Kanban Systems

Operations Strategy Kanban Systems

Kanban Systems: overview; pull systems

The ‘Pull’ Principle

ƒ Pull means replacing stock after consumption has occurred, ƒ i.e. what we do today is to replace what was used yesterday.

ƒ This is an old established principle in production control (see previous discussion of 2 bin, reorder level systems) ƒ Pull usually associated with Kanban ƒ what makes Kanban different, i.e. a key part of ‘Just In Time’ (JIT) system, is the way the factory is operated.

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Doug Love/Aston University

Kanban Systems: Overview

Kanban ƒ Material control system, plus ƒ Part of a related manufacturing philosophy ƒ Toyota Production System/Lean ƒ delivers Just-In-Time (JIT) performance.

ƒ There are different forms of Kanban ƒ Kanban square ƒ One card kanban ƒ Two card kanban ƒ Kanban-MRP

Doug Love/Aston University

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Kanban Systems: kanban squares

Control using Kanban Squares

Parts

Assembly line 1

Parts

Assembly line 2

Parts

Assembly line 3

Parts Manufacture Raw Material Stocks

Finished Goods Stocks

Cycle: consume - replenish - consume Kanban Square

Operator working kanban square empty

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Kanban Square

Operator stopped kanban square full Doug Love/Aston University

Kanban Systems; single stock point (1 card)

Kanban: single stock location or 1 card ƒ Kanban card & bin cycle round together ƒ Card specifies part number, qty, source, consumer ƒ Card authorises movement & production ƒ Number of ‘cards’ controls stock level. KK

Unit 2

K

M/c

M/c Make

Kanban Stock Position

KK KK KK

Unit 1

K

M/c M/c

M/c

Use KK KK KK KK

Doug Love/Aston University

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Kanban Systems; calculation of number of kanbans

Number of kanbans in circuit ƒ The number of kanbans has to be calculated (at least initially) to ensure smooth operation ƒ Number of kanbans controls material in system ƒ Need enough material in loop to ensure stock is always available to customer. ƒ Disruptions (scrap, breakdowns etc.) are covered by the safety factor in the formula

ƒ The formula is: Number of kanbans = Demand Rate x (Replenishment Time) x (1+Safety Factor) Container capacity 1 Where Replenishment Time includes all the times required to: Transport to cell & between machines + pass the card to the 1st operation + Queue at machine(s) + Set machine(s) + Process the kanban quantity Safety Factor is estimated/revised by users (can be statistically calculated) 1NB

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Container capacity can be simply the selected kanban card quantity

Doug Love/Aston University

Kanban Systems; dual stock (2 Card)

Kanban dual stock location or 2 card full

full

PP

Unit 1

M PP PP P Stock P

M/c Make PP PP PP

P

MM Stock

Use

M

M/c

SWOP CARDS

Unit 2

M

M

empty

ƒ Introduces the idea of ‘production’ and ‘move’ Kanbans. ƒ Customer empties container releases Move card + container ƒ Move card + bin taken to production cell ƒ Full container same part located and cards swapped (Move on full, Prod’n with empty). ƒ Move returns to customer on full bin ƒ Production joins queue at start of manufacturing process ƒ Production + full bin added to stock at supplier

Doug Love/Aston University

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Kanban Systems; dual stock (2 Card) cont.

MM Stock

PP

Unit 1

M/c Make PP PP PP

PP PP P Stock P

M

Use

M

M/c Unit 2

M

MM

SWOP CARDS

P

M MM

MM Stock

M

Use

M/c Unit 3

MM

ƒ Can handle multiple ‘customers’ ƒ Allows stock to be located at producer & user units ƒ Kanbans in each circuit calculated separately (different loop delays & risks) ƒ Good where units are a long distance apart & improves accountability. 8

Doug Love/Aston University

Kanban: factory environment

Kanban - Environment ƒ The system is very disciplined but it is also simple, so ƒ low stock operation depends on changes to; ƒ plant (productive maintenance, capability, setups), ƒ working practices (flexibility, ownership), ƒ supplier relationships (partnership, delivery, quality), ƒ customer relationships (level MPS). ƒ Levelled scheduling means building a schedule by making: ƒ runners every day/period ƒ repeaters when required (but as often as possible) ƒ strangers fill up the capacity available

ƒ These changes eliminate disruption to produce smooth, predictable material flow

Doug Love/Aston University

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Operations Strategy, Systems & Implementation; Kanban System Description

Kanban as part of Lean ƒ This old diagram shows some ‘building blocks’ to Just In Time operation J.I.T.

KANBAN

QUALITY ON LINE

RELIABLE MACHINES

SIMPLE CELL STRUCTURE

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QUALITY CIRCLES

SUPPLIER INTEGRATIO N

CAPABLE PROCESSES

EDUCATION AND TRAINING

LEVEL SCHEDULING

SHORT CHANGEOVER TIMES

COMPLETE ACCURATE DATA

FLEXIBLE PEO PLE

Doug Love/Aston University

Kanban Systems; The improvement cycle

Driving the Improvement cycle ƒ

The elimination of disruption is aided by the improvement cycle: 1.

2. 3. 4.

remove a card (or few) wait for a stoppage to occur, note cause if no stoppage, goto 1 otherwise fix the cause of the stoppage, then goto 1

ƒ

Note the direct link between the stock level and why it is needed, thus

ƒ

Kanban is used to focus improvement effort where it is most needed

ƒ

This process is a critical aspect of the success of the system

Doug Love/Aston University

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Kanban Systems MPS

Kanban Limitations ƒ Repetition of demand is required ƒ Why?

ƒ Demand must be (relatively) smooth ƒ Especially in terms of overall volume ƒ Some change in mix OK if setups are quick

ƒ Direct Kanban links and smooth demand ƒ Collaborating customers (customer development)

ƒ Otherwise an assembly schedule is used ƒ the MPS/assembly schedule uses levelled scheduling to minimise variability ƒ may require customer-facing stock to damp fluctuation

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Doug Love/Aston University

Kanban Systems MPS

Kanban – MRP ƒ These systems use both MRP and Kanban at the same time ƒ Each system is used for areas/cells/products that suit its characteristics ƒ In parallel systems some self-contained modules or product lines will be MRP controlled and others by Kanban MRP System

ƒ MRP for - ?

MRP Cells

ƒ Kanban for - ? Kanban Cells

Doug Love/Aston University

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Kanban Systems: Kanban-MRP Sandwich

Parts

Assembly line 1

Parts

Assembly line 2

Parts

Assembly line 3

Parts Manufacture Raw Material Stocks

MRP Control

Kanban Pull Control

Finished Goods Stocks

MRP Control

MRP Push linking Purchases with MPS

Kanban-MRP “Sandwich” ƒ MRP controls MPS and purchases ƒ Kanban controls all parts/assemblies made inside the factory ƒ MPS drives assembly/product manufacture 14

Doug Love/Aston University

Kanban Systems: Kanban-MRP Sandwich Parts

Parts

Parts

Kanban-MRP “Sandwich” ƒ MRP system changed: ƒ BOM: ƒ MRP does not control/stock any kanban controlled part ƒ Kanban parts entered as phantoms to implement this ƒ active BOM much simplified

ƒ Lead times ƒ include allowance for kanban controlled cycles ƒ Critical to synchronisation of supply

ƒ Inventory ƒ No tracking of kanban parts & no related transactions ƒ Backflushing widely used to avoid recording issues of purchased items

ƒ Levelled schedules required for viable operation ƒ Used where kanban links to customers & suppliers not feasible ƒ or may be an intermediate step in implementation

Doug Love/Aston University

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Kanban Tutorials: Review of principles

Kanban Tutorial – Review of Principles* 1. Is kanban a manufacturing philosophy or a control mechanism? 2. What directly & indirectly determines the level of work in progress in a kanban-controlled manufacturing system? 3. 2 bin systems are a special case of which kanban mechanism? 4. In comparison with 1 card systems, 2 card kanban can be said to provide a better fit to the causes of delay in the system(s). Why? 5. How can the number of kanbans in the conveyance circuit be said to depend on motorway traffic jams? 6. How can the number of kanbans in the production circuit depend on the handling system used? 7. How can the number of kanbans in the production circuit depend on the TPM? 8. What do you have to do to make a manufacturing system (that uses kanban) run with less work in progress and/or material stocks? 9. When designing a kanban system there is little point in trying to calculate the number of kanbans in the circuit very accurately – why? 10. Why can 2 card kanban be said to provide better accountability than 1 card? *based on material in the lecture, Control Systems text notes and other sources in the bibliography 16

Doug Love/Aston University

Kanban Calc Tutorial 1

Calculation Tutorial 1 – 2 card system design Demand Rate ( per 8hr day) =

1200

=

2.5

Production Circuit The production circuit includes both operations in the supplier cell.

/min

Container Size

Operation Data

Weight Each Size (LxWxD)

1 Kg 0.2x0.1x0.05m

Handling Method Manual Hand Truck Power Truck

Capacity Kg 40 500 2000

Container Tote Pan Bin Stillage

Volume m3 0.054 0.211 1.000

0.001

Machine (mins/each) Operator Perf variability +/- %

m3

Other Data Element

Operation (all times in Minutes) 10 20 Average Max. Average Max. 120 180 120 200 30 40 20 25 20 45 20 45

Queue wait machining Setting Transport to next stage

The above times include the effect of miscellaneous losses, breakdowns etc. The time to pass the kanban back to op 10 is estimated at = 20 mins

Move/Conveyance Circuit The supplier and customer cells are located on the opposite sides of a large industrial site. Transport Data Transport Element Time (Mins) Average Max Wait Truck 15 30 Customer->Supplier Cell 15 18 Swop Cards 2 2 Supplier -> Customer Cell 20 24

NB the formula is, No kanbans=

Operation 10 20 2 1 20 30

Demand Rate x(Sum of Times in circuit) x (1 + Safety Factor) container capacity

Doug Love/Aston University

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Kanban Calc Tutorial 2

Calculation Tutorial 2 – 1 or 2 card system? Cycle of activities Description of activity Empty => supplier Call truck Wait truck Transport to loading bay Wait loading/lorry or van available Load on lorry Transport to supplier Wait unloading Unload Wait truck Transport to manufacturing cell

Time (mins) 2 30 3 240 60 480 10 20 20 3

Swop cards

2

Production wait free operator transport card/bin to 1st op machine queue until parts for minimum run available set machine process run quantity move to next 2nd operation queue waiting machine free set machine process move to cell storage area

10 2 calculate 30 2 2 240 30 3 5

Wait truck Transport to loading bay

NB data continues on next page

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20 3

Comment put up signal next time on circuit

wait while other items loaded time depends on route

forklift drop off & place cards in rack

done by forklift driver

minimum run quantity =

2000

NB std time is 2 mins each 1st operator does this

NB std time is 3 mins each 2nd operator does this forklift

Doug Love/Aston University

Kanban Calc Tutorial 2

Calculation Tutorial 2 – 1 or 2 card system? cont. Supplier => return full Wait loading/lorry or van available Load on lorry Transport to customer Wait unloading Unload Wait truck Transport to assembly cell

Notes Variabilty of the times has been estimated at: Production Elements Transport Elements

30% 50%

Possible container capacities are (in units): Large Medium Small

2000 400 50

Demand Estimated demand is (units/hr)

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240 60 480 10 20 30 5

wait while other items loaded time depends on route

drop off by track

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Doug Love/Aston University