Research On Identification Of Kinematic Errors And Prediction Ofmachining Precision Particular To Five-Axis NC Machine Tool

With the development of technology, the requirement of mechanical parts withcomplicated shape and high dimensional precision increased dramatically in themanufacturing. Five-axis NC machine tools have been used for the machining ofmold and mechanical parts with complicated shape such as aerospace parts andturbine blades. Comparing to three-axis machine tools, five-axis machine tools have 2degree of freedom more. So five-axis machine tools could provide coordinates formachined workparts directly when machining sculptured surfaces, and optimize thecutting performance by changing the orientation of tools. The increase of complexityof machine tools'configuration also leads to the difficulty of enhancement ofmachining precision, because the complex mathematical model due to synchronousmotion of linear axes and rotary axes and coupling of kinematic errors in five-axis NCmachine tools makes that the application of relevant technology become very difficult.Based on this, the research on identification of the kinematic error in five-axis NCmachine tools was carried out in two approaches as follows in this paper.Firstly, the research on identification of kinematic error in five-axis NC machinetools based on telescoping double ball bar(DBB) was presented. The theory and thealgorithm of homogeneous transformation matrix (HTM) are applied in developingthe kinematic error model of five-axis NC machining tools with two-axis turntable,and a new method for the kinematic error separation in five-axis NC machining toolsis proposed based on the model. In this study, three kinds of simultaneouslycontrolled motions of three axes are designed for each rotary axis to identify thedeviations. In the measurement, two translational axes and one rotary axis aresimultaneously controlled keeping the distance between a tool and a worktableconstant. Telescoping double ball bar is used to measure the relative distance betweenthe spindle and the worktable in the motion of the machining tool. Finally, measuredvalue of telescoping double ball bar is substituted into the model to obtain kinematicerror of the machining tool. The simulation results demonstrated that the proposedmethod gives precision results and is able to apply to the measurement of five-axisNC machining tools effectively and conveniently.Secondly, new approaches on identification of the kinematic errors andprediction of machining precision in five-axis NC machine tools are presented basedon a cone frustum as specified in NAS979 standard. Kinematic error model of themachine tools is developed for the cone frustum based on the HTM method. Based onthe developed model, the roundness error of the cone frustum is simulated, and themachining error of the cone frustum can be predicted by this way. According to thesimulation results, it is easily to find out that each kinematic error has a differentimpact on the roundness error of the cone frustum, and the rotational errors affect theroundness more severely comparing to the translational errors. Further more, the research on identification of the kinematic error of five-axis NC machine tools basedon the characteristic of the surface of the cone frustum is carried out. The method ofnonlinear regression analysis is used to obtain the least-squares estimates of thekinematic errors based on roundness error and the angular errors of generatrix aroundthe cone frustum respectively. The simulation results demonstrated effectiveness ofthe proposed error identification algorithm for five-axis NC machine tools.The two approaches on identification of kinematic errors for five-axis NCmachine tools presented in the paper have high practical value, and each has their owncharacteristics. Any approaches can be chosen to identify the kinematic errors offive-axis NC machine tools in manufacturing of five-axis NC machine tools andfive-axis machining for the analysis of main error resources in five-axis machiningerror. And the machining precision of the machine tools can be enhanced throughimproving five-axis machine tools or compensating for errors.