This Month's Theme is Using Design FMEAs to Identify Key Product Characteristics
Next month's theme will be using Process FMEAs to identify key process characteristics
Every month in FMEA Corner, join Carl Carlson, a noted expert in the field of FMEAs and facilitation, as he addresses a different FMEA theme (based on his book Effective FMEAs) and also answers your questions.
Questions and answers are a great way to learn about FMEAs, for both experienced and less experienced FMEA practitioners. Please feel free to ask any question about any aspect of FMEAs. Send your questions to Carl.Carlson@EffectiveFMEAs.com, and your contact information will be kept anonymous. All questions will be answered, even if they are not featured in the FMEA Corner.
"The whole of science is nothing more than the refinement of thinking." - Albert Einstein
key [kee, noun]
The Oxford English dictionary defines "key" as "a thing that provides a means of achieving or understanding something."
characteristic [kar-ik-tuh-ris-tik, adjective]
The Oxford English dictionary defines "characteristic" as "a feature or quality belonging typically to a person, place, or thing and serving to identify them."
What is a product characteristic?
From chapter 6 of Effective FMEAs:
Are some product characteristics more important than others?
Dr. Joseph Juran taught the Pareto principle, sometimes called the 80/20 rule. The Pareto principle has its origin with Vilfredo Pareto, who, in 1896, observed that about 20% of the peapods in his garden contained 80% of the peas.
Certain product characteristics are more important than others, and have a disproportionate influence on product safety and performance. Identifying the most important characteristics is an important step in controlling them and achieving safe and reliable products.
Although there is no universal standard for defining product characteristics, many companies identify significant and key product characteristics.
What are significant and key product characteristics?
Two definitions are relevant to the application of product characteristics in FMEAs:
Significant Product Characteristics are unique product-related characteristics that can affect safety, regulatory compliance, appearance, function, performance or subsequent product manufacturing. They are the direct output of a given manufacturing operation. They may or may not be designated as Key Product Characteristics.
Key Product Characteristics (KPCs) are a subset of the significant product characteristics, and are designated by the company for highlighted attention. They require follow up in the Process Control Plan and usually have their own approval process.
How are significant or key product characteristics identified in FMEAs?
One of the objectives of a Design FMEA is to identify significant or key product characteristics. The classification column can be used to visually display where a significant or key characteristic is associated with a failure mode or cause. This column can also be used to highlight failure modes or causes for further discussion or for follow up action.
But this does not answer the question of how the Design FMEA team identifies significant or key product characteristics. To understand this question, we have to begin with the definition of "cause" in a Design FMEA.
By definition, a cause is the specific reason for the failure, preferably found by asking "why" until the root cause is determined. For Design FMEAs, the cause is the design deficiency that results in the failure mode. Think about that for a minute. What is a design deficiency? Deficient means insufficient or inadequate. A design deficiency is something inadequate or insufficient about the product design. This can often (not always) be related to a product characteristic: too large or too small, too hard or too soft, too much or too little of a key alloy, etc. If the Design FMEA team properly identifies causes as design deficiencies, and if the risk associated with the design deficiency is sufficient, the associated product characteristic can be a candidate for being designated as a significant or key product characteristic.
So, what makes a product characteristic "key"?
Designating a selected product characteristic as "key" is a matter of company policy. Some companies have criteria that are very rigorous, and some have more qualitative criteria. In some companies, the designation of "key" is related to the severity rating in the Design FMEA. In others, it is related to a combination of severity and occurrence ratings. In still others, it is left up to the design team, without objective criteria. It is important for this topic to be discussed and agreed upon by the individual company. There is no universal standard that defines the criteria for designating key product characteristics for all industries.
How are key product characteristics used in FMEA?
In order to understand how key product characteristics are used, we need to take up the subject of Process Control Plans. The following paragraph is from chapter 6 of Effective FMEAs.
Key product characteristics are input to the Process FMEA. This can be done by entering product characteristics into the Process Flow Diagram worksheet, which is part of the preparation for Process FMEAs. Key product characteristics are also input to the Process Control Plan, and are entered into one of the Process Control Plan columns.
For more information about Process Control Plans see the FMEA Corner article in issue #179, and stay tuned to the next article in FMEA Corner, which will explain how key process characteristics are used in Process FMEAs.
Example of application of key product characteristics
An FMEA team is doing a Design FMEA on a shaft, and one of the concerns is bending. Computer modeling shows that two characteristics are critical to avoid shaft bending under the required operating stresses — shaft material hardness and shaft diameter. The FMEA team may choose to enter a symbol, such as "KPC", into the classification column of the DFMEA next to the bending failure mode, and follow up in the recommended actions with formal KPC designation, and require further controls on both shaft material hardness and shaft diameter in the Process Control Plan.
For more examples of the use of key product and process characteristics in Process Flow Diagram worksheets, Process FMEAs and Process Control Plans, please refer to chapter 5 of Effective FMEAs.
Xfmea provides a number of features that support the application of key product and process characteristics. In this article, we'll show the application of the System/Design FMEA classification column to highlight key product characteristics. In the next issue of HotWire, we'll show the application of key process characteristics in the Process Flow Diagram Worksheet, Process FMEA and Process Control Plan.
The following is an excerpt from a hypothetical Design FMEA on a bicycle brake cable.
Notice the Classification column placed after the Cause column. In this example, the design deficiency is "inadequate cable thickness," and the FMEA team chose to make cable thickness a "KPC." They also added a Recommended Action, "Add cable thickness as KPC, and include in Process Flow Diagram worksheet, for use in Process FMEA and Process Control Plan." By adding this action, the Design FMEA can ensure that the information flows to the correct place in the Process Flow Diagram worksheet, for use in the Process FMEA and Process Control Plan. This placement in the Process Flow Diagram worksheet, Process FMEA and Process Control Plan will be explored further in the next FMEA Corner article.
To enable the Classification column in the worksheet, choose Project > Management > Configurable Settings > Interface Style, then click the FMEA Cause heading in the navigation panel and choose Yes to the Classification property.
The following is an excerpt from SAE J1739, (2009):
As you can see, the use of the Classification column and application of key product characteristics is left up to FMEA teams and individual companies. It is a good idea to discuss this topic within your company, in order to arrive at the best possible procedure to implement key characteristics.
I've always wanted to know about FMEAs
The important thing is not to stop questioning. - Albert Einstein
A reader submitted the following question to Carl Carlson. To submit your own question about any aspect of FMEA theory or application, e-mail Carl at Carl.Carlson@EffectiveFMEAs.com.
I am new to FMEA, and I find your articles on FMEA corner very useful. I am tasked with creating a DFMEA for a "die" used in a die-casting process. Should I consider the potential effects of die breakdown with the loss of manufacturing hours or with respect to the casting quality or with respect to the problem at the customer end, or should I include them all? Also should human errors be considered in DFMEA?
It would be helpful to me if you could give a clear picture of DFMEA.
Carl: I need to ask you a couple of clarifying questions, so that I can answer your question correctly. When you say that you are tasked with creating a DFMEA for a "die" used in die casting process, this could mean one of the following, and it will help to know which it is: A) the die is currently being developed and will be used in a die casting process and you are conducting an FMEA on the die equipment before it is used in the plant; B) the plant is using a die casting process and an FMEA is being done on the casting process using the die; C) a die is currently being used in a die casting process, and an FMEA will be done to support the maintenance and operability of the die. Any insight you can give me on which of these best describes your project, or any other description, will help me answer your question more accurately.
Reader: I am tasked with DFMEA for a new die in development, as well as for modifications of dies currently in use. Can you suggest a methodology and scope to develop a comprehensive FMEA document.
Carl: Since you are new to FMEA, I'll begin my answer with some general suggestions. If you have a copy of my book, Effective FMEAs, the procedure for Design FMEA is well covered in chapters 3, 5 and 6. If you don't have a copy of my book, you can study the FMEA Corner articles, as well as the SAE J1739 standard (for automotive applications) or the corresponding VDA FMEA document for European applications. You can also sign up for and take an FMEA course such as ReliaSoft's D470. There is no shortcut to successful FMEA projects, other than learning the fundamentals of FMEA and the proper procedure.
Regarding your questions: "Should I consider the potential effects of die breakdown with the loss of manufacturing hours or with respect to the casting quality or with respect to the problem at the customer end or should I include them all? Also, should human errors be considered in DFMEA?"
First of all, I want to be sure that you understand that FMEA is a team-based activity and the team is made up of cross-functional subject matter experts. It is not a document that is filled out by one person. So, when you consider the effects of die failure, it is done together with the team.
In a Design FMEA, "Effect" is defined as "the consequence of the failure on the system or end user." Since this is a die design, we would have to look at how the die is being used, its application. The die will be used in a manufacturing plant, which can be considered an "end user." There is also the possibility of the effect on the customer who uses the product that is made by the die. FMEA procedure allows the team to consider multiple effects. The team can consider the effect in manufacturing, as well as the effect on the customer, and should use the worst case. Be sure that the causes in the FMEA are described as die design deficiencies, so that the die design can be improved.
Once you have completed the die DFMEA functions and failure modes, identify the effects of failure (based on the definition of Effect above) for each failure mode. You would discuss the consequences of each die failure mode with the die Design FMEA team to determine the specific consequences on the system or end user. You can consider loss of manufacturing hours, as well as other potential consequences, including at the customer end.
Remember, it is important to get team consensus when establishing the verbiage for the effect(s) in an FMEA. FMEA procedure uses the most serious effect when assessing the severity risk ranking.
About the Author
Carl S. Carlson is a consultant and instructor in the areas of FMEA, reliability program planning and other reliability engineering disciplines. He has 35 years of experience in reliability testing, engineering and management positions, and is currently supporting clients from a wide variety of industries, including clients of HBM Prenscia. Previously, he worked at General Motors, most recently senior manager for the Advanced Reliability Group. His responsibilities included FMEAs for North American operations, developing and implementing advanced reliability methods and managing teams of reliability engineers. Previous to General Motors, he worked as a Research and Development Engineer for Litton Systems, Inertial Navigation Division. Mr. Carlson co-chaired the cross-industry team that developed the commercial FMEA standard (SAE J1739, 2002 version), participated in the development of SAE JA 1000/1 Reliability Program Standard Implementation Guide, served for five years as Vice Chair for the SAE's G-11 Reliability Division and was a four-year member of the Reliability and Maintainability Symposium (RAMS) Advisory Board. He holds a B.S. in Mechanical Engineering from the University of Michigan and completed the 2-course Reliability Engineering sequence from the University of Maryland's Masters in Reliability Engineering program. He is a Senior Member of ASQ and a Certified Reliability Engineer.
Selected material for FMEA Corner articles is excerpted from the book Effective FMEAs, published by John Wiley & Sons, ©2012. Information about the book Effective FMEAs, along with useful FMEA aids, links and checklists can be found on www.effectivefmeas.com. Carl Carlson can be reached at email@example.com.