[0001] The present invention relates to the setting of rivets. More particularly, the present
invention relates to a joining method for connecting at least two workpieces with
a rivet adapted to reduce the risk of cracking in the rivet.
[0002] Self-piercing riveting (SPR) is a spot-joining technique in which a self-piercing
rivet is driven, by a punch, into a layered workpiece supported on a die. The die
is shaped so that as the rivet is driven into the workpiece towards the die, the material
of the workpiece plastically deforms. This flow of workpiece material causes the annular
tip of the rivet to flare outwards and remain encapsulated by an upset annulus of
the workpiece material. The flared tip of the rivet interlocking with the upset annulus
of the workpiece prevents removal of the rivet or separation of the layers of the
workpiece.
[0003] Insertion of a rivet into the workpiece is performed using a drive unit with an actuator
(for instance a linear actuator such as a hydraulic cylinder or an electric linear
actuator) which is mounted on a support arm of a force reaction frame opposite a die
(or anvil) mounted on another support arm of the force reaction frame. The linear
actuator drives the punch and rivet towards a stationary workpiece and die. The large
forces on a workpiece which occur when a rivet is set or during punching must be compensated
by a counterforce. This is usually achieved by supporting the workpiece on a counterforce
structure, which preferably substantially has the shape of a C and is therefore also
usually designated as a C-frame.
[0004] For the precise setting of a rivet or accurate punching it is important to know how
deeply a rivet or a die has penetrated into the workpiece.
EP1228824A2 is directed to a method for riveting or punching dealing with this issue. More particularly,
the deformation of the counterforce structure during a riveting or punching process
is detected by a monitoring unit and a course of movement during the riveting or punching
process is corrected as a function of the bending.
[0005] Experience shows that some rivets tend to crack during a setting step. More particularly,
the rivet (or more particularly self-piercing rivet) may crack in the joining direction,
notably when the bore of the self-piercing rivet is filled. These cracks then widen
until the rivet reaches its final position. The tendency to crack formation depends,
among other things, on the following factors: rivet length, rivet coating, hardness
of the sheet to be riveted. Several developments have been made to overcome this issue.
EP2631022A1 and
EP3424611 for instance are directed to an improved die.
US2020261966A1 is directed to an improved self-piercing rivet.
[0006] However, a need still exists to improve the setting of rivets, and notably to provide
joining method which reduces the risk of damage in the fastener, like for instance
the risk of cracking in the self-piercing rivet, notably without significant impact
on cycle time and which works also for fasteners being mechanically coated. Examples
of the present disclosure aim to address the aforementioned problems.
[0007] According to an aspect of the present disclosure there is a joining method for connecting
at least two workpieces with a rivet comprising the steps of:
- Providing a setting tool comprising a frame, a punch that is displaceable on the frame
along a joining axis, a die that is mounted on the frame, and a clamping device with
which one or more components can be compressed in a joining direction, the punch being
guided in the clamping device by a drive unit;
- Defining a predetermined maximal rivet setting speed;
- Providing a rivet comprising a length inferior to the predetermined maximal rivet
length;
- Feeding the rivet into the setting tool;
- Setting the rivet such that in a first stage the punch moves at a first punch speed;
in a second stage the punch speed decreases until it reaches a second punch speed,
wherein the second punch speed is reached at the latest when the rivet first contacts
the workpiece, the second punch speed being lower than the first punch speed; in a
third stage the punch continues moving and the rivet speed is monitored to ensure
it does not exceed the predetermined maximal rivet setting speed.
[0008] Such method with a controlled speed decrease and the definition of a predetermined
maximal rivet setting speed which is not exceeded during the setting of the rivet
allows to reduce the risk of cracks in the rivet. More particularly, it must be guaranteed,
that the setting speed, from rivet contact point up to the end of the setting process
is lower or equal to the defined setting speed. The defined maximum rivet length allows
to keep the speed of the punch to a maximum up until it is reached, such that a faster
punch speed is mostly used during the joining method. A faster punch speed is indeed
advantageous for effective cycle times of the joining process. The speed reduction
allows to avoid or sensibly decrease the cracks. The rivet speed is monitored to ensure
an exact control of the process.
[0009] In an embodiment, the method further comprises the step of defining a predetermined
maximal rivet length, and wherein the second punch speed is reached when the punch
arrives at a position which corresponds to the predetermined maximal rivet length.
The maximal rivet length is a point which is always reached before the rivet contact
point and which can easily be indicated or defined by an operator. The maximal rivet
length can be adjusted by an operator.
[0010] In an embodiment, the first punch speed is within a range of 150-1000 mm/s. This
enables effective cycle times of the joining process.
[0011] In an embodiment, the second punch speed and/or the maximal predetermined rivet setting
speed is within a range of 10-300 mm/s. In an embodiment, the maximal predetermined
rivet setting speed is below 100 mm/s, or even below 50 mm/s. This allows to reduce
the risks of crack, notably for self-piercing rivets having a mechanical coating.
[0012] In an embodiment, the drive unit decreases the punch speed such that the predetermined
maximal rivet setting speed is reached when the predetermined maximal rivet length
is reached. For instance, the speed decrease is constant such that a smooth transition
is realized, the lower speed is reached only at the maximal rivet length or when the
rivet first contacts the workpiece and the effective cycle time is not affected. Indeed,
lower speed always increases the cycle time. The present method aims to have a minimal
effect on the cycle time. The method can thus be used for a wide range of rivets having
a length inferior to the pre-defined maximal rivet length.
[0013] In an embodiment, the second punch speed is correlated to the predetermined maximal
rivet setting speed so that the predetermined maximal rivet setting speed is reached
when the predetermined maximal rivet length is reached or when the rivet first contacts
the workpiece.
[0014] In an embodiment, the deformation of the frame during the setting step is detected
by a monitoring unit and a course of movement during the setting step is corrected
as a function of the deformation. Thus, the exact position of the rivet with regard
to the workpiece can be determine and therefore the rivet speed can be monitored and
controlled. In an embodiment, the rivet speed could also be monitored without the
knowledge of the workpiece position, but such monitoring is less accurate.
[0015] In an embodiment, a relative movement between the frame and clamping device is measured
by a first sensor and a relative movement between the frame and the punch is measured
by a second sensor. The two sensors can easily be implemented and allows to exactly
determine the rivet speed.
[0016] In an embodiment, the first sensor and/or the second sensor is a linear path recorder,
preferably a digital counter. For example, the digital counter counts stroke-shaped
markings on a kind of ruler. This enables fast and accurate processing of the signals
in a monitoring unit. In another embodiment, other systems like for instance laser
can be used.
[0017] In an embodiment, the first and/ or the second workpieces is a high strength steel
workpiece. Components made of high strength steel are known to be difficult to join
to each other or to other metals. Such high strength steel can, in some cases, have
a tensile strength of at least 400 MPa. In another embodiment, the workpieces or at
least one of the workpiece are/is made with thick (at least 3mm) mild steels or hard
aluminum alloys.
[0018] In another embodiment, three workpieces are joined, and the two upper layers are
high strength steel workpieces.
[0019] In an embodiment, the predetermined maximal rivet setting speed is adjustable. Thus,
depending on the rivet, the workpiece and the nature of the joint, the maximal rivet
setting speed can be adjusted by an operator to optimize the cycle time while avoiding
cracks.
[0020] Various other aspects and further examples are also described in the following detailed
description and in the attached claims with reference to the accompanying drawings,
in which:
Fig. 1 schematically shows a setting tool comprising a frame, a punch that is displaceable
on the frame along a joining axis, a die that is mounted on the frame, and a clamping
device with which one or more components can be compressed in a joining direction,
the punch being guided in the clamping device by a drive unit;
Fig. 2A shows a schematic diagram of the commanded and actual punch speed profile
during the joining method;
Fig. 2B shows the schematic diagram of Fig. 2A with the commanded punch speed profile
and the tool rectification force profile measured and used to command a precise setting;
Fig. 3A shows a schematic diagram of the punch speed in function of the relative punch
position;
Fig. 3B shows the schematic diagram of Fig. 3A with the punch speed in function of
the relative punch position and the tool rectification force profile measured and
used to command a precise setting.
[0021] The embodiments of the disclosure will be best understood by reference to the drawings,
wherein the same reference signs designate identical or similar elements. It will
be readily understood that the components of the disclosed embodiments, as generally
described and illustrated in the figures herein, could be arranged and designed in
a wide variety of different configurations. The steps of a method do not necessarily
need to be executed in any specific order, or even sequentially, nor need the steps
be executed only once, unless otherwise specified.
[0022] In some cases, well-known features, structures, or operations are not shown or described
in detail. Furthermore, the described features, structures, or operations may be combined
in any suitable manner in one or more embodiments. It will also be readily understood
that the components of the embodiments, as generally described and illustrated in
the figures herein, could be arranged and designed in a wide variety of different
configurations.
[0023] Fig. 1 schematically shows a joining arrangement 10 for carrying out the joining
method. The joining arrangement 10 comprises a setting tool 12. The setting tool 12
comprises a joining head which either can be mounted in a stationary fashion or can
be moved by means of a robot 14. A feeder arrangement with a feeder hose 16 can be
provided. The feeder hose 16 feeds singulated rivets 18 to the joining head, for example
by means of air blast. Alternatively, a feeder arrangement can also have a magazine
on the joining head, which is used to transfer singulated rivets automatically into
a holder.
[0024] The joining head further comprises a counterforce structure or frame 20. Typically,
such frame 20 is a C-shaped frame composed of a solid material. A punch 22 displaceable
on the frame 20 along a joining axis X is also provided. The punch 22 can be controlled
by a drive unit 24 and a control device 26. The drive unit 24 may comprise an actuator
to actuate the punch 22.
[0025] The joining head further comprises a die 28 that is mounted on the frame 20, and
a clamping device 30. The die 28 is mounted at one end of the frame 20, wherein the
clamping device 30 is mounted at the other end of the frame 20. The punch 22 is guided
in the clamping device 30 by the drive unit 24. The clamping device 30 is adapted
to be biased in the joining direction or in the direction of the die 28.
[0026] The joining arrangement 10 serves to connect a first workpiece 32 and a second workpiece
34 which, for example, can be designed as metal sheets, by a rivet 18. The setting
tool 12 can also be used to join together more than two workpieces 32, 34. For instance,
three workpieces are joined, and the two upper layers are high strength steel workpieces.
The rivet 18 can be for example a self-piercing rivet 18 or punch rivet which can
be a hollow or a semi-hollow punch rivet. The workpieces 32, 34 are for instance made
of high strength steel. The control device 26 can be programmed such that a predetermined
maximal rivet length can be entered. This predetermined maximal rivet length can be
adjusted by an operator and corresponds to the maximal length of a rivet used for
the joining method. The length of the rivet is the axial length and correspond to
the length between the head of the rivet and the foot of the rivet. The predetermined
maximal rivet length is given by an operator and can depend on the joining operations
to be executed. For example, for joining operations in the automotive industry, a
predetermined maximal rivet length of 8 mm can be used. The self-piercing rivet 18
can for instance be mechanically coated or galvanic coated for instance. The rivet
18 can be a self-piercing rivet as described in
EP3080463 A1 or
EP2470799 A1.
[0027] To set the rivet 18 in the workpieces 32, 34, the workpieces 32, 34 are placed on
the die 28. The punch 22 is then moved by means of the drive unit 24 in the direction
toward the die 28. In this case, the clamping device 30 first contacts a surface of
the uppermost workpiece 32 and presses the workpieces 32, 34 together in the joining
direction.
[0028] The punch 22 is driven in a first stage R1 at a first punch speed S1. The first punch
speed S1 is for instance between 150 and 1000 mm/s. The first punch speed S1 can be
constant or not. More particularly the first punch speed S1 can be raised to a particular
maximal value, for instance 300mm/s. The first punch speed S1 is commanded by the
control device 26. The punch 22 is driven at the first punch speed S1 by the drive
unit 24 and then the punch speed decreases until the punch reaches a position which
corresponds to the given maximal rivet length. The punch speed slowly decreases from
the first punch speed S1 to a second punch speed S2 such that as soon as the punch
reaches the position corresponding to the predetermined maximal rivet length, it reaches
the second punch speed S2. The punch speed reduction corresponds to a second stage
R2. The second punch speed S2 is for instance a constant speed or sensibly constant
speed. The second punch speed S2 is lower than the first punch speed S1.
[0029] During the motion of the punch 22 toward the die 28, the punch 22 contacts the head
of the rivet and therefore drive the rivet into the workpiece arrangement to realize
the connection. This stage corresponds to a third stage R3, during which the punch
22 drives the rivet 18 into the workpiece at the second punch speed and the rivet
speed is monitored to ensure it does not exceed a predetermined maximal rivet setting
speed. The predetermined maximal rivet setting speed is a data which is defined by
the operator before the joining method in the control device. The predetermined maximal
rivet setting speed and the second punch speed are correlated. The predetermined maximal
rivet setting speed can be adjusted by an operator depending on the joining operation,
the workpiece material, the rivet material, the rivet geometry, the die geometry,
... For instance, with a mechanically coated rivet, the predetermined maximal rivet
setting speed is within a range of 10-300 mm/s. The predetermined maximal rivet setting
speed can be below 100mm/s, or even below 50 mm/s in a particular embodiment.
[0030] The predetermined maximal rivet length allows to ensure that the speed is decreased
before or when the rivet contacts first the workpiece and therefore to determine when
the punch speed needs to be decreased such that the lower punch speed to avoid the
cracks on the rivet has a minimum impact on the cycle time.
[0031] In order to correctly monitor the rivet speed during the setting step, the setting
tool comprises a monitoring unit 36 adapted to detect the deformation of the frame
20 during the setting process and a course of movement during the setting step can
be corrected by the control device as a function of the deformation. Thus, the actual
displacement of the punch 22 and/ or the rivet 18 can be determined, and the actual
rivet speed correctly monitored. A first sensor 38 measures the relative movement
between clamping device 30 and frame 20. This sensor is preferably a linear path recorder
consisting of a kind of ruler which makes the same movement as the clamping device
and a counter which is fixed to the frame and counts markings on the ruler going past
it. A second sensor 40 measures the relative movement between frame and punch.
[0032] The first sensor 38 and the second sensor 40 are connected to the monitoring unit
36, which can thereby detect the bending of the frame during action of force by the
punch 22 and the clamping device 30 on the workpiece 32, 34. With the knowledge of
the bending of the frame 20 detected in this way the movement of the punch can be
adjusted in such a way that a constant penetration depth of the rivets is always ensured.
More particularly, the position of the rivet with regard to the workpiece can always
be measured. This allows to determine the exact punch position, but also the exact
rivet speed and to monitor correctly that the rivet speed does not exceed the predetermined
maximal rivet setting speed. The course of movement can thus be corrected by the control
device during the riveting or setting or joining process as a function of the bending.
[0033] Fig. 2A shows in more details the speed profile of the punch 22 during the setting
step. In Fig. 2A the X-axis represents the time and the Y-axis represents the punch
speed. Fig. 2A shows two curves. The plain line curve corresponds to the speed command.
It is the pre-programmed punch speed. The dotted line corresponds to the actual punch
speed. As visible in Fig. 2A, the two curve profiles are very similar, notably due
to the sensors and monitoring unit explained above. In an initial stage (not described
in detail above), the punch speed is constantly increased until it reaches a plateau.
The plateau corresponds to a maximal punch speed used to approach the rivet. The maximal
punch speed is for instance the first punch speed S1. The punch speed is then decreased
up to the second punch speed S2. The second punch speed S2 corresponds to the second
plateau. The inflexion point between the second plateau S1 and the curve portion corresponding
to the decreasing speed corresponds to the predetermined maximal rivet length. Indeed,
as mentioned below the predetermined maximal rivet length determines the instant when
the second punch speed S2 is reached. Once the rivet is set in the workpiece, the
punch 22 is then driven away from the workpiece and returns to its initial position
as quickly as possible, as visible in the last portion (right-handed portion) of the
curve shown in Fig. 2A. A new rivet can be fed and a new setting step can then be
undertaken on the same workpieces at a different area or on different workpieces.
[0034] Fig. 2B shows the pre-programmed punch speed as visible in fig. 2A in plain line
and the dotted line show the rectification tool force during the setting step. The
rectification tool force is calculated by the monitoring unit and the control device
via the measurement of the first and second sensors. The rectification tool force
allows to correct in real time the position of the punch and the rivet and their speed
such that the actual and target curve are sensibly similar, as visible in fig. 2A.
[0035] Fig. 3A and Fig. 3B show the punch speed in function of the relative punch position.
In Fig. 2A the X-axis represents the relative punch position, and the Y-axis represents
the speed. Unlike fig. 2A and Fig. 2, the curves in Fig. 3A and Fig. 3B only depicts
the first stage R1, second stage R2 and third stage R3 of the setting step. The curves
do not represent the return of the punch to its initial position after the setting
of the rivet.
[0036] In Fig. 3A, the plain line curve represents the commanded relative punch position,
and the dotted line curve represents the actual relative punch position. As visible,
due to the sensors 38, 40 and the monitoring unit 36, the command and actual curved
are sensibly similar. In the first stage R1, represented in the right-hand side of
Fig. 3A, the first plateau shows the first punch speed S1, which then slowly decreases
in the second stage to reach the second punch speed, lower than the first punch speed.
The second punch speed S2 forms the second plateau and corresponds to the third stage.
As seen in Fig. 3A, the punch speed reduction from the first punch speed S1 to the
second punch speed S2 can be sensibly constant. Fig. 3B shows the commanded relative
punch position in plain line, as visible in Fig. 3A and the dotted line show the rectification
tool force at the different position of the punch. The rectification tool force allows
to correct in real time the position of the punch and the rivet and their speed such
that the actual and target curve are sensibly similar, as visible in Fig. 3A.
joining arrangement 10
setting tool 12
robot 14
feeder hose 16
rivet 18
frame 20
punch 22
joining axis X
drive unit 24
control device 26
die 28
clamping device 30
first workpiece 32
second workpiece 34
first punch speed S1
second punch speed S2
first stage R1
second stage R2
third stage R3
monitoring unit 36
first sensor 38
second sensor 40
1. Joining method for connecting at least two workpieces with a rivet comprising the
steps of:
- Providing a setting tool (12) comprising a frame (20), a punch (22) that is displaceable
on the frame along a joining axis (X), a die (28) that is mounted on the frame, and
a clamping device (30) with which one or more components can be compressed in a joining
direction, the punch being guided in the clamping device by a drive unit (24);
- Defining a predetermined maximal rivet setting speed,
- Providing a rivet (18) comprising a length inferior to the predetermined maximal
rivet length,
- Feeding the rivet into the setting tool (12),
- Setting the rivet (18) such that:
∘ in a first stage the punch (22) moves at a first punch speed (S1);
∘ in a second stage the punch speed decreases until it reaches a second punch speed
(S2), wherein the second punch speed is reached at the latest when the rivet first
contacts the workpiece, the second punch speed being lower than the first punch speed,
∘ in a third stage the punch continues moving and the rivet speed is monitored to
ensure it does not exceed the predetermined maximal rivet setting speed.
2. Joining method according to claim 1, comprising the step of defining a predetermined
maximal rivet length, and wherein the second punch speed is reached when the punch
arrives at a position which corresponds to the predetermined maximal rivet length.
3. Joining method according to claim 1 or 2, wherein the first punch speed (S1) is within
a range of 150-1000 mm/s.
4. Joining method according to any of claims 1 to 3, wherein the second punch speed (S2)
and/or the maximal predetermined rivet setting speed is within a range of 10-300 mm/s.
5. Joining method according to any of claims 1 to 3, wherein the predetermined maximal
rivet setting speed is below 100 mm/s.
6. Joining method according to any of claims 1 to 5, wherein the drive unit (24) decreases
the punch speed such that the predetermined maximal rivet setting speed is reached
when the rivet first contacts the workpiece.
7. Joining method according to any of claims 1 to 6, wherein the second punch speed (S2)
is correlated to the predetermined maximal rivet setting speed so that the predetermined
maximal rivet setting speed is reached when the predetermined maximal rivet length
is reached.
8. Joining method according to any of claims 1 to 7, wherein the deformation of the frame
(20) during the setting step is detected by a monitoring unit and a course of movement
during the setting step is corrected as a function of the deformation.
9. Joining method according to any of claims 1 to 8, wherein a relative movement between
the frame (20) and clamping device (30) is measured by a first sensor (38) and a relative
movement between the frame and the punch is measured by a second sensor (40).
10. Joining method according to claim 9, wherein the first sensor (38) and/or the second
sensor (40) is a linear path recorder, preferably a digital counter.
11. Joining method according to any of the preceding claims, wherein the first and/ or
the second workpieces (32, 34) is a high strength steel workpiece.
12. Joining method according to any of the preceding claims, wherein three workpieces
are joined, and the two upper layers are high strength steel workpieces.
13. Joining method according to any of the preceding claims, wherein the predetermined
maximal rivet setting speed is adjustable.