Research On Directly CNC Machining Of Free-form Surfaces From Points Data

Directly Numerical control (NC) machining based on data points is one of reverse engineering methods studying unorganized points on the surface of the object, which not appending any geometrical and topological information (including normal vector, borderline information of surface).For the traditional reverse engineering, the tool-path is generated based on the surface fitting, surface offset and triangulation to construct the surface model. This method is not only fussy but also inaccurate especially. In order to improve precision and efficiency, many researchers begin to study how to generate NC tool-path from the data points directly.At first, the data points are pre-processed that acquired by LK G90C CMM and TDV 800 3D laser scanner, such as the probe radius compensation, the abnormal points elimination and smoothing etc.The projected left boundary of all the measured data points in the XY plane is considered as the primary cutter feed direction. In order to deal conveniently with the data points, those data are transformed according to coordinate system. An unique non-uniform CL-net is estabished by the least-squares plane fitting that is characterized with the optimal forward and side steps according to the machining error and scallop height constraints. To avoid the inaccurate fitting in the last one net of each cutting row, the double objective optimization model is established when the root mean square value of plane fitting residuals and the angle between the adjacent fitting plane are considered as the optimization variables.The established projected CL-net is represented by the CL-net nodes via their X and Y coordinates. Due to the absence of the continous surfaces representation, the machining error for a specific data point is defined as the Z deviation to the common tangent line of the two corresponding sectional ellipses from adjacent cylindrical tool swept surfaces, so the machining error of the each data point is a function about the location of the corresponding nodes. The Z coordinates are to be determinted by minimizing the machining errors of the corresponding CL-net cells by pattern search optimization algorithm. CL-net nodes for minimized machining errors from adjacent rows of CL-net cells are not well aligned in the common CL tool path. The CL points for machining are thus generated sequentially using the weighted averages of the CL-net node positions for the same tool path. According to the different of five-axis machine structure, adopt the relevant coordinate transform relationship. The G code generation procedures are compiled by Matlab 7.0 that is applied in three-axis and five-axis machining. Finally, some numerical calculating examples are given. The correctness and effectiveness of above NC machining methods are proved by the machining simulation with help of simulation software.