A cutting bed impeller comprises a body portion and a plurality of paddles projecting outwardly from the body portion, at least one of the paddles comprising a radially outer face extending between a leading edge and a trailing edge in a normal direction of rotation of the impeller about an axis of rotation of the cutting bed impeller, a leading face, and a trailing face. The radially outer face extends between the leading face and the trailing face. The leading face is inclined at a first angle &agr; to a first tangent T1 to the radially outer face at the leading edge. The trailing face is inclined at a second angle &bgr; to a second tangent T2 to the radially outer face at the trailing edge, the second angle &bgr; being less than the first angle. The radially outer face of one or more of the paddles may be provided with one or more replaceable wear elements.