US3018817A - Stretch leveling apparatus and method - Google Patents

Description

Jan. 30, 1962 w. 5. WHEELER ETAL 3,018,817

STRETCH LEVELING APPARATUS AND METHOD Filed March 1, 1960 INVENTORS WALTER (5. WHEELER OLAF B. WENDELL ATTORNEY rts 3,6 l 8,8 l7 Patented Jan. 30, 1&62

3,013,817 E'IRETQH LEVELWG APPARATUS AND METH'GD Walter G. Wheeler, Los Angeles, and Olaf B. Wendell,

El Segundo, Calif, assignors to The Huiford Qorporation, El Segundo, (Ialif, a corporation of California Filed Mar. 1, 1960, Ser. No. 12,194 9 Claims. (Cl. 153-35) This invention relates, in general, to metal forming methods and apparatus and, more particularly, to a method and apparatus for straightening, leveling and shaping of flat stock.

In the aircraft industry, as well as other industries, it has been found desirable to employ for use in certain products sheet metal material which has been subjected to a process of stretch forming, such as stretch leveling, in order to obtain certain desired qualities such as flatness to specified tolerances, freedom from residual stresses, increased or favored mechanical properties, etc. In the past, such sheet metal material has been stretched leveled by simply gripping the sheet metal material by opposed jaws and applying a load to the jaws to stretch the material therebetween. However, in a majority of cases, difficulties have been encountered when employing such stretch leveling techniques since the material has a tendency to shrink or close in upon itself in a lateral (transverse) direction when subjected to a pull longitudinally. If some prior condition exist in the sheet and does not exist uniformly over the sheet at any transverse section, such as localized differences in material strength, thickness, degree of flatness, or amount of induced or residual stress or strain, then the action of shrinking transversely will tend to create a longitudinal fold or wrinkle in the sheet making it unacceptable for use. For example, thin gage stainless and alloy steel, in a thickness range from .002 inch to .025 inch is vital to the manufacturing of missiles and high Mach number aircraft. In these products, it is imperative that the sheet stock be very flat, but the nature of the sheet manufacturing processes, such as rolling and heat treating, causes cans and wrinkles in the stock. Steel has a much greater tendency to form cans due to inner action of elongation and necking forces than softer metals such as aluminum for example. For aircraft and missile applications, elimination of these irregularities in the material is mandatory.

To obviate difficulties encountered with conventional stretch leveling apparatus, it is desirable to provide an apparatus and method which will prevent the development and formation of the longitudinal folds with as little restriction on size and thickness of sheet material as possible and capable of providing a uniform end product wherein all parts of the sheet material are subjected to the same conditions. In the apparatus of the present invention, means are employed for preventing the formation of longitudinal folds which provide the least amount of resistant or loading possible in areas outside of the longitudinal pull itself when the sheet material is subjected to the leveling or forming operation.

In accordance with the present invention, the workpiece or sheet material is gripped on opposite ends by two opposing jaws employed to apply longitudinal pull on the material workpiece. The area of the workpiece between the two opposing jaws is confined by two platen surfaces which confine the workpiece within a specific space, while allowing freedom of workpiece movement with no applied restraint on longitudinal or transverse motion. To achieve this, the platen halves are moved to a position against positive stops such as spacers or shims, or may be mechanically clamped or loaded to provide a gap, spacing or interval between the opposing platen halves equal to the thickness of the workpiece material plus some variable or specific amount of clearance determined by the workpiece material and the tolerances of the end result desired.

In general, it has been found desirable to employ a gap distance between the platen surfaces which permits engagement of these surfaces with the extreme apexes or high points of the workpiece irregularities which will bend such irregularities to semi-flatten the workpiece. However, it is understood that the engagement of the opposing platen surfaces with the heights of the irregularities is not tantamount to compressive loading forces which engage with substantially the entire surface area of the workpiece. Thereby, by imparting longitudinal stretching by means of the two opposing jaws, the workpiece will be stretched so that the heights of the irregularities will, in effect, disengage with the platen surfaces having a resultant workpiece which is substantially fiat when compared to its original condition. An optimum stretch leveled workpiece when stretched longitudinally will result in clearance or spacing between the surface areas of the workpiece and adjacent platen surfaces. It can be seen, that should a longitudinal fold or wrinkle tend to develop during the longitudinally stretching of the workpiece, that portion of the workpiece surface area involved in a fold or wrinkle will come into immediate engagement with one of the surfaces of the platen and thereby, the fold or wrinkle cannot develop beyond the width of the gap separating the platen surfaces. Since the workpiece or sheet is confined within the gap between the platen surfaces, the material of the workpiece developing in the longitudinal fold or wrinkle can develop only to the extent permitted by the gap and, thusly, by controlling the gap between the platen surfaces, the tolerances of the flatness required for the workpiece can be set. No limitation is placed on type or condition of material, thickness of material, on size of sheet or platen nor on the proportions of these elements. The apparatus and method of the present invention can be further employed to form parts of variable cross section and to those parts having at least one degree of curvature.

Therefore, it is a primary object of the present invention to level or form a sheet metal workpiece employing stretch leveling techniques including means for avoiding the development of longitudinal folds or wrinkles or hats, cans, etc. outside of predetermined tolerances.

Another object of the present invention is to provide a novel apparatus and method for stretch leveling or forming sheet metal material permitting normal shrinking transversally of the material during stretching without developing longitudinal folds, wrinkles, or hats outside predetermined tolerances.

Still a further object to the present invention is to provide a novel apparatus and method for stretch leveling sheet metal material which subjects the material to the least amount of resistant or loading possible from sources outside of the longitudinal pull per se.

Other objects of the invention, and the various advantages and characteristics of the present stretch leveling apparatus will be apparent from a consideration of the following detailed description when taken in connection with the accompanying drawings in which like numbers of reference denote corresponding parts throughout the several views; in which,

FIGURE 1 is a perspective drawing of the stretch leveling apparatus shown in accordance with the present invention;

FIGURE 2 is a sectional view of the platen assembly incorporated in the apparatus of FIGURE 1 taken in the directions of arrows 22; and

FIGURE 3 is an enlarged breakaway view in' section showing the platen assembly of FIGURE 2. h

In accordance with the present invention, FIGURE 1 shows a pair of opposing jaws and 11 suitably carried on the end of arms 12 and 13 which are activated by a suitable stretch press apparatus. For the purposes of the present invention, the conventional stretch press apparatus may take the form of the commonly referred to swing arm type machine or the stretch forming apparatus may be of the type known as the rotary type. In both types of apparatus, the pair of opposing jaws are powered by suitable piston and cylinder assemblages to apply longitudinal stretch to sheet metal material or workpiece such as workpiece 14. The opposite ends of workpiece 14 are suitably positioned within the jaws 10 and 11 so that sets of teeth 15 associated with each jaw converge upon the workpiece ends in gripping relationship when the arms of the stretch press move differentially to apply the stretching force.

During normal elongation of the material by stretching, the material has a tendency to shrink or close in upon itself in a transverse direction commencing in the center of the worksheet and progressing out towards the opposite ends according to Poissons ratio. The present invention includes a material confining means such as a platen assembly represented by the general reference numeral 16 which confines substantially the entire workpiece or portion thereof being leveled. The platen assembly may be supported 011 the floor below the jaws of the stretching apparatus or a combination of blocks, hydraulic bags, etc. may be employed for supporting the platen assembly about the workpiece. The platen assembly is conveniently arranged so that it may be installed or adjusted about the workpiece after the workpiece has been mounted in the jaws of the stretching apparatus.

With reference to FIGURE 2, the platen assembly 16 is shown installed about the workpiece 14 in such a manner that the workpiece is confined within a gap 17 defined by opposing fiat surfaces 20 and 21. The gap is of sufficient distance to permit a slight clearance between the surfaces and the workpiece so that no loading is placed upon the workpiece by the platen. Approximately .002 inch clearance has been found satisfactory. Consequently, there is no surface friction existing between the platen surfaces and the surface of the workpiece to prevent or inhibit elongation of the material during stretching. The gap may be regulated by means of a single shim or a plurality of shims or spacers such as shims 22 and 23 which are positioned along the outer extremities of the platen assembly on the top and bottom thereof.

The platen assembly, in general, comprises a pair of parallel plates represented by numerals 24 and 25 which are separated by means of a shim or shims 22 and 23 and which are held together by means of fasteners such as bolts 26. However, it is to be understood that the positioning or displacement of the platens relative to each other about the workpiece may be accomplished by other means than the incorporation of shims and fasteners. For example, the platens may be mounted on piston cylinder assemblages such as on a press and may be brought together and stopped to provide a specific clearance between the workpiece and platen surfaces.

In the event the workpiece is provided with irregularities prior to stretching, it is noted that the platens may be brought together so that the surfaces of the opposite platens will engage the heights of the irregularities and bend the sheet into a semi-flattened condition prior to the application of longitudinal pull. The contacting of the platen surfaces with such height is not to be considered a compressive force, but only bending movements are developed in the workpiece. These bending movements do not relieve the workpiece from residual stresses or strains and no permanent setting, forming or leveling of the workpiece is achieved.

With reference to FIGURE 3, it is noted that the shims incorporated in the present invention are provided with a plurality of spaced cut-outs 27 which mate with a plurality of holes 28 provided in each of the top and bottom marginal areas of the platens to accommodate passage of fasteners 26. Preferably the cut-outs are made to open at one side of the shims so that alignment of the platen holes and the shims can be easily facilitated.

In actual operation, it can be seen that when arms 12 and 13 are differentially actuated in the direction of arrows 30 and 31 a longitudinal pull will be developed on workpiece 14 to elongate the workpiece approximately two per cent to three per cent of elongation. As the workpiece is stretched, the center of the workpiece will tend to neck or close in transversely and the material gathered during this closing or necking will normally tend to cause the undesirable longitudinal fold or wrinkle. However, as the fold develops the workpiece will come into contact with the platen surfaces of the platen assembly 18. This engagement will deter or prevent the fold from developing any greater than the Width of the gap and the excess metal is caused to fiow into other areas of the panel instead of adding to the fold. In the event additional hats or cans develop in other areas of the workpiece while the workpiece is undergoing stretching contact of the workpiece with the platen surfaces will prevent the development of such hats or cans beyond tolerable limits as set by the gap width.

In essence, it can be seen through the employment of the apparatus and method of the present invention that the flow of metal which normally builds up at intermittent areas on the panel during elongation to provide folds, wrinkles, hats or cans is diverted into other areas of the workpiece so that the resultant workpiece is leveled to a degree heretofore unachievable. For example, it has been found that by employing the present invention when material such as stainless steel 157 or heat treated stain less steel 4340 have been employed, a degree of flatness 0r levelness can be achieved within .002 when the gap between the platen surfaces is .004" plus the thickness of the workpiece. The material employed in applicants operation has ranged in gages from .008" to .025" including material in an annealed condition.

Inasmuch as the flat plate surfaces defining the gap do not come into contact with the entire surface of the sheet or workpiece and inasmuch as no compressive load is placed on the workpiece during elongation, conventional stretch forming techniques can be employed including the employment of conventional stretch forming apparatus. In the present embodiment of the invention, the platen assembly may support itself on the floor of the workshop or the platen assembly may be supported on blocks or pads. The shims are employed only to provide the required gap width and the fasteners are employed to hold the shims and platen halves in place. Other types of fasteners may be employed such as clamps for example, and other stop means than shims may be employed such as electrical limit stretches which are actuated when the platens reach the specified gap width.

Various modifications may suggest themselves to those skilled in the art without departing from the spirit of our invention, and hence, we do not wish to be restricted to the specific form shown or uses mentioned herein, except to the extent indicated in the appended claims.

We claim:

1. Stretching apparatus for leveling sheet metal material, the combination comprising, movable means for engaging the opposite material edges, means in spaced relationship to the material to be leveled for housing the material and confining the material during leveling, and means for moving the movable means to stretch level the panel while so confined.

2. Stretching apparatus for stretch leveling a sheet metal workpiece, the combination comprising, gripping arrangement for engaging opposite edges of the workpiece, confining means arranged about the worpiece in spaced relationship thereto for limiting the extent of workpiece deformity, and means for moving the gripping arrangement in opposite direction to elongate the workpiece while so confined.

3. Stretching apparatus for leveling an elongated sheet metal workpiece, the combination comprising, gripping means engageable with opposite edge margins of the workpiece, workpiece confining means supported between the opposite edge margins of the workpiece having a pair of flat surfaces separated by the workpiece, each flat surface arranged relative to the workpiece to provide a minute clearance between the surface and the workpiece to confine the workpiece between the pair of surfaces, and means for moving the gripping means in opposite direction to stretch the workpiece longitudinally while confined between the pair of surfaces.

4. Stretching apparatus for leveling an elongated sheet metal workpiece, the combination comprising, gripping means engageable with opposite edge margins of the workpiece, a platen assembly disposed between the opposite edge margins of the workpiece including a pair of flat parallel platen surfaces separated by the workpiece, each platen surface arranged relative to the workpiece to provide a minute clearance between the surface and the workpiece to confine the workpiece between the pair of surfaces, means connected to platen assembly to adjust and maintain the platen surfaces constant relative to each other so that gap therebetween remains constant during workpiece elongation, and means for moving the gripping means to stretch the workpiece longitudinally while confined between the pair of surfaces.

5. Stretching apparatus for shaping an elongated sheet metal workpiece, the combination comprising, gripping means engageable with opposite edge margins of the workpiece, a platen assembly for confining the workpiece disposed between the opposite edge margins; the platen assembly comprising, a pair of parallel plates arranged on opposite sides of the workpiece to define a gap therebetween wherein the gap width is minutely greater than the workpiece gage, means for maintaining the plates stationary relative to each other to provide a constant gap width; and means for moving the gripping means difierentially to longitudinally elongate the workpiece within the gap while confined by the parallel plates of the platen assembly.

6. Stretching apparatus for shaping an elognated sheet metal workpiece, the combination comprising, gripping means engageable with opposite edge margins of the workpiece, a platen assembly disposed between the opposite edge margins for confining a major portion of the workpiece; the platen assembly comprising, a pair of parallel plates arranged on opposite sides of the workpiece to define a gap therebetween wherein the gap width is minutely greater than the workpiece gage, stop means of a predetermined thickness disposed between the plates along the top and bottom edge margins thereof for maintaining the plates stationary to provide a constant gap width; and means for moving the gripping means differentially to longitudinally elongate the workpiece within the gap while confined by the parallel plates of the platen assembly.

7. The method of stretch leveling sheet metal material, comprising the steps of, gripping the material on opposite edge margins, confining the material to be leveled in an area slightly wider than material thickness to restrict the amount of material deformation during stretching, and stretching the material longitudinally by differentially pull= ing on the material opposite edge mar-gins while confining the material within predetermined limits.

8. The method of stretch leveling sheet metal material comprising the steps of, gripping the material on opposite edge margins by grippers, confining the material to be leveled within a given area which is at least .004 inch greater than the gage of the mate-rial, and differentially pulling the grippers to longitudinally elongate the material while limiting the confined material deformity or deflection within the given area.

9. The method of stretch forming sheet metal material which comprises the steps of, surrounding a portion of the material to be leveled with a form assembly to define a forming area having a width at least .002 inch greater than the gage of the material so that material deformation or deflection will be held to a maximum tolerance of .001 inch and stretching the material longitudinally approximately two percent while so confined, thereby causing material to chow and distribute substantially evenly throughout the entire sheet of material.

References Cited in the file of this patent UNITED STATES PATENTS 933,778 Mueller Sept. 14, 1909 1,264,248 Yoder Apr. 30, 1918 1,863,073 Smythe June 14, 1932 2,441,858 Watter May 18, 1948

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