A surface plate, also known as a granite surface plate, is a large, flat, and precision-engineered slab made from granite or cast iron. It provides a stable and accurately leveled surface for measuring, inspecting, and marking workpieces in various industries, particularly in metalworking, engineering, and manufacturing.

Surface plates are used to gauge the flatness, straightness, and parallelism of other objects, such as machine parts, tools, and components. They serve as a reference surface against which the accuracy of other objects can be compared. The surface of a high-quality granite surface plate is meticulously ground and polished to ensure a high degree of flatness and smoothness.

Using specialized measuring tools, such as dial indicators, height gauges, or gauge blocks, operators can place and move workpieces on the surface plate to check for any deviations from perfect flatness or alignment. By doing so, they can identify imperfections and take corrective measures to ensure the quality and precision of the components being produced.

Surface plates come in various sizes and grades, with higher grades having tighter flatness tolerances and greater accuracy. They are a fundamental tool in metrology and quality control processes, helping to maintain high standards in manufacturing and ensuring the reliable performance of machined parts and assemblies.