Method and apparatus for expanding and contouring honeycomb core

A method and apparatus for expanding and contouring honeycomb preforms into compound curved honeycomb core sheets. Honeycomb preforms are provided having a plurality of face to face, contiguous, strips bonded together at selected narrow areas such that when strips along opposite edges are moved away from each other a sheet having a repeating array of regular hexagonal cells results. Opposite edge strips are secured to two spaced arms located adjacent to a freely rotatable crowned drum having a surface contour corresponding to the desired honeycomb sheet configuration. One of said arms is pivoted about the drum axis. As the arms are moved apart, a flat partially expanded honeycomb sheet initially develops. As arm movement continues, the sheet then comes into sliding contact with the drum surface. When cell expansion is completed, during the final shaping and setting phase, the honeycomb core sheet is in full contact with the drum surface. The resulting compound curved sheet is removed, trimmed and is ready for further processing, such as bonding face sheets thereto. Very uniform hexagonal cells result with no significant distortion. Non-compound, generally cylindrical expanded honeycomb sheets may be formed by utilizing only a slight drum crown.

BACKGROUND OF THE INVENTION 
This invention relates in general to the production of expanded metal 
honeycomb sheet material and, more specifically, to a method and apparatus 
for expanding and forming contoured metal honeycomb sheets. 
Honeycomb core panels have come into widespread use in a variety of 
applications that require a combination of high strength and light weight. 
Honeycomb sheets are formed from preforms that basically consist of a 
large number of narrow strips of metal material, such as aluminum, stacked 
and bonded together in small spaced areas, such as by adhesive or solder 
bonding, so that when the outside edge strips are pulled away from each 
other the preform expands into a sheet having a uniform pattern of a very 
large number of small hexagonal cells, resembling a honeycomb. Typical of 
the prior art methods of producing honeycomb core preforms is that 
described by Hartsell in U.S. Pat. No. 3,077,223. This core is 
conventionally formed into flat panels by stretching strips on opposite 
sides away from each other, as described by Steele et al in U.S. Pat. No. 
2,674,295. 
When face sheets, such as thin aluminum sheets, are bonded to the faces of 
the honeycomb core, a panel with a very high strength to weight ratio 
results. 
These honeycomb core panels are widely used in flat or cylindrical shapes 
for a variety of purposes. In some cases, complex curved shapes are 
required. For example, in aircraft engine casings or nacelles, fuselage 
panels, and other aerospace applications shapes, such as simple curves and 
compound curves, which are basically crowned cylindrical shapes are often 
required. 
In the past, such shapes have generally been formed by pulling a honeycomb 
preform into an expanded flat sheet or plank. The fully expanded sheet is 
then roll formed by passing the sheet through a series of forming rollers. 
Typical of this system is that disclosed by Curran in U.S. Pat. No. 
4,054,477. This method is generally suitable only for contouring the sheet 
in one direction. Unfortunately, this method often mutilates or distorts 
cells and areas of the sheet surface, so that generally only about 50% of 
the shaped honeycomb sheets are acceptable for use. Attempts to roll form 
in two directions generally result in unacceptable cell damage. Curved 
honeycomb core can also be formed by stretch forming the honeycomb core or 
on a fixed cylindrical curved surface as shown by Chester et al in U.S. 
Pat. No. 3,788,117. Since stretch forming often will severely distort the 
honeycomb cells through compression on the concave side of the core, 
Chester et al attempt to assure that the entire panel thickness is 
stretched, avoiding compression of the inner surface. This method is not 
capable of forming crowned curved surfaces and may cause severe distortion 
if the panel does not slip uniformly on the forming surface. Panels having 
a honeycomb core bonded to two face sheets have been formed into complex 
curved shapes by hydraulic methods, such as described by Fuchs in U.S. 
Pat. No. 3,373,460. This method can severely distort or crush the 
honeycomb core, as seen in Fuchs, FIG. 3, and is not suitable for shaping 
of honeycomb core sheets alone, since the face sheets are necessary to 
provide a surface to react the hydraulic forces. 
Thus, there is a continuing need for methods of forming metal honeycomb 
sheets into simple and complex curved shapes without damage to the 
honeycomb cells. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of this invention to provide an apparatus and 
method for expanding and contouring metal honeycomb sheets that overcome 
the above-noted problems. Another object is to provide an apparatus and 
method that can uniformly produce contoured honeycomb sheets without 
damaging the fully formed honeycomb cells. A further object is to provide 
an apparatus and method that is capable of producing a variety of high 
quality crowned cylindrical honeycomb sheet shapes. 
The above objects, and others, are accomplished in accordance with this 
invention by an apparatus and method that initially expands a honeycomb 
preform to a selected percentage of the final expansion, preferably from 
about 50 to 80%, then gradually brings the preform into gradually 
increasing sliding engagement with a crowned, freely rotatable, generally 
cylindrical, surface during further expansion, typically 10 to 30% of full 
expansion, and finally completes expansion, typically about 10-15% of full 
expansion with the honeycomb in full contact with the forming surface to 
set the final compound contour. 
In some cases, a cylindrical, non-compound curved, expanded honeycomb sheet 
is desired. If the preform is expanded around a simple cylindrical drum, 
the product will be bowed slightly toward the inside of the cylinder. To 
form a precise cylindrical expanded honeycomb product, the method and 
apparatus of the invention are used with a suitably slightly crowned drum. 
The basic apparatus includes a supporting structure on which a drum is 
mounted for unrestricted, free, rotation. The surface of the drum is 
crowned in a desired shape to provide a predetermined tool surface, which 
is a surface of rotation, to produce the desired honeycomb sheet compound 
contour. A fixed arm assembly includes a bar positioned adjacent to the 
drum surface. A second arm assembly is mounted so as to pivot about the 
drum axis with a second bar positioned adjacent to the drum surface. A 
honeycomb preform is secured between the fixed and movable bars by 
suitable connection means, such as wire loops, extending from each bar 
around the adjacent edge strip of the preform. 
In operation, the movable bar is moved away from the fixed bar to begin 
expanding the honeycomb. Initially, the preform does not contact the drum 
surface, so that the expanding honeycomb is initially essentially flat. 
Eventually, at a selected point in the expansion, the expanding sheet 
begins to contact the surface of the drum. Preferably, this contact begins 
after the preform has expanded from about 50 to 80% of full expansion. The 
honeycomb slides on the drum surface as further expansion brings more of 
the honeycomb into contact with the drum surface during a further 
expansion of about 10 to 30% of full expansion. The drum is completely 
free to rotate in response to friction forces between expanding honeycomb 
sheet and drum to accommodate expansion without "catching" cells on the 
drum surface and distorting the cells in a sliding contact that may 
involve a slight rotation of the drum. Finally, during the final expansion 
of about 10-15% of the total expansion, the honeycomb sheet is in full 
contact with the drum to set the final cell size and sheet compound 
contour. The connections between the arm assemblies and the sheet are then 
removed and the sheet is ready for further panel assembly processes. 
During this expansion and forming operation it is desirable for the 
honeycomb sheet to contact less than about 30% of the circumference of the 
drum. Greater contact may undesirably over expand and deform the end 
portions of the honeycomb sheet. 
If desired, both of the arms could be pivoted about the drum axis and 
movable away from each other, although the use of one fixed and one 
movable arm is preferred for simplicity and effectiveness. A variety of 
different connection means could be used between the arm assembly bars and 
the preform, including wires, openable rings, clamps and the like. 
The drum may have any suitable surface material. The surface material 
coefficient of friction is selected to provide a suitable resistance to 
sliding depending on the material from which the honeycomb is made. In 
general, the lowest possible resistance to sliding is preferred. A hard 
surface in relation to the hardness of the honeycomb core material is 
preferred. The surface should be very smooth. Typical surface materials 
include stainless steel cloth, fiberglass or graphite composite material, 
steel, stainless steel and other hard materials with low friction surface 
coatings. With aluminum honeycomb core, materials exhibiting smoothness, 
high hardness relative to aluminum, low sliding friction and durability 
are preferred.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
Referring now to FIG. 1, there is seen a support structure or base 10. A 
drum 12 is mounted for free, unrestricted, rotation on shaft 13 mounted in 
bearings 14 mounted at the top of uprights 16 (as best seen in FIG. 6). In 
FIG. 1, upright 16 on the near side is mostly enclosed within housing 18 
that shrouds the arm drive system, as detailed below. 
A fixed bar assembly 18 is mounted on support structure 10. The assembly 18 
includes a pair of brackets 20 positioned near each end of drum 12 and 
having a bar 22 running therebetween. Bar 22 is removably located in slots 
23 in brackets 20. 
A movable bar assembly 24 includes a pair of elongated arms 26 attached to 
sprocket 27 mounted to shaft 13 at the ends of drum 12 and opening 
allowing pivotably mounting on shaft 13. Drum 12 attaches to bearing 29 on 
shaft 13 attached to bearings 14. The other ends of elongated arms 26 are 
maintained in a spaced apart relationship by a spacer bar 28. Slots 30 at 
the ends of arms 26 are adapted to support a removable bar 32. A 
reversible drive system moves moveable arm assembly 24 between the 
positions shown in FIGS. 3 through 5 under the control of a conventional 
reversing switch 34. 
A suitable honeycomb preform 36 is positioned between fixed bar 22 and 
movable bar 32. A plurality of wires 38 are threaded through preform 
strips near the edges of preform 36, around bars 22 or 32, respectively 
and tied or twisted, as shown. Any other suitable connecting or clamping 
devices may be used, if desired to flexibly secure the preform edges to 
the fixed and movable bars 22 and 32 As seen in FIG. 1, the apparatus is 
ready to begin the honeycomb and expansion and forming operation. 
FIG. 2 shows the apparatus of FIG. 1 from a slightly different point of 
view, with the drum 12 removed. As seen in FIGS. 3 and 4, bars 22 and 32 
are spaced from the surface of drum 12, as bar 32 is pivoted around drum 
12 to initially preform the honeycomb core 36 and to expand it into a flat 
partially expanded honeycomb sheet as seen in FIG. 4. This initial 
expansion is preferably about 50 to 80% of full expansion. Eventually, 
depending on the position selected for fixed bar 22 on brackets 20 and the 
length of arms 26 and the position selected for arm 32 relative to the 
surface of drum 12, the partially expanded preform 36 will come into 
contact with the top of the crowned drum surface. 
Thereafter, the preform 36 will engage the drum surface, possibly rotating 
the drum as it slides over the drum surface. I have found that this 
combined rotation and sliding action avoids deformation of the expanding 
honeycomb cells. During this second step, further expansion of the 
honeycomb core takes place, preferably from about 10 to 30%, as well as 
initial forming over step it is desirable for the honeycomb sheet to 
contact less than about 30% of the circumference of the drum. Greater 
contact may undesirably over expand and deform the end portions of the 
honeycomb sheet. 
Eventually, the expanding preform reaches full expansion, as shown in FIG. 
5 to fully expand the core and to "set" the final deformation downwardly 
over the crowned drum 12. the expanded preform 36 is in full contact with 
the crowned drum surface. Wires 38 can then be removed and the expanded, 
shaped, honeycomb sheet can be removed. The edges of the sheet are trimmed 
to the desired dimensions and the sheet is ready for further processing, 
such as bonding face sheets thereto. Switch 34 is then activated to 
reverse the drive system to return the movable arm assembly 24 to the 
position shown in FIGS. 1 and 3. 
A conventional reversible electric motor 40 is mounted on support structure 
10. Sprockets 42 and 27 are drivingly connected to motor 40 and one of 
arms 26, respectively. Drive chain 46 runs between sprockets 42 and 27. 
Sprocket 27 is secured to an arm 26, such as by bolts or rivets. Drum 12 
rotates freely about the shaft while the shaft 13, arms 26 and sprockets 
27 are fixed to each other and rotate as a unit. Switch 34 (FIGS. 1 and 2) 
is a conventional reversing switch which selectively causes motor 40 to 
operate in either direction. Conventional limit switches can be mounted on 
support structure 10, if desired, to limit movement of arms to travel 
between the positions of FIGS. 3 and 5. 
Examples of typical honeycomb cell structure produced by the forming 
methods of the prior art and by the method and apparatus of this invention 
are provided in FIGS. 7 and 8, which are drawings based on photographs of 
actual honeycomb sheets. As seen in FIG. 7, the prior art roll forming 
method described above tends to distort the cell pattern of honeycomb 50, 
showing a number of damaged cells 52. Further, although not readily 
apparent in this view, the wall edges of the cells are often damaged, so 
that the cell walls will not meet and bond well to a face sheet which is 
later applied to the face of the honeycomb core sheet. As seen in FIG. 8, 
uniform, regular hexagonal honeycomb cells 54 result when the expanded 
honeycomb sheet is prepared in accordance with this invention. 
While certain specific relationships, arrangements and materials were 
specified in the above description of preferred embodiments, those may be 
varied, where suitable, with similar results. For example, honeycomb 
preforms made from a variety of materials may be formed using this method 
and apparatus, and various coatings, lubricants, etc. could be used on the 
crowned drum surface to optimize the surface coefficient of friction for 
specific honeycomb sizes and materials. 
Other applications, variations and ramifications of this invention will 
occur to those skilled in the art upon reading this disclosure. For 
example, certain arm and bar assemblies may be made to accommodate various 
application requirements. Those are intended to be included within the 
scope of this invention, as defined in the appended claims.