Cutting apparatus

Abstract

A cutting apparatus includes a support structure and first and second cutting assemblies. Each cutting assembly includes a rotatable cutting head and a mounting assembly for movably attaching the cutting head to the support structure. The cutting head is movable about a first pivot axis of the mounting assembly in a sideways direction relative to the support structure, and about a second pivot axis in an upwards-downwards direction. Each cutting head includes a plurality of cutting units, which each have a rotatable shaft and a cutter mounted thereon. The cutter includes a disc body and a plurality of buttons mounted in a radially peripheral portion of the disc body. At least some of the buttons have a central longitudinal axis that subtends an angle α to a reference axis, which extends perpendicularly outwards from an axis of the shaft, the angle α being ≥to 20° and ≤to 34°.

Claims

1. A cutting apparatus suitable for creating tunnels or subterranean roadways, the cutting apparatus comprising: a support structure having generally upward, downward, frontward and side facing regions; and first and second cutting assemblies, each of the first and second cutting assemblies including a rotatable cutting head and a mounting assembly, the mounting assembly attaching the cutting head to the support structure in a manner that enables the cutting head to move with respect to the support structure, said mounting assembly including 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 the support structure, said mounting assembly including 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, wherein each of the cutting heads includes a plurality of cutting units, each cutting unit includes a rotatable shaft having a central longitudinal axis and a cutter mounted on the shaft, said cutter including a disc body and a plurality of buttons for abrading rock, said buttons being mounted in a radially peripheral part of the disc body and protruding outwardly therefrom, wherein at least some of the buttons each have a central longitudinal axis that subtends a first angle with respect to a first reference axis, which extends perpendicularly outwards from the central longitudinal axis of the shaft, wherein the first angle is greater than or equal to 20° and less than or equal to 34°, wherein the disc body has a recessed underside to reduce frictional engagement between the disc and a rock face during a cutting operation, the underside of the disc body including a first sloping annular surface that slopes inwardly into the disc body from the radially peripheral part of the disc towards the central axis of the shaft, wherein the first sloping annular surface subtends a second angle with respect to a second reference axis, which extends perpendicularly outwardly from the central longitudinal axis of the shaft, wherein the second angle is greater than or equal to 2°, the radially peripheral part of the disc body including a first annular surface, the first annular surface sloping inwardly and upwardly towards the central longitudinal axis, and wherein the first annular surface subtends a third angle with a third reference axis that is parallel to the central longitudinal axis of the shaft, wherein the third angle is greater than or equal to 5°.

2. The apparatus according to claim 1, wherein the first angle is less than or equal to 29°.

3. The apparatus according to claim 1, wherein the first angle is greater than or equal to 24°.

4. The apparatus according to claim 1, wherein the third angle is less than or equal to 65°.

5. The apparatus according to claim 1, wherein the radially peripheral part includes a second sloping annular surface, wherein each of the buttons protrudes outwardly from the second sloping annular surface.

6. The apparatus according to claim 1, wherein at least some, or each, of the cutters are arranged to freely rotate.

7. The apparatus according to claim 1, wherein each mounting assembly includes: a support pivotally mounted relative to the support structure via 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 arranged 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 arranged to 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 rotatable cutting head is rotatable about a head axis orientated to extend substantially transverse to each arm pivot axis, wherein the cutting units provide an undercutting mode of operation.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) 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:

(2) 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 specific implementation of the present invention;

(3) FIG. 2 is a rear isometric view of the cutting apparatus of FIG. 1;

(4) FIG. 3 is a side elevation view of the apparatus of FIG. 2;

(5) FIG. 4 is a magnified front isometric view of the cutting unit of the apparatus of FIG. 3;

(6) FIG. 5 is a plan view of the cutting apparatus of FIG. 4;

(7) FIG. 6 is a side elevation view of the cutting apparatus of FIG. 5;

(8) FIG. 7 is a front end view of the cutting apparatus of FIG. 6;

(9) FIG. 8 is a longitudinal cross-sectional view of a cutting unit;

(10) FIG. 9 is an isometric view of a cutting disc included in the cutting unit of FIG. 8, showing a shaft engaging surface of the cutting disc;

(11) FIG. 10 is an isometric view of a cutting disc included in the cutting unit of FIG. 8, showing an underside of the cutting disc; and

(12) FIG. 11 is an enlarged cross-sectional view of part of the cutting disc of FIG. 9.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT OF THE INVENTION

(13) Referring to FIGS. 1 to 7, 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 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.

(14) 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.

(15) The support structure includes an undercarriage 109. The 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.

(16) 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.

(17) 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.

(18) The cutting apparatus 100 includes first and second cutting assemblies 900. 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 of the first and second mounting assemblies 902 includes a support 120. Each support 120 is pivotally mounted at, and projects 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 figured 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.

(19) Referring to FIGS. 4 and 5, as the respective pivot axes 400 are spaced 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°.

(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 disc like (generally cylindrical) configuration.

(21) Each cutting head 128 includes 12 cutting units 700. Details of the cutting units 700 are best seen in FIGS. 8 to 11. Each cutting unit 700 includes a casing 701, a shaft 703, a first bearing 705, a second bearing 707, a third bearing 709 and a cutter 127 comprising a disc body 711 and an arrangement of buttons 710. The shaft 703, and hence the disc, has a central longitudinal axis 704. 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 711 is fixed to the lower end 715 of the shaft, and rotates with the shaft. The disc 711 is attached to the shaft by bolts 717. The bolts 717 pass through holes 719 formed through the plane of the disc 711, and into threaded holes 721 in the flange 713. The disc 711 is annular. The disc 711 has a central through hole 723. The disc 711 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.

(22) The disc 711 includes an upper side 730, an underside 732, and a radially peripheral part 738.

(23) 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.

(24) The radially peripheral part 738 is generally the outer edge portion of the disc. 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.

(25) 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 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.

(26) Many holes 743 are bored into the annular tapering surface 735. The number of holes is selected according to the application. Typically around 30 to 50 holes 743 are formed in the disc 711. A button 710 is located in each of the holes 743. FIG. 10 shows the disc 711 including 45 buttons 710. Each button 710 protrudes outwardly from the disc beyond the maximum diameter 734 of the disc. Thus the circumscribed diameter of the cutting head 128 is defined by the extent to which the buttons 710 protrude beyond the disc. The buttons 710 are made from hard material, such as tungsten carbide, and are arranged abrade rock as the cutting head 128 rotates. Each button 710 has a cylindrical body part, which sits within the hole 743, and a rounded conical tip which protrudes outwardly from surface 735.

(27) Each button 710 has a central longitudinal axis 745. The central longitudinal axis of the button 745 subtends an angle α with a reference axis 746, which projects perpendicularly outwards from the central longitudinal axis of the shaft 704 (see FIG. 11). The reference axis 746 is aligned with the plane of the disc body. The angle α determines how the resultant cutting force acting on the tool will be split along the button 710 geometry, and perpendicular to it. An α=0° arrangement would be optimised for a pure shear up cutting movement, however this arrangement would not work in the sump phase. The inventors have determined that a must be larger than zero in order for the machine to operate. For at least some buttons 710, and preferably each button 710, on the disc 711 a 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 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.

(28) Other geometric aspects of the disc 711 are important for the purposes of strength and the effect of friction caused by rock during a cutting operation. It can be seen from FIG. 11 that the surface 739 subtends an angle γ to a reference axis 747. The reference axis 747 is perpendicular to the central longitudinal axis of the shaft 704. The reference axis 747 is aligned with surface 739. The reference axis 747 extends radially outwards from the central longitudinal axis 704 at a position substantially in line with lower edge 737. The inventors have determined that when γ is substantially equal to 0° the interaction between the surface 739 and the rock is too large and causes significant wear to the disc. However if γ is too large the amount of material that surrounds to the buttons 710 is significantly reduced thereby degrading the strength of the cutter 127. The inventors have determined, by significant testing, that γ should be greater than 0°, and ideally should be in the range 3° to 13° to balance friction reduction, while maintaining disc strength. A particularly preferred range is 6° to 10°, and a particularly preferred value is around 8°.

(29) Another geometric aspect of the disc 711 that is important for the purpose of determining the frictional force acting on the disc 711 during a cutting operation is the slope of the second tapered surface 733. It can be seen from FIG. 11 that the second tapered surface 733 subtends an angle β with a reference axis 749 which is arranged parallel with the central longitudinal axis 704. In FIG. 11, the reference axis 749 extends vertically upwards from surface 733, for example from the lower edge 734 of the surface, when the disc 711 is in a substantially horizontal orientation with the underside 732 facing downwards towards the ground. The inventors have determined that when 0 is substantially equal to 0° the interaction between the surface 733 and the rock generates large frictional forces, and there is significant wear to the disc 711. The inventors have determined, by significant testing, that 0 should be greater than 0°, and ideally should be in the range 15° to 55° to reduce the frictional forces generated, while maintaining sufficient strength in the vicinity of the buttons 710.

(30) A particularly preferred arrangement for the cutter 127 is for α=28°, γ=8° and β=39.5°. The inventors have determined that a cutter 127 having these values is a particularly well balanced for the full range of cutting movements undertaken by the cutting head 128.

(31) 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).

(32) Thus the plurality of generally annular or disc shaped roller cutters 127 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 are rotatably mounted independently relative to one another and head 128 and are generally free to rotate about their own axis. Each cutter 127 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 represent a lowermost part of the entire head 128 and arm 121 assembly.

(33) 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 by 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.

(34) 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 cutters 127 may be raised and lowered along the arcuate path 602 referring to FIG. 6. In particular, each arm 121, head 128 and cutters 127 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 and in particular head 128 is suspended in a declined orientation such that a forwardmost cutter 127 is positioned lower than a rearwardmost cutter 127. According to the specific implementation, this angle of declination is 10°. This is advantageous to engage the cutters 127 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.

(35) Thus the cutting movement of the apparatus 100 can be conceptualised as comprising two main sub movements. At first, there is a shallow interaction of the cutters 127 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 is approximately parallel with the floor, with the underside 732 facing towards the floor.

(36) The arms 128 then move the head 128 upwards across the rock face 1000. In this stage the disc bodies 711 are arranged substantially perpendicular to the floor, or a 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.

(37) 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 cutter 127 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.

(38) 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 cutters 127 between the extreme positions 600, 601.

(39) 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.

(40) 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.

(41) 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.

(42) 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 cutters 127 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 cutters 127 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 cutters 127 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 are then actuated according to the operation of the first arm 121 involving pivoting in both the vertical and horizontal planes.

(43) 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. The cutters 127 are optimised for the cutting action, and balancing low frictional engagement of the cutters 127 with the rock face 1000 and strength of the cutters 127.

(44) 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 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.

(45) Although the present invention has been described in connection with specific preferred embodiments, 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.

(46) 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.

(47) While the buttons 710 are shown in the diagrams as having a rounded conical protruding profile, other profiles are possible, such as chisel shaped profiles.