Machining of large components for the aerospace industry requires, in addition to sophisticated machining strategies a targeted monitoring of the machining quality due to the enormous demands on precision. For the acquisition of the as-is geometry after single machining steps, a non-contact system based on a confocal chromatic sensor was developed, with whose help the as-is geometry can be compared with the target geometry even in deep cavities at aspect ratios up to 40. The sensor system is coupled with the standard interfaces to a machine tool and performs measuring cycles based on modified NC programs. Data acquisition is done with a control-connected EDGE system. The result of the target/as-is comparison is the basis for CAM planning for subsequent machining. In addition to the design of the measuring system and the integration into the ecosystem of the machine system, a concept to calibrate the measuring system for operation on the machine tool was developed. Based on a simulation model of the measurement system various aspects of the calibration approach were analysed.