The Sampling Plan in Quality Control

April 3, 2019 Barbara Osborne

Saving Time and Cost in Small Metal Parts Inspection

We often talk 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, it is impractical to spend the amount of time and significant money required to look at 100% of the parts in every lot. For example, here at Metal Cutting Corporation, we routinely produce lots consisting of 100,000 parts.

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 — any of these can mean missing some defect that would have an impact on end 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%.

In quality control (QC), a sampling plan is a statistical method of determining whether to accept or reject a lot of material that is being produced. In a QC sampling, 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.

What Is AQL in Quality Control?

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).

Statistically, AQL is a measure of the maximum number of defective goods that would be considered acceptable in a particular QC sample size. It corresponds to the percentage of a production run that can be rejected before some corrective action must be taken.

Standards for QC Sampling Plans

The number of pieces to be inspected and the acceptable number of defectives pieces in a QC sample are usually based on AQLs and index values included in published standards such as ANSI/ASQ Z1.4-2008. You can also find lot and sampling size values and other technical information in the book 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 is AQL 1.0 c=0, which is a zero acceptance sampling plan from Squeglia’s book. 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 for the failed characteristic.

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 QC sampling at various stages. This includes 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.

We also do QC sampling 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 keep cost down while still providing the best quality for our customers’ products. In the end, with a statistically valid QC sampling plan, 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.

But remember, 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, your manufacturing partner can provide a more accurate and timely quote  — one that includes inspection in the total cost of your parts.

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