Is 100% Inspected Lot of Machined Parts requires Cp > 1.67

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#11
If the component is part of a assembly of components which the ultimate customer has placed a Capability requirements on for example the Stack of, then if the component is not capable then you might be committing your customer to 100% inspection on his assembly.
The OP's problem sounds like less capable parts (to the specification) work better. You are right, there may be a stack issue that is causing this, but the customer does not understand it well enough to relate it to an accurate specification. It also appears that they are trying to fix it empirically (just find something that works), rather than technically (find out what works, and specify it correctly).
 
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Bev D

Heretical Statistician
Staff member
Super Moderator
#12
Statistical Tolerancing is a difficult concept for many organizations to implement successfully and even harder to communicate to suppliers. IF this is the case with the OP's dilemma, then they need to have an open dialogue. and it's likely that the OP will have to sort to a tightened tolerance.
 
N

NANDAKUMAR_A

#13
In my view in your case the Cpk value is applicable for Machining process before Heat treatment.

As you are doing 100% inspection it all mean that the Non confirmning product will be filtered.

Furhter he requie the Cpk 1.67 means , he has to revise (squeeze) your tolerence and can try to see that his inward parts will meet Cpk 1.67 for the exisisting tolerence level if it is more critical for the function / assembly .
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#14
In my view in your case the Cpk value is applicable for Machining process before Heat treatment.

As you are doing 100% inspection it all mean that the Non confirming product will be filtered.

Further he require the Cpk 1.67 means , he has to revise (squeeze) your tolerance and can try to see that his inward parts will meet Cpk 1.67 for the existing tolerance level if it is more critical for the function / assembly .
In order to meet the intent of a 1.67 Cpk requirement for a final output, you would have to run the raw stock much tighter than the final specification. However, Cpk should not be applicable because properly controlled machining operations are not normal distributions, and the output is the sum of the variances, which would be the normal variation form heat treating summed with the non-normal distribution of machining - which does not net a normal distribution, either. Again, it is a customer rubber stamping a statistically lazy approach to vendor process control and lack of understanding of incoming product variation. But, it is a common misuse of statistics.
 

rnsvasan

Involved In Discussions
#15
We discussed with the customer and got no logical feedback. However, coming to squeezing the tolerance; we may have to accept as it became customer’s requirement.
On the other hand; my question is; are they not confident is their design engineers and application engineers who had accepted the tolerance for good quality. “Tolerance is that acceptable limit from the ideal dimension”. Why it is not being considered during desing stage?
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#16
We discussed with the customer and got no logical feedback. However, coming to squeezing the tolerance; we may have to accept as it became customer’s requirement.
On the other hand; my question is; are they not confident is their design engineers and application engineers who had accepted the tolerance for good quality. “Tolerance is that acceptable limit from the ideal dimension”. Why it is not being considered during design stage?
From the story you have laid out here in this thread, I would not seek to find logic in the situation. Your customer seems to have no idea what they want, do not wish to find out (maybe they do not have the resources to apply, or the statistical knowledge to figure it out).

“Tolerance is that acceptable limit from the ideal dimension” assuming you know all of the ideal dimensions. Often we do not have that luxury - especially if there are physical interactions. They do not know them, and it sounds like they have given up looking.
 
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