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Company ConfidentialCopyright NN, Inc. 2004
Poka-Yoke Systems
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Mistake Proofing
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Company ConfidentialCopyright NN, Inc. 2004
POKA YOKE
Inadvertent
Mistakes To Avoid( Yokeru )
The Poka-Yoke Concept was developed by Shigeo Shingo ,
who also developed the SMED System.
Poka-Yoke Systems
POKA-YOKE
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Human Errors are usually inadvertent.
Poka-Yoke devices help us to avoid defects
before they occur, even when human errorsare made.
Poka-Yoke is a too l that bui lds Qual i ty into th e
process and shi f ts the attent ion from detect ionand inspect ion to bui ld ing i t r ight the f irst t ime.
Poka-Yoke Systems
POKA-YOKE
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Two Arguments :
Errors are Inevitable:Humans always will make mistakes.
Where Do You Stand ?
Poka-Yoke Systems
ERRORS AND DEFECTS
Errors can be Avoided:Any kind of mistake people make
can be avoided one way or another!!
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Sampling Inspection vs. 100 Percent Inspection:
Sampling Inspection:Is an Average Quality Level (AQL) of0.10% good enough? One Customer in a thousand will get a
defective part !!
100 Percent Inspection:Can it efficiently become part of
your standard operation?? How??
INSPECTION
Poka-Yoke Systems
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Strategies for Zero Defects:
Dont Make Products You Do Not Need!!
The more parts you produce, the greater the opportunity for
defects to happen. Therefore, make only what you need,
when you need it, one piece at a time.
Build Quality into the ProcessPut safeguards into the production process to ensure quality.
Moreover, standardize the process and always think of
mistake proofing.
Cont inuous Flow
Make sure that continuously value is added to the product
until it is a finished product.
ZERO DEFECTS
Poka-Yoke Systems
This is where Poka-Yoke comes in!
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1. Forgetfulness: Forgetting a step or a part.
Safeguard:Checklist, Visual Standard Operating Procedure
2. Errors due to Misunderstanding: Not very familiar with
the required operation.
Safeguards:Continuous Training, Visual SOP
3. Errors in Identification: Problems in identification orclearness of required steps or parts.
Safeguards:Training, Visual Training, Standardization
Poka-Yoke Systems
TYPES OF ERRORS
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4. Errors due to Lack of Experience: New Employees.
Safeguard:Skill Building & Training, Work Standardization
5. Errors due to Lack of Standards: No clear way to perform
the task or job.
Safeguards:Standard Operations, Visual Instructions
6. Errors due to Machine Readability: Machine out of spec.Safeguards:TPM , Critical Parts List, Maintain Equipment
History List.
Poka-Yoke Systems
TYPES OF ERRORS
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Keep in Mind :Mistakes do happen for many reasons, but
almost all can be prevented if we take the timeto identify WHEN & WHY they happen and take
steps to prevent the recurrence of these errors
and mistakes by using a systematic approach
to Mistake Proofing aimed at eliminatingforeseeable errors.
Poka-Yoke Systems
TYPES OF ERRORS
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Defective Product
Material DefectsProcessing Defects
Processing
Omissions
Processing
Errors
Missing
Parts
Wrong
Items
Adjustment
Errors
Operation
Errors
Wrong
Piece
Wrong
Set-Up
Other
Errors
Poka-Yoke Systems
TYPES OF DEFECTS
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Information Material
People
Methodology / Procedure
The fou r elements of pro duc t ion determine whether a
produc t is correct ly manufactured or not .
Defect Free Products are a result of Contro l led Standardsof Operat ion in each of these pr incip le areas.
Poka-Yoke Systems
ELEMENTS OF PRODUCTION
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Omitted Processing
Processing Errors
Set-Up Errors
Missing Parts
Wrong Parts
Processing Wrong Work-Piece Adjustment Errors
Tools and Jigs improperly prepared
Poka-Yoke Systems
SOURCES OF DEFECTS
There are many sources o f defects ,
the fol lowing are a few to look for :
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Standardized Guiding Pins
Alarms
Lim i t Switches
Counters
Checkl is ts
Poka-Yoke Systems
FIVE BASIC CONCEPTS OF POKA-YOKE
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Detection
Prevention
Prediction
Warning
Have
Happened
About to
Happen
Poka-Yoke Systems
POKA-YOKE STRATEGY
A
ctions
Errors
Focus on this
first .
. and then
on this!
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Identify Items by their Characteristics:
By Weight: Establish weight standards to determine defective
parts by pre-defining the weight for a good part.
By Dimension: Length, Width, and Diameteretc.
Use jigs, fixtures, stoppers, and limit switches to eliminate errors
associated with positioning.
By Shape: Angles, Projections, Position of Holes, Curvature..etc
and Identify standards, limit switches, and fixtures.
Poka-Yoke Systems
POKA-YOKE HINTS
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There are three main methods for using Poka-Yoke Systems:
1. Contact Methods2. Fixed-Value Methods
3. Motion-Step Methods
Each of these methods can be used with eitherContro l
SystemsorWarning System. Focus on your end objective and
select the proper system accordingly.
Poka-Yoke Systems
METHODS FOR USING POKA-YOKE
SYSTEMS
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Contact Methods work by detecting whether a
product makes physical or energy contact with
a single detection device.
Micro Switches
Limi t Sw itches
Displacement Senso rs
Metal-Passage Sensors
A Variety of other Mechanical
& Electro -Mechanic al devic es
Poka-Yoke Systems
CONTACT METHODS
Contact Devices :
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Prox imi ty Switches: Respond to changes in distance from objects andto changes in magnetic force.
Posi t ion ing Sensors: Control cylinder strokes or determine screwheights.
Transm ission Pho toelectr ic Switch: Light beam between twoobjects detects any obstruction of beam.
Reflect ing Photoelectr ic Switch: Responds to light reflected from anobject to detect its presence.
Poka-Yoke Systems
CONTACT METHODS
Contact Methods may also be energy sensing devices that do
not physically touch the product but sense when something is
not in the desired position.
Energy Sensing Devices:
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Contact Methods do not have to be high-tech in order to be
effective. Some of the best contact devices are passive devices
SUCH AS :
Poka-Yoke Systems
CONTACT METHODS
Guide Pins Blocks
Jigs
Fixtures
Color Coded Devices
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Contact Devices often take advantage of parts that are designed
with uneven shape, such as a work-piece with a bump or a hole on
only one end. Use a Jig that matches the shape or a limit switch
positioned to detect an abnormal shape.
Poka-Yoke Systems
CONTACT METHODS
Rule of Thumb:
We can also make design recommendations based upon
adaptability to manufacturing.
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Fixed Value Methods are used when a fixed number
of parts must be attached to the product or when a
fixed number of repeated operations needs to be
done at a process station.
In this method, a device counts the number of times
something is done and signals or releases the product
only when the required number is fulfilled.
Poka-Yoke Systems
FIXED VALUE METHODS
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The Fixed-Value Method may use Limit Switches that are
tripped with each movement. The switch sends a signal to a
counter that detects when the correct number of movements
has taken place.
Poka-Yoke Systems
FIXED VALUE METHODS
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The Motion-Step Methods are used to sense whether a motion or
step in the process has been carried out within a certain
expected time, such as a machines Cycle Time.
Devices normally used in the Motion-Step Method:
Sensors
Photo-electric Switches
Timers
Counters
Poka-Yoke Systems
MOTION-STEP METHODS
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Poka-Yoke Sheet
POKA-YOKE SHEET
Department / Machine :
Assembly
Product :Bearing
Date :14 Dec. 2004
Problem :Ball missing.
Prevent ErrorX Shutdown
Solution :Sensing the difference of weight against standard
weight. Control
XDetect ErrorKey Improvement :Weighing scale installed on line.
Alarm
Before After
No device was present to detect Ball missing from the
bearing in the assembly cell.
Weighing machine installed on line after Etching station
which detects any Ball missing bearing and the transfer
plate automatically stops if any bearing comes even withone Ball missing.
Poka-Yoke Sheet
POKA-YOKE SHEET
Department / Machine :
Assembly
Product :Bearing
Date :14 Dec. 2004
Problem :Ball missing.
Prevent ErrorX Shutdown
Solution :Sensing the difference of weight against standard
weight. Control
XDetect ErrorKey Improvement :Weighing scale installed on line.
Alarm
Before After
No device was present to detect Ball missing from the
bearing in the assembly cell.
Weighing machine installed on line after Etching station
which detects any Ball missing bearing and the transfer
plate automatically stops if any bearing comes even withone Ball missing.
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Poka-Yoke Sheet (blank)
POKA-YOKE SHEET
Department / Machine : Product :
Date :
Problem :
Prevent ErrorShutdown
Solution :
Control
Detect ErrorKey Improvement :Alarm
Before After
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POKA-YOKE
8 MAIN PRINCIPLES
Bu i ld Quali ty into the pro cess.
All in advertent errors and defects can be el iminated.
Stop doing i t wrong and s tar t doing i t r ight - now !!!
Do not th ink up excuses, th ink about doing i t r ight .
An80% chance of success is good enough - focus
on implementat ion.
Attack d efects and errors as a team .
Ten heads are better than one - B rainsto rm id eas,Discus s, Think.
Seek out the root cause, use the 5 Whys.
Poka-Yoke Systems
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Poka-Yoke Systems
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The Sensing Devices used in the Poka-Yoke System can be
divided into three broad categories:
Physical Contact Sensing Devices
Energy Sensing Devices
Sensors that detect change in Physical Condition
- Pressure: Pressure Gauges
- Temperature: Thermometers
- Electrical Current: Meter Relays
Poka-Yoke Systems
TYPES OF SENSING DEVICES
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Physical Contact Sensing Devices
limit switches
microswitches
touch switches
trimetrons
Energy Sensing Devices
transmission types
reflection typestransmission types
reflection types
proximity switches
fiber sensors
area sensors
position sensors
dimension sensorsvibration sensors
displacement sensors
tap sensors
metal passage sensors
color-mark sensors
double-feed sensors
weld position sensors
photoelectric
switches
beam sensors
line plane
Passage
Presence
BreakConfirmation
ofPosition
Measurement
Overlap
Shape
Foreign
Matter
Damage
Color
Mismatch
What They Detect
Types of Sensors
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Physical Contact Sensing Deviceswork by physically touching the product
or machine part. In many cases, particularly in automated processes, physical
contact sensing devices send an electrical signal when they are touched. This
signal can be a command to start or stop a machine or a warning. Here are
some common examples:
Limit Switches and Microswitches are the most common devices. They areused to confirm the presence and position of objects that touch the small lever
on the switch.
Touch Switches have the same basic function as a limit switch, but they are
activated by a light touch on a thin antenna. Touch switches are more
sensitive than limit switches and can detect whether an object is present,properly positioned, broken, or of proper dimensions.
Trimetrons are needle-type gauges that send out signals to sound alarms or
stop machinery when the conditions they measure are not within acceptable
limits.
Poka-Yoke Systems
PHYSICAL CONTACT SENSING DEVICES
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Energy Sensing Devicesuse energy rather than physical contact todetermine whether an error is occurring. Here are some examples:
Photoelectric switches use beams of light to inspect transparent objects,judge welds, or verify conditions such as proper size and color.
Beam sensors use electron beams to verify conditions such as the
proper level of liquid in a container or the passage of an object on aconveyor.
Proximity switches respond to changes in distance from objects and tochanges in magnetic force.
Fiber sensors detect motor revolutions, color coding marks, or marks on
translucent objects or printed matter.
Area sensors detect random breaks in a fixed area, such as handsplaced in hazard areas or parts dropping from a conveyor.
Position sensors control cylinder strokes or determine screw heights.
Poka-Yoke Systems
ENERGY SENSING DEVICES
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Dimension sensors ensure correct product dimensions.
Vibration sensors detect product ejection areas, width distortion, seam
position, or start of processing.
Displacement sensors detect and measure warping, thickness, and fluid
level heights.
Tap sensors detect incomplete tap screw machining.
Metal passage sensors detect metal in motion.
Color-mark sensors detect colored marks or differences in color.
Double-feed sensors detect two products fed at the same time.
Weld position sensors detect joints such as weld lines in coil stock,
seams in pipes and cans, or splices in wires.
Poka-Yoke Systems
ENERGY SENSING DEVICES