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INTEGRATING INFORMATION FLOWWITH LINKED-CELL DESIGN
IN MANUFACTURING SYSTEM DEVELOPMENT
by
Anna W. Mierzejewska
B.S., Mechanical Engineering, 1998Massachusetts Institute of Technology
Submitted to the Department of Mechanical Engineeringin partial fulfillment of the requirement for the degree of
Master of Science in Mechanical Engineeringat the
Massachusetts Institute of Technology
June 2000
0 2000 Massachusetts Institute of TechnologyAll rights reserved
A uth or ........................................................ .. ......Department of Mechanical Ejgineering
ay 5, 2000
C ertified b y ..................................................................................David S. Cochran
Assistant Professor of Mechanical EngineeringThesis Supervisor
A ccepted by ............................................Ain A. Sonin
Chairman, Departme 7 on Graduate StudentsMASSACHUSETTS INSTITUTE
OFTECHNOLOGY
SEP 2 0 2000
I LIBRARIES
I
INTEGRATING INFORMATION FLOWWITH LINKED-CELL DESIGN
IN MANUFACTURING SYSTEM DEVELOPMENT
by
Anna W. Mierzejewska
Submitted to the Department of Mechanical Engineeringon May 5, 2000 in partial fulfillment of the requirement forthe degree of Master of Science in Mechanical Engineering
Abstract
In order to stay competitive, manufacturing companies have to face the challenge of beingresponsive to customer's needs by reducing cost, improving quality, and shrinking leadtimes. To achieve these objectives, a manufacturing system should be developed based on aclear understanding of the difference between value-adding and wasteful tasks.
In a linked-cell system, the flow of material and information through all the production unitsdetermines system performance in terms of efficiency and responsiveness. Although sourcesof waste in material flow are easier to observe and thus eliminate, information flow pattern isusually given less attention and the effect that the information flow has on the material flowhas been less understood. Thus, system development and implementation usually starts withdecisions about equipment purchases and their layout, and system information flow is usuallyforgotten until the end of the system development process.
This work attempts to show how information flow influences material flow and thus,performance of the overall system. The objective of this thesis is to present the integration ofinformation and material flow in the conveyance (withdrawal-replenishment) system and itsrole in design of the production system. Various types of conveyance systems are presentedand a discussion on their applicability is included. A case study is described to illustrate theneed for change and some of the difficulties in the implementation process.
Thesis Supervisor: David. S. CochranTitle: Assistant Professor of Mechanical Engineering
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Acknowledgements
I would like to thank Professor David S. Cochran for giving me the opportunity to do my
research in this area and to be part of the Production System Design Laboratory at MIT -
which I have greatly enjoyed. My thanks also go to Visteon Monroe for giving me the
motivation for research and for sponsoring my project. During my assignment at Monroe, I
have received much support and friendship from many people, but especially from Ed Umin,
for whom I have particular respect, Tim Rosengarten, Phil Wylie, Bill Nolan, Ed Patino, Ray
Carravallah, and Karen Smith, to name a few. I also enjoyed working on the project with
Jeff Smith and Steve Rupp - thank you for helping me learn. The brainstorming with
Brandon Carrus enriched my stay there and kept me sane.
I would also like to thank all the lab members for putting up with me and providing me with
valuable advice on survival and research, especially Jorge Arinez and Jim Duda. My words
of appreciation go to Pat Smethurst for her help in so many things and also for taking her
time to proof read my thesis.
I give my special thanks to Jose Israel Castafteda-Vega for his support, encouragement and
advice but most importantly his valuable time. Without you, it would not be the same.
Thank you for helping me through it and always giving me new ideas as well as your
patience. I hope you finish soon.
Finally, I would like to thank my entire family for their support but most especially my father
for sharing his passion for engineering and my mother for everlasting support and
encouragement. Thank you for your teachings and support. B6g zapla.
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Table of Contents
ABSTRACT............................................................................................................................. 3
ACKNOW LEDGEM ENTS ................................................................................................ 5
TABLE OF CONTENTS ................................................................................................... 7
1 INTRODUCTION...................................................................................................... 11
1.1 THESIS OBJECTIVE.................................................................................................. 12
1.2 THESIS OUTLINE ................................................................................................... 13
2 BACKGROUND ........................................................................................................ 15
2.1 IMPORTANCE OF MANUFACTURING AS COMPETITIVE ADVANTAGE ...................... 15
2.2 DEFINITION OF MANUFACTURING SYSTEM ........................................................... 17
2.3 HOLISTIC VIEW OF THE SYSTEM ............................................................................ 18
2.4 COMPONENTS OF MANUFACTURING SYSTEM.......................................................... 21
2.4.1 E lem ents..................................................................................................... . 2 1
2.4.2 C onnections................................................................................................. . 22
2.5 LINKED-CELL SYSTEM .......................................................................................... 23
3 DECOMPOSITION FRAMEWORK IN PRODUCTION SYSTEM DESIGN..... 25
3.1 AXIOMATIC DESIGN APPROACH IN PRODUCTION SYSTEM DESIGN DECOMPOSITION25
3.2 FR'S AND DP's OF INFORMATION AND MATERIAL FLOWS ................................... 28
3.2.1 Material Flow ............................................................................................... 30
3.2.2 Information Flow ........................................................................................ 31
3.2.3 G eneral ....................................................................................................... . 32
7
3.3 INTERRELATION BETWEEN MATERIAL AND INFORMATION FLOWS....................... 33
3.3.1 Controllability of the Production at the Line................................................ 33
3.3.2 Impact of the information and materialflows on the system performance..... 34
4 INFORMATION SYSTEM ...................................................................... 35
4.1 MODES OF MATERIAL FLOW ................................................................................... 38
4.2 INVENTORY STRATEGIES ...................................................................................... 41
4.3 PATTERNS OF INFORMATION FLOW UNDER VARIOUS CONTROL POLICIES ................. 43
5 CONVEYANCE METHODS IN LINKED-CELL SYSTEMS ............................. 47
5.1 CONVEYANCE: WITHDRAWAL, REPLENISHMENT ...................................................... 47
5.2 KANBAN AS A TOOL .............................................................................................. 50
5.2.1 Types of kanban .......................................................................................... 50
5.2.2 Kanban systems........................................................................................... 52
5.2.3 Specifications of kanban systems ................................................................. 54
5.3 CONVEYANCE METHODS ........................................................................................ 58
5.3.1 Mechanism of conveyance ........................................................................... 58
5.3.2 Pacemaker element withdrawal.................................................................. 60
5.3.3 Types of conveyance methods ...................................................................... 61
5.3.4 Examples of withdrawal systems ................................................................. 63
6 CASE STUDY ............................................................................................................... 67
6.1 INITIAL SYSTEM ..................................................................................................... 68
6.1.1 Information flow - the initial system .......................................................... 69
6.1.2 Material flow - the initial system.................................................................. 70
6.2 REDESIGNED SYSTEM - TOWARDS THE IDEAL STATE ............................................ 73
6.2.1 System level - redesigned system.................................................................. 74
6.2.2 Line level - redesigned system.................................................................... 78
6.3 PRESENT SYSTEM - TRANSITION STATE.................................................................. 81
6.4 FURTHER DEVELOPMENT ....................................................................................... 84
7 CONCLUSIONS ........................................................................................................ 87
REFERENCES ...................................................................................................................... 89
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1 Introduction
In today's world market, companies compete fiercely to gain and retain customers.
However, customers are becoming more finicky; ultimately they want to get their product for
free, they want it perfect, and they want it now [Hartman and Rodin, 1999]. In order to
compete, the manufacturing companies have to face this challenge with ever-increasing
dedication, by reducing cost, improving quality, and shrinking lead times. This challenge
places significant requirements on the manufacturing system to be responsive to customer
needs.
Responsiveness to customer needs has been the underlying objective of the Toyota
Production System (TPS) [Ohno, 1988], causing Toyota to be the most benchmarked
manufacturing company in the world today [Spear and Bowen, 1999]. The basic concept of
TPS is the elimination of activities that do not add value to the product, or elimination of
waste in the system. The following categories of waste have been identified [Ohno, 1988]:
" Overproduction - waste of resources needed to produce something that the customer
does not request at the moment; it will also cause additional waste in other areas, such
as transportation, and inventory.
" Waiting - waste of operator time.
- Transportation - waste of resources (including labor) on non-value adding activity.
" Additional processing - waste of labor and machine resources on additional
operations, such as rework.
- Inventory - waste of space and labor resources associated with inventory
management.
11
" Movement - waste of labor resources on activities that do not add value to the
product.
- Non-perfect yield - waste of resources used to make parts that cannot be sold because
they are defective or need to be reworked (requiring additional resources).
The elimination of sources of waste in the system improves its efficiency, in terms of the
quality of the products, the response time to the demand, and the cost [Ohno, 1988].
1.1 Thesis Objective
Because the performance of a manufacturing system depends to a great extent on the
underlying design of the system, the elimination of waste also places requirements on the
production system development to prevent waste from occurring in the first place. Designing
the value stream, i.e. all actions performed to manufacture a product [Rother and Shook,
1998], should be done to ensure that these actions in the value stream are in fact value
adding.
The value stream is comprised both of material flow, i.e. actions associated with the
processing and movement of material, and of information flow which dictates and controls
the flow of the material. Although sources of waste in material flow are easier to observe
and eliminate, the information flow pattern is usually given less attention and the effect that
the information flow has on the material flow has been less understood. Thus, the system
development and implementation usually starts with decisions about equipment purchases
and their layout, and the information flow in the system is usually forgotten until the end of
the system development.
This work attempts to show how information flow influences material flow and thus, the
performance of the overall system. The objective of this thesis is to present the integration of
information and material flow in the conveyance (withdrawal-replenishment) system and its
role in the production system design. Various types of conveyance systems are presented
and a discussion on their applicability is included.
12
1.2 Thesis Outline
This thesis starts with the background information in Chapter 2, which discusses the
importance of manufacturing and defines manufacturing system. This chapter also presents
the scope of the manufacturing systems analyzed and introduces the value stream mapping
method used throughout this work to illustrate material and information flows. Finally, the
linked-cell system, which is the focus of the scope of this thesis, is defined.
Chapter 3 presents the decomposition framework used in this thesis to analyze the
requirements that overall system design places on information and material flows. The
interdependence of information and material flows is discussed.
Information systems are the subject of Chapter 4. Various modes of material and
information flows are presented and their patterns under various control policies are
described. Chapter 5 focuses on conveyance methods. First, the conveyance is defined and
its mechanism is described in detail. Kanban systems are presented to illustrate how the
information authorizes and controls material flow in conveyance. Finally, various
conveyance methods are described in detail, and a discussion of their applicability is
provided.
The motivation for this work is presented in Chapter 6. This case study describes an initial
manufacturing system in an existing plant, displaying various types of waste and the need for
a system redesign. The redesigned system is then presented, along with the principles
underlying its design. Finally, the transition state is described, with a discussion of the
difficulties of implementation of the redesigned system and further recommendations
provided.
Finally, Chapter 7 concludes this thesis by providing a summary and the recommendation for
further work.
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2 Background
Manufacturing is the prime activity of many companies. Because of its role in value-adding
transformation of goods, it is critical in satisfying customer needs. This chapter discusses the
importance of manufacturing as a competitive advantage, defines what a manufacturing
system is, introduces the graphical representation of manufacturing systems used in this work
and specifies the scope of this work by presenting the manufacturing system types
considered.
2.1 Importance of Manufacturing as a Competitive Advantage
Any enterprise can be viewed as a chain of capabilities that enable it to satisfy the customer's
demand [Fine, 1999], as shown in Figure 2-1. This view goes beyond the organizational or
technological structure of a traditionally defined supply chain. Rather, it looks at activities
that are necessary under system development and not only system execution. As for any
chain, a prevalent principle holds true: a chain is only as strong as its weakest link.
Therefore, all components of the capability chain, whether owned by the company or
external to it, should be taken into consideration as key to gain advantage over the
competition. Manufacturing is one of those links.
Wheelwright and Hayes list four stages of the strategic role of manufacturing [Hayes and
Wheelwright, 1985]:
1. Internally neutral: minimize the negative effect of manufacturing. Outside experts are
called in to make decisions about strategic manufacturing issues. Internal, detailed
management control systems are the primary means for monitoring manufacturing
performance. Manufacturing is kept flexible and reactive.
15
2. Externally neutral: achieve parity with competitors. Industry practice is followed. The
planning horizon for manufacturing investment decisions is extended to incorporate a
single-business cycle. Capital investment is the primary means for catching up with
competition or achieving a competitive edge.
3. Internally supportive: provide support to the business strategy. Manufacturing
investments are screened for consistency with the business strategy. A manufacturing
strategy is formulated and pursued. Longer-term manufacturing developments and
trends are addressed systematically.
4. Externally supportive: manufacturing contributes significantly to competitive advantage.
Efforts are made to anticipate the potential of new manufacturing practices and
technologies. Manufacturing is involved "up front" in major marketing and engineering
decision. Long-range programs are pursued in order to acquire capabilities in advance
of needs.
Manufacturing, being one of the links in the chain of capabilities, can be therefore a source of
competitive advantage, provided that the manufacturing strategy is aligned with overall
business strategy.
Organizational Supply Chain
4th Tier 3rd TierSupplier Supplier
Technology Supply Chain
cheitry Casting Valve lifters
2nd Tier 1st Tier Assembler DealerSupplier Supplier
Cars
Capability Chain
Machine EquipmentControls DevelopmentDesign
Line DesignProduction
System DesignSupply ChainManufacturing Management
Figure 2-1. Supply chain mapping illustrating elements required to satisfy customer's needs(adapted from [Fine, 1999]).
16
engines
However, because the world we live in is constantly changing, especially in a business sense,
no single capability should be viewed as a sustainable advantage [Fine, 1999]. Because of
the increasing speed of change in industry, each company needs to reinvent itself over and
over again. This necessity places an important requirement on manufacturing strategy: not
only to provide a competitive advantage now, but also to develop new capabilities for the
future. In the case of manufacturing, this necessity relates not only to processes and
technology development but also to the organization and its ability to change. This flexibility
and anticipation of windows of opportunity to gain competitive advantage is the ultimate
core competency of business.
2.2 Definition of a Manufacturing System
Manufacturing can be defined as the transformation of material into something useful and
portable [Gershwin, 1994]. A manufacturing system, however, encompasses not only a
sequence of operations that add value to a product through transformation, but also some
other required tasks such as storage, inspection and transportation, which do not take part in
the transformation but are required to complete the process, as well as the policies governing
those processes.
Manufacturing systems can be classified according to their process structure [Hayes and
Wheelwright, 1979]:
- Job shops
" Disconnected flow lines - batching
- Connected flow lines
" Continuous flow processes
The process structure defines how the material flows through the plant and is usually related
to the type of product produced, as shown in Figure 2-2. The manufacturing systems
considered in this work are continuous flow lines, intended for repetitive production of
discrete items. They are characterized by a defined part routing and are connected by a
paced material handling system.
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Process structure
Jumbled flow(job shop)
Disconnectedline flow(batch)
Connectedline flow
(lines or cells)
Continuous flow
low s ardization Multiple products Few major products high stndardization
onelow volume higher volume commodity products
Ship building
Heavy equipment
Auto assembly
Sugar refinery
Figure 2-2. Matrix of manufacturing systems' process structure (adapted
Wheelwright, 1979]).
from [Hayes and
2.3 Holistic View of the System
In order to analyze the behavior of the entire manufacturing system it is important to take a
holistic view, rather than view it from a reductionist perspective [Hopp and Spearman, 1996].
This broad view allows for overall understanding of system behavior rather than one with
limited impact caused by a narrow focus. It prevents local optimization and instead seeks
overall system optimization. By relating customer requirements to system requirements, the
holistic view of the system allows concentrated analysis or design of the system based on the
parameters that have the greatest impact on overall system performance relevant to the
customer.
The system view recognizes that an understanding of entire system behavior requires
understanding its components. Therefore, any system, in this case manufacturing, is seen to
consist of various subsystems. Analyzing the parameters influencing the behavior of those
subsystems and the principles governing their interrelations provides an understanding of the
entire system. During the design of a system, the holistic view allows the overall system
18
objectives as seen by the customers to relate to the requirements for subsystem design. This
decomposition of the requirements and design parameters is further discussed in Chapter 3.
Viewing a system as a set of interacting subsystems also permits one to look at various levels
of detail, as shown in Figure 2-3. The overall system can be viewed at the inter-company
level of supply chain management, where decisions are aimed maximizing end-customer
satisfaction, at the same time maximizing profit shared over the entire chain beyond the
borders of the given company [Fine, 1999]. Those decisions which will greatly impact
customer satisfaction should not only include the supply chain design, including supplier
selection, relationships and transportation, but also issues such as product design and variety
portfolio. Furthermore, the supply chain view shows the impact that decisions at the
company level have on performance of the entire chain. Company level objectives can be
further broken down into the objectives of particular facilities, which should be aligned with
the company's business strategy. Furthermore, the level of detail can go down to line/cell,
and finally station/machine.
In this work, three levels of detail in manufacturing system design will be considered, as
shown in Figure 2-3:
- Plant - door-to-door view
- Cell/Line
* Station/Machine
The door-to-door view of the plant is chosen as the highest level of detail because of its
relevance to immediate implementation. However, it is important to remember that this level
should be looked at in the broader perspective of requirements specified by the supply-chain
system view. Because of this choice of the initial level of detail, certain characteristics of
manufacturing, such as customer relations, product design, manufacturing process selection
etc. are going to be looked at as fixed inputs to a design or redesign effort. This work
attempts to show how this higher level of detail impacts decisions on cell/line design and
further station/machine design.
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Supply Chain
7
Company
7
Facility
Raw material End CustomerSuC mBpr
Company C CmayB Company A NIA
Company B
Q LINE ASSEMBLY
Line or Cell
Station
Component Pr9diction InformationMarket
Incoming Material Finished Par
Material flow
Gomez, 1999
Figure 2-3. Various levels of detail for manufacturing system analysis.
20
f _No-_Am
2.4 Components of a Manufacturing System
A manufacturing system can be represented graphically by means of value stream mapping
[Rother and Shook, 1998], Figure 2-4. This simple method allows one to visualize the
material and information flows between components of a manufacturing system. The
following sections describe the different components of value-stream mapping and its
graphical representation.
4-
Manufacturing Process
Outside Sources
Manual Information Flow
Electronic Information Flow
Signal Kanban
- P' Material Flow
Truck Shipments 0 Physical Pull
Movement of FinishedGoods to Customer
Inventory
Part Market
Physical Push
-FIFO- Transfer of ControlledQuantities of Material in aFirst-In-First-Out Sequence
15 mi. Load Leveling of Volume and Mix
over a specified period of Time
Figure 2-4. Value stream mapping symbols [Rother and Shook, 1998].
2.4.1 Elements
The main elements of a manufacturing system are units of production, which keep a constant
amount of material in process (WIP). They could be stand alone machines or cells and lines,
given that all of the machines in the unit operate at a common production rate and operating
pattern. A sample manufacturing system is shown in Figure 2-5. In a manufacturing system,
21
ASSEMBLY
CUSTOMERS
'4 --- 7 ----
any such element has its suppliers (either internal suppliers - its upstream operations or
external - outside suppliers delivering raw material or components) as well as its customers
(either internal customers - its downstream processes, or external customers receiving end
products). At the same time, each element itself is a customer and a supplier to internal and
external elements. If a given element has multiple suppliers, it is generally an assembly
operation; if it has a single supplier, it is likely to be a machining line or a stand-alone
process such as a stamping press.
Connections
--- ROCEASSEMBLSUPPLIER -__ CUTOMEIRS
Figure 2-5. Sample manufacturing system with its elements and connections.
2.4.2 Connections
The connection between adjacent elements in a manufacturing system consists of two
components: material flow and information flow. Material connection describes how the
material is moved between various elements of the system. Its characteristics include:
" location and size of the inventory
" material movement prioritization policies (FIFO, FISFO, random, etc)
" material movement batch size (related to container size for transportation and the
capacity of the transport medium).
The information connection determines how the production schedule is communicated to all
elements of the system. This information consists of what needs to be produced and when it
needs to be produced. The main parameters describing the flow of information in a
manufacturing system are:
" number of the instruction points, where the information is released to the system
(either single or multiple)
" instruction points location (finished goods inventory, its components inventory, the
last or the first operation of the cell)
22
m information transfer medium (production authorization card, electronic)
m information transfer frequency
2.5 Linked-Cell System
A linked-cell system is a manufacturing system composed of manufacturing and assembly
cells linked by a pull system, thus providing a continuous flow and smooth movement of
materials through the plant [Black, 1991]. A linked-cell system has a single instruction
point, from which production requirement information is issued to upstream processes, in a
direction opposite to the material flow. Therefore, production is based on actual usage
downstream, rather than a forecast requirement.
The cells can be linked directly to each other by placing them directly at the point of use or
indirectly by the pull system of material control called kanban [Black, 1991], described in
detail in Section 4.3. The buffers between cells are used to protect downstream elements
from delays in upstream cells, providing a self-correcting mechanism for production
requirement information to compensate for variations in the system.
The linked-cell system requires a defined and constant internal customer - supplier structure
for a given product type; a given product type has a defined part routing - a sequence of
machines and a path it always takes in its manufacturing process, which is essential in the
flow line process structure. This type of the manufacturing system will be primarily
discussed in this work.
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3 Decomposition Framework in
Production System Design
As mentioned in the previous chapter, a holistic system view in production system design
gives the relation between system objectives, as seen by the customer, and the requirements
for subsystem design. Such a relation helps to prevent local sub-optimization. An approach
to relate these objectives is the decomposition method provided by the axiomatic design
theory [Suh, 1990], which can be understood as a top-down analysis of objectives and
requirements [Duda, 1999].
3.1 Axiomatic Design Approach in Production System Design Decomposition
The design decomposition approach using axiomatic design is a formalized methodology to
structure the design process. Its goal is to answer two questions:
" What are the objectives of the subsystem?
" How can given objective be achieved?
By answering these two questions, Functional Requirements (FR) and Design Parameters
(DP), respectively, are established. The pairs of FRs and DPs can be further decomposed into
lower levels of details by again answering the above questions. Additionally, the answer to
the question:
- How can performance against the given objective be measured?
appoints a performance measurable (PM), which can be used to evaluate how well the given
DP meets related FR.
25
In the decomposition of the design, it is important to adhere to the two Axioms that govern
the design process:
1. Independence Axiom: Maintain the independence of the Functional Requirements
2. Information Axiom: Minimize the information content of the design.
The independence axiom demands that any given DP satisfies a unique FR. Information
axiom states the necessity for simplicity in the design.
The Production System Design Decomposition (PSDD) [PSD, 2000] has been developed
using the axiomatic design approach to relate higher-level business objectives to the
requirements of production system elements. The complete PSDD is shown in the insert at
the end of this thesis. The initial decomposition, shown in detail in Figure 3-1, branches out
to the four fundamental dimensions of manufacturing [Hill, 1994]: quality, cost, response
time and reliability, which define classifications for the lower levels of the PSDD hierarchy.
The top-down analysis of the PSDD highlights the fact that a manufacturing system and the
specifications for its design should be seen as the result of a complex set of dependencies and
constraints and is influenced by the way its performance is measured. Consequently, there
are many options for making use of manufacturing resources; many ways to have time,
inventory, equipment and people work in the system, in order meet the requirements (quality,
cost, response time, response reliability) [Castafieda-Vega, et al., 2000]. Meeting those
requirements also establishes requirements for the information and material flow within the
system. The option selected should result naturally from the high-level performance metrics
employed and reflect the business strategy.
With the aid of the PSDD, the designer can make better decisions during the development or
improvement processes because of being able to see where a design parameter with its
related functional requirements and performance metrics is located in the entire system
hierarchy. The PSDD groups similar characteristics of production systems in a way that can
be used to formulate new system designs, evaluate current performance, guide the redesign
of an existing system or help make informed decisions regarding changes to be made [Carrus
and Cochran, 1998].
26
FR I
Maximize long-term return oninvestment
PM1
Return on investment oversystem lifecycle
DP I
Manufacturing System Design
FR 13
Minimize investmentover productionsystem lifecycle
PM 13
Investment oversystem Iifecycle
---------------------- ----- --[:::::====-,-- -DPI11 DPI12 DPI13
Production to Elimination of Investment basedmaximize customer non-value adding on a long-termsatisfaction sources of cost strategy
FR112 FR 113
re products Deliver products on Meet customer Investmentesign time expected lead timeons n 440 DP.E
PM 112PM11
PM 111 Percentage on-time Difference betweenProcess capability deliveries mean throughput
time and customer'sexpected lead time
DP111 DP112 DPI12
Production processes Throughput time Mean throughputwith minimal variation variation reduction time reductionfrom target
r - I--I
Figure 3-1. Highest levels of PSDD branching out to the four fundamental dimensions of
manufacturing.
27
FR 11
Maximize salesrevenue
PM 11
Sales revenue
FR 12
Minimize productioncosts
PM 12
Manufacturing costs
FR 111
Manufactuto target dspecificati
3.2 FRs and DPs of Information and Material Flows
A complete version of PSDD reveals the requirements placed on material and information
flows for a given production system. The location of these requirements in the overall PSDD
structure is highlighted in Figure 3-2. It can be seen that they mostly reside in two branches
of the decomposition: Predictable Output and Response Time. However, their influence on
the performance of the system, as it will be discussed in Section 3.3, spans many branches.
Figure 3-2. Location of requirements placed on material and information flows in the overall
PSDD structure [PSD, 2000].
The above mentioned requirements are shown in detail in Figure 3-3. They can be divided
into the following functional categories:
1. Material flow:
= Incoming material requirements
" Finished parts movement requirements
2. Information flow:
- Production information content and availability requirements
3. General:
" Requirements for replenishment interaction with other resources
" Simplicity requirement.
28
The applicability of these categories in the physical system is schematically shown in Figure
3-4. The requirements are described separately in sections below and their mutual influence
is discussed in Section 3.3.
Predctabe IDelayPreictble FR-Fl Reduction
Output IA1,
(S.1
PM-PuPM-P1
-P-
FR-P141 FR- Pl4
PM-11P M-PM14 l
-ktb d- W.
Ua.Pdt Pa .... d
ft, p.-o for a.,.
-- a--- a----------
.ya,. 1 D gP b
FR-fl FR-fl FRIJI
d... Iw ".)
PM-T2 PM-fl PU-TI
bP" - w"d Pa...Um
OP-TI OP-fl OP-TI
II IM th .. doobe din .a.Ow~~ UM9 e wv
FR-Tfl FR-fl1 FR-TSI FR-Tm3.. b a ba*. po A E w -
.q~b.* S, d-d
Ow V a - 1
PM-flS PM-TII PM-Tm3
.55. bk-Ta. f.ft ,- P.qn W
,WW 5 FShhi.bd- .I.-f
.*Mpf b.S.& p. a
-P a.. a-
55gb ~~N "" d..I"-
O5 a.
Figure 3-3. Branches of PSDD with requirements placed on material and information flows.
29
Indirect I ILabor FkR-12
PM-42
,,qk.b
Wo"I
-AMq)
General Requirements
FR-T51 FR-T53 FR-12Ensure that Ensure that Eliminatesupport support informationresources don't resources don't disruptionsinterfere with interfere withproduction one anotherresources
Componjnt ---
Marke FR-P14Ensurematerialavaility
Incoming Material
- -l
FR-T31FR-P11 Provide
Aq . Ensure knowledge of
/demandedproduct mix(part types andQuantities)
I---arrival rate isequal to servicerate (ra=r,)
Finished Parts>
I-------------------------------I
Figure 3-4. Physical system with applicable requirements on material and information flows.
3.2.1 Material Flow
The main necessity for material flow expressed by FR-P14 is that the material must be
available at the line, see Figure 3-5. This requirement is answered by the DP-P14, which
calls for a standard material replenishment system in whichFR-P14Ensure material availability replenishment worker(s) (replenishers) will deliver required
Number of disruptions due materials to the line. In order to achieve this designto material shortages,amount of interruption time parameter, its prerequisites are that the parts actually befor material shortages
available to the material handlers (FR-P141), which can be
Standard material achieved through market places linked with the line and theirre lenishment system
supplying upstream processes (DP-P141), and that part
FR-P141 FR-PI42 arrival is properly timed (FR-P142), according toEnsure that parts are Ensure proper timing ofavailable to the material part arrivals pitch (DP-P142). This initiates the definition of thehandlers
PM-P141 PM-P142 replenisher's or material handler's job, where he isNumber of occurrences of Parts demanded - partsmarketplace shortages delivered -responsible for delivering the material to the line,
DP-P141 DP-P42 according to its needs during the designated deliveryStandard work in process Parts moved tobetween sub-systems downstream operations pitch.
according to pitch
Figure 3-5. Requirements for incoming material.
30
Another part of the material flow is the movement of finished parts from the line, as well as
delivery of empty containers, if applicable, see Figure 3-6. This is addressed by FR-T23,
which asks that the part arrival rate to the downstream
FR-T23 customer - or part departure rate from the supplier process -Ensure that part arrival rate is be equal to the demand rate expressed by service rate. Thisequal to service rate (ra=rs)
PM-T23 can be achieved by movement of parts according to pitch
Difference between arrival and (DP-T23). The physical movement associated with this pitchservice rates
(retrieval pitch) will also act to pace the production, which is
especially important with the issue of controllability of theDP-T23Arrival of parts at downstream line. This topic will be further discussed in Section 3.3.operations according to pitch
Figure 3-6. Requirements for finished parts movement.
3.2.2 Information Flow
The PSDD expresses the importance of information flow, see Figure 3-7. Functional
Requirement FR-P 11 declares the necessity for relevant production information to be
available at the line. Designing and implementing a capable and reliable information system
(DP-P1 1) will allow that important prerequisite for predictable output of the line. PSDD also
specifies what kind of information is required by stating the need for knowledge of
demanded product mix to the
FR-P11 FR-T31 production line (FR-T3 1).Ensure availability of Provide knowledge ofrelevant production demanded product mix Therefore, information flow frominformation (part types and quantities)
the customer (DP-T3 1) will dictatePM-Ph1 PM-T31Number of occurrences of Has this information been not only the quantity of demandedinformation disruptions, provided? (Yes/No)Amount of interruption time products but also the mix specifyingfor information disruptions
111 part variety called for, as well as
DP-P1 1 DP-T31 timing of the order to ensure the
Capable and reliable Information expected response time.information system flow from downstream
customer
Figure 3-7. Requirements for flow of production information.
31
3.2.3 General
The PSDD also places requirements of a general nature on both the material and information
flows external to the line, as listed in Figure 3-8. Because material and information
movements involve indirect labor, FR-12 requires the elimination of information disruptions,
which will help reduce the indirect labor tasks (DP122). This can be achieved by
implementing a visual factory (DP-12), which simplifies the tasks performed by support
resources such as replenishers. FR-T51 and FR-T52 require that the support resources that
aid in the material and information flows do not interfere with production in the line and with
other support resources. DP-T51 ensures that the production line is designed to prevent
disruptive part delivery or disruptive part changeover, whereas DP-T53 proposes task
separation and coordination to prevent ambiguity in function performed by each support
person. This design parameter proves to be quite important in ensuring reliability of the
material and information flows as well as controllability of the line and, therefore, of the
entire system.
FR-T51 FR-T53Ensure that support Ensure that supportresources don't interfere resources don't interferewith production resources with one another FR-12
PM-T51 PM-T53 Eliminate information disruptions
Production time lost due to Production time lost due to PM-12support resources support resources Amount of indirect labor requiredinterferences with interferences with one to schedule systemproduction resources another
DP-T51 DP-T53 DP-12
Subsystems and Ensure coordination and Seamless information flow
equipment configured to separation of support work (visual factory)separate support and patternsproduction accessrequests
Figure 3-8. General requirements for replenishment.
32
3.3 Interrelation Between Material and Information Flows
Although the requirements for information and material flows are listed throughout the
PSDD branches, they seemed to be discussed separately. This section attempts to point out
their interdependence and mutual influence.
3.3.1 Controllability of the Production at the Line
As described in Section 3.2.1, the retrieval pitch - the frequency with which end products are
removed from the line to be delivered to the immediate customer - has an important role in
the performance of the system. Although it is directly related to material movement, it also
has information content; every time a material handler comes to pick up finished goods, an
evaluation of the production status can be made: are the parts ready to be picked up, or is the
line falling behind schedule? This assessment can be further substantiated if the same
material handler brings empty containers to the line. The equal exchange of empty
containers for filled ones validates the status of production. For example, if two empty
containers are brought to the line at the specified time but only one container is ready, it is
clear that the line is behind. The coupling of finished goods pickup and empty container
delivery also controls overproduction; since only a specified number of containers is
delivered, the line has to wait for the next retrieval pitch to continue production. Therefore,
this coupling is implementation of a self-controlling system to prevent overproduction.
The key point in the controllability of the line is, however, the above mentioned information
content of the material movement. The knowledge of the behind/ahead status of the line is
important information that can be used to react to system problems. If the material handler,
aside from material movement, is also responsible for bringing production information to the
line, the production status can be delivered to the scheduler with every retrieval pitch. This
brings the following benefits:
- Responsiveness: the scheduler can take action according to production status
information; for example, if the line is behind, additional resources can be sent to
solve the problem, line capacity can be increased (by reducing takt time), the
schedule can be re-sequenced according to priorities, arrangements for overtime can
be initiated, or customers can be contacted early on about possible delays.
33
x Reliability: since one person is obligated to deliver information and remove finished
goods, there is no ambiguity about who is accountable, and even a sense of ownership
can be developed, where the material handler is solely responsible for the connection
between the two elements in the system, shown in Figure 2-5.
For these reasons, it is beneficial to tie the material and information flows together, having a
single person perform both tasks at a given linkage in the system. A similar connection can
be done with line replenishment, where the replenisher brings necessary material to the line.
Here, the information content, which might easily get lost, is related to the quality of
incoming material. If material is delivered in prescribed quantities in every delivery pitch
interval, any quality problems are easily noticeable and can be tracked down and
communicated promptly to suppliers. This is especially important with internal suppliers, for
which immediate feedback can improve output quality. With external suppliers, the feedback
cannot be as rapid and, therefore, this approach might prove less valuable. In either case,
although the information content on incoming material quality might serve an important role,
it is also essential to design robustness against possible defects in incoming material into the
replenishment system to deal with problems that may arise and prevent down time of the line.
Section 5.3 provides a more detail description of possible replenishment options, as well as a
discussion of some of the factors that will influence the choice.
3.3.2 Impact of Information and Material Flows on the System Performance
From the above discussion, it can be seen that material and information flows might impact
many PSDD branches spanning all four fundamental dimensions of manufacturing. Since
the FRs discussed in Section 3.2 come from the Predictable Output, Delay Reduction and
Indirect Labor branches, their impact on those dimensions is obvious, see Figure 3-2.
However, as Section 3.3.1 suggests, the combined material and information flow strategy
will also have impact on the ability to identify and resolve problems of over- or under-
production by frequent information conveyance. Also, by providing robustness against
incoming material quality problems, a well-designed replenishment system will improve the
quality output of the manufacturing system. Thus, the material and information flow
requirements span all major branches of PSDD.
34
4 Information System Design
In general sense, information is a piece of knowledge represented by data that can be
interpreted and used by humans [Solvberg and Kung, 1993]. In the context of a
manufacturing system, not all of the information is actually used. Hence, information can be
divided into formal information -which is recognized, communicated and cultivated within
the organization, as well as informal information -which is comprehended or collected by
individuals, but not communicated, at least not in a formal manner, with others, not utilized
in the enterprise.
One of the goals of production system design is to recognize what information should be
formalized and how it should be communicated. Therefore, a complete information system
not only collects, stores, processes, and distributes information [Solvberg and Kung, 1993]
but also defines how and what information is created, how it flows in the physical system,
and how it is utilized and executed to impact production. Therefore, the information system
is not merely a computerized information system such as many companies implement, but
also the physical aspect of it, including human interaction in conveying the information, and
physical movement and interaction with other production resources. Indeed, information
system is intertwined with the physical part of the production system. These two subsystems:
the physical system - represented by the material flow, and information system - represented
by information flow, are integral parts of the entire production system and, therefore, greatly
impact one another in the design process. Hence, the layout of the physical elements will
influence how the information flows and vice-versa - how and what information is
communicated will influence the required physical system layout. This interconnection of
material and information systems also makes it difficult to discuss information flow patterns
35
without describing the material flow that it induces. Section 4.3 discusses both information
and material flows together under various control policies.
The complete information system of an enterprise has many subsystems, which encompass
all of the functional elements of the enterprise, as shown in Figure 4-1. The manufacturing
information system is one of those subsystems, interacting with other subsystems shown.
The goal of a manufacturing information system is, as shown in Figure 4-1, to plan, schedule,
control, monitor, and facilitate various aspects of production, which refer to various level of
detail, as discussed in Figure 2-3. This work will consider the aspects pertinent to the line and
facility level of detail. Those details are shown in a rectangle in Figure 4-1. The set of these
elements is going to be referred to in this work as the information system in the production
system.
Enterprise Information System
Engineering Manufacturing Human Resources Marketing Other ...Information System Information System Information System Information System
Planning, che ling, ontr 1, M itori , Facilitating of
Process Production / Material Material Inventory ProductionControl Capacity Movement Requisition Resources
Figure 4-1. Elements of enterprise information system.
36
In order to understand the impact of the manufacturing information system on the production
environment, it is useful to break it down along the time dimension. Time scale defines
planning horizon hierarchy in the decision-making process, shown in Figure 4-2. The three
decision levels - strategic, tactical, and operational, concern all aspects of manufacturing,
such as capacity, material movement, material requisition, and inventory from Figure 4-1.
For example, material requisition at the strategic level requires selecting suppliers and
defining the type of relationship. At the tactical level, purchasing policy - frequency and
method of material ordering - needs to be established. Finally, at the operational level,
facilitating delivery of ordered materials is required. At all of the above mentioned levels,
information is crucial in making the decisions.
CompetitiveStrategy
S e Strategic
Inventory Strategyand
Process Technology
Tactical
Production System Designand
Control & Scheduling
Operational
Figure 4-2. Planning horizon hierarchy in decision-making process (adapted from [Wein,2000]).
This work is mainly concerned with the operational and tactical level of decision-making.
This chapter presents issues such as mechanisms of material and information flows, selection
of instruction point in the system, inventory strategy and the basic types of information
patterns. Those decisions at the tactical level will impact the operation of the production
system. Therefore, this chapter discusses the information system in terms of the
requirements for information flow rather than as a computerized tool for information system
management.
37
4.1 Modes of Material Flow
In a production facility, the movement of material is often compared to the flow of a fluid
([Hopp and Spearman, 1996], [Rother and Shook, 1998], [Black, 1991], [Bonvik, 1996]).
This model is an accurate representation of material movement in continuous processes, but
even for production of discrete parts this model is a close approximation, especially on the
conceptual level. This comparison between material movement and fluid flow in discrete-part
production is discussed below.
Following this model, the flow is characterized by its velocity or by its volume flow rate, Q.In a production facility, the volume flow rate can be compared to the production rate, or
production throughput. When the flow accumulates in a tank, the concept of capacity is
introduced. Its equivalent in production is inventory, which is also characterized by its
capacity. Furthermore, the fluid can accumulate in terms of increased density, p i.e.
increased particle (mass) count in given volume, which in the production environment is
accumulation of Work-in-Process (WIP) within a production unit, causing material flow
congestion.
The fluid moves or flows because of a pressure difference across the length or as a result of
flow source. Three basic scenarios can be visualized: flow caused by low pressure source,
flow induced by high pressure source and flow initiated by flow source, shown in Figure 4-3.
In manufacturing, those three scenarios correspond to the pull, push and stream local flows
and they depend on the information flow in the system and more precisely how the
production is authorized.
In a pull mechanism, most typically a production unit will initiate production in response to
withdrawal of a part by its immediate customer. This situation is best illustrated by flow
caused by a low-pressure source, Figure 4-3(a). When a downstream process or customer
withdraws a part, a low pressure is created - the need to replenish that part. This condition
initiates the production and flow of parts. Through this mechanism, the output of the unit is
parallel to what has been requested, corresponding to the low pressure imposed.
38
Low pressurea) Pressure gradient source
High pressure) Congestionb) source
Flow Uniform flowC) source
Figure 4-3. Scenarios for fluid flow: a) low-pressure flow, b) high-pressure flow, c) flow
source flow.
Under the push mechanism, the production unit produces according to a schedule or available
incoming material. This condition corresponds to the high-pressure source flow scenario in
Figure 4-3(b), where the parts are pushed through the system "under pressure". This
situation often causes congestion of the flow resulting in WIP buildup and, similarly to the
compressible fluid flow, non-linearity in the production performance. Thus, the unit output
might significantly vary in quantity and time from the actual requirements. After the parts are
produced, they are pushed further downstream.
Another local material flow mechanism is stream flow, compared to the velocity source flow
in Figure 4-3(c). In this scenario, production is initiated by material coming in at a steady
rate, so no WIP buildup is possible. Parts are not pushed downstream; rather they flow
steadily downstream. Adhering to the First-In-First-Out (FIFO) policy for part processing
allows the production output to be predictable in terms of quantity, sequence and timing. This
mode of local material flow is used in CONWIP control policy as described in Section 4.3.
Which of these modes of local material flow will actually occur in a given production system
depends to a great extent upon the information flow in that system and more specifically the
production control policy adopted. Figure 4-4 presents three systems that demonstrate three
39
different material and information flow patterns. As mentioned in Section 2.4.2, one of the
main parameters characterizing the flow of information in a manufacturing system is the
number and location of instruction points.
Instruction point
a) pull pull
- ~ ~ -P R O E S A S S E M B L Y
UPPLIER F GCUSTOMERS
Instruction points Central
............ Schdln
push a ooCrs push ASSEMBLY
SUPPLIER f.PRCS t-CUSTOMERS
Instruction pointnt
pull pull sto c io
PROCESSASMLrSUPPLIER G CUSTOMERS
Figure 4-4. Various material flow patterns: a) pull flow, b) push flow, c) pull and stream
flows.
The instruction point specifies the element in the production system at which scheduling
information enters the system specifying quantity and time of required production. The
system shown in Figure 4-4(b) displays multiple instruction points to which the information
from the customer is distributed through central scheduling. This multiple location of
instruction points is responsible for creating the high-pressure sources, which initiate the
push flow mechanism of material movement. Systems in Figure 4-4(a) and (c), on the other
hand, have a single instruction point from the customer.
40
4.2 Inventory Strategies
Whereas the number of instruction points in a system determines to a great extent the mode
of material flow, the exact location of instruction points reflects the inventory strategy
chosen. Two major inventory strategies exist: build-to-stock (BTS) or build-to-order (BTO)
[Hopp and Spearman, 1996]. Figure 4-5 shows both of those strategies.
S PROCESS
SUPPIEJRS
ASSEMBLY
CUSTOMERS
bi)
S PROCESS
SUPPIERS
ASSEMBLY
CUSTOMERS
b2)
SUPER
PROCESS ASSEMBLY
CUSTOMERS
Figure 4-5. Manufacturing systems with different inventory strategies: a) build-to-stock
(BTS), bI) build-to-order (BTO), b2) assemble-to-order (ATO).
Build-to-stock strategy requires that the finished goods inventory (FGI) be kept at the end of
the value stream. That inventory allows delivery lead times shorter than manufacturing
throughput time, i.e. the time it takes one part to go through all processes. It also provides a
buffer against variations in production, demand forecast errors and enables batch production.
The BTS strategy also makes it possible to build ahead to cope with seasonality of demand
[Hopp and Spearman, 1996].
However, holding finished goods inventory is impractical or infeasible if there is a wide
product range, products are large and difficult to store or if the inventory is likely to become
41
obsolete. In this situation, build-to-order strategy can be implemented. Under this strategy,
the parts are made for a specific order received and directly after their production, they are
sent to the customer. A variation of this strategy is assembly-to-order, where the parts are
assembled to a specific order and shipped to the customer but the components themselves are
prefabricated or build-to-stock.
Table 1. Linking Manufacturing Strategy to inventory strategy choice (from [Hill, 1994]).Arrows signify the location in the spectrum between two characteristics.
Strategic Variables Inventory StrategyBuild-to-Order Assemble-to-Order Build-to-Stock
Type Custom-made Standard
PredeterminedRange Wide and narrow
Product volume Low-runner High-runnerper period Low-runner___High-runner
Speed Difficult Easy
Reliability Difficult Easy
Process choice Job shop Line flow
Managing changesMaaging hand x Order backlog WIP or FGI FGIS in sales and mix
Meeting delivery Through rescheduling Reduces process Eliminatesspeed requirement requirements lead time process lead time
Inventory strategy - decision at the tactical level - is an important influence on both material
and information flows. Table 1 presents some of the strategic variables that influence the
choice of inventory strategy.
Depending on product characteristics, if the product is highly customized, BTO inventory
strategy should be used (job shop process structure is often used in these circumstances). As
the product becomes more standardized, ATO or BTS strategy might become feasible (line
flow should be the choice for process structure). The choice of inventory strategy is also
affected by the range of products. If the product range is wide, it might be infeasible to hold
all types of parts in FGI and, therefore, BTO or ATO strategies are necessary. As the product
42
range becomes narrow and predetermined, BTS strategy becomes feasible. Oftentimes, the
product range consists of parts produced in high volume (so called high-runners) and parts
produced less frequently (low-runners). In order to prevent holding inventory that is not
needed for long periods of time, the low-runners should be managed under BTO strategy,
whereas the high-runners might be managed under BTS strategy.
BTS strategy generally offers short lead times and more reliable delivery, since the parts
requested by the customer can be simply withdrawn from the FGI. In the ATO and BTO
strategies, since the processing time is included in the lead time, fast and reliable delivery
becomes more difficult to achieve and the delivery speed requirement is met through
rescheduling. FGI present under the BTS strategy allows to easily manage minor changes in
demand volume and mix. Under the ATO strategy, the demand fluctuations might be
managed to some degree with WIP before assembly, whereas BTO strategy requires keeping
order backlog.
4.3 Patterns of Information Flow Under Various Control Policies
Both the number of instruction points in the system and the inventory strategy mentioned in
the previous sections are decisions which often are determined by the control policy of the
system. A control policy is a mechanism by which the production at the shop floor is
scheduled and controlled [Bonvik, 1996]. The information flow pattern dictated by the
chosen control policy will therefore affect the material flow as well.
One of the fundamental control policies used in industry is a push system [Hopp and
Spearman, 1996]. Under that policy the information is delivered to many locations, i.e. the
system has multiple instruction points. Material Requirement Planning or its later version -
Manufacturing Resources Planning - (MRP) is a tool used for implementation of push
policy, where it schedules the release of work based on demand or forecast. The push
control policy controls throughput and observes WIP and inventory levels [Hopp and
Spearman, 1996] and by doing so initiates the push mode of material flow.
43
In contrast, the pull system has a single instruction point in the system. This single
instruction point, which can initiate both pull and stream modes of material flow, has an
important role in system performance since it is the only point in the entire system at which
customer demand information is communicated. This point is called the pacemaker element,
because how production is controlled at this element sets the pace for all the upstream
processes [Rother and Shook, 1998], as shown in Figure 4-6. From that point in the system,
the demand information is issued to upstream elements based on actual downstream
consumption, following the direction opposite to that of the material flow. Hence, the control
information is self-compensated against variation amplification at every stage of production.
Kanban is a tool commonly used with pull systems. It authorizes the release of work based
on system status, therefore, it controls WIP and inventory levels and observes throughput
[Hopp and Spearman, 1996].
a) pull pull pull
PROCESS 1 PROCESS 2 PROCESS 3 ASSEMLY
-- -- CUTOMRS
b) pull pull pull
PROCESS 1 PROCESS 2 PROCESS 3 ASSEMBLY
CUSTOMERS
c) pull
PROCESS 1 PROCESS 2 PROCESS 3 ASSEMBLY
] FIFO, -FIFO+ USTMER
Figure 4-6. Different locations for pacemaker element and resulting control policies in a
linked-cell system: a) pure pull, b) sequenced pull, c) CONWIP, (adapted from
[Rother and Shook, 1998]).
44
Because the location of the pacemaker element determines the pattern of information flow in
the factory, it also sets the control policy in the system, such as pure pull, sequenced pull or
CONWIP [Rother and Shook, 1998] summarized below.
Pure pull control policy, Figure 4-6(a), locates the pacemaker element at the last stage of the
value stream, the finished goods inventory. This location of the pacemaker makes all
upstream processes to work to replenish actual consumption by implementing build-to-stock
inventory strategy. Customer lead time becomes the time it takes to withdraw and transport
material from the finished goods inventory. This control policy causes the material to flow
according to the pull mechanism.
When holding finished goods inventory becomes impractical, possibly due to a high number
or variations or large parts used infrequently, the sequenced pull could be a better control
policy [Rother and Shook, 1998]. Instead of holding the parts in the supermarket, the
pacemaker element (still at the end of the value stream) is instructed to make the parts to
order under ATO inventory strategy, Figure 4-6(b). This policy is feasible only if the
supplying process's lead-time is fast enough, as it will become part of the overall customer
lead time. Under this policy the material flows in both pull and stream flow modes, as shown
in Figure 4-6(b).
Another approach is locating the pacemaker element further upstream. The resulting control
policy is called CONWIP [Hopp and Spearman, 1996]. From the chosen pacemaker element,
the material flows in a FIFO sequence in the stream mode, limiting inventory between
downstream processes, Figure 4-6(c). The location of the pacemaker element will determine
the elements of the value stream that become part of the lead time of the system. This policy
requires a perfect quality yield from the processes downstream from the pacemaker, or a
system dealing with the quality problems by providing feedback information to achieve right
quantity in order to keep the FIFO sequence. Otherwise, quality problems will result in un-
met customer demand.
45
Although the effects of control policies have been studied extensively [Bonvik, 1996], the
choices for material withdrawal, the factors for its implementation and the impact of various
conveyance methods on the overall behavior of the manufacturing system seem to have
gotten less attention. The material withdrawal refers to the exact location of the instruction
point within the selected pacemaker element and how the information is translated into
material flow. Chapter 5 discusses withdrawal and replenishment methods in a linked-cell
system.
46
5 Conveyance Methods in Linked-
Cell Systems
A linked-cell system consists of multiple production units linked through local information
and material flow connections, Figure 4-6. Execution of those local connections is referred to
as conveyance. The goal of conveyance is to move the material between production units or
their storage market places and to transmit relevant production information. Those local
connections exist under various control policies in linked-cell system (pure pull, sequential
pull and CONWIP), as discussed in the previous chapter.
5.1 Conveyance: Withdrawal, Replenishment
Conveyance is the action of transmitting something between two elements - the origin and
the destination. In manufacturing, the entity being transmitted can be material or information
whereas the origin and destination elements can be either production units or inventory
market places, as Figure 5-1 shows.
As discussed in Section 3.3, information and material flow paces production, thus preventing
overproduction and allowing quick reaction to problems, thus counteracting underproduction.
This controllability of production elements, i.e. monitoring the performance in terms of
ahead/behind status and quick reaction to solving problems, is made possible by the feedback
loop created by coupling the material and information flow between two given elements of a
manufacturing system. This coupling is embodied in conveyance itself.
47
info
PROCESS 1
material
Pickp material transport Do f
Withdrawal Conveyance ReplenishmentFigure 5-1. Conveyance loop: withdrawal from origin and replenishment at the destination.
The main goal of the conveyance system is a frequent and cost-effective movement of
material and transmission of information [Monden, 1998]. The more frequent the
conveyance, the tighter control over production, with the optimal being a single piece
conveyance. However, these two objectives: increased frequency and cost effectiveness, can
be conflicting if increasing frequency requires additional resources. Therefore, the preferred
approach to increasing frequency is to shorten the conveyance time itself and thus release the
resources to perform more frequent conveyance. Another tactic for achieving frequent
conveyance is use of mixed loading, where instead of infrequent deliveries from each
supplier, more frequent joined supplier deliveries are practiced. The shortening of the
conveyance time is also important to shortening production lead time, especially because
conveyance is a non value adding task required in the production process. When single piece
conveyance is still not achievable, a less frequent "batch" conveyance is necessary. A
commonly chosen conveyance frequency within a given facility is one hour [Monden, 1998].
The action of conveyance consists of three stages shown in Figure 5-1. First, the entity
transmitted must be picked up from an element, its origin, then it is transported between the
elements, and finally it is dropped off at the destination element. However, it is important to
note that since the conveyance of material and information, which flow in opposite
48
directions, is often done in a loop by the one person, the drop-off point for the material is also
the origin of the information and the destination point for the information is also the origin of
the material for the given loop. In fact, the information conveyed in the loop is often the
instruction as to what material needs to be picked up at the upstream element. Therefore, in a
linked-cell system the pick-up of the material (and drop-off of the information) is referred to
as withdrawal and the drop-off of material (and pick up of information) is referred to as
replenishment.
Under a control policy such as CONWIP local information connections not always exist, see
Figure 4-6. In this case the given conveyance loop will transport material based on what is
available in FIFO sequence at the origin. However, there is still information that can be
conveyed: for example, the loop can be used to monitor the production status of the upstream
process and communicate developing problems, as discussed earlier.
Replenishment loop Withdrawal loop
info info
PROCESS
material maerial
Figure 5-2. Conveyance loops as seen from the particular element: upstream loop -
replenishment, downstream loop - withdrawal.
As described above, each conveyance loop performs withdrawal (pick-up of material at the
origin) as well as replenishment (drop-off of material at the destination). However, when
looking at a particular production element, such as a manufacturing cell, its upstream
conveyance loop can be labeled as replenishment (delivery of necessary material) and its
downstream conveyance loop as withdrawal (removal of finished parts), see Figure 5-2. This
definition of replenishment and withdrawal is mostly used in this work, whereas conveyance
49
refers to both actions of replenishment and withdrawal performed at the given loop between
two elements.
5.2 Kanban as a Tool
Kanban is a tool used in linked-cell systems to facilitate information flow under various
control policies. The principle behind the kanban system is to provide a direct and
standardized connection between elements that will prevent ambiguity in its outcome
resulting in a binary yes/no connection [Spear and Bowen, 1999]. This connection requires
conveyance of both the product and the information. The kanban system unambiguously
initiates release and production of parts, i.e. withdrawal and replenishment of parts.
5.2.1 Types of Kanban
Kanbans are essentially cards attached to part containers. They specify the part type,
quantity, destination and origin locations for the container with parts. Since the kanbans
perform many functions, there are many variations of kanbans. The most general
classification of kanban types is shown in Figure 5-3. Material withdrawal kanbans are used
to authorize pickup of material and initiate the conveyance of material. The kanban card
specifies what parts are needed, how many should be transported, where they should be
picked up from and where they should be delivered. Production authorization kanban, on the
other hand, signals to initiate production and thus constitutes information flow between
production elements. This kanban card specifies what part type is required, how many parts
are asked for, where the request is coming from (the local customer for the parts) and to
which manufacturing unit the information is going (the supplier for the parts).
The withdrawal kanban and production kanban are used for conveyance of material and
information for discrete parts in a relatively small container, or production run size. This
allows frequent conveyance of small quantities of material. For processes that require large
lot production, such as stamping press, material requisition and signal kanbans are used as
equivalent in function to withdrawal and production kanbans, respectively. The signal
kanban aggregates requests from individual production kanbans and authorizes production
50
when the required lot size is reached. Material requisition kanban is then used to request
material for the production of the entire run size.
Kanban FunctionMaterial Withdrawal Production Authorization
-4
D
Figure 5-3. Classification of kanban types based on its function and production lot size.
Other types of commonly used kanbans include supplier kanban, express kanban, emergency
kanban, through kanban, and job-order kanban [Monden, 1998]. Supplier kanban is a type of
a withdrawal kanban tailored to use of obtaining component parts from the suppliers. In
addition to the information every withdrawal kanban contains, the supplier kanban also
specifies the frequency of supplier delivery as well as the information that allows accounting
transactions to take place between supplier and the customer. Express kanban is used to
notify of unexpected shortage of a component and to prioritize its production, whereas
emergency kanban schedules production to make up defective units or unexpected down
time. Both express and emergency kanbans are one-time cards, which need to be removed
from the system after their use. When local information flow is not present, such as under
CONWIP control policy, a through kanban is used to identify the parts and specify their
movement between processes, restricting amount of WIP present by keeping it constant.
Finally, job-order kanban is used to schedule and authorize non-repetitive production or
services usually in support of production, such as tool or fixture procurement.
51
Withdrawal ProductionKanban Kanban
Supplier Kanban
Material Requisition Signal KanbanKanban
5.2.2 Kanban Systems
In normal circumstances, the flow of withdrawal and production kanbans is sufficient to fully
authorize and control production of the system. Such kanban system is called a two-kanban
system and it is shown schematically in Figure 5-4. It consists of two conveyance loops:
withdrawal loop (marked by W), which uses withdrawal kanbans and replenishment loop
(marked by R), which uses production kanbans.
Replenishment loop Withdrawal loop
Pro tionordering post
PROCESS
Coll etingpost
With rawal Ipost
PROCESS
Figure 5-4. Movement of kanbans in the two kanban system:
- - o production kanban, - . * withdrawal kanban.
The withdrawal kanbans are collected from the downstream production unit in a withdrawal
post, shown in Figure 5-4, step W1. They are picked up by the conveyance person in the
withdrawal loop and are taken to the component market, W2. At the market, the appropriate
containers, as specified by the withdrawal kanbans, are acquired and the production kanbans
attached to them are removed and placed in the collecting post. The withdrawal kanbans are
then attached to the containers, W3, which are then transported to the next production unit,
W4. At the production unit, as the parts from these containers are used up, the withdrawal
kanbans are removed from them and placed in the withdrawal post, W1, to complete their
cycle.
The production kanbans accumulating at the collecting post at the component market, R1, are
picked up and brought to the upstream production unit, R2. There, they are placed in the
52
production ordering post from which they are moved at a frequency set by a pace to the
production line to initiate the production, R3. As the production of the required parts is
completed, the production kanbans are attached to the appropriate containers, R4. The
containers are then taken to the downstream component market and placed there, R5, thus
completing the cycle of production kanbans.
The separation of the production kanbans from withdrawal kanbans allows autonomy of
replenishment and withdrawal loops, with the inventory market usually under the
administration of the production unit supplying it, the upstream unit. Each set of kanban
cards, production and withdrawal, can be managed separately, by defining the route,
frequency of the conveyance cycle as well as number of cards in the system. However, when
the production processes are a short distance from each other and they are under jurisdiction
of the same supervisor, a common kanban system can be used, thus simplifying the
management by the use of one set of cards, instead of two.
In the common kanban system, although there are still two separate conveyance loops of
replenishment and withdrawal, the same kanban is used for both of those actions. As shown
in Figure 5-4, kanbans are collected from the downstream production unit in a withdrawal
post, Wi. They are picked up by the conveyance person in the withdrawal loop and are taken
to the component market, W2. At the market, the appropriate containers, as specified by the
kanbans, are acquired and the kanbans brought are placed in the collecting post, Rl, where
they are picked up and brought to the upstream production unit by the replenishment person,
R2. There, as in the two-kanban system, they are placed in the production ordering post from
which they are moved at a pace to the line itself to initiate production, R3. As production of
the required parts is completed, kanbans are attached to the appropriate containers, R4, which
are then taken to the downstream component market, R5. From the market they are picked
up by the withdrawal loop conveyance person, W3, and are then transported to the following
production unit, W4. At the production unit, as the parts from these containers are used up,
the kanbans are removed from them and placed in the withdrawal post, Wi, to complete their
cycle.
53
In this common kanban system, since only one type of kanban card is used at any time, the
containers themselves can be used as kanbans, provided that they unambiguously specify all
necessary information, such as the part type, quantity, origin and destination location. An
empty container then signifies a production order, whereas a full container initiates material
delivery.
5.2.3 Specifications of Kanban Systems
The kanban system is an information system, which harmoniously controls the production
quantities in every process and manages the Just-In-Time (JIT) production method [Monden,
1998]. The system operates by circulation of cards described in previous section, which
authorize conveyance of both material and information, thus pacing and controlling
production.
The kanban system performs multiple functions in the manufacturing system [Monden,
1998]:
" Instruction
m Self-control
" Visual control
" Improvement
" Reduction of managerial costs
Those functions are discussed hereafter.
The flow of kanbans is the means of communication between various production elements
and instruction for the entire system at a local/micro level, through direct connections
between subsequent production units, as well as at a macro level, through the single
instruction point. The kanban cards provide instruction for production by specifying the part
type and quantity requested by the customer. The kanbans also instruct conveyance by
specifying where the parts are picked up and where they need to be delivered.
The circulation of kanbans in the system also provides a self-control function for the system.
By having a closed loop of kanban flow, the number of kanban cards in the system can be
controlled and kept constant. This automatically controls the amount of inventory in the
54
system and prevents overproduction. Also, since the system has a single instruction point, it
is able to respond quickly to demand changes in a self-controlled manner. In fact, the kanban
system is capable of handling ±10% fluctuations in demand volume, so called fine-tuning of
production [Monden, 1998]. More significant demand changes usually require an alteration
in the number of kanbans, which can be performed less frequently.
The kanban system also provides visual control of production levels and inventory counts.
Since each kanban card represents a container, the constant number of cards in circulation
allows easy and visual control of the production status. An accumulation of cards signals
problems, which can be quickly detected and counteracted. Because the kanban system is a
decentralized control system, the characteristic of being visual makes it transparent, which is
especially essential at the interface between local units, for example at the component market
place between the replenishment loop in its upstream unit and withdrawal loop in its
downstream unit.
The kanban system also serves to improve the process and manual operations. The close
control of production status through frequent conveyance of information in the kanban
system communicates problems quickly and allows the focus of additional resources to
overcome the obstacles. By increasing the frequency of conveyance, the instruction interval
for production and conveyance is greatly reduced at the same time, striving toward one piece
production and conveyance, which leads to true single piece flow of material as well as
information.
Finally, the single instruction point and the self-controlling property of the kanban system
allows for reduction of managerial costs. The kanban system, although requiring rigor in its
implementation, is simple to use. The role of the scheduler is greatly simplified, compared to
a conventional central control system; here, only one point needs to be informed of customer
demand and the demand information will propagate through the system via the kanban flow.
55
The implementation of the kanban system requires specifications of certain parameters to
completely characterize the operation of this production control system. Table 2 lists some
of those design factors.
Table 2. Kanban System Operation Design Factors [Berkley, 1992].
1. Kanban numbers2. Container processing (run) time distributions3. Setup time distributions4. Finished-goods demand5. Supply of purchased raw material6. Line configuration/Part routings7. Station blocking mechanism8. Number of part types9. Part yields10. Part container size11. Batch sizes (in containers)12. Station container sequencing rule13. Number of machines per station14. Machine reliability15. Worker flexibility16. Worker assignment rule17. Material handling trigger18. Material handling frequency19. Number of part-carriers20. Availability of part-carriers/Part-carrier assignment rule21. Part-carrier capacity22. Material-handling operation time distribution23. Material-handling operation blocking mechanism24. Material-handling operation container sequencing rule
By defining these parameters, the kanban system can be designed and implemented.
However, in order for it to perform satisfactorily, diligence is required. The following
application rules of a kanban system [Monden, 1998] depict this diligence:
A process should withdraw the necessary products from the preceding process in the
necessary quantities at the correct time. This rule is necessary to prevent ambiguity in the
connection, as mentioned at the beginning of Section 5.2. For this purpose the kanban card
explicitly specifies the part type, quantity and location (both of origin and destination). This
56
rule also requires that people performing the conveyance follow these instructions on the
card and their designated routes.
The preceding process should produce its products in quantities withdrawn by the subsequent
process. This rule, similar to the previous one, is necessary to prevent ambiguity in the
connection. It specifies that production should only be initiated in response to receiving a
production instruction for a specific part type and only the requested number of parts should
be produced, as specified by the received production kanban.
Defective products should never be passed to the next process. Otherwise, a shortage of
parts will be created downstream or defective product will be produced, resulting in costly
scrap or rework. This rule complements the previous two in preventing ambiguity in the
connections between production units. If a request for a given quantity of parts is sent, it
signifies the exact quantity of good parts that should be conveyed.
The number of kanbans should be minimized. Since the number of kanbans determines the
size of the inventory held, the reduction in the number of kanban will yield cost reductions.
Although the number of kanbans is calculated according to specific formulas based on
customer demand (described in Section 5.3.3), the line supervisor makes the final decision on
how many kanbans should be circulating, based on the knowledge of process capability of
the given area. Too small number of kanbans will lead to delivery shortages, whereas an
excessive number of kanbans will create inventory build up, material stagnation and slack in
the conveyance.
Kanbans should be used to adapt to small fluctuations in the demand (fine-tuning of
production). The self-control characteristic of the kanban system, therefore, apart from
managing the fluctuations in demand, should be used to detect problems with purpose of
resolving them, rather than their concealment. The system does need to be designed with
robustness against defects; this, however, should be done through error-proofing devices,
called poka-yoke, as well as express or emergency kanbans rather than compensation for
defects through delivery of extra parts. The kanban system is also capable of managing
57
delivery of parts that have by nature unstable usage, such as weights used to balance
irregularities in shaft rotation.
The actual quantity of parts contained in a box or packed in a load must be equal to the
quantity written on the kanban. This rule complements rule 1 in preventing ambiguity. Since
the kanban card explicitly specifies part quantity, each container for the given part type
should contain a constant quantity and no partially filled containers are allowed.
The essence of the above rules specifies the connection between production elements in a
direct and standardized manner and prevents ambiguity, thus providing a binary yes/no
connection in material and information flow. Adherence to these rules will ensure proper
performance of the kanban system to its full advantage, thus providing a complete production
control system.
5.3 Conveyance Methods
The conveyance between production elements and their market places, i.e. the replenishment
of used parts with parts withdrawn from the previous element, is critical to the performance
of the system as it executes material and information flow between them and provides
instruction for production throughout the system. The conveyance frequency determines the
management time frame for the system, i.e. the interval at which the performance can be
monitored and, if necessary, adjusted [Rother and Shook, 1998].
5.3.1 Mechanism of Conveyance
The execution of both withdrawal and replenishment is shown schematically in Figure 5-4.
The withdrawal loop starts at the withdrawal post, from which the conveyance person
performing the loop picks up accumulated kanban cards, Wi. The cards together with empty
containers, if applicable, are then taken to the upstream component market, W2, from which
the required parts are withdrawn and empty containers dropped off, W3, and at the same time
information to replenish them is sent upstream, Ri. The withdrawn parts are then taken to
the line requesting them, W3. The conveyance person then proceeds to the withdrawal post,
Wi, and continues another cycle of the withdrawal loop. One conveyance person usually
58
performs this entire loop, provided that the distance between the production element and the
market place is sufficiently short to allow for frequent conveyance. However, a withdrawal
in a given loop can be done for multiple types of components and the same conveyance
person can stop at multiple market places or even service multiple lines during a complete
cycle, as long as the loop route can be standardized.
The replenishment is initiated with the withdrawal of the parts from the component market
described above, W3, and the request to replenish them. This request signal is manifested by
cards accumulated in the collecting post, Ri, where the replenishment loop starts. Here the
cards are picked up and sequenced in the production ordering post, R2 from which they are
delivered in a specified interval (production ordering pitch) to the line, R3. At the end of the
line, the parts can then be picked up, R4 and delivered to the market place. The pickup of
parts is usually accompanied by delivery of empty containers to store produced parts. In
order to ensure constant WIP within the given production unit as well as monitor its
production status to prevent unexpected delays in production, the pickup of parts should be
done in terms of equal exchange of empty containers with full ones. Also, the interval at
which the withdrawal of parts from the production unit is performed (retrieval pitch) should
be equal to the production ordering pitch. This sometimes is reinforced by the above
mentioned equal exchange of empty and full containers, where an empty container is an
additional authorization to produce parts and its absence stops further production and thus
prevents overproduction.
It is important to mention that the market place is usually under the jurisdiction of its
immediate supplier and is therefore placed close to it. This allows for an easy visual control
and a sense of ownership, which helps promote process improvements leading to inventory
reduction. Because of the market location close to the supplying production unit, usually the
replenishment loop for the market is much shorter and thus a more frequent monitoring of the
production is possible. The withdrawal loop, however, is longer, which often advocates
withdrawals of multiple parts in a single route (mixed loading).
59
5.3.2 Pacemaker Element Withdrawal
An important element of a linked manufacturing system is a consistent, level production
scheduling of small quantities of work, called paced withdrawal [Rother and Shook, 1998],
which allows for frequent and regular monitoring the performance of the entire system to
customer demand. The pacemaker element of the system - the single instruction point in the
linked-cell system - receives the demand information and propagates it upstream through the
local connections between the elements. Thus, the pacemaker element sets the pace for
production in the entire system.
The pacemaker element receives leveled demand information through paced withdrawal.
Thus, the withdrawal loop for the pacemaker element is crucial for the performance of the
entire system. A tool used by some companies to implement level schedules in mix and
volume is called a heijunka box [Cochran, 1998], shown in Figure 5-5. The heijunka also
serves to instruct the paced withdrawal of material at a given frequency as defined by the
withdrawal pitch.
Figure 5-5. Heijunka: tool for leveled scheduling and paced withdrawal.
60
PACEMAKER SHIPPINGELEMENT
Figure 5-6. Paced withdrawal loop from heijunka to pacemaker element.
A withdrawal mechanism is shown in Figure 5-6. The paced withdrawal loop starts at the
heijunka box (1), from which the conveyance person performing the loop picks up the
kanban cards for the given pitch interval. The heijunka determines the frequency of
conveyance, as specified by the withdrawal pitch, the quantity conveyed, and the sequence of
the production during the next pitch interval. The remainder of the loop follows similarly to
any withdrawal loop from a production element (or replenishment to a market place as
described in Section 5.3.1) allowing replenishment of the parts market.
5.3.3 Types of Conveyance Methods
In order to manage minor ( 10%) fluctuations in demand, the withdrawal and replenishment
systems cannot be over-constrained; the conveyance frequency and the quantity conveyed in
each cycle cannot stay the same when the demand changes. This is a necessary characteristic
for production fine-tuning in the kanban system. Therefore, two general types of conveyance
methods are commonly used [Monden, 1998]:
" Constant cycle (non-constant quantity)
- Constant quantity (non-constant cycle)
In the constant cycle conveyance, the withdrawal or replenishment loop is performed always
at the same frequency, for example, once every hour. Depending on the demand, however, a
different number of kanbans will be picked up at the kanban post (W1, Ri, R2 in Figure 5-4),
resulting in conveyance of non-constant quantity (although always in accordance with the
number of cards picked up). This method is especially applicable when a single conveyance
61
loop services various production units. With constant cycle conveyance, when demand
fluctuation causes the production rate of one of the units to change, the number of the cards
picked up and quantity of parts conveyed every cycle will change, but the other units will not
be affected. Constant cycle conveyance is also required for parts with unstable usage, as
described in Section 5.2.3, which by nature have a varying quantity of consumption and
therefore conveyance.
In constant quantity conveyance, on the other hand, the conveyance will occur when a
predetermined quantity of parts is requested and thus when a predetermined number of
kanbans is collected. However, depending on the demand, the frequency with which the
conveyance takes place will change. This method is especially applicable if the conveyance
loop services a single production unit but many market places. If the production rate changes
due to demand fluctuations, the same quantity can still be conveyed but the frequency will
change based on the production rate change. This method, however, requires continuous
monitoring of inventory or number of collected kanbans, since the conveyance is initiated
when a specified number is reached.
The number of kanban required in the system in the constant cycle conveyance is given by
formula [Monden, 1998]:
Total Number of Kanbans = Dx(LT+CC+SP)CD
and for the constant quantity conveyance is given by formula:
Total Number of Kanbans = Dx(LT+SP)CD
where D is the average daily demand, LT is the lead time, CC is a conveyance cycle or
interval, SP is a safety factor, and CD is container density or capacity. Lead time, LT is
defined as:
LT = (processing + waiting + conveyance + kanban collecting) time.
The difference between these two formulas is the inclusion of the conveyance interval in the
calculation of number of kanbans for the constant cycle method, which will increase the
number of kanbans required for this method relative to the constant quantity method. The
reason for this difference is that whereas in the constant quantity method, as soon as the parts
62
are used, their replenishment is initiated, in the constant cycle method, the request has to wait
up to a full conveyance interval before being picked up and fulfilled.
For example if the average daily demand is D = 60000 pcs./day, LT = (processing + waiting
+ conveyance + kanban collecting) time = 3h + 0.5h + lh + 0.5h = 5h, and CD = 10 pcs.,
with SP = 3h, total number of kanbans for the constant quantity conveyance method = 2000,
and for the constant cycle conveyance method with CC = lh, total number of kanbans =
2250. The final number of kanbans, although based on the above formulas and calculated as
shown in the example above, is subject to change by the supervisor [Monden, 1998]. Those
adjustments are usually done through changing of safety factor, SP, which allows increase in
the number of kanbans depending on process capability.
5.3.4 Examples of Withdrawal Systems
The conveyance method - constant cycle vs. constant quantity - specifies the mechanism by
which the system adjusts to the changes in customer demand: either by varying the quantity
conveyed or varying the conveyance frequency. However, how the replenishment and
withdrawal is actually performed also depends on other issues, such as local inventory
strategy and the control policy adopted.
Figure 5-7 shows various examples of conveyance loops. Figure 5-7(a) depicts a system
similar to the one discussed in Section 5.3.1 and presented in Figure 5-4. This system
operates under pure pull control policy with build-to-stock (BTS) inventory policy. Figure
5-7(b) represents conveyance under sequential pull, where the inventory strategy is build-to-
order, BTO. Figure 5-7(c) shows CONWIP control policy with BTO inventory strategy. All
the above mentioned conveyance routes can be performed using either of the two conveyance
methods described in Section 5.3.3, either constant cycle or constant quantity. Also, it is
interesting to note, that customer lead time is different in each case, and is determined by the
length of the conveyance loop itself and, in case of sequential pull and CONWIP control
policies, also by the processing time which is included in the route.
63
PRCS SHIPPING
b)-
PROCESS PROCESS SHIPPING
PROCESS PROCESS SHIPPING
- FIFO --
Figure 5-7. Various conveyance loops: a) pure pull/BTS, b) sequential pull/ATO,
c) CONWIP/BTO.
Two commonly used withdrawal systems are later replenishment withdrawal and sequenced
withdrawal [Monden, 1998]. The later replenishment system operates under the BTS
inventory strategy, where the components are stored in a market place (as in Figure 5-7(a))
and then withdrawn based on a constant cycle withdrawal loop. The components specified
by the withdrawal kanbans brought from the local customer by the conveyance person will be
delivered to this customer just one cycle later, when the conveyance person finishes the loop.
The market place from which the parts were withdrawn will be replenished later, after the
production ordering kanbans make their loop. Due to the necessity for inventory in the
market place under this system, it is usually used for small components with few varieties.
64
When the size of inventory required in a market becomes an issue, making it infeasible to
implement BTS strategy, the sequenced withdrawal system is used. This system performs
under the constant quantity conveyance method and is usually used for components such as
engines or transmissions in the automotive industry [Monden, 1998] - large parts with many
variations. This system works under BTO strategy, as shown in Figure 5-7(b), (c). The
kanban cards are taken to the supplying production element; however, the corresponding
components are not brought back until a few conveyance cycles later, when the production of
those components is completed. This system avoids costly inventory, however it requires
that production lead time of the supplying production element is sufficiently short to supply
the parts on time. Another way to shorten the response time in this system is by sending the
requirements instantaneously via fax or electronically, so that the kanban conveyance time of
information between customer and supplier is shortened. This system, however, also
requires that from the moment the withdrawal loop is initiated, the production sequence of
the customer cannot change and thus the production sequence at the supplier does not have to
be readjusted.
65
66
6 Case Study
The case study project for this work took place at Visteon Automotive Systems in Monroe,
Michigan. Monroe plant is a facility of 1.5 million ft2 employing around 2200 workers with
a strong unions presence. It supplies various automotive components to 47 different
customers. The main product areas are steel wheels, coil springs, stabilizer bars, catalytic
converters, body components and hot stampings, with an annual production of 38 million
components.
The manufacturing system studied is a production line for catalytic converters for over 8500
F-series and Econoline trucks, supplying multiple customer plants. The product, shown in
Figure 6-1, weights up to 30 lb and is up to 5 ft long. It comes in 20 varieties and consists of
up to 11 different components. Some of those components are common to all types of
converters (mat mount and seal), some are complementary and not required on all types of
converters (hanger rod, upper shield), and the rest come in different varieties (three varieties
of inlet pipe, eight varieties of outlet pipe, five varieties of catalyst, etc.). The components
are delivered by various external suppliers, from few times daily to once a month, and a
single internal supplier twice a week.
The planned annual production volume for the line in 1998 was 350000 parts. The parts are
supplied to two main customers and few other customers on less regular basis. The line
produces components for current model cars - amounting to 91% of the total production, as
well as parts for service - 9% of the production.
67
MAT MOUNT (3) UPPER SHIELD
AIRADATERCATALIST (3) (6 UPPER SHELL
INLET PIPE
ID PLATE L[E SELOUTLET PIPE
LOWER SHIELD
Figure 6-1. Catalytic converter - product considered in the study and its major components.
This chapter describes the initial production system (circa June 1998), the redesigned system,
as well as the transition state in the implementation (circa August 1999), with focus on the
information system.
6.1 Initial System
The initial manufacturing system is illustrated in Figure 6-2. It shows information and
material flows between all elements that participate in the manufacturing process, from the
moment the raw material enters the plant from the suppliers to the moment the finished
converters leave the plant to be delivered to the customers - for the entire value stream.
The initial system consists of seven major elements: shipping, central scheduling, assembly
line, sub-assembly station, piercing operation, stamping press and purchasing department.
The material flow in the initial system is under a push mode, with a central production
control system (Figure 6-2: 4) that sends out information on what and when to produce. The
system has multiple instruction points; the control information is sent separately to different
stages in the material flow. Because of this, the different stages of the value stream are only
linked through the central production control system and scheduling. The material and
information flows between the various stages in the value stream are decoupled not only
68
through the information system but also through the use of a central hi-density inventory
(Figure 6-2: 11) where parts and components in various production stages are being stored.
PURCHASING SCHEDULING CENTRALy~)4 PRODUCTIONCONTROL
monthly and weeklproduction plan
weekly ad daiy (
SU -productio n sc ltle s 5~
ASSEMBLY
SUPPPLIERS ICUSTOMERS
G ASSEMBLY SHIPPING 2daily
Staging
STAMPING PIERCING
Figure 6-2. The initial manufacturing system at the facility level: information and material
flow [Br6te, et al. 1999].
6.1.1 Information Flow - the Initial System
Starting at the customer's side of the value stream, the customer sends its one-year forecast
schedule, one-month firm schedule, and its final one-week schedule for all products needed
to the central control system (Figure 6-2: 1). This information is sent to the plant - to its
scheduling and shipping departments. The one-year forecast is used for capacity planning for
the line at the tactical level. The one-month firm schedule and final one-week schedule are
used to schedule the production and introduce last minute changes to the schedule at the
operational decision making level.
The shipping department uses the customer demand information to schedule its shipping
windows, i.e. what and when to ship (Figure 6-2: 2). The shipping department pulls the
products to ship from the finished goods inventory or in some cases ship whatever is on
hand, in a push mode. In emergencies, when product is not ready, the shipping department
69
contacts the scheduling department (Figure 6-2: 3) to determine the status of the parts and to
let them know how many parts are needed. The scheduling department receives the same
information from the customer as the shipping department. The scheduling department
compares the release of the customer demand for the given week with the balance on hand
(BOH) - the finished goods inventory - for all different products (Figure 6-2: 4). Based on
differences between the customer release and the BOH, production in the required production
areas is scheduled for the entire week. The scheduling department manually distributes the
weekly schedule to the managers of the various production areas (Figure 6-2: 5). The area
managers are responsible for adjusting the production schedule to react to line downtime
(Figure 6-2: 7,8,9,10) caused by such problems as component shortage, machine failures, or
workers absenteeism. At the end of each day, the scheduling department is informed of the
on-time status of the production areas, enabling the scheduling department to further adjust
the schedule and plan for overtime.
The scheduling department is also responsible for scheduling the production or purchase of
necessary components based on the one-month customer schedule (Figure 6-2: 6). The
monthly component demands are calculated by breaking the customer monthly demand into
its components. If the balance on hand for a component is less than the monthly demand or if
the BOH reaches its required minimum level, production will be scheduled or the purchasing
department will be informed to purchase more parts. The same system is used for the
scheduling of the purchase of raw material. For the press area (Figure 6-2: 10) the
scheduling department determines and informs the purchasing department as to how many,
how large and when the new steel coils are needed. Because of the system of infrequent
ordering and a minimum level of BOH triggering the scheduling department to schedule
production and inform the purchasing department, the purchase of components is only
vaguely coupled to actual usage of components. Since the production of components is not
scheduled based on the actual usage, all manufacturing areas have to be scheduled separately
and lack predictability - thus often leading to component unavailability.
6.1.2 Material Flow - the Initial System
The material flow for the value stream selected for the study is described below, starting
downstream, with the customers. The finished goods are shipped to each customer once a
70
day. As described in the previous section, the shipping department pulls the products to ship
from the finished goods inventory (Figure 6-2: 2).
The assembly line (Figure 6-2: 7) delivers its finished goods, in containers holding between
76 and 98 parts, to the finished goods inventory where they are stored until the next shipping
window. The container size varies for different finished goods parts, with the result that it
cannot serve as a standard production unit. This inconsistent pack size creates several
problems with material flow in the plant. First, it introduces inconsistency of throughput
time into the system, since each container might take a different production lead time. This
causes unpredictable intervals between container deliveries among the elements of the
system downstream from the assembly cells, including the customer. Second, each part type
has a different pack quantity requiring a different pitch, or time interval between production
signals. Because of the central scheduling system, the important link between information
and material flow is lost throughout most of the value stream in the plant. Without constant
pack quantities across all finished goods, standards for pack quantities for purchased parts
and parts supplied in-house cannot be established. Thus, creating a pull system using tools
such as kanban is difficult.
TIPUP P UP
Material flow
WIP accumulation
Figure 6-3. The initial assembly line layout.
71
The assembly line (Figure 6-2: 7) operates at two shifts a day, five days a week. Based on
the demanded volumes from the customers, the line has to produce 90 parts an hour, or at a
takt time of 40 seconds. Since the line is designed for mass production and 90 parts an hour
is close to the maximum production rate of the line, each operator is tied to one machine
making the line inflexible to volume changes. The line layout, presented in Figure 6-3,
shows that the material flow through the line is complicated and difficult to follow making
defect detection and tracking difficult.
Since each machine in the assembly line has a different cycle time (ranging from 5 to 40
seconds) and the assembly line has not been balanced to a specific takt-time, there is a
substantial amount of work-in-process (WIP) between the stations that decouples the demand
from one station to another. Conveyors holding WIP parts between several of the stations
prevent reinforcing the principles of first-in-first-out (FIFO) which together with high WIP
levels makes it difficult to detect and prevent production of defects at their source, even
though the production runs with a batch size of one piece. Since there are 20 variations of the
final product, with different components and variations of components between them, there
are multiple decision points in the manufacturing process, which determine final product type
and where the information has to be delivered. Production instruction is partly controlled
through changeovers, where the new parts to be used are brought to the line and the parts not
to be used are taken away. Since there is no common container quantity for the components,
which vary in number between 250 and 1700, the parts taken away from the line are stored as
partials. Most of the parts and components being used are purchased parts, which upon
delivery from the suppliers are stored in hi-density storage (Figure 6-2: 11). The other
components being used come from sub-assembly (Figure 6-2: 8) and from stamping/piercing
operations (Figure 6-2: 9,10).
The sub-assembly station (Figure 6-2: 8) consists only of manual operations, but due to the
operating mode, it runs and produces the required quantity on a one-shift-per-day pattern.
The sub-assembly station is scheduled separately from the assembly line and has its own
local part-inventory for components and finished sub-assemblies (Figure 6-2: 12). Because
72
some of the components used in subassembly are easily damaged, the multiple transportation
and storage creates many quality problems.
The stamped parts used in the assembly line are brought from hi-density inventory (Figure
6-2: 11). The stamping area runs stampings twice a week (Figure 6-2: 10), limiting the
inventory level to a half-week's demand plus safety stock. Some of the parts are also
required to go through a piercing operation (Figure 6-2: 9). These parts are pushed through
the piercing operation prior to storing them in the hi-density storage. Besides the parts
produced for the assembly line, the stamping press runs eleven other parts - all shipped to
external customers, total of twelve parts. Among the other eleven parts, six are high runners
(high volume production) that are produced every week. The other five parts are scheduled
when needed to run either during one shift or the other in a week. The press runs at two shifts
per day and five days a week, with production scheduled in a sequence that minimizes
inventory. Since six of the seven high runners are produced for an external customer, the
production of these parts is sequenced in such a way that the parts are shipped when
produced with the schedule optimized to the shipping windows. The shipping windows for
the stamped parts are Monday, Tuesday and Friday.
6.2 Redesigned System - Towards the Ideal State
The initial system described in the previous section has been redesigned using the PSDD
framework. The overall goal of the new design has been to reduce waste and create a
manufacturing system with deterministic output, following the objective of minimizing mean
delivery time and its variation, and thus making the system responsive to customer
requirements. Such a system exhibits leveled production according to the actual demand as
specified by takt time and is capable of volume flexibility. It also has a reduced amount of
WIP, which shortens lead time. The goal is also to achieve single piece production and
conveyance at all points in the system - true single piece flow throughout. The redesigned
system described below is a first step towards this ideal.
73
In the newly designed linked-cell system, shown in Figure 6-4, the subassembly element
(Figure 6-2: 8) has been integrated into the assembly line (Figure 6-2: 7) and thus is
represented in a single unit as the assembly cells (Figure 6-4: 7). The assembly cells unit is
also a pacemaker element of the system.
rSUCUSTOMERS
dailyy
F C ~FIFO+- Staging
STAMPING _
PIERCING
@
Figure 6-4. Redesigned system at the facility level: information and material flow.
An important enabler to the redesigned system is the establishment of a constant pack density
of the finished goods, or a constant number of parts, regardless of type, in a container. A
quantity of 72 has been picked as it can accommodate all part types. This constant quantity
makes it possible to treat a container as a production unit with predictable production lead
time, enabling implementation of a kanban system.
6.2.1 System Level - Redesigned System
The information flow in the redesigned system starts with the customer. Based on the actual
demand, the customer sends the demand information to the plant via kanban cards, which
specify how many containers of what part type are requested (Figure 6-4: 1). This
information conveyance takes place daily when the parts from the previous order are
delivered (Figure 6-4: 2). Therefore, the system operates under BTO strategy with no
74
finished goods inventory. Upon their arrival in the plant, the cards are delivered to the
shipping/scheduling unit, where they are sorted and deposited in the leveling box (Figure 6-4:
3). The leveling box makes it possible to schedule the ordered parts into specific time slots
for the day.
It has been shown that the great demand variations in the initial system were mostly caused
by internal variability in production. The customer plant is designed to produce at a constant
pace, incapable of frequent rebalance, meaning that the customer is steady and paced and the
only way to increase production is through changing operating time. Volume fluctuations are
therefore easy to predict and plan for. The demand mix - the ratio of demand for different
part types - does fluctuate within a ±10% range depending on component availability from
other suppliers. Therefore, with leveled scheduling and simplified value stream, the demand
variations have been shown not to exceed ±10% in mix and volume. Thus, the leveling box
can be established once a month, based on the monthly forecast, but actual consumption,
communicated daily, is used to load the box and schedule the production, using the fine-
tuning characteristic of a kanban system to manage minor variations.
The pitch of the paced withdrawal is 1.5 hours, due to the large distance between the
assembly cells unit and the shipping unit. The conveyance operates under a constant cycle
method and is shown schematically in Figure 6-5. During this pitch, the conveyance person
picks up the kanban cards for the next pitch interval from the box and an equivalent number
of empty containers and delivers them to the assembly production unit drop off area. From
there, the equivalent number of full containers is picked up and delivered back to the
shipping, where they are staged waiting for delivery to the customer. The shipping staging
area uses visual monitoring of the production status, based on how many containers are
waiting. It is important to note that, since the conveyance person operates on a constant
cycle conveyance method, more than one production unit can be serviced by the same loop in
mixed loading, as shown in Figure 6-5 and thus increase efficiency of the conveyance loop.
75
Basket Staging Area
............. . ........
-.lu- 36 34
37 35
Shipping
3dSchedu in Box
Pitch Clock
h: min
1:30
Figure 6-5. Pacemaker conveyance loop from shipping to the assembly element.
The filled containers are brought to the drop off area by a forklift, one by one. The forklift
can service many production units similar to the above mentioned conveyance loop, and it
can operate on demand. As a line completes a container and starts to fill a new one, the
forklift is informed and it picks up the full container and leaves an empty one. This dual
conveyance: first by the external withdrawal loop and then by forklift and internal
replenishment loop (described below) - was chosen because of the great distance between
the shipping and production units. Hence the containers cannot be taken to shipping one by
one and need to be taken in a train, which is done in the external conveyance loop.
From the assembly production unit drop off area in Figure 6-5, the kanbans are then brought
to the assembly production unit itself. Here, the kanban cards instruct the material
replenisher as to what components are needed. Thus, the replenishment of material is
coupled with the conveyance of customer demand. Material replenishment, therefore,
provides production authorization and instructions to the workers by giving them the material
necessary to produce a specific product. Since the material replenishment loop is the
system's instruction point, downstream operations are performed in a stream flow mode.
76
Delivers informationFrom Heijunka in Shipping
- To Cells
Removes finished goods- Takes away full containers- Brings empties
This internal conveyance loop makes it possible to increase the conveyance frequency to the
cell and thus keep tighter control of production status. Every internal pitch (usually every 15
min.), the replenishment person delivers the required components in the required quantity.
This conveyance loop operates on a constant quantity method, with the same amount of
material always being delivered (components necessary for 18 parts which is % of the
finished goods container size), although frequency of the conveyance might vary depending
on the demand variation. The constant quantity method was used because of the great variety
of components and their size as compared to the available space at the cell. Due to the
number of parts needed to produce all the variations of the end item, the floor space needed
to hold even one container of each type would exceed the floor space available. By linking
the customer order point to material replenishment and supplying material to the cell in
constant quantities, only the parts that are needed will be in the cell racks. This also
eliminates possible quality problems caused by operators using incorrect components.
Delivering only the necessary types of components in small quantities also helps to eliminate
lengthy material changeovers (initially up to 30 minutes) between different part type runs.
However, in order to make the system robust against incoming material defect problems, an
emergency system is required to allow replenishment of additional parts, when need. This
can be done with an express kanban or an Andon display that communicates shortages and
instructs redelivery of certain parts. It should be noted, however, that incoming quality
problems were insignificant to begin with, compared to internal quality problems; this source
of defects was eliminated through the integration of the processes and reduced WIP in the
redesigned line as described later.
Components delivered to the assembly cells in the internal replenishment loop are picked
from a component line side market located close to the cells (Figure 6-4: 12). From that
market, as the components are consumed, the withdrawal kanbans instruct suppliers to
replenish them (Figure 6-4: 6). The internally procured parts (stampings) are delivered to the
assembly unit from the stamping press market (Figure 6-4: 11). From this market, a signal
kanban is used to schedule production when the level of a given component reaches a
77
minimum quantity. The signal kanban is sequenced together with signal kanbans for other
parts that are produced in that specific press. This sequence determines the operating pattern
at the press, i.e. which parts, what quantity, and in what order to produce. The sequencing
board also indicates when to make changeovers, as well as when and what kinds of coils are
needed. As the parts leave the press, they go through the piercing operation (Figure 6-4: 9),
if required, before they are placed in the market. The steel for the stampings is obtained from
the coil market using material requisition kanban. From that market, the information is sent
to the supplier to replenished used coils (Figure 6-4: 8).
6.2.2 Line Level - Redesigned System
The redesigned production unit layout is shown in Figure 6-6. As mentioned earlier, the sub-
assembly production unit has been integrated with the assembly line and rearranged into a U-
shaped cell configuration. The subassembly operation has been redesigned and balanced
with the cycle time of the rest of the cell to equalize the production operating pattern of both
of those production units.
Line side component market
PNL
IIc
Material flow
Figure 6-6. The redesigned assembly cells layout.
In order to design a value stream that is able to produce at the customers' demand cycle time,
the cell has been designed as a linked part of the value stream with a focus on the customers'
demand volumes and their variations for all parts produced within that cell [Br6te, et al.
78
FIFIFT-1
PW MR
iM-7
EF I F1 F-I F-I 1-1 El
1999]. Hence, the cell should enable a flow of parts at the ideal range of cycle time.
Consideration has also been given to volume fluctuations; the cell must be able to operate
within a range of demand fluctuations. The maximum level of expected demand puts a
requirement on the minimal takt time, a time at which all machines within the cell must be
able to operate. Since the product design has been held constant within the scope of the
project, the machines chosen for the project must be able to meet the requirements of the
existing product design, and at the same time be able to operate at the recommended
minimum takt time. If the cell is to operate and adjust to different demand levels efficiently,
it must be able to increase and decrease its capacity in small incremental steps. The ability to
increase and decrease production capacity within the cell corresponds to the need to operate
at different takt times in order to avoid the waste of overproduction. The cellular layout,
together with the decoupling of operators from the machines through SWIP, enables the cell
to be run at different takt times just by adding or reducing the number of operators in the cell.
In this way production volumes can be increased or decreased incrementally.
The decoupling of operators from the machines is done by autonomation. Autonomation is
used so that the operators start the manufacturing or assembly operation by loading the
machines, activate it by pushing the start button, and then walk to the next machine without
waiting for the machine to finish its cycle. The machines run and often even unload
themselves during the machine cycle time. This decoupling of operators from machines
allows for better utilization of the operators' time. An example of autonomation in the
redesigned cell is the seam welding station. After loading, the operator starts the cycle by
pushing a button and walks away with the previous part to the next station. Due to existing
equipment limitations, however, the seam welding machine is not equipped with the auto
unload feature. Upon return, therefore, the operator has to manually unload the previous part
before loading the next one. Since the machine cycle time is less than takt time, the cell is
able to always run at takt time. This way the throughput time - the time it takes one part to
go through all operations - is always the same and therefore predictable.
One of the goals for the system and cell design is to obtain predictable output in terms of
time and quality. The predictability of output requires a low scrap rate, which can be
79
obtained by visualizing the material flow and thus enables fast failure detection and quick
response. Alternative material flow should be avoided so that the machines that produce the
defect can be easily identified. The use of standard operation work sheets, which tell the
operators what tasks to perform to achieve various takt times, also describes the work content
for the different operations so that they will be performed in a consistent manner.
Two independent cells were formed for reason of increased capacity requirement, Figure 6-6.
Both cells have machines placed close one to another in a U-shaped configuration. The close
placement of the machines not only minimizes walk time for the operators but also reduces
the amount of WIP and reinforces FIFO flow in the cell. No conveyors are present between
stations, only a simple slide rail, which aids the operators as they move the parts between
stations, walking. The amount of incoming material is under tighter control. The parts are
delivered to the line in small batches (18 parts). Material delivery is performed through flow
racks from the outside of the cell so that material replenishment is non-disruptive to
production and workloop pathways. The parts delivery through flow racks also improves the
presentation of parts to the workers. They are easily accessible on the path between stations
or at the stations themselves.
Since the changeovers will be done sequentially as the new material is delivered to the cell,
there is a need for a visual changeover sign to inform that a changeover is in progress [Br~te,
et al. 1999]. Changeovers in this system are indicated in two ways. First, there is a
changeover card highlighting changeovers to the material replenisher as the material is
delivered. To instruct the operators that a changeover should take place, a wooden stick is
placed between parts being used for the previous product and parts being used for the new
product. In this way, the operators are given a signal to perform the changeover.
The location of the line side component market directly next to the line makes the
components easily accessible to the replenisher. The replenishment can be performed by a
single person, servicing both cells, and the component market can be visually managed from
the line.
80
6.3 Present System - Transition State
PURCHASING SCHEDULING PRODUCTON
CONTROL
Ar ~p ouc *ion sc ules
SUPPPLIERSt
daily XOX ASEBYSHIUING
-FIFO+ Staging
STAMPING PIERCING
Figure 6-7. Present system state at the facility level: information and material flow.
motly and weekiproduction plan
=CUSTOMERS
The present state is a description of the system in August 1999, and is only considered to be
one step in the process towards the designed system or the ideal. The implemented system is
shown in Figure 6-7. This section describes the implementation status of the system by
listing successfully executed elements as well as a discussion of issues that inhibited
progress. Table 3 compares the performance of the system before and after initial
implementation of changes.
Table 3. Comparison of system
implementation.
performance measurements before and after initial
Performance Measurements State of the SystemBefore Implementation After Implementation
Production volume 1600 parts per day 1600 parts per dayWork in Process ~1000 parts 15 partsInventory Large and variable 3 daysFloor space consumed 5000 ft' 4600 ft2
Throughput time (Dock-to-dock) 182 hours 92 hoursNumber of defects per month 300 parts 100 partsFirst time Through (no defects) 26% 52%Number of workers (direct/indirect) 14/6 11/4Material replenishment interval 2 hours 20 min.
81
One of the most successful implementations was reconfiguration of the assembly line and the
integration of the subassembly process into the new production unit, Figure 6-6. In addition,
the material flow within the production unit was examined and splitting the line into two
independent cells eliminated any parallel flow, allowing failure detection and quick response
to production disruption to be supported. Since there are two major customers for the
products, the best way to design the cells would be to have each cell dedicated to one
customer and responding to its needs. However, constrained by the equipment available and
due to financial restrictions, the cells had to be divided by type of process. This required both
cells to be scheduled together although the takt time for both cells is different and it is of
course a limitation to the flexibility of both cells. Having two cells instead of one increases
the takt time per cell since customer demand cycle time is divided between two cells. In the
case of this project, the split of the line gave production rates of 72 and 21 parts an hour for
each cell, respectively, equaling 90 parts per hour in total. The slower production rates -
longer takt time - makes it possible to run the machines at a slower machine cycle time,
which may improve product quality and reduce the wear on the machines and tools.
Both cells were installed using mostly existing equipment without additional investment.
The close configuration of the machines reduced greatly the amount of WIP, from over 1000
pieces down to 15. This WIP reduction not only shortened the lead time of the process from
hours down to minutes, but also improved detection of defects in the cell and communication
among workers. The result is improved quality in terms of overall defects as well as first
time through, as shown in Table 3. Overall, the reduced quantity of WIP made the
production output more predictable in terms of both the quality and lead time.
Predictable production resources regarding both human and machines are, of course,
necessary, [Br6te, et al. 1999]. Causes of unplanned downtime should be documented and
analyzed to prevent their reoccurrence. In general, solutions to downtime problems can be
classified as design change, preventative maintenance scheduling or repair when down. The
degree to which the system can respond quickly to downtime will greatly influence the
information and material flow side of the system design and its performance. With high rates
82
of downtime, the system becomes less responsive to customer demand and both functional
requirements of the system are negatively affected.
The configuration of the cell, with machines placed close together and absence of conveyors
between stations, allowed implementation of workloops in which some operators are able to
perform more than one operation during cycle time. However, there were some limitations in
the implementation. Firstly, since the cell consists of previously purchased equipment, the
distance between some of the stations is larger than optimal because of the size of some of
the machines. This increases the walking distance for the operators performing workloops
increasing non-value-adding movement. Secondly, because of the existing restrictions in
workers' task division, some operations could not be combined into a single workloop and
have to be performed by different operators. This restricted implementation to some degree,
preventing the full balance of the workloops in the cell. Overall, however, implementation of
workloops not only enabled volume flexibility, i.e. the ability of the cell to adjust to the
required production rate, but also allowed increased efficiency of the workers and required
fewer operators and support staff to operate the new system.
The implementation of flow racks delivering material to the cell not only reduced the
incoming material WIP in the cell but also greatly improved the ergonomics of handling of
the parts by operators. From the flow racks, they are easier to reach and are closer to the
stations at which they are used. Since the parts are delivered in small quantities,
replenishment can be performed more frequently and is non-disruptive to cell production.
The line side market placed in the vicinity of the cells also made frequent replenishment
possible. However, there were many limitations to its implementation. The design of the
new system called for all inventory for this production unit to be placed in a single location,
permitting visual management. In the vision of the future system design, the central market
should be eliminated and the suppliers would deliver their products directly to the line-side
market (Figure 6-4: 12). The line-side market would then be linked to the suppliers through
another information/kanban loop (Figure 6-4: 6), eliminating intermediate inventories and
allowing better control over the inventory levels of parts. However, due to the large initial
83
inventory size and limited space at the site of implementation, this was not carried out. Only
a single container of each type of component is staged at the line side market, and as the parts
are used up, a new container is brought from central inventory storage. Because of this, the
inventory coming from the external suppliers is managed as in the initial system - based on
the forecast of the part consumption from the central scheduling.
The future development for the system is to provide links between shipping and assembly
production unit, as well as continue those links with the rest of the system, such as presses
and suppliers. Currently, the production authorization for both the cells and the instruction of
the stamping press area are still done centrally and separately. The cells are still scheduled
based on the forecast demand. Although the internal replenishment loop has been
implemented, paced withdrawal has not. Therefore, the replenisher does not instruct the cell
based on leveled demand. In fact, the leveling box has not been implemented. The stamping
area is also scheduled separately, i.e. operates on push-production mode and the pull system
using signal kanban is not yet in place. A market of stamped parts should be located near the
press and conveyed to the assembly unit based on demand.
6.4 Further Development
The implementation thus far of the redesigned system has brought many benefits, as shown
in Table 3, such as improved quality and reduced lead time. However, much more is to be
gained through implementation of the other elements as well.
The biggest benefit will come from the control of purchased parts. Currently, they are placed
in a central storage location, which makes it difficult to control their levels and manage them
visually. They are ordered based on a forecast from central scheduling and delivered to the
line side market. In fact, the purchased components are a "hidden problem". They take up
floor space and require additional resources to transport them from the receiving dock to
storage and from storage to the production units, often many times back and forth. This
additional movement also increases the chance of creating defects. Some of those
components are also costly. Controlling inventory levels would bring, therefore, many
benefits.
84
However, before implementing the kanban pull system to order parts from suppliers based on
actual usage, first the production of the assembly needs to be leveled [Monden, 1998]. The
implementation of the leveling box and establishment of material and information
connections between shipping and assembly is required. The leveled production of small run
sizes of parts as well as their frequent conveyance will make it possible to reduce the
incoming material inventory, as mentioned earlier. This linkage, therefore, should be the
focus of immediate implementation because leveling will allow implementation of other
linkages in the system.
The implementation of paced withdrawal will also improve the ability of frequent monitoring
of systems performance. This, in turn, will help achieve predictable output. Another key
element of predictable output issue is the analysis of down time. Although the machines in
assembly are serviced for maintenance, it is mostly done in a "fire-fighting" mode, giving
attention to the unit only when one of the machines is down and production has stopped.
Little preventive maintenance is done. A dedicated support staff for this production system
would ensure better availability of resources and would allow continuity in the preventive
maintenance for the system and elimination of the root causes.
85
86
7 Conclusions
In manufacturing system development, in order to make the system efficient, it is important
to eliminate waste throughout the entire value stream. Thus, it has been shown, based on the
Production System Design Decomposition, that overall system design places important
requirements on material and information flows through the system. In fact, these two flows
significantly affect the performance of the system, in terms of cost, quality and response
time. However, it is also important to note the interdependence of the information flow and
material flow as discussed in Section 3.3.
Traditionally, information flow is designed after the physical elements have been laid out and
after the material flow has been established. However, some of the strategic decisions in
information flow design discussed in this thesis, such as the inventory strategy, pacemaker
element selection and control policies, will impact how the material flows. For example,
depending on the chosen location of the instruction points in the system, certain production
elements could or could not be integrated into a single unit - the information flow should
influence the relative placement of the elements, depending on the kind of connections
needed between them. Therefore, the design of the information flow will in fact affect the
physical layout of the system and both information and material flows should be designed
concurrently, at the system level.
In the linked-cell system, the interdependence of the material and information flows is
especially visible in the transmission of information and material between production units,
or conveyance. This thesis presents various conveyance methods and discusses the
circumstances in which they might be most useful. Because the conveyance is responsible
87
not only for delivery of the necessary material but also performs production control, it should
be regarded as a critical part of the manufacturing system development.
The role of conveyance in production control can be divided into two functions:
- Authorization of production
- Monitoring of performance
The conveyance also provides a feedback loop, which allows taking necessary actions and
maintaining controllability of the system, thus improving its performance. The frequency of
this feedback loop is a critical characteristic for controllability. However, little work has
been done so far to define controllability and to determine the optimal frequency of
conveyance in place of a commonly used rule of thumb, one-hour conveyance [Monden,
1998].
Although this thesis listed many factors that will influence the choice of information flow
and thus conveyance method used, it was not an intention to provide strict rules for
conveyance method selection or design. Instead, a discussion of the requirements some of
these factors place on the information flow has been provided as guidelines in decision-
making process. The information flow design is a strategic decision, which will greatly
impact the performance of the entire system.
88
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Generai notes:Ad xbrand siger arprom landsth putltnmSh= matres tor higher leels
Production System Design LaboratoryLaboratDry for Manuactuwing and ProductivtyMaSachsetmallls tiU TechnologyOtkctor Pe- David S. Codian
77 MASSACHUSETTS AVENUE, BUILDING 35-135CAMBRIDGE, MA 02139PHONE: (617) 231811 FAX: (817) 268-TM WES: hI:.d.rt.fd
'MANUFACTURING SYSTEMDESIGN DECOMPOSITION
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References
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