Abstract:
In a conventional color printer, especially an in-line type full color image forming apparatus, an aberration of optical axes caused by distortions of structures affect a performance of an image forming as a tilt aberration in a printed image. Accordingly, in a conventional color printer, distortions have to be prevented by strengthening rigidity of machine structures or parts of the machine. However, fine adjustment or assembling to strengthen rigidity generally causes an increase in cost and requires time-consuming adjustment. An apparatus in the present invention provides advantages of high accuracy and strengthened structure without increasing cost.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an image forming apparatus of a copying machine, a printer and the like, and more particularly, to an image forming apparatus for forming a color image using a plurality of image forming parts.  
           [0003]    2. Related Background Art  
           [0004]    As shown in FIGS. 12A and 12B, there is a conventional full color image forming apparatus of the in-line type which has a plurality of photosensitive drums arranged in line. There are mainly two kinds of such full color image forming apparatuses. One is an image forming apparatus  200 A of the horizontal arrangement type which has a plurality of photosensitive drums horizontally arranged in line as shown in FIG. 12A and the other is an image forming apparatus  200 B of the vertical arrangement type which has a plurality of photosensitive drums vertically arranged in line as shown in FIG. 12B. Their features are that the image forming apparatus  200 A of the horizontal arrangement type is short in height but requires a large area for installation, while that the image forming apparatus  200 B of the vertical arrangement type requires a small area for installation but is tall in height.  
           [0005]    In an in-line full color image forming apparatus, parallelism of each photosensitive drum  201  and accuracy in position between a scanner unit  202  and the drum are always considered to be very important in light of an image color aberration.  
           [0006]    Conventionally, as the image forming apparatus  200 B of the vertical arrangement type shown in FIG. 14, the photosensitive drum  201  is positioned by being abutted to a drum support block  204  mounted on each of right and left side plates  203   a  and  203   b  with accuracy.  
           [0007]    The scanner unit  202  is positioned by being mounted on a scanner stand  205  so as to bridge the right and left side plates  203   a  and  203   b  and being secured on the scanner stand  205  by screws with accuracy.  
           [0008]    In a conventional color printer, the most important problem has been a tilt aberration of each color in a printed image. As shown in FIG. 13, when black (Bk) and magenta (M) are recorded in superposed relation, a tilt between the photosensitive drums of the colors and a tilt of the scanner unit from the photosensitive drum directly appear on the image as the tilt aberration G. Namely, when the parallelism of four photosensitive drums is lost, the tilt aberration G of each color occurs on the printed image. In addition, when parallelism of an optical axis of the scanner unit and an axis of each photosensitive drum is lost, the tilt aberration G of each color occurs on the printed image likewise.  
           [0009]    Conventionally, as shown in FIG. 14, in order to obtain accuracy in position of a plurality of the photosensitive drums  201  and the scanner units  202 , the drum support blocks  204  are used in all four parts to assure parallelism of the photosensitive drums. For the scanner unit  202 , parallelism of the scanner stand  205  is strictly defined or the plate is thickened in order to strengthen rigidity or the like so as to improve accuracy and strength of components.  
           [0010]    In case where improving accuracy of the component is not sufficient, during assembly, a master tool which has an assured positional relationship is used to adjust a position of the scanner stand  205  whereto the drum support block  204  and the scanner  202  are mounted. There is another means such that the position is adjusted by an adjustment mechanism which can make fine adjustment to the assembly. Generally used as an adjustment method is a mechanical adjustment mechanism using a cam or a lever.  
           [0011]    In the above described means, however, there is a limit in strictly defining the accuracy of the component and assuring the position. It is generally said that amount of color aberration accepted in the color printer is 100 μm, and the color aberration in this configuration should be within tens of μm in spite of various factors of the color aberration. For this reason, each component has to be manufactured in the accuracy of some μm, which requires impracticable measurement defining in the drum support block  204  and the scanner stand  205 .  
           [0012]    Further, assembly by adjustment and fine adjustment in position cause an increase in cost and requires time-consuming adjustment, which is a serious disadvantage for products such as printers to be mass produced.  
         SUMMARY OF THE INVENTION  
         [0013]    The present invention is made in view of the above problems and an object of the present invention is to provide an image forming apparatus which forms images with good quality.  
           [0014]    An another object of the present invention is to provide an image forming apparatus with high accuracy in mutual position of a plurality of image forming portions.  
           [0015]    A still another object of the present invention is to provide an image forming apparatus comprising:  
           [0016]    a plurality of image bearing members;  
           [0017]    a plurality of image writing means for writing an image onto each of said image bearing member; and  
           [0018]    a frame for supporting a plurality of said image bearing members and a plurality of said image writing means, said frame having a plurality of first holding parts which hold both ends of each of said image bearing body and a plurality of second holding parts which hold both ends of each of said image writing means. 
       
    
    
       [0019]    Further objects of the present invention will become more apparent upon a reading of the following detailed description with reference to the drawings.  
       BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1 is a sectional view of an image forming apparatus according to a first embodiment of the present invention;  
         [0021]    [0021]FIG. 2 is a perspective view showing a configuration of a body frame of the image forming apparatus according to the first embodiment of the present invention;  
         [0022]    [0022]FIG. 3 is a side view partly showing a positioning structure of a photosensitive drum and a scanner unit of the image forming apparatus according to the first embodiment of the present invention;  
         [0023]    [0023]FIG. 4 is a side view partly showing a positioning portion of the photosensitive drum of the image forming apparatus according to the first embodiment of the present invention;  
         [0024]    [0024]FIG. 5 is a side view partly showing a positioning portion of the scanner unit of the image forming apparatus according to the first embodiment of the present invention;  
         [0025]    [0025]FIG. 6 is a sectional view of the photosensitive drum of the image forming apparatus according to the first embodiment of the present invention;  
         [0026]    [0026]FIG. 7 is a rear view of the body frame of the image forming apparatus according to the first embodiment of the present invention;  
         [0027]    [0027]FIG. 8 is a diagram showing temperature variations of each element in press work operation time;  
         [0028]    [0028]FIG. 9 is a view showing a machining procedure and a method of storage of right and left side plates of the image forming apparatus according to the first embodiment of the present invention;  
         [0029]    [0029]FIG. 10 is a sectional view partly showing a shaving work of right and left side plates of the image forming apparatus according to the first embodiment of the present invention;  
         [0030]    [0030]FIG. 11 is a perspective view showing a configuration of a body frame of the image forming apparatus according to a second embodiment of the present invention;  
         [0031]    [0031]FIGS. 12A and 12B are sectional views of a conventional full color image forming apparatus;  
         [0032]    [0032]FIG. 13 is a view showing an example of defects in an image; and  
         [0033]    [0033]FIG. 14 is a perspective view showing a frame structure of a conventional image forming apparatus. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0034]    An image forming apparatus according to the present invention will be described below with reference to the drawings.  
       Embodiment 1  
       [0035]    First, a whole configuration of a full color image forming apparatus will be described with reference to FIG. 1.  
         [0036]    [0036]FIG. 1 is a vertical sectional view showing the whole configuration of a full color image forming apparatus  100  (a full color laser beam printer). The illustrated full color image forming apparatus  100  is provided with four photosensitive drums  1  ( 1   a ,  1   b ,  1   c ,  1   d ) vertically arranged in line and each drum  1  is rotatably driven in a counterclockwise direction in FIG. 1 by driving means (not shown). Around the photosensitive drums  1 , there are provided charging apparatuses  2  ( 2   a ,  2   b ,  2   c ,  2   d ) for evenly charging a surface of the photosensitive drum  1 , scanner units  3  ( 3   a,    3   b,    3   c,    3   d ) for irradiating a laser beam based on an image information to form an electrostatic latent image on the photosensitive drum  1 , developing apparatuses  4  ( 4   a ,  4   b ,  4   c ,  4   d ) for having toner adhered on the electrostatic latent image to be developed as a toner image, an electrostatic transferring apparatus  5  for transferring the toner image on the photosensitive drum  1  to a transfer material S, and cleaning apparatuses  6  ( 6   a ,  6   b ,  6   c ,  6   d ) for removing remaining toner on a surface of the photosensitive drum  1  after transferring.  
         [0037]    Here, the photosensitive drums  1 , charging apparatuses  2 , developing apparatuses  4  and cleaning apparatuses  6  are formed integral with each other to be in a form of a cartridge to form process cartridges  7  ( 7   a ,  7   b ,  7   c ,  7   d ).  
         [0038]    Description will be made below in turn from the photosensitive drum  1 .  
         [0039]    The photosensitive drum  1  is configured as being in a layer by, for example, applying an organic photo-conductive material (an OPC photosensitive material) on an outer peripheral surface of an aluminum cylinder 30 mm in diameter. The photosensitive drum  1  is rotatably supported at its both ends by support members and rotatably driven in a counterclockwise direction in FIG. 1 by driving force transferred from driving motor (not shown) to its one end.  
         [0040]    As the charging apparatus  2 , the apparatus of the contact charging type can be used. A charging member is formed from an electro-conductive roller in the shape of a roller and the surface of the photosensitive drum  1  can be evenly charged by abutting the electro-conductive roller  2  on the surface of the photosensitive drum  1  and applying a charging bias voltage on the electro-conductive roller  2 .  
         [0041]    The scanner unit  3  is arranged substantially in a horizontal direction of the photosensitive drum  1  and an image light in response to an image signal is irradiated by a laser diode (not shown) to polygon mirrors  9  ( 9   a ,  9   b ,  9   c ,  9   d ) rotated at a high speed by a scanner motor (not shown). The image light reflected by the polygon mirror  9  is irradiated to the surface of the charged photosensitive drum  1  via image forming lenses  10  ( 10   a ,  10   b ,  10   c ,  10   d ) to form an electrostatic latent image on the photosensitive drum  1 .  
         [0042]    Each of the developing apparatus  4   a ,  4   b ,  4   c ,  4   d is configured by a developing apparatus which accommodates toner of yellow, magenta, cyan and black, respectively.  
         [0043]    An electrostatic transferring belt  11  is arranged which moves in circulation so as to contact opposite all the photosensitive drums  1   a ,  1   b ,  1   c ,  1   d  and the electrostatic transferring belt  11  is configured from film members approximately 150 μm thick having a volume resistivity of 10 11  to 10 14  Ω·cm. The electrostatic transferring belt  11  is supported by rollers  13 ,  14   a ,  14   b ,  15  at vertical four axes and moves in circulation so as to electrostatically attach the transfer materials S to an outer peripheral surface at a left side in FIG. 1 to contact the transfer material S to the photosensitive drum  1 . According to this, the transfer material S is transported to a transfer position by the electrostatic transferring belt  11  and the toner image on the photosensitive drum  1  is transferred thereto.  
         [0044]    Inside the electrostatic transferring belt  11 , transferring rollers  12  ( 12   a ,  12   b ,  12   c ,  12   d ) are arranged in line opposite four photosensitive drums  1   a ,  1   b ,  1   c ,  1   d . Positive charge from these transferring rollers  12  are applied to the transfer material S via the electrostatic transferring belt  11  and electric field by this charge transfers a negative toner image on the photosensitive drum  1  to the transfer material S contacting the photosensitive drum  1 .  
         [0045]    Here, the electrostatic transferring belt  11  is configured from an endless belt approximately 700 mm peripheral length and 150 μm thick, arranged around the driving roller  13 , driven rollers  14   a ,  14   b  and tension roller  15  and rotatably driven in the direction of arrow of FIG. 1. Then, the toner image is transferred to the transfer material S during circulation movement of the electrostatic transferring belt  11  such that the transfer material S is transported from the driven roller  14   a  side to the driving roller  13  side.  
         [0046]    A sheet feeding part  16  is provided for feeding and transporting the transfer material S to the image forming part and accommodates a plurality of transfer materials S in a sheet feeding cassette  17 . During image forming, a sheet feeding roller  18  (a hemispherical roller) and a pair of registration rollers  19  are rotatably driven in response to image forming operation and the transfer materials S in the feeding cassette  17  are separately fed one by one. The transfer material S stops when abutting at its tip against the pair of registration rollers  19 , and after forming a loop, it is fed to the electrostatic transferring belt  11  by the pair of registration rollers  19  such that the rotation of the electrostatic transferring belt  11  synchronizes with an image writing position.  
         [0047]    A fixing part  20  is provided for fixing toner images of plural colors transferred to the transfer material S, and consists of a rotating heating roller  21   a  and a pressurizing roller  21   b  welded with pressure thereto to heat and to pressurize the transfer material S.  
         [0048]    Accordingly, the transfer material S to which the toner image on the photosensitive drum  1  is transferred is transported by the heating roller  21   a  and pressurizing roller  21   b  during passing through the fixing part  20  and heated and pressurized by the heating roller  21   a  and pressurizing roller  21   b  , whereby the toner image with plural colors are fixed on the surface of the transfer material S.  
         [0049]    For image forming operation, the process cartridges  7   a ,  7   b ,  7   c ,  7   d in turn are driven in a timed relationship with printing, and the photosensitive drums  1   a ,  1   b ,  1   c ,  1   d  are rotatably driven in a counterclockwise direction. Then, the scanner units  3   a  to  3   d corresponding to the process cartridges  7   a  to  7   d in turn are driven, the charging rollers  2   a  to  2   d apply even charge on the peripheral surface of the photosensitive drums  1   a  to  1   d,  and the scanner units  3   a  to  3   d expose the peripheral surfaces of the photosensitive drums  1   a  to  1   d  in response to the image signal to form the electrostatic latent image on the peripheral surface of each of the photosensitive drums  1   a  to  1   d.  The developing rollers in the developing apparatuses  4   a  to  4   d transfer the toner to a low potential portion of the electrostatic latent image to form (develop) the toner image on the peripheral surface of the photosensitive drums  1   a  to  1   d.    
         [0050]    When the tip of the toner image on the peripheral surface of the most upstream photosensitive drum  1   a  is rotarily transported to an opposite point opposite the electrostatic transferring belt  11 , the pair of registration rollers  19  start rotating to feed the transfer material S to the electrostatic transferring belt  11  in such a manner that the printing start position of the transfer material S coincides with the opposite point.  
         [0051]    The transfer material S is welded with pressure on the periphery of the electrostatic transferring belt  11  such as to be nipped by an electrostatic attaching roller  22  and the electrostatic transferring belt  11 . A voltage is applied between the electrostatic transferring belt  11  and the electrostatic attaching roller  22 , which induces charge on the transfer material S which is a dielectric material and a dielectric material layer of the electrostatic transferring belt  11  so that the transfer material S is electrostatically attached to the outer periphery of the electrostatic transferring belt  11 . According to this, the transfer material S is stably attached to the electrostatic transferring belt  11  and transported to the most downstream transferring part.  
         [0052]    While the transfer material S is transported in such ways, the toner image of each of the photosensitive drums  1   a  to  1   d  in turn is transferred thereto by the electric field formed between the photosensitive drums  1   a  to  1   d  and the transferring rollers  12   a  to  12   d.    
         [0053]    The transfer material S having four color toner images transferred is separated from the electrostatic transferring belt  11  by curvature of the belt driving roller  13  to be transported in the fixing part  20 . The transfer material S is subjected to heat fixing of the toner image in the fixing part  20  and then discharged from the sheet discharging part  24  out of the apparatus by a pair of sheet discharging rollers  23  with the image surface down.  
         [0054]    Next, a configuration of a body frame will be described with reference to FIGS. 2 and 3 which is the characteristic part of the present invention. FIG. 2 is a perspective view of the body frame and FIG. 3 is a sectional view of part of the body frame.  
         [0055]    As shown in FIG. 2, a bearing  31  ( 31   a,    31   b ) is fitted to longitudinal both ends of the photosensitive drum  1 , which is rotatably mounted via the bearing  31  ( 31   a,    31   b ). The bearing  31  is axially defined by an E-ring (not shown). In FIG. 2, only the photosensitive drum  1  and the bearing  31  are shown in order to describe the configuration of the present invention to be easily understood.  
         [0056]    A left side plate  32   a  and a right side plate  32   b , which are bent outwardly at their lower portion and secured to the bottom plate  33  by screws from the above, are arranged in a position where they abut the outer peripheral surface of the bearing  31 . A pitch between the left and right side plates  32   a  and  32   b  is important and in order to define the pitch, a size in a wide direction and the parallelism of the positioning portions of the left and right side plates  32   a  and  32   b  are strictly defined on the bottom plate  33 . The bottom plate  33  is formed from a plate like the left and right side plates  32   a  and  32   b , and the bottom plate  33  is formed at their front with a sheet passing hole  33   a  through which the transfer material S passes.  
         [0057]    The left and right side plates  32   a  and  32   b  have bending also at their front (an inserting side of the photosensitive drum  1 ) to assure high rigidity as units. The left and right side plates  32   a  and  32   b  are formed with eight first openings (notches)  34  ( 34   a  to  34   h ) corresponding to the bending portions, and on the same surface as the first opening  34 , eight second openings  35  ( 35   a  to  35   h ) are similarly formed substantially in the horizontal direction.  
         [0058]    On a back side of the left and right side plates  32   a  and  32   b , a back stay  36  is positioned and secured by screws such as to bridge the left and right side plates  32   a  and  32   b , and the back stay  36  is formed with four third openings  37  ( 37   a  to  37   d ) substantially in the horizontal direction of the openings  34  and  35 .  
         [0059]    Accordingly, the body frame is configured by positioning the above described left and right side plates  32   a  and  32   b , bottom plate  33 , back stay  36  and stay (not shown) with accuracy and secured by screws.  
         [0060]    Next, a method of positioning image-formation systems (the photosensitive drum  1  and scanner unit  3 ) will be described.  
         [0061]    A photosensitive drum unit with the bearing  31  integrally incorporated is inserted from the direction of arrow of FIG. 2 to the first opening  34 .  
         [0062]    [0062]FIG. 4 is a partly enlarged view of a positioning portion of the photosensitive drum. As shown in this figure, the bearing  31  is configured by a ball bearing and positioned by being pressed on abutment surfaces  37  and  38  of the first opening  34 . Accordingly, defining pitches between the hatched abutment surfaces  37  and  38  and other three portions with accuracy can minimize tilt aberration of a printed image.  
         [0063]    As shown in FIG. 6, the same effect can be obtained by pressing the bearing (ball bearing)  31  in the both ends of the photosensitive drum  1 , rotatably fitting a through axis  50  in an inner diameter of the bearing  31  to be mounted thereto, and pressing both ends  50   a  and  50   b  of the through axis  50  against the abutment surfaces  37  and  38  of the first opening  34  to be positioned there. For the bearing  31 , a slide bearing can be adopted by using slidable resins such as polyacetal.  
         [0064]    Next, a method of pressing the photosensitive drum  1  will be described.  
         [0065]    As shown in FIG. 3, an axis  39  is secured to the left and right side plates  32   a  and  32   b  and a torsion coil spring  40  is supported by the axis  39 , and the torsion coil spring  40  is secured by its end  40   a  fitting into a hole  41  of the left and right side plates  32   a  and  32   b . In the absence of the photosensitive drum  1 , the rotational direction of the torsion coil spring  40  is controlled by a bent portion  42   a  from the left and right side plates  32   a  and  32   b . When the photosensitive drum  1  is inserted, the torsion coil spring  40  is rotated in the counterclockwise direction contrary to its force, and positioned as shown in FIG. 3 when having passed the bearing  31  to press the bearing  31  by approximately 1 kgf power in the direction of arrow.  
         [0066]    On the other hand, the scanner unit  3  is formed to be longitudinally longer than the pitch between the left and right side plates  32   a  and  32   b , and a projection  42  is mounted to be projected outwardly from the second opening  35 . At that time, pressing the projection  42  against hatched abutment surfaces  45  and  46  in FIG. 5 positions the scanner unit  3 . For this reason, defining positional relationship of the abutment surfaces  45  and  46  with corresponding abutment surfaces  37  and  38  of the photosensitive drum  1  with accuracy can minimize tilt aberration of a printed image.  
         [0067]    As shown in FIG. 3, the scanner unit  3  is pressed by a compression spring  43  such that inclined plane  44  of the projection  42  is pressed 45° downwardly by approximately 1 kgf power. This ensures the projection  42  pressed against the abutment surfaces  45  and  46  to thereby position the scanner unit  3 .  
         [0068]    Similar pressing is carried out in the back side of the scanner unit  3 . The detail thereof will be described in FIG. 7.  
         [0069]    [0069]FIG. 7 is a view of the image forming apparatus body seen from its back side (opposite the inserting direction of the photosensitive drum  1 ), and as shown in this figure, the back stay  36  is formed with the third openings  37  ( 37   a  to  37   d ) at four places and the scanner unit  3  is mounted such that a projection  47  formed at the back side thereof is projected outwardly from the openings  37  ( 37   a  to  37   d ). At that time, pressing the projection  47  against the abutment surface  48  shown in FIG. 7 positions the back side of the scanner unit  3 . The back side of the scanner unit  3  is pressed by the compression spring  49 . The compression spring  49  is mounted such that one end thereof is mounted on the end of the third opening  37  and that the other end is mounted on the top surface of the projection  47  of the scanner unit  3 , which presses the scanner unit  3  by approximately 1 kgf power.  
         [0070]    In this way, the scanner unit  3  is supported at three points by the image forming apparatus body and positioned by an urge of the compression springs  43  and  49  without securing the screws. Therefore, the scanner unit  3  is not at all influenced by distortion of the body frame, and when the apparatus body is distorted because of being installed on an uneven floor, the scanner unit  3  can perform stably.  
         [0071]    Next, a method of manufacturing the left and right side plates  32   a  and  32   b  will be described.  
         [0072]    The left and right side plates  32   a  and  32   b  are required to have positional accuracy in the order of 10 μm so that influence of the temperature variations of a workpiece, a pressing mold and a pressing machine cannot be ignored. The temperature variation of each element is shown in FIG. 8. The workpiece is kept in a work place so that its temperature tends to conform to the environmental temperature. Namely, as shown by a curve  51  in FIG. 8, the temperature is the lowest in the morning and keeps rising in compliance with the environmental temperature so that a temperature difference between the morning and evening is about 4.5° C. As shown by a curve  52 , it is confirmed by an experiment that the temperature of the pressing mold keeps rising by friction between the workpiece and mold due to a continuous press work and that the temperature rises approximately 8° C. in 5 hours. In addition, as shown by a curve  53 , the temperature of the pressing machine varies by heat from its moving part and electric control part. The temperature variation of the pressing machine is the smallest because of its large heat capacity and the presence of lubricant.  
         [0073]    In this way, there is always temperature variation in each part and each variation has a different pattern. All three elements are made of iron so that they have the same coefficient of linear expansion (0.00001116 mm/° C. mm) but their sizes go wrong in the order of 10 μm.  
         [0074]    For example, in the state shown in FIG. 8 which is obtained by the experiment, the temperature of the mold is approximately 4° C. different from the temperature of the workpiece. In this case, the hole of 300 mm pitch is 13 μm (=300 mm×4° C.×0.00001116 (coefficient of linear expansion)) shorter. The temperature difference of 4° C. usually occurs in the press work and the temperature difference per se always varies between the morning and evening, and the summer and winter.  
         [0075]    Next, machining procedure of the left and right side plates  32   a  and  32   b  is shown in FIG. 9.  
         [0076]    First, an item after a previous machining step except machining of the first openings  34  (4 places) and second openings  35  (4 places) is stocked for the left side plate  32   a . Similar item is stocked for the right side plate  32   b . The left and right side plates  32   a  and  32   b  actually have various openings machined, which are omitted in FIG. 9. In FIG. 9, a broken lines show openings to be formed in later machining step.  
         [0077]    Both stocked items after previous machining steps are stocked at room temperature approximately a day in order to have their temperature even. Stocking the items near the pressing machine to be used in the later machining steps can minimize the temperature variation which occurs during their flow.  
         [0078]    Then, the press work of the first opening  34  and second opening  35  is carried for the item which temperature is made even (accurate machining of openings). Since the left and right side plates  32   a  and  32   b  are machined using the same mold, the left side plate  32   a  is set in the pressing machine with the bent surface up and the  32   b  is set with the bent surface down to be machined. The press work is always carried out alternately one by one such that one left side plate  32   a  is machined, then that one right side plate  32   b  is machined, and then that one left side plate  32   a  is machined.  
         [0079]    The left side item after previous machining step and the right side item after previous machining step have the same temperature, and the temperature of the pressing mold and pressing machine is the same as that of the workpiece at the beginning of machining. As shown in FIG. 8, the temperatures of the workpiece, pressing mold, and pressing machine, respectively vary in different manner.  
         [0080]    Whenever the material is processed, however, two items continuously machined have almost the same temperature relationship of each element (the workpiece, pressing mold and pressing machine). According to this, the pair of left and right side plates  32   a  and  32   b  continuously machined have aberration about tens of μm a basic size by temperature factors, but there is no difference in size between the left and right side plates  32   a  and  32   b  continuously machined. In this embodiment, the difference is minimized by alternately machining, while depending upon the required difference in size between the right and left, the difference in size between the right and left can be minimized more remarkably than in a conventional machined item by alternately machining the items every 10 to 100 pieces.  
         [0081]    As shown in FIG. 9, the left and right side plates  32   a  and  32   b  alternately machined are packed in a packing box  55 . The packing box  55  is comparted by partitions  56 , and the left and right side plates  32   a  and  32   b  are stored in sets to be put in products. The order of machining is controlled by giving serial numbers 66 to the left and right side plates  32   a  and  32   b  in the order of being machined. This facilitates control before inputting products and enables correct products to be input by checking the number. Means for giving the serial numbers 66 is such as imprinting and stamping.  
         [0082]    Namely, the most important object of alternate pressing and controlling in pair is to eliminate the difference in size by machining the left and right side plates  32   a  and  32   b  under the same condition of temperature relationship between each element.  
         [0083]    The feature of the present invention is that the direction of punching burrs is the same at opening portions because the opening portions in the left and right side plates  32   a  and  32   b  are punched by a common pressing mold. In this embodiment, the punching burr of the left side plate  32   a  is projected outwardly of the apparatus body and the punching burr of the right side plate  32   b  is projected inwardly. The right side plate  32   b  is likely to be touched by a user so that the step of punching the burr is necessary for protecting the user. Generally, for coping with the necessity at low cost, a method of cutting to the face in the pressing step and squashing the punching burr is used.  
         [0084]    However, the cutting to the face work is likely to lower the accuracy in size. For this reason, in this embodiment, a resin component is arranged near the punching burr without cutting to the face work so that the user cannot touch the punching burr.  
         [0085]    An end surface of the opening is the surface for positioning the photosensitive drum  1  and the scanner unit  3 . In order to increase the surfaces for positioning, a shaving work is effective in accurate machining of openings.  
         [0086]    In FIG. 10, the sections of the first opening  34  and second opening  35  are shown in order to describe the shaving work. As shown in FIG. 10, also in this embodiment, prepared openings  57 ,  58  are machined in a previous machining step, and the shaving work wherein punching stocks  59  and  60  are shaved is carried out in a later machining step. The thickness of the punching stocks  59  and  60  are preferably 5 to 100% of that of the plate. In this embodiment, the metal plate 1.2 mm thick is used and the punching stocks  59  and  60  are set approximately 100 μm (approximately 8%). According to this, a shearing face 20 to 40% of thickness generally obtained by punching becomes more than 70% and reduction of surface pressure applied to the positioning portion and increased strength against shock can be attained. Further, reduction of load applied to the pressing mold extends a die life.  
       Embodiment 2  
       [0087]    Next, the second embodiment of the present invention will be described with reference to FIG. 11. FIG. 11 is a perspective view of a body frame of a full color image forming apparatus according to the present embodiment. In the description of this embodiment, the same members and the like as in the first embodiment are referred to by the same reference numerals and their descriptions will be omitted.  
         [0088]    As shown in FIG. 11, a left side plate  61  and right side plate  62  are secured to the bottom plate  33  by screws in the same way as in the first embodiment. A positioning plate  63  is secured to the left and right side plates  61  and  62  by the screws from outside at six places. The positioning plate  63  is a flat plate made of a metal plate 1.2 mm thick and has first openings  64  ( 64   a  to  64   d ) for positioning the photosensitive drum  1  and second openings  65  ( 65   a  to  65   d ) for positioning the scanner unit  3 . The methods of positioning and pressing the photosensitive drum  1  and the scanner unit  3  are the same as that in the first embodiment, but both photosensitive drum  1  and scanner unit  3  have escape openings a, b, c, d opening on the left and right side plates  61  and  62  and which are larger than the second openings  65  ( 65   a  to  65   d ) so as to abut the positioning plate  63 .  
         [0089]    In this embodiment, the machining procedures of the left and right side plates  61  and  62  are not controlled and the problem of machining condition is tried to be solved by controlling components of the positioning plate  63 .  
         [0090]    Namely, the positioning plate  63  is controlled in pair in the order of being pressed and the pair is input in the product. According to this, the positions of the first opening  64  and second opening  65  are not influenced by the machining condition and the difference in the positional relationship is minimized. This results in ensuring parallelism of each photosensitive drum  1  and positional relationship of the scanner unit  3  and the photosensitive drum  1  with accuracy.  
         [0091]    As the same as the first embodiment, the shearing face can be obtained by shaving the first opening  64  and second opening  65 , which enables increase in strength of the positioning portion and reduction of load of the pressing mold.  
         [0092]    Further, in this embodiment, the positioning plate  63  is added and the cost of components is raised, but the components can be easily stored since the components to be stored is small and of a simple shape. In addition, the simple shape of the positioning  63  increases efficiency in pressing process, namely, enables press work with high production ability of the transfer and progressive die type.