# Surface plates

Precision reference planes: overall and local flatness, grade selection, support, loading and calibration.

Reviewed: 2026-11-01

Tags: inspection, instruments, dimensional-metrology

## A reference plane in a measurement system

A surface plate supplies the reference for supports, stands and height comparisons. Local plate variation can change the result as a setup moves. [Starrett, Gaging Is Baseless Without Standards, 2 May 2023](https://www.starrett.com/news-events/gaging-is-baseless-without-standards)

Suppose a height comparison moves a workpiece from location A to location B while the indicator stand remains elsewhere. In an **original simplified example**, assume the effective support level at B is 3 µm higher than at A. With all other effects held constant, the comparison can shift by 3 µm even if the part is unchanged. Actual influence depends on base footprints, contact geometry and whether the work and reference share the same local region; the plate's overall tolerance is not automatically the error of every height measurement.

A plate is not automatically a dimensional datum for every face placed on it. Establish which workpiece surfaces contact the plate or fixtures, whether the part rocks, and how the setup realizes the drawing's datum requirement. Likewise, leveling a plate relative to gravity and determining its flatness are different tasks: a tilted plane can be flat, while a level setup can still have local departures.

## Overall flatness and local variation

Mitutoyo's **EDU-15004A, ©2018**, explains two separate characteristics under ASME B89.3.7-2013:

- **Overall flatness:** the full working surface's departure from a plane.
- **Local variation in flatness**, also called **repeat reading:** variation over a smaller area, subject to a separate tolerance.

Flatness is the separation of two parallel planes containing the surface, not a bilateral ± allowance to double. Grades **AA, A and B** provide different tolerances, with AA the tightest. The grade must be combined with plate size and the governing specification. [Mitutoyo, Surface Plates—Accuracy and Calibration, page 1](https://www.mitutoyo.com/webfoo/wp-content/uploads/15004A.pdf)

For a **24 × 36 inch** plate, that bulletin reproduces these ASME B89.3.7-2013 values:

| Grade | Local flatness | Overall flatness |
| --- | ---: | ---: |
| AA | 45 µin | 100 µin |
| A | 70 µin | 200 µin |
| B | 120 µin | 400 µin |

These are size-specific tolerance values, not the measured condition of a particular plate. For unit context, 100 µin = **2.54 µm**, using the exact inch-to-millimeter conversion. [Mitutoyo EDU-15004A, page 2](https://www.mitutoyo.com/webfoo/wp-content/uploads/15004A.pdf)

Use the edition required for the job. ASME's original B89.3.7-2013 preview gives an issuance date of **26 June 2013** and identifies separate tables for overall and local variation, support arrangements, testing and uncertainty guidance. The preview is not the complete test procedure. [ASME B89.3.7-2013 preview](https://www.asme.org/getmedia/f8122d5d-f158-4bc2-bdcb-4d7f33268d13/35000.pdf)

## Support and loading are part of the specification

Follow the manufacturer's support arrangement. Starrett's May 2023 guidance warns against supporting its pad-supported plates by ledges or four corners and describes different arrangements for very large plates. [Starrett, installation guidance](https://www.starrett.com/news-events/gaging-is-baseless-without-standards)

Check the assembled stand, floor support, working height, access and load arrangement. Keep the calibration configuration and in-service support configuration consistent. A stand with adequate gross carrying capacity does not establish that the plate is supported at its required points.

Loading requirements must identify both force and placement. A concentrated fixture, a broad workpiece and a load near an edge are not the same condition. Starrett's **15 January 2025** inspection-products article discusses load-induced deflection and correctly positioned stand crossbeams. Treat permissible load/deflection as a plate-and-support specification, not a general capacity inferred from granite thickness or color. [Starrett, Precision Granite Inspection Products](https://www.starrett.com/news-events/what-you-need-to-know-about-precision-granite-inspection-products)

Clamping is also consequential. Starrett cautions that excessive torque on work-clamping ledges can affect measurements near the edge. Specify engineered inserts, slots or other fixtures with the supplier when needed; do not assume modifications preserve an existing flatness qualification. [Starrett, ledges and custom configurations](https://www.starrett.com/news-events/gaging-is-baseless-without-standards)

## Local checks do not replace overall calibration

The named **Starrett Repeat Reading Gage** uses an indicator to detect local error. Its manufacturer explicitly says that it does **not** measure overall flatness. Sweeping such a gauge is useful for monitoring localized wear, but passing that check does not demonstrate that the full surface satisfies its overall tolerance. [Starrett Repeat Reading Gage](https://www.starrett.com/products/precision-granite-solutions/repeat-o-meter-reading-gage)

A separate named system is the **Starrett Planekator**: a calibrated granite straightedge, supports, indicator and carriage for overall-flatness work. Starrett specifies a straightedge at least the plate's full width and at least half its length; its example uses a 36-inch straightedge for a plate up to 36 × 72 inches. This is the manufacturer's system-sizing guidance, not proof that any ordinary straightedge and indicator can calibrate that plate. [Starrett Planekator kits](https://www.starrett.com/products/precision-granite-solutions/planekator)

A calibration plan should identify the method, measurement paths, working area and any exclusions, support condition, environmental conditions, resulting overall/local values, uncertainty and conformity rule. A sticker reading “Grade A” is less informative than the certificate and results supporting that conclusion. Preserve as-found condition when investigating damage or wear rather than cleaning, modifying or re-lapping away the evidence before assessment.

## Care and daily use

Clean the plate and contacting bases. Set work down gently and avoid concentrating repetitive inspection in one small area. Starrett recommends regular cleaning and covering the plate when unused. [Starrett, granite-plate care](https://www.starrett.com/news-events/gaging-is-baseless-without-standards)

The 2025 Starrett guidance recommends a granite surface-plate cleaner and a protective cover. Use the manufacturer's compatible cleaning method and let the setup stabilize before demanding measurements. Avoid treating a nominally stable granite plate as immune to heating, cooling, load changes or contamination. Review temperature differences among the plate, workpiece and reference artifacts as part of the measurement method. [Starrett, support and care](https://www.starrett.com/news-events/what-you-need-to-know-about-precision-granite-inspection-products)

A scratch or chip warrants assessment of its effect on support and the working area. Appearance alone cannot establish conformance. Re-lapping is a controlled restoration task followed by appropriate calibration, not an informal response to an inconvenient indicator reading.

## A bounded measurement example

Choose size and grade from the intended setup and uncertainty needs. Include instrument error, reference uncertainty, fixture geometry, repeatability, thermal effects and plate contribution in that evaluation. More digits on the indicator do not compensate for an unsuitable reference plane.

For purchasing or inspection planning, specify dimensions, material and standard/grade, usable area, support and stand, load placement, accessories, required calibration results and acceptance rule. For use, retain plate identity, calibration status, condition checks and the relevant setup. There is no one grade or universal recalibration interval suitable for every environment and workload.

## Related reference

[Inspection and quality documents](../../sourcing/documents/index.md)