Patent Publication Number: US-2006000312-A1

Title: Cutting machine with a sharpening unit for a blade, sharpening method and blade for said machine

Description:
TECHNICAL FIELD  
      The present invention relates to a method to sharpen disk-shaped cutting blades with a continuous cutting edge, and in particular to sharpen disk-shaped blades destined to cut rolls of web material such as paper, tissue paper, toilet tissue, kitchen towel and the like.  
      The present invention also relates to a sharpening unit for disk-shaped blades, in particular for cutting machines destined to cut rolls of web material or the like, and cutting machines comprising said sharpening unit.  
      Moreover, the invention relates to a blade particularly suitable to be used with the method of the present invention.  
     STATE OF THE ART  
      Cutting machines are commonly used in the paper converting industry to pro-duce small rolls from logs of wound paper, which have an axial length which is a multiple of the axial length of the finished products, corresponding to the axial dimension of the reels of paper coming from paper mills.  
      The cutting machines commonly used to cut logs of paper or other wound web materials are provided with a unit rotating about an axis usually parallel to the direction of feed of the logs to be cut or slightly slanting in respect of it These logs are fed along one or more channels parallel to one another to be subjected to the action of a rotating disk-shaped cutting blade carried by the rotating unit. The disk-shaped blade rotates about an axis in turn parallel to the direction of feed of the elongated products to be cut. Traditionally, machines of this type have intermittent or continuous feed (with variable or constant speed) of the logs. Examples of cutting machines of this type are described in U.S. RE-30,598, EP-B-0507750, U.S. Pat. No. 3,213,731, EP-B-0609668, U.S. Pat. No. 5,315,907.  
      The disk-shaped cutting blades used for this purpose are usually biconical in shape. That is, they are thicker in proximity to the axis and gradually decrease in thickness from the axis towards the edge. The cutting edge is formed of a bevel symmetrical in respect of the median plane orthogonal to the axis of the tool.  
      The blade must be sharpened frequently to restore the cutting edge especially as it is produced with steels of limited hardness and toughness, such as high speed steels. Pairs of grinding wheels, motorized or more frequently drawn by the movement of the tool, are used for sharpening; these act in an approximately symmetrical manner on the two sides of the cutting bevel of the blade. The diameter of the blade is in this way gradually reduced from the original dimension to a minimum dimension of diameter, beyond which the blade must be replaced. The cutting edge becomes blunted and damaged rather quickly and must be sharpened frequently, which causes relatively rapid consumption of the blade, due to the wear caused by each sharpening operation. This makes it necessary to use high initial diameters, in order to reduce the number of replacements required and above all to amortize the cost of each blade against a sufficient quantity of cut product.  
      The dimension of the product to be cut and of the hub to support the disk-shaped cutting blade make it impossible to go below a minimum dimension of diameter.  
      The biconical shape of the tool produces a great deal of friction between the tool and the material to be cut Moreover, to produce a biconically shaped tool, a large amount of initial raw material is required, as the biconical shape is obtained principally through grinding.  
      To decrease these drawbacks blades have been designed with a cutting edge defined by two asymmetrical sides, one of which is hardened by facing the cutting profile with hard oxides. A blade of this type is described in WO-A-0021722. The purpose of this known blade is to increase the quantity of logs cut during the life of the blade, to reduce the number of sharpenings required during the useful life of the blade and to reduce the variation in blade diameter due to wear caused by sharpening. However, this blade did not attain the expected results in terms of duration, decrease in wear and reduction of sharpening frequency.  
     OBJECTS AND SUMMARY OF THE INVENTION  
      The object of the present invention is to provide a circular blade with a continuous cutting edge, which overcomes the drawbacks of prior art blades. A further object of the present invention is to produce a sharpening unit that makes it possible to sharpen the blade efficiently decreasing wear and avoiding the need for substantial excursions of the sharpening grinding wheels to compensate for wear of the blade resulting from frequent sharpening operations.  
      Yet another object of the present invention is to produce a method of sharpening that is simpler and more efficient sharpening than prior art methods, and a cutting machine that implements said method.  
      Essentially, according to a first aspect, the present invention relates to a disk-shaped blade to cut logs of wound web materials, comprising an axis of rotation and a cutting bevel, with a continuous cuffing edge, defined by a first side and by a second side, the first side having a greater radial extension, and at least said first side having surface hardening treatment. Characteristically, according to the invention, the surface treatment has a penetration depth or a thickness of at least 30 micrometers and preferably more of less equal to or greater than 80 micrometers and even more preferably equal to or greater than 90 micrometers and even more preferably equal to around 100 micrometers or more.  
      In the context of the present description and of the attached claims, by surface treatment it is understood either a treatment which provides for the penetration of atoms or molecules of a material inside the base structure of the blade (such as a nitriding or carburizing, for example), or also a deposit—on the base material forming the blade—of a layer of harder material, such as a layer of carbides, a layer of ceramic material or of other hardening material, having a suitable thickness.  
      The penetration depth of the treatment or, generally speaking, the treatment thickness, is such as to ensure that the bevel formed by sharpening has a portion integrally formed in the thickness of the surface hardening, being it provided by penetration or by deposit.  
      Advantageously and preferably, at least the first side of the bevel has a surface face hardness equal to or greater than 70 HRC (Rockwell Hardness C) and preferably equal to or greater than around 72 HRC and even up to 73 HRC.  
      Advantageously, the blade is produced in alloy steel, such as molybdenum chrome steel, and at least the surface of the first side is treated by controlled nitriding, such as by Nitreg® surface treatment.  
      Nitreg® treatment is a special thermochemical nitriding treatment for hardening the surface of steels. For a description of the process see A. M. Staines, &lt;&lt; NI - TREG - A New Development in Gaseous Nitridin &gt;&gt;, published by Nitriding Services Ltd, Telford, Shropshire, UK. Nitriding treatments of this type are described in U.S. Pat. No. 4,391,654, U.S. Pat. No. 5,228,929, US-2002/0104587A1, EP-A-1229143. The treatment in question makes it possible to attain high degrees of hardness maintaining a high level of toughness in the material treated.  
      When the blade is new, that is before being sharpened for the first time, the two sides defining the cutting bevel may have symmetrical inclinations, that is they may define the same angle in respect of the lying plane of the cutting edge. However, it is preferable for the first side to have a lesser inclination than the second side in respect of the lying plane of the cutting edge. This makes it possible, as shall become clear hereunder, to position the sharpening grinding wheels in symmetrical positions and to produce, during the life of the blade, a cutting edge symmetrical in respect of the lying plane. For example, the difference in inclination between the first and the second side is at least 1° and preferably between around 1.50° and around 2.50° and even more preferably around 2°. According to an advantageous embodiment, the first side can have a zero inclination with respect to the median plane of the blade, i.e. with respect to the lying plane of the cutting edge of the blade.  
      When the blade is new, that is before wear determined by successive sharpenings, the cutting edge may lie on a lying plane that does not coincide with the plane of the center line of the blade and, in respect of this, is moved towards the second side.  
      The disk-shaped blade may be biconical in shape, that is with a body delimited by two opposed conical surfaces, with a wide aperture. Nonetheless, according to a preferred embodiment of the invention, the body of the blade is delimited by two planes essentially parallel to each other and essentially orthogonal to the axis of rotation of the blade.  
      According to a different aspect, the present invention relates to a sharpening unit to sharpen a disk-shaped blade, with a cutting bevel with a continuous circular cutting edge, and comprising a first grinding wheel and a second grinding wheel acting on a first side and on a second side of said bevel. Characteristically, according to the invention, the first grinding wheel has a finer grain than the second grinding wheel. Moreover, the inclination of the first grinding wheel is not parallel to the respective spective first side of the bevel of the blade, while the inclination of the second grinding wheel is essentially parallel to the second side of said bevel.  
      The two grinding wheels are advantageously equipped with a movement to move them towards and away from the blade in a direction essentially parallel to their respective rotation axis. The object of this movement is to position the grinding wheels in the operating and non-operating position respectively and also to recover wear of the blade caused by the successive sharpening operations.  
      The grinding wheel with larger grain is used to perforn the actual sharpening and acts on the side of the bevel that, following initial sharpening, loses the surface hardening treatment. On the contrary, the first grinding wheel, with extremely fine grain, acts on the side of the bevel destined to preserve the surface hardening treatment and acts simply to remove any burrs from the cutting edge, while also supporting the blade to prevent flexure caused by the pressure exerted by the second grinding wheel. The two grinding wheels may start to operate simultaneously. Nonetheless, to obtain optimal operation of the sharpening unit, the finest grinding wheel starts to operate before the grinding wheel with the larger grain and leaves the operating position with a delay. Preferably, the delay with which the first grinding wheel disengages from the blade in respect of the moment at which the second grinding wheel ceases to act on the bevel of the blade is equal to at least one complete turn of the blade. This ensures that any burrs are eliminated from the cutting edge.  
      The inclination of the first grinding wheel allows it to operate only in proximity to the cutting edge, that is at the tip of the bevel, and not along the entire extension of the side of the bevel. The thickness of surface treatment on the blade, the fact that the grinding wheel is not aggressive and its angular position in respect of the side of the bevel mean that the cutting edge, that is the line of intersection of the two sides and the surface area of the blade immediately adjacent to this line remain within the thickness of the material of the blade that has been subjected to the surface hardening treatment. Independently of whether it is formed by a deposit or by the penetration of particles—e.g. by thermal treatment—in the structure of the base material forming the blade.  
      The grinding wheels may be idle and hence drawn in rotation by the rotating blade. Nonetheless, they are preferably motorized. The motorized grinding wheels may be pressed against the blade with less pressure, thus making it possible to obtain a smoother ground surface. Mixed solutions may also be used in which one grinding wheel is drawn and the other motorized or in which there are more than two grinding wheels, some motorized and others drawn.  
      Preferably, according to a particularly advantageous embodiment of the invention, the inclinations of the two grinding wheels are equal and opposite in respect of the lying plane of the cutting edge of the blade.  
      According to another aspect, the present invention relates to a cutting machine to cut logs of wound web material, comprising: a feed path of the logs to be cut; at least a disk-shaped blade rotating about an axis of rotation and having a cutting bevel, with a continuous cutting edge, said bevel being defined by a first side and by a second side, the first side having a greater radial extension in respect of the second side, and at least said first side having a surface hardening treatment; a sharpening unit for the blade, with at least a first grinding wheel acting on the first side and a second grinding wheel acting on the second side. Characteristically, according to the invention, the fist grinding wheel has a finer grain than the second grinding wheel; and the inclination of the first grinding wheel is greater than the inclination of the first side of the bevel in respect of a lying plane of the cutting edge of the blade, while the inclination of the second grinding wheel is essentially parallel to the second side of said bevel. In this way the first grinding wheel acts only on the terminal part of the bevel, that is the part nearest to the cutting edge, while the second blade acts on the entire radial extension of the bevel.  
      Advantageously, the inclination of the first grinding wheel in respect of the first side of the bevel and the thickness of-the hardening treatment may allow the cutting edge of the blade to remain within the volume that has been subjected to the hardening treatment.  
      Yet another aspect of the present invention relates to a method to sharpen a disk-shaped blade rotating about an axis of rotation, said blade having a cutting bevel, with a continuous cutting edge, said bevel being defined by a first side and by a second side, the first side having a greater radial extension than the second side, and at least said first side having a surface hardening treatment. According to the method a first grinding wheel acts on the first side and a second grinding wheel acts on the second side. Characteristically, according to the invention: the first grinding wheel has a finer grain than the second grinding wheel; the first grinding wheel is moved against the first side of the blade with an inclination slightly greater than the inclination of the first side, in respect of a laying plane of the cutting edge of the blade; and the second grinding wheel is moved against the second side of the bevel with inclination essentially corresponding to the inclination of said second side in respect of said laying plane.  
      Further advantageous characteristics and embodiments of the blade, the sharpening unit, the cutting machine and the sharpening method according to the invention are indicated in the appended dependent claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention shall now be better understood by following the description and appended drawing, which shows a non-limiting practical example of the invention. In the drawing:  
       FIG. 1  shows a summary side view of a cutting machine according to the invention;  
       FIG. 2  shows a front view according to II-II in  FIG. 1 ;  
       FIG. 2A  shows a development in the plane of the control cam in  FIG. 2 ;  
       FIG. 3  shows a side view and part section according to III-III in  FIG. 2 ;  
       FIG. 4  shows a partly enlarged section of a detail of  FIG. 3 ;  
       FIG. 5  shows a front view of a cutting blade;  
       FIGS. 6A and 6B  shows two enlarged local sections according to a radial plane of the cutting bevel of the blade in  FIG. 5 , respectively of the new blade and of the completely worn blade;  
       FIG. 7  shows a-greatly enlarged-view of the cutting bevel and of the cutting edge of the blade during use and after at least a first sharpening;  
       FIG. 8  shows an enlarged view of the arrangement of the sharpening grinding wheels of one of the sharpening units of the cutting machine;  
       FIG. 9  shows a longitudinal section of one of the two grinding wheels with the respective. operating mechanism; and  
       FIG. 10  shows a part cross-section according to X-X in  FIG. 9 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION  
       FIG. 1  schematically shows limited to its front part) a cutting machine, as a whole indicated with  1 , to which the present invention is applied. The machine has a feed path of the logs to be cut, indicated with L, which are pushed by pushers  3  secured to a flexible chain element or the like  5 , driven about a driving wheel supported by a fixed structure  7 . Only one driving wheel, indicated with  9 , is visible in  FIG. 1 , while the other is at the rear end of the cutting machine, not shown. In actual fact, as known to those saddled in the art, there are more than one flexible elements  5  in parallel to feed several rows of logs L according to parallel paths. In the example shown four channels are provided for simultaneous feed of four logs L positioned side by side.  
      The flexible elements  5  associated with the various parallel feed channels of the logs may be motorized separately from one another to stagger the movement of logs in each feed channel.  
      The number  11  generically indicates a cutting head that, by means of a support  13 , carries a rotating unit  17 . The unit  17  rotates about a horizontal axis A-A parallel to the direction fL of feed of the logs L. In the example shown three disk-shaped shaped blades  19 A,  19 B and  19 C are mounted on the rotating unit  17 , disposed at 120° from one another about the axis A-A, as can be seen in particular in  FIG. 2 . Each of the rotating disk-shaped blades  19 A,  19 B and  19 C rotates about its own axis of rotation B-B parallel to the axis A-A and to the direction of feed fL of the logs L.  
      The number  21  indicates a motor that, by means of a belt  23 , transmits rotatory motion to the rotating unit  17 . A second motor  25  is positioned on the support  13  of the rotating unit  17  and, by means of a belt  27 , supplies rotatory motion to a shaft that drives disk-shaped blades  19 A,  19 B and  19 C in rotation by means of a transmission to be described hereunder. By means of a belt  31 , a third motor  29  drives the driving wheel  9  of the rotating element  5  in rotation. As mentioned above, as several parallel channels may be provided for feeding the logs L that are cut separately to form the small rolls R, a driving wheel  9  may be associated with each channel, with its own motor unit  29  suitably controlled as a function of the angular position of the rotating unit  17 . The number  35  indicates a programmable control unit that synchronizes the angular position of the unit  17  with the feed movement of the flexible element(s)  5  acting on the motor(s)  29 .  
       FIGS. 2 and 3  show how the rotating unit  17 , drawn in rotation by the hub  17 A, internally supports three toothed wheels, positioned at 120° from one another about the axis A-A, indicated with  41 A,  41 B and  41 C. Said wheels mesh with a central toothed wheel  43  keyed onto a shaft  45  that receives its motion from the motor  25  through the belt  27 .  
      The toothed wheels  41 A,  41 B and  41 C are keyed onto respective spindles  47 A,  47 B and  47 C onto which toothed pulleys  49 A,  49 B and  49 C are in turn keyed. Each of the toothed pulleys  49 A,  49 B,  49 C transmits the motion supplied by the motor  25 , through toothed belts  51 A,  51 B,  51 C, to the rotating disk-shaped cutting blades  19 A,  19 B e  19 C.  
      As can be seen in the detail in  FIG. 4  for the blade  19 C, the toothed belt  51 A,  51 B,  51 C transmits motion to a toothed pulley  53 A,  53 B,  53 C keyed onto an axle  55 A,  55 B,  55 C, on the opposite end of which the respective disk-shaped blade  19 A,  19 B, l 9 C is keyed.  
      Each of the shafts is supported by bearings  57  in a respective sleeve  59 A,  59 B,  59 C sliding on sliding bearings  61  mounted in a respective seat  63 A,  63 B,  63 C provided in the rotating unit  17 . The angular movement about the axis B-B of each sleeve  59 A,  59 B,  59 C is prevented by a tab  58  integral with the respective sleeve, cooperating with wheels  60  supported idle in the sliding seat of the sleeve.  
      At the rear, that is on the opposite side in respect of the disk-shaped blade  19 C, each sleeve  59 A,  59 B,  59 C has an enlarged area  65 A,  65 B,  65 C that houses the toothed pulley  53 A,  53 B,  53 C respectively, and mounted idle on which is a wheel  67 A,  67 B,  67 C that constitutes the feeler for a fixed cam  71  extending in an arc of circumference, shown in particular in  FIG. 2  and in its development in the plane in  FIG. 2A .  
      The arc of circumference along which the cam  71  extends has its center on the axis A-A of rotation- of the rotating unit  17  and extends in the lower part of the path of each disk-shaped blade  19 A,  19 B,  19 C, i.e. in the zone in which the blade is inserted into the product to be cut.  
      Through the effect of the cam  71  and of the feeler  67 A,  67 B,  67 C each sleeve  59 A,  59 B,  59 C associated with the respective disk-shaped blade  19 A,  19 B,  19 C travels with alternate motion according to the double arrow fl. Consequently, the respective disk-shaped blade  19 A,  19 B,  19 C are provided with the same motion. The movement according to the arrow f 1  is parallel to the direction of feed of the logs L or other elongated products to be cut. Contact of the feeler  67 A,  67 B,  67 C with the annular cam  71  is ensured by an arrangement of Belleville springs  72 A,  72 B,  72 C that act between the rotating unit  17  and the enlarged portion  65 A,  65 B,  65 C of the sleeve  59 A,  59 B,  59 C.  
      Along the lower arc of the circular trajectory taken by each disk-shaped blade  19 A,  19 B,  19 C, the blade is pushed forwards by the annular cam  71  that overcomes the compression strength of the respective Belleville springs  72 A,  72 B,  72 C. In this way the blade that is operating at that time, i.e. inserted in the material constituting the log(s) L to be cut, moves forward following the forward motion of the logs L along the feed path.  
      The forward movement is controlled by the ascending ramp  71 A of the cam  71  (see  FIG. 2A ). The forward motion starts before the respective blade  19 A,  19 B,  19 C penetrates the material constituting the first of the logs to be cut, so that at the time in which contact with the blade srts it is already moving forward at the same speed as the material to be cut according to the arrow fL.  
      When the blade emerges from the log(s) L, it is made to reverse by the springs  72  that maintain the feeler  67  in contact with the descending ramp  71 D of the circular cam  71 , which may be limited to a portion of the circumference followed by the feeler  67 C, as in the upper stretch of travel the blade  19 A,  19 B or  19 C does not require to follow the movement of the roll. Forward motion of the logs L is controlled in the same way described in EP-B-0507750.  
      The considerable length of the belts  51 A,  51 B and  51 C provides the toothed pulley  53 A,  53 B or  53 C with sufficient freedom of movement in the axial direction, so that the respective disk-shaped blades may advance and reverse without being obstructured by mechanical transmission of motion from the central axis. The axial extension of the toothed pulleys  53 A,  53 B,  53 C and  49 A,  49 B,  49 C may be greater than the height of the respective belts  51 A,  51 B,  51 C to allow any minor sliding of the belts on the driving pulleys.  
      Integral with each sleeve  59 A,  59 B,  59 C is a support  73 A,  73 B,  73 C, each carrying a sharpening unit  80  comprising a pair of grinding wheels  81 ,  83  to sharpen the respective rotating disk-shaped blades  19 A,  19 B,  19 C. Each grinding wheel of the pair of grinding wheels  81 ,  83  associated with each blade acts on one of the two sides of the cutting bevel of the blade, which will be described in detail with reference to FIGS.  5  to  7 .  
      The grinding wheels  81  and  83  may be motorized grinding wheels, that is drawn in rotation by specific motors such as pneumatic motors, although it is also possible to use grinding wheels mounted idle and drawn in rotation through the effect of contact friction with the disk-shaped blade. Feed of compressed air to the actuators associated with. the three pairs of grinding wheels  81 ,  83  may be supplied by an axial rotating distributor, not shown and of a per se known type.  
      The two grinding wheels  81 ,  83  of each sharpening unit  80  are also provided with a movement parallel to their axis of rotation to be brought alternately into contact with and moved away from the respective rotating disk-shaped blade, as sharpening is not continuous but performed only at regular intervals as the blade becomes blunt and thus requires sharpened. The structure of the mechanisns that make the grinding wheels rotate and cause them to move towards and away from the respective blade will be described with reference to  FIGS. 9 and 10 . The arrangement of the two grinding wheels  81 ,  83  of each sharpening unit  80  is shown in particular in the enlarged detail in  FIG. 8 .  
      Each of the blades  19 A,  19 B,  19 C is designed as shown in FIGS.  5  to  7 , which show any one of the three blades  19 A,  19 B,  19 C, indicated simply with the reference  19 .  
      The blade  19  has a disk-shaped body delimited by two flat faces  201 A,  201 B parallel to each other, and a circular cutting edge  203 . Therefore, essentially it has a continuous thickness in the range of 1.54 mm and preferably between 2 and 3 mm, in particular for example 2.5 mm.  
      The cutting edge  203  represents the final edge of a cutting bevel, indicated as a whole with  205 . This cutting bevel is delimited by two sides  207  and  209 . The first side  207  extends radially (i.e. in the direction of the radius of the disk-shaped blade) to a greater extent that the radial extension of the second side  209 , in any condition of wear of the blade.  
      At least the side  207  has been subjected to surface hardening treatment. IN a practical embodiment described here by way of example, said treatment is a controlled nitriding thermal treatment, such as in particular Nitreg® treatment. In actual fact, the entire surface of the blade may be subjected to this treatment, as it is simpler and less expensive to perform complete treatment than to mask the parts of the blade that do not require to be treated. Alternatively, the entire surface of the blade may be subjected to treatment, with the exception of the side of the cutting bevel on which the grinding wheel with the largest grain, destined to perform actual sharpening, acts. In this way the duration of the grinding wheel may be extended. The controlled thermochemical nitriding treatment penetrates the base material of the blade for a depth T ( FIG. 7 ), for example of around 100 micrometers. Alternatively, as set forth herein before, the surface hardening treatment can be provided in the form of a deposit of a harder material on the surface of the blade, or even in a depressed area of the blade body provided for the purpose of being filled up with said deposit.  
       FIG. 6A  shows the bevel of the blade before the first sharpening operation with continuous surface treatment along its entire surface. In this initial condition, the cutting edge  203  lies on a lying plane PG parallel to the median plane PM of the blade, which is represented by the plane orthogonal to the axis B-B of rotation and equidistant from the faces  201 A,  201 B of the body of the blade. The lying plane PG of the cutting edge  203  is shifted, in respect of the median plane PM of the blade, towards the second side  209  of the bevel  205 .  
      Still in the condition of a new blade, represented in  FIG. 6A , the side  207  is defined by a conical surface with axis coinciding with the axis B-B of rotation of the blade and with an inclination a in respect of the lying plane PG of the cutting edge  203 . The angle α may for example be around 8°. The side  209  also has a conical form coaxial to the axis B-B and an inclination β in respect of the plane PG. The angle β is slightly greater than the angle α and may be for example equal to 10°. The possibility is not excluded, however, that the angle α is 0°.  
      In  FIG. 6B  the blade is shown in its condition of maximum wear. The sides  207 ,  209  still have the same inclination, although the side  207  now extends in a radial direction for less than the side  209 . Moreover, the lying plane PG of the cutting edge  203  is moved in respect of the median plane PM towards the first side  207  and not towards the second side  209 . As the relative dimensions of the two sides vary as the blade becomes worn, unless otherwise specified in the present text and in the appended claims, reference is generally made to the dimensions of the new blade, that is before wear caused by initial sharpening.  
      The blade is produced in molybdenum chrome steel, such as X150CrMo12 steel, which with Nitreg® treatment even reaches a hardness equal to 72-73 HIRC for the penetration depth of the controlled nitriding treatment.  
      As shown in particular in the enlarged detail in  FIG. 8 , the two grinding wheels  81  and  83  are disposed with equal inclination and contrary in respect of the lying plane PG of the cutting edge  203  of the blade  19 , that is in respect of a plane orthogonal to the axis B-B of rotation of the blade  19 . More specifically, the two grinding wheels are inclined by an angle β in respect of the lying plane of the cutting edge of the blade. This means that the grinding wheel  83 , which acts on the side  209  inclined by an angle β in respect of the plane PG, operates parallel to the side and performs the actual sharpening of the blade. Removal of material from the side of the blade by the second grinding wheel  83  does not alter the conical shape of the side  209  of the bevel and its inclination in respect of the original inclination.  
      On the contrary, the first grinding wheel  81 , which acts on the side  207  of the bevel  205 , only touches the side in the area nearest the cutting edge, due to the difference in inclination between the side  207  (inclined by an angle ax in respect of the plane PG) and the grinding wheel (inclined by an angle β in respect of this plane). The contact conditions between the sides of the bevel  205  and the two grinding wheels are shown in the enlargement in  FIG. 7 , where the two grinding wheels  81 ,  83  are indicated with a dashed line. As can be seen in  FIG. 7 , due to the slight difference between the inclination of the side  207  and of the grinding wheel  81 , and due to the considerable depth T of the surface hardening treatment, the cutting edge  203  is produced wholly in a thickness of the material of the blade  19  subjected to this treatment. The cutting edge  203  and the portions of the sides of the bevel immediately lately adjacent to this cutting edge always remain within the thickness that has been subjected to hardening notwithstanding the amount of wear on the blade. Therefore, the cutting edge is hardened on both sides. Moreover, thanks to the symmetrical lay-out of the grindiig wheels  81 ,  83 , it has a symmetrical section in respect of its lying plane PG, with consequent advantages in terms of dynamic stresses on the blade.  
      The two grinding wheels  81  and  83  have markedly distinct abrasive characteristics. In fact, as mentioned hereinbefore, the second grinding wheel  83  is utilized for the actual sharpening operation and consequently has a grain size suitable for this purpose. On the contrary, the function of the grinding wheel  81  is to support the blade against the stresses exerted by the grinding wheel  83  and to eliminate any burrs produced along the cutting edge  203  by said grinding wheel  83 , although it does not actually perform any sharpening operations, but merely polishes the bevel. Therefore, it will have a much finer grain size than the grinding wheel  83  and will not abrade the surface layer of the side  207  that has been subjected to the hardening treatment, except to a negligible extent and only in proximity of the cutting edge, that is at the tip of the bevel  205 .  
      Typically, with reference to DIN standards and ISO 6106-1979 standards, the grinding wheel  81  may have diamond grains or equivalent and have an &lt;&lt;extremely fine&gt;&gt; grain, that is from 7 to 46 (ISO standards), and preferably in proximity or equal to the minimum value 7, corresponding to a dimension of the sieve from 37 to 44 micrometers. The grinding wheel  83  may, on the contrary, be produced with the same type of abrasive and &lt;&lt;fine&gt;&gt; grain according to DIN and ISO 6106-1979 classification, that is between 45 and 91 (ISO standards), corresponding to screen meshes with dimensions between 53 and 74 micrometers. Preferably the grain size of this grinding wheel is around 70-80 (ISO).  
      With this arrangement wear of the blade caused by sharpening is very limited and a considerably long useful life of the blade is obtained, greater than the useful life of traditional grinding wheels (in terms of number of cuts made), with a limited variation in the total diameter of the blade, for example of about 15-20 mm for blades with initial diameters usually between around 500 and 600 mm.  
      In addition to a lower cost for expendable materials, this also has the advantage of eliminating the need to provide the grinding wheels with a movement to move them gradually towards the axis of the blades in order to recover wear, and to adjust the position of the blade in respect of the axis of rotation of the unit carrying it, as the variation in diameter resulting from wear can be recovered by the simple movement towards the blade and away there from, with which the grinding wheels are periodically brought in the operating and non-operating position respectively.  
       FIG. 9  shows a longitudinal section of one of the grinding wheels  81  and of the relative axial supporting and traversing and rotation system. The grinding wheel  83  is mounted and rotation and traverse movement are controlled in the same manner.  
      The grinding wheel  81  is keyed onto a shaft  85  supported by bearings  87  in a bushing  89 . The bushing slides on the sliding bearings  91  inside a supporting sleeve  93  connected integral with the support  73 C. At the opposite end in relation to the position of the grinding wheel  81  the shaft  85  is connected to a hollow shaft  95  coupled by a spline coupling  97  to the motor shaft  99  of a pneumatic or equivalent motor  101 .  
      The bushing  89  has a helical groove  103  that extends through an arc of helix extremely reduced and inclined greatly in respect of the axis C-C of the shaft  85  of the grinding wheel  81 . A wheel  105  mounted idle on a spindle  106  supported by the support  93  engages in the helical groove  103 . The arrangement of the groove  103  and of the wheel  105  may be inverted, with the groove integral with the supporting sleeve  93  and the wheel integral with the bushing  89 .  
      With this arrangement angular oscillation about the axis C-C of the bushing  89  causes it to slide axially along the axis C-C through the effect of the wheel  105  that acts as a tappet inside the helical channel  103  that acts as a desmodromic cam.  
      Angular movement about the axis C-C of the bushing  89  is imparted by a pair of piston-cylinder actuators  108 A,  108 B parallel to each other, visible in particular in the section in  FIG. 10 . The cylinders of these actuators are integral with the supporting sleeve  93 , while the rods extend inside the sleeve and their ends rest on a leveled surface  110  produced on the bushing  89 . By extending one of the two actuators  108 A,  108 B and retracting the other this causes oscillation of the bushing  89  about the axis C-C and consequently axial movement of the bushing and of the grinding wheel supported by it.  
      With an arrangement of this type it is possible to control, with great precision, the pressure that each of the two grinding wheels  81 ,  83  exerts on the respective side of the corresponding disk-shaped blade  19 A,  19 B,  19 C. This is obtained by controlling the pressurized fluid inside the actuators  108 A,  108 B, In this way sharpening of the blades can be controlled accurately, limiting wear and at the same time maintaining optimal sharpening.  
      It is understood that the drawing merely shows a non-limiting example of the invention, which may vary in shapes and arrangements without however departing from the scope of the concept on which the invention is based. Any reference numbers in the appended claims are provided to facilitate their reading with reference to the description hereinbefore and the appended drawings, and do not limit the scope of protection.