Overview
ISO 8512-2:1990 specifies requirements for granite surface plates used as precision planar datums in inspection, layout and calibration work. The standard covers new, in-use and reconditioned rectangular or square granite plates in preferred sizes from 160 mm x 100 mm up to 2500 mm x 1600 mm, and defines four accuracy grades (0, 1, 2, 3) together with material, finish, marking, handling and testing guidance.
Key topics and technical requirements
- Material: Plates shall be made from stable, close-grained igneous rock (commonly called “granite”) with uniform texture, sound structure and no flaws or softer inclusions. Repair of defects in the top surface is not permitted.
- Preferred sizes: Examples include 160×100, 250×160, 400×250, 630×400, 1000×1000 and 2500×1600 mm (other sizes may be accepted but must meet the same requirements).
- Accuracy grades & flatness tolerances:
- Two flatness metrics: overall flatness and local flatness for any 250 mm × 250 mm area.
- Local flatness tolerances for a 250×250 mm area are defined as ≈3.5 µm (grade 0), 7 µm (grade 1), 15 µm (grade 2), and 30 µm (grade 3).
- Overall flatness tolerances depend on size and grade (tabulated in the standard).
- Surface finish: Grades 0 and 1 require lapped working surfaces; grades 2 and 3 may be lapped or ground.
- Support and rigidity: All plates must be supported on three feet (larger plates may have more supports and safety feet). For plates ≥ 400×250 mm, a concentrated load must not cause deflection exceeding 1 µm per 200 N in the loaded area relative to the remaining surface.
- Handling & clamping: Plates are not supplied with handles; tapped inserts or slots for clamping are optional and, if fitted, must be installed before finishing to avoid distortion. Projecting top edges for clamping are allowed to purchaser’s specification.
- Marking & protection: Each plate must carry a nameplate with manufacturer, the ISO number and the grade. Grade 0 and 1 plates are supplied with covers.
Applications and users
ISO 8512-2 is intended for:
- Calibration and metrology laboratories
- Inspection and quality-control departments in manufacturing (toolrooms, aerospace, automotive)
- Precision machine shops and toolmakers
- Surface plate manufacturers and reconditioning services
The standard helps ensure consistent surface plate tolerances, flatness, material quality, testing methods and safe use-critical for reliable dimensional measurement and reference surfaces in precision metrology.
Related standards
- ISO 8512-1: Surface plates - Part 1: Cast iron (companion standard covering cast-iron plates)
- Annexes in ISO 8512-2 provide informative guidance on care, testing methods (datum gauges, comparator and rigidity tests) and loading moderation.
Keywords: ISO 8512-2, granite surface plates, surface plate tolerances, flatness, accuracy grades, surface plate testing, precision metrology.
