There is no single AQL for engineering components. A common starting point is ISO 2859-1 at general inspection level II, with no critical defects accepted, a tight AQL for major defects and a looser one for minor defects.
The values you choose should reflect what a defect costs you. A burr on a hidden face and an oversized bearing seat are not the same problem, and your plan should not treat them alike.
What AQL actually means
AQL stands for acceptance quality limit. In ISO 2859-1 it is the worst average quality level that is still acceptable for a continuing series of lots. It is not a promise that each lot contains that many defects or fewer.
The standard gives you a table. You look up a sample size code letter from the lot size and inspection level. The code letter gives the number of parts to inspect. The AQL then gives the accept and reject numbers for that sample.
Classify defects first
- A critical defect could cause injury, a safety failure or a breach of law. Most buyers accept none in the sample.
- A major defect stops the part working or fitting as intended, such as an out-of-tolerance bore or a wrong thread.
- A minor defect does not affect function, such as a small cosmetic mark on a non-visible surface.
Values buyers commonly use
In practice, many engineering buyers set major defects at an AQL of 0.65 or 1.0, and minor defects at 2.5 or 4.0. These are common choices, not rules. Safety-related or precision parts often need tighter values, or 100% inspection of the critical feature.
Inspection level matters too. Level II is the default. Level III takes a larger sample and gives more confidence. The special levels S-1 to S-4 take very small samples and suit expensive tests, such as destructive hardness or salt spray checks.
ISO 2859-1 also sets rules for switching between normal, tightened and reduced inspection based on recent lot results. If a supplier fails lots, tightened inspection should follow automatically.
The limits of AQL
Sampling can pass a lot that still contains bad parts. For dimensions that must never be wrong, ask the factory for process control and 100% gauging, then use sampling to verify it. Our guide to dimensional inspection and sample size explains how to plan the measuring.
Write the AQL, inspection level and defect list into the purchase order. An AQL agreed only by email is easy to dispute when a lot fails. Our pre-shipment inspection guide shows where sampling fits.
Next step
We help you classify defects, set sampling plans and inspect lots to them. To agree a plan for your components, book a free sourcing review.