January 2011 Tube Products International
61
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.
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
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 8.7mm, speed 150mm/min
wall 4mm, speed 350mm/min
wall 11.1mm, speed 100mm/min
