Abstract:
A press forming process involves a stretching press to form a product with a curved portion from a workpiece by imparting a pad portion to the workpiece prior to starting the press forming process. The curved portion is prepared by spreading the pad portion mainly into the longitudinal direction of the workpiece, so that the curved portion is prepared so as to have a spring back of the product minimized.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a press forming process, more specifically, a press forming process involving stretching press to form a product with a curved portion from a workpiece by which a product with high dimensional accuracy can be easily produced.  
           [0003]    2. Discussion of the Related Art  
           [0004]    In recent years, there is a demand that many sorts of industrial machines including vehicles should have decreased weight and designed for safety. As a concrete measure to answer such demand, steel with high tensile strength is employed, which has excellent processing characteristics, weldability, and impact resistance in addition to the high tensile strength, whereby it has become possible to decrease weight of the product with maintaining the strength thereof.  
           [0005]    However, when steel with high tensile strength is employed as a material for obtaining a product therefrom such as a car frame, and is manufactured by a press forming involving stretching and/or bending, a so-called “spring back” occurs at a curved portion to a larger extent in comparison with commonly employed steel. Thus, it is still difficult to improve the dimensional accuracy of a product by the press forming.  
           [0006]    The reason why the “spring back” occurs is to be explained with referring to FIG. 6.  
           [0007]    [0007]FIG. 6 shows a frame  1  extended to include a curved portion, and having an approximate U-shaped cross-section formed with lateral walls  3  at both sides of a top face  2 . The curved portions  4  of the lateral walls  3  are obtained by a stretching press forming operation in order to prepare the frame  1  by the press forming. At this stage, the length of the frame I on an outer side of the bending is insufficient to retain a formed shape at the curved portion  4  due to a tensile force.  
           [0008]    The short length at the curved portion  4  causes a spring back of the product. In FIG. 6, the normal shape of the frame  1  shown by alternate long and two short dashed lines is deformed at the side under the tension to the direction shown by arrows X, whereby an undesirable product shown by a solid line is obtained. Consequently, the dimensional accuracy of the product is decreased.  
           [0009]    A new press forming process is proposed, for instance, as disclosed in Japanese Kokai Publication 7 (1995)-148527 wherein a product is neatly bent to prepare a corner by a part of a workpiece being bent once to have a corner with a depression, and then the corner with the depression is pressed again so as to have a sharp corner. In accordance with this method, the stress distribution on the bent portion can be well balanced in the plate thickness direction, so that the generation of the spring back phenomenon is suppressed.  
           [0010]    Furthermore, Japanese Kokai Publication 11 (1999)-226651 proposes a method wherein the shape of a die is prepared so as to partially differ from the shape of a product to be attained therefrom by estimating deformation of the product, e.g. spring back or warp, followed by performing press forming to give a product, thereby maintaining accuracy of the product.  
           [0011]    In the methods described in the above-mentioned Japanese Patent Kokai Publications 7 (1995)-148527 and 11 (1999)-226651, it is necessary to carry out press forming by the dies having shapes that are largely different from the products to be obtained. The deformation behavior of the products, however, does not always follow the estimation, so that it is necessary to repeatedly perform tests for determining the final shapes of dies, and many manufacturing steps are required also in the actual manufacture.  
         OBJECTS AND SUMMARY OF THE INVENTION  
         [0012]    It is therefore an object of the present invention to provide a press forming process involving a stretching press to form a product with a curved portion from a workpiece, wherein a product with high dimensional accuracy can be obtained by an occurrence of a spring back thereof being restrained without using many different dies by performing a step of providing a pad portion to the workpiece prior to starting the press forming process, the formation of the being completed by spreading the pad portion mainly in a longitudinal direction of the workpiece, whereby the curved portion is prepared so as to have a spring back of the product minimized.  
           [0013]    Another object of the present invention is to provide the above-mentioned press forming process involving a stretching press to form a product with a curved portion from a workpiece, by which the above-mentioned pad portion is easily created, by the pad portion being in a wave, a bead or a corrugation form in the above-mentioned press forming.  
           [0014]    A further object of the present invention is to provide the above-mentioned press forming process involving a stretching press to form a product with a curved portion from a workpiece, by which the occurrence of a spring back is effectively restrained, by the workpiece being made of a steel with a high tensile strength.  
           [0015]    A still further object of the present invention is to provide the above-mentioned press forming process involving a stretching press to form a product with a curved portion from a workpiece, by which press forming is smoothly carried out by preventing parts of the workpiece from catching or sticking to a die to be used therefor, by the pad portion having a shape which does not interfere with the die.  
           [0016]    A still further object of the present invention is to provide the above-mentioned press forming process involving a stretching press to form a product with a curved portion from a workpiece, by which the deformation of the product by the spring back is surely restrained by a portion of the workpiece to be stretched being substantially the same as that of a face of a plate holding member of a die used of the stretching press.  
           [0017]    A still further object of the present invention is to provide a product with a curved portion, which has high dimensional accuracy even at the curved portion without causing spring back, prepared by a press forming process from a workpiece, the press forming process comprising providing a pad portion to the workpiece prior to starting the press forming process, and completing formation of the curved portion by spreading of the pad portion mainly in the longitudinal direction of the workpiece, whereby the curved portion is prepared so as to have a spring back of the product minimized.  
           [0018]    A still further object of the present invention is to provide a workpiece appropriately used for the above-mentioned press forming process to obtain a product with a portion which has high dimensional accuracy even at the curved portion without causing spring back, by the provision of a pad portion at the a portion of the workpiece. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    A more complete appreciation of the invention and many advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:  
         [0020]    [0020]FIG. 1 is a perspective view of a product obtained by the press forming process according to the present invention;  
         [0021]    [0021]FIG. 2 is a perspective view of a product being pressed by means of a die by the aid of plate holding members;  
         [0022]    [0022]FIG. 3 is a perspective view of a workpiece for explaining an embodiment of a press forming process according to the present invention;  
         [0023]    [0023]FIG. 4 is a cross-section of a part of the product shown in FIG. 1 cut along I-I,  
         [0024]    [0024]FIG. 5 is a cross-section of the part shown in FIG. 4 cut along II; and  
         [0025]    [0025]FIG. 6 is a perspective view of a product prepared by a known press forming process for explaining a spring back phenomenon.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]    In accordance with the press forming process of the present invention, the pad portion is prepared to absorb deformation which will occur, by a material added for producing the pad portion being stretched evenly to parts of the curved portion which is otherwise would be completed under high tension, so that, in the present invention, spring back caused by such high tension can be minimized. A primary die used for the present invention has a simple shape only with the pad portion. Therefore, it has been possible to obtain a product by press forming using a die which is roughly in the same shape as that of the workpiece. This makes it easy to determine the shape of the die, thereby saving the cost and time for manufacturing the die, and shortening the operating time.  
         [0027]    Other feature of this invention will become understood in the course of the following description of embodiments, which are given for illustration of the invention and are not intended to be limiting thereof.  
         [0028]    In the present invention, a frame  1  as shown in the perspective view of FIG. 1 is obtained by a press forming process. The frame extends with a curved portion for instance as indicated by a reference numeral  4 , having an approximate U-shaped cross-section formed with a top face  2  and lateral walls  3  provided at both sides of the top face  2 . The press forming process for obtaining such a product will now be explained.  
         [0029]    The process of the invention was attained for minimizing the spring back at the curved portion  4 , which has occurred in the product produced by known methods. Such a spring back phenomenon occurs after the product with the curved portion under tension is prepared. The tensioned part of the product cannot maintain its shape and tends to inappropriately bend.  
         [0030]    The product  1  is produced by a die composed of a female die part  11  and a male die part  12  with plate holding members  10  as shown in FIG. 2. Because of the aforementioned reason, a short length at the curved portion  4  tends to appear, with respect to the corresponding length of the plate holding members  10 . Therefore, a workpiece of the present invention has a primary curved portion with a pad portion as will be described in detail below, which is for preparing a product  1  including a curve part with enough length to minimize the occurrence of the spring back.  
         [0031]    In an embodiment of the invention, drawing is carried out to obtain a primary formed article (i.e. a workpiece) with a pad portion  6  for the compensation of the short length to be appeared on a final product. The pad portion  6  is obtained as illustrated in FIG. 3 by drawing by a primary die for the primary formed article  1   a , which pad portion  6  has a plurality of concave portions  6   a  with space therebetween, each extends almost vertically at the primary curved portion  4   a  crossing with the running direction of the primary formed article  1   a . By the provision of a plurality of concave portions  6   a , the primary curved portion  4   a  has a cross-section in a wave shape as shown in the partially broken view. The product  1  to be obtained after the secondary press forming is shown by an alternate long and two short dashes line.  
         [0032]    Subsequently, the thus formed primary formed article  1   a  is subjected to press forming by a secondary die having a shape that is substantially the same as that of a product to be obtained. In the press forming, the primary curved portion  4   a  is subjected to a stretching press of the workpiece.  
         [0033]    The pad portion  6  is pressed out in the direction shown by an arrow F while maintaining a continuous shape to a primary formed lateral wall  3   a , whereby the primary curved portion  4   a  can be smoothly stretched, that is, without any irregularity, along the face of the above-mentioned plate holding member employed with the secondary die. As a result, the internal stress caused in the curved portion  4  is minimized, whereby the deformation of the obtained formed product by spring back can be satisfactorily eliminated. Finally, a frame  1  as shown in FIG. 1 is obtained with high dimensional accuracy.  
         [0034]    [0034]FIG. 4 shows a cross-section of the frame  1  as a product shown in FIG. 1 cut along I-I for comparing the shape with respect to a cross-section of the primarily formed article  1   a  shown in FIG. 3.  
         [0035]    Furthermore, FIG. 5 is a cross-section obtained by cutting the cross-section of FIG. 4 along II-II and seen from the top. The primary formed article  1   a  of which primary formed lateral wall  3   a  should be inwardly configured from the lateral wall  3  of the product  1  as shown in FIGS. 4 and 5. Since the primary formed article  1   a  in the present invention is prepared without having part protruded outwardly beyond the product  1  or the die, it is possible to avoid interference between the primary formed article  1   a  and the secondary die when the press forming is performed. Thus, the press forming is smoothly carried out.  
         [0036]    As described above, the primary formed article  1   a  is obtained in a simple shape having a plurality of concave portions  6   a  at the primary curved portion  4   a  by the die having a shape that is very close to that of the product. According to the present invention, it is possible to easily determine the shape of the die to be used since no deformation substantially occurs on the final product. Therefore, the cost for manufacturing the die is saved, the time period for manufacturing the die is shortened, and in addition, the forming operating time of minimized.  
         [0037]    Furthermore, it is preferable that the edge length and shape of the primary curved portion be made to be approximately the same as that of the face of the plate holding member. The deformation caused by the spring back can be completely eliminated by making the primary curved portion correspond to the face of the plate holding member.  
         [0038]    In the above embodiment, it is also possible to prepare the pad portion  6  in a bead or corrugation shape or the like instead of the above-mentioned wave shape.  
         [0039]    The present invention has been explained based on the embodiment for manufacturing a frame for a vehicle by press forming. In the alternative, the method of the invention can be applied for the manufacture of any other products obtained by press forming wherein a stretching press operation is carried out.  
         [0040]    As is understood from the above, the present invention provides a press forming process by which the product with high dimensional accuracy can easily be obtained, by the preparation of the pad portion at the part where a stretching press is carried out. By the process of the invention, the deformation of the product by spring back can be eliminated. In the present invention, it is not necessary to prepare a die which has a shape that is largely different from the product to be obtained. Additionally, it is possible to press the primary formed article by the die which has a similar shape to that of the product. The shape of the die employed for the press forming can be readily determined under the pre-assumption of a deformation behavior of the workpiece. Hence, it is possible not only to save the manufacturing cost of the dies and the preparation time period thereof, but also to shorten the operating time.  
         [0041]    The invention being thus described, it will be understood 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 modification as would be understood to one skilled in the art are intended to be included within the scope of the following claims.