Rosenhain and Sturney revisited: the 'tear' chip in cutting interpreted in terms of modern ductile fracture mechanics

摘要

Rosenhain and Sturney in 1925 identified the 'tear' chip, in addition to the well-known types of chip found in metal cutting, namely continuous with or without a built-up edge and discontinuous. Tear chips occur at deep uncut chip thicknesses and, since their formation results in undesirable surface finish, they have been largely ignored in subsequent analyses of machining. A recent paper by Atkins shows that metal cutting is from the class of ductile fracture problems where there is complete plastic collapse in the formation of the chip. It was demonstrated that incorporation of significant work of surface separation (ductile fracture toughness), in analyses of machining with continuous chip formation, explains many features of metal cutting which traditional 'plasticity and friction only' treatments cannot, in particular why the primary shear plane angle is material dependent. It also explained why finite element simulations of machining have to employ a 'separation criterion' at the tool tip. The new model is extended in this paper to predict quantitatively the conditions under which the tear chip forms. The production of other well-known types of chip is also considered, and the results are applied to diagrams relating combinations of tool rake angle and uncut chip thickness at which different types of chip are formed. It is demonstrated that the analysis has general applicability to the cutting of other materials such as plastics and wood for which similar diagrams exist.