Note: Descriptions are shown in the official language in which they were submitted.
~2~
1651.028 METHOD OF ~ORMING A THIN WALLED
ANNULAR CHANNEL
Field of the Invention
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The method or forming process of this invention relates generally
to sheet metal forming processes, wherein the work piece is forced between
opposed convex and concave spaced die surfaces. More particul~rly, the me~hod
of this invention is adapted to form a thin walled semi-toroidal shell from flatsheet stock, wherein the sheet is first formed into ~ cylindrical band, welded and
the band is then curled inwardly and compressed by the die surfaces, thus
eliminating tensile fracture of the weld and Accurately conforming the workpieceto the die surfaces.
Description of the Prior Art
The method of this invention is ad~pted to form thin w~lled
annular channels, generally U-shaped in cross-section or laterally bisected
toroid~l shells. Such shells are presently used in hydromatic couplings or
converters, wherein radially extending blades or fins are secured in slots in the
exterior surface of the shells to form a rotor. Two rotors sre used in an
automotive torque converter in open face-to-face relation, forming a finned
toroid. The exterior surface configuration of the semi-toroidal shells are thus
critical to proper functioning of the torque converter. For example, the exterior
tolerances of the semi-toroidal shell used in an automotive torque converter
must be maintained to plus or minus 5% cf the material thickness. Thus, in &
toroidal shell having a thickness of 0.040 inches, the tolerance must be
maint~ined to plus or minus 0.002 inches.
1651.028 Toroid~l shells are presently formed commercially by cutting or
stamping a washer-shaped blank from flat rnetal sheet stock, resulting in a
substantial waste of material~ which increases the cost of the shell. The blank is
then formed into a semi-toroiàal shell in a conventional die stamping process.
The method of this invention substantially eliminates scrap, thus
reducing the cost of the shells. United States Patent No. 3,851,517, assigned toGeneral Motors Corporation, discloses a process of forming semi-toroidal shells
from flat strip stock, which would have eliminated scrap, but was unsuccessful
commercially. The process disclosed in this patent, which is incorporated hereinby reference, utilizes a rectangular sheet metal blank, which is rolled into a
cylinder and welded. The end of the cylinder is then forced into a first pre-form
die set to initially turn outwardly or flare the top edge of the cylinder. The
flared end of the cylinder is then forced into a second curling die set having
spaced concave and matching convex outwardly curved die surfaces, curling the
cylinder outwardly under tension and forming a semi-toroidal shell. Finally, theshell is formed to its final configuration in a third re-strike die set.
The weldment had to be accurately controlled, planished and
smoothed prlor to forming because the weld is placed under extreme tensile
stress during the curl forming operation. Nevertheless, the process disclosed inthe above-referenced patent was unsuccessful commercially because the weld
failed or fractured repeatedly under the tensile stress created by curling the
cylinder outwardly and the method was finally abandoned. Further, the
workpiece tended to pull away from the concave die surface under the tensile
forces generated by the curling operation, making it difficult to control the
critical exterior tolerances of the shell.
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1651.G28 The method of this invention eliminates the problems inherent in
the above-referenced patent by curling the cyLindrical band inwardly, thereby
placing the weldment in compression. In fact, the semi-toroidal shell may be
formed by the method of this invention without welding or by joining the abutting
ends of the bQnd by other means, making the process suitable for other
applications. Further, the shell may be formed in one stroke of the press1
thereby eliminating ~he requirement for pre-forming a flared end on the cylinderas required in the abov~referenced patent.
SUMMARY OF THE INVENTION
The method of forming a thin walled semi~toroidal shell of this
invention includes forming a flat rectslinear sheet into a cylindrical band and
preferably joining the abutting free ends of the band, particularly for the above-
described applicstions of the semi-toroidal shell. One end of the cylindrical band
is then forced into a die set having a cylindrical opening smoothly blending into
an inwardly curving semi-toroidal space defined by opposed concave al!d convex
spaced die surfaces. The diameter of the cylindrical band is equal to the nominal
diameter of the die set opening which is located at the radial outer extent of the
die surfaces. As the cylindrical band is forced into the toroidal space betwee
the concave and convex die surfaces, the band is progressively curled inwardly
wlder compression to conform to the semi-toroidal space. The cylinder is thus
placed under compressive force and the outer toroid~l surface of the workpiece
accurately conforms to the concave die surface.
In the preferred method of forming a toroidal shell suitable for
hydromatic couplings or converters, such as automotive torque converters, the
free ends of the band sre preferably welded. The weld is thus wiped and
1651.028 compressed against the annular concave clie surf~e as the band is progressively
curled inwardly, accurately conforming the exterior welded surface to the
eoncave die surface.
The weldmen$ is thus not subiect to tensile fracture as in the prior
art method discussed hereinabove. As understood, a weld is not as pure as the
parent metal and a weld is subject to fracture in commercial ~pplications when
the weld is expanded approximately twenty percent. There is no known limit tG
compressi-/e forces. Further, the semi-toroid~ shell may be formed by the
method of this invention in one stroke of the press, eliminating the requirementfor pre-forming a nared end on the cylinder, although the semi-toroidal shell
may be restruck in a restrike die to form a more complex configuration. Finally,the critical exterior surface of the toroidal shell is easily held within close
tolerances because the exterior surface accurately conforms to the concave die
surface under the compressive curling forces.
Other advantages and meritorious features of the method of this
invention will be more fully understood from the following detailed description of
the method of this invention, the appended elaims and the drawings, a brief
description of which follows.
BRIEF I~ESCRIPTION OF THE DRAWINGS
Figure 1 is a side elevation of a semi-toroidal shell formed by the
method of this invention;
Figure 2 is a side elevation of a cylindric&l band which forms the
semi-toroidal shell of Figure 1;
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1651.028 Figure 3 is a side, partially cross~ectioned view of one
embodiment of a dle set used in the method of this invention with the ram
starting to form the cylindrical b~nd of Fi~ure 2;
Figure 4 is a side cross-sectionRl view of the die set shown in
Figure 3 with the ram lowered, forming the semi-toroidal shell shown in Figure 4;
Figure 5 is the semi-toroidal shell of Figure 1 after restriking;
Figure 6 is a partiRl side cross-sectional view of one embodiment
of a restrike die utilized to form the semi-toroidal shell of Figure 5;
~iigure ~ is a si~e cross-sectionP~ ~Tiew o~ the semi-toroidsl sh~ll
s)lown in ~igure ~; and
Figure 8 is a side cross-sectional view of the semi-toroid~ shell
shown in Figure 5.
DESCRIPTION OF THE PREFERRED METH(:)D OF THIS INVENTIC)N
Figure 1 illustrates Q thin walled annular channel or semi-toroidal
shel120 which may be formed by the method of this invention. The shell includes
an outer cylinricPl lip 22, A U-shaped mid portion 24, and an inner lip 26. The
shell is formed from a cylindrical band 30, as shown in Figure 2. The cylindrical
band may be formed from a flat rect inear strip which is rolled and welded as at28, which forms a weld in the semi-toroidal shell, as shown in Figure 1. The
method of forming a cylindrical band as shown in Figure 2 is described in more
detail in the abov~referenced patent assigned to General Motors Corporation.
d ~
1651.028 The semi-toroidal shell of Figure 1 may be formed by the die set
assembly 34 shown in Figures 3 and 4. The die set assembly includes an ~nnular
cylindrical ram 36 which is received in the cylindrical bore 40 of riser 38. Theriser includes a conical counte~bore 42 which receives the cylindrical band 30,
when the rflm 36 is raised above the position shown in Figure 3. The ram includes
an snnular lip 44 which engages the cylindrical band 30 and an end portion 46.
The cylindrical b~nd is received in a curl die 50 haYing an annular
cylindric~l s~face 52 which first receives the cylindricsl band and an ann~llar
concave surface 54 terminating in a cylindrical bore 56. An annular forming pos~~0 is received within the curl die 50 having a cylindrical exterior surface 62
opposite the cylindrical surfaee 52 of the curl die find an annular convex surface
64 opposite the concave surface 54 of the curl die. The forming post and curl die
thus form Q ll-shaped annular space which receives the cylindrical band 30, as
described hereinbelow.
The die set assembly nlso includes a static cylindrical center post
66, a spring pad 68 and a plurality of helical coil springs 70 which are biased
~gainst a cylindrical insert 58 in the forming post 60. A cente~ guide post 72
having a cylindrical exterior surface receives the ram in telescopic relation and
the ram includes ~ conical flange 74, which receives die pins 76. The die pins are
bolted or otherwise secured to the upper die platten, not shown. In the disclosed
ernbodiment, there are four die pins 76 and four springs 70. It will be understood
by those skilled in the art that only half of the die assembly is shown to illustrate
the method of this invention and the die assembly normally includes a lower die
pletten which supports the curl die 50 on a conventional bolster assembly, not
shown. Reference may also be made to the above-referenced Untied States
patent.
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1651.028 The method of this invention thus includes forming the cylindric~l
band 30 of Figure 2 from flat strip stock flnd welding the free ends of the strip,
as shown at 28. As described above, the free ends of the band m~y ~so be joined
by o$her means, including Tig welding, which includes fusing points of the band or
other means. In fact, the band may be formed into a U-shaped annular channel
with~ut joining the abutting free ends of the band because the method of this
invention compresses the b~nd during forming.
One end of the cylindrical band 30 is then forced into the semi-
toroidal space defined by the curl die 50 ~nd forming post 60. In the prefe~red
-embodiment, the opening to the space is cylindricsl, as defined by the opposed
cylindrical surf~ces 52 and 62. As shown, the cylindrical opening smoothly blends
into the semi-toroidal space defined by the annular concave and convex surfaces,54 and 64 respectively, of the curl die 50 and forming post 60, respectively. The
nominal diameter of the cylindrical band 30 is equal to the nominal diameter of
the cylindrical opening between the curl die 50 and the forming post 60. The
opposed end of the cylindric~l band is engaged by the annular lip 44 of the ram.Lowering of the ram 36 thus forces the cylindrical band downwardly in Figure 3,
progressively curling the band r~dially inwardly between the curl die and forming
post, compressing the cylindrical band and the weld 28, as shown in Figure 4.
The semi-toroidal shell 20 is thus formed in one stroke of the press, elimin~ting
the requirement of a pre-forming step, 8S described in the abov~referenced
patent.
As described above, the method of this invention results in ~ fifty
percent saviogs in material costs over the present method which includes die
forming the semi-toroidal shell from a washer-shaped blank. The exterior
surf~ce of the semi-toroidal shell accurately conforms to the concave surface 54
1651.028 of the curl die because the b~nd is curled inwardly, compressing ~he band and
conforming the band to the curl die. I~Iore inpo-rt~ntly, the weld 28 is placed in
compression, eliminating tensile fracture of the weld as experienced in the
method described in the above-referenced patent. Further, the exterior surf~ce
of the weld 28 is wiped against the ConCQve surface 54, accurately conforming
the weld to the concave surface. Because the band is placed in compression
during forming, r~ther than tension, the band does not tend to pull away from the
exterior die surfaces, maintaining the critical exterior surface within tolerance.
The semi-toroidsl shell 20 may be formed in a further operation
depending upon the application. For example, a core ring impeller for an
automotive hydromatic coupling or converter normally includes a cylindrical
inner surface which receives a shaft, as shown at 80 in Figure 5. This
configuration may be formed in a restrike die set, as shown in Figure 6. The
semi-toroidal shell 80 shown in Figure S includes a cylindrical inner lip portion 82
and an outer semi-toroid~l portion 84.
The restrike die ~ssembly includes a female die member 86 which
corresponds to the curl die 50 of Figures 3 and 4, having an annular concave
surface 88 which receives the st~tic cylindrical center post 92. The forming post
94 includes ~ annular ~onvex swrface 96 and ~ cylindricsl surf~ce 98. As
described ~bove in regard to Figures 3 ~nd 4, the female die 8~ and forming post94 are spaced to define a semi-toroidsl space therebetween and the she~l 80 is
formed by the die members.
ln the disclosed embodiment of the restrike die assembly shown in
Figure 6~ the shell is retained by an outer retainer ring 100 having an annular lip
102 snd an inner retainer ring 104 having an annul~r lip 106. The semi-toroida~
shell 20 shown in Figure 1 is inserted into the female die member 86 and forming
l ~ & X
1651.028 post 94 and retainer rings 100 and 104 are lowered into the femPle die to form
the final configuration of the semi toroidal shell 80 shown in Figure 5O The
change in the shell confi~uration is best understood by comparing Figures 7 and
8, wherein Figure 7 illustrates the cross--sectional configuration of the semi-
toroidal shell 20 of Figure 1 and Figure ~ illustrates the cross-sectional shape of
the semi-toroidal shell 80 OI Figure 5. The restrike die assembly expands the
shell, reforming the cylindric~ outer lip 22 into a continuous outer semi-toroidal
portion 84, as shown in Figure 8, and the inner lip 26 is reformed into a
cylindric&l lip 82, as shown in Figure 8.
The method of this invention thus substantially reduces the cost of
forming a se~mi-toroidfll sheLI by forming the shell from a rectangular strip ofsheet material, which is rolled and welded as shown in Figure 2. This results in Q
substantial saving in materiPl. Further, the shell is formed in one stroke of the
press, as shown in Figures 3 and 4, without imparting tensile stress to the weld28. The shell may be formed from various materials, depending upon the
application. For example, a core ring impeller may be formed from 1010 or 1018
shee~t steel having a thickness of, for example, 0.040 inches. As described above,
the weld 28 is placed in compression during the curl forming process shown in
Figures 3 and 4. The external lip 22 of the shell is expanded in the restrike die
set assembly of Figure 6, however, the expansion is less than ten percent to aYoid
fracture of the weld. The method of this invention thus avoids tensile forces onthe weld which would fracture the weld in production while forming the complex
semi-toroidsl shell configuration shown in Figure 5.
