Patent Description:
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 so as to create a desired tunnel cross sectional profile. To cut a lower profile tunnel with lower tunnel height that may be comparable to a diameter of a cutter head, the creation of the tunnel can be made by horizontal swinging operation of a cutter head, each time only a single layer is cut by pivoting movement of the cutter head in the lateral sideways direction. In order to be adapted for cutting hard rock, disc-like or roller-like form of cutters are considered in existing design for achieving undercut effect, the disc cutters are deposited on the cutter head such that the rotational axes of disc cutters are substantially parallel to the the rotational axes of the cutter head.

<CIT> describes an extraction machine for extracting hard rocks, in which disc or roller tools operating according to the undercut principle are provided with, wherein the disc or roller tools are mounted for rotation on a swivelling jib arm of the machine, with a head carrying the tools, the axis of rotation of which extends essentially in the direction of the jib arm axis, wherein the head carrying the tools on the machine frame is mounted for swivelling around a vertical axis. <CIT> describes a similar mining machine to the above described machine.

<CIT> describes a mobile mining machine <NUM> which comprises a movable machine base frame <NUM>, a rotatable tool drum <NUM> and including excavating tools <NUM>. Each excavating tool <NUM> comprises a rotatable tool holder <NUM> with a support head <NUM> located outside of the drum housing <NUM>. The support head <NUM> is fitted with a number of tool chisels <NUM> (see <FIG>&<NUM>).

<CIT> describes a milling device <NUM> comprising a milling drum <NUM> rotatable about a drum axis <NUM>; a first group of excavating heads <NUM> and a second group of excavating heads <NUM> arranged around the periphery of the milling drum <NUM>. The first group of excavating heads <NUM> includes a plurality of first excavating tools <NUM> configured to perform a first, cutting operation, and the second group of excavating heads <NUM> includes a plurality of second excavating tools <NUM> configured to perform a second excavating operation different from the first, cutting operation.

However, in the above described machine it is observed that a peak force is present on the individual cutting tools at initial phase of contacting rock (when disc or roller tools strike on rock face) and at end phase of leaving rock (when disc or roller tools leave the rock face), in particular there presents a peak value of reaction force from the rock, including normal force and lateral force, here a mean value of the reaction force in statistic sense is referred to. Such a peak force tends to result in extra wear to disc or roller tools. Therefore, conventional cutting machines are not optimised to cut hard rock whilst creating a tunnel or subterranean cavity efficiently with reduced wear and production costs. Accordingly, what is required is a cutting machine that addresses these problems.

It is an objective of the present invention to provide a cutter head and a mining machine suitable for cutting hard rock having a strength typically beyond <NUM> MPa in undercutting mode, particularly for achieving a tunnel with lower profile. It is a further specific objective to provide a cutter head with its cutting tools suffering from less wear during cutting operation. It is a further specific objective to provide a mining machine that, when advancing forward and in operation, creates a cut path with varying cut spacing.

It is an intention to overcome the negative effect of a conventional mining machine, in which the rotational axis of a disc cutter is substantially aligned with the rotational axis of the cutter head carrying the disc cutters thereon, during advancement of the cutter head, the individual disc cutters tend to have substantially equal cut spacing; further, when a disc cutter strikes into the rock and gets out of contact with the rock, the penetration approaches a minimal value of zero; the cutter head in this conventional configuration suffers a peak value of reaction force occurring at the initial phase of contacting rock also at the end phase of leaving rock. In undercutting model, such peak forces contribute less to the cutting performance, and more to causing significant wear to the disc cutter.

To overcome this above-mentioned negative effect, the group of disc cutters or disc-like roller cutters are arranged on the support portion in a manner that the respective rotational axis of each disc cutter is configured to be substantially transverse to the rotational axis of the cutter head, an individual disc cutter creates a groove or channel into the rock face as the head is driven about its rotational axis. The head may then be pivoted laterally 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. Advantageously, the individual disc cutter has a characteristically varying cut spacing over a single rotation of the cutter head, and no peak value of reaction force is present at the initial phase of contacting rock and at the end phase of leaving rock.

In order to achieve high cutting efficiency and to cope with the strength of hard rock (which requires a significantly large lateral force being applied to the rock face), it is generally required that each individual disc cutter comprises a single layer of an annular cutting edge - for example a cutting ring, or a single layer of an annular cutting arrangement defined by the outermost cutting tips of a plurality of cutting elements (such as cutting buttons) arranged on the outer periphery of the disc cutter. This corresponds to single-layer cutting mode, at each time of lateral slewing movement of the cutter arm, the cutter head removes one layer of rock. In this mode, multiple layers of rock are sequentially fractured one after another, where each layer is free of confinement at the free face of rock (since a neighbouring layer is already cracked by the previous cutting cycle), individual layers can be broken much easier, thus less energy is consumed, consequently the required overall cutting power decreases. On the contrary, in multiple-layer cutting mode, multiple layers of rock are cracked simultaneously in the same cutting cycle, an inner layer of rock is confined by the outer layer rock and is not easily cracked. An example for multiple-layer cutting mode is a conventional milling roller that includes multiple layers of cutting chisels or bit-like tools being arranged spirally over the carrier circumference or being distributed centrically about a rotational axis, for example being placed on a surface of a cylindrical or tapered or conical shaped cutter tool. Such a milling roller is not suitable or not practical for excavating hard rock in undercutting mode.

According to a first aspect of the present invention there is provided a cutter head for excavating hard rock materials in rock face, which comprises: a carrier that is attachable to a cutter arm of a cutter apparatus, and a drive shaft rotatably supported by the carrier, the drive shaft being rotatable about a drive axis and comprising at one end a support portion for mounting disc cutters; a plurality of disc cutters mounted on the support portion and configured to perform undercutting against the rock face; wherein each disc cutter is rotatable about a respective support axis, the disc cutters are attached on the support portion substantially in a manner that the support axes of the disc cutters extend to intersect with one another at the drive axis at an intersection point and lie within a common conical surface, wherein the disc cutters are of the same configuration in structure. In other words, the support axes extend substantially radially with respect to the intersection point, and form a cone-like shape. The disc cutters are attached on the support portion in a manner that the support axes of the disc cutters extend to essentially intersect with one another at the drive axis at an intersection point and essentially lie within a common conical surface.

The disc cutters have annular cutting edges, upon rotation of the cutter head, individual disc cutters may alternatively get contact with the rock face, and after a period of time leave the rock face sequentially, about half of the disc cutters are not in contact with the rock at each time instance. When a disc cutter strikes into the rock, the cut has a cut spacing of zero, as the cut continues, the cut spacing gradually increases to a maximum defined by the previous advance distance (sump) of the mining machine. After reaching the maximum the cut spacing decreases to zero until the the tool gets out of contact with the rock. During the cutting, the penetration of the disc cutter is however maintained more or less constant at maximal value.

The main beneficial effects include significantly reduced forces on the disc cutter due to the gradual changing of the cut spacing and and significantly reduced confinement to the disc cutter at the beginning and ending stages of the individual cuts. The reduced forces include reduced normal force perpendicular to the advancing direction of the tool and reduced lateral force parallel to the advancing direction of the tool, and advantageously result in less wear and longer tool lifetime, less frequently replacement of disc cutters indicates reduced extra machine down time. Consequently, not only the expense of wear parts is heavily reduced, but also the productivity of the machine is increased. Another benefit is the improved rock wall quality due to the gradually increasing cut spacing, especially on the floor, roof and face.

By the wording "substantially" it is meant to include the situations with a certain extent of deviation. For instance, one support axis may be slightly offset (for example by an offset of ± <NUM>) from a common conical surface defined by the other support axes, and/or do not strictly pass through a common vertex of the other support axes. Similarly, considering angular deviation, it refers to an angle offset in a range between about <NUM> degree and about ±<NUM> degrees, preferably in a range between about ±<NUM> degree and about ±<NUM> degrees, the support axes of the disc cutters being substantially transverse to the rotational axis of the cutter head, may include (or encompass) a perpendicular alignment.

The disc cutters represent all cutter tools placed on the cutter head, no other disc cutters placed in other orientation are included. The disc cutters are positioned at a same side of the carrier. They may be generally annular or disc shaped roller cutters and comprise a sharp annular cutting edge configured specifically for undercutting hard rock. In one implementation, each disc cutter may include a cutter ring or cutter disc in rigid connection to a cutter hub that is rotatably mounted at a disc shaft, each disc shaft is in rigid connection to the support portion (such as a cutter wheel). In another implementation, the cutter hub may be fixedly attached to the support portion, and the cutter disc is fixed to the disc shaft rotatable relative to the cutter hub.

Preferably, the disc cutters are spaced apart from the intersection point by the same offset.

Preferably, the disc cutters are of the same configuration in structure, preferably the disc cutters are uniformly distributed about a circumference in a plane perpendicular to the drive axis. Seen from the intersection point, the disc cutters are uniformly distributed in respective radial direction.

Preferably, the cutter head further comprises a flywheel coupled to the drive shaft, for example coupled indirectly via a gear mechanism to the drive shaft, the flywheel is configured for storing rotational energy, and helps to resist rapid changes in rotational speed by their moment of inertia.

Optionally, each disc cutter comprises a single layer of annular cutting edge, or a single layer of annular cutting arrangement defined by the cutting tips of a plurality of cutting elements arranged on the outer periphery of the disc cutter. Preferably, the cutting elements are in the form of cutting buttons consecutively distributed on the outer periphery of the disc cutter without interruption.

Optionally, the support axis of each disc cutter extends inclined relative to the drive axis by a disc inclination angle, preferably the disc inclination angle is in a range between <NUM> to <NUM> degrees, more preferably in a range from <NUM> to <NUM> degrees. The disc inclination angle may be set depending on the diameter of the disc cutters and the separation between an outermost cutting edge of the disc cutter and the drive axis.

Optionally, each disc cutter is independently rotatable about the respective support axis via a bearing.

Preferably, the cutter head further comprises a motor supported on the carrier, configured to actuate the drive shaft to rotate about the drive axis via a gear mechanism, preferably the gear mechanism comprises a first stage planetary gear coupled in series to a second stage planetary gear.

Preferably, the cutter head further comprises a plurality of material cleaning parts deposited between neighbouring disc cutters, and configured to clean material from the rock face.

Optionally, the gap between two neighbouring disc cutters is minimised so that the cutter head comprises as many disc cutters as possible, preferably the disc cutters has a diameter of <NUM> inches.

According to a further aspect of the present invention there is provided a cutter apparatus for creating a tunnel, comprising a main frame; a support mounted on the main frame and slidable relative to the main frame in the longitudinal direction of the cutter apparatus; a cutter arm mounted on the support and rotatable about a vertical axis; a cutter head according to any of above described embodiments and mounted at a distal end of the cutter arm.

Preferably, the cutter head is coupled to the cutter arm in a way such that a required angular offset of outermost cutting edge is satisfied, the angular offset of outermost cutting edge is defined by two rays starting from a rotation centre at the vertical axis, with one ray towards the outermost cutting edge, and the other ray perpendicular to the drive axis of the cutter head.

Preferably, the rotational axis of the cutter head extends substantially transverse to the longitudinal axis of the cutter arm which crosses the vertical axis.

Preferably, the cutter head is mounted at a distal end of the cutter arm in a manner that a free-cutting angle is between <NUM> to <NUM> degrees, preferably <NUM> degree. It is observed that the incident angle of a disc cutter has key influence on the cutting efficiency and/or the reacting force on the disc cutter, all disc cutters shall be configured to follow the same effective incident angle. It is important to maintain a free-cutting angle (or called a contacting angle) of a disc cutter at an optimal value, the free-cutting angle is defined by the tangent line of the rock face at a contacting point to the rock and a plane defined by the annular cutting edge of the disc cutter, the free-cutting angle is dependent on the disc inclination angle and the angular offset of the outermost cutting edge, and falls within the range of <NUM> to <NUM> degrees, preferably, the free-cutting angle is in a range of <NUM> to <NUM> degrees.

The rotation speed of cutter head and the slewing speed of the cutter arm shall be controlled so that a required penetration is met and the machine achieves high productivity. The slewing speed of the cutter arm is dependent on the rotation speed of cutter head, the amount of disc cutters, and required penetration.

Preferably, the cutter apparatus further comprises a loading means mounted on a lateral side of the cutter head, and configured to collect material that is cut off by the cutter head.

Optionally the cutter apparatus further comprises a slewing gear mechanism or a linear arm actuator to actuate the cutter arm to slew about the vertical axis, and/or a support actuator to actuate the support to slide relative to the main frame.

Optionally the cutter apparatus further comprises a plurality of floor and roof engaging means mounted at the main frame and/or at the support, extendible and retractable to raise and lower the cutter apparatus.

<FIG> illustrates a cutter head <NUM> with the front part in sectional view, the front side of which is indicated by arrow <NUM>, the cutter head <NUM> comprises a cylindrical shaped or drum-like body that may be fastened to a suitable holding arm or boom <NUM>, the body includes a housing or stationary holder <NUM> which may be tubular form having a chamber as a receptacle for shaft and gear, and a drive shaft <NUM> that is journaled to the housing <NUM> freely rotatably by means of bearings <NUM> such as tapered roller bearings arranged in an O arrangement or X arrangement, an electronic or hydraulic motor <NUM> can be mounted on the body for actuating the drive shaft <NUM> to rotate about a drive axis <NUM> via a speed reduction mechanism, motor <NUM> is connected to motor shaft <NUM> that is supported via bearings <NUM> on housing <NUM>. As shown in <FIG>, the speed reduction mechanism includes a bevel gear stage <NUM> in engagement with a first stage planetary gear <NUM> which is in series coupled to a second stage planetary gear <NUM>, the carrier of the first stage planetary gear introduces rotation to the sun gear of the second stage planetary gear <NUM>, a bevel gearwheel <NUM> may be shrink-fit connected to a gear shaft <NUM>, gear shaft <NUM> is supported via bearings <NUM> on housing <NUM>, at rear side the gear shaft <NUM> it is coupled to a flywheel <NUM>, the front side of the gear shaft <NUM> acts as input to the sun gear of the first stage planetary gear <NUM>. The rotation of bevel gear stage <NUM> is introduced by motor <NUM> and consequently transferred to shaft <NUM>, finally the carrier of the second stage planetary gear introduces rotation to the drive shaft <NUM>. The gear ratio for the bevel gear stage <NUM>, the first stage planetary gear <NUM> and the second stage planetary gear <NUM> may be set depending on the properties of motor <NUM> and a target rotational speed of the cutter head, and may be chosen such that the speed of the drive shaft <NUM> is in the range of <NUM>-<NUM> rpm.

Turn back to <FIG>, the drive shaft <NUM> projects out of a front end of the housing <NUM> and includes therein a cutter wheel <NUM> for mounting a group of disc cutters <NUM>, the cutter wheel <NUM> and the drive shaft <NUM> are connected fixedly in terms of rotation to one another or integrally formed in one piece. The group of disc cutters <NUM> are of the same kind of disk roller cutters and have the same design details, i.e. the same in dimension, in structure and in drive mechanism, in other words, they are structurally and functionally identical to each other. The arrangement of all disc cutters <NUM> disclosed herein may have a symmetrical or substantial symmetrical configuration with respect to the drive axis <NUM>. Referring to <FIG> the disc cutter <NUM> are mounted in a generally radial direction on the cutter wheel <NUM> facing outward, uniformly spaced apart from each other on a same outer circumference <NUM>.

Each disc cutter <NUM> is freely rotatable about a support axis <NUM>, the support axis <NUM> may intersect with one another at the drive axis <NUM> at intersection point <NUM>. The support axis <NUM> of each disc cutter runs inclined relative to the drive axis <NUM> by a disc inclination angle <NUM> which shall be substantially the same value for all disc cutters. Thus the respective support axes <NUM> define a conical surface with apex at the intersection point <NUM>. The disc inclination angle <NUM> is dependent on the diameter of the disc cutters and the separation <NUM> between a centre of cutter ring <NUM> and the drive axis <NUM>, preferably the disc inclination angle <NUM> is in a range between <NUM> to <NUM> degrees, more preferably the angle <NUM> is <NUM> degrees.

Further, the disc cutters <NUM> are spaced apart from the drum axis <NUM> by the same offset <NUM> in radial direction and positioned in the same altitude along the direction of drive axis <NUM>.

Each disc cutter may include a cutter disc or cutter ring <NUM> that is rigidly connected on one side to a cutter hub <NUM> that is in turn rotatably mounted at a disc shaft <NUM>, bearings <NUM> permit the cutter hub to be freely rotatable around the disc shaft <NUM>, a radially outer portion of each disc <NUM> by rotation of the disc configured to abrade rock and create a cut groove therein, each disc shaft <NUM> is of cylindrical shape and in rigid connection to the cutter wheel <NUM> e.g. via fastening screws.

Design details of a disc cutter <NUM> is partly shown in <FIG>, annular cutter ring <NUM> is mounted on the cutter hub <NUM> via shrink-fit or form-fit or screw bolt connection. A plurality of cutter buttons <NUM> made of diamond or carbide or other hard material are consecutively and uniformly embeded along the outer periphery of the cutter ring, the buttons are oriented to face obliquely outwards with the tips forming a general annular cutter edge. A radial outer face with respect to the support axis <NUM> is indicated by reference symbol <NUM>, the outer face is spaced apart from the intersection point <NUM> by an offset which is the same value for all disc cutters <NUM>.

The cutter head <NUM> further includes a set of shovels <NUM> mounted fixedly in terms of rotation to the cutter wheel <NUM>, each shovel extends in a respective plane across the drive axis <NUM>, and is positioned between a pair of neighbouring disc cutter <NUM>, by means of the shovel, released material can be loaded into a conveyor (not shown). For example the shovel can be a planar board suitable for scraping off rock deposits left on rock face.

<FIG> are for illustrating purpose, in another embodiment, depending on the amount of disc cutters <NUM>, support axis <NUM> of a disc cutter and that of an opposite disc cutter may not necessarily be in the same plane.

<FIG> illustrates one embodiment of a mining machine <NUM> for excavating hard rock, the machine comprises a main frame (chassis) <NUM> which is coupled to a pair of crawlers (or track wheels), the crawlers are driven via track gear to move the main frame within a tunnel, a support <NUM> is movably coupled to the main frame <NUM> and is actuated by linear drive <NUM> such as a hydraulic actuator to slide on the main frame <NUM> via a guide (not shown). The support <NUM> carries a pivoting mechanism <NUM> that is rotatable about a vertical axis <NUM>, the pivoting mechanism <NUM> in turn mounts an arm structure <NUM> which can be cranked or bent, the arm structure <NUM> at its distal end carries a cutter head <NUM>, optionally via a holder, a pair of actuators <NUM> such as hydraulic cylinders are coupled to the support <NUM> to rotate the pivoting mechanism <NUM> in horizontal plane, such that the cutter head <NUM> can be slew about an angle in range <NUM> to <NUM> degrees from initial position indicated as A (where the drive axis <NUM> runs substantially parallel to the longitudinal direction of the machine), to a position B.

The machine frame can be braced between the tunnel roof and floor by a plurality of jacking legs <NUM>, wherein the jacking legs are arranged on both sides of the longitudinal centre plane of the machine frame.

From <FIG> it is seen that, when the drive axis <NUM> of the cutter head is parallel to the longitudinal direction of the machine, the enveloping of the disc cutters is located in the front <NUM> relative to the rotational centre <NUM>, i.e. an angular offset <NUM> of outermost cutting edge of disc cutter is present, the angular offset <NUM> is defined by two rays starting from a rotation centre at the vertical axis <NUM>, with one ray <NUM> towards outermost cutting edge, and the other ray <NUM> perpendicular to the drive axis <NUM> of the cutter head. The angular offset <NUM> may be set in the range of <NUM> to <NUM> degrees.

It is important to maintain a free-cutting angle (or called a contacting angle) of a disc cutter at an optimal value, <FIG> illustrates a cutter head in cutting operation, the free-cutting angle <NUM> is defined by the tangent line of the rock face at contacting point to rock and a plane of outer face <NUM>, the plane of outer face <NUM> is formed by the annular cutting edge of the disc cutter. The free-cutting angle is preferably kept as a small value, it may be set in the range of <NUM> to <NUM> degrees, preferably, the free-cutting angle is in the range of <NUM> to <NUM> degrees.

During operation of a cutter head <NUM>, an individual disc cutter <NUM> is subjected to two rotational movements about two different rotational axes, i.e. in a first rotational movements about the drive axis <NUM>, and in a second rotation about support axis <NUM>. In addition, the disc cutter <NUM> is subjected to a pivoting movements about vertical axis <NUM>. The disc cutter <NUM> pierces into mining material, thereby causing cracks in the mining material and eventually creates an undercut or slot. A previous cutting path is indicated by reference symbol <NUM>, a succeeding path to be cut is indicated by reference symbol <NUM>, all shown in a horizontal plane. A disk cutter first cuts in the base rock along cutting path <NUM> to remove a free section <NUM>, a succeeding disk roller cutter comes to crush the base rock to remove a free section <NUM>. A maximal penetration <NUM> or undercut depth into the mining material, which is in radial direction with respect to the support axis <NUM>, may be set, for example, in a range between about <NUM> and about <NUM> for hard rock mining material. A cut spacing <NUM>, which is in radial direction with respect to the drive axis <NUM>, lies in a range of <NUM> to <NUM> preferably between <NUM> and <NUM>.

During cutting, the pivoting speed of the cutter arm is controlled in such a way that, the cutter ring of a succeeding disc cutter <NUM> comes into contact with the material to be removed at a point which is offset in a common horizontal plane from that of the cutter ring of the preceding disc cutter, wherein the offset corresponds to a required penetration <NUM>.

<FIG> illustrates another embodiment of a mining machine for excavating hard rock, the machine comprises a main frame <NUM>, a support <NUM> movably coupled to the main frame <NUM> via a drawer structure e.g. a rod within a sleeve, and is actuated by an actuator <NUM> to slide on the main frame <NUM>, a pivoting mechanism <NUM> carrying a cantilever arm <NUM> is mounted to the support <NUM>, a cutter head <NUM> is mounted at the distal end of the cantilever arm. In this design, the longitudinal axis of cantilever arm <NUM> is substantially perpendicular to the drive axis of the cutter head.

The pivoting mechanism <NUM> includes a rotary drive or slew drive train inside, in order to achieve a specific reduction ratio, the drive train may comprise a first stage planetary drive coupled in series to a second stage planetary drive (not shown). A motor <NUM> is provided as resource to the drive train. Jacking legs <NUM> are connected the main frame. Additional jacking legs <NUM> may be provided to support the pivoting mechanism <NUM>, optionally the additional jacking legs <NUM> may have rollers on foot. Other settings of the machine are similar to the machine of <FIG>.

In operation, the machine <NUM> is set in the required position in the tunnel, depending on needs, operating parameters such as the slewing speed of the cutter arm, rotational speed of the cutter head etc. may be set. Jacking legs <NUM> are actuated to stabilize the machine within the tunnel; then cutter heads <NUM> is rotated via the motor <NUM>, and cutter arm <NUM> is actuated to pivot about axis <NUM> to guide the cutter head to cut from position A to position B, thereafter cutter arm <NUM> is brought back to position A by pivoting of the arm in reverse direction. The support <NUM> together with the pivoting mechanism <NUM> is driven to slide forward by a distance corresponding to required sump depth, cutting is repeatedly performed from position A.

The sliding movement of support <NUM> and the succeeding cutting can be repeated many times until the maximal forward travel of the support <NUM> is achieved, then jacking legs <NUM> are retracted to engage the crawler <NUM> onto the ground. The machine <NUM> may then be advanced forward via crawler <NUM>. Jacking legs are extended again for repeating the cutting cycle.

Claim 1:
A cutter head (<NUM>) for excavating hard rock materials in rock face, comprising
a carrier (<NUM>) that is attachable to a cutter arm (<NUM>) of a cutter apparatus (<NUM>), and a drive shaft (<NUM>) rotatably supported by the carrier (<NUM>), the drive shaft (<NUM>) being rotatable about a drive axis (<NUM>) and comprising a support portion (<NUM>) for mounting disc cutters;
a plurality of disc cutters (<NUM>) mounted on the support portion (<NUM>) and configured to perform undercutting against the rock face;
each disc cutter (<NUM>) is rotatable about a respective support axis (<NUM>), the disc cutters (<NUM>) are attached on the support portion substantially in a manner that the support axes (<NUM>) of the disc cutters (<NUM>) extend to intersect with one another at the drive axis (<NUM>) at an intersection point (<NUM>) and lie within a common conical surface; characterized in that
the drive shaft (<NUM>) comprising at one end the support portion (<NUM>) for mounting disc cutters;
wherein the disc cutters (<NUM>) are of the same configuration in structure.