In ultra precision cutting of hard and brittle crystal materials, cutting conditions of an extremely small undeformed chip thickness are required to produce a ductile mode surface and/or super smooth surface uniformly in all crystal orientation. In this report, ultra precision cutting of hard and brittle crystal material with fly cutting, which is one of the interrupted cutting, was newly proposed and tested. In the basic experiment, plane of single crystal germanium was machined with proposed diamond fly cutting, to examine the influence of crystal orientation and the change of critical undeformed chip thickness in 0, -15, -30, -40 degrees of the tool rake angle. From the experimental results, the largest critical undeformed chip thickness was 235nm in -30 degrees of the tool rake angle. And finally aspherical lens were machined. As the result, the form accuracy of 0.19μmP-V and the surface roughness of 32nmRz were obtained.
