Deflectometry for Ultra Precision Machining - Measuring without Rechucking

By ultra precision turning it is possible to manufacture aspheric optical surfaces with a roughness of less than 10 nanometers. To achieve the highest possible shape accuracy, a sequence of iterative rework runs is necessary. However, this requires to measure the workpiece within the limited working space of the machine in order to avoid rechucking. On the other hand, typical workpieces can exhibit a very high angular dynamic which has to be covered by the measurement system. Until now, no optical measurement technique was able to meet these challenging demands. Phase Measuring Deflectometry (PMD), which has already become an established tool to measure specular freeform surfaces, offers a means to solve this task. However, some major difficulties have to be overcome: due to the limited working space, data has to be gathered from different observation directions. Combining this data requires an accurate knowledge about the directions involved. We solve these problems by exploiting the flexibility and inherent accuracy of the machine axes during the measurement process. We will present first results of high angular dynamic measurements without re-chucking the workpiece.