The Rockwell Hardness Chart measures the depth of indentation upon application of a total force under a preliminary force to determine the relative hardness of various metals and some plastics. The measurement is relative because the measurement has no units.
Invented by Stanley P. Rockwell and first used in 1919, the Rockwell Hardness Chart is based on the measurement of the depth of an indentation made by pressing a tiny diamond point into a material. This point is commonly spherical in shape. Scientists push the ball into the material for a certain amount of time and then measure the depth of the resulting indentation. After, the force is increased at a set rate until it reaches the value of total force before being reduced back to the preliminary force level. Typically, specialized machinery performs the task of applying this force to provide highly accurate results.
Separate equations exist for determining Rockwell hardness measurements through the use of spheroconical diamond points and balls. Additionally, separate equations exist for determining regular hardness and superficial hardness of tested materials. Scientists use separate charts to determine the relative regular and superfical hardness of materials, comparing them to the hardness ratings of common materials such as steel, cast iron, aluminum and lead.