In this paper, we present a PC-based retrofitting for the manufacture of free surfaces with built-in CAD/CAM feature. The retrofitting scheme is based on an integrated NC system (INCS) structure comprising modules for CAD, CAM, and CNC. The INCS scheme is implemented on a two-axis NC milling machine by developing an integrated software and reconfiguring the existing hardware. The retrofitted system allows users to perform the whole NC procedure including: (a) geometric modeling of free surfaces, (b) CL-data generation, (c) graphic simulation, and (d) direct machine control (without G-code conversion). The performance of the retrofitted system was tested via computer simulation and actual machining. Implementation methodology together with software and hardware development is given with adequate illustrations.