Novel selection system of ultra-precision machining tool for optical lens

The machining profile will always deviate from the designed profile, due to the finite tool radius of diamond turning tool. This deviation would occur theoretically before the commencement of actual machining process and introduce error in ultra-precision machining. Conventionally, tool path design will try to minimize the form errors which induced by the finite tool radius (FETR) geometrically. However, for optical components, the best geometrical approximation does not always match the best optical approximation. Thus, merely improving the form accuracies might not be an effective mean to attain a designated optical requirement which would be testified by comparing the designed profile with the measured profile. The measurement would be achieved by using a high precision interferometer. The optical lens would be machined again using tools with reduced tool radius, if the machined profile does not satisfy with the requirements. As tools with smaller radii are more vulnerable to tool wear, such tool will cause larger form errors. Thus, the optimisation process to look for an appropriate tool through trial and error is inevitably time-consuming. To this end the impact of FETR should be predicted and a guideline on selecting a suitable tool to machine an optical lens should be developed for saving time. In this paper, a novel selection system of single point diamond tool for ultra-precision machining of optical lens is proposed. By taking into account the FETR, this system will generate simulated machining profiles for different tools from a designed profile and then simulate the optical performance of these machining profiles based on an optical ray-tracing model. With the help of the ray-tracing model, the wave-front aberration of a simulated focal spot would be estimated. The optimal diamond tool would be selected based on two criteria. It must be able to produce a focal spot with an acceptable amount of aberration while the tool radius should be the larger the better. Based on this proposed selection system, the machining effect of the optical lens can be predicted and the trial-and-error cycle can be shortened.