Method and device for laser cutting

Abstract

The invention relates to a method for laser cutting (22) a self-contained cutting contour (32) into a workpiece (12) by means of a laser beam (20) emerging from a laser cutting head (22) at a laser power, said method comprising the following steps: dividing the cutting contour (32) into a main portion (34) and an end portion (36) which follows the main portion (34); laser cutting (22) the cutting contour (32) at a main power and detecting an actual distance (dIst) between the laser cutting head (22) and the workpiece (12), wherein a minimum actual distance (dIst,min) and a maximum actual distance (dIst,max) are detected in the main portion (34), and wherein a check is performed in the end portion (36) as to whether the actual distance (dIst) falls below a lower limit value (Gu) which characterises the minimum actual distance (dIst,min) and / or exceeds an upper limit value (Go) which characterises the maximum actual distance (dIst,min); and, if the actual distance (dIst) falls below the lower limit value (Gu) and / or exceeds the upper limit value (Go), reducing the laser power to at most 10% of the main power.

Description

[0001]Title: Method and Device for Laser Cutting Description The invention relates to a method and a device for laser cutting. From DE102015221243B4, a method for laser cutting closed contours is known, wherein the laser power is reduced or the laser is switched off before reaching the endpoint or returning to the starting point of the contour. In this method, the distance between a laser cutting head and a workpiece is controlled to a target distance in an end section of the contour in order to maintain a constant actual distance even with a moving slug. When cutting a slug from a workpiece, the slug tilts before the laser beam reaches the endpoint or returns to the starting point. It is a disadvantage that the laser continues to strike the slug with a main power output even then,when the slug has already tipped over or completely detached. This leads to irregularities, melt spatter, or lead-in marks, etc., at the cut end. It is therefore advantageous to perform cut end detection to reduce the laser power and to avoid irregularities at the cut end. The cut end detection known from the prior art has the disadvantage that vibrations of the workpiece occurring during the cutting process, which influence the actual distance, can trigger a false cut end detection. The invention is based on the objective of providing a method for laser cutting with reliable cut end detection. The objective of the invention is achieved by a method with the features of claim 1. The invention is directed to a method for laser cutting a closed cut contour into a, in particular metallic,The workpiece is cut using a laser beam exiting a laser cutting head with a laser power, comprising the following steps: dividing the cutting contour into a main section and a follow-up end section; laser cutting the cutting contour with a main power and detecting an actual distance between the laser cutting head and the workpiece, detecting a minimum actual distance and a maximum actual distance in the main section, and checking in the end section whether the actual distance falls below a lower limit characterizing the minimum actual distance and / or exceeds an upper limit characterizing the maximum actual distance; and, if the actual distance falls below the lower limit and / or exceeds the upper limit,Reduce the laser power to a maximum of 10% of the main power. By detecting a lower and / or upper limit for end-of-cut detection, the natural vibration band specific to the workpiece can be detected in real time during the laser cutting process in the main section. As long as the actual distance remains within this vibration band, it can be assumed that the slug has not yet tilted and is only vibrating naturally. Only when the actual distance is outside the vibration band can it be assumed that the slug has detached from the rest of the workpiece beyond its natural vibration or has tilted. Furthermore, it is advantageous to determine both a lower and an upper limit so that tilting towards and away from the laser cutting head can be taken into account. Consequently, more accurate end-of-cut detection is provided.which prevents false and premature cut end detection triggered by vibration. After the laser cutting process, the workpiece is divided into the slug and the blank, with the blank being held and the slug tipping or falling off as it separates from the blank. The cut contour preferably consists exclusively of the main section and the end section. The cut end, i.e., the falling or tipping of the slug, is to be expected in the end section. The ratio between the main section and the end section is preferably in a range between 95:5 and 70:30, particularly between 90:10 and 75:25. An advantageous aspect of the invention provides that, according to step c), the laser power is reduced to a maximum of 5%, particularly to a maximum of 2%, and preferably to 0%. This further ensures that no irregularities occur at the cut end of the blank. An advantageous aspect of the invention provides thatthat the lower limit corresponds to the minimum distance and / or the upper limit corresponds to the maximum distance. Accordingly, the limits are set precisely to the vibration band, which is a simple solution for accounting for the vibration. Alternatively, it can be provided that the lower limit results from a difference in the minimum distance. G, = A - ^Alternatively, the upper limit can be calculated as the sum of the maximum distance and an offset. G = A + ^ This allows for a safety factor to be considered, which absorbs any larger vibration in the end section that exceeds the vibration in the main section. An advantageous aspect of the invention provides that the offset lies in a range between 2% and 70%, particularly between 5% and 50%, preferably between 7% and 30%, and more preferably between 10% and 20%, of the actual distance. For example, with an actual distance of 4 mm, the offset is 0.6 mm. Another advantageous aspect of the invention provides that the actual distance between the laser cutting head and the workpiece is detected using a non-contact, particularly capacitive, measuring method. This allows the actual distance to be detected easily and without influencing the process.An advantageous aspect of the invention provides that, in the main section, if the actual distance deviates from a target distance, the actual distance is regulated by means of a distance control so that it approaches the target distance. Thus, a nearly constant actual distance is ensured in the main section, leading to an optimal cutting result. An advantageous aspect of the invention provides that the distance control is deactivated in the end section. This allows the end-of-cut detection to optimally identify the end of a cut. Alternatively, end-of-cut detection can also occur with the distance control activated. An advantageous aspect of the invention provides that, according to step c), the laser power is reduced before reaching a programmatic endpoint or before returning to the starting point of the cutting contour. This reliably minimizes irregularities at the end of the cut.An advantageous aspect of the invention provides that the cutting contour is divided into a main section and a final section depending on one, several, or all of the following parameters: - thickness of the workpiece, - density of the workpiece, - area of ​​the slug to be cut from the workpiece, - type of cutting process, - nozzle spacing, in particular actual and / or target nozzle spacing, - nozzle shape, - feed rate, - laser power, and / or - the gas force of a cutting gas acting on the workpiece, in particular on the slug. At the actual point of release, the moment M acting on the slug is greater than the holding moment M of the slug.M < M The acting moment M at the actual free-body point results from the weight force F and the gas force F in conjunction with the respective lever arm l between the respective point of application and the actual free-body point: M = (F * l ) + (F * l ) ^ (F + F ) * l The holding moment M is proportional to the section modulus W of the slug and can be determined according to the known formulas for rectangular, square and circular cross-sections, such as: M. ^ W = B * H / 6The holding torque M can be calculated from the section modulus W, the resulting bending stress σ, and the tensile strength R as follows: M = W * σ / R. Therefore, the following condition results for the critical bending stress σ at which the slug is actually cut free: σ = (F + F ) * l * R / W. The gas force preferably acts on the slug with approximately half the area of ​​the cutting nozzle opening. The gas force is calculated as follows, taking into account the density of the gas ^, the effective area of ​​the gas on the slug A, the velocity of the gas v, the radius of the cutting nozzle opening r, and the cutting width b: F = ^ * A * v A = 1 / 2 * (^ * r – b * 2r ). Thus, a preliminary cut end can be predicted with particular accuracy. Depending on the expected cut end, the cut contour can then be precisely divided into the main section and the end section. The main section preferably ends before the expected cut end.This ensures that the cut end occurs within the end section and can be detected by means of cut end detection. The problem underlying the invention is also solved by a device with the features of claim 11. The invention relates to a device for laser cutting a closed cut contour into a metallic workpiece, the device comprising: a laser source with a laser cutting head for generating a laser beam emerging from the laser cutting head with a laser power; a distance sensor system for detecting an actual distance between the laser cutting head and the workpiece; and a control unit for controlling the laser beam, wherein the control unit is configured such that the device performs the method described above.An advantageous aspect of the invention provides that the device includes a storage unit for storing a minimum actual distance and / or a maximum actual distance and / or a lower limit value and / or an upper limit value. Accordingly, the vibration band and the limit value can be stored in the storage unit by the control unit during the main section and subsequently recalled in the final section. An advantageous aspect of the invention provides that the device includes a control unit for regulating the actual distance in the main section and / or in the final section. An advantageous aspect of the invention provides that the distance sensor system is contactless, in particular capacitive. Further advantages, features, and details will become apparent from the following description, in which various embodiments of the invention are illustrated with reference to the drawing.The features mentioned in the claims and the description can each be essential to the invention individually or in any combination. The figures show: a diagram of the actual distance with end-of-cut detection without considering vibrations; a section of a slug according to the process sequence without considering vibrations; a section of a slug according to the process sequence taking vibrations into account; and a schematic view of a laser cutting device. The device 10 for laser cutting 22 of a workpiece 12, in particular for separating a slug 14 from a blank 16, has, according to the figure...4. A laser source 18 for generating a laser beam 20 with a laser power, a laser cutting head 22 for directing the laser beam 20 onto the workpiece 12, a control unit 24 for controlling the device 10, in particular the laser source 18 and its laser power, a distance sensor system 26 for detecting an actual distance d between the laser cutting head 22 and the workpiece 12, a storage unit 28 for storing distance data, and a control unit 30 for controlling the actual distance d with respect to a target distance d. Preferably, a gas 31 flows from the laser cutting head 22 towards a melt pool generated on the workpiece 12 by the laser beam 20, whereby material with a cutting width 38 is removed from the workpiece 12 by means of the laser beam 20. Fig. 1 shows the actual distance d between the laser cutting head 22 and the workpiece 12 over the process time.The laser cutting head 22 and the workpiece 12 approach each other until the actual distance d equals the target distance d. The laser cutting process then begins, with the laser beam 20 tracing a cutting contour 32, separating the slug 14 from the panel 16. With a closed cutting contour 32, the laser beam 20 can travel from the starting point to the endpoint, with the laser power remaining constant or only being switched off upon reaching the starting point again (see arrow P1 in Fig. 1). This leads to undesirable irregularities at the end of the cut. As a further development, a cut-end detection system can be used, in which the cutting contour 32, as shown in Figs. 1 to 3, is divided into a main section 34 and an end section 36. The control unit 24 activates cut-end detection at the transition from the main section 34 to the end section 36. This compares the actual distance d and the target distance d in the final section 36.If the actual distance d deviates sufficiently from the target distance d, or if the change in the actual distance d over time is sufficiently large, the control unit 24 detects a tilting or a cutting free of the slug 14 (see arrow P2 in Fig. 1). However, this does not take into account that the workpiece 12 vibrates during the process, particularly during the main section 34 and the end section 36. According to arrow P2, the end-of-cut detection therefore detected the end of the cut too early. The change in the actual distance d was a result of vibration of the workpiece 12, especially of the slug 14. This premature detection of the end of the cut leads to an unsatisfactory cutting result as shown in Fig. 2, where the slug 14 remains connected to the blank 16. Therefore, the vibration is taken into account in the end-of-cut detection according to the invention.During the main section 34, the actual distance d is detected by the distance sensor system 26, and at least the minimum and maximum actual distances d are stored in the memory unit 28 by the control unit 24 or by the distance sensor system 26. The cut-end detection is also activated during the transition from the main section 34 to the end section 36. This checks whether the actual distance d falls below a lower limit G or exceeds an upper limit G. The lower limit G characterizes the minimum detected actual distance d in the main section, and the upper limit G characterizes the maximum detected actual distance d in the main section. In a first embodiment, the lower limit G corresponds to the stored minimum actual distance d, and the upper limit G corresponds to the stored maximum actual distance d. Accordingly, as shown in Fig. 1, a cut end is detected at arrow P3.In a second embodiment, the lower limit value G corresponds to the difference between the stored minimum actual distance d and an offset ^, and the upper limit value G corresponds to the sum of the stored maximum actual distance d and the offset ^. Accordingly, as shown in Fig. 1, a cut end is detected at arrow P4. In both cases, an optimal cutting result can be achieved, with Fig. 3 showing such a slug 14. The cut end detection is activated in the end section 36 and detects the tipping or cutting of the slug 14 from the utility 16 at the actual cut point. From the cut point, the laser power is reduced to a maximum of 10% of the main power in the main section 34, or the laser beam 20 is switched off completely. Furthermore, in the main section 34, the actual distance d can be controlled to the target distance d by means of the control unit 30. In the end section 36, the control can be continued or switched off.Reference symbol list d Actual distance d Minimum actual distance in the main section d Maximum actual distance in the main section d Target distance G Lower limit G Upper limit F Actual clearance point 10 Device 12 Workpiece. 14 Butzen 16 Nutzen 18 Laser source 20 Laser beam 22 Laser cutting head 24 Control unit 26 Distance sensor system 28 Storage unit 30 Control unit 32 Cutting contour 34 Main section 36 End section 38 Cutting width

Claims

Claims 1. Method for laser cutting (22) a self-contained cutting contour (32) into a workpiece (12) by means of a laser beam (20) emerging from a laser cutting head (22) with a laser power, comprising the following steps: a) Dividing the cutting contour (32) into a main section (34) and a final section (36) following the main section (34); b) Laser cutting (22) of the cutting contour (32) with a main power and detection of an actual distance (d ) between the laser cutting head (22) and the workpiece (12), - wherein in the main section (34) a minimum actual distance (d ) and a maximum actual distance (d ) are detected, and - wherein in the final section (36) it is checked whether the actual distance (d ) falls below a lower limit (G) characterizing the minimum actual distance (d ) and / or exceeds an upper limit (G) characterizing the maximum actual distance (d );and c) If the actual distance (d) falls below the lower limit (G) and / or exceeds the upper limit (G), reduce the laser power to a maximum of 10% of the main power.

2. Method according to claim 1, wherein according to step c) the laser power is reduced to a maximum of 5%, in particular to a maximum of 2%, and preferably to 0%.

3. Method according to claim 1 or 2, wherein the lower limit (G) corresponds to the minimum actual distance (d) and / or the upper limit (G) corresponds to the maximum actual distance (d).

4. A method according to claim 1 or 2, wherein the lower limit value (G) results from the difference between the minimum actual distance (d) and an offset (^), and / or the upper limit value (G) results from the sum of the maximum actual distance (d) and an offset (^).

5. A method according to claim 4, wherein the offset (^) lies in a range between 2% and 70%, in particular between 5% and 50%, and preferably between 10% and 30%, of the actual distance (d).

6. A method according to any one of the preceding claims, wherein the actual distance (d) between the laser cutting head (22) and the workpiece (12) is detected by means of a non-contact, in particular capacitive, measuring method.

7. Method according to one of the preceding claims, wherein in the main section (34) in the event of a deviation of the actual distance (d ) from a target distance (d ), the actual distance (d ) is regulated by means of a distance control such that the actual distance (d ) approximates or corresponds to the target distance (d ).8.Method according to claim 7, wherein the distance control is switched off in the end section (36).

9. Method according to any of the preceding claims, wherein, according to step c), the reduction of the laser power takes place before reaching a programmatic endpoint of the cutting contour (32).

10. Method for laser cutting (22) a self-contained cutting contour (32) into a workpiece (12) by means of a laser beam (20) exiting a laser cutting head (22) with a laser power, in particular according to any of the preceding claims, wherein the cutting contour (32) is divided into a main section (34) and an end section (36). depending on one, several or all of the following parameters: - thickness of the workpiece (12), - density of the workpiece (12), - area of ​​the slug (14) to be cut out of the workpiece (12), - type of cutting process, - nozzle distance, - nozzle shape, - feed rate, - laser power and / or - gas force of a cutting gas acting on the workpiece (12), in particular on the slug (14).11.Device (10) for laser cutting (22) a closed cutting contour (32) into a workpiece (12), the device (10) comprising: - a laser source (18) with a laser cutting head (22) for generating a laser beam (20) emerging from the laser cutting head (22) with a laser power; - a distance sensor system (26) for detecting an actual distance (d) between the laser cutting head (22) and the workpiece (12); and - a control unit (24) for controlling the laser beam (20), wherein the control unit (24) is configured such that the method according to one of the preceding claims is carried out.

12. Device (10) according to claim 11, wherein the device (10) has a storage unit (28) for storing a minimum actual distance (d) and / or a maximum actual distance (d) and / or a lower limit (G) and / or an upper limit (G).

13. Device (10) according to claim 11 or 12, wherein the device (10) has a control unit (30) for controlling the actual distance (d) in the main section (34).

14. Device (10) according to any one of claims 11 to 13, wherein the distance sensor system (26) is contactless, in particular capacitive.

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