These plates are of close grained cast iron. They are amply ribbed to minimize deflection under load. The composition of the metal, the depth and cross-section of the ribs and the thickness of the plate are designed to conform to the deflection limits specified in IS 2285 -1963.

Surface plates are manufactured to two tolerance standards. - Grade-I and Grade-II. Grade-I plates are more precise than Grade-II plates. The deviation in flatness for Grade-I and  Grade -II  plates for sizes upto and including 2000 x 1000 mm are as prescribed in IS 2285-1963.

Grade-1 plates are artificially seasoned.They are hand scraped to achieve maximum local deviation from flatness of 5 microns over any length of 300 mm at any points on the working face , in plates of  630 X 400 mm and larger. Aspecial scraping technique is used to achieve a uniform distribution of load bearing points and oil pockets. An average of 15-20 load bearing points , after finish scraping , is standard. This finish scraping technique ensures that the area of the load bearing points increases  after the intial dulling of the high points. This guarantees a long working life, at the required accuracies.

Grade-II plates are accurately machined to get the requisite flatness.