Method of producing riveted joint

Abstract

To improve setting of a rivet.SOLUTION: A joining method for a self-piercing rivet comprises steps of: providing a setting tool, where a punch is guided in a clamping device by a drive unit; defining a maximal rivet setting speed; providing a self-piercing rivet having a length inferior to the predetermined maximal self-piercing rivet length; and setting the rivet. The step of setting the rivet is such that: in a first step, the punch moves at a first speed; in a second step, the punch speed decreases until it reaches a second punch speed, where the second punch speed is reached at latest when the rivet first contacts the workpiece, the second punch speed being lower than the first punch speed; and, in a third step, the rivet speed is monitored to ensure that it does not exceed the predetermined maximal rivet setting speed.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to setting rivets. More specifically, the present invention relates to a joining method for joining at least two workpieces having a rivet that is adapted to reduce the risk of the rivet cracking. [Background technology]

[0002] Self-piercing rivet (SPR) is a spot joining technique in which a self-piercing rivet is driven by a punch into a layer of a workpiece supported by a die. The die is configured so that the material of the workpiece plastically deforms as the rivet is driven into the workpiece and towards the die. This flow of workpiece material causes the annular tip of the rivet to flare outwardly and remain encased by an upset annulus of workpiece material. The flared tip of the rivet, which mates with the upset annulus of the workpiece, prevents stripping of the rivet or separation of the layers of the workpiece.

[0003] Insertion of the rivet into the workpiece is accomplished by using a drive unit having an actuator (e.g. a linear actuator such as a hydraulic cylinder or an electric linear actuator) mounted on a support arm of the force reaction frame opposite a die (or anvil) which is mounted on another support arm of the force reaction frame. A linear actuator drives the punch and rivet towards a stationary workpiece and die. The large forces on the workpiece that occur when the rivet is set or during punching must be compensated for by a repulsion force. This is usually achieved by supporting the workpiece on a counterforce structure, preferably having a substantially C-shape, and therefore usually also referred to as a C-frame.

[0004] For accurate setting of a rivet or accurate punching, it is important to know how deep the rivet or die penetrates into the workpiece. EP 1 228 824 A2 is directed to a method for riveting or punching which addresses this problem. More specifically, the deformation of the counterforce structure during the riveting or punching process is detected by a monitoring unit and the course of the movement during the riveting or punching process is corrected as a function of the bending.

[0005] Experience shows that some rivets have a tendency to crack during the setting stage. More specifically, rivets (or even more specifically self-piercing rivets) can crack in the direction of joining, especially when the hole in the self-piercing rivet is filled. These cracks are then propagated until the rivets are in their final position. The tendency for cracks to form depends, among other factors, on the rivet length, the rivet coverage, and the hardness of the sheets being riveted, which are described below. Several developments have been made to overcome this problem. For example, EP 2 631 022 A1 and EP 3 424 611 are directed to improved dies. U.S. Publication No. 2020261966A1 is directed to an improved self-piercing rivet. Summary of the Invention [Problem to be solved by the invention]

[0006] However, there is still a need to improve rivet setting, especially without a significant impact on cycle time, and in particular to provide a joining method that reduces the risk of damage to fasteners, such as, for example, the risk of cracking in self-piercing rivets. And there remains a need to improve rivet setting, particularly to provide a joining method that also works for mechanically coated fasteners. The examples of the present disclosure aim to address the above problems. [Means for solving the problem]

[0007] According to one aspect of the present disclosure, a joining method for joining at least two workpieces with a rivet includes the steps of: providing a setting tool having a frame, a punch displaceable on the frame along a joining axis, a die mounted on the frame, and a clamping device capable of compressing one or more members in a joining direction, the punch being guided in the clamping device by a drive unit; defining a predetermined maximum rivet setting speed; providing a rivet having a length less than a predetermined maximum rivet length; feeding the rivet into a setting tool; Setting a rivet, In a first stage, the punch moves at a first punch speed; In a second stage, the punch speed is decreased until a second punch speed is reached, the second punch speed being reached no later than when the rivet first contacts the workpiece, the second punch speed being slower than the first punch speed; In a third stage, the punch continues to move and the rivet speed is monitored to ensure that a predetermined maximum rivet setting speed is not exceeded. The rivet setting stage is as follows: Equipped with.

[0008] Such a method having a controlled speed reduction and the definition of a predetermined maximum rivet setting speed that is not reached during setting of the rivet makes it possible to reduce the risk of cracking of the rivet. More specifically, it must be ensured that the setting speed from the rivet contact point to the end of the setting process is lower than or equal to a defined setting speed. The defined maximum rivet length allows the punch speed to be kept up to a maximum. A faster punch speed is actually advantageous for the effective cycle time of the joining process. The reduction in speed makes it possible to avoid or significantly reduce cracking. The riveting speed is monitored to ensure precise control of the process.

[0009] In an embodiment, the method further comprises defining a predetermined maximum rivet length, and the second punch speed is reached when the punch reaches a position corresponding to the predetermined maximum rivet length. The maximum rivet length is a point that is always reached before the rivet contact point and can be easily indicated or defined by the operator. The maximum rivet length can be adjusted by the operator.

[0010] In an embodiment, the first punch speed is within the range of 150 to 1000 mm / s. This allows for efficient cycle times for the joining process.

[0011] In an embodiment, the second punch speed and / or the predetermined maximum rivet setting speed is within the range of 10 to 300 mm / s. In an embodiment, the predetermined maximum rivet setting speed is less than or equal to 100 mm / s, or even less than or equal to 50 mm / s. This can reduce the risk of cracking, especially for self-piercing rivets with mechanical coating.

[0012] In an embodiment, the drive unit reduces the punch speed when a predetermined maximum rivet length is reached, such that a predetermined maximum rivet setting speed is reached. For example, the speed reduction is constant so that a smooth transition is achieved, and the lower speed is reached only at the maximum rivet length, or when the rivet first contacts the workpiece, so that the effective cycle time is not affected. In fact, a slower speed always increases the cycle time. The method is intended to have a minimal effect on cycle time. The method can thus be used for a wide range of rivets having lengths less than a predetermined maximum rivet length.

[0013] In an embodiment, the second punch speed is related to a predetermined maximum rivet setting speed such that the predetermined maximum rivet setting speed is reached when the predetermined maximum rivet length is reached or when the rivet first contacts the workpiece.

[0014] In an embodiment, the deformation of the frame during the setting phase is detected by the monitoring unit and the course of the movement during the setting phase is corrected as a function of the deformation. In this way, the exact position of the rivet relative to the workpiece can be determined and the rivet speed can thereby be monitored and controlled. In embodiments, the rivet speed can be monitored without knowledge of the workpiece position, however, such monitoring lacks accuracy.

[0015] In an embodiment, the relative movement between the frame and the clamping device is measured by a first sensor and the relative movement between the frame and the punch is measured by a second sensor. Two sensors can be easily implemented and allow the rivet speed to be determined accurately.

[0016] In an embodiment, the first sensor and / or the second sensor is a linear path recorder, preferably a digital counter. For example, a digital counter counts stroke-shaped markings on a ruler. This allows for fast and accurate processing of the signal within the monitoring unit. In alternative embodiments, other systems such as lasers can be used.

[0017] In an embodiment, the first and / or second workpiece is a high strength steel workpiece. High strength steel components are notoriously difficult to join to each other and to other metals. Such high strength steels may in some cases have a tensile strength of at least 400 MPa. In another embodiment, the or at least one workpiece is made of thick (at least 3 mm) mild steel or hard aluminium alloy.

[0018] In another embodiment, three workpieces are joined, with the top two layers being high strength steel workpieces.

[0019] In an embodiment, the predetermined maximum rivet setting speed is adjustable. This allows, depending on the rivet, the nature of the workpiece and the joint, the maximum rivet setting speed to be adjusted by the operator to optimize cycle times while avoiding cracks.

[0020] Various other aspects and further examples are described in the following detailed description and appended claims, with reference to the accompanying drawings. [Brief description of the drawings]

[0021] [Figure 1] FIG. 1 shows a schematic diagram of a setting tool comprising a frame, a punch displaceable from the frame along a joining axis, a die mounted on the frame and a clamping device by which one or more members can be compressed together in the joining direction, the punch being guided in the clamping device by a drive unit. [Figure 2A] FIG. 2A shows a schematic diagram of the commanded and actual punch velocity contours during the joining process. [Figure 2B] FIG. 2B shows a schematic diagram of FIG. 2A and a contour of the commanded punch velocity and the contour of the tool commutation force that is measured and used for the precision setting command. [Figure 3A] FIG. 3A shows a schematic diagram of punch velocity as a function of relative punch position. [Figure 3B] FIG. 3B shows a schematic diagram of punch velocity in function of punch position relative to FIG. 3A and the contour of tool commutation force that is measured and used for the precision setting command. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] The disclosed embodiments are best understood by referring to the drawings, in which like reference numbers indicate identical or similar elements. It will be readily understood that the components of the disclosed embodiments, as generally described and illustrated in the figures herein, could be arranged and designed in a wide variety of different configurations. The steps of the methods need not be performed in any particular order or even sequence, unless specifically limited, or a step need not be performed only once.

[0023] In some embodiments, well-known features, structures, or operations are not shown or described in detail. Furthermore, the described features, structures, or operations may be combined in any suitable manner in one or more embodiments. It will be readily understood that the components of the embodiments, as generally described and illustrated in the figures herein, could be arranged and designed in a wide variety of different configurations.

[0024] FIG. 1 shows a schematic diagram of a bonding apparatus 10 for carrying out the bonding method. The joining apparatus 10 includes a setting tool 12 . The setting tool 12 includes a bonding head which can be fixedly mounted or can be moved by means of a robot 14 . A feeder device having a feeder hose 16 can be provided. A feeder hose 16 supplies singulated rivets 18 to the joining head, for example by means of an air blast. Alternatively, the feeder device may have a magazine on the joining head which is used to automatically transport the rivets to be singulated into the holder.

[0025] The bond head further includes a counter force structure or frame 20 . Typically, such a frame 20 is a C-frame constructed of solid material. A punch 22 displaceable on the frame 20 along the joint axis X is also provided. The punch 22 can be controlled by a drive unit 24 and a control device 26 . The drive unit 24 may include an actuator for operating the punch 22 .

[0026] The bond head further includes a die 28 mounted on the frame 20 and a clamping device 30 . A die 28 is attached to one end of the frame 20 and a clamping device 30 is attached to the other end of the frame 20 . The punch 22 is guided in a clamping device 30 by a drive unit 24 . The clamping device 30 is adapted to be biased towards the joining direction or the direction of the die 28 .

[0027] The joining device 10 serves to join by means of a rivet 18 a first workpiece 32 and a second workpiece 34 , which can be designed, for example, as metal sheets. The setting tool 12 can also be used to join two or more workpieces 32,34. For example, three workpieces are joined, with the top two being high strength steel workpieces. The rivet 18 can be, for example, a self-piercing rivet 18 or a punch rivet, which can be a hollow or semi-hollow rivet. The workpieces 32, 34 are made, for example, from high strength steel. The control device 26 can be programmed so that a predetermined maximum rivet length can be input. This predetermined maximum rivet length can be adjusted by the operator and corresponds to the maximum rivet length used for the joining method. The rivet length is the axial length corresponding to the rivet from the head to the foot. The predetermined maximum rivet length may be given by the operator and may depend on the joining operation being performed. For example, for joining operations in an automobile factory, a predetermined rivet maximum length of 8 mm may be used. The self-piercing rivet 18 can, for example, be metallically coated or, for example, galvanically coated. The rivet 18 may be a self-piercing rivet 18 as described in EP 3080463 A1 or EP 2470799 A1.

[0028] To set the rivet 18 in the workpieces 32,34, the workpieces 32,34 are placed over the die 28. The punch 22 is then moved by means of the drive unit 24 in a direction towards the die 28 . In this case, the clamping device 30 first contacts the surface of the top workpiece 32 and presses the workpieces 32, 34 together in the joining direction.

[0029] The punch 22 is driven at a first punch speed S1 in a first stage R1. The first punch speed S1 is, for example, between 150 and 1000 mm / s. The first punch speed S1 may or may not be constant. More specifically, the first punch speed S1 can be increased to a certain maximum value, for example 300 mm / s. The first punch speed S1 is commanded by the control device 26. The punch 22 is driven by the drive unit 24 at a first punch speed S1, after which the punch speed is reduced until the punch reaches a position corresponding to the maximum rivet length to be applied. The punch speed is slowly decreased from the first punch speed S1 to the second punch speed S2 such that the second punch speed S2 is reached as soon as the punch reaches a position corresponding to a predetermined maximum rivet length. The decrease in punch speed corresponds to the second phase, R2. The second punch speed S2 is, for example, a constant speed or a perceptually constant speed. The second punch speed S2 is lower than the first punch speed S1.

[0030] During movement of the punch 22 towards the die 28, the punch 22 contacts the head of the rivet, thereby driving the rivet into the workpiece arrangement to effect a bond. This phase corresponds to the third phase R3, in which the punch 22 drives the rivet 18 into the workpiece at the second punch speed while the rivet speed is monitored to ensure that it does not exceed a predetermined rivet setting maximum speed. The predetermined maximum rivet setting speed is a data defined by the operator prior to the joining method in the control device. The predetermined maximum rivet setting speed and the second punch speed are related to each other. The predetermined maximum rivet setting speed can be adjusted by the operator depending on the joining operation, the workpiece material, the rivet material, e.g., mechanically covered rivets, the rivet geometry, the die mechanical geometry, etc., for example, for mechanically covered rivets the predetermined maximum rivet setting speed is within the range of 10 to 300 mm / s. In certain embodiments, the predetermined maximum rivet setting speed may be less than 100 mm / s, or even less than 50 mm / s.

[0031] A predetermined maximum rivet length makes it possible to ensure that the speed is reduced when or before the rivet first contacts the workpiece, thereby defining when the punch speed needs to be reduced so that a slower punch speed to avoid cracking the rivet has minimal impact on cycle time.

[0032] In order to accurately monitor the rivet speed during the setting stage, the setting tool includes a monitoring unit 36 ​​adapted to detect deformations of the frame 20 during the setting process, so that the course of the movement during the setting stage can be corrected by the control device as a function of the deformations. Thus, the actual displacement of the punch 22 and / or the rivet 18 is defined and the actual rivet speed can be accurately monitored. The first sensor 38 measures the relative movement between the clamping device 30 and the frame 20 . The sensor is preferably a linear path recorder that includes some type of ruler that acts in a similar manner as the clamping device, and a counter that is fixed to the frame and counts the markings of the ruler as they pass over the ruler. A second sensor 40 measures the relative movement between the frame and the punch.

[0033] A first sensor 38 and a second sensor 40 are connected to the monitoring unit 36 ​​so as to be able to detect bending of the frame during the application of forces by the punch 22 and the clamping device 30 on the workpieces 32,34. With information of the bending of the frame 20 detected in this way, the movement of the punch can be adjusted in such a way that a constant penetration depth of the rivet is always guaranteed. More specifically, the position of the rivet with respect to the workpiece can be measured at all times. This allows not only the exact punch location to be determined but also the exact rivet speed, and allows the rivet speed to be precisely monitored to ensure that it does not exceed a predefined maximum rivet setting speed. The course of the movement can therefore be corrected by the control device as a function of the bending during the riveting or setting or joining process.

[0034] FIG. 2A shows in more detail the velocity profile of the punch 22 during the setting step. The X-axis in FIG. 2A represents time and the Y-axis represents punch speed. FIG. 2A shows two curves. The clear line curve corresponds to the command speed. This is the pre-programmed punch speed. The dotted line corresponds to the actual punch velocity. As can be seen in FIG. 2A, especially due to the sensors and monitoring units described above, the contours of the two curves are very similar. In the first stage (not described in detail above), the punch velocity is steadily increased until a plateau is reached. The plateau corresponds to the maximum punch speed used to approach the rivet. The maximum punch speed is, for example, the first punch speed S1. The punch speed is then decreased until it reaches a second punch speed S2. A second punch speed S2 corresponds to the second plateau. The inflection point between the second plateau S1 and the curve portion corresponding to the speed reduction corresponds to a predetermined maximum rivet length. In fact, as mentioned below, a predetermined maximum rivet length determines the moment at which the second punch speed S2 is reached. Once the rivet is set in the workpiece, punch 22 is then moved away from the workpiece and returned as quickly as possible to its starting position as shown in the final (right hand) portion of the curve visible in FIG. 2A. A new rivet can be supplied and a new setting step can then be performed in a different area of ​​the same workpiece or on a different workpiece.

[0035] FIG. 2B shows the predetermined punch speed as seen in FIG. 2A with a clear line, and the dotted line shows the rectification tool force during the setting stage. The commutation tool force is calculated by the monitoring unit and the control device via measurements of the first and second sensors. The commutation tool force allows the punch and rivet positions and their velocities to be corrected in real time so that the actual and target curves are perceptually the same, as can be seen in FIG. 2A.

[0036] 3A and 3B show punch velocity as a function of relative punch position. The X-axis in FIG. 3A represents punch position and the Y-axis represents velocity. Unlike FIGS. 2A and 2B, the curves in FIGS. 3A and 3B only show the first stage R1, the second stage R2, and the third stage R3 of the setting phase. The curve does not represent the return of the punch to its initial position after setting the rivet.

[0037] In FIG. 3A, the solid line curve represents the commanded relative punch position, and the dotted line curve represents the actual relative punch position. As can be seen, the commanded and actual curves, as measured by the sensors 38, 40 and the monitoring unit 36, are perceptually similar. In a first stage R1, represented on the right hand side of FIG. 3A, a first plateau is shown, which then slowly decreases in a second stage to reach a second punch speed S1, which is slower than the first punch speed. The second punch speed S2 forms a second plateau and corresponds to the third stage. As can be seen in FIG. 3A, the decrease in punch speed from the first punch speed S1 to the second punch speed S2 can be nominally constant. FIG. 3B shows the relative commanded punch positions as clear lines and the dotted lines show the commutating tool forces at different positions of the punch as seen in FIG. 3A. The commutation tool force allows the punch and rivet positions and their velocities to be corrected in real time so that the actual and target curves are perceptually the same, as seen in FIG. 3A. [Explanation of symbols]

[0038] Bonding equipment...10 Setting tools ···12 Robots 14 Feeder hose····16 Rivets 18 Frame·····20 Punch 22 Joint axis......X Drive unit 24 Control Devices 26 Die....28 Clamping Device···30 First workpiece···32 Second workpiece 34 First punch speed: S1 Second punch speed: S2 First stage: R1 Second stage: R2 Third stage: R3 Monitoring unit.....36 First sensor 38 Second sensor 40

Claims

1. 1. A joining method for joining at least two workpieces with a rivet, comprising the steps of: providing a setting tool (12) having a frame (20), a punch (22) displaceable on said frame along a joining axis (X), a die (28) mounted on said frame, and a clamping device (30) capable of compressing one or more members in a joining direction, said punch being guided in said clamping device by a drive unit (24); defining a predetermined maximum rivet setting speed; Providing a rivet (18); feeding the rivet into the setting tool (12); Setting the rivet (18), In a first stage, the punch (22) moves at a first punch speed (S1); in a second stage, the punch speed is reduced until a second punch speed (S2) is reached, said second punch speed being reached no later than when the rivet first contacts the workpiece, said second punch speed being slower than said first punch speed; In a third stage, the punch continues to move and the rivet speed is monitored to ensure that the predetermined maximum rivet setting speed is not exceeded. setting the rivet (18) such that A joining method comprising:

2. defining a predetermined maximum rivet length; the rivet has a length that is less than the predetermined maximum rivet length; the second punch speed is reached when the punch reaches a position corresponding to the predetermined maximum rivet length; The joining method according to claim 1 .

3. The first punch speed (S1) is within the range of 150 to 1000 mm / s; The bonding method according to claim 1 or 2.

4. the second punch speed (S2) and / or the predetermined maximum rivet setting speed is within the range of 10 to 300 mm / s; The bonding method according to any one of claims 1 to 3.

5. The predetermined maximum rivet setting speed is 100 mm / s or less; The bonding method according to any one of claims 1 to 3.

6. the drive unit (24) reduces the punch speed such that the predetermined maximum rivet setting speed is reached when the rivet first contacts the workpiece. The bonding method according to any one of claims 1 to 5.

7. said second punch speed (S2) being correlated with a predetermined maximum rivet setting speed such that said predetermined maximum rivet setting speed is reached when said punch reaches a position corresponding to said predetermined maximum rivet length; The bonding method according to any one of claims 1 to 6.

8. a deformation of the frame (20) during the setting step is detected by a monitoring unit and the course of movement during the setting step is modified as a function of the deformation; The bonding method according to any one of claims 1 to 7.

9. The relative movement between the frame (20) and the clamping device (30) is measured by a first sensor (38) and the relative movement between the frame and the punch is measured by a second sensor (40). The bonding method according to any one of claims 1 to 7.

10. the first sensor (38) and / or the second sensor (40) are linear path recorders, preferably digital counters; The bonding method according to claim 9.

11. the first and / or second workpiece (32, 34) is a high strength steel workpiece; The bonding method according to any one of claims 1 to 10.

12. Three work pieces are joined together, the top two layers being high strength steel work pieces; The bonding method according to any one of claims 1 to 11.

13. The predetermined maximum rivet setting speed is adjustable. The bonding method according to any one of claims 1 to 12.

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