Mechanics Of Machining Processes
Mechanics of MachiningProcessesIME 340/240
Types of Chips Four Types:(a)(b)(c)(e)(f)– Continuous– Built-up edge– Serrated orSegmented– Discontinuous Tool side ofchip is shiny orburnished(a) continuous chip withnarrow, straight primary shearzone; (b) secondary shearzone at the chip-tool interface;(c) continuous chip with largeprimary shear zone; (d)continuous chip with built-upedge; (e) segmented ornonhomogeneous chip and (f)discontinuous chip. Source:After M. C. Shaw, P. K.Wright, and S. Kalpakjian.(d)
Continuous Chips Ductile materials, High cutting speeds, High rake anglesNarrow (primary) shear zonePossible secondary shearzone at tool-chip interface(becomes thicker as frictionincreases)With soft metals and lowspeeds or rake angles, couldhave a wide primary shearzone with curved boundaries;leads to poor surface finishand residual surface stressesContinuous chips can causeproblems by becomingentangled in the tool holder,fixturing, workpiece, or chipdisposal systemChip breakers can help this problem
Built-Up Edge (BUE) Chips Layers of workpiece material are gradually deposited on the tool BUE eventually breaks off and is carried away by the chip and/ordeposited randomly on the workpiece surface(b) Large tool tip radius with BUE Produces rough surface finishes Generally undesirable but a thin, stable BUEcan reduce wear and protect the rake face Reduce probability of BUE forming by:(c) Decreasing depth of cutIncreasing rake angleUsing a sharp toolUsing an effective cutting fluidUsing cold-worked metals ratherthan annealed(b) Surface finish in turning 5130 steelwith a built-up edge. (c) surface finishon 1018 steel in face milling.Magnifications: 15X. Source: Courtesyof Metcut Research Associates, Inc.
Hardness of Built-Up Edge (BUE) Chips(a)Figure 20.6 (a) Hardnessdistribution in the cuttingzone for 3115 steel. Notethat some regions in thebuilt-up edge are as much asthree times harder than thebulk metal. Source:Courtesy of MetcutResearch Associates, Inc.
Serrated and Discontinuous Chips Serrated chips have a sawtooth-like appearance Also called segmented or nonhomogeneous chips Have zones of low and high shear strain Metals with low thermal conductivity or strength that decreases withtemperature (such as titanium) Discontinuous chips have firmly or loosely connected segments Brittle materials cannot withstand the high shear strains of cutting(thermoset plastics or ceramics) Workpiece materials with hard inclusions or impurities or structuressuch as graphite flakes in gray cast iron Very low or very high cutting speeds Large depths of cut or Low rake angles Lack of an effective cutting fluid or low machine tool stiffness Both chip types cause variations in cutting forces, and thus requirestiffness in the tool holder, work holder, and machine tool to avoidmachine vibration and chatter (which causes surface finish,dimensional accuracy, and tool wear problems)
Cutting Parameters and 2-dimensional Model Depth of cut – measured in mm or inchesFeed (rate) – distance tool travels per unit revolution (mm/rev, in/rev)Velocity, V – cutting tool velocityto – depth of cut (equiv. to feed in turning) a – rake angle f – shear angletc – chip thickness
Orthogonal (2D) Cutting Modeltosin fr tc cos(f a ) Cutting ratio, r (usually 1) Chip compression ratio, 1/r We can find shear angle, f, because to is a machine setting, and tcand a rake angle can be measured with micrometers or calipers Shear angle affects force and power requirements, chip thickness,and temperatureaa
Shear StrainShearStrain AB AO OB cot f tan(f a )OC OC OC Large shear strains due to low shear angles or rake angles that arelow or negative Shearing occurs at a very high rate
Velocity Based on mass continuity:Vc Vr V sin fcos(f a )Vt o VctcVVV s ccos(f a ) cos a sin fto Vcr tc V
Cutting Forces and Power Machine tools with enough power must be selected With known cutting forces, machine tools can be designed to avoidexcessive distortion and maintain desired tolerances Engineers can determine if the workpiece will withstand the cuttingforces without distortion The tool holder, workholding device, and machine tool must besufficiently stiff to minimize deflections caused by the thrust force sothat the tool is not pushed away from the workpiece (reducing depthof cut and causing dimensional inaccuracy) Cutting forces can be measured by mounting on the tool: Dynamometers Force transducers (piezoelectric crystals) Cutting forces can also be calculated if you know the powerconsumption of the machine Duller tools require higher forces and power
Cutting Forces and PowerResultant force, R decomposes into:1.2.Cutting force, Fc in the direction of VThrust force, Ft normal to V(both of the above are measurable)1.2.Friction force, FNormal force, NF R sin Fc sin a Ft cos aN R cos Fc cos a Ft sin a1.2.Shear force, FsNormal force, FnFs Fc cos f Ft sin fFn Fc sin f Ft cos fF Ft Fc tan a N Fc Ft tan a typically ranges from0.5 to 2 in metal cutting
Power Power is dissipated in the shear zone and along the tool rake facePower P FcV Specific energy for shearingShearingPower Ps FsVsFsVsus wtoV Specific energy for frictionFrictionPo wer Pf FVcFVcFruf wtoV wtoTotal specific energyRequired motor power,where em is efficiencyut us u fPPm em
Friction Analysis based on assumption that shear angle adjusts itself tominimize cutting energya Friction angle, tan f 45 2 2 Coefficient of friction, Thus, as rake angle decreases or as friction along the tool chipinterface (rake face) increases, the shear angle decreases and thechip becomes thicker Thicker chips dissipate more energy because the shear strain ishigher,thus thetemperatureVriseis higherc
Rake Face Friction
Chip Curl Chips develop a curvature as they leave the workpiece material formetals and plastics Reasons are not fully understood Affected by process variables, material properties, and cutting fluids Decreased depth of cut increases curliness which increases theeffective rake angle and decreases frictionFigure 20.8 Various chips produced in turning: (a) tightly curled chip; (b) chip hits workpiece andbreaks; (c) continuous chip moving away from workpiece; and (d) chip hits tool shank and breaksoff. Source: G. Boothroyd, Fundamentals of Metal Machining and Machine Tools. Copyright 1975; McGraw-Hill Publishing Company. Used with permission.
Oblique Cutting Most tools are three-dimensional (oblique)Cutting edge is now at an inclination angle, iChip moves away sideways, as with an angled snow plowan is the normal rake angle, a property of tool geometryac has been shown experimentally to be about equal to I 122Effective rake anglea e sin (sin i cos i sin a n )
Cutting Temperatures The energy dissipated is converted into heat which raises thetemperature in the cutting zone in the shear zonestrain rates in machining operations are very highalong the tool-chip interfacePossibly where a dull tool rubs against the machined surface Increased temperatures: Adversely affects strength, hardness, and wear resistance of cutting toolCauses dimensional changes in the workpieceInduces thermal damage to the machined surface, affecting its propertiesCauses distortion of the tool or machine, and thus poor dimensional control Temperature is proportional to cutting speed and feed (depth of cut),and dependent upon the materials of the tool and workpieceMeanTemp V a f b Temperatures are also affected by the use of cutting fluids (coolants!) High temperature workpieces and chips are a safety issue foroperators
Cutting Temperatures Most heat generated is carried away by the chip High speed machining has advantages due to economics and alsobecause as cutting speed increases, a larger proportion of the heatgenerated is carried away by the chipPercentage ofthe heatgenerated incutting goinginto theworkpiece,tool, and chip,as a functionof cuttingspeed.Typical temperature distribution the cutting zone. Note the steeptemperature gradients within the tool and the chip. Source: G. Vieregge.
Tool Wear Tool wear is gradual and depends on tool and workpiece materials,tool shape, cutting fluids, process parameters, and machine tools Two basic types of wear: Flank wear and Crater wear(b)(a)(c)RakeFaceFlankFace(d)(e)Figure 20.15 (a) Flankand crater wear in acutting tool. Tool movesto the left. (b) View ofthe rake face of a turningtool, showing noseradius R and crater wearpattern on the rake faceof the tool. (c) View ofthe flank face of aturning tool, showing theaverage flank wear landVB and the depth-of-cutline (wear notch). Seealso Fig. 20.18. (d)Crater and (e) flank wearon a carbide tool.Source: J.C. Keefe,Lehigh University.
Types of Tool Wear Abrasive wear – material loss due to hardparticles Adhesive wear – frictional contact betweensurfaces Diffusion related wear – degradation oftool material resulting from solid statediffusion
ToolFailureIn addition togradual wearproblems,cutting toolscan also failsuddenly bychipping,caused bymechanicalshock (impactdue tointerruptedcutting) orthermalfatigue.
Tool Wear Curve
Tool Life Flank wear is caused by the tool rubbing along the machined surface causing adhesive or abrasivewear High temperatures affecting tool-material properties and workpiece surface Taylor Tool Life Formula VT CnC is a constantV is cutting speedT is time (minutes) to develop a certain flank wear land (VB on previous slide)Exponent n depends on tool and workpiece materials and cutting conditionsC and n are determined experimentally Depth of cut and feed rate also influence tool lifeVT n d x f y C For constant tool life, increasing f or d means decreasing V Depending on exponents, a reduction in speed can result in anincrease in material removal
Tool Life CurvesTool Lifecurves shouldnot beextrapolated! Plots of experimental data for specificmaterials and conditionsGenerally plotted on log-log paper andused to determine nTABLE 20.3 Range of n Values for Eq.(20.20) for Various ToolMaterialsHigh-speed steels0.08–0.2Cast alloys0.1–0.15Carbides0.2–0.5Ceramics0.5–0.7
Wear Land Resharpen or replace tools when Workpiece surface finish deterioratesCutting forces increase significantlyTemperature rises significantlyAllowable wear land is exceeded Recommended cutting speed for high-speed steel tools yields a toollife of 60-120 minutes Recommended cutting speed for carbide tools yields a tool life of 3060 minutesTABLE 20.4 Allowable Average Wear Land (VB)for Cutting Tools in Various OperationsAllowable wear land (mm)OperationHigh-speed SteelsCarbidesTurning1.50.4Face milling1.50.4End milling0.30.3Drilling0.40.4Reaming0.150.15Note: 1 mm 0.040 in.
Crater Wear and Monitoring Tool Wear Crater wear occurs on the rake face of the tool and changes the chip-toolinterface geometryCrater wear is caused by Temperature at the chip-tool interface Chemical affinity between tool and workpiece materials Factors that affect flank wear, to a lesser extent Relationship between crater-wear rate and averagetool-chip interface temperature: (a) High-speed steel;(b) C-1 carbide; and (c) C-5 carbide. Note howrapidly crater-wear rate increases as the temperatureincreases. Source: B. T. Chao and K. J. Trigger.Monitor tool wear by Direct observation with atoolmakers microscope (offline) Indirectly monitoring forces,vibrations, torque, etc, Indirectly using the acousticemission technique, using apiezoelectric transducer attachedto the tool holder Using established tool-cycle times
Requirements of Tool Materials High temperature physical and chemicalstability – particularly strength andhardness High wear resistance Resistance to brittle fracture Obtaining high performance in all thesecharacteristics is difficult – hard strongmaterials tend to be brittle
Main Classes of Tool Material
Modern Cutting Tools
Materials Used in Cutting Tools
Cutting ToolsThe hardness of variouscutting-tool materials as afunction of temperature (hothardness). The wide range ineach group of materials is dueto the variety of toolcompositions and treatmentsavailable for that group.
Cutting Tool PropertiesTable 21.1CarbidesPropertyHardnessHigh-speedsteels83– 86 HRACast alloys82– 84 HRA46– 62 HRCWC90– 95 HRA1800– 2400 HKTiC91– 93 HRA1800– 3200 HKCompressive strengthMPa4100– 45001500– 23004100– 58503100– 38503psi x10600– 650220– 335600– 850450– 560Transverse rupture strengthMPa2400– 48001380– 20501050– 26001380– 19003psi x10350– 700200– 300150– 375200– 275Impact strengthJ1.35– 80.34– 1.250.34– 1.350.79– 1.24in.- lb12– 703– 113– 127– 11Modulus of elasticityGPa200–520– 690310– 450630–75– 10045– 65psi x10Density3kg/m86008000– 870010,000– 15,0005500– 58003lb/in.0.310.29– 0.310.36– 0.540.2– 0.22Volume of hard phase, %7– 1510– 2070– 90–Melting or decompositiontemperature1300–14001400 C F2370–25502550Thermal conductivity, W/30– 50–42– 12517mKCoefficient of thermal12–4– 6.57.5– 9–6expansion, x10 C*The values for polycrystalline diamond are generally lower, except impact strength, which is higher.Cubic boronnitride4000– 5000 HKSingle-crystaldiamond*7000– 8000 HK2750– 4500400– 6506900100069001000345– 95050– 1357001051350200 0.1 1 0.5 5 0.2 2310– 41045– 60850125820– 1050120– 1504000– 45000.14– 1300500– 20006– 8.54.81.5– 4.8Ceramics91– 95 HRA2000– 3000 HK
Cutting Tool InsertsTypical carbide inserts with various shapes andchip-breaker features; round inserts are alsoavailable. The holes in the inserts arestandardized for interchangeability. Source:Courtesy of Kyocera Engineered Ceramics,Inc., and Manufacturing EngineeringMagazine, Society of Manufacturing Engineers.Inserts with polycrystalline cubic boron nitride tips(top row) and solid polycrystalline cBN inserts(bottom row). Source: Courtesy of Valenite.
Machine Tool Cost and Failure RiskTABLE 21.6ToolHigh-speed steel tool bitsCarbide-tipped (brazed) tools for turningCarbide inserts, square 3/16"thickPlainCoatedCeramic inserts, squareCubic boron nitride inserts, squareDiamond-coated insertsDiamond-tipped inserts (polycrystalline)Size (in.)1/4 sq.x 2 1/2 long1/2 sq. x 41/4 sq.3/4 sq.Cost ( )1–23–7241/2 inscribed circle5–96–108–1260–9050–6090–100 11/2 inscribed circle1/2 inscribed circle1/2 inscribed circle1/2 inscribed circleRelative edgestrength andtendency forchipping andbreaking of insetswith various shapes.Strength refers to thecutting edge shownby the includedangles. Source:Kennametal, Inc.
Machine Tool CoatingsMultiphase coatings on a tungsten-carbide substrate. Three alternating layers of aluminum oxide areseparated by very thin layers ot titanium nitride. Inserts with as many as thirteen layers of coatingshave been made. Coating thicknesses are typically in the range of 2 to 10 m. Source: Courtesy ofKennametal, Inc., and Manufacturing Engineering Magazine, Society of Manufacturing Engineers.
Cutting Fluids Reduce friction and wear – improved toollife and surface finish; reduce forces andpower required Cool cutting zone – reducing temperaturesand improving tool life Flush away chips from the cutting zone Represent a significant cost – may be asmuch as 15-18% of overall costs Can be an environmental hazard
Types of Cutting Fluid Oils (straight oils) – mineral, animal,vegetable, synthetics – mainly slow speedoperations Emulsions (soluble oils) – mixture of oil,water and additives – coolants Semi-synthetics – chemical emulsionscontaining little oil diluted with water,including additives to reduce oil particlesize. Synthetics – chemicals with additives,diluted with water and containing no oil
Cutting Fluid Application Flooding – low pressure jets; mostcommon method of application Mist – atomized with air; good accessibility High pressure systems – for improvedheat dissipation; may be refrigerated Through the cutting tool – improvedlubrication and flushing; particularly drillingapplications
Cutting Fluid ApplicationSchematic illustration ofproper methods of applyingcutting fluids in variousmachining operations: (a)turning, (b) milling, (c) threadgrinding, and (d) drilling.
Chip Types in Machining
Basic Chip Breaker Types
Modern Chip Breaking Geometries
Surface Finish – Dull Tools and Feed MarksSchematic illustration of a dull tool inorthogonal cutting (exaggerated). Notethat at small depths of cut, the positiverake angle can effectively becomenegative, and the tool may simply rideover and burnish the workpiece surface.Turning and other cutting operationsleave a spiral profile on the machinedsurface, especially at higher feed rates(f) and smaller tool-nose radii (R)
Geometric Surface RoughnessUnderlying surface roughnessdepends upon tool geometry and feedRoughness is reduced by lowering thefeed rate
Cutting Speed and Surface RoughnessRoughness is improved at higher cutting speeds
Machine Tool Economics Cost Components (labor,overhead, and machine-tool costsin each category below) Nonproductive cost –during set-up,fixturing, advancing and retractingcutting tool Machining cost – while cutting isoccurring Tool-change cost – during toolchanges Cutting tool cost – only 5% of costsGraphs showing (a) costper piece and (b) timeper piece in machining.Note the optimumspeeds for both cost andtime. The range betweenthe two is known as thehigh-efficiencymachining range.
Tool wear is gradual and depends on tool and workpiece materials, tool shape, cutting fluids, process parameters, and machine tools Two basic types of wear: Flank wear and Crater wear Tool Wear (d) (e) (a) (b) (c) Figure 20.15 (a) Flank and crater wear in a cutting tool. Tool
There are different types of machining process used for sapphire material. The fig. 1 shows a graphical representation of sapphire machining processes i.e. laser machining process, grinding process, polishing process, lapping process, new developed machining process, compound machining process and electro discharge machining process. Fig.1.
Machining metals follows a predictable pattern with minimal creep. When machining plastics, quick adjustments must be made to accommodate substantial creep — not to mention that the material has a strong propensity for chipping and melting during machining. Simply stated, the basic principles of machining metals do not apply when machining
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Machining metals follows a predictable pattern with minimal creep. When machining plastics, quick adjustments must be made to accommodate substantial creep — not to mention that the material has a strong propensity for chipping and melting during machining. Simply stated, the basic principles of machining metals do not apply when machining
where the use of 5-axis simultaneous machining brings unequalled surface quality. Moreover, it is targeted at prototype machining, 5-axis trimming and special machining where full 5-axis machining is the requirement for quick and accurate manufacturing. Multi-Axis Surface Machining is also an add-on product to Prismatic Machining and Lathe .
a result, a new class of machining processes has evolved over a period of time to meet such demands, named non-traditional, unconventional, modern or advanced machining processes [1–3]. These advanced machining processes (AMP) become still more important when one considers precision and ultra-precision machining.
Advanced machining processes (AMPs) are widely utilized in industries for machining complex geome-tries and intricate profiles. In this paper, two significant processes such as electric discharge machining (EDM) and abrasive water jet machining (AWJM) are considered to get the optimum values of responses for the given range of process parameters.
The milling machining is limited to the XY plane and can thus follow 2-dimensional contours. The machining itself is limited to 2-dimensional contours. The third dimension is achieved by tilting and securing the machining plane. To machine the free -form surfaces, the 5 axes move dynamically and simultaneously. 2D machining . 2½D machining
