US3742745A

US3742745A - Metal forming method and apparatus

Info

Publication number: US3742745A
Application number: US00176519A
Authority: US
Inventors: P Swenson
Original assignee: Great Lakes Sports Mfg Co
Current assignee: Great Lakes Sports Mfg Co
Priority date: 1971-08-31
Filing date: 1971-08-31
Publication date: 1973-07-03
Anticipated expiration: 1990-07-03

Abstract

The specification and drawings disclose a method and apparatus for forming dish-shaped metal elements through the use of a combined fluid pressure and sheet tensioning process. The disclosed method is particularly intended for forming dish-shpaed sheet metal elements having a length substantially greater than their width and includes applying a longitudinal tension to the sheet while permitting the lateral edges to move toward one another substantially unconstrained while the fulid pressure acts to bow the sheet. The disclosed apparatus includes a flexible frame of generally eliptical shape which clamps to the edge of the sheet. The frame is arranged so that as the sheet bows outwardly, the lateral sides of the frame are pulled inwardly driving the ends of the frame apart.

Description

United States Patent 1 Swenson, Sr.

[ July 3,1973

[ METAL FORMING METHOD AND APPARATUS [75] lnventor: Paul F. Swenson, Sr., Cleveland,

Ohio

[73] Assignee: Great Lakes Sports Mfg. Co.,

Cleveland, Ohio [22] Filed: Aug. 31, 1971 {21] Appl. No.: 176,519

[52] US. Cl 72/54, 29/421, 72/350, 9/6 [51] Int. Cl. B2ld 24/04 [58] Field of Search 72/54, 60, 62, 350, 72/351; 29/421; 113/120 R; 9/6, 6.5

156] I References Cited UNITED STATES PATENTS 1,763,582 6/1930 Gulick 29/421 2,086,134 7/1937 vLudwick 29/421 3,164,894 1/1965 Johnson et al. 29/421 1,970,134 8/1934 Ferris 72/351 3.458.917 8/1969 Mueller 29/421 Primary Examiner-Richard .l. Herbst Attorney-Robert J. Fay et al.

[57] ABSTRACT shape which clamps to the edge of the sheet. The frame.

is arranged so that as the sheet bows outwardly, the lateral sides of the frame are pulled inwardly driving the ends of the frame apart.

14 Claims, 10 Drawing Figures METAL FORMING METHOD AND APPARATUS The subject invention is directed toward the art of metal forming and, more particularly, to an improved method and apparatus for stretch forming metal sheet.

The invention is especially suited for forming boat hulls and will be described with particular reference thereto; however, as will become apparent, the invention is capable of much broader application and could be employed for forming many different structures.

Typically, sheet metal elements and structures having compound curvatures must be formed by drawing operations. The die costs for such operations vary, of course, depending upon the size of the part, the amount of curvature, the depth of the draw, etc. Even for relatively small parts with simple, shallow draws, the die cost is substantial. However, when dealing with large, deep draw components such as boat hulls, fuel tanks or the like, the die and press costs become extremely high and can only be justified for very large production runs. For relatively small runs, other fabricating techniques involving much more extensive use of manual labor will prove to be less costly to employ.

Techniques for forming sheet metal into hollow or dish-shaped structures without the use of cooperating dies have been proposed in the past.-For example, heat exchange plates have been made by bonding two sheets of ductile metal together about an area and then pressurizing the unbonded area between the two sheets. The pressurizing caused the metal to be deformed outwardly beyond its elastic limit. In this system, the metal was constrained only by the surrounding welded metal and wrinkles and creases would invariably develop in certain areas. Further, the depth of the draw was limited by the percent of elongation which the metal could undergo without rupture. Although undesirable, these limitations were tolerable in some applications.

The subject invention provides a method by which large components of the type mentioned can be fabricated rapidly and inexpensively with a minimum of tooling- The method and the apparatus allow fabrication of sheet metal structures which could heretofore only be made by use of deep drawing dies and large, double action presses. Further, when employed as described herein, this method and apparatus permits the fabrication of the above-mentioned structures without the formation of wrinkles and creases.

According to one aspect of the invention, there is provided a method of forming sheet metal into dishshaped structures having a length greater than their width. The method includes the steps of:

a. providing a metal sheet with a length nearly as great as the actual length of the desired structure and a width substantially as great as the width of the structure measured over its surface;

b. sealing the sheet to a fluid impervious surface continuously about an area having a width substantially as great as the surface width of the desired structure and a length slightly less than the actual length of the desired structure; and,

c. thereafter, creating a pressure differential across the sheet and within the sealed area sufficiently to deform the sheet outwardly away from the surface while simultaneously applying a tension force longitudinally of the sheet.while permitting the lateral edges of the area to move toward one another.

Preferably, and in accordance with a more limited aspect of the invention, the air impervious surface is a second sheet of metal having dimensions generally corresponding to the first sheet. Also, the sealing is preferably accomplished by clamping the two sheets together about the area described.

Although the above described method could be carried out with a variety of structures and apparatus, the invention also concerns a particularly simple, flexible frame assembly which allows the required stretching or elongating forces to be generated by the pressurizing fluid. Broadly, the frame includes a pair -of generally elliptically-shaped clamp bars sized so as to encircle the noted area and engage on opposite sides of the two sheets. Means, such as mechanical clamps, are provided for tightly clamping the two frame sections to the sheets to seal the space between the sheets and within the area. Also, means are provided for supplying fluid pressure within the frame and between the sheets. The

lateral sides of the frames are sufficiently flexible so that as the sheets are moved apart by the fluid pressure, the sides of the frames are pulled toward one another. Because the circumference of the frame is constant, the ends of the frame must move outwardly or apart as the sides are pulled together. The outward movement of the ends of the frame automatically applies longitudinal tension to the two sheets.

Within limits, the frame assembly can replace extremely expensive forming dies and perform deep draw-type forming operations. The frame causes the resultant structure to have substantially no elongation of the metal in the transverse direction since the lateral sides of the frame merely move inwardly as the forming takes place. i

The invention also contemplates that the inward movement of the lateral sides can be limited at various points such as by fixed stops, to influencethe shape of the resulting structure.

Accordingly, a primary object of the invention is the provision of a method and apparatus particularly suited for forming dish-shaped sheet metal structures without the use of cooperating dies.

Yet another object is the provision of a forming method of the type described wherein the metal is subjected to little or no elongation in its transverse dimension.

A still further object is the provision of an extremely simple, flexible frame assembly which allows the method to be carried out very inexpensively.

The above and other objects and advantages will become apparent from the following description when read in conjunction with the accompanying drawings wherein:

FIGS. 1 through 4 are pictorial views somewhat diagrammatic showingthe sequence of operations used for forming metal in accordance with a preferred embodiment of the invention; 7

FIG. 5 is a plan view, somewhat diagramatic, showing the changes which take place in the dimensional relationships of the metal sheet during the forming operation shown in the FIGS. 1 through 4 embodiment;

FIGS. 6 and 6a are cross-sectional views taken on lines 6-6 and 6a--6a of FIG; 5;

FIG. 7 is a plan view of an apparatus which can be used for carrying out the methods illustrated in FIGS.

1 through 4;

sequence of steps could vary substantially and differ from that which will be shown and described; however, in the preferred form of the invention, the forming process begins by the provision of two sheets of metal and 12, each having an extent at least sufficient to form the blank for the structural component desired. The size of the starting sheets or blanks relative to the finished component will be described subsequently.

In the embodiment under consideration, the finished component to be formed is generally eliptical having a dish-shaped configuration and a length substantially greater than its width. In the FIG. 1 showing, the minimum length of the

blanks

10 and 12 are slightly less than the length of the finished component to be formed. Their minimum width is preferably as great as the actual surface width of the finished component. To explain, assume that it is desired to form a dish-shaped member having a length of approximately 16 feet and an actual width measured over its surface of approximately 30 inches. To form a component of this general configuration by the subject method requires that the minimum length of the sheet preferably be, for example, in the range of 14 to 16 feet long and that its width be at least nearly 30 inches wide.

In FIG. 1, a desired starting configuration for the structural component which is to be formed is illustrated on the sheet 10 with the dotted line 14. The

sheets

10 and 12 are illustrated as rectangular but they could be other shapes so long as they are larger than the area encompassed by line 14. Also, the sheets can be cut to the exact shape and size of the line 14. This will be illustrated with respect to the preferred form of apparatus shown in FIGS. 7 through 9.

Line 14 is empirically derived mathematically as a function of the final shape desired for the component being formed. To form a hull of the type under consideration, it generally is eliptical and its total length L is only slightly less than the resulting desired length of the hull structure. Its width D is preferably substantially equal to the actual width of the hull measured over the surface. That is, D equals the surface dimension of onehalf of the hull.

In the subject embodiment, the component to be formed is one-half of an elongated, hollow aluminum hull member which is intended for use as one of the hulls of a catamaran. The complete hull is formed by dishing or forming two sheets of aluminum and joining them along mating edges. In FIG. 1,

sheets

10 and 12 will be simultaneously formed to each form one-half of the hull structure.

In accordance with the invention, the

sheets

10 and 12 are joined together along line 14, preferably by clamping, so that they are in air tight sealed engagement along the line. FIG. 2 diagramatically illustrates clamping pressure applied to the sheets along line 14 by a series of arrows 15.

With the two

sheets

10 and 12 in sealed engagement along line 14, fluid pressure is introduced between the sheets and within the sealed area of line 14. The actual pressure required for carrying out the invention is relatively small and for the case of thin aluminum sheets of the size referred to, the pressure in the range of 5 to 10 psi will achieve the desired results.

While the sheets are tightly sealed along line 14 and during the time the fluid pressure is introduced, a substantial longitudinal tension is simultaneously applied as shown by the arrows 17 in FIG. 3. The tension generated must preferably be sufficient to cause the sheets to plastically deform and to be elongated to the desired final length. The tension required can be easily calculated from the width and thickness of the sheets and known parameters of the metal being formed. The percent change in length due to plastic deformation will be at least 5 percent when formed with the preferred apparatus but can, of course, be less or even substantially greater and up to the ultimate permissible for the metal.

During the inflation and longitudinal tensioning, the lateral edges of the sheets are constrained only slightly, or not at all and are permitted to draw toward one another as the metal deforms outwardly. They are permitted to move beyond the desired final spacing, i.e. the width of the structure, by an amount necessary to accommodate the spring back, inherent in forming metals. Further, the edge of the area, i.e. line 14, is preferably held in a single plane and is not permitted to bend or twist.

FIG. 4 shows the two sheets at the completion of the stretching and inflating process. Note that the sheet 10 has been dished outwardly within the area of line 14. (Although not shown, sheet 12 is similarly dished.) Additionally, the sheets have been elongated in the longitudinal direction. As mentioned, during the inflating process, the lateral edges are restrained or constrained only slightly so that the transverse dimension is not stretched but permitted to move inwardly as the inflation takes place. Thus, the metal in the resulting dishshaped structure has been strained and elongated in the longitudinal dimension but substantially unstrained in the transverse dimension.

FIG. 5 illustrates the plan view of one sheet within the line 14 before and after the forming process. The solid line shows the line 14 at the completion of the forming process. As best shown in FIGS. 6 and 6a, little or no elongation takes place in the sheet transversely to its longitudinal direction. However, in the longitudinal direction, the length of the sheet has been increased substantially. The amount of longitudinal stretch which the sheet can undergo is, of course, determined by the particular type of material being formed. For example, with aluminum, elongation of 12 percent to 25 percent can be achieved without tearing of the sheet, depending on the particular alloy employed.

Many different shapes have been formed by use of the described method. The best results have been achieved, however, when there are no sharp corners, re-entrant angles or curves and when the length is at least twice the width. Further, although it is preferred to not strain the sheet in the transverse dimension, good results can be obtained with some straining.

Many different types of structures and apparatus could be used for carrying out the described method. For example, the clamping or sealing could be accomplished by pneumatically or hydraulically actuated clamping jaws and the elongation can similarly be done through the use of fluid cylinders or mechanically. One

aspect of the subject invention, however, concerns the provision of an extremely simple, flexible frame assembly which permits the internal pressures acting within the sheets to produce the forces required for sheet elongation. FIGS. 7 through 9 show a preferred form of highly simplified apparatus in which this particular technique can be carried out.

Referring in particular to FIG. 7, the apparatus is shown as comprising a relatively flexible frame assembly 20 having an eliptical contour corresponding to the starting configuration of line 14 of FIGS. 1 through S.v

The frame assembly 20 includes a pair of

frame members

22 and 24 which, in the subject embodiment, are formed from steel bar bent to the eliptical shape illustrated and joined at their ends. As will become apparent, the required strength of the bars will vary depending upon the particular structural configuration being formed and the longitudinal compressive loads to which the bars will be subjected during a forming operation. In the embodiment shown, the bars have 1 a inch X 1% inch cross-section throughout their entire circumference. The bars are each of identical shape and are arranged to engage and grip the sheets and 12 along the line 14.

Many different types of gripping arrangements could be provided so that the sheets are tightly held and sealed along line 14. FIG. 9 shows a simple form of sealing and gripping jaw used in the subject embodiment. Note that bar or frame member 22 has a small groove 26 formed continuously about its lower edge surface. Bar 24 has a similar, aligned groove 28 formed about its top edge surface and a metal key or tongue member 30 is positioned within the groove to extend outwardly a short distance. Thus, when the bars are clamped on the

sheets

10 and 12, the sheets are sealed and tightly gripped by cooperation between the outwardly extending portion of strip 30 and the groove 26.

The required clamping force can be applied to the two

bars

22 and 24 in many different ways. For example, air or hydraulic cylinders can be used to draw the clamp bars together. In the embodiment under consideration, the clamping is accomplished by a large number of simple clamp assemblies 32. The clamp assemblies 32 are best shown in FIG. '8 and each comprise a pair of L-shaped bar members 34 which engage the clamp bars 22 and 24 in the manner shown. A bolt 36 extends through aligned openings in the members 34. By tightening the nuts 38, the frame assemblies can be tightly clamped together. It is important to note that the clamp assemblies 32 are arranged so that they do not affect the flexibility of the frame assembly. That is,

the lateral sides of the frame can flex inwardly during other of

sheets

10 and 12, or by a small fitting extend ing inwardly between the two sheets from the edge. During application of fluid pressure to the space between the sheets, the sheets tend to bow outwardly, i.e. toward and away from the viewer of FIG. 7. During this outward bowing of the sheets, the lateral sides of the frames are pulled inwardly toward one another because of the geometry of theframe and its flexibility. Simultaneously with the inward movement of the lateral sides,

the end portions of the frame are driven outwardly. That is, the length of the

sheets

10 and 12 must increase because the circumference of the frame remains constant while its width decreases. Thus, the frame automatically applies the required longitudinal stress while permitting the lateral edges to move inwardly substantially unconstrained.

Depending upon the final shape desired, one or the other or both of the lateral sides can be constrained at various points during the inflation process to influence the final shape of the structure. In the subject embodiment, it is desirable that the edge 40 of the structures have a somewhat straight configuration. For this reason, the frame assembly includes a rigid beam 42 in the form of a steel channel. The frame 42 is connected to the bar 22 by a plurality of tie rods 44. The tie rods 44 have nuts 46 adjusted so that as the lateral sides of the

frame sections

22 and 24 move inwardly, the nuts engage the beam at various points providing a stop to prevent further inward movement of the edge. Through the use of this arrangement, the amount of flexure which either one or both of the sides undergo can be varied. It should be appreciated that various other types of frame assemblies and motion limiting arrangements could be used for carrying out the invention. Additionally, although the length of the structures formed must be greater than their width to permit use of the inventive method, the variationscan be substantial.

The invention has been described in great detail sufficient to enable one of ordinary skill in the metal forming art to make and use the same. Obviously, modifications and alterations of the preferred embodiment will occur to othersupon a reading and understanding of the specification and it is my intention to include all such modifications and alterations as part of my invention insofar as they come within the scope of the appended claims.

What is claimed is: l

1. A method of forming a sheet of metal into a dishshaped structure having a length substantially greater than its width comprising the steps of:

a. providing a metal sheet having a length at least nearly as great as the actual length of the desired structure and a width which is at least nearly as great as the width over the surface of the desired structure; b. providing an air impervious surface; 0. sealing said sheet to said air impervious'surface continuously about an area having a width substantially as great as the width over the surface of the desired structure and a length less than the length over the surface of said desired structure; fluid pressurizing the space between said sheet and said surface within said sealed area sufficiently to cause said sheet to deform outwardly away from said surface while simultaneously permitting the lateral edges of said area to move inwardly so asto produce substantially no strain on said sheet in the direction of its width and simultaneously therewith applying a longitudinal tension force to said sheet to strain it sufficiently to elongate said sheet to the desired length of said structure.

2. The method as defined in claim 1 including-the step of maintaining the-lateral margins of said area sealed while permittingthem to move inwardly to the actual width of said desired structure.

3. The method as defined in claim 1 wherein said surface is defined by a second sheet of metal.

4. The method as defined in claim 1 wherein the perimeter of said area is maintained constant throughout said pressurizing.

5. The method as defined in claim 1 wherein said surface is defined by a second sheet of metal and wherein said two sheets are joined about said sealed area.

6. The method as defined in claim 5 wherein said sheets are joined about said sealed area after being pressurized.

7. The method as defined in claim 5 wherein said sheets are simultaneously elongated to the actual length of said desired structure.

8. The method as defined in claim 5 wherein at least a portion of one lateral edge of said area is prevented from moving during at least a portion of the time said pressurization takes place.

9. A method of forming a dish-shaped sheet metal structure having an actual length substantially greater than its actual width comprising the steps of:

a. providing a sheet of metal having a width at least nearly as great as the width over the surface of said desired structure and a length at least nearly as great as the surface length of said desired structure;

b. gripping said sheet continuously about an area having a width substantially equal to the surface width of said desired structure and a length slightly less than the actual length of said desired structure;

0. applying pressure uniformly over said area to deform said sheet while simultaneously applying a tension force to the ends of said area to strain said sheet and elongate said area to the actual length of said desired structure; and,

d. during application of said pressure, constraining the lateral sides of said area to move toward one another without appreciably straining said sheet across its width as said sheet is deformed until said lateral sides reach the actual width of said desired structure.

10. A method of forming a dish-shaped sheet metal structure comprising the steps of:

a. gripping a sheet of metal continuously along a line about an area having a length substantially greater than its width; pl b. applying s'ufficient pressure uniformly to said area to deform said sheet outwardly while maintaining said line in its original plane;

c. simultaneously applying a tension force with the application of said pressure applying force to the ends of said area to elongate the sheet; and,

d. constraining the lateral edges of said sheet in a manner to avoid substantial stretching of said sheet across the width of said area.

11. The method as defined in claim 10 wherein said area has a length at least twice as great as its width.

12. The method as defined in claim 10 wherein said force applied to the ends of said area is applied continuously during application of said pressure.

13. The method as defined in claim 12 wherein said force varies in relation to said pressure.

14. The method as defined in claim 10 wherein said force applied to the ends of said area is proportional to the pressure applied to said area.

Claims

1. A method of forming a sheet of metal into a dish-shaped structure having a length substantially greater than its width comprising the steps of: a. providing a metal sheet having a length at least nearly as great as the actual length of the desired structure and a width which is at least nearly as great as the width over the surface of the desired structure; b. providing an air impervious surface; c. sealing said sheet to said air impervious surface continuously about an area having a width substantially as great as the width over the surface of the desired structure and a length less than the length over the surface of said desired structure; d. fluid pressurizing the space between said sheet and said surface within said sealed area sufficiently to cause said sheet to deform outwardly away from said surface while simultaneously permitting the lateral edges of said area to move inwardly so as to produce substantially no strain on said sheet in the direction of its width and simultaneously therewith applying a longitudinal tension force to said sheet to strain it sufficiently to elongate said sheet to the desired length of said structure.

2. The method as defined in claim 1 including the step of maintaining the lateral margins of said area sealed while permitting them to move inwardly to the actual width of said desired structure.

9. A method of forming a dish-shaped sheet metal structure having an actual length substantially greater than its actual width comprising the steps of: a. providing a sheet of metal having a width at least nearly as great as the width over the surface of said desired structure and a length at least nearly as great as the surface length of said desired structure; b. gripping said sheet continuously about an area having a width substantially equal to the surface width of said desired structure and a length slightly less than the actual length of said desired structure; c. applying pressure uniformly over said area to deform said sheet while simultaneously applying a tension force to the ends of said area to strain said sheet and elongate said area to the actual length of said desired structure; and, d. during application of said pressure, constraining the lateral sides of said area to move toward one another without appreciably straining said sheet across its width as said sheet is deformed until said lateral sides reach the actual width of said desired structure.

10. A method of forming a dish-shaped sheet metal structure comprising the steps of: a. gripping a sheet of metal continuously along a line about an arEa having a length substantially greater than its width; b. applying sufficient pressure uniformly to said area to deform said sheet outwardly while maintaining said line in its original plane; c. simultaneously applying a tension force with the application of said pressure applying force to the ends of said area to elongate the sheet; and, d. constraining the lateral edges of said sheet in a manner to avoid substantial stretching of said sheet across the width of said area.