Interpretation of size effect in orthogonal machining of composite materials. Part II. Sheet moulding compound

Abstract

Long-term orthogonal cutting tests were carried out on sheet moulding compound, using high speed steel tools with 0° rake and 5° relief angle and a constant depth of cut, in order to verify the effect of tool wear on cutting forces. The tests were periodically interrupted, and the variation of cutting forces as a function of depth of cut at various degrees of wear was measured. It was observed that the thrust force is much more sensitive to wear than the principal one. The tool wear features suggested negligible sliding of the chip against the tool face, and intense friction along the tool flank, in agreement with previous results concerning the machining of unidirectional carbon-fibre-reinforced plastics. Accordingly, a force model previously proposed for unidirectional composites was applied to the cutting data. It was found that the increase in the principal force with wear is attributable to the increase in the thrust force generated at the tool flank, whereas the force arising at the tool face is substantially unaffected by tool wear. This allowed for the definition of a unit cutting force independent of both the depth of cut and the tool wear.