Research On Adaptive And Real-time Simulation Key Technologies For NC Machining Of Free-form Surface

The integration and intelligent development tendency of modern advancedmanufacturing technology puts forward higher requirements for thedevelopment of high-end Computer Numerical Control(CNC) system. Realisticdisplay of the CNC machining process will become one of the necessaryfeatures for high-end CNC system. The International Academy for ProductionEngineering(CIRP) has appointed a panel to evaluate the production mode ofcurrent and future manufacturing, the result show that simulation modelingproblem of the machining process has become one of the most importantresearch fields in manufacturing. NC machining simulation can not onlyprovide intuitive visual form of the machining process, but also present anefficient verification method for NC program. During the machining simulationprocess, the error,over-cut and under-cutting problems of the NC program willbe checked in time. Furthermore, the collision problems between the cutter andthe workpiece or other machine parts can be detected as well. Thus, NCmachining simulation has become a hot and difficult problem in advancedmanufacturing field. To this end, the NC machining process simulation andcollision detection technology is suggested in the project of The Major NationalS&T Program: High-grade CNC machine tools and basic manufacturingequipment and other important projects.On analyzing the research situation of CNC machining simulation at homeand aboard, in order to conquer the problems and limitations of the researchstatus, and satisfy the Major National S&T Program and high-end CNC systemdevelopment needs, this paper studied on the key technologies of real-timesimulation for NC machining of free-form surface. The main contents are asfollows:1. The new modeling method for NC machining real-time simulation offree-form surfaces.The current modeling methods are mainly suitable for the regular shapegeometries, and can't express the complex change characteristics of free-formsurface parts during the simulation process. Therefore,based on the advantagesof the Z-Map model and Level-of-Detail(LOD) technology, this paper presented a new modeling method that based on Advanced Z-Map and Binary Triangulartree LOD(BTLOD) model for NC machining simulation of free-form surface.First, the Advanced Z-Map structure(AZ-Map) was proposed to overcome theshortcomings of Z-Map structure. Then, we proposed the concept of BTLODmodel. The link relationship of AZ-Map and BTLOD model is established toform a kind of fast and efficient dynamic modeling method and descriptionmechanism for free-form surface, which is called AM-BTLOD. The newmodeling method which can describe the complex change characteristics offree-form surface parts during the simulation process will build a solid theoryfoundation for the further research work.2. Research on the real-time simulation key technologies based on theAM-BTLOD model of free-form surface NC machining.This paper proposed an adaptive algorithm for free-form surface NCmachining real-time simulation based on the proposed AM-BTLOD modelingmethod. Using the AZ-Map model to present the initial workpiece, the shape ofthe workpiece is dynamically changed by the control of BTLOD model duringthe simulation process. In order to improve the efficiency and accuracy of NCmachining simulation for free-form surface and make sure that the simulation isreal-time and realistic, a new cutting algorithm was designed based on BTLODmodel. Meanwhile, according to the characteristics of the free surface NCmachining, the triangles split and merge rules of BTLOD in simulation processare designed, and the corresponding triangles split and merge algorithms are putforward. Further,the dynamic frame rate adjustment strategy is adopted.According to different precision demand and different viewpoint, we can usedifferent LOD levels to control the number of triangle patches to be renderedand the frame rate, so as to decrease the computation time, and improve thegraph rendering speed. Therefore, the simulation speed and accuracy can becontrolled adaptively. According to several experiment examples, theeffectiveness, real-time and adaptability of the proposed algorithm have beenverified. Especially, the experiment results show that the real-time performanceof the proposed algorithm is superior to the existing algorithm obviously.3. Accurate modeling and computing method for generalized tool sweptvolume in five-axis NC Machining simulation. Due to two extra freedom of tool movement in5-axis NC machining, thecomputation and visualization of the tool swept volume is more difficult thanthat in three-axis NC simulation. The existing methods either limited to the tooltype and motion, only can be applied in three-axis NC simulation,or they are theapproximate swept computation methods in5-axis NC simulation, which can'tdescribe the actual swept that formed by the tool moving in space exactly, andNC machining simulation precision is not high as well. In order to acquire auniversal and widespread use algorithm, we proposed a new exact tool sweptvolume generation algorithm in5-axis NC simulation. It is based on the theoryof the envelope, designed for a generalized cutter, and suitable for any toolmovement. According to study the surface envelope theory and the differentialgeometry,and analyze the motion feature of the cutter in five-axis machining, ageneralized cutter model is established.Then, using rigid motion theory, thetool's moving frame is established, and the accurate modeling and computingmethod for generalized tool swept volume in five-axis NC Machiningsimulation is proposed. The experimental results show that the proposedalgorithm is correct and efficient, and the swept generated by the algorthm isrealistic and accurate.4. Research on real-time collision detection key technologies forfree-form surface NC machining simulation.The current collision detection algorithms are mainly designed fordetecting the collision problem between two general geometric models, and arenot suitable for the complex collision detection problem in free-form surfaceNC machining simulation, which collision may occur upon many objects. Thispaper analyzed and summarized the collision characteristics of the free-formsurface machining process. The dynamic collision problem in NC machining isstudied deeply. In order to improve the efficiency and the speed of collisionqueries, this paper proposed the multi-level detection strategy. Then, theselection principle of Bounding Volume and hierarchical tree is given,considering the advantages of binary tree and the octree, this paper put forwardsa new concept to describe the bounding volume hierarchical tree, which iscalled Bounding Volume Mixed-Hierarchical LOD Tree or BV-MLOD forshort. Then, the creation method of OBB BV-MLOD tree is given. On the basisof the BV-MLOD and interval arithmetic algorithm, the real-time collision detection algorithm for free-form surface NC machining is proposed, which canovercome the problem and limitations of the existing algorithms. Further, thetraversing algorithm of BV-MLOD is designed, the collision detection betweentwo BV-MLODs is executed during the BV-MLOD tree traversing. Meanwhile,the BV/BV intersection test and the triangle-triangle intersection test algorithmare designed. The typical free-form surface experiment example verified thatthe proposed algorithm is real-time and accurate. The collision detection speedand efficiency of the proposed method is superior to the existing methoddistinctly.