Laser cutting method for cutting out workpiece parts

Abstract

A laser cutting method for cutting a workpiece part (62) from a metal, in particular plate-shaped, workpiece (6) is provided, comprising the steps of cutting the workpiece (6) along a predefined cutting trajectory by means of a laser beam (5) and a cutting gas (7) and using predefined cutting parameters, changing the cutting parameters at an end section of the cutting trajectory in such a way that a material bridge (68) remains between the workpiece part (62) and a workpiece remainder (64) of the workpiece (6), wherein the material bridge (68) fixes the workpiece part (62) in the workpiece remainder (64), cooling the workpiece (6) in the area of the material bridge (68) by means of the cutting gas (7) and / or by means of a suitable cooling fluid, and completely singulating the workpiece part (62) from the workpiece remainder (64) by breaking the material bridge (68) by means of the laser beam (5) and the cutting gas (7). A laser cutting machine suitable for carrying out the method is also provided.

Description

TECHNICAL FIELD

[0001] The present invention relates to the field of laser cutting methods. In particular, the present invention relates to a laser cutting method for cutting a workpiece part from a metal, in particular a plate-shaped, workpiece. BACKGROUND

[0002] The initially mentioned methods for cutting metal workpieces, for example metal sheets, include laser combustion cutting and laser fusion cutting. Here, the workpiece is melted along a predefined cutting trajectory by means of a laser beam to produce a kerf and the melt is blown out of the workpiece by means of a cutting gas.

[0003] Some ferrous metals can be cut by means of laser combustion cutting using oxygen (O2) as cutting gas, the workpiece being heated by means of a laser beam and combusting in a jet of oxygen. The energy released during combustion promotes the cutting process. In contrast to laser combustion cutting, in laser fusion cutting, oxidation on the cutting edge when the hot workpiece contacts oxygen (O2) should be avoided. For this reason, in laser fusion cutting, an inert cutting gas, for example nitrogen (N2), is usually used. In this case, the cutting gas not only fulfils the task of blowing the molten material out of the kerf. The cutting gas simultaneously serves as a coolant for cooling the workpiece at the machining point under a protective gas atmosphere. In this way, oxidation on the cutting edge can be avoided. In particular when cutting stainless steel, oxidation increases the susceptibility to corrosion. In addition, temper colors on the cutting edge, which should be avoided for aesthetic reasons, are produced by oxidation.

[0004] Therefore, in principle, the complex and cost-increasing post-processing of the cutting edge can be avoided by cooling the cutting edge under the influence of a protective gas.

[0005] In connection therewith, when cutting a workpiece part from a workpiece, for example from a metal sheet, the cutting end forms a critical region, i.e. the region in which the workpiece part is completely separated from the remaining part of the workpiece, also referred to as the remaining grid. In this region, the supply of cutting gas is usually interrupted, DS147293191-cn

[0006] Because immediately after the workpiece part has been completely separated, the cutting head travels down the cutting contour. Since the supply of cutting gas is interrupted before the workpiece part has sufficiently cooled at the cutting end, the cutting edge in this region is exposed to oxygen in the air and oxidizes.

[0007] To avoid oxidation at the cut end, it is known that after completely cutting the workpiece portion, the cutting gas (or cutting gas jet) is directed at the kerf for a certain period of time to cool the workpiece portion in the cut end region under the influence of the protective gas. However, a drawback of this process is that the workpiece portion may fall off, sink, or slide within the remaining grid after complete separation, and the cut edge is no longer adequately covered by the protective gas jet. This drawback becomes more pronounced with increasing workpiece thickness. Whether and how the workpiece portion falls off the remaining grid depends particularly on whether and how the workpiece portion is supported by the machine's workpiece support.

[0008] Even when cutting internal contours within a workpiece to be finished (e.g., to form a hole), the portion of the workpiece to be cut will detach from the workpiece after complete slicing. It should be understood that in this case, the portion to be cut is not a finished part but a scrap, i.e., waste material to be cut away. Once the scrap falls downwards from the workpiece during complete slicing, the kerf no longer exists in its original form. This may cause the cutting gas flow to detach from the cutting edge and no longer adequately protect the cutting edge from the influence of oxygen in the surrounding air.

[0009] When cutting within thin sheet metal (workpiece thickness up to approximately 4 mm), microjoints or nanojoints are strategically created in the kerf to prevent the cut workpiece portion from sinking or falling off. Microjoints and nanojoints are material bridging points that connect the workpiece portion to the surrounding remaining grid; without them, the workpiece portion would be completely cut out. These microjoints and nanojoints can be created by not melting the workpiece at predetermined locations along the cutting path, or by not melting the workpiece across its entire thickness. Due to the small workpiece thickness, microjoints and nanojoints can be manually removed, thereby removing the workpiece portion from the remaining grid (e.g., by manually pushing the workpiece portion out of the remaining grid). Compared to microjoints, nanojoints have a height smaller than the workpiece thickness. Therefore, nanojoints can also have a smaller cross-section than microjoints and are easier to break.

[0010] DS147293191-cn

[0011] In the case of workpieces with a large thickness, parts of the workpiece can often no longer be manually removed from the remaining grid.

[0012] In addition to the unfavorable formation of oxidation on the cut edges, the different cooling conditions at the cut end compared to the rest of the cutting process can also have other negative effects on the cut workpiece portion. For example, the hot cut edges of the workpiece portion are more prone to undesirable deformation when it falls off the remaining grid. This type of adverse effect exists not only in laser melting cutting but also in laser burning cutting. Summary of the Invention

[0013] The purpose of this invention is to improve upon existing technology. In particular, when cutting workpieces of metal with a thickness of at least 4 mm, especially at least 10 mm, the uniformity of the cut edge and, in particular, the quality of the cut edge at the cutting end are improved.

[0014] To achieve the objectives of this invention, a laser cutting method is provided for cutting a portion of a workpiece from a metallic, particularly plate-shaped, workpiece. In a first step, the method includes cutting the workpiece along a predefined cutting trajectory using a laser beam and a cutting gas, and employing predefined cutting parameters. In a second step, the method includes changing the cutting parameters at the end section of the cutting trajectory in such a way that a material bridging portion is maintained between the workpiece portion and the remaining workpiece portion, wherein the material bridging portion secures the workpiece portion within the remaining workpiece portion. In a third step, the method includes cooling the workpiece in the region of the material bridging portion using the cutting gas and / or a suitable cooling fluid. In a fourth step, the method includes completely separating the workpiece portion from the remaining workpiece portion by disconnecting the material bridging portion using the laser beam and the cutting gas.

[0015] It should be understood that these method steps are performed in the order described.

[0016] Generally, the cutting path along which the workpiece portion is cut (or cut out) can form a closed profile. In the first step of cutting, this profile is not completely closed, so that the workpiece portion is connected by means of the retained material bridging portion DS147293191-cn

[0017] The part is fixed within the remaining portion of the workpiece. Only upon complete cutting is the contour fully closed, and the workpiece portion is completely cut off from the remaining portion. However, cutting out a workpiece portion also includes cases where the workpiece portion is located in the outer region of the workpiece, such that the workpiece and the workpiece portion share a common outer edge. In this case, to cut the workpiece portion from the workpiece, the cutting trajectory enters the workpiece from the common outer edge and terminates at another location on the common outer edge.

[0018] The laser beam and cutting gas can preferably be ejected from a shared cutting head during cutting. Therefore, the cutting gas can be easily carried along with the movement of the laser beam. Specifically, the cutting gas can be directed at the laser beam's processing position on the workpiece in the form of a directional jet at a predefined cutting gas pressure.

[0019] By cooling the workpiece while it is held in place within the remaining workpiece by the material bridging section, it is possible to prevent: the workpiece from falling out of the remaining workpiece (i.e., from the remaining grid), sinking or tipping over within the remaining workpiece, and slow cooling due to the influence of oxygen in the air. The kerf is retained due to the material bridging section. Because the position of the workpiece is fixed, the repeatability of cooling the workpiece is enhanced. Furthermore, active cooling of the workpiece at the end section of the cutting path enhances the uniformity of the cutting edge. In other words, in the end section of the cutting path, the variation in the cutting edge is reduced because cooling continues in that area.

[0020] Inert gases, particularly nitrogen (N2), can preferably be used as the cutting gas. Different inert gases can be used as supplementary or alternative cooling fluids. In particular, helium (He) can be used as a supplementary or alternative cooling fluid. Inert gases are characterized by their reactive inertness to the material of the workpiece to be cut at the primary process temperature. While nitrogen (N2) is also inert in many applications, helium (He) has better thermal conductivity than nitrogen (N2). Therefore, by using helium (He) as a supplementary or alternative cooling fluid, the cooling process can be further optimized.

[0021] According to one variation, cooling the workpiece may include turning off the laser beam for a predetermined period of time, preferably less than 5 seconds, and particularly less than one second. Turning off the laser beam interrupts the heating of the workpiece associated with laser radiation.

[0022] DS147293191-cn

[0023] Cooling the workpiece may also include: loading the workpiece region to be cooled with the cutting gas when the laser beam is turned off, wherein the cutting gas pressure during cooling is higher than or equal to the cutting gas pressure during cutting the workpiece. The workpiece cools down in the kerf region by blowing the cutting gas into the region to be cooled. In principle, the cooling process can be accelerated by increasing the cutting gas pressure. Further acceleration of the cooling process can be achieved by additionally loading the region to be cooled with water mist or any other suitable cooling fluid. For example, this additional cooling fluid can be directed towards the kerf through a transverse nozzle. In principle, the workpiece can also be cooled solely by using a cooling fluid different from the cutting gas through a transverse nozzle.

[0024] For economic reasons, a short cooling time is generally desirable. The goal is to visually make the cut edge of the workpiece portion in the final segment of the cutting trajectory identical to the rest of the cut edge. Depending on the individualized visual requirements of the cut edge, cooling the workpiece for less than one second, as described above, may be sufficient. In particular, with increasing workpiece thickness, it may be necessary to increase the cooling time.

[0025] According to a variation, the material bridging section has a predetermined minimum cross-section such that the workpiece portion remains fixed in its original position within the remaining workpiece portion during cooling. The cross-section of the material bridging section is determined by its height and width. The height of the material bridging section can correspond at most to the thickness of the workpiece. The width of the material bridging section corresponds to its extension dimension in the cutting direction. The material bridging section also has a length corresponding to the width of the kerf.

[0026] Because the workpiece portion is fixed or held within the remaining workpiece portion in its original position, it will not tip over or fall out of the remaining workpiece portion. The advantage of this is that the kerf is preserved. In this way, the cutting gas can still reach the cutting edge of the workpiece portion in the final section of the cutting trajectory. Therefore, the cross-section of the material bridging section should be large enough to ensure sufficient stability. On the other hand, the cross-section of the material bridging section should be as small as possible to minimize the energy input to the workpiece required to completely cut it. It should be understood that the minimum required cross-section of the material bridging section depends on several factors. These factors include the material of the workpiece, the volume (or weight) of the workpiece portion, the position of the center of gravity of the workpiece portion relative to the material bridging section, and whether or how many locations the portion to be completely cut is supported by the machine's workpiece support.

[0027] DS147293191-cn

[0028] According to one variation, changing the cutting parameters may include the following sub-step: turning off the laser beam before reaching the end of the cutting trajectory when cutting the workpiece.

[0029] The end of the cutting trajectory can be the point where the contour to be cut closes on the cutting trajectory.

[0030] By shutting off the laser beam slightly before the end of the cutting trajectory, the material bridging section can be configured as a so-called "micro-joint." In this case, the material bridging section can have a height corresponding to the thickness of the workpiece. The width of the material bridging section should be as small as possible. The optimal width of the material bridging section depends on the tilting moment that the material bridging section must withstand to hold the workpiece portion.

[0031] Microjoints can be generated in a simple way without the need for laborious adaptation of cutting parameters. To do this, the laser beam simply needs to be switched off at a certain position slightly before the end of the cutting trajectory is reached.

[0032] According to the alternative variant, changing the cutting parameters may include the following steps: changing the cutting parameters such that the workpiece is melted only to a depth less than the workpiece thickness; and cutting the workpiece using the changed cutting parameters until the end of the cutting trajectory, such that the height of the material bridging portion is less than the workpiece thickness.

[0033] In this way, the material bridging section can be configured as a so-called "nano-joint". Compared with microjoints, the cross-section of the material bridging section can be further reduced by decreasing its height.

[0034] For example, the applicant's patent application WO2019025327A2 describes changing the cutting parameters to produce nanojoints.

[0035] Modifying the cutting parameters to produce a nanojoint may include, for example, reducing laser power, increasing the cutting rate, and / or changing the focal position and / or focal diameter of the laser beam. To cool the workpiece in the nanojoint region, the cutting head can be positioned to move back along the cutting path across the width of the nanojoint while cutting gas is blown from the cutting head into the kerf. Alternatively, the cutting gas can be primarily directed towards the final cut end of the nanojoint DS147293191-cn. In this case, the cutting head does not need to move back.

[0036] Completely cutting the workpiece portion from the remaining portion of the workpiece may include the following sub-steps: turning on the laser beam; and using the laser beam and the cutting gas to completely cut the workpiece portion from the remaining portion of the workpiece by breaking the material bridging portion, wherein the cutting parameters are adjusted during the complete cutting of the workpiece portion so that less energy is coupled to the workpiece than in the previous cutting of the workpiece.

[0037] In principle, disconnecting the material bridging section for complete workpiece separation can be performed using the same principles as cutting the workpiece itself. It should be understood that, therefore, the same cutting parameters as when cutting the workpiece previously are used for complete separation. Even in this case, less heat is introduced into the workpiece than when cutting over a longer distance, due to the short duration of the laser beam when disconnecting the micro- or nano-joints (i.e., the material bridging section). Because the workpiece is heated to a lesser extent when completely separated, oxidation forming on the cut edges of the workpiece portion after complete separation can be reduced.

[0038] Adjusting the cutting parameters during the complete slicing of the workpiece may include reducing the laser power and / or increasing the cutting rate and / or changing the focal position and / or focal diameter of the laser beam. Additionally or alternatively, the pressure of the cutting gas used to completely slice the workpiece may be increased. The increased pressure of the cutting gas can also positively affect the cooling effect of the workpiece during complete slicing. It should be understood that the instructions for adjusting the cutting parameters are based on the corresponding cutting parameters used in previous workpiece cutting.

[0039] Depending on the position of the cutting head after the workpiece has cooled, the material bridging portion can be cut (i.e., disconnected) in the original cutting direction or in the opposite direction. In the case of microjoints, the cutting head can be positioned so that during cooling, it stops at the forward end of the microjoint along the cutting direction, i.e., at the position where the laser beam is turned off. In this case, complete severance can be performed in the same direction as the previously cut workpiece. Conversely, in the case of nanojoints, the cutting head can be positioned so that during cooling, it stops at the rear end of the nanojoint along the cutting direction. In this case, complete severance can be performed in the opposite direction to the previous cutting direction. This method correspondingly avoids additional displacement of the cutting head.

[0040] The workpiece can be at least 4 mm thick, preferably at least 10 mm. The workpiece can be, in particular, made of gold (DS147293191-cn).

[0041] The material is sheet metal. The advantageous effects of the method according to the invention are particularly evident when cutting within the range of thin sheet metal. This is especially relevant to the fact that "the heat input to the workpiece increases with the thickness of the workpiece." The thickness of the workpiece can, for example, be 40 mm or more.

[0042] To achieve the objectives of this invention, a laser cutting machine for cutting metal, particularly plate-shaped workpieces, is also provided, the laser cutting machine being configured to perform a laser cutting method according to one of the above variations. Attached Figure Description

[0043] The various aspects of the invention will now be explained in more detail with the aid of the accompanying drawings. In the drawings:

[0044] Figure 1 A schematic diagram of a laser cutting machine according to the present invention is shown;

[0045] Figure 2a This schematically illustrates a workpiece with a portion to be cut out from it.

[0046] Figure 2b : schematically shown Figure 2a The end region of the cutting trajectory in segment A;

[0047] Figure 3a : A schematic diagram illustrating a material bridging portion configured as a micro-joint in a cross-section; and

[0048] Figure 3b The diagram schematically illustrates a material bridging section configured as a nano-joint in a cross-section. Detailed Implementation

[0049] Figure 1 A laser cutting machine 1 suitable for performing the method according to the invention is illustrated by way of example. The laser cutting machine 1 includes a laser beam generator 2 (e.g., a CO2 laser, diode laser, or solid-state laser), a movable cutting head 3, and a workpiece holding portion 4. A laser beam 5 is generated in the laser beam generator 2 and guided from the laser beam generator 2 to the cutting head 3 by means of an optical fiber (not shown) or a deflector (not shown). A plate-shaped workpiece 6 is placed on the workpiece holding portion 4. The laser beam 5 is directed towards the workpiece 6 by means of a focusing optical unit arranged in the cutting head 3. The laser cutting machine 1 is further supplied with a cutting gas 7, such as nitrogen (N2). The use of the corresponding cutting gas 7 depends on the workpiece material and the quality requirements set for the cut edge. Furthermore, a suction device 8 is connected to a suction pipe 9 located below the workpiece holding portion 4. The cutting gas 7 is supplied to a cutting gas nozzle 10 of the cutting head 3, from which the cutting gas 7 is ejected along with the laser beam 5.

[0050] During laser beam cutting, predefined cutting parameters are used to guide the laser beam 5 along a predefined wheel DS147293191-cn.

[0051] The workpiece 6 is melted and blown downwards by cutting gas 7, thus creating a kerf. The laser cutting machine 1 also includes a control unit 15 for controlling the cutting parameters.

[0052] The following will use... Figure 2a and Figure 2b The various aspects of the method according to the invention will be explained in more detail. Figure 2a The diagram schematically shows a segment of a plate-shaped workpiece 6 viewed from above. A slit 66 is formed along the closed rectangular outline of the workpiece 6, dividing it into a workpiece portion 62 (or a cut part or finished product) and a remaining workpiece portion 64 (or remaining grid). However, the slit 66 does not completely close the outline. That is, the slit 66 is interrupted by a material bridging portion 68, which connects the workpiece portion 62 to the remaining workpiece portion 64 and secures the workpiece portion to the remaining workpiece portion. The method according to the invention can be applied to cutting the workpiece portion 62 from the workpiece 6. The cutting of the workpiece 6 (in...) Figure 2aThe cutting is performed using predefined cutting parameters (in a clockwise direction), which are particularly dependent on the thickness and material of the workpiece 6. The cutting parameters are changed at the end of the cutting path in a way that a material bridging portion 68 is created, fixing the workpiece portion 62 within the remaining portion 64. This can be achieved, in particular, by turning off the laser beam 5 before reaching the end of the cutting path to create the microjoint. Alternatively, the cutting parameters can be changed at the end of the cutting path (e.g., by reducing the laser beam power) so that the workpiece 6 is no longer cut across its entire thickness in that section. In this way, the material bridging portion 68 can be configured as a nanojoint.

[0053] Figure 2b Showing from Figure 2a Segment A. Using Figure 2b The cutting process can be reproduced. Laser beam 5 penetrates the workpiece 6 at position P1 and moves to the cutting contour. Laser beam 5 reaches the cutting contour at position P2. Subsequently, laser beam 5 moves along the contour of workpiece portion 62 and, through interaction with the cutting gas 7, produces a continuous kerf 66. When it reaches position P4, slightly ahead of the end of the cutting trajectory (position P3), the cutting parameters are changed so that the material bridging portion 68 is created as a micro-joint or nano-joint. The material bridging portion 68 is configured to be sufficiently stable to hold workpiece portion 62 within the remaining workpiece portion 64 and prevent workpiece portion 62 from tipping over relative to the remaining workpiece portion 64.

[0054] According to the present invention, after the material bridging portion 68 is generated, the laser beam 5 can be turned off, while the cutting gas 7 continues to be directed onto the material heated by the laser beam 5 in the region of the material bridging portion 68. Although the same inert protective gas, particularly nitrogen (N2), is used when cutting and cooling the workpiece 6, it can be DS147293191-cn

[0055] Advantageous, but not mandatory. Alternatively, nitrogen (N2) can be initially used as the process gas for cutting, while switching to another inert gas, such as helium (He), as the cooling fluid for cooling the cut edge at the end of the kerf. Since helium (He) has good thermal conductivity, using helium (He) as the protective gas can further improve the cooling effect and shorten the cooling time compared to nitrogen (N2). Without considering oxidation, it is also conceivable that the method according to the invention be performed during combustion cutting using oxygen (O2) as the cutting gas, solely to achieve uniform cooling of the cut edge of the workpiece portion 62. In this case, the advantage of the method according to the invention can be particularly significant in increasing the uniformity of the cut edge.

[0056] The cooling process can be accelerated by increasing the gas pressure.

[0057] Once workpiece 6 has cooled sufficiently, laser beam 5 is switched back on, and workpiece portion 62 is completely separated by disconnecting the material bridging portion 68. The target temperature of workpiece 6 after cooling depends on the purpose of cooling and varies particularly with the material of workpiece 6. In particular, oxide formation on the cutting edge can be intentionally reduced when cutting stainless steel to avoid tempering at the cutting edge. Depending on the thickness of the workpiece, a significant reduction in tempering at the cutting edge can be achieved in less than one second of cooling time compared to conventional methods where workpiece 6 is cut out (and thus completely separated) without a cooling pause.

[0058] Figure 3a and Figure 3b Correspondingly, schematically shown is a configuration as a micro-connector ( Figure 3a ) and nano-connectors ( Figure 3b A cross-sectional view of the material bridging section 68. Figure 3a and Figure 3b The drawing planes extend along the kerf 66 parallel to the cutting edge 63 of the workpiece portion 62 (and thus perpendicular to the workpiece surface). To ensure sufficient stability for the material bridging portion 68 to hold or secure the workpiece portion 62 within the remaining workpiece, the material bridging portion must have a minimum required cross-section. The cross-section of the material bridging portion 68 can be approximated by its height h and width b. It should be understood that, in practice, the cross-sectional shape of the material bridging portion 68 is typically determined according to... Figure 3a and Figure 3b The simplified rectangular shape is somewhat deviated. Methods for determining non-rectangular cross-sections are also known to those skilled in the art.

[0059] Figure 3a A material bridging portion 68 configured as a micro-connector is shown, the height h of which corresponds to the thickness H of the workpiece 6. For comparison, Figure 3b The material bridging portion 68 configured as a nano-connector is shown, DS147293191-cn

[0060] The height h of the material bridging portion is less than the thickness H of the workpiece 6. In principle, the cross-section of the material bridging portion 68 should be kept as small as possible so that the heat generated when the material bridging portion 68 is disconnected is also kept as small as possible. In microjoints ( Figure 3a In the case of material bridging portion 68, the cross-section of the material bridging portion 68 is controlled solely by the width of the material bridging portion 68. Especially when cutting workpieces 6 with a large thickness H (e.g., exceeding 10 mm), the material bridging portion 68 is configured as a nano-bridging portion (…). Figure 3bThis can be advantageous. In this case, in addition to the width b, the cross-section of the material bridging portion can also be controlled by the height h of the material bridging portion. This increases the possibilities in configuring the material bridging portion with optimal dimensions.

Claims

1. A laser cutting method for cutting out a workpiece portion (62) from a workpiece (6) of metal, the method comprising the steps of: cutting the workpiece (6) along a predefined cutting trajectory by means of a laser beam (5) and a cutting gas (7) and using predefined cutting parameters; changing the cutting parameters at an end section of the cutting trajectory in such a way that a material bridge (68) remains between the workpiece portion (62) and a workpiece remainder (64) of the workpiece (6), wherein the material bridge (68) fixes the workpiece portion (62) in the workpiece remainder (64); cooling the workpiece (6) in the region of the material bridge (68) by means of the cutting gas (7) and / or by means of a suitable cooling fluid; and completely separating the workpiece portion (62) from the workpiece remainder (64) by breaking the material bridge (68) by means of the laser beam (5) and the cutting gas (7).

2. The method of claim 1, wherein, an inert gas is used as the cutting gas (7).

3. The method of claim 1 or 2, wherein, the cooling of the workpiece (6) comprises switching off the laser beam (5) for a predetermined period of time.

4. The method of claim 3, wherein, the cooling of the workpiece further comprises: loading the region to be cooled with the cutting gas (7) during the period of time in which the laser beam (5) is switched off, wherein the cutting gas pressure during the cooling period is greater than or equal to the cutting gas pressure during the cutting of the workpiece (6).

5. The method of claim 1 or 2, wherein, the material bridge (68) has a predetermined minimum cross section, such that the workpiece portion (62) is fixed in the workpiece remainder (64) in the original position of the workpiece portion (62) exactly during the cooling period.

6. The method of claim 1 or 2, wherein, the change in the cutting parameters comprises: switching off the laser beam (5) before reaching the end of the cutting trajectory when cutting the workpiece (6).

7. The method of claim 1 or 2, wherein, the change in the cutting parameters comprises: changing the cutting parameters such that the workpiece (6) is only melted up to a depth which is less than the workpiece thickness; and cutting the workpiece (6) using the changed cutting parameters up to the end of the cutting trajectory, such that the height of the material bridge (68) is less than the workpiece thickness.

8. The method of claim 1 or 2, wherein, the complete separation comprises: switching on the laser beam (5); and completely separating the workpiece portion (62) from the workpiece remainder (64) by breaking the material bridge (68) by means of the laser beam (5) and the cutting gas (7), wherein the cutting parameters are set such that less energy is coupled into the workpiece (6) when completely separating the workpiece portion (62) than when previously cutting the workpiece (6).

9. The method of claim 8, wherein, the setting of the cutting parameters when completely separating the workpiece portion (62) comprises reducing the laser power and / or increasing the cutting speed and / or changing the focal point position and / or the focal point diameter of the laser beam (5).

10. The method of claim 1 or 2, wherein, the workpiece (6) has a thickness of at least 4 mm.

11. The method of claim 1 or 2, wherein, the workpiece (6) is plate-shaped.

12. The method of claim 2, wherein, the inert gas is nitrogen.

13. The method of claim 3, wherein, the laser beam (5) is switched off for less than 5 seconds.

14. The method of claim 3, wherein, the laser beam (5) is switched off for less than one second.

15. The method of claim 1 or 2, wherein, the workpiece (6) has a thickness of at least 10 mm.

16. A laser cutting machine (1) for cutting a workpiece (6) of metal, wherein The laser cutting machine (1) is configured to perform a laser cutting method as claimed in one of claims 1 to 15.

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