Assignation of RPN rating (Detection) on Visual Inspection where subsequent operation

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shallowmike

#11
We are currently involved in doing PFMEA'S and the team has reached a point of dissagrement regarding the setting criteria of the RPN number. Some of our customers use 50, some 55 as their high figure but I'm sure their criteria is different to ours as would be NASA for instance. So my question is how do you set a workable number and what do you need to underpin that decision.

Any help would be great

shalowmike
 
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Jim Wynne

Staff member
Admin
#12
shallowmike said:
We are currently involved in doing PFMEA'S and the team has reached a point of dissagrement regarding the setting criteria of the RPN number. Some of our customers use 50, some 55 as their high figure but I'm sure their criteria is different to ours as would be NASA for instance. So my question is how do you set a workable number and what do you need to underpin that decision.

Any help would be great

shalowmike
If you're talking about setting a "trigger" value for taking improvement actions (e.g., RPNs > x require action), then you shouldn't. Trigger values typically result in RPNs that are just shy of the appointed value. You should be using a combination of relative values and experience to determine when action is needed. Sometimes there might be a cheap fix for a potential failure mode when the RPN is relatively low, and other times there might be an item with a relatively high RPN for which the process has been optimized already and further action will amount to tampering and waste. IMO, trigger values are a way to keep from having to think about what you're doing.
 
S

shallowmike

#13
We have a process where we weld contacts using a resistant weld process. The welds are checked through a sampling rate at determined times by a destructive test. The contacts are welded to a backing material in strip form, exiting the welder and then processed into piece parts through a press. We can't check every weld, as you can imagine we would have no end product. There is no way we can run SPC on the process to give us a validated detection tool. This leaves us with sampling rates and our experience that our processes are robust with not many defects produced. It does however to a degree show the process is out of control. When doing the numbers a high RPN is generated. Because our process is robust would that in itself qualify as a reasonable detection? With regards to severity if a contact becomes detached in the end customer product who determines the severity number, as we are third tier suppliers do we liase with our customer with regards to this number. We have data to support our occurence numbers its the other two factors that are causing discussion. How would an auditor view this? I take your point about not using a trigger number as this could have an influence when doing the fmea. Would an auditor want to see some factual support to assess how you arrived at the number where we would deem necessary to start implementing corrective action. Auditors dont like low or high numbers. What do you think?
 

Jim Wynne

Staff member
Admin
#14
shallowmike said:
We have a process where we weld contacts using a resistant weld process. The welds are checked through a sampling rate at determined times by a destructive test. The contacts are welded to a backing material in strip form, exiting the welder and then processed into piece parts through a press. We can't check every weld, as you can imagine we would have no end product. There is no way we can run SPC on the process to give us a validated detection tool. This leaves us with sampling rates and our experience that our processes are robust with not many defects produced. It does however to a degree show the process is out of control. When doing the numbers a high RPN is generated. Because our process is robust would that in itself qualify as a reasonable detection? With regards to severity if a contact becomes detached in the end customer product who determines the severity number, as we are third tier suppliers do we liase with our customer with regards to this number. We have data to support our occurence numbers its the other two factors that are causing discussion. How would an auditor view this? I take your point about not using a trigger number as this could have an influence when doing the fmea. Would an auditor want to see some factual support to assess how you arrived at the number where we would deem necessary to start implementing corrective action. Auditors dont like low or high numbers. What do you think?
When you recommend actions--and why--in the PFMEA process is a function of experience and common sense. As I suggested earlier, there is no definite connection between RPN values and whether or not something can be prudently improved. I commend you for your desire to have data to support your decisions. When it comes to the Detection value, the criterion is the likelihood that the defect (or defective) in question will be detected A) before value is added, or B) before it is shipped to the customer. You should have historical data to indicate how likely it is that bad product will be shipped, and how often bad material is detected before subsequent operations. From you can estimate the likelihood, and then rate it on the FMEA 1-10 scale.
Severity is another story, and is not without controversy. IMO, job shops should not attempt to divine how a given defect will affect end use of the product. That information should be incorporated in the specifications. You should base the severity rating on how a given defect might affect your operations. If you make the part in accordance with the specifications and it doesn't work in the intended application, you have made a quality part, so it makes sense to let the customer worry about end use and for you to be most concerned with how nonconforming conditions will affect you.
 
S

shallowmike

#16
Thanks JS,

One other point that was raised during team meeting was generic pfmea, if the process was the same a member argued, to speed things up copy over lines for the next pfmea where the product is similar. I thought that was bad practice as although the overall process would remain the same and the same potential failures would be there, surely if you had a welding job with two different materials then the process does not alter how you produce the component but the input to the process has changed which could have an effect on the numbers. The danger is when resources are limited it can be easy when addressing several pfmea's at same time for similar products to be drawn into this trap.

Shallowmike
 

Jim Wynne

Staff member
Admin
#17
shallowmike said:
Thanks JS,

One other point that was raised during team meeting was generic pfmea, if the process was the same a member argued, to speed things up copy over lines for the next pfmea where the product is similar. I thought that was bad practice as although the overall process would remain the same and the same potential failures would be there, surely if you had a welding job with two different materials then the process does not alter how you produce the component but the input to the process has changed which could have an effect on the numbers. The danger is when resources are limited it can be easy when addressing several pfmea's at same time for similar products to be drawn into this trap.

Shallowmike
As with most of the PFMEA, let common sense be your guide. There's nothing basically wrong with "family" or generic documents, but if the reason for using them is laziness rather than pragmatism, then I think you know the answer already. If there's enough of a difference between two or more processes that you're concerned about missing something, then you should probably do separate PFMEAs.
 
M

Mark Paul

#18
let's get back to the original question

As a tier 1 to automotive mfrs., we make valve train components.
I have been trained that visual is an 8 detection rating. period.
I think that is loopy but if that's what detroit say's but then of course the customer is always right.

my contention is that when an operator refers to a router for instruction then they must be blind if they can't follow simple direction. In my mind, if an error is made by a material handler, that's an HR issue and a detection score of 1 or 2 since all tubs are color coded or stripped.

any response to what the # should be for this visual?
 

Helmut Jilling

Auditor / Consultant
#19
schmudy said:
As a tier 1 to automotive mfrs., we make valve train components.
I have been trained that visual is an 8 detection rating. period.
I think that is loopy but if that's what detroit say's but then of course the customer is always right.

my contention is that when an operator refers to a router for instruction then they must be blind if they can't follow simple direction. In my mind, if an error is made by a material handler, that's an HR issue and a detection score of 1 or 2 since all tubs are color coded or stripped.

any response to what the # should be for this visual?

I would say that 7 or 8 is entirely accurate. It is not that an operator can't read an instruction. Everyone knows that visual inspection is not 100% accurate for a variety of reasons. The higher score reflects that risk. An inherent pressure results which stresses mistake-proffing, poka-yokes, automatic sensing, etc. (when feasible) which would reduce the score.

That is the whole point of the exercise. Not to acheve artificially low scores, but to assess where the risks truly are. The same applies to the Occurance and Severity numbers.
 
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