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MIT 2 810 - Mechanical Assembly

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Assy-for-gutowski 1/45© Daniel E Whitney10/7/10Mechanical Assembly• Goals of this class:– Explain the basics of assembly as a manufacturingprocess– Show some examples– Explain Design for Assembly• Not included: (take 2.875)– Design processes for creating assemblies– Computer representations of assemblies– The “reach” of assembly into company operatingprocesses and strategies– Relationship to modularity and product architectureAssy-for-gutowski 2/45© Daniel E Whitney10/7/10Historical Aspects of Assembly• All assembly was manual until about 50 years ago• Little scientific knowledge existed about what happensduring assembly operations: people “just do it”• All fabrication techniques have been mechanized for 100 to5000 years and a lot is known about them• Assembly included fitting, adjustment, and selection until the1830s• Technology and methods to create interchangeable partsevolved during 1765-1900• Mass production requires interchangeable parts• Interchangeable parts enable use of low skill assemblers• Supply chain implementation of manufacturing requiresinterchangeable parts and supporting technologiesAssy-for-gutowski 3/45© Daniel E Whitney10/7/10Interchangeable PartsEnable Mass ConsumptionParts InterchangeAssembly Is quick andeasyLow skillPeople Can do itLots of AssembliesGet madeThey are cheapLots of People canAfford themThere is High demandLots of AssembliesGet soldThey are cheapEveryoneMakesmoneyIncludingThe Assemblers Investment in Tools andKnowledgeInvestment in equipmentAnd designAssy-for-gutowski 4/45© Daniel E Whitney10/7/10Technical Aspects• Assembly creates product functions or sub-functions• Results of assembly can be tested– Results of fabrication can be inspected but not tested• Assembly requires coordination of many parts, tools,fixtures, packages, people, companies• Assembly step times are short compared to manufacturingprocess step times– Non-assembly actions take proportionately much more time– Time is needed to move the assembly from one station to the nextor to change tools– People and space are needed for incoming parts and outgoingboxesAssy-for-gutowski 5/45© Daniel E Whitney10/7/10“Chain of Delivery” of QualityHood(Ford-Chicago)Inner Fenders(Budd-Philadelphia)Body Frame(Ford LAP)Radiator Support(DECO)Assembly Fixture for Fender:Fixture Vendor ACowl Top(Hawthorne)ContactChainAssembly Tooling (TESCO)Fender(Part:Budd-ShelbyvilleChecking fixture: M&M)Fender Skin(Budd-Shelbyville)Reinforcements(Wise, Metalform)CustomerFeature:Hood Fit toFenderFixture Vendor C)Check Fixture Vendor D)Parts Vendor BFixture Vendor FPart Vendor AClosure Panel Check Fixture: Fixture Vendor GAssembly Fixture for Hood: Fixture Vendor AN. Soman, M. ChangOrganizationalBoundaryPart Vendor CStructural Check Fixture:Fixture Vendor ED-pillar Assembly Station:Fixture Vendor BPART COUNT: 9PART SOURCES: 7TOOL COUNT: 5TOOL SOURCES: 4CHECK FIXTURE COUNT: 2CHECK FIXTURE SOURCES: 2DISPERSAL INDEX: 81%Assy-for-gutowski 6/45© Daniel E Whitney10/7/10More Technical Aspects of Assembly• Three methods are used– Manual (always involved for large items; almost always involvedfor small items)– Specialized equipment (used only for small items made in highvolumes - units/year in the millions)– Robots (used for small and medium sized items)• Low volume ~ big items: planes, ships• High volume ~ small items: cigarettes, small toys• Takt time for 777 airplane: 3 days• Takt time for Ford or GM car: 59 seconds• Takt time for a cigarette: 10 msAssy-for-gutowski 7/45© Daniel E Whitney10/7/10JapanKansasSeattleAssy-for-gutowski 8/45© Daniel E Whitney10/7/10Assy-for-gutowski 9/45© Daniel E Whitney10/7/10Typical Cam-operated Assembly MachineMultiple identical base modules bolted together as neededCams cut by NC to do the necessary operationsSynchronous parts transportAssy-for-gutowski 10/45© Daniel E Whitney10/7/10Cell Phone Assembly Machinehttp://www.modular.co.uk/pages/solutions.htmWork stops at each workstationThe phoneThe machineAssy-for-gutowski 11/45© Daniel E Whitney10/7/10Typical Dial MachineAssy-for-gutowski 12/45© Daniel E Whitney10/7/10Bottle Filling MachineWork does not stopAssy-for-gutowski 13/45© Daniel E Whitney10/7/10Machine Makes Washer TubsAssy-for-gutowski 14/45© Daniel E Whitney10/7/10Typical Small Parts AssemblyMachineAssy-for-gutowski 15/45© Daniel E Whitney10/7/10Economic Aspects• Assembly employs more people than any otherphase of manufacturing• Short assembly takt times mean that cost ofassembly is a small fraction of manufacturing cost• Each technical kind of assembly has its owneconomic featuresAssy-for-gutowski 16/45© Daniel E Whitney10/7/10Unit Cost Example - 2Unit Assembly Cost by Three Methods00.20.40.60.811.21.41.61.820 100000 200000 300000 400000 500000 600000Annual VolumeUnit CostMANUAL $/UNITFIXED $/UNITFLEX $/UNITfAC=0.38T=2sLH=$15/hrS$=50000$/tool = $10000N = 10 parts/unitw = 0.25 workers/staAssy-for-gutowski 17/45© Daniel E Whitney10/7/10Operational Organization of Assembly• One person or station does all assembly operations• Subassemblies are made and flow into a finalassembly process• Assembly is done in a small area by a team whereeach member does many operations• Assembly is done on a long line where eachperson does a small amount• As production rates and volumes rise, the linebecomes the only efficient wayAssy-for-gutowski 18/45© Daniel E Whitney10/7/10Operational Aspects - Line Balancing• Different operations take different lengths of time• When only one or a few ops are done at each station, largedifferences in station time can result• Slow stations make fast stations wait• Sometimes a different sequence will have better balance• Sometimes, extra stations in parallel are provided• Queues can build up behind slow stations• Fast stations can become starved• “A cycle lost on the bottleneck station is a cycle lost forever”Assy-for-gutowski 19/45© Daniel E Whitney10/7/10Assembly Sequence Options• Typical products have hundreds or thousands offeasible assembly sequences• Different sequences enable options for– Line balancing– In-line repair– Equipment feasibility– Human performance and fatigue– Product qualityAssy-for-gutowski 20/45© Daniel E Whitney10/7/10Two Alternator SequencesREAR HOUSINGSHAFT/ROTORBEARINGFRONT HOUSINGBEARING SPACERFANFAN SPACERPULLEYLOCKWASHERNUTMAIN ASSEMBLYFIXTUREASSEMBLYSEQUENCE


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MIT 2 810 - Mechanical Assembly

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