ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 20101Machining - Cutting Tool Lifever. 1ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 20102Overview• Failure mechanisms• Wear mechanisms• Wear of ceramic tools• Tool life• Machining conditions selectionME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 20103Tool Wear Zonesworkpiecechipcutting toolshear zoneCrater wearFlank wearME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 20104Chip / Tool InterfaceME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 20105Tool Wear(e)(d)(a)(b) (c)RakeFlankME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 20106Tool Wear Zones• Crater wear (crater)– tool-chip interface– predominant at high speeds– mitigated by efficient use of carbides• Flank wear (wear land)– tool-workpiece interface– predominant at low speedsME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 20107Failure MechanismsGross Fracture• High rupture strength needed– pure WC - 200,000 psi (1.4 GPa)– Al2O3+TiC - 125,000 psi (0.86 GPa)• Transverse rupture strength– max. tensile stress at failure of 3 pt. bendingME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 20108Failure MechanismsGross Fracture• Plastic deformation resistance needed– function of temperaturetensilestrengthT(oC)1100 1200ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 20109Failure Mechanisms• Fatigue• Abrasion• Chemical diffusion and convection• Chemical diffusionME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 201010Tool Wear MechanismsLow speed High speed Very high speedMechanicalpropertiesChemical diffusion andconvectionChemicaldiffusionME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 201011Wear at Low SpeedsMechanical Properties– flow induced crack nucleation and growth– micro fracture– fatigue– abrasion• Al2O3at all speeds – cutting steel and NiME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 201012Wear at High Speeds -Chemical Diffusion and ConvectionChipChip flowTool atomsToolInterphaseME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 201013Chemical Diffusion and Convection• Tool dissolves directly into chip• Convection - chip sliding on surface– transition between sliding and sticking begins– maximum heat generation point moves away from tool tip• Net flow of material away from interfaceME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 201014Wear at High Speeds• Carbides - super alloys, hard steels• Al2O3- Ti• Carbides, nitrides - steels• CBN - steelsME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 201015Wear at Very High Speeds -Chemical DiffusionChipChip flowTool atomsToolInterphaseME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 201016Chemical Diffusion• Transition from sliding to sticking moves from the nose– finally sticking occurs everywhere• Boundary layer builds up– no convection directly from tool to chip– only chemical diffusion through boundary layer of chip materialME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 201017Wear at Very High Speeds• Carbide, diamond - Ti at all speeds• CBN - super alloys, hard steelsME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 201018ChipChip flowTool atomsToolDissolution Controls Wear• Tool atoms diffuse up and are swept away by the chip at high temps.ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 201019Wear Velocity (vwear)• k =• c = equilibrium solubility • vy= bulk velocity of chip at chip-tool interface• D = chemical diffusivity• = concentration gradientyckDkcvvywearyc)()(ch iptoolVmaterialchipofvolumeMolarVmaterialtoolofvolumeMolarME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 201020Wear Velocity• Very difficult to determine from “first” principles• Estimate from experimentsME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 201021Bulk Velocity - Ex. 1.1vwear(HfC in Fe) 0.61 mm/mink = Vtool(HfC) / Vchip(Fe) =15.04 cm3/mole 7.11 cm3/mole = 2.12cHfC 2.75 x 10-5 vwear 0.61 mm/min = 2.12 x 2.75 x 10-5x vv(bulk velocity) 10,500 mm/min 1 cm/minME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 201022Relative Wear RatesNeed solubilities to solve: can be looked up22112121221121cVcVccVVVVckckvvchipchipwearwearME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 201023Relative Wear Rate - Ex. 2-1HfC vs. TiC in contact with steel (Fe+C) at 1600 K• Example to show methodME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 201024mCmHfHfCGGG Relative Wear Rate - Ex. 2-2• HfC in contact with steel (Fe+C)G = free energy of formation= affinity of Hf for C= affinity of Hf for Fe= affinity of C for FeHfCGmHfGmCGJosiah Willard Gibbs1839-1903Gibbs – A little history• Gibbs entered Yale University at the age of 15 graduating, in 1858, at the age of 18. • He then entered the new Yale graduate school earning the first PhD in engineering in the United States, completed in 1863. Gibbs' PhD thesis was “On the Form of the Teeth of Wheels in Spur Gearing.” • In 1871, two years after returning from a study abroad at various universities in Europe, Gibbs became Yale's first professor of mathematical physics. 25ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 2010ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 201026Relative Wear Rate - Ex. 2-3= excess free energy (enthalpy) of mixing= entropy of mixing x T =(TSi)c = concentrationT = absolute temperature (K)R = gas constant = 2 cal/mol/KixsimicRTGG lnxsiGicRT lnME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 201027Relative Wear Rate - Ex. 2-4= -49,000 cal/molecHf= cCfor HfC-49,000 = -2,100 + RT lncHf+ 7,600 + RT lncCHfCGCxsCHfxsHfcRTGcRTG lnln ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 201028Relative Wear Rate - Ex. 2-5-49,000 + 2,100 - 7,600 = 2 x 1600 (lncHf+ lncC) = 4 x 1600 x (lncHf)lncHf= - 8.52cHf= 2 x 10-4= cHfCME 6222: Manufacturing Processes and
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