Downloaded From : www.EasyEngineering.ne UNIT-1 INTRODUCTION Traditional or Conventional Machining ww w.E asy E ngi nee rin g.n e Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne Need for Unconventional Machining • Greatly improved thermal, mechanical and chemical properties of modern materials – Not able to machine thru conventional methods. (Why???) • Ceramics & Composites – high cost of machining and damage caused during machining – big hurdles to use these materials. ww • In addition to advanced materials, more complex shapes, low rigidity structures and micro-machined components with tight tolerances and fine surface finish are often needed. w.E asy E • To meet these demands, new processes are developed. • Play a considerable role in aircraft, automobile, tool, die and mold making industries. Need for Unconventional Machining ngi nee rin g.n e • Very high hardness and strength of the material. (above 400 HB.) • The work piece is too flexible or slender to support the cutting or grinding forces. • The shape of the part is complex, such as internal and external profiles, or small diameter holes. • Surface finish or tolerance better than that obtainable conventional process. • Temperature rise or residual stress in the work piece is undesirable. Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne Unconventional Machining Processes - Classification Electrical ww Mechanical Based Processes w.E asy E USM – through mechanical abrasion in a medium (solid abrasive particles suspended in the fluid) WJM – Cutting by a jet of fluid AWJM – Abrasives in fluid jet. IJM – Ice particles in fluid jet. ngi Abrasives or ice – Enhances cutting action. nee rin g.n e Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne Thermal based Unconventional Processes Thru – melting & vaporizing Many secondary phenomena – surface cracking, heat affected zone and striations. Heat Source: Plasma – EDM and PBM. Photons – LBM Electrons – EBM ww Ions – IBM w.E asy E Machining medium: different for different processes. ngi nee rin g.n e Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne Chemical & Electrochemical based Unconventional Processes CHM – uses Chemical dissolution action in an etchant. ECM – uses electrochemical dissolution action in an electrolytic cell. ww w.E asy E ngi nee rin g.n e The requirements that lead to the development of nontraditional machining. Very high hardness and strength of the material. The work piece: too flexible or slender to support the cutting or grinding forces. The shape of the part is complex, such as internal and external profiles, or small diameter holes. Surface finish or tolerance better than those obtainable conventional process. Temperature rise or residual stress in the work piece are undesirable. Conventional machining involves the direct contact of tool and work piece, whereas unconventional machining does not require the direct contact of tool and work piece. Conventional machining has many Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne disadvantages like tool wear which are not present in Non-conventional machining. Advantages of Non-conventional machining 1) High accuracy and surface finish 2) Less/no wear 3) Tool life is more 4 ) Quieter operation ww Disadvantages of non-conventional machining: w.E asy E 1) High cost 2) Complex set-up 3) Skilled operator required • • • • • • • • • • ngi Need for unconventional machining processes Machining – produces finished products with high degree of accuracy Utilizes cutting tools (harder than work piece material) Needs a contact between the tool and work piece. Needs a relative motion between the tool and work piece. The need for higher productivity, accuracy and surface quality Improve the capability of automation system and decreasing their sophistication (decreasing the investment cost) requirements Very hard fragile materials difficult to clamp for traditional machining When the work piece is too flexible or slender When the shape of the part is too complex Internal and external profiles, or small diameter holes. nee rin g.n e Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne Selection Process • Selection Process is based of following parameters – Physical Parameter – Shapes to be Machined – Process Capability – Economic consideration Physical Parameter ww Process w.E asy E Machines Holes ( Micro, Small, deep,Shallow) LBM, EBM,ECM, USM & EDM Precision Work USM & EDM Horning Etching Grinding Deburring ngi ECM nee rin g.n e ECM & EDM AJM & EDM USM & AJM Threading EDM Profile Cut PAM Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne Process Capability or Machining Characteristics MRR Accuracy ( mm3/s ) Surface Finish (μm) (μm) Power (kW/ cm3/ min LBM 0.10 0.4 – 6.0 25 2700 EBM 0.15 - 40 0.4 – 6.0 25 450 15 - 80 0.25 10 1.8 ECM 27 0.2 -0.8 50 7.5 PAM 2500 Rough 250 0.90 USM 14 0.2 – 0.7 7.5 9.0 AJM 0.014 0.5- 1.2 50 312.5 Process EDM ww w.E asy E ngi nee rin g.n e Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne PROCESS ECONOMY Process Capital Cost Tool & Fixtures Power Requirement Efficiency EDM Medium High Low High CHM Medium Low High Medium ECM V. High Medium Medium V. Low V. Low Low Low Low USM High High Low Medium EBM High Low Low V. High LBM Medium Low V. Low V. High PAM V. Low Low V. Low V. Low Conventional V. Low Low Low V. Low AJM ww w.E asy E ngi nee rin g.n e Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne UNIT-II MECHANICAL ENERGY BASED PROCESSES ABRASIVE JET MACHINING (AJM) In Abrasive Jet Machining (AJM), abrasive particles are made to impinge on the work material at a high velocity. The high velocity abrasive particles remove the material by micro-cutting action as well as brittle fracture of the work material. In AJM, generally, the abrasive particles of around 50 μm grit size would impinge on the work material at velocity of 200 m/s from a nozzle of I.D. of 0.5 mm with a standoff distance of around 2 mm. The kinetic energy of the abrasive particles would be sufficient to provide material removal due to brittle fracture of the work piece or even micro. ww w.E asy E ngi nee rin g.n e It is the material removal process where the material is removed by high velocity stream of air/gas or water and abrasive mixture .An abrasive is small, hard particle having sharp edges and an irregular shape .High velocity jet is aimed at a surface under controller condition . Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne Components Of Abrasive Jet Machining Abrasive delivery system Control system Pump Nozzle Mixing tube ww Motion system w.E asy E 1.Abrasive Delivery System Auto abrasive delivery system has the capability of storing abrasive & delivery the abrasive to the bucket . It’s works auto programming system by help of once measuring record & no adjustment or fine tuning system . High sensitive sensor gives extremely reliable & repeatable . 2.Control system ngi nee rin g.n e The control algorithm that computes exactly how the feed rate should vary for a given geometry in a given material to make a precise part .The algorithm actually determines desired variation in the feed rate & the tool path to provide an extremely smooth feed rate . 3.pump Crankshaft & intensifier pump are mainly use in the abrasive jet machine .The intensifier pump was the only pump capable of reliably creating pressures high .Crankshaft pumps are more efficient than intensifier pumps because they do not require a power robbing hydraulic system ultra high pressure & more stroke per minute . Advantages Extremely fast setup & programming Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne No start hole required There is only one tool Low capital cost Less vibration No heat generated in work piece Environmentally friendly Disadvantages ww Low metal removal rate Due to stay cutting accuracy is affected w.E asy E Abrasive powder cannot be reused Tapper is also a problem WATER JET MACHINING ngi nee rin g.n e Water jet cutting can reduce the costs and speed up the processes by eliminating orreducing expensive secondary machining process. Since no heat is applied on the materials, cut edges are clean with minimal burr. Problems such as cracked edge defects,Crystallization, hardening, reduced weald ability and machinability are reduced in this process. Water jet technology uses the principle of pressurizing water to extremely high pressures,and allowing the water to escape through a very small opening called “orifice” or “jewel”. Water jet cutting uses the beam of water exiting the orifice to cut soft materials. This method is not suitable for cutting hard materials. The inlet water is typically pressurized between1300 –4000 bars. This high pressure is forced through a tiny hole in the jewel, which is typically0.18 to 0.4 mm in diameter Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne ww w.E asy E ngi nee rin g.n e There are six main process characteristics to water jet cutting: 1. Uses a high velocity stream of Ultra High Pressure Water 30,000–90,000 psi (210–620 MPa) which is produced by an with possible abrasive particles suspended in the stream. 2. Is used for machining a large array of materials, including heat-sensitive, delicate or very hard materials. 3. Produces no heat damage to workpiece surface or edges. 4. Nozzles are typically made of sinter 5. Produces a taper of less than 1 degree on most cuts, which can be reduced or eliminated entirely by slowing down the cut process or tilting the jet. 6. Distance of nozzle from workpiece affects the size of the kerf and the removal rate of material. Typical distance is .125 in (3.2 mm) APPLICATIONS OF WJM Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne WJM is used on metals, paper, cloth, leather, rubber, plastics, food, and ceramics. It is a versatile and cost-effective cutting process that can be used as an alternative to traditional machining methods. It completely eliminates heat-affected zones, toxic fumes, recast layers, work hardening and thermal stresses. It is the most flexible and effective cleaning solution available for a variety of industrial needs. ww In general the cut surface has a sandblast appearance. w.E asy E Moreover, harder materials exhibit a better edge finish. Typical surface finishes ranges from 1.6 μm root mean square (RMS) to very coarse depending on the application. Tolerances are in the range of 25 µm on thin material. ngi Both the produced surface roughness and tolerance depend on the machining speed. ADVANTAGES It has multidirectional cutting capacity. No heat is produced. nee rin g.n e Cuts can be started at any location without the need for predrilled holes. Wetting of the workpiece material is minimal. There is no deflection to the rest of the workpiece. The burr produced is minimal. The tool does not wear and, therefore, does not need sharpening. The process is environmentally safe. Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne Hazardous airborne dust contamination and waste disposal problems that are common when using other cleaning methods are eliminated. There is multiple head processing. Simple fixturing eliminates costly and complicated tooling, which reduces turnaround time and lowers the cost. Grinding and polishing are eliminated, reducing secondary operation costs. LIMITATIONS ww Very thick parts can not be cut with water jet cutting and still hold dimensional accuracy. If the part is too thick, the jet may dissipate some, and cause it to cut on a diagonal, or to have a wider cut at the bottom of the part than the top. It can also cause a rough wave pattern on the cut surface. w.E asy E It is not suitable for mass production because of high maintenance requirements. ULTRASONIC MACHINING ngi nee rin g.n e Ultrasonic machining, or strictly speaking "Ultrasonic vibration machining", is a subtraction manufacturing process that removes material from the surface of a part through high frequency, low amplitude vibrations of a tool against the material surface in the presence of fine abrasive particles. The tool travels vertically or orthogonal to the surface of the part at amplitudes of 0.05 to 0.125 mm (0.002 to 0.005 in.). The fine abrasive grains are mixed with water to form a slurry that is distributed across the part and the tip of the tool. Typical grain sizes of the abrasive material range from 100 to 1000, where smaller grains (higher grain number) produce smoother surface finishes. Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne ww w.E asy E An ultrasonically vibrating mill consists of two major components, a transducer and a sonotrode attached to an electronic control unit with a cable. An electronic ngi oscillator in the control unit produces an alternating current oscillating at a high nee rin g.n e frequency usually between 18 and 40 kHz in the ultrasonic range. The transducer usually consists of a cylinder made of piezoelectric ceramic. The oscillating voltage is applied to electrodes attached to the transducer, which converts the electrical energy into mechanical vibrations. The transducer then vibrates the sonotrode at low amplitudes and high frequencies. The sonotrode is usually made of low carbon steel. A constant stream of abrasive slurry flows between t and work piece. This flow of slurry allows debris to flow away from the cutting area. The slurry usually consists of water (20 to 60% by volume) and boron carbide, aluminum oxide and silicon carbide particles.The sonotrode removes material from the work piece by abrasion where it contacts it, so the result of machining is to cut a perfect negative of the sonotrode's profile into the work piece. Ultrasonic Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne vibration machining allows extremely complex and non-uniform shapes to be cut into the workpiece with extremely high precision. Ultrasonic Machining Advantages and Disadvantages Get to know about the advantages and disadvantages of machining process in order to make the right decision: Advantages Machined all sorts of hard materials Produces fine finished and structured results Produces less heat Various hole cut shapes due to vibratory motion of the tool ww w.E asy E Disadvantages Requires a higher degree of integrity and skills No certified record of radiography Unnecessary large grain sizes causes defects Additional repairs might be required due to spurious signs and misunderstanding of the process ngi nee rin g.n e Ultrasonic machines are the future of machining which is used all over the world for creating hard and brittle forms of materials for the industrial uses. A lot of operations can be performed with the ultrasonic machine which can benefit the industrialists in a variety of ways. The advanced technology creates solution which helps in opening up the market opportunities and has made things easier. PRINCIPLE OF ULTRASONIC MACHINING • During one strike, the tool moves down from its most upper remote position with a starting speed at zero, then it speeds up to finally reach the maximum speed at the mean position. • Then the tool slows down its speed and eventually reaches zero again at the lowest position. • When the grit size is close to the mean position, the tool hits the grit with its full speed. Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne • The smaller the grit size, the lesser the momentum it receives from the tool. • Therefore, there is an effective speed zone for the tool and, correspondingly there is an effective size range for the grits. PIEZOELECTRIC TRANSDUCER • Piezoelectric transducers utilize crystals like quartz whose dimensions alter when being subjected to electrostatic fields. • The charge is directionally proportional to the applied voltage. • To obtain high amplitude vibrations the length of the crystal must be matched to the frequency of the generator which produces resonant conditions. ww ABRASIVE SLURRY w.E asy E • The abrasive slurry contains fine abrasive grains. The grains are usually boron carbide, aluminum oxide, or silicon carbide ranging in grain size from 100 for roughing to 1000 for finishing. • It is used to microchip or erode the work piece surface and it is also used to carry debris away from the cutting area. APPLICATIONS It is mainly used for (1) drilling (2) grinding, (3) Profiling ngi nee rin g.n e (4) coining (5) piercing of dies (6) welding operations on all materials which can be treated suitably by abrasives. Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne LIMITATIONS • Under ideal conditions, penetration rates of 5 mm/min can be obtained. • Power units are usually 500-1000 watt output. • Specific material removal rate on brittle materials is 0.018 mm cubic/Joule. • Normal hole tolerances are 0.007 mm and a surface finish of 0.02 to 0.7 micro meters. ww w.E asy E ngi nee rin g.n e Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne Unit-III: ELECTRICAL ENERGY BASED PROCESSES Part - A (2 Marks) ELECTRICAL DISCHARGE MACHINING (EDM) 1. What are functions of dielectric fluid used in EDM? 1. It acts as an insulating medium 2. It cools the spark region and helps in keeping the tool and work piece cool. ww 3. It maintains a constant resistance across the gap. w.E asy E 4. It carries away the eroded metal particles. 2. Basic requirement of dielectric fluid used in EDM? (Dec2005, AU) 1. Stable Dielectric strength. 2. It should have optimum viscosity. 3. It should have high flash point. ngi nee rin g.n e 4. It should be chemically stable at high temperature and neutral. 5. It should not emit toxic vapours. 3. What the dielectric fluids commonly used in EDM? 1. Petroleum based hydrocarbon fluids. 2. Paraffin, white sprite, transformer oil. 3. Kerosene, mineral oil. 4. Ethylene glycol and water miscible compounds. 4. What are the prime requirements of tool material in EDM? 1. It should be electrically conductive. Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne 2. It should have good mach inability. 3. It should have low erosion rate. 4. It should have low electrical resistance. 5. Name some of the tool material used in EDM? 1. Copper, brass, alloys of Zinc &tin. 2. Hardened plain carbon steel ww 3. Copper tungsten, silver tungsten, tungsten w.E asy E 4. Copper graphite and graphite. 6. What is the process parameter efficiency the MRR? 1. Energy discharge 2. Capacitance. 3. Size of work piece. 4. Machine tool design ngi nee rin g.n e 7. Write the formula for finding the energy discharge in EDM? W= (1/2) x EIT W-discharge energy I-Current T-time E-voltage 8. What is the effect of capacitance in EDM? Increasing the capacitance causes the discharge to increase and increase both the peak current and discharge time. 9. How do you increase the inductance of the circuit? Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne A piece of iron or steel be allowed to lodge between the leads it would increase the inductance of the circuit and reduce the M/C rate. 10. Define W/T ratio? It is the ratio of volume of work removed to the volume of tool removed. 11. What is cycle time? It is the sum of discharge time and waiting time. ww 12. Define over cut? It is the discharge by which the machined hole in the work piece exceeds the w.E asy E electrode size and is determined by both the initiating voltage and the discharge energy. 13. Define Rehardening? While metal heated to a temperature above the critical and then rapidly cooled by ngi the flowing dielectric fluid the metal is rehardened. 14. What is recast metal? nee rin g.n e Metal heated to a temperature above the melting point and which is not displaced by the action of the spark discharge, resoldifies as recast metal. 15. Explain electrode wear? A crater is produced in the electrode, which is likewise dependent on the electrode material and the energy of the discharge. 16. What are types of power supply circuits used in EDM? (Dec2007, AU) 1. R-Ccircuit. 2. Rotary impulse generator. 3. Controlled pulse (vacuum tube). 4. Oscillator controlled pulse. Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne 5. Transister pulsed circuit. 17. What are the design factors to be considered while selecting the machine tool? 1. Number of parts to be produced. 2. Accuracy. 3. Size of work piece. 4. Size of electrode. ww 5. Depth of cavity. w.E asy E 18. Why the servo controlled system is needed in EDM? (May 2011, Dec2006, AU) EDM requires that a constant arc gap be maintained between the electrode and the work piece to obtain maximum machining efficiency. Therefore EDM tool in corporate some form of servo control. ngi 19. Define wear ratio? nee rin g.n e Wear ratio=Work piece material removed/Loss of electrode material. Part – B (16 Marks) 1. Explain the process of electrical discharge machining, its process parameters and applications.(Dec2005,Dec2006, AU) (May/June 2015) (May2006, AU) (May 2011, May2007,AU) (May/June 2016) 2. What are the basic requirements of tool materials in EDM process? Name any four tool materials. (6) (May 2011. AU) 3. (i) Explain the different types of control circuits used in EDM process.(10) (May2011,AU) Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne Electro Discharge Machining (EDM) is an electro-thermal non-traditional machining process, where electrical energy is used to generate electrical spark and material removal mainly occurs due to thermal energy of the spark. EDM is mainly used to machine difficult-to-machine materials and high strength temperature resistant alloys. EDM can be used to machine difficult geometries in small batches or even on job-shop basis. Work material to be machined by EDM has to be electrically conductive Process In EDM, a potential difference is applied between the tool and workpiece. Both the tool and the ww work material are to be conductors of electricity. The tool and the work material are immersed in a dielectric medium. Generally kerosene or deionised water is used as the dielectric medium. A w.E asy E gap is maintained between the tool and the workpiece. Depending upon the applied potential difference and the gap between the tool and workpiece, an electric field would be established. Generally the tool is connected to the negative terminal of the generator and the workpiece is connected to positive terminal. As the electric field is established between the tool and the job, ngi the free electrons on the tool are subjected to electrostatic forces. If the work function or the bonding energy of the electrons is less, electrons would be emitted from the tool (assuming it to nee rin g.n e be connected to the negative terminal). Such emission of electrons are called or termed as cold emission. The “cold emitted” electrons are then accelerated towards the job through the dielectric medium. As they gain velocity and energy, and start moving towards the job, there would be collisions between the electrons and dielectric molecules. Such collision may result in ionisation of the dielectric molecule depending upon the work function or ionisation energy of the dielectric molecule and the energy of the electron. Thus, as the electrons get accelerated, more positive ions and electrons would get generated due to collisions. This cyclic process would increase the concentration of electrons and ions in the dielectric medium between the tool and the job at the spark gap. The concentration would be so high that the matter existing in that channel could be characterised as “plasma”. The electrical resistance of such plasma channel would be very less. Thus all of a sudden, a large number of electrons will flow from the tool to the job and ions from the job to the tool. This is called avalanche motion of electrons. Such movement of electrons and ions can be visually seen as Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne a spark. Thus the electrical energy is dissipated as the thermal energy of the spark. The high speed electrons then impinge on the job and ions on the tool. The kinetic energy of the electrons and ions on impact with the surface of the job and tool respectively would be converted into thermal energy or heat flux. Such intense localised heat flux leads to extreme instantaneous confined rise in temperature which would be in excess of 10,000oC. Such localised extreme rise in temperature leads to material removal. Material removal occurs due to instant vapourisation of the material as well as due to melting. The molten metal is not removed completely but only partially. As the potential difference is withdrawn, the plasma channel is no longer sustained. As the plasma channel collapse, it generates pressure or shock ww waves, which evacuates the molten material forming a crater of removed material around the site of the spark. Thus to summarise, the material removal in EDM mainly occurs due to formation of w.E asy E shock waves as the plasma channel collapse owing to discontinuation of applied potential difference. Generally the workpiece is made positive and the tool negative. Hence, the electrons strike the job leading to crater formation due to high temperature and melting and material removal. Similarly, the positive ions impinge on the tool leading to tool wear. In EDM, the ngi generator is used to apply voltage pulses between the tool and the job. A constant voltage is not applied. Only sparking is desired in EDM rather than arcing. Arcing leads to localised material nee rin g.n e removal at a particular point whereas sparks get distributed all over the tool surface leading to uniformly distributed material removal under the tool. Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne ww w.E asy E ngi nee rin g.n e Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne ww w.E asy E Characteristics of EDM ngi nee rin g.n e (a) The process can be used to machine any work material if it is electrically conductive (b) Material removal depends on mainly thermal properties of the work material rather than its strength, hardness etc (c) In EDM there is a physical tool and geometry of the tool is the positive impression of the hole or geometric feature machined (d) The tool has to be electrically conductive as well. The tool wear once again depends on the thermal properties of the tool material (e) Though the local temperature rise is rather high, still due to very small pulse on time, there is not enough time for the heat to diffuse and thus almost no increase in bulk temperature takes place. Thus the heat affected zone is limited to 2 – 4 μm of the spark crater Dielectric In EDM, as has been discussed earlier, material removal mainly occurs due to Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne thermal evaporation and melting. As thermal processing is required to be carried out in absence of oxygen so that the process can be controlled and oxidation avoided. Oxidation often leads to poor surface conductivity (electrical) of the workpiece hindering further machining. Hence, dielectric fluid should provide an oxygen free machining environment. Further it should have enough strong dielectric resistance so that it does not breakdown electrically too easily but at the same time ionise when electrons collide with its molecule. Moreover, during sparking it should be thermally resistant as well. Generally kerosene and deionised water is used as dielectric fluid in EDM. Tap water cannot be used as it ionises too early and thus breakdown due to presence of salts as impurities occur. ww Dielectric medium is generally flushed around the spark zone. It is also applied through the tool to achieve efficient removal of molten material. w.E asy E Electrode Material Electrode material should be such that it would not undergo much tool wear when it is impinged by positive ions. Thus the localis ngi ed temperature rise has to be less by tailoring or properly choosing its properties or even when temperature increases, there would be less melting. Further, the tool should be easily workable as intricate shaped geometric features are machined in EDM. nee rin g.n e Thus the basic characteristics of electrode materials are: •High electrical conductivity – electrons are cold emitted more easily and there is less bulk electrical heating •High thermal conductivity – for the same heat load, the local temperature rise would be less due to faster heat conducted to the bulk of the tool and thus less tool wear •Higher density – for the same heat load and same tool wear by weight there would be less volume removal or tool wear and thus less dimensional loss or inaccuracy •High melting point – high melting point leads to less tool wear due to less tool material melting for the same heat load •Easy manufacturability •Cost – cheap The followings are the different electrode materials which are used commonly in the industry: •Graphite Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne •Electrolytic oxygen free copper •Tellurium copper •Brass Equipment •Dielectric reservoir, pump and circulation system •Power generator and control unit •Working tank with work holding device •X-y table accommodating the working table •The tool holder ww •The servo system to feed the tool w.E asy E ngi nee rin g.n e Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne 4. DESCRIBE THE WIRE CUT EDM EQUIPMENT, ITS WORKING, APPLICATIONS AND ADVANTAGES (DEC2005, AU) . (MAY 2013, AU) (MAY 2011, MAY2007,AU) (16) The Wire Electric Discharge Machining (WEDM) is a variation of EDM and is commonly known as wire-cut EDM or wire cutting. In this process, a thin metallic wire is fed on-to the workpiece, which is submerged in a tank of dielectric fluid such as de-ionized water. This process can also cut plates as thick as 300mm and is used for making punches, tools and dies from hard metals that are difficult to machine with other methods. The wire, which is constantly ww fed from a spool, is held between upper and lower diamond guides. The guides are usually CNCcontrolled and move in the x–y plane. On most machines, the upper guide can move w.E asy E independently in the z–u–v axis, giving it a flexibility to cut tapered and ransitioning shapes (example: square at the bottom and circle on the top). The upper guide can control axis movements in x–y–u–v–i–j–k–l–. This helps in programming the wire-cut EDM,for cutting very intricate and delicate shapes. ngi Wire Cut Electric Discharge Machining (WEDM)The Wire Electric Discharge Machining nee rin g.n e (WEDM) is a variation of EDM and is commonly known as wire-cut EDM or wire cutting. In this process, a thin metallic wire is fed on-to the workpiece, which is submerged in a tank of dielectric fluid such as de-ionized water. This process can also cut plates as thick as 300mm and is used for making punches, tools and dies from hard metals that are difficult to machine with other methods. The wire, which is constantly fed from a spool, is held between upper and lower diamond guides. The guides are usually CNC-controlled and move in the x–y plane. On most machines, the upper guide can move independently in the z–u–v axis, giving it a flexibility to cut tapered and ransitioning shapes (example: square at the bottom and circle on the top). This helps in programming the wire-cut EDM,for cutting very intricate and delicate shapes. In the wire-cut EDM process, water is commonly used as the dielectric fluid. Filters and de-ionizing units are used for controlling the resistivity and other electrical properties. Wires made of brass are generally preferred. The water helps in flushing away the debris from the cutting zone. The flushing also helps to determine the feed rates to be given for different thickness of the materials. Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne ww w.E asy E ngi nee rin g.n e Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne ww w.E asy E ngi nee rin g.n e between the tool-electrode gap thereby creating a path for each discharge. The area wherein discharge takes place gets heated to very high temperatures such that the surface gets melted and removed. The cut particles (debris) get flushed away by the continuously flowing dielectric fluid. WEDM is a non-conventional process and is very widely used in tool steels for pattern and die making industries. The process is also used for cutting intricate shapes in components used for the electric erospace industries. Applications of Wire-Cut EDM Wire EDM is used for cutting aluminium, brass, copper, carbides, graphite, steels and titanium. A schematic of the cutting through wire EDM is shown. The wire material varies with the application requirements. Example: for quicker cutting action, zinc-coated brass wires are used while for more accurate applications, molybdenum wires are used. The process is used in the following areas: Aerospace, Medical, Electronics and Semiconductor applications Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne Tool & Die making industries. For cutting the hard Extrusion Dies In making Fixtures, Gauges & Cams Cutting of Gears, Strippers, Punches and Dies Manufacturing hard Electrodes. Manufacturing micro-tooling for Micro-EDM, Micro-USM and such other micro-machining applications. ww w.E asy E ngi nee rin g.n e Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne Unit-IV: CHEMICAL AND ELECTRO-CHEMICAL ENERGY BASED PROCESSES Part-A (2 Marks) CHEMICAL MACHINING (CHM) 1. What is the principle of Chemical Machining (CHM)? Chemical attacks metals and etch them by removing small amounts of material from the surface using reagents or etchants 2. What are the processing steps in CHM? I. Preparing: pre- cleaning II. Masking: application of chemically resistant material (if selective etching is desired) III. Etch: dip or spray exposure to the etchant IV. Remove Mask: strip remaining mask and clean V. Finish: inspection and post processing ww w.E asy E 3. What is etching factor? Ratio of undercut to depth of cut 4. What are the two major processes in CHM? Two major CHM processes are: I. Chemical milling - eroding material to produce blind details – pockets, channels etc., II. Chemical blanking – for producing details that penetrate the material entirely (holes, slots, etc.) 5. What is the purpose of etchant used in CHM? Give some examples. Purpose: To dissolve a metal by turning it into a metallic salt, which then goes into solution Many chemical are available as etchants:FeCl3, Chromic acid, FeNO3, HF, HNO3 6. What are the criteria used for selection of etchant? Required surface finish Removal rate Material type Etch depth Type of resist Cost 7. What is the purpose of maskant and how is it classified? (May/jun 2015,2016) Maskants (chemically resistant coatings) are used to cover the surfaces which are not to be machined – does not allow the etchant to react reach and react with work piece to dissolve it. 8. Based on the techniques of applying maskants, classify them (May/june 2013) ngi nee rin g.n e Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne Cut and peel - Involve the use of relatively thick material which is scribed and removed to create a selective exposure to the etchant - Neoprene, butyl or vinyl-based material is used as maskants Screen printing - A fine mesh silk or stainless steel screen, which has areas blocked-off to allow selective passage of the maskant is used Photo resist masks – materials that produce etchant resistant images by means of photographic techniques. 9. How are maskants selected? Selection of proper maskant for a particular application is accomplished by evaluation of the job with respect to six factors – chemical resistance, part configuration, quantity of parts, cost, ease of removal and required resolution 10. What are the applications for CHM and photochemical machining (PCM)? (Dec 2014/may-june 15) CHM is used extensively to etch preformed aerospace parts to obtain maximum strength to weight ratios: • Integrally stiffened Titanium engine ducts • Spray etching a rotating tube for cruise missile launch tubes • Thinning and sizing of a delta booster tank bulkhead • Chemical sizing of engine cowl inlet duct skins • Undercut on clad aluminium ww w.E asy E ngi 11. What are the advantages of CHM and PCM? • Metal removal is completely stress free • Complex shapes and deeply recessed areas can be uniformly chemically milled • Extremely thin sections can be chemically milled • Metal hardness or brittleness is not a factor 12. What are the limitations of CHM & PCM? • Fillet radius is approximately equal to depth of cut nee rin g.n e • Extremely deep cuts are usually not cost effective • A homogenous metal structure is normally required for good results • Suitable photographic facilities are not always available 13. Please identify the principle of ECM. How does it differ from electroplating?( May 2009/16) Principle of ECM - electrolysis. When a D.C potential is applied across two electrodes separated by a small gap and an electrolyte is pumped through the small gap, the constituents of the anode work piece material goes into the solution and not plate on the cathode tool. Electroplating is the reverse of ECM where the cathode is plated by the depleted metal from the anode. Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne 14. Define Etchants It is the chemical reagent (etchant) is used to remove the metal from the workpiece. The metal removed by the chemical conversion of the metal into metallic salt 15. What are the various etchant used for various materials? ww Sl Material No Etchant 1 Aliminium Caustic Soda 2 Steel HCl or Nitric acid 3 Stainless steel Iron chloride w.E asy E 4 Magnesium Nitric acid 5 Titanium Nitric acid 16. What are the various maskant used for various materials? Sl Material No maskant 1 Aluminium Butyl rubber, Neoprene rubber 2 Magnesium polymer 3 Nickel Neoprene 4 Titanium Translucent chlorinated polymers 5 Ferrous metals PVC, polythelene ngi nee rin g.n e 17. What are the various methods of masking? Scribed and peeled maskant Photoresists maskants 18. What is the classification of chemical machining process? Chemical blanking – material removed from entire area Contour machining – material removed from selected area only Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne ELECTRO CHEMICAL MACHINING (ECM) ww 1. Define ECM? (Dec,2004) w.E asy E It is the controlled removal of metals by the anodic dissolution in an electrolytic medium, where the work piece (anode) and the tool (cathode) are connected to the electrolytic circuit, which is kept, immersed in the electrolytic medium 2. Write the Faraday’s first law of electrolysis? ngi The amount of any material dissolved or deposited is proportional to the quantity of electrolyte passed. 3. Write the Faraday’s second law of electrolysis? nee rin g.n e The amount of different substances dissolved or deposited by the same quantity of electricity are proportional to their chemical equivalent weight. 4. Write Ohm’s law? Current, I = V/R Where, V = Voltage R = resistance 5. What are the factors that influence oxidation in ECM? (i) Nature of work piece. (ii) Type of electrolyte. (iii) Current density. (iv) Temperature of the electrolyte. Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne 6.What are the materials used to make the tool electrode? (May/June 2013) Copper and copper alloys, titanium, aluminum, brass, bronze, carbon, Monel and reinforced plastics. 7.What are the main functions of electrolysis in the ECM? (Dec 2006) i) For completing the electric circuit between the tool and the work piece and to allow the reaction to proceed efficiently. ii) To remove the products of machining from the cutting region. ww iii) To carry away the heat generated during the chemical reaction. w.E asy E iv) To avoid ion concentration at the work piece- tool gap. 6. What are the properties are expected from the electrolysis used in the ECM? (Dec 2007) i) High thermal conductivity. ii) Low viscosity and high specific heat. ngi nee rin g.n e iii) Should chemically stable even at high temperature. iv) Should be non-toxic and non-corrosive. 7. What is the electrolysis commonly used in ECM? 15 -20 % NaCl in water, sodium nitrate, potassium nitrate, sodium sulphate, sodium chromate and potassium chloride. 8. What are the results which are in improper selection of electrolyte in ECM? (i) Low machining rate. (ii) Over cut and stray cutting. 9. What are the methods generally used to filter the electrolyte? (i) Running the system until it is contaminated completely and replaces it. Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne (ii) Centrifugal separation. (iii) Sedimentation. (iv) Use of clarifiers 10. What are the characteristics of a good ECM tool? (i) It should be a good conductor of electricity and heat. (ii) Easily machinable. ww (iii) Resistant to chemical reaction. w.E asy E (iv) It offers resistance to the high electrolyte pressure 11. What are the problems that occur while improperly selecting the electrolyte flow? Cavitations, stagnation and vortex formation ngi 12. What are the parameters that affect the MRR? nee rin g.n e (i) Feed rate. (ii) Voltage. (iii) Concentration of the electrolyte. (iv) Temperature of the electrolyte. (v) Current density. (vi) Velocity of the electrolyte. 13. How the current density affect the MRR? Current density is controlled not only by the amount of current but also by the size of the gap between the tool and the work piece. A small gap results in high current density, which in turn produces more material removal. 14. What are the advantages of ECM? (i) ECM is simple, fast and versatile method. (ii) Surface finish can be extremely good. (iii) Fairly good tolerance can be obtained. 15. What are the limitations of ECM? (Dec 2007) Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne (i) Large power consumption and the related problems. (ii) Sharp internal corners cannot be answered. (iii) Maintenances of higher tolerances require complicated contours 16. What are the applications of ECM? (Dec,2004) ECM is used for sinking, profiling and contouring, multi hole drilling, trepanning, broaching, honing, steel mill applications, surfacing, sawing, contour machining of hand to hand machine materials. ww 17. What are the various tool materials that can be used effectively with ECM? w.E asy E Generally aluminium, copper, brass, titanium, cupro-nickel and stainless steel are used as tool materials. 18. What factors should be considered in selecting the tool materials in ECM? Good stiffness- Rigidity of the tool construction and material is important because Easy machinability- particularly important if complex shaped tools are requi High corrosion resistance- to protect itself from the highly corrosive electrolyte ngi solution 19. What are the different types ECM operations? Electro Chemical Milling (ECM) Electro Chemical Grinding (ECG) Electro Chemical Honing (ECH) Electro Chemical Deburing (ECD) Electro chemical turning (ECT) Electro chemical trepanning (ECTr) etc., nee rin g.n e ELECTO CHEMICAL GRINDING (ECG) 1. Define ECG. Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne ECG is the material removal process in which the material is removed by the combination of Electro- Chemical decomposition as in ECM process and abrasive due to grinding. 2. Which material is used to make the grinding wheel? Metal bonded diamond (or) Aluminum oxide. 3. What are the important functions of abrasive particles used in ECG? It acts as insulator to maintain a small gap between the wheel and work piece. ww They are electrolysis products from the working area. To cut chips if the wheel should contact the work piece particularly in the event of power failure. w.E asy E 4. What are the advantages of ECG? i) No thermal damage to work piece. ii) Wheel wear is negligible. iii) No distortion of the work piece. 5. What are the disadvantages of ECG? ngi nee rin g.n e High capital costs, because of the special wheel tool. Power consumption is quite high. Electrolyte is corrosive. 6. What are the limitations of ECG? 1. The work material must be conductive. 2. Nit suitable for machining soft material. 3. Require dressing tools for preparing the wheels. 7. What is the application of ECG? 1. Precision grinding of hand metals economically. 2. Grinding Carbide cutting tools inserts. Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne 3. To grind end mill cutters more precisely. 8. Explain the difference between the ECG and conventional grinding? In ECG, there is a grinding wheel similar to a conventional grinding wheel except that the bonding material is electrically conductive. Conventional grinding produces components with good surface finish and dimensional tolerances but such components are also associated with burrs, comparatively large heat affected zone (HAZ), and thermal residual stresses. These defects are not found in electrochemically ground work pieces. 9. What are the advantages of ECG over conventional grinding? ww Reduce cost of grinding (higher cost of equipment compensated by higher MRR, w.E asy E Impr ngi Reduced pressure of work against the wheel and so delicate parts can be manufactured without distortion 10. What is ECD? Identify its applications. ECD is a process for the removal of metal burrs by anodic dissolution, the same principle as ECM. Applications: Generally employed for far away located as well as inaccessible places where nee rin g.n e ECD process has been successfully applied in many different industries ranging from consumer appliances and automotive to biomedical and aerospace products The automotive industry, a heavy user of ECD, employs this process for deburring and radiusing the cross- Not only is ECD applicable to the high-volume environment of the automotive 11. How does ECD differ from ECM? ECD is a special version of ECM used exclusively to deburr or radius workpieces. It differs from ECM in that the electrolyte pressure, electrolyte flow, and current are all Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne relatively low. Another major difference is that the cathode (tool) is usually held stationary in ECD. 12. ECD is same as ECM except much simple to use. Justify the statement. ECD is same as ECM except much simple to use because, there is no feed mechanism needed for the ECD tool (the tool is held stationary). Moreover in ECD, the electrolyte pressure, electrolyte flow, and current are all relatively low. 13. What are the functions served by the electrolyte in the ECM process? ww Removes reaction products w.E asy E Part – B (16 Marks) 1. With the help of a simple schematic diagram, explain the working of Electro chemical machining process. Also show the block diagram of a typical power supply for an ECM ngi machine. (May 2011, AU) (Dec2005, AU) (May2010, AU) (May 2011) (May 2011, AU) (May nee rin g.n e 2013, AU) (May 2013, AU). (May 2013, AU) (May/June 2014) (May/June 2014) (Dec2014) (Dec2014) (May/June 2015) (May/June 2016)(May/June 2016) ELECTRO CHEMICAL MACHINING (ECM, Electrochemical machining (ECM) is a metal removal process base d on the principle of reverse electroplating. In this p rocess, particles travel from the anodic material (workpiece) toward the cathodic material (machining tool). Electrochemical machining (ECM) is a metal -removal process base d on the principle of reverse electroplating. In this p rocess, particles travel from the anodic material (workpiece) toward the cathodic material (machining tool). A current of electrolyte fluid carries away the depleted material before it has a chance to reach the machining tool. The cavity produced is the female mating image of the tool shape. Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne ww w.E asy E ngi nee rin g.n e Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne ww w.E asy E ngi nee rin g.n e . EQUIPMENTS:ECM machines come in both vertical and horizontal types. depending on the work requirements these machines are built in many different sizes as well. the vertical machine is comprised of a Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne base, column, table, and spindle head. the spindle head has a servo-mechanism that automatically advances the tool and controls the gap between the cathode (tool) and the workpiece. WORKING An electrode and work piece (conductor) are placed in an electrolyte, and a potential voltage is applied. on the anode (+ve) side the metal molecules ionize (lose electrons) break free of the work piece, and travel through the electrolyte to the electrode (a cathode; has a -ve charge; a surplus of electrons). in edm an arc was used to heat metal, here the metal dissolves chemically. ww variation in the current density will result in work taking the electrodes shape. the electrode is fed with a constant velocity, and the electrolyte is fed through the tool. the tool is designed to w.E asy E eliminate deposition of the ionized metal on the electrode. SUPPLY V = 8 TO 20V, I = >1000A. · ELECTRODE GAP IS TYPICALLY 0.1 TO 0.2 MM. ngi · MRR IS ABOUT 1600MM3/MIN. PER 1000A, OR 3KWHR FOR 16000 MM3 (NOT VERY EFFICIENT, 30 TIMES MORE THAN STANDARD MACHINING TECHNIQUES). · MRR IS INDEPENDENT OF MATERIAL HARDNESS. nee rin g.n e · GOOD FOR LOW MACHINABILITY, OR COMPLICATED SHAPES. · VERY LITTLE TOOL WEAR, · FORCES ARE LARGE WITH THIS METHOD BECAUSE OF FLUID PUMPING FORCES. · FARADAY'S LAWS STATE THAT, ADVANTAGES:- 1. the components are not subject to either thermal or mechanical stress. 2. there is no tool wear during electrochemical machining. 3. non-rigid and open work pieces can be machined easily as there is no contact between the tool and work piece. 4. complex geometrical shapes can be machined repeatedly and accurately 5. electrochemical machining is a time saving process when compared with conventional machining Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne 2) Describe the process of ECG with a neat sketch (8) (May/June 2015) Electro-chemical grinding (ECG) is a variant process of the basic ECM. It is a burr free and stress free material removal process, wherein material removal of the electrically conductive material takes place through mechanical (grinding) process and electro-chemical process. The abrasive laden grinding wheel is negatively charged and the workpiece is positively charged. They are separated by an electrolyte fluid. The fine chips of the material that is removed from the workpiece (debris) stays in the electrolyte fluid, which is further filtered out. Electrochemical grinding and electrochemical machining are similar processes with a difference that a wheel ww substitutes the tool used in ECM. The wheel shape is similar to the desired work shape w.E asy E ngi nee rin g.n e Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne ww w.E asy E ngi nee rin g.n e Process of Grinding in ECM The main feature of electrochemical grinding (ECG) process is the use of a metallic grinding wheel which is embedded with insulating abrasive particles such as diamond, set in the conducting material. Copper, brass, and nickel are the most commonly used materials while aluminum oxide is a typical abrasive used while grinding steels. The commutator is an electrolytic spindle having carbon brushes and holds the grinding wheel. It receives a negative charge from the DC power supply and the workpiece is given a positive charge. In ECG process, the grinding wheel slightly touches the workpiece. Electrolyte is supplied on-to the grinding wheel near the workpiece such that the wheel carries it through the cutting process thereby resulting in an electro-chemical action. A nozzle similar to the one which carries coolant in a conventional grinding process is provided, which enables the flow of Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne Process Characteristics The life of grinding wheel in ECG process is very high as around 90% of the metal is removed by electrolysis action and only 10% is due to the abrasive action of the grinding wheel. The ECG process is capable of producing very smooth and burr free edges unlike those formed during the conventional grinding process (mechanical). The heat produced in the ECG process is much less, leading to lesser distortion of the workpiece. The major material removal activity in ECG process occurs by the dissolving action through the chemical process. There is very little tool and workpiece contact and this is ideally suited for grinding of the following categories: ww Fragile work-pieces which otherwise are very difficult to grind by the conventional process w.E asy E The parts that cannot withstand thermal damages and The parts designed for stress and burr free applications Applications The applications of ECG process include the following: ngi In production of tungsten carbide cutting tools. In burr-free sharpening of hypodermic needles. In grinding of super-alloy turbine blades. In form grinding of aerospace honeycomb metals. nee rin g.n e In removal of fatigue cracks from steel structures that have been used for underwater applications. The ECG process can be applied to the following common methods of grinding. 1. Face Wheel Grinding. 2. Cone Wheel grinding. 3. Peripheral or Surface grinding. 4. Form Wheel or Square grinding. Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne Unit V: THERMAL ENERGY BASED PROCESSES Part –A (2Marks) LASER BEAM MACHINING (LBM) 1. What is Laser? It is acronym of light amplification by stimulated emission of radiation. 2. What is Maser? ww Laser can be melt diamond when focused by lens system. The energy density being of two order 100,000 KW/cm2. This energy is due to atoms that have light energy w.E asy E level. When such an atom impinge with electromagnetic waves having resonant frequency 3. What are the characteristics of Laser beam? 1. Material removal 2. Material shaping 3. Welding ngi 4. Thermo kinetic change. 4. What are the gases commonly used in LASER? nee rin g.n e The gases commonly used are: He, Ne, Argon, Co2 etc. 5. What are the advantages of Laser drilling? No physical contact between work root pair hence there is no possibility if breakage or wear of root. Precision location is ensured by focusing of the beam Large aspect ratio can be achieved. 6. What are the characteristics of Laser used in Laser machining? 1. Can be focused to maximum intensity or to lower intensity as needed. Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne 2. Can be moved rapidly on the work. 3. Remote cutting over long standoff distances. 7. What are the fundamentals of photons used in Laser? In the Laser the photons are in ground state at 0oC they are brought to the excited state by means of absorption of energy by temperature change, collision etc. 8. What are the emission lines? ww The atoms when these they are bringing down goes to the excited state by stimulated emission and emit photons within 10 nano seconds. They have the same wavelength as w.E asy E the excited photons. 9. What is the Maser principle? The atoms at this state will impinge with electrons waves having resonate frequency. This is known as maser. 10. What is population inversion? ngi nee rin g.n e If the atoms in the excited state are greater than that of the ground state then it is known as population inversion. 11. How does Laser melting works? It melts and vaporizes the unwanted material by means of narrow pulsed laser operating at 2 to 100pilses/sec Because of this high accuracy is not possible to micro sized holes. 12. What is solid state Laser? Solid state Laser is the Lasers, which consist of a hot nat, which may be crystalline solid/ glass, doped with an active material whose atoms provide the lasing action. 13. Write the properties of laser Monochromatic, coherence, divergence, intensity, mode structure Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne ELECTRON BEAM MACHINING (EBM) 1. Define EBM? It is the thermo-electrical material removal process on which the material is removed by the high velocity electron beam emitted from the tungsten filament made to impinge on the work surface, where kinetic energy of the beam is transferred to the work piece material, producing intense heat, which makes the material to melt or vaporize it locally. ww 2. What is the characteristic of the electron beam? (i) High concentrated energy. w.E asy E (ii) Deep penetration into the metals. (iii) Low distortion. (iv) Any material either conductive or non-conductive can be processed ngi 3. Write the application of electron beam? - Thin film machining. - Surface treatment. - Engraving metals and non-metals. - Cutting of materials. nee rin g.n e 4. What are the main elements of the EBM equipment? (i) Electron Gun. (ii) Beam focusing and deflecting units. (iii) Work Table. (iv) Vacuum chamber Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne 5. What is the function of magnetic lens used in EBM? It converges the beam into a narrow spot into the work piece. 6. What are the two types of EBM? (i) Thermal type. (ii) Non-thermal Type. 7. Explain the thermal type EBM? In this type the electron beam is used to heat the material up to the point where it ww is selectively vaporized. 8. Explain Non-thermal type EBM? w.E asy E In this type, the EBM produces a chemical reaction. 9. Write the advantage of EBM? (i) High accuracy. (ii) Any type of material can be processed. (iii) No mechanical or thermal distortion. (iv) No physical or metallurgical damage results. ngi 10. Write the disadvantages of EBM? (i) High cost of equipment. nee rin g.n e (ii) Skilled operator is required for operation. (iii) Limited to 10mm material thickness. 11. Write any four application of EBM? (i) Micro machining application on materials. (ii) Drilling of apertures for electron microscope. Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne (iii) Drilling of holes in ruby and diamond crystal. 12. Write the Richardson-Dushman Equation? J=At2 exp- (EW/KT) Where, J = Current Density A =constant (120 Amphere/cm2deg2) K =Boltzman Constant (1.3x10-23 J/K) ww T = Absolute temperature (Kelvin) w.E asy E W =work function (Volts) 13. Write general formula for focal length of a magnetic lens? f/(S + D) = 25V/(NT)2 ngi Where, V =Electron accelerating voltage nee rin g.n e NT =Ampere turns in the lens winding S =pole piece separation D =Bore diameter F =focal length 14. Why vacuum is needed in EBM? 1) To reduce corrosion 2) To get correct focusing 15. What is the drawback of electron beam machining? One major diameter of electron beam welding has been the requirement of high degree of vacuum essential or satisfactory operation of this process because of degassing. PLASMA ARC WELDING (PAW) Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne 1. Define plasma Plasma is defined as the gas, which has been heated to a sufficiently high temperature to become ionized. 2. What are the advantages of plasma arc welding? a. Exothermic oxidation takes place. b. DC power supply 3. What are the metals that can't be machined by plasma arc machining? ww a. Stainless steel b. Monel c. Super alloys 4. What is the basic heating phenomenon that takes place in plasma arc welding? w.E asy E The basic heating phenomenon that takes place at the work piece is a combination of anode heating due to direct electron bombardment recombination of molecules on the work piece. ngi 5. How the basic plasma does is generated. nee rin g.n e The basic plasma is generated by subjecting a stream of gas to the electron bombardment of the electric arc. 6. How the initial ionization is accomplished in plasma arc machining. A high voltage arc established between electrode and nozzle accomplishes initial ionization. 7. Why does gas formed in plasma do in P.A.M? This gas stabilizes the arc and prevents it from diverging. 8. How another source of heating achieved in P.A.M It is desirable to achieve a third source of heating by injecting oxygen into work area to take advantage of exothermic oxidation. 9. Write the principle of P.A.M Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne Once the material has been raised to molten point the high velocity gas stream blows the material away. 10. Write the circuitry details in PAM. + ve terminal connected to work piece and -- ve terminal connected to electrode. 11. Which type of power supply is used in P.A.M DC power supply is used. 12. Which part is constricted by plasma? ww Nozzle duct is constricted by plasma. Part – B (16 Marks) w.E asy E 1.EXPLAIN THE PRINCIPLE OF LBM WITH NEAT SKETCH (10) (MAY/JUNE 2016) (DEC2006, AU) (MAY 2011, AU) (DEC2014) (MAY/JUNE 2015) Laser Beam Machining – the lasing process Lasing process describes the basic operation of laser, i.e. generation of coherent (both temporal and spatial) beam of light by “light ngi amplification” using “stimulated emission”. In the model of atom, negatively charged lectrons nee rin g.n e rotate around the positively charged nucleus in some specified orbital paths.The geometry and radii of such orbital paths depend on a variety of parameters like number of electrons, presence of neighbouring atoms and their electron structure, presence of to magnetic field etc. Each of the orbital electrons is associated with unique energy levels. At absolutezero temperature an atom is considered to be at ground level, when all the electrons occupy their respective lowest potential energy. The electrons at ground state can be excited to higher state of energy by absorbing energy form external sources like increase in electronic vibration at elevated temperature, through chemical reaction as well as via absorbing energy of the photon. On reaching the higher energy level, the electron reaches an unstable energy band. And it comes back to its ground state within a very small time by releasing a photon. This is called spontaneous emission The spontaneously emitted photon would have the same frequency as that of the “exciting” photon. Sometimes such change of energy state puts the electrons in a metastable energy band. Instead of coming back to its ground state immediately (within tens of ns) it stays at the elevated energystate for micro to milliseconds. In a material, if more number of Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne electrons can be somehow pumped to the higher meta-stable energy state as compared to number of atoms at ground state, then it is called “population inversion”. LASING MEDIUM 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 0.7μm •Nd-glass lasers having a wavelength of 1.64 μm ww •Nd-YAG laser having a wavelength of 1.06 μm w.E asy E ngi nee rin g.n e Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne ww w.E asy E ngi nee rin g.n e Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne ww w.E asy E These solid-state lasers are generally used in material processing. The generally used gas lasers are •Helium – Neon •Argon •CO2. ngi nee rin g.n e Lasers can be operated in continuous mode or pulsed mode. Typically CO 2 gas laser is operated in continuous mode and Nd – YAG laser is operated in pulsed mode electrons, Laser Construction A typical Nd-YAG laser. Nd-YAG laser is pumped using flash tube. Flash tubes can be helical, as shown in or 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 LASER BEAM MACHINING – APPLICATION Laser can be used in wide range of manufacturing applications •Material removal – drilling, cutting and tre-panning •Welding Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne •Cladding •Alloying LASER BEAM MACHINING – ADVANTAGES •In laser machining there is no physical tool. Thus no machining force or wear of the tool takes place. •Large aspect ratio in laser drilling can be achieved along with acceptable accuracy or dimension, form or location •Micro-holes can be drilled in difficult – to – machine materials ww •Though laser processing is a thermal processing but heat affected zone specially in pulse laser processing is not very significant due to shorter pulse duration. w.E asy E ngi nee rin g.n e Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne ww w.E asy E ngi LASER BEAM MACHINING – LIMITATIONS •High initial capital cost •High maintenance cost nee rin g.n e •Not very efficient process •Presence of Heat Affected Zone – specially in gas assist CO2 laser cutting •Thermal process – not suitable for heat sensitive materials like aluminium glass fibre laminate Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne ww 1. Explain the principle of CBM with neat sketch (10) (May/June 2016) (Dec2006, AU) (May 2011, AU) (Dec2014) (May/June 2015) w.E asy E ELECTRON BEAM MACHINING – PROCESS Electron beam is generated in an electron beam gun. The construction and working principle of the electron beam gun would be discussed in the next section. Electron beam gun provides high ngi velocity electrons over a very small spot size. Electron Beam Machining is required to be carried out in vacuum. Otherwise the electrons would interact with the air molecules, thus they would nee rin g.n e loose their energy and cutting ability. Thus the workpiece to be machined is located under the electron beam and is kept under vacuum. The high-energy focused electron beam is made to impinge on the workpiece with a spot size of 10 – 100 μm. The kinetic energy of the high velocity electrons is converted to heat energy as the electrons strike the work material. Due to high power density instant melting and vaporisation starts and “melt – vaporisation” front gradually progresses Finally the molten material, if any at the top of the front, is expelled from the cutting zone by the high vapour pressure at the lower part. Unlike in Electron Beam Welding,the gun in EBM is used in pulsed mode. Holes can be drilled in thin sheets using a single pulse. For thicker plates, multiple pulses would be required. Electron beam can also be manoeuvred using the electromagnetic deflection coils for drilling holes of any shape ELECTRON BEAM MACHINING – EQUIPMENT Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne An electron beam gun, which is the heart of any electron beam machining facility. The basic functions of any electron beam gun are to generate free electrons at the cathode, accelerate them to a sufficiently high velocity and to focus them over a small spot size. Further, the beam needs to be manoeuvred if required by the gun. The cathode as can be seen is generally made of tungsten or tantalum. Such cathode filaments are heated, often inductively, to a temperature of around 25000C. Such heating leads to thermo-ionic mission of electrons, which is further enhanced by maintaining very low vacuum within the chamber of the electron beam gun. Moreover, this cathode cartridge is highly negatively biased so that the thermo-ionic electrons are strongly repelled away form the cathode. This cathode is often in the form of a cartridge so ww that it can be changed very quickly to reduce down time in case of failure Just after the cathode, there is an annular bias grid. A high negative bias is applied to this grid so that the electrons w.E asy E generated by this cathode do not diverge and approach the next element, the annular anode, in the form of a beam. The annular anode now attracts the electron beam and gradually gets accelerated. As they leave the anode section, the electrons may achieve a velocity as high as half the velocity of light. The nature of biasing just after the cathode controls the flow of electrons ngi and the biased grid is used as a switch to operate the electron beam gun in pulsed mode. After the anode, the electron beam passes through a series of magnetic lenses and apertures. The magnetic nee rin g.n e lenses shape the beam and try to reduce the divergence. Apertures on the other hand allow only the convergent electrons to pass and capture the divergent low energy electrons from the fringes. This way, the aperture and the magnetic lenses improve the quality of the electron beam. Then the electron beam passes through the final section of the electromagnetic lens and deflection coil. The electromagnetic lens focuses the electron beam to a desired spot. The deflection coil can manoeuvre the electron beam, though by small amount, to improve shape of the machined holes. Generally in between the electron beam gun and the workpiece, which is also under vacuum, there would be a series of slotted rotating discs. Such discs allow the electron beam to pass and machine materials but helpfully prevent metal fumes and vapour generated during machining to reach the gun. Thus it is essential to synchronize the motion of the rotating disc and pulsing of the electron beam gun. Electron beam guns are also provided with illumination facilityand a telescope for alignment of the beam with the workpiece. Workpiece is mounted on a CNC table so that holes of any shape can be machined using the CNC control and beam deflection in-built in the gun. One of the major requirements of EBM operation of electron beam gun is Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne maintenance of desired vacuum. Level of vacuum within the gun is in the order of 10-4 to 106Torr. {1 Torr = 1mm of Hg} Maintenance of suitable vacuum is essential so that electrons do not loose their energy and a significant life of the cathode cartridge is obtained. Such vacuum is achieved and maintained using a combination of rotary pump and diffusion pump. Diffusion pump is attached to the diffusion pump port of the electron beam gun Diffusion pump is essentially an oil heater. As the oil is heated the oil vapour rushes upward where gradually converging structure as The nozzles ww change the direction of motion of the oil vapour and the oil vapour starts moving downward at a high velocity as jet. Such high velocity jets of oil vapour entrain any air molecule is present w.E asy E within the gun. This oil is evacuated by a rotary pump via the backing line. The oil vapour condenses due to presence of cooling water jacket around the diffusion pump. ngi ELECTRON BEAM PROCESS – PARAMETERS The process parameters, which directly affect the machining characteristics in Electron Beam Machining, are: •The accelerating voltage •The beam current •Pulse duration •Energy per pulse •Power per pulse •Lens current nee rin g.n e •Spot size •Power density As has already been mentioned in EBM the gun is operated in pulse mode. This is achieved by appropriately biasing the biased grid located just after the cathode. Switching pulses are given to the bias grid so as to achieve pulse duration of as low as 50 μs to as long as 15 ms. Beam current is directly related to the number of electrons emitted by the cathode or available in the beam. Beam current once again can be as low as 200 μamp to 1 amp Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne ELECTRON BEAM PROCESS CAPABILITY EBM can provide holes of diameter in the range of 100 μm to 2 mm with a depth upto 15 mm, i.e., with a l/d ratio of around 10. The hole can be tapered along the depth or barrel shaped. By focusing the beam below the surface a reverse taper can also be obtained. Generally burr formation does not occur in EBM. A wide range of materials such as steel, stainless steel, Ti and Ni super-alloys, aluminium as well as plastics, ceramics, leathers can be machined successfully using electron beam. As the mechanism of material removal is thermal in nature as for example in electro-discharge machining, there would be thermal damages associated with EBM. However, the heat-affected zone is rather narrow due to shorter pulse duration in EBM. Some of ww the materials like Al and Ti alloys are more readily machined compared to steel. Number of holes drilled per second depends on the hole diameter, power density and depth of the hole as w.E asy E well as material type ELECTRON BEAM MACHINING – ADVANTAGES AND LIMITATIONS EBM provides very high drilling rates when small holes with large aspect ratio are to be drilled. It can machine almost any material irrespective of their mechanical properties. No mechanical cutting force, work holding and fixturing cost is very less. Fragile and ngi brittle materials can also be processed. nee rin g.n e The heat affected zone in EBM is rather less due to shorter pulses. EBM can provide holes of any shape by combining beam deflection using el Electromagnetic coils and the CNC table with high accuracy. Limitations high capital cost of the equipment and necessary regular maintenance applicable for any equipment using vacuum system. There is significant amount of non-productive pump down period for attaining desired vacuum. Though heat affected zone is rather less in EBM but recast layer formation cannot be avoided Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne ww w.E asy E ngi nee rin g.n e PLASMA ARC MACHINING (PAM) Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne Plasma arc machining is a nontraditional thermal process. Plasma is defined as a gas that has been heated to a sufficiently high temperature to become partially ionized and therefore electrically conductive. The term plasma, as employed in physics, means ionized particles. The temperature of plasma may reach as high as 28000 o C. Various devices utilizing an electric arc to heat gas to the Plasma The plasma arc produced by modern equipment is generated by a plasma torch that is constructed in such a manner as to provide an electric arc between an electrode and workpiece, a. A typical plasma torch consists of an electrode holder, an electrode, a device to swirl the gas, ww and a water-cooled nozzle. The geometry of the torch nozzle is such that the hot gases are constricted in a narrow column. w.E asy E Primary gasses, such as nitrogen, argon-hydrogen, or air, are forced through the nozzle and arc and become heated and ionized. Secondary gases or water flow are often used to help clean the kerf of molten metal during cutting. ngi The stream of ionized particles from the nozzle can be used to perform a variety of industrial nee rin g.n e jobs. The plasma arc, as an industrial tool, is a most heavy employed in sheet and plate cutting operations as an alternative to more conventional oxy-fuel torches or other cutting tools. Plasma arc is routinely used as an integral component of some modern punching machines. Plasma arc methods are also employed in special applications to replace conventional machining operations such as lathe turning, milling and planing, heat treatment and metal deposition operations, and plasma arc welding. PRINCIPLES OF OPERATION Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne ww w.E asy E ngi In PAM, constricting an electric arc through a nozzle, as shown in generates the basic plasma nee rin g.n e jet. Instead of diverging into an open arc, the nozzle constricts the arc into a small cross section. This action greatly increases the power of the arc so that both temperature and voltage are raised. After passing through the nozzle, the arc exists in the form of a high-velocity, well-columnated and intensely hot plasma jet. The basic heating phenomenon that takes place at the workpiece is a combination of heating due to energy transfer of electrons, recombination of dissociated molecules on the workpieces, and connective heating from the high-temperature plasma that accompanies the arc. In some cases, it is desirable to achieve a third source of heating by injecting oxygen into the work area and taking advantage of the exothermic oxidation reaction. Once the material has been raised to the molten point, the high-velocity gas stream effectively blows the material away. For an optimized PAM cutting or machining operation, up to 45% of the electrical power delivered to the torch is used to remove metal from the workpiece. Of the remaining power, approximately 10% go into the cooling water in the plasma generator and the rest is wasted in the hot gas and in heating the workpiece. Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne The jet stream of ionized gases exits at sonic speed and tends to maintain a slightly diverging columnar shape until deflected by solid material. This ionized jet serves as a conductor for the arc; it provides directional stability. The ionized gas may be further shielded from dispersion and heat loss, which result from impacting air molecules, as it exits from the nozzle by means of another annular stream of gas that surrounds the plasma as it leaves the orifice nozzle. The ionized plasma gas is usually inactive to protect the electrode from combustion and ensure long life. When oxygen is added as either the plasma ionized gas or the secondary enveloping gas, the speed of cutting steel is increased. Use of a secondary envelope of gas improves the kerf ww wall appearance on certain metals. This envelope also acts as a protective shield for the nozzle during extensive piercing operations. w.E asy E A major improvement in mechanized plasma arc cutting occurred in recent years with the development of so-called “water injection”. When the water injection technique is employed, the arc is constricted by a flow of water around the arc. This injection of water has many advantages, including: 1. A square cut can be made. 2. Arc stability is increased. ngi 3. Cutting speed can be increased. 4. Workpieces are heated less. 5. Less smoke and fumes are generated. 6. Nozzle life is increases. APPLICATIONS nee rin g.n e Cutting: With appropriate equipment and techniques, the plasma arc can be successfully employed to make cuts in electrically conductive metals. illustrates the general setup for cutting using a plasma arc torch. hole piercing, reproducible and high-quality holes are made rapidly in a variety of materials with the plasma arc. Holes can be pierced much faster than they can be drilled. Plasma arc hole piercing is performed with conventional plasma arc cutting equipment that has been Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne modified to produce a short, carefully controlled arc operating time, suitable arc current programming during the short operating cycle, and effective slag ejection. Almost instantly after ignition the arc rapidly penetrates through the plate forming a hole approximately the same size as the diameter of the arc stream. Continued plasma exposure increases the size of the hole to 4 or 5 times the arc stream diameter. Moving the torch or workpiece in a circular motion can produce larger holes. stack cutting, Plasma is effectively stack-cut stainless steel and aluminum. Plasma stack-cutting ww of thin carbon steel tends to weld the layers making them difficult to separate after cutting. During cutting, the layers should be clamped firmly enough to minimize gaps, but loose enough w.E asy E to permit slippage between layers due to differential expansion. The upper layer may buckle if clamping does not allow slippage. Machining: The plasma arc can be used for “machining” or removing the metal from the surface of a rotating cylinder to simulate a conventional lathe or turning ngi operation. The process As the workpiece is turned, the torch is moved parallel to the axis of the work. The torch is nee rin g.n e positioned so the arc will impinge tangentially on the workpiece and remove the outer layer of metal. Cutting can be accomplished with the workpiece rotating in either direction relative to the torch, but best results are obtained when the direction of rotation permits use of the shortest arc length for cutting. The flow of molten metal being removed must be in such a direction that it does not tend to adhere to the hot surface that has just been machined. Generally, the plasma-arc metal removal process has little if any advantage on easy-to-machine metals, but it has considerable economic advantage for rough metal removal of hard-to- machine metals. The process is generally considered to be a roughing operation, which should be followed by a conventional machine finishing cut. Metal removal rates with the plasma arc process on hard-to-machine metals are up to ten times faster than those achieved on a lathe using a tungsten carbide cutting tool. The principal disadvantages are the metallurgical alteration of the characteristic of the surface profile produced. OPERATING PARAMETERS Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne Cutting: Quality of cut and metal removal rate are largely dependent upon proper attention to operating variables. Several factors contribute to the quality and speed of cuts made by the plasma arc process, including cutting-tip nozzle selection, power level, gas type and mixture, gas flow rate, traverse speed, standoff distance, thickness of material, type of materials, impingement angle, and equipment design. Nozzle size: The highest quality plasma cut is usually obtained when maximum thermal intensity is used. To achieve this, the smallest, or next to the smallest, nozzle size that is capable of operating at a power level suitable for the speed and thickness involved is used. Nozzle size for ww cutting usually ranges from 0.80 to 6.30 mm in diameter. w.E asy E Power: Direct current up to 200 kW and 50-1000 A is employed in plasma arc cutting operations. Gas mixture composition: A proportion of 10% hydrogen and 90% nitrogen or argon usually gives good general-purpose results. Choice of plasma gas ngi composition can have considerable effect on the plasma flame since the ionized gas is the conductive path. The shielding gas or secondary gas can be nitrogen, nee rin g.n e oxygen, air, or carbon dioxide. Water is sometimes used as a shield. Gas flow rate: For any particular nozzle size, an increase in plasma gas (primary) flow permits an increase in current. This increases the power density of the flame and permits greater speed with less taper on the kerf walls. Primary gas flow rates usually range between 0.40-5.60 m3/hr. Secondary gasses are pressurized to flow at up to 11.3 m3/hr. Standoff distance: Due to the columnar shape of the plasma jet, a wide range of tip-toworkpiece spacing is allowable. This permits machine cutting along warped or irregular surfaces. General consideration include: Better quality cuts usually result from a short standoff distance since arc divergence is less and the thermal intensity of the arc is greater. Excessively close standoff distance can promote arcing due to the accumulation of slag drops on the tip. Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne Increased power input is necessary when the standoff distance is great. Standoff distance can range from 6.5-76.2 mm. Machining: In turning operations, torch variables include the electrical power delivered, the gases used to form the plasma, the flow rate of the gases through the torch, the orifice diameter through the nozzle duct, and any secondary gas streams. In general, there is an optimum exitorifice size for operation at a particular power level that produces a well-controlled, highvelocity plasma jet with maximized capacity for performing the material removal operation. Thus gas flow rate, orifice size, and power level are intimately related. ww Metal removal rate: In the physical orientation of PAM operations, such variables as torch w.E asy E standoff, angle to work, depth of cut, feed into the work, and speed of the work toward the torch are involved. Feed and depth of cut determine the volume of metal removed. illustrates the inter-relationship among some of the factors involved in plasma arc turning. Since the torch carriage speed and direction are not normally coupled to the work spindle, as they might be on an ordinary lathe, it is necessary to calculate the carriage speed in order to produce a turned ngi piece with a predetermined pitch (pitch-feed). When a 50kW power level on 50 mm rods is nee rin g.n e used, the maximum metal removal rate for satisfactory surface finish is about 114 cm3/min. Surface finish: Surface finish can vary anywhere from helical ridges along the surface to a depending upon the feed into the work optimization of the process. Plasma arc machining for maximum removal rate does produce a slight helical ridge as the cut progresses. Surface characteristics: One characteristics of the workpiece surface when it is not cooled during an operation is a gradual inward taper in the direction of the cut. This is believed to be due to accumulated heating of the workpiece as the cut progresses and should be minimized or eliminated by appropriate cooling methods. An oxidation scale normally forms behind the cut on an unprotected specimen, but this can be minimized or eliminated by proper shielding. METALLURGICAL EFFECTS Downloaded From : www.EasyEngineering.ne Downloaded From : www.EasyEngineering.ne Metallurgical effects of the PAM process are as widely varied as the materials used and there respective metallurgical histories. In general, the depth of the heat-affected zone is approximately 0.75 mm. For some operations, this hardened material may have to be removed; for other applications, a hardened surface such as this may be desirable. The nature of individual applications of the PAM process determines the metallurgical effects that can be tolerated. ww w.E asy E ngi nee rin g.n e Downloaded From : www.EasyEngineering.ne