Rasha machinery has introduced surface plates. These are made of close grained cast iron. These are amply ribbed to reduce deflection under load. The composition of the metal, the depth and cross-section of the ribs and the thickness of the plate are made as per is 2285-1963 specifications. These surface plates are made 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 up to and including 2,000 x 1,000 mm are as given in is 2285-1963. Grade-i plates are artificially seasoned. They are hand scraped to attain maximum local deviation from flatness of 5 microns over any length of 300. Mm at any point on the working face, in plates of 630 x 400 mm and larger. A special scraping technique is used to attain 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 assures that the area of the load bearing points increases after the initial dulling of the high points. This ensures a long working life, at the required accuracies. Grade-ii plates are accurately machined to get the requisite flatness. Standard accessories include handle and wooden cover. Optional accessories include fabricated mild steel stands with three leveling screws; cast iron column floor stands; a set of five, three for leveling and two for support for 2,000 x 1,000 mm plate; and a set of seven, five for leveling and two for support, for 3,000 x 1,000 mm plate.
