Method and device for processing a workpiece on a numerically controlled machine tool

Abstract

A method for processing a workpiece on a numerically controlled machine tool by a tool includes the steps of: controlling a relative movement of the tool relative to the workpiece for processing the workpiece, producing an ultrasonic vibration of the tool by an ultrasonic generator, detecting at least one sensor signal outputted from the ultrasonic generator and identifying a change in the material at the workpiece while controlling the relative movement of the tool relative to the workpiece on the basis of the at least one sensor signal outputted from the ultrasonic generator.

Claims

1. A method for machining a workpiece on a numerically controlled machine tool by a tool held in a tool receiving portion of a tool holder, the method comprising: controlling a relative movement of the tool relative to the workpiece for machining the workpiece based on predetermined processing parameters; producing an ultrasonic vibration of the tool by an ultrasonic generator provided in the tool holder, the tool holder being removably received in the machine tool; detecting at least one sensor signal outputted from the ultrasonic generator; identifying a change in a material of the workpiece while controlling the relative movement of the tool relative to the workpiece based on the at least one sensor signal outputted from the ultrasonic generator, and detecting a temporal change over time and a change in value of one or more parameters of the at least one sensor signal outputted from the ultrasonic generator; determining whether the temporal change over time and the change in the value of the one or more parameters of the at least one sensor signal correspondingly fall below a predetermined modification time and simultaneously exceed a predetermined modification value; and adjusting the predetermined processing parameters when the temporal change and the change in the value of the one or more parameters of the at least one sensor signal correspondingly fall below the predetermined modification time and simultaneously exceed the predetermined modification value.

2. The method according to claim 1, wherein the workpiece includes at least two different material regions, and a transition of the workpiece from a first material region of the at least two different material regions to a second material region of the at least two different material regions of the workpiece is identified during identifying the change in the material of the workpiece.

3. The method according to claim 2, wherein the workpiece includes a composite material including at least one of a carbon fiber-reinforced composite material, a glass, and a ceramic material.

4. The method according to claim 2, wherein the at least two material regions are layers of different materials or material characteristics.

5. The method according to claim 2, wherein the at least two material regions are material inclusions in the workpiece.

6. The method according to claim 2, wherein the at least two material regions are bores or recesses in the workpiece.

7. The method according to claim 1, wherein contact of the tool with a surface of the workpiece is identified during identifying the change in the material of the workpiece.

8. The method according to claim 1, wherein the adjustment of the predetermined processing parameters includes at least an adjustment of a speed or a feed of the relative movement of the tool.

9. The method according to claim 1, wherein the adjustment of the predetermined processing parameters includes at least adjusting a frequency or a power of the ultrasonic generator.

10. The method according to claim 1, wherein the ultrasonic generator is a piezo actuator system.

11. The method according to claim 1, wherein the tool has at least one geometrically determined cutting edge or at least one geometrically undetermined cutting edge.

12. A non-transitory computer-readable data storage medium having stored therein a computer program causing a computer of a numerically controlled machine tool or an external computer connected to the numerically controlled machine tool, to carry out the method according to claim 1 on the machine tool.

13. A device for use on a machine tool for machining the workpiece by the tool according to the method of claim 1, wherein the tool holder is removably received in the machine tool, and the machine tool includes (i) a controller configured to control the relative movement of the tool relative to the workpiece in order to process the workpiece, (ii) the ultrasonic generator configured to produce ultrasonic vibration of the tool, the ultrasonic generator being provided in the tool holder, and (iii) a sensor configured to detect the at least one sensor signal outputted from the ultrasonic generator, the device comprising: a processor configured to identify a change in the material of the workpiece while controlling the relative movement of the tool relative to the workpiece based on the at least one sensor signal outputted from the ultrasonic generator.

14. A machine tool for manufacturing the workpiece by the tool held in the tool receiving portion of the tool holder, the tool holder being removably received in the machine tool, the machine tool comprising: a controller configured to control the relative movement of the tool relative to the workpiece for manufacturing the workpiece, an ultrasonic generator configured to generate ultrasonic vibration in the tool, and the sensor configured to detect the at least one sensor signal outputted from the ultrasonic generator using the device according to claim 13.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a sectional view of a tool holder which can be used in embodiments of the method according to the invention;

(2) FIG. 2A shows schematically the processing of the multi-layered workpiece according to the invention with corresponding sensor signals of the ultrasonic generator, wherein the tool processes the workpiece in the region of material A;

(3) FIG. 2B shows schematically the processing of the multi-layered workpiece according to the invention with associated sensor signals of the ultrasonic generator, wherein the tool processes the workpiece in the region of material B;

(4) FIG. 3 shows a flowchart of an embodiment of the method according to the invention; and

(5) FIG. 4 shows schematically an embodiment of a machine tool according to the invention.

DETAILED DESCRIPTION OF THE DRAWINGS AND PREFERRED EMBODIMENTS OF THE PRESENT INVENTION

(6) Examples or embodiments of the present invention are described in detail below with reference to the enclosed drawings. The same or similar elements can be designated by the same reference signs in the drawings but sometimes also by different reference signs.

(7) It should be noted that the present invention is, however, by no means limited or confined to the below described embodiments and the design features thereof but additionally comprises modifications of the embodiments, in particular those which are comprised by modifications of the features of the described examples or by combination of individual or a plurality of the features of the described examples within the scope of protection of the independent claims.

(8) FIG. 1 shows an exemplary structure of a tool holder 10, which can be used in the method according to the invention.

(9) A tool receiving portion 11 for receiving a tool 90 (not shown in FIG. 1; see FIG. 4) is disposed at one end of the tool holder 10. A plurality, e.g. six, perforated disk-shaped first piezo elements 21 are arranged in the tool holder 10 e.g. as stacks, are connected to the tool receiving portion 11 e.g. via a transmission portion 12 and form e.g. an ultrasonic transducer 20 (ultrasonic generator) for transducing an electric voltage into a mechanical vibration (e.g. by means of a frequency in the ultrasonic range).

(10) As an example, the mechanical vibration of the first piezo elements 21 is transmitted to the tool 90 via the transmission portion 12. The first piezo elements 21 can be made e.g. as piezo ceramic disks with electrodes mounted therebetween.

(11) The energy of the ultrasonic transducer 20 is supplied e.g. via a transformer (first transformer) which on the machine side consists e.g. of a first pot core 31 and a primary winding 32 (not shown in FIG. 1; see FIG. 4) and on the tool side consists e.g. of a second pot core 33 and a secondary coil 34, which are arranged, by way of example, as annular elements on the outer side of the tool holder 10.

(12) For example, a perforated disk-shaped piezoelectric sensor element 40 is arranged on a side of the stack comprising first piezo elements 21, wherein said side faces away from the tool receiving portion 11. The sensor element consists e.g. of a piezo element 41 and two contacts 42 and is coupled e.g. mechanically to the first piezo elements 21 but is insulated electrically from the first piezo elements 21 by means of an insulating element 43, which can consist of a ceramic perforated hole disk. By means of another insulating element 43, the piezoelectric sensor element 40 is, by way of example, electrically insulated from a fastening element 13, e.g. a fastening nut.

(13) The fastening element 13 serves for fastening the piezoelectric sensor element 40 to the ultrasonic transducer 20 and for biasing the first piezo elements 21 owing to the dynamic load.

(14) The first piezo elements 21 and the piezoelectric sensor element 40 have the same orientation, on the one hand, so as to allow the generation and the detection of the vibration in the same direction and, on the other hand, to achieve a space-saving arrangement of the elements in the tool holder 10.

(15) The piezoelectric sensor element 40 converts the mechanical vibrations of the vibratory system, which consists of the tool 90, the transmission portion 12, the ultrasonic transducer 20 and the piezoelectric sensor element 40 into a sensor signal which, by way of example, is transferred as an electric voltage via a wire connection 50 from the piezoelectric sensor element 40 through the tool holder 10 to a sending element 61 and 62 on the outer side of the tool holder 10.

(16) The sensor signal is transmitted e.g. in contactless fashion from the sending element 61 and 62 to a receiving element 81 and 82 (not shown in FIG. 1; see FIG. 4) on the machine side.

(17) The sending element 61 and 62 is part of a further transformer (second transformer) and consists e.g. of a first ferrite core 61 and a primary winding 62; the receiving element 81 and 82 is also part of the second transformer and consists of a second ferrite core 81 and a secondary winding 82.

(18) Therefore, the sensor signal can be transferred inductively from the tool holder 10 to a sensor signal assessment device on the machine side.

(19) Alternatively, an optical transfer is also possible, wherein the sending element 61 and 62 is an LED and the receiving element 81 and 82 is a photodiode. The sending element 61 and 62 can be dimensioned and positioned in such a way that it fits into a bore 70 for a data chip for tool data according to the DIN 69893 standard. The tool holder 10 can be rotated in relation to a stationary part of the machine tool 1000 (not shown in FIG. 1; see FIG. 4).

(20) FIG. 2A shows schematically a processing operation of a machining tool 90 for the ultrasonic processing of a workpiece WS, which consists of a plurality of layers of different materials (material A and material B).

(21) The diagram beside it shows the corresponding sensor signals (amplitude, frequency, power) of the piezo elements 21, wherein the diagram already shows the difference between the system which can vibrate freely (i.e. without damping) and the dampedly vibratory system by entering into the material A.

(22) When entering into the material A, the vibratory system has to be provided with more power to produce a constant amplitude. However, at the same time the resonance frequency of the vibrator system changes towards a lower frequency since it is damped by the material A.

(23) FIG. 2B shows schematically the processing operation after advancing the machining tool 90 into the second material B for ultrasonic processing. In this process, the sensor signals of the piezo elements 21 change as shown in the diagram beside it.

(24) On the basis of the changed sensor signals, it is possible, for one thing, to identify the change in material in the workpiece, and it is also preferably possible to adjust the respective processing parameters, such as speed, cutting speed and/or tool feed, as well as the vibration parameters, such as parameters of the drive signals to the piezo elements 21 on the basis of the detected sensor signals and/or on the basis of the identification of the material change.

(25) FIG. 3 shows a flow diagram of an embodiment of the method according to the invention. Here, the tool 90 is operated on the basis of predetermined processing parameters at the beginning of step S2. In the next step (S3), the ultrasonic vibration of the tool 90 is generated by the piezo elements 21, wherein the signals of the piezoelectric sensor elements 40, such as amplitude, frequency and power, are simultaneously detected (step S4).

(26) In the next step, S5, the detected sensor signals are now assessed for the first time. It is here checked, for example, whether the detected signals are substantially constant (in the signal noise region). If this is not the case, a temporal change t of the respective sensor signal is detected in step S6, i.e. how quickly this signal changes, and the value change y of the respective sensor signal is simultaneously detected, i.e. how much or to what extent the corresponding signal changes.

(27) These detected values are now compared in the following two steps S7 and S8 with predetermined limit values t.sub.grenz and y.sub.grenz to be able to determine, by means of this comparison, whether or not there is a change in material.

(28) In step S7, the temporal change t is initially compared with the predetermined limit value t.sub.grenz for this purpose.

(29) If the detected temporal change t is below the predetermined limit value t.sub.genz, a comparison is conducted in the next step, S8, as to how much the value y has changed. For this purpose, the detected value change y is compared with the predetermined limit value y.sub.grenz. If the detected value change y exceeds the predetermined limit value y.sub.grenz, the detected sensor signals point to a change in the material of the workpiece WS.

(30) The above description can refer e.g. to the fact that the detected values comprise a frequency of the vibration generated on the tool holder and/or a detected power of the ultrasonic transducer 20.

(31) On this basis, the predetermined processing parameters are adjusted in step S9. This can include an adjustment or change in the feed rate of the tool and/or an adjustment or change in the cutting speed or rotary speed of the tool while the workpiece is processed.

(32) The parameter can here be adjusted as follows: Having detected the sensor signals of the piezoelectric sensor elements 40, they are compared, by way of example, with data sets which comprise already detected sensor signals of correspondingly known materials.

(33) These data sets can comprise resonance frequencies e.g. of a tool-material combination or a damping capacity of a material and are characteristic of every known material (as a kind of finger print). If a match is here determined, the present material is identified and the processing parameters can be adjusted to the corresponding material on this basis.

(34) However, if there is an unknown material, the kind of material concerned and/or the hardness degree and damping capacity of this material can be assessed on the basis of the detected sensor signals, and the processing parameters can be adjusted on this basis. Therefore, it is by way of example also possible to classify materials.

(35) In the case of workpieces having known material compositions, it is also possible that various processing parameters or processing parameter sets are already predetermined for various materials or material layers of the workpiece, and when the change in the material of the workpiece is identified at the position of the tool tip of the tool, the processing parameters are adjusted to the processing operation in accordance with the predetermined different processing parameters or processing parameter sets.

(36) However, if it is found in step S8 that the value change y of the corresponding sensor signal has not exceeded the predetermined limit value y.sub.grenz, the method is continued with unchanged processing parameters.

(37) FIG. 4 shows schematically a device according to the invention, by means of which the method according to the invention can be carried out.

(38) The device can be part of a machine tool 1000. This figure shows a tool holder 10 having a piezoelectric sensor element 40, the structure of which corresponds, by way of example, to the tool holder 10 shown in FIG. 1. The tool 90 is received at the tool holder 10 for the ultrasonic machining of workpieces.

(39) A generator 120 outputs a work signal A1 as the drive signal for the piezo drive in the tool holder 10. The work signal A1 has the work frequency f1 and is transferred in contactless manner with the power P1 via the energy generation device 30, which is made as a transformer consisting of primary winding 32 plus first pot core 31 and secondary winding 34 plus second pot core 33, to the rotating tool holder 10. Furthermore, the generator 120 outputs a test signal At of a power Pt<P1, which is superimposed to the work signal A1 and the frequency of which varies in a range of f1.

(40) On account of the signals A1 and At, the vibratory system in the tool holder 10 is induced to vibrate, the frequency spectrum of said vibration substantially having two frequencies.

(41) On account of the vibration of the vibratory system, the piezoelectric sensor element 40 also vibrates in the same way and thus produces an electric sensor signal A2, which contains the information on the frequency spectrum of the vibration.

(42) The sensor signal A2 is read out in contactless fashion e.g. via another transformer which consists of primary winding 62 plus first ferrite core 61 and secondary winding 82 plus second ferrite core 81, from a readout device 130 from the rotating tool holder 10 and is transferred to an analysis apparatus 140a.

(43) The analysis apparatus 140a identifies the frequencies contained in the frequency spectrum of A2, and therefore the frequency of the maximum peak in the spectrum (main frequency) can be associated with the work frequency f1 in an apparatus for determining the resonance frequency 140b, which can be realized as part of the analysis apparatus 140a, and the frequency of the smaller peak in the spectrum (auxiliary frequency) can be associated with the resonance frequency f2. The readout device 130, the analysis apparatus 140a and the apparatus for determining the resonance frequency 140b can also be combined into two apparatuses or be realized as a single device.

(44) The value of the determined resonance frequency f2 is transmitted to a first control device 150 which controls the generator 120 in such a way that the frequency f1 of the work signal A1 is adjusted to the value of the resonance frequency f2.

(45) Alternatively or additionally, the value of the determined resonance frequency f2 can be transmitted to a second control apparatus 160, which controls the generator 120 in such a way that the power P1, by means of which the work signal A1 is irradiated into the tool holder 10, is increased to a power P1, such that the mechanical vibration amplitude is achieved even in the case of an excitation where f1f2, wherein said amplitude would be attained as the maximum amplitude in an excitation with the resonance frequency f2.

(46) It is thus possible to stabilize the mechanical vibration amplitude of the tool tip to a certain value, which has a positive effect on the precision during machining with the tool 90. When the vibration amplitude is stabilized to the value which is the possible maximum in the case of a certain power, the efficiency of the workpiece processing is also increased.

(47) A user of the device can control the first control apparatus 150 and/or the second control apparatus 160 via a user interface 170 such that the work signal A1 is only adjusted at the command of the user or when a predetermined condition applies. The user can also determine that the work signal A1 is automatically adjusted at regular or irregular intervals on the basis of the last determined resonance frequency f2.

(48) The generator 120, the readout apparatus (or detection apparatus) 130, the analysis apparatus 140a and the first control apparatus 150 can be combined into a device 200 for outputting output signals and receiving input signals, wherein a first output signal of this device 200 corresponds to the work signal A1, a second output signal corresponds to the test signal At and an input signal corresponds to the sensor signal A2.

(49) In the above mentioned example, the vibration of the tool can be controlled by the respective resonance frequency of the vibration system.

(50) If the controlled resonance frequency or the resulting power changes when the tool or the tool tip is located at a boundary of two materials, this can be used according to embodiments of the invention to detect a change in the material.

(51) On the one hand, this can be the identification of a boundary between two material layers in the workpiece. However, this can also be a boundary between the material of the workpiece, e.g. in the case of entrapped air, cavities, bores, etc. in the workpiece or also on the surface of the workpiece (air to workpiece surface) e.g. for detecting the first contact with the workpiece.

(52) Examples and embodiments of the present invention and the advantages thereof are described in detail above with reference to the attached drawings.

(53) However, it should be noted once again that the present invention is by no means limited or confined to the above described embodiments and the design features thereof but also comprises modifications of the design features, in particular those which are comprised by modifications of the features of the described examples or by combination of individual or a plurality of features of the described examples within the scope of protection of the independent claims.