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PowerPoint PresentationSlide 2Fundamentals of cuttingIntroduction :Factors influencing cutting processMechanics of chip formation :Mechanism of chip formationSlide 8Slide 9Slide 10Types of chipsContinuous chipsSlide 13Built-up edges chipsDiscontinuous chipsSerrated chipsChip BreakersSlide 18Chip Formation in Nonmetallic MaterialsMechanism of Oblique CuttingTemperature In CuttingTemperature DistributionsTool Life: Wear and FailureWear and Tool Failures: Crater wearForces acting in 2-Dimensional cuttingTHE ENDChapter-20Fundamentals of cutting•Introduction•Mechanics of chip formation•Types of chips produced in meta cutting•Mechanics of oblique cutting•Cutting forces and power•Temperature in cutting•Tool life : Wear and failure•Surface finish and integrity•MachinabilityTOPICSFundamentals of cutting•Fig 20.3 Schematic illustration of a two-dimensional cutting process,also called orthogonal cutting.Note that the tool shape and its angles,depth of cut,to,and the cutting speed are all independent variables.Fig 20.1 Examples of cutting processFig 20.2 Basic principle of turning operationIntroduction :•Cutting process : Remove material from the surface of the work piece by producing chips•Turning operation : the work piece is rotated an a cutting tool removes a layer of material as it moves to the left•Cutting off: Cutting tool moves radially inwards and separated the right piece from the back of the blank.•Slab-milling rotating cutting tool removes a layer of material from the surface of the work piece•End-milling rotating cutter travels along a certain depth in the work piece and produces a cavityFactors influencing cutting processParameter Influence and interrelationshipCutting speed depth of cut,feed,cutting fluids.Tool anglesContinuous chipBuilt-up-edge chipDiscontinuous chipTemperature rise.Tool wearMachinabilityForces power,temperature rise,tool life,type of chips,surface finish.As above;influence on chip flow direction;resistance to tool chipping.Good surface finish;steady cutting forces;undesirable in automated machinery.Poor surface finish,thin stable edge can product tool surface.Desirable for ease of chip disposal;fluctuating cutting forces;can affect surface finish and cause vibration and chatters.Influences surface finish,dimensional accuracy,temperature rise,forces and power.Influences surface finish,dimensional accuracy,temperature rise,forces and power.Related to tool life,surface finish,forces and powerMechanics of chip formation :•Orthogonal cutting•Rake angle – Alpha•Relief angle ( clearance angle)•Shear angle ( Pi)•Thickness of a chip – Tc•Depth of cut- T0•Cutting ratio r = To / Tc = Sin Pi / Cos ( pi- Alpha )Mechanism of chip formationFig 20.4 (a) Schematic illustration of the basic mechanism of chip formation in metal cutting. (b) Velocity diagram in the cutting zone.Mechanism of chip formation•Chip compression ratio = 1 / r•Always > unity•On the basis of fig 20.4-a•Shear strain gama•Gama = AB/OC = AO/OC + OB/OC•Gama = Cot Pi + tan ( Pi – Alpha )•Note : for actual cutting operation shear strain > 5Mechanism of chip formation•Shear angle adjusts itself to minimize cutting force•Shear plane is the plane of maximum shear stress•Pi = 45 + Alpha / 2 – Beta / 2 •Beta : Friction angle •Mu – coefficient of friction•Mu = tan betaMechanism of chip formation•Mass continuity has to be maintained •So , we have •V To = Vc Tc•Vc = Vr•Vc = V Sin pi / Cos ( pi – Alpha )•Vc : Velocity of a chip•V : Cutting Speed •Vs : Velocity of shearing•From trigonometric relation•V / cos ( pi – Alpha ) = Vs / Cos ( Alpha ) = Vc / Sin ( pi )Types of chips•Continuous•Built up edge•Serrated or segmented •Discontinuous Fig20.5 Basic types of chips and their photomicrographs produced in metal cutting (a) continuous ship with a narrow,straight primary shear zone; (b) secondary shear zone at the chip tool interface;(c) continuous chip with large primary shear zone; (d) continuous chip with built-up-edge;(e) segmented or nonhomogeneous chip and (f) discontinuous chipsContinuous chips Fig :20.6 (a) Hardness distribution in the cutting zone for 3115 steel.Note that some regions in the built-up edge are as mach as three times harder than the bulk metal(b) Surface finish in turning 5130 steel with a built-up edge(c) Surface finish on 1018 steel in face milling•Continuous chips are usually formed at high rake angles and/or high cutting speeds.•A good surface finish is generally produced.•continuous chips are not always desirable, particularly in automated machine tools,•tend to get tangled around the tool•operation has to be stopped to clear away the chips.Continuous chipsBuilt-up edges chips•BUE consists of layers of material from the workpiece that are gradually deposited on the tool.•BUE then becomes unstable and eventually breaks up•BUE material is carried away on the tool side of the chip•the rest is deposited randomly on the workpiece surface.•BUE results in poor surface finish•reduced by increasing the rake angle and therefore decreasing the depth of cut.Discontinuous chips •Discontinuous chips consist of segments that may be firmly or loosely attached to each other•These chips occur when machining hard brittle materials such as cast iron.•Brittle failure takes place along the shear plane before any tangible plastic flow occurs•Discontinuous chips will form in brittle materials at low rake angles (large depths of cut).Serrated chips •Figure :20.5e•Segmented chips or non-homogeneous chips •Semi continuous chips with zones low and high shear strain•Low thermal conductivity and strength metals exhibit this behaviorFig 20.5 (e)segmented or nonhomogeneous chip andChip Breakers•Long continuous chip are undesirable•Chip breaker is a piece of metal clamped to the rake surface of the tool which bends the chip and breaks it•Chips can also be broken by changing the tool geometry,thereby controlling the chip flowFig 20.7 (a) Schematic illustration of the action of a chip breaker .(b) Chip breaker clamped on the rake of a cutting tool. (c) Grooves in cutting tools acting as chip breakersChip BreakersFig:Various chips produced in turning: a)tightly curled chip b)chip hits workpiece and breaks c)continuous chip moving away from workpiece;and d)chip hits tool shank and breaks offChip Formation in Nonmetallic MaterialsFig: a) cutting with an oblique


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UL Lafayette MCHE 365 - Fundamentals of cutting

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