Intermediate Process Model Construction For CNC Machining Of Aircraft Structural Parts

Aircraft structural parts are important components of plane body skeleton and aerodynamic shape and are widely used in the design process of the Aircraft. 3D process planning includes process route planning, machining resources selection, fixtures set-up design, NC program simulation and optimization, assembly route planning throughout the whole manufacturing process, the precision and efficiency of the process planning relates to the manufacturing precision and efficiency. The intermediate process model is the stock model after current machining process, it reflects the machining process from the stock to the product and it can be used to guide the next process planning and NC Programing, and then improves the efficiency of the process planning and speeds up the progress of process preparation and lays the foundation of high efficiency CNC machining for aircraft structural parts. Therefore, the research mainly concerns about the construction method and software development of the intermediate process model for aircraft structural parts.(1) Realized the construction of the intermediate process model for aircraft straight-wall structural parts. Firstly, based on the tessellation function provided by CATIA, researched the method of rapidly construction of the triangular mesh model. Then, the created mesh model was intersected with a set of parallel planes to extract contour loops of part features. Finally, offset the contour loops by machining allowance, drew sketches of the offset loops, extent the created sketches to construct of the intermediate process model. As the offset algorithm provided by CATIA runs slowly and sometimes cannot get right results, the plane loop offset algorithm based on small segments was put forward, by solving the local and global interference realized the stable plane loop offset by any offset distance. Analysed the tool accessibility in offset contour loops by the two-step offset method, in the first offset, offset the contour loops to obtain the tool center path without interference, and in the second offset, offset the tool center path to generate the processable contour loops.Simplified the orderly data points of contour loops by arc and spline fitting.(2) Realized the construction of the intermediate process model for aircraft structural parts with complex features. Firstly, geometrical features of the part were divided into machining features of holes, ribs, grooves and outer contour features. Then, according to the given allowance and the cutting tool, constructed the cutting envelope volume of single machining feature and the intermediate process model of current feature was created by Boolean operation of subtract with the stock.Finally, constructed the intermediate process model for the part by creating all the cutting envelope volumes of machining features and making Boolean operation of subtract with the created stock in order.(3) Based on CATIA secondary development interface and Visual Studio 2005 programming software, the system of "Construction of the intermediate process model" was developed using C++, rapidly and automaticly constructed the intermediate process model for aircraft structural parts. The system was evaluated by construction speed, construction precision and the accuracy of machining process, the result reflected that the system can construct the intermediate process model with rapid speed and high precision and can meet the process planning requirement. The system had been applied into Shenyang Aircraft Corporation and Harbin Aircraft Corporation with a good effect.