There are many factors that may affect the machining accuracy of machine tools, including geometric errors and thermal errors, cutting tool wear, workpiece thermal deformation and the error caused by the external environment [
1]. According to the study from Bryan [
2], 40–70% of machining errors are caused by thermal errors; hence, the thermal deformation of a machine tool is an important indicator of its machining accuracy. Chen et al. [
3] proposed a thermal-error model for a hydrostatic spindle by using a finite-element software at various rotational speeds; it indicated that a spindle that rotates faster experiences greater thermal deformation. Yang et al. [
4] used the ANSYS finite-element software to construct a finite-element model of the hydrostatic guideways of a three-axis milling machine. The study analyzed the thermal deformation of the guideway under different oil film thicknesses. Bo et al. [
5] studied the thermal characteristics of a hydrostatic spindle used in grinding machines. The results indicated that all of the thermal deformation of the spindle was seemingly caused by temperature changes in the hydrostatic bearings. Heng et al. [
6] used the computational fluid dynamics (CFD)-based numerical analysis software FLUENT to simulate the temperature distribution of an oil film and discussed the influence of rotational speed and lubricant viscosity on oil film heating. The results indicated that both of the aforementioned factors have significant effects on the temperature increase in the oil film. Moreover, the high-temperature zones were mainly concentrated at the outer edge of the oil sill; further, increases in rotational speed and viscosity caused the high-temperature zones to move closer to the outer edge of the oil sill. Ballscrews are important feed-drive components in precision machine tools. Since the ball rolls within a screw track, the contact between the ball and the track creates frictional heat, which, in turn, increases the temperature of the nut and screw, affecting the manufacturing accuracy of precision machine tools. Horejs [
7] examined the thermal stability of a ballscrew system by using a thermocouple to measure temperature. The temperature data was then imported into a finite-element software for use as thermal boundary conditions to obtain the distribution of the heat generated by the nut. Yu [
8] found that most researchers measure the surface temperature of nuts or screws using a thermocouple; therefore, gaps exist in the actual results when known temperature zones serve as the boundary conditions in finite-element thermal structures. This demonstrated that frictional heat accumulates in the screw track (contact zone of the screw) located between the nut and the ball. Based on the principles of the ball kinematics, Lin [
9] utilized different parameters to calculate the frictional heat flux produced by a ball moving along a screw track under different screw rotational speeds and preloads. The results were then imported into ANSYS as boundary conditions to simulate thermal and structural deformations. Experiments were also performed to compare the experimental and simulation results, where the heat flux derived from the actual sliding speed was imported as the heat source boundary condition. The results indicated that the error between the experimental and simulation results could be maintained to within 5%. This study adapted Lin’s method to estimate the heat flux of ballscrew nut at certain feed rate in the FEM model. Zheng and Dan [
10] studied a large machine tool equipped with a hydrostatic guideway and analyzed its dynamic characteristics and bearing stiffness by numerical solution and deduced that the bearing load capacity and stiffness were affected by the size of the rectangular oil recess design. They concluded that an appropriate increase in the lubricant viscosity will help to improve the dynamic performance of the hydrostatic guideway. The study of the dynamic characteristic analysis of a hydrostatic guideway conducted by Liu et al. [
11] included modal analysis by ANSYS software, analysis of natural frequencies of each mode and experimental verification of simulation parameters, and a further discussion on the deformation of oil recess on the clamp plate, which is fixed by bolts and screws, when the structure is subjected to recess pressure field. It pointed out that the deformation of the table has a certain influence on the straightness of the hydrostatic guideway, and the deformation of the clamp plate decreases bearing load capacity by 19%. Wang et al. [
12] proposed the interaction of static characteristics of the hydrostatic guideway between the fluid pressure field and the structure, in which the flow pressure field acted on the solid structure, and the deformation of the solid affected the characteristics of the flow pressure field. They mentioned that the final oil gap of the hydrostatic bearing was the stable oil film gap after the structural deformation. Yang et al. [
13] used the Fluent software to analyze the oil film flow field and obtained the static pressure distribution diagram and dynamic pressure distribution diagram of the oil film of the hydrostatic bearings at the moving feed rate of the hydrostatic guideway. In the contact between the oil recess and oil film, under the action of viscous friction between the oil film and the guideway fixed rail, a high temperature will be generated locally in the forward direction, and temperature rise will reduce the viscosity of the oil and affect the stiffness of the bearings. The dynamic characteristics analysis of the hydrostatic guideway proposed by Dong et al. [
14] included the analysis and simulation of the static characteristics of a single recess pad using the FLUENT software, optimization analysis of the dimensions of the orifice diameter, the recess depth and oil film thickness and a modal analysis using the ANSYS software. The simulation parameters were verified by experiments. Chen et al. [
15] studied an ultra-precision hydrostatic guideway and they compared its performance to a traditional rolling-contact guideway. They explored the three major performance indexes of a hydrostatic guideway, namely the load capacity, accuracy and stiffness and their relationship to three design parameters such as oil recess pressure, oil film thickness and the effective area of the bearing pad. They pointed out that the proper design of the oil recess was the primary consideration for the design of a hydrostatic guideway system. Gong et al. [
16] investigated the multi-objective optimization and flow field simulation analysis of a hydrostatic guideway. They had conducted a comprehensive comparison between closed-type and open-type hydrostatic guideways under a constant pressure oil supply system. From a fluid–solid coupled analysis they found that the oil recess pressure, which influenced the elastic deformation of the clamp plate on the closed-type hydrostatic guideway, might affect the straightness of the guideway. Jiang et al. [
17] proposed that the thermal deformation of a hydrostatic guideway might affect the machining accuracy of machine tools. They constructed a finite-element model of the hydrostatic guideway, and they simulated the thermal deformation analysis of the hydrostatic guideway under different working conditions. The results showed that under higher feed speed, the thermal deformation of the hydrostatic guideway was bigger, and this will have a great influence on the machining accuracy. Yang et al. [
4] built a finite-element model of a hydrostatic guideway with an average grid quality of 0.69 using ANSYS software and it pointed out that the grid quality criterion should be higher than 0.5 as the basic condition of a finite-element model for hydrostatic bearings. Other recent literature studied the thermal effect of lubricants for hydrostatic bearings under various service conditions and recess shapes for spindles and lathes [
18,
19,
20,
21,
22]. In the study, the thermal deformation and structural deformation of a hydrostatic guideway caused by different oil film thicknesses and ballscrew nuts were analyzed.
From the aforementioned literature, it is clear that, for a closed-type hydrostatic guideway system, the sliding heat generated between a small oil-film of hydrostatic bearings and the frictional heat flux in the ballscrew nut are the main heat sources of a vertical hydrostatic guideway system. This paper proposed a novel FE modeling method of a vertical hydrostatic guideway system for milling machine applications. It built a finite element model of a vertical hydrostatic guideway system by considering the heat generated from the hydrostatic bearings and ballscrew nut incorporated with thermal experiments to investigate the thermal characteristics of the system. In order to verify the setting of the temperature boundary conditions for the finite element model, thermal experiments at different feed speeds were conducted to validate the model as close as possible to the physical system. In the end, this FEM model of a vertical guideway system can be used to predict the positioning error of a worktable at the center point at any operational feed rate.