Granite surface plates are commonly classified as Grade AA, Grade A, or Grade B. These grades originated in the U.S. Federal Specification GGG-P-463C, Plate, Surface (Granite), first issued in 1973.[1] Although this federal specification is now inactive, its grading system remains widely used. The current U.S. standard for granite surface plates is ASME B89.3.7, Granite Surface Plates, which ASME describes as a revision of GGG-P-463C.[2]
The three traditional grades are:
| Grade | Common Name | Relative Flatness Tolerance | Typical Application |
|---|---|---|---|
| AA | Laboratory Grade | 1× | Precision metrology and laboratory work |
| A | Inspection Grade | 2× AA | Inspection and quality-control work |
| B | Toolroom Grade | 4× AA | General shop and production measurement |
The grade does not refer simply to the overall “quality” of the plate. GGG-P-463C specifically defines grade as the classification of a surface plate according to manufacturing tolerances on the flatness of its working surface.[1]
Overall Flatness
The primary difference among the grades is the allowable deviation from flatness. GGG-P-463C defines the flatness requirement by imagining two parallel planes that completely enclose the working surface. The distance between these planes is the total flatness deviation.
Grade A permits twice the flatness tolerance of Grade AA, while Grade B permits four times the tolerance of Grade AA.[1]
The allowable deviation also depends on the size of the plate. Larger plates are permitted greater total flatness deviation than smaller plates. For example:
| Surface Plate Size | Grade AA | Grade A | Grade B |
| 12 × 12 in. | 50 µin. | 100 µin. | 200 µin. |
| 18 × 24 in. | 75 µin. | 150 µin. | 300 µin. |
| 24 × 36 in. | 100 µin. | 200 µin. | 400 µin. |
| 36 × 48 in. | 200 µin. | 400 µin. | 800 µin. |
| 48 × 96 in. | 500 µin. | 1,000 µin. | 2,000 µin. |
A microinch (µin.) is one millionth of an inch, or 0.000001 inch. Thus, a flatness tolerance of 100 µin. is only 0.0001 inch.
Local Flatness and Repeat Reading
Overall flatness is not the only important characteristic of a surface plate. A plate could have acceptable overall flatness while still containing small local hills or valleys that affect measurements made over a small area.
GGG-P-463C therefore also specifies a repeat-reading tolerance. This is evaluated with a repeat-reading gage that is moved across the working surface to detect localized variations in height.[1]
For plates with a diagonal of 30 inches or less, the normal repeat-reading tolerances are:
| Grade | Repeat-Reading Tolerance |
| AA | 35 µin. |
| A | 60 µin. |
| B | 110 µin. |
More stringent limits of 25 µin., 50 µin., and 100 µin. respectively could also be specified.[1]
This distinction is important because a surface plate is a reference surface. When a height gage, indicator stand, or workpiece rests on the plate, localized variations in the surface can become part of the measurement error.
Surface Texture
GGG-P-463C also specifies the texture of the working surface. The average surface roughness is limited to 32 µin. for Grades AA and A and 64 µin. for Grade B. Interestingly, the specification also establishes a minimum roughness of 16 µin.[1]
A surface plate therefore should not be made as smooth as physically possible. Some surface texture is desirable to prevent precision objects from sticking to the plate and to allow them to slide freely.
Which Grade Is Needed?
A higher grade is not necessarily required for every measurement. Grade B plates are commonly appropriate for general shop and toolroom work, while Grade A plates provide tighter tolerances for inspection work. Grade AA plates are intended for applications requiring still greater accuracy, such as precision metrology laboratories.
The important consideration is whether the flatness and repeat-reading performance of the surface plate are sufficiently small relative to the tolerances of the measurements being made on it.
Surface plates also wear with use, particularly in frequently used areas. For this reason, the original grade marked on a plate should not simply be assumed to describe its present condition. Periodic calibration is necessary to determine the actual flatness and local variation of the working surface. GGG-P-463C itself recommends periodic recalibration and notes that plates exceeding their requirements may need to be downgraded, resurfaced, or replaced.[1]
A Note on the Current Standard
GGG-P-463C remains frequently referenced in industry, but it is no longer the current U.S. specification for new applications. The federal specification was made inactive in 2013.[3] The current standard is ASME B89.3.7, Granite Surface Plates. ASME states that B89.3.7 is a revision of GGG-P-463C and covers certification and recertification of granite surface plates as well as subjects such as measurement uncertainty and metrological traceability.[2]
References
[1] U.S. General Services Administration, Federal Specification GGG-P-463C: Plate, Surface (Granite) (Inch and Metric), September 10, 1973, with Amendment 1, June 15, 1977. U.S. Department of Defense ASSIST/Quick Search.
[2] American Society of Mechanical Engineers (ASME), ASME B89.3.7-2013 (R2023), Granite Surface Plates. ASME describes this standard as a revision of Federal Specification GGG-P-463C:1973.
[3] U.S. Department of Defense ASSIST, GGG-P-463, Plate, Surface (Granite) (Inch and Metric), Document ID 52805. GGG-P-463C was designated inactive for new use on January 11, 2013.