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 thermal error-temperature loop in a spindle of a CNC machine tool, using, initially, a bar and two displacement sensors to determine radial thermal errors of the spindle, wherein the radial thermal errors include radial thermal drift error, and radial thermal tilt error; using two temperature sensors; wherein, using one of the temperature sensors to determine temperature of an upper surface of spindle box, and using the other temperature sensor to determine temperature of a lower surface of the spindle box; drawing the thermal error-temperature loop based on the radial thermal drift error of the spindle and temperature difference between the upper surface and the lower surface of the spindle box; analyzing mechanism of radial thermal deformation of the spindle based on the thermal error-temperature loop in order to evaluate a level of the radial thermal errors; wherein steps of evaluating the level of the radial thermal errors are as follows: (1) arranging one of the temperature sensors on each of the upper surface of the spindle box and arranging the other temperature sensor on the lower surface of the spindle box, respectively; wherein the temperature sensor arranged on the upper surface of the spindle box is T.sub.1 and the other temperature sensor arranged on the lower surface of the spindle box is T.sub.2; (2) determining the radial thermal drift error along X- and Y-directions of the spindle by utilizing the bar and the two displacement sensors; selecting a direction corresponding to a high value of the radial thermal drift error; determining the radial thermal drift error of the spindle using the bar and the two displacement sensors arranged along a spindle axis; wherein the displacement sensor near a nose of the spindle is P.sub.2 and the other displacement sensor is P.sub.1; setting a test direction of the displacement sensors: the bar moves away from the displacement sensors as the radial thermal drift error increases; (3) test procedures for the radial thermal errors and temperature are as follows: letting the spindle initially run for M hours at a certain speed, and then stopping the run and letting the spindle remain at rest for N hours; determining a total test time as M+N hours; recoding data of the two temperature sensors during the test for the radial thermal errors of the spindle; (4) letting two groups of temperature data measured by the temperature sensors T.sub.1 and T.sub.2 be t.sub.1 and t.sub.2, respectively; letting two groups of displacement data measured by the displacement sensors P.sub.1 and P.sub.2 be e.sub.1 and e.sub.2, respectively; calculating the temperature difference ΔT between T.sub.1 and T.sub.2 using the formula:
ΔT(1)=[t.sub.1(1)]−[t.sub.2(1)−t.sub.2(1)], i=1,2, . . . ,n  (1) drawing a curve that is the thermal error-temperature loop with ΔT as abscissa and e.sub.1 as ordinate; (5) analyzing the radial thermal deformation of the spindle and evaluating the level of the radial thermal errors of the spindle based on the thermal error-temperature loop; evaluating the level of the radial thermal errors of the spindle is as follows: a) the larger the size of the thermal error-temperature loop is, the larger a radial thermal tilt and a radial thermal drift of the spindle are; b) the flatter the thermal error-temperature loop in a lateral direction is, the larger the radial thermal drift of the spindle is, and the smaller the radial thermal tilt of the spindle is; c) the flatter the thermal error-temperature loop in a longitudinal direction is, the larger the radial thermal tilt of the spindle is, and the smaller the radial thermal drift of the spindle is.

Description

DRAWINGS

(1) FIG. 1 is a schematic diagram of the “thermal error-temperature” loop.

(2) FIG. 2 shows a schematic diagram of the radial thermal error test of the spindle.

(3) FIG. 3 illustrates the measured thermal error-temperature loop at different speeds.

DETAILED DESCRIPTION

(4) 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.

(5) (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.

(6) (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 μm and 8.2 μm, 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.

(7) (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 10 s cycle during the test.

(8) (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 ΔT between T.sub.1 and T.sub.2 is calculated in accordance with formula (1). The curve drawn with ΔT as the abscissa and e.sub.1 as the ordinate is the thermal error-temperature loop, as shown in FIG. 3.

(9) (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:

(10) 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;

(11) 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;

(12) 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;

(13) 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.

(14) (6) According to the “thermal error-temperature” loop, the following conclusions are drawn:

(15) 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.

(16) 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.