Cutting processes - laser cutting
| Cut section of ellipse in flat plate |
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They offered an entirely new form of energy which in turn lent itself to uses in manufacturing, medicine and communications. Able to heat, melt and even vaporise material lasers are seen as the ideal medium for combining intense but controllable energy.
By far the most popular use of the laser, particularly the carbon dioxide laser, is for cutting.
Laser cutting
It is largely a thermal process in which a focused laser beam is used to melt material in a localised area. A co-axial gas jet is used to eject the molten material from the cut and leave a clean edge.A continuous cut is produced by moving the laser beam or workpiece under CNC control
The process also lends itself to automation with offline CAD/CAM systems controlling either 3-axis flat bed systems or 6-axis robots for three dimensional laser cutting.
The improvements in accuracy, edge squareness and heat input control means that other profiling techniques such as plasma cutting and oxy-fuel cutting are being replaced by laser cutting.
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What's the relationship between the lens used and the thickness of cut?
The laser cutting process involves focusing a laser beam, usually with a lens, to a small spot which has sufficient power density to produce a laser cut.The lens is defined by its focal length, which is the distance from the lens to the focused spot. However, the critical factors which determine the selection of the lens are the focused spot diameter, d, and the depth of focus, L.
The depth of focus is the effective distance over which satisfactory cutting can be achieved. It can be defined as the distance over which the focused spot size does not increase beyond 5%.
For a given beam diameter, as the focal length becomes shorter the focused spot diameter and the depth of focus also both become smaller. The size of the actual spot is also dependent on the raw beam diameter, D. As this increases, for a given lens, the focused spot size decreases.
To allow comparison between lasers with different beam diameters we therefore use a factor called the focus f-number, which is the focal length, F, divided by the incoming raw beam diameter, D.
As we are generally unable to alter the raw beam diameter we select the correct lens to give us a focus beam of the required type.
The requirements for cutting are high power density, and therefore small focused spot size but with a long depth of focus, and therefore the ability to process thicker materials with a reasonable tolerance to focus position variation.
These two requirements are in conflict with each other and therefore a compromise must be made. The only other consideration is that the shorter the focal length, the closer the lens is to the workpiece, and therefore more likely to be damaged by spatter from the cutting process.
For typical CO 2 laser cutting systems focal lengths can be selected in the range from 2 1/ 2 inches up to 10 inches, which are equivalent to f-numbers between two and ten, depending on the beam diameter.
In practice a 5 inch lens could cut up to around half inch thick steel before a longer lens would be required to provide a greater depth of focus. However on thin sheet material, for example 1mm, a shorter focal length may offer significantly higher cutting speeds, or allow more intricate detail to be produced.
In fact it would be possible to optimise focal length for each material thickness, but this would involve additional set up time when changing from one job to the next, which would have to be balanced against the increased speed. In reality lens changing is avoided and a compromise cutting speed used, unless a specific job has special requirements .
Just how flexible are they?
Most laser cutting machines are 3-axis systems, that is X-Y, two dimensional positioning control with a Z-axis height control.There are however a number of ways of achieving the X-Y movement, either moving the laser head, moving the workpiece or a combination of both.
The most popular approach is known as a 'flying optics' system where the workpiece remains stationary and mirrors are moved in both X and Y axes. The advantages of this approach are that the motors are always moving a known, fixed mass. This can often be much heavier than the workpiece, but it is easier to predict and control.
As the workpiece is not moved, this also means that there is no real limit to sheet weight. The disadvantage of flying optics is the variation in beam size, as a laser beam is never perfectly parallel, but actually diverges slightly as it leaves the laser.
This means that without controlling the divergence, there may be some variation in cutting performance between different parts of the table, due to a change in raw beam size. This effect can be reduced by adding a re-collimating optic, or some systems even use adaptive mirror control.
The alternative is a 'fixed optic' system where the laser head remains stationary and the workpiece is moved in both X and Y axes. This is the ideal situation optically, but the worst situation mechanically, especially for heavier sheets.
For relatively light sheet weights, a fixed optic system can be a viable option, but as the sheet weight increases, accurately positioning the material at high speed can be a problem.
The third option is known as a 'hybrid' system, where the laser head is moved in one axis and the material moved in the other axis. This is often an improvement over fixed optics, but still suffers from difficulties with heavier sheet weights.
What difficulties does reflection cause?
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Amada LCV laser cutting machine with autostorage and pallet changer system
Courtesy of Amada UK Ltd |
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Aluminium is more reflective than carbon manganese steel or stainless steel and has the potential to cause damage to the laser itself.
Most laser cutting machines use a laser beam aligned normal to a flat sheet of material. This means that should the laser beam be reflected by the flat sheet it can be transmitted back through the beam delivery optics, and into the laser itself, potentially causing significant damage.
This reflection does not come entirely from the sheet surface, but is caused by the formation of a molten pool which can be highly reflective. For this reason simply spraying the sheet surface with a non-reflective coating will not entirely eliminate the problem.
As a general rule the addition of alloying elements reduces the reflectivity of aluminium to the laser, so pure aluminium is harder to process than a more traditional 5000 series alloy.
With good, consistent cutting parameters the likelihood of a reflection can be reduced to almost zero, depending on the materials used. However it is still necessary to be able to prevent damage to the laser while developing the conditions or if something goes wrong with the equipment.
The 'aluminium cutting system' which most modern equipment uses is actually a way of protecting the laser rather than an innovative technique for cutting. This system usually takes the form of a back reflection system that can detect if too much laser radiation is being reflected back through the optics.
This will often automatically stop the laser, before any major damage is caused. Without this system there are risks with processing aluminium as there is no way of detecting if potentially hazardous reflections are occurring.
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