232) GATE-O-PEDIA Manufacturing Process 1_note BY REXODAS.pdf

Manufacturing Engineering Published By: Physics Wallah ISBN: 978-93-94342-39-2 Mobile App: Physics Wallah (Available on Play Store) Website: www.pw.live Email: [email protected] Rights All rights will be reserved by Publisher. No part of this book may be used or reproduced in any manner whatsoever without the written permission from author or publisher. In the interest of student's community: Circulation of soft copy of Book(s) in PDF or other equivalent format(s) through any social media channels, emails, etc. or any other channels through mobiles, laptops or desktop is a criminal offence. Anybody circulating, downloading, storing, soft copy of the book on his device(s) is in breach of Copyright Act. Further Photocopying of this book or any of its material is also illegal. Do not download or forward in case you come across any such soft copy material. Disclaimer A team of PW experts and faculties with an understanding of the subject has worked hard for the books. While the author and publisher have used their best efforts in preparing these books. The content has been checked for accuracy. As the book is intended for educational purposes, the author shall not be responsible for any errors contained in the book. The publication is designed to provide accurate and authoritative information with regard to the subject matter covered. This book and the individual contribution contained in it are protected under copyright by the publisher. (This Module shall only be Used for Educational Purpose.) Design Against Static Load INDEX 1. Theory of Metal Cutting...................................................................................................... 11.1 – 11.12 2. NTMM (NON-TRADITIONAL MACHINING METHOD)............................................................ 11.13 – 11.20 3. Casting................................................................................................................................. 11.21 – 11.34 4. Welding................................................................................................................................ 11.35 – 11.48 5. Metal Forming...................................................................................................................... 11.49 – 11.64 6. Metrology............................................................................................................................. 11.65 – 11.79 7. Advance Machining Method............................................................................................... 11.80 – 11.94 8. Machine Tool...................................................................................................................... 11.95 – 11.103 9. Material Science.............................................................................................................. 11.104 – 11.132 GATE-O-PEDIA MECHANICAL ENGINEERING 1 THEORY OF METAL CUTTING Fig. 1.1 Single point cutting tool Fig. 1.2 Continues Chip Fig. 1.3 Discontinues Chip 1.1 Machining Machining is an essential process of finishing by which jobs are produced to (a) The desired dimensions and (b) Surface finish GATE WALLAH MECHANICAL HANDBOOK 11.1 Theory of Metal Cutting 1.2 Orthogonal Machining Fig. 1.4. Machining 1.3 Types of Machining Fig. 1.5 Different types of Cutting Process 1.4 Orthogonal Cutting 1. Cutting edge of the tool is perpendicular to the direction of cutting velocity. 2. The cutting edge is wider than the workpiece width and extends beyond the workpiece on either side. Also, the width of the workpiece is much greater than the depth of cut. 3. The chip generated flows on the rake face of the tool with chip velocity perpendicular to the cutting edge 4. The cutting forces act along two directions only. GATE WALLAH MECHANICAL HANDBOOK 11.2 Theory of Metal Cutting 1.5 Geometry of single point turning tool Fig. 1.6 Single point cutting tool Fig. 1.7 Different type of views GATE WALLAH MECHANICAL HANDBOOK 11.3 Theory of Metal Cutting 1.6 Tool designation (ANSI) or ASA 1.6.1 To remember easily follow the rule Rake (αb αs), relief (γe γs), cutting edge (Ce Cs) Side will come last finish with nose radius (inch) α b − α s − γ e − γ s − Ce − Cs − R 1.6.2 Orthogonal Rake System (ORS) i − α − γ − γ 1 − Ce − λ − R Inclination angle (i) Orthogonal rake angle (α) Side relief angle (γ) End relief angle (γ1) End cutting edge angle (Ce) Principal cutting edge angle or Approach angle (λ = 90 – Cs) Nose radius (R) (mm) For Orthogonal cutting, i = 0 For Oblique cutting, i ≠ 0 1.6.3 Inter conversion between ASA & ORS tan α = tan α s sin λ + tan α b cos λ tan α= b cos λ tan α + sin λ tan i tan α s= sin λ tan α – cos λ tan i = tan i – tan α s cos λ + tan α b sin λ 1.7 Shear angle (φ) 1.7.1 Chip Thickness Ratio t0 lc Vc sin φ 1 r = = = = = tc l V cos(φ − α ) h r cos α and tan φ = 1 − r sin α Where r = chip thickness ratio or cutting ratio; r < 1 h = 1/r = Inverse of chip ratio or chip reduction factor or chip compression ratio; h > 1 GATE WALLAH MECHANICAL HANDBOOK 11.4 Theory of Metal Cutting Fig. 1.8 t0 t As sin φ = and cos(φ – α)= c AB AB t0 sin φ Chip thickness ratio (r) = = tc cos(φ − α) 1.7.2 Cutting shear strain (ϵ) i.e cutting strain The magnitude of strain, that develops along the shear plane due to machining action, is called cutting strain (shear). The relationship of this cutting strain, ϵ ε = cot φ + tan(φ − α) 1.8 Velocity diagram of cutting zone Need velocities to obtain power estimates VMaterial + V Chip = VChip Tool Material Tool = = VMaterial Cutting velocity V Tool = V Chip = Shear velocity Vs Material = = VChip Chip velocity Vc Tool Fig. 1.9 GATE WALLAH MECHANICAL HANDBOOK 11.5 Theory of Metal Cutting Fig. 1.10 Velocity Triangle Apply sin rule v vs vc = = π  π  sin ( φ ) sin  − φ + α  sin  − φ  2  2  v vs vc = = cos ( φ – α ) cos ( φ ) sin ( φ ) 1.9 Shear Strain Rate dε Vs ε = = dt thickness of shear zone (t s ) ts =1/10th or (10%) of shear plane length and its maximum value is 25 microns. 1.10 Determination of Un-deformed chip thickness in Turning: Fig. 1.11 Turning process GATE WALLAH MECHANICAL HANDBOOK 11.6 Theory of Metal Cutting = t0 f sin λ, d w= sin λ where f = feed (mm/rev) d = depth of cut (mm) t0 = uncut chip thickness W = width of chip λ = Approach angle (1) Turning is 3-D cutting ⇒ three force comes in picture (2) Turning is not Orthogonal cutting(2-D) (3) t0 = fsin λ (4) w = d/sin λ For orthogonal cutting t0 = d Width of chip = width of cut(w) 1.11 Types of Chip Continuous chip Discontinuous chip Continuous chip with BUE 1.12 Force & Power in Metal Cutting Fc and Ft The two orthogonal components (horizontal and vertical) Fc and Ft of the resultant force R can be measured by using a dynamometer. GATE WALLAH MECHANICAL HANDBOOK 11.7 Theory of Metal Cutting Merchant Force Circle Diagram (MCD) Fig. 1.12 Merchant Circle Diagram where fs = Shear force Fn = Normal to shear force Fc = Cutting component Ft = Thrust component F = Friction force N = Normal to friction β = Friction angle R = Actual cutting force For orthogonal cutting only The force relations F = Fc sin α + Ft cos α N = Fc cos α − Ft sin α Fn = Fc sinφ + Ft cosφ Fs = Fc cosφ – Ft sinφ (a) From Merchant Analysis π α β φ = + − 4 2 2 (b) Lee and Shaffer π φ = + α −β 4 (c) Stabler π α φ = + −β 4 2 GATE WALLAH MECHANICAL HANDBOOK 11.8 Theory of Metal Cutting 1.13 Metal Removal Rate (MRR) Metal removal rate (MRR) = Ac.v = wt0v(orthogonal cutting) = fdv(turning) Where Ac = cross-section area of uncut chip (mm2) v = cutting speed = πDN , mm / min f = feed (mm/rev) d = depth of cut (mm) w = width of cut t0 = uncut chip thickness 1.14 Heat Distribution in Metal Cutting Fig. 1.13 Heat distribution 1.15 Specific cutting pressure The cutting force, Fc, divided by the cross-section area of the undeformed chip gives the nominal cutting stress or the specific cutting pressure, Fc Fc Pc = = bt fd 1.16 Tool Wear, Tool Life 1.16.1 Tool Wear (i) Flank Wear, At low speed → Slow Death (ii) Crater Wear, At high speed → Slow Death (iii) Chipping off of the cutting-edge → Sudden Death GATE WALLAH MECHANICAL HANDBOOK 11.9 Theory of Metal Cutting 1.16.2 Flank Wear: (Wear land) Fig. 1.14 Tool Wear I = Primary wear zone II = Secondary wear zone III = Tertiary wear zone Note: (i) In Primary wear zone wear rate is constant. (ii) Wear gradually increases in tertiary wear zone. 1.17 Tool Life Taylor’s Tool Life Equation Causes Sliding of the tool along the machined surface Temperature rise VT n = C Where, V = cutting speed (m/min) T = Time (min) n = exponent depends on tool material C = constant based on tool and work material and cutting condition. 1.18 Extended or Modified Taylor’s equation VT n f a d b = C Where: d = depth of cut f = feed rate a 1 = n n1 b 1 = n n2 C1/ n or T = V 1/ n. f 1.n1.d 1/ n2 1 1 1 > > n n1 n2 i.e Cutting speed has the greater effect followed by feed and depth of cut respectively. GATE WALLAH MECHANICAL HANDBOOK 11.10 Theory of Metal Cutting 1.19 Economics of metal cutting Fig. 1.15 Economics of Metal Cutting Formula V0 T0n = C (a) Optimum tool life for minimum cost  Ct   1 − n  To =  Tc +   if Tc , Ct & Cm given  Cm   n  Ct  1 − n  =   if Ct & Cm given Cm  n  (b) Optimum tool life for Maximum Productivity (Minimum production time)  1− n  To = Tc    n  Units: Tc – min (Tool changing time) Ct – Rs./ servicing or replacement (Tooling cost) Cm – Rs/min (Machining cost) V – m/min (Cutting speed) Tooling cost (Ct) = tool regrind cost + tool depreciation per service/ replacement Machining cost (Cm) = labour cost + overhead cost per min GATE WALLAH MECHANICAL HANDBOOK 11.11 Theory of Metal Cutting 1.20 Surface Roughness 1.20.1 Ideal Surface (Zero nose radius) f Peak to valley roughness (h) = tan Cs + cot Ce h f and (Ra) = = 4 4 ( tan Cs + cot Ce ) 1.20.2 Practical Surface (with nose radius = R) f2 h = 8R f2 and Ra = 18 3R  GATE WALLAH MECHANICAL HANDBOOK 11.12 2 NTMM (NON-TRADITIONAL MACHINING METHOD) 2.1 Electro Chemical Machining (ECM) Fig. 2.1 Electro Chemical Machining GATE WALLAH MECHANICAL HANDBOOK 11.13 12 NTMM (NON-TRADITIONAL MACHINING METHOD) 2.1.1 Electrochemical Machining Electrochemical machining is the reverse of electro plating The work-piece is made the anode, which is placed in close proximity to an electrode (cathode), and a high-amperage direct current is passed between them through an electrolyte, such as salt water, flowing in the anode-cathode gap. MRR in ECM depends on atomic weight of work material Commercial ECM is carried out at a combination of low voltage high current ECM has the highest metal removal rate, among the NTMM. Fig. 2.2 ECM ECM Formula Atomic Weight E= Valancty Faraday’s laws state that, It E m= F Where m = weight (gm) of a material l = current (A) t = time (sec) E = gram equivalent weight of the material F = constant of proportionality – Faraday (96,500 coulombs) GATE WALLAH MECHANICAL HANDBOOK 11.14 NTMM (NON-TRADITIONAL MACHINING METHOD) ECM Calculations MRR for pure metal where, AI  cm3  EI  cm3  A = Atomic weight  =   ρvF  sec  ρF  sec  v = Valency MRR for Alloy E eq I  cm3    ρeq F  sec  100 x  100 xv  = ∑ i  and = ∑ i i  ρeq F i  ρi  E eq i  Ai  If the total over voltage at the anode and the cathode is ∆V and the applied voltage is V, the current l is given by, V − ∆V I= R JS= ( V − ∆V ) Y Fig. 2.3 ECM Principal I J = current density = A S = specific resistance of electrolyte I = applied of current A = Cross sectional area of electrode GATE WALLAH MECHANICAL HANDBOOK 11.15 NTMM (NON-TRADITIONAL MACHINING METHOD) Flow analysis To calculate the fluid flow, required, match the heat generated to the heat absorbed by the electrolyte. The heat generated in the gap, H is given by H = I2 × R where, I = Current R = Resistance of electrolyte in the gap Heat absorbed by the electrolyte, He is He = q ρe ce (Tf – Ti) Electing all the heat losses l2R = q ρe ce (Tf – Ti) where, q= Flow rate of electrolyte ρe = Density of electrolyte ce= Specific heat of electrolyte Ti = Initial temperature Tf = Final temperature 2.2 Electrochemical Grinding (ECG) In ECG: The tool electrode is a rotating, metal bonded, diamond grit grinding wheel. As the electric current flows between the workpiece and the wheel, through the electrolyte. a.The surface metal is changed to a metal oxide, b.Which is ground away by the abrasives. ECG is a low-voltage high-current electrical process. The abrasive particles are act as an insulating spacer. Fig. 2.4 Equipment setup and electrical circuit for electrochemical grinding. GATE WALLAH MECHANICAL HANDBOOK 11.16 NTMM (NON-TRADITIONAL MACHINING METHOD) 2.3 Electro Discharge Machining (EDM) Wear Ratio Tool wear is given in terms of wear ratio which is defined as, Volumeof metal removed work Wear ratio = Volumeof metal removed tool Relaxation circuit Fig-Relaxation circuit used for generating the pulses in EDM process  1  Vc = V0 1 − e RC    The time constant, τ of the circuit is given by Τ = Rc × C Fig. 2.5 Charging current can then be specified by 1 V0 τ ic = e Rc For maximum power Vc = 0.72 × V0   t  −   Vc = V0 1 − e  RC     V0 = Open circuit voltage R = Charging resistance C = Capacitance Vc = Instantaneous capacitor voltage during charging t = voltage at any time Spark energy 1 C ( Vc ) J / cycle 2 Es = 2 GATE WALLAH MECHANICAL HANDBOOK 11.17 NTMM (NON-TRADITIONAL MACHINING METHOD) 2.4 Ultrasonic Machining (USM) Tool of desired shape vibrates at an ultrasonic frequency (19 ~ 25 kHz) with an amplitude of around 15 – 50 μm The tool is pressed downward with a feed force, F. Fig. 2.6 Ultrasonic Machining Fig. 2.7 Variation of MRR w.r.t. different Parameters GATE WALLAH MECHANICAL HANDBOOK 11.18 NTMM (NON-TRADITIONAL MACHINING METHOD) 2.5 Water Jet Machining (WJM) Fig. 2.8 Water Jet Machining Narrow jet of water directed, at high pressure and velocity, against surface of workpiece Fig. 2.9 Variation of MRR w.r.t. different Parameters GATE WALLAH MECHANICAL HANDBOOK 11.19 NTMM (NON-TRADITIONAL MACHINING METHOD) 2.6 Abrasive Jet Machining (AJM) Fig. 2.10 Abrasive jet machining MRR ∝ QD3 Q = flow rate abrasives D = mean diameter of abrasives  GATE WALLAH MECHANICAL HANDBOOK 11.20 3 3.1 Casting CASTING Process in which molten metal flows by gravity or other force into a mold where it solidifies in the shape of the mold cavity 3.2 Steps in Sand Casting Fig. 3.1 Sand Casting GATE WALLAH MECHANICAL HANDBOOK 11.21 CASTING 3.3 Casting Terms Flask: Flasks have box-like structure made of rectangular walls (sometimes circular also) and without any bottom or top Cover: These are mostly made of cast iron although wood is also used sometimes Drag: Lower moulding flask. Cope: Upper moulding flask. Cheek: Intermediate moulding flask used in three-piece moulding. Fig. 3.2 Mould Box Pattern: Pattern is a replica of the final object to be made with some modifications. Parting line:This is the dividing line between the two moulding flasks that makes up the sand mould. Bottom board: This is a board normally made of wood, which is used at the start of the mould making. Fig. 3.3 Casting Terms Moulding Sand:The freshly prepared refractory material used for making the mould cavity. Typical mix: 90% sand, 3% water, and 7% clay Backing Sand:This is made up of used and burnt sand. Core: Used for making hollow cavities in castings. Pouring basin: A small funnel-shaped cavity at the top of the mould into which the molten metal is poured. Sprue: The passage through which the molten metal from the pouring basin reaches the mould cavity. Runner: The passage ways in the parting plane through which molten metal flow is regulated before they reach the mould cavity. Gate: The actual entry point through which molten metal enters the mould cavity in a controlled rate GATE WALLAH MECHANICAL HANDBOOK 11.22 CASTING Chaplet: Chaplets are used to support cores inside the mould cavity. Chill: Chills are metallic objects, which are placed in the mould to increase the cooling rate of castings. Riser: It is a reservoir of molten metal provided in the casting so that hot metal can flow back into the mould cavity when there is a reduction in volume of metal due to solidification. 3.4 Pattern A pattern is a replica of the object to be made by the casting process, with some modifications. The main modifications are 3.4.1 Pattern Allowances (1) Shrinkage or contraction allowance (2) Draft or taper allowance (3) Machining or finish allowance (4) Distortion or camber allowance (5) Rapping allowance/shaken allowance / Negative allowance 3.4.2 Shrinkage allowance Invar and Bismuth → shrinkage is Negligible. This is because of the inter-atomic vibrations which are amplified by an increase in temperature. 3.4.3 Liquid shrinkage and solid shrinkage Liquid shrinkage refers to the reduction in volume when the metal changes from liquid to solid state at the solidus temperature. To account for this, risers are provided in the moulds. Solid shrinkage is the reduction in volume caused, when a metal loses temperature in the solid state. The shrinkage allowance is provided to take care of this reduction. 3.4.4 Pattern Materials Wood: patterns are relatively easy to make. Wood is not very dimensionally stable. Commonly used teak, white pine and mahogany wood. Metal: patterns are more expensive but are more dimensionally stable and more durable. Commonly used CI, Brass, aluminium and white metal. Investment casting uses wax patterns. GATE WALLAH MECHANICAL HANDBOOK 11.23 CASTING 3.5 Types of Pattern Single Piece Pattern: These are inexpensive and the simplest type of patterns. Gated Pattern: Gating and runner system are integral with the pattern. This would eliminate the hand cutting of the runners and gates and help in improving the productivity of a moulding. Split Pattern or Two-Piece Pattern Pattern for intricate castings. When the contour of the casting makes its withdrawal from the mould difficult, Cope and Drag Pattern In addition to splitting the pattern, the cope and drag halves of the pattern along with the gating and riser systems are attached separately to the metal or wooden plates along with the alignment pins. Match Plate Pattern The cope and drag patterns along with the gating and the rise ring are mounted on a single matching metal or wooden plate on either side. Loose Piece Pattern This type of pattern is also used when the contour of the part is such that withdrawing the pattern from the mould is not possible. Follow Board Pattern This type of pattern is adopted for those castings where there are some portions, which are structurally weak and if not supported properly are likely to break under the force of ramming. Sweep Pattern These are used for generating large shapes, which are axi-symmetrical or prismatic in nature such as bell-shaped or cylindrical. Skeleton Pattern A skeleton of the pattern made of strips of wood is used for building the final pattern by packing sand around the skeleton GATE WALLAH MECHANICAL HANDBOOK 11.24 CASTING 3.6 Cooling curve Fig. 3.4 Cooling Curve 3.7 Core Used for making cavities and hollow projections. Core sand should be of higher strength than the moulding sand. Used clay free silica sand. Binders used are linseed oil, core oil, resins, dextrin, molasses, etc. The general composition of a core sand mixture could be core oil (1%) and water (2.5 to 6%) Net Buoyancy Force = Weight of Liquid Metal Displaced – Weight of Core P = Vgρm –Vgρc P = Vg(ρm-ρc) V = Volume of core ρm = Density of molten liquid metal ρc = Density of core material 3.8 Permeability Gases evolving from the molten metal and generated from the mould may have to go through the core to escape out of the mould. Hence cores are required to have higher permeability. VH R = pAT R = Permeability Number V = volume of air = 2000 cm3 GATE WALLAH MECHANICAL HANDBOOK 11.25 CASTING H = height of the sand specimen = 5.08 cm p = air pressure, g/cm2 = 10 g/cm2 (standard) A = cross sectional area of sand specimen = 20.268 cm2 T = time in minutes for the complete air to pass through 501.28 R = p.T 3.9 Grain size number ASTM (American Society for Testing and Materials) grain size number, defined as n–1 N=2 Low ASTM numbers mean a few massive grains; high numbers refer to many small grains. 3.10 Gate (Ingate) Design 3.10.1 Top Gate The velocity is more than time taking to fill the mould is minimum that’s why the temperature gradient is minimum. AH ttop = [ht = hs + hc] A g 2gh t hc = Cup height hs = Sprue height ht = Manometric height ttop = filling time in top gate Fig. 3.5 Top Gate 3.10.2 Bottom Gate 2A 1 tbottom = ( ht − ht − H ) Ag 2 g GATE WALLAH MECHANICAL HANDBOOK 11.26 CASTING Fig. 3.6 Bottom Gate Important result If ht = H in bottom Gate t bottom = 2ttop 3.10.3 Gating ratio Gating ratio is defined as: Sprue area: Runner area: Ingate area. 3.10.4 Sprue Design Sprue: Sprue is the channel through which the molten metal is brought into the parting plane where it enters the runners and gates to ultimately reach the mould cavity. To eliminate this problem of air aspiration, the sprue is tapered to gradually reduce the cross section as it moves away from the top of the cope as shown in Figure below(b). (a) Straight sprue (b) Tapered Fig. 3.7 Design of Sprue GATE WALLAH MECHANICAL HANDBOOK 11.27 CASTING 3.11 Risers and Riser Design Risers are added reservoirs designed to feed liquid metal to the solidifying casting as a means of compensating for solidification shrinkage. To perform this function, the risers must solidify after the casting. According to Chvorinov's rule, a good shape for a riser would be one that has a long freezing time (i.e., a small surface area per unit volume). Chvorinov’s rule n V  Total solidification time (ts) = B    A Where n = 1.5 to 2.0 [Where, B = mould constant and is a function of mould material, casting material, and condition of casting] n = 2 and triser = 1.25 tcasting. OR 2 2 V V   = 1.25    A  riser  A casting 3.11.1 Important Result Compare the solidification times for castings of three different shapes of same volume: (i) Cubic (Tcu) (ii) Cylindrical (with height equal to its diameter) (Tcy) (iii) Spherical (Tsp) Tsp : Tcy : Tcu = 1 : 0.763 : 0.649 3.11.2 Method of Riser Design (a) Modulus Method It has been empirically established that if the modulus of the riser exceeds the modulus of the casting by a factor of 1.2, the feeding during solidification would be satisfactory. MR = 1.2 Mc Modulus = Volume/Surface area GATE WALLAH MECHANICAL HANDBOOK 11.28 CASTING (b) Modulus of casting shape Shape Figure Modulus of Casting ab Long bar 2( a + b) D Cube 6 D Cylinder 6 D Sphere 6 Fig. 3.8 Modulus of different casting shapes (c) Caine’s Method Freezing ratio = ratio of cooling characteristics of casting to the riser. ( AV ) Casting ( AV ) X = Riser The riser should solidify last so x > 1 a According to Caine x = +c Y –b Vriser Y = and a, b, c are constant. Vcasting GATE WALLAH MECHANICAL HANDBOOK 11.29 CASTING (d) Naval research laboratory method (shape factor) This method, which is essentially a simplification of Caine’s method, defines a shape factor to replace the freezing ratio. The shape factor is defined as, Length + width SF = thickness Fig. 3.9 Shape Factor 3.12 Chills External chills are masses of high-heat-capacity, high-thermal-conductivity material that are placed in the mould (adjacent to the casting) to accelerate the cooling of various regions. Chills can effectively promote directional solidification or increase the effective feeding distance of a riser. Internal chills are pieces of metal that are placed within the mould cavity to absorb heat and promote more rapid solidification. Since some of this metal will melt during the operation, it will absorb not only the heat-capacity energy, but also some heat of fusion. Since they ultimately become part of the final casting, internal chills must be made from the same alloy as that being cast. 3.13 Casting Defects The following are the major defects, which are likely to occur in sand castings: 3.13.1 Gas Defects A condition existing in a casting caused by the trapping of gas in the molten metal or by mold gases evolved during the pouring of the casting. The defects in this category can be classified into blowholes and pinhole porosity. GATE WALLAH MECHANICAL HANDBOOK 11.30 CASTING 3.13.2 Shrinkage Cavities These are caused by liquid shrinkage occurring during the solidification of the casting. To compensate for this, proper feeding of liquid metal is required. For this reason, risers are placed at the appropriate places in the mold. 3.13.3 Molding material defects (i) Cut and washes These appear as rough spots and areas of excess metal, and are caused by erosion of molding sand by the flowing metal. This is caused by the: (a) Molding sand not having enough strength and (b) The molten metal flowing at high velocity. (ii) Scab This defect occurs when a portion of the face of a mould lifts or breaks down and the recess thus made is filled by metal. When the metal is poured into the cavity, gas may be disengaged with such violence as to break up the sand, which is then washed away and the resulting cavity filled with metal. (iii) Metal Penetration When molten metal enters into the gaps between sand grains, the result is a rough casting surface. (iv) Fusion This is caused by the fusion of the sand grains with the molten metal, giving a brittle, glassy appearance on the casting surface. (v) Swell Under the influence of met allostatic forces, the mold wall may move back causing a swell in the dimension of the casting. (vi) Inclusions Particles of slag, refractory materials sand or deoxidation products are trapped in the casting during pouring solidification. 3.13.4 Pouring metal defects (i) Mis-run A mis-run is caused when the metal is unable to fill the mold cavity completely and thus leaves unfilled cavities. (ii) Cold shut A cold shut is caused when two streams while meeting in the mold cavity, do not fuse together properly thus forming a discontinuity in the casting. 3.13.5 Mold Shift The mold shift defect occurs when cope and drag or molding boxes have not been properly aligned. GATE WALLAH MECHANICAL HANDBOOK 11.31 CASTING 3.14 Special Casting 3.14.1 Shell Casting The sand is mixed with a thermosetting resin is allowed to come in contact with a heated metal pattern (2000C). A skin (shell) of about 3.5 mm of sand and plastic mixture adhere to the pattern. Then the shell is removed from the pattern. The cope and drag shells are kept in a flask with necessary backup material and the molten metal is poured into the mold. Advantages Dimensional accuracy. Smoother surface finish. (Due to finer size grain used) Very thin sections can be cast. Very small amount of sand is needed. Process is EASY Limitations Expensive pattern Small size casting only. Highly complicated shapes cannot be obtained. More sophisticated equipment is needed for handling the shell moldings. 3.14.2 Investment Casting Investment casting refers to the ceramics formed around the wax patterns to create a casing for molten metal to be poured. Once the wax patterns are created, they are melted onto a gate system, dipped into slurry and sand to form a layered casing, then replaced with the melted metals such as stainless steel, aluminum, and much more Advantages Exceptional surface polish High dimensional precision Even the most complicated elements can be cast. Casting is possible with almost any metal. Limitations Costly patterns and moulds Labour costs can be high Limited size GATE WALLAH MECHANICAL HANDBOOK 11.32 CASTING 3.14.3 Hot Chamber Die Casting Die casting alloy is melted in a furnace located within the equipment Casting cycles are significantly shorter, thus has a higher production capacity Suitable for low melting point alloys Offers longer tool life Requires minimum safety measures Commonly used metal alloys include Zinc, lead and etc. 3.14.4 Cold Chamber Die Casting Dies casting alloy is melted in a separate furnace located outside the equipment. Has longer casting cycles; thus, the product capacity is less Suitable for high melting point alloys Has shorter tool life Requires more safety measures Commonly used metal alloys include Aluminum, Copper, Brass Magnesium, etc. 3.14.5 Centrifugal Casting (i) True centrifugal casting Process: Molten metal is introduced into a rotating sand, metal, or graphite mould, and held against the mould wall by centrifugal force until it is solidified A mold is set up and rotated along a vertical (rpm is reasonable), or horizontal (200-1000 rpm is reasonable) axis. The mold is coated with a refractory coating. During cooling lower density impurities will tend to rise towards the center of rotation. Important result: o Mechanical properties of centrifugally cast jobs are better compared to other processes o No cores are required for making concentric hole (ii) Semi-centrifugal Casting Centrifugal force assists the flow of metal from a central reservoir to the extremities of a rotating symmetrical mold, which may be either expendable or multiple-use Rotational speeds are lower than for true centrifugal casting. Cores can be used to increase the complexity of the product. (iii) Centrifuging Uses centrifuging action to force the metal from a central pouring reservoir into separate mold cavities that are offset from the axis of rotation. 3.14.6 Slush Casting Slush casting is a variation of the permanent mold process in which the metal is permitted to remain in the mold only until a shell of the desired thickness has formed. The mold is then inverted and the remaining liquid is poured out. GATE WALLAH MECHANICAL HANDBOOK 11.33 CASTING 3.14.7 Squeeze Casting Molten metal is poured into an open face die. A punch is advanced into the die, and to the metal. Pressure (less than forging) is applied to the punch and die while the part solidifies. The punch is retracted, and the part is knocked out with an ejector pin. 3.14.8 Plaster Casting A slurry of plaster, water, and various additives is additives is pouted over a pattern and allowed to set. The pattern is removed and the mould is baked to remove excess water. After pouring and solidification, the mould is broken and the casting is removed. 3.14.9 Loam Moulding Moulding loam is generally artificially composed of common brick-clay, and sharp sand. Loam means mud. Loam Moulding is restricted to forms which cannot be cast conveniently in any other process. 3.15 Type of Furnace 3.15.1 Cupola Cupola has been the most widely used furnace for melting cast iron. In hot blast cupola, the flue gases are used to preheat the air blast to the cupola so that the temperature in the furnace is considerably higher than that in a conventional cupola. Coke is fuel and Lime stone (CaCO3) is mostly used flux. 3.15.2 Electric Arc Furnace For heavy steel castings, the open-hearth type of furnaces with electric arc or oil fired would be generally suitable in view of the large heat required for melting. Electric arc furnaces are more suitable for ferrous materials and are larger in capacity. 3.15.3 Crucible Furnace Smaller foundries generally prefer the crucible furnace. The crucible is generally heated by electric resistance or gas flame. 3.15.4 Induction Furnace The induction furnaces are used for all types of materials, the chief advantage being that the heat source is isolated from the charge and the slag and flux get the necessary heat directly from the charge instead of the heat source. 3.16 Casting Cleaning (Fettling) Impurities in the molten metal are prevented from reaching the mould cavity by providing a (i) Strainer (ii) Bottom well (iii) Skim bob  GATE WALLAH MECHANICAL HANDBOOK 11.34 Design Against Static Load 4 4.1 Welding WELDING Welding is a process by which two materials, usually metals, are permanently joined together by coalescence, which is induced by a combination of temperature, pressure, and metallurgical conditions. 4.1.1 Types of Joints Fig. 4.1 Types of Joints 4.2 Electric Arc Welding Fig. 4.2 Electric Arc Welding An arc is generated between cathode and anode when they are touched to establish the flow of current and then separated by a small distance. 65% to 75% heat is generated at the anode. If DC is used and the work is positive (the anode of the circuit), the condition is known as straight polarity (SPDC). Work is negative and electrode is positive is reverse polarity (RPDC). GATE WALLAH MECHANICAL HANDBOOK 11.35 Welding Note: RPDC arc-welding maintain a stable arc and preferred for difficult tasks such as overhead welding Fig. 4.3 Overhead Welding Electrode Polarity Negative Positive AC Electron and Ion Flow Penetration Characteristics Yes-Once Every Oxide Cleaning Action No Yes Half Cycle Heat Balance In 70% At Work End 30% At Work End 50% At Work End The Arc (Approx.) 30% At Electrode End 70% At Electrode End 50% At Electrode End Penetration Deep; Narrow Shallow; Wide Medium Table 4.1 There are three modes of metal transfer (globular, spray and short-circuit). Fig. 4.4 Metal Transfer Bead is the metal added during single pass of welding. Bead material is same as base metal. In d.c. welding, the straight polarity (electrode negative) results in Lower deposition rate GATE WALLAH MECHANICAL HANDBOOK 11.36 36 Welding 4.3 Arc welding equipment’s 4.3.1 Droppers: Constant current welding machines Good for manual welding Iarc = I trnf Fig. 4.5 4.3.2 Constant voltage machines Good for automatic welding Varc = Vtrnf Fig. 4.6 4.4 Constant voltage machines Formula Fig. 4.7 OCV = open circuit voltage SCC = short circuit current V I + =1 OCV SCC OCV: It is the maximum rated voltage between open terminals under no loading conditions. SCC: It is the maximum rated current that is required during arc generation. GATE WALLAH MECHANICAL HANDBOOK 11.37 37 Welding Note: In arc welding, the arc length should be equal to Rod diameter In manual arc welding, the equipment should have drooping characteristics in order to maintain Current constant when arc length changes In arc welding, d.c. reverse polarity is used to bear greater advantage in Overhead welding. For maximum power applied current(I) and voltage (V) are: 𝑆𝑆𝑆𝑆𝑆𝑆 I= 2 𝑂𝑂𝑂𝑂𝑂𝑂 V= 2 4.4.1 Heat Input (Hin) Vl Heat input = (Joule/mm3) Abv Ab = weld bead area (mm2) v = velocity mm/sec H ηt = m H in Where Hm = Heat required for melting Hin = Heat input ηt = Melting efficiency 4.4.2 Duty Cycle Duty cycle is a welding equipment specification which defines the number of minutes, within a 10 minutes period, during which a given welder can safely produce a particular welding current. 2  I Required duty cycle Ta =   T  Ia  Where, T = rated duty cycle I = rated current at the rated duty cycle Ia = Maximum current at the rated duty cycle Note: For manual welding a 60% duty cycle is suggested and for automatic welding 100% duty cycle. 4.5 Electrode coating characteristic 1. Provide a protective atmosphere. 2. Stabilize the arc. 3. Provide a protective slag coating to accumulate impurities, prevent oxidation, and slow the cooling of the weld metal. 4. Reduce spatter. 5. Add alloying elements. 6. Affect arc penetration GATE WALLAH MECHANICAL HANDBOOK 11.38 38 Welding Note: The electrodes used in arc welding are coated. This coating is not expected to Prevents electrode from contaminate The coating material of an arc welding electrode contains which of the following? 1. Deoxidising agent 2. Arc stabilizing agent 3. Slag forming agent Note: Arc Length must be short because 1. Heat is concentrated. 2. More stable 3. More protective atmosphere. A long arc has following draw back 1. Large heat loss into atmosphere. 2. Unstable arc. 3. Weld pool is not protected. 4. Weld has low strength, less ductility, poor fusion and excessive spatter. 4.6 Arc blow in DC arc welding Fig. 4.8 Arc blow occurs during the welding of magnetic materials with DC. The effect is particularly noticeable when welding with bare electrodes or when using currents below or above. Again, the problem of arc blow gets magnified when welding highly magnetic materials Disadvantage of arc blow: 1. Low heat penetration. 2. Excessive weld spatter. 3. Pinch effect in welding is the result of electromagnetic forces Note: Arc blow is more common in D.C. welding with bare electrodes Pinch effect in welding is the result of Electromagnetic forces GATE WALLAH MECHANICAL HANDBOOK 11.39 39 Welding 4.7 Gas shields An inert gas is blown into the weld zone to drive away other atmospheric gases. Gases are argon, helium, nitrogen, carbon dioxide and a mixture of the above gases. 4.8 Tungsten Inert Gas welding (TIG) Arc is established between a non-consumable tungsten electrode and the workpiece. Arc length is constant, arc is stable and easy to maintain. With or without filler. Note: Gas tungsten arc welding process used non – consumable electrode In an inert gas welding process, the commonly used gas is Helium or Argon. 4.9 Gas Metal Arc Welding (GMAW)/MIG Fig. 4.9 MIG Welding Arc is generated between consumable electrode and workpiece. Electrode is in form of small ida (1-2.5mm) will and it will continually supply to workpiece through roller movement. (spray transfer) Liquid metal can be protected by inert gas Note: In MIG welding, the metal is transferred into the fine spray of metal. MIG welding process uses Consumable electrode D.C. power supply. 4.10 Submerged Arc welding (SAW) Joining of High thickness object in a single pass, this technique used. Arc is generated between consumable electrode & W/P through welding torch, solid form of granular flux (CaO, CaF2) will be continuously supply. Arc will be submerged under solid flux There is No Heat transfer loss, No splashing & weld spatter. Thickness-10-50mm, I = 200- 2000A, speed = 5m/min High weld deformation rate and High welding speed. Limited to Horizontal position. Use: Pressure vessel, ship bridges, LPG cylinder etc. GATE WALLAH MECHANICAL HANDBOOK 11.40 40 Welding Note: High welding speeds and High deposition rates are the major characteristics of submerged arc welding. 4.11 Gas Flame processes: Oxyacetylene welding, commonly referred to as gas welding, is a process which relies on combustion of oxygen and acetylene. When mixed together in correct proportions within a hand-held torch or blowpipe, a relatively hot flame is produced. Acetylene is the principal fuel gas employed. Combustion of oxygen and acetylene (C2H2) in a welding torch produces a temp. in a two-stage reaction. In the first stage C2 H 2 + O 2 → 2CO + H 2 + Heat In the second stage combustion of the CO and H2 occurs just beyond the first combustion zone. 2CO + O2 → 2CO2 + Heat H2 + O2 → H2O + Heat Note: Oxygen for secondary reactions is obtained from the atmosphere. Three types of flames can be obtained by varying the oxygen/acetylene ratio. 4.11.1 Neutral Flame The ratio (O2: C2H2) is about 1:1, all reactions are carried to completion and a neutral flame is produced. It is chemically neutral and neither oxidizes or carburizes the metal being welded. Fig. 4.10 4.11.2 Oxidizing flame A higher ratio (O2 > C2H2), such as 1.5:1, produces an oxidizing flame, hotter than the neutral flame (about 3300° C). Used when welding some nonferrous alloys such as copper-base alloys and zinc base alloys. GATE WALLAH MECHANICAL HANDBOOK 11.41 41 Welding Fig. 4.11 4.11.3 Carburizing flame Excess fuel, on the other hand, produces a carburizing flame. Carburizing flame can carburize metal also. The excess fuel decomposes to carbon and hydrogen, and the flame temperature is not as great (about 3000°C). Fig. 4.12 Note: OFW is fusion welding. No pressure is involved. Fluxes may be used Flux can be added as a powder, the welding rod can be dipped in a flux paste, or the rods can be pre-coated. The ratio between Oxygen and Acetylene gases for neutral flame in gas welding is 1:1. In Oxyacetylene gas welding, temperature at the inner cone of the flame is around 3200°C. 4.12 Oxygen Torch Cutting (Gas Cutting) Iron and steel oxidize (burn) when heated to a temperature between 800°C to 1000°C. High-pressure oxygen jet (300 KPa) is directed against a heated steel plate, the oxygen jet burns the metal and blows it away causing the cut (kerf). GATE WALLAH MECHANICAL HANDBOOK 11.42 42 Welding Fig. 4.13 Gas Welding Note: Final microstructure depends on cooling rate. Steels with less than 0.3 % carbon cause no problem. Cutting CI is difficult, since its melting temp. is lower than iron oxide. (a) Powder Cutting Difficult to cut metals by oxy-fuel cutting process are: Cast iron, stainless steel, and others high alloy steels. So, we can use powder cutting. By injecting a finely divided 200-mesh iron powder into the flame. (b) Plasma Cutting Uses ionized gas jet (plasma) to cut materials resistant to oxy-fuel cutting, the ionized gas is forced through nozzle (up to 500 m/s), and the jet heats the metal, and blasts the molten metal away. 1 1 HAZ is rd to th than oxyfuel cutting. 3 4 Maximum plate thickness = 200 mm 4.13 Resistance Welding 4.13.1 Spot Welding Resistance welding is the joining of metals by applying pressure and passing current for a length of time through the metal area which is to be joined. The key advantage of resistance welding is that no other materials are needed to create the bond, which makes this process extremely cost effective. GATE WALLAH MECHANICAL HANDBOOK 11.43 43 Welding Fig. 4.14 Spot Resistance Welding Note: Overall resistance is very low between the overlapping plate. Very high-current (up to 100,000 A) and Very low-voltage (0.5 to 10 V) is used. The maximum heat in resistance welding is at the Interface between the two plates being Joined. 4.13.2 Resistance seam welding Resistance Seam Welding is a subset of Resistance Spot Welding using wheel-shaped electrodes to deliver force and welding current to the parts. The difference is that the workpiece rolls between the wheel-shaped electrodes while weld current is applied. Fig. 4.15 Seam Resistance Welding Note: In resistance seam welding, the electrode is in the form of a circular disc. GATE WALLAH MECHANICAL HANDBOOK 11.44 44 Welding 4.13.3 Projection welding Like spot welding, the projection welding process relies on heat generated by an electric current to join metal pieces together. Projection electrodes are capable of carrying more current than spot welding electrodes and can, therefore, weld much thicker materials. Fig. 4.16 Projection Resistance Welding 4.13.4 Upset welding Upset welding or resistance butt welding is a welding technique that produces coalescence simultaneously over the entire area of abutting surfaces or progressively along a joint, by the heat obtained from resistance to electric current through the area where those surfaces are in contact. 4.13.5 Flash Welding It is similar to upset welding except the arc rather than resistance heating. 4.13.6 Percussion Welding Similar to flash welding except arc power by a rapid discharge of stored electrical energy. The arc duration is only 1 to 10 ms, heat is intense and highly concentrated. 4.14 Thermit Welding Thermit welding (TW) is a process that uses heat from an exothermic reaction to produce coalescence between metals. The name is derived from ‘thermite’ the generic name given to reactions between metal oxides and reducing agents. The thermite mixture consists of metal oxides with low heats of formation and metallic reducing agents which, when oxidized, have high heats of formation. The excess heats of formation of 8Al+ 3Fe3O4 → 9Fe + 4Al2O3 + heat 4.15 Electro Slag Welding Electroslag Welding is a welding process, in which the heat is generated by an electric current passing between the consumable electrode (filler metal) and the work piece through a molten slag covering the weld surface. Heat, generated by the arc, melts the fluxing powder and forms molten slag. The slag, having low electric conductivity, is maintained in liquid state due to heat produced by the electric current. GATE WALLAH MECHANICAL HANDBOOK 11.45 45 Welding Fig. 4.17 Electroslag Welding 4.16 Electron Beam Welding (EBW) A beam of electrons is magnetically focused on the work piece in a vacuum chamber. Fig. 4.18 Electron Beam Welding 4.17 Laser Beam Welding (LBW) Laser welding utilizes the heat from a high-power concentrated laser beam to melt thin or thick metal interfaces. It is generally used for producing narrow and deep joints of depth to width ratio ranging between 4 and 10 GATE WALLAH MECHANICAL HANDBOOK 11.46 46 Welding Note: Increasing order of Heat affected zone (HAZ) are Laser beam welding < MIG welding < Submerged arc welding < Arc welding Shielding method Welding Process A. Flux coating 1. Shielded metal arc welding B. Flux granules 2. Submerged arc welding C. CO2 3. Gas metal arc welding D. Vacuum 4. Electron beam welding 4.18 Friction Welding Friction welding (FRW) is a solid-state welding process that generates heat through mechanical friction between workpieces in relative motion to one another, with the addition of a lateral force called "upset" to plastically displace and fuse the materials. (a) (b) (c) (d) Fig. 4.19 Friction Welding 4.19 Ultrasonic Welding (USW) USW is a solid-state welding. High-frequency (10 to 200, KHz) is applied. Surfaces are held together under light normal pressure. Temp. do not exceed one-half of the melting point. The ultrasonic transducer is same as ultrasonic machining. 4.20 Explosion Welding Explosion Welding Done at room temperature in air, water or vacuum. Surface contaminants tend to be blown off the surface. Typical impact pressures are millions of psi. Well suited to metals that is prone to brittle joints when heat welded, such as, Aluminium on steel Titanium on steel Important factors are, Critical velocity Critical angle The cladding plate can be supported with tack welded supports at the edges, or the metal inserts. GATE WALLAH MECHANICAL HANDBOOK 11.47 47 Welding 4.21 Brazing and Soldering Fig. 4.20 Brazing and Soldering 4.21.1 Brazing Brazing is the joining of metals through the use of heat and a filler metal whose melting temperature is above 450°C; but below the melting point of the metals being joined. Fluxes used are combinations of borax, boric acid, chlorides, fluorides, tetra-borates and other wetting agents. Note: The strength of a brazed joint Increases up to certain gap between the two joining surfaces beyond which it decreases. 4.21.2 Soldering By definition, soldering is a brazing type of operation where the filler metal has a melting temperature below 450°C. Soldering is used for a neat leak-proof joint or a low resistance electrical joint.  GATE WALLAH MECHANICAL HANDBOOK 11.48 48 5 5.1 Recrystallisation Temperature (Rx) METAL FORMING “The minimum temperature at which the completed recrystallisation of a cold worked metal occurs within a specified period of approximately one hour”. Note: Rx varies between 1/3 to 1/2 melting point. Rx = 0.4 x Melting temp. Rx of Iron is 450°C and for steels around 1000°C 5.1.1 Grain growth Grain growth follows complete crystallization if the materials left at elevated temperatures. Heating beyond recrystallization temperature range causes the size of the recrystallized grains to increase, some of the grains grow by consuming others. Note: Grain growth is very strongly dependent on temperature. 5.2 Cold working Cold working of a metal is carried out below its recrystallisation temperature. Although normal room temperatures are ordinarily used for cold working of various types of steel, temperatures up to the recrystallisation range are sometimes used. In cold working, recovery processes are not effective. Advantages of Cold Working In cold working processes, smooth surface finish can be easily produced. Accurate dimensions of parts can be maintained. Strength and hardness of the metal are increased but ductility decreased. Since the working is done in cold state, no oxide would form on the surface and Consequently good surface finish is obtained. Cold working increases the strength and hardness of the material due to the strain hardening which would be beneficial in some situations. There is no possibility of decarburization of the surface. Better dimensional accuracy is achieved. GATE WALLAH MECHANICAL HANDBOOK 11.49 Metal Forming Disadvantages of Cold Working Some materials, which are brittle, cannot be cold worked easily. Since the material has higher yield strength at lower temperatures, the amount of deformation that can be given to is limited by the capability of the presses or hammers used. A distortion of the grain structure is created. Since the material gets strain hardened, the maximum amount of deformation that can be given is limited. Any further deformation can be given after annealing. 5.3 Hot working Plastic deformation of metal carried out at temperature above recrystallization temperature, is called hot working Recrystallization temperature = about one half of ‑ melting point on absolute scale In practice, hot working usually performed somewhat above 0.5Tm Metal continues to soften as temperature increases above 0.5Tm, enhancing advantage of hot working above this level Advantage of hot working Work part shape can be significantly altered Lower forces and power required Metals that usually fracture in cold working can be hot formed Strength properties of product are generally isotropic No strengthening of part occurs from work hardening Advantageous in cases when part is to be subsequently processed by cold forming. Dis-advantages of Hot Working Heat energy is needed It requires expensive tools. Poor surface finish of material due to scaling of surface due to the rapid oxidation Due to the poor surface finish, close tolerance cannot be maintained. 5.4 Rolling Rolling is the process of reducing the thickness or changing the cross section of a long workpiece by compressive forces applied through a set of rolls, as shown in figure. Most rolling is carried out by hot working Fig. 5.1 Rolling Process GATE WALLAH MECHANICAL HANDBOOK 11.50 Metal Forming (a) Continuity Equation hobovo = hfbfvf Fig. 5.2 Rolling process (b) Hot Rolling Done above the recrystallization temp. Results fine grained structure. Surface quality and final dimensions are less accurate. (c) Cold Rolling Done below the recrystallization temp. Products are sheet, strip, foil etc. with good surface finish and increased mechanical strength with close product dimensions (d) Defects in Rolling Defects What is Cause Wavy edges Strip is thinner along its edges than at its Centre. Due to roll bending edges elongates more and buckle. Alligatoring Edge breaks Non-uniform deformation (e) Draft/reduction(Δh) Rolling R = roll radius L = contact arc length Lp = projected arc length Hf = strip final thickness Vr = velocity of the roll Vf = velocity of the strip at roll exit GATE WALLAH MECHANICAL HANDBOOK 11.51 Metal Forming ho = strip initial thickness N = neutral point a = angle of bite Vo = velocity of strip at entrance to roll Fig. 5.3 Side view of flat rolling, indicating before and after thickness, work velocities, angle of contact with rolls, and other features. Maximum Draft Possible (Δh) max (∆h)max = µ2R αmax = tan–1(µ) Note: If αmax is larger than this value, the rolls begin to slip, Number of pass needed ∆h required n = ∆h max (f) Elongation Factor or Elongation Co-efficient (E/En) L1 A0 E = = for single pass L0 A1 Ln A0 En = = for n – pass L0 An GATE WALLAH MECHANICAL HANDBOOK 11.52 Metal Forming (g) Torque and Power Fig. 5.4 a a λ = = Lp R∆h Where, Lp = Projected length R = radius of roller Δh = Draft T = Torque per roller F = Roll Separating Force a = Radius from the center where roll separating force(F) is acting ω = Angular velocity Where λ is 0.5 for hot-rolling and 0.45 for cold-rolling. 5.5 Forging Process in which material is shaped by the application of localized compressive forces exerted manually or with power hammers, presses or special forging machines. Impression Die forging Here half the impression of the finished forging is sunk or made in the top die and other half of the impression is sunk in the bottom die. In impression die forging, the work piece is pressed between the dies. As the metal spreads to fill up the cavities sunk in the dies, the requisite shape is formed between the closing dies Open die forging in this, the work piece is compressed between two platens. There is no constraint to material flow in lateral direction. Open die forging is a process by which products are made through a series of incremental deformation using dies of relatively simple shape. Closed die forging Closed die forging is very similar to impression die forging, but in true closed die forging, the amount of material initially taken is very carefully controlled, so that no flash is formed Drop forging Drop forging utilizes a closed impression die to obtain the desired shape of the component. The shaping is done by the repeated hammering given to the material in the die cavity. The equipment used for delivering the blows are called drop hammers. GATE WALLAH MECHANICAL HANDBOOK 11.53 Metal Forming 5.5.1 Operations involved in forging Steps involved in hammer forging Fullering or swaging Edging or rolling Bending Drawing or cogging Flattening Blocking Finishing operation Trimming or cut off Fig. 5.5 Forging Operations 5.5.2 Die Materials Should have Thermal shock resistance Thermal fatigue resistance High temperature strength High wear resistance High toughness and ductility High hardenability High dimensional stability during hardening High machinability. Die materials: alloyed steels (with Cr, Mo, W, V), tool steels, cast steels or cast iron Note: 1. Carbon steels with 0.7-0.85% C are appropriate for small tools and flat impressions. 2. Medium-alloyed tool steels for hammer dies. 3. Highly alloyed steels for high temperature resistant dies used in presses and horizontal forging machines. GATE WALLAH MECHANICAL HANDBOOK 11.54 Metal Forming 5.5.3 Typical forging defects Incomplete forging penetration- should forge on the press. Microstructural differences resulting in pronounced property variation. Hot shortness, due to high sulphur concentration in steel and nickel. Pitted surface, due to oxide scales occurring at high temperature stick on the dies. Buckling, in upsetting forging. Subject to high compressive stress. Surface cracking, due to temperature differential between surface and center, or excessive working of the surface at too low temperature. Microcracking, due to residual stress. Flash line crack, after trimming-occurs more often in thin workpieces. Therefore, should increase the thickness of the flash. Cold shut or fold, due to flash or fin from prior forging steps is forced into the workpiece. Internal cracking, due to secondary tensile stress. 5.6 Sheet Metal Working Shearing is a cutting operation used to remove a blank of required dimension from a large sheet. Fig. 5.5 Punching and Blanking operation In blanking, the piece being punched out becomes the workpiece and any major burrs or undesirable features should be left on the remaining strip. In piercing (Punching), the punch-out is the scrap, and the remaining strip is the workpiece. GATE WALLAH MECHANICAL HANDBOOK 11.55 Metal Forming (a) Clearance Die opening must be larger than punch and known as ‘clearance’. Punching Punch = size of hole Die = punch size +2 clearance Note: In punching punch is correct size. (b) Blanking Die = size of product Punch = Die size –2 clearance Note: In blanking die size will be correct. Clearance formula = 1. c 0.0032t τ or 2. C = allowance (t) 3. C = (x%)t Where, t = sheet thickness (mm) τ = shear strength (N/mm2) C = Clearance (c) Punching Force and Blanking Force Fmax = Ltτ Where Fmax = Maximum force L = cutting parameter(mm) t = thickness of sheet (mm) τ = shear strength (N/mm2) The punching force for holes which are smaller than the stock thickness may be estimated as follows: πdt σ Fmax = 3 d t Where Fmax = Maximum force d = diameter of punch(mm) t = thickness of sheet (mm) σ = Tensile strength (N/mm2) GATE WALLAH MECHANICAL HANDBOOK 11.56 Metal Forming (d) Shear on Punch To reduce shearing force, shear is ground on the face of the die or punch. It distributes the cutting action over a period of time. Note: Shear only reduces the maximum force to be applied but total work done remains same. Fig. 5.6 Maximum shear force(Fs) Fmax ( Pt ) Fs = Pt + S If S > Pt then Fmax ( Pt ) Fs = S Where, Fmax = Maximum force P = percentage penetration t = thickness of sheet (mm) S = shear height (mm) 5.7 Drawing Drawing is a plastic deformation process in which a flat sheet or plate is formed into a three-dimensional part with a depth more than several times the thickness of the metal. Fig. 5.7 Drawing Operation GATE WALLAH MECHANICAL HANDBOOK 11.57 Metal Forming Blank Size D = d 2 + 4dh D = d 2 + 4dh − 0.5r when15r ≤ d ≤ 20r ( d − 2r ) + 4d ( h − r ) + 2π r ( d − 0.7 r ) 2 D = when d < 10r Fig. 5.8 5.8 Deep drawing Drawing when cup height is more than half the diameter is termed deep drawing. This can be achieved by redrawing the part through a series of dies. Note: Deep drawing - is a combination of drawing and stretching. 5.8.1 Deep Drawability The ratio of the maximum blank diameter to the diameter of the cup drawn. i.e. D/d. The average reduction in deep drawing, thumb rule for reduction d = 0.5 D  d Reduction =  1 −  × 100% = 50%  D Thumb rule: First draw: Reduction = 50% Second draw: Reduction = 30% Third draw: Reduction = 25% Fourth draw: Reduction = 16% Fifth draw: Reduction = 13% GATE WALLAH MECHANICAL HANDBOOK 11.58 Metal Forming 5.8.2 Defects in Drawing Wrinkle: An insufficient blank holder pressure causes wrinkles to develop on the flange, which may also extend to the wall of the cup. Fig. 5.9 Wrinkle Fracture: Further, too much of a blank holder pressure and friction may cause a thinning of the walls and a fracture at the flange, bottom, and the corners (if any). Fig. 5.10 Fracture Earing: While drawing a rolled stock, ears or lobes tend to occur because of the anisotropy induced by the rolling operation. Fig. 5.11 Earing Miss strike: Due to the misplacement of the stock, unsymmetrical flanges may result. This defect is known as miss strike. Fig. 5.12 Miss Strike Surface scratches: Die or punch not having a smooth surface, insufficient lubrication Fig. 5.13 Scratches GATE WALLAH MECHANICAL HANDBOOK 11.59 Metal Forming 5.9 Spinning Spinning is a cold-forming operation in which a rotating disk of sheet metal is shaped over a male form, or mandrel. Localized pressure is applied through a simple round-ended wooden or metal tool or small roller, which traverses the entire surface of the part. Relation between cone and blank tc = tb sinα Fig. 5.14 Spinning process Tc = cone thickness Tb = blank thickness Fig. 5.15 Spinning Process GATE WALLAH MECHANICAL HANDBOOK 11.60 Metal Forming 5.10 Stretch Forming A sheet of metal is gripped by two or more sets of jaws that stretch it and wrap it around a single form block. Because most of the deformation is induced by the tensile stretching, the forces on the form block are far less than those normally encountered in bending or forming. Fig. 5.16 Final thickness(t) formula For bi-axial stretching of sheets  li1  l  ε1 = In   ; ε 2 = In  i 2   l01   l02  Initial thickness(t) Final thickness = eε1 × eε 2 where li1 = final length in direction 1 li2 = final length in direction 2 lo1 = initial length in direction 1 lo2 = initial length in direction 2 ε1 = strain in direction 1 ε2 = strain in direction 2 5.11 Bending After basic shearing operation, we can bend a part to give it some shape. Bending parts depends upon material properties at the location of the bend. Fig. 5.17 Bending GATE WALLAH MECHANICAL HANDBOOK 11.61 Metal Forming Bend allowance (Lb), formula Lb = α(R + kt) where R = bend radius k = constant (stretch factor) t = thickness of material α = bend angle (in radian) 5.12 Extrusion Extrusion is a compression process in which the work metal is forced to flow through a die opening to produce a desired cross-sectional shape. As shown in figure Fig. 5.18 5.12.1 Extrusion Ratio Ratio of the cross-sectional area of the billet to the cross-sectional area of the product. 5.12.2 Direct Extrusion A solid ram drives the entire billet to and through a stationary die and must provide additional power to overcome the frictional resistance between the surface of the moving billet and the confining chamber. Fig. 5.19 Direct extrusion GATE WALLAH MECHANICAL HANDBOOK 11.62 Metal Forming 5.12.3 Indirect Extrusion A hollow ram drives the die back through a stationary, confined billet. Since no relative motion, friction between the billet and the chamber is eliminated. Fig. 5.20 Indirect Extrusion 5.12.4 Hydrostatic Extrusion In hydrostatic extrusion the container is filled with a fluid. Extrusion pressure is transmitted through the fluid to the billet. Friction is eliminated in this process because of there is no contact between billet and container wall. Brittle materials can be extruded by this process. Highly brittle materials can be extruded into a pressure chamber. Hydrostatic extrusion is a process in which the billet is completely circumscribed by a pressurized liquid in all the cases, with the exception being the case where billet is in the contact with die. This process can be carried out in many ways including warm, cold or hot but due to the stability of the used fluid, the temperature is limited. Hydrostatic extrusion has to be carried out in a completely sealed cylinder for containing the hydrostatic medium Fig. 5.21 Hydrostatic Extrusion 5.13 Wire Drawing A cold working process to obtain wires from rods of bigger diameters through a die. For fine wire, the material may be passed through a number of dies, receiving successive reductions in diameter, before being coiled. The wire is subjected to tension only. GATE WALLAH MECHANICAL HANDBOOK 11.63 Metal Forming Fig. 5.22 Drawing process 5.13.1 Extrusion Load (F) formula Extrusion load formula (Uniform deformation, no friction) “work – formula”  Ao  F = Aoσ o ln   A  f  For real conditions  Ao  F = KAo ln   A  f  A = initial cross-sectional area 0 A = final cross-sectional area f σ = yield strength of material 0 K = extrusion constant 𝜎𝜎 K = 0 (for plane strain) √3 𝜎𝜎0 = (for plane stress) 2 5.13.2 Wire or Tube drawing force(F) formula Wire or Tube drawing force formula (Uniform deformation, no friction) “work – formula”  Ao  F = A f σ 0 ln   A  f   GATE WALLAH MECHANICAL HANDBOOK 11.64 6 Design Against Static Load METROLOGY 6.1 Metrology and Inspection It is the measurement science that includes various aspects like design, manufacture, testing, and applications of various measuring instruments, devices, and techniques. Thus, it facilitates the proper application of the scientific principles in the accurate dimensional control of manufactured components. 6.1.1 Limit System Basic size: It is the size with reference to which upper or lower limits of size are defined. It is theoretical size of part as suggested by designer. Actual size: It is the size actually obtained by machining. It is found by actual measurement Tolerance: (a) The difference between the upper limit and lower limit. (b) It is the maximum permissible variation in a dimension. (c) The tolerance may be unilateral or bilateral. (d) It is always positive. Unilateral Limits occurs when both maximum limit and minimum limit are either above or below the basic size. Bilateral Limits occur when the maximum limit is above and the minimum limit is below the basic size. 6.1.2 Fit Fits: It is the relationship that exists between two mating parts, a hole and shaft with respect to their dimensional difference before assembly. Fig. 6.1 Fit GATE WALLAH MECHANICAL HANDBOOK 11.65 Metrology 6.1.3 Allowance It is Minimum clearance or maximum interference. It is the intentional difference between the basic dimensions of the mating parts. The allowance may be positive or negative. 6.2 Hole basis and Shaft basis System 6.2.1 Basis of Fits - Hole Basis In this system, the basic diameter of the hole is constant while the shaft size varies according to the type of fit. Fig. 6.2 Hole Basis Fits 6.2.2 Basis of Fits - Shaft Basis Here the hole size is varied to produce the required class of fit with a basic-size shaft. Fig. 6.3 Shaft Basis Fits GATE WALLAH MECHANICAL HANDBOOK 11.66 66 Metrology 6.3 Li

232) GATE-O-PEDIA Manufacturing Process 1_note BY REXODAS.pdf

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