Abstract:
A hollow stepped article is formed from a solid blank to reduce the material cost, and cracking is prevented in a stepped portion of large diameter when a portion of the blank is de-formed by its radial expansion. A hollow, stepped shaft is formed by holding an upper and a lower part axially of a solid rod-like blank with an upper and a lower die, respectively, which have a stepped recess of large diameter in a region where they are op-posed to each other; compressing the blank from both its axially opposite sides with an upper and a lower punch each of which is smaller in diameter than the blank, thereby extruding the blank so that an axial hollow is formed therein about its axis in each of its upper and lower parts and that a portion of the blank op-posed to the stepped recess of large diameter expands in diameter and deforms into that recess while leaving a solid plug-like portion between the punches; and thereafter further compressively moving one of the punches to shear the solid plug-like portion and force it out of the blank, whereby the blank is formed with a stepped portion of large diameter by radially expanding deformation in a region intermediate between its op-posed ends or at one of these ends and with a continuous axial hollow about its axis.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a Divisional of application Ser. No. 11/605,568 filed Nov. 29, 2006, which is a Divisional of application Ser. No. 10/803,231, filed Mar. 17, 2004 (now U.S. Pat. No. 7,171,837) which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a hollow stepped shaft which is formed in a region intermediate between its opposite ends or at one of its ends with a stepped portion larger in diameter than its axial portions and which is hollowed about its axis over its entire axial length or except for a portion thereof. The invention relates, inter alia, to a method of forming such a hollow stepped shaft and to a form or product made thereby. 
         [0004]    2. Description of the Prior Art 
         [0005]    A hollow shaft of this type has so far been formed by a method as described JP 2001-334317 A which uses a hollow tube as its starting blank material. The hollow tube is filled with a filler of a low melting point material and then loaded in an open die or a closed die in which the hollow tube together with the filler is compressed from both its axially opposite sides to cause its medial region to expand in diameter and to deform into an annular recess provided in the die. 
         [0006]    The unit cost of a tubular material as the blank amounts in weight unit cost to three to five times higher than that of a solid material (rod stock), however. For this reason, the conventional method using a tubular material as its starting blank has the problem that the material cost is high. 
         [0007]    Also, the axial compression of a blank that is already hollow to form a radial expansion as shown in  FIG. 22A  gives rise to the problem that a further axial compression of the radial expansion to increase its thickness causes a part of its inside to be bent and folded axially as shown in  FIG. 22B  and creates cracks in the grain flows which may become a critical internal defect. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    Made to solve the problems mentioned above, the pre-sent invention has for its object to provide a hollow stepped shaft and a method of forming it whereby material cost is made much lower than in the prior art and a critical internal defect due to cracking in a region of radial expansion can be prevented. 
         [0009]    In order to achieve the object mentioned above, there is provided in accordance with the present invention in a first form of implementation thereof a method of forming a hollow stepped shaft, characterized in that it comprises the steps of: holding an upper and a lower part axially of a solid rod-like blank with an upper and a lower die, respectively, which have a stepped recess of large diameter in a region thereof where they are opposed to each other; compressing the blank from both its axially opposite sides with an upper and a lower punch each of which is smaller in diameter than the blank and at least one of which is moving, thereby extruding the blank so that an axial hollow is formed therein about its axis in each of the upper and lower parts and that a portion of the blank opposed to the stepped recess of large diameter expands in diameter and deforms into the recess while leaving a solid plug-like portion between the punches; and thereafter further compressively moving one of the punches to shear the solid plug-like portion and force it out of the blank, whereby the blank is formed with a stepped portion of large diameter by radially expanding deformation in a region intermediate between its opposed ends or at one of these ends and with a continuous axial hollow about its axis, thereby forming a hollow stepped shaft. 
         [0010]    In the forming method mentioned above, the solid rod-like blank is loaded into the upper and lower dies which are in a closed die-fastened state and thereafter extrusion of the blank may be performed with the punches. Alternatively, the solid rod-like blank is loaded into the upper and lower dies which are in an open die-unfastened state and thereafter extrusion of the blank may be performed with the punches while the dies are being closed and fastened. 
         [0011]    The method mentioned above may further comprise the step wherein a hollow stepped shaft so formed as aforesaid is further formed in another die set to impart an additional outer contour thereto. Also in the forming method mentioned above, in the further step the additional outer contour may be imparted to the hollow stepped shaft with a mandrel inserted therein. 
         [0012]    The present invention also provides in a second form of implementation thereof a method of forming a hollow stepped shaft, characterized in that it comprises the steps of: supporting a solid rod-like blank at its first end with a bearer while its outer periphery is bound and extruding the blank about its axis from its second end with a first punch so as to form an axial hollow therein about the axis; and extruding the hollow blank forwards to backwards with a second and a third punch so as to form the hollow blank in a region thereof intermediate between the first and second ends or at one of these ends with a stepped portion enlarged in both diameter and thickness while simultaneously making the blank longer. 
         [0013]    In the forming method mentioned above, the blank may be extruded about its axis with the first punch to form the axial hollow while the bearer supporting the blank at the first end is resiliently supported by a hydraulic or pneumatic means. Alternatively, the blank may be extruded about its axis to form the axial hollow by rapidly advancing the first punch while the bearer supporting the blank at its first end is allowed to move back slowly by a servo mechanism. 
         [0014]    The present invention further provides in a third form of implementation thereof a method of forming a hollow stepped shaft, characterized in that it comprises the steps of: extruding a solid rod-like blank with its outer periphery bound, from its opposite sides about its axis with a first and a second punch so as to form a pair of axial hollows in its two axial parts, respectively, while leaving a solid plug-like portion of the blank between these two hollows; compressively moving one of the punches to shear the solid plug-like portion out of the blank whereby a single continuous axial hollow is formed from the axial hollows; and extruding the hollow blank forwards and backwards with a further punch so as to form the hollow blank in a region thereof intermediate between its opposite ends or at one of these ends with a stepped portion enlarged in both diameter and thickness while simultaneously making the blank longer. 
         [0015]    In the forming method mentioned above, the solid plug-like portion may be sheared out of the blank by one of the first and second punches after the other punch is extracted and while the blank is supported resiliently at one of its ends by a hydraulic or pneumatic means. Alternatively, the solid plug-like portion may be sheared out of the blank by extracting one of the first and second punches and thereafter rapidly advancing the other punch while one end of the blank is moved back slowly by a servo mechanism. 
         [0016]    In the forming method mentioned above, the solid rod-like blank may be made of carbon steel and may be hollowed at a rate of reduction in area of 25%. Then, the depth of the axial hollow in the blank may be set at a value that is 5 times or more larger than the inner diameter which is a criterion of stable working in a cold forging and its boring regions may be heated at a temperature ranging between a room temperature and 700° C. 
         [0017]    In the forming method mentioned above, the hollow stepped shaft may have those regions in axial portions where serrations are formed having a tooth form applied thereto by fitting or press-and-shrink fitting, which may be further drawn or made smaller in diameter by multistage pressure forming with upper punches and lower dies. 
         [0018]    According to the forming methods mentioned above in which a hollow stepped tube is formed from a solid blank such as a round rod as its starting material, the material cost can be sharply reduced compared with the conventional methods in which the starting material is a tubular blank. Further, since a solid blank is extruded with a punch or punches whereby an axial hollow is formed in the blank while a portion thereof in a medial area thereof is deformed so as to expand radially to form a stepped portion of large diameter, nothing is the case here that grain lines in the part deformed and enlarged in diameter may be axially folded and buckled as in the prior art. Thus, rather than broken in such a stepped portion as in the prior art, here the grain flows are streamlined and there can develop no defect such as cracking. 
         [0019]    The present invention also provides a hollow stepped shaft made by any one of the preceding methods. 
         [0020]    Since this hollow stepped tube has the hollow which except for the stepped portion of large diameter is shaped to conform in diameter to the outer contour and in other words having the axial portions uniformly thinned over their lengths, it is much lighter in weight than those made by cutting as in the prior art, namely in which the hollow is even in diameter and which thus must have been large in thickness. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    These and other objects, features and advantages of the present invention as well as other manners of its implementation will become more readily apparent, and the invention itself will also be better understood, from the following detailed description when taken with reference to the drawings attached hereto showing certain illustrative forms of implementation of the present invention. In the drawings: 
           [0022]      FIG. 1  is a cross sectional view illustrating a first step in a first process in a first embodiment of the present invention; 
           [0023]      FIG. 2  is a cross sectional view illustrating a second step in the first process in the first embodiment of the present invention; 
           [0024]      FIG. 3  is a cross sectional view illustrating a third step in the first process in the first embodiment of the present invention; 
           [0025]      FIG. 4  is a cross sectional view illustrating a second process in the first embodiment of the present invention; 
           [0026]      FIG. 5  is a cross sectional view illustrating a hollow stepped shaft formed by the first embodiment of the present invention; 
           [0027]      FIG. 6  is a cross sectional view illustrating an alternative second process in the first embodiment of the present invention; 
           [0028]      FIG. 7  is a cross sectional view illustrating another alternative second process in the first embodiment of the present invention; 
           [0029]      FIG. 8  is a cross sectional view illustrating a first step in a second embodiment of the present invention; 
           [0030]      FIG. 9  is a cross sectional view illustrating a second step in the second embodiment of the present invention; 
           [0031]      FIG. 10  is a cross sectional view illustrating a third step in the second embodiment of the present invention; 
           [0032]      FIG. 11  is a cross sectional view illustrating a fourth step in the second embodiment of the present invention; 
           [0033]      FIG. 12  is a cross sectional view illustrating a hollow stepped shaft formed by the second embodiment of the present invention; 
           [0034]      FIG. 13  is a cross sectional view illustrating a first step in a first process in a third embodiment of the present invention; 
           [0035]      FIG. 14  is a cross sectional view illustrating a second step in the first process in the third embodiment of the present invention; 
           [0036]      FIG. 15  is a cross sectional view illustrating a second process in the third embodiment of the present invention; 
           [0037]      FIG. 16  is a cross sectional view illustrating a third process in the third embodiment of the present invention; 
           [0038]      FIG. 17  is a cross sectional view illustrating a hollow stepped shaft formed by the third embodiment of the present invention; 
           [0039]      FIG. 18  is a cross sectional view illustrating a first step in a first process in a fourth embodiment of the present invention; 
           [0040]      FIG. 19  is a cross sectional view illustrating a second step in the first process in the fourth embodiment of the present invention; 
           [0041]      FIG. 20  is a cross sectional view illustrating a third step in the first process in the fourth embodiment of the present invention; 
           [0042]      FIG. 21  is a cross sectional view illustrating another hollow stepped shaft that can be formed by each of the embodiments of the present invention mentioned above; 
           [0043]      FIGS. 22A and 22B  are explanatory views illustrating grain flows in a stepped enlarged radial section according to the conventional forming method; and 
           [0044]      FIG. 23  is an explanatory view illustrating grain flows in such a stepped enlarged radial section according to the method of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0045]    Referring to  FIGS. 1 to 7 , an explanation is given in respect to a first embodiment of the method of the present invention. Here, a solid rod-like member is extruded to make it hollow and at the same time to deform and expand an axially medial region of it radially to form it there with a stepped portion of large diameter.  FIG. 5  shows an exemplary hollow stepped shaft  1  to be formed by the first embodiment of the present method. The hollow stepped shaft  1  comprises a stepped portion of large diameter  2  formed in an axially medial region of the shaft and larger in diameter than elsewhere thereof, and axial portions  3  and  4  at two opposite sides of the stepped portion of large diameter  2 . Further, the hollow stepped shaft  1  is made hollow by being formed about its axis with a bore or hollow  5 . 
         [0046]      FIGS. 1 to 3  show a first, a second and a third step, respectively, in a first process for forming the hollow stepped shaft  1 . A blank made of a solid round bar or rod is indicated at  6 . A first die set  7  comprises an upper and a lower die  8  and  9  formed with coaxial bores  8   a  and  9   a  for receiving the blank  6  and also formed with stepped bore or recesses of large diameter  8   b  and  9   b  where they are opposed to and here also contact with each other, the stepped bores of large diameter  8   b  and  9   b  being larger in diameter than the bores  8   a  and  9   a . An upper and a lower punch  10  and  11  are smaller in diameter than the blank  6  and inserted into the bores  8   a  and  9   a  of the upper and lower dies  8  and  9 , respectively. Indicated at  16  is a knockout in the form of a cylindrical sleeve inserted into the bore  9   a  of the lower die  9  while encircling the punch  11  therewith. 
         [0047]      FIG. 4  shows a second process in this embodiment of the present method. A second die set  12  includes an upper and a lower die  13  and  14  and a mandrel  15 . The upper and lower dies  13  and  14  have stepped forming recesses of large diameter  13   a  and  14   a  across their split face set to correspond in position to a center of the stepped portion of large diameter  2  of the hollow stepped shaft  1  for jointly forming this stepped portion of large diameter, and axial portion forming bores  13   b  and  14   b  for form ing the axial portions  3  and  4 , respectively. Here, the axial portion forming bore  14   b  of the lower die  14  is adapted to received and hold one of two axial portions of an intermediate form or product formed by the first process. 
         [0048]    Mention is next made of the forming method using the first and second die sets  7  and  12  with reference to  FIGS. 1 to 4 . 
         [0049]    In the first step shown in  FIG. 1  in the first process shown in  FIGS. 1 to 3 , the blank  6  is loaded into and set in the bores  8   a  and  9   a  of the first die set  7  as it is clamped. Then, supported by either the knockout  16  alone or both the punch  11  and knockout  16 , the blank  6  is positioned vertically. The vertical (axial) position of the blank  6  is set in accordance with where in its medial region the stepped portion of large diameter  2  of the hollow stepped shaft  1  (as a product) is to be positioned (see the left hand side in  FIG. 1 ). 
         [0050]    Next, in the second step shown in  FIG. 2 , the upper and lower punches  10  and  11  are moved towards to each other to extrude the blank  6  from its both sides axially. This by backward extrusion forces both upper and lower parts of material of the blank  6  to flow into cylindrical open spaces in the upper and lower dies  8  and  9  while by forward extrusion forces a medial part of it is forced and deformed into the stepped expansion forming recesses  8   b  and  9   b . Then, the knockout  16  which has supported the blank  6  is moved down with its lower backward extrusion. 
         [0051]    In the second step shown in  FIG. 2 , the extrusion with the punches  10  and  11  terminates when their ends reach positions where they are opposed across the stepped forming recesses of large diameter  8   b  and  9   b , respectively, whereby a pair of cylinder portions  18   a  (upper) and  18   b  (lower) are formed in axially opposite sides across a solid plug-like portion  17  positioned in an axially medial region of the blank  6  between the two punches  10  and  11 . And, the blank  6  is simultaneously formed in its medial region with a stepped portion of large diameter  18   c  deformed into the stepped recesses  8   b  and  9   b . Then, the stepped portion of large diameter  18   c  having been expanded and deformed stepwise from a solid state, there the lines of grain flow are continuous with no buckling created. 
         [0052]    Then, in a third step as shown in  FIG. 3 , by way of example the lower punch  11  is extracted and the upper punch  10  is moved down further whereby the solid plug-like portion  17  is sheared in the axial direction and forced out as an extract refuse piece. This completes the first process whereby an intermediate form or product  19  that is hollow and stepped is produced, in which grain flows in the stepped zone are streamlined in the absence of any break. 
         [0053]    The intermediate form or product  19  is finish-formed in the second process shown in  FIG. 4 . The intermediate form  19  is loaded into and set in the second die set  12  so that its lower cylinder portion  18   b  is received in the axial portion forming bore  14   b  (hung on its large-diameter rim) of the lower die  14 . In this embodiment, it is also seen that the mandrel  15  is inserted into the hollow (axial bore) of the intermediate form  19 . 
         [0054]    After that, the upper die  13  is moved down whereby the intermediate form  19  with its hollow held by the mandrel  15  has its axial portions  18   a  and  18   b  squeezed through the respective small-diameter rims of the axial portion forming bores  13   b  and  14   b , and the respective squeezed volumes of the axial portions  18   a  and  18   b  are forced out axially. Also, the stepped portion of large diameter  18   c  is axially compressed by the stepped forming recesses of large diameter  13   a  and  14   a  of the upper and lower dies  13  and  14  to expand and deform into them and the stepped portion is thereby formed into a shape complementary to a shape defined by the inner contours of the recesses of large diameter  13   a  and  14   a . As a result, there is formed a hollow stepped shaft  1  as shown in  FIG. 5  that is finished having an inner diameter sized to the mandrel  15  and an outer contour shaped to correspond to an inner contour of the second die set  12  as shown in  FIG. 4 . 
         [0055]      FIG. 6  shows a case in which the mandrel  15  is not inserted in the second process. In this case, portions of the blank formed by the small-diameter rims of the axial portion forming bores  13   b  and  14   b  of the upper and lower dies  13  and  14  are deformed inwards, reducing the diameter of the axial hollow there of the intermediate form  19 . If it is desired to set these axial portions reduced in inner diameter at a selected size, mandrels  15   a  and  15   b  so dimensioned are partially inserted as shown in  FIG. 7 . 
         [0056]    An explanation is given in respect of a second embodiment of the present method with reference to  FIGS. 8 to 11 . This embodiment is so designed that a hollow stepped shaft  20  of a selected shape as shown in  FIG. 12  is formed in the first process in the first-mentioned embodiment. This hollow stepped shaft  20  like that formed in the first embodiment is formed with a stepped portion of large diameter  21 , axial portions  22  and  23  at axially both sides of the stepped portion of large diameter  21 , and an axial hollow or axially penetrating bore  24 . 
         [0057]    In the Figures, there are shown a die set  25  and a blank  26  made of a solid round rod. The die set  25  comprises an upper and a lower die  27  and  28  with their split face corresponding in position to the stepped portion of large diameter  21  of the hollow stepped shaft  20 . The upper die  27  is formed with a bore  27   a  through which the blank  26  is received, and a stepped forming recess of large diameter  27   b  that is larger in diameter than the blank  26  while the lower die  28  is formed with a bore  28   a  through which the blank  26  is received. An upper and a lower punch  29  and  30  are shown inserted into and received through the bores  27   a  and  28   a  of the upper and lower dies  27  and  28 , respectively, and have extruder punches  29   a  and  30   b  smaller in diameter mounted coaxially therewith, respectively, for extruding the blank  26 . 
         [0058]    Mention is next made of a forming method in this second embodiment with reference to  FIGS. 8 to 11 . 
         [0059]    In the first step shown in  FIG. 8 , the blank  26  is inserted into the bore  28   a  of the lower die  28  in an open state. The blank  26  is then supported by the lower punch  30  and its extruder punch  30   a  to lie at a vertical position set to correspond to that of the stepped portion of large diameter  21  of the hollow stepped shaft  20  to be formed as a product from the blank  26  in the stepped forming recess of large diameter  27   b . After that, with the upper die  27  spaced away from the lower die  28  by a selected distance, its bore  27   a  is allowed to accept the blank  26 , and the upper punch  29  and its extruder punch  29   a  are brought into contact with the upper end of the blank  26 . 
         [0060]    This state shown in  FIG. 8  is followed by the second step shown in  FIG. 9  in which the upper die  27 , punch  29  and extruder punch  29   a  are moved down in a body. This causes a portion of the blank  26  in the upper die  27  to be forced down and a portion of the blank  26  intermediate between the punches  29 ,  29   a  and  30 ,  30   a  to be forced radially outwards and deformed into a space defined by the stepped forming recess of large diameter  27   b  of the upper die  27  and the lower die  28 . Then, the amount of expansion is set appropriately to be somewhat smaller than the size of the stepped portion in the formed product  20 . 
         [0061]    The state shown in  FIG. 9  is followed by the third step shown in  FIG. 10  in which the downward movement of the upper die  27  is continued to effect die clamping. During this further downward movement of the upper die  27  or after the die clamping is effected, the upper and lower punches  29  and  30  are freed whereupon the extruder punches  29   a  and  30   a  are moved towards each other to force to form the blank  26  from its both sides axially. This causes the upper and lower parts of the blank  26  to) be each extrude backwards into cylindrical open spaces of the bores  27   a  and  28   a  of the upper and lower dies  27  and  28 , respectively, while the axially medial part is extruded forwards to expand and deform into the stepped forming recess of large diameter  27   b.    
         [0062]    As shown in  FIG. 10 , this extrusion forming step by the extruder punches  29   a  and  30   a  terminates when their ends reach positions where they are opposed across the stepped expansion forming recess  27   b  or any appropriate positions whereby a pair of cylinder portions  32   a  and  32   b  are formed across a solid plug-like portion  31  at its axially opposite sides, the portion  31  lying between the opposed ends of the punches  29   a  and  30   a  in an axially medial region of the blank  26 , and at the same time in this medial region there is formed into the stepped forming recess of large diameter  27   b  the stepped portion of large diameter  21  as a continuous extension of the solid plug-like portion  31 . Hence, the stepped portion of large diameter  21  here is a continuous, radially expanded deformation deformed from a solid state along consecutive lines of grain flow while undergoing no buckling. 
         [0063]    Subsequently, in a fourth step as shown in  FIG. 11 , by way of example the lower extruder punch  30   a  is extracted and the upper extruder punch  29   a  is further moved down to continue to extrude. This causes the abovementioned solid plug-like portion  31  to be sheared axially and forced out and removed from the blank  26  as an extract refuse piece, thereby giving rise to a hollow stepped shaft  20  as shown in  FIG. 12 . 
         [0064]    Although in this second embodiment the blank  26  is shown as loaded in the upper die  27  open and this upper die  27  is shown as moved down together with the punch  29  and extruder punch  29   a , the blank  26  may be loaded in the upper die  27  closed, and then the upper punch  29  and extruder punch  29   a  may be moved down while the lower punch  30  and extruder punch  30   a  are moved up. 
         [0065]    An explanation is next given in respect of a third embodiment of the present method with reference to  FIGS. 13 to 17 . In this embodiment, a solid rod-like blank as it is shorter than a form or formed product to be formed is made both hollow and longer in a first process extrusion and the hollowed blank is then subjected to a second process of forward and backward extrusion designed to make its length still longer and the thickness in its upper and lower parts thinner while causing a medial region between them to radially expand stepwise, forming there a stepped portion enlarged in both outer diameter and thickness. The form eventually formed in this embodiment is a hollow stepped shaft  40 , as shown in  FIG. 17 , having a stepped portion of large diameter  41  and a pair of axial portions  42  and  43  lying at its axially opposite sides. The stepped portion of large diameter  41  is formed to be larger in thickness and formed on its outer periphery with teeth  44  and  45 , and the axial portions  42  and  43  are made to be thinner and smaller in diameter and are formed with serrations  46  and  47  which are each designed to have a tooth form (not shown) applied thereto by simple fitting or press-and-shrink fitting. 
         [0066]      FIGS. 13 and 14  show a first and a second step in the first process for forming the hollow stepped shaft  40 . In the Figures there are shown a first die set  48  and a blank  49  made of a solid round rod. The first die set  48  comprises an upper and a lower die  50  and  51 , and a bearer or pedestal  52  supporting them. The upper and lower dies  50  and  51  are formed with bores  50   a  and  51   a  in which the blank  49  is accepted. A punch  53  to be inserted into the bores  50   a  and  51   a  has an extruder punch  53   a  mounted therein coaxially therewith and that is smaller in diameter than the blank  49 . The bearer  52  is elastically or resiliently supported by a hydraulic or pneumatic unit (not shown) and is formed with a hole  52   a  into which the lower end of the extruder punch  53   a  can be accepted. 
         [0067]      FIG. 15  shows the second process in this embodiment. In the Figure, there are shown a second die set  54  which comprises an upper and a lower die  55  and  56 , a mandrel  57 , and an upper and a lower punch  58  and  59  which are each in the form of a cylindrical sleeve. The upper and lower dies  55  and  56  are formed with coaxial bores  55   a  and  56   a  into which a first intermediate form formed in the first process is accepted and into which the upper and lower punches  58  and  59  opposed each other are also to be inserted. The mandrel  57  has an outer diameter that is equal to that of an axial hollow of the first intermediate form, and each of the upper and lower punches  58  and  59  has an inner diameter that is smaller than the outer diameter of the first intermediate form. 
         [0068]      FIG. 16  shows a third process. In the Figure, there are shown a third die set  60  which comprises an upper and a lower die  61  and  62 , and an upper punch  63  which is in the form of a nearly cylindrical sleeve. The upper and lower dies  61  and  62  has their split face positioned at one end face of the stepped portion of large diameter  41  in the hollow stepped shaft  40  shown in  FIG. 17 , and the upper die  61  is formed with a bore  61   a  into which the upper punch  63  is to be inserted while the lower die  62  is formed with a stepped forming recess of large diameters  62   a  in which the stepped portion of large diameter  41  of the hollow stepped shaft  40  is to be formed and an axial portion forming bore  62   b  in which one axial portion  43  thereof is to be formed. The upper punch  63  is formed in a lower end of its axial bore with an axial portion forming bore  63   a  in which the other axial portion  42  of the hollow stepped shaft  40  is to be formed. Here, the axial portion forming bore  62   b  in the lower die  62  is so shaped that it can bear and support one (lower) axial portion of a second intermediate form formed in the second process. 
         [0069]    Mention is next made of the forming method in the third embodiment with reference to  FIGS. 13 to 16 . 
         [0070]    In the first step shown in  FIG. 13 , the blank  49  is inserted into the bore  51   a  in the lower die  51  in the open state to have its lower end supported by the bearer  52 . After that, the upper die  50  is moved down to close the die set  48 . Then, the punch  53  and extruder punch  53   a  are brought into contact with the upper end of the blank  49 . At this time, the punch  53  is set free. 
         [0071]    This state is followed by the second step shown in FIG.  14  in which moving the extruder punch  53   a  down forms an axial hollow  64   a  in the blank  49  about its axis and the same time forms from the blank  49  a hollow cylinder  64   b  that grows upwards by backward extrusion while leaving a solid plug-like portion which is finally axially sheared and forced out as an extract refuse piece  65 . A first intermediate form  64  that is hollow is thus formed. 
         [0072]    In the first process mentioned above, typically the solid rod-like blank  49  is made of carbon steel and is hollowed at a rate of reduction in area of 25%. The depth of the axial bore is set at a value that is 5 times or more larger than the inner diameter which is a criterion of stable working in a cold forging. To hollow the blank, its boring region is heated at a temperature ranging between a room temperature and 700° C. and its outer periphery is bound. While in this example the bearer  52  is mounted below the lower die  51  and the extruder punch  53   a  is moved down to hollow the blank  49  about its axis, it is also possible to mount a bearer  52  above the upper die  50  and use an extruder punch  53   a  that can be moved up to hollow the blank  49  about its axis. Alternatively, the bearer  52  may be controllably coupled to a servo mechanism so that the bearer  52  may recede or moved down slowly thereby while the extruder punch  53   a  is rapidly advanced to form a hollow in the blank about its axis. 
         [0073]    The first intermediate form  64  is further formed in the second process shown in  FIG. 15 . It is loaded in the bores  55   a  and  56   a  of the second die set  54  in the closed and fastened state. Then, the first intermediate form  64  is supported between the upper and lower punches  58  and  59  and vertically positioned. Further, the mandrel  57  is inserted into the axial hollow of the first intermediate form  64   
         [0074]    Subsequently, the upper and lower punches  58  and  59  are moved towards each other to form the first intermediate form  64  axially by forward and backward extrusion. This causes each of an upper and a lower part of the first intermediate form  64  to be extruded into each of open cylindrical spaces (defined between the upper punch  58  and the mandrel  57  and between the lower punch  59  and the mandrel  57 ) in the upper and lower dies  55  and  56 , respectively, and at the same time a medial portion of the form  64  to be radially expanded and deformed into a recess (defined among the lower end face of the upper punch  58 , the upper die  55 , the lower die  56  and the upper end face of the lower punch  59 ). This process of extrusion forming by both the punches  58  and  59  terminates when they reach positions where they are opposed to each other across a predetermined spacing whereby a second intermediate form  65  is formed having a pair of cylindrical portions  65   a  and  65   b  formed at its axially opposite sides and a stepped portion of radial expansion  65   c  formed at a medial region thereof. Here, the stepped portion of radial expansion  65   c  having been deformed by stepped portion of large diameter is a deformation in which the grain flow is continuous and having no buckling. 
         [0075]    The second intermediate form  65  is finish-formed in a third process as shown in  FIG. 16 . It is loaded into and set in the third die set  60  so that the lower cylindrical portion  65   b  of the second intermediate form  65  is supported by the axial portion forming bore  62   b  and accepted in its large-diameter bore part of the lower die  62  in the third die set  60 . 
         [0076]    After that, the upper punch  63  is moved down. This causes the axial portions  65   a  and  65   b  of the second intermediate form  65  to be draw-formed and deformed inwards while reducing their diameter by the small-diameter part of the axial portion forming bore  63   a  in the upper punch  63  and the small-diameter part of the axial portion forming bore  62   b  of the lower die  62 . And, the stepped portion of radial expansion  65   c  is extruded axially and expanded radially by the lower end of the upper punch  63  and the stepped forming recess of large diameters  62   a  of the lower die  62  to conform to the inner contour of the latter. Further, those regions in the axial portions  65   a  and  65   b  where the serrations are formed having the tooth form (not shown) applied thereto by fitting or press-and-shrink fitting may be further drawn or made smaller in diameter by multistage pressure forming with upper punches and lower dies. 
         [0077]    A hollow stepped shaft  40  is thus formed having a stepped portion of large diameter  41  and a pair of axial portions  42  and  43  located at its opposite sides. Since this hollow stepped tube  40  has the hollow which except for the stepped portion of large diameter  41  is shaped to conform in diameter to the outer contour and in other words having the axial portions  42  and  43  uniformly thinned over their lengths, it is much lighter in weight than those made by cutting as in the prior art, namely in which the hollow (axial bore) is even in diameter and which thus must have been large in thickness. Further, the stepped portion of large diameter  41  and the axial portions  42  and  43  may later be formed with teeth  44  and  45  and serrations  46  and  47  as shown in  FIG. 17 , by cutting or the like. 
         [0078]    An explanation is next given in respect of a fourth embodiment of the present method with reference to  FIGS. 18 to 20 . The embodiment differs from the third embodiment in the first process in which a solid rod-like blank is hollowed as it is shorter than its form, but is identical to the third embodiment in the second and third processes of extruding the hollowed blank forwards to backwards so as to form the hollow blank with a stepped portion enlarged in both diameter and thickness while simultaneously making the blank longer, thereby forming a hollow stepped shaft  40  as shown in  FIG. 17 . 
         [0079]      FIGS. 18 ,  19  and  20  show a first, a second and a third step in the first process for forming a hollow stepped shaft from a solid rod-like blank  49 . In the Figures, there are shown a first die set  66  which comprises an upper and a lower die  67  and  68 , and an upper and a lower punch  69  and  70 . The upper and lower dies  67  and  68  are formed with bores  67   a  and  68   a  coaxial with each other, respectively, into which the blank  49  is accepted. The upper and lower punches  69  and  70  are smaller in diameter than the blank  49  to enter the bores  67   a  and  68   a  in the upper and lower dies  67  and  68 , respectively. Also shown are an outer punch  71  in the form of a cylindrical sleeve inserted into the bore  67   a  and encircling the upper punch  69  and a knockout  72  in the form of a cylindrical sleeve inserted into the bore  68   a  and encircling the lower punch  70 . The knockout  72  is resiliently supported by an oil hydraulic or pneumatic means. 
         [0080]    Mention is next made of the forming method according to the fourth embodiment with reference to  FIGS. 18 to 20 . 
         [0081]    In the first step shown in  FIG. 18  of the first process shown in  FIGS. 18 through 20 , the blank  49  is loaded into and set in the bores  67   a  and  68   a  of the first die set  66  in its closed and fastened state. The blank  49  is then supported by either the knockout  72  alone or both the lower punch  70  and the knockout  72 . 
         [0082]    Next, in the second step shown in  FIG. 19 , the upper and lower punches  69  and  70  are moved towards each other to extrude the blank  49  from both its opposite sides axially. This by backward extrusion forces both upper and lower parts of material of the blank  49  to flow into cylindrical open spaces in the upper and lower dies  67  and  68 . In this course, the outer punch  71  is allowed to move up following the backward extrusion of the upper part of the blank  49  by the upper punch  69  and the knockout  72  to move down following the backward extrusion of the lower part of the blank  49  by the lower punch  70 . 
         [0083]    In the second step shown in  FIG. 19 , the extrusion with the punches  69  and  70  terminates when their ends reach positions where they are opposed to each other across a small spacing, leaving a solid plug-like portion  73  of the blank between the punches  69  and  79  in an axially medial region of the blank  49 . 
         [0084]    Then, in a third step as shown in  FIG. 20 , by way of ex-ample the lower punch  70  is extracted and the upper punch  69  is moved down further whereby the plug-like portion  73  is sheared in the axial direction and forced out as an extract refuse piece. This completes the first process whereby an intermediate form  64  that is hollow is produced. The second and third processes which then follow are identical to those mentioned in the third embodiment and hence repeated descriptions thereof are omitted. 
         [0085]    In the first process mentioned above, typically the solid rod-like blank  49  is made of carbon steel and is hollowed at a rate of reduction in area of 25%. The depths of the upper and lower axial hollows in the blank are each set at a value that is 5 times or more larger than the inner diameter which is a criterion of stable working in a cold forging. To hollow the blank, its boring regions are heated at a temperature ranging between a room temperature and 700° C. and its outer periphery is bound. The solid plug-like portion  73  of the blank may also be axially sheared and forced out as an extract refuse piece by extracting the upper punch  69  and moving the lower punch  70  up further. Alternatively, after one of the punches is extracted, a servomechanism may move the solid rod-like blank  49  back slowly while each of the punches is quickly advanced to shear the plug-like portion  73  out. 
         [0086]    In each of the embodiments described above, the blank  6 ,  26 ,  49  is heated in part or as a whole at a room temperature or a temperature ranging between 200 and 700° C. for forming at which an oxide film does not develop. It should be noted in this connection that if the blank is formed at a room temperature (by cold forging), its deformation raises its temperature to 200 to 700° C. 
         [0087]    In the embodiments mentioned above, a hollow stepped shaft with one of its ends closed as shown in  FIG. 21  may be obtained by leaving the solid plug-like portion  17 ,  31 ,  65 ,  73  in the shaft rather than forcing it out entirely with the punch  10 ,  29   a ,  53   a ,  69 . Also, a stepped portion of larger diameter may be located at one end of a hollow stepped shaft  1 ,  20 ,  40 . 
         [0088]    Although the present invention has hereinbefore been set forth with respect to certain illustrative embodiments thereof, it will readily be appreciated to be obvious to those skilled in the art that many alterations thereof, omissions therefrom and additions thereto can be made without departing from the essences of scope of the present invention. Accordingly, it should be understood that the invention is not intended to be limited to the specific embodiments thereof set forth above, but to include all possible embodiments that can be made within the scope with respect to the features specifically set forth in the appended claims and to encompass all the equivalents thereof.