US3096576A - parilla - Google Patents

Description

y 1963 A. R. PARILLA 3,096,576

FABRICATION OF BODIES HAVING COMPOUND CURVATURE Filed Sept. 24, 1958 5 Sheets-Sheet 1 INVENTOR.

ARTHUR R. PARILLA /7 MORGAN, FINNEGAN, DURHAM 8 PINE ii 20 ATTORNEYS July 9, 1963 A. R. PARILLA 3,095,575

FABRICATION OF BODIES HAVING COMPOUND CURVATURE Filed Sept. 24, 1958 3 Sheets-Sheet 2 FlG.-3

INVENTOR.

ARTHUR R. PARILLA BY IMORGAN, FINNEGAN, DURHAM a PINE ATTORNEYS FABRICATION OF BODIES HAVING COMPOUND CURVATURE Filed Sept. 24, 1958 A. R. PARlLLA July 9, 1963 3 Sheets-Sheet 5 L P Rm 0 a T P W W D ATTORNEYS United States Patent 3,096,576 FABRICATION 0F BODIES HAVING COMPOUND CURVATURE Arthur R. Parilla, 34 Crestview Road, Mountain Lakes, NJ. Filed Sept. 24, 1958, Ser. No. 763,114 17 Claims. (Cl. 29-421) This invention relates to new and improved methods for fabricating bodies having compound curvature. It is more specifically related to improved means for fabricating thin wall semi-elliptical and/or hemi-spher-ical head closures of high quality for use in ultra light weight, highly stressed pressure vessels, such as solid propellant rocket cases and/ or fuel tanks for missile use.

Such bodies have curvature in two or more planes and require extensive plastic flow of the material in order to form the desired shape, whereas bodies such as cylinders and cones having curvature in one plane only can be readily formed by rolling a flat sheet.

It is, therefore, the primary purpose of this invention to provide novel methods for forming bodies having compound curvature by first forming simple bodies having curvature in one plane only and then subsequently producing curvature in the second plane also.

Semi-elliptical and/or hemi-spherical heads may be readily formed by present fabrication methods such as deep-drawing or spinning when made in thick wall sections and using low alloy steels having good formability with high ductility, low yield strength and low workhardening characteristics. The resulting structure, how-. ever, is too heavy for missile application. These require very thin walled structures using more difficult to form ultra high strength steel alloys of higher carbon content, relatively low ductility, higher yield strength and higher work-hardening properties. In addition, the precision requirements are more severe; excessive local thinning of the metal during forming cannot be accepted because of the low margins of safety employed to achieve the weight reduction essential to missile performance.

As an example, the minimum wall thickness for a 54 inch diameter head which can now be economically fabricatecl commercially with the easier to form low alloy steels, is inch for elliptical heads and inch for hemi-spherical heads. In contrast, the requirements for missile application are for thicknesses of only 0.078 inch for the same diameter while using the more difficult to form super alloys such as tool steels, hot work die steels, or modifications of 4340 alloy steels, titanium alloys, magnesium alloys, and the like.

A major difficulty with deep-drawing such head closures is due to the high compressive stresses induced in the circumferential direction as the flat metal blank is drawn into the die. These compressive stresses require high radial tensile stresses, the combined effect being to thin the metal near the bottom of the die and thicken the metal near the top of the die. This leads to wide variation in wall thickness along any meridian.

The compressive stresses cause lateral buckling or wrinkling of the blank, this tendency to buckle increasing as the wall thickness decreases and diameter increases,

being a function of the second power of the ratio of diameter to wall thickness, thus aggravating the problem with thin wall structures. Hold-down fixtures or pressure plates are used on the blank to restrain buckling; this increases radial tensile stresses in the blank, sometimes causing tearing of the metal, while the application of pressure increases the capacity of the press required. Also, this requires a larger diameter blank, leaving a flange normal to the axis of the head which must be trimmed later; and aggravates the forming operation since the larger blank diameter again increases the percent of de- 3,096,576 Patented July 9, 1963 2 formation required to form the part, again increasing press capacity requirements.

While heads formed by spinning a flat sheet blank over a suitable mandrel of the desired curvature, is an alternate method successfully used in heavy wall, ductile material, it is unsatisfactory in thin gages. This method causes excessive thinning of the metal, requiring use of heavier stock in order to maintain the minimum wall, thus resulting in considerable variation in wall thickness along any meridian. The higher alloy material also work-hardened during the excessive deformation required to produce the final article.

Another fabrication method sometimes used is to form the segments of the head to the desired contour, then weld such segments into a unitized structure. This requires elaborate jigs and fixtures to position the segments, and high quality welding techniques to insure sound joints, leading to expensive fabrication including weld X-ray and other quality control techniques; it lacks reliability since even slight mis-match or eccentricities at welded joints can induce high stress raisers beyond the nominal membrane stresses for which the structure is usually designed.

In any of the above methods, the normal commercial tolerances on wall thickness of the original sheet as rolled causes further variation in thickness and hence weight of the completed part. This variation in weight from unit to unit in production is objectionable as it affects the reproducibility of performance of the complete missile.

When such head closures are used for solid rocket propellant cases, there is generally a requirement for central openings in such heads. The forward head generally requires a relatively small opening for attachment of an igniter flange; the aft head closure requires a substantial opening for a nozzle attachment flange. Present fabrication methods require forming the complete head and then removing portions for the required openings. Such methods do not utilize the advantage which may be taken of such openings to facilitate fabrication. An annular surface is all that is required in the final product.

It is, therefore, a primary object of this invention to provide novel fabrication methods for bodies having compound curvature which may be specifically adapted to produce thin walled bodies.

It is a further object of this invention to provide novel means for fabricating such bodies in which the material is subject to tensile stresses only, thus eliminating the difficult forming operations when high compressive stresses are induced, such as by deep-drawing.

Another object is to reduce the strain or percent of deformation required to form bodies having compound curvature by taking advantage of the need for central openings, and forming only the annular compound sur-' face.

Another object is to provide novel fabrication methods for bodies having compound curvature in which a uniform wall thickness may be maintained with close tolerances along any meridian.

Another object is to provide simple and flexible means whereby such bodies may be readily formed without the need for high capacity presses, such as by fluid forming or explosive forming; and in which design changes may be readily incorporated with a minimum of lead time.

Another object is to reduce the press capacity require ments for press forming such bodies in large quantities and at low cost, and of superior quality.

Another object is to provide means for fabricating such bodies which limit the amount of deflection occurring under tension to the ductility limits of the material.

The basic principles of this invention may be defined by the following further objects in which:

Another object is to fabricate bodies having compound a curvature, such as semi-elliptical or hemi-spherical heads, by first making a pre-form comprising a body having curvature in one plane only and thus may be easily rolled from fiat sheet, the pre-form approximating the final compound curvature only. The pre-form may then be inserted Within a die having a contour of the final desired shape. The pre-form is then stretched or expanded to the shape of the die contour to provide the desired curvature in the second plane. Since the metal in the pro-form is subjected only to .tensile stresses during forming, buckling or wrinkling due to high compressive stresses are eliminated.

Another object is to select the shape of the pre-form so that the amount of deformation or percent elongation during subsequent forming will be minimum, thus minimizing variation in wall thickness in the final article.

Another object is to vary the wall thickness of the preform so that the final part as formed will have a constant and uniform wall thickness along any meridian. Thus, the variable wall thickness in the pre-form anticipates the reduction in thickness due to Poissons ratio resulting from elongation of the material while stretching or expanding the pre-form.

A further object is to vary the wall thickness of the pre-form so that the final formed part may have a tapered or variable Wall thickness which varies along any meridian in any prescribed manner to suit design requirements; the variable wall thickness in the pre-form now anticipating both the eifect of Poissons ratio and the desired final thickness.

These and other objects will become apparent from the following detailed description read in connection with the annexed drawings, in which similar reference characters represent similar parts, and in which:

FIGURE 1 shows one embodiment of a method for forming a body having compound curvature which is approximately semi-elliptical in cross section.

FIGURE 2 shows a modification of FIGURE 1 whereby the body is approximately hemi-spherical in cross-section.

FIGURE 3 shows a method for providing constant wall thickness along any meridian in the final formed part.

FIGURE 4 shows a method whereby the final formed part may have a wall thickness along any meridian which may be tapered or varied in any manner to suit design requirements.

FIGURE 5 is a fragmentary view showing a detail of the forming operation occurring in either FIGURE 1 or 2.

FIGURE 6 shows an alternate method for assembly of components of FIGS. 1 and 2.

FIGURE 7 shows still another method of components of FIGS. 1 and 2.

FIGURE 8 shows a method for reducing the press capacity for forming thin-walled bodies having compound curvature with superior quality and low cost.

FIGURE 9 shows a method for fabricating light weight hemi-spherical heads with integral re-inforcement flanges.

Referring to FIGURE 1, two pro-forms 10 having a frusto-conieal shape are assembled within identical opposed dies 11, joined by the bolts 12 along the flanges 13. A rubber ring, 14, forms a seal between the bases 36 of the two cones and the dies 11. Two rubber discs 15 provide a seal between the upper cone ends 16 and the dies 11. A nut 17 provides initial compression of the rubber disc 15 by means of threads on the tube 18 and the cap 19. Fluid pressure admitted through the tube 18 by the valve 33 causes the pre-form to deflect until it reaches the wall of the disc 11 as shown by the dashed lines at 16'.

An alternate method for pressurizing the pre-form is also shown in FIGURE 1 wherein the nut 21 provides initial compression of the rubber disc 15 by means of threads on the tube 22 and the cap 23. Wires 24 and 2.5 are sealed and insulated within the tube 22 and connected to an explosive charge 26 mounted within the pro-form. When the wires 24 and are energized electrically from for assembly any external source, such as a battery (not shown), the gaseous products of combustion exert sufiicient pressure to deflect the walls of the pre-forms 10 until they reach the Walls of the dies 11 as shown by the dashed lines 10', and previously described. The valve 33 may then be used to release the pressure after forming. In either case, vents 34 are provided through the dies 11 to relieve the back pressure as the pre-form deflects against the dies 11.

It is understood that both methods of pressurization described above are shown in FIGURE 1 for illustrative purposes, but only one or the other method would be used as desired. The explosive charge method, while requiring greater development, is desirable since it causes a more rapid rate of loading, and also minimizes sealing problems.

It is also understood that any method of stretching the pre-form may be used, such as by insertion of a die within the pro-form, as described later, the pre-form replacing the flat sheet in the present press operation, the pre-form avoiding the buckling and wrinkling problems presently encountered by this method. The use of two opposed dies with fluid pressurization of the pre-form eliminates the need for large capacity presses and costly matched dies.

Since the opposed dies 11 for housing the pre-forms are heavily walled heads, they may be fabricated by present methods. The internal contour may be machined to various odd dimensions required for any application, while the dies are formed to standard dimensions available from existing tools. Design changes may thus be readily executed with a minimum of lead time. The machined contour of the internal surface of the dies 11 may also provide for spring-back allowance.

FIGURE 2 illustrates the same principles of FIGURE 1, the same numerals referring to the same functional parts, and shows suitable modifications for forming a hemi-spherical instead of semi-elliptical heads.

In forming hemi-spherical heads, the required total elongation in the pro-form material is substantially greater than in forming semi-elliptical heads and may Well exceed the allowable elongation permitted by the ductility of the material. This total elongation, however, may be split up into any number of progressive operations, each one of which controls the elongation well within the allowable limits of the material; intermediate annealing operations are then used between successive forming operations.

As illustrated in FIGURE 2, the elongation in any one operation is limited by spacers 31 fitted within the die 11. The spacers may be of any of the new plastic die materials, such as the epoxy resins, and cast within the die 11. As an example, the pre-form 19 may first be pressurized until its deflection engages the inner contour of the spacer 31 as shown by the dashed lines at 10. The spacer may then be removed, and after an intermediate anneal of the pre-form 10', re-pressurization will cause the pre-form to reach its final position as shown by the dashed lines 10".

The number of progressive operations may be determined by the geometry and property of materials to be used; also, it may be desirable to eliminate spacers and use integral intermediate dies having the desired internal contour.

FIGURE 3 illustrates in exaggerated form the modification to the pre-form of FIGURE 2 to provide for a constant and uniform wall thickness along any meridian. It is a well known property of materials that elongation in one plane will produce contraction in another plane normal to the first plane; the ratio of this contraction to elongation being known as Poissons ratio. If the pro-form is of constant thickness, it will have a minimum thickness where the elongation in the circumferential direction is maximum. Although the total elongation, and hence thinning, will be less than that occurring when forming the part from a flat sheet, this thinning may be anticipated and eliminated entirely by providing a variable wall thickness in the pre-form proportional to the elongation occurring ateach increment of length, as shown in FIG- URE 3.

An extension of this same principle is illustrated in FIGURE 4. The wall thickness of the pre-form may vary so as to intentionally introduce any desired variation in wall thickness of the final article after forming. While the illustration shows an increased wall thickness as the spherical head approaches the centerline, it is readily apparent that a reverse taper of the pre-form wall would produce a spherical head having a thicker flange portion.

The change in wall thickness resulting from the effect of Poissons ratio as the pre-forrn is expanded to the die contour may be readily calculated from the following relation:

where e is the strain or percent change in wall thickness; (I is the normal stress in the wall; E is the modulus of elasticity of the material; ,u. is Poissons ratio, :030 for steel; s is the strain in the material in a circumferential direction; and q is the strain in the material in a longitudinal direction or along the slant height of the pre-form.

Since o' is numerically equal to the pressure acting on the case, it is very small with respect to E, and the first term may be assumed equal to zero.

For the assemblies shown in FIGURES -l and 2, the ends 36 of the pre-form are unrestrained and therefore may move freely in a direction towards the cone apex during the forming operation. The seal 14 has suflicient length and flexibility to maintain engagement with the preform as this movement progresses. Neglecting small elastic strain, the value of 6 is also zero with no end restraint. The above relation for calculating change in wall thickness reduces to:

. 11' c a may be readily calculated by determining the loci of a number of points on the conical pre-form as deflection occurs, the upper or small diameter of the pre-form 16 reinaining in fixed position relative to the die 11. 6 is then found from Ru (3) where R is the final radius of any circumferential element andR is its initial radius on the cone.

From the above relations the contour or the pre-forrn for FIGURE 3 may be readily calculated. This increment may then be added to any desired taper or variation in Wall thickness desired by the designer, for example as shown in FIGURE 4.

' Referring to FIGURES l and 2, when the end 36 of the pre-form 10 is in close engagement with the wall of the die 11, the available local strain in this region may not exceed the yield point of the material. 'CNo plastic deformation will occur and the local deflection of the preform in this area is then illustrated in exaggerated form by FIGURE 5. The material over a small length, such as L of FIGURE 5 will then be stressed only elastically, with a gap as at 37 remaining after de-pressurization.

The length, L, is related to the thickness of the wall, h, of the pre-form by the following approximate expression:

where 0,, is the yield stress of the material, P is the maxi mum pressure acting on the pro-form.

For high strength steel alloys having a yield strength of 90,000 p.s.i., and for a maximum working pressure of 2,000 p.s.i., the length L will be approximately 7 .7h; and for h: 0.07 8, (typical) L is approximately 0.6 inch.

For the above case, the maximum gap at 37 will then be approximately 0.003 inch, resulting in an eccentricity of 3.8% of the wall thickness. In the event the margin 6 p of safety (allowable stress over actual stress) exceeds this amount, the end would not require trimming; if the margin of safety is less than this amount, the end should be trimmed at or above L, or higher pressure used.

The same type deflection curve will occur at the upper end of the pre-form at 16.

In the assembly shown in FIGURE 6, sufiicient clearance as at 34 may be allowed between the pre-form end 36 and the die 11 in the design of the pre-form so that the base 36 will exceed the elastic limit of the material before reaching the die wall. The seal .14 is modified to bear directly on the die between pre-forms. The upper end of the pre-form at 16 will continue to deflect in the manner illustrated by FIGURE 5.

Other modifications illustrated in FIGURE 6 include alternate methods for assembling the various components. The bolts '12 of FIGURES l and 2 may be replaced by two Ortrnan keys 32 which mate with suitable grooves in the dies 11 and in similar grooves in the continuous ring 38. Assembly and dis-assembly may be facilitated, it being necessary to remove only one of either of the two Ortman keys 32 in a manner well known in the art, to effect dis-assembly.

A plate 35, centered on the tube 22, or integrally machined as part of die 11, provides concentricity for the pre-form 10 relative to the die 11 and offers positive positioning of the upper or small end 16 of the pre-form 10, a similar arrangement (not shown) being used for the opposed pre-form and die at the opposite end.

FIGURE 7 illustrates another method of assembly in which the bases 36 of the opopsed pre-forms 10 are welded together, eliminating the seal ring M, the weld being performed after the seals 15, explosive charge 26 (if used) and associated components have been pre-assembled Within the pre-form. After forming, the parts are separated by cutting above and below the weld, such as by the distance L shown in FIGURE 5.

When this method is used, the variation in Wall thickness will be greater as determined by Equation 1 since the longitudinal strain, 5 is no longer Zero when the end 36 is restrained. This will require greater camber for the initial wall thickness of the pre-forrn of FIGURE 3 or 4. It will provide a longer straight flange on the formed part, or conversely a shorter pre-form may be used when longitudinal end restraint is applied as shown.

Another method for stretching the pre-form to its final compound curvature is shown in FIGURE 8. In this case, the part may be press formed by a novel method in which the press capacity required may be substantially reduced compared to deep drawing, cold forming, or other means, as described later. Simple. dies may be used, eliminating hold-down fixtures or pressure plates required for deep drawing.

Referring to FIGURE 8, a female die 41 is formed to have the desired contour as at 42, a portion of a semielliptical contour being shown. The die 41 has an integral land 43 corresponding to the diameter of the central opening, or the land may be formed by a separate plate, such as shown by 35 in FIGURE 6. The pre-form v10 is inserted in the die 41 with its small diameter 16 abutting on the land 43. The male die 44 has a contour 45 matched to the contour 42 of the female die with clearance for the metal thickness of the pre-form 10. A recess 46 in the male die 44 provides clearance for the land 43 of the female die, when in closed posit-ion. When the dies are closed, the conicalpre-form is stretched into the compound curvature of the die, as shown at 10'.

The press capacity is reduced for a number of reasons as follows:

(.1) Advantage is taken of the fact that the finished part frequently has a sizable opening or hole on its axis; present practice is to form a continuous head, then re move a portion from the completely formed part. By the method shown in FIGURE 8, only the annular surface remaining in the final article requires forming.

(2) The metal to be formed is already within the die and subject to tensile stresses only, requiring only moderate pressure on the pre-form. It is not necessary to overcome large compressive stresses in a blank as the material is drawn into the die as occurs in the deep draw operation; no hold down pressures are required to resist buckling or wrinkling, since the compressive stresses inducing this condition have been removed.

(3) Since the magnitude of the pressure required on the pre-forrn is less than that required on the continuous flat sheet in a deep draw, and since this reduced pressure also acts on the reduced area of only the annular surface being formed, it follows that much smaller press capacity is required compared to other processes.

FIGURE 9 shows modifications to the die 11 of FIG- UR-E 2 and the pre-form 10 of FIGURE 4, especially adapted to fabricate hemi-spherical head closures of minimum weight, other components being as shown previously.

It is well known by those familiar with the art that the membrane stress in hemi-spherical head closures is nominally one-half the tangential stress in the cylindrical portion of a pressure vessel. The wall thickness of the hemispherical head may then theoretically be one-half the wall thickness in the cylindrical portion. It is generally difiicult to achieve this in practice, since the straight flange portion of the head forms part of the cylindrical chamber and therefore requires the full thickness of the cylindrical wall. Also, it becomes more difiicult to perform the welding operation with dissimilar thickness increasing the risk of misalignment or mismatch, the result of such local eccentricities inducing high stress raisers which far exceed the nominal membrane stresses.

As shown in FIGURE 9, the frusto-conical pre-form may be fabricated to have an enlarged section 47 at its base, joined by a gradual transition section 48 to a thin section 49, which may be cambered to compensate for effect of Poissons ratio, as described previously, or have parallel walls, in either case providing a much reduced thickness of up to 50% compared with hemi-spherical heads of constant wall thickness.

A similar construction of the pre-form is shown at the small diameter opening at 50 to provide local re-inforcement for the connecting structure.

Both end re-inforcements at 47 and 50 are symmetrically disposed about the centerline to insure local eccentricities will not be present in the completed structure.

The normally spherical internal contour of the die 11 is modified by providing recesses at 51 and 52 to receive the re-inforced ends 47 and 50 of the pre-form, thereby again insuring concentricity of all sections of the completed structure.

Other features of the process, such as progressive forming with intermediate annealing, use of seals, etc., may be as previously described.

While the description and drawings show head closures having central openings, it can be seen that this process may be readily adapted to form closed surfaces. The major portion of the forming operation may be performed as described, with the central opening at the upper end of the frusto-conical pre-form reduced to a small diameter. Since connecting fittings are usually required, the small opening may be designed to receive a standard flange for such connections; or a spherically formed segment sometimes described as a dollar-plate, may be welded or otherwise attached to close the opening of the conical preform.

While this process is described to form head closures for cylindrical pressure vessels, such as rocket cases with reference to ultra high strength steel alloys, it is obvious it may also ofiier improvement for fabrication of other bodies of other materials, such as fuel tanks for missiles, accumulators of titanium, and the like. Nor need it be limited to semi-elliptical or hemi-spherical bodies; it may also be applied to various ogival shapes for missiles, contoured or hell shaped nozzles for super-sonic jets,

8 engine cowling, and a variety of parts having compound curvature.

Having now described the basic principles and theory of a novel fabrication method for forming thin walled bodies having compound curvature, together with various methods for performing the operations, the actual fabrication of a typical head closure will now be described with application of these principles.

Two semi-elliptical head closures will first be considered for use with solid propellant rocket cases.

(A) An aft head closure for a 54 inch diameter case with an 0.078 inch wall thickness and having a relatively large central opening of 75% of the case diameter (40.5 inches) for attachment of a nozzle assembly.

(B) A forward head closure also 54 inches in diameter by 0.078 inch wall thickness, having a relatively small central opening of 25% of the case diameter (13.5 inches).

In both cases a straight flange of two inches is assumed for the final formed part; and the forming operation will be without end restraint, as illustrated in FIGURE 1 or FIGURE 8.

For head (A), the large central opening results in a frusto-conical pre-form with a relatively small apex angle so that the maximum circumferential elongation during forming is approximately 9%. Using Equation 2, the wall thickness will reduce a maximum of 2.7%, or by 0.0021 inch; the minimum wall thickness becomes approximately 0.076 inch. Obviously, a flat sheet may be used for the pre-form without camber since there is little thinning of the metal.

For head (B), the smaller central opening results in a frusto-conical pre-form having a larger apex angle so that the maximum circumferential elongation becomes approximately 20%. Again using Equation 2, the wall thickness will reduce approximately 6%, or by 0.0047 inch; the minimum wall thickness becoming 0.073 inch. Again, a fiat sheet may be used for the pre-form without camber with less than 0.005 inch maximum variation in wall thickness due to forming.

It may thus be seen that semi-elliptical head closures of high quality may be fabricated in thin sections by this method, with only slight variation in wall thickness due to forming.

As case diameter and wall thickness increase, the same percent variation will cause a larger absolute tolerance requirement on the finished part; cambered walls may then be used to maintain closer tolerances if required.

The low value of 9% circumferential elongation for head (A) above may permit forming this head in a single operation without intermediate anneal, even with workhardening steels.

For comparison of hemi-spherical heads with semielliptical heads, it is assumed that head (B) above has a hemi-spherical shape; the maximum circumferential elongation, using a frusto-conical pre-form with no end restraint, then increases to approximately 30%; using Equation 2, the variation in wall thickness becomes 9%, or 50% greater than for the semi-elliptical head. For an 0.078 inch wall, the thickness reduces by 0.0072 inch; the minimum wall becomes approximately 0.071 inch. This occurs in the hemi-spherical section where the stress is nominally one-half of the tangential stress in the cylindrical case.

The maximum circumferential elongation in the hemispherical head fiange portion is only 4.5% resulting in a thickness reduction of only 1.35%; for an 0.078 inch Wall, the flange thickness will reduce only slightly more than 0.001 inch. A pre-form of flat sheet without camber may also be used for a hemi-spherical head in this diameter and thickness; however, the minimum weight for such a head would be achieved by the design and method described above in connection with FIGURE 9.

The design and fabrication of the pre form may now be described:

Since the frusto-conical pre-form has curvature in only one plane, it may readily be rolled from flat sheet. It either may be rolled and welded, in which case at least one longi-tudinal weld will be required along the slant height; or it may be roll-formed by *Hydrospinning or Flo-turning, in which case a weldless pre-lform can be made.

The large end, or base diameter of the preform, may be approximately the same as the shell diameter of the pressure vessel or rocket case; or, in the event it is desired to eliminate the small local eccentricity as discussed in connection with FIGURE 5, it may be made approximately 0.5% smaller than the shell diameter.

The upper diameter or small end of the frusto-com'cal preform is made equal to the central opening required in the finished piece, less any trimming allowance, if required.

For closed heads, this opening may be made as small as convenient for attachment of a dollar plate or standard fitting as required, and previously described.

The length of the pre-forrn is determined from the depth of head required, i.e. semi-elliptical or hemispherical; and is taken as the perimeter of the formed part from the central opening to the tangent point, plus the length of the straight flange, plus a trimming allowance, if required.

For rolled and welded pre-forms, the Weld overlay may be ground flush at least on its outer surface, with usual quality controls, such as X-ray inspection, to insure sound welds.

The use of hydrospun frusto-conicalpre-forms offers a more economical use of hydrospin machines since the cones can be rolled more quickly and less expensively from sheet metal, compared to rolling semi-ellipsoidal or hemi-spherical shapes; the cones may then be readily converted into the more difiicult shapes by stretching in the manner described herein, thereby increasing the productive capacity of the hydrospin machine.

With either the rolled and welded or hydrospun preforms, the material would be soft annealed for maximum ductility.

When extremely close tolerances are required on the final part, the pro-form may be machined to final dimensions thus eliminating variation in wall thickness due to commercial tolerances on light gages; when required, the desired camber may be provided, as by tracer attachment on the lathe or hydrospin machine.

Thus, novel fabrication methods have been described which make possible the fabrication of bodies having compound curvature, particularly adaptable to thin wall structures and materials difficult to form by known methods; and can produce an article of superior quality. While several embodiments and arrangements of my invention have been described, it is understood that changes and modifications may be made therein without departing from the spirit and scope of the invention. The limits of the invention are set forth in the following claims.

I claim:

1. The method for forming a desired thin-walled body of compound curvature from a flat body of hard-to-work thin sheet metal, which method comprises: deforming said flat sheet metal body into a curved preform body approximately the shape of the desired body of compound curvature and having a curvature in only one of at least two sets of transverse parallel planes; inserting said preform body into a die having a shaped curved surface corresponding to the shape of the desired body of compound curvature; sealing the joint between said preform body and said die; and then deforming said curved preform body into the desired thin-walled body of compound curvature, by expanding by fluid pressure said curved preform body in the set of planes of its original curvature only, to impart curvature in said curved preform body in the other of said sets, until said body conforms to said shaped curved surface.

2. The method as defined in claim 1 characterized by expanding the material of the preform in the set of planes of its original curvature only, and providing freedom of motion for displacement of the material of the preform in the other of said sets.

3. The method as defined in claim 1, in which the expanding force is applied through an incompressible fluid.

4. The method as defined in claim 1, in which the expanding force is applied through a fluid which comprises the gaseous products of combustion derived from an explosive charge.

5. The method for forming bodies having curvature in at least two planes which comprises forming a flat body into a preform body approximating a frustum of a cone in shape, inserting the frusto-conical preform into a die having a shaped surface generated by the rotation of a curved line about the central axis of said die, said preform being positioned with its apex encircling the central axis of said die and held fixed relative thereto, sealing the joint between said apex and said die, the base end of said frusto-conical body being positioned adjacent the shaped surface of said die and movable relative thereto, sealing the joint between said base and said shaped surface, closing the base of said die and increasing fluid pressure within said frusto-conical preform body to stretch the material thereof to engage the shaped surface of said die.

6. A die including a shaped surface generated by the rotation of a curved line about the central axis of said die, a preform body, holding means on said die and at the central axis thereof adapted to engage the apex of a generally frusto-conically shaped preform body to hold said body fixed in position relative to said die and to seal the space within said frusto-conical body from the space external thereof at said holding means, and sealing means at the periphery of said shaped surface adapted to engage the base of said generally frusto-conical body to seal the space within said frusto-conical body from the external space thereof at said periphery, means closing the die adjacent said periphery to form thereby a fluid tight container, and means for supplying high pressure fluid to the space within said frusto-conical body to cause stretching thereof to move it into contact with said shaped surface.

7. The method for forming semi-elliptical head closures for pressure vessels having a central opening on the minor axis of the ellipse, from a flat body of hard-to-work thin sheet metal, which method comprises: deforming said flat sheet metal body into a hollow frustum of a cone having a wall thickness equal to the desired thickness of the semi-elliptical head closure, and having open ends; inserting said frustum of a cone into a die having the desired semi-ellipsoidal shape; sealing the joint between said frustum of a cone and said die; and then deforming said frustum of a cone into the desired semi-elliptical head closure, by stretching by fluid pressure said frustum of a cone circumferentially in the set of planes of its original curvature only, until said frustum of a cone conforms to the shape of said die.

8. The method as defined by claim 7, in which the stretching of the frustum of a cone occurs only circumferentially, with freedom of motion of the frustum of a cone parallel to the minor axis of said semi-ellipsoidal die.

9. The method as defined by claim 7 characterized by the use of a frustum of a cone having cambered walls, the amount of camber being predetermined to compensate for local thinning of metal when stretched.

10. The method as defined by claim 7 characterized by the use of a frustum of a cone having variable wall thickness.

11. The method for forming hemi-spherical head closures for pressure vessels having a central opening on the longitudinal axis, from a flat body of hard-to-work thin sheet metal, which method comprises: deforming said flat sheet metal body into a hollow frustum of a cone having a wall thickness equal to the desired thickness of the hemi-spherical head closure, and having open ends; inserting said frustum of a cone into a die having the desired hemi-spherical shape; sealing the joint between said frustum of a cone and said die; and then deforming said frustum of a cone into the desired hemi-spherical head closure, by stretching by fluid pressure said frustum of a cone circumferentially in the set of planes of its original curvature only, until said frustum of a cone conforms to the shape of said die.

12. The method as defined by claim 11 in which the stretching of the frustum of a cone occurs only circumferentially, with freedom of motion of the frustum of a cone parallel to the symmetrical axis of said hemi-spherical die.

13. The method as defined by claim 11 characterized by the use of cambered walls, the amount of camber being predetermined to compensate for local thinning of the metal when stretched.

14. The method as defined by claim 11 characterized by the use of a frustum of a cone having variable wall thickness.

15. The method of claim 1, said shaped curved surface including curvature in said first mentioned set and also including curvature in the other of said two sets.

16. The method for forming bodies having compound curvature, which method comprises forming a first flat body into a first curved preform body having curvature in only one set of at least two sets of transverse parallel planes; forming a second fiat body into a second curved preform body having curvature in said only one set; positioning each of said curved preform bodies in 2. respective die, each of said dies having a shaped compound curved surface including curvature in said only one set and also including curvature in the other of said two sets; juxtaposing said dies so that said curved preform bodies face each other and provide a joint therebetween; sealing said joint; and stretching both of said curved preform bodies simultaneously, but each only in said only one set of planes, into contact with said shaped compound curved surface of the respective dies, to shape the curved preform bodies to the curvature of said compound curved surfaces, by increasing fluid pressure between said curved preform bodies.

17. The method for forming a desired thin-walled body of compound curvature from a flat body of hard-to-Work thin sheet metal, which method comprises: deforming said fiat sheet metal body into a preform body approximating a frustum of a cone in shape and also approximating the shape of the desired body of compound curvature; inserting said frusto-conical preform body into a die having a central axis and also having a shaped curved surface which corresponds to the shape of the desired body of compound curvature and has been generated by rotating a curved line about said central axis, said shaped curved surface including curvature in the same set of planes as said frustoconical preform body; sealing the joint between said frusto-conical preform body and said die; and, then deforming said frusto-conical preform body into the desired thin-walled body of compound curvature, by expanding said preform body by fluid pressure in the set of planes of its original curvature only, to impart curvature in said preform body in the other of said sets, until said body conforms to said shaped curved surface.

References Cited in the file of this patent UNITED STATES PATENTS 938,816 Bourgeois Nov. 2, 1909 939,702 Jones Nov. 9, 1909 1,329,969 Harrison Feb. 3, 1920 2,038,304 Middler Apr. 21, 1936 2,086,134 Ludwick July 6, 1937 2,317,869 Walton Apr. 27, 1943 2,498,275 Johnson Feb. 21, 1950 2,503,191 Branson Apr. 4, 1950 2,579,646 Branson Dec. 25, 1951 2,675,608 Vines Apr. 20, 1954 2,696,184 Demarest Dec. 7, 1954 2,752,675 Bauer July 3, 1956 2,763,917 Huet Sept. 25, 1956 2,763,924 Bellometti Sept. 25, 1956 2,841,865 Jackson July 8, 1958 2,849,977 Nielsen Sept. 2, 1958 2,861,530 Macha Nov. 25, 1958 FOREIGN PATENTS 451,056 Italy Aug. 23, 1949

Claims (1)

1. THE METHOD FOR FORMING A DESIRED THIN-WALLED BODY OF COMPOUND CURVATURE FROM A FLAT BODY OF HARD-TO-WORK THIN SHEET METAL, WHICH METHOD COMPRISES: DEFORMING SAID FLAT SHEET METAL BODY INTO A CURVED PERFORM BODY APPROXIMATELY THE SHAPE OF THE DESIRED BODY OF COMPOUND CURVATURE AND HAVING A CURVATURE IN ONLY ONE OF AT LEAST TWO SETS OF TRANSVERSE PARALLEL PLANES; INSERTING SAID PERFORMED BODY INTO A DIE HAVING A CURVED SURFACE CORRESPONDING TO THE DESIRED BIDY OF COMPOUND CURVATURE; SEALING THE JOINT BETWEEN SAID PREFORM BODY AND SAID DIE; AND THEN DEFORMING SAID CURVED

Images