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SSC Laser
9th February 2018

An Apprentice Engineer’s Quick Guide To Laser Cutting

What is Laser cutting?

The word ‘laser’ is an acronym of Laser Amplification of the Stimulated Emission of Radiation. A laser is a beam of radiation (monochromatic coherent) that has up until recently been produced in a resonator that emits radiation of in-step waves of identical frequency.

CO2 Laser – That is about as much as is required from the technical makeup of the laser, but suffice to say an extremely powerful beam is produced and focused and then focused by a system of mirrors and lenses, and assisted by a shield gas delivered through a lens loaded cutting head through a nozzle.

Fiber Laser-The laser beam is generated in the core of an optical fiber and is then delivered via fiber optic cables to the cutting head and as on the CO2 concentrated and focused through a lens to the nozzle

Advantages of Laser Cutting over traditional manufacturing methods

Two types of cutting are:

  1. Fusion Cutting (high pressure) in which the material is fused by the laser beam and high-pressure gas (nitrogen) is used to blow away the molten metal from the kerf area at (10-20 bar)
  2. Oxidation Cutting which is the material being heated to combustion temperature and the gas (oxygen) at medium pressure (o.5 – 5 bar) is used to oxidise the material to drive the molten metal out of the kerf area

Laser cutting cut quality depends on the following variables:

  1. Beam Generation
      1. Laser Power
  2. Beam Delivery
      1. Cutting Distance (nozzle & material)
      2. Focal Length
      3. Focal position
      4. Cut speed
  3. Cutting Gas
      1. Type and pressure of cutting gas
  4. Material
      1. Type of material
      2. Surface of material
      3. Thickness of material
      4. Shape of point

Two types of assist gas commonly used for laser cutting are… Oxygen and Nitrogen.

This has been historically determined by the type of material to be cut.

Oxygen is used to cut mild steel and nitrogen is used to cut aluminium and stainless steel.

With the onset of Fiber technology, much higher speeds can now be attained with the use of nitrogen in the mild steel cutting process. Historically it wasn’t cost effective to cut Mild Steel with nitrogen but now with the extra speed, we can achieve with fiber lasers the extra cost of nitrogen is negated.

The quality of both cut gases need to be above 99.95 purity, as anything below this can cause cut quality issues. The thickness of the material or profiling will point in the direction of the pressure of the gas required. This is usually determined by the parameters chosen for the material being processed.

Nozzles

Different sized nozzles are used for different cutting applications. For example, larger nozzle hole sizes are used for high pressure cutting (nitrogen). The roundness of the hole is critical as any deformation can have a negative impact on the quality of the cut.

Laser Optics

Traditionally on CO2 laser cutting the focusing optics enable the laser beam to be focused into a single spot that is typically 0.2mm wide. It is critical that the focal lens is kept clean, as a dirty lens absorbs more laser energy, which in turn heats up the lens and deforms it – this, in turn, can lead to poor cut quality or in extreme cases cause the lens to explode.

Though Fiber technology has changed the way the laser is generated and delivered the critical areas of lens cleanliness within the cutting head remain the same

Focal Length

Traditionally there have been two standard focal lengths – 5” for thin materials (0-3mm) and 7.5” for thicker materials (4mm-20mm). On traditional laser technology, 2 different laser cutting heads were used for this. Newer technology has seen the introduction of just one laser head which automates the whole process, reducing set-up time.

Focal Position

An Introduction to Laser CuttingAnother vital part of the cutting process where we must concentrate the beam in the most effective area of the material to give us the best cut quality and cutting efficiency. For caution, when cutting up to 6mm the focal point should be concentrated on top of the material (see f=0 in image right). For cutting 8mm and above, the focal point needs to be above the material (f>0 in image right). For stainless steel and aluminium, the focal point needs to be two thirds into the material (f<0 in image right).

Centering of the laser beam

This is also vital to the cut quality. Ideally, the beam needs to be central to the middle of the nozzle, failure to do this can cause poor quality cutting in some, if not all directions. This was always a very manual task involving sellotape and a mirror but is now mainly automated due to technological advancements.

Laser Cutting Materials

The compositions of the material influences how the laser cuts the materials. The main influences are thermal conductivity absorption and reflectivity. Material tension can also influence the cutting and reaction to the cutting process.

Material Surface

Laser cutting is controlled by optics, but is also an aerodynamic process, so surface reflectivity or defectivity and can negatively impact the cut. Scaly, dirty or shot blasted surfaces can also have a negative effect as the gas flow can be deflected on rough surfaces.

Sprayed and painted surfaces hinder the cut process, as does mill scale.  Warped or damaged surfaces cause issues with the gas flow around the beam, impacting on cut quality.

Sand or shot-blasted materials can deflect the focus of the laser beam and also affect the gas flow. Sand remnant can cause issues due to its silicon content.

Oiled surfaces –  this has no negative effect and tends to aid the cutting process and preventing slag from sticking.

Galvanised material can be cut successfully up to 6mm on CO2 using Nitrogen. One thing to consider here is that the laser cutting process will remove galvanising from the edges when it is cut, leading the cut edges open to atmospheric deterioration.

Poly/nitto coated materials can be cut using CO2 lasers directly, but the preferred method is to melt the poly coating on a ghost run as this gives the whole process a good stability.

Laser Cutting Rules of Thumb

CO2

0-25mm mild steel – hole size diameter the same as material thickness

0-12mm aluminium – hole size diameter the same as material thickness

0-15mm stainless steel – hole size diameter the same as material thickness

Fiber

CO2

0-25mm mild steel – hole size the same as material thickness

0-30mm aluminium – hole size dia ⅔ of the material thickness

0-30mm stainless steel – hole size dia ⅔  of the material thickness

These rules of thumb can be dramatically reduced but then further rules apply and come into effect. Long thin panels, for example, are difficult to process due to snaking and bowing of materials as it cuts.

Fiber Laser

Fiber laser has completely changed the face of laser cutting and a lot of the traditional rules have changed. The capabilities of the current 10KW class are on the whole, far and above what we have been used to for the last 20 years.

A lot of what has been previously discussed in this article, although still relevant in some areas, has now changed. The laser beam is now delivered directly to the head via fiber optic cables, and is far more efficient and stable, and does away with the costly and inefficient mirror systems.

This in turn changes the strictures on the machines abilities due to not having reflectivity issues. Thus opening the machine up to being able to cut thicker brass and also copper.

Servicing and electrical efficiencies are also changed with approximately 50% lower electric costs, and services require less downtime and larger intervals between services.

The main advantage the fiber laser gives is the speed of cut over thin to mid-range materials, particularly over the 4-8mm mild steel range, where speeds have increased up to 5 fold using nitrogen rather than traditional oxygen cutting methods. This also does away with the oxidised edge, improving secondary coating / painting process.

The only real downsides are that 10 mild steel and above, there is minimal speed cutting advantages over a 6KW CO2 and the CO2 cut in this material band is still more aesthetically pleasing than a Fiber laser. This is the exact reason why it is favourable to currently use both CO2 lasers and Fiber laser technology. However, as technology advances, direct diode lasers could offer the solution to both speed matching fiber lasers and quality matching that of a CO2 laser on thicker sheet steel. Direct diode lasers, although not a new advancement in the laser field, the harnessing of the technology to allow it to be used competitively and commercially is. Watch this space…

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