The test is done by measuring the penetration depth of a sphere under a large load compared to the penetration made by a reference preload.
The Rockwell scale is divided into 9 sub-scales marked by letters A through K. Each scale uses a different reference load and sphere sizes. In machining, the most common is the “C” scale called H R C. Machined metal is usually between 10 and 65 H R C.
Brinell is also a common unit used for listing the hardness of steel materials. The test is done with a 10 millimeters steel ball pressed with 3000 kilogram-force (About 6,600 Libras). The diameter of the resulting impression is measured under a microscope, and the Brinell value (also called H B) is calculated from it. Typical values for machined materials range from 100 H B for very soft materials up to 650 HB for heat-treated steels. The advantage of Brinell over Rockwell is that the whole range is covered on the scale, whereas in the Rockwell C scale when the hardness is below 180 HB, you must switch to Rockwell B.
Vickers is a common unit for measuring the hardness of cutting materials such as carbide grades, Ceramics, CBN, and PCD. The test is done by measuring the surface area of the indentation created by a diamond in the shape of a square-based pyramid. The HV number equals the force used divided by the area of the diamond impression. Carbide grades are usually in the range of 1,300 to 1,900 HV. Ceramics can reach a hardness of 2,000 HV, CBN up to 3,000 HV, and PCD up to 6,000 HV.