Machinability tests may be subdivided into two basic categories: those which do not require a machining process to take place, and those which do. A parallel subdivision gives two more categories: those tests which merely indicate, for a given set of conditions, the relative machinability of two or more work—tool combinations (ranking tests), and those which indicate the relative merits of two or more work—tool combinations for a range of cutting conditions (absolute tests). The results of the former, whilst extremely useful in many circumstances, have two main disadvantages. Firstly, even if as hoped, a particular test indicates that material A machines better than material B which in turn machines better than material C, in most cases there is no indication of the magnitude of the differences because the measure of machinability does not, in general, correlate on a predictable scale with, for example, the life of the cutting tool under a given set of conditions. Secondly, even if the test does attempt to compare work pieces for a given set of cutting conditions, there is no guarantee that when the cutting conditions change the ranking will remain the same. The results of absolute tests are usually applicable for changes in cutting speed, certainly over the practical range, and in some cases also take account of changes in other cutting conditions and tool geometry. Thus, a machinability test can, in theory, be one of four types but, in practice, a non—machining test is always a ranking test whereas a machining test can be either a ranking or an absolute test.
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