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Classification of flexible
manufacturing systemsBy Jim Browne, University College, Galway; Didier Dubois, Centre d'Etudes et
de Recherches de Toulouse: Keith Rathmill, Cranfield Institute of Technology:Suresh P. Sethi, Universityof Toronto; and Kathryn E. Stecke, The Universityof
Michigan.
There has been some uncertainty concerning the
conditions under which a manufacturing system may be
termed 'flexible'. To clarify this confusion eight types of
flexibilities are defined and described.
A FLEXIBLE Manufacturing System(FMS) is an integrated, computer-controlled complex of automatedmaterialhandling devices andnumerically controlled (NC) machinetools that can simultaneously process
medium-sized volumes of a variety ofpart types.
tslThis new productiontechnology has been designed to attaintheefficiency of well-balanced,machine-paced transfer lines, while
utilizing the flexibility that job shops
have 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 extent
of automation and thediversity of theparts that are important; some systemsare termed FMS just because they
contain automated material handling.
For example, dedicated, fixed, transferlines or systems containing onlyautomated storage and retrieval are
not FMSs. Other systems only containseveral (unintegrated) NC or CNCmachines. Still other systems use a
computer to control the machines, butoften require long set-ups or have no
automated 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 either
transfer 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 and
described. Examples or explanations
are provided when needed to illustratea particular flexibility type. Measure-ment and attainability of each are alsodiscussed.
qMachine Flexibility: the ease ofmaking the changes required toproducea givenset of part types.
Measurementof these changesinclude, for example, the time toreplace worn-out or broken cutting
tools, 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 for
a machine tool to switch from one parttype to another includes: cutting tool
preparation time; part positioning andreleasing time; and NC programchangeover time. This flexibility can
beattained by:(a) technological progress, such as
sophisticatedtool-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. -
qProcess Flexibility: the ability to
produce a given set of part types, eachpossibly using different materials, in
several ways. Buzacott [ 1982] calls this
job flexibility', which relates to the
mix of jobs which the system canprocess.' Gerwin [1982] calls this mixflexibility'.
Processflexibilityincreases 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 processed
without using batches. This flexibilitycan be attained by having:
(a) machineflexibility; and(b) multi-purpose, adaptable, CNC
machining centres.ElProduct Flexibility: the ability
changeover to produce a new (set o
product(s) very economically aquickly. Mandelbaum [ 1978] calls taction flexibility, the capacity
taking new action to meet new circustances.' Included in this concep
Gerwin's [ 1982] 'design-change flebility'. This flexibility heighten
company's potential responsivenescompetitive and/or market chang
Product flexibility can be measuredthe time required to switch from opart mix to another, not necessar
of the same part types. This flexibilcan be attained by having:
(a) an efficient and automated prodution planning and control systecontaining:
(i) automatic operation assigment procedures; and
(ii) automatic pallet distributi
calculation capability.(b) machineflexibility.ElRouting Flexibility:the abilityhandle breakdowns and to contin
producing the given set of part typ
This ability exists if either a part ty
can be processed via several routes, equivalently, each operation can
performed on more than one machinNote that this flexibility can be:
Potential:part routes are fixed, b
parts are automatically reroutwhen a breakdown occurs;
Actual:identical parts are actua
processed through different rout
independent of breakdown situ
tions_The main, applicable circumstanc
occurs when a system componesuch as a machine tool, breaks dow
This flexibility can be measured by robustness of the FMS when brea
downs occur the production rate do
not decrease dramatically and pa
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Relationships Among Flexibility Types
Product FlexibilityMachine Flexibility Process Flexibility
Operation Flexibility
continue to be processed. This flexi-
bility can beattained by allowing forautomated and automatic rerouting ofparts (potential routing flexibility), bypooling machines into machinegroups,
1 6 1which also allows machine
tool redundancy; and also by duplicat-ing operation assignments."' These
latter policies provide actual routingflexibility. The FMS would then bestate-driven by a feedback control
Policy.qVolume 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 labour
costs. If it is not economical to run a
particular 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 can
vary from week to week, resulting invariable machine and system utilisa-
tions. This flexibility can be measuredby how small the volumes can be for
all part types with the system stillbeing run profitably. The lower the
volume is, the more volume-flexiblethe system must be. This flexibility
can be attained by having:(a) multipurpose machines; and(b) a layout that is not dedicated to a
particular process; and(c)asophisticated,automated
materials handling system, such as(possibly intelligent) carts, and notfixed-route conveyors;and
(d)routingflexibility.qExpansion Flexibility: thecapability of building a system, and
expanding 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)anon-dedicated, non-process-
driven layout; and(b)aflexible materials handling
system consisting of, say, wire-guided carts; and
(c) modular, flexible machining cellswith pallet changers; and
(d)routingflex ibility.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 has
usually determined afixed ordering ofall operations, each on a particularmachine (type). However, keeping
the 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 currently
idle, busy, or bottleneck).O Production Flexibility: the universeof part types that the FMS canproduce. This flexibility is measured
by the level of existing technology. It is
attained by increasing the level oftechnology and the versatility of the
machine tools. The capabilities of all
. the previous flexibilities are required.
Not all of these flexibility types areindependent. The Figure displays the
relationships between the different
flexibilities.The arrows signifynecessary for'. An ideal FMS would
possess all of the defined flexibilities.However, the cost of the latest in hard-
ware and the most sophisticated (and
at present non-existent!) software to
plan 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 new
part numbers, since retooling costs arehigh and changeover time can be aday. Also, some flexible systems (such
as the SCAMP system in Colchester,
UK) include special-purpose, non-
CNC machines, such as hobbing and
broaching, which also require
(relatively) huge set-up times.This classification of flexibilities
can help categorize different types of
FMS.
Relationship among types of flex ibility.
The level of automation helpdetermine the amount of availaflexibility. Because of the diffechoices of various flexibility lev
there are different types of FMSs. Itherefore,- useful to classify thsystems in terms of their oveflexibility.
Towards a classification of flexmanufacturing systems, Groo[ 1 980) divided FMSs into two disttypes:
(i) Dedicated FMS;
(ii) Random FMS.A dedicated system machines a fi
set of part types with well-defimanufacturing requirements ov
known time horizon. The 'randFMS', on the other hand, machine
greater variety of parts in randsequence.
In addition to these basic, extre
types of FMSs, all FMSs are differin terms of the amounts of the flebilities that they utilize. In this sectia classification of FMSs accordin
their inherent, overall flexibilityprovided. Four general types of Fwill be defined.
The following standards are pvided based on FMS componenwhich will be used to describe aclassify the different types ofFMSs:1. Machine tools:
General-purpose or specialize Automatic tool changing capa
lities (increase flexibility)
Regarding tool magazines, th
capacity, removability, and tochanging needs (affect the fle
bility).2. Materials handling system: Types include: conveyor or on
way carousel; tow-line with ca
network of wire-guided carstand-alone robot carts
Partmovement equipmepalletized and/or fixtu red
Tool transportation systemmanual; or, automatically, w
parts.
ProductionFlexibility
Routing Flexibility Volume FlexibilityExpansion Flexibility
The FMS Maqazine April 1984
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Storage areas for in-process inven- machine tools, and the finished parts
tory:
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 to
cart assignment; cart traffic
regulation
Control of part mix: through
periodic input; through a feed-
back-based priority rule_
These `flexibility' standards for the
physical FMS components are used to
clarify differences and similarities
between the FMS types.
Although not typically considered
FMS, this classification scheme will
include the flexible assembly system
(FAS).
The simplest possible component ofan FMS or FAS is a flexible assembly
cell (FAC). It consists of one or more
robots and peripheral equipment,
such as an input/output buffer and
automated material handling. To date,
only about 6% of robot applications
are in assembly.
A flexible assembly system (FAS)
consists of two or more FACs. In the
future, as the technology develops to
allow the interface between manufac-
turing and assembly, an FAS could
also be a component of a flexible
system.The types of FMS described, are
categorized according to the extent of
use of their flexibilities. The classi-
fication of a particular FMS usually
results basically from its mode of
operation as well as the properties of
the four components described above.
Type I FMS: Flexible Machining
Cell
The simplest, hence most flexible
(especially with respect to five of the
flexibilities) type of FMS is a flexible
machining cell (FMC). It consists ofone general-purpose CNC machine
tool,interfaced with automated
material handling which provides raw
castings or semi-finished parts from an
input buffer for machining, loads and
unloads the machine tool, and trans-
ports the finished workpiece to an out-
put buffer for eventual removal to its
next destination. An articulated arm,
robot, or pallet changer is sometimes
used to load and unload. Storage
includes the raw castings area, the
input and output buffers of the
area.
Since an FMC contains only one
metal-cutting machine tool, one might
question its being called a system.
However, it has all of the components
of an FMS. Also, it is actually an
FMS component itself. With one
machine tool, it is the smallest, most
trivial FMS.
Type II FMS: Flexible MachiningSystem
The second type of FMS can have
the following features: It can have real-
time, on-line control of part produc-
tion. It should allow several routes for
parts, with small volume production
of each, and consists of FMCs of
different types of general-purpose,
metal-removing machine tools. Real-
time control capabilities can auto-
matically allowmultiple routes for
parts, which complicate scheduling
software. Because of real-time control,
however, the actual scheduling might
be easier. For example, the scheduling
rule might be to route randomly, or
route to the nearest free machine tool
of the correct machine type. The
scheduling rule could be some appro-
priate, system-dependent, dynamic
priority rule with feedback.
Sometimes, dedicated, special-
purpose machines tools, such as multi-
ple-spindle head changers, are used in
an FMS to increase production. The
machine tools are unordered in aprocess-independent layout. It is the
part types that are to be processed by
an FMS which define the necessary,
required machine tools.
A Type II FMS is highlymachine-
flexible,
process flexible, andproduct-flexible. It is also highlyrouting-flexible, since it can easily andauto-
matically cope with machine tool or
other breakdowns if machines are
grouped or operation assignments are
duplicated.
Within the Type II category, the
various kinds of material handling
provide a sub-range of flexibility. Inorder of increasing flexibility, various
material handling systems include:
power roller conveyors, overhead
conveyors, shuttle conveyors, in-floor
tow line conveyors, and wire-guided
carts. Some examples include:
(i) a network of carts and decentral-
ized storage areas, for shorter pro-
cessing times (Renault Machines
Outils, in Boutheon, France);
(ii) a tow line with carts and central-
ized storage areas, for longer
processing times (Sundstrand/
Caterpillar DNC Line, in Peo
Illinois, USA).
Type III FMS: Flexible TransferLine
The third type of FMS has t
following features. For all part typ
each operation is assigned to, a
performed on, only one machine. T
results ina fixed route for each pthrough the system. The layou
process-driven and hence ordere
The material handling systemusually a carousel or conveyor. T
storage area is local, usually betwe
each machine. In addition to gener
purpose machines, it can conta
special-purpose machines, robots, a
some dedicated equipment. Sched
ing, to balance machine workloads
easier. In fact, a Type III FMS is ea
tomanage because it operat
similarly to a dedicated transfer li
The computer control is more sim
and a periodic input of parts
realistic. Once set up, it is easy to r
and to be efficient. The difference
that it is set up often and relativ
quickly.
A Type III FMS is less Proceflexibleand less capable of aumatically handling breakdowns. Ho
ever, the system can adapt by
tooling and manually inputting
appropriate command to the com
puter, to re-route parts to the capa
machine tool. This takes more tim
than the automatic re-routing ava
able to a Type II FMS.
Type I V FMS: Flexible TransferMulti-LineThe fourth FMS type consists
multiple Type III FMSs that are int
connected. This duplication does
increaseprocess flexibility. Similar t
Type III FMS, scheduling and cont
are relatively easy, once the system
set up. The main advantage is t
redundancy that it provides in
breakdown situation, to increase
routing flexibility.Itattempts achieve the best of both FMS Type
and III.
Flexibility rangeAll things being equal, a Type
FMS is operated flexibly', whil
Type III FMS is operated in a mu
more fixed' manner. These typ
provide the extremes, say, the bounon flexibility. There is, of course
whole range of flexibilities betwe
the two general types. However, the
smaller variations in flexibility
defined by the versatilities a
capabilities of the machine too
which are dictated by the particu
7/27/2019 Browne- Clasification of FMS
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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's
mode ofoperation.In general, the FMSs of the United
States and the Federal Republic ofGermany tend to be more like theType II FMS, while those of Japan are
more similar to Type III. The second
floor of Fanuc's Fuji complex, consist-
ing of four flexible transfer lines, is anexample of an operating Type IVFMS. It consists of several identical
FACs, which are not all identicallytooled. Parts do have fixed routes, but
if an assembly cell is down, the parts
requiring it are automatically able to
be routed to another assembly cell,
which contains the correct tooling.
The first floor of this Fanuc plant, theMotor Manufacturing Division, is a
good 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. This
paper has provided a framework for
such strategic decisions.
Acknowledgements
Kathryn E. Stecke's research was supported inpart by a summer research grant from theGraduate School of Business Administration atThe University of Michigan as well as by a grant
by 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. 107-1( March-April 1982).
3. Mikell P. Groover, Automation, ProductiSystems, and Computer-Aided Manufactuing.Prentice-Hall, Englewood Cliffs N(1980).
4. MarvinMandelbaum, 'Flexibility Decision-Making: An Exploration and Unfication.' Ph.D. dissertation, Department
IndustrialEngineering, University Toronto, Ontario, Canada (1978).
5. Kathryn E. Stecke, 'Formulation anSolution of Nonlinear Integer Productio
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Machine Group Sizes for Flexible Manufaturing Systems,' Working Paper No. 29
Division of Research, Graduate School
Business Administration, The University
Michigan, Ann Arbor, MI (January 1982).
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1982).
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