Description

The present invention refers to an apparatus for dressing a grinding wheel and a method for controlling an apparatus for dressing a grinding wheel.

PRIOR ART

EP 0 876 875 A1 discloses a dressing apparatus having a CNC-control system. The dressing apparatus has a tool holder for accommodating a dressing tool, a CNC-controllable drive and a guide for moving the tool holder in such a manner, that the cutting position is movable in directions of at least two axes of a coordinate system . This dressing apparatus is used for the dressing of grinding wheels: Such grinding wheels are used for grinding several kinds of products. The quality of the created texture in particular deviates from the intended texture; improvements in the quality of the created texture are desirable. DISCLOSU RE OF I NVENTION

Thus, the invention has the object to create a texture of high quality on the surface of grinding wheels. This objective is achieved by an apparatus for dressing a grinding wheel taught by claim 1 and is achieved by a method for controlling an apparatus taught by claim 6. Preferred embodiments of the invention are defined by the sub-claims.

The invention provides an apparatus for dressing a grinding wheel mounted thereon by creating structures on the surface of a, in particular cylindrical, grinding surface of the grinding wheel, the apparatus comprising:

- a dressing tool holder arranged for accommodating a dressing tool in a dressing position , - a controllable actuator for adjusting the position of the tool holder within the apparatus in at least one direction ,

- a grinding wheel support for rotatably supporting a grinding wheel at the apparatus. The apparatus comprises further means for monitoring an individual structure, created by the tool on the surface of the grinding wheel, and means for monitoring a circumferential position and/or the rotary speed of the grinding wheel, and means for controlling the actuator, adapted to control the actuator as a function of the monitored structure and the monitored circumferential position and/or rotary speed.

Advantageously the invention provides the possibility to control the synchronisation between the circumferential position of a grinding wheel and the position of the dressing tool . The actuator may be a piezoelectric actuator, comprising a dressing tool holder associated in its free extremity. In particular a linear actuation of the actuator provides the contact between the dressing tool and the grinding wheel surface or a variation of the dressing depth . I n a predetermined circumferential position of the grinding wheel a contact between the dressing tool and the grinding wheel must be present to create the intended structures on the surface of the grinding wheel. Since the actuation of the actuator and thus the position of the tool is controlled as a function of the monitored structure and the monitored circumferential position and/or rotary speed , the structures can be created accurately on the predetermined position on the surface of the grinding wheel. Further a quality control of the surface created on the grinding wheel surface is given by the correction of the rotation and by control of the dressing tool feed. This includes the possibility of mapping of the obtained texture for quality assurance. The creation of the structures thus depends on the circumferential position of the grinding wheel.

Under the term "structure" small irregularities on the surface of the grinding wheels are meant. These irregularities causes cavities on surface of the workpiece, which is later treated by the grinding wheel. These cavities on the workpiece serve for accommodating small amounts of lubricant during use of the workpiece. The entirety of all structures on the grinding surface is called the "texture" of the grinding wheel surface within the scope of this invention .

The means for monitoring a circumferential position and/or the rotary speed of the grinding wheel can be a magnetic sensor, in particular a hall-sensor. It is immaterial whether the sensor actually measures the rotary speed or the circumferential position over time, since the rotary speed can be calculated by differentiating the circumferential position and the circumferential position can be calculated by integrating the rotary speed. In an embodiment the sensor can be an inductive, capacitive, magnetic, photoelectrical or optical rotary speed or circumferential position sensor.

In an embodiment an analysing unit is provided , adapted to reconstruct a structure created on the surface of the grinding wheel, based on the information provided by the means for monitoring an individual structure and the means for monitoring a circumferential position and/or the rotary speed of the grinding wheel. Said means use a synchronism reading signal which is provided by said means for monitoring a circumferential position and/or the speed. Additionally, the synchronism reading signal is used to generate and amplify an image of the created structure.

According to an embodiment of the invention the means for monitoring an individual structure are adapted to monitor the individual structure at the instant moment during creation of this respective structure. This allows a real time controlling of the actuator, since deviations at the time of creation can be immediately recognized and corrected . The tool actuation can thus be coordinated by the feedback, which is provided by said means at the instant moment during creation, thus correcting of errors and closer results of the initially desired structures can be obtained . The sensor for monitoring the structure can send the obtained signal to a structure representation system in real time. The structure reconstructed by the tool can be reconstructed based on historical data.

In an embodiment the means for monitoring the structure comprise an acoustic sensor for monitoring the noise, which is generated during creation of the respective structure, the acoustic sensor is in particular attached to the dressing tool and/or the dressing tool holder. This sensor records the noise, comprising in particular ultrasonic sound, which is generated when the tool immerses into the surface of the grinding wheel. The recorded noise is thus a real time representation of the structure, created at the time of recording. The recorded noise can be correlated to details of structures which are deposited in a database. By correlating the recorded noise and thus the obtained structure with the monitored circumferential position, the position of the created structure can be observed and monitored.

In an embodiment the means for controlling the actuator comprise numerical control means, enabling the input, compilation and transmission of information of machine commands without the intervention of an operator.

In an embodiment the apparatus comprises further means for comparing the monitored structure with a given predetermined structure and based on the result of the comparison adjusting at least one control pulse for controlling the actuator of the dressing tool and/or a motor for driving the grinding wheel. Said means can be integrated in the means for controlling the actuator. By comparing the monitored structure with a given predetermined structure a control circuit, in particular a real time control circuit can be established. The objective is further solved by a method for controlling an apparatus as already described . The method comprises the following steps:

monitoring an individual structure created on the surface of the grinding wheel, and

monitoring the circumferential position and/or the rotary speed of the grinding wheel,

controlling the actuator of the dressing tool as a function of the monitored structure and the monitored circumferential position and/or rotary speed .

The advantages and further embodiments described with reference to the apparatus are also applicable to the claimed method . EXAMPLE EMBODI MENT OF TH E I NVENTION

Further embodiments of the invention are subsequently disclosed with reference to the accompanied figure. The figure show a block diagram of an apparatus according to the invention.

The apparatus 1 comprises a grinding wheel support 14 for rotatably supporting a grinding wheel 2 to be dressed within the apparatus 1 . A motor 20 drives the grinding wheel 2 into rotation. A magnetic circumferential position sensor 8 is provided to monitor the actual circumferential position 19 of grinding wheel 2. By differentiating the several monitored circumferential position values a rotary speed of the grinding wheel is obtained.

A dressing tool 4 comprise a diamond tip 5, wh ich during the dressing process gets in contact with the surface 3 of the grinding wheel 2. The dressing tool 4 is supported by a dressing tool holder 6. The position of the dressing tool holder 6 can be manipulated by an actuator 7 in various directions. I n particular by varying the position of the dressing tool holder 6 in x-direction , the depth of immersion of the diamond tip 5 into the surface 3 of the grinding wheel 2 is adjusted .

A control unit 1 0 provides control pulses 16 for the dressing tool actuator 7. The control unit 10 is provided with data concerning the predetermined surface 3 _P , which has to be created on the grinding wheel 2. Several predetermined structures 1 1 are provided within these data. Synchronization with the actual circumferential position of the grinding wheel 2 is provided by the circumferential position signal 19 of the grinding wheel 2.

An acoustic sensor 13 is provided for recording noise 1 8 occurring during immersion of the diamond tip 5 into the surface 3. Based on the immersion depth , the force, which is applied on the tip 5, the duration of the immersion and the area of the immersion , noises of different spectra 18 are generated . So when different structures are created on the surface noises of different frequencies and volumes, subsequently summarized under the term noise spectra 1 8, occur. A library of different noise spectra 18 is deposited in a database 1 5, along with details of structures 12 assigned thereto. By comparing the recorded noise spectra 18 with a noise spectra 18 deposited in the database 15 under consideration of the rotary speed signal 19 of the grinding wheel 2, a structure 12 can be determined , wh ich is similar to the structure which was created during the recording of the respective noise 18. This determination is performed by an analysing unit 9, wh ich has access to the data base 1 5. As a result the analysing unit 9 establishes an observed surface image 3 ₀ comprising the structures 12 as monitored .

Due to wear of the dressing tool 4 or wear of bearing of the grinding wheel support 14 it may occur, that the accuracy of the dressing process is reduced in the course of time. As an example figure 1 shows in the observed surface 3 ₀ a structure 12', which deviates from the predetermined structures 1 1 . This indicates an undue wearout of the dressing tool 4. This wearout can be compensated by a respective change in the controlling of the actuator 7. For example the loss of diamond particles at the tip 5 can be compensated by an increased feed rate of the actuator 7 in x-direction . The apparatus and method according to the present invention provides the possibility of adapting the feed rate within a control circuit in real-time.

L I ST O F R E F E R E N C E S I G N S

1 apparatus

2 grinding wheel

3 surface of grinding wheel

4 dressing tool

5 diamond tip

6 dressing tool holder

7 dressing tool actuator

8 circumferential position sensor

9 analysing unit

10 control unit

11 predetermined structure

12 monitored structure

13 acoustic sensor

14 grinding wheel support

15 database

16 control pulse for dressing tool actuator

17 control pulse for motor

18 noise spectrum

19 circumferential position and/or rotary speed

20 motor