Research On NC Machining Simulation Based On STL Model And Its Key Technologies

The tradeoff between simulation accuracy and simulation speed is an unavoidable contradiction in NC machining simulation.NC machining simulation based on STL model has the advantages of high simulation accuracy and easy exchange of model data,however,the simulation speed is slow because Boolean oprations between STL model is complex and computationally expensive.To improve simulation speed while not sacrificing simulation accuracy,an NC machining simulation approach based on STL model is proposed and implemented in this paper,and the methods to improve simulation efficiency is studied.The main contents are as follows:The cutter swept volume is a prerequisite for NC machining simulation and a fast modeling approach of the cutter swept volume is proposed.The approach is suitable for any rotary cutter and can be used for multi-axis NC machining simulation.Based on the envelope theory and the geometric and kinematic characteristics of the cutter,the grazing points are calculated by interpolation method,discrete method and binary search method.Then grazing curves are extracted to approximate the cutter swept volume.Besides,the methods of detecting and overcoming the problem of self-intersection which may occur during the modeling of cutter swept volume are presented.An algorithm of exact Boolean operations on STL models is realized,which is the key algorithm in the NC machining simulation approach based on STL model.The simulation of machining process is realized via consecutive Boolean subtractions between the updated workpiece and the cutter swept volume.The accuracy of NC machining simulation can be ensured by the exact Boolean operation algorithm.Degenerate cases such as coplanar triangles and invalid intersection lines which may occur in the Boolean operation algorithm is studied and dealed with to improve the robustness of the algorithm.The Boolean operation algorithm consists of four steps:(1)performing triangle-triangle intersection test,(2)partitioning for intersecting triangles,(3)carrying out inside-outside test,and(4)building a new solid.In the NC machining simulation based on STL model,the major factor affecting the simulation speed is the large amount of triangle-triangle intersection tests needed in the Boolean operation algorithm.Two spatial decomposition methods including workpiece division and background grid are applied to accelerate the intersection test.First,the workpiece is divided into several sub-workpieces before simulation starts.Then a background grid is employed to limit the searching range for triangles that may intersect.Experimental results indicate that simulation efficiency can be improved by several times and even more when spatial decomposition methods are applied.NC machining simulation based on STL model is implemented after studying the above mentioned algorithms.And a geometric accuracy verification algorithm is presented to quantitatively analysis the error between the simulated workpiece and the designed workpiece.The algorithm consists of two steps:the sampling points calculation and the signed distances calculation.