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. Poke-yoke, or mistake proofing, is one key aspect of ZQC. Poke-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-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 into 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-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.

About the author
Darren Dolcemascolo is an internationally recognized lecturer, author and consultant. As senior 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. To learn more, visit www.emsstrategies.com.