Patent Publication Number: US-2021162567-A1

Title: Method for clamping of workpieces as well as embossing device and clamping device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. § 119 to the following German Patent Application No. DE 10 2019 132 276.6, filed on Nov. 28, 2019, the entire contents of which are incorporated herein by reference thereto. 
     TECHNICAL FIELD 
     The invention refers to a method for clamping of workpieces consisting of ductile material, as well as an embossing device for use in this method as well as a clamping device also for use in this method. 
     BACKGROUND 
     For processing or machining of workpieces in machine tools, particularly for chip-creating machining, the workpieces have to be clamped and held in suitable clamping units. The clamping must be carried out in a manner such that during the machining processes that have to be carried out on the workpiece, also larger forces can be supported without movement of the workpiece in the clamping device or without releasing of the workpiece therefrom. On the other hand, the workpiece must be accessible as far as possible, in order to be able to carry out as many machining processes on the workpiece as possible with one single clamping setting. 
     For this EP 1 071 542 B1 proposes a method for clamping of workpieces in which regularly spaced depressions are provided on a workpiece in a preparing process step, wherein these depressions do only serve as coupling elements during clamping with a respective clamping device, but are without any function apart therefrom. After this preparing process step the workpieces are placed in chucks that have abutment surfaces at their clamping jaws for friction-fit clamping and that have form-fit elements for form-fit positioning and securing of position that are complimentary to the depressions of the workpiece. Thus the workpiece is clamped in a form-fit as well as friction-fit manner. This clamping method has proved itself in general. 
     Further a clamping method is known from DE 10 2009 052 334 A1 in which the workpiece is provided with a clamping structure in the proximity of its base surface. Two grooves provided at the flanks of the workpiece form part thereof in which corresponding bar-like projections of two clamping jaws of a chuck engage. The grooves are, for example, introduced by means of a milling process and provide a form-fit connection between the clamping jaws and the workpiece. The wall remaining at the workpiece and limiting the groove must be resistant against bending and breaking, which requires a certain minimum wall thickness of the groove wall. This determines the space requirement of the clamping structure. 
     When configuring clamping systems, it has to be expected that users desire to machine a range of different workpieces from different materials without being limited to one certain type of workpiece and material. This has to be considered when clamping systems are configured. On one hand the clamping device must reliably hold the workpieces, also if they are subject to high machining forces, wherein on the other hand damaging the workpieces by means of the clamping device has to be avoided, such as unallowed deformations or excavations. 
     BRIEF SUMMARY 
     Starting therefrom it is the object of the invention to provide a clamping method with which a wide range of workpieces consisting of different materials can be clamped, wherein the space required by the clamping device on the workpiece shall be as small as possible. 
     The clamping method according to the invention is based on that in a preparing process step deformations are provided in a defined grid on the workpieces to be clamped, e.g. in the shape of a row of regularly distanced depressions. These depressions (or other deformations) serve as positioning and coupling elements during clamping in a respective clamping device, whereby they do not fulfill any other function on the workpiece apart therefrom. For clamping of the workpieces the clamping jaws comprise clamping surfaces for the friction-fit holding of the workpiece. In addition, the clamping jaws are provided with form-fit elements, preferably at the abutment surfaces that serve to form-fit positioning and securing of the workpiece position. The workpiece is thus clamped in a combined friction-fit and form-fit manner. 
     According to the invention, the deformations provided on the workpiece are arranged with a center distance of 2.5 mm to 3.5 mm from each other. It has been shown that by means of such a grid an optimum in terms of the holding force and the area required on the workpiece for holding can be obtained. Local stress peaks in the workpiece are reduced to an amount supportable for most materials and a uniform holding force transmission between the clamping jaws and the workpiece is obtained. Excavations, crack formations on the workpiece or other workpiece damages are avoided. A grid dimension of 3 mm (center distance) forms an optimum of the ratio of reachable holding force to required surface area that applies for most of the ductile materials such as plastics, particularly plastically deformable plastics, aluminum, aluminum alloys, as well as other metal alloys and metals. 
     The depressions are preferably arranged in a row that extends along the bottom edge of the workpiece next to the base surface of the workpiece. Preferably the row is a straight row. The depressions can, however, also be arranged in two or more rows, preferably arranged parallel to each other. The depressions of two rows can be arranged adjacent to one another in pairs or can be alternatively offset from each other. In at least one of the two (or more) rows, preferably in all of the rows, the grid distance is defined to a value in a range from 2.5 mm to 3.5 mm, preferably 3 mm. 
     Preferably the deformations that have to be provided on the workpiece are depressions that are introduced in the workpiece by plastic deformation by means of an embossing device. Each depression can be created by material displacement by means of an embossing tooth. The depressions are preferably arranged in a straight row and with constant distances to each other. Between the depressions non-depressed or also slightly elevated areas are formed that separate the individual depressions from each other. 
     The depressions have preferably a rectangular cross-section with rounded corners, wherein the cross-section decreases toward the bottom of the depression. The rectangular depressions have preferably a length in the direction of the row in which they are arranged that is at least as long as the length of the areas between two adjacent depressions respectively that is measured in the same direction. 
     The material displacement that occurs during creation of the depressions, leads to a flow of the material of the workpiece, whereby below the depression and around the depression a zone of hardened material can be created. Particularly when using metals and metal alloys, this zone can have an increased strength so that it is particularly suitable for support and distribution of forces in the workpiece. 
     Preferably a distance between the embossing tool and the workpiece is left open between two adjacent embossing teeth in which displaced material can enter. The workpiece surface that is smooth prior to the embossing, e.g. cylindrical or planar, thus obtains the desired deformations, e.g. depressions, during the preparing process step. Thereby wavy or elevated deformations of the workpiece surface can be created between these depressions, due to material displacement. The workpiece surface resulting therefrom that does not only contain depressions for reception of the form-fit elements of the clamping jaw, but in addition is curved multiple times. In other words the formerly smooth workpiece surface can comprise certain regular deformations, particularly in the areas between the depressions or around these depressions after the formation of the depressions. 
     In the inventive method the clamping surfaces of the clamping jaws are preferably brought in complete contact with the workpiece surface during clamping of the workpiece. In other words the clamping surface of the clamping jaw is pressed with high force on the workpiece surface that was potentially slightly deformed in the preparing working step. In doing so, the workpiece surface can be smoothed in that material displaced out of the workpiece surface is (elastically or plastically) deformed again until—in the ideal condition—the planar clamping surface abuts against the workpiece completely in a two-dimensional manner. This can be combined with a slighter additional plastic deformation of the workpiece, whereby the workpiece is held in the clamping device between the clamping jaws particularly reliably. 
     The inventive embossing device serves for carrying out of the inventive method. At least one embossing die is part of the embossing device, wherein the embossing die comprises an embossing structure that defines a grid, wherein the grid dimension is in a range of 2.5 mm to 3.5 mm. The embossing die can be a linearly movable die, a roller die or the like. The embossing device comprises a counter support for reception of the workpiece, wherein the counter support is arranged opposite the embossing die. The counter support itself can be configured as embossing die, such that on the workpiece on two opposite sides facing away from each other, the desired positioning and coupling elements can be provided in one single working step. 
     For embossing the one embossing die or the more embossing dies are pressed against the workpiece, preferably by means of a force generating device. The force generating device is preferably configured to have the embossing die or embossing dies to act on the workpiece with a predefined force. In doing so, the embossing depth is defined by the ductility of the workpiece material. However, in any case damage of the workpiece, due to an excessive force acting on the workpiece, is avoided. 
     The depressions are created by means of the embossing device preferably while measuring the penetration depth of the embossing teeth. In doing so, it can be ensured that the depressions reach a desired depth during the embossing process, but do not exceed a maximum depth. Mechanical means for limiting the penetration depth, as e.g. abutment teeth provided at the embossing jaws (between the embossing teeth) are not provided. Rather a free space is respectively formed between two embossing teeth, the limitation of which does not get into contact with the workpiece. The penetration depth of the embossing teeth is preferably defined to an amount that is deeper than the tooth height of the holding teeth. In doing so, an uncontrolled workpiece deformation during clamping as well as an excessive wear of the holding teeth is avoided. 
     The embossing device comprises preferably multiple embossing teeth, the center distance of which corresponds to the grid dimension. Preferably each embossing tooth is provided with at least one, preferably with two or more flanks curved in a concave manner such that the flank angle continuously decreases with the progressive penetration of the embossing tooth during embossing at the penetration location of the workpiece. In this manner similar penetration depths are provided also in different ductile materials such that the workpieces embossed in this manner fit on the clamping device in any case. A small variability of the penetration depth of the embossing teeth can be achieved in case of a large variability of the ductility of the different workpiece materials. In doing so, it is guaranteed that the form-fit elements of the clamping device fit in the depressions provided on the workpiece independent from the used material of the workpiece. This applies at least to a wide range of workpiece materials. 
     As mentioned, at least slightly different penetration depths of the embossing teeth and thus slightly different cross-sections of the depressions are obtained in different ductile materials. Due to the control or regulation of the embossing force, however, it can be achieved that the embossed depressions always have a depth that is deeper than the tooth height of the holding teeth. 
     The holding teeth of the clamping jaws are so small that they fit into the smallest depressions that have to be expected in any case. For this reason they can engage with a slightly lateral play in the depressions, if the workpiece is made of a very ductile material and the depressions are rather large. Due to the pressing force that the clamping surfaces of the clamping jaws effect on the workpiece surface and thus on the surrounding area of each depression, however, the depressions can be slightly narrowed again such that the holding teeth finally engage in the depressions without play. 
     The embossing teeth of the embossing die limit between each other an interstice in which workpiece material can flow during embossing. The interstice preferably comprises limitation or boundary that follows the contour of a cylinder. In addition, the embossing teeth are preferably formed on a bar projection of the embossing die. This ensures that during embossing no planar surface outside the embossing teeth get in contact with the workpiece surface. In doing so, the individual depressions can be surrounded by a more or less large ring-shaped elevation depending on the workpiece ductility. The workpiece surface can freely deform outside of the depressions. This is achieved in that the embossing force acts only and exclusively between the embossing teeth and the workpiece. 
     The inventive clamping device comprises at least one, preferably multiple clamping jaws, the workpiece clamping surface of which is provided with form-fit elements that are arranged in the predefined grid of the depressions, i.e. comprising a uniform center distance in a range from 2.5 mm to 3.5 mm, preferably 3 mm. The form-fit elements are preferably teeth, the shape of which is similar to the shape of the embossing teeth, wherein the holding teeth are preferably smaller than the embossing teeth. For this reason the dimension and shape of a depression in materials with low ductility can correspond to the shape and dimension of a holding tooth. In high ductile materials the depressions created by the embossing teeth can be indeed also wider and deeper such that the holding teeth are first engaging the depressions with some play. In order to compensate potentially different embossing depth, the holding teeth (or other form-fit elements) are preferably less high than the depressions embossed in the workpiece are deep. When clamping the workpiece, however, a part of the material displaced by the embossing tooth can flow back and finally surround the holding teeth of the clamping device completely and preferably without any gaps. In addition, a two-dimensional contact between the workpiece clamping surface and the workpiece can be achieved. In doing so, the form-fit and the friction-fit are maximized. 
     Embossing jaws with embossing teeth of a different dimension can be provided for different workpiece materials. In addition or as an alternative, different embossing forces can be used for different workpiece materials. For this the embossing device can be configured such that the embossing force can be adjusted accordingly. In addition or as an alternative, a measurement or monitoring device can be provided that is configured for measuring or monitoring the penetration depth of the embossing teeth in the workpiece. The embossing device can be configured such that it terminates the embossing process, if the desired depth of the depressions is reached. In all presented embodiments the depth of the depressions is preferably in a range of 0.2 mm to 2 mm. Preferably the depth of the embossed depression is larger than the height of the holding tooth by an amount in the range of 0.02 mm to 0.05 mm. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Further details and advantages of the inventive clamping system are apparent from the drawings and the claims. The drawings show: 
         FIG. 1  a prepared workpiece during the clamping in an inventive clamping device, 
         FIG. 2  an embossing die forming part of an embossing device in a perspective overview illustration, 
         FIG. 3  the embossing die during an embossing process in its relation to the workpiece in a schematic illustration, 
         FIG. 4  an embossing tooth during embossing and during penetration into the workpiece, 
         FIG. 5  the workpiece after execution of the preparing embossing process in a schematic perspective illustration, 
         FIG. 6  the workpiece and an embossing die in a slightly modified embodiment after the embossing process in a schematic illustration, 
         FIG. 7  the workpiece and a clamping jaw during clamping of the workpiece in a sectional schematic illustration, 
         FIG. 8  a relation between grid dimension and scaled holding force in form of a diagram, 
         FIG. 9  an embossing device in a schematic top view, 
         FIG. 10  a modified embodiment of an embossing die and an assigned workpiece after the embossing process in a schematic side view, 
         FIGS. 11 and 12  further embodiments of the inventive clamping system and assigned workpieces. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a method for clamping a workpiece  10 . In the exemplarily and schematically illustrated workpiece  10  deformations  11 , e.g. in the form of a row of depressions  12 , have been provided in a preparing process step. They serve as position and coupling elements during clamping in a respective clamping device  13 . The clamping device  13  comprises at least one movable clamping jaw  14  and a counter bearing  15  assigned thereto that can also be configured as clamping jaw in a mirror-symmetrical manner compared with the clamping jaw  14 . Also other counter bearings, e.g. clamping jaws having smooth clamping surfaces can be used. 
     The subsequent description of the clamping jaw  14  applies for the present and all further embodiments, for the counter support  15  configured as clamping jaw or additional clamping jaws and counter supports accordingly. 
     The clamping jaw  14  comprises a row of form-fit elements  16  that can have the shape of holding teeth  17  that fit with the depressions  12  in terms of shape, position and dimension. The holding teeth  17  are, e.g. arranged in a straight row that is arranged in a distance to a support surface  18  formed on the clamping jaw  14 . The holding teeth can, however, also be arranged in another pattern, e.g. in a zigzag row or in two or more rows. The preferably planar support surface  18  is arranged orthogonal to a clamping surface  19  from which the holding teeth  17  project. The clamping surface  19  is preferably a planar surface. Preferably the holding teeth  17  are configured uniformly. 
     The depressions  12  as well as the holding teeth  17  are arranged in a corresponding grid R that is symbolically marked on the workpiece  10  in  FIG. 1 . The grid R defines the center distances of the depressions  12  as well as the center distances of the holding teeth  17  to a uniform value within a range between 2.5 mm and 3.5 mm. Preferably the center distance of the holding teeth  17  as well as the depressions  12  is defined to 3 mm in the grid R. 
     During clamping of workpiece  10  between the clamping jaws  14 ,  15  the holding teeth  17  engage in the depressions  12  and the clamping surface  19  gets in contact with the workpiece surface  20  surrounding the depressions  12 . Thereby the clamping surface  19  applies a pressure force on the workpiece surface  20 . Concurrently the holding teeth  17  are positioned in the depressions  12  without play. In doing so, the workpiece  10  is clamped in a friction-fit and form-fit manner. Due to the low grid dimension of preferably 3 mm, a quasi-continuous form-fit holding of the workpiece  10  with low local force peaks is obtained thereby. Concurrently due to the latching of the holding teeth  17  in the depressions  12 , the workpiece position parallel to the clamping jaws  14 ,  15  is defined. 
       FIG. 2  illustrates a part of the tool for creation of the depressions  12  on the workpiece  10 . The depressions  12  are thereby not part of the desired workpiece geometry, but only serve to clamp the workpiece  10 . They are provided in an area of the workpiece  10  in which no machining processes are required in the selected setting. 
       FIG. 9  illustrates an embossing device  31  for creation of depressions  12  at the workpiece  10 . The embossing device  31  preferably comprises two embossing dies  21 ,  21 ′ that can be moved and clamped toward each other by means of a force creation device  30 . For this the force creation device is connected with the two embossing dies  21 ,  21 ′ and is configured to apply a controlled force on a workpiece  10  held between the two embossing dies  21 ,  21 ′. The two embossing dies  21 ,  21 ′ are preferably configured identically and are arranged mirror-symmetrically with regard to each other. 
     The embossing die  21  illustrated in  FIG. 2  comprises a support surface  22  on which the workpiece  10  can be placed before executing an embossing process. The support surface  22  can be a planar surface or also a surface that is disrupted multiple times. It is also possible to omit such a support surface  22 . 
     A bar  23  is configured on the embossing die  21  parallel to the support surface  22 . The bar  23  is preferably provided with curved or rounded cavities  24 . These cavities  24  have preferably a shape corresponding to a cylindrical surface section in each case and limit the embossing teeth  25  between each other, the embossing teeth  25  being arranged in the grid R. In other words the center distances are defined in the preferred grid of 2.5 mm to 3.5 mm and have, for example, a uniform amount of 3 mm. 
     Apart therefrom, the bar  23  can have parallel flanks or can be wedge-shaped, i.e. extending toward the tips of the embossing teeth  25  in a wedge-shaped manner. In addition the embossing teeth can also be configured in a curved or rounded manner on these flanks. 
       FIG. 3  illustrates the ratio of the dimension of the embossing teeth to the depressions  12  created in the workpiece  10  for complimentary clarity. As apparent, the embossing teeth are pushed in the material of the workpiece  10  only partly during the embossing process, i.e. their respective height H (compare  FIG. 3 ) is longer than the depth T of the created depressions  12 . The height H of the embossing teeth  25  can be measured originating from a dashed illustrated virtual connection line of the deepest positions of the depressions  12  shown in  FIG. 3  up to the tooth tip and can have an amount of, e.g. 3 mm. The depth T of the depressions  12  then correspond to the penetration depth of the teeth. 
     During an embossing process the embossing dies  21  are preferably moved toward each other in a force-actuated manner, wherein the embossing process is executed preferably in a force-controlled manner, however, at least in a force-limited manner. This means that depth T results from a cooperation of the material ductility of workpiece  10  and the applied embossing force. In doing so, and contrary to distance-controlled embossing processes that have a defined penetration depth, damage of workpiece  10  is avoided. In addition or as an alternative, the penetration depth of the embossing teeth can be measured during the embossing process. The penetration depth is the distance that the embossing jaws travel subsequent to the first contact of the workpiece with the faces of the embossing teeth. The embossing device can be configured to terminate the embossing process as soon as the desired penetration depth is reached and thus the desired depth of the depression is achieved. The depth T of the depression  12  is preferably at least slightly longer than the height of holding teeth  17 . The difference can be, e.g. in a range from 0.02 mm to 0.05 mm and can also have a higher or less amount if applicable. 
       FIG. 4  illustrates the embossing process with a well-flowable and thus ductile material, as e.g. ductile aluminum. The embossing tooth  25  penetrates in the workpiece surface  20  locally, wherein the material of the workpiece  10  is displaced and deformed. An influenced zone  26  is created in which the material of the workpiece  10  is compacted and can also be hardened, due to the deformation. In addition, a non-planar deformation of the workpiece surface  20  can be formed around the created depression  12 , e.g. in the form of a wall-like bulge or elevation  27  or in another slight lifting of the workpiece surface  20  counter to the penetration direction of the embossing tooth  25 . This is in slightly exaggerated illustration also shown in  FIGS. 5 and 6 . 
     As also apparent from  FIG. 4 , the cross-section of the embossing tooth increases originating from its face  25   a . Due to the preferably present curvature of its flanks  24   a ,  24   b , the penetration resistance of the embossing tooth increases disproportionally with increasing penetration depth. 
     As shown in  FIG. 6 , the workpiece surface  20  has multiple curves, particularly between the depressions  12  after the creation of the depressions  12 —also, if the workpiece surface  20  was planar prior thereto—i.e. it bulges around different centers of curvature and thus deviates from the planar shape as well as from other simple geometric shapes, such as for example a cylindrical shape. 
     The embossing teeth  25  have preferably two or also four flanks  24   a ,  24   b  that are curved in a concave manner, as particularly shown in  FIGS. 2 and 6 , such that the resistance of each embossing tooth  25  during the embossing process increases non-linearly with increasing penetration depth. For this reason depressions are obtained during the embossing of materials with different ductility having a depth T that is in any case sufficient in order to receive the holding teeth  17 . The penetration depth is still deeper in more ductile material. The variation of the depth T is, however, substantially lower than the variability of the ductility of the different materials that can be used. In addition, the penetration depth can be monitored during embossing and can be feed-back controlled to a desired value. Workpieces  10  can consist, e.g. of aluminum, aluminum alloys, different other metals and metal alloys or plastic. By means of the curvature of the surface  24  it can be achieved that also in softer materials a complete penetration of the embossing tooth  25  in the workpiece  10  is not to be expected. 
     As already mentioned, the clamping surface  19  and the workpiece surface  20  are brought into contact during clamping while the holding teeth  17  engage the depressions  12 . As apparent from  FIG. 7 , preferably the height h of a holding tooth measured in clamping direction S is smaller than the depth T of the depression  12 . During the clamping process the wall-like elevation  27  is at least partly planed such that material of the workpiece, particularly zone  26  is further deformed and tightly huddles around the holding tooth  17 . Thereby the material may flow again, such that additional deformation areas  28 ,  29  result that are illustrated with hatches in  FIG. 7 . Thus, a large section of the clamping surface  19 , preferably the whole clamping surface  19  and the outer surface ( 24   a ,  24   b ) of the holding teeth  17  is used as holding force transmitting surface. The material of the workpiece  10  is in abutment against the flanks with pretention and potentially also against the faces of the holding teeth  17 . Preferably the depth of the depression  12  is so deep, such that the face of the holding tooth  17  does not contact the bottom of the depression  12 , if the workpiece  10  is clamped (compare  FIG. 7 ). 
     Due to the predefined grid distance of preferably 3 mm, it is achieved that the influenced zones  26  of the different embossing teeth  25  abut or overlap in the workpiece  10 . In doing so, a quasi-continuous clamping of the workpiece  10  is allowed. Tests thereby show that longer as well as shorter grid distances lead to reduced workpiece holding forces.  FIG. 8  illustrates this in a diagram. The vertical axis (ordinate) shows the ratio of achievable holding force F to the provided clamping surface area A. In order to obtain comparable curves for different materials, the force F is scaled on the maximum holding force without holding teeth. 
     The horizontal axis (abscissa) shows a grid dimension of the grid R. The grid dimension is a center distance of the embossing teeth  25  and concurrently the center distance of the depressions  12  as well as the center distance of the holding teeth  17 . It has been shown that the maximum achievable holding force F related to the area A reaches a maximum with a grid dimension of 3 mm, wherein in the range from 2.5 mm to 3.5 mm still good holding force values can be achieved. The decrease with regard to the maximum holding force has an amount in this area of mostly less than 30%, frequently less than 10%. This applies to nearly all of the at least somehow ductile and thus embossable materials practically used. 
     Thereby the holding force F is a force that is effective orthogonal to the support surface  18  and thus tends to pull the workpiece  10  that is clamped between the clamping jaws  14 ,  15  out of the clamping jaws  14 ,  15  (vertical to the top in  FIG. 1 ). 
     Surprisingly it has shown that the grid distance of 2.5 mm to 3.5 mm is an optimum for a wide range of workpieces and materials such that no determination on specific materials and workpiece geometries is necessary for the presented clamping system. A universal clamping system can be offered having a wide application in practice. 
     As illustrated in  FIG. 10 , modifications are possible. For example, the embossing teeth and accordingly also the holding teeth  17  can have substantially planar flanks, wherein apart therefrom the above description applies accordingly. 
     In addition,  FIG. 11  illustrates another modification of the invention as described above, wherein the workpiece  10  is held between four jaws  14 ,  15 ,  14   a ,  15   a  that comprise holding teeth  17  in each case and for which the description of the clamping jaw  14  applies accordingly. Clamping jaws  14 ,  15 ,  14   a ,  15   a  arranged opposite each other are moved by clamping drives toward each other or away from each other and can, therefore, clamp the workpiece  10  at four sides. 
       FIG. 12  shows the clamping of a workpiece  10 ′ at the cylindrical section thereof by means of corresponding cylinder section shaped adapted clamping jaws  14 ,  14   a ,  15 , wherein the workpiece  10 ′ is provided with the necessary depressions prior to clamping in a preparing working step, preferably by embossing, as also the case in all other embodiments of the invention. 
     In the inventive clamping method a workpiece  10  is first provided with depressions  12 , wherein these depressions are, for example, arranged in one row or in a field having a grid dimension with preferably 3 mm grid distance. The grid dimension is thereby the center distance of the depressions  12 . The depth T of the depressions  12  is preferably less than the length of the depressions  12  measured in the direction of the row of the depressions  12 . The distances between the depressions  12  preferably correspond approximately to the length of the depressions  12 . 
     Preferably these depressions  12  are formed with embossing tools that comprise embossing teeth  25  with rounded flanks  24   a ,  24   b . Preferably the flanks  24   a ,  24   b  are rounded in a concave manner. 
     The embossing process is preferably carried out such that elevated areas  27  are formed between the depressions  12  that first get into contact with the planar clamping surface  19  of clamping jaws  14 ,  15  during clamping of the workpiece  10  between the clamping jaws  14 ,  15 . A deformation of these elevated areas  27  during the clamping process increasing the holding force. 
     The grid distance of 2.5 mm to 3.0 mm has proven to be an optimum for a wide range of usable workpieces and materials. 
     LIST OF REFERENCE SIGNS 
     
         
           10 ,  10 ′ workpiece 
           11  deformations 
           12  depressions 
           13  clamping device 
           14  clamping jaws 
           15  counter support 
           16  form-fit elements 
           17  holding teeth 
           18  support surface 
           19  clamping surface 
         R grid 
           20  workpiece surface 
           21  embossing die 
           22  support surface 
           23  bar 
           24  cavities 
           24   a, b  flanks of embossing tooth  25   
           25  embossing teeth 
           25   a  face of embossing tooth  25   
         H height of embossing teeth  25   
         T depth of cavities  12   
           26  influenced zone 
           27  wall-like elevation 
         R grid 
         S clamping device 
         h height of holding tooth  17   
         F holding force 
         A area of clamping surface  20   
           28 ,  29  deformation areas 
           30  force creation device 
           31  embossing device