In this study, varied cooling oil volume (VOV) control was developed for the oil coolant of a machine tool. This allows adjustment of the oil circulation flow rate in terms of the machining loads and rotational speeds of the spindle to remove the generated heat effectively from the spindle. A mathematical model of the cooling oil flow rate in terms of the rotating speed and torque of the spindle for VOV method is developed. From the thermal deformation experiments with the VOV method, the thermal deformations in both the Y-axis and Z-axis can be greatly reduced, by 70.1 % and 73.5 %, respectively, in variable rotational speed operation with a short operational period (10 minutes). Moreover, the VOV method was applied to shorten the required warm-up time of the spindle. The required warm-up time of the spindle can be shortened by 50 %, while the three axes of the spindle attain stable thermal conditions. In practical machining experiments, the machining accuracy with the VOV method can be greatly enhanced by 34 % to 62 % in comparison with the current case of constant cooling oil volume (COV). The VOV control system in the machine tool spindle can effectively reduce the thermal deformation and shorten the required warm-up time. In addition, the machining accuracy can be greatly enhanced. INDEX TERMS Thermal deformation, thermal suppression, varied cooling oil volume, machining accuracy.

In this study, the thermal deformation of a machine tool structure due to the heat generated during operation was analyzed, and embedded cooling channels were applied to exchange the heat generated during the operation to achieve thermal error suppression. Then, the finite volume method was used to simulate the effect of cooling oil temperature on thermal deformation, and the effect of thermal suppression was experimentally studied using a feed system combined with a cooler to improve the positioning accuracy of the machine tool. In this study, the supply oil temperature in the structural cooling channels was found to significantly affect the position accuracy of the moving table and moving carrier. If the supply oil temperature in the cooling channels is consistent with the operational ambient temperature, the position accuracy of the moving table in the Y direction and the moving carrier in the X and Z directions has the best performance under different feed rates. From the thermal suppression experiments of the embedded cooling channels, the positioning accuracy of the feed system can be improved by approximately 25.5 % during the dynamic feeding process. Furthermore, when the hydrostatic guideway is cooled and dynamic feeding is conducted, positioning accuracy can be improved by up to 47.8 %. The machining accuracy can be improved by approximately 60 % on average by using the embedded cooling channels in this study. Therefore, thermal suppression by the cooling channels in this study can not only effectively improve the positioning accuracy but also enhance machining accuracy, proving that the method is effective for enhancing machine tool accuracy.

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