Severity of Effect on Process (Manufacturing/Assembly Effect) Scrap or Reworking

[Carmine]

dear all.
I want just show you a pratical example in evaluating the Severity of an effect of failure on the process.
Immage In an assembly process, I have to assembly some small mechanical components (for example O'ring ,a small screw, plastic components) on a main component the "big part" . This main component goes through the line and position by position station by station the operators add small components to the main big one.
The operator is doing now this operation of assembling at a particular step of my process: assembling this small component on the main big component. This operation fails in meeting the requirements. When the operation fails, my small component loses its integrity so the operator has to take another one and repeat the operation.
So at the end I have the small component scrapped, but the main big one preserves its integrity. The operator can take another one and repeat the operation in station.
Now the question is about evaluating the effect on the process: AIAG 4Th Ed. gives
4 = 100% of production run may have to be reworked in station before it is processed.
8 = 100% of product may have to be scrapped. Line shutdown or stop ship.
Because of I can take another small component and repeat the operation on the main component at station (even the small tiny component is scrapped), I think the right number to give to rapresent the effect on the process is 4 = 100% of production run may have to be reworked in station before it is processed.
Is 4 right?
Last question: when the AIAG talk about product scrapped or production run. Are they reffering to the main component assembled at current station like a steering gear or a pump for example?
This question might seems very simple and banal, but when you have to apply some guideline on many different process like assembly, turning, forming or molding even somenthing that seems banal can be difficult to explain.
Thanks everyone in advance

Carmine

 

[Stijloor]

A Quick Bump!

Can someone help Carmine?

Thank you very much!!

 

[Jen Kirley]

Good day Carmine,

I hope to understand you correctly: the small part fails (an o-ring perhaps) but the unit it is installed in does not have to return to previous assembly steps to replace this o-ring; the o-ring can be replaced at this same station, the same unit with its new o-ring can then move on to further processing.

If that is, in fact your scenario then I agree that as per Table Cr1 on page 88 of the FMEA Manual 4th ed., the Severity can be assigned at 4. A severity of 8 is meant for an issue that may cause the entire unit to be scrapped, or its failure stops production somehow.

As for question #2: we should take care not to cross-apply the severity of effect on end customer and effect on process. Notice that on page 37, in the Design FMEA section of the manual, this same table is referred to for product failure severity classifications. The design part is the left column: the pump does not operate, thus the vehicle does not run. The right side of Cr1 is for effect on process. In this context entire pump may need to be scrapped. "Line shutdown" implies the issue is severe enough to potentially cause more than one unit failure, so the line is stopped in order to resolve the issue and stop the nonconformity.

I hope this helps!

 

[Chennaiite]

Because of I can take another small component and repeat the operation on the main component at station (even the small tiny component is scrapped), I think the right number to give to rapresent the effect on the process is 4 = 100% of production run may have to be reworked in station before it is processed.
Is 4 right?
Carmine

It appears to be 4. However, if this rework means you are losing production quite significantly, I would go for much higher Severity. That's why I normally look into the subjective part of the criteria given in AIAG manual i.e. major/significant/moderate disruption in this case. Irrespective of whether it is scrap or rework, it is imporatnt to understand the damage done by the non-conformity.