Research On Key Technology Of STEP-NC-based NC Mill Machining

Conventional programming of numerical controlled machine tools is based on G-Codes (ISO 6983), no high level information (such as 3D geometry, process technology, and so on) has been included in the NC program which will be transferred to the CNC. And no information can be feed back to the CAD/CAM/CAPP, so that the empirical knowledge of the workers can't be used.ISO 14649 (STEP-NC) extends the current STEP standard into the application area of manufacturing. STEP-NC, which is based on the STEP standard and uses the same geometry description method , increases the efficiency and flexibility of NC programming by means of a structured, object-oriented machining description. Therefore, STEP-NC-based NC program will not only transfer the 3D geometry of the machined workpiece into CNC, but also the process technology and machining information, which will overcome lots of disadvantage of the conventional ISO 6983-based NC program.The outstanding characteristic of STEP-NC-based NC program is that it is neutral and notspecific to any CNC system. Based on the newest international standard--STEP-NC, theauthor studies the key technology of NC mill machining, the main contents are described as follows:1. The shortcomings of conventional NC programming data interface--ISO 6983 waspointed out. The develop course of product data exchange has been analyzed, and it was pointed out that STEP-NC would be used as new NC programming data interface replacing ISO 6983, which has been used more than 50 years.2. The rationale and framework of STEP-NC data model was introduced, and the relationship between STEP-NC and STEP was analyzed. The principle difference of STEP-NC, which was used as Application Reference Model, and AP 238, which was used as Application Interpret Model, was pointed out. And the difficulty of using STEP-NC or AP 238 as implementation method was studied. Three kind of different implementation method were introduced and compared, and ISO Part 21 physical file format was adopted as implementation method in this paper.3. The generating process of STEP-NC Part 21 physical file was studied, which was composed of three parts: feature recognition, macro process planning and mapping into Part 21 physical file. STEP-NC manufacture feature was recognized from the input AP 203 physical file, and was used as the input of macro process planning.4. Based on open architecture and modular idea, functional framework of STEP-NC millingCNC system was proposed. The main functional parts included NC program interpreter, micro process planning, tool path generation and motion control card. In the micro process planning module, after analyzing the logical relationship of the main entities in the STEP-NC NC program, arithmetic of extracting and updating mill_technology and mill_machine_function were proposed.5. STEP-NC numerical controlled program interpreter was studied, including mapping from EXPRESS to C++, grammar and syntax analysis, etc. A kind of memory-based NC program interpreting method was proposed. Workingstep was used as basic unit, arithmetic of generating process sequence tree was studied.6. Two kinds of two5D_manufacturing_feature: planar_face and open_pocket were studied, and methods of tool path generating were proposed when different machining strategy were used. Firstly, the method of extracting the useful information of STEP-NC manufacture feature was proposed;Secondly, arithmetic of matching mill tool which was recommended in the STEP-NC NC program with the tools that are exist in the current tool library;Finally, methods of generating tool path for planarface and open_pocket were studied, including approach and retract route, coordinates of cutter location, and connection method of two workingstep.The emergence of STEP-NC as a new technology will bring weighty influence on the CNC system. STEP-NC-oriented open architecture CNC will be the trend of CNC development, it will lead to re-partition of function and framework of CAD.CAM and CNC, and this will promote the integration of design and manufacture.