Laser cutting: Definition from Answers.com

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Laser cutting is a technology that uses a laser to cut materials, which is used in the production line and is typically used for industrial manufacturing applications. Laser cutting works by directing the output of a high power laser, by computer, at the material to be cut. The material then either melts, burns, vaporizes away, or is blown away by a jet of gas,^[1] leaving an edge with a high quality surface finish. Industrial laser cutters are used to cut flat-sheet material as well as structural and piping materials.

Comparison to mechanical cutting

Advantages of laser cutting over mechanical cutting vary according to the situation, but two important factors are the lack of physical contact (since there is no cutting edge which can become contaminated by the material or contaminate the material), and to some extent precision (since there is no wear on the laser). There is also a reduced chance of warping the material that is being cut, as laser systems have a small heat-affected zone. Some materials are also very difficult or impossible to cut by more traditional means. One of the disadvantages of laser cutting includes the high energy required.

Types

A diffusion cooled resonator

There are three main types of lasers used in laser cutting. The CO₂ laser is suited for cutting, boring, and engraving. The neodymium (Nd) and neodymium yttrium-aluminum-garnet (Nd-YAG) lasers are identical in style and differ only in application. Nd is used for boring and where high energy but low repetition are required. the Nd-YAG laser is used where very high power is needed and for boring and engraving. Both CO₂ and Nd/ Nd-YAG lasers can be used for welding.^[2]

Common variants of CO₂ lasers include fast axial flow, slow axial flow, transverse flow, and slab.

CO₂ lasers are commonly "pumped" by passing a current through the gas mix (DC Excited) or using radio frequency energy (RF excited). The RF method is newer and has become more popular. Since DC designs require electrodes inside the cavity, they can encounter electrode erosion and plating of electrode material on glassware and optics. Since RF resonators have external electrodes they are not prone to those problems.

In addition to the power source, the type of gas flow can affect performance as well. In a fast axial flow resonator, the mixture of carbon dioxide, helium and nitrogen is circulated at high velocity by a turbine or blower. Transverse flow lasers circulate the gas mix at a lower velocity, requiring a simpler blower. Slab or diffusion cooled resonators have a static gas field that requires no pressurization or glassware, leading to savings on replacement turbines and glassware.

Lasing Materials	Applications
CO₂	Boring Cutting/Scribing Engraving
Nd	High energy pulses Low repetition speed (1kHz) Boring
Nd-YAG	Very high energy pulses Boring Engraving Trimming

Process

Generation of the laser beam involves stimulating a lasing material by electrical discharges or lamps within a closed container. As the lasing material is stimulated, the beam is reflected internally by means of a partial mirror, until it achieves sufficient energy to escape as a stream of monochromatic coherent light. The coherent light then passes through a lens that focuses the light into a highly intensified beam generally less than 0.0125 in (0.3175 mm). in diameter. Depending upon material thickness, kerf widths as small as 0.004 in (0.1016 mm). are possible.^[3] In order to be able to start cutting from somewhere else than the edge, a pierce is done before every cut. Piercing usually involves a high power pulsed laser beam which slowly (taking around 5-15 seconds for half-inch thick stainless steel, for example) makes a hole in the material.

There are many different methods in cutting using lasers, with different types used to cut different material. Some of the methods are vaporization, melt and blow, melt blow and burn, thermal stress cracking, scribing, cold cutting and burning stabilized laser cutting.

Beam Geometry

The parallel rays of coherent light from the laser source may be 1/16 in. to 1/2 in. (1.5875 mm to 12.7 mm) in diameter. This beam is normally focused and intensified by a lens or a mirror to a very small spot of about 0.001 in. (0.0254 mm) to create a very intense laser beam. Recent investigations reveal that the laser beam has a distinctive polarization. In order to achieve the smoothest possible finish during contour cutting, the direction of polarization must be rotated as it goes around the periphery of a contoured workpiece. For sheet metal cutting, the focal length is usually between 1.5 in. and 3 in. (38.1 mm and 76.2 mm)^[4]

Vaporization cutting

In vaporization cutting the focused beam heats the surface of the material to boiling point and generates a keyhole. The keyhole leads to a sudden increase in absorptivity quickly deepening the hole. As the hole deepens and the material boils, vapor generated erodes the molten walls blowing ejecta out and further enlarging the hole. Non melting material such as wood, carbon and thermoset plastics are usually cut by this method.

Melt and blow

Melt and blow or fusion cutting uses high pressure gas to blow molten material from the cutting area, greatly decreasing the power requirement. First the material is heated to melting point then a gas jet blows the molten material out of the kerf avoiding the need to raise the temperature of the material any further. Materials cut with this process are usually metals.

Thermal stress cracking

Brittle materials are particularly sensitive to thermal fracture, a feature exploited in thermal stress cracking. A beam is focused on the surface causing localized heating and thermal expansion. This results in a crack that can then be guided by moving the beam. The crack can be moved in order of m/s. It is usually used in cutting of glass.

Burning stabilized laser gas cutting

Burning stabilized laser cutting is essentially oxygen cutting but with a laser beam as the ignition source. This process can be used to cut very thick steel plates with relatively little laser power.

Tolerances and Surface Finish

This process is capable of holding quite close tolerances, often to within 0.001 in. (0.0254 mm) Part geometry and the mechanical soundness of the machine have much to do with tolerance capabilities. The typical surface finish resulting from laser beam cutting may range from 125 to 250 micro-inches (0.003175 mm to 0.00635 mm).^[5]

Setup and Equipment

The laser machining system consists of a power supply for producing a laser beam (Power requirements below), a workpiece positioning table, laser material, a method of stimulation, mirrors, and a focusing lens. The workpiece is held stationary by clamps, straps, hold down tabs, pressure blocks, positioning tabs, magnets, or suction cups. The focusing unit moves around the workpiece to cut the desired shape.

Machine configurations

There are generally three different configurations of industrial laser cutting machines: Moving material, Hybrid, and Flying Optics systems. These refer to way that the laser beam is moved over the material to be cut or processed. For all of these, the axes of motion are typically designated X and Y. axis. If the cutting head may be controlled, it is designated as the Z-axis.

Moving material lasers have a stationary cutting head and move the material under it. This method provides a constant distance from the laser generator to the workpiece and a single point from which to remove cutting effluent. It requires fewer optics, but requires moving the workpiece.

Hybrid lasers provide a table which moves in one axis (usually the X-axis) and move the head along the shorter (Y) axis. This results in a more constant beam delivery path length than a flying optic machine and may permit a simpler beam delivery system. This can result in reduced power loss in the delivery system and more capacity per watt than flying optics machines.

Flying optics lasers feature a stationary table and a cutting head (with laser beam) that moves over the work piece in both of the horizontal dimensions. Flying-optics cutters keep the workpiece stationary during processing, and often don't require material clamping. The moving mass is constant, so dynamics aren't affected by varying size and thickness of workpiece. Flying optics machines are the fastest class of machines, with higher accelerations and peak velocities than hybrid or moving material systems.^{[citation needed]}

Dual Pallet Flying Optics Laser

Flying optic machines must use some method to take into account the changing beam length from near field (close to resonator) cutting to far field (far away from resonator) cutting. Common methods for controlling this include collimation, adaptive optics or the use of a constant beam length axis.

The above is written about X-Y systems for cutting flat materials. The same discussion applies to five and six-axis machines, which permit cutting formed workpieces. In addition, there are various methods of orienting the laser beam to a shaped workpiece, maintaining a proper focus distance and nozzle standoff, etc.

Pulsing

Pulsed lasers which provide a high power burst of energy for a short period are very effective in some laser cutting processes, particularly for piercing, or when very small holes or very low cutting speeds are required, since if a constant laser beam were used, the heat could reach the point of melting the whole piece being cut.

Most industrial lasers have the ability to pulse or cut CW (Continuous Wave) under NC program control.

Effects on Work Material Properties

The effects on the workpiece materials is rather minimal due to the small zone of metal affected by the laser beam. However, the effects are due to the high temperature of the laser that change the hardness and the creation of a narrow heat-affected zone.

Work material properties	Effects of laser beam cutting
Mechanical	May affect hardness Narrow heat-affected zone
Physical	Grain size may change
Chemical	No change

Typical Workpiece Materials

Materials with a low thermal conductivity and reflectivity can be cut with a laser. Currently, mild steel, titanium, paper, wax, plastics, wood, and fabrics are typical workpiece materials.

Factors Affecting Process Results

The cutting speed, feed rate, beam geometry, beam intensity, beam focusing, beam/workpiece material, type of workpiece material, and positioning accuracy all effect the tolerances and surface finish of a process.

Production and Cutting Rates

The production rate is determined by using the metal thickness and the metal type. For a typical laser beam, the metal must be within the bounds of 0.020 in. and 0.5 in. (0.508 mm and 12.7 mm) in thickness. For all intents and purposes, a laser can be up to thirty times faster than standard sawing.

Workpiece Material	Material Thickness
	0.02	0.04	0.08	0.125	0.25	0.5	in.
	0.508	1.016	2.032	3.175	6.35	12.7	mm
Stainless Steel	750	550	325	10	20	-
Aluminum	800	350	150	100	40	30
Mild Steel	-	177	70	40	-	-
Titanium	300	300	100	80	60	40
Plywood	-	-	-	-	180	45
Boron/epoxy	-	-	-	60	60	25

Using a CO₂ laser
Units are in (ipm)

Cooling

It is necessary to keep the medium that generates the laser, and the lens cool to a safe working temperature. In both cases the cooling is done by water. In each case, water is constantly pumped around the heated object. By so doing, the heated water is pushed out and recycled for another use.

Power requirements

The amount of power required, known as heat input, to cut through a workpiece is determined by the thickness and type of material used. The amount of kilowatts required to run the laser is related to the heat input.

Amount of heat input required for various material at various thicknesses using a CO₂ laser (watts)^[6]
Material	Material thickness (in)
Material	0.02	0.04	0.08	0.125	0.25
Stainless steel	1000	1000	1000	500	250
Aluminum	1000	1000	1000	3800	10000
Mild steel	-	400	-	500	-
Titanium	250	210	210	-	-
Plywood	-	-	-	-	650
Boron/epoxy	-	-	-	3000	-

Cost Element

When calculating the cost elements of a project, include things like setup time, load/ unload times, cutting time, direct labor rate, overhead rate, and amortization of equipment and tooling.

Time Calculation

$\begin{array}{lcl} \text {Length of cut (in.)} & = & L \\ \text {Material thickness (in.)} & = & T \\ \text {Feed Rate (ipm)} & = & F \\ \text {Width of kerf (in.)} & = & W \\ \text {Cutting Time} & = & L / F \\ \text {Removal Volume} & = & L * W * T \\ \end{array}$

Safety Factors

The following risks should be taken into consideration.

Personal

Contact with hot tools and workpieces
Eye contact with beam radiation
Inhalation of toxic fumes

Environmental

Smoke
Fumes and dust particles

References

^ Oberg, p. 1447.
^ Todd, p. 186.
^ Todd, p. 185.
^ Todd, p. 188.
^ Todd, p. 186.
^ Todd, Allen & Alting 1994, p. 188.

Bibliography

Oberg, Erik; Franklin D. Jones, Holbrook L. Horton, Henry H. Ryffel (2004). Machinery’s Handbook (27th edition ed.). New York, NY: Industrial Press Inc. ISBN 978-0831127008.
Steen, Wlliam M. (1998). Laser Material Processing (2nd edition ed.). Great Britain: Springer-Verlag. ISBN 3-540-76174-8.
Todd, Robert H.; Allen, Dell K.; Alting, Leo (1994), Manufacturing Processes Reference Guide, Industrial Press Inc., ISBN 0-8311-3049-0, http://books.google.com/books?id=6x1smAf_PAcC .

External links

Wikimedia Commons has media related to: Laser cutting

Laser cutting steel parts for a miniature Live steam locomotive
Zero width laser cutting (glass)
Laser Glass Cutting Good Further Information Resource with White Paper

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