Patent Publication Number: US-2005115298-A1

Title: Method and device for producing a steering rack

Description:
FIELD OF THE INVENTION  
      The present invention relates to a method and a device for manufacturing a toothed rack, for use, e.g., in steering gear mechanisms for motor vehicles.  
      The present invention relates to a method that includes a noncutting shaping process in which the desired toothing of the toothed rack is formed on a blank using shaping tools. For example, the following may be provided in this process: the teeth of the toothed rack are formed by corresponding negative teeth which are provided in one of the two shaping tools (for example in the upper die). The back of the toothed rack is formed by a negative back which is provided in the other shaping tool (for example in the lower die). During the shaping process, two lateral burrs are formed from excess material between the two shaping tools.  
     BACKGROUND INFORMATION  
      According to German Published Patent Application No. 32 02 254, the negative back which is formed in the lower die is shaped such that the cross-section of the finished toothed rack has an approximately y-shaped section. However, a toothed rack which has a round section is also conventional. An example embodiment of the present invention may be applied in both cross-sectional shapes and also with other possible cross-sectional shapes.  
      According to German Published Patent Application No. 32 02 254, the length of the negative teeth in the upper die is equal to the desired tooth width in the finished toothed rack. In practice, attempts have been made, by applying orbital pressing, to influence the flow of the material such that the negative teeth are filled as completely as possible with material. In this manner, the desire was to obtain a clean sectional shape over the entire tooth width, as far as possible without subsequent working. However, it has become apparent that such an ideal profile of the shaping process is generally not achievable. In particular, it has not been possible to bring about a situation in which the material in the individual negative tooth penetrates completely into the two end faces of the negative tooth. As a result, only a reduced effective tooth width could therefore be achieved. In other words, the toothing could not be shaped completely in the lateral direction. This has resulted (for example when a conventional toothed rack is applied in a vehicle steering gear mechanism) in a situation in which the compressive load per unit area on the tooth edges increased unacceptably, and there was thus the risk of premature wear.  
     SUMMARY  
      An example embodiment of the present invention may provide a method for manufacturing a toothed rack in which the noncutting shaping process may be used to shape the teeth of the toothed rack completely, i.e., the usable tooth width of the toothing is to extend as far as possible over the entire available cross-section of the finished toothed rack. In this context, as in the past, a rod-shaped blank is to be shaped in a noncutting fashion in a single procedure.  
      An example embodiment of the present invention may provide a device which is suitable for carrying out the method.  
      According to an example embodiment of the present invention, the noncutting shaping process is configured such that a larger flow of material than in the past occurs on both sides into the lateral burrs. A feature here is that a sufficient quantity of material is pressed into elongations of the negative teeth. In comparison with the past, an enlarged tooth width which extends into the regions of the lateral burrs is thus formed. If the elongated negative teeth are not filled with material completely as far as their end faces, the usable tooth width is nevertheless sufficiently large. Generally, it will extend at least over the entire cross-section of the toothed rack, or even beyond it. If the two burrs with the protruding toothing regions which are not completely formed are disruptive during the later use of the toothed rack, they may be removed, for example, by a cutting processing procedure.  
      An aspect of an example embodiment of the present invention is that during operation the compressive load per unit area on the tooth edges of the toothed rack assumes at maximum the normal, acceptable value, and the previously existing risk of premature wear may therefore be eliminated.  
      The device which is suitable for carrying out the previously described method according to an example embodiment of the present invention may have the feature that, viewed in cross-section through the upper die, the length of the negative teeth extends on both sides as far as into the regions of the lateral burrs which are formed.  
      The free flow of material which may be achieved according to an example embodiment of the present invention (on both sides into the burrs) may be improved even further by pressing some of the material into a widened portion of the negative back. For this purpose there may be provided in the lower die for the negative back to have, on each side in the region of the pressing plane, a recess which extends into the region of the lateral burr which is formed. As a result, thicker lateral burrs than before may be formed, but primarily the material may penetrate into the negative teeth better than before. A the device according to an example embodiment of the present invention, according to which the central axis of the toothed rack is lower in the lower die than the pressing plane, also serves this purpose.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIGS. 1A and 1B  show the manufacture of a toothed rack by shaping a blank.  
       FIG. 2A  is a cross-sectional view taken along the line II-II in  FIG. 1B .  
       FIG. 2B  shows the toothed rack illustrated in  FIG. 2A  after finishing.  
       FIGS. 3A, 3B  and  4 A,  4 B show different exemplary embodiments. 
    
    
     DETAILED DESCRIPTION  
      The rod-shaped blank  11  (with length L 1 ) which is illustrated in  FIG. 1  is shaped in a non-cutting fashion according to  FIGS. 1B and 2A  between two shaping tools, e.g., an upper die  16  and a lower die  17 . On the one hand the teeth  13  and on the other hand the back  14  of the toothed rack are formed, the toothed rack (now designated by  12 ) being lengthened, according to  FIG. 2 , to the length L 2 . It may be provided to clamp the toothed rack between two end shaping tools  18 . These form a centering element  19  at each end of the toothed rack.  
      In the upper die  16 , negative teeth are provided for shaping the teeth  13  of the toothed rack  12 . In a similar manner, a negative back is provided in the lower die  17  for shaping the back  14  of the toothed rack  12 . If the blank  11  already has a shape which corresponds to the desired shape of the back  14 , the lower die  17  serves, during the shaping of the teeth  13 , essentially only to support the blank, and possibly to increase the surface quality of the toothed rack back  14 .  
      The lateral burrs which form between the dies  16  and  17  during the noncutting shaping process are designated by  15  in  FIG. 2A . According to an example embodiment of the present invention, the negative teeth  20  extend far over the circular section of the blank as far as into the regions of the lateral burrs  15 . The tooth width B which is increased in this manner (for example, measured at the base of the tooth) is greater than the tooth width B required in the finished toothed rack (designated by  22  according to  FIG. 2B ). As a result, allowance is made for the fact that during the noncutting shaping process the teeth  13  are usually not formed completely, i.e. the negative teeth do not fill completely with material. The burrs  15  are removed after the shaping process by cutting processing (for example, by milling). As a result, the finished toothed rack  22  is provided with completely shaped teeth with, for example, planar, oblique end faces  23 .  
      The noncutting shaping process may take place using the orbital pressing process described in German Published Patent Application No. 32 02 254. The orbital angle is designated by w in  FIG. 2A .  
      In the toothed rack  12   a  and  22   a  according to  FIGS. 3A and 3B , there is in turn provision for the negative teeth  20  which serve to form the teeth  13   a  to extend on both sides as far as into the regions of the lateral burrs  15 . However, in comparison with  FIG. 2A , an additional measure is taken in  FIG. 3A . In the lower die  17   a , the negative back which serves to form the back  14   a  has a recess  24  on each side in the region of the pressing plane E. These recesses  24  fill with material during the shaping process so that here the lateral burrs  15   a  may be formed in a more satisfactorily defined manner. In this exemplary embodiment, the arrangement may be made such that in the lower die  17   a  the central axis of the toothed rack is lower by the dimension m than the pressing plane E. The upper die  16   a  corresponds essentially to the upper die  16  in  FIG. 2A . After the shaping process, the burrs  15   a  are in turn removed, for example, by turning, so that a circular section is obtained, as illustrated in  FIG. 3B .  
      The manufacturing method for the toothed rack  12   b  or  22   b  illustrated in  FIGS. 4A and 4B , respectively, with teeth  13   b , is largely similar to that in  FIGS. 2A and 2B . However, instead of a round section, the toothed back  14   b  has a section which is triangular in a rough approximation. The section of the toothed rack which is finished according to  FIG. 4B  is also referred to as a “y section”.  
     Reference Symbols  
     
         
           11  Blank  
           12 ,  12   a ,  12   b  Toothed rack  
           13 ,  13   a ,  13   b  Teeth  
           14 ,  14   a ,  14   b  Back  
           15 ,  15   a ,  15   b  Burr  
           16 ,  16   a  Upper die  
           17 ,  17   a  Lower die  
           18  End shaping tool  
           19  Centering element  
           20  Negative teeth  
           22 ,  22   a ,  22   b  Finished toothed rack