Relating Cpk to Sampling Plans - AS9100C Para 8.2.4 & AS9103 Para 4.4 d.

R

rob-dog64

#1
Hello Forum
This is my first post.

I'm trying to understand the requirements for applying sample inspection in lieu of 100% inspection for aerospace components.

AS9100C Para 8.2.4 States
"When the organisation uses sampling inspection as a means of product acceptance, the sampling plan shall be justified on the basis of recognised statistical principles and appropriate for use (i.e. matching the sampling plan to the criticality of the product and to the process capability).

and

AS9103 para 4.4 d. states
"When process capability is used to justify reduced frequency of inspection, the process capability (probability of nonconformance) shall be determined using recognized industry statistical methods."

Both of these standard requirements apply.

My question is how, in general terms, do I match a sampling plan for a process with a Cpk of 1.33 or greater. Please note the criticality of the product is best described as low.

Thanks Rob
:confused:
 
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Bev D

Heretical Statistician
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#2
As a start: If the Cpk is truly > 1.33 then you would have a very low defect rate - in real terms probably zero until something changes. If the criticality is low then sample inspection provides a very low 'risk'. (very high severity characteristics are difficult to justify for sampling even with high Cpk values due to the severity of even one defect. Typically one must resort to mistake proofing)

Sample size will be the biggest issue and is usually dependent on the defect rate that is clearly unacceptable to ship (known as the RQL or LTPD) and the defect rate that is 'acceptable' to ship.

When your customer wants zero defects shipped and you have high capability you can utilize SPC to identify when a significant change in quality has occurred and invoke 100% inspection for the effected material until the assignable cause has been corrected then resume some sampling plan. This plan should have a sampling frequency that monitors the process at all process, material and other change points.

Others will no doubt add to this...
 
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