The metal sheets of cut parts are off-loaded by an operator. Thereafter, cut pieces are picked up out of the nest to divulge clean edges and the sheets are ready for the next operation. Isn’t it looking ideal? Quite true, but, burrs will remain. Such defects remain, its intensity may vary from minor to major, but an operator can reduce or avoid such burrs by adjusting the cutting parameters correctly. To discover perfect settings, operators have to know about the laser cutting processes in which assist gas and workpiece adjusted to create the perfect cut edges.
It is like revealing the secret of burr-free laser cutting. But is there any big secret behind it? No, of course not. Instead of that, there are strategies that revolve around one single component of laser cutting. This element is under the control of the operator and these elements stand for gas flow dynamics and its assistance through the kerf.
Understanding which parameter to change
Generally, the modern machine controls the laser beam elements like the beam profile and beam power (at the maximum). The beam focus that is used should consist of a particular grade, standards, and thickness based on optical focus. In order to match the true focus position on the workpiece for each lens, modern systems, automated machine and technicians usually check the ample number of parameters that includes beam delivery system, beam alignment, centering the nozzle, and precisely focusing the position and calibrating the nozzle. Under some circumstances, overflushing can occur, if focus spot is set too high in the cut and it can leave spiky dross, while too low cut can reduce the cutting speed, leaving beads.
Remaining parameters consist of gas pressure, commanded laser power-frequency duty, nozzle standoff, and cutting frequency. In modern systems, maximum parameters are automated which includes altering the nozzle to a larger or smaller diameter. It simply means that gas pressure, focus position and cutting speed are adjusted by the operator standing by the machine plant. It so happens that some of them may do whatever they find necessary to complete the job, and forget to adjust the parameters in the right direction.
If an operator observes a burr on the bottom of the stainless steel part, then firstly, the operator would slow down the cutting speed of the laser beam. This reaction is justified because in his perception cutting speed is too high and a problem arises from it. But after slowing down the gas pressure, the operator generally finds a bigger burr. So, here the creation of a burr depends on how the beam, gas, and material interact.
How are burrs formed?
To start with the basics, the intense energy emerged out of laser beam brings the metal above its melting point and thereby metal exceeds the melting temperature. During this process, a strong action of the assist gas removes the metal from the kerf. While using nitrogen, which is an inert gas, the cutting procedure totally depends on the beam’s energy to melt the metal. But when oxygen assist gas is used to cut the carbon steel, it seems that hot metal interacts with oxygen gas which creates an exothermic reaction and adds heat.
In both the cases, burrs are created due to molten metal which solidifies before it can be removed. That solid material becomes tougher at the bottom of the kerf, forming a burr.
Dynamics of the gas
Particularly, when it is concerned with nitrogen assist gas, the operator should ideally alter the efficiency, quality, and cost, respectively. Nitrogen assist gas can reach up to 35 to 50 per cent of the variable costs, so it is important to control the consumption of the gas while laser cutting. Hence, the first cutting parameter that should be considered is to minimise the nozzle diameter. It means you have to select the smallest nozzle diameter in order to achieve the desired quality of performance.
It should be noted that when it comes to assist gas flow, nozzle size and diameter, it makes a big difference. If the nozzle diameter is increased by a factor of 2, the flow rate of the gas will increase by a factor of 4. When you decide the smallest nozzle diameter size then you can easily determine the minimum pressure required to get a good quality of laser cutting with no burrs. Further, you can proportionally increase your flow rate with good molten-metal separation without going too high on pressure.
Slower doesn’t mean better
The ‘smaller and lower’ logic for nozzle diameter and gas pressure doesn’t apply to cutting speed. Hence, when operator slows the cutting speed, you end up injecting more heat than required in the kerf. Along with this, temperature also rises and causes vaporisation that disturbs the gas flow. As a result, this disturbance creates more burrs and the operator makes the quality worse instead of making it better.
But, the operator can actually save the material from burrs by precisely increasing the cutting speed. This increase in speed would minimise the heat and the ablation while restoring the gas flow dynamics to its appropriate state.
Oxygen cutting considerations
While shifting to oxygen cutting, the exothermic reaction should be accurately considered. Because oxygen-cutting carbon steel benefits from higher oxygen gas purity levels. It has been proven many times that, with both CO2 and fibre lasers, increasing oxygen global purity to 99.95% or above, you can increase the cutting speed in production significantly, sometimes between 30 and 40%.
It is necessary to check entire parameters set for achieving clean cut edge and avoid burrs in the metal sheet. It highly relies on the gas dynamic and the laser beam parameters. Hence, burr-free laser cutting is all about making sure that the beam parameters and gas dynamics work proportionally together to assure that the correct amount of molten metal evacuates from the kerf and the assist gas work simultaneously. ☐