| Measuring casting state precisely and quickly, analyzing machining allowance and compensating processing allowance accurately have a great significance for improving machining process design and product quality. Based on digital measurement technique and reverse engineering theory, this paper realizes the CAD model reconstruction of complex casting blank, and it completes the optimal blank allowance allocation under constraint conditions by allowance optimization and ananysis technique. It could be listed as following:(1) Based on the3D digital measurement technique, some experimentes were taken by the photoelectronic measurement technique, such as ligting point measuring, marker photogrammetry, portable light pen contact measurement technique and so on. A suitable profile measuring method is proposed for complex structural casting blank. The3D measurement uncertainty is less than0.1mm in the range of1M in all directions.(2) CAD model reconstruction of complex casting blank could be finished based on the reverse modeling technique. According to the base of blank contour, the method about the reconstruction of actual blank modelis is researched, including the preprocess of measuring base, classifing the structural features, reconstructing of free form surface, modelling whole structural part etc. Based on it, CAD model of blank is constructed in accordance with the experimental base.(3) The blank allowance optimal allocation under constraint comditions is realized based on the model comparative analysis. The technique of machining allowance allocation optimization is proposed. Allowance analysis module of the software is developed by the initial relationship between the processing model and the measuring base, which could be optimized by blank measuring base and processing model under allowance constraint automatically. It could give visual output and report of allowance allocation. Allowance analysis uncertainty is less than0.3mm.(4) The real attitude of blank in actual coordinate system of machine tool is analyzed and the machining coordinate transformation is canculated by using auto-positioning function of machine tool. The optimal machining positioning base is finished by software system. Coordinate transformation of machining positioning base is realized by the acquiring technique of actual coordinate system of machine tool and the method about how to feed back the positioning base. The requirements are the basis of the alignment optimization for measuring base and processing model. Processing base could be obtained authomatically by software. The real machining of processing base could be obtained reasonably according to the analysis results of software.As application of machining allowance analysis and processing compensation of casting blank, the comprehensive and verification experiments were conducted for a typical part. The experiments includes the whole process of projection, such as blank measuring, base processing, model reconstruction, allowance analysis, coordinate base positioning, machining simulation and post processs.The results show that:(1) The integrated optical measurement techniques could solve the problems about how to measure the full-size complex castings with high precision and effectivity. The application of using measurement base to reconstructe CAD model could meet the accuracy requirement.(2) The blank allowance allocation optimization techniques could accurately allocate blank allowance under the constraints to meet the needs of the special structure of parts, and provide data to support the process compensation.(3) By the machine self-positioning function, the blank real attitude of the machine coordinate system in the actual processing could be obtained. Through adaptive machining programming, the precision and efficiency of machining complex structures part could be improved. |
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