U.S. patent application number 15/098826 was filed with the patent office on 2017-07-20 for self-piercing rivet.
The applicant listed for this patent is Henrob Limited. Invention is credited to Christopher James Clarke, Russell John Trinick.
Application Number | 20170204893 15/098826 |
Document ID | / |
Family ID | 43414674 |
Filed Date | 2017-07-20 |
United States Patent
Application |
20170204893 |
Kind Code |
A1 |
Trinick; Russell John ; et
al. |
July 20, 2017 |
SELF-PIERCING RIVET
Abstract
A self-piercing rivet (10) for forming joints in stacked sheets
of high or low strength, light alloy metals such as aluminium and
magnesium. The rivet (10) comprises a head (11) and a substantially
cylindrical shank (12) that is at least partially hollow so as to
define a bore (13) that extends along at least part of its length.
The outside diameter (D1) of the shank (12) is at least 6 mm, the
effective length of the rivet is at least 1.3 times the diameter of
the shank and the bore (13) has a volume that is at least 38% of
the effective solid volume of the rivet (10). The rivet geometry is
such that it has enhanced column strength to withstand the high
insertion forces required and a high bore volume to accommodate
displaced sheet material.
Inventors: |
Trinick; Russell John;
(Flintshire, GB) ; Clarke; Christopher James;
(Cheshire, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Henrob Limited |
Flintshire |
|
GB |
|
|
Family ID: |
43414674 |
Appl. No.: |
15/098826 |
Filed: |
April 14, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13884906 |
May 10, 2013 |
9316243 |
|
|
PCT/GB11/01566 |
Nov 9, 2011 |
|
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15098826 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B21J 15/025 20130101;
Y10T 29/49956 20150115; F16B 19/086 20130101; Y10T 29/49943
20150115; Y10T 29/5377 20150115; F16B 5/04 20130101 |
International
Class: |
F16B 19/08 20060101
F16B019/08; B21J 15/02 20060101 B21J015/02; F16B 5/04 20060101
F16B005/04 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 11, 2010 |
GB |
1019021.3 |
Claims
1. A self-piercing rivet comprising a head and a substantially
cylindrical shank that is at least partially hollow so as to define
a bore that extends along at least part of its length, wherein the
outside diameter of the shank is at least 6 mm, an effective length
of the rivet is at least 1.3 times the diameter of the shank and
the bore has a volume that is at least 38% of an effective solid
volume of the rivet.
2. A self-piercing rivet according to claim 1, wherein the bore
volume is in the range 38-50% of the effective solid volume of the
rivet.
3. A self-piercing rivet according to claim 1, wherein the bore
volume is in the range 38-42% of the effective solid volume of the
rivet.
4. A self-piercing rivet according to claim 1, wherein the bore
extends along the full length of the rivet.
5-14. (canceled)
Description
[0001] The present invention relates to a self-piercing rivet of
the kind that is inserted into sheet material without full
penetration such that a deformed end of the rivet remains
encapsulated by an upset annulus of the sheet material. It also
relates to a method for forming a joint with such a rivet and to a
rivet insertion system.
[0002] A self-piercing rivet generally has a head and a partially
hollow shank. It is driven by a punch into the sheet material such
that it pierces the top sheet and forms a mechanical interlock with
the bottom sheet with the head often (but not always) flush with
the upper surface of the top sheet. Since the bottom sheet is not
pierced there is a reduced risk of corrosion occurring in the
completed joint. Using self-piercing rivets in a joining process
reduces the number of production steps as compared to conventional
riveting in which a hole first has to be drilled into the sheet
material before the rivet is inserted and then its projecting ends
upset.
[0003] Self-piercing riveting has been used to great commercial
success in the automobile industry where light-weight materials,
such as aluminium, have been adopted for vehicle body panels in the
interests of weight reduction and therefore reduced energy
consumption. Aluminium is difficult or not feasible to spot weld,
particularly to steel, owing to its high thermal conductivity, low
melting range and propensity to form oxide surface film.
[0004] More recently in the automotive industry there has been a
move towards using high strength sheet metals. Our European patent
no. 2024651 describes a self-piercing rivet particularly suitable
for joining high strength, thick stack steels. Since such steels
have a high ultimate tensile strength the insertion forces applied
to the rivet are necessarily high there is thus a significant risk
of rivet collapse. The rivets must be treated to give them a
medium/high hardness value (e.g. 450-510 Hv) so that they have
sufficient strength. It has been established that such a rivet is
not always suitable for use with thick stack, high strength light
metal alloys such as magnesium and aluminium alloys especially
where the material combination may have three or more layers.
Moreover, other conventional rivets are not generally suitable for
joining such materials.
[0005] Aluminium alloy sheet material generally exhibits superior
ductility and so dies with relatively deep cavities tend to be used
but the joints suffer from the tendency for the middle sheets to
push through the lowermost sheet in the insertion process. This
leaves a weakened joint which is often prone to corrosion and it
may not be possible to produce a satisfactory joint repeatedly in a
mass production environment.
[0006] It is one object of the present invention to obviate or
mitigate the aforesaid disadvantages. It is also an object of the
present invention to provide for an improved or alternative
self-piercing rivet.
[0007] According to a first aspect of the present invention there
is provided a self-piercing rivet comprising a head and a
substantially cylindrical shank that is at least partially hollow
so as to define a bore that extends along at least part of its
length, wherein the outside diameter of the shank is at least 6 mm,
the effective length of the rivet is at least 1.3 times the
diameter of the shank and the bore has a volume that is at least
38% of the effective solid volume of the rivet.
[0008] The effective length of a rivet in this context is the
length of the rivet that is intended to be embedded in the final
joint. For example, where the head of the rivet is designed to
stand proud of the upper surface of the top sheet material in the
finished joint, the effective length is the overall length of the
rivet minus the thickness of any part of the rivet head that is
intended to remain above the upper surface of the finished joint.
The effective solid volume of a rivet is the volume of that part of
the rivet that is intended to be embedded in the final joint i.e.
the solid volume of the rivet excluding any part of the rivet head
that is intended to remain above the upper surface of the finished
joint. For a rivet with a substantially cylindrical shank that is
hollow or partially hollow, the solid volume is determined as if
the shank were solid i.e. it includes the volume of the bore.
[0009] Some rivet types have a head that is designed to be embedded
in the sheet material such that its flat upper surface is
substantially flush with the upper surface of the top sheet of
material in which case the effective length is equivalent to the
overall length of the rivet. Other rivet types have a head which is
intended to stand proud of the upper sheet of material, such as for
example pan head or domed head rivets. In the latter types the
underside of the head is intended to abut the surface of the top
sheet of material such that the full length of the shank is
embedded in the final joint.
[0010] In one preferred embodiment the bore has a volume that is in
the range 38% to 50% of the effective solid volume of the rivet.
More preferably the range is 38% to 42%
[0011] The outside diameter of the shank has no upper limit.
However, it will be appreciated that as the rivet diameter gets
larger so will the weight of the rivet. Moreover, larger rivets
mean that the equipment required to insert the rivets becomes more
bulky and expensive and consume more power. Larger rivets are thus
likely to have limited applications. Furthermore, the force
required to insert the rivet increases with the outside diameter in
view of the increase surface area of the outside of the rivet. In
one embodiment the outside diameter of the shank exceeds 8 mm.
[0012] The length of the shank is limited only by the manufacturing
process. Rivets that are particularly long are difficult to
extrude.
[0013] The hardness of the rivet may be in the range 250 Hv to 650
Hv.
[0014] The rivet preferably has a piercing end opposite the head
and the bore may taper outwardly at the piercing end. There may be
an arcuate transition between the head and the shank.
[0015] In a second aspect of the present invention there is
provided a self-piercing rivet comprising a head and a
substantially cylindrical shank that is at least partially hollow
so as to define a bore that extends along at least part of its
length, wherein the outside diameter of the shank is at least 8 mm,
and the bore has a volume that is at least 38% of the effective
solid volume of the rivet.
[0016] According to a third aspect of the present invention there
is provided a method for forming a joint in a stack of at least two
sheets of light metal alloy having an ultimate tensile strength in
the range 50 to 600 MPa, the stack having a thickness of at least
6.0 mm, using a self-piercing rivet comprising the steps of:
positioning the material over a die; providing a self-piercing
rivet having a head and a substantially cylindrical shank that is
at least partially hollow so as to define a bore that extends along
at least part of its length; positioning the rivet over the sheet
material at a position opposite the die; using a punch to set the
rivet and force it into the sheet material such that it pierces at
least an uppermost sheet of the stack and such that the shank
deforms outwardly to interlock with the material but without
penetration of the lowermost sheet in the stack; wherein the
outside diameter of the shank is at least 6 mm, the effective
length of the rivet is at least 1.3 times the diameter of the shank
and the bore has a volume that is at least 38% of the effective
solid volume of the rivet.
[0017] A light metal alloy is a term used to refer to alloys based
on low density metals and in particular metals having a density
lower than that of steel. It includes in particular, aluminium
alloys and magnesium alloys.
[0018] For a rivet shank having an outside diameter, the die may
have a die cavity with a maximum depth in the range 0.5 to 3.5 mm.
However, the die cavity may fall outside of the upper limit of this
range for dies with a larger shank outside diameter.
[0019] According to a fourth aspect of the present invention there
is provided a method for manufacturing a component including
forming a joint in accordance with the method defined above.
[0020] According to a fifth aspect of the present invention there
is provided a rivet insertion system comprising a punch for
applying an insertion force to a self-piercing rivet, a die into
which the material being riveted is deformed and a rivet as defined
above.
[0021] The system may further comprise a rivet feed for feeding
rivets to the punch from a bulk store. The punch may be part of a
rivet insertion tool, the rivets being fed to a nose of the
tool.
[0022] The system may further comprise a C-frame to which the tool
is mounted. The C-frame may have a first limb to which the tool is
mounted and a second limb on which an upsetting die is supported.
The C-frame may be supported for movement by a robot handler.
[0023] The system may also comprise a controller for controlling
the operation of the tool and/or the feed.
[0024] Specific embodiments of the present invention will now be
described, by way of example only, with reference to the
accompanying drawings in which:
[0025] FIG. 1 is a sectioned view of a rived joint using a
self-piercing rivet in accordance with a first embodiment of the
present invention;
[0026] FIG. 2 is a side view of the self-piercing rivet used in
FIG. 1;
[0027] FIG. 3 is a sectioned view taken along the central axis of
the rivet of FIG. 2; and
[0028] FIG. 4 is an isometric perspective view of the rivet of FIG.
2;
[0029] FIGS. 5a and 5b show sectioned views through two riveted
joints prepared for comparative purposes: one using a prior art
self-piercing rivet and the other using a self-piercing rivet in
accordance with a second embodiment of the present invention;
[0030] FIGS. 6a and 6b show sectioned views through two riveted
joints for comparative purposes: one using a prior art
self-piercing rivet and the other using a self-piercing rivet in
accordance with a third embodiment of the present invention;
[0031] FIGS. 7a and 7b show sectioned views through two riveted
joints prepared for comparative purposes: one using a prior art
self-piercing rivet and the other using a self-piercing rivet in
accordance with a fourth embodiment of the present invention;
[0032] FIGS. 8a, 8c and 8d show sectioned views through three
riveted joints prepared for comparative purposes: the first two
using a prior art self-piercing rivet and the third using a
self-piercing rivet in accordance with a fifth embodiment of the
present invention;
[0033] FIG. 8b is an underneath view of the joint of FIG. 8a;
[0034] FIG. 9 is an embodiment of a rivet insertion system for use
with the rivet of FIGS. 1 to 8.
[0035] Tests performed by the inventors have established that a
self-piercing rivet suitable for high strength, thick stack steels
such as that described in our European patent No. 2024651 is not
always suitable for thick stack, light metal alloys such as
magnesium or higher strength aluminiums. These materials generally
exhibit reduced ductility and thus have a tendency to fracture when
used with conventional dies. The "button" of material that is
deformed into the die tears during the rivet insertion process.
This is undesirable as the finished joint is weakened and prone to
corrosion. It has been realised by the inventors that such material
should be joined by using a die with a relatively small volume die
cavity (i.e. a shallow die) in order to avoid tearing. However, a
reduction in the die depth serves to increase the force experienced
by the rivet during insertion. Moreover, it has been found that
there is a tendency for the end of the rivet shank to drag sheet
material down through the joint to such an extent that it is pushed
through and out of the lowermost sheet, resulting in a joint that
is prone to corrosion and may also be considered unacceptable in
aesthetic appearance.
[0036] It has been established by the inventors that other existing
rivets are also not generally suitable for joining relatively thick
stacks of higher strength, light metal alloys. The higher strength
and low ductility of such material generally means that the rivet
experiences higher stress during the joining operation and this is
compounded when a shallow die is used. Conventional self-piercing
rivets are not capable of withstanding these high stresses in such
a manner that the deformation of the rivet shank remains
controllable to ensure the final joint is of satisfactory quality.
Simply manufacturing the rivet from a higher strength material does
not generally achieve the desired results as the corresponding
reduced ductility can cause the rivet shank to crack as it attempts
to deform during insertion. In order to form a suitable joint with
satisfactory strength and corrosion resistance the shank of the
rivet needs to have sufficient column strength to pierce the top
sheet of material without buckling but yet flare outwardly during
insertion in a repeatable and predictable manner without tearing or
cracking in order to form a satisfactory joint.
[0037] Similarly, when such rivets are used to join thick stacks of
lower strength, higher ductility light metal alloys the inventors
have noticed that there is a tendency for the lowermost sheet to
thin and material from the sheet immediately above is dragged down
and often pushes through the lowermost sheet, resulting in a joint
that is prone to corrosion.
[0038] One typical approach to strengthening the shank of a
self-piercing rivet is to increase the thickness of the shank wall
but this increases the tendency of the shank to crack during
insertion of the rivet. Another approach is to increase the depth
of material below the head (known as the rivet "web") thus reducing
the length of the unsupported hollow part (the bore) of the shank
but this is counter-productive as the volume of sheet material
displaced by the rivet is less readily accommodated within the bore
leading to the detrimental effects discussed above. The relatively
low ductility of the rivet material only allows for limited
deformation and displacement of material before it tends to crack
rendering it susceptible to fatigue. In view of this self-piercing
rivets are not successfully used in thick stack, high strength
light alloy applications. A further approach is to increase the
hardness of the rivet material but this only increases the tendency
of the rivet to fracture as the shank deforms outwardly during the
joining process.
[0039] Referring now to FIGS. 1 to 4 of the drawings the exemplary
self-piercing rivet 10 is substantially cylindrical with a head 11
that extends radially outwards from a depending shank 12 that is
hollow so as to define a central bore 13. The shank has a piercing
end 14 distal from the head. This particular rivet is made from
carbon-manganese boron steel conforming to BS EN 10263:2001 steel
grade 36MnB4. It will be appreciated that rivets of other suitable
steel compositions may be used.
[0040] The head 11 of the rivet 10 has a substantially constant
diameter that is integrally formed with an upper end of the shank
12 and a substantially planar upper surface 15 to which a force is
applied to insert the rivet into the workpiece. The rivet is
configured such that in the final joint the head stands proud of
the upper surface of the workpiece. However, other rivet
embodiments in accordance with the invention may have a head
portion that is designed to have its upper surface substantially
flush with the surrounding surface 15 in the finished joint.
[0041] The upper end of the shank 12 flares outwardly with a radius
R1 to meet with the underside of the head 11. This radius serves as
a transition surface between the head 11 and the shank 13. The
shank has an outside diameter D1, an inside diameter D2 and an
axial length L. The rivet has an overall length H+L where H is the
depth of the head.
[0042] In the embodiment shown in FIGS. 1 to 4 the rivet has the
following dimensions:
D1=6.5mm+/-0.1 mm
D2=4.0mm+/-0.1 mm
L=10 mm+/-0.1 mm
H=2.5 mm+/-0.1 mm
R1=0.75+/-0.25 mm.
[0043] The piercing end 14 of the rivet comprises a flat annular
piercing edge 16 that extends in a plane substantially parallel to
that occupied by the upper surface 15 of the head 11. The central
bore 13 in the shank 12 is cylindrical and extends from a position
just under the head 11 to the piercing edge 16. It has a
substantially constant diameter along it length but as it
approaches the piercing end 14 the central bore 13 increases in
diameter by virtue of an outward conical taper 17 on the inner
surface of the shank 12. In the exemplary embodiment the taper 17
has an inclusive angle of 60.degree. but it will be appreciated
that other angles may be adopted.
[0044] The bore 13 terminates just short of the underside of the
head 11 at a base surface 18 that is slightly conical. There is an
arcuate transition of radius R2 between the base surface 18 and the
inside surface of the shank 12. The minimum thickness of the web of
material between the base surface 18 of the bore 13 and the upper
surface 15 of the head 11 is labelled as W in FIG. 4 and in the
particular embodiment shown is 3 mm+/-0.1 mm such that the base
surface 18 is offset from the underside of the head 11 by 0.5 mm.
The bore depth is 9.5 mm.
[0045] The rivet is heat treated to provide a hardness of 250
Hv-650 Hv and is inserted into sheet material by a punch of a known
rivet setting tool over a die with a relatively shallow die cavity
(1.4 mm in joint of FIG. 1). In FIG. 1 the rivet 10 is shown after
insertion into three sheets 20, 21, 22 of high strength aluminium
alloy (Ac300 T61) each of 2.45 mm thick. As can be seen from the
figure, the rivet exhibits sufficient column strength to prevent
significant compression. The geometry of the rivet allows the
displaced sheet material to be accommodated within the central bore
13 of the rivet 10 without creating excessive stress. This allows
the shank 11 of the rivet 10 to deform in such a way that it
provides a symmetric interlock in the lower sheet 22 as
illustrated.
[0046] It has been realised by the inventors that sufficient column
strength of the rivet can be advantageously achieved by using an
increased outside diameter rather than simply increasing the
thickness of the shank or increasing the thickness of the rivet
web. Moreover, it has been realised that the bore volume needs to
be a significant proportion of the volume of the rivet that is
embedded in the joint to ensure that the displaced sheet material
can be accommodated particularly as the depth of the die
deliberately has to be made relatively shallow in view of the low
ductility of the sheet material that only permits relatively small
deformation before tearing. It has been established that in order
to be effective in this context the bore volume should be greater
than 38% of the effective solid volume of the rivet (that is the
solid volume of the rivet that is embedded in the final joint,
including the volume of the bore but not including the head if that
is designed to stand proud of the upper surface of the sheet
material in the finished joint). The rivet should have a relatively
long shank for use with thick sheet material or thick stacks of
such material. It is envisaged that the present invention applies
to rivets that have an effective length that is at least 1.3 times
the outside diameter of the shank.
[0047] The rivet geometry thus provides adequate column strength to
withstand the high stress encountered during insertion and a high
bore volume to accommodate displaced sheet material. The rivet
geometry is somewhat counter-intuitive as conventional approach to
improving column strength is to make increase the thickness of the
shank wall and the web, thus reducing the available bore
volume.
[0048] FIGS. 5 to 8 show the results of tests performed by the
inventors on both prior art self-piercing rivets and self-piercing
rivet embodiments in accordance with the present invention. In each
instance a joint formed in sheets of aluminium alloy with a prior
art rivet is shown alongside a joint in the same sheet material but
formed with a rivet in accordance with the present invention. In
each of the figures the lowermost sheet has been chemically treated
after cutting through the joint to provide a contrast with the next
sheet so that the deformation can be clearly observed.
[0049] FIG. 5a is an illustration of a joint produced by inserting
a prior art self-piercing rivet of high hardness (circa 530-580 Hv)
into three overlying sheets of high strength aluminium Ac300 T61,
each of 2.45 mm thickness, using a die having a cavity depth of 1.4
mm. The rivet has a shank outer diameter of 5.3 mm, an inner
diameter of 3.2 mm and an effective length of 10.0 mm. This is
shown alongside FIG. 5b in which a self-piercing rivet in
accordance with the present invention is inserted into the same
sheet material using a die having the same depth. It can be seen
that the rivet of FIG. 5a has not deformed consistently around the
shank and has partially collapsed as a result of insufficient
column strength in the shank to support the forces experienced
during insertion. There is also insufficient mechanical interlock
between the rivet and the lowermost sheet. It has been realised by
the inventors that the rivet has insufficient bore volume to
accommodate the displaced sheet material and as a result a
satisfactory interlock is not achieved. In FIG. 5b the rivet has a
shank outer diameter of 6.5 mm, an inside diameter of 4.0 mm, an
effective length of 10.0 mm and is heat treated to a medium
hardness (450-510 Hv). The effective length of the rivet is the
same as the shank length in this instance as the head of the rivet
is designed to remain above the exposed upper surface of the joint.
As in the embodiment of FIGS. 1 to 4, the central bore terminates
just short of the underside of the head at a base surface.
[0050] The self-piercing rivet of FIG. 5b has sufficient column
strength to prevent significant compression even with a large bore
volume. The displaced sheet material is readily accommodated by the
bore thereby reducing the stress experienced by the rivet during
insertion. This allows the rivet to pierce the upper sheets and to
provide a symmetric interlock with the lowermost sheet. It was
established that the geometry of the rivet provides sufficient
column strength such that a rivet of lower hardness could be used
thereby allowing the rivet shank to deform radially outwards
without risk of fracture.
[0051] FIG. 6a is an illustration of a joint produced by the
inventors by inserting a prior art self-piercing rivet of medium
hardness (circa 450-510 Hv) into three overlying sheets of high
strength aluminium (611 T4), each of 3 mm thickness, using a die
having a maximum cavity depth of 2.0 mm. The rivet has a shank
outside diameter of 5.3 mm an inside diameter of 2.9 mm and an
effective length of 12.0 mm. The die has a central "pip" extending
upwardly from the bottom of the cavity which explains the
undulating form of the underside of the joint. Examination of the
joint established that the rivet had exhibited adequate column
strength but the middle sheet had pushed through the lowermost
sheet producing a crevice which is prone to corrosion. The middle
sheet has not been pierced and as a result it had been dragged down
and through the lowermost sheet preventing adequate interlock. It
has been realised by the inventors that the rivet has insufficient
bore volume to accommodate the displaced sheet material thus
preventing effective interlock.
[0052] The self-piercing rivet of FIG. 6b is a tubular rivet in
which the central bore extends through the head. The effective
length of the rivet is 12.0 mm, the outside diameter of the shank
6.00 mm and the inside diameter of the shank 3.65 mm. The rivet has
a medium/high hardness of 450-510 Hv. The joint has been produced
using the same die as that used for the FIG. 6a joint. The geometry
of the rivet helps to reduce the compressive stress experienced by
the lowermost sheet during rivet insertion. As a result the
lowermost sheet has not been reduced in thickness to a degree where
the middle sheet has penetrated. The button formed on the underside
of the joint is completely encapsulated by the lowermost sheet thus
eliminating the risk of corrosion in the joint. The increased bore
volume allows the middle sheet to be pierced successfully with an
adequate and symmetric interlock in the lowermost sheet.
[0053] FIG. 7a is an illustration of a joint produced by inserting
a prior art self-piercing rivet of medium hardness (circa 450-510
Hv) into three overlying sheets of lower strength, higher ductility
aluminium (NG 5754), each of 3.0 mm thickness, using a die having a
cavity depth of 2.8 mm. The rivet has a countersunk style head, an
outside shank diameter of 5.3 mm, an inside shank diameter of 3.2
mm and the effective length of the rivet is 12 mm. The lowermost
sheet has been subjected to excessive thinning to the extent that
the middle sheet has been pushed through. Fracture of the lower
sheet provides a direct path for moisture ingress to reach the
interface between the rivet and sheet material thus producing a
potential for corrosion. The rivet also exhibited some compression
and asymmetry. It has been realised by the inventors that even with
the high volume die cavity the rivet bore exhibited insufficient
volume to accommodate the displaced sheet material. Moreover, it
has been established that the compression and asymmetry is due in
part to the additional stress imposed when the bore is filled by
the displaced upper and middle sheet material.
[0054] The joint shown in FIG. 7b was produced using a
self-piercing rivet in accordance with the invention and which has
an outside shank diameter of 6.00 mm, an inside shank diameter of
3.65 mm, an effective length of 12 mm and a hardness of 450 Hv to
510 Hv. The central bore in the rivet terminates just short of the
underside of the head to provide an increased volume (and a
relatively thin rivet web). The die was the same as that used in
the production of the joint of FIG. 7a. Inspection of the joint
shows that the rivet has sufficient column strength and an
effective symmetric interlock with the lower sheet was achieved.
The lowermost sheet has maintained sufficient thickness for the
button to be completely encapsulated so that the risk of corrosion
is significantly reduced. The high bore volume readily accommodates
the displaced upper and middle sheet material and the rivet
geometry helps to reduce the stress experienced by the lowermost
sheet during rivet insertion.
[0055] FIG. 8a shows a joint produced by inserting a prior art
self-piercing pan head rivet into four stacked sheets of high
strength aluminium alloy (three sheets of 6111-T4 and one sheet of
6111 PFHT) each 3.0 mm thick using a flat bottom die with a cavity
depth of 2.8 mm. The rivet has a hardness of 530-580 Hv, an outside
shank diameter of 5.5 mm, an inside shank diameter of 2.9 mm and an
effective length of 14 mm. The middle sheets have not all been
pierced by the rivet and as a result there is not an effective
interlock with the lowermost sheet. It has been determined by the
inventors that this deficiency is as a result of inadequate rivet
bore volume. The relatively deep die prevents the middle sheet
material being pushed through the lowermost sheet but the button
formed on the underside of the lowermost sheet has several tears as
indicated by arrows in FIG. 8b.
[0056] FIG. 8c shows a joint produced using a prior art
self-piercing rivet identical to that used in the joint of FIGS. 8a
and 8b and the same material for the sheet stack. In this instance
the die has a cavity with an upstanding pip and a maximum depth of
2.0 mm. Although the shallower die prevents tearing of the
lowermost sheet, the sheet immediately above has been pushed
through the lowermost sheet rendering the joint prone to corrosion.
It has been realised that this is as a result of inadequate rivet
bore volume.
[0057] FIG. 8d shows a joint produced in an identical stack of
material as used in FIGS. 8a to 8c using a tubular rivet in
accordance with the present invention. In this instance the central
bore extends through the full length of the rivet by penetrating
through the rivet head. The rivet has a hardness of 530-58-Hv, an
outside shank diameter of 6.5 mm, an inside shank diameter of 3.9
mm and an effective length of 15 mm. The die is the same as that
used in the joint of FIG. 8c. The low die volume prevents tearing
in the region of the button and the high bore volume allows the
rivet to pierce the middle sheets and provide an effective
interlock with the lowermost sheet without the lower middle sheet
being pushed through.
[0058] In the embodiment of FIGS. 1 to 4 the bore volume is 38% of
the effective solid volume of the rivet.
[0059] Other exemplary rivet embodiments of the present invention
are shown in the table below:
TABLE-US-00001 Bore RIVET L H W D1 D2 volume % A 7.0 2.5 3 6.5 4.0
38.21 B 8.0 2.5 3 6.5 4.0 38.17 C 9.0 2.5 3 6.5 4.0 38.13 D 12.0
2.5 3 6.5 4.0 38.07
[0060] The end column "Bore volume %" is the volume of the bore
expressed as a percentage of the effective solid volume of the
rivet as discussed above.
[0061] The invention has application to forming self-piercing
riveted joints in thick stack (6 mm or over), high strength light
metal alloys including for example, aluminium and magnesium alloys,
which generally have a relatively low ductility. For the types of
aluminium relevant to the invention, an ultimate tensile stress of,
for example, over 300 MPa may be considered high strength and for
magnesium alloys over 200 MPa. The rivet is suitable for stacks
having a minimum thickness of 6.0 mm. In such applications the die
is relatively shallow (the die cavity is generally less than
approximately 2.0 mm in depth) to obviate the risk of the tearing
of the lowermost sheet
[0062] The invention also has application to forming self-piercing
riveted joints in thick stacks (6.0 mm or over) lower strength
light metal alloys. In order to produce such joints a deeper die
cavity may be used such as for example up to 3.5 mm but larger
depths may be used for particularly thick stacks.
[0063] It will be appreciated that numerous modifications to the
above described design may be made without departing from the scope
of the invention as defined in the appended claims. For example,
the rivet head may take any suitable form depending on the joint
required and may also perform a secondary function such as
providing a male or female thread for attachment of a further
component. Moreover, the particular dimensions specified may be
varied depending on the application provided.
[0064] It is to be understood that the term "sheet" is used herein
to refer to material produced by any process including for example
casting, extruding or rolling. Such a sheet may be an integral part
of a larger component which is not sheet-like in overall
appearance.
[0065] An exemplary rivet insertion system for inserting the rivets
in accordance with the described method is shown in FIG. 9.
[0066] A rivet setting tool 101 is mounted on an upper jaw of
conventional C-frame 102 above a rivet-upsetting die 103 disposed
on the lower jaw. Rivets 10 (not shown in FIG. 9) are inserted by
the tool into a workpiece (not shown) supported over the die 103 as
is well known in the art. It is to be appreciated that whilst the
specific embodiment described herein relates to the feeding and
insertion of rivets it has application to other fasteners. The
C-frame is mounted on a robot manipulator (not shown) via a
mounting bracket 102a such that it is movable with the tool 1 by
the robot towards and away from the workpiece as required.
[0067] Rivets are delivered to the tool by feed apparatus 110 that
comprises two principal sections 111, 112 releasably connectible
together at a stationary floor-mounted docking stand 113
intermediate the tool 101 and a bulk source 214 of rivets. A first
section 111, downstream of the docking stand 113, is carried on the
C-frame 102 with the tool 101 and transports rivets from a
tool-side docking interface 115 to a nose 104 of the tool 101 for
insertion into the workpiece. A second section 112, which is
principally upstream of the docking stand 113, is connected between
the bulk source 114 of rivets stored in a cabinet 116 and a
stand-side docking interface 117 supported on the docking stand 13.
The two sections 11, 112 are releasably connectable at the docking
stand 113 by bringing the docking interfaces 115, 117 into
register
[0068] The rivet insertion tool 101 contains a reciprocal punch
(not shown) by which the rivet is driven into the workpiece.
[0069] As is known, the cabinet 116 not only houses the bulk source
of rivets (e.g. vibratory bowls with rivet orientation mechanisms)
but also the compressed gas (e.g. air) deliver}' systems required
to propel rivets in the feed apparatus. It may house a programmable
controller in the form of microprocessor-based hardware and
operational software for controlling the operation of the feed
apparatus and the rivet insertion apparatus, although this may be
housed separately and connected by suitable cabling or other
communication means to the cabinet. Such gas delivery and control
systems are well known and will not therefore be described in
detail herein.
[0070] The first and second sections 111, 112 dock together at the
docking stand 113 at predetermined intervals in the riveting
operation to collect rivets for the next riveting cycle, such
rivets being temporarily stored in a buffer magazine 105 that is
integral with the first section 111 of the feed apparatus 110. The
docking operation brings together the tool-side and stand-side
docking interfaces 115, 117 of the rivet feed apparatus 110 and
allows rivets to flow from the bulk sources 114 across the
interfaces to the nose 104 of the setting tool 1.
[0071] The described and illustrated embodiments are to be
considered as illustrative and not restrictive in character, it
being understood that only the preferred embodiments have been
shown and described and that all changes and modifications that
come within the scope of the inventions as defined in the claims
are desired to be protected. It should be understood that while the
use of words such as "preferable", "preferably", "preferred" or
"more preferred" in the description suggest that a feature so
described may be desirable, it may nevertheless not be necessary
and embodiments lacking such a feature may be contemplated as
within the scope of the invention as defined in the appended
claims. In relation to the claims, it is intended that when words
such as "a," "an," "at least one," or "at least one portion" are
used to preface a feature there is no intention to limit the claim
to only one such feature unless specifically stated to the contrary
in the claim. When the language "at least a portion" and/or "a
portion" is used the item can include a portion and/or the entire
item unless specifically stated to the contrary.
* * * * *