GB/T 18779.4-2020 in English
VALIDGeometrical product specifications (GPS)—Inspection by measurement of workpieces and measuring equipment—Part 4:Background on functional limits and specification limits in decision rules
- Issued on:2020-12-14
- Implemented on:2021-07-01
- File Format:PDF
- Delivery:Via email within 1~3 business days
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《GB/T 18779.4-2020产品几何技术规范（GPS） 工件与测量设备的测量检验 第4部分：判定规则中功能限与规范限的基础》由TC240（全国产品几何技术规范标准化技术委员会）归口，主管部门为国家标准化管理委员会。
Introduction
Analysis of the core content of the standard
| Elements | Functional Limit (FL) | Specification Limit (SL) |
|---|---|---|
| Definition basis | Actual functional requirements of the product | Technical requirements agreed in the contract |
| Determination method | Experimental research/functional degradation curve | Functional limit + safety margin |
| Legal effect | Technical reference basis | Contract execution standard |
Key terms
Functional degradation curve: A mathematical model that characterizes the relationship between characteristic values and functional levels. When the characteristic value exceeds the functional limit, the product function gradually decays (as shown in Figure 2). A typical example is the reduction in matching performance caused by diameter deviation in shaft-hole matching.
Specification operation set: A complete system that includes the implementation guidelines, measurement methods, and judgment rules for determining specification limits, which directly affects the conversion accuracy from functional limits to specification limits.
Key points of technical implementation
Establishment of limit relationship
The standard proposes three typical relationship modes:
- Ideal mode: specification limit = functional limit (Figure 1/Figure 7)
- Safety margin mode: the specification limit is inside the functional limit (Figure 6/Figure 10)
- Contract limit mode: a stricter contract acceptance limit is added inside the specification limit (Figure 11)
Method for determining functional limit
| Method | Advantages | Limitations |
|---|---|---|
| Advanced model method | Data directly comes from actual workpieces | Easily affected by sample representativeness |
| Reverse engineering | Applicable to analysis of existing products | Constrained by feature selection and measurement errors |
| Trial and error method | Simple operation | Low efficiency and unstable results |
Standard application recommendations
Acceptance case of measuring equipment
For the maximum permissible error (MPE) specification of coordinate measuring machine:
- The functional limit should be determined based on the functional level of the equipment's metrological characteristics (Article 3.6)
- Considering the influence of measurement uncertainty, the specification limit should be appropriately stricter than the functional limit
- It is recommended to adopt the double-sided specification limit determination rule in Section 4.3
Special Tips: When using non-GB/T18779.1 standard judgment rules, it is necessary to clearly state it in the technical documents and obtain the approval of the trading partners (see Article 9.2).
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