65.2 LASER CUTTING

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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.

65.2.1 References

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

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