Production Engg. Theory


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Production Engg. Theory

  1. 1. Metal Cutting, Metal Forming & Metrology Theory for IES, GATE & PSUs Section‐I: Theory of Metal Cutting Chapter-1: Basics of Metal Cutting Chapter-2: Force & Power in Metal Cutting Chapter-3: Tool life, Tool Wear, Economics and Machinability Note down in the class Note down in the class Page-1 Section‐II: Metal Forming Chapter-4: Cold Working, Recrystalization and Hot Working Chapter-5: Rolling Chapter-6: Forging Chapter-7: Extrusion & Drawing Chapter-8: Sheet Metal Operation Chapter-9: Powder Metallurgy Page-6 Page-8 Page-13 Page-16 Page-21 Page-30 Section‐III: Metrology Chapter-10: Limit, Tolerance & Fits Chapter-11: Measurement of Lines & Surfaces Chapter-12: Miscellaneous of Metrology Page-34 Page-38 Page-45 For‐2013 (IES, GATE & PSUs)    For IES, GATE, PSUs Page 1 of 49 Bhopal -2014
  2. 2. Tool Failure Tool Wear, Tool Life & Machinability Tool Wear, Tool Life & M hi bili T lW T l Lif & Machinability By  S K Mondal l Tool Wear Tool failure is two types 1. Slow‐death: The gradual or progressive wearing g p g g away of rake face (crater wear) or flank (flank wear) of g the cutting tool or both. 2. Sudden‐death: Failures leading to premature end  of the tool  The sudden‐death type of tool failure is difficult to predict. Tool failure mechanisms include plastic deformation, brittle fracture, fatigue fracture or edge pp g p chipping. However it is difficult to predict which of these processes will dominate and when tool failure will occur. Flank Wear: (Wear land) Reason Abrasion b h d particles and i l i Ab i by hard i l d inclusions i the work in h k piece. Shearing off the micro welds between tool and work material. material Abrasion by fragments of built‐up‐edge ploughing against the clearance f i h l face of the tool. f h l At low speed flank wear predominates. p p If MRR increased flank wear increased. Flank Wear: (Wear land) Stages Flank Wear Fl k W occurs i three stages of varying wear rates in h f i Flank Wear: (Wear land) Primary wear The Th region where the sharp cutting edge i quickly i h h h i d is i kl broken down and a finite wear land is established. Secondary wear y The region where the wear progresses at a uniform rate. For IES, GATE, PSUs Page 2 of 49 Tool Wear (a) Flank Wear ( ) Fl k W ( ) (b) Crater Wear (c) Chipping off of the cutting edge Flank Wear: (Wear land) Effect Flank Fl k wear di directly affect the component di l ff h dimensions i produced. Flank wear is usually the most common determinant of tool life life. Flank Wear: (Wear land) Tertiary wear The Th region where wear progresses at a gradually i h d ll increasing rate. In the tertiary region the wear of the cutting tool has become sensitive to increased tool temperature due to high wear land. Re‐grinding i recommended b f R i di is d d before they enter this h hi region. Bhopal -2014
  3. 3. Crater wear Tool life criteria ISO (A certain width of flank wear (VB) is the most common  (A  i   id h  f fl k   (VB) i   h       criterion) Uniform wear: 0.3 mm averaged over all past Localized wear: 0.5 mm on any individual past Localized wear: 0 5 mm on any individual past More common in ductile materials which produce continuous chip. h Crater wear         Contd….. Crater depth exhibits linear increase with time. It increases with MRR MRR. Crater wear occurs on the rake f C h k face. At very hi h speed crater wear predominates high d t d i t For crater wear temperature is main culprit and tool defuse into the chip material & tool temperature is maximum at some distance from the tool tip. Wear Mechanism 1. Abrasion wear 2. Adhesion wear 3. 3 Diffusion wear 4. Chemical or oxidation wear Why chipping off or fine cracks  developed at the cutting edge d l d h d Tool material is too brittle Crater wear has little or no influence on cutting forces forces, work piece tolerance or surface finish. Notch Wear Notch wear on the trailing edge is to a great extent an oxidation wear mechanism occurring where th cutting id ti h i i h the tti edge leaves the machined workpiece material in the feed Weak design of tool, such as high positive rake angle direction. As a result of crack that is already in the tool But abrasion and adhesion wear in a combined effect can contribute to the formation of one or several notches. Excessive static or shock l di of the tool. E i i h k loading f h l List the important properties of cutting tool  materials and explain why each is important. t i l d l i h hi i t t Hardness at high temperatures ‐ this provides longer life of the cutting tool and allows higher cutting speeds. Toughness ‐ to provide the structural strength needed to resist impacts and cutting forces Wear resistance ‐ to prolong usage before replacement doesn’t chemically react ‐ another wear factor Formable/manufacturable ‐ can be manufactured in a useful geometry For IES, GATE, PSUs Tool Life Criteria Tool life criteria can be defined as a predetermined numerical value of any type of tool deterioration which can be measured. Some of the ways Actual cutting time to failure. Volume of metal removed. Volume of metal removed Number of parts produced. p p Cutting speed for a given time Length of work machined. Page 3 of 49 Taylor’s Tool Life Equation  based on Flank Wear Causes Sliding of the tool along the machined surface Temperature rise VT n = C Where, V = cutting speed (m/min) T = Time (min) T   Time (min) n = exponent depends on tool material C = constant based on tool and work material and cutting  condition. Bhopal -2014
  4. 4. Values of Exponent ‘n’ Tool Life Curve l f Extended or Modified Taylor’s equation n = 0.08 to 0.2 for HSS tool = 0.1 to 0.15 for Cast Alloys = 0.2 to 0.4 f carbide tool for bid l [IAS 1999; IES 2006] [IAS‐1999; IES‐2006] = 0.5 to 0.7 for ceramic tool 5 7 [NTPC‐2003] i.e Cutting speed has the greater effect followed by feed  g p g y and depth of cut respectively. Cutting speed used for different  tool materials  Effect of Rake angle on tool life ChipEquivalent(q) = Engaged cutting edge length Plan area of cut It is I i used f controlling the tool temperature. d for lli h l 2. Carbide   3. Ceramic Effect of Clearance angle on tool life If clearance angle increased it reduces flank wear but weaken the cutting edge so best compromise is 80 for edge, HSS and 50 for carbide tool. HSS (min) 30 m/min < Cast alloy < Carbide  < Cemented carbide 150 m/min < Cermets  < Ceramics or sintered oxide (max) 600  m/min Chip Equivalent 1. HSS    Effect of work piece on tool life With hard micro‐constituents in the matrix gives poor tool life. With larger grain size tool life is better. • The SCEA alters the length of the engaged cutting E i f t l tti Economics of metal cutting edge without affecting the area of cut. As a result, the chip equivalent changed. When the SCEA is increased, the chip equivalent is increased, without significantly h h l d h f l changing th cutting f h i the tti forces. • I Increase i nose radius also i in di l increases th value of th the l f the chip equivalent and improve tool life life. For IES, GATE, PSUs Page 4 of 49 Bhopal -2014
  5. 5. l Formula Vo To = C n Optimum tool life for minimum cost ⎛ C ⎞⎛ 1− n ⎞ To = ⎜ Tc + t ⎟ ⎜ ⎟ Cm ⎠ ⎝ n ⎠ ⎝ C ⎛ 1− n ⎞ = t ⎜ ⎟ Cm ⎝ n ⎠ if Tc , Ct & Cm given if Ct & Cm given g Optimum tool life for Maximum Productivity p y (minimum production time) ⎛ 1− n ⎞ To = Tc ⎜ ⎟ ⎝ n ⎠ g g 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 + over head cost per  ) = labour min Minimum Cost Vs Production Rate Machinability‐Definition Machinability can be tentatively defined as ‘ability of M hi bili b i l d fi d ‘ bili f being machined’ and more reasonably as ‘ease of machining’. Such ease of machining or machining characters h f h h h of any tool‐work pair is to be judged by: y p j g y Tool wear or tool life Magnitude of the cutting forces Surface finish Magnitude of cutting temperature g g p Chip forms. Vmax.production > Vmax.profit > Vmin. cost Machinability‐‐‐‐‐‐‐‐‐‐‐‐‐Contd……. Machinability will be high when cutting forces, M hi bilit ill b hi h h tti f temperature, surfaces roughness and tool wear are less, tool life is long and chips are id ll uniform and short. t l lif i l d hi ideally if d h t The addition of sulphur lead and tellurium to non‐ sulphur, ferrous and steel improves machinability. Sulphur i added t steel only if th S l h is dd d to t l l there i sufficient is ffi i t manganese in it. Sulphur forms manganese sulphide which exists as an i l t d phase and act as i t hi h i t isolated h d t internal l lubrication and chip breaker. If insufficient manganese is there a low melting iron sulphide will formed around the austenite grain boundary. Such steel is very weak and brittle. For IES, GATE, PSUs Free Cutting steels Addition of lead in low carbon re‐sulphurised steels and also in aluminium copper and their alloys help reduce aluminium, their τs. The dispersed lead particles act as discontinuity and solid lubricants and thus improve machinability by reducing friction, cutting forces and temperature, tool wear and BUE f d formation. i It contains less than 0.35% lead by weight . 35 y g A free cutting steel contains C‐0.07%, Si C % Si‐0.03%, M % Mn‐0.9%, P % P‐0.04%, S % S‐0.22%, Pb % Pb‐0.15% % Page 5 of 49 Machinability Index  Or    Machinability Rating The machinability index KM is defined by KM = V6 /V6 R 60 60R Where V60 is the cutting speed for the target material that ensures tool lif of 6 min, V60R i the same f the h l life f 60 i is h for h reference material. If KM > 1, the machinability of the target material is better that this of the reference material and vice versa material, Bhopal -2014
  6. 6. Role of microstructure on Machinability Coarse microstructure leads to lesser value of τs. C   i  l d    l   l   f  Therefore, τs can be desirably reduced by Proper heat treatment like annealing of steels P  h    lik   li   f  l Controlled addition of materials like sulphur (S), lead  p ( ), (Pb), Tellerium etc leading to free cutting of soft ductile  metals and alloys. metals and alloys ff f l k l ( ) Effects of tool rake angle(s) on machinability As Rake angle increases machinability increases. But too much increase in rake weakens the cutting edge. Effects of Cutting Edge angle(s) on  machinability The Th variation i th cutting edge angles d i ti in the tti d l does not affect t ff t cutting force or specific energy requirement for cutting cutting. Increase in SCEA and reduction in ECEA improves surface finish sizeably in continuous chip formation hence Machinability. Brittle materials are relatively more machinable. Effects of  clearance angle on machinability Proper tool nose radiusing improves machinability to some extent through increase in tool life by increasing mechanical strength and reducing temperature at the tool tip d d i h l i reduction of surface roughness, hmax g , Inadequate clearance angle reduces tool life and surface finish by tool – work rubbing, and again too large clearance reduces the tool strength and tool life hence g machinability. Cutting fluid Cutting fluid The cutting fluid acts primarily as a coolant and secondly as a lubricant, reducing the friction effects at dl l bi t d i th f i ti ff t t the tool‐chip interface and the work‐blank regions. Cast Iron: Machined dry or compressed air, Soluble oil for high speed machining and grinding Brass: Machined dry or straight mineral oil with or without EPA ih EPA. Aluminium: Machined dry or kerosene oil mixed with y mineral oil or soluble oil Stainless steel and Heat resistant alloy: High performance soluble oil or neat oil with high concentration with chlorinated EP additive. i i h hl i d ddi i For IES, GATE, PSUs Surface Roughness Effects of Nose Radius on machinability hmax f2 = 8R 8R Ideal Surface ( Zero nose radius) f tan SCEA + cot ECEA h f and (Ra) = = 4 4 ( tan SCEA + cot ECEA ) Peak to valley roughness (h) = Practical Surface ( with nose radius = R) h= f2 8R and Ra = f2 18 3R Change in feed (f) is more important than a change in nose radius g ( ) p g (R) and depth of cut has no effect on surface roughness. IAS 2009 Main IAS ‐2009 Main What are extreme pressure lubricants? What are extreme‐pressure lubricants? [ 3 – marks] Where hi h pressures and rubbing action are Wh high d bbi i encountered, hydrodynamic lubrication cannot be maintained; so E i i d Extreme P Pressure (EP) additives must b ddi i be added to the lubricant. EP lubrication is provided by a number of chemical components such as b b f h i l h boron, phosphorus, sulfur, chlorine, or combination of these. The Th compounds are activated b the hi h temperature d i d by h higher resulting from extreme pressure. As the temperature rises, EP molecules b i l l become reactive and release i d l derivatives such as iron chloride or iron sulfide and forms a solid protective coating. f lid i i Page 6 of 49 Bhopal -2014
  7. 7. Four Important forming techniques are: Rolling  Metal Forming Sheet Metal Operation Sh t M t l O ti Powder Metallurgy P d  M ll Forging g g Extrusion Drawing D i By  S K Mondal Terminology Ingot Plastic Deformation Mill product Deformation beyond elastic limits. Plate is the product with thickness > 5 mm Sheet is the product with thickness < 5 mm and width > 600 Due to slip, grain fragmentation, movement of atoms  p, g g , and lattice distortion. mm Strip is the product with a thickness < 5 mm and width < 600 mm Rx depends on the amount of cold work a material has already received. The higher the cold work, the lower would b the Rx. ld be h Terminology Semi‐finished product Ingot: is the first solid form of steel. I i h fi lid f f l Bloom: is the product of first breakdown of ingot has square p g q cross section 6 x 6 in. or larger Billet: is hot rolled from a bloom and is square 1 5 in on a square, 1.5 in. side or larger. Slab: is the hot ll d ingot or bl Sl b i th h t rolled i t bloom rectangular cross t l section 10 in. or more wide and 1.5 in. or more thick. Billet slab Recrystallisation Temperature (Rx) “The minimum temperature at which the completed “Th i i hi h h l d recrystallisation of a cold worked metal occurs within a specified period of approximately one hour”. Rx decreases strength and increases ductility ductility. If working above Rx, hot‐working process whereas working b l ki below are cold‐working process. ld ki It involves replacement of cold‐worked structure by a t vo ves ep ace e t o co d o ed st uctu e new set of strain‐free, approximately equi‐axed grains to replace all the deformed crystals crystals. Contd. Contd Grain growth h Grain growth follows complete crystallization if the materials  left at elevated temperatures. p Bloom Strain Hardening Strain Hardening When metal is formed in cold state there is no state, Grain growth does not need to be preceded by recovery and  recrystallization; it may occur in all polycrystalline materials. ll ll l ll l Rx = 0 4 x Melting temp (Kelvin) 0.4 temp. (Kelvin). Rx of lead and Tin is below room temp. p recrystalization of grains and thus recovery from y g y In contrary to recovery and recrystallization, driving force   for this process is reduction in grain boundary energy. Rx varies between 1/3 to ½ melting paint paint. place. grain distortion or fragmentation does not take In practical applications, grain growth is not desirable. As grain deformation proceeds, greater resistance to this ti t thi action results i i lt in increased h d d hardness and d Rx of Iron is 450oC and for steels around 1000°C Incorporation of impurity atoms and insoluble second phase  particles are effective in retarding grain growth. Finer is the initial grain size; lower will be the Rx Grain growth is very strongly dependent on temperature. Rx of Cadmium and Zinc is room temp. For IES, GATE, PSUs Contd. Page 7 of 49 strength i.e. strain hardening. Bhopal -2014
  8. 8. Strain Hardening St i H d i Malleability ll b l Strain hardening (cold Working) Malleability is the property of a material whereby it can σ o = Kε n be h b shaped when cold b h d h ld by hammering or rolling. ll Strain rate effect (hot Working) ( g) σ o = Cε Where ε= A malleable material i capable of undergoing plastic ll bl i l is bl f d i l i m deformation without fracture fracture. A malleable material should be plastic but it is not 1 dh v Platen Velocity = = h dt h Instantaneous height g Cold Working Working below recrystalization temp. W ki  b l   li i   essential to be so strong. g Lead, soft steel, wrought iron, copper and aluminium are some materials in order of diminishing malleability. Advantages of Cold Working d f ld k Disadvantages of Cold Working d f ld k Equipment of higher forces and power required 1. Better accuracy, closer tolerances 1. 2. Better surface finish Hot Working 2. S f Surfaces of starting work piece must be free of scale and    f  t ti   k  i   t b  f   f  l   d  3. Strain hardening increases strength and hardness 4. Grain flow during deformation can cause desirable directional properties in product 5. 5 No heating of work required (less total energy) dirt 3. Ductility and strain hardening limit the amount of forming  that can be done 4. In some operations, metal must be annealed to allow  further deformation 5 5. Some metals are simply not ductile enough to be cold  py g Working above recrystalization temp. Working above recrystalization temp worked. Advantages of Hot Working Dis‐advantages of Hot Working 1. The porosity of the metal is largely eliminated. 2. 2 The grain structure of the metal is refined refined. 3. The impurities like slag are squeezed into fibers and distributed h di ib d throughout the metal. h h l 4 4. The mechanical properties such as toughness, p p g , percentage elongation, percentage reduction in area, and resistance to shock and vibration are improved due to the refinement of grains. 1. It requires expensive tools. 2. 2 It produces poor surface finish due to the rapid finish, oxidation and scale formation on the metal surface. 3. D Due to the poor surface fi i h close tolerance h f finish, l l cannot be maintained. For IES, GATE, PSUs Page 8 of 49 Micro‐Structural Changes in a Hot  Mi St t l Ch i H t Working Process (Rolling) Working Process (Rolling) Bhopal -2014
  9. 9. Annealing g •Annealing relieves the stresses from cold working – three stages: recovery, recrystallization and grain growth. recovery growth •During recovery, physical properties of the cold‐worked material are restored without any observable change i i l d ih b bl h in microstructure. Warm Forming Isothermal Forming h l Deformation produced at temperatures intermediate to During hot forming, cooler surfaces surround a hotter hot d ld forming is k h and cold f known as warm f forming. interior, interior and the variations in strength can result in non non‐ Compared to cold f C d ld forming, i reduces l d i i it d loads, increase material ductility ductility. uniform deformation and cracking of the surface. For temp.‐sensitive materials deformation is performed under isothermal conditions. Compared to hot forming it produce less scaling and forming, The dies or tooling must b h Th di li be heated to the workpiece d h k i decarburization, better dimensional precision and p temperature, sacrificing die life for product quality. p , g p q y smoother surfaces. Close tolerances, low residual stresses and uniform metal flow. Rolling Definition: The process of plastically deforming metal by b passing it b between rolls. ll g Rolling Most id l M widely used, hi h production and close tolerance. d high d i d l l Friction b t F i ti between th rolls and th metal surface the ll d the t l f produces high compressive stress stress. Hot working Hot‐working (unless mentioned cold rolling. By  S K Mondal Metal will undergo bi‐axial compression. g p Hot Rolling Done above the recrystallization temp. Results fine grained structure. Surface quality and fi l di S f lit d final dimensions are l accurate. i less t Breakdown of ingots into blooms and billets is done by hot‐rolling. This is followed by further hot‐rolling into g y g plate, sheet, rod, bar, pipe, rail. Hot rolling is terminated when the temp. falls to about For IES, GATE, PSUs (50 to 100°C) above the recrystallization temp. Page 9 of 49 Bhopal -2014
  10. 10. Cold Rolling Ring Rolling Done below the recrystallization temp.. Ring rolls are used for tube rolling, ring rolling. Products are sheet, strip, foil etc. with good surface As the rolls squeeze and rotate, the wall thickness is finish fi i h and i d increased mechanical strength with close d h i l h ih l reduced and the di d d d h diameter of the ring i f h i increases. product dimensions dimensions. Shaped rolls can b used t produce a wide variety of Sh d ll be d to d id i t f Performed on four‐high or cluster‐type rolling mills four high cluster type mills. cross section profiles. cross‐section profiles ( (Due to high force and power) g p ) Ring rolls are made of spheroidized graphite bainitic and pearlitic matrix or alloy cast steel base. Sheet rolling In sheet rolling we are only attempting to reduce the cross section thickness of a material. h k f l Roll Forming Roll Bending A continuous form of three‐point bending is roll bending, where plates, sheets, and rolled shapes can be bent to a desired curvature on forming rolls. Upper roll being adjustable to control the degree of curvature. t For IES, GATE, PSUs Page 10 of 49 Bhopal -2014
  11. 11. Shape rolling Pack rolling Thread rolling Pack rolling involves hot rolling multiple sheets of Used to produce threads in substantial quantities. material at once, such as aluminium f l l h l foil. This is a cold‐forming process in which the threads are A thin surface oxide fil prevents their welding. hi f id film h i ldi formed b rolling a thread bl k b f d by lli h d blank between h d hardened di d dies that cause the metal to flow radially into the desired shape. p No metal is removed, greater strength, smoother, harder, g g and more wear‐resistant surface than cut threads. Thread rolling                    contd…. Manufacture of gears by rolling Major diameter is always greater than the diameter of the The straight and helical teeth of disc or rod type external blank bl k ( steel gears of small to medium d l f ll d diameter and module are d d l Blank diameter i li l l Bl k di is little larger than the pitch di h h i h diameter of f generated by cold rolling rolling. the thread thread. High accuracy and surface integrity integrity. Restricted to ductile materials materials. Employed for high productivity and high quality (costly quality. machine) ) Larger size gears are formed by hot rolling and then finished by machining. Roll piercing ll Fig. Production of teeth of spur gears by rolling For IES, GATE, PSUs Page 11 of 49 It is a variation of rolling called roll piercing. , The billet or round stock is rolled between two rolls, both of them rotating in the same direction with their axes at an angle of 4.5 to 6.5 degree. These rolls have a central cylindrical portion with the sides tapering slightly There are two small side rolls slightly. rolls, which help in guiding the metal. Because of the angle at which the roll meets the metal, it gets in addition to a rotary motion, an additional axial advance, which brings the metal into the rolls. This cross‐rolling action makes the metal friable at the g centre which is then easily pierced and given a cylindrical shape by the central‐piercing mandrel. central piercing Bhopal -2014
  12. 12. Planetary mill Consist of a pair of heavy backing rolls surrounded by a large number of planetary rolls. Each planetary roll gives an almost constant reduction to the slab as it sweeps out a circular path between the backing rolls and the slab. As each pair of planetary rolls ceases to have contact with the work piece, another pair of rolls makes contact and repeat that reduction. h d i The overall reduction is the summation of a series of small reductions b each pair of rolls. Th f d ti by h i f ll Therefore, th planetary mill the l t ill can reduce a slab directly to strip in one pass through the mill. mill The operation requires feed rolls to introduce the slab into the mill, and a pair of planishing rolls on the exit to improve the surface finish. Camber Defects in Rolling Lubrication for Rolling Hot rolling of ferrous metals is done without a lubricant. Hot rolling of non‐ferrous metals a wide variety of Defects f Surface Defects compounded oils, emulsions and f d d il l i d fatty acids are used. id d Cold C ld rolling l b i lli lubricants are water‐soluble oils, l t t l bl il low‐ Wavy edges viscosity lubricants such as mineral oils emulsions lubricants, oils, emulsions, Alligatoring p paraffin and fatty acids. y What is h Cause Scale, rust, scratches, pits, cracks Strip is thinner along its edges than at its centre. Edge breaks Inclusions and impurities in the materials Due to roll bending edges elongates more and buckle. Non‐uniform deformation Camber can be used to correct the roll deflection (at only one value of the roll force). Geometry of Rolling Process Draft Total reduction or “draft” taken in rolling. T l  d i    “d f ”  k  i   lli Δh=h - h =2(R- Rcos α) =D(1- cos α) 0 f Usually, the reduction in blooming mills is about 100  y, g mm and in slabbing mills, about 50 to 60 mm. Maximum Draft Possible For IES, GATE, PSUs ( ΔhPage 12 of = μ 2 R )max 49 Torque and Power The power is spent principally in four ways Th i i i ll i f ) gy 1) The energy needed to deform the metal. 2) The energy needed to overcome the frictional force. 3) Th power l ) The lost i the pinions and power‐transmission in h i i d i i system. 4) Electrical losses in the various motors and generators. Remarks: Losses in the windup reel and uncoiler must p also be considered. Bhopal -2014
  13. 13. Torque and Power Assumptions in Rolling 1. Rolls are straight, rigid cylinders. R ll i h i id li d 2. Strip is wide compared with its thickness, so that no p p , [For IES Conventional Only] Will be b discussed in class Stress Equilibrium of an Element in Rolling Considering the thickness of the element perpendicular to the plane of paper to be unity We get equilibrium unity, equation in - σ x h + (σ x +dσ x ) (h + dh) - 2pR dθ sin θ x‐direction as, + 2 τ x R dθ cos θ = 0 I= 2Rθdθ = 2 f + Rθ ∫h ∫ Now h / R = or 2Rθdθ = h 2θdθ ∫h/R ⎛h⎞ = ln ⎜ ⎟ ⎝R⎠ hf + θ2 R d ⎛h⎞ = 2θ θ dθ ⎜ R ⎟ ⎝ ⎠ 2Rμ R II = ∫ dθ h f + Rθ2 2μ dθ =∫ h f / R + θ2 = 2μ ⎛ R ⎞ R .tan −1 ⎜ .θ ⎟ ⎜ h ⎟ hf f ⎝ ⎠ For IES, GATE, PSUs For sliding friction, τ x = μp Simplifying and neglecting second order terms, sin θ ≅ θ and cos θ = 1 we get d d t i d 1, t d (σ x h ) = 2 pR (θ ∓ μ ) dθ 2 p −σ x = σ 0 = σ 0' 3 d ' ⎡ h ( p − σ 0 ) ⎤ = 2 pR (θ ∓ μ ) ⎦ dθ ⎣ ⎞⎤ d ⎡ ' ⎛ p ⎢σ 0 h ⎜ ' − 1 ⎟ ⎥ = 2 pR (θ ∓ μ ) dθ ⎣ ⎝σ0 ⎠⎦ ∴ ⎛h⎞ ln p / σ '0 = ln ⎜ ⎟ ∓ 2μ ⎝R⎠ ( ) R .tan −1 hf R .tan −1 hf ' d ( p /σ0 ) ( p /σ ) ' 0 ⎛ R ⎞ .θ ⎟ + ln C ⎜ ⎜ h ⎟ f ⎝ ⎠ ⎛ R ⎞ .θ ⎟ ⎜ ⎜ ⎟ ⎝ hf ⎠ Now at entry ,θ = α Hence H = H0 with θ replaced by ∝ in above equation At exit θ = 0 Therefor p = σ '0 Page 13 of 49 ' thus σ 0 h nearly a constant and itsderivative zero. h = h f + 2 R (1 − cos θ ) ≈ h f + Rθ 2 ⎛h⎞ p = C σ '0 ⎜ ⎟ e∓ μH ⎝R⎠ where H = 2 ' Due to cold rolling, σ 0 increases as h decreases, d ( p / σ 0' ) 2R dθ = (θ ∓ μ ) ' p /σ0 h ⎞ d d ⎛ p ⎞ ⎛ p σ 0' h (σ 0' h ) = 2 pR (θ ∓ μ ) ⎜ ' ⎟ + ⎜ ' − 1⎟ dθ ⎝ σ 0 ⎠ ⎝ σ 0 ⎠ dθ ∴ widening of strip occurs (plane strain conditions). 3. 3 The arc of contact is circular with a radius greater than the radius of the roll. 4. The material is rigid perfectly plastic (constant yield st e gt ). strength). 5. The co‐efficient of friction is constant over the tool‐ work i t f k interface. = 2R (θ ∓ μ ) dθ h f + Rθ 2 Integrating both side 2 Rθ dθ ' ln ( p / σ 0 ) = ∫ ∓ h f + Rθ 2 ∫h 2 Rμ dθ = I ∓ II ( say ) 2 f + Rθ ⎛h ⎞ In the entry zone, p = C.σ '0 ⎜ o ⎟ e− μHo y , ⎝R⎠ R μHo and C = .e ho p = σ '0 h μ H −H . e ( 0 ) h0 In the it I th exit zone ⎛ h ⎞ p = σ '0 ⎜ ⎟ .eμH ⎝ hf ⎠ At the neutral po int above equations will give same results Bhopal -2014
  14. 14. hn h μ H −H . e ( 0 n ) = n . eμ Hn h0 hf or p = ( σ′ − σ b ) o ho μ H − 2H = e ( 0 n) hf or Hn = ⎛ h0 ⎞ ⎤ 1⎡ 1 ⎢H0 − ln ⎜ ⎟ ⎥ 2⎢ μ ⎝ hf ⎠⎥ ⎣ ⎦ ⎛ R ⎞ .θ ⎟ ⎜ ⎜ h ⎟ f ⎝ ⎠ ⎛ h f Hn ⎞ hf ∴ θn = .tan ⎜ . ⎜ R 2 ⎟ ⎟ R ⎝ ⎠ and h n = h f + 2R (1 − cos θn ) From H = 2 If back tension σ b is there at Entry Entry, R .tan −1 hf h μ H −H . e ( 0 ) h0 Forging If front tension σ f is there at Exit, p = ( σ′ − σ f ) o h . eμ H hf By  S K Mondal y Forging Draft f Because of the manipulative ability of the forging B f h i l i bili f h f i process, it is possible to closely control the grain flow in the specific direction, such that the best mechanical p p properties can be obtained based on the specific p application. The draft provided on the sides for withdrawal of the forging. Adequate draft should be provided‐at least 3o for provided at aluminum and 5 to 7o for steel steel. Internal surfaces require more draft than external surfaces. Flash l h The excess metal added to the stock to ensure complete Th l dd d h k l filling of the die cavity in the finishing impression is called Flash. For IES, GATE, PSUs Flash l h Contd… A flash acts as a cushion for impact blows from the fl h hi f i bl f h finishing impression and also helps to restrict the outward flow of metal, thus helping in filling of thin ribs and bosses in the upper die. pp The amount of flash depends on the forging size and may ar ma vary from 10 to 50 per cent 0 cent. The forging load can be decreased by increasing the flash thickness. Page 14 of 49 Gutter In addition to the flash, provision should be made in the I ddi i h fl h i i h ld b d i h die for additional space so that any excess metal can flow and help in the complete closing of the die. This is called g gutter. Bhopal -2014
  15. 15. Gutter           Contd…. Without a gutter, a flash may become excessively thick, not allowing the d to close completely. ll h dies l l l Gutter d h and width should b sufficient to G depth d id h h ld be ffi i accommodate the extra material extra, material. Fullering or swaging       Contd… ll A forging method for f i h d f reducing the diameter of a bar and in the process making it longer is termed g g as Fullering. Operations involved in forging l d f Steps involved in hammer forging  S  i l d i  h  f i   Fullering or swaging g g g Edging or rolling Bending B di Drawing or cogging g gg g Flattening Blocking  l k Finishing operation Trimming or cut off Edging or rolling d ll Fullering or swaging ll It is the operation of reducing the stock between the two I i h i f d i h kb h ends of the stock at a central place, so as to increase its length. Edging or rolling d ll Contd…. Gathers the material as required in the final forging. The pre‐form shape also helps in proper location of stock in h blocking impressions. i the bl ki i i The Th area at any cross section should b same as th t of t ti h ld be that f the corresponding section in the component and the flash allowance. Bending d Bending operation makes the longitudinal axis of the stock in two or more places. This operation is d k l h done after f Blocking l k Imparts to the forging it’s general but not exact or final I h f i i’ lb fi l shape. This operation is done just prior to finishing operation. Flattening l This operation is used to flatten the stock so that it fits Thi i i d fl h k h i fi properly into the finishing impression of a closed die. the stock has been edged or fullered and edged so that the stock is brought into a proper relation with the shape of the finishing impression. For IES, GATE, PSUs Page 15 of 49 Bhopal -2014
  16. 16. Finishing h Drop Forging The dimensions of the finishing impression are same as The drop forging die consists of two halves. The lower that of the f l f h f h final forging d desired with the necessary d h h half f h die fixed h lf of the d is f d to the anvil of the machine, while h l f h h hl allowances and tolerances tolerances. the upper half is fixed to the ram The heated stock is ram. A gutter should be provided in the finishing impression impression. kept in the lower die while the ram delivers four to five Cut off Cut‐off blows on the metal, in quick succession so that the metal A pair of blades used to cut away a forging from the bar after the finishing blow. spreads and completely fills the die cavity. When the two die halves close, the complete cavity is formed. Drop forging is used to produce small components. Press Forging Advantages of Press Forging over Drop Forging Force is a continuous squeezing type applied by the Press forging is faster than drop forging hydraulic h d l presses. Alignment of the two die halves can be more easily Die Materials Should have l h ld h Good hardness, toughness and ductility at low and  Good hardness  toughness and ductility at low and  elevated temperatures  p Adequate fatigue resistance Sufficient hardenability Low thermal conductivity Amenability to weld repair A bili     ld  i Good machinability Material: Cr‐Mo‐V‐alloyed steel and Cr‐Ni‐Mo‐alloyed  y y steel. Machine Forging g g p g g Unlike the drop or p press forging where the material is drawn out, in machine forging, the material is only upset to get the desired shape. Upset Forging i maintained than with h i i d h i h hammering. i Structural St t l quality of th product i superior t d lit f the d t is i to drop Increasing the diameter of a material by compressing its forging. forging length. l th With ejectors in the top and bottom dies, it is possible to Employs split dies that contain multiple positions or handle reduced die drafts. cavities. Roll Forging ll When the rolls are in the open position, the heated stock Roll Forging               Contd…. ll A rapid process. id Forging Defects f Unfilled Sections: Die cavity is not is advanced up to a stop. As the rolls rotate, they grip and d d h ll h d completely f ll d d l l filled, due to improper roll down the stock The stock is transferred to a second stock. design of die set of grooves. The rolls turn again and so on until the Cold Shut or fold: A small crack at piece is finished. the corners of the forging. Cause: g g improper design of the die For IES, GATE, PSUs Page 16 of 49 Bhopal -2014
  17. 17. Forging Defects f Contd…. Scale Pits: Irregular depressions on the surface due to S l Pi I l d i h f d improper cleaning of the stock. Die Shift: Due to Misalignment of the two die halves or making the two halves of the forging to be of improper shape. Flakes: Internal ruptures caused b the improper l k l d by h cooling. Improper Grain Flow: This is caused by the improper design of the die which makes the flow of metal not die, flowing the final intended directions. Forging Defects f Lubrication for Forging b f Contd…. Forging Laps: These are folds of metal squeezed Lubricants influence: friction, wear, deforming forces together d h during f forging. They h h have irregular contours l and fl d flow of material in d f l die‐cavities, non‐sticking, k and occur at right angles to the direction of metal flow flow. thermal barrier barrier. Hot tears and thermal cracking: These are surface For hot forging: graphite MoS2 and sometimes molten graphite, cracks occurring due to non‐uniform cooling from the g g g glass. forging stage or during heat treatment. For cold forging: mineral oil and soaps. g g p In hot forging, the lubricant is applied to the dies, but in cold forging, it is applied to the workpiece. Assumption Extrusion & Drawing Forging force is maximum at the end of the forging. forging Coefficient of friction is constant between workpiece and dies (platens). IES Conventional Only Details will be discussed in the Class Extrusion The extrusion process is like squeezing toothpaste out of a tube. For IES, GATE, PSUs Thickness of the workpiece i small compared with other Thi k f h k i is ll d ih h dimensions, and the variation of stress field along y‐ , g y direction is negligible. Length is much more than width, problem is plain strain type. type The entire workpiece is in the plastic state during the p p g process. Metal is compressed and forced to flow through a suitably shaped die to form a product with reduced but constant cross section. Metal will undergo tri‐axial compression. Hot extrusion is commonly employed employed. Lead, copper, aluminum, magnesium, and alloys of these metals are commonly extruded. Steels, Steels stainless steels and nickel based alloys are steels, nickel‐based difficult to extrude. (high yield strengths, welding with wall). Use phosphate‐based and molten glass lubricants . Page 17 of 49 By  S K Mondal Extrusion Ratio Ratio of the cross‐sectional area of the billet to the cross‐ sectional area of the product. l f h d about 40: 1 f h extrusion of steel b for hot i f l 400: 1 f aluminium for l i i Bhopal -2014
  18. 18. Advantages of Extrusion d f Any cross‐sectional shape can be extruded from the nonferrous metals. f t l Limitation of Extrusion Limitation of Extrusion Cross section must be uniform for the entire length of the product. p Many shapes (than rolling) No draft od a t Huge reduction in cross section. Conversion from one product to another requires only a single die change Good dimensional precision. Hot Extrusion Process The temperature range for hot extrusion of aluminum is 430‐480°C Used U d to produce curtain rods made of aluminum. d i d d f l i Application A li ti Working of poorly plastic and non ferrous metals and alloys. Manufacture of sections and pipes of complex co gu a o . configuration. Medium and small batch production. Manufacture of parts of h h d f f f high dimensional accuracy. l Direct Extrusion A solid ram drives the entire billet to and through a lid di h i bill d h h stationary die and must provide additional power to overcome the f h frictional resistance b l between the surface of the h f f h moving billet and the confining chamber. Indirect Extrusion Indirect Extrusion A hollow ram drives the die back through a stationary, confined billet. billet Design f die is D i of di i a problem. bl Either direct or indirect method used used. Since no relative motion, friction between the billet and the chamber i eliminated. h b is li i t d Required force is lower (25 to 30% less) Low process waste Cold Extrusion ld Backward cold extrusion k d ld Used with low‐strength metals such as lead, tin, zinc, The metal is extruded through the gap between the and aluminum to produce collapsible tubes f d l d ll bl b for punch and d opposite to the punch movement. h d die h h toothpaste, medications, toothpaste medications and other creams; small "cans" cans For f F softer materials such as aluminium and i alloys. i l h l i i d its ll for shielding electronic components and larger cans for Used for U d f making collapsible t b ki ll ibl tubes, cans f li id and for liquids d food and beverages. Impact Extrusion similar articles articles. Now‐a‐days also been used for forming mild steel parts. For IES, GATE, PSUs Page 18 of 49 The extruded parts are stripped by the use of a stripper plate, because they tend to stick to the punch. Bhopal -2014
  19. 19. Hooker Method    k h d Hooker Method k h d The ram/punch has a shoulder and acts as a mandrel. Th / hh h ld d t d l A flat blank of specified diameter and thickness is placed in a suitable di and i f i bl die d is forced through the opening of the di with d h h h i f h die i h the punch when the punch starts d h h h downward movement. P d Pressure i is exerted by the shoulder of the punch, the metal being forced to flow th t fl through th restricted annular space b t h the ti t d l between th the punch and the opening in the bottom of the die. In l I place of a fl solid bl k a h ll slug can also b used. f flat lid blank, hollow l l be d If the tube sticks to the punch on its upward stroke, a stripper will strip it f ll from the punch. h h Small copper tubes and cartridge cases are extruded by this method. Hydrostatic Extrusion   Contd…. d Hydrostatic Extrusion   Contd…. d Temperature is limited since the fluid acts as a heat sink T i li i d i h fl id h i k and the common fluids (light hydrocarbons and oils) burn or decomposes at moderately low temperatures. The metal deformation is performed in a high‐ high compression environment. Crack formation is suppressed, suppressed leading to a phenomenon kno n as known pressure‐induced ductility. Relatively brittle materials like cast iron, stainless steel, molybdenum, tungsten and various inter‐metallic inter metallic compounds can be plastically deformed without fracture, fracture and materials with limited ductility become highly plastic. Lubrication for Extrusion b f For hot extrusion glass is an excellent lubricant with F h i l i ll l bi ih steels, stainless steels and high temperature metals and alloys. For cold extrusion lubrication is critical especially with extrusion, critical, steels, because of the possibility of sticking (seizure) between bet een the workpiece and the tooling if the lubrication orkpiece breaks down. Most effective lubricant is a phosphate conversion coating on the workpiece. h k Wire Drawing Hydrostatic Extrusion d Another type of cold extrusion process. High‐pressure fluid applies the force to the workpiece through a di h h die. It i f is forward extrusion, b t th fl id pressure d t i but the fluid surrounding the billet prevents upsetting upsetting. Billet chamber Billet‐chamber friction is eliminated, and the die. Application Extrusion of nuclear reactor fuel rod E t i   f  l   t  f l  d Cladding of metals Making wires for less ductile materials  Wire Drawing   Contd…. A cold working process to obtain wires from rods of bigger d b diameters through a d h h die. Same process as b d S bar drawing except that i i i h it involves l smaller‐diameter material material. At the start of wire drawing the end of the rod or wire to drawing, enters the die orifice and sticks out behind the die. Page 19 of 49 the pressurized fluid acts as a lubricant between the billet be drawn is pointed (by swaging etc.) so that it freely p ( y g g ) y For IES, GATE, PSUs and Bhopal -2014
  20. 20. Wire Drawing   Contd…. Wire getting continuously wound on the reel. Cleaning and Lubrication in wire Drawing Wire Drawing Die Cleaning is done to remove scale and rust by acid pickling. Cleaning is done to remove scale and rust by acid pickling Lubrication boxes precede the individual dies to help reduce For fine wire, the material may be passed through a friction drag and prevent wear of the dies. number of di b f dies, receiving successive reductions i i i i d i in Sulling: The wire is coated with a thin coat of ferrous diameter, diameter before being coiled coiled. hydroxide which when combined with lime acts as filler for The wire is subjected to tension only But when it is in only. contact with dies then a combination of tensile, the lubricant. Phosphating: A thin film of Mn, Fe or Zn phosphate is applied on the wire wire. compressive and shear stresses will be there in that Electrolytic coating: For very thin wires, electrolytic coating y g y , y g portion only. of copper is used to reduce friction. Rod and Tube Drawing d d b Die materials: tool steels or tungsten carbides or polycrystalline diamond (for fine wire) Rod and Tube Drawing   Contd… d d b Rod drawing is similar to wire drawing except for the fact R dd i i i il i d i f h f that the dies are bigger because of the rod size being larger than the wire. The tubes are also first pointed and then entered through the die where the point is gripped in a similar way as the bar dra ing and pulled through in the form a drawing desired along a straight line. When the final size is obtained, the tube may be annealed and straightened. The practice of drawing tubes without the help of an internal mandrel i called t b sinking. i t l d l is ll d tube i ki Swaging or kneading  Contd… k d Moving Mandrel Extrusion Load Approximate method (Uniform deformation, no friction)  A i   h d (U if  d f i    f i i )  “work – formula” The hammering of a rod or tube to reduce its diameter where the d itself acts as the h h h die lf h hammer. ⎛A P = Aoσ o ln ⎜ o ⎜A ⎝ f Repeated bl R d blows are d li delivered f d from various angles, i l causing the metal to flow inward and assume the shape ⎞ ⎟ ⎟ ⎠ For real conditions  F   l  di i   ⎛A P = KAo ln ⎜ o ⎜A ⎝ f of the die. It is cold working. The term swaging is also applied to g g g pp processes where material is forced into a confining die to reduce its diameter. For IES, GATE, PSUs Fixed Plug Drawing  Floating plug Drawing Swaging or kneading k d Tube Sinking ⎞ ⎟ ⎟ ⎠ K = extrusion constant. Page 20 of 49 Bhopal -2014
  21. 21. Wire Drawing Wire Drawing Force required in Wire or Tube drawing Approximate method (Uniform deformation, no friction)  Approximate method (Uniform deformation  no friction)  “work – formula” ⎛A P = Af σ o ln ⎜ o ⎜A ⎝ f σd = σ o (1 + B ) ⎡ ⎛r ⎞ ⎤ ⎛r ⎞ ⎢1 − ⎜ f ⎟ ⎥ + ⎜ f ⎟ .σ b ⎢ ⎝ ro ⎠ ⎥ ⎝ ro ⎠ ⎣ ⎦ B 2B 2B Maximum Reduction per pass With back stress, σ b σo = ⎞ ⎟ ⎟ ⎠ σ o (1 + B ) ⎡ 2B 2B ⎛ rf ⎞ ⎤ ⎛ rf ⎞ ⎢1 − ⎜ ⎟ ⎥ + ⎜ ⎟ .σ b ⎢ ⎝ ro ⎠ ⎥ ⎝ ro ⎠ ⎣ ⎦ B Without back stress, σ b σo = f Wire Drawing Analysis (Home Work) Wire Drawing Analysis (Home Work) The equilibrium equation in x-direction will be (σ x + dσ x ) π ( r + dr ) 2 dx ⎞ ⎛ − σ xπ r 2 + τ x cos α ⎜ 2π r ⎟ cos α ⎠ ⎝ dx ⎞ ⎛ + Px sin α ⎜ 2π r ⎟=0 cos α ⎠ ⎝ or Bσ x − (1 + B ) σ o = ( rC ) 2B B.C at r = ro ,σ x = σ b σ o (1 + B ) ⎡ Dividing by r 2 dr and taking dx/dr = cotα we get dσ x 2 2τ + (σ x + Px ) + x cotα = 0 dr r r Vertical component of Px ≅ Px due to small half di i l f d ll h lf die angles and that of τ x can be neglected neglected. Thefore, Thefore two principal stresses are σ x and − Px Both Tresca's and Von-Mises criteria will give g σ x + Px = σ o and τ x = μ Px = μ (σ o − σ x ) Extrusion Analysis (Home Work) Extrusion Analysis (Home Work) ∴ Bσ x − (1 + B ) σ o = ( rC ) ⎤ ⎛ r ⎞2 B ⎥ + ⎜ ⎟ .σ b or σ x = B ⎥ r ⎦ ⎝ o⎠ 2B 2B σ o (1 + B ) ⎡ ⎛ rf ⎞ ⎤ ⎛ rf ⎞ ⎢1 − ⎜ ⎟ ⎥ + ⎜ ⎟ .σ b ∴ Drawing stress (σ d ) = B ⎢ ⎝ ro ⎠ ⎥ ⎝ ro ⎠ ⎣ ⎦ ⎛r⎞ ⎢1 − ⎜ ⎟ r ⎢ ⎣ ⎝ o⎠ dσ x 2σ o 2 μ (σ o − σ x ) + + cotα = 0 dr r r Let μ cotα = B dσ x 2 = ⎡ Bσ x − (1 + B ) σ o ⎤ ⎦ dr r⎣ dσ x 2 or = dr ⎡ Bσ x − (1 + B ) σ o ⎤ r ⎣ ⎦ Integrating both side ln ⎡ Bσ x − (1 + B ) σ o ⎤ × ⎣ ⎦ σ xo = same equation except B.Cs 1 2B 2B For IES, GATE, PSUs B 2B ⎛r ⎞ ⎤ ⎢1 − ⎜ f ⎟ ⎥ ⎢ ⎝ ro ⎠ ⎥ ⎣ ⎦ 1 = 2 ln ( rC ) B {Cis integration cont.} at r = ro For a round bar both wire drawing and extrusion will give g g s ⎡ Bσ b − (1 + B ) σ o ⎤ ⎦ ∴C = ⎣ ro or σ x 2rdr + dσ x r 2 + 2rτ x dx + Px 2rdx tan α = 0 σ o (1 + B ) ⎡ B.C s at r = rf , σ x = 0 ⎡ − (1 + B ) σ o ⎦ ⎤ ∴C = ⎣ rf or σ x = σ o (1 + B ) ⎡ B 2B (at exit stress is zero) σ o (1 + B ) ⎡ B ⎞ ⎟ ⎟ ⎠ 2B ⎤ ⎥ ⎥ ⎦ 2 A ⎛r ⎞ Extrusion ratio, R = o = ⎜ o ⎟ for round bar , A f ⎜ rf ⎟ ⎝ ⎠ 1 2B 2B ⎛r ⎞ ⎤ ⎢1 − ⎜ ⎟ ⎥ ⎜ ⎟ ⎢ ⎝ rf ⎠ ⎥ ⎣ Page 21 of 49 ⎦ ⎛r ⎢1 − ⎜ o ⎢ ⎜r ⎣ ⎝ f σ xo = σ o (1 + B ) B ⎛h =⎜ o ⎜h ⎝ f ⎞ ⎟ for flat stock ⎟ ⎠ ⎡1 − R 2 B ⎤ ⎣ ⎦ Bhopal -2014
  22. 22. If effect of container friction is considered Sheet Metal p f = ram pressure required by container friction τ i = uniform interface shear stress between billet and container wall 2τ L p f .π r0 = 2π r0τ i L or p f = i ro 2 Product has light weight and versatile shape as compared to forging/casting Sheet Metal Operation Most commonly used – low carbon steel sheet (cost, strength, formability) Aluminium and titanium for aircraft and aerospace ∴ Total Extrusion Pressure(Pt ) = σ xo + p f Sheet metal has become a significant material for, and Extrusion Load = pt .π r0 ‐ automotive bodies and frames, 2 ‐ office furniture By  S K Mondal y ‐ frames for home appliances Piercing (Punching) and Blanking Piercing (Punching) and Blanking ( h ) d l k Piercing and blanking are shearing operations. In blanking, the piece being punched out becomes the workpiece and any major b h k i d j burrs or undesirable d i bl features should be left on the remaining strip strip. In piercing (Punching) the punch‐out is the scrap (Punching), punch out and the remaining strip is the workpiece. g p p Both done on some form of mechanical press. Clearance (VIMP) l ( ) Clearance         Contd…. l Die opening must be larger than punch and known as Di i b l h h d k ‘clearance’. Punching Punch = size of hole Die = punch size +2 clearance Remember: I punching punch i correct size. R b In hi h is t i Blanking Bl ki Die = size of product Punch = Die size ‐2 clearance Blanking Punching Remember: In blanking die size will be correct. For IES, GATE, PSUs Page 22 of 49 Bhopal -2014
  23. 23. Punching Force and Blanking Force h d l k Clearance in % Clearance in % If th allowance f th material i a = 0.075 given th the ll for the t i l is i then C = 0 075 x thickness of the sheet 0.075 Fm ax = Ltτ Capacity of Press for Punching and Blanking Press capacity will be =  F ax ×C m If clearance is 10 % given then The punching force for holes which are smaller than the stock  thickness may be estimated as follows: thi k    b   ti t d   f ll C = 0 01 x thickness of the sheet 0.01 Fmax = π dtσ 3 Minimum Diameter of Piercing f d t Energy and Power for Punching and Blanking Ideal E Id l Energy (E in J) = maximum force x punch travel = Fmax × ( p × t ) i i f h l π τs πd.t Piercing pressure,            = Strength of punch, σc × 4 d2 (Unit:Fmax in kN and t in mm othrwise use Fmax in N and t in m) a a Where p is percentage penetration required for rupture E×N 60 [Where N = actual number of stroke per minute] Ideal power in press ( P inW ) = Actual Energy ( E in J ) = Fmax × ( p × t ) × C Where C is a constant and equal to 1.1 to 1.75 depending upon the profile E×N Actual power in press ( P i W ) = A l i inW 60 ×η WhereE is actual energy and η is efficiency of the press Force required with shear on Punch F= [Where C is a constant and equal to 1.1 to 1.75 depending  upon the profile]  th   fil ] Fmax (tp) Lτ t(tp) = S S Shear on Punch h h To reduce shearing force, shear is ground on the face of the d or punch. h die h It distribute the cutting action over a period of time. I di ib h i i i d f i Shear only reduces th maximum f Sh l d the i force t b applied b t to be li d but total work done remains same same. Fine Blanking l k Fine Blanking ‐ dies are designed that have small Fi Bl ki di d i d h h ll clearances and pressure pads that hold the material while it is sheared. The final result is blanks that have extremely close tolerances. y Where p = penetration of punch as a fraction  S   shear on the punch or die, mm S = shear on the punch or die, mm For IES, GATE, PSUs Page 23 of 49 Bhopal -2014
  24. 24. Slitting ‐ moving rollers trace out complex paths during Trimming ‐ Cutting unwanted excess material from the Lancing – A hole is partially cut and then one side is bent cutting (like a can opener). periphery of a previously formed component. down to form a sort of tab or louver. No metal removal, no Shaving ‐ Accurate d h dimensions of the part are obtained b f h b d by scrap. Perforating: Multiple holes which are very small and close together are cut in flat work material. removing a thin strip of metal along the edges edges. Notching: Metal pieces are cut from the edge of a sheet, strip or bl k ti blank. Squeezing ‐ Metal is caused to flow to all portions of a die cavity under the action of compressive forces. Dinking k Steel Rules ‐ soft materials are cut with a steel strip shaped so that the edge is the pattern to b cut. h d h h d h be Nibbling Nibbli ‐ a single punch i moved up and d i l h is d d down rapidly, idl Used to blank shapes from low‐strength materials, such as U d bl k h f l h i l h rubber, fiber, or cloth. The shank of a die is either struck with a hammer or mallet or the entire die is driven downward by some form of y mechanical press. Elastic recovery or spring back  l b k Total deformation = elastic deformation + plastic deformation. d f each time cutting off a small amount of material This material. At th end of a metal working operation, when th the d f t l ki ti h the allows a simple die to cut complex slots. p p pressure is released there is an elastic recovery and the released, total deformation will get reduced a little. This g phenomenon is called as "spring back". Elastic recovery or spring back      Contd.. l b k More important in cold working. Punch and Die material Punching Press h Commonly used – tool steel For high production ‐ carbides It d depends on th yield strength. Hi h th yield d the i ld t th Higher the i ld strength, greater spring back. To compensate this, the cold deformation be carried beyond the desired limit by an amount equal to the spring back. For IES, GATE, PSUs Page 24 of 49 Bhopal -2014
  25. 25. Bolster plate l l Bolster plate     Contd.... l l Punch plate h l When many dies are to run in the same press at different Used to locate and hold the times, the wear occurring on the press b d is h h The h h bed high. h punch in position. h bolster plate is incorporated to take this wear wear. This is Thi i a useful way of f l f Relatively cheap and easy to replace replace. mounting, mounting Attached to the press bed and the die shoe is then small punches. p especially for attached to it. Stripper Stripper       Contd.... The stripper removes the stock from the punch after a piercing or blanking operation. Ps = KLt Where Ps = stripping force, kN  stripping force  kN L = perimeter of cut, mm  t = stock thickness, mm      k  hi k     Knockout k Knockout is a mechanism, usually connected to and K k i h i ll d d operated by the press ram, for freeing a work piece from a die. K = stripping constant,  = 0.0103 for low‐ carbon steels thinner than 1.5 mm with     the cut at the edge or near a preceding cut  = 0.0145 for same materials but for other cuts     f     i l  b  f   h     = 0.0207 for low‐ carbon steels above 1.5‐mm thickness = 0.0241 for harder materials  f h d l Pitman Dowel pin l It is a connecting rod which is used to transmit motion from the main d f h drive shaft to the press slide. h f h ld Drawing For IES, GATE, PSUs Page 25 of 49 Bhopal -2014
  26. 26. Drawing Drawing Drawing is a plastic deformation process in which a flat Hot drawing is used for thick‐walled parts of simple sheet or plate is f h l formed into a three‐dimensional part d h d l geometries, thinning takes place. h k l with a depth more than several times the thickness of Cold drawing uses relatively thin metal, changes the C ld d i l i l hi l h h the metal. thickness very little or not at all and produces parts in a all, As a punch descends into a mating die, the metal p g , wide variety of shapes. y p assumes the desired configuration. Blank Size Blank Size D = d + 4dh 2 D = d 2 + 4dh − 0.5r D= Drawing Force ⎡D ⎤ P = π dtτ ⎢ − C ⎥ ⎣d ⎦ When d > 20r when15r ≤ d ≤ 20r ( d − 2r ) + 4d ( h − r ) + 2π r ( d − 0.7r ) 2 when d < 10r Deep drawing d Drawing when cup height is more than half the diameter is termed deep drawing. p g Easy with ductile materials. Blank Holding Force Blank holding force required depends on the wrinkling t d i kli tendency of th cup. Th maximum f the The i g y g limit is generally to be one‐third of the drawing force. Draw Cl D Clearance Punch diameter = Die opening diameter – 2 5 t 2.5 Stresses on Deep Drawing Stresses on Deep Drawing In flange of blank: Bi‐axial tension and compression A cylindrical vessel with flat bottom can be deep drawn by The ratio of the maximum blank diameter to the diameter of the cup d d f h drawn . i.e. D/d. d There i a li i i d Th is limiting drawing ratio (LDR) after which the i i (LDR), f hi h h Due to the radial flow of material, the side walls increase in thickness as the height is increased. Deep Drawability bl punch will pierce a hole in the blank instead of drawing drawing. In wall of the cup: simple uni axial uni‐axial tension This ratio depends upon material amount of friction material, double action deep drawing drawing. p present, etc. Deep drawing ‐ is a combination of drawing and stretching. Limiting drawing ratio (LDR) is 1.6 to 2.3 For IES, GATE, PSUs Page 26 of 49 Bhopal -2014
  27. 27. Limiting Drawing Ratio (LDR) The average reduction in deep drawing  d =05 0.5 D d ⎞ ⎛ Reduction = ⎜ 1 − ⎟ × 100% = 50% D⎠ ⎝ Thumb l Th b rule: First draw:Reduction = 50 % Second draw:Reduction = 30 % Third draw:Reduction = 25 % Fourth draw:Reduction = 16 % Fifth draw:Reduction = 13 % Progressive piercing and blanking die for making a simple washer. making a simple washer Defects in Drawing ‐ wrinkle f kl An insufficient blank holder pressure causes wrinkles to A i ffi i bl k h ld i kl develop on the flange, which may also extend to the wall of the cup. Flange Wrinkle For IES, GATE, PSUs Wall Wrinkle Die Design Progressive dies Compound dies Combination dies Progressive dies Perform two or more operations simultaneously in a single stroke of a punch press, so that a complete component is k f h h l obtained for each stroke. Compound dies All the necessary operations are carried out at a single station, in a single stroke of the ram. To do more than one set of operations, a compound die consists of the necessary sets of punches and di f h d dies. Combination di C bi i dies A combination die is same as that of a compound die with the th main diff i difference th t h that here non‐cutting operations such as tti ti h bending and forming are also included as part of the operation. operation Method for making a simple washer in a compound piercing and blanking die. Part is blanked (a) and subsequently pierced  (b) The blanking punch contains the die for piercing. Defects in Drawing ‐ Fracture f Further, too much of a blank holder pressure and friction F h h f bl k h ld df i i may cause a thinning of the walls and a fracture at the flange, bottom, and the corners (if any). Page 27 of 49 Lubrication b In drawing operation, proper lubrication is essential for I  d i   i    l b i i  i   i l f p 1.  To improve die life. 2. To reduce drawing forces. 3. T   d   To reduce temperature. 4 4.  To improve surface finish. p Defects in Drawing ‐earing f While drawing a rolled stock, ears or lobes tend to occur Whil d i ll d k l b d because of the anisotropy induced by the rolling operation. Bhopal -2014
  28. 28. Defects in Drawing – miss strike  f k Defects in Drawing – Orange peel  f l Due to the misplacement of the stock, unsymmetrical D h i l f h k i l flanges may result. This defect is known as miss strike. A surface roughening (defect) encountered in forming f h i (d f ) d i f i products from metal stock that has a coarse grain size. It is due to uneven flow or to the appearance of the overly large grains usually the result of annealing at too high a temperature. Stretcher strains (like Luders Lines) St t h t i (lik L d Li ) Caused by plastic deformation due to inhomogeneous C d b l ti d f ti d t i h yielding. These lines can criss‐cross the surface of the workpiece and p may be visibly objectionable. Low carbon steel and aluminium shows more stretcher strains. Surface scratches Surface scratches Spinning Die or punch not having a smooth surface, insufficient  lubrication Spinning Spinning i a cold‐forming operation i which a S i i is ld f i ti in hi h 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 Spinning 1. A mandrel (or die for internal pieces) is placed on a d l ( di f i l i )i l d rotating axis (like a turning center). 2. A blank or tube is held to the face of the mandrel. 3. 3 A roller is pushed against the material near the center of rotation, and slowly moved outwards, pushing the bl k against the mandrel. h blank h d l 4. The part conforms to the shape of the mandrel (with e pa t co o s t e s ape o t e a d e ( t some springback). 5. Th process i stopped, and th part i removed and The is t d d the t is d d trimmed. For IES, GATE, PSUs tc = tb sinα Page 28 of 49 Bhopal -2014
  29. 29. Underwater  explosions. HERF High Energy Rate Forming, also known as HERF or explosive forming can b utilised t f f i be tili d to form a wide variety of metals, f id i t f t l from g gy g( ) High Energy Rate Forming(HERF) aluminum to high strength alloys. Applied a large amount of energy in a very sort time interval. Electro‐magnetic  Electro magnetic  (the use of  rapidly formed  magnetic fields). HERF Underwater spark  discharge (electro‐ discharge (electro hydraulic). HERF makes it possible to form large work pieces and difficult‐to‐form metals with less‐expensive equipment and Internal  combustion of  g gaseous  mixtures. tooling required. No N springback i b k Underwater Explosions U d E l i Underwater explosions U d l i Electro‐hydraulic Forming l h d l A shock wave in the fluid medium (normally water ) is generated b d d by detonating an explosive charge. l h TNT and d d dynamite f hi h energy and gun powder f i for higher d d for lower energy is used used. Used for parts of thick materials materials. Employed in Aerospace, aircraft industries Pneumatic‐ P i mechanical  means and automobile related components. An operation using electric discharge in the form of sparks to generate a shock wave in a fluid is called electrohydrulic forming. A capacitor bank is charged through the charging circuit, subsequently, a switch i closed, resulting i a spark b tl it h is l d lti in k within the electrode gap to discharge the capacitors. g p g p Energy level and peak pressure is lower than underwater explosions but easier and safer. Used for bulging operations in small parts. Electromagnetic or Magnetic Pulse Forming Based on the principle that the electromagnetic field of B d h i i l h h l i fi ld f an induced current always opposes the electromagnetic field of the inducing current. A large capacitor bank is discharged, producing a current surge through a coiled conductor. h h l d d For IES, GATE, PSUs If the coil has been placed within a conductive cylinder, around a cylinder, or adjacent th fl t sheet of metal, th d li d dj t the flat h t f t l the discharge induces a secondary current in the workpiece, causing it to be repelled from the coil and conformed to a die or mating workpiece.29 of 49 Page Bhopal -2014
  30. 30. Stretch Forming h Electromagnetic or Magnetic Pulse Forming The process is very rapid and is used primarily to expand or contract tubing, or to permanently assemble b l bl component parts parts. This process is most effective for relatively thin materials ( 5 (0.25 to 1.25 mm thick). 5 ) Produce large sheet metal parts in low or limited P d l h t t l t i l li it d quantities. 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 g, tensile stretching, the forces on the form block are far less than those normally encountered in bending or o g. forming. There is very little springback, and the workpiece conforms very closely to the shape of the tool tool. Because the forces are so low, the form blocks can often be b made of wood, l d f d low‐melting‐point metal, or even lti i t t l plastic. Stretch Forming   Contd...... h Stretch Forming   Contd...... h Popular in the aircraft industry and is frequently used to form aluminum and stainless steel f l d l l Low‐carbon steel can b stretch f L b l be h formed to produce l d d large panels for the automotive and truck industry industry. Stretch Forming   Contd...... h Ironing The process of thinning the walls of a drawn cylinder by passing it b between a punch and d whose separation is h d die h less than the original wall thickness thickness. The walls are thinned and lengthened while the lengthened, thickness of the base remains unchanged. g Examples of ironed products include brass cartridge p p g cases and the thin‐walled beverage can. Ironing        Contd.... Embossing b Coining It is a very shallow drawing operation where the depth of Coining is essentially a cold‐forging operation except for the d h draw is l limited to one to three times the thickness of d h h h k f the f h fact that the fl h h flow of the metal occurs only at the top f h l l h the metal and the material thickness remains largely metal, layers and not the entire volume volume. unchanged. Coining is used for making coins medals and similar coins, articles. For IES, GATE, PSUs Page 30 of 49 Bhopal -2014
  31. 31. Bending After basic shearing operation, we can bend a part to give it some  shape. h Bending parts depends upon material properties at the location of  the bend. h  b d At bend, bi‐axial compression and bi‐axial tension is there. Bending Bending The strain on the outermost fibers of the bend is Bend allowance, Lb = α(R+kt)  α(R+kt) ε= where R = bend radius k = constant (stretch factor) k     ( h f ) For R > 2t k = 0.5 For R < 2t 1 2R +1 +1 t k = 0.33 t = thickness of material     hi k   f  i l α = bend angle (in radian) g ( Bending Force Bending Force Klσ ut t 2 F= w Where l =Bend length = width of the stock, mm Powder Metallurgy σ ut = Ulti t tensile strength, MPa (N/mm 2 ) Ultimate t il t th MP (N/ t = blank thickness, mm w = width of die-opening, mm idth f di i K = die-opening factor , (can be used followin table) Condition V-Bending U-Bending Edge-Bending W < 16t 1.33 2.67 0.67 W > = 16t 1.20 2.40 0.6 For U or channel bending force required is double than V  For U or channel bending force required is double than V – bending For edge  bending  it will be about one‐half that for V ‐ bending By  S K Mondal Manufacturing of Powder Manufacturing of Powder Atomization using a gas stream Powder Metallurgy Powder Metallurgy Powder metallurgy is the name given to the p process by which fine powdered materials are y p blended, pressed into a desired shape (compacted), and then heated (sintered) in a controlled atmosphere to b d the contacting ll d h bond h surfaces of the particles and establish the desired p p properties. For IES, GATE, PSUs Molten metal is forced th f d through a h small orifice and is disintegrated by a jet of compressed air air, inert gas or water jet,. jet It is used for low melting point materials, brass, materials brass bronze, Zn, Tn, Al, Pb etc. Manufacturing of Powder Manufacturing of Powder Reduction Metal oxides are turned to pure metal powder when exposed to below melting point gases results in a product of cake of sponge metal. The i h irregular sponge‐like particles are soft, readily l lik i l f dil compressible, compressible and give compacts of good pre‐sinter (“green”) strength g g Used for iron, Cu, tungsten, molybdenum, Ni and Page 31 of 49 Cobalt. Bhopal -2014
  32. 32. Manufacturing of Powder Manufacturing of Powder Manufacturing of Powder Electrolytic Deposition Used for iron, copper, silver Process is similar to electroplating electroplating. For making copper powder, copper plates are placed as anode in the tank of electrolyte, whereas the aluminium plates are placed i th electrolyte t act as cathode. l t l d in the l t l t to t th d p pp g p When DC current is passed, the copper gets deposited on cathode. The cathode plated are taken out and powder i scrapped off. Th powder i washed, d i d and d is d ff The d is h d dried d p pulverized to the desired grain size. g The cost of manufacturing is high. Granulations ‐ as metals are cooled they are stirred rapidly Machining ‐ coarse powders such as magnesium Milling ‐ crushers and rollers to break down metals. Used for g brittle materials. Shooting ‐ drops of molten metal are dropped in water, used for low melting point materials materials. Condensation – Metals are boiled to produce metal vapours and then condensed to obtain metal powders. Used for Zn, Characteristics of metal powder: Ch i i f l d Fineness: refers to particle size of powder, can be p p , determined either by pouring the powder through a sieve or by microscopic testing A standard sieves with mesh size testing. varies between (100) and (325) are used to determine particle size and particle size di t ib ti of powder i a ti l i d ti l i distribution f d in certain range. Particle size distribution: refers to amount of each particle size in the powder and have a great effect in determining flowability, apparent density and final porosity of product. Mg, Cd. Mg Cd Blending l d Blending or mixing operations can be done either dry or wet. Bl di i i ti b d ith d t Lubricants such as graphite or stearic acid improve the flow characteristics and compressibility at the expense of reduced strength. Binders produce the reverse effect of lubricants. Thermoplastics or a water‐soluble methylcellulose binder is water soluble used. Most lubricants or binders are not wanted in the final product and are removed ( volatilized or burned off) Compacting C ti Compacting Sintering Powder is pressed into a “green compact” Controlled atmosphere: no oxygen 40 to 1650 MPa pressure (Depends on materials, Heat to 0.75*T melt Particles bind together, diffusion, recrystalization  P ti l  bi d t th  diff i   t li ti   product complexity) and grain growth takes place. g g p Still very porous, ~70% density Part shrinks in size  May be done cold or warm (higher density) Density increases, up to 95% Strength increases, Brittleness reduces, Porosity  St th i  B ittl   d  P it   decreases. Toughness increases. g For IES, GATE, PSUs Page 32 of 49 Bhopal -2014
  33. 33. H t I t ti P i (HIP) Hot Isostatic Pressing (HIP) Cold Isostatic Pressing (CIP) ld ( ) Is carried out at high temperature and p g p pressure using a g The powder is contained in a flexible mould made of gas such as argon. rubber or some other elastomer material bb h l l The flexible mould is made of sheet metal. (Due to high The flexible Th fl ibl mould i then pressurized b means of ld is h i d by f temperature) high‐pressure water or oil (same pressure in all oil. Compaction C i directions) ) simultaneously. simultaneously No lubricant is needed U Used in the production of billets of super‐alloys, high‐ p p y, g High and uniform density can be achieved speed steels, titanium, ceramics, etc, where the integrity and d sintering i i are completed l d of the materials is a prime consideration Features of PM products f d For high tolerance parts, a sintering part is put back into F hi h l i i i b ki a die and repressed. In general this makes the part more accurate with a better surface finish. A part has many voids that can be impregnated One impregnated. method is to use an oil bath. Another method uses vacuum first then impregnation acuum first, impregnation. A part surface can be infiltrated with a low melting point metal to increase density, strength, hardness, ductility and impact resistance. Plating, heat treating and machining operations can also be b used. d Advantages      Contd…. d Physical properties can be controlled Variation from part to part is low Hard to machine metals can be used easily H d t   hi   t l    b   d  il No molten metals No need for many/any finishing operations Permits high volume production of complex shapes g p p p Production of magnets d f 50:50 Fe‐Al alloys is used for magnetic parts  F Al  ll  i   d f   i   Al‐Ni‐Fe is used for permanent magnets p g Sintering is done in a wire coil to align the magnetic  poles of the material H2 is used to rapidly cool the part (to maintain magnetic  alignment) Total shrinkage is approximately 3‐7% (for accurate parts  an extra sintering step may be added before magnetic  alignment) li t) The sintering temperature is 600°C in H2 g p Disadvantages d Metal powders deteriorate quickly when stored  M l  d  d i   i kl   h   d  improperly Fixed and setup costs are high Part size is limited by the press, and compression of the  Part size is limited by the press  and compression of the  powder used. Sharp corners and varying thickness can be hard to  p oduce produce Non‐moldable features are impossible to produce. Allows non‐traditional alloy combinations Good control of final density For IES, GATE, PSUs Page 33 of 49 Advantages d Good tolerances and surface finish G d  l   d  f  fi i h Highly complex shapes made quickly g y p p q y Can produce porous parts and hard to manufacture  materials (e.g. cemented oxides) materials (e g  cemented oxides) Pores in the metal can be filled with other  materials/metals Surfaces can have high wear resistance Porosity can be controlled Low waste Automation is easy A li ti Applications Oil impregnated bearings made from either iron or Oil‐impregnated copper alloys for home appliance and automotive applications li ti P/M filters can be made with pores of almost any size. p y Pressure or flow regulators. Small S ll gears, cams etc. t Products where the combined properties of two or more p p metals (or both metals and nonmetals) are desired. Cemented carbides are produced by the cold‐ Cemented carbides are produced by the cold compaction of tungsten carbide powder in a binder, such  as cobalt ( 5 to 12%), followed by liquid‐phase sintering.   b lt (   t   %)  f ll d b  li id h   i t i Bhopal -2014
  34. 34. Pre ‐ Sintering Repressing Infiltration fl If a part made by PM needs some machining, it will be Repressing is performed to increase the density and Component is dipped into a low melting‐temperature rather very d ff l if the material is very h d and h difficult f h l hard d improve the mechanical properties. h h l alloy l ll liquid d strong. strong These machining operations are made easier by Further improvement i achieved b re‐sintering. F h i is hi d by i i The liquid Th li id would fl ld flow i into the voids simply b capillary h id i l by ill the pre‐sintering operation which is done before pre sintering action, action thereby decreasing the porosity and improving sintering operation. the strength of the component. g p The process is used quite extensively with ferrous parts p q y p using copper as an infiltrate but to avoid erosion, an alloy of copper containing iron and manganese is often used. Impregnation Oil‐impregnated Porous Bronze Bearings Impregnation is similar to infiltration I i i i il i fil i PM component is kept in an oil bath. The oil penetrates p p p into the voids by capillary forces and remains there. The oil is used for lubrication of the component when necessary. During the actual service conditions, the oil is released slowly to provide the necessary l b l d l l d h lubrication. The components can absorb between 12% and 30% oil by e co po e ts ca abso b bet ee %a d o volume. It i b i is being used on P/M self‐lubricating b d lf l b i ti bearing i components since the late 1920's. For IES, GATE, PSUs Page 34 of 49 Bhopal -2014
  35. 35. Terminology Nominal size: Size of a part specified in the drawing Basic size: Size of a part to which all limits of variation (i.e. tolerances) are applied. ( ) pp t, o e a ce & ts Limit, Tolerance & Fits Actual size: Actual measured dimension of the part. p The difference between the basic size and the actual size should not exceed a certain limit, otherwise it will interfere with the interchangeability of the mating parts. By  S K Mondal Terminology Terminology            C td Contd.... Limits of sizes: There are two extreme permissible sizes for a dimension of the part. The largest permissible size for a dimension is called upper or high or maximum limit, whereas the smallest size is known as lower or minimum limit. Tolerance The difference between the upper limit and lower limit. It is the maximum permissible variation in a dimension. The tolerance may be unilateral or bilateral. Terminology Terminology            C td Contd.... g p g Zero line: A straight line corresponding to the basic size. The deviations are measured from this line. Deviation: Is the algebraic difference between a size (actual, max. etc.) and the corresponding basic size. Actual deviation: Is the algebraic difference between an actual size and the corresponding b i size. l i d h di basic i Upper d i i U deviation: I the algebraic diff Is h l b i difference b between Terminology            Contd.... e o ogy Unilateral Limits occurs when both maximum limit and minimum limit are either above or below the basic size. +0.18 e.g. Ø25 +0 18 +0.10 Basic Size = 25 00 mm 25.00 Upper Limit = 25.18 mm Lower Limit = 25.10 mm Tolerance = 0.08 mm 0.10 e.g. e g Ø25 -0 10 -0.20 Basic Size = 25.00 mm Upper Limit = 24.90 mm Lower Limit = 24.80 mm Tolerance = 0.10 mm Terminology Terminology            C td Contd.... Lower deviation: Is the algebraic difference between the minimum size and the basic size. Mean deviation: Is the arithmetical mean of upper pp and lower deviations. Terminology Terminology            Contd Contd.... For Unilateral Limits, a case may occur when one of the Limits limits coincides with the basic size, e.g.  Ø25 +0.20     , Ø25  0 0   ‐0.10 0.10 Bilateral Limits occur when the maximum limit is above  and the minimum limit is below the basic size. e.g. Ø25 ±0.04 Basic Size = 25 00 mm 25.00 Upper Limit = 25.04 mm Lower Li it = 24.96 mm L Limit 6 Tolerance = 0.08 mm Fit Fits: (assembly condition between “Hole” & “Shaft”) Hole – A feature engulfing a component Shaft – A feature being engulfed by a  component p Fundamental deviation: This is the deviation, either the upper or the lower deviation, which is nearest one to zero line for either a hole or shaft. the maximum size and the basic size size. For IES, GATE, PSUs Page 35 of 49 Bhopal -2014
  36. 36. Transition Fits Clearance Fits Interference Fits Hole Hole Max C Hole Max C Min C Tolerance zones never meet T l       Tolerance zones always  overlap Shaft Max I Shaft Tolerance zones never meet  but crosses each other Min I Max I Shaft Max. C = UL of hole - LL of shaft Min. C = LL of hole - UL of shaft The clearance fits may be slide fit, easy sliding fit, running  Th   l  fit    b   lid  fit     lidi  fit   i   fit, slack running fit and loose running fit. Max. C = UL of hole - LL of shaft Max. I = LL of hole - UL of shaft The transition fits may be force fit, tight fit and push fit. Max. I = LL of hole - UL of shaft Min. Min I = UL of hole - LL of shaft The interference fits may be shrink fit, heavy drive fit and  The interference fits may be shrink fit  heavy drive fit and  light drive fit. 5.  Basis of Fits ‐ Hole Basis Tolerance  Zone µ µm • It is defined graphically by the magnitude of the Tolerance Zone tolerance and by its position in relation to the zero line. 55 20 Allowance In this system, the basic  diameter of the hole is constant  while the shaft size varies  according to the type of fit. It is Minimum clearance or maximum interference. It is the intentional difference between the basic dimensions of the mating parts. The allowance may be gp y positive or negative. I T C Hole Basis Fits Basic Size Legends: Hole Shaft Tolerance C - Clearance T-T Transition ii I - Interference • This system leads to greater economy of production, as a single drill or reamer size can be used to produce a variety of fits by merely altering the shaft limits limits. • The shaft can be accurately produced to size by turning and grinding. • Generally it is usual to recommend hole-base fits except where fits, temperature may have a detrimental effect on large sizes. Basis of Fits ‐ Shaft Basis Limits and Fits Limits and Fits •Here the hole size is varied to produce the required class of fit with a basic-size shaft. C T I Shaft Basis Fits Legends: Hole Shaft Tolerance C - Clearance T-T Transition ii I - Interference A series of drills and reamers is  required for this system,  therefore it tends to be costly.  It may, however, be necessary  It may  however  be necessary  to use it where different fits are  required along a long shaft. For  example, in the case of driving  example  in the case of driving  shafts where a single shaft may  have to accommodate to a  variety of accessories such as  couplings, bearings, collars,  etc., it is preferable to maintain  a constant diameter for the  permanent member, which is  the shaft, and vary the bore of  the accessories.   For IES, GATE, PSUs Limits and fits comprises 18 grades of fundamental tolerances for both shaft and hole, designated as IT01, IT0 and IT1 to IT16. These are called standard tolerances. (IS‐919) But ISO 286 specify 20 grades upto IT18 There are 25 (IS 919) and 28 (ISO 286) types of fundamental deviations deviations. Hole: A, B, C, CD, D, E, EF, F, FG, G, H, J, JS, K, M, N, P, R, S, T, U, V, X, Y, Z, ZA, ZB, ZC. R S T U V X Y Z ZA ZB ZC Shaft : a, b, c, cd, d, e, ef, f, fg, g, h, j, js, k, m, n, p, r, s, t, u, v, x, y, z, za, zb, zc. A unilateral hole basis system is recommended but if y necessary a unilateral or bilateral shaft basis system may Page 36 of 49 also be used Tolerance Designation (ISO) Tolerance on a shaft or a hole can also be calculated by using the formulas provided by ISO ISO. T = K ×i where, where T is the tolerance (in µm) i = 0.453 D + 0.001D (unit tolerance, in µm) D = D1D2 (D1 and D2 are the nominal sizes marking the beginning and the end of a range of g g g sizes, in mm) K = 10(1.6)( ITn − IT 6 ) [For IT6  to IT16] Bhopal -2014
  37. 37. Diameter Steps Diameter Steps Above  (mm) ( ) Upto and including  (mm) ( ) ‐ ‐ 3      ‐ 6       6      ‐ 10      ‐ 18 ‐ 8 30     ‐ 5 50      ‐ 80      ‐ 120      ‐ 180      ‐ 250      ‐ 315      ‐ 400      ‐ 3 6 10 18 30 50 80 120 180 250 315 400 500 Fundamental Deviation Value of the Tolerance  IT01 IT0 IT1 IT3 3 ar2 IT4 ar3 IT5 5 ar4 = 7i IT7 IT8 0.3 + 0.008D 0.5 + 0.012D 0.8 + 0.02D =a 10(1.6)(ITn -IT6) ( ) = 16i IT11 10(1.6)(ITn -IT6) = 100i IT15 10(1.6)(ITn -IT6) = 640i 10(1.6)(IT 0( 6) n -IT6) = 25i IT12 10(1.6)(ITn -IT6) = 160i IT9 ) 10(1.6)( 10(1 6)(ITn -IT6) = 40i IT13 10(1.6)(ITn -IT6) = 250i IT2 ar r = 101/5 IT6 6 10(1.6)(ITn -IT6) = 10i Grades of Tolerance It is an indication of the level of accuracy. IT01 to IT4 measuring i i instruments IT10 IT6) 10(1.6) 10(1 6)(ITn -IT6) = 64i IT14 10(1.6)(ITn -IT6) = 400i IT16 ‐ For production of gauges, plug gauges, IT5 to IT t IT 7 ‐ F fit i precision engineering applications For fits in i i i i li ti IT8 to IT11 – For General Engineering IT12 to IT14 – For Sheet metal working or press working IT15 to IT16 – For processes like casting general cutting casting, 10(1.6)(ITn -IT6) = 1000i work Fundamental Deviations is chosen to locate the tolerance zone w.r.t. the zero line Calculation for Upper and Lower Deviation For Shaft Holes are designated by capital letter: Letters A to G - oversized holes Letters P to ZC - undersized holes ei = es – IT es = ei + IT For Hole F  H l EI = ES – IT ES = EI + IT Shafts are designated by small letter: Letters m to zc - oversized shafts Letters a to g - undersized shafts es = upper deviation of shaft pp ei = lower deviation of shaft ES = upper deviation of hole EI= lower deviation of hole H is used for holes and h is used for shafts whose fundamental deviation is zero For hole, H stands for a dimension whose lower deviation f d i ti refers t th b i size. Th h l H f which to the basic i The hole for hi h the lower deviation is zero is called a basic hole. Similarly, for shafts, h stands for a dimension whose upper deviation refers to the basic size. The shaft h for which the upper deviation is zero is called a basic shaft. A fit is designated by its basic size followed by symbols representing the limits of each of its two components, the hole being quoted first. For example 100 H6/g5 means basic size is 100 mm example, and the tolerance grade for the hole is 6 and for the shaft is 5 5. For IES, GATE, PSUs Recommended Selection of Fits Basic size Hole Tolerance Zone Shaft Tolerance Zone Fundamental Deviation F d t l D i ti IT# Page 37 of 49 Bhopal -2014
  38. 38. Interchangeability Selective Assembly All the parts (hole & shaft) produced are measured and graded into a range of dimensions within the tolerance groups. Reduces the cost of production d h f d Term employed for the mass production of identical items within the prescribed limits of sizes. If the variation of items are within certain limits, all parts of equivalent size will be equally fit for operating in machines and mechanisms and the mating parts will give the required fitting. This facilitates to select at random from a large number of parts f an assembly and results i a considerable f for bl d l in id bl g p saving in the cost of production. Tolerance Sink A design engineer keeps one section of the part blank (without tolerance) so that production engineer can dump all the tolerances on that section which b d ll h l h i hi h becomes most inaccurate dimension of the part part. Position of sink can be changing the reference point point. Tolerance for the sink is the cumulative sum of all the tolerances and only like minded tolerances can be added i.e. either equally bilateral or equally unilateral. Limit Gauges Limit Gauges Allocation of manufacturing tolerances ll i f f i l holes. Plug gauge: used to check the holes The GO plug gauge is the size of the low limit of the hole while the NOT GO plug gauge corresponds to the high limit of the hole hole. Snap, Gap or Ring gauge: used for gauging the shaft and male components. Th G snap gauge i of a size l The Go is f i corresponding to the high (maximum) limit of the shaft, while the NOT GO gauge corresponds to the l hil h d h low (minimum limit). Unilateral system: gauge tolerance zone lies t l li entirely within the work tolerance zone. work tolerance zone becomes smaller by the sum of the gauge tolerance tolerance. Example Size of the hole to be checked 25 ± 0.02 mm Here, Hi h limit of hole = 25.02 mm H Higher li it f h l 25 02 Lower limit of hole = 24 98 mm 24.98 Work tolerance = 0.04 mm ∴ Gauge tolerance = 10% of work tolerance = 0.004 mm +0.004 mm −0 000 0.000 +0.000 0.000 Dimension of 'NOT GO' Plug gauge = 25.02 mm −0.004 ∴ Dimension of 'GO' Plug gauge = 24.98 Fig. Plug gauge Fig. Ring and snap gauges • Taking example of above: • Bilateral system: in this ∴Wear Allowance = 5% of work tolerance = 0.002 mm system, the GO and NO GO gauge tolerance zones are bisected by the high and low limits of the work f tolerance zone. Taking example as above: ∴ Dimension of 'GO' Plug gauge = 24.98 +0.002 −0 002 0.002 Dimension of 'NOT GO' Plug gauge = 25.02 For IES, GATE, PSUs mm +0.002 +0.002 mm −0.002 g g y Wear allowance: GO gauges which constantly rub  against the surface of the parts in the inspection are  subjected to wear and loose their initial size. The size of go plug gauge is reduced while that of go  snap gauge increases.    i To increase service life of gauges wear allowance is  g g added to the go gauge in the direction opposite to  wear. Wear allowance is usually taken as 5% of the  work tolerance. Wear allowance is applied to a nominal diameter  W   ll  i   li d      i l di   before gauge tolerance is applied. Page 38 of 49 Nominal size of GO plug gauge = 24.98 + 0 002 mm 24 98 0.002 ∴ Di Dimension of 'GO' Plug gauge = 24.982 i f Pl 24 982 +0.004 mm −0.000 gg g Dimension of 'NOT GO' Plug gauge = 25.02 +0.000 −0 004 0.004 Bhopal -2014 mm
  39. 39. T l ’ Pi i l Taylor’s Principle Linear measurements This principle states that the GO gauge should always be so d designed that it will cover the maximum metal d h ll h l condition (MMC) of as many dimensions as possible in the same limit gauges, whereas a NOT GO gauges to Measurement of Lines & Surfaces cover the minimum metal condition of one dimension only. Some of the i S f h instruments used f d for the li h linear measurements are: Rules Vernier Micrometer Height gauge Bore gauge B Dial indicator Slip gauges or gauge blocks By  S K Mondal Vernier Caliper A vernier scale is an auxiliary scale that slides along the main scale. The vernier scale is that a certain number n of divisions on the vernier scale is equal in length to a different number (usually one less) of main‐scale divisions main scale divisions. nV = (n −1)S where n = number of d h b f divisions on the vernier scale h l V = The length of one division on the vernier scale g and S = Length of the smallest main‐scale division Least count is applied to the smallest value that can be read directly by use of a vernier scale. Least count = S − V = 1 S n M t i Mi t Metric Micrometer A micrometer allows a measurement of the size of a body. It is one of the most accurate mechanical devices in common use. It consists a main scale and a thimble Method of Measurement Step‐I: Find the whole number of mm in the barrel Step‐I: Find the reading of barrel and multiply by 0.01 Vernier Caliper Bore Gauge: used for measuring bores of different g g g sizes ranging from small‐to‐large sizes. Provided with various extension arms that can be added for different sizes sizes. Micrometer  For IES, GATE, PSUs Page 39 of 49 Step‐III: Add the value in Step‐I and Step‐II Dial indicator: Converts a linear displacement into a radial movement to measure over a small range of movement f the ll f for h plunger. The typical least count that can be obtained with suitable gearing dial indicators is 0.01 mm to 0.001 mm. mm It is possible to use the dial indicator as a comparator by mounting it on a stand at any g y suitable height. Principle of a dial indicator Bhopal -2014
  40. 40. pp cat o s o d a d cato c ude: Applications  of dial indicator include: centering workpices to machine tool spindles offsetting lathe tail stocks aligning a vice on a milling machine checking dimensions To make up a Slip Gauge pile to 41.125 mm A Slip Gauge pile is set up with the use of simple Sli G il i t ith th f i l maths. Decide what height you want to set up, in this g y p case 41.125mm. Take away the thickness of the two wear gauges, and then use the gauges in the set to remove each place of decimal in turn starting with the turn, lowest. A M t i li t (88 Pi ) A Metric slip gauge set (88 Pieces) Slip gauges size or  range, mm 1.005 1.001 to 1.009 1.010 to 1.490 0.500 to 9.500 0 500 to 9 500 10 to 100 Increment, mm Increment  mm ‐ 0.001 0.010 0.500 0 500 10.000 For IES, GATE, PSUs Number of  Pieces 1 9 49 19 10 Slip Gauges or Gauge blocks These are small bl k of alloy steel. Th ll blocks f ll l Used in the manufacturing shops as length standards. g p g Not to be used for regular and continuous measurement. measurement Rectangular blocks with thickness representing the dimension of the block. The cross‐section of the block is usually 32 mm x 9 mm. s usua y 3 . Are hardened and finished to size. The measuring surfaces of th gauge bl k are fi i h d t a very hi h f f the blocks finished to high degree of finish, flatness and accuracy. Come in sets with different number of pieces in a given set t suit th requirements of measurements. t to it the i t f t A typical set consisting of 88 pieces for metric units is yp g p shown in. To build T b ild any given di i dimension, it i necessary t i is to y , p g identify a set of blocks, which are to be put together. Number of blocks used should always be the smallest. Generally the top and b G ll h d bottom Sli G Slip Gauges i the pile in h il g g y are 2 mm wear gauges. This is so that they will be the only ones that will wear down, and it is much cheaper to replace two gauges than a whole set. l h h l To make up a Slip Gauge pile to 41.125 mm 41.125 -4.000 ______ 37.125 -1.005 1 00 _______ 36.120 -1.020 1 020 _______ 35.100 -1.100 1 100 _______ 34.000 -4.000 4 000 _______ 30.000 -30.000 30 000 _______ 0.000 Comparators Comparator is another form of linear measuring method, which is quick and more convenient for checking l h ki large number of id ti l di b f identical dimensions. i During the measurement, a comparator is able to give g p g the deviation of the dimension from the set dimension. Cannot measure absolute dimension but can only compare two dimensions. Highly reliable. To magnify the deviation, a number of principles are used such as mechanical, optical, pneumatic and electrical. electrical Page 40 of 49 Fig. Principle of a comparator Bhopal -2014
  41. 41. Mechanical Comparators Mechanical Comparators Limit Gauges Feeler Gauge Gauge Snap Gauge External Dimensions Plug Gauge g g Internal Dimensions Taper Plug Gauge Taper hole Ring Gauge External Diameter Gap Gauge G  G Gaps and Grooves G   d G Radius Gauge Gauging radius Thread pitch Gauge p g The Mikrokator principle greatly magnifies any deviation i size so th t d i ti in i that even small deviations produce l d large d fl deflections of f the pointer over the scale. p For Measuring External Thread Sigma Mechanical Comparator Mechanical Comparators Mechanical Comparators The Sigma Mechanical Comparator uses a partially The Eden‐Rolt Reed system uses a y wrapped b d wrapped about a d d band d b driving d drum to turn a pointer attached to the end of two pointer needle The assembly provides a frictionless needle. reeds. One reed is pushed by a movement with a resistant pressure provided by the plunger, while the other is fixed. As springs. one reed moves relative t th other, d l ti to the th the pointer that they are commonly attached to will deflect. Sigma Mechanical Comparator Optical Comparators These d i devices use a plunger to rotate a mirror. A li h light Th l i beam is reflected off that mirror, and simply by the virtue of distance, the small rotation of the mirror can be converted to a significant translation with little g friction. Pneumatic Comparators Pneumatic Comparators Flow type: The float height is essentially proportional to the air that escapes f h from the gauge h d h head Master M t gauges are used t fi d calibration points on d to find lib ti i t the scales The input pressure is regulated to allow magnification adjustment For IES, GATE, PSUs Page 41 of 49 Bhopal -2014