Machinability of metals and machining costs

THE MACHINABILITY of a metal may be defined in various ways, but none of the existing definitions seems to rate a metal properly in this respect. The values of the cutting speeds for 60 min tool-life, Vr0, of different metals, which are quite often used as machining characteristics of one metal relative to another, may suffice when making very rough estimates of this factor. However, with the present day requirements of the metal cutting industry it is not only necessary to investigate whether or not a 60 min tool-life is a good economic average value, but also to know if the V60-value stated corresponds to an economic combination of feed and depth of cut. Besides, it is quite evident that such a combination cannot be the same comparing one metal with another. Limiting factors, such as surface finish or tolerance requirements, available power, vibrations, inconvenient forms of chips and so on, also contribute to make it difficult for the metal producer or the production shop to obtain unique values of the machinability of a metal. However, tending to rough machining operations in turning, shaping, milling and grinding the above mentioned limiting factors are less important as to the metallurgical and the mechanical properties of a metal than the time of machining it, i.e. the cost of machining a unit volume of a particular metal. It is this cost or this production rate, which, in general, is the most important machinability characteristic of a metal. In this paper the concept of machinability is defined by a term called the productivity p referring to machining with optimum cutting data such that either cost is a minimum, or production rate is a maximum, i.e. when only the time for tool replacement is taken into account. This productivity is a function of the tool-life equation of the particular tool-metal combination and also of the general level of the cost components forming the machining cost in a particular company. Its absolute value thus depends on the magnitude of costs such as cost of machine, operator, regrinding and overhead cost. A relationship between productivity and machining cost is derived and a nomogram for the computation of cost per part Q is shown. This nomogram is particularly suitable for examining the relative influence of tool-life, tool regrinding cost, and the sum of handtime, set-up time and delay-time, when machining time is held constant. Comparisons between the cost due to varying tool-lives from different metals using constant machining and tooling data illustrate one example of its usage. In the previous theories of metal cutting generalized tool-life equations of the Taylor type are used as a first approximation of tool-life curves. This approximation is good enough for many purposes, but there are many phenomena for which these equations yield invalid conclusions or erroneous results, e.g. in machining economics studies. Such generalized Taylor equations have been developed for turning, where tool-life T, cutting speed V, feed s and depth 'of cut t are the considered variables. In milling, similar types of