Study On Comprehensive Errors Modeling Of Workpiece-Fixture System And Fixture Optimization Design Methods

Machine tool fixture is a clamping equipment, which is widely used in the process of manufacturing. It is used for locating and clamping of workpiece in the machine tool table. The manufacturing and installation errors of the fixture locating elements, clamping force and cutting force will cause the workpiece position deviation, namely there are errors between the actual position and the theory position, and eventually led to the workpiece machining errors. By modeling comprehensive errors of workpiece-fixture system, we can obtain the workpiece machining errors caused by the known error source, and then we can optimize the fixture design. In this paper, the machine tool fixture was studied and the following work has been completed:First of all, the relationship between the manufacturing and installation errors of fixture locating elements and the workpiece position errors was analyzed using the homogeneous coordinate transformation method, and the model of the relationship between the hole position error of workpiece and the fixture locating elements errors was established. We can obtain the hole position error of workpiece caused by the known fixture locating elements. Then, taking the minimum position error of workpiece hole axis as the optimization objective, the layout of fixture was optimized using the genetic algorithm. By optimizing the fixture layout design, the hole position error of workpiece was effectively reduced.Secondly, based on the minimum complementary energy principle, the influence relationship between contact force and friction force in the process of the clamping was established. Also, the workpiece position errors caused by different clamping sequences were analyzed. Then, the workpiece position errors caused by the dynamic cutting force in the process of manufacturing were analyzed. The workpiece position error caused by dynamic cutting force was obtained using the Lagrange energy method and Newton Euler’s formula. Based on the optimal fixture layout, the clamping force was optimized taking the minimum machining error as the optimization objective. The machining accuracy of key manufacture characteristics was improved by optimizing the fixture layout and clamping force.Thirdly, the comprehensive error modeling of high rigidity workpiece and low rigidity workpiece were established respectively, and then two examples were analyzed. Based on the error propagation modeling in multi-operational machining processes, a new method was proposed to optimize fixture using fixture layout similarity. After optimization, the fixture layout in different machining stages may be same, and the machining precision of workpiece still meet the design requirements. Therefore, the numbers of fixture design, manufacture, installation and deployment times were reduced, which will effectively reduce the costs of fixture design and manufacturing.Finally, the above methods were validated by experiment. In the experiment, the fixture locating elements errors were extracted firstly, using the Hexagon Global three coordinate measuring machine. Then, the workpiece was processed on the SKVH850 vertical machining center. At the end of the experiment, the workpiece was measured using the Hexagon Global three coordinate measuring machine. The methods proposed in this paper were verified to be effective and feasible by contrasting the experimental results and the predicted results.