Ultraprecision turning of aspherical profiles with deep sag

This paper reports on the technology required to produce deep-sag aspherical profiles, such as those found in optical mold inserts. To fabricate such inserts, the diamond tool has to be tilted sufficiently to achieve an effective flank angle of around 30 degrees. The maximum flank angle should be kept lower than 15 degrees to ensure adequate strength of the tool cutting edge. Due to the tool tilt, two types of form errors have to be addressed: the form error arising from the presented tool nose changing from circular to elliptical curvature, and the cone form error caused by the tool flank interfering with the center portion of the machined profile. The first type of form error can be eliminated by using a tool path correction procedure. For the second type of form error, theoretical and experimental investigations were conducted to determine the effects of the diamond tool tilt, the rotated angle and the tool nose radius on the center cone form error. This produces information on the correct tool radius and rotated angle required to minimize the center cone form error. With the above two procedures, the form error of deep-sag aspherical profiles can be improved significantly from 22 μm to 0.11 μm.