Patent Publication Number: US-2010116110-A1

Title: Sheet processing apparatus and sheet processing method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Division of co-pending U.S. patent application Ser. No. 11/533,947 filed on Sep. 21, 2006, which is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2005-276592 filed on Sep. 22, 2005 and Japanese Patent Application No. 2005-276593 filed on Sep. 22, 2005, the entire contents of all of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet processing apparatus for correcting skews and performing punching processes on the sheets delivered from an image forming apparatus 
     2. Description of the Related Art 
     Image forming apparatus including MFPs (multi function peripherals), copying machines and printers are provided at the sheet delivery section of the image forming apparatus main body with a post-processing unit for performing post-processing operations including an operation of sorting sheets where an image is formed, that of stapling sheets and/or that of punching sheets. 
     In such a post-processing unit, the sheet of paper delivered from the image forming apparatus can move aslant and become skewed relative to the sheet delivering direction. When the skewed sheet of paper is subjected to a punching process (and punched), one or more than one holes can be produced at wrong positions by the punching process to give rise to a problem for a succeeding filing operation. For this reason, the post-processing unit is equipped with a skew correcting means for correcting the skew (if any) of the sheet of paper before the punching process. 
     Jpn. Pat. Appln. Laid-Open Publication No. 2002-274699 describes a sheet processing apparatus. The apparatus described in the above-cited Patent Document comprises a punch unit for performing a punching process on the sheet of paper being delivered to it on which an image has already been formed and the sheet of paper is made to abut the nipper of the inlet roller arranged upstream relative to the punch unit in order to correct the skew, if any, of the sheet of paper. However, with the apparatus described in the above-cited Patent Document, the sheet of paper to be corrected for the skew, if any, can be pinched by the nipper of the inlet roller to prevent the skew, if any, from being corrected. 
     Jpn. Pat. Appln. Laid-Open Publication No. 2002-193493 describes a punching processing apparatus. In the punching processing apparatus described in the above-cited Patent Document, a registration roller is arranged downstream relative to the punching means of the apparatus and the front end of the sheet of paper is made to abut the registration roller by the sheet conveying force of the sheet delivering roller of the image forming apparatus main body to correct the skew, if any. However, since the sheet conveying fore of the sheet delivering roller of the image forming apparatus main body is utilized, the positional relationship between the image forming apparatus main body and the post-processing apparatus need to be precisely defined so that the mechanical configuration of the post-processing apparatus is disadvantageously subjected to limitations. 
     Jpn. Pat. Appln. Laid-Open Publication No. 2002-302332 describes an image forming apparatus having a punch device. In the image forming apparatus described in the above-cited Patent Document, a stop member is made to project across the sheet conveyance route and one or more than one holes are cut in the sheet of paper stopped by the stop member by means of the punch. However, the above-cited Patent Document does not describe anything about skew correction. 
     Jpn. Pat. Appln. Laid-Open Publication No. 2004-217337 described a post-processing apparatus having an alignment means for aligning a sheet of paper. In the post-processing apparatus described in the above-cited Patent Document, the skew, if any, of the sheet of paper is corrected by the alignment means and then forced to abut an abutment member before it is subjected to a punching process. However, the post-processing apparatus requires an alignment plate and other parts to make the structure of the apparatus a complicated one. 
     In view of the above-identified circumstances, it is therefore the object of the present invention to provide a sheet processing apparatus having a simple configuration that can correct the skew, if any, of a sheet of paper and accurately execute a punching process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of an image forming apparatus equipped with a sheet processing apparatus according to the present invention, showing the overall configuration thereof; 
         FIG. 2  is a schematic illustration of an embodiment of the sheet processing apparatus according to the present invention, showing the configuration thereof; 
         FIG. 3  is a schematic block diagram of the control system of the sheet processing apparatus according to the present invention; 
         FIGS. 4A through 4E  are schematic illustrations of the operation of the sheet processing apparatus of  FIG. 2 ; 
         FIGS. 5A and 5B  are schematic plan views of the sheet processing apparatus of  FIG. 2 , illustrating the skew correcting operation thereof; 
         FIG. 6  is a schematic illustration of the second embodiment of the sheet processing apparatus according to the present invention, showing the configuration thereof; 
         FIG. 7  is a schematic perspective view of the punch unit to be used in the sheet processing apparatus of  FIG. 6 , illustrating the specific configuration thereof; 
         FIGS. 8A and 8B  are schematic perspective views of the punch unit of  FIG. 7 , illustrating the skew correcting operation thereof; 
         FIGS. 9A through 9E  are schematic illustrations of the operation of the sheet processing apparatus of  FIG. 6 ; 
         FIGS. 10A and 10B  are schematic plan views of the sheet processing apparatus of  FIG. 6 , illustrating the skew correcting operation thereof; and 
         FIG. 11  is a schematic illustration of an image forming apparatus provided with another embodiment of the sheet processing apparatus according to the present invention, showing the overall configuration thereof. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Throughout this description, the embodiments and examples shown should be considered as exemplars, rather than limitations on the apparatus of the present invention. 
     Now, the present invention will be described by referring to the accompanying drawings that illustrate preferred embodiments of the invention. Throughout the drawings, the same or similar components are denoted respectively by the same reference symbols. 
       FIG. 1  is a schematic illustration of an image forming apparatus equipped with a sheet processing apparatus according to the present invention, showing the overall configuration thereof.  FIG. 2  is a schematic illustration of a principal part of an embodiment of the sheet processing apparatus according to the present invention, showing the configuration thereof. 
       FIG. 1  shows an MFP (multi function peripheral) that operates as an image forming apparatus. The image forming apparatus  10  comprises a scanner section  11 , an automatic document feeder (ADF)  12  and an operation section  13  in an upper part thereof. A printer section  14  and a sheet feeding section  15  are arranged respectively in a central part and in a lower part of the image forming apparatus  10 . A finisher  20  is arranged contiguously relative to the image forming apparatus  10  for punching operations and stapling operations. 
     The printer section  14  typically has a tandem type color laser printer. The printer section  14  includes a laser  141  and a photosensitive drum  142 . A charging device  143 , a developing device  144  and a transferring device  145  are arranged around the photosensitive drum  142 . Toner is supplied to the developing device  144  from a toner cartridge. 
     Now, the operation of the printer section  14  will be summarily described below. The surface of the photosensitive drum  142  is scanned for exposure by the laser beam from the laser  141  to form an electrostatic latent image on the photosensitive drum  142 . The electrostatic latent image is developed by the developing device  144  to form a toner image on the photosensitive drum  142 . The toner image is then transferred onto a sheet of paper P by the transferring device  145 . For a color printing process, toner cartridges of four colors including cyan, magenta, yellow and black are arranged in a row and photosensitive drums and optical units of the respective colors are provided. 
     The sheet feeding section  15  has a plurality of sheet feeding cassettes for containing sheets of paper of various different sizes. A sheet of paper P is sent from one of the sheet feeding cassettes to conveyor belt  16  and an image is formed on the sheet of paper P. The sheet of paper P on which an image is formed is then delivered and conveyed to the finisher  20  by way of sheet delivery section  17 . 
     The finisher  20  performs post-processing operations such as stapling and punching on the sheet of paper P and delivers it to either a sheet delivery tray  21  or a sheet delivery tray  22 . The sheet delivery trays  21 ,  22  are driven to move up and down by a sheet delivery tray drive section so that the sheet of paper P is delivered to either of the trays. 
     The finisher  20  is provided with a sheet processing apparatus  30  according to the present invention. The sheet processing apparatus  30  comprises a punching unit  40  for punching the sheet of paper P conveyed from the image forming apparatus  10  and a skew correcting section  50  for correcting the skew, if any, of the sheet of paper P before punching the sheet. 
       FIG. 2  schematically illustrates the specific configuration of the sheet processing apparatus  30 . Referring to  FIG. 2 , reference symbol  40  denotes the punching unit. The punching unit  40  includes a hole cutting blade  41 , an eccentric cam  42  for driving the hole cutting blade  41  to move up and down, a punch motor  43  for driving the eccentric cam  42  to rotate and a punch die  44 . 
     The hole cutting blade  41  is constantly urged in the direction of the eccentric cam  42 . The punch die  44  includes an upper die  441  and a lower die  442  and the sheet of paper P is conveyed between the upper die  441  and the lower die  442 . The sheet of paper P is punched as the hole cutting blade  41  is reciprocated in the direction of the punch die  44  (that is orthogonal relative to the direction of conveying the sheet of paper P). While the eccentric cam  42  is described above as means for reciprocating the hole cutting blade  41 , it may be replaced by some other means for reciprocating the hole cutting blade  41 . 
     On the other hand, a first conveyer roller pair  51  and a second conveyer roller pair  52  are arranged respectively at the sheet feeding side and at the sheet discharging side of the punching unit  40  to convey the sheet of paper along a conveyance route. The conveyer roller pairs  51 ,  52  are driven to rotate respectively by conveyor motors  53 ,  54 . 
     The first conveyer roller pair  51  includes upper and lower conveyer rollers  511 ,  512 , which are driven to rotate by the conveyer motor  53  to convey a sheet of paper P, pinching it between them. Similarly, the second conveyer roller pair  52  includes upper and lower conveyer rollers  521 ,  522 , which are driven to rotate by the conveyer motor  54  to convey a sheet of paper P, pinching it between them. 
     The first conveyer roller pair  51  can change the condition of contact of the rollers  511 ,  512  by means of a load controller  55 . More specifically, the first conveyer roller pair  51  has a functional feature of registration roller where the rollers  511 ,  512  are brought into tight contact with each other and a functional feature of slipping roller where the rollers  521 ,  522  are brought into loose contact with each other. It operates as registration roller when conveying a sheet of paper, whereas it operates as slipping roller when correcting the skew, if any, of the sheet of paper. 
     A sensor  56  is arranged between the sheet feeding side of the punching unit  40  and the first conveyer roller pair  51  to detect the condition of the sheet of paper P being fed in and conveyed. In this embodiment of the invention, the sheet of paper P is turned upside down (with the image carrying surface facing downward) when it is conveyed in the direction of arrow X. Any of a variety of mechanisms for turning the sheet of paper P upside down may be used for the purpose of the present invention. 
       FIG. 3  is a schematic block diagram of the control system of a sheet processing apparatus  30  according to the present invention and an image forming apparatus  10  to be used with the sheet processing apparatus  30 . Referring to  FIG. 3 , reference symbol  61  denotes a control section, which is a microprocessor that includes a CPU. In  FIG. 3 , reference symbol  62  denotes a motor drive circuit for driving the punching motor  43  and reference symbol  63  denotes a motor drive circuit for driving the conveyer motor  53 , whereas reference symbol  64  denotes a motor drive circuit for driving the conveyer motor  54 . The control section  61  controls the load controller  55  and the outcome of the detecting operation of the sensor  56  is input to it. 
     In  FIG. 3 , reference symbol  65  denotes a control section for controlling the image forming apparatus  10 , which is a microprocessor that includes a CPU. The control section  61  and the control section  65  exchange information with each other to coordinate the image forming operation of the image forming apparatus  10  and the operation of the finisher  20 . Various directives are input by the user to the control section  65  by way of the operation section  13  and the control section  65  controls the operation of the printer section  14  and that of the sheet feeding section  15  according to the directives so as to convey the sheet of paper P on which an image is formed to the finisher  20 . 
     Now, the operation of the sheet processing apparatus  30  according to the present invention will be described by referring to  FIGS. 4A through 4E . The sheet of paper P on which an image is formed by the printer section  14  of the image forming apparatus  10  is then conveyed to the sheet processing apparatus  30 . Since the sheet of paper P flows from the printer section  14  toward the tray  21  or the tray  22 , the side of the printer  14  is referred to as upstream side and the side of the tray  21  or the tray  22  is referred to as downstream side in the following description. 
       FIG. 4A  illustrates a condition where the sheet of paper P on which an image is formed by the printer section  14  is being conveyed toward the punching unit  40 . In the initial stages, the first conveyer roller pair  51  and the second conveyer roller pair  52  are driven respectively by the conveyer motors  53 ,  54  and the sheet of paper P is conveyed in the first direction (as indicated by arrow X) from the upstream side toward the downstream side. The hole cutting blade  41  of the punching unit  40  is separated from and located above the punching die  44  in this condition. 
     As the sheet of paper P is conveyed and sensor  56  detects that the leading end of the sheet of paper P passes the first conveyer roller pair  51 , the punching motor  43  starts rotating to drive the eccentric cam  42  to rotate and lower the hole cutting blade  41  in the direction toward the punching die  44 . As a result, the hole cutting blade  41  projects to block the sheet conveyance route. In other words, the hole cutting blade  41  operates as a stopper member that retractably projects across the sheet conveyance route so that it can temporarily stop the sheet of paper P being conveyed. 
     The first conveyer roller pair  51  is controlled by the load controller  55  in such a way that the rollers  511 ,  512  thereof lightly contact with each other and operate as slipper roller. Then, as a result, the first conveyer roller pair  51  conveys the sheet of paper P downstream with force weaker than before and makes the sheet of paper P abut the projecting hole cutting blade  41 . 
     Thus, the rollers  511 ,  512  of the first conveyer roller pair  51  lightly touch the sheet of paper P and push it against the hole cutting blade  41  so that, if the sheet of paper P is skewed, the skew is corrected. 
     Then, as shown in  FIG. 4C , the eccentric cam  42  of the punching unit  40  is driven to rotate by the punching motor  43  with the sheet of paper P corrected for the skew, if any, and the hole cutting blade  41  is lifted to move away from the sheet conveyance route. Subsequently, the first conveyer roller pair  51  is controlled by the load controller  55  so as to operate as registration roller and driven to rotate by the motor  53  and convey the sheet of paper P that is corrected for the skew, if any, in the direction of arrow X. 
     As the sheet of paper P is conveyed by a predetermined distance and the sensor  56  detects that the tail end of the sheet of paper P passes the first conveyer roller pair  51  as viewed in the sheet conveying direction, the conveyer motor  54  stops rotating and hence the second conveyer roller pair  52  also stops rotating as shown in  FIG. 4D . Then, the eccentric cam  42  of the punching unit  40  is driven to rotate by the punching motor  43  and the hole cutting blade  41  once again projects across the sheet conveyance route and executes a punching process on the sheet of paper P. 
     As the punching process is completed, the eccentric cam  42  is driven to rotate by the punching motor  43  in a manner as shown in  FIG. 4E  to raise the hole cutting blade  41 . Then, the first and second conveyer roller pairs  51 ,  52  are driven to rotate respectively by the conveyer motors  53 ,  54  and the sheet of paper P is conveyed away. Thereafter, the steps of operation of  FIGS. 4A through 4E  are repeated so that sheets of paper P are sequentially subjected to a punching process and then delivered to the delivery tray  21  or  22 . 
     The sensor  56  detects the leading end of the sheet of paper P when the sheet of paper P is fed in by way of the first conveyer roller pair  51 . It also detects that the tail end of the sheet of paper P passes the position vis-à-vis the sensor  56 , and notifies the control section  61  of what it detects. Then, the control section  61  controls the motor drive circuits  62 ,  63 ,  64  and the load controller  55  according to what is detected by the sensor  56 . 
     Note that, as for the timing of lowering the hole cutting blade  41 , it may be so arranged that the punching motor  43  is driven to rotate after the elapse of a predetermined time period since the time when the sensor  56  detects a sheet of paper P being fed in. Alternatively, it may be so arranged that a stepping motor is used as the punching motor  43  and a predetermined number of pulses are supplied to the stepping motor from the time when a sheet of paper P being fed in is detected by the sensor  56  to control the rotary angle of the punching motor  43 . 
     As for the timing of restarting the conveyer roller  52  after stopping it, it may be so arranged that the conveyer motor  56  is driven to rotate after the elapse of a predetermined time period since the time when the sensor  56  detects a sheet of paper P being fed in. Alternatively, it may be so arranged that the conveyer motor  54  is driven to rotate when the rise of the hole cutting blade  41  is detected. 
     While the motor drive circuit  62 ,  63 ,  64  are controlled according to the outcome of detection of the sensor  56  in the above description, it may be alternatively be so arranged that they are controlled according to the information received from the image forming apparatus  10 . For example, the delivery of a sheet of paper P may be detected by way of the delivery section  17  of the image forming apparatus  10  and the elapse of a predetermined time period may be gauged by referring to the time of detection to drive the motor drive circuit  62 ,  63 ,  64 . 
       FIGS. 5A and 5B  are schematic plan views of the sheet processing apparatus of  FIG. 2 , illustrating the skew correcting process and the punching process thereof.  FIG. 5A  shows the condition of the sheet processing apparatus when a sheet of paper P passes the first conveyer roller pair  51  and abuts the hole cutting blade  41 . This condition corresponds to that of  FIG. 4B . If the sheet of paper P is skewed as indicated by solid lines in  FIG. 5A , the skew is corrected as indicated by dotted chain line in  FIG. 5A  because the first conveyer roller pair  51  operates as slipping roller at this time and the sheet of paper is continuously and lightly pushed by the rotary motion of the first conveyer roller pair  51 . 
     After the skew is corrected, the sheet of paper P is conveyed in the X direction by the first conveyer roller pair  51  and the second conveyer roller pair  52 . When the tail end p 0  of the sheet of paper P passes the first conveyer roller pair  51 , the sensor  56  detects it and the operation of conveying the sheet of paper P is stopped and the sheet of paper P is subjected to a punching process by the punching unit  40  as shown in  FIG. 5B . This condition corresponds to that of  FIG. 4D . As a result, the sheet of paper P is punched at the right position and conveyed away. 
     The hole cutting position of the punching process is defined typically by the predetermined distance L 1  from the tail end p 0  of the sheet of paper P as viewed in the sheet conveying direction (e.g., 12 mm). If the gap between the tail end p 0  and the hole cutting position of the sheet of paper P is L 1 , the control section  61  performs its control operation in such a way that the conveyance of the sheet of paper is temporarily stopped at a position where the gap between the hole cutting blade  41  and the tail end p 0  of the sheet of paper becomes equal to L 1  to carry out a hole cutting operation before the tail end p 0  of the sheet of paper passes the punching unit. 
     In this way, with the sheet processing apparatus  30  according to the present invention, a sheet of paper P is conveyed from the upstream side to the downstream side and the first conveyer roller pair  51  is made to operate as slipping roller when the sheet of paper P is fed in so that the leading end of the sheet of paper P is made to abut the hole cutting blade  41  of the punching unit  40  in order to correct the skew, if any, of the sheet of paper. 
     Additionally, the hole cutting blade  41  of the punching unit  40  is utilized as stopper member that can retractably project across the sheet conveyance route. In other words, the number of components is minimized because the hole cutting blade  41  of the punching unit  40  operates both in the skew correction mode and the hole cutting mode of operation. Still additionally, this invention provides ease of designing the finisher  20  and can simplify the configuration thereof because the first and second conveyer roller pairs can be arranged totally in the inside of the finisher  20 . Thus, with a sheet processing apparatus according to the present invention, it is possible to correct the skew, if any, of the sheet of paper being processed and accurately execute a punching process. 
     Now, the second embodiment of the sheet processing apparatus  30  according to the present invention will be described by referring to  FIG. 6 . 
       FIG. 6  is a schematic illustration of the second embodiment of the sheet processing apparatus  30  according to the present invention, showing the principal configuration thereof. 
     Referring to  FIG. 6 , reference symbol  40  denotes a punching unit. The punching unit  40  includes a rotary shaft  45 , eccentric cams  42   a ,  42   b  fitted to the rotary shaft  45 , a hole cutting blade  41  driven to move up and down by the rotary motion of the eccentric cam  42   a , a shutter  46  driven to move up and down by the rotary motion of the eccentric cam  42   b , a punching die  44  and a motor  43  for driving the rotary shaft  45  to rotate. 
     The eccentric cams  42   a ,  42   b  are fitted to the rotary shaft  45  and separated from each other by a predetermined gap. The eccentric cams  42   a ,  42   b  are arranged in such a way that their front ends are angularly separated from each other by an angle of 180°. The eccentric cams  42   a ,  42   b  are adapted to rotate and operate in a symmetric manner. The hole cutting blade  41  and the shutter  46  are arranged so as to be respectively held in contact with the eccentric cams  42   a ,  42   b  and the hole cutting blade  41  is driven to move up and down by the rotary motion of the eccentric cam  42   a , whereas the shutter  46  is driven to move up and down by the rotary motion of the eccentric cam  42   b . Thus, the hole cutting blade  41  and the shutter  46  projects and retreat relative to the sheet conveyance route in a manner inverted relative to each other. 
     In this embodiment, the shutter  46  operates as stopper member that can retractably project across the sheet conveyance route. 
     The punching die  44  includes an upper die  441  and a lower die  442  and a sheet of paper P is conveyed between the upper and lower dies. Both a punching operation and a skew correcting operation are performed as the hole cutting blade  41  and the shutter  46  are driven to reciprocate in the direction of the punching die  44  (that is orthogonal relative to the direction of conveying the sheet of paper P), the operations will be described in greater detail hereinafter. 
     On the other hand, first and second conveyer roller pairs  51 ,  52  are arranged respectively at the sheet feeding side and the sheet discharging side of the punching unit  40  to convey the sheet of paper from the upstream side to the downstream side. The conveyer roller pairs  51 ,  52  are driven to rotate respectively by conveyer motors  53 ,  54 . 
     The first conveyer roller pair  51  includes upper and lower conveyer rollers  511 ,  512 , which are driven to rotate by the conveyer motor  53  to convey a sheet of paper P, pinching it between them. Similarly, the second conveyer roller pair  52  includes upper and lower conveyer rollers  521 ,  522 , which are driven to rotate by the conveyer motor  54  to convey a sheet of paper P, pinching it between them. 
     The first conveyer roller pair  51  can change the condition of contact of the rollers  511 ,  512  by means of a load controller  55 . More specifically, the first conveyer roller pair  51  has a functional feature of registration roller where the rollers  511 ,  512  are brought into tight contact with each other and a functional feature of slipping roller where the rollers  511 ,  512  are brought into loose contact with each other. The pair of rollers  511 ,  512  operate as registration roller when conveying a sheet of paper, whereas they operate as slipping roller when correcting the skew, if any, of the sheet of paper (as will be described in greater detail hereinafter). A sensor  56  is arranged between the sheet feeding side of the punching unit  40  and the first conveyer roller pair  51  to detect the condition of the sheet of paper P being fed in and conveyed. 
     The control system of the sheet processing apparatus  30  of  FIG. 6  has a circuit configuration similar to that of  FIG. 3  and includes a control section  61  along with a motor drive circuit  62  for driving the motor  43 , a motor drive circuit  63  for driving the conveyer motor  53  and a motor drive circuit  64  for driving the conveyer motor  54 . The control section  61  controls a load controller  55  and receives the outcome of detection of the sensor  56  as input. 
     In the sheet processing apparatus  30  of  FIG. 6 , the rotary motion of the motor  43  is controlled by the motor drive circuit  62  of  FIG. 3  and the rotary shaft  45  is driven to rotate so that the eccentric cams  42   a ,  42   b  are by turn driven to rotate. 
       FIG. 7  is a schematic perspective view of the hole cutting blade  41  and the shutter  46  and their drive systems of the punch unit  40  of the sheet processing apparatus of  FIG. 6 , illustrating the specific configuration thereof. 
     Referring to  FIG. 7 , as a matter of fact, a pair of hole cutting blades  41  and a pair of shutters  46  are provided. The gap separating the pair of hole cutting blades  41  is equal to the gap separating a pair of holes punched through a sheet of paper P. The hole cutting blades  41  are fitted to respective support members  47 . The shutters  46  are also fitted to respective support members  48 . 
     On the other hand, two pairs of eccentric cams  42   a ,  42   b  are fitted to the rotary shaft  45  with a predetermined gap separating them to form a cam structure. The eccentric cams  42   a  are driven to rotate in order to move the support members  47  up and down respectively, whereas the eccentric cams  42   b  are driven to rotate in order to move the support members  48  up and down respectively. A frame  400  is arranged vis-à-vis the support members  47 ,  48  and springs  49  are arranged between the frame  400  and the support members  47 ,  48  to constantly urge the support members  47 ,  48  so as to be pushed against the eccentric cams  42   a ,  42   b  respectively. 
     The eccentric cams  42   a ,  42   b  are arranged in such a way that their front ends are angularly separated from each other by an angle of 180°. Therefore, the support members  47 ,  48  are driven to move up and down relative to each other as the rotary shaft  45  rotates. In other words, the hole cutting blades  41  and the shutters  46  are driven to move up and down in synchronism with the movements of the support members  47 ,  48  and the shutters  46  rise to their highest points when the hole cutting blades  41  fall to their lowest points, whereas the shutters  46  falls to their lowest points when the hole cutting blades  41  rise to their highest points. The support members  47 ,  48  and the cam structure concertedly form a reciprocating structure for reciprocating the hole cutting blades  41  and the shutters  46  in the direction of the sheet conveyance route. 
     The hole cutting blades  41  are provided respectively with vertically oblong holes  411 , while the shutters  46  are also provided respectively with vertically oblong holes  461 . A shaft S is made to run through the oblong holes  411 ,  461  as guide member and the opposite ends of the shaft S are rigidly secured in position. As a result, the hole cutting blades  41  and the shutters  46  are guided by the respective oblong holes  411 ,  461  and the shaft S so that they reciprocate accurately in the direction of the sheet conveyance route. 
       FIGS. 8A and 8B  schematically illustrate the operation of the hole cutting blades  41  and the shutters  46 . Both of the paired hole cutting blades  41  operate in the same way. Similarly, both of the paired shutters  46  operate in the same way. Therefore, only one of the hole cutting blades  41  and one of the shutters  46  are illustrated in  FIGS. 8A and 8B . 
       FIG. 8A  shows a condition where the hole cutting blade  41  is at the lowest position and the shutter  46  is at the highest position. In this condition, the front end of the eccentric cam  42   a  faces downward and the support member  47  is pressed downward by the spring  49 . On the other hand, the front end of the eccentric cam  42   b  faces upward and the support member  48  is raised upward against the urging force of the spring  49 . 
       FIG. 8B  shows a condition where the hole cutting blade  41  is inversely at the highest position and the shutter  46  is at the lowest position. In this condition, the front end of the eccentric cam  42  faces upward and the support member  47  is raised upward against the urging force of the spring  49 . On the other hand, the front end of the eccentric cam  42   b  faces downward and the support member  48  is pressed downward by the spring  49 . Thus, as the rotary shaft  45  rotates, the hole cutting blade  41  and the shutter  46  reciprocate in a direction orthogonal relative to the direction of conveying a sheet of paper P. 
     Now, the operation of the sheet processing apparatus  30  of  FIG. 6  will be described below by referring to  FIGS. 9A through 9E . Note that, in  FIG. 9A through 9E , the support members  47 ,  48  are omitted and show only how the hole cutting blade  41  and the shutter  46  move in synchronism with the movements of the eccentric cams  42   a ,  42   b.    
       FIG. 9A  illustrates how a sheet of paper P is fed in by way of the image forming apparatus  10 . In the initial stages, the first and second conveyer roller pairs  51  are driven to rotate respectively by the motors  53 ,  54  and the sheet of paper P is conveyed in the first direction (indicated by arrow X) from the upstream side toward the downstream side. Under this condition, the front ends of the eccentric cams  42   a ,  42   b  face sideways and the hole cutting blade  41  and the shutter  46  of the punching unit  40  are at the respective retracted positions located high above the punching die  44 . 
     As the sheet of paper P is fed in and the sensor  56  detects that the leading end of the sheet of paper P passes the first conveyer roller pair  51 , the motor  43  is driven to rotate to by turn drive the rotary shaft  45  to rotate. As a result, the front end of the eccentric cam  42   a  is made to face upward while the front end of the eccentric cam  42   b  is made to face downward as shown in  FIG. 9B . The hole cutting blade  41  is moved to its highest position and the shutter  46  (stopper member) is lowered in the direction toward the punching die  44  to project and block the sheet of paper P being conveyed. 
     The first conveyer roller pair  51  is controlled by the load controller  55  in such a way that the paired rollers  511 ,  512  lightly contact with each other and operate as slipping roller. Therefore, the first conveyer roller pair  51  conveys the sheet of paper P with force weaker than before and makes the front end of the sheet of paper P abut the projecting shutter  46 . In other words, the paired rollers  511 ,  512  press the sheet of paper P against shutter  46 , while lightly contacting with each other so that the skew, if any, of the sheet of paper P is corrected. 
     Then, as shown in  FIG. 9C , the motor  43  of the punching unit  40  is driven to operate to by turn drive the rotary shaft  45  to rotate while the sheet of paper P has been corrected for the skew, if any. As the rotary shaft  45  is driven to rotate, the front ends of the eccentric cams  42   a ,  42   b  are made to face sideways. The hole cutting blade  41  is lowered slightly from the highest position and the shutter  46  is raised slightly to an intermediate position. As a result, both the hole cutting blade  41  and the shutter  46  retreat from the sheet conveyance route. Additionally, the first conveyer roller pair  51  is controlled by the load controller  55  so as to operate as registration roller and driven to rotate by the motor  53  and convey the sheet of paper P that has been corrected for the skew, if any, in the direction of arrow X. 
     Then, as the sheet of paper P is conveyed by a predetermined distance and the sensor  56  detects that the tail end of the sheet of paper P passes the first conveyer roller pair  51  in the sheet conveying direction, the conveyer motor  54  stops its rotation and the second conveyer roller pair  52  also stops rotating as shown in  FIG. 9D . As the second conveyer roller pair  52  stops rotating, the motor  43  of the punching unit  40  starts rotating to make the front end of the eccentric cam  42   a  face downward. Then, the hole cutting blade  41  falls to its lowest position to project across the sheet conveyance route to carry out a punching process on the sheet of paper P. At this time, the front end of the eccentric cam  42   b  also faces upward and the shutter  46  is at its highest position. 
     As the punching process is completed, the motor  43  starts rotating to make the eccentric cams  42   a ,  42   b  face sideways and the hole cutting blade  41  rises slightly from the lowest position while the shutter  46  falls slightly to get to an intermediate position as shown in  FIG. 9E . Then, as a result, both the hole cutting blade  41  and the shutter  46  retreat from the sheet conveyance route. Then, the first and second conveyer roller pair  51 ,  52  are driven to rotate respectively by the conveyer rollers  53 ,  54  and the sheet of paper P is conveyed away. 
     Subsequently, the steps of operation of  FIG. 9A  through  FIG. 9E  are repeated so that sheets of paper P are sequentially subjected to a punching process one by one and delivered to the delivery tray  21  or  22 . 
     The sensor  56  detects the leading end of the sheet of paper P when the sheet of paper P is fed in by way of the first conveyer roller pair  51 . It also detects that the tail end of the sheet of paper P passes the position vis-à-vis the sensor  56  and notifies the control section  61  of what it detects. Then, the control section  61  controls the motor drive circuits  62 ,  63 ,  64  and the load controller  55  by referring to the outcome of detection of the sensor  56 . 
       FIGS. 10A and 10B  are schematic plan views of the second embodiment sheet processing apparatus according to the present invention, illustrating the skew correcting operation and the punching operation thereof.  FIG. 10A  shows a condition where a sheet of paper P passes the first conveyer roller pair  51  and abuts the shutter  46 . In other words, this condition corresponds to that of  FIG. 9B . At this time, the first conveyer roller pair  51  operates as slipping roller and hence, if the sheet of paper P is skewed as indicated by solid lines in  FIG. 10A , the skew is corrected as indicated by dotted chain lines in  FIG. 10A  as the sheet of paper P is continuously and lightly pushed by the rotary motion of the first conveyer roller pair  51 . 
     After the skew is corrected, the sheet of paper P is conveyed by the first conveyer roller pair  51  and the second conveyer roller pair  52  and, as the tail end pa of the sheet of paper P passes the first conveyer roller pair  51 , the sensor  56  detects it to suspend the sheet conveying operation. Then, the hole cutting blade  41  carries out a punching process in a manner as shown in  FIG. 10B  so that a hole H is cut reliably at the right position on the sheet of paper P. Thus, this condition corresponds to that of  FIG. 9D . 
     The hole cutting position of the punching process is defined typically by the predetermined distance L 1  from the tail end p 0  of the sheet of paper P as viewed in the sheet conveying direction (e.g., 12 mm). If the gap between the tail end p 0  and the hole cutting position of the sheet of paper P is L 1 , the control section  61  performs its control operation in such a way that the conveyance of the sheet of paper is temporarily stopped at a position where the gap between the hole cutting blade  41  and the tail end p 0  of the sheet of paper becomes equal to L 1  to carry out a hole cutting operation before the tail end p 0  of the sheet of paper passes the punching unit  40 . 
     In this way, with the sheet processing apparatus  30  of the second embodiment according to the present invention, a sheet of paper P is conveyed from the upstream side to the downstream side and, when the sheet of paper P is fed in, the leading end of the sheet of paper P is made to abut the shutter  46  to correct the skew, if any. Since a punching process is executed by means of the hole cutting blade  41  on a sheet of paper whose skew, if any, has been corrected, the hole cutting operation is reliably carried out at the right position. 
     The finisher  20  is arranged contiguously relative to the image forming apparatus  10  and provided with the punching unit  40  in the instance of the above-described instance of  FIG. 1 . However, the punching unit  20  may alternatively be contained in the image forming apparatus  10 . More specifically, the printer section  14  and the sheet processing section  30  may be arranged in a main body of the image forming apparatus  10  and the sheet of paper delivered from the printer section  14  may be subjected to a punching process by the punching unit  40  and corrected for the skew, if any. 
     Although exemplary embodiments of the present invention have been shown and described, it will be apparent to those having ordinary skill in the art that a number of changes, modifications or alternations to the invention as described herein may be made, none of which depart from the spirit of the present invention. All such changes, modifications and alterations should therefore be seen as within the scope of the present invention.