Thermal errors of a rotary axis of a 5-axis machine tool
The investigated machine tool, shown in the figure below, has a rotary table (C-axis) of 500 mm of diameter. The main purpose of the rotary axis is positioning the workpiece during the manufacturing process. Additionally, the rotational speed of the torque motor can be up to 1,200 rpm, enabling the possibility to perform turning operations using the table as the spindle. However, the main disadvantage is that the rotary table at high rotational speeds dissipates a large amount of heat, leading to thermal errors. Therefore, we study how the heat losses during the operation of the rotary table affects the thermal response of a 5-axis precision machine tool using a physical model.
In order to describe the thermal behavior of the system, we estimated the heat losses associated with the rotation of the C-axis. The main heat inputs are the heat losses of the stator of the torque motor and the frictional losses at the bearings. The cooling system surrounding the torque motor removes part of the heat originated during the rotation.
We performed measurements of the power consumption of the different elements in order to estimate the heat losses. We performed the power measurements with an in-house measurement system, shown in the figure.
Once we set up the model and considered all different thermal inputs, we compared the simulated and measured thermal errors. The figure illustrates the measured (full line) and simulated (dashed line) displacements between the tool and precision sphere located at the table during the rotation of the C-axis at 600 rpm over 6 h.
After the validation of the thermo-mechanical model, we proceeded to investigate the response of the machine tool. Our main objective was improving the thermal error compensation models. These models aim at predicting the thermally-induced errors and offsetting the linear axes accordingly. The figure shows the frequency response function (FRF), being the input the heat losses and the output the linear displacements between tool center point (TCP) and table. This type of analyses visualizes the dynamics of the thermal response of the machine tool and can be used as compensation models.
Another interesting conclusion from this project was to determine the origin of the thermal displacements. When turning on the rotary table, the heat losses warm up the structure of the C- and B-axis. The air inside the working space removes part of this heat, resulting in an increase of the air temperature inside the working space. In fact, the air temperature increases up to 3 K during the rotation of the C-axis. The air transfers the heat to the tool-sided axes, resulting in a deformation of the Z-axis.
The developed thermo-mechanical model enabled us to separate the displacements of the tool- and workpiece axes. This is particularly important for compensating the thermal errors at different indentations of the swiveling axis (B-axis).
