The scale of relative hardness, developed by Stanley F. Rockwell around 1920, runs from C20 to C80.
Each point on the Rockwell scale represents 80 millionths of an inch of depth when the steel is tested with a sharp point and a standardized weight.
The higher the number, however, the harder the steel. In general, the harder the steel in the knife, the sharper the edge it takes.
There are other hardness scales, but the Rockwell scale is especially suited to the rapid inspection of parts.
In one device, the part to be tested is put under a diamond penetrator, ground to a curved conical point.
First the minor load, a weight of 10 kilograms, is applied, and the zero point is determined. Then the major load, a weight of 150 kilograms, is applied for two seconds.
The device displays the hardness number, based on how far the point penetrates.
The minor load is applied first to get down into the material a little bit, to get beyond surface irregularities that might influence the hardness test.
