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Chapter6Traditional ManufacturingSystemsManufacturing companies in the twenty-first century face increasingly frequentlychanging andunpredictable marketimperativescausedbyincreasedcompetitionandglobalization. These include rapid introduction of new products, constantly varyingproduct demand, and changing government regulations. To stay competitive, com-panies must use manufacturing systems that not only produce their goods with highproductivity but also allow for rapid response to market pressures and changingconsumer needs. Can traditional manufacturing systems, many of them designed inthe 1980s, cope with these new requirements?6.1 MANUFACTURING SYSTEMSA manufacturing system is defined as a collection of manufacturing machines (orstations)thatareintegratedtoperformacontrolledsetofrepeatableoperationsonrawmaterials, which alter that material to achieve a desired final form, or to assemblea final product (Figure 6.1). Assembly systems and machining systems are particularcases of manufacturing systems. In machining systems every operation is done bya machine tool that has many cutting tools deployed from either large tool holders oratoolmagazine.Inassemblysystems,themanipulativeoperationsmaybeperformedby robots, people, or combinations of both.Examples of manufacturing machines include milling, turning, drilling, andgrinding machines for machining, robots and welders for assembly, presses forThe Global Manufacturing Revolution: Product-Process-Business Integration and Reconfigurable SystemsBy Yoram KorenCopyright ? 2010 John Wiley & Sons, Inc.148metal forming, laser processing equipment for metal cutting and heat treatment,injection molding for plastic manufacturing, and more. Machining systems typicallyremove material in order to shape parts into a finished or refined state. Examples ofmachined parts include engine blocks, pump housings, and compressors. Assemblysystems are used to fit parts together in order to make a finished product. They areutilized, for example, to assemble computers or build automobiles from given setsof parts and sub-assemblies. Chemical manufacturing systems apply complexchemical processes to produce, for example, pharmaceuticals and semi-conductorswafers.When building complex products, many manufacturing processes are needed. Forexample, one machining system will receive rough castings of transmission cases asinput and mill and bore the finished shells. These are then transferred to an assemblysystem where the various gears, shafts, and other components are installed into thetransmission case. The assembled transmission is then sent to a car assembly systemthat assembles the whole car.Multi-stage Manufacturing Systems: Manufacturing systems for production atmedium and high quantities are composed of multiple stages, with each stagecontaining a piece of equipment that performs a given set of operations. When theoperationsinonestagearecompleted,thepartiallyprocessedproductistransferredtothenextstage,andsoforthuntilallneededoperationsarecompletedandtheproductisfinished. Examples of multi-stage systems may be for machining, assembly, semi-conductor fabrication, paper production, etc.Multi-stage manufacturing systems can be configured in many different ways,definedby(1)thewaythatthemachinesarearrangedinthestagesand (2)thewaythatthe part is transferred between machines (with the aid of material transport systems,such as conveyors or overhead gantries). Many manufacturing systems are arrangedsequentially, in serial lines, as depicted at the top of Figure 6.2. Serial assembly linesareverycommoninmanyindustries.Whenlargequantitiesoftheproductareneeded,or when a set of operations takes an especially long time to complete, multiplemachines (or assembly stations) may be installed at a stage to perform identicaloperations (Fig. 6.2, bottom).Figure 6.1A manufacturing system converts raw material to a useful part or product.MANUFACTURING SYSTEMS1496.2 PRODUCTION OF COMPLEX PRODUCTSIn this section, we present two examples of manufacturing operations flow in theproduction of complex products. Both examples involve several manufacturingprocesses that must be synchronized to avoid large buffers (inventories) betweenoperations and between processes.6.2.1 Production of AutomobilesYourcarortruckisthemostcomplexproductthatordinarypeoplebuy.ItisbyfarmorecomplexthanyourcomputerorTVset.Theproductionofautomobilesismultifacetedandrequiresexpertiseinavarietyofmanufacturingareas:stamping,casting,injectionmolding, welding, machining and assembly, as shown in Figure 6.3.*Automobile production begins at the stamping plant where huge presses stampsheet metal into metal dies to create the panels that make up the automobile. Thevarious panels are shipped to an auto-body automated assembly plant, in whichwelding robots join the metal panels and create the overall shape of the car, what iscalled the body-in-white. In parallel, the doors are manufactured in another stampingplant and then transferred to the auto-body assembly plant and assembled manuallyonto the body-in-white. The automobile is then sent on a moving line for spraypainting, which is performed by robots.Simultaneous with the auto-body fabrication, powertrain plants manufacture themajor drive componentsengine blocks, cylinder heads, crankshafts, and transmis-sioncasesbymachiningoperations(theleftsideofFigure6.3).Therawpartsforthemachining operations arrive from casting plants. The powertrain plant primarilyuses high-precision machine tools arranged in multi-stage systems (often 50100Figure 6.2Examples of multi-stage manufacturing systems: a serial line (top), and afour-stage hybrid system (bottom).*The flowchart of automobile production was composed by the author.150TRADITIONAL MANUFACTURING SYSTEMSmachines per system), and inspection equipment to measure the machined parts totolerances as precise as 10mm. The powertrain plants are the most expensive plantsin the auto industry, more expensive than assembly plants by far. In parallel to themanufacturing of the major powertrain components, first-tier suppliers make otherpowertrain components, such as pistons. These parts are added to the engine block,together with the electrical wiring, fuel lines, and belts, in the powertrain assemblysection of the engine production plant.At the same time, other first-tier suppliers build the entire front instrument panel(IP)sub-assembly,includingair-conditioningandtemperaturecontrol,audiosystem,airbags, gauges, etc. The components in the IP are supplied to the first-tier supplierby second-tier suppliers. Other first-tier suppliers build the car seats, the exhaustsystem, etc.Afterpainting,thevehicleshellenterstheassemblyshop.Atypicalassemblyshophas three sections: trimming, chassis assembly, and final assembly. The cars entertrimmingonmovingplatforms,eachcaronaseparateplatform,whichtraveloneafteranother in a line. Trimming is a manually intensive assembly process; workers,arrangedinteams,standontheslowlymovingplatformandassemblevariouspartsinthe car interior, such as the IP and steering column, as well as the windshield, lamps,bumpers, and exterior molding. Each team of workers is responsible for a limited setof tasks that takes about 50seconds. They are doing these tasks repetitively all day.Figure 6.3A flowchart of automobile production.PRODUCTION OF COMPLEX PRODUCTS151Notethatthe(quitecomplex)IP,forexample,isshippedfromthefirst-tiersupplierasone sub-assembly and installed in the car at the line cycle speed of about 50secondsincluding all of its various connections.Next,thechassisassemblyisdone.Inchassisassembly,thebottomofthecarisputtogether. There are plants in which the whole chassis, including all of the powertrainand exhaust system components, is pre-assembled and attached to the car at the linecycle speed, again within 50seconds. Finally, the car moves to the final assemblysection where the car seats and wheels are installed, and fluids are added. The enginestarts,andthecarrollsofftheassemblylineonitsownpower,atanaveragerateofonecarperminute.Keepinmindthatcarsrollingoutatarateofoneperminutemustalsobe sold at a rate of one per minute.Our student, Craig Ashmore, described the manufacturing process of the IP.“I was recently at an IP manufacturing plant and saw this process first hand. The IP beingbuiltwasaskinandfoampanel.Theskinswerecreatedaboutevery34minutes.Theskinsthen went over to the next line where foam was put between the retainer and skin and thensenttoawaterjetline.Thewaterjetcutoutalltheextraskinmaterial.TheIPwasthensenttoanareawhereACventswereinstalled.Immediatelythetoppanelwassenttoalinewhereitwasmatedtothecross-carbeamandalltheotheritemswereadded.ThecompleteIPwasthen loaded onto racks and loaded on a truck to be shipped that day to the plant. The IP isthen installed into a car the day it is received at the manufacturing plant. I was veryimpressedattheefficiencythatwasusedintheplant.ThewholeIPlinewascontainedinanarea about 1000 square feet.”6.2.2 Production of AppliancesThe appliance industry is a growing global manufacturing sector. It includes theproduction of washers and dryers, refrigerators, freezers, dishwashers, ranges,compactors, room air conditioners, and microwaves, together with portable appli-ances such as stand mixers, hand mixers, and blenders. The 100 top global appliancecompanies earned combined sale revenues of more than $900 billion U.S. in 2005.1Whirlpool Corporation, for example, accounts for over $14.3 billion of these sales,and its brand names are marketed in more than 170 countries worldwide.*One of the most common appliances in the global market place is the refrigerator.In 2007 there were 10 million refrigerators exported from the United States bydomestic home appliance manufacturers. The manufacturing of a refrigerator isa multifaceted process and requires the expertise of many skilled workers as well asmanufacturing processes such as extrusion, thermoforming, stamping, welding, andinjection molding.*This section was written by Kate Presnell, Lukasz Skalski, and Katherine Warner, former students of theglobal manufacturing class, who are Whirlpools employees; see also .152TRADITIONAL MANUFACTURING SYSTEMSLike most manufacturing processes, the refrigerator begins with raw materials;most notably in the form of plastic pellets and coils of steel. A number ofpreparation steps and sub-assemblies occur before the product enters the finalassembly line. The first of these sub-assemblies is the door and can be found on theleft side of Figure 6.4. The plastic pellets are extruded into a sheet, which is thenthermoformed. The thermoforming process includes heating up the plastic linersheet, forming it over a mold, and cooling the product in a temperature-controlledenvironment. The liner is then assembled to the metal doors. The doors are formedby straightening the coils of steel into flat sheets, then cutting, stamping, andwelding them accordingly.Betweentheplasticlinerandthemetaldoor,theappropriateinternaliceandwatersub-system components are attached. These components typically consist ofconduits for the wiring harness and water inlet, as well as the reinforcing structurefor the exterior elements of the system added later in the assembly process.Expanding adhesive foam insulation is then sprayed into the vacant space betweenFigure 6.4A flowchart of refrigerator production.PRODUCTION OF COMPLEX PRODUCTS153the inner and outer doors. This foam serves as the insulating agent as well as adds tothestructuralstabilityofthe door. This sub-assemblythen proceeds downaconveyorto where the external door features including dispenser electronics and housing,handles, and water inlet hose are added.The other major sub-assembly consists of the main refrigerator cabinet and itscomponents. The cabinet is formed much like that of the door described earlier, of aninner liner and metal exterior. These two parts are assembled and foam insulation isagain injected into the vacant space between the two pieces and allowed to expand.Once again, the foam serves to both insulate the cabinet and add structural rein-forcement. The cabinet is then equipped with the cooling system that consists ofacondenser,compressor,evaporator,andnecessaryconduits.Theseconduitsarethencharged with refrigerant.Thecabinetthentravelstothemainassemblyline.Here,theremainingfeaturesareinstalled: shelves, drawers, bins, etc. Once the inside of the refrigerator is assembled,thedoorsareattached.Thereisthenaqualitychecktoensuretherefrigerationsystemissealedwithnoleaks.Somerefrigeratorsmayproceed toperformancetestingforanadditionalqualitycheckwhilethemajorityoffinishedgoodsarepackagedandsenttothe distribution centers.6.3 THE STATE OF ART AT THE END OF THE TWENTIETH CENTURYMulti-stage manufacturing systems are made up of many machines or assemblystationsconnectedviaamaterialtransfermechanism.Thetransfermechanismmaybeanoverheadgantrysystem,aconveyor,oranautonomous-guidedvehicle(AGV).Themachinesmaybededicatedtoonefixedsetofoperations,flexiblesuchasrobotsinanassembly system, or programmable computerized numerically controlled (CNC)machinetools.CNCcontrollersareveryversatileandcanbeeasilyappliedinmilling,turning, drilling, grinding, shaping, laser processing, etc.To be regarded as a true manufacturing “system,” the operation of at least thecritical elements of the line must be coordinated through a supervising centralcommand station where a computer controls and synchronizes the operations ofthemachines.Thetwomostcommonmanufacturingsystemtypesthatwereavailableatthe end ofthe twentiethcenturywere the dedicatedmanufacturingline (DML)andflexible manufacturing system (FMS). Many manufacturing industries combinea portfolio of dedicated and FMSs to produce their products. This approach hasevolvedbecauseneitherapproach,onitsown,isadequatetomeettheever-expandingchallenges of a global economy of rapidly changing demands and consumer tasteswhile maintaining low prices.6.3.1 Dedicated Manufacturing LinesDedicated lines (also referred to as “transfer lines”) are designed to produce just oneproduct, butatveryhighthroughput.(Theaverageoutputofaproductionmachineor154TRADITIONAL MANUFACTURING SYSTEMSsystem, measuredinpartspertimeunit, iscalled throughput.)TheDML isbased ona collection of relatively simple machines arranged sequentially in a line, where theprocessed part moves in a synchronous manner from one machine (or station) tothe next. The high throughput of DML is cost-effectivewhenvery large quantities ofthe product are needed.An example of a dedicated machining line composed of 10 dedicated machiningstations is shown in Figure 6.5. This system was built to produce just one part. Anoperator loads the raw parts and unloads the finished parts. The part moves along theconveyor and stops at each station for processing. In some stationsseveral tools workon the part simultaneously. Two stations in the system of Figure 6.5 even havemachines that do machining operations on both sides of the part simultaneously. TheDML control is done by fixed automation (called hard automation) that in practicecannot be changed. This production line has very high throughput of identical parts.Once in place, DMLs operate reliably and arewell suited to produce high volumes ofidentical parts at great efficiency.Each station or machine in a DML performs a single unchangeable operation onevery part passing through. Special multi-spindle tools that drill or tap a fixed patternofseveral holesat onestroke contributetothe highthroughput ofDMLsthat producemachined parts. The pattern of holes fits only certain part geometry, as shown inFigure 6.6.Figure 6.5A dedicated machining line.THE STATE OF ART AT THE END OF THE TWENTIETH CENTURY155DMLs have been a basic element of high-volume manufacturing for generations.True to the mass production paradigm, since their output volume is high, the cost perpart is relatively low. Once fully in place, DMLs perform at a constant designed rate,outputting identical high-quality parts at large quantities. However, with increasingpressure from global competition, the demand for specific parts can vary widely.Situations oftenoccur where dedicated lines donotoperate atfullcapacity,andwhenthat happens the cost per part is much higher.For example, a dedicated line can produce an engine block every 30seconds. Aslong as the dedicated line operates at its planned capacity, it produces many parts atvery attractivecosts. However, if engines are produced at a rate of 30seconds each, itstands to reason that they have to be assembled into corresponding numbers ofvehicles that will be sold at a rate of a car every 30seconds. What happens when carscannot be sold at a rate of one every 30seconds? Then the rate needed has beenreduced, say to one every three minutes. When that happens the dedicated line isunderutilized,andlosesitseconomyofscale.Thecostperproductbecomeshigher.Infact, because of increased market volatility and competitive pressures, DMLs areincreasingly underutilized. A report published in Italy in 19982indicated that theaverage utilization of the surveyed DMLs in a European auto manufacturer was only53%! One common reason for this low average utilization is that in the introductorystages of new products, or at the end of a product life cycle, fewer parts are required,considerably lower than optimal volumes. But because of global competition, evenproducts in the mature phase do not always reach the production volumes forecastwhen the line was designed.Attheotherextreme,DMLsalsofailwhendemandgoesabovethedesigncapacity(Figure 6.7). If a products popularity exceeds all market expectations, or when newFigure 6.6A multi-spindle head.156TRADITIONAL MANUFACTURING SYSTEMSuses are found for existing products, the DML is powerless to respond, resulting ina loss of sales.If market demand for a product increases quickly, the fixed maximum capacity ofthe DML does not allow the manufacturer to grab the opportunity to produce and sellmore products.Thisresults in substantial losses in potential sales and actually entailsmuch larger economic losses than those from unused capacity.In the quote below we cite Ronald Zarrella, a former President of General Motors(GM),whodescribedhowashortageofengineblocks(acomponentproducedlargelyon DMLs) caused a loss in sales of entire vehicles.General Motors Corp.s top executive said that a sharp rebound in market sharewouldnotbepossibleiftheindustrysalescontinueattheirtorridpacebecauseGMcould not produce enough full-sized pickup trucks to meet demand. “GM simplycannot make enough big V8 engines to build all the full-sized pickup trucks itneeds to meet the market share goal,” said Ronald Zarrella, the president of GMsNorth American Operations. But Zarrella said low gasoline prices, which helpsalesofsportutilityvehiclesandpickuptrucks,werestillbetterthanhighgasolineprices, which help sales GMs low-profit cars, for which it has extra productioncapacity. “If gasoline prices rose, he said, thered been positive in terms ofmarket share and lots of negative in terms of profitability.” (The New York Times,February 12, 1999).The main drawback of DMLs in the globalization era is that these lines are notdesigned to change, and therefore they cannot be converted to produce new productseasily. In the globalization era, the marketable lifespan of a product becomes shorterandshorterwithcompetingproductsbeingintroducedfasterandfaster.ThisallmakesFigure 6.7A DML system is an excellent choice when it is operating at its designedthroughputcapacity,whichcannotbeeasilychanged.Lossescanoccurinunmetdemandwhen a product is popular, as well as in unused capacity as its popularity wanes.THE STATE OF ART AT THE END OF THE TWENTIETH CENTURY157the building of DMLs uneconomical, and for this reason they are vanishing in manymanufacturing industries.6.3.2 Flexible Manufacturing SystemsAs we have said, DMLs cover the high-volume/low-variety work and stand-alonecomputer numerically controlled (CNC) machines are for low- to medium-volume,high-variety production on the other end of the spectrum. The middle ground (mid-volume/mid-variety manufacturing) is taken by FMS and by reconfigurablemanufacturingsystems.TheFMSisdefinedas:Anintegratedgroupofprocessingunits, such as CNC machine tools, linked by an automated material handlingsystem, whose operation is controlled by a supervisory computer.Historyof Flexible Manufacturing Systems: One of the firstfirms to developanintegrated manufacturing system was Molins Company in the United Kingdom. In1967 this company presented the “Molins System 24,” a flexible and integratedsystem demonstrating a novel way to increase productivity. In this system severalmachining stations were linked by an automated handling system for transferringparts that were mounted on pallets.Fouryearslater,in1971,Sundstrand(inIllinois,USA)developedthe“ShuttleCarSystem”, a rail-type pallet transfer system on which parts flow to and from themachining stations, located along the rail track.3This system, however, was suitableonly for long and variable machining times.At the Leipzig Spring Fair in 1972, Auerbach, a German machine tool builder,presented their manufacturing system “M250/02 CNC”. It was quipped with twothree-axis machining centers, three two-arm changers, and one four-arm robot; thissystemenabledacompletefive-facemachiningofprismaticparts.Acentralcomputerwas used to control the machining centers, but the part handling was done manuallyfrom an operators station.In the mid-1970s FMS emerged for producing small batches of many differentparts.CincinnatiMilacron,amachinetoolbuilderinOhio,wasanindustrypioneerinthedevelopmentofFMSs.4Productioncellslinkedwithautomatedmaterialhandlingsystems emerged in the 1980s (e.g., by Max M uller and Fritz Werner). In the last20 years of the twentieth century FMSs proliferated throughout the industrializedworld,althoughthetrendinsomeindustrieshasbeentoutilizesmaller,lessexpensiveflexible manufacturing cells.FMS is a major enabler of mass customization because it can produce a variety ofcomponentsorproducts withinitsstated capability.FMSalsoenables theredesign ofproducts to meet new market requirements without adding substantial investments inthe manufacturing system. Most of the experience gained with FMS installations hasbeen with machining systems.5,6ThebuildingblocksofFMSsareCNCmachines(inmachiningsystems),orrobots(in assembly systems and automatic welding lines). Both have sophisticated oper-ational controllers integrated with a material handling system that transfers the partsbetween machines and/or assembly stations. CNC machines in flexible machiningsystems include machining centers, drilling machines, and laser cutters, and158TRADITIONAL MANUFACTURING SYSTEMSsometimes automated inspection machines. Typical material handling systems areconveyors,overheadgantries,andAGVs.ThesuccessfuloperationofawholeFMSisbased on coordinating the operation of its flexible pieces of equipment.In order to perform various operations quickly, CNC machines are equipped withrotatingtoolmagazinesandautomatictoolchangers(ATCs),asdepictedinFigure6.8.Thevariety of tools in the magazine enables CNC machine tools to perform differentmillinganddrillingoperationsonthepartononemachine.IfthetoolmagazineoftheCNCmachineislargeenough tocontainallthetoolsneededtoproduceseveralparts,the CNC can even be programmed to produce a variety of parts.In machining systems parts are located and clamped on fixtures, which allowstransferring the part for succeeding machining operations. The fixtures are built toprecisely fit the part geometry, and their clamps must be located such that they allowfor cutting tool accessibility. The fixtures with their parts are mounted on pallets,which are transferred between machines through the whole system.The data stream sent to the CNCs, and the robots that constitute the FMS, isasequenceoftasksthattheyhavetoperform(e.g.,drill,changetool,mill,weld,movea part, etc.) in order to manufacture a particular part. This data stream is called a partprogram. The machines and robots are designed to accurately follow the series ofnumerically defined steps over and over. Producing a new part type requires loadinganewpartprogramaswellashavingtherighttoolsinthetoolmagazineandusingtheproper fixtures to hold the new parts.If the tool magazine of each CNC machine contains the tools needed to produceseveraltypesofparts,theFMScanrapidlyswitchitsproductionfromoneparttypetoanother.TheoperationofanFMSproducesseveraltypesofparts,eachtypeinabatchofmultipleidenticalparts.Forexample,ifthreepartsA,B,andChavetobeproduced,Figure 6.8A machining center with a chain-type tool magazine and an automatic toolchanger.THE STATE OF ART AT THE END OF THE TWENTIETH CENTURY159then a certain number (called the lot size, or batch) of parts A are produced first, thenabatchofpartB,followedbyabatchofpartC,andthenabatchofpartAisproducedagain.The size of the tool magazine and the total number of operations needed on eachpart are the two important factors in deciding the number of parts that a machine canproduce without replacing tools in the magazine. However, when the tool magazinesinasystemaretoosmalltocontainallthetoolsneededtoproducealltypesofparts,anoptimization algorithm that balances the workload per machine can be utilized toallocate the operations (and their cutting tools) of a selected subset of the parts thatwill be processed.7Adding sensors to the machines, and transmitting their data continuously to theCNCcontroller,greatlyenhancesmachinediagnostics.Typically,accelerometersandvibration sensors are used, although other sensor types such as acoustic emissionmonitoring and force/torque sensors are sometimes utilized. Cutting tools may bechanged out based on these sensor readings, the number of parts cut by the tool, or atthe end of each work shift.6.3.3 Is a Pure-parallel FMS Practical?ItisacommonassumptionthatFMSshouldbeabletoproduce(1)anypart(withinthemachine envelope), or (2) any mix of parts produced at any sequence (namely, non-batch operation).But only a purely parallel FMS can fulfill these requirementseffectively without losing time because of balancing processing-time issues betweenstages. A purely parallel FMS, however, is not designed for multi-stage operations; itisratheragroupofCNCmachinesarrangedinparallel,whereeachmachineproducesthe entire part (Figure 6.9).This parallel configuration provides the highest flexibility and efficiency inproducing a variety of parts or products simultaneously. Parallel FMS may be foundin the aerospace and opto-mechanical industries, for example.Figure 6.9A parallel FMS.160TRADITIONAL MANUFACTURING SYSTEMSHowever, significantly increasing the number of fully equipped CNC machinesconnected in parallel creates a reverse economy of scale in the system. First, eachCNCmachinemustincludetheentiresetoftoolsneededtoproducethewholeparteven though only one tool operates at a time, meaning that many expensive tools sitidle as they wait their turn to be used. Second, large magazines of tools must beattached to each CNC, which increases each machines cost. These two points areamplifiedwhenthemachinesproduceseveraldifferentpartsthateachrequiresitsownset of cutting tools.Paying for unused machining horsepower can also be a cost issue. While highhorsepower might be required for a few operations (e.g., large surface milling), it iswastedonalltheotherswhereitisnotneeded(suchasdrillingofholesinthecasting,whichrequireslowpower).AnadditionalcostfactoristhateveryCNCmachineinthesystem must be designed with a wide geometric range of motions to accommodatethelargestandmostcomplexoperationneeded.Thesefourreasons(alargenumberoftools, large tool magazines, high machine power, and machine geometric size)increase the cost of each CNC machine, and thus the entire cost of the parallel FMScan become very large.CaseStudyParallelFMS:Atthe1996InternationalMachineToolShow(IMTS)inChicago,MakinodemonstratedthefullmachiningofaGMengineblockwithasinglefive-axis CNC machining center. After the initial machining in this demonstration, thesemi-machined block wastakenautomatically out of the machine,flipped 90?,or180?,(byaspecialmechanism),andtheninsertedbacktothemachineworktableforcontinuedmachining. The Makino staff in the booth told me that GM bought 96 five-axes CNCmachines,tobeinstalledatGMplantsinapure-parallelarrangement.Eachmachine,theysaid, could produce an entire engine block or an engine head (except for line boringoperations) in the same way that was demonstrated at IMTS.These 96 machines were installed at a GM engine plant in Michigan to produceboth engine blocks and heads of a particular six-cylinder engine. That machiningsystemwentintoproductioninearly1998,andhadacapacityof600engines/daywitha very good uptime. However, the machines were not installed in a purely parallelconfiguration,astheMakinofolksannouncedatIMTS.GMconcludedthatitwasnotpractical to machine an entire block or entire head on a single machine in one setup.For one thing, the part has to be gripped somewhere, and therefore all of its sidescannot be accessed in one setup.A single machine doing two setups that are switched using a special dedicatedmechanism is also impractical. In fact, it is actually impossible to do it and stillachieve the required precision. Although the fewest possible stations (or setups)requiredformachininganentireblockorheadwouldbetwo,thisisstillonlypossibleif one can perform engine assembly operations (e.g., inserting valves) as well asmachiningoperationsinthesamesetup.Butperformingmachiningandassemblyonasingle station is very impractical and risky because one has to find a way to wash thepart and keep it free of contamination before the assembly.Furthermore, when a complex part is machined, some operations must come afterothers. In cylinder heads, for example, one must machine a valve seat pocket beforepressing in a powdered-metal valve seat. But machining thevalve seat itself can onlyTHE STATE OF ART AT THE END OF THE TWENTIETH CENTURY161befinishedafterithasbeenpressedinplace.Thismeansthatthecylinderheadmustbetaken off the machine for valve seat insertion, and then put back on a machine forfinish valve machining.For all these reasons, the GM cylinder machining system was installed in sixparallel lines, each with eight machines to run eight different machining operations(i.e., eight stages). This system contained 48 of the Makino machines. GM could runthe system with or without crossover among these parallel lines. Throughputincreased tremendously, however, when crossover was allowed. The engine blockmachining system had a similar structure.Theconclusionisthatforcomplexpartswithseveralfacestobemachined,apurelyparallelsystemofCNCmachinesisabsolutelyimpractical.TheGMsystemproducedthese engines (blocks and heads) until 2002, when the product was phased out.To conclude, whether arranged in parallel or not, FMSs are expensive. They areexpensive for several reasons; most particularly because the equipment, whichpossesses features enabling general flexibility, is expensive to build and maintain.They are also expensive in the sense that companies sometimes purchase morefunctionality than they really need and the extra functionality is never utilized, or isonly used after several years (see the survey in Section 6.5). Nevertheless, due todramatic reductions in the price of CNC equipment in recent years, FMS is still afrequent optimal choice for producing parts in medium to high quantities.6.3.4 Comparing DML and FMSDMLs are based on fixed(or hard) automation and produce a single part invery largequantities. Dedicated equipment tends to be relatively inexpensive to buy and simpletomaintain.However, altering anyelementofaDMLin ordertoadd anewoperationrequires a lot of downtime and expense to return the line to optimal efficiency.Changing the line in order to make a new product will take a couple of years and iscompletely uneconomical for runs shorter than several years.On the other end of the spectrum FMSs are composed of computer numericallycontrolled (CNC) machines and other programmable automation, and are able toperform many different operations. FMSs can produce a variety of products on thesame system. The production capacity of FMSs is much lower than that of dedicatedlines and their initial capital cost is higher. The spectrum of products that are beingproduced on FMS is quite large, from optical parts for missiles, to aircrafts, car engineblocks, and even mass-customized shoes.The common denominator for both DML and FMS-type systems is that they usefixed hardware. FMS and CNC operate with fixed software. Although part programscan be changed on CNC machines, neither the core software nor the controlalgorithms can be altered by the user.The one practical advantage the DML has over FMSs is throughput. Flexiblemachining systems composed of CNC machines are more expensive and slowercompared to dedicated machines, to produce the same number of parts. Consider, asan example, that on a CNC, a cluster of drilling operations is preformed with a singletool rather than applying a multi-tool “gang” drilling device such as a dedicated162TRADITIONAL MANUFACTURING SYSTEMSmachine could use. Furthermore, even though the CNC is capable of differentmachining operations, each one requires a different tool and there is a delayfor each tool change. The distances each tool must travel to and from the toolchanger (i.e., tool positioning) are not a value-added operation and add time tocomplete production. These extra motions needed for tool positioning and toolchanges make the CNC much slower than dedicated equipment. In contrast, in aDML,awhole blockfulloftoolsmightbeoperatingat thesametime,performingtheprocess at very high rates of speed. Because they are slow, more CNC machines areneeded and the entire FMS becomes more expensive than the DML.In summary, typical DMLs can produce a single part at very high productionvolume,andforthisreasontheycanstillbeefficientforbasicworkoncoreproductsofthe company. In the spectrum of manufacturing systems, DMLs are at the highestvolume per product and the lowest functionality position, as depicted in Figure 6.10.High functionality means a system includes many functions (and tools) that enable itto move very quickly from the production of one product to another. This is the areawhere FMSs excel. FMSs are highly flexible in converting from one part operation toanother,butbecauseoftheirslowerthroughputandhighercost,theyarebetterappliedto those operations that change frequently.The comparison between the dedicated and flexible systems, shown in Table 6.1,identifies key limitations in both types of systems.Theworkforceskilllevelrequiredbythetwosystemsisverydifferent.Dedicated,transfer lines require basic skills from most of the operators. CNC machines (usedin FMS and RMS) require much higher skill levels. FMSs require skilled labor for(1) loading part programs, (2) changing worn cutting tools in the tool magazines ofthe machining centers, and (3) performing maintenance, diagnostics, and repairs onall components.The operators must possess some familiarity in computers anda knowledge in operating machines via computer screens, including reading basictrouble-shooting instructions and reacting accordingly.Figure 6.10Volume-functionality spectrum.THE STATE OF ART AT THE END OF THE TWENTIETH CENTURY163The system design focus is perhaps the most important difference between DMLand FMS. A designer cannot just design a dedicated line if the part to be processed isnotgiven.ThatmeansthatwhenconsideringaDML,thesystemdesignfocusisonthepartfirstand foremost. Thefocus onthepart atthe designstageenablesopportunitiesfor simultaneous multiple-tool operation (e.g., gang drilling) that, in turn, enhancessystem productivity, which creates even more cost-effective DML systems.Incontrast,thedesigneffortsformostFMSarefocusontheequipmentstandardCNC machines and robots. Unlike DML stations, CNC machines are not designedaroundthepartorevenapartfamily.Rather,general-purposeCNCsarebuiltaroundageneralizedoperational envelope,designed beforethe manufacturer determinestheproduct to be built. Only when CNCs are selected to make up a system, is processplanning undertaken to adapt the process to the part. In most cases, CNC machinebuilders do not know ahead of time what specific applications their machines will beused for when they design them. That is why flexible systems and machines aretypically constructed with more features than are ever really needed to produce theparts they will manufacture. Customers, in turn, pay for things that they do not reallyneed and the extra functionality amounts to a waste of capital.A serious drawback of the DML system comes from the lead time required forproduct design. Basically, a specific product design needs to freeze many monthsbefore production is to begin so that there is time to design and tune the DML. Withproduct development times shrinking, implementing a DML becomes problematic.Toconclude,Table6.2summarizesthemainfeaturesofDMLandFMSaswehavediscussed above. Further discussion on convertibility and scalability is presentedbelow.System Convertibility is defined as the ease of rapidly adjusting a systemsproduction functionality, changing the production of one product to a new one beingintroducedtothemarket.Systemconvertibilitymusttakeintoaccount(1)therangeofconvertibility of the machines in the system, (2) the arrangement of the machines inthe system, and (3) the material handling devices that connect the machines. As arule, better convertibility usually makes the capital investment in a system moreexpensive.Deprived convertibility is a major limiting factor of how quickly new productscan be introduced. For example, in a serial line (Figure 6.11a) the configuration has aTABLE 6.1Comparison Between DML and FMSDMLFMSLimitationsNot flexiblefor a single partExpensiveFixed capacitynot scalableSlowsingle-tool operationAdvantagesLow costConvertible for new productsFastmulti-tool operationScalable capacityWorkforce skillsBasicRequire computer knowledgeHardwareFixedFixedSoftwareNoneFixed164TRADITIONAL MANUFACTURING SYSTEMSminimum increment of conversion of 1.00 or 100%. That is, in order to introduce anew product, the entire line must be shut down, changed over, reconfigured (ifpossible at all), and restarted. The configuration in Figure 6.11(b), however, can bepartially converted to a new product after only 50% of the machines have been shutdown and reconfigured (a conversion factor of 0.50).The convertibility of the completely parallel configuration (Figure 6.11e) is thebest; this one can be converted to a little production of a new product after only 16%of the machines have been shut down and reconfigured. This pure parallel config-uration is valuable when a company wants to introduce new products to the marketas quickly as possible, and then later ramp up to full production. In contrast, DMLsdo not have any degree of cost-effective convertibility because they are not designedfor change.SystemScalabilityisdefinedasthe ease ofrapidly adjusting productioncapacity,or changing a givensystems throughput from oneyield to another as needed to meetchanges in market demand. We define system scalability asSystem scalability 100?smallest incremental capacity in percentageScalabilityisthecapacityincrementbywhichthesystemoutputcanbeadjustedtomeet new market demand. For example, in configuration (a) in Figure 6.11 theTABLE 6.2Features of DML and FMSDedicatedFMS/CNCStructureFixedFixedSystem design focusPartMachineConvertibility/flexibilityNoYes (general flexibility)Volume scalabilityNoYes in parallel FMSMulti-tool operationYesNoProductivityHighLowLifetime investment costLowReasonablewhen fully utilizedfor production of many partsFigure 6.11Five scalable configurations.THE STATE OF ART AT THE END OF THE TWENTIETH CENTURY165minimumincrementofaddingproductioncapacityis100%ofthesystem(i.e.,addinga whole new line); we define its increment as: Scalability100?1000%. Thus,zero scalability means that in order to increase the system capacity beyond itsmaximum, the entire line must be duplicated.Similar calculations show that Configuration b has a scalability of 50%,Configuration c has 67%, and Configurations d and e have a scalability of84%. That means that a minimum increment of one sixth of the systemin this case,one machinecan be added to increase the system capacity (e.g., a machine can beadded to stage 2 of configuration d).ScalableCapacity:Dedicatedlinesdonothavescalablecapacityandcannotcopewithlargefluctuations inproductdemand.ThischallengecantheoreticallybemetbyFMSs that are scalable, when designed with CNC machines that operate in parallel,but the economics of implementing a parallel FMS is questionable.The system designers have to weigh the amount of capacity they need to add. Forexample,toaddcapacitytoConfigurationa,100%oftheequipmentwouldneedtobeduplicated. To add to Configuration e, only 16% of the equipment needs to beduplicated. However, adding any capacity to Configuration a doubles the actualoutput. Instantly doubling capacity is rarely appropriate and so, in most cases, is notjustified.In summary, one can see how system responsivenessconvertibility and scal-abilityisa criticalconcern ina manufacturingsystem. It playsa keyrole ingaininga competitive advantage for a company in the globalization era.Two of our students, who work at the auto industry, shared their practicalexperience:“The discussion on dedicated manufacturing lines and the comparison to flexiblemanufacturing systems matches exactly to my own experience (in 2003). I am workingwiththreeplantstoimplementaproductchange.TwositeshaveFMS-typeequipment,andonesitehasadedicatedmanufacturingline.Thedifferenceincostandtimetoimplementachangeissignificant! Whatcanbe accomplishedinsixmonthson the FMSequipmentwilltake18monthsontheDMLequipment,andcostmuchmore.Butthecostperunitproducedis lowest at the DML site. Therefore, the DML site generates high profits, but only if themarket is stable. The longer changeover could result in lower profit from lost sales.”Dan Gulledge“I have good experience with flexible manufacturing systems and its odd to hear of anFMS as being traditional. But then, if you look into dedicated lines, there is somewhata factor of flexibility about them today. I currently (2004) supervise a dedicated line thatproduceslost-foamengineblockandheadcastings,andeventhoughthatisallweproduce,the tooling can be switched from block to head on each machine. Although the castingsector is dedicated, it too can be outfitted with the others tooling to produce the oppositeproduct. Even engine block machining lines are flexible enough to be altered for differentblocksandarejustascapableasputtingupthenumbersasdedicatedlinesdo.Isupposethedifferenceswouldbeinthetoolingtoaccomplish,suchasgorgetomill,butotherthanthat,systems even 15-20 years old now are flexible somewhat.” Greg Wood166TRADITIONAL MANUFACTURING SYSTEMS6.4 ASSEMBLY SYSTEMSAssembly systems are utilized invirtually all types of durable goods manufacturing.There are three basic types of assembly systems: (1) manual assembly, which iscarried out by human assemblers, usually with the aid of simple power tools. This isthe most flexible assembly system, since humans are very “flexible” and can easilyadapt to perform new tasks. This type of assembly is the norm in assembly of anycomplexproductsandespeciallyinautomotivefinalassembly. (2)Assemblysystemsthatcombine humanassemblersandautomated mechanisms.Thistype iscommoninassembly of mass-customized personal computers as described below. (3) Fullyautomated assembly systems for mass-produced parts, and particularly in hazardousenvironments such as in welding of auto body panels (an assembly operation).Theinventionofprogrammableindustrialrobotsintheearly1980sacceleratedthedevelopment of automated assembly systems.8Assembly robots are equipped withvarious “end effectors” that can perform simple operations such as inserting screwsand grasping and placing parts. Simple grippers containing two or three fingers canholdpartsandplacethemintheassembly;morecomplexendeffectorsmayincludeanautomatic bolt screwing device or a fast tool-changing device. The geometry andworking-envelope of the robot must fit all members of the particular product familyfor which the assembly system is designed. Because robots have CNC-type con-trollers, multiple task programs may be stored in the controller, and that program isexecuted whenaparticular productofthe familyisassembled. Therobotendeffectertypically must bechangedto assemble each different product, similar totool changesin CNC machining centers.A typical automated assembly station has two basic functions: (1) the transfer andfeeding of components into the assembly station and (2) the insertion of componentsinto the product assembly.In fully automated assembly systems the feedingmechanisms usually consist of magazines (or buffers) into which the componentsare staged, and multiple material-flow sub-systems that feed the components auto-matically to the assembly station. The actual insertion of the components (i.e., theassembly itself) is done by assembly robots and automated (often pneumatic)equipment, aided by sensors. The types of sensors used depend on the applicationand include computer vision, force sensing, proximity, and photoelectric sensing.The design features of the product and its components determine if it can beassembled on an automated assembly system or if it must be assembled manually.Note that many products require at least some insertion operations that are toocomplex to be automated. Such products must be at least partially assembledmanually.Inmulti-stationautomated assembly systems,pallets carrythe product throughallassembly stations (one pallet for each product). The pallets ride on a conveyor alongthe conveyor until encountering a stop gate at an assembly station or another pallet.The product assembly is done sequentially in the stations along the conveyor. Anautomated assembly station for an engine is shown in Figure 6.12.Assembly systems are designed in stages where each stage rigidly transmits itsoutput to the immediate successor stage. There are serial, synchronous assemblyASSEMBLY SYSTEMS167systems, in which the product is partially assembled at each stage, allocating exactlythe same amount of time at every stage. Office chairs, for instance, are assembledmanually on serial synchronous lines of about 20 stations, and the chair stays for30seconds at each assembly stage. That means that the line throughput is two chairsper minute.Butthereareothermulti-stageassemblysystemsinwhicheachstagerequirespartsthat have been assembled from several preceding stages. These complex systems areasynchronous and usually require buffers between the stages. Figure 6.13 showsa typical assembly line*with assembly stations, M, and buffers, B. The highlightedstations form the main assembly line, and the others are sub-assemblies that providecomponentstothemainlinestations.Forexample,threesub-assemblylinesfeedthreedifferent components to station Ma. The output of station Ma moves to buffer Baawaiting transfer to station Mb, where new parts as well as a component from anothersub-assemblyline (Mb1-Bb1)are addedon.Station Mb1 isanautomated station thatassemblestwopartsandplacesthesub-assemblyinbufferBb1.Theassemblytasksonstation Mc take very long time. To balance the assembly line flow and ensure eventhroughput, three identical assembly stations are arranged in parallel at this stage(Mc1, Mc2, Mc3). A second buffer Bb distributes output to all three, and buffer Bccollects them again for the next stage Md. The final assembly station, Me, completesthe product and moves it off the line.It is common now for personal computers to be custom assembled to satisfya customers particular order. The assembly line for these consists of robots andautomatedmechanismsforkitting,testing,andpackaging,butpeoplestillperformtheactual computer assembly. Because of the huge variety of customers orders, humanassemblers are the most practical option to accommodate the various needs.Figure 6.12An engine assembly station (courtesy of GM R&D Center).*Thisexample,broughtbyDr.WencaiWang,isfromtheautoindustry.StationMb1isanautomatedstationthat puts a needle bearing onto a shaft. The other stations in this example are all manual.168TRADITIONAL MANUFACTURING SYSTEMSFigure 6.14, depicts the computer assembly line starting with a series of kittingstations where trays move along a line and amass the components for each computerorder. Each order has its own tray. A robot selects the desired component at eachstation and puts it on the tray in the appropriate order. For example, a robot picksaparticularhard diskfromamagazine ofseveralchoicesandplacesit onthetray.Allcomponents are selected in the same way. At the end of the kitting line, all the partsneeded for a given order are on the tray, arranged appropriately for assembly. Eachtray is then conveyed to a manual assembly station where two workers, facing eachother,assemblethecomputer.Thetopmostcomponent,thecomputershell,comesofffirst and then each internal component is inserted as required. The time required fortheassemblyofeachcomputerorderisdifferentdependingonthenumberandtypeofcomponentsrequired.Tomeetdemandandtomaintainthepaceoftheserialportionofthe line, there may be 1020 assembly tables arranged in parallel. When eachassembly is completed, the computer is sent for automated software loading andtesting stations. When that phase is successfully completed, the computer is put backon a conveyor, sent to the automated packaging station, and then shipped to thecustomerwhoorderedit.Typically,computersarerolledoutoftheassemblysystemata rate of 4seconds each.Figure 6.14A typical flexible assembly system for personal computers.Figure 6.13A typical multi-stage assembly system.ASSEMBLY SYSTEMS169There are also reconfigurable assembly systems. A key feature of these systems isa modular conveyor system that can operate asynchronously, and is reconfigurable toaccommodate a large variety of component choices according to the application andthe productbeing assembled. Areconfigurableconveyor allows quick rearrangementto alter process flow, adding or bypassing assembly stations according to the desiredproduct. It also allows for serialparallel configurations to balance the assembly lineflowasnecessarytoensureeventhroughput.Anexampleofthistypeofconveyorcanbe seen at this reference website.96.5 INDUSTRY EXPERIENCE WITH FMSA SURVEYThe development of flexible automation enablers (e.g., robots and CNC machiningcenters) during the 1970s has resulted in many successful FMS systems since the1980s.10Nevertheless, in the early 1990s we also began to hear about cases of FMSfailure. These stories were not popular, and information about them was not easilyavailable. One case was reported in Japan, where a large manufacturing companydecided to get rid of its FMS because it was too complex to operate, and itsproductivity was much lower than expected. An engine manufacturer in Michiganbought a flexible machining system composed of 12 CNC machines to augment itsdedicated lines, but after 2 years of unsuccessful attempts to operate the system, thecompanygotridofthewholeFMS.InanothercaseinMichigan,awell-knownsystemintegrator sold a FMS consisting of 15 machining centers to an engine producer, butcouldnotachievetherequiredpartquality.Italsosufferedfromgreaterthanexpecteddowntimes due to operators training issues. The system integrator finally had to takeback its system, and the case was settled out of court.These stories motivated us to investigate the reasons for success and failures ofFMS in the mechanical industries (machine builders, automobile producers, enginemanufacturers, etc.). Our research was based on a survey that was conducted by aCIRP*Working Group on “Flexible AutomatonAssessment and Future” heldjointly with the ERC for reconfigurable manufacturing systems during fall 2001through summer 2002.Atthattimethemanufacturingindustrywasjuststartingtofacenewchallenges offluctuatingmarketsandtheneedforproductionofhighquantitiesofamixofproductsfromthesameproductfamilyonasingleFMS.Inourstudy,industryexpertswereaskedtoevaluatetheir current FMSsandsuggestenablingtechnologiesthatwere neededtoimprove the performance of flexible manufacturing. The survey findings indicatedsome valuable results, the most important of which was that reconfigurablemanufacturingsystemshademergedasthemajoroperationalpriorityacrosstheboard.Wereceived27responsestoourquestionnaire;14responsesfromcompaniesintheUnited States, and 13 from Europe (seven from Germany, four from Italy, and two*CIRPistheInternationalAcademyforProductionEngineeringthathasheadquartersinParis,France.Thesurveywasconductedby the Manufacturing ParadigmsWorkingGroupof CIRP. Thesurvey leaderswere:Y. Koren in the United States, U. Heisel in Germany, C. Boer in Italy, and D. Dumor in France.170TRADITIONAL MANUFACTURING SYSTEMSfrom France). The range of respondents covered a wide spectrum from VicePresidents and Executive Directors in the auto industry, to R&D Managers andsystem designers in the machine tool industry.The objectives of the survey were to.Understand the reasons for success and failures of FMS in the mechan-ical industry.EvaluatepossibletechnologiesthatmaycontributetothesuccessofFMS.Thesystemconfigurationtypesutilizedbythesurveyedindustry,asreportedtous,are shown in Figure 6.15.Regarding the system design phase, the survey conclusions were.New flexible systems should be designed to produce both existing and futureproducts.In many cases, the purchased systems had over-capacity and excess function-ality. Purchasing over-capacity is more common than purchasing over-functionality. That extra capacity was never utilized in 20% of all cases, andthe extra functionality was never utilized in 30% of them.Thetimetodesignandinstall(rampup)largemanufacturingsystemsisconsideredvery critical by most experts. Regarding