EP3448644B1

EP3448644B1 - Log saw machine with grinding wheels and grinding method

Info

Publication number: EP3448644B1
Application number: EP17718877.8A
Authority: EP
Inventors: Romano Maddaleni; Mario Gioni Chiocchetti
Original assignee: Fabio Perini SpA
Current assignee: Fabio Perini SpA
Priority date: 2016-04-27
Filing date: 2017-04-14
Publication date: 2020-02-12
Anticipated expiration: 2037-04-14
Also published as: ES2788683T3; ITUA20162916A1; BR112018071770A2; WO2017186520A1; CN109311177A; EP3448644A1

Description

TECHNICAL FIELD

The present invention relates to the field of machines for processing logs of web material, in particular, but without limitation, logs of paper, for instance, although without limitation, tissue paper or the like for producing toilet paper, kitchen towels and the like.

BACKGROUND ART

In many industrial fields, for the production of rolls of wound web material, logs of significant axial dimensions are produced and are subsequently cut into smaller rolls, i.e. into rolls of smaller axial dimension intended for packaging and sale. Typical examples of this kind of processing are in the field of tissue paper converting, for producing rolls of toilet paper, kitchen towels and the like. In this field, plies of cellulose material are produced by means of continuous machines and wound into so-called parent reels. These latter are then unwound and wound again in rewinding machines, to form logs, the axial length whereof corresponds to the width of the ply produced by the paper mill and is equal to a multiple of the axial length of the products to be marketed.
Then, saw machines are used to cut the logs into rolls of smaller axial dimensions, intended for packaging and marketing. Examples of saw machines of this type are disclosed in US6786808 , US5522292 , US20060000312 , US20060011015 , and WO201603125 . The saw machines for logs of wound web material, especially tissue paper, typically comprise a feeding path for the logs to be cut and a cutting head arranged along the feeding path. The cutting head comprises one or more disc-shaped cutting blades, which rotate around their axis and are also provided with a cyclical movement (a rotating or oscillating movement, for instance) to cut sequentially the logs of greater axial dimensions, fed along the feeding path, into single subsequent rolls.
The disc-shaped cutting blades are subject to wear and therefore require to be often ground. When it is ground, the disc-shaped cutting blade is gradually eroded, with a consequent reduction of its diameter. When the minimal diameter dimension is achieved, the disc-shaped cutting blade shall be replaced.
The cutting blade is usually ground by means of two grinding wheels pressed against opposite sides of a circular cutting bevel of the disc-shaped cutting blade. The grinding wheels may be motorized, i.e. driven into rotation by means of respective motors, for example electric or pneumatic motors. In other embodiments, the grinding wheels are supported idle and driven into rotation by friction between the active surface of the grinding wheel and the blade surface to be ground.
Grinding is crucial, as it affects both the useful life of the blade and the cutting quality. For example, a non-optimal repeated grinding causes a nonuniform consumption of the disc-shaped cutting blade, a consequent worse cutting quality and the deformation of the disc-shaped cutting blade, the cutting edge whereof tends to lose its round shape and to become polygonal. The useful life of a disc-shaped cutting blade significantly decreases as the cutting edge thereof becomes polygonal in shape.
WO-A-01/36151 discloses a grinding device for a log saw according to the preamble of claim 1. The grinding device comprises two grinding wheels which are arranged on opposite sides of a cutting edge of the disc-shaped cutting blade. The two grinding wheels are driven into rotation by contact friction with the disc-shaped blade and by air jets generated by air nozzles.
US-A-2006/0162522 discloses a log saw provided with three disc-shaped cutting blades. Each cutting blade is in turn provided with a grinding unit, comprised of two motorized grinding wheels.
GB-A-2171345 discloses a sharpening device for a disc knife for use in a cigarette manufacturing machine. The sharpening device is comprised of two grinding members. The grinding members are motor-driven and have different shapes and dimensions. They are arranged with their rotation axes falling outside the circular cutting edge of the disc knife. A first grinding member has an annular cup form and is provided with a grinding surface at its rim. The rim surrounds a hollow. The second grinding member is a grinding wheel, which has its axis of rotation inclined to that of the cup-shaped annular grinding member. The diameter of the grinding wheel is substantially smaller than the diameter of the grinding rim of said cup-shaped grinding member. Thus, a portion of the grinding wheel enters the a hollow of the cup formed grinding member when the grinding wheel enters the a hollow of the cup formed grinding member when the two grinding members are in contact with the two opposite sides of the cutting edge of the disc knife.
Grinding is provided for also in other fields, e.g. in food slicers. WO-A-00/40367 discloses a sharpener assembly for a food slicer. The sharpener comprises two grinding wheels. The two grinding wheels are made of different materials such that one wheel is more aggressive and the other is less aggressive and is used for truing. To properly perform their operation, the two grinding wheels are arranged with respect to the disc blade such that the speed vectors of the two wheels are oriented in opposite directions with respect to the cutting edge of the disc blade. Specifically, one of the wheels is arranged such that the relative velocity vector is oriented outwards with respect to the cutting edge.
It is therefore useful to provide improvements to the saw machines and, especially, to the grinding systems thereof, as well as to the grinding methods.

SUMMARY OF THE INVENTION

According to the invention a log saw machine is provided, comprising a feeding path for the logs to be cut and a cutting head arranged along the feeding path. The cutting head comprises a disc-shaped cutting blade rotating around a rotation axis and provided with a cyclical movement for cutting the logs. The cutting head also comprises two grinding wheels, which are arranged and configured to grind two sides of a cutting edge of the disc-shaped cutting blade, and each of which rotates around its rotation axis. The grinding wheels are so arranged, with respect to the disc-shaped cutting blade, that, in a contact area between each grinding wheel and the disc-shaped cutting blade, the grinding wheel has a velocity directed towards the inside of the cutting edge. In other words, the contact area between the grinding wheel and the respective side of the cutting edge has a velocity represented by a vector directed towards the inside of the cutting edge instead of towards the outside thereof. If this relative motion condition occurs for both the grinding wheels, a more regular grinding is provided, as well as a better cutting quality.
The grinding wheels are so arranged, with respect to the disc-shaped cutting blade, that the contact areas between the grinding wheels and the disc-shaped cutting blade are arranged between two approximately parallel planes, each of which contains the rotation axis of one of the grinding wheels. The planes are approximately parallel to a plane containing the rotation axis of the disc-shaped cutting blade.
The grinding wheels are supported idle and driven into rotation by friction with the disc-shaped cutting blade thanks to the torque generated on the grinding wheels by the friction between these latter and the disc-shaped cutting blade.
In advantageous embodiments, the two contact areas between the grinding wheels and the disc-shaped cutting blade are symmetrical with respect to a plane containing the rotation axis of the cutting blade and extending between the rotation axes of the two grinding wheels.
According to some embodiments, the two grinding wheels may be so arranged that, when they are in an operative position into contact with the disc-shaped cutting blade, the rotation axis of a first of said two grinding wheels is arranged outside the cutting edge of the disc-shaped cutting blade, and the rotation axis of the other of said two grinding wheels is arranged inside the cutting edge so as to intersect the disc-shaped cutting blade.
According to the invention, a method of grinding a disc-shaped cutting blade of a saw machine for cutting logs of web material is provided. The method is defined in claim 5.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention shall now be more apparent by following the description and accompanying drawing which shows a practical embodiment of the invention. More in particular, in the drawing:

Figure 1 is a partial schematic side view of the main members of an embodiment of a saw machine;
Figure 2 is a sectional view, according to a vertical plane, of the head of the saw machine of Figure 1, with the disc-shaped cutting blade and the grinding wheels;
Figure 3 is an enlarged cross-section of the grinding wheels near the blade cutting edge according to the plane III-III of Figure 4; and
Figure 4 schematically shows the position of the grinding wheels and the disc-shaped cutting blade according to the arrow IV of Figure 3.

DETAILED DESCRIPTION OF EMBODIMENTS

The following detailed description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Additionally, the drawings are not necessarily drawn to scale. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims.
Reference throughout the specification to "one embodiment" or "an embodiment" or "some embodiments" means that the particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrase "in one embodiment" or "in an embodiment" or "in some embodiments" in various places throughout the specification is not necessarily referring to the same embodiment(s). Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments in accordance with the appended claims.
Figure 1 schematically shows the main members of a saw machine 1, that may comprise the novel features described herein for the grinding system. It should be understood that the structure of the saw machine can be different from the one briefly described herein. For instance, different drive means can be provided for transmitting the advancing motion of the logs and of the disc-shaped cutting blade. This latter can be provided with a reciprocating motion, for instance a pivoting motion, instead of a continuous motion. Moreover, the cutting head of the saw machine may comprise more disc-shaped cutting blades.
The saw machine 1 illustrated herein comprises a feeding path schematically indicated with P, along which the logs L, that shall be cut into rolls R of smaller axial dimension, move forward. The rolls are then fed to packaging machines, not shown. The saw machine is arranged downstream of a rewinding machine and of further processing stations, not shown and known by those skilled in the art.
In some embodiments, the saw machine 1 comprises a cutting station 3, which in turn comprises a cutting head schematically indicated with number 5. The cutting head 5 may comprise an orbital head 7 rotating around the rotation axis A-A, which may be generally oriented approximately in the same direction as the feeding path P for the logs L to be cut. The orbital head 7 of the cutting head 5 carries at least one disc-shaped cutting blade 9, which can rotate around a rotation axis B-B. The rotation axis B-B of the disc-shaped cutting blade 9 may be generally oriented approximately in the direction of the axis A-A. As known by those skilled in the art, there are saw machines where the axes A-A and B-B are not perfectly parallel to each other and/or to the feeding direction along the feeding path PA for the logs to be cut. This is due to some features of the saw machine that are not relevant to the present description and relate to the relative movements between the cutting head 5 and the forward movement of the logs L to be cut.
The saw machine 1 can comprise a motor 11 providing the disc-shaped cutting blade 9 with the rotary motion, and a further motor 13, providing the cutting head 5 and the orbital head 7 with the rotary motion around the axis A-A.
The logs L can move forward according to the feeding path PA along one or more lanes parallel to one another, so as to cut simultaneously more logs and increase the productivity of the saw machine 1, as known by those skilled in the art.
The forward movement of the logs L may be provided, for instance, by means of a continuous flexible member 15, such as a chain or a belt, driven by a motor 17. Advantageously, the motors 11, 13, and 17 can be controlled by a central control unit 19, in a manner known to those skilled in the art and not described in greater detail.
The flexible member 15 can comprise pushers 16 arranged at preferably regular intervals along the extension of the flexible member 15, to push each single log L along the feeding path PA through the cutting station 3.
In some embodiments, the forward movement of the logs can be continuous, at constant or variable speed. In other embodiments, the forward movement can be intermittent. During stops, the log is cut by the disc-shaped cutting blade 9. The orbital head 7 and/or the disc-shaped cutting blade 9 can have a forward and backward movement along the path PA to cut the log L while this latter moves along the feeding path PA without stopping, as known by those skilled in the art. In some embodiments, clamps can be provided; they close during the cutting step to hold the log, thus ensuring a better cut quality, and open when the log shall move forward. Preferably, two clamps re provided: one upstream of the cutting plane, to hold the log, and one downstream of the cutting plane, to hold the part of the log that is cut to form a roll.
During operation, the rotation of the orbital head 7 around the axis A-A transmits a cyclic movement to the disc-shaped cutting blade 9, so that between subsequent cuts of the log(s) L, the log(s) L can move forwards.
Figure 2 illustrates an embodiment of the cutting head 5 and, especially, of the orbital head 7.
In this embodiment, the cutting head 5 comprises a slide 21 mounted on the orbital head 7 so as to move according to the double arrow 21 for the purposes that will be better described below. In some embodiments, the slide 21 is guided on guides carried by the orbital head 7. A gear motor 25 moves the slide 21 according to the double arrow f21. The motion can be transmitted by means of a system with a threaded bar 27 and a nut screw 29, for instance a recirculating ball screw. The nut screw 29 can be fastened to the slide 21.
In advantageous embodiments, two grinding units, indicated with 31 and 33, can be arranged on the slide 21. Each grinding unit 31, 33 comprises a respective grinding wheel for grinding the disc-shaped cutting blade 9, and a controlled approach system to move the respective grinding wheel towards the cutting edge of the disc-shaped cutting blade, so as to move periodically the grinding wheel towards the disc-shaped cutting blade 9. The grinding wheels of the two grinding units 31 and 33 are labelled 32 and 34 respectively. The grinding wheel 32 rotates around an axis C-C and the grinding wheel 34 rotates around an axis D-D. The two grinding wheels 32 and 34 are so arranged that each of them can approach one of the two sides of the cutting edge or bevel 9B of the disc-shaped cutting blade 9, so that each of them grinds one of the two sides of the cutting bevel 9B.
The two grinding wheels 32, 34 of the two grinding units 31, 33 can be equal to each other, for instance when the disc-shaped cutting blade is symmetrical with respect to its median plane, indicated with T in Figure 2. However, this is not necessary, but only preferred in some embodiments. As known, according to some embodiments the disc-shaped cutting blade 9 can have an asymmetrical bevel. In this case, the grinding wheels of the two grinding units can be different from each other and/or they can be adjusted differently from each other.
The two grinding wheels are supported in supports 36, 38 by means of shafts 40, 42. In the embodiment described herein, the shafts 40, 42, and therefore the grinding wheels 32, 34, are supported idle in the supports 36, 38; the rotation of the grinding wheels 32, 34 is therefore due to the friction between the active surface of each grinding wheel and the respective side of the cutting edge 9B on which the wheel 32, 34 acts.
Reference numbers 32A and 34A indicate the abrasive surfaces of the two grinding wheels 32 and 34, acting on the sides of the cutting bevel or cutting edge 9B. Each grinding wheel 32, 34 may have two opposite abrasive surfaces, and may be mounted in two different positions, in order to allow the wheel a longer useful life, by using the two abrasive surfaces until they are exhausted and are no longer able to grind.
Moreover, the cutting head 5 comprises a coupling for the disc-shaped cutting blade 9, indicated as a whole with number 35. The coupling 35 can comprise a mandrel or rotary shaft 37, driven into rotation by a toothed wheel or pulley 39, around which a toothed belt 41 can be driven, controlled by a motor 11 or other suitable motion source, not shown in detail.
For a better understanding of the innovative aspects of the grinding system and method described herein, reference shall be made now to Figures 3 and 4. The grinding wheel 32 is mounted on the slide 21 so that the rotation axis C-C of the grinding wheel 32 is external with respect to the disc-shaped cutting blade 9, i.e. external with respect to the edge or bevel 9B of the disc-shaped cutting blade 9. Moreover, the grinding wheel 32 is so mounted as to touch the respective side of the cutting edge 9B of the disc-shaped cutting blade 9 in the area Z32 (Figure 4). The area Z32 is arranged between a plane P containing the rotation axis B-B of the disc-shaped cutting blade and a plane P32 (Figure 4) containing the axis C-C of the grinding wheel 32 and approximately parallel to the plane P. Practically, the plane P is orthogonal to the median plane (plane T, Figure 3) of the disc-shaped cutting blade 9.
In Figure 4, F0 indicates the resultant of the friction force generated by the contact between the grinding wheel 32 and the disc-shaped cutting blade 9 acting on the grinding wheel 32. The vector F0 is tangent to the cutting edge 9B in the area Z32. It can be divided into the components F1 and F2, orthogonal to each other, wherein the component F1 is orthogonal to the radius R of the grinding wheel 32. Therefore, the component F1 gives the torque driving into rotation the grinding wheel 32 rotating according to the arrow f32 when it is in contact with the disc-shaped cutting blade 9. The component F1 of the friction force F0 is directed in the same direction as the vector representing the tangential velocity of the grinding wheel 32. As it can be easily understood from Figure 4, the direction of rotation of the grinding wheel 32 is therefore "entering" the cutting edge 9B. "Entering" means that the surface 32S of the wheel in the area Z32 moves from the edge or bevel 9B towards the axis B-B of the disc-shaped cutting edge 9. The vectorial velocity of the grinding wheel in the area of contact with the disc-shaped cutting blade 9 is therefore directed towards the inside of the cutting edge. The smaller one α of the angles between the vectors representing the tangential velocities of the disc-shaped cutting blade 9 and the grinding wheel is less than 90°.
The "entering" direction of the movement of the grinding wheel 32 with respect to the disc-shaped cutting blade 9 in the area of reciprocal contact between the grinding wheel 32 and the disc-shaped cutting blade 9 is optimal for the cleaning of the edge of bevel 9B; namely, any chips or filaments are moved away from the cutting edge 9B towards the axis of the disc-shaped cutting blade 9; in this way they do not project from the cutting edge 9B, what would negatively affect the cutting quality.
The grinding wheel 34 is so arranged as to touch the cutting edge 9B in an area Z34. The area Z34 is arranged in an intermediate position between the plane P defined above and a plane P34, approximately parallel to the plane P and containing the rotation axis D-D of the grinding wheel 34. Also in this case, F0 indicates the resultant of the friction forces between the grinding wheel 34 and the disc-shaped cutting blade 9 acting on the grinding wheel 34. The vector F0 can be divided into components F3 and F4, wherein the vector F3 (similarly to the vector F1) transmits the rotation torque to the grinding wheel 34 and is directed in the same direction as the vector representing the peripheral velocity of the grinding wheel 34 in the area Z34, the grinding wheel 34 rotating according to the arrow f34.
Therefore, similarly to what occurs for the grinding wheel 32, in the area Z34 of contact with the disc-shaped cutting blade 9 also the grinding wheel 34 has a peripheral velocity directed towards the inside of the cutting edge 9B; this avoids the formation of chips or filaments that may project outside the cutting edge 9B, and more precisely outside the joining line of the two sides defining the cutting edge 9B.
The condition described above is due to the fact that the grinding wheel 34 is arranged with the rotation axis D-D thereof close to the rotation axis B-B of the disc-shaped cutting blade 9. More in particular, the rotation axis D-D of the grinding wheel 34 is so arranged as to intersect the disc-shaped cutting blade 9, at a radial distance DR with respect to the line defining the edge of the cutting chamfer 9B.
The arrangement of the grinding wheels 32, 34 described above allows, for both the grinding wheels, such a direction of rotation that, in the contact area between the grinding wheel 32, 34 and the disc-shaped cutting blade 9, the peripheral velocity of the grinding wheel 32, 34 is directed towards the inside of the cutting edge, i.e. it has a velocity component directed radially towards the rotation axis B-B of the disc-shaped cutting blade 9, so as to have optimal grinding conditions, preventing the formation of filaments or chips projecting outside the disc-shaped cutting blade 9.
Moreover, the grinding wheels 32, 34 are in such a position that the contact areas Z32 and Z34 between the grinding wheels and the disc-shaped cutting blade 9 are near each other. Namely, the two contact areas are located between the two planes P32 and P34 (Figure 4) containing the rotation axes C-C and D-D of the two grinding wheels 32, 34. This condition is particularly advantageous as it limits bending of the disc-shaped cutting blade 9 during grinding due to the pressure of the grinding wheels 32, 34 against the disc-shaped cutting blade 9.
Preferably, the contact areas Z32 and Z34 between each grinding wheel 32, 34 and the disc-shaped cutting blade 9 are symmetrical with respect to the plane P, i.e. with respect to the plane containing the rotation axis B-B of the disc-shaped cutting blade 9 and intermediate between the rotation axes C-C and D-D of the grinding wheels 32, 34.
In Figures 1 to 4, the supports 36, 38 of the grinding wheels 32, 34 are arranged on the same side of the plane T where there is the circular line defining the edge of the cutting bevel 9B, i.e. the line formed by the intersection of the two sides of the cutting edge 9B. This is particularly advantageous for accessing and replacing the disc-shaped cutting blade 9. In other embodiments, the two supports 36, 38 may be arranged on opposite sides of the plane T. In this case, advantages may result from the possibility of moving the rotation axes C-C and D-D of the two grinding wheels 32, 34 closer to each other, consequently moving the areas Z32 and Z34 closer to each other.
Moreover, in the embodiments described above, the grinding wheels 32 and 34 are mounted idle and driven into rotation by friction between each grinding wheel 32, 34 and the disc-shaped cutting blade 9. This is particularly advantageous as it results in the grinding device being simple and reliable. The lack of rotation motors prevents the grinding wheels from malfunctioning and stopping due to faults of the motors.

Claims

A saw machine (1) for cutting logs of web material, comprising:
a feeding path (PA) for the logs (L) to be cut;

a cutting head (5) arranged along the feeding path (PA);

a disc-shaped cutting blade (9), which is mounted on the cutting head (5), rotates around a rotation axis (B-B) and is provided with a cyclic movement for cutting the logs (L);

two grinding wheels (32, 34), which are arranged and configured to grind two sides of a cutting edge (9B) of the disc-shaped cutting blade (9), and each of which rotates around its own rotation axis (C-C; D-D); wherein the grinding wheels (32, 34) are supported idle and driven into rotation by friction with the disc-shaped cutting blade (9); characterized in that the grinding wheels (32, 34) are arranged with respect to the disc-shaped cutting blade (9) such that, in a contact area (Z32, Z34) between each grinding wheel (32; 34) and the disc-shaped cutting blade (9), the grinding wheel (32; 34) has a velocity directed towards the inside of the cutting edge; the grinding wheels (32, 34) are arranged with respect to the disc-shaped cutting blade (9) such that the contact areas (Z32, Z34) between the grinding wheels (32, 34) and the disc-shaped cutting blade (9) are arranged between two planes (P32; P34), each of which contains the rotation axis (C-C, D-D) of a respective one of the grinding wheels (32; 34); said planes are approximately parallel to each other and to a further plane (P) containing the rotation axis (B-B) of the disc-shaped cutting blade (9), said plane containing the rotation axis (B-B) of the disc-shaped cutting blade (9) being arranged between the two planes (P32, P34) containing the rotation axes (C-C; D-D) of the grinding wheels (32, 34).
Saw machine according to claim 1, wherein the contact areas (Z32, Z34) are symmetrical with respect to a plane (P) substantially orthogonal to the disc-shaped cutting blade (9) and containing the rotation axis (B-B) of the disc-shaped cutting blade (9).
Saw machine (1) according to claim 1 or 2, wherein, when the two grinding wheels (32, 34) are in an operative position into contact with the disc-shaped cutting blade (9), a first (32) of said two grinding wheels (32, 34) is arranged with the rotation axis (C-C) thereof outside the cutting edge (9B) of the disc-shaped cutting blade (9), and the other (34) of said two grinding wheels (32, 34) is arranged with the rotation axis (D-D) thereof inside the cutting edge (9B) so as to intersect the disc-shaped cutting blade (9).
Saw machine (1) according to one or more of the previous claims, wherein said two grinding wheels (32, 34) are supported by two respective supports (36, 38) which are arranged on the same side of a median plane (T) of the circular blade, on which the cutting edge (9B) of the disc-shaped cutting blade (9) is arranged.
A method of grinding a disc-shaped cutting blade (9) of a saw machine (1) for cutting logs (L) of web material, the method comprising the steps of:
- placing a first grinding wheel (32) into contact with a first side of a cutting edge (9B) of the disc-shaped cutting blade (9) in a first contact area (Z32);

- placing a second grinding wheel (34) into contact with a second side of the cutting edge (9B) of the disc-shaped cutting blade (9) in a second contact area (Z34);

- positioning the first contact area (Z32) and the second contact area (Z34) between a first plane (P32), containing the rotation axis (C-C) of the first grinding wheel (32), and a second plane (P34), containing the rotation axis (D-D) of the second grinding wheel (34), said first plane (P32) and said second plane (P34) being approximately parallel to each other and approximately parallel to a further plane (P) containing the rotation axis (B-B) of the disc-shaped cutting blade (9) and arranged between the first plane (P32) and the second plane (P34);

- rotating the first grinding wheel (32) and the second grinding wheel (34) by friction between the disc-shaped cutting blade (9) and said first grinding wheel (32) and said second grinding wheel (34), around respective rotation axes (C-C, D-D) in such a direction that, in each said first contact area (Z32) and second contact area (Z34), the first grinding wheel (32) and the second grinding wheel (34) have a respective rotation velocity directed towards the inside of the cutting edge (9B).
Method according to one or more of claims 5, wherein the first contact area (Z32) and the second contact area (Z34) are symmetrical with respect to a plane (P) substantially orthogonal to the disc-shaped cutting blade (9) and containing the rotation axis (B-B) of the disc-shaped cutting blade (9).
Method according to one or more of claims 5 or 6, wherein the first grinding wheel (32) is arranged with the rotation axis (C-C) thereof outside the cutting edge (9B) of the disc-shaped cutting blade (9) and the second grinding wheel (34) is arranged with the rotation axis (D-D) thereof inside the cutting edge (9B) so as to intersect the disc-shaped cutting blade.
Method according to one or more of claims 5 to 7, wherein the first grinding wheel (32) and the second grinding wheel (34) are supported by two respective supports (36, 38) which are arranged on the same side of a median plane (T) of the circular blade, on which the cutting edge (9B) of the disc-shaped cutting blade (9) is arranged.