Note: Descriptions are shown in the official language in which they were submitted.
CA 022~72~1 1998-12-04
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Press-joining method and device for connecting
sheet metal parts
The invention relates to a press-jolning method
S according to the precharacterizing clause of Patent
Claim 1 and to a device for implementing the said
method in accordance with the precharacterizing clause
of Patent Claim 8.
EP 0 330 061 Bl discloses a method and a device
for press-joining two or more even sheet metal parts
lying one upon another, in which a tool set consisting
of punch and die is used, whose die has a central anvil
and die mould pieces which project vertically with
respect to an anvil working face, which can preferably
move out sideways during upsetting and delimit a mould
cavity into which the join is made.
In the case of known press-joining, the sheet
metal parts to be connected are simultaneously
displaced at an optional joint location and then upset
without heating whilst applying such pressures that
positive and force-fitting seating of the sheet metal
parts in one another in the form of a cup-like joint
element is produced as a result of lateral material
flow. The volume of material that is located at the
joint location under a die active face is in this case
initially only displaced. To this end, the sheet metal
material is displaced out of the plane of the sheet
metal parts, the base thickness of the joint produced
still corresponding approximately to the overall
initial sheet metal thickness. During further press-
joining, the base of the cup of the joint element is
clamped in between the punch active face and the anvil
and, by means of an increasingly rising punch force, an
upsetting operation with radial material flow in the
bottom region is effected, the so-called spreading. The
thickness of the bottom of the metal sheets is reduced
by upsetting with respect to the initial sheet metal
thickness, which is necessary in order to produce the
necessary interlock by means of spreading.
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In order to make this upsetting operation
easier, the sheet metal parts to be joined can be cut
into partially in the direction of displacement at the
joint location, since then, during the upsetting
operation, the base of the cup of the joint element
remains largely free of the bond to the remaining
material, or this bond is at least reduced. However, it
is disadvantageous that a cut portion at the joining
operation reduces the strength of the press-joint, and
the joint is not gastight, so that joint elements which
are produced without local cutting are preferred.
Until now, it has not been economically
possible to use this press-joining of sheet metal parts
in the case of sheet metal parts made of stainless
steel with a predominantly austenitic basic structure.
The same is true in the case of other metals with a
particularly pronounced tendency to work-hardening
during a progressive deformation process, for example
highly alloyed stainless steels, titanium, or of metals
that are already pre-hardened, for example higher-
strength steels. Here, the material-induced processing
forces for joining by metal forming lie above the load
limit of the joining tool or lead to the latter failing
as early as after a few joining operations.
However, if too low processing forces are used,
the load capacity of the joints is inadequate.
Furthermore, it has been established that, even given
the greatest possible cold upsetting, only an
inadequate interlock in the joint element is achieved.
Press-joining such sheet metal materials is therefore
only possible at all when accompanied by cutting of the
sheet metal parts, and at the expense of the
disadvantages associated with this, the reduction of
the processing forces which is achieved by the clltting
not leading to any noticeable increase in the tool
service life either.
A first object of the invention is therefore to
provide a press-joining method of the generic type
mentioned at the beginning with which an adequately
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strong joint can be produced even in the case of parts
to be joined which consist of materials with a
particular tendency to work-hardening, or pre-hardened
materials.
A second object is to provide a device for this
method.
The first object is achieved by means of the
features of the characterizing part of Patent Claim 1.
The second object is achieved by means of the
features of the characterizing part of Patent Claim 8.
By this means, a method is provided in which an
effective interlock is achieved with the application of
a reduced joining force. This applies, in particular,
to the materials with a particular tendency to work-
hardening. The joining forces which have to be appliedto this extent are reduced such that the tools can
withstand this mechanical loading.
According to the invention, it has been
established that, in particular in the case of severely
work-hardening sheet metal materials, spreading is made
more difficult by the fact that the material attempts
to move out under pressure stresses, and flows back
counter to the direction of action of the punch. By
pressing on the parts to be joined during the joining
operation, this reverse flow is prevented, and the
sheet metal material is forced, by means of the
spreading initiated in this way, to form an
interlocking joining element in the active range of the
die. In the case of materials with a particular
tendency to work-hardening, the method therefore also
leads to a good interlock which was previously not
achieved.
Because the sheet metal parts located around a
joint location are pressed onto the die with a force
which is so great as to press the material onto a
support (the die here) in such a way that the volume of
material to be joined can no longer flow back counter
to the direction of action of the punch, the situation
is specifically achieved in which the upsetting force
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applied is used in an early phase of this upsetting to
form an effective interlock. Consequently, a
sufficiently strong joint results in an advantageous
way, even when, whilst taking care of the tools, the
necessary deformation degree at the joint location
takes place with the application of a reduced force. As
a result, press-joining in particular becomes more
economical, since the number of joints which can be
carried out with one joining tool is increased
considerably.
The pressing force acting on the sheet metal
parts around the joint location is a passive force,
which must be higher than the reverse-flow force of the
material of the parts being joined during the joining
operation. According to the invention, it has been
established that the pressing force exceeds the
reverse-flow force of the material if, preferably, the
pressing force is set to be at least so high that,
during the joining operation, the sheet metal parts do
not gape in the area surrounding the punch.
Furthermore, preferred values for the pressing
force, for example for flat, even sheet metal parts,
lie at around five times to eight times the forces
which are needed merely to strip the joint parts or to
prevent them bending up during the displacement phase.
In addition, the pressin~ force is preferably
applied to an effective contact area of such a size
around the joint location that the pressing force to be
applied leaves behind no impression in the metal
sheets, and the mould pieces of the die are able to
move away sideways during upsetting. The pressing-force
active face can therefore be dimensioned to be
sufficiently large, so that the resulting pressure
loading cannot lead to plastic deformation of the
clamped parts to be joined, and to be as small as
possible, in order that joints can even be made at
poorly accessible locations.
It has been found that metal forming may be
improved still further if the sheet metal parts are
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joined at a temperature which is increased above the
ambient temperature or that of the tools; in this case,
a temperature difference of 10~C to 50~C already
results in an improvement of the load capacity of the
joint, given the same joining force, or in the case of
the same load capacity as in the case of joint elements
produced at ambient temperature, the joining force may
be reduced by up to 15%. One reason for this resides in
a reduction in the work-hardening, such as occurs in
particular in the case of austenitic sheet metal
materials.
Using the press-joining device according to the
invention, it is also possible for sheet metal
materials with a pronounced tendency to work-hardening
to be press-joined, without plastic deformations of
punch and anvil or failure of the tool basic body
occurring. In this case, the modular construction of
the punch and die permits the tool set to be loaded
more highly, since cracks are forestalled.
Consequently, the tool set is able to absorb higher
joining pressures.
Preferred high-strength materials for the punch
and anvil pin are hard metal and high-strength ceramic.
In order to support the punch and anvil pin in
a manner suitable for their stress, these may each be
supported on a planar supporting plate.
The anvil face preferably has a shape which is
suitable for material flow in the form of a
circumferential, flat chamfer, with which an improved
interlock is achieved with the same sheet metal bottom
thickness out of sheet metal. At the same time, such a
chamfer leads to the pressure on the edge being
relieved, as a result of which the service life is
increased.
The active faces of punch and anvil pin may
have a star-shaped grinding pattern, as a result of
which their service life is further increased and the
material flow is improved.
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Further refinements of the invention are to be
taken from the following description and the subclaims.
The invention will be explained in more detail
below with reference to exemplary embodiments
illustrated in the appended figures, in which:
Fig. 1 shows, in schematic form, a partly sectioned
side view of a first exemplary embodiment of a
press-joining device,
Fig. 2 shows a partly sectioned side view of a die
with a pressing element according to a further
exemplary embodiment of a press-joining device,
Fig. 3 shows a plan view of an active face of a punch
according to Fig. 2,
Fig. 4 shows a side view of an anvil according to a
further exemplary embodiment of a press-joining
device,
Fig. 5 shows a plan view of an active face of an anvil
pin according to Fig. 4,
Figs. 6, 7 and 8 are enlarged side views of further
embodiments of the area marked with a dashed
circle in Fig. 4,
Fig. 9a shows, in partial section, a press-joining
element produced using the device according to
Fig. 1,
Fig. 9b shows, in partial section, a press-joining
element produced using a device without a
pressing element, given the same bottom
thickness of the joint element as in Fig. 9a,
Fig.lOa shows the press-joining element produced
according to Fig. 9a,
Fig.lOb shows, in partial section, a press-joining
element produced using a device without a
pressing element, given a bottom thickness of
the joint element that is reduced as a result
of a greater joining force than in Figs. 9a, 9b
and lOa.
Fig. 1 shows a first exemplary embodiment of a device
for press-joining, comprising a tool set with a punch 1
CA 022~72~1 1998-12-04
and a die 2, which are each arranged in a press-joining
tool holder 3, 4.
As its central area, the die 2 has an anvil 5
which at the top forms an upsetting table 6 (anvil
working face) of a press-joining mould cavity 7 in the
die 2. The press-joining mould cavity 7 delimits the
displacement of a volume of material by means of die
elements, projecting vertically with respect to the
upsetting table 6, in the form of mould pieces 8, 9,
which are able to move out sideways during an upsetting
operation. The mould pieces 8, 9 additionally form,
with their respective tops 10, 11, a die rest for
sheet-metal parts 12, 13 to be joined. In order that,
following a respective joining operation, the mould
lS pieces 8, 9 which have move out can be put back into
position once more and close the die 2, two restoring
springs 42, 43 are preferably combined in a spring cap.
The die 2 can be fastened to the tool holder 4 by means
of a fastening screw 14.
The punch 1 has a punch pin 15 with a punch
active face 16, which penetrates into the press-joining
mould cavity 7 with a selectable joining force for a
press-joining operation. During this joining operation,
a volume of material of the sheet metal parts 12, 13
that is located at a joint location under a punch
active face 16 is initially displaced, that is to say
displaced out of the plane of the sheet metal parts 12,
13, until a bottom 17 (cf. Figs. 9a and lOa) of the
joint being produced rests on the anvil 5, and is then
upset with spreading.
The punch 1 is enclosed by a pressing element
18, which permits force to be applied to the sheet
metal parts 12, 13 around a joint location, that is to
say adjacent to the punch active face 16. For this
purpose, the pressing element 18 presses the sheet
metal parts 12, 13 lying around the die active face 16
against the die rest. The pressing element 18 is
therefore intended to exert a counter-action to a
reverse-flow force of a material. The level of the
CA 022~72~1 1998-12-04
application of force respectively the pressing force
can be controlled. This control may be separate from
the application of the joining force or linked with the
latter.
Spring elements 19, 20 for applying the
pressing-element force are preferably provided. In
addition, the pressing element 18 is preferably guided
via the spring elements 19, 20 on the tool holder 3 for
the punch 1. Such guidance makes it possible in a
simple way to increase the application of force by the
pressing element 18 during the joining operation. To
this end, the pressing element 18 can be provided with
an active face 21, against which the punch 1 operates,
whilst compressing the spring elements 19, 20 and
increasing the pressing force. The pressing element 18
is preferably designed as a pressure plate and can
additionally also be used as a stripping element at the
end of a joining operation.
In order to apply the pressing-element force,
it is also possible for pressure transmitters other
than springs to be used to produce the pressure forces.
In addition, it is not necessary for the pressing
element 18 to be guided on the punch. It is also
possible for the pressing element 18 to carry out its
function by means of an external guide and an external
drive.
The active face 21 of the pressing element 18
is dimensioned to be sufficiently large when the
resulting pressure loading does not lead to plastic
deformation of the surface of the sheet metal parts 12,
13 to be joined, and the mould pieces 8, 9 of the die 2
are also able to open during the joining operation.
The punch 1 and the die 2 are preferably
modularly constructed. To this end, the punch
comprises the punch pin 15, at whose top end the die
active face 16 is constructed, and which is pressed
into a basic body 23. For this purpose, the basic body
23 preferably has a through hole 24. This modular
construction permits the use of different materials for
.... .. .
CA 022572~l l998-l2-04
the punch pin 15 and the basic body 23. The punch pin
may consist of a first material, specifically a
material of relatively high hardness or relatively high
strength. Preferred materials for this are hard metal
and high-strength ceramic. The basic body 23 may
consist of a second material, which is less hard but
for this reason, for example, is more ductile than the
first material. Thermal treated tool steel is preferred
as the second material.
At its end facing away from the working active
face 16, the punch pin 15 iS in addition preferably
supported on a plate 25, which may consist of the same
rnaterial as the punch pin 15 and which, for its part,
is supported in the tool holder 3 of a press (not
illustrated). The transverse dimensions of the plate 25
are selected such that the load acting on the tool
holder 3 can be absorbed by the latter without risk of
fracture. The punch 1 can be fastened to the tool
holder 3 by means of a fastening screw 26.
The die 2 can be correspondingly modularly
constructed, like the punch 1. The anvil 5 is then
formed by an anvil pin 22 made of a first material,
specifically a material of relatively high hardness or
relatively great strength, such as hard metal or high-
strength ceramic, for example. The anvil pin 22 iS
pressed into a basic body 27 made of more ductile
~aterial, for which purpose the latter has a through
hole 29. The material pairing can be selected as in the
case of the punch. The anvil 5 may also be supported on
the tool holder 4 via a plate 28. The plate 28 can be
designed corresponding to the plate 25 for the punch 1.
Figs. 2 to 5 show a second exemplary embodiment
of a press-joining device, which differs from the
exemplary embodiment described above only in the
differences cited below. Otherwise, the abovementioned
designs apply in a corresponding manner, so that the
same reference symbols can be used for identical parts.
Fig. 2 shows a punch 1 of modular design having
a punch pin 15, on which the die active face 16 iS
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designed, said pin being pressed into a basic body 23
and being supported on a base plate. Part
circumferential grooves 30 are used for changing the
tool rapidly, and hence for fastening the punch 1 to a
tool holder. As in the case of the exemplary embodiment
according to Fig. 1, the active end of the punch is
surrounded by a pressing element 18, which is supported
on the basic body 23 via springs 19, 20. Via this
pressing element 18, which encloses the punch 1 as
closely as possible, but with a sliding fit, and the
springs 19, 20, the pressing forces according to the
invention are now applied, and press the pressing
element 18 so forcefully onto the die that the sheet
metal material is clamped firmly around the punch 1,
and the material of the sheet metal on the punch side
is thus prevented from flowing upward, that is to say
counter to the active direction of the punch 1, during
upsetting process. It is possible to press on the sheet
metal on the die rest before the start of displacement,
so that no bending of the metal sheets during
displacing needs to be feared either.
The pressing element 18 according to Fig. 2
differs from that of Fig. 1 in that the active face 21
of the pressing element 18 runs continuously.
Fig. 4 shows the associated die 2 with an anvil
5, which is pressed into a basic body 27 and is
supported on a base plate 28. The basic body 27 has, on
its side facing the punch 1, oblique surfaces 32, on
which die elements (not illustrated) are supported,
resting with spring prestress on the anvil 5 and,
together with the active face 6 of the anvil, limiting
the die cavity.
In order to prevent the risk that the anvil 5
and/or the die pin 15 will break out at the edge under
high pressure loading, a circumferential chamfer 31 is
provided, whose cross-sectional shape, as illustrated
in Figs. 6 to 8, may be variable: rounded (Fig. 6),
flat bevelled (Fig. 7) or double bevelled (Fig. 8).
This chamfer also promotes material flow.
.
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The mutually facing active faces 16, 6 of punch
1 and/or anvil 5 are provided with a surface finish
which promotes the flow of the material to be joined,
as can be seen on the star-shaped grinding pattern in
Figs. 3 and 5. The active faces 16 and 6 in the case of
the device according to Fig. 1 may also be designed
correspondingly.
A press-joining method for connecting sheet
metal parts 12, 13 lying one upon another comprises a
joining operation in which, by means of a punch 1 and a
die 2, at a joint location a volume of material of the
sheet metal parts 12, 13 is displaced locally and, as a
result of upsetting perpendicular to the plane of the
sheet metal, is joined as the material flows. During
this joining operation, the sheet metal parts 12, 13
surrounding the joint location are pressed onto the die
2 with a pressing force which is sufficient to prevent
any flow of material counter to the direction of action
of the punch 1.
The pressing force can be kept constant during
the entire joining operation. Alternatively, the
pressing force can be increased during the joining
operation, in order that, at least during upsetting, a
pressing force which exceeds the reverse-flow force
acts in the area surrounding the joint location,
whereas the said force can be selected to be lower
before this. It is also possible for a first pressing
force to be applied around the active face of the punch
before or shortly before the joining operation.
The sheet metal parts 12, 13 are pressed in an
essentially stationary manner on the die 2, to be
specific with such a pressing force that the reverse
flow of material is prevented. This applies, in
particular, for materials with a particular tendency to
work-hardening, or pre-hardened materials.
The pressing force is in particular selected
such that, during the joining operation, the sheet
metal parts 12, 13 do not gape in the area surrounding
_ _
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the punch 1, that is to say the sheet metal parts 12,
13 lie flat one upon another.
In addition, the pressing force for flat, even
sheet metal parts can be selected such that it is five
times to eight times the forces which are needed merely
to prevent the joint parts bending up during the
displacement phase. A guide value, mentioned by way of
example, for the force applied in the case of a joint
element diameter of 5 mm is 3000 N, if, for example,
austenitic stainless steel sheets are being joined.
In this case, the pressing force is preferably
applied to an effective contact area of such a size
around the joint location that the pressing force to be
used leaves behind no impression in the sheet metal
parts, and the die can open sideways.
Furthermore, the temperature of the sheet metal
parts 12, 13 can be increased above the ambient
temperature. Preference is given to temperatures that
are 10~C to 50~C higher than the ambient temperature.
2 0 Fig. 9a shows, by way of example, a joint
produced using the above-described press-joining
devices, the pressing force exerted by the pressing
element 18 during the joining operation being indicated
by the arrows and the letter F. Sheet metal parts 12,
13 made of a particularly work-hardening material and
having an initial sheet metal thickness of about 1 mm
in each case have been connected with interlock by a
joint element whose bottom 17 has been upset between
the active face 16 of the punch and the working face 6
of the anvil in the mould cavity 7, whilst reducing the
overall initial thickness. A spreading 33 has been
formed as a result of material flow, for which purpose
the mould pieces 8, 9 have moved outwards. The result
is a joint with a good interlock. At the end of the
joining operation, the sheet metal parts 12, 13 do not
gape in the area surrounding the punch, so that these
sheet metal parts lie flat one upon another and without
any gap areas.
CA 022~72~1 1998-12-04
By comparison with this, Fig. 9b shows a joint
with a punch 1 and a die 2 with a punch assembly but
without a pressing element. In the case of achieving
the same bottom thickness, that is to say approximately
equal joining forces, an effective interlock is still
not achieved.
Figs. lOa and lOb serve to clarify the
inventive method further. The joint according to
Fig. lOa corresponds to that illustrated in Fig. 9a.
The joint of Fig. lOb differs from that of Fig. 9b in
that increased joining forces have been used, which
have led to a higher upsetting degree of the bottom 17
of the joint. However, the spreading 34 achieved in
this way still does not exhibit an adequate interlock.
Figs. 9a, 9b and lOa, lOb make it clear that
the inventive method permits the use of lower joining
forces, since the effective interlock is already
achieved with lower upsetting rate.
.
