⚡ Key Takeaways
- Standard: GB/T 90.1-2003, acceptance inspection for fasteners.
- Three classes of characteristic: A (critical — hardness, tensile, proof load) 100% or type-tested; B (dimensions, thread) sampled; C (appearance, finish) sampled.
- AQL values: 1.0 for class B, 2.5 for class C.
- Decision: Ac means accept, Re means reject.
- Consequence: a rejected lot is returned in full or 100% sorted.
📑 Contents
1. The three inspection classes
2. AQL sampling plans
3. Reading Ac and Re
4. Handling nonconforming lots
1. The three inspection classes
2. AQL sampling plans
3. Reading Ac and Re
4. Handling nonconforming lots
1. The three inspection classes
| Class | Characteristics | Method | AQL |
|---|---|---|---|
| A (critical) | Hardness, tensile strength, proof load, decarburisation | 100% or type test | 0 |
| B (major) | Go/no-go thread gauges, principal dimensions, mechanical properties | Sampling | 1.0 |
| C (minor) | Appearance, finish, marking, secondary dimensions | Sampling | 2.5 |
2. AQL sampling plans
GB/T 90.1 uses general inspection level II with single sampling under normal inspection:
| Lot size N | Code letter | Sample size n | Class B Ac/Re (1.0) | Class C Ac/Re (2.5) |
|---|---|---|---|---|
| 51–90 | A | 8 | 0/1 | 0/1 |
| 91–150 | B | 13 | 0/1 | 1/2 |
| 151–280 | C | 20 | 1/2 | 1/2 |
| 281–500 | D | 32 | 1/2 | 2/3 |
| 501–1200 | E | 50 | 2/3 | 3/4 |
| 1201–3200 | F | 80 | 3/4 | 5/6 |
3. Reading Ac and Re
Example: a lot of 5,000 bolts, class B sample n = 80, Ac = 5, Re = 6:
- Inspect 80 pieces; 5 or fewer nonconforming → accept the lot;
- 6 or more nonconforming → reject the lot.
Important: AQL is an acceptable quality level, not an average quality target. A few nonconforming pieces in the sample are tolerated, but the lot as a whole must be far better than the AQL.
4. Handling nonconforming lots
- Reject the lot and return it to the supplier;
- Or agree a 100% sort, keep the good parts and return the rest;
- A class A nonconformity is a serious quality incident and requires an 8D report;
- Three consecutive rejections trigger tightened inspection.
FAQ
Q: What happens after two consecutive rejections? Inspection switches to tightened, which doubles the sample size. Five consecutive tightened lots must pass before normal inspection resumes.
Q: How different are AQL 1.0 and 2.5? 1.0 is stricter: for a 1,000-piece sample Ac = 2 against Ac = 5 at 2.5, so more defects are tolerated at 2.5.
Q: Can a customer specify stricter AQL? Yes. Automotive customers often ask for 0.65 or 0.4 on class B, with a corresponding price adjustment.
🛠️ Engineering support: Toprecision issues an inspection report to GB/T 90.1 for every lot, and can tighten to a customer-specified AQL.
👉 Engineering services ｜ 👉 Request a technical quote
👉 Engineering services ｜ 👉 Request a technical quote