Abstract:
A method for finish-machining a workpiece surface includes moving the workpiece surface relative to an active area of the finishing tool in a rotation direction about a workpiece axis, and superimposing on the relative movement of the workpiece surface and the active area an additional oscillatory movement with an oscillation frequency lower than 20 kHz in a direction perpendicular to the workpiece surface.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims the priority of European Patent Application, Serial No. 12 164 131.0-2302, filed Apr. 13, 2012, pursuant to 35 U.S.C. 119(a)-(d), the content of which is incorporated herein by reference in its entirety as if fully set forth herein. 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to a method for finishing a workpiece surface with a finishing tool. 
         [0003]    The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention. 
         [0004]    A hybrid technology is known from the project “Ultrasound-assisted Superfinishing of cylindrical precision components (SoFi—Sonic Finish)” wherein a conventional finishing process is combined with an ultrasound machining for precision machining a workpiece. The conventional finishing process includes a rotational movement of the workpiece relative to the finishing tool and a low-frequency, oscillatory relative movement of the workpiece and finishing tool in a direction parallel to a rotation axis of the workpiece. The ultrasound machining includes a radial movement of the finishing tool relative to workpiece, which oscillates at the ultrasound frequency. 
         [0005]    It has been found that the aforedescribed hybrid technology is problematic in practice. For example, the loads operating on the finishing tools are so high that the tools need to be replaced after a relatively short time. Furthermore, a high noise level is produced, requiring complicated sound abatement measures. Moreover, aerosols may be produced by nebulization of coolants or lubricants, which in the worst case may cause an explosion risk. Finally, a complicated drive with a sonotrode is required to produce a movement of the finishing tool at ultrasound frequencies, which can be designed only for a particular ultrasound frequency and for a certain mass of the finishing tool. 
         [0006]    It would therefore be desirable and advantageous to obviate prior art shortcomings and to provide an improved method and device for finish-machining a workpiece surface. 
       SUMMARY OF THE INVENTION 
       [0007]    According to one aspect of the present invention, a method for finish-machining a workpiece surface with a finishing tool includes moving the workpiece surface relative to an active area of the finishing tool in a rotation direction about a workpiece axis, and superimposing on the relative movement of the workpiece surface and the active area an additional oscillatory movement with an oscillation frequency lower than 20 kHz in a direction perpendicular to the workpiece surface. 
         [0008]    With the method according to the invention, the active area of the finishing tool is moved periodically in and opposite to the direction of the workpiece surface to be machined. This results in “hammering” processing of the workpiece surface and contact between the active area and the workpiece surface with alternatingly a higher pressing force and with a lower pressing force, commensurate with the oscillation frequency (wherein the active area can also “lift” off the workpiece surface.) 
         [0009]    The movement of the active area of the finishing tool occurs along an additional movement axis, which is oriented perpendicular relative to the workpiece surface to be machined. The oscillation frequency of the additional movement is lower than ultrasound. It is lower than 20 kHz, preferably lower than 16 kHz and in particular lower than 1 kHz. 
         [0010]    The “hammering” processing of the workpiece surface is advantageous in that the active constituents of the active area, such as the cutting grains, penetrate deeper into the material of the workpiece than would otherwise be the case with conventional finish-machining. This supports the formation of chips and increases the removal rate compared to conventional finish-machining. 
         [0011]    The “hammering” processing of the workpiece surface has the additional advantage that the active ingredients of the active area, i.e. the cutting grains, are briefly exposed to an increased pressure load. This supports the formation of chips and thus produces a self-sharpening effect, which in turn contributes to an increase in the material removal rate. 
         [0012]    The additional movement according to the invention is also accompanied by a periodic interruption of the cut or of the chip formation and thus causes an interrupted ground structure. In classical finish-machining, continuous, groove-like depressions are produced, which carry away coolants or lubricants used during machining of the workpiece surface. 
         [0013]    Due to the interruption of the ground structure, a larger proportion of coolants or lubricants remain on the workpiece surface to be machined. This allows the active area of the finishing tool to better penetrate into the workpiece surface to be machined. Periodically lifting the finishing tool, i.e. periodically separating the active area of the finishing tool from the workpiece surface, allows the coolant to more easily enter the contact zone and removed material to be better washed out and transported away. The kinetic energy introduced into the finishing tool during the additional movement also promotes cleaning of the finishing tool from abraded material embedded in or on the active area. Overall, the cutting behavior of the active ingredients of the finishing tool is significantly improved. 
         [0014]    Moreover, the periodic pressing contact of the active area with the workpiece surface causes an increase in the residual compressive stresses in the near-surface portions of the workpiece, so that the fatigue strength of the workpiece (for example, of a rolling bearing part or crankshaft) can be increased. 
         [0015]    The increase in the residual compressive stress in the near-surface portions of the workpiece caused by the additional machining of the workpiece causes a reduction in the notch effect and a reduction in the tensile stress which occur in Hertzian pressing. This also increases the service life of the workpieces machined according to the present invention. 
         [0016]    Lastly, the above-mentioned interruption of the ground structure can advantageously reduce a drainage effect in a finished machined workpiece significantly. This is particularly advantageous when the workpiece is a bearing ring. Rolling of a rolling element on the bearing surface of the bearing ring then will no longer cause a lubricant to be displaced. Corresponding benefits are attained for hydrodynamic slide bearings, where a better retention of the lubricant (in particular oil) is ensured. 
         [0017]    According to an advantageous feature of the present invention, the oscillation frequency of the additional movement may be higher than about 50 Hz, for example higher than about 100 Hz. Advantageously, an oscillation frequency range of between about 100 Hz and about 1 kHz may be employed, for example an oscillation frequency of 200 Hz. This refers particularly to the frequency of the movement of the active area of the finishing tool along the axis of the additional movement. The movements in the aforementioned frequency range can be readily controlled; at the same time, the advantages described above with reference to the “hammering” processing of the workpiece surface can be achieved. 
         [0018]    An amplitude of the additional movement may, for example, be only 0.1 to 5 micrometers. However, in order to achieve a significant increase of a material removal rate, it is proposed that an amplitude of the additional movement (corresponding to half the stroke of the active area) is at least about 5 micrometers. This allows, for a typical grain size of the finishing material (about 10 micrometers), the entire extent of a grain to penetrate into the material of the workpiece. 
         [0019]    According to another advantageous feature of the present invention, an amplitude of the additional movement may be, for example, 0.2 mm to several millimeters. For optimal controllability of the finishing process, an amplitude of the additional movement may advantageously be at most about 200 micrometers (a favorable amplitude value is 50 micrometers). This can also prevent the macro geometry of the workpiece to be machined from deteriorating when using the inherently advantageous effects of the inventive method. An advantageous value for the amplitude of the additional movement is 100 micrometers. 
         [0020]    According to another advantageous feature of the present invention, the workpiece surface and the active area not move relative to each other in a direction parallel to the axis of the workpiece. Here, a relative movement in a direction parallel to the workpiece axis used in a conventional finishing process is thus expressly eliminated. The relative movement between the workpiece surface and the active area is then based exclusively on the rotation of the workpiece surface about the workpiece axis and on the movement of the active area of the finishing tool in a direction perpendicular to the workpiece surface. Advantageously, a comparatively complex drive for the oscillatory movement of the finishing tool and/or of the workpiece in a direction parallel to the workpiece axis may then be omitted, while still attaining a sufficiently high material removal rate for a variety of applications. 
         [0021]    In an alternative embodiment of the invention, the workpiece surface and the active area may move back and forth relative to each other in a direction parallel to the workpiece axis. A conventional oscillatory drive is provided in this case. Such oscillatory drive is advantageous for realizing particularly high material removal rates. 
         [0022]    To achieve a high material removal rate and to introduce into the workpiece a basic structure and increased residual compressive stress, a workpiece surface may advantageously also be initially machined using the inventive method (that is, with a rotary movement of the workpiece and with an additional movement perpendicular to the workpiece surface and possibly additionally with an oscillatory movement parallel to the workpiece axis). Subsequent to this processing operation, the workpiece can then be further processed with a conventional finishing process (i.e., with rotary movement of the workpiece and without an additional movement perpendicular to the workpiece surface and with an oscillatory movement parallel to the workpiece axis) so as to produce a particularly fine workpiece surface. 
         [0023]    When an aforementioned oscillatory drive for generating a relative movement in a direction parallel to the workpiece axis direction is provided, the oscillation frequency of the back-and-forth movement in the direction parallel to the workpiece axis direction may advantageously be at least about 1 Hz. 
         [0024]    When an aforementioned oscillatory drive for generating a relative movement in a direction parallel to the workpiece axis direction is provided, the oscillation frequency of the back-and-forth movement in the direction parallel to the workpiece axis direction may advantageously be at least about 50 Hz. 
         [0025]    According to another advantageous feature of the present invention, advantageous oscillation frequencies for a finishing tool in the form of a finishing belt (in the direction parallel to the workpiece axis) may be between 1 and 21.67 Hz, for example 5 Hz. 
         [0026]    Advantageous oscillation frequencies for a finishing tool in the form of a finishing stone (in the direction parallel to the workpiece axis) may be between 5 and 50 Hz, preferably 33.33 Hz. 
         [0027]    Advantageously, the oscillation frequency of the additional movement may be greater by a factor between 1 and 1000, in particular by a factor between 6 and 40, than the oscillation frequency of the back-and-forth movement in direction parallel to the workpiece axis. These frequency ratios produce an optimal combination of a high material removal rate, an increase in the residual compressive stress in near-surface layers of the workpiece and a reduced drainage effect compared to a conventional cross-hatch structure. 
         [0028]    According to another advantageous feature of the present invention, an amplitude of a back-and-forth movement in the direction parallel to the workpiece axis (corresponding to one half of the total stroke) may be between about 0.1 mm and about 3 mm. Such amplitude range contributes to an increased material removal rate while maintaining a high dimensional stability of the workpiece to be machined. A preferred amplitude for a finishing tool in the form of a finishing belt is 0.5 mm; for a finishing tool in the form of finishing stone at least 0.5 mm, preferably 1 mm. 
         [0029]    According to another advantageous feature of the present invention, the amplitude of the additional movement may be smaller by a factor of 5 to 600, in particular by a factor of 10 to 20, than the amplitude of the back-and-forth movement in the direction parallel to the workpiece axis. These amplitude ratios result in an optimum combination of a high material removal rate, an increase in the residual compressive stress of near-surface workpiece layers and reduced drainage effect compared to a conventional cross-hatch structure. 
         [0030]    Advantageously, the amplitude of the additional movement may be smaller by a factor of 1 to 5 than the amplitude of the back-and-forth movement in the direction parallel to the workpiece axis. These factors are, for example, particularly well suited when a laterally delimited workpiece surface that is only slightly wider than the finishing tool (for example the large end bearing of a crankshaft) needs to be machined. In extreme cases, even factors from 0.5 to 1 (ratio of the amplitude of the additional movement to the amplitude of the back-and-forth movement in the direction parallel to the workpiece axis) or even smaller factors may be suitable. 
         [0031]    According to another aspect of the invention, a device for finish-machining a workpiece surface includes a finishing tool having an active area, a rotary drive for generating a rotary movement of the workpiece surface relative to the active area of the finishing tool in a rotation direction about a workpiece axis, and an additional drive constructed to generate an additional oscillatory movement with an oscillation frequency lower than 20 kHz in a direction perpendicular to the workpiece surface, wherein the additional oscillatory movement is superimposed on the relative movement of the workpiece surface and the active area. 
         [0032]    The device according to the invention shares the advantages described above in conjunction with the inventive method. 
         [0033]    According to an advantageous feature of the present invention, the additional drive includes a piezoelectric actuator. Such an actuator is particularly suitable for generating an oscillatory movement of a working surface of a finishing tool. 
         [0034]    It will be understood that other types of actuators may be used instead of a piezoelectric actuator, for example, hydraulic, pneumatic or electric drives as well as drives based on magnetostriction. 
         [0035]    When using a piezoelectric actuator, the piezoelectric actuator may advantageously be aligned along an additional movement axis for a simple construction, and more particularly piezoelectric elements stacked along the additional movement axis may be employed. 
         [0036]    Advantageously, the piezoelectric actuator and finishing tool may be directly coupled with one another for movement, so that the movement of the piezoelectric actuator along the additional movement axis is identical to a movement of the active area along the additional movement axis. This means that an expansion of the piezoelectric elements in a direction parallel to the additional movement axis is directly converted into a corresponding movement of the active area of the finishing tool, thereby producing a “1:1 conversion” without employing a gear having a step-up or step-down gear ratio. 
         [0037]    Alternatively, other gear devices, such as levers, may be provided, which convert a movement of the piezoelectric actuator into a (preferably greater) movement of the active area of the finishing tool. 
         [0038]    For a particularly simple transfer of the movement of the piezoelectric actuator to the finishing tool, it is proposed that a drive surface of the piezoelectric actuator and the finishing tool are rigidly connected with each other. 
         [0039]    Alternatively, the piezo actuator may have a force transmitting surface for transmitting a pressing force generated by the piezoelectric actuator to a force-receiving surface of the finishing tool. This enables a “tappet-like” transmission of forces direction towards the workpiece. A movement of the finishing tool in the opposite direction can, for example, be generated by an elastic return deformation of a holder of the finishing tool or by additional springs. 
         [0040]    Within the context of the invention, the finishing tool may be constructed as a finishing stone. 
         [0041]    Within the context of the invention, the finishing tool may be constructed as a finishing belt. In this case, a pressing shell is preferable used to press the finishing belt against the workpiece surface, wherein the pressing shell has a pressing surface acting transversely to the running direction of the finishing belt, wherein at least a portion of the pressing surface is formed by a pressing section, which is movable relatively to a stationary shell section along an additional movement axis. Such pressing shell enables defined guidance and positioning of finishing belt, and simultaneously allows a workpiece surface to be machined by “hammering” in the region of the pressing section. 
         [0042]    According to another advantageous feature of the present invention, the pressing section and the stationary shell section may be integrally formed as one piece and interconnected via a connecting section, wherein the connecting section is formed so that driving forces of the additional drive cause the pressing section to move along the additional movement axis. In this way, the surfaces of the pressing section and of the stationary shell section facing the workpiece may be produced in one operation and thus precisely geometrically matched. Simultaneously, the pressing section and the stationary shell section can be positioned relative to each other with high accuracy, since a relative movement between these sections occurs only through an (elastic) deformation of the connecting section, starting from an undeformed initial position. 
         [0043]    According to another advantageous feature of the present invention, an oscillatory drive may be provided for generating a relative back-and-forth movement of the workpiece surface and the active area in a direction parallel to the workpiece axis. 
         [0044]    In an alternative embodiment of the invention, an oscillatory drive for generating a relative back-and-forth movement of the workpiece surface and the active area in a direction parallel to the workpiece axis is explicitly not provided. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0045]    Other features and advantages of the present invention will be more readily apparent upon reading the following description of currently preferred exemplified embodiments of the invention with reference to the accompanying drawing, in which: 
           [0046]    The drawings show in: 
           [0047]      FIG. 1  a side view of an embodiment of a device according to the present invention for finish-machining a workpiece surface; 
           [0048]      FIG. 2  an enlarged detail marked in  FIG. 1  with II; 
           [0049]      FIG. 3  a front view of the detail of  FIG. 2 ; 
           [0050]      FIG. 4  a detail corresponding to  FIG. 2  of another embodiment of a device for finish-machining a workpiece surface; 
           [0051]      FIG. 5  a view of the detail of  FIG. 4  corresponding to the view of  FIG. 3 ; 
           [0052]      FIG. 6  a schematic view of a workpiece surface produced with a conventional finishing process; 
           [0053]      FIG. 7  a schematic view of a workpiece surface produced with a finishing process according to the present invention; 
           [0054]      FIG. 8  a perspective view of another embodiment of a device for finish-machining a workpiece surface; 
           [0055]      FIG. 9  a perspective view of another embodiment of a device for finish-machining a workpiece surface; 
           [0056]      FIG. 10  side view of a part of the device designated  FIGS. 8 and 9  with VI, VII; 
           [0057]      FIG. 11  a detail marked in  FIG. 10  with XI in an enlarged scale; 
           [0058]      FIGS. 12-16  side views of embodiments of pressing shells for use in devices according to  FIGS. 8 to 11 ; 
           [0059]      FIG. 17  a side view of an embodiment of a device for finish-machining a workpiece surface; 
           [0060]      FIG. 18  a plan view of another embodiment of a device for finish-machining a workpiece surface; 
           [0061]      FIG. 19  a side view of the device according to  FIG. 18 ; and 
           [0062]      FIG. 20  a schematic view of a workpiece surface produced with the devices according to  FIGS. 17 to 19 . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0063]    Throughout all the figures, same or corresponding elements may generally be indicated by same reference numerals. These depicted embodiments are to be understood as illustrative of the invention and not as limiting in any way. It should also be understood that the figures are not necessarily to scale and that the embodiments are sometimes illustrated by graphic symbols, phantom lines, diagrammatic representations and fragmentary views. In certain instances, details which are not necessary for an understanding of the present invention or which render other details difficult to perceive may have been omitted. 
         [0064]    Turning now to the drawing, and in particular to  FIG. 1 , there is shown a device for finish-machining a workpiece surface, with the device being designated by the reference numeral  10 . The device  10  includes a machine frame  12  for placing the device  10  on a supporting surface  14 . The frame has a workpiece holder  16  for receiving a workpiece  18  to be finish-machined. 
         [0065]    The workpiece  18  has a central workpiece axis  20 . The workpiece  18  is, for example, a bearing ring. 
         [0066]    The device  10  includes a rotary drive  22  for rotating the workpiece  18  held on the workpiece receptacle  16  about the workpiece axis  20 . The workpiece axis  20  extends coaxially with the rotation axis of the rotary drive  22 . 
         [0067]    In particular, the workpiece  18  has a workpiece surface  24 , which is finish-machined with a finishing tool  26  as described below, extending concentrically with the workpiece axis  20 . 
         [0068]    The finishing tool  26  is, for example, a finishing stone  28 . The finishing tool  26  is supported on a finishing tool holder  30  and can be driven in an oscillatory fashion relative to the finishing tool holder  30  along an additional movement axis  32  (see  FIG. 2 ). As a result, an active area  34  of the finishing tool  26  facing the workpiece  24  is moved towards and away from the workpiece surface  24 . 
         [0069]    For generating the movement of the finishing tool  26 , the device  10  includes an additional drive  36 , in particular in the form of a piezoelectric actuator  38 . The additional drive  36  generates an oscillatory movement of the active area  34  along the additional movement axis  32 . 
         [0070]    For example, a transmission member  40  is provided, which is connected to a clamping device  42 , for coupling the movement of the additional drive  36  and the finishing tool  26 . 
         [0071]    The clamping device  42  includes, for example, a sleeve  44 , which is set in motion by the transmission member  40  of the additional drive  36 . The sleeve  44  is slideably received in a housing  46  of the finishing tool holder  30  for movement along the additional movement axis  32 . 
         [0072]    The clamping device  42  further includes a clamping element  48 , which is connected to the sleeve  44  by a screw connection, allowing the finishing stone  28  to be clamped with the clamping element  48  and sleeve  44 . 
         [0073]    The finishing tool holder  30  can be positioned with a positioning device  50  relative to the frame  12  along a positioning axis  52  (see  FIG. 1 ). The positioning axis  52  is parallel to the workpiece axis  20 . The positioning device  50  includes a holder  54  which is movable on the frame  12  along a tool axis  53  on which a carriage  56  is supported for movement along the positioning axis  52 . 
         [0074]    The carriage  56  and the finishing tool holder  30  are connected to each other in such a way that the finishing tool holder  30  can be positioned relative to the carriage  56  in a direction perpendicular to the workpiece axis  20 . To this end, a finishing tool guide  57  is provided, with which the finishing tool holder  30  can be positioned parallel to the tool axis  59 . This allows compensation of the finishing tool  26  for wear and simplifies handling of the finishing tool  26  in setup or tool change operations. 
         [0075]    The carriage  56  and the finishing tool holder  30  can be connected to each other so that the finishing tool holder  30  is unable to move relative to the carriage  56  in a direction parallel to the workpiece axis  20 . 
         [0076]    Alternatively, the device  10  includes an oscillatory drive  58  for generating a back-and-forth movement of the tool holder  30  in a direction parallel to the workpiece axis  20 . 
         [0077]    The oscillatory drive  58  has, for example, a conventional eccentric drive  60  which will not be explained in detail and which is driven for rotation about an eccentric axis  62  and generates an oscillatory movement of a driven element  66  designated by a double-headed arrow  64 . The driven element  66  is fixedly connected to the finishing tool holder  30 , allowing an oscillatory movement of the driven element  66  to be transmitted to the finishing tool holder  30  and thus to the finishing tool  26 . 
         [0078]    As an alternative to a (hydrodynamic or hydrostatic) sliding bearing of the sleeve  44  in the housing  46  shown in  FIGS. 2 and 3 , the clamping device  42  may also be supported in the housing  46  by at least one linear rolling guide. 
         [0079]    The clamping device  42  may also be supported for movement relative to the housing  46  of the finishing tool holder  30  by at least one membrane element  68  (see  FIGS. 4 and 5 ). The membrane element  68  preferably extends in a direction perpendicular to the additional movement axis  32 . The membrane element  68  is preferably formed as an annular disk, which is connected radially outwardly to the housing  46  and radially inwardly to the sleeve  44 . 
         [0080]    Preferably, two membrane elements  68  are provided, which are arranged in relation to the additional movement axis  32  on opposite sides of the sleeve  44 . 
         [0081]    When the workpiece  18  is machined in a conventional finishing process, the active area  34  does not move along the additional movement axis  32 . In the conventional process, a relative movement between the workpiece surface  24  and active area  34  is composed of a rotation of the workpiece surface  24  about the workpiece axis  20  and an oscillatory movement  64  of the active area  34  parallel to the workpiece axis  20 . This produces a cross-hatch structure  70  characteristic for a conventional finishing process, which is schematically shown in  FIG. 6 . The cross-hatch structure  70  includes a plurality of grooves  72  which are continuous and substantially parallel to each other at least in partial areas, wherein these grooves  72  intersect with likewise continuous grooves  74 . The continuity of the grooves  72  and  74  causes the grooves  72  and  74  to be interconnected at intersections  76  for fluid flow. This produces in a conventional finishing process an increased drainage effect, wherein coolants or lubricants are prematurely removed and must therefore be continuously replenished in comparatively large quantities. 
         [0082]    When another movement, namely the additional movement of the finishing tool  26  along the additional movement axis  32 , is superimposed on the relative movement between the workpiece  18  and finishing tool  26  described above with reference to  FIG. 6 , a surface structure  78  shown in  FIG. 7  is formed. 
         [0083]    The surface structure  78  also includes grooves  80  and  82  extending at an angle relative to one another. However, the grooves  80  and  82  are not continuous, but have breaks  84 , forming mutually separated grooved portions  86 . The grooved portions  86  serve as a storage space for coolants and lubricants, which in contrast to the cross-hatch structure  70  illustrated in to  FIG. 6  is not prematurely removed. This not only improves the cooling and lubrication of the finishing tool  26 , but in particular also reduces the drainage effect of the workpiece surface  24  when using the workpiece  18 . 
         [0084]      FIGS. 8 to 11  show additional embodiments of devices  10  for finish-machining a workpiece surface  24 . These devices  10  include a finishing tool  26  in the form of a finishing belt  88  (see  FIG. 10 ). 
         [0085]    The device  10  of  FIG. 8  includes a frame  12  that can be placed on a supporting surface  14 . The frame  12  is used for arranging an oscillatory drive designated overall with the reference numeral  58  and capable of generating an oscillatory movement of a workpiece holder  16  and a workpiece  18  designated by a double-headed arrow  64 . This oscillatory movement is parallel to a workpiece axis  20  of the workpiece  18 . 
         [0086]    The workpiece holder  16  is part of the rotary drive  22 , with which the workpiece  18  can be driven to rotate about the workpiece axis  20 . The rotary drive  22  includes a headstock  90  and a tailstock  92 . In the embodiment illustrated in  FIG. 8 , the headstock  90  and the tailstock  92  are mounted on a driven member  66  of the oscillatory drive  58 . 
         [0087]    The device  10  shown in  FIG. 9  does not include an oscillatory drive  58 . The headstock  90  and the tailstock  92  are mounted directly on the frame  12  of the device  10 . 
         [0088]    The devices  10  illustrated in  FIGS. 8 and 9  have an identical construction except for the aforedescribed difference (oscillatory drive  58  available or not available). The following description therefore applies to both the device  10  of  FIG. 8  and the device  10  of  FIG. 9 . 
         [0089]    The workpiece surface  24  of the workpiece  18  to be machined is, for example, a large end bearing surface of a crankshaft which has a radial offset from the workpiece axis  20 . This workpiece surface  24  then moves in a circle about the workpiece axis  20 . The finishing tool  26  must then be able to also follow this movement of the workpiece surface  24 . 
         [0090]    Therefore, a bearing device  94  is provided for supporting the finishing tool  26  on the frame  12 , wherein the bearing device  94  has two degrees of freedom and allows a movement of the finishing tool  26  in a plane perpendicular to workpiece axis  20 . 
         [0091]    The bearing device  94  includes a pivot portion  96 , which is held on a frame part  102  of the frame  12  by a pivot bearing  98  for pivoting about a pivot axis  100 . The pivot axis  100  extends parallel to the workpiece axis  20 . 
         [0092]    The pivot portion  96  is used to arrange at least one linear guide  104  (see  FIG. 10 ), with which a bearing member  106  is supported for movement relative to the pivot portion  96  along a guide axis  108  of the linear guide  104 . 
         [0093]    The bearing portion  106  extends substantially in a plane perpendicular to the workpiece axis  20 . 
         [0094]    The bearing member  106  has an opening  108  through which the pivot bearing  98  passes. 
         [0095]    The bearing member  106  has a bearing portion end  110  facing the workpiece  18  for arranging a pressing device  112 . 
         [0096]    The pressing device  112  includes at least two gripper arms  114 . The gripper arms  114  can be pivoted about gripper arm axes  116  relative to the bearing part  106  (see  FIG. 10 ). The gripper arm axes  116  extend parallel to the pivot axis  100  of the pivot member  96 . 
         [0097]    The gripper arms  114  have at their end facing the work piece  18  a unit  118  which will be described in more detail below with reference to  FIG. 11 . 
         [0098]    For generating a pressing force, a conventional pressing drive  119 , which will not be described further, is provided which applies to the units  118  of the gripper arms  114  forces  120  acting in the direction toward the workpiece  18 . 
         [0099]    The units  118  have a holder  122  which is fixedly connected to the gripper arms  114  and is configured for arranging a clamping device for the finishing belt  88 . 
         [0100]    The device  10  includes an additional drive  36  in the form of a piezoelectric actuator  38 . The piezoelectric actuator  38  includes a plurality of piezoelectric elements (“stack”) which are stacked consecutively along the additional movement axis  32 . 
         [0101]    The additional drive  36  is rigidly connected to a drive housing  126  with the gripper arms  114 . The front side  128  of the piezoelectric actuator  38  is connected to a force transmitting element  130 , which has a force transmitting surface  132  that transmits the pressing force produced by the piezoelectric actuator  38  to a force receiving surface  134  of a driven element  136 . The force transmitting surface  132  and the force receiving surface  134  may also be fixedly interconnected, thereby allowing tensile forces to be transmitted from the piezoelectric actuator  38  to the driven element  136 . 
         [0102]    For pressing the finishing belt  88  against the workpiece surface  24 , the units  118  each include a corresponding pressing shell  138 , which each have a curved pressing surface  140 . 
         [0103]    The pressing shells  138  include a stationary shell portion  142 , which is, for example, fixedly connected to the gripper arm  114  by a screw connection  144 . The stationary shell portion  142  is used for arranging a pressing section  146 , which is movable relative to the stationary shell portion  142 , namely along the additional movement axis  32 . 
         [0104]    The pressing section  146  has a curved surface  148 , which forms a portion of the pressing surface  140  (the other portion of the pressing surface  140  is formed by the stationary shell portion  142 ). The pressing section  146  is formed as a single piece with the stationary shell portion  142  and is connected thereto via at least one connecting portion  150 . 
         [0105]    For example, the connecting portion  150  is formed as a thin web  152  which extends transversely, in particular perpendicular, to the additional movement axis  32 . The pressing section  146  is fixed connected to the driven element  136 , so that an expansion of the piezoelectric actuator  38  operates on the force receiving surface  134  via the force transmitting surface  132  and is thus converted by the driven element  136  directly into a movement of the pressing section  146  and hence of the curved surface  146 . 
         [0106]    Several embodiments of pressing shells  138  will now be described with reference to  FIGS. 12 through 16 . The curved surface  148  of the pressing shell  138  of  FIG. 11  formed by the pressing section  146  is comparatively short, as seen in the direction of the finishing belt  88 , so that the curved surface  148  is smaller than half of the total pressing surface  140 . 
         [0107]    In the embodiment of a pressing shell  138  illustrated in  FIG. 12 , the pressing section  146  is enlarged, so that the curved surface  148  formed by the pressing section  146  is greater than half of the total pressing surface  140 . 
         [0108]    The pressing shell  138  shown in  FIG. 13  has the special feature that the connecting portion  150  in the form of a thin web with a surface  154  also forms a part of the pressing surface  140 . The pressing surface  140  is thus composed of a curved surface  148  formed by the connecting portion  146 , at least one surface portion  154  formed by one or more of the connecting portions  152 , and optionally by an additional surface portion  156  formed by the stationary shell portion  142 . 
         [0109]    In an extreme situation, the entire pressing surface  140  may be formed by the pressing section  146 , which is illustrated in  FIG. 14 . 
         [0110]    In the embodiments of pressing shells  138  shown in  FIGS. 15 and 16 , the pressing surface  140  is likewise formed entirely by the curved surface  148  of the pressing section  146 . Additionally, the stationary shell portion  142  has arms  158 , which are provided at their free ends with pressing elements  160 , for example in the form of pressure rollers. The pressing elements  160  are used for support on the workpiece  18  so that the workpiece surface  24  to be machined can be accurately positioned relative to the pressing surface  140 . 
         [0111]    When the forces  120  are introduced into the workpiece  18  by way of the pressing elements  160 , a region of the workpiece surface  24  to be machined by “hammering” remains unaffected by the forces  120 . The forces  120  generated with the pressing drive  119  and the surface machining forces generated by the piezoelectric actuator  38  can thus be adjusted independently of one another. 
         [0112]    The pressing elements  160  may act substantially in a direction parallel to the direction of forces  120  (see  FIG. 10 ), as shown in the embodiment illustrated in  FIG. 15 . 
         [0113]    Alternatively, the pressing elements  160  may act substantially in a direction transverse to the direction of forces  120  (see  FIG. 10 ), as shown in the embodiment illustrated in  FIG. 16 . 
         [0114]      FIGS. 17 to 19  illustrate embodiments of devices  10  for finish-machining a workpiece surface  24 , wherein an additional movement axis  32  is not perpendicular to a workpiece surface  24 , but instead parallel thereto (see  FIG. 17 ), or tangentially thereto (see  FIG. 18 ). 
         [0115]    In the device  10  of  FIG. 17 , an additional movement of the active area  34  of the finishing tool  26  in a direction parallel to the workpiece axis  20  is superimposed on a rotational movement of a workpiece  18  about the workpiece axis  20 , as indicated in  FIG. 17  by a small double-headed arrow  162 . The additional movement  162  is generated, for example, by a piezoelectric actuator  38 , which imparts an additional oscillatory movement  162  on a finishing stone holder  30  and hence on a finishing stone  28 . 
         [0116]    A conventional oscillatory movement generated by a conventional oscillatory drive (in indicated  FIG. 17  by a larger double-headed arrow  64 ) can also be superimposed on the additional movement  162 . 
         [0117]    In the device  10  illustrated in  FIGS. 18 and 19 , an additional movement  162  of the active area  34 , which is tangential to the workpiece surface  24 , is superimposed on the rotary movement of the workpiece surface  24  relative to the active area  34  of the finishing tool  26 . For this purpose, an additional drive  36  in the form of a piezoelectric actuator may be provided, which drives a finishing stone holder  30  with a movement aligned with the additional movement axis  32 . A conventional oscillatory movement parallel to the workpiece axis  20  may here also be optionally provided (see double-headed arrow  64  in  FIG. 19 ). 
         [0118]    In a conventional finishing process known in the prior art, an active component of the active area  34  of a finishing tool  26 , for example a grain, produces a sinusoidal active line  164  extending around the workpiece axis  20  on the workpiece surface  24 , as shown in  FIG. 20 . The device  10  shown in  FIGS. 18 and 19  is capable of producing a generally sinusoidal active line  166 , which is different from the active line  164  in that it is wavelike on a smaller scale. The active line  166  is essentially composed wave segments oriented along the course of active line  164 . 
         [0119]    When using a device  10  according to  FIG. 17 , an active line  168  different from the active line  164  can be produced, which has a coarse path similar to that of the active line  164 , but has wave segments oriented substantially perpendicular to the course of active line  164 . 
         [0120]    While the invention has been illustrated and described in connection with currently preferred embodiments shown and described in detail, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit and scope of the present invention. The embodiments were chosen and described in order to explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. 
         [0121]    What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims and includes equivalents of the elements recited therein: