MXPA98004698A - Apparatus and method for curving vine leaves - Google Patents

Abstract

The present invention provides an apparatus and method for a forming sheet that includes a support frame and a forming rail supported on the frame. The forming rail has a sheet forming surface that adapts in elevation and contour to the desired final shape of a marginal edge of a sheet of glass to be shaped. An auxiliary rail having a sheet forming surface generally corresponding to a preliminary shape of a selected marginal edge portion of the sheet is positioned along a section of the forming rail with a corresponding sheet forming surface portion. in general to the desired final shape of the selected marginal edge portion of the sheet. The auxiliary rail is mounted for movement relative to the forming rail section from a first position, where portions of the sheet forming surface of the auxiliary rail are above the sheet forming surface portion of the forming rail section, and a second section, wherein the sheet forming surface of the auxiliary rail is positioned below the surface forming portion of the forming rail section. When the auxiliary rail is in its first position, it is capable of supporting the selected marginal edge portion of the sheet above the forming rail section and of preliminarily shaping the sheet. When the auxiliary rail is in its second position, the surface forming portion of the forming rail section is able to support and conform the selected marginal edge portion of the sheet to the final shape desired.

Description

APPARATUS AND METHOD FOR CURVING GLASS LEAVES BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to the buckling curvature of glass sheets on bending molds and, in particular, to an apparatus and method for controlling buckling curvature along selected portions of glass. the glass sheets while the sheets pass over molds to bend through a heating furnace. 2. Technical Considerations In effecting the buckling curvature to form shaped glass windows for automobiles and the like, as described in U.S. Patent 4,375,978 issued to Reese, a glass sheet is supported on a structural bending mold. . The forming rail of the mold has a shape and configuration similar to that of the glass sheet formed in a slightly inward position of its peripheral edge. The bending molds are then transported in succession through a heating furnace where the glass sheet is heated to its deformation temperature in such a manner that it begins to undergo gravity buckling until the glass sheet conforms to the configuration of the glass sheet. conformador lane. After forming the glass sheet, the mold is transported through an annealing zone, where the glass sheet is cooled in a controlled manner from its deformation temperature through its annealing band to temper the glass sheet . The technique of curvature by gravity buckling has been used to simultaneously form two sheets of glass, or doublets, sheets that are then laminated together to form a car windshield. As car stylists strive for more aerodynamic designs, windshields are taking on more complex and deeper curvature configurations, which are becoming increasingly difficult to form through conventional buckling curvature operations. It has been found that, by producing glass doublets shaped with a deep curvature along their center line, because of the amount of heat and the time period necessary to achieve such deep buckling, some portions of the glass near the rails of conformation are prone to curvature by excessive buckling. Several devices have been developed to facilitate the control of the shape of the glass sheets. In U.S. Patents 4,265,650 and 4,894,080 to Reese et al., The glass sheets are pressed against a press face or between two opposing press faces. In U.S. Patents 4,084,397 to Stas et al. And 5,049,178 to Pareman et al., Partial presses are used to contact and press selected portions of the glass sheets. These shaping methods positively form the sheets in the desired configuration, but, because the press faces contact the glass surfaces, there may be some marking. U.S. Patent 5,167,689 issued to Weber controls the curvature at the corners of the glass sheets supported in an outline mold by buckling the sheets in a preliminary curved configuration and then lifting peripheral portions of the sheet from the forming rails curved, the additional forming rails having the desired final elevation configuration. This assembly reduces the reverse curvature at the corners of the glass sheets, but does not address the problem of excessive glass buckling near the forming rails. It would be advantageous to develop an apparatus and method for forming glass sheets while controlling the excessive deformation of the glass near the peripheral forming rails. SUMMARY OF THE INVENTION The present invention provides an apparatus and method for a forming sheet including a support frame and a forming rail supported on the frame. The forming rail has a sheet forming surface that adapts in elevation and contour to the desired final shape of a marginal edge of a sheet of glass to be shaped. An auxiliary rail having a sheet forming surface generally corresponding to a preliminary shape of a selected marginal edge portion of the sheet, is positioned along a section of the forming rail with a sheet shaping surface portion generally corresponding to the desired final shape of the selected marginal edge portion of the sheet. The auxiliary rail is mounted for movement relative to the forming rail section from a first position, where portions of the sheet forming surface of the auxiliary rail are above the surface forming portion of the forming rail section, and a second position, wherein the sheet forming surface of the auxiliary rail is positioned below the surface forming portion of the forming rail section. When the auxiliary rail is in its first position, it is capable of supporting the selected marginal edge portion of the sheet above the forming rail section and of preliminarily shaping the sheet. When the auxiliary rail is in its second position, the sheet forming surface portion of the forming rail section is capable of supporting and shaping the selected marginal edge portion of the sheet to the desired final shape. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cut-away perspective view of a heating furnace representing glass sheets supported on an annular contour mold embodying the present invention, with portions removed for reasons of clarity. Figure 2 is an enlarged perspective view of a pivot wing section of the annular mold illustrated in Figure 1, with portions removed for reasons of clarity. Figure 3 is an end view of the pivot wing section of the annular mold illustrated in Figure 1 depicting an auxiliary rail in a raised and lowered position, with portions removed for reasons of clarity. Figures 4, 5 and 6 are schematic sectional views of the pivot section of the annular mold illustrated in Figure 1 taken along the line AA at different stages during a glass sheet buckling operation, with portions removed for reasons of clarity. Figure 7 is an end view of an alternative embodiment of the annular mold of the present invention, with portions removed for reasons of clarity, depicting an auxiliary rail in a raised and lowered position. Figure 8 is a side view of another embodiment of the annular mold of the present invention, with portions removed for reasons of clarity, representing an auxiliary rail in a raised and lowered position. Figures 9, 10 and 11 are schematic perspective views corresponding in general to Figures 4, 5 and 6, respectively, representing the sequence of forming a sheet supported on the contour rail of the present invention, with portions removed by reasons for clarity. DETAILED DESCRIPTION OF THE INVENTION The invention disclosed herein is presented in combination with a conventional stopping and starting type heating furnace, where the glass sheets are supported on forming rails within separate heating chambers that are transported sequentially to through the furnace to heat and shape the glass sheets, in a manner known in the art. It will be appreciated that the present invention can be incorporated in the curvature operation by buckling glass sheets using other types of heating ovens, for example, conventional tunnel type ovens. In the process of forming glass sheets by bending by gravity buckling, one or more glass sheets G are placed on a transport frame 10 at a loading station (not shown) and passed through a heating oven 12, where they are heated to their heat softening temperature and allowed to undergo gravity buckling to the desired shaped configuration. After shaping, the sheets G and the frame 10 are transported through annealing and cooling zones of the oven 12 to minimize the stresses on the glass and consolidate the shape of the glass. With reference to figure 1, a conveyor belt composed of a plurality of short rollers 14 arranged in longitudinally spaced, transversely opposite relationship, extends over the entire length of the furnace 12 and defines a path of movement of the frame 10 along a longitudinal reference line through from the oven. Each short roller 14 is mounted on an axle extending through a side wall of the furnace 12 and is connected to conveyor drive means (not shown). The conveyor belt can be divided into several sections, each moved by its own driving means, or the sections of the conveyor belt can be moved by a common drive mechanism by clutches, in a manner known in the art. While not limiting the present invention, the frame 10 illustrated in Figure 1 includes an annular mold 16, similar to the mold described in U.S. Patent 4,626,267 to Reese and U.S. Patent 4,804,397 to Stas et al. whose ideas are incorporated by reference to the present specification, having a sheet forming surface 18 that adapts in elevation and contour to the final desired shape of the marginal edge of the glass sheets G to be shaped. The frame 10 may include other insulating structures (not shown) enclosing the frame 10 and isolating it from adjacent frames. In this particular embodiment, the mold 16 is an articulated mold with pivoting end sections; however, it should be appreciated that the present invention can be used in combination with a non-articulated annular mold. The mold 16 is provided with opposed spaced central rails 20 (only one is shown in Figure 1) and two pivoting end sections 22, each of which includes side rails 24 and an end rail 26. Each end section 22 it is supported by a cantilever beam 28 positioned below the corresponding rails 24 and 26. The cantilever beam 28 protrudes from the end sections 22 towards a pivot in a post 30 and is attached to a lever arm with ballast 32. As the glass sheets G supported in the annular mold 16 are heated, the lever arm 32 descends by the force of gravity overcoming the lesser counterforce of the glass sheets G as they are softened by heat to pivot the end sections 22 upwards to a closed position as shown in Figure 1. In this closed position, the rails 20, 24 and 26 form the shaping surface 18 and the glass sheets G undergo gravity buckling. to contact and conform by the surface 18 slightly within its periphery. The following explanation relates to the use of an additional shaping rail in one of the pivoting end sections 22 to control the shape of the glass during a gravity buckling operation, but it should be understood that, if necessary, the additional rails they can be similarly positioned along portions of the central rails 20 or the end section rails 24 and 26 at the opposite end of the mold 16. With reference to Figure 2, the end section 22 of the mold 16 includes further an auxiliary rail 34 extending the length and is positioned adjacent to the end rail section 26 of the end section 22. The auxiliary rail 34 is supported on the frame 10 to move between a raised position, where the surface The upper sheet forming portion 36 of the auxiliary rail 34 is generally positioned above the upper sheet forming surface 38 of the adjacent rail 26 as illustrated in FIGS. Figures 1, 2 and 3, for preliminarily supporting and shaping a marginal edge portion 40 of the glass sheets G, and a lowered position, where the surface 36 of the auxiliary rail 34 is positioned below the surface 38 of the adjacent end rail 26 as illustrated by the lines of transparency 42 in Figure 3 in such a way that the rail 26 can support and shape the marginal edge portion 40 in its desired final configuration during a portion of the buckling process. In particular, the elevation profile of the shaping surface 36 is configured to have a curvature that is less than that of the surface 38 of the adjacent rail 26 so that, while the marginal edge portion 40 is supported in the auxiliary rail 40, can not experience buckling to its final configuration. Delaying the curvature by final buckling of the sheets G and, in particular, the marginal edge portion 40 and the portions of the sheets near the edge portion 40 within the end section 22, until late in the bending operation by buckling, as will be explained later in more detail, the amount of time available for said areas of the glass sheets G to experience buckling by gravity is reduced so that the sheets can not experience excessive buckling in said critical zones. While not limiting the present invention, in the particular embodiment illustrated in Figures 1-3, the shaping surface 36 of the auxiliary rail 34 has a straight profile, i.e., there is no vertical curvature along its length and is positioned in a manner which is aligned slightly above the highest points of the adjacent rail 26. The auxiliary rail 34 can be moved from its raised position to its lowered position by gravity or by a mechanical or electromechanical device. Without limiting the scope of the present invention, Figures 2 and 3 illustrate one embodiment of a gravity type assembly. More specifically, the end 44 of the rail 34 is mounted so as to pivot about the post 45. A bracket 46, which includes a projection 48, is pivotally fixed to the cantilever beam 28 or a guide 50 of the pivot section 22. When it is in its raised position, the rail 34 is supported at one end by the post 45 and its opposite end sits on the projection 48. At a predetermined moment during the process of curvature by buckling of the glass sheet, the bracket 46 is pivoted moving away from rail 34, causing rail 34 to exit protrusion 38 and slide downward, as shown by lines of transparency 42 in Figure 3, pivoting about post 45. Guide 50 maintains the position of auxiliary rail 34 generally along the adjacent lane 26. In the particular embodiment illustrated in FIG. 2, the guide 50 includes a plate 52 with a slot 54 through which the lane 34 extends and along which it slides. . While not limiting the present invention, the pivoting action of the bracket 46 occurs when the end section 22 of the mold 16 pivots upward by a predetermined amount. More particularly, with reference to Figures 4 to 6, the arm 56 extends from the bracket 46 and includes a pivot rod 58 that extends through a portion of the frame 10. The stop 60 is fixed to the rod 58 in such a way that, when the end section 22 of the mold 16 is pivoted down to an open position, the stop 60 is separated from the frame 10, as illustrated in Figure 4. As the glass sheets G supported in FIG. lanes 20, 24 and 34 are heated and softened, the pivot end section 22 begins to rotate upwards, as indicated by arrow 62 in figure 5, to slowly obtain the desired final upward contour of the sheet. The bracket 46 moves with the section 22 as it pivots and continues to hold the rail 34 in its raised position as shown in Figure 5. As the end section 22 pivots, the rod 58 moves upward, approaching the stop 60 to the frame 10. When the end section 22 has pivoted upward by a predetermined amount, the stop 60 will contact the frame 10. As the end section 22 continues to pivot upwardly, the bracket 46 is prevented from maintaining its orientation with in relation to the rail 34 and pivoted, as indicated by the arrow 64, which causes the rail 34 to come out of the projection 48 and fall to its lowered position, as illustrated in Figure 6, and deposit the sheet on it. end rail 26. Although in the embodiment of the invention illustrated in Figures 1-3 the bracket 46 is positioned at one end of the rail 34, it should be appreciated that it can be placed at any other position along the rail 34, by ej Emplo, at its center. It should also be appreciated that other types of devices can be used to move the rail 34 from its raised position to its lowered position. For example, the bracket 46 can be replaced by a linear actuator, such as an electric motor, ball screw, electric motor, or the like, for raising and lowering one end of the rail. It is also contemplated that instead of pivotally mounting the auxiliary rail 34 at one end and using a support device to move its opposite end, the rail 34 can be vertically rotated between its up and down positions. More specifically, with reference to Figure 7, the opposite ends of the rail 34 can be supported by linear actuators 66, as explained above, to raise both ends of the rail 34 to their raised position and lower them to their lowered position, as indicated the lines of transparency 68. These types of actuators may be connected to a controller (not shown) controlling the rise and fall of the rail 34 during the curvature operation by gravity buckling. It is also contemplated that, instead of vertically alternating the rail 34, the rail 34 can be moved from its upward position to the downhill by a pivoting action. More specifically, with reference to Figure 8, the rail 34 may include additional sections 70 (only one is shown in Figure 8) which are pivotally mounted in the frame 10 so that, instead of simply falling from its raised position to the descent, lane 34 and additional sections 70 turn away from the adjacent lane 26, as indicated by transparency lines 72. Lane 34 and section 70 can be raised and lowered in the manner previously explained. When the rail moves from its upward to downward position, the marginal edge portion 40 of the glass sheet G is transferred from the upper shaping surface 36 of the auxiliary rail 34 to the upper shaping surface 38 of the rail 26. As has been explained above, the movement of the rail 34 to its lowered position can take place at any desired time during the buckling cycle. However, in determining when the auxiliary lane 34 can be lowered, it should be remembered that there must be sufficient time after the glass sheets G have been transferred from lane 34 to lane 26 so that the glass G undergoes buckling by gravity in contact with the shaping surface 38 of the rail 26 and assume its final configuration. More specifically, at a buckling curvature temperature of between about 571 and 677 ° C (1,060-1,250 ° F), it is estimated that glass G will take approximately 10 to 20 seconds to experience buckling in contact with lane 26 after being deposited. above, depending on the depth of the curvature provided by the shaping surface 38. In addition, care must be taken not to allow bending by excessive buckling in the inward areas of the rail 26 after having experienced buckling the marginal edge portions 40 a contact with the shaping surface 38. It is considered that the rapid descent of the auxiliary rail 34 from its upward to downward position can accelerate the final shaping of the marginal edge portion 40 from its preliminary shape to its desired final shape. In operation, the annular mold 16 of the present invention forms the sheets first in one direction, to preliminarily form the sheets in general in the longitudinal direction, and then in a second direction, to impart a transverse curvature and to form the sheets in their desired final configuration. More specifically, the end sections 22 of the annular mold 16 are pivoted downwards and the auxiliary rails 34 (one in each section 22) are moved to their raised position, as shown in figures 4 and 9, manually or with a mounting automatic lane placement. The glass sheets G (only one shown in Figures 4-6 and 9-11) are placed on the annular mold 16 and are supported on rails 34 and end sections 74 of the side rails 20. When the sheets are heated , soften and experience buckling by gravity. The marginal edge portions 76 along the longitudinal sides of the glass sheets G undergo buckling in contact with and are formed by central rails 20 and the rails 24 in the end section 22 when the arms with ballast 32 (represented only in Figure 1) pivot each end section 22 upwards, as shown in Figures 5 and 10. The marginal edge portions 40 of the glass sheets G are supported on the auxiliary rail 34, which is initially placed on top of the rail 26 of end section 22 and remains undeformed. As a result, the glass sheets G deform initially assuming their longitudinal curvature, that is, a generally cylindrical curvature from one end section 22 to the other. During this preliminary conformation, there may be a certain curvature by transverse buckling of the glass, ie, between the central rails 20, but the curvature by transverse buckling of the leaves G is limited because the shaping surface 36 of each auxiliary rail 34 is configured to limit the buckling curvature of the marginal edge portions 40 of the sheets G immediately inside the auxiliary rails 34, as shown in figures 5 and 10. At the desired time in the curvature cycle, the rails 34 are lowered, depositing the marginal edge portions 40 on the rails 26 of the sections of end 22, as shown in Figs. 6 and 11. The glass sheets G continue to experience gravity buckling for the time required for the marginal edge portions 40 to contact the forming surfaces 38 of the rail 26 and complete the transverse shaping of the glass. G in the pivoting sections 22.

After completion of the shaping, the frame 10 and the shaped glass sheet G are transported through the annealing and cooling sections (not shown) of the oven 12 where the glass sheets G are controllably cooled. The shaped sheets G are removed after the mold 16 for further processing. During the buckling curvature operation, additional sheet forming assemblies may be used to further deform the sheets. More specifically, partial or full surface press faces may contact the upper and / or lower main surfaces of the glass sheets G to contact and shape selected portions of the glass in the desired configuration, for example, as described in the United States 4,804,397. When using additional press molds in combination with an articulated ring mold as described herein to form the glass, it is desirable to incorporate a locking device into the pivoting end sections 22 of the mold 16 so that the end sections 22 do not turn down when glass G is contacted and pressed into shape, as described in U.S. Patent 4,804,397. As explained above, in the embodiment of the invention illustrated in FIGS. 1 and 2, the auxiliary rail 34 is a straight rail with a straight blade forming surface 36 positioned outside the rail 26 and moves vertically from its raised position. on the way down. The possibility of modifying the position and shape of the lane is contemplated. For example, the auxiliary rail 34 may extend only along a portion of the rail 26 so as not to support the entire marginal edge 40 of the leaves G. In addition, the auxiliary rail 34 may be placed within the rail 26 rather than that was. Further, although it is preferred that the sheet forming surface 36 of the auxiliary rail 34 be placed on top of the sheet forming surface 38 of the rail 26 when the rail 34 is in its raised position, it is contemplated that the surface 36 can be placed over only a portion of the surface 38 so that the marginal edge portion 40 of the glass sheets G is supported on and preliminarily formed by both the auxiliary rail 34 and the rail 26. The auxiliary rail 34 may also be horizontally curved to conform to the horizontal contour of the adjacent rail 26. As explained above, although it is preferred that the elevation contour of the ho-forming surface 36 of the auxiliary rail 34 be straight in order to control the buckling curvature in the lateral portions of the glass sheets G, especially near the marginal edge portions 40, however, it is contemplated that the The surface 36 may be curved, provided that its curvature is smaller than the desired final contour of the surface 38 of the adjacent rail 26, that is, it has a larger radius of curvature. For example, the surface 36 of the auxiliary rail 34 may have a radius of curvature of the order of approximately 450 cm (approximately 177 inches), while the surface 38 of the adjacent rail 26 has a radius of curvature of the order of approximately 225 cm (approximately 89 inches). Since the surface 36 of the rail is "flatter" than the surface 38 of the adjacent rail 26, the auxiliary rail 34 may preliminarily shape the marginal edge portion 40 and allow some curvature by preliminary buckling near the marginal edge portions 40 during most of the buckling curvature operation and the surface 38 of the adjacent rail 26 may conform the marginal edge portion 40 and the remaining portions of the glass sheets G to their desired final shape during the last portion of the curvature operation by buckling. The present invention controls the curvature by excessive gravity buckling in glass sheets by limiting the amount of buckling of the glass in selected areas of the glass sheets during a portion of the buckling curvature operation. The glass sheets are pre-formed around an axis and then formed around a second axis. This is done by letting the opposite marginal edge portions 74, extending longitudinally, of the glass sheets G undergo buckling in contact with the opposite rails 2.0 and 24, while supporting the marginal edge portions 40, which extend transversely, of the glass sheets G on auxiliary rails. 34. As a result, the leaves G assume their general longitudinal curvature. The rails 34 are then lowered to allow the edge portions 40 to be supported by and experience buckling in contact with the rails 26 of the end sections 22 to facilitate the buckling curvature of the glass sheets in the transverse direction, especially near between the rails 24 of the end sections 22, and completing the shaping of the glass. By maintaining the marginal edge portions 40 in a generally planar configuration and by retarding the curvature by transverse buckling in said selected areas until a late moment in the buckling process, the contoured shape of the glass in the pivoting sections 22 is controlled and minimized the curvature due to excessive buckling. In addition, the reduction of the curvature by excessive buckling by controlling the speed and form in which the glass is formed, will also reduce the distortion in the glass sheet due to the undesired inverse curvature of the sheet, especially in its corners. The invention described and illustrated herein represents a description of its illustrative preferred embodiments. It is understood that several changes can be made without departing from the essentials of the invention defined in the following claims.

Claims (19)

1. CLAIMS 1. An apparatus for a forming glass sheet, including: a support frame; a forming rail supported on said frame and having a sheet forming surface that performs the conformation in elevation and contour to a desired final shape of a marginal edge of a glass sheet to be formed; and at least one auxiliary rail positioned along a section of said shaping rail, wherein said shaping rail section has a sheet shaping surface portion generally corresponding to said desired end shape of a selected marginal edge portion of said sheet and said auxiliary rail has a sheet forming surface generally corresponding to a preliminary form of said selected marginal edge portion of said sheet, said auxiliary rail being mounted for movement relative to said forming rail section from a first position, wherein portions of said sheet forming surface of said auxiliary rail are above said sheet forming surface portion of said forming rail section and are capable of supporting said selected marginal edge portion of said sheet above said rail section conformador and to conform preliminarily dic The selected marginal edge portion, and a second position, wherein said sheet forming surface of said auxiliary rail is positioned below said portion of said sheet forming surface of said forming rail section such that said portion of said sheet forming surface of said forming rail section being able to support and conform said selected marginal edge portion of said sheet to said desired final shape. The apparatus according to claim 1, wherein said shaping rail portion is a pivoting portion of said shaping rail. The apparatus according to claim 2, wherein said auxiliary rail is mounted from said pivoting portion. The apparatus according to claim 1, further comprising an auxiliary rail support assembly mounted on said frame for movement from a first position, wherein said support assembly supports said auxiliary rail in said first auxiliary rail position, to a second position wherein said support assembly allows said auxiliary rail to fall from said first position of the auxiliary rail to its second position. The apparatus according to claim 1, further including elevators for vertically alternating said auxiliary rail between its first and second positions. The apparatus according to claim 1, wherein said sheet forming surface of said auxiliary rail has a straight profile. The apparatus according to claim 6, wherein said auxiliary rail is positioned outside said forming rail section and said sheet forming surface of said auxiliary rail is completely above said said sheet forming surface portion of said adjacent forming rail. when said auxiliary lane is in its first position. The apparatus according to claim 7, further comprising an auxiliary rail support assembly mounted on said frame for movement from a first position, wherein said support assembly supports said auxiliary rail in said first auxiliary rail position, to a second position wherein said support assembly allows said auxiliary rail to fall from said first position of the auxiliary rail to its second position. The apparatus according to claim 1, wherein said auxiliary rail has a horizontal curvature that generally corresponds to a horizontal curvature of said shaping rail portion. The apparatus according to claim 1, wherein said auxiliary rail is positioned outside said forming rail section. 11. The apparatus according to claim 1, wherein said auxiliary rail is positioned within said auxiliary rail section. The apparatus according to claim 1, wherein said sheet forming surface of said auxiliary rail is completely above said sheet forming surface portion of said adjacent forming rail when said auxiliary rail is in its first position. The apparatus according to claim 1, wherein said auxiliary rail is mounted between its first and second positions. 14. A method of forming a glass sheet by curvature by gravity buckling, including the steps of: providing a shaping rail with a top shaping surface having an outline in elevation and profile generally corresponding to the desired final shape of a marginal edge portion of a sheet to be shaped; supporting an auxiliary rail having an upper forming surface generally corresponding to a preliminary shape of a marginal edge portion selected from said sheet, in a first position along an adjacent section of said forming rail, wherein said upper forming surface of said auxiliary rail is above at least a portion of said upper shaping surface of said adjacent shaping rail section; placing said sheet on said shaping rail and said auxiliary rail in such a way that said selected edge edge portion of said sheet is supported by said upper shaping surface of said auxiliary rail and above said portion of said upper shaping surface of said cross section. adjacent shaping rail; heating said sheet to its heat softening temperature in such a manner that a first portion of said sheet undergoes buckling by gravity in contact with portions of said upper shaping surface of said shaping rail and said selected edge portion of said sheet contacts said surface top shaper of said auxiliary rail, for preliminarily shaping said sheet; and lowering said auxiliary rail to deposit said selected marginal edge portion of said sheet on said adjacent forming rail section such that said selected edge portion of said sheet may undergo buckling in contact with said portion of said upper forming surface of said sheet. adjacent forming rail section and said sheet may undergo buckling to said desired final configuration. The method according to claim 14, further comprising the step of providing said upper shaping surface with said auxiliary rail with a straight elevation profile. The method according to claim 14, wherein said auxiliary rail is positioned outside said adjacent forming rail section. The method according to claim 14, wherein the upper shaping surface of said auxiliary rail is above the entire upper shaping surface of said adjacent shaping rail section when in said first position. The method according to claim 14, further comprising the step of contacting at least a portion of a major surface of said sheets with a press face having a sheet forming surface generally corresponding to said desired final configuration. 19. A method of forming a glass sheet by bending by gravity buckling, including the steps of: providing a forming ring having opposed longitudinally extending rails and transversely extending opposite rails, said rails having a superior forming surface with an outline in elevation that corresponds in general to the desired final shape of a marginal edge portion of a sheet to be shaped; placing auxiliary rails, each having an upper forming surface with a straight elevation profile, in a first position along a corresponding transversely extending rail, wherein said upper forming surface of said auxiliary rail is above said forming surface top of said corresponding rail extending transversely; placing said sheet on said forming ring in such a manner that at least the first marginal edge portions selected from said sheet are supported by said upper shaping surface of said auxiliary rails and above said upper shaping surface of said transversely extending rails; heating said sheet to its heat softening temperature in such a manner that second marginal edge portions of said sheet undergo gravity buckling in contact with said upper shaping surface of said longitudinally extending opposite rails to impart a generally cylindrical curvature to said sheet and preliminarily forming said sheet; and moving said auxiliary rails to a second position for depositing said first marginal edge portions selected from said sheet on said transversely extending rails so that said first marginal edge portions selected from said sheet may undergo buckling in contact with said forming surface. top of said rails that extend transversely and experience buckling to said desired final configuration. SUMMARY OF THE INVENTION The present invention provides an apparatus and method for a forming sheet including a support frame and a forming rail supported on the frame. The forming rail has a sheet forming surface that adapts in elevation and contour to the desired final shape of a marginal edge of a sheet of glass to be shaped. An auxiliary rail having a sheet forming surface generally corresponding to a preliminary shape of a selected marginal edge portion of the sheet is positioned along a section of the forming rail with a sheet forming surface portion that corresponds in general to the desired final shape of the selected marginal edge portion of the sheet. The auxiliary rail is mounted for movement relative to the forming rail section from a first position, where portions of the sheet forming surface of the auxiliary rail are above the surface forming portion of the forming rail section, and a second position, wherein the sheet forming surface of the auxiliary rail is positioned below the surface forming portion of the forming rail section. When the auxiliary rail is in its first position, it is capable of supporting the selected marginal edge portion of the sheet above the forming rail section and of preliminarily shaping the sheet. When the auxiliary rail is in its second position, the sheet forming surface portion of the forming rail section is capable of supporting and shaping the selected marginal edge portion of the sheet to the desired final shape.