Helmut Jilling
Auditor / Consultant
Hi all,
I'm new to this site, and forums in general, but feel lucky to have come across this wealth of information. I've been involved with and facilitated FMEAs in the past, and my current employer has me looking at their pFMEA methods to extract more value and to streamline the process (pFMEA). I need to make this tool practical and valuable for the product/process development efforts I'm involved in.
This thread hits on an item that continues to nag me as I try to clarify and standardize certain aspects of our pFMEAs. I'd appreciate any help to get to a workable solution. I hope you'll forgive my wordiness here to try to express my complete thoughts.
It seems that failure mode definition is very key in getting the pFMEA started in the right direction. For a dFMEA, I think failure mode definition is more clear than for a pFMEA. A pFMEA has a potential dual path for FM definition: 1) a symptom/defect on the product or 2), failure of a process step to perform its intended function.
If I choose #1 (a symptom/defect on the product), I seem to be more in line with how most pFMEAs are done (is this true)?
Possible Benefits of #1 Approach:
> Failure mode generation seems more straight forward (if requirements are well defined) basing them on the ways in which the built product can vary from the product requirements - dimensional, visual, design..
> FM at the product makes Effect and SEV easier to tie to critical design characteristics and customer impacts.
> Causes in the process can be expressed at a higher level to help with efficiency of the pFMEA, but still identify areas of concern that may require further root cause investigation.
Potential Problems with #1:
> It seems to me, this approach makes identifying internal effects and SEV meaningless. Since the severity assessment is supposed to be based on the failure, without consideration of frequency or probability of occurance, I can't see how to tie a non-conforming product into a quantifyable internal impact. For instance, often I've seen generic process impacts of scrap, downtime, rework in these types of FMEAs. Sure, but in order to assign a SEV ranking to these, I need to understand the nature of the cause, or failure in the process, to determine how much - scrap, downtime, etc.
> I saw in an earlier post, and agree, that working from a product based symptom to a process cause can miss failure potentials in the process because the focus is not on the process based requirements/functions, but on the product based requirements.
#2 FM defined as failure of a process step to perform its intended function.
Possible Benefits:
> This method focuses on the process requirements (such as - hold energy at XmJ +/- YmJ; clamp part; spin part at Xrpm, etc), which need to stem from the product requirements, and can capture more ways in which the process can fail by focusing on how the process requirements could not be met.
> The causes for these FMs are closer to, or are, the root causes of the problems, getting the team closer to proper mitigation action.
> The internal effects can be assessed at the failure mode stage of the analysis because the nature of the process failure is more clear. Also, the external effect can still be assessed. Note: I think a tie to the external is always needed to put failures into context so appropriate actions and priorities can be set. All product requirements are not equal - some can kill if not met, others make the product less pretty.
Possible Problems:
> This seems to me a deeper level FMEA. We have moved down one notch in the cause-effect; or effect-cause in this case, hierarchy by choosing process failure - which is the cause of the product defects - as our FM. A deeper FMEA requires more up front time, better understanding of product/process relationships, better defined steps and functions, more specific process SME involvement - maybe not bad things? But this may not be as familiar and will take more time - which management won't like, (but I can sell it if it's the right thing to do).
Lastly, I've seen FMEAs where both #1 and #2 types of failure modes are used which would be very confusing - one FM is a defect and another FM can cause the same defect.
I hope you can see how this has been bothering me, and that I've given it some significant thought. I believe that I need to choose one path here to help avoid some of the pitfalls and get a more functional system Please send your thoughts and recommendations. Thank You!!
Welcom to the Elsmar Cove. You might be overthinking this. It may be helpful to read the sections by the Tables again.
For Severity, you are to consider both columns, and select the higher rating. For example, a missing mounting bracket thread would show a moderate Severity on the Assembly line, but nothing on the end user. But a defective sensor on an airbag could score low on the assembly line, but would be very severe to the end user.
Frequency and Amounts would factor into Occurance. Volume of something is a little less clear. You would need to make a judgement as to whether it would factor into Occurance or Severity.