Process FMEA - Need help to clarify O (Occurrence) and D (Detection) rankings

A

astudent

#11
JSW05, about (O), I've not understood a thing.
It would seem that (O) is referred to cause, but why you wrote: "How likely is it that this particular failure (either the mode or the cause) will occur?".
What you mean "either the mode or the cause"?
thank you
 
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Jim Wynne

Staff member
Admin
#12
astudent said:
JSW05, about (O), I've not understood a thing.
It would seem that (O) is referred to cause, but why you wrote: "How likely is it that this particular failure (either the mode or the cause) will occur?".
What you mean "either the mode or the cause"?
thank you
As I said in an earlier post in this thread, it's possible for the cause to occur without producing a defect:
It's possible for a potential cause to occur without resulting in defective output. For example, in a machining operation, failure to change tools at a prescribed interval might be a process failure mode, but it might not result in defects every time it happens.
Thus it's possible for the likelihood of occurence of the cause to be different from the likelihood of occurence of a defect.
 
B

Bill Ryan - 2007

#13
Al Dyer said:
Maybe this doesn't have to be dredged back up but I think this list of Detection ratings is fine for a Machine FMEA. To me there is a distinction (a big one using our methodology) between a machine and process FMEA. We perform MFMEAs mostly in the Design of a machining center or assembly machine and I think Marc's table works well. But in the Process FMEA the Detection rating should be more in tune with capturing defective product.
 
A

astudent

#14
- Let's see if now it's clear.
If in my FMEA I have 1 failure mode and 2 causes, say every time there's the first cause the failure mode happens,
50% of times the second one occurs producind the same failure mode.
In the first case (O) should be referred to the cause, in the second one to the mode (combination of the second cause + the failure mode).
If what I understood is correct we can say that (O) is generally referred to the likelihood of the cause+mode event.

- about (D).
In our PFMEA for each failure mode we have several line items (one for every cause).
So if (D) is associated with the mode, we should have the same value in every row. Isn't it?
 

Jim Wynne

Staff member
Admin
#15
astudent said:
- Let's see if now it's clear.
If in my FMEA I have 1 failure mode and 2 causes, say every time there's the first cause the failure mode happens,
50% of times the second one occurs producind the same failure mode.
In the first case (O) should be referred to the cause, in the second one to the mode (combination of the second cause + the failure mode).
If what I understood is correct we can say that (O) is generally referred to the likelihood of the cause+mode event.
I'm not sure if I understand the question, but the "Occurence" factor should always refer to the likelihood that the defect will occur, whether the defect is in the process or the product. Earlier in this thread we discussed the classification of failure modes as either failures of the process, or manifestations of process failures in the parts (part defects). Whichever method you choose, you should stick with it to avoid confusion.

astudent said:
- about (D).
In our PFMEA for each failure mode we have several line items (one for every cause).So if (D) is associated with the mode, we should have the same value in every row. Isn't it?
The "D" value should be the same for each distinct defect, because how the defect occurs has no bearing on how easy or difficult the defect is to detect before shipment or before additional value is added.
 
B

bgwiehle

#16
JSW05 said:
The "D" value should be the same for each distinct defect, because how the defect occurs has no bearing on how easy or difficult the defect is to detect before shipment or before additional value is added.
The Severity (of the defect) is the same regardless of cause, but Detection is not -- that depends on the control applied.

Per the FMEA manual, controls may detect either the cause or the defect and are rated depending on how effective they are (visual, gauging, poka-yokes).

B.G. Wiehle
 

Jim Wynne

Staff member
Admin
#17
bgwiehle said:
The Severity (of the defect) is the same regardless of cause, but Detection is not -- that depends on the control applied.

Per the FMEA manual, controls may detect either the cause or the defect and are rated depending on how effective they are (visual, gauging, poka-yokes).

B.G. Wiehle
The question was, if I understood it correctly, "If there are two different potential causes for the same defect (in the same operation), should the detection values be different?" The answer is "no" because there's no reason to assume that there would be different controls for the same defect in the same operation. If what you're saying is that there might be different detection values for the cause than for the result, then I agree, but I also said that it's not a good idea to mix those things up in the PFMEA, and this sort of confusion is the reason for that advice.
 
B

bgwiehle

#18
JSW05 said:
The question was, if I understood it correctly, "If there are two different potential causes for the same defect (in the same operation), should the detection values be different?" The answer is "no" because there's no reason to assume that there would be different controls for the same defect in the same operation. If what you're saying is that there might be different detection values for the cause than for the result, then I agree, but I also said that it's not a good idea to mix those things up in the PFMEA, and this sort of confusion is the reason for that advice.
Here's an example:
A part can be built with the wrong components. The cause could be due to wrong set-up and controlled with set-up verification (First-off).

However, it might be caused by random mixed components. Different occurrance and different detection (unless there are poka-yokes in place that will detect wrong set-up as well as the random defect).

B.G. Wiehle
 

Jim Wynne

Staff member
Admin
#19
bgwiehle said:
Here's an example:
A part can be built with the wrong components. The cause could be due to wrong set-up and controlled with set-up verification (First-off).
So in this case, is the "detection" value is based on likelihood of detecting "wrong setup" or "wrong components"? If the former, you're using inspection (wrong components) to detect the bad setup. How is that materially different from just using detection of wrong components for the detection value?

bgwiehle said:
However, it might be caused by random mixed components. Different occurrance and different detection (unless there are poka-yokes in place that will detect wrong set-up as well as the random defect).
But you're still using the discovery of bad output to tell you something's wrong with the process, even if there's a poke-a-yoke control involved.
 
R

RAHarpster

#20
Occurrence and Detection Ratings

Your questions are very good ones. Let me deal with each one individually.

Question #1:
The Occurrence (O) is the frequency of failure mode or the frequency of failure cause?

Answer #1:
The occurrence rating is the probability that the the failure mode will occur due to the cause. In order to calculate this rating, you must multiply the probability of the cause occurring times the probability of the failure mode occuring when you assume the cause is present. This may sound a little complicated so let me explain. If you ask most companies if they ever run with worn tooling, they would say yes. If you then ask them if the worn tooling always results in a defect or failure mode, they will say no. Consequently, if you want to know the true probablity of the defect or failure mode happening due to a particular cause, you must perform the calculation above.

Unfortunately, in 24 years of doing Process FMEAs I have never seen any company who has the required data. The lack of this data leads to countless arguments that are a tremendous waste of time. There is good news in that there is a way to overcome this problem and I will begin to provide the solution by answering your second question.

Question #2:
If the O) rating is for the frequency of failure casue, how can we use the statistic data to count it because normally, we monitor ppm or % defect for only failure mode.

Answer #2:
As described in the answer to Question #1, the occurrence is the probability of the failure mode due to the cause. The ppm or %defect data that you have represents the probability of the failure mode due to all of the causes together. Although this is not what the Process FMEA is asking for, you can use the ppm and %defect data that you have to establish a maximum occurrence rating for any cause linked to the failure mode.

Question #3:
Also, for the Detection (D), this is for detection of failure mode or detection of failure cause?

Answer #3: It depends on the type of control that you are talking about. If it is a prevention control, it is about detection of the cause. If it is a detection control, it is about detection of the failure mode.

Here in lies the problem with most of the literature published about Process FMEAs today. They tell you to concentrate on Type 1 controls or prevention controls. However, when you look at their detection rating tables they only talk about detecting failure modes. They do not tell you how to rate prevention controls. Some books go as far as telling you that if you do not have a detection control you must put a 10 in the detection rating column. This simply does not make sense if you take the time to consider that if you ever created the perfect process were all sources of variation were controlled through prevention controls, you would never inspect the product. However if you follow the majority of the current Process FMEA publications in print, you would have to put a 10 on every row thus sending your RPNs through the roof.

I want to congratulate you on your excellent questions. I have seen literally hundreds of people struggle with them over the years. There is way too much time wasted when people try to assign ratings to their Process FMEAs.

Rich Harpster
Harpco Systems, Inc.
 
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