Abstract:
A flanging or hemming device ( 1 ) is provided with an external flanging unit ( 6 ) and/or an internal flanging unit ( 7 ) as well as a clamping device ( 3 ). The clamping device ( 3 ) has one or more leading pressing and clamping elements ( 71 ) which are movable or deformable and can be adjusted in accordance with the place or time and/or force with which they act on the workpiece or work-pieces ( 9 ). The pressing and clamping elements ( 71 ) are notably elastically deformable. They can also comprise a spring ( 72 ) or be fluidically pre-stressed.

Description:
FIELD OF THE INVENTION 
     The present invention pertains to a hemming apparatus with an outer hemming device and/or an inner hemming device as well as a clamping device with pressing and clamping elements 
     BACKGROUND OF THE INVENTION 
     Such a hemming apparatus has been known from DE-A 43 30 683. It has a hemming bed that can be raised and lowered and two hemming units, each of which has two different hemming tools for prehemming and finishing hemming. The hemming drive is arranged on the hemming bed, which performs the hemming stroke by a lifting movement. The hemming units can be pivoted only onto the hemming bed, and the prehemming and finishing hemming tools are brought into the working position by a vertical adjustment of the hemming bed. The workpiece is clamped on the hemming tools, which grasp the workpiece together and simultaneously, via spring-loaded holding-down devices. 
     SUMMARY AND OBJECTS OF THE INVENTION 
     The object of the present invention is to provide a better hemming apparatus. 
     According to the invention, a hemming apparatus is provided with an outer hemming device and/or an inner hemming device as well as a clamping apparatus with pressing and clamping elements. The clamping apparatus has one or more leading pressing and clamping elements. 
     The hemming apparatus according to the present invention can operate with different clamping apparatus. The use of leading pressing and clamping elements, which make possible a predeterminable clamping sequence of any freely selectable points of the workpiece or workpieces, is particularly advantageous. These pressing and clamping elements are mounted in such a way that they are deformable or can yield, so that their leading function can be eliminated at the end of the clamping movement so that a simultaneous clamping of the workpiece or workpieces takes place at all intended points. 
     The hemming apparatus according to the present invention require little design effort and have freely settable and adjustable kinematics. Retooling operations and tool change can be performed rapidly and simply. The hemming unit can be controlled very easily and accurately. The drive is uncomplicated. The entire hemming unit is insensitive to disturbances and also has a high reliability in operation due to the small number of moving parts. This also leads to advantages for simple and inexpensive maintenance. 
     The basic components of the hemming units are standardized and independent from the shape of the workpiece. They can therefore be manufactured in advance and can be reused in the case of a tool change. 
     The hemming apparatus according to the present invention makes it possible, in particular, to change the hemming tools independently from the hemming stroke. It is particularly advantageous in this connection that the hemming stroke can be very short and can be carried out with a high force. In particular, the hemming head according to the present invention does not require an additional height and feed height in the hemming area. This is especially true in the case of the preferred design as a rotating roll. If a plurality of hemming units are used, their hemming tools can be changed independently from one another. 
     The entire hemming unit is compact and can be placed especially favorably and simply on the hemming apparatus as a result. In addition, the small overall height offers advantages concerning the other components. For example, the rotating roll makes possible tool change under the clamping apparatus, which can remain as a result in position at the workpiece and holds same during the tool change. The small overall height makes it possible, in addition, to arrange the hemming unit obliquely in order to thus optionally achieve a better direction of the hemming force at right angles to the surface of the workpiece. If the hemming unit according to the present invention is arranged obliquely on the outside of the workpiece, the present invention has the advantage that the interference distance from a possible inner hemming device is greater because of the small overall height of the hemming unit. 
     The design of the hemming unit according to the present invention also makes it possible to use hemming tools of any desired shape and size. A longer or multipart hemming head, which is arranged on two or more hemming units and is supported via intermediate supports in a suitable manner, may be optionally used for this purpose. 
     The hemming head may carry a plurality of hemming tools. In the preferred embodiment, it carries two hemming tools for prehemming and finishing hemming. The design of the hemming head as a rotating roll has the advantage that an especially simple kinematics with reliable guiding and setting is guaranteed for adjusting the tool. In particular, the hemming head can be set and positioned completely independently from the hemming drive, which generates the hemming stroke. The pivoting drive for the hemming head preferably has a crank drive. This makes it possible to make do with weaker driving forces for adjusting the hemming head and to nevertheless generate strong, positive-locking holding forces in the working position in order to thus transmit correspondingly strong hemming forces. 
     The hemming tools may have one or more centering inserts, which have a stop and centering function for the workpiece or workpieces. These centering stops are also advantageous for holding and positioning the workpiece during tool change and during the turning of the hemming head. Special advantages arise when the hemming apparatus is designed as a combination device with an outer and inner hemming device. 
     The centering inserts facilitate the staggered feed in time of the hemming unit. The final shape of the workpiece can be influenced as a result. In addition, errors due to warping or dimensional errors can be compensated. 
     The hemming unit may have one or more movable and driven tensioners or clamps, which can be actuated independently from the hemming tools. It is advantageous in this connection for the clamps to be arranged behind the hemming heads and to extend over them. As a result, the hemming heads can be rotated under the clamps for tool change, while the workpiece or workpieces can remain clamped during a tool change staggered in time. The clamps associated with the hemming unit or hemming units may replace an internal clamping device, e.g., a vertical or pivotable clamping frame. They may alternatively also complement same. 
     The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a side view of a hemming apparatus with outer and inner hemming device and a pivotable clamping frame; 
     FIG. 2 is a front view of the hemming apparatus according to arrow II in FIG. 1; 
     FIG. 3 is a top view of the hemming apparatus according to arrow III in FIG. 1; 
     FIG. 4 is a side view of a hemming unit with a hemming head in an operating position; 
     FIG. 5 is a side view of a hemming unit with a hemming head in another operating position; 
     FIG. 6 is a side view of a hemming unit with a hemming head in another operating position; 
     FIG. 7 is an enlarged side view of the hemming unit according to FIGS. 4 through 6; 
     FIG. 8 is a variant of the hemming drive for a hemming unit according to FIGS. 4 through 7 in three operating positions; 
     FIG. 9 is a side view of a hemming apparatus with individual clamps on the hemming units; 
     FIG. 10 is a variant to FIG. 1 with individual clamps on the clamping frame; 
     FIG. 11 is a perspective view of the hemming heads and hemming tools on the hemming bed; 
     FIG. 12 is a top view of the arrangement according to FIG. 1; 
     FIG. 13 is an enlarged side view of the hemming tools with integrated centering inserts for the workpieces; 
     FIG. 14 is a view showing a variant of the centering device according to FIG. 13; 
     FIG. 15 is a view showing a variant of the centering device according to FIG. 13; 
     FIG. 16 is an enlarged side view of the centering insert according to FIG. 14; 
     FIG. 17 is an enlarged front view of the centering insert according to FIG. 14 according to arrow XVII; 
     FIG. 18 is a section through the centering insert according to FIG. 17 along section line XVIII—XVIII; 
     FIG. 19 is an enlarged side view of a clamping contour with a holding-down device and a pressing and clamping element; 
     FIG. 20 is a view showing a design variant of a leading pressing and clamping element according to FIG. 19; and 
     FIG. 21 is a view showing a design variant of a leading pressing and clamping element according to FIG.  19 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 through 3 show general views of a hemming apparatus  1 . This comprises a machine frame  2 , on which a plurality of outer hemming devices  6  and one or more inner hemming devices  7  are arranged. In addition, a pivotable clamping apparatus  3  with a clamping frame  69  is arranged on the machine frame  2 . Moreover, a lifter  8  is located on the machine frame  2 . 
     The hemming apparatus  1  is intended for processing one or more workpieces  9 , in which prepared hemming flanges  63  see FIG. 16 are first prehemmed and then subjected to finishing hemming. The workpieces  9  may be any parts. In the preferred embodiment, the hemming apparatus  1  is used to process vehicle body parts, especially doors. The workpieces  9  may have one or more openings  10 . These openings may be, e.g., window cutouts or the like. 
     Prehemming and finishing hemming of the workpieces  9  can be carried out with the hemming apparatus  1  within one station. In addition, the hemming apparatus  1  makes it possible to carry out inner and outer hemming in the same station. The inner and outer hemming may happen simultaneously or staggered in time. The inner hemming takes place in the area of the openings  10 , especially window cutouts. 
     The workpieces  9  are fed in in the correct position by apparatus of a suitable conveying apparatus. In the preferred embodiment, the feed is performed by apparatus of a multiaxial industrial robot, which also removes the workpieces  9  after the hemming. 
     The machine frame  2  has a hemming bed  5  with suitable supports for receiving one or more workpieces  9 . The hemming bed  5  may also have one or more positioning or centering pins for positioning the workpiece or workpieces  9  in the correct position. The workpieces  9  are pressed onto the hemming bed  5  by the clamping frame  69 , which will be described in greater detail below, and held in the desired position. The clamping frame  69  may have on its underside a clamping contour  70 , whose shape is adapted to the workpieces  9 , and optionally one or more pressing and clamping elements  71 . One or more centering pins  41  or other positioning aids may also be present on the clamping apparatus  3 . 
     The lifter  8  is arranged obliquely and essentially in parallel to the inner hemming device  7 , which will be described in greater detail below. It is located in the inner area and essentially under the machine frame  2  and the hemming bed  5 . It is used to receive and position the workpieces  9 . Due to the pivotable clamping frame  69 , a laterally and upwardly wide open access opening  44  is obtained in the open position of the clamping frame, and the feed apparatus or the above-mentioned industrial robot feeds and removes the workpieces  9  through the said access opening. 
     For feeding the workpieces  9 , the lifter  8  is in an extended position (not shown). The lifter  8  has a plurality of suitable mounts for receiving and supporting the workpieces  9 . Centering pins  42  or other positioning elements, which may be designed, e.g., as an accurate centering apparatus for the workpieces  9  fed in on the lifter  8 , may be arranged on the mounts. 
     The feed apparatus or the industrial robot places the workpiece or workpieces  9  onto the extended lifter  8  in the correct and correctly centered position. The lifter  8  then moves back and positions the workpiece or workpieces  9  on the hemming bed  5 . The lifter  8  may form with its mounts a part of the hemming bed  5  and preferably remains connected to the workpiece or workpieces during hemming. The workpieces  9  are thus held in the correct position in the hemming apparatus  1  and can be processed in the manner to be described below. After hemming, the lifter  8  again extends obliquely upward and makes the workpieces  9  available to the feed apparatus for being removed. 
     The inner hemming device  7  has hemming tools  16 ,  17 , which can be extended and withdrawn, so that the workpieces  9  with the opening  10  can be placed on the hemming bed  5  past the inner hemming device  7 . The outer hemming device  6  can be returned into a resting position and makes free the access to the lifter  8  and the workpieces  9  for feeding and removal. 
     The outer hemming device  6  and the inner hemming device  7  can be positioned and aligned independently from the workpieces  9  independently from one another and actuated separately. They have for this purpose a plurality of hemming units  11  that can be actuated separately with hemming drives  14  of their own. The inner hemming device  7  has hydraulic drives for withdrawing and extending the hemming tools  16 ,  17  and for the hemming stroke. It is preferably designed corresponding to DE-U 296 12 192. 
     The outer hemming device  6  is shown in greater detail in FIGS. 4 through 7. It comprises one or more hemming units  11 , which are mounted preferably hanging on the machine frame  2  or the hemming bed  5  and together with their hemming tools  16 ,  17  that surround the outer contour of the workpieces  9  to be hemmed. The hemming units  11  have a hemming head  12  and a hemming drive  14  each. The hemming head  12  carries the hemming tools  16 ,  17 . These are, e.g., hemming tools for prehemming and finishing hemming. 
     The hemming tools  16 ,  17  usually have the same length as the hemming heads  12 . In a variant, it is also possible for the lengths to be different and for divisions to be formed by, e.g., one hemming head  12  having excessive length and carrying two or more shorter prehemming and finishing hemming tools  16 ,  17  arranged axially one behind the other. This arrangement is shown in FIG.  11 . 
     The hemming tools  16 ,  17  are fastened replaceably on prepared recessed mounts via suitable inserts. The mounts are dimensioned such that they can grasp inserts and tools of different sizes. The prehemming and finishing hemming tools  16 ,  17  are now located in different locations of the hemming head  12 . 
     The hemming head  12  is independently adjustable in relation to the hemming drive  14  to be described below. It brings the hemming heads  16 ,  17  into the particular working position by a movement of the head. FIG. 4 shows the hemming head  12  in the prehemming position. FIG. 5 shows the finishing hemming position and FIG. 6 shows the resting position, in which the entire hemming unit  11  is pivoted away from the machine frame  2 . 
     The hemming head  12  may be movable in different ways. In the preferred embodiment, the hemming head  12  is designed as a rotating roll, which is mounted pivotably around an essentially horizontal axis on end bearings  23 . 
     The hemming head  12  or the rotating roll may also have a greater length and be divided into a plurality of hemming head sections  13 , which are driven together. The hemming head sections  13  are now mounted and supported rotatably on the inner contact point or contact points via intermediate bearings  22 . This arrangement is shown in FIGS. 11 and 12. 
     The hemming head  12  or the individual hemming head section  13  is moved by a pivoting drive  18 . The prehemming and finishing hemming tools  16 ,  17  are brought into the working position by the rotary movement. They are arranged diagonally close to one another on the hemming head  12  and form an angle of preferably about 90° to 120° with one another. The hemming head  12  now performs an alternating rotary movement through a corresponding angle of rotation. 
     The pivoting drive  18  comprises a suitable drive member, e.g., a pneumatic cylinder, and a suitable movement transmission apparatus, e.g., a crank drive  19 . The crank drive  19  is connected via one arm to the piston rod of the cylinder, which is arranged essentially vertically. The other crank arm is connected to the hemming head  12  via a crank  20 . The crank arm now acts on a sliding block, which is held movably to and fro in a suitable crank guide on the hemming head  12 . 
     The angular position of the crank arms is between about 90° and 140° and is preferably selected to be such that the crank arm leading to the hemming head  12  is essentially at right angles to the crank guide  20  in the prehemming position shown in FIG.  4  and it applies the necessary holding forces for the hemming stroke with the cylinder withdrawn. In the finishing hemming position shown in FIG. 5 with the cylinder extended, the crank arm leading to the hemming head  12  with its mount for the crank  20  is between the finishing hemming tool  17  and the pivot axis  15 . The crank arm assumes a relatively flat and slightly obliquely upwardly directed position. It is now again essentially at right angles to the crank guide  20 . 
     The end positions of the crank drive  19 , which are also the working positions of the hemming tools  16 ,  17  at the same time, are limited and defined by apparatus of pivot stops  21 . The cylinder  18  is only withdrawn and extended. Adjustable stop screws, which cooperate with relatively stationary stop heads, are located on the crank arms. In addition, hemming stroke stops  37  may be present on the crank head from which the two crank arms originate. These are arranged offset in different planes extending at right angles to the bearing axis of the crank drive  19  and around the pivot angle of the crank drive  19  on the circumference. In the two working positions shown in FIGS. 4 and 5, one of the two hemming stroke stops  37  points vertically downward and cooperates with respective, relatively stationary, adjustable stops. The prehemming and finishing hemming strokes can be set independently from one another and limited in the downward direction by apparatus of stop screws  65  or the like. 
     The hemming head  12  is fastened and mounted on a bracket  25  together with the pivoting drive  18 . The bracket  25  is in turn mounted longitudinally movably on a swivel arm  24  via a bracket guide  26 . The hemming stroke is carried out by the movement of the bracket  25  along the swivel arm  24 . The bracket  25  forms the hemming drive  14  together with the swivel arm  24  and a hemming cylinder  30 . The hemming drive  14  is separate from the pivoting drive  18  of the hemming head  12 . 
     FIGS. 4 through 7 show one embodiment of the hemming drive  14 . FIG. 8 shows a variant to this. The hemming drive  14  generates both the feed motion and the hemming movement, doing so independently from one another and in a time sequence that can be set separately or in a separate time sequence. The hemming tools  16 ,  17  are brought with the feed for this purpose into a defined and statically determined operation, from which the hemming stroke is performed. To do so, all hemming units  11  are brought consecutively or simultaneously into the starting or working position, in which they can wait for one another, by apparatus of a corresponding displacement or time monitoring or control. From this position, they can perform the hemming stroke in any desired time sequence, especially together and simultaneously. 
     In both cases shown in FIGS. 4 through 7 and in FIG. 8, the swivel arm  24  is mounted rotatably on the machine frame  2  or a frame attachment via a bearing  29 . The connection point between the bearing  29  and the machine frame  2  can be adjusted multiaxially and fixed. It is preferably used for the fine adjustment of the hemming unit  11  in relation to the hemming bed  5  and the workpiece  9 . In the working positions shown in FIGS. 4 and 5, the swivel arm  24  is engaged. In the resting position shown in FIG. 6, it is pivoted off to the rear. The arrangement may be such that the hemming stroke is carried out according to FIGS. 4 and 5 with an essentially vertical movement component. 
     FIG. 1 shows a variant to this, in which the hemming units  11  are in an oblique position in the working position and the hemming stroke is performed in a correspondingly oblique direction of movement. The hemming stroke is preferably directed obliquely to the outside. Such an arrangement has advantages because the hemming tools  16 ,  17  come into contact with the opposing surface of the hemming bed  5  in a direction that is as close to the vertical as possible, because of the space requirement and because of the absence of interference in relation to the inner hemming device  7 . 
     A spring  27 , whose travel can preferably be limited by apparatus of a setting device  28 , is located between the bracket  25  and the swivel arm  24 . In addition, the spring  27  is pretensioned. The spring  27  is preferably designed as a compression spring and acts against the hemming cylinder  30 . The spring  27  is directed in parallel to the carriage-like bracket guide  26 . The setting device  28  comprises, e.g., an adjusting screw with clamping. 
     The upper position and the starting hemming position of the hemming tools  16 ,  17  and of the hemming head  12  are set by apparatus of the setting device  28 . The above-described hemming stroke stops  37  define the lower hemming end position and limit the hemming stroke. The adjusting screws  65  for the hemming stroke stop are arranged on the swivel arm  24 . The angular position of the hemming tools  16 ,  17  on the hemming head  12  is coordinated with the rotary movement of the pivoting drive  18  and the shape of the crank drive  19  such that the hemming tools  16 ,  17  are located essentially at the same height and position in the working position. 
     The hemming stroke is performed by the hemming cylinder  30 , which pulls the bracket  25  downward against the force of the spring  27  on the swivel arm  24 . In the embodiment according to FIGS. 4 through 7, the hemming cylinder  30  acts via a toggle lever  31 . In the embodiment according to FIG. 8, the hemming cylinder  30  acts directly. 
     The hemming stroke is preferably in the same direction during prehemming and finishing hemming. This also applies to the outer hemming device  6  and the inner hemming device  7 . The direction of the stroke is vertical or oblique. It is usually in the direction of the hemming bed  5  and downward in the exemplary embodiment being shown. 
     In the variant according to FIGS. 4 through 7, the hemming cylinder  30  with its housing is mounted rotatably on the bracket  25  via a cylinder bearing  35 . The piston rod acts on the central bearing of the toggle lever  31  via a connection  32 . One end of the lever is mounted stationarily on the machine frame  2  via a stationary lug  34 . The other lug  33  of the swivel arm is arranged on the swivel arm  24 . 
     FIG. 6 shows the pivoted-down resting position of the hemming unit  11 . The hemming cylinder  30  is withdrawn and the toggle lever  31  is correspondingly buckled in this position. To pivot the hemming unit  11  into the working position shown in FIGS. 4 and 5, the hemming cylinder  30  extends and the toggle lever  31  is brought into the stretched position shown in FIGS. 4 and 5. The extending movement and the stretched position are limited and defined by a pivot stop and lever stop  36 . The piston rod with the connection  32  strikes this stop  36 . The hemming unit  11  is first pivoted to come into contact by this stretching movement. The cylinder force needed in this process is weaker than the opposing force of the spring  27 . As a result, the force of the cylinder is deflected into the pivoting movement. There is no relative movement as yet between the swivel arm  24  and the bracket  25  and consequently there is no hemming stroke. 
     The force of the spring  27  is approx. 10% of the maximum force of the cylinder. For example, the spring  27  has a tensioning force of 1 ton and the hemming cylinder  30  has a force of 10 tons. In addition, the spring force is stronger than the kinetic resistances of the swivel arms  24 , i.e., the inertia of masses, frictional forces, etc. As soon as the pivoted position to the machine frame  2  shown in FIGS. 4 and 5 has been reached and the hemming cylinder  30  extends farther, the cylinder force supported on the lever stop  36  is deflected into a downward movement of the bracket  25 . The cylinder housing mounted in a rotatably movable manner via the cylinder bearing  35  carries the bracket  25 . This movement forms the hemming stroke, during which the spring  27  is compressed by the stronger force of the cylinder. The hemming stroke is limited downward by the hemming stroke stop in the above-described manner. 
     After hemming, the hemming cylinder  30  again withdraws, supported by the spring  27  and brings the bracket  25  into the starting position in relation to the swivel arm  24  and the setting device  28  to come into contact with the swivel arm  24 . The further withdrawing movement induces the buckling in of the toggle lever  31  and the pivoting off of the hemming unit  11  into the resting position shown in FIG.  6 . 
     In the embodiment according to FIGS. 4 through 7, the swivel arm bearing  29  arranged hanging on the underside of the machine frame  2  is above the stationary lever bearing  34 . It is preferably located above it in the vertically aligned position. In the stop position, the toggle lever  31  preferably extends horizontally and essentially at right angles to the line connecting the bearings  29 ,  34 . The horizontal forces of reaction and the moments of reaction from the hemming are supported on the lever bearing  34  in the working position. 
     The toggle lever  31  is missing in the variant according to FIG.  8 . The hemming cylinder  30  is mounted, instead, with its connection  32 , preferably the piston rod connection, rotatably on the machine frame  2 . The connection  32  has an effect similar to that of the bearing  34  and supports the forces of reaction and the moments of reaction from the hemming on the machine frame  2 . With its opposite cylinder bearing  35 , the hemming cylinder  30  is connected to the bracket  25  as in the first exemplary embodiment. In this variant, the bearing  29  of the swivel arm  24  is, when viewed in the horizontal plane, between the cylinder connection  32  and the cylinder bearing  35  or the bracket guide  26 . 
     FIG. 8 shows different operating positions of the hemming unit  11  in three views. In the resting position shown on the left, the hemming cylinder  30  is directed essentially vertically and the hemming unit  11  is pivoted off similarly to FIG.  6 . 
     The view in the center of FIG. 8 shows the starting hemming position and the view on the right shows the hemming or working position of the hemming unit  11  at the end of the hemming stroke. The hemming cylinder  30  is directed obliquely in both positions. The horizontal force component stabilizes the pivoted in hemming or working position and is supported via the bracket guide  26 . In addition, the pivoting movement of the swivel arm  24  is limited in relation to the machine frame  2  by apparatus of a pivot stop  36 . 
     This arrangement is reversed in the exemplary embodiment according to FIGS. 4 through 7, and the hemming cylinder  30  is in an oblique direction in the resting position and is directed essentially vertically in the working or hemming position. The kinematics with the hemming movement of the bracket  25 , swivel arm  24  and spring  27  is otherwise the same in both exemplary embodiments. 
     The arrangement of the bearings and the kinematics of the hemming unit  11  are selected to be such in both exemplary embodiments according to FIGS. 4 through 7 and in FIG. 8 that an engaging moment of reaction, which turns the hemming head  12  to the workpiece  9 , is generated during hemming. As long as this condition is met, the design, the mounting and the kinematics of the hemming unit  11  and of its parts may be varied as desired. 
     FIGS. 11 and 12 illustrate the outer hemming device  6  and the arrangement of the hemming units  11  with their hemming heads  12  in a perspective view and in a top view, respectively. The figures also show the design and the arrangement of the hemming tools  16 ,  17 . The hemming heads  12  form with one another an essentially closed hemming contour extending around the workpiece  9  in the working position. To change the hemming tools  16 ,  17 , the hemming units  11  are pivoted back into the resting position shown in FIG.  6 . The hemming heads  12  can be rotated in this position by apparatus of the pivoting drive  18 . 
     A plurality of pivoting drives  18 , which are synchronized with one another via the intermediate bearing  22  and move the hemming head sections  13  simultaneously, may be arranged under certain circumstances in the case of excessively long hemming heads  12  with two or more hemming head sections  13 . The intermediate bearings  22  offer the necessary support and prevent sagging and other deformations. 
     The clamping frame  69 , on the one hand, and, on the other hand, optionally one or more separate clamps  4  are provided for fixing the workpiece or workpieces  9  on the hemming bed  5 . These clamps  4  and their arrangement are illustrated schematically and as examples in FIGS. 9 and 10. In the exemplary embodiment according to FIG. 9, one or more such clamps or tensioners  4  are associated with the hemming units  11  of the outer hemming device  6 . They extend over the hemming heads  12  and are supported on the swivel arm  24  in a suitable manner. They are preferably designed as pivoting clamps and have a tensioner bearing or clamp bearing  46  and a suitable tensioner drive or clamp drive  45 , e.g., a cylinder, which is supported on the swivel arm  24  and acts on the tensioner or clamp  4  via an extension arm with the articulated bearing  49 . 
     In the variant according to FIG. 10, one or more similar clamps  4  may be arranged on the clamping frame  69  and/or on its pivoting frame  48 . The tensioner bearing  46  and the tensioner drive  45  are likewise located on the clamping frame  69  and/or on the pivoting frame  48 . 
     The clamps  4  are designed such in both exemplary embodiments that they extend over the hemming heads  12  at spaced locations and press them onto the workpieces  9  in the immediate vicinity of the hemming points. 
     In the variant according to FIG. 10, the clamps  4  are shaped such on the clamping frame  69  and/or on the pivoting frame  48  that they leave sufficient free space for the hemming units  11  for being pivoted back into the resting position and for rotating the hemming head  12 . As a result, the workpieces  9  remain fixed in their position and in the pressing position during tool change. 
     In the variant according to FIG. 9, the clamps  4  are opened for tool change before the hemming units  11  are pivoted back and closed again after pivoting into the working position. If the workpiece  9  is still to remain clamped, the tool change of the individual hemming units  11  can be carried out one after another staggered in time. To feed and remove the workpieces  9 , the clamps  4  can always be moved sufficiently to the rear. 
     In the case of the arrangement of the clamps  4  with tensioner drive  45  and/or tensioner bearing  46  on the clamping frame  69  and/or on the pivoting frame  48  in the inner area of the outer hemming device  6 , the specifications for FIGS. 9 and 10 must be taken into account only conditionally, because the clamps  4  do not now usually extend over the outer hemming device  6 , nor do they interfere during the feed and removal of the workpieces  9 . 
     FIGS. 1 through 3,  9 ,  10 , and  19  through  21  illustrate the clamping apparatus  3 , which may have, in particular, at least one movable clamping frame  69  and/or one or more individual clamps  4 . The clamping frame  69  is mounted pivotably on the machine frame  2  by apparatus of pivoting frame bearings  38  via a frame-like pivoting apparatus  48 . The pivoting frame drive  39  is preferably designed as a pivotable cylinder, which is mounted rotatably with its housing on a support column  40  of the machine frame  2 . The piston rod also acts rotatingly on the pivoting frame  48 . In an alternative embodiment, the clamping frame  69  may be fed in any other desired and suitable matter, e.g., by a vertical translatory movement with a corresponding drive. 
     On the underside, the clamping frame  69  carries the clamping contour  70  and one or more suitable pressing and clamping elements  71  for specifically pressing the workpieces  9  against the hemming bed  5 . Such pressing and damping elements  71  may also be arranged on the clamps  4 . Various centering pins  41  and similar other positioning elements may also be present here on the clamping frame  69  and/or on the pivoting frame  48  in the manner described in the introduction. The pressing and clamping elements may be adapted to the shape of the workpieces  9  and be rigid or movable or deformable. 
     In addition, some of the pressing and clamping elements  71  may be leading elements in relation to other pressing and clamping elements  71 . The leading pressing and clamping elements  71  are longer than other pressing and clamping elements  71  or are positioned in areas where pressing and clamping elements  71  come into clamping contact with the workpiece or workpieces  9 . As a result, the leading pressing and clamping elements  71  intentionally first press the workpiece or workpieces  9  at certain selectable points. A defined order of clamping is obtained as a result, which counteracts the undesired deformations and/or displacements of the workpiece or workpieces  9 . 
     The leading pressing and clamping elements  71  are movable in themselves, e.g., they are spring-loaded or elastically deformable, and yield in the course of the feed in order to make possible the uniform contact of all pressing and clamping elements  71  at the end of the clamping process. In addition, the pressing and clamping elements  71  can act intentionally with different pressures on the workpiece or workpieces  9 . 
     FIGS. 19 through 21 show an enlarged side view of such pressing and damping elements  71  on the underside of the clamping frame  69  in the clamped position on the workpiece  9 . They fittingly imitate the contour of the workpiece and have one or more contact or pressure surfaces for holding and clamping the workpiece or workpieces  9  in a defined manner. 
     The pressing and clamping elements  71  may be, e.g., the lower part of the clamping contour  70  here. They are a component of the clamping contour  70  that is either made in one piece with it or is attached to it. However, the pressing and clamping elements  71  may also be arranged on holding-down devices  67 , which are located on the clamping frame  69  or on the clamping contour  70 . The holding down devices  67  may be designed as replaceable inserts and changed for adaptation to other workpieces  9 . They are detachably fastened for this purpose on the underside in a recess of the clamping contour  70 , e.g., by apparatus of a screw or the like. 
     In a variant of the embodiment shown, the holding-down devices  67  may also be located at the ends of the clamps  4  and/or on other clamping elements of the clamping apparatus  3 . The holding-down devices  67  may be one-part or multipart holding-down devices. 
     The pressing and clamping elements  71  may be designed as movable and optionally leading pressure pieces  68 , which are permanently or detachably fastened to the holding-down devices  67  or in other points of the clamping apparatus  3 . The pressure pieces  68  may consist, e.g., according to FIG. 19, of a plastic or another elastic material. Corresponding to FIGS. 20 and 21, they may also be spring-mounted  72 . The elasticity or the spring action can also be affected by the shaping, in addition to the selection of the material. For this purpose, the pressure pieces  68  have, e.g., one or more areas of weakened material due to recesses or perforations, which specifically allow deformations to take place in one or more directions. 
     As is illustrated in FIGS. 20 and 21, the elastic or spring-loaded pressure pieces  68  yield against the force of the spring  72  or due to the intrinsic elasticity of their material over the feed path of the clamping apparatus  3  after coming into contact with the workpieces  9 . At the end, all pressing and clamping elements  71  are uniformly in contact with the workpieces  9 , coming into the stop position on their carriers and their mobility is eliminated. The clamping force is now applied by apparatus of the feed drive of the clamping apparatus  3 . 
     The elasticity or spring force of the pressure pieces  68  may differ. As a result, the pressure pieces  68  can act on the workpiece or workpieces  9  with difference forces or pressures. The pressure pieces  68  are particularly suitable for use as leading pressing and clamping elements  71 . Their action on the workpiece or workpieces  9  in terms of location and time and/or force or pressure can be specifically set due to the possibilities of variation. The elasticity or mobility of the pressing and clamping elements may also be present without leading function. 
     As an alternative, the pressure pieces  68  may also consist of a hard and solid material, e.g., steel or another metal. The pressure pieces  68  are themselves adapted to the shape of the workpiece  9  in a suitable manner. FIG. 19 shows on the holding-down device  67  two pressing and clamping elements  71 , of which the rear one is rigid and the front one is designed as a movable and preferably leading pressure piece  68 . 
     In another embodiment variant, not shown, the pressing and clamping elements  71  may have a fluidic, e.g., pneumatic or hydraulic pretension. In particular, they may be designed as inflatable tube elements or the like. Furthermore, they may consist of multiple parts and be designed as movable elements in themselves e.g., telescopically or be mounted movably. 
     The pivoting frame  48  has multiaxially adjustable pivoting frame bearings  38 . For example, the pivoting frame bearings  38  may be designed as spherical or crowned bearings with three translatory axes of adjustment. 
     Due to the adjustable pivoting frame bearings  38 , the pivoting frame  48  and consequently the clamping frame  69  are aligned in relation to the hemming bed  5  and the workpieces  9 . The adaptation is performed by the adjustment of the clamping frame  69  with the pivoting frame  48  as a whole. The mounts and the centering pins  41  or the other positioning aids are positioned individually on the pivoting frame  48  and/or on the clamping frame  69  depending on the application and are arranged adjustably and their position is preferably not changed. The clamping frame  69  itself may also comprise a plurality of parts depending on the application. 
     The pivoting frame bearings  38  are located close to the table surface of the machine frame  2 . The pivoting frame  48  is correspondingly buckled for this purpose. In a preferred embodiment, the pivoting frame bearings  38  are located in or close to a workpiece plane  43 , which is formed and defined by the centering openings on the workpieces  9 . In such an alignment, the centering pins  41  dip essentially vertically into the centering openings on the workpieces  9  and exert essentially a centering action in the workpiece plane  43 . Arranging the pivoting frame bearings  38  close to the table also makes it possible to have a large access opening  44 . 
     The pivoting frame  48  and/or the clamping frame  69  may preferably have two to four or more vertically adjustable supports  64 , which may be preferably arranged on the periphery or in the hemming bed area  5 . These supports  64  make possible the supporting of the clamping force of the clamping apparatus  3  on the machine frame  2  and consequently a specifically partially variable clamping action on the workpiece  9  in order to permit or prevent clamping- and shape-equalizing movements of the workpiece  9  before or during the hemming operation, depending on the application. 
     In another embodiment, not shown, the pivoting frame bearings  38  may be arranged at a spaced location above the machine frame table  2  by apparatus of corresponding brackets or supports. Such an arrangement may be necessary, depending on the design and the arrangement of the workpieces  9 , to allow the pivoting frame  48  or the clamping frame  69  to dip in the arc between the outer and optionally inner hemming devices  6 ,  7 . 
     The arc-shaped clamping movement may also be replaced completely or partially or combined with straight guiding. Different shapes and positions of the workpieces  9  as well as arrangements of the hemming units  11  can thus be taken into account. 
     FIGS. 13 through 18 show different views of a centering apparatus for the workpiece or workpieces  9  in the hemming apparatus  1 . One or more centering inserts  50 ,  51 ,  58 ,  59 ,  60  are arranged for this purpose, preferably on the hemming tools  16 ,  17 . The drawings show different variants of this. 
     In the exemplary embodiment according to FIG. 13, such a centering insert  50  is arranged on the prehemming tool  16  and is adjustably fastened preferably by apparatus of one or more fitting plates  52  and a clamping element  62 , preferably a screw connection. The centering insert  50  is arranged obliquely in relation to the mount or the prehemming tool  16 . Its oblique position corresponds to the prehemming angle  56  assumed by the hemming flank  66  of the prehemming tool  16 . The possibility of adjustment is given as a result in the direction of the slope of the hemming flank. 
     The centering insert  50  is inserted frontally into a corresponding recess in the prehemming tool  16 . It slightly projects over the normal hemming tool contour  53  in the manner to be described below. In preferably all embodiments, the centering inserts  50 ,  51 ,  58 ,  59 ,  60  have a contour that essentially corresponds to the contour of the hemming tool and consequently essentially also to the workpiece contour  57 . 
     FIG. 13 shows another variant of the centering insert  51  on the finishing hemming tool  17 . This insert is aligned here essentially in parallel to the base of the tool mount on the hemming head  12  and is also adjustable by apparatus of fitting plates  52  and a clamping element  62  in the above-described manner. Adjustment in a plurality of directions may be possible in both embodiments by apparatus of a plurality of fitting plates  52  arranged at an angle. 
     The centering insert  51  contains only the tool contour located under the hemming nose or the hemming flank  66 . 
     FIG. 13 also shows a third variant of a centering insert  58 , which is arranged, e.g., on the prehemming tool  16  of the inner hemming device  7 . The centering insert  58 , which is likewise adjustable, is arranged above the prehemming nose. 
     The centering inserts  50 ,  51  act on the workpieces  9  in the area of the hemming flanges  63  and center the outer contour of the workpieces  9 . The centering insert  58  acts on another part of the inner contour of the workpieces  9 , which may remain rigid under certain circumstances and is not hemmed. The workpiece  9  is shown in FIG. 13 as a workpiece section  55  which shows, e.g., a section through parts of the workpiece  9 , especially through a window guide frame. The centering insert  58  acts on this. The window guide frame  55  and the inner part of the workpiece  9  connected thereto can be held rigidly or elastically on the hemming bed  5  by holding-down devices  67  in the clamping frame  69  and preferably by cooperation with the centering inserts  58  in the inner hemming tools  16 ,  17  (see FIGS.  2  and  19 ). 
     FIG. 14 shows a variant to FIG. 13, in which the centering inserts  50 ,  51  are also arranged in a corresponding form and arrangement on the prehemming and finishing hemming tools  16 ,  17  of the inner hemming device  7 . The centering insert  50  on the prehemming tool  16  is in an oblique position here as well and is aligned corresponding to the prehemming angle  56 . Due to this oblique position, the centering insert  50  can be placed especially accurately and adjusted in the direction of the hemming flank. The support and centering function is particularly good as a result and adjustment is necessary only one-dimensionally. 
     FIG. 15 shows another variant with centering inserts  59 ,  60 , which have a larger form here than in the preceding exemplary embodiments and are also fastened by a plurality of clamping elements  62 . They encompass nearly the entire cross-sectional contour of the hemming tools  16 ,  17 . Moreover, the centering insert  60  on the finishing hemming tool  17  comprises, unlike in FIGS. 13 and 14, the hemming flank  66 . The centering inserts  59 ,  60  are arranged on the inner hemming device  7  here. They may also be arranged on the hemming heads  12  of the outer hemming device  6  in a corresponding manner. 
     FIG. 16 illustrates the shape and the function of the centering inserts  50 ,  51 ,  58 ,  59 ,  60  in an enlarged view. The hemming tools  16 ,  17  are normally located at a certain laterally spaced location from the workpiece  9 ,  55  in their starting hemming position before the beginning of the hemming stroke. The centering inserts  50 ,  51 ,  59 ,  60  may slightly project over the above-mentioned normal hemming tool contour  53  with their centering contour  54  located under the hemming flank  66  and are as a result in contact with the workpiece  9 ,  55  with one or more surfaces. The hemming bed  5  may have one or more fitting recesses  61  for this purpose. FIG. 16 also illustrates the position of the hemming flange  63  during prehemming and finishing hemming. 
     The centering inserts  50 ,  51 ,  58 ,  59 ,  60  are arranged in some areas on the hemming tools  16 ,  17 . As a result, the workpiece  9 ,  55  is guided and centered in a punctiform manner at a plurality of points on the outer and/or inner circumference. 
     The centering inserts  50 ,  51 ,  58 ,  59 ,  60  make it possible to affect the shape of the workpiece during the feed of the hemming tools  16 ,  17  during the inner and outer hemming and provide for this the necessary centering and holding functions, which may also be important especially during the change of the prehemming and finishing hemming tools  16 ,  17 . 
     When the outer hemming device  6  is in operation and performs the prehemming, the workpiece  9 ,  55  is supported against the hemming forces acting on the outer circumference on the centering inserts  50 ,  58 ,  59  of the prehemming tools  16  of the inner hemming device  7 . These are moved for this purpose at least into a readiness position in which the hemming flanks  66  are located at a closely spaced location above the hemming flanges  63 . The centering contour  54 , which preferably projects under the hemming flanks  66 , will then support the workpiece  9 ,  55  on the inner circumference in the manner described. The projecting centering contour  54  may now bring about, e.g., a shape correction of the workpiece  9 . 
     The inner prehemming may take place after the outer prehemming. As an alternative, the outer and inner prehemming may take place simultaneously or in a changed sequence. The supporting and centering function is equally given during the inner prehemming on the inner circumference of the workpiece  9 ,  55  in the area of its opening  10 . 
     During the inner prehemming, the workpiece  9 ,  55  is supported on the outside on the centering pins  50 ,  59  of the outer hemming device  6 . In the prehemming position of the outer hemming device  6 , holding up is also ensured via the already bent, oblique hemming flanges  63 . There is flat contact now between the hemming flange  63  and the hemming flank  66  and/or the centering contour  54 . 
     The finishing hemming tools  17  of the outer and inner hemming device  6 ,  7  can be subsequently brought into position simultaneously or one after another. These now support the workpiece  9 ,  55  on the inside and on the outside via their centering inserts  51 ,  60 . The projecting centering contour  54  now bridges over the free space formed during the prehemming and the rolling in of the hemming flange  63 . The finishing hemming can then take place inside and outside one after another or simultaneously. 
     FIG. 17 shows a detail view of the centering apparatus according to arrow XVII in FIG.  14 . It is seen here how, e.g., a centering insert  51  is fitted to the workpiece contour  57  and how it fills the area up to the hemming flank  66 . 
     FIG. 18 shows a section along section line XVIII—XVIII in FIG.  17 . The projection of the centering contour  54  in relation to the normal hemming tool contour  53  is recognized from this view on the example of a centering insert  51 . In addition, the association and the shape of the recess  61  in the hemming bed  5  are seen. FIG. 18 also shows the limited width of the centering inserts  50 ,  51 ,  58 ,  59 ,  60 . 
     If the centering and/or support function of the centering inserts  50 ,  51 ,  59 ,  60  is not desired or necessary due to the projecting adjustment of the centering contour  54 , the centering contour  54  remains flush with the hemming tool contour  53 , which may optionally continue to be present on the hemming tool  16 ,  17  and may also have support and/or centering function. 
     Various modifications of the embodiment shown are possible. On the one hand, the hemming heads  12  may be designed, mounted and driven differently. Instead of a crank drive, it is also possible to use any other power and movement transmission apparatus. The drive for the hemming head  12  may also contain any other drive element instead of the cylinder. In addition, the hemming heads  12  may be combined with any other hemming drive  14  of a different design. Besides fluidic, i.e., hydraulic or pneumatic, drives, electric drives with suitable motors, transmissions and spindles may also be used as the hemming drive  14 . In addition, separate drives may be used for feeding the hemming unit and for the hemming stroke. The various units may be combined, expanded or reduced as desired depending on the task. For example, the lifters  8  may be eliminated altogether in the case of the direct feed of the workpiece  9  onto the hemming bed  5 . 
     In another variant, the hemming apparatus  1  may have only one outer hemming device  6 . The centering inserts  50 ,  51 ,  58 ,  59 ,  60  may also be arranged on the outer hemming units  11  only. Furthermore, the clamping apparatus  3  and the other machine parts of the hemming apparatus  1  are variable as well. In particular, the clamping apparatus  3  may be movable linearly. The clamps  4  may also be mounted stationarily on the machine frame  2  instead of on the movable hemming units  11 . 
     While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.