STRETCH-FORMING DEVICE AND METHOD FOR STRETCH FORMING A WORKPIECE AND STRETCH-FORMED PART

Abstract

A stretch-forming device is provided for stretch forming a workpiece, which has a first bending device having a first bending stamp and a first bending shoe and a second bending device having a second bending stamp and a second bending shoe. In addition, the stretch-forming device has a first shaft and a second shaft, the first shaft mounted so it is rotatable around a first axis and providing a first support surface for the workpiece, and the second shaft mounted so it is rotatable around a second axis and providing a second support surface for the workpiece. The first bending stamp and the second bending stamp are components of a first part of the stretch-forming device, and the first bending shoe and the second bending shoe are components of a second part of the stretch-forming device. The first part and the second part are movable relative to one another. The second part has a first recess and a second recess, the first bending shoe and the first shaft situated on opposite sides of the first recess and the second bending shoe and the second shaft situated on opposite sides of the second recess.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to German Patent Application No. 102009056315.6, filed Dec. 1, 2009, which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The technical field relates to a stretch-forming device for stretch forming a workpiece, a method for stretch forming a workpiece using a stretch-forming device, and a stretch-formed part of an outer skin of a motor vehicle.

BACKGROUND

A stretch-forming tool having a tool lower part and a tool upper part, which is movable relative thereto, a forming stamp, and a stretching device, which can be lowered in relation to the forming stamp, clamps the sheet-metal board to be deformed at the edges, and forms it under tensile stress until it reaches the plastic state on the forming stamp, is known from DE 102 22 314 A1. In this stretch-forming tool, a kinematically simple, compact construction and precise control during processing of the stretch forming is achieved in that stretching slides, which work together in pairs via terminal clamping surfaces running transversely to the stroke direction of the tool parts, are provided, and these are each situated so they are displaceable in the tool lower part or upper part on linear guides inclined at equal angles diagonally to the tool interior and are movable in the retraction direction opposite to a defined counter force, which causes the clamping of the board between the clamping surfaces.

A device for planar stretch forming of workpieces is known from DE 10 2006 047 484 A1, which has a tool upper part and a tool lower part engaged with the tool upper part, which are movable relative to one another. Both the tool upper part and also the tool lower part each have at least two reforming beads, which are spaced apart from one another, on at least two sides, which each engage in corresponding recesses of the tool lower part or the tool upper part.

Outer skin parts that are produced by stretch forming according to the prior art have a relatively slight pre-stretching, for example, 0.5% to 2% pre-stretching. A higher pre-stretching typically causes an increased bulging resistance of the workpiece, for example, the outer skin part.

It is therefore at least one object to specify a stretch-forming device for stretch forming a workpiece, a method for stretch forming a workpiece using a stretch-forming device, and a stretch-formed part, which allow higher pre-stretching values and/or higher pre-stretching of a workpiece. In addition, other objects, desirable features and characteristics will become apparent from the summary and subsequent detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.

SUMMARY

A stretch-forming device is provided for stretch forming a workpiece that has a first bending device, implemented for bending a first end area of the workpiece. The first bending device has a first bending stamp and a first bending shoe. In addition, the stretch-forming device has a second bending device, implemented for bending a second end area of the workpiece. The second bending device has a second bending stamp and a second bending shoe. Furthermore, the stretch-forming device has a first shaft, the first shaft being mounted so it is rotatable around a first axis and providing a first support surface for the workpiece. In addition, the stretch-forming device has a second shaft, the second shaft being mounted so it is rotatable around a second axis and providing a second support surface for the workpiece. The first bending stamp and the second bending stamp are components of a first part of the stretch-forming device and the first bending shoe and the second bending shoe are components of a second part of the stretch-forming device. The first part and the second part are movable relative to one another. The second part has a first recess and a second recess, the first bending shoe and the first shaft being situated on opposite sides of the first recess and the second bending shoe and the second shaft being situated on opposite sides of the second recess.

The stretch-forming device advantageously allows, through the two rotatably mounted shafts in the form of the first shaft and the second shaft, which each provide a support surface for the workpiece, a nearly frictionless force deflection during stretch forming of the workpiece. Friction losses during the stretch forming are thus reduced and increased force introduction into the part geometry in the form of the workpiece is made possible, whereby increased pre-stretching can be achieved, which lies in a range of approximately 5% to approximately 7%, for example.

The higher pre-stretching causes increased bulging resistance of the workpiece. The achieved higher bulging resistance advantageously allows a reduction of the plate thickness of the workpiece and thus lower material costs and a reduction of the part weight. In addition to allowing a lower material use and a lower part weight for the workpiece, the stretch-forming device according to the invention has the further advantage that in contrast to typical deep drawing, no sheet-metal holder or holders are required. The sheet-metal holder guide and pressurized-gas springs or lifting pins are thus dispensed with and the overall tool height can be reduced. Overall, the mentioned advantages additionally cause a cost savings both in the stretch-forming device and also in the workpiece to be formed.

The first part of the stretch-forming device can be implemented as stationary and the second part of the stretch-forming device can be implemented as movable. In a further embodiment, both the first part and also the second part of the stretch-forming device are movable. The second part of the stretch-forming device is particularly preferably implemented as stationary and the first part of the stretch-forming device is implemented as movable.

In a further embodiment of the stretch-forming device, the first bending stamp and the second bending stamp are situated on opposite sides of the first part and the first bending shoe and the second bending shoe are situated on opposite sides, corresponding thereto, of the second part. In this embodiment, the first bending device and the second bending device are therefore situated on opposite sides of the first and second part of the stretch-forming device.

In a further embodiment, the stretch-forming device additionally has a third bending device having a third bending stamp and a third bending shoe and a fourth bending device having a fourth bending stamp and a fourth bending shoe. The first bending device and the second bending device are situated on opposite sides of the first part and the second part and the third bending device and the fourth bending device are situated on opposite sides, which are complementary thereto, of the first part and the second part.

The aforementioned embodiments advantageously allow reliable clamping of the workpiece between the first bending stamp and the first bending shoe and the second bending stamp and the second bending shoe and optionally the third bending stamp and the third bending shoe and the fourth bending stamp and the fourth bending shoe on at least two or optionally four sides of the workpiece.

In a further embodiment of the stretch-forming device, the height of the first bending stamp varies over its length and/or the height of the second bending stamp varies over its length. The first bending stamp and/or the second bending stamp may thus be adapted optimally to the workpiece.

In a further embodiment, the orientation of the first bending stamp and/or the second bending stamp is adapted to the geometry of the workpiece. This embodiment also allows a precise adaptation of the first bending stamp and/or the second bending stamp to the workpiece.

In a further embodiment of the stretch-forming device, a lateral distance between the first bending stamp and the first bending shoe and/or a lateral distance between the second bending stamp and the second bending shoe is adapted in such a manner that a predetermined clamping force for the workpiece is settable. A clamping force to be set for the workpiece, in order that it does not slip out of the clamping, can thus advantageously is provided, without further components being required for this purpose.

The stretch-forming device can further have a deep-drawing device having a forming stamp and a matrix corresponding thereto. The forming stamp is preferably a component of the first part of the stretch-forming device and the matrix is a component of the second part of the stretch-forming device. The aforementioned embodiments thus allow, in addition to the planar stretch forming of the workpiece, deep drawing processing thereof. Particularly preferably, the first part is an upper part of the stretch-forming device and the second part is a lower part of the stretch-forming device.

In a further embodiment of the stretch-forming device, the first bending shoe has a first clamping device and the second bending shoe has a second clamping device, the first clamping device being situated adjacent to the first recess and the second clamping device being situated adjacent to the second recess. By providing the first clamping device and the second clamping device, the required clamping, i.e., a predetermined clamping force for the workpiece can be provided very precisely, variations in the thickness of the workpiece additionally being able to be compensated for using the clamping devices.

The first clamping device and/or the second clamping device preferably have a plate and a clamping insert. The plate can comprise a polyurethane plastic and/or a hard rubber. The polyurethane plastic particularly advantageously has a flat modulus of elasticity. In a further embodiment, the first clamping device and/or the second clamping device have compressed-gas springs.

In a further embodiment of the stretch-forming device, the second part has a first brake shoe and a second brake shoe, the first brake shoe being situated below the first shaft and adjacent to the first recess and the second brake shoe being situated below the second shaft and adjacent to the second recess. Uncontrolled running on of the workpiece during the forming thereof can thus advantageously be avoided. The first brake shoe and the second brake shoe preferably comprise a polyurethane plastic and/or a hard rubber and/or compressed-gas springs.

A method is also provided for the stretch forming of a workpiece using a stretch-forming device according to one of the embodiments, the method having the following steps: the workpiece is laid at least on the first support surface, the second support surface, the first bending shoe, and the second bending shoe. In addition, the first part is moved relative to the second part in such a manner that the first end area of the workpiece is clamped between the first bending stamp and the first bending shoe and the second end area of the workpiece is clamped between the second bending stamp and the second bending shoe. Furthermore, the workpiece is pre-stretched. The method has the advantages already mentioned in connection with the stretch-forming device according to the embodiments. These are not listed once again at this point to avoid repetitions. The workpiece is preferably a deep-drawn plate or an aluminum plate. In one embodiment of the method, after the pre-stretching of the workpiece, the workpiece is deep drawn if the stretch-forming device has a deep-drawing device. This method is particularly suitable for workpieces which are to have a small drawing depth.

Furthermore, a stretch-formed part of an outer skin of a motor vehicle is provided that has pre-stretching VR in a range of approximately 4%≦VR≦8%, preferably approximately 5%≦VR≦7%, and more preferably approximately 6%≦VR≦7%. The stretch-formed part has the advantages already mentioned in connection with the stretch-forming device, which are not listed once again at this point to avoid repetitions. The stretch-formed part is particularly preferably produced using a method for stretch forming a workpiece according to one of the above-mentioned embodiments. In one embodiment, the stretch-formed part is a deep-drawn plate or an aluminum plate, which preferably has a small drawing depth.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and:

FIG. 1 shows a stretch-forming device for stretch forming a workpiece according to an embodiment;

FIG. 2A shows a schematic perspective illustration of the first part of the stretch-forming device according to FIG. 1;

FIG. 2B shows a schematic perspective illustration of the second part of the stretch-forming device according to FIG. 1;

FIG. 3A through FIG. 3E show a method for stretch forming a workpiece according to an embodiment; and

FIG. 4A through FIG. 4D shows a method for stretch forming a workpiece according to a further embodiment.

DETAILED DESCRIPTION

The following detailed description is merely exemplary in nature and is not intended to limit application and uses. Furthermore, there is no intention to be bound by any theory presented in the preceding background or the following detailed description.

FIG. 1 shows a stretch-forming device 1 for stretch forming a workpiece 2 according to an embodiment of the invention in cross-section. The stretch-forming device 1 has a first bending device 3, which is implemented to bend a first end area 4 of the workpiece 2. For this purpose, the first bending device 3 has a first bending stamp 5 and a first bending shoe 6. In addition, the stretch-forming device 1 has a second bending device 7, which is implemented to bend a second end area 8 of the workpiece 2. For this purpose, the second bending device 7 has a second bending stamp 9 and a second bending shoe 10.

Furthermore, the stretch-forming device 1 has a first shaft 11, the first shaft 11 being mounted so it is rotatable around a first axis and providing a first support surface 12 for the workpiece 2. In addition, the stretch-forming device 1 has a second shaft 13, the second shaft 13 being mounted so it is rotatable around a second axis and providing a second support surface 14 for the workpiece 2. The respective axes around which the first shaft 11 and the second shaft 13 are mounted so they are rotatable are situated perpendicular to the plane of the drawing in the embodiment shown and are not shown in greater detail in FIG. 1.

The first bending stamp 5 and the second bending stamp 9 are components of a first part 15 of the stretch-forming device 1 and the first bending shoe 6 and the second bending shoe 10 are components of a second part 16 of the stretch-forming device 1. The first part 15 and the second part 16 are movable relative to one another. In the embodiment shown, the second part 16 is stationary and the first part 15 is movable.

The first part 15 is an upper part of the stretch-forming device 1 in the embodiment shown and the second part 16 is a lower part of the stretch-forming device 1. The second part 16 has a first recess 17 and a second recess 18. The first bending shoe 6 and the first shaft 11 are situated on opposite sides of the first recess 17 and the second bending shoe 10 and the second shaft 13 are situated on opposite sides of the second recess 18. In the embodiment shown, the first bending shoe 6 forms a lateral surface of the first recess 17 and the second bending shoe 10 forms a lateral surface of the second recess 18.

The first bending stamp 5 and the second bending stamp 9 are situated on opposite sides of the first part 15 and the first bending shoe 6 and the second bending shoe 10 are situated on opposite sides, corresponding thereto, of the second part 16. The first end area 4 of the workpiece 2 is situated on the same side as the first bending stamp 5 and the first bending shoe 6 and the second end area 8 of the workpiece 2, which is opposite to the first end area, is situated on the same side as the second bending stamp 9 and the second bending shoe 10.

Furthermore, the stretch-forming device 1 has a deep drawing device 19 having a forming stamp 20 and a matrix 21 corresponding thereto. In the embodiment shown, the forming stamp 20 is a component of the first part 15 of the stretch-forming device 1 and the matrix 21 is a component of the second part 16 of the stretch-forming device 1. The workpiece 2 is, for example, a board or a plate, in particular a deep-drawn plate or an aluminum plate.

FIG. 2A shows a schematic perspective illustration of the first part 15 of the stretch-forming device 1 according to FIG. 1 and FIG. 2B shows a schematic perspective illustration of the second part 16 of the stretch-forming device 1 according to FIG. 1. Components having the same functions as in FIG. 1 are identified by the same reference numerals and are not explained in greater detail hereafter. As may be seen in FIG. 2A and FIG. 2B, the first bending stamp 5 and the second bending stamp 9 as well as the first bending shoe 6 and the second bending shoe 10 each have a diagonal shape in their longitudinal directions in the embodiment shown. The orientation of the first bending stamp 5, the second bending stamp 9, the first bending shoe 6, and the second bending shoe 10 is thus adapted to the geometry of the workpiece (not shown in FIGS. 2A and 2B). In addition, it is also possible that the height of the first bending stamp 5 varies over its length and/or the height of the second bending stamp 9 varies over its length.

Furthermore, plain bearings 24, 26, 27, and 28 are shown in FIG. 2B, in which the first shaft 11 and the second shaft 13 are mounted so they are rotatable. The plain bearings 24 and 26 are situated on the two opposite ends of the first shaft 11 and the plain bearings 27 and 28 are situated on the two opposite ends of the second shaft 13. Furthermore, in addition to the respective plain bearing situated at the ends of the first shaft 11 and the second shaft 13, support bearings may be provided, which are situated between the two opposite ends of the respective shaft.

FIG. 3A through FIG. 3E show a method for stretch forming a workpiece 2 using the stretch-forming device 1 according to an embodiment. Components having the same functions as in the preceding figures are identified by the same reference numerals and are not explained in greater detail hereafter. In FIG. 3A through FIG. 3E, only a subarea of the stretch-forming device 1 is shown in each case, in particular the part which contains the first bending device 3 and the first shaft 11. The second bending device (not shown) and the second shaft (also not shown) are situated as shown in FIG. 1.

In a first step of the method according to an embodiment, the workpiece 2, for example, in the form of a board, is laid on the first support surface 12 of the first shaft 11 and the second support surface of the second shaft and the first bending shoe 6 and the second bending shoe. FIG. 3A shows the stretch-forming device 1 after the just-mentioned method step.

In a further step, the first part 15 is lowered in the direction toward the second part 16 and the workpiece 2 is thus clamped using a part of the first end area 4 between the first bending stamp 5 of the first part 15 in the form of an upper part and the first bending shoe 6 of the second part 16 in the form of a lower part. In addition, the workpiece 2 is clamped using a part of the second area between the second bending stamp and the second bending shoe. FIG. 3B shows the stretch-forming device 1 after the just-mentioned method step.

In a further step, the first part 15 is lowered further. As the first bending stamp 5 is moved lower, which is connected thereto, the clamped workpiece 2 is also drawn downward and pre-stretched via the first shaft 11 and the second shaft. Since the first shaft 11 and the second shaft are mounted so they are rotatable, nearly frictionless force deflection is possible, whereby the highest possible pre-stretching, for example, in a range from approximately 5% to approximately 7%, can be achieved. In order that the workpiece 2 does not slip out of the clamping, a clamping force ascertained depending on the material quality of the workpiece 2, for example, depending on plate quality, is provided. The clamping force is set via the distance between the first bending stamp 5 and the first bending shoe 6 or the second bending stamp and the second bending shoe. For this purpose, this distance is adapted in such a way that the corresponding clamping force can be provided via a segmented surface pressure. FIG. 3C shows the stretch-forming device 1 after the just-mentioned method step.

After the pre-stretching to be achieved has been reached, the static friction between the workpiece 2 and the first bending stamp 5 or the second bending stamp is overcome and the workpiece 2 can run on. The clamping effect is thus reduced because of a decreased clamping surface, which results through the material retraction, and the forming of the part into the form of the workpiece 2 begins. The workpiece 2 can run on further because of the reduced clamping force. The corresponding deep-drawing process is shown in FIG. 3D and FIG. 3E.

FIG. 4A through FIG. 4D show a method for stretch forming a workpiece 2 using a stretch-forming device 23 according to a further embodiment. Components having the same functions as in the preceding figures are identified by the same reference numerals and are not explained in greater detail hereafter. Only a subarea of the stretch-forming device 23 is shown in each of FIG. 4A through FIG. 4D, in particular the part which contains the first bending device 3 and the first shaft 11. The second bending device (not shown) and the second shaft (also not shown) are situated as shown in FIG. 1.

The method according to this embodiment is performed essentially corresponding to the method which was shown in FIG. 3A through FIG. 3E. The stretch-forming device 23 differs from the stretch-forming device 1 in that the first bending shoe 6 has a first clamping device 22 and the second bending shoe has a second clamping device, the first clamping device 22 being situated adjacent to the first recess 17 and the second clamping device being situated adjacent to the second recess. In the embodiment shown, the first clamping device 22 is implemented as a clamping insert 30, which is pre-tensioned using a plate 29 made of polyurethane plastic. The second clamping device is also implemented as a clamping insert pre-tensioned using a plate made of polyurethane plastic. The clamping force which is used depending on the quality of the workpiece 2 is set using the pre-tensioned clamping inserts. Furthermore, possibly existing thickness tolerances of the workpiece 2 may be compensated for using the clamping insert.

In addition, the second part 16 has a first brake shoe 25 and a second brake shoe, the first brake shoe 25 being situated below the first shaft 11 and adjacent to the first recess 17 and the second brake shoe being situated below the second shaft and adjacent to the second recess. In the embodiment shown, the first brake shoe 25 is implemented as an insert 32 which is pre-tensioned using a plate 31 made of polyurethane plastic. The second brake shoe is also implemented as an insert which is pre-tensioned using a plate made of polyurethane plastic. On the one hand, the controlled intake during the forming is achieved using the pre-tensioned inserts; on the other hand, the material of the workpiece 2 is pre-stretched up to a defined drawing depth. Furthermore, possibly existing thickness tolerances of the workpiece 2 may be compensated for using the inserts.

In a first step of the method according to this embodiment, the workpiece 2, for example, in the form of a board, is laid on the first support surface 12 of the first shaft 11 and the second support surface of the second shaft as well as the first bending shoe 6 and the second bending shoe. FIG. 4A shows the stretch-forming device 23 after the just-mentioned method step.

In a further step, the first part 15 is lowered in the direction toward the second part 16 and the workpiece 2 is thus clamped using a part of the first end area 4 between the first bending stamp 5 of the first part 15 in the form of an upper part and the first clamping device 22 in the form of a pre-tensioned clamping insert of the second part 16 in the form of a lower part. In addition, the workpiece 2 is clamped using a part of the second end area between the second bending stamp and the second clamping device in the form of a pre-tensioned clamping insert. FIG. 4B shows the stretch-forming device 23 after the just-mentioned method step.

In a further step, the first part 15 is lowered further. As the first bending stamp 5 moves lower in connection therewith, the clamped workpiece 2 is also drawn downward and pre-stretched via the first shaft 11 and the second shaft. Since the first shaft 11 and the second shaft are mounted so they are rotatable, nearly frictionless force deflection is possible, whereby the highest possible pre-stretching, for example, in a range of approximately 5% to approximately 7%, can be achieved. In order that the workpiece to does not slip out of the clamping, a clamping force is provided, which is ascertained depending on the material quality of the workpiece 2, for example, depending on the plate quality. The clamping force is provided via the pre-tensioned plates made of polyurethane plastic behind the clamping insert. FIG. 4C shows the stretch-forming device 23 after the just-mentioned method step.

After the pre-stretching to be achieved has been reached, the first bending stamp 5 and the second bending stamp run over the respective clamping inserts, whereby the clamping force is reduced, and the forming of the part into the form of the workpiece 2 begins. The workpiece 2 can run on because of the reduced clamping force. In order to prevent uncontrolled running on, pre-tensioned brake shoes as already described above are installed below the first shaft 11 and the second shaft. The corresponding deep-drawing process is shown in FIG. 4D.

While at least one exemplary embodiment has been presented in the foregoing summary and detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration in any way. Rather, the foregoing summary and detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope as set forth in the appended claims and their legal equivalents.

Claims

1. A stretch-forming device for a stretch forming of a workpiece, comprising:

a first bending device adapted to bend a first end area of the workpiece, the first bending device comprising a first bending stamp and a first bending shoe;

a second bending device adapted to bend a second end area of the workpiece, the second bending device comprising a second bending stamp and a second bending shoe;

a first shaft mounted and rotatable around a first axis and adapted to provide a first support surface for the workpiece;

a second shaft mounted and rotatable around a second axis and adapted to provide a second support surface for the workpiece;

a first part comprising the first bending stamp and the second bending stamp; and

a second part formed of a first bending jaw and the second bending shoe, the second part movable relative to the first part and the second part comprising a first recess and a second recess,

wherein the first bending shoe and the first shaft are situated on opposite sides of the first recess and the second bending shoe and the second shaft are situated on opposite sides of the second recess.

2. The stretch-forming device according to claim 1, wherein the first bending stamp and the second bending stamp are situated on opposite sides of the first part, and the first bending shoe and the second bending shoe are situated on opposite sides of the second part.

3. The stretch-forming device according to claim 1, wherein a height of the first bending stamp varies over a length of the first bending stamp.

4. The stretch-forming device according to claim 1, wherein a height of the second bending stamp varies over a length of the second bending stamp.

5. The stretch-forming device according to claim 1, wherein an orientation of the first bending stamp is adapted to a geometry of the workpiece.

6. The stretch-forming device according to claim 1, wherein an orientation of the second bending stamp is adapted to a geometry of the workpiece.

7. The stretch-forming device according to claim 1, wherein a lateral distance between the first bending stamp and the first bending shoe is adapted such that a predetermined clamping force is settable for the workpiece.

8. The stretch-forming device according to claim 1, wherein a lateral distance between the second bending stamp and the second bending shoe is adapted such that a predetermined clamping force is settable for the workpiece.

9. The stretch-forming device according to claim 1, further comprising a deep-drawing device that comprises a forming stamp and a matrix.

10. The stretch-forming device according to claim 9, wherein the forming stamp is a first component of the first part of the stretch-forming device and the matrix is a second component of the second part of the stretch-forming device.

11. The stretch-forming device according to claim 1, wherein the first part is an upper part and the second part is a lower part.

12. The stretch-forming device according to claim 1, wherein the first bending shoe comprises a first clamping device and the second bending shoe comprises a second clamping device, the first clamping device is situated adjacent to the first recess and the second clamping device is situated adjacent to the second recess.

13. The stretch-forming device according to claim 12, wherein the first clamping device comprises a plate and a clamping insert.

14. The stretch-forming device according to claim 12, wherein the second clamping device comprises a plate and a clamping insert.

15. The stretch-forming device according to claim 13, wherein the plate comprises a polyurethane plastic.

16. The stretch-forming device according to claim 14, wherein the plate comprises a rubber.

17. The stretch-forming device according to claim 13, wherein the plate comprises a compressed-gas spring.

18. The stretch-forming device according to claim 1, wherein the second part comprises a first brake shoe and a second brake shoe, the first brake shoe situated below the first shaft and adjacent to the first recess, and the second brake shoe situated below the second shaft and adjacent to the second recess.

19. The stretch-forming device according to claim 18, wherein the first brake shoe and the second brake shoe comprise a polyurethane plastic.

20. The stretch-forming device according to claim 18, wherein the first brake shoe and the second brake shoe comprise a rubber.

21. The stretch-forming device according to claim 18, wherein the first brake shoe and the second brake shoe comprise a compressed-gas springs.

22. A method for stretch forming a workpiece with a stretch-forming device, the stretch-forming device comprising:

a first bending device adapted to bend a first end area of the workpiece, the first bending device comprising a first bending stamp and a first bending shoe;

a second bending device adapted to bend a second end area of the workpiece, the second bending device comprising a second bending stamp and a second bending shoe;

a first shaft mounted and rotatable around a first axis and adapted to provide a first support surface for the workpiece;

a second shaft mounted and rotatable around a second axis and adapted to provide a second support surface for the workpiece;

a first part comprising the first bending stamp and the second bending stamp; and

a second part formed of a first bending jaw and the second bending shoe, the second part movable relative to the first part and the second part comprising a first recess and a second recess,

wherein the first bending shoe and the first shaft are situated on opposite sides of the first recess and the second bending shoe and the second shaft are situated on opposite sides of the second recess, the method comprising: placing the workpiece at least on the first support surface, the second support surface, the first bending shoe and the second bending shoe; moving the first part relative to the second part such that the first end area is clamped between the first bending stamp and the first bending shoe and the second end area is clamped between the second bending stamp and the second bending shoe; and pre-stretching the workpiece.

23. The method according to claim 22, wherein the pre-stretching the workpiece produces a stretch-formed part of an outer skin of a motor vehicle and the stretch-formed part has a pre-stretching VR in a range approximately 4%≦VR≦8%.