Research On Error Analysis And Prediction In Workpiece-Fixture-Cutter System

The quality of the machined part mainly depends on the complicated process system which consists of machine tools, fixture, workpiece, cutter and process parameters etc. At specific machine tools, once process parameters are selected, the machining quality of part depends on the performance of workpiece-fixture-cutter system. Therefore, in this paper, workpeice-fixture-cutter is taken as the investigated subject so that the rules of error propogration and prediction methods can be found out. Specific work includes following four aspects:(1). Location scheme design based on the machining featureA machining feature requirement driven workpiece holding scheme is proposed to constrain the degrees of freedom (DOFs) of the workpiece. In the locating scheme, the machining feature on workpiece is used to determine those DOFs which have to be constrained in machining. And then a kinematic model is used to determine the number of the constrained DOFs in a real locating scheme. By means of kinematics and the virtual displacement principle, Homogeneous solution of the linear space of non-homogeneous linear equations of location system is constructed to determine the number of the constrained DOFs of the workpiece which are unrelated to dimensional accuracy. Finally, an evaluation criterion is given to judge the correctness of the locating scheme.(2). Dimension devation evaluation and adjustment of the machining featureA model is built to integrate position and orientation errors of cutter and workpiece displacement into the relative displacement of the cutter with respect to the workpiece by means of the differential motion theory. Based on the model, a deviation adjustment model is constructed to adjust the position and orientation of the workpiece with respect to the cutter in order that the machining accuracy of workpiece can be improved. Therefore, a compensation technique which can utilize one error source to cancel out multiple error sources is proposed. Three examples are used to validate the feasibility of the proposed models.(3). Dimensional deviation propagation model and adjustment modelIn multi-source multi-processing machining operations, a machining feature from a previous machining operation is taken as the machining datum at the current machining operation. The devation from upstream will be accumulated on the current machining feature. Therefore, a comprehensive dimensional deviation evaluation framework is developed which it can be utilized to predict process deviation. At the same time, in order to obtain the displacement of workpiece due to the geometric defects of workpiece-fixture sytem, cutting forces and fixturing forces, the solution problem of contact deformation between the fixture elements and workpiece is transformed into a nonlinear programming, and solve the nonlinear programming under geometric and physical constraints and obtain the deformation magnigudes. In addition, the dynamic changing process of the deformation is evaluated in machining processs. Based on the predicted deviation, a model is built to adjust the tool path by means of the cutter source file from CAM in order that the machining accuracy of the machining feature can be improved.(4). Surface topography prediction.A general kinematic model which integrates the deflection of the cutting tools with respect to workpiece caused by cutting force and process parameters into one model is presented to build kinematic relationship between the arbitrary point of the cutter edge and the arbitrary point of the machining feature. According to the properties of moving frame, the solution problem of the machining surface scallop is transformed into a nonlinear programming problem. By means of solving the nonlinear programming problem, these scallops along the normals of nominal surface can be obtained, and then construct the surface topography and calculate the roughness values. At the same time, the stability lobe figure is constructed to select the process parameters for machining thin-walled cavity parts. Finally, the planar surface and circular arc surface texture are simulated by means of calculating a scallop value along a normal direction of an arbitrary point on nominal part surface in terms of the proposed model, three dimension profile can also be simulated. The feasibility of the proposed models is verified.