Front page / How do I set up incoming inspection with AQL sampling (ANSI/ASQ Z1.4)?
How do I set up incoming inspection with AQL sampling (ANSI/ASQ Z1.4)?
AQL sampling tells you how many parts to pull from a lot and how many defects you can find before rejecting it. With ANSI/ASQ Z1.4, you look up the lot size and inspection level, usually general level II, to get a code letter. The code letter gives the sample size, and the AQL you chose gives the accept and reject numbers. For example, a 3,500-piece lot at level II gives code letter L and a sample of 200; at AQL 2.5 for major defects, up to 10 defects are accepted.
The three inputs
- Lot size. The quantity presented for inspection. GQC advises treating each different product as its own lot.
- Inspection level. General levels I, II and III, with II the usual default. Level I samples fewer parts and level III more. Special levels S-1 to S-4 give small samples for slow or destructive tests.
- AQL per defect class. Typically separate limits for critical, major and minor defects.
Excerpt of the code letter table, general level II
| Lot size | Code letter | Sample size |
|---|---|---|
| 281 to 500 | H | 50 |
| 501 to 1,200 | J | 80 |
| 1,201 to 3,200 | K | 125 |
| 3,201 to 10,000 | L | 200 |
| 10,001 to 35,000 | M | 315 |
| 35,001 to 150,000 | N | 500 |
Values are from Table I of Z1.4 as reproduced by Netray. Sample size grows more slowly than lot size, so larger lots are inspected at a lower percentage.
Worked examples
From GQC: 3,500 units, level II, AQL 2.5 major and 4.0 minor. Code letter L gives a sample of 200; the lot passes with no more than 10 major and 14 minor defects.
Our own moulding example: a lot of 2,000 housings at level II falls in the 1,201 to 3,200 band, giving code letter K and a sample of 125. Netray's figures for letter K are: at AQL 1.0, accept up to 3 and reject at 4; at AQL 2.5, accept up to 7 and reject at 8. So a plan of AQL 1.0 for major defects and 2.5 for minor defects means pulling 125 parts, rejecting the lot at 4 majors or at 8 minors.
Writing the plan for moulded parts
- Define defects with pictures. Short shots, flash on sealing faces and cracked bosses are usually major; light flow marks on non-cosmetic faces may be minor. Agree limit samples with the supplier.
- Decide what counts as critical. Anything that affects safety or regulatory compliance. Many buyers set a zero-defect rule for critical defects.
- Separate attribute checks from measurements. Z1.4 is an attribute plan: parts pass or fail. Key dimensions are usually checked on a smaller set and tracked over time.
- Sample across the lot. Pull from several cartons and, for multi-cavity moulds, record cavity numbers so a problem cavity shows up.
- Record and react. Note the edition of the standard, level, AQLs, sample size and result on each report.
Summary of public guidance from inspection service providers. Use the edition of the standard named in your contract, and remember that passing an AQL inspection does not guarantee a defect-free lot, as GQC also notes.
Questions buyers ask
Does AQL 2.5 mean 2.5 percent of my parts can be bad?
Not exactly. It sets the accept and reject numbers for the sample, but a lot with a defect rate above the AQL can still pass by chance, and vice versa.
Should a new supplier get a higher level?
GQC suggests level I only when you trust the manufacturer and quantities are large, so many buyers start new suppliers at level II or III.
Is ISO 2859-1 the same?
GQC lists ISO 2859-1, ANSI/ASQ Z1.4 and MIL-STD-105E together as the sampling standards its inspectors apply. Check which one, and which edition, your contract names.
Where does first article inspection fit?
First article inspection checks the first parts against every requirement once; AQL sampling then checks routine lots.