LASER CUTTING PROCESS FOR CUTTING OUT A WORKPIECE PART

Description

field of technology

[0001] The invention relates to the field of laser cutting methods. In particular, the invention relates to a laser cutting method for cutting a workpiece part from a metallic, particularly plate-shaped, workpiece. This type of method is disclosed in EP 0 646 434. State of the art

[0002] Common methods for cutting metallic workpieces (e.g., sheet metal) include laser flame cutting and laser fusion cutting. To create a cutting gap along a predetermined cutting path, the workpiece is melted using a laser beam, and the melt is blown out of the workpiece using a cutting gas.

[0003] Some ferrous metals can be cut using laser cutting by using oxygen (O 2 ) as the cutting gas. The material is heated by the laser beam and burns in the oxygen beam. The energy released during combustion supports the cutting process. In contrast to laser cutting, the aim of laser fusion cutting is to avoid oxidation at the cutting edges, which occurs when the hot workpiece comes into contact with oxygen (O 2 ). For this reason, an inert cutting gas, e.g. nitrogen (N 2 ), is usually used in laser fusion cutting. In this case, the cutting gas not only fulfills the task of blowing the molten material out of the cutting gap, but also serves as a coolant for cooling the workpiece at the processing location in a protective gas atmosphere. This way, oxidation at the cutting edge can be avoided.Especially when cutting stainless steel, oxidation increases the susceptibility to rust formation. Oxidation also causes tarnishing on the cut edges, which should be avoided for aesthetic reasons.

[0004] By cooling the cut edges under the influence of protective gas, complex and costly post-processing of the cut edges can be avoided.

[0005] When cutting a workpiece part out of a workpiece (e.g., from a metal sheet), the cut end represents a critical area in this context. This is the area where the workpiece part is cut free from the remaining workpiece (also known as the residual skeleton). The cutting gas supply is usually interrupted in this area, as the cutting head moves to the next cutting contour immediately after the workpiece part has been cut free. If the cutting gas supply is interrupted before the workpiece part has cooled sufficiently at the end of the cut, the cut edges in this area are exposed to atmospheric oxygen, resulting in oxidation.

[0006] To avoid oxidation at the end of the cut, it is known to direct the cutting gas (or cutting gas jet) onto the cutting gap for a certain period of time after the workpiece part has been cut free in order to cool the workpiece part in the area of ​​the cut end under the influence of the shielding gas. A disadvantage of this approach, however, is that the workpiece part can fall out of the residual skeleton (residual workpiece) after cutting free, or can sag or slip within the residual skeleton, meaning the cut edge is no longer adequately covered by the shielding gas jet. This disadvantage increases with the increasing thickness of the workpiece. Whether and how the workpiece part falls out of the residual skeleton depends, among other things, on whether and how the workpiece part is supported by the machine's workpiece support.

[0007] Even when cutting an internal contour in a workpiece to be manufactured, e.g., to create a hole in the workpiece, the part to be cut out can fall out of the workpiece after the free cutting process. It goes without saying that in this case, the part to be cut out is not a good part, but a slug, i.e., a waste part to be cut out. As soon as the slug falls downwards out of the workpiece during the free cutting process, the cutting gap no longer has its original shape. This can lead to the cutting gas flow detaching from the cutting edge and no longer adequately shielding it from the surrounding atmospheric oxygen.

[0008] When cutting thin sheet metal (up to approximately 4 mm workpiece thickness), microjoints or nanojoints are deliberately created in the cutting gap to prevent cut-out workpiece parts from sagging or falling out. Microjoints and nanojoints are material webs that connect an otherwise completely cut-out workpiece part to the surrounding scrap skeleton. They can be created by not melting the workpiece at predetermined points along the cutting path or by not melting it across the entire thickness of the workpiece. Due to the low workpiece thickness, the microjoints and nanojoints can be removed manually and the workpiece part can thus be removed from the scrap skeleton (e.g., by manually pressing the workpiece part out of the scrap skeleton). Compared to microjoints, nanojoints have a height that is less than the workpiece thickness. Their cross-section can therefore be even smaller than that of microjoints and they are even easier to cut through.

[0009] For workpieces of greater thickness, the workpiece parts can often no longer be removed manually from the scrap skeleton.

[0010] Apart from the adverse effects of oxidation on the cut edge, the altered cooling conditions at the end of the cut compared to the rest of the cutting process can also have other negative effects on the cut workpiece. For example, the hot cut edges of the workpiece part are more susceptible to undesirable deformation when the workpiece part falls out of the scrap skeleton. Such adverse effects can occur not only in laser fusion cutting but also in laser flame cutting.

[0011] The object of the present invention is to improve the state of the art. In particular, when cutting out metallic workpieces with a thickness of at least 4 mm, especially at least 10 mm, the homogeneity of the cut edge and, in particular, the quality of the cut edge at the cut end are to be improved. The invention

[0012] To achieve the object underlying the invention, a laser cutting method for cutting out a workpiece part from a metallic, in particular plate-shaped, workpiece is provided. The method comprises, in a first step, cutting the workpiece along a predetermined cutting path using a laser beam and a cutting gas and using predetermined cutting parameters. In a second step, the method comprises changing the cutting parameters in an end section of the cutting path such that a material web remains between the workpiece part and a residual workpiece, wherein the material web fixes the workpiece part in the residual workpiece. In a third step, the method comprises cooling the workpiece in the region of the material web using the cutting gas and / or a suitable cooling fluid.In a fourth step, the process comprises cutting the workpiece part free from the remaining workpiece by separating the material web using the laser beam and the cutting gas.

[0013] It is understood that the procedural steps take place in the order described.

[0014] The cutting path along which the workpiece part is cut (or cut out) can form a closed contour overall. In the first cutting step, this contour is not completely closed in order to fix the workpiece part in the remaining workpiece by means of the remaining material web. Only during free cutting is the contour completely closed and the workpiece part completely separated from the remaining workpiece. However, cutting out the workpiece part also includes a case in which the workpiece part is located in an edge area of ​​the workpiece, so that the workpiece and the workpiece part have a common outer edge. To cut out the workpiece part from the workpiece, the cutting path then leads from the common outer edge into the workpiece and ends at a different point on the common outer edge.

[0015] Preferably, the laser beam and cutting gas exit from a common cutting head during cutting. This allows the cutting gas to be easily traversed along with the movement of the laser beam. The cutting gas can be directed in a directional beam, particularly at a predetermined cutting gas pressure, onto the laser beam's processing location on the workpiece.

[0016] By cooling the workpiece while the workpiece part is held in the remaining workpiece by the material web, it can be prevented that the workpiece part falls out of the remaining workpiece (i.e., from the residual skeleton), sags, or tilts and slowly cools under the influence of atmospheric oxygen. The cutting gap is kept open by the material web. The fixed position of the workpiece part increases the reproducibility when cooling the workpiece part. Active cooling of the workpiece part in the final section of the cutting path can also increase the homogeneity of the cutting edge. In other words, changes in the cutting edge in the final section of the cutting path are reduced by the continued cooling of the cutting edge in this area.

[0017] Preferably, an inert gas, in particular nitrogen (N 2 ), can be used as the cutting gas. An inert gas that is different from the cutting gas can be used as an additional or alternative cooling fluid. In particular, helium (He) can be used as an additional or alternative cooling fluid. An inert gas is characterized by its low reaction speed with respect to the material of the workpiece to be cut at the prevailing process temperatures. Although nitrogen (N 2 ) is also inert in many applications, helium (He) has better thermal conductivity than nitrogen (N 2 ). The use of helium (He) as an additional or alternative cooling fluid can therefore further optimize the cooling process.

[0018] According to one variant, cooling the workpiece can comprise switching off the laser beam for a predetermined period of time, preferably for less than 5 seconds, in particular for less than one second. Switching off the laser beam interrupts the heating of the workpiece associated with the laser irradiation.

[0019] Cooling the workpiece can further comprise: exposing the area of ​​the workpiece to be cooled to the cutting gas while the laser beam is switched off, wherein the cutting gas pressure during cooling is greater than or equal to the cutting gas pressure during cutting of the workpiece. By blowing the cutting gas into the cutting gap in the area to be cooled, the workpiece cools in this area. Increasing the cutting gas pressure can generally accelerate the cooling process. The cooling process can be further accelerated by additionally exposing the area to be cooled to water mist or another suitable cooling fluid. Such additional cooling fluid can, for example, be directed onto the cutting gap via a cross-jet nozzle. In principle, the workpiece can also be cooled exclusively using a cooling fluid other than the cutting gas via a cross-jet nozzle.

[0020] For economic reasons, it is generally desirable to keep the cooling time short. One goal is to visually match the cutting edge of the workpiece part in the final section of the cutting path to the rest of the cutting edge. Depending on the individual requirements for the appearance of the cutting edge, it may be sufficient to cool the workpiece for less than one second as described above. Increasing the cooling time may be necessary, especially with increasing workpiece thickness.

[0021] According to one variant, the material web can have a predetermined minimum cross-section so that the workpiece part is just fixed in its original position in the remaining workpiece during cooling. The cross-section of the material web is determined by the height and width of the material web. The height of the material web can correspond to a maximum of the thickness of the workpiece. The width of the material web corresponds to the extension of the material web in the cutting direction. The material web also has a length that corresponds to the width of the cutting gap.

[0022] By fixing or holding the workpiece part in its original position in the remaining workpiece, the workpiece part cannot tilt within the remaining workpiece or fall out of it. This has the advantage of keeping the cutting gap open. This means that the cutting edge of the workpiece part remains accessible to the cutting gas in the final section of the cutting path. The cross-section of the material web should therefore be large enough to ensure sufficient stability of the material web. On the other hand, the cross-section of the material web should be as small as possible in order to minimize the energy input into the workpiece required to cut the workpiece free. It is understood that the minimum required cross-section of the material web depends on several factors. These factors include the material of the workpiece, the volume of the workpiece part (orits weight), as well as the center of gravity of the workpiece part in relation to the material web and whether or at how many points the part to be cut free is supported by the workpiece support of the machine.

[0023] According to a variant, changing the cutting parameters may include the following sub-step: switching off the laser before the end of the cutting path is reached when cutting the workpiece.

[0024] The end of the cutting path can in particular be the position on the cutting path in which the contour to be cut is closed.

[0025] By switching off the laser beam shortly before the end of the cutting path, the material bridge can be formed as a so-called "microjoint." In this case, the material bridge can have a height corresponding to the thickness of the workpiece. The width of the material bridge should be as small as possible. The optimal width of the material bridge depends on the tilting moment that the material bridge must withstand to hold the workpiece part.

[0026] A microjoint can be created easily, without requiring complex adjustment of the cutting parameters. The laser beam simply needs to be switched off at a point just before the end of the cutting path is reached.

[0027] According to an alternative variant, changing the cutting parameters may comprise the following steps: changing the cutting parameters such that the workpiece is only melted to a depth that is less than the workpiece thickness; and cutting the workpiece with the changed cutting parameters to the end of the cutting path so that the material web has a height that is less than the workpiece thickness.

[0028] In this way, the material bridge can be formed as a so-called "nanojoint." By reducing the height of the material bridge compared to the microjoint, the cross-section of the material bridge can be further reduced.

[0029] Changing cutting parameters to create a nanojoint is described, for example, in the applicant's patent application WO 2019025327 A2.

[0030] Changing the cutting parameters to create a nanojoint can include, for example, reducing laser power, increasing cutting speed, and / or changing the focus position or focus diameter of the laser beam. To cool the workpiece in the area of ​​the nanojoint, the cutting head can be moved back along the cutting path across the width of the nanojoint while the cutting gas is blown from the cutting head into the cutting gap. Alternatively, the cutting gas can be directed primarily at the last cut end of the nanojoint. In this case, the cutting head does not need to be moved back.

[0031] The cutting of the workpiece part from the remaining workpiece can comprise the following sub-steps: switching on the laser; and cutting, by means of the laser beam and the cutting gas, the workpiece part from the remaining workpiece by separating the material web, wherein the cutting parameters when cutting the workpiece part are adjusted so that less energy is coupled into the workpiece than during the previous cutting of the workpiece.

[0032] Separating the material web to free the workpiece part can essentially be done according to the same principle as cutting the workpiece itself. It goes without saying, therefore, that the same cutting parameters are used to free the workpiece part as for the previous cutting of the workpiece. Due to the short exposure time of the laser beam when severing the micro- or nanojoint (i.e. the material web), less heat is introduced into the workpiece in this case than when cutting the workpiece over longer distances. By heating the workpiece less during free cutting, the formation of oxidation at the cut edge of the workpiece part after free cutting can be reduced.

[0033] Adjusting the cutting parameters when freeing the workpiece part can include reducing the laser power and / or increasing the cutting speed and / or changing the focus position and / or the focus diameter of the laser beam. Additionally or alternatively, the pressure of the cutting gas for freeing the workpiece part can also be increased. The increased pressure of the cutting gas can positively influence the cooling effect on the workpiece part even during freeing. It is understood that the relative specifications for adjusting the cutting parameters refer to the corresponding cutting parameters during the previous cutting of the workpiece.

[0034] Depending on the position of the cutting head after the workpiece has cooled, the material web can be cut off (i.e. separated) either in the original cutting direction or opposite to the original cutting direction. In the case of a microjoint, the cutting head can be arranged to remain stationary at the front end of the microjoint in the cutting direction during cooling, i.e. at the position at which the laser beam was switched off. In this case, the free cutting can take place in the same direction as the previous cutting of the workpiece. In the case of a nanojoint, on the other hand, the cutting head can be arranged to remain stationary at the rear end of the nanojoint in the cutting direction during cooling. In this case, the free cutting can take place in a direction opposite to the previous cutting direction. In this way, an additional travel path of the cutting head can be avoided.

[0035] The workpiece can have a thickness of at least 4 mm, preferably at least 10 mm. The workpiece can, in particular, be a metal sheet. The advantageous effects of the method according to the invention are particularly evident when cutting thick sheet metal. This is due, among other things, to the fact that the heat input into the workpiece increases with increasing thickness. For example, the workpiece can also have a thickness of 40 mm or more.

[0036] To achieve the object underlying the invention, a laser cutting machine for cutting metallic, in particular plate-shaped, workpieces is further provided, which is designed to carry out the laser cutting method according to one of the variants described above. Examples of implementation

[0037] Aspects of the invention are explained in more detail below with reference to the drawings. They show: Figure 1: a schematic representation of a laser cutting machine according to the invention; Figure 2a: schematically shows a workpiece with a workpiece part to be cut out therefrom; Figure 2b: schematically shows the end region of a cutting path in section A of Figure 2a ; Figure 3a: schematically shows a cross-section of a material web designed as a microjoint; and Figure 3b: schematically shows a cross-section of a material web designed as a nanojoint.

[0038] Figure 1shows an example of a laser cutting machine 1 that is suitable for carrying out a method according to the invention. The laser cutting machine 1 comprises a laser beam generator 2 (e.g. a CO2 laser, diode laser or solid-state laser), a movable cutting head 3 and a workpiece support 4. In the laser beam generator 2, a laser beam 5 is generated, which is guided from the laser beam generator 2 to the processing head 3 by means of a fiber optic cable (not shown) or deflecting mirrors (not shown). A plate-shaped workpiece 6 is arranged on the workpiece support 4. The laser beam 5 is directed onto the workpiece 6 by means of focusing optics arranged in the processing head 3. The laser cutting machine 1 is also supplied with cutting gas 7, for example nitrogen (N2). The use of the respective cutting gas 7 depends on the workpiece material and on the quality requirements for the cut edges.Furthermore, an extraction device 8 is provided, which is connected to an extraction channel 9 located beneath the workpiece support 4. The cutting gas 7 is fed to a cutting gas nozzle 10 of the cutting head 3, from which it exits together with the laser beam 5.

[0039] During laser beam cutting, the workpiece 6 is melted along a predetermined contour using the laser beam 5 and blown downward using the cutting gas 7, creating a cutting gap. To control the cutting parameters, the laser cutting machine 1 further comprises a control unit 15.

[0040] Based on the Figures 2a and 2b Aspects of a method according to the present invention are explained in more detail below. In Figure 2aA section of a plate-shaped workpiece 6 is shown schematically in plan view. In the workpiece 6, a cutting gap 66 is formed along a closed, rectangular contour, which divides the workpiece 6 into a workpiece part 62 (or cut part or good part) and a remaining workpiece 64 (or residual skeleton). However, the cutting gap 66 does not completely close the contour. Thus, the cutting gap 66 is interrupted by a material web 68, which connects the workpiece part 62 to the remaining workpiece 64 and fixes it therein. A method according to the present invention can be used to cut out the workpiece part 62 from the workpiece 6. The cutting of the workpiece 6 (in Figure 2aclockwise) is carried out using predetermined cutting parameters which depend, among other things, on the thickness and material of the workpiece 6. In an end section of the cutting path, the cutting parameters are changed such that the material web 68 is created, which fixes the workpiece part 62 in the remaining workpiece 64. In particular, this can be achieved by switching off the laser beam 5 to create a microjoint before the end of the cutting path is reached. Alternatively, the cutting parameters can be changed in the end section of the cutting path, e.g. by reducing the laser beam power, such that the workpiece 6 is no longer cut across its entire thickness in this section. In this way, the material web 68 can be formed as a nanojoint.

[0041] Figure 2b shows section A from Figure 2a . Based on Figure 2bThe course of the cutting process can be traced. The laser beam 5 penetrates the workpiece 6 at position P1 and moves to the contour to be cut, which it reaches at position P2. The laser beam 5 then traces the contour of the workpiece part 62 and, in interaction with the cutting gas 7, creates the continuous cutting gap 66. Upon reaching position P3, shortly before the end of the cutting path (position P4), the cutting parameters are changed such that the material web 68 is created as a microjoint or nanojoint. The material web 68 is sufficiently stable to hold the workpiece part 62 in the remaining workpiece 64 and prevent the workpiece part 62 from tilting relative to the remaining workpiece 64.

[0042] After the material web 68 has been created, the laser beam 5 can be switched off according to the invention, while the cutting gas 7 continues to be directed onto the material heated by the laser beam 5 in the region of the material web 68. While it can be advantageous to use the same inert shielding gas, in particular nitrogen (N 2 ), both when cutting the workpiece 6 and when cooling, this is not absolutely necessary. Alternatively, it is possible, for example, to initially cut with nitrogen (N 2 ) as the process gas and to switch to another inert gas, e.g. helium (He) (as the cooling fluid), for cooling the cut edges at the end of the cutting gap. Due to its good thermal conductivity, the use of helium (He) as the shielding gas can further increase the cooling effect compared to nitrogen (N 2 ) and reduce the cooling time.In order to achieve uniform cooling of the cutting edge of the workpiece part 62, regardless of oxidation, it is also conceivable that the method according to the invention is carried out in a flame cutting process using oxygen (O 2 ) as the cutting gas. In this case, the advantages of the method according to the invention can lie in particular in an increase in the homogeneity of the cutting edge.

[0043] The cooling process can be accelerated by increasing the gas pressure.

[0044] After the workpiece 6 has cooled sufficiently, the laser beam 5 is switched on again, and the workpiece part 62 is cut free by severing the material web 68. The target temperature of the workpiece 6 after cooling depends on the cooling objective and varies, among other things, depending on the material of the workpiece 6. In particular, the intention may be to reduce oxide formation at the cutting edge when cutting stainless steel in order to prevent tarnishing of the cutting edge. Depending on the thickness of the workpiece, a significant reduction in tarnishing at the cutting edge can be achieved even with a cooling time of less than one second, compared to a conventional method in which the workpiece 6 is completely cut out (i.e., also cut free) without a cooling break.

[0045] The Figures 3a and 3b show schematically a sectional view through a material web 68, which is used as a microjoint ( Figure 3a ) or as a nanojoint ( Figure 3b ) is formed. The drawing plane of the Figures 3a and 3b extends along the cutting gap 66 parallel to the cutting edge 63 of the workpiece part 62 (i.e. perpendicular to the workpiece surface). In order for the material web 68 to have sufficient stability to hold or fix the workpiece part 62 in the remaining workpiece, it must have a minimum required cross-section. The cross-section of the material web 68 can be approximately determined via the height h and the width b of the material web 68. It is understood that the cross-sectional shape of the material web 68 in practice differs from the simplified rectangular shape according to the Figures 3a and 3b usually deviates. Methods for determining non-rectangular cross-sections are also familiar to the person skilled in the art.

[0046] Figure 3a shows a material web 68 designed as a microjoint, the height h of which corresponds to the thickness H of the workpiece 6. Figure 3bshows, in comparison, a material web 68 formed as a nanojoint, the height h of which is less than the thickness H of the workpiece 6. In principle, the cross-section of the material web 68 should be kept as small as possible so that the heating of the workpiece when separating the material web 68 can be kept as low as possible. In the case of a microjoint ( Figure 3a ), the cross-section of the material web 68 can only be regulated via the width of the material web 68. Especially when cutting workpieces 6 with a large thickness H, for example, beyond 10 mm, it can be advantageous to design the material web 68 as a nanojoint ( Figure 3b ). In this case, the cross-section of the material web can be regulated not only by its width (b) but also by its height (h). This increases the possibilities for forming a material web of optimal size.

Claims

1. A laser cutting method for cutting out a workpiece part (62) from a metallic, in particular plate-shaped, workpiece (6), the method comprising the following steps: cutting the workpiece (6) along a predefined cutting path, using a laser beam (5) and a cutting gas (7), and using predefined cutting parameters; changing the cutting parameters in an end portion of the cutting path so that a material web (68) remains between the workpiece part (62) and a workpiece remainder (64) of the workpiece (6), wherein the material web (68) fixes the workpiece part (62) in the workpiece remainder (64); and characterized by the following steps: cooling the workpiece (6) in the region of the material web (68) using the cutting gas (7) and / or a suitable cooling fluid; and snipping out the workpiece part (62) from the workpiece remainder (64) by separating the material web (68) using the laser beam (5) and the cutting gas (7).

2. The method according to claim 1, wherein an inert gas, in particular nitrogen (N2), is used as the cutting gas (7).

3. The method according to one of the preceding claims, wherein the cooling of the workpiece (6) comprises: switching off the laser beam (5) for a predetermined period of time, preferably for less than 5 seconds, in particular for less than one second.

4. The method according to claim 3, wherein the cooling of the workpiece further comprises: impinging the region to be cooled with the cutting gas (7) while the laser beam (5) is switched off, wherein a cutting gas pressure during the cooling is higher than or equal to a cutting gas pressure during the cutting of the workpiece (6).

5. The method according to one of the preceding claims, wherein the material web (68) has a predetermined minimum cross-section such that the workpiece part (62) is fixed in its original position in the workpiece remainder (64) during the cooling.

6. The method according to one of the preceding claims, wherein changing the cutting parameters comprises: switching off the laser beam (5) before the end of the cutting path is reached, while cutting the workpiece (6).

7. The method according to claims 1 to 5, wherein changing the cutting parameters comprises: changing the cutting parameters in such a way that the workpiece (6) is only fused down to a depth that is less than the workpiece thickness (H); and cutting the workpiece (6) using the changed cutting parameters up to the end of the cutting path such that the material web (68) has a height (h) that is less than the workpiece thickness (H).

8. The method according to one of the preceding claims, wherein the snipping-out step comprises: turning on the laser beam (5); and snipping out the workpiece part (62) from the workpiece remainder (64) using the laser beam (5) and the cutting gas (7) by separating the material web (68), wherein the cutting parameters are adjusted while snipping out the workpiece part (62) such that less energy is coupled into the workpiece (6) than in the previous cutting of the workpiece (6).

9. The method according to claim 8, wherein the adjusting of the cutting parameters while snipping out the workpiece part (62) comprises a reduction of a laser power and / or an increase in a cutting speed and / or a changing of a focus point and / or a focus diameter of the laser beam (5).

10. The method according to one of the previous claims, wherein the workpiece (5) has a thickness (H) of at least 4 mm, preferably of at least 10 mm.

11. A laser cutting machine (1), comprising a laser beam generator (2), a movable cutting head (3) and a workpiece support (4), wherein a focusing optics system arranged in the processing head (3) is used to direct the laser beam (5) onto a workpiece (6) arranged on the workpiece support, and wherein the laser cutting machine (1) is supplied with a cutting gas (7) which is fed to a cutting gas nozzle (10) of the cutting head (3), the gas exiting the nozzle together with the laser beam (5), characterized by a control unit (15) that controls the cutting parameters for carrying out a method according to claim 1.