EP1946864B1 - Online determination of the quality characteristics during self-piercing riveting or clinching - Google Patents

Description

The present invention relates to an on-line determination of compression measure and and Nietkopfendlage a rivet in a punch riveting process.

Punch riveting is a joining process performed with rivet elements. These rivet elements include solid rivets and semi-hollow punch rivets.

After the punch riveting, the punched rivet joint is subjected to quality control. A distinction is made between non-destructive and destructive quality control. As a means of non-destructive quality control, the visual inspection, the control of the outer joining geometry and the process monitoring are used on an industrial scale. However, the visual inspection only provides general information about a manufactured rivet connection, since only external features of the punched rivet connection are available. These include, for example, in a half-holed rivet joint, the flush of the rivet head, the condition of the die-side sheet, the damage of adherend surfaces by the hold-down, and the orientation of the rivet with respect to the die.

Even in the control of the outer joining element geometry, only the externally visible sizes of the produced joint connection are available. These are the Nietkopfendlage, the upsetting measure in the punch riveting with semi-hollow punch rivet and the embossing depth in punch riveting with solid punch rivet.

Furthermore, the quality control process monitoring is used, which is based on force-displacement data of the joining process. To evaluate the joining processes, the force-displacement curve of a manufactured optimum joint connection is used as a reference curve. For example, envelopes, tolerance bands or process windows are placed around this reference curve in order to be able to determine a deviation of the force-displacement data from the reference curve during a joining process.

Another alternative to quality control is the above-mentioned destructive examination of the joint connection produced. For destructive quality control, macrosections of the joint connection are made and / or strength tests of the joints Joined joint performed. From a macrosection is a flatness of the joining parts in the joining zone, a gap formation between the joining parts, a flush of the rivet head with a punch side plate, an undercut and a crack-free joint connection evaluable. The mentioned strength test allows statements about the carrying capacity of the punched rivet joint under shear, peel and Kopfzugbelastungen.

Usually, the joining parameters and geometric parameters for the joint connection are determined in preliminary tests in practice. On this basis, the characteristics Nietkopfendlage and upsetting dimension of an optimal joint connection are assumed as reference variables, since they can be determined non-destructive. As a result, the cost of destructive quality inspection can be reduced. But even these reference variables must be measured individually after each joining process. This involves a lot of time and is not suitable for mass production. Another alternative is the random control of the above reference quantities.

It is therefore the object of the present invention to provide a method, which is improved in comparison with the prior art, for checking the quality characteristics of joint connections.

The above object is achieved by the method according to independent claim 1. Further developments and advantageous embodiments of the present invention will become apparent from the following description, the accompanying drawings and the appended claims.

The method according to the invention discloses an online determination of compression measure x _ST and rivet head end position K _{HS of} a semi-hollow punch rivet having a length L in a punch riveting operation with the aid of a movable punch and a rigid die. The on-line determination comprises the following steps: detecting a path traveled by the movable punch during the punch riveting operation with the aid of a displacement pickup, detecting a force applied to the semi-hollow punch rivet by the movable punch during the punch riveting operation in dependence on the distance covered, determining an attachment point x ₂ of Rivet on a joining part and a relief point x ₄ , which identifies a relieving of the stamp after the punch riveting process, from the detected force-displacement data and calculating the Nietkopfendlage K _HS according to K = x ₂ + Lx ₄ and the upsetting measure x _ST according to x _ST = xx ₄ , while x describes the maximum distance between mutually facing sides of punch and die.

The present invention is based on the acquisition and evaluation of force-displacement data of each individual joining operation. During the punch riveting operation, on the one hand, the path traveled by the punch and, on the other hand, the force applied by the punch to the semi-hollow punch rivet are recorded and evaluated together. If the recorded force-displacement data of the punch riveting process are represented as a curve in a force-displacement diagram, relevant variables for calculating compression dimension x _ST and rivet head end position K _HS from this illustration or even from typical changes in force-displacement data are without Curve representation derivable. The attachment point x _{2 of} the Halbhohlstanzniets on joining part can be detected, for example, in the force-displacement data on detecting a missing change in the detected movable path of the punch despite a punch feed. According to a further alternative, the attachment point x ₂ in the force-displacement data is identifiable as the path at which the detected force exceeds a holding force of a setting head or hold-down by a certain threshold. If no setting head or hold-down is used, it is also conceivable to set the threshold at any other initial power value.

The detected force-displacement data are detected and evaluated according to an embodiment in a data processing unit, in particular in a computer. For this purpose, for example, the data of the transducer and the force sensor are transmitted directly or via an analog-to-digital converter to the data processing unit.

It is furthermore preferred to calculate a reference variable Δx _c for a machine rigidity of the joining machine according to Δx _C = x ₃ -x ₄ . This reference value indicates how yielding the constructive connection between punch and die is. If, for example, the punch riveting process is carried out with the aid of a C-frame, it can be determined from the reference variable Δx _C whether material fatigue occurs due to the joining processes in the C-frame. To calculate this reference quantity, the point x _{3 is} detected from the force-displacement data as the path at which the maximum force F _{max of} the punch is reached during the joining operation.

According to a further embodiment, the force-displacement data of the joining process are displayed as a curve in a force-displacement diagram. After reaching the maximum force F _{max of} the punch, the punch is returned, resulting in a mechanical relief of the punch and the rivet connection. This retraction of the punch is called return in the force-displacement data of the joining operation. Immediately after reaching the maximum force F _{max of} the punch, the return at the beginning of an approximately linearly sloping course. A point x ₄ can be identified within this retrace by applying a tangent to the approximately linear force-displacement data at the beginning of the retrace, such that deviating the force-displacement data by a predetermined value from the tangent to the point x ₄ indicates.

With the acquisition of force-displacement data during the joining process and the immediate evaluation in the computer can thus be an online determination of compression measure x _ST and Nietkopfendlage K _HS perform as quality control. With these automatically documented quality characteristics, process capability studies are carried out and quality control charts are written. Furthermore, statements can be made about geometric sizes and bearing behavior of the joint connection achieved, which were previously to be determined only by the destructive examination of the joint connection. The correlations and correlations of the quality parameters that can be managed by neural networks are used.

Analogous to the online determination of quality parameters in the punch riveting of semi-hollow rivets, this method is also applicable to the punch riveting of solid punch rivets and clinching. The essential method steps for the on-line determination of embossing depth h _d and rivet head end position K _{VS of} a solid punch rivet with a length L in a punch riveting operation with the help of a movable punch and a die can be summarized as follows: detecting a path traveled by the movable punch during the punch riveting process with the aid of a Wegaufnehmers, detecting a force applied to the Vollstanzniet of the movable punch F during the punch riveting operation in dependence on the distance traveled, determining a Ansetzpunktts x _{2 of} the Vollstanzniets with stamp on a mating part and a relief point x ₄ from the detected force-displacement data, during the Relief point x ₄ relieving the punch after the punch riveting process and calculating the rivet head end position K _VS according to K _VS = x ₂ + Lx ₄ and the embossing depth h _d according to h _d = t- [x- (x ₂ + L)], while x is the maximum distance between facing sides of the die and Die and t describes a thickness of the parts to be joined.

During clinching, the following steps are carried out for the online determination of the quality parameter floor thickness t _b : detection of a path traveled by the movable punch during the clinching process with the aid of a displacement transducer, detection of a force F applied to a joining part by the movable punch during the clinching operation as a function of the distance traveled Way, determining a relief point x ₄ from the detected force-displacement data that identifies a relief of the punch after the clinching operation, and calculating the ground thickness t _b according to t _b = xx ₄ , while x is the maximum distance between facing sides of the punch and Matrix describes.

Figur 1

eine teilweise Explosionsdarstellung einer Ausführungsform einer Anordnung zur Durchführung des Stanznietens,

Figur 2

eine schematische Teilansicht eines Ausschnitts aus Figur 1,

Figur 3

eine Darstellung der Größen Nietkopfendlage K_HS und Stauchmaß x_ST beim Fügen eines Halbhohlstanzniets,

Figur 4

ein Kraft-Weg-Diagramm, das die während eines Stanznietvorgangs aufgezeichneten Kraft-Weg-Daten sowie markante Positionen während des Fügevorgangs von Halbhohlstanznieten enthält,

Figur 5

die Kraft-Weg-Daten eines Stanznietvorgangs eingetragen in einem Kraft-Weg-Diagramm sowie die markanten Punkte der Kurve, aus denen sich verschiedene geometrische Größen zur Qualitätsbestimmung der hergestellten Stanznietverbindung ergeben,

Figur 6

ein Flussdiagramm zur Darstellung der Verfahrensschritte zum Stanznieten und Clinchen,

Figur 7

eine schematische Darstellung einer Vorrichtung zur Durchführung des Vollstanznietens,

Figur 8

eine Darstellung der Größen Nietkopfendlage K_VS und Prägetiefe h_d beim Vollstanznieten,

Figur 9

eine schematische Darstellung einer Vorrichtung zur Durchführung des Clinchens und

Figur 10

eine Darstellung der Größe Bodendicke t_b beim Clinchen.

Preferred embodiments of the present invention will be explained in detail with reference to the accompanying drawings. Show it:

FIG. 1

a partially exploded view of an embodiment of an arrangement for performing the punch rivet,

FIG. 2

a schematic partial view of a section of FIG. 1 .

FIG. 3

a representation of the sizes Nietkopfendlage K _HS and Stauchmaß x _ST when joining a semi-hollow rivet,

FIG. 4

a force-displacement diagram which contains the force-displacement data recorded during a punch riveting operation as well as prominent positions during the joining operation of semi-hollow rivets,

FIG. 5

the force-displacement data of a punch riveting process entered in a force-displacement diagram as well as the distinctive points of the curve, from which different geometric parameters result for determining the quality of the produced riveted joint,

FIG. 6

a flow chart illustrating the process steps for punch riveting and clinching,

FIG. 7

a schematic representation of an apparatus for performing the punch-riveting,

FIG. 8

a representation of the sizes Nietkopfendlage K _VS and embossing depth h _d in the case of full punch riveting,

FIG. 9

a schematic representation of an apparatus for performing the clinching and

FIG. 10

a representation of the size floor thickness t _b when clinching.

The online determination of compression measure x _ST and rivet head end position K _{HS of} a rivet is described below using the example of a punch riveting operation of a semi-hollow rivet. In analogy to the following description, the online determination of quality characteristics for the semi-hollow rivet is also applicable to the punch riveting of a full punch rivet or to clinching (see below).

An embodiment of a joining device for punch riveting a semi-hollow punch rivet is in FIG. 1 shown. It comprises a punch 10 and a die 20, which are arranged opposite one another with the aid of a C-frame 30. The force applied by the punch 10 is detected by means of a force sensor 40, for example a load cell (step A in FIG FIG. 6 ). The distance covered by the stamp 10 is detected by a displacement transducer 50 of known design (compare step B in FIG FIG. 6 ). The force data detected by the force sensor 40 and the path data detected by the displacement transducer 50 are transmitted to a data processing unit 60, such as a computer, where they are stored as force-displacement data of the punch riveting operation. In addition to the preferred representation of the force-displacement data in a force-displacement diagram (see step C in FIG FIG. 6 ) is carried out in the data processing unit 60 in parallel to the joining process, the online evaluation of the detected force-displacement data.

FIG. 2 schematically shows an enlarged section FIG. 1 in which various components of the half-hollow punch rivet are shown. In semi-hollow punch riveting, joining parts 5 are first pressed against the die 20 via a setting head or hold-down 12 with a predetermined hold-down force. The punch 10 then moves a semi-hollow punch rivet 3 toward the die 20 to make the joint. The path traveled by the punch 10 during this movement is detected with the aid of the displacement sensor 50. In the same way, the force applied to the rivet 3 during this movement of the punch 10 is detected by the force sensor 40. It is also preferred to record the holding forces of the hold-down 12 for the joining parts 5 via the force sensor 40 and to include them in the force-displacement data of the joining operation to be evaluated later. The thus determined force-displacement data of the joining process, which both the lead of the punch 10 to the pressing of the semi-hollow punch rivet 3 in the joining parts 5 (see. Solid curve in FIG. 4 ) as well as the return of the punch 10 and blank holder 12 to their original positions (see dashed line in FIG FIG. 4 ) are evaluated online for the joining process in the data processing unit 60.

FIG. 3 shows a schematic section through the joint connection consisting of Halbhohlstanzniet 3 and Fügeteilen 5. The joint connection can be characterized by the quality characteristics Stauchmaß x _ST and Nietkopfendlage K _HS whose geometric significance in a joint in FIG. 3 is shown. The rivet head end position K _HS denotes the distance between the rivet head surface of the half-hollow punch rivet 3 and the surface of the joining part 5. The upsetting dimension x _ST denotes the distance between the rivet head surface of the half-hollow punch rivet 3 and the bottom surface of the joining part 5 below the semi-hollow punch rivet 3.

By analogy with the joining of semi-hollow punch rivets, quality parameters can also be determined online when joining solid punch rivets and clinching. Fig. 8 shows a joint connection consisting of joining parts 5 and a solid punch rivet 4. This connection is characterized by the Nietkopfendlage K _VS as a distance between the Nietkopfoberfläche of Vollstanzniets 4 and the upper surface of the joining part 5. Another quality parameter is the embossing depth h _d , which is a Einpresstiefe a Die 20 (cf. Fig. 7 ) in the lower joining part 5 describes. In addition to this, the quality characteristic floor thickness t _b can be determined online during clinching Fig. 10 is shown.

In the process monitoring of the joining process, ie the online determination and evaluation of the above-described force-displacement data, the travel signals of the punch 10 are recorded (step A). The setting head 12 leads the punch 10 ahead of the punch stroke length. First, the setting head 12 sits on the joining parts 5 and presses the joining parts 5 on the die 20. This moment is in the force-displacement curve of the joining operation according to FIG. 4 at the point P1, to which the path x _{1 has been covered} by the stamp 10. The punch 10 retraces the path corresponding to the punch stroke minus a length L of the rivet 3, and sets the half-hollow punch rivet 3 on the adherends 5 (see point P2 in FIG FIG. 4 ). This point is referred to as the attachment point, which is described by the path x ₂ . Upon further increase of the pressing force of the punch 10, the half-hollow punch rivet 3 is pressed into the joining parts 5 and deformed by the counterforce of the die 20. Upon reaching a predefined maximum force F _{max of} the joining process or a predefined path of the stamp, the semi-hollow punch rivet 3 is sunk in the joining parts 5 (compare P3 after the path x ₃ in FIG FIG. 4 ). During this process, the C-frame is bent up by the pressing together of punch 10 and die 20 due to its elastic material properties and construction. The force-displacement curve up to point P3 is in FIG. 4 is described by the solid line and is referred to as the lead of the punch 10. Starting at point P3 to a zero punching force, the return stroke of the punch 10 is shown by a dashed line. This return of the punch 10 begins by reducing the force applied by the punch 10 so that the bowing of the C-frame 30 decreases. During the reduction of the punch force at the beginning of the return, the force of the punch 10 decreases linearly until the punch 10 at point P4 after the path x _4, the Nietkopfoberfläche only with a minimum force compared to the maximum force F _max during the previous run of the punch 10 touched. The path difference between the points P3 and P4 is attributable to the bending of the C-frame 30. After reaching point P4 in FIG. 4 drive punch 10 and hold-down or setting head 12 back to their normal position.

The process described above can thus be read from the detected force-displacement data of the joining process. In order to carry out the online determination of the quality characteristic upsetting dimension x _ST and rivet head end K _HS , the maximum distance x between the underside of the punch 10 and the upper side of the die 20, preferably the Matrizendorns, to be known. This quantity x results from the construction of the joining device as a constant value. It can be measured manually or is the result of a reference movement of the punch 10 until it touches the die mandrel or the die bottom. The position of the attachment point of the setting head x ₁ at the point P1, the attachment point of Halbhohlstanznietsx ₂ at point P2, the distance traveled stamp path x ₃ when reaching the maximum joining force F _max at point P3, the Nietkopfposition x ₄ after relieving the C frame at point P4 will be out of the FIGS. 4 and 5 shown exemplary process curve or the force-displacement curve or automatically determined in the data processing unit based on certain mathematical criteria from the force-displacement data. To properly detect these positions, the transducer 50 must be calibrated accordingly.

Referring to FIG. 5 the distance x ₁ up to the position P _{1 can} be determined by the fact that, at the position P ₁ , the force applied by the ram 10 exceeds a predetermined threshold value. The exceeding of the threshold value indicates that a pressing force is exerted on the joining parts 5 in the direction of the die 20 by the setting head or holding-down device 12. After the force has reached a preset value with which the set head or hold-down 12 presses against the joining parts 5, this is held over a certain path between the points P1 and P3.

During the transition from the point P1 to the point P2, the punch 10 moves with the semi-hollow punch rivet 3 in the direction of the die 20 until the half-hollow punch rivet 3 touches the top side of the joining parts 5 at the point P2. The attachment point x ₂ at point P2 of the punch 10 at the joining parts 5 can be identified by detecting a missing change in the detected movable travel of the punch 10 despite a punch feed. The missing path change preferably takes place via a punch feed of 1 to 20 increments. The preferred displacement sensor 50 measures, for example, a measuring range of 0-100 mm, 0-150 mm or 0-200 mm. Depending on the detected path, it provides an output signal in a range of 0-10 V. With a resolution of 12 bits, this voltage range is subdivided into 4096 increments. If this is applied to a measuring range of 150 mm, an increment corresponds to a distance of 0.036 mm and an output signal of 0.0024 V. If, according to another alternative, a digital displacement sensor with a 16-bit resolution is used, the measuring range of the Displacement sensor in 65536 increments. For a measuring range of 150 mm, therefore, an increment corresponds to a path change of 0.00229 mm.

According to another alternative, the starting point x ₂ at the point P2 in the force-displacement data can be identified as the way in which the detected force of the punch 10 exceeds the holding force of the setting head / hold-down 12 by a certain threshold. Furthermore, it is conceivable to computationally determine the path x ₁ from the relationship x ₁ = x ₂ - (punch stroke + L), while L designates the length of the semi-hollow punch rivet. The punch stroke designates the distance between the underside of the punch 10 and the underside of the setting head / hold-down 12.

The path x ₃ to the point P3 is identified by reaching the maximum force F _{max of} the punch 10. This maximum force F _max is adjustable according to the components 3, 5 to be joined before the joining process and therefore known.

During the return of the punch 10 (see dashed line in the FIGS. 4 and 5 ), the path x ₄ to point P4 is identifiable as follows (step D). At point P3 after the path x ₃ , a tangent to the approximately linearly running return (see dashed curve in FIGS FIGS. 4 and 5 ), so that a deviation of the force-displacement curve by a predetermined value from the tangent provides the point P4 after the path x ₄ . Here one defines a threshold value for the maximum permissible path change or deviation of the path from the tangent with Δx≤1-20 increments. If the maximum permissible deviation .DELTA.x is exceeded by the tangent, this defines the point P4 and the way x _4mm.

It is also conceivable to read the point x ₄ from the force-displacement data without representing a curve. In this case, starting at point P3, one would assume a linear change in the force-displacement data during the return of the punch 10 until it is unloaded. As soon as the change in force-displacement data assumed to be linear deviates from its linearity, this point of deviation determines the path x ₄ .

According to a further alternative, a reference variable Δx _C for the rigidity of the C-frame 30 was determined in preliminary tests. With the help of this reference quantity Δx _C , x ₄ results from the difference of x ₃ and Δx _C according to x ₄ = x ₃ - Δx _C. If x ₃ and x ₄ from the force-displacement curve Δx _{c can} also be calculated from the difference of the paths x ₃ and x ₄ according to Δx _C = x ₃ -x ₄ (step F).

Based on the variables determined from the force-displacement data, the compression dimension x _ST and the rivet head end position K _HS can be calculated in accordance with the following equations (step E). The upsetting dimension x _ST is given by x _ST = xx ₄ , while x is the maximum distance between the underside of the die and the top of the die and x _{4 describes} the position of the rivet head after relieving the C frame 30 at the point P ₄ .

The rivet head end position K _HS results from the equation K _HS = (x ₁ + Δx _s + L) -x ₄ = x ₂ + Lx ₄ . In this formula, x ₁ designates the setting point of the setting head 12 on the joining parts 5 at the point P1, x ₂ = Δx _s + x ₁ the attachment point of Halbhohlstanzniets3 on the joining parts 5 at the point P2, L the length of Halbhohlstanzniets3, x ₄ the position of rivet head after releasing the C-frame 30 and Ax _{S is} the difference between the variables x ₂ and x ₁ as the distance traveled Ax _S of the die 10 after the attachment of the placement head / down device 12 at the joining portions 5 at point P1 to the attachment of the Rivet 3 at the point P2 at the joining parts 5.

By analogy with the calculations described above, the quality characteristics of the rivet head end position K _VS and the embossing depth h _d for the punch rivet with solid punch rivet and the bottom thickness t _b during clinching can also be determined.

The components for joining a solid punch rivet 4 are shown schematically in FIG Fig. 7 shown. The solid punch 4 with the length L is driven by means of a punch 10 in the joining parts 5. During joining, the joining parts 5 are pressed against a die 20. In the same way as when joining the half-hollow punch rivet 3, the force-displacement data are detected and evaluated during the joining process. In the force-displacement data of the joining with solid punch rivet 4, the paths x ₂ to the attachment point of the punch 10 on the solid punch rivet 4 and x ₄ after relieving the punch 10 at the point P 4 can be seen, as described with respect to the joining of a semi-hollow punch rivet 3 ( see. Fig. 4 . 5 ). Furthermore, the point P3 with the path x ₃ on reaching the maximum joining force F _max and the value Δx _C = x ₃ -x _{4 can be} derived from the force-displacement data. Thus, the rivet head end position K _VS in accordance with K _VS = x ₂ + Lx can ₄ = x ₂ + L (x ₃ -Δx _C) into account. The embossing depth h _d results from h _d = t- [x- (x ₂ + L)], while t describes the common thickness of the joining parts 5 at the joint (see Fig. 7 ).

When clinching, that in Fig. 9 is shown schematically, presses a punch 10, the joining parts 5 against a die 20. In this process, the force-displacement data are detected and evaluated in the same manner as in the joining of semi-hollow punch rivets 3. In these force-displacement data, the quantities x ₃ , x ₄ and Δx _{c are} identifiable, as has already been described above. In addition, the maximum distance x between the bottom of the punch 10 and the top of the die 20 is known. On this basis, the bottom thickness t _b in accordance with t _b = xx ₄ = x- (x ₃ -Δx _C) calculated to characterize the clinch connection generated between the joining parts. 5

Claims (12)

1. Online determination of bulge/upset dimension x_ST and rivet head end position K_HS of a half-hollow punch rivet (3) having a length L in a punch rivet process by means of a moveable punch (10) and a die (20), comprising the following steps:

   a. capturing (A) a path covered by a moveable punch (10) during the punch rivet process with the help of a travel sensor (50),

   b. capturing (B) a force F applied to the half-hollow punch rivet (3) by the moveable punch (10) during the punch rivet process depending on the covered path,

   c. determining an attachment point x₂ of the half-hollow punch rivet (3) with punch (10) to a joint part (5) and a release point x₄ from the captured force/path data (D), while the release point x₄ identifies a release of the punch (10) after the punch rivet process and

   d. calculating (E) the rivet head end position K_HS in accordance with K_HS=x₂+L-x₄ and bulge/upset dimension x_ST in accordance with x_ST=x-x₄, where x is the maximum distance between facing sides of punch (10) and die (20).
2. Online determination of embossing depth h_d and rivet head end position K_VS of a full punch rivet having a length L in a punch rivet process by means of a moveable punch (10) and a die (20), comprising the following steps:

   a. capturing (A) a path covered by a moveable punch (10) during the punch rivet process with the help of a travel sensor (50),

   b. capturing (B) a force F applied to the full punch rivet by the moveable punch (10) during the punch rivet process depending on the covered path,

   c. determining an attachment point x₂ of the full punch rivet with punch to a joint part (5) and a release point x₄ from the captured force/path data (D), while the release point x₄ identifies a release of the punch (10) after the punch rivet process and

   d. calculating (E) the rivet head end position K_VS in accordance with K_VS=x₂+L-x₄ and the embossing depth h_d in accordance with h_d=t-[x-(x₂+L)], where x is the maximum distance between facing sides of punch (10) and die (20) and t is a thickness of the joint parts (5).
3. Online determination according to claim 1 or 2, comprising the further steps:

   capturing the applied force with a force sensor (40) and

   storing the force/path data in a data processing unit (60), in particular a computer.
4. Online determination according to any of the preceding claims, comprising the further step:

   identifying the attachment point x₂ in the force/path data via a detection of a missing change in the captured moveable path despite a punch infeed, preferably a missing change over 1-20 increments during the punch infeed or

   identifying the attachment point x₂ in the force/path data as the path, on which the captured force exceeds a certain threshold value, preferably a holding force of a set head or a hold-down device (12).
5. Online determination according to any of the preceding claims, comprising the further step:

   identifying the point x₃ as the path, on which the maximum force F_max of the punch (10) is reached.
6. Online determination according to claim 5, comprising the further step:

   calculating (F) a reference variable Δx_C for a machine rigidity in accordance with Δx_C=x₃-x₄, which specifies the flexibility of the constructive connection between the punch (10) and the die (20), preferably a C frame (30).
7. Online determination according to claim 1 or 2, comprising the further steps:

   representing (C) the captured force/path data in the form of a curve and

   identifying (D) the point x₄ through the creation of a tangent on the almost linearly running force/path data after a maximum force F_max of the punch (10) is reached so that a deviation of the force/path data by a specified value from the tangent gives point x₄.
8. Online determination of a base thickness t_b in a clinch process by means of a moveable punch (10) and a die (20), which has the following steps:

   a. capturing (A) a path covered by a moveable punch (10) during the clinch process with the help of a travel sensor (50),

   b. capturing (B) a force F applied to a joint part (5) by the moveable punch (10) during the clinch process depending on the covered path,

   c. determining a release point x₄ from the captured force/path data (D), which identifies a release of the punch (10) after the clinch process and

   d. calculating (E) the base thickness t_b in accordance with t_b=x-x₄, where x is the maximum distance between facing sides of punch (10) and die (20).
9. Online determination according to claim 8, comprising the further steps:

   capturing of the applied force by means of a force sensor (40) and

   storing the force/path data in a data processing unit (60), in particular a computer.
10. Online determination according to claim 8 or 9, comprising the further step:

    identifying the point x₃ as the path, on which the maximum force F_max of the punch (10) is reached.
11. Online determination according to claim 10, comprising the further step:

    calculating (F) a reference variable Δx_C for a machine rigidity in accordance with Δx_C=x₃-x₄, which specifies the flexibility of the constructive connection between the punch (10) and the die (20), preferably a C frame (30).
12. Online determination according to one of the claims 8 to 11, comprising the further steps:

    representing (C) the captured force/path data in the form of a curve and

    identifying (D) the point x₄ through the creation of a tangent on the almost linearly running force/path data after a maximum force F_max of the punch (10) is reached so that a deviation of the force/path data by a specified value from the tangent gives point x₄.

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