Abstract:
A button bit is disclosed of the type that is rotatable in a predetermined direction of rotation (R) around a centre axis (C) and includes a number of spaced-apart peripheral buttons, which protrude forward from a front end surface to which an envelope surface connects having a rotationally symmetrical shape, the individual peripheral button being tilted partly in relation to the centre axis (C) of the drill bit, partly in the forward direction of rotation (R) in relation to the surface from which the button protrudes. The hole for the individual peripheral button mouths in a countersunk entering surface which is planar, ring-shaped and extends perpendicularly to the centre axis (C 1 ) of the hole. By tipping the button in the forward direction of rotation, the same will operate aggressively and efficiently. Also, the peripheral button may be mounted in accurate drilled apertures which are simple to provide, despite the complex space geometry predicament.

Description:
RELATED APPLICATION DATA 
       [0001]    This application claims priority under 35 U.S.C. §119 and/or §365 to Swedish patent application No. 0602559-7, filed Nov. 29, 2006, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The present disclosure relates to a button bit intended for percussive rock drilling and of the type that is rotatable in a predetermined direction of rotation around a centre axis, and comprises a number of spaced-apart peripheral buttons, which protrude forward from a front end surface to which an envelope surface connects having a rotationally symmetrical basic shape, the individual peripheral button is tilted partly in relation to the centre axis C, partly in the forward direction of the rotational direction R in relation to the surface from which the button protrudes. 
       BACKGROUND 
       [0003]    In the discussion of the background that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art. 
         [0004]    Button bits can be realized in most varying embodiments depending on the specifics of the field of application. Thus, the different embodiments differ not only as a consequence of varying diameters of the holes to be drilled, but also depending on the nature of the rock or the soil type to be machined, e.g., in respect of the hardness thereof. However, common to previously known button bits is that they comprise a generally rotationally symmetrical basic body that is manufactured from steel or the like and equipped with a number of buttons of a material, e.g., cemented carbide, that has greater hardness and wear resistance than the material of the basic body. Said buttons are placed in different configurations on a front end surface, which via a circular borderline transforms into an envelope surface having a rotationally symmetrical basic shape, which partly tapers in the backward direction. More precisely, the front end surface is included in a head having a greater diameter than a shank behind, which may be in the form of a sleeve or skirt into which a drill rod can be inserted and interconnected with the drill bit. The head is strongly dimensioned in order to withstand above all considerable impulsive forces. The individual button is usually formed with a cylindrical base part and a tapering top or tip, which effects the proper machining of the rock. The button is permanently united to the drill bit by a shrink-fit joint, which is provided by inserting the base part of the button into a drilled, cylindrical hole in a heated drill bit, which is allowed to cool down in order to clamp the button in the hole. The button may also be applied by pressing into a cold steel body. The buttons are grouped centrally as well as peripherally along the front end surface of the drill bit. More precisely, a certain number of centre buttons are placed in different locations along a circular, central front surface, while a usually greater number of peripheral buttons are placed tangentially spaced apart along a peripheral, ring-shaped surface between the central end surface and the envelope surface. This peripheral ring surface is conical, having cone angles within the range of 90-140°, which means that the centre axes of the different peripheral buttons commonly meet the centre axis of the drill bit at equal angles of 20-45°. 
         [0005]    Examples of known button bits of the kind in question are disclosed in SE 9001081-0, SE 8305048-4, SE 607972, FR 1514998 and FR 2646875. 
         [0006]    Previously known rock-drilling equipment for top-hammer drilling allows machining of the rock at a relatively moderate rotational speed and moderate percussion frequency. Thus, the rotational speed has previously been limited to about 200 r/min (slightly more than 3 revolutions per second), and the percussion frequency to about 100 Hz (i.e., 100 percussions per second). Under these conditions, the individual button performs repeated forwardly directed impact motions, which results in rock being crushed, at the same time as the same slowly rotates around the centre axis of the drill bit, the entire drill bit fairly slowly being fed axially into the rock. Therefore, because the speed of rotation is moderate in comparison with the percussion frequency, the rock is in all essentials machined by crushing. 
         [0007]    However, rock-drilling equipment has recently been developed that allows rotation of the drill bit at a considerably higher rotational speed and also at a higher percussion frequency. During drilling by means of such equipments, it has turned out that the rock is machined not only by being crushed as a consequence of the axial percussion motions of the button, but also by being cut into pieces as a consequence of the rotation of the button. In other words, the rock is disintegrated by a combination of percussive crushing and rotating, cutting machining. 
         [0008]    FR 1514998 and FR 2646875 disclose drill bits. In practice such constructions are troublesome to produce, because the drilling of the individual hole must be performed in a geometrically complicated position in relation to a cone surface. Thus, the drill tip may easily glide along the cone surface during the initial drilling and thereby deviate from the determined entering point. 
       SUMMARY 
       [0009]    The presently disclosed button bits aim at eliminating the above mentioned problems and at further developing previously known button bits so far that they are particularly suitable for drilling at high rotational speed and high percussion frequency. Therefore, one object is to provide a button bit having peripheral buttons that may be mounted in accurately drilled apertures which are simple to provide, despite the complex space geometry predicament. Another object is, by the utilization of the understanding that high-speed rotating buttons fragmentize the rock through a combination of percussive crushing and rotary cutting into pieces, to provide an efficiently and aggressively operating button bit, by means of which the times for the requisite drilling operation per hole meter are reduced. An additional object is to provide a high-speed rotating drill bit having long service life, in particular such as this is determined by the capacity of the peripheral buttons to resist wear. 
         [0010]    The above mentioned objects are at least partly attained by a mouth of the hole for the individual peripheral button including a countersunk entering surface which is planar and ring-shaped and extends perpendicularly to a centre axis of the hole. 
         [0011]    For example, an exemplary embodiment of a button bit that is rotatable in a predetermined direction of rotation around a centre axis comprises a body, and a number of spaced-apart peripheral buttons each positioned in a hole, wherein the buttons protrude forward from a front end surface to which an envelope surface connects, wherein the body has a rotationally symmetrical shape, wherein at least one individual peripheral button is tilted partly in relation to a centre axis of the bit, and partly in a forward direction of rotation in relation to a surface from which the button protrudes, wherein a mouth of the hole for the individual peripheral button includes a countersunk entering surface which is planar and ring-shaped, and wherein the countersunk entering surface extends perpendicularly to a centre axis of the hole. 
         [0012]    Another exemplary embodiment of a button bit of the type that is rotatable in a predetermined direction of rotation around a centre axis comprises a body having a front end surface and an envelope surface rearward of the front end surface, and a number of spaced-apart peripheral buttons each positioned in a hole in the front end surface, wherein the buttons protrude forward from the front end surface, and wherein at least one individual peripheral button is tilted partly in relation to a centre axis of the bit, and partly in a forward direction of rotation in relation to a surface from which the button protrudes. 
         [0013]    A further exemplary embodiment of a button bit of the type that is rotatable in a predetermined direction of rotation around a centre axis comprises a body having a front end surface and an envelope surface rearward of the front end surface, and a number of spaced-apart peripheral buttons each positioned in a hole in the front end surface, wherein the buttons protrude forward from the front end surface, wherein at least one individual peripheral button is tilted partly in relation to a centre axis of the bit, and partly in a forward direction of rotation in relation to a surface from which the button protrudes, wherein the buttons have an apex at a first end, the first end exposed when mounted in the hole, and wherein a first generatrix from the apex to a rotationally leading end point along the hole edge has a first length and a second generatrix from the apex to a rotationally trailing end point along the hole edge has a second length, and the first length is less than second length. 
         [0014]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0015]    The following detailed description can be read in connection with the accompanying drawings in which like numerals designate like elements and in which: 
           [0016]      FIG. 1  is a perspective exploded view showing an exemplary embodiment of a button bit, an individual peripheral button being shown spaced-apart from the basic body of the drill bit. 
           [0017]      FIG. 2  is a central longitudinal section through the drill bit of  FIG. 1 . 
           [0018]      FIG. 3  is an enlarged end view from the front of the drill bit according to  FIG. 1 . 
           [0019]      FIG. 4  is a schematic section through only the front head of a previously known drill bit (PRIOR ART). 
           [0020]      FIGS. 5-8  are a series of schematic illustrations, which in two dimensions explain the difference between an exemplary embodiment of a disclosed button bit ( FIGS. 7-8 ) and the previously known technique ( FIGS. 5-6 ). 
           [0021]      FIGS. 9-10  are detailed sections showing how a hole for an individual peripheral button can be created. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    In  FIG. 1 , an exemplary embodiment of a button bit is shown, which comprises a basic body  1  as well as a number of buttons  2 ,  3  that, in this case, are in the form of separately manufactured bodies, which are permanently connected to the basic body  1 . The buttons consist of peripheral buttons  2  as well as centre buttons  3 . In practice, the button bodies are manufactured from cemented carbide or another material that is harder and more wear-resistant than the material of the basic body, which usually consists of steel. 
         [0023]    The basic body  1  comprises a front head  4  and a rear shank  5  that is thinner than the head and, in the example, is sleeve-shaped. The shape of the basic body is generally rotationally symmetrical so far that the envelope surface thereof comprises a cylindrical surface  7 , which forms the outside of the shank  5 , and a slightly conical clearance surface  8  on the head  4 . Said cone surface converges in the backward direction and is spaced-apart from the cylinder surface  7  via a ring-shaped neck  9 . A circular borderline  10  forms a transition between the clearance surface  8  and the front end surface of the head  4 . Said front end surface comprises also two part surfaces, viz. a central front surface  12  and a peripheral cone surface  13 . The clearance surface  8  is interrupted by a number of chipways  14 . Furthermore, at least one flushing-liquid channel  15  terminates in the clearance surface  8 . Other flushing-liquid channels  16  can terminate in the front surface  12 . 
         [0024]    The drill bit is rotatable in a predetermined direction of rotation R, around a centre axis designated C. 
         [0025]    From the enlarged end view in  FIG. 3 , it is seen that in the example, the number of peripheral buttons  2  amounts to six, and the number of chipways  14  to three, the peripheral buttons being placed in pairs between adjacent chipways. In the example, the number of centre buttons  3  amounts to four. Of these four, three are situated peripherally along the central front surface  12 , in the immediate vicinity of a circular borderline  11  between the surfaces  12  and  13 , while a fourth one (designated  3   a ) is situated closer to the centre axis C of the drill bit. 
         [0026]    The three outermost centre buttons  3  are equidistantly spaced-apart by a partition of 120°. The button  3   a  is placed in such a way inside a pair of outer centre buttons, that the areas swept by the buttons cover each other. Radially inside two pairs of outer buttons, there terminate two channels  16  for flushing liquid that can be flushed from the interior of the drill bit and has, among other things, the purpose of evacuating the debris broken off from the drill hole. 
         [0027]    The number and arrangement of buttons, chipways and channels are non-limiting examples; other suitable numbers and arrangements of these features are within the scope of the disclosure. 
         [0028]    As is seen in  FIG. 3 , the individual button comprises a cylindrical base part  18  and a tapering tip or top  19  that, in the example according to  FIGS. 1 and 2 , has a so-called ballistic shape so far that the same is delimited by one or more convex surfaces. The base part  18  is applied in a hole  20  in the head of the drill bit and is permanently united to the same in a suitable way, e.g., via a shrinkage fit, which is provided by applying the button in the hole when this is widened by heating of the drill bit, and then the drill bit is allowed to cool down so that the button is clamped in the hole. 
         [0029]    Reference is now made to  FIG. 4 , which illustrates the basic geometry of previously known button bits of the kind in question. In the example, the angle α between the peripheral cone surface  13  and an imaginary plane P in the extension of the front surface  12  amounts to 30°, i.e., the cone angle β amounts to 120° (2×60°). The centre axis C 1  of the individual hole  20  and the button  2 , respectively, extends perpendicularly to the cone surface  13 , such as this is represented by the line L in  FIG. 4 , the centre line C 1  of all buttons meeting each other in a common point “O” along the centre axis C of the drill bit. In other words, the individual centre axis C 1  and the line L may be said to be situated in a common plane that is traversed by the centre axis C of the drill bit. A consequence of this geometry is that the borderline  21 , which forms a transition between the cone surface  13  and the cylindrical inside of the hole  20 , obtains a circular shape as viewed along the axis C 1  (even if the same is deflected in a plane as a consequence of the conicity of the surface  13 ). 
         [0030]    In  FIG. 2 , it is shown how the rear shank part  5  of the drill bit in this case delimits a hollow space  22  in which the front end of a drill rod or drill string can be inserted and interconnected with the drill bit, e.g., via a threaded joint. From this hollow space  22 , flushing liquid can be fed out via the channels  15 ,  16 . 
         [0031]    As far as the shown drill bit has been described hitherto, the same is in all essentials previously known. 
         [0032]    Reference is now made to  FIGS. 5-9 . Because the general idea of the invention is difficult to clearly perceive in the three-dimensional illustrations according to  FIGS. 1-3 ,  FIGS. 5-9  have been made in two dimensions, so far that the in reality conical surface  13  of the head of the drill bit here is shown extended in a plane. 
         [0033]    In  FIGS. 5 and 6 , the placement of the peripheral buttons  2  according to prior art is shown, i.e., with the centre axes C 1  of the buttons orientated perpendicularly to the surface  13  (even if the axes—thanks to the conicity of the surface  13 —in reality extends at the angle α to the centre axis C of the drill bit). If the button is regarded from outside in the direction of the axis C 1 , such as in  FIG. 6 , it is realized that the leading half  19   a  of the protruding tip  19  of the button, which first impinges against the rock during rotation, is identical to the trailing half  19   b  facing rearward. In relation to the front head of the drill bit, hence the individual button is tilted only at a primary tip angle, viz. the angle α according to  FIG. 4 . 
         [0034]    In  FIGS. 7 and 8 , the placement of the peripheral buttons according to exemplary embodiments of the presently disclosed button bits are illustrated. In this case, the individual button is tilted not only at the primary tip angle α (not shown here), but also at a secondary tip angle ε. This is effected by locating the individual button hole  20 , which in practice is manufactured by chip removing machining, such as drilling or possibly milling, with the centre axis C 1  thereof directed in the desired angle, without because of this needing to modify the design of the button. Instead of directing, for instance, a drill linearly inward toward the centre axis C of the drill bit (only at the angle α), the same is directed toward a point situated beside the centre axis C. The cone angle for the cone surface  13  amounts to 120° (2×60°). The individual peripheral button  2  is tilted partly in relation to the centre axis C (30° in the example), partly in the forward direction in relation to the cone surface  13  seen in the rotational direction R. 
         [0035]    In such a way, a double inclination may be said to arise, which also may be described such that the imaginary plane (see  FIG. 4 ), in which the line L and the centre axis C 1  are mutually situated, is turned in the backward/upward direction around the line L. Thus, one example measure to realize the invention during practical manufacture of the drill bit is to alter the entering angle of the drill to the desired one, wherein buttons of standard design, e.g., conical or ballistic, can still be used. 
         [0036]    In order for the aggressiveness and efficiency of the buttons to increase in comparison with conventionally mounted buttons, the angle ε has of course to be greater than 0 (zero). In order to improve the effect of cutting into pieces upon rotation considerably, the angle ε should, however, amount to at least 5°. Upward from this limit value, the angle ε may vary most considerably. However, the same should not be more than 25°, because if a larger inclination would be chosen, risk arises that the button cracks or is damaged in the percussive phase of the operation. Suitably, the angle ε should be within the range of 10-20°. Thus, the same amounts to 15° in the embodiment shown in  FIGS. 1-3 , and to 20° in the schematic embodiment according to  FIG. 7 . 
         [0037]    If the button hole  20  terminates directly in the cone surface  13  (see  FIG. 3  as well as  FIGS. 7 and 8 ), the endless borderline  21 , which forms a transition between the surface  13  and the cylindrical inside of the hole, will obtain an oval, more precisely elliptical shape, the major axis of the ellipse extending tangentially and the minor axis radially along the cone surface. However, in practice, it is cumbersome to enter a drill in the desired, secondary tip angle ε to the cone surface  13 . For this reason, such as is shown in  FIGS. 9 and 10 , a particular entering surface  23  may be milled or formed in another way in the cone surface  13  before the drilling of the hole takes place. Said entering surface  23  is planar and located perpendicularly to the centre axis C 1  of the hole to be made. In the example according to  FIGS. 9 and 10 , the entering surface is ring-shaped and delimited by a shallow cylinder surface  24 , the depth of which decreases in the direction from a rotationally leading end toward a trailing end. The entering of the conical surface  13  at the desired angle by means of, for instance, a shank-end mill, presents no practical difficulties. As soon as the planar entering surface has been provided, a gimlet can be pressed against the same and with high accuracy drill the hole at the desired angle. 
         [0038]    In addition to the individual peripheral button, as a consequence of the forwardly tipped position thereof, operating aggressively and efficiently during the rotating, cutting-machining phase, an increased service life of the same is also gained. In  FIGS. 7 and 8 , MP designates a centre plane that is orientated perpendicularly to the surface  13  and intersects the hole mouth at a point halfway between the (rotationally) leading and trailing end points  25 ,  26  along the hole edge or borderline  21 . Such as is clearly shown to the right in  FIG. 7 , the quantity or mass of material (cemented carbide) in the buttons situated to the left, i.e., rotationally in front, of the plane MP is considerably greater than the mass of material situated behind the same. This means that the part of the wear of the button caused by the rotary cutting into pieces of the rock (isolated from the crushing) takes long time in comparison with the rotation wear of a button mounted in a conventional way, the two material masses of which on opposite sides of the centre plane are the same. 
         [0039]    In  FIG. 7 , a and b designate two imaginary, generatrix-like lines extending from a common apex point on the top of the button to the two end points  25 ,  26  of the hole mouth. As a consequence of the forwardly tipped position of the button, the generatrix line a becomes always shorter than the line b. More precisely, the ratio a:b decreases with increasing tilt angle ε. 
         [0040]    The invention is not limited only to the embodiment described above and shown in the drawings. Thus, it is feasible to tilt only some of the peripheral buttons in the forward direction of rotation, instead of all, such as is shown in the preferred embodiment example. For instance, every second peripheral button could be tipped forward, but not the other ones. It is also feasible to tip forward different peripheral buttons at different angles. Within the scope of the invention, it is also feasible to tip forward one or more of the centre buttons. Neither is the invention limited to such bits that have the peripheral buttons mounted in an outer cone surface of the head. Thus, within the scope of the invention, it is feasible to mount the peripheral buttons in question in a substantially planar, ring-shaped surface, which extends perpendicularly to the centre axis of the drill bit. Neither is the invention limited to such bits that are composed of a basic body of steel and a number of separately manufactured buttons of another material. Thus, the invention envisages the possibility of integrating the buttons with at least the front head of the drill bit. This could be realized by forming button-like projections integrally with the rest of the head in accordance with the known MIM-technique (Metal Injection Moulding). Although such projections would come to lack geometrical centre axes in a proper sense, the same may, however, be tilted forward in the sense that a leading generatrix line a (see  FIG. 7 ) becomes shorter than a corresponding trailing generatrix line b. 
         [0041]    Although described in connection with preferred embodiments thereof, it will be appreciated by those skilled in the art that additions, deletions, modifications, and substitutions not specifically described may be made without department from the spirit and scope of the invention as defined in the appended claims.