FMEA – Determine the scope of the FMEA (FMECA) - Step 2
Having formed the FMEA team you now need to determine the scope of the FMEA.
Introduction
The scope of the FMEA describes, very precisely, what will and what will not be included in this FMEA. That’s not to say that items or interactions that are excluded will not be subject to an FMEA but just not in this one.
Defining the scope of the FMEA uses a tool called a boundary diagram. This is a simple tool based upon a block diagram and is explained later in this article.
This activity requires the core team to contribute as consideration will need to be given to not only the target item (the item that is the target of the FMEA) but also any direct or indirect interactions with other components, sub-assemblies or systems.
Constructing the Boundary Diagram
The Boundary diagram is constructed in three phases: -
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Draw a block diagram of the item and it’s directly connecting items
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Determine the interactions between the target item and all connecting parts
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Agree the boundary to be considered in this FMEA
For the purposes of this article I will use a the design of spark plugto demonstrate how to construct the Boundary Diagram.
1. Draw the Block Diagram
The first step is to draw a block diagram including all of the directly connecting parts, sub-assemblies or systems.
I have used a generic example of a spark plug assembly to demonstrate how this might be done.
In this example I have simplified the blocks to show the main components that have a functional relationship to the spark plug assembly.
2. Determine the interactions
The second step requires the FMEA Team to consider the relationships between all the items in the block diagram.
In this example I have suggested some linkages.
E.g. The relationship between the plug assembly and the gasket ring is the retaining fit, the relationship between the spark plug assembly and the fuel mixture is the spark gap in the correct position.
I have kept this diagram fairly simple for demonstration purposes. In most cases at this point we would also consider the robustness issues as well.
Robustness - introduced by Genichi Taguchi and refers to the operation of a component, sub-assembly or system in the presence of the operating environment and all of the internal and external variations.
3. Agree the Boundary of this FMEA
Having agreed the block diagram and the connections or linkages between each of the blocks, the team now determine the scope of the FMEA. The scope of a design FMEA (DFMEA) is restricted to:-
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Failures attributed to design only (i.e. does not include manufacturing , fitting or other induced errors unless through design issues) E.g. If a spark plug was fitted but cross threaded, this would not be considered unless the relationship between the cylinder head and the fitting tool induced the error.
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Analysis of items within the boundary diagram
In this example the boundary diagram shows that this FMEA will include the spark plug assembly, the provision for retaining the spark plug connector, the provision for retaining the retained gasket ring, the retained gasket itself, positioning of the spark gap and provision of a hexagonal shape for the fitting and removal tools. However we won’t include the plug removal tool, spark plug connector, fuel mixture or the cylinder head in this FMEA.
What the boundary diagram does for the FMEA Team is to provide a focus for the analysis. This does not mean that all the other linkages and interactions are not subjected to analysis, just not in this instance.
FMEA is like a jig-saw puzzle. Each FMEA fits into the jig-saw puzzle to build the complete picture of the design
Using the Boundary Diagram
Once completed, the boundary diagram becomes a visual aid for the FMEA team to use at each meeting to ensure that they are focussed on the agreed scope of the FMEA.
FMEA Template
My previous article discussed forming the FMEA Team and included a free FMEA Template (FMEA Form). If you would like a copy please click on the link at the begining of this paragraph.
Graham Cripps
Results Consortium Ltd