GB2442793A

GB2442793A - Forming curved panels

Info

Publication number: GB2442793A
Application number: GB0620035A
Authority: GB
Inventors: Paul Weston
Original assignee: APOGEE ANTENNAS Ltd; APOGEE RMS Ltd
Current assignee: APOGEE ANTENNAS Ltd; APOGEE RMS Ltd
Priority date: 2006-10-10
Filing date: 2006-10-10
Publication date: 2008-04-16
Also published as: ATE451982T1; EP2076341B1; WO2008044021A3; US20100031726A1; DE602007003861D1; WO2008044021A2; EP2076341A2; GB0620035D0

Abstract

Aluminium panels curved in mutually orthogonal x, y and z dimensions are made by stretch forming. A former 34 for use in this process comprises a plurality of steel bars 60 extending generally in the y direction and mutually spaced apart in the x direction, The bars 60 are held in side rails 62 and 64, which are formed with holes 66 to receive the bars 60, and flexed over an intermediate rail 68. Thus each bar 60 is curved in a yz plane. The holes 66 and the seats for the bars on the intermediate rail 68 each lie on a curve in an xz plane. So located, the bars 60 combine to define an envelope which is curved in mutually orthogonal x, y and z dimensions. When an aluminium sheet is stretched over the former 34, it takes up the shape of this envelope.

Description

FORMING CURVED PANELS

This invention concerns the forming of curved panels from sheet material, for the construction of reflectors for antennas including satellite communications antennas, and other purposes such as lighting reflectors and solar collectors.

A satellite communications antenna, for instance one used to for TV signals, comprises a reflective parabolic dish several metres across arranged to collect an incoming signal and focus it at the focal point of the dish. Such dishes are typically constructed from a number of curved panels, and for effective communications each panel must be extremely accurate in form -typically an average error of not more than 0.5 mm over the panel. The panels are commonly made by means of a stretch forming machine in which a ductile aluminium sheet is stretched beyond its elastic limit (or "pulled") over a former.

The "kin" so formed is relatively delicate and therefore before use it is usually ".. stiffened by "stringers" secured to the skin. * ***

* ** Heretofore the manufacture of antenna panels by stretch forming has * ** been difficult and expensive for three reasons as follows. First, conventional S. *.

stretch forming machines are designed principally for large-scale applications such as the manufacture of aircraft wings and fuselage sections and such machines are therefore needlessly massive (and hence unnecessarily costly) for the manufacture of antenna panels. For example, many sections typically manufactured by the stretch form process require considerable curvature in the panel, and therefore the conventional machine drives the former into the ductile sheet by means of a massive ram. The cost problem is exacerbated by the need for conventional stretch forming machines to be versatile, and therefore relatively complex, in order to accommodate a large variety of jobs and sustain the investment in the machine. Second, the former for such a machine is conventionally made from a solid block of material accurately machined to the required shape. Formers of this kind are very expensive to make, and more so to modify, which in turn means that non-standard reflectors cannot generally be afforded. It will also be understood that these formers are awkward to store.

Third, the stringers supporting the panel are themselves conventionally pre-formed and secured to the skin by means of a special assembly fixture, adding two more cost elements to the manufacture of the antenna.

It is an object of the present invention to reduce the cost of manufacturing curved panels for use in the construction of satellite communications antennas and the like.

Thus according to a first aspect of the invention there is provided apparatus for forming from ductile sheet material a panel curved in mutually * ** :. orthogonal x, y and z dimensions, which apparatus comprises two side rails and an intermediate rail between the two side rails, all said rails extending generally in the x direction, and a plurality of bars held by the side rails to extend generally in the y direction and be mutually spaced apart in the x direction, characterised in that the side rails and the intermediate rails are relatively configured and arranged so that the bars are flexed thereby to form a plurality of curves in yz planes and the apices of the curves in the yz planes form a curve in the xz plane.

Each bar may be held at or adjacent its ends in holes formed in the side rails. Each hole is preferably formed to extend along the curve in the yz plane of the bar it holds, and the holes mutually spaced apart in the y direction along a curve in the xz plane. (

Preferably each bar is flexed over a seat on the intermediate rail, and the seats mutually spaced apart in the y direction along a curve in the xz plane.

There may be more than one said intermediate rail.

Those skilled in the art will appreciate that the use of multiple bars as a former is much cheaper to manufacture than the solid formers in general use heretofore, because instead of accurately machining a complete surface it is necessary only to provide accurate locations for the bars by means of the rails that hold them. Further, if a panel requires some modification, it is at most necessary to adjust the rails rails, which is much cheaper than reshaping a solid former. Also the assembly of rails and bars can be readily dismantled for convenient storage when not in use. *. **

According to a second aspect of the invention there is provided apparatus for forming a panel from ductile sheet material, which apparatus comprises a former defining a surface curved in mutually orthogonal x, y and z dimensions and two clamps extending generally in the y direction and mutually spaced apart in the x direction to clamp said sheet material at opposite ends of said former, characterised in that said apparatus comprises a jack operable to drive said clamps apart and stretch the clamped sheet material over said surface.

Preferably each said clamp is pivoted about an axis extending generally in the y direction and rearward of said former (which is to say, in a direction opposite to the convexity of the former) whereby when the clamps are driven apart the sheet material is pulled rearwards onto said surface. The jack, which may be operable by fluid pressure, is preferably arranged between said clamps to bear on each thereof; or there may be two jacks, one on each clamp, bearing on each other.

This aspect of the invention takes advantage of the fact that items such as antenna panels have small curvatures in comparison with most stretch formed components such as aircraft parts. The action of the pivoting clamps draws the material down onto the former, as the material is strained, obviating the need for a massive ram to drive the former as heretofore. The pivoting arrangement of ***. the clamps is a simple and therefore relatively inexpensive way of drawing the S...

*::: workpiece sheet onto the former. And by arranging one jack to act on both clamps, or two jacks to act against each other, there is no need for a particularly *5 massive frame to resist the jacking force.

Each clamp may comprise first and second members arranged to receive said sheet material therebetween and a source of pressurised fluid arranged to drive the second member towards the second member thereby to clamp the sheet material, and there may be a flexible membrane between the second member and the source of pressurised fluid.

According to a third aspect of the invention there is provided a method of forming from ductile sheet material a panel curved in mutually orthogonal x, y and z dimensions, characterised in that said method comprises arranging a plurality of bars to extend generally in the y direction and be mutually spaced apart in the x direction, flexing the bars to form a plurality of curves in the yz plane, locating the bars so that the apices of the curves in the yz planes form a curve in the xz plane and stretching said sheet material over said bars.

A panel formed as aforesaid may be reinforced by stringers applied to the convex face of the panel while its concave face is on said former, and the stringers may be made from lengths of box section made flexible by kerfing before application to the panel. This means that the stringers are cheaply and easily made and their application to the panel does not require a special assembly fixture.

.. Panels included within the invention may be formed from aluminium and . may be used in the construction of a communications antenna, or another * ** product such as a lighting reflector or a solar heating reflector. ** **

Other features of the invention will be apparent from the following description, which is made by way of example only and with reference to the accompanying schematic drawings, in which -Figures 1 to 3 show in third angle projection three corresponding views of an antenna panel of the kind made by means of the invention, Figure 1 being a front elevation, Figure 2 being a side elevation and Figure 3 being a plan view from below; Figure 4 is an isometric view of a stretch forming machine embodying the Invention; Figures 5 and 6 illustrate the operation of the stretch forming machine of Figure 4; Figure 7 is an isometric view of a former for the stretch forming machine of Figure 4; Figure 8 is a cross-sectional view of a clamp of the machine of Figure 4; and Figure 9 illustrates a stringer for application to an antenna panel.

Referring first to Figures 1 to 3, it should first be noted that the curvature of the antenna panel indicated at 10 therein is shown exaggerated for clarity of illustration. The panel 10 has a top 12 and a bottom 14 and two sides 16 and 18 and is curved so as to have a concave face 20 and a convex face 22.

The panel 10 is formed from aluminium alloy I 050A, which is ductile, and I. its concave face 20 acts as a focussing reflector for electromagnetic signals. Thus in use the panel 10 is put together with others (as indicated in broken lines at 24, 26 and 28) to construct a satellite communications antenna. The antenna'is of parabolic form, but the details of its construction are not germane to the present invention, save that the concave face of each panel must have an average error of not more than 0.5 mm across its surface.

As can be seen (exaggerated) in Figures 1 to 3, and as will be understood by those skilled in the art, the panel 10 is curved in three mutually orthogonal dimensions, identified in Figures 1 to 3 and elsewhere herein as x, y and z. Figure 4 shows indicated generally at 30 a stretch forming machine according to the present invention.

A sheet 32 of aluminium is placed on a former 34 of the machine 30. The former 34, which will be described in more detail hereinafter, is configured and arranged to present a generally convex forward face (ie the upper face as seen in Figure 4). This face is curved in mutually orthogonal x, y and z dimensions.

The sheet 32 is gripped at each end by a clamp 36 secured to a swinging frame 38 mounted on pivots 40 on the main frame 42 of the machine 30. The swinging frames 38 are each thus rotatable about an axis through their pivots 40, which are rearward of the convex face of the former 34 (ie below the former 34, as seen in Figure 4). In use the swinging frames are driven by a hydraulic jack 50 (not shown in Figure 4) as will now be described with reference to the diagrammatic Figures 5 and 6. *

The hydraulic jack 50 is connected to each of the swinging frames 38.

Before use, as shown in Figure 5, the swinging frames 38 are positioned close up to the ends of the main frame 42 of the machine 30. In this position the aluminium sheet 32, held at each end by the clamps 36, rests on the convex face of the former 34. Pressurised fluid (from a source not shown) is admitted to the jack 50 which then drives the frames 38 apart, each rotating about its pivots 40.

Thus the clamps 36 move arcuately and the sheet 50 is stretched and simultaneously pulled down onto the former 34, as shown in Figure 6.

It should be noted that rotation of the swinging frames 38 and hence the stretching of the sheet 32 is exaggerated in Figures 5 and 6, for clarity of illustration, but in practice it is sufficient for the sheet 32 to be stretched beyond its elastic limit. In this way the sheet 32 is conformed to the shape of the convex face of the former 34. It should also be noted that each swinging frame 38 is cranked adjacent its pivots 40, as indicated at 38a, which has the effect reducing the amount of rotation required to draw the sheet 32 onto the former 34.

Referring now to Figure 7, the former 34 comprises a plurality of steel bars 60 extending in the y direction and mutually spaced apart in the x direction.

The bars 60 are held in side rails 62 and 64, which are formed with holes 66 to receive the bars 60, and flexed over an intermediate rail 68. Thus each bar 60 is *:* curved in a yz plane. Each hole 66 is bored to align with the flexure of the bar 60 * ** * . * it holds. * I.

As can be seen from Figure 7, the holes 66 lie on a curve in an xz plane. S. *.

Similarly the seats (not detailed) for the bars on the intermediate rail 68 lie on a curve in an xz plane. -By these means the bars 60 combine to define an envelope which is curved in mutually orthogonal x, y and z dimensions. Therefore, when an aluminium sheet 32 is stretched over the former 34 as aforedescribed, it takes up the shape of this envelope. More particularly, the sheet 32 is given a convex ace closely conforming to the envelope defined by the bars 60. It can then be released from the clamps 36 and trimmed (by means of a plasma cutter) along top, bottom and sides to provide an antenna panel as shown in Figures 1 to 3.

This arrangement is substantially cheaper than the conventional use of a solid former, even with a large number of bars 60. Further cost reductions can be effected by using fewer bars 60, but some consequent reduction in accuracy may limit the application of panels so made.

The accuracy of the panel (for any given application) is checked by means of a measurement system comprising commercial off-the-shelf (COTS) units that it is considered unnecessary to illustrate or describe. If the resulting shape is not exactly as required, the bars 60 can be removed and relocated, or new rails made.

After use the former 34 may be stored as an assembly, or alternatively **,* dismantled for more compact storage. If it is dismantled the bars are removed and, being only elastically flexed, may then be used to make other formers. The rails 62, 64 and 68 may be retained for reuse in the event that a panel to the same * design is required again.

Figure 8 illustrates the construction of a clamp 36. An edge of the sheet 32 is located between a first member 70 and a second member 72. The first member is a fixed part of the clamp 36 and the second member 72 is a movable part thereof. The second member 74 is backed by a flexible membrane 74.

Pressunsed fluid 76 (from a source not shown, but which may be the same as that for the jack 50) is delivered behind the membrane 74 and thus the movable second member 72 is driven towards the edge of the sheet 32 which is then firmly held against the fixed first member 70.

The formed and trimmed panel 10 is reinforced by a stringer 80. The panel 10 is conveniently supported on the former 34 during this process. The stringer 80 is manufactured from a length of straight box section material with a plurality of kerfs 82 cut through three sides thereof so as to be flexible. The side of the stringer 80 opposite the uncut side of the box section is placed on the panel 10 and conformed thereto by the application of weights or other means such as by air pressure. The stringer 80 is then secured to the panel 10 by any appropriate mean such as glue, rivets, bolts, screws or welding, or a combination of these means.

Whilst the bars shown in the drawings are of generally circular cross-section, the bars may be of any appropriate shape. Some material other than *:** aluminium may be used for the panels. The single jack 50 shown in the drawings may be replaced by two jacks carried respectively by the two clamps and bearing ** upon each other. Further, panels may be made for applications other than satellite communications antennas, eg radar and microwave antennas, reflectors for high power lighting units of the kind used in sports stadiums and solar heating reflectors. Other possible modifications and adaptations will be apparent to those skilled in the art.

It will also be apparent that the tool described herein is of general utility, and can be used for methods other than stretch forming, for example in processes where the material is kept within its elastic limit and held in shape by strong stringers, or in the manufacture of items made from composite material.

Finally it should be noted that the x, y and z dimensions referred to herein and so annotated in the drawings are not intended to designate any specific orientation with respect to any forming machine or any former for such a machine or any panel made by such a machine. Thus, for instance, whilst Figure 7 is described as showing a former with its bars 60 extending in the y direction (ie widthwise) and mutually spaced apart in the x direction (ie lengthwise), this is done purely for clarity of illustration. It should be understood that the invention does not call for an x dimension to be defined with reference to the length of any former or a y dimension to be defined with reference to its width. Similarly, ". with regard to Figure 4 neither the lengthwise axis nor the widthwise axis of the *s * machine shown therein defines a specific x or y dimension. And with regard to Figure 1 to 3, whilst the distance from head 12 to tail 14 of the panel 10 is *. ..

shown aligned with the x dimension, this does not mean that the x dimension is defined with reference to the head and tail of any panel 9 (and nor is the y dimension defined with reference to the sides of a panel). Rather, the x, y and z dimensions are simpiy conventional designations of three mutually orthogonal -12-dimensions, and the panel 10, the former 34 and the machine 30 may be orientated in any way relative to those dimensions. It follows that the bars 60 may extend generally in the x direction as well as or instead of the y direction as described hereinbefore, and the bars may be flexed in an xz plane as well as or instead of a yz plane as described. Whilst the z dimension herein accords with the conventional reference to up-down dimensions, no such constraint is to be inferred. The x, y and z dimensions hereof can refer to any three mutually orthogonal dimensions. * . * I.. ***. * I * *1

** I S I *. * I. * ** *** I *I * * * I **

Claims

-13 -

1 Apparatus for forming from sheet material a panel curved in mutually orthogonal x, y and z dimensions, which apparatus comprises two side rails and an intermediate rail between the two side rails, all said rails extending generally in the x direction, and a plurality of bars held by the side rails to extend generally in the y direction and be mutually spaced apart in the x direction, characterised in that the side rails and the intermediate rails are relatively configured and arranged so that the bars are flexed thereby to form a plurality of curves in yz planes and the apices of the curves in the yz planes form a curve in the xz plane.

2 Apparatus for forming a panel as claimed in Claim I characterised in that each bar is held at or adjacent its ends in holes formed in the side rails. S...

S..... 3 Apparatus for forming a panel as claimed in Claim 2 characterised * U. 15 in that each hole in the side rails is formed to extend along the curve in the yz plane of the bar it holds. S. **

: 4 Apparatus for forming a panel as claimed in Claim 3 characterised in that said holes are mutually spaced apart in the y direction along a curve in the xz plane.

5 Apparatus for forming a panel as claimed in any preceding claim characterised in that each bar is flexed over a seat on the intermediate rail.

-14 - 6 Apparatus for forming a panel as claimed in Claim 5 characterised in that said seats are mutually spaced apart in the y direction along a curve in the xz plane.

7 Apparatus for forming a panel as claimed in any preceding claim characterised in that there is more than one said intermediate rail.

8 Apparatus for forming a panel from sheet material, which apparatus comprises a former defining a surface curved in mutually orthogonal x, y and z dimensions and two clamps extending generally in the y direction and mutually spaced apart in the x direction to clamp said sheet material at opposite ends of said former, characterised in that said apparatus comprises a jack operable to drive said clamps apart and stretch the clamped sheet material over said surface.

9 Apparatus for forming a panel as claimed in Claim 8 characterised * * in that one or each said clamp is carned on a frame pivoted about an axis *S*.

extending generally in the y direction and rearward of said former, whereby when the clamps are driven apart the sheet material is pulled rearwards onto said *. ** * * * * surface. ** * * S S

* 10 Apparatus for forming a panel as claimed in Claim 9 characterised in that each said frame is cranked adjacent its pivotal axis.

11 Apparatus for forming a panel as claimed in Claim 9 or Claim 10 characterised in that said jack is arranged between said clamps to bear on each thereof.

-15 - 12 Apparatus for forming a panel as claimed in Claim 9 or Claim 10 characterised in that each clamp carries a said jack and the two jacks bear upon each other.

13 Apparatus for forming a panel as claimed in Claim 11 or Claim 12 characterised in that the or each said jack is operable by fluid pressure.

14 Apparatus for forming a panel as claimed in any of Claims 8 to 13 characterised in that each said clamp comprises first and second members arranged to receive said sheet material therebetween and a source of pressurised fluid arranged to drive the second member towards the second member thereby to clamp the sheet material.

Apparatus for forming a panel as claimed in Claim 14 characterised in that each said clamp includes a flexible membrane between the second member and the source of pressurised fluid.

16 Apparatus for forming a panel substantially as hereinbefore S...

15 described with reference to and as shown in the accompanying drawings. * S.

17 A method of forming from sheet material a panel curved in mutually 5* orthogonal x, y and z dimensions, characterised in that said method comprises arranging a plurality of bars to extend generally in the y direction and be mutually spaced apart in the x direction, flexing the bars to form a plurality of curves in the yz plane, locating the bars so that the apices of the curves in tbe yz planes form a curve in the xz plane and stretching said sheet material over said bars. -16-

18 A method of forming a panel as claimed in Claim 17 characterised in that the bars are located so that their ends are mutually spaced apart in the y direction along a curve in the xz plane.

19 A method of forming a panel as claimed in Claim 18 characterised in that the sheet material is clamped along edges extending in the y direction at opposite ends of said former and the clamped edges are driven apart to stretch the clamped sheet material over said surface.

A method of forming a panel as claimed in Claim 19 characterised in that one or each said clamped edge is driven arcuately about an axis extending generally in the y direction and rearward of said former, whereby the clamped sheet material is pulled rearwards onto said surface.

21 A method of forming a panel as claimed in any of Claims 17 to 20 characterised in that said former is adjusted by repositioning the bars.

22 A method of forming a panel as claimed in any of Claims 17 to 21 *.. 15 characterised in that said panel is trimmed to shape after being formed.

23 A method of forming a panel as claimed in any of Claims 17 to 22 *. ..

characterised in that the panel is reinforced by stringers applied to the convex face of the panel while its concave face is on said former.

24 A method of forming a panel as claimed in Claim 23 characterised in that each said stringer is made from a length of box section made flexible by kerfing before application to the panel. -17-

A method of forming a panel as claimed Claim 24 characterised in that each said stringer is secured to the panel by an adhesive or by rivets or by bolts or by screws or by welding.

26 A method of forming a panel substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

27 A panel formed by a method of forming as claimed in any of Claims 17 to 26.

28 A panel as claimed in Claim 27, characterised in that said panel is formed from aluminium.

29 A sateffite communications antenna comprising a panel as claimed in Claim 27 or Claim 28.

A radar antenna comprising a panel as claimed in Claim 27 or Claim 28.

s's... 31 A microwave antenna comprising a panel as claimed in Claim 27 or *.. 15 Claim 28. * *.

32 A lighting reflector comprising a panel as claimed in Claim 27 or ** .. * 5 5

* Claim 28. ** * S S*

* 33 A solar heating reflector comprising a panel as claimed in Claim 27 or Claim 28.