Patent Publication Number: US-7721405-B2

Title: Joining method for operating a fastening tool

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a continuation of prior application Ser. No. 11/201,667, filed Aug. 11, 2005, now U.S. Pat. No. 7,475,473 the entirety of which is hereby incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to a method for joining at least two plate-shaped workpieces of similar or different materials by means of a fastening tool, and a device for operating such a fastening tool. Preferably the fastening tool is a riveting tool for setting the self-piercing rivets or a clinching tool for performing a clinching operation. 
   Various types of drives for fastening tools such as self-piercing riveting and clinching tools have become known. The most common type of drives comprises a hydraulic piston-cylinder-assembly for actuating a punch to perform the joining operation and a further hydraulic piston-cylinder-assembly for actuating a clamp to exert a clamping force upon the workpieces during the joining operation, cf: for example WO93/24256 and EP 0675774. In the method and device of EP 0675774 a “substantial” clamping force is exerted prior to and during the joining operation; as said in this publication the clamping force may be up to 1.5 tons. In practise clamping forces in the order of e.g. 8 to 10 kN are used. Even though this method does have its advantages, exerting a high clamping force prior and during the joining operation does have also some drawbacks. For example the high clamping force exerted during the joining operation may prevent free deformation of the self-piercing rivet. Furthermore, exerting a high clamping force prior to the joining operation may also suffer from some drawbacks in a combined method of using rivets and adhesive to join the workpieces because the high clamping force may detrimentally affect compression and flow of the adhesive from the joining area. 
   In a prior self riveting apparatus of applicant a small clamping force was exerted by means of a spring during the joining operation, and the clamping force was increased towards the end of the joining operation by having the piston of the hydraulic cylinder of the punch transfer a part of the punch force to the clamp via abutment means. The above mentioned EP 0675774 B1 discloses a similar apparatus wherein before and during the joining operation a separate hydraulic cylinder exerts a “substantial” clamping force which is momentarily increased at the end of the riveting operation to about 5 tons by abutment means between the piston of the punch and the piston of the clamp. 
   By WO 00/29145 it has become known to withdraw the punch after the joining operation and, when the punch has been withdrawn, to exert a clamping force upon the workpieces so as to reduce deformations of the plate-shaped workpieces (sheets) out of their plane. To this end this document discloses two basic principles for achieving this result. According to one principle a retaining device for the clamp is provided to prevent yielding of the clamp when the lower leg of the C-shaped frame as used in rivet setting machines springs back as a result of the punch having been withdrawn. According to the other principle the punch and the clamp, during the joining operation, are pressurized simultaneously by a high pressure via abutment means, and after the joining operation the punch is withdrawn whereupon the clamp is again pressurized so as to exert a relatively high clamping force. Furthermore, this document mentions that during the joining operation a high or small clamping force or no clamping force at all may be exerted. 
   Finally, it has become known to exert, prior to the joining operation, at least a small clamping force so as to urge the workpieces against each other and to prevent them from sliding relative to each other and in particular to perform rivet and workpiece checking operations, cf. for example WO 93/24258. 
   SUMMARY OF THE INVENTION 
   It is a primary object of the present invention to provide a method for joining at least two plate-shaped workpieces of similar or different materials by a fastening tool and a device for operating such a fastening tool which is of simple structure and operation and compact design. 
   It is a further object of the present invention to have the joining operation prevented from being detrimentally affected by high clamping forces. 
   It is a further object of the present invention to provide a method and a device for joining at least two plate-shape workpieces, which method and device are able to obtain a high quality joint and minimal deformation of the workpieces at and round the joining area. 
   According to the method of the present invention, during the joining operation a high punch force is exerted to perform the joining operation 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. 
   The joining operation is defined as the operation between the beginning and termination of material deformations necessary for making the joint. In the case of self-piercing riveting, the joining operation is the operation between the beginning and termination of the self-piercing rivet penetrating into the workpieces to be joined. In the case of clinching the joining operation is the operation between the beginning and termination of material deformation of the workpieces at the joining area. 
   The high clamping force exerted after the joining operation is to be selected such that deformations of the workpieces at and around the joining area as caused by the joining operation are reduced and that some compression or compacting effect in the joining area of the workpieces is obtained so as to enhance the quality of the joint and to obtain sufficient final strength of the joint. It is important that, after the joining operation, a high punch force is exerted at the joining area at the same time when the high clamping force is exerted in order to prevent the high clamping force from detrimentally affecting the joint. So the total surfaces at and around the joining area are subjected to high forces after the joining operation whereby the above-mentioned advantages are obtained and furthermore reaction and deflection movements of the C-shaped frame of such fastening tools are reduced. A further advantage of the present invention is that the C-frame may be designed so as to be of reduced weight and strength because substantially no clamping force is exerted during the joining operation so that the C-frame is subjected only to the punch force. 
   A preferred device of the present invention includes only one hydraulic cylinder which is divided into a piston rod remote work chamber and a piston rod adjacent work chamber by the main piston for the punch. The clamping piston for operating the clamp is disposed in the piston rod work chamber of the hydraulic cylinder, and the sections of the piston rod adjacent work chamber on axially opposite sides of the clamp piston communicate to each other by fluid flow passage means such that the clamping piston can be pressurized on its axially opposite sides by the pressure prevailing in the piston rod adjacent work chamber. 
   This structural design of the device is suited to perform the method of the invention. To this end the device preferably has an operating position wherein the piston rod remote work chamber is pressurized by a pressure sufficient for performing the joining operation, and the piston rod adjacent work chamber is depressurized; furthermore, the device preferably has a post operating position wherein both work chambers are pressurized by high pressure such that, after the joining operation, a high clamping force and a high punch force are exerted at the joining area at the same time. 
   Since only one hydraulic cylinder with only two work chambers is required for operating the punch and the clamp, this device of the present invention needs only two fluid pressure ports and only two fluid pressure conduits so that the structural expenditure is minimal. Furthermore, the present invention allows for a compact design of the device because the main piston and the clamping piston are “intercalated”. Therefore, in this device of the present invention increasing the stroke of the device by a certain amount will result in the length of the device being increased by a similar amount whereas, in a drive having two separate hydraulic cylinders, increasing the stroke of the device by a certain amount will result in the length of the device being increased by twice the amount. 
   In another embodiment of the present invention there are provided two hydraulic cylinders. Since however two work chambers of the two cylinders are permanently communicated with each other by flow passage means, many advantages of the first mentioned embodiment will be present also in this second embodiment. For example this second embodiment requires only two pressure ports which again provides for reduced structurable expenditure and a simplified hydraulic control system. 
   An important advantage of the present invention is that the joining operation is not detrimentally affected by clamping forces. When the invention is used e.g. in a tool for setting self-piercing rivets, the self-piercing rivet is enabled freely to be deformed while penetrating into the workpieces. Deformation of the rivet occurs in a precise manner, and the rivet may be spread more than in a conventional method where high clamping forces are exerted during the riveting operation. Furthermore cracking or fissuring of the rivet is avoided by the present invention. 
   The method of the present invention may be used in combination with adhesive for joining the workpieces. In such a method, an adhesive layer is provided between the workpieces at and around the joining area before the clamp is urged against the workpieces. Since in the method of the present invention only a small clamping force is exerted prior to the joining operation and substantially no clamping force is exerted during the joining operation, the adhesive may be freely pressed and may freely flow from the joining area during the joining operation. Furthermore, exerting substantially no clamping force during the joining operation results in reduced formation of air bubbles between the workpieces as will be explained in more detail below. 
   The risk of forming air bubbles between the workpieces may be further reduced by using self-piercing rivets with rivet stems having peripheral surfaces which are tapered, preferably of conical shape. The tapered rivet stem exerts, during the joining operation, a force upon the upper workpiece such that re-bouncing of the upper workpiece is prevented or at least reduced such that the workpieces remain in contact to each other at the joining area. This reduces the risk of formation of air bubbles as will be explained in more detail below. 
   As already mentioned, the present invention allows to avoid or at least reduce reactive movements or deflections of the C-frame whereby the joining quality and final strength of the joint are enhanced. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     With reference to the drawings preferred embodiments of the present invention will be described in detail. In the drawings: 
       FIG. 1  is a longitudinal section of a fastening tool of the present invention in a basic position; 
       FIG. 2  is a longitudinal section similar to  FIG. 1  of the fastening tool in a pre-operating position; 
       FIG. 3  is a longitudinal section similar to  FIG. 1  of the fastening tool in an operating position and post-operating position; 
       FIG. 4  is a longitudinal section of the fastening tool of  FIGS. 1 to 3  including a valve assembly for hydraulic control of the fastening tool in its basic position; 
       FIGS. 5 to 8  are representations similar to  FIG. 4  of modified embodiments of the fastening tool and the valve assembly; 
       FIG. 9  is a special self-piercing rivet to be used in the method and device of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The fastening tool  2  shown only schematically in the  FIGS. 1 to 3  is arranged to make a joint between two (or more) plate-shaped workpieces A and B (metal sheets). In the embodiment as shown the fastening tool  2  is a rivet setting device for setting self-piercing rivets N even though the fastening tool  2  could be another tool such as for example a clinching device. Since self-piercing rivets and self-piercing rivet joints are basically known, they will not be described any further. 
   The fastening tool  2  includes a drive comprising a hydraulic cylinder  4 , and a die  6 , with the workpieces A, B being supported thereagainst. The cylinder  4  is supported on a C-shaped frame (not shown) as is usual in fastening tools of this type. 
   The hydraulic cylinder  4  has a cylindrical cavity  10  receiving a main piston  12  with a piston rod  14  so as to be displaceable therein. The piston rod  14  of the main piston  12  is integrally connected to a punch  16  for setting the rivet N. 
   As is shown in  FIGS. 1 to 3 , the main piston  4  divides the cylindrical cavity  10  into a piston rod remote upper work chamber  10   a  and a piston rod adjacent lower work chamber  10   b . The lower work chamber  10   b  receives a clamping piston  18  which is integrally connected to a sleeve-shaped clamp  22  (clamping nose) via a piston rod  20 . The clamping piston  18  and its piston rod  20  coaxially surround the piston rod  14  of the main piston  12  and are slidingly guided along the cylindrical inner wall of the cylinder  4  such that they are axially displaceable relative to the main piston  12 . 
   The upper work chamber  10   a  has a fluid pressure port  24  for selectively being pressurized by a controlled fluid pressure (hydraulic pressure) or depressurized, while the lower work chamber  10   b  has its own fluid pressure port  26  for selectively being pressurized by a controlled fluid pressure or being depressurized. As schematically shown in the figures, the sections of the work chamber  10   b  on opposite sides of the clamping piston  18  communicate with each other via fluid flow passage means  28  such that the axially opposite sides of the clamping piston  18  are always pressurized by the same fluid pressure. The fluid flow passage means  28  may comprise one or several bores through the clamping piston  18  or may be provided otherwise, for example by one or several longitudinal grooves at the outer periphery of the clamping piston  18 . 
   Furthermore, between the piston rod  14  of the main piston  12  and the sleeve-shaped piston rod  20  of the clamping piston  18  there is provided an abutment means  30  which acts only in one axial direction, i.e. when the main piston  12  moves in a direction away from the lower work chamber  10   b  (upwards in the figures). 
   Operation of the device described above is as follows: 
   In  FIG. 1  the device is in its basic position wherein the upper work chamber  10   a  is depressurized via the fluid pressure port  24  and the lower work chamber  10   b  is pressurized by a relatively small fluid pressure via its fluid pressure port  26 . The fluid pressure prevailing in the lower work chamber  10   b  urges the main piston  12  to its upper end position. The fluid pressure prevailing in the lower work chamber  10   b  acts upon the axially opposite sides of the clamping piston  18 . Since the upper surface area of the clamping piston  18  exceeds its lower surface area for an amount similar to the cross-sectional surface area of the piston rod  20 , a downwardly directed pressure difference acts upon the clamping piston  18 . However, the abutment means  30  between the two piston rods  14  and  20  retains the clamping piston  18  in its basic position which is an axially intermediate position as shown in  FIG. 1 . 
   The operation for setting the self-piercing rivet N and therefore for making the joint is performed in three steps: 
   In a first step, prior to the joining operation, the main piston  12  and the clamping piston  18  are moved to a pre-operating position wherein the clamp  20  and the rivet N are urged against the workpieces A and B by a relatively small force, see  FIG. 2 . To this end both work chambers  10   a ,  10   b  are each pressurized by a small fluid pressure via its respective fluid pressure port  24  and, respectively,  26 . Since the upper surface area of the main piston  12  exceeds its lower surface area for an amount similar to the cross-sectional surface area of the piston rod  14 , the punch  18  exerts a corresponding downwardly directed punch force upon the rivet N at a joining area  8  including the rivet N and the workpiece area between the clamp  22  and the die  6 . Also the clamping piston  18  is urged downwards by the fluid pressure due to the mentioned surface difference so that the clamp  20  exerts a corresponding clamping force upon the workpieces A, B adjacent and around the rivet N at the joining area  8 . The punch force and the clamping force need to be only of a size sufficient to prevent movements of the workpieces A, B relative to each other and relative to die  6  and to allow for performing rivet and workpiece checking operations. The clamping force is preferably less than 7.8 kN and may be for example in the order of 3 to 5 kN. Generally a clamping force of less than 3.9 kN is sufficient. The punch force is preferably less than 5 kN and may be in the order of 1 to 3 kN. 
   In a second step), the joining operation proper, the device is in the operating position shown in  FIG. 3 , wherein the upper work chamber  10   a  is pressurized by the fluid pressure necessary for the joining operation via the fluid pressure port  24 , while the lower work chamber  10   b  is depressurized via its fluid pressure port  26 . The main piston  12  exerts, via the piston rod  14  and the punch  16 , a punch force upon the rivet N, at the joining area  8  the punch force being of a value sufficient for making the joint, see  FIG. 3 . Since the lower work chamber  10   b  is depressurized, the clamp  20  does not perform any function during the joining operation. It goes without saying that in hydraulic systems there is always a residual pressure at the low pressure side such that the lower workchamber cannot be depressurized so as to be completely pressureless. However, the resulting clamping force which may be in the order of 0.3 to 0.5 kN, is negligible. Depending on the material of the workpieces as used, a clamping force of e.g. less than 4 kN has not any measurable influence on the workpiece properties. In any case this is true for clamping forces of less than 1.5 kN. 
   As mentioned above, the lack of significant clamping forces enables the rivet N to be freely deformed into the workpieces A, B and the die  6  during the setting operation so that deformation of the rivet occurs in a precisely defined manner. As compared to conventional methods using high clamping forces, a somewhat increased bottom of the rivet N will result so as to avoid the risk of cracking or fissuring thereof. 
   The punch force needed for setting the self-piercing rivet N is in the usual order of e.g. 30 to 80 kN. 
   In a third step following the joining operation when the device is in a post-operating position also shown in  FIG. 3 , both work chambers  10   a  and  10   b  are pressurized by high fluid pressures via their fluid pressure ports  24  and  26 . Preferably, this is performed by maintaining the high fluid pressure prevailing in work chamber  10   a  while work chamber  10   b  is pressurized by the same fluid pressure or a similar high fluid pressure via fluid pressure port  26 . 
   To ensure that deformation of the self-piercing rivet during the joining operation is continued to its end without any disturbances, pressurisation of work chamber  10   b  and, accordingly, increase of the clamping force are initiated only a brief duration after termination of the joining operation. This duration is for example 0.2 to 0.3 seconds and preferably exceeds 0.1 seconds. 
   Pressurization of work chambers  10   a  and  10   b  as described will result in the punch  6  being urged against rivet N by a high punch force due to the above mentioned surface difference of the main piston  12 . At the same time the clamp  20  exerts a high clamping force upon the workpieces A, B due to the above mentioned surface difference of the clamping piston  18 . The high clamping force is intended to reduce potential deformations of the workpieces A, B at the joining area  8  from their planes and furthermore to perform some compacting action upon the workpieces A, B in this area so as to enhance final strength and quality of the joint. Furthermore, since not only the clamp  20  but also the punch  16  exerts a high force upon the joint, a certain equalizing effect with respect to the forces acting at the joining area will be obtained. Finally, the punch force and the clamping force prevent or at least reduce reactive movements or re-bouncing of the C-shaped frame (not shown). 
   The punch force and the clamping force are to be selected to obtain these functions depending on the specific application. Preferably, the clamping force exceeds 5 kN and in particular 7.8 kN. The punch force preferably exceeds 5 kN and in particular 6.5 kN. 
   The punch force and the clamping force are maintained during a predetermined duration. Thereafter the device is returned to its basic position shown in  FIG. 1  by depressurizing the upper work chamber  10   a  via its fluid pressure port  24  and by pressurizing the lower work chamber  10   b  by low fluid pressure via its fluid pressure port  26 . 
     FIG. 4  schematically shows a fastening tool  2  as described above with reference to  FIG. 1 to 3  in connection with a valve-assembly of a (not shown) hydraulic control system for operating the fastening tool. The valve-assembly consists of a pair of separate directional valves  32  and  34  of which the directional valve  32  communicates with the work chamber  10   a  via the fluid pressure port  24  and the directional valve  34  communicates with the work chamber  10   b  via the fluid pressure port  26  in order to pressurize and depressurize the work chambers  10   a  and  10   b . Accordingly, the directional valves  32  and  34  control communications between each of the work chambers  10   a ,  10   b  and a (not shown) pressure control device (e.g. a proportional valve) and a low pressure area of the hydraulic control system. 
   Operation of the hydraulic control system is similar to the operation as described above with reference to  FIGS. 1 to 3 . Additionally the following is to be noted: 
   In the basic position of the device as shown in  FIG. 5 , the directional valve  32  is in a pressure relief position, and the directional valve  34  is in a pressurization position. When the joining tool is in the pre-operating position, the directional valves  32 ,  34  are in their pressurization positions, wherein both work chambers  10   a ,  10   b  are pressurized by low fluid pressure. When the joining tool is in its operating or joining position, the directional valve  32  is in its pressurization position, and the directional valve  34  is in its pressure relief position in order to have the punch exert a high punch force and, respectively, have the clamp exert substantially no clamping force. When the joining tool is in its operating position, both directional valves  32 ,  34  are in their pressurization positions in order to have both the punch and the clamp exert high forces. 
   The embodiment shown in  FIG. 5  differs from that of  FIG. 4  with regard to the structure of the cylinder assembly of the joining tool. Components similar to those in  FIG. 4  are designated by the same reference numerals with the addition of the letter a. The joining tool of  FIG. 5  differs from that of  FIG. 4  by the fact that instead of a common hydraulic cylinder  4 , there are provided two cylinders  4   a  and  5   a  of which the cylinder  4   a  receives the main piston  12   a  for the punch  16   a  and the cylinder  5   a  receives the clamp piston  18   a  for the clamp  22   a  A partition  29  between the cylinders  4   a  and  4   b  together with the clamp piston  18   a  define a work chamber  31  which has its own fluid pressure port in the embodiment as shown. 
   As described so far this assembly is similar to conventional self-piercing riveting tools having separate cylinders for the punch and clamp. In contrast to these conventional self-piercing riveting tools, the embodiment of  FIG. 5  is of a design such that the piston rod adjacent work chamber of the cylinder  4   a  and the work chamber  31  of the cylinder  5   a  are permanently in communication with each other by a fluid flow passage  33 . The fluid flow passage  33  may extend, as shown, externally of the cylinder or through the partition  29 . 
   As a result of the permanent communication between the piston rod adjacent work chamber of the cylinder  4   a  and the work chamber  31  of the cylinder  5   a  operation of the assembly in  FIG. 5  is similar of that of the assembly in  FIG. 4 . So also in the assembly of  FIG. 5  two simple directional valves  32  and  34  are sufficient to control pressurization and pressure relief of the individual work chambers. Therefore, as to operation of the assembly of  FIG. 5  attention is drawn to the description of the operation of the assembly in  FIG. 4 . 
   Referring to  FIG. 6 , the structure of the joining tool including the hydraulic cylinders  4   a  and  5   a  shown therein is identical to that of  FIG. 5 .  FIGS. 5 and 6  differ from each other only in that, there are provided a bypass valve  36  and a directional valve  38  connected in series, instead of the directional valves  32  and  34  connected in parallel. 
   The directional valve  38  is displaceable between a basic position (shown in  FIG. 6 ) and an operative position. When the directional valve  38  is in its basic position, it isolates a pair of pressure fluid lines  37 ,  39  from the pressure source and the low pressure area of the hydraulic system. When the directional valve  38  is in its operative position, it communicates the pressure fluid line  37  with the pressure source and the pressure fluid line  39  with the low pressure area. 
   The bypass valve  36  is displaceable between a basic position (shown in  FIG. 6 ) and a bypass position. When the bypass valve  36  is in its basic position, it communicates the fluid pressure line  37  with the fluid pressure port  24   a , and the fluid pressure line  39  with the fluid pressure port  26   a . When the bypass valve  36  is in its bypass position, it communicates both pressure ports  24   a ,  26   a  with the fluid pressure line  37 . 
   Operation is as follows: 
   In  FIG. 6  the bypass valve  36  and the directional valve  38  are in their basic positions. To set the joining tool into its pre-operating position, the directional valve  36  is displaced into its operative position where it remains when the joining tool will be in its operating and post-operating positions. 
   When the joining tool is in its pre-operating position, the bypass valve  36  is in its bypass position wherein it communicates the piston rod remote and piston rod adjacent work chambers of the cylinder  4   a  and the work chamber  31  of the cylinder  5   a  with the (controllable) pressure source of the hydraulic system via the fluid pressure line  37  and the directional valve  38 . To move the joining tool to its operating position, the bypass valve  36  is returned to its basic position where it communicates the piston rod remote work chamber of the cylinder  4   a  with the pressure source, and the piston rod adjacent work chamber and the work chamber  31  with the low pressure side of the hydraulic system. When the joining tool is in its post-operating position, the bypass valve  36  will be again in its bypass position wherein all work chambers are communicated with the pressure source. 
   It is to be noted that the valve assembly of  FIG. 6  can be used also in a joining tool as shown in  FIG. 4 . 
   The joining tool shown in  FIG. 7  is of a structure which is essentially similar to that shown in  FIG. 6 . Corresponding components have been designated by similar reference numerals wherein the letter a has been replaced by the letter b. A difference with respect to  FIG. 6  is that the fluid flow passage  33  has been omitted. Rather the work chamber  31   b  of the cylinder  5   b  has a pressure fluid port  27   b  separate from the pressure fluid port  24   b ,  26   b ; the fluid pressure port  27   b  may be pressurized and depressurized via a pressure control valve  40  (e.g. a proportional valve). The joining tool including the two cylinders  4   b  and  5   b  for actuating the punch and the clamp, therefore, is similar to that of a conventional joining tool. 
   The fluid pressure ports  24   b  and  26   b  of the work chambers of the cylinder  4   b  are adapted to be controlled via a bypass valve  36  and a directional valve  38  which are identical to the respective valves in  FIG. 6 . 
     FIG. 7  shows the valves in their basic positions. As to actuation of the bypass valve  36  and the directional valve  38  prior to, during and after the joining operation, reference is made to the description of  FIG. 6 . The pressure control valve  40  is operated such that the work chamber  31   b  is pressurized by a relatively low pressure in the pre-operating position, by a negligible pressure in the operating position, and by a high pressure in the post-operating position of the joining tool. 
   The arrangement of  FIG. 7  enables the piston rod adjacent work chamber of the cylinder  4   b  to be depressurized via the bypass valve  36  in the post-operating position so that the punch force exerted by the main piston  12   b  can be increased correspondingly. 
   The arrangement shown in  FIG. 8  differs from that in  FIG. 7  only in that the bypass valve  36  of  FIG. 7  has been replaced by a bypass valve  36   b  which when in the bypass position communicates the piston rod remote work chamber and piston rod adjacent work chamber of the cylinder  4 , however, isolates them with respect to the pressure source of the hydraulic system. When the joining tool is in the pre-operating position, both work chambers of the cylinder  4   b  are pressureless so that the punch does not exert any punch force. Therefore, this embodiment does not allow for rivet detection when the joining tool is in its pre-operating position. In all other embodiments of the invention, when the joining tool is in the pre-operating position the punch exerts a punch force which can be used for rivet detection. Apart therefrom, operation of the arrangement of  FIG. 6  is similar to that of the arrangement in  FIG. 7 . 
   In the embodiments of  FIGS. 1 to 7  the first working step prior to the joining operation can be performed in two stages such that initially the clamp is actuated to exert the required relatively small clamping force and thereafter the rivet is urged against the workpieces by the punch by a relatively small punch force. This allows to depressurize the clamp already a brief duration prior to setting the rivet because the workpieces are held in contact to each other by the rivet. 
   As mentioned above the riveting method of the present invention can be combined with a method using adhesive between adjacent surfaces of the workpieces at least at the joining area. More particularly, to this end an adhesive layer K is provided between the workpieces A and B, cf.  FIGS. 1 to 3 . 
   In a conventional riveting method as disclosed e.g. in EP 0675774 wherein a high clamping force is exerted prior to and during the joining operation, the adhesive is enclosed within the central joining area by the clamping pressure before the joining operation. As a result free compression and flow of the adhesive from the joining area radially outwards is not possible during the joining operation. On the other hand, a high clamping force enhances re-bouncing of the upper workpiece when it has been penetrated by the self-piercing rivet. This may result to formation of air bubbles and channels within the adhesive layer. 
   In contrast thereto, in the method of the present invention where before the joining operation only a relatively small clamping force is exerted and during the joining operation substantially no clamping force is exerted, the adhesive may be freely compressed and may freely flow from the joining area radially outwards during the joining operation. Furthermore, exerting substantially no clamping force during the joining operation will reduce re-bouncing of the upper workpiece A which also reduces the risk of air bubbles and channels within the adhesive layer K. 
   In order further to reduce this risk, self-piercing rivets N′ as shown in  FIG. 9  are used along with adhesive in the combined method. The self-piercing rivet N′ comprises, a rivet head N 1  and a rivet stem N 2 . In contrast to conventional rivets, the peripheral surface N 3  of the rivet stem N 2  is not cylindrical but slightly tapered so as to be divergent towards the rivet head N 1 . More precisely, the peripheral surface N 3  is of conical shape having a cone angle α. Therefore, when the rivet N′ with its conical peripheral surface N 3  is driven so as to penetrate into the workpieces A, B ( FIG. 3 ), the rivet stem N 2 , due to its tapered peripheral surface, exerts upon the upper workpiece A a downward force which prevents the upper workpiece A from re-bouncing, i.e. from being deflected upwards. As a result the workpieces A, B remain in contact to each other in the joining area so as to avoid any gap between the workpieces A, B and to prevent the formation of air bubbles and channels within the adhesive layer K. 
   The cone angle α exceeds 0 in each case and is preferably in the range between 0.5 to 10°, in particular in the range from 1 to 5°. As shown, the peripheral surface of the rivet stem N 2  may be tapered along its total length. As an alternative, only a certain portion of the peripheral surface could be tapered while the remaining portion of the peripheral surface would be cylindrical. In particular, it would be possible to make the peripheral surface N 3  in a bottom portion cylindrical while only the portion between the bottom portion and the head N 1  would be tapered. The axial length of this bottom portion preferably would be selected such that it does not exceed the thickness of the upper workpiece A. 
   The use of such self-piercing rivets N′ in a combined rivet-adhesive-method for joining workpieces is particularly effective in avoiding any gaps between the workpieces A, B in the joining area and in preventing the formation of air bubbles and channels within the adhesive layer K.