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Cutting

1 Basic Principles

Figure 1 is a schematic of a laser cutting head. The mechanism by which a cut is achieved is extremely simple:

  1. The laser beam is focused down to a small spot on, or just below, the surface of the workpiece.
  2. The intense heat of the focused beam melts the workpiece all the way through over a very narrow area (typically 0.3 mm or 12 thousandths of an inch).
  3. A gas jet acting with the laser pushes the melt out of the bottom of the cut zone, as shown in Figure 2. (Note: When cutting certain materials the gas jet can be chosen to react chemically with the workpiece to produce heat and accelerate the cutting speed. The most common application of this is the cutting of mild steel with an oxygen jet).
  4. The cut zone is moved around the workpiece to produce the desired profile. Either the laser head or the workpiece or both can be moved, depending on the machine design.

 

 

Cutting figure 1

Figure 1. A schematic of laser cutting
The lens mount or the nozzle (or both) can be adjusted from left to right or into and out of the sketch plane. This allows for centralization of the focused beam with the nozzle. The vertical distance between the nozzle and the lens can also be adjusted. Source: J. Powell ‘The LIA Guide to Laser Cutting’ – Pub; Laser Institute Of America.

 

cutting figure 2

Figure 2. The basic mechanism of laser cutting
The laser generates a localized melt and the liquid is ejected from the cut zone by a gas jet. In some cases (e.g. when cutting steel with oxygen) the gas jet reacts chemically with the workpiece to produce extra heat which speeds up the cutting process. Source: J. Powell 'CO2 Laser Cutting', 2nd Ed., ISBN 1-852-33047-3.

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2 Benefits of Laser Cutting

  1. The process cuts at high speed compared to other profiling methods. For example a typical 2000 W CO2 laser will cut 2 mm thick (0.08 in) mild steel at ~6m/min (~240 in/min). The same machine will cut 5mm (0.2 in) thick acrylic sheet at ~12m/min (~480in/min).
  2. In most cases (e.g. the two examples given above) the cut components will be ready for service immediately after cutting without any subsequent cleaning operation.
  3. The cut width (kerf width) is extremely narrow (typically 0.1 to 1.0 mm {0.004 in to 0.04 in}). Very detailed work can be carried out without the restriction of a minimum internal radius imposed by milling machines and similar mechanical methods.
  4. The process is fully CNC controlled. This, combined with the lack of necessity for complex jigging arrangements, means that a change of job from cutting component 'A' out of steel to cutting component 'B' out of a polymer (plastic) can be carried out in seconds.
  5. Although laser cutting is a thermal process, the actual area heated by the laser is very small and most of this heated material is removed during cutting. Thus, the thermal input to the bulk of the material is very low, heat affected zones are minimized and thermal distortion is generally avoided.
  6. It is a non-contact process which means that material needs only to be lightly clamped or merely positioned under the beam. Flexible or flimsy materials can be cut with great precision and do not distort during cutting as they would when cut by mechanical methods.
  7. Owing to the CNC nature of the process, the narrowness of the kerf width and the lack of mechanical force on the sheet being cut, components can be arranged to 'nest' very close together. Hence, material waste is reduced to a minimum. In some cases this principle can be extended until there is no waste material at all between similar edges of adjacent components. The laser cut separates the two components which therefore 'share' the cut line. This is, of course, an extremely effective use of material and laser time but is generally only applicable where two components can be arranged to share a straight line cut.
  8. Although the capital cost of a laser cutting machine is substantial, the running costs are generally low. Many industrial cases exist where a large installation has paid for itself in under a year.
  9. The process is extremely quiet compared to competing techniques, a factor which improves the working environment and the efficiency of the operating staff.
  10. Laser cutting machines are extremely safe to use in comparison with many of their mechanical counterparts.

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3 Cutting Metals

Stainless Steel and Non-Ferrous metals (Aluminium, Copper etc.)

Stainless steels and non-ferrous metals such as aluminium and copper alloys are cut by the melt shearing method described in figure 2. For most metals a high pressure jet of Nitrogen is used to remove the melt completely from the cut zone – this leaves a cut edge which is ready for use in most engineering applications (although high tolerance holes might need to be reamed out). Note also that: (i) Materials that are susceptible to heat-treatment will have a narrow Heat Affected Zone (HAZ) next to the cut edge, though this effect is unimportant in most applications; and (ii) Titanium alloys react chemically with Nitrogen and so Argon is used as a cutting gas in this case.

Mild and Carbon Steels

Mild and carbon steels are cut by a combination of melt shearing (see Figure 2) and a chemical reaction with the cutting gas jet, which in this case is pure oxygen. Inside the cut zone the oxygen jet reacts with the iron in the steel to produce iron oxide. The reaction has two beneficial effects on the cutting process:

  1. The chemical reaction produces heat which speeds up the cutting process.
  2. The chemical reaction produces an oxidized liquid, which does not stick to the solid steel and is easily blown away by the oxygen jet. This results in a dross free cut edge and cut components that are ready for immediate use.

These two effects were largely responsible for the rapid growth of laser cutting in the 1980s because they greatly reduced the cost of the process for mild steel. Mild steel is still the most commercially important material for the laser cutting industry.

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4 Cutting Non-metals

Polymers

Polymers or plastics as they are more commonly called can be divided up into two main groups - Thermoplastics and Thermosets:

Thermoplastics

These are polymers which can be repeatedly melted down and cast into new shapes. They include Polypropylene, Polystyrene, Polyethylene (polythene), Polyamide (Nylon) and others. Figure 2 is a good description of how most thermoplastics are cut by a CO2 laser, and in this case the cutting gas jet will be nitrogen or air. The resulting cut edge is of good quality but covered in microscopic ripples. Thermoplastics cut at high speed and relatively thick sections can be profiled.
There are two important thermoplastics that do not cut by the melt shearing mechanism:

  1. Polyvinyl Chloride (PVC): this material degrades chemically when heated by the laser. The fumes given off contain high levels of Hydrogen Chloride which is extremely corrosive and very toxic. For this reason PVC must never be cut by laser.
  2. Polymethyl Methacrylate (Acrylic, Plexiglass etc): this material boils rather than melts during the cutting process and, under the correct conditions can produce a polished cut edge. This process is known as cutting by vaporization. As the boiling vapour is blown away from the cut zone, it leaves a thin liquid layer on the cut edges. If the gas jet blowing the vapour away flows gently over this liquid layer, it will dry like paint to produce a glossy edged cut. If the air jet is turned up the edge will have a frosted appearance.

Thermosets

These materials cannot be remelted once they have been made into their initial shape. They sometimes involve the mixing of two liquids which then set hard. This group includes epoxy, phenolic resins, and most natural rubber products. These materials are not cut by the melt shearing mechanism shown in Figure 2 for the simple reason that they cannot melt. In this case the laser burns the workpiece, reducing the plastic to a smoke made up of carbon and all the other chemicals which were contained in the original material. This process is known as cutting by chemical degradation. Because this process takes more energy than simple melting, cut speeds and maximum thicknesses for thermosets are lower than for thermoplastics. The cut edge of such materials is generally flat, smooth and covered with a thin layer of carbon.

Wood Based Products

Wood and wood based products are cut by the same mechanism as thermoset plastics (see previous section). The laser burns through the material to produce a cut and the carbon based smoke is blown out of the cut zone by a gas jet which is usually air. The top and bottom surfaces of the workpiece retain their original appearance but the cut edge is covered in a layer of carbon which darkens it.

Other Materials

The range of materials which can be laser cut is enormous and is growing all the time. If you have a potential application it is easy to get samples produced at a laser cutting jobshop.

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