Direct correction of thermo-elastic displacements

 

According to VDI/DGQ 3441, manufacturing accuracy, working accuracy and geometric precision are relevant for the entirety of machine tools (WZM). The geometric accuracy of the machine tool is the accuracy with which any preselected position in the workspace can be approached. In turn, it depends on the positional deviations of the individual axes relative to each other as well as on the geometric accuracy of the axes themselves. The Cartesian deviations of the Tool Center Point (TCP) from the nominal position can be clearly described by means of the error parameters defined in ISO 230-1 and a model of the kinematic chain.

To increase the geometric accuracy of VCMs, their geometry is calibrated after commissioning and compensated by the control system. The calibrated geometric errors of the VCM are, however, superimposed by static, dynamic and transient thermo-elastic errors during operation. The thermo-elastic errors can account for up to 75% of the total error. They are the focus of the project outlined here. In addition to machine-internal influences, thermal environmental influences also act on the machine and cause heat to be introduced into the structure. The induced heat flows lead to an uneven and unsteady temperature distribution. Their characteristics depend on the thermo-elastic material properties such as heat capacity and thermal conductivity, the mass distribution and the location of the heat sources. Together with the effective expansion lengths and their coefficients of expansion, the component structure, the position of the components relative to each other and the type and attachment of the position measuring system, this temperature field results in displacements and inclinations of the machine structure. This ultimately results in a deviation of the machining point from its nominal position. The resulting displacements on the TCP are thus usually highly pose dependent.

If the thermal state of the machine changes significantly after calibration, the errors can increase, which leads to a deviation of the TCP from the nominal position. As a result, the component quality decreases. This can be remedied by a process-actual recording of the geometric errors. In the context of this application, measured machine errors are process-current if they were measured a few minutes before the final cut. According to the current state of the art in research and industry, there is no measuring method that can record the geometric condition of small and medium-sized machine tools in a process-actual manner. Therefore, the aim of the research project is the validation of a measuring method for the actual process mapping of the change of the geometric accuracy in small and medium-sized machining centres. Based on this, a method for the direct correction of the occurring displacements on the TCP is to be validated within the project.