Mechanics of Machining

Investigators in the metal cutting field have attempted to develop an analysis of cutting process which gives a clear understanding of the mechanism involved and which enables the prediction of the important cutting parameters, without the need for empirical testing. Since most practical cutting operations are geometrically complex, we shall first consider the simple case of orthogonal cutting (i.e., cutting with the cutting edge perpendicular to the relative velocity between work and tool), and extend the theories to more complicated processes. Because of its simplicity and fairly wide application, the continuous chip without built-up edge has been most thoroughly studied and will be the major topic in this chapter.

 

 

In the last thirty years many papers on the basic mechanics of metal cutting have been written. Several models to describe the process have been developed; some have been fairly successful in describing the process, but none can be fully substantiated and definitely stated to be the correct solution. Thus, while none of the analyses can precisely predict conditions in a practical cutting situlation, the analyses are worth examining because they can qualitatively explain the phenomena observed and indicate the direction in which conditions should be changed to improve cutting performance.

 There is conflicting evidence about the nature of the deformation zone in metal cutting. This has led to two basis schools of thought in the approach to analysis. Many workers, such as Piispaneu, Merchant, Kobayashi and Thomsen, have favored the thin-plane (or thin-zone) model, as shown in following Fig (a). Others such as Palmer and Oxley, and Okushima and Hitomi, have based analyses on a thick defermation region as in following Fig (b).

 



The available experimental evidence indicates that the thick-zone model may describe the cutting process at very low speeds, but at higher speeds most evidence indicates that a thin shear plane is approached. Thus it seems that the thin-zone model is likely to be the most useful for practical cutting conditions.In addition, it leads to far simpler mathematical treatment than does the thick-zone model. For these two reasons the analysis of the thin zone has received far more attention and is more complote than that of the thick zone.