Abstract:
A stretch roll forming apparatus has multiple roll forming stations arranged serially so that a sheet metal workpiece can be advanced through the stations in sequence. Each station includes a pair of roller dies supported by an inboard support station and by an outboard support station, in spaced apart relationship, so that the roller dies are each rotatable about a horizontal axis. The roller dies are configured to shape a sheet metal workpiece which is advanced between them. At least one of the roll forming work stations is a stretch roll forming work station. In this station, the roller dies each have a frusto-concial shape. The roller dies are positioned such that the taper of one extends from the inboard support to the outboard support while the taper of the other extends from the outboard support to the inboard support. When a sheet metal workpiece is advanced between the roller dies of the stretch roll forming station, one longitudinal edge of the workpiece is stretched relative to the other longitudinal edge. This gives the workpiece a longitudinal curvature.

Description:
RELATED APPLICATION 
     This patent application claims priority of Provisional Patent Application 60/110,735 filed Dec. 3, 1998. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for stretch roll forming. More particularly, the present invention provides a method and apparatus for producing a roll formed shape which is curved along its length. 
     BACKGROUND OF THE INVENTION 
     Generally, automotive body panels are formed from metal sheet using various forming methods to create the desired shape. For example, to form a roof header for a truck cab, processes such as contour roll forming and stretch forming may be used. Two separate processes are generally required because the profile of a roof header has a compound angle and a non-uniform cross section that cannot be formed by a single operation. 
     Contour roll forming is one process used to form metal sheet into a desired shape. Contour roll forming is a cold rolling process in which an elongated metal sheet is fed through a series of longitudinally aligned roll stations to progressively shape of sheet. Contour roll forming may be used for symmetrical and asymmetrical shapes of uniform cross section. 
     A typical roll forming station includes a set of roller dies which are contoured for forming the metal sheet into the desired shape. The roller dies are set opposite each other along a parallel axis, for creating the upper and lower profile of the metal sheet. The roller dies may be positioned side by side or above and below each other depending on the shape forming requirements. 
     Roll forming produces lengths of material having a constant crosssection; however, many articles, such as roof headers for pickup trucks, require an additional longitudinal curvature. Therefore, the roll formed stock must then be stretched in a separate operation to create a curved outer edge resulting in a non-uniform cross section. In stretch forming, the metal sheet is positioned over a form block or mating dies having the desired profile. The sheet is then held in tension and stretched beyond its yield point, causing the metal to take the desired shape. Unlike contour roll forming, stretch forming may be used to create shapes with compound curves, twists and bends in multiple planes. A bending machine including a form block having adjustable grippers on either side is generally used to form a parabolic curve along the outer edge of the roll formed stock. Alternatively, the metal sheet may be placed between upper and lower dies and the parabolic curve formed with the mating dies. Adjustable grippers on either edge of the dies hold the metal sheet in place and stretches the material between the dies. 
     A disadvantage of this manufacturing process is that it involves two forming steps, there by requiring a multiplicity of machinery and abundance of time. Each forming method requires its own specific machinery in addition to the auxiliary equipment required for production operation. Stretch forming using a bending machine has the further disadvantage that is slow and not well suited to high volume production. Also, stretch forming requires extra metal sheet to allow for gripping the work piece during the forming process. Additional material must also be provided because of the possibility of spring back after the stretch forming process. 
     Clearly, it is desirable to consolidate or eliminate the requirement of using separate roll forming and stretching operations in the manufacture of complexly contoured articles such as truck roof headers. However, it has not heretofore been possible to roll form sheet metal while simultaneously stretching the sheet metal to introduce a longitudinal curvature in the plane of the sheet. As will be detailed hereinbelow, the present invention provides a novel roll forming process which also stretches a workpiece. 
     SUMMARY OF THE INVENTION 
     According to the present invention a stretch roll forming apparatus is provided that includes a plurality of roll forming stations. The roll forming stations are serially disposed so that a sheet metal workpiece may be advanced through each of the stations in sequence. Each station includes a pair of roller dies supported by an inboard support station and an outboard station, in a spaced apart relationship, so as to each be rotatable about a horizontal axis. The roller dies are configured to shape a sheet metal workpiece which is advanced therebetween. At least one of the roll forming stations comprises a stretch roll forming station wherein the roller dies each have a frusto-conical shape. The roller dies are disposed so that the taper of one of the roller dies extends from the inboard support to the outboard support, while the taper of the other die extends from the outboard support to the inboard support. When a sheet metal workpiece is advanced between the roller dies of the stretch roll forming work station, a first longitudinal edge of the workpiece is stretched relative to a second longitudinal edge such that a longitudinal curvature is imposed on the workpiece. 
     In some embodiments of the present invention, one of the roll forming stations is a stretch roll forming station while the other stations are non-stretch roll forming stations. The stretch roll forming station may be the final roll forming station or one or more non-stretch roll forming stations may be positioned after the stretch roll forming station. In other embodiments of the present invention a sweep station is provided which is operative to impose a curvature on the sheet metal workpiece out its original plane. The sweep station may be provided downstream of the roll forming stations. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a preferred embodiment of the present invention; 
     FIG. 2 is a side view of the stretch forming roll station of the present invention; 
     FIG. 3 is a cross-sectional view of the stretch forming roll station depicted in FIG. 2 taken along Lines  2 — 2 ; 
     FIG. 4 is a top plan view of a sweep station which may be utilized in conjunction with the present invention; 
     FIG. 5A is a perspective view of a roll formed workpiece which has not been stretched; and 
     FIG. 5B is a perspective view of a roll formed workpiece which has been stretched through the use of the present invention so as to provide a longitudinal curvature thereto. 
    
    
     DETAILED DESCRIPTION 
     The present invention overcomes the disadvantages of the prior art by providing a roll forming station that also stretch forms a workpiece thereby eliminating a step in the metal sheet forming process. The implementation of an additional roll station to perform the stretch forming eliminates the need for additional stretch forming equipment while decreasing production time. Deletion of the separate stretch forming process also eliminates the scrap metal sheet resulting from the need to grip the metal while forming the curvature about a form block. 
     One embodiment of the roll forming method and apparatus of the present invention is depicted in FIG. 1 at  10 , and comprises a series of six roll forming stations, specifically configured to produce a truck cab roof header, it being understood that the apparatus may be configured to produce other items. One of the stations performs the stretch roll forming method of the present invention and eliminates the need for stretch forming the roof header. A first roll forming station  12  is aligned in a series with three other roll forming stations  14 ,  16 ,  18 . A stretch roll forming station  20  is provided after the first four roll forming stations  12 ,  14 ,  16 ,  18  and is aligned longitudinally with the stations. A final roll forming station  22  is aligned longitudinally with stretch roll forming station  20  and is placed in the longitudinal series of stations at the end of the contour roll forming process generally shown at  10 . 
     In the preferred embodiment of the present invention, a truck cab roof header is formed by initially passing a metal sheet  24  longitudinally through a series of roll stations  12 ,  14 ,  16 ,  18 ,  20 ,  22 . A typical roll station  12  is equipped with a set of roller dies  26 ,  28  for forming the shape of the metal sheet  24 . Roller dies  26 ,  28  are set opposite each other along a parallel axis, creating the upper and lower profile of the metal sheet  24 . Roller dies  26 ,  28  are supported in their horizontal position by inboard stand  30  and outboard stand  32 . Typically, roll station  12  is referred to as a universal contour roll forming machine with outboard support for roll shafts  34 ,  36 . Outboard stand  32  can be adjusted horizontally by sliding it inwardly toward the inboard stand  30  or it may be removed to change roll shafts  34 ,  36  by sliding it outwardly away from inboard stand  30 . Outboard stand  32  may also be adjusted vertically by an adjusting screw (not shown) on top of the stand. Alternatively, adjustment may be accomplished by the use of shims or the like. Such non-stretch, roll forming stations are known in the prior art. Roll stations  14 ,  16 ,  18  and  22  are also universal contour roll forming machines with outboard support for roll shafts as described above. 
     The stretch roll forming station  20  of the present invention is similarly designed as a universal contour roll forming machine having outboard support for roll shafts. As shown in FIGS. 1 and 2, stretch roll forming station  20  includes an inboard stand  40  and an outboard stand  42 . Contoured roll dies  44  and  46  each have a frusto-conical shape, rather than the cylindrical shape of non-stretch forming roll dies  26 ,  28 , and perform the stretch roll forming process of the present invention. 
     As best shown in FIG. 3, frusto-conical roll dies  44 ,  46  oppose each other and are supported in a horizontal plane by inboard stand  40  and outboard stand  42 . Upper roll die  44  preferably has the frustum  48  extending from inboard stand  40  toward outboard stand  42 . Lower roll  46  preferably has frustum  50  extending from outboard stand  42  toward inboard stand  40 . Metal sheet  24  is then stretched roll formed between upper roll  44  and lower roll  46 . The circumference of opposed portions of the two tapered roller dies  46 ,  48  differ, and when the web of workpiece material is advanced therebetween, the rollers  46 ,  48  differentially stretch the material across its width. This differential stretching curves the workpiece along its length. The curvature is in the plane of the workpiece. As shown in FIG. 3, the lower roll  46  preferably has a raised central portion which acts as a forming ridge  47 . The upper roll  44  has a corresponding lowered central portion which acts as a forming groove  45 . Obviously, the groove  45  and ridge  47  may take other shapes depending on the application. 
     With reference now to outboard stand  42 , like roll station  12 , adjusting screws, shims or the like (not shown) may be provided under base  52  to adjust outboard stand  42  along a vertical plane. While the upper roll  48  is stationary about roll shaft  54 , the lower roll  46  is supported by a stationary ball joint assembly  56  within inboard stand  40  and an adjustable ball joint assembly  58  within outboard stand  42 . Lower roll  46  is completely adjustable about the horizontal and vertical plane of travel of metal sheet  24  by adjustable ball joint assembly  58 . Roller bearing assemblies  60 ,  62  are typically provided about roll shaft  64  for rotation of lower roll  46 . 
     With reference to FIGS. 1 and 2, the preferred process of the present invention is illustrated in the context of the forming of metal sheet  24  into a truck cab roof header by longitudinally passing metal sheet  24  through the series of six roll stations. Initially, metal sheet  24  is fed through first roll station  12  having a first set of contoured roll dies  26 ,  28 . Metal sheet  24  is bent 22½° at either end  66 ,  68  along a horizontal plane. Metal sheet  24  continues toward roll station  14  where contoured roll dies  70 ,  72  continue to shape metal sheet  24  by additionally bending either end  66 ,  68  by 22½° for a total bend of 45° Metal sheet  24  continues through roll stations  16  and  18  where contoured roll dies  74 ,  76  and  78 ,  80  continue bending ends  66 ,  68  an additional 22½°, respectively, to create a full 90° bend before passing through stretch roll forming station  40 . It is to be understood that the number of stations and the angle of bending achieved in each may be varied, depending on the particular item being fabricated, as is known in the art; and the principles of the present invention can be adapted to any roll forming process. 
     Once each end  66 ,  68  of metal sheet  24  is bent 90°, metal sheet  24  is passed through stretch roll forming station  20 . Opposing frusto-conical roll dies  44 ,  46  then shape an outer radial curvature within the horizontal plane of metal sheet  24 . In the preferred embodiment, end  68  is stretched formed by frusto-conical roll dies  44 ,  46  so that metal sheet  24  has an outer radius  82  greater than inner radius  84  along end  66  (see FIG.  2 ). 
     Once metal sheet  24  has passed through the first four roll stations  12 ,  14 ,  16 ,  18  and stretch roll forming station  20 , roll station  22  is provided to reshape the 90° bends formed along ends  66 ,  68  if necessary. Specifically, roll station  22  is provided with contoured roll dies  86 ,  88  having the same contour as roll dies  78 ,  80  provided in fourth roll station  18 . Roll station  22  reshapes metal sheet  24  if necessary where the 90° bends may be deformed by stretch roll forming station  20 . 
     Although not illustrated in FIG. 1, the roll forming system may also include a sweep stand disposed downstream of the last roll station  22 . As is known in the art, a sweep stand, also referred to as a sweep head, operates to impose a further curvature on the workpiece passing therethrough. In the illustrated embodiment, as depicted for the formation of a truck cab roof header, the sweep stand curves the stretch and roll formed workpiece in a direction generally perpendicular to the longitudinal curvature imposed by the stretch roll forming stand  40 . This curvature is in a direction generally perpendicular to the planar surface of the workpiece. 
     Sweep stands are well known in the art, and FIG. 4 is a top plan view of a sweep station which may be utilized in the present invention. As illustrated, the sweep head  100  is operative to receive the roll formed web  24 , and to impose a curvature on that web in a direction generally perpendicular to the illustration. The sweep head includes a support frame  102  which may be further mounted on a support stand (not illustrated) generally similar to the support stands used for the roll forming stations. The support frame  102  retains at least one set of sweep forming dies. Each die set includes an upper and a lower die member, and these dies have a profile generally corresponding to the roll formed profile of the web  24 . The illustrated embodiment  100  includes three of such die sets,  104   a ,  104   b ,  104   c . The illustration only the top member of each die set is shown, it is understood that the bottom members are generally similar and aligned therewith; however, there is a slight offset between the top and bottom members of each die set, and it is this offset which produces the sweep curvature. Preferably, the members of the die set are formed from hard bronze material, and one preferred material is an alloy sold under the designation Ampco® 21. Adjustment screws, for example screws  106  and  108 , permit adjustment of the dies relative to one another. The sweep station is an optional component of the systems of the present invention, and may be eliminated or otherwise configured depending on the particular items being fabricated, 
     Various characteristics of workpieces fabricated in accord with the present invention are better explained by reference to FIGS. 5A and 5B. FIG. 5A is a perspective view of a roll formed workpiece  110  which has not been subjected to the stretch roll forming process of the present invention. The workpiece  110  is a generally planar body of sheet metal having a first contoured ridge  112  projecting upwards from the plane of the workpiece and a second contoured ridge  114  projecting downward therefrom. The two ridges  112 ,  114  are generally parallel to one another, and run the entire length of the roll formed workpiece  110 . The two edges  116 ,  118  of the workpiece  1   10  are straight and generally parallel to one another; although it is to be understood that in some roll forming processes, edges are distorted and hence non-parallel. FIG. 5B depicts a workpiece  120  which is generally similar to the workpiece  110  of FIG. 5A in its contour, but which has been subjected to a stretch roll forming process according to the present invention, and hence is curved along its length. The workpiece  120  of FIG. 5B has a cross-section with ridges  112 ,  114  generally similar to those in FIG. 5A; however, the edges  122 ,  124  are both curved along their lengths, in a plane corresponding generally to the plane of the workpiece  120 . It is to be understood that workpiece  120  is not entirely planar, since it includes curved features; however, in the context of this disclosure, the plane of the workpiece is understood to be the plane which was defined by the sheet of workpiece material prior to its deformation in the roll forming process. Although not shown in FIG. 5B, the workpiece may be further curved in a direction generally perpendicular to the plane of the workpiece through the use of a sweep station as discussed above. 
     Although the present example describes roll forming of metal sheet  24  having symmetrical bends of 90° along ends  66 ,  68 , stretch roll forming through roll form station  20  may also be performed on asymmetrical profiles. Also, while this invention has been described in connection with the fabrication of a header element for a vehicle roof, it may be used in conjunction with the roll forming of other members. 
     A significant advantage of the present invention is the elimination of the separate stretch forming process to create the desired profile of metal sheet  24 . Elimination of the separate stretch forming process cuts down on cost and scrap metal while simultaneously increasing production. The ability to feed metal sheet  24  continuously through a series of roll stations while simultaneously bending and stretch forming metal sheet to its desired profile is novel to the art, eliminates multiple machinery, decreases production time and also eliminates scrap. Thus, the present invention provides a simple and easy way to form a metal sheet into a symmetrical or asymmetrical radially curved profile. 
     Having described the various embodiments of the present invention with reference to the accompanying figures, it will be appreciated that various changes and modifications can be made without departing from the scope or spirit of the invention.