EP1640081B2 - Joining method and device for driving a joining tool - Google Patents

Abstract

The stamping machine (2) has a stamping tool (16) pushing a rivet against a top sheet (A). The top sheet rests on a bottom sheet (B). The river deforms the sheets and forcing them into a matrix (6). A cylindrical holder piece (22) presses the sheets down against the rim of the matrix. A cylinder (4) contains a first piston (12) with a piston rod pressing down on an intermediate piece and the stamp. There are hydraulic fluid connections (24,26) to chambers (10a) above and (10b) below the first piston. A second piston slides on the piston rod of the first piston and pushes on the holder piece. It has throttle channels (28) allowing hydraulic fluid to flow through it slowly.

Description

The present invention relates to a method for joining at least two plate-shaped workpieces and a device for actuating a joining tool for joining at least two plate-shaped workpieces of similar or different materials according to the preamble of claim 18 or 19, in particular for punch riveting or clinching, as well as a method of operating such a device.

Numerous embodiments of drives for joining tools such as punch riveting and clinching devices are known. The most common drive form consists of a hydraulic cylinder for actuating a punch for the joining operation and a further hydraulic cylinder for actuating a hold-down, which during the joining operation a hold-down force on the workpieces around the joint around, for example WO 93/24258 and EP 0675774 , In the method and the device according to the EP 0675774 Already before and during the joining process, a "substantial" hold-down force is exerted, which according to the information in this document can amount to 1.5 tons. In practice, holding-down forces of the order of, for example, 8-10 kN are used. Although this method has its advantages, the use of a large hold-down force before and during the joining process also has its disadvantages. For example, the large hold-down force during the joining process prevents unimpeded deformation of the punch rivet. The use of a large hold-down force before the joining process causes certain disadvantages in the combined stamped rivet bonding method, since the large hold-down force hinders the pressing and flow of the adhesive from the joining zone.

By an obvious prior use of the applicant, it is already known to exert a small hold-down force during the joining process by means of a spring and to increase the hold-down force by the end of the joining process, that the plunger of the hydraulic cylinder transmits a portion of the setting force via a stop connection on the hold-down. A similar arrangement is from the already mentioned EP 0675774 B1 Known in the already before and during the joining process by its own hydraulic cylinder a large hold-down force is exerted, which is then increased towards the end of the riveting process by a stop connection between the plunger and the hold-down piston according to information in this document in the short term to values up to about 5 tons.

From the WO 00/29145 It is already known to withdraw the punch after the joining operation and, when the punch is retracted, to exert a hold-down force on the workpieces to reduce deformations of the plate-shaped workpieces (sheets) out of their plane. For this purpose, this document discloses two basic ways to achieve this. According to one possibility, a retaining device is provided for the hold-down device, which prevents yielding of the hold-down after the joining operation, when the lower leg of the usual in riveting C-frame springs back after returning the punch. According to the other possibility, the punch and the hold-down are simultaneously subjected to a high pressure during the joining operation via a stop connection, and after the joining operation, the punch is slightly retracted, whereupon the hold-down is again subjected to hydraulic pressure to exert a relatively large hold-down force. It is also mentioned in this document that a great or little or no hold-down force can be exerted during the joining process.

Furthermore, it is already known in the prior art to apply at least one small hold-down force before the joining operation in order to press the workpieces against each other before the joining operation and to hold them immovably and in particular to perform a rivet and workpiece control, see, for example WO 93/24258 ,

The present invention has for its object to provide a method for joining and a device for actuating a joining tool for joining at least two plate-shaped workpieces of similar and different materials, which despite great simplicity and compact design offer the possibility of an impairment of the joining process by the To avoid hold-downs and still ensure a high-quality joint with minimal deformation of the workpieces outside the joint.

The method according to the invention is defined in claim 1.

In the method according to the invention, a high punching force is exerted during the joining operation for the joining process and substantially no hold-down force to allow unimpeded deformation of the joining region and simultaneously a high punching force and a high holding force for reducing material deformations of the workpieces and for producing a compacting effect after the joining process exercised in the joining area.

Under joining process is here, as usual, to understand the process between the beginning and end of the required to make the joint connection deformations. In the case of punch riveting, this is the process between the beginning and end of the penetration of the punch rivet into the workpieces to be joined. In the case of clinching, this is the process between the beginning and the end of the deformation of the workpieces into the joining zone.

The hold-down force exerted after the joining process must be selected such that bulges of the workpieces around the Joint is reduced around and also a certain compression effect is exerted on the workpieces in order to increase the connection quality and to achieve a sufficient final strength of the connection. Important in this context is that at the same time a high punch force is exerted on the joint to prevent the high hold-down force affecting the joint. The entire surface of the joint and of the surrounding workpiece area is thus subjected to high forces after the joining process, whereby, moreover, springback of the C-frame usually provided in such tools is also reduced. Another advantage of the method according to the invention is that the C-frame can be built more easily than in the prior art, since during the joining process essentially no hold-down force is exerted and the C-frame therefore only has to absorb the stamping force.

An embodiment of a device according to the invention is defined in claim 17.

According to the invention, this device has only a single hydraulic cylinder, which is subdivided by the main piston for the punch in a piston rod remote working chamber and in a piston rod side working chamber. The holding-down piston serving for operating the hold-down is disposed in the piston rod-side working chamber of the cylinder, and the portions of the piston-rod-side working chamber located on axially opposite sides of the hold-down piston are connected to each other by fluid communication, so that the hold-down piston on its axially opposite sides with that in the piston rod side Working chamber prevailing pressure can be applied.

This structural design of the device makes it possible to carry out the method according to the invention explained above. For this purpose, it is provided in particular that the device has a joining position in which the piston rod facing away from the working chamber is subjected to a pressure for performing the joining process and the piston rod side working chamber is depressurized, and that it has a post-processing position in which both working chambers are subjected to a high pressure , So that after the joining process, a high hold-down force and a high punching force are exerted on the joint and the surrounding workpiece area.

Since thus only a single hydraulic cylinder with only two working chambers are required to operate the punch and the blank, the device comes with only two ports and two hoses, which reduces the design effort accordingly and in particular the hydraulic control is simplified. In addition, the invention allows a compact design of the device, since the main piston and the hold-down piston in a sense interlock. An increase in stroke therefore has only a simple effect on the device designed according to the invention, whereas in the case of an attachment with two separate hydraulic cylinders, a stroke magnification has a double effect.

Another embodiment of a device according to the invention is defined in claim 18.

In this embodiment, although two hydraulic cylinders are provided. However, since two working chambers of the two cylinders are constantly connected to one another by a flow connection, the same advantages result in part as in the embodiment described above. So only two pressure medium connections are required, which in turn reduces the design effort and simplifies the hydraulic control.

An important advantage of the invention is that the joining process itself is not affected by any hold-down force. When setting punched rivets for example, this leads to the fact that the punch rivet in the workpieces can deform unhindered and informal. This results in a defined forming behavior of the rivet, which leads to a slightly larger bottom of the rivet than the punch riveting with high hold-down force. Cracks in the rivet can be avoided.

The method according to the invention can be used with particular advantage in conjunction with an adhesive method. In a further embodiment of the invention, it is therefore provided that an adhesive layer is provided between the workpieces, at least in the joint area, before the hold-down device is pressed against the workpieces. Since only a small hold-down force and during the joining process substantially no hold-down force are exerted in the inventive method before the joining process, the adhesive can be pressed freely during the joining process and flow out of the joining zone. The absence of a hold-down force during the joining process to a reduction of air bubbles between the workpieces, as will be explained in more detail.

A further reduction of the risk of air bubble formation can be achieved in a further embodiment of the invention in that punch rivets are used, in which the peripheral surface of the rivet shank is divergent, preferably cone-shaped, at least in a predetermined region in the direction of the rivet head. The rivet shank formed in this way exerts a force on the punch-side workpiece during the joining operation, which prevents or at least reduces the spring-back of the punch-side workpiece, so that the workpieces remain in abutment in the joining zone. This also reduces the risk of air bubbles, as will be explained in more detail.

As already mentioned, the invention also reduces springback of the usually provided C-frame, whereby the connection quality and final strength of the joint connection are improved accordingly.

Further advantageous embodiments of the invention are defined in the dependent claims.

• Fig. 1 einen Längsschnitt durch ein Fügewerkzeug in seiner Grundstellung;
• Fig. 2 einen der Fig. 1 entsprechenden Längsschnitt durch das Fügewerkzeug in der Vorbearbeitungsstellung;
• Fig. 3 einen der Fig. 1 entsprechenden Längsschnitt durch das Fügewerkzeug in der Fügestellung und Nach-Bearbeitungsstellung;
• Fig. 4 einen Längsschnitt durch das Fügewerkzeug der Fign. 1 bis 3 mit einer Ventilanordnung für die hydraulische Steuerung des Fügewerkzeuges in seiner Grundstellung;
• Fign. 5 bis 8 der Fig. 4 entsprechende Darstellungen abgewandelter Ausführungsformen des Fügewerkzeuges und der Ventilanordnung.
• Fig. 9 einen in spezieller Weise ausgebildeten Stanzniet zur Verwendung mit dem Verfahren und der Vorrichtung gemäß der Erfindung.

Reference to the drawings, an embodiment of the invention will be explained in more detail. It shows:

• Fig. 1 a longitudinal section through a joining tool in its normal position;
• Fig. 2 one of the Fig. 1 corresponding longitudinal section through the joining tool in the Vorbearbeitungsstellung;
• Fig. 3 one of the Fig. 1 corresponding longitudinal section through the joining tool in the joining position and post-processing position;
• Fig. 4 a longitudinal section through the joining tool of FIGS. 1 to 3 with a valve arrangement for the hydraulic control of the joining tool in its basic position;
• FIGS. 5 to 8 of the Fig. 4 corresponding representations of modified embodiments of the joining tool and the valve assembly.
• Fig. 9 a specially formed punch rivet for use with the method and apparatus of the invention.

The joining tool 2 shown only schematically in the drawing serves for producing a joint connection between two (or more) plate-shaped workpieces A and B (sheets). In the illustrated embodiment, the joining tool 2 is a riveting tool for setting punch rivets N, although the joining tool 2 and another tool such. b. a device for clinching can be. Since punch rivets and punched rivet joints are basically known, this will not be discussed in detail.

The joining tool 2 comprises an actuating device in the form of a hydraulic cylinder 4 and a die 6, on which the workpieces A, B are supported. The cylinder 4 is supported on a C-frame (not shown), as is common in such joining tools.

The cylinder 4 has a cylindrical inner space 10, in which a main piston 12 with a piston rod 14 is slidably disposed. The piston rod 14 of the main piston 12 is fixedly connected to a punch 16 for setting the rivet N.

Like from the FIGS. 1 to 3 As can be seen, the main piston 4 divides the cylindrical inner space 10 into a piston rod-facing upper working chamber 10a and into a piston rod-side lower working chamber 10b. In the lower working chamber 10b, a hold-down piston 18 is arranged, which is fixedly connected via a piston rod 20 with a tubular holding-down device 22. The hold-down piston 18 and its piston rod 20 surround the piston rod 14 of the main piston 12 coaxially and are slidably guided on the cylindrical inner wall of the cylinder 4 so that they are axially displaceable relative to the main piston 12.

The upper working chamber 10a can be pressurized or relieved of pressure via a pressure medium connection 24, optionally with a controlled fluid pressure (hydraulic pressure), while the lower working chamber 10b can also be acted upon or relieved of pressure via its own pressure medium connection 26 with a controlled fluid pressure. As indicated schematically in the figures, the portions of the working chamber 10b located on both sides of the hold-down piston 18 are connected to one another by a flow connection 28, so that the axially opposite sides of the hold-down piston 18 are always subjected to the same fluid pressure. The flow connection 28 may consist of one or more of the hold-down piston 18 penetrating holes or in other ways, for. B. by one or more longitudinal grooves on the outer periphery of the hold-down piston 18, are formed.

Further, a stopper connection 30 is provided between the piston rod 14 of the main piston 12 and the sleeve-shaped piston rod 20 of the hold-down piston 18, which is effective only in one axial direction, with movements of the main piston 12 in the direction away from the lower working chamber 10b (ie upwards in the figures).

The operation of the device described is as follows:

In Fig. 1 the device is in its normal position, in which the upper working chamber 10 a is held without pressure via the pressure medium connection 24 and the lower working chamber 10 b is acted upon via the pressure medium connection 26 with a relatively small fluid pressure. By prevailing in the lower working chamber 10b fluid pressure of the main piston 12 is pressed into its upper end position. On the hold-down piston 18 acts on its axially opposite sides of the prevailing in the lower working chamber 10b fluid pressure. Since the upper side of the hold-down piston 18 is larger than its underside by the cross-sectional area of the piston rod 20, a downwardly acting pressure difference acts on the hold-down piston 18. By acting between the two piston rods 14 and 20 stop connection 30 of the hold-down piston 18 is, however, held in a basic position in which he the in Fig. 1 shown axially middle position occupies.

The machining process for setting the punch rivet N and thus for producing the joint connection takes place in three steps:

In a first step, before the joining operation, the main piston 12 and the hold-down piston 18 are moved to a Vorbearbeitungsstellung in which the hold-down 20 and the rivet N are pressed against the workpieces A and B with a comparatively low force, s. Fig. 2 , For this purpose, both working chambers 10a, 10b are each acted upon via their pressure medium connection 24 and 26 with a low fluid pressure. Because the top of the main piston 12 is greater than its underside about the cross-sectional area of the piston rod 14, the punch 8 exerts a corresponding downward punching force on the rivet N. Also, the hold-down piston 18 is pressed down due to the mentioned surface difference by the fluid pressure, so that the hold-down 20 exerts a corresponding hold-down force on the workpieces A, B. The punch force and the hold-down force need only be so large that the workpieces A, B are held immovably in contact with each other and with the die 6 and a rivet and tool control can be performed. The hold-down force is preferably less than 7.8 kN and may for example be of the order of 3 to 5 kN. In general, a hold-down force of less than 3.9 kN is sufficient. The stamping force is preferably less than 5 kN and may be of the order of 1 to 3 kN.

In the second processing step, the joining process, the device takes in Fig. 3 shown joining position, in which the upper working chamber 10a is acted upon via the pressure medium connection 24 with the required for the joining fluid pressure, while the lower working chamber 10b is made depressurized via the pressure medium connection 26. The main piston 12 then exerts via the piston rod 14 and the punch 16 the stamping force required for producing the joint connection on the joint 8 or the rivet N, see Fig. 3 , Further, since the lower working chamber 10b is depressurized, the hold-down 20 has no function during the joining process. It is understood that in hydraulic systems on the low pressure side there is always a residual pressure, so that the lower working chamber 10b can not be made completely depressurized. However, the resulting holding-down force, which is for example in the order of 0.3 to 0.5 kN, is negligible. Depending on the material of the workpieces used, a hold-down force of, for example, less than 4 kN has no measurable influence on the workpiece properties. In any case, this applies to hold-down forces of less than 1.5 kN.

As already mentioned, dispensing with a significant hold-down force has the advantage that the punch rivet N can deform unhindered during the setting process, so that a defined forming behavior of the punch rivet occurs. Compared to a procedure with high hold-down force then comes to a slightly larger bottom of the punch rivet, which reduces the risk of cracking.

The required for setting the punch rivet N stamping force is in a usual order of z. B. 30 to 80 kN.

In the subsequent to the joining process third processing step, in which the device in a likewise by the Fig. 3 illustrated post-processing position, both working chambers 10 a and 10 b respectively acted on their pressure fluid ports 24 and 26 with a high fluid pressure. This is expediently carried out such that the high fluid pressure prevailing in the working chamber 10a is maintained, while the working chamber 10b is subjected to the same high fluid pressure or a similarly high fluid pressure via the pressure medium connection 26.

To ensure that the deformation of the punch rivet during the setting process can end without hindrance, the pressurization of the working chamber 10b and thus the structure of the hold-down force takes place only a short period of time after completion of the setting process. This period of time is for example 0.2 to 0.3 seconds and is preferably greater than 0.1 seconds.

Due to the described pressurization of the working chamber 10a and 10b of the punch 16 is pressed against the rivet N due to the above-mentioned surface difference of the main piston 12 with a high punch force. At the same time, the hold-down 20 is pressed against the workpieces A, B due to the surface difference of the hold-down piston 18 with a large hold-down force. The high hold-down force has the purpose of reducing any deformations of the workpieces A, B around the riveting out of their plane and also in this area a certain compression effect on the workpieces A, B exercise to increase their final strength and thereby the quality of the connection improve. Further, since not only the hold-down 20, but also the punch 16 exerts a high force on the joint connection, a certain equalization of the force occurs. Furthermore, by the stamping force and the hold-down force, a return springing of the C-frame (not shown) is prevented or at least reduced.

The punch force and the holding force must be selected depending on the application to achieve these functions. Preferably, the hold-down force is greater than 5 kN and in particular greater than 7.8 kN. The stamping force is preferably greater than 5 kN and in particular greater than 6.5 kN.

The punching force and the holding force are maintained for a predetermined period of time. Subsequently, the device is returned to its in Fig. 1 shown reset by the upper working chamber 10a is made depressurized via the pressure medium connection 24 and the lower working chamber 10b is acted upon by a low fluid pressure.

Fig. 4 shows schematically a joining tool 2, as described above with reference to FIGS. 1 to 3 has been described in connection with a valve arrangement of a (not shown otherwise) hydraulic control for operating the joining tool. The valve arrangement consists of two separate directional control valves 32 and 34, of which the directional control valve 32 via the pressure medium connection 24 of the working chamber 10 a and the directional control valve 34 is assigned to the working chamber 10 b via the pressure medium connection 26, in order to either pressurize the working chambers 10 a and 10 b or relieve pressure. The directional control valves 32 and 34 thus control the connection of the working chambers 10a and 10b respectively with a pressure control device (not shown) (eg, proportional valve) and a pressure relief section of the hydraulic system.

With regard to the operation of the hydraulic control can on the above-described operation with reference to the FIGS. 1 to 3 Be referred. In addition, please note:

In the in Fig. 5 illustrated basic position, the directional control valve 32 assumes its pressure relief position, and the directional control valve 34 its pressurization position. In the pre-processing position of the joining tool, both directional valves 32, 34 are in their pressurization position, in which both working chambers 10a, 10b are subjected to a low pressure. In the joining position, the directional control valve 32 is in its pressurization position and the directional control valve 34 is in its pressure relief position in order to exert a high punching force and (essentially) no hold-down force. In the post-processing position, both directional valves 32, 34 are in their pressurization position to exert both a high punch force and a high hold down force.

In the Fig. 5 illustrated embodiment differs from that of Fig. 4 with regard to the structural design of the cylinder assembly of the joining tool, wherein the Fig. 4 corresponding components of Fig. 5 are denoted by the same reference numerals, supplemented by the letter a. The joining tool of Fig. 5 is different from the one of Fig. 4 in that instead of a common hydraulic cylinder 4, two cylinders 4a and 5a are provided, of which the cylinder 4a receives the main piston 12a for the punch 16a and the cylinder 5a the hold-down piston 18a for the hold-down 22a. A transverse wall 29 between the cylinders 4a and 4b, together with the hold-down piston 18a, defines a working chamber 31, which in the illustrated embodiment has its own pressure medium connection.

In that regard, the arrangement corresponds to conventional punch riveting tools with separate cylinders for the punch and hold-down. In contrast to the prior art punch riveting tools, however, in the embodiment of the Fig. 5 the piston rod-side working chamber of the cylinder 4a and the working chamber 31 of the cylinder 5a by a flow connection 33 constantly connected to each other. The flow connection 33 may, as shown, outside of the cylinder or through the transverse wall 29 extend therethrough.

Due to the permanent connection between the piston rod side working chamber of the cylinder 4a and the working chamber 31 of the cylinder 5a results in the arrangement of Fig. 5 the same operation as in the arrangement of Fig. 4 , So in the case of the Fig. 5 two simple way valves 32 and 34 for controlling the pressurization and discharge of the individual working chambers. With regard to the operation of the arrangement of Fig. 5 can therefore refer to the description Fig. 4 to get expelled.

Regarding Fig. 6 is the structural design of the joining tool shown there with the hydraulic cylinders 4a and 5a identical to that of Fig. 5 , The only difference is that instead of the two (parallel) directional control valves 32 and 34, a bypass valve 36 and a directional control valve 38, which are connected in series, are provided.

The directional control valve 38 is between a (in Fig. 6 shown) basic position and a working position adjustable. In the basic position, the directional control valve 38 blocks two pressure medium lines 37 and 39 with respect to the pressure source and the pressure relief area of the hydraulic system. In the working position, the directional control valve 38 connects the pressure medium line 37 to the pressure source and the pressure medium line 39 to the pressure relief area.

The bypass valve 36 is between a (in Fig. 6 illustrated) basic position and a bypass position adjustable. In the basic position, the bypass valve 36 connects the pressure medium line 37 to the pressure medium connection 24a and the pressure medium line 39 to the pressure medium connection 26a. In the bypass position, the bypass valve 36 connects both pressure medium connections 24a and 26a to the pressure medium line 37.

The operation is as follows: In the basic position take the bypass valve 36 and the directional control valve 38 in Fig. 6 shown positions. For the Vorbearbeitungsstellung the joining tool, the directional control valve 38 is moved to its working position, in which it remains for the joining and post-processing position.

In the pre-processing position of the joining tool, the bypass valve 36 assumes its bypass position, in which it connects the piston rod side and piston rod side working chamber of the cylinder 4 a and the working chamber 31 of the cylinder 5 a via the pressure medium line 37 and the directional control valve 38 to the (controllable) pressure source of the hydraulic system. For the joining position, the bypass valve 36 is moved back into its basic position, in which it now connects the piston rod facing away from the working chamber of the cylinder 4a with the pressure source and the piston rod side working chamber and the working chamber 31 with the pressure relief area. Finally, in the post-processing position, the bypass valve 36 resumes the bypass position in which all the working chambers are connected to the pressure source.

It should be noted that the valve arrangement of Fig. 6 even with a joining tool the Fig. 4 can be used.

This in Fig. 7 shown joining tool largely corresponds in terms of its structural design the in Fig. 6 represented, wherein corresponding components with corresponding reference numerals, but in which the letter a is replaced by the letter b, are called. Different from each other Fig. 6 is, however, that the flow connection 33 is missing. Rather, the working chamber 31b of the cylinder 5b has a pressure medium connection 27b which is separate from the pressure medium connections 24b and 26b and which can be pressurized or depressurized via a pressure control valve 40 (eg proportional valve). The joining tool with the two cylinders 4b and 5b for actuating the punch and the blank holder therefore corresponds to a conventional joining tool.

The pressure medium connections 24b and 26b of the working chambers of the cylinder 4b are controllable via a bypass valve 36 and a directional control valve 38 which communicate with the corresponding valves of the Fig. 6 are identical.

In the basic position, the valves take the in Fig. 7 shown basic position. For the operation of the bypass valve 36 and the directional control valve 38 before, during and after the joining process, the description of the Fig. 6 to get expelled. The pressure control valve 40 is operated to apply a relatively low pressure to the working chamber 31b in the pre-machining position, to a negligible pressure in the joining position, and to a high pressure in the post-processing position.

The arrangement of Fig. 7 provides the ability to depressurize the piston rod side working chamber of the cylinder 4b in the post-processing position via the bypass valve 36, so that the punch force exerted by the main piston 12b can be correspondingly increased.

In the Fig. 8 The arrangement shown differs from that of Fig. 7 only in that the bypass valve 36 of the Fig. 7 has been replaced by a bypass valve 36b, which connects in the bypass position, the piston rod-facing and piston rod side working chambers of the cylinder 4 with each other, but it shuts off from the pressure source of the hydraulic system. In the Vorbearbeitungsstellung therefore both working chambers of the cylinder 4b are depressurized, so that no stamping force is generated. In this embodiment, therefore, no rivet detection is possible in the pre-processing position, whereas in all other embodiments, the ram in the pre-processing position exerts a punching force that can be used for rivet detection. Moreover, the operation of the arrangement corresponds to the Fig. 8 the re Fig. 7 described operation.

In the embodiments of the FIGS. 1 to 7 Incidentally, the first operation before the joining process can be performed in two stages in such a way that first the hold-down with the required comparatively low hold-down force is applied and only then the rivet on the stamp with a relatively low punch force is applied to the workpieces. This then makes it possible to depressurize the hold-down already a short period of time before setting the rivet, since the workpieces are pressed together by the attached rivet.

As already mentioned, the riveting method according to the invention can be combined particularly advantageously with the bonding to the hybrid joining technique "punch riveting-bonding". In this hybrid joining technique, an adhesive layer K is provided between the workpieces A and B, see FIGS. 1-3 :

In the above-described riveting after EP 0675774 in which a large hold-down force is exerted already before and during the joining process, the adhesive is enclosed by the hold-down pressure before the joining process within the inner joining zone. During the joining process, therefore, unimpeded pressing and flowing of the adhesive from the joining zone radially outwards is not possible. On the other hand, the large hold down force promotes springback of the punch side workpiece after being punched by the punch rivet. This in turn can lead to the formation of air pockets and channels in the adhesive layer.

In the method according to the invention, however, in which only a relatively small hold-down force and during the joining process substantially no hold-down force is exerted before the joining process, an unimpeded pressing and flowing of the adhesive from the joining zone radially outward during the joining process is substantially possible. In addition, the absence of a hold-down force during the joining process leads to a reduced springback of the punch-side workpiece A, which also reduces the risk of trapped air and channels in the adhesive layer K.

To further reduce this risk, punch rivets N 'are used in the combined punch rivet bonding as in Figure 9 shown used. The punch rivet N 'has in the usual way a rivet head N1 and a rivet N2. In contrast to conventional punch rivets, however, the peripheral surface N3 of the rivet shank N2 is not cylindrical, but slightly divergent in the direction of the rivet head N1. More specifically, the peripheral surface N3 has the shape of a cone with the cone angle α. Is therefore the punch rivet N 'driven with its conical peripheral surface N3 in the joining parts A, B ( Fig. 3 ), so the rivet N2 exerts due to its conicity on the punch side workpiece A from a downward force, which prevents a rebound of the punch-side workpiece A. The workpieces A, B therefore remain in the joining zone in abutment, so that a gap formation between the workpieces A, B and thus the formation of air inclusions and channels are prevented in the adhesive layer K.

The cone angle α must in any case be greater than 0 and is preferably in the range of 0.5 to 10 °, particularly preferably in the range of 1 to 5 °. As illustrated, the entire circumferential surface of the rivet shank N2 may be divergent. However, it is It is also possible to make the peripheral surface N3 cylindrical in a foot region and diverging only in the region between the foot region and the head N1. The axial length of this foot region is expediently chosen so that it corresponds to the thickness of the punch-side workpiece A.

The use of a stamped rivet N 'formed in this way in conjunction with the stamped rivet adhesive method designed according to the invention permits a particularly effective suppression of a gap formation between the workpieces A, B in the joining region and thus the formation of air bubbles and channels in the adhesive layer K.

Claims (25)

1. A method for joining at least two plate-shaped workpieces of similar or different materials by means of a fastening tool which comprises a punch (16) for performing a joining operation at a joining area (18) and a clamp (22) for exerting a clamping force upon said workpieces (A, B) supported against a die, in which method:

   during the joining operation a high punch force required for the joining operation is exerted and substantially no clamping force is exerted to enable free material deformation at the joining area, and

   after the joining operation a high punch force and a high clamping force are exerted at the same time for reducing deformations of the workpieces and to provide for compression of the workpieces at the joining area wherein said high clamping force is exerted only a short time after termination of the joining operation.
2. The method of claim 1, characterized in that said high punch force exerted after the joining operation and said high clamping force exerted after the joining operation are maintained during a predetermined duration.
3. The method of any of the preceding claims, characterized in that said high clamping force exerted after the joining operation exceeds 5 kN, preferably 7,8 kN.
4. The method of claim 1, wherein said high punch force exerted after the joining operation exceeds 5 kN, preferably 6,5 kN.
5. The method of any of the preceding claims, characterized in that prior to the joining operation the clamp (22) is urged against the workpieces (A, B) by a small clamping force to prevent relative displacement of the workpieces (A, B) with respect to each other.
6. The method of claim 5, characterized in that said small clamping force is less than 7.8 kN, preferably less than 3.9 kN.
7. The method of any of claims 1 to 6, characterized in that prior to the joining operation a small punch force is exerted to perform a checking operation at the joining area.
8. The method of claim 7, characterized in that said small punch force exerted prior to the joining operation is less than 5.0 kN, preferably less than 2.5 kN.
9. The method of any of claims 1 to 6, characterized in that prior to the joining operation no punch force is exerted.
10. The method of any of the preceding claims, characterized in that an adhesive layer is provided between the workpieces (A, B) at least in the joining area before the clamp (22) is urged against the workpieces (A, B).
11. The method of any of claims 1 to 10, characterized in that said joining tool is a riveting tool for setting self-piercing rivets (N).
12. The method of claims 10 and 11, characterized in that self-piercing rivets (N) are used where there peripheral surface (N3) of the rivet shaft (N2) is divergent towards the rivet head (N1) at least in a predetermined area.
13. The method of claim 12, characterized in that the peripheral surface of the rivet stem (N2) of the used rivets (N3) is of conical shape in the predetermined area.
14. The method of any of claims 1 to 10, characterized in that the joining tool is a clinching tool.
15. The method of any of the preceding claims, characterized in that the punch (16) and the clamp (22) are actuated by a common hydraulic piston-cylinder-assembly (4).
16. The method of any of claims 1 to 14, characterized in that the punch (16) and the clamp (22) each are actuated by its own piston-cylinder-assembly (4; 5).
17. A device for operating a fastening tool for joining at least two plate-shaped workpieces (A, B) of similar or different materials, comprising:

    a hydraulic cylinder (4) having a cylindrical cavity (10),

    a main piston (12) having a piston rod (14) integrally connected to a punch (16) for performing the joining operation,

    the main piston (12) dividing the cavity (10) of said cylinder (4) into a piston rod remote work chamber (10a) and a piston rod adjacent work chamber (10b) such that the main piston (12) has one side pressurized by pressure prevailing in said piston rod remote work chamber (10a) and has an axially opposite side pressurized by pressure prevailing in said piston rod adjacent work chamber (10b),

    a clamp piston (18) having a piston rod (20) integrally connected to a clamp (22) for exerting a clamping force upon the workpieces (A, B),

    said clamp piston (18) along with its piston rod (20) coaxially extending about the piston rod (14) of said main piston (12) and being displaceable with respect thereto, and

    each of said work chambers (10a, b) having a fluid pressure port (24; 26) selectively to be pressurized or depressurized,

    characterized by:

    the clamp piston (18) being disposed in said piston rod adjacent work chamber (10b) of said hydraulic cylinder (4) and said piston rod adjacent work chamber (10b) has sections disposed on axially opposite sides of said clamp piston (18) and communicating with each other permanently by fluid flow passage means (28) such that the axially opposite sides of said clamp piston (18) both are pressurized by pressure prevailing in said piston rod adjacent work chamber (10b).
18. A device for operating a joining tool for joining at least two plate-shaped workpieces of similar or different materials, comprising:

    a first hydraulic cylinder (4a) with a main piston (12a) having a piston rod (14a) integrally connected to a punch (16a) for performing a joining operation, and dividing the cylindrical cavity of said cylinder (4a) into a piston rod remote work chamber and a piston rod adjacent work chamber each of which has a fluid pressure port (24a, 24b) selectively to be pressurized or depressurized,

    a second hydraulic cylinder (5a) including a clamp piston (18a) connected via a piston rod (20a) to a clamp (22a) for exerting a clamping force, and confining a work chamber (31) selectively to be pressurized by pressure fluid or depressurized,

    characterized in that:

    said work chamber (31) of said second hydraulic cylinder (5a) and said piston rod adjacent work chamber of said first cylinder (4a) are permanently connected to each other by fluid communication means (33).
19. The device of claim 17 or 18, characterized in that said piston rod remote work chamber and said piston rod adjacent work chamber are adapted selectively to be pressurized or depressurized each via a separate directional valve (32, 34).
20. The device of claim 17 or 18, characterized in that said piston rod adjacent work chamber and said piston rod remote work chamber are adapted to selectively to be pressurized or depressurized via a common bypass valve (36) and a common directional valve (38) which are connected in series.
21. The device of any of claims 17 to 20, characterized in that said main piston (12) and said clamp piston (18) are connectable to each other by one-way abutment means (30) acting only in one axial direction such that movement of said main piston (12) from a work position to an end position drives said clamp piston (18) to a corresponding basic position.
22. A method for operating a device of any of claims 17 to 21, characterized in that, in a joining position during the joining operation, the piston rod remote work chamber (10a) is pressurized by a pressure for performing the joining operation, and the piston rod adjacent work chamber (10b) is made substantially pressureless such that the clamp (22) exerts substantially no clamping force.
23. The method of claim 22, characterized in that, in a post-operating position after the joining operation, the piston rod remote and the piston rod adjacent work chambers (10a, 10b) each are pressurized by a high pressure such that the punch (16) and the clamp (22) exert a high force upon the joining area (8) and the workpieces (A, B) after the joining operation.
24. The method of claim 22 or 23, characterized in that, in a pre-operating position prior to the joining operation, the piston rod remote and the piston rod adjacent work chambers (10a, 10b) are pressurized by a low pressure such that the punch (16) and the clamp (22) each exert a small force upon the joining area (8) and the workpieces (A, B).
25. The method of any of claims 22 to 24, characterized in that, in a basic position, the piston rod adjacent work chamber (10b) is pressurized by a low pressure and the piston rod remote work chamber (10a) is made substantially pressureless such that the main piston (12) is urged into an end position serving as basic position.

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