Is there a Grade 000 for granite surface plates? Core Conclusion
In the current national standard GB/T 20428-2021 “Flatness of Rock Plates”, there is no Grade 000; it only defines: Grades 00, 0, 1, and 2.
The old national standard GB 4987-85 did have a Grade 000, which has now been abolished, but the industry, manufacturers, and old verification procedures still use the term “Grade 000” (Douyin Baike).
International ISO8512 also does not have a Grade 000; the AA grade of US ASME B89.3.7 is functionally equivalent to the domestic Grade 000; the highest grade of German DIN876 is Grade 00.
- Evolution of Standards
Old national standard GB 4987-85 (obsolete): Clearly divided into four grades: 000, 00, 0, and 1. Grade 000 is the highest. Formula:
Grade 000 flatness: (1 \times (1 d/1000) \mu m)
Grade 00: (2 \times (1 d/1000) \mu m)
d = diagonal in mm, at 20°C (Douyin Baike).
Replacement new version GB/T 20428-2006 / 2021: Removed Grade 000; the official grades are only: 00, 0, 1, and 2.
Verification procedure JJG117-2013 (Verification of Flatness of Surface Plates): The procedure only goes up to Grade 00, with no verification criteria for Grade 000.
👉 In other words: Third-party metrology institutes cannot issue a “Grade 000” verification certificate; at most, they can verify it to Grade 00, and the report will only state the actual measured flatness value. - Then what is the “Grade 000 granite surface plate” on the market? It is an industry colloquialism / enterprise internal control grade, not a legally mandated national grade:
It means achieving the flatness tolerance of Grade 000 from the old GB4987-85 (the k=1 tier);
Widely used for metrology bases, CMM bases, semiconductors, and optical bases;
It must be in a strictly constant temperature environment of 20±0.5°C with vibration isolation; ordinary workshops cannot achieve Grade 000 performance at all.
Example: A 1000×1000 plate, diagonal ≈ 1414mm
Industry colloquial Grade 000 tolerance: ≈ 2.4 μm
National standard official Grade 00 tolerance: ≈ 4.8 μm
- Critical pitfalls in procurement and contract signing (Practical focus)
Do not just write “Grade 000”, as metrology agencies do not recognize this grade name;
Write the flatness tolerance value the referenced calculation formula directly in the contract / technical agreement, for example:
Working surface diagonal d, flatness ≤ (1 \times (1 d/1000) \mu m) (at 20°C);
Third-party certificates will only print the actual measured flatness in μm, not the words “Grade 000”;
Many small factories falsely label products as Grade 000, while actually only achieving Grade 00 or even Grade 0, because there is no mandatory standard constraint. - Simple Summary
✅ This industry term exists, originating from the abolished old national standard GB4987-85;
❌ Current national standards and national verification procedures do not have “Grade 000” as a legally defined accuracy grade;
✅ Products can achieve the flatness indicators corresponding to Grade 000, but the certificate will not be labeled “Grade 000”.
