Patent Application
20250170689
The invention relates to a finishing method for the finish machining of workpieces having at least one raceway for rolling elements which runs in encircling fashion about a raceway axis, and to a finishing apparatus suitable for carrying out the finishing method.
Finishing, which is referred also to as superfinishing, microfinishing or short-stroke honing, is a cutting machining method using geometrically undefined cutting edges according to DIN 8580. One characteristic is the use of multi-cut tools (finishing tools) which have cutting grains bound in a bond and execute, relative to a moving workpiece, a cutting movement superposed from two velocity components. Relatively speaking, one component of the cutting movement always alternates or oscillates, and therefore the machined surface has defined cross traces (“cross hatch”). Depending on the type of finishing tool (finishing stone or finishing tape), a further subdivision into stone finishing and tape finishing is customary. A further division is performed according to the shaped elements to be machined.
Finishing is used on a large scale, inter alia, in the production of components for rolling bearings. 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 raceway (rolling element raceway) running in encircling fashion on its inner side, an inner ring, which has a raceway running in encircling fashion on its outer side, a cage and the rolling elements. The rolling elements are located between the two rings and ensure the rolling friction. In this case, they roll with point contact or linear contact on the raceways. The cage ensures uniform distribution of the rolling elements in the bearing. The names of the bearing types are derived from the form of the rolling elements (for example ball bearing, needle bearing, cylindrical roller bearing, spherical roller bearing, tapered roller bearing). Each of these rolling bearings can absorb different forces in an axial and radial direction depending on the type of rolling elements and cross-sectional profile of the rolling element raceways.
In the case of ball bearings, the raceways for the balls are usually referred to as ball raceways. Many ball raceways are formed as grooves which run in encircling fashion around the periphery of a workpiece and have a cross-sectional profile that is curved partially or completely in the shape of a circular arc. In the case of a standard ball bearing, annularly closed ball raceways are formed on the outer side of inner rings and on the inner side of outer rings. The cross-sectional profile is generally in the shape of a circular arc, for example approximately semicircular. Special designs may also have ring halves which are split normal to the axis of rotation, and are offered, for example, as what are known as 4-point contact bearings in order to increase the axial force absorption capability.
Ball raceways are also found on ball screw drives. A ball screw drive, also referred to as ball screw, is a screw drive containing inserted balls in order to transmit the force between ball screw spindle (threaded rod) and ball screw nut. Both parts have a respective helically running groove which functions as ball raceway and together form a helical tube filled with balls. The ball raceways of ball screw spindles often have a Gothic profile (also called pointed arch profile) made up of two circular-arc-shaped portions which converge in the bottom region of the groove so as to form an angle and each have their own radius center. This achieves an increased axial load-bearing capacity, as in the case of the 4-point contact bearing constructions.
The wording “substantially semicircular cross section” used in this application is intended to encompass more or less semicircular cross sections and, more generally, cross-sectional profiles that are curved continuously in the shape of a circular arc and Gothic cross-sectional profiles.
Ball raceways on inner rings 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 which oscillate back and forth at high vibration frequency about a pivot axis, whilst the workpiece rotates about its workpiece axis. Here, the finishing tool used is usually a finishing stone; finishing tape may also be used where appropriate. The pivot axis lies in the vicinity of the contact region between workpiece surface and finishing tool. The pivot axis should ideally lie at the curvature center of the groove profile to be machined or in the vicinity thereof. The combination of the oscillating pivoting movement of the finishing tool and the intrinsic rotation of the workpiece about the raceway axis leads to an optimal workpiece surface.
Document DE 10 2014 222 848 B4 describes a finishing machine designed for machining processes of this kind.
A problem in the finish machining of ball bearing rings is what is known as the “groove offset”, that is to say the tolerance of the raceway middle (middle of the ball raceway in the axial direction of the workpiece) with respect to the ring contact side, i.e. with respect to a planar surface or another reference surface which serves as axial reference surface when clamping the workpiece. In practice, the pre-machining (usually by grinding) is used to try to keep this tolerance as small as possible. Nowadays, position accuracies in the range from 5 μm to 10 μm can be achieved in series production with correspondingly high effort. If, however, the finishing tool in a finishing machine is then intended to be used to further improve the geometry within the micrometer range or even below that, a groove offset is inhibiting.
The problem can be at least partially compensated for by operation being performed with relatively soft finishing tools which can adapt rapidly to the workpiece geometry. Operation may also be performed with flexible finishing tool holders which are able to distort. However, both auxiliary measures are not conducive to a high geometric quality.
Patent specification DD 234 388 A1 describes different solution approaches for dealing with the groove offset. What is proposed is a pneumatic setting apparatus for setting the axial and radial position of a honing tool and the pivot axis thereof on honing machines for machining inner and outer rings of rolling bearings. The pneumatic setting apparatus is based on the nozzle/baffle plate principle and is characterized in that a measuring head is tightened into the tool holder for a honing stone, in that the measuring head has a doubly curved end face similar to the raceway of a rolling bearing ring, in that four measuring nozzles are introduced into the end surface, wherein the measuring nozzles each lie opposite one another symmetrically with respect to the end face center on the axes of symmetry of the end face, and in that cutouts are introduced into the end face in order to discharge the air exiting the measuring nozzles. This makes it possible to set the axial and radial position of the honing tool exactly to the groove middle prior to the finish machining.
Document DE 30 30 703 A1 discloses apparatuses for checking the dimensions of the race of a bearing ring using sensors.
Ball screw drives usually have to be optimized with regard to wear and noise emission. Owing to the relatively low loading in contrast to permanently rotating rolling bearings, the wear inhibition is achieved already by a comparatively low removal of material (removal of the soft skin). However, the noise behavior is influenced significantly by the microgeometry and macrogeometry of the raceway profile. Pitch errors may also occur.
Against the background of this prior art, the invention addresses the problem of providing a finishing method and a finishing apparatus for a finishing process which make it possible, on workpieces having at least one raceway for a rolling element which runs in encircling fashion about a raceway axis, for the raceways to be finished within short cycle times with high geometric precision and high surface quality.
To solve this problem, the invention provides a finishing method having the features of claim 1. Furthermore, a finishing apparatus which is suitable for carrying out the finishing method and has the features of claim 10 is provided. Advantageous developments are specified in the dependent claims. The content of all the claims is incorporated in the description by reference.
In the finishing method, a workpiece having at least one raceway for rolling elements (rolling element raceway) which runs in encircling fashion about its raceway axis is fine-machined by means of the machining method finishing or superfinishing. This is preferably the finish machining of the region of a ball raceway intended to serve as rolling surface for balls. Balls are ideally intended to be only in point contact with the rolling surface. For this machining task, use is usually made of finishing tools in the form of finishing stones which have an abrasive working surface provided for engagement on the workpiece. The form thereof can be approximated to the form of the workpiece by pre-machining. Such finishing stones can adapt well to the ball raceway to be machined after a short machining time as a result of possibly uneven abrasion.
In some cases, use may also be made of finishing tools in the form of a finishing tape which is pressed onto the workpiece surface by means of a pressing device which is possibly adapted to the workpiece geometry.
When setting up the finishing apparatus, the finishing tool, for example a finishing stone, is mounted on a tool holder of the finishing apparatus. To this end, the tool holder has, at an end close to the workpiece, a corresponding finishing tool receptacle on or in which the finishing tool can, for example, be clamped or mounted in some other way and possibly fixed or movably guided.
For the material-removing machining, an intrinsic rotation of the workpiece about a raceway axis is generated. The raceway axis is the axis leading through the center of the encircling raceway. For this purpose, the finishing apparatus has a workpiece receiving device having an assigned rotary drive. The intrinsic rotation is effected at a predefinable rotational frequency or rotational speed, which should be substantially constant at least in phases, about a workpiece axis of rotation which ideally coincides with the raceway axis. In the case of completely rotationally symmetrical workpieces, the raceway axis coincides with the axis of symmetry thereof. However, a raceway axis may also be offset parallel to a workpiece axis.
Furthermore, an oscillating movement of the tool holder or of the finishing tool held therein, said oscillating movement being superposed on the intrinsic rotation of the workpiece, is generated. This oscillating movement is effected at a predefinable oscillation frequency, which should be kept as constant as possible at least in phases. In the case of a varying oscillation frequency, the variance should be known.
The oscillating movement may, for example, be an oscillating pivoting movement of the tool holder or of the finishing tool held therein about a pivot axis, the pivoting movement being effected at a predefinable pivoting frequency. This may be expedient in particular when machining ball raceways having a substantially semicircular cross-sectional profile.
When the finishing apparatus is properly set up, the pivot axis should lead more or less exactly through a curvature center of the substantially semicircular profile of the ball raceway.
The oscillating movement may also be a linear movement of the tool holder or of the finishing tool held therein along an oscillation axis, said linear movement oscillating back and forth rectilinearly at an oscillation frequency. This may be expedient in particular when machining raceways having cylindrical or frustoconical portions for roller-shaped or tapered rolling elements.
To generate the oscillating movement passing back and forth over a limited angle range or over a limited movement range, an oscillation device a having controllable oscillatory drive is provided.
To generate cutting force for the material removal during the operation, the finishing tool, in particular the finishing stone, is pressed with a press-on force in a press-on direction onto the workpiece surface in the region of the raceway in such a way that the abrasive working surface can engage in the region of the raceway under an effective force. For the pressing onto the point of action, the finishing apparatus has a press-on device which generates the press-on force. In the case of a pivoting movement which oscillates about a pivot axis, the press-on direction of the finishing tool runs perpendicular to the pivot axis and oscillates synchronously with the tool holder about the pivot axis. In the case of an oscillating linear movement, the press-on direction of the finishing tool runs perpendicular to the straight movement axis oscillates synchronously with the tool holder.
In a finishing method according to the claimed invention, a force signal caused by the press-on force is continuously detected at least during a detection time period which partially or completely overlaps the machining time period of the finish machining. The force signal may be detected throughout the entire finish machining operation or within a shorter time window which at least partially overlaps the machining time. The detection and evaluation of the force signal may begin before the finishing tool is fed into the engagement position, that is to say at a point in time at which no press-on force acts. The force curve when the workpiece is contacted during the feeding can then also be detected and analyzed.
For the force measurement and generation of a force signal, the finishing apparatus has at least one force transducer, which in this application is also referred to synonymously as force sensor. The force transducer may, for example, have at least one strain gauge (SG), which can be mounted on an elastically deformable portion of a part lying in the force flow for example by adhesive bonding. As an alternative or in addition, use may for example be made of a piezo force transducer for generating the force signal.
Here, the term “force signal” refers to a preferably electrical signal which is in a defined functional relationship with a state detected by the force transducer or force sensor. The force signal may, for example, have an amplitude which is substantially proportional to the force or a force component acting in a sensitivity direction of the force transducer. It may be transmitted in an electrically wired and/or wireless manner from the force sensor for further processing and may possibly be filtered or smoothed prior to evaluation in the context of signal conditioning.
The at least one force transducer is preferably arranged in the force flow between the abrasive working surface of the finishing tool and the press-on device. An arrangement at a different point is also possible, for example in the force flow between a base frame of the finishing apparatus and the housing of the oscillatory drive mounted thereon or between the latter and the press-on device borne thereby.
A force transducer may be arranged in particular in the region of a finishing tool receptacle of the tool holder between the finishing tool (in particular finishing stone) and the tool holder. The arrangement of a force transducer between a finishing tool and the tool holder affords the advantage in the case of finishing inter stones, alia, that substantially only the flexurally stiff or rigid material of the finishing stone is located between the point of action and the force sensor, and therefore the forces and torques relevant to the measurement pass over a short distance largely undamped to the force sensor.
As an alternative or in addition, a force transducer may, for example, be installed on or in a component which is subjected to bending and/or torsional loading due to the machining forces during the machining. In this case, forces caused by elastic deformation act on the force sensor. Where appropriate, both tensile and thrust forces can be measured by elastic deformation. A force transducer may, for example, be arranged on or in the tool holder between the finishing tool receptacle and the connection to the drive device. Depending on the type and arrangement of the force transducer, shear forces may also be detected. This is particularly important when the construction does not make it possible to introduce the press-on force perpendicularly with respect to the raceway axis of the workpiece, or flexibilities cannot be compensated for via proportional factors.
A force transducer is used to measure forces acting on the force transducer. There are one-component force transducers which have only a single sensitivity direction, that is to say can only measure forces or force components acting in this sensitivity direction. For the force measurement, a one-component force transducer may be used, for example on or in a portion of a component which is subjected to bending and/or torsional loading. In some cases, the use of a multi-component force transducer, which has two or three or more sensitivity directions transverse to one another, in particular orthogonal to one another, may be advantageous. In this way, a multidimensional force measurement is possible, which provides even more precise and more easily interpretable information regarding spatial force distributions and the process-related changes thereof. A group of two, three or more separate force transducers may also be provided, which are preferably installed with two, three or more different sensitivity directions.
The force signal is evaluated in temporally resolved fashion. This evaluation is used to determine at least one geometry parameter. In this context, a geometry parameter is a parameter which is determined significantly by the workpiece geometry in the region of the machined raceway. A geometry parameter may be exclusively workpiece-specific and represent the workpiece geometry, that is to say the macroscopic shape of the workpiece in the region of the machined raceway. The workpiece geometry may be described by means of coordinates of the workpiece coordinate system which is fixed in relation to the workpiece. A geometry parameter may, for example, describe the nature and scope of any waviness of the raceway in the peripheral direction.
As an alternative or in addition, a geometry parameter may represent a geometric relationship between the workpiece geometry in the region of the machined raceway and a position of the oscillation axis (pivot axis or translatory axis). As a result, a relationship between the workpiece coordinate system and the machine coordinate system may be established. This makes it possible, for example, to carry out an automatic centering operation, in order to center the finishing tool in relation to a raceway, in particular to a groove or a ball raceway.
In this case, it is important that the time curve of the force signal is detected and evaluated, that is to say the curve of the force signal as a function of time. This time dependency of the force signal is then analyzed and used to determine the at least one geometry parameter. The geometry information is thus derived from information regarding whether, and possibly in which way, the force signal changes over time.
The invention is based, inter alia, on the finding that temporally resolved detection and analysis of force signals makes it possible to ascertain geometry information regarding the machining process and/or regarding the positioning of the workpiece relative to the finishing apparatus and/or regarding the workpiece. This makes it possible, for example, to determine the position of the finishing tool and/or of the pivot axis and/or of a translatory axis relative to the workpiece, in particular the axial position, that is to say the position in the axial direction of the workpiece.
This makes it possible, for example when machining ball raceways, to quantitatively determine the extent of any axial offset of the axial position of the pivot axis in relation to the axial position of the ball raceway to be machined. As an alternative or in addition, it is also possible to determine and evaluate any geometry deviations present on the workpiece, for example a waviness at the periphery of the machined workpiece in the region of a raceway.
The continuous detection of the press-on force may additionally also be used in order to determine the force which is essential for the material removal and acts in the contact region between the abrasive working surface and the workpiece.
It is important for the understanding of the invention that statements relating to the time dependency of force signals are determined, which is also possible without ascertaining or using the absolute values of the force. The basis is thus an analysis of force curves of possibly changing forces or of corresponding force signal curves in the time domain.
The invention thus enables quantitative ascertainment of geometric properties of the workpiece and/or of geometric conditions relating to the relationship between the machine coordinate system and the workpiece coordinate system on the basis of a force measurement effected over a certain time period and the information regarding force changes that is derived therefrom.
In preferred embodiments, it is provided that, in addition to the temporally resolved evaluation of the force signal, a spatially resolved evaluation of the force signal is effected, by virtue of force signals detected during the machining at locations of a workpiece surface that are successively passed over being assigned to corresponding spatial coordinates in a workpiece coordinate system. As a result, the workpiece itself becomes the observation system. An additional spatially resolved evaluation makes it possible to considerably improve the significance of the measurement, to prevent the generation of pseudo-errors and, as a result, to provide traceability of the measurement results in terms of measurement technology.
In order achieve a spatially resolved to evaluation, in addition to the force signal, with accurate timing associated rotary encoder signals of the rotary drive of the workpiece holding apparatus and rotary encoder signals of the pivoting device or encoder signals of a translatory oscillation device and possibly position encoder signals of a translatory machine axis for displacements parallel to the workpiece axis of rotation can be evaluated.
According to one development, force-controlled closed-loop axial position control is realized. In this case, an axial relative position between the pivot axis or the pivoting device or another oscillation device and the workpiece is changed as required in dependence on the at least one geometry parameter or on the force signal, by virtue of the workpiece and/or a pivoting device or oscillation device bearing the tool holder being displaced axially, that is to say in a direction running parallel to the workpiece axis of rotation, in dependence on the force signal. To this end, an operator can obtain corresponding setting instructions. Preferably, the force-controlled closed-loop axial position control functions automatically, i.e. without intervention of an operator.
To this end, provision is preferably made for the control unit of the finishing apparatus to be configured to control at least one controllable device of the apparatus in dependence on the (at least one) force signal or the geometry parameter derived therefrom. In this way, knowing the at least one geometry parameter makes it possible to directly influence the machining process predefined by the machine controller. Closed-loop-controlled finishing or operation sensitive finishing operation is thus possible, in which machining parameters of the finishing apparatus can be set or changed automatically in dependence on the current force conditions in the engagement region between the finishing tool and the workpiece.
According to the above-mentioned development, provision may be made for the controllable device to generate an automatic change of an axial relative position between the pivot axis or the pivoting device or another oscillation device and the workpiece in dependence on the at least one geometry parameter, by virtue of the workpiece and/or the finishing apparatus being displaced in a direction running parallel to the workpiece axis. Where appropriate, for example when machining angular contact bearings or ball axial bearings, the displacement direction between an oscillation axis and the workpiece may also run obliquely or perpendicularly with respect to the workpiece axis.
In one exemplary embodiment of the finishing apparatus, the latter 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 running parallel to the workpiece axis and the control unit is configured, in one operating mode, to control this machine axis in dependence on the geometry parameter. A numerically controlled machine axis suitable for this is already present in many conventional finishing apparatuses, for example in order to be able to adapt the apparatus via control signals of the control device for example to ball bearing components of different diameter and/or to different raceway positions or, in the case of finishing apparatuses for machining ball screw drive components, for the continuous changing of the axial relative position when following the pitch of the helical profile of the ball raceway during the finish machining. This translatory machine axis can thus be used for a further functionality, namely force-controlled closed-loop position control for the axial position.
In preferred embodiments having a pivoting device, automatic centering or optimization of the axial position of the pivot axis in relation to the machined ball raceway is effected on the basis of the geometry parameter. The automatic centering may be realized by changing the axial relative position between the workpiece and the tool by displacing the workpiece and/or the pivoting device parallel to the workpiece axis. To this end, the above-described linear machine axis for changing an axial relative position between the pivot axis and the workpiece in a displacement direction running parallel to the workpiece axis can be used.
In this way, it is for example possible for the problem of the groove offset mentioned in the introduction to mitigated, be without requiring intervention of an operator. An apparatus according to this embodiment “finds” the groove middle on the basis of the force detection and the temporally resolved analysis of the force curve independently in a start phase of the machining and can thus operate with high precision almost from the beginning, and specifically independently of how great the groove offset was originally. It is then no longer necessary to discuss compromises with respect to the tool hardness and/or a deliberately introduced flexibility in the region of the components bearing the finishing tool.
Such a finishing apparatus thus has devices for automatic groove middle identification.
It is thus possible to use finishing tools (finishing stones or finishing tapes) with a hard bond and a correspondingly low tool wear and the tool holder does not require any flexibility, but rather can be of ideally stiff design. In this way, important prerequisites for a considerable improvement in the workpiece quality are provided.
In addition, there is a time saving when setting up the finishing apparatus since the finishing tool has to be positioned only roughly in relation to the workpiece in such a way that the finishing tool is arranged in the region of the ball raceway and can “find” the latter automatically. Any deviation (groove offset) is then corrected automatically on the basis of the ascertained geometry parameters.
When machining ball screw spindles etc., this centering functionality can be used to identify any pitch errors and to automatically compensate for them in the process. A device for compensating for pitch errors on ball screw drives is thus also provided.
Such a finishing apparatus thus has devices for identifying and compensating for pitch errors on ball screw drives.
Some preferred designs for the evaluation or the evaluation unit are explained below.
During a real machining process, a force sensor detects not only the forces and force changes that are attributed to the machining process and are intended to be measured but also forces and force changes that are attributed to events in the environment and possibly particular features in the finishing machine which may influence the force signal and make it difficult to interpret.
In the context of the temporally resolved evaluation, in preferred embodiments a frequency-specific evaluation of the force signals is provided, preferably at different intervals of the vibration periods. “Frequency-specific evaluation” means in particular that force the signals (possibly after corresponding filtering in the context of signal conditioning) are analyzed with regard to signal components that may be assigned to specific frequencies. The further steps of the analysis are then carried out on signal curve components that owing to their frequencies likely describe the desired observation object.
In the context of the evaluation, in preferred embodiments spectral distribution function methods, but at least several partially evaluated data of the discrete Fourier transform (DFT), are used. With a DFT, a time-discrete signal, here the time-dependent force signal, can be decomposed into its frequency components and be partially analyzed as a result, in order to evaluate changes in dependence on the workpiece and the tool engagement conditions. This makes it possible to avoid or considerably reduce ambiguities in the evaluation.
In the context of the evaluation, in preferred embodiments a fast Fourier transform (FFT) method is used. With a fast Fourier transform, a time-discrete signal, here the time-dependent force signal, can be decomposed into its frequency components and be analyzed as a result. Thus, in the evaluation of the time-dependent force signal, the latter can be decomposed into its frequency components and those components which correspond to the pivoting frequency and/or to the rotational frequency can be further processed to determine the geometry parameter. Other signal components which have a potentially disturbing effect and/or lead to result corruption are thus explicitly excluded from the evaluation via correspondingly adapted filter methods.
Accordingly, an FT module for ascertaining a signal curve component of the force signal, said signal curve component varying substantially periodically at the pivoting frequency and/or at the rotational frequency, and modules for analyzing this signal curve component are provided in the evaluation unit of preferred exemplary embodiments. Here, the abbreviation “FT module” stands for a sub-module of the evaluation, said sub-module being realized using corresponding software and being able to perform Fourier transformations of the initially time-dependent force signal.
Some embodiments are characterized in that the evaluation of the force signal comprises ascertainment of a signal curve component of the force signal, said component signal curve varying substantially periodically at the oscillation frequency, in particular at the pivoting frequency, and (temporally resolved) analysis of this signal curve component. This is based, inter alia, on the consideration that recurring rectified signals which can be assigned with a certain degree of probability to the same location on the workpiece can be attributed with a high degree of probability to a geometry deviation of the workpiece from the target geometry. Other signal curve components having a time curve which does not correlate with the pivoting frequency or with another process-specific oscillation frequency may have other causes.
Preferably, in the case of a pivoting oscillating movement, the pivoting position is additionally detected, such that, for the evaluation, it is at all times known whether a particular feature in the signal curve is generated during pivoting in clockwise or the counterclockwise direction. Analogously, the position of a linear oscillating movement may also be detected and taken into account.
It is possible to achieve particularly meaningful results according to one embodiment with an oscillating pivoting movement or a linear oscillating movement in that at least one asymmetry parameter (asymmetry indicator) is ascertained during the analysis and represents a systematic asymmetry of the signal curve component with respect to a zero point position of the oscillating movement (pivoting movement linear movement) of the finishing tool.
In this way, it is for example possible to detect whether, and possibly to what extent, the pivot axis is not sufficiently well centered above the middle of the ball raceway, such that during the pivoting back and forth the finishing tool is systematically pressed more strongly onto one of the flanks of the groove than onto the opposite flank. This becomes noticeable in a characteristic asymmetry of the periodically varying signal curve component, can be quantitatively analyzed and be used as a basis for the automatic centering of the pivot axis, by virtue of the axial relative position between the pivot axis and the workpiece being changed such that the asymmetry is reduced, ideally until there is no longer any asymmetry.
When an assignment of the asymmetry characteristics to the pivoting position is possible, a compensation movement can be directed in the correct direction from the beginning, which is conducive to the speed of the compensation.
Analogously, an incorrectly set angle could be identified for example when machining a tapered roller bearing ring by means of a linearly oscillating finishing tool.
It is also possible to ascertain geometry information regarding the workpiece on the basis of the intrinsic rotation of the workpiece by means of the force detection. In some exemplary embodiments, provision is made for the evaluation to comprise ascertainment of a signal curve component of the force signal, said signal curve component varying substantially periodically at a rotational frequency of the intrinsic rotation of the workpiece, and analysis of this signal curve component. Preferably, this for example makes it possible to ascertain a geometry parameter in the form of a waviness the parameter. Here, term “waviness” relates to deviations of the macroscopic cross-sectional shape from a circle. Here, wavinesses of smaller order such as ovality (2nd order waviness) or trefoil or quatrefoil may be of interest. However, wavinesses of higher order can also be detected, for example fine wavinesses with ten or more waves or fifteen or more waves at the periphery. Such fine wavinesses may be of importance for example with regard to the problem of noise generation.
In preferred embodiments, the rotational position of the workpiece is ascertained during the intrinsic rotation and spatially dependent analysis of the force signal is carried out on the basis of the rotational position. When the rotational position of the workpiece is known, the force signals can be assigned to the associated locations at the periphery of the workpiece, such that, for example, given an ascertained waviness, not only the number of waves but also the position thereof in the peripheral direction in relation to a reference rotational position of the workpiece can be determined.
An approximate unambiguous assignment of particular geometric features to the location along the periphery is thus possible. As an alternative or in addition, in the case of a helical profile of a raceway, the axial position of particular geometric features may also be determined.
In some embodiments, at least one operating parameter of the apparatus is changed on the basis of the at least one geometry parameter. When, for example, the force measurement-based analysis shows that there is no waviness lying outside the tolerances in the peripheral direction, even though the provided maximum machining time has not yet been reached, it is thus possible to determine whether or not, and possibly when, there is a part of acceptable quality prior to the expected process end. For example, the cycle time may possibly be optimized individually for each workpiece. In some cases, a contact time extension may be performed until the target value for example for the waviness is reached. However, it is also possible to end a machining process early and to, as a result, shorten the cycle time when the analysis determines that the workpiece geometry already lies within the tolerance.
Further advantages and aspects of the invention will become apparent from the claims and from the description of exemplary embodiments of the invention, which are explained below on the basis of the figures.
The finishing machine 100 is a computer numerically controlled fine machining machine having multiple controllable machine axes, a drive system having multiple partially electrical drives for driving the machine axes and a control device 200 for coordinated actuation of working movements or other actions of the machine axes. To illustrate construction and function, a rectangular machine coordinate system MK denoted by lower-case letters x, y and z and having a vertical z-axis and horizontal x-and y-axes is shown.
The machine axes described below should be differentiated from the coordinate axes which are fixed in relation to the machine. Here, the term “machine axis” refers to a movable device which can be moved by at least one drive, for example an electromechanical, electro-hydraulic or electro-pneumatic drive, in at least one mechanical degree of freedom. It may be a translatory machine axis which moves for example a linearly movable carriage, or a rotational machine axis, for example having a spindle which is rotatable about a spindle axis. A machine axis may move either a tool or a workpiece. Reference is also made to NC axes in the case of computer numerically controlled machines. Preferably, these are machine axes that can be operated with closed-loop control. To this end, encoder systems (for example position encoders on translatory machine axes, rotary encoders for rotary machine axes) are present, which can use corresponding encoder signals to signal the currently available position of each machine axis to the control unit 200 via a feedback channel, and therefore machine data regarding the current state of the monitored and controlled components finishing apparatus are available at all times in the control unit.
In the case of the example, the workpiece 110 is an inner ring for a ball bearing. An annularly closed groove 115 running in the peripheral direction and having a substantially semicircular cross-sectional profile is formed on the radial outer side of the annular tool body. More specifically, the groove has a continuous concave cross-sectional profile in the shape of a circular arc, which forms slightly less than an exact semicircle. The curvature center KM lies outside the workpiece centrically with respect to the groove (cf.
The workpiece 110 is rotationally symmetrical in relation to its workpiece axis 112 and is clamped with its workpiece axis 112 oriented vertically on the flat upper side of a workpiece receiving device 120, which has a centering pin. The central axis of the circularly encircling ball raceway 115, which is referred to here as raceway axis, coincides with the workpiece axis 112. A lateral planar surface 113 of the workpiece rests on the flat upper side. This planar surface of the workpiece serves as reference surface with respect to the finishing apparatus 100 or with respect to the finishing machine. An electromotive rotary drive (servomotor) can be used to rotate the workpiece receiving device without limitation about a vertical axis of rotation ROT oriented parallel to the z-direction. The rotational speed or rotational frequency and the direction of rotation of this rotational machine axis can be predefined by the control unit 200 on the basis of operator inputs.
There are also non-rotationally symmetrical workpieces, in the case of which the central axis of the rolling element raceway (i.e. the raceway axis) does not coincide with a workpiece axis. In general, the workpiece is received in such a way that the middle of the encircling rolling element raceway lies on the axis of rotation of the workpiece receptacle.
For the machining, a finishing tool 180 in the form of a finishing stone 180 is used. The finishing stone 180, which is formed for example by a sintered material, contains a multiplicity of abrasive grains which, in the case of the example, are homogeneously distributed within a matrix composed of a binder. Abrasive grains may consist, for example, of corundum or silicon carbide; where appropriate, they may also be diamond grains or grains of cubic boron nitride (CBN). Suitable binders include, for example, a ceramic or metallic material.
The finishing stone has an abrasive working surface 182 which is intended to face the workpiece 110 and at which cutting grains, still bound in the bond, protrude from the bond. The form of the working surface of a fresh, as yet unused finishing stone can be adapted to the machining task such that relatively large-area contact with the workpiece is quickly possible during the machining. To this end, the working surface is convexly curved in a first direction which is intended to be oriented substantially parallel to the workpiece axis 112 or to the axis of rotation ROT, the radius of curvature lying in the orders of magnitude of the radius of curvature of the groove formed in the workpiece but not being identical thereto. In a second direction which is perpendicular to the first direction and runs tangentially with respect to the workpiece during the machining, the abrasive working surface is concavely curved such that it can adapt to the peripherally running curvature of the ball raceway. Owing to the oscillating movement, further areas on the finishing tool rapidly develop, which then form the actual contact surface together with the main curvatures.
For receiving the finishing stone on the finishing apparatus, a tool holder 170 is provided, which can also be referred to as stone holder, is approximately U-shaped overall and has, at the free end of a substantially downwardly directed limb, a tool receptacle 172 in the form of a clamping holder 172 for receiving the finishing stone 180.
The other limb is fastened by means of four screws to the end side of a for example pneumatically or hydraulically drivable carriage 162 belonging to a linear machine axis of the finishing apparatus. This linear machine axis is an essential component of a press-on device 160, which will be explained in more detail below.
A base part of the carriage is fastened to the end side of the spindle of a servomotor 152, which serves as oscillatory drive 152 of an oscillation device 150. Here, the oscillating movement is a pivoting movement, therefore the oscillatory drive is referred to as pivoting drive 152 and the oscillation device as pivoting device 150. During operation, the latter generates an oscillating pivoting movement of the stone holder 170 and thus of the finishing tool 180 fastened thereto about a pivot axis SWA. The horizontal spindle axis of the servomotor, said axis being oriented parallel to the y-direction, defines the pivot axis SWA of the finishing apparatus. The pivoting drive 152 is designed, and actuated, in such a way that the stone holder 170 can execute an oscillating pivoting movement about the pivot axis SWA. In this case, the zero position of the pivoting movement is oriented parallel to the x-direction, the pivot angles in the clockwise and counterclockwise direction are adapted to the machining task and may lie, for example, in the range of up to about ±20° around the zero position, for example be about ±3° to ±6° around the zero position.
The above-mentioned carriage 162, which bears the stone holder 170, is a component of a press-on device 160 for pressing the finishing stone 180 onto the workpiece 110. The carriage axis defines the press-on direction ADR able to be generated thereby. The press-on direction is oriented perpendicularly with respect to the pivot axis SWA and oscillates with the oscillating movement of the spindle of the pivoting drive 152.
The pivoting drive 152 is fastened to a vertically displaceable carriage (NC vertical carriage) 132 of a linear machine axis 130, which makes it possible to precisely set the height of the pivot axis SWA in the z-direction by means of control signals of the control unit 200.
The components of the NC vertical carriage are mounted on a carriage (horizontal carriage) 142, which is displaceable horizontally parallel to the x-direction, of a further linear machine axis 140. In this way, the position of the stone holder or of the finishing stone can be changed in the x-direction, in particular for adapting the apparatus to workpieces having different groove diameters.
When setting up the finishing machine 100 for a finishing process, a finishing stone 180 is installed in the stone holder by means of the components of the tool receptacle 172 which has a clamping action, and the horizontal and vertical machine axes are displaced in such a way that the finishing stone 180 or its abrasive working surface 182 comes into engagement with the workpiece in the region of the groove to be machined.
For the finish machining, the workpiece is then set into an intrinsic rotation about the vertical workpiece axis of rotation 112, which in this case is identical to the central axis of the encircling ball raceway. This relatively rapidly rotating workpiece movement (rotational speeds may lie for example in the range from 1000 rpm to 6000 rpm) is superposed on a working movement of the finishing tool 180. This tool movement is a back-and-forth oscillating pivoting movement of the stone holder 170 about the pivot axis SWA over a predefinable angle range around the zero position. The pivoting frequency may lie for example in the range of a few hundred Hz.
In order to achieve sufficient material removal, the finishing stone 180 is pressed with a press-on force that is predefinable via the control device onto the workpiece outer surface by means of the press-on device 160. It is possible for the press-on force to be nominally substantially constant. Since the pneumatic drive for the pressing-on of the finishing stone is fastened to the end side of the spindle of the pivoting drive 150, the press-on direction ADR oriented perpendicularly with respect to the pivot axis SWA oscillates together with the finishing stone 180.
The finishing apparatus is equipped with a force transducer 190 or force sensor 190 which makes it possible to continuously detect the reaction forces, generated by pressing of the finishing tool 180 onto the workpiece, in the vicinity of the region of action, that is to say in the vicinity of the contact zone between the workpiece and the finishing tool, and to generate corresponding force signals which 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 receptacle 172 equipped with a clamping device. One example of a clamping holder having a multi-component force transducer connected between the clamping device and the finishing stone will be explained in more detail in connection with
An evaluation device 210 integrated into the control device 200 is configured, by means of corresponding hardware and/or software, to receive the force signals of the force transducer and to subject them to temporally resolved evaluation in one operating mode. In this case, the evaluation speed is so great that a multiplicity of data points can be received and processed per vibration period duration of the pivoting drive (time for a single back-and-forth pivoting operation). The Nyqvist-Shannon sampling theorem is reliably satisfied; the sampling frequency is usually at least one order of magnitude (factor of 10) above the oscillation frequency.
The evaluation device 210 is configured, by means of corresponding software and/or hardware, to ascertain at least one geometry parameter therefrom. A geometry parameter may, for example, describe information regarding the workpiece geometry in the region of the machined ball raceway 115. As an alternative or in addition, a geometry parameter may parameterize a geometric relationship between the workpiece geometry in the region of the machined ball raceway and the position of the pivot axis SWA. Position errors between the workpiece and the tool can possibly be quantitatively detected therefrom and subsequently corrected.
Explanations for better understanding of the functioning of the exemplary embodiment and some of its advantages will now be provided on the basis of the following schematic figures.
(Ideal) force curves of this of kind occur essentially only if there is an ideal symmetry of the workpiece and also an ideal axial positioning of the finishing tool in relation to the ball raceway to be machined. Such situations occur only rarely in practice, for example if a highly experienced operator has enough time for the setting up. In principle, however, it is more frequently the case that there are deviations, to a lesser or greater extent, from this ideally symmetrical situation. The reasons for such deviations include workpiece tolerances, the dressing cycles of grinding tools used for the pre-machining of the workpiece, wear phenomena, temperature-related changes and, last but not least, the precision when setting up the machine. In practice, it is thus necessary to take account of considerable deviations from the ideal configuration.
In the case of the example in
The force arrows FA and FR have the same meaning as in the ideal case. The reaction forces at the lateral flanks are unequal.
This asymmetry in the force curve occurs because the machine-side pivot axis SWA is offset to the left in relation to the ideal position (curvature center KM of the ball raceway). The asymmetry in the force curve is an indicator of the presence of a groove offset, the extent of the asymmetry correlating with the size of the groove offset. By ascertaining a geometry parameter which quantitatively describes the asymmetry (asymmetry parameter), quantitative information regarding the groove offset is obtained.
The orientation of the finishing stone upon occurrence of the relatively greater value of the reaction force makes it possible to identify, directly from the force curve signal, the direction in which the pivot axis is offset in relation to the ball raceway to be machined. A correction can then be effected in the correct direction right from the start.
The force signals are processed in the evaluation device 210, on the basis of which control signals for correcting the groove offset are output. The finishing apparatus carries out automatic centering of the axial position of the pivot axis SWA in relation to the machined ball raceway on the basis of the geometry parameter or the asymmetry parameter. To this end, in the case of the example, the NC vertical carriage 132 is displaced parallel to the axis of rotation ROT, in order to automatically set the position of the pivot axis SWA to the axial groove middle to minimize a groove offset. The machining is then continued under optimized conditions.
Advantageous possibilities for the signal processing and evaluation of the force signals will be explained below on the basis of
Since during a real machining process the force transducer detects all the forces that occur at its installation location, the curve of the raw analog force signal (raw signal) is generally able to be reliably interpreted only with great difficulty. The raw signal is therefore subjected to signal conditioning SV, in order to eliminate or suppress technically implausible signal components through suitable filtering. In the context of signal conditioning, appropriate filtering can eliminate or suppress for example signal components which are attributed to control processes for the axis movement or to artefacts of the sampling of the raw signal, and therefore filtered force signals SIG are available for the further analysis, which essentially represent the force curve of interest, that is to say form a plausible dataset and, as a result, describe a logical state. This filtering may be carried out for example in the manner of low-pass filtering. The lower partial figure shows the time curve of the filtered force signal, which is taken as a basis for the further evaluation.
The signal evaluation comprises frequency-specific analysis of the signals which are detected by measurement technology (and filtered). In the case of the example, the evaluation device 210 comprises, for this purpose, a software module which, over the machining time, carries out a multiplicity of fast Fourier transformations (FFT) of the filtered force signal at different points in time and is therefore also referred to as FT module. This manner of evaluation implements a spectral distribution function method and also detects the gradual change in the FFT amplitudes over time as a result of the ongoing material removal.
As a result, the time-dependent force signal is analyzed with regard to signal components that occur at certain frequencies. The frequencies may in particular be the pivoting frequency and/or the rotational frequency.
For illustration,
In the lower partial figure of
It can be seen that frequency filtering in the context of signal conditioning suppresses those frequency components in the spectrum which produce with a high degree of probability disturbances and artefacts, for example due to sampling effects. After this filtering, signal components which in the case of the example are characterized by two dominant frequencies remain. The lower frequency fD corresponds to the rotational speed or the rotational frequency of the workpiece rotation and the higher frequency corresponds to the pivoting frequency fSW of the oscillating tool movement about the pivot axis.
For simplistic illustration, in the example that follows only the signal components belonging to a frequency which corresponds to the pivoting frequency of the finishing stone are taken as a basis for the further analysis. A signal component which varies substantially periodically and whose frequency corresponds to the pivoting frequency is thus identified. This is based on the inventors' finding that recurring rectified signals which vary at the same rate as the pivoting movement can be assigned with a high degree of probability to the geometry at the periphery of the workpiece. A geometry parameter can be obtained therefrom.
On the basis of
The left-hand partial figure of
This asymmetry in the forces at the edges of the groove becomes noticeable in the frequency spectrum due to the fact that further amplitudes appear at the pivoting frequency compared with the symmetrical case. More specifically, at the dominant frequency fSW of the pivoting movement, a twin pair ZW of unequal twins, that is to say two amplitudes of different height, is produced in the region of the associated frequency. (For illustrative purposes, the twins are illustrated next to one another, they actually lie one on top of the other at fSW)
The production of such twins having amplitudes of unequal height in the frequency-based illustration is considered to be a signature or fingerprint for the presence of a middle offset. If there is a middle offset, this twinning necessarily appears in the signal analysis. Such twinning (further amplitudes at the same frequency fSW) is considered in the evaluation to be a necessary condition for the presence of a groove offset.
In other words: a force imbalance at opposite flanks of a ball raceway owing to a middle offset becomes systematically noticeable in that (i) a further amplitude appears in the region of the corresponding vibration frequency in the spectrum, and therefore a twin pair having two amplitudes is produced, and in that (ii) these related amplitudes have different intensities or amplitudes.
These features of the spectrum can be quantitatively evaluated. They contain information regarding the force imbalance and the extent and direction of the middle offset. From this, parameters are calculated which indicate an asymmetry and are therefore also referred to as asymmetry parameters. The corresponding data are made available to the closed-loop control system.
The asymmetry is quantitatively evaluated and provides information regarding the extent and direction of the groove offset. From this, the system can calculate a correction value which specifies the extent to which, and the direction in which, the workpiece and/or the tool has to be displaced parallel to the workpiece axis of rotation relative to the other part in order to compensate for the groove offset and, as a result, to achieve the most symmetrical machining conditions. In the case of the example, the axial relative position between the tool (pivot axis) and the workpiece is changed by actuation of the vertical carriage 132 such that the pivot axis SWA is centered above the ball raceway 115 and thus the force imbalance disappears.
The automatic centering described here of the pivot axis SWA in relation to the ball raceway 115 to be machined is based on geometry parameters which result from analysis of the time curve of the force signal. In this example, the geometry parameters represent the in each case current groove offset in terms of extent and direction. The centering operation takes place fully automatically without intervention of an operator. Owing to the automatic detection of the groove middle, the setting-up process can lead in a rapid and targeted manner to a workpiece that is optimally centered in the context of the tolerances. Even during the machining, the tool can always be automatically centered with respect to the ball raceway as soon as significant asymmetries start to present in the force curve, as a result of which the geometric tolerances of the ball raceway can be systematically adhered to precisely. 100% force control also increases the machine safety. Lastly, the tool costs can also be reduced, since the necessary self-dressing of the finishing tool when a groove offset is present is not necessary or not to the extent if the finishing tool is optimally centered with respect to the raceway.
The functionality of the automatic centering of a finishing tool in relation to the ball raceway to be machined is an example of force-controlled closed-loop position control. In this case, the position of the workpiece in relation to the (unchanged) position of the pivot axis in the machine is changed on the basis of the force signal by actuation of the linear machine axis 130 with vertical carriage 132 such that the ball raceway running at the periphery of the workpiece is centered in relation to the finishing tool.
The force signal can also be evaluated with regard to signal components belonging to a frequency which corresponds to the rotational frequency of the workpiece. In this way, it is for example possible to ascertain a geometry parameter which quantifies any waviness of the ball raceway profile in the peripheral direction. Preferably, an absolute rotary encoder signal is considered to be retrieved, from which the current rotational position of the workpiece becomes apparent. In this way, the force signals can also be assigned to their point of origin along the periphery of the ball raceway.
In an idealized frequency observation, a trefoil over the periphery would become noticeable in that the rotation generates not only an amplitude at the rotational frequency but also a further amplitude at triple this frequency, that is to say 3*fD, since the force theoretically passes through three maxima (in regions with bulging) and three interposed minima of the force in the case of a full rotation. This is indicated in
A considerable improvement in the reliability and significance of measured values can be achieved by virtue of a spatially resolved evaluation of the force signal being carried out in addition to the temporally resolved evaluation. This makes it possible, inter alia, to take account of the fact that, during the finish machining, the ball raceway to be machined is passed over by the oscillating finishing tool not only once but multiple times. In the case of ball bearing components, several hundred or several thousand passes can typically be provided; in the case of ball screw drives, the number of passes is usually lower, for example in the order of magnitude of ten to twenty. In this case, multiple force signals are theoretically detected 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 which are systematically produced at the same surface location in each pass can then be attributed to an actually present local irregularity of the surface. If, by contrast, in the case of hundreds of passes a force maximum signal is generated at a certain location only once or twice approximately, whilst the point appears unobtrusively smooth in the case of the multiplicity of other passes, the force maxima are most probably not expected to be due to a surface irregularity, but rather are attributed to other causes. A spatially resolved evaluation can thus exclude pseudo-events and, as a result, make the measurement result traceable overall.
In order to obtain a database for the spatially resolved evaluation, force signals detected during the machining at locations of a workpiece surface that are successively passed over are assigned to the respectively corresponding spatial coordinates in a workpiece coordinate system. This is possible since information regarding the rotational position of the workpiece during the intrinsic rotation is available at all times by way of the rotary encoder signals of the rotary drive, and in the case of the machining of ball screw drives, the axial position of the finishing tool during the machining can be retrieves by way of a position encoder a linear machine axis.
In the case of the example, the elongate workpiece 310 is a toothed steering rack with a ball screw portion 314. Here, it is clamped with a horizontal workpiece axis 312 between tips of a workpiece receiving device (not illustrated in any more detail). The workpiece has a toothing portion (not illustrated) which transitions into a substantially cylindrical portion adjoined by a ball circulating portion in which a ball raceway 315, formed in the manner of a groove, runs helically about the workpiece axis 312 at the workpiece periphery. In contrast to the ball raceway of a rolling bearing ring, the ball raceway here has a pitch, that is to say does not run in a plane perpendicular to the workpiece axis of rotation, but rather runs obliquely with respect thereto.
Such ball raceways generally have a Gothic cross-sectional profile in order to produce a point contact between the ball raceway and the balls merely at each flank, such that very good running properties with precise guidance and an increase in the axial force absorption capability can be achieved. A Gothic profile (also called pointed arch profile, cf.
In contrast to the first exemplary embodiment, this finishing apparatus has two pivoting devices 350-1, 350-2, the pivot axes SWA1 and SWA2 of which are oriented at an angle of about 40° to 60° relative to one another. These are mounted in a horizontally displaceable base frame 305 fastened to an NC horizontal carriage (not illustrated). Whilst the first pivot axis SWA1 runs obliquely with respect to the horizontal plane, the second pivot axis SWA2 runs in the horizontal plane. In this way, the helical encircling ball raceway can be finish-machined equally at two points that are offset slightly in the peripheral direction and axial direction. The axial distance between the pivot axes may be set manually by means of a handwheel during the setting up.
The pneumatically operating press-on devices 360-1, 362-2 are of similar or identical configuration to the press-on device 160. The pivoting devices and the drives thereof and the pneumatically operating press-on devices 360-1, 362-2 have substantially the same construction as those in the first exemplary embodiment, which is why reference is made to the description there. There is an angle setting device which is manually adjustable by means of a handwheel and makes it possible in each of the finishing units to move the orientation of the pivot axis out of the y-z plane, in order to adapt the finishing stone orientation to the oblique profile of the helical groove.
Since helically encircling ball raceways 315 on ball screw drives can extend over relatively large axial portions of the workpiece, in order to track the pitch during the workpiece rotation there is a translatory NC machine axis 330 with a displacement direction oriented parallel to the axis of rotation of the workpiece receptacle and a correspondingly great linear stroke. This NC machine axis comprises a horizontal carriage 332 borne by the base frame 305 in which the components of the pivoting device, of the press-on devices, etc. are mounted.
This finishing apparatus is also configured for force-controlled closed-loop position control, which operates similarly to in the first exemplary embodiment. After the finishing machine has been manually set up, the finishing process is initiated. At the beginning, the finishing stones are fed in and, as described above, an automatic centering operation takes place in order to center the finishing stones as best as possible in the facing portion of the ball raceway on the basis of force signals.
As the workpiece rotates further, the finishing stones then follow the pitch profile synchronously with one another, whereby the NC horizontal carriage 332 displaces slowly in the axially parallel direction (x-direction). If the pitch corresponds to the expected pitch, the centering of the finishing stones remains unchanged. In the case of any deviations, for instance because a workpiece of a different batch with a slightly different screw geometry was inserted, significant changes in the force signal occur, the evaluation of which can lead to the warning signal thereof. In this way, automatic identification of workpiece type is also possible. Furthermore, analogously to the above example, the workpiece geometry is checked in the peripheral direction for example for wavinesses. Any defects in the ball raceway can also be detected.
One example of the type and arrangement of a force sensor 400 is now explained on the basis of
In the reinforced wall portion 374-2 of the opposite side, a cutout, in which a widened press-on foot of a clamping screw 378 sits, is provided on the inner side. The foot is supported on the outer side of the clamping plate 373-2, the opposite surface of which is supported on the finishing stone. Tightening of the clamping screw 378 clamps the finishing stone 380 between the clamping plates 371-1, 371-2.
The clamping force FK acts parallel to a first sensitivity direction SR1 of the force sensor 400, which can thus measure, inter alia, the clamping force.
On the opposite side, there is only a low friction coefficient between the outer side of the clamping plate 373-2 and the foot 377 of the clamping screw, such that a relative displacement in the press-on direction ADR is easily possible. In other embodiments, the clamping screw 377 is supported on the second clamping plate 373-2 by way of rollers, since only a low rolling resistance acts in the press-on direction.
The force sensor 400 is a multi-channel force sensor. A second sensitivity direction SR2 runs perpendicularly with respect to the first sensitivity direction SR1 parallel to the direction of the press-on force FA. In this way, shear force components acting in this direction can be detected. In the first sensitivity direction SR1, it is also possible to detect force components attributed to tilting moments caused by tilting of the finishing stone 380 relative to the stone holder 370 about a direction running perpendicular to the pivot axis.
The force sensor 400 is inserted in the force flow between the zone of action of the finishing stone (contact zone between the finishing stone and the workpiece) and the press-on device 360, specifically between the finishing stone 380 and the stone holder 370. This position very close to the point of origin of the reaction forces allows force changes and force change components in the engagement region between the workpiece and the tool to be detected with high precision virtually without delay and without substantial damping between the point of origin and the measurement location.
This force measurement is not dependent on a substantial deformation of the stone holder, which can correspondingly be of very torsionally stiff and flexurally stiff design, which in turn is beneficial to the directness of the force transmission between the press-on device 360 and the finishing stone 380.
Instead of a multi-channel force sensor, it is also possible for two, three or more one-channel force sensors to be provided, which can detect the force changes of interest in different directions with high precision.
Generally, the press-on force should preferably be generated in such a way that no transverse forces are produced. As a result, a directly measurable reaction force is produced in a force direction at a measurement point in the force flow, which is preferably arranged in the direction of action for this purpose. As an alternative, the reaction force may also be measured indirectly or in a manner not aligned with the direction of action. To this end, three or more force measurement points that can be evaluated synchronously should be provided, and therefore the individual force directions can be converted to a force vector via known lever lengths. The effective force then results via geometric calculations from the individual measured forces. Damping induced by the construction and any lever stiffnesses can be compensated for via proportional factors.
Important aspects of some embodiments of the invention have been explained hitherto on the basis of the machining of ball raceways having a substantially semicircular cross-sectional profile. However, the applicability of the invention is not restricted thereto. Rolling element raceways having a different cross-sectional form can also be finished, the rolling elements do not have to be spherical. For example raceways for angular contact bearings (cf.
Angular contact bearings are ball bearings in which the bearing cross section in the region of the ball raceway LB is non-symmetrical. Angular contact bearings are intended to be able to absorb forces whose line of action does not run exactly perpendicular to the axis, but rather obliquely at a certain angle with respect to the perpendicular axis. To this end, the shoulder is stronger around the balls in the region of the line of action (cf.
In contrast to ball bearings, roller bearings have cylindrical rolling elements which are particularly suitable for higher loads at relatively lower rotational speeds. The rollers offer a larger contact area between the rolling body surface and the raceway than in the case of ball bearings. As shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2022 202 259.9 | Mar 2022 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2023/055036 | 2/28/2023 | WO |
