31. Laser Cutting

31.1 Lasers

• Light Amplification by Stimulated Emission of Radiation.

• When are they best used?

when highly focused energy is required (light or heat)

when contact forces must be eliminated

when a small geometry is required

• What do LASERs do?

Produce collimated light: all of the light rays are (nearly) parallel. This means the light doesn’t diffuse quickly like normal light.

Monochromatic: because the light is generated using specific gases, the frequency (wavelength) has a specific value. Normal white light tends to contain a wide mixture of different frequencies (a wide spectrum), but laser light is very specific.

The light has significantly less power than a normal light bulb, but it is highly focussed, thus delivering a significantly higher light intensity.

• The principle behind lasers are

1. Excitation of light emission by electrical discharge.

 

2. Resonance: the laser chamber has reflecting ends separated by a multiple of half wavelengths one end is completely reflecting, and the other end is partially reflecting. The result is a reflection that leads to resonance.

 

• The height of the orbit the electron is in determines the wavelength of the photon. Larger atoms have higher orbits, therefore longer wavelengths (infrared). Smaller atoms have shorter falls, therefore shorter wavelengths (Ultraviolet).

• The electrons are caused to jump by a discharge of electrons with a potential charge in the range of KV.

 

• Various gases are used in Lasers. The contents of a laser can be a single gas, or a combination of gases.

e.g. in a CO2 laser, CO2 is used to produce light with a 10.6 micrometer wavelength. Nitrogen is used to maintain electron populations in the upper valence shell of the CO2 molecules. Helium is used as an intracavity cooling agent.

• Lasers are very inefficient and build up excessive heat. If this heat becomes high enough it will effect the performance, and eventually damage the laser. To counteract this, heatsinks, water, and other forms of heat dissipation are used.

• The lasers often have sensors to shutdown when the temperatures become too high.

• 1 Angstrom A = 10-10m

 

• Energy of a photon

 

• Absorption is when energy causes an electron to accept enough energy to jump up one or more energy levels.

• Spontaneous emission is the drop of the electron to a lower energy orbit, and the release of the energy change as a photon.

 

• Absorption can be caused by energy sources, such as light, but it is also caused by the heat of an object. (as with incandescent lights)

 

• We can draw out a spectrum for frequencies emitted.

 

• Fluorescence is light of one color that causes emission of light of another color. (a shorter wavelength).

 

• In a laser the energy levels are increased to move more than 50% of the electrons (in the lasing material) to a higher energy state.

• The usual population inversion allows incoming photons to cause a new photon to be emitted without being absorbed itself. The two photons have,

the same frequency

the same phase

the same direction

Note: This effect is also a 2 times amplification

• How a laser works,

1. The electrical/light discharges are used to cause electron population inversion and cause a few spontaneous emissions of photons.

2. The new photons travel in all directions, but some travel toward the mirrors, where they are reflected back and forth between the mirrors.

3. As the photons travel, they cause the generation of other photons travelling in the same direction.

4. This builds until the laser has a high intensity output.

5. The output beam escapes through one end of the laser that has a half silvered mirror.

• Laser light is polarized

 

• Various lasers are suited to different applications.

 

• Efficiency of lasers is often about 0.1% for gas, but CO2 can be as high as 18%.

31.1.1 References

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31.1 Harry, J. E., Industrial Lasers and Their Applications, McGraw-Hill, London, 1974.

31.2 Hugel, H., Lasers in Manufacturing, Proceedings of the 5th, International Conference, Sept., 1988, Stuttgart, West Germany.

31.3 Ready, J. F., Lasers in Modern Industry, Society of Manufacturing Engineers, Dearborn Michigan, 1979.

31.4 Soares, O. D. D., and Perez-Amor, M., Applied Laser Tooling, Martinus Nijhoff Publishers, Lancaster UK, 1987.

31.2 Laser Cutting

• Good for,

thin work pieces that would be greatly effected by contact force (e.g. soft or brittle)

parts that are too complex for saws and other cutters

materials normally too hard to machine with traditional methods. The laser effects the thermal, not hardness cutting conditions.

• Used for,

cutting

welding

scribing

drilling

heat treating

• Results,

less part deformation

reduced part grinding and deburring

lighter fixtures

• Can be used for 2D or 3D workspace.

• The cutters typically have a laser mounted, and the beam is directed to the end of the arm using mirrors.

 

• Mirrors are often cooled (water is common) because of high laser powers.

• The light focuses on the surface, and vaporizes it. The basic process is,

1. Unheated material.

2. Heating begins and metal becomes reflective.

3. Heating continues and reflectivity decreases.

4. A molten zone is established.

5. Material vaporizes, consuming most of the laser energy. Very little energy goes into heating the surrounding material.

6. Outgoing vapor is struck by the laser and further energizes, producing plasma.

• The cuts look like,

 

• Dross is metal that has been collected on the underside of the sheet and protrudes as a burr would.

• Laser cutting is often assisted with a gas,

oxygen is used to help when cutting metals. This happens because the oxygen initiates the exothermic reactions to increase cutting rates, and it cools surrounding material. The user must be aware that the oxygen reacts with the heated metals and forms an oxide layer on the cutting edge.

• Gas flow tends to “blow” vaporized metal away from the cutting zone, and minimize the beam absorption in the vapor.

• Slag-collectors and vacuums are used to clear debris and vapors in these systems.

• Cutting speeds are related to material thickness, and laser power.

• Typical laser components are,

laser tube/laser power supply/controls

mirrors to direct laser to end of tool

focusing optics

nozzle with optics, gas delivery, etc.

workpiece fixture

nozzle/fixture positioner

fume extractors for vapors

slag collectors

enclosure

safety interlock system

• Additional laser cutter components are,

CNC machine/robot

diagnostic software/sensors for beam condition

beam splitter for multiple operations.

 

• The major design decisions are,

a) Move the workpiece and maintain fixed optics.

b) Move the “flying optics” and keep the workpiece steady. Large parts are easier to deal with when they don’t move, but the changes in the optics system can cause focussing problems.

• Laser speed example

5KW laser, 5mm thick carbon steel cut at 1m/min.

 

• Some commercial specifications for the Trumpf L5000 are given below,

 

• Summary of Laser machining;

mechanics of material removal: melting, vaporization

medium: normal atmosphere

tool: high power laser

maximum mrr = 5 mm3/min

specific power consumption 1000W/mm3/min

critical parameters: beam power intensity, beam diameter, melting temperature

material application: all materials

shape application: drilling fine holes

limitations: very large power consumption, cannot cut materials with high heat conductivity and high reflectivity.

31.2.1 References

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31.5 Ghosh, A., Manufacturing Science, Ellis Horwood Ltd., Chichester, UK, 1986.

31.6 Weiss, N., “Laser Cutting?”, A Project report submitted for MEC732 in the fall of 1993.

31.3 Problems

Problem 31.1 If the frequency of light from a laser was to be halved, what physical changes would have to be made to the laser?

Problem 31.2 Describe the basic process of laser cutting.

Problem 31.3 What is the major limiting factors for depth of cuts using lasers?

Answer 31.3 The diffusion of the beam, and distortion due to gases limit laser depth to a reasonable range of about one inch.

Problem 31.4 We are to make a cut, that is 1/4in thick and 10in long, using a laser. If the laser kerf (cutting slot) is 1/16in wide, what is the cutting time?

 

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