Abstract:
A finisher includes a folding device which folds a sheet having an image formed surface, a reversing mechanism which reverses the sheet, and a conveying path for conveying the sheet bypassing the reversing mechanism. The finisher decides whether to pass the sheet through the reversing mechanism based on whether the sheet is to be folded. The finisher adjusts both folded sheets and unfolded sheets in the order of page numbers and staples a sheaf of the sheets easily and accurately, and also utilizes space efficiently and prevents the finisher size from enlarging.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a finisher, which is connected to an image forming apparatus such as a printer or a copying machine, applying such additional-workings as sorting, binding, creasing, folding, and punching to a recording medium (hereinafter referred to briefly as “sheet”) such as a recording paper outputted from the image forming apparatus. More particularly, it relates to a finisher with a folding device in which a stopper comes in contact with the leading end of the sheet to form a loop and a pair of rollers nips the loop to fold the sheet. 
     2. Description of the Prior Art 
     Recently, various finishers, which apply various additional-workings to a sheet with an image formed surface which are outputted from such image forming devices as printers and copying machines, have been proposed (U.S. patent application Ser. No. 08/821,444). The term “additional-workings” as used herein means various working processes such as sorting sheets according to the number of copies, filing sheets with staples, folding sheets in two (hereinafter referred to as “double-folding”), folding sheets in three or in a cross section like a letter Z (hereinafter referred to as “Z-folding”), and punching sheets for filing. 
     A finisher, which is illustrated in FIG. 18, has been known as one example of the above finishers. The finisher is provided with a folding mechanism  700 . The finisher gives the Z-folding to sheets as follows. First, the folding mechanism  700  receives a sheet with a copied image on the upper surface, which is fed from the right in the diagram. A first folding stopper  704  comes in contact with the sheet to form a loop in the sheet. Then, a pair of folding rollers  701 ,  702  nips the loop to effect the first folding at a position separated by about one quarter of the size of the sheet in the conveying direction from the leading end of the sheet. Next, the second folding stopper  705  comes in contact with the first fold to form a loop in the sheet. Last, a pair of folding rollers  702 ,  703  nips the loop to effect the second folding at a roughly central position of the sheet in the conveying direction. Accordingly, the sheet is folded into three parts or in a cross section like a letter Z. 
     The above conventional folding devices adopt a so-called last page system which copies a plurality of original documents sequentially in reverse order from the last page forward. The last page system, which is provided with the folding mechanism  700  disposed below the finisher as illustrated in the diagram, can change the sheets to be folded in the order of page numbers. 
     Incidentally, multifunction machines, which work as a printer, facsimile equipment and the like, have been proposed with the popularization of digital copying machines. Such a multifunction machine is utilized for various purposes of copying, printing, facsimile receiving, facsimile transmitting, etc. A user can be confused when all of the functions in a series of documents to be printed do not have the same printing order, such as the order from the first page onward or in reverse order from the last page backward. 
     The application programs, which are executed by a computer, normally give a printer instructions for printing in the order from page 1 onward. Accordingly, a large number of multifunction machines adopt a so-called first page system in which a printing starts from the first page onward as the common printing order applied to all functions. 
     The first page system, however, requires a sheet having a copied image in a reverse side be discharged in order to adjust the order of page numbers. A conventional folding device as illustrated in FIG. 18, provided with a reversing mechanism  710   a  on the front of the folding mechanism  700  as illustrated in FIG. 19 (on condition that a reversing mechanism  710   b  is not incorporated), folds a sheet with respect to a surface opposite from an image formed surface C as illustrated in FIG.  2 A and FIG.  20 B and does not fold properly when discharging a sheet having a copied image on a reverse side. A conventional folding device as illustrated in FIG. 18, provided with a reversing mechanism  710   b  on the front of the discharging unit as illustrated in FIG. 19 (on condition that a reversing mechanism  710   a  is not incorporated), folds a sheet with respect to the image formed surface C. However, the folding device has to staple a sheaf of temporarily stored sheets as illustrated in FIG.  21 A and FIG. 21B, at a stapling position S being the farthest position in a conveying direction of the sheaf when the folding device takes the sheaf out in a direction opposite to a receiving direction of the sheets for temporarily storage. Namely, the folding device has the problem that the stapling is difficult. 
     In the first page system, it is preferable to store sheets in such a manner as illustrated in FIG.  22 A and FIG. 22B for the purpose of fixing the stapling position S in the conveying direction for all the sizes of sheets. It results in shortening the conveying distance of sheets or a sheaf necessary for stapling on the downstream side, securing the accuracy of positioning and reducing the deviation of sheets during the conveyance, for example. 
     An arrangement as illustrated in FIG. 23, which reverses a sheet P, irrespective of the necessity for a folding, may store sheets in such a manner as illustrated in FIG.  22 A and FIG. 22B 
     In the arrangement, the conveying path is not easily laid out unless the folding mechanism  700  is disposed above the finisher. In general, a finisher stores sheets P temporarily, staples a sheaf of stored sheets and discharges a stapled sheaf into another receiving tray unit. A position of the receiving tray unit which stores a stapled sheaf is preferably lower than a position of the stapler for stapling a sheaf for the purpose of attaining the steady conveyance of the sheaf. A tray, which temporarily receives sheets, must inevitably be disposed on the upper section for the purpose of enlarging a storage capacity for stapled sheaves. A finisher including the folding mechanism  700  disposed on the upper section as well as the tray is of large size as a whole, and loses balance because a component density is high in the upper section. 
     A finisher with folding rollers having a changed layout, which gives the first folding of the Z-folding to a sheet at the position separated by about three quarters of the size of the sheet from the leading end, namely about one quarter of the size from the trailing end, and not at the position separated by about one quarter of the size of the sheet from the leading end in the conveying direction, can store folded sheets in a desired form. However, the finisher has the problem that an image formed surface of an unfolded sheet faces outward and the order of page numbers is confused. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide a finisher which adjusts both folded sheets and unfolded sheets in the order of page numbers and staples a sheaf of the sheets easily and accurately, and also utilizes a space efficiently and prevent the size from enlarging. 
     The present invention accomplishing the object concerns a finisher which comprises a folding device which folds a sheet having an image formed surface, a reversing mechanism which reverses the sheet, and a conveying path for conveying the sheet with bypassing the reversing mechanism, wherein a passage of the sheet through the reversing mechanism is decided on a judgment whether or not the sheet is to be folded. This finisher preparatorily judges whether or not the sheets are to be folded by the folding device and, based on the result of the judgment, actuates the reversing mechanism to reverse the sheet. The finisher according to the so-called first page system can adjust both the folded sheets and the unfolded sheets in the order of page numbers, and only staples the sheets at a position falling on the side of a regulating device disposed in the conveying direction on condition that the sheets are temporarily stored in the tray, and obviates the necessity for changing the stapling position in the conveying direction in response to the size of sheet and the mode of operation. 
     The finisher reduces the conveying distance for stapling, and obtains the accuracy of the stapling position, and represses the deviation of sheets to only a small extent. 
     Besides, the folding device is disposed below the finisher. It results in enabling the finisher to utilize space efficiently and prevent the size from enlarging and be in a compact construction. The finisher, when operated in the first page system, inevitably requires a mechanism for reversing sheets. However, the finisher fulfills the requirement simply by incorporating a conveying path for discharging sheets without reversing. Accordingly, the finisher entails neither any notable addition to the size nor any notable increase in cost as compared with the conventional folding device. 
     This invention also concerns a finisher which comprises a folding device which folds a sheet having an image formed surface and a reversing mechanism which reverses the sheet, wherein the sheet is conveyed to the folding device without passing through the reversing mechanism. 
     This invention further concerns a method for folding a sheet having an image formed surface, which comprises a step of making a choice between a first mode of folding the sheet and a second mode of not folding the sheet and a step of folding and discharging the sheet in a non-reversed state when the first mode is chosen, and reversing and discharging the sheet without folding when the second mode is chosen. 
     The objects, characteristics and features of this invention other than those set forth above will become apparent from the following description based on preferred embodiments, which are illustrated in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of an embodiment having a finisher according to this invention connected to a copying machine as an image forming device; 
     FIG. 2 is a cross-sectional view of the essential section of the finisher; 
     FIG. 3 is a cross-sectional view of the construction of a folding device; 
     FIG. 4 is a cross-sectional view of the folding device which is jammed; 
     FIG.  5 A and FIG. 5B are cross-sectional views of the essential section of a mechanism for regulating the first folding position in the folding device; 
     FIG. 6 is a bottom view of the mechanism for regulating the first folding position in the folding device; 
     FIG. 7 is a perspective view of the essential section of a first folding stopper; 
     FIG. 8 is a cross-sectional view of the state of the folding device under the A3 Z-folding mode; 
     FIG. 9 is a cross-sectional view of the state of the folding device under the A3 double-folding mode; 
     FIG. 10 is a cross-sectional view of the state of the folding device under the creasing mode; 
     FIG. 11 is a flow chart of a process for setting a conveying path; 
     FIG. 12 is a cross-sectional view of the construction of an additional-work tray unit and a stapler disposed in the downstream side; 
     FIG. 13A is a diagram illustrating the normal staple mode; 
     FIG. 13B is a diagram illustrating the fold staple mode functions; 
     FIG. 13C is a diagram illustrating the mixed staple mode functions; 
     FIG. 14 is a cross-sectional view of stapler together with a first and a second sheaf-conveying rollers; 
     FIG. 15 is a perspective view of the construction of the stapler, 
     FIG. 16 is a cross-sectional view of the operation of positioning for the staple mode; 
     FIG. 17 is a block diagram of the construction of a control system which controls a copying machine and a finisher; 
     FIG. 18 is a cross-sectional view of the conventional folding device; 
     FIG. 19 is a cross-sectional view of the addition of a reversing mechanism to the conventional folding device shown in FIG. 18; 
     FIG.  20 A and FIG. 20B are perspective views of a Z-folding sheet produced by a folding device according to the folding device shown in FIG. 19, comprising a reversing mechanism in front of the folding mechanism; 
     FIG.  21 A and FIG. 21B are perspective views of a Z-folding sheet produced by a folding device according to the folding device shown in FIG. 19, comprising a reversing mechanism in front of a discharging unit; 
     FIG.  22 A and FIG. 22B are perspective views of a Z-folding sheet in a form suitable for storage of sheets in the first page system; and 
     FIG. 23 is a cross-sectional view of a folding mechanism, which is disposed above the finisher, produces the form suitable for storage of sheets illustrated in FIG.  22 A and FIG.  22 B. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The preferred embodiments of this invention will be described below with reference to the accompanying drawings. 
     FIG. 1 is a cross-sectional view of an embodiment having a finisher  100  according to this invention connected to a copying machine  10  as an image forming device and FIG. 2 is a cross-sectional view of the essential section of the finisher  100 . 
     In this specification, the direction of conveyance of a sheet will be referred to as “conveying direction” and the direction perpendicular to the conveying direction as “orthogonal direction.” Then, the orientations of a sheet are defined as follows relative to the conveying direction. The orientation of the sheet whose longitudinal direction falls along the conveying direction will be referred to as “longitudinal” and the orientation of the sheet whose longitudinal direction perpendicularly crosses the conveying direction as “lateral.” 
     The illustrated copying machine  10  to which the finisher  100  is connected is a digital copying machine. The digital copying machine reads and temporarily stores in a memory an image on the surface of a document and, when necessary, executes various image processings. Then, it forms the image on a sheet by the well-known electrophotographic method and outputs sheets with the copied image one by one from a sheet output section  10   b.    
     The copying machine  10  has an automatic document feeder  12  (hereinafter referred to as “ADF”)on the upper section. The ADF  12  feeds one document or a plurality of documents (group of documents) set on a tray  14  one by one onto a platen glass (not shown) of the copying machine  10  and, after scanning the image, outputs and stacks the document onto a tray  16 . 
     The copying machine  10  of the present embodiment is a so-called first page system which starts a copying motion from the first page onward of the group of documents. On the tray  14  of the ADF  12 , the group of documents are set, with the first page turned upward. The copying machine of the first page system obviates the necessity for inputting or detecting the number, odd or even, of the documents in the group as when an image on one side of the document is copied on the obverse and reverse sides of one sheet. It produces advantages such as a quick copying motion. 
     As the document is set on the platen glass by the ADF  12 , the image on the document is read by an image reader (not shown) built in the copying machine  10 , converted into digital data, and stored in a memory of the control unit. The copying operation, after read out of the image data, is executed as combined with such necessary editorial processings as, for example, changing the order of pages, inverting an image, or producing copied images on both sides of a sheet. 
     This copying machine  10  is provided near the sheet output section  10   b  with a turn-back mechanism  20  for turning a sheet with copied image upside down. This mechanism will be described more specifically herein below. 
     &lt;&lt;General Construction and General Operation of Finisher  100 &gt;&gt; 
     [General Construction] 
     The finisher  100  of the present embodiment performs, either selectively or as suitably combined, such folding work as folding the sheets outputted from the sheet output section  10   b  of the copying machine  10  and conveyed one by one, in two or three (Z-folding in a cross section like a letter Z) as occasion demands, a punching work for forming holes for filing in the edges of the sheets, and a stapling for binding a sheaf with staples. Further, in this finisher  100  the mode of conveyance of sheets, the mode of stacking of sheets, or the mode of folding of sheets are designed on the assumption that it will be used as connected to the copying machine or a printer as an image forming device of the first page system. 
     The finisher  100 , as illustrated in FIG. 2, comprises a feed channel section  150  through which a sheet P outputted from the sheet output section  10   b  is fed, a folding device  200  which folds or creases the sheets conveyed one by one, a punching device  300  which forms holes for filing in the sheets P conveyed one by one, an additional-work tray unit  400  which stacks and aligns the sheets before a stapling work, a stapler  500  disposed on the downstream side of the additional-work tray unit  400  for stapling a sheaf of stacked and aligned sheets, an accumulating tray unit  600  which is capable of receiving a stapled sheaf or an unstapled sheet, and an output tray unit  110  which receives the sheets outputted from the finisher  100 . 
     The feed channel section  150  is provided with a conveying roller  101  and a guide plate. The folding device  200  is provided with a plurality of folding rollers  207 ,  208 , and  209  and is adapted to nip a sheet P between the folding rollers  207 ,  208 , and  209  and folds or creases the sheet P. The stapler  500  is so constructed as to be moved in the two directions, i.e. the conveying direction and the orthogonal direction of the sheaf stacked and aligned in the additional-work tray unit  400 . 
     For the purpose of conveying the sheet to various sections in the finisher  100 , conveying rollers  104 ,  106 ,  111 , and  121  are disposed along the sheet conveying paths. For the purpose of conveying the sheaf, sheaf-conveying rollers  114  and  115 ,  116  and  117 , and  119  and  120  are disposed along the conveying paths of the sheaf. A discharge roller  109  for discharging the sheet P into the output tray unit  110 , a discharge roller  113  for discharging the sheet P into the additional-work tray unit  400 , and discharge rollers  122  and  123  for discharging the sheet P or the sheaf into the accumulating tray unit  600  are respectively disposed at the terminal positions of the conveying paths. 
     For the purpose of changing the destination of the sheet being conveyed, a plurality of switch claws  201 ,  103  and  107  are disposed on the sheet conveying paths. The switch claw  201 , which is disposed between the feed channel section  150  and the folding device  200 , decides whether or not the sheet P is fed into the folding device  200 . The punching device  300  is disposed on the downstream side of the switch claw  201  and is enabled to punch the sheet conveyed from the feed channel section  150  or the sheet conveyed from the folding device  200 . The punching device  300  is provided with a punch blade  303  and a resist roller  308  for determining a punching position. The switch claw  103  disposed on the downstream side of the punching device  300  decides whether the sheet P is conveyed to the output tray unit  110  or to the additional-work tray unit  400  or the sheet P is directly conveyed to the accumulating tray unit  600 . The switch claw  107  disposed on the downstream side of the switch claw  103  decides whether the sheet P is conveyed to the output tray unit  110  or to the additional-work tray unit  400 . 
     For the purpose of timing the driving or stopping the various components in the finisher  100 , a plurality of sensors  102 ,  105 ,  108 ,  112 ,  118 ,  124  and  225  for detecting the sheet are disposed on the sheet and sheaf conveying paths. 
     The finisher  100  of the present embodiment is further provided with a guide unit  160  for preventing the sheaf bound by stapling like a weekly magazine from being defectively discharged into the accumulating tray unit  600 . The guide unit  160  illustrated in the diagram is composed of an auxiliary guide  125  which supports the lower side of the sheaf discharged from a space between discharge rollers  122  and  123 , and is allowed freely to advance and retract. This construction permits the leading end of the sheaf being discharged to fall toward the downstream side along the discharging direction further than the peak of the preriously discharged center bound sheaf even when the sheaves of sheets are stacked such that the bound sections project upward like a mountain. It results in precluding the possibility of the leading ends of the successively discharged sheaves being caught in the neighborhood of the peaks of the already stacked sheaves. 
     The finisher  100  is capable of performing a plurality of additional-workings (folding, punching and stapling) on the sheets. The user of the finisher  100  may select freely these works by the use of a control panel of the copying machine  10 . 
     When the user selects a mode excluding a stapling, the sheet P discharged from the sheet output section  10   b  of the copying machine  10  is worked by the folding device  200  and the punching device  300  in response to instructions of the user and conveyed by means of rollers to the output tray unit  110  or the accumulating tray unit  600  for storage. 
     When the user selects a mode including a stapling, first the sheet P is worked by the folding device  200  and the punching device  300  in response to instructions of the user similar to the mode excluding stapling. Then, a certain number of sheets P which have been folded and/or punched are conveyed to the additional-work tray unit  400  and sequentially stacked and aligned. Thereafter, the sheets which have been stacked and aligned are fed as one sheaf by rollers to the stapler  500 . 
     After the stapler  500  has bound the sheaf by driving staples into the sheaf at the positions selected by the user, the stapled sheaf is conveyed by the rollers to the accumulating tray unit  600  and is stored. 
     In this finisher  100 , the folding device  200  and the punching device  300  (as means for working the incoming sheets one by one) are disposed on the upstream sides of the position of the switch claw  103 , or on the upstream sides of the branching points of the conveying paths to a plurality of receiving tray units (referring collectively to the output tray unit  110 , the additional-work tray unit  400 , and the accumulating tray unit  600 ). The sheets which have undergone the works (folding and punching in this embodiment) one by one, therefore, can be discharged to any of the receiving tray units. 
     The main mechanisms of the finisher  100  will be sequentially described in detail below. 
     FIG. 3 is a cross-sectional view of the construction of the folding device  200 . FIG. 4 is a cross-sectional view of the folding device  200  which is jammed. FIGS. 5A and 5B and FIG. 6 are respectively cross-sectional views and a bottom view illustrating the essential section of a mechanism for regulating a first folding position in the folding device  200 . FIG. 7 is a perspective view illustrating the essential section of a first folding stopper. 
     The folding device  200  is built in the finisher  100  so as to be drawn out toward the front side of the finisher  100  (the foreground side of FIG. 1) and is supported as mounted to a rail (not shown) extended in the longitudinal direction of the finisher  100 . 
     The folding device  200 , as illustrated in FIG. 3, is composed of a feed channel section  251  for inside feeding a sheet for folding, a adjusting section  252  for correcting the sheet fed into the folding device  200  by removing a deviation, a first conveying section  253  for regulating the first folding position of the sheet conveyed from the adjusting section  252 , a folding section  254  for creasing or folding the sheet, a second conveying section  255  for regulating the second folding position, and a discharging section  256  for conveying the folded sheet from the folding device  200  to the punching device  300 . 
     The feed channel section  251  comprises the switch claw  201  which selectively guides the sheet to the folding device  200 , conveying rollers  202 ,  203  which convey the sheet fed into the folding device  200 , a solenoid (not shown) which rotates the switch claw  201 , and a sheet sensor  225  which detects the sheet fed into the folding device  200 . 
     The adjusting section  252  comprises resist rollers  205 ,  206  disposed on the downstream side of the feed channel section  251 , a drive motor (not shown) which drives the resist rollers  205 ,  206  for folding a sheet, and a solenoid clutch (not shown) which selectively cuts the connection of the motor to the resist rollers  205 ,  206 . The resist rollers  205 ,  206  are a pair of rollers composed of straight rollers. The surface friction coefficient μ of the roller  205  is set at a level lower than that of the other roller  206 . A guide  260  which is disposed on the upstream side of the resist rollers  205 ,  206  is shaped such that the leading end of the sheet is made to contact infallibly to the roller  205  having a lower surface friction coefficient. 
     The procedure for correcting a deviated sheet is as follows. 
     First, the sheet sensor  225  detects the leading end of an incoming sheet. At this time, the solenoid clutch is in the OFF state and the driving force of the motor for sheet folding is not transmitted to the resist rollers  205 ,  206 . 
     Then, after the elapse of the time (t+t 1 ) in seconds, the solenoid clutch is turned on to transmit the driving force to the resist rollers  205 ,  206  to convey the sheet to the downstream side. Here, the letter “t” refers to the time in seconds required for the leading end of a given sheet to reach the nip part of the resist rollers  205 ,  206 . 
     In consequence of the operation, a loop, V×t 1  [mm] (in which V stands for the sheet conveying speed [mm/second]) in length, is formed on the sheet between the conveying rollers  202 ,  203  and the resist rollers  205 ,  206 . Owing to the formation of this loop, the leading end of the sheet is caused by the intensity of the nerve of the sheet to conform to the contour of the nip part and the deviation of the sheet is adjusted. 
     The first conveying section  253  disposed on the downstream side of the adjusting section  252  comprises first folding stoppers  215 ,  216 ,  217  and  223  which move into and out of the sheet conveying paths in accordance with the sheet size and the folding form and regulate the first folding position of the sheet by contacting to the leading end of the sheet, cams  211 ,  212  and  213  which actuate the first folding stoppers  215 ,  216  and  217 , a stepping motor  210  which rotates the cams  211 ,  212  and  213 , and anti-deviation devices  226  of an elastic material which are disposed where the first folding stoppers  215 ,  216 ,  217  and  223  are come with the leading end of the sheet. 
     The first folding stoppers  215 ,  216 ,  217  and  223  will be described more specifically herein below. The first folding stopper  217  especially has the function of regulating the first folding position for sheets of two kinds with one stopper. 
     The three cams  211 ,  212  and  213  are fixed to a cam shaft  224  as shifted in angle such that the three first folding stoppers  215 ,  216  and  217  are each moved in and out of the sheet conveying path just once each time the cam shaft  224  produces one complete rotation. 
     The folding section  254  disposed between the downstream positions of the resist rollers  205 ,  206  and the upstream position of the first folding stopper  215  has three folding rollers  207 ,  208  and  209 . These folding rollers  207 ,  208  and  209  have a straight shape. 
     The folding rollers  208  and  209  are each pressed against the folding roller  207 . Namely, the folding rollers  207 ,  208  and the folding rollers  207 ,  209  are respectively in pairs. The folding rollers  207 ,  208  which are paired will be referred to hereinafter as “paired folding rollers  207 ,  208 ” and the folding rollers  207 ,  209  as “paired folding rollers  207 ,  209 .” The paired folding rollers  207 ,  208  are disposed such that the nip part continues into the first conveying section  253 . 
     The second conveying section  255  is disposed between the downstream positions of the paired folding rollers  207 ,  208  and the upstream positions of the paired folding rollers  207 ,  209 . The second conveying section  255  comprises a second folding stopper  219  which regulates the second folding position of the sheet by contacting the leading end of the sheet, a solenoid (not shown) which switches the position of the second folding stopper  219  contacting to the sheet in conformity with the sheet size, a switching mechanism  218  which selectively guides the leading end of the sheet which has undergone the first folding by the paired folding rollers  207 ,  208  in the direction of the nip part of the paired folding rollers  207 ,  209  or in the direction of the second folding stopper  219 , and a solenoid (not shown) which rotates the switching device  218 . 
     The discharging section  256  is disposed on the downstream side of the paired folding rollers  207 ,  209  and is possessed of discharging rollers  203  and  204 . The roller  203  constitutes one of the conveying rollers  202 ,  203 . 
     In the construction, the discharging section  256  is disposed between the conveying path on the upstream side for conveying the sheet in the direction of the first folding stopper  215  for the sake of the first folding and the conveying path in the second conveying section with the second folding stopper  219 . Consequently, the paired folding rollers  207 ,  209  are disposed at the initial point of the conveying path in the discharging section  256 . The folding roller  207  which is used commonly by the two pairs of folding rollers is disposed on the upstream side in the conveying direction of the sheet during the first folding. 
     The mechanism of restoring from a sheet jam which occurs in the folding section  254  of the folding device  200  will be described with reference to FIG.  4 . 
     The folding rollers  207 ,  208  and  209  in the folding section  254  have relatively high pressing force because they are required to fold the sheet strongly. The pressing force, for example, is 10 kg per roller. When the sheet happens to be wrapped tightly around any of the folding rollers  207 ,  208  and  209 , it is a very difficult to remove the stuck sheet or solve the jam. 
     The folding device  200  of the present embodiment, therefore, releases either of the two folding rollers  208 ,  209  from being pressed against the folding roller  207  and opens the folding section  254  in order to improve the operational efficiency of restoring from the jam in the vicinities of the folding rollers  207 ,  208  and  209 . This construction will be described below. 
     An open unit  222  is formed by integrally retaining the second conveying section  255 , the single folding roller  209  and a guide  261  of the discharging section  256 . This open unit  222  is supported as being freely rotated around a fulcrum  262  provided on a frame of the folding device  200 . 
     Further, a lock lever  220  constructed to encircle the periphery of the most section of the open unit  222  from the fulcrum  262  (as the upper end of the diagram) is supported as being freely rotated around a fulcrum  263  provided on the frame. Lock shafts  227  are provided one each in the front and rear portions of the lock lever  220  extending in the direction perpendicular to the face of the sheet bearing an image. When the open unit  222  is closed, the lock shafts  227  are each engaged with recess  22   a  formed in the open unit  222  and the open unit  222  is infallibly locked to the folding device  200 . 
     The lock lever  220  and the open unit  222  are connected through a link device  221 . The link device  221  enables the open unit  222  to be retained and rotated as synchronized with the rotation of the lock lever  220  and can preclude the falling of the open unit  222  during opening of the lock. 
     As illustrated in FIG. 5A, FIG.  5 B and FIG. 6, the first folding stoppers  215 ,  216 ,  217  and  223  as devices for regulating the leading end of the sheet, the cams  211 ,  212  and  213 , the stepping motor  210 , and the cam shaft  224  are integrally held by a stopper unit frame  228 . 
     Except for the stopper  223 , which is disposed on the most downstream side in the conveying direction of the sheet, the first folding stoppers  215 ,  216  and  217  are constructed as being freely rotated around respective fulcrums provided on the stopper unit frame  228 . The first folding stopper  223  is fixed to the stopper unit frame  228  and retained as constantly projected into the sheet conveying path. 
     The first folding stoppers  215 ,  216  and  217  are driven to move into and out of the sheet conveying path by the rotation of the cams  211 ,  212  and  213  and the cam shaft  224  which are disposed on the lower side of the frame  228 . The cams  211 ,  212  and  213  are attached at different angles to the cam shaft  224 . The first stoppers  215 ,  216  and  217  each move into and out of the sheet conveying path while the cam shaft  224  produces one complete rotation. The stepping motor  210  rotationally drives the cam shaft  224 . One of the first folding stoppers  215 ,  216  and  217  is moved into and out of the sheet conveying path by actuating the stepping motor  210  in a desired angle proper for the folding mode or the sheet size. 
     The cam shaft  224  is provided with a light stop or gobo  231 . The gobo  231  is moved into and out of the detecting area of a home position sensor  230  in consequence of the rotation of the cam shaft  224 . The position at which the home position sensor  230  detects the gobo  231  is the home position for the cam shaft  224 . At the home position, all of the first folding stoppers  215 ,  216  and  217  that are capable of moving into and out of the sheet conveying path are not in a projecting state except the first folding stopper  223 . 
     The first folding stopper  217  is designed to have the function of regulating two kinds of folding positions. To be specific, it is approximately shaped like a letter U having the opposite ends projected toward the upstream side in the conveying direction of the sheet as clearly shown in FIG.  6 . This shape is applicable only when the position for regulating the leading end of a sheet of a small width relative to the orthogonal direction falls on the downstream side in the conveying direction from the position for regulating the leading end of a sheet of a large width. Naturally, in this case, the stopper for the sheet of a large width must be disposed on the outer side along the orthogonal direction than the stopper for the sheet of a small width. In other words, the first folding stopper  217  is required to form, at the upstream position in the conveying direction, a notch of a width larger than the width of that of the two kinds of sheets which has a smaller width. The edges of the notch, or the edge located on the upstream side in the conveying direction and the edge located on the bottom, function as stoppers which come in contact with the leading edges of the two different kinds of sheets, respectively 
     In the illustrated embodiment, the first folding stopper  217  is constructed by integrating stoppers  217   a  disposed on the opposite outer sides used in double-folding of an A3 sheet with a stopper  217   b  disposed on the further downstream side than the stopper  217   a  and used in Z-folding of a B4 sheet. 
     The anti-deviation device  226  is mounted where the first folding stoppers  215 ,  216 ,  217  and  223  come in contact with the leading end of the sheet as illustrated in FIG.  7 . The anti-deviation device  226  is provided for the purpose of precluding the inconvenience that the leading end of the sheet slides laterally on the contacting face of a stopper and induces deviation of the folding position. This fact explains why the anti-deviation device  226  is made of an elastic material with a high surface friction coefficient and a low hardness. The anti-deviation device  226  is also effective in abating the noise which is made when the leading end of the sheet comes in contact with the stopper. 
     The advantages of the construction are as follows. 
     Firstly, the deviation of positions occurring when the leading end of a sheet is regulated is slight, because the devices for regulating the leading end of a sheet, or stoppers  215 ,  216 ,  217  and  223  are disposed one each at the plurality of positions used or required for regulating the leading end of a sheet. 
     Secondly, one motor  210  suffices as a drive source, because the plurality of devices for regulating the leading end of a sheet can be actuated by the single cam shaft. 
     Thirdly, the components for actuation can be simplified, because a device for regulating the leading end of a sheet, or stopper  217  has the function of regulating the leading ends of two kinds of sheet and a device for regulating the leading end of a sheet on the most downstream side, or stopper  223  has a stationary structure. Namely, the function of regulating the leading end of a sheet can be accomplished with high accuracy by means of simple and inexpensive construction. 
     It is, when necessary, allowable to divide the drive system into two and add the cam shafts, etc. though one cam shaft and one motor suffice to actuate the plurality of devices for regulating the leading end of a sheet. 
     The folding device  200  has the three folding modes, (1) Z-folding, (2) double-folding, and (3) creasing. When the folding mode is inputted through a control panel provided in the copying machine  10 , the folding device  200  is controlled in the inputted mode. 
     FIG. 8 is a cross-sectional view illustrating the state of the folding device  200  under the A3 Z-folding mode. In the diagram, the states which the sheet P assumes at different points of time are simultaneously indicated in the folding device  200  as well as in FIGS. 9 and 10. 
     The term “Z-folding mode” refers to a mode of folding a sheet of a large size (A3 or B4) in a cross section like a letter Z, or in the sheet in a size approximately one half of the original length of the sheet along the conveying direction. 
     The sheet P outputted from the sheet output section  10   b  of the copying machine  10  is conveyed in the “longitudinal” direction to the switch claw  201 , with the image-formed face held on the upper side. The sheet P is fed into the folding device  200  by the rotation of the switch claw  201  and then nipped by the conveying rollers  202 ,  203 . The sheet P is further conveyed to the adjusting section  252  wherein the leading end of the sheet is corrected by removal of a deviation. Thereafter, the sheet P is conveyed toward the first folding stoppers  215 ,  216 ,  217  and  223 . 
     Immediately after the command of a copy start is inputted, the stepping motor  210  is rotated by a fixed number of steps proper for the sheet size and the folding mode to set the position of the first folding stopper  215   216  or  217  (projecting position or retracting position). All the three first folding stoppers  215 ,  216  and  217  are retracted and the fixed first folding stopper  223  alone is projected when the sheet has the size of A3 and is in the longitudinal direction under the Z-folding mode as illustrated in the diagram. The first folding stopper  217  is moved to the projected position when the sheet has the size of B4 and is in the longitudinal direction. 
     After the leading end of the sheet has come with the first folding stopper  223 , the conveyance of the sheet is further continued. As a result, the sheet forms a loop in the neighborhood of the nip of the paired folding rollers  207 ,  208  and the loop is finally gripped by the nip of the paired folding rollers  207 ,  208 . Consequently, the first folding is effected on the sheet. 
     A guide  264  near the nip of the paired folding rollers  207 ,  208  is naturally constructed in a shape such that the loop in the sheet P is infallibly formed steadily as directed to the nip of the paired folding rollers  207 ,  208 . 
     The first folding position is separated by approximately ¾ of the total length of the sheet in a given sheet size from the edge of the sheet, or the leading end side in entering the folding device  200 . In this specification, for the sake of convenience of description, the first fold will be defined as “three-quarter (¾) fold.” The first fold at the position separated by approximately ¼ of the total length of the sheet from the edge of the sheet will be defined as “one-quarter (¼) fold.” 
     In response to the command “Z-folding” from the copying machine  10 , the switching device  218  is moved to the position for leading the sheet P in the direction of the second folding stopper  219 . The leading end of the sheet P conveyed by the paired folding rollers  207 ,  208  comes in contact with the second folding stopper  219  which has been switched in accordance with the sheet size. 
     When the conveyance of the sheet P is continued by the paired folding rollers  207 ,  208  after the leading end has come with the second stopper  291 , the sheet P forms a loop near the nip of the paired folding rollers  207 ,  209 . This loop is finally gripped by the nip of the paired folding rollers  207 ,  209 . The second folding position is at a distance of approximately ½ of the total length of the sheet. 
     Here again, a guide  265  near the nip of the paired folding rollers  207 ,  209  is naturally constructed in a shape such that the loop in the sheet P is infallibly formed steadily as directed to the nip of the paired folding rollers  207 ,  209 . 
     The sheet P on which the Z-folding has been completed by the second folding is conveyed toward the discharging section  256  by the paired folding rollers  207 ,  209  and discharged from the folding device  200  by the discharging rollers  203 ,  204 . 
     The Z-folding mode can do a so-called mixed working, i.e. an additional-working on a mixture of folded sheets and unfolded sheets. To be specific, Z-folding mode can achieve the mixed working of A3 Z-folding in the longitudinal direction and unfolded A4 sheets in the lateral direction or the mixed working of B4 Z-folding in the longitudinal direction and unfolded B5 sheets in the lateral direction. 
     Under the mixed mode, sheets for folding can be fed at a standard interval into the finisher  100  when the sheets follow sheets for no folding into the finisher  100 . Conversely, feeding of the sheets for no folding at the standard interval into the finisher  100  possibly causes such inconveniences as disruption of the order of pages or the contact between the sheets when the sheets follow sheets for folding into the finisher  100 . The present embodiment, therefore, precludes in the latter case the occurrence of such inconveniences as the disruption of the order of pages by loading a weight on the conveyance of the sheets for no folding and preventing these sheets from entering the finisher  100  until the folded sheets are discharged from the folding device  200 . 
     In consideration of the appearance of the product of the mixed working, the second crease or fold is preferably prevented from jutting out of the unfolded sheets. For this reason, the second folding position preferably deviate slightly from the ½ position of the total length of the sheet toward the edge of the sheet as the leading end side in entering the folding device  200 . 
     Namely, Z-folding is done as follows. A sheet is conveyed as the surface with a formed image opposes to the paired folding rollers  207 ,  208 . The first folding is done at the position separated by approximately three quarters of the total length of the conveying direction from the leading end of the sheet on the side of the first folding stopper  217 . And the sheet is conveyed as led by the crease of the first folding. The second folding is done by gripping, with the paired folding rollers  207  and  209 , a loop formed in consequence of the contact with the second folding stopper  291 . Then, the sheet is conveyed through the conveying path of the discharging section  256  which is disposed between the conveying path in the vicinity of the adjusting section  252  and the conveying path of the second conveying section  255 . The conveyance of this mode achieves the discharge of the sheet wherein the sheet is advanced as led by the crease and the folded section of the sheet falls on the side bearing the formed image and is directed downward. Therefore, the sheaf including Z-folding sheets is smoothly stacked without disruption of the order of pages in the first page system. Moreover, the sheets can be received such that the sides for stapling opposite to the folded sections approximate closely to the stapler  500  disposed on the downstream side in the conveying direction of the sheet as will be described herein below. 
     FIG. 9 is a cross section illustrating the state of the folding device  200  under the A3 double-folding mode. 
     The term “double-folding mode” refers to the mode of folding a sheet in two or in the central section. 
     The sheet P discharged from the sheet output section  10   b  of the copying machine  10  undergoes the same process as under the Z-folding mode and conveyed toward the first folding stoppers  215 ,  216 ,  217  and  223 . 
     Likewise under the double-folding mode, the stepping motor  210  is controlled to move only the first folding stopper  217  to the projecting position when the sheet has the size of A3 and is in the longitudinal direction, as illustrated in the diagram. The first folding stopper  216  is only moved to the projecting position when the sheet has the size of B4 and is in the longitudinal direction. The first folding stopper  215  is only moved to the projecting position when the sheet has the size of A4 and is in the longitudinal direction. The sheet P, after undergoing the same process as under the Z-folding mode, is gripped by the nip of the paired folding rollers  207 ,  208  and then given the first folding. 
     In response to the command “double-folding” from the copying machine  10 , the switching device  218  is moved to the position for guiding the sheet P toward the nip of the paired folding rollers  207 ,  209 . Then, the sheet P conveyed by the paired folding rollers  207 ,  208  is gripped on the crease by the nip of the paired folding rollers  207 ,  209  and conveyed per se to the paired discharging rollers  203 ,  204  and discharged from the folding device  200 . 
     FIG. 10 is a cross-sectional view illustrating the state of the folding device  200  under the creasing mode. 
     The term “creasing mode” refers to the mode of preparatorily creasing the central section of sheet for stapling the central crease of the sheaf like a weekly magazine. 
     The sheet P discharged from the sheet output section  10   b  of the copying machine  10  is conveyed toward the first folding stoppers  215 ,  216 ,  217  and  223 , similarly to the Z-folding mode or the double-folding mode. 
     The folding position under the creasing mode is identical with that under the double-folding mode. The motions of the first folding stoppers  215 ,  216  and  217  are controlled in the same manner as under the double-folding mode. And the sheet P is gripped by the nip of the paired folding rollers  207 ,  208  and given the first folding. 
     In response to the command “creasing mode” from the copying machine  10 , the switching device  218  is moved to the position for guiding the sheet P toward the second folding stopper  219 . The sheet P which has undergone the first folding is conveyed by the paired folding rollers  207 ,  208  toward the second folding stopper  219 . 
     The driving direction of the rollers  202 ,  205  and  207  in the folding device  200  is switched from the normal rotation (the direction of the arrow a in the diagram) to the reverse rotation (the direction of the arrow b in the diagram) after the elapse of the period of the time t 2  in seconds which follows the detection of the trailing edge of the sheet P having undergone the first folding by the sheet sensor  225  in the feed channel section  251 . The term “t 2 ” refers to the length of time satisfying the following condition: 
     
       
         ( y/V )&gt; t   2 &gt;( x/V ) 
       
     
     in which V stands for the rate of conveyance of a sheet, x for the distance between the sheet sensor  225  and the lower edge of the switch claw  201 , and y for the distance between the leading end of the sheet and the second folding stopper  219  after the detection of the trailing end of the sheet and the completion of the first folding. 
     The crease formed in the central section of the sheet P is released from the paired folding rollers  207 ,  208  in consequence of the reverse rotation of the rollers  202 ,  205  and  207 . The edge, which has been the trailing edge during the feed of the sheet into the folding device  200 , is now the leading edge. And the sheet is led to the switch claw  201  held in the same state as during the feed of the sheet, passed through the path indicated by the arrow W, and discharged from the folding device  200 . In this manner, the sheet P with the central crease can be conveyed in an opened posture toward the downstream side. 
     Incidentally, all the three folding modes are invariably accepted only when the sheet has a length of not less than twice the length of the sheet of the smallest size that is available for conveyance. 
     A turn-back mechanism  20 , which turns a sheet with a copied image upside down, is installed near the sheet output section  10   b  of the copying machine  10 . This turn-back mechanism  20  comprises a path for switchback conveyance of a sheet and a pair of reversible rollers provided in the path. The turn-back mechanism promotes compactness of the finisher and reduction in the cost. The arrangement of the turn-back mechanism  20  does not need to be limited to the vicinity of the sheet output section  10   b  of the copying machine  10 . This mechanism  20  may be disposed closely to the feed channel section  150  of the finisher  100  instead. 
     The copying machine  10  further comprises three paths  21 ,  22  and  23  used as selectively switched. The first path  21  is applied to discharge the sheet turned by the turn-back mechanism  20  from the sheet output section  10   b.  The second path  22  is applied to rotate the sheet turned by the turn-back mechanism  20  within the copying machine  10  for two-sided copies or copying an image on the side opposite to the side with the copied image. The third path is applied to directly discharge the sheet from the sheet output section  10   b  without passing the sheet through the turn-back mechanism. 
     The copying machine  10 , based on the operating mode set by the user and the size of the sheet selected for copying, judges whether or not the sheet for copying is subsequently folded and inputs the information resulting from this judgment to the finisher  100 . 
     FIG. 11 is a flow chart illustrating the process for setting a sheet conveying path. 
     When the copy mode is not a two-sided copying mode (“N” at Step S 11 ) and the judgment is “sheet for folding” (“Y” at Step S 12 ), the copying machine  10  switches the conveying path to the third path  23  (Step S 13 ). Then, the sheet is discharged from the sheet output section  10   b  without passing through the turn-back mechanism. In contrast, when the judgment is “sheet for no folding” (“N” at Step S 12 ), the copying machine  10  switches the path to the first path  21 . Then, the sheet is passed through the turn-back mechanism  20  and discharged in a reversed state from the sheet output section  10   b  (Step S 14 ). The finisher  100 , based on the information inputted from the copying machine  10 , controls the rotation of the switch claw  201  disposed on the upstream side of the folding device  200  and the positions of the first and second folding stoppers  215 ,  216 ,  217 ,  223  and  219  in conformity to the relevant folding mode. 
     When the copy mode is a two-sided copying mode (“Y” at Step S 11 ), the conveying path is temporarily switched to the second path  22  (“N” at Step S 15 , S 16 ) after the first copy is completed on one side. After the second copy is completed on the other side (“Y” at Step S 15 ), the operation described above is executed, depending on the result of the judgment whether or not the sheet folding is necessary. 
     In the first page system the first folding is done at the position separated by approximately three quarters of the length of the sheet in the conveying direction from the leading end of the sheet by means of the paired folding rollers and the folding stoppers in order to stack Z-folding sheets, for example, in a desired form. This construction is only applied to a sheet requiring folding. A sheet for no folding is turned back and a side with a copied image is directed downward the sheet to conform to the sheets which have been folded and stacked. Both the folded sheets and the unfolded sheets are in a correct sequence of pages and are stacked in a form permitting easy stapling. 
     It, therefore, suffices to perform the stapling at the position on the side of a leading end stopper  409  (FIG. 12) as a regulating device disposed in the direction of conveying such sheets as are temporarily stored in the tray. Thus, it is unnecessary to change the stapling position relative to the conveying directions in accordance with the sheet size and the operating mode. 
     Further, the conveying distance of the sheaf for stapling is reduced. As a result, it is possible to secure the stapling position with high accuracy and repress such inconveniences as the sheet deviation. 
     Besides, the folding device can be mounted in the lower section of the interior of the finisher. Thus, the finisher can be produced in a compact construction avoiding an addition to size and enjoying efficient use of space. The first page system inevitably necessitates a turn-back mechanism. In short, an additional device is merely a path which is capable of discharging a sheet as it is. And the folding device of this construction neither noticeably enlarges the size nor increases the cost as compared with the conventional folding device. 
     It is only natural that the digital data of images stored in the memory of the control unit are read out such that sheets are arranged in a correct sequence of pages when a sheaf like a weekly magazine is centrally creased or the sheets are copied on two sides. 
     FIG. 12 is a cross-sectional view illustrating the construction of the additional-work tray unit  400  and the stapler  500  disposed on the downstream side. 
     For the sake of convenience of the description, the alignment along the conveying direction from the additional-work tray  401  to the stapler  500  (FD-direction) will be referred to as “FD-alignment” and the alignment along the width direction of conveying sheet, i.e. the orthogonal direction (CD-direction), as “CD-alignment” hereinafter. 
     The additional-work tray unit  400  comprises the additional-work tray  401  which temporarily stores, in a face-down state, the sheet which is reversed upside down in the upstream section and then discharged by the discharging roller  113 , a leading end stopper  409  which is disposed in the sheet discharging outlet  401   a  of the additional-work tray  401  and effects the FD-alignment of the sheet, a pair of lateral aligning plates  402  which effects the CD-alignment of the sheet discharged by the discharging roller  113 , a trailing end stopper  403  which stabilizes the FD-alignment done with the leading end stopper  409  by contacting to the leading end of the sheet discharged by the discharging roller  113 , and the first sheaf-conveying rollers  114 ,  115  which convey a certain number of sheets stored in the additional-work tray  401  as one sheaf to the stapler  500 . 
     The additional-work tray  401  is set up such that the sheet-discharging outlet  401   a  is inclined downward by a certain angle. The pair of lateral aligning plates  402  is disposed such that they are freely moved symmetrically along the CD-direction. The pair of lateral aligning plates will be occasionally referred to hereinafter otherwise as “paired lateral aligning plates.” The trailing end stopper  403  is disposed so as to move along the FD-direction freely. The CD-alignment is effected each time that the additional-work tray  401  receives a sheet. Besides, the FD-alignment is effected each time that the additional-work tray  401  receives a sheet or a certain number of sheets. The first sheaf-conveying rollers  114 ,  115  constitute a pair of the lower roller  114  and the upper roller  115 . The upper roller  115  can move substantially in the vertical direction to press the lower roller  114  or depart from the lower roller  114 . 
     The paired lateral aligning plates  402 , are composed of plates having a height (L 1 ) greater than the largest height of the sheaf that can be stored on the additional-work tray  401 . The paired lateral aligning plates  402  are each mounted on a pair of racks  420  provided on the reverse side of the additional-work tray  401  along the CD-direction. The paired racks  420  are mounted opposed to each other across a gear  421  which is rotatably driven by a stepping motor  408 . The rotation of the gear  421  causes the paired lateral aligning plates  402  to move symmetrically along the CD-direction. To be specific, the paired lateral aligning plates  402  synchronously move toward each other during the normal rotation of the stepping motor  408  and synchronously move away from each other during the reverse rotation of the stepping motor  408 . 
     The paired lateral aligning plates  402  have two waiting positions, i.e. a first waiting position and a second waiting position. The first waiting position is a place occupied before the discharging roller  113  discharges the sheet. The second waiting position, as altered by the size of the sheet to be discharged, occupies a slightly wider area than the size of the sheet and is a place for awaiting the discharge of the sheet by the discharging roller  113 . The paired lateral aligning plates  402  are freely moved between the three positions, i.e. the first waiting position, the second waiting position, and the position for the CD-alignment of the sheet discharged by the discharging roller  113 . 
     A plurality of sensors for positioning the paired lateral aligning plates  402  are provided on the lower face of the additional-work tray  401 . The gobos, or stops for intercepting the light from the sensors  410  are integrally mounted on the paired lateral aligning plates  402 . Positioning of the first and second waiting positions are based on that the gobos intercept the light from the sensors  410 . The positioning of the paired lateral aligning plates  402  for the alignment is done by controlling the number of pulses inputted the stepping motor  408  to actuate the gear  421 . 
     The leading end stopper  409  is roughly shaped like a letter L and is composed of a bottom plate  409   a  and a blocking plate  409   b  raised from the leading end of the bottom plate  409   a.  The leading end stopper  409  is so mounted on the lower face of the additional-work tray  401  to freely rotate about a fulcrum  430  provided on the bottom plate  409   a.  The leading end stopper  409  is urged by the elastic force of a spring to come in contact with the lower face of the additional-work tray  401 . The blocking plate  409   b  of the leading end stopper  409  forms a base plane when the FD-alignment is effected on the sheet to be stored in the additional-work tray  401 . The blocking plate  409   b  of the leading end stopper  409  is moved downward as indicated by a phantom line in FIG. 12, by actuating a solenoid to pull a link arm (not shown) pivotally supported on a rotary fulcrum  430 . It results in opening the sheet-discharging outlet  401   a  for feeding a sheaf to the stapler  500 . 
     The trailing end stopper  403  comprises a plate  412 , a sponge  411  attached to one face of the plate  412  to which the sheet contacts, and a framer  413  supporting the plate  412 . Roughly the upper half of the plate  412  is rounded, or radius-shaped by being projected as slightly curved from the direction perpendicular to the upper face of the additional-work tray  401  toward the leading stopper  409  located on the sheet discharging outlet  401   a.    
     The plate  412  of the trailing end stopper  403  with the rounded shape produces the following advantages. The trailing end of the sheet along the conveying direction from the additional-work tray  401  to the stapler  500  (corresponding to the leading end of the sheet being discharged from the discharging roller  113 ) always steadily contacts the plate  412  of the trailing end stopper  403  without reference to the number of sheets stacked on the additional-work tray  401 , the size of the sheet, or the presence or absence of folding. In consequence of this contact, the sheet is repelled in the direction opposite the discharging direction and the leading end of the sheet along the conveying direction infallibly comes in contact with the leading end stopper  409  and the FD-alignment is further ensured. The Z-folding sheet, owing to the crease, has the trailing end along the conveying direction in a slightly lifted state. However, the sheaf including Z-folding sheets can be uniformly pushed in and brought into contact with the leading end stopper  409  by using the plate  412  having the radius-shaped upper part. Thus, the additional-work tray unit  400  can infallibly eliminate the deviation in the conveying direction possibly produced in the sheaf including Z-folding sheets during the conveyance to the stapler  500 . 
     The framer  413  of the trailing end stopper  403  is engaged with a spiral shaft  404  which is installed as extended along the conveying direction at the center of the lower face of the additional-work tray  401 . This spiral shaft  404  is connected to a motor  406 , such as a DC motor, through a transmission device (not shown), such as a gear train. The trailing end stopper  403  is moved forward or backward by a necessary distance along the conveying direction by actuating the motor  406  properly in the normal or reverse direction to rotate the spiral shaft  404 . 
     FIGS. 13A-13C are diagrams illustrating the states of various staple modes. The stapler has three staple modes, i.e. normal staple mode (FIG.  13 A), fold staple mode (FIG.  13 B), and mixed staple mode (FIG.  13 C), which are selectively adopted. The normal staple mode is a mode for stapling a sheaf solely of unfolded sheets, the fold staple mode is a mode for stapling a sheaf solely of folded sheets, and the mixed staple mode is a mode for stapling a sheaf of unfolded and folded sheets. 
     Without reference to the kind of staple mode, the folded and/or unfolded sheets are stacked on the additional-work tray  401  prior to the relevant stapling, subjected to the CD-alignment by the paired lateral aligning plates  402 , and then subjected to the FD-alignment performed jointly by the trailing end stopper  403  and the leading end stopper  409 . 
     After the CD-alignment and the FD-alignment are completed in the additional-work tray  401 , the sheaf is nipped by the first sheaf-conveying rollers  114 ,  115  and passed through the sheet discharging outlet  401   a  opened in consequence of the rotation of the leading end stopper  409 . 
     [Construction of Stapler  500 ] 
     FIG. 14 is a cross-sectional view illustrating the stapler  500  together with the first and second sheaf-conveying rollers  114 - 117  and FIG. 15 is a perspective view illustrating the construction of the stapler  500 . 
     The stapler  500  performs a stapling at certain positions of a sheaf nipped and conveyed by the first sheaf-conveying rollers  114 ,  115  on the upstream side of the stapler  500  relative to the conveying direction. The stapler  500  comprises a head unit  501 , an anvil unit  502 , a supporting mechanism  520  which supports the units  501 ,  502  such that the units  501 ,  502  are freely moved in the orthogonal direction and rotated, a first drive mechanism  521  which moves the units  501 ,  502 , and a second drive mechanism  522  which rotates the units  501 ,  502 . In the stapler  500 , devices which engage or connect the head unit  501  with the anvil unit  502  do not transverse the sheet conveying path. 
     Further, the second sheet-conveying rollers  116 ,  117  which convey the stapled sheaf and the second sensor  118  for fixing the stapling position of the sheaf (as will be specifically described herein below) are installed on the downstream side of the stapler  500 . 
     The head unit  501  separates one staple from a cartridge held within a cartridge case (not shown), bends the separated staple in the shape nearly resembling a letter U, and transfixes the sheaf with the bent staple. This unit  501  is provided with a sensor which detects the presence or absence of staple in the cartridge case. 
     The anvil unit  502  inwardly bends shanks of the staple which has penetrated through the sheaf and receives the shock of stapling performed by the head unit  501 . This unit  502  comprises a receiving plate, which inwardly bends the shanks of the staple, and a supporting plate, which receives the shock of the stapling action. 
     The supporting mechanism  520 , as illustrated schematically in FIG. 15, comprises a frame  510  provided with a pair of lateral wall  509   a,    509   b  and supporting shafts  503 ,  506  extending along the orthogonal direction and supported by the frame  510 . The distance between the lateral wall  509   a,    509   b  of the frame  510  is set to surpass at least the length of a sheet in the orthogonal direction, which is passable. The supporting shafts  503 ,  506  are each formed of a round bar. The supporting shaft  503  is inserted through the head unit  501  and the supporting shaft  506  is inserted through the anvil unit  502 . The units  501 ,  502  are freely moved in the orthogonal direction along the supporting shafts  503  and  506  and are freely rotated respectively about the supporting shafts  503  and  506 , respectively. 
     The first drive mechanism  521  comprises a spiral shaft  504  inserted through the head unit  501  and a spiral shaft  507  inserted through the anvil unit  502 . The spiral shafts  504 ,  507  extend along the orthogonal direction and supported by the frame  510 . In consequence of the rotation of the spiral shaft  504 , the head unit  501  is moved in the orthogonal direction as guided by the supporting shaft  503 . In consequence of the rotation of the spiral shaft  507 , the anvil unit  502  is moved in the orthogonal direction as guided by the supporting shaft  506 . 
     The second drive mechanism  522  comprises a drive shaft  505  inserted through the head unit  501  and a drive shaft  508  inserted through the anvil unit  502 . The drive shafts  505 ,  508  extend along the orthogonal direction and supported by the frame  510 . In consequence of the rotation of the drive shaft  505 , the driving force for transfixing a sheaf is transmitted to the head unit  501  and the head unit  501  is rotated about the supporting shaft  503  as a center. In consequence of the rotation of the drive shaft  508 , the driving force for bending shanks of a staple is transmitted to the anvil unit  502  and the anvil unit  502  is rotated about the supporting shaft  506  as a center. The drive shafts  505 ,  508  include a shaft possessed of a rectangular cross section incapable of generating slippage for the purpose of infallibly transmitting the driving force to the units  501 ,  502 . When the drive shafts are formed of a round bar, the slippage between the drive shafts and the units  501  and  502  may be precluded by means of a key or a key groove, for example. 
     The units  501 ,  502  can be linearly moved independently and parallel along the orthogonal direction with the aid of the plurality of shafts  503 - 505  and  506 - 508 , which are inserted respectively. 
     The head unit  501  and the anvil unit  502  are moved along the orthogonal direction by the rotation of the spiral shafts  504 ,  507  which have the same phases. A timing belt  511  is suspended as passed around the spiral shafts  504 ,  507 . This belt  511  is connected to a drive motor  512 . The drive motor  512  is formed of a DC motor and enabled by a pulse disc sensor  513  to produce a controlled rotation. Owing to the construction, the units  501 ,  502  can be each moved in an equal distance. The first drive mechanism  521  is composed of the spiral shafts  504  and  507 , the timing belt  511 , the drive motor  521 , etc. 
     A light-permeable sensor  516  is mounted on the frame  510  for detecting the home positions of the units  501 ,  502 . After detecting the gobos provided on the head unit  501  by the sensor  516 , the units  501 ,  502  are both moved to the respective home positions. The distances of movement of the units  501 ,  502  are set on the basis of the home positions. 
     The head unit  501  and the anvil unit  502  are actuated to produce the transfixing motion by the rotation of the drive shafts  505 ,  508 . A belt  514  is suspended as passed around the drive shafts  505 ,  508 . This belt  514  is connected to a drive motor  515 . Owing to this construction, the units  501 ,  502  are each driven to transfix a sheaf at positions arbitrarily selected in the orthogonal direction. The second drive mechanism  522  is composed of the drive shafts  505  and  508 , the belt  514 , the drive motor  515 , etc. 
     The head unit  501  and the anvil unit  502  of the stapler  500  at first stand at rest at the home positions for intercepting the light from the sensor  516 . The sheets outputted from the copying machine  10  are conveyed to the additional-work tray  401  and are stacked and aligned. When as many sheets as suffice for one job are stacked on the additional-work tray  401 , the stacked sheet are conveyed as a sheaf in the direction of the stapler  500 . 
     The first sheaf-conveying rollers  114 ,  115  as a conveying device for nipping and conveying the sheaf to the stapler  500  can control the conveying distance of the sheaf by the amounts of their rotation. The first sheaf-conveying rollers  114 ,  115  convey the sheaf at a position such that the stapling position arbitrarily selected on the sheaf coincides with the transfixing position. 
     Thereafter, the drive motor  512  is actuated to rotate the spiral shafts  504 ,  507  through the belt  511  while the pulse disc sensor  513  detects the amount of rotation. The units  501 ,  502  are each moved over an equal distance in the direction of the stapling positions selected arbitrarily. When the units  501 ,  502  are stopped at the selected stapling positions, the drive motor  515  is actuated to rotate the drive shafts  505 ,  508  through the belt  514 . The units  501 ,  502  are rotated to transfix a sheaf. 
     When the stapling is performed at a plurality of points falling on a straight line along the orthogonal direction, the units  501 ,  502  are moved to the next transfixing point by the operation of the motor  512  after completing the transfixing work at the first point. Then, the motor  515  is actuated to perform the transfixing work. By repeating this process, the stapling work at the plurality of points is wholly completed. 
     As shown in FIG. 14, first sheaf-conveying rollers  114 ,  115  which are composed of a pair of rollers (upper and lower rollers) are disposed in the upstream section and second sheaf-conveying rollers  116 ,  117  which are composed of a pair of rollers (upper and lower rollers) are disposed in the downstream section of a stapler  500 . The distance between the nip position of the first sheaf-conveying rollers  114 ,  115  and the nip position of the second sheaf-conveying rollers  116 ,  117  is set at a size slightly smaller than the smallest of the sizes of sheets to be conveyed. 
     A first DC motor drives the first sheaf-conveying rollers  114 ,  115  to be moved toward each other until pressure contact or separated away from each other. A stepping motor rotates the rollers  14 ,  15 . The conveying distance of the sheaf is adjusted by controlling the revolving speed of the stepping motor. The second sheaf-conveying rollers  116 ,  117  are constructed similarly to the first sheaf-conveying rollers  114 ,  115 . A second DC motor drives the second sheaf-conveying rollers  116 ,  117  to be moved toward each other until pressure contact or separated away from each other, independently of the first sheaf-conveying rollers  114 ,  115 . The stepping motor, which drives the first sheaf-conveying rollers  114  and  115 , also rotates the second sheaf-conveying rollers  116 ,  117  and controls the conveying distance of the sheaf. The rollers  114 - 117  are invariably formed of an identical material with low hardness and in a geometrical similar shape. The rollers  116 ,  117  have a smaller diameter than the rollers  114 ,  115 . 
     A first sensor  137  which detects the edge of a sheaf being fed is disposed near the downstream side of the first sheaf-conveying rollers  114  and  115 , and a second sensor  118  is disposed near the downstream side of the second sheaf-conveying rollers  116 ,  117  as illustrated in FIG.  14 . The sensors  118 ,  137  are set at a position separated by a certain distance from the stapling position. 
     The conveying path at least between the first sheaf-conveying rollers  114 ,  115  and the second sensor  118  is formed of a straight conveying guide. 
     The leading end of the sheaf has been aligned by a leading end stopper  409  during the temporary storage of sheets. In this state, the first sheaf-conveying rollers  114 ,  115  begin movement toward each other until pressure contact. Thus, the first sheaf-conveying rollers  114 ,  115  nip the leading end of the sheaf in the aligned state. 
     The conveying path between the first sheaf-conveying rollers  114 ,  115  and the stapling position has a straight shape. The leading end of the sheaf retains the aligned state intact even when the sheaf is nipped and conveyed by the first sheaf-conveying rollers  114 ,  115  to the stapling position. 
     If the conveying path in the downstream side in the conveying direction from the first sheaf-conveying rollers  114 ,  115  is bent like an arc, a sheaf of sheets will become long along a guide plate having an arc of a small radius and short along a guide plate having an arc of a large radius such that the leading end of the sheaf is slanted relative to the guide plate. If the stapler staples the sheaf in the direction perpendicular to the guide plates, it will inevitably bind the sheaf obliquely. 
     In conclusion, the conveying path between the first sheaf-conveying rollers  114 ,  115  and the stapling position must be in a straight shape when the stapler  500  staples a sheaf being nipped by the first sheaf-conveying rollers  114 ,  115 . 
     The present embodiment, as will be described herein below, is constructed such that the first sheaf-conveying rollers  114 ,  115  nip and convey a sheaf, and the second sheaf-conveying rollers  116 ,  117  nip and convey the sheaf additionally, and the first sheaf-conveying rollers  114 ,  115  release the sheaf, and the second sheaf-conveying rollers  116 ,  117  nip and convey the sheaf exclusively, and the stapler  500  staples the sheaf. The finisher must keep the aligned leading end of the sheaf, which is nipped and conveyed by the first sheaf-conveying rollers  114 ,  115  solely, intact until the second sheaf-conveying rollers  116  nip and convey the sheaf additionally. Thus, the conveying path between the first sheaf-conveying rollers  114 ,  115  and the second sensor  118  which is located at the position of the sheaf at which the second sheaf-conveying rollers  116 ,  117  begin to nip the sheaf, must be in a straight shape. 
     The finisher includes the second sheaf-conveying rollers  116 ,  117  which nip the sheaf on the downstream side from the stapling position. Therefore, the conveying path extending in the downstream side of the second sensor  118  does not need to be in a straight shape but may be bent like an arc, for example. The bending of the conveying path can prevent the whole finisher from growing in size. 
     When the staple mode is selected, sheets are stacked on the additional-work tray  401 . At this time, the first sheaf-conveying rollers  114 ,  115  are separated from each other. After the temporary stacking or storing of the sheets is completed, the first sheaf-conveying rollers  114 ,  115  are shifted to a mutually pressed state to nip a sheaf of the sheets and the leading end stopper  409  retracts outside the conveying path. Then, the sheaf is conveyed by rotating the first sheaf-conveying rollers  114 ,  115  and the stapling position is located along the conveying direction. The present embodiment contemplates three staple modes. The first mode is “leading end bind” which binds the leading end of the sheaf along the conveying direction. The second mode is “center bind” which binds the central section of the sheaf along the conveying direction. The third mode is “trailing end bind” which binds the trailing end of the sheaf along the conveying direction. The operation of the positioning depends on these modes. The each operation of the location for modes will be described below with reference to FIG.  16 . 
     The leading end of the sheaf has already undergone the FD-alignment during the temporary stacking of sheets with the blocking plate  409   b  of the leading end stopper  409  used as a regulating face. In the mode of leading end bind, it suffices for the location of the stapling position to convey the sheaf in a certain distance without reference to the size of sheet even when the sheaf have been given a Z-folding, for example. To be specific, it is only required that the first sheaf-conveying rollers  114 ,  115  convey the sheaf in the distance resulting from adding the length from the leading end of the sheaf to the desired stapling position (normally about 10 mm) to the length from the blocking plate  409   b  of the leading end stopper  409  to the stapler  500 . 
     Thereafter, the rollers  114 ,  115  are stopped and the stapler  500  is actuated to staple the sheaf. The conveyance of the sheaf is resumed after the completion of the stapling. The conveyance of the sheets is stopped when the leading end completely reaches the second sheet-conveying rollers  116 ,  117 . The second sheet-conveying rollers  116 ,  117  are shifted to a mutually pressed state to nip the leading end of the sheaf. Then, the second sheet-conveying rollers  116 ,  117  are rotated to start the conveyance of the sheaf again. 
     The first DC motor is actuated with continuing the conveyance of the sheaf and exclusively shifts the first sheaf-conveying rollers  114 ,  115  to a mutually separated state. The sheaf is subsequently conveyed and nipped by the second sheet-conveying rollers  116 ,  117  toward the accumulating tray unit  600 . 
     The stepping motor rotates the first and second sheaf-conveying rollers  114 - 117 . The conveying distance of the sheaf is controlled by regulating the pulses of the stepping motor. 
     In the mode of center bind, the stapling is done in the central section of the sheaf along the conveying direction. Naturally, the conveying distance of the sheaf for the stapling varies with the size of sheet. The conveying distance is long as compared with that involved in the mode of leading end bind. 
     The stepping motor conveys the sheaf. It is theoretically possible to control, by simply changing pulses, the conveying distance even when the conveying distance is long. However, the diameters of the sheaf-conveying rollers  114 - 117  and the widths of the nips cannot be thoroughly freed from dimensional dispersions. Namely, the inaccuracy in the actual conveying distance enlarges in proportion as the conveying distance lengthens. To reduce the inaccuracy, the conveyance of the sheaf in the mode of center bind is effected as follows. 
     First, a sheaf is nipped and conveyed by the first sheaf-conveying rollers  114 ,  115 . After the second sensor  118  disposed in the downstream side of the second sheet conveying roller  116 ,  117  has detected the leading end of the sheaf, the sheaf is further conveyed in a distance proper for the sheet size and is stopped. Then, the sheaf is stapled. 
     At this time, the leading end of the sheaf has completely reached the second sheet-conveying rollers  116 ,  117 . The second sheet-conveying rollers  116 ,  117  nip the sheaf. Then, the second sheet-conveying rollers  116 ,  117  are rotated to resume the conveyance of the sheaf. Meanwhile the first DC motor is actuated to shift the first sheaf-conveying rollers  114 ,  115  alone to a mutually separated state, continuing the conveyance of the sheaf. Thereafter, the sheaf is conveyed and nipped by the second sheet-conveying rollers  116 ,  117  toward the accumulating tray unit  600 . 
     Incidentally, in the mode of center bind, sheets having a length not less than twice the length of a sheet of the smallest size to be conveyed are only applicable. 
     The following steps are adopted in the leading end bind and center bind modes in order to shorten the total time required for the conveyance of the sheaf and improve the productivity. Namely, the first sheaf-conveying rollers  114 ,  115  positioned in the upstream side and the second sheaf-conveying rollers  116 ,  117  positioned in the upstream side of the stapler  500  nip and convey the sheaf together, and then the first sheaf-conveying rollers  114 ,  115  are switched to a state of mutual separation while the conveyance is in process. 
     In the mode of trailing end bind, first the sheaf is nipped and conveyed by the first sheaf-conveying rollers  114 ,  115 . When the leading end of the sheaf completely reaches the second sheet-conveying rollers  116 ,  117 , the conveyance is stopped and the sheaf is nipped by the second sheet-conveying rollers  116 ,  117 . 
     After the completion of the nipping by the second sheet-conveying rollers  116 ,  117 , the first DC motor is actuated to shift the first sheaf-conveying rollers  114 ,  115  to a mutually separated state. At this time, the conveyance of the sheaf is not proceeding. 
     The reason for the suspended conveyance is that the sheaf has not yet been stapled by the time that it is nipped by the second sheaf-conveying rollers  116 ,  117  unlike in the leading end bind mode or the center bind mode, and the individual sheets of the sheaf are inevitably deviated when the conveyance of the sheaf begins again without waiting the completion of separating the first sheaf-conveying rollers  114 ,  115  mutually and a deviation or difference happens to occur in the timing for starting or in the speed of conveyance between the first sheaf-conveying rollers  114 ,  115  and the second sheaf-conveying rollers  116 ,  117 . In short, the suspended conveyance can preclude the deviation in the sheaf. 
     After the completion of the operation for mutually separating the first sheaf-conveying rollers  114 ,  115 , the second sheet-conveying rollers  116 ,  117  are rotated to resume the conveyance of the sheaf. When the second sensor  118  detects the leading end of the sheaf, the sheaf is stopped after conveyed in a certain distance proper for the sheet size. Then the sheaf is stapled. 
     The stapled sheaf resumes being conveyed and nipped by the second sheet-conveying rollers  116 ,  117  toward the accumulating tray unit  600 . 
     In the above mode of conveyance, the conveying distance is set based on the position of the second sensor  118 . Optionally, the conveying distance in the mode of trailing end bind may be set based on the position of the first sensor  137  which is disposed in the downstream side of the first sheaf-conveying rollers  114 ,  115 . In the present mode, the sheaf is conveyed in a certain distance after the first sensor  137  has detected the trailing end of the sheaf. Namely, the sheaf has only to be conveyed in a prescribed distance without reference to the size of sheet. The first sensor  137  approximates closely the stapling position. Advantageously, it results in shortening the conveying distance and improving the positioning accuracy. 
     The sheet discharge unit which discharges sheets to the accumulating tray unit  600  as illustrated in FIG. 2, comprises the third sheet-conveying rollers  119 ,  120  which conveys the sheaf, the conveying roller  121  disposed in the downstream side of the switch claw  103  and conveys a lone sheet, and discharging rollers  122 ,  123  which outputs the sheaf or the single sheet into the accumulating tray  601  in addition to the first and second sheaf-conveying rollers  114 ,  115  and  116 ,  117 . 
     Namely, the accumulating tray unit  600  is so constructed as to receive a sheaf of sheets, which is discharged from the additional-work tray  401  and stapled by the stapler  500 , and an unstapled single sheet, which is conveyed through the other conveying path. 
     The system for controlling the various processing will be explained below. FIG. 17 is a block diagram of the control system for executing the various processing. 
     The control system is composed of a CPU  910  which controlling the copying machine, a CPU  950  which controls the ADF  12 , and a CPU  980  which controls the finisher  100 . These CPUs are provided respectively with ROM  911 ,  951  and  981 , which store the control programs, and RAM  912 ,  952  and  982 , which function as relevant working areas. 
     The CPU  910  for the copying machine is provided with an image memory  825  which stores a scanned image data and an image data processing unit  820  which executes such image processing as rotation, enlargement, and reduction of the image based on the image data stored in the image memory  825 . A CCD line sensor  822  of the image reader is connected to the image data processing unit  820  through an A/D converter  821  which converts the scanned analog signal into a digital signal. Further, the imaged at a processing unit  820  controls a laser device  832  of an image forming device (not shown) through a D/A converter  831  which converts a digital signal as a digital image data to an analog signal as an analog image data for outputting. 
     Various driven units and sensors are connected to the CPU  980  for the finisher for controlling and actuating the various units or devices of the finisher. The driven units include the motors and the solenoids. The sensors include the sheet sensor  225  provided in the conveying path and the home position sensor  230  provided in the folding section  254 . 
     The ROM  981  connected to the CPU  980  for the finisher stores the number of sheets as thresholds for determining leading end bind and training end bind. The CPU  980  is constructed to be able to make a choice between the leading end bind and the trailing end bind in consideration of the following point. The deviation of sheets enlarges in proportion as the conveying distance increases (corresponding to in the trailing end bind mode) and the number of sheets of sheaf increases when rollers convey the sheaf. The sheaf continues to remain in the additional-work tray unit which is used for temporary storage during the stapling and thus the productivity in the leading end bind mode is lower than that in the trailing end bind mode. The present embodiment automatically makes the choice, depending on the question whether or not the number of sheets of sheaf is larger than the set value as the threshold. Of course, it may be constructed such that the user optionally makes the choice. 
     The CPU  910  for the copying machine calculates the number of output sheets besides the basic operations proper for a copying machine (such as reading an image data on a document, storing the image data in memory, editing or processing the image data, forming an edited image on a paper, and outputting the paper). Specifically, the CPU  910  controls the document feeding of the ADF  12 , obtains the number of documents from the ADF  12 , and calculates the number of output sheets based on the number of documents and the copy mode inputted through the control panel. The result of the calculation is inputted to the CPU  980  for the finisher. The CPU  980  effects the choice between the leading end bind and the trailing end bind. In case of the trailing end bind, the CPU  980  inputs an instruction for rotating an image to the CPU  910  for the copying machine. In the above manner, the leading end bind or the trailing end bind is automatically selected. 
     It is obvious that this invention is not limited to the particular embodiments shown and described above but may be variously changed and modified by any person of ordinary skill in the art without departing from the technical concept of this invention. 
     The entire disclosure of Japanese Patent Application No. 09-058118 filed on Mar. 12, 1997, including the specification, claims, drawings and summary are incorporated herein by reference in its entirety.