Process and machining system for optical surfaces

Conventional manual polishing is the most common finishing process step in an otherwise completely automated process chain for free formed dies and moulds made of steel. As constant process parameters cannot be provided by the worker, manual polishing carefully approaches the goal to obtain fine surfaces down to 5 nm Ra and form accuracies better than 1 m at the same time and requires up to 30 minutes per square centimetre. Therefore, a manual process cannot be implemented efficiently in terms of economic aspects. For steel, ultra precision diamond machining cannot be applied because of high diamond tool wear. The material removal of processes based on indefined cutting edges such as grinding, lapping and polishing are mainly driven by the combination of materials and the process parameters cutting speed, contact pressure and specific duration [1]. In order to perform a deterministic fine machining, thus controlling and keeping these parameters constant is essential. Fu rthermore, the investigation on fine machining of steel surfaces identified micro chipping as most responsible for material removal [2]. For this reason, a modular force controlled machining system, which is guided by a 6-axis robot arm, was developed at Fraunhofer IPT. The machine can perform both rotational and oscillating removal motions and applies an additional rotation for multidirectional surface treatment. With regard to first polishing attempts the system enables the automated fine machining down to 10 nm Ra starting with 400 nm Ra in less than 5 minutes per square centimetre