# Vertical Machining Center Feed Axis Thermal Error Compensation Strategy Research
## 3 Test Experiments and Results  
### Test Experiment of Thermal Error of Machine Feed Axis under Variable Motion  
#### 3.1.1 Thermal Error Testing Program for the Vertical Machining Center  
The temperature detection device for this experiment is a numerical temperature sensor with a measurement accuracy of \(\pm\)0.2 \({}^{\circ}\)C and a maximum resolution of 0.0625. Displacement data are collected using a Renishaw XL-80 laser interferometer, which uses the principle of laser interference to detect phase changes between interfering beams to measure distance with a resolution of 0.1 \(\upmu\)m.  
The experiment follows the national standard GB/T-17421.2 to detect the machine tool from cold to thermal equilibrium. The thermal deformation state of the machine tool in the default state and the thermal deformation state after the implementation of compensation are detected, and the positioning error of the machine tool from the cold state operation to the thermal equilibrium state is obtained as the input thermal deformation data. A vertical machining center with a detection stroke of 0\(\sim\)650 mm is used as the research object, and the vertical machining center is connected to the compensator via RJ45. A laser interferometer collects the position error of the machine tool. Its experimental site setup is shown in Figure 18.  
During displacement detection, the reference point was set at zero, the measurement spacing was set to 50 mm, and 14 measurement points were assigned. During each measurement, the machine table was stationary for 4 s (the sampling time of the interferometer was set to 2 s). The reverse return distance was set to 2 mm to avoid measurement errors caused by gaps. Acquisition of the initial machine positioning error requires five detections. The configuration of the interferometer combination method needs to ensure that there is no motion interference and that the receiving aperture can capture two laser rays. The X-axis measuring mirror assembly of the laser interferometer is shown in Figure 19.  
Figure 18: Machine positioning error acquisition solution. Temperature sensors are located as close to the heat source as possible to obtain the temperature data required for modeling. Moreover, considering that the vertical machining center feed screw axis temperature variation cannot be collected directly in real-time, the temperature values near the heat source and the ambient temperature values are mainly collected as the basis for temperature modeling. Since screw bearings, screw nut subs, and servo feed motors are the primary heat sources of the machine tool, all are placed with magnetic suction-type temperature sensors in their housings. Temperature sensors are also placed around the machine's support feet and at the ends of the feed axis as reference data. The sensors use a wire harness for signal transmission and are secured with a reel or tape to avoid interference with the machine's motion. The general layout of the test system temperature sensors is shown in Figure 20.  
The specific compensation verification test procedure is as follows:  
1. The CNC machine tool is in a cold state after the test starting point to open the thermal error compensation function, and the laser interferometer displays zero. 2. In the case of setting the feed rate of 800 mm/min, the mechanical coordinates in the range of 0-650 mm are repeatedly measured five times to obtain the fundamental positioning error of the machine tool and record the overall temperature value of the CNC machine tool. Figure 19: Combined laser interferometer X-axis measurement methods. Figure 20: Overall layout of the temperature sensor. 3 Thermal error testing of the machine according to the parameters of different positions and strokes. Before reaching thermal equilibrium, record machine tool positioning error and temperature values are recorded every 30 min. 4. The above three steps are performed cyclically to collect the nine data sets required for its modeling.