Grinding machine and grinding method for grinding coated brake discs

Abstract

A grinding machine (100) for grinding substantially plane-parallel, annular workpiece surfaces on a disc-shaped workpiece section (BA) of a workpiece (WS1, WS2) comprises a grinding unit (121) in the manner of a double-disc grinding unit with two tool spindles (132-1, 132-2) which are arranged coaxially to one another and each carry a grinding disc (130-1, 130-2), wherein the grinding discs are arranged with abrasive side faces (135-1, 135-2) which face one another and delimit a grinding space (133), and at least one workpiece spindle (154) having a workpiece holder (155) for holding the workpiece (WS1, WS2) in a rotationally fixed manner such that the workpiece section (BA) of the held workpiece extends arcuately through the grinding space for grinding. The grinding machine comprises a first position data determination system for determining workpiece position data which represent an axial position of a workpiece surface (O1, O2) at at least one surface location in relation to a reference coordinates system (RKS) which is fixed to the machine, and a second position data determination system for determining tool position data which represent an axial position of an abrasive side face (135-1, 135-2), facing the workpiece surface (O1, O2), in relation to the same reference coordinates system (RKS). A control unit (190) of the grinding machine is configured, in at least one operating mode, to control at least one grinding parameter in at least one phase of the grinding operation depending on the workpiece position data and / or the tool position data. The grinding machine can be used to grind workpiece surfaces of an annular braking section of a brake disc.

Description

[0001] Grinding machine and grinding process for grinding coated brake discs

[0002] FIELD OF APPLICATION AND STATE OF THE ART

[0003] The invention relates to a grinding machine and a grinding method for grinding essentially plane-parallel, circular workpiece surfaces on a disc-shaped workpiece section of a workpiece. A preferred area of ​​application is the grinding of surfaces of a circular brake section of a brake disc, in particular a coated brake disc.

[0004] A brake disc is the part of a disc brake that is non-rotatably connected to the wheel, upon which the brake pads attached to a brake caliper act to decelerate rotational movement. A brake disc is usually point-symmetric or rotationally symmetrical to an axle center through which the axis of rotation of the brake disc passes. A brake disc has a central hub section and a circular brake section enclosing the hub section, which has two axially opposing free surfaces that serve as the friction surface of the brake disc.

[0005] Conventional brake discs for the mass market are usually made of gray cast iron. Gray cast iron brake discs are now reaching their limits in terms of corrosion and wear behavior, among other things. While there are also brake discs made of ceramic materials, these are expensive and currently only available for high-end vehicles.

[0006] Upcoming tightening of regulations on fine particle emissions from motor vehicles suggests that future brake discs for motor vehicles will have to be designed to release fewer fine particles during braking. One approach to this is to coat the brake discs, or rather the surface sections intended as friction surfaces, with a thin functional layer made of a more wear-resistant material. In coated brake discs, the surfaces of the annular braking section each bear a functional layer that is rotationally symmetrical with respect to the rotation axis, the free surface of which is designed as the friction surface of the brake disc.

[0007] The manufacturing process for a coated brake disc comprises one or more coating operations for coating the surfaces of the braking section of a brake disc with a functional layer that, due to its relatively high mechanical hardness, can have a wear-reducing function. Alternatively or additionally, a corrosion-inhibiting effect can also be present. Such functional layers often consist essentially of metal; they can consist of a single layer or multiple layers with different properties. Such coatings can be applied, for example, by flame spraying or laser cladding. Typical layer thicknesses can range, for example, between 50 μm and 350 μm. The coatings are usually applied on both sides. The documents EP 2 746 613 A2 and WO 2019 / 021161 A1 disclose examples of coated brake discs.

[0008] Depending on the coating process and the coating material, the exposed surfaces of the finished coatings can exhibit different properties. The functional layers are generally relatively hard mechanically and relatively rough on the surface after coating.

[0009] A subsequent grinding process is intended to create a sufficiently flat surface on the coating that is optimized for braking function. For example, the specifications for the friction surfaces of a brake disc can be such that the mean roughness Ra determined according to DIN EN ISO 4288 should be in the range of 1 pm to 3 pm — 3.2 pm, and the flatness deviation should not exceed 20 pm (cf. WO 2021 / 224308 A).

[0010] EP 3 789 512 A1 discloses a system for coating and subsequent grinding of brake discs.

[0011] TASK AND SOLUTION

[0012] Against this background, the object of the invention is to provide a grinding machine and a grinding method which can be used, for example, in the production of workpieces in the form of brake discs, in particular coated brake discs, and which systematically enable the production of ground workpieces within narrow manufacturing tolerances even in small batch sizes, while allowing the grinding process to be optimized as required, for example with regard to costs and / or processing time.

[0013] To achieve this object, the invention provides a grinding machine having the features of claim 1. Furthermore, a grinding method having the features of claim 14 is provided. Preferred developments are specified in the dependent claims. The wording of all claims is incorporated into the description by reference. According to one aspect of the invention, a grinding machine is provided for grinding essentially plane-parallel, circular-ring-shaped workpiece surfaces on a disk-shaped workpiece section of a workpiece. In a preferred field of application, the workpieces are brake disks whose circular-ring-shaped braking sections are to be ground on both sides.

[0014] Due to its special design features and the layout of the operating control system with the control unit and the hardware components (including sensors and actuators) and software components (including control and evaluation software) communicating with it, the grinding machine is configured, or can be configured by providing a corresponding operating mode, in such a way that special grinding processes can be carried out that were not feasible with conventional grinding machines.

[0015] The grinding machine is designed like a double-side surface grinder. The grinding machine comprises a grinding unit with two tool spindles arranged or capable of being arranged coaxially to one another on a machine frame, each supporting a grinding wheel. The grinding wheels are arranged with facing abrasive side surfaces that define a grinding chamber of variable axial height in the axial direction. Each of the grinding wheels can be rotated about its associated rotational axis by means of an associated rotary drive, independently of the other grinding wheel, with a predefined speed profile. A feed drive can be used to advance it axially by linear displacement parallel to the associated rotational axis with predefined feed parameters, such as a predefined feed speed. If necessary, a grinding wheel can also be advanced perpendicular to the rotational axis, i.e., radially to it.

[0016] In the case of a "coaxial" arrangement of the tool spindles within the meaning of this application, the axes of rotation coincide within the framework of manufacturing tolerances; slight deviations from the mathematically exact coaxiality due to parallel offset or tilting of one or both tool spindles are possible, e.g. under one-sided loading during grinding.

[0017] The grinding machine further comprises at least one workpiece spindle with a workpiece holder for rotationally fixedly holding a workpiece, wherein the workpiece holder is rotatable by means of a rotary drive about a rotation axis running essentially parallel to the rotation axes of the grinding wheels. The workpiece spindle is arranged in a working position at least during one phase of a grinding operation such that the workpiece section of the held workpiece to be machined runs in a circular arc through the grinding space. Depending on the design of the grinding machine, the workpiece spindle can be permanently in the working position or movable. For example, the workpiece spindle can be moved into the working position to carry out a grinding operation and moved out of the working position again after completion of a grinding operation.The rotating workpiece spindle can be stationary during a material removal phase while the grinding wheels are advanced. This allows for variants of cross-side double-face grinding in which the advance movement runs transversely, particularly perpendicularly, to the workpiece surfaces to be machined.

[0018] Alternatively or additionally, in some cases it is also possible for the workpiece spindle and the grinding unit to be moved relative to each other during a phase of a grinding operation, e.g., by a drive-controlled displacement of the workpiece spindle perpendicular to its rotational axis, while the grinding wheels are in engagement with the workpiece surfaces. This allows variants of a longitudinal-lateral double-surface grinding process to be implemented in which an infeed movement occurs along the workpiece surface, i.e., parallel to it, e.g., in a direction radial to the rotational axis of the tools and / or the workpiece.

[0019] A grinding operation can include both variants, e.g. in such a way that in a preceding phase, roughing takes place by means of longitudinal side double face grinding, through which a larger part of the material is removed at a high removal rate, and then in a subsequent phase, finishing takes place by means of transverse side double face grinding, through which the final dimension and the desired surface quality are achieved at a lower removal rate.

[0020] The grinding machine also features an operating control system with a control unit for controlling the operation of the grinding machine. The control unit uses appropriate control signals to control the connected actuators, including the drives for the workpiece and tool movements. The control unit also receives sensor signals from connected sensors and processes them, among other things, for the purpose of targeted control of actuators in a control loop.

[0021] In this application, the term "axial direction" refers to the direction in which the axes of rotation run on the workpiece side and the tool side. The axial direction can be oriented, for example, vertically (parallel to the direction of gravity) or horizontally or obliquely to the horizontal and vertical. The grinding wheels are advanced in the axial direction. The suitability of the grinding machine for carrying out particular grinding processes can be recognized by the fact that the grinding machine has a first position data determination system for determining workpiece position data and a second position data determination system for determining tool position data and that the control unit is configured in at least one operating mode to control at least one grinding parameter of a grinding operation in at least one phase of the grinding operation as a function of the workpiece position data and / or the tool position data.

[0022] The workpiece position data are data that represent the axial position of a workpiece surface to be ground at at least one surface location relative to a machine-fixed reference coordinate system. In contrast, the tool position data are data that represent the axial position of an abrasive side surface facing the workpiece surface to be machined relative to the same reference coordinate system.

[0023] The term “axial position” refers to the position of the corresponding surfaces or surface locations in the axial direction, which runs parallel to the axes of rotation of the grinding wheels and the workpiece.

[0024] Such a grinding machine thus has two separately operable devices that can determine the axial workpiece position and the axial tool position, respectively, with reference to the same reference coordinate system. This allows the corresponding position data to be directly offset against each other during data evaluation to determine a spatial relationship in the axial direction between the abrasive side surfaces and the respective associated workpiece surfaces.

[0025] This possibility of calculating workpiece-related and tool-related axial position data in the same reference coordinate system results in numerous advantages, which will be explained later using examples.

[0026] The invention takes into account, among other things, the realisation that a grinding process or grinding operation should always be adapted to specific input conditions and user requirements, and that these can vary. For example, the dimensions of the workpieces at the input (before grinding begins) can differ from workpiece to workpiece, even if the workpieces are nominally identical (in terms of their specification) and the pre-processing (e.g. coating process) was carried out nominally unchanged from workpiece to workpiece. When changing from one workpiece type to the next, adjustments to the grinding process are generally necessary. It is also possible that the abrasives used do not always have identical properties, which can influence the grinding process. Finally, user requirements can vary.Particularly when grinding coated brake discs with mechanically very hard and initially very rough surfaces in the brake section to be ground, there is a sensitive interplay between tool wear and the efficiency of material removal during grinding. From a user's perspective, it may therefore be desirable to always be able to set the optimal operating point. The invention creates the conditions for this.

[0027] According to a further development, the control unit is configured to control, depending on the workpiece position data and / or the tool position data in at least one operating mode, (i) the feed rate of one grinding wheel or both grinding wheels, (ii) the rotational speed of one grinding wheel or both grinding wheels, and / or (iii) the rotational speed and / or direction of rotation of the workpiece spindle that carries the workpiece for the grinding operation. Each of these grinding parameters can be used to specifically influence the material removal rate and / or the wear on the tools occurring during grinding and / or the quality of the grinding result. Frequently, two or more of these grinding parameters are changed simultaneously by a control intervention.

[0028] At least during one phase of the grinding operation, the workpiece spindle supporting the workpiece is arranged in a working position characterized by the workpiece section to be machined extending in a circular arc through the grinding chamber. To perform a grinding operation, the workpiece and the grinding wheels are set in rotation about their rotational axes. During one phase of a grinding operation, the abrasive side surfaces are brought into engagement with the associated surface by axially advancing the grinding wheels.

[0029] The term "grinding operation" within the meaning of this application includes, among other things, those phases in which material removal occurs, i.e., in which at least one tool is in material-removing engagement with the workpiece. Furthermore, a grinding operation also includes phases without contact between the tool and the workpiece, in particular a start-up phase preceding the material removal. The determination of workpiece position data and / or tool position data can be performed in various ways.

[0030] Preferably, suitable measurements are performed. According to a corresponding development, the first position data determination system is configured as a first position measuring system for the metrological determination of the workpiece position data, and the second position data determination system is configured as a second position measuring system for the metrological determination of the tool position data in the same reference coordinate system. A grinding machine configured for such measurements thus has a first position measuring system for the metrological determination or measurement of the workpiece position data and a second position measuring system for the metrological determination or measurement of the tool position data.The position measuring systems are configured to determine the axial positions of the surfaces to be measured (workpiece surfaces to be ground in the first position measuring system or abrasive side surfaces in the second position measuring system) with sufficiently high accuracy and with reference to the reference coordinate system.

[0031] External measurement of the workpiece prior to installation, i.e., outside the grinding machine and before mounting it on the grinding machine's workpiece holder, would be fundamentally possible. A reference to the machine's internal reference coordinate system could then be established during installation.

[0032] However, according to a further development, the workpiece is measured in an already installed state, i.e., after being attached to a tool holder of a workpiece spindle of the grinding machine. Accordingly, the first position measuring system and the second position measuring system can be designed so that the workpiece can be measured while installed in the workpiece holder. By measuring the workpiece internally within the machine, some sources of error can be avoided. For example, it cannot be ruled out that the position of the workpiece may change after it has been inserted and held down. Furthermore, geometric deviations, such as shielding, must be taken into account.

[0033] In particular, the first position measuring system and the second position measuring system can be configured such that position measurements can be performed when the workpiece holder with the workpiece is in the working position. This can be achieved, among other things, by appropriately arranging sensors for position measurement. This eliminates sources of error that could arise between a measurement taken on the workpiece outside the working position during transfer to the working position.

[0034] Preferred embodiments are characterized in that the grinding machine is configured to simultaneously measure workpiece position data and tool position data during at least one phase of a grinding operation. In a grinding process with simultaneous acquisition of workpiece position data and tool position data of the grinding wheels intended for grinding the workpiece, real-time monitoring and, if necessary, real-time control of the sequences of a grinding operation is possible.

[0035] According to a further development, the surface position measurement is performed contactlessly, for which at least one contactless (non-tactile) sensor can be used. This prevents mechanical damage to the sensor and, if necessary, to the workpiece surfaces when measuring on the rotating workpiece and / or the rotating grinding wheel.

[0036] To determine the workpiece position data and / or the tool position data, a distance measurement is preferably performed between at least one distance sensor of the respective position measuring system and the surface to be measured. The axial position of the distance sensor in the reference coordinate system is known with a correspondingly high degree of accuracy.

[0037] According to a further development, a distance sensor can be calibrated from time to time against a reference standard. For this purpose, the grinding machine has an internal calibration system with reference elements. The distance sensors can be moved in a controlled manner within an individual working range. Each distance sensor is assigned a reference element that is located within the working range of the distance sensor and can thus be approached for a reference measurement. A reference element can be assigned to multiple distance sensors. The distance sensors can be calibrated regularly or on an ad hoc basis and adjusted as needed to ensure consistently high measurement accuracy. This allows traceable measured values ​​to be generated. Traceability is known to be a property that allows a measurement result to be related to a standard or is related to it.

[0038] Preferably, a pneumatic measuring system with at least one measuring nozzle acting as a distance sensor is used for distance measurement. Such measuring systems, which operate according to the nozzle-baffle plate principle, are also referred to as air measuring systems. Compressed air flows from the measuring nozzle toward the surface to be measured, and a property of the compressed air that depends on the distance between the measuring nozzle and the surface is measured and evaluated to determine the distance. In some variants, the dynamic pressure is measured; in others, the volume flow. For this application, an air measuring system has the advantage, among other things, that it can operate with relatively high measurement accuracy in the range of a few micrometers, even on relatively rough surfaces.For measurements that take place during a “wet” grinding operation, it is advantageous that the measuring point can be blown clean locally using compressed air, so that the distance measurement records the distance to the surface to be measured, even if it is otherwise covered with a layer of liquid.

[0039] Alternatively, other distance measuring systems can be used, for example optical measuring systems, such as laser measuring systems, or measuring systems whose sensors work with radar radiation, or measuring systems with capacitive or inductive sensors.

[0040] According to a further development, an axial actuator is assigned to a distance sensor, preferably to each of the distance sensors, for the controllable change of an axial position of the distance sensor. For this purpose, for example, a measuring nozzle of a pneumatic measuring system can be mechanically coupled to a probe element of a tactile sensor, allowing fine adjustment of the axial position of the measuring nozzle. If a tactile sensor is used to adjust the axial position of a measuring nozzle, the tactile sensor can simultaneously function as an axial displacement sensor, whose encoder signals can be used to precisely determine the axial position of the associated distance sensor.

[0041] Some examples of available operating modes are described below.

[0042] According to a further development, an incoming inspection of the workpiece to be ground is provided by measuring the workpiece in the grinding machine. For this purpose, it can be provided that after the workpiece has been fastened to the workpiece holder and before a grinding operation on the workpiece begins, workpiece position data are determined for each of the surfaces of the workpiece section to be ground. Because the measurement takes place after the workpiece has been fastened to the workpiece holder, a relationship can be easily established between the determined position data and a machine-fixed reference coordinate system. During this measurement, it is preferably provided that workpiece position data are determined for a plurality of surface locations offset in the circumferential direction and / or for a plurality of surface locations offset in the radial direction.A spatially resolved determination of axial positions takes place, in particular a two-dimensional spatially resolved measurement. This allows important workpiece geometry data to be determined for further processing, for example the thickness of the workpiece section between the essentially parallel workpiece surfaces, any curvature of the workpiece section, which in the case of brake discs is often also referred to as shielding, and any waviness in the circumferential and / or radial direction. This allows the subsequent grinding process to be optimized individually, i.e. separately for each workpiece, by setting the appropriate grinding parameters. The measured surface locations or measuring points can, for example, lie on concentric circles around the axis of symmetry of the workpiece or on a spiral path and this.

[0043] In a corresponding manner, an initial inspection of the ground workpiece can be carried out within the grinding machine before it is unloaded from the tool holder.

[0044] According to a further development, a contactless approach control is implemented in which an abrasive side surface of a grinding wheel is advanced toward the workpiece surface to be ground until it reaches a starting position of the next infeed phase, which is located at a safety distance from the workpiece surface to be machined. Initial workpiece position data and initial tool position data are calculated before the start of material removal to determine the starting position. This can make an important contribution to efficient process control during grinding with the least possible time loss while simultaneously protecting grinding tools and workpieces.

[0045] This contactless approach control system takes advantage of the fact that the workpiece position data and the tool position data are available in the same reference coordinate system and can therefore be directly offset against each other. This allows the axial position to which the grinding wheel will initially be advanced to be determined even before the grinding wheel begins its feed movement. The design of the safety distance ensures that contact between the abrasive side surface and the workpiece surface is absolutely certain to be completely eliminated at the end of this initial feed movement. In many designs, the safety distance is ideally in the range of approximately 10 μm to approximately 100 μm.Preferably, the grinding wheel is advanced at a first infeed speed until the starting position is reached, and the infeed speed is automatically reduced to a lower second infeed speed upon reaching the starting position. The first infeed can therefore be performed at rapid traverse in a relatively short time, even with a longer feed distance, before the second infeed phase begins in the immediate vicinity of the workpiece surface, which is performed at a significantly lower infeed speed. This can be extended, if necessary, until contact is made between the grinding wheel and the tool.

[0046] The second infeed operation can be maintained at least until a nominal contact position is reached. The nominal contact position is an axial position at which the current tool position (i.e., the axial position of the abrasive grinding surface) corresponds to the initial workpiece position (i.e., the axial position of the workpiece surface to be ground). This makes it possible to change the infeed speed and / or another grinding parameter again at the beginning of contact between the grinding wheel and the workpiece surface, or shortly before or after the initial contact, in order to achieve an optimized material-removing grinding process. The second infeed operation can also be continued continuously beyond the time of the initial contact without changing the infeed speed.

[0047] In some embodiments, particularly good adaptation of the grinding parameters of a grinding process to the conditions of the workpiece can be achieved by determining tool position data continuously or intermittently as a function of time, i.e., in a time-resolved manner, during the grinding operation. The resulting time-dependent tool position data is processed to control the infeed of the grinding wheels and / or at least one other grinding parameter. The real-time observation of the axial position of the grinding surface allows conclusions to be drawn, among other things, about the grinding behavior of the workpiece and tool.

[0048] Alternatively or additionally, it can be provided that during the grinding operation, workpiece position data are determined continuously or intermittently as a function of time and the resulting time-dependent workpiece position data are processed to control the feed of the grinding wheels and / or at least one other grinding parameter.

[0049] In some embodiments, the control unit of the grinding machine is configured to relate time-dependent tool position data determined in an operating mode during the grinding operation to time-dependent feed position data. The time-dependent feed position data can be determined from the control program for the grinding process and / or optionally also measured using at least one displacement sensor. In some embodiments, this operating mode is also referred to as a grinding ratio optimization mode, as it creates an information base for the grinding ratio at which the grinding process is operating at the time of observation. Among other things, a comparison can be made between the feed and the resulting achievable material removal. A comparison of these time dependencies can then be used to determine wear and, accordingly, to optimize wear.

[0050] The control unit can be configured to determine information about the current grinding ratio G and make it available for use, e.g., in the form of a G-value or corresponding data. The grinding ratio is characteristic for each grinding process. In the definition used here, the grinding ratio G expresses the relationship between material removal and wheel wear in volume units (e.g., cm 3 / cm 3). If a phase of the grinding process is intended to achieve the best possible dimensional and shape accuracy, the feed ratios can be adjusted to minimize wheel wear. However, if, for example, a high material removal rate is desired in order to remove a large amount of material thickness in a relatively short time, the feed rate can be set comparatively higher. This provides the user with an important tool for optimizing "their" grinding process thanks to the ability to determine the workpiece position data and the tool position data.

[0051] Some embodiments offer the option of entering a desired target grinding ratio for the grinding process or the desired parameters required to determine a target grinding ratio via an operating unit. The control unit can be configured so that the current grinding ratio or the current value is calculated continuously or at short intervals based on the data supplied by the sensors and compared with the target grinding ratio. As long as the current G value remains within a tolerance range around the target grinding ratio, the grinding process is proceeding satisfactorily. The control system can monitor compliance with the G value and control operation based on the result of the comparison.For example, an acoustic, visual, and / or haptic warning signal can be generated if the current G-value leaves the tolerance range or approaches the tolerance limit at a certain rate of change, indicating that a limit violation is imminent. The operator can then intervene and attempt to determine the causes of the undesired process change and, if necessary, eliminate them. It is also possible to design the control unit so that the process is automatically regulated to maintain the G-value. The machine control then automatically ensures, within certain limits, that the process runs approximately at the desired target grinding ratio without operator intervention.

[0052] It is possible for the grinding machine to have only a single workpiece spindle arranged in the working position. The workpiece holder can be loaded and unloaded, for example, by means of a robot or other handling device, or manually. However, a grinding machine is often designed as a multi-station grinding machine having a transport system comprising a spindle carrier unit movable relative to a machine base, which carries at least one workpiece spindle which is rotatably mounted about a rotation axis relative to the spindle carrier unit and has a workpiece holder. The grinding machine has at least two work stations accessible via the workpiece spindle, which work stations comprise a loading station and at least one grinding station with at least one grinding unit.This allows loading and unloading on the one hand and grinding on the other to be spatially separated, so that these steps can be carried out at the same time. This can significantly increase the piece output. In some variants, the spindle support unit is rotatable and can be constructed using components from a conventional rotary table. Alternatively, at least one translationally movable spindle support unit can be provided, which can, for example, be moved back and forth in a straight line between the loading station and the work station on the grinding unit. If required, two mutually orthogonal, alternately usable linear systems can be provided, e.g. to provide two loading stations.

[0053] BRIEF DESCRIPTION OF THE DRAWINGS

[0054] 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.

[0055] Fig. 1 shows a schematic side view of an embodiment of a grinding machine configured for double-sided grinding of coated brake discs;

[0056] Fig. 2 shows a workpiece-side part of a schematic representation of the system setup for workpiece measurement; Fig. 3 shows a tool-side part of a schematic representation of the system setup for tool measurement;

[0057] Fig. 4 shows a measuring nozzle to which a tactile sensor is assigned as an axial actuator for fine adjustment of the axial position with respect to the carrier of the measuring nozzle;

[0058] Fig. 5 shows schematically an arrangement for measuring the axial position of the side surface of the upper grinding wheel;

[0059] Fig. 6 and 7 show two possibilities for spatially resolving position measurement of workpiece surfaces, each with a large number of measuring points;

[0060] Fig. 8A to 8D show diagrams illustrating measured value evaluations;

[0061] Fig. 9 shows a path-time diagram of a contactless start-up control;

[0062] Fig. 10 shows a schematic overview diagram with time-dependent distance values ​​and values ​​for the infeed of a grinding wheel during a grinding operation;

[0063] Fig. 11 shows schematically in 11A to 11C gradient curves representing the time dependencies during a grinding operation;

[0064] Fig. 12 shows a diagram showing the relationship between wear and grinding ratio (G-value)

[0065] DETAILED DESCRIPTION OF THE EMBODIMENTS

[0066] Fig. 1 shows a schematic side view of an embodiment of a grinding machine 100 designed for grinding essentially plane-parallel, annular workpiece surfaces 01, 02 on a disc-shaped workpiece section of workpieces WS1, WS2 in the form of brake discs WS1, WS2. In the example, the grinding machine is configured for grinding surfaces on both sides of an annular brake section BA of coated brake discs.

[0067] Each of the brake discs has a base body made of grey cast iron, for example, with a central hub section NA, which is used to attach the brake disc to a

[0068] REVISED SHEET (RULE 91) ISA / EP vehicle axle, as well as a circular brake section BA that encloses the hub section. The mass distribution of the base body is generally rotationally symmetrical to the rotational axis of the brake disc. The brake section has two axially opposite, mutually parallel surfaces (upper workpiece surface 01 and lower workpiece surface 02).

[0069] In an upstream phase of the manufacturing process, these were provided with a rotationally symmetrical coating or functional layer with respect to the rotation axis, the exposed surface of which will ultimately serve as the friction surface of the brake disc. The coating can contain, for example, tungsten, titanium, and / or niobium carbide and be very hard. In this example, both sides were coated using a special variant of laser cladding, namely a variant of extreme high-speed laser cladding, also known as the "EHLA process." The coating can also be applied by other methods, such as high-velocity oxygen spraying (HVOF) or cold gas spraying.

[0070] The grinding machine 100 is configured as a numerically controlled rotary transfer machine with two work stations, namely a loading station 110 for loading and unloading and a grinding station 120. All functions are implemented via control commands from a control unit 190 of the operating control system, which can be arranged locally (on or next to the machine) or remotely, e.g., in another room, and in the example case can be operated via a connected display and control unit 195 with a graphical user interface.

[0071] For transporting the brake discs between the work stations 110, 120, an internal machine transport system with a rotary table or turntable 150 is used, which is mounted on or in the machine base 102 so that it can rotate about a vertical turntable axis 152 and can be rotated indefinitely about the turntable axis by means of a rotation drive 105.

[0072] The loading station 110 is located on the left side in Fig. 1. There, a first brake disc (here WS1) is held on a workpiece spindle 154 in a horizontal orientation, i.e. with a vertically aligned axis of rotation (“turntable arrangement”). Loading can be carried out, for example, by means of a robot or other handling device or manually. The fastening can be carried out, for example, by clamping it to a workpiece holding device or workpiece holder 155 of the workpiece spindle in such a way that the brake disc is clamped or mounted in a rotationally fixed manner and the rotation axis of the brake disc is coaxial with the rotation axis 156. A rotationally fixed connection to the workpiece spindle can also be achieved by holding it down or securing it from above. In the example, vertically movable hold-down devices 158 are provided.

[0073] A brake disc loaded in this way is then transported by rotating the rotary table 180° clockwise to a working position 125 in the area of ​​the grinding station 120. There, the grinding steps (one or more) described later take place fully automatically. At the loading station 110, a previously ground brake disc can be removed simultaneously, and a new, not yet ground brake disc can be clamped in. After grinding is completed, the brake disc, which has been ground on both sides, is transported by rotating it 180° to the loading station 110, from where it can be unloaded, for example, by means of a robot or other handling device, or manually.A new brake disc to be ground can be clamped onto the workpiece spindle that is then freed up, so that, with the exception of the changeover times, both workpiece spindles are occupied with brake discs and are each in different phases of handling.

[0074] The grinding machine machines the workpiece surfaces using the double-side face grinding process. At the grinding station 120, the grinding machine 100 has a grinding unit 121 with two tool spindles (upper tool spindle 132-1 and lower tool spindle 132-2), ideally arranged coaxially to one another on a frame part, each carrying a grinding wheel (upper grinding wheel 130-2 and lower grinding wheel 130-2). The grinding wheels have facing abrasive side surfaces 135-1 and 135-2, respectively, and are arranged such that these grinding surfaces axially define a grinding chamber 133. Each of the grinding wheels can be rotated about the associated rotational axis 136-1 or 136-2 independently of the other grinding wheel by means of an associated spindle drive or rotational drive (upper rotational drive 134-1 or lower rotational drive 134-2) and can be moved by means of its own feed drive (upper feed drive 131-1 orlower feed drive 131-2) can be fed or advanced parallel to the assigned rotation axis with a predeterminable feed speed profile.

[0075] The grinding wheels 130-1, 130-2 are designed as circumferentially segmented cup wheels with individually replaceable and individually adjustable strip-shaped grinding segments.

[0076] A special feature of the grinding machine 100 is that it has two integrated position measuring systems, the results of which can be linked and evaluated together to control the operations of the grinding machine. The grinding machine has a first position measuring system 200, which is configured to determine workpiece position data by means of contactless distance measurements, which represent the axial positions of the workpiece surfaces 01, 02 of the braking section BA (top and bottom) to be ground at at least one surface location with respect to a machine-fixed reference coordinate system RKS. Associated hardware components (including distance sensors S1-1 and S2-1 in the form of measuring nozzles of an air measuring system) are arranged in the area of ​​the working position 125 of the workpiece spindle in the process zone of the grinding station 120, but outside the grinding chamber 133.

[0077] Furthermore, hardware components of a second position measuring system 300 are arranged in the area of ​​the grinding station 120. This second position measuring system 300 is configured to determine tool position data by means of non-contact distance measurements. These data represent the axial positions of the facing abrasive side surfaces 135-1, 135-2 of the two grinding wheels with respect to the same reference coordinate system RKS. Hardware components close to the tool (including distance sensors S1-2 and S2-2 in the form of measuring nozzles of an air measuring system) are arranged within the grinding chamber 133.

[0078] The "axial position" of a surface is the position of the surface at the measurement location in an axial direction, which here corresponds to the Z direction of the reference coordinate system (RKS). In an ideally configured grinding machine, the rotation axes of the workpiece spindles 154 and the tool spindles 136-1, 136-2 are aligned parallel to the axial direction. The infeed movements or feed movements of the grinding wheels run in the axial direction.

[0079] The first position measuring system 200 can also be referred to as workpiece position data measuring system 200 and correspondingly the second position measuring system 300 as tool position data measuring system 300.

[0080] The measurement results from both position measuring systems are processed jointly in an evaluation unit. The evaluation unit can be a functional component of the control unit 190 and implemented using appropriate evaluation software. Because the position data related to the workpiece and the position data related to the tool are available with reference to the same reference coordinate system (RKS), the spatial relationship between the abrasive side surfaces 135-1, 135-2 of the grinding wheels and the respective associated workpiece surfaces 01, 02 can be determined mathematically at any time. In the configuration shown, both position measuring systems are designed as pneumatic distance measuring systems. Measuring nozzles serve as distance sensors, which, during operation, emit compressed air toward the surface to be measured.The measuring signal is measured from a property of the compressed air that depends on the distance between the measuring nozzle and the surface and is evaluated to determine the distance.

[0081] In the grinding machine of Fig. 1, the grinding station 120 simultaneously functions as a workpiece measuring station. The first position measuring system 200 comprises (at least) one upper measuring nozzle S1-1 and (at least) one lower measuring nozzle S2-1. The upper measuring nozzle S1-1 is mounted with the nozzle opening facing downward and measures the distance to the top side O1 of the braking section BA. The lower measuring nozzle S2-1 is mounted with the nozzle opening facing upward and measures the distance to the bottom side O2 of the braking section BA. The sensors can each be moved in an automatically controlled manner within an assigned working range both in the radial direction relative to the rotational axis 156 of the workpiece spindle and in the axial direction. Each of the sensors is also assigned an axial position actuator for fine adjustment of the axial position relative to the carrier of the respective measuring nozzle (see Fig. 4).

[0082] The second position measuring system 300 provides a similar arrangement for tool measurements. There, the upper sensor S1-2 and the lower sensor S2-2 are each mounted with nozzle openings facing away from each other in the area of ​​the grinding chamber 133 between the abrasive side surfaces 135-1, 135-2, in a radial direction such that the abrasive surfaces of the cutting bars or grinding wheels are measured at the cup edge.

[0083] The grinding machine further comprises an internal calibration system with reference elements for calibrating the distance sensors and for their adjustment. The first position measuring system 200 comprises a reference element RE1, which is arranged in the lateral travel range of the measuring nozzles S1-1, S2-1. On each side (top and bottom), the reference element has two parallel, flat measuring surfaces separated by a step, whose absolute positions are known with high accuracy in the reference coordinate system RKS. Similar reference elements RE2-1 and RE2-2 are provided on the opposite side for calibrating the tool-side measuring nozzles and distance sensors S1-2, S2-2.

[0084] A schematic representation of the system structure with relevant components and parameters is shown in Fig. 2 (workpiece measurement) and Fig. 3 (tool measurement). The upper side of the workpiece holder 155, on which the inner surface of the hub section NA rests, acts as the basis for the support of the brake disc and also defines the axial reference level or the axial reference A of the reference coordinate system RKS. All distances (axial distances) in the Z-axis refer to this reference A. A basic idea of ​​the measurement strategy implemented in the grinding machine is that the distances detected by sensors are always recorded in relation to the reference A. All measurement signals determined during the distance measurements and the data derived from them are available to the central control system or the control unit 190 for processing.

[0085] Using the reference elements RE1 on the workpiece side or RE2-1 and RE2-2 on the tool side, the pneumatic distance sensors can be aligned with the reference A. Since each reference element has two flat measuring surfaces offset from each other in the Z direction with a defined axial distance, calibration of the measuring sensors is also possible using these reference elements.

[0086] The non-contact sensors (distance sensors) S1-1, S1-2, etc., formed by the measuring nozzles of the pneumatic measuring system, are mechanically coupled to tactile sensors ST1-1, ST1-2, etc. More precisely, the measuring nozzles are each attached to the axially adjustable probe element of the tactile sensor. This coupling is shown schematically in Fig. 4. This design offers advantages, among other things, with regard to the possibility of extending the measuring range (by axially displacing the measuring nozzle over longer travel distances), and calibrating and adjusting the axial positions of the measuring nozzle. This configuration thus features a tactile distance sensor that is mechanically coupled to the non-contact distance sensor (measuring nozzle).

[0087] This combination of two sensors is advantageous for various applications, but can also be implemented differently, for example, by combining a reference plate with a one-time measurement with an external measuring system. This can also ensure the traceability of the measurement results.

[0088] The design and functionality of the sensors on the tool side is similar to that on the workpiece side. One difference is that the measuring nozzles S1-2 and S2-2 are arranged in a different orientation (with the nozzle openings facing away from each other). Accordingly, there is an upper reference element RE2-1 and a lower reference element RE2-2 on the tool side.

[0089] In non-contact pneumatic distance measurement, i.e., the sensor-based detection of the distance between the measuring sensor and the appropriate surface, the distance from the sensor to the measured surface is measured without contact. Figure 5 shows an example of the situation in the area of ​​sensor S1-2 for measuring the axial position of the side surface 135-1 of the upper grinding wheel 130-1. The axial position of the side surface of the lower grinding wheel is measured in a similar manner.

[0090] The contactless distance sensors (measuring nozzles) are movably mounted and can be moved via an assigned machine axis in the Z direction (axial direction) and perpendicular to this in the X direction (radial direction in relation to the workpiece rotation axis) in order to be able to assume different working positions. Fig. 2 shows different working positions of the same sensor S1-1. Working position P1 corresponds to a first reference position for calibration and / or adjustment on the first flat reference surface of the reference element RE1. Position P2 corresponds to a second position for calibration / adjustment at a further axially offset axial position. Position P3 shows the measurement of the upper workpiece surface 01 for distance recording in the process and also for an upstream measurement (e.g. as part of an incoming goods inspection). Position P4 is a reference position for comparison with the workpiece reference (reference A).The references are redundant for security and verification purposes.

[0091] It is evident that a multitude of geometric variables on the workpiece side and on the tool side can be used here. All distances shown are known to the central control system 190 and can, for example, be stored digitally in a memory accessible to the control unit. Some of the values ​​can be generated from measurements using external measuring systems or from operator inputs. Another group of measured values ​​comes from measurements with the integrated sensors. These include both the non-contact sensors Sx of the position measuring system (measuring nozzles) and the tactile sensors STx used for their axial displacement. Further values ​​can come from measurements from the actuators, e.g., from encoders of drives for machine axes.

[0092] Using the first position measuring system 200 and the second position measuring system 300, the workpiece position data (axial positions of the surfaces to be ground on the braking section) and the tool position data (axial positions of the abrasive side surfaces of the grinding wheels relative to the same reference coordinate system) can be acquired as needed and made available to the evaluation unit. The two position measuring systems are operated simultaneously in phases. The evaluation unit can evaluate the generated data and process it such that the control unit 190 can control the feed of the grinding wheels and / or other grinding parameters 130-1, 130-2 during various phases depending on these workpiece position data and tool position data.The overall high measuring accuracy is also enhanced by the fact that the measurements can be carried out on the workpiece, which is already in its working position 125 near the grinding wheels.

[0093] Some examples of useful operating modes are explained below.

[0094] In one operating mode, an incoming inspection of a newly loaded workpiece to be ground can be carried out in the grinding machine. To determine the incoming quality of the new workpiece, the workpiece (the brake disc) is first transported from the loading station to its working position 125 in the grinding station 120 after being inserted and held down. There, it is rotated once at a defined peripheral speed. The angular positions of this rotary movement are recorded by rotary encoder signals on the rotary drive of the workpiece rotation. The workpiece holder 155 is equipped with an anti-twist device so that the workpiece holder has a defined zero position known to the control unit. The current workpiece position can then be evaluated in comparison to a reference disc or a reference reference. An ideal reference is initially measured during the process.All distance values ​​are recorded with the location reference of the angular position, as well as the X-distance (radial distance from the rotational axis 156) and the Z-distance (distance in the axial direction) and stored in a reference matrix. The workpiece is then read in and compared to the reference. This makes it possible to evaluate relevant geometric characteristics of the brake disc, such as any circumferential shielding of the brake disc, and to consider them in the subsequent process if necessary.

[0095] In a similar way, an outgoing inspection of the finished, ground workpiece can also be performed after the grinding process has been completed in grinding station 120. The measured values ​​obtained on the workpiece can be compared with the reference values. If necessary, a reference wheel can also be measured again to provide comparison values.

[0096] For each of these measurements, the workpiece position data, i.e. the local axial positions of the workpiece surface, are recorded with spatial resolution for a large number of surface locations (or measuring points) in order to generate a sufficient number of measurement data in both the circumferential and radial directions to evaluate the quality. Two possible measurement strategies are shown schematically in Fig. 6 and 7. In the variant shown in Fig. 6, only a single distance sensor S1-1 is used, which is gradually moved in the radial direction from the inside to the outside or from the outside to the inside as the workpiece WS1 rotates. As a result, the measured measurement locations MO lie on a spiral path. In the case of a fine-meshed spiral, the results can be interpolated to form circular rings if necessary.

[0097] Alternatively, it is also possible to use a sensor that is moved step by step to various radial positions and remains stationary during rotation of the workpiece. In this case, the measured surface locations or measuring points MO are each located on concentric rings, whose radial spacing can be selected according to the desired spatial resolution in the radial direction. The X-resolution of this method is variable and depends solely on the size of the measuring spot of an individual measurement.

[0098] It is also possible to distribute multiple distance sensors evenly or unevenly in the radial direction across the track width of the brake disc (see Fig. 7). This may require only a single rotation of the workpiece. The radial resolution depends on the distribution distance and the measuring spot.

[0099] A possible evaluation of the results is explained using Fig. 8. Fig. 8A shows a schematic representation of the spatial references, Fig. 8B shows signal curves of various sensors S1, S2, S3 during a rotation U of 360°, with the time plotted on the x-axis of the diagram and the axial position or a distance value ABZ plotted on the y-axis. Fig. 8C shows the calculation of the measured values ​​for the zero-degree position P o , Fig. 8D a corresponding calculation for another position Pj. Reference symbol SP denotes the track width covered by the three sensors in the radial direction.

[0100] Each distance sensor has a unique location in the reference coordinate system at a given time t. In this example, the X location (radial position relative to the workpiece's rotational axis) is a constant position for each distance sensor. The Y position changes due to rotation (rotation angle RW) and is generated by the angle encoder of the rotary drive for the workpiece rotation. This is then converted into a Cartesian value. The Z position corresponds to the measured sensor distance, which represents the workpiece position data. This provides a location-specific measurement point on the workpiece for each sensor, dependent on time.

[0101] The evaluation or utilization in the sense of a quality control can then be carried out, for example, by checking for each angular position whether the measured values ​​lie within a permissible tolerance range between an upper limit value (ULW) and a lower limit value (ULW). The information on the input quality can be used to control the behavior of the grinding machine during machining. In another operating mode, a contactless start-up control is implemented, which is explained in more detail using the travel-time diagram in Fig. 9.In this case (in phase PO) the abrasive side surface of a grinding wheel on each side (top and bottom) is first advanced in rapid traverse with a slightly decreasing infeed speed during the infeed of the grinding wheels in the direction of the workpiece surface to be ground until it reaches a position without workpiece contact, in which the side surface is still at a safety distance (e.g. from 10 pm to 100 pm) in front of the workpiece surface to be machined. Starting from this start position SP the tool is then advanced at a reduced infeed speed in a next infeed phase (phase P1). This approach control is made possible by the fact that the current position (Z position) of the workpiece and the current position of the tool are known at all times due to the continuous distance measurements on the workpiece and on the tool. This means that both tools can initially be advanced simultaneously in rapid traverse.The current axial positions of the abrasive side surfaces are recorded using non-contact distance measurement. The axial positions of the workpiece surfaces were initially recorded during the incoming inspection and can continue to be determined continuously.

[0102] The operator can now specify a safety distance to which the abrasive side surfaces can be advanced at rapid traverse before switching from rapid traverse to a lower feed rate. The switchover from phase PO (rapid traverse) to phase P1 (feed up to workpiece contact and beyond, with material removal) is based on the contactless measured values ​​continuously recorded by the sensors. In phase P1, which begins after reaching the starting point SP at the safety distance, the further feed of the tools can then be controlled via the control device 190.

[0103] In phase PO, the tools or tool spindles are in their home position. The tools and the workpiece rotate. Both tools (upper and lower grinding wheels) then move simultaneously towards the workpiece. Since the axial positions of the workpiece surfaces to be ground are known or can be measured and the axial positions of the abrasive side surfaces are permanently known through measurement, the distance between the workpiece surface and the abrasive side surface can be determined at any time so that the starting position SP with safety distance can be approached with high accuracy. For switching to phase P1, the desired value for the safety distance for switching to phase P1 is known in the machine control system via operator input. The safety distance can, for example, be in the order of magnitude of approximately 10 pm to approximately 100 pm.The starting point SP of phase P1 can thus be determined from the operator input and the current geometry of the workpiece, as well as from the measured values ​​for the tool position. Fig. 9 shows a schematic path-time diagram for a possible speed curve of the infeed movement within the approach control. The switchover point between phase PO and phase P1 is where the specified safety distance is reached. This is the starting point SP for the controlled feed rate according to the programming.

[0104] This contactless approach control offers significant advantages over conventional approach controls, which rely on physical contact between the workpiece surface and the tool. If the tool is advanced at rapid traverse until contact is made between the rotating tool and the rotating workpiece, significant material removal occurs immediately after the abrasive side surface of the tool engages the workpiece surface, which simultaneously leads to significant tool wear. Tool wear can be significant, especially when machining potentially very hard coatings on brake discs.The contactless approach control avoids this situation with particularly high tool wear without significantly slowing down the overall process, since the feed can be carried out in rapid traverse until immediately before contact is reached, but the contact is only reached a short time later at a significantly lower feed speed and correspondingly less wear.

[0105] Tool wear can become a significant cost factor, especially when grinding potentially very hard coatings on brake discs. However, thanks to the real-time position information acquired using position measuring systems, grinding processes with particularly low tool wear can be implemented. There are also advantages with regard to wear compensation. The described start-up control system ensures that the axial position of the cutting tool is known immediately before the start of the material-removing machining process, thus already providing compensation.

[0106] During grinding, the axis positions of the workpiece spindles and the distances between the tools and the workpiece can be recorded and evaluated in such a way that information about tool wear can be derived at any time. For example, to determine the current coating height of the cutting medium on the grinding wheels, the operator can enter the original coating height after setup into the control system. By recording the positions of the machine axes used for infeed and the distances between tools and workpieces during machining, wear can be calculated directly.

[0107] Further advantages include the ability to shut down the grinding process, phase switching during the grinding process, wear monitoring and, if necessary, wear compensation, as well as the ability to determine real-time information on the process-specific grinding ratio and thus optimize the process flow. This is explained primarily with reference to Figs. 10 to 12.

[0108] During the start-up phase and during the material-removing grinding process, the distance sensors are permanently active and record, among other things, the time-dependent processing, i.e. distance values ​​as a function of time.

[0109] Some relationships that are helpful for understanding are explained using Fig. 10. This uses a schematic diagram to show the time dependence of certain parameters during a grinding operation. The abscissa represents time t and the ordinate represents the value of the parameter ABZ, which indicates the distance measured in the axial direction between a distance sensor and the workpiece surface being monitored by it. In the example, this is sensor S2-1, which measures its distance to the underside of the braking section BA outside the grinding area. This distance increases as the material removal on this underside increases. The horizontal line ABS represents the distance value that will be present when the specified final dimension for the axial position of the underside of the brake disc is reached.

[0110] The process or grinding operation goes through a sequence of phases, with phase PO being the start-up phase. Phase P1 is the first phase of the grinding tool feed at a preprogrammed initial feed rate. Phase Pn, which follows phase P1 after a programmed changeover time, is carried out at a lower feed rate until the final dimension is reached. After completion of the first phase, there can be a single or several consecutive phases with different grinding parameters.

[0111] First, the workpiece is placed on the workpiece holder and pressed firmly onto it with the hold-down device. Distance changes can occur during this process. By holding it down, a fixed reference to the machine-fixed reference coordinate system is established. This is followed by the approach phase, in which there is still no contact between the tool (grinding wheel) and the workpiece surface. Accordingly, in this phase PO the distance is constant (apart from small periodic fluctuations due to azimuthal waviness in the workpiece). The first phase (phase P1) begins in this example when workpiece contact is established. It can be seen that in the first phase the distance increases more or less continuously (apart from the superimposed waviness). After switching to the following phase Pn at the switchover point UP the distance increases less rapidly, which corresponds to less material removal per unit of time.With this material removal, the switch-off point AP is then reached.

[0112] This shutdown variant can be implemented in such a way that the operator first specifies a component-dependent dimension. The specified dimensions are then calculated to obtain corresponding values ​​for distances at the sensors. The grinding machine continuously executes its operating program during grinding. The position measuring system is the master for deciding the switchover between phases and the shutdown at the end of the grinding process. Thus, the machine control system makes the decision regarding the shutdown time or the time when the desired dimension is reached based on the specified dimensions and the measured values ​​from the position measuring system.

[0113] Further possibilities for obtaining important information for controlling the grinding process arise in some operating modes by relating time-dependent tool position data determined during the grinding operation with time-dependent feed position data (ZPOS). In other words, the evaluation can be expanded by considering, in addition to the data resulting from the distance measurements, data or values ​​that include the current axis positions of the respective tool spindles.

[0114] Additional process-relevant information can be obtained, for example, if the distance measurement values ​​are offset against data that represent the infeed of the tools, i.e. the machine-side specification for the Z position of the grinding wheel. This information can be obtained, for example, from position sensor signals of a feed drive 131-1, 131-2 or from the control program. The time dependence of the material removal or the distance values ​​and the infeed can be specified, for example, in the form of gradients. In Fig. 10, the gradient line ST1 corresponds to the gradient of the distance ABZ during phase P1. This can, for example, correspond to a material removal in the range of approximately 2 pm / s to approximately 5 pm / s. The lower material removal after switching to phase n can, for example, be on the order of magnitude of approximately 1 pm / s and is represented by a gradient line ST2 with a lower gradient.

[0115] For the same time window, corresponding time-dependent data can also be determined for the programmed infeed of the grinding wheels as well as for the distances between the sensors of the second position measuring system 300 and the respective appropriate tool side surfaces (for determining the tool position data). For illustration, Fig. 11 shows three path-time diagrams, where Fig. 11A represents measured values ​​of the SSWS sensor system on the workpiece side and their temporal change, the middle diagram (Fig. 11B) represents the gradient resulting from the programmed infeed speed ZPOS of the tools, and the right-hand diagram (Fig. 11C) represents the temporal development of the distances ABZ of the second position measuring system (of the SSWZ sensor system on the tool side). Significant statements about the grinding process can be drawn from the relationships between these gradients.

[0116] As mentioned above, the gradient line ST1 represents the change in the distance between the sensor S2-1 and the underside of the workpiece over time and should therefore essentially represent the material removal on this underside of the workpiece. If the programmed feed rate corresponds to a straight line with the same gradient, this can (ideally) mean that the entire feed rate is converted into workpiece removal without loss, which would correspond to a grinding process without wear on the grinding wheels.

[0117] In a more realistic case, however, the feed rate (middle diagram, Z-axis ZA) will tend to be greater than the achievable material removal rate. For illustration, a gradient line ST3 is plotted in Fig. 10, whose gradient is significantly greater than the gradient resulting from the temporal observation of the distance value (gradient line ST1). This discrepancy can have at least two different causes: wear and deformation of the tools.

[0118] As a rule, the grinding wheels wear more or less quickly, so that only a portion of the feed path is actually converted into material removal in the z-direction, while another portion compensates for the material wear. A clear borderline case can be considered a grinding process in which the mechanical hardness of the workpiece surface to be ground is significantly greater than the hardness of the grinding wheels, so that practically no material removal occurs despite the feed rate. In this case, the gradient for material removal would be almost zero, whereas the continuous feed rate would result in a significantly larger gradient for the infeed.

[0119] A second cause of a deviation in the gradients between the measured distance value and the change in the feed position over time is possible deformation of the grinding unit components. While the grinding unit should be constructed as rigidly as possible, some flexibility will be unavoidable. In this case, it is possible that some of the forces acting during the feed of the grinding wheels are not used for material removal, but instead lead to deformation of the grinding unit components.

[0120] According to the inventors' experience, the portion of the discrepancy A due to wear is significantly higher than the portion due to deformation, so that, as a first approximation, the tool wear can be inferred from the discrepancy between the pitch line ST 1 and the pitch line ST3 (for the infeed).

[0121] In order to also be able to determine the deformation component, in some embodiments there are one or more deformation sensors in the grinding unit that can measure the deformation of the unit consisting of the tool spindle and grinding wheel on both sides, so that the deformation component of the discrepancy can be taken into account in the evaluation.

[0122] In another operating mode, the control unit is configured to determine and provide quantitative information about the current grinding ratio G (also known as the G-value). The grinding ratio G corresponds to the relationship between material removal and wheel wear, each in volume units. The grinding ratio G therefore provides meaningful information about the nature of the current grinding process, particularly with regard to tool wear and the efficiency of the grinding operation, or rather the relationship between material removal and tool wear. The grinding ratio can be used to describe the process in terms of productivity and performance. The grinding machine offers the option of calculating the grinding ratio (G-value) using the values ​​of the existing actuators, sensors and operator inputs directly while the machine is running. The G-value orCorresponding data can be displayed to the operator in a suitable numerical and / or graphical form. Alternatively or additionally, a warning signal can be generated if the current grinding ratio deviates too significantly from a preset target value or is about to deviate. Alternatively or additionally, a closed-loop control can be implemented using the control unit, e.g., to adjust the grinding operation so that the grinding ratio remains within a preset value range window. To determine the G value, the following information is processed in one embodiment.

[0123] • G1 : Total area of ​​the cutting bars | ASG or the area of ​​each individual bar

[0124] • G2: Covering height of the cutting bars | BHs

[0125] • G3: The current axis position and / or current distance to the sensor system tool side

[0126] • G4: Track width of the workpiece

[0127] • G5: Start and current height of the workpiece top / bottom

[0128] The current volume of the cutting bar VGS(t) can be determined from the area of ​​the cutting bars G5 and the height of the cutting bars (G2 / G3). From the track width (ring area | G4) and the distances from the sensor system to the workpiece side (G5), the volume V G w(t). The G-value is determined by V G w(t) / V GS (t). With decreasing wear VS (in volume units) at a constant machining volume, the G-value G increases; an example curve is shown in Fig. 12. This functionality allows the operator and / or the control system to set the optimal grinding process for the respective machining task.

Claims

Patent claims 1. Grinding machine (100) for grinding essentially plane-parallel, annular workpiece surfaces on a disk-shaped workpiece section (WA) of a workpiece (WS1, WS2), in particular for grinding workpiece surfaces of an annular brake section of a brake disk, comprising a grinding unit (121) with two coaxially arranged tool spindles (132-1, 132-2), each carrying a grinding disk (130-1, 130-2), wherein the grinding disks are arranged with mutually facing abrasive side surfaces (135-1, 135-2) which axially delimit a grinding space (133), wherein each of the grinding disks is rotatable about the associated rotational axis (136-1, 136-2) independently of the other grinding disk by means of an associated rotational drive (134-1, 134-2) and is movable by means of a feed drive (131-1, 131-2) can be delivered parallel to the associated rotation axis,at least one workpiece spindle (154) with a workpiece holder (155) for rotationally fixedly holding the workpiece (WS1, WS2), wherein the workpiece holder is rotatable by means of a rotation drive (157) about a rotation axis (156) running parallel to the rotation axes of the grinding wheels and is arranged in a working position at least during a phase of a grinding operation such that the workpiece section (WA) of the held workpiece extends in a circular arc through the grinding space; an operation control system with a control unit (190) for controlling the operation of the grinding machine, characterized by a first position data determination system for determining workpiece position data representing an axial position of a workpiece surface (01, 02) at at least one surface location with respect to a machine-fixed reference coordinate system (RKS), and a second position data determination system for determining tool position data,the axial position of an abrasive side surface (135-1, 135-2) facing the workpiece surface (01, 02) in relation to the same, Represent a reference coordinate system (RKS), wherein the control unit (190) is configured in at least one operating mode to control at least one grinding parameter in at least one phase of the grinding operation as a function of the workpiece position data and / or the tool position data.

2. Grinding machine according to claim 1, characterized in that the control unit (190) is configured to input at least one grinding parameter from the following group into Depending on the workpiece position data and / or the tool position data, to control: a feed speed of a grinding wheel; a rotational speed of a grinding wheel; a rotational speed of the workpiece spindle that carries the workpiece for the grinding operation; a rotational direction of the workpiece spindle that carries the workpiece for the grinding operation.

3. Grinding machine according to claim 1 or 2, characterized in that the first Position data determination system as a first position measuring system (200) for the metrological determination of the workpiece position data and the second Position data determination system as a second position measuring system (300) for the metrological determination of the tool position data in the same Reference coordinate system (REF), wherein preferably the first Position measuring system (200) and / or the second position measuring system (300) is a contactless position measuring system.

4. Grinding machine according to claim 3, characterized in that the first position measuring system (200) is designed to measure the workpiece (WS2) in a state installed on the workpiece holder (155), preferably in such a way that position measurements can be carried out when the workpiece holder (155) with the workpiece is in the working position (125).

5. Grinding machine according to claim 3 or 4, characterized in that the first position measuring system (200) and the second position measuring system (300) are designed such that position measurements can be carried out when the workpiece holder with the workpiece is in the working position, wherein preferably the grinding machine (100) is configured to measure workpiece position data and tool position data simultaneously during at least one phase of a grinding operation.

6. Grinding machine according to one of claims 3 to 5, characterized in that the first position measuring system (200) and / or the second position measuring system (300) is designed as a distance measuring system for a distance measurement between at least one distance sensor (Sx) of the position measuring system and a surface to be measured, wherein preferably the distance measuring system is a pneumatic measuring system with distance sensors in the form of measuring nozzles (Sx).

7. Grinding machine according to one of claims 3 to 6, characterized in that the grinding machine has a machine-internal calibration system with reference elements (REx) for calibrating sensors, in particular distance sensors (Sx), wherein distance sensors are preferably movable in a controlled manner within an individual working area and each distance sensor is assigned a reference element (REx), which is located within the working area of ​​the distance sensor and can be approached for a reference measurement and / or wherein a reference element has a first measuring surface and at least one second measuring surface on a side accessible for a distance measurement, wherein the measuring surfaces are located at different axial positions.

8. Grinding machine according to one of claims 6 or 7, characterized in that an axial actuator (STx) for controllably changing an axial position of the distance sensor is assigned to a distance sensor, preferably to each of the distance sensors (x), and / or that an axial displacement sensor for detecting an axial position of the distance sensor is assigned to a distance sensor, preferably to each of the distance sensors, wherein preferably a distance sensor, preferably each of the distance sensors, has a measuring nozzle of a pneumatic measuring system and the measuring nozzle is attached to an axially movable probe element of a tactile sensor.

9. Grinding machine according to one of the preceding claims, characterized in that the grinding machine (100) is designed as a multi-station grinding machine comprising: a transport system with a spindle support unit (150) which is movable relative to a machine base (102) and which carries at least one workpiece spindle (154) which is rotatably mounted about a rotation axis (156) relative to the spindle support unit and has a workpiece holder (155); at least two work stations (110, 120) which can be approached by the workpiece spindle and which comprise a loading station (110) and at least one grinding station (120) with at least one grinding unit (121), wherein preferably the spindle support unit is rotatable in the manner of a turntable or has at least one translationally movable spindle support unit which can be moved back and forth between the loading station and the work station on the grinding unit.

10. Grinding machine according to one of the preceding claims, characterized in that the control unit (190) is configured in an operating mode for carrying out an initial inspection of the workpiece to be ground by measuring the workpiece in the grinding machine, wherein after the attachment of the workpiece to the workpiece holder and before At the start of a grinding operation on the workpiece, workpiece position data are determined for each of the surfaces of the workpiece section to be ground, preferably spatially resolved for a plurality of surface locations offset in the circumferential direction and / or for a plurality of surface locations offset in the radial direction, wherein workpiece geometry data are determined in particular from the workpiece position data.

11. Grinding machine according to one of the preceding claims, characterized in that the control unit (190) is configured in an operating mode for carrying out a contactless start-up control, wherein an abrasive side surface of a grinding wheel is fed in the direction of the workpiece surface to be ground up to a starting position (SP) located at a safety distance from the workpiece surface to be machined, wherein to determine the starting position (SP), initial workpiece position data and initial tool position data are offset against one another, wherein preferably the grinding wheel is fed at a first feed speed until the starting position (SP) is reached, and the feed speed is automatically reduced to a lower second feed speed upon reaching the starting position,wherein preferably the second feed rate is maintained at least until a nominal contact position is reached at which the current tool axial position corresponds to the initial workpiece axial position., 12. Grinding machine according to one of the preceding claims, characterized in that the control unit (190) is configured in an operating mode such that during the grinding operation tool position data are determined continuously or intermittently as a function of time and resulting time-dependent tool position data are processed to control the feed of the grinding wheels and / or at least one other grinding parameter.

13. Grinding machine according to one of the preceding claims, characterized in that the control unit (190) is configured in an operating mode such that time-dependent tool position data determined during the grinding operation are related to time-dependent feed position data.

14. Grinding machine according to one of the preceding claims, characterized in that the control unit (190) is configured in an operating mode to determine information about a current grinding ratio G, which is defined as the ratio between material removal and wheel wear in volume units, wherein preferably a target grinding ratio desired for a grinding process or the parameters required to determine a target grinding ratio can be specified and the control unit is configured to calculate the current grinding ratio permanently or at short intervals on the basis of data provided by sensors, to compare it with the target grinding ratio and to control the operation of the grinding machine depending on the result of the comparison, in particular by one of the following measures: a) generating a warning signal, in particular an electrical, acoustic, optical and / or haptic warning signal, when the current grinding ratio leaves a tolerance range around the target grinding ratio or approaches the tolerance limit at a certain rate of change and a limit violation is imminent;b) Controlling the grinding operation to ensure compliance with the target grinding ratio in such a way that a grinding operation is carried out without operator intervention within tolerances and approximately with the desired target grinding ratio.

15. Grinding method for grinding essentially plane-parallel, circular-ring-shaped workpiece surfaces on a disc-shaped workpiece section of a workpiece, in particular for grinding surfaces of a circular-ring-shaped brake section of a brake disc, by means of a grinding machine having two grinding wheels with mutually facing abrasive side surfaces which axially delimit a grinding space of variable height, wherein the grinding wheels are rotatable independently of one another about coaxial axes of rotation and can be advanced by feeding in an axial direction oriented parallel to the axes of rotation, wherein the workpiece is held in a rotationally fixed manner on a workpiece holder rotatable about a rotation axis and arranged, at least during a grinding operation, in a working position such that the workpiece section extends in a circular arc through the grinding space,wherein, to carry out a grinding operation, the workpiece and the grinding wheels are set in rotation about their axis of rotation and the abrasive side surfaces are brought into engagement with the associated workpiece surface by axial feed of the grinding wheels, wherein the grinding method comprises the following steps for each workpiece surface to be ground: Determining workpiece position data representing an axial position of a workpiece surface to be ground at at least one surface location with respect to a reference coordinate system, Determining tool position data representing an axial position of an abrasive side surface facing the workpiece surface with respect to the same reference coordinate system, Controlling at least one grinding parameter of the grinding operation in at least one phase of a grinding operation as a function of the workpiece position data and / or the tool position data.

16. Grinding method according to claim 15, characterized in that in order to determine the workpiece position data and / or the tool position data, a preferably contactless distance measurement is carried out between at least one distance sensor and the workpiece surface to be measured, wherein a pneumatic measuring system with at least one measuring nozzle acting as a distance sensor is preferably used for the distance measurement.

17. Grinding method according to claim 15 or 16, characterized in that the workpiece (WS2) is measured in a state installed on the workpiece holder (155), in particular when the workpiece holder (155) with the workpiece is in the working position (125).

18. Grinding method according to one of claims 15 to 17, characterized in that in an operating mode an initial inspection of the workpiece to be ground is carried out by measuring the workpiece in the grinding machine, wherein after the workpiece has been fastened to the workpiece holder and before the start of a grinding operation on the workpiece, workpiece position data are determined for each of the surfaces of the workpiece section to be ground.

19. Grinding method according to one of claims 15 to 18, characterized by a spatially resolved determination of workpiece position data in that workpiece position data are determined for a plurality of surface locations offset in the circumferential direction and / or for a plurality of surface locations offset in the radial direction, wherein workpiece geometry data are preferably determined from the workpiece position data, in particular a thickness of the workpiece section between the workpiece surfaces that are essentially parallel to one another, a curvature of the workpiece section in the radial direction and / or waviness in the circumferential direction and / or in the radial direction.

20. Grinding method according to one of claims 15 to 19, characterized in that in one operating mode a contactless start-up control takes place, in which an abrasive The side surface of a grinding wheel is advanced in rapid traverse during the advance in the direction of the workpiece surface to be ground to a start position (SP) located at a safety distance from the workpiece surface to be machined, and the advance speed is automatically reduced to a lower second advance speed when the start position is reached, wherein the safety distance is preset and in particular lies in the range from 10 pm to 100 pm.

21. Grinding method according to one of claims 15 to 20, characterized in that during at least one phase of a grinding operation, workpiece position data and tool position data are measured simultaneously.

22. Grinding method according to one of claims 15 to 211, characterized in that a distance sensor is calibrated from time to time during operation of the grinding machine against a reference element of a machine-internal calibration system arranged in a working area of ​​the distance sensor and, if necessary, is adjusted by controlled variation of the axial position of the distance sensor.

23. Grinding method according to one of claims 15 to 22, characterized in that during the grinding operation tool position data are determined continuously or intermittently as a function of time and time-dependent tool position data are processed to control the feed of the grinding wheels.

24. Grinding method according to one of claims 15 to 23, characterized in that time-dependent tool position data determined in an operating mode during the grinding operation are related to time-dependent feed position data.

25. Grinding method according to one of claims 15 to 24, characterized in that a grinding ratio G is determined in an evaluation mode, wherein preferably at least one grinding parameter of the grinding process is controlled as a function of the grinding ratio G.

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