Sub-scale Machining of Large Components

A post-processing algorithm for CNC machine NC code was developed for altering NC code based on measurements from a stand-alone coordinate metrology system that measures the orientation and offset of the part and machine coordinate systems (CS). With this post-processing algorithm, the ability to machine a monolithic part larger than the working area of a machine in multiple sections becomes possible. A series of test parts were machined with a Haas Vertical Milling Machine (VMM) and a Leica laser tracker as proof of concept and to determine the errors introduced into the machining process by adding the additional metrology system. Monte Carlo simulations confirm that adding the stand-alone metrology system increases the magnitude of errors to approximately the accuracy of the metrology system itself, about 50 micrometers. The initial set of test parts machined showed errors at a factor of 25 larger than expected. It was determined that a component used to determine the machine's spindle location was introducing significant error into the process. The component was replaced and new parts machined that showed a significant improvement but still with errors larger than expected. Machine squareness errors showed the importance of measuring CS features within the same plane. Finally, it was determined that the definition of the part CS is sensitive to how the part is fixtured to the machine. Sub-scale machining is a viable process for machining large components with a slight increase in feature location error.