Guide
Surface plate grades AA, A and B: which grade does your inspection actually need?
Surface plate grades are the three flatness classes for granite inspection plates, AA (laboratory), A (inspection) and B (toolroom), defined today in ASME B89.3.7 and before 2013 in Federal Specification GGG-P-463c, with Grade A allowed twice and Grade B four times the flatness error of Grade AA. The grade matters to a buyer because any height-gauge or indicator measurement made on the plate inherits its error, so the plate must be flat enough for the tolerance being checked and calibrated recently enough to prove it.
By Saola Manufacturing engineering team · Published · Updated
Most buyers never see a supplier’s surface plate, yet it is the reference for a large share of the dimensions on an inspection report: heights, step dimensions, parallelism and flatness checks made with a height gauge or a dial indicator on a stand. This guide explains what the grades specify, how to read the tolerance table, and how to judge whether a plate is good enough for a given drawing tolerance. The general rule for gauge accuracy against tolerance is covered in the 10-to-1 gauge accuracy guide; here it is applied to the plate.
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Which standard defines surface plate grades today?
ASME B89.3.7-2013, Granite Surface Plates, which replaced Federal Specification GGG-P-463c and kept its grades and, apart from minor rounding, its tolerance values.
Mitutoyo’s metrology training bulletin on surface plates states that GGG-P-463c was made inactive in 2013 when ASME B89.3.7 was published, and that much of its content, including the standardized grades and tolerance values, moved into the new standard unchanged (Mitutoyo EDU-15004A, 2018). Many drawings, purchase orders and calibration certificates still cite GGG-P-463c, which is acceptable in practice for the grades, but a new quality document should cite ASME B89.3.7.
The federal specification also set material, construction, surface texture and repeat-reading requirements, not just flatness, as Sandia National Laboratories summarized in its paper on calibrating large plates to it (Sandia, SAND2008-2571C). For a buyer, the two characteristics that matter are the overall flatness of the work surface and the local variation in flatness, usually called the repeat reading.
What flatness does each grade allow?
For a common 24 by 36 inch plate, Grade AA allows 100 microinches of overall flatness error, Grade A 200 and Grade B 400, per ASME B89.3.7; the local repeat-reading limits are much tighter and grow more slowly with size.
The table reproduces selected rows from the Mitutoyo summary of the ASME B89.3.7 tolerances. One microinch is one millionth of an inch, so the 100-microinch Grade AA value is about 2.5 µm, per ASME B89.3.7 as tabulated there.
| Plate size (in) | AA local | AA overall | A local | A overall | B local | B overall |
|---|---|---|---|---|---|---|
| 12 × 18 | 35 | 50 | 60 | 100 | 110 | 200 |
| 18 × 24 | 35 | 80 | 60 | 160 | 110 | 320 |
| 24 × 36 | 45 | 100 | 70 | 200 | 120 | 400 |
| 36 × 48 | 45 | 200 | 70 | 400 | 120 | 800 |
| 48 × 96 | 75 | 500 | 100 | 1000 | 200 | 2000 |
For sizes not listed, the federal specification gave a formula for Grade AA based on the plate diagonal, with the A and B values doubled and quadrupled, and excluded a band along the edges from the flatness tolerance, as described in the Sandia paper (Sandia). The listed values govern where a size is in the table.
What is the difference between overall flatness and the repeat reading?
Overall flatness is the total departure of the whole work surface from a plane; the repeat reading is the local variation measured with a small gauge swept around the plate, and it is what matters when the part and the reference sit close together.
Mitutoyo describes the overall check as a series of measurement lines made with a laser interferometer, autocollimator, electronic levels or a master straightedge and indicator, combined into a three-dimensional map. The repeat reading uses a gauge with a small three-point base and a fourth contact point; the range of the indicator as it is swept around the plate is the local variation (Mitutoyo EDU-15004A).
In practice: when a height gauge is zeroed on a gauge block next to the part, only the local flatness between the block and the part enters the result. When a long part is checked for flatness or parallelism by sweeping an indicator across it, or when the gauge is zeroed at one end of the plate and used at the other, the overall flatness enters. Good practice is to work in a small area of the plate and zero close to the feature.
How do you decide if a plate grade is good enough for a tolerance?
Compare the plate error that actually enters the measurement, local or overall, with the tolerance band, and keep it to a small fraction of it, typically no more than a tenth.
A worked example per ASME B89.3.7 values: a step height with a total tolerance band of 0.002 in, or 2,000 microinches, is checked with a height gauge. If the part is measured across a 24 by 36 inch Grade B plate, the overall tolerance of 400 microinches per ASME B89.3.7 is a fifth of the band, which fails a 10-to-1 rule. If the gauge is zeroed next to the part, the Grade B local limit of 120 microinches per ASME B89.3.7 is about 6% of the band, which passes. The same plate can therefore be adequate or inadequate depending on how it is used.
The standard itself applies a decision rule to plate calibration: Mitutoyo notes that ASME B89.3.7-2013 uses simple acceptance with a test uncertainty ratio of at least 4 for Grades A and B and at least 2 for Grade AA (Mitutoyo). That means a certificate showing a plate “in tolerance” still carries calibration uncertainty, which matters most when you rely on a Grade AA plate near its limit.
A rough guide that follows from the table: Grade B plates suit general machining tolerances and layout; Grade A is the usual inspection-room plate; Grade AA belongs where gauge blocks are compared or tolerances are a few ten-thousandths of an inch.
How often should a surface plate be calibrated, and what should the certificate show?
No interval is fixed by the old federal specification; it depends on use and wear, and should be backed by repeat-reading checks between full calibrations.
Sandia notes that GGG-P-463c specifies no interval, that a busy shop might calibrate every six months and a laboratory a year or longer, and that the specification recommends monitoring flatness with a repeat reading gauge and recalibrating when the readings change from the last calibration (Sandia, 2008). Plates need time to reach room temperature before calibration and must be cleaned first, the same paper notes.
When you audit a supplier or review a first-article package, ask for the plate size and grade, the date of the last calibration, the overall and local results against the grade, and the standard used. A plate that is worn in the middle, where most work is done, will show it in the local result first. Re-lapping on site can restore a plate.
What should a buyer ask a machining supplier about surface plates?
Ask which dimensions on your drawing are measured on a plate, which grade and size that plate is, and when it was last calibrated; then check that the plate error is small against those tolerances.
At our lead CNC plant in Vietnam, parts made on CNC milling and CNC turning are inspected with equipment that includes a height gauge and a 2D optical (vision) measuring instrument; there is no CMM. Heights, steps and parallelism are therefore height-gauge measurements, and the question in this guide applies directly. For a drawing with tight height or flatness tolerances, ask at drawing review how each one will be measured. Every new part gets a first-article inspection report and every lot a dimensional inspection record. Send the drawing for a quote.
Frequently asked
Is GGG-P-463c still valid?
It is inactive, replaced by ASME B89.3.7 in 2013. Because the grades and values carried over, a plate calibrated to the old grades is still meaningful, but new documents should cite the ASME standard.
Is a black granite plate better than a pink one?
The grade, not the color, sets the flatness. Stone types differ in stiffness, wear and moisture absorption, which affect how long a plate holds its grade, but two plates of the same grade and size have the same tolerance.
Can a Grade B plate be used for final inspection?
Yes, for tolerances it is adequate for. Work close to the reference, use the local flatness in the error budget, and keep tighter features on a better plate.
Does a cast iron plate use the same grades?
No. ASME B89.3.7 and GGG-P-463c cover granite plates. Cast iron plates have their own specifications; check what the supplier’s certificate cites.
Sources
- Surface Plates – Accuracy and Calibration, EDU-15004A, Mitutoyo Institute of Metrology, Mitutoyo America Corporation, 2018
- Comparison of Calibration Methods for a Surface Plate, SAND2008-2571C, O. C. Espinosa et al., Sandia National Laboratories, NCSL International Workshop and Symposium, 2008