| 1. IntroductionBased on a comprehensive analysis of the research status at home and abroad, several key technologies of geometrical simulation and physical simulation of NC machining are studied deeply with the theory and method of virtual manufacturing. The research work is of great significance for assuring the machining quality, for making fully use of NC machine potential and improving the machining efficiency.2. The establishment of virtual machining environmentVirtual machining environment is the foundation and platform of NC machining simulation, which include virtual machine, virtual fixtures, virtual cutting tools, virtual workpiece and virtual control panels etc. The virtual machine is the key part of the virtual machining environment.The geometric model of virtual machine is a digital description of the actual machine, namely is the mapping of the actual machine in the information domain. From the view of the geometric analysis, the virtual machine is actually an assembly including many parts, and there are certain constraint relationship, fit relationship, location relationship and connection relationship among them. During the establishment of geometric model of the virtual machine, each component (including the bed, spindle box, column and platforms and so on ) should be modeled respectively ,these components can be considered as an assembly with some parts which are formed by set operation of intersection, combination and subtraction of some objects with some regular shapes or their 2D extension.The 3D geometric model of virtual machine and the fixture are established by the OpenGL graphics library with the method of Construction of Solid Geometry(CSG) and Boundary Representation(B-Rep), and some realistic effects treatments (including material, light, texture mapping etc) are applied. The established virtual machine can be zoomed, rotated, translated in the view area. The movement of the virtual machine is driven by the NC code or tool position documents, the inverse kinematics solving method is used for establishing the kinetic model of the virtual machine, the movement rule of midst organization are calculated according to the movement requirement of the performing component.3. Geometric simulation of NC machiningGeometric simulation of NC machining including the technologies of workpiece model dispersing, cutting tool scanning body creating, material removal simulating, third dimension graphics creating, graphics blanking, animation displaying, impact and interference inspecting, geometric precision examining and so on. The essential of material removal simulation is the process of subtracting the scanning body formed by the cutting tool movement from the workpiece. The establishment of geometric model of workpiece and cutting tool is the foundation and prerequisite of the material removal simulation. The triangular mesh model is used to construct the discrete model of the workpiece, for the area in which the curvature is big, the partial subdivision method is presented in this paper ,in order to avoide the whole Boolean operation in material removal simulation, the model is simplified, the display speed and the simulating accuracy are all improved.Material removal simulation is operated through animation technology by using the technology of computer graphics, the workpiece model is dispersed according to the precision requirements, then calculate the next cutting tool coordinates generate the cutting tool scanning model and identify the possible discrete cutting points in scanning region by the NC Code Translator, the intersection coordinate values z of discrete points vector z and cutting tool scanning body are calculated, then update the data model of workpiece and call the displaying procedures to display the machined workpiece model on the screen.Aimed at the defect of algorithm complexity, poor Real-time, unable overall showing the workpiece model during graphic blanking and updating in the process of material removal, a new rapid partial blanking algorithm is proposed, a lot of resolving intersection calculations in the general blanking algorithm is avoided, the running speed is improved, which is realized by the functions of OpenGL color cache, depth cache and moldboard cache by reason that during each cutting in the NC machining process, only the partial section of workpiece graphic, which is cut by tool is changed. The partial update technology is adopted in this paper, it need only to calculate and show the scope of the changed part of workpiece and cutting tool model. The real-time animated display in the simulation process is achieved by OpenGL-caching technology which adoptes two alternating caches to realize the smooth animation display in cutting process.The differences between the z coordinates values of various triangular net points of the machined workpiece curved surface and the relavant z coordinate values of various triangular net points of the theoritical curved surface are used directly to examine whether there are existed excessive cutting or owing cutting between the cutting tool and the machined curved surface, the shortcomings of repeated modeling and massive computations can be avoided.In this paper, the impact and interference inspection is divided into following three steps: namely cursory, careful and accurate examination. The improved surrounding box method based on FDH is used for careful inspection of impact and interference by establishing and searching all over the surrounding box level tree and by the intersecting tests of the surrounding boxes and basic units, so as to confirme above two objects whether intersecte or not. Not only the raised polyhedrons but also the concave polyhedrons can be examined by this method ,it possecesses the advantages of quick examination speed and good real time.4. Forecast of NC machining precisionThe widely used CK6136H type CNC lathe is selected as the research object, the influence of the machine geometry error, the force deformation error and the thermal deformation error on machining precision is mainly studied, according to the cause of formation, the respective coordinates of every error are established and the respective error valus are calculated, then let all these error values transforme into technical coordinate though coordinate transformation, and then calculate the comprehensive error values, the comprehensive model of precision prediction is established based on obove analysis and calculation. The cutting force, cutting heat, rigidity of the machining system and other related parameters are obtained by combing theoretical calculations with experiments. The actual example examined results as follows: the forecast value of shaft diameter is 49.160 mm,the measuring value is 49.129 mm,and the forecast error value is 0.031mm;the forecast value of roundness error of shaft is 0.035 mm,the measuring value is 0.034 mm;the forecast value of cylindricity error of shaft is 0.118 mm,the measure value is 0.129 mm。5. Conclusions(1) Not only the geometry models of the cutting tool and the workpiece, but also the geometric models of machine, the kinetic model, the geometric model of the fixture and the virtual control panel are all included in the established virtual machining environment. The established virtual machining environment can be zoomed, rotated, translated in the visible area, the simulation environment is really true to practice and simple operation.(2) The triangular mesh model is used to establish the workpiece model, and the method of partial subdivision is applied to transform the uniform dispersion model. The simulation precision as well as the display speed is improved, meanwhile the workpiece model can be used in the material removal simulation, geometric accuracy tests and impact tests, which can avoide the repeate modeling and calculating, and improve the simulation efficiency.(3) A new rapid partial blanking algorithm is proposed, the OpenGL color cache, depth cache and moldboard cache functions are used to realize the graphic blanking and updating in the process of material removal. The shortcomings of algorithm complexity, Real-time poor, unable overall showing the workpiece model can be all avoided successfully.(4) The improved surrounding box method based on the FDH is applied to inspection of impact and interference by establishing and searching all over the surrounding box level tree and by the intersecting tests of the surrounding boxes and the basic units, which can improve the inspection efficiency effectively.(5)The comprehensive precision forecast model is established based on analyzing the influence of machine geometric (movement) error, the fixture error, the force deformation of the processing system, the thermal deformation of the cutting tool and the workpiece, the cutting tool wear on machining precision. The actual example examined results show that the measure values of shaft diameter error, the roundness error, the cylindricity error of the shaft are on the whole accord with the simulated forecast valus, so the established forecast model for machining precision is correct and practicable.
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