Abstract:
An apparatus for processing workpieces by relative displacement of a plurality of simultaneously operating functional elements in relation to the workpieces being worked on is provided. The present apparatus includes a device for the mounting of the functional elements, characterized in that the device has apparatus for keeping the functional elements constantly in their operative position in relation to their workpiece section presently to be processed on the processing path by altering their position during a relative displacement with any desired change of direction. A method for tracing a processing path is also provided. The method includes tracing a processing path by a plurality of functional units arranged in succession one behind another in the path direction and moved along the path. The functional units are aligned on the path independently of each other by positioning means individually assigned to them.

Description:
Applicants hereby claim foreign priority benefits under 35 U.S.C. § 119 of PCT Patent Application No. PCT/CH01/00627 filed Oct. 22, 2001; Swiss Patent Application No. 2083/00 filed Oct. 24, 2000 and Swiss Patent Application No. 1932/01 filed Oct. 12.2001, the disclosures of which are herein incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The invention relates to an apparatus for processing workpieces by relative displacement of a plurality of simultaneously operating functional elements in relation to the workpieces which are being worked on, with a device to hold the functional elements. 
     2. Background Information 
     Such apparatuses are known in the field of tailored blanks or in the production of tubes for internal high-pressure forming; and they are also known in other fields. 
     “Tailored blanks” (Platinen) are metal sheets produced by welding different sheet-metal pieces together. The individual sheet-metal pieces are laid side by side and welded together; e.g., by roller seam welding with the adjacent sheets overlapping, or by laser welding with the sheets butted together. 
     It is also possible to form tubular parts from different sheet-metal pieces by closing the tube by subsequent welding and by then welding several such tubular pieces together to produce a tube made up of different sheet-metal pieces. 
     Tailored blanks or tubes of this type have applications; e.g., in the motor-vehicle industry. Tailored blanks for some years; tubes are making their debut. Tailored blanks are deep drawn and formed e.g. into beams for body framework, with defined dissimilar mechanical properties of the beam produced derived from the dissimilar materials or thicknesses of the individual sheet-metal pieces. Tubes are likewise formed into parts for body framework, by internal high-pressure forming (IHU), with the advantage that complicated shapes no longer have to be built up from individual parts, and costly tolerance problems upon assembly disappear. 
     It is evident from the foregoing that very exacting requirements have to be imposed on the weld seams, since these are already highly stressed by the forming process. It is also evident that no structure-weakening weld flaws possibly leading to defects in the finished product that could have serious consequences can be allowed to occur. 
     European Patent Publication No. EP 0770445 (hereinafter “EP 0770445” indicates a method and an apparatus for assuring and monitoring the quality of the welding process in the fabrication of inter alia tailored blanks (Reference is hereby expressly made to the total disclosure of EP 0770445). The said document discloses an apparatus for the fabrication of tailored blanks wherein the individual sheet-metal pieces are fixed on a carriage with the welding edges butted together and are led through and under a stationary welding station in a single straight pass. This relative displacement between the carriage (with its straight absolute motion through the apparatus) and the stationary welding station results in a linear processing path on the workpieces. 
     In principle, individual sheets of any size can be welded together to produce tailored blanks of any desired size. Nowadays, finished tailored blanks can attain dimensions of several meters. For instance, the side-panel of the Jeep Cherokee® is a tailored blank. Lengths of weld seam in the range of a few meters are possible in certain applications. 
     On the other hand, the effective laser focus has a diameter of 0.2 mm and has to be kept on the processing path with a precision commensurate with this diameter, otherwise weld defects will ensue. Although the mechanics of the apparatus do allow very precise guidance of the carriage through and under the welding station, tolerances are unavoidable. Moreover, further dimensional changes may arise in operation; e.g., due to heating of the laser beam path. Therefore in accordance with EP 0770445 a corrective adjustment is used for the line of the laser beam, the necessary measurements or correction data being obtained by image processing (corrective adjustment of a laser beam is known per se and is effected, e.g., by alterations to the tilt of the mirror in the lasers optical system). According to EP 0770445, the correction data are obtained by detecting the actual position of the edges before the welding point by image processing, and are used to control the lasers optical system. Deviations from the required edge-position of down to a few tenths of a millimeter can be compensated in this way. The weld seam produced then runs along the straight processing path required with a maximum deviation of those few tenths of a millimeter. 
     But this compensation will work only if the laser beam guide system produces the effective laser focus at the required spot, which is not always the case, owing to, e.g., heating of the beam guide as previously mentioned. 
     Therefore, another functional element after the welding point detects the actual position of the weld seam, again by image processing. If the weld seam is not at the spot to which the laser focus has been corrected, there is an unpermitted tolerance in the beam guide. Thus, a further correction signal for the laser optical system can be obtained. The result is a control maintaining the correct position of the beam on the straight processing path, thus assuring high weld quality. 
     Besides the quality assurance afforded by the three functions of (i) first sensor for image processing, (Y) laser optical system and (W) second sensor for image processing, further functions may be critical for the welding process, such as for example a material feed for the welding point (in the form of welding wire or metal powder) known in itself in welding technology. Further or other functions may be adopted as the need arises, such as for example when cutting rather than welding is taking place, as cutting may likewise involve precise guiding of the cutting element (such as a laser, water jet, etc.). Further functions may also arise when workpieces other than tailored blanks are worked with. 
     In addition to the trend which has been described towards ultra-high requirements in respect of weld quality, increasing use of tailored blanks is also resulting in a growing demand not just for straight welded joints but also for the ability to produce non-linear welded joints—and this for increasingly large production runs. Currently, the stringent requirements that have to be imposed on the welding process are achievable for linear weld seams, but not for seams that are curved or crooked. 
     It is true that it is known to mount tailored blanks requiring non-linear joints on a carriage and to propel the carriage through the apparatus in the longitudinal direction as the welding station travels to and fro on a transverse bridge; with suitable control of the two motions of carriage and welding station (each in a straight line), processing paths of any desired curvature can be traversed. Such apparatuses work with a single functional element—the focusing head for a laser beam. Hence they fail to realize a welding process of high quality, since other functions, e.g. as disclosed in EP 0770445, are absent. 
     Hence, the problem of the present invention is to provide an apparatus, which enables several functional elements to be used simultaneously on a curved processing path of an apparatus for processing workpieces. 
     DISCLOSURE OF THE INVENTION 
     The problem is solved by the present apparatus for processing workpieces by relative displacement of a plurality of simultaneously operating functional elements in relation to the workpieces being worked on. The present apparatus includes a device for the mounting of the functional elements characterized in that the device has means for keeping the functional elements constantly in their operative position in relation to their workpiece section presently to be processed on the processing path by altering their position during a relative displacement with any desired change of direction. 
     The problem is also solved by the present method for tracing a processing path by a plurality of functional units arranged in succession one behind another in the path direction and moved along the path, wherein the functional units are aligned on the path independently of each other by positioning means individually assigned to them. 
     By modifying the position of functional elements, it is possible on the one hand, for elements acting in succession one after the other to be individually kept in their standard position (e.g., vertically over the workpiece surface) on curved sections of the processing path at all times, independently of the absolute motions in the apparatus; but on the other hand, for individual elements also to be deflected from their standard position and thus for radii of curvature to be executed which could not be executed in the standard position because of the dimensions of the elements themselves or, owing to the high accelerations involved, could only be executed at high cost. 
     For the purposes of the invention it does not matter how the relative motion between the workpieces and the functional elements is produced, whether by functional elements that are fixed in position in combination with a carriage for the workpieces that can be moved in all directions, or vice versa; or by a hybrid solution such as transverse motion of functional elements with respect to the longitudinal motion of the carriage. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The invention will now be described in detail with reference to the figures, in which: 
         FIGS. 1   a  to  1   c  show schematically three functional elements, in each case on a curved processing path with position modified accordingly. 
         FIGS. 2   a  to  2   c  show three functional elements on the processing paths corresponding to  FIGS. 1   a  to  1   c , shown with regard to the mutual change in position. 
         FIG. 3  shows a cross-section through one embodiment of a device for positioning the functional elements. 
         FIG. 4   a  is a view from above of a further embodiment. 
         FIG. 4   b  shows a cross-section through the embodiment of  FIG. 4   a.    
         FIG. 5   a  is a view of a further embodiment. 
         FIG. 5   b  shows a section through part of the embodiment of  FIG. 5   a.    
         FIG. 5   c  is a side view of the embodiment of  FIG. 5   a.    
         FIG. 6  shows schematically the suspension of an orientatable functional element. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1   a  shows two workpieces  10  and  11 , which have been butted together for processing (in this case, welding). The juxtaposed edges of the workpieces form a processing path  4 . Three functional units  1 ,  2  and  3  are arranged in a device  13  for holding the functional units, and are located above the processing path  4  in the operative position to weld the workpieces  10  and  11 ; e.g., in the manner proposed in EP 0770445. This is performed by a laser beam whose position is controlled with the aid of two sensors for image processing. An energy beam of a different type, such as an electron beam for example, could also be used. The functional unit  1  (a head with a laser optical system) is located between a precursor functional unit  2  and a follower functional unit  3  (both sensors for image processing). 
     It is evident from the figure that the functional units have modified their position in relation to a straight processing path: 
     The relative position of the three functional units  1 ,  2  and  3  alters continually in accordance with the changing curvature of the processing path  12 . If only two functional units are operative, they still alter their position in relation to the workpieces  10 ,  11  (and/or to the device  13  as the case may be) even though the distance between them remains constant. 
       FIG. 1   b  shows a circular processing path  4 . It can be seen from the figure that with the functional units  1 ,  2  and  3  arranged over the processing path the smallest possible distance between two functional units (here functional units  2  and  3 ) determines the minimum radius of curvature. Were the circle formed by the processing path  12  smaller than the distance between the units  2  and  3 , one of the units could no longer be kept on the processing path  12 . 
       FIG. 1   c  shows a processing path  4  through a rounded right-angled corner with a very small radius of curvature. If the precursor functional unit  2  deviates sideways from the processing path  4  and stays on the track  5  drawn as a broken line, a simpler movement cycle results. First of all, large accelerations of the unit  2  are prevented, which is in keeping with the need to keep the rate of deposition as uniform as possible, and hence to keep the rate of travel of the functional unit  1  in relation to the processing path  4  as uniform as possible. Then, there is a larger radius of curvature of the track to be followed by the individual functional units, which can be a decisive factor in the case of a processing path  4  with a complex course. The track curve  5  in the figure presupposes that the position of the functional unit  2  can be modified so that it is able to operate on the processing path  4 , and thus stay in the operative position, even when laterally offset, as indicated by the arrow  6 . This contrasts with the arrangement of  FIG. 1   a  where the functional units  1  to  3  are shown in an arrangement directly over the processing path  4 . 
       FIG. 2   a  shows the processing path  4  of two workpieces  10  and  11  and the functional units  1 ,  2  and  3 , the device  13  being omitted so as not to clutter the drawing. A reference point  14  marked with a cross does however denote the position of the device  13  in relation to the workpieces. In this instance this point coincides with the position of the precursor functional unit  1 . As indicated in  FIG. 2   a , the position of the functional units  2  and  3  are defined from the reference point  14 : firstly by the shorter vector  7  (distance between units  1  and  2 ), secondly by the longer vector  8  (distance between units  1  and  3 ). 
       FIG. 2   b  shows the arrangement of  FIG. 2   a , with the position of the functional unit  3  in relation to the functional unit  1  (and the reference point  14 ) defined by the position of the functional unit  2 . The sum of the vectors  7  and  9  gives the mutual position of the functional units  1  and  3 . 
       FIG. 2   c  again shows the arrangement of  FIG. 2   a , with the functional unit  1  (and the reference point  14 ) lying between the functional units  2  and  3  whose position is thus defined by the vectors  7  and  9  (distance between units  2  and  3 ). 
     Although all variants in  FIGS. 2   a  to  2   c  are feasible depending on the purpose for which the apparatus is used, the arrangement of  FIG. 2   c  is especially appropriate. If the reference point  14  (which preferably, though not necessarily, coincides with one of the functional units  1  to  3 ) is the basis for computation by the control unit of the respective position of the functional units  1  to  3 , or for the mechanical traversing of the track curve  4  by holders arranged movably in the vectors  6  to  9 , rounding errors, tolerances etc. are inevitable. The effects of angular errors grow larger as the distance between the reference point  14  and the respective functional unit  1 ,  2  or  3  increases. For this reason, the arrangement of  FIG. 2   c  with the short vectors shown is preferable to the arrangement of  FIG. 2   a . The arrangement of  FIG. 2   c  is also preferable to that of  FIG. 2   b , as in the latter the errors between the reference point  14  and the functional unit  3  are cumulative. 
       FIG. 3  shows a cross-section through an embodiment of the device  13  according to the invention with the functional units  1 ,  2  and  3 . The figure also shows schematically a processing path  4 / 4 ′. The reference point  14  defines the position of the device  13  in relation to the workpieces  10  and  11 , enabling the overriding control of the apparatus to make certain of the predetermined relative displacement by modifying the direction and/or rate of displacement between the workpieces  10 ,  11  and the device  13 . 
     The functional unit  1  is configured as a sensor for image processing, and detects e.g., the position of the edges of the juxtaposed workpieces  10 ,  11  in a defined section, now about to be processed, of the path  4 . The functional unit  2  is configured as a focusing head for an energy beam, e.g., the beam of a jag laser, and functions as a welding unit to weld the workpieces  10 ,  11  together. The functional unit  3  in turn is configured as a sensor for image processing, and detects the position and/or quality of the weld seam. EP 0770445 shows a possible way in which these functional units can work together. 
     A baseplate  12  forms the base for the arrangement of the functional units  1 ,  2  and  3  in the device  13 , and is preferably arranged displaceably on a bridge  15  (see  FIG. 4   a ). This sets up the arrangement previously described in the apparatus configured in accordance with the invention in which workpieces  10 ,  11  fixed to a carriage advanced longitudinally and so pass under a device  13  held displaceably in the transverse direction on the bridge  15 . However, the invention is not limited to such an arrangement and can be used wherever any kind of relative displacement is created between the workpieces  10 ,  11  and the device  13 . 
     A holder  16  whose orientation can be varied is arranged on the base  12 . It is configured as a body of revolution and has an axis of symmetry  16 ′. It is mounted rotatably on the base  12  by the schematically indicated ball bearing  17 . It has a central opening  18  with an insert  19  enabling the functional element  1 , which is fixed in position with respect to the base  12 , to retain its orientation independently of any rotation of the holder  16 . The axis of symmetry  16 ′ of the holder  16  coincides with the operative line  1 ′ of the functional element  1 . The position of the operative line  1 ′ on the processing path constitutes the reference point  14 . The position of the base  13  in relation to the workpieces  10 ,  11  is thus defined, as is the position of the functional element  1  in relation to the workpieces  10 ,  11 . 
     A motor  20  attached to the base  12  acts via a belt drive  21  on a belt pulley  22  fixedly arranged on the holder  16  so that the holder  16  can be turned in relation to the base  12 , thus altering the position of the functional element  2  mounted in the holder  16 . 
     Also arranged on the holder  16  is a ring structure  25 , which in turn serves as holder for the functional element  3 . The ring structure  25  and holder  16  are connected to each other by a schematically shown ball bearing  26 , with the result that the ring  25  can be rotated in relation to the holder  15  (and also, of course, in relation to the base  14 ). The drive for the ring  25  is provided by a pinion  27  acting on internal toothing  28  on the inner circumference of the ring  25 . The pinion  27  is mounted on a shaft  24  which extends through an opening  29  in the holder  16 . Shown schematically is a drive  30  mounted on the holder  16  for the shaft  24 , by means of which the relative position of the ring  25  in relation to the holder  16  can be altered. 
     The mounting of the ring  25  on the holder  16  and, in turn, the mounting of the latter on the base  12  are in effect a cascade arrangement, the ring  25  being mounted on the base  12  indirectly through the holder  16 . 
     The processing path  4  and the reference point  14  are indicated in the lower part of the figure. The configuration of the functional units on the processing path  4  corresponds to that of  FIG. 2   a . However, the configuration of  FIG. 2   c  can be created simply rotating the holder  12 . In the figure, this would give the arrangement running on the processing path  4 ′ drawn as a broken line. 
     The functional element  1  for its part can be mounted rotatably about its own axis on the base  12 , for example where for image processing purposes the same defined orientation in relation to the viewed section of the processing path  4  must be maintained at all times. This also applies to the mounting of the functional element  2  on the holder  16 , which may be fixed or rotatable. A fixed arrangement is possible if a round laser focus is used for welding the workpieces  10 ,  11 . For an oval focus, a constant, defined orientation of the focus relative to the section of processing path is necessary, hence the functional element  2  will need to be rotatable as indicated by the arrow  31 . The corresponding drive has been omitted to avoid unduly encumbering the drawing. What has just been stated in relation to the mounting of the functional elements also applies to the functional element  3 . Here again, the corresponding drive (or fixed mounting as the case may be) has been omitted from the figure for the sake of simplicity. 
     The mounting and rotational drive of the functional elements can be of conventional design and construction and will therefore not be described in detail here. 
     The pinion  27  co-operating with internal toothing  28  can of course also be arranged on an outer circumference  32  of the ring  25 . 
       FIG. 4   a  shows a device  13  for mounting functional elements  1 ,  2  and  3 , arranged on a bridge  15 . This means that the device  18  is held slidably in the transverse direction with respect to the longitudinal feed of the workpieces  10  and  111  indicated by the doubleheaded arrow  35 . A disc-shaped holder  40  is mounted rotatably on the baseplate  12  of the device  13 . A motor  42  mounted on the base  12  imparts the required rotation via a belt drive  41 . At the center of the holder  40  in turn, and likewise rotatable about its own axis, the functional element  1  is mounted and is thus located in a defined and spatially fixed position in relation to the base  12 . 
     Rotation of the functional element  1  is by means of a shaft  43  driven by a motor  44 . The motor  44  is mounted on the base  12 , but the corresponding arrangement has been omitted from the drawing in the interest of clarity. 
     The functional element  2  is also mounted on the holder  40 , but not centrally, and so changes its position in relation to the base  12  as the holder  40  rotates. A secondary holder  45  follows the rotation of the holder  40  and carries a drive  46  which in turn, a schematically shown belt drive  47 , causes the element  2  to rotate about its own axis. 
     A further holder  50 , which mounts the functional element  3 , is arranged on the mounting of the functional element  2 . A schematically indicated drive  51  acts via a pinion  52  on a toothed quadrant  53  to rotate the functional element  3  about its own axis. A glass fiber-cable  39  for a jag laser arranged in the element  2  is merely suggested schematically. 
     The holder  40  is configured as a body of revolution or ring. Its axis of symmetry  48  coincides with the operative line  49  of the functional unit  1 . Axis of symmetry  48  and operative line  49  pass through the workpieces at the location of the reference point  14 . 
     The device  13  illustrated in  FIG. 4   a  has the configuration of  FIG. 2   b.    
       FIG. 4   b  shows part of the arrangement of  FIG. 4   a  in cross-section. 
     The holder  40  is mounted on the base  12  via a schematically shown ball bearing  55 . The functional unit  1  is mounted by a bearing element  56  at the centre of the holder  40  so that it does not itself participate in the latter&#39;s rotation but is only altered in its position in relation to the base  12  by the drive  44 . The sweeping holder  50  is mounted in the holder  40  and undergoes a change in position when the latter rotates. This change in position is also imparted to the functional element  2 . The functional element  2  can be rotation about its own axis by the drive  46 ,  47  shown in  FIG. 4   a . The functional element  3  is mounted on the sweeping holder  50 . The drive  52 ,  53 , shown in  FIG. 4   a , for rotating the functional element  3  has been omitted to clarify the figure. 
       FIG. 5   a  shows a further embodiment of a device  13  for mounting the functional elements  1 ,  2  and  3 . This shows a view from above of the device  13  and the workpieces  10 ,  11  and processing path  4  underneath. The contour of the base  12  is schematically suggested by the broken outline, as is that of the bridge  15 . In the figure, a ring-shaped holder  60  conceals similarly configured holders  61  and  62  arranged beneath it. Thus the holders  60 ,  61  and  62  are located over the base  12 , vertically one above the other, and their axes of symmetry coincide and are perpendicular to the base  12 . The holder  60  is mounted on the base via guide rollers  63 ,  63 ′and  63 ″ (see  FIG. 5   b ). The holders  61  and  62  are mounted via guide rollers  64 ,  64 ′ and  64 ″ in the same way as the holder  60  is mounted, and as shown in  FIG. 5   b . The rings  60 ,  61  and  62  are thus arranged rotatably about their axis of symmetry on the base  12 . Belt drives  66 ,  67  and  68  each act upon one of the holders  60 ,  61  and  62 . Pinions  69 ,  70  and  71  of the drives  66 ,  67  and  68  act on external toothing  72 ,  73  and  74  on the corresponding ring  60 ,  61  and  62 . The diameter of the pinion  69 ,  70 ,  71  is preferably made small in proportion to the diameter of the rings  60 ,  61  and  62 . With this gear reduction, the rotation of the rings can be very precisely controlled, which can be crucial for proper operation of the functional elements. 
     The ring  60  has a segment  75 , which supports the functional unit  2 . The ring  61  has a segment  76  for mounting the functional unit  2 , and the ring  62  has a segment  77  for mounting the functional unit  3 . The operative line of the functional unit  1  passes through the processing path  4  at the reference point  14 , which has been omitted to alleviate the figure. 
     From the configuration shown in  FIG. 5   a  it can be seen that the functional elements  1 ,  2 , and  3  are arranged in the region inside the rings  60 ,  61  and  62 , which means that the distances between them are unaffected by the design of the rings  60 ,  61  and  62  (apart from their diameter, which can easily be increased if need be). The ring structure also means that guide rollers and drives for the rotation of the rings can be arranged in the outer region with the result that the whole of the inner space is available for the functional elements. 
     Consequently the structure shown is universally usable for simple and complex processing paths, and for a single functional unit or a larger number of functional units. The minimum distance between the functional units is not governed by the holder structure but solely by the construction of the elements themselves. 
     Data exchange and power supply to the functional units can be effected via cable or slip rings. Slip rings may be arranged, e.g., on an outer circumference, and have the advantage that the rings can be rotated as desired without having to take cable twisting into account. Wireless data-transmission is of course also feasible. 
     The configuration of the device  13  shown in  FIG. 5   a  corresponds to the arrangement of  FIG. 2   c.    
       FIG. 5   b  shows a cross-section through the ring  60 , the guide rollers  63 ,  63 ′ (shown opposite one another in the figure) and the base  12 . The guide rollers run on the outer circumference of the holder  60 , thus supporting it radially. At the same time, axial support is given through the beveled flanks by the matched contact faces of the rollers  63 ,  63 ′, with a form-fitting connection. The other rings  61  and  62  are omitted to simplify the figure, as are the drives  66 ,  67  and  68 . Also shown schematically are the functional unit  2  with its operative line  2 ′, and the axis of symmetry  78  of the ring  60 . The workpieces  10 ,  11  also appear. The guide rollers  63  are freely rotatably arranged on the schematically shown guide roller holders  64 ,  64 ′. These holders  64 ,  64 ′ are mounted in turn on the base  12 . With this configuration, any number of rings can be mounted one above the other on the base  14  by means of the same mounting  64 , by arranging a number of guide rollers  63 ,  64  or  65 . 
     Alternatively, the guide rollers may have toothing co-operating with the external toothing of the rings  60  to  63 , with the advantage that separate drives  66  to  68  are no longer necessary. 
       FIG. 5   c  is a side view of the arrangement shown in  FIGS. 5   a  and  5   b . Parts are referred to by the same numbers as before. Bridge  15  and base  12  are omitted, to relieve the drawing. 
     Further embodiments can be configured as follows: 
     For example, the arrangement as shown in  FIG. 3  with ring  25  omitted so that only two functional elements can be used. The element  2  is arranged as in  FIG. 3  without modification, whereas the element  1  is not placed centrally in the holder  16 , but offset, like the element  2 . When a change in position occurs, the relative situation of the elements  1 ,  2  is not altered, but their position is. Thus, again, any desired processing path can be traversed as relative movement occurs between the workpieces  10 ,  11  and the device  13 . 
     For example the arrangement as shown in  FIG. 4   a , a number of holders  50  being provided, with these holders linked at a common point and preferably having a common sweep axis. 
     For example, the arrangement as shown in  FIG. 4   a , with the element  1  arranged on its shaft  43  with provision for vertical adjustment and/or for tilting.  FIG. 6  shows schematically the element  1  in the configuration of  FIG. 4   a , albeit not directly connected to the shaft  43  but arranged on an auxiliary holder  80  so as to be tiltable at a tilting point  83  (see double-headed arrow  85 ). Tilting drive is provided by a motor  81  via a pushrod  82 . In addition to the shaft  43 , a threaded spindle  43 ′ is provided which runs in a mating part  43 ″ of the auxiliary holder  80  and whose rotation causes a vertical shift of the auxiliary holder  80  as indicated by the double-headed arrow  84 . Rotation of the auxiliary holder  80  can be effected, as before, by the motor  44 , here indicated in broken lines. 
     Although this invention has been shown and described with respect to the detailed embodiments thereof, it will be understood by those of skill in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed in the above detailed description but that the invention will include all embodiments falling within the scope of the appended claims.