12.008-spring-2004 S.Kim 12.008 Design & Manufacturing IISpring 2004Metal Cutting II2.008-spring-2004 S.Kim2Cutting processes Objectives Product quality: surface, tolerance Productivity: MRR , Tool wear Physics of cutting Mechanics Force, power Tool materials Design for manufacturing 2.008-spring-2004 S.Kim3Orthogonal cutting in a latheRake angleShear angleTo: depth of cutShear planeAssume a hollow shaft2.008-spring-2004 S.Kim4Velocity diagram in cutting zone()() ()φsincVαcossVαφcosV==−()()αφcosφsinrctotVcV−===()()αφcosφVsincV−=()()αφcosφsinrctotVcV−===Cutting ratio: r <12.008-spring-2004 S.Kim5E. Merchant’s cutting diagramαφRFtFcFnFsFNαβSource: KalpajkianFtFcβ−α2.008-spring-2004 S.Kim6() ()α)βRcos(φφsintFφcoscFsF −+=⋅−⋅=() ()φcostFφsincFnF ⋅+⋅=()βsinRF ⋅=()βcosRN ⋅=()βtanµ =AngleFrictionβ =2µ0.5:Typcially <<FBD of Forces()()αtantFcFαtancFtFNFµ⋅−⋅+==()α-βsinRFt ⋅=()α-βcosRFc ⋅=22.008-spring-2004 S.Kim7Analysis of shear strain What does this mean: Low shear angle = large shear strain Merchant’s assumption: Shear angle adjusts to minimize cutting force or max. shear stress Can derive:2β2α45φo−+=()αφtancotφaccdbcγ −+=+=2.008-spring-2004 S.Kim8Shear Angle2β2α45φo−+=FsFcφwsinφAFsAsFsτ ==σsAsFs ⋅=() ()α)βRcos(φφsintFφcoscFsF −+=⋅−⋅=()α-βcosRFc ⋅=Maximize shearstressMinimize Fc0ddτ=ϕ0ddFc=ϕ2.008-spring-2004 S.Kim9PowerssVF ⋅:shearingfor Power VFc⋅:inputPower cVF ⋅:friction viadissipatedPower VtwVFuosss⋅⋅⋅=:shearingfor energy SpecificVtwVFuocf⋅⋅⋅=:frictionfor energy SpecificVtwVFVtwVFuuossocfs⋅⋅⋅+⋅⋅⋅=+:energy specific TotalMRR=> shearing + frictionExperimantal dataMRR (Material Removal Rate) = w.to.V2.008-spring-2004 S.KimKalpakjian 10Cutting zone picturescontinuous secondary shear BUEserrated discontinuous2.008-spring-2004 S.Kim11Chip breaker- Stop and go- millingContinuous chip: bad for automation2.008-spring-2004 S.Kim12Cutting zone distributionHardness Temperature770500316Mean temperature: CVafbHSS: a=0.5, b=0.37532.008-spring-2004 S.Kim13Built up edge What is it? Why can it be a good thing? Why is it a bad thing? Thin BUE How to avoid it…•Increasing cutting speed•Decreasing feed rate•Increasing rake angle•Reducing friction (by applying cutting fluid)2.008-spring-2004 S.Kim14ToolsHSS (1-2 hours)Inserts-High T-High σ-Friction-Sliding on cut surface2.008-spring-2004 S.Kim15Source: KalpajkianTool wear up closeCrater wearDepth of cut lineFlank wearWear land2.008-spring-2004 S.Kim16Source: KalpajkianTaylor’s tool wear relationship (flank wear)CTVn=⋅) fpm ( velocity cuttingV =(min) failure totimeT =F. W. Taylor, 1907Workpiece hardnessfpmCTVn=⋅⋅⋅yxfdrate feedfcut of depthd==41-77fdVCT−−⋅⋅⋅=Ex.Optimum for max MRR?2.008-spring-2004 S.KimKalpakjian 17Taylor’s tool life curves (Experimental) Coefficient nvaries from:SteelsCeramics0.1 0.7 As nincreases, cutting speed can be increased with less wear. Given that, n=0.5, C=400, if the V reduced 50%, calculate the increase of tool life?Log scale2.008-spring-2004 S.Kim18Source: KalpajkianWhat are good tool materials? Hardness wear temperature Toughness fracture42.008-spring-2004 S.KimSandvik Coromant, Kalpakjian 19History of tool materialsTrade off: Hardness vs Toughnesswear vs chipping2.008-spring-2004 S.Kim20HSS High-speed steel, early 1900 Good wear resistance, fracture resistance, not so expensive Suitable for low K machines with vibration and chatter, why? M-series (Molybdenum) Mb (about 10%), Cr, Vd, W, Co Less expensive than T-series Higher abrasion resistance T-series (Tungsten 12-18%) Most common tool material but not good hot hardness2.008-spring-2004 S.Kim21Carbides Hot hardness, high modulus, thermal stability Inserts Tungsten Carbide (WC) (WC + Co) particles (1-5 µ) sintered WC for strength, hardness, wear resistance Co for toughness Titanium Carbide (TiC) Higher wear resistance, less toughness For hard materials Uncoated or coated for high-speed machining TiN, TiC, TiCN, Al2O3 Diamond like coating CrC, ZrN, HfN2.008-spring-2004 S.Kim22Crater wear Diffusion is dominant for crater wear A strong function of temperature Chemical affinity between tool and workpiece Coating?Crater wear2.008-spring-2004 S.Kim23Multi-phase coatingTiN low frictionAl2O3 thermal stabilityTiCN wear resistanceCarbide substratehardness and rigidityCustom designed coating for heavy duty, high speed, interrupted, etc.2.008-spring-2004 S.Kim24Ceramics and CBN Aluminum oxide, hardness, high abrasion resistance, hot hardness, low BUE Lacking toughness (add ZrO2, TiC), thermal shock Cold pressed and hot sintered Cermets (ceramic + metal) Al2O3 70%, TiC 30%, brittleness, $$$ Cubic Boron Nitride (CBN) 2ndhardest material brittle Polycrystalline Diamond52.008-spring-2004 S.Kim25Range of applicationsVfHighHigh2.008-spring-2004 S.Kim26Chatter Severe vibration between tool and the workpiece, noisy. In general, self-excited vibration (regenerative) Acoustic detection or force measurements Cutting parameter control, active control2.008-spring-2004 S.Kim27Turning parameters MRR = π Davg. N. d . f N: rotational speed (rpm), f: feed (in/rev), d: depth of cut (in) l; length of cut (in) Cutting time, t = l / f N Torque = Fc (Davg/2) Power = Torque. Ω 1 hp=396000 in.lbf/min = 550 ft.lbf/secExample 6 inch long and 0.5 in diameter stainless steel is turned to 0.48 in diameter. N=400 rpm, tool in traveling 8 in/min, specific energy=4 w.s/mm2=1.47 hp.min/in3 Find cutting speed, MRR, cutting time, power, cutting force.2.008-spring-2004 S.Kim28Sol. Davg=(0.5+0.48)/2= 0.49 in V=π. 0.49.400 = 615 in/min d=(0.5-0.48)/2=0.01 in F=8/400=0.02 in/rev MRR=V.f.d=0.123 in3/min Time to cut=6/8=0.75 minP=1.47 x 0.123 = 0.181 hp=Torque x ω1hp=396000 in-lb/minT=P/ω=Fc. (Davg/2)Then, Fc=118 lbs2.008-spring-2004 S.Kim29Drilling parameters MRR: Power: specific energy x MRR Torque: Power/ω A hole in a block of magnesium alloy, 10 mm drill bit, feed 0.2 mm/rev, N=800 rpm Specific power 0.5 W.s/mm2 MRR TorqueNf4 DπMRR2⋅⎟⎠⎞⎜⎝⎛=2.008-spring-2004 S.Kim30Sol MRR=π (10x10/4 ) . 0.2 . 800 =210 mm3/s Power= 0.5