Patent Publication Number: US-11047235-B2

Title: Cutting apparatus

Description:
RELATED APPLICATION DATA 
     This application is a § 371 National Stage Application of PCT International Application No. PCT/EP2018/058273 filed Mar. 30, 2018 with priority to EP 17166804.9 filed Apr. 18, 2017. 
     FIELD OF INVENTION 
     The present invention relates to rock cutting apparatus suitable for creating tunnels or subterranean roadways and in particular, although not exclusively, to undercutting apparatus in which a plurality of rotating heads are capable of being slewed laterally outward and raised in the upward and downward direction during forward cutting. 
     BACKGROUND ART 
     A variety of different types of excavation machines have been developed for cutting drifts, tunnels, subterranean roadways and the like in which a rotatable head is mounted on an arm that is in turn movably mounted at a main frame so as to create a desired tunnel cross sectional profile. WO2012/156841, WO 2012/156842, WO 2010/050872, WO 2012/156884, WO2011/093777, DE 20 2111 050 143 U1 all described apparatus for mill cutting of rock and minerals in which a rotating cutting head forced into contact with the rock face as supported by a movable arm. In particular, WO 2012/156884 describes the cutting end of the machine in which the rotatable heads are capable of being raised and lowered vertically and deflecting in the lateral sideways direction by a small angle in an attempt to try enhance the cutting action. 
     WO 2014/090589 describes a machine for digging roadways tunnels and the like in which a plurality of cutting heads are movable to dig into the rock face via a pivoting arcuate cutting path. US 2003/0230925 describes a rock excavator having a cutter head mounting a plurality of annular disc cutters suitable to operate in an undercutting mode. 
     However, conventional cutting machines are not optimised to cut hard rock having a strength typically beyond 120 MPa whilst creating a tunnel or subterranean cavity safely and reliably of desired cross sectional configuration. WO2016/055087 describes a type of machine that is addresses some of these problems, however the inventors have determined that the arrangement of the cutting heads on these devices is not optimised. For example, the inventors have determined that cutters used on the current cutting head suffer from a so called “tracking” problem. That is, the buttons on a following cutter tend to follow the grooves formed in a rock face by the cutter immediately preceding it rather than cutting their own new grooves. The consequence of this is that the number of load cycles required to remove a given amount of rock is significantly higher than it should be. Furthermore since there are a high number of load cycles this tends to fatigue and wear the cutters more than is necessary. 
     Accordingly it is desirable to provide a solution to the tracking problem. It is also desirable to provide a solution to the tracking problem that has a relatively low cost and that is easy to manufacture and assemble. 
     SUMMARY OF THE INVENTION 
     It is an objective of the present invention to provide cutting apparatus suitable to form tunnels and subterranean roadways being specifically configured to cut hard rock, say beyond 120 MPa, in a controlled and reliable manner, that is, apparatus capable of mine development work. It is a further objective to provide a cutting apparatus capable of creating a tunnel with a variable cross sectional area within a maximum and a minimum cutting range. It is a further objective to provide cutting (excavator) apparatus operable in an ‘undercutting’ mode according to a two stage cutting action. It is a further objective to provide a cutter that has an optimised cutting geometry for the cutting apparatus. It is a further object to provide a cutter that has an optimised geometry for balancing cutter strength and reducing cutter wear. 
     At least some of the objectives are achieved by providing a cutting head including a plurality of different types of cutter units. The cutter units are distributed circumferentially around a perimeter of each head so as to create a groove or channel into the rock face as the heads are driven about their respective rotational axes. Each cutter includes a disc body and an arrangement of hard buttons for abrading the rock. The different types of cutters are arranged on the cutting head in a manner that addresses the tracking problem. At least some of the objectives can be achieved by mounting cutting units on the cutting head body such that, in the direction of rotation of the cutting head, each immediate successive cutter has a different arrangement of buttons from the arrangement of buttons from its immediate preceding cutter. 
     At least some of the objectives are achieved by providing a cutting apparatus having a plurality of cutting assemblies, each including a rotatably mounted cutting head that is attached to a support structure by a mounting assembly. Each mounting assembly is arranged to enable its respective cutting head to be pivoted in an upward and downward direction and a lateral side-to-side direction, with respect to the support structure. In particular, each mounting assembly comprises a support pivotally mounted to the support structure and carrying an arm via a respective additional pivot mounting such that each cutting head is capable of pivoting about two pivoting axes. The desired range of movement of each head is provided as the dual pivoting axes are aligned transverse (including perpendicular) to one another and are spaced apart in the longitudinal direction of the apparatus between a forward and rearward end. The heads may then be raised vertically so as to overcome the relatively low tensile strength of the overhanging rock to provide breakage via force and energy that is appreciably lower than a more common compressive cutting action provided by cutting picks and the like. 
     According to a first preferred embodiment of the invention there is provided a cutting head for a cutting apparatus suitable for creating tunnels or subterranean roadways and the like. The cutting head having: a rotatable cutting head body; a plurality of cutting units mounted on the cutting head body, said cutting units including at least first and second different types of cutting units, the first type of cutting unit having a first rotatable cutter comprising a first disc body and a first arrangement of buttons for abrading rock, said first buttons are mounted in a radially peripheral portion of the disc body and protrude outwardly therefrom, and the second type of cutting unit having a second rotatable cutter comprising a second disc body and a second arrangement of buttons for abrading rock, said second buttons are mounted in a radially peripheral portion of the disc body and protrude outwardly therefrom; wherein the different types of cutting units are different from one another by at least the arrangements of their buttons. 
     The arrangements of buttons can differ, for example by means of the characteristics of the buttons, such as the size and shape of the buttons, and/or the way in which the buttons are mounted on the disc body, such as the number of buttons, the distribution of buttons on the body, and the mounting orientation of buttons. 
     The invention significantly mitigates the “tracking” problem and helps to ensure that the majority, of buttons of each subsequent cutter, in the direction of cutting, do not follow the grooves cut into the rock by the respective preceding cutters. Instead the majority, of the buttons of the subsequent cutter abrade new rock and follow different paths across the rock face than the buttons of the preceding cutter in the direction of cutting. This occurs since each subsequent cutter, in the cutting direction, has a different arrangement of cutting buttons from its respective preceding cutter. This has the effect of reducing the number of load cycles required to remove the same amount of rock as a prior art device having only one type of cutter, and reduces fatigue and wear of the cutters. The effect can be achieved by having at least two different types of cutter. The majority of the benefit of the invention can be achieved by having just two different types of cutter. However it will be appreciated that the invention can comprise three or more different types of cutter. Having a low number of different types of cutter (for example two or three different types) simplifies the manufacturing and assembly processes, and reduces cost. 
     In preferred embodiments the cutting units are mounted on the cutting head body such that, in the direction of rotation of the cutting head, each immediate successive cutter has a different arrangement of buttons from the arrangement of buttons from its immediate preceding cutter. That is, for a given cutter, the cutter immediately preceding it is a different type of cutter from the given cutter. For example, a cutter of type B can be immediately preceded by a cutter of type A. The cutter immediately following the given cutter is a different type of cutter from the given cutter. For example, a cutter of type A can be followed by a cutter of type B. This helps to ensure that the buttons on a following cutter does not track the groves formed by the immediately preceding cutter. 
     In preferred embodiments the cutting units are mounted on the cutting head body sequentially such that the cutting units alternate in the direction of rotation of the cutting head. For example in the following arrangement A, B, A, B; or A, B, C, A, B, C. This provides a simple and easy to manufacture arrangement that only requires two or more different types of cutting units to address the tracking problem. 
     In preferred embodiments the cutting head includes at least three cutting units of the first type of cutting unit. In preferred embodiments the cutting head includes at least three cutting units of the second type of cutting unit. Typically, a cutting head includes at least four cutting units. Typically, a cutting head includes less than or equal to 20 cutting units. A particularly preferred embodiment includes 12 cutting units: six of the first type and six of the second type. 
     In preferred embodiments each first cutter includes a different number of buttons from the number of buttons included in each second cutter. This provides a simple way of addressing the tracking problem. Typically an odd number of buttons is included in each cutter. Each first cutter includes n buttons, each second cutter includes m buttons, wherein n≠m. For example, in one embodiment each first cutter includes 39 buttons and each second cutter includes 45 buttons. Preferably the difference between n and m is at least three buttons, and more preferably at least 5 buttons. In some embodiments a prime number of buttons can be selected for each cutter. 
     In preferred embodiments the spacing between adjacent buttons on each first cutter is different from the spacing between adjacent buttons on each second cutter. This provides a simple way of addressing the tracking problem. In preferred embodiments the buttons on each first cutter are substantially evenly spaced about the peripheral portion of their respective disc bodies. Therefore the spacing between adjacent buttons is substantially equal. In preferred embodiments the buttons on each second cutter are substantially evenly spaced about the peripheral portion of their respective disc bodies. Therefore the spacing between adjacent buttons is substantially equal. Cutters having this arrangement are easy to manufacture. Of course, the spacing between adjacent buttons on at least one of the first and second cutters can vary around the periphery of the disc. If varying spaces are used, each first cutter has a different pattern of varying spaces from the pattern of varying spaces used on each second cutter. 
     In preferred embodiments at least some, and preferably each, of the buttons on each first cutter have a different shape from the shape of at least some, and preferably each, of the buttons on each second cutter. In particular, the parts of the first cutter buttons that engage the rock face can be differently shaped from the parts of the second cutter buttons that engage the rock face. For example, one of the first and second buttons can have a rounded conical engagement part and the other of the first and second buttons can have a chiselled or pointed engagement part. 
     In preferred embodiments at least some, and preferably each, of the buttons on each first cutter have a different size from the size of at least some, and preferably each, of the buttons on each second cutter. For example, the parts of the first cutter buttons that engage the rock face can have a different height, width, length and/or volume than the height, width, length and/or volume of the parts of the second cutter buttons that engage the rock face. 
     In preferred embodiments each cutting unit includes a rotatable shaft having a central longitudinal axis and the cutter is mounted on the shaft. At least some, and preferably each, of the first buttons each have a central longitudinal axis that subtends an angle α A  with respect to a reference line that extends perpendicularly from the central longitudinal axis of the shaft. At least some, and preferably each, of the second buttons each have a central longitudinal axis that subtends an angle α B  with respect to a reference line that extends perpendicularly from the central longitudinal axis of the shaft. Preferably α A  is different from α B . 
     Preferred embodiments include at least one further type of cutting unit. The at least one further type of cutting unit can include a further rotatable cutter comprising a further disc body and a further arrangement of buttons for abrading rock. The further buttons are mounted in a radially peripheral portion of the further disc body and protrude outwardly therefrom. The further arrangement of buttons is different from the first and second arrangements of buttons. 
     In preferred embodiments the cutting units are mounted to a radially peripheral part of the cutting head body. 
     In preferred embodiments the cutting units are distributed around a pitch circle on the cutting head body. Preferably the central longitudinal axis of each cutting unit rotatable shaft is positioned on the pitch circle. 
     In preferred embodiments the cutting head body is annular. 
     In preferred embodiments the cutting head body has a central axis arranged substantially perpendicularly to the plane of the body. Each disc body has a central axis that is arranged substantially perpendicular to the plane of its respective disc body. The central axes of the disc bodies are arranged substantially parallel with the central axis of the cutting head body. Thus the plane of each disc body is approximately parallel with the plane of the cutting head body. 
     In preferred embodiments each cutter is freely rotatable relative to the cutting head. That is, each cutter is not directly driven. Each cutter rotates in reaction to engagement with the rock face. 
     According to another preferred embodiment of the invention there is provided cutting apparatus suitable for creating tunnels or subterranean roadways and the like. The apparatus includes: a support structure having generally upward, downward, frontward and side facing regions; first and second cutting assemblies, each of the first and second cutting assemblies including a cutting head according to any configuration described herein and a mounting assembly. The mounting assembly attaches the cutting head to the support structure in a manner that enables the cutting head to move with respect to the support structure. The mounting assembly includes a first pivot axis wherein the cutting head is movable about the first pivot axis thereby enabling the cutting head to move in a generally sideways direction relative to support structure. The mounting assembly includes a second pivot axis wherein the cutting head is movable about the second pivot axis thereby enabling the cutting head to move in a generally upwards-downwards direction relative to the support structure. 
     In preferred embodiments the cutting units provide an undercutting mode of operation. 
     In preferred embodiments each mounting assembly includes: a support pivotally mounted relative to the support structure via a the first pivot axis, which is aligned generally upright relative to the upward and downward facing regions such that each support is configured to pivot laterally in a sideways direction relative to the side facing regions; at least one support actuator to actuate independent movement of each of the supports relative to the support structure; an arm assembly pivotally mounted to the support via the second pivot axis aligned in a direction extending transverse including perpendicular to each support pivot axis to enable the arm to pivot independently relative to the support in an upward and downward direction relative to the upward and downward facing regions; at least one arm actuator actuate independent pivoting movement of the arm relative to the support; wherein each rotatable cutting head is mounted towards a free end of its respective arm, and each cutting head is rotatable about a head axis orientated to extend substantially transverse to each arm pivot axis. 
     In preferred embodiments each arm actuator comprises a planetary gear assembly mounted at the junction at which each arm pivots relative to each support. 
     In preferred embodiments each arm actuator includes at least one first drive motor to drive the pivoting movement of the arm relative to the support. 
     In preferred embodiments each support actuator comprises a hydraulic linear actuator. 
     In preferred embodiments each cutting assembly includes at least one second drive motor to drive rotation of the cutting head relative to the arm. 
     In preferred embodiments the support structure includes a main frame and a powered sled movably mounted at the main frame to be configured to slide in a forward cutting direction of the apparatus relative to the main frame, and each cutting head is mounted at the sled via its respective arm and support so as to be configured to advance in the forward cutting direction. 
     In preferred embodiments each arm is configured to pivot in the upward and downward direction by up to 180°; and each support is configured to pivot in the lateral sideways direction by up to 90°. Optionally, each arm may be configured to pivot over a range of up to 155°. Optionally, the first and second supports are configured to pivot in the lateral sideways direction by up to 90°. Optionally, the supports may be configured to pivot up to 20° in the lateral sideways direction. Such a configuration provides control of the profile shape and avoids any cuts or ridge that would otherwise remain on the roof and floor of the as-formed tunnel. 
     Preferably, the apparatus comprises tracks or wheels mounted at the main frame to allow the apparatus to move in a forward and rearward direction. The tracks or wheels enable the apparatus to be advanced forwardly and rearwardly within the tunnel both when maneuvered into and from the cutting face between cutting operations and to be advanced forwardly during cutting operations as part of the cut-and-advance cutting cycle that also utilises the sliding sled. 
     Preferably, the apparatus further comprises floor and roof engaging members mounted at the main frame, at least the floor engaging members being extendable and retractable to respectively raise and lower the apparatus in the upward and downward direction. The engaging members are configured to wedge the apparatus in position between the roof and floor of the tunnel to provide points of anchorage against which the machine may be braced to allow the cutters to be forced against the rock face. 
     Preferably, the apparatus further comprises a first material discharge conveyor to convey cut material rearwardly from the first and second cutting head; and a gathering head to direct cut material onto the conveyor, the gathering head positioned rearwardly behind at least one of the first and second cutting heads. The apparatus is accordingly configured to transport rearwardly material from the cut face to provide unhindered forward cutting movement into the rock. 
     Preferably, the apparatus further comprises a control unit demountably connectable to the apparatus, the control unit comprising operational components to power at least the first and second support and arm actuators, the control unit further comprising a second conveyor to receive material from the first conveyor and to discharge the material at a position rearward of the apparatus and the control unit. Preferably, the control unit is demountably coupled to the apparatus so as to be capable of being advanced and refracted in the forward and rearward directions with the cutting apparatus. Preferably, the control unit is suspended above the tunnel floor by suitable couplings to the apparatus. The control unit may comprise ground engaging support members provided at a rearward and/or forward regions. Optionally, the control unit may be attachable at its rearward end to a material collection and discharge vehicle and to be connectable at its forward end to the cutting apparatus. 
     According to another preferred embodiment of the invention there is provided cutting apparatus suitable for creating tunnels or subterranean roadways and the like, the cutting apparatus including: a main frame having generally upward, downward and side facing regions; a powered sled movably mounted at the main frame and configured to slide in a forward cutting direction of the apparatus relative to the main frame; first and second arms pivotally mounted to the sled by respective pivot arm axes aligned in a direction extending transverse including perpendicular to a longitudinal axis of the main frame to allow each arm to pivot independently of one another in an upward and downward direction relative to the upward and downward facing region of the main frame; at least one first and second arm actuator to actuate independent pivoting movement of the first and second arms relative to one another and the main frame; each of the first and second arms having a cutting head according to any configuration described herein, each cutting head is moveable in the upward and downward direction and can be advanced in the forward cutting direction, each cutting head being rotatable about a head axis orientated to extend substantially transverse to respective pivot arm axes. 
     Optionally, each of the first and second arms is respectively mounted at a first and second support slidably mounted relative to the main frame via a common or respective slidable means such that each first and second support is configured to slide laterally in a sideways direction relative to the side facing regions. 
     Optionally, each rotatable cutting head comprises a generally annular roller cutter each having a generally annular cutting edge or layered cutting edges to provide an undercutting mode of operation. 
     In preferred embodiments each of the roller cutters is independently rotatably to its respective cutting head. 
     Optionally, the plurality of roller cutters are generally annular roller cutters each having a generally annular cutting edge or layered cutting edges to provide an undercutting mode of operation. 
     Optionally, each of the first and second arm actuator comprises a planetary gear assembly mounted at the junction at which each arm pivots relative to each support. 
     According to another preferred embodiment of the invention there is provided a cutting head for cutting apparatus suitable for creating tunnels or subterranean roadways and the like, said cutting head having: a rotatable cutting head body; a first set of a first type of cutting unit mounted on the cutting head body, each first type of cutting unit having a first rotatable cutter comprising a first disc body and a first arrangement of buttons for abrading rock, said buttons are mounted in a radially peripheral portion of the disc body and protrude outwardly therefrom; and a second set of a second type of cutting unit mounted on the cutting head body, each second type of cutting unit having a second rotatable cutter comprising a second disc body and a second arrangement of buttons for abrading rock, said buttons are mounted in a radially peripheral portion of the disc body and protrude outwardly therefrom; wherein said second arrangement of buttons is different from the first arrangement of buttons, and the cutting units are mounted on the cutting head body sequentially such that the first and second types of cutting units alternate in the direction of rotation of the cutting head. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       A specific implementation of the present invention will now be described, by way of example only, and with reference to the accompanying drawings in which: 
         FIG. 1  is a front isometric view of a mobile cutting apparatus suitable for creating tunnels or subterranean roadways having a forward mounted cutting unit and a rearward control unit according to a first embodiment of the present invention; 
         FIG. 2  is a rear isometric view of the cutting apparatus of  FIG. 1 ; 
         FIG. 3  is a side elevation view of the apparatus of  FIG. 2 ; 
         FIG. 4  is a magnified front isometric view of the cutting unit of the apparatus of  FIG. 3 ; 
         FIG. 5  is a plan view of the cutting apparatus of  FIG. 4 ; 
         FIG. 6  is a side elevation view of the cutting apparatus of  FIG. 5 ; 
         FIG. 7  is a front end view of the cutting apparatus of  FIG. 6 ; 
         FIG. 8  is a longitudinal cross-sectional view of a cutting unit, including a cutter for abrading rock; 
         FIG. 9  is a view of an underside of a first cutter (the cutting unit of  FIG. 8  including the first cutter is referred to as a first cutting unit); 
         FIG. 10  is a cross-sectional view of the first cutter shown in  FIG. 9 ; 
         FIG. 11  is a view of an underside of a second cutter (the cutting unit of  FIG. 8  including the second cutter is referred to as a second cutting unit); 
         FIG. 12  is a cross-sectional view of the second cutter shown in  FIG. 11 ; and 
         FIG. 13  is a front end view of a mobile cutting apparatus suitable for creating tunnels or subterranean roadways having a forward mounted cutting unit and a rearward control unit according to a second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENT OF THE INVENTION 
     Referring to  FIG. 1 , cutting apparatus  100  comprises a support structure  800  mounting a plurality of cutting components configured to cut into a rock or mineral face  1000  to create tunnels or subterranean roadways. Apparatus  100  is configured specifically for operation in an undercutting mode in which a plurality of rotatable roller cutters  127 A,  127 B may be forced into the rock to create a groove or channel and then to be pivoted vertically upward so as to overcome the reduced tensile force immediately above the groove or channel and break the rock. Accordingly, the present cutting apparatus is optimised for forward advancement into the rock or mineral utilising less force and energy typically required for conventional compression type cutters that utilise cutting bits or picks mounted at rotatable heads. However, the present apparatus may be configured with different types of cutting head to those described herein including in particular pick or bit type cutting heads in which each pick is angularly orientated at the cutting head to provide a predetermined cutting attack angle. 
     Referring to  FIGS. 1 to 3 , the support structure  800  includes a main frame  102 . The main frame  102  comprises lateral sides  302  to be orientated towards the wall of the tunnel; an upward facing region  300  to be orientated towards a roof of the tunnel; a downward facing region  301  orientated to be facing the floor of the tunnel; a forward facing end  303  intended to be positioned facing the cutting face and a rearward facing end  304  intended to be positioned facing away from the cutting face. 
     An undercarriage  109  is mounted generally below main frame  102  and in turn mounts a pair of crawler tracks  103  driven by a hydraulic (or electric) motor to provide forward and rearward movement of apparatus  100  over the ground when in a non-cutting mode. A pair of rear ground engaging jacking legs  106  are mounted at frame sides  302  towards rearward end  304  and are configured to extend and retract linearly relative to frame  102 . Frame  102  further comprises a forward pair of jacking legs  115  also mounted at each frame side  302  and towards forward end  303  and being configured to extend and retract to engage the floor tunnel. By actuation of legs  106 ,  115 , main frame  102  and in particular tracks  103  may be raised and lowered in the upward and downward direction so as to suspend tracks  103  off the ground to position apparatus  100  in a cutting mode. A pair of roof engaging grippers  105  project upwardly from main frame  102  at frame rearward end  304  and are extendable and retractable linearly in the upward and downward direction via control cylinders  116 . Grippers  105  are therefore configured to be raised into contact with the tunnel roof and in extendable combination with jacking legs  106 ,  115  are configured to wedge apparatus  100  in a stationary position between the tunnel floor and roof when in the cutting mode. 
     The support structure  800  includes a sled  104 . The sled  104  is slidably mounted on top of main frame  102  via a slide mechanism  203 . Sled  104  is coupled to a linear hydraulic cylinder  201  such that by reciprocating extension and retraction of cylinder  201 , sled  104  is configured slide linearly between frame forward and rearward ends  303 ,  304 . 
     A pair of hydraulically actuated bolting units  107  are mounted at main frame  102  between sled  104  and roof gripping unit  105 ,  116  relative to a lengthwise direction of the apparatus. Bolting units  107  are configured to secure a mesh structure (not shown) to the roof of the tunnel as apparatus  100  is advanced in a forward cutting direction. Apparatus  100  also comprises a mesh support structure (not shown) mounted generally above sled  104  so as to positionally support the mesh directly below the roof prior to bolting into position. 
     The cutting apparatus  100  includes first and second cutting assemblies  900  (see  FIGS. 4 and 2 ). The first cutting assembly  900  includes a first cutting head  128  and a first mounting assembly  902 . The second cutting assembly  902  includes a second cutting head  128  and a second mounting assembly  902 . Each mounting assembly  902  includes a support  120 . Each support  120  is pivotally mounted at, and project forwardly from, sled  104  immediately above frame forward end  303 . Supports  120  are generally spaced apart in a lateral widthwise direction of the apparatus  100  and are configured to independently pivot laterally outward from one another relative to sled  104  and main frame  102 . Each support  120  comprises a forward end  503  and a rearward end  504  referring to  FIG. 5 . A first mount flange  118  is provided at support rearward end  504  being generally rearward facing. A corresponding second mount flange  119  projects laterally outward from a side of sled  104  immediately behind the first flange  118 . A pair of linear hydraulic cylinders  117  are mounted to extend between flanges  118 ,  119  such that by linear extension and retraction, each support  120  is configured to pivot in the generally horizontal plane and in the lateral sideways direction relative to frame sides  302 . Referring to  FIG. 4 , each support  120  is mounted at sled  104  via a pivot rod  404  extending generally vertically (when apparatus  100  is positioned on horizontal ground) through sled  104  and being suspended generally above the main frame forward end  303 . Each support  120  is therefore configured to pivot or slew about pivot axis  400 . Referring to  FIG. 5 , each support  120  is further coupled to a respective inner hydraulic cylinder  500  mounted at an inner region of sled  104  to cooperate with side mounted cylinders  117  to laterally slew each support  120  about pivot axis  400 . 
     Referring to  FIGS. 4 and 5 , as the respective pivot axes  400  are space apart in the widthwise direction of apparatus  100 , supports  120  are capable of being slewed inwardly to a maximum inward position  501  and to be slewed laterally outward to a maximum outward position  502 . According to the specific implementation, an angle between the inner and outer slewing positions  501 ,  502  is 20°. 
     Referring to  FIGS. 1 to 3 , each mounting assembly  902  includes an arm  121 . Each arm is pivotally mounted generally at the forward end  503  of each support  120 . Each cutting head  128  is rotatably mounted at a free distal end of each arm  121 . Each cutting head  128  comprises a disk like (generally cylindrical) configuration. 
     Each cutting head  128  includes a body  131  and 12 cutting units: six of a first type of cutting unit  700 A and six of a second type of cutting unit  700 B (see  FIGS. 1 and 7 ). Details of the cutting units  700  are best seen in  FIGS. 8 to 11 . Each cutting unit  700 A,  700 B includes a casing  701 , a shaft  703 , a first bearing  705 , a second bearing  707 , and a third bearing  709 . Each first type of cutting unit  700 A includes a first cutter  127 A comprising a first disc body  711 A and a first arrangement of buttons  710 A. Each second type of cutting unit  700  includes a second cutter  127 B comprising a second disc body  711  and a second arrangement of buttons  710 B. 
     Preferably the shaft  703 , bearings  705 ,  707 ,  709  and casing  701  are similar for both types of cutting unit  700 A,  700 B. Thus the following description is applicable to both types of cutting units  700 A,  700 B, unless indicated otherwise. The shaft  703  has a central longitudinal axis  704 , and since each disc body  711 A,  711 B is mounted on its respective shaft  703 , the disc body  711 A,  711 B shares this axis. The central axis  704  is arranged substantially perpendicular to the plane of the disc. The shaft  703  is journalled in the first, second and third bearings  705 ,  707 ,  709  and is arranged to rotate freely in the bearings. The shaft  703  includes a flange  713  towards a lower end  715  of the shaft. The disc body  711 A,  711 B is fixed to the lower end  715  of the shaft, and rotates with the shaft. The disc  711 A,  711 B is attached to the shaft by bolts  717 . The bolts  717  pass through holes  719  formed through the plane of the disc  711 A,  711 B, and into threaded holes  721  in the flange  713 . The disc  711 A,  711 B is annular. The disc  711 A,  711 B has a central through hole  723 . The disc  711 A,  711 B is mounted onto the shaft  703  such that the lower end  715  of the shaft protrudes through the central through hole  723 . A collar assembly  725  sits in an annular space between an outer surface  727  of the lower end of the shaft and an inner surface  729  of the annular disc. 
     The disc  711 A,  711 B includes an upper side  730 , an underside  732 , and a radially peripheral part  738 . 
     The upper side  730  faces generally towards arms  121 , and away from the rock face  1000 , during an undercutting operation. The upper side  730  includes an annular upper surface  731 , which is substantially planar. The upper surface  731  abuts against the flange  713 . 
     The radially peripheral part  738  generally comprises the outer circumferential edge portion of the disc  711 A,  711 B. The radially peripheral part  738  includes a first (upper) annular tapering surface  733 , which tapers upwardly and inwardly towards the upper surface  731 . The first tapering surface  733  has a maximum diameter at its lower edge  734  and a minimum diameter at its upper edge  736 . The radially peripheral part  738  includes a second (lower) annular tapering surface  735 , which tapers downwardly and inwardly from the lower edge  734  of the first tapering surface, to its own lower edge  737 . Thus the second annular tapering surface  735  has a maximum diameter at edge  734  and a minimum diameter at edge  737 . The edge  734  is the maximum diameter of the disc  711 A,  711 B. 
     The underside  732  faces generally towards the rock face  1000  during an undercutting operation. The underside  732  is recessed to reduce the amount of friction between the disc  711 A,  711 B and the rock face  1000 . It will be appreciated that the recessed underside  732  can take many different forms, for example the recessed underside  732  can have a substantially concave formation. A particularly preferred arrangement is for the underside  732  to include an annular tapering surface  739  which tapers inwardly and upwardly from lower edge  737  to upper edge  741 . Thus the annular tapering surface  739  has a maximum diameter at lower edge  737  and a minimum diameter at upper edge  741 . 
     Many holes  743  are bored into the annular tapering surface  735 . In the preferred arrangement of the invention, the number of holes  743  formed in the first disc  711 A is different from the number of holes  743  formed in the second disc  711 B. Typically around 30 to 60 holes  743  are formed in each disc  711 A,  711 B. For example, the first disc  711 A can include 39 holes formed therein. The second disc  711 B can include 45 holes formed therein. A button  710 A,  710 B is located in each of the holes  743 . Therefore, the number of buttons  710 A,  710 B mounted to the first disc  711 A is different from the number of buttons  711 B mounted to the second disc  711 B. Comparing  FIGS. 9 and 11 ,  FIG. 9  shows the first disc  711 A having 39 buttons  710 A and  FIG. 11  shows the second disc  711 B having 45 buttons  711 B. Preferably an odd number of buttons  710 A,  710 B is mounted to each disc  711 A,  711 B, and in some instances a prime number of buttons  710 A,  710 B is used. Preferably the buttons  710 A on each of the first cutters  127 A are substantially evenly spaced about the peripheral portion of their respective disc bodies  711 A. Preferably the buttons  710 B on each of the second cutters  127 B are substantially evenly spaced about the peripheral portion of their respective disc bodies  711 B. This arrangement is easy to manufacture, and provides a low cost approach to mitigating the tracking problem. Each button  710 A,  710 B protrudes outwardly from the disc  711 A,  711 B beyond the maximum radius  734  of the disc. Thus the circumscribed diameter of the cutter  127 A,  127 B is defined by the extent to which the buttons  710 A,  710 B protrude beyond the edge of the disc  711 A,  711 B. The buttons  710 A,  710 B are made from hard material, such as tungsten carbide, and are arranged abrade rock as the cutting head  128  rotates. 
     Each button  710 A,  710 B has a central longitudinal axis  745 . The central longitudinal axis of the button  745  subtends an angle α A , α B  with a reference axis  746 , which projects perpendicularly outwards from the central longitudinal axis of the shaft  704  (see  FIGS. 10 and 12 ). The reference line  746  is aligned with the plane of the disc body. The angle α A , α B  determines how the resultant cutting force acting on the tool will be split along the button  710  geometry, and perpendicular to it. An α A , α B =0° arrangement would be optimised for a pure shear up cutting movement, however this arrangement would not work well in the sump phase. The inventors have determined that α A , α B  should be larger than zero in order for the machine to operate properly. For at least some buttons  710 A,  710 B, and preferably each button  710 A,  710 B, on the disc  711 A,  711 B α A , α B  is set in the range 20° to 34°, preferably between 24° and 28°, and more preferably still at around 28°. The inventors have determined, after significant testing, that these ranges provide the best overall cutting effect for cutters  127 A,  127 B for this type of boring machine. In particular, taking into account the range of movement of the cutting heads  128  that is undertaken by this type of rock cutting apparatus. 
     The size of the cutting disc  711  is selected for the application. A preferred maximum diameter of the disc is typically around 17″ (431.8 mm). 
     Thus the plurality of generally annular or disc shaped roller cutters  127 A,  127 B are mounted at the circumferential perimeter of each head  128  and comprise a sharp annular cutting edge configured specifically for undercutting the rock. Cutters  127 A,  127 B are rotatably mounted independently relative to one another, and to the head  128 , and are generally free to rotate about their own axis. Each roller cutter  127 A,  127 B projects axially beyond a forwardmost annular edge of head  128  such that when arms  121  are orientated to be extending generally downward, roller cutters  127 A,  127 B represent a lowermost part of the entire head  128  and arm  121  assembly. 
     The cutting units  700 A,  700 B are mounted on the cutting head body  131  sequentially around a pitch circle in an alternating fashion, that is in the following sequence: A, B, A, B, A, B, A, B, A, B, A, B. It will be apparent that as the cutting head body  131  rotates, each successive cutter will have a different arrangement of buttons from the cutter immediately preceding it. This significantly mitigates the tracking problem. It also provides a solution to the tracking problem that is easy to manufacture since it requires just two different types of cutting unit  700 A,  700 B, and the different types of cutting units  700 A,  700 B differ in that theirs cutters  127 A,  127 B have a different number of buttons  710 A,  710 B. 
     Each arm  121  may be considered to comprise a length such that arm  121  is mounted at each respective support  120  at or towards a proximal arm end and to mount each head  128  at a distal arm end. In particular, each arm  121  comprises an internally mounted planetary gear indicated generally be reference  122 . Each gear  122  is preferably a Wolfrom type and is coupled to a drive motor  130  via a drive train indicated generally by reference  123 . A pair of drive motors  125  are mounted at the lateral sides of each arm  121  and are orientated to be approximately parallel with the rotational axis of each respective cutting head  128  as shown in  FIG. 7 . Each arm  121  further comprise an internal drive and gear assembly  124  coupled to a gear box  126  mounted at one end of each of the drive motors  125 . Each cutting head  128  is driveably coupled to the drive motors  125  via the respective gear assembly  124  to provide rotation of cutting head  128  about axis  402 . 
     As shown in  FIG. 7 , each arm  121  is coupled to a respective motor  130  mounted at a forward end of sled  104 . Each planetary gear  122  is centred on a pivot rod  405  having a pivot axis  401  referring to  FIG. 4 . Each axis  401  is aligned to be generally horizontal when apparatus  100  is positioned on horizontal ground. Accordingly, each arm  121  is configured to pivot (relative to each support  120 , sled  104  and main frame  102 ) in the upward and downward direction (vertical plane) by actuation of each motor  130 . As such, each cutting head  128  and in particular the roller cutters  127 A,  127 B may be raised and lowered along the arcuate path  602  referring to  FIG. 6 . In particular, each arm  121 , head  128  and roller cutters  127 A,  127 B may be pivoted between a lowermost position  601  and an uppermost raised position  600  with an angle between positions  600 ,  601  being approximately 150°. When in the lowermost position  601 , each roller cutter  127 A,  127 B and in particular head  128  is suspended in a declined orientation such that a forwardmost roller cutter  127 A,  127 B is positioned lower than a rearwardmost roller cutter  127 A,  127 B. According to the specific implementation, this angle of declination is 10°. This is advantageous to engage the cutters  127 A,  127 B into the rock face at the desired attack angle to create the initial groove or channel during a first stage of the undercutting operation. Additionally, the extensive range of movement of the cutting heads  128  over the rock face is possible due, in part, to axis  401  being separated and positioned forward relative to axis  400  by a distance corresponding to a length of each support  120 . 
     Thus the cutting movement of the apparatus  100  can be conceptualized as comprising two main sub movements. At first, there is a shallow interaction of the cutters  127 A,  127 B with the rock face towards the mine floor level (often referred to as “sump in”). Here the cut depth is increased from zero to a few millimetres. At this stage each disc body  711 A,  711 B is approximately parallel with the floor, with the underside  732  facing towards the floor. 
     The arms  128  then move the head  128  upwards across the rock face  1000 . In this stage the disc bodies  711 A,  711 B are arranged substantially perpendicular to the floor, or are moving towards that orientation, with the underside  732  facing towards the rock face  1000 . At this stage, the cut thickness reaches it maximum. This is typically referred to as “shear up”. The shear up phase lasts longer in the cutting cycle. 
     Referring to  FIG. 4 , each support pivot axis  400  is aligned generally perpendicular to each arm pivot axis  401 . Additionally, a rotational axis  402  of each cutting head  128  is orientated generally perpendicular to each arm pivot axis  401 . A corresponding rotational axis  704  of each roller cutter  127 A,  127 B is angularly disposed relative to the cutting head axis  402  so as to taper outwardly in the downward direction. In particular, each roller cutter axis  704  is orientated to be aligned closer to the orientation of each cutting head rotational axis  402  and support pivot axis  400  relative to the generally perpendicular arm rotational axis  401 . 
     Accordingly, each support  120  is configured to slew laterally outward in a horizontal plane about each support axis  400  between the extreme inner and outward positions  501 ,  502 . Additionally and referring to  FIG. 6 , each respective arm  121  is configured to pivot in the upward and downward direction about arm pivot axis  401  to raise and lower the roller cutters  127 A,  127 B between the extreme positions  600 ,  601 . 
     A gathering head  129  is mounted at main frame forward end  303  immediately rearward behind each cutting head  128 . Gathering head  129  comprises a conventional shape and configuration having side loading aprons and a generally inclined upward facing material contact face to receive and guide cut material rearwardly from the cutting face (and cutting heads  128 ). Apparatus  100  further comprises a first conveyor  202  extending lengthwise from gathering head  129  to project rearwardly from frame rearward end  304 . Accordingly, material cut from the face is gathered by head  129  and transported rearwardly along apparatus  100 . 
     Referring to  FIGS. 1 to 3 , a detachable control unit  101  is mounted to the frame rearward end  304  via a pivot coupling  200 . Control unit  111  comprises a personnel cabin  110  (to be occupied by an operator). Unit  111  further comprises an electric and hydraulic power pack  114  to control the various hydraulic and electrical components of apparatus  100  associated with the pivoting movement of supports  120  and arms  121  in addition to the sliding movement of sled  104  and the rotational drive of cutting heads  128 . 
     Control unit  101  further comprises a second conveyor  112  extending generally lengthwise along the unit  101  and coupled at its forwardmost end to the rearwardmost end of first conveyor  202 . Unit  101  further comprises a discharge conveyor  113  projecting rearwardly from the rearward end of second conveyor  112  at an upward declined angle. Accordingly, cut material is capable of being transported rearwardly from cutting heads  128  along conveyors  202 ,  112  and  113  to be received by a truck or other transportation vehicle. 
     In use, apparatus  100  is wedged between the tunnel floor and roof via jacking legs  106 ,  115  and roof grippers  105 . Sled  104  may then be displaced in a forward direction relative to main frame  102  to engage roller cutters  127 A,  127 B onto the rock face. Cutting heads  128  are rotated via motors  125  that create the initial groove or channel in the rock face at a lowermost position. A first arm  121  is then pivoted about axis  401  via motor  130  to raise roller cutters  127 A,  127 B along path  602  to achieve the second stage undercutting operation. The first support  120  may then be slewed in the lateral sideways direction via pivoting about axis  400  and combined with the raising and lowering rotation of roller cutters  127 A,  127 B creates a depression or pocket within the rock immediately forward of the first arm  121  and support  120 . The second arm  121  and associated head  128  and cutters  127 A,  127 B are then actuated according to the operation of the first arm  121  involving pivoting in both the vertical and horizontal planes. This sequential dual pivoting movement of the second arm  121  is independent of the initial dual pivoting movement of the first arm  121 . A phasing and sequencing of the pivoting of arms  121  about axes  401  and supports  120  about axes  400  is controlled via control unit  111 . 
     When the maximum forward travel of sled  104  is achieved, jacking legs  106 ,  115  are retracted to engage tracks  103  onto the ground. Tracks  103  are orientated to be generally declined (at an angle of approximately 10° relative to the floor) such that when ground contact is made, the roller cutters  127 A,  127 B are raised vertically so as to clear the tunnel floor. The apparatus  100  may then be advanced forward via tracks  103 . Jacking legs  106 ,  115  may then be actuated again to raise tracks  103  off the grounds and grippers  105  moved into contact with the tunnel roof to repeat the cutting cycle. A forwardmost roof gripper  108  is mounted above sled  104  to stabilise the apparatus  100  when sled  104  is advanced in the forward direction via linear actuating cylinder  201 . 
     Although the present invention has been described in connection with a specific preferred embodiment, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. Furthermore, it will be apparent to the skilled person that modifications can be made to the above embodiment that fall within the scope of the invention. 
     For example, the number of cutting units  700  included in a cutting head  128  can be different. Typically a cutting head  128  includes between 6 and 18 cutting units, and preferably between 8 and 16 cutting units. 
     The cutter head  128  can include at least one further cutting unit  700 C, which is different from the first and second cutting units  700 A,  700 B. This is shown in  FIG. 13 . In particular, the arrangement of cutting buttons in the further cutting unit  700 C is different from the arrangement of cutting buttons  710 A,  710 B in the first and second types of cutting units  700 A,  700 B. Otherwise, the cutting unit  700 C is similar to at least one of the cutting units  700 A,  700 B. It will be appreciated that this enables many different arrangements of cutting units  700 A,  700 B,  700 C around the cutting head  131  that significantly mitigates the tracking problem. For example, the cutting units can be mounted on the cutting head body  131  sequentially around a pitch circle in one of the following arrangements: A, B, C, A, B, C, A, B, C; A, B, C, B, A, C, A, B, C, B, A, C; or A, B, A, C, A, B, A, C, A, B. In preferred arrangements, as the cutting head body  131  rotates, each immediately successive cutter  127  has a different arrangement of buttons  710  from the cutter  127  immediately preceding it. That is, for any given cutting unit  700  mounted on the cutting head body  131 , the cutting units  700  immediately adjacent to it on the pitch circle are different from the given cutting unit  700 . Thus the cutting unit  700  immediately preceding the given cutting unit in the direction of rotation of the cutting head body  131 , and the cutting unit  700  immediately following the given cutting unit  700  in the direction of rotation of the cutting head body, each have a different arrangement of buttons  710  than the given cutting unit  700 . This helps to mitigate the tracking problem since the buttons  710  on the successive cutter  127  tend to cut their own paths in the rock face  1000  rather than following paths formed in the rock face  1000  by the cutter  127  immediately preceding it. It will be appreciated that the immediately preceding cutting unit  700  and the immediately following cutting unit  700  can be similar to each other or can be different from one another. 
     While the buttons  710 A, B are shown in the diagrams as having a rounded conical protruding profile, other profiles are possible, such as chisel shaped profiles. In addition, or as an alternative, to the first and second cutters  127 A,  127 B having a different number of buttons  710 A,  710 B, the tracking problem can be mitigated by at least one of the following:
         The spacing S A  is the distance between the tips of adjacent first buttons  710 A (see  FIG. 9 ). The spacing S B  is the distance between the tips of adjacent second buttons  710 B (see  FIG. 11 ). The spacing S A  between adjacent first buttons  710 A on the first cutter  127 A is different from the spacing S B  between adjacent second buttons  710 B on the second cutter  127 B. In preferred arrangements the spacing S A  between adjacent first buttons  710 A is substantially equal around the circumference of the first cutter  127 A. In preferred arrangements the spacing S B  between adjacent second buttons  710 B is substantially equal around the circumference of the second cutter  127 B. This provides cutters  127 A,  127 B that are easy to manufacture.   The spacings S A , S B  between adjacent buttons  710 A,  710 B on their respective discs  711 A,  711 B can be uneven. The spacing S A  between adjacent first buttons  710 A can vary around the circumference of the first disc  711 A. The spacing S B  between adjacent second buttons  710 B can vary around the circumference of the second disc  711 A.   Each first cutter button  710 A can comprise a first shape. Each second cutter button  710 B can comprise a second shape. The first shape is different from the second shape. For example, one of the first and second buttons can have a rounded conical engagement part and the other of the first and second buttons can have a chiselled or pointed engagement part.   The first cutter buttons  710 A can have a different size from the size of the second cutter buttons  710 B. For example, the parts of the first cutter buttons  710 A that engage the rock face  1000  can differ from equivalent parts of the second cutter buttons  710 B that engage the rock face  1000  in at least one of the following parameters: height, width, length and volume.   Buttons  710 A on each first cutter  127 A can have a different angle α A  from the angle α B  for the buttons  710 B on each second cutter  127 B. In this instance, the angle α A , α B  is set differently for each different type of disc. Preferably the angle α A  for each first type of disc  711 A is substantially the same around the circumference of the first disc  711 A. Preferably the angle α B  for each second type of disc  711 B is substantially the same around the circumference of the second disc  711 B. This makes the cutters  127 A,  127 B easy to manufacture. Preferably the angles α A , α B  are in the range 20° to 34°.   The angle α A  can vary around the circumference of the first disc  711 A. That is, every button  710 A does not have the same cutting angle α A  on the first cutter  127 A, though of course some buttons  710 A may have the same angle α A . The angle α B  can vary around the circumference of the second disc  711 B. That is, every button  710 B does not have the same cutting angle α B  on the second cutter  127 B, though of course some buttons  710 B may have the same angle α B . Thus the first disc  711 A can have a different pattern of angles α A  than the pattern of angles α B  for the second disc  711 B. This overcomes the tracking problem, but is more complex to manufacture.   Any combination of the above features.       

     It will be appreciated that the arrangement of buttons in each further cutting unit  700 C can differ from the arrangement of buttons  710 A,  710 B in the first and second cutting units  700 A,  700 B, by way of any characteristic described herein, or any combination of characteristics.