Research On The Interference Detection In 5-Axis NC Machining

In 5-axis NC machining of free-form surfaces, interference detection is a critical problem. Based on the hierarchical oriented bounding box (OBB) which is used in virtual reality to detect spatial collisions between 3D objects, a new interference detection method is developed in this dissertation. In this method, in order to simplify the computation process of updating tool positions and orientations in 5-axis machining, the cutter and cutter holder are modeled by a hierarchical OBB structure. In order to apply the separating axis theory, which is effective in solving 3D object collision problems in computer graphics, an octree is used to approximate the freeform surfaces. With this strategy, interference detection is conducted between the tool OBBs and the gray octants of the surface octree using the separating axis theorem. When interference is detected between the bounding volumes, the discretized surface points stored in the leaf octants of the surface octree are further checked with vector calculation to determine whether these points really collide with the tool.In the detection process, besides the simplifications used in OBB interference testing, the hierarchical structure of the octree also makes it possible to significantly reduce the need for extensive interference checking. Interference detection is first conducted between tool OBBs and the octants in the first level of the octree. If collision occurs, the detection process continues to the octants in the subsequent levels. This iteration process terminates when the colliding leaf nodes of octree are located, or when it is confirmed that there is no collision in the sub-octants. In this process, only the collision octants are traversed. The sub-octants of those collision-free octants are pruned. Similarly, interference detection is first conducted between octants and tool OBBs in the first level. When interference is found in any OBB, its four sub-OBBs in the second level are further tested. Otherwise it could be determined with certainty that there is no interference between the tool and the octant.Furthermore, based on collision space-time relativity, next collided octants are closer to that of last iteration. Before detection iteration, neighbor octants of last collided octant are pre-check to reduce the whole iteration load.Analysis and actual implementation show that this algorithm could be effectively used in detecting both local and global interferences in 5-axis machining of free-from surfaces with significantly reduced computation loads.