Laser processing method for processing texture or cavity on workpiece

Abstract

The invention relates to a laser machining method for engraving a texture or a cavity on a workpiece by a machine tool, comprising: calculating a machining path based on the geometry and the texture or the cavity of the workpiece prior to machining, where the machining path defines a series of relative positions of a laser head with respect to the workpiece, wherein calculating the machining path comprises the steps of calculating a plurality of machining layers and calculating a plurality of laser tool paths for each layer; receiving, by the control unit, the machining path for ablating the workpiece; ablating the workpiece layer by layer; adjusting the machining path after machining the at least one layer includes scanning a defined machining region on the surface of the workpiece by a measuring device to obtain machining depths at a plurality of locations on the machining region, comparing the measured machining depths at the plurality of locations on the machining region with a defined target depth to determine a depth deviation, and at least partially recalculating the machining path based on the determined depth deviation; and ablating the remaining layer of the workpiece using the recalculated machining path.

Description

TECHNICAL FIELD

[0001] The present invention relates to a laser machining method for machining a texture or a cavity on a workpiece. Furthermore, the present invention relates to a laser machine tool for engraving a texture or a cavity on a workpiece. BACKGROUND

[0002] Laser ablation methods are widely used to engrave textures on the surface of physical objects by sublimating the material on the surface of the object. For example, the principle used to produce a desired structure or texture on the surface of an object is described in document DE 42 09 933 A1. This process can be considered as an "inverted stereolithography": instead of building by adding a coating, the material is sublimated layer by layer by laser machining, as described for example also in another publication EP 1189724 A1.

[0003] Machine tools for machining objects by laser ablation are also well known. For example, European patent application EP 2 301 706 A1 describes a possible technical design of such a device.

[0004] The complete texture machining of the surface of an object consists in ablating a plurality of machining layers, each layer having a set of positions that the machining head must reach in order to machine a patch for each position of the relevant layer. This software always takes into account the actual and real three-dimensional shape of the surface of the object and the texture to be applied thereon.

[0005] Therefore, before machining, a machining file comprising machining data must be generated based on the geometry of the object and the texture or cavity to be machined thereon. The control unit of the machine tool can read the machining file to control the movement of the laser beam emitted by the laser head, since the machining file defines the positions where the material should be removed. The machining file generated essentially consists of a machining path comprising a series of positions that the laser machining head must occupy with respect to the object, and for each position, a series of ablation operations corresponding to the scans that the laser beam must perform from this position. This result directly affects both the machining time and the quality of completion of the texture.

[0006] The preparation of the machining file comprises the modeling of the geometry of the object and the analysis of the image file defining the texture. The machining file must be generated by taking into account the texture, the corresponding image and the geometry of the object. Therefore, the object is normally numerically modeled by a usually triangular mesh to generate a 3-D modeling file, for example, the object geometry data is a mesh file. Additionally, the texture or cavity cannot be engraved on the object in one go, since the laser beam can only reach a limited depth. Therefore, the machining is carried out layer by layer. During the preparation of the machining file, the object is modeled and the model is divided into a plurality of so-called machining layers.

[0007] Additionally, the texture defined by the image file is processed by the software application and the mesh file of the modeled 3D surface of the object, respectively. The grey scale image representing the three-dimensional texture consists of a plurality of individual pixels, whereby the depth of a position in the texture is defined as the corresponding grey scale of the corresponding pixel. The brighter the pixel, the shallower the texture at the specific point. The darker the pixel, the deeper the texture at the specific position. Preferably, the number of grey scales corresponds to the number of layers applied. In this way, the grey scale image defines for each layer whether a specific point needs to be ablated during the processing of this layer. If the pixel on the grey scale image is equal to or darker than the grey scale of a specific processing layer, the corresponding point needs to be ablated. If the pixel in the grey scale image is brighter than the grey scale of this specific layer, the corresponding point must not be ablated. Thus, white pixels or areas represent points or areas on the textured surface that have no depth. This means that the textured point corresponds to an unprocessed surface at this position and no laser ablation has to take place there. Based on the mapping of the texture on the modeled object, so-called laser toolpaths are determined for each processing layer.

[0008] If a cavity has to be processed, a CAD file can be prepared to model the cavity. Based on the CAD file, the volume to be ablated to form the cavity can be calculated. Additionally, the processing layers can be calculated based on the CAD file. For laser processing, the removal rate of the laser parameters has to be known precisely in order to reach the precise depth of the area to be ablated. If the removal rate is not precise enough or if the removal is not uniform, the bottom cavity of the processing is not accurate. One problem that often occurs in laser processing is the depth inaccuracy. The processing depth deviates from the target depth. Another problem is the flatness of the processed surface. Especially, since the processing is done layer by layer, inaccuracies that already occur on one processing layer can accumulate to the next layer and lead to more inaccuracies.

[0009] So far, the processing file is determined before the processing and applied for the whole processing without any adjustment during the processing. Therefore, it is not possible to correct inaccuracies of the part that is already processed on site during the processing.

[0010] EP 3 875 206 discloses a processing system and a processing method. The processing system includes a housing that accommodates an object, a processing device that is disposed within the housing and that processes the object, a measuring device that is disposed within the housing and that measures the object processed by the processing device, and a control device that sets a processing condition by using a measurement result of the object.

[0011] However, the measurement result of the object is not applied to improve the accuracy of the processed area. SUMMARY

[0012] It is an object of the present invention to provide a laser machining method for engraving a texture or a cavity on a workpiece by a machine tool, wherein the accuracy of the machined part is improved. It is an object of the present invention to provide a laser machining method for engraving a texture or a cavity on a workpiece by a machine tool, which enables real-time adjustment of the machining path.

[0013] According to the invention, these objects are achieved by the features of the independent claims. Further advantageous embodiments result from the dependent claims and the description.

[0014] The present invention relates to a laser machining method for engraving a texture or a cavity on a workpiece by a machine tool, in particular a 5-axis laser machine tool. The laser machine tool comprises a machine table for mounting the workpiece thereon and a laser head for emitting a laser beam on the workpiece. The laser beam is guided by a laser galvanometer scanner integrated in the laser head to reach a defined position on the workpiece. The machine tool further comprises a control unit configured to control the movement of the laser head and the laser galvanometer scanner.

[0015] The method comprises the steps of: prior to machining, calculating a machining path based on the geometry of the workpiece and the texture or cavity; receiving by the control unit an initial machining path for ablating the workpiece; and ablating the workpiece layer by layer. The machining path defines a series of relative positions of the laser head with respect to the workpiece. Calculating the initial machining path comprises the steps of: calculating a plurality of machining layers to be successively machined, and calculating for each layer a plurality of laser tool paths, the laser tool paths defining positions to be ablated by the laser beam.

[0016] The method further comprises adjusting the machining path after machining at least one layer. Adjusting the machining path comprises the steps of: scanning by a measuring device a defined machining area on the surface of the workpiece to obtain a machining depth at a plurality of positions on the machining area; comparing the measured machining depth with a defined target depth to determine a depth deviation for each measured position; and re-calculating at least partially the machining path based on the determined depth deviations.

[0017] Additionally, the method comprises the step of applying the re-calculated machining path to ablate the remaining layers.

[0018] The present invention can not only be applied to machining a texture on a workpiece, but also to machining one or more cavities on a workpiece. The size of the cavity can vary in a large range, from small cavities with a volume of a few mm 3 to large cavities with a volume of several cm 3 or more. For most applications, the machining of the cavity should reach a high accuracy, for example in the range of a few micrometers.

[0019] Prior to the machining, a machining file is prepared to determine the machining path and the machining parameters to be applied to the ablation of the workpiece. Normally, the machining file is determined by a processing unit arranged outside the machine tool and is transmitted into the control unit of the machine tool. The control unit is configured to control the mechanical axes and the laser galvanometer scanner to direct the laser beam to ablate the workpiece layer by layer.

[0020] In the known method, the machining path is not adjustable during the entire machining. That is, when the program describing the machining path is loaded into the control unit, the program is executed by the control unit without any changes. Therefore, any inaccuracies or even errors occurring on the machined workpiece during the machining cannot be compensated or corrected. It is an object of the present invention to provide the possibility to monitor the machining process in real time and to react on the monitoring results. In particular, the quality of the machined part is monitored during the machining such that real-time error correction is allowed. In this way, an adaptive machining is achieved. This provides the advantage that certain inaccuracies or errors occurring in previous phases of the machining can be compensated or corrected during the remaining phases of the machining. In particular, geometrical inaccuracies or geometrical errors of the engraved surface can already be detected during the early phase of the machining and be corrected or compensated during the remaining phases of the machining. Thus, the geometrical accuracy of the engraved texture or cavity can be improved since the real machined surface is considered during the subsequent machining periods.

[0021] The laser ablation machining is performed in a plurality of processing steps, wherein the surface structure is machined layer by layer. Thus, the machining path comprises a plurality of machining layers to be successively machined. On each layer, an ablation area is determined. Additionally, a plurality of laser tool paths is determined on each layer to define the position where material should be removed. There are different types of laser machining, such as the so-called vector laser machining and sandblasting. For the vector laser machining, the laser tool path to be ablated comprises a plurality of laser vectors. The laser vectors define the position at which the laser beam should be emitted. For ablation of the corresponding area, the laser beam is emitted along the laser vector from one end of the vector to the other end and is emitted vector by vector.

[0022] The adjustment of the machining path is performed after the machining of one or more layers. Preferably, the ablation is first stopped before the machining path adjustment is performed. A part of the machined surface of the workpiece is scanned by the measuring device. It is possible to scan the entire surface or a defined area of the surface.

[0023] Since the texture or cavity to be engraved on the workpiece is normally three-dimensional, the surface topology represents the spatial structure of the texture or cavity formed on the workpiece. The target surface topology is specified before machining. The target surface topology is associated with the texture or cavity. The measured surface topology scanned by the measuring device shows the intermediate state of the texture or cavity machined on the workpiece. The goal is to have the final machined part have a surface topology corresponding to the specified target surface topology. Therefore, the measured surface topology is compared to the target surface topology in order to determine whether the remaining machining can be performed by applying the initially determined machining path or whether the initially determined machining path needs to be modified for the remaining machining. The deviation between the measured surface topology and the target topology provides information about the geometrical inaccuracy of the machined part at this stage of machining. Since this is done before the entire material is removed by definition, this inaccuracy can still be corrected during the remaining machining process by removing more or less material than initially designed. This is achieved by recalculating the machining path and applying the recalculated machining path to the remaining machining process.

[0024] The target surface topology defines the target depth at different locations on the surface. The measuring unit is configured to measure the depth of the engraved texture or cavity at different locations of the surface to obtain a surface topology. The scanning allows to obtain the machining depth information for all scanned areas. This information can be analyzed to determine whether the machining depth corresponds to the specified target depth at each location of the surface. The measured depth is compared to the target depth at each location to determine the depth deviation. The depth deviation represents the inaccuracy of the machining, in particular the inaccuracy of the material removal at this stage. This provides an input for the correction.

[0025] In one variant, recalculating the machining path comprises recalculating at least one machining layer and recalculating at least partially the laser tool path on this layer.

[0026] If the measured depth deviates from the target depth at a location on the machined area, it means that too much material has been removed or too little material has been removed at this location. Therefore, in order to correct this error, the corresponding machining layer is recalculated so that the error can be compensated at the subsequent machining period. Adapting the machining layers for the remaining machining is a way to correct the inaccuracy that occurred during the previous machining process. How many layers have to be adjusted depends on the depth deviation. The greater the depth deviation, the more layers have to be adjusted. In addition, the laser tool path on these machining layers is adjusted.

[0027] In one variant, if the depth deviation indicates under-engraving, the machining path is recalculated by adding at least one machining layer to the remaining machining layers and recalculating the laser tool path on this layer. Under-engraving occurs when the measured machining depth is less than the target depth at a location. Under-engraving means that at this machining stage, less material is removed from the workpiece than intended. If the measured surface topology at a location has a smaller depth than the target surface topology, this location is identified as under-engraved. To compensate for the under-engraving, more material must be removed during the remaining machining period, so the machining tool path must be adapted. In particular, the machining layers must be recalculated by adding additional machining layers in the remaining machining layers. For example, a scan is made after N number of machining layers have been machined. A remaining number M of machining layers are to be ablated. If under-engraving occurs, a number L of additional layers are added to the M machining layers, so that material that should have been removed during machining of the N layers but has not yet been removed can be ablated during machining of these additional layers L. This means that the recalculated remaining layers include a total of M+L layers, instead of the initially designed M layers.

[0028] Advantageously, machining layers are only added to the areas where under-engraving is identified. To optimize machining efficiency, additional L layers are only added in areas where under-engraving is identified.

[0029] In another variant, if the depth deviation indicates over-engraving, the machining path is recalculated by at least partially removing the laser tool path on at least one machining layer. Over-engraving means that at this machining stage, more material is removed from the workpiece than intended. If the measured machining depth at a location is greater than the target depth, this location is identified as over-engraved. To compensate for the over-engraving, less material must be removed during the remaining machining, so the machining tool path must be adapted. This is achieved by removing the initially determined laser tool path in the subsequent layers. How many layers must be adapted depends on the depth deviation. In particular, the laser tool path of the corresponding machining area on at least one of the remaining machining layers must be removed. It can be necessary to recalculating the machining layers by removing the machining layers of the corresponding machining area. For example, a scan is made after N number of machining layers have been machined. A remaining number M of machining layers are to be ablated. If under-engraving occurs, a number L of additional layers are removed from the M machining layers, so that material that should not have been removed during machining of the M layers but has already been removed must not be ablated again during machining of these additional layers L. This means that the layers recalculated for over-engraved locations include a total of M-L layers.

[0030] Preferably, machining layers are only removed from areas where over-engraving is identified.

[0031] In particular, under-engraving and over-engraving can occur at the same processing stage but at different areas of the surface. The re-computed processing layers can differ at different areas of the surface. For under-engraved areas, the re-computed processing layers comprise M+L layers. For over-engraved areas, the re-computed processing layers comprise M-L layers. For other areas where no depth deviation is identified, the re-computed processing layers remain unchanged from before the scan.

[0032] Considering the defects detected during the scan, the method allows modifying the remaining lines of the program defining the processing tool path. When not enough material has been ablated, it specifically adds layers in the zones intended to be ablated. When too much material has been ablated, it prunes the remaining processing lines on the zones that have been over-engraved. The number of layers pruned / added depends on the ablation rate of the laser parameters, which is calculated from the scan and indicates the depth of each layer.

[0033] In an advantageous variant, the re-computed processing path is performed by modifying at least one processing layer. It is thus not necessary to re-compute all the processing layers in a new way to complete the processing. This can save computing power and save computing time.

[0034] In particular, if under-engraving occurs, the re-computed processing path comprises copying at least one processing layer, removing the laser tool path intended to ablate the areas where under-engraving is not identified, and adding the modified processing layer to the remaining processing layers. By copying a processing layer that has already been determined and modifying it, the computing time and power can be greatly reduced. The computation can thus be performed by a processing unit integrated in the machine tool.

[0035] In some embodiments, determining the processing tool path comprises defining, for each processing layer, a plurality of small areas intended to be processed from a single position of the laser head, and adjusting the processing tool path comprises re-computing the small areas of at least one processing layer. Each small area defines an area that can be ablated by keeping the laser head in one position. This can further optimize the processing time and the quality of the processed part.

[0036] The frequency of adjustment of the processing path has a direct impact on the geometric accuracy of the final part. The higher the frequency of scanning the processing surface and adjusting the processing path, the higher the accuracy that can be achieved by the final part. On the other hand, the scanning is preferably performed during processing pauses, which lengthens the processing time by the scanning. The frequency of adjustment is therefore dependent on the processing priorities, which include the processing time and the accuracy. In one variant, the processing tool path adjustment is performed after processing each processing layer. If corrections can be performed after processing each layer, the accuracy of the processed part can be maximized, as each processing error that occurs during the processing of each layer can be immediately corrected.

[0037] In one variant, the machining tool path adjustment is made in an irregular manner, wherein the timing of the adjustment depends on the initial machining path, in particular the number of adjustments during the second half of the machining layer is higher than the number of adjustments during the first half of the machining layer. This is a compromise between machining time and accuracy of the machined part. The machining path depends on the shape of the texture or cavity. For example, some parts of the texture or cavity require a high accuracy and some parts of the texture or cavity require a lower accuracy. This method offers the possibility to achieve the accuracy requirements without unnecessarily increasing the machining time.

[0038] Preferably, the timing of the adjustment of the machining tool path is determined based on the geometry of the texture or the geometry of the cavity.

[0039] In particular, the timing of the adjustment of the machining tool path is determined based on the results of the previous scans.

[0040] In a preferred variant, the scan is made after machining the last machining layer. This measurement offers on the one hand the possibility to correct inaccuracies that occur during machining the last layer and on the other hand also the quality of the final part of the machining.

[0041] Preferably, the scan is made before machining. This allows to obtain the initial positions of the surface of the workpiece. The workpiece is mounted on the machine table for machining. However, the workpiece is not always perfect. The surface of the workpiece is not completely flat. Therefore, the measurement of the scan can be inaccurate. If the surface is scanned before machining, these initial position errors can be compensated afterwards.

[0042] Advantageously, the machining depth is calculated with respect to the top surface of the workpiece. For example, the Z position of the measurement surface is measured and the Z position of the machining area is measured. The difference between these two positions provides information about the machining depth. This has the advantage that the measured depth is not influenced by the positioning of the workpiece on the machine table or by the unevenness of the surface of the workpiece.

[0043] In particular, the scan is controlled by the control unit of the machine based on the machining path and / or the recalculated machining path. In particular, only the area to be machined is scanned.

[0044] In an advantageous variant, cleaning is made before the scan. In order to achieve a good scan of the surface, the part should be clean enough. Dust of ablated material often contaminates the surface or the cavity. The deeper and the smaller the cavity, the most difficult it is to clean the dust in the cavity.

[0045] In one variant, a dust removal system is applied for cleaning. Alternatively, a blower system is applied for cleaning. Also a gas or fluid can be applied for cleaning.

[0046] In some embodiments, the laser source parameters are adapted based on the determined depth deviation. In this way, the machining can be further optimized. For example, the scan results can show which machining process has been carried out, including rough machining, semi-finish machining or finish machining. Depending on the machining process, the laser source parameters can be adapted.

[0047] In one variant, a plurality of sets of laser source parameters are predefined. After the scan, one of the predefined sets of laser source parameters is selected according to the scan results. Many parameters in the laser source need to be tuned to find ablation parameters that give interesting results in terms of productivity. For many applications, a balance between ablation productivity and ablation quality has to be achieved. For example, the ablation productivity is defined by the ablation depth in pm / piece. For example, the ablation quality is defined by the surface roughness S a defined. During the machining process, the program can be adapted according to the results of the surface scan and can adapt not only the machining layers and the laser tool paths, but also the laser ablation parameters between the groups of different parameter sets.

[0048] The invention relates to a laser machine tool for engraving textures or cavities on a workpiece, in particular a 5-axis laser machine tool. The machine tool comprises a machine table for mounting a workpiece thereon and a laser head for emitting a laser beam on the workpiece, wherein the laser beam is guided by a laser galvanometer scanner integrated in the laser head to reach defined positions on the workpiece, the machine tool further comprising a processing unit, a control unit and a measuring device.

[0049] The processing unit is configured to determine a machining path before machining by the processing unit, the machining path comprising a series of relative positions of the laser head with respect to the workpiece, wherein a plurality of machining layers aimed at continuous machining are calculated and for each machining layer a plurality of laser tool paths are calculated, the laser tool paths defining positions to be ablated by the laser beam. The control unit is configured to apply the defined machining path for ablating the workpiece, wherein the workpiece is ablated layer by layer. The measuring device is configured to scan a defined area on the surface of the workpiece to obtain a machining depth at a plurality of positions on the machined area. The control unit is further configured to compare the measured depth with a target depth to determine a depth deviation and to recalculate the machining path based on the depth deviation.

[0050] In one variant, the measuring device is a mechanical sensor, such as a contact touch probe. In another variant, the measuring device is an optical sensor, such as a 3-D camera, a structured light sensor, a focus variation sensor, a laser sensor, an interferometer, an optical coherence tomography.

[0051] In particular, the measuring device is a distance sensor, a depth sensor, in particular a confocal sensor.

[0052] Advantageously, the measuring device is a point sensor, a line sensor or a field sensor.

[0053] Preferably, the measuring device is arranged on the machining head and controlled by the control unit. The measuring device is moved by the machine's mechanical axes, which are controlled by the control unit for controlling the movement of the laser beam. Therefore, no additional control unit is required to control the measuring device.

[0054] In particular, the measuring device is attached in the vicinity of the optical scanner used for machining. The measuring device is positioned in such a way that a distance is maintained between the sensor axis and the laser machining axis.

[0055] Alternatively, the measuring device is positioned in such a way that it can be aligned with the main beam path of the machine. This means that the movement of the measuring device is achieved by moving the optical laser scanner. This can reduce the scanning time, since the measuring device is moved by the optical axis instead of the mechanical axis.

[0056] A dedicated sensor optical scanner can be provided for moving the measuring device to scan the surface. BRIEF DESCRIPTION OF DRAWINGS

[0057] A more particular description of the principles described briefly above will be rendered by reference to specific embodiments thereof, which are illustrated in the appended drawings. These drawings depict example embodiments of the present disclosure and are not therefore to be considered limiting of its scope. The principles described herein are described and explained with additional specificity and detail through the use of the accompanying drawings in which:

[0058] Figure 1 Fig. 1 illustrates a laser machine;

[0059] Figure 2 Fig. 2 illustrates a laser head of the laser machine;

[0060] Figure 3 Fig. 3 illustrates an image to be ablated on a workpiece;

[0061] Figure 4 Fig. 4 illustrates an ablation area on a machining layer and a plurality of laser tool paths;

[0062] Figure 5 Fig. 5 illustrates an example of one of the calculated machining layers with corresponding laser tool paths;

[0063] Figure 6 Fig. 6 illustrates a machining layer;

[0064] Figure 7a Fig. 7 illustrates a theoretical pre- and post-machining workpiece; Figure 7b

[0065] Fig. 8 illustrates a machining method of the prior art; Figure 8a Figure 8b Fig. 9 illustrates a machining method of the prior art; and

[0066] Fig. 10 illustrates a machining method of the prior art.Figure 9a , Figure 9b , Figure 9c The diagram illustrates how the processing path can be adjusted based on the scan results during processing.

[0067] Figure 10 , Figure 11 , Figure 12 , Figure 13 , Figure 14 The diagram illustrates the recalculation of the laser tool path;

[0068] Figure 15 The schematic map shows a machine tool with measuring equipment;

[0069] Figure 16 The figure shows an example of measuring depth in a machine area;

[0070] Figure 17 and Figure 18 The figure shows a machine tool in which workpieces are arranged on the machine tool table. Detailed Implementation

[0071] Figure 1 An example of a laser machine tool 1 including a laser head 2 is schematically shown. The laser head and the workpiece to be processed can be positioned relative to each other according to three or five mechanical axes, which makes it possible to orient the direction of the emitted laser beam and position the focal point of the laser beam on the surface of the workpiece (not shown) placed in the machine. Figure 1 In the example shown, the laser head can be displaced in three dimensions (X, Y, and Z) in a Cartesian reference frame. Advantageously, the head can also rotate about two axes of rotation to further enhance accuracy and flexibility. The laser head includes devices that allow it to rotate about the axes, a laser source, optical devices, and a device commonly referred to as a galvanometer scanner.

[0072] Figure 2 The operation of a laser galvanometer scanner is schematically illustrated. A laser source 3 emits a laser beam 9, or more specifically, a pulsed laser beam. The laser beam 2 is reflected by a mirror 4, which allows the position of the projection point of the laser beam onto the surface of the workpiece 5 to be defined according to the X and Y axes of a Cartesian reference system, respectively. An actuator 8 enables control of the angular position of the mirror 4. The laser beam also passes through a lens 6, commonly referred to as an F-theta lens. Therefore, the device allows the point of impact between the laser beam and the surface 7 of the workpiece 5 to be defined in a plane within the considered focal length range. Focal lengths up to 430 mm are common in existing machines. Further details of the construction of the laser head 2 are described in document EP 2 301 706.

[0073] Figure 3The figure illustrates a texture example which has to be applied by laser ablation to the workpiece surface defined by the grey scale image.

[0074] Therefore, the set of machining layers is usually calculated from a 3D modeling file representing the shape of the workpiece and a grey scale texture file. For each machining layer, a plurality of laser tool paths is calculated to define the positions where the laser beam should be directed to remove the material on the corresponding layer.

[0075] To process the workpiece surface, the laser beam is always moved along a predefined parallel laser vector 41 over the displayed ablation area 31 as Figure 4 illustrated. At the border of the area, it jumps to the next position. Figure 4 An example is displayed where the whole area has to be ablated.

[0076] Figure 5 An example of a calculated machining layer 21 with corresponding laser tool paths 42 is displayed. The area 34 covered by the laser vectors is the ablation area, i.e. the material of this area has to be removed. The white areas are non-ablation areas 32 and i.e. no material in these areas has to be ablated.

[0077] Figure 6 The figure illustrates a plurality of defined machining layers. This is a simplified illustration, therefore, the workpiece is illustrated as a cube with a rectangular shape and a flat surface. However, the method of the invention is not limited to be applied to machining workpieces having such a simple shape. It can be applied to machining workpieces having an arbitrary shape as well as curved surfaces. The model of the workpiece 10 is divided into e.g. 30 machining layers LI to L30. The scanning of the machining surface can be performed in a regular way or in an irregular way. In a regular way, the scanning is performed after machining a defined number of machining layers, e.g. 10 layers. The machining starts with ablating the first layer LI and then continues with ablating the consecutive layers L2 to L10. After machining the first 10 layers, the machining surface is scanned by a measuring device. Based on the measured surface geometry, the machining path is adjusted. Then the machining continues until another 10 layers are machined using the adjusted machining path. Then the scanning of the machining surface is performed again and the machining tool path is adjusted again. If the scanning is performed in an irregular way. For example, the scanning is performed after machining the 5th layer, the 7th layer, the 11th layer, the 20th layer and the 28th layer.

[0078] Figure 7a and Figure 7b The figure illustrates an example of machining a cavity 20 on a workpiece 5 in theory. Figure 7a The workpiece before machining is displayed and Figure 7b The machined workpiece is displayed. According to the specification, the machining surface in the cavity 21 should be flat as Figure 7b illustrated. However, in reality, due to the limitations of the machining accuracy, the machining surface is not flat.

[0079] Figure 8a and Figure 8b The prior art shows how to machine such a cavity. The workpiece is machined layer by layer from LI to Lfinal. The machined surface 22 in the cavity is not flat, because during machining each position on a machined layer is not ablated to the same depth due to the interaction of the laser with the material. By applying the method of the present invention, this inaccuracy can be corrected even during machining.

[0080] Figure 9a 、 Figure 9b 、 Figure 9c The figures show how the machining path is adjusted during machining based on the scan results. For the sake of clarity, the figures are illustrated in a simplified manner. For example, the workpiece is depicted in a two-dimensional view and the layers are depicted by lines. Each line represents a machined layer.

[0081] Figure 9a The situation before machining is shown. A number of machining layers from LI to Lfinal are calculated. Preferably, it is also calculated when the surface should be scanned, for example after machining the fourth layer L4 and the layer Lx. In particular, as indicated by the symbol Lscan in Figure 9a the surface is scanned before machining. However, scanning the surface before machining is not mandatory.

[0082] Figure 9b The situation is shown where the first four layers LI, L2, L3 and L4 are machined and a scan is performed immediately after machining the fourth layer L4. The fifth layer L5 is the next layer to be machined. Before machining this layer, a surface scan is performed and the machining path is adjusted. The machined surface of the workpiece is scanned by a measuring device. The curve M is a simplified illustration of the measured surface profile, i.e. the measured machining depth. For each measured position, the measured depth is compared to the target depth defined before machining. These positions are depicted in the figure in one dimension, i.e. the ablation depth in the z-direction. However, the scan is a three-dimensional scan. Therefore, the measurement data comprises all three coordinates of each position, i.e. X, Y and Z.

[0083] The remaining layers comprise the fifth layer L5, the sixth layer L6 and all subsequent layers. The machined surface profile indicated by M is also shown in Figure 9b . Ideally, the surface profile should be flat over the entire surface. However, the scan results show a deviation from the ideal profile. The machining depth is not equal over the entire surface.

[0084] As Figure 9bThree cases can occur as shown. In zones A and C, the Z position depicted by curve M is higher than the target depth at different locations, which means that in these two zones the machining depth is smaller than the target depth. This indicates under-engraving. For example, the measured position Pm1 in the Z direction is higher than the target position Pt1 because less material is removed as targeted. In zone D, the Z position depicted by curve M is lower than the target depth, which means that the machining depth is larger than the target depth in this zone. This indicates over-engraving. For example, the measured position Pm2 is lower than the target position Pt2 because more material is removed as targeted. In zone B, no under-engraving or over-engraving occurs.

[0085] Figure 9c It is shown how the depth errors occurring after machining the first four layers are corrected. To compensate for the under- or over-engraving in zones A, C and D, the machining program is recalculated based on the measurement data obtained by the surface scan. Depending on the scan results, one or several layers have to be recalculated. In this example, several layers have to be recalculated, namely the second layer L2’, the third layer L3’, the fourth layer L4’ and the fifth layer L5’. This example shows that not only the remaining layer L5 which has not been machined has to be recalculated, but also three already machined layers, namely the second, third and fourth layer. For clarity, the recalculated layers are indicated as L2’, L3’, L4’ and L5’ in Figure 9c . The remaining layers Lx up to Lfinal are not adjusted. In Figure 9c , the laser tool path calculated before the scan without adjustment is depicted by a dotted line and the laser tool path recalculated after the scan is depicted by a dot-dashed line.

[0086] On the adjusted layers, the laser tool path is adapted with a simple tool path pruning operation, which means that the laser tool path is removed in areas which do not require correction. On the second layer L2’, the laser tool path is pruned in such a way that only the laser tool path at zone A remains unchanged, because this area has to be ablated again to correct the under-engraving. On the third layer L3’, the laser tool path is pruned in such a way that the laser tool path at both zones A and C remains unchanged, because these areas have to be ablated again to correct the under-engraving. On the fourth layer L4’, the laser tool path is pruned in such a way that the laser tool path at both zones A and C remains unchanged, because these areas have to be ablated again to correct the under-engraving. The laser tool path for ablating zone A is included on all three layers, but the size of the area to be ablated in zone A can be different on the different layers. This can also be seen in Figure 10 , Figure 11 and Figure 12The fifth layer L5' is shown in Figure 5. At the zone D, the laser tool path is removed to compensate for the over-engraving. Since the over-engraving at the zone D is too deep, the sixth layer L6' also has to be corrected by removing the laser tool path at the zone D. However, as shown in Figures 6 and 7, the non-ablated area on the sixth layer L6' at the zone D is smaller than the non-ablated area on the fifth layer L5' at the zone D. Figure 13 and Figure 14 The non-ablated area on the sixth layer L6' at the zone D is smaller than the non-ablated area on the fifth layer L5' at the zone D.

[0087] Figure 15 The machining area of the machine tool with the integrated measuring device 50 is shown. The measuring device is arranged in the vicinity of the laser head. During the scan, the measuring device moves together with the laser head, in particular in the horizontal direction. Figure 16 The measured depth of the machining area is shown. At positions S1, S2 and S3, the measured depth is different. The machining depth at S1 is greater than the depth at S3, but less than the depth at S2.

[0088] Figure 17 and Figure 18 The problem of the surface orientation is shown. If the unprocessed workpiece is perfectly prepared, it can be positioned perfectly on the machine table 51 as shown in Figure 8. However, in many cases, the workpiece is not perfectly prepared, for example as shown in Figure 9, because the upper surface of the workpiece mounted on the machine table has a tilt. To solve this problem, this tilt can be included in the measurement data and can be taken into account. To compensate for this error, before machining, the top surface of the workpiece is first scanned. The measurement of this initial surface is stored and applied as a reference for calculating the depth deviation. The machining depth is not an absolute value, but a relative value from the scanned top surface of the workpiece. Figure 17 Figure 18 The surface of the workpiece, in particular the surface to be ablated, can be chosen as a reference, or the axis of the machine tool, for example the top surface of the machine table, can be chosen as a reference.

[0089] The variant of choosing the top surface of the workpiece as a reference is shown. Figure 17 The variant of choosing the top surface of the machine table as a reference is shown. Figure 18 The variant of choosing the top surface of the machine table as a reference is shown.

[0090] List of references

[0091] 1 Machine tool

[0092] 2 Laser head

[0093] 3 Laser source

[0094] 4 Mirror

[0095] 5 Workpiece

[0096] 6 Lens

[0097] 7 Surface of the workpiece​

[0098] 8 actuator

[0099] 9 laser beam

[0100] 10 workpiece model

[0101] 20 cavity

[0102] 21 surface of the workpiece to be machined in theory

[0103] 22 surface of the workpiece to be machined

[0104] 32, 33 non-ablation region

[0105] 34 ablation region

[0106] 41, 42 laser tool path

[0107] 50 measuring device

[0108] 51 machine tool table

Claims

1. A laser processing method for engraving textures or cavities on a workpiece (2) using a machine tool (1), wherein the machine tool includes a machine tool table (13) for mounting the workpiece thereon and a laser head (2) for emitting a laser beam (9) onto the workpiece, wherein the laser beam is guided by a laser galvanometer scanner integrated in the laser head to a defined position on the workpiece, wherein the machine tool further includes a control unit configured to control the movement of the laser head and the laser galvanometer scanner, the method comprising: a. Calculate a processing path based on the geometry and texture of the workpiece or the cavity prior to processing, wherein the processing path defines a series of relative positions of the laser head relative to the workpiece, wherein calculating the processing path includes the following steps: calculating multiple processing layers to be processed sequentially, and calculating multiple laser tool paths for each layer, wherein the laser tool paths define the locations to be ablated by the laser beam; b. The control unit receives the processing path for ablating the workpiece; c. Ablating the workpiece layer by layer; d. Adjusting the processing path after processing at least one layer, including: i. Scan the defined machining area on the surface of the workpiece using a measuring device (50) to obtain the machining depth at multiple locations on the machining area; ii. Compare the measured machining depth with the defined target depth to determine the depth deviation at each measurement location; and iii. Recalculate the machining path at least partially based on the determined depth deviation; and e. Apply the recalculated machining path to ablate the remaining machining layers.

2. The laser processing method according to claim 1, wherein, Recalculating the machining path includes recalculating at least one machining layer and at least partially recalculating the laser tool path on that layer.

3. The laser processing method according to claim 1 or 2, wherein, If the depth deviation indicates insufficient engraving, the machining path is recalculated by adding at least one machining layer to the remaining machining layers and recalculating the laser tool path on that layer, wherein insufficient engraving occurs when the measured machining depth at a location is less than the target depth.

4. The laser processing method according to claim 3, wherein, If the depth deviation indicates insufficient engraving, the recalculation of the machining path includes copying at least one machining layer, removing the laser tool path used to ablate areas where insufficient engraving was not identified, and adding the modified machining layer to the remaining machining layers.

5. The laser processing method according to any one of claims 1 to 4, wherein, If the depth deviation indicates over-engraving, the machining path is recalculated by at least partially removing the laser tool path on at least one machining layer, wherein the over-engraving occurs when the measured machining depth at a location is greater than the target depth.

6. The laser processing method according to any one of claims 1 to 5, wherein, The processing path is adjusted after each processing layer is processed.

7. The laser processing method according to any one of claims 1 to 6, wherein, The processing path is adjusted in an irregular manner, wherein the timing of the adjustment depends on the processing path, and in particular, the number of adjustments during the second half of the processing is higher than the number of adjustments during the first half of the processing.

8. The laser processing method according to any one of claims 1 to 7, wherein the timing of the adjustment depends on the geometry of the texture or the geometry of the cavity.

9. The laser processing method according to any one of claims 1 to 8, wherein, Perform an initial scan before processing and / or a final scan after processing.

10. The laser processing method according to any one of claims 1 to 9, wherein, The processing area is cleaned before it is scanned by the measuring device.

11. The laser processing method according to any one of claims 1 to 10, wherein, The laser source parameters are adapted based on the determined depth deviation.

12. A laser machine tool, particularly a 5-axis laser machine tool, for engraving textures or cavities on a workpiece, wherein the machine tool includes a machine tool table for mounting the workpiece thereon and a laser head for emitting a laser beam onto the workpiece, wherein the laser beam is guided by a laser galvanometer scanner integrated in the laser head to a defined position on the workpiece, the machine tool further comprising: a. A processing unit configured to determine a processing path prior to processing, the processing path including a series of relative positions of the laser head relative to the workpiece, wherein multiple processing layers to be processed sequentially are calculated, and for each processing layer, multiple laser tool paths are calculated, the laser tool paths defining the locations to be ablated by the laser beam; b. A control unit configured to apply a defined machining path to ablate the workpiece, wherein the workpiece is ablated layer by layer; Features are The machine tool further includes a measuring device configured to scan a defined area on the surface of the workpiece to obtain machining depths at multiple locations on the machining area, wherein the control unit is further configured to compare the measured machining depths with a target depth to determine a depth deviation, and to recalculate the machining tool path based on the determined depth deviation.

13. The laser machine tool according to claim 12, wherein the measuring device is a depth sensor, particularly a confocal sensor.

14. The laser machine tool according to claim 12 or 13, wherein the measuring device is arranged on the machine head and controlled by the control unit.

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