KR20240138078A - Double-sided grinder and method for grinding brake discs - Google Patents

Abstract

The present invention relates to a double-sided grinding machine (10) comprising a first machine part (16) having a first grinding disc (18), a second machine part (20) having a second grinding disc (22), and a workpiece holding device (30) for holding a workpiece (26), wherein the distance between the first grinding disc and the second grinding disc can be adjusted along the machine axis (14), and wherein the workpiece holding device comprises at least one force sensor for detecting a force acting on the workpiece holding device during a grinding process of the workpiece. The present invention also relates to a method for grinding a brake disc.

Description

Double-sided grinder and method for grinding brake discs

The present invention relates to a double-sided grinding machine comprising a first machine part having a first grinding disc, a second machine part having a second grinding disc, and a workpiece holding device for holding a workpiece, wherein a distance between the first grinding disc and the second grinding disc can be adjusted along a machine axis.

The object of the present invention is to provide a double-sided grinder particularly suitable for grinding a workpiece having a very hard surface but relatively low dimensional stability.

The above task is achieved in a double-sided grinding machine of the aforementioned type, wherein the workpiece holding device includes at least one force sensor for detecting a force acting on the workpiece holding device during the grinding process of the workpiece.

By detecting the forces acting on the workpiece holding device, conclusions can be drawn about the forces acting on the workpiece itself. Knowing these forces, the machining parameters of the grinding or double-sided grinding machine of the workpiece can be controlled according to the detected forces.

In particular, the detection of forces detected in the workpiece holding device is a prerequisite for being able to equalize the forces acting on the workpiece from both sides of a double-sided grinding machine during workpiece grinding.

It is preferred that at least one force sensor is coupled to the control unit of the double-sided grinder. This allows the signals of the force sensor to be fed back directly so that these signals can be taken into account in the control unit.

In particular, it is preferable that the first machine component has a first feed drive for moving a first grinding disc along the machine axis, and that the second machine component has a second feed drive for moving a second grinding disc along the machine axis, wherein the feed drives can be controlled by the control device as a function of a signal from at least one force sensor such that the machining forces acting on the workpiece by the first grinding disc and the second grinding disc can be coordinated with one another. In this way, it is possible to equalize the machining forces acting on the workpiece from the two grinding discs by using the force acting on the workpiece holding device as an input variable for the feed drive control.

The workpiece holding device preferably comprises a workpiece holder for connection to the workpiece to be ground. This workpiece holder can be formed as an adapter having a mechanical interface for a workpiece having a specific interface geometry.

The workpiece holder is preferably designed as a brake disc holder. Brake discs for automobiles have a relatively low stability when viewed along their central rotational axis (i.e. axially), especially in the radially inner transition area between the annular brake disc section and the central brake disc port. Grinding the surface of the annular brake disc section is particularly challenging if the surface of said section has a very hard wear-protecting layer. This is especially true for brake discs which are designed for low wear in order to prevent fine dust. For this purpose, a coating with a carrier material for embedding hard carbide particles is used. The carrier material is, for example, a stainless steel alloy for reasons of corrosion protection, which alloy is relatively soft and flexible and therefore makes it difficult to exert high machining forces on the carbide particles, at least to the extent that high machining forces would lead to deformation of the annular brake disc section relative to the brake disc port.

When grinding the brake disc as described above, the effective surface of the grinding disc must be pressed with a great force against the brake disc coating on one surface of the brake disc section so as to grind the hard carbide particles. However, as described above, since the brake disc is flexible in the direction parallel to the central axis of the brake disc, the brake disc is deformed in the axial direction, i.e. parallel to the machine axis, during grinding between the first grinding disc and the second grinding disc, and the grinding of the two brake pad coatings can be performed while the brake disc is deformed. In this way, it is possible to grind the brake pad coatings with a satisfactory quality; however, after the grinding is completed, the brake disc is deformed again and has insufficient axial runout.

The force sensor according to the present invention detects the aforementioned deformation force and influences the machining parameters of the double-sided grinder so that this force is minimized. In this way, the brake disc can be ground so that the ground brake disc coating maintains a maximum roughness value and the brake disc has a high axial runout accuracy after the grinding is finished, despite the high grinding force being applied to the brake disc coating.

The workpiece holding device preferably has a spindle drive for rotating the workpiece holder. This spindle drive may be arranged coaxially with the axis of rotation of the workpiece or offset from the axis of rotation of the workpiece.

Additionally, it is preferable that the workpiece holding device has a holding structure for supporting the workpiece holder. Such a holding structure is particularly suitable for arranging at least one force sensor, since the holding structure is fixed during the grinding of the workpiece or at least does not move around its rotational axis.

The double-sided grinding machine preferably also comprises a feed drive, by means of which the holding structure can be moved transversely, in particular vertically, relative to the machine axis. This simplifies the positioning of the workpiece in the working space formed between the grinding discs of the double-sided grinding machine. The feed can be carried out, for example, radially, relative to the axis of rotation of the grinding discs, or along a pivot path around a pivot axis which is parallel to the machine axis and spatially offset.

At least one force sensor can be designed as a strain gauge, for example. This makes it possible to detect the stresses acting on the section of the workpiece holding device due to tensile and/or compressive stresses. It is particularly advantageous for the force sensor to be arranged on the holding structure. This is particularly advantageous when the holding structure is very rigid, so that the forces exerted from the grinding disc to the workpiece and from the workpiece to the workpiece holding device result in negligibly small deformations of the holding structure. Nevertheless, the forces resulting from the grinding of the workpiece can be detected by measuring the stresses within the holding structure or on the surface of the holding structure.

Using force sensors designed as strain gauges, the machining forces can be detected indirectly. Additionally or alternatively, it is also possible to directly measure the forces acting on the workpiece holding device.

For this purpose, it is preferable that the workpiece holding device comprises a linear guide, the linear guide axis of which is parallel to the machine axis, the linear guide having two guide elements, the first guide element being connected to the holding structure and the second guide element being connected to the frame, and at least one force sensor being connected to the first and second guide elements and detecting the forces acting between the guide elements. The holding structure receives a resulting force, the amount and direction of which is given by the difference between the machining forces exerted by the first and second grinding discs. This resulting force is transmitted from the workpiece via the workpiece holder to the holding structure and from there to one of the guide elements. From there, the force is transmitted to the force sensor, which is supported on the frame via the second guide element.

Preferred guide elements include at least one rail and at least one carriage guided by the rail.

Furthermore, it is preferred that the sensor axis of at least one force sensor is parallel to the machine axis. This allows machining forces acting parallel to the machine axis, which are caused by the grinding discs, to be detected particularly simply.

Particularly desirable force sensors are piezoelectric force sensors that can detect very high forces with minimal deformation and convert them into high-resolution electrical signals.

The force sensor may be a pressure sensor or a prestressed pressure sensor or a sensor for detecting compressive and tensile forces. For economic reasons, it is preferable to use a pressure sensor or a prestressed pressure sensor.

The choice of a suitable sensor depends on the orientation of the machine axis of the double-sided grinder. If the double-sided grinder has a vertical machine axis, at least part of the workpiece holding device loads the force sensor by its weight, which can be subjected to additional load (higher pressure) or reduced load (lower pressure) depending on the machining forces exerted by the first and second grinding discs. If the weight of the workpiece holding device is insufficient, a pressure sensor that has been prestressed and calibrated at the factory can be used, or a pressure sensor that has been prestressed (e.g. by an expansion screw) and then calibrated only when installed on the double-sided grinder. Via the prestressing of the pressure sensor, not only compressive forces but also tensile forces can be detected.

If the machine axis of a double-sided grinder is oriented horizontally, prestressed pressure sensors or sensors for detecting compressive and tensile forces may be used in particular. In the case of a horizontally oriented machine axis, the feed drives of the grinding discs can be dimensioned comparatively weakly, since the associated machine components for rotating the grinding discs and the rotary drives integrated therein do not have to be lifted against gravity along the machine axis.

The present invention also relates to a method for grinding a brake disc, particularly an automobile brake disc, using the above-mentioned double-sided grinder, the method comprising:

a) positioning a partial section of the brake disc in a working space arranged between a first grinding disc and a second grinding disc;

b) feeding the first grinding disc in the direction of the first brake disc surface towards the first grinding disc until contact is detected by at least one force sensor and/or an additional sensor, and returning the first grinding disc by a return stroke distance;

c) feeding the second grinding disc in the direction of the second brake disc surface towards the second grinding disc until contact is detected by at least one force sensor and/or an additional sensor, and advancing the first grinding disc by a return stroke distance;

d) a step of advancing the two grinding discs at the same feed speed in a direction facing each other;

e) a step of adjusting the feed speed of at least one grinding disc among the grinding discs according to the signal of the force sensor.

In the method according to the invention, bringing the two brake disc surfaces into contact ensures that the initial removal takes place simultaneously on both brake disc surfaces. Thus, the possibility of the brake disc being severely deformed in the axial direction due to the previously described brake disc instability is avoided, as only one of the grinding discs comes into contact with the brake disc.

The contact between the grinding disc and the brake disc surface can be detected using at least one force sensor which detects the contact force resulting from such contact. Additionally or alternatively, the contact can be detected using additional sensors (e.g. structure-sound sensors).

By bringing the oppositely facing brake disc side faces into contact, the initial reference position of the grinding discs can be defined. In order that the determination of the reference position is not influenced by the other grinding disc, after the first grinding disc has come into contact with the first brake disc surface, the first grinding disc is returned by a certain return stroke distance. This allows the second grinding disc to come into contact with the oppositely facing second brake disc surface without interference of the first grinding disc, thereby determining the reference position of the second grinding disc. The first grinding disc is then fed back to the first brake disc surface by the return stroke distance, so that when grinding of the brake disc is started, the two grinding discs each come into contact with the two brake disc surfaces. From this point on, the two grinding discs advance in the opposite direction at the same feed rate.

If the surface qualities of the two brake disc surfaces are identical, at least one force sensor of the workpiece holding device does not receive a force that requires an adjustment of the feed rate of the grinding disc. However, if the geometry and/or the quality of the two brake disc surfaces are different, one of the grinding discs can exert a greater machining force on the relevant brake disc surface than the other grinding disc exerts on the other brake disc surface. This difference between the machining forces can be detected by at least one force sensor, which can then generate a signal that can be used to control the grinding disc that exerts too high a machining force on the brake disc surface to a lower feed rate. In this way, too high machining forces can be reduced.

The same feed rate according to d) is the maximum feed rate, and the adjustment of the feed rate according to e) preferably entails a reduction in the feed rate of one of the grinding discs. This means that in the ideal case the brake disc is machined from both sides at the maximum feed rate, but if necessary the feed rate of the grinding disc that presses the relevant brake disc surface with too high a machining force is reduced.

Additional features and advantages of the present invention are the subject of the following description and graphical representations of the examples.

Figure 1 illustrates a perspective view of one embodiment of a double-sided grinder.
Figure 2 shows a perspective view of an assembly of a double-sided grinder according to Figure 1.
Figure 3 illustrates a perspective view of another embodiment of a double-sided grinder.
Figure 4 illustrates a side view of the double-sided grinder according to Figure 3.
Figure 5 illustrates a perspective view of another embodiment of a double-sided grinder.
Figures 6a to 6e illustrate front views of the double-sided grinder according to Figure 5.
Figures 7a and 7b illustrate the force curves of the workpiece holding device and the feed speeds of the grinding discs of the double-sided grinder according to Figure 5.

An embodiment of a double-sided grinder is indicated in the drawings generally by reference numeral 10. The grinder (10) comprises a machine frame (12) having a vertical machine axis (14), along which a first machine part (16) having a first grinding disc (18) and a second machine part (20) having a second grinding disc (22) are movably arranged.

The first grinding disc (18) and the second grinding disc (22) can each be designed as cup wheels having an annular effective surface.

A working space (24) is formed between the opposing effective surfaces of the grinding discs (18, 22), and a workpiece (26), particularly in the form of a brake disc (28), can be partially inserted into the working space (24).

The workpiece (26) is held in a workpiece holding device (30) that can move along a horizontal feed axis (32) so as to move the workpiece (26) into and out of the working space (24).

The workpiece (26) is connected to the workpiece holder (34) by, for example, a screw connection. The workpiece holder (34) can rotate around the central axis (38) of the workpiece (26) by the spindle drive (36), so that the workpiece (26) connected to the workpiece holder (34) also rotates around the central axis (38). The central axis (38) is parallel to the machine axis (14).

The workpiece holder (34) is mounted on a holding structure (40) which can be moved along the feed axis (32) by a feed drive (not shown as it is known in itself).

The holding structure (40) is shown as part of the assembly of the double-sided grinder (10) shown in FIG. 2 as seen from below.

In the illustrated embodiment, the holding structure (40) comprises a housing (42) of the spindle drive (36). The holding structure (40) has at least one wall section (44) that undergoes expansion or compression as a result of a force acting along the central axis (38). A force sensor (46) designed as a strain gauge is arranged in said wall section (44).

The embodiment of the double-sided grinder (10) illustrated in FIGS. 3 and 4 has a similar structure to the double-sided grinder (10) described above with reference to FIGS. 1 and 2. Only the differences from the double-sided grinder (10) according to FIGS. 1 and 2 are described below.

A double-sided grinder (10) according to FIGS. 3 and 4 comprises a workpiece holding device (30) having a linear guide (48) defining a linear guide axis (50) parallel to the machine axis (14). The linear guide (48) comprises two guide elements (52 and 54) which are guided along the linear guide axis (50) with each other. The first guide element (52) is assigned to a holding structure (40) which also comprises a housing (42) of the spindle drive (36). The second guide element (54) is assigned to a frame (56) which cannot move perpendicularly to the feed axis (32) and parallel to the machine axis (14).

The double-sided grinder (10) comprises a force sensor (46), in particular in the form of a piezoelectric force sensor, which is connected at one end to a holding structure (40) along a sensor axis (without drawing symbol, parallel to the linear guide axis (50)) and at the other end to a frame (56). For example, this force sensor (46) is a pressure sensor or a prestressed pressure sensor.

In principle, the holding structure (40) is mounted on the frame (56) so that it can move along the linear guide axis (50). However, since the relatively rigid force sensor (46) is connected to both the holding structure (40) and the frame (56), the actual travel distance of the holding structure (40) relative to the frame (56) and along the linear guide axis (50) can be neglected. This is intended, since the force sensor (46) needs to detect the forces introduced into the workpiece (26), in particular in the form of a brake disc (28), by grinding with the first and second grinding discs (22), with minimal deformation.

The double-sided grinder (10) illustrated in Fig. 5 has a similar structure to the double-sided grinder (10) according to Figs. 3 and 4, but unlike this, it has a horizontal machine axis (14). The double-sided grinder (10) according to Fig. 5 has a linear guide (48) having a linear guide axis (50) that is parallel and horizontal to the machine axis (14).

The linear guide device (48) comprises guide elements (52, 54) each designed as a guide rail and a carriage guided on the rail. These guide elements are also provided in the linear guide (48) of the double-sided grinder (10) according to FIGS. 3 and 4.

The force sensor (46) of the double-sided grinder (10) according to Fig. 5 is a piezoelectric sensor. For example, the force sensor (46) is a prestressed pressure sensor or a sensor designed to detect compressive and tensile forces.

A method for grinding a workpiece (26) in the shape of a brake disc (28) is described below with reference to a double-sided grinder according to FIG. 5.

Figures 6a to 6e illustrate a workpiece holding device (30) having a brake disc (28) and grinding discs (18, 22). These have respective effective surfaces (56 and 58) that interact with the associated brake disc surface (60 or 62) of the brake disc.

In the initial state (see Fig. 6a), the two effective surfaces (56, 58) of the grinding discs (18, 22) are spaced from the two brake disc surfaces (60, 62).

The brake disc (28) is partially introduced into the working space (24) (see Fig. 6a). From there, the first grinding disc (18) is fed along the machine axis (14) in the direction of the brake disc (28) by means of the first feed drive of the first machine component (16) until the effective surface (56) of the first grinding disc (18) comes into contact with the first brake disc surface (60) of the brake disc (28) (see Fig. 6b). During this contact, the brake disc (28) is rotationally driven by the spindle drive (36). The grinding disc (18) also rotates and is driven around the machine axis (14) by means of a rotary drive which is known in itself and therefore not shown.

The contact of the effective surface (56) of the first grinding disc (18) with the first brake disc surface (60) causes a compressive force to be applied to the first brake disc surface (60), which is transmitted via the tool holder (34) to the holding structure (40) of the workpiece holding device (30) and from there acts on the force sensor (46). The force sensor (46) is supported on the frame (56) at the other end and receives a compressive force, which is depicted as force (64) in FIGS. 7a and 7b.

Subsequently (see FIG. 6c), the first grinding disc (18) is returned by the first feed drive of the first machine component (16) by a return stroke distance (66), so that the first grinding disc (18) is again separated from the brake disc (28). This leads to a load reduction of the force sensor (46) corresponding to the force curve region (68) of FIGS. 7a and 7b.

Next (see FIG. 6d), the second grinding disc (22) is moved along the machine axis (14) by the second feed drive of the second machine component (20) until the effective surface (58) of the second grinding disc (22) comes into contact with the second brake disc surface (62) of the brake disc (28). During this contact, the brake disc (28) is rotationally driven by the spindle drive (36). The grinding disc (22) also rotates and is driven about the machine axis (14) by a rotational drive which is known per se and therefore not shown. The contact causes the holding structure (40) to apply a tensile force corresponding to the force curve region (70) of FIGS. 7a and 7b to the force sensor (46).

Afterwards, the first grinding disc (18) is fed again for a return stroke distance (66) until the first grinding disc (18) comes into contact with the brake disc (28) again (see FIGS. 6d and 6e). This state corresponds to the force curve region (72) according to FIGS. 7a and 7b, where the force sensor (46) is ideally not subjected to a tensile or compressive force.

From the state according to Fig. 6e, the actual grinding of the brake disc (28) begins, during which the grinding discs (18 and 22) are moved towards each other at the same feed rate in order to remove the surface coating of the brake disc surfaces (60 and 62). However, deformation forces resulting from different machining forces must be avoided. If the quality and geometry of the brake disc surfaces (60, 62) are identical, the same feed rate of the two grinding discs can be maintained without the force sensor (46) being subjected to compressive or tensile forces. This ideal state is illustrated in Fig. 7a, where the feed rate (74) corresponds to the feed rate of the first grinding disc (18) and the feed rate (76) corresponds to the feed rate of the second grinding disc (22).

Fig. 7b shows the force curves leading to an adjustment of the feed rates (74, 76) away from the ideal state. Starting from the state according to Fig. 6e and from the force sensor (46) without compressive and tensile forces, during the machining of the brake disc (28), for example, the effective surface (56) of the first grinding disc (18) comes into contact with the surface coating of the first brake disc surface (60) over a larger contact area than the effective surface (58) of the second grinding disc (22) with the surface coating of the second brake disc surface (62). This causes the first grinding disc (18) to exert a greater machining force on the brake disc (28) than the second brake disc (22). This is detected by the force sensor (46) in particular as a non-zero pressure (78) (see Fig. 7b).

By maintaining the feed rates (74 and 76) of the two grinding discs (18 and 22), the brake disc (28) is deformed towards the second grinding disc (22) with respect to the workpiece holder (34) due to the increased pressure via the grinding disc (18). To prevent this, the feed rate of the first grinding disc (18) is reduced until the force sensor (46) is deactivated again (see area marked 80 in Fig. 7b).

Then, machining can be performed again at maximum feed rates (74 and 76). For example, if the second grinding disc (22) acts on the brake disc (28) with a greater force than the first grinding disc (18), the force sensor (46) is subjected to a tensile force (see drawing symbol 82 in Fig. 7b). In order to prevent deformation of the brake disc (28) in the direction of the first grinding disc (18), in this case the feed rate of the second grinding disc (22) is reduced until the force sensor (46) is no longer activated (see area marked 84 in Fig. 7b).

As illustrated in FIG. 7b, the feed speed change described above can be performed multiple times; for example, the feed speed (74) of the first grinding disc (18) and/or the feed speed (76) of the second grinding disc (22) can be decreased and increased multiple times.

The feed rate (74 and/or 76) can also be reduced to “0” if required, i.e. until the grinding disc (16 and/or 18) comes to a standstill along the machine axis (14).

10: Double-sided grinder
16: 1st machine part
18: 1st grinding disc
20: Second machine part
22: Second grinding disc
24: Workspace
26: Workpiece
28: Brake disc
30: Workpiece holding device
34: Workpiece holder
36: Spindle drive
40: Holding structure
46: Force sensor
48: Linear guide
52, 54: Guide elements
56: Frame
60: 1st brake disc surface
62: Second brake disc surface
66: Return stroke distance

Claims (17)

It comprises a first machine part (16) having a first grinding disc (18), a second machine part (20) having a second grinding disc (22), and a workpiece holding device (30) for holding a workpiece (26).
The distance between the first grinding disc (18) and the second grinding disc (22) can be adjusted along the machine axis (14),
A double-sided grinding machine (10), characterized in that the workpiece holding device (30) includes at least one force sensor (46) for detecting a force acting on the workpiece holding device (30) during a grinding process of the workpiece (26). A double-sided grinder (10), characterized in that in the first paragraph, said at least one force sensor (46) is coupled to a control device of said double-sided grinder (10). A double-sided grinding machine (10) characterized in that in the second paragraph, the first machine component (16) has a first feed drive for moving the first grinding disc (18) along the machine axis (14), and the second machine component (20) has a second feed drive for moving the second grinding disc (22) along the machine axis (14), and that the feed drives can be controlled by the control device as a function of a signal of the at least one force sensor (46) in such a way that the machining forces acting on the workpiece (26) by the first grinding disc (18) and by the second grinding disc (22) can be coordinated with one another. A double-sided grinding machine (10) characterized in that in any one of claims 1 to 3, the workpiece holding device (30) includes a workpiece holder (34) for connecting to a workpiece (26) to be ground. A double-sided grinding machine (10), characterized in that in claim 4, the workpiece holder (34) is designed as a brake disc holder. A double-sided grinding machine (10) characterized in that in claim 4 or claim 5, the workpiece holding device (30) has a spindle drive (36) for rotating the workpiece holder (34). A double-sided grinding machine (10) characterized in that in any one of claims 4 to 6, the workpiece holding device (30) has a holding structure (40) for supporting the workpiece holder (34). A double-sided grinder (10) characterized in that in claim 7, the double-sided grinder (10) comprises a feed drive, and the holding structure (40) can be moved transversely, in particular vertically, to the machine axis (14) by the feed drive. A double-sided grinder (10) characterized in that in any one of claims 1 to 8, said at least one force sensor (46) is designed as a strain gauge. A double-sided grinder (10) according to claim 9, citing claim 7 or 8, characterized in that the force sensor (46) is arranged on the holding structure (40). A double-sided grinding machine (10) characterized in that in any one of claims 1 to 10, the workpiece holding device (30) comprises a linear guide (48), the linear guide axis (50) of the linear guide (48) being parallel to the machine axis (14), the linear guide (48) having two guide elements (52, 54), the first guide element (52) being connected to the holding structure (40), the second guide element (54) being connected to the frame (56), and the at least one force sensor (46) being connected to the holding structure (40) and the frame (56) to detect a force acting between the holding structure (40) and the frame (56). A double-sided grinder (10) characterized in that in claim 11, the guide element (52, 54) comprises at least one rail and at least one carriage guided on the rail. A double-sided grinder (10) characterized in that in any one of claims 1 to 12, the sensor axis of at least one force sensor (46) is parallel to the machine axis (14). A double-sided grinder (10) characterized in that in any one of claims 1 to 13, said at least one force sensor (46) is a piezoelectric force sensor. A double-sided grinder (10), characterized in that in any one of claims 1 to 14, the at least one force sensor (46) is a pressure sensor, a prestressed pressure sensor, or a sensor for detecting compressive force and tensile force. A method for grinding a brake disc (28), particularly an automobile brake disc, using a double-sided grinder (10) according to any one of claims 1 to 15,
a) a step of positioning a partial section of the brake disc (28) in a working space (24) arranged between a first grinding disc (18) and a second grinding disc (22);
b) a step of supplying the first grinding disc (18) in the direction of the first brake disc surface (60) towards the first grinding disc (18) until contact is detected by at least one force sensor (46) and/or an additional sensor, and returning the first grinding disc (18) by a return stroke distance (66);
c) feeding the second grinding disc (22) in the direction of the second brake disc surface (62) towards the second grinding disc (22) and advancing the first grinding disc (18) by a return stroke distance (66) until contact is detected by at least one force sensor (46) and/or an additional sensor;
d) a step of advancing the two grinding discs (18, 22) in the direction facing each other at the same feed speed;
e) A method comprising a step of adjusting the feed speed of at least one of the grinding discs (18, 22) according to the signal of the force sensor (46). A method characterized in that in claim 16, the same feed rate according to d) is a maximum feed rate, and that the adjustment of the feed rate according to e) entails a reduction in the feed rate of one of the grinding discs (18, 22).