The Sampling Plan in Quality Control

Barbara Osborne

Saving Time and Cost in Small Metal Parts Inspection

We have talked before about how important inspections are to the manufacture of small metal parts, helping to ensure the quality of components that make up thousands, if not millions, of products. Yet, 100% inspection eats up significant time and cost — plus it still does not guarantee 100% compliance. And that is why manufacturers and their customers rely on a sampling plan in quality control.

Good Reasons to Do a Sampling Inspection

Visual inspections — whether with the naked eye or using sophisticated optical tools — are done to detect a variety of surface finish issues in metal parts, from corrosion and contamination to cracks and surface irregularities. But given the large quantities that most manufacturers work with — for example, here at Metal Cutting Corporation, we routinely produce lots consisting of 100,000 parts — it is impractical to spend the amount of time and significant money required to look at 100% of the parts in every lot.

Inspecting such a large lot, it is also easy to make an error; put a part in the wrong bin, have a micrometer placed slightly off when measuring, or just blink, and you might miss some defect that would have an impact on ultimate product performance. In addition, even with a range of very precise tools available for inspecting parts, it is impossible to examine every tiny segment of a part’s surface. That means even if you do 100% inspection, it is not precisely 100%.

A sampling plan in quality control is a statistical method of determining whether to accept or reject a lot of material that is being produced. In a sampling inspection, a relatively small number of pieces from the lot are inspected to determine if the entire lot will be accepted or rejected based on the number of defective pieces in the sample.

At Metal Cutting, experience has shown us that inspecting a small portion of the parts in a lot is not only faster and more cost effective — it also allows us to pay closer attention to the inspection process. With every piece in the sample being more carefully inspected, the likelihood of errors or of simply missing something is reduced. Best of all, a sampling plan in quality control provides statistically valid results and high confidence that if the sample is defect-free, the entire lot will meet the customer’s requirements for an Acceptable Quality Level (AQL).

Standards for Sampling Plans

The number of pieces to be inspected and the acceptable number of defectives pieces in a sampling plan in quality control are usually based on AQLs and Index Values included in published standards such as ANSI/ASQ Z1.4-2008. (You can find lot and sampling size values and other technical information in Zero Acceptance Number Sampling Plans, Fifth Edition, by Nicholas L. Squeglia.) The higher the AQL, the fewer the number of parts that will be inspected. This means with a lot of 100,000 pieces, for example, 123 pieces would be inspected at AQL 0.4; only 29 pieces would be inspected at AQL 4.0.

Metal Cutting’s preferred sampling plan in quality control is AQL 1.0 c=0, which is a zero acceptance sampling plan; the associated AQL determines how many randomly selected parts will be inspected, based on the total lot size. For instance, with a lot size of 100,000 pieces, at AQL 1.0 we would inspect a sample of 74 parts. Using this zero acceptance sampling plan, if even one randomly selected part in the sample fails inspection, then the entire lot is rejected or 100% inspection must be performed.

Sometimes a customer will ask for a different AQL level or sampling plan, and of course we accommodate them. However, we find that our preferred standard (AQL 1.0 c=0) helps us deliver on customer-requested quality levels while also reducing the cost of inspection, by allowing us to inspect fewer parts without increasing the statistical probability of rejectable parts.

When to Use a Sampling Plan in Quality Control

At Metal Cutting, where we produce thousands of small metal parts every day, we like to say that inspecting parts for quality is part of our DNA. Because of our quality commitment, we don’t just inspect final parts at the end of manufacturing, prior to packaging and delivery. Our process includes a sampling plan in quality control at various stages, including receiving — where we take in and inspect not just our raw material but also any customer-supplied materials that we will be using to make a part — and at a designated step (or steps) during production, as well as at the end. This enables us to be sure that quality is tightly controlled throughout the process.

Of course, sampling plans are not perfect either, so there may be one or two imperfect parts in a lot of 100,000. That’s where other statistical tools can be brought into the process. For example, a customer might ask for a Cpk value of 1.33 or better, to ensure that (statistically speaking) the parts are close to the nominal value rather than the upper or lower limits of the specifications for tolerance. If the customer has very tight tolerances and is using the very maximum capabilities of a machine, then Cpk is not the best option, since the entire tolerance range will need to be used to meet the tight specification; however, with a looser tolerance range, the Cpk statistic can robustly be used to show conforming process and outcomes.

Statistical Validity Wins the Day

At Metal Cutting, we are always striving to keeps cost down while still providing the best quality for our customers’ products. In the end, with a statistically valid sampling plan in quality control, we can inspect a smaller number of pieces more carefully and give customers a high level of confidence that if the sample is acceptable, the entire lot is acceptable.

Whatever your preferred sampling plan may be, it is important to include this information in your specifications and your request for a project estimate. That way, Metal Cutting can make recommendations and provide a more accurate and timely quote that includes inspection in the total cost of your parts. 



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