Interpretation of size effect in orthogonal machining of composite materials. Part I: Unidirectional glass-fibre-reinforced plastics

Orthogonal cutting tests were carried out on unidirectional glass-fibre-reinforced plastics, using high-speed steel tools. During the tests, the tool rake and relief angle, and the depth of cut t were varied, whereas the cutting direction was held parallel to the fibre orientation. The trend of the principal forces recorded during cutting clearly showed the so-called `size effect': the unit cutting force (also known as `specific energy') was a decreasing function of the depth of cut, tending to infinity for vanishingly small values of t. A previous force scheme, assuming a significant influence of the friction generated by the work material sliding along the tool flank on the overall cutting forces, was utilized to interpret the cutting data, and to differently define the unit cutting force. According to the model, the unit cutting force and the coefficient of friction were independent of both the relief angle and the depth of cut, although the former, as expected, sensibly decreased with increasing the rake angle.