Patent Publication Number: US-7216863-B2

Title: Sheet processing apparatus above image forming means and image forming apparatus

Description:
This is a divisional of U.S. patent application Ser. No. 09/791,775, filed Feb. 26, 2001now U.S. Pat. No. 6,581,922, and allowed on Jan. 28, 2003. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a sheet processing method of performing a predetermined process on sheets, a sheet processing apparatus for implementing the sheet processing method, and an image forming apparatus having the sheet processing apparatus. 
   2. Description of Related Art 
   Heretofore, some image forming apparatuses, such as printers, are provided with a sheet processing apparatus for aligning a plurality of sheets having images formed (printed) thereon and, then, performing a process on the sheets, such as stapling (driving needles into) the end parts of the sheets. 
   Such a sheet processing apparatus is, in many cases, provided as an option unit which is detachably attached to a printer or a copying machine, and is mounted in such a way as to be connected directly with a sheet discharge port of the body of the image forming apparatus. Then, sheets which have been subjected to printing at the image forming apparatus body are sequentially supplied from the sheet discharge port to the sheet processing apparatus, at which the sheets are aligned and are, then, subjected to a predetermined process. 
   However, in such a conventional image forming apparatus, there are such inconveniences that, in order to discharge and stack, in the order of page numbers, the sheets subjected to printing at the image forming apparatus body, it is necessary to provide the sheet processing apparatus with an inverting mechanism for inverting sheets, or it is necessary to assure a wide interval between the sheets so as to allow an inverting action on the sheets. 
   Further, since the sheet processing apparatus is disposed at the side of the sheet discharge port of the image forming apparatus body, not only the area of installation of the whole image forming apparatus is caused to increase, but also the production cost of the image forming apparatus is caused to rise disadvantageously. 
   BRIEF SUMMARY OF THE INVENTION 
   The invention has been made in view of the problems mentioned above, and, in accordance with an aspect of the invention, there is provided a sheet processing apparatus, which comprises a first sheet stacking portion for temporarily stacking a sheet discharged thereon, alignment means for aligning the sheet discharged on the first sheet stacking portion, sheet processing means for performing a predetermined process on the sheet stacked on the first sheet stacking portion, and a second sheet stacking portion located substantially vertically below the first sheet stacking portion, wherein the alignment means acts to cause the aligned sheet to drop to the second sheet stacking portion. 
   The above and further aspects and features of the invention will become apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       FIG. 1  is a sectional view showing in outline the arrangement of an image forming apparatus having a sheet processing apparatus mounted thereon according to an embodiment of the invention. 
       FIGS. 2(   a ) and  2 ( b ) are sectional views for explaining the operation of the sheet processing apparatus according to the embodiment. 
       FIGS. 3(   a ) and  3 ( b ) are sectional views for explaining the operation of slide guides in the embodiment, showing the state in which the slide guides are located at their standby positions. 
       FIGS. 4(   a ) and  4 ( b ) are sectional views for explaining the operation of the slide guides in the embodiment, showing the state in which sheets have been aligned by the slide guides. 
       FIGS. 5(   a ) and  5 ( b ) are sectional views for explaining the operation of the slide guides in the embodiment, showing the state in which the slide guides are located at their home positions and the sheets are dropping. 
       FIGS. 6(   a ) and  6 ( b ) are sectional views for explaining the arrangement and operation of a slide guide and a fixed guide in another embodiment of the invention, showing the state in which the slide guide is located at its standby position. 
       FIGS. 7(   a ) and  7 ( b ) are sectional views for explaining the operation of the slide guide in the embodiment shown in  FIGS. 6(   a ) and  6 ( b ), showing the state in which sheets have been aligned by the slide guide and the fixed guide. 
       FIGS. 8(   a ) and  8 ( b ) are sectional views for explaining the operation of the slide guide in the embodiment shown in  FIGS. 6(   a ) and  6 ( b ), showing the state in which the slide guide is located at its home positions and the sheets are dropping. 
       FIG. 9  is a sectional view showing in outline the arrangement of an image forming apparatus having a sheet processing apparatus mounted thereon according to a further embodiment of the invention. 
       FIG. 10  is an enlargement view showing a reference wall and parts therearound. 
       FIGS. 11(   a ) and  11 ( b ) are sectional views for explaining the operation of a sheet processing apparatus in which side wall parts are provided at the reference wall, showing the state in which the slide guides are located at their standby positions. 
       FIGS. 12(   a ) and  12 ( b ) are sectional views for explaining the operation of the sheet processing apparatus in which the side wall parts are provided at the reference wall, showing the state in which sheets have been aligned by the slide guides. 
       FIGS. 13(   a ) and  13 ( b ) are sectional views for explaining the operation of the sheet processing apparatus in which the side wall parts are provided at the reference wall, showing the state in which the sheets abut on the side wall part and are then dropping. 
       FIG. 14  is a front view for explaining the connection between the sheet processing apparatus and the image forming apparatus according to the invention. 
       FIG. 15  is a front view for further explaining the connection between the sheet processing apparatus and the image forming apparatus according to the invention. 
       FIG. 16  is a view showing the sheet processing apparatus and the image forming apparatus as viewed from the side opposite to the side shown in  FIG. 14 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Hereinafter, preferred embodiments of the invention will be described in detail with reference to the drawings. In the following description, as an embodiment of the invention, there is employed a sheet processing apparatus that is mountable on a printer apparatus, which is represented by a laser beam printer. 
   First, the outlines of the invention will be described with reference to  FIG. 1  to  FIGS. 5(   a ) and  5 ( b ).  FIG. 1  is a sectional view showing in outline the whole arrangement of a sheet processing apparatus and an image processing apparatus (printer) according to the embodiment of the invention. 
   In  FIG. 1 , reference numeral  100  denotes a printer body serving as the image forming apparatus. When connected solely to a computer or when connected to a network, the printer body  100  is arranged to form an image (print) on a sheet by a predetermined image forming process on the basis of image information, a printing signal or the like received from the computer or the network. 
   On the other hand, the sheet processing apparatus, which is denoted by reference numeral  300 , is arranged to scoop up, with a flapper  301 , a sheet discharged outside from the printer body  100 , onto the side of the sheet processing apparatus  300 . The sheet is caused to pass through a conveying part provided inside the sheet processing apparatus  300  and is stacked on a first sheet stacking portion in a state in which a surface having an image formed thereon faces downward, i.e., in the so-called face-down state. Then, the sheets as stacked are aligned by an alignment means and are bundled for every predetermined job. The sheets as bundled are subjected to a predetermined process by a sheet processing means. The detailed description of the sheet processing apparatus  300  will be made later herein. 
   The sheet processing apparatus  300  and the printer body  100  are electrically connected with each other by a cable connector (not shown). 
   Further, the sheet processing apparatus  300  is provided with a casing part  300 A, which contains the various members of the sheet processing apparatus  300 . The casing part  300 A of the sheet processing apparatus  300  is detachably attached to a casing part  100 A of the printer body  100 , which will be described later. 
     FIG. 14  is a view obtained by simplifying the view of  FIG. 1  showing the outline arrangement of the printer body  100  on which the sheet processing apparatus  300  is mounted. In addition,  FIG. 15  is a view showing a section E—E of a connection part between the sheet processing apparatus  300  and the printer body  100  shown in  FIG. 14 . 
   As is understandable from  FIGS. 14 and 15 , the mounting of the sheet processing apparatus  300  on the printer body  100  is effected by snap-fitting a connection part  380  of the sheet processing apparatus  300  to a recessed part formed at a joint between an exterior cover  150  and an exterior cover  151  of the printer body  100 . 
   As shown in  FIG. 16 , the arrangement of the sheet processing apparatus  300  and the image forming apparatus on the side opposite to the side shown in  FIG. 14  is the same as that shown in  FIG. 14 . According to such an arrangement, even in a case where the sheet processing apparatus  300  is mounted, as an option, on the printer body  100 , exterior covers of the printer body, which are conventionally provided, can be utilized without making a special alteration to the printer body  100 , so that it is possible to reduce the cost of the printer body  100  and to prevent the fine appearance in design of the printer body  100  from being impaired. 
   Detachment of the sheet processing apparatus  300  from the printer body  100  can be effected by releasing the connection part  380  from snap-fitting. 
   Further, as shown in  FIG. 16 , support members  360  and  370  of the sheet processing apparatus  300 , which abut on the printer body  100 , are arranged such that, in consideration of the position of a stapling part (stapler) H serving as a sheet processing means, the support member  360 , which is provided on the side for supporting the stapling part H being heavy, is disposed at a position nearer to the stapling part H than the support member  370 , which is provided on the side not for supporting the stapling part H. 
   Thus, when the sheet processing apparatus  300  is viewed from the side as shown in  FIG. 16 , the support member  360  and the support member  370  are arranged in the state of shifting from each other by a distance L. This arrangement is provided for compensating for such a construction that the stapling part H in the present embodiment is disposed to staple the sheets on one end on the discharge direction side thereof and, therefore, the balance of weight between the right and left ends is not made. Accordingly, if the balance of weight is made symmetrical between the right and left ends by adjusting the position of the stapling part H or by additionally providing a weight member or the like, the disposition of the support members  360  and  370  should not be limited to the above-mentioned disposition. 
   Further, the sheet processing apparatus  300  may be arranged to be surely supported by adjusting the strength or the like of the supporting members. 
   In addition, in the present embodiment, the support member  370  is kept in contact with the casing part  100 A of the printer body  100  at the position on a display panel  390  of the printer body  100 . This arrangement prevents the visibility of the display panel  390  from being hindered by the support member  370 . 
   Next, the outlines of the printer body  100  will be described along the conveying path for a sheet to be subjected to image formation. 
   As shown in  FIG. 1 , a feed cassette  200  is capable of accommodating a plurality of sheets to be subjected to image formation, and is arranged to sequentially feed the accommodated sheets one by one with the various rollers. 
   In addition to the sheet feeding action, a toner image is formed on a photosensitive member at an image forming part  101  disposed inside the printer body  100 , on the basis of a printing signal transmitted from a computer or a network. The toner image formed on the photosensitive member is transferred onto the sheet S which has been fed from the feed cassette  200 . Then, the toner image is semipermanently fixed to the sheet S at a fixing part  120 . 
   The sheet S having an image thus fixed thereto is turned up at an approximately-U-shaped sheet conveying path leading to a discharge roller  130 , so that the top and bottom of the image-formed surface of the sheet S are reversed. Then, the sheet S is discharged outside from the printer body  100  by the discharge roller  130  in a state in which the image-formed surface faces downward. 
   In the present embodiment, the position of the flapper  301  in the sheet processing apparatus  300  is decided on the basis of a control signal supplied from a control part (not shown), so that, selectively, the sheet S is discharged to a face-down discharge part (a second sheet stacking portion)  125  provided on the upper surface part of the printer body  100  or the sheet S is conveyed to the side of the sheet processing apparatus  300 . 
   Incidentally, in a case where a power source of the sheet processing apparatus  300  is not yet turned on or an accident occurs to the sheet processing apparatus  300 , the flapper  301  is controlled in such a way as to be brought to the position for discharging the sheet S from the discharge roller  130  to the second sheet stacking portion  125 , so that a sheet having an image formed thereon can be discharged without hindrance. 
   Further, while an image forming apparatus utilizing an electrophotographic process is used as the image forming apparatus according to the present embodiment, the invention is not limited to such an image forming apparatus, but is also applicable to an image forming apparatus of the ink-jet type in which an image is formed on a sheet by jetting ink. Thus, any image forming processes are applicable. 
   Further, while an image forming apparatus for forming an image on one side of the sheet is shown in the present embodiment, the invention is applicable to an image forming apparatus of the type having a construction for forming images on two sides of the sheet. 
   Next, the arrangement of the sheet processing apparatus  300  and the operation of each part of the sheet processing apparatus  300  in a case where the sheet S transported by the discharge roller  130  is conveyed to the sheet processing apparatus  300  will be described with reference to  FIGS. 2(   a ) and  2 ( b ) and  FIGS. 3(   a ) and  3 ( b ). 
   Here,  FIGS. 2(   a ) and  2 ( b ) show the sections of the discharge roller  130  and the sheet processing apparatus  300 .  FIG. 3(   a ) shows the section A—A of the sheet processing apparatus  300  shown in  FIG. 1 .  FIG. 3(   b ) shows the section B—B of the sheet processing apparatus shown in  FIG. 3(   a ). 
   In  FIGS. 2(   a ) and  2 ( b ), reference numeral  320  denotes a conveying roller, reference numeral  321  denotes a discharge sensor, reference character M denotes a jogger motor, reference numeral  322  denotes a sheet return member, and reference numeral  323  denotes a reference wall for abutting thereon the rear end of the sheet. These members will be described later. 
   As shown in  FIGS. 2(   a ) and  2 ( b ), the conveying roller  320  is disposed above the flapper  301 , which serves as a switching means as mentioned above, on the downstream side in the sheet conveying direction, and is arranged to be driven to rotate by a driving motor (not shown). The discharge sensor  321  is disposed near the conveying roller  320  on the downstream side in the sheet conveying direction, and is arranged to detect the passage of the front end and rear end of the sheet as conveyed by the conveying roller  320 . The jogger motor M is a motor capable of rotating forward and backward for driving slide guides  310  and  311 , which serve as guide members, and is a stepping motor in the case of the present embodiment. 
   The sheet return member  322  is disposed, as shown in  FIGS. 2(   a ) and  2 ( b ), on the most downstream side in the sheet conveying direction in the sheet processing apparatus  300 , and is arranged to be swingable around a pivot shaft part  322   a .  FIG. 2(   a ) shows the initial position of the sheet return member  322 .  FIG. 2(   b ) shows a state in which the sheet return member  322  is pushed up by the sheet S which has been conveyed up to the sheet return member  322 . 
   The sheet return member  322  has a predetermined value of weight. When having being pushed up counterclockwise, as viewed in  FIG. 2(   b ), by the sheet S, the sheet return member  322  is caused to swing in the direction of an arrow shown in  FIG. 2(   b ) (clockwise) by being urged by a spring (not shown). Such a swinging force of the sheet return member  322  causes the rear end in the sheet conveying direction of the sheet S to abut on the reference wall  323 , so that the alignment action in the sheet conveying direction is performed on the sheet S. 
   Incidentally, if the weight of the sheet return member  322  itself is arranged to be adjustable, the alignment action on the sheet S may be performed without utilizing the urging force of the spring. 
   Subsequently, as shown in  FIGS. 3(   a ) and  3 ( b ), the sheet processing apparatus  300  is provided with the slide guide (R)  310  and the slide guide (L)  311 , which will be described later, as guide members for aligning the sheet S in the width direction thereof. 
   In a case where a control means (not shown) performs control over a stapling operation in response to a command outputted beforehand from a computer or the like, the sheet processing apparatus  300  performs the stapling operation in the following manner. Before the sheet to be stapled is discharged by the discharge roller  130 , the fore end side of the flapper  301  is made to be located at a lower position than that of a nip portion of the discharge roller  130  by a solenoid (not shown) through a link mechanism (not shown). Accordingly, as shown in  FIG. 2(   a ), the sheet S discharged outside from the discharge roller  130  is led upward along the flapper  301 , and is conveyed to the inside of the sheet processing apparatus  300 . Then, the sheet S is transported to the first sheet stacking portion  300 B, which is arranged to temporarily stack sheets. 
   The first sheet stacking portion  300 B is composed of the slide guide (R)  310  and the slide guide (L)  311 . There are provided no members for touching and supporting the sheet at a space between the slide guide (R)  310  and the slide guide (L)  311 . In other words, the sheet S discharged to the first sheet stacking portion  300 B is stacked with the right end part of the sheet S supported by the slide guide (R)  310  and the left end part of the sheet S supported by the slide guide (L)  311 . 
   In this instance, in the sheet processing apparatus  300 , as shown in  FIG. 3(   a ), the slide guide (R)  310  and the slide guide (L)  311 , which are disposed respectively on the right-hand side and the left-hand side with respect to the sheet discharging direction (an arrow T shown in  FIG. 3(   a )), retreat to their respective positions each of which is located outside by a predetermined amount with respect to the end of the width of the sheet S, so as not to interfere with the sheet S being conveyed, thus, waiting for the sheet S to come in. 
   Then, in the sheet processing apparatus  300 , when the sheet S for the first time is discharged from the discharge roller  130  of the printer body  100 , the sheet S is transported by the flapper  301  to the inside of the casing part  300 A, and is discharged onto the guide surface of the first sheet stacking portion  300 B, which is composed of the slide guide (R)  310  and the slide guide (L)  311 , by the discharge roller  320 , which is driven to rotate by a driving motor (not shown). 
   The guide surface of the first sheet stacking portion  300 B is, as shown in  FIG. 2(   a ), inclined by a predetermined angle with respect to the horizontal direction, and the angle of inclination differs with the upstream side and the downstream side of the guide surface of the first sheet stacking portion  300 B in the sheet discharging direction. More specifically, there is formed a bend part  300 C which is bent by an angle of inclination α between a predetermined section on the upstream side and a predetermined section on the downstream side. 
   With the bend part  300 C thus provided, the guide surface of the first sheet stacking portion  300 B is arranged to prevent a middle portion of the sheet S, which is not guided by the slide guides  310  and  311 , from bending, with the rigidity of the sheet S utilized. 
   Incidentally, since the angle of inclination α depends on an angle of inclination of the slide guides  310  and  311 , an angle which the second sheet stacking portion  125  makes with a horizontal plane, etc., it is not always necessary that the angle of inclination α differs with the upstream side and the downstream side in the sheet discharging direction. Thus, the angle of inclination α may be made zero. 
   Then, when detected by the discharge sensor  321  disposed near the discharge roller  320  on the downstream side, a fore end of the sheet S conveyed to the inside of the casing part  300 A of the sheet processing apparatus  300  causes a flag  321   a  of the discharge sensor  321  to swing counterclockwise as viewed in  FIG. 2(   a ). 
   Subsequently, when a rear end of the sheet S passes through the discharge roller  320 , as shown in  FIG. 2(   b ), the flag  321   a  swings, by its own weight, clockwise as viewed in  FIG. 2(   b ), and the rear end of the sheet S is pushed downward by the flag  321   a , so that the sheet S can be surely dropped to the guide surface composed of the slide guide (R)  310  and the slide guide (L)  311 . At this time, the discharge sensor  321  turns off. 
   Further, as mentioned in the foregoing, although the fore end of the sheet S stacked on the first sheet stacking portion  300 B tries to push up the sheet return member  322  counterclockwise as viewed in  FIG. 2(   b ), the sheet return member  322 , which is caused to swing in the direction of the arrow shown in  FIG. 2(   b ) (clockwise) by being urged by the spring (not shown) causes the rear end of the sheet S to abut on the reference wall  323 . Accordingly, the alignment action in the sheet conveying direction (in the longitudinal direction) is performed on the sheet S stacked on the first sheet stacking portion  300 B. 
   The present embodiment is arranged such that, when the discharge sensor  321  turns off, only the slide guide (R)  310  on the right-hand side acts to start the alignment action in the width direction of the sheet S discharged on the first sheet stacking portion  300 B. 
   More specifically, the slide guide (R)  310  is driven by the jogger motor M to move in the direction of an arrow L shown in  FIG. 3(   a ), so that reference pins (R)  330  which are projections provided on the slide guide (R)  310  abut on the right side surface of the sheet S. Then, the slide guide (R)  310  pushes the sheet S, with the reference pins (R)  330 , toward the slide guide (L)  311 . 
   The left side surface of the sheet S pushed by the slide guide (R)  310  comes to abut on reference pins (L)  331  which are projections provided on the slide guide (L)  311 . Accordingly, the sheet S is moved to a predetermined position. 
   Here, the construction of the slide guides  310  and  311  will be described further in detail.  FIGS. 3(   a ) and  3 ( b ) show the section A—A of the sheet processing apparatus  300  shown in  FIG. 1 .  FIGS. 4(   a ) and  4 ( b ) and  FIGS. 5(   a ) and  5 ( b ) are sectional views for explaining the operation of the slide guides  310  and  311 . In addition,  FIG. 3(   a ) shows the sheet processing apparatus  300  as viewed from the side of the jogger motor M shown in  FIG. 3(   b ), with a frame F shown in  FIG. 3(   b ) removed. 
   The slide guides  310  and  311  are arranged to move to the right and to the left in  FIG. 3(   a ) (in the width direction of the sheet), i.e., in directions perpendicular to the sheet conveying direction (the arrow T shown in  FIG. 3(   a )), by receiving a driving force transmitted from the jogger motor M, while being guided by guide pins  314   a ,  314   b ,  314   c  and  314   d  provided on the frame F of the sheet processing apparatus  300 . 
   Further, in the state shown in  FIG. 3(   a ), the slide guide (L)  311  is restrained from moving further in the direction of the arrow R because of abutting on the guide pin  314   c . The position of the guide pin  314   c  is decided with respect to the position of the stapling part H. Since the stapling part H in the present embodiment is fixed to the sheet processing apparatus  300 , it is necessary to perform the alignment action on the sheet S with the stapling part H always used as a point of reference. The reason for this is that, if the slide guide (L)  311  is moved to the side in the direction of the arrow R beyond the stapling part H at the time of the alignment action, the stapling operation becomes impossible. 
   Therefore, the moving range of the guide slide (L)  311  is restricted by the guide pin  314   c.    
   As shown in  FIG. 3(   b ) when viewed from the sheet conveying direction, each of the slide guides  310  and  311  is composed of a wall part arranged to guide each side surface of the sheet S and a guide part arranged to support the upper and lower surfaces of the sheet S. Since the sheet S is supported by the lower surface of the guide part of each of the slide guides  310  and  311 , a middle portion of the sheet S in the width direction thereof is not supported. 
   The first sheet stacking portion  300 B is provided with a stepped gear  317 , which is arranged to receive a driving force from the jogger motor M. Then, the slide guide (R)  310  is provided with a rack part  310   a  having an open toothed part, which meshes with the stepped gear  317 . 
   On the other hand, at a position opposite to the rack part  310   a  across the stepped gear  317 , there is provided a slide rack  312 . The slide rack  312  has also an open toothed part, which meshes with the stepped gear  317 . 
   The slide rack  312  is arranged to be relatively movable with respect to the slide guide (L)  311  via a coiled spring  313 . The spring  313  has its one end engaging with the slide guide (L)  311  and its other end engaging with the slide rack  312 . Then, the spring  313  is arranged to have its spring force acting in such a way as to extend the space between the slide guide (R)  311  and the slide rack  312 . 
   Further, the slide rack  312  has an embossed part  312   a , which moves inside a rectangular hole part  311   a  formed on the slide guide (L)  311  as a slot extending in the width direction of the sheet, and is thus arranged to fit into the slide guide (L)  311 . 
   Further, the slide guide (R)  310  and the slide guide (L)  311  have their positions in the height direction regulated by the stepped gear  317  and a height regulating member  315 . 
   The side wall of the slide guide (R)  310  is provided with two reference pins (R)  330 , and the side wall of the slide guide (L)  311 , too, is provided with two reference pins (L)  331 . Then, when the alignment action on the sheets in the width direction thereof is performed, the slide guide (R)  310  moves to cause the reference pins (R)  330  and the reference pins (L)  331  to abut on the right and left end surfaces of the sheet S, respectively. Accordingly, the sheets S stacked on the first sheet stacking portion  300 B are aligned in respect of the position in the width direction. 
   Incidentally, the reference pins (R)  330  and the reference pins (L)  331  are made of material having high abrasion resistance (rigidity). The reason for this is that, if a portion abutting on the sheet is abraded when the sheet processing apparatus  300  has performed the alignment action on the sheets a great number of times, it becomes impossible to perform a high-accurate alignment action. 
   Next, the operation of the slide guides (R)  310  and (L)  311  will be described. 
   When a power source of the sheet processing apparatus  300  is turned on, the discharge roller  320  is driven by the driving motor to start rotating. Subsequently, when the jogger motor M is rotated to cause the stepped gear  317  to rotate, a driving force is transmitted to the rack part  310   a  of the slide guide (R)  310 , so that the slide guide (R)  310  retreats outside (in the direction of the arrow R in  FIG. 3(   a )). 
   Further, similarly, the slide rack  312  is moved in the direction of the arrow L. when the embossed part  312   a  of the slide rack  312  abuts on the left-side end surface (as viewed in  FIG. 3(   a )) of the rectangular hole part  311   a  of the slide guide (L)  311 , the slide guide (L)  311  is pushed by the embossed part  312   a  to retreat outside (in the direction of the arrow L in  FIG. 3(   a )). 
   The slide guide (R)  310  is provided with a flag part  310   f . When the flag part  310   f  is moved up to a predetermined retreat position, as shown in  FIG. 5(   a ), the flag part  310   f  blocks light incident on a photo-sensor  316 , thereby turning off the photo-sensor  316 . At this point of time, the jogger motor M comes to a stop. This position is referred to as the “home position” of the sheet processing apparatus  300 . 
   After the above-stated initial operation is performed on the sheet S, when a signal indicating that the sheet S enters the sheet processing apparatus  300  is inputted from the printer body  100  to the sheet processing apparatus  300 , the jogger motor M rotates in the direction reverse to the direction employed for the initial operation, so that the slide guide (R)  310  and the slide guide (L)  311  move inward. Then, each of the slide guide (R)  310  and the slide guide (L)  311  comes to a stop at such a position as to be wider by a predetermined amount “d” than the width of the sheet S discharged to the first sheet stacking portion  300 B, as shown in  FIGS. 3(   a ) and  3 ( b ). In this position, the slide guide (L)  311  abuts on the guide pin  314   c  and is thus prevented from moving further in the direction of the arrow R. In the present embodiment, the position shown in  FIGS. 3(   a ) and  3 ( b ) is referred to as the “standby position”. In the standby position, the reference pins (L)  331  of the slide guide (L)  311  are used as the reference position for the alignment action. 
   In the present embodiment, the standby position of each of the slide guide (R)  310  and the slide guide (L)  311  is set in such a manner that, in a case where the size (width) of the sheet S is the largest of sizes of sheets which can pass through the sheet processing apparatus  300 , each of gaps appearing on the both sides of the sheet S has the predetermined amount “d”. 
   Incidentally, in a case where sheets which are narrower in width than the sheet of the largest size are aligned in the sheet processing apparatus  300 , the slide guide (R)  311  moves in the direction of the arrow L by an amount corresponding to such a difference in width, so that a gap between the sheet and the slide guide (R)  310  in the standby position has always the predetermined amount “d”. In this case, a gap between the sheet and the slide guide (L)  311  becomes wider by an amount corresponding to half of the difference in sheet width than the predetermined amount “d”. 
   Now, when the sheet S for the first time is discharged from the discharge roller  130  of the printer body  100 , the sheet S is transported to the inside of the sheet processing apparatus  300  with the transporting direction of the sheet S controlled by the flapper  301 . Then, the sheet S is discharged onto the slide guides  310  and  311  by the discharge roller  320 . 
   In this instance, after the fore end of the sheet S is detected by the discharge sensor  321 , the sheet S is conveyed along the supporting surfaces of the slide guides  310  and  311  (the lower surface parts of guide parts), so that the left-side corner part of the fore end (the left and bottom end part shown in  FIG. 3(   a )) of the sheet S enters an opening part of the stapling part H. 
   Further, the fore end of the sheet S abuts on the sheet return member  322 , and, then, the rear end part of the sheet S is aligned with respect to the reference wall  323  by the action of the sheet return member  322 . 
   Further, when the rear end of the sheet S comes off the discharge roller  320  to turn off the flag  321   a  of the discharge sensor  321 , the rear end side of the sheet S is pushed downward by the flag  321   a , as mentioned in the foregoing, so that the sheet S is caused to surely drop to the supporting surfaces of the first sheet stacking portion  300 B, which is composed of the slide guides (R)  310  and (L)  311 . 
   In the present embodiment, when the discharge sensor  321  has turned off, the jogger motor M starts rotating, so that the slide guides (R)  310  and (L)  311 , which have been in the standby position, begin the alignment action in the following manner. 
   First, the jogger motor M rotates in such a direction as to cause the slide guide (R)  310  to move inward (in the direction of the arrow L). Accordingly, the slide guide (R)  310  moves in the direction of the arrow L to abut on the right-side end part of the sheet S. 
   Further, the rotation of the jogger motor M is transmitted to the slide rack  312 , so that the slide rack  312  moves inward (in the direction of the arrow R). At this time, the spring  313  is compressed by the slide rack  312 . Since the end part of the spring  313  as compressed, which part is opposite to the side thereof engaging with the slide rack  312 , is engaging with the slide guide (L)  311 , a force with which the spring  313  as compressed tries to expand acts on the slide guide (L)  311 . Therefore, the slide guide (L)  311  tires to move in the direction of the arrow R. 
   However, since, in the standby position, the slide guide (L)  311  is in the state of abutting on the guide pin  314   c , the slide guide (L)  311  is restrained from moving inward (in the direction of the arrow R). Therefore, during the alignment action, only the slide guide (R)  310  moves for aligning the sheet S. 
   In the alignment action, first, with the slide guide (R)  310  moving in the direction of the arrow L, the reference pins (R)  330  abut on the right-side end surface of the sheet S, and then push the sheet S toward the slide guide (L)  311 . Then, when the left-side end surface of the sheet S abuts on the reference pins (L)  331 , there is obtained the state shown in  FIGS. 4(   a ) and  4 ( b ). Incidentally, in consideration of any bending of the sheet S, etc., the slide guide (R)  310  may be moved up to a position where the interval between the reference pins (R)  330  and (L)  331  becomes narrower than the length of the sheet S in the width direction thereof. 
   The jogger motor M temporarily stops when there has been obtained the state shown in  FIGS. 4(   a ) and  4 ( b ) in which the both side end parts of the sheet S abut on the slide guides  310  and  311 . After that, the jogger motor M starts rotating reversely and then stops when the slide guides (R)  310  and (L)  311  have come again to the standby position shown in  FIGS. 3(   a ) and  3 ( b ). The control action on the amount of movement of the slide guide (R)  310  in such a series of operations is managed on the basis of the number of driving pulses for the jogger motor M, being a stepping motor, with the home position, where the photo-sensor  316  is light-blocked, taken as a reference point. 
   In addition, during the operation in which the slide guide (R)  310  returns to the standby position, while the slide rack  312  also moves in such a direction as to expand the spring  313 , the slide guide (L)  311  itself does not move, being kept in the standby position. Accordingly, the left-side end part of the sheet S, as viewed in  FIG. 4(   a ), is kept in the state of abutting on the slide guide (L)  311 . 
   Next, when a sheet S 2  for the second time is conveyed to the sheet processing apparatus  300  similarly to the sheet S for the first time, and the rear end of the sheet S 2  passes through the discharge sensor  321 , the sheet S 2  is superposed on the sheet S. Then, with such a state, the alignment action is started similarly to the case of the sheet S for the first time. 
   More specifically, with the jogger motor M rotating, the slide guide (R)  310  moves and the reference pins (R)  330  abut on the right-side end surface of the sheet S 2 . The slide guide (R)  310  moves further up to a position where the left-side end surface of the sheet S 2  abuts on the reference pins (L)  311  provided on the slide guide (L)  311 . Accordingly, the sheet S 2  is aligned similarly to the sheet S for the first time. After that, the slide guide (R)  310  moves up to the standby position and then stops. 
   The above operation is performed until the last sheet Sn (for the n-th time) in one job is aligned. Then, in the state in which the reference pins (R)  330  provided on the slide guide (R)  310  cause the left-side end surfaces of the sheets to abut on the reference pins (L)  331  of the slide guide (L)  311 , i.e., in the state in which the alignment action has been performed as shown in  FIGS. 4(   a ) and  4 ( b ), the sheets are stapled by the stapling part H, which is located on the left side of the fore end of the sheet bundle. 
   Incidentally, the stapling action of the stapling part H is performed in such a manner that, since the sheets are stacked serially from the first page with an image-formed surface of each sheet facing downward, needles are driven into the sheet bundle upward from below. 
   According to the above-described construction and operation, during the alignment action on each sheet, the slide guide (L)  311  is stopped in the standby position without moving, and only the slide guide (R)  310  moves to cause the left-side end part of each sheet to align to the reference position for stapling. Accordingly, the stapling part H, which is disposed on the side of the slide guide (L)  311 , is enabled to perform the sheet stapling action accurately and simply. 
   Further, even in a case where sheets conveyed to the sheet processing apparatus  300  for one job vary in width, or even in a case where the sheet size varies from the size “LTR” to the size “A4” or the like, the left-side end part of each sheet is aligned to the constant position in respect of the width of each sheet. Therefore, the stapling part H is enabled to perform a stapling process excellent in accuracy and precision. 
   Subsequently, when the stapling action is completed as described above, the jogger motor M is driven to rotate, so that the slide guide (R)  310  and the slide guide (L)  311  move in the direction of the arrow R and in the direction of the arrow L, respectively, from the state shown in  FIG. 4(   a ). Incidentally, since, at the time of start of rotation of the jogger motor M, the slide rack  312  first moves to the left as viewed in  FIG. 4(   a ), the slide guide (L)  311  itself does not immediately move. 
   When the slide guide (R)  310  passes over the standby position shown in  FIG. 3(   a ), the embossed part  312   a  of the slide rack  312  abuts on the end surface of the rectangular hole part  311   a  of the slide guide (L)  311 . Then, the slide guide (L)  311  starts moving in the direction of the arrow L by being pushed by the embossed part  312   a , so that both the slide guides  310  and  311  move. 
   When the interval between the slide guides  310  and  311 , which are supporting the sheet bundle as stapled, becomes equal to or wider than the sheet width, the sheet bundle drops downward as shown in  FIGS. 5(   a ) and  5 ( b ). Accordingly, the sheet bundle drops to the face-down discharge part (the second sheet stacking portion)  125 , which is provided on the upper surface of the casing part  100 A of the printer body  100 , and is stacked there. 
   As mentioned above, in the present embodiment, the face-down discharge part  125  of the printer body  100  is used also as a stacking part for the sheet bundle discharged from the sheet processing apparatus  300 , without providing any dedicated stacking part for the sheet bundle. Therefore, the size of the sheet processing apparatus  300  can be reduced. 
   Further, in the present embodiment, the sheet processing apparatus  300  is mounted on the upper portion of the casing part  100 A of the printer body  100 , and the conveying path for a sheet discharged from the printer body  100  in the face-down manner is changed over by the flapper  301 . This arrangement obviates the necessity of provision of an inverting mechanism for discharging and stacking image-formed sheets in the order of page numbers, which mechanism is required in conventional processing apparatuses. Therefore, the size of the sheet processing apparatus  300  can be reduced with space saving and at low cost. 
   Incidentally, if, after the sheet bundle as stapled is made to drop to the face-down discharge part  125  of the printer body  100 , a sheet is discharged directly to the face-down discharge part  125  from the discharge roller  130  of the printer body  100 , there is the possibility that, depending on the position of the sheet bundle stacked on the face-down discharge part  125 , a fore end of the thus-discharged sheet is caught by a stapled portion of the sheet bundle and the sheet is then damaged, the alignment of sheets or sheet bundles is impaired, or jamming occurs in sheets. 
   In order to prevent the occurrence of such inconveniences, the operation of the printer body  100  and the sheet processing apparatus  300  is controlled as follows. After a sheet bundle stapled by the stapling part H is discharged to the face-down discharge part  125 , at least the first sheet which is discharged next is made to drop to the face-down discharge part  125  through the sheet processing apparatus  300  without being discharged directly to the face-down discharge part  125  from the printer body  100 . 
   This operation makes it possible to cover the stapled portion of a sheet bundle as earlier discharged, with a sheet which is next dropping. Therefore, even if, after that, a sheet is discharged directly to the face-down discharge part  125 , the above-mentioned inconveniences can be solved. 
   Further, while, in the present embodiment, during the alignment action on sheets, only the slide guide (R)  310  moves and the slide guide (L)  311  does not move, the slide guide (L)  311  may be made to move during the alignment action on sheets. In such a case, for example, this arrangement can be realized by making the slide guide (L)  311  have the same construction as the slide guide (R)  310 . Incidentally, in a case where the alignment action on sheets is performed by moving both the slide guides  310  and  311 , it goes without saying that the construction and control operation for appropriately aligning sheets to the position of the stapling part H become necessary. 
   Further, while, in the present embodiment, both the slide guides are made to move so as to cause the sheets subjected to the alignment action to drop, only one of the slide guides may be made to move so as to cause the sheets to drop. This arrangement will be described later as another embodiment of the invention. 
   Further, while a processing means is exemplified by the stapling part H for stapling sheets, the invention is applicable, with the similar construction and control operation, to a means for performing a process after aligning sheets, such as a punching means for punching sheets, a binding means for fixing sheets with paste or the like, etc. 
   Further, while, in the present embodiment, a predetermined process is performed on sheets as aligned, the invention is not limited to this arrangement and may be arranged to cause sheets which are not subjected to a process after being aligned to drop to the second sheet stacking portion  125 . For example, if a sheet stacking position obtained by discharging sheets directly to the second sheet stacking portion  125  from the discharge roller  130  and a sheet stacking position obtained by aligning sheets at the first sheet stacking portion  300 B and causing the sheets to drop are made to shift from each other in the sheet width direction or in the sheet discharging direction, it becomes possible to perform a sorting control operation. 
   As has been described above, in the present embodiment of the invention, a sheet processing apparatus is disposed above a sheet discharge part of an image forming apparatus, sheets discharged onto a sheet stacking portion of the sheet processing apparatus are aligned and subjected to a predetermined process, and, after that, the sheets are dropped and stacked on the sheet discharge part of the image forming apparatus by moving the sheet stacking portion. This arrangement makes it possible to realize the simplification and cost reduction of the sheet processing apparatus and to save space in mounting the sheet processing apparatus on the image forming apparatus or the like. 
   Next, a sheet processing apparatus according to another embodiment of the invention will be described with reference to  FIGS. 6(   a ) and  6 ( b ) to  FIGS. 8(   a ) and  8 ( b ).  FIG. 6(   a ) shows the sheet processing apparatus  300  as viewed from above, and  FIG. 6(   b ) shows the section C—C of the sheet processing apparatus  300  shown in  FIG. 6(   a ). 
   While, in the first-mentioned embodiment, both the slide guides on the right and left sides are made to move when causing sheets which have been temporarily stacked on the first sheet stacking portion  300 B and have been aligned to drop, the present embodiment is arranged such that, for the purpose of furthering the simplification and cost reduction of the sheet processing apparatus  300 , one guide member (L)  411  (hereinafter referred to as the fixed guide (L)  411 ) is fixed and only the other guide member (R)  410  (hereinafter referred to as the slide guide (R)  411 ) is made to move. 
   In the following, a first sheet stacking portion  400 B of the sheet processing apparatus  300  according to the present embodiment will be described. It is to be noted that the members similar to those described in the foregoing description are omitted from the description here. 
   The slide guide (R)  410  is provided with a rack part  410   a  having an open toothed part which meshes with the stepped gear  317 . On the other hand, the fixed guide (L)  411  is fixed to the frame F, and the position of the fixed guide (L)  411  is the standby position in the first-mentioned embodiment (i.e., the reference position for stapling). Accordingly, a gap between the left-side end of the sheet conveyed and discharged onto the first sheet stacking portion  400 B and the fixed guide (L)  411  becomes the predetermined amount “d”. 
   The slide guide (R)  410  is arranged to be movable to the right and to the left as viewed in  FIG. 6(   a ), i.e., to be capable of making a reciprocating motion in the sheet width direction which is perpendicular to the sheet conveying direction. The slide guide (R)  410  is driven to move by the jogger motor M. Further, the slide guide (R)  410  has its position in the height direction regulated by the stepped gear  317  and a height regulating member  415 . 
   Next, the operation of the slide guides (R)  410  will be described. When a power source of the sheet processing apparatus  300  is turned on, the discharge roller  320  is driven by a conveying motor (not shown) to start rotating. Subsequently, when the jogger motor M is rotated to cause the stepped gear  317  to rotate, a driving force is transmitted to the rack part  410   a  of the slide guide (R)  410 , so that the slide guide (R)  410  retreats outside (in the direction of the arrow R in  FIG. 6(   a )). 
   The slide guide (R)  410  is provided with a flag part  410   f . When the flag part  410   f  is moved up to a predetermined retreat position, as shown in  FIG. 8(   a ), the flag part  410   f  blocks light incident on a photo-sensor, thereby turning off the photo-sensor. At this point of time, the jogger motor M comes to a stop. This position is referred to as the “home position” of the sheet processing apparatus  300 . 
   After the above-stated initial operation is performed on the sheet S, when a signal indicating that the sheet S enters the sheet processing apparatus  300  is inputted from the printer body  100  to the sheet processing apparatus  300 , the jogger motor M rotates in the direction reverse to the direction employed for the initial operation, so that the slide guide (R)  410  moves inward (in the direction of the arrow L in  FIG. 6(   a )). Then, the slide guide (R)  410  comes to a stop at such a position as to be wider by the predetermined amount “d” than the width of the sheet S discharged to the first sheet stacking portion  400 B, as shown in  FIGS. 6(   a ) and  6 ( b ). 
   Now, when the sheet S for the first time is discharged from the discharge roller  130  of the printer body  100 , the sheet S is transported to the inside of the sheet processing apparatus  300  with the transporting direction of the sheet S controlled by the flapper  301 . Then, the sheet S is discharged by the discharge roller  320  onto the first sheet stacking portion  400 B, which is composed of the slide guide (R)  410  and the fixed guide (L)  411 . 
   In this instance, after the fore end of the sheet S is detected by the discharge sensor  321 , the sheet S is conveyed along the supporting surfaces of the slide guides (R)  410  and the fixed guide (L)  411  (the lower surface parts of guide parts), so that the left-side corner part of the fore end (the left and bottom end part shown in  FIG. 6(   a )) of the sheet S enters an opening part of the stapling part H. 
   Further, the fore end of the sheet S abuts on the sheet return member  322 , and, then, the rear end part of the sheet S is aligned with respect to the reference wall  323  by the action of the sheet return member  322 . 
   Further, when the rear end of the sheet S comes off the discharge roller  320  to turn off the flag  321   a  of the discharge sensor  321 , the rear end side of the sheet S is pushed downward by the flag  321   a , as mentioned in the foregoing, so that the sheet S is caused to surely drop to the supporting surfaces of the first sheet stacking portion  400 B, which is composed of the slide guide (R)  410  and the fixed guide (L)  411 . 
   In the present embodiment, when the discharge sensor  321  has turned off, the jogger motor M starts rotating, so that the slide guide (R)  410 , which has been in the standby position, begins the alignment action in the following manner. 
   First, at the time of start of the alignment action, the jogger motor M rotates in such a direction as to cause the slide guide (R)  410  to move in the direction of the arrow L. Accordingly, the slide guide (R)  410  moves to cause reference pins (R)  430  thereof to abut on the right-side end part of the sheet S. Further, the slide guide (R)  410  moves in the direction of the arrow L so as to cause the left-side end surface of the sheet S to abut on reference pins (L)  431  of the fixed guide (L)  411 . 
   The state obtained after the alignment action has been performed is shown in  FIGS. 7(   a ) and  7 ( b ). In this instance, in consideration of any bending of the sheet S, etc., the slide guide (R)  410  may be moved up to a position where the interval between the reference pins (R)  430  and (L)  431  becomes narrower than the length of the sheet S in the width direction thereof. 
   The jogger motor M temporarily stops when there has been obtained the state shown in  FIGS. 7(   a ) and  7 ( b ). After that, the jogger motor M starts rotating reversely and then stops when the slide guide (R)  410  has come again to the standby position. The control action on the amount of movement of the slide guide (R)  410  is managed on the basis of the number of driving pulses for the jogger motor M, similarly to the first-mentioned embodiment. 
   A sheet for the second time or for the subsequent time is subjected to the alignment action in the similar manner. The above operation is performed until the last sheet Sn (for the n-th time) in one job is aligned. Then, in the state in which the reference pins (R)  430  and the reference pins (L)  431  provided on the slide guide (R)  410  and the fixed guide  411  abut on the right and left end parts of the sheet, the sheets are stapled by the stapling part H, which is located on the left side of the fore end of the sheet bundle. 
   According to the above-described construction and operation, during the alignment action on each sheet, the fixed guide (L)  411  is fixed to the reference position, and only the slide guide (R)  410  moves to cause the left-side end part of each sheet to align to the reference position for stapling. Accordingly, the stapling part H, which is fixedly disposed on the side of the fixed guide (L)  411 , is enabled to perform the sheet stapling action surely and precisely. 
   Further, even in a case where sheets conveyed to the sheet processing apparatus  300  for one job vary in width, or even in a case where the sheet size varies from the size “LTR” to the size “A4” or the like, the left-side end part of each sheet is aligned to the constant position in respect of the width of each sheet. Therefore, the stapling part H is enabled to perform a stapling process excellent in accuracy. 
   In the present embodiment, when the stapling action is completed, the jogger motor M is driven to rotate, so that the slide guide (R)  410  moves in the direction of the arrow R from the state shown in  FIG. 7(   a ). Then, when the end part of the supporting surface of the slide guide (R)  410  has moved in the direction of the arrow R beyond the position of the right-side end part of the sheet bundle as stapled, the sheet bundle drops downward as shown in  FIGS. 8(   a ) and  8 ( b ). Accordingly, the sheet bundle drops to the face-down discharge part (the second sheet stacking portion)  125 , which is provided on the upper surface of the casing part  100 A of the printer body  100 , and is stacked there. 
   As described in the foregoing, the sheet processing apparatus according to the present embodiment is arranged such that, in addition to the advantageous effect of the first-mentioned embodiment, one of the guides is fixed. Therefore, it becomes possible to further the simplification and cost reduction of the sheet processing apparatus. 
   Incidentally, while the guide to be fixed is a guide disposed on the side where the stapling part H is located, the invention is not limited to this arrangement. 
   Next, a sheet processing apparatus according to a further embodiment of the invention will be described with reference to  FIG. 9 . 
   In the sheet processing apparatus  300  shown in  FIG. 9 , a stacking tray  325  serving as a second sheet stacking portion for stacking thereon sheets discharged from the sheet processing apparatus  300  and sheet bundles subjected to the stapling process is disposed above the face-down discharge part  125  of the printer body  100 . 
   In the present embodiment having such a construction, a sheet bundle subjected to the stapling process by the stapling part H is necessarily stacked on the stacking tray  325 . Therefore, it becomes unnecessary to perform such a complicated control operation as described in the first-mentioned embodiment, i.e., a control operation for causing at least the next sheet after the sheet bundle subjected to the stapling process by the stapling part H is discharged to drop from the sheet processing apparatus  300 , without discharging that sheet to the face-down discharge part  125  from the discharge roller  130  of the printer body  100 . 
   Further, since it becomes possible to stack sheets or sheet bundles on the stacking tray  325  in addition to the face-down discharge part  125  of the printer body  100 , the sheet processing apparatus  300  according to the present embodiment is very convenient for users to discharge a great number of sheets. 
   In addition, since, in the construction shown in  FIG. 9 , the stacking tray  325  is added to the sheet processing apparatus  300  and the printer body  100  shown in  FIG. 1 , the flapper  301 , etc., are configured and controlled in the same manner as in the first-mentioned embodiment. However, if, instead of the construction in which the conveying path for a sheet having passed through the discharge roller  130  is changed over to the side of the sheet processing apparatus  300  or the face-down discharge part  125 , the construction in which the conveying path for a sheet can be changed over before the sheet passes through the discharge roller  130  is adopted, the invention is advantageous even for the construction in which the second sheet stacking portion (the stacking tray)  325  is provided separately from the face-down discharge part  125 . 
   Next, an operation for causing a sheet to drop, which is characteristic of the invention, will be described in detail with reference to  FIG. 10  to  FIGS. 13(   a ) and  13 ( b ).  FIG. 11(   a ) shows the sheet processing apparatus  300  as viewed from above, and  FIG. 11(   b ) shows the section D—D of the sheet processing apparatus  300  shown in  FIG. 11(   a ). 
   In the first-mentioned embodiment, a sheet bundle subjected to the stapling process by the stapling part H is made to drop by moving both the slide guides to their respective outsides of the sheet processing apparatus  300 . 
   However, there are some cases where the sheet S sticks to the slide guide (R)  310  or (L)  311  due to static electricity caused by the alignment action on the slide guide  310  or  311  or due to a frictional state of the surface of the slide guide  310  or  311 , so that a normal dropping operation of the sheet S is not performed. In view of such a case, the reference wall  323  is provided with a member for causing the sheet to drop correctly. 
     FIG. 10  is an enlarged sectional view showing parts around the reference wall  323  in the present embodiment. The reference wall  323  is provided with side wall parts  323   a , which are projections indicated by hatching. As is understandable from  FIG. 11(   a ), the side wall parts  323   a  are disposed in a protruded manner at the respective positions of the surface of the reference wall  323 , which are separate from each other at an interval wider than the width of the sheet. 
   In the first-mentioned embodiment, there is the possibility that, even when the slide guides  310  and  311  are spread to the right and to the left, respectively, so as to cause a sheet stacked on the first sheet stacking portion  300 B to drop, the sheet sticks to the slide guide  310  or  311 , thereby deteriorating the property of dropping of the sheet, deteriorating the stacked state of the sheet after dropping, or, in some cases, causing jamming of the sheet. 
   Therefore, in order to cause the sheet to drop normally, the reference wall  323  is provided with the side wall part  323   a . The advantageous effect of the side wall part  323   a  will be described below. 
   As mentioned in the foregoing, if, when the slide guides  310  and  311  are spread to the right and to the left, respectively, so as to cause the sheet to drop, the sheet sticks to any one of the slide guides  310  and  311 , the sheet would follow the movement of the slide guide  310  or  311 . 
   However, as shown in  FIG. 13(   a ), the rear end part of the sheet which is in the state of sticking to the slide guide  311  abuts on the side wall part  323   a , so that the sheet is prevented from following the movement of the slide guide  311 . Accordingly, it becomes possible to cause the sheet to drop to the face-down discharge part  125  at an appropriate location. Since there is the possibility that the sheet sticks to either one of the slide guides  310  and  311 , the side wall part  323   a  is provided on each side of the reference wall  323 . 
   In addition, in a case where only one of the guide members is arranged to move as described with reference to  FIG. 6(   a ), etc., the above-stated advantageous effect can be obtained if the side wall part  323   a  is provided only on the side of the guide member arranged to slide. 
   As has been described in the foregoing with the various embodiments, the invention enables the space saving and cost reduction to be realized with the more simplified construction than in the conventional sheet processing apparatus. 
   Incidentally, while the invention has been described on the basis of the embodiments in which the sheet processing apparatus  300  is disposed above a printer serving as the image forming apparatus, the sheet processing apparatus according to the invention may be mounted on any kind of apparatus, without limiting to the image forming apparatus, as long as it is arranged to perform a stapling process, a punching process, or the like, on the sheet. 
   Further, while the invention has been described on the basis of the construction in which one sheet processing apparatus  300  is provided, a plurality of sheet processing apparatuses may be provided in piles. For example, assuming that two sheet processing apparatuses are disposed one on top of the other, a sheet processed by the upper sheet processing apparatus is made to drop to the upper surface of the lower sheet processing apparatus. Therefore, it is preferable to provide the upper surface of the sheet processing apparatus with a stacking part for stacking sheets thereon. With a plurality of sheet processing apparatuses thus provided, it becomes possible to perform the various processes and to perform a process coping with a great number of jobs. 
   Further, the printer body  100  in each of the embodiments is assumed to be an apparatus of the so-called center reference type in which a sheet of any size is conveyed with the center of a conveying path taken as a reference. Therefore, a sheet which has been conveyed to the sheet processing apparatus  300  from the printer body  100  is discharged to the position where the center of the interval between the right and left guides is taken as a reference. However, even in a case where the printer body  100  is arranged to perform the so-called one-side reference conveying operation in which the sheet is conveyed with one side of the conveying path taken as a reference, it is of course possible to provide the sheet processing apparatus. 
   Further, the invention may be modified such that a sheet which has been conveyed to the sheet processing apparatus  300  is subjected to the alignment action and, after that, is made to drop to the face-down discharge part  125  on the upper surface of the printer body  100  without being subjected to any predetermined process.