Method for cutting closed contours into a platelike workpiece

Abstract

The invention relates to a method for cutting closed contours (11, 12, 13) into a platelike workpiece (10) by means of a laser beam, which is moved over the workpiece (10) by means of a machining head, the advancement speed of the machining head being reduced at a definable distance before the cutting of the contour (11, 12, 13) is completed.

Description

[0001] Method for cutting closed contours into a panel-shaped workpiece

[0002] Background of the invention

[0003] The invention relates to methods for cutting self-contained contours into a panel-shaped workpiece using a laser beam moved over the workpiece by means of a machining head.

[0004] Such self-contained contours can be incorporated into the workpiece, for example, as recesses for internal holes or the like. In this case, the good part is the remaining workpiece, while the cut-out contours are waste parts. Conversely, the cut-out parts are the good parts, and the remaining skeleton forms the waste part.

[0005] The path of the machining head is usually programmed so that the starting point and end point of the contour cut are identical. Once the end point is reached, the laser is switched off. However, the contour is cut free before the laser path reaches the starting point. Due to the weight of the cut-out parts, these can tip downward while the laser is still active. This results in irregularities in the contour in the area of ​​the starting and end points of the contour cut. These irregularities can be discoloration of the material due to excessive heat input, splashes of molten material, or contour defects. These irregularities are undesirable and may also require remachining of the finished part.

[0006] To mitigate these problems, DE 102015221 243 A1 proposes switching off the laser beam as soon as the cut-out part tilts after free cutting, i.e., before the starting point of the cutting path is reached again. However, due to the circular diameter of the laser beam, switching off the laser beam immediately after free cutting of the contour can lead to the formation of a nose, i.e., a small piece of material protruding beyond the cutting edge of the contour that is not captured by the laser beam if the laser beam is switched off before the starting point of the cut is reached again.

[0007] Object of the invention

[0008] The invention is based on the object of providing a method with which the irregularities occurring in the area of ​​the cut end when cutting closed contours can be further reduced.

[0009] Description of the invention

[0010] This object is achieved according to the invention by a method for cutting self-contained contours into a panel-shaped workpiece with a laser beam moved over the workpiece by means of a machining head, which is characterized in that the feed rate of the machining head is reduced at a predeterminable distance before reaching the cutting end of the contour.

[0011] The subclaims relate to preferred embodiments.

[0012] By reducing the feed rate of the machining head at a predefined distance from the programmed cutting end, the cutting front lag of the laser cut is reduced, so that the cutting front assumes an angle of almost 90° to the workpiece surface. This can reduce material discoloration in the area of ​​the cutting end. The programmed cutting end can preferably coincide with the starting point of the contour cut. The reduction of the feed rate can be initiated, for example, at a distance of 2 mm to 8 mm before reaching the cutting end. The feed rate of the machining head can preferably be reduced to 0.5 m / min, in particular to 0.1 m / min.

[0013] Preferably, the laser beam can also be pulsed and / or operated at reduced power at a predefined distance before reaching the cutting end of the contour. This measure can also further reduce discoloration of the material. It has proven advantageous to operate the laser beam at a pulse frequency of 100 Hz and a power between 400 W and 1500 W at the predefined distance before reaching the cutting end of the contour.

[0014] If the feed rate of the processing head and the pulse frequency and power of the laser beam are reduced simultaneously and linearly, material discoloration can be almost completely avoided due to the strong reduction in the energy per unit length of the laser cut.

[0015] Complete cutting of the contour can be detected by a distance measurement between the machining head and the workpiece and / or by an optical sensor device. If complete cutting is indicated by the distance measurement and / or the optical sensor device, the laser beam can be switched off. This prevents the laser beam from hitting a workpiece part that has already been cut free and tilted, which could lead to melt splashes or contour damage. The distance between the machining head and the workpiece can be measured capacitively, for example.

[0016] Process reliability can be increased if both a distance measurement and an optical control are used to check that the contour has been cut free, and to trigger the laser beam shutdown. If there is an abrupt change in the distance between the machining head and the workpiece and a simultaneous change in the intensity of the scattered light detected by the optical sensor device, the contour has been cut free can be reliably detected and the laser beam switched off. If a distance measurement signal is used alone, the laser beam may be switched off incorrectly, for example due to vibrations in the panel-shaped workpiece. To avoid these incorrect shutdowns, the scattered light in the area of ​​the cut end can be detected. Due to the reduced feed rate of the machining head in this area, the intensity of the scattered light increases.An increase in the intensity of the scattered light can thus be used to verify the plausibility of the detection of the contour being cut free by an increase in the distance measurement signal.

[0017] To avoid a nose at the cutting edge, at the start of the contour cut the laser beam can be moved a fraction of its diameter and perpendicular to the programmed cutting path from the machining head into the contour if this is the finished part to be produced, or into the remaining workpiece if this is the finished part, the machining head can be moved back to the starting point of the programmed cutting path, and the contour can be cut. While the machining head is moving back to the starting point of the programmed cutting path the laser beam can be switched off, but is switched on again before the subsequent cutting of the contour. Instead of a material nose, this creates a small depression in the cutting edge, which is generally easier to tolerate visually than a material protrusion.

[0018] Additionally or alternatively, before the contour is completely cut free, the laser beam can be moved a fraction of its diameter from the processing head into the contour if this is the finished part to be produced, or into the remaining workpiece if this is the finished part, and the contour cutting can be completed in this position. This measure also creates a small hollow in the cutting edge. Preferably, the laser beam can be moved into the finished part at an angle of 45° 5 mm before reaching the end point of the cut-free cutting. The movement of the laser beam into the finished part can be, for example, 0.1 to 0.2 mm, so that the hollow created in the cutting edge is barely noticeable.

[0019] The invention also relates to a laser cutting machine for machining panel-shaped workpieces, comprising a machining head for guiding a laser beam and a control device for regulating the feed of the machining head and the laser beam, wherein the laser cutting machine is configured to carry out a method according to the invention.

[0020] In particular, the laser cutting machine can be equipped with a device for measuring the distance between the processing head and a workpiece and with an optical sensor device for detecting scattered light.

[0021] Further features and advantages of the invention will become apparent from the description, the claims, and the drawings. According to the invention, the above-mentioned and further-described features can be used individually or in combination in any convenient way. The embodiments shown and described are not intended to be exhaustive, but rather are exemplary in nature for describing the invention.

[0022] Detailed description of the invention and drawing

[0023] Fig. 1 shows a schematic representation of a panel-shaped workpiece with bore cutouts produced by a laser beam;

[0024] Fig. 2 shows a schematic representation of the laser beam at the beginning and end of the cutting of a bore section of the workpiece from Fig. 1; Fig. 3 shows a diagram of the distance measurement of the machining head of a laser cutting machine for detecting the free cutting of a bore section;

[0025] Fig. 4 shows a schematic representation of the laser beam at the beginning and end of cutting a bore section with a measure to avoid / reduce a starting nose in the bore section; and

[0026] Fig. 5 shows the entire path of the laser beam for cutting a

[0027] Hole section with the measure from Fig. 4.

[0028] Fig. 1 shows a panel-shaped workpiece 10, for example a steel sheet, from which two circular bore cutouts 11, 12 have been cut out. In addition, the path 14, 15 of a laser beam 17 (Fig. 2) for cutting a third bore cutout 13 into the workpiece 10 is drawn. For this purpose, the laser beam 17 is first pierced in the center of the bore cutout 13 and then the laser beam 17 is moved outwards on a radial path 14 until the desired outer diameter of the bore cutout 13 is reached at point 16. From point 16 onwards, the laser beam 17 is guided on a circular path 15 until the material of the bore cutout 13 is completely cut free shortly before reaching point 16 and falls out of the workpiece 10.

[0029] At point 16, the workpiece 10 experiences a relatively high heat input from the laser beam 17, as the laser beam initially strikes point 16 from the radial path 14 and is then deflected onto the circular path 15. At the end of the circular path 15, heat is again input from the laser beam 17 in the area of ​​point 16. In addition, the bore cutout 13 can tilt downward after cutting. This leads to irregularities 11.1, 12.1 in the cutting edges 11.2, 12.2 of the bore cutouts 11 to 13, as shown in the example of the bore cutouts 11 and 12. These irregularities can be material discoloration, splashes of melt, or noses 18 (Fig. 2).

[0030] To minimize these irregularities 11.1, 12.1, the feed rate of the laser beam 17 is reduced from V to V' before the circular path 15 is completed, i.e. before the bore cutout 13 is completely cut free, as shown in Fig. 2. The speed V is preferably only 0.5 m / min. The greatly enlarged and schematic illustration in Fig. 2 shows the circular cross-section of the laser beam 17 at the start of the circular path 15 and 17' at the end of the circular path 15. When the position 17' is reached, the bore cutout 13 is completely cut free, and the laser beam 17' can be switched off at this position. However, a so-called approach nose 18 of the workpiece 10 remains and projects into the bore cutout 13. This nose 18 is caused by the circular cross-section of the laser beam 17, 17' and the switching off of the laser beam 17' before reaching the starting position 17 again.

[0031] In addition to reducing the feed rate V, the power of the laser beam 17' can also be reduced, for example, to 1500 W to 400 W. Furthermore, the pulse frequency of the laser beam 17' can also be reduced from, for example, 20,000 Hz to 100 Hz. These measures lead to a reduction in the energy per unit length and thus to a further reduction of the irregularities 11.1, 12.1 at the cutting edges 11.2, 12.2.

[0032] To reliably detect the cutting of the bore section 13 and thus the possibility of switching off the laser beam 17' in time to avoid unnecessary heat input into the workpiece 10, the distance 19 between the processing head for guiding the laser beam 17 and the workpiece 10 can be monitored in an end section of the path 15 of the laser beam 17, as shown in Fig. 3. When the bore section 13 has been completely cut free, it falls out of the workpiece 10. This increases the distance between the workpiece 10 and the laser processing head. If a threshold value SA is exceeded by the distance 19, the laser beam 17' is switched off. The distance measurement can also be verified by an optical measurement signal. By reducing the feed rate from V to V, the intensity of the scattered light on the laser path 15 increases, which can be detected, for example, by a photodiode.If an increase in distance 19 above threshold SA coincides with an increase in the intensity of the scattered light, it can be safely assumed that the hole section 13 has been cut free and the laser beam 17' can thus be switched off. The programmed end of the path of the machining head is reached at time tA, i.e., after the laser beam is switched off.

[0033] Fig. 4 and Fig. 5 illustrate a measure with which the approach nose 18 from Fig. 2 can be reduced to a smaller nose 18'. For this purpose, the laser beam 17' is moved into the workpiece 10 in an end section 15.1 of the laser path 15 by a distance d of, for example, 0.1 to 0.2 mm. Fig. 4 shows the end position 17' of the laser beam when switched off and the reduced approach nose 18'. Fig. 5 shows the entire programmed laser path 15' with the end section 15.1' running obliquely into the workpiece 10. The end point 20 of the laser beam 17' reached in this way is located radially outside the starting point 16 of the laser path 15'. This creates a small depression in the workpiece 10. The approach nose 18, 18' can even be completely avoided if the radial laser path 14 shown in Fig. 1 is additionally extended to the radial distance of the point 20 before cutting the circular path 15 begins.

[0034] In the embodiment shown in the figures, the workpiece 10 is the good part, while the bore cutouts 11, 12 and 13 form waste parts. The method according to the invention with the various measures for reducing the irregularities 11.1 and 12.1 can be carried out in an analogous manner if the cutouts 11, 12 and 13 are the good parts and the remaining workpiece 10 is the waste part. In this case, the laser beam 17 is fed radially from the outside onto the circular path 15 in Fig. 1. To reduce approach noses 18, before the cutouts 11, 12 and 13 are cut free, the laser beam 17 is moved into these good parts 12 to 13 and not into the workpiece 10. The changes to the laser beam parameters such as feed rate, power and pulse frequency are independent of which parts are the good parts.Likewise, the detection of the free cutting of cutouts 11 to 13 via a distance measurement with optional optical plausibility check and thus the automatic switching off of the laser beam remains unchanged if cutouts 11 to 13 are the good parts.

Claims

Patent claims 1. Method for cutting self-contained contours (11, 12, 13) into a panel-shaped workpiece (10) with a laser beam (17, 17') moved over the workpiece (10) by means of a machining head, characterized in that the feed rate (V') of the machining head is reduced at a predeterminable distance before reaching the cutting end of the contour (11, 12, 13).

2. Method according to claim 1, characterized in that the feed rate (V') of the machining head is reduced to 0.5 m / min, in particular to 0.1 m / min.

3. Method according to claim 1 or 2, characterized in that at a predeterminable distance before reaching the cutting end of the contour (11, 12, 13) the laser beam (17, 17') is pulsed and / or operated with reduced power.

4. Method according to claim 3, characterized in that the laser beam (17, 17') is operated at the predeterminable distance before reaching the cutting end of the contour (11, 12, 13) with a pulse frequency of 100 Hz and a power between 400 W and 1500 W.

5. Method according to one of the preceding claims, characterized in that the feed rate (V') of the machining head and the pulse frequency and power of the laser beam (17, 17') are reduced simultaneously and linearly.

6. Method according to claim one of the preceding claims, characterized in that the complete cutting of the contour (11, 12, 13) by a distance measurement between the machining head and the workpiece (10) and / or by an optical sensor device.

7. Method according to claim 6, characterized in that the distance (19) between the machining head and the workpiece is measured capacitively.

8. Method according to claim 6 or 7, characterized in that in the event of an abrupt change in the distance (19) between the machining head and the workpiece (10) and a simultaneous change in the intensity of the scattered light detected by the optical sensor device, the free cutting of the contour (11, 12, 13) is detected and the laser beam (17, 17') is switched off.

9. Method according to one of the preceding claims, characterized in that at the start of the contour cut (15) the laser beam (17) is moved a fraction of its diameter and perpendicular to the programmed cutting path from the machining head into the contour (13) if this is the good part to be produced, or into the remaining workpiece (10) if this is the good part, and the machining head is moved back to the starting point of the programmed cutting path (15) and the contour (13) is cut.

10. Method according to one of the preceding claims, characterized in that before the contour (13) is completely cut free, the laser beam (17') is moved a fraction of its diameter from the machining head into the contour (13) if this is the good part to be produced, or into the remaining workpiece (10) if this is the good part, and in this position the cutting free of the contour (13) is completed.

11. Method according to claim 10, characterized in that the laser beam is moved into the workpiece (10, 13) at an angle of 45° 5 mm before reaching the end point of the free cutting.

12. Method according to one of claims 9 to 11, characterized in that the laser beam is moved 0.1 to 0.2 mm into the good part (10, 13).

13. Laser cutting machine for machining panel-shaped workpieces (10) with a machining head for guiding a laser beam (17, 17') and a control device for regulating the feed rate (V, V') of the machining head and the laser beam (17, 17'), characterized in that it is designed to carry out a method according to one of claims 1 to 12.

14. Laser cutting machine according to claim 13, characterized in that it is equipped with a device for measuring the distance (d) between the processing head and a workpiece (10) and with an optical sensor device for detecting scattered light.

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