Patent Publication Number: US-2006011015-A1

Title: Sharpening unit and cutting machine comprising at least one blade and said sharpening unit

Description:
DESCRIPTION  
      1. Technical Field  
      The present invention relates to a cutting machine for cutting elongated products, such as and in particular logs of web material for the production of small rolls destined for packaging and sale.  
      More specifically, the invention relates to a cutting machine with at least a cutting blade and at least a sharpening unit associated with said cutting blade.  
      The invention also relates to a sharpening unit for cutting machines or other machines provided with a blade that must be constantly or periodically sharpened.  
      2. State of the Art  
      In the paper converting industry rolls or logs of substantial axial length are produced by winding a predetermined quantity of paper, for example tissue paper, to subsequently produce small rolls of toilet tissue, kitchen towels and the like. For this purpose the logs are cut orthogonal to their axis and divided into a plurality of small rolls of a suitable length, which are then packaged for distribution and sale. The logs are cut by means of special “cutting” machines, which have one or more cutting blades and one or more sharpening-units for each blade.  
      Analogous requirements are found in other technological sectors, where it is necessary to cut an elongated product into smaller portions, in particular logs of wound web material.  
      U.S. Pat. No. 3,213,731 describes a cutting machine for logs of web material, wherein a disk-shaped blade rotates about its axis supported by a unit in turn rotating about a principal axis parallel to the direction of feed of the logs to be cut, which are fed along a feed path towards the cutting area.  
      An analogous cutting machine, but with two disk-shaped cutting blades, is described in U.S. Pat. No. RE 30,598. In this case the axis of rotation of each disk-shaped blade is parallel to the axis of the logs to be cut, but skew in respect of the principal axis of rotation of the unit carrying the blades, to obtain a component of motion of the blade in a direction parallel to the direction of feed of the logs to be cut, so that these can advance with continuous movement at constant speed. A sharpening unit with two sharpening grinding wheels for each blade periodically sharpens the respective blade that loses its cutting edge.  
      EP-A-507750 describes a cutting machine wherein the rotating unit carrying the disk-shaped blade(s) is provided with alternate translatory movement to allow the logs to be cut to move forwards with continuous movement. The feed speed of the logs is variable and not constant, to obtain a series of advantages in terms of flexibility and reduction in stresses and in the overall dimensions of the machine. In this case too a sharpening unit is provided, with two grinding wheels to restore the cutting edge of the blades.  
      EP-A-609668 describes a cutting machine with a rotating unit carrying two disk-shaped blades rotating about respective axes parallel to the logs to be cut, but skew in respect of the axis of rotation of the rotating unit. The logs are fed at a variable speed as in EP-A-507750 to obtain the same advantages of flexibility.  
      EP-A-0555190 describes a cutting machine with a helical cutting blade and a sharpening unit with two grinding wheels.  
      U.S. Pat. No. 5,038,647 describes a cutting machine that uses a band blade rather than a disk-shaped blade, particularly suitable for cutting rolls with a large diameter. Two sharpening units with different functions are associated with the blade. A first sharpening unit, with motorized grinding wheels, produces and sharpens the principal bevel of the blade, while a second unit with idle grinding wheels keeps a counter-bevel or secondary bevel sharpened.  
      WO-A-0136151 describes a sharpening unit for the blade of a cutting machine, with tools to dress the grinding wheels.  
      In prior art machines the grinding wheels are carried by a grinding wheel unit that is gradually moved towards the blade to be sharpened in order to offset the reduction in diameter (in the case of disk-shaped or helical blades) or in width (in the case of band blades) due to wear caused by repeated sharpening operations. The movement towards the blade is set by the operator normally as a function of the number of sharpenings performed on the blade. In other words, the grinding wheel unit is moved towards the blade by a predetermined extent after a predefined number of sharpening operations, assuming that this corresponds to a wear and thus a reduction in the dimension of the blade that are always constant. The movement towards the blade is calculated so that contact is always guaranteed with sufficient pressure of the grinding wheel on the blade, even if there should accidentally be more wear than expected. This means that there is often more pressure of the grinding wheels on the blade than necessary and consequently also excessive wear on the blade. On the contrary, there may be insufficient sharpening pressure, caused by the blade and the grinding wheels not having been moved close enough together. In this case sharpening is not performed efficaciously.  
      Moreover, due to the rigidity of the grinding wheel unit the pressure with which the two grinding wheels operate on the two sides of the blade is not equal, due to unavoidable errors in positioning, tolerances and any uneven wear on the grinding wheels.  
     OBJECTS AND SUMMARY OF THE INVENTION  
      The object of the present invention is to provide a sharpening unit, in particular although not exclusively for cutting machines to cut elongated products, which allows more efficient sharpening in respect of prior art sharpening units.  
      Another object of the invention is to provide a cutting machine to cut products, especially although not exclusively logs of web material, comprising at least one particularly efficient sharpening unit and which on the one hand sharpens the blades accurately and on the other causes limited wear on the blade(s).  
      For this object, according to a first aspect of the present invention, a sharpening unit is provided comprising a grinding wheel unit with at least two opposed grinding wheels to act on two sides of a blade, characterized in that said grinding wheel unit is equipped with at least a degree of freedom to center the grinding wheels in respect of a lying surface of the portion of the cutting edge of the blade on which the grinding wheels act This allows a balanced and uniform sharpening action on the two sides of the blade. Moreover, when the grinding wheel unit is equipped with a movement towards the blade to recover any decreases in the dimension of the blade caused by wear, with self-centering of the grinding wheels the pressure exerted can be controlled more accurately, avoiding pressures and thus excessive wear.  
      The blade can be a flat disk-shaped blade, in which case centering is performed in practice in respect of a lying plane of the cutting edge. However, the blade can also have other forms, for example it can extend helically with a corresponding helical form of the cutting edge. In this case centering of the two grinding wheels takes place in respect of the lying surface of the portion of cutting edge or cutting bevel of the blade on which the grinding wheels are temporarily acting and this surface can vary according to the position of the grinding wheels along the blade. In the case of band blade, the lying surface of the cutting edge, in respect of which the grinding wheels are centered, is a plane parallel to the portion of the band forming the blade on which the area of the cutting edge is found that is instantaneously sharpened.  
      A further aspect of the present invention relates to a cutting machine for cutting elongated products, comprising: at least one path for the products to be cut; at least one device to feed the products along said path; at least one blade provided with a cutting movement to cut said products; at least one sharpening unit for said blade, which comprises a grinding wheel unit with at least two opposed grinding wheels to act on two sides of said blade. According to the invention, the machine is characterized in that the grinding wheel unit is provided with at least a degree of freedom to center the grinding wheels in respect of a lying surface of the portion of the cutting edge, that is of the cutting bevel of the blade on which the grinding wheels act. The grinding wheel unit can be taken to a fixed position in respect of the blade, when the dimensions of the latter vary slightly due to wear and said wear can be recovered for example by moving the grinding wheels towards the blade without also moving the grinding wheel unit. Nonetheless, the sharpening unit normally comprises a system to move the grinding wheel unit towards the blade along a direction of forward movement, to recover wear on the blade. In this case, self-centering of the grinding wheels is particularly important and advantageous as it prevents the onset of excessive sharpening pressures, or—on the contrary—conditions of insufficient pressure and thus insufficient sharpening.  
      According to a particular embodiment, the grinding wheel unit is provided with a further degree of freedom, partly restricted, to center the grinding wheels in respect of the blade. Partly restricted degree of freedom is intended as possible movement restricted, for example, through the effect of a return spring and/or an actuator that limits the freedom of movement of the grinding wheel unit according to this degree of freedom. For example, the grinding wheel unit is free to move in one direction, but its movement is limited in the other direction, or the movement is contrasted by a return spring. This guarantees that the movement according to this further degree of freedom always brings the grinding wheels into contact with the blade to be sharpened, preventing movement away from the cutting edge.  
      According to an advantageous embodiment, this second degree of limited or restricted freedom is represented by the fact that the grinding wheel unit can rotate or oscillate about an axis of oscillation. In this way the grinding wheel unit revolves about an axis of oscillation disposed generically in an intermediate position between the axes of rotation of the grinding wheels and in substance lying on the lying plane of the portion of cutting edge on which the grinding wheels acts. When the blade has a cutting edge that does not lie on the plane but on a lying surface of a different shape, such as the case of a helical blade, the axis of oscillation can lie on a plane that approximates the lying surface of said portion of cutting edge.  
      The grinding wheels are generally disposed with their respective axes of rotation skew. According to a preferred embodiment of the invention, the axis of oscillation of the grinding wheel is advantageously disposed in a position of minimum distance between said axes of rotation. In practice, the axis of oscillation can also be the axis of symmetry of the grinding wheels, i.e. these are disposed in a substantially symmetrical way in respect of the axis of oscillation. The movement of the grinding wheel unit about the axis of oscillation must be restricted, so that the grinding wheels are effectively stressed to come into contact with the blade to be sharpened, rather than tending to move to a non-operating position. For this purpose an elastic return element, an actuator element or another device or means to control the pressure may be provided with which the grinding wheels are pushed against the blade. In substance, therefore, oscillation of the grinding wheel unit represents a further degree of freedom in the movement of the grinding wheel unit, although this movement is not strictly completely free, but restricted so that it takes place in the direction that brings the grinding wheels effectively into the operating position against the blade.  
      The axis of oscillation of the grinding wheel unit may be essentially parallel to the direction of feed of the grinding wheel unit in respect of the blade and be essentially orthogonal to the direction of feed of the products to be cut towards the blade.  
      According to a further particularly advantageous characteristic of a possible embodiment of the invention, the grinding wheel unit is free to translate along a direction of translation not parallel to the lying plane of the blade, to center the grinding wheel in respect of the lying plane, that is the median plane of the blade. The direction of translation is in practice essentially orthogonal to the lying plane of the blade and preferably approximately substantially parallel to the direction of feed of the products to be cut. The axis of oscillation of the grinding wheel unit is therefore advantageously orthogonal to the direction of translation of the unit. According to this preferred embodiment of the invention, therefore, the grinding wheel unit has a first degree of freedom consisting in the fact that it can translate in the direction of translation, and a second degree of freedom consisting in the fact that it can oscillate about the axis of oscillation, the latter movement being limited or restricted in the manner and for the reasons explained above.  
      The machine according to the invention may have one or more blades. Moreover, a single sharpening unit or even more than one sharpening units may be associated with the blade or with each blade. In this case the two or more sharpening units advantageously have different characteristics and functions. For example, a first unit can have motorized grinding wheels to sharpen the principal bevel of the blade and the second can have idle grinding wheels to sharpen the counter-bevel of the blade. The grinding wheels of the two units can typically have different inclinations. One blade that has two sharpening units to sharpen bevel and counter-bevel (or secondary bevel) is described in U.S. Pat. No. 5,038,647. In the case of a blade with a single sharpening unit, this may have motorized grinding wheels or idle grinding wheels, which are drawn in rotation by frictional force with the blade.  
      Further advantageous features and embodiments of the invention are set forth in the appended dependent claims, and shall be described hereunder with reference to some examples of embodiment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention shall be better understood by following the description and the accompanying drawing, showing non-limiting practical embodiments of the invention.  
      More specifically, in the drawing:  
       FIG. 1  shows a schematic side view of a cutting machine for cutting rolls of web material, with a rotating unit carrying a disk-shaped blade, to which a sharpening unit according to the present invention is applied;  
       FIG. 2  shows a front view and partial section of the sharpening unit in a first embodiment, according to II-II in  FIGS. 1 and 3 ;  
       FIG. 3  shows a side view and partial section according to III-III in  FIG. 2 ;  
       FIG. 4  shows a plan view according to IV-IV in  FIG. 2 ;  
       FIG. 5  shows a partial side view of a sharpening unit in a different embodiment;  
       FIG. 6  shows a plan view according to VI-VI in  FIG. 5 ; and  
       FIG. 7  shows a section according to VII-VII in  FIG. 5 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION  
       FIG. 1  schematically shows a cutting machine according to the present invention.  
      In this case it is a cutting machine for rolls of paper or other wound web material, wherein the cut is performed by a-disk-shaped blade rotating about its axis, carried by a unit in turn rotating about a principal axis of rotation, parallel or approximately parallel to the direction of feed of the rolls to be cut. Advantageously, a sharpening unit of the type shown in FIGS.  5  to  7  with motorized grinding wheels is applied to a machine of this type.  
      However, a sharpening unit with idle grinding wheels, of the type shown in FIGS.  2  to  4 , may be applied (if need be in combination with or alternatively to it). The sharpening units, which shall be described hereunder with particular reference to FIGS.  2  to  7 , may also be applied to different machines, such as cutting machines with a helical blade or with a band blade. It must therefore be understood that the machine shown in  FIG. 1  must be intended purely as an example of a possible machine to which sharpening units according to the present invention may be applied.  
      Moreover, the sharpening grinding wheels of the unit shown in FIGS.  2  to  4  may also be motorized grinding wheels and, on the contrary, the grinding wheels of the sharpening unit shown in FIGS.  5  to  7  may be idle grinding wheels.  
       FIG. 1  schematically shows (limited to its front part) the cutting machine as a whole, indicated with  1 . 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 driving wheels 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 from prior art, there may be more than one flexible element  5  in parallel to feed several rows of logs L according to parallel paths.  
      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, a disk-shaped blade  19  is mounted on the rotating unit  17 , rotating about its own axis of rotation B-B parallel to the axis A-A and to the direction of feed ff, of the logs L. Two or more disk-shaped blades rotating about their axes of rotation distributed about the axis A-A may be provided on the rotating unit  17 . In a per se known way the rotating unit  17  can be equipped with an alternate translatory movement parallel to the direction if,, or the blade  19  can be provided individually with this movement and in this case translate in respect of the unit  17 . In either of these cases the logs may be fed with continuous rather than intermittent movement.  
      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 the rotating disk-shaped blade  19  in rotation. By means of a belt  31 , a third motor  29  drives the guiding wheel  9  of the continuous flexible element  5  in rotation. As mentioned above, as several parallel channels may be provided for feed of the logs L that are cut separately to form the small rolls R, a guiding 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 forward movement of the flexible element(s)  5  through the motor(s)  29  with the angular position of the rotating unit  17  driven in rotation by the motor  21 .  
      A sharpening unit generically indicated with  50  is disposed on the rotating unit  17 , to sharpen the blade  19 . The sharpening unit  50  has two grinding wheels  51  and  53 , which act on two sides of the cutting edge of the blade  19 .  
      FIGS.  2  to  4  show a first embodiment of the sharpening unit  50 . In this embodiment idle grinding wheels are used, which are drawn in rotation through the effect of the frictional force exerted during contact between each grinding wheel and the blade  19 . The sharpening unit  50  has a pair of plates  55 ,  57  secured to the rotating unit  17  (or to another part of the cutting machine if this does not have a rotating unit, for example to the supporting frame of a band blade). Between the two plates  55 ,  57  bars  59  with a circular section extend to form sliding guides for the same number of bushings  61 . The bushings are integral with a carriage indicated as a whole with  63 .  
      The carriage  63  moves along the guides formed by the bars  59  according to the arrow f 63  to move gradually against the axis B-B of the blade  19 , in order to hold the grinding wheels in the operating position offsetting the decrease in dimension of the blade  19  due to wear caused by sharpening.  
      Movement of the carriage  63  according to the arrow f 63  is controlled by a free wheel mechanism  65  keyed on a threaded bar  67  parallel to the guide bars  59 . A nut screw  69  integral with the carriage  63  engages with the threaded bar  67 . Rotation of the free wheel mechanism  65  is controlled by a piston-cylinder actuator  71  with a short stroke that acts on a bracket  73  integral with the free wheel mechanism  65 . The number  75  indicates a return spring of the bracket  73 . Each stroke to extend the actuator  71  causes the threaded bar  67  to rotate by an angular step and therefore a forward movement by a controlled extent in the direction of the arrow f 63  of the carriage  63 .  
      A cantilevered bracket  77  is integral with the carriage  63  and supports a pair of guides  79  essentially parallel to the axis B-B of the blade  19  and essentially orthogonal to the direction f 63  of translation of the carriage  63 . A slide  81  that supports a grinding wheel unit  80  is free to translate along the guides  79 . A shaft  85  is supported idle by bearings  83  inside the slide (see in particular  FIG. 2 ). The shaft  85  is free to rotate about its axis C-C, parallel to the axis of the threaded bar  67  and therefore to the direction f 63  in which the carriage  63  and the sharpening grinding wheels move against the blade  19 .  
      Fixed to the bottom of the shaft  85  is a plate  87  that carries integrally secured to it two blocks  89  supporting the grinding wheels  51 ,  53 . As shown in particular in  FIG. 3  for the grinding wheel  53 , the grinding wheels are carried by spindles  91  supported idle by bearings  93  in the blocks  89 . Rotation of the grinding wheels is produced by the frictional force between them and the sides of the blade  19 .  
      While the plate  81  is completely free to translate along a. direction of translation according to the double arrow f 81  parallel to the guides  79 , the plate  87  carried by the shaft  85  is elastically stressed in a predetermined angular position (defined by a stop schematically indicated in  FIG. 2  with  88 ) by a pulling spring  95 , the ends of which are fixed on one side to the plate  87  and on the other to a suitable point of the slide  81  (for example in the point  90  indicated in  FIG. 3 ). Alternatively, the spring  95  could be fastened to a fixed point in respect of the bracket  77 . Coupling to the slide  87  is preferable as in this case the stress of the spring does not tend to produce flexure of the blade.  
      With this arrangement the grinding wheel unit  80  is provided with a degree of freedom along the direction of translation f 81  and with a degree of freedom limited by the presence of the pulling springs  95 ) constituted by the fact that the plate  87  can rotate. The grinding wheels  51 ,  53  integral with the grinding wheel unit  80  are thus provided with a double movement that allows them to be centered in respect of the lying plane of the blade  19 , that is the median plane of the blade, or in any case the lying plane of the cutting bevel. The first movement is a movement according to the guides  79  in a direction orthogonal to the lying plane of the blade and, therefore, in the layout of the machine in  FIG. 1 , parallel to the direction fL of feed of the products L to be cut. The second movement is an oscillatory movement about the axis C-C orthogonal to the axis B-B of the blade, and therefore to the direction of feed of the products. As can be seen in particular in  FIGS. 2 and 3 , the axis C-C lies on the median plane of the blade, or more generically on the lying plane of the cutting edge of the blade, in an intermediate position between the two grinding wheels  51 ,  53 . More specifically, the axis C-C is in a barycentric position in respect of the axes A 1 -A 1  and A 2 , A 2  of the two grinding wheels  51 ,  53 .  
      Thanks to this arrangement the grinding wheels can be centered on the blade  19  and the pressure they exert on the blade can be controlled to prevent excessive pressure. In fact, when the grinding wheel unit  80  is moved, through the action of the free wheel mechanism  65 , according to the arrow f 63 , towards the axis B-B of the blade  19  by a predetermined extent, the grinding wheels  51 ,  53  react against the blade  19  which wedges in the space between the grinding wheels. Being free to translate with the slide  81  and the plate  87  along the guides  79  according to the arrow f 81 , this movement takes the grinding wheels  51 ,  53  to a position that is centered at all times in respect of the centerline plane of the blade. At the same time, the fact that the grinding wheel unit can rotate the grinding wheels  51 ,  53  about the axis C-C means that the two grinding wheels exert on the blade the same pressure, determined by the force of the pulling spring  95 .  
      As the draw spring  95  extends by a very short extent as a result of the modest oscillations of the grinding wheels about the axis C-C, it may be considered that its traction force is essentially constant and therefore the pressure exerted by the grinding wheels on the blade will also be essentially constant, irrespective of extent of the angle of oscillation about the axis C-C. Therefore, by setting a forward movement step along the direction f 63  approximate to the radial wear of the blade  19 , even if the effective wear of the blade is less than estimated, thanks to the fact that the grinding wheels  51 ,  53  can be centered in respect of the blade and to the presence of the pulling spring  95 , essentially the same contact force can always be obtained between the grinding wheels and the blade and consequently the pressure strictly necessary to obtain sharpening is not exceeded, thus reducing wear on the blade and increasing its duration.  
      Instead of a spring  95  another system may be used to apply controlled stress to the plate  87  and to the grinding wheels  51 ,  53  about the axis C-C, for example a piston-cylinder actuator with a device to control stress.  
      Moreover, a position sensor may also be provided to detect the angular position of the plate  87  and of the grinding wheels  51 ,  53  to control forward movement of the grinding wheel unit  80  as a function of the wear on the blade. In fact, as the blade becomes worn and its diameter decreases, if the carriage  63  with the grinding wheel unit  80  does not move forward along the direction f 63 , the decrease in diameter is offset with rotation of the plate  87  and therefore of the grinding wheels  51 ,  53  about the axis C-C. Offset can be obtained up to a certain point, beyond which the plate  87  meets the stop  88 . By detecting the angular position of the plate  87  the carriage  63  can be moved forward by a predeterminable extent when the plate  87  has reached a predetermined angular position, to recover wear on the blade by forward movement of the carriage.  
      As in the example shown the unit  17  moves with an alternate translatory movement parallel to the axis A-A of rotation to allow continuous feed of the products L to be cut, in order to prevent the onset of inertial forces on the grinding-wheel unit  80  and on the slide  81  carrying it, which would tend to make the unit translate along the guides  79 , a counterweight can be secured to the slide  81  and restricted to move along the direction of the guides in the opposite direction to the direction in which the grinding-wheel unit  80  and the slide  81  move. This arrangement is indicated with a dashed line and schematically in  FIG. 3 . The counterweight is indicated with  101 . It is guided along guides parallel to the guides  79  and not shown and linked by a pinion  103  to a rack  105  integral with the slide  81 . The pinion, supported idle about a fixed axis in respect of the structure  77 , also meshes with a rack not shown integral with the counterweight  101 . In this way the inertial forces applied simultaneously to the counterweight  101  and to the assembly comprising the grinding wheel unit  80  and the slide  81  cause no translation of these elements along the direction f 81 .  
      Moreover, to prevent accelerations deriving from the alternate movement of the unit  17  from producing a torque on the grinding wheel unit  80  that tends to make the unit and therefore the grinding wheels  51   53  rotate about the axis C-C, the grinding wheel unit  80  is dimensioned and balanced so that its center of gravity falls on the axis C-C, or at least so that the center of gravity of the elements free to rotate about this axis, namely the shaft  85 , the blocks  89 , the plate  87  and the grinding wheels  51 ,  53 , falls on it.  
      The device described with reference to FIGS.  2  to  4  is particularly suitable to produce a counter-bevel on the blade  19  or the like, for example a band blade. In this case the sharpening unit in question will be associated with another sharpening unit that produces the principal bevel.  
      This further sharpening unit may be produced in the same way as described, or as shown in the example of embodiment in FIGS.  5  to  7 . This further embodiment may be also adopted to produce a single sharpening unit to sharpen blades without a counter-bevel.  
      FIGS.  5  to  7  do not show the system to move the grinding wheels towards the blade, which may be produced as in the previous example. Equivalent numbers increased by 200 indicate equivalent or corresponding parts to those in the previous example of embodiment. The number  277  indicates the cantilevered bracket that supports the slide  281  carrying the grinding wheel unit  280 . The slide  281  carrying the grinding wheel unit  280  translates freely along guides  279  orthogonal to the lying plane of the blade, indicated once more with  19 , and therefore parallel to its axis of rotation B-B. The slide  281  supports rotatingly about the axis C-C the shaft of the grinding wheel unit  280  on which a plate  287  is fixed, integral with which are blocks  289  carrying the grinding wheels indicated with  251  and  253 . In this case the grinding wheels are motorized and the number  254  indicates the respective motors that can be, in a per se known way, pneumatic motors or the like.  
      The axis C-C is again on the lying plane of the cutting edge of the blade and in a central position in respect of the axes A 1 -A 1  and A 2 -A 2  of the two grinding wheels  251 ,  253 .  
      Other elements common to the previous example of embodiment such as the moving counterweight are not shown but may be present.  
      When the grinding wheels  251 ,  253  are pressed against the blade  19 , for example by a forward movement according to the arrow f 63 , the oscillations of the blade can stress the grinding wheels  251 ,  253  making them oscillate about the axis C-C moving them away from the blade. This may occur due to the considerable flexural deformations to which the blade  19  may be subjected. This would cause vibrations and defects in sharpening.  
      In order to prevent this drawback, a device, indicated as a whole with  350  and described hereunder, is associated with the grinding wheel unit  280 .  
      The device  350  comprises a slider  351  housed in a support  353  made integral (for example through a bracket not shown for clarity of the drawing) with the slide  281 . The back end of the slider  351  is secured by means of a bracket  355  to a piston-cylinder actuator  357 , in turn secured to the support  353 . Extension and retraction of the piston-cylinder actuator  357  causes rotation of the slider  351  about its axis D-D. The slider  351  has a front rod  351 A that cooperates with an appendix  359  integral with the plate  287 .  
      The slider  351  has a channel  361  (see in particular  FIG. 7 ) that extends along a short arc of helix coaxial with the axis D-D of the slider  351 . A roller  363  carried by a spindle  365  integral with the support  353  engages in the channel  361 . The channel  361  and the roller  363  form a cam mechanism that obliges the slider  351  to move along the axis D-D when the actuator  357  causes a rotation of the slider about said axis. An axial thrust on the slider  351  does not cause an axial movement due to inclination of the channel  361 , chosen so that the mechanism is irreversible.  
      In this way, when the grinding wheels  251 ,  253  are required to operate, they are first moved against or towards the blade  19  by the forward movement device not shown that produces a movement according to f 63 . The blade  19  is inserted into the space between the grinding wheels  251 ,  253 .  
      In this position the grinding wheels may not be in contact with the blade  19 . They are pushed and forced into the operating position with the required pressure against the sides of the blade  19  by extension of the piston-cylinder actuator  357  that brings the slider  351  into a predetermined position corresponding to an angular position of the plate  287  and of the blocks  289  and therefore of the grinding wheels  351 ,  353  in respect of the axis C-C. This position is maintained even if flexural deformations of the blade  19  exert an axial force on the cursor  351 , thanks to the irreversibility of the cam mechanism  361 ,  363 . The movement of oscillation about the axis C-C produced by extension of the actuator  357  produces translation of the slide  281  along the direction f 281  to bring -the axis C-C to the lying plane, i.e. to the centerline of the blade  19 . Therefore, also in this case the grinding wheels system is self-centering in respect of the blade.  
      The grinding wheels remain locked in their angular position in respect of the axis C-C to press with the due pressure against the blade  19  until the actuator  357  is operated again in the opposite direction to allow oscillation of the grinding wheels about the axis C-C and move them away from the sides of the blade  19 , bringing them finally to a non-operating position Oscillation can be controlled by a return spring, not shown.  
      The embodiment in FIGS.  5  to  7  therefore, allows both self-centering of the grinding wheels and movement of the grinding wheels alternately to an operating position and to a non-operating position.  
      This second example of embodiment may also be provided with systems, analogous to those described with reference to FIGS.  2  to  5 , to prevent the effect of inertia on the grinding wheels.  
      It is understood that the drawing merely shows practical embodiments 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 purely to facilitate their reading with reference to the description hereinbefore and the appended drawings, and do not limit the scope of protection whatsoever.