Ch 21 Machining

THEORY OF METAL MACHINING

2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Material Removal ProcessesAlthough Machining is a general term used to describe removal of material from a work piece, it cover several processes, which are usually divided into following broad categories.Machining/Cutting which generally involves single-point or multi-point tools and processes, such as turning, boring, drilling, milling, sawing and broaching;

Abrasive processes material removal by hard, abrasive particles, e.g., grinding, honing, lapping and super finishing;

Non-Traditional processes which uses various sources of energy to remove material from the work piece surface


Cutting action involves shear deformation of work material to form a chip As chip is removed, new surface is exposedFigure. (a) A crosssectional view of the machining process, (b) tool with negative rake angle; compare with positive rake angle in (a).Machining


Why Machining is ImportantVariety of work materials can be machinedMost frequently used to cut metalsVariety of part shapes and special geometric features possible, such as:Screw threadsAccurate round holesVery straight edges and surfacesGood dimensional accuracy and surface finish


Disadvantages with MachiningWasteful of material Chips generated in machining are wasted material, at least in the unit operation Time consuming A machining operation generally takes more time to shape a given part than alternative shaping processes, such as casting, powder metallurgy, or forming


Machining in Manufacturing SequenceGenerally performed after other manufacturing processes, such as casting, forging, and bar drawing Other processes create the general shape of the starting workpartMachining provides the final shape, dimensions, finish, and special geometric details that other processes cannot create


Machining OperationsMost important machining operations:TurningDrillingMillingOther machining operations:Shaping and planingBroachingSawing


Fundamentals of cuttingFig 20.1 Examples of cutting process


Single point cutting tool removes material from a rotating workpiece to form a cylindrical shape Figure 21.3 Three most common machining processes: (a) turning,Turning


Used to create a round hole, usually by means of a rotating tool (drill bit) with two cutting edgesFigure 21.3 (b) drilling,Drilling


Rotating multiple-cutting-edge tool is moved across work to cut a plane or straight surfaceTwo forms: peripheral milling and face millingFigure 21.3 (c) peripheral milling, and (d) face milling.Milling


Cutting Tool ClassificationSingle-Point ToolsOne dominant cutting edgePoint is usually rounded to form a nose radiusTurning uses single point toolsMultiple Cutting Edge ToolsMore than one cutting edgeMotion relative to work achieved by rotating Drilling and milling use rotating multiple cutting edge tools


Figure 21.4 (a) A singlepoint tool showing rake face, flank, and tool point; and (b) a helical milling cutter, representative of tools with multiple cutting edges.Cutting Tools


Cutting Conditions in MachiningThree dimensions of a machining process: Cutting speed v primary motionFeed f secondary motionDepth of cut d penetration of tool below original work surfaceFor certain operations, material removal rate can be computed as RMR = v f d where v = cutting speed; f = feed; d = depth of cut


Cutting Conditions for TurningFigure 21.5 Speed, feed, and depth of cut in turning.


Roughing vs. FinishingIn production, several roughing cuts are usually taken on the part, followed by one or two finishing cuts Roughing - removes large amounts of material from starting workpartCreates shape close to desired geometry, but leaves some material for finish cuttingHigh feeds and depths, low speedsFinishing - completes part geometryFinal dimensions, tolerances, and finishLow feeds and depths, high cutting speeds


Machine Tools A powerdriven machine that performs a machining operation, including grinding Functions in machining:Holds workpartPositions tool relative to workProvides power at speed, feed, and depth that have been set The term is also applied to machines that perform metal forming operations


Simplified 2-D model of machining that describes the mechanics of machining fairly accurately Figure 21.6 Orthogonal cutting: (a) as a threedimensional process. Orthogonal Cutting Model


Chip Thickness Ratiowhere r = chip thickness ratio; to = thickness of the chip prior to chip formation; and tc = chip thickness after separationChip thickness after cut always greater than before, so chip ratio always less than 1.0


Determining Shear Plane AngleBased on the geometric parameters of the orthogonal model, the shear plane angle can be determined as: where r = chip ratio, and = rake angle


Figure 21.7 Shear strain during chip formation: (a) chip formation depicted as a series of parallel plates sliding relative to each other, (b) one of the plates isolated to show shear strain, and (c) shear strain triangle used to derive strain equation.Shear Strain in Chip Formation


Shear StrainShear strain in machining can be computed from the following equation, based on the preceding parallel plate model:

= tan( - ) + cot

where = shear strain, = shear plane angle, and = rake angle of cutting tool


Figure 21.8 More realistic view of chip formation, showing shear zone rather than shear plane. Also shown is the secondary shear zone resulting from toolchip friction.Chip Formation


Four Basic Types of Chip in MachiningDiscontinuous chipContinuous chipContinuous chip with Built-up Edge (BUE)Serrated chip


Types of chipsContinuousBuilt up edgeSerrated 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 chips


Brittle work materialsLow cutting speedsLarge feed and depth of cutHigh toolchip friction

Figure 21.9 Four types of chip formation in metal cutting: (a) discontinuousDiscontinuous Chip


Ductile work materialsHigh cutting speedsSmall feeds and depthsSharp cutting edgeLow toolchip friction

Figure 21.9 (b) continuousContinuous Chip


Continuous chips Continuous chips are typically forms at high cutting speeds and/or high rake anglesThe deformation of the material takes place along a very narrow shear zone called the primary shear zoneA good surface finish is generally produced.continuous chips are not always desirable, particularly in automated machine tools


Discontinuous chips Discontinuous 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).


Ductile materialsLowtomedium cutting speedsTool-chip friction causes portions of chip to adhere to rake faceBUE forms, then breaks off, cyclicallyFigure 21.9 (c) continuous with builtup edgeContinuous with BUE


Semicontinuous - saw-tooth appearanceCyclical chip forms with alternating high shear strain then low shear strain Associated with difficult-to-machine metals at high cutting speeds Serrated ChipFigure 21.9 (d) serrated.


Serrated chips Segmented chips or non-homogeneous chips Semi continuous chips with zones low and high shear strainLow thermal conductivity and strength metals exhibit this behavior


Friction force F and Normal force to friction N Shear force Fs and Normal force to shear Fn Figure 21.10 Forces in metal cutting: (a) forces acting on the chip in orthogonal cuttingForces Acting on Chip


Resultant ForcesVector addition of F and N = resultant RVector addition of Fs and Fn = resultant R' Forces acting on the chip must be in balance:R' must be equal in magnitude to R R must be opposite in direction to RR must be collinear with R


Coefficient of FrictionCoefficient of friction between tool and chip: Friction angle related to coefficient of friction as follows:


Shear StressShear stress acting along the shear plane: where As = area of the shear planeShear stress = shear strength of work material during cutting


F, N, Fs, and Fn cannot be directly measuredForces acting on the tool that can be measured:Cutting force Fc and Thrust force Ft Figure 21.10 Forces in metal cutting: (b) forces acting on the tool that can be measuredCutting Force and Thrust Force


Forces in Metal Cutting Equations can be derived to relate the forces that cannot be measured to the forces that can be measured:F = Fc sin + Ft cosN = Fc cos Ft sin Fs = Fc cos Ft sinFn = Fc sin + Ft cosBased on these calculated force, shear stress and coefficient of friction can be determined


The Merchant Equation Of all the possible angles at which shear deformation can occur, the work material will select a shear plane angle that minimizes energy, given by

Derived by Eugene MerchantBased on orthogonal cutting, but validity extends to 3-D machining


What the Merchant Equation Tells Us To increase shear plane angle Increase the rake angle Reduce the friction angle (or coefficient of friction)


Higher shear plane angle means smaller shear plane which means lower shear force, cutting forces, power, and temperatureFigure 21.12 Effect of shear plane angle : (a) higher with a resulting lower shear plane area; (b) smaller with a corresponding larger shear plane area. Note that the rake angle is larger in (a), which tends to increase shear angle according to the Merchant equationEffect of Higher Shear Plane Angle


Power and Energy Relationships A machining operation requires powerThe power to perform machining can be computed from: Pc = Fc v where Pc = cutting power; Fc = cutting force; and v = cutting speed


Power and Energy Relationships In U.S. customary units, power is traditional expressed as horsepower (dividing ftlb/min by 33,000) where HPc = cutting horsepower, hp


Power and Energy Relationships Gross power to operate the machine tool Pg or HPg is given by

or where E = mechanical efficiency of machine tool Typical E for machine tools 90%


Unit Power in Machining Useful to convert power into power per unit volume rate of metal cutCalled unit power, Pu or unit horsepower, HPu

orwhere RMR = material removal rate


Specific Energy in MachiningUnit power is also known as the specific energy UUnits for specific energy are typically Nm/mm3 or J/mm3 (inlb/in3)


Cutting Temperature Approximately 98% of the energy in machining is converted into heatThis can cause temperatures to be very high at the toolchip The remaining energy (about 2%) is retained as elastic energy in the chip


Cutting Temperatures are ImportantHigh cutting temperatures Reduce tool lifeProduce hot chips that pose safety hazards to the machine operatorCan cause inaccuracies in part dimensions due to thermal expansion of work material


Cutting TemperatureAnalytical method derived by Nathan Cook from dimensional analysis using experimental data for various work materialswhere T = temperature rise at toolchip interface; U = specific energy; v = cutting speed; to = chip thickness before cut; C = volumetric specific heat of work material; K = thermal diffusivity of work material


2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/eCutting TemperatureExperimental methods can be used to measure temperatures in machining Most frequently used technique is the toolchip thermocouple Using this method, Ken Trigger determined the speedtemperature relationship to be of the form: T = K vm where T = measured toolchip interface temperature, and v = cutting speed


Ch 21 Machining

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