Push vs. Pull Process Control

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Push vs. Pull Process Control

• IE 3265 POM
• Slide Set 9
• R. Lindeke, Sp 2005

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Basic Definitions

• MRP (Materials Requirements Planning). MRP is the
  basic process of translating a production
  schedule for an end product (MPS or Master
  Production Schedule) to a set of time based
  requirements for all of the subassemblies and
  parts needed to make that set of finished goods.
• JIT Just-in-Time. Derived from the original
  Japanese Kanban system developed at Toyota. JIT
  seeks to deliver the right amount of product at
  the right time. The goal is to reduce WIP
  (work-in-process) inventories to an absolute
  minimum.

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Why Push and Pull?

• MRP is the classic push system. The MRP system
  computes production schedules for all levels
  based on forecasts of sales of end items. Once
  produced, subassemblies are pushed to next level
  whether needed or not.
• JIT is the classic pull system. The basic
  mechanism is that production at one level only
  happens when initiated by a request at the higher
  level. That is, units are pulled through the
  system by request.

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Comparison

• These methods offer two completely different
  approaches to basic production planning in a
  manufacturing environment. Each has advantages
  over the other, but neither seems to be
  sufficient on its own. Both have advantages and
  disadvantages, suggesting that both methods could
  be useful in the same organization.
• Main Advantage of MRP over JIT MRP takes
  forecasts for end product demand into account. In
  an environment in which substantial variation of
  sales are anticipated (and can be forecasted
  accurately), MRP has a substantial advantage.
• Main Advantage of JIT over MRP JIT reduces
  inventories to a minimum. In addition to saving
  direct inventory carrying costs, there are
  substantial side benefits, such as improvement in
  quality and plant efficiency.

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Comparisons (cont.)
6
Comparisons (cont.)
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Focusing on JIT

• JIT (Just In Time) is an outgrowth of the Kanban
  system developed by Toyota.
• Kanban refers to the posting board (and the
  inventory control cards posted there) where the
  evolution of the manufacturing process would be
  recorded.
• The Kanban system is a manual information system
  that relies on various types of inventory control
  cards.
• Its development is closely tied to the
  development of SMED Single Minute Exchange of
  Dies, that allowed model changeovers to take
  place in minutes rather than hours.
• The Fundamental Idea of JIT and Lean
  Manufacturing Systems in General (an
  Americanization of the Toyota P. S.) is to
  empower the workers to make decisions and
  eliminate waste wherever it is found

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The Tenets of JIT/Lean

• Empower the workers
• Workers are our intelligent resources allow
  them to exhibit this strength
• Workers ultimately control quality lets them do
  their job correctly (Poka-Yoke)
• Dont pit workers against each other eliminate
  piece-work disconnected from quality and allow
  workers to cooperate in teams to design jobs and
  expectations

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The Tenets of JIT/Lean

• Eliminate Waste
• Waste is anything that takes away from the
  operations GOAL (to make a profit and stay in
  business!)
• Reduce inventory to only what is absolutely
  needed
• Improve Quality scrap and rework are costly and
  disrupt flow
• Only make what is ordered
• Make setups and changes quickly and efficiently
• Employ only the workers needed
• Eliminate Clutter it wastes time

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Features of JIT Systems

• Small Work-in-Process Inventories.
• Advantages
• 1. Decreases Inventory Costs
• 2. Improves Efficiency
• 3. Reveals quality problems (see Figure 7-10)
•  
• Disadvantages
• 1. May result in increased worker idle time
• 2. May result in decreased throughput rate

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River/Inventory Analogy Illustrating the
Advantages of Just-in-Time
Revealing fundamental problems is the noted
competitive advantages of JIT/Lean
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Features of JIT Systems (continued)

• Kanban Information Flow System
•   Advantages
• 1. Efficient tracking of lots
• 2. Inexpensive implementation of JIT
• 3. Achieves desired level of WIP based on
  Number of Kanbans in the system
•  
• Disadvantages
• 1. Slow to react to changes in demand
• 2. Ignores predicted demand patterns (beyond 2
  months or so)

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Focus on The Kanban

• Typically it is a 2-card system
• The P (production) Card and W (withdrawal) Card
• Limits on product inventory (number of P W
  cards) are set by management policy
• The count is gradually lowered until problems
  surface
• The actual target level (card count) is based on
  short term forecasting of demands

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Focus on The Kanban
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Focus on The Kanban the worker as manager

• P cards cycle from their accumulation post at
  Center 1 to product (when a defined trigger point
  is reached) and then to output queue
• When trigger level is reached, Ct 1 worker pulls
  product from Ct 1 Wait point queue and replaces
  the Ct 1 W-cards with Ct 1 P-Cards which then are
  loaded to the Ct 1 processors the worker puts
  Ct 1 W-Cards to his/her acc. Post for W-cards
• Finished Product is pushed into the Ct 1 output
  queue

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Focus on The Kanban the worker as manager

• A second worker (Ct 2s worker) watches for
  accumulation of Ct 2 W-Cards
• When it reaches their trigger level, he/she pulls
  product into Ct 2 Holding area after replacing Ct
  1 P-Cards with their W-Cards and returns Ct 1
  P-Cards to their Acc. Post for Ct. 1 workers
  benefit
• They also watch for accumulation of Ct. 2 P-Cards
  on their acc. Post and when trigger count is
  reached they pull product from holding area and
  replace Ct 2 W-Cards w/ Ct 2 P-Cards then push it
  into the processors
• And around and around they go!

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Focus on The Kanban

• So how many cards? speaking of which, a card is
  associated with a container (lot) of product so
  the number of P W cards at a station determines
  the inventory level of a product!

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Focus on The Kanban

• Lets look at an example
• 950 units/month (20 productive days) ? 48/day
• Container size a 48/10 4.8 ? 5
• L data
• A. setup is 45 minutes (.75 hour)
• B. Setup is 3 minutes (.05 hr)
• Wait time .3 hr/container
• Transport time .45 hr/container
• Prod Time 0.09 hr/each .45 hr/container

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Focus on The Kanban
Requires 32 65 30 pieces in inventory
also, with 45mins set up 10 times a day means
that we consume 450 min or 7.5 hours/day just
setting up!
Here only 22 45 20 pieces and also only
.0510 50 min for setup (.833 hr) per day
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So, setup reduction impacts Factory Capabilities
Inventory

• Lets look at the effect of studies comparing cost
  of setup vs. inventory cost like EOQ
• Then lets see what we can invest to reduce
  inventory levels
• We will spend money on reducing setup cost (time)
  and see if reduced inventory will offset our
  investment
• This is the driving force for SMED

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Focus on the Penalty Factor

• We can effectively model this a(K) function as
  a logarithmic investment function
• By logarithmic we imply that there is a an
  increasing cost to continue to reduce setup cost
• We state, then, that there is a sum of money that
  can be invested to yield a fixed percentage of
  cost reduction
• That is (for example) for every investment of
  200 the organization can get a 2 reduction in
  Setup cost

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Focus on the Penalty Factor

• Lets say that the investment is ? buys a fixed
  percent reduction in K0
• If we get actually get 10 setup cost reduction
  for ?, then an investment of ? will mean
• Setup cost drops to 0.9K0
• A second ? investment will lead to a further 10
  reduction or
• .9K-.1.9K .81K0
• This continues K3? .729K0
• Generalizing

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Focus on the Penalty Factor

• With that shape we can remodel the a(K)
  logarithmically
• a(K) bln(K0) ln(K)
• where
• Reverting back to G(Q,K) function and
  substituting Q

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Focus on the Penalty Factor

• Finding the K after the minimization
• To determine what we should do, determine G(K)
  using K0 and K

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Lets try one

• K0 1000
• ? 95 for each 7.5 reduction in setup cost
• Annual quantity 48000
• Holding cost 4.50
• MARR is 13

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Continuing

• Investment to get to K
• Testing for decision
• No investment (K K0)
• At Min K

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SMED
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Some terms

• SMED single minute exchange of dies which means
  quick tooling change and low setup time (cost)
• Inside Processes ? setup functions that must be
  done inside the machine or done when the
  machine is stopped
• Minimally these would include unbolting departing
  fixtures/dies and positioning and bolting new
  fixture/dies to the machine

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More Terms

• Outside Setup ? activities related to tooling
  changes that can be done outside of the machine
  structure
• These would include
• Bringing Tooling to Machine
• Bringing Raw Materials to Machine
• Getting Prints/QC tools to machine
• Etc.

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Focus on SMED

• When moving from No Plan or Step 1 to Step 2
  (separating Inside from Outside activities)
  investments would be relatively low to accomplish
  a large amount of time (cost) saving
• Essentially a new set of change plans and a small
  amount of training to the Material Handlers so
  that they are alerted ahead of time and bring the
  tooling out to the machine before it is needed

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Moving to Step 3 and Step 4

• Require investments in Tooling
• Require Design Changes
• Require Family tooling and adaptors
• Require common bolstering attachments
• In general requiring larger and larger
  investments in hardware to achieve smaller and
  smaller time (cost) savings in setup

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Therefore, we can say SMED is

• In reality the essence of a Logarithmic setup
  reduction plan!

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Lets Look into Line Balancing

• This is a process to optimize the assignment of
  individual tasks in a process based on a planed
  throughput of a manufacturing system
• It begins with the calculation of a system Takt
  or Cycle time to build the required number of
  units required over time
• From takt time and a structured sequential
  analysis of the time and steps required to
  manufacture (assembly) a product, compute the
  number of stations required on the line
• Once station count is determined, assign feasible
  tasks to stations one-at-a-time filling up to
  takt time for each station using rational
  decision/assignment rules

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Line Balancing

• Feasible tasks are ones that have all
  predecessors completed (or no predecessors) and
  take less time that the remaining time at a
  station
• Feasibility is also subject to physical
  constraints
• Zone Restriction the task are physically
  separated taking to much movement time to
  accomplish both within cycle (like attaching
  tires to front/back axles on a bus!)
• Incompatible tasks the Grinding/Gluing
  constraint

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Some of the Calculations

• Takt (Cycle) Time
• Minimum Workstations reqr

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Efficiency Calculations
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Lets Try One
Times A 25s B 33sC 33s D 21s E 40s F40s G
44s H 19s
A
B
C
Production Requirement is 400/shift
G
H
D
E
F
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Calculation of Takt Time Optimal Station count
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To Perform Assignment we need Assignment Rules

• Primary Rule
• Assign task by order of those having largest
  number of followers
• Secondary Rule
• Assign by longest task time

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Primary Assignment Rule
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Line Balancing Assignments
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The Line Balance
A
B
C
WS 5
WS 3
G
H
WS 1
WS 6
D
E
F
WS 2
WS 4
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Checking Efficiencies
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Dealing with Efficiencies

• We investigate other Rules application to
  improve layout
• 1st by followers then by longest time then most
  followers
• Alternating!
• Consider line duplication (if not too expensive!)
  which lowers demand on a line and increases Takt
  time
• The problem of a long individual task
• In Koeln, long time stations were duplicated
  then the system automatically alternated
  assignment between these stations