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Boundary Diagram – How to construct an FMEA boundary diagram.

Posted by Graham Cripps on Mon, Sep 23, 2013 @ 01:00 PM

Boundary Diagram - How to construct an FMEA boundary diagram


"A boundary diagram provides a focus for the FMEA Team".

FMEA Boundary diagram - step 1

Constructing a boundary diagram for an FMEA is a relatively simple task.

However, in my experience so many people get it wrong. This can have a major impact on the complexity and quality output of the FMEA.

 

Let me explain why and for this I will use Design FMEA as an example.

Photo Copier

In an organisation, the FMEA’s conducted all form part of a large jigsawpuzzle. For example, if we were to consider a copier machine then to carry out a Design FMEA on the whole product would be a daunting, if not impossible task. At a macro level, a photo copier has a number of sub-systems that are made up of sub-assemblies that have individual controls and management protocols.

So, we need to be able to manage our FMEA activity in a way that considers a number of factors including: -

  • Customer critical functions
  • Reliability
  • System and sub-system levels
  • Critical systems, sub-systems and components

 

FMEA Jigsaw

 

As stated in other articles, the target FMEA then forms just one small part of the FMEA puzzle.

The FMEA boundary (what will be included and will not be included in this FMEA) describes the size and shape of this piece of the puzzle and will have an impact on the boundary of subsequent FMEA’s

  

There are some key foundations for a successful FMEA:

Adopt a team approach - the skills, knowledge and experience in the design team is unlikely to rest with one person. Also part of the process is to challenge and test all assumptions which need more than one person. Most FMEA's require expertise from multiple disciplines

Determine the prime functions that are to be the target of the FMEA - use of a Function Tree can help to identify these 

Clearly understand and describe the prime functions of the design. The whole purpose of an FMEA is to establish what could go wrong (design does not provide the intended function) and ensure that the risk of failure is minimised by having controls in place to prevent or detetct the failure mechanism

Determine the FMEA scope (use the boundary diagram). Keeping within the scope of each FMEA is key to getting good quality outcomes from the FMEA process
Be clear about what is a failure mode, an effect of the failure and a cause of the failure by:

Establishing what could possibly go wrong, the failure mode (potential failure modes are limited)
Establishing the effects of the failure of the design to perform the intended functions (use the effects list)
Determine the causes of potential failure modes

 

The constructing of the boundary diagram is a critical step in the management of the FMEA effort.

Constructing a Robust Boundary Diagram

In previous articles I have explained how to construct a boundary diagram. I want to now look at some of the considerations and understanding that underpin boundary diagram construction in our effort to achieve robust FMEA’s.

Identify the subject of the FMEA – make sure that the team clearly understand the subject of the FMEA, describing the system, sub-system, assembly or component.

Example: If the FMEA is to be carried out on a car braking system, make sure you determine each sub-assembly and concentrate on just one at a time. These sub-assemblies could include:-

  • Master cylinder sub-assembly
  • Front disc brake assembly
  • Slave cylinder sub-assembly
  • Hydraulic actuation assembly (including delivery and reservoir sub-system
  • Rear disc brake assembly
  • Hand brake sub-assembly
  • Sensor system (various depending upon complexity)

Understand the Robustness linkages – in considering our product design, we will have considered how it will perform in the hands of the customer and in all of the environments that it is likely to be subjected to. Remember that there are unintended functions that the customer will subject the product to as well.

In understanding the robustness linkages we also need to understand the influence of neighbouring systems or sub-systems that can also have a direct impact upon the operating environment of the target of the FMEA.

Example:  a car steering wheel has primary functions that generically include: providing housing for air bag, horn etc.; provide mechanical advantage (turning motion); provide comfortable grip; facilitate access to stick controls. The steering wheel is also used to get into and out of the car by some customers (unintended function) – we need to design for this.

Establish the interactions – how are the various components related to each other, directly or indirectly? Are they bolted together, welded, fixed to the same substrate, connected electrically, etc.?

The Process

If we now look at the process for constructing the Boundary Diagram, we can then start to think about the tools and techniques available to us to support this activity.

From previous articles the process was described as: 

  • Draw a block diagram 
  • Determine the interactions 
  • Establish and agree of this FMEA

For the purposes of this article I will use a the design of an automotive spark plug to demonstrate how to construct the Boundary Diagram

Step 1 - Draw the Block Diagram  

FMEA Boundary Diagram - Step 1

 

The first step is to draw a block diagram including all of the directly connecting parts, sub-assemblies or systems.  

 

 

 

 

Step 2 - Determine the interactions

The second step requires the FMEA Team to consider the relationships between all the items in the block diagram.

FMEA Boundary Diagram - Step 2

 

In this example I have suggested some linkages.

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. These variations are often refered to as noise.

Step 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:-

  • 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.
 

Boundary Diagram - Step 3

 

The completed 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 plug fitting and removal

  

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.

Perhaps simple is not the right word, but this is not a difficult concept. 

So, the inputs to the construction of the boundary diagram are:

  • Clear description of the subject of the FMEA
  • Robustness linkages
  • Design brief including prime functions (a function tree would be very useful)
  • Multi-disciplined team with the appropriate level of training in FMEA to be able to contribute

For further reading to see where the block diagram fits in the FMEA process go to Articles

 

Graham CrippsGraham Cripps - Director

Results Consortium Ltd

www.resultsresults.co.uk

Topics: Design FMEA, FMECA, DFMEA, Boundary Diagram, How to construct a boundary diagram

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