Laser Beam Machining (LBM)

Prepared by,

Vaishali Desai

Yaminiba Chudasama

M. Tech (CAD/CAM)

Content

Introduction.

Basic Principle.

Laser Concept.

Actual LBM process.

Mechanism of material removal.

Advantages of LBM.

Disadvantages of LBM.

Introduction:-

The term “laser” is an stands for Light Amplification by

Stimulated Emission of Radiation. LBM is nontraditional thermal

process.

In thermal removing processes, thermal energy, provided by a heat

source, melts or vaporizes the volume of the material to be

removed.

Among thermal removal methods,

Electrical discharge machining or EDM is the oldest and most widely

used.

Electron-beam (EBM) and laser beam machining (LBM) are newer

thermal techniques also widely accepted in industry today.

Principle;-

Laser beam machining

is based on the

conversion

of electrical energy

into light energy and

then to thermal

energy.

• Laser Concept

The electrons are charged particles, they carry some energy. The

energy related with the orbit in which the electrons revolve.

Generally the electrons are present in the outer most orbit of the

atom take part in the process of energy absorption or emission.

Ground state, the state with lowest energy is the most stable state for

the electrons. After absorbing the energy electron jump to the higher

energy state and staying there for some seconds jump to the ground

state and release the absorbed energy.

This jump may be in two stages:-

Electron from higher state may not come directly to the ground state but

may halt for some micro seconds on some intermediate state before finally

coming to the ground state.

The period for which electron stays in a higher energy state is known as the

lifetime of that energy state.

Lasing action:-

The emission of photon is not done by only one atom at upper energy level

but on the influence of external light, a sort of chain reactions occurs and

one after other atoms start emitting photons. Thus whole avalanche dumps

down together. This is called lasing action.

Not all the materials are suitable for producing laser beam.

One of the most common laser materials and also one capable of

delivering high power is the chromium on a ruby crystal.

Crystal ruby is aluminum oxide.

Generally the ruby rod 1 cm diameter and 10 cm long with ends

polished fully is used.

Actual LBM Process 1) In the beginning in atom all the crystals are in ground state

2) When the light is flash over the crystal, most of the atoms are raised to

the excited state. Some light waves incline to the axis of the crystal will

leave the box either after only a few reflections or without strike on

mirror.

3) Some of the waves that travel parallel to the axis of the crystal, will

spontaneously emit photon from chromium ions. These photon stimulate

another atom to contribute a second photon.

This process continues as the photons are reflected to and fro between the

mirror.

4) At the each reflection a certain loss occurs

Fig. Laser Mechanism

It is very interesting that laser has to be used on materials where it absorbs

laser energy.

Upon absorption of the laser energy, there is rapid rise in the temperature

leading once again to melting and vaporization and material removal.

Although several types of laser exist, all lasers produce (emit) intense,

coherent, highly collimated beam of single wavelength light. In material

processing applications, this narrow beam is focused by an optical lens to

produce a small, intense spot of light on the work piece surface, as shown

in figure.

Optical energy is converted into heat energy upon impact and temperatures

generated can be high to melt and/or vaporize any material.

Absorb the laser energy

Rapid rise in temperature

Melting & evaporation

Material Removal

Gas Assist Mechanism of

Material

Removal

In

LBM

Advantages:

Excellent control of the laser beam with a stable motion system achieves

an extreme edge quality. Laser-cut parts have a condition of nearly zero

edge deformation, or roll-off

It is also faster than conventional tool-making techniques.

Laser cutting has higher accuracy rates over other methods using heat

generation, as well as water jet cutting.

There is quicker turnaround for parts regardless of the complexity,

because changes of the design of parts can be easily accommodated.

Laser cutting also reduces wastage.

Disadvantages:

The material being cut gets very hot, so in narrow areas, thermal

expansion may be a problem.

Distortion can be caused by oxygen, which is sometimes used as an assist

gas, because it puts stress into the cut edge of some materials; this is

typically a problem in dense patterns of holes.

Lasers also require high energy, making them costly to run.

Lasers are not very effective on metals such as aluminum and copper

alloys due to their ability to reflect light as well as absorb and conduct

heat. Neither are lasers appropriate to use on crystal, glass and other

transparent materials.

Thank You…..!!