Abstract:
A sheet processing apparatus which is cable of moving a sheet without applying extra load to the movement of the sheet in the case of laterally moving the sheet for sorting after completion of punching of holes in the sheet. A sheet is conveyed toward an abutment member. The sheet comes into abutment with the abutment member, whereby skew of the sheet is corrected. A punching unit punches holes in the sheet in abutment with the abutment member. A shift unit shifts the punched sheet that has been punched by the puncher in a direction intersecting with a conveying direction of the conveying unit. A controller causes the shift unit to start shifting the sheet in response to separate between the punched sheet and the abutment member.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet processing apparatus for performing post-processing on sheets having images formed thereon, a method of controlling the sheet processing apparatus, and an image forming apparatus equipped with the sheet processing apparatus. 
     2. Description of the Related Art 
     In recent years, an image forming apparatus for forming images on sheets has been generally equipped with a sheet processing apparatus called a finisher. The finisher aligns the edges of sheets discharged from the image forming apparatus, using a sheet alignment device, and then carries out post-processing including punching for punching holes in each of the sheets, stapling sheets stacked in a bundle form into one bundle, sheet sorting, etc. The sheet processing apparatus for performing such post-processing is demanded to execute the post-processing, such as pinching and stapling, with high accuracy so as to enhance the quality of products. 
     For example, in the case of punching, it is required to highly accurately position holes to be punched, so as to prevent dislocation of holes. However, a sheet conveyed into a sheet processing apparatus from an image forming apparatus can be laterally displaced in a direction orthogonal to a sheet conveying direction or skewed. In order to punch holes in such a sheet with high accuracy, the lateral displacement or skew of the sheet is corrected before punching. 
     For example, there has been proposed a device configured to punch holes in a sheet after correcting displacement of the sheet and then offset the sheet for sorting after completion of the punching (see e.g. Japanese Patent Laid-Open Publication No. 2003-226464). 
     Further, there has been proposed a device configured to punch holes in a sheet after correcting skew of the sheet by bringing the sheet into abutment with an abutment member (see US Patent Publication No. 2007/0029719). 
     The sheet processing apparatus is further demanded to perform post-processing with high accuracy without reducing productivity. In other words, the sheet processing apparatus is demanded to perform post-processing with high accuracy and in a short time. 
     However, the conventional sheet processing apparatus suffers from the following problem: In the conventional sheet processing apparatus, the skewed side of a sheet is brought into abutment with the abutment member, whereby the skew of the sheet side is corrected. Then, punching is performed on the sheet held in abutment with the abutment member, and the punched sheet is shifted laterally for sorting in the direction orthogonal to the sheet conveying direction. In this case, the sheet is moved in a state held in contact with the abutment member, and hence a frictional force is generated between the sheet and the abutment member. As a consequence, extra load due to the frictional force is applied to a motor for offsetting sheets. To withstand the increased load, the motor requires an increased size, which results in an increase in the manufacturing costs of the sheet processing apparatus. 
     SUMMARY OF THE INVENTION 
     The present invention provides a sheet processing apparatus which is cable of moving a sheet without applying extra load to the movement of the sheet in the case of laterally moving the sheet for sorting after completion of punching of holes in the sheet, a method of controlling the sheet processing apparatus, and an image forming apparatus equipped with the sheet processing apparatus. 
     In a first aspect of the present invention, there is provided a sheet processing apparatus comprising an abutment member configured to correct skew of a side of a sheet, a conveying unit configured to convey the sheet toward the abutment member so as to bring the sheet into abutment with the abutment member, a puncher configured to punch holes in the sheet in abutment with the abutment member, a shift unit configured to shift the punched sheet that has been punched by the puncher in a direction intersecting with a conveying direction of the conveying unit, and a control unit configured to cause the shift unit to start shifting the sheet in response to separation between the punched sheet and the abutment member. 
     In a second aspect of the present invention, there is provided a method of controlling a sheet processing apparatus including a conveying unit configured to convey a sheet, an abutment member configured to correct skew of the sheet, a punching unit configured to punch holes in the sheet, and a shift unit configured to shift the punched sheet in a direction intersecting with a conveying direction, comprising causing the conveying unit to convey the sheet toward the abutment member and bring the sheet into abutment with the abutment member so as to correct skew of the sheet, causing the punching unit to punch holes in the sheet in abutment with the abutment member, and causing the shift unit to shift the punched sheet that has been punched by the puncher in the direction intersecting with the conveying direction of the conveying unit in response to separation between the punched sheet and the abutment member. 
     In a third aspect of the present invention, there is provided an image forming apparatus comprising an image forming unit configured to form an image on a sheet, and a sheet processing apparatus configured to perform post-processing on the sheet having the image formed thereon by the image forming unit, wherein the sheet processing apparatus comprises an abutment member configured to correct skew of a sheet, a conveying unit configured to convey the sheet toward the abutment member so as to bring the sheet into abutment with the abutment member, a puncher configured to punch holes in the sheet in abutment with the abutment member, a shift unit configured to shift the punched sheet that has been punched by the puncher in a direction intersecting with a conveying direction of the conveying unit, and a control unit configured to cause the shift unit to start shifting the sheet in response to separate between the punched sheet and the abutment member. 
     In a fourth aspect of the present invention, there is provided a sheet processing apparatus comprising an abutment member configured to correct skew of a side of a sheet, a conveying unit configured to convey the sheet toward the abutment member so as to bring the sheet into abutment with the abutment member, a puncher configured to punch holes in the sheet in abutment with the abutment member, a shift unit configured to shift the punched sheet that has been punched by the puncher in a direction intersecting with a conveying direction of the conveying unit, and a control unit configured to cause the shift unit to start shifting the sheet when a predetermined time period elapses after an operation for separating between the punched sheet and the abutment member is started. 
     According to the present invention, in the case of laterally moving a sheet for sorting after completion of punching of holes in the sheet, it is possible to move the sheet without applying extra load to the movement of the sheet. This eliminates the need to increase the size of a drive system for moving sheets for sorting. 
     The features and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic longitudinal cross-sectional view of an image forming apparatus equipped with a sheet processing apparatus according to a first embodiment of the present invention. 
         FIG. 2  is a schematic longitudinal cross-sectional view of essential parts of the sheet processing apparatus appearing in  FIG. 1 . 
         FIG. 3  is a perspective view of the appearance of a shift unit appearing in  FIG. 2 . 
         FIG. 4  is a view of the shift unit as viewed in a direction indicated by an arrow K in  FIG. 3 . 
         FIG. 5  is a view of a punching unit appearing in  FIG. 2 , as viewed from an upstream side of a sheet conveying path. 
         FIG. 6  is a view showing a status of the punching operation of the punching unit in  FIG. 5 . 
         FIG. 7  is a schematic cross-sectional view of an abutment member appearing in  FIG. 2  as viewed from an upstream side in a sheet conveying direction. 
         FIG. 8  is a schematic view showing a status of sheet conveyance in a case where the sheet processing apparatus according to the first embodiment performs punching, provided that sheet sorting is to be executed. 
         FIG. 9  is a schematic view showing another status of the sheet conveyance. 
         FIG. 10  is a schematic view showing another status of the sheet conveyance. 
         FIG. 11  is a schematic view showing another status of the sheet conveyance. 
         FIG. 12  is a block diagram of respective controllers of a copying machine and the sheet processing apparatus, appearing in  FIG. 1 . 
         FIG. 13  is a flowchart of a control process executed by the controller of the sheet processing apparatus according to the first embodiment in a case where the sheet processing apparatus punches holes in a sheet and then sorts the punched sheet. 
         FIG. 14  is a schematic view of an abutment member provided in a sheet processing apparatus according to a second embodiment of the present invention. 
         FIG. 15  is a schematic view showing a status of sheet conveyance in a case where the sheet processing apparatus according to the second embodiment punches holes in a sheet and then sorts the punched sheet. 
         FIG. 16  is a schematic view showing another status of the sheet conveyance. 
         FIG. 17  is a schematic view showing another status of the sheet conveyance. 
         FIG. 18  is a schematic view showing another status of the sheet conveyance. 
         FIG. 19  is a flowchart of a control process executed by a controller of the sheet processing apparatus according to the second embodiment in a case where the sheet processing apparatus executes a punching process for punching holes in a sheet and then sorting the punched sheet. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The present invention will now be described in detail below with reference to the accompanying drawings showing embodiments thereof. 
       FIG. 1  is a schematic longitudinal cross-sectional view of an image forming apparatus equipped with a sheet processing apparatus according to a first embodiment of the present invention. 
     As shown in  FIG. 1 , the image forming apparatus of the present embodiment is comprised of a color copying machine (hereinafter referred to as “the copying machine”)  300  and a sheet processing apparatus  100  which is connected to the copying machine  300 . Here, the copying machine  300  includes a document feeder  500 , a scanner  905 , a plurality of cassettes  909   a  to  909   d , a plurality of image forming units  914   a  to  914   d , a fixing device  904 , and a controller  950 . The copying machine  300  has an console section  308 . The console section  308  includes a plurality of keys for configuring various functions for image forming operation, and a display section for displaying information indicative of the configurations. 
     The document feeder  500  sequentially feeds set originals onto a platen glass  906 . The scanner  905  reads an original fed onto the platen glass  906 , and outputs image data of the original, which is obtained by the reading. The output image data is converted into image data of respective colors of yellow, magenta, cyan, and black. 
     Each of the image forming units  914   a  to  914   d  receives image data of an associated color, and forms a toner image of the associated color based on the input image data. The toner images formed by the respective image forming units  914   a  to  914   d  are transferred onto a sheet fed from one of the cassettes  909   a  to  909   d , in superimposed relation. Thus, a full-color toner image is transferred on the sheet, and the sheet is conveyed to the fixing device  904 . 
     The fixing device  904  heats and presses the sheet having the toner image transferred thereon, to thereby fix the toner image on the sheet. Thus, a full-color image is formed on the sheet, and the sheet is conveyed to the sheet processing apparatus  100 . 
     The sheet processing apparatus  100  includes a saddle stitch processing unit (saddle processing unit)  135  and a side-stitching processing unit  136 . Each of the saddle stitch processing unit  135  and the side-stitching processing unit  136  is capable of processing sheets discharged from the copying machine  300  online. The side-stitching processing unit  136  is capable of stacking the sheets as a bundle, and performing stapling on the sheet bundle using staples. 
     The controller  950  of the copying machine  300  controls not only the copying machine  300 , but also the sheet processing apparatus  100 . 
     The copying machine  300  can be used alone, and the sheet processing apparatus  100  is an optional device which is connected to the copying machine  300  as required. Alternatively, the image forming apparatus may integrally comprise the copying machine  300  and the sheet processing apparatus  100 . 
     Next, a description will be given of the essential parts of the sheet processing apparatus  100  with reference to  FIG. 2 .  FIG. 2  is a schematic longitudinal cross-sectional view of the essential parts of the sheet processing apparatus  100  in  FIG. 1 . 
     As shown in  FIG. 2 , the sheet processing apparatus  100  includes an inlet roller pair  102  which receives a sheet having an image formed thereon from the copying machine  300  and conveys the same toward a conveying path  103 . An inlet sensor  101  is disposed at a location upstream of the inlet roller pair  102 , and sheet receiving timing is detected based on an output from the inlet sensor  101 . 
     At a location downstream of the inlet roller pair  102  of the conveying path  103 , there are arranged an abutment member (stopper)  151 , a punching unit  150 , a shift unit  108 , a conveying roller  110  and a separation roller  111 , a flapper  114 , and a buffer roller pair  115  in the mentioned order. Further, a lateral displacement sensor  104  is disposed in the vicinity of the inlet of the shift unit  108 , and a buffer sensor  109  is disposed between the shift unit  108  and the conveying roller  110 . 
     The abutment member  151  is moved, as described hereinafter, to its retreat position for retreat from the conveying path  103  or to its appearance position for appearance on the conveying path  103 . By moving to its appearance position, the abutment member  151  functions as a member for correcting skew of the trailing end side of a sheet to be punched, by abutment with the trailing end side of the sheet. The construction and operation of the abutment member  151  will be described in detail hereinafter. 
     The punching unit  150  punches holes in sheets. The construction and operation of the punching unit  150  will also be described in detail hereinafter. 
     The lateral displacement sensor  104  detects an end of a sheet in a transverse direction orthogonal to the sheet conveying direction. An output from the lateral displacement sensor  104  is used to detect the amount of shift (lateral displacement) from a reference position (central position in the conveying path  103 ) in the transverse direction. 
     The shift unit  108  is provided with two shift roller pairs  105  and  106 . The shift unit  108  is moved, with the shift roller pairs nipping a sheet, in the direction orthogonal to the conveying direction by a distance of travel that offsets a shift amount detected based on an output from the lateral displacement sensor  104 . Thus, the sheet is returned to the reference position. The construction of the shift unit  108  will be described in detail hereinafter. A sheet sensor  107  is disposed between the shift roller pairs  105  and  106 . 
     The conveying roller  110  and the separation roller  111  are configured such that they can come into contact with each other and separate from each other. The conveying roller  110  and the separation roller  111  convey a sheet having passed through the shift unit  108  toward the buffer roller pair  115  via the flapper  114 . 
     The sheet conveyed by the buffer roller pair  115  is guided into an upper conveying path  117  or a bundle conveying path  121  via a flapper  118 . The sheet conveyed into the conveying path  117  is discharged on an upper tray  139  by an upper discharge roller pair  120 . A sheet sensor  119  for detecting a sheet jam is disposed on the upper conveying path  117 . 
     The sheet guided into the bundle conveying path  121  is conveyed by a buffer roller pair  122  and a bundle conveying roller pair  124 , and is guided into a saddle path  133  or a lower conveying path  126  by a flapper  125 . 
     The sheet guided into the saddle path  133  is conveyed to the saddle stitch processing unit  135  by a saddle inlet roller pair  134 . The construction of the saddle stitch processing unit  135  is well known, and hence a description thereof is omitted. 
     The sheet guided into the lower conveying path  126  is conveyed through a lower discharge roller pair  128  to the side-stitching processing unit  136 . The side-stitching processing unit  136  has an intermediate processing tray  138 . Sheets sequentially discharged on the intermediate processing tray  138  by the lower discharge roller pair  128  are stacked one upon another to form a bundle. At this time, alignment processing for aligning ends of the respective stacked sheets is performed by the operations of a bundle discharge roller pair  130 , a paddle  131 , and so forth. Then, when a number of sheets required to form one set of copies are stacked as a bundle on the intermediate processing tray  138 , the sheets are stapled into one bundle by a stapler  132 , as required. The sheet bundle stapled by the stapler  132  or the unstapled sheet bundle is discharged on a lower discharge tray  137  by the bundle discharge roller pair  130 . 
     Next, the construction of the shift unit  108  will be described with reference to  FIGS. 3 and 4 . 
       FIG. 3  is a perspective view of the appearance of the shift unit  108  appearing in  FIG. 2 .  FIG. 4  is a view of the shift unit  108  as viewed in a direction indicated by an arrow K in  FIG. 3 . “Front Side” indicated in  FIG. 3  or  4  corresponds to the front (side toward the viewer, as viewed in  FIG. 1 ) of the sheet processing apparatus  100 , and “Rear Side” indicated in the same corresponds to the rear (side remote from the viewer, as viewed in  FIG. 1 ) of the sheet processing apparatus  100 . 
     As shown in  FIGS. 3 and 4 , the shift unit  108  has a frame  108 A. A conveying motor M 2  and the two shift roller pairs  105  and  106  are mounted on the frame  108 A. The conveying motor M 2  causes rotation of the shift roller pair  105  via a drive belt  209  (see  FIG. 4 ). The rotation of the shift roller pair  105  is transmitted to the shift roller pair  106  via a drive belt  213 , whereby the shift roller pairs  105  and  106  rotate in unison with each other. When the shift roller pairs  105  and  106  are driven for normal rotation, a sheet S conveyed into the shift unit  108  is conveyed in a direction C indicated by an arrow C (i.e. in the downstream direction along the conveying path  103 ). On the other hand, when the shift roller pairs  105  and  106  are driven for reverse rotation, the sheet S conveyed into the shift unit  108  is conveyed in an opposite direction to the direction C (i.e. in the upstream direction along the conveying path  103 ). The shift roller pairs  105  and  106  and the conveying motor M 2  function as a unit for conveying sheets upstream or downstream along the conveying path  103 . 
     Further, the frame  108 A has a plurality of slide bushes  205   a ,  205   b ,  205   c , and  205   d  mounted thereon for having guide rails  246  and  247  extended therethrough. The guide rail  247  extends through the slide bushes  205   a  and  205   d , and the guide rail  246  through the slide bushes  205   b  and  205   c . The guide rails  246  and  247  extend parallel with each other in a direction J indicated by an arrow J in  FIGS. 3 and 4 , and the ends of the respective guide rails  246  and  247  are rigidly secured to a frame body (not shown) of the sheet processing apparatus  100 . Thus, the frame  108 A can reciprocate in the direction J while being guided by the guide rails  246  and  247 . The direction J corresponds to the transverse direction orthogonal to the sheet conveying direction. 
     The movement of the frame  108 A in the direction J is caused by a shift motor M 3 . More specifically, a drive belt  211  extend around pulleys  220  supported respectively on an output shaft of the shift motor M 3  and the frame body of the sheet processing apparatus  100 , and the frame  108 A is secured to the drive belt  211  via a connecting member  212 . With this construction, when the shift motor M 3  is driven, the frame  108 A is moved in the direction J in accordance with the motion of the drive belt  211 . The movement of the frame  108 A in the direction J is caused while the shift roller pairs  105  and  106  are nipping the sheet S. 
     The lateral displacement sensor  104  disposed upstream of the shift unit  108  is kept on standby at a predetermined position (home position). Then, when the sheet sensor  107  (see  FIG. 2 ) disposed between the shift roller pairs  105  and  106  detects the sheet S, the lateral displacement sensor  104  is moved from its home position by a sensor moving motor M 4  in a direction E indicated by an arrow E in  FIG. 4 . The direction E is identical to the direction J. Then, when the lateral displacement sensor  104  detects a side end (sheet end extending along the sheet conveying direction) of the sheet S, the movement of the lateral displacement sensor  104  is temporarily stopped. At this time, the distance of travel of the lateral displacement sensor  104  (i.e. the number of drive pulses of the sensor moving motor M 4 ) is detected, and the amount of lateral displacement of the sheet S is calculated based on the sensed travel distance. 
     The shift unit  108  is moved in the direction J by a distance that offsets the detected amount of lateral displacement of the sheet S, whereby the sheet S is returned to the reference position on the conveying path  103 . Thus, the lateral displacement of the sheet S is corrected. 
     The shift unit  108  also functions as an offset mechanism for moving each sheet S in the direction orthogonal to the conveying direction so as to stack one sheet S on another on the lower discharge tray  137  in a sorted state. When an offset mode is set, the shift unit  108  is moved in the direction J by a set offset amount by the shift motor M 3 . This causes lateral shift (offset) of a sheet S punched by the punching unit  150 . The offset mode is a mode in which a sheet discharge position is laterally shifted on a job-by-job basis or whenever one set of sheets is output, so as to enable sheet sorting, and sheets are stacked e.g. on the lower discharge tray  137 . 
     Next, the construction of the punching unit  150  will be described with reference to  FIGS. 5 and 6 .  FIG. 5  is a view of the punching unit  150  appearing in  FIG. 2 , as viewed from the upstream side of the conveying path  103 .  FIG. 6  is a view showing a status of the punching operation of the punching unit  150  in  FIG. 5 . 
     The punching unit  150  is rigidly secured on the frame body of the sheet processing apparatus  100  such that the lateral center of the punching unit  150  coincides with the central position of the conveying path  103 . As shown in  FIGS. 5 and 6 , the punching unit  150  is provided with a punch  712  and a die  711  for simultaneously forming a plurality of holes arranged in the direction orthogonal to the sheet conveying direction. The punch  712  is moved to a position for mating engagement with the die  711  by a drive mechanism having a punch motor M 5  (not shown in  FIGS. 5 and 6 ) as a drive source (see  FIG. 6 ). After having been moved to the position for mating engagement with the die  711 , the punch  712  is returned to its former position (see  FIG. 5 ). 
     By thus moving the punch  712  to the position for mating engagement with the die  711 , a plurality of holes arranged in the direction orthogonal to the sheet conveying direction are formed in a sheet between the punch  712  and the die  711 . The movement of the punch  712  is controlled based on a rotational speed of the punch motor M 5  detected by a punch motor rotational speed sensor  713  (not shown in  FIGS. 5 and 6 ). 
     Next, the construction of the abutment member  151  will be described with reference to  FIG. 7 .  FIG. 7  is a schematic cross-sectional view of the abutment member  151  as viewed from upstream in the sheet conveying direction. 
     As shown in  FIG. 7 , the abutment member  151  has an abutment surface  151   a  for abutment with the trailing end of a sheet. The abutment member  151  is attached to a rotating shaft  720  extending in a direction orthogonal to the conveying path  103 . The rotating shaft  720  is rotatably supported on the frame body of the sheet processing apparatus  100  and is driven for rotation by an abutment motor M 6  (not shown in  FIG. 7 ). With this construction, the abutment member  151  is rotated about the rotating shaft  720  by the abutment motor M 6  to be selectively moved to its retreat position PA and to its appearance position PB. 
     The retreat position PA is a position (home position) for retreat of the abutment member  151  from the conveying path  103 . When the abutment member  151  is at its retreat position PA, the conveying path  103  is kept open such that a sheet can be conveyed. On the other hand, the appearance position PB is a position for appearance of the abutment member  151  on the conveying path  103 . When the abutment member  151  is moved to its appearance position PB, the conveying path  103  is closed by the abutment member  151 , whereby passage of a sheet is blocked. At this time, the abutment surface  151   a  of the abutment member  151  has projected on the conveying path  103  in orthogonal relation to the same. 
     Whether the abutment member  151  is at its retreat position PA is detected based on an output from a home position sensor (hereinafter abbreviated as “the HP sensor”)  153 . The HP sensor  153  is disposed such that it can detect the abutment member  151  when the abutment member  151  is at its retreat position PA. Control for moving the abutment member  151  from the retreat position PA to the appearance position PB or vice versa is performed based on the output (retreat position PA) from the HP sensor  153  and the number of drive pulses of the motor M 6 . 
     Next, a punching operation carried out using the punching unit  150  will be described with reference to  FIGS. 8 to 11 . Each of  FIGS. 8 to 11  is a schematic view showing a status of sheet conveyance in a case where the sheet processing apparatus  100  according to the first embodiment performs punching, provided that sheet sorting is to be executed. 
     As shown in  FIG. 8 , a sheet S received from the copying machine  300  is conveyed toward the shift unit  108  along the conveying path  103  by the inlet roller pair  102 . Then, the sheet S is guided into the shift unit  108  via the punching unit  150 . The sheet S is nipped and conveyed by the shift roller pairs  105  and  106  of the shift unit  108 . At this time, when the sheet sensor  107  detects the sheet S, movement of the lateral displacement sensor  104  is started, and when the lateral displacement sensor  104  detects the side end of the sheet S, the movement of the lateral displacement sensor  104  is stopped. Then, the amount of lateral displacement of the sheet S is detected based on the distance of travel of the lateral displacement sensor  104 , and the shift unit  108  is moved in the distance J by a distance that offsets the detected lateral displacement amount. Thus, the lateral displacement of the sheet S is corrected. 
     After the correction of the lateral displacement of the sheet S, the sheet S is conveyed by the shift roller pairs  105  and  106  to a position where the trailing end of the sheet S passes through the punching unit  150 . Time taken to convey a sheet S to the position where the trailing end thereof passes through the punching unit  150  after detection of the leading end of the sheet S by the sheet sensor  107  can be calculated based on the length of the sheet S in the sheet conveying direction, a conveying speed for conveying the sheet S, and the distance between the sheet sensor  107  and the punching unit  150 . Information on the length of the sheet S in the sheet conveying direction has already been received from the controller  950 . The sheet conveying speed and the distance between the sheet sensor  107  and the punching unit  150  are fixed values. Therefore, it is possible to determine, based on a time period having elapsed after a time point when the sheet sensor  107  detected a sheet S, whether or not the sheet S has been conveyed to the position where the trailing end thereof passes through the punching unit  150 . 
     When the sheet S is conveyed to the position where the trailing end thereof passes through the punching unit  150 , the abutment member  151  is moved from its retreat position PA to its appearance position PB as shown in  FIG. 9 . In timing synchronous with the movement of the abutment member  151 , the shift roller pairs  105  and  106  are temporarily stopped and then reversely rotated by the conveying motor M 2 . As a consequence, the sheet S is conveyed not downstream along the conveying path  103 , but in a direction opposite thereto, i.e. toward the punching unit  150  upstream along the conveying path  103 . The reverse rotation of the shift roller pairs  105  and  106  is continued over a predetermined time period even after the trailing end of the sheet S (i.e. the leading end of the sheet S as viewed in the reverse conveying direction) has come into abutment with the abutment surface  151   a  of the abutment member  151 . 
     Time taken for conveyance of the sheet S from the start of the reverse rotation of the shift roller pairs  105  and  106  to the abutment of the trailing end of the sheet S on the abutment surface  151   a  of the abutment member  151  can be calculated based on the length of the sheet S in the sheet conveying direction, a conveying speed for conveying the sheet S, and the distance between the punching unit  150  and the abutment member  151 . Therefore, it is possible to obtain the amount of warpage of the sheet S after the abutment of the trailing end of the sheet S on the abutment surface  151   a  of the abutment member  151 , based on a time period that has elapsed after the start of the reverse rotation of the shift roller pairs  105  and  106 . The above-mentioned predetermined time period corresponds to driving time of the shift roller pairs  105  and  106  required to moderately warp the sheet S with the trailing end of the sheet S held in abutment with the abutment surface  151   a  of the abutment member  151 . 
     When the sheet S is further conveyed by the shift roller pairs  105  and  106  over the predetermined time period with the trailing end of the sheet S in abutment with the abutment member  151 , the shift roller pairs  105  and  106  are stopped, whereby the conveyance of the sheet S is stopped. This causes the sheet S to become warped with its trailing end held in abutment with the abutment member  151 , as shown in  FIG. 10 . More specifically, the trailing end of the sheet S is pressed against the abutment surface  151   a  of the abutment member  151 , whereby skew of the trailing end of the sheet S is corrected. Then, the punching unit  150  punches holes in the skew-corrected trailing end-side portion of the sheet S. 
     After completion of the punching of the sheet S, the abutment member  151  is moved from its appearance position PB to its retreat position PA as shown in  FIG. 11 . This releases the abutment between the trailing end of the sheet S and the abutment member  151  to separate the former from the latter. At the same time, the warpage of the sheet S is eliminated. 
     In a case where the offset mode for sheet sorting is set, the shift unit  108  is moved in the direction J by a set offset amount, with the shift roller pairs  105  and  106  nipping the sheet S. Thus, the sheet S is shifted (offset) for sorting. At this time, the trailing end of the sheet S is not in contact with the abutment member  151 . Therefore, no frictional force is generated between the trailing end of the sheet S and the abutment member  151  during movement of the shift unit  108 , which makes it possible to reduce load applied to the shift motor M 3 . 
     Then, the shift roller pairs  105  and  106  of the shift unit  108  are driven for normal rotation by the conveying motor M 2 , whereby the sheet S is conveyed toward the conveying roller  110  (i.e. downstream along the conveying path  103 ). 
     Next, the configuration of control in the present embodiment will be described with reference to  FIG. 12 .  FIG. 12  is a block diagram of the controller  950  of the copying machine  300  and a controller  501  of the sheet processing apparatus  100  appearing in  FIG. 12 . 
     As shown in  FIG. 12 , the controller  950  of the copying machine  300  includes a CPU  305 , a ROM  306  which stores control programs executed by the CPU  305 , and a RAM  307  which provides a work area for the CPU  305 . A document feeder controller  301 , a scanner controller  302 , an image signal processing section  303 , a printer controller  304 , and an console section  308  are connected to the controller  950 . Further, the controller  950  is provided with an interface, not shown, for connection to the controller  501  of the sheet processing apparatus  100 , and the controller  950  and the controller  501  are communicably connected to each other via the above-mentioned interface. The CPU  305  of the controller  950  controls the above-mentioned blocks to carry out respective associated operations according to the control programs stored in the ROM  306 . 
     The document feeder controller  301  controls the operation of the document feeder  500  (see  FIG. 1 ) based on instructions from the controller  950 . The scanner controller  302  controls the operation of the scanner  905  (see  FIG. 1 ) based on instructions from the controller  950 . 
     The image signal processing section  303  converts RGB analog image signals output from the scanner  905  into digital image signals based on instructions from the controller  950 , and performs processing on each of the digital image signals. The digital image signals are converted into video signals and are delivered to the printer controller  304 . 
     The printer controller  304  controls the operations of the respective image forming units  914   a  to  914   d , the operation of the fixing device  904  (see  FIG. 1 ), and so forth, based on instructions from the controller  950 , so as to print out the video signals from the image signal processing section  303 . 
     The console section  308  includes a plurality of keys for configuring various functions for image forming operation, and a display section for displaying information indicative of settings. A key signal associated with each key operation of the console section  308  is input to the controller  950 . Further, on the display section of the console section  308  is displayed information, such as apparatus status information, set mode information, and warning information, output from the controller  950 . 
     The controller  501  of the sheet processing apparatus  100  is incorporated in the sheet processing apparatus  100 , and controls the operation of the sheet processing apparatus  100  based on instructions from the controller  950 . The controller  501  includes a CPU  401 , a ROM  402 , and a RAM  403 . The CPU  401  controls the operation of each solenoid of a solenoid group  405  and that of each motor of a motor group  406  according to control programs stored in the ROM  402 , while monitoring output from each sensor of a sensor group  404 . The RAM  403  provides a work area for the CPU  401 . 
     The sensor group  404  includes a plurality of sensors, such as the lateral displacement sensor  104 , the sheet sensor  107 , the punch motor rotational speed sensor  713 , and the HP sensor  153  for detecting the home position of the abutment member  151 . The sensor group  404  further includes other sensors than these, but they are not shown in  FIG. 12 . 
     The solenoid group  405  includes solenoids (not shown) for causing respective operations of the flappers  114 ,  118 , and  125 . 
     The motor group  406  includes a conveying motor M 1  for causing rotation of the inlet roller pair  102 , the conveying roller  110 , etc., and the conveying motor M 2  and the shift motor M 3  of the shift unit  108 . Further, the motor group  406  includes the sensor moving motor M 4  for moving the lateral displacement sensor  104 , the punch motor M 5  for driving the punching unit  150 , and the abutment motor M 6  for driving the abutment member  151 . The other motors than these are not shown in  FIG. 12 . 
     Next, a control process executed by the controller  501  in a case where holes are punched in a sheet and then the punched sheet is conveyed for sorting will be described with reference to  FIG. 13 .  FIG. 13  is a flowchart of the control process executed by the controller  501  of the sheet processing apparatus  100  according to the first embodiment in the above-mentioned case where the sheet processing apparatus  100  punches holes in a sheet and then sorts the punched sheet. The process based on the flowchart in  FIG. 13  is executed by the CPU  401  according to a program stored in the ROM  402  of the controller  501 . 
     Now, it is assumed that a mode for punching holes in a sheet having an image formed thereon in the copying machine  300  and then sorting the sheet has been set, and the controller  501  performs control according to the set mode. 
     Upon reception of a sheet having an image formed thereon from the copying machine  300 , the controller  501  (CPU  401 ) of the sheet processing apparatus  100  performs control such that conveyance of the sheet is started. This causes the sheet to be conveyed along the conveying path  103 . Then, the sheet is guided into the shift unit  108  via the punching unit  150 , and is conveyed while being nipped by the shift roller pairs  105  and  106  of the shift unit  108 . 
     During this conveyance, when the sheet sensor  107  in the shift unit  108  detects the sheet, the controller  501  detects the amount of lateral displacement of the sheet (step S 1001 ). In the step S 1001 , the lateral displacement sensor  104  is moved by the sensor moving motor M 4  until the side end of the sheet is detected, and the amount of lateral displacement of the sheet is detected based on the distance of travel of the lateral displacement sensor  104 . Then, the controller  501  causes the shift motor M 3  to move the shift unit  108  in the direction J (i.e. the direction orthogonal to the sheet conveying direction) by a distance that offsets the detected lateral displacement amount, to thereby correct the lateral displacement of the sheet (step S 1002 ). At this time, the shift roller pairs  105  and  106  of the shift unit  108  are nipping and conveying the sheet. 
     Then, the controller  501  awaits passage of the trailing end of the sheet through the punching unit  150  (step S 1003 ). More specifically, the controller  501  waits for a time period that has elapsed after detection of the sheet (leading end thereof) by the sheet sensor  107  to reach a conveying time period required to convey the sheet until the trailing end of the sheet has passed through the punching unit  150 . 
     When the trailing end of the sheet passes through the punching unit  150 , the controller  501  causes the abutment motor M 6  to move the abutment member  151  from its retreat position PA to its appearance position PB (step S 1004 ). Then, the controller  501  temporarily stops the shift roller pairs  105  and  106  to stop conveyance of the sheet (step S 1005 ). Thereafter, the controller  501  causes the conveying motor M 2  to drive the shift roller pairs  105  and  106  for reverse rotation (step S 1006 ). This conveys the sheet upstream toward the punching unit  150  along the conveying path  103 . 
     Then, the controller  501  waits until the shift roller pairs  105  and  106  have been driven over a predetermined time period after abutment of the trailing end of the sheet on the abutment member  151  through the punching unit  150  (step S 1007 ). More specifically, the controller  501  waits for a time period that has elapsed after the start of the reverse rotation of the shift roller pairs  105  and  106  to reach a time period required to cause the sheet to become moderately warped after abutment of the trailing end of the sheet on the abutment member  151 . 
     When the shift roller pairs  105  and  106  are driven over the predetermined time period after abutment of the trailing end of the sheet on the abutment member  151 , the controller  501  stops the shift roller pairs  105  and  106  to thereby stop the conveyance of the sheet (step S 1008 ). Thus, skew of the trailing end of the sheet is corrected. 
     Then, the controller  501  causes the punching unit  150  to punch holes in the sheet (step S 1009 ). In the step S 1009 , the punch motor M 5  is actuated to move the punch  712  to the position for mating engagement with the die  711 . Thereafter, the punch  712  is moved to its former position from the position for mating engagement with the die  711 . Then, the controller  501  causes the abutment motor M 6  to move the abutment member  151  from its appearance position PB to its retreat position PA (step S 1010 ). 
     Then, the controller  501  waits for the HP sensor  153  to detect the abutment member  151  (step S 1011 ). More specifically, the controller  501  awaits completion of the movement of the abutment member  151  to its retreat position PA. When the movement of the abutment member  151  to its retreat position PA is completed, the controller  501  causes the shift motor M 3  to move the shift unit  108  in the direction  3  by the set offset amount to thereby perform offsetting for sheet sorting (step S 1012 ). During the movement of the shift unit  108 , the trailing end of the sheet is kept away from the abutment member  151 . In place of starting the movement of the shift unit  108  based on an output from the HP sensor  153 , it is possible to start the movement of the shift unit  108  when a time period required to eliminate the warpage of the sheet has elapsed after the start of driving of the abutment motor M 6 . It is assumed in this case that data on this time period is stored in the ROM  402  in advance. 
     Then, the controller  501  awaits completion of the offsetting by the shift unit  108  (step S 1013 ). When the offsetting is completed, the controller  501  causes the conveying motor M 2  to drive the shift roller pairs  105  and  106  for normal rotation to start conveyance of the sheet (step S 1014 ). Thus, the sheet is conveyed toward the conveying roller  110 . 
     Although in the present embodiment, conveyance of the sheet is started after completion of the movement of the shift unit  108  (i.e. the offsetting for sorting), conveyance of the sheet may be started by the shift roller pairs  105  and  106  before or upon the start of the movement of the shift unit  108  in the direction J so as to achieve higher productivity. Alternatively, conveyance of the sheet may be started in the middle of the movement of the shift unit  108  in the direction J. 
     When a separation operation for separating the abutment member  151  from the sheet is started, the frictional force between the abutment member  151  and the sheet becomes smaller than before the start of the separation operation. This means that even if the sheet processing apparatus starts a shift operation by the shift unit  108  in a state where the sheet and the abutment member  151  are not fully separated from each other, it is possible to produce an effect of reducing load applied to the shift motor M 3 . Therefore, the shift operation by the shift unit  108  may be started when a predetermined time period has elapsed after the start of the separation operation. The predetermined time period is set to be shorter than a time period taken from the start of the separation operation to completion of the same, and is regarded as a time period taken before the frictional force between the sheet and the abutment member  151  becomes negligible. It is also assumed in this case that the predetermined time period is stored in the ROM  402  in advance. 
     Next, a second embodiment of the present invention will be described with reference to  FIGS. 14 to 19 .  FIG. 14  is a schematic view of an abutment member provided in a sheet processing apparatus according to the second embodiment. Each of  FIGS. 15 to 18  is a schematic view showing a status of sheet conveyance in a case where the sheet processing apparatus according to the second embodiment punches holes in a sheet and then sorts the punched sheet.  FIG. 19  is a flowchart of a control process executed by the controller  501  in a case where the sheet processing apparatus according to the second embodiment executes a punching process for punching holes in a sheet and then sorting the punched sheet. The control process based on the flowchart in  FIG. 19  is executed by the CPU  401  according to a program stored in the ROM  402  of the controller  501 . 
     The present embodiment is distinguished from the above-described first embodiment only in that an abutment member is different in construction from the abutment member in the first embodiment, and is configured similarly to the first embodiment in the other respects. Therefore, members and blocks identical to those of the first embodiment are denoted by identical reference numerals, and description thereof is omitted. 
     In the present embodiment, the abutment member  730  is disposed in the punch unit  150  as shown in  FIG. 14 . The abutment member  730  is formed by a thin plate of an elastic material bent through approximately 110 degrees. The bending angle is not limited to 110 degrees, but it may be set to any angle within a range of 100 to 120 degrees. The bent abutment member  730  has one side portion  730   a  rigidly secured to a casing  150   a  of the punch unit  150  and the other side portion  730   b  obliquely projecting downstream in a manner closing the conveying path  103 . The side portion  730   b  is elastically deformable from a boundary portion  730   c  between the side portions  730   a  and  730   b  (see two-dot chain lines in  FIG. 14 ) in a retreat direction from the conveying path  103 . The position of the boundary portion  730   c  is slightly offset from a position on the conveying path  103  so as to prevent a conveyed sheet S from colliding with the side portion  730   a.    
     In a case where the sheet S is conveyed from upstream toward the side portion  730   b , the sheet S comes into contact with the side portion  730   b  and pushes the same aside as shown in  FIG. 15 . At this time, the side portion  730   b  is deformed from the boundary portion  730   c  between the side portions  730   a  and  730   b  to be retreated from the conveying path  103 , and the sheet S is conveyed downstream along the conveying path  103  while passing by the side portion  730   b . Then, when the trailing end of the sheet S has passed by the side portion  730   b , the side portion  730   b  returns to its position for closing the conveying path  103 . 
     Then, the amount of lateral displacement of the sheet S is detected, and the lateral displacement of the sheet S is corrected by the shift unit  108  as in the first embodiment. After completion of the correction of the lateral displacement of the sheet S, the shift unit  108  conveys the sheet S upstream toward the punching unit  150 . In this case, the trailing end of the sheet S comes into abutment with the side portion  730   b  projecting in a manner closing the conveying path  103  or the side portion  730   a , as shown in  FIG. 16 , but the side portion  730   b  is hardly deformed. Then, when the sheet S is further conveyed over a predetermined time period with the trailing end thereof held in abutment with the side portion  730   b  or the side portion  730   a , the trailing end of the sheet S is moderately warped as shown in  FIG. 17 . Thus, similarly to the abutment member  151  in the first embodiment, the abutment member  730  functions as a member for correcting skew of the trailing end side of a sheet, whereby skew of the trailing end side of the sheet S is corrected. 
     After completion of the correction of the skew of the trailing end side of the sheet S, the sheet S is conveyed downstream along the conveying path  103  as shown in  FIG. 18 . In accordance with the downstream conveyance of the sheet S, the trailing end of the sheet S is separated from the side portion  730   b , whereby the warpage of the sheet S is eliminated. 
     Next, a control process executed by the controller  501  in a case where holes are punched in a sheet and then the punched sheet is conveyed for sorting will be described with reference to  FIG. 19 . The control process based on the flowchart in  FIG. 19  is executed by the CPU  401  according to a program stored in the ROM  402  of the controller  501 . 
     Now, as in the first embodiment, it is assumed that the controller  501  performs control according to the mode for punching holes in a sheet having an image formed thereon in the copying machine  300  and then sorting the sheet. 
     A sheet received from the copying machine  300  is conveyed along the conveying path  103 . Then, the sheet is guided into the shift unit  108  via the punching unit  150 , and is conveyed while being nipped by the shift roller pairs  105  and  106  of the shift unit  108 . 
     During this conveyance, when the sheet sensor  107  in the shift unit  108  detects the sheet, the controller  501  (CPU  401 ) detects the amount of lateral displacement of the sheet based on the distance of travel of the lateral displacement sensor  104  (step S 2001 ). Then, the controller  501  causes the shift motor M 3  to move the shift unit  108  in the direction J (i.e. the direction orthogonal to the sheet conveying direction) by a distance that offsets the detected lateral displacement amount, to thereby correct the lateral displacement of the sheet (step S 2002 ). 
     Then, the controller  501  awaits passage of the trailing end of the sheet through the punching unit  150  (step S 2003 ). When the trailing end of the sheet passes through the punching unit  150 , the controller  501  temporarily stops the shift roller pairs  105  and  106  to stop conveyance of the sheet (step S 2004 ). Thereafter, the controller  501  causes the conveying motor M 2  to drive the shift roller pairs  105  and  106  for reverse rotation to thereby start switch-back conveyance of the sheet (step S 2005 ). This conveys the sheet upstream toward the punching unit  150 . 
     Then, the controller  501  waits until the shift roller pairs  105  and  106  have been driven over a predetermined time period after abutment of the trailing end of the sheet on the abutment member  730  through the punching unit  150  (step S 2006 ). When the shift roller pairs  105  and  106  are driven over the predetermined time period after abutment of the trailing end of the sheet on the abutment member  730 , the controller  501  stops the shift roller pairs  105  and  106  to thereby stop the conveyance of the sheet (step S 2007 ). Thus, skew of the trailing end side of the sheet is corrected. Then, the controller  501  causes the punching unit  150  to punch holes in the sheet (step S 2008 ). 
     Then, the controller  501  causes the conveying motor M 2  to drive the shift roller pairs  105  and  106  for normal rotation to thereby start conveying the sheet at a conveying speed V 1  (step S 2009 ). It is assumed that the conveying speed V 1  is set to a speed that makes it possible to complete offsetting by the shift unit  108  before the sheet reaches the conveying roller  110  which cannot be laterally moved (see  FIG. 2 ). Assuming that a time period from completion of punching by the punching unit  150  to completion of offsetting by the shift unit  108  is represented by T 1 , and a time period taken before a sheet conveyed at the conveying speed V 1  reaches the conveying roller  110  after completion of the punching is represented by T 2 , the following relationship is satisfied between the time period T 1  and the time period T 2 :
 
 T 2− T 1&gt;0
 
     Therefore, the conveying speed V 1  is set to a value that satisfies the relationship. The reason why the conveying speed V 1  is set is that in the second embodiment, timing in which the trailing end of a warped sheet S is separated from the abutment member delays in comparison with the separation timing in the first embodiment. 
     Then, the controller  501  waits until the distance of conveyance of the sheet at the conveying speed V 1  reaches a preset conveyance distance D 1  (step S 2010 ). The distance of conveyance of the sheet at the conveying speed V 1  is calculated based on the conveying speed V 1  and a time period that has elapsed after the start of the conveyance of the sheet at the conveying speed V 1 . The preset conveyance distance D 1  corresponds to a conveyance distance required to separate the trailing end of the warped sheet from the abutment member  730 . 
     When the distance of conveyance of the sheet at the conveying speed V 1  reaches the preset conveyance distance D 1 , the controller  501  causes the shift motor M 3  to move the shift unit  108  in the direction J by a set offset amount to thereby perform offsetting for sheet sorting (S 2011 ). Then, the controller  501  awaits completion of the offsetting by the shift unit  108  (step S 2012 ). 
     When the offsetting by the shift unit  108  is completed, the controller  501  switches the sheet conveying speed of the shift roller pairs  105  and  106  from the conveying speed V 1  to a conveying speed V 2  and conveys the sheet at the conveying speed V 2  (step S 2013 ). The conveying speed V 2  corresponds to a conveying speed in a case where offsetting is not performed by the shift unit  108 , and is set to be higher than the conveying speed V 1 . The reason why the conveying speed V 1  is switched to the conveying speed V 2  is that the offsetting of the sheet is completed before the sheet reaches the conveying roller  110 . 
     According to the present embodiment, after completion of punching of a sheet, the sheet is conveyed downstream at the conveying speed V 1 , and when the distance of conveyance of the sheet at the conveying speed V 1  reaches the conveyance distance D 1  set as a conveyance distance required to separate the trailing end of the sheet from the abutment member  730 , offsetting by the shift unit  108  is started. This makes it possible to prevent frictional force from being generated between the trailing end of the sheet and the abutment member  730  during the offsetting, to thereby reduce load applied to the shift motor M 3 . Further, by setting the conveying speed in the offset mode to be lower than that in the non-offset mode, it is possible to prevent the leading end of the sheet from reaching the conveying roller  110  downstream before completion of the offsetting. 
     In the case where sheet conveyance is started before completion of movement of the shift unit  108  in the first embodiment, the method of switching between the conveying speed V 1  and the conveying speed V 2  may be employed as in the second embodiment. 
     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 modifications, equivalent structures and functions. 
     This application claims priority from Japanese Patent Application No. 2008-245984 filed Sep. 25, 2008, which is hereby incorporated by reference herein in its entirety.