On the origin of cutting forces in machining unidirectional composite materials

Short-term and wear cutting tests were carried out on unidirectional graphite fibre reinforced composites, holding the cutting direction coincident with fibre orientation. During the tests, performed with high speed steel tools, the tool rake and relief angle, and the depth of cut were varied. The observation of tool worn surfaces after wear tests revealed absence of friction along the face; on the contrary, an intense sliding of the work against the tool flank was suggested by the flank surface morphology. Consequently, a force scheme consistent with the information gathered from tool wear examination was built. It is shown that, when the experimental cutting forces are treated according to the new force scheme, the coefficient of friction, f, can be considered independent of the tool geometry and the depth of cut, t. Besides, the unit cutting force X is unaffected by the depth of cut, although it is a decreasing function of the rake angle. On the contrary, if the simplified hypotheses generally adopted in metal cutting are used, both f and X strongly vary with the depth of cut. In particular, the unit cutting force notably increase with decreasing t, undergoing the well known `size effect'.