<<< Back to Index
This Month's Article >>>

FMEA Corner 
This Month's Theme is Using Process FMEAs to Identify Key Process Characteristics
Next month's theme will be "to scribe or not to scribe"

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

 

 

Note: This article is complementary to FMEA Corner Issue 196 "Key Product Characteristics" and Issue 179 "Using Process FMEAs to Improve Process Control Plans."

 
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 process characteristic?

From chapter 5 of Effective FMEAs:

Process Characteristics are process variables and parameters that have a cause and effect relationship with the variation found in product characteristics. Examples include mold temperature, cycle time, pressure, flow rate, tool speed, etc.

Are some process characteristics more important than others?

Yes, certain process characteristics are more important than others, and have a disproportionate influence on product safety and performance. Identifying the most important characteristics is an essential step in controlling them, and achieving safe and reliable products.

Although there is no universal standard defining process characteristics, many companies identify significant and key process characteristics.

What are significant and key process characteristics?

Two definitions are relevant to the application of process characteristics in FMEAs:

Significant Process Characteristics are unique process-related characteristics that can affect the ability of the manufacturing process to meet significant product characteristics. They are input to a given manufacturing operation. They may or may not be designated as Key Process Characteristics.

Key Process Characteristics  are a subset of the significant process 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 process characteristics identified in FMEAs?

One of the objectives of a Process FMEA is to identify significant or key process 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 Process FMEA team identifies significant or key process characteristics. To understand this question, we have to begin with the definition of "cause" in a Process FMEA.

By definition, a cause is the specific reason for the failure, preferably found by asking "why" until the root cause is determined. For Process FMEAs, the cause is the manufacturing or assembly deficiency (or source of variation) that results in the failure mode. Think about that for a minute. What is a process deficiency? Deficient means insufficient or inadequate. A process deficiency is something inadequate or insufficient about the manufacturing or assembly process. This can often (not always) be related to a process characteristic: process variables and parameters that have a cause and effect relationship with the variation found in product characteristics.

If the Process FMEA team properly identifies causes as process deficiencies, and if the risk associated with the process deficiency is sufficient, the associated process characteristic can be a candidate for being designated as a significant or key process characteristic.

So, what makes a process characteristic "key"?

Designating a selected process 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 FMEA. In others, it is related to a combination of severity and occurrence ratings. In still others, it is left up to the process team, without objective criteria. There is no universal standard that defines the criteria for designating key process characteristics for all industries.

How are key process characteristics used in FMEA?

In order to understand how key process characteristics are used, we need to take up the subject of Process Flow Diagrams and Process Control Plans. The following paragraphs are from chapter 5 of Effective FMEAs.

A Process Flow Diagram is a graphical representation of all of the process operations that result in the manufactured or assembled product, and are within the scope of the Process FMEA project. This is essentially the process hierarchy, including manufacturing and assembly operations, shipping, incoming parts, transporting of materials, storage, conveyors, tool maintenance, and labeling, and any other steps of the operations that are within the scope of the Process FMEA. Each of the process operations is represented by a symbol representing the type of operation, such as Fab, Move, Store, Get, Inspect, Rework, Scrap or Contain, and the symbols are connected in the precise sequence of the operations in the manufacturing or assembly process. Use of such symbols is easily tailored to company needs and policy.

A Process Control Plan (PCP) is a "summary description" of the methods used in the manufacturing environment to minimize variation and control product and process characteristics in order to ensure capability and stability of the manufacturing process. It is a structured approach for the design, selection and implementation of control methods, and reactions to problems with the manufacturing and assembly operations when they do occur.

One of the most useful worksheets that pulls together key product and process characteristics and connects them to the process steps is called a Process Flow Diagram Worksheet. In the next couple of sections, we’ll provide an example of the application of key process characteristics, and show how to use Xfmea to organize and display the important information relating to key characteristics and map them to the Process FMEA and Process Control Plan.

Example of application of key product and process 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.

The Process FMEA team will fill out the Process Flow Diagram Worksheet, including significant/key product and process characteristics (see Xfmea example below). They will proceed to the Process FMEA and identify key process characteristics. In the example of shaft material hardness, temperature of water quench and duration of water quench may be key process characteristics, as they affect the ability of the manufacturing process to meet the required shaft material hardness.

In this example, both the key product characteristic (shaft material hardness) and the corresponding key process characteristics (temperature and duration of water quench) are entered in the Process Control Plan, along with other information that assures the manufacturing process is stable and capable.

Using Xfmea to manage key characteristics

In this section, we’ll explore how to use Xfmea to manage key characteristics in Process FMEAs, using a fictitious example from the bicycle front wheel subassembly.

The Front Wheel Design FMEA identified a key product characteristic, "correct number of wheel spokes," when analyzing the failure mode "bent wheel."  The team believes that insufficient number of spokes is a problem potentially leading to a bent wheel, and designated "correct number of wheel spokes" as a KPC.

Process Step 1.2.6, "Orient and place wheel spokes in wheel assembly fixture," is one of the operations in assembly of the front wheel.

System Hierarchy

Select the step in the System panel and choose Analyses > Other > Add PFD Worksheet. In this example, only operation number 1.2.6 is shown. Note that the Significant Process Characteristics column can be filled out at this time, if the information is available. If the info is not available, the team can proceed to the Process FMEA in order to identify any significant process characteristics, and back fill to the PFD Worksheet.

PFD Worksheet

The Significant Product Characteristics (outputs) from the PFD Worksheet should be considered input to the description of the corresponding failure mode of the Process FMEA. The Significant Process Characteristics (inputs) from the PFD Worksheet should be considered input to the description of the corresponding cause of the Process FMEA.

Select the step in the System panel and choose Analyses > FMEA > Add FMEA. The Process FMEA team proceeds to analyze the assembly process.

FMEA Worksheet

When the team has completed the Process FMEA, they can generate a Process Control Plan. Select the step in the System panel and choose Analyses > Other > Add Control Plan.

Control Plan

The Process Control Plan contains the key product and process characteristics and the control methods from the Process FMEA. The team can fill out the rest of the Process Control Plan in order to control the operations.

!FMEA Tip

One of the important inputs to a Process FMEA is key product characteristics that are associated with the item being manufactured or assembled. The Process FMEA team should ensure that the Design FMEA team provides this information, before proceeding.

?Something 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.

My question has to do with how to use concept FMEAs. We are more experienced with Design FMEA.

We are currently redesigning a complete packaging line from scratch. We are frequently selecting different architecture, design, and technology, and I’m supporting the concept selection decisions with FMEAs.

My concern is that we are making so many changes in a short time, that there’s no time (and no reason) to fully complete the FMEA.

Obviously the new changes are made consciously avoiding highlighted effects discuss during review meetings. How would you highlight such changes and justify among different designs that we are going in the right direction?

Carl: Great question about Concept FMEAs. Let me begin my answer with an excerpt from my book.

A Concept FMEA is a short version of FMEA to aid in selecting optimum concept alternatives or to determine changes to system design specifications. It increases the likelihood that potential failure modes and resulting effects of a proposed concept are considered before the final concept is determined and actual design work proceeds. An example of this is analyzing the risk of failures of each of the new all-terrain bicycle concept alternatives, in support of the decision to select the optimum concept that is most reliable and will meet program objectives. The Concept FMEA also identifies system level testing requirements and helps to determine if hardware system redundancy may be required within a design proposal.

There is no standard for the exact procedure for a Concept FMEA. It depends on the information that is available at the time, and which of the columns of the FMEA are most relevant to the concept selection decision. You can start with the following for the concepts that are being considered:

The following are a couple of tips to save time when doing Concept FMEAs. If you have many functions, you can prioritize the functions, so you focus on the most critical ones. If you have a multitude of concepts being considered, you can limit Concept FMEAs to the concepts that are under most serious consideration, such as the final 3 or 4 candidates.

The "Concerns" column is based on the risk associated with the concept. It provides a good input to the concept selection decision.

Once the concept has been determined, you can continue the FMEA to completion, including Controls, Detection Ranking, RPNs, and Recommended Actions.

I’ve attached an example. In this example, you do not have to fill out the Controls, Detection and RPN columns.

Concept FMEA


About the Author

Carl S. CarlsonCarl S. Carlson is a consultant and instructor in the areas of FMEA, reliability program planning and other reliability engineering disciplines. He has 30 years of experience in reliability testing, engineering and management positions, and is currently supporting clients of ReliaSoft Corporation with reliability and FMEA training and consulting. Previous to ReliaSoft, 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.

Effective FMEAsMaterial for the FMEA tips, problems and solutions 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.

 
ReliaSoft