Laser Beam Machining (LBM)

The process of material removal using a focused mono – frequencycollimated light beam which heats, melts and vaporize work material at thefocused area, is known as Laser Beam Machining. A laser is an optical transducerthat converts electrical energy into a highly coherent light beam. One mustknow the full name of laser, it stands for “light amplification of stimulatedemission of radiation”. Laser being coherent in nature has a specific property,if it is focused by conventional optical lenses, can generate high powerdensity.

Working Principle of LBM: According toEinstein, there could be two types of emissions:

1. Spontaneous Emission: It is thatprocess of emission in which no light is present at the higher energy level andthe atom in an exited state could fall to a lower state emitting a photon andthe process is independent of light intensity.

2. Stimulated or induced emission: It isthat process of emission in which, emission is affected by the presence of light.If a light of right frequency acts on an atom, an increased rate of photon willbe emitted and such emissions would be proportional to the intensity of light.This stimulated emission is of importance for LBM. However, in the presence oflight of appropriate frequency, stimulated emission will occur in the upperenergy level when the atoms in this energy level will begin to emit and a chainreaction will take place by causing more to emit.

Process: Figure shows a schematicarrangement of the LBM process. The process may be explained as follows:

Process: Figure shows a schematic arrangement of the LBM process.The process may be explained as follows:

 The job is held in a fixture below the laser system.

 The Xenon filled flash lamps are switched ON. The energy getsadded to the system to excite electrons through the generation of a veryintense light flux operated in a pulsed manner.

 Crystalline ruby, placed in a reflecting box acts as a lasermaterial. The reflecting box in which ruby is placed, consists of two smallmirrors facing each other. One of the mirrors is fully reflective while themirror at the other end of ruby is partially reflecting.

 Before photon cascade begins to form a laser beam, the atoms inthe laser crystal are in the ground state. Pumping of light raises most of theatoms to the excited state.

 The light waves that travel parallel to the axis of the crystalspontaneously, emit a photon from the excited atoms. These photons stimulateanother atom to contribute a second photon.

 This process continues as the photons are reflected back and forthbetween the ends of two mirrors. Finally, amplification is large enough, thelight energy becomes so powerful that a laser beam bursts out through partiallymirrored end of the crystal causing lasing action. The light beam emitted bythe laser is highly collimated and has high power density.

 The laser beam focused through a lens is directed on the workpiece. The work piece is fed past this beam according to requirement.

LBM uses the light energy of a laser beam to remove material byvaporization. The working principle and the process details (setup) areindicated in the following figure.

Process Details of LBM:

Laser Tube and Lamp Assembly: This isthe main part of LBM setup. It consists of a laser tube, a pair of reflectors,one at each end of the tube, a flash tube or lamp, an amplification source, apower supply unit and a cooling system. This whole setup is fitted inside anenclosure, which carries good quality reflecting surfaces inside. In this setupthe flash lamp goes to laser tube, that excites the atoms of the inside media,which absorb the radiation of incoming light energy. This enables the light totravel to and fro between two reflecting mirrors. The partial reflecting mirrordoes not reflect the total light back and apart of it goes out in the form of acoherent stream of monochromatic light. This highly amplified stream of lightis focused on the work piece with the help of converging lens. The converginglens is also the part of this assembly.

Work piece: The range of work piecematerial that can be machined by LBM includes high hardness and strengthmaterials like ceramics, glass to softer materials like plastics, rubber wood,etc. A good work piece material must have high light energy absorption power,poor reflectivity, poor thermal conductivity, low specific heat, low meltingpoint and low latent heat.

Cooling Mechanism: A cooling mechanismcirculates coolant in the laser tube assembly to avoid its over heating in longcontinuous operation.

Tool Feed Mechanism: There is no toolused in the LBM process. Focusing laser beam at a pre – decided point in thework piece serve the purpose of tool. As the requirement of being focusedshifts during the operation, its focus point can also be shifted gradually andaccordingly by moving the converging lens in a controlled manner. This movementof the converging lens is the tool feed mechanism in LBM process.

Applications of LBM: LBM is used toperform different machining operations like drilling, slitting, slotting,scribing operations. It is used for drilling holes of small diameter of theorder of 0.025 mm. It is used for very thin stocks. Other applications arelisted below:

(a). Making complex profiles in thin andhard materials like integrated circuits and printed circuit boards (PCB).

(b). Machining of mechanical componentsof watches.

(c). Smaller machining of very hardmaterial parts.

Advantages of LBM:

(a) Materials which cannot be machined byconventional methods, are machined by LBM.

(b) There is no tool so no tool wear.

(c) Application of heat is very muchfocused, so rest of the work piece is least affected by the heat. (d)Drills very hard and precise holes and cavities.

Disadvantages of LBM: Majordisadvantages of LBM process are given below:

(a) High capital investment is involved.Operating cost is also high.

(b) Recommended for some specificoperations only as production rate is very slow.

(c) Cannot be used comfortably for highheat conductivity materials light reflecting materials.

(d) Skilled operators are required.

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