Precision-folded, high strength, fatigue-resistant structures and sheet therefor   

Abstract: A sheet of material formed for bending along a bend line comprises a plurality of slits positioned proximate and along the bend line. The slits each have opposite end portions which diverge away from the bend line. The slits are configured and positioned to produce bending of the sheet of material along the bend line. The diverging slit end portions reduce stress in the sheet of material during bending.

This application is a Continuation of U.S. patent application Ser. No. 11/384,216 filed Mar. 16, 2006, which claims the benefit of U.S. Provisional Application No. 60/663,392 filed Mar. 17, 2005, and the \'216 application is a Continuation-in-Part of U.S. patent application Ser. No. 10/672,766 filed Sep. 26, 2003 and now U.S. Pat. No. 7,152,449 B2, which is a Continuation-in-Part of U.S. patent application Ser. No. 10/256,870 filed Sep. 26, 2002 and now U.S. Pat. No. 6,877,349 B2, which is a Continuation-in-Part of U.S. patent application Ser. No. 09/640,267 filed Aug. 17, 2000 and now U.S. Pat. No. 6,481,259 B1. All the above applications are incorporated herein for all purposes by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates, in general, to the designing and precision folding of sheets of material and the manufacture of structures therefrom. More particularly, the present invention relates to processes of designing, preparing and manufacturing, including, but not limited to, ways of preparing sheet material, in order to enable precision folding and to the use of such processes for rapid two-dimension- to- three-dimensional folding of high strength, fatigue-resistant structures or assemblies.

2. Description of Related Art

A commonly encountered problem in connection with bending sheet material is that the locations of the bends are difficult to control because of bending tolerance variations and the accumulation of tolerance errors. For example, in the formation of the housings for electronic equipment, sheet metal is bent along a first bend line within certain tolerances. The second bend, however, often is positioned based upon the first bend, and accordingly, the tolerance errors can accumulate. Since there can be three or more bends which are involved to create the chassis or enclosure for the electronic components, the effect of cumulative tolerance errors in bending can be significant. Moreover, the tolerances that are achievable will vary widely depending on the bending equipment, and its tooling, as well as the skill of the operator.

One approach to this problem has been to try to control the location of bends in sheet material through the use of slitting or grooving. Slits and grooves can be formed in sheet stock very precisely, for example, by the use of computer numerically controlled (CNC) devices which control a slit or groove farming apparatus, such as a laser, water jet, punch press, knife or other tool.

Referring to FIG. 1, a sheet of material 21 is shown which has a plurality of slits or grooves 23 aligned in end-to-end, spaced apart relation along a proposed bend line 25. Between pairs of longitudinally adjacent slits or grooves are bending webs, splines or straps 27 which will be plastically deformed upon bending of sheet 21. Webs 27 hold the sheet together as a single member. When grooves that do not penetrate through sheet 21 are employed, the sheet of material is also held together by the web of material behind each groove.

The location of grooves or slits 23 in sheet 21 can be precisely controlled so as to position the grooves or slits on bend line 25 within relatively close tolerances. Accordingly, when sheet 21 is bent after the grooving or slitting process, the bend occurs at a position that is very close to bend line 25. Since slits can be laid out on a flat sheet of material precisely, the cumulative error is much less in such a bending process, as compared to one in which bends are formed by a press brake, with each subsequent bend being positioned by reference to the preceding bend.

Nevertheless, even a grooving-based or slitting-based bending of sheet material has its problems. First, the stresses in bending webs or straps 27, as a result of plastic deformation of the webs and slitting at both ends of webs 27, are substantial and concentrated. For grooving, the stresses on the material behind or on the back side of the groove also are substantial and very concentrated. Thus, failures at webs 27 and/or behind grooves 23 can occur. Moreover, the grooves or slits do not necessarily produce bending of webs 27 directly along bend line 25, and the grooving process is slow and inconsistent, particularly when milling or point cutting V-shaped grooves. Grooving, therefore, is not in widespread commercial use.

As can be seen in FIGS. 1A and 1B, if sheet 21 is slit, as is shown at 23a and/or grooved, as shown at 23b, and then bent, bending webs 27a and 27b will experience plastic deformation and residual stress. For slit 23a, of course, material will be completely removed or severed along the length of the slit. For V-shaped groove 23b, there will be a thin web 29 between groove 23b and the convex outside of the bend, but it also will be plastically deformed and highly stressed. The bend for V-shaped grooving will normally be in a direction closing groove 23b so that the side faces come together, as shown in FIG. 1B. Loading of the bent structure of FIGS. 1A and 1B with a vertical force FV and/or a horizontal force FH will place the bend, with the weakening slits and/or grooves and the plastically deformed straps or webs 27a, 27b, as well as thin web 29, under considerable stress. Failure of the structure will occur at lower force levels than if a non-slitting or non-grooving bending process was used.

Another scheme for sheet slitting to facilitate bending has been employed in the prior art. The slitting technique employed to produce bends, however, was designed primarily to produce visual or decorative effects for a sculptural application. The visual result has been described as “stitching,” and the bends themselves have been structurally reinforced by beams. This stitched sculpture was exhibited at the New York Museum of Modern Art by at least 1998, and the sheet slitting technique is described in Published U.S. Patent Application U.S. 2002/0184936 A1, published on Dec. 12, 2002, (the “Gitlin, et al Application.”). The sculpture is also shown and described in the publication entitled “Office dA” by Contemporary World Architects, pp. 15, 20-35, 2000. FIGS. 2, 2A and 2B of the present drawing show one example of the stitching technique employed.

One embodiment of the Office dA or Gitlin, et al. Application is shown in FIG. 2. A plurality of slits 31 is formed in a sheet material 32. Slits 31 are linear and offset laterally of each other along opposite sides of a bend line 33. The slits can be seen to longitudinally overlap so as to define what will become bending splines, webs, straps or “stitches” 34 between the overlapped slit ends. FIGS. 2A and 2B show an enlarged side elevation view of one end of one slit in sheet 32, which has been bent along bend line 33 by 90 degrees, and sheet portions 35 and 36 on opposite sides of the bend line are interconnected by the twisted straps or “stitches” 34, which twist or stitch between the 90 degree sheet portions 35, 36. The architects of the New York Museum of Modern Art sculpture recognized that the resulting bend is not structurally very strong, and they have incorporated partially hidden beams welded into the sculpture in the inner vertices of each of the stitched bends.

Since slits 31 are parallel to bend line 33, straps 34, which also have a constant or uniform width dimension, are twisted or plastically deformed in torsion over their length, with the result that at the end of a 90° bend a back side of the strap engages face 38 on the other side of slit 31 at position 37. Such engagement lifts sheet portion 35 up away from face 38 on sheet portion 36, as well as trying to open end 40 of the slit and producing further stress at the slit end. The result of the twisting of straps 34 and the lifting at the end of the bend is a gap, G, over the length of slit 31 between sheet portion 35 and face 38. Twisted straps or stitches 34 force sheet portion 35 off of face 38 and stress both slit ends 40 (only one slit end 40 is shown but the same stress would occur at the other slit end 40 of the slip 31 shown in FIGS. 2A and 2B).

Gaps G are produced at each slit 31 along the length of bend line 33 on alternative sides of the bend line. Thus, at each slit a sheet portion is forced away from contact with a slit-defining face instead of being pulled into contact with, and thus full support by, the face.

Moreover, and very importantly, the slitting configuration of FIG. 2 stresses each of straps 34 to a very high degree. As the strap length is increased (the length of overlap between the ends of slits 31) to attempt to reduce the stress from twisting along the strap length, the force trying to resiliently pull or clamp a sheet portion against an opposing face reduces. Conversely, as strap length 34 is decreased, twisting forms micro tears in the constant width straps with resultant stress risers, and the general condition of the twisted straps is that they are overstressed. This tends to compromise the strength of the bend and leaves a non-load bearing bend.

A vertical force (Fv in FIG. 2B) applied to sheet portion 35 will immediately load twisted and stressed strap 34, and because there is a gap G the strap will plastically deform further under loading and can fail or tear before the sheet portion 35 is displaced down to engagement with and support on face 38. A horizontal force FH similarly will tend to crush the longitudinally adjacent strap 34 (and shear strap 34 in FIG. 2B) before gap G is closed and the sheet portion 35 is supported on the opposing slit face 38.

Another problem inherent in the slitting scheme of FIGS. 2-2B and the Gitlin, et al. Application is that the constant strap width cannot be varied independently of the distance between slits, and the strap width cannot be less than the material thickness without stressing the straps to the extreme. When slits 31 are parallel to each other and longitudinally overlapping, the strap width, by definition, must equal the spacing or jog between slits. This limits the flexibility in designing the bends for structural loading of the straps. Still further, the slits terminate with every other slit end being aligned and directed toward the other. There is no attempt, therefore, to reduce stress risers and micro-crack propagation from occurring at the ends of the slits, and aligned slit ends can crack under loading.

The sheet slitting configuration of FIGS. 2-28, therefore, can be readily employed for decorative bends, but it is not optimally suited for bends which must provide significant structural support and fatigue resistance.

The Gitlin et al. Application also teaches the formation of curved slits (in FIGS. 10a, 10b), but the slits again parallel a curved bend line so that the width of the bending straps is constant, the straps extend along and parallel to the bend line, not across it, the straps are twisted in the extreme, the slit ends tend to direct micro-cracks and stress concentrations to the next slit, and the application teaches employing a slit kerf which results in engagement of the opposite side of the slit, at 37, only at the end of the bend.

A simple linear perforation technique also was used by the same architects in an installation of bent metal ceiling panels in a pizza restaurant in Boston. Again, the bent sheet components by linear perforation were not designed to bear significant unsupported loads along the bends.

Slits, grooves, perforations, dimples and score lines also have been used in various patented systems as a basis for bending sheet material. U.S. Pat. No. 5,225,799 to West et al., for example, uses a grooving-based technique to fold up a sheet of material to form a microwave wave guide or filter. In U.S. Pat. No. 4,628,161 to St. Louis, score lines and dimples are used to fold metal sheets. In U.S. Pat. No. 6,210,037 to Brandon, slots and perforations are used to bend plastics. The bending of corrugated cardboard using slits or die cuts is shown in U.S. Pat. No. 6,132,349 and PCT Publication WO 97/24221 to Yokoyama, and U.S. Pat. Nos. 3,756,499 to Grebel et al. and 3,258,380 to Fischer, et al. Bending of paperboard sheets also has been facilitated by slitting, as is shown in U.S. Pat. Nos. 5,692,672 to Hunt, 3,963,170 to Wood and 975,121 to Carter. Published U.S. Patent Application No. US 2001/0010167 A1 also discloses a metal bending technique involving openings, notches and the like and the use of great force to produce controlled plastic flow and reduced cracking and wrinkling.

In most of these prior art bending systems, however, the bend forming technique greatly weakens the resulting structure, or precision bends are not capable of being formed, or bending occurs by crushing the material on one side of the bend. Moreover, when slitting is used in these prior art systems, in addition to structural weakening and the promotion of future points of structural failure, the slitting can make the process of sealing a bent structure expensive and difficult. These prior art methods, therefore, are less suitable for fabricating structures that are capable of containing a fluid or flowable material.

The problems of precision bending and retention of strength are much more substantial when bending metal sheets, and particularly sheets of substantial thickness. In many applications it is highly desirable to be able to bend metal sheets with low force, for example, by hand with only hand tools, or with only moderately powered tools. Such bending of thick metal sheets, of course, poses greater problems.

In another aspect of the present invention the ability to overcome prior art deficiencies in slitting-based bending of sheet material is applied to eliminate deficiencies in prior art metal fabrication techniques and the structures resulting therefrom.

A well known prior art technique for producing rigid three-dimensional structures is the process of cutting and joining together parts from sheet and non-sheet material. Jigging and welding, clamping and adhesive bonding, or machining and using fasteners to join together several discrete parts has previously been extensively used to fabricate rigid three-dimensional structures. In the case of welding, for example, a problem arises in the accurate cutting and jigging of the individual pieces; the labor and machinery required to manipulate a large number of parts, as well as the quality control and certification of multiple parts. Additionally, welding has the inherent problem of dimensional shape warping caused by the heat-affected zone of the weld.

Traditional welding of metals with significant material thickness is usually achieved by using parts having beveled edges often made by grinding or single point tools, which add significantly to the fabrication time and cost. Moreover, the fatigue failure of heat-affected metals is unpredictable for joints whose load-bearing geometries rely entirely on welded, brazed or soldered materials. Fatigue failure of welds usually is compensated for by increasing the mass of the components, which are welded together and the number and depth of the welds. The attendant disadvantage of such over design is, of course, excessive weight.

With respect to adhesively bonding sheet and non-sheet material along the edges and faces of discrete components, a problem arises from the handling and accurate positioning the several parts and holding or clamping them in place until the bonding method is complete.

Another class of prior art techniques related to the fabrication of three-dimensional structures are the Rapid Prototyping methods. These include stereo lithography and a host of other processes in which a design is produced using a CAD system and the data representation of the structure is used to drive equipment in the addition or subtraction of material until the structure is complete. Prior art Rapid Prototyping techniques are usually either additive or subtractive.

The problems associated with subtractive Rapid Prototyping methods are that they are wasteful of materials in that a block of material capable of containing the entire part is used and then a relatively expensive high-speed machining center is required to accurately mill and cut the part by removal of the unwanted material.

Problems also exist with prior art additive Rapid Prototyping techniques. Specifically, most such techniques are optimized for a very narrow range of materials. Additionally, most require a specialized fabrication device that dispenses material in correspondence with the data representing the part. The additive Rapid Prototyping processes are slow, very limited in the scale of the part envelope and usually do not make use of structurally robust materials.

Generally in the prior art, therefore, sheet slitting or grooving to enable sheet bending has produced bends, which lack the precision and strength necessary for commercial structural applications. Thus, such prior art sheet bending techniques have been largely relegated to light gauge metal bending or decorative applications, such as sculpture.

In a broad aspect of the present invention, therefore, it is an important object of the present invention to be able to bend sheet material in a very precise manner and yet produce a bend, which is capable of supporting substantial loading and is resistant to fatigue failures.

Another object of this aspect of the present invention is to provide a method for precision bending of sheets of material using improved slitting techniques, which enhance the precision of the location of the bends, the strength of the resulting structures and reduce stress-induced failures.

Another object of the present invention is to provide a precision sheet bending process and a sheet of material which has been slit or grooved for bending and which can be used to accommodate bending of sheets of various thicknesses and of various types of non-crushable materials.

Another object of the present invention is to provide a method for slitting sheets for subsequent bending that can be accomplished using only hand tools or power tools which facilitate bending but do not attempt to control the location of the bend.

Another object of the present invention is to be able to bend sheet material into high strength, three-dimensional structures having precise dimension tolerances.

It is another object of the present invention to be able to bend sheet materials into precise three-dimensional structures that are easily and inexpensively sealed thus enabling the containment of fluid or Plowable materials.

In a broad aspect of the present invention relating to the use of slit-based bending to enhance fabrication and assembly techniques, it is an object of the present invention to provide a new Rapid Prototyping and Advanced Rapid Manufacturing technique that employs a wide range of materials including many that are structurally robust, does not employ specialized equipment other than what would be found in any modem fabrication facility, and can be scaled up or down to the limits of the cutting process used.

It is another object of this aspect of the present invention to provide features within the sheet of material to be bent that assist in the accurate additive alignment of components prior to and after the sheet material is bent.

A further object of the present invention is to provide a fabrication method that serves as a near-net-shape structural scaffold for multiple components arranged in 3D space in the correct relationship to each other as defined by the original CAD design process.

It is a further object of the present invention to provide a method of fabricating welded structures that employs a smaller number of separate parts and whose edges are self jigging along the length of the bends and whose non-bent edges provide features that facilitate jigging and clamping in preparation for welding. In this context it is yet another object of the present invention to provide a superior method of jigging sheet materials for welding that dramatically reduces warping and dimensional inaccuracy caused by the welding process.

Yet another object of the present invention is to provide a novel welded joint that provides substantial load-bearing properties that do not rely on the heat affected zone in all degrees of freedom and thereby improve both the loading strength and cyclical, fatigue strength of the resulting three-dimensional structure.

Still another object of the present invention is to provide a superior method for: 1) reducing the number of discrete parts required to fabricate a strong, rigid, dimensionally accurate three-dimensional structure, and 2) inherently providing a positioning and clamping method for the various sides of the desired three-dimensional structure that can be accomplished through the bent and unbent edges of the present invention resulting in a lower cost, higher yield fabrication method.

It is a further object of the present invention to provide a method of fabricating a wide variety of fluid containing casting molds for metals, polymers, ceramics and composites in which the mold is formed from a slit, bent, sheet of material which can be either removed after the solidification process or left in place as a structural or surface component of the finished object.

Still another object of the present invention is to provide a sheet bending method that is adaptable for use with existing slitting devices, enables sheet stock to be shipped in a flat or coiled condition and precision bent at a remote location without the use of a press brake, and enhances the assembly or mounting of components within and on the surfaces in the interior of enclosures formed by bending of the sheet stock after component affixation to the sheet stock.

Still another object of the present invention is to provide a precision folding technique that can be used to create accurate, precise, load-bearing folds in sheets of material, including but not limited to, metals, plastics, and composites.

Another object of the present invention is to provide a precision folding technique that allows folding around a virtual bend line and requires considerably less force to accomplish the fold than conventional bending techniques.

Another object of the present invention is to provide a precision folding technique that is essentially linearly scalable independently of the thickness or microstructural characteristics of the material

Another object of the present invention is to form the geometries described herein whether by a slitting/removal process, a severing process or by an additive process, and arrive at the advantages herein described by any route.

Yet another object of the present invention is to provide a precision folding technique for folding a non-crushable material in which the microstructure of the material remains substantially unchanged around the fold.

The methods and discrete techniques for designing and precision folding of sheet material, the fabrication techniques therefor, and the structures formed from such precision bending of the present invention have other features and objects of advantage which will become apparent from, or are set forth in more detail in the following detailed description and accompanying drawings.

SUMMARY

In a broad aspect, a sheet of material for bending along a desired bending line includes bending strap-defining structures formed in the sheet. The strap-defining structures are positioned to define at least one bending strap in the sheet; the strap having a longitudinal strap axis that is oriented and positioned to extend across the bend line. Moreover, the strap defining structures are configured and positioned to produce bending of the sheet of material along the bend line.

In another aspect, a hollow beam includes two sheets of material. The first sheet of material is formed for bending along a plurality of first sheet bend lines by having a plurality of bending strap-defining structures positioned proximate each of the bend lines, with the bending strap-defining structures configured to produce bending along the bend lines. A hollow beam is formed by securing the first sheet of material, being bent along first sheet bend lines, to a second sheet of material.

In yet another aspect, an exoskeletal framework includes a single sheet of material formed for bending along a plurality of bend lines. The sheet of material is formed with a plurality of bending strap-defining structures positioned proximate each of the bend lines, and the bending strap-defining structures are configured to produce bending. Bending the sheet of material along the bend lines results in a framework of structural members.

The precision-folded, high strength, fatigue-resistant structures and sheet therefor of the present invention has other features and advantages which will be apparent from or are set forth in more detail in the accompanying drawings, which are incorporated in and form a part of this specification, and the following Detailed Description of the Invention, which together serve to explain the principles of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary, top plan view of a sheet of material having slits and grooves formed therein in accordance with one prior art technique.

FIG. 1A is an enlarged, fragmentary view, in cross section, taken substantially along the plane of line 1A-1A in FIG. 1, of the sheet of FIG. 1 when in a bent condition.

FIG. 1B is an enlarged, fragmentary view, in cross section, taken substantially along the plane of line 1B-1B of FIG. 1, of the sheet of FIG. 1 when in a bent condition.

FIG. 2 is a fragmentary, top plan view of a sheet of material having a plurality of slits formed therein using an alternative configuration known in the prior art.

FIG. 2A is an enlarged fragmentary side elevation -view of the sheet of FIG. 2 bend by about 90 degrees.

FIG. 2B is a cross-sectional view taken substantially along the plane of line 2B-2B in FIG. 2A.

FIG. 3 is a fragmentary, top plan view of a sheet of material slit in accordance with one embodiment of the present invention.

FIGS. 4A-4D are fragmentary, top plan views of a sheet of material which has been slit according to the embodiment of FIG. 3 and which is in the process of being bent from a flat plane in FIG. 4A to a 90 degrees bend in FIG. 4D.

FIGS. 5A-5A are fragmentary, cross-sectional views, taken substantially along the planes of lines 5A-5A, in FIGS. 4A-4D during bending of the sheet of material.

FIG. 6 is a top plan view of a sheet of material slit in accordance with a second embodiment of the present invention.

FIG. 7 is a top plan. view of the sheet of FIG. 6 after being bent by about 90 degrees.

FIG. 8 is an end view of the sheet of material of FIG. 7.

FIG. 8A is an enlarged, end elevation view, in cross section, of the sheet of material of FIG. 7 taken substantially along the plane of 8A-BA in FIG. 7 and rotated by about 45 degrees from FIG. 8.

FIG. 8B is an enlarged, end elevation view, in cross section, of the sheet of material of FIG. 7 taken substantially along the plane of 8B-8B in FIG. 7 and rotated by about 45 degrees from FIG. 8.

FIG. 9 is a fragmentary top plan view of a sheet of material slit according to a further alternative embodiment of the present invention.

FIG. 10 is a side elevation view of the sheet of FIG. 9 after bending by about 90 degrees.

FIG. 10A is a fragmentary cross-sectional view taken substantially along the plane of line 10A-10A in FIG. 10.

FIG. 11 is a fragmentary, top plan view of a schematic representation of a further alternative embodiment of a sheet of material having strap-defining structures constructed in accordance with the present invention.

FIG. 11A is a fragmentary top plan view of a slit of the configuration shown in FIG. 11 which has been formed using a rapid piercing laser cutting technique.

FIG. 12 is a fragmentary, top plan view of one sheet of material before bending and assembly into a curved box beam.

FIG. 13 is a side elevation view of a curved box beam constructed from two sheets of material each being slit as shown in FIG. 12.

FIG. 14 is an end elevation view of the beam of FIG. 13.

FIG. 15 is a top plan view of a sheet of material formed with strap-defining structures and configured for enclosing a cylindrical member.

FIG. 16 is a top perspective view of the sheet of material of FIG. 15 as bent along bend lines and mounted to enclose a cylindrical member.

FIG. 17 is a top perspective, exploded view of a corrugated assembly formed using a sheet of material formed in accordance with the present invention.

FIG. 18 is a top perspective, exploded view of an alternative embodiment of a sheet of material formed in accordance with the present invention.

FIG. 19 is a top plan view of the slit sheet used to construct an alternative embodiment of a corrugated deck prior to bending or folding.

FIG. 20 is a top perspective view of a corrugated sheet or deck constructed using the slit sheet material of FIG. 19.

FIG. 21 is an enlarged, fragmentary perspective view substantially bounded by line 21-21 in FIG. 20.

FIG. 21A is an enlarged, fragmentary, top plan view substantially bounded by line 21A-21A in FIG. 19.

FIG. 22 is a schematic, end elevation view of a cylindrical member constructed using a corrugated sheet similar to that of FIGS. 19 and 20, scaled to define a cylindrical form.

FIG. 23 is an enlarged, fragmentary, side elevation view of a sheet of material slit in accordance with the present invention and having a tongue or tab displaced to ensure predictable bending.

FIG. 23A is a reduced, end elevation view of the sheet of FIG. 23 during bending.

FIG. 24 is a fragmentary, end elevation view of a sheet of material slit at an oblique angle to the plane of the sheet and shown during bending a to a complimentary angle.

FIG. 25 is a side elevation, schematic representation of a reel-to-reel sheet slitting line arranged in accordance with the present invention.

FIG. 26 is a top perspective view of a coiled sheet of material which has been slit, for example, using the apparatus of FIG. 25 and is in the process of being rolled out and bent into a three-dimensional structure.

FIGS. 27A-27G are top perspective views of a sheet of material constructed in accordance with the present invention as it is being bent into a cross-braced box beam.

FIGS. 28A-28E are top perspective views of a sheet of material constructed in accordance with the present invention as it is being bent into a chassis for support of components such as electrical components.

FIG. 29 is a top perspective, schematic representation of one embodiment of equipment suitable for low-force bending or folding of the slit sheet of the present invention.

FIG. 30 is a top perspective, schematic representation of another embodiment of sheet bending or folding process of the present invention.

FIG. 31 is a flow diagram of one aspect of the interactive design, fabrication and assembly processes for slit sheet material bending of the present invention.

FIGS. 32A-32E are top perspective views of a sheet of material constructed in accordance with the present invention as it is being bent into a stud wall/ladder.

FIG. 33 is a top perspective view of a curved corrugated deck or panel constructed in accordance with the present invention.

FIG. 34A-34E are top perspective views of a sheet of material including swing-out bracing and shown as it is being bent into a swing-out braced box-beam.

FIG. 35 is a top plan view of a sheet of material slit in accordance with the present invention and including a single slit embodiment

FIG. 36 is a top perspective view of the sheet of FIG. 35 as bent into a roller housing.

FIG. 37 is a fragmentary top plan view of a sheet of material having differing bend line termination slit configurations.

FIG. 38A is a top perspective view of a sheet of material constructed in accordance with the present invention before being bent into a chassis.

FIG. 38B is a schematic, top perspective view of a sheet of material as in FIG. 38A after being bent into a chassis.

FIG. 38C is a top perspective view of several sheets of material as in FIG. 38A after being bent into a transitionary form of a chassis and stacked.

FIG. 39A is a top view of two sheets of material constructed in accordance with the present invention before being farmed and joined into a curved beam.

FIG. 39B is a top perspective view of a curved channel constructed in accordance with the present invention from a sheet similar to that shown in FIG. 39A.

FIG. 39C is a top perspective view of a closed, hollow curved beam constructed in accordance with the present invention from two sheets similar to those shown in FIG. 39A.

FIG. 40A-H are perspective views of a sheet of material constructed in accordance with the present invention before and in phases of being folded into a skeletal structure.

FIG. 4I is a perspective view of a corner portion of a skeletal structure according to the present invention before and in phases of being folded.

FIG. 42A is a perspective view of a corner portion as shown in FIG. 4I.

FIG. 42B is a side view of an edge slot as shown in FIG. 42A

FIG. 42C is a side view of an alternate embodiment of an edge slot.

FIG. 43A is a top view of a sheet of material constructed in accordance with the present invention before being formed into a curved exoskeletal structure.

FIG. 43B is a perspective view of a sheet of material as shown in FIG. 43A after being formed into a curved exoskeletal structure.

FIG. 43C is a top view of a portion of a sheet of material similar to that shown in FIG. 43A before being formed into a curved exoskeletal structure.

FIG. 44 is a perspective view of another sheet of material formed into a three dimensional structure in accordance with the present invention.

DETAILED DESCRIPTION

Reference will now be made in detail to the preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the preferred embodiments, it will be understood that they are\'not intended to limit the invention to those embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims. Num The present method and apparatus for precision bending of sheet material is based upon the slitting geometries disclosed in prior applications, U.S. patent application Ser. No. 09/640,267, filed Aug. 17, 2000, now U.S. Pat. No. 6,481,259 B1, and entitled METHOD FOR PRECISION BENDING OF A SHEET OF MATERIAL AND SLIT SHEET THEREFOR, and U.S. patent application Ser. No. 10/256,870, filed Sep. 26, 2002 and entitled METHOD FOR PRECISION BENDING OF SHEET OF MATERIALS, SLIT SHEETS AND FABRICATION PROCESS, now U.S. Pat. No. 6,877,349 B2, which are incorporated herein by reference in their entirety.

One embodiment of the precision and high strength bending process and apparatus of the present invention can be described by reference to FIGS. 3-5. In FIG. 3 a sheet of material 41 is formed with a plurality of bending strap-defining structures, in this case slits, generally designated 43, along a bend line 45. Slits 43, therefore, are longitudinally extending and in end-to-end spaced relation so as to define bending webs or straps 47 between pairs of slits 43. In FIG. 3, slits 43 are provided with stress reducing structures at ends thereof; namely openings 49, so as to effect a reduction in the stress concentration in bending webs 47. It will be understood from the description below, however, that stress reducing structures, such as enlarged openings 49 in FIG. 3, are not required for realization of the benefits of the precision bending system of the present invention.

For the embodiment of slits 43 shown in FIG. 3, however, each longitudinally extending slit between the slit ends is laterally or transversely stepped relative to bend lines 45. Thus, a slit, such as slit 43a, is formed with a pair of longitudinally extending slit segments 51 and 52 which are positioned proximate to, and preferably equidistant on opposite sides of; and substantially parallel to, bend line 45. Longitudinal slit segments 51 and 52 are further connected by a transversely extending slit segment 53 so that slit 43a extends from enlarged opening 49a to enlarged opening 49b along an interconnected path which opens to both of the enlarged openings and includes both longitudinally extending slit segments 51, 52 and transverse slit segment 53.

The function and advantages of such stepped slits can best be understood by reference to FIGS. 4A-4D, and the corresponding FIGS. 5A-5C to 5A-5C, wherein the bending or folding of a sheet of material 41, such as shown in FIG. 3 is illustrated at various stages. In FIG. 4A, sheet 41 is essentially slit as shown in FIG. 3. There is a difference between FIGS. 3 and 4A in that in FIG, 3 a kerf width or section of removed material is shown, while in FIG. 4A the slit is shown without any kerf; as would be produced by a slitting knife or punch. The effect during bending, however, is essentially the same if the kerf width is small enough that the material on the opposite sides of the slit interengage during bending. The same reference numerals will be employed in FIGS. 4A-5C as were employed in FIG. 3.

Thus, sheet 41 is shown in a flat condition before bending in FIG. 4A. Longitudinally extending slit segments 51 and 52 are shown in FIG. 4A and in the cross sections of FIGS. 5A-5C. The positions of the various cross sections of the sheet are also shown in FIG. 4A.

In FIG. 4B, the sheet has been bent slightly along bend line 45, which can best be seen in FIGS. 5A-5C. As can be seen in FIGS. 5.6. and 513, slits 51 and 52 have opened up along their top edges and the portion of the sheet which extends beyond bend line 45 was referred to in U.S. Pat. No. 6,481,259 B1 and U.S. patent application Ser. No. 10/256,870 (now U.S. Pat. No. 6,877,349 B2) as a “tab” 55, but for the sake of consistency with later embodiments in this application shall be referred to as “lip” 55. The lower or bottom side edges 51a and 52a of lips 55 have moved up slightly along supporting faces 51b and 52b of the sheet on the opposite sides of the slit opposite to lips 55. This displacement of lip edges 51a and 52a may be better seen in connection with the sheet when it is bent to a greater degree, for example, when bent to the position shown in FIG. 4C.

In FIG. 4C it will be seen that edges 51a and 52a have moved upwardly on supporting faces 51b and 52b of sheet 41 on opposite sides of bend line 45. Thus, there is sliding contact between edges 51a and 52a and the opposing supporting faces 51b and 52b of the slit during bending. This sliding contact will be occurring at locations which are equidistant on opposite sides of central bend line 45 if longitudinal slit segments 51 and 52 are formed in equally spaced positions on opposite sides of bend line 45, as shown in FIG. 4A. Sliding contact also can be facilitated by a lubricant or by adhesives or sealants prior to their setting up or bonding.

The result of this structure is that there are two actual bending fulcrums 51a, 51b and 52a, 52b spaced at equal distances from, and on opposite sides of, bend line 45. Lip edge 51a and supporting face 51b, as well as lip edge 52a and supporting face 52b, produce bending of bending web 47 about a virtual fulcrum that lies between the actual fulcrums and will be understood to be superimposed over bend line 45.

The final result of a 90 degree bend is shown in FIG. 4D and corresponding cross sections 5A-5C. As will be seen, sheet edge 52a and bottom side or surface 52c now are interengaged or rest on, and are supported in partially overlapped relation to, supporting face 52b (FIG. 5A). Similarly, edge 51a and bottom surface 51c now engages and rests on face 51b in an overlapped condition (FIG. 5B). Bending web 47 will be seen to have been plastically deformed or extended along an upper surface of the web 47a and plastically compressed along a lower surface 47b of web 47, as best illustrated in FIG. 5C.

In the bent condition of FIG. 4D, the lip portions of the sheet, namely, portions 55, which extend over the center line when the sheet is slit, are now resting on supporting faces 51b and 52b. This edge-to-face engagement and support during the bend, which alternates along the bend line in the configuration shown in the drawing, produces greater precision in bending or folding and gives the bent or folded structure greater resistance to shear forces at the bend or fold in mutually perpendicular directions. Thus a load La (FIG. 5A) will be supported between bending webs 47 by the overlap of the edge 52a and bottom surface 52c on supporting edge 52b. Similarly, a load Lb (FIG. 5B) will be supported by overlap and engagement of the edge 51a and surface 51e on supporting face 51b intermediate bending webs 47.

This is referred to herein as “edge-to-face” engagement and support of the material along substantially the entire length of one side of the slit by the material along substantially the entire length of the other side of the slit. It will be appreciated that, if sheet 41 were bent or folded by more than 90 degrees, edges 51a and 52a would lift up off the faces 51b and 52b and the underneath surfaces 51c and 52c would be supported by the lower edges of face 51b and 52b. If the sheet is bent by less than 90 degrees the edge still comes into engagement with the face almost immediately after the start of bending, but only the edge engages the face. This support of one side of the slit on the other shall be deemed to be “edge-to-face” engagement and support as used in the specification and the claims. As will be described hereinafter, non-ninety degree bends with full support of edges 51a and 52a by faces 51b can be achieved by slitting the sheet at angles which are not at 90 degrees to the sheet.

While bending straps or webs 47 have residual stresses as a result of plastic deformation, and while the slits cause a substantial portion of the bend not to be directly coupled together in the slit-based bending system of the present invention, the slits are formed and positioned so as to produce an edge-to-face overlap which provide s substantial additional strength to the bent structure over the strength of the structures of FIGS. 1, 1A and 18 and 2A and 2B, which are based upon conventional slitting or grooving geometries. The bending straps of the present invention, in effect, pre-load the bend so as to pull or clamp the sides of the slit into edge-to-face engagement over substantially the entire bending process, and at the end of the bend, over substantially the entire slit length. Pre-loading of the bend by the residual tension in the strap also tends to prevent vibration between the slit edge which is pre-loaded against the face which acts as a bed on the other side of the slit.

Moreover, since the edges are interengaged with the faces over a substantial portion of the length of the slits, loads La and Lb will not crush or further plastically deform bending straps 47, as is the case for the prior art slitting configuration of FIGS. 2, 2A, 2B. Loading of the present bend is immediately supported by the edge-to-face engagement produced by the slitting technique of the present invention, rather than merely by the cross-sectional connecting area of a twisted and highly stressed strap, as results in the prior art configuration of FIGS. 2, 2A, 2B and the Gitlin et al. application.

The embodiment employing laterally stepped or staggered slits of the present invention, therefore, result in substantial advantages. First, the lateral position of the longitudinally extending slit segments 51 and 52 can be precisely located on each side of bend line 45, with the result that the bend will occur about a virtual fulcrum as a consequence of two actual fulcrums equidistant from, and on opposite sides of, the bend line. This precision bending reduces or eliminates accumulated tolerance errors since slit positions can be very precisely controlled by a cutting device which is driven by a CNC controller.

It also should be noted, that press brakes normally bend by indexing off an edge of a sheet or an existing bend, or other feature(s). This makes bending at an angle to the sheet edge feature(s) difficult using a press brake. Bending precisely at angles to any feature(s) of the sheet edge, however, can be accomplished readily using the present slitting process. Additionally, the resulting bent sheet has substantially improved strength against shear loading and loading along mutually perpendicular axes because the overlapped edges and faces produced by the present slit configurations support the sheet against such loads.

As can be seen, the embodiment of the present invention, as shown in FIGS. 3-5C produces precision bending of straps 47 which are substantially perpendicular to the bend line. Such an orientation of the bending straps produces significant plastic elongation along the outside or top surface of the strap, as well as significant compression along the inside or bottom surface of the strap. The bend occurs on the relatively short perpendicular straps in a manner similar to the bends of the perpendicular straps of FIGS. 1-1B, but in FIGS. 3-5C′″ the lip 55 of one plane is tucked into interlocking or interengaged relationship with the face of the other plane for increased bend strength.

The prior art approach shown in FIGS. 2-2B orients the connecting straps 34 parallel to the bend line and results in significant plastic twisting deformation of the straps. Also this plastic twisting deformation significantly changes the microstructure of the material around the bend line. Moreover the straps do not fully tuck or clamp the opposite sides of the sheet into interengaged relation over the length of the slits. Still further in the embodiment of FIGS. 3-5C″′ the strap width can be varied independently of the jog distance between slits 51 and 52 so that greater flexibility in design of the bend strength can be achieved.

While bending of sheet material by 90 degrees has been illustrated in the drawing, it will be understood that most of the advantages described in all embodiments of the present invention also can be realized if the slit sheet is bent by more or less than 90 degrees. The lip which extends across the bend line will slide onto and engage the opposite face beginning at small bend angles, and such support and engagement will continue at large, 90 degree plus, bend angles.

It has been found that the embodiment of FIGS. 3-5C″′ is best suited for use with relatively ductile sheet materials. As the material becomes harder and less ductile, a second embodiment is preferred.

In the embodiment of the present invention shown in FIGS. 6-8B, a slitting configuration is employed which tucks or clamps the sheet material into interengaged relation on both sides of the slits, and also reduces bending strap plastic deformation and the residual stress in the straps. Moreover, this embodiment also allows the strap width to be varied independently of the jog distance between slits and to have the strap width increase in both directions from the bend line for less stress concentration in the connected portions of the sheet of material on opposite sides of the bend line.

A bending strap which is oblique to the bend line is employed, which allows the strap length to be increased, as compared to the shorter bending straps of FIGS. 3-5C″. Plastic deformation also is accomplished in part by twisting, rather than purely by bending, as is the case in FIGS. 3-5C″\', but the amount of twisting is greatly reduced, as compared to the parallel straps of FIGS. 2-2B. Moreover, the material lips on opposite sides of the slit are tucked into interengagement with the faces over virtually the entire length of the slit so that substantial additional strap stress on loading does not occur.

Additionally, in the embodiment shown in FIGS. 6-8B, the slit configuration produces a continuous sliding interengagement between material on opposite sides of the slits during bending, which interengagement progresses along the slit from the middle toward the ends. The faces on one side of the slits act as beds for sliding support during the bend, which results in a more uniform and a less stressful bending of the bending straps. The embodiment as shown in FIGS. 6-8B, therefore, can be used with sheet material that is less ductile, such as heat treated 6061 aluminum or even some ceramics, and with thicker sheets of material.

Referring specifically to FIGS. 6-8B, a sheet of material 241 to be bent or folded is formed with a plurality of longitudinally extending bending strap-defining structures, such as slits 243, along a bend line 245. Each of slits 243 optionally may be provided with enlarged stress-relieving end openings 249, or a curved end section 249a, which will tend to cause any stress cracks to propagate back into slits 243, depending on the loading direction of the sheet. As will be seen, the slits of the embodiment of FIGS. 6 and 8B are not stepped, but they are configured in a manner producing bending and twisting of obliquely oriented bending straps 247 about a virtual fulcrum superimposed on bend line 245. The configuration and positioning of the slits, including selection of the jog distance and kerf width, also causes the sheet material on opposite sides of the slits to tuck or to move into an edge-to-face interengaged relationship during bending. Most preferably edge-to-face interengagement occurs throughout the bend to its completion. But, the jog distance and kerf can be selected to produce edge-to-face interengagement only at the start of the bend, which will tend to insure precise bending. Thus, as used herein, the expression “during bending” is meant to include edge-to-face interengagement at any stage of the bend.

While the embodiments shown and described in FIGS. 6-8B and 9-10A are not stepped, the oblique straps of the embodiments of 6-BB and 9-10A can be combined with the stepped slit configuration of FIGS. 3-5C. Thus, one or both of the ends of the stepped slits can be oblique or curved.

As shown in FIG. 6, pairs of elongated slits 243 are preferably positioned on opposite sides of and proximate to bend line 245 so that pairs of longitudinally adjacent slit end portions 251 on opposite sides of the bend line define a bending web, spline or strap 247, which can be seen to extend obliquely across bend line 245. “Oblique” and “obliquely,” as will be explained in more detail below in connection with FIG. 11, shall mean that the longitudinal central axis of the strap crosses the desired bend line at an angle other than 90 degrees. Thus, each slit end portion 251 diverges away from bend line 245 so that the center line of the strap is skewed or oblique and bending, as well as twisting of the strap, occurs. Although not an absolute requirement to effect bending in accordance with the present invention, it will be seen that slits 243 are longitudinally overlapping along bend line 245.

Unlike slits 31 in FIGS. 2-2B and the prior art Gitlin, et al. Application, which are parallel to the bend line in the area defining bending straps 34, the divergence of the slits 243 from bend line 245 results in oblique bending straps that do not require the extreme twisting present in the prior art of FIGS. 2-23 and Gitlin et at Application. Moreover, the divergence of slits 243 from bend line 245 results in the width dimension of the straps increasing as the straps connect with the remainder of sheet 241. This increasing width enhances the transfer of loading across the bend so as to reduce stress concentrations and to increase fatigue resistance of the straps.

As was the case for the first embodiment, slit kerfs 243 preferably have a width dimension , and the transverse jog distance across the bend line between slits is dimensioned, to produce interengagement of sheet material on opposite sides of the slits during bending. Thus, slits 243 can be made with a knife and have essentially a zero kerf, or they can have a greater kerf which still produces interengagement, depending upon the thickness of the sheet being bent. Most preferably the kerf width is not greater than about 0.3 times the material thickness, and the jog distance is not greater than about 1.0 times the material thickness.

As was the case for the embodiment of FIGS. 3-5C, a lip portion 253 extends across bend line 245 to slit 243. Lip 253 slides or rides up a face 255 of a tongue 260 on the other side of slit 243 if the kerf width and jog distance, relative to the thickness of the material, are not so large as to prevent contact between the two sides of the slit during bending.

If the kerf width and jog distance are so large that contact between the lip portion 253 and face 255 of tongue 260 does not occur the bent or folded sheet will still have some of the improved strength advantages of oblique bending straps, but in such instances there are no actual fulcrums for bending so that bending along bend line 245 becomes less predictable and precise. Similarly, if the strap-defining structures are grooves 243 which do not penetrate through the sheet of material, the grooves will define oblique, high-strength bending straps, but edge-to-face sliding will not occur during bending unless the groove is so deep as to break-through during bending and become a slit. Thus, arcuately or divergently grooved embodiments of the bending straps will have improved strap strength even if edge-to-face bending does not occur.

Another problem which will be associated with a kerf width that is too wide to produce interengagement of lips 253 with faces 255 of tongues 260 is that the resultant bent sheet material will not have a lip edge supported on a slit face, unless the bend is relatively extreme so as to define a small arcuate angle between the two sides of the bent sheet. As noted in connection with the prior art slitting approach, this will result in immediate further stressing of the bending straps upon loading. The problem would not be as severe in the strap configuration of FIGS. 6-8B as in the prior art, but the preferred form is for the kerf width and jog distance to be selected to insure interengagement of the lip and tongue face substantially Throughout the bending process.

It is also possible for the slits 243 to actually be on the bend line or even across the bend line and still produce precise bending from the balanced positioning of the actual fulcrum faces 255 and the edges of lips 253 sliding therealong. A potential disadvantage of slits 243 being formed to cross the bend line 245 is that an air-gap would remain between edge 257 and face 255. An air-gap, however, may be acceptable in order to facilitate subsequent welding, brazing, soldering, adhesive filling or if an air-gap is desired for venting. Slit positioning to create an air-gap is a desirable feature of the present invention when subsequent bend reinforcement is employed. Unfilled, however, an air-gap will tend to place all of the load bearing-requirements of the bend in all degrees of freedom, except rotation, on the connected zone or cross-sectional area of plastically deformed strap 247. It is also possible to scale slits that cross the bend line that produce edge-to-face engagement without an air gap.

FIGS. 7, 8, 8A and 8B illustrate the sheet 241 as bent to a 90 degree angle along bend line 245. As best may be seen in FIGS. 8A and 8B, an inside edge 257 of lip 253 has slid up on face 255 of tongue 260 on the opposite side of the slit and is interengaged and supported thereon. A vertical force, F, therefore, as shown in FIG. 8A is supported by the overlap of edge 257 on face 255. A horizontal force, FH, as shown in FIG. 8B similarly will be resisted by the overlap of edge 257 on face 255. Comparison of FIGS. 8A and 8B to the prior art FIGS. 1A, 1B and 2A and 2B will make apparent the differences which the present bending method and slit configuration have on the strength of the overall structure. The combination of alternating overlapping edge-to-face support along the slits and the oblique bending straps, which are oblique in oppositely skewed directions, provides a bend and twist which is not only precise but has much less residual stress and higher strength than prior slitting configurations will produce.

However, skewing of the bending straps in opposite directions is not required to achieve many of the advantages of the present invention. When sheet 241 is an isotropic material, alternate skewing of the strap longitudinal central axes tends to cancel stress. If the sheet material is not isotropic, skewing of the oblique straps in the same direction can be used to negate preferential grain effects in the material. Alternatively, for isotropic sheet material, skewing of the straps in the same direction can produce relative shifting along the bend line of the portions of the sheet on opposite sides of the bend line, which shifting can be used for producing a locking engagement with a third plane such as an interference fit or a tab and slot insertion by the amount of side shift produced.

The geometry of the oblique slits is such that they bend and twist over a region that tends to reduce residual stress in the strap material at the point where the slit is terminated or the strap connected to the rest of the sheet Thus, crack propagation is reduced, lessening the need for enlarged openings or curls at the slit ends. If the resultant structure is intended primarily for static loading or is not expected to be loaded at all, no stress reducing termination is required in the arcuate slit that produces the oblique strap.

Moreover, it will be understood that slits 243 can be shifted along bend line 243 to change the width of straps 247 without increasing jog distance at which the slits are laterally spaced from each other. Conversely, the jog distance between slits 243 can be increased and the slits longitudinally shifted to maintain the same strap thickness. Obviously both changes can be made to design the strap width and length to meet the application.

Generally, the ratio of the transverse distance from slit to slit, or twice the distance of one slit to the bend line is referred to as the “jog”. The ratio of the jog distance relative to the material thickness in the preferred embodiments of the present invention will be less than 1. That is, the jog distance usually is less than one material thickness. A more preferred embodiment makes use of a jog distance ratio of less than 0.5 material thickness. A still more preferred embodiment makes use of a jog distance ratio of approximately 0.3 material thickness, depending upon the characteristics of the specific material used and the widths of the straps, and the kerf dimensions.

The width of bending straps 247 will influence the amount of force required to bend the sheet and that can be varied by either moving slits 243 farther away from the bend line 245 or by longitudinally shifting the position of the slits, or both. Generally, the width of oblique bending straps 247 most preferably will be selected to be greater than the thickness of the material being bent, but strap widths in the range of about 0.5 to about 4 times the thickness of the material may be used. More preferably, the strap width is between 0.7 and 2.5 times the material thickness.

One of the advantages of the present invention, however, is that the slitting configuration is such that bending of sheets can normally be accomplished using hand tools or tools that are relatively low powered. Thus, the bending tools need only so much force as to effect bending and twisting of bending straps 247; they do not have to have sufficient power so as to control the location of the bend. Such control is required for powered machines, such as press brakes, which clamp the material to be bent with sufficient force so as to control the location of the bend. In the present invention, however, the location of the bend is controlled by the actual fulcrums, namely edges 257 pivoting on face 255 on opposite sides of the bend line. Therefore, the bending tool required need only be one which can effect bending of straps 247, not positioning of the bend. This is extremely important in applications in which high strength power tools are not readily available, for example, in outer space or in the field fabrication of structures or at fabricators who do not have such high-powered equipment. It also allows low-force sheet bending equipment, such as corrugated cardboard bending machines, bladders, vacuum bending, hydraulic pulling cylinders with folding bars, and shape-memory bending materials, to be used to bend metal sheets, as will be set forth in more detail below. Additionally, strong, accurate bends are important in the fabrication of structures in which physical access to power bending equipment is not possible because of the geometry of the structure itself. This is particularly true of the last few bends required to close and latch a three-dimensional structure.

The most preferred configuration for slit end portions 251 is an arcuate divergence from bend line 245. In fact, each slit may be formed as a continuous arc, as shown in FIGS. 9, 10 and 10A and described below. An arc causes the material on the side of the slit to smoothly and progressively move up the face side of the tongue along an arcuate path beginning at center of the slit and progressing to the ends of the slit. This reduces the danger of hanging up of edge 257 on face 255 during bending and thereby is less stressful on the bending straps. Additionally, large radii of cut free surfaces are less prone to stress concentration. In the configuration of FIGS. 6-8B, the central portion of slits 243 is substantially parallel to bend line 245. Some non-parallel orientations, particularly if balanced on either side of the bend line, may be acceptable and produce the results described herein.

It also would be possible to form end portions 251 to diverge from bend line 245 at right angles to the bend line and the center of slits 243. This would define a bending strap that could be non-oblique, if the slits did not longitudinally overlap. The disadvantage of this approach is that the bending straps 247 tend not to bend as uniformly and reliably and thereby influence the precision of the location of the bend. Additionally, such a geometry eliminates twisting of the strap and induces severe points of stress concentration on the inner and outer radii of the bend and may limit the degree of edge-to-edge engagement

The bending straps in all the embodiments of the present invention are first elastically deformed and in plastic/elastic materials thereafter plastically deformed. The present slitting invention also can be used with elastically deformable plastics that never plastically deform. Such materials would be secured in a bent or folded condition so that they do not resiliently unbend. In order to make it more likely that only elastic deformation occurs, it is preferable that the bending straps be formed with central longitudinal strap axes that axe at a small angle to the bending line, most preferably, 26 degrees or less. The lower the angle, the higher the fraction of twisting that occurs and the lower the fraction of bending that occurs. Moreover, the lower the angle, the higher the bending radius that occurs. Rigid materials that do not gracefully deform plastically, such as rigid polymers, rigid metal, the more flexible ceramics and some composites, can tolerate a large bending radius in the elastic regime. They can also tolerate a torsion or twisting spring action that is distributed over a long strap of material. Low angle straps provide both aspects.

At the end of the bend of a plastically deformed sheet, however, there will remain a certain resilient elastic deformation tending to pull edge 257 down against face 255 and resulting in residual resilient clamping force maintaining the interengagement between material on opposite sides of the slits. Thus, the elastic resiliency of the sheet being bent will tend to pre-load or mug down the overlapping sheet edges against the supporting faces to ensure strength at the bend and reduce bending strap incremental stress on loading of the bend.

The embodiment shown in FIGS. 9, 10 and 10A is a special case of the oblique strap embodiment described in connection with FIGS. 6-8B. Here the oblique straps are formed by completely arcuate slits 443. This slit configuration, shown as a circular segment, is particularly well suited for bending thicker and less ductile metal sheets, for example, titanium and ¼ inch steel plate and up.

When arcuate or circular slits 443 are formed in sheet 441 on opposite sides of bend line 435, lip portions 453 of the sheet, which extend over bend line 445 to slits 443, begin tucking or sliding onto face 455 of the tongues 470 at a center of each arcuate slit at the start of bending. Lip portions 453 then slide from the center of each slit partially up onto tongue faces 455 progressively toward the slit ends as straps 447 are twisted and bent. The progressive tucking of the lips onto the opposing faces is less stressful on the slit ends 449, and therefore more suitable for bending of less ductile and thicker materials, than say the embodiment of FIGS. 6-8B, in which the slits have straight central portions and simultaneously slide up onto the faces over the entire straight portion.

Slit ends 449 in FIG. 10 do not have the stress-relieving openings 249, nor radiused ends 249a of FIGS. 6-8 nor the curved ends of FIG. 11, but slits 443 are more economical to cut or form into most sheet stock Moreover, the deformation of straps 447 is more gradual during bending so that stress concentration will be reduced. This, of course, combines with increasing strap width to transfer loading forces and bending forces more evenly into the remainder of the sheet with lower stress concentration.

The various embodiments of the present sheet slitting and grooving invention allow designing manufacturing and fabrication advantages to be achieved which have not heretofore been realized. Thus, the full benefits of such design and fabrication techniques as CAD design, Rapid Prototyping and “pick and place” assembly can be realized by using sheet stock formation techniques in accordance with the present invention. Moreover, standard fabrication techniques, such as welding, are greatly enhanced using the strap-defining configurations of the present invention.

The many advantages of using sheets formed in accordance with the present invention can be illustrated in connection with a manufacturing technique as basic as welding. Sheet bending using the present method, for example, avoids the manufacturing problems associated with handling multiple parts, such as jigging.

Additionally, the bent sheets of the present invention in which slitting is employed can be welded along the slits. As can be seen in FIG. 10A, for example, face 455 and end surface 457 of tab 453 form a V-shaped cross section that is ideal for welding. No grinding or machining is required to place a weld 460 (broken lines) along slits 443 as shown in FIG. 10A. Moreover, the edge-to-face engagement of the sides of the sheet on opposite sides of the slits, in effect, provides a jig or fixture for holding the sheet portions together during the weld and for reducing thermally induced warping. Set up time is thereby greatly reduced, and the dimensional accuracy achieved by the present slitting Process is maintained during the welding step. The arcuate slits also provide an easily sensed topographic feature for robotic welding. These advantages also accrue in connection with soldering, brazing and adhesive filling, although thermal distortion is usually not a serious issue for many adhesives.

Filling of the slits by welding, brazing, soldering, potting compound or adhesives allows the bent sheets of the present invention to be formed into enclosures which hold fluids or flowable materials. Thus, bent sheet enclosures can even be used to form fluid-tight molds, with the sheeting either being removed or left in place after molding.

One of the significant advantages of using oblique, and particularly curved, grooves or slits is that the resulting bending straps are diverging at the point at which they connect to the reminder of the sheet material. Thus, area 450 of strap 447 in FIG. 10 is transversely diverging between slit end 449 and the next slit 443. This divergence tends to deliver or transfer the stresses in strap 447 at each end into the remainder of the sheet in a diffused or unconcentrated manner. As the arc or radius of the slits is reduced the divergence increases, again allowing a further independent tailoring of the strap stress transfer across the bend. Such tailoring can be combined with one or more of changes to strap width, jog distance and slit kerf to further influence the strength of the bend. This principle is employed in. the design of the slits on grooves of FIG. 11.

While the oblique bending straps of the embodiments of FIGS. 6-8 and FIGS. 9-10 result in substantial improvements of the overall strength and fatigue resistance of the bent structure, it has been found empirically that still further improvements, particularly in connection with fatigue, can be achieved if the strap-defining Structure takes the form of an arcuate slit. As used herein, “arcuate” shall mean and include a circular arc and a series of longitudinally connected, tangential arcs having differing radii. Preferably, the arcuate slits or grooves have relatively large radii (as compared to the sheet thickness), as illustrated in FIG. 11. Thus, a sheet of material 541 can be provided with a plurality of connected, large radii, arcuate slits, generally designated 542, along bend line 543. Arcuate slits 542 preferably are longitudinally staggered or offset (by an offset distance measured between the centers of adjacent slits along bend line 543 and alternatively are on opposite sides of the bend line 543, in a manner described above in connection with other embodiments of the present invention. Arcuate slits 542 define connected zones, which are bending straps 544, and disconnected zones, which are provided by slits 542. Only the right hand slit 542 in FIG. 11 shows a kerf or slit thickness, with the remainder of the slits 542 being either schematically shown or taking the form of a slit form by a knife resulting in no kerf.

Longitudinally adjacent slits 542 defined therebetween bending straps 544, which are shown in this embodiment as being oblique to bending line 543 and skewed in alternating directions, as also described above. Each slit 542 tends to have a central arcuate portion 546 which diverges away from bending line 543 from a center point 547 of the arcuate slit. End portions 548 also may advantageously be arcuate with a much smaller radius of curvature that causes the smiles to extend back along arc portion 549 and finally terminated in an inwardly arc portion 551.

It will be seen, therefore, that bending strap 544 is defined by the arc portions 546 on either side of bending line 543 and at the end of the straps by the arcuate end portions 548. A minimum strap width occurs between the arcuate slit portions 546 at arrows 552 (shown in FIG. 11 at the left hand pair of longitudinally adjacent slits). If a center line 553 is drawn through arrows 552 at the minimum width of the strap, it would be seen that the center line crosses bend line 543 at about the minimum strap width 552. Strap 544 diverges away from longitudinal strap axis 553 in both directions from minimum strap width 552. Thus, a portion 554 of the sheet on one side of bend line 543 is connected to a second portion 556 of the sheet on the opposite side of bend line 543 by strap 544. The increasing width of strap 544 in both directions from the minimum width plane 552 causes the strap to be connected to the respective sheet portions 554 and 556 across the bend line in a manner which greatly reduces stress and increases fatigue resistance.

For purposes of further illustration, strap 544a has been cross hatched to demonstrate the increasing width of the strap along its central longitudinal strap axis 553. Coupling of sheet portion 554 by an ever-increasing strap width to sheet portion 556 by a similarly increasing strap width tends to reduce stress. Orienting the central longitudinal axes 553 of straps 554 at an oblique angle to bend line 543 results in the straps being both twisted and bent, rather than solely twisted, which also reduces stresses in the straps. Stresses in the sheet flow across the bend through the connected material of the strap. Cyclical stress in tension, the primary cause of fatigue failure, flow through the twisted and bent strap and generally parallel to large radii arcs 546 and 549. The smaller radii of arcs 551 and 548 provide a smooth transition away from the primary stress bearing free surfaces of 546 and 549 but do not themselves experience significant stress flow. In this way, the arcuate slits are like portions of very large circles joined together by much smaller circles or arcs in a way that positions only the large radii arcs (compared to the material thickness) in the stress field flow, and uses smaller radii arcs as connectors to minimize the depth into the parent plane away from the fold line that the slit is farmed. Thus; slit ends, at which stress caused micro cracking is most likely to occur, will tend not to be propagated from one slit to another down the length of the bend, as can possibly occur in a failure condition in the embodiments of FIGS. 6-8 and 9-10.

The bending strap shape also will influence the distribution of stresses across the bend. When the bending strap diverges relatively rapidly away from the narrowest strap width dimension, e.g., width dimension 552 in FIG. 11, there is a tendency for this minimum dimension to act as a waist or weakened plane at the center of the strap. Such rapid narrowing will allow localized plastic deformation and stress concentration in the strap, rather than the desired distribution of the stresses over the full length of the strap and into the sheet material 554 and 556 on either side of the strap.

As shown in FIG. 11, and as is preferred, strap 544 preferably a minimum width dimension 552 providing the desired strap strength and then gradually diverge in both directions along the strap with any rapid divergence taking place as the strap terminates into the sheet portions 554 and 556. This construction avoids the problem of having an unduly narrow strap waist at 552 which will concentrate bending and twisting forces and produce failure, rather than distributing them evenly along the length of the strap and into sheet portions 554 and 556.

The tongue side of a slit, that is, the portion of the parent plane defined by the concave side of the arcuate slit, tends to be isolated from tensile stress. This makes the tongue ideal for locating features that cut into the parent plane. Attachment or alignment holes, or notches that mate with other connecting geometry are examples. FIG. 11A illustrates positioning of water-jet cut or laser cut, rapid piercing holes 560 and 565 on the tongue 555 of slit 546. Rapid pierce holes are somewhat irregular and elsewhere might initiate a crack failure in fatigue. In FIG. 11A two alternative locations of rapid piercing holes are shown. Rapid pierce holes are important to reduce the total cost of laser or water-jet cutting because slow piercing is very time consuming.

One of the most beneficial aspects of the present invention is that the design and cutting of the material to form the straps and the edge-to-face engagement of the lips and tongues of the slits is accomplished in a manner in which the microstructure of the material around the bend or fold is essentially unchanged in comparison to the substantial change in the microstructure of materials bent or folded to the same angle or degree of sharpness using conventional bending techniques, as described in the prior art. It is the relationship of the straps and the edge-to-face engagement of the slits which provides a combination of twisting and bending deformation when the material is bent that greatly reduces the stress around the bend and leaves the microstructure of the material around the bend essentially unchanged. When conventional bending techniques of the prior art are used there is a substantial change in the microstructure of the material around the bend if the bend is made to be sharp (for example, 90 degrees on the inside of the bend, as shown for example in FIGS. 5A, 8, 8A, 8B and 10A.

As was generally described in connection with other embodiments of the present invention, slits 542 can have their geometries altered to accommodate a wide range of sheet characteristics. Thus, as the type of sheet material which is bent is altered, or its thicknesses changed or strength characteristics of the bend are to be tailored, the geometry of smile slits 542 can also change. The length, L, of each slit can change, as can its offset distance, O.D., or longitudinal spacing along bend line 543. The height, H, of the slits can also be changed, and the jog distance, J, across the bend line between slits on opposite sides of the bend line can be altered. These various factors will have an effect on the geometry and orientation of straps 544, which in turn will also effect the strength of the bend and its suitability for use in various structures. Of equal importance is the shape of the arcuate slit in conjunction with the aforementioned sealing and positioning variable.

It is a feature of the present invention, therefore, that the strap-defining slits or grooves can be tailored to the material being bent or folded and the structure to be produced. It is possible, for example, to empirically test sheets of a given material but having differing thicknesses with arc slit designs in which the geometries have been changed slightly, but the designs comprise a family of related arc geometries. This process can be repeated for differing materials, and the empirical data stored in a database from which designs can be retrieved based upon input as to the sheet of material being bent and its thickness. This process is particularly well suited for computer implementation in which the physical properties of the sheet of material are entered and the program makes a selection from the computer database of empirical data as to the most appropriate arc geometry for use in bending the material. The software can also interpolate between available data when the sheet is of a material for which no exact data is stored or when the sheet has a thickness for which there are no exact stored data.

The design or configuration of the arcs, and thus the connecting straps, also can be varied along the length of a bend line to accommodate changes in the thickness of the sheet of material along the bend line. Alternatively, strap configurations along a bend line can change or be tailored to accommodate non-linear loading. While not as important as the strength and fatigue-resistance improvements of the present invention, the slit or strap configurations also can be varied to provide different decorative effects in combination with improved strength and fatigue resistance.

Another advantage which accrues from the various embodiments of the sheet slitting system of the present invention is that the resulting bends or fold are relatively sharp, both internally and externally. Sharp bends enable strong coupling of one bent structure to another structure. Thus, a press brake bend tends to be rounded or have a noticeable radius at the bend. When a press brake bent structure is coupled to a plate, for example, and a force is applied tending to rotate the bent structure about the arcuate bend, the bent structure can decouple from the plate. Such decoupling can occur more easily than if the bend were sharp, as it will be for the bends resulting from using the present slitting scheme.

The ability to produce sharp or crisp bends or folds allows the process of the present invention to be applied to structures which had heretofore only been formed from paper or thin foils, namely, to the vast technology of origami or folded paper constructions. Complex three-dimensional folded paper structures, and a science or mathematics for their creation, have been developed after centuries of effort. Such origami structures, while visually elegant, usually are not capable of being formed from metal sheets of a thickness greater than a foil. Thus, origami folded sheets usually cannot support significant loading. Typical examples of origami are the folded paper constructions set forth in “ADVANCED ORIGAMI” by Dedier Boursin, published by Firefly Books, Buffalo, N.Y. in 2002, and “EXTREME ORIGAMI” by Kunihiko Kasahara, published by Sterling Publishing Company, NY, N.Y. in 2002. The present invention thus enables a new class of origami-analog designs in which the slitting and bending methods described herein are substituted for origami creases. mom The sheet slitting or grooving process of the present invention produces sharp bends and even allows the folding of metal sheets by 180 degrees or back on itself. Thus, many structurally interesting origami constructions can be made using sheet metal having a thickness well beyond that of a foil, and the resulting origami-based structure will be capable of supporting significant loads.

Another interesting design and fabrication potential is realized by using the present slitting configurations in connection with Rapid Prototyping and Rapid Manufacturing, particularly if automated “Pick and Place” component additions are employed: Rapid Prototyping and Rapid Manufacturing are broadly known and are comprised of the use of CAD (computer-assisted design) and CAM (computer-assisted manufacturing) design, respectively, to enable three-dimensional fabrication. The designer begins with a desired virtual three-dimensional structure. Using the current invention to enable Rapid Prototyping, the CAD software unfolds the three-dimensional structure to a two-dimensional sheet and then locates the slit positions for bending of the sheet to produce the desired structure. The same can be done in Rapid Manufacturing using CAM. Other types of software for performing similar tasks. The ability to precisely bend, and to tailor the bend strength, by selecting jog distances and bending strap widths, allows the designer to layout slits in the unfolded two-dimensional sheet drawing in the design process, which thereafter can be implemented in the manufacturing process by sheet grooving or slitting and bending to produce complex three-dimensional structures, with or without add-on components.

Broadly, it is also known to assemble components onto circuit boards for electronic devices using high speed “pick and place” automated component handling techniques. Thus, assembly robots can pick components from component supply devices and then place them on a circuit board or substrate or chassis. The robotics secure the components to the substrate using fasteners, soldering plug-ins or the like. Such “pick and place” assembly has been largely limited to placing the components on a flat surface. Thus, the circuit boards must be placed in a three-dimensional housing after the “pick and place” assembly has been completed.)

An electronic housing, usually cannot be folded or bent into a three-dimensional shape after components are secured to the walls of the housing. Moreover, prior techniques for bending have lacked the precision possible with the present invention and necessary to solve component or structural alignment problems. Pre-folding or bending up the housing has, therefore, limited the ability for pick and place robotics to be used to secure electronic components in the housings.

It also should be noted that the straps present between slits can be advantageously used as conductive paths across bends in electronic applications, and the precision possible allows conductive paths or components on the circuit board to be folded into alignment when the three-dimensional chassis is formed, or when circuit boards themselves are folded into a more dense conformation.

The design and manufacturing processes of the present invention, however, enable precision bends to be laid out, slit and then formed with relatively low forces being involved, as is illustrated in FIGS. 28A-28E. Thus, a housing can be designed and cut from a flat sheet 821 and high-speed pick and place robotics used to rapidly secure components, C, to any or all six walls of a cube enclosure, and the housing or component chassis can be easily bent into a three-dimensional shape after the pick and place process is completed. toms) As shown in FIG. 28A, sheet 821 has component C secured thereto before bending, preferably by high-speed robotic techniques. Sheet 821 is formed by laser cutting, water jet cut, die cutting or the like with the designed cutout features 822, component-receiving openings 823, tabs 824 and support flanges 826 and tab-receiving slots 827. In FIG. 28E sheet 821 has been bent along bend line 831, causing a tab 824 to be displaced outwardly. The sheet is next bent along bend line 832 in FIG. 28C and then bent over component C along bend line 833 in FIG. 28D, while side flange 826 has been bent along bend line 834. Finally, chassis end portion 836 is bent upwardly along bend line 837 and tabs 824 are inserted into slots 827 so as to enable rigid securement of the sheet into a three-dimensional electronics chassis 838 around component C.

Obviously, in most cases a plurality of components C would be secured to sheet 821 before bending, and components C also can be secured to chassis 838 at various steps in the bending process and to various surfaces of the chassis.

FIGS. 28A-28E also illustrate a fundamental design process which is implemented by the sheet bending method of the present invention. One of the most space-efficient ways of supporting components is to mount them on sheet stock. Using conventional sheet stock bending techniques, however, does not enable tight bends and intricate inter-leaved sheet portions. The bending process of the present invention does, however, by reason of the ability to lay out slits extremely accurately that will produce bend in precise locations so that openings, cutouts, slots, tabs and the like will precisely align in the bent structure, as well as mounted components and the coupling to other structures.

Moreover, the precise layout of bending lines and chassis dr enclosure features is only part of the advantage. The structure itself can be bent using relatively low force, and even by means of hand tools. The combination of precision location of bend lines and low-force bending enables a design technique which was only heretofore partially realized. The technique involves selecting components having the desired functions and positioning them in space in a desired arrangement Thereafter, a chassis is designed with supporting thin sheet portions of the chassis necessary to support the components as positioned being designed, for example, using CAD techniques. The bend lines are located to produce the supporting sheet portions, and the chassis unfolded graphically to a flat sheet with the necessary feature and fold lines, as shown in FIG. 28A.

While such techniques have been described before in CAD design literature, and CAD and CAM software programs, they have not heretofore been effectively implemented in anything but the most simple designs because precision, low-force bending of sheet metals was not practical. The present slitting-based invention enables practical fabrication of this theoretical CAD or CAM design technique. Prior art CAD or CAM designs could not previously be physically realized in real materials to the same accuracy as the theoretical CAD or CAM model because, for example, conventional bending tolerances could not be held. The precision of bending possible with the present invention dramatically increases the correspondence between the CAD or CAM model and the achievable physical form for bent sheet materials.

Moreover, the bending need not take place at the pick and place or rapid prototyping site. The sheet with attached components can be transported with the components being formed and selected to act as dunnage for the transport process. Once at the fabrication site, which may be remote from the design and cutting site, the chassis or housing sheet will be bent precisely, even by hand if desired, and the bent housing secured into a three-dimensional structure, with a plurality of selected components being secured thereto internally and/or externally.

Moreover, three-dimensional chassis and other structures also can have panels therein which are attached by straps along a bend line to provide doors in the chassis or structure for periodic or emergency access to the interior of the structure. Separate door hinge assemblies are thereby eliminated.

Using the various embodiments of the sheet slitting or grooving techniques described herein, an extremely wide range of products can be formed. Without limitation by enumeration, the following are examples of products which can be folded from sheet material using the slitting and grooving schemes of the present invention: trusses, beams, curved beams, coiled beams, beams within beams, enclosures, polyhedrons, stud walls, beam networks, enveloped beams, flanged beams, indeterminate multiple-piece flanged beams, machines, works of art and sculpture, origami three-dimensional structures, musical instruments, toys, signs, modular connections, packages, pallets, protective enclosures, platforms, bridges, electrical enclosures, RE shield enclosures, EMI shields, microwave guides and ducts. A few examples of such structures are shown in FIGS. 12-30 and 32.

Formation of a curved box beam using the slitting process and slit sheet of the present invention can be described by reference to FIGS. 12, 13 and 14. A sheet of material 561 is shown in FIG. 12 that has two bend lines 562 and 563. Bend line 562 has a plurality of arcuate slits 563 on opposite sides of bend line 562. Also positioned along bend line 562 are smaller arcuate slits 564. The slits 563 and 564 have the general configuration as described and shown in connection with slits 542 in FIG. 11, but the length of slits 564 is reduced relative to the length of slits 563, and slits 564 will be seen to be positioned at the apex 566 of notches 567 which are provided in the edges 568 of the sheet of material. The bending straps 569 defined by longitudinally adjacent end portions of slits 563 and longitudinally adjacent end portions of slits 563 and 564 are essentially the same in configuration, notwithstanding differences in the length of the slits 563 and 564. There will be some slight shape difference due to arcuate segment differences, but bending straps 569 will be essentially uniform in their strength and fatigue-resistant capabilities along the length of bending line 562.

One of the advantages of the placement of slits 564 is that they tend to contain any stress crack propagation, which could occur at apexes 566 of notches 567. The various leaves or fingers 571 defined by notches 567 can be bent, for example, into or out of the page to a 90 degree angle, or to other angles if the structure should require. The central portion 572 can remain in the plane of the sheet on which FIG. 12 is drawn.

A plurality of slits 576 and 577 are positioned along second bending line 563. These slits have much tighter end curve portions 578 than the arc-like slits shown proximate first bend line 562. Generally, the tight curved end portions 578 are not as desirable as the more open-ended portions used in connection with slits 563 and 564. Nevertheless, for ductile materials that do not tend to stress fracture, slits of the type shown for slits 576 and 577 are entirely adequate. Again, the difference between slits 576 and 577 is that the smaller slits have been used at the apexes 566 of notches 567.

Once slit, sheet 561 can be bent along bend line 563 so that the leaves 571 can be bent to an angle such as 90 degrees relative to the central portion 572. It should be noted that normally the slits along bend line 562 and 563 will have the same shape, that is, they will either be slits 563 and 564 or slits 576 and 577. It is possible to mix slit configurations, but normally there will be no advantage from mixing them as shown in FIG. 12. The purpose of the illustrated embodiment of FIG. 12 is to show different slit configurations that are suitable for use in the bending of sheet material in accordance with the present invention.

The design and formation of a curved box beam using two sheets slit, as shown in the flat in FIG. 12, can be described in connection with FIGS. 13 and 14. The design would be accomplished on a CAD or CAM system, as described earlier, and the slits made in sheet 561 identically as laid out in the design process on the CAD, CAM or other systems. A curved box beam, generally designated 581, is shown in which one designed, cut and bent U-shaped sheet 572a is secured to a second designed, cut and bent U-shaped sheet 572b. As will be seen from FIGS. 13 and 14, the fingers or leaves 571a have been folded down over the outside of the fingers or leaves 571b. In both cases, the apexes 566 are closely proximate the fold lines 562a, 563a, 562b and 563b. This placement of the apexes allows bending of the sheet, by permitting notches 567a to have the included angle of the notches increase, while the included angle of notches 567b decrease in the area 582 of the longitudinal bending of beam 581. The central portions 572a and 572b of the sheet material have a thickness that will accommodate bending without buckling, at least in radii that are not extreme.

The folded sheets can be secured together by rivets 583 or other suitable fasteners, adhesives or fastening techniques such as welding and brazing. Openings for the fasteners can be pre-formed as shown in FIG. 12 at 580. The location of the openings 580 can be precisely set if the exact curved configuration is determined or known in advance of bending, or openings 580 can be positioned in central locations and thereafter used with later drilled holes to join the two bent sheets together in a curvature that is indeterminate or established in the field.

One application for indeterminate curved box beams, for example, is in the aircraft industry. Difficult to bend 4041 T-6 or 6061 T-6 aluminum is designed with the desired layout of slits and then provided in completed slit sheets as shown in FIG. 12. The sheets are then formed in the field to provide a box beam having a curvature which is determined in the field, for example, by the curvature of a portion of an airplane which must be repaired. The two sheets that form the box beam are curved to fit under a portion of the skin of the airplane which has been damaged, and then the skin is thereafter attached to the central section 572 of the curved box beam.

Bending of the leaves or fingers 571 can be done with simple hand tools, or even by hand, and field riveting used to hold the curvature of the box beam by using the pre-formed holes 58 as guides for holes that are drilled in the leaves or fingers of the underlying folded sheet. Thus, with a simple hand drill and pliers, a high-strength structural 4041 T-6 aluminum box beam can be custom formed and positioned as an airplane structural component for subsequent fastening of the skin of the airplane thereto. This can enable, for example, field repairs under even combat conditions so that the plane can be flown to a site at which permanent repairs can be made.

When the longitudinally curved box beam has a predetermined or known longitudinal curvature, leaves or fingers 571a and 571b can be defined by notches in which the fingers interdigitate or mesh with each other in the same plane. This will produce beam side walls that are smooth and without openings.

As shown in FIGS. 12-14 a longitudinally curved box beam 681 is produced by bending the sheet material along straight fold lines 562 and 563. It is also possible to produce a longitudinally curved box beam by slitting or grooving along curved bend lines. “Longitudinally” refers to a direction transverse to the bend line and/or original plane of the sheet as shown, for example, in. FIGS. 13-14. “Longitudinal bend” refers to bending in a transverse direction. to a bend line and is generally used interchangeably with “longitudinal curvature.”

In addition to the curved beam embodiments described above, other examples of curved structural members are immediately apparent as a result of simply laying out bending strap-defining structures along bend lines having non-linear portions. On folding or bending along such bend lines, or curves, the sheet becomes a curved three-dimensional structure.

Turning now to FIGS. 15 and 16, a sheet of material designed and slit or grooved for folding and a three-dimensional structure made from the same, respectively, are shown. Sheet 611 has been designed to be slit or grooved along longitudinally extending fold lines 612 and 613. Further slitting and grooving has taken place on transversely extending fold lines 614, 615, 616 and 617. Opposed side edges 618 of sheets 611 are circular, and a plurality of notches 619 are formed in opposite side edges of the sheet. A coupling tab or flange 621 is formed at one end of the sheet and preferably has fastener receiving openings 622 therein which will align with opening 623 in the opposite end of sheet 611. Slits or grooves 624 of the type shown in the embodiment of FIGS. 9 and 10 have been positioned along fold lines 612-617. It will be understood that slits or grooves of the type shown in other embodiments could be employed within the scope of the present invention.

The sheet of material shown in FIG. 15 is designed to envelop or enclose a cylindrical member, such as a rod, post or column 631 shown in FIG. 16. By bending sheets 616 along fold lines 612-617, sheet 611 can be folded around to enclose cylindrical member 631 as shown in FIG. 16. The circular arcuate portion 618 of the sheet are dimensioned to have a radius which mates with that of column 631. Notches 619 close up and the edges defining the notches abut each other, while the fold lines 614-617 allow the sheet to be folded into a square configuration around the column 631. The bent three-dimensional structure which results has a plurality of planar panels 636-639 which provide surfaces against which other members or structures can be easily attached. Folded sheet 611 may be secured in Place around column 631 by fasteners through openings 622 and 623. The configuration of the grooves or slits 624 causes the folded sheet 611 to become a high-strength, rigid structure around column or post 631. Securement of folded sheet 611 to post 631 against vertical displacement can be the result of an interference fit between arcuate edges 618 and the post, and/or the use of fasteners, adhesives, welding, brazing or the like, and the assembly has many applications which solve the problem of subsequent coupling of structural members to a cylindrical structure. The example of FIGS. 15 and 16 is not only a potential cosmetic cladding, it is a structural transition piece between cylindrical and rectilinear forms.

The designed and manufactured slit or grooved sheet and method of the present invention also may be used to design and form corrugated panel or deck assemblies. FIGS. 17 and 18 illustrate two corrugated panel assemblies that can be designed and constructed using the apparatus and methods of the present invention. Such assemblies are particularly effective in providing high-strength-to-weight ratios, and the sheet folding techniques of the present invention readily accommodate both folding of the corrugated sheet and the provision of attachment tabs.

In FIG. 17 attachment tabs are provided which can extend through slits to couple the corrugated sheet to the planar sheet, while in FIG. 18 tabs having fastener receiving openings are provided, toms) In FIG. 17, a sheet of material 641 has been slit or grooved along longitudinally extending fold lines 642-647 in accordance with the teaching of the present invention. Additionally, a plurality of tabs 649 have been formed along fold line 643, 645 and 647. Tabs 649 are cut in sheet 641 at the same time as formation of the slits or grooves 651 along the fold lines. Thus, a U-shaped cut 652 is formed in sheet 641 so that when the sheet is folded to the corrugated condition shown in FIG. 17, the tabs will protrude upwardly. Tabs 649 will extend at an angle from the vertical when folding occurs to form the corrugations, but tabs 649 can be bent from an angled position to a near vertical position, as shown in 617, by a subsequent step.

The folded or corrugated sheet 641 shown in FIG. 17 can be attached to a second planar sheet 656 which has a plurality of slits 657 formed therein. Slits 657 are positioned and dimensioned to matingly receive tabs 649 therethrough. When sheet 656 is lowered down over corrugated folded sheet 641, tabs 649 will extend up through slits 657. Tabs 649 can be in interference fit with slits 657 to secure the sheets together, or tabs 649 can be bent to a horizontal position or twisted about a vertical axis to secure the two sheets together. Tab 649 also may be beat down and secured to sheet 656 by adhesives, welding, brazing or the like.

Optionally, a second sheet of material, not shown, can be attached to the lower side of folded or corrugated sheet 641 using tabs (also not shown) which are formed out of sheet 641 during the slitting or grooving process. The second sheet would be secured to the bottom of folded corrugated sheet 641 in a manner described in connection with sheet 656.

The result is a high-strength, fatigue-resistant and lightweight corrugated panel or deck assembly which can be used in numerous applications.

A corrugated panel assembly similar to FIG. 17 can be constructed as shown in connection with the assembly of FIG. 18. Folded corrugated sheet 661 includes a plurality of fold lines 662 and a plurality of tabs 663. Tabs 663 are formed from sheet 661 in a manner similar to that described in connection with tab 649, only tabs 663 include fastener receiving openings 664. Additionally, tabs 663 are folded down to a near horizontal position, rather than up to a near vertical position, as described in connection with tabs 649. In the horizontal position, tab 663 can be used to couple a second sheet of material 666 having fastener receiving openings 667 therein. Sheet 666 is positioned so that opening 667 align with opening 664, and fasteners are used to secure the two sheets together. As described in connection with FIG. 17, a third sheet can be secured to the bottom of the corrugated sheet 666, although the figure does not show the securement tabs 664 on the bottom side of the corrugated sheet 61.

Again, by employing a plurality of grooves or slits 668 formed in accordance with the present invention, as above described, a corrugated deck or panel assembly can be fabricated which is very high in strength, has good fatigue resistance and is lightweight

FIGS. 19-22 illustrate a further embodiment of a continuous corrugated panel or deck which can be formed using the slit sheet and method of the present invention. Moreover, the panel of FIGS. 19-22 illustrates the strength advantages which can be obtained by reason of the ability to make sharp bends or folds that have significant load carrying capabilities. Still further, the embodiment of FIGS. 19-22 illustrates the use of tabs to interlock a folded sheet into a high strength three-dimensional structure.

Prior art techniques forming corrugated panels or decks often have suffered from an inability to achieve a desired high level or percentage of chord material to the overall panel material. Generally, the purpose of the webbing is to separate the chords with the minimal web mass required to accomplish that task. I-beams are rolled or welded forms that use thicker top and bottom chords relative to the connecting web between them. The present invention enables a class of corrugated structures that provide for wide design flexibility in creating rigid, strong, low weight structures that can be manufactured from continuous coils, transported in a compact coil form, and easily formed on site. The interlocking nature of this enabled embodiment avoids welding at the corners where welding is especially subject to failure.

Sheet material 721 has been slit using the present invention and is shown in FIG. 19 in a flat state before bending or folding. As will be seen, a plurality of substantially parallel bend lines 722 have a pattern of alternating arcuate slits 723 positioned on opposite sides of the bend lines to define obliquely extending straps skewed in opposite directions. Slits 723 can take the form of the slits in FIG. 6 or 9, for example. Also formed in sheet 721 are a plurality of tabs 724 which extend outwardly of the tongue portions of slits 723, and a plurality of key-hole like openings 725. Openings 725 are positioned in aligned relation to tabs 724.

In FIG. 21A. tabs 724 will be seen to extend across bend line 722 from slits 723. Tabs 724 are, therefore extensions of the tongue side of slits 723. Key hole openings 725 is a cut-out or negative tab in the tongue side of slits 723 which have a configuration dimensioned to receive tabs 724. In order to prevent the neck of tabs 724 from being interfered with by the upwardly displaced face on the opposite side of the slits, a notch 730 is provided in the lip side of the slits 723. Thus, the entire area of 725 and 730 is cut and falls out or is removed from the sheet so that tabs 724 can be inserted into notches 725/730.

In FIG. 20 the flat sheet 721 of FIG. 19 has been folded into a continuous corrugated panel or deck 726. Panel 726 includes web portions 727 and chord portions 728. As will be seen in panel 726, chords 728 are in end-to-end abutting relation over the full length of the panel on both the upper side and the lower side of the panel to provide continuous deck or chord surfaces. This construction affords panel 726 greatly enhanced strength, for example, in bending, over panels in which all the transverse webs are not joined by chords on both the top and bottom side of the panel. The deck or panel can be further reinforced by adding a sheet of additional material (not shown) which would further improve the ratio of chord material mass to the mass of the entire deck or panel for superior strength/stiffness-to-weight ratio.

FIG. 21 illustrates in greater detail the bending or folding scheme employed for panel 726. Commencing, for example, with end flange 729, web 727a can be bent down and back at bend line 722a down to a lower side of the panel. Sheet material 721 is then bent forward at bend line 722b and chord 728a extends in a longitudinal direction of the panel parallel to flange 729. At bend line 722c web 727b is bent to extend up and back to bend line 722a, at which point chord 728b is bent forward and extends to bend line 722b. Web 727 is then bent back at bend line 722d to bend line 722c. The bending continues along the length of panel 726 so as to produce a folded corrugated panel in which there are a plurality of end-to-end chords on both the top and bottom of the panel which are separated by connecting webs. The mass of the chord material in the panel to the overall panel mass is relatively high for a high strength-to-weight ratio.

The ability to fold a sheet 721 in sharp or crisp folds using the slitting process of the present invention allows the apexes 731 between the webs 727 and chords 728 to be relatively sharp and to be positioned in close, abutting relation. As illustrated, the panel of FIGS. 19-21 has webs and chords of equal length creating equilateral triangles in which each apex is about 120 degrees. As will be understood, many other corrugation geometries are equally possible.

While there are numerous ways in which folded panel 726 can be secured in a three-dimensional configuration, a preferred method is to employ tabs 724 and mating keyhole openings 725 cut into sheet 721 during formation of the bending slits.

Tabs 724a, for example, are provided by laser or water jet cutting of the tabs to extend outwardly of slit tongues from flange 729 into web 727a. When web 727a is bent downwardly and rearwardly to bend line 722b, tabs 724a remain. in the horizontal plane of flange 729. As best seen in FIG. 21A, a mating opening 725 cut into chord 728b and aligned with tab 724a will allow tab 724a to be positioned in opening 725. If each tab 724 has an enlarged head or end 734, the tabs will lock or be captured by their mating openings 725, much as a jigsaw piece can capture or interlock with an adjacent piece. This interlocking resists separation of the tabs from the mating openings in the top and bottom planes of the panel. The tabs and openings do not need to be, and preferably are not, dimensioned to produce an interference fit.

Interlocking of tabs 724 and openings 725 also occurs along the bottom side of panel 726, and the result is securement of the folded panel in the form as shown. in FIG. 20, even without additional securement techniques, such as adhesives, welding, brazing or the like, which optionally also can be used.

In FIG. 22, the sheet slitting and bending process of FIGS. 19-21 is schematically shown as applied to the formation of a cylindrical member 741. Again, webs 742 and chords 743 are formed about bend lines and the locations of the bend lines selected so that the chords on the inner radius 744 are shorter in their length than the chords on the outer radius 746 of cylinder 741. Tabs and mating opening may be used to lock the chords and webs in. the desired configuration, depending on the thickness of the material and the radii of cylinder 741. The resulting cylindrical structure can be used, for example, as a lightweight, high-strength column or post.

In most embodiments of the present invention, and particularly those in which the sheet of material has a substantial thickness, commencement of bending will automatically cause the tongue or tab portion of the slit to begin to slide in the correct direction against the face on the opposite side of the slit. When the sheet material is relatively thin and the kerf of the slit is small or zero, however the tab portions of the slit sheet occasionally will move in the wrong direction and thereby effect the precision of the bend. In order to remedy this problem, it is possible for the tongue portion of the slit to be biased in a direction producing predictable proper bending. This solution is shown in FIGS. 23 and 24A.

A sheet of material 681 is formed for bending about a plane of bend line 682 using the design and sheet slitting technique of the present invention. Arcuate slits 683 are formed which define tongues 684 that will slide along opposing faces during bending of the sheet about bend line 682.

In FIG. 23a, sheet of material 681 can be seen as it is being bent in a down-ward direction, as indicated by arrows 687, about bend line 682. Because tongues 684 are downwardly displaced, the lower edges or corners 688 of lips 689 will tuck up and engage faces 690 of tongues in a manner which will produce sliding of edges 688 along faces 690. The edges 68B on each side of bend line 682 will be displaced upwardly to slide on the downwardly pm-set tongues 684 so that bending about bend line 682 predictably produces sliding of the edges along the faces of the tongues in the desired direction during the bending process.