Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of PCT International Application No. PCT/US2008/080282 filed on Oct. 17, 2008, which claims the benefit of U.S. Provisional Patent Application 60/980,531 filed on Oct. 17, 2007, which is incorporated by reference herein. 
    
    
     STATEMENT CONCERNING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     FIELD OF THE INVENTION 
     The invention relates to forging die tool sets and particularly to forging with core rods used to form voids in forged components. 
     BACKGROUND OF THE INVENTION 
     Forging is a metal forming process used to shape and strengthen many types of components. For example, forging is used to manufacture engine connecting rods, cam shafts, gear blanks, bushings, hammers, wrenches, golf clubs and other well known objects. Forging is advantageous over other metal forming processes since it provides components with increased strength relative to the original material. Strengthening occurs due to change in the grain structure of the material during component shaping. Forging can be performed at various temperatures. Cold forging is typically performed with a work piece at room temperature. This process is used for relatively small components or when a small amount of material flow is required. Hot forging is typically performed with the work piece at an elevated temperature but below the material&#39;s melting point. This process is used for relatively large components or when a large amount of material flow is required. 
     Forging presses are typically driven by mechanical components, such as eccentric shafts, cranks, and screws, or hydraulic actuators. A forged component takes the shape of a die tool set cavity on the forging press. When annular components are forged, the die tool set typically includes a die, upper and lower punches, and core rods. The die surrounds the work piece in a radially outward direction. The upper and lower punches compress the work piece in an axial direction. The core rods hold and complete internal voids in the work piece. 
     Forging is typically used for steel or steel alloy components. However, processes for forging other materials, such as aluminum, copper, and titanium, are also known in the art. Forging processes can also be used to shape sintered powder metal blanks. After a sintering process, a powder metal blank has the approximate shape of the final component. However, a forging process is typically required for the component to meet manufacturing tolerances. 
     In hot forging operations, core rods are used to create and shape internal void shapes. The core rods are subjected to extreme heat and pressures and tend to wear significantly as the number of press cycles increases. Eventually, the core rods need to be replaced to make parts that are within specifications. In addition, sharp corners are often required for components which include internal splines. Wear of the core rod occurs even more rapidly on these sharp corners. Considering the limitations of the previous forging core rods, a need exists for a core rod that is resistant to wear compounded by heat and pressure, yet is capable of producing components with high precision. 
     SUMMARY OF THE INVENTION 
     The present invention provides a forging die tool set that defines a cavity and includes a core rod in the cavity for shaping a void in a work piece. The core rod extends in a direction in which the work piece is introduced, compressed, and ejected from the cavity. The core rod includes an upper portion and a lower portion. The upper portion has a cross sectional shape that forms a certain shape in the work piece and a radially tapered section. The lower portion also has a cross sectional shape that forms a certain shape in the work piece, and the cross sectional shape of the upper portion differs from the cross sectional shape of the lower portion. 
     In another aspect, the upper portion cross sectional shape may be a final shape, and the lower portion cross sectional shape may be an intermediate shape between the final shape and the initial shape of the work piece. In addition, the lower portion cross sectional shape may be more rounded than the upper portion cross sectional area. For example, both the lower portion cross sectional shape and the upper portion cross sectional shape may be spline shapes. 
     Preferably, the void in the forging blank is sized and shaped so that it can pass by the upper portion of the core rod without substantial deformation by the core rod on the way into the die. When the blank reaches the bottom of the die and is subjected to pressure, the void is collapsed inwardly against the lower portion of the core rod so that the shape of the lower portion of the core rod is forged into the void. When the blank is ejected, the void is further deformed by the upper portion of the core rod to finish the forged shape of the void as the forged part is slid by the upper portion. 
     The foregoing and other objects and advantages of the invention will appear in the detailed description which follows. In the description, reference is made to the accompanying drawings which illustrate a preferred embodiment of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference is hereby made to the following figures in which: 
         FIG. 1  is a cross sectional schematic view of a forging die tool set of the present invention; 
         FIGS. 2   a - 2   h  are cross sectional schematic views of the forging die tool set of  FIG. 1  which illustrate the forging process; 
         FIGS. 3   a - 3   c  are alternative embodiments of a core rod according to the present invention; 
         FIGS. 4   a  and  4   b  are examples of a square internal shape and a rounded internal shape, respectively, of a work piece forged by the present invention; and 
         FIG. 5  is a sketch illustrating differences between a rounded internal shape of a lower portion of the core rod and a more squared internal shape of an upper portion of the core rod. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In  FIGS. 1 ,  2   a - 2   h , and  3   a - 3   c , the illustrated components are symmetric about an axis passing vertically through the center of the apparatus. For simplicity, the components are only numbered on one side of the axis of symmetry. 
       FIG. 1  illustrates a forging die tool set  10  according to the present invention. The forging die tool set  10  includes a die  12 , an upper punch  14 , a lower punch  16 , a support shaft  18 , a support surface  20 , and a core rod  22 . The forging die tool set  10  forges a work piece  24 . The work piece  24  may be an annular powder metal blank such as a helical gear, a spur gear or the like. The die  12  surrounds the work piece  24  in a radially outward direction and contacts an outer surface  26  on the work piece  24 . The upper punch  14  and the lower punch  16  contact an upper surface  28  and a lower surface  30 , respectively, on the work piece  24 . The core rod  22  is located in the central void of the work piece  24 . A threaded fastener  32  passes through the core rod  22  and is screwed into an internal thread  33  in the support shaft  18 . The core rod  22  contacts an inner surface  34  on the work piece  24 . 
     The upper punch  14  and the lower punch  16  are moved by independent actuators (not shown). These actuators may be mechanical, hydraulic, or the like. The die  12  and the support shaft  18  may also be moved by independent actuators to reduce cycle time. In addition, automatic component insertion and extraction mechanisms may also be used in the system. Such mechanisms are well known in the art. 
     According to the present invention, the core rod  22  includes two portions, upper core rod portion  36  and lower core rod portion  38 . Lower core rod portion  38  is preferably made from a material which is resistant to deformation at high temperatures and pressures, such as high temperature steel. Other materials which are resistant to deformation at high temperatures and pressures may also be used. Such materials are well known in the art. Using any such material is advantageous since the work piece  24  transfers a large amount of heat to the lower core rod portion  38 . Additionally, forging dies are commonly used to create components with internal splines, or the like. In this case, the lower core rod portion  38  does not provide the final internal shape to the work piece  24 . Instead, the lower core rod portion  38  includes rounded edges (relatively larger radii at the corners) instead of relatively more angled or squared corners of smaller radii in the final forged shape to provide additional resistance to wear and deformation compounded by heat and pressure during forging. For example, the distance between a sharp edge and the nearest point on a rounded edge in  FIG. 5  should be approximately 0.02 in. However, the size of the rounded edges may be increased to further provide resistance to wear and deformation compounded by heat and pressure. The rounded profile is sized relative to the squared profile so that the cross-sectional areas of the forging chamber adjacent to the upper and lower core rod portions are substantially the same, with only the shape changing so that the material of the workpiece can be displaced in equal volumes. 
     Referring again to  FIG. 1 , the upper core rod portion  36  is also preferably made from high temperature steel. Alternatively, the upper core rod portion  36  may be made from carbide, ceramic, or other materials known in the art. Additionally, the upper core rod portion  36  includes two sections, a sizing section  40  and a tapered section  42 . The sizing section  40  has similar geometry to the lower core rod portion  38  and contacts the work piece  24  during ejection from the die as explained below. The tapered section  42  separates the lower core rod portion  38  from the sizing section  40  and does not contact the work piece  24 . The tapered section  42  is relatively short compared to the height of the entire core rod  22 . For example, the tapered section  42  may be 0.25 inches in height. The tapered section  42  limits heat transfer between the lower core rod portion  38  and the sizing section  40 . Limited heat transfer results in less deformation of the sizing section  40 . Advantageously, the service life of the sizing section  40  and the core rod  22  is increased. Additionally, when the forging die tool set  10  is used to create components with internal splines, or the like, the sizing section  40  of the upper core rod portion  36  provides the final internal shape to the work piece  24 . The process of using the forging die tool set  10  is explained in further detail below. 
       FIG. 4   a  illustrates an example of the final internal shape of the work piece  24 . The inner surface  34  of the work piece  24  includes a plurality of involute spline surfaces  44 . The involute spline surfaces  44  permit torque transmission and independent axial motion between the work piece  24  and an adjacent shaft (not shown). The number of involute spline surfaces  44  and the spline size may be selected as appropriate for a particular application. For example, the spline size may be a standard size as published by ANSI. Alternatively, the final internal shape may be any spline shape known in the art. In any case, the sizing section  40  of the upper core rod portion  36  includes the negative of the final internal shape of the work piece  24  after the work piece is ejected from the forging die tool set. 
       FIG. 4   b  illustrates an example of the rounded internal shape of an unfinished work piece  124 , after having been forged against the lower core rod portion  38  but prior to being refined by the upper core rod portion  36 . The inner surface  134  of the unfinished work piece  124  includes a plurality of rounded involute spline surfaces  144 . The lower core rod portion  38  includes the negative of the final internal shape with rounded corners. The shape imparted to the workpiece by the upper core rod portion  36  is said to be more refined than the shape imparted by the lower core rod portion  38  because the upper core rod portion  36  changes the shape imparted by the lower core rod portion  38  to be closer to the shape of the finished forged work piece  124 . In most cases, the more refined shape will have sharper corners, as is the case comparing  FIGS. 4   a  and  4   b.    
     In addition and referring again to  FIG. 1 , the components of the forging die tool set  10  may form chamfers between the upper surface  28  and the inner surface  34  and between the lower surface  30  and the inner surface  34 . 
     In addition, the upper core rod portion  36  and the lower core rod portion  38  should be designed such that the cross-sectional area of the cavity adjacent to each portion is equal. Equivalently, the solid line in  FIG. 5  should enclose equal areas on both sides of the dashed line. If the cross-sectional area of the cavity adjacent to the lower core rod portion  38  is smaller than that adjacent to the upper core rod portion  36 , the work piece  24  will not occupy all of the sharp corners of the cavity adjacent to the upper core rod portion  36 . If the cross-sectional area of the cavity adjacent to the lower core rod portion  38  is larger than that adjacent to the upper core rod portion  36 , a burr will form on the work piece  24  or excessive tooling wear will occur. 
     In addition, some forged components become deformed due to temperature and cooling rate differences between areas of the forged material. This deformation, or “lobing”, causes the final shape of a forged component to differ from the intended shape. Lobing can be predicted using well-known finite element analysis computer programs. Therefore, the shape of the core rod sections can be designed such that forged components meet manufacturing tolerances despite lobing. 
     The lower punch  16  is used to push the work piece  24  out of the die  12 , as will be explained in further detail below. Accordingly, the lower punch  16  is used to support the lower surface  30  of the work piece  24  without contacting either the lower core rod portion  38  or the upper core rod portion  36  when it ejects the work piece  24  from the die  12 . That is, the lower punch  16  may include the same internal cross sectional shape as the final shape of the work piece  24 , but radially enlarged to prevent interference with the core rod  22 . Accordingly, the support shaft or core rod base  18  has an external cross sectional shape that may be the negative of the internal cross sectional shape of the lower punch  16  and fit closely with the lower punch  16 . Also, the upper core rod portion  36  and the lower core rod portion  38  are sized and shaped to clear the unforged work piece when it is placed in the die  12 . The upper punch  14  is sized and shaped to clear the upper core rod portion  36  as the upper punch  14  moves past the upper core rod portion  36 . That is, the upper punch  14  includes the same internal cross sectional shape as the final shape of the work piece  24 , but slightly larger radially to prevent interference with the upper core rod portion  36 . Accordingly, a small height of the lower core rod portion  38  at the top of the lower portion  38  may have the final internal shape of the work piece  24  to prevent contact with the upper punch  14  during the forging process. 
     The process for forging the work piece  24  in the forging die tool set  10  is as follows. As shown in  FIG. 2   a , the forging die tool set  10  initially does not include the work piece  24  and the upper punch  14  is in a retracted position. Next, the work piece  24  is placed in the die  12  as shown in  FIG. 2   b . The upper punch  14  then moves downward to contact the work piece  24  as shown in  FIG. 2   c . The upper punch  14  continues to move downward after initial contact with the work piece  24 . The work piece  24  is compressed between the upper punch  14  and the lower punch  16  as shown in  FIG. 2   d . The work piece  24  expands radially outwardly and inwardly to contact the die  12  and the lower core rod portion  38 , respectively. After the work piece  24  has been compressed, the upper punch  14  moves to its initial position as shown in  FIG. 2   e . In  FIG. 2   f , the lower punch  16  moves upward to shape the inner surface  34  of the work piece  24  using the sizing section  40  of the upper core rod portion  36 . After this step, deformation of the work piece  24  is complete and the work piece  24  is in a position to be removed from the forging die tool set  10 , as shown in  FIG. 2   g . In  FIG. 2   h , the lower punch  16  moves downward to its initial position. The process may be repeated by returning to the step shown in  FIG. 2   a.    
     In addition, if the work piece  24  is a helical gear, the process may include rotation of the work piece  24  during ejection and shaping of the inner surface  34 . Such processes for rotating helical gears are well known in the art. In this process, the lower core rod portion  38  may have a circular cross section, and the upper core rod portion  36  may have a spline shape for forming splines on the work piece  24 . 
       FIGS. 3   a  through  3   b  illustrate several alternative embodiments of the core rod  22 . In  FIG. 3   a , a core rod  122  includes an upper core rod portion  136  and a lower core rod portion  138 , but is created from a single piece of material. In the embodiment of  FIG. 1 , the upper core rod portion  36  is one piece and the lower core rod portion  38  is a second, separate piece. The upper core rod portion  136  includes a sizing section  140  and a tapered section  142 . The sizing section  140 , the tapered section  142 , and the lower core rod portion  138  features are formed by machining an original piece of material. In addition, a threaded fastener  32  passes through the core rod  122  and is threadably attached to an internal thread  33  in the support shaft  18 . 
       FIG. 3   b  illustrates a core rod  222  that does not require a separate fastener. Instead, an upper core rod portion  236  includes an integral threaded section  232  which attaches to an internal thread  235  in a lower core rod portion  238 . The lower core rod portion  238  includes an integral threaded section  237  which attaches to an internal thread  33  in the support shaft  18 . Like other embodiments of the invention, the upper core rod section  236  also includes a sizing section  240  and a tapered section  242 . 
       FIG. 3   c  illustrates a core rod  322  that also does not require a separate fastener. Instead, an upper core rod portion  336  includes an integral threaded section  332  which passes through a lower core rod portion  338  and attaches to an internal thread  33  in the support shaft  18 . Again, the upper core rod section  336  also includes a sizing section  340  and a tapered section  342 . 
     The upper core rod portion and the lower core rod portion of any embodiment may be made using well known machining processes, such as turning and milling. The manufacturing process may be modified depending on the type of fastener to be used and the number of pieces of material used to create the core rod. 
     A preferred embodiment of the invention has been described in considerable detail. Many modifications and variations to the preferred embodiment described will be apparent to a person of ordinary skill in the art. Therefore, the invention should not be limited to the embodiment described, but should be defined by the claims that follow.

Technology Category: 7