ANALYSIS: FIBRE LASERS


Improving sheet metal cutting using a high-power fibre laser with variable beam quality


Paul Harrison, chief engineer for product applications at SPI Lasers, describes a new mode of operation for fibre lasers that enables beam quality to be varied on demand


The laser cutting of sheet metals is a well- established production method, with many thousands of cutting systems in operation worldwide. In the last 10 years solid-state fibre-coupled lasers such as fibre and disk systems have become the laser of choice, replacing the CO2


systems. Machines using either fibre or disk lasers


operate in a similar manner – a cutting head is used to focus the laser beam onto the sheet metal through a conical nozzle with co-axial high-pressure assist gas. Until now, such laser sources have used a fixed beam quality, and the cutting performance has been characterised over a wide range of output powers, identifying the range of suitable focal spot sizes which produce good cuts. The selection of a particular focal spot size for a given laser cutting system can be made using a ‘One Spot Size Fits All’ approach, where one lens combination is used to process all metal types and thicknesses. For a low-cost, straightforward machine this can work acceptably well, and the cutting performance for a given laser power can be well-defined for a range of sheet metals. However, this approach always involves


compromise because of the different metals being processed. Fusion cutting, typically using nitrogen assist gas, is used for cutting stainless steel and requires a smaller focal spot with low divergence. On the other hand, reactive gas cutting using oxygen assist gas is used for mild steel cutting, and requires a large focal spot with high divergence. It is difficult to optimise for both fusion


and reactive cutting when using the ‘One Spot Size Fits All’ approach. One option


16 LASER SYSTEMS EUROPE SUMMER 2019 @LASERSYSTEMSMAG | WWW.LASERSYSTEMSEUROPE.COM @researchinfo | www.researchinformation.info lasers used in many older


available to cutting machine builders is to use a more capable cutting head with a motorised zoom collimator and variable beam expander, which enables the focal spot size to be modified. However, as shown in figure 1, this does not provide the right combination of spot size and divergence as long as the laser beam quality is fixed, since a small focal spot will always have a higher divergence compared to a larger focal spot. In order to improve the cutting process for both fusion and reactive cutting, the laser beam quality needs to be variable. SPI Lasers has therefore developed a new option for its high-power lasers enabling variable beam quality to be added, known as variMODE. This is achieved by controlling the output beam divergence and thus beam quality. This is all done inside the laser, with no need for any external optics. Initially the laser has been fitted with a Ø100µm delivery


fibre, and the beam quality can be switched from 3.2mm.mrad (M2 = 9.5) to 5.8mm.mrad (M2 = 17), as shown in figure 2. The lower beam quality has been set to match the numerical apeture of standard commercially available cutting heads, to ensure that all of the laser power is delivered to the sheet metal and that beam clipping does not occur within the cutting head. The switching time from low to high beam quality is typically 30ms, which is fast enough to be changed between piercing and cutting.


Stainless steel cutting Cutting trials were made with a 3kW laser, comparing the variMODE performance to the 4.5mm.mrad beam quality of a standard laser. The variMODE laser was set to high beam quality (3.2mm.mrad)


“For the same laser power, the variable beam quality laser can out-perform the fixed beam quality system”


Figure 1: Focal spot size vs divergence for a laser with 4.5mm.mrad beam quality delivered through a Ø100µm fibre.


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