Thin-walled metal duct having integral reinforced coupling ends

Thin-walled metal duct particularly useful for heating and air-conditioning purposes is usually made by grooving, spiral wrapping, and seaming strips of thin metal sheet. The resultant tubular form provides the necessary rigidity to resist collapse of the tubular form. In the past, at least one end of the ribbed tubular ducts have been reformed to permit the ducts to be joined together. The end reforming techniques of the past have resulted in reformed ends having insufficient rigidity and strength to resist the abuse the ducts frequently receive in shipping, storing, handling, and installation. The present product, method, and apparatus provide a ribbed thin-walled tubular duct having reformed ends with rigidity and strength superior to the prior art products. This achievement is accomplished in the present invention by moving more metal into the end portion of the duct prior to reforming the end portion, thus reinforcing the end portion of the duct.

The present invention is related to grooved or ribbed thin-walled metal 
ducts having reformed ends with improved strength and rigidity, and to the 
method and apparatus used to manufacture such ducts. 
BACKGROUND OF THE INVENTION 
Round, thin-walled, spirally grooved or ribbed metal ducts and their method 
of manufacture are known as evident by U.S. Pat. Nos. 3,435,852 and 
3,621,884. Round ducts of this type have many uses, e.g., as cold or warm 
air ducts for residential, commercial, and industrial construction. 
It is also known to reform at least one end of such ducts to facilitate 
joining two ducts together as evidenced by U.S. Pat. Nos. 3,708,867, 
3,758,139, and 3,794,364. These methods involved rolling one end of the 
duct to flatten the spiral ribs sufficiently to permit the threading of 
the rolled duct end into an unrolled duct end, or rolling the ribs on both 
ends of the duct down with a narrow roller and then crimping one of the 
ends to make it small enough to fit into a rolled but uncrimped end. The 
problem with each of these methods is that the ends are fragile, 
particularly when the ducts are made from very thin metal, such as 15 mils 
or less, and particularly when aluminum metal is used. The fragile duct 
ends become damaged in shipment, handling, or installation rendering the 
ducts either useless or requiring expensive repair. As a result the 
tendency has been to use thicker gage metal to provide for stronger ends 
or to use heavier gage metal coupling members. Both alternatives are less 
than desirable. 
The object of the present invention is to provide metal ducts having ribs 
with reformed ends that permit joining without auxiliary coupling members, 
which ends are much stiffer and significantly more resistant to damage 
than the prior art reformed ends. 
SUMMARY OF THE INVENTION 
The present invention provides tubular metal duct sections having at least 
one integral stiffened and reinforced end portion having at least three 
layers of metal over a majority of the area of the end portion. Preferably 
both end portions of the duct are reformed and one of the end portions is 
also tapered to enable that end portion to be inserted into a non-tapered 
end portion of a second duct section. 
The improved duct sections of the present invention are made by compressing 
the duct lengthwise from at least one end, and preferably from both ends, 
to produce at least one end portion in which the lengthwise distance 
between the ribs in the end portion is significantly less than the 
lengthwise distance between the ribs in an uncompressed intermediate 
portion of the duct adjacent to the end portion of the duct, at least 
partially removing the ribs from at least one end portion of the duct by 
working the walls of the end portion to at least partially collapse the 
ribs and to form a cylindrical end portion, and preferably finally 
tapering an outer extremity of at least one end portion to reduce the 
outside diameter of the tapered portion to less than the inside diameter 
of the cylindrical end portion. Preferably, the amount of lengthwise 
compression at the ends of said duct is sufficient to produce end portions 
having three layers of the thin-walled metal over a majority of the area 
of the walls of the end portions after the rib collapsing step. 
A suitable apparatus for practicing the method of the present invention to 
produce the ducts of the present invention comprises means for supporting 
a duct, means for compressing the duct lengthwise from at least one end to 
produce at least one end portion having a greater number of ribs per 
linear dimension of duct than an intermediate portion of the duct adjacent 
to the end portion, and means for at least partially collapsing the ribs 
in the end portion of the duct without changing the generally cylindrical 
shape of the end portion.

DETAILED DESCRIPTION AND PREFERRED EMBODIMENTS 
FIG. 1 shows two adjacent helically or spirally ribbed duct sections having 
reformed ends in accordance with the present invention and joined together 
without the need for separate coupling members. The lefthand duct 2 and 
the righthand duct 1 both comprise a thin metal wall having spiral ribs 4 
running therearound with spiral grooves 6 occuring between the ribs 4. 
Since this duct was made by spirally wrapping and seaming a strip of metal 
around a mandrel, spiral seams 8 also appear periodically along the 
lengths of the ducts. 
Metal duct useful in the present invention can be made in a variety of 
ways, such as the methods described in U.S. Pat. Nos. 3,753,363 and 
3,753,367. The seams 8 can also be made in a variety of ways including 
those disclosed in U.S. Pat. Nos. 3,621,884 and 3,865,146. 
Duct 2 in FIG. 1 has a reformed male end comprising a reinforced portion 10 
and a reinforced and tapered portion 12. The tapered portion 12 can be of 
the type and can be formed in the manner disclosed in U.S. Pat. Nos. 
3,708,864 and 3,758,139. The reformed end of the righthand duct 1 in FIG. 
1 is shown in cross-section and comprises a reinforced portion 14. The 
tapered portion 12 is tapered to a diameter at the extremity sufficiently 
small to allow the tapered portion 12 to be inserted inside the female 
reformed portion 14 as shown in FIG. 1. 
The reformed portion 10 and the reformed portion 14 form flat surfaces for 
the application of any approvel duct tape, usually pressure-sensitive, for 
sealing the connection between the two ducts. Screws, rivets, or other 
fastening means can also be used in a conventional manner to prevent the 
ducts from separating at the joint. 
FIG. 2 shows a typical helically ribbed duct prior to reforming the ends in 
accordance with the present invention. While this duct is the type used in 
the preferred embodiment, other ducts could be used with the present 
invention. For example, smooth-walled ducts having helical or 
circumferential ribs only in the end portions of the duct or ducts having 
circumferential ribs throughout the length of the duct can be used. 
While the process of the present invention can be used to reform and 
reinforce an end portion metal duct with walls of any thickness, the 
invention is particularly useful on ribbed duct made from thin metal such 
as aluminum, stainless steel, steel, galvanized steel, copper and other 
metals, either coated, laminated or uncoated, conventionally used in the 
manufacture of ducts. For example, ribbed ducts made from sheet metal 
having a thickness of 15-18 mils or less are frequently used to make a 
duct of the type shown in FIG. 2. The invention is particularly useful 
when the sheet metal is about 12 mils or less, preferably 10 mils or less 
and most preferably less than 8 mils, for example 6 mil aluminum or alloys 
thereof. These thicknesses refer to ducts having diameters of about 3 to 
22 inches. When larger diameter ducts are used, thicker sheet metal is 
required because of the greater radius of the metal and the resultant 
lower rigidity of the duct walls. 
FIG. 9 shows an apparatus in accordance with the present invention 
comprising a main frame 20 which is usually in the vertical position and 
which is supported by a conventional metal base (not shown). Attached to 
the base 20 is a mandrel holder 22 which, by the aid of bearings 24, 
support a mandrel 26 in a rotatable relationship. The mandrel 26 is 
surrounded by a sprocket 28 which can be driven by a conventional drive 
means (not shown) with the aid of a chain 30. 
Also connected to the base 20 is a conventional air or hydraulic cylinder 
32 having an extended rod 34 passing along the axis of the mandrel 26 and 
through a hole in an end 36 of the mandrel 26. A layer 38 of an 
elastomeric material, having a hole in its center and having a relaxed 
diameter essentially the same as that of mandrel 26, is positioned 
adjacent the end 36 of the mandrel 26 and surrounds the rod 34. A round, 
rigid, usually metal, plate 40 having a diameter essentially the same as 
the diameter of the mandrel 26 and having a hole in its center is 
positioned around the rod 34 and adjacent to the elastomeric layer 38. A 
thrust bearing 42 is positioned around rod 34 and adjacent to plate 40 and 
is snuggley maintained in position by a washer 44 and a nut 46 threaded 
onto the end of the rod 34. 
Also attached to the base plate 20 are a plurality of cylinders 48, 
preferably four cylinders located at 12 o'clock, 3 o'clock, 6 o'clock and 
9 o'clock positions. Rod ends 50 of the cylinders 48 are connected in a 
conventional manner to a forming plate 52. The forming plate 52 has a hole 
in its center through which the mandrel 26 passes with a slight clearance. 
The clearance between the outer surface of the mandrel 26 and the plate 52 
is only sufficiently great to allow rotation of the mandrel 26 without 
causing friction with the forming plate 52. Located on a working face 54 
of the plate 52 is a circular groove 56 located next to the outer surface 
of the mandrel 26. The groove 56 extends completely around the opening in 
plate 52 for the mandrel 26 except in the area behind an edge forming 
wheel 58. Forming plate 52 is shown partially broken away to better 
illustrate the location of the edge forming wheel 58 mounted in a 
rotatable manner on a shaft 60 which is supported by bearings 62. The edge 
forming wheel 58 has a concave working surface 64. 
An upper frame member 66 is attached to the frame member 20 and is 
supported in a cantilevered position. Attached to the upper frame member 
66 is an arm 68 to which is attached in a vertically slideable 
relationship a dog leg support 70. A hard metal roller 72 is rotatably 
attached by a conventional shaft and conventional bearings (not shown) to 
the dog leg support 70. Also attached to the dog leg support 70 in a 
conventional manner is a rod end 74 of a hydraulic or pneumatic cylinder 
76 which in turn is mounted to the upper frame member 66. Activation of 
the cylinder 76 to extend the rod end 74 moves the dog leg support 70 and 
the roller 72 downwardly and actuation of the cylinder 76 to retact the 
rod end 74 moves the dog legged support 70 and the roller 72 upwardly. 
The apparatus shown in FIG. 9 operates as follows. A duct such as the type 
shown in FIG. 2, is pushed over the mandrel 26 and up against the groove 
56 in the forming plate working face 54 and against the concave surface 64 
of the forming wheel 58 in the manner shown in FIG. 9. Cylinder 32 is then 
actuated to retract the rod 34 which causes the plate 40 to be pulled in 
the direction of the mandrel end 36. This action causes the elastomeric 
layer 38 to be compressed, (see FIG. 10) thus increasing its diameter and 
pressing the outer surface of the layer 38 against the inside of the 
ribbed duct. This action holds the portion of the ribbed duct in contact 
with layer 38 in place on the mandrel 26. 
In the next step, shown in FIG. 10, cylinders 48 are actuated to extend 
rods 50. Extension of the rods 50 move the forming plate 52 to the right 
compressing linearly the end portion of the duct, increasing the density 
of the ribs 4 in the end portion of the duct between the working face of 
the forming plate 52 and the elastomeric gripping member 38 to produce a 
configuration as shown in FIG. 3. The distance between the ribs 4 in the 
end portion after plate 52 has been moved sufficiently to contact, or 
almost contact, the end of roller 72 is substantially less than the 
distance between the ribs 4 in the non-compressed portion of the duct. 
Next, the mandrel 26 is rotated by actuating a conventional drive means 
(not shown) which, through drive chain 30, rotates the mandrel 26 and the 
sprocket 28 attached thereto. Since the duct is gripped by the elastomeric 
member 38 rotation of the mandrel 26 also causes the elastomeric gripping 
member 38, the plate 40, and the ribbed duct to rotate. The rod 34 does 
not rotate because of the thrust bearing 42. Rotation of the duct in the 
groove 56 causes the leading edge of the duct to be turned outwardly, or 
sometimes inwardly. When the turned edge of the duct contacts the working 
surface 64 of roller 58 the working surface 64 tends to further turn the 
edge back upon itself as will be better seen in later figures. 
Referring to FIG. 11, showing only a portion of the apparatus shown in FIG. 
10, the next step is to activate cylinder 76 extending rod 74 and lowering 
the dog leg support 70 and the roller 72 such that the roller 72 
forcefully bears against the ribs on the rotating duct. The working face 
of the roller 72 can be a smooth surface, a knurled surface, a surface 
having a plurality of angular shallow grooves, e.g., about 5 mils deep, or 
any surface that will grip the duct metal during rolling. Preferably the 
surface of the roller is knurled like that shown in FIGS. 4 and 5 of U.S. 
Pat. Nos. 3,708,867 and 3,758,139, the disclosures of which are 
incorporated herein by reference. Sufficient force is applied by the 
cylinder 76 and the roller 72 to cause the ribs to be flattened, as shown 
in better detail in FIGS. 12, 13, and FIG. 5. 
In the apparatus shown in FIGS. 9-12, a wall configuration such as that 
shown in FIGS. 5 and 13 is produced. This configuration comprises a series 
of generally squatty T-shaped members which result in a layered wall 
having three layers of metal along a majority of its length. These extra 
layers of metal stiffen and reinforce the end portion of the duct. In wall 
portions shown FIGS. 5 and 13, at least three layers of metal preferably 
exist over at least 85% of the length of the reformed end. This can be 
varied by varying the amount of lengthwise compression in the end portion 
prior to rolling, depending upon the degree of stiffness and strength 
desired in the reformed end. 
In the area of the seams, see FIG. 4, the number of layers of metal may 
exceed three, depending upon the type of seam used. As shown in FIG. 13, 
the leading end of the duct comprises two layers resulting from the end 
being turned in by the groove 56 and rolled back by the surface 64 of the 
forming wheel 58 and then flattened down by roller 72. This produces a 
leading end which is smooth, eliminating any sharp edges that could make 
manual handling hazardous. 
The next step is to deactivate cylinder 76 which raises roller 72 
vertically, to deactivate the cylinders 48 to withdraw the forming plate 
52, to deactivate the driving means rotating the mandrel, and to 
deactivate cylinder 32 allowing the elastomeric gripping layer 38 to 
collapse and allowing the duct to be removed from the mandrel 26. Assuming 
that the apparatus shown in FIGS. 9-11 has made a female reformed end 
which might typically be about 11/4 inches long, the next step preferably 
would be to reform the opposite end of the duct to form a male end. It is 
desired that the male end portion be longer than the female end portion, 
e.g. about 21/2 inches long typically. 
Reforming of the longer male end could be done on a second machine 
identical to the machine shown in FIG. 9 except having a wider roller 72, 
or the roller 72 on the machine in FIG. 9 could be replaced with a wider 
roller needed for reforming the male ends. After the male end has been 
reformed in the same manner as the female end, it is desirable to taper 
the extremity of the male end to facilitate its entry into a female 
reformed end of a second duct. In the preferred embodiment, the male end 
can be tapered by crimping in the manner disclosed in U.S. Pat. Nos. 
3,708,867 and 3,758,139 to produce a reformed male end as shown in FIG. 8. 
The crimped portion 12 is preferably about 11/4 inches long and the 
flattened reformed male end portion 10 is also about 11/4 inches long. 
These dimensions are merely a matter of choice and could be varied to suit 
any particular installation. 
FIGS. 14 and 15 show the preferred apparatus of the present invention. FIG. 
14 is a partial cross-sectional and partially broken elevational view of a 
portion of the apparatus. The remainder of the apparatus is shown in end 
view in FIG. 15. 
Referring to FIG. 14, the apparatus comprises a main stationary frame 82 
and a second movable frame 84. The second movable frame 84 is carried on 
two rods 90 by way of supports 86 and bushings 88. The rods 90 are 
supported on the main frame 82 by rod holders 92. The movable frame 84 is 
moved back and forth along the length of rods 90 by moving the arm 94. 
Connected to the arm 94 is a threaded tubular member 96 which surrounds a 
spirally grooved shaft 98 rotatably mounted in bearings 100 and 102. The 
spirally grooved shaft 98 can be rotated clockwise or counter-clockwise by 
rotating a sprocket 104 fixedly attached to the shaft 98. The sprocket 104 
is rotated by a drive chaim 106 connected to a reversible drive 108. 
Preferably the tubular threaded member 96 and shaft 98 are a conventional 
Saginaw screw and a tubular recirculating ball housing assembly. The 
sprocket 104 preferably is a combination sprocket and torque limiter. 
Mounted on the movable frame 84 is a removable and rotatable mandrel 110. 
The mandrel 110 is held in the proper position, on a mandrel hub 112, by 
pins 114 rigidly attached to the hub 112 and extending through centering 
holes 116 in a back plate 118 of the mandrel 110. The back plate 118 is 
held against the hub 112 by any suitable means such as a pneumatic or 
hydraulic cylinder 120 acting through a rod 122, a thrust bearing 124, and 
a nut 126 threaded onto a threaded end portion 128 of rod 122. The back 
plate 118 can also be properly positioned and held against the hub 112 by 
any suitable fastening means such as conventional bolts, screws, etc. The 
fastening means should be readily removable to allow the mandrel to be 
removed and a different sized mandrel inserted quickly. 
The mandrel 110 and the mandrel hub 112 are held in place and rotated by a 
mandrel support assembly 130. The mandrel support assembly comprises a 
support frame 132 which is mounted to the linearly movable frame 84 by 
brackets 134. The mandrel support frame 132 supports a hollow tubular 
member 136 which is fixedly attached to the hub 112 in a rotatable manner 
using conventional bearing assemblies 138 and conventional seals 140. 
Preferably, the rotatable tubular member 136 has removable machined 
members 142 on each end which act as smooth surfaces for the seals 140 to 
ride against and also to help hold the bearings 138 in place. 
The mandrel 110 is rotated by a drive 144 which is mounted on the movable 
frame 84. The drive 144 rotates the tubular member 136 by rotating a 
sprocket 146 mounted on the end of the tubular member 136 by way of a 
conventional drive chain 148. 
An elastomeric gripping layer 148 is held against a front plate 150 of the 
mandrel 110 by an outer rigid plate 152 in a similar manner as the 
apparatus previously described and shown in FIGS. 9-11. 
A rigid heavy plate 153 is attached to the main frame 82 by way of angled 
supports 154 and bolts 156. A removable edge forming face 158 is mounted 
to the heavy plate 153 by any suitable manner, such as counter-sunk 
flathead machine screws. The forming plate 158 contains a groove of the 
same type as groove 56 in plate 52 shown in FIG. 9. Forming plate 158 
should be easily removable because forming plates having different inside 
diameters are necessary when different sized mandrels are used to 
accommodate different diameter ducts. 
All of the reforming equipment is not shown on FIG. 14 in order that the 
portion of the apparatus shown in FIG. 14 could be better illustrated. The 
remainder of the reforming equipment is shown in FIG. 15, which is an end 
view of the apparatus shown in FIG. 14 looking from left to right. Mounted 
on the heavy plate 153 is at least one, and preferably four, end portion 
rolling assemblies 159, preferably mounted 90.degree. from one another 
around the periphery of the mandrel. Each roller assembly 159 comprises a 
roller 160 which is like, and for the same purpose as, roller 72 shown in 
FIG. 11. The roller 160 is free to rotate on a shaft 162 extending through 
a clevis support 164. The clevis support is guided and maintained in 
proper alignment by the guide members 166. The roller 160 is moved towards 
or away from the mandrel surface by activation of a cylinder 170 which 
extends or withdraws a rod 168 attached to the clevis 164. 
The cylinder 170 is fixedly attached to a mounting plate 172 which contains 
two grooves 174 for mounting bolts 176. These grooves allow the roller 
assemblies 159 to be moved towards or away from the axis of the mandrel to 
compensate for different diameter mandrels and different diameter ducts. 
Only portions of the other roller assemblies 159 are shown because they 
are identical to the roller assembly shown in full. 
Also mounted on the heavy plate 153 is a leading edge forming roller 
assembly 177 comprising a roller 178 which is identical to the roller 58 
shown in FIG. 9. Only one leading edge forming roller assembly 177 is 
necessary and, except for the roller 178, it is constructed and mounted in 
an identical manner as the end portion rolling assembly 159. 
In operation of the apparatus shown in FIGS. 14 and 15, the drive 108 is 
actuated in the proper direction to move the frame 84 and the mandrel 110 
to the left (FIG. 14) a desired distance. A duct of the type shown in FIG. 
2 is then pushed over the mandrel and against the forming face 158. The 
other end of the duct can be supported in any conventional means to permit 
rotation of the duct around its horizontal axis, for example a vertically 
adjustable set of cradle rolls. Next, cylinder 120 is actuated to pull the 
plate 152 against the elastomeric gripping layer 148 to cause it to expand 
in diameter and securely grip the inside of the duct to prevent that 
portion of the duct adjacent the end portion from moving along the mandrel 
surface in the next step. Drive 108 is then actuated to cause the frame 
84, the mandrel 110 and the duct secured thereto to move to the right 
compressing the end portion of the duct as shown in FIG. 14. This latter 
action linearly compresses the end of the duct against the forming plate 
158 forming a duct of the type shown in FIG. 3. The drive 108 is 
inactivated as soon as the desired amount of linear compression is 
achieved. 
Drive 144 is now actuated causing the mandrel 110 and the duct thereon to 
rotate, typically at about 10 to 12 rpms. The thrust bearing 124 permits 
the mandrel 110 and the plate 152 to rotate while the rod 122 remains 
stationary. Rotation of the duct and its leading edge against the wear 
plate 158 and the groove therein, like groove 56 in FIG. 9, causes the 
leading end of the duct usually to be flared outwardly, but because of the 
nature of the thin-walled ducts, part of the circumference may be turned 
inwardly. As long as the leading edge is bent over and rolled down the 
direction of the bend is not critical. Cylinder 180 is then actuated to 
extend rod 182 and clevis 186 guided by guides 184 which lowers the 
leading edge forming roller 178 against the flared out or turned in end of 
the duct while the roller 178 rotates on the shaft 188. This causes the 
flared out or turned in end of the duct to be turned back upon itself as 
shown in FIGS. 10 and 13. Either simultaneously or after roller 188 has 
been lowered into its functional position, one or more units 159 are 
actuated in a similar manner to lower rollers 160 against the ribs of the 
duct to reform the duct end in the manner shown in FIG. 12. After 
sufficient revolutions of the duct to produce the desired flattened duct 
end, usually about two revolutions, the units 159 are actuated to withdraw 
the rollers 160, and the cylinder 180 is actuated to withdraw the roller 
178. The drive 144 is stopped, stopping the rotation of the mandrel 110, 
cylinder 120 is deactuated allowing the elastomeric layer 148 to assume 
its original diameter thus releasing the duct, and the duct having a 
reformed end portion is removed from the mandrel. 
If the reformed end is intended for a female end the reforming of that end 
is complete. If the reformed end is intended as a male end, it is 
necessary to further reform the extremity of the reformed end to achieve a 
male end as shown in FIG. 8 on another apparatus as disclosed in U.S. Pat. 
Nos. 3,708,867 and 3,758,139. 
The above described operation of the apparatus of FIGS. 14 and 15 is 
typical and can be varied. For example, the rotation speed of the mandrel 
will depend to a certain extent on the diameter of the duct being formed, 
with smaller duct permitting a faster rotational speed than larger duct to 
achieve the same linear reforming speed on the surface of the duct. Also, 
the same machine can be used to form female ends and the wider male ends 
by placing male rollers on two of the opposed reforming assemblies 159 and 
female rollers on the remaining two opposed assemblies 159 and actuating 
only the appropriate opposed assemblies. 
The apparatus shown in FIGS. 14 and 15 can be operated in a different 
manner to produce a different wall structure of the type shown in FIGS. 6 
and 7. In the wall structure shown in FIGS. 6 and 7, the compressed ribs 
are bent over and flattened instead of being mashed down and flattened in 
the embodiment previously described and shown in FIG. 5. 
To produce the embodiments shown in FIGS. 6 and 7, operation of the 
apparatus of FIGS. 14 and 15 is as follows. After the duct has been 
linearly compressed a sufficient amount by actuation of the drive 108, 
drive 144 is actuated to begin rotation of the mandrel and the duct 
thereon. Cylinder 180 is actuated to lower the roller 178 into position to 
roll the flared or tuned in leading edge of the duct back upon itself. 
After this objective is accomplished, cylinder 180 is activated to raise 
roller 178 to its original position. Drive 108 is then actuated to move 
the mandrel 110 to the left, looking at FIG. 14, a sufficient distance to 
permit rolles 160 to be lowered against the surface of the mandrel 110 
without contacting the duct. Units 159 are then actuated to extend rollers 
160 so that the roller surface is spaced from the surface of the mandrel 
an amount essentially equal to the desired thickness of the wall in the 
reformed end portion of the finished duct. With the mandrel 110 rotating 
and the rollers 160 in this position, drive 108 is then actuated to move 
the mandrel to the right, looking at FIG. 14. The edge of the roller 160 
facing the ribs in the end portion of the duct has a radius, such as that 
shown on roller 72 in FIG. 13. As the rounded edge of the roller contacts 
the ribs as the mandrel moves to the right, the ribs are first bent over 
in a gradual but progressive manner and finally flattened by the flat 
portion of roller 160. The mandrel is continued to be moved to the right 
by drive 108 until all of the ribs in the linearly compressed end portion 
of the duct have been bent over and flattened in the manner shown in FIGS. 
6 and 7. In the cross-section shown in FIG. 7 the three layers of metal 
form a series of generally flattened Z-shaped members. Drive 108 is then 
stopped, assemblies 159 are actuated to retract rollers 160, cylinder 120 
is deactuated to release the duct from thwe mandrel, and the duct is 
removed from the mandrel. 
In describing the invention certain embodiments and reformed metal 
configurations have been used to illustrate the invention and the practice 
thereof. However, the invention is not limited to these specific 
embodiments and configurations as other embodiments or configurations, and 
modifications within the spirit of the invention will readily occur to 
those skilled in the art on reading this specification. The invention is 
thus not intended to be limited to the specific embodiments disclosed, but 
instead is to be limited only by the claims intended hereto.