Abstract:
The invention relates to a method of finishing a metal preform. The preform is green machined in a first method step. Next, the quality surfaces, which, on the finished component part are to meet stringent quality requirements, are roller burnished. Finally, the preform is hardened.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a method of finishing a metal preform. More specifically, the invention is directed to the finishing of preforms or component parts of constant velocity joints.  
         [0003]     2. Related Technology  
         [0004]     Many metal component parts need to maintain, in the finished state, a required diameter tolerance and straightness. The surfaces are to be absolutely smooth, meaning their roughness must be below maximum values. These requirements apply for instance to bearing surfaces on which balls or needles are rolling.  
         [0005]     The known state of the art, and in the further course of the description the invention as well, will be understood better upon reading the description of a three-legged spider for a constant velocity joint given by way of example only; the invention is not restricted to such type component parts, though.  
         [0006]     A three-legged spider is utilized in a tripod joint. Tripod joints in turn are constant velocity joints that are utilized for instance in the drivetrain of automotive vehicles having front-wheel drive. Such type joints must permit both the deflection of the wheels and the length accommodation resulting therefrom as well as their steering angle.  
         [0007]     A three-legged spider includes an annular hub body that is adjoined with pivots oriented radially with respect to the longitudinal axis of the hub body. The pivots each include surfaces for bearing the roller bearings that are disposed on the circumference of the pivot axes. Rollers carried on needles with cylindrical axial hole into which the pivots engage with a substantially zero clearance fit are positioned on a respective one of the pivots. Accordingly, the roller carriers are axially displaceable with respect to the pivots. As a result, the tripod rollers, which are supported by the roller carriers, may be guided so as to execute a rolling motion when the outer part of the joint and the three-legged spider are rotating at an angular position with respect to one another.  
         [0008]     The hub body includes a through hole in the inner periphery of which has a toothed surface.  
         [0009]     To manufacture a three-legged spider, usually one first forms the toothed surface feature in the inner surface of the hole in the hub body. Then, hardening is performed using a suited hardening method. Examples of suited methods include quench hardening, induction hardening, marquenching or nitration. After hardening, the pivots are hard machined. This hard machining step is needed to remove the dimensional excess and because changes in shape such as warping during hardening are unavoidable. Also, the component parts grow, increase in volume, during hardening because they are enriched in carbon. This growth also is irregular.  
         [0010]     For hard machining, grinding and turning are particularly suited methods. The disadvantage of the approach as described is that hard machining places high demands on tool and machine. Accordingly, hard machining tools and machines are extremely cost intensive both in terms of acquisition and of maintenance. This applies more specifically for hard turning.  
         [0011]     Therefore, an alternative to the approach described involves premachining the preform prior to hardening by grinding it to already achieve so high a quality that the hard machining step subsequent to hardening can be eliminated. Grinding however also suffers from considerable disadvantages. The dust generated or emulsion of grinding swarf and water or coolant that is inevitably produced is harmful to health and has an impact on the environment. Also, after grinding, careful washing is needed. In addition, the grinding process takes quite a lot of time. It has also been found out that the inevitable growth of the component part that occurs during hardening is very irregular so that surface differences due to warping of up to 30 μm are almost inevitable.  
         [0012]     Cold extruded, expensive near netshape preforms are used for all of the manufacturing methods. If less expensive hot-forged parts are utilized, the pivots must be turned in a preliminary step.  
         [0013]     It is the object of the present invention to provide a method of finishing a metal preform that may be carried out at low cost. More specifically, the hard machining step subsequent to hardening is to be excluded. Also, the new method should have minimal impact on the environment and should be simple and fast to perform.  
       SUMMARY OF THE INVENTION  
       [0014]     In accordance with the invention, this object is solved by a method of finishing a metal preform that is characterized by the following method steps: 
        1. green machining the preform,     2. roller burnishing quality surfaces which, on the finished component part, are to meet stringent quality requirements,     3. hardening the preform.        
 
         [0018]     The method of the invention permits to machine the preform prior to hardening in such a manner that hard machining after hardening may be eliminated. In accordance with the invention, it is not necessary to grind the surfaces on which the most stringent quality requirements are places. Accordingly, all the disadvantages arising from grinding are eliminated. The method of the invention is ecological, low in cost and easy to perform.  
         [0019]     The term “quality surfaces” designates the surfaces on which the most stringent quality requirements are places upon completion of the component part. These are for example bearing surfaces, gasket seats, ball hubs and the like.  
         [0020]     The concomitant disadvantages of the prior art hardening method are almost completely eliminated by roller burnishing the surface. It has been found out that the growth occurring during hardening is many times more uniform if the corresponding regions have been roller burnished before. This again results in a considerably reduced roughness of the corresponding surfaces on the one side but also, thanks to the uniform compressive strain exerted during roller burnishing, in a substantially reduced warping of these surfaces over surfaces that have been turned or ground prior to hardening.  
         [0021]     The process of roller burnishing is also referred to as burnishing. Roller burnishing is a surface finishing technique in which highly polished steel rollers are brought into pressure contact with a softer material. As the pressure generated through the rollers exceeds the yield point of the material, the margin of the machined surface is plastically deformed by the thus generated cold flowing process.  
         [0022]     Deformation results in a mirror-like surface with superior load-carrying characteristics which make the burnished surface superior to finishes obtained by abrasive metal-removal methods. Burnished surfaces are smoother and more wear-resistant than surfaces obtained by abrasive metal removal methods.  
         [0023]     Roller burnishing permits to achieve good size control, superior load-carrying characteristics, short machining times and is quite easy to handle. Another advantage is that standard machines may be utilized for the roller burnishing process.  
         [0024]     A major advantage further is that, as contrasted with all the other known methods, the manufacturing method of the invention allows utilization of warm forged parts. This brings a significant cost saving.  
         [0025]     Increasing the load-carrying characteristics means that the smoother surface of a body that is to rest on a surface such as a needle of a needle bearing has a larger surface by which it is supported, this provision reducing the load or rather the load peaks. This significantly influences the life span of such bearings.  
         [0026]     In manufacturing the three-legged spiders described herein above, it has been found out that the surfaces carrying the roller bearings, which are arranged on the pivots, comprise a warp of less than 4 μm after hardening. If, by contrast, a grinding step has been performed prior to the hardening process, warping is 20-30 μm. The pivots machined using the roller burnishing technique of the invention had in parts a maximum uniform growth of 0.02 mm. In the hard state, ovality was nearly non-existent and was 0.005 mm, maximum. Accordingly, a certain growth potential during roller burnishing is possible.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]     The invention will be better understood upon reading the following description of the figures with reference to a three-legged spider. Further advantages and exemplary embodiments of the invention will become apparent from the claims.  
         [0028]      FIG. 1  is a perspective view of an inner joint part of a constant velocity joint (of a three-legged spider);  
         [0029]      FIG. 2  is a top view of the three-legged spider of  FIG. 1  when viewed in the axial direction;  
         [0030]      FIG. 3  shows a measurement curve of a surface measurement of a pivot surface carrying the roller bearing manufactured according to prior art; and  
         [0031]      FIG. 4  shows a measurement curve of a surface measurement of a pivot surface carrying the roller bearing manufactured according to the method of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0032]     The invention is explained with reference to a three-legged spider  10  but is not to be limited to the manufacturing thereof. As can be seen from the  FIGS. 1 and 2 , the three-legged spider  10  comprises three pivots  2  that are circumferentially disposed about a hub body  14 . In the embodiment as shown, each pivot  12  comprises a surface  15  for carrying a roller bearing on its outer periphery. The pivots  12  are disposed, or devised, so as to fit into an inner ring of a roller bearing.  
         [0033]     As very stringent requirements in terms of roughness ard ovality are placed on the surface  16  carrying the roller bearings, these surfaces are called quality surfaces.  
         [0034]     The hub body  14  comprises a through hole  20  having a toothed inner surface feature  22 .  
         [0035]     Usually, a three-legged spider  10  is made from a preform. This preform is first green machined, with the toothed surface feature  22  being formed first, prior to forming the desired final shape of the three-legged spider  10  by broaching, turning or grinding. Next, the preform is hardened. During hardening, warping and growth of the preform are unavoidable as the preform enriches in carbon, this being the reason why the preform usually is hard machined in order to form the final shape within prescribed tolerances. A turning process, namely what is termed hard turning, is also gained acceptance as a hard machining technique. It is merely known that it is possible to dispense with hard machining if the preform has been ground prior to hardening. However, as already explained herein above, grinding has so many disadvantages that it has been replaced, in accordance with the invention, with a roller burnishing process. This means that, after the toothed surface feature  22  has been formed, the basic shape of the three-legged spider  10 , or of the pivots  12  respectively, is first obtained by turning or broaching and the final shape is produced next, prior to hardening, by roller burnishing the surfaces  16  carrying the roller bearings.  
         [0036]     The pivot surfaces  16  carrying the roller bearings may for example be burnished using a conically configured roller cage having for example  7  roller needles. The rotating pivot  12  is thereby introduced into the conically configured roller cage. The pressure applied onto the roller needles exceeds the yield point of the material of the three-legged spider  10  or of the pivot  12 , which causes the margin of the machined surface to be plastically deformed by the thus generated cold flowing process.  
         [0037]     It has been found out that, prior to the roller burnishing process, a certain roughness of the surface, which is also referred to as burnishing, is quite advantageous. The reason is that “protruding” material may be pressed into also existing “valley”.  
         [0038]     The  FIGS. 4 and 5  show results of a surface measurement of a surface  16  carrying the roller bearing that has been hard turned after hardening ( FIG. 3 ) and of a surface  16  carrying the roller bearing that has been roller burnished prior to hardening ( FIG. 4 ). The roughness of the surface  16  carrying the roller bearing is measured in the axial direction, meaning from the spider body  14  toward the free end. The surface variations are indicated in μm on the y-axis, whereas the occurrence rate of a certain variation is plotted in percent on the x-axis. This means that the shape of a measurement curve  24  obtained is indicative of the material fraction of the surface, or rather of the load carrying characteristics thereof. It is desirable that the measurement curve  24  be oriented as soon as possible approximately in parallel to the x-axis. For this signifies that the surface mainly varies about a certain roughness thanks to which bearings or rollers resting on said surface are carried by as large a surface as possible.  
         [0039]     It appears from the two diagrams that a surface that has been roller burnished prior to hardening yields a measurement curve  24  that is quite straight and constant at about 10% already. This is not the case with a pivot  12  manufactured according to prior art where the curve is quite steep over the entire bandwidth, meaning from 10%.  
         [0040]     It also appears from the two FIGS. that a component part manufactured in accordance with the invention, in the instant case a pivot  12  given by way of example only, grows very uniformly and hardly warps during the hardening process. If this were not the case, the curve would be much steeper, i.e., the roughness variations would be more marked.