Influence of tooth passing frequency, feed direction, and tool overhang on the surface roughness of curved surfaces of hardened steel

The task of milling hardened steel with complex-shaped surfaces (e.g., for dies and molds) differs from that of traditional milling. The tool is usually long with a small diameter, and therefore vibrates easily. Its trajectory must follow the surface shape, causing its effective diameter in contact with the workpiece to change as the cutter travels along the surface. Consequently, cutting speed also varies, which may affect tool life and surface roughness. Since the tool is not rigid, the tooth passing frequency, which is the main frequency of the cutting forces, may be close to the natural frequency of the tool, often leading to high vibration and even chatter. This study seeks to contribute to a better understanding and use of this type of process. Several experiments were performed, using tool overhang, lead angle, tooth passing frequency and feed direction as input variables. Three cutting force components and surface roughness were measured throughout the experiments, and the tool’s frequency response functions for the two tool overhang lengths used in the experiments were calculated to better analyze the results. The main conclusion was that feed direction was the input variable that most affected surface roughness, since the use of descendant milling led to lower variations in radial force, and hence, less vibration.