Granite surface plates are critical precision tools in metrology and manufacturing. To ensure the highest levels of accuracy and longevity, they must adhere to strict technical specifications. Here are the essential requirements:
Material Composition: The plate must be manufactured from fine-grained, densely structured igneous rock, such as Gabbro, Diabase, or Granite (with biotite content less than 5%). The material must possess a modulus of elasticity greater than 0.6 × 10^4 kg/cm² and a water absorption rate below 0.25%.
- Surface Hardness: The working surface must have a minimum hardness of 70 HS (Shore Scleroscope), ensuring exceptional resistance to wear and indentation.
- Surface Integrity: The critical working surface must be entirely free from defects that impair function and appearance, including sand holes, porosity, cracks, slag inclusions, shrinkage cavities, deep scratches, bumps, and rust stains.
- Surface Finish: The working surface must achieve a superior micro-finish with a maximum allowable roughness average (Ra) between 0.32 μm and 0.63 μm. Non-functional side surfaces may have an Ra up to 10 μm.
- Permissible Repairs: Minor imperfections on non-working surfaces, edges, or corners may be repaired with high-strength epoxy filler, provided the repair does not affect the plate's structural integrity or accuracy.
- Embedded Features: Any threaded holes, inserts, or T-slots must be machined such that they do not protrude above the level of the working surface.
- Geometric Tolerances: The perpendicularity of the working face to the sides, and the perpendicularity between adjacent sides, must meet the stringent Grade 12 tolerance as specified in the GB/T 1184 standard (or equivalent international standards like ISO 2768).
- Handling Provisions: To protect their accuracy, precision grades 000 and 00 should not be fitted with lifting handles. Alternative handling methods must be provided.
- Accuracy Grade: The plate must be manufactured and calibrated to meet internationally recognized accuracy grades (e.g., Grade 000, 00, 0, 1) based on its intended application, with strict adherence to flatness tolerances.
- Thermal Stability: The granite material must be naturally aged and stabilized to minimize thermal expansion, ensuring consistent accuracy in varying workshop or lab environments.
Investing in a granite surface plate that meets these comprehensive technical requirements is fundamental to guaranteeing reliable, high-precision measurement results and achieving a superior return on investment.