HHIP 12 X 12 X 2" Cast Iron Hand Scraped Surface Plate Made from close grained castings stress relieved, precision ground and hand scraped for high accuracy. Used in various industries for tool making, inspection, spotting, gauging etc. Very sturdy, heavy duty construction. Holds magnetic bases easily. Flatness accuracy of 8.0 microns refers to a geometric tolerance that specifies the maximum allowable deviation of a surface from a perfectly flat plane. In simpler terms, it means the surface must stay within a tolerance zone of 8 micrometers (µm) or 0.008 mm.