APPLICATION METHOD OF THE THERMAL ERROR-TEMPERATURE LOOP IN THE SPINDLE OF A CNC MACHINE TOOL

Abstract

An application method of the thermal error-temperature loop in the spindle of a CNC machine tool. This uses a bar and two displacement sensors to determine radial thermal errors of the spindle. Meanwhile two temperature sensors are used to determine the temperature of the upper and lower surfaces of the spindle box. Then, the thermal error-temperature loop is drawn with the temperature difference between two temperature sensors as the abscissa and the radial thermal error of the spindle as the ordinate. Finally, the loop is employed to analyze the mechanism of the radial thermal deformation of the spindle and the thermal error level is evaluated. Since the method is based on measured data, the results of the analysis are closer to the reality, compared to those from the numerical simulations.

Claims

1. An application method of the thermal error-temperature loop in the spindle of a CNC machine tool, wherein a bar and two displacement sensors are initially used to determine the radial thermal errors of the spindle, which include thermal drift error and thermal tilt error; meanwhile, two temperature sensors are used to determine the temperature of the upper and lower surfaces of the spindle box; then, based on the radial thermal drift error of the spindle and the temperature difference between the upper and lower surfaces of the spindle box, the thermal error-temperature loop is drawn; finally, the mechanism of the radial thermal deformation of the spindle is analyzed based on the thermal error-temperature loop so that the level of the thermal error can be evaluated; wherein the steps are as follows: (1) arranging one temperature sensor on each of the upper and lower surfaces of the spindle box respectively; the temperature sensor near the surface of the spindle motor is T.sub.1 and the other temperature sensor is T.sub.2; (2) determining the radial thermal drift error along the X- and Y-directions of the spindle by utilizing a bar and two displacement sensors; selecting a direction with higher radial thermal drift error; then, using the bar and two displacement sensors arranged along the spindle axis to determine its thermal error; the displacement sensor near the nose of the spindle is P.sub.2 and the other one is P.sub.1; the test direction of the displacement sensor is set as: the bar moves away from the displacement sensors along with the radial thermal drift error increasing; (3) the test procedure for the thermal error and temperature: the spindle initially runs for M hours at a certain speed and then the spindle stops running and remains at rest for N hours; therefore, the total test time is M+N hours; recoding the data of two temperature sensors during the test for the radial thermal error of the spindle; (4) letting two groups of temperature data measured by temperature sensors T.sub.1 and T.sub.2 be t.sub.1 and t.sub.2; letting two groups of displacement data measured by displacement sensors P.sub.1 and P.sub.2 be e.sub.1 and e.sub.2; the formula for calculating the temperature difference T between T.sub.1 and T.sub.2 is as follow:
T(i)=[t.sub.1(i)t.sub.1(1)][t.sub.2(i)t.sub.2(1)],i=1,2, . . . ,n(1) the curve drawn with T as the abscissa and e.sub.1 as the ordinate is approximately a loop, which is called the thermal error-temperature loop; (5) based on the thermal error-temperature loop, the radial thermal deformation of the spindle is analyzed and the radial thermal error level of the spindle is evaluated; the evaluation method is as the following: a) the bigger the size of the thermal error-temperature loop is, the greater the radial thermal tilt and the thermal drift of the spindle are; b) the flatter the thermal error-temperature loop in the lateral direction is, the lager the radial thermal drift of the spindle is, and the smaller the thermal tilt is; c) the flatter the thermal error-temperature loop in the longitudinal direction is, the larger the radial thermal tilt of the spindle is, and the smaller the thermal drift is.

Description

DRAWINGS

[0022] FIG. 1 is a schematic diagram of the thermal error-temperature loop.

[0023] FIG. 2 shows a schematic diagram of the radial thermal error test of the spindle.

[0024] FIG. 3 illustrates the measured thermal error-temperature loop at different speeds.

DETAILED DESCRIPTION

[0025] In order to make the objects, technical solutions and advantages of the proposed invention more clear, a specific embodiment of the invention with the reference to a certain type of the vertical machining center is described as below.

[0026] (1) The temperature sensors entitled by T.sub.1 and T.sub.2 are arranged on the upper and lower surfaces of the spindle box, respectively.

[0027] (2) A bar and two displacement sensors are utilized to determine the thermal drift error along the X- and Y-directions of the spindle. The spindle continuously rotates at 2000 rpm for 1 hour during the test. It is found that the thermal drift errors in the X- and Y-directions are 1.2 m and 8.2 m, respectively. Therefore, it is concluded that the radial error along the Y-direction is the governing error. Moreover, after shutting down the machine for 3 hours, the Lion spindle error analyzer is used to test the radial thermal drift and thermal tilt error along the Y-direction of the spindle. The upper and lower displacement sensors are P.sub.1 and P.sub.2 respectively. It is observed that when the bar is close to the displacement sensor, the test value is positive, while when the bar is far away from the displacement sensor, the test value is negative. FIG. 2 shows the configuration of sensors in the proposed scheme.

[0028] (3) In order to contrast the thermal error level of the spindle at different speeds, three tests are carried out at 1000 rpm, 2000 rpm and 4000 rpm respectively. The spindle continuously runs for 4 hours and then remains at rest for 3 hours. Moreover, record the data from the displacement sensor and the temperature sensor in 10s cycle during the test.

[0029] (4) Let two groups of temperature data measured by temperature sensors T.sub.1 and T.sub.2 be t.sub.1 and t.sub.2. Let two groups of displacement data measured by displacement sensors P.sub.1 and P.sub.2 be e.sub.1 and e.sub.2. The temperature difference T between T.sub.1 and T.sub.2 is calculated in accordance with formula (1). The curve drawn with T as the abscissa and e.sub.1 as the ordinate is the thermal error-temperature loop, as shown in FIG. 3.

[0030] (5) Based on the thermal error-temperature loop, the radial thermal deformation of the spindle in the Y-direction is divided into the following 4 stages:

[0031] a) Stage 1: The spindle starts to rotate and T.sub.1 heats up rapidly due to heat sources, such as the spindle motor. On the other hand, T.sub.2 is far from these heat sources and the temperature rise lags behind T.sub.1, which results in an abrupt increment in the temperature difference between T.sub.1 and T.sub.2. Meanwhile, the radial thermal error of the spindle is mainly the thermal tilt, and the dip is rapidly increased. Therefore, the bar is close to the displacement sensor and the error value is positive;

[0032] b) Stage 2: As the spindle runs, the temperature difference between T.sub.1 and T.sub.2 gradually stabilizes, the thermal tilt of the spindle also stabilizes and the thermal drift increases gradually. Thereby, the bar gradually moves away from the displacement sensor so that the error value gradually becomes negative and changes in the negative direction;

[0033] c) Stage 3: The spindle stops rotating and cools down. Since the temperature value of T.sub.1 is higher than that for T.sub.2, its temperature drop is faster than that for T.sub.2. Therefore, the temperature difference between T.sub.1 and T.sub.2 rapidly decreases so that the radial thermal tilt of the spindle rapidly decreases. Meanwhile, the bar is still far away from the displacement sensor and the error value still changes in the negative direction;

[0034] d) Stage 4: After a period of cooling, the temperatures of T.sub.1 and T.sub.2 decrease towards the ambient temperature so that the radial thermal tilt and thermal drift of the spindle decrease and the bar gradually approaches the displacement sensor. Therefore, the error value changes in the positive direction.

[0035] (6) According to the thermal error-temperature loop, the following conclusions are drawn:

[0036] a) The higher the spindle speed is, the larger the thermal error-temperature loop is. This indicates that the higher the spindle speed is, the greater the thermal tilt and thermal drift are.

[0037] b) The thermal error-temperature loop is not closed at the end because the cooling time is not enough so that the spindle does not return to the initial thermal equilibrium.