Abstract:
A method of improving the surface finish of a sheared end face of a cylindrical blank in a progressive forming machine comprising deforming the end face out of its original as sheared transverse plane into a first shape concentric with the axis of the blank that slightly departs from the original plane a distance that increases with proximity to the axis of the blank, thereafter deforming the end face from the first shape into a second shape in a direction opposite a direction the first shape was displaced from the original shear plane, the second shape slightly departing from a transverse plane a distance that increases with proximity to the axis of the blank.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to metal forming and, in particular, to a method of improving the surface finish of a sheared end face of a blank being formed in a progressive forming machine. 
       PRIOR ART 
       [0002]    Typically, a progressive forming machine shears wire stock into cylindrical blanks that are transferred from workstation to workstation in the machine. Ordinarily, the blank is incrementally reshaped with punches and dies at the successive workstations to achieve a desired final part configuration. The end faces of the original blank are usually characterized by a relatively rough surface finish. As the blank end faces are struck or bolstered by the tooling in the workstations, some of the original surface roughness is eliminated. However, even when the blank is subjected to high compressive forces in reshaping blows, the sheared end surface can have a residual surface roughness. The resulting surface roughness may be unsatisfactory for some applications because of the intended function of the finished part or for aesthetic reasons. There has existed a long felt need for a simple, effective process to obtain a smooth surface finish on the sheared end surface areas of metal blanks formed in a progressive forming machine. 
       SUMMARY OF THE INVENTION 
       [0003]    The invention provides a method of greatly improving the surface finish of a sheared blank end in a progressive forming machine. The process involves a sequence of steps in which an end surface is forced into a non-planar configuration, is then driven into a reverse configuration and then, is optionally flattened. In the preferred embodiment, the first blow forces the end face into a concave configuration and at a successive station the end face is forced into a convex configuration. 
         [0004]    As disclosed, auxiliary steps are performed to facilitate the transition of the end face between the concave and convex stages. The blank adjacent its end is tapered so that it is smallest at the end face prior to being formed into a convex shape. The taper advantageously ensures that the blank end material is not radially restricted and caused to flash around the punch when it is being reshaped. The tool or punch that converts the end face from concave to convex has a unique central vent for exhausting oil/coolant and/or air, thereby preventing these fluids from obstructing the blank end material from closely conforming to the face of the tool. After the blank end surface is reshaped from the original shear plane to a concave shape and then to a convex shape, it is preferably finally flattened to achieve a quality finish. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  illustrates a blank, in side view, as sheared from a wire coil or bar; 
           [0006]      FIG. 2  illustrates the blank after a first forming blow; 
           [0007]      FIG. 3  illustrates the blank after a second forming blow; 
           [0008]      FIG. 4  illustrates the blank after a third forming blow; and 
           [0009]      FIG. 5  is a somewhat schematic fragmentary plan view of several stations of a progressive forming machine used to perform the inventive method. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0010]      FIG. 5  illustrates parts of a progressive forming machine  10  including a cutoff station  11  and several regularly spaced workstations  12 - 14 . Each workstation  12 - 14  has a pair of opposed punch and die holders  16 ,  17 . The die holders  17  are stationarily mounted on a die breast or bolster diagrammatically illustrated at  18  and the punch holders  16  are mounted on a ram or slide diagrammatically indicated at  19 . 
         [0011]    Referring to  FIG. 1  and the extreme right in  FIG. 5 , a generally cylindrical metal blank  20  of steel, copper, brass, aluminum or other metal is sheared at the cutoff station  11  from a supply of coiled wire or, less frequently, from an elongated bar. The leading and trailing sheared end faces  21  of the blank  20  are nominally planar and transverse to the blank axis. As is customary, the blank  20  is oriented such that its axis is parallel to axes of successive workstations  12 - 14  to which it is transferred and to the axis of reciprocation of the slide  19 . As is also customary, a mechanical transfer device, not shown, transfers a blank  20  from the cutoff station  11  and the workstations  12 - 14  during a cycle of reciprocation of the ram  19 . 
         [0012]    The sheared end face  21  of the blank  20  deviates from a true plane, as is somewhat exaggerated in  FIG. 1 , owing to a slight round-over where the blank is engaged by cutting edges of the shear. Additionally, in the nature of a solid, the main area of the sheared end face  21  is somewhat rough since it is the result of a fracture of the blank material. 
         [0013]    In normal prior art practice, the rough surface finish of the sheared end faces  21  is somewhat improved as the blank is progressively deformed in successive workstations into a desired ultimate shape. This incidental surface finish improvement can satisfy many applications but such a finish can be unsatisfactory where the sheared end face area needs a high quality surface finish or smoothness. This can be true even when the applied forming pressures are extreme. Experience has shown that even when forming pressures are very high, a sheared surface can resist conforming perfectly to the tooling surface and will retain at least some degree of its original surface irregularity. 
         [0014]    From the cutoff station  11 , the blank  20  is transferred to the first station  12 . Punch and die assemblies  26 ,  27  are configured to taper both ends  28  of the blank  20  in the manner of a barrel so that the outside diameter of the blank at both ends is progressively smaller with increasing proximity to the blank end face  21 . Where the taper is a simple angle, the angle α ( FIG. 2 ) at each side of the blank  20  can range between about 3 degrees to about 15 degrees and is preferably about 4.5 degrees. Additionally, the punch and die assembly tools  26 ,  27  make the end faces  20  concave when the ram  19  reaches the forward dead center position of  FIG. 5 . In the illustrated embodiment, the concave end faces are caused by the punch and die assemblies  26 ,  27  to take the form of shallow cones concentric with the blank axis. The cone angle β ( FIG. 2 ) can range from about 3 degrees to about 10 degrees and is preferably about 5 degrees. The forming action at the first station  12  is a combination of an upset resulting in an increase in diameter of the mid-section of the blank  20  and an extrusion in which the end portions of the blank are reduced in diameter from the original diameter of the cylindrical blank  20 . Preferably, the tooling  26 ,  27  at front dead center enclose a space that is slightly greater than the volume of the blank so that the tooling space is not completely filled and the corners between the blank end faces and the blank sidewalls have a small radius. 
         [0015]    The blank  20  is transferred to the second workstation  13  where punch and die tools  31 ,  32  are shaped to reverse the concave configuration of the blank end faces. The tools  31 ,  32  form the blank end faces  20  into shallow convex shapes. Preferably, these shapes take the form of shallow cones concentric with the blank axis, for example extending along a line, at each side, at an angle θ ( FIG. 3 ) of about 3 degrees to about 10 degrees and preferably about 5 degrees. 
         [0016]    The cavity volume formed by the punch and die tools  31 ,  32  at front dead center at this station  13 , like that of the first station  12 , is slightly larger than the volume of the blank  20 . The tapered blank ends formed at the first station enable the blank end faces  21  to freely upset radially outward at this second station  13 , mostly without confinement, so that flash between the tools confining the end faces  21  and the tools surrounding the blank sidewall is avoided. 
         [0017]    A unique feature of the punch and die tools  31 ,  32  is the provision of a small central axial vent  33  disposed at the center of the blank end faces, that is, on the central axis of the blank and tools. The vents  33  allow for the escape of air, coolant and/or lubricant from the initial space between the tools  31 ,  32  and respective blank end faces  20 . The vents  33  are less than 1/32 inch and preferably are about 0.6 mm or 0.024 mm in diameter. These fluids, if otherwise trapped between a tool and blank because of a seal that is formed at the periphery of the end face  20 , would prevent a blank end from being properly reformed by the tool. 
         [0018]    The blank  20  with the convex end faces is transferred to the third workstation  14  where the end faces are flattened by punch and die elements  36 ,  37  into a plane perpendicular to the blank axis ( FIG. 4 ). As before, the cavity space afforded by the tooling  36 ,  37  is preferably slightly greater than the volume of the blank. 
         [0019]    The disclosed process of first making the blank end faces concave, then convex, and then preferably finally flat, has been found to greatly improve the surface smoothness of a blank end face. It has been found that the disclosed process can, for example, achieve a surface finish with steel of R z =16 or less under ISO 468-1982(F). The functionality and/or aesthetics of the surface is thereby significantly improved over that which ordinarily occurs when a blank is directly formed into a desired shape. 
         [0020]    While the invention has been disclosed as applied to both ends of a blank some products being formed in a forming machine may only require or be benefitted by treating one end face of the blank to the disclosed concave/convex surface reconfiguration. The workstations in such instances can be used to shape the opposite end of the blank towards a final configuration. While not as effective, the sequence of reforming an end face from its original flat sheared condition to slightly non-planar configurations can be reversed such that the blank is initially worked to make the sheared end face convex and is then worked to make the end face concave and thereafter flattened. 
         [0021]    It should be evident that this disclosure is by way of example and that various changes may be made by adding, modifying or eliminating details without departing from the fair scope of the teaching contained in this disclosure. The invention is therefore not limited to particular details of this disclosure except to the extent that the following claims are necessarily so limited.