Granite Surface Plate Grades Explained: DIN 876, ASME B89.3.7, GB/T 20043 And How To Choose

The grade of a granite surface plate is a flatness tolerance class, and choosing it correctly means matching the plate's tolerance to the accuracy of the work you inspect on it. A plate that is too coarse hides errors, while a plate that is too fine adds cost with no benefit.

This guide explains how grades work under the main international standards, shows typical flatness tolerances, and gives a simple method for choosing the right grade for a laboratory, workshop or production line.

What a Surface Plate Grade Actually Measures

A surface plate grade states how far the working surface may depart from a perfect plane, and a plate only meets its grade when every part of the surface stays inside that limit. Flatness is defined as the distance between two parallel planes that contain all points on the surface.

Two separate checks usually apply:

Overall flatness covers the whole working surface. It grows with plate size, because a longer plate has more room to deviate.

Local flatness (repeat reading) covers a small area, such as a 250 × 250 mm square. It is always tighter than overall flatness and protects you from sudden local dips or waves that overall flatness alone would not catch.

A plate can pass the overall check and still have a poor local area, so ask for a certificate that states both values. Flatness is also temperature sensitive: standards use 20 °C as the reference, and a plate that sits in a warm spot or under strong lighting can measure differently from the certificate.

The Main Standards Compared

Five standards cover most export orders, and their grade names differ even though the idea is the same.

For orientation, buyers often treat ASME Grade AA as roughly equal to DIN Grade 00, Grade A to Grade 0 and Grade B to Grade 1. This is an approximation only: the tolerance formulas, plate sizes and test rules differ, so always state the standard, the grade and the plate size together on your purchase order.

Under DIN 876 and GB/T 20428 the permitted flatness grows with plate length in a formula of the form t = K × (1 + L / 1000) µm, where L is the plate length in mm and K depends on the grade. Check the edition of the standard you are buying to for the exact constants.

Flatness Tolerances by Grade and Plate Size

The ASME B89.3.7-2013 values below show how tolerance grows with plate size and how each grade doubles the one before it. Figures are converted from microinches to micrometres and rounded.

The overall tolerance for Grade AA follows the formula (40 + d² / 25) µin, where d is the plate's diagonal in inches. Grade A is twice that value and Grade B is four times that value. The local tolerance changes much more slowly, which is why large plates can still have a very smooth surface.

For DIN 876 and GB/T 20428, ask the supplier for the tolerance table for your exact plate size and grade, and have it written on the quotation. Sources: Mitutoyo technical bulletin on ASME B89.3.7-2013 and Willrich Precision FAQ.

How to Choose the Right Grade

Choose the grade by the tolerance of the parts you measure, not by the highest grade available. A common rule of thumb is that the plate should be at least three times more accurate than the tolerance you are checking, and four times or better is preferred.

Four practical points help you avoid over- or under-buying:

Match the grade to the room. A very fine grade needs a stable temperature near 20 °C. In an uncontrolled workshop, the plate will not hold its certified accuracy, so a coarser grade costs less and performs the same.

Check size and thickness. Larger plates need more thickness and correct support points, or they sag under load and lose flatness.

Specify extras up front. Threaded inserts, through holes, T-slots, stands and surface finish should all be on the purchase order, because they change manufacturing and testing.

Ask for a certificate with both overall and local flatness, the standard, the grade and the plate serial number.

Calibration, Recalibration and Care

A surface plate wears with use, so its certificate only describes its accuracy on the day it was measured. Many laboratories recalibrate plates in daily use about every 12 months, and plates in clean, light-duty rooms about every 24 months, shortening the interval if the plate shows wear or is checked often.

Overall flatness is typically measured with electronic levels, an autocollimator, a laser interferometer or a master straightedge. Local flatness is checked with a repeat reading gauge. A plate that has worn out of tolerance can often be re-lapped on site by a skilled technician.

Simple habits extend the plate's life:

Keep the surface clean and covered when not in use.

Set the plate on its designated support points and keep it level.

Avoid dropping tools or dragging heavy parts across the surface.

Keep the plate away from heat sources, direct sunlight and strong overhead lighting.

Rotate the areas you use most, so wear spreads evenly.

UNPARALLELED® Granite Surface Plates and Measuring Tools

UNPARALLELED® makes granite surface plates, square rulers, straight rulers, V blocks and parallels from high-density black granite with a density of about 3,100 kg/m³. Our team works to DIN, ASME, JIS and GB requirements, and our master lappers have decades of hand-lapping experience.

Every plate is verified with Mahr dial indicators (0.5 µm resolution), WYLER electronic levels and Renishaw laser interferometers. Our instruments carry calibration certificates from the Jinan and Shandong metrology institutes, traceable to the national metrology institute. Customers can request inspection videos before shipment.

Frequently Asked Questions

What is the difference between Grade 00 and Grade 0? Grade 00 is the finer grade. Under the common formulas, its flatness tolerance is about half of Grade 0 for the same plate size.

Is ASME Grade AA the same as DIN Grade 00? They are close in purpose and often treated as equivalent, but the tolerance formulas differ. State the standard, grade and plate size on your order.

Do I need both overall and local flatness on the certificate? Yes. Overall flatness shows the whole surface is within tolerance, and local flatness shows there are no sharp waves or dips in a small area.

How often should I recalibrate a surface plate? About every 12 months for daily use and about every 24 months for light use in a clean room, or sooner if the plate shows wear.

Can you supply plates with inserts, T-slots or a stand? Yes. Send your drawing and we will quote a complete plate with the features you need.

Get a Quote for Your Granite Surface Plate

Send your size, grade, standard and quantity to info@unparalleled-group.com and our engineers will reply within 24 hours. You can also visit www.unparalleled-group.com to see our granite measuring tools and precision granite components.