Abstract:
To record images accurately regardless of rigidity and size of a recording medium, each rear end clamp has a clamp rubber element for engaging a variable margin of the recording medium mounted peripherally of the recording drum. The margin is varied according to the rigidity of the recording medium. The margin is enlarged when the recording medium is highly rigid, and reduced when the recording medium is low in rigidity.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates to an image recording apparatus for recording images by rotating a recording drum with a recording medium mounted peripherally thereof. 
         [0003]    2. Description of the Related Art 
         [0004]    In such an image recording apparatus, the recording medium mounted peripherally of the recording drum is fixed to the peripheral surface of the recording drum by forward end clamps for fixing the forward end of the recording medium to the peripheral surface of the recording drum, and by rear end clamps for fixing the rear end of the recording medium to the peripheral surface of the recording drum. 
         [0005]    On the other hand, in such an image recording apparatus, it is necessary to attach recording media of various sizes to the peripheral surface of the recording drum. For this purpose, the rear end clamps are constructed movable along recesses formed to extend circumferentially of the recording drum. Thus, the rear end clamps may be set to varied positions according to the lengths of recording media to be mounted peripherally of the recording drum. 
         [0006]    In such an image recording apparatus, depending on the size of a recording medium mounted peripherally of the recording drum, an edge of the recording medium axially of the recording drum may be placed over one of the recesses extending circumferentially of the recording drum. In such a case, unless the rear end clamps apply an appropriate pressure to the recording medium, the edge of the recording medium will be lifted from the surface of the recording drum by a centrifugal force produced by rotation of the recording drum, or will yield to the pressure and sink in the recess. When such a phenomenon occurs, a laser beam emitted to the recording medium will go out of focus on the surface of the recording medium, thereby failing to record images properly. 
         [0007]    To cope with the above situation, an image recording apparatus described in Japanese Patent No. 3595229 is constructed to adjust clamp rubber elements of rear end clamps to proper positions. 
         [0008]    The rigidity of a recording medium to be mounted peripherally of the recording drum varies according to the type of the recording medium. Also, depending on the length of the recording medium lying over a recess formed to extend circumferentially of the recording drum, unless the rear end clamps apply to the recording medium an appropriate pressure according to that length, the edge of the recording medium will be lifted from the surface of the recording drum by a centrifugal force produced by rotation of the recording drum, or will yield to the pressure and sink in the recess. With the image recording apparatus described in the above U.S. Pat. No. 3,595,229, it is difficult to solve such a problem. 
       SUMMARY OF THE INVENTION 
       [0009]    The object of this invention, therefore, is to provide an image recording apparatus capable of recording images accurately regardless of the rigidity and size of a recording medium. 
         [0010]    The above object is fulfilled, according to this invention, by an image recording apparatus comprising a recording drum for supporting a recording medium mounted peripherally thereof; forward end clamps for fixing a forward end of the recording medium mounted peripherally of the recording drum, to a peripheral surface of the recording drum; and rear end clamps for fixing a rear end of the recording medium mounted peripherally of the recording drum, to the peripheral surface of the recording drum, the rear end clamps being movable along recesses formed to extend circumferentially of the recording drum; the recording medium being mounted peripherally of the recording drum, with an edge of the recording medium axially of the recording drum overhanging one of the recesses; wherein a margin of the recording medium for gripping by the rear end clamps is changeable according to one of rigidity of the recording medium mounted peripherally of the recording drum and a length of the recording medium overhanging the one of the recesses. 
         [0011]    This image recording apparatus can record images accurately regardless of rigidity and size of the recording medium. 
         [0012]    In a preferred embodiment, positions of the recording medium gripped by the rear end clamps are shiftable circumferentially of the recording drum according to the rigidity of the recording medium mounted peripherally of the recording drum, or the length of the recording medium overhanging the one of the recesses. 
         [0013]    In another embodiment, an area of contact between the recording medium and the rear end clamps over the recesses is changed according to one of the rigidity of the recording medium mounted peripherally of the recording drum, and the length of the recording medium overhanging the one of the recesses. 
         [0014]    In an image recording apparatus according to another aspect of the invention, each of the rear end clamps has an elastic element for contacting a surface of the recording medium; the elastic element having a pair of projections for contacting the recording medium over one of the recesses when the recording medium is gripped; the rear end clamps being arranged in positions where both of the projections contact the recording medium when the recording medium is highly rigid, and in positions where one of the projections contacts the recording medium when the recording medium is low in rigidity. 
         [0015]    With this image recording apparatus, the area of contact between the rear end clamps and recording medium can be changed easily by changing the positions of gripping by the rear end clamps. 
         [0016]    Other features and advantages of the invention will be apparent from the following detailed description of the embodiments of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    For the purpose of illustrating the invention, there are shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangement and instrumentalities shown. 
           [0018]      FIG. 1  is a schematic side view of an image recording apparatus according to this invention; 
           [0019]      FIG. 2  is a perspective view showing a recording drum along with forward end clamps and rear end clamps; 
           [0020]      FIG. 3  is a developed view of the peripheral surface of the recording drum; 
           [0021]      FIG. 4A  is a schematic view of a rear end clamp; 
           [0022]      FIG. 4B  is a schematic view of the rear end clamp; 
           [0023]      FIG. 5A  is a schematic view of the rear end clamp; 
           [0024]      FIG. 5B  is a schematic view of the rear end clamp; 
           [0025]      FIG. 6A  is a schematic view showing a relationship between a clamping bolt and a clamp groove; 
           [0026]      FIG. 6B  is a schematic view showing the relationship between the clamping bolt and clamp groove; 
           [0027]      FIG. 6C  is a schematic view showing the relationship between the clamping bolt and clamp groove; 
           [0028]      FIG. 7A  is an explanatory view showing a clamp rubber element of a rear end clamp for fixing single-mounting and double-mounting plates; 
           [0029]      FIG. 7B  is an explanatory view showing the clamp rubber element of the rear end clamp for fixing single-mounting and double-mounting plates; 
           [0030]      FIG. 8A  is an explanatory view showing a clamp rubber element of a rear end clamp for fixing single-mounting and double-mounting plates; 
           [0031]      FIG. 8B  is an explanatory view showing the clamp rubber element of the rear end clamp for fixing single-mounting and double-mounting plates; 
           [0032]      FIG. 9  is a flow chart of a plate mounting operation; 
           [0033]      FIG. 10  is a flow chart of a clamp position reading operation; and 
           [0034]      FIG. 11  is a flow chart of another clamp position reading operation. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0035]    An embodiment of this invention will be described hereinafter with reference to the drawings.  FIG. 1  is a schematic side view of an image recording apparatus according to this invention. 
         [0036]    The image recording apparatus includes a recording drum  11  drivable by a motor  25  to rotate in a direction indicated by an arrow A in  FIG. 1 , with a printing plate P acting as a recording medium mounted peripherally of the drum  11 , a rotary encoder  26  for detecting a rotational angular position of the recording drum  11 , a recording head  12  for recording images on the printing plate P mounted on the recording drum  11 , a transport unit  13  for transporting the printing plate P to the recording drum  11  and removing the printing plate P from the recording drum  11 , a punch unit  14  for punching positioning holes in the printing plate P transported from the transport unit  13  toward the recording drum  11 , a clamp unit  17  for opening and closing forward end clamps  15  and rear end clamps  16  arranged peripherally of the recording drum  11 , and moving the rear end clamps  16  along the peripheral surface of the recording drum  11 , a controller  20 , an input unit  21 , and a table  22  storing predetermined data. The recording drum  11  has positioning pins (not shown) erected on the peripheral surface thereof for fitting into the positioning holes to determine a forward end position of the printing plate P. 
         [0037]    The recording head  12  opposed to the recording drum  11  has numerous laser diodes not shown. The recording head  12  records a two-dimensional image on the printing plate P mounted peripherally of the recording drum  11 , by emitting laser light according to image signals outputted from the controller  20 , while moving axially of the recording drum  11  (i.e. in directions perpendicular to the plane of  FIG. 1 ) synchronously with rotation of the recording drum  11 . 
         [0038]    The transport unit  13  disposed above the recording drum  11  is tiltable as indicated by an arrow B in  FIG. 1 , and defines therein a transport path  18  for transporting the printing plate P to the recording drum  11  and a transport path  19  for discharging the printing plate P. 
         [0039]    The clamp unit  17  is rockable as indicated by an arrow C in  FIG. 1 , and has, arranged at a distal end thereof, a forward end clamp presser  23  for opening and closing the forward end clamps  15 , and a rear end clamp mounter  24  for mounting the rear end clamps  16 . The rear end clamps  16  arranged peripherally of the recording drum  11  are movable along clamp grooves  30  formed in the peripheral surface of the recording drum  11  as described hereinafter. The rear end clamp mounter  24  also performs an operation to move the rear end clamps  16 . 
         [0040]      FIG. 2  is a perspective view showing the recording drum  11  along with the forward end clamps  15  and rear end clamps  16 . 
         [0041]    The recording drum  11  has fourteen recessed clamp grooves  30  extending circumferentially thereof. The forward end clamps  15 , fourteen in number, are arranged in positions corresponding to the respective clamp grooves  30 . On the other hand, the rear end clamps  16  are seven in number, each bridging a pair of clamp grooves  30 . The rear end clamps  16  are positionally adjustable circumferentially of the recording drum  11 , along the clamp grooves  30 . The rear end clamps  16  are mounted peripherally of the recording drum  11  in a required number according to the size of a printing plate P to be mounted on the recording drum  11 . 
         [0042]      FIG. 3  is a developed view of the peripheral surface of the recording drum  11 . 
         [0043]    As seen, the recording drum  11  has fourteen clamp grooves  30   a - 30   n  formed therein (which are collectively referred to as “clamp grooves  30 ”). The rear end clamps  16  are fixed peripherally of the recording drum  11 , respectively, by using pairs of clamp groove  30   a  and clamp groove  30   b , clamp groove  30   c  and clamp groove  30   d , clamp groove  30   e  and clamp groove  30   f , clamp groove  30   g  and clamp groove  30   h  clamp grooves  30   i  and clamp groove  30   j , clamp groove  30   k  and clamp groove  30   l  and clamp groove  30   m  and clamp groove  30   n.    
         [0044]    This recording drum  11  accommodates both a single-mounting mode for supporting one printing plate P and a double-mounting mode for supporting two printing plates P. In the single-mounting mode, one printing plate P is mounted peripherally of the recording drum  11 , in bilateral symmetry with respect to a center line Y located in an axially middle of the recording drum  11 . In the double-mounting mode, two printing plates P are mounted peripherally of the recording drum  11 , each in bilateral symmetry with respect to a center line X of the left half or a center line Z of the right half of the recording drum  11 . 
         [0045]    Printing plates P having various degrees of rigidity are mounted on the recording drum  11 . That is, printing plates P vary in rigidity according to different thicknesses or materials. In this image recording apparatus, printing plates P are handled as classified into two categories according to whether or not the plates P have rigidity of a predetermined value and above. That is, plates P having rigidity of the predetermined value and above are regarded as “thick plates”, and those having rigidity less than the predetermined value as “thin plates”. The printing plates P regarded as “thick plates” generally correspond to single-mounting plates P. The printing plates P regarded as “thin plates” generally correspond to double-mounting plates P. 
         [0046]    However, the degrees of rigidity of printing plates P may be classified more finely. Printing plates P for single mounting, but having rigidity less than the predetermined value, may be classified as “thin plates”. Printing plates P for double mounting, but having rigidity of the predetermined value and above, may be classified as “thick plates”. 
         [0047]    In  FIG. 3 , P 1  denotes a single-mounting plate of minimum size, P 2  denotes a single-mounting plate of intermediate size, and P 3  denotes a single-mounting plate of maximum size. P 11 , P 12  and P 13  denote double-mounting plates of typical sizes mounted on the left half of the recording drum  11 . P 21 , P 22  and P 23  denote double-mounting plates of typical sizes mounted on the right half of the recording drum  11 . P 11  and P 21  denote double-mounting plates of minimum size, P 13  and P 23  those of maximum size, and P 12  and P 22  those of intermediate size. 
         [0048]    This recording drum  11  has the clamp grooves  30   a - 30   n  arranged in bilateral symmetry with respect to the center line Y. That is, the pairs of clamp groove  30   a  and clamp groove  30   n , clamp groove  30   b  and clamp groove  30   m , clamp groove  30   c  and clamp groove  30   l , clamp groove  30   d  and clamp groove  30   k , clamp grooves  30   e  and clamp groove  30   j , clamp groove  30   f  and clamp groove  30   i , and clamp groove  30   g  and clamp groove  30   h , respectively, are arranged in bilateral symmetry with respect to the center line Y. On the other hand, the clamp grooves  30  are not in bilateral symmetry with respect to the center line X or Z of the left or right half. 
         [0049]    In time of double-mounting printing plates P, therefore, only one of the opposite edges in the direction of width (i.e. axially of the recording drum  11 ) of each plate P overhangs a clamp groove  30 . For example, a plate P 13  of maximum size that can be mounted in the left-hand region on the recording drum  11  will have one edge thereof overhanging the clamp groove  30   g , while the other edge will not be present on the clamp groove  30   a.    
         [0050]    In this case, patterns of plates P arranged on the clamp grooves  30  can be classified into the following four patterns. 
         [0051]    The first pattern is a case where the opposite edges of a single-mounting plate P and each double-mounting plate P are arranged on clamp grooves  30 . In this case, since the single-mounting plate P generally corresponds to a thick plate P and each double-mounting plate P to a thin plate P, the edges of the thick and thin plates P are arranged on the clamp grooves  30 . The second pattern is a case where only the edges of a single-mounting plate P are arranged on clamp grooves  30 . In this case, the edges of a thick plate P are arranged on clamp grooves  30 . The third pattern is a case where only the edges of double-mounting plates P are arranged on clamp grooves  30 . In this case, the edges of thin plates P are arranged on clamp grooves  30 . The fourth pattern is a case where the edges of plates P are not arranged on clamp grooves  30 . 
         [0052]    Plates P may be mounted over the above clamp grooves  30   a ,  30   b ,  30   m  and  30   n  in the state of the first pattern. Plates P may be mounted over the clamp grooves  30   c ,  30   d ,  30   k  and  30   l  in the state of the second pattern. Plates P may be mounted over the clamp grooves  30   f  and  30   i  in the state of the third pattern. Further, the edges of plates P are not arranged over the clamp groove  30   e ,  30   g ,  30   h  or  30   j.    
         [0053]    Thus, the edges of thick and thin plates P may be arranged over the clamp grooves  30   a ,  30   b ,  30   m  and  30   n . Therefore, when the edges of plates P are arranged over these clamp grooves  30 , the margin of each such plate P for gripping by the rear end clamps  16 , for example, is changed according to whether it is a thick plate P or a thin plate P. Only the edges of a single-mounting plate P can overlap the clamp grooves  30   c ,  30   d ,  30   k  and  30   l , and only the edges of double-mounting plates P can overlap the clamp grooves  30   f  and  30   i.    
         [0054]    Next, the construction of the rear end clamps  16  will be described.  FIGS. 4A ,  4 B,  5 A and  5 B are schematic views of one rear end clamp  16 .  FIGS. 4A and 4B  show the rear end clamp  16  being detached from the recording drum  11 .  FIGS. 5A and 5B  show the rear end clamp  16  being attached to the recording drum  11 .  FIGS. 4A and 5A  are top views, and  FIGS. 4B and 5B  bottom views. 
         [0055]    As shown in these figures, the rear end clamp  16  has a main body  41 , a pair of clamping bolts  42  for engaging with clamp grooves  30  of the recording drum  11 , and a drive block  43  for operating the clamping bolts  42 . The main body  41  has bores  44  formed therein for engaging with holding pins of the rear end clamp mounter  24  shown in  FIG. 1 . 
         [0056]    Each clamping bolt  42  has projections formed at opposite ends thereof for engaging with a clamp groove  30 . The drive block  43  is driven by an operating pin of the rear end clamp mounter  24  to turn the clamping bolts  42  between an unlock state shown in  FIG. 4  where the projections of the clamping bolts  42  extend parallel to the direction of extension of the clamp groove  30 , and a lock state shown in  FIG. 5  where the projections of the clamping bolts  42  extend perpendicular to the direction of extension of the clamp groove  30 . 
         [0057]      FIGS. 6A ,  6 B and  6 C are schematic views showing a relationship between a clamping bolt  42  and a clamp groove  30  in time of fixing the rear end clamp  16  to the recording drum  11 . 
         [0058]    First, as shown in  FIG. 6A , the recording drum  11  is rotated while the rear end clamp  16  is held by the rear end clamp mounter  24  shown in  FIG. 1 , to place the clamping bolts  42  of the rear end clamp  16  over the clamp grooves  30 . At this time, each clamping bolt  42  is in the unlock state shown in  FIGS. 4A and 4B . 
         [0059]    In this state, as shown in  FIG. 6B , the rear end clamp mounter  24  advances each clamping bolt  42  into the clamp groove  30 . 
         [0060]    Subsequently, as shown in  FIG. 6C , the clamping bolt  42  is turned to the lock state where the projections of the clamping bolt  42  extend in the direction perpendicular to the direction of extension of the clamp groove  30 . As a result, the rear end clamp  16  is fixed to the surface of the recording drum  11 . 
         [0061]    The main body  41  of each rear end clamp  16  has a clamp rubber element  51  secured by an adhesive or the like to a back surface of a forward end thereof in the direction of drum rotation, for pressing and fixing a plate P to the peripheral surface of the recording drum  11 . The clamp rubber element  51  has a different shape according to the clamp groove  30  to which the rear end clamp  16  is attached. Specifically, a different rear end clamp  16  has a different shape in the portion, opposed to the clamp groove  30 , of the clamp rubber element  51  shown in circle E in  FIGS. 4B and 5B . 
         [0062]      FIGS. 7A and 7B  are explanatory views showing a clamp rubber element  51  of a rear end clamp  16  for fixing single-mounting and double-mounting plates P.  FIGS. 8A and 8B  are explanatory views showing a clamp rubber element  51  of a rear end clamp  16  for fixing a single-mounting or double-mounting plate P. 
         [0063]    As noted hereinbefore, the edges of thick and thin plates P may be arranged over the clamp grooves  30   a ,  30   b ,  30   m  and  30   n . On the other hand, only the edges of a single-mounting plate P can overlap the clamp grooves  30   c ,  30   d ,  30   k  and  30   l , and only the edges of double-mounting plates P can overlap the clamp grooves  30   f  and  30   i.    
         [0064]    Thus, rear end clamps  16  placed opposite the clamp grooves  30   a ,  30   b ,  30   m  and  30   n  use the clamp rubber element  51  shown in  FIGS. 7A and 7B , so that changes may be made in the area of contact between the plate P and the clamp rubber element  51  of the rear end clamps  16  over the clamp grooves  30 . Rear end clamps  16  placed opposite the clamp grooves  30   c ,  30   d ,  30   k  and  30   l  and rear end clamps  16  placed opposite the clamp grooves  30   f  and  30   i  use the clamp rubber element  51  shown in  FIGS. 8A and 8B . As shown in  FIGS. 8A and 8B , the rear end clamps  16  shown in  FIG. 8A  may grip positions different in the axial direction of the recording drum  11  from positions gripped by the rear end clamps  16  shown in  FIG. 8B . 
         [0065]    Specifically, as shown in  FIGS. 7A and 7B , the clamp rubber element  51  of the rear end clamps  16  placed opposite the clamp grooves  30   a ,  30   b ,  30   m  and  30   n  has a pair of projections  51   a  and  51   b  that project over the clamp grooves  30 . When a thick plate P of high rigidity is used, as shown in  FIG. 7A , the rear end clamp  16  is placed in a position where an end of the clamp rubber element  51  is slightly spaced from the rear end of the plate P, to set a large margin for gripping by the clamp rubber element  51  of the rear end clamp  16 . In this case, the plate P contacts both of the projections  51   a  and  51   b  of the clamp rubber element  51  over the clamp groove  30 , thereby providing a large area of contact between the plate P and the clamp rubber element  51  of the rear end clamp  16  over the clamp groove  30 . 
         [0066]    On the other hand, when a thin plate P of low rigidity is used, as shown in  FIG. 7B , the rear end clamps  16  opposite the clamp grooves  30   a ,  30   b ,  30   m  and  30   n  are placed in a position where an approximately middle portion of the clamp rubber element  51  coincides with the rear end of the plate P, to set a small margin for gripping by the clamp rubber element  51  of the rear end clamp  16 . In this case, the plate P contacts only the projection  51   a  of the clamp rubber element  51  over the clamp groove  30 , thereby providing a small area of contact between the plate P and the clamp rubber element  51  of the rear end clamp  16  over the clamp groove  30 . 
         [0067]    As opposed to the above, rear end clamps  16  placed opposite the clamp grooves  30   c ,  30   d ,  30   k  and  30   l  are used for fixing a thick plate P of high rigidity. In this case, as shown in  FIG. 8A , a clamp rubber element  51  having a projection  51   c  is used. This clamp rubber element  51  has a gripping position set axially of the recording drum  11  so that the entire region of the projection  51   c  may be disposed over a clamp groove  30 . As shown in  FIG. 8A , the rear end clamp  16  is placed in a position where an end of the clamp rubber element  51  is slightly spaced from the rear end of the plate P, to set a large margin for gripping by the clamp rubber element  51  of the rear end clamp  16 . 
         [0068]    Rear end clamps  16  placed opposite the clamp grooves  30   f  and  30   i  are used for fixing plates P of low rigidity. In this case, as shown in  FIG. 8B , the clamp rubber element  51  having the projection  51   c  is used. This clamp rubber element  51  has a gripping position set axially of the recording drum  11  so that only part of the projection  51   c  may be disposed over a clamp groove  30 . Further, as shown in  FIG. 8B , the rear end clamp  16  is placed in a position where an approximately middle portion of the clamp rubber element  51  coincides with the rear end of the plate P, to set a small margin for gripping by the clamp rubber element  51  of the rear end clamp  16 . 
         [0069]    As noted hereinbefore, the clamping rubber element  51  shown in  FIGS. 7A and 7B  has a pair of projections  51   a  and  51   b  spaced a predetermined distance apart circumferentially of the recording drum  11 . Thus, even when a thin plate P is used and an error occurs in a stopping position of the recording drum  11 , a constant contact area between the plate P and clamp rubber element  51  over the clamp groove  30  is secured as long as the edge of the plate P is disposed between the projections  51   a  and  51   b . Therefore, even with a slight error in the mounting position of the rear end clamp  16 , it is possible to secure a constant contact area between the plate P and clamp rubber element  51  over the clamp groove  30 . 
         [0070]    Next, operation of the above image recording apparatus for mounting a printing plate P on the recording drum  11  will be described.  FIG. 9  is a flow chart of the plate mounting operation. 
         [0071]    First, information on the plate P is inputted (step S 1 ). This information indicates a size of the plate P and whether this plate P is a thick plate or thin plate. This information is inputted from the input unit  21  shown in  FIG. 1 . 
         [0072]    Next, the controller  20  shown in  FIG. 1  reads a position of the rear end clamps  16  (step S 2 ).  FIG. 10  is a flow chart of this clamp position reading operation. 
         [0073]    When reading a clamp position, a clamp position r corresponding to the size of plate P is set first (step S 21 ). This clamp position r is a position corresponding to a thick plate P, which allows the plate P to be gripped by a large gripping margin. The clamp position r is a position expressed as a distance from a predetermined reference position circumferentially of the recording drum  11 , which is expressed, for example, as a length measured from the forward end clamps  15  counterclockwise in  FIG. 1 , along the peripheral surface of the recording drum  11 . Alternatively, the clamp position r is expressed as a rotational angular position of the recording drum  11 , in which the counterclockwise direction of the recording drum  11  in  FIG. 1  is regarded as positive. This clamp position r is stored beforehand in the table  22  shown in  FIG. 1 . 
         [0074]    Next, it is determined whether the plate P to be mounted is a thin plate (step S 22 ). When the plate P is not a thin plate but a thick plate, this subroutine is ended with no further action. On the other hand, when the plate P to be mounted is a thin plate, an amount of correction α is added to r, to shift the clamp position by the amount of correction a counterclockwise on the recording drum  11 , to a position for gripping the plate P by a small gripping margin (step S 23 ). 
         [0075]    Referring to  FIG. 9  again, the plate P is fed out of the transport path  19  of the transport unit  13  shown in  FIG. 1 , and the punch unit  14  punches positioning holes in the plate P (step S 3 ). The punched plate P is fed toward the recording drum  11  (step S 4 ). 
         [0076]    When the forward end of the plate P reaches the position of the forward end clamps  15  on the recording drum  11 , the forward end clamp presser  23  of the clamp unit  17  opens and closes the forward end clamps  15  to fix the forward end of the plate P to the recording drum  11 . Then, the motor  25  rotates the recording drum  11 , to apply the entire length of the plate P to the peripheral surface of the recording drum  11  (step S 6 ). Rotational angular positions of the recording drum  11  at this time are constantly monitored by the rotary encoder  26 . 
         [0077]    When the recording drum  11  has rotated to a rotational angular position where rear end clamps  16  can grip the rear end of the plate P by a gripping margin set beforehand, the rotation of the recording drum  11  is stopped (step S 7 ). Then, the rear end clamp mounter  24  of the clamp unit  17  attaches the rear end clamps  16  to the recording drum  11 , thereby attaching the rear end of the plate P to the recording drum  11  (step S 8 ). 
         [0078]    When the plate P mounted peripherally of the recording drum  11  is a thick plate, the clamp rubber element  51  of each of the rear end clamps  16  placed opposite the clamp grooves  30   a ,  30   b ,  30   m  and  30   n  presses the plate P in the state shown in  FIG. 7A , and the clamp rubber element  51  of each of the rear end clamps  16  placed opposite the clamp grooves  30   c ,  30   d ,  30   k  and  30   l  presses the plate P in the state shown in  FIG. 8A . 
         [0079]    When the plate P mounted peripherally of the recording drum  11  is a thin plate, the clamp rubber element  51  of each of the rear end clamps  16  placed opposite the clamp grooves  30   a ,  30   b ,  30   m  and  30   n  presses the plate P in the state shown in  FIG. 7B , and the clamp rubber element  51  of each of the rear end clamps  16  placed opposite the clamp grooves  30   c ,  30   d ,  30   k  and  30   l  presses the plate P in the state shown in  FIG. 8B . 
         [0080]    Next, another embodiment of this invention will be described.  FIG. 11  is a flow chart of a clamp position reading operation according to another embodiment. 
         [0081]    In the foregoing embodiment, when the plate P is a thick plate, a large margin for gripping by the rear end clamps  16  is provided to compensate for an insufficient pressing force due to the rigidity of the plate P. In this embodiment, on the other hand, when a large length of plate P overhangs a clamp groove  30 , a margin for gripping by the rear end clamps  16  is reduced in order to prevent the plate P from sinking in the clamp groove  30 . This embodiment is different from the foregoing first embodiment only in the clamp position reading step (step S 2 ) of the plate mounting operation shown in  FIG. 9 . 
         [0082]    As shown in  FIG. 11 , when reading a clamp position in this embodiment, a clamp position r corresponding to the size of plate P is set first (step S 24 ). This clamp position r is a position corresponding to a case where a relatively small length of plate P overhangs the clamp groove  30 , which allows the plate P to be gripped by a large gripping margin. This clamp position r is stored beforehand in the table  22  shown in  FIG. 1 . 
         [0083]    Next, it is determined whether the plate P overhangs the clamp groove  30  by a length equal to or more than a threshold (step S 25 ). The threshold has different values set beforehand whether the plate P to be mounted peripherally of the recording drum  11  is a thick plate or thin plate. The threshold values are stored beforehand in the table  22  shown in  FIG. 1 . 
         [0084]    When the plate P overhangs the clamp groove  30  by a length equal to or larger than a threshold, an amount of correction β is added to r, to shift the clamp position by the amount of correction β clockwise on the recording drum  11 , to a position for gripping the plate P by a small gripping margin (step S 26 ). 
         [0085]    In both of the foregoing embodiments, the margin of the plate P for gripping by the rear end clamps  16  is changed by changing the mounting position of the rear end clamps  16  on the recording drum  11 , that is by changing the position of the plate P gripped by the rear end clamps  16 . Alternatively, the positional relationship between the rear end of the plate P and the rear end clamps  16  may be changed by shifting the mounting position of the plate P circumferentially of the recording drum  11  according to the rigidity or width of the plate P, or the length of the plate P overhanging a clamp groove  30 . If, for example, positions of the positioning holes for engaging with the positioning pins erected on the peripheral surface of the recording drum  11  or a plate edge distance of the positioning holes are/is changed circumferentially of the recording drum  11 , the positional relationship between the rear end of the plate P and the rear end clamps  16  can be changed without changing the mounting position of the rear end clamps  16 , thereby to adjust the gripping margin of the plate P. 
         [0086]    In the foregoing embodiments, the size of the gripping margin is changed according to whether the plate P to be mounted on the recording drum  11  is a thick plate or thin plate, or whether the plate P overhangs a clamp groove  30  by a large length or small length. The gripping margin may be changed by taking both criteria into account. 
         [0087]    This invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification, as indicating the scope of the invention. 
         [0088]    This application claims priority benefit under 35 U.S.C. Section 119 of Japanese Patent Application No. 2006-097316 filed in the Japanese Patent Office on Mar. 31, 2006, the entire disclosure of which is incorporated herein by reference.