Research & Development On The 3D Simulation Software System Of Numerical Control Mills

I. IntroductionGeneraly,the component which passed through the CAD/CAM system shouldbe performed the step of Tape Tryout before the offical process in an actualmanufacture system. The process of Tape Tryout aims at verifying the NCprograms. The NC porgrams become more longer and complex because of thegrowth of the component's complexity, this makes the corresponding rate of errorof the NC code grows too.The statistical data indicates the NC code's rate of errorwill be more higher to 3 axis or 5 axis milling . The incorrect NC code will resultin over cutting, less cutting,bad product, even generating the interference andcollision between part and tools , tools and fixtures, tools and worktable, this willbe dangerous and influence the cost directly.The technology of numericla control machining simulation is graduallydeveloped along with computer technology, CAD/CAM technology ,computergraphics,computer simulation. Using this technology can simulate the wholeprocess of tool cutting path and part cutting, people can easily find out the problemmay be occured at actual cutting process ,they can modify the NC code repeatedlyuntil the correct result is produced. and these operation do not consume the realmachining times and meterials.It is very simply and reliably by simulating the machine process to verify NCcode by using computer simulation technology, and many problems which usetraditional machine method can be solved using it. The NC programmer can use itto view clearly the result of the NC code from the computer screen and to make ajudgement whether the NC code be correct or not. then they can modify the NCcode and optimize the toolpath. And this makes the machine process reliabilitybe good and makes efficiency be high.In domestic, the machine tools construction enterprise have already makevery big progress through the unceasing development, but be compared with theoverseas similar enterprise's level,we have the very big disparity in the dynamicdesign, the structure optimization as well as the new product development abilityand so on , in related software product development, research and developmentability also is relative insufficiency. The machine process simulation softwarewhich act as the numerical control machine's necessary software has the quiteimportant status in the entire software. Our goal is to research the core question ofthe algorithm of the milling process simulation module and develop it. Thisresearch project that its objective is to develop CNC plus NC simulation systemcomes from the Singapore country controlled joint-stock company of STL, someworks just like lathe process module have been developed by other people in ourcollege.II. Geometry Modeling Technology and Boolean Operation for NC simulationNC machining simulation research on international starts at 19 century 70's,in the early time, researchers usually use the line diagrams to realize themachining simulation and verification. The effect using that method changes verybadly when the model was complex component geometry model. The appearanceof Solid Geometric Model System makes the simulation and verification forcomplex component become possible, One typical method used in this kind ofsystem is CSG,which has the good Boolean calculation ability,but waste more timein calculation and the simulation complexity is high.After that ,many method liketriangular faceted method,voxel modeling, etc. have been developed, all thesemethod based on the descrete principle,and domestic research on NC machiningprocess simulation also based on it typically.The Voxel Model may be regard as three dimensional expansion from thetwo-dimensional raster image, it is constructive volume data compositive by 3Dregular grid. Every grid is one element of the constructive regular volumedata,which be named as voxel. When using this model to describe geometry,if onepoint being inside the voxel model ,it's value is 1,otherwise 0 which indicates thatpoint is outside the voxel modle. this model method gives out only every voxel'spositional information and the topology relation between two voxels is determinedby the (i,j,k,m) data construction,the value of m indicates the position to geometry ,1 inside, 0 outside.The Dexel Model method based on the principle of Z-buffer was devised byVan Hook. in this method, storing every voxel was abbreviated, Instead only theinterpoints between the beam and geometry surface along with designationdirection was stored. this may massively save the storage space,but it has thefollowing shortcoming: First you must choice the viewing direction according tothe geometry shape, it is very difficult to determine the direction when the shape iscomplex;Second the entire NC machine process rehas carried on and thesimulation data need to be reproduced when changing the viewing direction;Insome peculiar circumstance the NC simulation result cannot express the actualprocessing result.The compressed voxel modelong technology is one kind of modelingtechnology which absorbs the method both voxel model method and dexel modelmethod, In this method, 3 mutual vertical Dexel beam group is used, they areseparately vertical to xoy, yoz, the xoz coordinates plane. In each Dexel beamgroup, each beam is located in the interpoint of the square grid and distributedregularly according to the distance of the voxel size. We can express the positionof the surface points precisely using the float in the Dexel beam direction beacausethe dexel model record the information of the interpoint between the surface ofmodel and the Dexel beam.In the numerical control milling simulation process, the most important anddifficulty step is the Boolean operation which is processed between the cuttingtool sweeping model and the wordpiece. To simulate the milling process andverify the toolpath of the NC code, the process that the cutting tool cuts theworkpiece needs to be considered the process of the Boolean difference operation.cotinuous boolean difference operation results in the model which becomes themachined part,the simulation working then be finished.The NC maching simulation system which be developed using thecompressed voxel modeling technology has the benifit of simplified thecomplexity of the boolean operation,timeliness in simulation process,highoperating efficiency. all these benifits are made because of the characters that themodel is discrete which can simplify the boolean operation from the complex 3Dspace to the 1D.III. Dynamic Graphic Display Technology and Optimal MethodIn the numerical control milling simulation process, another important step isthe visualization of the model.in this process, a image which displayed on thescreen was created based on the part which was descriped using the computer data .Usually two methods are used to generate the image of the voxel model,which arethe direct volume rendering and the indirect volume rendering. Among the directvolume rendering method, the primitive voxel in 3D space are transformeddirectly to 2 dimensional pictures. But the simple demonstration to the object canonly be carried on using this method , and a time wsated operation will be carriedon, and it was difficult with use of the capability of traditional graphics hardwareaccelaration. Among the indirect volume rendering method, additional geometryprimitives such as triangle mesh or polygon mesh were generated first at 3D spaceand then the image which were generated using the method of the traditionalcomputer graphics.The traditional method to extract triangle mesh from the surface of the voxelmodel is base on the method named Marching-Cubes.In this paper a algorithmbased on the Marching Cubes is presented. this algorithm fully utilizes theinformation provide by the compressed voxel model to extract the triangle meshdirectly from the surface of the model and it can not only stride over the outervacant voxel of the model ,but also stride over the inner full voxel of the model.and the times of checking the outer voxel will be reduced remarkably.It's another important key point to optimizate the algorithm of the graphicaldisplay process in any numerical control machining simulation system.in thispaper two method to enhance the simulation process speed is represented, one is toseperate the voxel model to piece and and another is to use the display listtechnology in OpenGL. In the simulation process, it will be time-consuming if were-extract the entire triangle mesh of the voxel model at each simulation step. atsame time only a small part of the model is changed,the majority of the modelhave not changed. so extracting these part of the model belongs to the duplicatedprecess. Through using the method of dividing the model to some small cubeterritories and extracting the triangle mesh in these erea,the simulation speed canbe improved efficiently.Through the use of above theory, the object-oriented programmingtechnology and the dynamic link library technology, the numerical controlprocessing simulation software system have been developed meeting therequriment.