BACKGROUND OF THE INVENTION One example of a self-piercing rivet setting apparatus is described in Japanese Patent Laid-Open No. 08-505087. Fig. 1 of this publication shows one example of a self-piercing rivet. The self-piercing rivet comprises a large- diameter head and a cylindrical hollowed leg extending downward from the head. When the self-piercing rivet is driven into workpieces, such as two body panels, by a punch and a die of the setting apparatus, the leg is driven to pierce the workpieces while allowing the front end of the leg to be expanded and deformed to connect the workpieces with one another by the expanded leg and the head. The self-piercing rivet is suitable for connecting aluminum body panels to which welding is not applicable. The demand for the self- piercing rivet is increasing because aluminum bodies are increasingly employed to drive forward weight reduction in automobile bodies. In particular, since the self-piercing rivet is driven to allow the front end of the leg to pass through a punch-side workpiece but to stay in a receiving-side workpiece adjacent to the die without passing therethrough, the rivet does not form any opening in the surface of the receiving-side workpiece. This provides an advantage of maintaining a sealing performance and good appearance of the receiving-side workpiece.

In a conventional self-piercing-rivet driving operation, if the punch-side workpiece has a greater thickness in a rivet-driving direction than that of the receiving-side workpiece adjacent to the die, a radial piercing length, that is, an undercut amount of the leg of the rivet obliquely piercing the receiving-side workpiece can be reduced, resulting in insufficient connecting strength. Such a condition will be described in conjunction with Fig. 1. Fig. 1 shows the condition when a self-piercing rivet 1 is driven into two workpieces 2 and 3 to connect the punch-side workpiece 2 (a plurality of punch-side workpieces can be provided) with the receiving-side workpiece 3 adjacent to the die. The self- piercing rivet 1 has a large-diameter head 5 and a hollowed leg 6 extending from the head. As shown in Fig. 1, when the punch-side workpiece 2 has a greater thickness than that of the receiving-side workpiece 3, a radial piercing length, that is, an undercut amount 7 of the leg 6 of the self-piercing rivet obliquely piercing the receiving-side workpiece 3 is reduced, and thereby the workpiece 3 cannot be sufficiently connected to the workpiece 2.

Japanese Patent Laid-Open No. 07-503665 discloses a method for connecting two panels with each other using a punch, a die and a right- cylindrical rivet, in which the panels are protruded and deformed to allow the protruded and deformed portions to be received into a cavity of the die while squeezing a punch-side workpiece in the protruded and deformed portions by a receiving-side panel adjacent to the die in the protruded and deformed portion. This panel connecting method uses a right-cylindrical hollowed rivet rather than a self-piercing rivet. The right-cylindrical hollowed rivet does not intend to connect a plurality of panels with each other in combination with any self-piercing rivet. In another embodiment of this publication, the protruded and deformed portions are firstly squeezed (or clinched), and secondly a piercing rivet is driven into the protruded and deformed portions by the punch.

Neither this method nor the above method intends to perform the squeezing operation in conjunction with any self-piercing-rivet driving operation.

Japanese Patent Laid-Open No. 09-506153 discloses a self-piercing rivet setting apparatus for connecting a plurality of workpieces with each other by using a self-piercing rivet, and a punch and a die. In the self-piercing rivet

setting apparatus, in order to solve a problem in which thin panels as the workpieces might be either partially damaged or entirely cut at a self-piercing- rivet driving position, a member for defining a cavity of the die is movable in the radial outward direction of the cavity so that protruded portions of the thin panels deformed toward the die during a self-piercing-rivet driving operation can escape in the radial outward direction. This self-piercing rivet setting apparatus is directed to connect the plurality of panels with each other by using the self-piercing rivet, but it is not directed to squeeze the punch-side panel by the receiving-side panel or perform the squeezing operation in conjunction with the self-piercing-rivet driving operation.

It is therefore an object of the present invention to provide a self- piercing rivet setting apparatus capable of deforming workpieces to compensate an insufficient connecting force resulting from an inadequate undercut amount of a self-piercing rivet.

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention provides a self-piercing rivet setting apparatus comprising a punch and a die associated with the punch to drive a self-piercing rivet into a plurality of workpieces including a receiving-side workpiece adjacent to the die, the self-piercing rivet having a large-diameter head and a hollowed leg extending from the head wherein when the self-piercing rivet is driven into the workpieces, the leg is driven to pierce the workpieces while allowing the front end of the leg to be expanded and deformed in its radial outward direction and to be retained in the receiving-side workpiece without passing therethrough to connect the plurality of workpieces with each other by the expanded leg and the head.

In the self-piercing rivet setting apparatus, the die includes a cavity for receiving therein protruded portions of the workpieces which are protruded and deformed into the die due to the pressure of the self-piercing rivet pressed by the punch, the cavity is formed to have a diameter that progressively increases from an inlet portion to the bottom of the cavity, so that when the protruded workpiece portions are received in the cavity, the

protruded receiving-side workpiece portion of the protruded workpiece portions has a configuration to squeeze (or clinch) the remaining protruded workpiece portion (s) at the inlet portion of the cavity.

According to the above self-piercing rivet setting apparatus, when the protruded workpiece portions are received in the cavity, the protruded receiving-side workpiece portion in the protruded workpiece portions acts to squeeze (or clinch) the remaining protruded workpiece portion (s) at the inlet portion of the cavity. Thus, a connection force based on the squeezing can be added to a connecting force based on the self-piercing rivet to reinforce the connection between the plurality of workpieces. Further, the larger-diameter bottom portion of the cavity allows the front end of the leg of the self-piercing rivet to be further expanded in the radial outward direction to minimize inadequacy in the undercut amount and provide enhanced connecting strength in combination with the squeezing operation at the inlet portion of the cavity.

In the above the above self-piercing rivet setting apparatus, the cavity may be formed so that after the rivet setting, the squeezing configuration of the protruded receiving-side workpiece portion is effectively maintained due to its spring back even when the protruded workpiece portions are released from the cavity. The cavity may be formed with a raised portion protruding toward the punch at the center of the bottom surface thereof to thereby facilitate the leg of the self-piercing rivet to be expanded and deformed in its radial outward direction while piercing the workpieces. This makes it possible to compensate for the undercut amount and thereby provide further enhanced connecting strength of the workpieces. Furthermore, the die may comprise a one-piece member or integrally assembled members capable of maintaining a given configuration of the cavity (or preventing the configuration of the cavity from being changed). This makes it possible to reliably maintain the function of deforming the protruded workpiece portions as being protruded and deformed by driving the self-piercing rivet.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional view showing connected workpieces with a self- piercing rivet driven by a conventional setting apparatus.

Fig. 2 is a front view of a self-piercing rivet setting apparatus according to one embodiment of the present invention.

Fig. 3 is a vertical sectional view of a die used in the self-piercing rivet setting apparatus in Fig. 2.

Fig. 4 is a vertical sectional view of another die used in the self-piercing rivet setting apparatus in Fig. 2.

Fig. 5 is a vertical sectional view of workpieces released from a die after the completion of a self-piercing-rivet setting operation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT With reference to the drawings, an embodiment of the present invention will now be described. Fig 2 schematically shows the entire structure of a self-piercing rivet setting apparatus 9 according one embodiment of the present invention. In Fig. 2, the self-piercing rivet setting apparatus 9 includes a C-shaped frame 11 having a coupling portion 10 to be coupled with an articulated robot arm (not shown). The C-shaped frame 11 is an integral rigid body including an upper horizontal arm, a vertical arm having the coupling portion 10 attached thereto, and a lower horizontal arm. A setting assembly 13 of the self-piercing rivet setting apparatus is attached to the end of the upper horizontal arm or one end of the C-shaped frame 11. The setting assembly 13 is provided with a punch 14 movably attached to the front-end (the lower end in Fig. 2) side thereof, and a receiver unit 15 extending from the punch 14 to the front-end side. A self-piercing rivet (for example, see the self-piercing rivet 1 in Fig. 1) fed to and held in the receiver unit 15 is driven by the punch 14. A spindle type driving unit 17 is provided on the upper side of the punch 14. The spindle type driving unit 17 is operable to press the punch 14 to drive the self-piercing rivet held in the receiver unit on the lower side of the punch. A die 18 is attached to the end of the lower horizontal arm or the other end of the C-shaped frame 11. For example, the spindle type

driving unit 17 comprises an electric driving motor 19, a reduction gear assembly 21 and a gear assembly 22 for transmitting a rotation force of the motor, and a spindle 23 adapted to move vertically while rotating according to the rotation force from the gear assembly 22. When the spindle moves downward according the rotation force of the electric driving motor, this movement is transmitted to the punch 14, and then the punch 14 strongly presses the self-piercing rivet held in the receiver unit 15 toward the die 18. A plurality of workpieces (for example, see the workpieces 2 and 3 in Fig. 1) are placed on the die 18. According to the downward movement of the punch 14, the self-piercing rivet is driven into the plurality of workpieces to connect these workpieces with each other. The C-shaped frame 11 elastically supports the setting assembly 13 and the die 18 to absorb an impact force during the rivet- driving operation.

In the present invention, as shown in Fig. 3, the die 18 includes a cavity 25 having a specific configuration. In Fig. 3, the cavity 25 of the die 18 is adapted to receive therein protruded portions of the workpieces which are protruded and deformed into the die due to the pressure of the self-piercing rivet which is pressed by the punch 14. The cavity allows the protruded workpiece portions received therein to be protruded and deformed in conformity to the configuration of the cavity 25. The cavity 25 is formed to have a diameter that progressively increases from an inlet portion 26 to a bottom surface 27 of the cavity. Thus, when the protruded workpiece portions are received in the cavity, a protruded receiving-side workpiece adjacent to the die (for example, of the receiving-side workpiece 3 in Fig. 3) in the protruded workpiece portions has a configuration that squeezes or clinches the remaining protruded workpiece portion (s) of the workpieces (for example, the punch-side workpiece 2 in Fig. 3) at the inlet portion 26. Further, the angle and/or curvature of the inlet portion 16 of the cavity 25 is designed to allow the squeezed configuration of the protruded workpiece portions or the squeezing (clinching) configuration of the receiving-side workpiece to be effectively maintained due to its spring back even when the protruded workpiece

portions, which have been rivet-set and squeezed in the cavity, are released from the cavity.

The bottom surface 27 of the cavity 25 of the die 18 may be substantially flat, as shown in Fig. 3. As shown in Fig. 4, however, the bottom surface 27 is more preferably formed with a raised portion 29. In Fig. 4, the raised portion 29 protruding toward the punch is provided at the center of the bottom surface 27 of the cavity 25. Thus, when the self-piercing rivet pressed onto the workpieces by the punch is driven to pierce the workpieces, the rigidity of the raised portion 19 facilitates allowing the leg of the self-piercing rivet to be expanded and deformed in its radial outward direction. Both in the dies of Figs. 3 and 4, the die 18 is made from a one piece member or integrally assembled members capable of maintaining a certain configuration of the cavity 25 to prevent the configuration of the cavity from being deformed.

For example, the cavity 28 may be made of a solid cylindrical member by cutting the cavity 25. Alternatively, the cavity 25 may be formed by splitting the solid blank into two halves, forming one half of the cavity 25 in each of the two halves to provide the cavity, and the two halves are firmly connected with one another, for example, by means of bolt/nut or welding.

Fig. 5 is a sectional view showing the punch-side workpiece 2 and the receiving-side workpiece 3 after the completion of a self-piercing-rivet setting operation. In the setting operation, a self-piercing rivet (see Fig. 1) is automatically fed from a feeding unit (not shown) to the receiver unit 15, and held in the receiver unit 15 to locate the rivet below the punch 14. The workpieces 2 and 3 (see Fig. 1) to be connected with one another are placed between the die 18 and the punch 14. It is to be understood that the number of the workpieces may be two or more. The punch 14 is moved downward by receiving the pressing force from the spindle type driving unit 17 (Fig. 2) to drive the self-piercing rivet 1 into the punch-side workpiece 2. Through this rivet-driving operation, the hollowed leg 6 of the self-piercing rivet 1 progressively pierces the workpiece 2, and reaches the receiving-side workpiece 3 adjacent to the die 18. As the self-piercing rivet is driven, a portion of each of the workpieces 2 and 3 located below the rivet is protruded

and deformed into the die, and the resulting protruded workpiece portions will be received in the cavity 25. The cavity 25 is narrowed at the inlet portion and widened at the bottom surface 25. The protruded workpiece portions will be deformed in conformity to the configuration of the cavity 25.

The above condition is shown in Fig. 5. The receiving-side workpiece 3 adjacent to the die 18 is deformed to have a configuration to squeeze the remaining workpiece potion (or a potion of the workpiece 2) by a neck portion 30 at the inlet portion 26. This deformation allows the receiving-side workpiece 3 to clinch or squeeze the punch-side workpiece 2. Through the squeezing operation, the receiving-side workpiece 3 is connected to the punch-side workpiece 2. In addition, the front end of the leg 6 of the self- piercing rivet 1 passes through the punch-side workpiece 2, and then pierces the receiving-side workpiece 3 while being expanded and deformed in the radial outward direction to firmly connect the workpieces 2 and 3 with one another. When the leg 6 is driven to pierce the receiving-side workpiece 3 while allowing the front end of the leg to be expanded and deformed in the radial outward direction, the front end of the leg can be largely expanded in the radial outward direction at the larger-diameter bottom portion because the cavity 25 is formed to have a diameter that progressively increases from the inlet portion 26 to the bottom portion 27, or toward the bottom surface 27. This makes it possible to increase the undercut amount and provide enhanced connecting strength of the workpieces in combination with the squeezing operation at the inlet portion of the cavity. The rivet-driving operation is completed in the condition where the front end of the leg 6 is retained in the receiving-side workpiece 3 without passing therethrough.

While the operation of connecting the workpieces through the self- piercing-rivet driving operation can be achieved without the raised portion protruding toward the punch at the center of the bottom surface 27 of the cavity 25 as shown in Fig. 4, the cavity 25 having the raised portion 29 at the center of the bottom surface 27 can provide further enhanced connecting strength. When the leg of the self-piercing rivet pressed onto the workpieces 2 and 3 is driven into the workpieces while deforming the workpieces,

particularly, the protruded workpiece portions toward the inside of the cavity 25, the rigidity of the raised portion 29 formed at the center of the bottom surface of the cavity 25 effectively prevents the leg of the self-piercing rivet from moving in a straight direction, so as to facilitate allowing the leg to be expanded and deformed in the radial outward direction. In this manner, the raised portion 29 facilitates allowing the leg of the self-piercing rivet to be expanded and deformed in the radial outward direction when the leg is driven into the workpieces. This makes it possible to increase the outward expansion of the leg and further increase the undercut amount to provide further enhanced connecting strength of the workpieces. In Fig. 5, the two-dot chain line (or phantom line) indicates a concave portion to be deformed by the raised portion 29 if it is provided.

The workpieces 3 and 2, more specifically, the protruded workpiece portions deformed in the cavity 25 have the smaller-diameter neck portion 30 at the inlet portion 26 of the cavity 25, and the larger-diameter portion at the bottom portion. Thus, when the protruded workpiece portions are released from the die 18, it is required to strongly pull the workpieces to get the protruded workpiece portions out of the cavity 25 of the die 18. In this case, the cavity is formed to allow the squeezing configuration of the protruded receiving-side workpiece portion to be effectively maintained due to its spring back even when the protruded workpiece portions, which have been rivet-set and squeezed in the cavity, are released from the cavity. Thus, as shown in Fig. 5, the workpieces 2 and 3 released from the cavity 25 remain in the squeezed shape by the neck portion 30, and the protruded portion of the receiving-side workpiece 3 in the protruded workpiece portions still clinches the protruded punch-side workpiece portion to maintain the connection according to the squeezing operation. Thus, even after the workpieces are released from the cavity, their connecting strength is not deteriorated.

According to the present invention, when the protruded workpiece portion is received in the cavity, the protruded receiving-side workpiece portion included in the protruded workpiece portions acts to squeeze the remaining protruded workpiece portion (s) at the inlet portion of the cavity.

Thus, a connection force based on the squeezing can be added to a connecting force based on the self-piercing rivet to reinforce the connection between the plurality of workpieces. Further, the larger-diameter bottom portion of the cavity allows the front end of the leg of the self-piercing rivet to be further expanded in the radial outward direction so as to minimize the inadequacy in the undercut amount and provide enhanced connecting strength in combination with the squeezing operation at the inlet portion of the cavity.