locking said button with 10 respect to said bit head while allowing it to freely rotate. According to another embodiment of the invention said female member is an annular groove in said hole walls, and said engaging means comprises a ring of elastic material. This 15 allows easy mounting of said button in said hole by pushing it thereinto while compressing said ring, 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. 20 According to another embodiment of the invention said engaging means comprises a pin configured to be pushed into 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 pin will also by simple means provide a 25 reliable locking of said button in said hole of the bit head while allowing it to freely rotate. The invention also relates to a drilling assembly for percussive rock drilling according to the appended independent claim for 30 such an assembly. Such a drilling assembly makes it possible to carry out rock drilling with a high penetration rate over a longer time and while replacing the rock drill bit thereof more seldom with respect to such drilling assemblies utilizing known rock drill bits. 35 6771586 1 (GHMatters) P93026.AU WILSONC 13096WO_1 2011-08-31 8 The invention also relates to a use of a rock drill bit according to the invention for percussive rock drilling into earth material, such as rock. 5 Further advantages as well as advantageous features of the in vention will appear from the following description. BRIEF DESCRIPTION OF THE DRAWINGS 10 With reference to the appended drawings, below follows a spe cific description of embodiments of the invention cited as exam ples. In the drawings: 15 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 20 prior art after some time of operation, Fig 3 is a perspective view illustrating the principle of a rock drill bit according to the present invention, 25 Fig 4 shows a longitudinal section through a part of a rock drill bit according to a first embodiment of the inven tion in operation, Fig 5 is an exploded view of the rock drill bit according to 30 Fig 4, Fig 6 is a view corresponding to Fig 4 of a rock drill bit ac cording to a second embodiment of the invention, 35 Fig 7 is an exploded view of the rock drill bit according to Fig 6, 6771586 1 (GHMatters) P93026.AU WILSONC 13096WO_1 2011-08-31 9 Fig 8 is a simplified view corresponding to Fig 4 of a rock drill bit according to a third embodiment of the inven tion, 5 Fig 9 is a simplified view of a button allowed to rotate in a bit head of a rock drill bit according to a fourth em bodiment of the invention, and 10 Fig 10 is a very simplified view of a drilling assembly for per cussive rock drilling according to an embodiment of the present invention in operation. DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVEN 15 TION Fig 3 shows very schematically the principle of a rock drill bit according to the present invention, in which all gauge buttons 20 and all front buttons 21 are configured or allowed to rotate 20 about their own symmetry axis by being received in holes 22 in a substantially circumferential ring surface 23 defining a substantially frusto-conical shape as seen in the intended drilling direction and in a front surface 24, respectively. The buttons are preferably of tungsten carbide. 25 A rock drill bit 30 according to a first embodiment of the present invention will now be described while making reference to Figs. 4 and 5. The rock drill bit comprises a first member 31 having a substantially circumferential ring surface 32 defining a substan 30 tially frusto-conical shape as seen in the intended drilling direc tion. This first member 31 is provided with means configured to secure this member to a drill element 33, in which this securing means is formed by a sleeve-like portion 34 of the first member 31 provided with engagement means in the form of an internal 35 thread 35 configured to engage engagement means in the form of an external thread 36 on the drill element. 6771586 1 (GHMatters) P93026.AU WILSONC 13096WO_1 2011-08-31 10 The rock drill bit further comprises a second member 37 defining a front end 38 of a bit head 39 of the rock drill bit. This second member is provided with a plurality of through holes 40 5 receiving the gauge buttons 41 and front buttons 42 while allowing these to rotate about their own symmetry axis. The through-holes 40 each have a diameter slightly exceeding (suitably by a diameter difference in the order of 30-80 pm) the diameter of the respective button received therein for allowing 10 the button to move with respect to walls 43 in the second member 37 defining said hole when rotating. However, this difference in diameter has been exaggerated in this figure and also in the embodiment shown in Fig 6 and described below for better illustrating this feature. The gauge buttons as well as the 15 front buttons are provided 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. 20 A gauge button 41 rests by the base portion 44 thereof on said ring surface 32 configured to transfer impact forces to the gauge button and allow the base portion to move thereon when rotat ing. This means that impact forces are transferred to the gauge buttons from a surface 32 located inside the drill bit. The first 25 member has also surfaces 45 directed in an intended drilling di rection for supporting base portions of front buttons and transferring impact forces thereto while allowing these base portions to move on these surfaces 45 when rotating. Further more, the bit head 39 will through a shoulder 47 on the first 30 member 31 provide a clearance C with respect to this member 31, so that the button 41 may rotate freely without jamming. Particular measures are taken for flushing the surfaces and spaces surrounding the button, which will be explained more in detail below. 35 6771586 1 (GHMatters) P93026.AU WILSONC 13096WO_1 2011-08-31 11 The rock drill bit comprises means 46 configured to secure the second member 37 to the first member 31. The securing means is preferably configured to releasably secure these members to each other, for instance by mutually securing them by engage 5 ment of threads. 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 10 easier to accomplish. When carrying out percussive drilling 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 15 the gauge buttons 41 will 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 will be reduced more slowly than would the gauge buttons be fixedly arranged in the bit head. 20 Figs. 6 and 7 illustrate a rock drill bit 50 according to a second embodiment of the invention. This rock drill bit has a first mem ber 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 having a ring 25 surface 54 forming a support for a base portion 55 of each gauge button 56 in the same way as the corresponding surface 32 in the embodiment shown in Figs. 4 and 5. Thus, impact forces will through this surface be transferred to the gauge but tons while the base portions thereof are allowed to move 30 thereon when rotating. A second member 57 of the rock drill bit has through holes 58 receiving said gauge buttons and allowing them to move with respect to walls of these holes when rotating. The front buttons 35 59 are, as an example, in this embodiment fixedly secured to a front end 60 of the second member 57. 6771586 1 (GHMatters) P93026.AU WILSONC 13096WO_1 2011-08-31 12 The second member 57 is in this embodiment provided with means for securing this member to a drill element 53 by having a sleeve-like portion 61 designed to receive a drill element and 5 having engagement means in the form of an internal thread 62 for engaging with engagement means in the form of an external thread 63 on the drill element for releasably securing said sec ond 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 10 provided with a collar 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 flushing of a button allowed to rotate is also important. It is indicated in Fig 6 that the rock drill bit has a conventional 15 flush channel 67 extending through the bit head. The flush channel has also at least one flushing 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 20 clear and eliminates wear problems while 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 alia the first embodiment of the present invention. 25 A part of a rock drill bit according to a third embodiment of the invention is very schematically shown in Fig 8. This rock drill bit is provided with alternative means to lock a button 80 to a drill bit head 81 while allowing the button to rotate. A blind hole 82 in the bit head designed to receive the button 80 is provided 30 with an annular groove 83, and the button 80 is provided with a corresponding annular groove 84 receiving an elastic lock ring 85, for example a ring, such as a C-ring, made from steel. When the button 80 is pushed into the hole 82 the lock ring will first be compressed until reaching the groove 83 in the bit head. It will 35 then expand outwards into that groove and lock the button to the bit head 81 while allowing the button to rotate. 6771586 1 (GHMatters) P93026.AU WILSONC 13096WO_1 2011-08-31 13 Fig 9 illustrates an alternative way of locking a button 90 to a bit head not shown in a rock drill bit according to a fourth embodi ment of the invention while allowing the button to rotate. This is 5 achieved by providing the button 90 with an annular groove 91 as in the embodiment shown in Fig 8. However, a lock pin 92 is used instead of a lock ring, and this lock pin is after pushing the button 90 into a corresponding hole in the bit head pushed into the groove 91 while locking the button in place and still allowing 10 it to rotate about its own symmetry axis. Fig 10 illustrates very schematically a drilling assembly for per cussive rock drilling according to the present invention having a rock drill bit 70 according to an embodiment of the invention 15 provided with gauge buttons 71. This drilling assembly is a so called top hammer drill acting upon the rock drill bit from a loca tion above the ground and has power means 72, such as diesel engine and hydraulic pump, configured to drive the rock drill 76, which in turn makes said drill element 73 and the rock drill bit to 20 rotate and carry out percussions and by that crush the rock. A design of the drilling 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 25 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 30 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 particularly well suited to be used in drilling assemblies controlled to have frequencies above 250 Hz, preferably above 35 350 Hz and most preferred in the range of 350 Hz - 1000 Hz. 6771586 1 (GHMatters) P93026.AU WILSONC 13096WO_1 2011-08-31 14 Drilling with a drilling assembly according to Fig 8 with a rock drill bit according to the present invention will be more efficient than with rock drill bits already known, since the penetration rate may be kept at a high level longer and the stops needed for 5 replacing the rock drill bit or parts thereof will be less frequently occurring. It is preferable that the base portion of each rotatable button rests against or contacts the bottom of the hole to transfer 10 impact forces to the button and while allowing the base portion to move thereon when rotating. The invention is of course not in any way restricted to the em bodiments described above, but many possibilities to modifica 15 tions thereof would be apparent to a person with skill in the art without departing from the scope of the invention as defined in the appended claims. The number and positions of the buttons of the rock drill bit may 20 differ a lot with respect to the embodiments shown in the fig ures. "Substantially" used in the expressions "substantially a frusto conical shape" and "substantially circumferential ring" also 25 cover the case when cutting recesses or grooves and/or gauge buttons intersect the ring, as shown in the figures. The disclosures in EP Patent Application No. 10178387.6, from which this application claims priority, are incorporated herein by 30 reference. 6771586 1 (GHMatters) P93026.AU WILSONC