2TOPICS Introduction Mechanics of chip formation Types of chips produced in meta cuttingMechanics of oblique cuttingCutting forces and powerTemperature in cuttingTool life : Wear and failureSurface finish and integrityMachinability
3Fundamentals of cutting Fig 20.1 Examples of cutting processFig 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 20.2 Basic principle of turning operation
4Introduction :Cutting process : Remove material from the surface of the work piece by producing chipsTurning operation : the work piece is rotated an a cutting tool removes a layer of material as it moves to the leftCutting 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 pieceEnd-milling rotating cutter travels along a certain depth in the work piece and produces a cavity
5Factors influencing cutting process ParameterInfluence 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.Related to tool life,surface finish,forces and power
6Mechanics of chip formation : Orthogonal cuttingRake angle – AlphaRelief angle ( clearance angle)Shear angle ( Pi)Thickness of a chip – TcDepth of cut- T0Cutting ratio r = To / Tc= Sin Pi / Cos ( pi- Alpha )
7Mechanism of chip formation Fig 20.4 (a) Schematic illustration of the basic mechanism of chip formation in metal cutting. (b) Velocity diagram in the cutting zone.
8Mechanism of chip formation Chip compression ratio = 1 / rAlways > unityOn the basis of fig 20.4-aShear strain gamaGama = AB/OC = AO/OC + OB/OCGama = Cot Pi + tan ( Pi – Alpha )Note : for actual cutting operation shear strain > 5
9Mechanism of chip formation Shear angle adjusts itself to minimize cutting forceShear plane is the plane of maximum shear stressPi = 45 + Alpha / 2 – Beta / 2Beta : Friction angleMu – coefficient of frictionMu = tan beta
10Mechanism of chip formation Mass continuity has to be maintainedSo , we haveV To = Vc TcVc = VrVc = V Sin pi / Cos ( pi – Alpha )Vc : Velocity of a chipV : Cutting SpeedVs : Velocity of shearingFrom trigonometric relationV / cos ( pi – Alpha ) = Vs / Cos ( Alpha ) = Vc / Sin ( pi )
11Types of chips Continuous Built up edge Serrated or segmented DiscontinuousFig20.5 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 chips
12Continuous chips(b) Surface finish in turning 5130 steel with a built-up edgeFig :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(c) Surface finish on 1018 steel in face milling
13Continuous chipsContinuous 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 tooloperation has to be stopped to clear away the chips.
14Built-up edges chipsBUE consists of layers of material from the workpiece that are gradually deposited on the tool.BUE then becomes unstable and eventually breaks upBUE material is carried away on the tool side of the chipthe rest is deposited randomly on the workpiece surface.BUE results in poor surface finishreduced by increasing the rake angle and therefore decreasing the depth of cut.
15Discontinuous chipsDiscontinuous chips consist of segments that may be firmly or loosely attached to each otherThese chips occur when machining hard brittle materials such as cast iron.Brittle failure takes place along the shear plane before any tangible plastic flow occursDiscontinuous chips will form in brittle materials at low rake angles (large depths of cut).
16Serrated chips Figure :20.5e Segmented chips or non-homogeneous chips Semi continuous chips with zones low and high shear strainLow thermal conductivity and strength metals exhibit this behaviorFig 20.5 (e)segmented or nonhomogeneous chip and
17Chip 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 itChips 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 breakers
18Chip 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 off
19Chip Formation in Nonmetallic Materials Fig: a) cutting with an oblique tool b) Top view showing the inclination angle, i. c) Types of chips produced with different inclination
20Mechanism of Oblique Cutting The cutting edge is at an angle i, called inclination angle.The chip movement is in lateral directionFig: a)right hand cutting tool.Although these tools have traditionally been produced from solids tool-steel bars,they have been largely replaced by carbide or other inserts of various shapes and sizes,as shownin b).The vcarious angles on these tools and their effects on machining are described
21Temperature In Cutting Fig:Percentage of the heat generated in cutting going into the workpiece,tool,and chip,as a function of cutting speed.Fig:Typical temperature distribution in the cutting zone.
22Temperature Distributions Fig:Temperatures developed in turning steel: a) flank temperature distribution;and b)tool-chip interface temperature distribution
23Tool Life: Wear and Failure Flank wear :It occurs on the relief face of the tool and the side relief angle.Crater wear:It occurs on the rake face of the tool.Chipping :Breaking away of a small piece from the cutting edge of the tool .Fig (a) Flank and crater wear in a cutting tool.tool moves to the left. (b) View of the rake of a turning tool,showing nose radius R and crater wear pattern on the rake face of the tool c)View of the flank face of a turning tool,sowing the average flank wear land VB and the depth-of-cut line (wear notch)
24Wear and Tool Failures: Crater wear Fig (a) Schematic illustrations of types of wear observed on various types of cutting tools .(b) Schematic illustrations of catastrophic tool failures.A study of the types and mechanism of tool wear and failure is essential to the development of better tool materials
25Forces acting in 2-Dimensional cutting Cutting forces can be measured by using suitable dynamometers or force transducers mounted on the machine toolThey can also be calculated from the amount of power consumption,that occurs during cutting.Fig :Forces acting on a cutting tool in a two dimensional cutting .Note that the resultant force,R,must be collinear to balance the forces