Abstract:
The present disclosure relates to a method for forming an end portion of a tube. The method includes rotating the tube about a longitudinal axis, heating the end portion of the tube, and forming the end portion of the tube by reciprocating a forming member along a succession of curved forming paths. Each curved forming path is tangent to and curves away from a corresponding one of a succession of angularly spaced apart, substantially straight reference lines. A forming reduction is provided between each curved forming path and its corresponding one of the reference lines.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to forming techniques. More particularly, the present invention relates to spin forming techniques for shaping metallic tubes. 
     BACKGROUND OF THE INVENTION 
     Spin forming techniques are used to manufacture tanks, drums and other vessels. For example, spin forming techniques are used to manufacture vessels ranging in size from fire extinguishers to heads for concrete truck drums. Spin forming techniques commonly include rotating a cylindrical tube about its longitudinal axis, while concurrently heating an end portion of the tube. The tube is formed by applying pressure to the heated end portion to either constrict or expand the end portion of the tube. In this regard, U.S. Pat. No. 2,408,596 (Bednar et al.) discloses a method for forming cylindrical ends by torch heating, rotating, and applying pressure to a cylindrical work piece. Pressure is applied by a tool moving in arcuate paths that curve toward a desired contour of the work piece. Similarly, U.S. Pat. No. 5,235,837 (Werner) discloses an apparatus for producing thin-walled cylindrical pressure vessels or tanks through metal spinning operations. A cylindrical work tube is rotated about its longitudinal axis, and the end of the work tube is heated by heating torches. Forming rollers are moved along a plurality of arcuate stroking paths to shape the end of the work tube. The arcuate stroking paths curve toward a desired final shape of the work tube. 
     U.S. Pat. No. 5,598,729 (Hoffmann et al.) discloses another method for spin forming a metallic tube. The metallic tube is rotated about its longitudinal axis, and inductive heating elements are used to heat an end portion of the tube. Forming rollers are used to form the heated end portion of the tube. The forming rollers are moved along a succession of angularly spaced apart, substantially straight forming passes. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention relates to a method for forming an end portion of a tube. The method includes rotating the tube about a longitudinal axis, heating the end portion of the tube, and forming the end portion of the tube by reciprocating a forming member along a succession of curved forming passes. Each curved forming path is tangent to a corresponding one of a succession of angularly spaced apart, substantially straight reference lines. A forming reduction is provided between each curved forming path and its corresponding one of the reference lines. 
     Another aspect of the present invention relates to a method for constricting an end portion of a tube. The method includes rotating the tube about a longitudinal axis, heating the end portion of the tube and constricting the end portion of the tube by reciprocating a forming member along a succession of curved forming passes. Preferably, at least some of the curved forming paths curve away from the longitudinal axis of the tube. 
     A further aspect of the present invention relates to a method for forming an end portion of a tube from an initial shape to a desired final shape. The method includes rotating tube about a longitudinal axis, heating the end portion of the tube, and forming the end portion of the tube by reciprocating a forming member along a succession of curved forming paths. Preferably, at least some of the curved forming paths curve away from the desired final shape. 
     A variety of advantages of the invention will be set forth in part in the description that follows, and in part will be apparent from the description, or may be learned by practicing the invention. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several aspects of the invention and together with the description, serve to explain the principles of the invention. A brief description of the drawings is as follows: 
     FIGS. 1-6 schematically show a succession of curved forming paths suitable for forming an end portion of a tube in a manner consistent with the principles of the present invention; 
     FIG. 7 schematically illustrates a curved forming path in accordance with the principles of the present invention. 
     FIG. 8 schematically illustrates multiple orientations that a forming roller can be oriented in as an end portion of a tube is formed; and 
     FIG. 9 schematically illustrates an apparatus suitable for forming a tube in a manner consistent with the principles of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference will now be made in detail to exemplary aspects of the present invention that are illustrated in the accompanying drawings. Wherever possible, these same reference numbers will be used throughout the drawings to refer to the same or like parts. 
     FIGS. 1-6 schematically illustrate a method in accordance with the principles of the present invention for forming an end portion of a cylindrical tube  20 . For ease and clarity of explanation, only five separate forming paths have been illustrated. However, it will be appreciated that in actual practice, multiple intermediate forming paths will be made between the five representative paths depicted in the figures. Consequently, many more than five paths would generally be used to form a particular cylindrical tube into a desired shape. Additionally, for ease of explanation, the curvatures of the forming paths have been greatly exaggerated. 
     FIG. 1 illustrates an end portion  22  of the cylindrical tube  20  prior to forming. The tube  20  extends along a central longitudinal axis  24 . Preferably, the tube  20  is made of a metal material, and the end portion  22  of the tube  20  is inductively heated prior to and during the forming steps of FIGS. 2-6. U.S. Pat. No. 5,598,729 to Hoffmann et al., which is hereby incorporated by references, discloses exemplary techniques that can be used to inductively heat the end portion  22  of the tube  20 . 
     The tube  20  is preferably rotated about the longitudinal axis  24  by suitable techniques such as a lathe or a spinning table. Preferably, rollers  26  are used to form or shape the end portion  22  of the tube  20 . Each roller  26  preferably has a central axis of rotation for allowing the rollers  26  to rotate as they contact the tube  20 . As shown in FIGS. 2-5, the rollers  26  include a pair of rollers positioned on opposite sides of the longitudinal axis  24 . However, it will be appreciated that a single roller, or more than two rollers, could also be used. Also, multiple rollers can be used along common axes of rotation, or along parallel axes of rotation. 
     It will be appreciated that the forming technology disclosed in the present specification can be used for tubes having various different sizes. For example, the technology can be used for tubes having diameters as small as 10-16 inches to as large as 8-10 feet. Of course, the technology can also be used for tubes having diameters other than those specifically described. 
     FIGS. 1-6 illustrate a method for constricting the tube  20  to form a hemispherical end cap. However, it will be appreciated that the various aspects of the present invention can be used in forming applications that either constrict or expand a tube. Additionally, the present invention is useful in forming various different types of shapes. For example, the present invention is applicable to forming exemplary end shapes such as hemispherical ends, semi-elliptical ends, conical ends, toriconical ends, torospherical ends, combined shapes, non-concentric shapes, and user specified arbitrary shapes. 
     As described above, FIGS. 2-6 illustrate five forming paths for constricting the end portion  22  of the tube  20  to form a ellipsoidal cap. It will be appreciated that during each of the forming paths of FIGS. 2-6, the tube  20  is continuously rotated about the longitudinal axis  24 , and the end portion  22  of the tube  20  is preferably continuously heated. 
     In FIGS. 2-6, the rollers  26  are shown traversed along five separate forming paths. As the rollers  26  are traversed along the forming paths, the rollers  26  contact the end portion  22  of the tube and the metal of the end portion  22  of the tube  20  follows and conforms to the paths traversed by the rollers  26 . Consequently, each of the forming paths shown in FIGS. 2-6 is also representative of the shape of the end portion  22  of the tube  20  after each path has been completed. 
     Referring now to FIG. 2, the forming rollers  26  are traversed along first curved forming paths  28 . Each curved forming path  28  is tangent to a corresponding substantially straight reference line  30 . The reference lines  30  are aligned at oblique angles with respect to the longitudinal axis  24  of the tube  20 . 
     Each first curved forming path  28  curves away from its corresponding reference line such that a forming reduction is provided between each first curved forming path  28  and its corresponding reference line  30 . The term “forming reduction” is intended to mean that less metal is displaced by the rollers  26  by following the first curved paths  28  than would have been displaced had the rollers  26  followed the reference lines  30 . For example, as shown in FIG. 2, the first curved paths  28  provide a forming reduction distance D 1  relative to their corresponding reference lines  30 . The forming reduction distance is measured from the end of each curved path to the location on the reference line where the tube would have ended had the roller been moved along the corresponding reference line. It has been determined that the forming reduction provides the advantage of reduced forming force requirements at the edge of the tube, and more uniform metal flow from knuckle  31  (i.e., the boundary between the portion of the tube being formed and a portion of the tube that has already been formed to a final desired shape) to tube edge  33 . The forming reduction also assists in controlling the wall thickness of the end portion  22  of the tube  20 . 
     FIG. 3 illustrates the rollers  26  being moved along second curved paths  32  that are tangent to second reference lines  34 . Because the tube  20  is being constricted, the second reference lines  34  are angled closer to perpendicular relative to the longitudinal axis  24  as compared to the first reference lines  30 . 
     The second curved paths  32  are curved relative to their corresponding second reference lines  34  to provide forming reductions having distances D 2 . Because the paths  32  start from radially inward positions and move radially outward, the forming reductions are located at the starting points of the paths  32 . Preformed regions  36  are not contacted by the rollers  26  as they are moved along the second curved paths  32 . The preformed regions  36  were formed to their desired final shape during subsequent paths of the rollers  26  and are separated from the forming regions by a knuckle  37 . Because the preformed regions  36  have already been formed to their desired final shape and need not be modified by the rollers  26 , the second curved paths  32  are preferably shorter than the first curved paths  28 . 
     FIG. 4 illustrates the forming rollers  26  being moved along third curved paths  38  that are tangent with respect to corresponding third reference lines  40 . The third reference lines  40  are closer to perpendicular relative the longitudinal axis  24  as compared to the second references lines  34 . The third curved paths  38  curve away from the third reference lines  40  to provide forming reductions having distances D 3 . Preformed regions  42  are not contacted by the third curved paths  38 . A knuckle  39  forms a boundary between the curved paths  38  and the preformed regions  42 . The preformed regions  42  have been previously formed to their final desired shape and are longer than the preformed regions  36  of FIG.  3 . Consequently, the third curved paths  38  have shorter lengths than the second curved paths  32 . 
     FIG. 5 illustrates the rollers  26  being moved along fourth curved paths  44  that are tangent to corresponding fourth reference lines  46 . The fourth reference lines  46  are aligned closer to perpendicular relative to the longitudinal axis  24  as compared to the third reference lines  40 . The fourth curved paths  44  curve relative to their corresponding reference lines  46  to provide forming reductions having distances D 4 . The forming reductions are provided at the beginnings of the paths. As shown in FIG. 5, the end portion  22  of the tube  20  includes preformed regions  48  that are not contacted by the forming rollers  26  during the fourth curved paths  44 . The preformed regions  48  have previously been formed to a final desired shape during subsequent paths of the rollers  26 . Referring to FIGS. 4 and 5, the preformed regions  48  of FIG. 5 have lengths that are longer than the preformed regions  42  of FIG.  4 . Consequently, the fourth curved paths  44  of FIG. 5 are shorter than the third curved paths  38  of FIG.  4 . 
     Referring now to FIG. 6, the ellipsoidal shape of the end portion  22  is completed by moving at least one of the rollers  26  along a linear path  60  that is tangent to the top of the ellipse. In this manner, the roller  26  closes the ellipse and completes the forming process. 
     The aspects of the present invention can be used to form complete heads having curved shapes such as hemispherical ends, semi-elliptical ends and other curved ends. The present invention can also be used to form heads that are not complete or enclosed such as conical heads. In forming a head such as a conical head, a work-out routine can be used to flatten or otherwise work out undesired curvature along the head. 
     In the method of FIGS. 1-6, the curved paths  28 ,  32 ,  38  and  44  comprise circular arcs. While the curved paths  28 ,  32 ,  38  and  44  have been shown as circular arcs, it will be appreciated that other types of curvatures can also be used. For example, parabolic, hyperbolic, elliptical or even random curvatures can be used. 
     FIGS. 1-6 show a method for constricting the end portion  22  of the tube  20 . In performing the constriction, the curved paths  28 ,  32 ,  38  and  44  curve away from the longitudinal axis  24  of the tube  20  and also curve away from the desired final shape of the end portion (e.g., the ellipsoidal shape of FIG.  6 ). It will be appreciated that when a tube is expanded, the curved paths preferably curve towards the longitudinal axis of rotation of the tube being expanded, but still curve away from the desired final shape. 
     FIG. 7 shows a particular curved forming path  70  in accordance with the present invention that corresponds to a reference line  72 . The reference line  72  intersects a side wall  74  of a tube  76  desired to be formed at point  78 . The reference line  72  forms an angle θ relative to the side wall  74 . The curved path  70  is tangent to the point  78  and intersects a forming reduction line  80  at point  82 . An angle α is formed between the forming reduction line  80  and the reference line  72 . In certain cases, the angle α has a value that is less than 5% of the angle θ. For many applications, the angle α has a value that is about 1% or 2% of the angle θ for forming passes made early in the forming process. Due to a cumulative effect, the angle α can be as much as 50-100% of the angle θ later in the forming process. The curved path  70  is shown as a circular arc and extends between a first radius R 1  perpendicular to the reference line  72  at the tangent point  78 , and a second radius R 2 . 
     FIG. 8 shows a forming roller  90  that is being moved along a curved forming path  92 . The forming roller  90  has a curved nose  94 . The curved nose  94  is in the shape of a circular arc centered about a center point  96 . During movement of the roller  90  through the curved forming path  92 , the roller  90  can be pivoted about the center point  96  of the nose  94 . Such rotation changes the part of the nose  94  that contacts the piece  98  being formed, but does not change the location at which the piece  98  is contacted by the roller  90 . Pivoting of the roller  90  about the center point  96  of the nose  94  provides the advantage of increased mechanism flexibility providing for easier collision avoidance (e.g., between rollers and heating elements). In this manner, mechanism limitations can be overcome or circumvented without compromising forming paths, etc. The process could also be used in other applications using members having radiused noses such as grinding processes having radiused grinding wheels. Further, the processors could be used with straight or curved forming paths. 
     FIG. 9 illustrates a system  120  for forming a tube  122  in accordance with the principles of the present invention. The system includes a rotation table  124  adapted to hold the tube  122  in a vertical orientation. Of course, the tube  122  can also be rotated in a horizontal orientation. The rotation table  124  is adapted to rotate the tube  122  about a vertical axis of rotation  126 . Forming rollers  128  are used to shape an end portion of the tube  122 . The forming rollers  128  are preferably rotatably connected to robotic mechanisms  130 . The robotic mechanisms preferably have multiple joints, articulations and pivot locations and are adapted for three-dimensionally moving the forming rollers  128 . One suitable type of robotic mechanism having model S-400 is sold by Fanuc Ltd., of Japan. 
     The tube  122  is preferably heated by an inductive heating element  132 . The inductive heating element  132  is preferably mounted on a mechanism  134  similar to the robotic mechanisms  130 . 
     The rotation table  124  and the mechanical mechanisms  130  and  132  are preferably controlled by a central or integrated processing units such as a computer  138 . A user interface  140  is used to input data into memory associated with the computer  138 . The computer preferably includes a software driven processing unit for controlling or orchestrating the movement of the inductive heating elements  132  and the forming rollers  128 . For example, a user can input data such as the diameter of the tube  122 , and the desired final shape of the tube  122 . Based on such information, the computer calculates coordinates for moving the forming rollers  128  and the inductive heating element  132  in an orchestrated manner such that the desired shape is formed. 
     With regard to the foregoing description, it is to be understood that changes may be made in detail, especially in matters of the construction materials employed and the shape, size and arrangement of the parts without departing from the scope of the present invention. It is intended that the specification and depicted aspects be considered exemplary only, with a true scope and spirit of the invention being indicated by the broad meaning of the following claims.