LASER CUTTING METHOD

Abstract

In a method for laser fusion cutting in particular a plate-shaped workpiece, preferably with a thickness D of at least 1 mm, a laser beam and a cutting gas, in particular nitrogen, at a cutting gas pressure are directed at the workpiece surface by a convergent cutting nozzle. The laser power is at least 6 kW and the cutting nozzle has a nozzle end face on the workpiece side. A distance A between the nozzle end face and the workpiece surface during the cutting operation is 2 to 8 mm. The cutting nozzle has a nozzle channel with a diameter d.sub.D at the nozzle end face on the workpiece side of 1.5 to 4 mm. The cutting gas pressure before emergence from the cutting nozzle is 15 to 30 bar. This makes it possible to achieve high productivity along with a reduced risk of collision, i.e. higher process reliability.

Claims

1. A method for laser fusion cutting a workpiece, which comprises the steps of: directing a laser beam and a cutting gas at a cutting gas pressure at a workpiece surface by means of a convergent cutting nozzle, the convergent cutting nozzle having a nozzle end face on a workpiece side; setting a laser power to be at least 6 kW; setting a distance between the nozzle end face and the workpiece surface during the laser fusion cutting to be 2 to 8 mm; setting a diameter of a nozzle channel of the convergent cutting nozzle at the nozzle end face on the workpiece side to be 1.5 to 4 mm; and setting the cutting gas pressure before emergence from the convergent cutting nozzle to be 15 to 30 bar.

2. The method according to claim 1, which further comprises maintaining the distance between the nozzle end face and the workpiece surface throughout a cutting process.

3. The method according to claim 1, which further comprises providing a single-channel nozzle or an annular die as the convergent cutting nozzle.

4. The method according to claim 1, wherein the cutting gas pressure before emergence from the convergent cutting nozzle is more than 18 bar.

5. The method according to claim 1, which further comprises moving the convergent cutting nozzle relative to the workpiece at least at times at a cutting speed of at least 60 m/min.

6. The method according to claim 1, wherein a focal position of the laser beam lies on the workpiece surface or in a workpiece half facing towards the convergent cutting nozzle.

7. The method according to claim 1, which further comprises setting the laser power during a cutting operation to be at least 10 kW.

8. The method according to claim 1, which further comprises carrying out the method on workpieces with a workpiece thickness of at least 4 mm.

9. The method according to claim 1, wherein the laser beam punctures the workpiece surface at least at one puncture point while the convergent cutting nozzle and the workpiece are being moved relative to one another.

10. The method according to claim 9, which further comprises reducing an advancement speed to a puncture speed at the at least one puncture point.

11. The method according to claim 10, which further comprises reducing the advancement speed to the puncture speed over a displacement distance of less than 2 mm with a result that the puncture speed is reached at the at least one puncture point.

12. The method according to claim 10, wherein after the laser beam punctures the workpiece surface, the puncture speed is maintained for a few milliseconds and then the advancement speed is increased back up to a cutting speed.

13. The method according to claim 1, wherein: the workpiece is a plate-shaped workpiece; the cutting gas is nitrogen; and the distance between the nozzle end face and the workpiece surface during the laser fusion cutting is 4 to 8 mm.

14. The method according to claim 1, wherein a focal position of the laser beam lies on the workpiece surface or in a workpiece half facing towards the convergent cutting nozzle and lies between 0.2 mm and 1.5 mm below a top side of the workpiece.

15. The method according to claim 9, which further comprises reducing an advancement speed to the puncture speed, by 10%-90% of a cutting speed, at the at least one puncture point.

16. The method according to claim 10, which further comprises reducing the advancement speed to the puncture speed over a displacement distance of less than 0.5 mm, with a result that the puncture speed is reached at the at least one puncture point.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0028] FIG. 1 is a diagrammatic, longitudinal section view through a cutting nozzle and through a plate-shaped workpiece during the laser fusion cutting operation;

[0029] FIG. 2 is a perspective view of a workpiece with a multiplicity of cut contour portions that has been processed on-the-fly;

[0030] FIG. 3 is a graph showing a change over time of an advancement speed and of laser power in a vicinity of a puncture point during the on-the-fly puncturing operation; and

[0031] FIG. 4 is a perspective view of a laser cutting machine for carrying out the laser fusion cutting method according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0032] In the following description of the figures, identical reference signs are used for identical or functionally identical components.

[0033] Referring now to the figures of the drawings in detail and first, particularly to FIG. 1 thereof, there is shown a convergent cutting nozzle 1 for laser cutting a plate-shaped metal workpiece 2 (a sheet) with a thickness D by means of a laser beam 3 and a cutting gas 24 (cf. FIG. 4). The cutting nozzle 1 contains a nozzle channel 5, which has a relatively small diameter d.sub.D of 1.5 to 4 mm at a nozzle end face 8 on the workpiece side. The cutting gas 24 and the laser beam 3 both emerge together from the nozzle channel 5 of the cutting nozzle 1. The laser beam 3 has a beam direction 6 which runs in the negative Z direction of an XYZ coordinate system. In the present case, the laser cutting process is a fusion cutting process, which makes use of nitrogen as the cutting gas 24.

[0034] According to the invention, a distance A between the nozzle end face 8 on the workpiece side and the workpiece surface 9 facing towards the cutting nozzle 1 is at least 2 mm, preferably at least 4 mm, in particular up to 8 mm. According to the invention, a focal position F of the laser beam 3 in the beam direction 6 is located within the thickness D of the workpiece 2, in the upper half of the workpiece 2 that faces towards the cutting nozzle 1 or on the workpiece surface 9 (the latter not being shown). Expressed differently, the focal position F of the laser beam 3 in the beam direction 6 is located in the workpiece 2 at a depth that is less than half D/2 of the thickness D of the workpiece 2.

[0035] The cutting nozzle 1 is moved over the workpiece 2 at a cutting speed in a cutting direction 7, which corresponds to the X direction of the XYZ coordinate system, in order to produce a kerf 4 in the workpiece 2.

[0036] FIG. 2 shows a workpiece 2 with many straight contour portions 11 (edges of a square) arranged in a line. To fusion cut such contours, at the start of each contour portion 11 the laser beam 3 punctures the workpiece surface 9 at a puncture point 10. To that end, the laser beam 3 is switched on at the puncture point 10 of the respective contour portion 11, moved along the contour portion 11 and switched off at the end of the contour portion 11.

[0037] When this operation is to be carried out on-the-fly, that is to say without stopping the cutting nozzle 1 at the puncture point 10, in the case of thick workpieces 2 it is advantageous to reduce the advancement speed of the cutting nozzle 1 upstream of the puncture point 10 (in the cutting direction). To that end, a laser cutting head with the cutting nozzle 1 is continuously moved linearly over the workpiece 2, wherein the advancement speed is reduced upstream of the puncture points 10 and increased back up again downstream of the puncture points 10.

[0038] FIG. 3 shows a possible sequence for adjusting the advancement speed and the corresponding laser power during the on-the-fly puncturing operation. The cutting speed v.sub.C can be, for example, 14.5 m/min at a laser power of 10 kW, and, for example, 25 m/min at a laser power of 20 kW. After the cutting I of a first contour portion 11 in the period of time t0 to t1, with the laser beam 3 switched on and the cutting speed v.sub.C as advancement speed, in order to subsequently position II the cutting nozzle 1 the cutting speed v.sub.C of the cutting nozzle 1 is first of all maintained (period of time t1 to t2). Just upstream of the next puncture point 10, the advancement speed of the cutting nozzle is reduced to a puncture speed v.sub.P for a period of time t2 to t3. When cutting construction steel with a workpiece thickness of 5 mm using a laser power of 10 kW, the puncture speed can be, for example, approximately 5 m/min, and using a laser power of 20 kW, the puncture speed can be, for example, approximately 10 m/min. The point in time t2 is preferably selected such that the distance between the point at which the reduction in the advancement speed is commenced (position of the cutting nozzle 1 at the point in time t2) and the next puncture point is at most 2 mm, preferably at most 0.5 mm. At the point in time t3, the cutting nozzle 1 reaches the puncture point at the puncture speed v.sub.P and the laser beam is switched on again to puncture the workpiece 2. The puncturing operation III takes place in the period of time t3 to t5. During the puncturing operation III, the puncture speed v.sub.P should be maintained preferably for as short as possible a time (period of time t3 to t4). After that, the advancement speed of the cutting nozzle 1 is increased back up to the cutting speed v.sub.C within the period of time t4 to t6. At the point in time t5, the puncturing operation III has ended, i.e. the laser beam has penetrated through the entire thickness of the workpiece. At the point in time t6, the cutting speed v.sub.C is reached again and the contour portion 11 can be fully cut at the cutting speed v.sub.C. The advancement speed is preferably reduced and increased linearly.

[0039] FIG. 4 shows a laser cutting machine 20 that is suitable for carrying out the laser fusion cutting method described above.

[0040] The laser cutting machine 20 comprises, for example, a solid-state laser or a diode laser as laser beam generator 21. The laser cutting machine 20 further has a displaceable (laser) cutting head 22, together with which the cutting nozzle 1 is moved, and a workpiece rest 23, on which the workpiece 2 is arranged. The laser beam 3 which is guided from the laser beam generator 21 to the cutting head 22 is generated in the laser beam generator 21. The laser beam 3 is directed at the workpiece 2 by means of a focusing optical unit arranged in the cutting head 22.

[0041] Moreover, the laser cutting machine 20 is supplied with cutting gas 24, in this instance nitrogen. To carry out the laser fusion cutting method according to the invention that is described above, nitrogen as cutting gas 24 is supplied to the cutting nozzle 1 of the cutting head 22, to be precise at an overpressure of approximately 15-30 bar (before the emergence of the cutting gas 24 from the cutting nozzle 1).

[0042] Further, the laser cutting machine 20 contains a machine controller 25 which is programmed to move the cutting head 22, together with its cutting nozzle 1, relative to the stationary workpiece 2 in accordance with a cutting contour. The machine controller 25 also controls the power of the laser beam generator 21, which is more than 6 kW and in particular is more than 10 kW in the case of the fusion cutting process described above. In this way, for example, given a workpiece thickness of 1.5 mm at 6 kW, a cutting speed (advancement) of 60 m/min or even higher can be reached, with the cutting speed increasing as the laser power increases.

[0043] The following is a summary list of reference numerals and the corresponding structure used in the above description of the invention.

LIST OF REFERENCE SIGNS

[0044] 1 Cutting nozzle

2 Workpiece

[0045] 3 Laser beam

4 Kerf

[0046] 5 Nozzle channel
6 Beam direction of the laser beam
7 Cutting direction
8 Nozzle end face
9 Workpiece surface
10 Puncture points
11 Contour portion
20 Laser cutting machine
21 Laser beam generator
22 Cutting head
23 Workpiece rest

24 Cutting gas

[0047] 25 Machine controller
F Focal position
D Workpiece thickness

A Distance

[0048] d.sub.F Laser beam diameter
d.sub.D Nozzle channel diameter