Machining processes2.810 Fall 2007Professor Tim GutowskiOutlineBasic Machine Configuration1Basic Mechanics1Geometry1,3Production machining1,2,3,4,5Environmental IssuesReadingsKalpakjian Ch 21-27 (Ch 20-26, ed 4)“Simplified Time Estimation Bookletfor Basic Machining Operations”Design for Machining handoutSingle minute exchange of dies(SMED) handout“A Job Shop” handoutOutline Basic Machine Configuration1 Single point machining Turning, boring, trepanning, planing Multiple point machining Drilling, milling, reaming, sawing, broaching,grindingMachining processesHorizontal Slab milling Face milling End millingCutterArborArborSpindleSpindleEnd millShankTurningMilling* Source: Kalpakjian, “ Manufacturing Engineering and Technology”** GrindingGrindingwheelDGrainsWorkpiecevVHorizontal-spindle surface grinderMachine ToolsColumnBaseHeadTableSaddleKnee*SpindlespeedselectorFeedchangegearboxCompound rest andslides (swivels)ApronBedLead screwFeedrodHeadstockSpindleCross slideWay sCarriageCenterTailstock quillTailstockBasic LatheVertical-Spindle Mill** Source: Kalpakjian, “ Manufacturing Engineering and Technology”*OutlineBasic Mechanics1 Power, Forces Heat, Tool materials, Rate limits New Technology to reduce these effectsSee Video on Plastic DeformationBasic Machining MechanismApproximationus ~ H (Hardness) t0tcφShear planeShear angleToolVChipWorkpieceα+-Rakeangle)42(61 u4 2 d uu 80%) to(65 u uu energy specific volworkwork dtd(work) Power VFppfriction workplasticSS!"##$$#=+======%&•••H'(('()*Specific energy, uSHence we have the approximation;Power ≈ us X MRRMRR is the Material Removal Rate or d(Vol)/dtSince Power isP = F * Vand MRR can be written as,d(Vol)/dt = A * VWhere A is the cross-sectional area of the undeformed chip, we canget an estimate for the cutting force as,F ≈ us × ANote that this approximation is the cutting force in the cutting direction.Basic Machining MechanismCutting Force Directions in MillingFpFcnFcFcnFpFcFcnFcFpFcnFpFcFc ~ H × Ac(Tangential Cutting Force ~ Chip Cross-section × Hardness)Feed per Tooth and MRRf = feed per tooth (m)w = width of cut (m)v (m/s)W = rotational rate (rpm)Consider the workpiece moving into the cutter at rate “v”. The travel in time t’is v*t’. During the same time, the cutter would rotate W*t’ times and theworkpiece would see 4W*t’ cutter teeth. In general, a cutter may have “N”teeth, so the feed per tooth isf = v / NWThe material removal rate (MRR) is,MRR = v w dwhere “d” is the depth of the tool into the workpiece.Top view of face millingWith 4 tooth cutterSide viewEx) Face milling of Al Alloywdf =DvwN = 4 (number of teeth)D = 2” (cutter diameter)Let w = 1” (width of cut), d=0.1” (depth of cut)f = 0.007” (feed per tooth),vs = 2500 ft/min (surface speed; depends oncutting tool material; here, we must have acoated tool such as TiN or PCD)The rotational rate for the spindle isW = vs / pD = 4775 rpmNow, we can calculate vw, workpiece velocity,f = vw / N W => vw= 134 [in/min]Material removal rate, MRR = vw*w*d = 13.4 [in3/min]Power requirement, P = us*MRR = 5.36 [hp]Cutting force / tooth, F ~ us*d*f = 111 [lbf]us from Table 21.2 (20.2 ed 4); Note 1 [hp min/in3] = 3.96*105 [psi]Ex) Turning a stainless steel barfD=1”dToolRecommended feed = 0.006” (Table 23.4 (22.4))Recommended surface speed = 1000 ft/minW = 1000 ft/min = 3820 rpm p*1” * 1ft/12”Material removal rate, MRR = 0.1*0.006* (p*1* 3820) = 7.2 [in3/min]Power requirement, P = us*MRR = 1.9*7.2 = 13.7 [hp]Cutting force / tooth, F ~ us*d*f = (1.9*3.96*105)*(0.1*0.006) = 450 [lbf]us from Table 21.2 (20.2 ed 4); Note 1 [hp min/in3] = 3.96*105 [psi]Let d = 0.1”Temperature Rise in CuttingAdiabatic Temperature Rise:r cp DT = uSNote : uS ~ H, HardnessDTadiabatic > ½ Tmelt (Al & Steel)Interface Temperature:DT = 0.4 (H / r cp)(v f / a)0.33v = cutting speedf = feeda = thermal diffusivity of workpieceNote v f / a = Pe = convection/conductionTypical temperature distributionin the cutting zone* Source: Kalpakjian, and Schmidt 5th ed** Reference: N. Cook, “ Material Removal Processes”Cutting tool materials & process conditionsTemperature (°F )Hardness (HRA )HRCF eed (in/rev )C utting speed (ft/min)m/minYearMachining time (min)* Source: Kalpakjian, “ Manufacturing Engineering and Technology”C utting Speed (ft/min)Tool life (min)Limits to MRR in MachiningSpindle Power – for rigid, well supported partsCutting Force – may distort part, break delicatetoolsVibration and Chatter – lack of sufficient rigidity inthe machine, workpiece and cutting tool mayresult in self-excited vibrationHeat – heat build-up may produce “welding”, poorsurface finish, excessive work hardening; can bereduced with cutting fluidSee Video on Rate Limits In MachiningTypical Material Removal Rate10-410-310-210-11 10 102EBM1EDM1,2Grinding3MachiningCreep Feed2GrindingLASER3Chem. Milling2[cm3/sec]25A, 6um RMS1Rough millingof Al > 35hp1m X 1m areaNote: 1cm3/sec = 3.67 in3/min* References: 1. Advanced Methods of Machining, J.A.McGeough, Chapman and Hall, 1988 2. Manufacturing Engineering and Technology, S. Kalpakjian, Addison-Wesley, 1992 3. Laser Machining, G. Chryssolouris, Springer-Verlag, 1991High speed Machining and AssemblyHigh Speed Machined aluminum parts are replacing built-upparts made by forming and assembly (riveting) in theaerospace industry. The part below was machined on a 5-axisMakino (A77) at Boeing using a 8-15k rpm spindle speed, and afeed of 240 ipm vs 60 ipm conventional machining. This partreplaces a build up of 25 parts. A similar example exists for theF/A-18 bulkhead (Boeing, St. Louis) going from 90 pieces(sheetmetal build-up) to 1 piece. High speed machining is ableto cut walls to 0.020” (0.51mm) without distortion. Part can befixtured using “window frame” type fixture.MRR = f d * N W wOutlineGeometry1,3 Micro-geometry: tolerance, surface finish Macro-geometry: 5 axes, form tools vs. softwareMicro-geometryExcellent surface finish and dimensionprecision are possible.Compare machining tolerances with otherprocesses. See figure 23.14 (22.15) and 35.19 (35.22) ofKalpakjianCompare surface finish with other processes,and various applications. See figure 23.13 (22.14) and 35.20 (35.23) ofKalpakjianWhy is machining so good?Variation Vs Part SizeMacro-geometry Machine tool
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