UNIT I MECHANICAL PROCESSES

Ultrasonic Machining- Elements of process, cutting tool system design, effect ofparameters, economic considerations, applications, limitations of the process,advantages and disadvantages. Abrasive Jet Machining- Variables in AJM, metalremoval rate in AJM. Water Jet Machining- Jet cutting equipments, process details,advantages and applications.

ULTRASONIC MACHINING

Ultrasonic machining is a non-traditional machining process. USM is grouped underthe mechanical group Non Traditional Machining processes.

In ultrasonic machining, a tool of desired shape vibrates at an ultrasonic frequency(19000 ~ 25000 Hz) with an amplitude of around 15 – 50 μm over the workpiece.Generally the tool is pressed downward with a feed force, F. Between the tool andworkpiece, the machining zone is flooded with hard abrasive particles generally inthe form of a water based slurry. As the tool vibrates over the workpiece, theabrasive particles act as the indenters and indent both the work material and thetool. The abrasive particles, as they indent, the work material, would remove thesame, particularly if the work material is brittle, due to crack initiation, propagationand brittle fracture of the material. Hence, USM is mainly used for machining brittlematerials which are poor conductors of electricity and thus cannot be processed byElectrochemical and Electro-discharge machining.

Effect of Process ParametersThe process parameters which govern the ultrasonic machining process are

Amplitude of vibration (ao) 15 – 50 μm

Frequency of vibration (f) 19 – 25 kHz Feed force (F) related to tool dimensions Feed pressure (p) Abrasive size 15 μm – 150 μm

Abrasive material – Aluminium Oxide (Al2O

3), Silicon Carbide (SiC), Boron

Carbide (B4C), Boronsilicarbide, Diamond.

Flow strength of work material Flow strength of the tool material Contact area of the tool – A Volume concentration of abrasive in water slurry – C

Effect of machining parameters on MRR

Ultrasonic Machining EquipmentThe basic mechanical structure of an USM is very similar to a drill press. It hasadditional features to carry out USM of brittle work material. The workpiece ismounted on a vice, which can be located at the desired position under the tool usinga 2 axis table. The table can further be lowered or raised to accommodate work ofdifferent thickness. The typical elements of an USM are Slurry delivery and return system Feed mechanism to provide a downward feed force on the tool during machining The transducer, which generates the ultrasonic vibration The horn or concentrator, which mechanically amplifies the vibration to the

required amplitude of 15 – 50 μm and accommodates the tool at its tip.

The ultrasonic vibrations are produced by the transducer. The transducer is drivenby suitable signal generator followed by power amplifier. The transducer for USMworks on the following principle Piezoelectric effect Magnetostrictive effect Electrostrictive effect

Magnetostrictive transducers are most popular and robust amongst all. The horn orconcentrator of the Magnetostrictive transducers is a wave-guide, which amplifiesand concentrates the vibration to the tool from the transducer. The horn orconcentrator can be of different shapes like Tapered or conical Exponential Stepped

Applications Used for machining hard and brittle metallic alloys, semiconductors, glass,

ceramics, carbides etc. USM is used in machining round, square, irregular shaped holes and surface

impressions. Wire drawing, punching or small blanking dies can be fabricated by ultrasonic

machining.

Limitations Low Material Removal Rate, which results in higher time for machining Rather high tool wear hence increased setup time and tool cost Low depth of hole. Cannot be used for producing high depth holes.

ABRASIVE JET MACHINING

In abrasive jet machining (AJM), a focused stream of abrasive particles, carried byhighpressure air or gas is made to impinge on the work surface through a nozzle andthework material is made to impinge on the work surface through a nozzle andworkmaterial is removed by erosion by high velocity abrasive particles.

Process:Abrasive particles of around 50 microns grit size are made to impinge on workmaterial athigh velocity (200 m/s or more) with a stand-off distance of about 2 - 3mmbetween nozzle and work. Jet of abrasive particles is carried by carrier gas or air.The highvelocity stream of abrasives is generated by converting the pressure energyof carriergas or air into Kinetic energy and thereby producing a high velocity jet. TheNozzle directs abrasivejet in a controlled manner onto work material. The highvelocity abrasive particlesremove the material by micro-cutting action as well asbrittle fracture of the work material.

Equipment:A schematic layout of AJM is shown below. The gas stream is passed to thenozzlethrough a connecting hose. The velocity of the abrasive stream ejectedthrough thenozzle is generally of the order of 330 m/sec.

Abrasive jet machining consists of

Gas propulsion system Abrasive feeder Machining Chamber (Mixing) AJM Nozzle Abrasives

Gas Propulsion SystemSupplies clean and dry air. Air, Nitrogen and carbon dioxide to propel theabrasiveparticles. Gas may be supplied either from a compressor or a cylinder. Incase of acompressor, air filter cum drier should be used to avoid water or oilcontamination ofabrasive powder. Gas should be non-toxic, cheap, easily available.It should notexcessively spread when discharged from nozzle into atmosphere. Thepropellantconsumption is of order of 0.008 m3/min at a nozzle pressure of 5 bar andabrasiveflow rate varies from 2 to 4 gm/min for fine machining and 10 to 20 gm/minforcutting operation.

Abrasive Feeder.Required quantity of abrasive particles is supplied by abrasive feeder. Thefilletedpropellant is fed into the mixing chamber where in abrasive particles are fedthrougha sieve. The sieve is made to vibrate at 50-60 Hz and mixing ratio iscontrolled bythe amplitude of vibration of sieve. The particles are propelled by carrier

gas to amixing chamber. Air abrasive mixture moves further to nozzle. The nozzleimpartshigh velocity to mixture which is directed at work piece surface.

Machining chamberIt is well closed so that concentration of abrasive particles around theworkingchamber does not reach to the harmful limits. Machining chamber isequipped withvacuum dust collector. Special consideration should be given to dustcollectionsystem if the toxic material (like beryllium) are being machined.

AJM nozzleAJM nozzle is usually made of tungsten carbide or sapphire ( usually life – 300hoursfor sapphire , 20 to 30 hours for WC) which has resistance to wear. The nozzleismade of either circular or rectangular cross section and head can be head canbestraight, or at a right angle. It is so designed that loss of pressure due to thebends,friction etc is minimum possible. With increase in wear of a nozzle, thedivergence ofjet stream increases resulting in more stray cutting and highinaccuracy.

AbrasivesAluminum oxide (Al2O3), Silicon carbide (SiC) Glass beads, crushed glassandsodiumbicarbonate are some of abrasives used in AJM. Selection of abrasivesdepends onMRR , type of work material , machining accuracy.

Abrasives Grain Sizes ApplicationAluminum oxide(Al2O3) 12, 20, 50 microns Good for cleaning, cuttingand

deburring

Silicon carbide (SiC) 25,40 micron Used for similarapplication but forhardmaterial

Glass beads 0.635 to 1.27mm Gives matte finishDolomite 200 mesh Etching and polishing

Sodium bi carbonate 27 micros Cleaning, deburring and cutting ofsoft material Light finishing below50°C

Process parametersThe followingare the process criteriafor abrasive jet machining.1. Mass flow rate2. Abrasive grain size3. Velocity of abrasive particles4. Mixing ratio5. Nozzle pressure

Process criteria are generally influenced by the process parameters asdetailedbelow:

Abrasives material – Al2O3, SiC, Glass beads Crushed glass Sodium bi carbonate shape – irregular/regular Size – 10 to 50 microns Mass flow – 2 – 20 gm/min

Carrier Gas Composition – Air, CO2, N2

b) Density – 1.3 kg/m3

c) Velocity - 500 to 700 m/s d) Pressure - 2 to 10 bar e) Flow rate - 5 to 30 microns

Abrasive Jet Velocity - 100 to 300 m/s Mixing ratio – Volume flow rate of abrasives/Volume flow rate of gas Stand-off distance – 0.5 to 15mm. Impingement angle – 60° to 90°

Nozzle Material – WC/Sapphire Diameter – 0.2 to 0.8 mm Life – 300 hours for sapphire, 20 to 30 hours for WC

Process capability Material removal rate – 0.015 cm3/min Narrow slots – 0.12 to 0.25mm± 0.12mm Surface finish -0.25 micron to 1.25 micron Sharp radius up to 0.2mm is possible It is possible to cut Steel up to 1.5mm, Glass up to 6.3mm thick

Effect of abrasive flow rate and grain size on MRRIt is clear from the figure that at a particular pressure MRR increase with increaseofabrasive flow rate and is influenced by size of abrasive particles. But afterreachingoptimum value, MRR decreases with further increase of abrasive flow rate.This isowing to the fact that Mass flow rate of gas decreases with increase ofabrasive flowrate and hence mixing ratio increases causing a decrease in materialremoval ratebecause of decreasing energy available for erosion.

Effect of exit gas velocity and abrasive particle densityThe velocity of carrier gas conveying the abrasive particles changes considerablywith the change of abrasive particle density. The exit velocity of gas can beincreased to critical velocity when the internal gas pressure is nearly twicethepressure at exit of nozzle for theabrasive particle density is zero. If the densityofabrasive particles is gradually increased exit velocity will go on decreasing forthesame pressure condition. It is due to fact that Kinetic energy of gas is utilizedfortransporting the abrasive particle

Effect of mixing ratio on MRRIncreased mass flow rate of abrasive will result in adecreased velocity of fluid andwill thereby decreasesthe available energy for erosion and ultimately theMRR. It isconvenient to explain to this fact by termMIXING RATIO. Which is defined as

Mixing ratio = Volume flow rate of carrier gas / Volume flow rate of carrier gas

The effect of mixing ratio on the material removal rate is shown below.

The material removal rate can be improved byincreasing the abrasive flow rateprovided the mixingratio can be kept constant. The mixing ratio isunchanged only bysimultaneous increase of both gasand abrasive flow rate.

Effect of nozzle pressure on MRRThe abrasive flow rate can be increased byincreasing the flow rate of the carriergas.This is only possible by increasing the internalgas pressure as shown in thefigure. As theinternal gas pressure increases abrasive massflow rate increase andthus MRR increases.

Applications

This is used for abrading and frosting glass more economically as comparedtoetching or grinding

Cleaning of metallic smears on ceramics, oxides on metals, resistive coatingetc.

AJM is useful in manufacture of electronic devices , drilling of glass wafers,deburring of plastics, making of nylon and Teflon parts permanent markingonrubber stencils, cutting titanium foils

Deflashing small castings, engraving registration numbers on toughenedglass used for car windows

Used for cutting thin fragile components like germanium, silicon etc. Used for drilling,cutting, deburring etching and polishing of hard and brittle

materials Most suitable for machining brittle and heat sensitive materials like glass,

quartz, sapphire,mica, ceramics germanium, silicon and gallium. It is also good method for deburring small hole like in hypodermic needles

andfor small milled slots in hard metallic components.

Advantages High surface finish can be obtained depending upon the grain sizes Depth of damage is low ( around2.5 microns) It provides cool cutting action, so it can machine delicate and heat sensitive Material Process is free from chatter and vibration as there is no contact between the

tool and work piece Capital cost is low and it is easy to operate and maintain AJM. Thin sections of hard brittle materials like germanium, mica, silicon, glass

andceramics can be machined. It has the capability of cutting holes of intricate shape in hard materials.

Limitations

Limited capacity due to low MRR. MRR for glass is 40 gm/minute Abrasives may get embedded in the work surface, especially while machining

softmaterial like elastomers or soft plastics. The accuracy of cutting is hampered by tapering of hole due to unavoidable

flaringof abrasive jet. Stray cutting is difficult to avoid A dust collection system is a basic requirement to prevent atmospheric

pollutionand health hazards. Nozzle life is limited (300 hours) Abrasive powders cannot be reused as the sharp edges are worn and

smallerparticles can clog the nozzle. Short stand-off distances when used for cutting, damages the nozzle.

WATER JET MACHINING

Water Jet Machining (WJM) and Abrasive Water Jet Machining (AWJM) are two non-traditional or non-conventional machining processes. They belong to mechanicalgroup of non-conventional processes like Ultrasonic Machining (USM) andAbrasiveJet Machining (AJM). In these processes the mechanical energy of waterandabrasive phases are used to achieve material removal or machining.

WJM and AWJM can be achieved using different approaches andmethodologies asenumerated below:

WJM - Pure WJM - with stabilizer AWJM – entrained – three phase – abrasive, water and air AWJM – suspended – two phase – abrasive and water

However in all variants of the processes, the basic methodology remains thesame.Water is pumped at a sufficiently high pressure, 200-400 MPa usingintensifiertechnology. An intensifier works on the simpleprincipleofpressureamplification usinghydrauliccylindersofdifferent crosssectionsas used in “JuteBellPresses”.When water at such pressure isissuedthrough a suitable orifice(generallyof 0.2- 0.4 mm dia),the potentialenergyof water is converted intokineticenergy, yielding a highvelocityjet(1000m/s). Such high velocitywater jet canmachine thin sheets/foils of aluminium,leather, textile,frozen food etc.

In pure WJM, commercially pure water (tap water) is used for machining purpose.However as the high velocity water jet is discharged from the orifice,the jet tends toentrain atmospheric air and flares out decreasing its cuttingability.Hence, quite oftenstabilisers (long chain polymers) that hinder thefragmentation of water jet are addedto the water.

In AWJM, abrasive particles like sand (SiO), glass beads are added to thewater jet toenhance its cutting ability by many folds. AWJ are mainly of twotypes – entrainedand suspended type as mentioned earlier. In entrained typeAWJM, the abrasiveparticles are allowed to entrain in water jet to formabrasive water jet with significantvelocity of 800 m/s. Such high velocityabrasive jet can machine almost any material.

Process parametersThe following are the process criteria for abrasive jet machining.1. Material removal rate2. Geometry and surface finish of work piece3. Wear rate of the nozzle

Material removal rate is directly proportional to the reactive force (F) of the jet.MRR α FMRR αm x v

Where m – mass flow rate andv – jet velocity

Geometry and surface finish of work material is affected by nozzle design, jetvelocity, cutting speed and depth of cut.

Wear rate of the nozzle depends upon hardness of nozzle material, pressureexerted by the water jet, velocity of the jet and nozzle design.

Commercial CNC water jet machining system and cutting heads

Materials that can be processed by WJM Steels Non-ferrous alloys Ti alloys, Ni- alloys Polymers Metal Matrix Composite Ceramic MatrixComposite Metal Polymer Laminates Glass Fibre Metal Laminates

MachineAny standard abrasive water jet machining (AWJM) system using entrainedAWJMmethodology consists of following modules.

LP booster pump Orifice

6

5

4

3

21

Hydraulic unit Mixing Chamber Additive Mixer Focussing tube or inserts Accumulator

1. LP Booster2. Hydraulic drive3. Additive mixer4. Direction control5. Intensifier6. Accumulator

Mechanism of material removalThe general domain of parameters in entrained type AWJ machining system is givenbelow:

Orifice – Sapphires – 0.1 to 0.3 mm Focussing Tube – WC – 0.8 to 2.4 mm Pressure – 2500 to 4000 bar Abrasive – garnet and olivine - #125 to #60 Abrasive flow - 0.1 to 1.0 Kg/min Stand-off distance – 1 to 2 mm Machine Impact Angle – 60° to 90° Traverse Speed – 100 mm/min to 5 m/min Depth of Cut – 1 mm to 250 mm

Mechanism of material removal in machining with water jet and abrasive water jet israther complex. In AWJM of ductile materials, material is mainly removed by lowangle impact by abrasive particles leading to ploughing and micro cutting. In case ofAWJM of brittle materials, other than the above two models, material would be

Intensifier On-off valve Catcher CNC table Abrasive metering device

removed due to crack initiation and propagation because of brittle failure of thematerial.Applications

Paint removal Cleaning Cutting soft materials Cutting frozen meat Textile, Leather industry Mass Immunization Surgery Turning Nuclear Plant Dismantling

Advantages Extremely fast set-up and programming Machine virtually any 2D shape on any material Very low side forces during the machining Almost no heat generated on the part Machining thick plates is possible

Limitations Initial cost is high Difficult to machine hard material Noisy operation

Unit IIElectro Chemical Machining

Instructional Objectives

(i) Identify electro-chemical machining (ECM) as a particulartype of non-tradition processes

(ii) Describe the basic working principle of ECM process(iii) Draw schematically the basics of ECM(iv) Draw the tool potential drop(v) Describe material removal mechanism in ECM(vi) Identify the process parameters in ECM(vii) Develop models for material removal rate in ECM(viii) Analyse the dynamics of ECM process(ix) Identify different modules of ECM equipment(x) List four application of ECM(xi) Draw schematics of four such ECM applications

1. Introduction

Electrochemical Machining (ECM) is a non-traditional machining (NTM)process belonging to Electrochemical category. ECM is opposite ofelectrochemical or galvanic coating or deposition process. Thus ECM can bethought of a controlled anodic dissolution at atomic level of the work piece thatis electrically conductive by a shaped tool due to flow of high current atrelatively low potential difference through an electrolyte which is quite oftenwater based neutral salt solution.Fig. 1 schematically shows the basic principle of ECM.

TOOL

Fig. 1 Schematic principle of Electro Chemical Machining (ECM)

2. Process

During ECM, there will be reactions occurring at the electrodes i.e. at theanode or workpiece and at the cathode or the tool along with within theelectrolyte.Let us take an example of machining of low carbon steel which is primarily aferrous alloy mainly containing iron. For electrochemical machining of steel,generally a neutral salt solution of sodium chloride (NaCl) is taken as theelectrolyte. The electrolyte and water undergoes ionic dissociation as shownbelow as potential difference is applied

NaCl ↔ Na+ + Cl-H2O ↔ H+ + (OH)-

As the potential difference is applied between the work piece (anode) and thetool (cathode), the positive ions move towards the tool and negative ionsmove towards the workpiece.Thus the hydrogen ions will take away electrons from the cathode (tool) andfrom hydrogen gas as:

2H+ + 2e- = H2 ↑ at cathodeSimilarly, the iron atoms will come out of the anode (work

piece) as: Fe = Fe+ + + 2e-

Within the electrolyte iron ions would combine with chloride ions to form ironchloride and similarly sodium ions would combine with hydroxyl ions to formsodium hydroxide

Na+ + OH- = NaOHIn practice FeCl2 and Fe(OH)2 would form and get precipitated in the form ofsludge. In this manner it can be noted that the work piece gets graduallymachined and gets precipitated as the sludge. Moreover there is not coatingon the tool, only hydrogen gas evolves at the tool or cathode. Fig. 2 depictsthe electro-chemical reactions schematically. As the material removal takesplace due to atomic level dissociation, the machined surface is of excellentsurface finish and stress free.

WFe++

OH− Cl- H+ Na+

TOH−

ONa+

OH−

Fe+

+ H2

OR Fe+

+O

K + LOH− H -

e FeCl2−

eCl

Fe(OH)2

OH

Fig. 2 Schematic representation of electro-chemical reactions

The voltage is required to be applied for the electrochemical reaction toproceed at a steady state. That voltage or potential difference is around 2 to30 V. The applied potential difference, however, also overcomes the followingresistances or potential drops. They are:

• The electrode potential• The activation over potential• Ohmic potential drop• Concentration over potential• Ohmic resistance of electrolyte

Fig. 3 shows the total potential drop in ECMcell.

Anode potential VoltageAnodic Activation over potentialovervoltage ohmic potential

concentration potential

Voltageconcentration potential

Ohmic dropcathodic overpotentialactivation overpotential

cathodic potential

anode cathode

3. EquipmentThe electrochemical machining system has the following modules:

• Power supply• Electrolyte filtration and delivery system• Tool feed system• Working tank

Fig. 4 schematically shows an electrochemical drilling unit.

Flow controlPressure

gaugevalve

Flow meter Constant feed to

Pressure Toolthe tool

-verelief valve

PS Lowvoltage +vehighcurrent

power supply

Pump Spent electrolyte

Material removal rate (MRR) is an important characteristic to evaluate efficiency of anon-traditional machining process.In ECM, material removal takes place due to atomic dissolution of work material.Electrochemical dissolution is governed by Faraday’s laws.The first law states that the amount of electrochemical dissolution or deposition isproportional to amount of charge passed through the electrochemical cell, which may beexpressed as:

m∝Q ,where m = mass of material dissolved or deposited Q =

amount of charge passedThe second law states that the amount of material deposited or dissolved furtherdepends on Electrochemical Equivalence (ECE) of the material that is again the ratio ofthe atomic weight and valency. Thus

m α ECE α A/V

Thus m α QA/ν

where F = Faraday’s constant= 96500 coulombs

∴ m = ItA/Fν

∴ MRR = m / tρ

where I = currentρ= density of the material

The engineering materials are quite often alloys rather than element consisting ofdifferent elements in a given proportion.

6. Applications

ECM technique removes material by atomic level dissolution of the same byelectrochemical action. Thus the material removal rate or machining is not dependenton the mechanical or physical properties of the work material. It only depends on theatomic weight and valency of the work material and the condition that it should beelectrically conductive. Thus ECM can machine any electrically conductive workmaterial irrespective of their hardness, strength or even thermal properties. Moreover

as ECM leads to atomic level dissolution, the surface finish is excellent with almoststress free machined surface and without any thermal damage.ECM is used for

• Die sinking• Profiling and contouring• Trepanning

• Grinding• Drilling• Micro-machining

Tool

Work3D profiling

Different applications of Electro Chemical Machining

(drilling)

UNIT III

ELECTRIC DISCHARGE MACHINING (EDM)

It is also known as spark erosion machining or spark machining. Material of workpiece removed due to erosion caused by electric spark. Working principle isdescribed below.

Working Principle of Electric Discharge MachiningElectric discharge machining process is carried out in presence of dielectric

fluid which creates path for discharge. When potential difference is created acrossthe two surfaces of die electric fluid, it gets ionized. An electric spark/discharge isgenerated across the two terminals. The potential difference is developed by apulsating direct current power supply connected across the two terminals. One of theterminal is positive terminal given to work piece and tool is made negative terminal.Two third of the total heat generated is generated at positive terminal so work pieceis generally given positive polarity. The discharge develops at the location where twoterminals are very close. So tool helps in focusing the discharge or intensity ofgenerated heat at the point of metal removal. Application of focused heat raise thetemperature of work piece locally at a point, this way two metal is melted andevaporated.

Electric Discharge Machining Process DetailsThe working principle and process of EDM is explained with the help of line

diagram in Figure 3.1. The process details and components are explained belowserially.

Fig 3.1 : Line Diagram Indicating Working Principle and Process Details of

EDM

Base and Container

A container of non-conducting, transparent material is used for carrying outEDM. The container is filled with dielectric solution. A base to keep work piece isinstalled at the bottom of container. The base is made of conducting material andgiven positive polarity.Tool

Tool is given negative polarity. It is made of electrically conducting materialline brass, copper or tungsten. The tool material selected should be easy tomachine, high wear resistant. Tool is made slightly under size for inside machiningand over sized for cut side machining. Tool is designed and manufactured accordingto the geometry to be machined.Dielectric Solution

Dielectric solution is a liquid which should be electrically conductive. Thissolution provides two main functions, firstly it drive away the chips and prevents theirsticking to work piece and tool. It enhance the intensity of discharge after gettingionized and so accelerates metal removal rate.Power Supply

A DC power supply is used, 50 V to 450 V is applied. Due to ionization ofdielectric solution an electrical breakdown occurs. The electric discharge so causeddirectly impinges on the surface of work piece. It takes only a few micro seconds tocomplete the cycle and remove the material. The circuit cam be adjusted for auto offafter pre-decided time interval.

Tool Feed Mechanism

In case of EDM, feeding the tool means controlling gap between work pieceand the tool. This gap is maintained and controlled with the help of servomechanism. To maintain a constant gap throughout the operation tool is movedtowards the machining zone very slowly. The movement speed is towards themachining zone very slowly. The movement speed is maintained by the help of gearand rack and pinion arrangement. The servo system senses the change in gap dueto metal removal and immediately corrects it by moving the tool accordingly. Thespark gap normally varies from 0.005 mm to 0.50 mm.

Work piece and Machined Geometry

The important point for work piece is that any material which is electricalconductor can be machined through this process, whatever be the hardness of thesame. The geometry which is to be machined into the work piece decides the shapeand size of the tool.

Application of Electric Discharge MachiningThis process is highly economical for machining of very hard material as

tool wear is independent of hardness of work piece material. It is very useful in toolmanufacturing. It is also used for broach making, making holes with straight or

curved axes, and for making complicated cavities which cannot be produced byconventional machining operations. EDM is widely used for die making as complexcavities are to be made in the die making. However, it is capable to do all operationsthat can be done by conventional machining.

Advantages of EDM

This process is very much economical for machining very hard material. Maintains high degree of dimensional accuracy so it is recommended for tool

and die making. Complicated geometries can be produced which are very difficult otherwise. Highly delicate sections and weak materials can also be processed without any

risk of their distortion, because in this process tool never applies direct pressureon the work piece.

Fine holes can be drilled easily and accurately. Appreciably high value of MRRR can be achieved as compared to other non-

conventional machining processes.

Disadvantages and Limitations of EDM Process

There are some limitations of EDM process as listed below:

This process cannot be applied on very large sized work pieces as size of workpiece is constrained by the size of set up.

Electrically non-conducting materials cannot be processed by EDM. Due to the application of very high temperature at the machining zone, there are

chances of distortion of work piece in case of this section. EDM process is not capable to produce sharp corners. MRR achieved in EDM process is considerably lower than the MRR in case of

conventional machining process so it cannot be taken as an alternative toconventional machining processes at all.

PROCESS PARAMETERS

The following factors influences the process parameters in EDM processes

1. Operating parameters:

Operating process involves the removal of metal from the work piece and toolas a measure of electrical energy input.

2. Taper:

Tapering Effect is observed due to the side sparks .under high dielectricpollution, side sparks are more pronounced as compared to frontal sparks.

3. Surface finish:

The surface finish of the material depends upon the following factorsi. Energy of the pulse andii. Frequency of operation

WIRE CUT ELECTRIC DISCHARGE MACHINING (WCEDM)

This is a special type of electric discharge machining that uses a small diameter wireas a cutting tool on the work. Working a principle of wire cut electric dischargemachining is same as that of electric discharge machining.

Process Details of WCEDMProcess details of WCEDM are almost similar to EDM with slight difference.

The details of the process are indicated in the line diagram shown in Figure 5.2. Itsmajor difference of process details with EDM process details are described below.

Tool DetailsThe tool used in WCEDM process is a small diameter wire as the electrode to

cut narrow kerfs’ in the work piece. During the process of cutting the wire iscontinuously advanced between a supply spoil and wire collector. This continuousfeeding of wire makes the machined geometry insensitive to distortion of tool due toits erosion. Material of wire can be brass, copper, tungsten or any other suitablematerial to make EDM tool. Normally, wire diameter ranges from 0.076 to 0.30 mmdepending upon the width of kerf.

Tool Feed MechanismTwo type of movements are generally given to the total (wire). One is

continuous feed from wire supply spoil to wire collector. Other is movement of thewhole wire feeding system, and wire along the kerf to be cut into the work piece.Both movements are accomplished with ultra accuracy and pre-determined speedwith the help of numerical control mechanism.

Wire supply spool

Continuouslyfed wire

Dialectic spraySmallwidth

nozzleelectrode (tool)

KerfWork piece

Wire collectorKerf

Machine portion

Fig 3.2 : Line Diagram for Process Details of Working of Wire Cut ElectricDischarge Machining

Dielectric Fluid and Spray MechanismLike EDM process dielectric fluid is continuously sprayed to the machining

zone. This fluid is applied by nozzles directed at the tool work interface or workpiece is submerged in the dielectric fluid container. (Rest of the process details in

case of WCEDM process are same as that in case of EDM process).

Applications of WCEDMWCEDM is similar to hand saw operation in applications with good precision.

It is used to make narrow kerf with sharp corners. It does not impose any force toworkpiece so used for very delicated and thin work pieces. It is considered ideal formaking components for stamping dies. It is also used to make intricate shapes inpunch, dies and other tools.

Advantages of WCEDMAdvantages are listed below:

Accuracy and precision of dimensions are of very good quality. No force is experienced by the work piece. Hardness and toughness of work piece do not create problems in

machining operation.

Disadvantages and Limitations of WCEDMThe major disadvantages of this process are that only electrically conducting

materials can machine. This process is costly so recommended for use specificallyat limited operations.

LASER BEAM MACHINING (LBM)

Laser beam have wide industrial applications including some of the machiningprocesses. A laser is an optical transducer that converts electrical energy into ahighly coherent light beak. One must know the full name o amplification ofstimulated emission of radiation” specific property, if it is focused by conventionaloptical lenses can generate high power density.

Working Principle of LBMLBM uses the light energy of a laser beam to remove material by vaporization

and ablation. The working principle and the process details (setup) are indicated inFigure 5.6. In this process the energy of coherent light beam is focused optically forpredecided longer period of time. The beam is pulsed so that the released energyresults in an impulse against the work surface that does melting and evaporation.Here the way of metal removing is same as that of EDM process but method ofgeneration of heat is different. The application of heat is very finely focused in caseof LBM as compared to EDM.

Process Details of LBMProcess details of LBM are shown in line diagram shown in Figure 5.6,

description of the details is given below.

Laser Tube and Lamp AssemblyThis is the main part of LBM setup. It consists of a laser tube, a pair of

reflectors, one at each end of the tube, a flash tube or lamp, an amplification source,a power supply unit and a cooling system. This whole setup is fitted inside aenclosure, which carries good quality reflecting surfaces inside. In this setup the

flash lamp goes to laser tube, that excites the atoms of the inside media, whichabsorb the radiation of incoming light energy. This enables the light to travel to andfro between two reflecting mirrors. The partial reflecting mirror does not reflect thetotal light back and apart of it goes out in the form of a coherent stream ofmonochromatic light. This highly amplified stream of light is focused on the workpiece with the help of converging lens. The converging lens is also the part of thisassembly.

Work pieceThe range of work piece material that can be machined by LBM includes high

hardness and strength materials like ceramics, glass to softer materials like plastics,rubber wood, etc. A good work piece material high light energy absorption power,poor reflectivity, poor thermal conductivity, low specific heat, low melting point andlow latent heat.

Cooling MechanismA cooling mechanism circulates coolant in the laser tube assembly to avoid its

over heating in long continuous operation.

Tool Feed MechanismThere is no tool used in the LBM process. Focusing laser beam at a pre-

decided point in the work piece serve the purpose of tool. As the requirement ofbeing focused shifts during the operation, its focus point can also be shiftedgradually and accordingly by moving the converging lens in a controlled manner.This movement of the converging lens is the tool feed mechanism in LBM process.

Total reflector

Flash Lamp

Laser tube

Reflector(partial)

Parallel LASERBeam

Conversing lense

machining zone

WorkpieceTable

Fig 3.3 : Working Principle and Process Details of LBM

Applications of LBMLBM is used to perform different machining operations like drilling, slitting, slotting,scribing operations. It is used for drilling holes of small diameter of the order of 0.025mm. It is used for very thin stocks. Other applications are listed below :

Making complex profiles in thin and hard materials like integrated circuits and

printed circuit boards (PCBS). Machining of mechanical components of watches. Smaller machining of very hard material parts.

Advantages of LBM Materials which cannot be machined by conventional methods are machined by

LBM. There is no tool so no tool wear. Application of heat is very much focused so rest of the workpiece is least

affected by the heat. Drills very find and precise holes and cavities.

Disadvantages of LBMMajor disadvantages of LBM process are given below :

High capital investment is involved. Operating cost is also high. Recommended for some specific operations only as production rate is very slow. Cannot be used comfortably for high heat conductivity materials light reflecting

materials. Skilled operators are required.

QUESTIONS:

1.Explain the principle of working of EDM process with sketch.

2.What are the advantages and disadvantages of the EDM process?

3.Explain the principle of working of wire cut EDM process with a sketch.

4.What are the tools(electrodes) used in EDM process?

5.what are the function of dielectric fluids used in EDM process?

6. Explain the principle of working of LBM process with sketch.

7. What are the advantages and disadvantages of the LBM process?

UNIT IV

Plasma Arc Machining (PAM)

Plasma arc machining is a nontraditional thermal process. Plasma is defined as agas that has been heated to a sufficiently high temperature to become partially ionizedand therefore electrically conductive. The term plasma, as employed in physics, meansionized particles. The temperature of plasma may reach as high as 28000°C. Variousdevices utilizing an electric arc to heat gas to the Plasma State have been in existencesince the early 1900’s. However, the development of such apparatus into commercialplasma arcs equipment for metal cutting applications dates back to only about 1955.

The plasma arc produced by modern equipment is generated by a plasma torchthat is constructed in such a manner as to provide an electric arc between an electrodeand workpiece, as shown in Fig. 1. A typical plasma torch consists of an electrodeholder, an electrode, a device to swirl the gas, and a water-cooled nozzle. Thegeometry of the torch nozzle is such that the hot gases are constricted in a narrowcolumn.

Figure 1 Plasma Torch

Primary gasses, such as nitrogen, argon-hydrogen, or air, are forced through thenozzle and arc and become heated and ionized. Secondary gases or water flow areoften used to help clean the kerf of molten metal during cutting.

The stream of ionized particles from the nozzle can be used to perform a varietyof industrial jobs. The plasma arc, as an industrial tool, is a most heavy employed insheet and plate cutting operations as an alternative to more conventional oxy-fueltorches or other cutting tools. Plasma arc is routinely used as an integral component ofsome modern punching machines. Plasma arc methods are also employed in special

applications to replace conventional machining operations such as lathe turning, millingand planing, heat treatment and metal deposition operations, and plasma arc welding.

Principle of operation

In PAM, constricting an electric arc through a nozzle, as shown in Fig. 1generates the basic plasma jet. Instead of diverging into an open arc, the nozzleconstricts the arc into a small cross section. This action greatly increases the power ofthe arc so that both temperature and voltage are raised. After passing through thenozzle, the arc exists in the form of a high-velocity, well-columnated and intensely hotplasma jet.

The basic heating phenomenon that takes place at the workpiece is acombination of heating due to energy transfer of electrons, recombination of dissociatedmolecules on the workpieces, and connective heating from the high-temperatureplasma that accompanies the arc. In some cases, it is desirable to achieve a thirdsource of heating by injecting oxygen into the work area and taking advantage of theexothermic oxidation reaction. Once the material has been raised to the molten point,the high-velocity gas stream effectively blows the material away.

For an optimized PAM cutting or machining operation, up to 45% of the electricalpower delivered to the torch is used to remove metal from the workpiece. Of theremaining power, approximately 10% go into the cooling water in the plasma generatorand the rest is wasted in the hot gas and in heating the workpiece.

The jet stream of ionized gases exits at sonic speed and tends to maintain aslightly diverging columnar shape until deflected by solid material. This ionized jetserves as a conductor for the arc; it provides directional stability. The ionized gas maybe further shielded from dispersion and heat loss, which result from impacting airmolecules, as it exits from the nozzle by means of another annular stream of gas thatsurrounds the plasma as it leaves the orifice nozzle.

The ionized plasma gas is usually inactive to protect the electrode fromcombustion and ensure long life. When oxygen is added as either the plasma ionizedgas or the secondary enveloping gas, the speed of cutting steel is increased. Use of asecondary envelope of gas improves the kerf wall appearance on certain metals. Thisenvelope also acts as a protective shield for the nozzle during extensive piercingoperations.

A major improvement in mechanized plasma arc cutting occurred in recent yearswith the development of so-called “water injection”. Figure 2 illustrates the principle ofwater swirl injection. When the water injection technique is employed, the arc isconstricted by a flow of water around the arc. This injection of water has manyadvantages, including:

1. A square cut can be made.2. Arc stability is increased.3. Cutting speed can be increased.4. Workpieces are heated less.5. Less smoke and fumes are generated.6. Nozzle life is increases.

Figure 2 Water Swirl Injection

Applications

Cutting: With appropriate equipment and techniques, the plasma arc can besuccessfully employed to make cuts in electrically conductive metals. Figure3 illustrates the general setup for cutting using a plasma arc torch.

Figure 3 Cutting using Plasma Arc Torch

In hole piercing, reproducible and high-quality holes are made rapidly in avariety of materials with the plasma arc. Holes can be pierced much faster than they

can be drilled. Plasma arc hole piercing is performed with conventional plasma arccutting equipment that has been modified to produce a short, carefully controlled arcoperating time, suitable arc current programming during the short operating cycle, andeffective slag ejection.

Almost instantly after ignition the arc rapidly penetrates through the plate forminga hole approximately the same size as the diameter of the arc stream. Continuedplasma exposure increases the size of the hole to 4 or 5 times the arc stream diameter.Moving the torch or workpiece in a circular motion can produce larger holes.

In stack cutting, Plasma is effectively stack-cut stainless steel and aluminum.Plasma stack-cutting of thin carbon steel tends to weld the layers making them

difficult to separate after cutting. During cutting, the layers should be clamped firmlyenough to minimize gaps, but loose enough to permit slippage between layers due todifferential expansion. The upper layer may buckle if clamping does not allow slippage.

Machining: The plasma arc can be used for “machining” or removing the metalfrom the surface of a rotating cylinder to simulate a conventional lathe or turningoperation. The process is shown schematically in Fig. 4.

Figure 4 Metal Removing in a rotating cylinder

As the work piece is turned, the torch is moved parallel to the axis of the work.The torch is positioned so the arc will impinge tangentially on the workpiece andremove the outer layer of metal. Cutting can be accomplished with the workpiece

rotating in either direction relative to the torch, but best results are obtained when thedirection of rotation permits use of the shortest arc length for cutting. The flow of moltenmetal being removed must be in such a direction that it does not tend to adhere to thehot surface that has just been machined.

Generally, the plasma-arc metal removal process has little if any advantage oneasy-to-machine metals, but it has considerable economic advantage for rough metalremoval of hard-to-machine metals. The process is generally considered to be aroughing operation, which should be followed by a conventional machine finishing cut.Metal removal rates with the plasma arc process on hard-to-machine metals are up toten times faster than those achieved on a lathe using a tungsten carbide cutting tool.The principal disadvantages are the metallurgical alteration of the characteristic of thesurface profile produced.

Operating Parameters

Cutting: Quality of cut and metal removal rate are largely dependent upon properattention to operating variables. Several factors contribute to the quality and speed ofcuts made by the plasma arc process, including cutting-tip nozzle selection, power level,

gas type and mixture, gas flow rate, traverse speed, standoff distance, thickness ofmaterial, type of materials, impingement angle, and equipment design.

Nozzle size: The highest quality plasma cut is usually obtained when maximumthermal intensity is used. To achieve this, the smallest, or next to the smallest,nozzle size that is capable of operating at a power level suitable for the speedand thickness involved is used. Nozzle size for cutting usually ranges from 0.80to 6.30 mm in diameter.

Power: Direct current up to 200 kW and 50-1000 A is employed in plasma arccutting operations.

Gas mixture composition: A proportion of 10% hydrogen and 90% nitrogen orargon usually gives good general-purpose results. Choice of plasma gascomposition can have considerable effect on the plasma flame since the ionizedgas is the conductive path. The shielding gas or secondary gas can be nitrogen,oxygen, air, or carbon dioxide. Water is sometimes used as a shield.

Gas flow rate: For any particular nozzle size, an increase in plasma gas(primary) flow permits an increase in current. This increases the power density ofthe flame and permits greater speed with less taper on the kerf walls. Primarygas flow rates usually range between 0.40-5.60 m3/hr. Secondary gasses arepressurized to flow at up to 11.3 m3/hr.

Standoff distance: Due to the columnar shape of the plasma jet, a wide rangeof tip-to-workpiece spacing is allowable. This permits machine cutting alongwarped or irregular surfaces. General consideration include:

Better quality cuts usually result from a short standoff distance since arcdivergence is less and the thermal intensity of the arc is greater.

Excessively close standoff distance can promote arcing due to theaccumulation of slag drops on the tip.

Increased power input is necessary when the standoff distance is great. Standoff distance can range from 6.5-76.2 mm.

Machining: In turning operations, torch variables include the electrical power delivered,the gases used to form the plasma, the flow rate of the gases through the torch, theorifice diameter through the nozzle duct, and any secondary gas streams. In general,there is an optimum exit-orifice size for operation at a particular power level thatproduces a well-controlled, high-velocity plasma jet with maximized capacity forperforming the material removal operation. Thus gas flow rate, orifice size, and powerlevel are intimately related.

Metal removal rate: In the physical orientation of PAM operations, suchvariables as torch standoff, angle to work, depth of cut, feed into the work, andspeed of the work toward the torch are involved. Feed and depth of cut

determine the volume of metal removed. Since the torch carriage speed anddirection are not normally coupled to the work spindle, as they might be onan ordinary lathe, it is necessary to calculate the carriage speed in order toproduce a turned piece with a predetermined pitch (pitch-feed). When a 50kWpower level on 50 mm rods is used, the maximum metal removal rate forsatisfactory surface finish is about 114 cm3/min.

Surface finish: Surface finish can vary anywhere from helical ridges along thesurface to a completely smooth surface with approximately 0.75 μm Ra finish,depending upon the feed into the work optimization of the process. Plasma arcmachining for maximum removal rate does produce a slight helical ridge as thecut progresses.

Surface characteristics: One characteristics of the workpiece surface when it isnot cooled during an operation is a gradual inward taper in the direction of thecut. This is believed to be due to accumulated heating of the workpiece as thecut progresses and should be minimized or eliminated by appropriate coolingmethods. An oxidation scale normally forms behind the cut on an unprotectedspecimen, but this can be minimized or eliminated by proper shielding.

Metallurgical effects

Metallurgical effects of the PAM process are as widely varied as the materials used andthere respective metallurgical histories. In general, the depth of the heat-affected zoneis approximately 0.75 mm. For some operations, this hardened material may have to beremoved; for other applications, a hardened surface such as this may be desirable. Thenature of individual applications of the PAM process determines the metallurgical effectsthat can be tolerated.

Advantages of PAM Process

It gives faster production rate. Very hard and brittle metals can be machined. Small cavities can be machined with good dimensional accuracy.

Disadvantages of PAM Process Its initial cost is very high. The process requires over safety precautions which further enhance the

initial cost of the setup. Some of the workpiece materials are very much prone to metallurgical

changes on excessive heating so this fact imposes limitations to thisprocess.

It is uneconomical for bigger cavities to be machined.

ELECTRON BEAM MACHINING

Electron beam machining is a thermal process used for the metal removal during themachining process. In the electrical beam machining, electrical energy is used togenerate the electrons with high energy.

Electron Beam Machining Process:In the electron beam gun the electric beam is generated. Electron beam consists

of a small spot size, from which it provides the high velocity electrons. The electronbeam machining process is carried out in the vacuum as shown in Figure 5. This is dueto the electrons present in the process react with the air molecules so they lose theenergy and ability of cutting. The work piece material must be placed under the electronbeam, and where the equipment is placed under the vacuum. With the spot size of 10to 100 , the high energy absorbed electron beam is ready to show impact on the work

piece material. The high velocity electron consists of kinetic energy, the energy isconverted into the heat energy, where the electrons strikes the work material. Becauseof the high energy present in the electrons it starts to melting and vaporization of thework piece material. The process is done from top to the bottom of the work piecematerial. In the electrical beam machining the gun is used in the pulsed mode. By usingthe single pulse holes, and is drilled on the thin sheets. Multiple passes are required forthe thicker plates.

Figure 5 Electron Beam Machining

Cathode, Bias grid, anode, electromagnetic lens, electromagnetic coils, deflectorcoils, telescope, vacuum gauge, throttle valve, diffusion pump.

The electron beam machine consists of an electron beam gun used to producefree electrons at the cathode. The high velocity particles are moving through thesmall spot size. The cathode (tool) is made of tantalum or tungsten material. Thecathode filaments are heated to a temperature of 2500to 3000and the heatingleads to thermo – ionic emission of electrons. The magnitude varies from the 25mA to 100 mA. The solidities lies between 5 Ac to 15 Ac .The emission current isinfluenced by the voltage that is nearly 150kV, and the current is applied betweenthe anode and cathode to release the electrons in the direction of work piece.

Bias grid: It is also known as grid cup. The grid cup is a negative that issubjected with respect to the filament. So, the electrons generated with the helpof the cathode will directly flow towards the anode. During the flow of theelectrodes no diversions are seen. The anode attracts the electrons and getsaccelerated; the electrons will gain a high velocity.

The cathode controls the flow of the electrons, and the grid cup used to operatethe gun in pulsed mode only.

After the anode the electron beam passing through the magnetic lens and theapertures are connected in series. The magnetic lens is used to shape theelectron beam and reduce the diversion factor.

The apertures allow the convergent electrons to permit and caught the lowenergy divergent electrons from the fringes.

Finally the electron beam passes through the electromagnetic lens and deflectioncoil. Then the deflection coil sends the electron beam through the hole, toimprove the shape to machine a hole.

The vacuum is created between the work piece and the electron beam gun, andthere is a series of rotating disc with slots.

The disc allows the electron beam to pass over the material for machining, and itprevents from the fumes and vapors generated during the machining.

Work piece is placed on the CNC bench. Then holes of any shape are made onthe work piece material. In the gun beam flection and CNC control are used toshape.

Vacuum is maintained in gun, and the vacuum ranges from Suitable vacuum ismaintained because the electron as it does not lose their energy, and where thelife of the cathode is obtained. By using the diffusion pump and rotary pump thevacuum is maintained.

Diffusion pump should act as an oil heater. If the oil is heated then the oil vaporrushes upwards. The nozzle changes the direction of the oil vapor and startsmoving in the downward direction at high velocity. The oil vapors are reduces inthe diffusion pump; this is because of the presence of the cooling water cover.

Parameters in the Electron discharge machining: We already know that the electron gun is works within the pulse mode. The bias

grid is located after the cathode. Then pulse is given to the grid cup, where thepulse duration ranges from 50to 15 ms.

Beam current is related to the electrons that are emitted from the cathode oravailable in the beam. Beam current is ranges from the 200micro amps to 1 amp.If the beam current increases, simultaneously there is also an increase in theenergy per pulse. High pulse energy is used to machine thicker plates and makethe holes larger.

The power and energy density is ruled by the energy per pulse and the nozzlespot size. With the help of the electromagnetic lens the spot size is controlled.For lower spot size they require a high energy density. The metal removal mustbe high; this is when compared to the holes size where the hole must be similar.

The plane of focusing must be above or below the surface of the work piecematerial.

Capability of Electron Beam machining process Electrical beam machining makes a hole ranges from 100 to 2 mm. The depth of cut must be 15 mm with a length to diameter ratio of nearly 10. Holes can be elongated along with the barrel shape or depth. Reverse tapper can also be performed below the surface of the work piece

material. In the electron discharge machining Cut formation is not observed With the help of the electron discharge machining we can machine the wide

range of materials like stainless steel, aluminum, steel, plastics, ceramics etc. In EBM the heat affected zone is narrow; this is because of the short pulse

occurrence. The heat affected zone is nearly 20 to 30 Compares to the steels aluminum and titanium is freely machined. Based upon the type of the material, power density, depth of cut holes diameter,

which are the reasons for the number of holes drilled per second on the material. The EBM does not apply any cutting forces on the material. During the process very simple investment is required for work EBM process allows machining of brittle and fragile materials. Holes are drilled at an angle of 20 to 30

Advantages of Electron beam machining There is no contact between the tool material and work piece material Very small holes are also machined on different type of work piece materials with

high accuracy Drilling is also done on the work piece material with a diameter of nearly 0.002

inches Drilling parameters are changed automatically during the machining Distortions are not observed to the work piece material This process is proficient in attaining high accuracy along with repeatability. Compare with the other process, formation of holes is easy with the other

process.

Disadvantages: The cost of the equipment is very high Metal removal rate during the process is low Small cut operations are performed on the work piece material with the help of

EBM machine Vacuum requirements boundaries the dimensions of the work piece material Need for secondary backing materials.

Applications of EBM: EBM is mainly used for micro machining operations on thin materials. These

operations include drilling,perforating,slotting,and scribing etc. Drilling of holes in pressure differential devices used in nuclear reactors, air craft

engines etc. It is used for removing small broken taps from holes.

PROCESS PARAMETERS:

The process parameters, which directly affect the machining

characteristics in Electron Beam Machining, are:

• The accelerating voltage

• The beam current

• Pulse duration

• Energy per pulse

• Power per pulse

• Lens current

• Spot size

• Power density

As has already been mentioned in EBM the gun is operated in pulse

mode. This is achieved by appropriately biasing the biased grid located just after

the cathode. Switching pulses are given to the bias grid so as to achieve pulse

duration of as low as 50 μs to as long as 15 ms. Beam current is directly related

to the number of electrons emitted by the cathode or available in the beam.

Beam current once again can be as low as 200 μ amp to 1 amp.

Increasing the beam current directly increases the energy per pulse.

Similarly increase in pulse duration also enhances energy per pulse. High-energy

pulses (in excess of 100 J/pulse) can machine larger holes on thicker plates. The

energy density and power density is governed by energy per pulse duration and

spot size. Spot size, on the other hand is controlled by the degree of focusing

achieved by the electromagnetic lenses. A higher energy density, i.e., for a lower

spot size, the material removal would be faster though the size of the hole would

be smaller.

QUESTIONS:

1.Explain plasma arc machining process with sketch.

2.Discuss about PAM process parameters.

3.List the advantages,disadvantages and applications of PAM.

4. Explain EBM process with sketch.

5.Discuss about EBM process parameters.

6.List the advantages,disadvantages and applications of EBM.

CONCURRENT ENGINEERING Introduction to concurrent engineering: Concurrent engineering (CE) is a method that is used in the product development process. It is different than the traditional approach from the product development in which it uses simultaneous, something that sequential, processes. By finishing the tasks in paralelamente, the product development can be obtained more efficiently and in substantial saving in costs. In the traditional approach finishing all the physical manufacture of a prototype before realizing any test, but In the concurrent engineering it allows to design and multiple analyses to happen at the same time, and at different times, before the real unfolding. This multidisciplinary approach accentuates work in equipment with the use of cross-functional equipment, and allows so that the employees work in the end of collaboration in all the aspects of a project of the beginning. Collaboration between all individuals, groups and departments within a company: Customer research, Designers, Marketing, Accounting, and Engineering. Also known like the iterative method of the development, concurrent engineering requires the continuous revision of the progress of equipment and the frequent revision of the plans of the project. The analysis reasoned behind this creative, modern approach is that whichever previous those errors can be shortages, easiest and less expensive they are to correct. The concurrent engineering professionals explain from their experience that this system of management and design offers several advantages, including the quality of the increasing product for the end user, faster times of the product development, and lower costs for the manufacturer and the consumer. There are some disadvantages associated to the putting in initial practice of concurrent engineering, including the necessity of the considerable reconstruction of organization and the extensive retraining of workers. Such potentially breaking changes and requisite aggregates of work can be fulfilled resistance of in charge and other employees. Also, there are generally considerable difficulties in data of transference between employees in diverse departments that can require additional pursuit of computer software applications. Besides these significant initial investments, the organizations whom they adopt a concurrent model of the work of engineering must typically wait for several years before considering the advantages of this transition.

Concurrent Engineering

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---------------------------------------------------------------------------------------------------------------------------------------------- Why Concurrent Engineering?

• Pace of market change has increased • Decisions made sooner rather than later • Reduces/eliminates repetition of tasks • Reduces waste and reworking of design • Product quicker to market • Maximises company profit • Company operates more efficiently

To make decisions concurrently: • Team knows the design goals/objectives • Team is aware of the interrelationships between all aspects of the design process • Superior communication between all sections of the company

AGILE MANUFACTURING Introduction

• Manufacturing industry is on the verge of a major paradigm shift. This shift will take us away from mass production, way beyond lean manufacturing, into a world of Agile Manufacturing.

What is Agile Manufacturing? • Agile manufacturing is a method for manufacturing which combine our organization, people and

technology into an integrated and coordinated whole. Why do we need to be agile?

• Global Competition is intensifying. • Cooperation among companies is becoming necessary, including companies who are in direct

competition with each other. • Customers are expecting: Low volume products, High quality products, Custom products • Very short product life-cycles, development time, and production lead times are required. • Customers want to treated and individuals.

Keys to agility and flexibility • To determine customer needs quickly and continuously reposition the company against it’s

competitors. • To design things quickly based on those individual needs. • To put them into full scale, quality, production quickly. • To respond to changing volumes and mix quickly. • To respond to a crisis quickly.

Agile manufacturing in our company • Customer-integrated process for designing, manufacturing, marketing, and supporting all products • Decision making at functional knowledge points not in centralized management • Stable unit costs, no matter what the volume. • Flexible Manufacturing-ability to increase or decrease production volumes at will. • Easy access to integrated data whether it is customer-driven, supplier-driven, or product and

process-driven. • Modular production facilities that can be organized into ever changing manufacturing nodes. • Data that is rapidly changed into information that is used to expand knowledge. • Mass customized product verses mass produced product.

Four core concepts • A strategy to become an Agile Manufacturing enterprise. • A strategy to exploit agility to achieve competitive advantage. • Integration of organization, people and technology into a coordinated interdependent system

which is our competitive advantage. • An interdisciplinary design methodology to achieve the integration of Organization, people and

technology. Nuts and Bolts

• Enriching the customer: Replace large centralized with distributed clusters of mini-assembly plants located near customers.

• Cooperating to enhance competition: Internal—cross-functional teams, empowerment & External—managing the supply chain.

• Organizing to manage change and uncertainty: Rapid reconfiguration of plant and facilities & Rapid decision making-shallow empowered.

• Leveraging people and information: Distribution of authority, resources, and rewards. Interdisciplinary Design

• Interdisciplinary design will form the basis of designing Agile Manufacturing systems in the new knowledge intensive era.

• Interdisciplinary design is one of the most important challenges to those managers and systems designers and integrators will face in the years ahead, it leads us to new approaches and new ways of working and of thinking.

• To successfully adopt an interdisciplinary design method, we need to: • Challenge our accepted design strategies and develop new and better approaches. • Question our established and cherished beliefs and theories, and develop new ones to replace

those that no longer have any validity. • Consider how we address organization, people and technology, and other issues in the design of

manufacturing systems, so we can have systems that are better for performance, better for the environment, and better for the people.

• Go beyond the automation paradigm of the industrial era, to use technology in a way that makes human skill, knowledge, and intelligence more effective and productive, and that allows us to tap into the creativity and talent of all our people.

Transition to Agile manufacturing • Make the break with the things that are wrong with the way we do things today. • Examine and define the underlying conceptual framework on which Agile Manufacturing

enterprises will be built. • Explore and understand the nature of the mass production paradigm and the nature of the cultural

and methodological difficulties involved in the transition to Agile Manufacturing. • Define a methodology for designing a 21st century manufacturing enterprise.

Real world example: • The Industry: Japanese car makers • The goal: To produce the three day car, (three days from customer order for a customized car to

dealer delivery). • The challenges:

• Break dependency on scale and economies of scale (reducing setup costs in key). • Produce vehicles in low volumes at a reasonable cost. • Guarantee the three day car. • Replace large centralized with distributed clusters of mini-assembly plants located near

customers. • Be able to reconfigure components in many different ways. • Make work stimulating. • Turn the customer into a “procurer,” an ugly neologism that means proactive something;

the idea is that the customer will take an active role in the product design by, for example, configuring options at a computer in a dealer showroom.

• Streamline ordering systems and establish close relationships with suppliers. • Manage the massive volumes of data generated by the production system so as to be able

to analyze that data quickly and agilely.

Mass Production Agile Manufacturing

Standardized products Long market life expected

Produce to forecast Low information content

Single time sales Pricing by production cost

Customized products Short market life expected

Produce to order High information content

Continuing relationship Pricing by customer value

Summary Agile Manufacturing enterprises will be capable of rapidly responding to changes in customer demand. They will be able to take advantage of the windows of opportunities that appear in the market place. With Agile Manufacturing we will be able to develop new ways of interacting with our customers and suppliers. Our customers will not only be able to gain access to our products and services, but will also be able to easily access and exploit our competencies, so enabling them to use these competencies to achieve the things that they are seeking.

Introduction to Computer Integrated Manufacturing(CIM)

1. Flexible Manufacturing System (FMS)2. Variable Mission Mfg. (VMM)3. Computerized Mfg. System (CMS)

Four-Plan Concept of Manufacturing

CIM System discussed:

Computer Numerical Control (CNC) Direct Numerical Control (DNC) Computer Process Control Computer Integrated Production Management Automated Inspection Methods Industrial Robots etc.

A CIM System consists of the following basic components:

I. Machine tools and related equipmentII. Material Handling System (MHS)III. Computer Control SystemIV. Human factor/labor

CIMS Benefits:

1. Increased machine utilization2. Reduced direct and indirect labor3. Reduce mfg. lead time4. Lower in process inventory5. Scheduling flexibility6. etc.

CIM refers to a production system that consists of:

1. A group of NC machines connected together by2. An automated materials handling system3. And operating under computer control

Why CIMS?In Production Systems

1. Transfer Lines: is very efficient when producing "identical" parts in largevolumes at high product rates.

2. Stand Alone: NC machine: are ideally suited for variations in work partconfiguration.

In Manufacturing Systems:

1. Special Mfg. System: the least flexible CIM system. It is designed to produce a

CIM System

TransferLines

Stand AloneNC Machine

Part Variety (# of different parts)

ProductionVolumn(part/yr)

15,000

15

Part Variety (# of different parts)

ProductionVolumn(part/yr)

15,000

15

SpecialSystem

ManfuacturingCell

FlexibleManufacturingSystem

8001002

very limited number of different parts (2 - 8).2. Mfg. Cell: the most flexible but generally has the lowest number of different parts

manufactured in the cell would be between 40 - 80. Annual production rates roughfrom 200 - 500.

3. Flexible Mfg. System: A typical FMS will be used to process several part familieswith 4 to 100 different part numbers being the usual case.

General FMS

Conventional Approaches to Manufacturing

Conventional approaches to manufacturing have generally centered around machineslaid out in logical arrangements in a manufacturing facility. These machine layoutsare classified by:

1. Function - Machines organized by function will typically perform the samefunction, and the location of these departments relative to each other is normally

arranged so as to minimize interdepartmental material handling. Workpieceproduced in functional layout departments and factories are generally manufacturedin small batches up to fifty pieces (a great variety of parts).

2. Line or flow layout - the arrangement of machines in the part processing order orsequence required. A transfer line is an example of a line layout. Parts progressivelymove from one machine to another in a line or flow layout by means of a rollerconveyor or through manual material handling. Typically, one or very few differentparts are produced on a line or flow type of layout, as all parts processed require thesame processing sequence of operations. All machining is performed in onedepartment, thereby minimizing interdepartmental material handling.

3. Cell - It combines the efficiencies of both layouts into a single multi-functional unit.It referred to as a group technology cell, each individual cell or department iscomprised of different machines that may not be identical or even similar. Each cellis essentially a factory within a factory, and parts are grouped or arranged intofamilies requiring the same type of processes, regardless of processing order.Cellular layouts are highly advantageous over both function and line machinelayouts because they can eliminate complex material flow patterns and consolidatematerial movement from machine to machine within the cell.

Manufacturing Cell

Four general categories:

1. Traditional stand-alone NC machine tool - is characterized as a limited-storage,automatic tool changer and is traditionally operated on a one-to-one machine tooperator ratio. In many cased, stand-alone NC machine tools have been groupedtogether in a conventional part family manufacturing cell arrangement andoperating on a one-to-one or two-to-one or three-to-one machine to operator ratio.

2. Single NC machine cell or mini-cell - is characterized by an automatic workchanger with permanently assigned work pallets or a conveyor-robot arm systemmounted to the front of the machine, plus the availability of bulk tool storage.There are many machines with a variety of options, such as automatic probing,broken tool detection, and high-pressure coolant control. The single NC machinecell is rapidly gaining in popularity, functionality, and affordability.

3. Integrated multi-machine cell - is made up of a multiplicity of metal-cuttingmachine tools, typically all of the same type, which have a queue of parts, eitherat the entry of the cell or in front of each machine. Multi-machine cells are eitherserviced by a material-handling robot or parts are palletized in a two- orthree-machine, in-line system for progressive movement from one machining

station to another.

FMS - sometimes referred to as a flexible manufacturing cell (FMC), is characterizedby multiple machines, automated random movement of palletize parts to and fromprocessing stations, and central computer control with sophisticated command-drivensoftware. The distinguishing characteristics of this cell are the automated flow of rawmaterial to the cell, complete machining of the part, part washing, drying, andinspection with the cell, and removal of the finished part.

I. Machine Tools & Related Equipment

Standard CNC machine tools Special purpose machine tools Tooling for these machines Inspection stations or special inspection probes used with the machine tool

The Selection of Machine Tools1. Part size2. Part shape3. Part variety4. Product life cycle5. Definition of function parts6. Operations other than machining - assembly, inspection etc.

II. Material Handling System

A. The primary work handling system - used to move parts between machine toolsin the CIMS. It should meet the following requirements.

i). Compatibility with computer controlii). Provide random, independent movement of palletized work parts between

machine tools.iii). Permit temporary storage or banking of work parts.iv). Allow access to the machine tools for maintenance tool changing & so on.v). Interface with the secondary work handling systemvi). etc.

B. The secondary work handling system - used to present parts to the individualmachine tools in the CIMS.

i). Same as A (i).ii). Same as A (iii)iii). Interface with the primary work handling systemiv). Provide for parts orientation & location at each workstation for processing.

III. Computer Control System - Control functions of a firm and the supportingcomputing equipment

Control Loop of a Manufacturing System

IV. Functions of the computer in a manufacturing organization

V. Functions of Computer in CIMS1. Machine Control –CNC

NCProgramming

Micro Computer(Software Function

&NC Program Storage)

Feedback

MachineCenter

Hardware(interface

&Servo)

2. Direct Numerical Control (DNC) - A manufacturing system in which a number ofm/c are controlled by a computer through direct connection & in real time.

Consists of 4 basic elements:

Central computer Bulk memory (NC program storage) Telecommunication line Machine tools (up to 100)

3. Production Control - This function includes decision on various parts onto thesystem.

Decision are based on: red production rate/day for the various parts Number of raw work parts available Number of available pallets

4. Traffic & Shuttle Control - Refers to the regulations of the primary & secondarytransportation systems which moves parts between workstation.

5. Work Handling System Monitoring - The computer must monitor the status ofeach cart & /or pallet in the primary & secondary handling system.

6. Tool Control Keeping track of the tool at each station Monitoring of tool life

7. System Performance Monitoring & Reporting - The system computer can beprogrammed to generate various reports by the management on systemperformance.

Utilization reports - summarize the utilization of individual workstation as wellas overall average utilization of the system.

Production reports - summarize weekly/daily quantities of parts produced froma CIMS (comparing scheduled production vs. actual production)

Status reports - instantaneous report "snapshot" of the present conditions of theCIMS.

Tool reports - may include a listing of missing tool, tool-life status etc.

Central

Computer

Bulk memory(NC Program)

Satellit

MinicomputerBulk

memory

m/c m/c

sends instructions & relieves data (ethernet)

Tele-Communication Lines

Up to 100 m/c tools

8. Manufacturing data base

Collection of independent data bases Centralized data base Interfaced data base Distributed data base

Production StrategyThe production strategy used by manufacturers is based on several factors; the twomost critical are customer lead time and manufacturing lead time.Customer lead time identifies the maximum length of time that a typical customer iswilling to wait for the delivery of a product after an order is placed.Manufacturing lead time identifies the maximum length of time between the receipt ofan order and the delivery of a finished product.Manufacturing lead time and customer lead time must be matched. For example,when a new car with specific options is ordered from a dealer, the customer is willingto wait only a few weeks for delivery of the vehicle. As a result, automotivemanufacturers must adopt a production strategy that permits the manufacturinglead-time to match the customer's needs.The production strategies used to match the customer and manufacturer lead times aregrouped into four categories:

1. Engineer to order (ETO)2. Make to order (MTO)3. Assemble to order (ATO)4. Make to stock (MTS)

Engineer to OrderA manufacturer producing in this category has a product that is either in the first stageof the life-cycle curve or a complex product with a unique design produced insingle-digit quantities. Examples of ETO include construction industry products(bridges, chemical plants, automotive production lines) and large products withspecial options that are stationary during production (commercial passenger aircraft,ships, high-voltage switchgear, steam turbines). Due to the nature of the product, thecustomer is willing to accept a long manufacturing lead time because the engineeringdesign is part of the process.

Make to OrderThe MTO technique assumes that all the engineering and design are complete and theproduction process is proven. Manufacturers use this strategy when the demand is

unpredictable and when the customer lead-time permits the production process to starton receipt of an order. New residential homes are examples of this production strategy.Some outline computer companies make personal computer to customer specifications,so they followed MTO specifications.

Assemble to OrderThe primary reason that manufacturers adopt the ATO strategy is that customer leadtime is less than manufacturing lead time. An example from the automotive industrywas used in the preceding section to describe this situation for line manufacturingsystems. This strategy is used when the option mix for the products can be forecaststatistically: for example, the percentage of four-door versus two-door automobilesassembled per week. In addition, the subassemblies and parts for the final product arecarried in a finished components inventory, so the final assembly schedule isdetermined by the customer order. John Deere and General Motors are examples ofcompanies using this production strategy.

Make to StockMTS, is used for two reasons: (1) the customer lead time is less than themanufacturing lead time, (2) the product has a set configuration and few options sothat the demand can be forecast accurately. If positive inventory levels (the store shelfis never empty) for a product is an order-winning criterion, this strategy is used. Whenthis order-winning criterion is severe, the products are often stocked in distributionwarehouses located in major population centers. This option is often the last phase ofa product's life cycle and usually occurs at maximum production volume.

Manufacturing Enterprise (Organization) In most manufacturing organizations the functional blocks can be found as: A CIM implementation affects every part of an enterprise; as a result, every

block in the organizational model is affected.

Sales and Promotion The fundamental mission of sales and promotion (SP) is to create customers.To achieve this goal, nine internal functions are found in many companies: sales,customer service, advertising, product research and development, pricing,packaging, public relations, product distribution, and forecasting.

sales and promotion interfaces with several other areas in the business: The customer services interface supports three major customer functions:

order entry, order changes, and order shipping and billing. The order changeinterface usually involves changes in product specifications, change inproduct quantity (ordered or available for shipment), and shipment dates andrequirements.

Sales and marketing provide strategic and production planning information tothe finance and management group, product specification and customerfeedback information to product design, and information for masterproduction scheduling to the manufacturing planning and control group.

Product/Process Definition Engineering The unit includes product design, production engineering, and engineering

release. The product design provides three primary functions: (1) product design and

conceptualization, (2) material selection, and (3) design documentation. The production engineering area establishes three sets of standards: work,

process, and quality. The engineering release area manages engineering change on every

production part in the enterprise. Engineering release has the responsibility ofsecuring approvals from departments across the enterprise for changes madein the product or production process.

Manufacturing Planning and Control (MPC) The manufacturing planning and control unit has a formal data and

information interface with several other units and departments in theenterprise.

The MPC unit has responsibility for:1. Setting the direction for the enterprise by translating the management

plan into manufacturing terms. The translation is smooth iforder-winning criteria were used to develop the management plan.

2. Providing detailed planning for material flow and capacity to supportthe overall plan.

3. Executing these plans through detailed shop scheduling and purchasingaction.

MPC Model for Information Flow

Shop Floor Shop floor activity often includes job planning and reporting, material

movement, manufacturing process, plant floor control, and quality control. Interfaces with the shop floor unit are illustrated.

Support Organization The support organizations, indicated vary significantly from firm to firm. The functions most often included are security, personnel, maintenance,

human resource development, and computer services. Basically, the support organization is responsible for all of the functions not

provided by the other model elements.Production Sequence :one possibility for the flow required to bring a product to acustomer

2.0 FLEXIBLE MANUFACTURING SYSTEMS (FMS)

A Flexible Manufacturing System (FMS) is a “reprogrammable” manufacturing system

capable of producing a variety of products automatically. Conventional manufacturing

systems have been marked by one of two distinct features:

The capability of producing a variety of different product types, but at a high cost (e.g., job

shops).

The capability of producing large volumes of a product at a lower cost, but very inflexible in

terms of the product types which can be produced (e.g., transfer lines).

An FMS is designed to provide both of these features.

Figure 2.1: Flexible Manufacturing System

FMS Components

• Numerical Control (NC) machine tools

• Automated material handling system (AMHS)

– Automated guided vehicles (AGV)

– Conveyors

– Automated storage and retrieval systems (AS/RS)

• Industrial Robots

• Control Software

2.1 Equipment of FMS

Primary equipment

Work centers

• Universal machining centers (prismatic FMSs)

• Turning centers (rotational FMSs)

• Grinding machines

• Nibbling machines

Process centers

• Wash machines

• Coordinate measuring machines

• Robotic work stations

• Manual workstations

Secondary equipment

Support stations

• Pallet/fixture load/unload stations

• Tool commissioning/setting area

Support equipment

• Robots

• Pallet/fixture/stillage stores

• Pallet buffer stations

• Tools stores

• Raw material stores

• Transport system(AGVs,RGVs,robots)

• Transport units(pallets/stillages)

2.2 Classification of FMS-related Problems

• Strategic analysis and economic justification, which provides long-range, strategic

business plans.

• Facility design, in which strategic business plans are integrated into a specific facility

design to accomplish long-term managerial objectives.

• Intermediate-range planning, which encompasses decisions related to master

production scheduling and deals with a planning horizon from several days to several

months in duration.

• Dynamic operations planning, which is concerned with the dynamic, minute-to-

minute operations of FMS.

2.3 Types of FMS

Sequential FMS

Random FMS

Dedicated FMS

Engineered FMS

Modular FMS

2.4 Application of FMS

Metal-cutting machining

Metal forming

Assembly

Joining-welding (arc , spot), glueing

Surface treatment

Inspection

Testing

2.5 FMS different approaches

The capability of producing different parts without major retooling

A measure of how fast the company converts its processes from making an old line of

products to produce a new product

The ability to change a production schedule, to modify a part, or to handle multiple

parts

2.6 Advantages of using FMS

To reduce set up and queue times

Improve efficiency

Reduce time for product completion

Utilize human workers better

Improve product routing

Produce a variety of Items under one roof

Improve product quality

Serve a variety of vendors simultaneously

Produce more product more quickly

2.7 Disadvantage of using FMS

Limited ability to adapt to changes in product or product mix (ex:machines are of

limited capacity and the tooling necessary for products, even of the same family,

is not always feasible in a given FMS)

Substantial pre-planning activity

Expensive, costing millions of dollars

Technological problems of exact component positioning and precise timing

necessary to process a component

Sophisticated manufacturing systems

Figure 2.2: Illustration example of a FMS cell

2.8 FMS Layouts

Progressive Layout:

o Best for producing a variety of parts

Closed Loop Layout:

o Parts can skip stations for flexibility

o Used for large part sizes

o Best for long process times

Ladder Layout:

o Parts can be sent to any machine in any sequence

o Parts not limited to particular part families

Open Field Layout:

o Most complex FMS layout

o Includes several support stations

2.9 FMS Problems

• Part type selection - selecting parts that will be produced in the FMS over some

relatively long planning horizon.

• Part selection - from the set of parts that have current production requirements and

have been selected for processing in the FMS, select a subset for immediate and

simultaneous processing.

• Machine grouping - partition machines into groups where each machine in a group

can perform the same set of operations.

• Loading - allocate the operations and required tools of the selected part types among

the machine groups.

• Control - provide instructions for, and monitor the equipment in the FMS so that the

production goals identified by the above problems are met.

SIMULATION OF MANUFACTURING SYSTEMS 1 INTRODUCTION: One of the largest application areas for simulation modeling is that of manufacturing systems, with the first uses dating back to at least the early 1960’s. Detailed discussions of simulation, in general, may be found in Banks, Carson, and Nelson (1996) and Law and Kelton (1991). A practical discussion of the steps in a sound simulation study is given in Law and McComas (1990). 2 MANUFACTURING ISSUES ADDRESSED BY SIMULATION: The following are some of the specific issues that simulation is used to address in manufacturing:

· The need for and the quantity of equipment and personnel number and type of machines for a particular objective.

· Number, type, and physical arrangement of transporters, conveyors, and other support equipment (e.g., pallets and fixtures)

· Location and size of inventory buffers · Evaluation of a change in product volume or mix · Evaluation of the effect of a new piece of equipment on an existing manufacturing system · Evaluation of capital investments · Labor-requirements planning Performance evaluation · Throughput analysis · Time-in-system analysis · Bottleneck analysis Evaluation of operational procedures · Production scheduling · Inventory policies · Control strategies [e.g., for an automated guided vehicle system (AGVS)] · Reliability analysis (e.g., effect of preventive maintenance) · Quality-control policies.

3 FOLLOWING ARE SOME OF THE PERFORMANCE MEASURES COMMONLY ESTIMATED BY SIMULATION:

Throughput

Time in system for parts

Times parts spend in queues

Queue sizes

Timeliness of deliveries

Utilization of equipment or personnel 4 SIMULATION PROCESS FOR MANUFACTURING SYSTEMS ANALYSIS: The process of simulating manufacturing systems involves the following phases and steps: A. Model Design: 1. Identify the issues to be addressed. 2. Plan the project. 3. Develop conceptual model. B. Model Development 4. Choose a modeling approach. 5. Build and test the model. 6. Verify and validate the model. C. Model Deployment: 7. Experiment with the model. 8. Analyze the results 9. Implement the results for decision-making. Opportunities for efficiency improvements for each phase will be discussed below.

Model Design: The model design phase is a very important, but often overlooked, part of the simulation process. In this phase, the project participants are identified, the project goals clearly delineated, and the basic project plan developed. If these activities are not done well, the model developed will likely be a very detailed model. While incorporation of detail may increase the credibility of the model, excessive levels of detail may render a model hard to build, debug, understand, deploy, and maintain. The determination of how much detail to add to the model is a primary goal of the design stage. Experienced simulationists seem to instinctively know the proper amount of detail. Perhaps an expert system or at least a system that keeps track of previous analyses and the level of detail used could be developed to assist in the model design phase. The issue of determining the appropriate level of detail becomes even more important when one models a manufacturing supply chain. While a lot of work has been done to model operations at the machine and factory levels, considerably less has been done at the supply chain level. For the most part, models are used at just one of these levels and little information is shared between them. In the future, it will become increasingly necessary for these models to be used in conjunction with one another. In order to do this, several key questions remain: what is the right level of abstraction for each model? Can parallel and distributed simulation capabilities be employed? What is the right way to share information between the levels? While manufacturing was one of the earliest simulation application areas, simulation of manufacturing supply chains is just starting to become widely used. Supply chains can be very complex, spanning multiple manufacturing sites in various locations around the globe. For simplicity, most researchers have limited the scope of their analyses to only a few broad categorical links in the chain and have taken an aggregate view of the supply chain making various assumptions about the internal workings of each chain member. Supply chain analyses are often restricted to a single representative product. Barker indicates that it is the lack of models and frameworks for analyzing the value adding capability throughout the supply chain that represents the greatest weakness in our supply chain knowledge base and current literature. The dearth of supply chain models is not industry specific, but rather seems endemic to all industries due to a lack of overall comprehension of the intricacies of the supply chain. Traditionally, simulation models of these systems have been either:

a discrete event simulation model that tracks lots through each factory in the supply chain by considering the queuing at various workcenters;

a high level continuous simulation model that does not track individual lots through the factories but simply considers the gross output of each factory. The first approach can be quite accurate, but it generally takes a long time to build the model and the execution of the model is extremely slow, making the exploration of many different scenarios prohibitive.

Execution is much faster, but a large amount of accuracy is generally lost Model Development: After the preliminary work is done and the conceptual model designed, the next phase is to develop the model. This involves choosing the modeling approach, building the model, and doing verification and validation of the model. While there typically is not a lot of efficiency to be gained in choosing the modeling approach, the choice of approach can make a large difference in the subsequent model building and model execution times. Most conventional simulation software packages used for modeling manufacturing systems take a "job-driven" worldview (also called a process interaction worldview). In this approach, manufacturing jobs are the active system entities while system resources such as machines are passive. The simulation model is created by describing how jobs move through their processing steps seizing available resources whenever they are needed. A separate record for every job in the system is created and maintained for tracking wafers or lots through the factory. A lot of execution time can be consumed when sorting lists of these jobs in a given queue or in searching the queue for a given job. Therefore, the speed and space complexity of these simulations must be at least on the order of some polynomial of the number of jobs in the factory.

Model Deployment: In the model deployment phase, the main area for efficiency improvement is in executing the model. While simulations of some manufacturing systems may not take much time to run, models of complex manufacturing systems may take several hours for a single replication, particularly when details of automated material handling systems (AMHS) are included. Anecdotal evidence suggests that these run times are not uncommon for models of wafer fabrication facilities. Clearly, simulation run times of the magnitudes mentioned above limit the number of problem solving cycles that can be achieved. In fact, the IMTI Integrated Manufacturing Technology Road mapping Project report on Modeling and Simulation identifies a key goal to be the need for “fast, accurate exploration of many more product and process design options” and the “requirement for simulation techniques that enable complex simulations to run orders of magnitude faster and more cost effectively than today”. The time to analyze the results of the simulation can be shortened by automatic generation of graphs, charts, confidence intervals, paired t-tests, etc. However, ultimately human judgment is critical, and this part of the analysis process does not provide opportunity for significant time savings.