Method for determining the quality of a joint, and control method for a process of joining a plurality of metal sheets by means of a joining device

Abstract

A method for determining the quality of a joint fabricated in a plurality of metal sheets by a joining device and a control method. The joining device comprises a drive element, and a hold-down device The method may have the following steps: moving the drive element to move the punch and, via at least one first and one second spring, the hold-down device; recording of a force applied by the drive element with a first sensor and of a distance covered by the drive element with a second sensor during a movement of the drive element in a mating direction and opposite thereto as a force/displacement curve; and comparing a linear relief range in the recorded curve with a reference curve.

Claims

1. A method for determining the quality of a joint fabricated in a plurality of metal sheets by means of a joining device, said device comprising a drive element in combination with a punch which can be moved with it and a hold-down device which can be moved with it, wherein the method has the following steps: a. moving the drive element in a mating direction, wherein the drive element transfers the movement to the punch and, via at least one first and one second spring, to the hold-down device, b. recording of a force applied by the drive element with a first sensor and of a distance covered by the drive element with a second sensor during a movement of the drive element in the mating direction and opposite thereto as a recorded force/displacement curve, and, c. after the completion of a joining process, comparing a linear relief range in the recorded force/displacement curve with a reference curve of the joining device, which is determined as a reference force/displacement curve with the movement of the punch and the hold-down device via the drive element toward and away from a counter-support without any sheet arranged between and without a joining element, wherein the comparison ensures an evaluation of a head end position of a joining element or a remaining bottom thickness in the plurality of metal sheets, wherein the reference force/displacement curve and the recorded force/displacement curve each represents a deformation of the first spring of the hold-down device and a deformation of the second spring of the hold-down device in a first linear increase range and in a second linear increase range separated via a step, and the method has a further step: d. generating a difference of a displacement value W.sub.RS, 0 of the step in the reference force/displacement curve and a displacement value W.sub.ES, 0 of the step in the recorded force/displacement curve, which represents a thickness of the plurality of metal sheets to be joined.

2. The method according to claim 1, wherein the linear relief range, upon a match when compared to a reference relief range, identifies a specified head end position of a punch rivet during placement of the punch rivet in the plurality of metal sheets or a predefined remaining bottom thickness in a clinch process, with an arrangement of the linear relief range displaced toward smaller displacement values compared to the reference relief range, a larger head projection than specified with reference to the plurality of metal sheets or a greater remaining bottom thickness than predefined in the clinch process, and with an arrangement of the linear relief range displaced toward larger displacement values compared to the reference relief range, the joining element pressed too deeply into the plurality of metal sheets or a remaining bottom thickness smaller than predefined in the clinch process.

3. The method according to claim 1, wherein the linear relief range in the force/displacement curve extrapolated up to an intersection point with a displacement axis or a straight line to a defined force value represents a flush or specified head end position of a punch rivet during placement of the punch rivet in the plurality of metal sheets or a specified remaining bottom thickness in a clinch process if the intersection point matches a reference intersection point of a linear reference relief range extrapolated to the displacement axis or to the straight line for the defined force value, wherein a deviation of the intersection point from the reference intersection point R toward smaller displacement values compared to the reference relief range represents a larger head projection than specified with reference to the plurality of metal sheets or a greater remaining bottom thickness than predefined in the clinch process, and wherein a deviation of the intersection point from the reference intersection point displaced toward larger displacement values represents the joining element pressed too deeply into the plurality of metal sheets or a remaining bottom thickness smaller than predefined in the clinch process.

4. The method according to claim 1, with the further step of generating a difference between an initial displacement value S.sub.0 of a non-linear increase range bordering on the second linear increase range and a starting displacement value S.sub.02 of the bordering second linear increase range, which represents a length of the joining element.

5. The method according to claim 4, with the further step of comparing the determined length of the joining element with saved rivet lengths and signaling a misaligned rivet or a non-specified rivet if the determined length of the joining element is shorter than that of a minimum rivet length stored or it is longer than a maximum rivet length.

6. A control method for joining a plurality of metal sheets by means of a joining device, said device comprising a drive element in combination with a punch which can be moved with it and a hold-down device which can be moved with it, wherein the method has the following steps: a. specifying a maximum joining force with a force setpoint value F.sub.soll, b. recording of a force applied by the drive element with a first sensor and of a distance covered by the drive element with a second sensor during a movement of the drive element in a mating direction and opposite thereto as a recorded force/displacement curve, c. moving the drive element in the mating direction, wherein the drive element transfers the movement to the punch and, via at least one first and one second spring, to the hold-down device until the force setpoint value F.sub.soll is reached, and after that moving the drive element into an initial position, d. offsetting the force setpoint value F.sub.soll by at least one correction value after comparing a linear relief range in the recorded force/displacement curve with a reference curve of the joining device, which is determinable as a reference force/displacement curve with the movement of the punch and the hold-down device via the drive element toward and away from a counter-support without any metal sheet arranged between and without a joining element, in order to achieve a specified head end position in a subsequent joining process during punch riveting or a predefined remaining bottom thickness in a clinch process, wherein the reference force/displacement curve and the recorded force/displacement curve each represents a deformation of the first spring of the hold-down device and a deformation of the second spring of the hold-down device in a first linear increase range and in a second linear increase range separated via a step, and the method has a further step: generating a difference of a displacement value W.sub.RS, 0 of the step in the reference force/displacement curve and a displacement value W.sub.ES, 0 of the step in the recorded force/displacement curve, which represents a thickness of the plurality of metal sheets to be joined, and wherein the step of offsetting includes the step of offsetting of the force setpoint value F.sub.soll to a corrected force setpoint value F.sub.soll,k as a function of the thickness determined for the plurality of metal sheets to be joined.

7. The control method according to claim 6 with the further step of generating a difference between an initial displacement value S.sub.0 of a non-linear increase range bordering on the second linear increase range and a starting displacement value S.sub.02 of the bordering second linear increase range, which represents a length of the joining element, comparing the determined length of the joining element with saved rivet lengths and aborting the joining process if the determined length of the joining element is shorter than that of a minimum rivet length stored or it is longer than a maximum rivet length.

8. The control method according to claim 6, wherein the linear relief range in the force/displacement curve is extrapolated up to an intersection point SP with a displacement axis or to a straight line at a defined force value and the step of offsetting includes the further step of maintaining the force setpoint value F.sub.soll if the intersection point matches a reference intersection point of a linear reference relief range extrapolated to the displacement axis or to the straight line at the defined force value and, thus, indicates the flush or specified head end position or the predefined remaining bottom thickness, increasing the force setpoint value F.sub.soll by the at least one correction value if a deviation of the intersection point from the reference intersection point toward smaller displacement values indicates a greater head projection than specified or a larger remaining bottom thickness than specified in the clinch process, and reducing the force setpoint value F.sub.soll by the at least one correction value if a deviation of the intersection point from the reference intersection point toward larger displacement values indicates a head of a punch rivet pressed too deeply into the plurality of metal sheets or a remaining bottom thickness smaller than specified in the clinch process.

9. The control method according to claim 8, with the further step of generating a difference between an initial displacement value S.sub.0 of a non-linear increase range bordering on the second linear increase range and a starting displacement value S.sub.02 of the bordering second linear increase range, which represents a length of the joining element, comparing the determined length of the joining element with saved rivet lengths and aborting the joining process if the determined length of the joining element is shorter than that of a minimum rivet length stored or it is longer than a maximum rivet length.

10. The control method according to claim 6, which further includes maintaining the force setpoint value F.sub.soll if the linear relief range matches a reference relief range, increasing the force setpoint value F.sub.soll by the at least one correction value if the linear relief range is displaced toward smaller displacement values compared to the reference relief range, and reducing the force setpoint value F.sub.soll by the at least one correction value if the linear relief range is displaced toward larger displacement values compared to the reference relief range.

11. The control method according to claim 10, wherein the linear relief range in the force/displacement curve is extrapolated up to an intersection point SP with a displacement axis or to a straight line at a defined force value and the step of maintaining includes the step of maintaining the force setpoint value F.sub.soll if the intersection point matches a reference intersection point of the linear reference relief range extrapolated to the displacement axis or to the straight line at the defined force value and, thus, indicates the flush or specified head end position or the predefined remaining bottom thickness, wherein the step of increasing includes the step of increasing the force setpoint value F.sub.soll by the at least one correction value if a deviation of the intersection point from the reference intersection point toward smaller displacement values indicates a greater head projection than specified or a larger remaining bottom thickness than specified in the clinch process, and wherein the step of reducing includes the step of reducing the force setpoint value F.sub.soll by the at least one correction value if a deviation of the intersection point from the reference intersection point toward larger displacement values indicates a head of a punch rivet pressed too deeply into the plurality of metal sheets or a remaining bottom thickness smaller than specified in the clinch process.

12. The control method according to claim 10, with the further step of generating a difference between an initial displacement value S.sub.0 of a non-linear increase range bordering on the second linear increase range and a starting displacement value S.sub.02 of the bordering second linear increase range, which represents a length of the joining element, comparing the determined length of the joining element with saved rivet lengths and aborting the joining process if the determined length of the joining element is shorter than that of a minimum rivet length stored or it is longer than a maximum rivet length.

Description

4. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

(1) Representative embodiments of the present disclosure are explained in greater detail with reference to the accompanying drawings. These show in:

(2) FIG. 1A is a sectional view of an embodiment of a joining device from the prior art, which is described in DE 100 21 781 B4 and the design and manner of function of which is referenced herewith,

(3) FIG. 1B is a sectional enlargement from the sectional view in FIG. 1A,

(4) FIG. 2 is a force/displacement curve (blue) and a reference force/displacement curve (red) compared,

(5) FIG. 3 the curves according to FIG. 2, wherein these are superimposed based on the step for evaluation,

(6) FIG. 4 is a schematic view for interpreting the head end position and remaining bottom thickness and for correcting the force setpoint value,

(7) FIG. 5 is a schematic view of a punch rivet turned in the joining channel, and

(8) FIGS. 6a, 6b and 6c are schematic views (6a) of the head end position of a semi-hollow punch rivet, (6b) the remaining bottom thickness of a clinch joint and (6c) the head projection of a solid punch rivet with head contact.

5. DETAILED DESCRIPTION

(9) The control method for joining a plurality of metal sheets by means of a joining device and the method for determining the quality of such a joint were developed for joining devices as are known from the prior art. The design and manner of function for such a joining device is described, for example, in DE 100 21 781 B4, the content of which is incorporated in this context by reference. FIGS. 1A and 1B show the referenced joining device as a preferred embodiment of a joining device for the use of the present disclosure.

(10) The basic structure of the joining device F comprises a drive element 1 which is powered here via a toggle joint. As an alternative to this, according to further embodiments a hydraulic piston-cylinder drive or an electromotive spindle drive and a pneumatic piston-cylinder drive can be used. The drive used moves the drive element 1 which moves a punch 9 and a hold-down device 13 via a linear movement in the direction of a plurality of metal sheets 14 or away from them. Thus the drive element 1 produces a controlled linear movement of the punch 9 and hold-down device 13 in a known way parallel to the mating direction R.sub.F. The hold-down device 13 is preferably able to be mechanically pretensioned against the plurality of metal sheets 14 via at least one first spring 7 and one second spring 6. During the joining process for joining the plurality of metal sheets 14, the plurality of metal sheets 14 is supported on a counter-support 15. As part of a clinch process, this counter-support 15 is constituted by a correspondingly shaped die which supports the deep-drawing process. When punch riveting a semi-hollow punch rivet or a solid punch rivet, the counter-support also consists of a die which supports the formation of a closing head. An anvil may be used as a counter-support during the determination of the reference force/displacement curve.

(11) According to the disclosure, the movement of the drive element 1 is preferably recorded by a travel sensor (not shown). The recorded travel sensor data is transmitted to a microcontroller or industrial computer for monitoring and controlling the joining device F and is processed further there. The force transferred by the drive element 1 to the elements upstream in the mating direction R.sub.F—the hold-down device 13, springs 6, 7 and punch 9—is recorded by means of a force sensor (not shown). Such force sensors are known in their type of construction and arrangement in a joining device. The recorded force sensor data is transmitted to the microcontroller or industrial computer for monitoring and controlling the joining device F and is processed further there.

(12) According to the disclosure, the first spring 7 may have a smaller spring constant compared to the second spring 6. Thus the first spring 7 is the week spring, while the second spring 6 is the strong spring. The first spring 7 is for positioning the die of the hold-down device 13 in an initial position. Accordingly, the force of spring 7 guides the hold-down device 13 back to its initial position after the joining process is ended and the joining device F is lifted from the metal sheets 14. To implement this function, the spring 7 may be mechanically pretensioned between the upper side of the hold-down device sleeve 13.sub.H and the interior upper side of the drive element 1.

(13) The second spring 6 is for applying a retaining force to the metal sheets 14. The retaining force may be set by pre-tensioning the second spring 6 between the interior upper side of the drive element 1 and a stop 16. According to a further embodiment, the stop 16 is able to be displaced with the aid of an axially displaceable threaded sleeve 17 in or opposite to the mating direction F.sub.R for this purpose, which relieves the second spring 6 or compresses it.

(14) The drive element 1 is now moved in the mating direction and in the process the displacement W of the drive element 1 and the force F applied by the drive element 1 to the hold-down device 13 and the punch 9 are recorded. The recorded displacement S and the recorded force F are able to be represented in the force/displacement diagram, as is the case, for example, in FIGS. 2, 3 and 4. If the drive element 1 moves in the mating direction R.sub.F and has not yet contacted the metal sheets 14, the force F is almost zero. As soon the die of the hold-down device 13 is placed onto the metal sheets 14, the recorded force jumps to the value of the pre-set pretension of the first spring 7. If the drive element 1 moves further in the mating direction R.sub.F after placement onto the metal sheets 14, the first spring 7 is compressed further via a displacement S.sub.F (see FIG. 1B) adjustable in its length until a shoulder 13.sub.S of the hold-down device 13 abuts the stop 16. In the force/displacement diagram, this can be recognized by a first step based on a first linear increase L.sub.1 of the force/displacement curve. The first step was offset to the point of origin in FIGS. 2 to 4 according to a further embodiment of the present disclosure, i.e. to displacement S=0. Therefore the force/displacement curve also does not begin with the force at zero but rather with the pre-tension force stored in the spring 7. This offset/displacement is represented in FIG. 2, in which the force/displacement curve continues in the range of the negative displacement axis (the x-axis; see the red and blue curves).

(15) With the shoulder 13.sub.S abutting the stop 16, a further movement of the drive element 1 in the mating direction R.sub.F now compresses the second spring 6 in combination with the first spring 7. The hold-down device 13 correspondingly applies at least the pretension preset in the second spring 6 to the metal sheets 14 as a retaining force. In the force/displacement curve, the effect of the second spring 6 is recognizable by a second step S.sub.1 or a second jump and a subsequent linear increasing range L.sub.2.

(16) While the drive element 1 moves in the mating direction R.sub.F, the aforementioned distance S.sub.F is covered until the hold-down device 13 applies its retaining force to the metal sheets 14. While the hold-down device 13 is displaced by this pre-settable distance S.sub.F or also free space within the joining device F, the punch 9 moves coaxially in the mating direction R.sub.F within the hold-down device 13. The distance S.sub.S (see FIG. 1B) which the punch 9 covers within the hold-down device 13 until placement on the metal sheets 14 is selectively adjustable in its length within the joining device F. The distance S.sub.S is set greater than the distance S.sub.F to be covered by the hold-down device 13 to bridge the free space. The difference in length U.sub.L between S.sub.S and S.sub.F, i.e. S.sub.S−S.sub.F=U.sub.L>0, is thus a configurable parameter of the joining device F and therefore a known parameter. The difference in length U.sub.L ensures first of all that the hold-down device 13 is placed onto the metal sheets 14 and applies its full retaining force by means of the springs 6, 7 to the metal sheets before the punch 9 itself or a punch rivet moved by the punch 9 in the mating direction R.sub.F acts upon the metal sheets 14 with a force. Only when the punch 9 or a punch rivet moved by the punch 9 in the mating direction R.sub.F acts upon the metal sheets 14 with a force does the joining process begin, which is recognizable in the recorded force/displacement curve or in the force/displacement curve respectively by a non-linear section NL.

(17) While the aforementioned methods are generally applicable to joining devices, they will be described based on the penetration of a punch rivet into the plurality of metal sheets. Therefore the joining device F of FIGS. 1A and 1B also shows as an example a rivet feeder 12 which has fed a punch rivet 11, in particular a semi-hollow rivet or solid punch rivet, under the punch 9. In an analogously performable clinch process, the punch 9 would perform a deep-drawing process in a correspondingly adapted die 15 to join the plurality of metal sheets 14 to one another.

(18) Similar suitable joining devices are described in EP 1 294 505 B1, U.S. Pat. No. 4,365,401, DE 100 31 073 A1 and DE 10 2004 015 568 A1; reference is made herein to their designs.

(19) FIG. 2 shows two curves in a force/displacement diagram. The solid line (blue here) describes the penetration or placement of a punch rivet with the joining device F described above. The springs 6, 7 are mechanically pre-tensioned for this. The drive element 1 is moved by controlled force from its initial position in the mating direction R.sub.F up to a specified force setpoint value F.sub.soll and then moved back opposite the mating direction R.sub.F to its initial position. During this controlled movement of the drive element 1, which is based on the specification of correctable force setpoint values F.sub.soll and uses no controller or real-time control, the distance S covered by the drive element 1 and the force F applied by the drive element 1 are recorded and processed.

(20) The solid force/displacement curve of a joining process may comprise a first linear increase range L.sub.1, a second linear increase range L.sub.2 following thereupon, a non-linear increase range NL following thereupon and a first linear relief range E.sub.1 with a first negative slope and a second linear relief range E.sub.2 with a second negative slope. The first negative slope is absolutely greater than the second negative slope, so that the first relief range E.sub.1 decreases more sharply than the second relief range E.sub.2. Moreover, the second relief range E.sub.2 may run antiparallel to the increase range L.sub.2 and then also transitions and runs antiparallel to the increase range L.sub.1. The increase ranges L.sub.1 and L.sub.2 may be separated from one another by a step S. It is also preferred that the increase ranges L.sub.1, L.sub.2 transition directly into one another without a step S. In this case, the spring 6 of the hold-down device 13 is preferably not pre-tensioned. If, in addition to the springs 6, 7 further holding springs engage one after the other, this would be recognizable based on further linear increase ranges L in the force/displacement curve of the drive element 1. The respective increase of a linear increase range L may be determined by the spring constant of the spring currently being compressed.

(21) Before the weak spring 7 is compressed, despite the movement of the drive element 1 in the mating direction R.sub.F, the recorded force is equal to zero or nearly zero within the range of fluctuations to be tolerated. In order to interpret the recorded force/displacement curve with greater ease and be able to be evaluated in combination with a reference force/displacement curve, a starting point of the first linear increase range L.sub.1 may be set at the displacement zero point in the force/displacement diagram, as shown in FIG. 2. The same may be applicable to the reference force/displacement curve discussed below. The recorded force/displacement data are preferably evaluated for this purpose in the industrial computer with respect to the beginning of the increase range L.sub.1. After this has been recognized using mathematical criteria, the recorded force/displacement curve and the reference force/displacement curve are offset to negative displacement values in such a way that the rise of the increase range L.sub.1 begins at the displacement value equal to zero.

(22) In the same way, it is alternatively possible to offset the beginning of the second linear increase range L.sub.2 to negative displacement values until the rise of the increase range L.sub.2 begins at the displacement value equal to zero. In this way, the influence of possible errors in the determination of measured values or generally in the force/displacement phase L.sub.1 is reduced.

(23) The second linear increase range L.sub.2 transitions to the non-linear increase range NL. At the cited transition point between the range L.sub.1 and the range NL the punch rivet, in particular a solid punch rivet or a semi-hollow punch rivet with or without a head, or a punch in the clinch process or another joining element moved by the punch 9 rests on the plurality of metal sheets 14. Therefore if one generates the difference between the initial displacement value S.sub.0 of the non-linear increase range NL bordering on the second linear increase range L.sub.2 and the starting displacement value S.sub.02 of the bordering second linear increase range L.sub.2, then a length segment proportional to the length of the joining element used results from this. In order to determine the length of the joining element from this, the length segment 1 determined from the force/displacement curve, i.e. 1=S.sub.0—S.sub.02, is to be subtracted from the constant U.sub.L typical for a setting head already discussed above.

(24) Since the joining device used works with controlled force, the drive element 1 is moved in the mating direction R.sub.F until the specified force setpoint value F.sub.soll is reached. Reaching the force setpoint value F.sub.soll means that the joining process, i.e. the joining of the plurality of metal sheets, is ended.

(25) After reaching this force setpoint value F.sub.soll specified and stored in the memory area of the controlling microcontroller or industrial computer, the drive element 1 is moved back opposite to the mating direction R.sub.F to its initial position. Thus the first relief range E.sub.1 results in the force/displacement curve. This makes it clear that a C-frame used with the joining device returns with this relief to its initial shape, since it was elastically bent open during the joining process. As soon as the C-frame has almost completely achieved its initial shape, the first relief range E.sub.1 transitions into the second relief range E.sub.2. Only the forces applied by the hold-down device still effect a slight bending open of the C-frame. These retaining forces and the associated bending open of the C-frame are considered to be negligible, so they are not considered in the evaluation. Within this relief range E.sub.2 the spring 6 of the hold-down device 13 relaxes. The relief range E.sub.2 transitions to the relief range E.sub.3 via the step S, and in the latter range the spring 7 of the hold-down device 13 relaxes in the same manner until the drive element 1 has reached its initial position.

(26) A reference force/displacement curve is determined in order to be able to evaluate the recorded force/displacement curve and optimize the joining process as part of the control method. This is represented in FIG. 2 as a red line. The drive element 1 is displaced in the mating direction R.sub.F for this and the punch is thus displaced against the counter-support without a plurality of metal sheets and without a joining element located between the punch and the counter-support. Based on this configuration, the reference force/displacement curve is comprised only of the contributions from the elastically deformable components of the joining device F, which transition directly into one another. These are the compressions of the holding springs 6, 7 in the linear increase ranges L.sub.1, R and L.sub.2, R and a bending open of the C-frame in the linear increase range L.sub.3, R. After reaching the specified force setpoint value F.sub.soll, the drive element 1 is moved back to its initial position. This relieving return movement takes place in the linear ranges L.sub.3, R, L.sub.2, R, L.sub.1, R or the reference force/displacement curve. Due to the lack of a plastic deformation when recording the reference curve, the movement of the drive element 1 in the mating direction R.sub.F and opposite thereto takes place on the same reference force/displacement curve.

(27) The reference force/displacement curve described above is recorded as a characteristic parameter for each joining device. In order to be able to use it in the later evaluation of actual joining processes, the reference force/displacement curve is preferably first offset to negative displacement values such that the first linear increase range L.sub.1, R also begins its rise at the displacement value 0. As an alternative to this, it is likewise preferred that the second linear increase range L.sub.2, R begin its rise at the displacement value 0 (see above). If one uses this position as a reference point for all reference force/displacement curves to be evaluated, i.e. each initial point of the first linear increase range L.sub.1 or of the second linear increase range L.sub.2 is placed at the displacement value 0, then the force/displacement curves can be evaluated with the help of the reference force/displacement curve and the joining processes can be optimized in this way.

(28) As already described above, the thickness of the metal sheets 14 to be joined to one another results from the distance of the step S.sub.1 in the force/displacement curve and S.sub.R, 1 in the reference force/displacement curve, specifically from the difference of the values W.sub.RS,0 and W.sub.ES,0. To determine now whether an optimal joint has been achieved with the specified force setpoint value F.sub.soll, the force/displacement curve and the reference force/displacement curve are preferably offset relative to one another parallel to the displacement axis in such a way that the steps S.sub.1 and S.sub.R, 1 overlap. This relative offset can take place by calculation and/or graphically in the controlling microcontroller or industrial computer.

(29) As soon the steps S.sub.1, S.sub.R,1 are made to overlap, the linear relief ranges E.sub.1 of the force/displacement curve and E.sub.R of the reference force/displacement curve are able to be compared with one another and evaluated. After the setpoint value of the joining force F.sub.soll has been reached, the drive element 1 is relieved by moving it opposite to the mating direction R.sub.F. In this first relief phase, the C-frame, which is bent open elastically, returns almost completely to its initial shape, i.e. with negligible elastic deformations from the holding springs, so that the linear relief range E.sub.1 runs parallel to the linear relief range E.sub.R. If this parallel trend cannot be recorded, this confirms a fault in the joining device or its sensors.

(30) If the punch rivet is placed with a head end position K.sub.HS (see FIGS. 6a and c) in the preferred interval of 0≤K.sub.HS≤0.5 mm, then the linear relief range E.sub.1 lies on the linear relief range E.sub.R of the reference force/displacement curve. Accordingly, the head is flush with the uppermost sheet metal layer of the plurality of metal sheets (see FIG. 6a) or with the head projection corresponding to its head height for semi-hollow or solid punch rivets with head contact (see FIG. 6c). According to a further embodiment, the upper side of the rivet head is situated at a depth of K.sub.HS=0.1 mm or according to a further embodiment with a maximum head projection above the component or metal sheet surface of 0.1 mm (see FIG. 6a). In this manner, an optimal setting of the predefined force setpoint value F.sub.soll of the joining force is confirmed preferably by calculation and/or graphically. The same applies for a joining force with which the optimal remaining bottom thickness to (see FIG. 6b) is achieved.

(31) If the actuated force setpoint value F.sub.soll is not sufficiently high, the punch rivet, in particular a solid punch rivet or semi-hollow punch rivet without head contact, is placed with a head projection K.sub.HS or the punch rivet with head contact placed with a head end position K.sub.VS greater than specified or a clinch joint is produced with too large a remaining bottom thickness. Accordingly, with a relief of the drive element 1, the linear relief range E.sub.1 is situated displaced toward smaller displacement values in comparison to the relief range E.sub.R of the reference force/displacement curve.

(32) If the actuated force setpoint value F.sub.soll is too high, the punch rivet, in particular a solid punch rivet or semi-hollow punch rivet without head contact, is pressed too deeply into the plurality of metal sheets or the punch rivet with head contact is placed with a head end position K.sub.VS smaller than specified or a clinch joint is produced with too small a remaining bottom thickness. Accordingly, with a relief of the drive element 1, the linear relief range E.sub.1 is situated displaced toward larger displacement values in comparison to the relief range E.sub.R of the reference force/displacement curve.

(33) A correction value is proposed in order to be able to optimize the force setpoint value F.sub.soll to improve the joint to be fabricated. This correction value is stored in the industrial computer as part of a characteristic map. The correction value is preferably adapted dependent on characteristic values with reference to the respective conditions of the joining process. With a punch rivet pressed in too deeply, the correction value K corrects the force setpoint value F.sub.soll toward smaller values, whereas with a punch rivet with a head projection or too great a head projection or too great a remaining bottom thickness, the punching force F.sub.soll stored is increased by means of the correction factor K. Accordingly, the joining forces F.sub.soll,K to be actuated then result (see FIG. 4).

(34) A tolerance band may be defined around the relief range E.sub.R of the reference force/displacement curve. Based on this it can be determined both graphically and by calculation with which head end position or remaining bottom thickness the joining process is concluded with the specified force setpoint value F.sub.soll. If the linear relief range E.sub.1 of the determined force/displacement curve does not lie within the tolerance band, then the force setpoint value F.sub.soll is offset by a correction factor K according to the result of joining. Preferably the correction factor K is saved as a characteristic map. Depending on whether there is a deviation of the linear relief range E.sub.1 toward smaller or larger displacement values with regard to the linear relief range E.sub.R and how large the absolute value of this deviation is, the characteristic map provides a correspondingly calibrated correction factor K. The corresponding characteristic map is preferably saved in the memory area of the industrial computer and can be accessed there.

(35) According to a further alternative of the control program and the program for determining the quality of the joint, an intersection point SP is determined between the linear relief range E.sub.1 and a straight line with a selected comparative force value F.sub.V. Analogously, an intersection point SP.sub.R is generated between the linear relief range ER and the comparative force value F.sub.V. It is also preferred that these intersection points be determined and compared with one another on the displacement axis, i.e. with a force value of zero. If the intersection points SP and SP.sub.R match, then the force setpoint value F.sub.soll is set at an optimum. In this case, the match signals a flush head end position of the punch rivet or that the predefined remaining bottom thickness has been achieved in a clinch process. If the intersection point SP is offset to smaller displacement values, then this indicates a head projection of the punch rivet or a remaining bottom thickness greater than specified in a clinch process. In this case, the force setpoint value F.sub.soll is increased by the at least one correction value K. If the placed punch rivet is pressed too deeply into the metal sheet layers or too small a remaining bottom thickness is achieved in the clinch process, then the force setpoint value F.sub.soll is reduced by the at least one correction value K.

(36) As already explained above, the length of the joining element is determined from the difference of the initial displacement value S.sub.0 of the non-linear increase range NL bordering on the second linear increase range L.sub.2 and a starting displacement value S.sub.02 of the bordering second linear increase range L.sub.2.

(37) According to the disclosure, the rivet lengths and punch rivet geometries used are saved in the memory area of the controlling industrial computer. It can be recognized based on FIG. 1 that the punch rivet 11 is fed to the joining channel below the punch 9. The joining channel, which preferably also serves as a hold-down device 13, has an inner diameter which approximately corresponds to a head diameter of the punch rivet 11 or is formed somewhat larger. Now if the punch rivet has a head diameter and a length of its rivet shaft in a ratio greater than or equal to 2:1, preferably 8:3.3 or 12:5 or 8:4, then these punch rivets 11 can misalign in the joining channel. In this case, the rivet shaft would no longer be oriented parallel to the mating direction R.sub.F, but instead would be at an angle or nearly perpendicular to it, so that the punch rivet would be situated edgewise in the joining channel. As can be seen from the ratios above, a misaligned rivet is only possible with relatively short punch rivets. This is because as soon as the rivet shaft is longer than half the rivet head diameter, the diameter of the joining channel no longer permits misalignment. Short rivets of this kind have a head diameter to shaft length ratio of 8:3.5 mm or 8:4 mm. Therefore if it is recognized according to the determination above of the rivet length and the comparison with saved rivet lengths that the rivet length determined is greater than the stored maximum rivet length, this signals a misaligned rivet. The joining process in progress is then aborted accordingly.