| NC technology has achieved great development since it was born more than 50 years ago. Conventional programming for numerical controlled machine tools is based on ISO 6983( G&M Codes) which is process-oriented, so high level information such as 3D geometry, tolerance information and surface roughness, will lose when NC program is transferred to the CNC. Information can not be completely transferred between CNC system and CAD/CAPP/CAM. STEP-NC is an extension of STEP standard in manufacturing field, which has the same geometric description as STEP. STEP-NC has broken the bottle-neck of the information transmission between CAX and CNC in digital manufacturing, and become the foundation of CIMS(computer integrated manufacturing system).Firstly, the architecture of the STEP standard was described, and STEP-NC data model was analysed. Then the techniques to generate STEP-NC programs were researched, which included the establishment of mapping rules from EXPRESS data types to C++ language and SQL Server data types, feature recognition technology of STEP AP 203 files, macro process planning methods, STEP-NC file output, and CAD / CAPP integration.Based on open architecture and modular methodology, functional framework of STEP-NC standard-based milling CNC system was proposed, which included NC program interpreter, micro process planning module, motion control module. The interpreter mapped the information in the entity instances of program into memory. The micro process planning module was used to display, match and specify process parameters, machine tool functions, and on-line generate tool path intelligently. The motion control module called the interpolation algorithm according to the cutter location file,and drove machines in steps.Tool paths for planar faces, holes and cavities based on two5D manufacturing features in STEP-NC were investigated. Based on .NET platform, STEP-NC-compliant CNC milling system prototype was developed in Visual C++ language. STEP-NC file read, syntax check, micro process planning and tool path simulation were illustrated for the prototype software by examples. Tests were carried out in a three-axis CNC milling machine, and the feasibility of the prototype software and CAPP / CNC integration were verified. |
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