Laser cutting is slowly becoming a cornerstone of the metalworking industry. Unlike conventional machining, it has many advantages. However, setting up a laser cutting machine correctly requires some knowledge. Different metals have different properties. Aluminum reacts very differently than mild steel. Sheet metal is very different than 3D workpieces.
So the operator does need some knowledge to adjust when cutting is not done as expected. In this article we are going to talk about the most common problems in laser cutting and their solution. We divided these problems into 2 groups: problems if there is a cut and problems if the laser beam does not (or not sufficiently) cut through the material.
If you are not familiar with the terminolgy used, be sure to check out this article first, where we explain the most common terms and principles of laser cutting:
Table of Contents
- Cutting quality at different process parameters.
- Problems if there is a cut
- Problems with no cut
- Problems with inadequate cutting
Cutting quality at different process parameters.
First of all, it is important to know what process parameters affect cut quality and what to expect if this parameter is set too high or too low. That’s the first step in putting you on the right path to solving your problem.
If the power delivered by your laser is set too high, you will notice this first of all in an increase in the width of the cut. You will suffer from burrs, taper and see wavy ridges (striations) in the cut.
If the power is too low, the cut width will decrease or there will be no cut.
If you set your cutting speed too high there will not be enough time for the laser beam to do its job. You will see a decrease in the cutting width and the roughness will increase (striations). Sometimes there is no cut at all.
If the cutting speed is too low, you will again see an increase in the cutting width and there will be burr formation and taper.
The focus position is also important when cutting. If it is too high you will see a decrease in the cutting width or no cutting. The roughness will also increase. If the focus position is too low, burr formation and taper occur. The cut width increases.
Pressure cutting gas
If the pressure of your cutting gas is too high, you will have to deal with deep ridges (striations) in the cutting surface. A wide and/or burned cut on the underside is one of the most important characteristics. If the pressure of the cutting gas is too low, the cut material is not discharged properly, resulting in burr formation. Sometimes there is no cut.
Nozzle plate distance
If the distance between the nozzle and the plate is too high, burr formation occurs. If it is too low, however, you are faced with a high level of roughness (deep striations).
Diameter nozzle opening
If the diameter is too high, you get burr formation and very high gas consumption. If the diameter is too low, alignment of the nozzle to the laser beam becomes more difficult. This may result in insufficient expulsion of the molten gas. Sometimes this causes the lack of a cut.
Problems if there is a cut
Cutting width too large
If the cutting width is too large, there will usually also be burr formation. In this case, be sure to check the following things:
- Is the focus position optimal? If the focus point is placed too deep below the product surface, you will get a cut width that is too large at the top. If the focus point is placed too high above the product surface, the cut width will be too large at the bottom.
- Is the average laser power too high?
- Is the cutting speed too low?
The cut makes an angle with the surface
In this case, you should take a close look at the alignment of the laser beam with the focusing optic. It should be aligned coaxially. If a focusing laser is used to check this, make sure it is coaxially aligned with the laser beam.
Too much burr
If you find that too much burr is occurring, be sure to look at the above points first. Too much cutting width very often results in burr. If this is not the problem, then the problem is probably due to your cutting gas, which does not expel the molten metal sufficiently. Check the following parameters:
- Cutting pressure too high or too low? Check the cylinder pressure and the setting of the pressure regulator (reducing valve). Check that there is no leak in the gas hoses or tubes. Laser welding often creates spatters and sparks, these can always cause damage. Also check for loose fittings.
- Too large a nozzle-plate distance? This means that not enough gas is blown through the cut, thus creating burr.
- Nozzle diameter too large (or became too large due to wear). As a result, the gas will not be blown through the cut precisely enough. A lot of gas will be lost as a result.
Irregular burr (on one side of the cut)
If you only have burrs on one side of the cut, this will usually also change sides if the cutting direction changes. Check the following parameters:
- Laser beam misaligned with nozzle opening?
- Check to see if the nozzle opening is partially clogged (or blocked) by splashes or other contaminants.
- Is the nozzle opening damaged? Collisions with workpieces or fixtures can damage the nozzle opening or cause misaligned laser beams.
- Is the intensity distribution of the laser beam sysmmetric?
Cutting edge too rough (deep striations)
A cut that is too rough is usually characterized by deep striations. These occur when the molten metal is driven out of the cut by a too turbulent gas flow. Workings are thus created and striations will form as a result. Check the following points:
- Is the gas pressure too high?
- Is the nozzle-plate distance too small?
Combustion of the cut
If there is too much oxygen reacting with the molten metal, burning of the cut will occur. Check the following points:
- Too high a gas pressure? If you use a non-reactive gas, too much oxygen from the environment can still be sucked into the cut because the high gas flow creates turbulence. Lower your gas pressure.
- Is the cutting speed too low? In that case the molten metal is exposed to oxygen for too long
Problems with no cut
If no cut is created at all, some general points should be checked as well as some aspects of the bundle transport.
- Too low (average) laser power? This means there is not enough energy to generate a cut at a given welding speed. The laser power can be set incorrectly, but can also be reduced by contamination of the gas in the resonator (CO2 laser), wear of lamps and diodes (fiber laser or Nd:YAG laser), incorrect alignment or contamination or wear of the optical elements in the resonator. Use a power meter to check the emitted laser power.
- Cutting speed too high? This means there is not enough energy available to cut at a given laser power.
- A non-optimal focus position. If the focus point is positioned too deep below or too high above the product surface, the diameter of the laser spot on the product surface will be too large. As a result, the intensity of the laser will be too low to melt the material. If the focusing optic has recently been replaced, it may be necessary to redetermine the optimal focus position.
- Focal length combined with the right optics. Optics with too long a focal length will cause too much focus. Another lens with a short focal length to determine the optimal focus position is then appropriate.
Control points for bundle transportation
The laser beam is transported through mirrors or glass fibers to the focusing optic. Errors in this transport can lead to loss of power, resulting in the failure of the cut. Check the following points:
- Are the mirrors clean? Also check the flow rate of the gas baffle and the filters of these mirrors if they are present.
- Does the laser beam have a free path from source to optics? If the beam hits the beam shield somewhere you get an incomplete mode burn or a mode burn with interference patterns.
- Is the aiming laser aligned with the laser beam? Are both beams centered with respect to the mirrors of the beam transport system and the focusing optics?
- Is the laser power properly coupled into the glass fiber (see Figure 2.8)? To do this, compare the set laser power with the (measured) power leaving the fiber. The latter should not be lower than approximately 90% of the set laser power.
Problems with inadequate cutting
- Is the nozzle-plate distance constant over the entire length of the cutting contour? This can be checked by using distance plates between the nozzle and the product. Measure at different places along the contour. If your system has a distance sensor, check the adjustment.
- Check for obstructions that obstruct the laser beam. Clamps or parts of the product itself may be in the way. Sticking a piece of tape on a suspected obstacle and watching for burn marks is the easiest way to be sure.
Control points for bundle transport and focusing
- If there has been a decrease in cut quality for several days or weeks, this may be due to thermal focusing of the lens. Check to see if the service life of the lens has been (almost) exceeded. Is there an apparent shortening of the focal length? You can do a mode burn to check and compare it to expectations. Also check the condition of the resonator (purity of the gas in the CO2 laser, quality of the cooling water, etc.).
- Is the cooling of the beam transport system and focusing optics adequate? The optics housing and cooling water should not be more than hand warm.
- Check the optics. Is it dirty or damaged (scratches and/or pits). Is it free of dirt (splashes, soot)? When the optics are dirty, absoprtion of the laser energy occurs and there is heating of the lens. This causes an apparent shortening of the focal length. This is especially important when using short focal length readers.
We’ve talked here about the most common problems in laser cutting. But maybe the problem doesn’t lie with your machine at all. Maybe it lies with the material you wish to cut. Therefore, be sure to check out the following article about the properties of different metals and how they affect the laser cutting process.