USN. Hosur : 6A/6B/6C 10ME665. Discuss briefly. 1 a.

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1 USN 1 P E PESIT Bangalore South Campus Hosur road, 1km before Electronic City, Bengaluru -100 Department of Mechanical Engineering INTERNAL ASSESSMENT TEST 3 Solutions Subject & Code : NTM 10ME665 Name of faculty : DS/MSA/CA Max Marks : 50 Section : 6A/6B/6C Time :90 mins 1 a. PART A Discuss briefly the elements of plasma arc machining. 7 The basic principle is thatt the arc formed between the electrode and the workpiece is constricted by a fine bore, copper nozzle. This increases the temperature and velocity of the plasma emanating from the nozzle. The temperature of the plasma is in excess of C and the velocity can approach the speed of sound. When used for cutting, the plasma gas flow is increased so

2 that the deeply penetrating plasma jet cuts through the material and molten material is removed. The power source required for the plasma arc process must have a drooping characteristic and a high voltage. Although the operating voltage to sustain the plasma is typically 50 to 60V, the open circuit voltage needed to initiate the arc can be up to 400V DC. On initiation, the pilot arc is formed within the body of the torch between the electrode and the nozzle. For cutting, the arc must be transferred to the workpiece in the so called 'transferred' arc mode. The electrode has a negative polarity and the workpiece a positive polarity so that the majority of the arc energy (approximately two thirds) is used for cutting. The plasma gas flow is critical and must be set according to the current level and the nozzle bore diameter. If the gas flow is too low for the current level, or the current level too high for the nozzle bore diameter, the arc will break down forming two arcs in series, electrode to nozzle and nozzle to workpiece. The effect of 'double arcing' is usually catastrophic with the nozzle melting. In the conventional system using a tungsten electrode, the plasma is inert, formed using either argon, argon H 2 or nitrogen. Water injection Nitrogen is normally used as the plasma gas. Water is injected radially into the plasma arc to induce a greater degree of constriction. The temperature is also considerably increased, to as high as 30,000 C. b. List the functions of dielectric in EDM. Dielectric should remain electrically non conductive until the required breakdown voltage is reached, i.e. they should have high dielectric strength. Dielectric should breakdown electrically in the shortest possible time once the breakdown voltage has reached. Dielectric should remain quench the spark after the discharge. Dielectric should provide an effective cooling medium. Dielectric should carry out machined particles from suspension away from the working region. Dielectric should have good degree of fluidity, easily available & economical. 3

3 2 With a neat sketch explain the setup of electric discharge machining 10 Consider the case of a discharge between two electrodes (tool cathode and work anode) through a gaseous or liquid medium. As soon as suitable voltage is applied across the electrodes. The potential intensity of the electric field between them builds up, until at some predetermined value, the individuals electrons break loose from the surface of the cathode and move towards the anode under the influence of field forces. While moving in the inter electrode space, the collide with the neutral molecules of the dielectric, detaching electrons from them and causing ionization. The discharge leads to generation of extremely causing fusion or partial vaporization of the metal and the dielectric fluid at the point of discharge. The metal in the form of liquid drops is dispersed into the space surrounding the electrodes by the explosive pressures of the gaseous products in the discharge in the work piece. Comparatively less metal is removed from cathode(tool) due to following reasons. The momentum at which the positive ions strike the cathode surface is much less than the momentum at which electrons impinge on the work piece surface. A compressive force is generated on the cathode surface by the spark which reduces the tool wear. During metal removal process dielectric would be contaminated by conductive particles. They form colloidal suspensions around the electrodes. Spark discharge leads to an intense ejection of anode particles into the

4 surrounding space. This principle is used in drilling, die sinking, tool preparation. 3 With a neat sketch explain the setup of solid state laser beam machining. 10 Nd YAG laser is pumped using flash tube. Flash tubes can be helical, as shown in Fig. they can be flat. Typically the lasing material is at the focal plane of the flash tube. Though helical flash tubes provide better pumping, they are difficult to maintain. The flash tube is operated in pulsed mode by charging and discharging of the capacitor. Thus the pulse on time is decided by the resistance on the flash tube side and pulse off time is decided by the charging resistance. There is also a high voltage switching supply for initiation of pulses. One end of the glass is blocked with a 100% reflective mirror and the other end is having a partially reflective mirror. Population inversion can be carried out by exciting the gas atoms or molecules by pumping it with flash lamps. Then stimulated emission would initiate lasing action. Stimulated emission of photons could be in all directions. Most of the stimulated photons, not along the longitudinal direction would be lost and generate waste heat. The photons in the longitudinal direction would form coherent, highly directional, intense laser beam. This laser beam is used to remove the material from workpiece. 4 a. Explain electric discharge grinding. 8

6 Another application in EDM is grinding. This process is similar to EDM except that the electrode is rotating wheel of graphite or brass.the wheel can be easily formed to any complex shape and the desired profiles can be obtained on workpiece. EDG wheel rotates at rpm. The table is under servo control to maintain constant gap. Standard EDG machines are available and also conventional grinding machines can be converted and adopted to this process. EDM is advantageously adopted in grinding steel and carbide, thin and fragile sections, grinding of brittle materials and complex forms etc., The obtained surface finish of order of 0.4 micro meter, and an accuracy of± ±0.002mm is quite common. Corner radii can be controlled about 0.1m m b. List few of laser materials in LBM. Many materials can be used as the heart of the laser. Depending on the lasing medium lasers are classified as solid state and gas laser. Solid state lasers are commonly of the following type Ruby which is a chromium alumina alloy having a wavelength of 0.7 μm Nd glass lasers having a wavelength of 1.64 μm Nd YAG laser having a wavelength of 1.06 μm The generally used gas lasers are Helium Neon Argon CO2 etc. 2

7 PART B 5 Explain the process parameters involved in plasma arc machining. Those associated with the design and operation of the torch. Those associated with the physical configuration of the set up. Environment in which the work is associated 10 6 Explain suction flushing, side flushing, pulsed flushing synchronized with electrode movement with sketch. Flushing is defined as the correct circulation of dielectric fluid between the electrodes & work piece. At the start of machining, dielectricc would freee from eroded particles and carbon residue which possesses high insulation property. With successive discharges dielectric gets contaminated, reducing its insulation strength. If density of particles becomes high it may lead to dielectric bridge whichh will lead to abnormal discharges damagingg the tool and work piece. This build up of wear debris is eliminated by flushing. It is achieved by the following ways: Injection Flushing Suction Flushing Side flushing Flushing by dielectric pumping The dielectricc fluid is injected continuously into the working gap either through the work piece or tool. A hole is provided in the work piece or the tool for this purpose Pressure flushing: 10

: In the first case the workpiece is pre drilled and mounted on a pot connected to the flushing

The pressure employed is of the order of 1.5 2.

fig. this is due to the particles being forced up the sides of the electrode producing lateral

Suction Flushing: In this method, the fluid is sucked either through the work piece or the tool

8 : In the first case the workpiece is pre drilled and mounted on a pot connected to the flushing supply. In the second case the electrode is drilled and the dielectric is directly fed through it. The pressure employed is of the order of kg/cm2 The components machine using pressure flushing are always slightly tapered as shown in fig. this is due to the particles being forced up the sides of the electrode producing lateral discharge. Suction Flushing: In this method, the fluid is sucked either through the work piece or the tool electrode. Compared with injection flushing, suction avoids taper effects due to sparking via particles along the sidess of the electrode. Suction flushing through the tool rather than through the work piece has proved to be more efficient.

9 In this method, the fluid is sucked either through the work piece via pot or through the tool electrode. The taper effect is absent in this case. The vacuum pressure of order of kg/cm2. Side flushing: When flushing holes cannot be drilled either in the work piece or the tool this type of flushing is employed. For the entire working area to be evenly flushed, special precautions have to be taken for pumping of dielectric. 7 Discuss the advantages, disadvantages and applications of laser beam machining. Advantages : There is direct contact with tool and workpiece. Machining of any material including non metals is possible, irrespective of their hardness and brittleness 10

10 Welding, drilling and cutting of areas not readily accessible are possible Heat effected zone is small(0.1 mm) Extremely small holes can be machined There is easy control of beam configuration and size of exposed area There is no tool wear problem Soft material like rubber and plastics can be machined Limitations: Its overall efficiency is extremely low (10 15%) The process is limited to thin sheet plates and low material removal rate The machined holes are not round and straight Certain materials like fiber glass reinforced materials, vinyl, etc., cannot be machined as these material burn Life of the lamp is short Effective safety procedures required cost is high Application: Mass micro machining production Sheet metal trimming, blanking and resistor trimming In addition to cutting and drilling laser beam is additionally used in welding Laser welding can be done in inaccessible places such as the inside of sealed glass containers. Dynamic balancing of rotating parts by removing metal at appropriate spots without stopping the part rotation 8 Write short notes on transferred arc and non transferred arc in PAM with sketches. 10

11 Non transferred arc: The arc is formed between the electrode (negative) and the water cooled constricting nozzle (positive). Arc plasma comes out of the nozzle as a flame. The arc is independent of the workpiece and the workpiece does not form a part of the electrical circuit. Just as an arc flame, it can be moved from one place to another and it can be better controlled. This mode is used for plasma spraying or for very low current applications as in non metals. The non transferred arc plasma possesses comparatively less energy density as compared to transferred arc plasma and it is employed for welding and in applications involving ceramics or metal plating. Transferred arc mode: Current is transferred from the tungsten electrode (negative) through the orifice to the workpiece (positive) and back to the power supply. The anode nozzle is connected to positive side of the supply through a suitable resistor to limit current through the nozzle to 50 amps. A transferred arc possesses high energy density and plasma jet velocity. This is the mode most commonly used for welding. So it is employed to cut and melt metals. It can also be used for welding at high arc travel speeds.

PES INSTITUTE OF TECHNOLOGY, BANGALORE SOUTH CAMPUS DEPARTMENT OF MECHANICAL ENGINEERING LESSON PLAN NON-TRADITIONAL MACHINING

PES INSTITUTE OF TECHNOLOGY, BANGALORE SOUTH CAMPUS DEPARTMENT OF MECHANICAL ENGINEERING LESSON PLAN NON-TRADITIONAL MACHINING Faculty name : JAGADEESH BAGALI Subject code : 10ME665 Total No. of Lecture