Abstract: In developing an integrated CAD/CAM system for multi axis NC machining, especially for the design and manufacturing of ruled surface, a kind of curved suface used frequently in engineering is defined as twisted ruled surface. Its main feature can be depicted by twist angle and degree of twisting. For finding the tool axis vector, a feature offset method is proposed to minimize machining tolerance under the condition of circumferential milling. In order to improve the machining process,the inter relation ship among degree of twisting,cutter radius and tolerance is discussed.