Machining processes2.810 Fall 2006Professor Tim GutowskiOutlineBasic Machine Configuration1Basic Mechanics1Geometry1,3Production machining1,2,3,4,5Environmental IssuesReadings1. Kalpakjian Ch 21-27 (Ch 20-26, ed 4)2. “Simplified Time Estimation Booklet for Basic Machining Operations” 3. Design for Machining handout4. Single minute exchange of dies (SMED) handout5. “A Job Shop” handoutOutlineBasic Machine Configuration1n Single point machiningw Turning, boring, trepanning, planingn Multiple point machiningw 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*Spindlespeed selectorFeed change gearboxCompound rest and slides (swivels) ApronBedLead screwFeed rodHeadstockSpindleCross slideWaysCarriageCenterTailstock quillTailstockBasic LatheVertical-Spindle Mill** Source: Kalpakjian, “Manufacturing Engineering and Technology”*OutlineBasic Mechanics1n Power, Forcesn Heat, Tool materials, Rate limitsn 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−×≅≅≤≤≅=+======⋅∫•••HτγγτγεσSpecific energy, uSHence we have the approximation;Power ˜ usX 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 can get 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)Ω = 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 Ω∗t’ times and the workpiece would see 4Ω∗t’ cutter teeth. In general, a cutter may have “N” teeth, so the feed per tooth isf = v / NΩThe 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 Alloywd φ=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 on cutting tool material; here, we must have a coated tool such as TiN or PCD) The rotational rate for the spindle isΩ = vs/ πD = 4775 rpmNow, we can calculate vw, workpiece velocity,f = vw/ N Ω => 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]usfrom 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/minΩ = 1000 ft/min = 3820 rpmπ∗1” ∗ 1ft/12”Material removal rate, MRR = 0.1∗0.006∗(π∗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]usfrom 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: ρ cp∆T = uSNote : uS~ H, Hardness∆Tadiabatic> ½ Tmelt(Al & Steel)Interface Temperature: ∆T = 0.4 (H / ρ cp)(v f / α)0.33v = cutting speedf = feedα = thermal diffusivity of workpieceNote v f / α = Pe = convection/conductionTypical temperature distribution in the cutting zone* Source: Kalpakjian, “Manufacturing Engineering and Technology”** Reference: N. Cook, “Material Removal Processes”Cutting tool materials & process conditionsTemperature (°F)Hardness (HRA)HRCFeed (in/rev)Cutting speed (ft/min)m/minYearMachining time (min)* Source: Kalpakjian, “Manufacturing Engineering and Technology”Cutting Speed (ft/min)Tool life (min)Limits to MRR in Machining1. Spindle Power – for rigid, well supported parts2. Cutting Force – may distort part, break delicate tools3. Vibration and Chatter – lack of sufficient rigidity in the machine, workpiece and cutting tool may result in self-excited vibration4. Heat – heat build-up may produce “welding”, poor surface finish, excessive work hardening; can be reduced 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 milling of Al > 35hp1m X 1m areaNote: 1cm3/sec = 3.67 in3/min* References: 1. Advanced Methods of Machining, J.A.McGeough, Chapman and Hall, 19882. Manufacturing Engineering and Technology, S. Kalpakjian, Addison-Wesley, 19923. Laser Machining, G. Chryssolouris , Springer-Verlag, 1991High speed Machining and AssemblyHigh Speed Machined aluminum parts are replacing built-up parts made by forming and assembly (riveting) in the aerospace industry. The part below was machined on a 5-axis Makino (A77) at Boeing using a 8-15k rpm spindle speed, and a feed of 240 ipm vs 60 ipm conventional machining. This part replaces a build up of 25 parts. A similar example exists for the F/A-18 bulkhead (Boeing, St. Louis) going from 90 pieces (sheetmetalbuild-up) to 1 piece. High speed machining is able to cut walls to 0.020” (0.51mm) without distortion. Part can be fixturedusing “window frame” type fixture.MRR = f d * N Ω wOutlineGeometry1,3n Micro-geometry: tolerance, surface finishn Macro-geometry: 5 axes, form tools vs. softwareMicro-geometryExcellent surface finish and dimension precision are possible.Compare machining tolerances with other processes.n See figure 23.14 (22.15) and 35.19 (35.22) of KalpakjianCompare surface finish with other processes, and various applications.n See figure 23.13 (22.14) and 35.20 (35.23) of KalpakjianWhy is machining so good?Variation Vs Part SizeMacro-geometryMachine tool
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