Measurement-Machining Integrated Manufacturing Method And Technology Of Large Irregular Thin-walled Parts

There are a number of large irregular thin-walled parts with high-performance machining requirements in national major projects, including aerospace, carrying and defense. Rocket nozzles, rocket propellant isolation layer structure and rocket fuel tank panel are three typical parts, which share some common manufacturing features, such as large size, complex shape, low structural stiffness, difficult-to-machine materials, etc. As a result, the structural deformation error is easily caused at semi-finishing stage, which is much larger than the tolerance. If the codes in accordance with the original design model are directly adopted in NC machining, the machined parts can not satisfy the accuracy and performance requirements. Therefore, it is very difficult to machine those parts. The dissertation is carried out according to the large/heavy liquid rocket engine development project from China Aerospace Science and Technology Corporation. The subject has also been funded by Natural Science Foundation of China (key program grant No.50835001) and Advanced Research Foundation (key program grant No.9140××××××××××××0902).In this dissertation, the machining problems of large irregular thin-walled parts are analyzed. An efficient and precision digital machining theory and some key technologies are researched.A troublesome problem arises for the large irregular thin-walled parts. It is the actual target surface which is inconsistent with the design model. To obtain the finishing target surface, a novel surface generation method is developed taking dimension error or structure deformation into consideration. It can satisfy the digital machining method which integrates measurement, surface redesign and machining. The spatial pose of measurable reference surface, datum surface and target surface are initially analyzed. A modeling technique for target surface redesign is developed based on E.Cartan moving frame theory and surface accompanied theory. And a computing method for surface redesign is then researched utilizing the measured data of geometrical parameters. On the base of the analysis mentioned above, the finishing target surface can be redesigned in accordance with performance constrains.In order to imrpove the coordinate extracting speed and accuracy of large complex geometric profile, a fast and precision non-contact laser scanning method is developed based on iso-planar principle. Both the geometric information extraction technology for irregular surface with four sides and the measuring error prediction of laser triangulation method are researched. To avoid losing key geometric information as equal step strategy, the calculation of measuring control step is expanded from2D space to3D space. Further, coordinate points are adaptively distributed according to curvature characteristics. Sampling control points are planned using design model of model-known surface. But for the model-unknown surface, besides the measured information, a hybrid extrapolation method and a quadratic function are essential for optimizing feeding step and side interval. In the following, the impact of incident angle on measurement accuracy is quantitatively analyzed, which is on the base of strong correlation of displacement detection error and surface geometric features.The milling dynamics process is modeled and the stablility is predicted against low machining efficincy and chatter of stainless steel material. A mechanics model to predict three-dimensional cutting forces with runout effect is developed, as well as a two-degree-of-freedom milling dynamics model of’spindle-cutter-workpiece’system is also established. The present cutting force models mainly focus on some special end mills. Based on oblique mechanism and material constitutive relations, a general3D steady-state cutting forces model for parameterized mills is formulated. Then, the stability is predicted using multi-frequency method which takes high harmonics effect into account. As a result, cutting parameters can be optimized according to stability lobe.To solve the problems of the existing copying horizontal milling method for liquid rocket engine nozzle, such as bulging uneven, process dispersion, low machining efficiency and etc, a vertical machining method for straight cooling channel of rocket nozzle is invented in this dissertation. The digital slot milling strategy is designed for rocket nozzle. A high stiffness fixture is developed for vertical clamping of nozzle. Further, a multi-stage machining system is developed. Geometry parameters measurement and slot milling can be automatically accomplished under the condition of one-time setup. Finaly, a special machine with dual-spindle and double-column structure for slot milling is manufactured.The proposed measurement-machining integrated manufacturing method and the special digital machining equipment with high efficiency and high reliability has been successfully applied to the new generation of liquid rocket engine nozzle, which has made a positive contribution to the development of China’s aerospace industry.