Calibration standard for granite flatness and parallelism

Full Calibration Standards for Flatness and Parallelism of Granite (Stone) Surface Plates

I. Core Execution Standards (Currently Effective in China)

1. Product Manufacturing Acceptance Standard
   GB/T 20428-2006 “Stone Surface Plates” (Equivalent to ISO 8512-2)
   Applicable to: Newly manufactured, in-use, and repaired granite measuring platforms, divided into 4 accuracy grades: 0, 1, 2, and 3.
2. Metrological Verification / Calibration Procedures (Basis for Third-Party Verification)
   JJG 117-2013 “Surface Plates” (General Verification for Cast Iron Granite Surface Plates)
   Specifies flatness layout, measurement methods, pass/fail criteria, and flatness fluctuation; mandatory basis for issuing calibration certificates.
3. General Reference for Parallelism Geometric Tolerances
   GB/T 1184-1996 “Geometrical Product Specifications (GPS) – Standard Tolerances”
   GB/T 1958-2017 “Geometric Tolerance Inspection and Verification”
4. Commonly Used International Standards
   DIN 876 (Germany), ISO 8512-2 (International Stone Surface Plates)

II. Flatness Standards (Core Tolerances Calculation Formulas)

1. Maximum Permissible Error for Flatness
   General Formula (JJG117, GB/T20428):
   (F=K\times\left(1 \frac{d}{1000}\right) \quad (\mu\text{m}))
   d: Diagonal length of the surface plate (mm)
   K: Grade coefficient

Industry colloquialisms “00/000 grade” refer to old versions or internal enterprise sub-grades; the official national standard does not include these sub-grades, and metrology institutes uniformly issue reports according to grades 0/1/2/3.

2. Local Flatness Fluctuation (250mm × 250mm Area)
   Mandatory requirement of JJG117:

3. Example Calculation
   For a 1000×1000mm surface plate, diagonal d=1414mm:
   Grade 0 flatness: (4\times(1 1414/1000)≈9.66\ \mu\text{m})

III. Parallelism Calibration Standards (Divided into Two Scenarios)
Scenario 1: Single Granite Platform — Parallelism of Top and Bottom Working Surfaces (Thickness-Reference Platform)

1. Tolerance Determination Basis

• Foundation: Flatness error is the upper limit for parallelism; parallelism error ≤ flatness tolerance of the same grade;
• Parallelism of side/bottom surfaces relative to the working surface: According to GB/T1184, unstated geometric tolerances for Grade 12;
• General industry internal control: Parallelism error ≤ 1.5 times the flatness tolerance is considered a pass.

1. Calibration Method

• Level the platform, fix the dial indicator stand on standard height blocks;
• Contact the indicator tip with the bottom surface, move uniformly across the entire XY area, and record the maximum and minimum difference as the parallelism error;
• For high-precision Grade 0 platforms, a laser interferometer with 0.1μm accuracy is recommended.

Scenario 2: Granite Parallel Bars / Square Boxes / Matching Reference Blocks
Parallelism of Two Working Surfaces
GB/T 24761-2009 “Steel Straightedges and Stone Straightedges”
Supporting requirements:

• Grade 0 stone parallel bar parallelism ≤3μm / full length;
• Grade 1 ≤6μm / full length;
• Inspection: Stack on a standard Grade 0 surface plate, scan the gap variation with a dial indicator.

Scenario 3: Relative Parallelism of Two Granite Platforms Used in Pairs
Stack the two plates, measure the gap range with a dial indicator; the range must not exceed the sum of the flatness errors of the two surface plates.

IV. Mandatory Calibration Environment Conditions (Prerequisites for Certificate Validity)
Standard temperature: 20℃±2℃, temperature fluctuation ≤1℃ per hour;
Humidity: 40%～60%, to prevent stone moisture absorption and deformation;
No vibration, no direct airflow, vibration isolation pads placed on the tabletop;
Platform must be kept at a constant temperature and static for ≥4h before measurement.
V. Standard Calibration Methods for Flatness (Three methods specified in JJG117)
Diagonal Point Layout Method (Preferred General Method)
8 diagonal lines in a cross pattern, bridge plate electronic level (resolution 0.001mm/m), least squares method fitting to calculate flatness; standard method for small and medium-sized platforms.
Grid Point Layout Method (Large Platforms)
50～100mm grid evenly distributed across the entire working surface, providing more complete data, suitable for large plates over 2000mm.
Laser Interferometer Method (Grade 0 Ultra-precision)
Nanometer-level measurement, used for calibrating standard reference platforms in metrology institutes.
VI. Complete Calibration Steps for Parallelism
First complete the flatness calibration of the working surface and confirm it meets the requirements;
Level the platform using three-point support to eliminate self-weight deflection;
Dial gauge resolution 0.001mm, probe must be vertically flush with the bottom surface;
Perform full coverage scanning in both transverse and longitudinal directions, recording maximum and minimum values;
Difference = parallelism error, compare with grade tolerance to determine compliance;
Multi-thickness segmented platforms require re-measurement by region.
VII. Calibration Cycle and Judgment Rules
Calibration Cycle
Grade 0 precision metrology platform: once every 6 months;
Grade 1/2 workshop inspection platform: once every 12 months;
Grade 3 marking platform: once every 18 months.
Compliance Judgment
Measured errors of flatness and parallelism ≤ maximum allowable error of the standard; plane fluctuation must also meet the standard to be judged compliant.
Out-of-tolerance handling: Re-calibrate after returning to the factory for grinding and repair.
VIII. Basic Requirements for Supplementary Materials (Prerequisites for Platform Factory Release)
Stone: Gabbro / Black Granite (Jinan Green), Shore hardness HS≥70;
Working surface roughness Ra: 0.32～0.63μm;
No sand holes, cracks, or bumps, to avoid local edge collapse affecting accuracy.