Manufacturing Science, Lecture notes of Mechanical Engineering

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 ## MACHININ6~ t ## Contents **(1)** Classi fic ation of Machining (2) C ut ting Too l Geom etry 1.1. **-..........** (^). ::;:: "". ## Gate Forum ## D ate: K an pur Chaptcr- 3 (rvlanufacluring Science) Machini ng- GATE Syllllhus <P M ·.h:lIl.i c.s of ttl :lchinin $ Single >I ud ll\ !I Ii-point cUlling [ 00 1 -, ## $- Tool geom etry :llld Jl)a[cri~l s, $- Tool life and wcat; -$- Ec ono m ics of machining; ~ Prin~ip lc s of non-traditional mach!ning processes;. ## -$ Princi ll es of work holding, _tit_ Princi ples of design of jjgs and Uxi.ures, Year Wise Distribu tion of Marks 1- ## Year 2003 Mark'1±l lxl lxO 1x3 lxO ## 2x3 2x 2 2x2 20 --- ## lxO 2x lxl 2x --.. _.•.. _. . 7 4 3 Ixl 1x1 lx1^1 x^1 2x4 2x3 2x3^ 2x^2 - ....-. (^). 5 ~ Net (^9 7) ___ L ___ -^ -^ ---'--.^ _..J^ I In tbe broad est se ns e, the re arc two dishuc t forms of soJid-st:ltc manufacruring Ulvoluing plastic deforma tion; (i) Forming : Bulk deformation processes in wbich the .naterial is movd but v01ume is conserved (ll) Machining :Pl astic deformation p.ro~ess followed bv fracture that C:luses material temoval for s hape generation , In this process vo l ume varies d ~pen cling upon amount of chip gCi1etated. Finished part (1) Introd ucti on of Mach ining .:. Machining i~ op erated hy s electively removing the metal from '.vork piece to prodl1CC the required shape. - ~ Removal of me ta l p arts is accomplished by s tra ining a local region of the work piece to fracture by the relative motion of the tool and the work pi ece. (B) Abrasive proc esses - Removal of the metal from the work piece using tool made from abrasive particles of irregular geometry. In this process material is removed in the form of tiny particles. Examples of abrasive machining are lappin g, honing and super-finishing. (C) Non- tradi tional process es - Various forms of energy other than sharp cutting tool is used to remove material from work piece. Depending up on these energy types it can be classified in several categories as ECM, EDM, USM, AJM etc. ## (3) Me~hanics of C uttin g Pro cess Secon.d.aty Shear Zone .:. In the cutting process, metal ahead of the cutting tool is compressed. - :. Material in a nattow zone e.xteD.ding from the tool tip to the free surface of work piece gets plastically deformed. .:. Deformation occurs due to intense s hearing in the narrow ~one called the shear zone. This narrow shear ## zone is approximated by a plane called the shear plane. - :. Higher shear angle means smaller shear plane which means lower shear force that giveseasier cutting ## conditions by lower cutting forces, power, and temperature. .' Three mo st important cutting tool geometrical parameters can be given as; ## (i) Rake Angles (0() It is the angle made by rack surface with normal to cutting direction. It influences cutting forces, power, "tool life and product surface finjsh. ,.' ., :. ~ ' ' ..^ ,' ,.^ " ',.-^ ... ::^ ..^ ". ::.'j'^ 1j '^ ,^ "..,.... .... •^.^ ',^.^ ,^ ". ## (a) Positive rak e angl. u (6-300) (b)^ Nega^ tive^ rake^ angle^ ex^ (0)^ Negatlve^ rake^ angle tool^ with ## leads to low cutting forces but prod^ uces^ higher cutting forces^ chIp^ breaker^ - a useful ## tragUe tools and more^ robust tools.^ compromise. ## Case-l Large rack angle (IX) ## .:. Good surface quality, lowercutting forces and power consumptiondue to sharper cutting. In general, ## power consumption de cteas~ by ..... 1 % for l°change in ex. - :. Primary shear can be related to the rake angle ex and increasing O(reduces shear plane sizetha t affects cutting parameters as well as cutting quality. - :. Higher rack angle causes adverse effe ct on tool strength because less metal is available to support the tobl and hence reduces tool life. - :. Greatly reduces capacity to c onduct he at away from the cutting edge and hence 0 or negative rake angles employed on carbide, CetattllC and similar ''hard'' tools. - :. Lower for brittle material. and dec reases with increasing material hardness (strength) as 8-25· for ## Cupper, 3-10· for Brass, 0-.4" for Titanium. MaterIal being cut a Cestlron 0 Free-ouftlng brass 0 Ductile brass 14" Till bronze 8" ## Aluminium aIIooJ 300 Mild steel 2&' MedllJlTKaJbon steel 20" HlgtH!adxm S'ieel 12" TufnoI plastlc 0" ## Case-IINegative rack a.ttgle ## .:. Gives poor surface quality, higher cut ting forces and power consumption due to blunt cutting. If ex is negative, the ttlaterial is forced back on itself, thus requiring higher cutting forces. - :. Higher tool strength because sufficient metal is available to support the tool and hence improves tool life. ## Prefetted for brittle material. machining und cases where intermittent cutting is to apply (grinding). - :. Increases shear pla11e size th at causes more defo.ttnation. ## Case-II1The tool has a negative ex but a sttlall area of positive rake just behind the cutting edge. This system is ## termed as chip breaker system. R~ma rk-3: Effe ct of rake face contact length on chip thickness and shear plane angle .:. The deformed chip is flowing over a static tool, leading to fri ctional force similar to friction hilL ## .:. If fl. is greater than 0.5, sticky friction will result and flow will occur only within the work piece but not at the t~ol work piece intetface. - :. Sticky friction is the issue .in cutting due to difficulty in applying lubricant Problem: Chip thickness (i) ~ Fotce to move the chip (I) ## ~ Cutting power (t) Solution: Change shear angle tp -+ Change rack angle r:J. workpiice •^ l~d , II .>- _t_ - ## (5) Types of Chips More realistic view of chip f otma tion can be understood by showing shear zones ratherthan shear plane. ## This shear zone can be divided in two parts as primary shear zone and secondary shear zone.Based on the characteristics of these two zones, chips can be classified in three categories: (b ) Chip with a built up ## edge, SUE ## (a) Continuous chip^ (c)^ Discontinuous^ chip ## (i) Continuous Chips Continuous chip _is_ characteristic of cutting ductile materials under steady stage conditions. However, long continuous chips present handling and removal problems and in practi t:al op eration requires chip breaker. - :. Ductile work materials ## .:. High cutting speeds ## .:. Small feeds and depths .:. Sharp cutting edge (zero nose radius) .:. Low tool-chip friction ## (ii) Discontinuous Chips ## Discontinuous chip is fo tmed in brittle mate rials which cannot withstand the high shear strains imposed in the trutchining process before fmctme. In case of discontinuous chips, fractu1"e occurs within primary shear zone while in continuous chip5 ftacnu:e is assumed after secondary zone. **_.:t_** (^) Brittle work materials (^) DIsconti nuous chip **.:.** (^) Low cutting speeds Discontinuous^ chip^ formation **.:.** (^) Sm all rack (^) angle causes chip fracture (^) follo·lateral.... ing a c^ deforrn:tllon r;)ck **.:.** (^) Large feed and depth cutt ing tool of cut **.:.** (^) High tool-chip friction (^) Irregular surface due (^) workpiece crack to chi discontinuities (iii) Continuous with a built-up edge (BU E) Under high tettlperatute conditio~ whe,re th e friction between the chip and the rake face, of the tool is high, ## the chip may weld to the tool face.The accumulation of the chip material is known asa built-up edge (BUE). The fonnation ofBUE is due to wotk hardening in the secondary shear zone at low speed (since heat is transferred to the t oo~.(Fotmation Rnd brake up mechanism ofhuilt-up edge) ## The built-up edge fottnati on is a transient and unstable phenomenon and occurs at a critical speed range.If speed increases tendency of built-up edge fonnation reduces due to contraction in built-up heap size. Usually, ## in this case chip fracture occurs in secondary zone. The BUE act as a substitute cuttit;tg edge (blunt tool (^) Formation of Built-up Edge (SUE) ## with a low rake angle) and hence gives p oor surface quality. .:. Occurs in ductile materials .:. Low-to-medium cutting speeds .:. Tool-chip friction causes portions of chip to adhere to rake face (^) workpiece (iv) Semi-continuous Chips (piispanen's Card Model of Conti.o.uous ChipFonnation) .:. Also known as Serrated Chip (saw to oth appearance) .:. Cyclical chip fotms with alternating hlgh shear strain to low shear strain .:. Associated with metals at high cu~ speeds -...11...-_-depositBUE Higher speeds Segmented chip Rem ark-1: Why should we know? » Power requirement for the machine tool can be calculated » D esign of stiffness, etc. for the machine tolerances » W'h ether work piece can withstand the cutting force ## (i) Force Analysis Ernst and Merchant (1941) did the first scientific analysis based on known parameters. As Fc and FT can be measured by using force dynamometer, hence they have developed a theory which can relate all other forces in ## terms of known parameters. This theory is based on Merchant Circle Theory. ## (a) Calculation of Forces ## In this theory, it is assumed that continuous chip fonnation takes place without any BUB. All material properties ## were remains constant during the cutting process. In their analysis, they found following results for unknown forces as F (friction force) and N (normal force on rack face); _F =Fc_ sina+Frcosa^ " _N:= Fc_ cosa-Frsina ## Similarly Fs (shear force) and Fn (n onnal force on shear face) can be given as; _F$_ = _Fc_ COS; - Fr sin; _Fa_ =Fcsin;+Frcos¢ ## Here the back calculation can be performed for calculation of known parameters as Fc (cutting force) and Ft (thrust force); _Fc_ = _FS cos;+FNsin;_ .Fj. = _FN_ cos;- _Fs_ sin _t/J_ Similarly resul ta nt for each pair of forces can be given as R (resultant force). It can be calculated by applying vector addition rule. Some advanced relation between resultant and basic cutting forces can be easily. calculated by using Merchant circle; _Fs =R_ cos(¢ + it- _a) Fe_ = _Rcos(A-a) Fr_ =Rsin(lt-a) Fs = shear force Fa = shear uorm..1 l force a. = tool rake angle $ = silear angle i.. = friclion angle ## (b) Determination of Coefficient of Fricti on and Friction Angle (1) Friction is a very important paratneter in cutting because it affects power consumption as well as quality of ## cutting. Mainly it decided the type of chip generation, relative parameters of cutting forces etc. Madlematically it can be given as; Coefficient of friction in metal cutting ranges from 0.5 to 2.0. ## (c) Determination of Sttess, Strain and *Suain Rate Using these patameters respective stresses can be calculated as; (. Average Shear SUess ## Where Fs is the shear force and As is the s he ar plane area. It is the area where the shear stress acts. Math~tically it can be given as, _A_ - _11ltO S_ - sinfS ~ As can be increased by increasing ~ ~ Shear stress is independent of rake angle ## .) Average Normal Stress (7) Power Cons umptio n in Cutting .:. The total energy for cutting can be divided into a number of components as; ~ Ener gy required for producing the gro ss deformation in the shear zone. ~ Frictional energy resulting from the chip sliding over the tool face. ~ Energy required for curling the chip. ~ Momen tum energy associated with the m omentum change as the metal crosses the shear plane. ~ The energy required to produce the new surface area. •:. Where do es c utting energy go? ~ 90% to chip ~ 5% to tool ~ 5% to workpiece .:. A machining operation requires power an d th at power to perform machining can be computed from: Where Pc = cutting power; Fe = cutting force; a nd v = cutting speed. - :. The specific cutting energy (e nergy per uni t volume) Uc can be given as, Where b is the width of the chip and t is the uncut chip thickness. So in terms of specific energy it can be represented as; IPo wer = Uc: *MRR I .:. The specific cutting energy Uc depends ' on the material being machined and also on the cutting speed, feed, rake angle, and other machining parameters. - :. Total specific cutting energy can be divi ded in two parts as specific frictional energy (UJand specific shear energy (UJ. The specific frictional energy consumed at the tool-chip interface howeverspecific shear energy consumed along the shear plan e. Thes e co mponents can be given as; _Fv_ c _F r_ (^) _lu F,v,_ I _btv bt_^ _S_^ _btv_ So for a simple example question, it c an be given as; _F ri ction e nergy_ _ _Uf_ _ _Fvc F r T otal energy' U_ Pc v ~