Measurement Of The Error On A3-axis CNC Milling Machine Based On A Specially-designed-machined Artifact

At the present time, precision/ultra-precision machining products are widely usedin the field of aerospace, biomedical, micro communications, and play an increasinglycritical role in these areas. At the same time, the rapid development of these fields raisesincreasingly demanding requirements for precision products, thus, how to improve theaccuracy of the products will be the enormous challenges faced by the machiningindustry. As processing machines, Machine tools directly affect the accuracy of theprocessed components product, so in order to obtain the component and productaccuracy, reasonable measures should be taken to improve the accuracy level of themachine tools. Numerous practices show that the error compensation is the mosteconomical and effective method for the improvement of the machine tool precision,and the identification/prediction of various machine tool errors is the first condition forthe realization of the error compensation. Despite the current many methods ofmeasuring machine errors, the urgent need of the industrial sector still cannot be met.This subject proposes an error measuring methods based on a special artifact in order toprovide a novel and unique machine tool error measuring approach. Because of theactual machining of the special artifact, the measured machine tool error can betterreflect the dynamic characteristics of the machine tool in processing. Also, for itsconvenience and efficiency, this method will obtain broad potential applications in theindustry. Consequently, based on the special artifact, this paper investigates the keyfundamental problems aiming at3-axis CNC milling machine error identification andmeasurement.Due to the presence of various machine tool errors, there are a variety of form andposition errors such as straightness and position accuracy on the work-piece. In order toestablish the analytical relationship between the form and position errors on thework-piece and the21errors on the three-axis milling machine, the machine tool ismodeled in this paper based on the kinetic theory of the multi-body system. Three-axismilling machine topology is described by means of multi-sequence body array and listof freedom degrees, and using the multiplication of ideal characteristic matrix and errorcharacteristic matrix, spatial error model of tool’s forming point relative to thework-piece is established. Based on the definition of perpendicularity error of machinetools, the21motion errors of the three-axis milling machine are equivalently simplifiedto18independent error parameters.Take FXYZ3-axis milling as an example, certain geometry elements sets and theirrelative relations of position parameters are solved on the basis of the spatial model.18equivalent errors of the three-axis milling machine are identified and analyticexpression of the equivalent errors is given. Considering the non-unified coordinate systems of the measuring machine and the work-piece, the spatial position andorientation model between the coordinate systems are established, and the conversion ofthe coordinate values in the two coordinate systems is realized. At this point, a completeerror measurement theory chain is established based on the special artifact of thethree-axis milling machine. At the same time, on the basis of the series special artifacts,an artifact, i.e. the XY plane special artifact and Z axis special artifact, which canidentify21errors of the three-axis milling machine, is designed.In order to verify the correctness of the theory chain, MATLAB simulation systemis built. And in order to test the robustness of the models and to ensure the objectivity ofthe simulation process, the key parameters of the machine tool error, coordinate systemconversion angles are all generated by the random function, and at the same time toattribute the random error existed in measuring process, normal distribution function isintroduced. The simulation results show that the radius of the tool, forming pointposition parameters, position parameters of the geometric elements and the randomerror in the measuring process all affect both the special artifact identification accuracyand a reasonable choice of these parameters can make the equivalent error excellentlyapproximate to the actual machine error.Last, this paper conducts a preliminary experiment in XYFZ3-axis CNC millingmachine and takes the X-guide straightness as a validation index. In the experiment, thelaser tracker is used to measure the X-guide actual position and orientation duringoperation and the general straight line analytical equation is given based on laser tracker.Additionally, general straightness is extracted from the processed special artifact, andcomparative analysis shows that the special artifact based machine tool error measuringmethod is feasible.