MRO Today - Error proofing

Error proofing

by Subramaniam “Mani” Manivannan

Error Proofing is a process improvement designed to prevent a specific defect from occurring. It is a process improvement system that prevents personal injury, promotes job safety, prevents faulty products and prevents machine damage. As you will see, this is a powerful tool for creating and sustaining reliability in any operation, be it production or maintenance.

Shigeo Shingo’s poka-yoke
Shigeo Shingo introduced the concept of poka-yoke in 1961, when he was an industrial engineer at Toyota Motor Corporation. The initial term was baka-yoke, which means “fool-proofing.” In 1963, a worker at Arakawa Body Company refused to use baka-yoke mechanisms in her work area because of the term’s dishonorable and offensive connotation. Hence, the term was changed to poka-yoke, which means “error-proofing” or “mistake-proofing.”

The Error Proofing triangle
There are three components of poka-yoke; physical, operational and philosophical, and each must be present in the solution for the system to work.

Physical Error Proofing involves installing components like fixtures or sensors to eliminate conditions that may lead to an error.

Operational Error Proofing involves making modifications or installing devices that reinforce the correct procedure sequence.

Philosophical Error Proofing involves identifying situations that cause defects and doing something about it – which includes empowering the workforce.

Approaches to Error Proofing
Prevention: Prevents errors from creating defects
Detection: Detects defects and immediately initiates
corrective action to prevent multiple defects from forming

Definition of a defect:
A defect is the result of any deviation from product specifications that may lead to customer satisfaction.

To classify as a defect:
1. The product has deviated from manufacturing or
design specifications
2. The product does not meet internal and/or external
customer expectations

Definition of an Error:
An error is any deviation from specified manufacturing process. There can be an error without a defect, but there cannot be a defect without an error.

Error Proofing as it is understood and practiced today is an outgrowth of the quality movement, specifically the “Zero Defects initiative.” It is a team-based plant floor improvement strategy that focuses on production processes and operations. Error Proofing aims to prevent errors and deviations from standards of all kinds that can impact quality, safety, manufacturing costs, and customer satisfaction.

The zero defects system

All the manufacturing/process issues are prioritized to help identify opportunities for greatest impact for customers and return for investment. The most common tool used to identify/prioritize the issues is Process Failure Mode and Effects Analysis (PFMEA). The Process FMEA method is used by cross functional team approach to answer all process related questions and to quantify the results in the form of a Risk Priority Number (RPN).

Question	PFMEA Column(s)
What steps/processes are important?	Function/Requirements
What can go wrong?	Potential Failure Mode
How could the failure affect the customer (internal/external)?	Effects of Failure
How serious is the effect of the failure mode on the customer?	Severity (rating from 1-10)
Why would the failure occur?	Cause of Failure
How often will it happen?	Occurrence
What are the controls used by the process?	Current Process Control
What is the probability that current process control will detect a problem?	Detection	Error Proofing
What can be done to improve the process?	Recommended Action(s)	Error Proofing

The PFMEA tool helps the team to ask the key questions, also to identify and implement the proper Error Proofing to improve the process.

Why use Error Proofing?
Competitive advantage: In a global market the cost of quality is part of the competitive advantage. It costs far less to prevent defects from occurring in the first place than to catch them later through inspection and have to rework or repair them.

Knowledgeable workers: When every employee understands the principles of Error Proofing, work teams can see more easily how defects are generated and can then effectively eliminate them. They can participate in the design and improvement of parts processing and assembly operations in order to prevent defects from occurring. These methods can be employed in our offices as well to eliminate errors in paper processes.

Predictability: If our machines (manual or robotic) include error-proofing devices, then we are assured that the end product will be defect free. This eliminates inspection and rework operations, as well as scrap, which all increase manufacturing costs.

Reduced variation: Error Proofing devices also ensure that subassembly and assemblies are exactly the same. There will be little chance of part-to-part variation if the machines are designed or modified to prevent errors and their resulting defects.

Human error is natural. But sometimes when errors can be traced back to the operator’s interaction with the process, there is a tendency to blame the operator. We encourage the operator to try harder NOT to make mistakes. But the root cause of the error is usually failure to account for the possibility of human errors or omissions – by people who design machinery, layouts or operating procedures. Error Proofing can correct this.

One of the most important facts to realize about human error is that errors are inevitable – they are part of human nature. Few workers make errors intentionally; most strive to prevent errors. Error Proofing alters the work environment to reduce the opportunity for human errors.

When incorporating Error Proofing into the work environment, understanding human limits is essential. These limits include:
• Vision: People vary in ability to distinguish details, colors, or adjust vision to lighting.
• Hearing: Individual upper and lower thresholds of hearing change when background noise is added.
• Repetition ability: Muscular efficiency and mental tracking decrease as rates of repetition increases.

Six Sigma and Error Proofing
In a DMAIC (Define-Measure-Analyze-Improve-Control) project, this step is usually performed in the Control phase to prevent a specific defect from occurring. It is next to impossible to reach Six Sigma type results without applying Error Proofing concepts. The steps involved in initiating and driving a Six Sigma project in your plant are the subject of the next article in this series.

The following checklist is a very good tool for checking the Error Proofing devices in your plant and verifying that they are in good working order.

Subramaniam “Mani” Manivannan is a Quality Coach/Assessor – PTO Quality manufacturing process and product support engineer for Ford Motor Company in Dearborn, Michigan. He can be reached at 313-323-7719; e-mail: smanivan@ford.com.