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Why Kanban Systems Fail and What You Can Do About it Saifallah Benjaafar Center for Manufacturing Logistics Department of Mechanical Engineering University of Minnesota Minneapolis, MN 55455. Kanban.

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slide1

Why Kanban Systems Fail and What You Can Do About it

Saifallah Benjaafar

Center for Manufacturing Logistics

Department of Mechanical Engineering

University of Minnesota

Minneapolis, MN 55455

kanban
Kanban
  • A “kanban” is a sign-board or card in Japanese and is the name of the flow control system developed by Toyota.
outline
Outline
  • 1. The JIT context
  • 2. Basic mechanics
  • 3. Advantages
  • 4. Limitations
  • 5. Alternatives to Kanban
  • 6. CONWIP systems
  • 7. PFB systems
  • 8. MTO/MYS systems
  • 9. Conclusions
slide4
Role

Kanban is a tool for realizing just-in-time. For this tool to work fairly well, the production process must be managed to flow as much as possible. This is really the basic condition. Other important conditions are leveling production as much as possible and always working in accordance with standard work methods.

  • --- Ohno 1988
just in time jit
Just-in -Time (JIT)
  • A collection of principles aimed at reducing inventory, increasing throughput, and minimizing manufacturing lead times.
origins of jit
Origins of JIT
  • Japan’s attempt to become competitive with US and European manufacturers after WW II
  • The Toyota Production System
jit principles
JIT principles
  • Reduce setup times
  • reduce batch sizes
  • Reduce variability
  • Reduce material handling
  • Reduce defects and rework
  • Reduce breakdowns
  • Increase capacity
  • Smooth production schedules
jit principles cont
JIT principles (cont.)
  • Maintain constant WIP (limit WIP buffer sizes)
  • Limit finished goods inventory and raw materials
  • Synchronize operations within the factory
  • Synchronize material delivery with suppliers and customers
  • Improve worker flexibility and empower worker to make improvements
  • Simplify workflow
the seven zeros
The Seven Zeros
  • Zero (excess) lot sizes
  • Zero setups
  • Zero breakdowns
  • Zero lead times
  • Zero surging
  • Zero material handling
  • Zero defects
kanban systems
Kanban Systems
  • A shop floor control strategy aimed at (1) reducing inventory, (2) simplifying workflow, (3) increasing throughput, (4) reducing cycle time, (5) improving customer lead times, and (5) improving quality.
mechanics of kanban
Mechanics of Kanban
  • Push vs. Pull: Kanban is a “pull system”
    • Push systems schedule releases
    • Pull systems authorize releases
  • Synchronous manufacturing: Pull signals ensure tight coupling between operations throughout the system
mechanics of kanban cont
Mechanics of Kanban (cont.)
  • One card systems
  • Two card systems
mechanics of kanban cont1
Mechanics of Kanban (cont.)

Work Center

Buffer

Card Flow

one card kanban

Outbound stockpoint

Outbound stockpoint

Completed parts with cards enter outbound stockpoint.

Production

cards

When stock is removed, place production card in hold box.

Production card authorizes start of work.

One-Card Kanban
two card kanban

Inbound stockpoint

Outbound stockpoint

Move stock to inbound stock point.

Move card authorizes pickup of parts.

When stock is removed, place production card in hold box.

Remove move card and place in hold box.

Production cards

Move cards

Production card authorizes start of work.

Two-Card Kanban
mrp versus kanban
MRP versus Kanban

MRP

Lover Level Inven-tory

Assem-bly

Kanban

Lover Level Inven-tory

Assem-bly

Kanban Signals

Full Containers

signaling
Signaling
  • Cards
  • Lights & sounds
  • Electronic messages
  • Automation
the key issue
The Key Issue
  • How many Kanbans should we have at each stage of the process and for each product?
tradeoffs
Tradeoffs
  • Too many Kanbans Too much WIP and long cycle times
  • Too few Kanbans Lower throughput and vulnerability to demand and process variability
little s law
Little’s Law

Cycle time = WIP/Throughput

WIP = (Cycle time)(Throughput)

slide21

Example

  • Four identical tools in series.
  • Each takes 2 hours per piece.
  • No variability.
  • Constant WIP.
slide23

.5

.4

.3

Throughput (Jobs/hr)

.2

.1

0

0 2 4 6 8 10 12 14

WIP (Jobs)

Throughput and Cycle Time vs. WIP

slide24

Throughput and Cycle Time vs. WIP (cont.)

20

16

12

Cycle time (Hours)

8

4

0

0 2 4 6 8 10 12 14

WIP (Jobs)

slide25

The Case of an Unbalanced Line

  • The maximum feasible throughput rate is the processing rate of the bottleneck
  • Critical WIP = (Bottleneck rate)(Total processing time)
slide26

The Impact of Variability

  • Processing times are subject to variability, material handling is not instantaneous, processes are subject to breakdowns, demand is subject to fluctuation
  • Longer cycle times and lower throughput
slide27

Number of Kanbans at Toyota

  • Number of cards = WIP
  • Number of cards = D/QL(1 + a)
  • D: Demand
  • Q: Container
  • L: Raw processing time
  • a: safety (variability) factor
guidelines for allocating kanbans
Guidelines for allocating Kanbans
  • Allocate Kanbans evenly in a balanced system
  • Allocate more Kanbans to slower processes or processes with higher workloads
  • Always protect the bottleneck
guidelines for allocating kanbans cont
Guidelines for allocating Kanbans (cont.)
  • Start with current averages
  • Gradually reduce Kanbans at stations that are always full
  • Increase Kanbans at stations that are always empty
advantages of kanban
Advantages of Kanban
  • Simplifies workflow
  • Synchronizes manufacturing
  • Reduces WIP accumulation at all processes stages
  • Improves performance predictability and consistency
  • Fosters communication between neighboring processes
  • Reduces defects and enables 100% inspection
advantages of kanban cont
Advantages of Kanban (cont.)
  • Encourages line balancing and process variability reduction
  • Encourages focused and cellular manufacturing
  • Y2K robust
limitations of kanban systems
Limitations of Kanban Systems
  • Ideal for high volume and low variety manufacturing
  • Vulnerable to fluctuations in demand volume and product mix
  • Vulnerable to process variability and machine breakdowns
  • Inefficient in handling infrequent orders or expediting special orders
  • Vulnerability to raw material shortages and variability in supplier lead times
alternatives to kanban
Alternatives to Kanban
  • Constant Work-in-Process (CONWIP)
  • Pull from the Bottleneck (PFB)
  • Hybrid Make-to-stock/Make-to-order system
conwip
CONWIP

CONWIP Cards

Production Line

Inbound

Stock

Outbound

Stock

basic configurations
Basic Configurations
  • Constant work-in-process
  • Input/output control
  • Asynchronous operation
  • Dispatching list
  • Example: Flow lines
slide36

Dispatching list

DD PN Quant

–— –— –––––

–— –— –––––

–— –— –––––

–— –— –––––

–— –— –––––

–— –— –––––

–— –— –––––

–— –— –––––

–— –— –––––

–— –— –––––

–— –— –––––

–— –— –––––

–— –— –––––

–— –— –––––

LAN

R

G

PC

PC

. . .

Work Centers

CONWIP Control

conwip configurations
CONWIP Configurations

Basic CONWIP

Multi-Loop CONWIP

Kanban

Work Center

Buffer

Card Flow

conwip mechanics
CONWIP Mechanics
  • A new job is introduced whenever one completes
  • The next job is selected from a dispatching list based on current demand
  • The mix of jobs is not fixed
  • Priorities can be assigned to jobs in the dispatching list
  • WIP level can be dynamically reduced
slide39

Advantages of CONWIP-based Control

  • Accommodates multiple products and low production volumes
  • Allows expediting and infrequent orders
  • Less vulnerable to demand and process variability
  • Less vulnerable to breakdowns
  • Protects throughput and prevents bottleneck starvation
  • Simpler to implement and manage
slide40

Coupled and Uncoupled CONWIP Loops

Bottleneck

CONWIP Loop

Buffer

Job

CONWIP Card

Material Flow

Card Flow

slide41

Splitting Loops at Shared Resource

Routing A

Routing A

Routing B

Routing B

CONWIP Loop

Card Flow

Material Flow

Buffer

slide42

Modifications of Basic CONWIP

  • Multiple Product Families:
    • Capacity-adjusted WIP
    • CONWIP Controller
    • Running a card deficit
  • Assembly Systems:
    • CONWIP achieves synchronization naturally
    • WIP levels must be sensitive to “length” of fabrication lines
slide43

B

Card Deficits

Jobs without Cards

Jobs with Cards

Bottleneck Process

Failed Machine

slide44

CONWIP Assembly

Processing Times

for Line A

2

1

4

1

Processing Times

for Line B

3

3

2

3

Assembly

Buffer

Card Flow

Material Flow

slide45

Pull From the Bottleneck

  • Problems with CONWIP/Kanban:
    • Bottleneck starvation due to downstream failures
    • Premature releases due to CONWIP requirements
  • PFB Remedies:
    • PFB ignores WIP downstream of bottleneck
    • PFB launches orders when bottleneck can accommodate them
  • PFB Problem:
    • Floating bottlenecks
slide46

B

Simple Pull From the Bottleneck

Material Flow

Card Flow

slide47

Make-to-order/Make-to-stock Configurations

  • Build components and subassemblies to stock
  • Build final assemblies to order
  • Pull system for MTS
  • Push System for MTO
slide48

Conclusion

  • There are Pros and Cons to everything
  • Pull systems are one piece of the puzzle
  • Change throughout the organization is essential
  • An integrated supply chain strategy is critical
slide49

References and Additional Reading

  • Factory Physics, W. J. Hopp and M. Spearman, Irwin, 1996 (and teaching notes)
  • The Race, E. M. Goldratt and R. E. Fox, North-River Press, 1986
  • QRM Revisited: Don't Push or Pull - POLCA, APICS Magazine, Vol. 8, No. 11, 1998.
slide50

Additional Resources at U of M

  • Industrial Engineering Graduate Program
  • Master of Science in Manufacturing Systems
  • Graduate Student Internships
  • Center for Manufacturing Logistics