Textbook of Production Technology PDF

394 A Textbook of Production Technology Electrodes = 8 to 15% Power and Equipment = 2% On the other hand, for RW processes, the cost may be mostly for power and equipment and litt...

394 A Textbook of Production Technology Electrodes = 8 to 15% Power and Equipment = 2% On the other hand, for RW processes, the cost may be mostly for power and equipment and little for labour and non for consumable electrodes. 5.10. DESIGN ASPECTS The following points should be kept in mind when designing a weldment : 1. Weldments should be designed to require a minimum of weld metal. 2. Thermal contraction of metal, which has been heated by welding, may cause internal residual stresses and distortion. These can be controlled or reduced by : (a) Preheating (b) Minimum number of welds (c) Smallest size of weld that fulfills requirements (d) Maximum use of intermittent welds (e) Slow after cooling. 3. Sharp discontinuities in metal should be kept at a minimum since these cause stress concentration. 4. An important strength weld should not be located where much of it may be removed later by machining. 5. Welds should be located so that adequate strength will be provided at the proper places on a structure or part. 6. As far as possible, a straight line force pattern should be provided. 7. Laps, straps and stiffening angles should be avoided except as required for strength. 8. Lap welds and lap strap welds are not recommended for elements over 10 mm thick. 9. Where ever possible, use butt welds. 10. The ends to be welded should be of equal thickness. 11. Welds at the vulnerable cross-sections should be avoided. 12. The use of welding fixtures should be avoided as far as possible. 13. Welds should not be subjected to bending. 14. A weld should not be located at the point of maximum deformation. 15. Ribs should be designed correctly and these should be used with care. 16. Provide for easy access to welds so that they are accessible for inspection. 17. Distribute heavy loading over long welds in the longitudinal direction. 18. Avoid large flat walls, which tend to bulge and flex. 19. The joint should have properly prepared grooves. 20. If alternating stresses are involved, avoid running a weld at right angles to the direction of maximum principal stress owing to the low fatigue resistance offered by welds. 21. Whenever possible, the design should provide for welding in the flat or horizontal position, not overhead. 22. Some design can avoid or minimise edge preparations. 23. Weld bead size should be kept to a minimum to conserve weld metal. 24. Components should fit properly before welding. 5.10.1. Heat input and Efficiency Calculations (a) Electric Arc Welding :– We know that the power input by the heat source is given as, P = V × I, watts, where The Welding Process 395 V = potential of the power source, volts I = Current, amperes. Heat input into the workpiece = P × efficiency of heat input/transfer  Hi = P ×  t , watts (J/s) Now heat needed for melting of the workpiece, Hm = Heat needed to melt a unit volume of workpiece × volume of workpiece melted per unit time. Hm  Melting efficiency, m  H i H = Heat energy input (J/mm) = P/v , where v = velocity of heat source (mm/s) (b) Electric Resistance Welding :– We have already noted that the amount of heat generated at the contacting area of the elements to be welded, is given by Joule's law as, Q = I2 Rt, Joules where, I = Current in amperes R = Resistance of the circuit at the contacting area of the elements, in ohms t = time during which the current flows, in seconds. Example 1 :– Calculate the melting efficiency in the case of arc welding of steel with a potential of 20 V and current of 200 A. The travel speed is 5 mm/s and the cross-sectional area of the joint is 20 mm2. Heat required to melt steel may be taken as 10 J/mm3 and the heat transfer efficiency as 0.85. (PTU Dec. 2004) Solution : The power input by the heat source is, P = V × I, watts = 20 × 200 = 4000 watts Heat input into the workpiece, Hi = P ×  , watts t = Heat transfer efficiency = 0.85  Hi = 4000 × 0.85 = 3400 watts Now heat needed to melt the workpiece material, Hm = volume of material melted per unit time × Heat needed to melt a unit volume of the material Now heat needed to melt a unit volume of material (here steel) is given as = 10J/mm3 Again volume of material melted per unit time = 20 × 5 = 100 mm3/s Note :– Heat needed to melt a unit volume of material is also given approximately as, (Tm  273) 2 H vol.  , J/mm3 300, 000 where Tm = Melting point of the material, °C Now, Hm = 10 × 100 = 1000 J/s = 1000 watts 396 A Textbook of Production Technology Hm 1000  m    29.41% Hi 3400 Example 2: Two steel plates each 1 mm thick are spot welded at a current of 5000 A. The current flow time is 0.1 s. The electrodes used are 5 mm in diameter. Determine the heat generated and its distribution in the weld zone. The effective resistance in the operation is 200  . Solution. Heat generated, Q = I2. R.t. joules = (5000)2. 200 × 10–6.0.1= 500 J In spot welding, there is a depression on the free surface of the plates, so the actual weld area can be taken as a cylinder of 5 mm diameter and height less than (1 + 1 = 2 mm), say 1.5 mm.  2  Weld nugget volume   5 1.5  30 mm3 4  Heat required for melting = 30 × 10 (from example 1) = 300 J  Heat dissipated into the metal surrounding the nugget = 500 – 300 = 200 J. Note :– Heat required for melting can also be determined as follows :– It has been seen that the heat needed to melt 1 gm of steel = 1400 J Now taking the density of steel as 7800 kg/m3, the mass of nugget = 0.0078 × 30 = 0.234 gm  Heat needed for melting = 0.234 × 1400 = 327.6 J  Heat dissipated = 500 – 327.6 = 172.4 J Example 3:– How much heat would be generated in the spot welding of two sheets of 1 mm thick steel that required a current of 10000 A for 0.1 seconds ? An effective resistance of 100   is assumed. Solution : – Heat generated is given as, Q = I2. R.t, Joules = (10,000)2 × 100 × 10–6 × 0.1 = 1000 J Example 4 :– Two 1.2 mm thick, flat copper sheets are being spot welded using a current of 6000 A and a current flow time of t = 0.18 s. The electrodes are 5 mm in diameter. Estimate the heat generated in the weld zone. Take effective resistance as 150  . Solution :- Q = I2 Rt, joules = 6000 × 6000 × 150 × 10–6 × 0.18 = 972 joules Example 5:– Calculate the temperature rise in the Example 4, assuming that the heat generated is confined to the volume of material directly between the two electrodes and the temperature is distributed uniformly. Solution :– Nugget can be taken as a cylinder of 5 mm diameter and 2 mm height.   Volume of material   25  2  40 mm3 4 Now density of copper  9000 kg/m3  Mass of material = 0.36 gm Now Q  C.m.T The Welding Process 397 C = specific heat, J/gm K = 0.3936 for Cu. (For steel, it is  0.46 )  972 = 0.3936 0.36 × T  T  6860C Example 6 : What amount of heat would be required to melt 200 mm3 of metal, whose melting temperature is 1050°C. Solution :– Heat needed to melt a unit volume of material is (Tm  273)2 given as, H vol.  , J/mm3 300, 000 Now, Tm = 1050°C 13232   5.83 J/mm3 300,000 3  Amount of heat needed to melt 200 mm of material 5.83 × 200 = 1166 J General Characteristics of Common Welding Processes are Given in Table 5.5, Below : Table 5.5. Characteristics of Welding Processes Materials to be Min. Costs operator Opera- Welding welded Thick- skill tion Process Gene- Pre ness Equip-ment Labour Fini- ral ferred mm shing OAW All but C.I., 0.6 D-E* A A A Manual refractory Steels SMAW All but Zn Steels 1.5 D A A-B A -do- - 2.0 FCAW All Steels L-C 1.5 B-D A-D A-C A-D All Steels SAW All Steels -do- 5 B-C B-D A-C C-D Auto- matic MIG All but Zn Steels, 0.5 B-C A-C B-D A-D All Al, Cu TIG -do- All but 0.2 B-C A-C B-E A-D All Zn EBM -do- -do- 0.05 A A-D C-E A-D All * A indicates the highest value and E indicates the lowest value. 5.11. HARD FACING OF METALS Hard facing or surfacing is the technique of depositing a layer of hard material on a component to increase the hardness, strength and wear resistance of the base metal. The technique is widely used in bearings, camshafts, valves and valve seats, hot extrusion dies, closed dies especially for abrasive powders, earth handling and mining equipment of many types such as rock drills, stone crushers etc., hammer mills, shear blades, and many types of trimming and cutting dies. 398 A Textbook of Production Technology The composition of the surfacing metal differs from that of the base metal. Hard facing materials include stellite and other cutting and wear-resistant alloys. Tips or rods from 5 to 10 mm thick, cast from stellite alloy, are used in the hard facing of tools by welding techniques. The cutting tool materials with very hard phases (mostly carbides) have such a high alloying-element concentration that they can not be manufactured into welding rods. The ingredients are incorporated in the flux coating or packed inside tubular rods, and the alloy is formed in the welding process itself. Hard facing is done by means of gas, arc, or shielded arc welding techniques. Gas and shielded arc welding ensures a more uniform composition of the deposited layer. Surfacing by ordinary arc welding is faster and cheaper, but there is greater danger of dilution of the surfacing metal with the base metal. Deposition of tungsten carbide by an electric arc is called ‘‘spark hardening’’, useful for cutting tools. When thick layers are deposited, one speaks of ‘weld overlays’. However, the thickness of the deposit should not, as a rule, exceed 2 mm, because the susceptibility to cracking increases with thicker layers. The hard facing techniques and conditions should ensure a strong bond of the deposit with the base metal, restrict their mixing and avoid the formation of cracks and other defects in the deposited layer. Parts to be hard faced are first preheated to 3500 to 500°C, the hard faced parts are to be cooled slowly. Hard facing increases the service life of certain parts by 3 or 4 times and enables worn parts to be repeatedly restored. 5.12. CHARACTERISTICS OF WELDING PROCESSES The characteristics of the main welding processes are given next page in Table 5.6. Table 5.6. Main Welding Processes and their Characteristics S. Welding Welded Recommended Type of Methods of No. process Materials thickness or welded joint cleaning cross-section of components components welded before welding 1. Gas welding Steel, Al < 2 mm – 10 mm Butt, Edge By steel-wire alloys, Cu brush alloys, hard alloys 2. PGW Steel < 25,000 mm2 Butt Machining ends with cutting 3. SMAW Steel, Al > 1.5 – 2 mm Butt, Lap, T-, By steel-wire Alloys and Edge brush 4. Carbon-arc Low C-Steel, > 4 – 12.0 mm Butt and -do- Welding Al, Cu Edge 5. SAW Steel > 2 – 2.5 mm Butt, Lap, T-, -do- or gas and Edge flame 6. Argon-shield Stainless < 4 mm Butt, T and By steel wire ed arc welding steel, Edge brush Al and Mg alloys The Welding Process 399 7. Resistance Low-carbon, < 12 mm, < 10 mm, Lap Cold-drawn spot welding alloyed and < 6 mm, < 2.5 mm steel without stainless cleaning; hot steels; rolled steel Al and Cu cleaning by alloys etching, sand blasting or machining 8. Flash butt Steel and Butt By welding alloys steel-brush 9. Upset Butt Steel, Al upto 1000 mm2 -do- Ends Welding Alloys and cleaning Cu alloys by machining 10. Atomic Alloyed steels Bars < 10 mm Butt, T- and By steel-wire hydrogen Edge brush welding 11. Cold welding Low C-, < 8 mm, < 2 mm Lap Sand blasting (Roll welding) alloyed, and or machining stainless steels; Al and Cu alloys 12. LBW Chemically < 1 mm Lap Careful aggressive cleaning and and degreasing refractory not metals and essential alloys 13. EBW -do- Butt -do- 14. USW Similar and < 10 mm, Lap -do- dissimilar < 0.05 – 0.5 mm metals and alloys 15. Friction Carbon and Round section Butt By steel-wire Welding alloys steels, components < 40 mm brush or sand non-ferrous dia. blasting. alloys 16. ESW Carbon and 20 – 600 mm Butt -do- alloyed steels 400 A Textbook of Production Technology PROBLEMS 1. Define the ‘‘welding process’’. 2. Give the applications of the “welding process”. 3. Write the advantages and drawbacks of the “welding process”. 4. How the “welding process” may be classified ? 5. Differentiate between “autogeneous”, “homogeneous”, and “hetro- geneous” welding processes. 6. Sketch the various weld joints. 7. Sketch the various types of welds used in making a joint. 8. Sketch and write on the various edge preparations used for welded joints. 9. Sketch and write on the various “welding positions”. 10. Why the cleaning of a joint is important before welding ? 11. What is meant by “fluxing” ? Why it is done ? What are the properties which a good flux should possess ? 12. Define : OAW and PGW processes. 13. Sketch and compare the two systems of OAW process. 14. Write on the “Gas welding equipment”. 15. Sketch the three types of flames used in OAW process. Give the uses of each. 16. Write the steps in lighting up the OAW flame. 17. Write the closing down procedure of OAW flame. 18. Define power of blow pipe and nozzles. 19. Sketch and compare the two welding techniques used in OAW process. 20. Sketch and explain PGW process. 21. Write about the other gases used in OW process, in place of acetylene. 22. Write on “oxy-acetylene flame cutting”. How the cutting tip differs from a welding tip ? 23. Define “electric arc welding”. 24. List the principal advantages of : (a) Arc welding over gas welding. (b) Gas welding over arc welding. (c) D.C. arc welding over a.c. arc welding. 25. Sketch the two polarities of d.c. supply and compare these for welding process. 26. Write on the different types of electrodes used in arc welding. 27. What is the purpose of coating on an arc welding electrode ? 28. Write the constituents of a “coating” and write the function of each. 29. Write on “coding” of electric arc electrodes. 29. (a) In a resistance welding process, the applied voltage is 5V. Determine the rate of heat generated per unit area with 25 bridges/cm2, each brdige having a radius of 0.1mm. The resistivity of the material is given to be 2 × 105 Ohm-cm. (PTU; Ans. 1.136 × 105 J/s/cm2) 30. Name the ten methods of arc welding. Of these methods, which is the most widely used ? 31. Explain the following electric arc welding processes with the help of neat sketches : a. SMAW b. FCAW c. GTAW d. GMAW f. Atomic hydrogen welding g. Electro-slag welding h. PAW i. Stud welding e. SAW The Welding Process 401 32. Name the shielding gases used in “Inert gas arc welding” methods and compare them. 33. What metals can be joined by “shielded arc welding” ? What are some of the most important advantages gained by this type of welding ? 34. Write a note on “electric arc cutting”. 35. Define “Resistance welding process”. 36. Name six types of resistance welding methods. For what kind of production is resistance welding mainly employed ? 37. With the help of a neat sketch explain the “spot - welding method”. 38. What metals may be spot - welded ? Can dissimilar metals be spot - welded ? 39. Sketch and write on the various spot welding machines. 40. How does “seam welding” differ from “spot welding” ? 41. From what materials are spot welding electrodes usually made ? 42. What are the special features of “resistance projection welding” ? 43. With the help of neat sketches explain the following welding methods: a. Upset butt welding b. Flash butt welding c. Percussion butt welding. 44. What is the difference between “flash” and “upset welding” ? 45. With the help of neat sketches explain the following welding methods: a. Forge welding b. Induction welding c. Friction welding d. Ultra-sonic welding e. Explosive welding. f. Cold welding g. Electron-beam welding h. Laser-beam welding i. Thermit welding 46. For what commercial applications can the EBW process be economical? 47. What are “flow welding” and “diffusion welding” processes ? 48. Distinguish between “welding”, “brazing” and “soldering” processes. 49. Which method of resistance welding is used to join dissimilar metals ? 50. Distinguish between “brazing” and “braze welding”. 51. Write the steps to be taken in brazing process. 52. Write about the various fluxes used in brazing process. 53. Write about the filler materials used in brazing process. 54. Write a note on the various brazing methods. 55. Write the advantages and limitations of brazing process. 56. Write the common uses of brazing process. 57. Distinguish between “soft solder” and “hard solder”. 58. Write about the various soldering techniques used 59. Define ‘‘weldability”. 60. What effect does carbon content of steel have on weldability ? 61. What are the effects of the following elements on weldability ? Mn, Si, P, and S. 62. What is the purpose of preheating a part to be welded ? How would you select a preheat temperature? 63. Write briefly on “Testing and Inspection of welded joints”. 402 A Textbook of Production Technology 64. What causes weldments to crack ? Explain the reasons and suggest the remedies. 65. Write the various welding defects. Give their reasons and suggest the remedies. 66. Write a note on welding costs. 67. Give the design aspects of a weldment. 68. Write on “hard facing of metals”. 69. List the safety measures to be taken in welding shop. 70. Explain the principle of Arc welding. 71. List the work preparation for welding operation. 72. Identify various welded joints by BIS symbols. 73. List the major advantages and limitations of OAW. 74. Write about selection of filler rod and flux for OAW. 75. List the functions performed by the electrode coatings. 76. How electrodes are selected for a job ? 77. List the common solders used in the soldering method. 78. Write the various fluxes used in the soldering method. 79. State applications of Soldering process. 80. State the advantages and limitations of soldering pocess. 81. Explain fusion as it relates to welding processes. 82. Explain the chemical reaction that takes place in an OAW torch. 83. What is the level of temperature obtained in the three types of flames of an OAW torch ? 84. Why for welding Brasses and Bronzes, an oxidizing flame is desirable ? 85. Why is SMAW a commonly used method of electric arc welding ? Why is it also called stick welding and MMAW ? 86. Why is the quality of SAW very good ? 87. Why a flux is not needed in GTAW and GMAW ? 88. Why is tungsten the preferred material for non-consumable electrodes ? 89. What is the advantage of EBW and LBW as compared to arc welding ? 90. What are : Weld Zone, Fusion Zone, Weld metal Zone and Heat affected Zone ? 91. Discuss the matalurgical effects due to thermal gradients in HAZ. 92. Why is OAW limited to rather thin sections ? 93. What is Arc stability ? How is it achieved ? 94. What is Arc blow ? How is it avoided ? 95. What are the sources of weld spatter ? How can it be controlled ? 96. List a few products that can be fabricated by resistance welding processes. 97. Explain the principle of Electric Resistance Welding/ 98. Describe the features of a fusion weld. Identify the different zones. 99. Differentiate between Hot welding and Cold welding. 100. How can cracking in weldments be aboided ? 101. The voltage – arc length characteristic of a D.C. arc is given by V = (20 + 40L) volts, where L is the arc length in cm. The static – volt ampere characteristic of the power source is approximated by a striaght line with no load voltage of 80V and a short circuit current of 1000 A. Determine the optimum are length and the corresponding Arc power. (GATE, 1991) (Ans. 0.5 cm, 20 kVA) The Welding Process 403 102. The voltage – arc length of a.d.c. source is given by V = (20 + 4L) volts, where L = length of the arc in mm. The arc length is expected to vary between 4 mm and 6 mm and it is desired to limit the current in the range 450 to 550 A. Assuming a linear power source characteristic determine the open circuit voltage and the short circuit current of the power source. (Ans. 80 V, 1000 A) 102. (a) In a given are welding operation, the power source is at 20V and current at 300A. If the electrode travel speed is 6mm/s, calculate the cross-sectional area of the joint. The heat transfer efficiency may be taken as 0.80 and melting efficiency as 0.30. Heat required to melt the steel is 10 J/mm3. (PTU, Ans. 24 mm2) 103. Describe the formation of slag in welding. 104. Discuss the effect of atmospheric gases on welding. 105. Name a few of coating materials and their properties. 106. Which of the following gases is not present in the atmosphere : (a) Argon (b) CO2 (c) H2 (d) He (DU, AMIE) 107. Which of the following gases is removed by the process of reduction : (Ans. c) (a) He (b) H2 (c) O2 (d) N2 (Ans. c) 108. Name a few of the inert gases and their functions in welding. (AMIE, MU, UT) 109. What is the technical meaning of :– (a) Hot cracking (b) Cold cracking (c) Grain growth (d) Recrystallization (L.U.) 110. Which of the following would help to reduce distortion ? (a) Concentration of welding to one area. (b) Increasing the input of welding heat. (c) Use of single V-preparation (d) Use of welding sequence. (Ans. d) 111. Which one of the following statements is incorrect. (a) The greater the distortion, the less the residual stress. (b) The greater the distortion, the greater the residual stress. (c) The greater the restraint, the greater the residual stress. (d) The greater the weld concentrations, the greater the residal stress. (Ans. b) 112. Describe the effects of distortion on welded structure. 113. The type of crystal normally found in a single run are weld in the as welded condition is :- (a) Equi-axed (b) Columnar (c) Polycrystalline (d) Dendritic (Ans. b) 114. When weld metal refinement takes place in a multi-run deposit, it is known by the term : (a) Weld annealing (b) Weld refining (c) Weld Normalizing (d) Weld recrystallization (Ans. c) 115. The first sub-zone in the HAZ of the present metal nearest the weld metal deposit will consist of : (a) Large crystal grains (b) Small crystal grains (c) Elongated crystal grains (d) Distorted crystal grains (Ans. a) 116. Which one of the following statements is correct ? (a) Preheating increases hardness (b) Preheating increases cooling (c) Preheating increases dilution (d) Preheating increases shrinkage stress. (Ans. c) 117. During welding, the parent metal in HAZ undergoes certain changes, Discuss these changes. 118. Discuss the effects of preheating. 119. Describe the welding of Cast Iron. (DU, AMIE, UPSC) 120. What is the effect of Carbon in welding of plain carbon steels. 404 A Textbook of Production Technology 121. Which one of the following NDT would be used to examine a completed weld for surface defects : (a) Ultrasonics (b) Dye-penetrant (c) Radiography (d) Acoustics (I. Mech E., AMIE) (Ans. b) 122. If a fabricated vessel is to be pressure tested using water, it will come under the heading of : (a) Ultrasonics (b) Pneumatics (c) Hydraulics (DU) (Ans. c) 123. Describe the testing of weldments with penetrants (a) Dye (b) Fluorescent 124. State some of the NDT used for testing weldments. Give their advantages and disadvantages. 125. The voltage- arc length characteristics of a power source is V = 20 + 40 L, where V = operating voltage, L = arc length, mm. Determine OCV and short circuit current for arc length ranging from 3 to 5 mm and the current from 400 to 500 A during welding operation. (GATE, 1993) Solution : – Voltage - arc length characteristic is :— V (voltage drop across arc) = 20 (electrode drop) + 40 L (column drop). Power source characteristic is :— V I  1 OCV SCC Now V1 = 20 + 40 × 3 = 140 V V2 = 20 + 40 × 5 = 220 V 140 500 200 400   1 and  1 OCV SCC OCV SCC From here, OCV = 540 V and SCC = 675 A Note :— For welding arc V = a + bL.... (1) For Power source, ⎛ OCV ⎞ V = OCV − ⎜ I... (2) ⎝ SCC ⎟⎠ where I = Arc current, V = Arc voltage OCV = open circuit voltage, SCC = short circuit current For stable arc, (1) = (2)  Chapter 6 Machining Process 6.1. GENERAL Metal cutting or ‘‘Machining’’ is the process of producing a workpiece by removing unwanted material from a block of metal, in the form of chips. This process is most important since almost all the products get their final shape and size by metal removal, either directly or indirectly. The major drawback of the process is loss of material in the form of chips. Inspite of these drawbacks, the machining process has the following characteristics : 1. They improve the dimensional accuracy and tolerances of the components produced by other processes. 2. Internal and external surface features which are difficult or not possible to be produced by other processes, can be produced by machining processes. 3. Specified surface characteristics or texture can be achieved on a part or whole of the component. 4. It may be economical to produce a component by machining process. In this chapter, we shall have a fundamental understanding of the basic metal cutting process. 6.2. THE MECHANICS OF CHIP FORMATION A typical metal cutting process can be schematically represented as shown in Fig. 6.1. A wedge-shaped tool is made to move relative to the workpiece. As the tool makes contact with the metal, it exerts a pressure on it resulting in the compression of the metal near the tool tip. This induces shear-type deformation within the metal and it starts moving upward along the top face of the tool. As the tool advances, the material ahead of it is sheared continuously along a plane called the ‘‘Shear plane’’. This shear plane is actually a narrow zone (of the order of about 0.025 mm) and extends from the cutting edge of the tool to the surface of the workpiece. The cutting edge of the tool is formed by two intersecting surfaces. The surface along which the chip moves upwards is called ‘‘Rake surface’’ and the other surface which is relieved to avoid rubbing with the machined surface, is called ‘‘Flank’’. The angle between the rake surface and the normal is known as ‘‘Rake angle’’,  (which may be positive or negative), and the angle between the flank and the horizontal machined surface is known as the ‘‘relief or clearance angle’’, . Most cutting processes have the same basic features as shown in Fig. 6.1, where a single point cutting tool is used (a milling cutter, a drill, and a broach can be regarded as several single-point tools joined together and are known as multi-point tools). 6.3. SINGLE POINT CUTTING TOOL A single point cutting tool consists of a sharpened cutting part called its point and the shank, (Fig. 6.2). The point of the tool is bounded by the face (along which the chips slide as they are cut 405 406 A Textbook of Production Technology tc Tool Chip Lip (wedge) – + angle Rake V  Cutting angle B Flank t   A Shear Workpiece Plane  = Shear angle : t = uncut chip thickness: tc = chip thickness after the metal is cut Fig. 6.1. Schematic Representation of Machining Process. by the tool), the side flank or major flank the end flank, or minor flank and the base. The side cutting edge, a-b, is formed by the intersection of the face and side flank. The end cutting edge a-c is formed by the intersection of the face and the end flank. The chips are cut from the work piece by the side-cutting edge. The point ‘a’ where the end and side-cutting edges meet is called the ‘‘nose’’ of the tool. Fig 6.2 is for a right hand tool. Below, we give the definitions of the various tool elements and tool angles :– Shank. It is the main body of the tool. Tool Axis Shank Major cutting edge Cutting Part Minor c a cutting b Edge Base Corner Minor Flank Major Flank Face Fig. 6.2. A Single Point Cutting Tool. Flank. The surface or surfaces below and adjacent to the cutting edge is called flank of the tool. Face. The surface on which the chip slides is called the face of the tool. Heel. It is the intersection of the flank and the base of the tool. Nose. It is the point where the side cutting edge and end cutting edge intersect. Machining Process 407 Cutting edge. It is the edge on the face of the tool which Primary removes the material from the workpiece. The total cutting edge Cutting Edge consists of side cutting edge (major cutting edge), end cutting edge (minor cutting edge and the nose). A single point cutting tool may be either right or left hand cut tool depending on the direction of feed. In a right cut tool, the side cutting edge is on the side of the thumb when the right hand is placed on the tool with the palm downward and the fingers pointed towards the tool nose (Fig. 6.3 b). Such a tool will cut when fed from right to left as in a lathe in which the tool moves from tailstock to headstock. A left-cut tool is one in Feed Feed which the side cutting edge is on the thumb side when the left hand is applied (Fig. 6.3 a). Such a tool will cut when fed from (a) (b) left to right. Fig. 6.3. Left and Right Cut Tools. The various types of surfaces and planes in metal cutting Cutting are explained below with the help of Fig. 6.4, in which the Surface Work Machined basic turning process is shown. The three types of surfaces are Surface surface :– X (1) the work surface, from which the material is cut. X (2) the machined surface which is formed or generated Cutting after removing the chip. Plane Basic Plane (3) the cutting surface which is formed by the side cutting edge of the tool. Y The references from which the tool angles are specified are the ‘cutting plane’ and the `basic plane' or the ‘principal Fig. 6.4. Principal Surfaces and plane’. The cutting plane is the plane tangent to the cutting Planes in Metal Cutting. surface and passing through and containing the side cutting edge. The basic plane is the y plane parallel to the longitudinal and cross feeds, that is, this plane lies along and normal S to the longitudinal axis of the workpiece. In a lathe tool, the basic plane concides with the base of the tool. S Section B – B 6.3.1. Designation of Cutting Tools. A By designation or nomenclature of a cutting tool is meant the designation of the shape of x x the cutting part of the tool., The two systems B B to designate the tool shape, which are widely e used, are :– Ce b 1. American Standards Association Feed System (ASA) or American National Standards Institute (ANSI). Cs Y 2. Orthogonal Rake System (ORS). ASA System. In the ASA system, the A angles of tool face, that, is its slope, are defined in two orthogonal planes, one parallel Fig. 6.5. ASA System. 408 A Textbook of Production Technology to and the other perpendicular to, the axis of the cutting tool, both planes Side being perpendicular to the base of End Cutting Rake Angle ank the tool. For simple turning Edge Sh Axis Angle Face operation, this system is illustrated Side Relief Nose Angle in Fig. 6.5. Radius Back Rake The typical right hand single Angle End point cutting tool terminology is Cutting Edge given in Fig. 6.6 (a). Fig. 6.6 (b) End Side Relief Angle gives the three views of the single Flank point cutting tool, with all the details Base Axis Side Flank Side Cutting Edge Angle marked on it. Clearance or end The various tool angles are (a) c Relief angle defined and explained below : e Side Cutting Edge Angle F (SCEA). Side cutting edge angle, Cs Nose Radius e R e also known as lead angle, is the d cs angle between the side cutting edge and the side of the tool shank. Top Face The complimentary angle of s b SCEA is called the ‘‘Approach Lip Flank Shank angle’’ Angle End Cutting-Edge Angle (ECEA). This is the angle between s Heel e Lip Angle the end cutting edge and a line (b) normal to the tool shank Ce. Fig. 6.6. Tool Terminology. Side Relief Angle (SRA). It is the angle between the portion of the side flank immediately below the side cutting edge and a line perpendicular to the base of the tool, and measured at right angle to the side flank s. End Relief Angle (ERA). It is the angle between the portion of the end flank immediately below the end cutting edge and a line perpendicular to the base of the tool, and measured at right angle to the end flank e. Back-Rake Angle (BRA),  b. It is the angle between the face of the tool and a line parallel to the base of the tool and measured in a plane (perpendicular) through the side cutting edge. This angle is positive, if the side cutting edge slopes downwards from the point towards the shank and is negative if the slope of the side cutting edge is reverse. So this angle gives the slope of the face of the tool from the nose towards the shank. Side-Rake Angle (SR),  s. It is the angle between the tool face and a line parallel to the base of the tool and measured in a plane perpendicular to the base and the side cutting edge. This angle gives the slope of the face of the tool from the cutting edge. The side rake is negative if the slope is towards the cutting edge and is positive if the slope is away from the cutting edge. Importance of Tool Angles : 1. Side Cutting-Edge Angle, Cs. It is the angle which prevents interference as the tool enters the work material. The tip of the tool is protected at the start of the cut, Fig. 6.7, as it enables the tool to contact the work first behind the tip. This angle affects tool life and surface finish. This angle can vary from 0° to 90°. The side cutting edge at increased value of SCEA will have more of Machining Process 409 its length in action for a given depth of cut Side Cutting and the edge lasts longer. Also, the chip edge Angle (SCEA) produced will be thinner and wider which will distribute the cutting and heat produced over more of the cutting edge. On the other hand, the larger this angle, the greater the component of force tending to separate the work and the tool. This promotes chatter. Satisfactory values of SCEA vary from 15° Approach ECEA Setting to 30°, for general machining. The shape of Angle Angle the workpiece will also determine the SCEA. To produce a shoulder, zero degree SCEA is needed. No SCEA is desirable when TOOL machining castings and forgings with hard and scaly skins, because the least amount of tool f edge should be exposed to the destructive Fig. 6.7. SCEA and ECEA. action of the skin. 2. End Cutting-Edge Angle, Ce. The ECEA provides a clearance or relief to the trailing end of the cutting edge to prevent rubbing or drag between the machined surface and the trailing (non- cutting) part of the cutting edge. Only a small angle is sufficient for this purpose. Too large an ECEA takes away material that supports the point and conducts away the heat. An angle of 8° to 15° has been found satisfactory in most cases on side cutting tools, like boring and turning tools. Sometimes, on finishing tools, a small flat (1.6 to 8 mm long) is ground on the front portion of the edge next to the nose radius, to level the irregular surface produced by a roughing tool. End cutting tools, like cut off and necking tools often have no end cutting-edge angle. 3. Side Relief Angel, (SRA) and End Relief Angle (ERA). These angles (denoted as  s and e in the figure) are provided so that the flank of the tool clears the workpiece surface and there is no rubbing action between the two. Relief angles range from 5° to 15° for general turning. Small relief angles are necessary to give strength to the cutting edge when machining hard and strong materials. Tools with increased values of relief angles penetrate and cut the workpiece material more efficiently and this reduces the cutting forces. Too large relief angles weaken the cutting edge and there is less mass to absorb and conduct the heat away from the cutting edge. 4. Back and Side Rake Angle ( b, s). The top face of the tool over which the chip flows is known as the rake face. The angle which this face makes with the normal to the machined surface at the cutting edge is known as ‘‘Back-rake angle,  b ’’, and the angle between the face and a plane parallel to the tool base and measured in a plane perpendicular to both the base of the tool holder and the side cutting edge, is known as ‘‘Side-rake angle,  s ’’ The rake angles may be positive, zero, or negative. Cutting angle and the angle of shear are affected by the values for rake angles. Larger the rake angle, smaller the cutting angle (and larger the shear angle) and the lower the cutting force and power. However, since, increasing the rake angle decreases the cutting angle, this leaves less metal at the point of the tool to support the cutting edge and conduct away the heat. A practical rake angle represents a compromise between a large angle for easier cutting and a small angle for tool strength. In general, the rake angle is small for cutting hard materials and large for cutting soft ductile materials. An exception is brass which is machined with a small or negative rake angle to prevent the tool form digging into the work. The use of negative rake angles started with the employment of carbide cutting tools. When we use positive rake angle, the force on the tool is directed towards the cutting edge, tending to 410 A Textbook of Production Technology chip or break it, Fig. 6.8 (a). Carbide being brittle lacks shock resistance and will fail if positive rake angles are used with it. Using negative rake angles, directs the force back into the body of the tool away from the cutting edge, Fig. 6.8 (b), which gives protection to the cutting edge. The use of negative rake angle, increases the cutting force. But at higher cutting speeds, at which carbide cutting tools can be used, this increase in force is less than at normal cutting speeds. High cutting speeds are, therefore, always used with negative rakes, which requires ample power of the machine tool. Cutting Cutting Force Force – + (a) With Positive Rake. (b) With Negative Rake. Fig. 6.8. Cutting with Positive and Negative Rake Tools. The use of indexable inserts has also promoted the use of negative rake angles. An insert with a negative rake angle has twice as many cutting edges as an equivalent positive rake angle insert (as will be discussed ahead). So, to machine a given number of components, smaller number of negative rake inserts are needed as compared to positive rake inserts. The use of positive rake angles is recommended under the following conditions : 1. When machining low strength ferrous and non-ferrous materials and work-hardening materials. 2. When using low power machines. 3. When machining long shafts of small diameters. 4. When the set up lacks strength and rigidity. 5. When cutting at low cutting speeds. The use of negative rake angles is recommended under the following conditions : 1. When machining high strength alloys. 2. When there are heavy impact loads such as in interrupted machining. 3. For rigid set ups and when cutting at high speeds. Recommended rake angles are given in Table 6.1. Table 6.1. Recommended Rake Angles Tool Material Work H.S.S. and Cemented Carbide + Material Cast Alloys Brazed Throw away Back Side Back Side Back Side Free Machining Steels 10 12 0 6 – 5 – 5 Mild Steel 8 10 0 6 – 5 – 5 Med. Carbon Steels 0 10 0 6 – 5 – 5 Alloy tool Steels 0 10 –5 –5 – 5 – 5 Stainless Steels 0 10 0 6 – 5 – 5 Cast Iron 5 5 to 10 0 6 – 5 – 5 Machining Process 411 Aluminium Alloys 20 15 3 15 0 5 Copper Alloys 5 10 0 8 0 5 Magnesium Alloys 20 15 3 15 0 5 Titanium Alloys 0 5 0 6 –5 –5 5. Nose Radius. Nose radius is favourable to long tool life and good surface finish. A sharp point on the end of a tool is highly stressed, short lived and leaves a groove in the path of cut. There is an improvement in surface finish and permissible cutting speed as nose radius is increased from zero value. Too large a nose radius will induce chatter. The use of following values for nose radius is recommended : R = 0.4 mm, for delicate components.  1.5 mm for heavy dpeths of cut, interrupted cuts and heavy feeds. = 0.4 mm to 1.2 mm for disposable carbide inserts for common use. = 1.2 to 1.6 mm for heavy duty inserts. The rules of thumb for selection of Nose radius are :- (i) For a strong cutting edge, select the largest prossible Nose radius. (ii) A large nose radius permits higher feeds. (iii) Select a smaller nose radius if there is a tendency to vibrate. For rough machining, the most commonly used nose radii are 1.2 to 1.6 mm Tool Designation. The tool designation or tool signature, under ASA system is given in the order given next : Back rake, Side rake, End relief, Side relief, End cutting edge angle, Side cutting edge angle, and nose radius that is,  b   s  e   s  Ce  Cs  R If tool designation is : 1 8 – 14 – 6 – 6 – 6 – 15 – , it means that, 8 b  8,  s  14 e  6, s  6 Ce = 6°, Cs = 15° 1" and R. 8 In ASA system of tool angles, the angles are specified independently of the position of the cutting edge. It, therefore, does not give any indication of the behaviour of the tool in practice. Therefore, in actual cutting operation, we should include the side cutting edge (principal cutting edge) in the scheme of reference planes. Such a system is known as Orthogonal Rake System (ORS). Orthogonal Rake System (ORS). As mentioned above, in this system the planes for designating tools are the plane containing the principal or side cutting edge and the plane normal to it. In the plane NN which is normal to the principal cutting edge and is known as Orthogonal plane 412 A Textbook of Production Technology or the chief plane, we have the following angles : side relief angle the side rake angle (known as Orthogonal rake angle) wedge (lip angle) and the cutting angle (see Fig. 6.9).   1  A N Section at N-N 1 90° M Cs Section At M-M  M N Basic Plane i View Facing Arrow A Fig. 6.9. ORS of Tool Angles. The side relief angle is the angle between the side (main) flank and the cutting plane. The side rake angle, , is the angle between the toolface and a plane normal to the cutting plane and passing through the main cutting edge. This angle is positive when the face slopes downward from the plane perpendicular to the cutting plane (as shown in Fig. 6.9), equal to zero when the face is perpendicular to the cutting plane and negative when the face slopes upwards. The ‘‘wedge angle,  ’’ is the angle between the tool face and the main flank. The ‘‘cutting angle,  ’’ is the angle between the tool face and the cutting plane. When  is positive, we have,     wedge angle = 90°     The usual values of  and  are :   10 to + 15°,   6 to 12 In the ORS, the back rake angle is the inclination angle (i) between the principal cutting edge and a line passing through the point of the tool parallel to the principal plane. This angle is measured in a plane passing through the main cutting edge and perpendicular to the basic plane. In Fig. 6.9, the angle i is negative with tool nose being the highest point of the cutting edge. It will be zero when the cutting edge is parallel to the basic plane and positive if the cutting edge is towards the right (Fig. 6.9) of the line passing through the point of the tool and parallel to the principal (basic) plane, that is, the tool nose is the lowest point of the cutting edge. In addition to the angles discussed above, angles are also measured in the plane MM (known as Auxiliary reference plane) which is normal to the projection of the end cutting edge on the basic plane. These angles are the end relief angle 1 , and the back rake angle 1 (also called auxiliary rake angle). The plan angles are the Approach angle or entering angle  which is equal to (90° – Cs ) and the end cutting edge angle, Ce. 1  8 to 10,   30 to 70°, Ce = 10° to 15° Machining Process 413 The tool designation under ORS is : i      1  Ce    R A typical tool designation (signature) is : 0 – 10 – 6 – 6 – 8 – 90 – 1 mm Interconversion between ASA system and ORS tan   tan  s sin   tan b cos  tan b  cos  tan   sin  tan i tan  s  sin  tan   cos  tan i tan i   tan  s cos   tan b sin  In the second and third equations above, the values of angles  and i are taken with their signs. 6.4. METHODS OF MACHINING In the metal cutting operation, Fig. 6.1, the tool is wedge-shaped and has a straight cutting edge. Basically, there are two methods of metal cutting, depending upon the arrangement of the cutting edge with respect to the direction of relative work-tool motion : 1. Orthogonal cutting or two dimensional cutting. 2. Oblique cutting or three dimensioning cutting. In orthogonal cutting, Fig. 6.10, the cutting edge of the tool is arranged perpendicular to the cutting velocity vector, V, whereas in oblique cutting, it is set at some angle other than to the cutting velocity vector, which gives an ‘‘inclination angle i’’. The analysis of oblique cutting being very complex, the relatively simple arrangement of orthogonal cutting is, therefore, widely used in theoretical and experimental work. Tool Chip-Flow Chip Chip Angle Tool Workpiece Workpiece V V 90° 90° i Cutting Edge Inclination Fig. 6.10. Methods of Machining. In pure orthogonal cutting, i = 0°, Ce = 0°, and   90. This is also known as orthogonal system of second kind. When i = 0, and 0    90. 0, it is called as orthogonal system of first kind. A common example of pure orthogonal cutting process is the turning of a thin pipe with a straight edged tool set normal to the longitudinal axes. 6.5. TYPES OF CHIPS Whatever the cutting conditions can be, the chips produced may belong to one of the following three types, (Fig. 6.11) : 414 A Textbook of Production Technology 1. Discontinuous Chips. 2. Continuous Chips 3. Continuous Chips with build-up-edge (BUE). Discontinuous Chips. These types of chips are usually produced when cutting more brittle materials like grey Feed Feed cast iron, bronze and hard brass. These materials lack the ductility necessary for appreciable plastic chips formation. The material ahead of the tool edge fails in a brittle fracture manner along the shear (a) Continuous Chip (b) Discontinuous Chip zone. This produces small fragments of discontinuous chips. Since the chips break up into small segments, the friction Built up between the tool and the chips reduces, Feed Edge resulting in better surface finish. These on Tool Built up on work chips are convenient to collect, handle and dispose of. Discontinuous chips are also produced when cutting more ductile materials under the following conditions : (c) Built up chip (i) large chip thickness. (ii) low cutting speed. (iii) small rake angle of the tool. Fig. 6.11. Types of Chips (iv) cutting with the use of a cutting fluid. Continuous Chips. These types of chips are produced when, machining more ductile materials. Due to large plastic deformations possible with ductile materials, longer continuous chips are produced. This type of chip is the most desirable, since it is stable cutting, resulting in generally good surface finish. On the other hand, these chips are difficult to handle and dispose off. The chips coil in a helix (chip curl) and curl around the work and the tool and may injure the operator when break loose. Also, this type of chip remains in contact with the tool face for a longer period, resulting in more frictional heat. These difficulties are usually avoided by attaching to the tool face or machine on the tool face, a ‘chip breaker’, (Fig. 6.12). The function of chip breaker is to reduce the radius of curvature of the chip and thus break it. The following cutting conditions also help in the production of continuous chips : (i) small chip thickness. (ii) high cutting speed. (iii) large rake angle of the cutting tool. (iv) reducing the friction of the chip along the tool face, by : imparting high surface finish to the tool face, use of tool material with low co-efficient of friction, and use of a good cutting fluid. Continuous Chips with Built-up-edge (BUE). When machining ductile materials, conditions of high local temperature and extreme pressure in the cutting zone and also high friction in the tool-chip interface, may cause the work material to adhere or weld to the cutting edge of the tool forming the built-up edge. Successive layers of work material are then added to the built-up edge. When this edge becomes larger and unstable, it breaks up and part of it is carried up the face of the tool alongwith the chip while the remaining is left over the surface being machined, which contributes to the roughness of the surface. The built-up edge changes its size during the cutting operation. It Machining Process 415 first increases, then decreases, then again increases etc. This cycle is a source of vibration and poor surface finish. Although, the built-up edge protects the cutting edge of the tool, it changes the geometry of the cutting tool. Low cutting speed also contributes to the formation of the built- up edge. Increasing the cutting speed, increasing the rake angle and using a cutting fluid contribute to the reduction or elimination of the built-up edge. Chip Breaker Before Chip After Tool Workpiece Fig. 6.12. Chip Breaker. From the above discussion, we can summarize the factors that are likely to influence the formation of various types of chips. Factors Types of Chips Continuous Continuous with B.U.E. Discontinuous Material Ductile Ductile Brittle Tool :- Rake angle Large Small Small Cutting edge Sharp Dull – Cutting conditions : Speed High Low Low Feed Low High High Friction Low High – Cutting Fluid Efficient Poor – 6.6. PRINCIPAL ELEMENTS OF METAL MACHINING The principal elements of metal machining are : (a) Cutting Speed (b) Feed (c) Depth of Cut Cutting Speed. The cutting speed can be defined as the relative surface speed between the tool and the job. It is a relative term, since either the tool or the job or both may be moving during