NURBS based ultra-precision free-form machining

The machining of ultra-precision optical components with form tolerances in the sub-micron range requires a close interaction between the machine tool, the process and the procedure for the NC tool path generation. Especially for the optical free-form machining the choice of a data- format for the surface description as well as the calculation of the tool path is crucial for the overall achievable quality of the work piece. This paper presents the layout of a tool path calculation based on the NURBS (Non-Uniform-Rotational-B-Spline) data format that has been developed at the Fraunhofer IPT. In addition the interfaces and the hardware and software for the realisation of a NURBS based control unit for Fast Tool Servo (FTS) turning and local corrective polishing operations are described. In a second implementation step NURBS have been used for multi-axis machining. For this purpose a double-V-parallel kinematic system has been developed for NC controlled local polishing. A base machine fulfils a continuous 3D-movement during the polishing process. The NC polishing module realises the superimposing of eccentric movements and angular alignments of the polishing pad. The NURBS data format is used during CAD/CAM operation for computing a G-code (base machine) as well as splines (polishing module). Actual development activities include the implementation of a NURBS control unit with an online trajectory generator for simultaneous multi-axis diamond machining improving computational power in combination with lean algorithms for the NURBS interpolation.