Abstract:
The invention relates to a joining device for the joining, in particular the welding, of two parts one to the other, having a joining head comprising a lift device for linear transport of a joining unit along a lift axis relative to the joining head, said joining device executing a joining operation by transporting the joining unit out of a lift position towards a base position, and said joining unit comprising a boom extending outside of the joining head, at the free end of which the joining operation takes place offset parallel to the lift axis, wherein the joining unit is so oriented relative to the joining head that the joining unit is run out relative to the joining head in the lift position.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of International Application Serial No. PCT/EP2004/007472, filed on Jul. 8, 2004, which claims priority to German Application Serial No. DE 10333415.7, filed on Jul. 17, 2003, both of which are incorporated by reference herein. 
     
    
     BACKGROUND AND SUMMARY  
       [0002]     The present invention relates to a joining device for the joining, in particular the welding, of two parts one to the other, having a joining head comprising a lifting device for linear transport of a joining unit along a lift axis relative to the joining head, said joining device performing a joining operation by transporting the joining unit out of a lift position towards a base position, and the joining unit comprising a boom extending outside of the joining head, at the free end of which the joining operation takes place offset parallel to the lift axis. The invention relates further to a process for the joining, in particular the welding, of two parts one to the other, comprising the following steps: transport of a joining unit relative to a joining head into a lift position by means of a lifting device in order to join the parts one to the other towards a base position of the joining unit, and transport of the joining unit into the base position.  
         [0003]     Such a joining device and such a joining process are disclosed in EP 0,367,414 B1. By joining, in the production engineering sense, is meant a process of positive geometrical, dynamic or material connection of two or more parts, or of parts to an amorphous material, by juxtaposition (e.g. insertion, encasement), pressing on or into (e.g. screwing, wedging, shrinking), material connection (e.g. welding, soldering, bonding), molding (e.g. fusing, casting), transforming (e.g. rolling) or filling (e.g. impregnating an electrical winding).  
         [0004]     In short-time electric arc welding, an element is welded onto a part. Thus, an electric arc is set up between the element and the part, incipiently fusing the frontal surfaces. Then the element and the part are moved towards each other so that the melts mingle. The arc is short-circuited and the whole melt solidifies. It is possible to draw the electric arc (lift ignition). Here, the element is first placed on the part (base position). Then a pilot current is switched on, and the element is lifted relative to the part up to a desired level (lift position), thus drawing an arc. Only then, the welding current is switched on and the element is replaced on the part, whereupon the melts mingle. Then the welding current is switched off, so that the entire melt solidifies.  
         [0005]     To obtain uniformly good welding results, it is important, among other things, to find the relative position between element and part, in particular to lift the element to the correct level before the welding current is switched on. For this purpose, as a rule each welding operation is preceded by a measurement of the relative position, particularly in the form of a zero position determination. This is especially important in the case of robot-based systems. True, present-day robots are generally able to position with comparative precision. High precision, however, especially owing to the large masses in motion, is not available with very high dynamics. Ordinarily, the robots have a scope of motion in three coordinates. In the simplest case, a robot is an automatically driven linear guide (carriage) on which a welding head is mounted. As a rule, such a carriage is attached to the arm of a multi-axial (bent arm) robot.  
         [0006]     In the automotive industry, bolt welding systems are employed. There they serve primarily to weld elements such as threaded or unthreaded bolts, eyes, nuts, etc. onto the body sheet-metal. These elements then serve as holding anchors for attachment, for example, of interior trim, cable trees or the like. In the automotive industry, speed of production is an essential consideration. Within a few minutes, hundreds of elements are to be welded on in various positions automatically by means of robots. The robots must consequently move with high dynamics.  
         [0007]     Thus, it is known that a welding head base bearing a carriage may be attached to the arm of a robot. The carriage is high-dynamically movable, commonly by means of a pneumatic or hydraulic system. On the carriage, the welding head proper is mounted, in turn possessing a lifting device to move the element with high precision. To determine the relative position between the element and the part, it is known that a so-called support foot may be attached to the welding head (for example, “ Neue TUCKER - Technologie. Bolzenschweissen mit System,”  Emhart Tucker, September 1999).  
         [0008]     The spatial size of a welding head, however, represents a natural spatial barrier within which bolt welding with welding heads is possible. So that bolts can be welded in locations of difficult access as well, there are welding heads known in the prior art that are connected to a welding gun by way of an extension cable. Such a welding head is disclosed in DE G 94 17 371. The welding head of DE G 94 17 371 is much smaller than the welding gun connected to it. This welding head is suitable for applications where not much room is available. A disadvantage of this type of bolt welding is that the welding device must be operated by hand. Both the welding gun and also the welding head connected by way of an extension must be positioned and actuated manually by a worker. This is time-consuming. It is also reflected in high production costs, operating labor being costly as a rule.  
         [0009]     Hence, one object of the invention is to create a joining device and a process whereby automated joining can be accomplished even in positions difficult of access. This object is accomplished by a joining device of the kind initially mentioned, wherein the joining unit is so oriented relative to the joining head that the joining unit is extended in the lift position. Further, the object is accomplished by a process for joining of the kind initially mentioned, wherein the joining unit is extended into the lift position relative to the joining head.  
         [0010]     In this way, it is brought about that even positions of difficult access can be reached automatically with a joining head for purposes of joining. The invention makes it possible, by contrast with the prior art, to join ‘rearward.’ The ‘rearward’ orientation will be explained in the following. Besides, time and expense can be saved. The device and the process according to the present invention open up a multitude of applications in which but little space is available for joining.  
         [0011]     It is preferred as well for the joining unit to be so oriented relative to the joining head that the joining device is run into the base position. According to a preferred embodiment, the joining head comprises a holding device for holding one of the parts. Further, it is preferred for a held part to be a bolt and the holding device a bolt holder. Also, it is advantageous if the joining head is a welding head. Through these measures, it is possible to utilize the joining device as an arc welding device, capable in particular of welding bolts to a part.  
         [0012]     According to another preferred embodiment, the joining head comprises a support foot. An additionally provided support foot facilitates the positioning of the joining head relative to the part to which the other part is attached. Further, it is preferred for the joining head to be attached to a carriage. Thus, it is possible rapidly to transport the joining head, and with it the joining unit, into the base position.  
         [0013]     According to another embodiment, the joining device further comprises a part feed, in particular an element feed. This likewise will enhance the speed of an operating cycle, since parts can be supplied automatically rather than manually. It has also proved advantageous for the lifting device to comprise a compression spring, to be wound by means of a linear motor. Moreover, the linear motor is extended in the lift position. Through these measures, it is possible to weld ‘rearward.’ It will be understood that the features aforementioned and those yet to be illustrated below may be employed not only in the particular combination specified but also in other combinations, or alone, without departing from the scope of the present invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     Embodiments of the invention are represented by way of example in the drawings and will be illustrated in more detail in the description to follow. In the drawings:  
         [0015]      FIG. 1  shows a schematic view of a welding head according to the prior art, having a boom welding forward;  
         [0016]     FIGS.  2  to  5  show schematic views of a joining operation using a joining device according to the present invention; and  
         [0017]      FIG. 6  shows another embodiment of the joining device according to the invention. 
     
    
     DETAILED DESCRIPTION  
       [0018]      FIG. 1  shows a welding head  10  known in the prior art, welding ‘forward’ and suitable for use in applications where there is not enough room to position the complete welding head  10  directly above a weld location. For example, it may be required to weld a bolt  12  into an opening of a part  14 , say a tube, into which the welding head  10  will not fit. For automated welding of the bolt  12  with the welding head  10  despite the lack of space, the top of the welding head  10  is fitted with a boom  16  connected to a bolt holder  18 . By means of the boom  16 , it is possible for a welding axis SA to be offset parallel to a lift axis HA. As a result, the welding head  10  need not be introduced into the aperture of the tube  14  in order to weld the bolt forward along the welding axis SA, as indicated by the arrowhead at the welding axis SA, but it may be positioned outside of the tube  14 .  
         [0019]     Further, the welding head  10  comprises a lift device  20  by means of which a welding unit  22  is transportable along the lift axis HA. In  FIG. 1 , the welding unit  22  comprises the boom  16 , the bolt holder  18  and an element transportably lodged in the lift device  20 . In  FIG. 1 , a dashed line partly represents the interior of the lift device  20 . The lift device  20  can transport the welding unit  22  in its interior, for example with a linear motor (not shown), along the lift axis HA.  
         [0020]      FIG. 1  shows the situation in which the welding unit  22  is run all the way ‘forward.’ ‘Forward’ means upward in the plane of the drawing of  FIG. 1 . In the interior of the lift device  20 , there is enough room rearward to transport the welding unit  22  out of a base position GS into a lift position HS. The base position GS and the lift position HS are indicated in  FIG. 1  by horizontal dotted lines.  
         [0021]     In the following, with reference to  FIG. 1 , the course of a welding operation will be roughly outlined. The welding head  10 , for example by means of a robot  24  of the kind originally mentioned, may first be run into a position whence the welding head, with the aid of a carriage  26  capable of electric or pneumatic operation, can be run into the base position GS (cf. also positioning of the welding head  10  with respect to the carriage  26  in  FIGS. 2 and 3 ). Also, a nozzle  28  may be attached to the welding head  10 . The nozzle  28  in  FIG. 1  is fixedly connected to the welding head  10  and comprises an opening  29  through which the bolt holder  18  reaches. Before the nozzle  28  is seated on the inside of the top segment  14   a  of the tube  14 , it is possible that the bolt  12  may protrude from another opening of the nozzle  28  (cf. also  FIG. 2 ). The welding unit  22 , and with it the bolt  12 , held by the bolt holder  18 , moves at the instant when the nozzle  28  is seated on the part  14   a , downward or rearward by a suitable amount, whereby both the nozzle  28  and the welding unit  22  are located in base position.  
         [0022]     A nozzle is commonly used when working with protective gas in order to obtain better welding results. For that purpose, the nozzle forms a space around the weld location to collect the protective gas. The nozzle  28  may at the same time serve as a support foot. With the aid of a support foot, a welding head is run into its base position.  
         [0023]     After the welding unit  22  has been brought into the base position GS, the lift device  20  transports it into the lift position HS. When the bolt  12  and the tube  14  have been incipiently fused sufficiently, the bolt  12  is transported in the welding direction, i.e., from the lift position HS towards the base position GS. However, the size configuration pictured here is not to scale. As a rule, the welding head  10  is much larger than the bolt  12  and the bolt holder  18 . With a device according to  FIG. 1 , therefore, automated welding in ‘forward direction’ at places of difficult access is possible, the welding axis SA lying offset parallel to the lift axis HA. However, this mode of welding is not possible when the segment  14   b  of part  14  shown in  FIG. 1  at bottom extends so far to the left in  FIG. 1  that working from ‘below’ with a welding head  10  is no longer possible.  
         [0024]     Referring to FIGS.  2  to  5 , the operation of joining two parts one to the other will be explained. In  FIG. 2 , a schematic joining device is generally designated  30 . The joining device  30  comprises a joining unit  22 , a joining head  10  and a lift device  20 . The joining device  30  is to be described in terms of an arc welding device. However, this does not represent any limitation to arc welding. Still other possible applications will be discussed towards the end of the description.  
         [0025]     The part in  FIG. 2  may for example be a trunk cover  32  of a motor vehicle, to which bolts are to be attached. Into the plane of the drawing of  FIG. 2 , the lengthwise direction of the trunk cover  32  extends. In  FIG. 3 , a schematic sectional side view may be noted.  
         [0026]     The bolt  12  is to be attached at the center of the lengthwise direction of the trunk cover  32 . This cannot be done with a welding head according to  FIG. 1 , because this is usable only for weld locations at the outside lateral edge of the cover  32 . Access from the side at the top in  FIG. 2  is not possible with a welding head according to  FIG. 1 , because this would have to pierce the trunk cover  32  in order to be able to weld ‘forward.’ Access from below does not make sense, since the bolt is not to be welded to the opposed side in the cover  32 .  
         [0027]     The welding head  10  of  FIG. 2  according to the invention comprises a lift means  20  transportable along a lift axis HA. To the lift means  20  a boom  16  is attached, to which a bolt holder  18  couples mechanically. A nozzle  28  is provided additionally. The welding head  10  may be mounted on a robot arm (not shown; cf.  FIG. 1 ). In order to be run into the welding position, the robot arm can transport the welding head  10  dynamically into a position in the neighborhood of the weld position. This situation is illustrated in  FIG. 2 . The welding head  10  is maximally extended relative to a carriage  26  to which it is connected, i.e. it is run maximally upward in  FIG. 2 . The bolt  12  projects beyond the wider aperture of the nozzle  28  and does not touch the trunk cover  32 .  
         [0028]     In  FIG. 3 , the welding head  10  has been run downward by means of the carriage  26 , so that the bolt  18  is seated in the desired weld position on the inside of the trunk cover  32 . By way of the bolt  12 , the welding unit  22  is run a small distance relative to the welding head  10 . We see that compared to  FIG. 2 , a longer portion of the welding unit  22  protrudes from the welding head  10  in the direction of the lift axis HA. The bolt  12  and hence also the welding unit  22  are in a base position in  FIG. 3 .  
         [0029]     To bring the bolt  12  into a lift position, the welding unit  22  is run out relative to the welding head  10  by means of the lift device  20 . The welding unit may but need not necessarily be fully extended. For that purpose, the lift device may, for example, comprise a linear motor (not shown) moving the welding unit  22  against the force of a spring (not shown). The lift position is illustrated in  FIG. 4 . The welding unit  22  has been run still farther out of the welding head  10  than in  FIG. 3 . The bolt  12  no longer touches the trunk cover  32 . However, the nozzle  28  is seated on the cover  32 . The welding current may now be switched on for welding.  
         [0030]      FIG. 5  shows the situation in which the bolt  12  has again been moved towards the trunk cover  32  by means of the welding unit  22 . Compared to  FIG. 4 , it is clearly seen that the portion of the welding unit  22  protruding from the welding head  10  is definitely shorter. Furthermore, in  FIG. 5 , the nozzle  28  of FIGS.  2  to  4  has been exchanged for a support foot  34 . However, this does not affect the course of the welding operation. The support foot  34 , like the nozzle  28 , merely fixes the position of the welding head  10  relative to the cover  32 .  
         [0031]     It is clear that the invention is not employable for arc welding only.  FIG. 6  schematically shows a spot welding device  40 . With the spot welding device  40 , two parts  42  and  44  can be welded together. The device  40  comprises a welding head  10 , a lift device  20  and a welding unit  22  transportable into a lift position out of the welding head  10 . Further, a counterelectrode  46  is provided, arranged opposed to the welding unit  22  in the neighborhood of a weld location. The transportability is represented by the dotted lines on the welding unit  22 .  
         [0032]     There are the further possibilities of welding with or without support foot, with or without nozzle, and with or without protective gas. Reversing the direction of welding leaves all functions and advantages of the welding head unchanged. However, the invention may also be employed in the fields of riveting, e.g. blind riveting, or punching.