Patent Publication Number: US-7900493-B2

Title: Closed forging die and forging method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a closed forging die and a forging method. 
     2. Description of the Related Art 
     In manufacturing a product having a boss portion radially provided with shaft portions, such as a trunnion for a constant velocity joint or a cross spider for a universal joint, by closed forging, a closed forging die is used. 
     As shown in  FIG. 8 , the closed forging die includes openable dies  1  and  2 , and punches  4  and  5  arranged so as to be capable of being driven along center axes of the dies  1  and  2 , respectively. That is, by pressurizing a material therein by the punches  4  and  5  while the dies  1  and  2  are in a closed state, a cavity  9  corresponding to configurations of shaft portions  7  and a boss portion  8  of a product  6  is molded. Thus, as shown in  FIG. 9A , in a case where, after introducing a billet (material)  10  (refer to  FIG. 9A ) in the dies  1  and  2 , clamping is performed and then the billet  10  is pressurized by the punches  4  and  5 , the billet  10  plastically deforms, to thereby configure the product  6  including the boss portion  8  and the shaft portions  7  as shown in  FIG. 9B . 
     That is, as shown in  FIG. 9A , in a case where the cylindrical billet  10  having a radius of curvature R 2  is introduced in the die and forged, the product  6  having the shaft portions  7  each of whose tip portion  7   a  has a radius of curvature R 2 ′ larger than the radius of curvature R 2  can be molded. 
     Incidentally, in a case where a sealed state is established in the closed forging die, a processing load drastically increases, which leads to a fear in that the die may be damaged or short-lived. Thus, a related art describes that a length of each shaft molding portion is set larger than a required length of each shaft portion, to thereby provide a clearance to each shaft tip portion (JP 2003-343592 A). 
     In the related art die formed with the clearance portion in each shaft molding portion, when the billet is pressurized by the punches, the material is extruded to mold the shaft portions. At a tip surface of each shaft portion, a center portion of the extruded material readily flows and a peripheral portion thereof does not readily flow. Thus, as shown in  FIG. 4 , a shaft portion having a tip surface having a radius of curvature R 1 ′ smaller than a radius of curvature R 1  of the tip surface of a normal shaft portion is molded. In this manner, in the conventional die formed with a clearance in each shaft molding portion, “sagging” occurs by which a circumferential surface side is retracted toward a base end portion side of the shaft portion in an axial direction thereof compared to the tip portion of the normal shaft portion. 
     Thus, in a case of securing the length of the shaft portion accurately molded using the die, the material is additionally required by an amount corresponding to the “sagging.” Incidentally, the forged product molded by using the closed forging die is included in an inner joint member of a constant velocity joint or a universal joint. Thus, in order to make the constant velocity joint or the universal joint employing the product compact and lightweight, it is necessary to machine a tip of the shaft portion to be removed. 
     In addition, in order to extend a lifetime of the constant velocity joint or the universal joint including the product incorporated therein and to suppress vibration and noise in use, it is necessary that, after increasing strength and hardness of the product by heat treatment, the shaft-portion outer circumferential surface of the product be molded higher in accuracy than that molded by the forging. Thus, it is necessary to finish the product by machining after the heat treatment. The shaft tip may be removed by the machining prior to the heat treatment in order to facilitate the machining after the heat treatment, and a coupling surface of the removed surface and the shaft-portion outer circumferential surface may be used as a reference for phase matching in the case where the shaft-portion outer circumferential surface is subjected to highly-accurate machining. Thus, the coupling portion is required to be formed with high accuracy. 
     SUMMARY OF THE INVENTION 
     In view of the above problems, it is an object of the present invention to provide a closed forging die and a forging method with which sagging can be reduced, which can make a constant velocity joint or a universal joint compact and lightweight, which do not require a shaft tip to be removed by machining prior to heat treatment, and which can reduce material costs and machining costs. 
     According to the present invention, there is provided a closed forging die for molding a product having shaft portions radially formed, the closed forging die including: dies which are openable; and punches, which move in an opening/closing direction of the dies to pressurize a material in the dies, in which: a clearance is provided to a tip surface of each of the shaft portions molded; and the dies are each provided with abutting portions abutting against at least a tip side of an outer circumferential surface of each of the shaft portions. 
     According to the closed forging die of the present invention, during the pressurization by the punch, the material abuts against the abutting portions, so the partial or entire configuration of the outer circumference of each shaft tip is secured by the dies. The portion thus secured can be used as a referential surface for phase matching in a case where the shaft-portion outer circumferential surface is subjected to highly-accurate machining. 
     According to the present invention, there is provided a forging method of molding a product having shaft portions radially formed, by using a closed forging die including dies which are openable and punches, which move in an opening/closing direction of the dies to pressurize a material in the dies, the forging method including molding the material to be introduced in the closed forging die such that a radius of curvature of a surface of the material, which is to be molded into a tip surface of each of the shaft portions, is larger than a radius of curvature of the tip surface of each of the shaft portions to be molded. 
     According to the forging method of the present invention, in the material to be introduced in the closed forging die, the radius of curvature of the surface, which is to be molded into the tip surface of each of the shaft portions, is larger than the radius of curvature of the tip surface of each of the shaft portions to be molded. Thus, in the process of molding the product configuration using the closed forging die (referred to as principal molding), even when the peripheral portion of the tip surface of the shaft portion less easily flows than the center portion thereof, “sagging” (an amount by which a circumferential surface side is retracted toward a base end portion side of the shaft portion in an axial direction thereof) can be reduced. In other words, prior to the principal molding, there is performed a preliminary molding process of molding the material such that the radius of curvature of the portion to be molded into the tip surface of each of the shaft portions is larger than the radius of curvature of the tip surface of each of the shaft portions to be molded. In the case of molding by using the closed forging die a product from the material which has been subjected to the preliminary molding process, even though a clearance is formed in the closed forging die, the “sagging” in the shaft portion can be reduced. 
     In the closed forging die according to the present invention, since the portion secured by the dies can be used as a referential surface for phase matching in the case of the highly-accurate machining, the shaft tip is not necessarily to be removed by machining in order to form a referential surface (referential portion) prior to heat treatment, to thereby reduce material costs and machining costs. 
     According to the present invention, the “sagging” can be reduced in the shaft portion, and thus a constant velocity joint or a universal joint employing the forged product can be made compact and lightweight. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a cross-sectional view showing a closed forging die according to an embodiment of the present invention; 
         FIG. 2  is a cross-sectional view showing the closed forging die viewed from a direction other than a direction from which  FIG. 1  is viewed; 
         FIG. 3  is a cross-sectional plan view showing the closed forging die; 
         FIG. 4  is an enlarged cross-sectional view showing a main portion of a product molded by using the closed forging die; 
         FIG. 5  is a cross-sectional view showing a mold apparatus used in preliminary molding; 
         FIG. 6  is a cross-sectional plan view showing the mold apparatus; 
         FIGS. 7A to 7D  are diagrams for explaining processes of the preliminary molding; 
         FIG. 8  is a cross-sectional view showing a conventional closed forging die; and 
         FIGS. 9A and 9B  are diagrams for explaining conventional forging method. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Hereinafter, an embodiment of the present invention will be described with reference to  FIGS. 1 to 7 . 
       FIG. 1  shows a closed forging die according to an embodiment of the present invention. The closed forging die includes openable dies  11  and  12  and punches  14  and  15  which is driving along an opening/closing direction of the dies  11  and  12  to pressurize a material in the dies  11  and  12 , respectively, and molds a product (e.g., trunnion for a constant velocity universal joint)  16  which is radially formed with shaft portions  17 . Note that the product  16  being the trunnion includes a boss portion  18  and three shaft portions  17  externally extending from the boss portion  18  in a diameter direction thereof. 
     Accordingly, the dies  11  and  12  are provided with guide holes  21   a  and  21   b  at axial center portions thereof, respectively. In the guide holes  21   a  and  21   b , the punches  14  and  15  are fit-inserted, respectively. Further, in opening portions of the guide holes  21   a  and  21   b  on contact surfaces  11   a  and  12   a  side of the dies  11  and  12 , three concave portions  22  and three concave portions  23  extending in the diameter direction of the guide holes  21   a  and  21   b  are arranged at pitches of 120°, respectively. 
     In a state where the dies  11  and  12  are superimposed with each other as shown in  FIG. 1 , voids  24  for forming the shaft portions  17  of the product  16  are formed by the opposing concave portions  22  and  23 . In this case, in each void  24 , abutting portions  25  are provided in the outer side of the diameter direction so as to swell toward the inside of the void  24 , and the abutting portions  25  abut on the molded shaft portion  17  at a tip portion side of an outer circumferential surface of the shaft portion  17 . In addition, a gap (clearance)  26  is formed at a tip portion  17   a  of the shaft portion  17  to be molded. 
     Further, on a lower surface  14   a  of the upper punch  14 , a swelling portion  27  is provided at a center portion thereof, and on a lower surface  15   a  of the lower punch  15 , a swelling portion  28  is provided at a center portion thereof. 
     Next, a forging method using the die shown in  FIGS. 1 to 3  will be described. First, the upper die  11  and the lower die  12  are relatively spaced apart from each other to establish a die-opened state. In this case, the upper punch  14  is raised and the lower punch  15  is lowered. In this state, in the guide hole  21   b  of the lower die  12 , a billet (material)  20  (refer to  FIG. 5 ) is introduced. Note that the billet  20  can be fit-inserted into the guide holes  21   a  and  21   b , and corresponds to a volume of a product to be molded. 
     After that, clamping is performed so as to make the upper die  11  and the lower die  12  relatively close to each other. Next, the upper punch  14  is lowered and the lower punch  15  is raised. Thus, the billet  20  is vertically pressurized so that the voids  24  for forming the shaft portions  17  are formed. The billet  20  is caused to partially flow into the voids  24 , to thereby mold the product  16  (tripod member) including the three shaft portions  17  radially extending from the circumference of the boss portion  18 . 
     In this case, during the pressurization by the punches  14  and  15 , the material abuts against the abutting portions  25 , so the partial or entire configuration of the outer circumference of each shaft tip is secured by the dies. Secured portions  40  (refer to  FIG. 4  etc.) thus formed can serve as referential surfaces (referential portions) for phase matching in a case where the outer circumferential surface of the shaft portion is subjected to highly-accurate machining. In addition, since the clearance  26  is formed at the tip surface  17   a  of the molded shaft portion  17 , a surface pressure load with respect to the die can be reduced, to thereby prevent the die from being damaged. 
     Further, as shown in  FIG. 7C , a material  20 A has a configuration in which a radius of curvature R 1  of each surface  30 , which is to be molded into the tip surface  17   a  (refer to  FIG. 4 ) of the shaft portion  17 , is made larger than a radius of curvature R 1 ′ (refer to  FIG. 4 ) of the tip surface  17   a  of the shaft portion  17  to be molded. 
     The material  20 A is manufactured by using a mold apparatus  31  shown in  FIGS. 5 and 6 . The mold apparatus  31  includes a preliminary molding die  32 , and a preliminary molding punch  33  and an ejector  34  which are fit-inserted into a hole portion  32   a  of the preliminary molding die  32 . 
     The hole portion  32   a  of the preliminary molding die  32  is a hexagonal hole whose cross-sectional configuration is as shown in  FIG. 6 . In this case, the hole portion  32   a  is formed with three surfaces  37  each having the radius of curvature R 1  same as the radius of curvature R 1  of each surface  30  of the material  20 A. That is, the surfaces  37  having the radius of curvature R 1  are provided at pitches of 120°, and surfaces  37   a  each having a radius of curvature smaller than the radius of curvature R 1  of each surface  37  are provided between the adjacent surfaces  37 . 
     Further, a swelling portion  35  is formed at a center portion of a lower surface  33   a  of the preliminary molding punch  33 , and a swelling portion  36  is formed at a center portion of an upper surface  34   a  of the ejector  34 . The swelling portion  35  of the preliminary molding punch  33  has the same diameter and configuration as those of the swelling portion  27  of the upper punch  14 , and the swelling portion  36  at the center portion of the upper surface  34   a  of the ejector  34  has the same diameter and configuration as those of the swelling portion  28  of the lower punch  15 . Note that a reinforcing member (reinforcing ring; not shown) is externally fitted in the preliminary molding die  32  by press fitting or shrink fitting. 
     Subsequently, a molding method of the material  20 A by using the mold apparatus  31  will be described. First, as shown in  FIG. 7A , a disk-like billet  20 B having a radius of curvature R 2  of outer-circumferential-surface is introduced in the mold apparatus  31  in a opened state. In this case, the opened state refers to a state where the preliminary molding punch  33  is raised, which allows the billet  20 B to be introduced in the hole portion  32   a  of the preliminary molding die  32 . Alternatively, although not shown in the drawings, an outer circumferential surface of the billet  20 B may be subjected to ironing, to thereby eventually obtain the material  20 A. 
     At this time, the radius of curvature R 2  of outer-circumferential-surface of the billet  20 B is set smaller than the radius of curvature R 1  of each surface  37  of the hole portion  32   a . Further, the billet  20 B is inserted into the hole portion  32   a  while maintaining a gap of φ0.005 to φ0.3. Alternatively, in the case where the circumferential surface of the billet  20 B is formed by ironing, there is provided a guide portion which allows the billet  20 B to be inserted into the billet-introducing side of the preliminary molding die  32  while maintaining the above-mentioned gap. 
     In this state, the preliminary molding punch  33  is lowered, and the billet  20 B is pressurized by the preliminary molding punch  33  and the ejector  34 . As a result, the billet  20 B plastically deforms so as to fill a cavity  38  defined by the hole portion  32   a  of the preliminary molding die  32 , the preliminary molding punch  33 , and the ejector  34 , whereby the material  20 A as shown in  FIG. 7B  is molded. That is, the material  20 A having the three surfaces  30  each having the radius of curvature R 1  can be molded. Note that the surfaces  30   a  each having the radius of curvature corresponding to that of each surface  37   a  of the mold apparatus  31  is molded between the adjacent surfaces  30  each having the radius of curvature R 1 . 
     After that, as shown in  FIG. 7C , the material  20 A is introduced in the closed forging die. Subsequently, as described above, the dies  11  and  12  are subjected to clamping, and then the material  20 A is pressurized by the punches  14  and  15 . As a result, as shown in  FIG. 7D , a product in which the boss portion  18  protrudingly provided with the shaft portions  17  can be molded. At this time, the three surfaces  30  of the material  20 A are extruded into the voids (cavities)  24 , to thereby mold the tip surfaces  17   a  of the shaft portions  17 . 
     As described above, in the case of using the mold apparatus  31 , prior to the process of molding the product configuration (referred to as principal molding), there is performed a preliminary molding process of molding the material  20 A having the radius of curvature R 1  of each portion to be molded into the tip surface  17   a  of the shaft portion  17  in the principal molding larger than the radius of curvature of the tip surface  17   a  of the shaft portion  17  to be molded. In the principal molding, the peripheral portion of the portion to be molded into the tip surface  17   a  of the shaft portion  17  less easily flows than the center portion thereof. However, owing to the provision of the preliminary molding process, as shown in  FIG. 4 , “sagging” can be reduced even though the clearances  26  are provided in the closed forging die. In other words, each surface  30  of the billet  20 B has the radius of curvature R 1  while the tip surface  17   a  of the molded shaft portion  17  has the radius of curvature R 1 ′, i.e., the “sagging” is reduced. The reduction of the “sagging” of the tip portion  17   a  of each shaft portion  17  allows a constant velocity joint or a universal joint employing the forging product to be made compact and lightweight. 
     The embodiment of the present invention has been described in the above. However, the present invention is not limited to the embodiment but can be diversely modified. For example, each abutting portion  25  may be formed over the entire circumference of the void  24 , while in the closed forging die according to the embodiment, the plurality of abutting portions  25  are arranged at predetermined pitches in the circumferential direction. In addition, the sectional configuration and the size of the abutting portions  25  can be arbitrarily changed as long as the outer circumferential configuration of each shaft tip is secured by the dies  11  and  12 , and as long as each secured portion  40  thus molded can serve as the referential surface in the highly-accurate machining. 
     Further, in the closed forging die shown in  FIG. 1 , the configuration of the swelling portion  27  of the upper punch  14  is different from that of the swelling portion  28  of the upper punch  15 , but they may be the same with each other. Also in this case, in the mold apparatus  31  shown in  FIGS. 5 and 6 , the configurations of the swelling portions  35  and  36  of the preliminary molding punch  33  and the ejector  34  are required to be the same as those of the swelling portions  27  and  28  of the upper and lower punches  14  and  15  shown in  FIG. 1 , respectively. 
     Further, in the case of performing the preliminary molding process as shown in  FIGS. 7A to 7D , the closed forging die may not be provided with the abutting portions  25 , since the preliminary molding process enables reducing the “sagging” in the tip portions  17   a  of the shaft portions  17 , to thereby make a constant velocity joint or the like compact and lightweight. Note that the surface  30   a  may have the radius of curvature R 1 , while in the material  20 A of the embodiment, the three surfaces  30  each having the radius of curvature R 1  are arranged at pitches of 120° in the circumferential direction, and the radius of curvature of each surface  30   a  between the adjacent surfaces  30  has the radius of curvature different from the radius of curvature R 1 . In other words, all the six surfaces may each have the radius of curvature R 1 . In the case where every surface has the radius of curvature R 1  as described above, positioning of the material  20 A with respect to the closed forging die is readily performed when introducing the material  20 A in the closed forging die, which is advantageous. 
     Example 
     A state of “sagging” in the case of performing the preliminary molding as shown in  FIGS. 7A to 7D  was compared to a state of “sagging” in a case of not performing the preliminary molding. Table 1 shows the result. In Table 1, “billet radius of curvature R 2 ” represents the radius of curvature of the material  20 B before the preliminary molding (i.e., radius of curvature of conventional material  10  shown in  FIG. 9 ), “premolding radius of curvature R 1 ” represents the radius of curvature of the preliminary-molded material  20 A, “shaft end radius of curvature R 3 ” represents the radius of curvature of the tip surface of the molded shaft portion  17 , and “sagging” represents a difference between an outermost apex of the tip surface of the molded shaft portion  17  and an outer circumferential rim thereof 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Premolding 
                 Premolding not 
               
               
                   
                 performed 
                 performed 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Billet radius of curvature 
                 16.0 
                 16.0 
               
               
                   
                 R2 
               
               
                   
                 Premolding radius of 
                 47.8 
                 — 
               
               
                   
                 curvature R1 
               
               
                   
                 Shaft end radius of 
                 30.5 
                 22.1 
               
               
                   
                 curvature R3 
               
               
                   
                 Sagging 
                 1.4 
                 2.1 
               
               
                   
                   
               
            
           
         
       
     
     As apparent from Table 1, in the case of inserting and processing the material  20 A in the principal-molding die without performing the premolding, the amount of sagging was 2.1 mm, while in the case of performing processing in the principal-molding die after performing the premolding, the amount of sagging was 1.4 mm, i.e., the sagging was reduced.