posbul1a
Drum –Buffer-Rope

•Based on : R. Holt, Ph.D., PE

Traditional Approach:
Divide and Conquer
* Division of Labor breaks down linkages  complex systems into manageable chunks.
* Which is harder to manage?  Left or Right?
•Left
•Right

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We Measure Operational  Efficiency
* Work flows from left to right through processes with capacity shown.
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•Process A B C D E
•RM
•FG
•Capability
•Parts 7 9 5 8 6
per Day
•Excellent Efficiency--Near 100%
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•Chronic Complainer
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•Too Much Overtime
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•Market
Request
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•RM=Raw Material
•FG=Finished Goods

Reward Based on Efficiency
* Work flows from left to right.
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•Process A B C D E
•Capability
•P/D 7 9 5 8 6
•Both found ways to  look busy and
appear to have a capacity of 5 parts/day.
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•RM
•FG
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In reality...
•Process A B C D E
•Potential
•P/D 7 9 5 8 6
•Reality 5 5 5 5 5
* Processes A and B won’t produce more than Process C for long.
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•RM
•FG
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Then Variability Sets In
* Processing times are just AVERAGE Estimates = (7+9+5+8+6)/5=35/5=7
•Process A B C D E
•Reality        5±2       5±2        5±2         5±2        5±2
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•RM
•FG
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•7 9 5 8 6

What’s an Average?  50%
•Process A B C D E
•Reality        5±2       5±2        5±2         5±2        5±2
•Probability 0.5 0.5 0.5 0.5 0.5
* Half the time there are 5 or more per day at each process--Half the time less
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•Two at a time:      0.25
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•0.25
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•Over all: 3% Chance of 5 per day !!!
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•RM
•FG
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Previous Solution: Inventory
•WIP       5       5      5      5     5         Total 25
•Process A B C D E
•Variable 5±2       5±2        5±2         5±2        5±2
•Process
* Put a day of inventory before each process!
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•RM
•FG
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•WIP = Work  in Process (sometimes Work in Progress )

System Variability Takes Over--Chaos
•Variable      5±2       5±2        5±2         5±2        5±2
•Process
•WIP       3              0       10               8       4     Total 25
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•RM
•FG
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•Process      A B C D E
•Inventory (WIP) quickly shifts position.
•Inventory manager/expediter tries to smooth it out.
•Distribution problems result. Costs go up.

System Variability Takes Over--Chaos
•WIP       3              0       10               8       4     Total 25
•Variable      5±2       5±2        5±2         5±2        5±2
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•Process
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•Shifting work-in-process creates large queues at some
locations.   This makes work wait longer to be processed.
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•RM
•FG
•An Average of 5 means sometimes 3 (5-2) and some times 7(5+2)
•Process      A B C D E

System Variability Takes Over--Chaos
•WIP       3             0       10           8       4     Total 25=3+10+8+4
•Variable     5±2       5±2        5±2         5±2        5±2
•Process
•Shifting work-in-process (WIP)creates large queues at some
locations (e.g. before  C station). This makes work wait longer
•to be processed.
Other workstations can be starved for work.  The work they
could be doing is delayed because it is not there. They
•can’t take advantage of their extra capability.  So...
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•RM
•FG
•Process      A B C D E

System Variability Takes Over--Chaos
•WIP       3             5       10            8       4     Total 25
•Variable     5±2       5±2        5±2         5±2        5±2
•Process
•So…  Management Helps!  Management puts in more work
•(Inventory) to give everyone something to do!
•Result:  It takes longer and longer from time of release
•until final shipping.  More and more delay!!!!!!!!!!!
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•RM
•FG
•Process      A B C D E
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•->X 30

Operation’s Dilemma
•Injection:  Put a large inventory where its needed and low everywhere else!
•Manage production
•effectively
•Produce a lot
•Costs & delivery in control
•Increase work-in- process
•Decrease work-in-process
•Assumption:
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•We can’t both increase WIP and decrease WIP at the same time.

TOC Steps to
Continuous Improvement
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Step 1.  Identify the system’s constraint.
Step 2. Exploit the system’s constraint.
Step 3.  Subordinate everything else to the above decision.
Step 4.  Elevate the system’s constraint.
Step 5.  If a constraint is broken (that is, relieved or improved), go back to  Step 1. But don’t
allow inertia to become a constraint.

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Five Steps Applied to Flow Operations
•   A B C D E
• 7 9 5 8 6
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•WIP                Total
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•Step 3.  Subordinate Everything Else (Rope)
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•Step 4.  Elevate the Constraint ($?)
•X 5.5
•Step 5.  If the Constraint Moves, Start Over
•XXX
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•Five Focusing Steps
•RM
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•Step 1.  Identify the Constraint (The Drum)
•FG
•Step 2.  Exploit the Constraint (Buffer the Drum)
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•12
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•FG
Understanding Buffers
•   A B C D E
• 7 9 5 8 6
•RM
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•The “Buffer” is Time!
•In general, the buffer is the total time from work release until the work arrives at the
constraint.
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•WIP                        Total 12parts/5parts per day=2.5 Days
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•Rope
•Constraint=drum

We need more than one Buffer
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•FG
•   A B C D E
• 7 9 5 8 6
•RM
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•There is variability in the Constraint.
To protect our delivery to our customer we
need a finished goods buffer.
•Finished Goods
Buffer (1 day)
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•There is variability in our suppliers.
We need to protect ourselves from unreliable
delivery.
•Raw Material
Buffer
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Buffer Time is Constant-Predictable
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•FG
•   A B C D E
• 7 9 5 8 6
•RM
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•Finished Goods
Buffer
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•Constraint
Buffer
2.5 Days=(4+5+3)/5
•Raw Material
Buffer
•Finished Goods
Buffer
1 Day
•Processing Lead Time is Constant
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•Raw Material
•Buffer
•2 Days=10/5

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•FG
Buffer Management
•   A B C D E
• 7 9 5 8 6
•RM
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•Constraint Buffer WIP         Total 12/5=2.5 Days
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•Time until Scheduled at Constraint
•0
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•2.5 Days
•WO17
•WO14
•WO15
•WO16
•WO10
•WO11
•WO12
•WO13
•WO18
•WO19
•The Constraint is scheduled very carefully
•Buffer Managed by location
•Individual activities in the buffer are not scheduled
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•WO21
•WO20
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•   A B C D E
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Problem Identification
• 7 9 5 8 6
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•Time until Scheduled at Constraint
•0
•2.5 Days
•WO10
•WO20
•WO12
•WO13
•WO21
•WO15
•WO16
•WO17
•WO18
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•WO19
Delayed
Parts
•WO11
•WO14
•WO19
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WO19 OK (Green)
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Watch WO14 (Yellow)
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Constraint schedule
is in jeopardy!
(Red Zone Hole)
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Additional Buffers
* Constraint Buffer (as we discussed)
•Protects the Constraint from running out of work
* Finished Goods Buffer
•Protects customer delivery from Constraint variation
* Raw Material Buffer
•Protects the Release of material from suppliers
* Assembly Buffer
•Facilitates speedy flow of products

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