Sampling Plan for Incoming Raw Material
A receiving inspection is performed on a lot of 500 identical machined brackets. The inspection level is II, AQL 1.0%, normal inspection, single sampling. The inspector uses ANSI/ASQ Z1.4 (ISO 2859-1) tables. The lot size is 500, so the sample size code letter is 'H'. The sample size for code letter H is 50. The acceptance number (Ac) is 1 and the rejection number (Re) is 2. The inspector randomly selects 50 brackets and inspects them for a critical dimension. Two brackets are found to be nonconforming. The inspector must decide whether to accept or reject the lot and what actions to take. The inspector also notes that one of the nonconforming brackets has a dimension that is only slightly out of tolerance, while the other is significantly out. The inspector is tempted to accept the lot because the nonconformities are minor.
1. Determine the lot acceptance decision based on the sampling plan. Explain the reasoning and the implications of the decision.
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For sample size 50, Ac=1 and Re=2. Since 2 nonconforming units were found, which is equal to the rejection number, the lot must be rejected. The decision is based on the number of nonconforming units, not the severity of the nonconformity. Even though one nonconformity is slight, the sampling plan does not differentiate; any nonconforming unit counts. Therefore, the lot is rejected. The inspector should not accept the lot based on subjective judgment. The proper action is to reject the lot and initiate disposition procedures, such as 100% inspection, sorting, or return to supplier.
2. If the inspector decides to perform 100% inspection of the lot after rejection, what are the considerations and potential pitfalls?
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100% inspection can be effective in removing nonconforming items, but it is subject to human error, fatigue, and the limitations of the inspection method. The inspector must ensure that the inspection process is capable of detecting the nonconformity (e.g., gauge capability, lighting, training). Also, 100% inspection may not guarantee that all nonconformities are found, especially if the nonconformity is subtle. Additionally, the inspector should consider the cost and time. After 100% inspection, the lot may be accepted if all remaining items conform, but the inspector should document the results and may need to adjust the sampling plan for future lots. If the nonconformity rate is high, the supplier should be notified for corrective action.