Finishing method and finishing device for finish machining of rolling element raceways

Abstract

The invention relates to a finishing method for finish machining of a workpiece having at least one raceway for rolling elements that extends around a workpiece axis, the method comprising the following steps: providing a finishing tool which has an abrasive working surface; applying the finishing tool to a workpiece holder; generating spinning of the workpiece about a raceway axis at a rotational frequency; generating an oscillating movement of the tool holder, which is superimposed on the spinning of the workpiece and oscillates at an oscillating frequency; and pressing the finishing tool onto the workpiece surface in a pressing direction with a pressing force so that the abrasive working surface acts on the workpiece in the region of the raceway. The finishing method is characterised by continuous detection of a force signal, which is caused by the pressing force, within a detection time period which partly or completely overlaps a machining time period of the finish machining, and by time-resolved evaluation of the force signal in order to determine at least one geometry parameter which represents (i) a workpiece geometry in the region of the machined raceway and / or (ii) a geometric relationship between the workpiece geometry in the region of the machined raceway and a position of an oscillating apparatus.

Description

[0001] Finishing processes and Finishyoi MBH

[0002] FIELD OF APPLICATION AND STATE OF THE ART

[0003] The invention relates to a finishing method for finishing workpieces having at least one raceway for rolling elements rotating around a raceway axis, and to a finishing device suitable for carrying out the finishing method.

[0004] Finishing, also known as superfinishing, microfinishing, or short-stroke honing, is a machining process with geometrically undefined cutting edges according to DIN 8580. One characteristic is the use of multi-edged tools (finishing tools) that have cutting grains bound in a bond and execute a cutting motion, often consisting of two superimposed speed components, relative to a moving workpiece. Relatively speaking, one component of the cutting motion always alternates or oscillates, so that the machined surface exhibits defined cross-cut marks (“cross-cutting”). Depending on the type of finishing tool (finishing stone or finishing belt), a further subdivision into stone finishing and belt finishing is common. A further classification is made according to the form elements to be machined.

[0005] Finishing is used extensively in the manufacture of components for rolling bearings, among other things. Rolling bearings are the connecting component between a rotating part (shaft, axle) and a stationary part (housing, frame). The main components of a rolling bearing are an outer ring, which has a rotating raceway on its inside, an inner ring, which has a rotating raceway on its outside, a cage, and the rolling elements. The rolling elements are located between the two rings and provide rolling friction. They roll along the raceways with point or line contact. The cage ensures that the rolling elements are evenly distributed in the bearing. The names of the bearing types are derived from the shape of the rolling elements (e.g. ball bearing, needle bearing, cylindrical roller bearing, spherical roller bearing, spherical roller bearing, tapered roller bearing).Each of these rolling bearings can absorb different forces in the axial and radial directions depending on the type of rolling elements and the cross-sectional profile of the rolling element raceways.

[0006] In ball bearings, the raceways for the balls are usually referred to as ball raceways. Many ball raceways are designed as grooves running around the circumference of a workpiece with a cross-sectional profile that is partially or completely circularly curved. In a standard ball bearing, closed, annular ball raceways are formed on the outside of the inner rings and on the inside of the outer rings. The cross-sectional profile is usually circular, e.g., approximately semicircular. Special designs can also have ring halves split perpendicular to the axis of rotation and are offered, for example, as so-called 4-point contact bearings, to increase the axial force absorption capacity.

[0007] Ball raceways are also found on ball screws. A ball screw, also known as a recirculating ball screw, is a helical gear containing inserted balls to transmit the force between the ball screw spindle (threaded rod) and the ball screw nut. Both parts each have a helical groove that acts as a ball raceway and together form a helical tube filled with balls. The ball raceways of ball screws often have a Gothic profile (also called a pointed arch profile), consisting of two circular arc-shaped sections that converge at an angle in the bottom area of ​​the groove and each have their own radius center. This, as with 4-point bearing designs, achieves increased axial load capacity.

[0008] The term ‘substantially semicircular cross-section’ used in this application is intended to cover more or less semicircular cross-sections and, more generally, continuously circularly curved cross-sectional profiles and Gothic cross-sectional profiles.

[0009] Ball raceways on inner or outer rings of ball bearings and other similar profiles, such as ball raceways on ball screws for a ball screw drive, are machined with finishing tools that oscillate back and forth around a pivot axis at a high oscillation frequency while the workpiece rotates around its workpiece axis. A finishing stone is usually used as the finishing tool here, but finishing tape can also be used if necessary. The pivot axis is close to the contact area between the workpiece surface and the finishing tool. Ideally, the pivot axis should be at or near the center of curvature of the groove profile to be machined. The combination of the oscillating pivoting movement of the finishing tool and the natural rotation of the workpiece around the raceway axis results in an optimal workpiece surface.

[0010] Document DE 10 2014 222 848 B4 describes a finishing machine designed for such machining processes. One problem when finishing ball bearing rings is the so-called “groove offset”, i.e. the tolerance of the raceway center (center of the ball raceway in the axial direction of the workpiece) to the ring contact side, i.e. to a flat surface or another reference surface that serves as the axial reference surface when clamping the workpiece. In practice, attempts are made to keep this tolerance as small as possible through pre-machining (usually by grinding). Nowadays, with correspondingly high expenditure of effort, position accuracies in the range of 5 pm to 10 pm can be achieved in series production. However, if the finishing tool in a finishing machine is to be used to improve the geometry in the micrometer range or even smaller, a groove offset is a hindrance.

[0011] This problem can be at least partially compensated for by using relatively soft finishing tools that can quickly adapt to the workpiece geometry. Flexible finishing tool holders, which can twist, can also be used. However, neither of these measures is conducive to high geometric quality.

[0012] Patent specification DD 234 388 A1 describes various other approaches to dealing with groove misalignment. It proposes a pneumatic adjustment device for adjusting the axial and radial position of a fine drawing grinding tool and its oscillation axis on fine drawing grinding machines for machining inner and outer rings of rolling bearings. The pneumatic adjustment device is based on the nozzle-baffle plate principle and is characterized by a measuring head clamped into the tool holder for a fine drawing grinding stone. The measuring head has a doubly curved end face similar to the raceway of a rolling bearing ring. Four measuring nozzles are incorporated into the end surface. The measuring nozzles are positioned symmetrically to the end face center on the end face's symmetry axes. Recesses are incorporated into the end face to divert the air escaping from the measuring nozzles.This allows the axial and radial position of the fine drawing tool to be adjusted exactly to the groove center before finishing begins.

[0013] Document DE 30 30 703 A1 discloses devices for checking the dimensions of the race of a bearing ring using measuring sensors.

[0014] Ball screws usually require optimization with regard to wear and noise emissions. Due to the relatively low load compared to continuously rotating rolling bearings, wear inhibition is achieved through comparatively minimal material removal (removal of the soft skin). However, noise behavior is significantly influenced by the micro- and macrogeometry of the raceway profile. Pitch errors can also occur.

[0015] TASK AND SOLUTION

[0016] Against the background of this prior art, it is an object of the invention to provide a finishing method and a finishing device for a finishing process which allow the raceways of workpieces which have at least one raceway for a rolling element rotating around a raceway axis to be finished within short cycle times with high geometric precision and high surface quality.

[0017] To achieve this object, the invention provides a finishing method having the features of claim 1. Furthermore, a finishing device suitable for carrying out the finishing method is provided, having the features of claim 10. Advantageous further developments are specified in the dependent claims. The content of all claims is incorporated into the description by reference.

[0018] In the finishing process, a workpiece that has at least one raceway for rolling elements (rolling element raceway) running around its raceway axis is finely machined using the finishing or superfinishing machining process. This preferably involves the finish machining of the area of ​​a ball raceway that is to serve as a rolling surface for balls. Ideally, balls should only have point contact with the rolling surface. For this machining task, finishing tools in the form of finishing stones are usually used. These have an abrasive working surface intended for engagement with the workpiece. Their shape can be approximated to the shape of the workpiece through pre-machining. Such finishing stones can adapt well to the ball raceway to be machined after a short machining time due to possibly uneven abrasion.

[0019] In some cases, finishing tools in the form of a finishing belt can also be used, which is pressed onto the workpiece surface using a pressing device that may be adapted to the workpiece geometry.

[0020] When setting up the finishing device, the finishing tool, e.g. a finishing stone, is attached to a tool holder of the finishing device. The tool holder has a corresponding finishing tool holder at an end close to the workpiece, on or in which the finishing tool can be clamped or attached in some other way and, if necessary, fixed or guided so that it can move. For material removal, the workpiece rotates naturally around a track axis. The track axis is the axis leading through the center of the rotating track. For this purpose, the finishing device has a workpiece holder with an associated rotary drive. The natural rotation occurs at a predeterminable rotational frequency or speed, which should be essentially constant at least in phases, around a workpiece rotation axis that ideally coincides with the track axis.For completely rotationally symmetrical workpieces, the raceway axis coincides with their symmetry axis. However, a raceway axis can also be offset parallel to a workpiece axis.

[0021] Furthermore, an oscillating motion of the tool holder or the finishing tool held therein is generated, superimposed on the workpiece's rotation. This oscillating motion occurs at a predefined oscillation frequency, which should be as constant as possible, at least in phases. If the oscillation frequency varies, the variance should be known.

[0022] The oscillation movement can, for example, be an oscillating pivoting movement of the tool holder or the finishing tool held therein around a pivot axis, with the pivoting movement occurring at a predefined pivoting frequency. This can be particularly useful when machining ball raceways with a substantially semicircular cross-sectional profile.

[0023] If the finishing device is properly set up, the pivot axis should pass more or less exactly through a center of curvature of the essentially semicircular profile of the ball raceway.

[0024] The oscillation movement can also be a linear movement of the tool holder or the finishing tool held therein, oscillating back and forth at an oscillation frequency along an oscillation axis. This can be particularly useful when machining raceways with cylindrical or frustoconical sections for roller-shaped or conical rolling elements.

[0025] To generate the oscillating movement back and forth over a limited angular range or over a limited travel range, an oscillation device with a controllable oscillation drive is provided. To generate cutting force for material removal during operation, the finishing tool, in particular the finishing stone, is pressed against the workpiece surface in the area of ​​the track with a pressing force in a pressing direction such that the abrasive working surface in the area of ​​the track can engage under an effective force. To press it against the effective point, the finishing device has a pressing device which generates the pressing force. The pressing direction of the finishing tool runs perpendicular to the pivot axis during a pivoting movement oscillating about a pivot axis and oscillates synchronously with the tool holder about the pivot axis.In an oscillating linear movement, the pressing direction of the finishing tool runs perpendicular to the straight movement axis and oscillates synchronously with the tool holder.

[0026] In a finishing method according to the claimed invention, a force signal caused by the contact force is continuously recorded at least during a recording period that partially or completely overlaps with the finishing processing period. The force signal can be recorded during the entire finishing process or within a shorter time window that at least partially overlaps with the processing time. The recording and evaluation of the force signal can begin before the finishing tool has been advanced to the engagement position, i.e., at a time when no contact force is yet acting. The force curve upon reaching workpiece contact during the advance can then also be recorded and analyzed.

[0027] The finishing device has at least one force transducer for measuring force and generating a force signal, which is also referred to synonymously as a force sensor in this application. The force transducer can, for example, have at least one strain gauge (DMS), which can be attached, for example, by adhesive, to an elastically deformable section of a part located in the force flow. Alternatively or additionally, a piezo force transducer can be used to generate the force signal.

[0028] The term "force signal" here refers to a preferably electrical signal that has a defined functional relationship with a state detected by the force transducer or force sensor. The force signal can, for example, have an amplitude that is essentially proportional to the force or a force component that acts in a sensitivity direction of the force transducer. It can be transmitted electrically and / or wirelessly from the force sensor for further processing and, if necessary, filtered or smoothed before evaluation as part of signal preparation. The at least one force transducer is preferably arranged in the force flow between the abrasive working surface of the finishing tool and the pressing device. An arrangement at a different location is also possible, e.g.in the power flow between a base frame of the finishing device and the housing of the oscillation drive mounted thereon or between this and the pressing device carried by it.

[0029] A force transducer can be arranged, in particular, in the area of ​​a finishing tool holder of the tool holder, between the finishing tool (especially the finishing stone) and the tool holder. The arrangement of a force transducer between a finishing tool and the tool holder offers, among other advantages, that essentially only the rigid or stiff material of the finishing stone is located between the effective point and the force sensor, so that the forces and moments relevant for the measurement are transmitted to the force sensor over a short distance and largely undamped.

[0030] Alternatively or additionally, a force transducer can be installed, for example, on or in a component that is subjected to bending and / or torsion by the machining forces during machining. Forces caused by elastic deformation act on the force sensor. If necessary, both tensile and shear forces can be measured due to elastic deformation. A force transducer can, for example, be arranged on or in the tool holder between the finishing tool holder and the connection to the drive device. Depending on the type and arrangement of the force transducer, shear forces can also be measured. This is particularly important if, due to design reasons, the contact pressure cannot be applied perpendicular to the workpiece's track axis, or if compliance cannot be compensated for using proportional factors.

[0031] A force transducer measures the forces acting on it. Single-component force transducers exist, which have a single sensitivity direction, meaning they can only measure forces or force components acting in this sensitivity direction. A single-component force transducer can be used to measure forces, e.g., on or in a section of a component subject to bending and / or torsion. In some cases, it can be advantageous to use a multi-component force transducer, which has two or three or more transverse, particularly orthogonal, sensitivity directions. This enables multi-dimensional force measurement, which provides even more precise and easier-to-interpret information about spatial force distributions and their process-related changes.A group of two, three or more separate force transducers can also be provided, which are preferably installed with two, three or more different sensitivity directions. The force signal is evaluated with time resolution. This evaluation is used to determine at least one geometric parameter. A geometric parameter in this sense is a parameter that is largely determined by the workpiece geometry in the area of ​​the machined raceway. A geometric parameter can be exclusively workpiece-related and represent the workpiece geometry, i.e. the macroscopic shape of the workpiece in the area of ​​the machined raceway. The workpiece geometry can be described using coordinates of the workpiece-fixed workpiece coordinate system. A geometric parameter can, for example, describe the type and extent of any waviness of the raceway in the circumferential direction.

[0032] Alternatively or additionally, a geometry parameter can represent a geometric relationship between the workpiece geometry in the area of ​​the machined raceway and a position of the oscillation axis (swivel axis or translational axis). This allows a relationship to be established between the workpiece coordinate system and the machine coordinate system. This allows, for example, an automatic centering operation to be performed to center the finishing tool with respect to a raceway, in particular a groove or a ball raceway.

[0033] It is important that the temporal progression of the force signal is recorded and evaluated, i.e., the progression of the force signal as a function of time. This temporal dependence of the force signal is then analyzed and used to determine at least one geometry parameter. The geometry information is thus derived from information about whether and, if so, how the force signal changes over time.

[0034] The invention is based, among other things, on the finding that time-resolved detection and analysis of force signals makes it possible to determine geometric information about the machining process and / or the relative positioning of the workpiece to the finishing device and / or about the workpiece itself. This makes it possible, for example, to determine the position of the finishing tool and / or the pivot axis and / or a translational axis relative to the workpiece, in particular the axial position, i.e., the position in the axial direction of the workpiece.

[0035] This allows, for example, the extent of any axial offset of the pivot axis's axial position relative to the axial position of the ball track being machined to be quantitatively determined when machining ball tracks. Alternatively or additionally, any geometric deviations present on the workpiece can also be determined and evaluated, for example, waviness on the circumference of the machined workpiece in the area of ​​a track.

[0036] The continuous recording of the contact force can also be used to determine the force that is decisive for material removal and acts in the contact area between the abrasive work surface and the workpiece.

[0037] Important for understanding the invention is that statements about the time dependence of force signals can be determined, which is also possible without determining or using the absolute values ​​of the force. The basis is therefore an analysis of force curves of possibly changing forces or corresponding force signal curves in the time domain.

[0038] The invention thus enables a quantitative determination of geometric properties of the workpiece and / or geometric relationships for the relationship between the machine coordinate system and the workpiece coordinate system on the basis of a force measurement carried out over a certain period of time and the information on force changes derived therefrom.

[0039] In preferred embodiments, in addition to the time-resolved evaluation of the force signal, a spatially resolved evaluation of the force signal is performed by assigning force signals detected at successively traversed locations on a workpiece surface during machining to corresponding spatial coordinates in a workpiece coordinate system. This makes the workpiece itself the observation system. With the aid of an additional spatially resolved evaluation, it is possible to significantly improve the significance of the measurement, prevent the generation of pseudo-errors, and thus ensure traceability of the measurement results in terms of metrology.

[0040] In order to achieve a spatially resolved evaluation, in addition to the force signal, time-accurately associated encoder signals of the rotary drive of the workpiece holding device as well as encoder signals of the swivel device or encoder signals of a translatory oscillation device and, if necessary, position encoder signals of a translatory machine axis for displacements parallel to the workpiece rotation axis can be evaluated.

[0041] According to a further development, a force-controlled axial position control is implemented. In this case, an axial relative position between the swivel axis or the swivel device or another oscillation device and the workpiece is changed as needed depending on the at least one geometric parameter or the force signal by displacing the workpiece and / or a swivel device or oscillation device carrying the tool holder axially, i.e., in a direction parallel to the workpiece rotation axis, depending on the force signal. An operator can receive corresponding setting instructions for this purpose. The force-controlled axial position control preferably functions automatically, i.e., without operator intervention.

[0042] For this purpose, it is preferably provided that the control unit of the finishing device is configured to control at least one controllable device of the device as a function of the (at least one) force signal or the geometric parameter derived therefrom. In this way, knowledge of the at least one geometric parameter can be used to directly influence the machining process specified by the machine control system. This enables controlled finishing operation or sensitive finishing operation, in which machining parameters of the finishing device can be automatically adjusted or changed depending on the current force ratios in the engagement area between the finishing tool and the workpiece.

[0043] According to the above-mentioned development, it can be provided that the controllable device generates an automatic change in an axial relative position between the pivot axis or the pivot device or another oscillation device and the workpiece as a function of the at least one geometric parameter by displacing the workpiece and / or the finishing device in a direction parallel to the workpiece axis. If necessary, e.g., when machining angular contact ball bearings or axial ball bearings, the direction of displacement between an oscillation axis and the workpiece can also run obliquely or perpendicularly to the workpiece axis.

[0044] In one embodiment of the finishing device, it comprises a linear machine axis for changing an axial relative position between the pivot axis (or another oscillation axis) and the workpiece in a displacement direction parallel to the workpiece axis, and the control unit is configured in an operating mode to control this machine axis as a function of the geometry parameter. A suitable numerically controlled machine axis is already present in many conventional finishing devices, e.g. in order to be able to adapt the device to ball bearing components of different diameters and / or to different raceway positions via control signals from the control unit, or, in the case of finishing devices for machining ball screw components, for continuously changing the axial relative position when following the pitch of the helical course of the ball raceway during finishing.This translational machine axis can thus be used for an additional functionality, namely a force-controlled position control for the axial position.

[0045] In preferred embodiments with a swivel device, the axial position of the swivel axis relative to the machined ball track is automatically centered or optimized based on the geometry parameter. Automatic centering can be achieved by changing the axial relative position between the workpiece and tool by moving the workpiece and / or the swivel device parallel to the workpiece axis. For this purpose, the linear machine axis described above can be used to change the axial relative position between the swivel axis and the workpiece in a displacement direction parallel to the workpiece axis.

[0046] In this way, for example, the problem of groove misalignment mentioned in the introduction can be mitigated without operator intervention. A device according to this embodiment independently "finds" the groove center based on force detection and time-resolved analysis of the force profile in the initial phase of machining and can thus operate with high precision almost from the very beginning, regardless of the original groove misalignment. Compromises regarding tool hardness and / or deliberately introduced flexibility in the area of ​​the components supporting the finishing tool are then no longer necessary.

[0047] Such a finishing device thus has devices for automatic groove center detection.

[0048] This allows the use of finishing tools (finishing stones or finishing bands) with hard bonds and correspondingly reduced tool wear, and the tool holder does not require any flexibility, but can be designed to be ideally rigid. This creates important prerequisites for a significant improvement in workpiece quality.

[0049] Additionally, setting up the finishing fixture saves time, as the finishing tool only needs to be roughly positioned relative to the workpiece so that it is positioned in the area of ​​the ball track and can "find" it automatically. Any deviation (groove misalignment) is then automatically corrected based on the determined geometry parameters. When machining ball screws, etc., this centering functionality can be used to detect any pitch errors and automatically compensate for them during the process. This also provides a device for compensating pitch errors on ball screws.

[0050] Such a finishing device thus has devices for detecting and compensating pitch errors on ball screws.

[0051] Some preferred designs of the evaluation or evaluation unit are explained below.

[0052] During a real machining process, a force sensor not only records the forces and force changes that are due to the machining process and are to be measured, but also forces and force changes that are due to processes in the environment and, if applicable, special features in the finishing machine that can influence the force signal and make it difficult to interpret.

[0053] Within the scope of the time-resolved evaluation, preferred embodiments provide for a frequency-specific evaluation of the force signals, preferably at different intervals of the oscillation periods. "Frequency-specific evaluation" means, in particular, that the force signals (if necessary after appropriate filtering during signal preparation) are analyzed for signal components that can be assigned to specific frequencies. The further steps of the analysis are then performed on those signal waveform components that, based on their frequencies, likely describe the desired object of observation.

[0054] In preferred embodiments, spectral distribution function methods, or at least several sectionally evaluated discrete Fourier transform (DFT) data, are used for the evaluation. With a DFT, a time-discrete signal, in this case the time-dependent force signal, can be decomposed into its frequency components and thus analyzed section by section to evaluate changes depending on the workpiece and the tool engagement conditions. This allows ambiguities in the evaluation to be avoided or significantly reduced.

[0055] In preferred embodiments, a fast Fourier transform (FFT) method is used for the evaluation. Using a fast Fourier transform, a time-discrete signal, in this case the time-dependent force signal, can be broken down into its frequency components and analyzed. Thus, during the evaluation of the time-dependent force signal, it can be broken down into its frequency components, and those components corresponding to the swivel frequency and / or the rotation frequency can be further processed to determine the geometry parameter. Other signal components that may interfere and / or distort the results are explicitly excluded from the evaluation using appropriately adapted filter methods.

[0056] Accordingly, the evaluation unit of preferred embodiments includes an FT module for determining a signal waveform component of the force signal that essentially varies periodically with the pivoting frequency and / or the rotational frequency, as well as modules for analyzing this signal waveform component. The abbreviation "FT module" here stands for a sub-module of the evaluation implemented with appropriate software, which can perform Fourier transformations of the initially time-dependent force signal.

[0057] Some embodiments are characterized in that the evaluation of the force signal comprises determining a signal curve component of the force signal that varies substantially periodically with the oscillation frequency, in particular with the pivoting frequency, and a (time-resolved) analysis of this signal curve component. This is based, among other things, on the consideration that recurring rectified signals that can be assigned with a certain degree of probability to the same location on the workpiece are highly likely to be attributable to a geometric deviation of the workpiece from the target geometry. Other signal curve components, whose temporal progression does not correlate with the pivoting frequency or another process-specific oscillation frequency, may have other causes.

[0058] In the case of a pivoting oscillatory movement, the pivot position is preferably also recorded, so that it is always known for the evaluation whether a special feature in the signal curve is generated during clockwise or counterclockwise pivoting. Analogously, the position of a linear oscillatory movement can also be recorded and taken into account.

[0059] Particularly meaningful results can be achieved according to an embodiment with oscillating pivoting movement or linear oscillation movement by determining at least one asymmetry parameter (asymmetry indicator) during the analysis, which represents a systematic asymmetry of the signal curve component with respect to a zero point position of the oscillating movement (pivoting movement or linear movement) of the finishing tool. This can be used, for example, to detect whether and, if so, to what extent the pivot axis is not sufficiently centered over the center of the ball track, so that the finishing tool is systematically pressed more strongly against one of the flanks of the groove than against the opposite flank when pivoting back and forth.This becomes noticeable in a characteristic asymmetry of the periodically varying signal waveform component, can be quantitatively analyzed and used as a basis for automatic centering of the swivel axis by changing the axial relative position between the swivel axis and the workpiece in such a way that the asymmetry is reduced, ideally until no asymmetry is present anymore.

[0060] If it is possible to assign the asymmetry characteristics to the swivel position, a compensating movement can be guided in the right direction from the beginning, which is beneficial for the speed of the compensation.

[0061] Similarly, when machining a tapered roller bearing ring using a linear oscillating finishing tool, an incorrectly set angle could be detected.

[0062] It is also possible to determine geometric information about the workpiece based on the workpiece's rotation using force detection. In some embodiments, the evaluation includes determining a signal curve component of the force signal that essentially varies periodically with a rotational frequency of the workpiece's rotation, and analyzing this signal curve component. Preferably, this can, for example, determine a geometric parameter in the form of a waviness parameter. The term "waviness" here refers to deviations of the macroscopic cross-sectional shape from a circular shape. Smaller-order wavinesses such as ovality (two-wavelength), three-wavelength, or four-wavelength may be of interest here. However, higher-order wavinesses can also be detected, e.g., fine wavinesses with ten or more waves, or fifteen or more waves on the circumference. Such fine wavinesses can, for example,be important with regard to the problem of noise generation.

[0063] In preferred embodiments, the rotational position of the workpiece is determined during its own rotation, and a location-dependent analysis of the force signal is carried out based on the rotational position. If the rotational position of the workpiece is known, the force signals can be assigned to the corresponding locations on the circumference of the workpiece. This means that, for example, if waviness is detected, not only the number of wavinesses but also their position in the circumferential direction with respect to a reference rotational position of the workpiece can be determined. This makes it possible to approximately unambiguously assign geometric features to a location along the circumference. Alternatively or additionally, if a ball track follows a helical path, the axial position of geometric features can also be determined.

[0064] In some embodiments, at least one operating parameter of the device is changed based on the at least one geometry parameter. If, for example, the analysis based on force measurement shows that there is no waviness outside the tolerances in the circumferential direction, even though the intended maximum machining time has not yet been reached, this can be used to determine whether and, if so, when a good part will be produced before the assumed end of the process. For example, the cycle time can be optimized individually for each workpiece if necessary. In some cases, the contact time can be extended until the target value, for example for the waviness, is reached. However, it is also possible to end a machining process prematurely and thereby shorten the cycle time if the analysis determines that the workpiece geometry is already within the tolerance.

[0065] BRIEF DESCRIPTION OF THE DRAWINGS

[0066] Further advantages and aspects of the invention emerge from the claims and from the description of embodiments of the invention, which are explained below with reference to the figures.

[0067] Fig. 1 shows an oblique perspective overview of a finishing machine for machining annular ball bearing components according to an embodiment;

[0068] Fig. 2 shows a schematic view of the engagement area between workpiece and finishing stone in the area of ​​a ball track;

[0069] Fig. 3A and 3B show a pivoting oscillation movement with ideal positioning of the pivot axis of the finishing tool in the groove center;

[0070] Fig. 4 shows schematic force-time curves for the machining operations in Fig. 3A and 3B;

[0071] Fig. 5 schematically shows a configuration with groove offset; Figs. 6A and 6B show a pivoting oscillation movement with non-ideal positioning of the pivot axis of the finishing tool with an offset from the groove center;

[0072] Fig. 7 shows schematically an asymmetric force-time curve for the processing in Figs. 6A and 6B;

[0073] Fig. 8A shows a schematic time-based representation of an unfiltered force signal curve (top) and a filtered force signal curve (bottom), Fig. 8B shows a frequency-based representation of the signal curves in Fig. 8A;

[0074] Fig. 9A and 9B show an application of a frequency-specific evaluation for the case of detection of a groove offset, where Fig. 9A shows the theoretical ideal case without center offset and Fig. 9B shows corresponding diagrams for a situation with groove offset;

[0075] Fig. 10 shows an oblique perspective overview of a finishing machine for machining workpieces with helically rotating ball tracks (e.g. ball screws or steering racks) according to an embodiment;

[0076] Fig. 11 shows an embodiment with a force transducer arranged between the finishing stone and the tool holder;

[0077] Fig. 12A and 12B schematically show the finishing of an inner ring of an angular contact ball bearing and in 12B the finishing of an inner ring of a roller bearing.

[0078] DETAILED DESCRIPTION OF THE EMBODIMENTS

[0079] Fig. 1 shows an oblique perspective overview of an embodiment of a finishing machine 100 which is designed and equipped for machining rolling element raceways in the form of ball raceways on components of ball bearings.

[0080] The finishing machine 100 is a computer-numerically controlled precision machining machine with several controllable machine axes, a drive system with several, partly electric drives for driving the machine axes, and a control device 200 for the coordinated control of working movements or other actions of the machine axes. To illustrate the structure and function, a rectangular machine coordinate system MK, labeled with lowercase letters x, y, and z, with a vertical z-axis and horizontal x- and y-axes is shown. The machine axes, which are explained below, must be distinguished from the machine-fixed coordinate axes. The term "machine axis" refers to a movable device that can be moved in at least one mechanical degree of freedom by at least one drive, e.g., an electromechanical, electrohydraulic, or electropneumatic drive.It can be a translatory machine axis, which moves a linearly movable carriage, for example, or a rotary machine axis, for example with a spindle that can rotate about a spindle axis. A machine axis can move either a tool or a workpiece. Computer-numerically controlled machines are also referred to as NC axes. Preferably, these are machine axes that can be operated under closed-loop control. For this purpose, encoder systems (e.g. position encoders on translatory machine axes, rotary encoders for rotary machine axes) are present, which can report the current position of a machine axis to the control unit 200 via a feedback channel using corresponding encoder signals, so that machine data on the current status of the monitored and controlled components of the finishing device are available in the control unit at all times.

[0081] In the example, the workpiece 110 is an inner ring for a ball bearing. On the radial outer side of the ring-shaped tool body, a circumferentially closed, ring-shaped groove 115 is formed with an essentially semicircular cross-sectional profile. More precisely, the groove has a continuously circular, concave cross-sectional profile that forms slightly less than an exact semicircle. The center of curvature KM lies outside the workpiece, central to the groove (cf. Fig. 2). The groove is intended to serve as the ball raceway 115 of the ball bearing and was pre-machined in a preceding process before the start of finishing, e.g. by grinding or hard turning, in order to specify the macro shape. The aim of finishing is essentially to improve the surface quality of the ball raceway, among other things to increase the contact area of ​​the roughness profile, and if necessary,minor improvements in macro shape (especially roundness, waviness and transverse shape) can also be achieved.

[0082] The workpiece 110 is rotationally symmetrical with respect to its workpiece axis 112 and is clamped with the workpiece axis 112 vertically aligned on the flat upper side of a workpiece holding device 120, which has a centering mandrel. The central axis of the circular ball track 115, referred to here as the track axis, coincides with the workpiece axis 112. The workpiece rests on the flat upper side with a lateral flat surface 113. This flat surface of the workpiece serves as a reference surface for the finishing device 100 or the finishing machine. The workpiece holding device can be rotated indefinitely about a vertical rotation axis ROT oriented parallel to the z-direction using an electric motor-driven rotary drive (servo motor). The speed or rotational frequency and the direction of rotation of this rotary machine axis can be specified by the control unit 200 based on operator inputs.

[0083] There are also non-rotationally symmetrical workpieces, where the center axis of the rolling element raceway (i.e., the raceway axis) does not coincide with a workpiece axis. Generally, the workpiece is mounted so that the center of the rotating rolling element raceway lies on the rotation axis of the workpiece holder.

[0084] A finishing tool 180 in the form of a finishing stone 180 is used for machining. The finishing stone 180, formed, for example, from a sintered material, contains a plurality of abrasive grains, which in the example case are homogeneously distributed within a matrix of a binder. Abrasive grains can consist, for example, of corundum or silicon carbide; if necessary, they can also be diamond grains or grains of cubic boron nitride (CBN). A ceramic or metallic material, for example, can be used as a binder.

[0085] The finishing stone has an abrasive working surface 182 facing the workpiece 110, on which cutting grains still bound in the bond protrude from the bond. The shape of the working surface of a fresh, unused finishing stone can be adapted to the machining task in such a way that a relatively large-area contact with the workpiece is possible quickly during machining. For this purpose, the working surface is convexly curved in a first direction oriented essentially parallel to the workpiece axis 112 or the rotation axis ROT, wherein the radius of curvature is of the order of magnitude of the radius of curvature of the groove formed in the workpiece, but is not identical to it. In a second direction perpendicular to the first direction, which runs tangentially to the workpiece during machining, the abrasive working surface is concavely curved so that it can adapt to the circumferential curvature of the ball raceway.Due to the oscillation movement, additional surfaces quickly form on the finishing tool, which then, together with the main curvatures, form the actual contact surface.

[0086] To hold the finishing stone on the finishing device, a tool holder 170 is provided, which can also be referred to as a stone holder, is designed approximately U-shaped overall and has a tool holder 172 in the form of a clamp holder 172 for holding the finishing stone 180 at the free end of a leg that is directed downwards.

[0087] The other leg is attached by four screws to the front of a pneumatically or hydraulically driven slide 162, which belongs to a linear machine axis of the finishing device. This linear machine axis is an essential component of a pressing device 160, which will be explained in more detail below.

[0088] A base part of the carriage is attached to the end face of the spindle of a servomotor 152, which serves as the oscillation drive 152 of an oscillation device 150. The oscillation movement here is a pivoting movement, therefore the oscillation drive is referred to as the pivot drive 152 and the oscillation device as the pivot device 150. During operation, this generates an oscillating pivoting movement of the stone holder 170 and thus of the finishing tool 180 attached to it about a pivot axis SWA. The horizontal spindle axis of the servomotor, oriented parallel to the y-direction, defines the pivot axis SWA of the finishing device. The pivot drive 152 is designed and controlled in such a way that the stone holder 170 can execute an oscillating pivoting movement about the pivot axis SWA.The zero position of the swivel movement is oriented parallel to the x-direction, the swivel angles in the clockwise and counterclockwise direction are adapted to the machining task and can, for example, be in the range up to approx. ± 20° around the zero position, e.g. approx. + 3° to ± 6° around the zero position.

[0089] The aforementioned carriage 162, which carries the stone holder 170, is a component of a pressing device 160 for pressing the finishing stone 180 against the workpiece 110. The carriage axis defines the resulting pressing direction ADR. The pressing direction is oriented perpendicular to the pivot axis SWA and oscillates with the oscillating movement of the spindle of the pivot drive 152.

[0090] The swivel drive 152 is attached to a vertically movable slide (NC vertical slide) 132 of a linear machine axis 130, which allows the height of the swivel axis SWA to be precisely adjusted in the z-direction by means of control signals from the control unit 200.

[0091] The components of the NC vertical slide are mounted on a slide (horizontal slide) 142 of another linear machine axis 140, which can be moved horizontally parallel to the x-direction. This allows the position of the stone holder or the finishing stone to be changed in the x-direction, particularly to adapt the device to workpieces with different groove diameters. When setting up the finishing machine 100 for a finishing process, a finishing stone 180 is installed in the stone holder using the components of the clamping tool holder 172, and the horizontal and vertical machine axes are moved so that the finishing stone 180 or its abrasive working surface 182 engages the workpiece in the area of ​​the groove to be machined.

[0092] For finishing, the workpiece is then set into a self-rotation around the vertical workpiece rotation axis 112, which here is identical to the center axis of the revolving ball track. This relatively fast rotating workpiece movement (speeds can be in the range of 1000 rpm, for example) 1 up to 6000 u mim 1 A working movement of the finishing tool 180 is superimposed on the workpiece (see figure 1). This tool movement is a back-and-forth oscillating pivoting movement of the stone holder 170 about the pivot axis SWA over a predeterminable angular range around the zero position. The pivoting frequency can, for example, be in the range of several hundred Hz.

[0093] To achieve sufficient material removal, the finishing stone 180 is pressed against the outer surface of the workpiece by means of the pressing device 160 with a pressing force that can be specified via the control device. The pressing force can be nominally essentially constant. Since the pneumatic drive for pressing the finishing stone is attached to the front end of the spindle of the swivel drive 150, the pressing direction ADR, oriented perpendicular to the swivel axis SWA, oscillates together with the finishing stone 180.

[0094] The finishing device is equipped with a force transducer 190 or force sensor 190, which allows the reaction forces generated by pressing the finishing tool 180 against the workpiece in the vicinity of the effective area, i.e., in the vicinity of the contact zone between the workpiece and the finishing tool, to be continuously recorded and corresponding force signals to be generated that represent these reaction forces and are transmitted to the control device. The force transducer is arranged close to the finishing stone 180, between the latter and the tool holder 172 equipped with a clamping device. An example of a clamping holder with a multi-component force transducer connected between the clamping device and the finishing stone will be explained in more detail in connection with Fig. 11. This arrangement can also be provided here. Alternatively, a force transducer, e.g., a single-component force transducer, can be arranged at a different location, e.g.on or in the tool holder or between the oscillation drive and the frame of the device.

[0095] An evaluation device 210 integrated into the control device 200 is configured by appropriate hardware and / or software to receive the force signals from the force transducer in an operating mode and evaluate them with time resolution. The evaluation speed is sufficiently high that a large number of data points can be recorded and processed per oscillation period of the rotary actuator (the time required for a single back-and-forth swivel). The Nyqvist-Shannon sampling theorem is reliably fulfilled; the sampling frequency is usually at least one order of magnitude (a factor of 10) higher than the oscillation frequency.

[0096] The evaluation device 210 is configured, using appropriate software and / or hardware, to determine at least one geometry parameter therefrom. A geometry parameter can, for example, describe information about the workpiece geometry in the area of ​​the machined ball track 115. Alternatively or additionally, a geometry parameter can parameterize a geometric relationship between the workpiece geometry in the area of ​​the machined ball track and the position of the swivel axis SWA. From this, any positioning errors between the workpiece and the tool can be quantitatively recorded and subsequently corrected.

[0097] The following schematic figures are used to provide explanations for a better understanding of the operation of the embodiment and some of its advantages.

[0098] Fig. 2 shows a schematic view of the engagement area between workpiece 110 and finishing stone 180 in the region of a ball track or groove 115 in a viewing direction parallel to the pivot axis SWA of the finishing stone 180 and perpendicular to the rotation axis 112 of the workpiece. Figs. 2, 3A and 3B refer to an ideal case of a perfectly configured finishing device in which the axial position of the pivot axis SWA is centered in the axial direction of the workpiece above the ball track such that the pivot axis, i.e. the central point of the pivoting movement of the finishing tool, essentially coincides with the center of curvature KM of the circular cross-sectional profile P1 of the ball track. The arrow FA represents the pressing force FA, which always acts perpendicular to the pivot axis SWA of the finishing stone and can be adjusted, for example, via the pressure of the pneumatic drive of the pressing device 160.The arrow FR illustrates the reaction force FR generated by contact with the workpiece, the effect of which can be continuously recorded by the force sensor.

[0099] Fig. 3A illustrates the conditions in this ideal case during the angularly limited pivoting movement of the finishing stone. The finishing stone oscillates with its pivoting frequency between the left oscillation end position and the oscillation end position shown on the right, with a pivoting angle range of, for example, + 10° or less, around the zero position of the pivoting movement, which lies in an xy plane. Due to the symmetry of the engagement conditions on the two sides or flanks and in the center, approximately equal reaction forces arise in the left and right end ranges of the pivoting movement as well as in the center. A slight fluctuation can occur because the size of the contact zone between tool and workpiece can vary. Under these perfect conditions, the finishing stone essentially generates almost constant or only slightly varying reaction forces for all pivoting angles, which can be recorded in a time-resolved manner by the force transducer.

[0100] Figure 3B illustrates the machining of a ball raceway with a Gothic profile P2. Here, the reaction forces on the left and right flanks are greater than in the center.

[0101] Fig. 4 shows a schematic force-time diagram of force curves of the reaction force in the case of a semicircular profile P1 according to Fig. 3A (curve FP1) and a Gothic profile P2 according to Fig. 3B (curve FP2). Shown are the resulting temporal curves of that signal component of the force signal which fluctuates with the swivel frequency fsw over time t. The associated period T - 1 / fsw can, for example, be in the range of a few milliseconds (ms), e.g., approximately 2 ms. Relative to the zero position of the swivel movement (pressing force perpendicular to the rotation axis of the workpiece), the maximum amplitudes of the force signal on the left and right are essentially the same, and the temporal force curve KR is approximately sinusoidal. The fluctuation range of the force is greater for the Gothic profile than for the circular arc profile.

[0102] Such (ideal) force curves essentially only occur when there is ideal symmetry of the workpiece and also ideal axial positioning of the finishing tool in relation to the ball race to be machined. Such situations rarely occur in practice, for example when a very experienced operator has enough time for setup. In principle, however, more or less significant deviations from this ideally symmetrical situation occur more frequently. The reasons for such deviations include workpiece tolerances, the dressing cycles of grinding tools used for pre-machining the workpiece, wear, temperature-related changes and, last but not least, the precision of machine setup. In practice, therefore, significant deviations from the ideal configuration can be expected.

[0103] Fig. 5 illustrates a frequently occurring problem, namely the "groove offset" RV. This means that the axial position of the swivel axis SWA (i.e., the position of the swivel axis SWA in a direction parallel to the workpiece rotation axis) does not coincide with the center of curvature KM of the ball raceway to be machined with sufficient accuracy. Such groove offsets can, for example, be in the order of 5p to 15p or more.

[0104] In the example shown in Fig. 5, an offset of the finishing stone 180 relative to the ball track 115 to the left is shown. This leads, among other things, to the finishing stone initially engaging the flank of the groove shown on the left during infeed. This problem can be quickly reduced by uneven wear on the finishing stone, but undesirable geometric deviations remain.

[0105] Using Figs. 6A, 6B, and 7, we will now explain the influence of the non-ideal configuration of Fig. 5 (groove offset) during finishing on the force curve recorded using the force transducer 190. Fig. 6A shows the finishing stone in its left oscillation end position, and Fig. 6B shows it in its right oscillation end position. When moving up to the left end position (Fig. 6A), the forces increase faster and more strongly than on the opposite side, resulting in an asymmetry in the force curve.

[0106] The force arrows FA and FR have the same meaning as in the ideal case. The reaction forces on the lateral flanks are unequal.

[0107] This asymmetry in the force curve arises because the machine-side swivel axis SWA is offset to the left relative to the ideal position (center of curvature KM of the ball raceway). The asymmetry in the force curve is an indicator of the presence of groove misalignment, with the extent of the asymmetry correlating with the size of the groove misalignment. By determining a geometric parameter that quantitatively describes the asymmetry (asymmetry parameter), quantitative information about the groove misalignment is obtained.

[0108] By observing the orientation of the finishing stone when the relatively larger reaction force value occurs, the force profile signal can be used to directly determine the direction in which the pivot axis is offset relative to the ball track being machined. A correction can then be made in the right direction right from the start.

[0109] The force signals are processed in the evaluation device 210, and control signals for correcting the groove offset are output based on them. The finishing device automatically centers the axial position of the swivel axis SWA relative to the machined ball raceway based on the geometry parameter or the asymmetry parameter. In the example, the NC vertical slide 132 is moved parallel to the rotation axis ROT to automatically adjust the position of the swivel axis SWA to the axial groove center to minimize groove offset. Machining then continues under optimized conditions.

[0110] Advantageous options for signal processing and evaluation of the force signals are explained below using Figures 8 and 9. The upper part of Figure 8A schematically shows the temporal progression of the analog force signal SIG generated by the force transducer in a force-time diagram.

[0111] Since the force sensor records all forces occurring at its installation location during a real machining process, the curve of the raw analog force signal (raw signal) is generally difficult to reliably interpret. The raw signal is therefore subjected to signal preparation (SV) in order to eliminate or suppress technically implausible signal components through suitable filtering. During signal preparation, appropriate filtering can be used to eliminate or suppress signal components that are due to control processes during axis movement or to artifacts from sampling the raw signal, so that filtered force signals SIG are available for further analysis. These signals essentially represent the force curve of interest, i.e. form a plausible data set and thus describe a logical state. This filtering can be carried out, for example, using low-pass filtering.The lower part of the figure shows the time course of the filtered force signal, which is used as the basis for further evaluation.

[0112] The signal evaluation comprises a frequency-specific analysis of the measured (and filtered) signals. In the example case, the evaluation device 210 comprises a software module that performs a plurality of fast Fourier transforms (FFTs) of the filtered force signal at different times over the processing time and is therefore also referred to as an FT module. This type of evaluation implements a spectral distribution function method and also records the gradual change in the FFT amplitudes over time as a result of the progressive material removal. Figure 8B illustrates the final collected result.

[0113] This allows the time-dependent force signal to be analyzed for signal components that occur at certain frequencies. These frequencies can, in particular, be the swivel frequency and / or the rotation frequency.

[0114] For illustration, Fig. 8B shows, in each of the two sub-figures, a simplified frequency-based representation of the signals shown in Fig. 8A in the time-based representation. The frequency-based representation (the frequency spectrum) can be obtained from the time-based signals by Fourier transformation. The frequency f, i.e., the inverse of the period T, is plotted on the abscissa, and the amplitude A of the signal component that contributes to the overall signal with the corresponding frequency is plotted on the ordinate.

[0115] In the lower part of Fig. 8B, the time-t-dependent force signal SIG is also plotted on a third axis. This illustrates that, with this spectral distribution function method, the Fourier spectrum A(f) results from a multitude of fast Fourier transformations FFT1(t1), FFT2(t2), FFT3(ts), etc., which were determined at successive times t1, ts, t1a, etc.

[0116] It can be seen that frequency filtering during signal preparation suppresses those frequency components in the spectrum that are most likely due to interference and artifacts, such as sampling effects. After this filtering, signal components remain that, in the example, are characterized by two dominant frequencies. The lower frequency f Dcorresponds to the speed or rotational frequency of the workpiece rotation and the higher frequency corresponds to the swivel frequency fsw of the oscillating tool movement around the swivel axis.

[0117] For simplified illustration, the following example will only consider the signal component that corresponds to the swivel frequency of the finishing stone. Thus, a signal component is identified that essentially varies periodically and whose frequency corresponds to the swivel frequency. This is based on the inventors' finding that recurring, rectified signals that vary in rhythm with the swivel movement can be assigned with a high degree of probability to the geometry on the circumference of the workpiece. A geometry parameter can be derived from this.

[0118] The application of a frequency-specific evaluation for the detection of a groove offset will now be explained in more detail with reference to Fig. 9. Fig. 9A, in the adjacent sub-figures, represents the theoretical ideal case without center offset, while Fig. 9B shows the corresponding diagrams for a situation with groove offset.

[0119] The left part of Fig. 9A shows a time window of the force curve of the filtered force signal. The X symbols correspond to the situation at the left reversal point of the swivel movement, while the circle symbols correspond to the positions at the opposite edge. The total fluctuation range AN1 of the forces is relatively small. The frequency spectrum shown on the right is characterized by two dominant amplitudes, with the lower frequency fo corresponding to the workpiece rotation and the higher frequency fsw corresponding to the oscillating swivel movement.

[0120] Figure 9B illustrates the case with groove offset and the correspondingly asymmetric force distribution. The measured force is significantly higher when reaching the flank closest to the pivot axis (circled symbols) than on the opposite side, so that the force signals corresponding to these pivot positions are at two distinctly different levels (with a fluctuation range of AN2 > AN1).

[0121] This asymmetry of forces at the edges of the groove is noticeable in the frequency spectrum by the fact that, compared to the symmetrical case, wider amplitudes appear at the pivoting frequency. More precisely, at the dominant frequency fsw of the pivoting movement, a pair of unequal twins ZW is created, i.e., two amplitudes of different magnitudes in the range of the corresponding frequency. (The twins are shown side by side for illustrative purposes; in fact, they are located one above the other at fsw.)

[0122] The formation of such twins with unequal amplitudes in the frequency-based representation is considered a signature or fingerprint for the presence of a center offset. If a center offset occurs, this twinning must be evident in the signal analysis. Such twinning (a further amplitude at the same frequency fsw) is considered a necessary condition for the presence of a groove offset during evaluation.

[0123] In other words: a force imbalance on opposite flanks of a ball track due to a center offset is systematically noticeable in that (!) another amplitude appears in the range of the corresponding oscillation frequency in the spectrum, so that a twin pair with two amplitudes is created and in that (ii) these related amplitudes have different intensities or amplitudes.

[0124] These features of the spectrum can be quantitatively evaluated. They provide information on the force imbalance and the extent and direction of the centerline misalignment. From these, parameters are calculated that indicate asymmetry and are therefore also referred to as asymmetry parameters. The corresponding data is made available to the control system. The asymmetry is quantitatively evaluated and provides information on the extent and direction of the groove misalignment. From this, the system can calculate a correction value that indicates the extent and direction in which the workpiece and / or the tool must be moved parallel to the workpiece rotation axis relative to the other part in order to compensate for the groove misalignment and thus achieve the most symmetrical machining conditions possible.In the example case, the axial relative position between the tool (swivel axis) and the workpiece is changed by controlling the vertical slide 132 in such a way that the swivel axis SWA is centered over the ball track 115 and thus the force imbalance disappears.

[0125] The automatic centering of the swivel axis SWA relative to the ball raceway 115 to be machined, described here, is based on geometric parameters resulting from the analysis of the temporal progression of the force signal. In this example, the geometric parameters represent the current groove offset in terms of magnitude and direction. The centering operation runs fully automatically without operator intervention. Due to the automatic detection of the groove center, the setup process can quickly and accurately lead to a tool that is optimally centered within the tolerances. Even during machining, the tool can always automatically center itself relative to the ball raceway as soon as significant asymmetries in the force progression become apparent, allowing the geometric tolerances of the ball raceway to be systematically and precisely maintained. 100% force control also increases machine reliability.Finally, tool costs can also be reduced, since the necessary self-dressing of the finishing tool in the presence of groove misalignment is not necessary, or not to the same extent, if the finishing tool is optimally centered to the orbit.

[0126] The functionality of automatically centering a finishing tool relative to the ball track to be machined is an example of force-controlled position control. In this case, based on the force signal, the position of the workpiece relative to the (unchanged) position of the swivel axis in the machine is changed by controlling the linear machine axis 130 with vertical slide 132 such that the ball track running along the circumference of the workpiece is centered relative to the finishing tool.

[0127] The force signal can also be evaluated with regard to signal components belonging to a frequency corresponding to the rotational frequency of the workpiece. This allows, for example, a geometric parameter to be determined that quantifies any waviness of the ball track in the circumferential direction. Preferably, an absolute encoder signal is taken into account, which indicates the current rotational position of the workpiece. This also allows the force signals to be assigned to their location along the circumference of the ball track.

[0128] In an idealized frequency analysis, a three-wave pattern across the circumference would be noticeable in that the rotation generates not only one amplitude at the rotational frequency fo, but also another amplitude at three times this frequency, i.e., at 3*fo, since the force theoretically passes through three maxima (in regions with bulging) and three intermediate minima during a full rotation. This is indicated in Fig. 9B.

[0129] A significant improvement in the reliability and significance of measured values ​​can be achieved by performing a spatially resolved evaluation of the force signal in addition to the time-resolved evaluation. This makes it possible, among other things, to take into account the fact that during finishing the ball raceway to be machined is traversed by the oscillating finishing tool not just once, but multiple times. For ball bearing components, typically several hundred or several thousand passes can be provided; for ball screws, the number of passes is usually lower, e.g., in the order of ten to twenty. Theoretically, several force signals are recorded for each location on the workpiece surface. The spatially resolved evaluation can evaluate these many force signals in a location-specific manner. For example,Force peaks that systematically occur at the same surface location during each overflow can be attributed to an actual local irregularity in the surface. However, if, among hundreds of overflows, a force maximum signal is generated only once or twice at a specific location, while the location appears unremarkably smooth during the numerous other overflows, the force maxima are most likely not due to a surface irregularity, but rather to other causes. A spatially resolved analysis can thus exclude pseudo-events and thus make the measurement result traceable overall.

[0130] In order to obtain a database for spatially resolved evaluation, force signals that are recorded during machining at successive locations on a workpiece surface are assigned to the corresponding spatial coordinates in a workpiece coordinate system. This is possible because information about the rotational position of the workpiece during its own rotation is available at any time via the encoder signals of the rotary drive and, in the case of ball screw machining, the axial position of the finishing tool can also be queried during machining via a position encoder on a linear machine axis. Figure 11 shows an example of a workpiece coordinate system WK. In this system, the spatial coordinates of a point on the workpiece surface are given by the axial position in the z-direction, the radial distance R of a surface point from the workpiece axis, and the position in the circumferential direction by the angle Phi.A Cartesian coordinate system would also be possible.

[0131] With reference to Fig. 10 and Fig. 11, an embodiment of a finishing machine 300 will now be explained. This finishing machine is configured for machining workpieces 310 that have a helical ball track over at least part of their length. These include, for example, recirculating ball screws or steering racks that have a toothed section with steering toothing and a spindle section in the manner of a recirculating ball screw. For reasons of clarity, components that are structurally and / or functionally similar to corresponding components of the first exemplary embodiment are designated by the same reference numerals, increased by 200.

[0132] In the example, the elongated workpiece 310 is a steering rack with a ball screw section 314. It is clamped here with the workpiece axis 312 horizontal between the centers of a workpiece holding device (not shown in detail). The workpiece has a toothed section (not shown) that transitions into a substantially cylindrical section, followed by a ball recirculating section in which a ball race 315, designed like a groove, runs helically around the workpiece axis 312 on the workpiece circumference. In contrast to the ball race of a rolling bearing ring, the ball race here has a gradient, i.e., it does not run in a plane perpendicular to the workpiece rotation axis, but rather at an angle to it.

[0133] Such ball raceways typically have a Gothic cross-sectional profile to ensure that only point contact occurs between the ball raceway and the balls on each flank, thus enabling excellent running characteristics with precise guidance and increased axial force absorption. A Gothic profile (also called a pointed arch profile, see Fig. 3B) has two concave, circularly curved flanks that converge at the base of the ball raceway, forming a pointed discontinuity. The centers of curvature RML and RMR of the left and right flanks, respectively, are close together in a central region; for simplicity, this region can be considered the center of curvature of the ball raceway. Gothic profiles are also referred to in this application as "essentially semicircular profiles."In contrast to the first example, this finishing device has two swivel devices 350-1, 350-2, whose swivel axes SWA1 and SWA2 are aligned at an angle of approximately 40° to 60° to each other. These are mounted in a horizontally movable base frame 305, which is attached to an NC horizontal slide (not shown). While the first swivel axis SWA1 runs at an angle to the horizontal plane, the second swivel axis SWA2 lies in the horizontal plane. This allows the helical, rotating ball track to be finished at two points slightly offset in the circumferential and axial directions. The axial distance between the swivel axes can be adjusted manually during setup using a handwheel.

[0134] The pneumatically operated pressing devices 360-1, 362-2 are configured similarly or identically to the pressing device 160. The pivoting devices and their drives, as well as the pneumatically operated pressing devices 360-1, 362-2, have essentially the same structure as those in the first embodiment, which is why reference is made to the description there. There is an angle adjustment device, manually adjustable via a handwheel, which allows the orientation of the pivot axis to be moved out of the yz plane for each of the finishing units in order to adapt the finishing stone orientation to the inclined course of the helical groove.

[0135] Since helical ball raceways 315 on ball screws can extend over large axial sections of the workpiece, a translatory NC machine axis 330 with a displacement direction oriented parallel to the rotation axis of the workpiece holder and a correspondingly large linear stroke is provided for pitch tracking during workpiece rotation. This NC machine axis comprises a horizontal slide 332, which supports the base frame 305, in which the components of the swivel devices, the pressure devices, etc., are mounted.

[0136] This finishing device is also configured for force-controlled position control, which operates similarly to the first embodiment. After manual setup of the finishing machine, the finishing process is initiated. Initially, the finishing stones are advanced, and as described above, an automatic centering operation takes place to center the finishing stones as accurately as possible in the facing section of the ball track based on force signals.

[0137] As the workpiece continues to rotate, the finishing stones then follow the pitch curve synchronously with each other, with the NC horizontal slide 332 moving slowly in a direction parallel to the axis (x-direction). If the pitch corresponds to the expected increase, the centering of the finishing stones should be maintained. Any deviations, for example, because a workpiece from a different batch with a slightly different screw geometry was inserted, will result in significant changes in the force signal, the evaluation of which can lead to a warning signal. This also enables automatic workpiece type recognition. Furthermore, analogous to the above example, the workpiece geometry is checked in the circumferential direction, e.g., for waviness. Any defects in the ball race can also be detected.

[0138] An example of the type and arrangement of a force sensor 400 will now be explained with reference to Figure 11. The figure shows a section through a tool holding device in the form of a clamping device 372 in the region of the free end of a stone holder 370. The sectional plane of the illustration is perpendicular to the pivot axis SWA, and the pressing force FA acting on the finishing stone 380 acts in this plane in the longitudinal direction of the finishing stone. In the solid end section of the stone holder, a rectangular recess is provided which runs from top to bottom and is delimited on the sides perpendicular to the clamping force direction FK by solid wall sections 374-1, 374-2. On the inside of the wall section 373-1 visible on the right, which faces the finishing stone, there is a recess which is approximately circular in cross-section and into which a force sensor 400 is inserted.Its height (measured in the clamping direction) is greater than the depth of the recess, so that the side facing the finishing stone protrudes beyond the inner wall. This side of the force sensor rests on the right clamping plate 373-1, preventing it from slipping. The opposite side of the clamping plate rests on one side of the finishing stone 380. A high coefficient of friction exists between the first clamping plate 373-1 and the facing side of the force sensor, so that this clamping plate cannot slip relative to the force sensor. A clamping plate can also be omitted or designed as an integral component of the force sensor.

[0139] In the reinforced wall section 374-2 on the opposite side, a recess is provided on the inside, in which a widened pressure foot of a clamping screw 378 is seated. The foot rests on the outside of the clamping plate 373-2, the opposite surface of which rests on the finishing stone. By tightening the clamping screw 378, the finishing stone 380 is clamped between the clamping plates 371-1, 371-2.

[0140] The clamping force FK acts parallel to a first sensitivity direction SR1 of the force sensor 400, which can thus measure, among other things, the clamping force. On the opposite side, there is only a low coefficient of friction between the outer side of the clamping plate 373-2 and the base 377 of the clamping screw, so that a relative displacement in the pressing direction ADR is easily possible. In other embodiments, the clamping screw 377 is supported by rollers on the second clamping plate 373-2, so that only a low rolling resistance acts in the pressing direction.

[0141] The force sensor 400 is a multi-channel force sensor. A second sensitivity direction SR2 runs perpendicular to the first sensitivity direction SR1 and parallel to the direction of the contact force FA. This allows shear force components acting in this direction to be detected. In the first sensitivity direction SR1, force components can also be detected that are due to tilting moments caused by the tilting of the finishing stone 380 relative to the stone holder 370 in a direction perpendicular to the pivot axis.

[0142] The force sensor 400 is inserted in the force flow between the effective zone of the finishing stone (contact zone between the finishing stone and the workpiece) on the one hand and the pressing device 360 ​​on the other hand, namely between the finishing stone 380 and the stone holder 370. This position very close to the point of origin of the reaction forces makes it possible to record force changes and force change components in the engagement area between the workpiece and the tool with high precision almost without delay and without substantial damping between the point of origin and the measuring point.

[0143] This force measurement does not depend on a substantial deformation of the stone holder, which can be designed to be very torsionally and flexurally rigid, which in turn benefits the immediacy of the force transmission between the pressing device 360 ​​and the finishing stone 380.

[0144] Instead of a multi-channel force sensor, two, three or more single-channel force sensors can be provided, which can detect the force changes of interest in different directions with high precision.

[0145] In general, the contact force should be generated in such a way that no transverse forces are generated. This creates a directly measurable reaction force in one force direction at a measuring point in the force flow, which is preferably arranged in the direction of action. Alternatively, the reaction force can also be measured indirectly or not aligned with the direction of action. For this purpose, three or more synchronously evaluated force measuring points should be provided so that the individual force directions can be converted into a force vector over known lever lengths. The effective force is then determined from the individual measured forces using geometric calculations. Damping caused by the design and any lever stiffness can be compensated for using proportional factors.

[0146] Important aspects of some embodiments of the invention have so far been explained with reference to the machining of ball raceways with a substantially semicircular cross-sectional profile. However, the applicability of the invention is not limited to this. Rolling element raceways with other cross-sectional shapes can also be finished; the rolling elements do not have to be spherical. For example, raceways for angular contact ball bearings (see Fig. 12A), roller bearings (see Fig. 12B), or tapered roller bearings can be machined using the advantages of the invention.

[0147] Angular contact ball bearings are ball bearings in which the bearing cross-section is asymmetrical in the area of ​​the ball raceway LB. Angular contact ball bearings are designed to absorb forces whose line of action is not exactly perpendicular to the axis, but rather obliquely at a certain angle to the vertical axis. For this purpose, the shoulder around the ball is more strongly designed in the area of ​​the line of action (see Fig. 12A). Obliquely acting loads can therefore be absorbed better. The opposite bearing shoulder is significantly weaker to facilitate assembly. Here, machining can be carried out using a pivoting finishing tool 190; however, the zero position NL of the pivoting movement is preferably oriented not perpendicular, but obliquely to the raceway axis LBA (e.g. at an angle of 30° to 60° to it).

[0148] In contrast to ball bearings, roller bearings have cylindrical rolling elements, which are particularly suitable for higher loads at relatively lower speeds. The rollers offer a larger contact area between the rolling element surface and the raceway than in ball bearings. As shown in Fig. 12B, the raceway LB has a more or less wide cylindrically curved section ZT. In tapered roller bearings, the raceways have a more or less wide conically curved section. In these cases, oscillation devices are generally used that can generate a linear oscillating movement of the finishing tool WZ (finishing stone or finishing band) parallel to the surface line (see double arrow OSZ) of the cylindrical or frustoconical part.

Claims

Patent claims 1. Finishing process for finishing a workpiece with at least one raceway for rolling elements rotating around a raceway axis, comprising the following steps: Providing a finishing tool which has an abrasive working surface; Attaching the finishing tool to a tool holder; Generating a self-rotation of the workpiece around a raceway axis at a rotational frequency; Generating an oscillatory movement of the tool holder superimposed on the workpiece's own rotation and oscillating at an oscillation frequency; Pressing the finishing tool against the workpiece surface with a pressing force in a pressing direction such that the abrasive working surface engages the workpiece in the area of ​​the raceway; characterized by Continuous acquisition of a force signal caused by the contact force within a acquisition period that partially or completely overlaps with a processing period of the finishing operation, and time-resolved evaluation of the force signal to determine at least one geometric parameter that (!) represents a workpiece geometry in the area of ​​the machined track and / or (ii) represents a geometric relationship between the workpiece geometry in the area of ​​the machined track and a position of an oscillation device.

2. Finishing method according to claim 1, characterized in that the workpiece has a raceway in the form of a ball raceway for spherical rolling elements, in particular a ball raceway with a substantially semicircular cross-sectional profile, and that the oscillation movement comprises an oscillating pivoting movement of the tool holder with a predefinable pivoting frequency about a pivot axis which substantially passes through a center of curvature of a profile of the ball raceway, wherein the contact force is preferably oriented perpendicular to the pivot axis.

3. Finishing method according to claim 1 or 2, characterized in that, in addition to the time-resolved evaluation of the force signal, a spatially resolved evaluation of the force signal is carried out by assigning corresponding spatial coordinates in a workpiece coordinate system to force signals recorded at successively traversed locations on a workpiece surface during processing, wherein preferably a rotational position of the workpiece during its own rotation is determined and a spatially dependent analysis of the force signal is carried out on the basis of the rotational position.

4. Finishing method according to one of the preceding claims, characterized by a force-controlled axial position control, wherein an axial relative position between an oscillation device and the workpiece, in particular between the pivot axis and the workpiece, is automatically changed as a function of at least one geometric parameter by displacing the workpiece and / or an oscillation device carrying the tool holder holder, in particular a pivot device, as a function of the force signal in a direction parallel to the track axis.

5. Finishing method according to claim 2, 3 or 4, characterized by automatic centering of the axial position of the pivot axis relative to the machined ball raceway based on the geometry parameter, wherein preferably the position of the pivot axis is automatically set to an axial groove center to minimize groove offset and / or, when machining ball screw spindles, any pitch errors are automatically detected and compensated during machining.

6. Finishing method according to one of the preceding claims, characterized in that the time-resolved evaluation comprises a frequency-specific evaluation of the force signal, wherein methods of the spectral distribution function are preferably used in the evaluation.

7. Finishing method according to claim 6, characterized in that the evaluation comprises determining a signal component of the force signal which varies substantially periodically with the oscillation frequency, in particular the pivoting frequency, and analyzing this signal component, wherein preferably at least one asymmetry parameter is determined in the analysis which represents a systematic asymmetry of the signal component with respect to a zero point position of the oscillating movement, in particular a pivoting movement, of the finishing tool.

8. Finishing method according to claim 6 or 7, characterized in that the evaluation comprises determining a signal component of the force signal which varies substantially periodically with a rotational frequency of the workpiece's own rotation and analyzing this signal component, wherein preferably at least one ripple parameter is determined during the analysis.

9. Finishing method according to one of the preceding claims, characterized by a direction-selective detection of force components in two or more transverse, in particular perpendicular to each other, sensitivity directions.

10. Finishing device (100, 300) for finishing a workpiece (110) with at least one raceway (115, 315) for rolling elements rotating about a raceway axis, comprising: a workpiece holding device (120) for receiving the workpiece; a rotary drive for generating a self-rotation of the workpiece received in the workpiece holding device about the raceway axis at a predefinable rotational frequency; a tool holder (170, 370) for receiving a finishing tool (180, 380) having an abrasive working surface (182, 382); an oscillation device (150, 350) with an oscillation drive (152) for generating an oscillation movement of the tool holder at a predefinable oscillation frequency; a pressure device (160) for pressing the finishing stone against the workpiece with a predefinable pressure force (FA) in a pressure direction (ADR);a control unit (200) for controlling the rotary drive, the oscillation drive, the pressure device and other controllable devices of the apparatus; characterized by a force sensor (400) for continuously detecting reaction forces generated by pressing the finishing tool (180, 380) against the workpiece and for generating a force signal representing the reaction forces; an evaluation device (210) for time-resolved evaluation of the force signal to determine at least one geometric parameter that (i) represents a workpiece geometry in the area of ​​the machined track (115, 315) and / or (ii) represents a geometric relationship between the workpiece geometry in the area of ​​the machined track and a position of the oscillation device.

11. Finishing device (100, 300) according to claim 10, characterized in that the oscillation device has a pivoting device (150, 350) with an oscillation drive in the form of a pivoting drive (152) for generating an oscillating pivoting movement of the tool holder with a predefinable pivoting frequency about a pivot axis (SWA) and the pressure device is designed to generate a pressure force oriented perpendicular to the pivot axis.

12. Finishing device (100, 300) according to claim 10 or 11, characterized in that the control unit (200) is configured to control at least one controllable device of the finishing device depending on the geometry parameter, wherein the finishing device preferably has a linear machine axis (130) for changing an axial relative position between the oscillation device, in particular the pivot axis (SWA), and the workpiece has a displacement direction parallel to the workpiece axis and the control unit (200) is configured to control this machine axis depending on the geometry parameter.

13. Finishing device (100, 300) according to claim 11 or 12, characterized in that the finishing device has devices for automatic groove center detection and / or devices for detecting and compensating for pitch errors on ball screw drives.

14. Finishing device (100, 300) according to one of claims 10 to 13, characterized in that the evaluation unit (210) has an FT module for determining a signal component of the force signal which varies substantially periodically with the oscillation frequency, in particular the pivot frequency, and / or with the rotation frequency, and for analyzing this signal component.

15. Finishing device (100, 300) according to one of claims 10 to 14, characterized in that a force transducer (400) is arranged in the area of ​​a finishing tool receptacle of the tool holder between the finishing tool, in particular a finishing stone (380), and the tool holder (380).

16. Finishing device (100, 300) according to one of claims 10 to 15, characterized in that a multi-component force sensor or a group of force sensors with two, three or more different sensitivity directions is provided for direction-selective detection of force components in two or more transverse, in particular perpendicular to each other.

17. Finishing device (100, 300) according to one of claims 10 to 16, characterized in that the control unit (200) is configured in at least one operating mode to control the finishing device such that a finishing process according to one of claims 1 to 9 is carried out.

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