Laser Beam Machining (LBM)

The process of material removal using a focused mono – frequency collimated light beam which heats, melts and vaporize work material at the focused area, is known as Laser Beam Machining. A laser is an optical transducer that converts electrical energy into a highly coherent light beam. One must know the full name of laser, it stands for “light amplification of stimulated emission of radiation”. Laser being coherent in nature has a specific property,if it is focused by conventional optical lenses, can generate high power density.

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

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

2. Stimulated or induced emission: It is that 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 will be emitted and such emissions would be proportional to the intensity of light.This stimulated emission is of importance for LBM. However, in the presence of light of appropriate frequency, stimulated emission will occur in the upper energy level when the atoms in this energy level will begin to emit and a chain reaction will take place by causing more to emit.

Process: Figure shows a schematic arrangement 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 gets added to the system to excite electrons through the generation of a very intense light flux operated in a pulsed manner.

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

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

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

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

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

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

Process Details of LBM:

Laser Tube and Lamp Assembly: This is the 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, a power supply unit and a cooling system. This whole setup is fitted inside an enclosure, which carries good quality reflecting surfaces inside. In this setup the 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 to travel to and fro between two reflecting mirrors. The partial reflecting mirror does not reflect the total light back and apart of it goes out in the form of a coherent stream of monochromatic light. This highly amplified stream of light is focused on the work piece with the help of converging lens. The converging lens is also the part of this assembly.

Work piece: The range of work piece material that can be machined by LBM includes high hardness and strength materials 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 melting point and low latent heat.

Cooling Mechanism: A cooling mechanism circulates coolant in the laser tube assembly to avoid its over heating in long continuous operation.

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

Applications of LBM: LBM is used to perform different machining operations like drilling, slitting, slotting,scribing operations. It is used for drilling holes of small diameter of the order of 0.025 mm. It is used for very thin stocks. Other applications are listed below:

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

(b). Machining of mechanical components of watches.

(c). Smaller machining of very hard material parts.

Advantages of LBM:

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

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

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

Disadvantages of LBM: Major disadvantages of LBM process are given below:

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

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

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

(d) Skilled operators are required.

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