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| Mistake Proofing OverviewNovember 1, 2004 Mistake
proofing is critical to the lean organization for creating and maintaining
process stability. Process
instability is one of the biggest problems organizations encounter when
attempting to implement lean. Among
the tools available to the lean practitioner to improve process stability,
mistake proofing is one of the simplest, yet most effective. For
the manufacturer, mistake proofing techniques can be applied to the
manufacturing process or the product design itself to prevent manufacturing
errors. They can also be used
outside of manufacturing: hospitals, financial institutions, and other service
organization have successfully used mistake proofing techniques.
While
mistake proofing in some forms has been around for a very long time, it was
Toyota that formalized a system. Toyota’s
Shigeo Shingo developed an approach called Zero Quality Control (ZQC).
ZQC, sometimes referred to as “Zero Defects,” is based on the
principle that defects are prevented by controlling the performance of a process
so that it cannot produce defects even when a machine or person makes a mistake.
Poka-Yoke or Mistake Proofing is one key aspect of ZQC.
Poka-Yoke or Mistake Proofing systems use sensors or other devices that
make it nearly impossible for an operator to make an error.
They regulate the production process and prevent defects in one of two
ways: Control
System –
stops the equipment when an irregularity happens or locks a clamp on the
workpiece to keep it from moving on when it is not completely processed.
This is the better system since it is not operator dependent. Warning
System –
signals the operators to stop the machine or address the problem. This type of
system is operator dependent. Basic
Methods There
are three types of poke-a-yoke methods: contact methods, fixed-value methods,
and motion-step methods. Contact
Methods detect whether a product makes physical or energy contact with a sensing
device. An example of physical
contact would be a limit switch that is pressed when screws are attached to a
product. An energy contact example
would be photoelectric beams that sense when something is not in the correct
position. Some of the best contact
methods are passive devices such as guide pins or blocks that don’t allow a
product to be positioned incorrectly. Many
times such simple devices can be designed in to a product. Fixed-value
methods should be used when a fixed number of parts much be attached to a
product or a fixed number of operations need to be done at a workstation.
Under this method, a device counts the number of times something is done
and signals or releases the product when the value is reached. Motion-step
methods detect whether a motion or step has happened within a certain period of
time. Also, they can be used to ensure that events happened according to a
certain sequence. These methods
generally utilize sensors and devices like a photoelectric switch connected to a
timer. An example would be a label
dispenser that uses a photoelectric switch to stop the line if it does not
detect removal of a label within the machine’s cycle time. Types
of Sensing Devices There
are three categories of sensing devices used in Poke-a-Yoke applications: ·
Physical Contact Sensing Devices ·
Energy Sensing Devices ·
Sensors that detect changes in physical conditions Physical
Contact Sensing Devices This
type of device works by physically touching a product or part of a machine. In most automated applications, the device sends an
electrical signal when contact is made. Some
examples of such devices include: Limit
Switches –
These confirm the presence and position of objects that touch the small lever on
the switch. These are among the
most common and least expensive devices. Touch
Switches – These
are similar to limit switches; however, they are activated by a light touch on a
thin “antenna.” Trimetron
– These are
sensitive needle gauges that send signals to sound an alarm or stop equipment
when a measurement is outside the acceptable range. Energy
Sensing Devices This
type of device uses energy rather than physical contact to determine whether an
error is occurring. Examples of
such devices include: Proximity
Switches –
These devices use beams of light to inspect transparent objects, judge welds,
and verify conditions such as (a) proper color or size of an object, (b) passage
of objects on a conveyor, (c) proper supply of parts, or (d) proper feeding of
parts. Beam
Sensors –
These sensors are similar to proximity switches; however, they use beams of
electrons to detect. Other
types of energy sensing devices include: ·
Fiber sensors ·
Area sensors ·
Position sensors ·
Dimension sensors ·
Vibration sensors ·
Displacement sensors ·
Top sensors ·
Metal passage sensors ·
Color mark sensors ·
Double-feed sensors ·
Weld position sensors Sensors
That Detect Changes in Physical Position This
type of sensor detects condition changes such as pressure, temperature, or
electrical current. Examples
include pressure gauges, thermostats, and meter relays. Mistake proofing is a powerful tool for the lean enterprise. It can be applied to almost any process to create more stability. However, it is important to implement mistake proofing systems where they are needed most. To find out how to best implement these systems, read our article entitled “Implementing Mistake Proofing Systems.” Click here to subscribe to our free e-newsletter Learning to Lean and receive three articles like this one each month. About the Author Darren Dolcemascolo is an internationally recognized lecturer, author, and consultant. As Sr. Partner and co-founder of EMS Consulting Group, he specializes in productivity and quality improvement through lean manufacturing. Mr. Dolcemascolo has written the book Improving the Extended Value Stream: Lean for the Entire Supply Chain, published by Productivity Press in 2006. He has also been published in several manufacturing publications and has spoken at such venues as the Lean Management Solutions Conference, Outsourcing World Summit, Biophex, APICS, and ASQ. He has a BS in Industrial Engineering from Columbia University and an MBA with Graduate Honors from San Diego State University. | ||||||||||||||
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