US2738824A - Method of and apparatus for stretch forming sheet metal - Google Patents

Description

March 20, 1956 HQLOPCHAK 2,738,824

METHOD OF AND APPARATUS FOR STRETCH FORMING SHEET METAL Filed June 11, 1952 3 Sheets-Sheet l INVEN TOR.

GEORGE HOLOPCHAK Agent March 1956 G. HOLOPCHAK 2,738,824

METHOD OF AND APPARATUS FOR STRETCH FORMING SHEET METAL Filed June 11, 1952 3 Sheets-Sheet 2 IN VENTOR. GEORGE HOLOPGHAK BY March 20, 1956 HOLOPCHAK METHOD OF AND APPARATUS FOR STRETCH FORMING SHEET METAL 3 Sheets-Sheet 3 Filed June 11, 1952 INVENTOR. GEORGE HOLOPCHAK gent METHOD OF AND APPARATUS FOR STRETCH FORMING SHEET METAL George Holopchak, Los Angeles, Calif., assignor to Lockheed Aircraft Corporation, Burbank, Calif.

Application June 11, 1952, Serial No. 292,917

8 Claims. (Cl. 15332) This invention relates to the shaping or forming of sheet metal and relates more particularly to contour clamps for avoiding the formation of unwanted wrinkles in sheet metal when stretch forming the same.

Stretch presses have gone into extensive use, particu larly in aircraft production, to shape or form sheet metal parts to the desired contours. It is a common occurrence for severe wrinkles to develop in the sheet parts during the stretch forming operations, particularly where the parts are formed with a large or substantial radius of curvature in the longitudinal direction. The tendency for the wrinkles to develop is accentuated in the case of 75S. aluminum alloys and the other sheet metals that are inherently difiicult to form. The wrinkles must, of course, be removed by subsequent hammering operations, or the like, which materially add to the cost of production and where the Wrinkles are excessively large or severe the parts may have to be discarded.

It is a general object of this invention to provide simple, practical, and inexpensive floating contour clamps for greatly reducing or eliminating the formation of unwanted wrinkles in sheet metal parts during the stretch forming of the same. The clamps of the invention thus avoid the necessity of subsequent hammering operations to remove such wrinkles and avoid the possibility of irreparably damaging the sheets.

Another object of theinvention is to provide clamps or devices that are applied to the sheet metal part immediately prior to the stretching of the same and that serve to pre-form the sheet by imparting a transverse curvature to the same so that upon stretch forming the part or sheet each longitudinal strip or region of the sameis given substantially the same percentage or amount of elongation as every other region, thus avoiding or reduc ing to a minimum the wrinkles that would otherwise develop during the stretch forming of the part.

A further object of the invention is to provide clamps or devices of this kind that are simple and inexpensive to manufacture and easy and convenient to use. Each clamp is a simple device including two hingedly connected sections of wood, or other suitable material, readily hung or engaged on the sheet and then clamped together beyond and adjacent the ends of the die to preshape the sheet with the desired or necessary transverse curvature. The arrangement of the clamps in position and the closing andtightening of the same are operations that may be simply and easily performed.

Other objectives and features of the invention will become apparent from the following detailed description of typical preferred embodiments, throughout which reference will be made to the accompanying drawings, where- 1n:

Figure l is a front perspective view of a stretch press showing a sheet metal part engaged in the jaws thereof, and illustrating one form of contour clamps of the invention associated with the sheet, one clamp being closed and the other clamp being operated to the closed position;

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Figure 2 is a front elevation of the stretch press showing another form of clamps of the invention arranged on the sheet or part;

Figure 3 is an enlarged side or end view of one of the clamps appearing in Figure 2;

Figure 4 is an enlarged side or end view of one of the clamps shown in Figure 1 illustrating the clamp in the closed position on a work part or sheet;

Figure 5 is an enlarged fragmentary front elevation illustrating one of the clamps, such as shown in Figure 1, connected or suspended from one of the jaws of the stretch press; and

Figure 6 is a fragmentary enlarged perspective view illustrating the manner of mounting a clamp of the invention on an end of a die.

The devices of the invention may, of course, be constructed for use in forming or shaping sheet metal parts of various kinds and may be employed with stretch presses of different types and sizes. The configuration and sizes of the devices or clamps will depend too upon the size and character of the workpieces to be shaped or formed. In the drawings I have shown typical devices or clamps of the invention engaged on sheet metal workpieces which are being shaped or formed in a stretch press of a conventional type. It is to be understood that the invention is not to be considered as restricted for use on work parts of this kind or for use with any particular type of stretch press.

The stretch press shown in the drawings includes a frame or body 10 and the die 11 is mounted or secured on the front of this body. The die 11 presents an active forward face curved both longitudinally and transversely. The machine or press further includes two jaws 12 for gripping the ends of the sheet or work W. The jaws 12 are carried and operated by cylinder and piston devices 13 operable to move the jaws outwardly and thus exert a pull on the work sheet W to stretch the same. The assemblies of the jaws 12 and cylinder and piston devices 13 are carried by triangular frames 14 hinged or pivoted to the body 10 of the press at 15 to swing about vertical axes. Cylinder and piston devices 16 are connected with these frames 14 and are operable to swing or pivot the same and thus wrap the workpiece W onto the die 11 before the workpiece is stretched. The stretch forming machine or press of course includes other parts and instrumentalities common to devices of this 7 kind.

Figures 2 and 3 of the drawings illustrate a simple embodiment of the invention suitable for use on relatively narrow sheets or workpieces W. As shown in Figure 2, two devices or clamps C of the invention are engaged on the work W, one clamp C being positioned on the work beyond but adjacent each end of the die 11. The clamps C may be identical and each includes two jaws or sections 29 and 21, see Figure 3. The sections 20 and 21 are formed of wood, such as hard wood plywood, or any other appropriate material. In making the clamp sections 20 and 21, a piece of plywood, or other selected material, of rectangular, or other shape, is cut with a saw, or the like, into the two sections 20 and 21 in such a manner that the cut thus made is approximately the same as the transverse curvature of the end portions of the die 11. Thus, as shown in Figure 3, the saw cut provides the clamp section 20 with a convex inner surface or edge 22 for engaging the inner surface of the work W, and provides the clamp section 21 with a concave inner edge 23 for engaging the outer surface of the work W and the curvature of these edges 22 and 23 may approximate the transverse curvature of the die 11 adjacent its ends. However, the curvature of the surfaces 22 and 23 need not beexactly or even substantially the same as the curvature of any given die as I have found that clamps having surfaces 22 and 23 with radii of curvature somewhat greater or somewhat smaller than those of given die ends operate entirely satisfactory with such dies and therefore a .given pair of clamps C may be used successfully with several different dies for forming sheet metal parts having transverse curvatures of different radii.

The clamp sections and 21 are suitably hinged together at one end. For example, as shown in Figure 3, a hinge 24 connects the lower ends of he sections 2 and 21 at or adjacent their active edges 22 and 23. The hinging together of the sections 20 and 21 facilitates the ready arrangement or positioning of the clamp on the work W. Clamp means is provided at the opposite end of the clamp C for drawing the sections 2% and 21 to gether to give the work W its pro-formed transverse curvature. This means may comprise a yoke-like clamp body 25 that is slipped over the upper portions of the sections 20 and 21 after the same have been swung toward one another and a clamp screw 26 threaded through a nut 27 on the body 25 to engage against the edge of the seeti on 21.

in employing the contour clamps C of Figures 2 and 3, the work W is first arranged in the stretch press with its opposite ends gripped in the jaws 12. The clamps C are then arranged. in place on the sheet or work W adjacent the ends of the die 11. This is done by hinging or swinging the sections 20 and 21 together or toward one another so that the edge surfaces 22 and 2-3 bend the work W to have a transverse curvature and the clamps 2.5 are arranged in place and operated. Upon tightening down the clamp screws 26 the clamp sections 2t and 21 bend the sheet or work W to have a transverse curvature corresponding to the curvature of the edges 22 and 23 and therefore substantially corresponding to the transverse curvature of the die ends. As the clamps C of the invention are arranged on the work W at both ends of the die 11, the entire portion of the workpiece extending across the face of the die is given the initial transverse curvature by the clamps C, at least immediately supplying tension to the work. After the clamps C have been positioned and actuated, as just described, the cylinder and piston devices 16 are operated to wrap the sheet or work onto the die 11 and the cylinder and piston devices 13 are then actuated to stretch the work and set the work to final contour by stretching or tensioning it beyond the yield point, these operations being typical or conventional in operating the type of press. During the stretch forming of the workpiece W the clamps C preserve the transverse curvature at or beyond the ends of the die 11 so that the longitudinal fibres or regions of the work W are substantially uniformly elongated or stretched. This uniform stretch forming of the part W avoids or reduces the development of unwanted wrinkles that would otherwise be formed in the work, especially where the die 11 has both the transverse curvature and a substantial longitudinal curvature, as shown.

Figures 1, 4 and 5 illustrate a somewhat different type of clamp of the invention suitable for use on wider sheets or workpieces W. Each of these clamps C2 includes two jaws or sections 30 and 31 which may be constructed in the same manner as the above described clamp sections 20 and 21 and which may be substantially identical therewith. These sections 30 and 31 have the curved edges 32 and 33 corresponding with the edges 22 and 23, above described, and the sections are hinged together at their lower edges by a hinge 34. In this form of the invention the clamps C2 are carried by the jaws 12 of the stretch press. I have shown a channel projecting inwardly from the top of each jaw 12 and a plate or bracket 41 is attached to the upper end of the section 30 or" each clamp C2. Plates 42 are shiftable in the channels 40 and carry hooks or hangers 43. Coil springs 44 are connected between the hangers 43 and the'brackets 41. With this arrangement the clamps C2 are carried by the jaws 12 for easy handling and yet are free to move with the work W as the same is wrapped about the ends of the die 11 and is elongated during the stretch forming operations. Thus the plates 42 are shiftable in the channels 40 to allow the clamps C2 to be secured on the work W at the desired locations and the springs 44 are adapted to yield to allow free movement of the clamps C2 with the work W as it is wrapped and stretch formed.

The clamps C2 of Figures 1, 4 and 5, have a drum and cable means for tightening or clamping the sections 30 and 31 onto the work 'W to give the work the desired initial transverse curvature. Each of these means includes a winding drum 45 rotatably mounted on a side of a clamp section 31 adjacent its upper end. Cables 46 are wound on the drums 45 and have hooks 47 on their ends engageable in eyes 48 secured on the companion clamp sections 39. After the sections 30 and 31 have been partially closed against the sheet or work W, the hooks 47 are engaged in the eyes 48 and the drums 45 are rotated by suitable cranks 49 to tighten the clamp sections onto the work and thus bend or shape the same to the desired initial transverse curvature. Ratchet means 50 are associated with the drums 45 to retain the clamp sections 30 and 31 in the actuated positions. Upon manually releasing the ratchet means 50 after the stretch forming operations have been completed, the hooks 47 are released from the eyes 48 and the sections 30 and 31 are swung open to permit removal of the work W.

Figure 6 illustrates still another arrangement or embodiment of the invention. In this case the clamps C2, which may be the same as shown in Figures 1, 4 and 5, are mounted on the ends of the die 11. Two vertically spaced horizontal arms 60, are secured to each end of the die 11 by vertical hinge pins 61 removably engaged through lugs 62 on the die and through connecting tubes 63, secured to the ends of the arms 60. This mounting or attachment of the arms permits the arms to swing about vertical axes. Brackets 64 are attached to the sections 3t! of the clamps C2 and have sleeves 65 on their outer ends which are slidably engaged on the pivoted arms 60. The pivotal mounting of the arms 60 on the pins 61 allows the clamps C2 to move or swing with the end portions of the work W when the same is wrapped on the die and the sleeves 65 are free to move on the arms 60 so that the clamps C2 may move with the work W as it is stretched during the stretch forming operations.

From the foregoing detailed description it will be seen that I have provided simple, effective clamps readily applied to the sheet or work W adjacent the ends of the die 11. The contour clamps are operable to bend the work transversely into substantial conformance with the end portions of the die 11 and to retain the end parts of the work in the bent or contoured condition during the stretch forming operations. This pre-bending or initial contouring of the work equalizes the strain in the material or Work so that each longitudinal strip or region of the sheet is given substantially the same percentage of elongation as the adjacent strips or regions. This strain equalizing effect has been found to avoid or eliminate the wrinkles that would otherwise develop, particularly where the die 11 has a substantial longitudinal curvature as well as a transverse curvature. It will be observed that in each form or embodiment of the invention the contour clamps are free floating, that is they are carried by or associated with the work W itself so as to move therewith as the work is stretch formed. The contour clamps are simple and inexpensive to manufacture and easy and convenient to use. The employment of the contour clamps does not complicate or interfere with the normal stretch forming operations.

Having described only typical forms of the invention I do not wish to be limited to the specific details herein set forth, but wish to reserve to myself any variations or modifications that may appear to those skilled in the art and fall within the scope of the following claims.

I claim:

1. In combination, a die having a die having an end portion with a transverse curvature and against which a sheet part is to be stretch formed, gripping means for gripping the part in an area spaced beyond said end of the die, two clamp sections in the region between said end of the die and the gripping means, one having a convex surface for engaging one side of the part adjacent said end portion of the die, the other having a concave.

surface for engaging the opposite side of the part adjacent said end portion of the die, the curvature of said surfaces conforming generally with the transverse curvature of said end portion of the die, and means for actuating the two clamp sections toward one another to clamp said surfaces of the sections against the sides of the part adjacent but outwardly of said end of the die to give the part an initial transverse curvature corresponding generally with said curvature of the die.

2. In combination; a die having an end portion with a transverse curvature and against which a sheet part is to be stretch formed, jaw means for gripping the part in a region spaced beyond said end of the die; two clamp sections, one jaw means for gripping the part in a region spaced beyond said end of the die; two clamp sections,

one having a convex surface for engaging one side of the part in a region between the jaw means and said end of the die, the other having a concave surface for engaging the opposite side of the part in said region between the jaw means and said end of the die, the curvature of said surfaces conforming generally with the transverse curvature of said end portion of the die, means hingedly connecting together corresponding ends of the, sections, and means for actuating the sections against the part to clamp said surfaces of the sections against the sides of the part adjacent but outwardly of said end of the die to give the part an initial transverse curvature.

3. In combination; a die having an end portion with a transverse curvature and against which a sheet part is to be stretch formed, jaw means for gripping the part in a region spaced beyond said end of the die; two

clamp sections in the region between said end of the:

die and the jaw means, one having a convex surface for engaging one side of the part in the region between the jaw means and said end of the die, the other having a concave surface for engaging the opposite side of the part in said region, the curvature of said surfaces conforming generally with the transverse curvature of said end portion of the die, means movably supporting the sections so as to be movable with the part, and means for clamping the sections onto the part so that said surfaces give the part an initial transverse curvature.

4. In combination; a die having an end portion with a transverse curvature and against which a sheet part is to be stretch formed, jaw means for gripping the part some distance beyond said end of the die, two clamp sections in the region between the jaw means and said end of the die, one having a convex surface for engaging one side of the part, the other having a concave surface for engaging the opposite side of the part, the curvature of said surfaces conforming generally with the transverse curvature of said end portion of the die, means on the die movably supporting the sections adjacent said end thereof so as to be movable with the part as it is stretch formed, and means for clamping the sections onto the part to have said surfaces give the part an initial transverse curvature. I

5. In combination; a die having an end portion with a transverse curvature and against which a sheet part is to be stretch formed, jaw means for gripping the part some distance beyond said end of the die, two clamp sections, one having a convex surface for engaging one side of the part, the other having a concave surface for engaging the opposite side of the part, the curvature of said surfaces conforming generally with the transverse curvature of said end portion of the die, means movably supporting the sections adjacent said end of the die so as to move with the part as the same is stretch formed, means hinging together corresponding ends of the sections, and winding drum and cable means associated with the other ends of the sections for clamping the sections on the part to have said surfaces give the part an initial transverse curvature.

.6. In combination; a die having an end portion with a transverse curvature and against which a sheet part is to be stretch formed, jaw means for gripping the part some distance beyond said end of the die, two clamp sections arranged in the region between the jaw means and said end of the die, one having a convex surface for engaging one side of the part, the other having a concave surface for engaging the opposite side of the part, the curvature of said surfaces conforming generally with the transverse curvature of said end portion of the die, means hingedly connecting corresponding ends of the sections, and screw means at the other ends of the sections for clamping them onto the part so that said surfaces give the part an initial transverse curvature.

7. In a stretch press having a jaw for gripping an end portion of a sheet metal part the combination of; a die having a transversely curved end portion and against which the part is to be formed, two clamp sections, hinge means connecting corresponding ends of the sections, means for movably supporting the sections on the jaw to be adjacent said end of the die, one section having a convex surface for engaging one side of the part, the other having a concave surface for engaging the other side of the part, the curvature of said surfaces substantially following the curvature of said end portion of the die, and means for clamping the sections onto the part so that said surfaces give the part an initial transverse curvature.

8. The method of stretch forming a sheet metal part which includes engaging a side of the part against the curved face of a die, gripping the ends of the part with jaws, giving the part an initial transverse curvature corresponding generally with the transverse curvature of the end portions of the die by gripping and transversely bending the part in regions between the ends of the die and the jaws, and then moving the jaws to subject the part to elongating forces, while said regions of the part remain transversely bent, to stretch form the part to a contour determined by the shape of the die.

References Cited in the file of this patent UNITED STATES PATENTS 20,128 Pitcher Apr. 27, 1858 28,901 Pierson June 26, 1860 174,398 Wright Mar. 7, 1876 417,603 Sullivan Dec. 17, 1889 569,927 Laney Oct. 20, 1896 2,378,413 Lermont et a1. June 19, 1945 2,446,089 Hochreiter July 27, 1948 2,609,860 Kindelberger Sept. 9, 1952 FOREIGN PATENTS 494,702 Germany Mar. 27, 1930

Claims (2)

1. IN COMBINATION, A DIE HAVING A DIE HAVING AN END PORTION WITH A TRANSVERSE CURVATURE AND AGAINST WHICH A SHEET PART IS TO BE STRETCH FORMED, GRIPPING MEANS FOR GRIPPING THE PART IN AN AREA SPACED BEYOND SAID END OF THE DIE, TWO CLAMP SECTIONS IN THE REGION BETWEEN SAID END OF THE DIE AND THE GRIPPING MEANS, ONE HAVING A CONVEX SURFACE FOR ENGAGING ONE SIDE OF THE PART ADJACENT SAID END PORTION OF THE DIE, THE OTHER HAVING A CONCAVE SURFACE FOR ENGAGING THE OPPOSITE SIDE OF THE PART ADJACENT SAID END PORTION OF THE DIE, THE CURVATURE OF SAID SURFACES CONFORMING GENERALLY WITH THE TRANSVERSE CURVATURE OF SAID END PORTION OF THE DIE, AND MEANS FOR ACTUATING THE TWO CLAMP SECTIONS TOWARD ONE ANOTHER TO CLAMP SAID SURFACES OF THE SECTIONS AGAINST THE SIDES OF THE PART ADJACENT BUT OUTWARDLY OF SAID END OF THE DIE TO GIVE THE PART AN INITIAL TRANSVERSE CURVATURE CORRESPONDING GENERALLY WITH SAID CURVATURE OF THE DIE.

8. THE METHOD OF STRETCH FORMING A SHEET METAL PART WHICH INCLUDES ENGAGING A SIDE OF THE PART AGAINST THE CURVED FACE OF A DIE, GRIPPING THE ENDS OF THE PART WITH JAWS, GIVING THE PART AN INITIAL TRANSVERSE CURVATURE CORRESPONDING GENERALLY WITH THE TRANSVERSE CURVATURE OF THE END PORTIONS OF THE DIE BY GRIPPING AND TRANSVERSELY BENDING THE PART IN REGIONS BETWEEN THE ENDS OF THE DIE AND THE JAWS, AND THEN MOVING THE JAWS TO SUBJECT THE PART TO ELONGATING FORCES, WHILE SAID REGIONS OF THE PART REMAIN TRANSVERSELY BENT, TO STRETCH FORM THE PART TO A CONTOUR DETERMINED BY THE SHAPE OF THE DIE.

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