TPi January 2011

The system used in this work is unusual in that its focusing lens had a long focal length of 500mm. The effect of this was to produce a very narrow beam of light, with a large depth of focus. This large depth of focus is a major contribution to the process of single-sided tube cutting. The laser beam was enclosed by a cutting nozzle and a nozzle tip with an exit diameter of about 5mm. In contrast to conventional laser cutting, for tube cutting, the laser beam focus is positioned about 90mm below the tip of the nozzle, allowing tubes up to 170mm in diameter to be cut from one side. The cutting process was also assisted by a jet of air at high pressure, which exited the nozzle concentric with the laser beam. This compressed air was necessary to blow away material in the kerf of the cut, melted by the laser beam, and is also particularly important for single-sided cutting in order to achieve separation of the tube. The cutting head can be seen in Figure 1. This cutting system could also be equipped with a camera, which looks directly through the cutting nozzle, focusing at approximately the same point as the laser beam. This is useful in remotely positioning the cutting head above the tube to be cut. For the work described here, the cutting head was manipulated by an articulated arm robot, although other deployment systems are possible. All movement of the process head and hence the laser beam, switching of the compressed air and control of the laser, was achieved through the robot controller. Using this equipment various options for single-sided tube cutting were possible. Stainless steel tubes from 25 to 170mm in diameter, with a range of wall thicknesses from 1.5 to 11mm, were cut using single pass, two pass and multiple pass techniques. Generally speaking, a two pass technique proved the most efficient for all but the smallest diameter tubes. If the cut edge of a tube is examined, it is clear that the quality of the cut at the side closest to the cutting head is much cleaner than at the opposite side. This is because, on the first

Figure 2: Cutting speed as a function of laser power for a 155mm diameter tube with a 1.5mm wall. Two pass cutting, assist gas pressure 8bars

Single-sided laser cutting Laser cutting of flat plate and tube are very well established manufacturing processes which account for the largest industrial use of high power lasers. The majority of work performed is the cutting of material up to about 20mm thick, with exceptional quality of the resulting edge. In conventional tube cutting, the tube rotates under the laser beam. For single-sided tube cutting, where the tube is fixed in space, alternative systems are required. The laser light arriving at the cutting head via the optical fibre first expands as it leaves the fibre and is then made parallel by a lens. Below this lens, a second lens then focuses the laser light to a very small spot to create the power density needed for cutting.

Figure 3: Results of cutting 60mm diameter tube at 4.6kW laser power and 8bar assist gas pressure

Figure 4: 170mm diameter stainless steel pipe with a 7mm thick wall, cut in a time of 7 minutes using a three pass technique

wall 1.5mm, speed 1,000mm/min

wall 4mm, speed 350mm/min

wall 8.7mm, speed 150mm/min

wall 11.1mm, speed 100mm/min

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January 2011 Tube Products International

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