Abstract:
A sheet finisher for an image forming apparatus that executes a precise and continuous punching at a specific position on a sheet while maintaining the conveyance speed of the sheet ejected from the image forming apparatus. The puncher is provided with the first and second conveyance rollers disposed along the conveyance direction of the sheet, in which the drive speed of the second conveyance roller disposed upstream in the conveyance direction is temporarily increased to form a loop of the sheet, and the punching device is driven so as to synchronize with the timing that the trailing edge of the sheet is detached from the second conveyance roller and brought into a temporal halt, thereby executing a punching precisely at the specific position on the sheet.

Description:
This application is based on application No. 11-81249 filed in Japan, the contents of which is hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet finisher that executes a finishing processing, such as hole punching, etc., to a sheet ejected from an image forming apparatus. 
     2. Prior Art 
     There have been disclosed various types of a sheet finisher that receives sheets ejected from the image forming apparatus and punches binding holes (punching holes) at specific positions, and is used in installation close to the image forming apparatus. 
     Such a sheet finisher includes two types: one that temporarily collects sheets conveyed into a tray and then punches binding holes on the sheets collectively, and another one that punches binding holes on the sheet one by one on the way of conveyance. 
     The former holds an inconvenience that the collective punching will not permit such a processing as collation, sorting, paper jogging, etc., after punching. On the other hand, the latter holds an inconvenience being unsuitable for a high-speed punching, because the punching on the way of the conveyance of the sheet temporarily stops the conveyance thereof, and accordingly the subsequent sheet cannot be conveyed during the punching. 
     In an attempt to dissolve such inconveniences, a finisher by the rotary puncher has been proposed which can execute to a sheet the finishing, such as collation, sorting, paper jogging, etc., and which can punch the sheet without stopping the conveyance (refer to Japanese Laid Open Patent Publication No. Hei 6-135620). 
     In the finisher that temporarily stops the conveyance of a sheet on the way thereof and punches the sheet, the leading edge of the sheet is blocked by the resist roller so as to temporarily halt the conveyance, and the sheet is punched. Since the trailing edge of the sheet is still under conveyance during the punching, the sheet makes a loop as a whole during this period; and when the conveyance of the sheet is restarted, the loop of the sheet is dissolved. 
     Therefore, the spacing S between the preceding sheet and the succeeding sheet to be punched needs more than the spacing that corresponds to the total time T of the time t 1  required for punching the preceding sheet and the time t 2  required for the loop of the preceding sheet dissolving (T=t 1 +t 2 ). If the conveyance speed of the sheet is given by V, the spacing will be expressed by S=V×T. That is, the preceding sheet and the succeeding sheet need to secure at least the foregoing spacing S between them. 
     Accordingly, to enhance the processing capacity needs to increase the conveyance speed, and to shorten the time required for the hole punching; however, the conveyance speed is subjected to the restriction by the speed of the sheet ejected from the image forming apparatus, which cannot be changed. Accordingly, to enhance the processing capacity needs to increase the punching speed, but to increase the speed involves size enlargement of the apparatus, creation of noises, and dispersion of hole positions, which are inconvenient. 
     And, in the finishing by the foregoing rotary puncher, it is not needed to give a spacing between the preceding sheet and the succeeding sheet, but since the leading edge of the sheet is not made to be blocked by the resist roller, the leading edge thereof becomes irregular, so that the hole positions become dispersed, which is also inconvenient. 
     SUMMARY OF THE INVENTION 
     A major object of the invention is to provide a novel sheet puncher that executes a punching to a sheet ejected from an image forming apparatus on the way of the conveyance while maintaining the conveyance speed of the ejected sheet. 
     Another object of the invention is to provide a novel image forming apparatus that executes a finishing processing such as a punching at a high speed on the way of the conveyance of the sheet while maintaining the conveyance speed of the sheet that has completed an image recording and is ejected from the image forming apparatus. 
     Another object of the invention is to provide a novel sheet finisher that executes a finishing processing such as a punching on the way of the conveyance to a sheet ejected from the image forming apparatus while maintaining the conveyance speed of the ejected sheet. 
     Another object of the invention is to provide a novel sheet puncher that temporarily increases the speed of one conveyance roller disposed upstream in the conveyance direction, of a pair of conveyance rollers disposed along the conveyance direction of the sheet, operates a punching mechanism when the trailing edge of the sheet to be conveyed is brought in a temporal halt, and executes a precise punching at a specific position on the sheet. 
     Other objects and features of the invention will become apparent from the detailed description of the invention in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view illustrating a construction of a sheet finisher of one embodiment of the present invention; 
     FIG. 2 is a side view illustrating a construction of a major part of a punching assembly (sheet conveying state); 
     FIG. 3 is a side view illustrating a construction of a major part of a punching assembly (punching state); 
     FIG. 4 is a plan view illustrating a construction of the drive mechanism of a resist roller and a conveyance roller; 
     FIG. 5 is a front view illustrating a construction of the punching unit drive mechanism; 
     FIG. 6 is a block diagram illustrating the configuration of a control circuit; 
     FIG. 7 is a flowchart explaining the operation to control the sheet finisher; 
     FIG. 8 is a flowchart explaining the control operation of the punching processing; 
     FIG. 9 is a sectional view illustrating an outline of an image forming apparatus configured with the finisher coupled to a copying machine; and 
     FIG. 10 is a block diagram of a control circuit of the image forming apparatus configured with the finisher coupled to the copying machine. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiment of the invention will be described with reference to the accompanying drawings. 
     FIG. 1 is a sectional view illustrating a construction of a sheet finisher relating to the embodiment of the invention. In FIG. 1,  100  denotes the sheet finisher,  200  a sheet folding unit,  300  a punching assembly,  400  a sheet accumulation unit,  500  a stapling unit, and  600  a second ejection tray. Also,  11  denotes a sheet inlet which a sheet P is carried into,  50  a first ejection tray, and  80  a drive control unit. 
     The punching assembly  300  includes a punching unit  301 , a first punching conveyance roller  303  disposed on the downstream of the punching unit  301 , a second punching conveyance roller (resist roller)  302  disposed on the upstream of the punching unit  301 , and the other components, the detail of which will be described later. Here, the sheet folding unit  200  and the stapling unit  500  take on the well-known construction, and the detail is omitted. 
     The outline of the sheet finisher  100  will be explained. Along a sheet conveyor from the sheet inlet  11  until the downstream of the punching assembly  300  are disposed a conveyance roller  12 , a conveyor switching claw  13 , and a sheet sensor  14 ; and on the downstream of the punching assembly  300  is disposed a conveyor switching claw  15 . 
     The conveyor switching claw  13  is to switch the conveyor so that the sheet is conveyed to the sheet folding unit  200 , and the conveyor switching claw  15  disposed on the downstream of the punching assembly  300  is to switch the conveyor into the first ejection tray  50  or the second ejection tray  600 . 
     Along a sheet conveyor from the conveyor switching claw  15  toward the first ejection tray  50  are disposed a conveyance roller  16 , a sheet sensor  17 , a conveyance roller  18 , and a conveyor switching claw  19 , in which the sheet conveyor is designed to be switched by the conveyor switching claw  19  into the first ejection tray  50  or the sheet accumulation unit  400 . 
     Along a sheet conveyor toward the first ejection tray  50  are disposed a sheet sensor  20  and a conveyance roller  21 . And, along a sheet conveyor toward the sheet accumulation unit  400  are disposed a conveyance roller  25 , a sheet sensor  26 , and a conveyance roller  27 . 
     Along a sheet conveyor toward the second ejection tray  600  are disposed a conveyance roller  34 , a sheet sensor  35 , and a conveyance roller  36 . 
     In order to bind the accumulated sheets in the sheet accumulation unit  400 , the stapling unit  500  is disposed on the downstream of the sheet accumulation unit  400 , and sheet bundle pressure rollers  31 ,  32  are disposed before and after the stapling unit  500 . Further, a bundle conveyance roller  33  is disposed on the downstream thereof, and the sheet conveyor in this part communicates with the sheet conveyor toward the foregoing second ejection tray  600 . 
     Next, the operation of the sheet finisher will briefly be explained. The operation of the sheet finisher  100  is controlled by the drive control unit  80  described later, and the conveyance speed of the sheet by the conveyance rollers is controlled to be constant, except for the punching by the punching assembly  300  described hereafter. 
     When a sheet folding instruction is issued from the operation panel on the copying machine, the conveyor switching claw  13  is switched so that the sheet P carried in the sheet inlet  11  can be carried to the sheet folding unit  200 , where the sheet P is folded and carried toward the downstream (the punching assembly  300 ). 
     When the sheet folding instruction is not issued, the conveyor switching claw  13  is switched so that the sheet P carried in the sheet inlet  11  can be carried to the punching assembly  300 . 
     When a punching instruction is issued from the operation panel on the copying machine, the sheet P is carried by the first punching conveyance roller  303  and the second punching conveyance roller (having a function as a resist roller)  302 , and it passes through the punching assembly  300 . At this moment, the trailing edge of the sheet P is positioned by the second punching conveyance roller  302 , as described later; and the sheet P is punched by the punching unit  301  and is carried downstream. 
     When a non-punching instruction is issued from the operation panel on the copying machine, the sheet P is carried by the first punching conveyance roller  303  and the second punching conveyance roller  302 , and is carried downstream without punching. 
     When a stapling instruction is issued from the operation panel on the copying machine, the conveyor switching claw  15  and the conveyor switching claw  19  are switched so that the sheet P is carried to the sheet accumulation unit  400 , in which the sheet P is accumulated by one set each. One set of the accumulated sheets is pressurized into one bundle by the sheet bundle pressure rollers  31 ,  32  disposed before and after the stapling unit  500 ; and then the bundle is stapled by the stapling unit  500 , and ejected into the second ejection tray  600  through the bundle conveyance roller  33  and the conveyance roller  36 . 
     The second ejection tray  600  is designed so that the tray can be shifted up and down, right and left by a driving source not illustrated to carry out the sorting of the sheets (called as shift sorting). Further, the second ejection tray  600  is made so as to shift the vertical position of the tray in accordance with the number of the sheets ejected. 
     When a sorting instruction is issued from the operation panel on the copying machine, the conveyor switching claw  15  is switched so that the sheet P is carried to the second ejection tray  600 . The sheets P carried in the sheet inlet  11  are ejected into the second ejection tray  600 , and the sheets are sorted in the second ejection tray  600 . 
     When a non-sorting instruction is issued from the operation panel on the copying machine, the conveyor switching claw  15  and the conveyor switching claw  19  are switched so that the sheet P is carried to the first ejection tray  50 . The sheets P carried in the sheet inlet  11  are ejected into the first ejection tray  50 . 
     Next, the construction and operation of the punching assembly  300  of this embodiment will be explained. FIG.  2  and FIG. 3 are side views illustrating the construction of the major part of the punching assembly  300 . FIG. 2 illustrates a state in which the sheet P is conveyed, and FIG. 3 illustrates a state in which the sheet P is punched. FIG. 4 is a plan view explaining the construction of the drive mechanism of the second punching conveyance roller  302  and the first punching conveyance roller  303 , and FIG. 5 is a front view explaining the construction of the drive mechanism of the punching unit  301 . 
     As shown in FIG. 1, FIG. 2, and FIG. 3, the punching assembly  300  includes the punching unit  301 , the second punching conveyance roller  302  disposed on the upstream thereof, and the first punching conveyance roller  303  disposed on the downstream thereof. On the upstream of the second punching conveyance roller  302  is disposed the sheet sensor  14 , and between the punching unit  301  and the first punching conveyance roller  303  is provided a space such that the sheet P can form a loop R of a specific size therein. 
     The second punching conveyance roller  302  is provided with a construction that drives the conveyor at a first conveyance speed V 1  being the normal sheet conveyance speed and at a second conveyance speed V 2  which is higher than V 1 . FIG. 4 is a plan view explaining the drive mechanism of the first punching conveyance roller  303  and the second punching conveyance roller  302 , in which a drive shaft  313  of the first punching conveyance roller  303  is driven by a motor  311  being a power source through a sprocket wheel  311   a  connected to the motor  311 , a transmission chain  312 , and a sprocket wheel  312   a.    
     The drive shaft  313  has a sprocket wheel  313   a  and a sprocket wheel  313   b  attached there. On the other hand, a drive shaft  316  of the second punching conveyance roller  302  has a sprocket wheel  316   a  connected through a one-way clutch  319 . Further, the drive shaft  316  has a sprocket wheel  316   b  whose number of teeth is fewer than that of the sprocket wheel  313   b  connected through a clutch  318  controlled by the drive control unit  80  described later. 
     The first punching conveyance roller  303  is driven at the first conveyance speed V 1  by the motor  311  through the sprocket wheel  311   a , the transmission chain  312 , the sprocket wheel  312   a , and the drive shaft  313 . 
     In the drive at the first conveyance speed V 1  of the second punching conveyance roller  302 , which is the normal conveyance state of the sheet, the rotation of the sprocket wheel  313   a  on the drive shaft  313  driven through the foregoing transmission passage is transmitted to a transmission chain  314 , the sprocket wheel  316   a , the one-way clutch  319 , and the drive shaft  316 , thus driving the second punching conveyance roller  302 . In this state, the clutch  318  is not operational, and the sprocket wheel  316   b  is not coupled with the drive shaft  316 . 
     In the drive at the second conveyance speed V 2 , which is higher than the speed V 1 , of the second punching conveyance roller  302 , the clutch  318  is operational, and the sprocket wheel  316   b  is coupled with the drive shaft  316 . Thereby, the rotation of the sprocket wheel  313   b  on the drive shaft  313  driven through the foregoing transmission passage is transmitted to a transmission chain  315 , the sprocket wheel  316   b , the clutch  318 , the drive shaft  316 , and the second punching conveyance roller  302  is driven at the second conveyance speed V 2 . 
     In this state, the rotation speed of the drive shaft  316  is higher than that of the sprocket wheel  316   a , and the one-way clutch  319  is operational; and accordingly, the sprocket wheel  316   a  is separated from the drive shaft  316 . 
     FIG. 5 is a front view explaining a construction of the drive mechanism of the punching unit  301 . The punching unit  301  includes a clutch  321  that transmits the rotational driving force from a power source not illustrated to a drive shaft  322 , an eccentric cam  305  rigidly connected to the drive shaft  322 , and punch  304  driven up and down by the eccentric cam  305 , which is driven by the control of the drive control unit  80  described later. Further, the drive shaft  322  is provided with a punch position sensor  323  in order to detect whether the punch is set at a home position (return position) outside the conveyor of the sheet. 
     Also, the eccentric cam  305  and the punch  304  are laid out each in plural sets in the direction perpendicular to the conveyance direction of the sheet, whereby a desired number of holes can be punched. 
     Next, the operation of the first punching conveyance roller  303  and the second punching conveyance roller  302 , and the punching unit  301  will be explained. 
     When the sheet P is detected by the sheet sensor  14  disposed on the upstream in the conveyance direction of the sheet, the drive control unit  80  described later switches the conveyance speed of the second punching conveyance roller  302  from the speed V 1  into the speed V 2  higher than V 1 . 
     Since the sheet conveyance speed of the first punching conveyance roller  303  is the speed V 1 , by the conveyance speed difference of the first and second punching conveyance rollers, the loop R of the sheet P is formed between the first punching conveyance roller and the second punching conveyance roller (FIG.  3 ). When the trailing edge of the sheet P passes through the second punching conveyance roller  302 , the trailing edge of the sheet P is brought into contact with the nip of the second punching conveyance roller  302  by a repulsion of the looped sheet. This state is maintained until the loop is dissolved, and the trailing edge of the sheet P is positioned accordingly. 
     In the state that the trailing edge of the sheet P is positioned, the punching unit  301  is driven to punch the sheet within a time until the loop is dissolved. Since the trailing edge of the sheet P is positioned, the punching position is not dispersed. And, since the sheet P is continuously conveyed by the first punching conveyance roller  303 , the loop is dissolved with the passage of time, and the sheet P can be punched in continuous conveyance. 
     When the punching operation of the sheet P is completed, and the trailing edge of the sheet P is detached from the nip of the second punching conveyance roller  302 , the loop is dissolved. At this moment, since the leading edge of the succeeding sheet can be brought to the nip on the entrance side of the second punching conveyance roller  302 , a spacing is not necessary to be made between the preceding sheet and the succeeding sheet. 
     Now, the punching unit operation time will be explained. 
     Provided that the conveyance speed of the first punching conveyance roller is given by V 1 , the conveyance speed of the second punching conveyance roller is given by V 2 , and the distance between the trailing edge of the sheet (being also the position where the sheet sensor  14  is disposed) and the nip of the second punching conveyance roller is given by L, the loop formation time t 1  (from the time the trailing edge of the sheet is detected by the sheet sensor  14  to the time the trailing edge of the sheet is brought into contact with the nip of the second punching conveyance roller, and the trailing edge is positioned) is given by the following. 
     
       
         t 1 =L/V 2   (1) 
       
     
     The size of the loop R at this moment is: 
     
       
         R=t 1 ×(V 2 −V 1 )  (2) 
       
     
     The time t 2  until the loop is dissolved, namely, the punching unit operation permissible time is: 
     
       
         t 2 =R/V 1   (3) 
       
     
     From the foregoing equations (1), (2), (3), the distance L between the trailing edge of the sheet (being also the position where the sheet sensor  14  is disposed) and the nip of the second punching conveyance roller can be expressed as follows. 
     
       
         L=t 2 ×V 1 ×V 2 /(V 2 −V 1 )  (4) 
       
     
     Since the punching unit operation time is determined by the construction, and the conveyance speed V 1  is specified by the image forming apparatus, for obtaining a necessary loop R, it is only needed to determine the conveyance speed V 2  and the disposition of the sheet sensor  14  through alignment. 
     FIG. 6 is a block diagram illustrating the configuration of the drive control unit  80  that controls the operation of the finisher  100 . The drive control unit  80  includes a CPU  81 , to the input/output ports of which are connected the motor  311  being the power source for the conveyance rollers of the sheet and the punching assembly, etc., and in addition, the sheet folding unit  200 , the stapling unit  500 , and the second ejection tray  600 . The second ejection tray  600  is controlled by the CPU  81  in order for executing the shift sorting. 
     Further, to the input ports of the CPU  81  are connected the sheet sensors  14 ,  17 ,  20 ,  26 ,  35 , and the punch position sensor  323  of the punching assembly as well. To the output ports thereof are connected the conveyor switching claws  13 ,  15 ,  19 , and in addition, the clutch  318  for the drive shaft  316  that drives the second punching conveyance roller, and the clutch  321  for the drive shaft  322  that drives the punching assembly  300 . 
     Next, referring to the flowcharts in FIG.  7  and FIG. 8, and FIG.  1  through FIG. 5, the control operation of the drive control unit  80  will be described. 
     FIG. 7 is a flowchart explaining the total operation of the finisher  100 . First, the conveyance speed of the sheet P by the conveyance rollers in the finisher  100  is controlled so that the specific conveyance speed V 1  can be maintained except for the punching in the punching assembly  300  described hereunder (step P 1 ). 
     The state of the sheet sensor  14  is judged (step P 2 ), and if the copied sheet P carried in the sheet inlet  11  is detected, whether or not the folding instruction is present is judged (step P 3 ). If the folding instruction is present, the conveyor switching claw  13  is switched so that the sheet P is conveyed to the folding unit  200 , where the sheet P is folded (step P 4 ), and the folded sheet P is conveyed downstream. 
     Whether the punching instruction is present is judged (step P 5 ), and if the punching instruction is present, the punching operation is carried out by the punching assembly  300  (step P 6 ), and the sheet P is conveyed downstream. 
     Whether the stapling instruction is present is judged (step P 7 ), and if the stapling instruction is present, the conveyor switching claws  15  and  19  are switched so that the sheet P is conveyed to the accumulation unit  400 , where the sheet P is accumulated (step P 8 ). The accumulated sheets are stapled by the stapling unit  500 , and the stapled sheet bundle is ejected into the second ejection tray  600  (step P 9 , P 10 ). 
     Whether the sheet P to be finished is present is judged (step P 11 ), if it is present, the processing returns to step P 2 , and if not, the processing is terminated. 
     At the judgment at step P 7 , if it is non-stapling instruction, whether it is the sorting instruction is judged (step P 12 ); and if it is the sorting, the sheet P is carried to the second ejection tray  600  to carry out the sorting (step P 13 , P 14 ), and the processing transfers to the step P 11  and after. At the judgment at step P 12 , if it is not the sorting, namely, it is the non-sorting, the conveyor switching claws  15  and  19  are switched so that the sheet P is carried to the first ejection tray  50 , and the sheet P is ejected into the first ejection tray  50  (step P 15 ), and the processing transfers to the step P 11  and after. 
     FIG. 8 is a flowchart explaining the control of the punching operation illustrated at step P 6  in FIG.  7 . Here, in the initial state, the second punching conveyance roller  302  and the first punching conveyance roller  303  are assumed to be controlled to carry the sheet P at the specific conveyance speed V 1 . 
     First, the processing waits for the trailing edge of the sheet P being detected by the sheet sensor  14  (step P 21 ). Here, the disposition of the sheet sensor  14  is set such that the distance between the trailing edge of the sheet P and the nip of the second punching conveyance roller  302  becomes equal to the specific value L, when the trailing edge of the sheet P is detected. 
     When the trailing edge of the sheet P is detected, the processing starts to count the time T representing the feed length of the sheet P by the second punching conveyance roller  302  (step P 22 ). At the same time as starting to count the time T, it operates the clutch  318  (see FIG.  4 ), switches the conveyance speed of the second punching conveyance roller  302  into the speed V 2 , and starts conveying the sheet P at the higher speed (step P 23 ). 
     It is judged whether the time T representing the feed length of the sheet P counts up to the time t 1  corresponding to the specific loop formation (step P 24 ); and if the time T does not count up to the time t 1 , the processing returns to step P 24  to continue the conveyance of the sheet P at the higher speed, but if it counts up to t 1  (T=t 1 ), the conveyance speed of the second punching conveyance roller  302  is switched into the speed V 1  (step P 25 ), and at the same time the punching is started. 
     In the execution of the punching, first, the clutch  321  is operated to rotate the drive shaft  322 , which executes the punching operation by the punch  304  (step P 26 ). The punch position sensor  323  detects whether the punch  304  is returned to the home position outside the conveyor of the sheet (step P 27 ); and if they are returned, the clutch  321  is disconnected and stops the rotation of the drive shaft  322  (step P 28 ), thus terminating the punching operation and returns to the main routine. 
     FIG. 9 is a schematic sectional view of an image forming apparatus being one embodiment of the invention, which is configured with the finisher  100  coupled to a copying machine  700 . 
     The copying machine  700  to which the finisher  100  is coupled is the so-called digital copying machine, in which document images are read and stored in an image memory, the images are edited in various ways as needed, thereafter the images are formed on a sheet by the well-known electrophotographic technology, and the copied sheets are ejected one by one from a sheet ejection unit  724  to the finisher  100 . 
     The copying machine  700  has an automatic document feeder  750  (hereunder, mentioned as ADF) incorporated on the top of copying machine  700 . This ADF  750  feeds one or more documents set in a document tray  751  one by one onto the platen glass (not illustrated) of the copying machine  700 , and ejects to pile up the original documents whose image readout has been finished into an ejection tray  752 . 
     The copying machine  700  of this embodiment starts copying operation from the first page of the documents, which is the so-called the first page system, in which the documents are set in the document tray  751  of the ADF  750  with the first page facing upward. In case of the double sided copying, for example, that copies the images on single sided document to both sides of one sheet, the copying machine of the so-called the first page system is not necessary to designate or detect whether the documents are made of an odd number of sheets or an even number, and the copying operation can be made rapidly, which is advantageous. 
     The images on the document set on the platen glass of the copying machine by the ADF  750  are read by the image reader (not illustrated) incorporated in the copying machine  700 , and converted into the digital data to be stored in the memory. The copying operation is executed by reading out the image data, in which a necessary editing, for example, alteration of page order, inversion of images, or double sided copying, is added. 
     Further, a sheet reversing mechanism  720  for reversing the front and back sides of a copied sheet is installed near the sheet ejection portion  724  of the copying machine  700 . 
     FIG. 10 is a block diagram illustrating a control circuit of the image forming apparatus configured with the finisher coupled to the copying machine. 
     The major part of this control circuit is a copying machine CPU  910  that controls the operation of the copying machine, an ADF CPU  950  that controls the operation of the ADF  750 , and the finisher CPU  81  that controls the operation of the finisher. These CPUs  910 ,  950 ,  81  are provided with ROMs  911 ,  951 ,  981 , respectively, which store the corresponding control programs, and RAMs  912 ,  952 ,  982  being the work areas. 
     The copying machine CPU  910  is provided with an image memory  825  that stores read images, and an image signal processing unit  820  that executes, on the basis of the image information stored in this image memory  825 , the image processing such as an image rotation, image enlargement, image contraction, etc. 
     Further, a CCD line sensor  822  of the image reader is connected to the image signal processing unit  820  through an A/D converter  821 . The A/D converter  821  converts the analog signals read by the CCD line sensor  822  into the digital signals, and the converted results are inputted to the CPU  910 . 
     Further, the image signal processing unit  820  is connected to a D/A converter  831  that converts the digital signals of the image information into the analog signals, and the converted analog signals drive a laser device  832  of an image forming unit (not illustrated). 
     To the finisher CPU  81  are connected, as described above, various drive means such as the motor or solenoids to execute the operations of various units in the finisher  100 , and various sensors installed along the sheet conveyor and in the folding unit  200  of the finisher. 
     The copying machine CPU  910  outputs the signal to the finisher CPU  81 , which instructs or inhibits the execution of folding, punching, stapling, sorting, or the like in accordance with the input signal from an operation panel  701 . 
     As described above, the finisher of this invention is provided with the first conveyance roller and the second conveyance roller disposed on the upstream side from the first, and when the sheet sensor installed on the upstream in the conveyance direction of the sheet detects the sheet, a loop of the sheet is formed between the first and second conveyance rollers by the conveyance speed difference obtained by switching the conveyance speed of the second conveyance roller into the higher conveyance speed. Thereby, the trailing edge of the sheet is positioned, and the punching can be carried out to the sheet thus positioned during the conveyance of the sheet. Therefore, compared with the conventional apparatus, the finisher of this invention will significantly restrict the dispersion of the punching positions and execute the punching at specific positions precisely and continuously. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.