Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND DEVICE FOR PRODUCING
A SCREW CONNECTION POINT
The invention relates to a method for producing a screw connection point,
according to the preamble of claim 1, and to a device for executing the
method,
according to the preamble of claim 7.
A method of the generic type, or a device of the generic type, is known from
DE
197 33 477 Al. In that case, a punch having a central bore, in which a
stamping
punch is guided, is integrated into a hydroforming tool. The free end of the
stamping punch projects beyond the end of the punch in which it is received,
an
attachment having a through opening being threaded onto the end of the
stamping punch. When the stamping punch is in the position of non-use, the
free
end terminates flush with the side of the attachment that faces away from the
receiving punch. The punch, for its part, is displaceably guided in a guide
bore in
the hydroforming tool, which guide bore opens into the die relief of the
forming
tool, which die relief delimits a forming cavity. There is inserted in the
forming
cavity a hollow profile blank, which, after the hydroforming tool has been
closed, is
expanded, through the generation of a fluidic inner high pressure, until it
bears on
the die relief of the hydroforming tool in a true-to-contour manner, thereby
creating
the wanted form of the hollow profile. The punch, together with the piercing
punch
and the attachment, is then moved towards the hollow profile, as a result of
which
the latter becomes impressed, such that there is realized a pocket having
approximately the dimensions of the attachment. Owing to the continued action
of
the fluidic inner high pressure, the wall of the hollow profile bears
intimately on the
outside of the attachment, and binds the latter in the wall of the hollow
profile.
Finally, in the position assumed by the receiving punch, the piercing punch is
moved further forwards, as a result of which the hollow profile is pierced
through
by the attachment, which has an internal thread. The hole slug falls into the
interior of the hollow profile.
An attachment joined in such manner to a hollow profile is subject to
relatively
large tensile forces, however, it being possible for this attachment to become
detached from the pocket. In this case, the screw connection point is
destroyed.
The invention is based on the object of so developing a method of the generic
type that the production of a durable screw connection point on the periphery
of a
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hollow profile is rendered possible in a manner that is simple and reliable in
respect of the process. Furthermore, a device for executing the method is to
be
specified.
The object is achieved, according to the invention, by the features of claim 1
in
respect of the method, and by the features of claim 7 in respect of the
device.
The resistance welding of the attachment, which carries an internal thread,
onto
the hollow profile creates an extremely durable screw connection point, which
also
withstands large mechanical stresses in the tensile and compressive
directions.
Owing to the fact that the electrode simultaneously guides the attachment up
to
the site where the welded connection point is to be effected, production of
the
screw connection point is rendered simpie in respect of the economy of the
process. Owing to the attachment being arranged in a receiving cavity
constituted
by the electrode and its guide bore, it can be positioned on the hollow
profile in a
defined, reliable, precise and, above all, reproducible manner. This applies
irrespective of the production tolerances of the attachment, since the
electrode is
integrated within the hydroforming tool, and the electrode thus always
approaches
the same point on the hollow profile upon each new welding operation. The
forming by expansion results in the hollow profile bearing on the attachment
in a
true-to-contour manner, as a result of which the joint surface extends over
the
entire side of the attachment that faces towards the hollow profile, and thus
guarantees maximum hold. The mutual bearing contact is thus not localized, but
planar. Furthermore, in the case of mechanical stresses the introduction of
force
is distributed, as a result of which these stresses can be better absorbed by
the
hollow profile, and the screw connection point remains undamaged. Since the
operation of guiding-up and positioning the attachment is assumed by the
electrode, there is no need for additional tools respectively serving one of
these
purposes. Owing to the electrodes of the resistance welding facility being
integrated into the hydroforming tool, the amount of required equipment is
reduced and there is a substantial saving in structural space. In addition,
there is
no need for the attachment to be clasped mechanically by the hollow profile,
which would result in a thinning of material on the hollow profile.
Consequently, in
the case of the screw connection point according to the invention, there is no
susceptibility to cracking, particularly on the hollow profile, during the
joining
operation, a high process reliability being ensured in the case of the
production
method according to the invention.
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In a particularly preferred development of the invention, according to claim
2, the
welding is effected before the piercing. Pressure fluid is thereby prevented
from
reaching the welding point, which can impair the quality of the welding
operation.
Additionally achieved is the fact that, in the production of the screw
connection
point, there is no premature drop in pressure, which might possibly occur upon
piercing and, owing to the pressure of the electrode, result in a sink point
on the
hollow-profile. Such a sink point is disadvantageous to full-surface welding
of the
attachment onto the hollow profile. Moreover, the attachment has already been
immovably located on the hollow profile at a particularly early point in time,
such
that there can be no difficulties whatsoever in positioning the attachment in
relation to the hollow profile. In the case of very rapidly succeeding process
steps
of welding and piercing, there additionally ensues the advantage that the
welding
heat following the welding operation is at least partially concentrated on the
welding point, as a result of which the hollow profile material is still
readily
formable and workable in the region of the screw connection point. The result
of
this is that the process step of piercing is facilitated, owing to the
relative softness
of the hollow profile material caused by the heat.
In a further particularly preferred development of the invention, according to
claim
3, the hollow profile is pierced through the bore of the attachment. Achieved
thereby is a taught guidance of the piercing punch, as a result of which the
latter
always impacts the hollow profile at the correct point. Very high production
accuracies are thereby achieved for the screw connection points.
In an alternative preferred development of the method according to the
invention,
according to claim 4, the attachment is pressed into the hollow profile wall
by
means of the electrode. The attachment, which has an end-face cutting
geometry, itself produces the hole in the hollow profile wall. Welding to the
hollow
profile is then effected. In the case of this variant of the invention, the
attachment
attains its exact position relative to the hollow profile as it is pressed
into the
hollow.profile wall. The preliminary, but precise locating of the attachment
on the
hollow profile and the simultaneous creation of a hole by the attachment
simplify
the entire production process, since there is almost no more need for the
attachment to be held in its pressing-in location and pressed onto the hollow
profile. The production process is accelerated, since creation of the hole is
already effected at the point in time at which the attachment is positioned on
the
hollow profile. Since the attachment has already been pressed into the hollow
profile wall, there is no need for any additional mechanical welding pressures
to
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be applied upon the subsequent, final locating of the attachment to the hollow
profile by resistance welding, this in turn simplifying the welding equipment,
since
there are no corresponding pressing devices. In order to keep the hollow
profile
as tight as possible upon being pierced, the stub, realized on the attachment,
that
is pressed in has a shape having a slight conical taper towards the interior
of the
hollow profile. In the case of the corresponding advantageous development of
the
device, according to the invention, as claimed in claim 9, the electrode
itself
constitutes the punch, the attachment bearing on the end face of the latter.
As
already stated, the attachment has a cutting geometry that faces towards the
hollow profile. The electrode in this case does not constitute a piercing
punch per
se, but a punch that can press the attachment into the hollow profile. There
is no
separate piercing punch whatsoever, since the cutting geometry is
advantageously provided on the attachment itself, and the latter thereby
assumes
the function of the piercing punch. The absence of the piercing punch
additionally
simplifies the control of the device for producing the screw connection point.
In a preferred development of the invention, according to claim 5, the
piercing is
effected from the outside inwards, through material displacement of the hollow
profile material. Based correspondingly thereon, according to the preferred
development of the device according to the invention, according to claim 11,
the
punch has an arrow-shaped tip at its free end. Owing to the material
displacement of the hollow profile that is effected through the inner high
pressure
acting in combination with the arrow-shaped tip of the punch penetrating the
hollow profile for the purpose of piercing, no hole slug is produced, removal
of
which would entail resource expenditure. At the same time, the punched
displacement bead constitutes an extension of the guide for a screw that is
screwed into the attachment and the hollow profile. This screw, for example by
means of its thread, can then cut an internal thread into the displacement
bead as
its is screwed in.
In a further preferred development of the invention, according to claim 6,
during
piercing a hole slug is parted out from the hollow profile wall, from the
outside
inwards. The production of a hole slug results in the achievement of a highly
precise hole size. Also prevented thereby is the occurrence of dimensional
reductions that reduce the bearing contact surface of the hollow profile on
the
attachment. With the therewith associated maintenance of the entire available
surface of the hollow profile for the joining connection to the attachment,
this
connection, if already effected, is not damaged, and if not yet effected is
afforded
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a particularly good durability with the welding operation, owing to the
bearing
contact surface having remained maximal.
In a further particularly preferred development of the device according to the
invention, according to claim 8, the electrode arranged in the guide . bore
surrounds the punch concentrically. This results in the punch being delivered
accurately to the bore of the attachment, which is located in the receiving
cavity
that is itself coaxial in relation to the electrode. The punch is thereby
prevented
from colliding with the attachment. Owing to the punch being enclosed by the
electrode almost without play, the punch is guided in a taught and reliable
manner
in the electrode.
In a further particularly preferred development of the invention, according to
claim
10, one of the electrodes is constituted by the hydroforming tool itself. This
reduces quite considerably the amount of equipment required for arrangement of
the device, since it is not necessary for the second electrode then to be
additionally introduced into the device, or into the hydroforming tool. Since
the
hydroforming tool consists of a conductive metal, in particular iron, or of a
steel in
any case, it is easy to connect this hydroforming tool to an electric pole.
Advantageously, this pole can be attached to any given location of the forming
tool, such that the electrical connection can be arranged at the location that
appears most suitable to the constructor of the installation. Owing to the
absence
of a separate electrode, the number of components of the device is reduced,
which again simplifies the latter.
Otherwise, it remains to be noted here that, owing to the attachment being
welded
onto the hollow profile before the piercing operation, the hollow profile is
reinforced by the attachment, countering an impressing movement of the hollow
profile wall upon the impingement by the piercing punch. Consequently, the
hollow profile wall does not sink-in in an unwanted manner.
Advantageously, capacitor discharge welding is to be used for resistance
welding.
This method is distinguished by a rapid current rise with a high-current
pulse, of
several hundred thousand amperes, lasting only for milliseconds, a short
welding
time and high welding currents. The application of this method for connecting
the
attachment to the hollow profile has the consequence that no cooling is
required
for the welding operation, and welding is effected without annealing and
delay,
such that the finished, worked parts retain their dimensional accuracy
following
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welding. The welding method renders possible the use, reliably in respect of
the
process and with the achievement of a joining quality that can be subjected to
large mechanical stresses, of heat-sensitive parts, coated metals, steels
having a
carbon content greater than 0.2 per cent, and differing material thicknesses
of the
two elements to be joined. Quality monitoring, with recording of the welding
data
(force, energy, welding current and melt) is very simple. At the same time,
there
is only a small demand upon the electrical power network, owing to the use of
storage capacitors.
The invention is explained more fully in the following with reference to a
plurality
of exemplary embodiments represented in the drawings, wherein:
Fig. I shows a cross-sectional representation of a device according to the
invention during the piercing operation, with the production of a hole
slug,
Fig. 2 shows a cross-sectional representation of a dev.ice-according tothe _
invention during hole formation by material displacement by a punch,
Fig. 3 shows a cross-sectional representation of a device according to the
invention, with an attachment pressed into a hollow profile wall, with
realization of a hole slug.
Represented in Figure 1 is a device for producing a screw connection point 1
on
the periphery 34 of a hollow profile 7. The device includes a hydroforming
tool 2,
which consists of a top die 3 and a bottom die 4. Worked-in on the mutually
facing sides of the dies 3 and 4 is a die relief 5, which, when the tool 2 is
closed,
i.e. when the top die 3 bears on the bottom die 4, delimits a forming cavity
6. The
hollow profile 7 is received in this forming cavity 6, for the purpose of
forming and
piercing. Worked into the mutually facing sides of the top die 3 and the
bottom die
4 is a guide bore 9, the axis 35 of which lies in the plane of the parting
line 8 of the
two dies 3 and 4. The guide bore 9 leads, on the one hand, out of the
hydroforming tool 2 and, on the other hand, it opens into the die relief 5 of
the
forming tool 2. Inserted in the guide bore 9 is an electrically insulating
sleeve 10,
which extends over the entire guide bore 9 and bears closely on its wall 36.
An
electrode 11 of an electric resistance welding facility of the device
according to the
invention is guided displaceably, with little play, in the sleeve 10. The
electrode
11, with its end face 14 and the inside 15 of the sleeve 10, delimits a
receiving
cavity 16, in which an attachment 17 is arranged coaxially in relation to the
electrode 11. The receiving cavity 16 is fashioned such that it is open
towards the
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die relief 5, or towards the hollow profile 7. The electrode 11 has a central
bushing 12, through which there extends a punch 13. The electrode 11 in this
case surrounds the punch 13 concentrically, the latter being displaceably
driven in
the bushing 12.
For the purpose of producing the screw connection point 1, a hollow profile
blank
is first inserted in the forming cavity 6 of the hydroforming tool 2 and
expanded
through generation of a fluidic inner high pressure in the interior 23 of the
hollow
profile blank, until the latter is pressed onto the die relief 5 of the
hydroforming tool
2 in a true-to-contour manner. The receiving cavity 16 is thereupon provided
with
an attachment 17, which has a central through bore 18 having an intemal
thread.
In a feed movement of the electrode 11 together with the punch 13, the
attachment 17, which is threaded onto the end 37 of the punch 13 that projects
out of the electrode 11, is moved up to the hollow profile 7 such that it is
enclosed
between the latter and the end face 14 of the electrode 11, the attachment 17
being pressed, on its end face 19 that faces towards the electrode 11, onto
the
end face 14 of the electrode 11, and on its end face 20 that faces towards the
hollow profile 7, onto the periphery 34 of the latter. In this state, the
electric
resistance welding facility is put into operation, an electric pole being
applied to
the hydroforming tool 2, i.e. either the top die 3 or the bottom die 4, such
that the
respective tool part constitutes the other electrode of the resistance welding
facility. In the flow of electric current that is generated, the attachment 17
is
thereby welded onto the periphery 34 of the hollow profile 7, with a welded
connection 24 being formed. As soon as the attachment 17 has been firmly
attached to the hollow profile 7 by the welded connection 24, the punch 13,
which
is realized as a stamping punch, travels forwards in the direction of the
hollow
profile 7, traveling through the bore 18 of the attachment 17 and stamping out
of
the hollow profile 7 a piece of wall, which constitutes a hole slug 22,
forming the
hole 21 into the interior 23 of the hollow profile 7. This is effected while
there is
inner high pressure in the interior 23 of the hollow profile 7. After
formation of the
hole, the fluidic inner high pressure in the hollow profile 7 is then removed,
and
the punch 13 is withdrawn from the forming cavity 6, or from the interior 23
of the
hollow profile 7. The hole slug 22 in this case falls into the interior 23 of
the
hollow profile 7. The hydroforming tool 2 can now be opened, and the hollow
profile 7, with the screw connection point 1 constituted by the attachment 17
and
the hole region of the pierced hollow profile wall, can be removed from the
forming
tool 2. In order to effect the purpose of-the screw connection point 1, a
screw or a
pin having an external thread can be screwed into the latter.
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In divergence from the preceding exemplary embodiment, in a new variant of the
invention the punch 25, which is comparable to the punch 13, is realized with
an
arrow-shaped tip 27 at the free end 26 (Figure 2). For the purpose of
producing a
hole 38, after the attachment 17 has been welded onto the hollow profile 7 the
punch 25 is moved onto the hollow profile 7, the punch penetrating the wall of
the
hollow profile 7 by means of its tip 27. Under the counteracting force of the
inner
high pressure prevailing in the interior 23 of the hollow profile 7, the
hollow profile
material impinged upon by the punch 25 is displaced away sideways, forming a
hollow profile bead 28. At the same time, a sink cavity 29 is produced, which
forms on the outer periphery 34 of the hollow profile 7, beneath the
attachment
17. The method described in this exemplary embodiment thereby produces a
hole 38 without a hole slug 22. To remove the hollow profile 7, as in the case
of
the stamping punch 13 of the preceding exemplary embodiment the punch 25 is
drawn back out of the hollow profile 7, being so drawn that it is also at a
distance
from the bore 18 of the attachment 17. Clearly, the fluidic inner high
pressure is
removed before the hydroforming tool 2 is opened, and the pressure fluid is
taken
away.
A further variant of the invention is shown by Figure 3. There, the electrode
30 is
of a solid design, and has no bore. The attachment 31 is realized according to
the
model of a stamping nut. In contrast to the two preceding exemplary
embodiments, here it is the piercing of the hollow profile 7, not the welding-
on of
the attachment 31, that is performed first. The electrode 30, which in this
case
assumes the role of a punch, presses the attachment 31 against the wall of the
hollow profile 7 in such a way that a stub 32 of the attachment 31, which stub
has
a cutting geometry 33, is pressed into the hollow profile wall, a hole slug 22
and
the hole 39 being produced at the same time. The attachment 31, fixed thus in
the hollow profile wall, is now resistance-welded to the latter. In order to
ensure
the outward tightness of the hollow profile 7 under the counteracting inner
high
pressure, the stub 32 is realized with a slight conical taper towards the
interior 23
of the hollow profile, such that wedge faces, which effect a particularly good
tightness capability with the conical shape of the stub 32, are produced at
the
edges of the produced hole 39.
In all three stated exemplary embodiments, it is conceivable for the
electrically
insulating sleeve 10 to be likewise displaceable in the guide bore 9. This is
particularly helpful when the hollow profile 7, with the produced screw
connection
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point 1, is to be removed without impediment from the hydroforming tool 2. In
this
case, the hollow profile 7 can be released from the forming cavity 6 in a
lifting
movement when the hydroforming tool 2 is open, the sleeve 10 remaining in a.
position drawn back from the screw connection point 1, or from the attachment
17,
31.
