Grinding method and grinding machine

Abstract

In a grinding method for grinding a substantially flat workpiece surface of a workpiece, a grinding machine is used which has an abrasive wheel with an abrasive side surface, wherein the abrasive wheel can be rotated about an axis of rotation by means of a rotary drive and can be advanced by means of an advancing drive in an advancing direction oriented parallel to the axis of rotation. In order to carry out a grinding operation, the workpiece is arranged in a working position, the abrasive wheel is set in rotation about the axis of rotation, the abrasive side surface is brought into contact with the workpiece surface by advancing of the abrasive wheel, and the abrasive wheel is advanced proceeding from a starting position with workpiece contact, according to an advancing function, in order to produce material removal. The advancing of the abrasive wheel takes place according to a workpiece-specific advancing function in accordance with a material proportion curve, the material proportion curve representing a distribution of workpiece material in a surface profile according to a cutting depth measured parallel to the advancing direction.

Description

[0001] Grinding process and grinding machine

[0002] SCOPE OF APPLICATION AND STATE OF THE ART

[0003] The invention relates to a grinding method for grinding a substantially flat workpiece surface using a grinding machine comprising a grinding wheel with an abrasive side surface, wherein the grinding wheel is rotatable about a rotational axis by means of a rotary drive and can be moved in a feed direction parallel to the rotational axis by means of a feed drive. The invention also relates to a grinding machine suitable for carrying out the grinding method.

[0004] One preferred application is the grinding of the surfaces of annular brake sections of coated brake discs.

[0005] Conventional brake discs for the mass market are mostly made of gray cast iron. Gray cast iron brake discs are now reaching their limits, particularly with regard to corrosion and wear resistance. In the future, brake discs for motor vehicles should be designed to release fewer fine particles during braking. One approach to reducing particulate emissions from motor vehicles is to coat the brake discs, or rather the surface sections intended as friction surfaces, with a thin functional layer of more wear-resistant material. In coated brake discs, the surfaces of the annular braking section each carry a functional layer or coating that is rotationally symmetrical with respect to the axis of rotation, and the free surface of this layer serves as the friction surface of the brake disc.

[0006] The manufacturing process of a coated brake disc comprises one or more coating operations to apply a functional layer to the surfaces of the brake section of the disc. Due to its relatively high mechanical hardness, this layer can reduce wear and, if necessary, inhibit corrosion. Such functional layers can consist of a single layer or multiple layers with different properties. For example, a coating might have a thin metal adhesive layer topped with a wear-resistant wear layer containing hard particles, such as carbide particles, in a metallic matrix. These coatings can be applied by methods such as flame spraying or laser cladding. Typical layer thicknesses can range from well over 50 µm to 350 µm or more. The coatings are generally applied to both sides of the disc.Documents EP 2 746 613 A2 and WO 2019 / 021161 A1 reveal examples of coated brake discs.

[0007] Depending on the coating process and the coating material (one or more), the finished coatings can exhibit different properties. The functional layers are generally relatively hard mechanically, at least in the area of ​​the free surface of the coating, and relatively rough and superficially porous after coating. Typical roughness values ​​can range, for example, from 30 µm to 80 µm or higher.

[0008] A subsequent grinding process is intended to create a sufficiently flat surface on the coating, optimized for braking function. For example, the specifications for friction surfaces of a brake disc may stipulate that the mean roughness Ra, determined according to DIN EN ISO 4288, should be in the range of 1 pm to 3.2 pm and that the flatness deviation should not exceed 20 pm (see WO 2021 / 224308 A).

[0009] EP 3 789 512 A1 discloses a system for coating and subsequently grinding brake discs.

[0010] Side surface grinding machines, often in the form of double-sided surface grinding machines, are frequently used for grinding brake discs.

[0011] Side-face grinding is a grinding process for grinding a substantially flat workpiece surface using a grinding machine that has a grinding wheel with an abrasive side surface. The grinding wheel is rotatable about a rotational axis by means of a rotary drive and can be moved in a feed direction parallel to the rotational axis by means of a feed drive. To perform a grinding operation, the workpiece is held in or on a workpiece holder such that it is in a working position at least during the grinding operation. The grinding wheel is set into rotation about the rotational axis. The workpiece is preferably rotated about a workpiece rotational axis at least for the duration of the grinding operation. The workpiece and tool can rotate in the same or opposite directions.The abrasive side surface of the grinding wheel is brought into contact with the workpiece surface by moving the grinding wheel. Starting from a point of contact with the workpiece, the grinding wheel is then moved according to a feed function, via corresponding control signals from a control unit of the grinding machine, to generate material removal. The feed function represents the functional relationship between time and the feed position of the tool (grinding wheel) along the feed direction, or the temporal movement of the tool in the assigned feed direction.

[0012] In many well-known grinding processes, grinding programs are designed so that the entire allowance is divided into individual passes such as "roughing", "finishing", "fine finishing" and "firing out", each with adjusted parameters for cutting speed and feed (feed position).

[0013] Document EP1947538A1 discloses a method for controlling a movable grinding tool by means of a feed function that specifies the tool's movement over time in at least one axis. For programming, an initial feed function is displayed on a monitor in the form of at least one initial feed curve. From the initial feed function and inputs made via a pointing device, a feed function is calculated and displayed on the monitor as a modified initial feed curve. This is intended to simplify and make the input of a feed function more flexible.

[0014] Document DE 102021 132468 B3 provides a detailed description of fine dust problems caused by brakes and specific problems encountered during the grinding of brake discs with a difficult-to-machine coating. It describes a device for grinding the flat surfaces of a coated brake disc for a motor vehicle, wherein the device comprises at least two grinding wheels, a first grinding wheel for rough grinding the flat surface by a large allowance and a second grinding wheel for finish grinding the flat surface by a small allowance. The two grinding wheels are designed as axially displaceable, nested cup wheels.

[0015] TASK AND SOLUTION

[0016] The invention is based on the objective of providing a grinding process of the type mentioned in the introduction, which makes it possible to grind nominal planar surfaces of workpieces, even with initially relatively large microscopic and, if applicable, macroscopic shape deviations, in such a way that ground planar surfaces with high machining quality can be achieved within relatively short cycle times. A further objective is to provide a grinding machine suitable for carrying out the grinding process. To achieve this objective, the invention provides a grinding process with the features of claim 1. Furthermore, a grinding machine with the features of claim 16 is provided. Advantageous embodiments are specified in the dependent claims. The wording of all claims is made clear by reference to the content of the description.

[0017] According to one aspect of the invention, a grinding method for grinding a substantially flat workpiece surface is provided, which is carried out using a grinding machine having a grinding wheel with an abrasive side surface. The grinding wheel is rotatable about an axis of rotation by means of a rotary drive and can be moved in a feed direction parallel to the axis of rotation by means of a feed drive. The feed is thus a linear movement parallel to the axis of rotation of the grinding wheel; this direction is also referred to in this application as the "axial direction". A feed movement can be performed both in the direction of the workpiece and in the opposite direction.

[0018] To perform a grinding operation, the workpiece is positioned in a working position in which the surface to be ground is in a specific axial position. Typically, to generate material removal during the grinding operation, both the tool (grinding wheel) and the workpiece are rotated at a predefined speed. To enable this, the grinding machine can have a workpiece spindle with a workpiece holder for the rotationally fixed mounting of the workpiece. This workpiece holder is rotatable about an axis of rotation that runs essentially parallel to the axis of rotation of the grinding wheel by means of a rotary drive. To perform a grinding operation, the grinding wheel is set into rotation about this axis of rotation. The workpiece is also typically set into rotation, either in the same direction as the grinding wheel or in the opposite direction.By moving the grinding wheel towards the workpiece, the abrasive side surface is brought into contact with the workpiece surface. This phase of the grinding operation is also known as the approach phase. During the grinding operation, the grinding wheel is moved from a starting position with workpiece contact to generate material removal according to a feed rate function.

[0019] The term "grinding operation" as used in this application thus encompasses both the start-up phase, in which there is still no contact between the grinding wheel and the workpiece, and those phases in which the grinding wheel or the abrasive side surface is in material-removing engagement with the workpiece and in which material removal takes place. A special feature of the grinding process is that the infeed of the grinding wheel is determined according to a workpiece-specific infeed function, depending on a material fraction curve.

[0020] The material fraction curve represents the distribution of workpiece material in a surface profile as a function of a cutting depth measured parallel to the feed direction. The material fraction can be expressed as a percentage value, indicating the ratio of material-filled profile sections to a reference line. The material fraction curve can be determined from the surface profile of a workpiece by drawing lines parallel to a reference line and measuring the proportion of the total line length that lies within the surface profile, i.e., the section that already passes through workpiece material. Mathematically, the material fraction curve can also be described as the cumulative probability density function of the surface height profile. The material fraction curve can be determined in the same way as the so-called Abbott curve.The Abbott-Firestone curve is used in some areas of machining to qualify a workpiece surface achieved through surface treatment. According to the ASME B46.1 standard, the Abbott-Firestone curve is also referred to as the bearing area curve (BAG). The terms "bearing area curve" or "Abbott bearing area curve" are also frequently used.

[0021] The feed rate between the starting position and the end position of the material-removing grinding operation is thus determined by the material fraction curve, either directly or indirectly. This approach utilizes the understanding that a functional relationship exists or can be established between the material fraction curve and the feed rate function, and that the feed rate can be optimized with regard to its temporal progression, tool wear, and grinding result by considering workpiece properties related to the material fraction curve.

[0022] According to the inventors' findings, such an orientation of the feed to the material fraction curve offers considerable advantages compared to the prior art, especially with regard to the cycle times of the grinding process, i.e., the total times required to move from a relatively rough surface profile to the desired functional surface, which is practically free of pores or other material-free zones, i.e., passes through the solid material.The feed rate can be optimized so that a relatively high feed rate can be used immediately after the start of the cutting action, since the material content in a given area is still relatively low at that stage. The feed rate is then gradually reduced to a degree that achieves a favorable compromise between material removal rate, tool wear, and stress on the grinding system. The grinding operation then ends promptly after the full cut is achieved. This ensures that, on the one hand, the entire near-surface area that was part of the surface profile before the start of the grinding operation is reliably removed, while on the other hand, no more material is removed than is necessary to eliminate the surface profile area.

[0023] A grinding machine according to the claimed invention is, due to its design features and the design of the operating control system with a control unit and the hardware components (including sensors and actuators) and software components (including control and evaluation software) communicating with it, configured or configurable by providing a corresponding operating mode, so that a grinding process according to the claimed invention can be carried out, i.e., a grinding process in which the feed of the grinding wheel is carried out as a function of the material content curve.

[0024] The orientation of the delivery to the material proportion curve can be direct or indirect.

[0025] Direct or immediate orientation to the material fraction curve can be implemented, for example, by controlling the infeed of the grinding wheel according to a workpiece-specific infeed function, based on a material fraction curve whose shape was determined before the grinding operation began. Such embodiments include determining a material fraction curve representative of the workpiece or of a group of similar workpieces containing the workpiece, and controlling the infeed according to an infeed function calculated based on the determined material fraction curve. The infeed during the grinding operation can then be based, for example, on a feedforward control system, possibly in combination with a control system built upon it.

[0026] An indirect orientation towards the material fraction curve can be implemented, for example, by determining, through simulation, calculations, experiments, or other means, which characteristic response (at least) a selected process parameter would exhibit for a given material fraction curve and a predefined feed function, and then by controlling or regulating the feed in such a way that the process parameter exhibits a profile corresponding to the material fraction curve. The selected process parameter could be, for example, the torque of a rotary drive of a workpiece spindle and / or a tool spindle, or the electrical power or current consumption of the rotary drive.

[0027] There are various methods for determining a material fraction curve suitable for characterizing the workpiece to be machined. According to one advanced method, a surface profile of at least a section of the workpiece surface is measured using a roughness measurement operation, and the material fraction curve is calculated based on this measured surface profile. The roughness measurement operation can be performed immediately before the grinding operation begins on the workpiece that is to be ground next. For this purpose, the grinding machine can incorporate components of a roughness measurement system, which are, for example, located in the area of ​​a loading station of the grinding machine, in order to measure the roughness of the workpiece surface to be ground immediately after the workpiece holder has been loaded.The roughness measurement system can be equipped with at least one tactile sensor and / or at least one optical sensor to determine a surface roughness profile in a selected measurement zone of the workpiece surface. For example, a 3D laser microscope can be used. Measurements could also be performed at multiple locations on a workpiece surface to obtain more robust quantitative information about the surface profile.

[0028] It is also possible to perform the roughness measurements (one or more) outside the grinding machine, for example, in a separate measuring room, and to determine the material content curve based on the measured values ​​obtained there. When using roughness measurements, an individual material content curve can be assigned to each workpiece, allowing the grinding process to operate with workpiece-specific feed functions. It is also possible to determine a representative material content curve on a representative workpiece from a batch, or on several workpieces from the same batch or group of similarly pre-machined workpieces, representing a large number of workpieces from the same batch, and then use this curve as the basis for the control system.

[0029] It is also possible to determine a representative material fraction curve without performing a roughness measurement, for example, by means of simulation calculations that take into account the manufacturing process and the workpiece's production parameters before the grinding operation begins, in order to calculate a model of the surface profile and thus a representative material fraction curve. According to a further development, the grinding process is used to grind a workpiece that has a surface profile with a profile depth of at least 50 pm on the workpiece surface to be ground before the grinding operation begins. Accordingly, at least 50 pm of material must be removed from the workpiece to reliably expose the area of ​​solid material. The initial profile depth can also be significantly greater, for example, more than 80 pm, more than 100 pm, or more than 150 pm.On the other hand, the surface roughness should not be too pronounced, so that preferably the profile depth should not be greater than 150 pm in order to derive optimal benefit from the application of the grinding process.

[0030] The novel grinding process was developed and optimized within the context of developing processes for manufacturing coated brake discs. However, its application is not limited to such workpieces. According to a further development, the grinding process is used to grind a workpiece that has a base body with at least one flat surface on which a single-layer or multi-layer functional layer is applied, the free surface of which forms the workpiece surface to be ground. The functional layer is preferably produced by a laser-based additive coating process using at least one filler material heated and / or melted by laser radiation. The functional layer can, for example, be produced by a laser cladding process, sometimes also referred to as "laser metal deposition" (LMD).One or more additive materials can be supplied in powder form and melted using a laser beam, thus fusing them to the base material. The functional layer adjacent to the exposed workpiece surface can consist of a composite material containing hard particles, such as carbides or similar substances, bonded in a metallic manner. This allows for the creation of hard, wear-resistant workpiece surfaces. Furthermore, such heterogeneous layers often exhibit relatively rough surfaces, making grinding processes ideally suited for machining.

[0031] For the practical implementation of the concept into a feed strategy that is easily controllable from a control engineering perspective, preferred embodiments proceed as follows: the grinding operation is controlled according to a sequence of immediately successive phases, each with different combinations of grinding parameters, in particular with a predefinable feed rate and predefinable material removal depth. Preferably, transition points between the phases can lie on the material fraction curve. The material fraction curve thus provides, so to speak, a stable framework for the design of the individual phases, so that the feed strategy is directly oriented to the shape of the material fraction curve. If a feed rate and the resulting material removal depth are specified, the machining time required per phase is derived as a parameter.This allows the grinding parameters to be combined in such a way that a specific total machining time can be worked towards as a target value across all phases.

[0032] It has proven useful to divide the workpiece surface into a peak area (R) based on the material fraction curve. pk ), a core area (R k ) and a grooved area (R vkThe grinding process is divided into phases, with at least one phase for removing the tip area, at least one phase for removing the groove area, and several phases for removing the core area. Close adherence to the material fraction curve can be achieved if at least two phases with different grinding parameter combinations are provided in each of the areas. It is often advantageous to provide three or more phases with different grinding parameter combinations in the core area. Overall, it has proven beneficial to control the grinding operation according to a sequence of five or more immediately consecutive phases, each with different grinding parameter combinations, where, for example, five, six, seven, eight, nine, or ten different phases can be provided.

[0033] Such a detailed subdivision of the total machining time into phases with different grinding parameter combinations differs significantly from the conventional approach to designing grinding programs. In conventional grinding processes, the total stock removal is often divided into a maximum of four different phases: roughing, finishing, fine finishing, and spark-out. Each of these phases has adapted parameters for cutting speed and longitudinal feed. Roughing is the mode with the greatest material removal per unit of time; finishing and subsequent fine finishing improve the surface finish with less material removal; and spark-out relieves stresses within the grinding system without significant further material removal.While the state of the art generally provides for at most these four different phases, and some grinding machine controls offer a maximum of these setting options, the aforementioned further training offers the possibility to define significantly more phases with different grinding parameter combinations in order to optimally adapt the grinding process to the specific workpiece properties.

[0034] One method variant provides that a cutting speed is varied during the grinding operation, preferably with different cutting speeds being set for selected different phases, in particular such that a relatively lower cutting speed is specified in a phase for removing the tip area in relation to a mean cutting speed and a relatively higher cutting speed is specified for a spark-out phase in relation to a mean cutting speed.It has been shown that the relatively lower cutting speed at the beginning of machining results in relatively long chips, leading to a better self-sharpening effect on the cutting edge. Furthermore, at the relatively higher cutting speed during spark-out, the contact pressure exerted by the grinding wheel on the workpiece material gradually decreases, allowing the system to relax overall. Average cutting speeds can be in the range of 15 to 20 m / s.

[0035] According to further training, the feed rate during the core grinding phases can be controlled in such a way that the torque of the rotary drive remains essentially constant. This means that the torque preferably fluctuates within a constant torque range by no more than 10% around a mean value. This allows for a particularly efficient yet material- and tool-friendly grinding process within the core area, achieving a good compromise between achievable material removal per unit of time and tool wear.

[0036] To achieve even better adaptation of the feed kinematics or feed function to the conditions present on the workpiece in the area of ​​the surface profile, some embodiments provide that, in addition to the material ratio curve, at least one of the following parameters is taken into account to determine a desired material removal depth of a phase: a total profile height (R max) of the surface profile before the start of the grinding operation; a deviation of the workpiece surface from a plane; a compliance or stiffness in the force flow of the grinding machine; a compliance of the cutting material of the grinding wheel. Preferably, several or all of these parameters are taken into account when determining the workpiece-specific feed function. This ensures that, in any case, the entire surface profile area is removed at the end of the grinding operations and that work is carried out in the area of ​​the bulk material ("full cut"). The significance of these individual contributions will be explained in more detail in connection with the description of exemplary embodiments.

[0037] A grinding operation can be controlled entirely based on a feed function that takes some or all of the aforementioned influencing factors into account. By orienting itself to the material fraction curve, a very good adaptation of the feed function's timing to the workpiece's specific characteristics can already be achieved. However, pure control cannot fully account for unforeseen disturbances or exceptional deviations from the assumed machining conditions. To enable further improvements in this area, some embodiments are characterized by a control of at least one grinding parameter influencing material removal, superimposed on the feed control. In this concept, the feed function, which is based solely on the pre-determined material fraction curve, is thus used like a feedforward control system with a control function superimposed on it.Pure feedforward control based on the material fraction curve already works so well in many cases that additional control interventions would only lead to minor improvements. In such instances, pure control can be used, which is generally faster than a control system, enabling particularly short cycle times. In other cases, control is combined with a control system. Fast feedforward control brings the control loop close to the target value. The control loop then ensures that the target value is achieved and that disturbances are compensated for promptly.

[0038] In some embodiments, the superimposed control is a force control to regulate the contact force exerted by the grinding wheel on the workpiece, or a feed rate control to regulate the feed rate, which determines the material removal achievable during a unit of time.

[0039] According to a further training, force control is achieved by monitoring whether the actual machining time required to complete a phase lies within or outside a tolerance range around the target machining time for that phase. If the actual machining time is outside the target machining time for that phase, the contact force for grinding operations in the same phase on a subsequently machined second workpiece is adjusted so that any deviation of the machining time from the target machining time remains within the tolerance range. This is an example of post-process control, where subsequently machined workpieces benefit from target deviations detected in the previously machined workpiece. It is also possible to control the spindle current.

[0040] Variants of in-process regulation are also possible.

[0041] As mentioned above, the feed rate can also be aligned indirectly with the material distribution curve, which in this context means that the workpiece-specific material distribution curve does not necessarily have to be determined beforehand, for example, by measurement or simulation. According to a further development approach, the grinding wheel feed is adjusted according to a workpiece-specific feed function, depending on the material distribution curve, such that a characteristic measurement parameter for the interaction between the abrasive side surface, the grinding wheel, and the workpiece is recorded during the grinding operation, and the feed rate is controlled in such a way that the measurement parameter follows a predefinable time course related to the material distribution curve.

[0042] Preferably, the measured variable is proportional to the torque of the rotary drive of the tool spindle, in particular a power consumption (current) of the rotary drive, or a measured variable proportional to the contact force of the grinding wheel. Alternatively, the workpiece drive can also be controlled.

[0043] Preferably, for a grinding operation, the workpiece is also set into rotation, which can be in the same direction as the rotation of the grinding wheel or in the opposite direction. For this purpose, the workpiece is or is fixedly mounted on a workpiece holder of a workpiece spindle, which is rotatable about a rotational axis by means of a rotary drive. This axis of rotation is parallel or approximately parallel to the rotational axis of the grinding wheel. In such cases, the measured variable can alternatively or additionally be a measured variable proportional to the torque of the rotational drive of the workpiece spindle.

[0044] In a further training process, the feed of the grinding wheel is controlled such that a measured value proportional to the torque of the rotary drive initially increases from the starting position in a rising phase, in particular essentially linearly, and that the measured value then transitions in a short transition phase into a constant phase in which the measured value remains essentially constant. "Essentially constant" here means, in particular, that the measured value fluctuates by no more than 10% around a mean value.

[0045] The progression of the ramp-up phase can vary from case to case; in particular, ramp-up phases with different gradients can be used to represent those phases of material removal in a surface profile where the material removal has not yet reached the core area. In the subsequent constant-torque phase, the core area can then be removed with a more or less constant torque, which is particularly advantageous with regard to a good compromise between removal rate and tool wear.

[0046] The invention also relates to a grinding machine which, due to its structural and control configuration, is suitable for carrying out the grinding process. Such a grinding machine for grinding a substantially flat workpiece surface comprises a grinding unit which has at least the following components: at least one tool spindle for carrying a grinding wheel with an abrasive side surface; a rotary drive for rotating the grinding wheel about an axis of rotation; a feed drive for feeding the grinding wheel in a feed direction oriented parallel to the axis of rotation; a workpiece holder for receiving a workpiece such that the workpiece is arranged in a working position for at least one grinding operation; and an operating control system with a control unit for controlling the operation of the grinding machine.The operating control system has at least one operating mode in which the control unit is configured such that the grinding machine performs a grinding process according to the claimed invention.

[0047] Preferably, it is a double-sided surface grinding machine.

[0048] The grinding machine can be configured for grinding essentially plane-parallel, annular workpiece surfaces on a disc-shaped workpiece section, in particular for grinding workpiece surfaces of an annular brake section of a brake disc. For this purpose, it can have two coaxially or substantially coaxially arranged tool spindles, each carrying a grinding wheel. The grinding wheels are arranged with their abrasive side surfaces facing each other, axially defining a grinding chamber. Each grinding wheel can be rotated independently of the other grinding wheel about its associated axis of rotation by means of an associated rotary drive and fed parallel to its associated axis of rotation by means of a feed drive.Furthermore, the grinding machine preferably has at least one workpiece spindle with a workpiece holder for the rotationally fixed mounting of the workpiece, wherein the workpiece holder is rotatable by means of a rotary drive about an axis of rotation running parallel to the axes of rotation of the grinding wheels and is arranged in a working position at least during one phase of a grinding operation such that the workpiece section of the mounted workpiece passes through the grinding chamber in a circular arc shape.

[0049] BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Further advantages and aspects of the invention will become apparent from the claims and from the description of exemplary embodiments of the invention, which are explained below with reference to the figures. Fig. 1 shows a schematic side view of an exemplary embodiment of a

[0051] Double-sided surface grinding machine designed for grinding coated brake discs and controlling the feed of the grinding wheels according to a workpiece-specific feed function depending on a material ratio curve;

[0052] Fig. 2 schematically shows a roughness measurement operation on the surface of a coated brake disc;

[0053] Figs. 3A and 3B show in 3A a roughness profile and in 3B a material distribution curve derived from it as well as different phases of a feed function adapted to it;

[0054] Fig. 4 shows a brake disc with a shape deviation referred to as shielding; and

[0055] Fig. 5 shows a schematic diagram of the dependence of the torque of a rotary drive on time.

[0056] DETAILED DESCRIPTION OF THE EXECUTION EXAMPLES

[0057] Fig. 1 shows a schematic side view of an embodiment of a grinding machine 100, which is designed for grinding substantially 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 this example, the grinding machine is configured for grinding both sides of the surfaces of an annular brake section BA of coated brake discs.

[0058] Each brake disc has a base body, made of, for example, gray cast iron, with a central hub section NA, which serves to attach the brake disc to a vehicle axle, and an annular brake section BA, which surrounds the hub section. The mass distribution of the base body is rotationally symmetrical about the axis of rotation of the brake disc. The brake section has two axially opposite, parallel surfaces (upper workpiece surface 01 and lower workpiece surface 02).

[0059] In a preliminary stage of the manufacturing process, these components were provided with a rotationally symmetrical coating or functional layer FS (see Fig. 2) with respect to the axis of rotation. The free surface of this coating is intended to serve as the friction surface of the brake disc. The coating can, for example, contain a stainless steel alloy with tungsten, titanium, and / or silicon 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. The coating can also be applied by other methods, such as high-velocity oxygen fuel spraying (HVOF) or cold gas spraying.

[0060] The grinding machine 100 is configured as a numerically controlled rotary transfer machine with two workstations, 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 located locally (on or next to the machine) or remotely, e.g. in another room, and in this example can be operated via a connected display and control unit 195 with a graphical user interface.

[0061] For transporting the brake discs between the workstations 110, 120, an internal transport system is used with a rotary table or turntable 150, which is rotatably mounted on or in the machine base 102 about a vertical rotary table axis 152 and can be rotated without limit about the rotary table axis by means of a rotary drive 105.

[0062] On the left side of Fig. 1 is the loading station 110. There, a first brake disc (here WS1) is mounted on a workpiece spindle 154 in a horizontal orientation, i.e., with its axis of rotation vertically aligned (“turntable configuration”). Loading can be carried out, for example, by a robot or other handling device, or manually. The brake disc can be secured, for example, by clamping it to a workpiece holding device or workpiece fixture 155 on the workpiece spindle in such a way that it is clamped or mounted in a rotationally fixed manner and its axis of rotation is coaxial with the axis of rotation 156. A rotationally fixed connection to the workpiece spindle can also be achieved by holding the brake disc down or clamping it from above. In this example, vertically movable hold-down devices 158 are provided.

[0063] 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 operation (one or more steps) takes place fully automatically. At the same time, a previously ground brake disc can be removed from the loading station 110 and a new, unground one can be clamped in. After completion of a grinding operation, the brake disc, ground on both sides, is transported by rotating 180° to the loading station 110, from where it can be unloaded, for example, by a robot or other handling device, or manually. A new brake disc to be ground can then be clamped onto the workpiece spindle that is now free, so that, except during changeover times, both workpiece spindles are occupied with brake discs and are each in different phases of the handling process.

[0064] The grinding machine processes the workpiece surfaces using a double-sided surface grinding process. At 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 each other on a frame component, each carrying a grinding wheel (upper grinding wheel 130-2 and lower grinding wheel 130-2, respectively). The grinding wheels have abrasive side surfaces 135-1 and 135-2 facing each other and are arranged such that these grinding surfaces axially define a grinding chamber 133. Each of the grinding wheels can be rotated independently of the other grinding wheel about the assigned axis of rotation 136-1 or 136-2 by means of an associated spindle drive or rotary drive (upper rotary drive 134-1 or lower rotary drive 134-2) and can be adjusted by means of its own feed drive (upper feed drive 131-1 or lower rotary drive 134-2).lower feed drive 131-2) parallel to the associated axis of rotation with a predefinable feed speed profile.

[0065] The grinding machine 100 comprises a first position measuring system 200 equipped with pneumatic distance sensors S1-1 and S2-1 for determining workpiece position data representing the axial position of a workpiece surface 01, O2 at at least one surface location with respect to a machine-fixed reference coordinate system RKS, and a second position measuring system 300 equipped with pneumatic distance sensors S1-2 and S2-2 for determining tool position data representing the axial position of an abrasive side surface 135-1, 135-2 facing the workpiece surface 01, O2 with respect to the same reference coordinate system RKS. The control unit 190 can 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 sensors can be calibrated as needed via integrated reference elements RE1, RE2-1, and RE2-2.Details of these components and their function are described, for example, in DE 202023 100 514 U1, to whose disclosure reference is made in this respect.

[0066] The grinding wheels 130-1 and 130-2 are designed as circumferentially segmented cup wheels with individually replaceable tool segments. A special feature of the grinding machine is that, in one operating mode, it is configured to control the infeed of the grinding wheels according to a workpiece-specific infeed function, depending on a material fraction curve. This curve—simply put—describes the distribution of workpiece material from the surface down to the depth of the near-surface material. More precisely, in this example, the material fraction curve represents the distribution of near-surface workpiece material in a surface profile as a function of the cutting depth, which describes the depth of the material measured parallel to the infeed direction from the workpiece surface.

[0067] A preferred method for determining the material fraction curve starts with a measured surface profile. The determination of the material fraction curve is carried out using a roughness measurement operation, in which a surface profile of at least a section of the workpiece surface is measured using a roughness measuring system, and the material fraction curve is calculated based on the surface profile determined by measurement in a manner known from surface metrology.

[0068] In the illustrated embodiment, the grinding machine has an integrated roughness measuring system RM, the workpiece-adjacent components of which are arranged in the area of ​​the charging station 110. A separate roughness sensor RS-1, RS-2 is provided for each of the surfaces 01, 02. In this example, the roughness sensors are tactile roughness sensors, the measuring tips of which are shown schematically.

[0069] The determination of the surface profile by a roughness measurement can also be carried out outside the grinding machine, for example in a separate measuring room, either individually for each workpiece to be ground or, for example, batch-wise for a group of workpieces that were coated under the same coating conditions.

[0070] According to the inventors, workpiece-specific feed is particularly advantageous when grinding coated brake discs, as it helps to optimize the grinding of the initially rough and partially porous near-surface coating regions. This ensures that, on the one hand, the areas characterized by porosity and surface roughness are reliably and completely removed immediately adjacent to the free surface, and on the other hand, only as much material is removed as is necessary to achieve a virtually pore-free free surface of the coating with the desired roughness. Figure 2 illustrates a roughness measurement operation with a sectional panorama through a brake section of a brake disc whose base body (GK) is made of gray cast iron.A bonding layer HS, consisting primarily of stainless steel and approximately 100 pm thick, was first applied to the gray cast iron surface by laser cladding or a comparable process. Subsequently, a wear-resistant wear layer VS, approximately 250 to 300 pm thick in this example, was applied to this bonding layer. The bonding layer HS and the wear layer VS together form a two-layer functional layer FS. The wear layer contains hard particles, particularly carbide particles, in a metallic matrix. According to the inventors, this heterogeneity of the layer material near the exposed workpiece surface contributes to the relatively rough and porous nature of these layers in the vicinity of the exposed surface.

[0071] Roughness measurement can be performed outside the grinding machine or using the integrated roughness measurement system RM. This system uses a tactile roughness measurement sensor RMS, which operates via a measuring section MS.

[0072] One method for determining the Abbott material fraction curve (ABB) from the results of a roughness measurement is explained with reference to Figures 3A and 3B. To quantify the depth distribution of the workpiece material, characteristic values ​​according to ISO 13565 Part 2, the so-called Rk parameters, are determined. These characteristic values ​​are defined in the so-called Abbott curve and allow the surface to be divided into a peak region (R). pk ), a core region extending in the direction of depth (R k ) and an adjoining grooved area (R vkSuch a description was specifically developed for the functional description of mechanically highly stressed surfaces, for example, to describe the surface structure of cylinder bores after honing or to characterize bearing surfaces. In the present grinding process, however, this characterization is not used to describe the result or the final outcome of a machining operation, but rather to describe the workpiece properties before the material removal process begins, in order to optimize the tool feed for the specific workpiece.

[0073] To illustrate the construction of the Abbott curve, or the material fraction curve ABB, Fig. 3A shows a section of a surface profile over a measuring section. Fig. 3B shows the corresponding Abbott curve, where the cutting depth c (in micrometers) is plotted on the y-axis and the corresponding material fraction Mr(c) on the x-axis. For standardized measurements, measuring sections of 4 mm or 12.5 mm are frequently used; in this example, measurements over a measuring section MS of 4 mm have proven sufficient. To calculate the Rk parameters, a secant with a minimum slope over a 40% material fraction difference is first fitted to the Abbott curve. A regression line is then calculated over this central region. The perpendicular distance between the points of intersection of this line at Mr = 0% and Mr = 100% is considered the core roughness depth R. kThe perpendicular from the points of intersection of the regression lines at Mr = 0% and Mr = 100% onto the Abbott curve intersects it at the values ​​Mr1 and Mr2. The reduced peak height Rpk and the reduced groove depth Rvk are each calculated as the side length of a triangle whose area is equal to the peak area A1 and the groove area A2, respectively. Figure 3B schematically shows an Abbott curve and the parameters calculated from it.

[0074] To ensure that, in every case, the material removal at the end of the grinding operation is sufficient at all points of the surface to be ground to remove the entire area of ​​the original surface profile, further aspects are taken into account when determining the material removal to be achieved.

[0075] For example, the workpiece surface may exhibit a macroscopic deviation from a plane resulting from pre-machining before the grinding operation begins. In coated brake discs, for instance, thermal influences during coating can lead to a so-called shielding SM, i.e., a slight tilt or curvature relative to a plane orthogonal to the axis of rotation. In such cases, the actual axial position (position in the Z-direction) of the workpiece surface may vary radially relative to the workpiece's axis of rotation RA when the brake disc is clamped. For example, the workpiece surface may be slightly higher near the hub section than at the edge. Figure 4 illustrates this with measurement results of the axial positions (Z-direction). This contribution should be taken into account to ensure complete material removal from the edge regions of the workpiece surface.The corresponding shape deviation parameter can, for example, be defined as the difference in axial positions between the highest and lowest points and be on the order of 10 to 50 pm.

[0076] Furthermore, tool wear occurs in a real grinding process, and the elements subjected to stress in the grinding machine's force flow can yield. These circumstances mean that not the entire feed distance is converted into material removal. These factors can be accounted for with appropriate compliance and wear parameters. Additionally, a further parameter that influences material removal is preferably defined, taking into account that only a relatively small area of ​​the workpiece surface was measured during roughness measurement, and therefore the surface profile may be deeper in other areas. To account for this effect as well, a total profile height parameter is determined, which is derived from the total profile height (R). max(cf. Fig. 3A) in the measured roughness profile. The sum of the contributions of these parameters is later used in the grinding process in the form of offset parameters to specify the material removal per side. The offset values ​​are taken into account in addition to the values ​​resulting from the material ratio curve.

[0077] Figure 3B will now be used to illustrate how the infeed kinematics of the grinding wheel can be determined in a specific machining application. The procedure is based on the experimentally confirmed approach that aligning the infeed with the material fraction curve significantly contributes to workpiece-specific optimization of the infeed kinematics. Based on this, the total grinding time is divided into a sequence of consecutive phases P1, P2, P3, P4, ..., P6 ..., each characterized by different combinations of grinding parameters: infeed speed and infeed depth. Phase P1 corresponds to the approach phase, which ends when the grinding wheel reaches a position of workpiece contact. This can be considered the starting position.

[0078] With respect to the material fraction curve, the switching points between the phases correspond to a sequence of k-values ​​k-, k-, k-2, etc. The corresponding R values ​​can be derived from these k-values. mr - Determine the values ​​at the end of each phase. From this, the feed rate ZG can be derived. Based on this, the feed depth a* of the corresponding machining phase can then be determined. This results from the feed depth of the phase according to the calculation from the material ratio curve and allowances or offset values ​​for form deviations as well as tool wear and system compliances.

[0079] In serial experiments on the grinding of coated brake discs, the grinding process was carried out in several different phases. Total material removal depths on the order of 150 pm to 200 pm were achieved within total machining times of less than 2 minutes. This involved considering contributions from form deviations (e.g., brake disc shielding on the order of 30 to 60 pm (see Fig. 4) and offset contributions due to compliance of the loaded elements on the order of 20 to 40 pm). The feed rate decreased from phase to phase, while the material removal depths in the core area remained constant across several phases. The machining times per phase ranged from approximately 5 seconds to approximately 30 seconds, with the machining time per phase tending to increase towards the last phases.

[0080] Orienting the workpiece-specific feed function directly to the pre-determined material fraction curve generally resulted in characteristic power consumption profiles of the rotary drive. Specifically, the torque, or the proportional power consumption L, increased more or less linearly in the initial phases (in the Rpk range). After reaching the core Rpk range of the surface profile, a longer phase with essentially constant spindle torque followed. Figure 5 shows a typical power consumption profile L of the rotary drive of the tool spindle. Such a profile is considered characteristic of an optimized feed for grinding relatively rough-surface workpieces. Initially, the peaks of the profile are removed, so that relatively little torque is required to rotate the grinding tool.Once the core area is reached, the subsequent feed rate stabilizes with essentially constant torque (typical fluctuations of a maximum of ± 10% around a mean value). This is considered an indication of a good compromise between feed rate and achievable material removal and also has a positive effect on the quality of the finished ground surfaces.

[0081] Figure 5 thus shows what the inventors consider to be a typical curve for the spindle torque or the power consumption of the rotary drive of a tool spindle when the feed rate is strictly aligned with the workpiece-specific material fraction curve. Based on this finding, an efficient grinding process can potentially be achieved even without prior determination or knowledge of the workpiece-specific material fraction curve by controlling the feed rate in such a way that this spindle torque curve is established. This variant is referred to in this application as indirect orientation of the feed rate to the material fraction curve. Direct orientation, particularly based on roughness measurements, systematically yields very good grinding results, but also requires greater effort in the process design.The indirect method has the advantage of requiring less effort and can deliver acceptable grinding results in many cases.

[0082] In this embodiment, the workpiece is fixedly mounted on a workpiece holder that can rotate about an axis of rotation and is positioned during a grinding operation such that the workpiece section runs through the grinding chamber in a circular arc. To perform a grinding operation, the workpiece and the grinding wheels are set into rotation in the same or opposite directions about their axes of rotation, and the abrasive surfaces of the grinding wheels are brought into contact with the corresponding workpiece surface by axially feeding the grinding wheels. The feed of each grinding wheel is controlled according to the feed function specific to the respective side, depending on a material ratio curve. The start-up phases can be staggered to ensure that the brake disc is held force-free except by gravity.This enables highly accurate measurement of the initial axial positions of the workpiece surfaces. In contrast, the grinding wheels are simultaneously engaged with the corresponding workpiece surface at least during one phase of the grinding operation, so that each grinding wheel can absorb a portion of the forces applied by the opposing grinding wheel, preventing unnecessary deformation of the workpiece.

Claims

Patent claims 1. Grinding method for grinding a substantially flat workpiece surface using a grinding machine having a grinding wheel with an abrasive side surface, wherein the grinding wheel is rotatable about an axis of rotation by means of a rotary drive and can be moved in a feed direction oriented parallel to the axis of rotation by means of a feed drive, wherein, to carry out a grinding operation, the workpiece is arranged in a working position; the grinding wheel is set into rotation about the axis of rotation; the abrasive side surface is brought into contact with the workpiece surface by moving the grinding wheel; and the grinding wheel is moved from a starting position with workpiece contact to generate material removal according to a feed function;characterized in that the feed of the grinding wheel is carried out according to a workpiece-specific feed function as a function of a material proportion curve, wherein the material proportion curve represents a distribution of workpiece material in a surface profile as a function of a cutting depth measured parallel to the feed direction.

2. Grinding method according to claim 1, characterized by determining a material proportion curve representative of the workpiece or of a group of similar workpieces containing the workpiece and by controlling the feed according to a feed function calculated on the basis of the determined material proportion curve.

3. Grinding method according to claim 2, characterized in that, in order to determine the material fraction curve of a workpiece, a surface profile of at least a section of the workpiece surface is measured using a roughness measurement operation and the material fraction curve is calculated on the basis of the surface profile, wherein preferably the roughness measurement operation is carried out using a roughness measurement system integrated into the grinding machine and / or wherein the roughness measurement operation is carried out immediately before the start of the grinding operation on the workpiece which is to be ground next.

4. Grinding method according to one of the preceding claims, characterized in that the grinding method is used for grinding a workpiece which, prior to the start of the grinding operation, has a surface profile on the workpiece surface to be ground with a profile depth of at least 50 pm and / or that more than 50 pm of workpiece material is removed by grinding during the grinding operation.

5. Grinding method according to one of the preceding claims, characterized in that the grinding method is used for grinding a workpiece which has a base body having at least one planar surface on which a single-layer or multi-layer functional layer is applied, the free surface of which forms the workpiece surface to be ground, wherein the functional layer is preferably produced by a laser-based additive coating process using at least one additive material heated and / or melted by laser radiation and / or wherein the workpiece is a coated brake disc.

6. Grinding method according to one of the preceding claims, characterized in that the grinding operation is controlled according to a sequence of immediately successive phases with different grinding parameter combinations, in particular with a predefinable feed rate and predefinable material removal depth, wherein preferably transition points between the phases lie on the material content curve.

7. Grinding method according to claim 6, characterized in that the surface is divided into a peak area (Rpk), a core area (Rk) and a groove area (Rvk) based on the material fraction curve and that at least one phase is designed for removing the peak area, at least one phase for removing the groove area and several phases for removing the core area and / or that the grinding operation is controlled according to a sequence of five or more immediately successive phases, each with different grinding parameter combinations, wherein preferably five, six, seven, eight, nine or ten different phases are provided.

8. Grinding method according to one of claims 6 or 7, characterized in that a cutting speed is varied during the grinding operation, wherein preferably different cutting speeds are set for selected different phases, in particular such that in a phase for removing the tip area a relatively lower cutting speed is specified in relation to a mean cutting speed and for a spark-out phase a relatively higher cutting speed is specified in relation to a mean cutting speed.

9. Grinding method according to one of the preceding claims, characterized in that the feed in the phases for removing the core area is controlled in such a way that a torque of the rotary drive remains essentially constant (fluctuates by no more than 10% around a mean value) 10. Grinding method according to one of claims 6 to 9, characterized in that, in order to determine a material removal depth of a phase, at least one of the following parameters is taken into account in addition to the material fraction curve: (i) A total profile height (Rmax) of the surface profile; (ii) A deviation of the workpiece surface from a plane, in particular a skimming; (iii) A flexibility / stiffness in the force flow of the grinding machine; (iv) Compliance of the cutting material of the grinding wheel 11. Grinding method according to one of the preceding claims, characterized by a control of at least one grinding parameter influencing material removal superimposed on the feed control, wherein the superimposed control preferably comprises a force control for controlling the contact force exerted by the grinding wheel on the workpiece or a feed speed control for controlling the feed speed.

12. Grinding method according to one of the preceding claims, characterized in that the feed of the grinding wheel is carried out according to a workpiece-specific feed function as a function of the material fraction curve, by recording a measured variable characteristic of the interaction between the abrasive side surface of the grinding wheel and the workpiece during the grinding operation and by controlling the feed in such a way that the measured variable follows a predefinable (temporal) profile which is related to the material fraction curve.

13. Grinding method according to claim 12, characterized in that the measured variable is a measured variable proportional to the torque of the rotary drive of the tool spindle or a measured variable proportional to the torque of a rotary drive of a workpiece spindle, in particular a power consumption of the rotary drive, or a measured variable proportional to the contact force of the grinding wheel, wherein preferably the feed of the grinding wheel is controlled such that a measured variable proportional to the torque of the rotary drive is initially generated from the starting position in an increase phase in the It increases essentially linearly and then transitions into a constant phase in which the measured quantity remains essentially constant.

14. Grinding method according to one of the preceding claims, characterized in that the grinding method is used for grinding substantially plane-parallel, annular workpiece surfaces on a disc-shaped workpiece section of a workpiece, in particular for grinding surfaces of an annular brake section of a brake disc, wherein a grinding machine is used which has 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 moved by feed in an axial direction oriented parallel to the axes of rotation, the workpiece is mounted non-rotatably on a workpiece holder rotatable about an axis of rotation and is arranged or is arranged in a working position at least during a grinding operation,that the workpiece section runs in a circular arc through the grinding chamber, wherein, in order to carry out a grinding operation, the workpiece and the grinding wheels are set into a 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 feed of each of the grinding wheels is carried out according to a workpiece-specific feed function as a function of a material fraction curve.

15. Grinding method according to claim 14, characterized in that the grinding wheels are in engagement with the associated workpiece surface at least during one phase of the grinding operation.

16. Grinding machine (100) for grinding a substantially flat workpiece surface of a workpiece (WS1, WS2), the grinding machine comprising a grinding unit (121) comprising: at least one tool spindle (132-1, 132-2) for carrying a grinding wheel (130-1, 130-2) with an abrasive side surface (135-1, 135-2), a rotary drive (134-1, 134-2) for rotating the grinding wheel about a rotational axis; a feed drive (131-1, 131-2) for feeding the grinding wheel in a feed direction oriented parallel to the rotational axis; a workpiece holder (155) for receiving a workpiece such that the workpiece is arranged in a working position at least during one grinding operation; an operating control system with a control unit (190) for controlling the operation of the grinding machine (100), characterized in that the operating control system has at least one operating mode in which the control unit is configured such that the grinding machine performs a grinding process according to one of the preceding claims.

17. Grinding machine according to claim 16, characterized in that the grinding machine (100) is configured for grinding substantially plane-parallel, annular workpiece surfaces on a disc-shaped workpiece section (WA) of a workpiece (WS1, WS2), in particular for grinding workpiece surfaces of an annular brake section of a brake disc, wherein the grinding unit (121) comprises: two coaxially arranged tool spindles (132-1, 132-2), each carrying a grinding wheel (130-1, 130-2), wherein the grinding wheels are arranged with mutually facing abrasive side surfaces (135-1, 135-2) that axially delimit a grinding chamber (133), wherein each of the grinding wheels is rotated independently of the other grinding wheel about the associated axis of rotation (136-1, 134-2) by means of an associated rotary drive (134-1, 134-2). 136-2) rotatable and by means of a feed drive (131-1 ,131-2) is adjustable parallel to the associated axis of rotation, at least one workpiece spindle (154) with a workpiece holder (155) for the rotationally fixed mounting of the workpiece (WS1 , WS2), wherein the workpiece holder is rotatable about an axis of rotation (156) running parallel to the axes of rotation of the grinding wheels by means of a rotary drive (157) and is arranged in a working position at least during one phase of a grinding operation such that the workpiece section (WA) of the mounted workpiece passes through the grinding chamber in a circular arc.

18. Grinding machine according to claim 16 or 17, characterized in that the grinding machine (100) has components of a roughness measuring system (RM) which are preferably arranged in the area of ​​a loading station (110) of the grinding machine in order to measure the roughness of the workpiece surface to be ground immediately after loading a workpiece holder.

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