Abstract: Aiming at the data model and programming character of object oriented of STEP-NC, with design thinking of software module applied, an interpreting module including four sub-modules is built. ① Inputting sub-module is responsible for parsing STEP-NC program, and a special data structure is designed to store the entity information of program line. ② Recognizing sub-module can recognize all kinds of entities in the machining program, and a fixed recognizing order is pro-posed to guarantee the accurate result. ③ Modeling sub-module establishes corresponding C++ class model by entity mapping according to entity type. ④ Planning sub-module plans the tool path according to the class model parameters, and its run-ning process is explained. A complicated problem involved in path planning is coordinate transformation. The hierarchy frame of coordinate of STEP-NC is summarized and the coor-dinate reference principle is elaborated. By visual tool path in a simulation system for 3-D milling machining based on STEP-NC, the path planning is proved to be right. At last the interpreting module is planted into an open CNC and confirmed to be valid and correct by real machining experiment.

Key words: Coordinate system, Interpreting module, Path planning, STEP-NC