INTERPRETATION OF SIZE EFFECT IN ORTHOGONAL MACHINING OF COMPOSITE-MATERIALS - PART II - SHEET MOLDING COMPOUND

Long-term orthogonal cutting tests were carried out on sheet moulding compound, using high speed steel tools with 0 degrees rake and 5 degre es relief angle and a constant depth of cut, in order to verify the ef fect of tool wear on cutting forces. The tests were periodically inter rupted, and the variation of cutting fords as a function of depth of c ut at various degrees of wear was measured. It was observed that the t hrust force is much more sensitive to wear than the principal one. The tool wear features suggested negligible sliding of the chip against t he tool face, and intense friction along the tool flank, in agreement with previous results concerning the machining of unidirectional carbo n-fibre-reinforced plastics. Accordingly, a force model previously pro posed for unidirectional composites was applied to the cutting data. I t was found that the increase in the principal force with wear is attr ibutable to the increase in the thrust force generated at the tool fla nk, whereas the force arising at the tool face is substantially unaffe cted by tool wear. This allowed for the definition of a unit cutting f orce independent of both the depth of cut and the tool wear. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.