Work material becomes a workpiece when it is fixed in the machine tool (lathe, machining center, milling machine, etc.) and is being machined. Traditionally, guidelines for selection of cutting tools (geometry and grades) are based on tool application considerations with only limited attention to workpiece mechanical properties. The most important mechanical properties from the machining viewpoint are hardness and tensile strength. Knowledge of hardness, tensile strength and any relationship between them, adds to the guidelines for the application of cutting tools, which, in turn, increases the effectiveness of machining (estimating cutting forces, torque values, and required power).
Hardness of metals is a measure of their resistance to deformation. From the machining viewpoint, hardness is the resistance to cutting. Various static indentation and dynamic methods are used to measure hardness. Typical static indentation methods are Rockwell, Brinell, Vickers, and Knoop hardness testing, which use different loads and indenters. A typical dynamic (rebound) method is Scleroscope hardness testing, also known as the Shore method.
Rockwell hardness testing is applied to most metals and alloys, from annealed steels and copper alloys to cemented carbides. There are two types of tests: Rockwell standard and Rockwell superficial. The correct way to indicate Rockwell hardness is with three letters. The "H" indicates hardness, the "R" stands for Rockwell and the last letter indicates the scale in use. There are 15 standard hardness scales: A, B, C, D, E, F, G, H, K, L, M, P, R, S,...
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