Patent Publication Number: US-11649133-B2

Title: Sheet post-processing apparatus

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a sheet post-processing apparatus that performs post-processing such as punch hole forming processing and binding processing on a sheet on which an image is formed by an image forming apparatus such as a copier, a printing machine, and a laser beam printer. 
     Description of the Related Art 
     In recent years, known sheet post-processing apparatuses cause a sheet on which an image is formed by an image forming apparatus such as a copier and a laser beam printer to invert (switchback) in a conveyance path to make a preceding sheet and a following sheet overlap each other to form a sheet bundle of a plurality of sheets and perform prescribed post-processing. Examples of the post-processing include binding processing to perform binding with a staple, punch processing to form a boring hole (punch hole) with a boring apparatus (punch hole forming apparatus), sort processing to perform a sorting operation, or the like. As shown in Japanese Patent Application Laid-open No. 2017-197314, a sheet post-processing apparatus that forms a punch hole along an end edge extending in a sheet conveyance direction in which a sheet is vertically conveyed has been put on the market in recent years. 
     SUMMARY OF THE INVENTION 
     If a following sheet in which a protruding curl is caused is conveyed to a punch hole of a preceding sheet when a sheet bundle is formed by sheets on which a punch hole is formed as shown in Japanese Patent Application Laid-open No. 2017-197314, there is a possibility that the corner portion of the leading end of the following sheet gets snagged on the punch hole. At this time, the subsequent snagged sheet repels with a repulsive force due to its stiffness and may cause the pushing out or skewing of the preceding sheet. If such pushing-out or skewing is caused, there is a problem that the distance of a deviation between the leading end positions of the respective sheets of a sheet bundle or a skew amount does not fall within a certain range. Therefore, the sheet bundle may not be aligned satisfactorily. 
     The present invention has an object of providing a technology that allows post-processing with excellent alignment in a sheet post-processing apparatus that performs punch hole forming processing on a sheet on which an image is formed and performs post processing to form a sheet bundle. 
     In order to achieve the above object, a sheet post-processing apparatus according to the present invention that performs post-processing on a sheet on which an image is formed by an image forming apparatus includes: 
     a punch hole forming portion that forms a punch hole on a sheet; and 
     a sheet bundle forming portion that has a conveyance portion to convey a sheet on which a punch hole is formed and makes a plurality of sheets on which a punch hole is formed overlap each other to form a sheet bundle, the sheet bundle forming portion stopping a first sheet on which a punch hole is precedingly formed on a conveyance path and conveying a second sheet on which a punch hole is formed after the first sheet to a position overlapping the first sheet to form the sheet bundle, wherein 
     the sheet bundle forming portion has a shift portion to make a shift so that a relative position between the first sheet and the second sheet in a width direction orthogonal to a conveyance direction becomes a relative position at which the second sheet does not overlap a punch hole formed on the first sheet to make the second sheet overlap the first sheet, and forms a sheet bundle in which sheets overlapped with each other are deviated in the width direction. 
     In order to achieve the above object, a sheet post-processing apparatus according to the present invention that performs post-processing on a sheet on which an image is formed by an image forming apparatus includes: 
     a punch hole forming portion that forms a punch hole on a sheet; and 
     a sheet bundle forming portion that has a conveyance portion to convey a sheet on which a punch hole is formed and makes a plurality of sheets on which a punch hole is formed overlap each other to form a sheet bundle, the sheet bundle forming portion stopping a first sheet on which a punch hole is precedingly formed on a conveyance path and conveying a second sheet on which a punch hole is formed after the first sheet to a position overlapping the first sheet to form the sheet bundle, wherein 
     the sheet bundle forming portion sets a stop position of the first sheet at a stop position at which the second sheet overlaps the first sheet while preventing a leading end of the second sheet from contacting a punch hole formed on the first sheet to make the second sheet overlap the first sheet. 
     As described above, it is possible to perform post-processing with excellent alignment in a sheet post-processing apparatus that performs punch hole forming processing on a sheet on which an image is formed and performs post-processing to form a sheet bundle. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a cross-sectional view of an image forming apparatus  1  connected to a sheet post-processing apparatus according to the present invention; 
         FIGS.  2 A to  2 D  are schematic views of the boring apparatus of the sheet post-processing apparatus according to the present invention; 
         FIG.  3    is a cross-sectional view of a buffer portion according to the present invention; 
         FIGS.  4 A and  4 B  are outline views of a shift portion according to the present invention; 
         FIGS.  5 A to  5 J  are operation views of the buffer portion according to the present invention; 
         FIGS.  6 A to  6 J  are overhead views of the operation of the buffer portion according to the present invention; 
         FIG.  7    is a view of a transverse position detection sensor according to the present invention; 
         FIG.  8    is a hardware configuration diagram according to the present invention; 
         FIG.  9    is a control block diagram according to the present invention; 
         FIGS.  10 A and  10 B  are views for describing a time at which a downwardly-curled sheet is conveyed to a sheet subjected to boring processing in the buffer portion; 
         FIGS.  11 A to  11 C  are views for describing the buffer operation of a sheet subjected to boring processing in a conventional buffer portion; 
         FIGS.  12 A and  12 B  are flowcharts showing the operation of a sheet post-processing apparatus  4  according to the first embodiment; 
         FIGS.  13 A to  13 D  are upper surface views showing the state of a sheet bundle subjected to buffer processing according the first embodiment; 
         FIG.  14    is a flowchart showing the operation of a sheet post-processing apparatus  4  according to a second embodiment; 
         FIGS.  15 A to  15 D  are upper surface views showing the state of a sheet bundle subjected to buffer processing according to the second embodiment; 
         FIG.  16    is a flowchart showing the operation of a sheet post-processing apparatus  4  according to a third embodiment; 
         FIGS.  17 A to  17 E  are cross-sectional views showing the state of a sheet bundle subjected to buffer processing according to the third embodiment; 
         FIGS.  18 A to  18 E  are upper surface views showing the state of a sheet bundle subjected to buffer processing according to the third embodiment; 
         FIGS.  19 A and  19 B  are views showing the state of an intermediate loading portion  71  according to the present invention when seen from the upper surface of a sheet. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, a description will be given, with reference to the drawings, of embodiments (examples) of the present invention. However, the sizes, materials, shapes, their relative arrangements, or the like of constituents described in the embodiments may be appropriately changed according to the configurations, various conditions, or the like of apparatuses to which the invention is applied. Therefore, the sizes, materials, shapes, their relative arrangements, or the like of the constituents described in the embodiments do not intend to limit the scope of the invention to the following embodiments. 
     First Embodiment 
       FIG.  1    shows a cross-sectional view of an image forming apparatus  1  and a sheet post-processing apparatus  4  that have carried out the present invention. After the description of the simple operations of the respective apparatuses, the detailed operation of the sheet post-processing apparatus  4  will be described. The sheet post-processing apparatus  4  has a boring apparatus  60  that subjects a sheet to boring (punch hole) processing. The sheet punched by the boring apparatus  60  is caused to switchback on a conveyance path and subjected to buffer processing to overlap in which a plurality of sheets are overlapped with each other. Next, a sheet bundle formed by the buffer processing is conveyed on the conveyance path and loaded into an intermediate loading portion  71 . After that, the end of the sheet bundle in a conveyance direction and the end of the sheet bundle in a direction orthogonal to the conveyance direction are aligned by alignment means inside the intermediate loading portion. Then, the sheet bundle aligned in the conveyance direction and the direction orthogonal to the conveyance direction is allowed to be pushed out from the intermediate loading portion  71  and dischargeable to an outside as it is by a bundle sheet discharging roller  36 . 
     A plurality of sheet feeding apparatuses  6  that accommodate a plurality of sheets and feed the sheets one by one with a prescribed sheet feeding interval are connected to the image forming apparatus  1 . The skew of a sheet fed from the sheet feeding apparatuses  6  is corrected by a resist roller  7  and conveyed to a photosensitive drum  9  rotatably supported by an image forming cartridge  8  and a transfer roller  10  to which prescribed charges are applied. Inside the image forming cartridge, a toner image is formed on the surface of the photosensitive drum  9  after the photosensitive drum  9  is subjected to the respective processes of exposure, charging, latent image formation, and development. The latent image formation is carried out by a laser scanner unit  15  that scans the sheet with the blinking of laser light in a conveyance direction and a perpendicular direction using a polygon mirror and a lens to perform image formation. The sheet on which a toner image is formed is fed to a horizontal conveyance portion  14  via a fixing unit  11  that fixes toner on the sheet by heating and pressing. When both sides of the sheet are to be printed, the sheet is temporarily conveyed to an inversion roller  12 , fed to a sheet re-feeding conveyance portion  13  after being subjected to switchback conveyance to switch the leading end and the rear end of the sheet, and conveyed to the resist roller  7  again at a prescribed timing to be subjected to image formation for the second time. 
     On the other hand, the sheet conveyed from the horizontal conveyance portion  14  is delivered to the sheet post-processing apparatus  4  by an entrance roller  21  of the sheet post-processing apparatus  4 . In order to absorb a difference in conveyance speed between a conveyance speed inside the sheet post-processing apparatus  4  and a conveyance speed inside the horizontal conveyance portion  14 , a one-way clutch (not shown) is embedded in a driving portion (not shown). 
     When the discharging destination of the sheet is a sheet discharging upper tray  25  that is a first sheet discharging port, the sheet on which an image is formed is subjected to boring processing and then conveyed on the conveyance path. The leading end of the sheet is delivered from a pre-buffer roller  22  to a sheet discharging inversion roller  24 . The sheet is discharged to the sheet discharging upper tray  25  without being caused to switchback. At this discharging destination, the sheet is not subjected to buffer processing in which a sheet bundle is formed. 
     Next, the buffer processing of a sheet performed when the discharging destination of the sheet is a sheet discharging lower tray  37  that is a second sheet discharging port will be described using  FIG.  3   . The sheet is conveyed from the entrance roller  21  via the pre-buffer roller  22 , and the rear end of the sheet passes through a backflow prevention valve  23  that is urged in a clockwise direction in  FIG.  3    by a spring (not shown). A conveyance path through which the sheet passes at this time is defined as a first conveyance path  91 . The backflow prevention valve  23  plays a role in preventing the sheet that is caused to switchback from being conveyed to the first conveyance path again. Then, the sheet is held by the sheet discharging inversion roller  24  at the timing, and the rotating direction of the sheet discharging inversion roller  24  is inverted after the sheet is temporarily stopped. When the sheet is caused to switchback, the conveyance direction of the sheet is reversed. That is, in a traveling direction in which the sheet is conveyed, the upper and lower surfaces of the sheet remain the same, but the end that served as the leading end of the sheet before the switchback serves as the rear end of the sheet after the switchback. Note that a path through which the sheet passes before the switchback and a path through which the sheet passes after the switchback after passing through the first conveyance path  91  are defined as a switchback path  92   a  and a switchback path  92   b , respectively, and the switchback paths  92   a  and  92   b  are collectively defined as a switchback path  92 . Then, the sheet that is caused to switchback is conveyed to an inside sheet discharging roller  26 . In the present embodiment, configurations relating to the switchback of a sheet such as the pre-buffer roller  22 , the backflow prevention valve  23 , the sheet discharging inversion roller  24 , the inside sheet discharging roller  26 , and the switchback path  92  correspond to switchback means according to the present invention. Note that specific configurations constituting the switchback means are not limited to the configurations shown in the present embodiment. A conveyance path through which the sheet passes after the switchback path  92  at this time is defined as a second conveyance path  93 . Then, at a stage at which the leading end of the sheet reaches the inside sheet discharging roller  26 , the sheet discharging inversion roller  24  cancels its nip and prepares for the reception of a following sheet directed to the sheet discharging inversion roller  24 . After that, the driving of the inside sheet discharging roller  26  is temporarily stopped in a state in which the sheet is held by the inside sheet discharging roller  26 . The following sheet is conveyed from the first conveyance path  91  to the switchback path  92  as described above, and the leading end of the following sheet partially overlaps so as to protrude with respect to the rear end of the preceding sheet. Then, the sheet discharging inversion roller  24  is nipped again at the timing, whereby it is possible to integrally convey the preceding sheet and the following sheet to a position at which the preceding sheet and the following sheet fall within the switchback path  92 . The inside sheet discharging roller  26  conveys the preceding sheet again in an inversion direction. At the same time, the pre-buffer roller  22  conveys the following sheet. The preceding sheet conveyed in the inversion direction from the inside sheet discharging roller  26  and the following sheet conveyed from the pre-buffer roller  22  are integrally overlapped with each other as described above, whereby it is possible to perform a sheet buffer to form a sheet bundle. Further, it is possible to perform a sheet buffer of a plurality of sheets regardless of the lengths or the number of the sheets by repeatedly performing the switchback of the inside sheet discharging roller  26 . In addition, in the present embodiment, configurations relating to the conveyance of a sheet such as the entrance roller  21 , the pre-buffer roller  22 , the sheet discharging inversion roller  24 , the inside sheet discharging roller  26 , the first conveyance path  91 , the switchback path  92 , and the second conveyance path  93  correspond to conveyance means according to the present invention. Note that specific configurations constituting the conveyance means are not limited to the configurations shown in the present embodiment. Moreover, in the present embodiment, configurations relating to the overlap of a sheet such as the entrance roller  21 , the pre-buffer roller  22 , the backflow prevention valve  23 , the sheet discharging inversion roller  24 , the inside sheet discharging roller  26 , the first conveyance path  91 , the switchback path  92 , and the second conveyance path  93  correspond to sheet bundle forming means according to the present invention. Note that specific configurations constituting the sheet bundle forming means are not limited to the configurations shown in the present embodiment. 
     After the sheet buffer, the sheet conveyed from the inside sheet discharging roller  26  is fed to a kicking-out roller  29  via an intermediate conveyance roller  28  on the conveyance path and conveyed to the intermediate loading portion  71  including an intermediate loading upper guide  31  and an intermediate loading lower guide  32 . A longitudinal alignment reference plate  39  is arranged on the most downstream side of the intermediate loading portion  71 . The longitudinal alignment of a sheet bundle is performed in such a manner that the end of the sheet in the conveyance direction is abutted on the longitudinal alignment reference plate  39 . Further, a flexible pressing guide  56  is fixed to the intermediate loading upper guide  31  and contacts a sheet inside the intermediate loading portion  71  with a prescribed pressing force. Further, a half-moon roller  33  (pressing member) having a contact portion that conveys a sheet passing through the kicking-out roller  29  to the longitudinal alignment reference plate  39  is rotatably supported by the intermediate loading upper guide  31  on the downstream side of the pressing guide  56 . As alignment means for aligning a sheet bundle in the conveyance direction, the half-moon roller  33  conveys a conveyed sheet of the sheet bundle after the rear end of the conveyed sheet passes through an intermediate loading front sensor  38 . The half-moon roller  33  rotates while causing its contact portion that is a part of the half-moon roller  33  to contact the sheet bundle in order from the sheet of the lowermost surface of the sheet bundle at a prescribed timing. Thus, a conveyance force toward the longitudinal alignment reference plate  39  is applied in order from the sheet of the lowermost surface of the sheet bundle, the sheets of the sheet bundle sequentially contact the longitudinal alignment reference plate  39 , and the end of the sheet bundle is aligned. Note that as previously described in the mechanism of the buffer portion, the sheets of the sheet bundle are overlapped with each other from below in order of their conveyance to form the sheet bundle. Further, the conveyance pressure of the half-moon roller  33  is adjusted so that the half-moon roller  33  slips on the sheets after the sheets contact the longitudinal alignment reference plate  39 . In addition, on the downstream side of the kicking-out roller  29 , a bundle pressing flag  30  that suppresses the lift of the rear end of a sheet to prevent interference between the rear end of a sheet loaded into the intermediate loading portion  71  and the leading end of a following sheet is rotatably supported. The lower surface of the bundle pressing flag  30  presses the upper surface of the rear end of a sheet previously discharged into the intermediate loading portion  71  and has the function of causing the leading end of a sheet subsequently discharged by the kicking-out roller  29  to pass through a place above the rear end of the previously-discharged sheet. 
     Further,  FIGS.  19 A and  19 B  show views of the intermediate loading portion  71  when seen from the upper surface of a sheet. In order to align the ends of the sheets in a direction (transverse direction) orthogonal to the conveyance direction of a sheet bundle, the intermediate loading lower guide  32  is provided with a fixed transverse alignment reference plate  81  and a transverse alignment jogger  82  movable in a sheet width direction with respect to the transverse alignment reference plate. As shown in  FIG.  19 A , a sheet S 1  is separated from the kicking-out roller  29  and conveyed in an N direction by the rotation of the half-moon roller  33 . After that, the transverse alignment jogger  82  moves in an M1 direction that is a direction toward the transverse alignment reference plate  81 . Thus, as shown in  FIG.  19 B , the end (lateral end) of the sheet S 1  in the direction (transverse direction) orthogonal to the conveyance direction of the sheet bundle is abutted on the transverse alignment reference plate  81 , whereby the transverse position of the sheet S 1  is aligned (sheet bundle transverse alignment means). 
     After the alignment of a prescribed number of sheets is finished in the manner described above, a binding operation is performed by a stapler (not shown) and a sheet bundle is pushed out when a bundle sheet discharging guide  34  connected to a guide driving portion  35  moves in parallel in the direction of the bundle sheet discharging roller  36  from a standby position. At a stage at which the leading end of the sheet bundle reaches the bundle sheet discharging roller  36 , the bundle sheet discharging guide  34  stops and returns to the standby position again. The bundle sheet discharging roller  36  discharges the sheet bundle received from the bundle sheet discharging guide  34  to the sheet discharging lower tray  37 . Then, the sheet discharging upper tray  25  and the sheet discharging lower tray  37  sequentially detect the positions of sheet surfaces with a sheet surface detection sensor (not shown) and move in an A2 direction and a B2 direction, respectively, when sheets are piled up. 
     Next, the boring apparatus  60  will be described in detail using  FIGS.  2 A to  2 D . The boring apparatus  60  is a rotary-type boring apparatus that bores a sheet with a rotating punch  61 . As shown in  FIG.  2 A , the boring apparatus  60  has the punch  61  that is rotatably supported about a punch shaft  65  and a dice  62  that rotates about a dice shaft  66 . An entrance sensor  27  is arranged on the upstream side of the boring apparatus  60  on the conveyance path. The dice  62  has a dice hole  64  capable of meshing with the punch  61 , and the punch shaft  65  and the dice shaft  66  are driven by a punch motor  646  and mesh with gears (not shown). When the punch motor  646  serving as a driving source rotates, the punch  61  rotates in a clockwise direction shown by an arrow and the dice  62  rotates in a counterclockwise direction in  FIG.  2 A .  FIG.  2 A  is a schematic view showing a state in which the punch  61  is on standby at a home position that is a previously-set position before boring starts. The punch  61  and the dice  62  are controlled to be located at the home position when an image forming job for forming an image on a sheet starts and ends. Further, the punch  61  and the dice  62  are also located at the home position during a period in which no job is input and during a job performed without a punch. When located at the home position, the punch  61  and the dice  62  are arranged so as not to hinder the conveyance of a sheet.  FIG.  2 B  is a view showing a state in which the punch  61  has rotated to a position at which the punch  61  contacts a sheet. At the position, the boring of the sheet starts.  FIG.  2 C  is a schematic view showing the punch  61  and the dice  62  placed at a meshing position. When the punch  61  and the dice  62  are located at the meshing position, the punch  61  meshes with the dice hole  64  of the dice  62  to bore the sheet.  FIG.  2 D  is a schematic view showing the punch  61  and the dice  62  located at a boring end position. At this position, the punch  61  separates from the sheet. 
     As described above, the punch  61  is on standby at the home position until a sheet is conveyed. After that, on the basis of the fact that the entrance sensor  27  has detected the leading end of the sheet, the rotational driving of the punch shaft  65  and the dice shaft  66  is started at a prescribed timing by the punch motor  646 , and the boring of the sheet is made possible while the conveyance of the sheet is continued without stopping the sheet. 
     When a plurality of holes are bored on a sheet, the punch  61  and the dice  62  continue their rotation to repeatedly perform the operations of  FIGS.  2 A to  2 D  again by the number of the holes. An interval at which the punch  61  rotates from the position shown in  FIG.  2 D  to the position shown in  FIG.  2 A  is an interval at which the punch  61  does not contact a sheet, and the punch  61  and the dice  62  are allowed to rotate at any speed. By controlling the rotation speeds of the punch  61  and the dice  62  and adjusting the time of the rotational movement of the punch  61  and the dice  62 , it is possible to achieve a standard in which a distance between boring is required, e.g., 80 mm or 108 mm. Thus, a rotary-type boring apparatus is allowed to perform boring processing complying with various standards according to the destination of the apparatus without replacing a punch. 
     Next, the buffer portion and shift means constituting a part of the buffer portion will be described using  FIGS.  3 ,  4 A,  4 B, and  5 A to  5 J .  FIG.  3    is a cross-sectional view of the buffer portion,  FIGS.  4 A and  4 B  are outline views of the shift portion, and  FIGS.  5 A to  5 J  are operation views of the buffer portion. 
     The configuration of the buffer portion will be described using  FIG.  3   . A portion between the entrance roller  21  and the pre-buffer roller  22  is constituted by an entrance upper guide  40  and an entrance lower guide  41 . A sheet is conveyed on a conveyance path constituted by both the conveyance guides. Further, an entrance sensor  27  is arranged at the entrance upper guide  40 . The backflow prevention valve  23  is rotatably arranged by a rotation shaft  23   a  with respect to an inside sheet discharging upper guide  46  on the downstream side of the pre-buffer roller  22  and urged at all times in a C2 direction by a spring (not shown). The leading end of the backflow prevention valve  23  overlaps an inversion upper guide  42  in a comb-tooth shape. The backflow prevention valve  23  rotates in a C1 direction and causes the sheet to pass therethrough only when the sheet is fed by the pre-buffer roller  22 . When the rear end of the sheet passes through the backflow prevention valve  23 , the backflow prevention valve  23  rotates with the above-described spring (not shown) in the C2 direction and restores to its original position. Note that as conveyance paths, the entrance upper guide  40  and the entrance lower guide  41  form the first conveyance path  91 , the inversion upper guide  42  and an inversion lower guide  43  form the switchback path  92  on which the conveyance direction of the sheet is switched according to a switchback, and the inside sheet discharging upper guide  46  and an inside sheet discharging lower guide  47  form a second conveyance path  93  on which the sheet is conveyed after the switchback. 
     The sheet discharging inversion roller  24  is constituted by an inversion upper roller  24   a  and an inversion lower roller  24   b , and driving is supplied to both the rollers. Further, a separation lever  44  is connected to the inversion upper roller  24   a . The separation lever  44  is rotatably supported by a lever support-point shaft  44   a  with respect to the inversion upper guide  42  and rotatably connected to a plunger solenoid  45  by a solenoid connection shaft  44   b . When a current flows through the plunger solenoid  45 , the separation lever  44  rotates in an E1 direction since the core of the plunger solenoid  45  moves in a D1 direction. As a result, the sheet discharging inversion roller  24  is put into a separated state. Further, when the current flowing through the plunger solenoid  45  stops, the inversion upper roller  24   a  moves in an E2 direction with a sheet discharging inversion roller pressing spring  48  and the plunger solenoid  45  moves in a D2 direction. 
     Next, the shift means will be described using  FIGS.  4 A and  4 B . The shift means is configured to perform shift movement to move a sheet in a direction (width direction) orthogonal to the conveyance direction. The shift means is fixed to the sheet discharging inversion roller  24  as shown in  FIG.  4 A  and transmits the rotational motion of a shift motor  645  to a slider  73  in a holding member  76  arranged to hold the slider  73  as linear motion using a shift crank mechanism  72 . The holding member  76  includes holes ( 76   a ,  76   b ,  76   c , and  76   d ) through which the inversion upper roller  24   a  and the inversion lower roller  24   b  are allowed to penetrate. With a configuration in which the inversion upper roller  24   a  and the inversion lower roller  24   b  penetrate the through-holes, it is possible to shift the inversion upper roller  24   a  and the inversion lower roller  24   b  in F1 and F2 directions. Further, with the formation of a shaft hole penetrating the slider  73  as shown in  FIG.  4 B , it is also possible to cause the inversion upper roller  24   a  to contact and separate from the inversion lower roller  24   b  while maintaining the rotation state of the sheet discharging inversion roller  24 . The shift crank mechanism  72  has a shift-home position sensor  74  that detects the home position of a shift position and a light-shielding flag  75  attached to the slider  73 . Further, on the basis of the home position detected by the shift-home position sensor  74 , the shift motor  645  is driven to control a shift amount. 
     The outline of shift and buffer operations will be described using  FIGS.  5 A to  5 J and  6 A to  6 J . Conveyed sheets are shown as S 1 , S 2 , and S 3  and conveyed in order of S 1 , S 2 , and S 3 . 
     (1) The leading end of the sheet S 1  passes through the switchback path  92  via the first conveyance path  91 . Then, at a timing at which the rear end of the sheet S 1  has passed through the pre-buffer roller  22 , the shift motor  645  is driven to shift the sheet to a position at which the lateral end of the sheet S 1  that is a preceding sheet (first sheet) aligns with the lateral end of the sheet S 2  that is a following sheet (second sheet) in the direction (width direction) orthogonal to the conveyance direction ( FIGS.  5 A and  6 A ). 
     (2) The conveyance of the sheet S 1  temporarily stops at a position at which the sheet S 1  has passed through the backflow prevention valve  23  ( FIGS.  5 B and  6 B ). 
     (3) The sheet S 1  is caused to switchback by the sheet discharging inversion roller  24  of which the rotating direction is inverted and conveyed toward the inside sheet discharging roller  26 . Note that the spot of the sheet S 1  that served as the leading end in the conveyance direction is conveyed as the rear end of the sheet S 1  in turn by the switchback. In the switchback, the conveyance direction of the sheet is switched, while the upper and lower surfaces of the conveyed sheet remain the same before and after the switchback ( FIGS.  5 C and  6 C ). 
     (4) The sheet S 1  is held by the inside sheet discharging roller  26  and stops at a position at which the sheet S 1  is conveyed by a prescribed amount. Further, the inversion upper roller  24   a  separates at a timing at which the sheet S 1  is held by the inside sheet discharging roller  26 . Then, after the separation of the inversion upper roller  24   a , the leading end of the following sheet S 2  passes through the switchback path  92  via the first conveyance path  91  and passes through the sheet discharging inversion roller  24 . Further, after the separation of the inversion upper roller  24   a , the sheet discharging inversion roller  24  changes its rotating direction ( FIGS.  5 D and  6 D ). 
     (5) At a timing at which the rear end of the following sheet S 2  is conveyed by a prescribed amount after passing through the entrance sensor  27 , the conveyance of the sheet S 1  held by the inside sheet discharging roller  26  starts again toward the sheet discharging inversion roller  24 . Then, at a timing at which the relative speed of the preceding sheet S 1  conveyed toward the sheet discharging inversion roller  24  becomes approximately equal to that of the following sheet S 2  conveyed toward the sheet discharging inversion roller  24 , the inversion upper roller  24   a  contacts a sheet bundle and holds the sheets S 1  and S 2  at the same time with their transverse positions aligned ( FIGS.  5 E and  6 E ). 
     (6) The sheet bundle of the sheets S 1  and S 2  held by the sheet discharging inversion roller  24  is integrally conveyed in the downstream direction of the first conveyance path  91  (the upstream direction of the second conveyance path  93 ). Note that the sheet S 2  before the switchback has its leading end in the downstream direction of the first conveyance path  91  and the sheet S 1  after the switchback has its leading end in the downstream direction of the second conveyance path  93  (the sheet S 1  has its rear end in the upstream direction of the second conveyance path  93 ) at this time. The integrated sheet bundle of the sheets S 1  and S 2  is conveyed as described above. After the respective rear ends of the sheets S 1  and S 2  pass through the backflow prevention valve  23 , the sheet discharging inversion roller  24  stops, and the sheet bundle is conveyed to a position at which the sheet bundle falls within the switchback path  92 . At this time, the position of the leading end of the following sheet S 2  is arranged so as to overlap the position of the rear end of the preceding sheet S 1  ( FIGS.  5 F and  6 F ). 
     (7) The shift position of the sheet bundle of the sheets S 1  and S 2  held by the sheet discharging inversion roller  24  is set at an ideal position. Note that the ideal position represents a prescribed shift position other than a home position when it is assumed that a position at which a following sheet is conveyed is the home position. The shift of the sheet bundle is started as described above to move the sheet bundle in the direction orthogonal to the conveyance direction. Then, the sheet bundle is caused to switchback toward the inside sheet discharging roller  26  and conveyed in the direction of the second conveyance path  93  ( FIGS.  5 G and  6 G ). 
     (8) At a timing at which the leading end of the sheet bundle of the sheets S 1  and S 2  is held by the inside sheet discharging roller  26 , the inversion upper roller  24   a  is separated to prepare for the reception of a following sheet S 3  ( FIGS.  5 H and  6 H ). 
     (9) After the separation of the inversion upper roller  24   a , a shift mechanism is moved to the home position ( FIGS.  5 I and  6 I ). 
     (10) At a timing at which the rear end of the sheet bundle of the sheets S 1  and S 2  conveyed in the direction of the second conveyance path  93  passes through the sheet discharging inversion roller  24 , the sheet discharging inversion upper roller  24   a  is caused to contact the sheet S 3  entering the switchback path  92  from the first conveyance path  91  and hold and convey the same. Note that the sheet S 3  is a following sheet (second sheet) while the sheet S 2  is a preceding sheet (first sheet) in this process ( FIGS.  5 J and  6 J ). 
     With the repetition of the above operations, it is possible to continuously carry out a buffer operation. Further, in the conveyance direction of the buffer portion, sheets overlap each other in a state in which the distance of a deviation between the rear end of a preceding sheet S 1  and the leading end of a following sheet S 2  that are caused to switchback falls within a certain range. It is possible to form a sheet bundle in which the position of the leading end of the following sheet S 2  overlaps the position of the rear end of the preceding sheet S 1  by making the sheets overlap each other as described above. It is also possible to realize the shift and the buffer of at least three sheets by the repetition of the operations (1) to (5). Note that the direction in which the sheets are shifted is shown as F1 and the movement to the home position is shown as F2 in  FIGS.  5 A to  5 J  but are not limited to the above example. 
     Next, transverse position detection means according to the present embodiment will be described using  FIGS.  1  and  7   . In the present embodiment, a transverse position detection sensor  70  is provided as the transverse position detection means. As shown in  FIG.  7   , the transverse position detection sensor  70  is arranged to be capable of detecting all the transverse positions of the sheets of a maximum-size sheet to a minimum-size sheet in the conveyance direction. In the present embodiment, a line sensor is used as the detection portion of the transverse position detection sensor and determines a sheet transverse position with a boundary that detects the presence or absence of a sheet on the line sensor. Further, since the position of the line sensor is fixed inside the sheet post-processing apparatus  4 , the line sensor calculates the position of a sheet conveyed inside the sheet post-processing apparatus  4  from the position of a pixel that has detected a sheet transverse position on the line sensor. The sheet post-processing apparatus  4  detects the transverse position of a sheet with the transverse position detection sensor  70  when a prescribed time elapses after the leading end of the sheet has passed through the entrance sensor  27 . 
     A hardware configuration according to the present embodiment will be described using  FIG.  8   .  FIG.  8    is a diagram showing a hardware configuration in an image forming system in which the sheet post-processing apparatus  4  is attached to the image forming apparatus  1  shown in  FIG.  1   , mainly the configuration of the sheet post-processing apparatus  4  relating to the control of the present embodiment. Symbol  601  shows a video controller that exercises control over the image forming apparatus  1  and the sheet post-processing apparatus  4 . Symbol  602  shows an engine control portion that controls the image forming apparatus  1 . Symbol  603  shows a main control portion that controls the sheet post-processing apparatus  4 . Symbol  604  shows a serial command transmission signal line that transmits a command from the video controller  601  to the engine control portion  602  through serial communication. Symbols  605  shows a serial command transmission signal line that transmits a command from the video controller  601  to the main control portion  603  through serial communication. Symbol  606  shows a serial status transmission signal line that transmits status data from the engine control portion  602  to the video controller  601  instead of a command through serial communication. Symbol  607  shows a serial status transmission signal line that transmits status data from the main control portion  603  to the video controller  601  instead of a command through serial communication. In performing an image forming operation, the video controller  601  transmits a serial command to the engine control portion  602  and the main control portion  603  while receiving status data from the engine control portion  602  and the main control portion  603  to perform control. When a plurality of apparatuses are connected and operated as described above, the video controller  601  manages the control or the states of the respective apparatuses and maintains operation consistency between the respective apparatuses. 
     The main control portion  603  is a control IC configured so that a CPU  608  or the like that will be described later is connected to an I/O port  613  with which control signals are input to and output from various units inside the sheet post-processing apparatus  4  through a bus  614 . Note that portions connected to the I/O port  613  through the bus  614  are the CPU  608  that controls the various operations of the sheet post-processing apparatus  4 , a RAM  609  that temporarily stores control data, a ROM  610  that stores a control table necessary for a program or an operation in a non-volatile manner, communication means  611  for performing communication processing with the video controller  601 , and a system timer  612  that generates a timing necessary for various control. An entrance sensor input circuit  615  receives a signal from an entrance sensor  27  and transmits the received signal to the main control portion  603 . A shift-home position sensor input circuit  616  receives a signal from a shift-home position sensor  74  and transmits the received signal to the main control portion  603 . A transverse position detection sensor input circuit  617  receives a signal from a transverse position detection sensor  70  and transmits the received signal to the main control portion  603 . Then, each of an entrance motor driving circuit  618 , a pre-buffer motor driving circuit  619 , a sheet discharging inversion motor driving circuit  620 , an inside sheet discharging motor driving circuit  621 , a shift motor driving circuit  622 , a plunger solenoid driving circuit  623 , and a punch motor driving circuit  624  receives a control signal from the main control portion  603 . After receiving the control signal, each of the entrance motor  641 , the pre-buffer motor  642 , the sheet discharging inversion motor  643 , the inside sheet discharging motor  644 , the shift motor  645 , the plunger solenoid  45 , and the punch motor  646  drives. 
     Function blocks according to the present embodiment will be described using  FIG.  9   . Here, only portions relating to the present embodiment will be extracted and described. In  FIG.  9   , symbol  603  shows the function of the main control portion of the sheet post-processing apparatus  4  that performs sheet conveyance control. The main control portion  603  is constituted by the communication means  611 , the system timer  612 , sheet conveyance control means  701 , shift control means  702 , sensor control means  720 , motor control means  721 , solenoid control means  722 , buffer control means  707 , and punch control means  708 . The sensor control means  720  is means for inputting signals from the entrance sensor  27 , the shift-home position sensor  74 , and the transverse position detection sensor  70  to the sheet conveyance control means  701 . The sheet conveyance control means  701  controls the shift control means  702 , the buffer control means  707 , the motor control means  721 , the solenoid control means  722 , and the punch control means  708  on the basis of instructions from the sensor control means  720 . By these control means, the entrance motor  641 , the pre-buffer motor  642 , the sheet discharging inversion motor  643 , the inside sheet discharging motor  644 , the shift motor  645 , the plunger solenoid  45 , and the punch motor  646  are driven. Note that the driving target of the entrance motor  641  is the entrance roller  21 , the driving target of the pre-buffer motor  642  is the pre-buffer roller  22 , the driving target of the sheet discharging inversion motor  643  is the sheet discharging inversion roller  24 , and the driving target of the inside sheet discharging motor  644  is the inside sheet discharging roller  26 . Further, the driving target of the shift motor  645  is the shift crank mechanism  72 , and the driving target of the plunger solenoid  45  is the separation lever  44 . The driving targets of the punch motor  646  are the punch  61  and the dice  62 . 
     The shift control means  702  is constituted by shift amount calculation means  703 , timing management means  706 , alignment position movement control means  704 , and shift-home movement means  705 . The shift amount calculation means  703  calculates a buffer shift amount for aligning and buffering a sheet on the basis of a plurality of information. The plurality of information includes sheet width information instructed from the video controller  601 , environment information such as temperature and humidity, sheet buffer information, and information on the transverse position of a sheet received from the sensor control means  720 . Further, the shift amount calculation means  703  is means for, when inverting and conveying a plurality of buffered sheets, calculating inversion shift amounts to align the transverse positions of the plurality of sheets with each other at a prescribed position, notifying the alignment position movement control means  704  of these shift amounts, and indicating the rotating direction of a motor to the shift-home movement means  705 . The timing management means  706  is means for notifying the shift-home movement means  705  of a timing at which the movement of the shift means to a home is needed. The timing management means  706  notifies the alignment position movement control means  704  of a timing at which the shift control is performed on the basis of the signal information of the entrance sensor  27  received from the sensor control means  720 . Further, the timing management means  706  includes the buffer control means  707  and performs motor control or solenoid control for the buffer control of a plurality of sheets with the buffer control means  707 . The alignment position movement control means  704  is means for calculating the driving amount of a motor from a shift amount when receiving the notification of a timing at which shift movement is needed and the shift amount, controlling the respective motors using the motor control means  721 , and performing the shift of a necessary amount of sheets. The shift-home movement means  705  is means for moving the shift means to a home position when receiving the notification of a timing at which movement to the home is needed. The movement means controls the motor control means  721  and the solenoid control means  722  according to a rotating direction instructed from the shift amount calculation means  703  and performs the movement on the basis of the signal information of the shift-home position sensor  74  received from the sensor control means  720 . In addition, the punch control means  708  is means for performing the control of the boring apparatus. 
     Here, when a boring hole Pa is formed on a preceding sheet S 1  as shown in  FIGS.  10 A and  10 B , there is a case that the leading end of a following sheet S 2  contacts the boring hole Pa of the preceding sheet S 1 .  FIG.  10 A  shows a time at which the downwardly-curled following sheet S 2  contacts the boring hole of the sheet S 1 .  FIG.  10 B  shows a state in which the leading end of the downwardly-curled sheet S 2  jumps up with a repulsive force due to its stiffness and pushes out the sheet S 1  to cause a skew. Note that the downwardly-curled sheet here represents a sheet curled in a protruding shape against the sheet S 1 . Alternatively, the downwardly-curled sheet may be expressed as a curved sheet with its left and right ends positioned below the center of the sheet in a vertical direction. One of reasons causing a curl in a sheet as described above is an amount of heat applied to the sheet by the fixing unit  11 . When an amount of heat is applied from the fixing unit  11  to fix a toner image on a sheet, the sheet shrinks due to the evaporation of water contained in the sheet. At this time, there is a case that the amount of heat applied is different between the front surface and the rear surface of the sheet depending on the configuration of the fixing unit  11 . In this case, a downward curl is caused in the sheet since there is a difference in a shrinkage degree between the front surface and the rear surface of the sheet. In addition, when humidity is high as a surrounding environment, a downward curl is easily caused since the moisture absorption of the sheet accelerates and water contained in the sheet increases. 
       FIGS.  11 A to  11 C  are views showing a state in which the buffer operation of a bored sheet S 1  and a bored sheet S 2  is performed and a state in which pushing out is caused during buffer processing.  FIG.  11 A  shows a state in which the corner portion of the leading end of the following sheet S 2  contacts a boring hole Pa of the sheet S 1  in a state in which the sheet S 1  is placed in the buffer portion. Next,  FIG.  11 B  shows a state in which the corner portion of the leading end of the sheet S 2  is held as a sheet bundle while contacting the boring hole Pa of the sheet S 1 . Further,  FIG.  11 C  shows a state in which the sheet bundle of the bored sheet S 1  and the bored sheet S 2  is stopped to perform a switchback toward the inside sheet discharging roller  26  as described in  FIG.  5 F . At this time, the retention of the inversion upper roller  24   a  to hold the sheet bundle of the sheets S 1  and S 2  is not strong since the inversion upper roller  24   a  is urged by a spring. Accordingly, when the corner portion of the leading end of the sheet S 2  contacting the boring hole Pa of the sheet S 1  jumps up at a certain timing, there is a case that the sheet S 1  is pushed out in the conveyance direction. If the pushing out of the sheet S 1  is caused, the protruding amounts of the sheets S 1  and S 2  after the end of the buffer operation are different from expected amounts. 
     After the sheet bundle is conveyed to the intermediate loading portion  71  inside the sheet post-processing apparatus  4 , the half-moon roller  33  conveys the sheets of the sheet bundle to the longitudinal alignment reference plate  39  at a prescribed timing to align the ends of the sheets with each other in the conveyance direction of the sheet bundle. At this time, the half-moon roller  33  rotates while contacting the sheets in order from the sheet of the lowermost surface of the sheet bundle through its contact portion that is a part of the half-moon roller  33 . Thus, a conveyance force is applied toward the longitudinal alignment reference plate  39  in order from the sheet of the lowermost surface of the sheet bundle, the sheets sequentially contact the longitudinal alignment reference plate  39 , and the ends of the sheet bundle are aligned with each other. Note that as described in the mechanism of the buffer portion, the sheets of the sheet bundle overlap each other from below in order of their conveyance to form the sheet bundle. Therefore, if the overlap of the sheets of the sheet bundle is different from an expected one as in a case in which the preceding sheet is pushed out by the following sheet, there is a possibility that the leading ends of the sheets do not contact the longitudinal alignment reference plate  39  and the sheet bundle inside the intermediate loading portion  71  is not aligned. Accordingly, in order to solve a problem that protruding amounts during a buffer operation are different from expected amounts as described above, the following control is performed as the first embodiment to avoid the occurrence of pushing out caused by the boring hole of the preceding sheet and the corner portion of the leading end of the following sheet. 
       FIGS.  12 A and  12 B  are flowcharts showing the operation of the sheet post-processing apparatus  4  according to the present embodiment.  FIG.  12 A  is a control flowchart of the shift amount calculation means  703 . The shift amount calculation means  703  activates at a timing at which a sheet width and environment information are notified from the video controller  601  and waits for the detection of the leading end of a sheet conveyed from the image forming apparatus  1  by the entrance sensor  27  (S 1201   a ). Using a timing at which the leading end of the sheet has turned on the entrance sensor  27  as a starting point, the shift amount calculation means  703  waits for a timing until the transverse position of the sheet is detected by the transverse position detection sensor  70  (S 1202   a ). The shift amount calculation means  703  acquires information on the transverse position of the detected sheet (S 1203   a ) and information on the sheet width and the environment information instructed from the video controller  601  (S 1204   a ). On the basis of these information, the shift amount calculation means  703  calculates a buffer shift amount to move the preceding sheet to a position at which the leading end of a following sheet does not contact a boring hole of the preceding sheet (S 1205   a ). Then, the shift amount calculation means  703  notifies the alignment position movement control means  704  of the buffer shift amount (S 1206   a ). Similarly, the shift amount calculation means  703  calculates an inversion shift amount (S 1207   a ) and notifies the alignment position movement control means  704  of the inversion shift amount ( 51208   a ). Finally, the shift amount calculation means  703  notifies the shift-home movement means  705  of the rotating amount of the shift motor  645  to return to a home position ( 51209   a ) and ends the processing. 
       FIG.  12 B  is a control flowchart of the sheet conveyance control means  701 . Since the sheet conveyance control means  701  includes the shift control means  702  and the buffer control means  707 , the present control flow is represented as including the control flow of the shift control means  702  and the buffer control means  707 . The shift control means  702  moves the shift means to the home position (S 1201   b ) and performs buffer processing on the sheet conveyed with the calculated buffer shift amount. Then, it is shifted in the opposite direction to the home position (S 1202   b ). When the position of the shifted sheet reaches the upper limit of the operation range of the shift means (Yes in S 1203   b ), the sheet conveyance control means  701  feeds the sheet bundle into the intermediate loading portion  71  (S 1204   b ) and ends a series of the processing. When the position of the shifted sheet does not reach the upper limit of the operation range of the shift means (No in S 1203   b ), the sheet conveyance control means  701  receives a following sheet without moving the position of the shift means and performs buffer processing and shift processing with the calculated buffer shift amount. 
       FIGS.  13 A to  13 D  are views showing the state of a sheet bundle in the buffer portion of the sheet post-processing apparatus  4  in a case in which the operation processing of  FIGS.  12 A and  12 B  is performed. After moving the shift means to the home position, the buffer portion receives a bored sheet S 1  over the first conveyance path  91  and the switchback path  92  ( FIG.  13 A ). Next, the buffer portion performs buffer processing and shift processing with the sheet discharging inversion roller  24 . As a result, the sheet S 1  is arranged at a shift position at which the sheet S 1  has moved in a direction orthogonal to the conveyance direction, and put over the switchback path  92  and the second conveyance path  93  ( FIG.  13 B ). Then, a subsequent curled sheet S 2  is conveyed to the buffer portion, and the buffer portion receives the sheet S 2  over the first conveyance path  91  and the switchback path  92 . At this time, the buffer portion receives the sheet S 2  with the position of the shift means unchanged ( FIG.  13 C ). As described above, the following sheet S 2  is conveyed to the home position while the conveyed preceding sheet S 1  is shifted. Therefore, it is possible to overlap the sheets with each other in a state in which the respective relative positions of the preceding sheet S 1  and the following sheet S 2  are deviated in a width direction. After that, the buffer portion performs buffer processing and shift processing with the sheet discharging inversion roller  24 , and the sheet S 2  is arranged at the shift position and put over the switchback path  92  and the second conveyance path  93  ( FIG.  13 D ). By performing the buffer processing and the shift processing in combination as described above, it is possible to form a sheet bundle while preventing the leading end of the sheet S 2  from contacting the boring hole of the sheet S 1 . Note that a sheet bundle in which the leading end of the following sheet S 2  overlaps the rear end of the preceding sheet S 1  caused to switchback in the conveyance direction is formed during buffer processing in the present embodiment as well. 
     As described above, it is possible to form a sheet bundle that is free from an unexpected and sudden deviation caused when the leading end of a sheet contacts a boring hole even when the sheet subjected to boring processing is conveyed to the buffer portion according to the present embodiment. The present embodiment shows an example of the buffer of two sheets. However, when sheets are overlapped with each other in the buffer processing of at least three sheets, the sheets may be overlapped with each other in a direction orthogonal to the conveyance direction in such a manner that the leading end of a following sheet avoids a boring hole over the rear end of a preceding sheet caused to switchback in the conveyance direction in the same direction. Sheet bundles are formed in a staircase pattern when the sheets are overlapped with each other in one direction as in the present embodiment. 
     Note that although the above embodiment assumes a case in which sheets forming a sheet bundle in the buffer portion have the same size and a downward curl is caused in a following sheet, the present invention is applicable so long as a relationship in which sheets forming a sheet bundle have different sizes but the leading end of a following sheet contacts a boring hole of a preceding sheet is established. Further, shift control to avoid a boring hole is applied to all papers in the present embodiment. However, the shift control to avoid a boring hole may be applied only to a sheet that is easily curled (such as a thin paper and a moisture absorption sheet) or a sheet in which a protruding curl is caused and may not be applied to a sheet that is not curled. Further, a setting like a curled sheet conveyance mode to change an operation when a curled sheet is conveyed may be provided in the image forming system or the sheet post-processing apparatus, and the shift control to avoid a boring hole may be employed only when the setting becomes valid. 
     Second Embodiment 
     Next, a second embodiment of the present invention will be described. In the present embodiment, a preceding sheet is not deviated in the same direction unlike the first embodiment but buffer processing and shift processing are alternately performed so as not to contact a boring hole. Since a hardware configuration and a control block diagram are the same as those of the first embodiment and the operation of a sheet post-processing apparatus  4  is also the same as that of the first embodiment in the present embodiment, their descriptions will be omitted. 
       FIG.  14    is a control flowchart of the sheet post-processing apparatus  4  according to the present embodiment. Shift control means  702  moves shift means to the upper limit of an operation range (S 1401 ). The shift control means  702  receives a sheet after completing the movement of the shift means and performs buffer processing (S 1402 ). Next, the shift control means  702  moves the shift means to a shift-home position while receiving the sheet (S 1403 ). The shift control means  702  receives a following sheet and performs buffer processing (S 1404 ). Then, the shift control means  702  determines whether the buffer processing of a prescribed number of sheets has been completed. When the buffer processing has been completed (Yes in S 1405 ), the shift control means  702  feeds a sheet bundle into an intermediate loading portion (S 1406 ). When the buffer processing of the prescribed number of sheets has not been completed (No in S 1405 ), the shift control means  702  performs the processing of S 1401  to S 1404  to generate a sheet bundle. 
       FIGS.  15 A to  15 D  are views showing the state of a sheet bundle in the buffer portion of the sheet post-processing apparatus  4  in a case in which the operation processing of  FIG.  14    is performed. After moving the shift means to the upper limit position of the operation range (moving the shift means to the lowermost position in  FIG.  15 A ), the shift control means  702  receives a sheet S 1  over a first conveyance path  91  and a switchback path  92  ( FIG.  15 A ). Next, the shift control means  702  holds the sheet S 1  with a sheet discharging inversion roller  24  and moves the shift means to the shift-home position ( FIG.  15 B ). Then, when the shift control means  702  performs buffer processing, the sheet S 1  is arranged over a switchback path  92  and a second conveyance path  93 . Then, after receiving a following sheet S 2  over the first conveyance path  91  and the switchback path  92 , the shift control means  702  performs buffer processing to form a sheet bundle ( FIG.  15 C ). Then, the shift control means  702  shifts the shift means to the upper limit position of the operation range (shifts the shift means to the lowermost position in  FIG.  15 D ) and performs buffer processing ( FIG.  15 D ). After completing the buffer processing of a prescribed number of sheets, the shift control means  702  feeds a sheet bundle into the intermediate loading portion. By performing buffer processing and shift processing in combination as described above, it is possible to form a sheet bundle while preventing the leading end of the following sheet S 2  from contacting a boring hole of the preceding sheet S 1 . In the first embodiment, the sheets of a sheet bundle are overlapped with each other in the same direction. In the second embodiment, shift processing in one direction and shift processing in the other direction are alternately performed, whereby sheets are alternately overlapped with each other at the upper limit position of the shift operation range and the home position. Therefore, the present embodiment is characterized in that the formation of a sheet bundle having a constant width in a direction orthogonal to a conveyance direction is allowed. Note that a sheet bundle in which the leading end of the following sheet S 2  overlaps the rear end of the preceding sheet S 1  caused to switchback in the conveyance direction is formed during buffer processing in the present embodiment as well. 
     As described above, it is possible to form a sheet bundle that is free from an unexpected and sudden deviation while preventing the leading end of a sheet from contacting a boring hole even when the sheet subjected to boring processing is conveyed to the buffer portion according to the present embodiment. The present embodiment shows an example of the buffer of two sheets. However, when the buffer of at least three sheets is performed, the sheets may be alternately deviated from each other so as to avoid a boring hole. Further, the movement range of the shift means is set at the upper limit position of the operation range in the present embodiment. However, the present invention is applicable so long as the movement range is a prescribed amount with which it is possible to avoid a boring hole. The present invention is applicable so long as a relationship in which sheets forming a sheet bundle have different sizes but the leading end of a following sheet contacts the boring hole of a preceding sheet is established. In the present embodiment, shift control to avoid a boring hole is applied to all papers. However, the shift control to avoid a boring hole may be applied only to a sheet that is easily curled (such as a thin paper and a moisture absorption sheet) or a sheet in which a protruding curl is caused and may not be applied to a sheet that is not curled. Further, a setting like a curled sheet conveyance mode to change an operation when a curled sheet is conveyed may be provided in the image forming system or the sheet post-processing apparatus, and the shift control to avoid a boring hole may be employed only when the setting becomes valid. 
     Third Embodiment 
     In addition, a third embodiment of the present invention will be described. In the present embodiment, a sheet is not deviated in a direction orthogonal to a conveyance direction but a timing at which a preceding sheet and a following sheet are overlapped with each other is adjusted to perform buffer processing so as to prevent the leading end of a sheet from contacting a boring hole. 
       FIG.  16    is a control flowchart of a sheet post-processing apparatus  4  according to the present embodiment. The conveyance speed of a horizontal conveyance portion  14  is represented as V 1 , and the speed of acceleration inside the apparatus of a pre-buffer roller  22 , a sheet discharging inversion roller  24 , and an inside sheet discharging roller  26  is represented as V 2 . Sheet conveyance control means  701  starts the rotation of an entrance roller  21  and the pre-buffer roller  22  at a speed V 1  (S 1601 ). The sheet conveyance control means  701  determines whether a conveyed sheet S 1  is the first sheet of a sheet bundle subjected to buffer processing (S 1602 ). When the sheet S 1  is the first sheet (Yes in S 1602 ), the sheet conveyance control means  701  causes the sheet discharging inversion roller  24  to contact the sheet S 1  and start rotation at the speed V 1  in the direction of a sheet discharging upper tray  25  (hereinafter represented as an F2 direction) (S 1603 ). The sheet conveyance control means  701  waits until the rear end of the sheet S 1  passes through an entrance sensor  27  (S 1604 ). After the rear end of the sheet S 1  passes through the entrance sensor  27 , the sheet conveyance control means  701  accelerates the speed of the pre-buffer roller  22  and the sheet discharging inversion roller  24  to the speed V 2  and sets a sheet interval necessary for a switchback at a place between the sheet S 1  and a following sheet (S 1605 ). When the conveyed sheet is not a last sheet loaded into a buffer portion (No in S 1611 ), the sheet conveyance control means  701  waits until the rear end of the sheet S 1  passes through a backflow prevention valve  23  by a prescribed amount (S 1613 ). When the rear end of the sheet moves by the prescribed amount, the sheet conveyance control means  701  stops the sheet discharging inversion roller  24  and the inside sheet discharging roller  26  (S 1614 ). Next, the sheet conveyance control means  701  starts the rotation of the sheet discharging inversion roller  24  and the inside sheet discharging roller  26  at the speed V 2  and in an F1 direction (S 1615 ). The sheet conveyance control means  701  waits until the leading end of the sheet passes through the inside sheet discharging roller  26  (S 1616 ) and separates an inversion upper roller  24   a  (S 1617 ). The sheet S 1  stops at a position at which the sheet S 1  is conveyed by a prescribed amount from the inside sheet discharging roller  26  (S 1618 ). 
     When a sheet other than the first sheet is conveyed (No in S 1602 ), the sheet conveyance control means  701  waits until the rear end of the second or following sheet passes through the entrance sensor  27  (S 1606 ). When the rear end of the second or following sheet passes through the entrance sensor  27 , the sheet conveyance control means  701  accelerates the speed of the pre-buffer roller  22  and the sheet discharging inversion roller  24  to the speed V 2  (S 1607 ). The sheet conveyance control means  701  monitors the elapse of a time at which the leading end of the second or following sheet (for example, a sheet S 2 ) contacts the sheet S 1  (S 1608 ). The sheet conveyance control means  701  causes the inside sheet discharging roller  26  to start rotation again at the speed V 2  and in the F2 direction toward the sheet discharging inversion roller  24 , and the sheet S 1  held by the inside sheet discharging roller  26  is conveyed (S 1609 ). At a timing at which the relative speed of the sheet S 1  and the relative speed of the sheet S 2  become equal, the inversion upper roller  24   a  contacts the sheet S 2  in an E2 direction (S 1610 ). When the sheet is not a last sheet loaded into a buffer portion like the first sheet (No in S 1611 ), the sheet conveyance control means  701  performs the processing of S 1603  to S 1618 . When the conveyed sheet is the last sheet loaded into the buffer portion (Yes in S 1611 ), the sheet conveyance control means  701  feeds a sheet bundle into an intermediate loading portion (S 1612 ). 
       FIGS.  17 A to  17 E and  18 A to  18 E  are views showing the state of a sheet bundle in the buffer portion of the sheet post-processing apparatus  4  in a case in which the operation processing of  FIG.  16    is performed. A sheet S 1  is conveyed until the sheet S 1  passes through the backflow prevention valve  23  by a prescribed amount via a first conveyance path  91  and caused to switchback by the sheet discharging inversion roller  24 , and the leading end of the sheet S 1  is held by the inside sheet discharging roller  26  ( FIGS.  17 A and  18 A ). When the leading end of the sheet S 1  moves by a prescribed amount, the sheet discharging inversion roller  24  and the inside sheet discharging roller  26  are stopped and wait for the conveyance of a following sheet S 2  ( FIGS.  17 B and  18 B ). At this time, the stop position of the sheet S 1  caused to switchback is arranged at a stop position at which the leading end of the subsequently-conveyed sheet S 2  is allowed to contact a position closer to a rear end side than a punch hole of the sheet S 1 . When the following sheet S 2  contacts the preceding sheet S 1 , the preceding sheet S 1  is conveyed ( FIGS.  17 C and  18 C ) as the inside sheet discharging roller  26  starts rotation again at the speed V 2  and in the F2 direction toward the sheet discharging inversion roller  24 . Then, at a timing at which the relative speed of the sheet S 1  and the relative speed of the sheet S 2  become equal, the inversion upper roller  24   a  contacts the sheet S 2  in the E2 direction ( FIGS.  17 D and  18 D ). The preceding sheet S 1  and the following sheet S 2  are conveyed by a prescribed amount as they are. As a result, a sheet bundle is formed in which the distance of a deviation between the rear end of the sheet S 1  caused to switchback and the leading end of the sheet S 2  that are caused to switchback falls within a certain range in a buffer portion ( FIGS.  17 E and  18 E ). Note that a sheet bundle in which the leading end of the following sheet S 2  overlaps the rear end of the preceding sheet S 1  caused to switchback in the conveyance direction is formed during buffer processing in the present embodiment as well. In the present embodiment, when landing on (contacting) the sheet S 1 , the leading end of the sheet S 2  is configured to land on a position closer to a leading end side than the first boring hole of the sheet S 1  and avoid the boring hole. By this configuration, the sheet S 1  is prevented from being pushed out by the sheet S 2 . In the present embodiment, the deviation of a relative position is not caused in a direction orthogonal to the conveyance direction of a sheet unlike the first and second embodiments. 
     As described above, it is possible to form a sheet bundle that is free from an unexpected and sudden deviation while preventing the leading end of a sheet from contacting a boring hole by changing the timing of buffer processing even when the sheet subjected to boring processing is conveyed to the buffer portion according to the present embodiment. The present invention is applicable so long as a relationship in which sheets forming a sheet bundle have different sizes but the leading end of a following sheet contacts a boring hole of a preceding sheet is established. In the present embodiment, buffer processing timing change control to avoid a boring hole is applied to all sheets. However, the buffer processing timing change control to avoid a boring hole may be applied only to a sheet that is easily curled (such as a thin paper and a moisture absorption sheet) or a sheet in which a protruding curl is caused and may not be applied to a sheet that is not curled. Further, a setting like a curled sheet conveyance mode to change an operation when a curled sheet is conveyed may be provided in the image forming system or the sheet post-processing apparatus, and the buffer processing timing change control to avoid a boring hole may be employed only when the setting becomes valid. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2020-217249, filed on Dec. 25, 2020, which is hereby incorporated by reference herein in its entirety.