Method for Laser Cutting with Optimized Gas Dynamics

Abstract

This disclosure relates to methods for cutting metal workpieces in sheet form with a thickness of at least 2 mm. A laser beam is positioned in a nozzle opening of a cutting gas nozzle configured to cut via the laser beam and a cutting gas so that a beam axis of the laser beam along a direction of propagation of the laser beam is at least a distance of 3 mm from a rear opening wall portion of the nozzle opening. Cutting gas configured for concurrently exiting the nozzle opening with the laser beam is emitted through the nozzle opening at a cutting gas pressure (p) of at most 10 bar.

Claims

1. A method of cutting a sheet metal workpiece having a thickness (d) of at least 2 mm, the method comprising positioning a laser beam in a nozzle opening of a cutting gas nozzle configured to cut via the laser beam and a cutting gas so that, at the nozzle end face, a beam axis of the laser beam along a direction of propagation of the laser beam is at least a distance of 3 mm from a rear opening wall portion of the nozzle opening; and causing the cutting gas configured to concurrently exit the nozzle opening with the laser beam to be emitted through the nozzle opening at a cutting gas pressure (p) of at most 10 bar.

2. The method according to claim 1, wherein positioning the laser beam in the nozzle opening for workpieces with the thickness (d) greater than 10 mm comprises positioning the beam axis of the laser beam at a distance of at least 4 mm from the rear opening wall portion of the nozzle opening.

3. The method according to claim 1, further comprising selecting an opening area of the nozzle opening on the workpiece side, a distance (A) of the nozzle end face from the upper surface of the workpiece surface, and the cutting gas pressure (p) such that the maximum speed of the gas flow when it enters the cutting gap does not exceed the speed of sound.

4. The method according to claim 1, wherein an opening diameter on the workpiece side or a length (a) of a long axis of the nozzle opening corresponds to 10 times to 30 times a cutting gap width (b).

5. The method according to claim 1, wherein an opening diameter on the workpiece side or a length (a) of a long axis of the nozzle opening is at least 7 mm.

6. The method according to claim 1, wherein a distance (A) from an upper surface of the workpiece to a nozzle end face is 0 mm to 0.5 mm.

7. The method according to claim 1, wherein a focus diameter of the laser beam is at least 1/30 of the workpiece thickness (d).

8. The method according to claim 1, wherein a focus diameter of the laser beam is at least 150 m.

9. The method according to claim 1, wherein the laser beam is focused on the workpiece surface.

10. The method according to claim 1, further comprising selecting a beam parameter product and a focus diameter of the laser beam such that the resultant Rayleigh length corresponds to 0.5 times to 1.0 times the workpiece thickness (d).

11. The method according to claim 1, wherein the method comprises a fusion cutting process causing at least one of nitrogen and a nitrogen-oxygen mixture to be emitted as the cutting gas.

12. The method according to claim 1, further comprising generating the laser beam via a diode laser that generates a multiple wavelength laser beam.

Description

DESCRIPTION OF DRAWINGS

[0026] FIG. 1 shows a longitudinal section through a cutting gas nozzle moved over a workpiece in sheet form during the laser cutting according to the invention.

[0027] FIGS. 2A and 2B show a view from below of the workpiece in sheet form and of the cutting gas nozzle from FIG. 1. The cutting gas nozzle shown in FIG. 2A has a round nozzle opening cross-section and the cutting gas nozzle shown in FIG. 2B has an oval nozzle opening cross-section.

[0028] FIG. 3 shows a laser-cutting machine for carrying out the methods according to the certain embodiments of the invention for the laser cutting of workpieces in sheet form.

DETAILED DESCRIPTION

[0029] In the following description of the drawings, identical reference signs are used for components that are the same or functionally the same.

[0030] The cutting gas nozzle 1 shown in FIG. 1 is used for the laser cutting of metal workpieces 2 in sheet form with a thickness d of at least 2 mm via a laser beam 3 and a cutting gas 4 exiting concurrently through a nozzle opening 5 of the cutting gas nozzle 1. The laser cutting method may be a fusion cutting process in which nitrogen or a nitrogen-oxygen mixture, such as compressed air, is used as the cutting gas 4.

[0031] The cutting gas nozzle 1 is moved over the workpiece 2 in the cutting direction 6, to cut an opening 7 in the workpiece 2. A distance A (not shown to scale) between a nozzle end face 8 disposed on the workpiece side and the workpiece surface 9 can be between 0 mm and 0.5 mm, in certain embodiments. The cutting gas 4 exiting through the nozzle opening 5a has a cutting gas pressure p of at most 10 bar. The beam axis of the laser beam 3 is denoted by reference 10. The focus of the laser beam 3 is advantageously arranged at a height above the workpiece surface 9 or on the workpiece surface 9 and the focus diameter of the laser beam 3 is at least of the workpiece thickness d, in particular embodiments. The beam parameter product and the focus diameter of the laser beam 3 are chosen such that the resultant Rayleigh length corresponds to 0.5 times to 1.0 times the workpiece thickness d, in some embodiments. The opening area of the nozzle opening 5 on the mouth side, the distance A of the nozzle end face 8 from the workpiece surface 9, and the cutting gas pressure p are chosen such that the maximum speed of the gas flow when it enters the cutting gap 7 does not exceed the speed of sound, in particular embodiments.

[0032] FIG. 2A shows the view from below of a cutting gas nozzle with a round opening cross-section, and FIG. 2B shows the view from below of a slot cutting gas nozzle with an oval opening cross-section. Alternatively, the opening cross-section of the slot cutting gas nozzle can also be rectangular. At the height A of the nozzle end face on the workpiece side, the beam axis 10 of the laser beam 3 is at a distance of at least 3 mm from the rear opening wall portion 11a, 11b of the nozzle opening 5a, 5b, respectively, in the cutting direction 6. Seen in the direction of the beam axis 10, the cutting gap 7 extending from the cutting front 12 is therefore covered or overlapped by the nozzle opening 5a, 5b of the cutting gas nozzle 1 over a length L of at least 3 mm. In the case of the round opening cross-section that is shown in FIG. 2A, the opening diameter of the nozzle opening 5a preferably corresponds to 10 times to 30 times the cutting gap width b and is at least 7 mm. In the case of the slot cutting gas nozzle 1 with an oval opening cross-section that is shown in FIG. 2B, the length a of the long axis of the nozzle opening 5b extending in the cutting direction 6 is preferably 10 times to 30 times the cutting gap width b, and is at least 7 mm.

[0033] A cutting gas pressure below 10 bar causes a small expansion of the cutting gas 4 exiting through the nozzle opening 5a. The resultant reduced speed of the flow has the effect of reducing the number and intensity of pressure impacts in the cutting gap 7. Due to the great coverage of the cutting gap 7 by the nozzle opening 5a, 5b, a breakaway of the cutting gas flow from the surface of the cutting front 12 occurs onlyif at allafter a greatest possible extent of penetration into the cutting gap 7. These effects make a higher cutting speed possible with at the same time a high quality of the cut edges or corners and also an increase in the maximum workpiece thickness d that can be cut. The great coverage of the cutting gap 7 by the nozzle opening 5a, 5b has the effect of reducing the formation of burrs and oxidations when there are abrupt changes of direction. The soft large-area gas flow from the large opening diameter of the cutting gas nozzle 1 makes it possible still to reliably expel even melt that is produced further behind in the cutting gap 7 downwards out of the cutting gap 7, and thereby avoid the formation of burrs when moving around curves with a lower rate of advancement.

[0034] FIG. 3 shows a laser cutting machine 20 suitable for carrying out the laser cutting method described.

[0035] The laser cutting machine 20 has, for example, a CO.sub.2 laser or solid-state laser and includes a diode laser as the laser beam generator 21, in certain embodiments. The laser cutting machine 20 includes a movable laser cutting head 22 and a workpiece support 23, on which the workpiece 2 is arranged. The laser beam 3 is generated in the laser beam generator 21 and is guided by means of a fiber-optic cable (not shown) or deflecting mirrors (not shown) from the laser beam generator 21 to the laser cutting head 22. The laser beam 3 is directed onto the workpiece 2 by means of a focusing optical system arranged in the laser cutting head 22. The laser-cutting machine 1 is additionally supplied with cutting gases 24, for example oxygen and nitrogen. The cutting gas 24 is fed to the cutting gas nozzle 1 of the laser cutting head 22, from which it leaves together with the laser beam 3. The laser-cutting machine 20 also comprises a machine controller 25, which is programmed to move the laser cutting head 22 together with its cutting gas nozzle 1 in relation to the workpiece 2 in a way corresponding to a cutting contour.

Other Embodiments

[0036] A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.