PFMEA Severity - What is Process FMEA Severity estimation based on?

  • Thread starter Matthew_Hopkins
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Matthew_Hopkins

#1
Dear all,
When performing a PFMEA would you recommend to base the severity estimation on the product non-conformance (as potentialy recieved by customer) or effect on the process (the "internal" severity of the non-conformance, i.e. re-work/scrap/production stop etc.)?

In my opinion it's difficult and inappropriate to merge the severity estimation for the product and process effects in the same sentence and number. In addition, the effect on the process will only occur if the product non-conformace is actually detected. Is it just me being confused?

//MH
 

Jim Wynne

Super Moderator
#2
Re: Severity estimation in a PFMEA is a bit confusing

Dear all,
When performing a PFMEA would you recommend to base the severity estimation on the product non-conformance (as potentialy recieved by customer) or effect on the process (the "internal" severity of the non-conformance, i.e. re-work/scrap/production stop etc.)?

In my opinion it's difficult and inappropriate to merge the severity estimation for the product and process effects in the same sentence and number. In addition, the effect on the process will only occur if the product non-conformace is actually detected. Is it just me being confused?

//MH
My own opinion is that process FMEAs should be concentrated on processes, not products. There will be disagreement in this regard, but to me, it's fairly simple. If a customer has done a DFMEA, and the output of that process is reflected in the product specifications, and the product is manufactured in accordance with the specifications, everyone should be happy, all else being as it should be. This means that I, as the producer of the product, should be able to concentrate on designing a process that will produce product that meets the specifications, and I shouldn't need to concern myself with what might blow up in end use if the product doesn't conform--that's the designer's responsibility.

So to answer your question, I think that the PFMEA Severity factor should be based on the risks of the producer, not the customer.
 
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Matthew_Hopkins

#3
Re: Severity estimation in a PFMEA is a bit confusing

Thanks for your prompt reply Jim.
It's the disagreement that bothers me, have yet to choose a side. I'm working for a medical device company where patient safety has to be the main quality concern. We're asked to perform the PFMEA with focus on the risks of "manufacturing products out of spec." To me its sounds like I, as a producer, need to know how critical each specification item are and base severity levels on that knowledge.

In your opinion, the severity level of a process failure resulting in misplaced labels will be higher than a process failure resulting in unsterile products, if the re-work effort/scrap amount/production delay etc. are worse for the cosmetic defect?
 
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Duke Okes

#4
Re: Severity estimation in a PFMEA is confusing - What is Severity estimation based o

Severity for a pFMEA should focus on impact on customer/user. Matter of fact the rankings for Severity in pFMEA should come from the dFMEA. You could also include effect on manufacturing, but if you do that you'll either need to have two Severity numbers (which will cause the FMEA to expand phenomenally), or you'll use the higher of the two (e.g., effect on customer vs. effect on process).
 
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tac123

#5
Re: Severity estimation in a PFMEA is confusing - What is Severity estimation based o

Dear all,
When performing a PFMEA would you recommend to base the severity estimation on the product non-conformance (as potentialy recieved by customer) or effect on the process (the "internal" severity of the non-conformance, i.e. re-work/scrap/production stop etc.)?

In my opinion it's difficult and inappropriate to merge the severity estimation for the product and process effects in the same sentence and number. In addition, the effect on the process will only occur if the product non-conformace is actually detected. Is it just me being confused?

//MH
I am not sure why you would consider it difficult or inappropriate to merge the severity estimation. When considering effects you should consider all customers. Customer could include end user, downstream operation etc... List all potential effects and use the worst case (highest) severity of the effects listed as the number in the severity column.

In most cases the severity of a end user effect is greater than that of an internal effect.

Remember that effects are what could happen if you don't have process controls. So potentially all failure modes "could" get to the customer.

I strongly disagree with Jim. Design FMEAs focus and design issues not on problems that could occur in the manufacturing process.
 

Jim Wynne

Super Moderator
#6
Re: Severity estimation in a PFMEA is a bit confusing

Thanks for your prompt reply Jim.
It's the disagreement that bothers me, have yet to choose a side. I'm working for a medical device company where patient safety has to be the main quality concern. We're asked to perform the PFMEA with focus on the risks of "manufacturing products out of spec." To me its sounds like I, as a producer, need to know how critical each specification item are and base severity levels on that knowledge.

In your opinion, the severity level of a process failure resulting in misplaced labels will be higher than a process failure resulting in unsterile products, if the re-work effort/scrap amount/production delay etc. are worse for the cosmetic defect?
I knew there would be disagreement. The problem with the other responses thus far is that people are reading the AIAG/SAE manual as if it were sacred scripture, and it's far from it. True enough, if you have automotive customers who demand that you go according to the book, you're stuck. But if you don't, you can do what makes sense for you.

I should have stipulated in my original post that I was speaking from the perspective of a job shop or contract manufacturer, and not a captive operation where a DFMEA document and its team might be accessible.
 
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Matthew_Hopkins

#7
Re: Severity estimation in a PFMEA is confusing - What is Severity estimation based o

Thanks. Nice to have the other standpoint represented as well.

Even though I would have a hard time "ignoring" the end user in a PFMEA, I still recognize and understand Jims point of view as well. For example, I don't believe many of our component suppliers have an idea about the DFMEA of our final product. I wouldn't expect all of them to have the risks of using our product as input to their PFMEA, yet their manufacturing processes is contributing to considerable risks to our end users.

tac123, I like your way of looking at downstream operation as one of my customers and I will probably end up estimating severity of both "internal" and "external" consequences in one way or another. It just felt like a confusion of ideas when I was about to combine this with detect abilitity. An additional process control would reduce the "external" customer RPN but somehow increase the "internal" RPN since the rejection/re-work/lead time increases....
 

Jim Wynne

Super Moderator
#8
Re: Severity estimation in a PFMEA is confusing - What is Severity estimation based o

Severity for a pFMEA should focus on impact on customer/user. Matter of fact the rankings for Severity in pFMEA should come from the dFMEA.
It depends on the circumstances. In job shops, it's unlikely that a customer DFMEA will ever be available, and there's no point in guessing if it's not. I'll say it again: process FMEAs should focus on potential process failures. It doesn't make any sense to do it any other way.
 

Jim Wynne

Super Moderator
#9
Re: Severity estimation in a PFMEA is confusing - What is Severity estimation based o

I strongly disagree with Jim. Design FMEAs focus and design issues not on problems that could occur in the manufacturing process.
One of the outputs of the design process is specifications. If the specifications reflect end-use requirements, there is no need to consider end-use requirements per se in production. If we make it like the drawing says, it will have met design intent. If the specifications don't include enough information, there's a different problem that won't be helped by a PFMEA.
 

Helmut Jilling

Auditor / Consultant
#10
Re: Severity estimation in a PFMEA is confusing - What is Severity estimation based o

It depends on the circumstances. In job shops, it's unlikely that a customer DFMEA will ever be available, and there's no point in guessing if it's not. I'll say it again: process FMEAs should focus on potential process failures. It doesn't make any sense to do it any other way.

I agree that it focuses on processing problems, but also the effect of that processing problem on the resulting product.

The poster should use the Severity table in the FMEA book as a guide.

For example, a dull stamping die could leave a sharp burr. That burr could have a significant impact on either the assembly plant or possibly the end user, when that part is assembled on the vehicle. So, it would get a fairly high Severity number.
 

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