Setting a standard rate/percentage for inspection

At our warehouse, we check for dimensions and function when we conduct our quality check on incoming product. I would like to get your thoughts on how can we set a standard rate/percentage of how many pieces we need to inspect, for example let’s say AQL 10% for dimensions and function test?

Manufacturing Guru Whit Kelly was kind enough to provide some feedback. AQL is explained below as well.

There is no one answer to that question that I am aware of. But let me give you some options.

The AQL system in general is the “least worst” option for deciding the frequency of inspections. AQL was developed almost 100 years ago during World War I, because prior to that no formal system existed to control quality inspections.

For products where you have history and data of past inspections of parts coming from a certain vender, were it up to me, I would start a SPC (statistical process control) system, as it incorporates actual data and allows you to create statistical models that tailor your inspection plans to the variability of the operations and products. A highly variable operation or supplier would warrant more stringent inspections, while one that operated within tolerances (calculated by making periodic checks to gather data) would eventually be downgraded to smaller sample batches or in the case of a very stable process or supplier, inspections might be eliminated altogether.

For example, my company in the USA, SVI, has some suppliers who have not delivered a defective part in 25 years. Their shipments are never inspected, and parts go straight to the assembly line. Others warrant 100% inspection – which we also conduct at the assembly line, as we have negotiated a deal whereby the supplier credits us for any defective parts.

I think common sense is what is needed. If you order 20,000 machined parts, I would absolutely perform AQL II, as machining is highly variable and the tolerances are typically very tight. I might even recommend a more stringent AQL level if that particular supplier is new.

But if you order 20,000 cardboard boxes, I might check a dozen with a tape measure, fold a few up to make sure they erect properly and set an inspection quantity appropriately. If the printing on the box is critical, then an AQL inspection is required.

In each case, use your best judgment, and if you unsure, don’t be afraid to ask your project team what parts are critical and know that you can engage 3rd party experts to assist. Click here for engineering and sourcing support.

Note to reader: The Acceptable Quality Level (AQL) is defined as the maximum percent defective (or the maximum number of defects per hundred units) that, for purpose of sampling inspection, can be considered satisfactory as a process average. The sampling plans most frequently used by the department of Defense are based on the AQL.

Learn more about AQL tables.

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