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To determine hardness on the Rockwell "C" scale, the test machine is fitted with a diamond cone called a brale. The machine presses the brale into the sample with a load of 150 kg for a fixed time.
Results
When the load is removed, a dimple is left in the sample. The test engineer measures the depth of the indentation and applies the formula of HR = E - e, where "HR" is the Rockwell Hardness number, "E" is a constant of 100 for tests done using the brale and "e" is depth of penetration into the sample measured in increments of 0.002 millimeters.
Interpretation of Test Results
The meaning of the Rockwell C Hardness test depends on the application. A quality engineer might test a sample of parts for hardness consistency. A manufacturing engineer might test a sample of feedstock against a standard hardness. Finally, finished goods, such as tools, can be tested to rate their durability.
The "C" scale is unitless. However, for perspective, the score on the "C" scale can be cross-referenced to the Mohs Hardness scale, which rates hardness against items in nature. A Rockwell "C" hardness score of 65 is approximately a Mohs score of 8, which means the item is as hard as a topaz.