Classification of flexiblemanufacturing systemsBy Jim Browne, University College, Galway; Didier Dubois, Centre d'Etudes etde Recherches de Toulouse: Keith Rathmill, Cranfield Institute of Technology:Suresh P. Sethi, Universityof Toronto; and Kathryn E. Stecke, The UniversityofMichigan.

There has been some uncertainty concerning theconditions under which a manufacturing system may betermed 'flexible'. To clarify this confusion eight types offlexibilities are defined and described.

A FLEXIBLE Manufacturing System(FMS) is an integrated, computer-controlled complex of automatedmaterial handling devices andnumerically controlled (NC) machinetools that can simultaneously processmedium-sized volumes of a variety ofpart types. tsl This new productiontechnology has been designed to attainthe efficiency of well-balanced,machine-paced transfer lines, whileutilizing the flexibility that job shopshave to simultaneously machinemultiple part types.

Recently, many new manufacturingfacilities have been labelled FMS. Thishas caused some confusion about whatconstitutes an FMS. Flexibility andautomation are the key conceptualrequirements. However, it is the extentof automation and the diversity of theparts that are important; some systemsare termed FMS just because theycontain automated material handling.For example, dedicated, fixed, transferlines or systems containing onlyautomated storage and retrieval arenot FMSs. Other systems only containseveral (unintegrated) NC or CNCmachines. Still other systems use acomputer to control the machines, butoften require long set-ups or have noautomated parts transfer.

Some systems are called flexiblebecause they produce a variety of parts(of very similar type, using fixedautomation). In most of theseexamples, the operating mode is eithertransfer line-like or based on produc-ing batches of si mi lar part types.

To help clarify the situation, eighttypes of flexibilities will be defined anddescribed. Examples or explanationsare provided when needed to illustratea particular flexibility type. Measure-ment and attainability of each are alsodiscussed.

q Machine Flexibility: the ease ofmaking the changes required toproduce a given set of part types.Measurement of these changesinclude, for example, the time toreplace worn-out or broken cuttingtools, the time to change tools in a toolmagazine to produce a different subsetof the given part types, and the time toassemble or mount the new fixturesrequired. The set-up time required fora machine tool to switch from one parttype to another includes: cutting toolpreparation time; part positioning andreleasing time; and NC programchangeover time. This flexibility canbe attained by:(a) technological progress, such as

sophisticated tool-loading andpart-loading devices;

(b) Proper operation assignment, sothat there is no need to change thecutting tools that are in the toolmagazines, or they are changed lessoften;

(c) having the technological capabilityof bringing both the part andrequired cutting tools to themachine tool together. -

q Process Flexibility: the ability toproduce a given set of part types, eachpossibly using different materials, inseveral ways. Buzacott [ 1982] calls this`job flexibility', which `relates to themix of jobs which the system canprocess.' Gerwin [1982] calls this `mixflexibility'. Process flexibilityincreases as machine set-up costsdecrease. Each part can be machinedindividually, and not necessarily inbatches. This flexibility can bemeasured by the number of part typesthat can simultaneously be processedwithout using batches. This flexibilitycan be attained by having:(a) machineflexibility; and(b) multi-purpose, adaptable, CNC

machining centres.El Product Flexibility: the ability tochangeover to produce a new (set of)product(s) very economically andquickly. Mandelbaum [ 1978] calls thisaction flexibility, the capacity for

taking new action to meet new circum-stances.' Included in this concept isGerwin's [ 1982] 'design-change flexi-bility'. This flexibility heightens acompany's potential responsiveness tocompetitive and/or market changes.Product flexibility can be measured bythe time required to switch from onepart mix to another, not necessarilyof the same part types. This flexibilitycan be attained by having:(a) an efficient and automated produc-

tion planning and control systemcontaining:

(i) automatic operation assign-ment procedures; and

(ii) automatic pallet distributioncalculation capability.

(b) machineflexibility.El Routing Flexibility: the ability tohandle breakdowns and to continueproducing the given set of part types.This ability exists if either a part typecan be processed via several routes, or,equivalently, each operation can beperformed on more than one machine.Note that this flexibility can be:

Potential: part routes are fixed, butparts are automatically reroutedwhen a breakdown occurs;Actual: identical parts are actuallyprocessed through different routes,independent of breakdown situa-tions_The main, applicable circumstances

occurs when a system component,such as a machine tool, breaks down.This flexibility can be measured by therobustness of the FMS when break-downs occur the production rate doesnot decrease dramatically and parts

Relationships Among Flexibility Types

Product FlexibilityMachine Flexibility Process Flexibility

Operation Flexibility

continue to be processed. This flexi-bility can be attained by allowing forautomated and automatic rerouting ofparts (potential routing flexibility), bypooling machines into machinegroups, 161 which also allows machinetool redundancy; and also by duplicat-ing operation assignments."' Theselatter policies provide actual routingflexibility. The FMS would then bestate-driven by a feedback controlPolicy.q Volume Flexibility: the ability tooperate an FMS profitably at differentproduction volumes. A higher level ofautomation increases this flexibility,partly as a result of both lowermachine set-up costs and lowervariable costs such as direct labourcosts. If it is not economical to run aparticular system at its usual volume,say during a decrease in marketdemand or a recession, then there areless personnel problems concerningthe idling of labour. Perhaps alterna-tive uses of the FMS could befound. Also, production volumes canvary from week to week, resulting invariable machine and system utilisa-tions. This flexibility can be measuredby how small the volumes can be forall part types with the system stillbeing run profitably. The lower thevolume is, the more volume-flexiblethe system must be. This flexibilitycan be attained by having:(a) multipurpose machines; and(b) a layout that is not dedicated to a

particular process; and(c) a sophisticated, automated

materials handling system, such as(possibly intelligent) carts, and notfixed-route conveyors;and

(d) routingflexibility.q Expansion Flexibility: thecapability of building a system, andexpanding it as needed, easily andmodularly. This is not possible withmost assembly and transfer lines. Thisflexibility can be measured accordingto how large the FMS can become.This flexibility is attained by having:(a) a non-dedicated, non-process-

driven layout; and(b) a flexible materials handling

system consisting of, say, wire-guided carts; and

(c) modular, flexible machining cellswith pallet changers; and

(d) routingflexibility.q Operation Flexibility: the ability tointerchange the ordering of severaloperations for each part type. There isusually some required partial pre-cedence structure for a particular

part type. However, for someoperations, their respective ordering isarbitrary. Some process planner hasusually determined afixed ordering ofall operations, each on a particularmachine (type). However, keepingthe routing options open and not pre-determining either the 'next' opera-tion or the 'next' machine increasesthe flexibility to make these decisionsin real-time. These decisions shoulddepend on the current system state(which machine tools are currentlyidle, busy, or bottleneck).O Production Flexibility: the universeof part types that the FMS canproduce. This flexibility is measuredby the level of existing technology. It isattained by increasing the level oftechnology and the versatility of themachine tools. The capabilities of all.the previous flexibilities are required.

Not all of these flexibility types areindependent. The Figure displays therelationships between the differentflexibilities. The arrows signifynecessary for'. An ideal FMS wouldpossess all of the defined flexibilities.However, the cost of the latest in hard-ware and the most sophisticated (andat present non-existent!) software toplan and control adequately would bequite high on some of these measuresand low on others. For instance,processing a particular group ofproducts may be made possiblethrough the use of head indexershaving multiple-spindle heads. How-ever, they hinder both adding new parttypes to the mix and introducing newpart numbers, since retooling costs arehigh and changeover time can be aday. Also, some flexible systems (suchas the SCAMP system in Colchester,UK) include special-purpose, non-CNC machines, such as hobbing andbroaching, which also require(relatively) huge set-up times.

This classification of flexibilitiescan help categorize different types ofFMS.

Relationship among types offlexibility.

The level of automation helps todetermine the amount of availableflexibility. Because of the differentchoices of various flexibility levels,there are different types of FMSs. It is,therefore,- useful to classify thesesystems in terms of their overallflexibility.

Towards a classification of flexiblemanufacturing systems, Groover[ 1 980) divided FMSs into two distincttypes:

(i) Dedicated FMS;(ii) Random FMS.

A dedicated system machines a fixedset of part types with well-definedmanufacturing requirements over aknown time horizon. The 'randomFMS', on the other hand, machines agreater variety of parts in randomsequence.

In addition to these basic, extremetypes of FMSs, all FMSs are differentin terms of the amounts of the flexi-bilities that they utilize. In this section,a classification of FMSs according totheir inherent, overall flexibility isprovided. Four general types of FMSwill be defined.

The following standards are pro-vided based on FMS components,which will be used to describe andclassify the different types ofFMSs:1. Machine tools:

• General-purpose or specialized• Automatic tool changing capabi-

lities (increase flexibility)• Regarding tool magazines, their

capacity, removability, and tool-changing needs (affect the flexi-bility).

2. Materials handling system:• Types include: conveyor or one-

way carousel; tow-line with carts;network of wire-guided carts;stand-alone robot carts

• Part movement equipment:palletized and/or fixtu red

• Tool transportation system:manual; or, automatically, withparts.

ProductionFlexibility

Routing Flexibility Volume FlexibilityExpansion Flexibility

The FMS Maqazine April 1984 115

Storage areas for in-process inven- machine tools, and the finished partstory:• Central buffer storage• Decentralised buffer at each

machine tool• Local storage.

4. Computer control:• Distribution of decisions• Architecture of the information

system• Types of decisions: input

sequence; priority rules; part tocart assignment; cart trafficregulation

• Control of part mix: throughperiodic input; through a feed-back-based priority rule_

These `flexibility' standards for thephysical FMS components are used toclarify differences and similaritiesbetween the FMS types.

Although not typically consideredFMS, this classification scheme willinclude the flexible assembly system(FAS).

The simplest possible component ofan FMS or FAS is a flexible assemblycell (FAC). It consists of one or morerobots and peripheral equipment,such as an input/output buffer andautomated material handling. To date,only about 6% of robot applicationsare in assembly.

A flexible assembly system (FAS)consists of two or more FACs. In thefuture, as the technology develops toallow the interface between manufac-turing and assembly, an FAS couldalso be a component of a flexiblesystem.

The types of FMS described, arecategorized according to the extent ofuse of their flexibilities. The classi-fication of a particular FMS usuallyresults basically from its mode ofoperation as well as the properties ofthe four components described above.

• Type I FMS: Flexible MachiningCell

The simplest, hence most flexible(especially with respect to five of theflexibilities) type of FMS is a flexiblemachining cell (FMC). It consists ofone general-purpose CNC machinetool, interfaced with automatedmaterial handling which provides rawcastings or semi-finished parts from aninput buffer for machining, loads andunloads the machine tool, and trans-ports the finished workpiece to an out-put buffer for eventual removal to itsnext destination. An articulated arm,robot, or pallet changer is sometimesused to load and unload. Storageincludes the raw castings area, theinput and output buffers of the

area.Since an FMC contains only one

metal-cutting machine tool, one mightquestion its being called a system.However, it has all of the componentsof an FMS. Also, it is actually anFMS component itself. With onemachine tool, it is the smallest, mosttrivial FMS.

• Type II FMS: Flexible MachiningSystem

The second type of FMS can havethe following features: It can have real-ti me, on-line control of part produc-tion. It should allow several routes forparts, with small volume productionof each, and consists of FMCs ofdifferent types of general-purpose,metal-removing machine tools. Real-time control capabilities can auto-matically allow multiple routes forparts, which complicate schedulingsoftware. Because of real-time control,however, the actual scheduling mightbe easier. For example, the schedulingrule might be to route randomly, orroute to the nearest free machine toolof the correct machine type. Thescheduling rule could be some appro-priate, system-dependent, dynamicpriority rule with feedback.

Sometimes, dedicated, special-purpose machines tools, such as multi-ple-spindle head changers, are used inan FMS to increase production. Themachine tools are unordered in aprocess-independent layout. It is thepart types that are to be processed byan FMS which define the necessary,required machine tools.

A Type II FMS is highly machine-flexible,

process flexible, and product-flexible. It is also highly routing-flexible, since it can easily and auto-matically cope with machine tool orother breakdowns if machines aregrouped or operation assignments areduplicated.

Within the Type II category, thevarious kinds of material handlingprovide a sub-range of flexibility. Inorder of increasing flexibility, variousmaterial handling systems include:power roller conveyors, overheadconveyors, shuttle conveyors, in-floortow line conveyors, and wire-guidedcarts. Some examples include:(i) a network of carts and decentral-

ized storage areas, for shorter pro-cessing times (Renault MachinesOutils, in Boutheon, France);

(ii) a tow line with carts and central-ized storage areas, for longerprocessing times (Sundstrand/

Caterpillar DNC Line, in Peoria,Illinois, USA).

• Type III FMS: Flexible TransferLineThe third type of FMS has the

following features. For all part types,each operation is assigned to, andperformed on, only one machine. Thisresults in a fixed route for each partthrough the system. The layout isprocess-driven and hence ordered.The material handling system isusually a carousel or conveyor. Thestorage area is local, usually betweeneach machine. In addition to general-purpose machines, it can containspecial-purpose machines, robots, andsome dedicated equipment. Schedul-ing, to balance machine workloads, iseasier. In fact, a Type III FMS is easierto manage because it operatessimilarly to a dedicated transfer line.The computer control is more simpleand a periodic input of parts isrealistic. Once set up, it is easy to runand to be efficient. The difference isthat it is set up often and relativelyquickly.

A Type III FMS is less Process-flexible and less capable of auto-matically handling breakdowns. How-ever, the system can adapt by re-tooling and manually inputting theappropriate command to the com-puter, to re-route parts to the capablemachine tool. This takes more timethan the automatic re-routing avail-able to a Type II FMS.• Type I V FMS: Flexible Transfer

Multi-LineThe fourth FMS type consists of

multiple Type III FMSs that are inter-connected. This duplication does notincrease process flexibility. Similar to aType III FMS, scheduling and controlare relatively easy, once the system isset up. The main advantage is theredundancy that it provides in abreakdown situation, to increase itsrouting flexibility. It attempts toachieve the best of both FMS Types IIand III.

Flexibility rangeAll things being equal, a Type II

FMS is operated `flexibly', while aType III FMS is operated in a muchmore `fixed' manner. These typesprovide the extremes, say, the boundson flexibility. There is, of course, awhole range of flexibilities betweenthe two general types. However, thesesmaller variations in flexibility aredefined by the versatilities andcapabilities of the machine tools,which are dictated by the particular

FMS application, i.e., the part types tobe machined. The types of materialhandling system also provides sub-groups of flexibility. The overall flexi-bility, however, is defined by an FMS'smode ofoperation.

In general, the FMSs of the UnitedStates and the Federal Republic ofGermany tend to be more like theType II FMS, while those of Japan aremore similar to Type III. The secondfloor of Fanuc's Fuji complex, consist-ing of four flexible transfer lines, is anexample of an operating Type IVFMS. It consists of several identicalFACs, which are not all identicallytooled. Parts do have fixed routes, butif an assembly cell is down, the partsrequiring it are automatically able tobe routed to another assembly cell,which contains the correct tooling.The first floor of this Fanuc plant, theMotor Manufacturing Division, is agood example of Type II.

All FMSs consist of similar com-ponents. The numbers and typesof machine tool may differ. Whatreally defines the flexibility of aninstallation is how it is run. The levelof desired flexibility is an importantstrategic decision in the developmentand implementation of an FMS. Thispaper has provided a framework forsuch strategic decisions.

AcknowledgementsKathryn E. Stecke's research was supported in

part by a summer research grant from theGraduate School of Business Administration atThe University of Michigan as well as by a grantby the Ford Motor Company, Dearborn,Michigan.

ReferencesI. J. A. Buzacott,'The Fundamental Principles

of Flexibility in Manufacturing Systems',Proceedings of the 1st International Con-ference on Flexible Manufacturing Systems,Brighton, UK. (20-22 October 1982).

t. Donald Gerwin, 'Do's and Don'ts of Com-puterized Manufacturing', Harvard Business

Review, Vol. 60, No. 2, pp. 107-116( March-April 1982).

3. Mikell P. Groover, Automation, ProductionSystems, and Computer-Aided Manufactur-ing. Prentice-Hall, Englewood Cliffs NJ(1980).

4. Marvin Mandelbaum, 'Flexibility inDecision-Making: An Exploration and Uni-fication.' Ph.D. dissertation, Department ofIndustrial Engineering, University ofToronto, Ontario, Canada (1978).

5. Kathryn E. Stecke, 'Formulation andSolution of Nonlinear Integer ProductionPlanning Problems for Flexible Manufactur-ing Systems,' Management Science, Vol. 29,No. 3, pp. 273-288 (March 1983).

6. Kathryn E. Stecke and James J. Solberg, 'TheOptimality of Unbalanced Workloads andMachine Group Sizes for Flexible Manufac-turing Systems,' Working Paper No. 290,Division of Research, Graduate School ofBusiness Administration, The University ofMichigan, Ann Arbor, MI (January 1982).

7. D. M. Zelenovic, 'Flexibility — A Conditionfor Effective Production Systems,' Inter-national Journal of Production Research,Vol. 20, No. 3, pp. 319-337 (May-June1982).

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