U.S. patent application number 13/303065 was filed with the patent office on 2013-05-23 for die with multi-sided cavity for self-piercing riveting process.
The applicant listed for this patent is Aindrea McKelvey Campbell, Michael William Danyo. Invention is credited to Aindrea McKelvey Campbell, Michael William Danyo.
Application Number | 20130125611 13/303065 |
Document ID | / |
Family ID | 48222233 |
Filed Date | 2013-05-23 |
United States Patent
Application |
20130125611 |
Kind Code |
A1 |
Danyo; Michael William ; et
al. |
May 23, 2013 |
DIE WITH MULTI-SIDED CAVITY FOR SELF-PIERCING RIVETING PROCESS
Abstract
A die member is provided for a self-piercing riveting process,
the die member comprising a die cavity having an axis and a
plurality of sides positioned about the axis, wherein each side
extends along a plane which includes a chord connecting two points
along a circle centered on the axis.
Inventors: |
Danyo; Michael William;
(Trenton, MI) ; Campbell; Aindrea McKelvey;
(Beverly Hills, MI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Danyo; Michael William
Campbell; Aindrea McKelvey |
Trenton
Beverly Hills |
MI
MI |
US
US |
|
|
Family ID: |
48222233 |
Appl. No.: |
13/303065 |
Filed: |
November 22, 2011 |
Current U.S.
Class: |
72/470 |
Current CPC
Class: |
B21J 15/025 20130101;
Y10T 29/49915 20150115; Y10T 29/53735 20150115; Y10T 29/5343
20150115; Y10T 29/49835 20150115; Y10T 29/49837 20150115; B21J
15/36 20130101; Y10T 29/49956 20150115 |
Class at
Publication: |
72/470 |
International
Class: |
B21D 37/14 20060101
B21D037/14 |
Claims
1. A die member for a self-piercing riveting process, the member
comprising a die cavity having an axis and a number N of flat sides
positioned about the axis, wherein each side lies along a plane
including a chord connecting two points along a circle centered on
the axis, and wherein an included angle between radii extending
from the axis to opposite ends of any chord is equal to 360/N
degrees.
2. The die member of claim 1 wherein each plane along which an
associated side extends is parallel to the axis.
3. The die member of claim 1 wherein the die member further
comprises a bearing surface, wherein the die cavity has a floor,
and wherein each plane along which an associated side extends is
perpendicular to a plane defined by the bearing surface and is also
perpendicular to a plane defined by cavity floor.
4. The die member of claim 1 wherein the die member further
comprises a bearing surface, wherein the die cavity has a floor,
and wherein a plane along which at least one side of the plurality
of sides extends is angled toward the axis in a direction
proceeding from the bearing surface toward the floor.
5. A die member for a self-piercing riveting process, the die
member comprising a die cavity formed in the die member, a
perimeter of the cavity being formed by a plurality of sides and a
plurality of fillet radii, each end of each side being connected by
a fillet radius to an adjacent side at an end of the adjacent side,
wherein each radius r has a value within the range 0.25 mm-3.25 mm
inclusive.
6. The die member of claim 1 wherein the number of cavity sides is
within the range of 3-20 inclusive.
7. The die member of claim 6 wherein each side has a length, and
wherein the lengths of all of the sides are equal.
8. The die member of claim 1 wherein the number of cavity sides is
equal to twelve.
9. The die member of claim 16 wherein the number of cavity sides is
within the range of 3-20 inclusive.
10. The die member of claim 5 wherein the number of cavity sides is
within the range of 3-20 inclusive.
11. The die member of claim 5 wherein each of the sides is
straight.
12. A die member for a self-piercing riveting process, the die
member comprising: a bearing surface; a die cavity formed in the
bearing surface, the die cavity including a floor and a central
axis extending through the floor; a plurality of flat sides
extending between the floor and the bearing surface, wherein at
least one of the sides is sloped away from the axis in a direction
proceeding from the floor toward the bearing surface.
13. The die member of claim 12 wherein the number of cavity sides
is within the range of 3-20 inclusive.
14. The die member of claim 12 wherein the number of straight
cavity sides is equal to twelve.
15. The die member of claim 12 wherein each of the sides is
straight.
16. A die member comprising comprising a die cavity having an axis
and a plurality of flat sides disposed about the axis, each side
intersecting a chord connecting two points along a circle centered
on the axis, the cavity being structured to receive therein a
portion of a die button responsive to driving of a self-piercing
rivet into a first panel of a plurality of panels stacked over the
die cavity.
17. The die member of claim 5 wherein all of the wall portions are
flat wall portions.
18. The die member of claim 16 wherein a distance from a plane
defined by the bearing surface to a plane defined by the cavity
floor and measured along a plane extending parallel to the central
axis is within the range of 1.95 mm to 3.30 mm inclusive.
19. The die member of claim 16 wherein the die cavity has six flat
sides disposed about the axis.
20. The die member of claim 5 wherein the number of sides is equal
to six.
Description
BACKGROUND OF THE INVENTION
[0001] The embodiments of the present invention relate to a
self-piercing riveting process and, more particularly, to a die
member for use in a self-piercing riveting process.
[0002] In the joining of components used in high volume vehicle
production, it may be desirable to use mechanical fasteners to help
achieve the required strength and durability of joints. One type of
mechanical fastener used in vehicle production is a self-piercing
rivet (SPR).
[0003] The general principles of self-piercing rivet technology are
known in the art. To apply a self-piercing rivet to workpieces to
be joined, a portion of a first workpiece or panel is placed upon a
bearing surface of a die member, so as to overlie a die cavity
formed in the die member. Portions of one or more additional panels
are then stacked on the portion of the first panel overlying the
die cavity. The panels are secured in position with respect to each
other and with respect to the die member, to prevent relative
movement of the parts during application of the rivet. The die
cavity may also contain a die post which assists in forcing a
portion of the rivet to spread or deflect radially outwardly when
pressure sufficient to pierce the first workpiece is applied to the
rivet. The rivet also pierces surfaces of the second panel
overlying the first panel. In a known manner, up to four layers of
material may be joined using existing SPR processes.
[0004] During application of the rivet to the workpieces to be
joined, a feature known as an SPR "button" is produced. This SPR
button is in the form of a protrusion in a surface of the second
panel along a side of the second panel opposite the side pierced by
the rivet. One of the challenges encountered during SPR joining is
the nucleation and propagation of cracks on the "button" side of
the joint, along corners of the button shaped by the floor and
walls of the die cavity during the SPR operation. The presence and
size of these cracks can affect the quality of the joint and the
viability of SPR technology as a fastening option.
[0005] Thus, a need exists for a die geometry in which crack
formation in the rivet material along the SPR button during
formation of the button is reduced or minimized.
SUMMARY OF THE INVENTION
[0006] In one aspect the embodiments of the present invention, a
die member for a self-piercing riveting process includes a die
cavity having an axis and a plurality of sides positioned about the
axis. Each side of the die cavity extends along a plane which
includes a chord connecting two points along a circle centered on
the axis.
[0007] In another aspect the embodiments of the present invention,
a die member for a self-piercing riveting process includes a die
cavity formed in the die member. A perimeter of the cavity is
formed by a plurality of sides and a plurality of fillet radii.
Each end of each side of the die cavity is connected by a fillet
radius to an adjacent side of the cavity at an end of the adjacent
side.
[0008] In another aspect the embodiments of the present invention,
a die member for a self-piercing riveting process includes a
bearing surface and a die cavity formed in the bearing surface. The
die cavity includes a cavity floor and a central axis extending
through the cavity floor. A plurality of cavity sides extends
between the cavity floor and the bearing surface. At least one of
the sides is sloped away from the axis in a direction proceeding
from the floor toward the bearing surface.
[0009] In another aspect the embodiments of the present invention,
a die member for a self-piercing riveting process includes six wall
portions, each end of each wall portion being connected to an
adjacent wall portion at an end of the adjacent wall portion.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] In the drawings illustrating embodiments of the present
invention:
[0011] FIG. 1 is a cross-sectional view of a self-piercing rivet
usable with a die member in accordance with an embodiment of the
present invention for joining portions of a pair of stacked
panels.
[0012] FIG. 2 is a perspective view of a portion of a multi-sided
die member in accordance with one embodiment of the present
invention.
[0013] FIG. 3 is a cross-sectional view of the portion of the
multi-sided die member shown in FIG. 2.
[0014] FIG. 4 is a plan view of the portion of a die member shown
in FIG. 2, showing positions of die cavity sides or wall portions
along a perimeter of the die cavity.
[0015] FIG. 5 is a plan view of a portion of a multi-sided die
member in accordance with another embodiment of the present
invention.
[0016] FIGS. 6-9 show a sequence of operations in applying a
self-piercing rivet to a pair of panels to join the panels.
[0017] FIG. 10 is a plan view of a portion of a multi-sided die
member in accordance with another embodiment of the present
invention.
[0018] FIG. 11 is a perspective view of a portion of a multi-sided
die member in accordance with another embodiment of the present
invention.
DETAILED DESCRIPTION
[0019] The exemplary embodiments described herein provide detail
for illustrative purposes and are subject to many variations in
structure and design. It is to be understood that the phraseology
and terminology used herein are for the purpose of description and
should not be regarded as limiting.
[0020] The terms "a" and "an" herein do not denote a limitation as
to quantity, but rather denote the presence of at least one of the
referenced items. Also, use herein of the terms "including,"
"comprising," "having" and variations thereof is meant to encompass
the items listed thereafter and equivalents thereof as well as
allowing for the presence of additional items. Further, the use of
terms "first", "second", and "third", and the like herein do not
denote any order, quantity, or relative importance of the items to
which they refer, but rather are used to distinguish one element
from another.
[0021] Unless limited otherwise, terms such as "configured,"
"disposed," "placed", "coupled to" and variations thereof herein
are used broadly and encompass direct and indirect attachments,
couplings, and engagements. In addition, the terms "attached" and
"coupled" and variations thereof are not restricted to physical or
mechanical attachments or couplings.
[0022] Unless noted otherwise, similar reference numerals appearing
in views of different embodiments of the present invention refer to
similar elements. For example, reference numeral 59 in FIG. 3
refers to a side or wall portion of the die cavity 52 shown in FIG.
3, while reference numeral 59' in FIG. 5 refers to a side or wall
portion of the die cavity 52' shown in FIG. 5.
[0023] FIGS. 1-9 show one embodiment of an exemplary self-piercing
rivet 20 and a die member 50 usable in a self-piercing riveting
operation for securing a pair of stacked panels together. The
self-piercing rivet and die structure described herein may be
utilized in any application where rivets are presently used, such
as securing together panels and closures. As used herein, "panel"
refers to any plate, panel or metal sheet having a thickness
suitable for permitting piercing of the panel or a surface of the
panel with the rivet as described herein.
[0024] A self-piercing rivet and associated die member of the
embodiments of the present invention may be adapted for mass
production applications, including automotive applications.
Embodiments of the self-piercing rivet and die member disclosed
herein are suitable for installation and use in a conventional die
press, such as utilized by the automotive industry to join sheet
metal parts, including body panels and structural components. In
such applications, the press applies one or more self-piercing
rivets with each stroke of the press.
[0025] FIG. 1 shows an example of a self-piercing rivet 20 of known
construction. In the embodiment shown in FIG. 1, rivet 20 includes
a head portion 22 and a body portion 24 extending from the head
portion. Body portion 24 is at least partially hollow and includes
a base surface 24a spaced apart a distance d from the head portion
22, and an annular wall 24b surrounding the base surface 24a. Base
surface 24a and wall 24b combine to define a cavity 24c. In the
embodiment shown in FIG. 1, base surface 24a is concave.
[0026] An end 24d of wall 24b is formed into a cutting or piercing
surface configured to pierce a panel or workpiece in a manner known
in the art, when the wall end 24d is forced into contact with the
workpiece by application of a pressing force on the rivet 20. If
desired, an inner portion of wall 24b adjacent the wall end 24d may
be chamfered as shown in FIG. 1. Similarly, if desired, an outer
portion of wall 24b adjacent the wall end 24d may also be
chamfered. As is known in the art, self-piercing rivet 20 may be
formed from steel or any other suitable material, and may be
heat-treated for surface hardness, ductility, etc.
[0027] FIGS. 2-4 show various views of a die member 50 in
accordance with one embodiment of the present invention. The die
member 50 includes bearing surface 51 and a die cavity 52 formed in
the bearing surface. Bearing surface 51 supports portions of
workpieces 100 and 102 (FIG. 3) being joined by the riveting
operation. Cavity 52 includes an annular floor or die surface 56
surrounding a center die post 58. A central axis X of the die
cavity 52 extends through center post 58 and floor 56. If desired,
the die member may include a relief port (not shown) which permits
outflow of air which would otherwise be entrapped between the
second panel 102 and die cavity floor 56 during a riveting
operation, as described below. An outer surface 64 of the die post
tapers radially outwardly as it extends into the cavity toward
floor 56. Also, in the embodiment shown in FIGS. 2-3, surface 64
blends smoothly into the die cavity floor 56.
[0028] In one particular embodiment, die post outer surface 64
forms an angle J in the range of 9.5 degrees to 30.5 degrees
inclusive with respect to axis X. In another particular embodiment,
the die post is omitted from the die cavity. In this embodiment,
deformation of the rivet wall 24b is produced by pressure of the
wall against die surface 56.
[0029] In particular embodiments, a multi-sided or polygonal die
cavity 52 in accordance with the present invention has a plurality
of wall portions or sides 59 extending between die surface 56 and
bearing surface 51. Wall portions 59 are straight within the limits
of manufacturing tolerances.
[0030] In a particular embodiment, the depth of the die cavity as
measured from a plane defined by bearing surface 51 to a plane
defined by die surface 56 and along a plane extending parallel to
axis X is within the range of 1.95 mm to 3.30 mm inclusive.
[0031] Referring to FIG. 4, in a particular embodiment, a span S of
the die cavity between opposite straight sides when measured at the
bearing surface 51 is within the range of 6.95 mm to 12.05 mm
inclusive.
[0032] Referring to FIG. 4, in particular embodiments, the
arrangement of sides 59 along the die cavity perimeter for a given
number of sides may be defined by forming on the die member a
circle C' having a center C and a radius R, and extending a
plurality of angularly evenly spaced lines 200 outwardly from
center C to intersect the circle at intersection points P2. The
number of lines 200 extending from center C will be equal to the
number of sides 59 desired for the perimeter of cavity 52. Each
side 59 then extends along a plane which includes a chord C2 of
circle C' connecting adjacent points of intersection P2. As used
herein, the term "chord" is defined as a single straight line
segment joining two points on a curve. In FIG. 4, the curve is
circle C'. The particular embodiment in FIG. 4 illustrates the
layout of a six-sided or hexagonal die cavity having sides of equal
length.
[0033] In these embodiments, central axis X of the die cavity
extends through circle center C. Thus, axis X is spaced an equal
distance R from each point P2 at which adjacent chords C2
intersect, as shown in FIG. 4. In addition, as shown in FIG. 5,
equal angles .theta. facing into the die cavity are formed between
adjacent chords C2.
[0034] In the view shown in FIG. 4, and also in the embodiment
shown in FIG. 3, the plane along which the side 59 extends is
parallel to axis X and extends between a plane defined by bearing
surface 51 and a plane defined by floor 56. Also, in this
embodiment, it may be seen that a line L1 connecting the central
axis X with a point on the side 59 closest to the axis is
perpendicular to the side 59 at the point.
[0035] In a particular embodiment, the plane along which the side
59 extends perpendicular to a plane defined by the bearing surface
51 and is also perpendicular to a plane defined by cavity floor
56.
[0036] Referring to FIG. 10, in another embodiment, a plane along
which at least one of sides 59 extends is angled inwardly toward
axis X in a direction proceeding from bearing surface 51 toward
floor 56. This sloping of wall 59 facilitates extraction of the SPR
button from the die member 50. Sloping of the cavity wall(s) or
sides 59 from bearing surface 51 toward floor 56 may also be used
to reduce the radial distances from the axis X to the portions of
the wall(s) residing along or proximate the floor of the die cavity
(relative to the distances from axis X to portions of bearing
surface 51), thereby shortening the radial deformation or "spread"
of the SPR button within the cavity during button formation. It is
believed that this aids in avoiding or reducing the occurrence of
microcracks.
[0037] The procedure set forth above may be used to provide a die
cavity having any desired number of cavity sides of equal length
(taking into account manufacturing tolerances relating to the
lengths of the sides).
[0038] In addition, a fillet radius r is formed at each
intersection of adjacent wall portions 59 and extends along each of
the wall portion intersections between die surface 56 and bearing
surface 51. In one embodiment, each radius r has a value within the
range 0.25 mm-1.0 mm inclusive. In one particular embodiment, the
radii r have a value in the range of 0.75 mm to 3.25 mm
inclusive.
[0039] In one embodiment, sides 59 have equal lengths with equal
angles .theta. (again, within the limits of manufacturing
tolerances) formed between each two adjacent sides and facing into
the die cavity.
[0040] In one embodiment, as shown in FIGS. 2-4, a perimeter of
cavity 52 is in the shape of a six-sided polygon, or hexagon. In
the particular embodiment of a hexagon shown in FIGS. 2-4, sides 59
have equal lengths with equal angles of 120.degree. formed between
each two adjacent sides.
[0041] FIG. 5 shows a die member 50'' in accordance with another
embodiment of the present invention. In this embodiment, a
perimeter of die cavity 52'' is in the shape of an eight-sided
polygon, or octagon. In the particular embodiment of an octagon
shown in FIG. 5, sides 59'' have equal lengths with equal angles of
.theta.=135.degree. formed between each two adjacent sides. In this
embodiment, bearing surface 51'', die post 58'', and cavity floor
56'' are structured as previously described.
[0042] Referring to FIG. 11, in another particular embodiment 850,
the die cavity has twelve straight sides.
[0043] In alternative embodiments, rather than six or eight
straight sides, the die cavity 52 may have a greater number of
straight sides or a lesser number of straight sides, according to
the requirements of a particular process. Thus, while the above
examples described hexagonal and octagonal die cavities, a cavity
in accordance with an embodiment of present invention may have any
desired number of sides of substantially equal length, depending on
the properties and thicknesses of the materials to be joined, the
number of sheets to be joined, and other pertinent factors. In
particular embodiments, cavities having anywhere from three to
twenty sides, inclusive, are contemplated.
[0044] In addition, a radius r2 is formed at the intersection
between die surface 56 and each of wall portions 59. In one
embodiment, each radius is has a value within the range 0.25 mm-1.0
mm inclusive. In one particular embodiment, the radii r2 have
values in the range of 0.75 mm to 3.25 mm inclusive.
[0045] Referring to FIG. 10, in another particular embodiment, the
die member 50' includes bearing surface 51' and die cavity 52'
formed in the bearing surface. Die cavity 52' includes cavity floor
56' and central axis X' extending through the cavity floor. A
plurality of cavity wall portions or sides 59' extends between the
cavity floor 56' and the bearing surface 51'. A portion of at least
one of the sides 59' adjacent the bearing surface 51' is spaced a
first distance dl apart from the axis X. A portion of the at least
one of the sides 59' adjacent the floor 56' is spaced a second
distance d2 apart from the axis X. In this embodiment, the first
distance dl is greater than the second distance d2. Thus, in this
embodiment, one or more of sides 59' is sloped relatively outwardly
(i.e., away from axis X) in a direction proceeding from floor 56'
toward bearing surface 51'. This sloping of wall 59' facilitates
extraction of the SPR button from the die member 50'.
[0046] In a particular embodiment, all of the sides 59' of the
cavity are sloped outwardly as described above.
[0047] In the embodiment shown in FIG. 10, one or more of sides 59'
is sloped such that the a plane defined by the side forms an angle
Q with a plane K extending parallel to axis X' and along a line
defined by an intersection of the side plane and bearing surface
51'. In a particular embodiment, angle Q has a value within the
range of 0 degrees to 15.5 degrees inclusive.
[0048] Any of the embodiments of the die member described herein
may be formed from steel or any other suitable material or
materials.
[0049] FIGS. 6-9 are perspective views illustrating an assembly
sequence for joining portions of a pair of stacked panels 100 and
102 using a self-piercing rivet and complementary die member, in
accordance with one embodiment of the present invention. Where the
self-piercing rivets 20 are applied by a die press, the rivets may
be fed to an installation head (not shown) which is attached to one
platen of the die press. The installation head may include a punch
42 which having a bore or cavity (not shown) which receives the
head portion 22 of the rivet. The punch includes a driving surface
46 which is driven against the rivet head portion. Die member 50
may be attached to the opposite die platen (not shown) with the die
cavity 52 in coaxial alignment with the punch 42.
[0050] FIG. 6 shows the rivet 20 prior to contact with a first
panel or workpiece 100. Referring to FIGS. 1-4 and 6-9, in
operation, the rivet body portion 24 is driven into the first panel
100 in coaxial alignment with the central die post 58 of the die
cavity 52. In actual operation, the panels 100 and 102 may be
securely clamped to prevent movement of the panels relative to each
other and to prevent movement of panel 102 relative to bearing
surface 51.
[0051] FIG. 7 shows the rivet being driven into first panel 100. As
the body portion 24 is driven into the panel, the piercing surface
along annular wall 24b deforms and then pierces the surface of
first panel 100. Wall 24b also forces the unsupported portion of
second panel 102 into die cavity 52 and into engagement with die
post 58.
[0052] Referring to FIG. 8, when the unsupported second panel
portion contacts die post 58, further deflection of the second
panel portion is prevented, and the portion of the second panel
residing within the die cavity is now supported. Thus, further
motion of the rivet in the direction of arrow "A" causes the rivet
wall 24b to deflect radially outwardly as the wall 24b engages the
supported portion of second panel 102. As seen in FIG. 8, continued
downward deflection and radial spreading of the rivet wall 24b
produces a corresponding downward and radially outward deflection
of the portion of the second panel not supported by the die post
58, along the floor of the die cavity 52. This action produces a
"die button" or SPR button 150, which is defined as a protrusion in
a surface of the second panel along a side of the second panel
opposite the side along which the rivet is applied.
[0053] The rivet design, die member design, and process parameters
are specified so that rivet wall portion 24b does not pierce
completely through the thickness of second panel 102 during
formation of the die button. The portion of the second panel
deflected into die cavity 52 expands radially until it abuts cavity
wall portions 59. FIG. 9 shows the finished riveted joint after
withdrawal of the punch 42.
[0054] It is believed that crack nucleation in the rivet is related
to the lack of ductility which often exists in high strength alloys
(including aluminum based materials) from which the rivet may be
formed. It is believed that the cracks observed in SPR buttons
nucleate and grow after a critical stress or cumulative strain is
achieved in a given material. During self-piercing rivet processes,
material is displaced and is subjected to significant multi-axial
stresses and strains during SPR button formation within the die
cavity. Often, if cracks are initiated in the SPR button, the
cracks are observed along the button edge and surface. It is
believed that the largest cumulative strains in the rivet material
occur along surfaces of the button located the greatest distance
from the central axis of the die cavity, due to significant
material displacements required and due to the need for the die
cavity to accommodate the volume of the deformed rivet.
[0055] It has been found that the geometry of the die cavity can
play a significant role in controlling displacement of the rivet
material during formation of the SPR button. It is believed that an
SPR button formed in a multi-sided die cavity 52 defined as
described above using a circle C' with a radius R will experience
less crack formation than an SPR button formed in a circular die
cavity having the radius C'. The material of second panel 102 is
prevented from deforming uniformly radially outwardly by the
straight wall portions 59. Thus, rather than deforming to a
circular configuration having the uniform radius R of circle C',
the outer boundary of the SPR button acquires the shape of the
multi-sided die cavity 52. Thus, it is believed that use of
straight wall portions 59 in restricting or confining deformation
of the SPR button material aids in mitigating crack formation and
crack propagation along the outer surfaces of the SPR button
150.
[0056] It is also seen that, as the number of straight wall
portions forming the sides of die cavity 52 increases, the area of
the floor 56 of cavity 52 increases, more closely approaching the
floor area that would be provided with a circular cavity having the
radius R. This increase in floor area allows a relatively greater
radial expansion of the material forming the die button. Thus, in a
self-piercing rivet application in which the area or space that may
be occupied by the riveted joint is restricted, the die cavity
floor area available for expansion of the die button can be
maximized within a permissible circular joint area or die button
area .pi.R.sup.2 of circle C' while eliminating or mitigating crack
formation that would otherwise occur during uniform radial
expansion of the die button material.
[0057] The number of die cavity sides may also be specified so as
to take into account the cavity volume needed to accommodate a
given rivet size while still minimizing cumulative strain during
deformation of a rivet material having a given ductility. This
design flexibility with regard to die cavity dimensions also aids
in eliminating or mitigating crack formation.
[0058] The optimum configuration of wall portions 59 can be
determined iteratively and/or analytically to meet the requirements
of a particular application, based on factors such as rivet design,
panel materials and thicknesses, permissible SPR button area, and
other pertinent factors.
[0059] It will be understood that the foregoing description of the
present invention is for illustrative purposes only, and that the
various structural and operational features herein disclosed are
susceptible to a number of modifications, none of which departs
from the spirit and scope of the present invention. The preceding
description, therefore, is not meant to limit the scope of the
invention. Rather, the scope of the invention is to be determined
only by the appended claims and their equivalents.
* * * * *