Abstract:
A rotary cutter for trimming printed products conveyed in an overlapping flow. The rotary cutter including a first knife that rotates and a second knife that rotates counter to the first knife. The second knife is arranged to operate jointly with the first knife. At least one of the knives includes a plurality of blades distributed over a circumference. A conveying device conveys the overlapping flow between the first knife and the second knife. A control unit controls the rotational speed of at least one of the first knife and the second knife. The rotational speed of at least one of the knives is predetermined based on at least one of the speed value for the conveying device or a thickness value of the overlapping flow.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the priority of European Patent Application No. 07405027.9, filed on Feb. 2, 2007, the subject matter of which is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    The invention relates to a device for trimming printed products conveyed in an overlapping flow, the device comprising a rotating first knife that operates jointly with a counter-rotating second knife, between which an overlapping flow is conveyed by a conveying device, wherein at least one of the knives is embodied so as to have a plurality of blades distributed along the circumference of the knife and wherein the device is provided with a control unit for controlling the operation of the knives. A rotary cutter of this type is described in the U.S. Pat. No. 3,813,981. The geometric arrangement of the blades along the circumference of one of the two rotating knives makes it possible to prevent the printed products from being displaced during the trimming operation. For that reason, the circumferential speed of the knife provided with the blades must be adapted precisely to the conveying speed of the device for conveying the printed products. To be able to process even thick printed products in an overlapping flow with high production capacity and good trimming quality, the knives of rotary cutters are embodied as shown in the European patent document EP-A-1 510 288. The blades must be re-sharpened regularly because any wear on the knives reduces the trimming quality. This re-sharpening operation is comparatively involved and expensive. A replacement of one or more of the knives furthermore results in a shutdown period, for example ranging from 10 to 30 minutes. 
       SUMMARY OF THE INVENTION 
       [0003]    It is therefore an object of the present invention to create a rotary cutter of the aforementioned type, provided with blades that do not show wear as quickly, thereby making it possible to have a longer service life. 
         [0004]    The above and other objects are achieved according to the invention, wherein there is provided, in one embodiment, a rotary cutter for trimming printed products, conveyed in an overlapping flow, the rotary cutter comprising: a first knife that rotates; a second knife that rotates counter to the first knife, wherein the second knife is arranged to operate jointly with the first knife; at least one of the first knife or the second knife including a plurality of blades distributed over a circumference of at least one of the first knife or the second knife; a conveying device to convey the overlapping flow between the first knife and the second knife; and a control unit coupled to control the rotational speed of at least one of the first knife and the second knife, wherein the rotational speed of at least one of the knives is predetermined based on at least one of the speed value for the conveying device or a thickness value of the overlapping flow. 
         [0005]    The idea behind the invention is based on the finding that rotary cutters of this type frequently utilize only the tips of the blades for the trimming. The individual blades consequently wear down faster in a small partial region, resulting in a less than optimum use of the blade. For an optimum use, approximately 70% of the blade would be utilized which, as a rule, is the case only if the trimming occurs at maximum thickness and at maximum speed of the product flow. With extremely thin products or low conveying speeds, however, only the tip of a blade is engaged, resulting in the aforementioned, extremely rapid wear. By controlling the speed as disclosed in the invention, the circumferential speed of at least one knife can always be adapted optimally to the products to be trimmed, even if the characteristics change. The thickness of the product flow and the conveying speed may be used as the product flow characteristics in this case. However, other characteristics can also be used for controlling the product flow, for example the type of material and especially the type of paper used for the products. 
         [0006]    According to one embodiment of the invention, at least one of the knives is driven by a speed-controlled motor, thereby resulting in a particularly simple and precise control of the rotational speed for the respective knife. 
         [0007]    According to a different embodiment of the invention, each of the two knives is driven by a speed-controlled motor. In that case, the two knives can be adapted precisely, independent of each other, and comparatively easily to the characteristics of the product flow or the products to be controlled. The service life of the two knives can thus be extended even more. 
         [0008]    According to yet another embodiment of the invention, the force that must be generated at the driven knife shaft is measured and the resulting value is then used for checking the trimming condition. It is thus possible to ensure that the blades are respectively re-sharpened or replaced at the optimum point in time, which furthermore avoids any unfavorable trimming quality as a result of worn blades. 
         [0009]    The characteristics used for controlling the at least one knife include the thickness of the product flow and the conveying speed. The use of one or more additional characteristics can reduce the blade wear even further. For example, the type of paper in particular can be used as another characteristic of the products. 
         [0010]    According to yet another embodiment of the invention, a measuring device is used if the thickness of the products or the product flow is used as a variable for controlling the at least one knife, for example a sensor which determines the thickness of the product flow and subsequently supplies this value to the respective control or regulating device. The conveying speed can also be determined with a suitable measuring device. However, it is also possible in principle to take over the speed directly from the conveying device, for example a conveying belt. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    These and other features and advantages of the invention will be further understood from the following detailed description with reference to the accompanying drawings, which show in: 
           [0012]      FIG. 1  A schematic, three-dimensional view of a rotary cutter according to the invention; 
           [0013]      FIG. 2  A schematic partial side view of two blades and the product flow, designed to illustrate the trimming operation; and 
           [0014]      FIG. 3  A partial block diagram illustrating the control unit of the rotary cutter according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    Referring to  FIG. 1 , there is shown a rotary cutter  1  used for trimming products  13  conveyed in a product flow  12 , in particular printed products such as newspapers, magazines, folded sheets and signatures, as well as individual sheets. The products  13  in particular are conveyed in an overlapping flow  12 . The products  13  arrive, for example, from a rotary printing machine that is not shown herein and are conveyed in a conveying device  11  in the direction of the arrow  14 , as shown in  FIG. 2 . The conveying device  11  can be embodied in a manner known per se, for example in the form of a belt conveyor. The conveying device  11  generally operates at a constant speed that is predetermined, for example, by upstream-connected machines and in particular a printing machine. The speed of the conveying device  11  can vary considerably, depending on the product  13 , and can thus be comparatively slow or extremely fast, wherein the speed can range up to about 1.2 m/s. The products  13  are trimmed in a longitudinal direction of the overlapping flow  12  by trimming off an edge of the product  13 , which is not shown in further detail herein. 
         [0016]    The rotary cutter  1  has a machine frame  2  with a rotating first knife  4  and second knife  5  positioned on the machine frame. In the embodiment of  FIG. 1 , the first knife  4  is illustrated above second knife  5 . The knife  4  has a knife shaft, known per se and not shown herein, which is positioned in the machine frame  2 . This knife shaft extends parallel to a knife shaft for the second knife  5 , which is also positioned inside the machine frame  2 . The knife shaft for the second knife  5  can be embodied to be displaceable in a longitudinal direction for adjusting the cutting gap between the first knife  4  and the second knife  5 , as disclosed in EP 1 637 295 A. 
         [0017]    The first knife  4  is a so-called segment knife, comprising a disc  7  as a carrier, which is attached with a flange  28  to a knife shaft that is not shown herein. A plurality of blades  6  of hard metal or another suitable material are clamped onto the knife in circumferential direction of the disc  7 , wherein these blades  6  are also called knife cutters. According to  FIG. 2 , they are respectively provided with a cutting edge  16  for trimming the products  13 . These cutting edges  16  must be sharpened periodically, for example as described in EP 1 510 288 A. Each cutting edge  16  extends between a tip  18  and corner  20 . 
         [0018]    The first knife  4  is driven by a motor M that acts via a gear  3  upon the knife shaft of the first knife  4 , wherein this gear is only indicated herein. The first knife  4  is driven in the direction of arrow  9 . The motor M may be an electric motor and further may be a speed-controlled servo-motor. 
         [0019]    The second knife  5  consists of a ring  8  which is attached to a disc-shaped support  29 , wherein the ring  8  in particular is made of hard metal and can be glued to the support  29 . The ring has an inside area, not shown herein, which can be re-sharpened. A cutting gap ranging from about 0.03 mm to about 0.035 mm exists between this inside area and the blades  6 . The second knife  5  is driven in the direction of arrow  10 . The drive can be a passive drive, meaning the speed of the second knife  5  automatically adjusts to the speed of the first knife  4 . However, the second knife  5  can also be actively driven by the motor M, with the aid of a gear that is not shown herein. Finally, the second knife  5  can also be driven by a separate motor M′ which acts via a gear, not shown herein, upon the knife shaft for the second knife  5 . This motor M′, which is indicated in  FIG. 1 , is also an electric motor and may be a speed-controlled servo-motor. 
         [0020]    With the rotary cutter  1  according to  FIG. 1 , the first knife  4  is installed on the top and the second knife  5  is installed on the bottom. However, a design is also conceivable where the second knife  5  is embodied as the segment knife and the overlapping flow  12  is trimmed from below. The overlapping flow  12  in that case is formed such that the edges of the products  13  are on the bottom. 
         [0021]    The thickness H for the overlapping flow  12 , indicated in  FIG. 2 , can vary considerably, depending on the products  13  and the arrangement of the products in the overlapping flow. Prior to the trimming operation, the thickness H is detected with the sensor  24 , indicated in  FIG. 3 , or with the aid of another suitable measuring device. The sensor  24  can be a non-contacting distance measuring device, which measures in the direction of arrow  21  as shown in  FIG. 3 , or it can be a mechanical scanning device. The measured value for the thickness H is then supplied to a control unit S, which is connected to a speed regulator  25  for the motor M. If the second knife  5  is driven by the motor M′, a control signal is also supplied to a speed regulator  26  for the motor M′. In addition to the thickness H, the speed of the overlapping flow  12  that is indicated with arrow  22  in  FIG. 3 , can be determined with a different sensor  23 . The sensor  23  can operate non-contacting or can be a known mechanical speed indicator. One skilled in the art is familiar per se with method of measuring the speed of overlapping flows. The sensor  23  can also be integrated into the conveying device  11 . The measured speed value is also supplied to the control unit S, which then supplies the corresponding signals to the speed regulator  25  and, if applicable, also to the speed regulator  26 . 
         [0022]    Based on the value for the measured thickness H and/or the measured conveying speed, the control unit S predetermines the rotational speed for the first knife  4 , wherein this speed is calculated to ensure an optimum use of the cutting edge  16  during each trimming operation. An optimum use exists if the area  17 , shown in  FIG. 2 , corresponds to approximately 70% of the total length of the cutting edge  16 . The area  17  starts at the tip  18  and extends to the point of impact P that is shown in  FIG. 2 . This point of impact P is the point at which the cutting edge  16  impacts with the product  13  to be cut. The cutting edges  6  are thus utilized in the area  17  and, over time, will become dull in that area. The area  17  can also correspond to less than 70%, e.g. 60% or 50%, of the cutting edge  16  length. The shape of the cutting edge  16  is preferably straight between the tip  18  and the corner  20 , but can also be non-straight. 
         [0023]    If a motor M′ is provided, then the control unit S can also calculate the speed of the second knife  5  based on a characteristic and in particular based on the thickness H and/or the speed of the overlapping flow  12 , wherein the two motors M and M′ are advantageously controlled separately. In that case, the rotational speeds of the two knives  4  and  5  can be adjusted optimally with respect to thickness and/or conveying speed of the overlapping flow  12 . In principle, it is also possible to have a control that solely depends on the conveying speed or the thickness H. However, both variables may be taken into consideration. Of course, if the conveying speed of the overlapping flow  12  remains constant and unchanged, even for differently thick products  13 , then the adjustment is based solely on the thickness H. If the thickness H remains constant and the conveying speed varies, then the adjustment is accordingly based on the conveying speed. 
         [0024]    To compute the optimum speed for the first knife  4  and/or the second knife  5 , additional characteristics of the overlapping flow  12  and/or the printed products can be supplied to the control unit S, either through manual input or by linking it to additional measuring devices. 
         [0025]    With an optimum adjustment, each of the blades  6  intersects at an optimum angle with a plane representing the top  27  ( FIG. 2 ) of a product  13 , as explained above. During the trimming operation, the cutting edges  16  form a cutting curve  19  in the area  17 .  FIG. 2  shows that the cutting edges  16  move in the direction of arrow  15 , and that the products  13  move in the direction of arrow  14 . The blades  6  and the products  13  consequently move simultaneously during the trimming operation.  FIG. 2  shows a blade  6  in the position where it hits the product  13  and a blade  6 ′ which is in the process of leaving the product  13  following the trimming operation. If the speed of the products  13  and/or the thickness H changes, then the adjustment made by the aforementioned control unit S will adjust the rotational speed of the first knife  4  and, if applicable, also the rotational speed of the second knife  5 . The circumferential speed of the first knife  4  is critical for the optimum positioning of the point of impact P, which in this case depends directly on the rotational speed. 
         [0026]    The following table contains a plurality of possible adjustment cases. 
         [0000]    
       
         
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                   
                 Rotational 
                 Rotational 
               
               
                 Case 
                   
                 Conveying 
                 speed of 
                 speed of 
               
               
                 No. 
                 Thickness H 
                 speed v 
                 first knife 
                 second knife 
               
               
                   
               
             
             
               
                 1 
                 larger/smaller 
                 Same 
                 higher/lower 
                 same 
               
               
                 2 
                 larger/smaller 
                 Same 
                 same 
                 higher/lower 
               
               
                 3 
                 larger/smaller 
                 Same 
                 higher/lower 
                 higher/lower 
               
               
                 4 
                 Same 
                 higher/lower 
                 higher/lower 
                 same 
               
               
                 5 
                 Same 
                 higher/lower 
                 same 
                 higher/lower 
               
               
                 6 
                 Same 
                 higher/lower 
                 higher/lower 
                 higher/lower 
               
               
                 7 
                 larger/smaller 
                 higher/lower 
                 higher/lower 
                 same 
               
               
                 8 
                 larger/smaller 
                 higher/lower 
                 same 
                 higher/lower 
               
               
                 9 
                 larger/smaller 
                 higher/lower 
                 higher/lower 
                 higher/lower 
               
               
                   
               
             
          
         
       
     
         [0027]    The conveying speed as well as the rotational speed of the second knife  5  remains the same for Case  1  in the above Table. If the thickness H of the overlapping flow  12  increases, then the rotational speed of the first knife  4  increases. However, if the thickness H decreases, the rotational speed of the first knife  4  decreases as well. For the adjustment, the blades  6  always start the trimming operation at the optimum point of impact P. In that case, only the rotational speed of the first knife  4  is adjusted based on the thickness H. For the above case  9 , the thickness H and the conveying speed function as parameters for adjusting the rotational speed of the first knife  4  and the second knife  5 . Still other cases are the result of using additional parameters for the control, e.g. the paper quality mentioned in the above. The optimum adjustment results in a longer service life as well as a uniform, optimum cutting quality. 
         [0028]    Also considered for the adjustment can be the force that must be generated at the knife shafts, or at least at one knife shaft. This force can be measured by the speed regulators  25  and  26 , for example by measuring the current consumption at the motors M and/or M′, and can be supplied to the control unit S for an evaluation. Conclusions can be drawn from this with respect to the state of the blades  6 , the cutting edges  16  and/or the rings  8 . With the aid of a corresponding indicator, it is possible to ensure that the blades  6  or the hard metal ring  8  are re-sharpened or replaced in time. 
         [0029]    It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.