METHOD AND DEVICE FOR FINISHING A WORKPIECE SURFACE

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

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

The present invention relates to a method for finishing a workpiece surface with a finishing tool.

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.

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.

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.

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

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.

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.)

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

The device according to the invention shares the advantages described above in conjunction with the inventive method.

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.

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.

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.

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.

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.

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.

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.

Within the context of the invention, the finishing tool may be constructed as a finishing stone.

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.

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.

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.

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

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:

The drawings show in:

FIG. 1 a side view of an embodiment of a device according to the present invention for finish-machining a workpiece surface;

FIG. 2 an enlarged detail marked in FIG. 1 with II;

FIG. 3 a front view of the detail of FIG. 2;

FIG. 4 a detail corresponding to FIG. 2 of another embodiment of a device for finish-machining a workpiece surface;

FIG. 5 a view of the detail of FIG. 4 corresponding to the view of FIG. 3;

FIG. 6 a schematic view of a workpiece surface produced with a conventional finishing process;

FIG. 7 a schematic view of a workpiece surface produced with a finishing process according to the present invention;

FIG. 8 a perspective view of another embodiment of a device for finish-machining a workpiece surface;

FIG. 9 a perspective view of another embodiment of a device for finish-machining a workpiece surface;

FIG. 10 side view of a part of the device designated FIGS. 8 and 9 with VI, VII;

FIG. 11 a detail marked in FIG. 10 with XI in an enlarged scale;

FIGS. 12-16 side views of embodiments of pressing shells for use in devices according to FIGS. 8 to 11;

FIG. 17 a side view of an embodiment of a device for finish-machining a workpiece surface;

FIG. 18 a plan view of another embodiment of a device for finish-machining a workpiece surface;

FIG. 19 a side view of the device according to FIG. 18; and

FIG. 20 a schematic view of a workpiece surface produced with the devices according to FIGS. 17 to 19.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

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.

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.

The workpiece 18 has a central workpiece axis 20. The workpiece 18 is, for example, a bearing ring.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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).

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.

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.

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.

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.

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.

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.

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.

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.

The bearing portion 106 extends substantially in a plane perpendicular to the workpiece axis 20.

The bearing member 106 has an opening 108 through which the pivot bearing 98 passes.

The bearing member 106 has a bearing portion end 110 facing the workpiece 18 for arranging a pressing device 112.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

In an extreme situation, the entire pressing surface 140 may be formed by the pressing section 146, which is illustrated in FIG. 14.

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.

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.

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.

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.

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).

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.

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.

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).

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.

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.

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.

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:

Claims

1. A method for finish-machining a workpiece surface with a finishing tool, comprising:

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.

2. The method of claim 1, wherein the oscillation frequency of the additional movement is lower than 16 kHz.

3. The method of claim 1, wherein the oscillation frequency of the additional movement is lower than 1 kHz.

4. The method of claim 1, wherein the oscillation frequency of the additional movement is greater than 50 Hz.

5. The method of claim 1, wherein the oscillation frequency of the additional movement is greater than 100 Hz.

6. The method of claim 1, wherein an amplitude of the additional movement is at least about 5 micrometer and at most about 200 micrometer.

7. The method of claim 1, wherein the workpiece surface and the active area are not moved relative to each other in a direction parallel to the workpiece axis.

8. The method of claim 1, wherein the workpiece surface and the active area are moved back and forth in a direction parallel to the workpiece axis.

9. The method of claim 8, wherein the oscillation frequency of the back-and-forth movement in the direction parallel to the workpiece axis is at least about 1 Hz.

10. The method of claim 8, wherein the oscillation frequency of the back-and-forth movement in the direction parallel to the workpiece axis is at most about 50 Hz.

11. The method of claim 8, wherein the oscillation frequency of the additional oscillatory movement is greater by a factor of 1 to 1000 than an oscillation frequency of the back-and-forth movement in the direction parallel to the workpiece axis.

12. The method of claim 8, wherein the oscillation frequency of the additional oscillatory movement is greater by a factor of 6 to 40 than an oscillation frequency of the back-and-forth movement in the direction parallel to the workpiece axis.

13. The method of claim 8, wherein an amplitude of the back-and-forth movement in the direction parallel to the workpiece axis is between about 0.1 mm and about 3 mm.

14. The method of claim 8, wherein an amplitude of the additional movement is smaller by a factor of 5 to 600 than an amplitude the back-and-forth movement in the direction parallel to the workpiece axis.

15. The method of claim 8, wherein an amplitude of the additional movement is smaller by a factor of 10 to 20 than an amplitude the back-and-forth movement in the direction parallel to the workpiece axis.

16. A device for finish-machining a workpiece surface, comprising:

a finishing tool comprising 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.

17. The device of claim 16, wherein the additional drive comprises a piezoelectric actuator.

18. The device of claim 17, wherein the piezoelectric actuator is aligned along an additional movement axis and comprises piezoelectric elements which are stacked along the additional movement axis.

19. The device of claim 16, wherein the finishing tool is constructed as a finishing stone.

20. The device of claim 16, wherein the finishing tool is constructed as a finishing belt, the dice further comprising a pressing shell for pressing the finishing belt against the workpiece surface, wherein the pressing shell has a pressing surface operating transversely to a running direction of the finishing belt, wherein at least a portion of the pressing surface is formed by a pressing section that is movable relative to a stationary shell section along an additional movement axis.

21. The device of claim 20, wherein the pressing section and the stationary shell section are integrally formed with each other and interconnected via a connecting portion, wherein the connecting portion is constructed such that driving forces of the additional drive cause the pressing section to move along the additional movement axis.