DE102023200881A1 - Grinding machine and grinding process for grinding coated brake discs - Google Patents

Abstract

A grinding machine (100) for grinding essentially plane-parallel, annular workpiece surfaces on a disk-shaped workpiece section (BA) of a workpiece (WS1, WS2), comprising a grinding unit (121) in the manner of a double-side surface grinding unit with two tool spindles (132-1, 132-2) arranged coaxially to one another, each of which carries a grinding wheel (130-1, 130-2), wherein the grinding wheels are arranged with abrasive side surfaces (135-1, 135-2) facing one another, which axially delimit a grinding space (133), and at least one workpiece spindle (154) with a workpiece holder (155) for rotationally fixedly holding the workpiece (WS1, WS2) such that the workpiece section (BA) of the received workpiece runs in a circular arc through the grinding space for grinding. The grinding machine comprises a first position data determination system for determining workpiece position data that represent an axial position of a workpiece surface (O1, O2) at at least one surface location in relation to a machine-fixed reference coordinate system (RKS), and a second position data determination system for determining tool position data that represent an axial position of an abrasive side surface (135-1, 135-2) facing the workpiece surface (O1, O2) in relation to the same reference coordinate 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 for grinding workpiece surfaces of an annular brake section of a brake disk.

Description

FIELD OF APPLICATION AND STATE OF THE ART

The invention relates to a grinding machine and a grinding method for grinding essentially plane-parallel, circular ring-shaped workpiece surfaces on a disk-shaped workpiece section of a workpiece. A preferred field of application is the grinding of surfaces of a circular ring-shaped brake section of a brake disk, in particular a coated brake disk.

A brake disc is the part of a disc brake that is connected to the wheel in a rotationally fixed manner and on which the brake pads attached to a brake calliper act to slow down a rotational movement. A brake disc is usually point-symmetrical or rotationally symmetrical to an axis center through which the axis of rotation of the brake disc passes. A brake disc has a central hub section and a circular ring-shaped brake section that encloses the hub section and has two axially opposite free surfaces that serve as the friction surface of the brake disc.

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. There are also brake discs made of ceramic materials. However, these are expensive and have so far only been available for high-priced vehicles.

Upcoming tightening of the regulations on fine particle emissions from motor vehicles means that future brake discs for motor vehicles will have to be designed in such a way that fewer fine particles are released during braking. One approach to this is to coat the brake discs or their surface sections intended as friction surfaces with a thin functional layer made of more wear-resistant material. In coated brake discs, the surfaces of the circular braking section each have a functional layer that is rotationally symmetrical with respect to the axis of rotation, the free surface of which is designed as the friction surface of the brake disc.

The manufacturing process of 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 which can have a wear-reducing function due to its relatively high mechanical hardness. Alternatively or additionally, a corrosion-inhibiting effect can also be present. Such functional layers often consist essentially of metal; they can have a single layer or several layers with different properties. Such coatings can be applied, for example, by flame spraying or laser cladding. Typical layer thicknesses can be in the range 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 reveal examples of coated brake discs.

Depending on the coating process and the coating material, the free surfaces of the finished coatings can have different properties. The functional layers are usually mechanically relatively hard and have a relatively rough surface after coating.

A subsequent grinding process is used to create a sufficiently flat surface on the coating that is optimized for the braking function. The specifications for the friction surfaces of a brake disc can be such that a EN ISO 4288 The determined mean roughness Ra should be in the range of 1 µm to 3 µm - 3.2 µm and the flatness deviation should not exceed 20 µm (cf. WO 2021/224308 A ).

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

TASK AND SOLUTION

Against this background, the invention is based on the object of providing 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 enable the systematic 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.

To solve this problem, the invention provides a grinding machine with the features of claim 1. Furthermore, a grinding method with the features of claim 14 is provided. Preferred developments are specified in the dependent claims. The wording of all claims is incorporated into the content of the description by reference.

According to one aspect of the invention, a grinding machine for grinding essentially surfaces of plane-parallel, circular ring-shaped workpiece surfaces on a disk-shaped workpiece section of a workpiece. In a preferred area of application, the workpieces are brake disks whose circular ring-shaped brake sections are to be ground on both sides.

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 an appropriate operating mode so that special grinding processes can be carried out that were not possible with conventional grinding machines.

The grinding machine is designed like a double-side surface grinding machine. The grinding machine comprises a grinding unit with two tool spindles arranged or able to be arranged coaxially to one another on a machine frame, each of which carries a grinding wheel. The grinding wheels are arranged with abrasive side surfaces facing one another, which delimit a grinding space of variable axial height in the axial direction. Each of the grinding wheels can be rotated about its associated axis of rotation using an associated rotary drive independently of the other grinding wheel with a predeterminable speed profile and can be axially fed using an infeed drive by linear displacement parallel to the associated axis of rotation with predeterminable infeed parameters, for example a predeterminable infeed speed. If necessary, a grinding wheel can also be fed perpendicular to the axis of rotation, i.e. radially to it.

In a "coaxial" arrangement of the tool spindles within the meaning of this application, the axes of rotation coincide within the scope 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.

The grinding machine further comprises at least one workpiece spindle with a workpiece holder for holding a workpiece in a rotationally fixed manner, wherein the workpiece holder can be rotated 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 workpiece held in the grinding chamber 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 can be moved. For example, the workpiece spindle can be moved into the working position to carry out a grinding operation and then moved out of the working position again after the grinding operation has been completed. The rotating workpiece spindle can be stationary during a material-removing phase while the grinding wheels are fed. This makes it possible to implement variants of transverse side double-face grinding in which the feed movement runs transversely, in particular perpendicularly, to the workpiece surfaces to be machined.

Alternatively or additionally, it is also possible in some cases for the workpiece spindle and the grinding unit to be moved relative to one another during a phase of a grinding operation, e.g. by a drive-controlled displacement of the workpiece spindle perpendicular to its axis of rotation, while the grinding wheels are in engagement with the workpiece surfaces. This makes it possible to implement variants of a longitudinal-lateral double-face grinding process in which a feed movement takes place along the workpiece surface, i.e. parallel to it, e.g. in a direction running radially to the axis of rotation of the tools and/or the workpiece.

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, while 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.

The grinding machine also has 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 movements and the tool movements. The control device 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.

The term “axial direction” in this application refers to the direction in which the rotation axes run on the workpiece side and the tool side. The axial direction can be, for example, vertical (parallel to the direction of gravity) or horizontally or at an angle to the horizontal and vertical. The grinding wheels are fed 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 depending on the workpiece position data and/or the tool position data.

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

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.

Such a grinding machine therefore has two devices that can be operated separately and which can determine the axial workpiece position and the axial tool position in relation to the same reference coordinate system. In this way, the corresponding position data can be directly offset against one another during data evaluation in order to determine a spatial relationship in the axial direction between the abrasive side surfaces and the respective associated workpiece surfaces.

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.

The invention takes into account, among other things, the knowledge that a grinding process or a grinding operation should always be adapted to certain input conditions and user requirements and that these can vary. It is therefore possible that the dimensions of the workpieces at the input (before grinding begins) can differ from workpiece to workpiece, even if the workpieces are nominally identical (with regard to 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 usually indicated. It may also be the case that the abrasives used do not always have identical properties, which can influence the grinding process. Finally, user requirements may vary. In particular, 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 point of view, it may therefore be desirable to always be able to set the optimal operating point from the user's point of view. The invention creates the conditions for this.

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 speed of a grinding wheel or both grinding wheels, (ii) the speed of a grinding wheel or both grinding wheels and/or (iii) the speed and/or the direction of rotation of the workpiece spindle that carries the workpiece for the grinding operation. The material removal rate and/or the wear on the tools that occurs during grinding and/or the quality of the grinding result can be specifically influenced via each of these grinding parameters. Two or more of these grinding parameters are often changed simultaneously by a control intervention.

At least during one phase of the grinding operation, the workpiece spindle carrying the workpiece is arranged in a working position which is characterized by the fact that the workpiece section to be machined runs in a circular arc through the grinding space. To carry out a grinding operation, the workpiece and the grinding wheels are set in rotation about their axes of rotation. In one phase of a grinding operation, the abrasive side surfaces are brought into engagement with the associated surface by axially feeding the grinding wheels.

The term “grinding operation” in the sense of this application includes, among other things, those phases in which material removal takes place, i.e. in which at least one tool is in material-removing engagement with the workpiece. In addition, 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 done in different ways.

Preferably, suitable measurements are carried out. According to a corresponding development, the first position data determination system is designed as a first position measuring system for the metrological determination of the workpiece position data and the second position data determination system is designed 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.

An external measurement of the workpiece before installation, i.e. outside the grinding machine and before mounting on the workpiece holder of the grinding machine, would in principle be possible. During installation, a reference to the machine's internal reference coordinate system could then be established.

According to a further development, however, 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 in such a way that the workpiece can be measured in a state installed on the workpiece holder. By measuring the workpiece internally in 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. In addition, geometric deviations, such as shielding, must be taken into account.

In particular, it can be provided that the first position measuring system and the second position measuring system are designed in such a way that position measurements can be carried out 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 the transfer to the working position.

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

According to a further development, the measurement of the surface position is carried out without contact, for which at least one contactless (non-tactile) sensor can be used. This means that mechanical damage to the sensor and possibly to the workpiece surface can be avoided when taking measurements on the rotating workpiece and/or on the rotating grinding wheel.

Preferably, in order to determine the workpiece position data and/or the tool position data by measurement, a distance measurement is carried out between at least one distance sensor of the relevant 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.

According to a further development, a distance sensor can be calibrated from time to time against a reference standard. 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 area. Each distance sensor is assigned a reference element that is located in the working area of the distance sensor and can therefore be approached for a reference measurement. A reference element can be assigned to several distance sensors. The distance sensors can be calibrated regularly or on an ad hoc basis and adjusted if necessary in order to ensure permanently high measurement accuracy. This allows traceable measured values to be generated. The traceability As is well known, adaptability is a property that allows a measurement result to be related to a standard or is in relation to it.

Preferably, a pneumatic measuring system with at least one measuring nozzle acting as a distance sensor is used for distance measurement. Such measuring systems that work according to the nozzle-baffle plate principle are also referred to as air measuring systems. Compressed air flows from the measuring nozzle in the direction of 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 other variants the volume flow. One of the advantages of an air measuring system for this application is that it can work 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.

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.

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 so that a fine adjustment of the axial position of the measuring nozzle is possible. 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, the sensor signals of which can be used to precisely determine the axial position of the assigned distance sensor.

Some examples of available operating modes are described below.

According to a further development, an initial 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 attached 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 attached to the workpiece holder, a relationship can be easily established between the determined position data and a machine-fixed reference coordinate system.

In this measurement, it is preferably provided that workpiece position data are determined for a large number of surface locations offset in the circumferential direction and/or for a large number of surface locations offset in the radial direction. A spatially resolved determination of axial positions therefore takes place, in particular a two-dimensional spatially resolved measurement. This makes it possible to determine workpiece geometry data that is important for further processing, for example the thickness of the workpiece section between the workpiece surfaces that are essentially parallel to one another, any curvature of the workpiece section, which in the case of brake disks is often also referred to as shielding, and any waviness in the circumferential direction and/or in the radial direction. The subsequent grinding process can therefore 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.

In a similar way, an initial inspection of the ground workpiece can be carried out within the grinding machine before it is unloaded from the tool holder.

According to a further development, a contactless start-up control is implemented in which an abrasive side surface of a grinding wheel is fed in the direction of the workpiece surface to be ground to a start position of a next feed phase that is a safety distance away from the workpiece surface to be machined, whereby initial workpiece position data and initial tool position data are offset against each other before material removal begins to determine the start position. This makes an important contribution to efficient process control during grinding with the least possible loss of time while simultaneously protecting grinding tools and workpieces.

This contactless approach control takes advantage of the fact that the workpiece position data and the tool position data are in the same chen reference coordinate systems and can therefore be directly offset against one another, so that the axial position to which the grinding wheel is initially fed can be determined before the feed movement of the grinding wheel begins. The design of the safety distance ensures that at the end of this first feed movement there can be no contact between the abrasive side surface and the workpiece surface. In many embodiments the safety distance is expediently in the range of approx. 10 µm to approx. 100 µm. It is preferably provided that the grinding wheel is fed at a first feed speed until the start position is reached and that the feed speed is automatically reduced to a lower second feed speed when the start position is reached. The first feed can therefore take place in rapid traverse in a relatively short time even with a longer feed distance, before the second feed phase begins in the immediate vicinity of the workpiece surface, which is carried out at a significantly lower feed speed. This can be extended if necessary until contact is made between the grinding wheel and the tool.

The second feed operation can be maintained at least until a nominal contact position is reached. The nominal contact position is an axial position in 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 means that at the start of contact between the grinding wheel and the workpiece surface or shortly before or shortly after the initial contact, the feed speed and/or another grinding parameter can be changed again in order to achieve an optimized material-removing grinding process. The second feed operation can also be continued continuously beyond the time of the initial contact without changing the feed speed.

In some embodiments, a 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. time-resolved, during the grinding operation and processing the resulting time-dependent tool position data to control the feed 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 about the grinding behavior of the workpiece and tool, among other things.

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.

In some embodiments, the control unit of the grinding machine is configured in such a way that it relates time-dependent tool position data determined during the grinding operation to time-dependent feed position data in an operating mode. The time-dependent feed position data can be determined from the control program for the grinding process and/or possibly also measured with at least one displacement sensor. In some embodiments, the said operating mode is also referred to as grinding ratio optimization mode, since it creates an information basis for the grinding ratio with which the grinding process is working at the time of observation. Among other things, a comparison can be made between the feed and the removal that can be achieved with it. A comparison of these time dependencies can then be used to determine wear and accordingly to optimize wear.

The control unit can be configured in such a way that it determines information about the current grinding ratio G and makes it available for use, e.g. in the form of a G value or corresponding data. The grinding ratio is characteristic of 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 to lead to the best possible dimensional and shape accuracy, the feed ratios can be set so that wheel wear is as small as possible. If, on the other hand, a high metal removal rate is desired in order to remove a large amount of material thickness in a relatively short time, the feed speed can be set comparatively higher. Due to the options for determining the workpiece position data and the tool position data, a user therefore has an important tool for optimizing "his" grinding process.

Some embodiments offer the possibility of using a control unit to set a target grinding ratio for the grinding process or the parameters required to determine a target grinding ratio. nit. The control unit can be configured so that the current grinding ratio or the current value is calculated permanently 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 running sufficiently well. The control system can monitor compliance with the G value and control operation depending on the result of the comparison. For example, an acoustic, optical 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 and a limit violation is imminent. The operator can then intervene and try to determine the causes of the undesirable process change and eliminate them if necessary. It is also possible to design the control unit so that the process is automatically regulated to ensure compliance with the G value. The machine control then automatically ensures, within certain limits and without operator intervention, that the process runs approximately with the desired target grinding ratio.

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 another handling device or manually. However, a grinding machine is often designed as a multi-station grinding machine that has a transport system that has a spindle carrier unit that is movable relative to a machine base and that carries at least one workpiece spindle that is mounted so that it can rotate about an axis of rotation relative to the spindle carrier unit and has a workpiece holder. The grinding machine has at least two work stations that can be approached by the workpiece spindle and that include a loading station and at least one grinding station with at least one grinding unit. This makes it possible to spatially separate loading and unloading on the one hand and grinding on the other, so that these steps can be carried out at an overlapping time. This can significantly increase the piece output. In some variants, the spindle carrier unit is rotatable and can be built using components from a conventional rotary table. Alternatively, at least one translationally movable spindle carrier unit can be provided, which can be moved back and forth in a straight line between the loading station and the work station on the grinding unit, for example. If required, two mutually orthogonal, alternately usable linear systems can be provided, e.g. to provide two loading stations.

BRIEF DESCRIPTION OF THE DRAWINGS

• 1 zeigt eine schematische Seitenansicht eines Ausführungsbeispiels einer Schleifmaschine, die zum beidseitigen Schleifen von beschichteten Bremsscheiben konfiguriert ist;
• 2 zeigt einen werkstückseitigen Teil einer schematischen Darstellung des Systemaufbaus für die Werkstückvermessung;
• 3 zeigt einen werkzeugseitigen Teil einer schematischen Darstellung des Systemaufbaus für die Werkzeugvermessung;
• 4 zeigt eine Messdüse, der ein taktiler Sensor als Axial-Aktor zur Feineinstellung der Axialposition in Bezug auf den Träger der Messdüse zugeordnet ist;
• 5 zeigt schematisch eine Anordnung zur Messung der Axialposition der Seitenfläche der oberen Schleifscheibe;
• 6 und 7 zeigen zwei Möglichkeiten zur ortsauflösenden Positionsmessung von Werkstückoberflächen jeweils mit einer Vielzahl von Messstellen;
• 8A bis 8D zeigen Diagramme zur Illustration von Messwertauswertungen;
• 9 zeigt ein Weg-Zeit-Diagramm einer kontaktlosen Anfahrsteuerung;
• 10 zeigt ein schematisches Übersichtsdiagramm mit zeitabhängigen Abstandswerten und Werten für die Zustellung einer Schleifscheibe während einer Schleifoperation;
• 11 zeigt in 11A bis 11D schematisch Steigungsverläufe, die die Zeitabhängigkeiten während einer Schleifoperation darstellen;
• 12 zeigt ein Diagramm zum Zusammenhang zwischen Verschleiß und Schleifverhältnis (G-Wert)

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.

• 1 shows a schematic side view of an embodiment of a grinding machine configured for grinding coated brake discs on both sides;
• 2 shows a workpiece-side part of a schematic representation of the system structure for workpiece measurement;
• 3 shows a tool-side part of a schematic representation of the system structure for tool measurement;
• 4 shows a measuring nozzle to which a tactile sensor is assigned as an axial actuator for fine adjustment of the axial position in relation to the carrier of the measuring nozzle;
• 5 shows schematically an arrangement for measuring the axial position of the side surface of the upper grinding wheel;
• 6 and 7 show two possibilities for spatially resolving position measurement of workpiece surfaces, each with a large number of measuring points;
• 8A to 8D show diagrams to illustrate measured value evaluations;
• 9 shows a path-time diagram of a contactless start-up control;
• 10 shows a schematic overview diagram with time-dependent distance values and values for the feed of a grinding wheel during a grinding operation;
• 11 shows in 11A to 11D schematically gradient curves representing the time dependencies during a grinding operation;
• 12 shows a diagram of the relationship between wear and grinding ratio (G-value)

DETAILED DESCRIPTION OF THE EXAMPLES

1 shows a schematic side view of an embodiment of a grinding machine 100 which is used for grinding essentially plane-parallel, circular ring-shaped workpiece surfaces O1, O2 on a disk-shaped workpiece section of workpieces WS1, WS2 in the form of brake discs WS1, WS2. In the example case, the grinding machine is configured for double-sided grinding of surfaces of a circular brake section BA of coated brake discs.

Each of the brake discs has a base body made of gray cast iron, for example, with a central hub section NA, which is used to attach the brake disc to a vehicle axle, and a circular brake section BA, which encloses the hub section. The mass distribution of the base body is overall rotationally symmetrical to the axis of rotation of the brake disc. The brake section has two axially opposite, parallel surfaces (upper workpiece surface O1 and lower workpiece surface O2).

In an upstream phase of the manufacturing process, these were provided with a coating or functional layer that is rotationally symmetrical with respect to the axis of rotation, the free surface of which is ultimately intended to serve as the friction surface of the brake disc. The coating can contain tungsten, titanium and/or niobium carbide, for example, and can be very hard. In the example case, both sides were coated using a special variant of laser cladding, namely a variant of extreme high-speed laser cladding, which is also known as the "EHLA process". The coating can also be applied in other ways, e.g. by high-velocity flame spraying (HVOF) or cold gas spraying.

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 operating unit 195 with a graphical user interface.

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

At the 1 The loading station 110 is located on the left-hand side. There, a first brake disk (here WS1) is mounted 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 another handling device, or manually. 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 disk is clamped or mounted in a rotationally fixed manner and the axis of rotation of the brake disk is coaxial with the axis of rotation 156. A rotationally fixed connection to the workpiece spindle can also be achieved by holding it down or holding it from above. In the example, vertically movable hold-down devices 158 are provided.

A brake disk loaded in this way is then transported to a working position 125 in the area of the grinding station 120 by rotating the rotary table 180° clockwise. There, the work steps of the grinding operation (one or more) described later take place fully automatically. In the loading station 110, a previously completely ground brake disk can be removed at the same time and a new, not yet ground one can be clamped in. After grinding has been completed, the brake disk, which has been completely ground on both sides, is transported to the loading station 110 by rotating it 180°, from where it can be unloaded, for example by means of a robot or another handling device, or manually. A new brake disk 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 by brake disks and are each in different phases of handling.

The grinding machine processes the workpiece surfaces using the double-side face grinding method. 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) that are ideally arranged coaxially to one another on a frame part, each of which carries a grinding wheel (upper grinding wheel 130-2 and lower grinding wheel 130-2). The grinding wheels have abrasive side surfaces 135-1 and 135-2 facing one another and are arranged such that these grinding surfaces axially delimit a grinding space 133. Each of the grinding wheels can be rotated independently of the other grinding wheel about the associated rotation axis 136-1 or 136-2 by means of an associated spindle drive or rotation drive (upper rotation drive 134-1 or lower rotation drive 134-2) and can be moved parallel to the associated rotation axis with its own feed drive (upper feed drive 131-1 or lower feed drive 131-2) delivered or advanced using a specified delivery speed profile.

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

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 set up to determine workpiece position data by means of contactless distance measurements, which represent the axial positions of the workpiece surfaces O1, O2 of the braking section BA (top and bottom) to be ground at at least one surface location in relation 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 space 133.

In the area of the grinding station 120, hardware components of a second position measuring system 300 are also arranged, which is set up to determine tool position data by means of contactless distance measurements, which represent the axial positions of the mutually facing abrasive side surfaces 135-1, 135-2 of the two grinding wheels in relation 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 space 133.

The "axial position" of a surface is the position of the surface at the measuring location in an axial direction, which here corresponds to the Z direction of the reference coordinate system RKS. In an ideally set up 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.

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.

The measurement results of the two position measuring systems are processed together in an evaluation unit. The evaluation unit can be a functional component of the control unit 190 and can be implemented using appropriate evaluation software. Because the position data relating to the workpiece and the position data relating to the tool are available in relation 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 respectively assigned workpiece surfaces O1, O2 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 emit compressed air in the direction of the surface to be measured during operation. The measurement signal is measured from a property of the compressed air that depends on the distance between the measuring nozzle and the surface and evaluated to determine the distance.

The grinding machine of the 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 pointing downwards and measures the distance to the top O1 of the braking section BA. The lower measuring nozzle S2-1 is mounted with the nozzle opening pointing upwards and measures the distance to the bottom O2 of the braking section BA. The sensors can each be moved in an automatically controlled manner within an assigned working area both in the radial direction to the axis of rotation 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 in relation to the carrier of the respective measuring nozzle (cf. 4 ).

In the second position measuring system 300, a similar arrangement is provided for the measurements on the tools. 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 so that the abrasive surfaces of the cutting strips or grinding bodies are measured on the edge of the pot.

The grinding machine also has an internal calibration system with reference The first position measuring system 200 has reference elements for calibrating the distance sensors and for adjusting them. The first position measuring system 200 has a reference element RE1, which is arranged in the lateral travel range of the measuring nozzles S1-1, S2-1. The reference element has two parallel flat measuring surfaces separated by a step on each of the sides (top and bottom), the absolute positions of which 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 or distance sensors S1-2, S2-2.

A schematic representation of the system structure with relevant components and parameters is shown in 2 (workpiece measurement) and 3 (tool measurement). The top 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 disk 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 based on the fact that the distances recorded by sensors are always recorded in relation to the reference A. All measurement signals determined during the distance measurements or the data derived from them are available to the central control system or the control unit 190 for calculation.

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, it is also possible to calibrate the measuring sensors on these reference elements.

The contactless sensors (distance sensors) S1-1, S1-2 etc. of the pneumatic measuring system formed by the measuring nozzles 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. In 4 This coupling is shown schematically. This design offers advantages, among other things, with regard to the possibility of extending the measuring range (by axially moving the measuring nozzle over longer travel distances), the calibration and adjustment of the axial positions of the measuring nozzle. This configuration therefore has a tactile distance sensor that is mechanically coupled to the contactless distance sensor (measuring nozzle).

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

The structure and functionality of the sensors on the tool side is comparable to that on the workpiece side. One difference is that here the measuring nozzles S1-2, 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.

In non-contact pneumatic distance measurement, i.e. the sensory detection of the distance between the measuring sensor and the appropriate surface, the distance from the sensor to the measured surface is detected without contact. In 5 The situation in the area of the sensor S1-2 for measuring the axial position of the side surface 135-1 of the upper grinding wheel 130-1 is shown as an example. The axial position of the side surface of the lower grinding wheel is measured in an analogous manner.

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 it in the X direction (radial direction in relation to the workpiece rotation axis) in order to be able to assume different working positions. In 2 different working positions of the same sensor S1-1 are shown. 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 on an axially offset axial position. Position P3 shows the measurement of the upper workpiece surface O1 for distance detection 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.

It is clear that a large number 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 be stored digitally, for example, in a memory accessible to the control unit. Some of the values can be derived from measurements using external measuring systems generated or by operator inputs. Another group of measured values comes from measurements with the integrated sensors. These include both the contactless sensors Sx of the position measuring system (measuring nozzles) and the tactile sensors STx used for their axial displacement. Other values can come from measurements of the actuators, e.g. from encoders of drives for machine axes.

With the help of 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 in relation to the same reference coordinate system) can be recorded as required 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 prepare it in such a way that the control unit 190 can control the feed of the grinding wheels and/or other grinding parameters 130-1, 130-2 during different phases depending on these workpiece position data and tool position data. The overall high measurement accuracy is also promoted by the fact that the measurements can be carried out on the workpiece that is already in its working position 125 near the grinding wheels.

Some examples of useful operating modes are explained below.

In one operating mode, an input inspection of a newly loaded workpiece to be ground can be carried out in the grinding machine. In order to record the input quality of the new workpiece, the workpiece (the brake disk) 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. A current workpiece position can then be evaluated in comparison to a reference disk or a reference reference. In the process, an ideal reference is initially measured. All distance values are recorded with the location reference of the angular position as well as with the X distance (radial distance to the rotation 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, for example any shielding of the brake disc around the circumference, and to take this into account in the subsequent process if necessary.

In a similar way, an output inspection of the finished, ground workpiece can be carried out after the grinding process has been completed in the grinding station 120. The measured values determined on the workpiece can be compared with the reference values. If necessary, a reference disk can also be measured again in order to provide comparative values.

In 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. In the 6 and 7 Two possible measurement strategies are shown schematically. In the variant of 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. This means that the measured measuring points MO lie on a spiral path. With a fine-meshed spiral, the results can be interpolated to form circular rings if necessary.

Alternatively, it is also possible to use a sensor that is moved step by step to different radial positions and does not move while the workpiece is rotating. The measured surface locations or measuring points MO are then each located on concentric rings, the radial distance of which can be selected according to the desired spatial resolution in the radial direction. The X-resolution of this method is variable and depends only on the size of the measuring spot of an individual measurement.

It is also possible to distribute several distance sensors evenly or unevenly in the radial direction on the track width of the brake disc (see Fig. 7 ). In this case, only a single rotation of the workpiece may be required. The radial resolution depends on the distribution distance and the measuring spot.

A possible evaluation of the results is based on 8th explained. 8A a schematic representation of the location references, 8B shows signal curves of various sensors S1, S2, S3 during a rotation U of 360°, with the time on the x-axis of the diagram and the The axial position or a distance value ABZ is plotted on the y-axis. 8C shows the calculation of the measured values for the zero-degree position P ₀ , 8D a corresponding calculation for another position P _i . Reference symbol SP denotes the track width covered by the three sensors in the radial direction.

Each distance sensor has a unique location in the reference coordinate system at a given time t. In this example, the location X (radial position relative to the axis of rotation of the workpiece) is a constant position for each distance sensor. The Y position changes due to the rotation (rotation angle RW) and is generated by the angle encoder of the rotary drive for the workpiece rotation. This is then converted into the Cartesian value. The Z position corresponds to the measured sensor distance, which represents the workpiece position data. This gives a location-related measuring point on the workpiece for each sensor depending on the time.

The evaluation or utilization in the sense of a quality check can then be carried out, for example, by checking for each angular position whether the measured values are within a permissible tolerance range between an upper limit value UGW and a lower limit value UGW or not. The information on the input quality can be used to control the behavior of the grinding machine during processing.

In another operating mode, a contactless start-up control is implemented, which is based on the path-time diagram in 9 is explained in more detail. In this case (in phase P0), on each side (top and bottom), the abrasive side surface of a grinding wheel is initially advanced (in phase P0) before the material-removing grinding operation begins in the direction of the workpiece surface to be ground at rapid traverse with a slightly decreasing infeed speed to a position without workpiece contact, in which the side surface is still at a safety distance (e.g. from 10 µm to 100 µm) from 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 at rapid traverse. The current axial positions of the abrasive side surfaces are recorded by the contactless distance measurement. The axial positions of the workpiece surfaces have already been recorded for the first time during the incoming inspection; they can continue to be determined continuously.

The operator can now specify a safety distance up to which the abrasive side surfaces can be advanced in rapid traverse before switching from rapid traverse to a lower feed speed. The switch from phase P0 (rapid traverse) to phase P1 (feed up to workpiece contact and beyond with material removal) is based on the contactless measured values that are 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.

In phase P0, 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 in the direction of 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 by measurement, the distance between the workpiece surface and the abrasive side surface can be determined at any time, so that the starting position SP can be approached with a safety distance 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 through operator input. The safety distance can, for example, be in the order of magnitude of approx. 10 µm to approx. 100 µm. 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. In 9 A path-time diagram is shown schematically for a possible speed curve of the feed movement within the framework of the start-up control. The switchover point between phase P0 and phase P1 is where the specified safety distance is reached. This is the starting point SP for the controlled feed according to the programming.

This contactless approach control has significant advantages over conventional approach controls, which rely on physical contact between the workpiece surface and the tool. If the tool is fed in rapid traverse until contact is made between the rotating tool and the rotating workpiece, the After the abrasive side surface of the tool comes into contact with the workpiece surface, a large amount of material is removed, which also leads to high wear on the tool. Tool wear can be significant, particularly when machining coatings on brake disks that may be very hard. The contactless start-up control avoids this situation with particularly high tool wear without significantly slowing down the overall process, as rapid feed can be carried out until immediately before contact is reached, but contact is only reached a little later at a much lower feed speed and correspondingly less wear.

Tool wear can become a significant cost factor, particularly when grinding very hard coatings on brake disks. However, thanks to the real-time position information recorded using position measuring systems, grinding processes can be carried out with particularly low tool wear. There are also advantages in terms of wear compensation. The described start-up control means that the axial position of the cutting tool is known immediately before the material-removing machining process begins, so that compensation is already achieved in this respect.

During grinding, the axis positions of the workpiece spindles and the distances between the tools and the workpiece can be recorded and used 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 that existed after setup into the control system. Because the positions of the machine axes used for infeed and the distances between tools and workpieces are recorded during processing, wear can be calculated immediately.

Further advantages include switching off the grinding process, phase switching during the grinding process, wear monitoring and, if necessary, wear compensation, as well as the possibility of determining real-time information on the process-specific grinding ratio and thus optimizing the process flow. This is mainly achieved by means of the 10 to 12 explained.

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.

Some helpful connections for understanding are explained using 10 explained. This uses a schematic diagram to show the time dependence of some parameters during a grinding operation. The time t is entered on the abscissa and the value of the parameter ABZ on the ordinate, which indicates the distance measured in the axial direction between a distance sensor and the workpiece surface observed by it. In the example case, this is sensor S2-1, which measures its distance to the underside of the brake section BA outside the grinding area. This distance increases as the amount of material removed 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 brake disc underside is reached.

The process or grinding operation goes through a sequence of phases, where phase P0 is the start-up phase. Phase P1 is the first phase of the advance of the grinding tools at a pre-programmed first 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 phase or several consecutive phases with different grinding parameters.

First, the workpiece is placed on the workpiece holder and pressed firmly onto it with the hold-down device. This can result in changes in distance. By holding it down, a fixed reference to the machine-fixed reference coordinate system is established. This is followed by the start-up phase, in which there is still no contact between the tool (grinding wheel) and the workpiece surface. Accordingly, in this phase P0 the distance is constant (apart from small periodic fluctuations due to azimuthal undulations in the workpiece). The first phase (phase P1) begins in this example when workpiece contact is established. It is clear that in the first phase the distance increases more or less continuously (apart from the superimposed undulations). After switching to the following phase Pn at the switching point UP, the distance increases less quickly, which corresponds to less material removal per unit of time. With this material removal, the switch-off point AP is then reached.

This variant of shutdown can be carried out in such a way that the operator first dependent dimension. The specified dimensions are calculated to give corresponding values for distances on the sensors. The grinding machine continuously carries out its operating program during grinding. The position measuring system is the master for deciding when to switch between the phases and when to switch off at the end of the grinding process. The machine control system therefore makes the decision about when to switch off or when the desired dimension is reached based on the specified dimensions and the measured values of the position measuring system.

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

Additional process-relevant information can be obtained, for example, if the distance measurement values are calculated with 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 dependencies of the material removal or the distance values and the infeed can be specified, for example, in the form of gradients. In 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 approx. 2 µm/s to approx. 5 µm/s. The lower material removal after switching to phase n can, for example, be in the order of approx. 1 µm/s and is represented by a gradient line ST2 with a lower gradient.

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

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 the 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 is converted into workpiece removal without loss, which would correspond to a grinding process without wear on the grinding wheels.

In a more realistic case, however, the feed rate (middle diagram, Z-axis ZA) will tend to be higher than the achievable material removal rate. To illustrate this, 10 Another gradient line ST3 is entered, the gradient of which 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, namely wear and deformation of the tools.

As a rule, the grinding wheels wear out more or less quickly, so that only a portion of the feed path is actually converted into material removal in the z-direction and another portion compensates for the material wear. 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 can be seen as an illustrative borderline case, so that practically no material removal takes place despite the feed. In this case, the gradient for the material removal would be almost zero, while the continuous feed for the feed would result in a significantly greater gradient.

A second reason for a deviation in the gradients between the measured distance value and the change in the feed position over time is a possible deformation of the components of the grinding unit. Although this should be constructed as rigidly as possible, a certain degree of flexibility will be unavoidable. It may then be the case that part of the forces acting when the grinding wheels are fed in are not used for material removal, but lead to the deformation of the components of the grinding unit.

According to the inventors' experience, the portion of the discrepancy Δ due to wear is significantly higher than the portion due to deformation, so that as a first approximation, the tool wear can be determined from the discrepancy between the pitch line ST1 and the pitch line ST3 (for the infeed).

In order to 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.

• • G1: Gesamtfläche der Schneidleisten | ASG bzw. die Fläche jeder einzelnen Leiste
• • G2: Belagshöhe der Schneidleisten | BHs
• • G3: Die aktuelle Achsposition und/oder aktueller Abstand zum Sensor-System Werkzeugseite
• • G4: Spurbreite des Werkstückes
• • G5: Start und aktuelle Höhe des Werkstückes Oberseite / Unterseite

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 ratio 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 process or the ratio between material removal and wear on the tool. 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 or corresponding 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 much from a predefined target value or is about to deviate shortly. Alternatively or additionally, a control system can be implemented using the control unit, e.g. to adjust the grinding operation so that the grinding ratio remains within a predeterminable value range window. In order to determine the G value, the following information is processed in one embodiment.

• • G1: Total area of the cutting bars | ASG or the area of each individual bar
• • G2: Covering height of the cutting bars | BHs
• • G3: The current axis position and/or current distance to the sensor system tool side
• • G4: Track width of the workpiece
• • G5: Start and current height of the workpiece top / bottom

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). The volume V _{GW (t) can be determined from the track width (ring area | G4) and the distances from the sensor system on the workpiece side (G5). The G value is determined by V GW (} t) / V _GS (t). With decreasing wear VS (in volume units) at a constant cutting volume, the G value G increases; an example curve is shown in 12 This functionality gives the operator and/or the control system the opportunity to set the optimal grinding process for the respective machining task.

QUOTES INCLUDED IN THE DESCRIPTION

This list of documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions.

Cited patent literature

• EP 2746613 A2 [0005]
• WO 2019/021161 A1 [0005]
• WO 2021/224308 A [0007]
• EP 3789512 A1 [0008]

Cited non-patent literature

• EN ISO 4288 [0007]

Claims (25)

Grinding machine (100) for grinding essentially plane-parallel, circular ring-shaped workpiece surfaces on a disk-shaped workpiece section (WA) of a workpiece (WS1, WS2), in particular for grinding workpiece surfaces of a circular ring-shaped brake section of a brake disk, comprising a grinding unit (121) with two tool spindles (132-1, 132-2) arranged coaxially to one another, each of which carries a grinding disk (130-1, 130-2), wherein the grinding disks are arranged with abrasive side surfaces (135-1, 135-2) facing one another, which axially delimit a grinding space (133), wherein each of the grinding disks can be rotated about the associated rotation axis (136-1, 136-2) independently of the other grinding disk by means of an associated rotation drive (134-1, 134-2) and can be moved parallel to the associated rotation axis (136-1, 136-2) by means of an infeed drive (131-1, 131-2). the associated axis of rotation can be fed, 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 rotary drive (157) about a rotation axis (156) running parallel to the axes of rotation 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 runs 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 which represent an axial position of a workpiece surface (O1, O2) at at least one surface location in relation to a machine-fixed reference coordinate system (RKS), and a second position data determination system for determining tool position data which represent an axial position of an abrasive side surface (135-1, 135-2) facing the workpiece surface (O1, O2) in relation to the same 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 depending on the workpiece position data and/or the tool position data. Grinding machine according to Claim 1 , characterized in that the control unit (190) is configured to control at least one grinding parameter from the following group depending on the workpiece position data and/or the tool position data: a feed speed of a grinding wheel; a rotational speed of a grinding wheel; a rotational speed of the workpiece spindle which carries the workpiece for the grinding operation; a direction of rotation of the workpiece spindle which carries the workpiece for the grinding operation. Grinding machine according to Claim 1 or 2 , characterized in that the first position data determination system is designed as a first position measuring system (200) for metrologically determining the workpiece position data and the second position data determination system is designed as a second position measuring system (300) for metrologically determining 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. 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). 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. Grinding machine according to one of the Claims 3 until 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). Grinding machine according to one of the Claims 3 until 6 , characterized in that the grinding machine has an internal calibration system with reference elements (REx) for calibrating sensors, in particular distance sensors (Sx), wherein preferably distance sensors are movable in a controlled manner within an individual working range and each distance sensor is assigned a reference element (REx) which is located within the working range 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. Grinding machine according to one of the 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. 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 carrier unit (150) which is movable relative to a machine base (102) and which carries at least one workpiece spindle (154) which is mounted so as to be rotatable about an axis of rotation (156) relative to the spindle carrier 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 the spindle carrier unit is preferably rotatable in the manner of a turntable or has at least one translationally movable spindle carrier unit which can be moved back and forth between the loading station and the work station on the grinding unit. 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 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, 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. 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 start position (SP) located at a safety distance from the workpiece surface to be machined, wherein initial workpiece position data and initial tool position data are offset against one another to determine the start position (SP), wherein the grinding wheel is preferably fed at a first feed speed until the start position (SP) is reached and the feed speed is automatically reduced to a lower second feed speed when the starting position is reached, wherein the second feed speed is preferably maintained at least until a nominal contact position is reached at which the current tool axial position corresponds to the initial workpiece axial position. 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. 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. 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 using 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) regulating the grinding operation to ensure compliance with the target grinding ratio in such a way that a grinding operation runs approximately with the desired target grinding ratio within tolerances without operator intervention. Grinding method for grinding essentially plane-parallel, circular workpiece surfaces on a disk-shaped workpiece section of a workpiece, in particular for grinding surfaces of a circular brake section of a brake disk, by means of a grinding machine which has two grinding disks with abrasive side surfaces facing one another, which axially delimit a grinding space of variable height, wherein the grinding disks can be rotated independently of one another about coaxial rotation axes and can be fed by feeding in an axial direction oriented parallel to the rotation axes, wherein the workpiece is held in a rotationally fixed manner on a workpiece holder which can be rotated about a rotation axis and is arranged in a working position at least during a grinding operation such that the workpiece section runs through the grinding space in a circular arc, wherein, in order to carry out a grinding operation, the workpiece and the grinding disks are set in rotation about their rotation axis and the abrasive side surfaces are brought into engagement with the associated workpiece surface by axial feeding of the grinding disks, wherein the grinding method comprises the following steps for each workpiece surface to be ground: Determining workpiece position data, which represent an axial position of a workpiece surface to be ground at at least one surface location in relation to a reference coordinate system, Determining tool position data which represent an axial position of an abrasive side surface facing the workpiece surface in relation 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 depending on the workpiece position data and/or the tool position data. Grinding process according to Claim 15 , characterized in that 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. Grinding process 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). Grinding process according to one of the Claims 15 until 17 , characterized in that in an operating mode an input 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 starting a grinding operation on the workpiece, workpiece position data are determined for each of the surfaces of the workpiece section to be ground. Grinding process according to one of the Claims 15 until 18 , characterized by a spatially resolved determination of workpiece position data in which 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. Grinding process according to one of the Claims 15 until 19 , characterized in that in an operating mode a contactless start-up control is carried out, in which an abrasive side surface of a grinding wheel is moved in rapid traverse in the direction of the workpiece surface to be ground up to a distance at a safety distance from the workpiece surface to be machined. distant start position (SP) and the delivery speed is automatically reduced to a lower second delivery speed when the start position is reached, whereby the safety distance can be specified and is in particular in the range of 10 µm to 100 µm. Grinding process according to one of the Claims 15 until 20 , characterized in that during at least one phase of a grinding operation, workpiece position data and tool position data are measured simultaneously. Grinding process according to one of the Claims 15 until 21 , characterized in that a distance sensor is calibrated from time to time during operation of the grinding machine on 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 change of the axial position of the distance sensor. Grinding process according to one of the Claims 15 until 22 , characterized in that during the grinding operation tool position data are determined continuously or intermittently depending on time and time-dependent tool position data are processed to control the feed of the grinding wheels. Grinding process according to one of the Claims 15 until 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. Grinding process according to one of the Claims 15 until 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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