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Machining ProcessMachining Process

R. I. R. I. SujithSujithDept. of Aerospace EngineeringDept. of Aerospace Engineering

IIT MadrasIIT Madras

Fabrication of detailed partsFabrication of detailed parts

There are two broad groups of processes There are two broad groups of processes for fabricating detail parts of an a/cfor fabricating detail parts of an a/c

metal cutting or machining processmetal cutting or machining processdesired shape is obtained by removal of metal desired shape is obtained by removal of metal from the work piecefrom the work piece

metal forming processmetal forming processmetal is conserved, metal is conserved, shape is obtained by plastic deformation of the shape is obtained by plastic deformation of the materialmaterial

Metal Cutting OperationsMetal Cutting Operations

Also referred to as machining operations includeAlso referred to as machining operations includeturningturningreamingreamingboringboringmilling milling BroachingBroachingGrindingGrindingHoningHoningLappingLappingdrillingdrilling

MachiningMachining

Machining is resorted to depending on the Machining is resorted to depending on the shape of the component and when close shape of the component and when close dimensional tolerances/surface finishes dimensional tolerances/surface finishes are requiredare required

PrinciplePrinciple

By the shearing action caused by relative motion By the shearing action caused by relative motion between the work piece and a cutting tool with between the work piece and a cutting tool with sharp edge.sharp edge.The work piece is generally held in a fixture or The work piece is generally held in a fixture or clamped on to the machine tool chuck/tableclamped on to the machine tool chuck/tableThe relative motion can be controlled by an The relative motion can be controlled by an operator or in the case of CNC machines, by operator or in the case of CNC machines, by computer controlcomputer control

Other process for metal removalOther process for metal removal

Electric discharge machining (EDM) for eroding hard Electric discharge machining (EDM) for eroding hard materialmaterialElectro Chemical Milling (ECM) for fabricating complex Electro Chemical Milling (ECM) for fabricating complex profiles as in turbine bladesprofiles as in turbine bladesElectrochemical finishing (ECF) for superior surface finishElectrochemical finishing (ECF) for superior surface finishChemical milling for making pockets or grooves in sheet Chemical milling for making pockets or grooves in sheet metal panels/bulkheads.metal panels/bulkheads.Laser drilling for drilling holesLaser drilling for drilling holesWater jet machining for cutting of composite materialWater jet machining for cutting of composite material

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MachinabilityMachinability & Parameters & Parameters Affecting MachiningAffecting Machining

A measure of A measure of machinabilitymachinability is is ““specific cutting specific cutting energyenergy”” which is equal to F/BTwhich is equal to F/BT

F is the horizontal cutting forceF is the horizontal cutting forceB is the width of the chipB is the width of the chipT is the thickness of the chipT is the thickness of the chip

The chip is the layer of metal which is removed The chip is the layer of metal which is removed from the workfrom the work--piece just in front of the tool piece just in front of the tool cutting edge (due to shearing action)cutting edge (due to shearing action)

Factors affecting cutting energyFactors affecting cutting energy

Specific cutting energy depends on Specific cutting energy depends on the workthe work--piece mechanical properties, piece mechanical properties, the tool material & tool geometry, the tool material & tool geometry, cutting speeds and feeds, type of cuttingcutting speeds and feeds, type of cutting

continuous as in turning, or continuous as in turning, or intermittent as in milling), intermittent as in milling),

types of cutting fluid, types of cutting fluid, rigidity of machine tool rigidity of machine tool machining fixture or device machining fixture or device

in which the work piece is clamped.in which the work piece is clamped.

Speed & FeedSpeed & Feed

Speed is the surface velocity of relative Speed is the surface velocity of relative motion between the tool and the workmotion between the tool and the work--piece, measured in piece, measured in metresmetres/minute/minute

Feed or depth of cut is the advancement Feed or depth of cut is the advancement of the tool per revolution (of the workof the tool per revolution (of the work--piece/tool), measured in mmpiece/tool), measured in mm

Typical Speeds and FeedsTypical Speeds and Feeds

Al alloy using HSS toolAl alloy using HSS toolSpeed 300Speed 300--500 m/min, 500 m/min, Feed 0.3 mm for rough cut, 0.05 mm for Feed 0.3 mm for rough cut, 0.05 mm for finish cutfinish cut

Al alloy with carbide toolAl alloy with carbide toolSpeed 800Speed 800--1200 m/min1200 m/minFeed 0.2 mm for rough cut, 0.07 mm for Feed 0.2 mm for rough cut, 0.07 mm for finish cutfinish cut

Speed & Feed for SteelSpeed & Feed for Steel

Steel with HSS toolSteel with HSS toolSpeed 12Speed 12--17 m/min17 m/minFeed 0.15 mm for rough cut and 0.05 mm for Feed 0.15 mm for rough cut and 0.05 mm for finish cutfinish cut

Steel with carbide toolSteel with carbide toolSpeed 70Speed 70--100 m/min100 m/minFeed 0.15 mm for rough cut & 0.05 mm for Feed 0.15 mm for rough cut & 0.05 mm for finish cutfinish cut

Al Vs. SteelAl Vs. Steel

Material removal rate for Al is much higher Material removal rate for Al is much higher than for steelthan for steel

Metal removal rates for both materials are Metal removal rates for both materials are higher when machining with carbide tools higher when machining with carbide tools in comparison with HSS tool.in comparison with HSS tool.

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Cutting FluidsCutting Fluids

Heat is generated during cutting and the tool tip Heat is generated during cutting and the tool tip temperature rises, temperature rises,

which weakens the tool materials andwhich weakens the tool materials andshortens the tool lifeshortens the tool life

Amount of heat generated is a function of rate Amount of heat generated is a function of rate of metal removalof metal removalUse of cutting fluid helps to Use of cutting fluid helps to

conduct heat away from the tool tip & also conduct heat away from the tool tip & also reduces the friction of the chip with the tool face, reduces the friction of the chip with the tool face, and hence reduces tool wearand hence reduces tool wear

Striking a BalanceStriking a Balance

Machining cost, in general, reduces with Machining cost, in general, reduces with increasing cutting speedsincreasing cutting speedsHowever, with increasing speeds, the tool However, with increasing speeds, the tool life is reduced and the tool has to be life is reduced and the tool has to be removed for reremoved for re--sharpening/replacement sharpening/replacement which adds to the costwhich adds to the costA balance has to be struck between these A balance has to be struck between these two opposing factors.two opposing factors.

Typical Tolerances & Surface Typical Tolerances & Surface FinishesFinishes

Turning/boringTurning/boringTolerance Tolerance ++2020--30 30 µµm, Surface Finish 0.4m, Surface Finish 0.4--0.6 0.6 µµmm

DrillingDrillingTolerance Tolerance ++5050--75 75 µµm, Surface Finish 1.6m, Surface Finish 1.6--3.2 3.2 µµmm

ReamingReamingTolerance Tolerance ++88--12 12 µµm, Surface Finish 0.8m, Surface Finish 0.8--3.2 3.2 µµmm

MillingMillingTolerance Tolerance ++3030--50 50 µµm, Surface Finish 0.8m, Surface Finish 0.8--6 6 µµmm

BroachingBroachingTolerance Tolerance ++55--8 8 µµm, Surface Finish 0.8m, Surface Finish 0.8--3.2 3.2 µµmm

GrindingGrindingTolerance Tolerance ++44--5 5 µµm, Surface Finish 0.1m, Surface Finish 0.1--0.2 0.2 µµmm

Lapping/HoningLapping/HoningTolerance Tolerance ++44--5 5 µµm, Surface Finish 0.05m, Surface Finish 0.05--0.4 0.4 µµmm

Machining OperationsMachining Operations

TurningTurning

Turning operations generate external Turning operations generate external surfaces of revolution on rotating worksurfaces of revolution on rotating work--pieces using a traversing tool.pieces using a traversing tool.Turning machines Turning machines --> lathes.> lathes.Lathes are versatile machines and can be Lathes are versatile machines and can be adapted for drilling and boring operations adapted for drilling and boring operations also.also.

TurningTurning

Typical parts which are machined by turningTypical parts which are machined by turningshafts, axles, housings, conical parts, and as a first shafts, axles, housings, conical parts, and as a first operation prior to boringoperation prior to boring

Turning is more economical than milling and a Turning is more economical than milling and a better tolerance and finish is achieved.better tolerance and finish is achieved.Single point cutting tools are used for turning.Single point cutting tools are used for turning.

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DrillingDrilling

Is a process of producing holes, Is a process of producing holes, countersinking and spot facingcountersinking and spot facing

The cutting tool, called drill is usually held The cutting tool, called drill is usually held in the machine rotating spindle and the in the machine rotating spindle and the drill is forced into the workdrill is forced into the work--piece.piece.

ReamingReaming

Reaming is a Reaming is a finishing operationfinishing operation for for enlarging a drilled hole as to obtain close enlarging a drilled hole as to obtain close tolerance and good surface finishtolerance and good surface finish

The cutting tool is called a reamerThe cutting tool is called a reamer

BoringBoring

Boring is a process in which the internal surfaces of Boring is a process in which the internal surfaces of revolution are generated using a traversing tool.revolution are generated using a traversing tool.If boring is done in a lathe, the workIf boring is done in a lathe, the work--piece is held in the piece is held in the rotating chuck.rotating chuck.Where higher accuracy is required, machines specially Where higher accuracy is required, machines specially designed for boring are useddesigned for boring are used

In such machines, the workIn such machines, the work--piece is clamped to a table and the piece is clamped to a table and the cutting tool rotates and traverses the depth of the borecutting tool rotates and traverses the depth of the bore

Internal bores of undercarriage struts and hydraulic Internal bores of undercarriage struts and hydraulic system jacks are examples of components bored in system jacks are examples of components bored in horizontal boring machines.horizontal boring machines.

Jig BoringJig Boring

Jig boring machine is a vertical boring Jig boring machine is a vertical boring machine machine

designed for precision boring of components, designed for precision boring of components, mainly used for tooling applications and mainly used for tooling applications and for certain aerofor certain aero--engine components such as engine components such as casingscasings

MillingMilling

In this metal is removed by rotating a multiple In this metal is removed by rotating a multiple tool cutter.tool cutter.The cutting action is INTERMITTANT unlike in The cutting action is INTERMITTANT unlike in turning or boring.turning or boring.The workThe work--piece is clamped to the table of the piece is clamped to the table of the machine and the milling cutter is fixed to the machine and the milling cutter is fixed to the rotating spindle.rotating spindle.Milling operations are used for machining Milling operations are used for machining

flat surfaces or flat surfaces or ““cubic componentscubic components””curved surfaces of complex shapes.curved surfaces of complex shapes.

BroachingBroaching

Metal is removed by a rotating multiple Metal is removed by a rotating multiple tool cutter called a broach, against the tool cutter called a broach, against the workwork--piece surfacepiece surface

The cutter is pushed through or pulled The cutter is pushed through or pulled through a hole or across a surfacethrough a hole or across a surface

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BroachingBroaching

Close dimensional tolerances and good Close dimensional tolerances and good surface finish are achieved by broachingsurface finish are achieved by broaching

Slots, keyways and serrations in levers, Slots, keyways and serrations in levers, shafts are usually broachedshafts are usually broached

GrindingGrinding

Metal is removed by mechanical action of Metal is removed by mechanical action of abrasive particles bonded together in a abrasive particles bonded together in a grinding wheel.grinding wheel.Grinding is essentially a finishing process Grinding is essentially a finishing process for obtaining close tolerances and good for obtaining close tolerances and good surface finishsurface finish

Honing & LappingHoning & Lapping

These are finishing processes which are used to These are finishing processes which are used to obtain very good surface finish,obtain very good surface finish,even better than what can be achieved by grinding.even better than what can be achieved by grinding.

The peaks and valleys on a machined The peaks and valleys on a machined component are smoothed out in the component are smoothed out in the honing/lapping processhoning/lapping processBy the action of an abrasive medium suspended By the action of an abrasive medium suspended in a fluid or paste form which is caused to scrub in a fluid or paste form which is caused to scrub the surface of the component by relative motion the surface of the component by relative motion of the honing/lapping tool called a of the honing/lapping tool called a ‘‘stonestone’’ over over the surface of the component.the surface of the component.

Computer Numerical Control Computer Numerical Control MachinesMachines

NC MachinesNC Machines

Numerical Control (NC) machine tools were Numerical Control (NC) machine tools were developed in the early 1950s in the USA developed in the early 1950s in the USA to meet the critical developments for machining to meet the critical developments for machining aerospace components which call for aerospace components which call for

close dimensional tolerances, close dimensional tolerances, repeatable accuracy and repeatable accuracy and require considerable removal of stock and several require considerable removal of stock and several hundred positioning movements of the machine tool hundred positioning movements of the machine tool slides.slides.

Why NC?Why NC?

Manual operation under these conditions Manual operation under these conditions is tedious and are prone to error.is tedious and are prone to error.NC was developed to automate machine NC was developed to automate machine tools using electronic digital technology.tools using electronic digital technology.The information to actuate and control the The information to actuate and control the machine tool slides was coded numerically machine tool slides was coded numerically and hence the name and hence the name ‘‘numerical controlnumerical control’’machine.machine.

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CNC MachinesCNC Machines

With further development of NC With further development of NC technology, mini computers and technology, mini computers and microprocessors were incorporated in NC microprocessors were incorporated in NC machine tools machine tools

much of the control logic was realized through much of the control logic was realized through computer hardware and software.computer hardware and software.

Such improved machines are called Such improved machines are called Computer Numerical Machines (CNC).Computer Numerical Machines (CNC).

Driving the toolDriving the tool

In CNC machine, the machine tool slides In CNC machine, the machine tool slides are driven by AC or DC servomotorsare driven by AC or DC servomotors

Software drivenSoftware driven

A CNC program (software) provides A CNC program (software) provides instructions to the CNC controller, which instructions to the CNC controller, which sends signals to the servomotors driving sends signals to the servomotors driving the machine tool slides.the machine tool slides.The machine tool slides move in The machine tool slides move in accordance with the positioning accordance with the positioning information contained in the CNC information contained in the CNC program.program.

Feedback ControlFeedback Control

Feedback devices mounted in the machine Feedback devices mounted in the machine tool slides tool slides

measure the linear/angular displacement of measure the linear/angular displacement of the slidesthe slidesthis information is sent to the CNC controller this information is sent to the CNC controller which in turn sends the requisite signals to which in turn sends the requisite signals to the servomotors to ensure the desired the servomotors to ensure the desired positions of the machine tool slides are not positions of the machine tool slides are not overshotovershot

CNC Control & AccuracyCNC Control & Accuracy

Following the servomechanism principles, Following the servomechanism principles, the machine tool slides are caused to the machine tool slides are caused to move to the exact positions as per the move to the exact positions as per the instructions set in the CNC instructions set in the CNC programmeprogramme..

The positioning accuracy and repeatability The positioning accuracy and repeatability is of the order of 5 to 10 is of the order of 5 to 10 µµm.m.

CNC ControlCNC Control

The selection of spindle speeds, feeds, coolant, The selection of spindle speeds, feeds, coolant, fitment of required cutting tools in the machine fitment of required cutting tools in the machine tool spindle are under CNC control.tool spindle are under CNC control.The majority of CNC machines used for The majority of CNC machines used for machining are either machining centers or machining are either machining centers or turning centers.turning centers.

Machining centre: essentially a milling machine, but Machining centre: essentially a milling machine, but can also undertake boring, drilling & tapping can also undertake boring, drilling & tapping operationsoperationsTurning centre: A lathe, but can also undertake Turning centre: A lathe, but can also undertake drilling, tapping and milling operations.drilling, tapping and milling operations.

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Tool MagazineTool Magazine

A number of tools are stored in a tool magazine, A number of tools are stored in a tool magazine, attached to the CNC machine.attached to the CNC machine.The required cutting tools can be The required cutting tools can be

selected & fitted into the spindle selected & fitted into the spindle after removing the cutting tool used for the previous after removing the cutting tool used for the previous operation operation in a few seconds in a few seconds automatically as per the instructions given in the automatically as per the instructions given in the program.program.

All these features ensure that setup time is All these features ensure that setup time is reduced to a very great extent.reduced to a very great extent.

Automation of machine toolsAutomation of machine tools

The CNC principle of automation of machine The CNC principle of automation of machine tools have been extended totools have been extended to

Jig boringJig boringGrindingGrindingCoordinate measuring (inspection) machines Coordinate measuring (inspection) machines Drilling & routingDrilling & routingBlankingBlankingPipe bendingPipe bendingElectron beam weldingElectron beam weldingElectric discharge machiningElectric discharge machining

Programming for CNC MachinesProgramming for CNC Machines

Essentially consists of giving a set of Essentially consists of giving a set of instructions to the CNC control instructions to the CNC control which will cause the machine tool slides to which will cause the machine tool slides to move in such a manner that the cutting move in such a manner that the cutting tool is suitably tool is suitably

positioned, oriented, and moves in the path positioned, oriented, and moves in the path required to machine the component as per required to machine the component as per drawing dimensions.drawing dimensions.

Where to cut?Where to cut?

To do this, it is necessary, in the first instance, to define To do this, it is necessary, in the first instance, to define the location and path of the cutting tool in relation to the the location and path of the cutting tool in relation to the surface of the component to be machined throughout surface of the component to be machined throughout the process of machining.the process of machining.Thereafter, it is necessary to arrange to coordinate the Thereafter, it is necessary to arrange to coordinate the

movement of the machine tool slides with the cutter movement of the machine tool slides with the cutter location/path, taking due account of the location/path, taking due account of the

cutting tool shape,cutting tool shape,its dimensions, its dimensions,

method of clamping the component to the machine table and method of clamping the component to the machine table and limits of travel of the machine tool slides.limits of travel of the machine tool slides.

Machining ParametersMachining Parameters

CNC parameters also take into account CNC parameters also take into account various machining parameters, such as various machining parameters, such as spindle speeds, feeds, coolant usage, tool spindle speeds, feeds, coolant usage, tool wear compensation etc.wear compensation etc.

ProgrammingProgramming

1.1. ManuallyManually2.2. With the assistance of computersWith the assistance of computers3.3. Following the CAD/CAM route using a Following the CAD/CAM route using a

workstation with interactive graphic workstation with interactive graphic simulation and other facilities (such as simulation and other facilities (such as FEM) for design analyses.FEM) for design analyses.

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Manual ProgrammingManual Programming

Useful for very simple jobs, in which Useful for very simple jobs, in which machining is required in 2 axis only.machining is required in 2 axis only.Most aerospace components require Most aerospace components require control over 3 or 4 or 5 axis and it will not control over 3 or 4 or 5 axis and it will not be practical to do manual programming be practical to do manual programming for such componentsfor such components

Machine Operator Machine Operator –– does does whaaatwhaaat??

Load the components on to the machine before Load the components on to the machine before commencing the CNC operations and unload the commencing the CNC operations and unload the component after CNC operations are overcomponent after CNC operations are overSwitch on the machine after loading the Switch on the machine after loading the component and switch off the machine after component and switch off the machine after CNC operations are overCNC operations are overSupervise operations carried out automatically Supervise operations carried out automatically under CNC control to ensure that no untoward under CNC control to ensure that no untoward damage occurs, such as tool breakage in which damage occurs, such as tool breakage in which the operator can override the control and stop the operator can override the control and stop the machine.the machine.

Skills of the operatorSkills of the operator

CNC operator does not have to have the high CNC operator does not have to have the high manual skills required of machine operators of manual skills required of machine operators of conventional machines (with manual tool slide conventional machines (with manual tool slide controls).controls).However, he needs to be suitably trained to However, he needs to be suitably trained to understand the principles of programming and understand the principles of programming and operation of CNC machines and be alert while operation of CNC machines and be alert while machining is going on.machining is going on.

Advantages and Disadvantages Advantages and Disadvantages of CNC Machiningof CNC Machining

AdvantagesAdvantages

Improved qualityImproved qualityRepeatable accuracy as human error is Repeatable accuracy as human error is reducedreduced

Manufacturing flexibilityManufacturing flexibilityTo accommodate frequent job changes To accommodate frequent job changes

as setas set--up times are shortup times are short

design changes design changes CNC programs can easily be modified.CNC programs can easily be modified.

AdvantagesAdvantages……..

Significant reduction in requirement of expensive Significant reduction in requirement of expensive jigs and fixturesjigs and fixtures

Simple holding fixtures are sufficientSimple holding fixtures are sufficient

Manufacturing lead time cut down drasticallyManufacturing lead time cut down drasticallyDue to reduction in scale of jigs and fixtures and due Due to reduction in scale of jigs and fixtures and due to short setup timesto short setup timesConsequently, reduced inventory of tools and raw Consequently, reduced inventory of tools and raw materialmaterialSatisfying customer demand in a short periodSatisfying customer demand in a short period

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AdvantagesAdvantages

Reduction in number of machine operatorsReduction in number of machine operatorsOne CNC machine can do the work of more One CNC machine can do the work of more than one type of conventional machinethan one type of conventional machine

E.g., one machine centre can replace 3 E.g., one machine centre can replace 3 conventional machines, a milling machine, a conventional machines, a milling machine, a drilling machine and a boring machinedrilling machine and a boring machine

DisadvantagesDisadvantages

High initial (capital) costs of acquisition of CNC High initial (capital) costs of acquisition of CNC machinesmachinesAssociated capital costs of CNC programming Associated capital costs of CNC programming aids, precision tool setting equipment and CNC aids, precision tool setting equipment and CNC regrinding machinesregrinding machinesCutting tools have to be reCutting tools have to be re--sharpened accuratelysharpened accuratelyHigher overheads costsHigher overheads costs

Due to costlier maintenance and requirement for air Due to costlier maintenance and requirement for air conditioning for the CNC shopconditioning for the CNC shop

Which way to go?Which way to go?

The advantages offset the disadvantages. The advantages offset the disadvantages. Use of CNC machines results in overall Use of CNC machines results in overall economyeconomy

If one can raise the capital funds for If one can raise the capital funds for purchasing the CNC machinespurchasing the CNC machines