pull-push system and jit and lean

Computer Integrated manufacturing Systems ENGR 4440

What is Inventory?

Inventory (n) - 1: an itemized list of current assets; 2: the quantity of goods or materials on hand: STOCK

Merriam Webster’s Collegiate Dictionary, 10th ed.

Inventory - any of the materials used directly in the manufacture of a finished product


Types of Inventory

$ Invested

$ $ $Flexibility

high medium low

1 2 4 53 ...

Finished Goods Raw

Materials

Work-In-Process


Why Have Inventory?

Variability

Large Inventories

High Costs

- Customers- Suppliers- Competitors- Mfg. Process

- “Just-In-Case”

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Why Have Inventory?In

ven

tory

Lev

el

Work-In-Process


Managing Inventory

Two ways to attack the problem...

ReduceSources ofVariability

ImplementInventory

ManagementSystem

...best solution: Do Both!


Identify “Rock”

Form Teams

ReceiveTraining

Document/Analyze Process

ImplementImprovements

Adjust Process or System

EnhancedPerformance

Inventory Management: Variability Reduction


Inventory Management: System

A systematic method of purchasing, controlling, using,and distributing the three classes of inventory: - Raw Materials - Work - In - Process (WIP) - Finished Goods

1 2 4 53 ...

Finished Goods Raw

Materials

Work-In-Process


To build the right product,

at the right time,

in the right quantity

to meet customer demand

subject to

At the right cost

To meet company profitability goals

Purpose of Inventory Management Systems


Inventory Management Philosophies

I. Push System: Manufacturing Resource Planning (MRP II) Links demand for parts to demand for finished goods through standard lead times and bills-of-material via computer systems;Production is triggered by a schedule

II. Pull System: Just-In-Time (JIT) Links demand for parts to demand for finished goods through series of calculated buffers via simple visual cards/containers known as kanban; Production is triggered by demand


Inventory Costs

Opportunity cost

Carrying costs

Obsolescence

Design changes

Operating costs Handling Space People Paperwork Equipment Damage

Excess inventory is the root of all evil.

K. Suzaki, The New Manufacturing Challenge


Key Inventory Measure

Inventory Turns = Gross Annual Sales

Avg. On-Hand Inventory

Inventory Turns: Harley Davidson

05

10152025

1 2 3 4 5 6Year

Annu

al

Turn

sA

nn

ual

T

urn

s


Key Inventory Measure

Company Inventory Turns Turn Days

Avg. U. S. manufacturer 7 - 8 45 - 52

Harley Davidson 21 17

Black & Decker 37 10

Avg. U. S. grocery store 50 7

Toyota 80-100 3.7 - 4.7

Southland (7-11 stores) 300+ ~1


Push Systems

Product is pushed through the manufacturing process in batches large enough to: A. Meet present and future demand B. Compensate for problems in process

Trigger: Schedule, based on Forecast & Actual Customer Orders

“Just build it, They’ll buy it!”

Note: MRP II is a Push System

customer


MRP II Strengths

Well suited for variable demandWorks in both Make-To-Stock and

Make-To-OrderProvides integrated manufacturing

management systemHandles large number of parts easilyWidely used & accepted


MRP II Weaknesses

Assumes infinite plant capacity Requires accurate lead times Requires accurate forecasts No limit to WIP inventory Lack of robustness (i.e., easily “disturbed”) Requires accurate Bill-of-Materials


Pull Systems

Product is pulled through the manufacturing processat the rate of customer demand; Finished Goods arereplenished through a linked series of tactical buffers (kanban) that offer control of Work-In-Process inventory.

Trigger: Customer Purchase of Goods

“If they buy it, We’ll build it!”

customer


One Piece Flow - Pull System

One order at a time

Order in = Order out

First in - first out

Produce what you need - not what you can

Lot size reduction

Simplified scheduling


Cross Training and Load Leveling

Opens system constraints or bottle necks

Better utilizes all resources

Improves communication

IMPROVES productivity


Pull System

A Trigger to Signal:

• What to Build

• When to Build

• How Much to Build

Kanban


Pull System Strengths

Continual visibility Simple, low cost production scheduling tools Controlled / Calculated buffers Reduced dependence on specific

item forecast


Pull System Weaknesses

Requires relatively steady demand Not Useful for Make-To-Order: FG Relies on manufacturing process Not as widely understood


Which System is Best?

There is no “Silver Bullet”!

You must find the appropriate systemfor your business environment


Ch 26 Just-In-Time and Lean Production

Sections:

1. Lean Production and Waste in Manufacturing

2. Just-in-time Production Systems

3. Autonomation

4. Worker Involvement


What is Lean Production?

Lean production means doing more work with fewer resources

Adaptation of mass production in which work is accomplished in less time, smaller space, with fewer workers and less equipment

Based on the Toyota Production System The term "lean production" was coined by researchers

at Massachusetts Institute of Technology


Structure of Lean Production System

Taiichi Ohno's structure of the Toyota Production System


Activities in Manufacturing

1. Actual work - activities that add value to the product

2. Auxiliary work - activities that support the value-adding activities

3. Muda (waste) - activities that neither add value nor support the value-adding activities


Muda (Waste)

Taiichi Ohno’s seven forms of waste:

1. Production of defective parts

2. Production of more parts than needed (overproduction)

3. Excessive inventories

4. Unnecessary processing steps

5. Unnecessary movement of people

6. Unnecessary handling of materials

7. Workers waiting


Keys to Eliminating Waste

1. Just-in-time production

2. Autonomation (automation with a human touch)

3. Worker involvement


Just-In-Time Production

Production and delivery of exactly the required number of each component to the downstream operation in the manufacturing sequence just at the moment when the component is needed

Minimizes: Work-in-process Manufacturing lead time


Requisites for JIT

1. A pull system of production control

2. Setup time reduction for smaller batch sizes

3. Stable and reliable production operations


Pull System of Production Control

A system in which the order to make and deliver parts at each workstation in the production sequence comes from the downstream station that uses those parts

JIT is based on a pull system of production control Alternative is a push system in which parts are produced

at each station irrespective of the immediate need for those parts at the downstream station


Kanban System

Toyota’s way of implementing a pull system of production control

Kanban means “card” in Japanese Two types of kanbans:

1. Production kanban – authorizes upstream station to produce a batch of parts

2. Transport kanban – authorizes transport of the parts to the downstream station


Operation of a Kanban System

1. Station i + 1 removes next P-kanban from dispatching rack. This P-kanban authorizes it to process a container of part b. A material handling worker removes the T-kanban from incoming container of part b and takes it back to station i.



1. At station i, the material handling worker finds the container of part b, removes the P-kanban and replaces it with a T-kanban. He then puts the P-kanban in the dispatching rack at station i.



1. The container of part b that was at station i is moved to station i + 1 as authorized by the T-kanban. The P-kanban for part b at station i authorizes station i to process a new container of part b, but it must wait its turn in the dispatching rack. Scheduling of work at each station is determined by the order of P-kanbans.


Setup Time Reduction

Starting point in setup time reduction is recognition that the work elements in setup are of two types:

1. Internal elements – can only be done while the production machine is stopped

2. External elements – do not require the machine to be stopped


External Work Elements

Can be accomplished while previous job is still running Strategy:

Design the setup tooling and plan the changeover procedure to permit as much of the setup as possible to consist of external elements

Examples: Retrieve tooling for next job from tool crib Assemble tools for next job Reprogram machine for next job


Internal Work Elements

Use time & motion study and methods improvement to minimize the sum of the internal work element times

Use two workers rather than one to accomplish the changeover

Eliminate adjustments in the setup Use quick-acting fasteners rather than bolts and nuts Use U-shaped washers instead of O-shaped washers Design modular fixtures consisting of a base plus insert

tooling that can be quickly changed for each new part style Base part remains attached to production machine


Examples of Setup Reduction

Setup time

Equipment type Before After Reduction

1000 ton press 4 hr 3 min 98.7%

Transfer line 9.3 hr 9 min 98.4%

Punch press 2 hr 3 min 97.5%

Machine tool 6 hr 10 min 97.2%

45 ton press 50 min 2 min 96.0%


Stable and Reliable Production Operations

Production leveling - distribute changes in product mix and quantity as evenly as possible over time

On-time delivery of components Defect-free components and materials Reliable production equipment Workforce that is cooperative, committed, and cross-

trained Dependable supplier base


Autonomation

“Automation with a human touch” Production machines operate autonomously as long as

they are functioning properly When they do not function properly (e.g., they produce a

defect), they are designed to stop Autonomation topics:

1. Stop the process

2. Error prevention

3. Total Productive Maintenance (TPM)


Stop the Process

“Jidoka” Japanese word meaning machines that are designed to

stop automatically when something goes wrong Stop the process when:

Defective parts are produced Required production quantity has been completed

Avoids overproduction


Error Prevention

“Poka-yoke” Japanese word meaning prevention of errors using low

cost devices to prevent or detect them Common mistakes in manufacturing:

Omitting processing steps Incorrectly locating a part in a fixture Using the wrong tool Neglecting to add a part in assembly


Poka-Yoke Functions

Performs 100% inspection for the following: Workpart deviations Processing and methods deviations Counting and timing functions Verification of steps during work cycle

When an error or other exception is identified, the poka-yoke responses are either or both of the following: Stops the process when an error or problem is detected Provides an audible or visible warning to alert operator

and other workers


Total Productive Maintenance

Goal: zero breakdowns TPM = integration of preventive and predictive

maintenance to avoid emergency maintenance Emergency maintenance = repair equipment that

breaks down Preventive maintenance = routine repairs to avoid

breakdowns Predictive maintenance = anticipating malfunctions

before they occur


Equipment Availability Curve

Typical U-shaped availability curve for a piece of equipment during its life


Overall Equipment Effectiveness

Measure that includes availability (reliability), equipment utilization, yield of good product, and operating capability

OEE = A U Y ros

where OEE = overall equipment effectiveness

A = availability (proportion uptime)

U = equipment utilization (time equipment is used relative to available time)

Y = yield of good product = 1 - q, where q = fraction defect rate

ros = operating capability (actual speed / design speed)


Worker Involvement

Components: Continuous improvement Visual workplace Standard work procedures Total productive maintenance


Continuous Improvement

“Kaizen” Japanese word meaning continuous improvement of

production operations Usually implemented by worker teams, sometimes called

“quality circles” Encourages worker sense of responsibility Allows workers to gain recognition among colleagues Improves worker’s technical skills


Visual Management and 5S

Principle: the status of the work situation should be evident just by looking at it Objects that obstruct the view are not allowed Build-up of WIP is limited to a specific height Andon boards located above the assembly line indicate

the status of the workstations Worker training includes use of photos and diagrams to

document work instructions


Worker Involvement through 5S

Japanese word

Seiri

Seiton

Seiso

Seiketsu

Shitsuke

English equivalent

Sort

Set in order, simplify access

Shine, sweep, scrub

Standardize

Self-discipline, sustain


Standardized Work Procedures

Three components:

1. Cycle time – actual time required “Takt time” – reciprocal of demand rate adjusted for

available shift time

2. Work sequence Basically the same as a standard method

3. Standard work-in-process Minimum number of parts to avoid waiting of

workers


Takt Time and Cycle Time

Takt time defined

Ttakt = EOT / Qdd

where Ttakt = takt time

EOT = effective daily operating time

Qdd = daily quantity demanded

In the Toyota Production System, the work must be designed so that the operation cycle time is consistent with the takt time


Standard Operations Routine Sheet

Shows the machines that must be visited by the worker during each work cycle


U-shaped Work Cell

Allocation of work at nine machines between three workers in a production work cell


Operations Routine Sheets

Allocation of work at nine machines for three workers in a production work cell


Standard Work-In-Process Quantity

Defined as the minimum number of parts necessary to avoid workers waiting

Factors that affect the standard WIP quantity: If quality inspections must be performed as distinct

steps, then additional parts must be provided If processing includes heating of parts, then additional

parts must be provided for heating and cooling time If the worker's work sequence is in the opposite

direction of the part processing sequence, then at least one workpart must be held between machines to avoid waiting time


HW # 6

26.2, 26.3, 26.4, 26.12

pull-push system and jit and lean

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