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●—≺ Preface ≻—●
Precision CNC machine tools are an important yardstick to measure the level of a country's manufacturing industry, and the stability of machine tool processing accuracy is one of the important indicators to measure the performance of machine tools.
The stability of machining accuracy refers to the consistency of part dimensional accuracy and shape and position error when batch machining parts. When the machining accuracy of the part enters the micron level, the thermal changes of the spindle and feed shaft during machine processing will become an important factor affecting the accuracy.
Therefore, the study of the thermal characteristics of the feed shaft has always been a key issue in the field of precision CNC lathes. By establishing a thermosteady state simulation model for the heat source and thermal boundary conditions of the ball screw system, the thermosteady state temperature field distribution is obtained, and verified by experiments.
Through the study of the Lelo polygonal groove, the Lelo pentagonal section filament with high screw transmission efficiency and low friction heat generation was explored to calculate and analyze the lead screw of the laser cladding feed system through ANSYS software, and the temperature and stress changes of the lead screw were obtained by considering the influence of various factors such as laser heat source.
By establishing the temperature field prediction model of ball screw system of servo system with time correction factor, the temperature change of ball screw is effectively predicted.
Taking a high-precision CNC horizontal lathe as the research object, aiming at the problem of unstable accuracy caused by thermal changes during machining of the feed shaft of precision CNC lathe, combined with the ball screw, bearing seat and lead screw bearing feed system.
From the assembly process method to the thermal steady-state analysis of the feed system to the application in the actual precision lathe product testing, the change law is compared and obtained, which provides a theoretical basis for the improvement of precision CNC lathe and the improvement of processing accuracy.
It can be seen from Table 5 that during the continuous rapid moving operation of the X axis for 2 h, the lead screw temperature is close to the fluctuation trend of the oil cooler, which can prove that the cooling capacity of the oil cooler can completely cover the heat generation of the feed system, and after a period of operation, the lead screw temperature has been fluctuating with the temperature of the lead screw inlet, but the fluctuation of the lead screw temperature does not exceed 0.6 °C.
When measuring a fixed position in the X-axis, with the fluctuation of the lead screw temperature, the displacement error also fluctuates, and after continuous operation for 2h, the displacement error fluctuates by no more than 2 um, realizing the expected effect of stable feed accuracy.
●—≺ Feed shaft structure and thermal deformation direction≻—●
The feed axes of precision horizontal CNC lathes are X and Z axes respectively, and both axes are directly connected to the ball screw through servo motors through couplings. The ball screw adopts the method of fixing one end and supporting the other end.
As can be seen from Figure 1, the angular contact ball bearing in the left end housing is close to the screw screw end face, that is, the housing end is a fixed end, and the lead screw nut at the housing end will be locked first during assembly to ensure that the end face is tight.
The angular contact ball bearing in the right end of the motor seat has a reserved gap of 3 mm with the lead screw end face, i.e. the motor seat end is the support end, which is pre-stretched at this end during assembly, and the lead screw is also extended in the direction of the motor end when the machine is machined.
The advantage of this structural design is that the direction of thermal elongation of the feed shaft is away from the main axis, that is, the positive direction of the X axis in Cartesian coordinates.
Lead screw thermal elongation influencing factors, in the lathe cutting process, feed shaft lead screw nut is affected by the cutting force transmitted by the saddle or skateboard, and the ball screw produces rolling friction, resulting in the temperature of the lead screw rise, and the lead screw produces thermal elongation in the axial direction due to the increase in temperature.
For every 1°C increase in the temperature of the lead screw, the lead screw elongates by 12 um every 1 m. Taking the X-axis of the precision CNC lathe studied in this paper as an example, the lead screw lead is 12mm and the maximum speed is 2500r/min.
In the process of fast moving and cutting, greater heat will be generated due to friction, which will increase the temperature of the lead screw, resulting in thermal elongation of the lead screw, reduced positioning accuracy, and reduced precision stability of batch processing. Under normal processing conditions, the temperature rise of the lead screw due to heat is 3~6 °C.
The effective length of the X-axis lead screw is 430mm, and the thermal elongation is 15.48~30.96um, which is elongated away from the main axis, and the thermal elongation of the lead screw increases by 5.16 um for every 1 °C increase in temperature rise.
Reflected in the product, the change in the diameter of the machined parts is twice the amount of X-axis thermal elongation, so for the stability of batch processing of precision parts, lead screw thermal elongation is an important influencing factor.
●—≺ Improve thermal elongation effects ≻—●
To compensate for thermal elongation, the lead screw can be pre-stretched during assembly. In the cold state of the machine tool, the amount of pre-stretching should be slightly greater than the amount of thermal elongation. After the machine tool processes the lead screw to heat, the thermal elongation cancels part of the pre-stretching amount, so that the tensile stress in the lead screw decreases, but the length does not change.
For lead screws to be pre-stretched, the target stroke of the lead screw at the time of manufacture (length of the threaded part at 20 °C) is equal to the nominal stroke (the theoretical length of the threaded part is equal to the nominal lead multiplied by the number of thread turns on the lead screw) minus the amount of pre-stretching. The amount subtracted is called the "stroke compensation value".
When pre-stretching the lead screw assembly, it is required that the verticality of the end of the wire mother seat facing the reference rail slide must not be greater than 0.005/100. Under the premise that the dial gauge is pressed at both ends of the lead screw to ensure that the bearing face of the bearing housing (fixed end) is tight.
Tighten the nut in the motor seat (support end), tighten the precision nut at the right end of Figure 2 to stretch the lead screw so that the change of the number of thousandths of the support end l, minus the change of the number of thousandths of the fixed end is not greater than the required stroke compensation value Al, that is, 1 ≤Al7.
The amount of change shall not be greater than 0.01 mm, as shown in fig. 2, the radial runout of the 3 points shall not be greater than 0.01 mm, and if there is a slight deviation, it can be adjusted according to the adjustment method of the inclination of the angular contact ball bearing. At the same time, the periodic axial channeling of the lead screw shall not be greater than 0. 005 mm, the axial channeling directly affects the position accuracy of the lead screw feed system and must be strictly controlled.
Due to the preload of the lead screw itself and the lead screw bearing, the temperature change in the stroke compensation value generally does not exceed 3 °C. Therefore, when the temperature rise of the lead screw exceeds 3°C during machine machining, the pre-stretching method cannot be completely compensated.
At the same time, the target stroke of the lead screw is the natural length of the lead screw at an ambient temperature of 20 °C, so there are certain requirements for the constant temperature environment of the lead screw assembly workshop.
Therefore, lead screw pre-stretching can be used as one of the conditions to improve the thermal error of the feed shaft, and other methods are needed to solve the problems that pre-stretching cannot solve.
●—≺ thermal error compensation function≻—●
At present, the new CNC system is also equipped with the corresponding feed shaft thermal error compensation function. For batch processed parts, you can first collect the thermal data of the machine tool itself, and then simulate the processing of batch parts for processing data collection.
The collected data is sorted out, input into the compensation module of the system, and the feed shaft is compensated by the internal calculation of the system, so as to improve the influence of thermal error. Take the X-axis of the precision CNC lathe studied in this paper as an example to provide test description.
X-axis limit data acquisition measurement process. Measures the maximum elongation that can be achieved by the lead screw at a fast movement speed of the servo axis. During rapid X-axis movement (24 000 mm/min), the displacement error at a fixed position in the X-axis is measured.
During the measurement process, the X-axis moves rapidly for 2 h from the cold state of the machine, during which the X-axis displacement error is continuously measured and the X-axis stops running, and the X-axis displacement error is continuously measured for 1 h. X-axis limit data acquisition is shown in Figure 3. Measuring tools include dial indicators, laser displacement sensors, and precision infrared thermometers.
Measurement results and analysis. X-axis in the 2h continuous rapid movement operation, the screw temperature has a significant rise, after a period of operation, the screw temperature is stable at about 24 °C When measuring the displacement error of a fixed position of the X-axis, with the rise of the lead screw temperature, the displacement error increases, after continuous operation for 1.5 h, the displacement error of this position reaches the maximum and stabilizes at 14 u.m. Detailed data are provided in Table 1.
Batch parts processing simulates test data acquisition and measurement process. Simulating the machining program of batch parts, the X-axis displacement error is measured 23 times at the end of each run, taking about 1.5 h.
Measurement results and analysis. After about 1 h of operation, the X-axis displacement error reached the maximum and stabilized at the maximum displacement error of 4 um. Detailed data are shown in Table 2.
Through the above two tests, find the time point when the temperature rise of the lead screw is stable, take the FANUC system as an example (see Figure 4), and input the above test data and machine tool information into the system compensation module to complete the compensation of thermal error.
The system temperature compensation module improves the limitations of the influence of thermal error, because the system temperature compensation module is based on the data collection information to compensate, so when the customer replaces the processing parts or the environmental conditions of the machine tool have a large change, it will cause the compensation conditions to change, resulting in compensation failure. Therefore, this method is suitable for batch processing of single parts and the equipment environment is basically stable.
●—≺ adopts hollow oil cold feed structure≻—●
Since the temperature rise of the lead screw is completely positively correlated with the thermal elongation, if the temperature change of the lead screw can be stably controlled within 3 °C during machine processing, the thermal error caused by the hot elongation can be eliminated by the method of lead screw pre-stretching, and the accuracy stability of batch processing can be greatly improved.
Based on the above ideas, the hollow oil-cooled feed system is designed to take away the heat generated by the lead screw in time through the oil circuit circulation to achieve the stability of the feed shaft temperature.
Hollow oil cold feed shaft structure design, based on the thermal analysis results, the lead screw adopts hollow structure to take away the heat generated by the friction between the wire mother and the lead screw, and the bearing chamber of the bearing seat and the motor seat also needs to have cooling oil circulation, the heat generated by the high-speed operation of the lead screw bearing and the heat transmitted by the feed shaft servo motor to the lead screw are taken away.
Taking the X-axis of a precision CNC lathe as an example, the structure of the hollow oil cold feed shaft is shown in Figure 6.
Both the bearing housing and the motor housing are designed with a cooling oil circulation chamber and are connected to the hollow screw oil circuit. After the oil circuit cycle begins, the bearing chamber is filled with cooling oil to lubricate diagonally contact ball bearings. The cooling oil grade is VG10 spindle oil, which takes into account the cooling and lubrication functions. When the feed shaft moves fast, the lead screw rotates at high speed, so the sealing performance of the circulating oil chamber is critical.
Taking the fixed end of the bearing housing as an example, there are three types of seals used, namely static seal, rotary seal and wiper. Among them, the static O-ring at the end cover has no relative displacement, and the dust ring can be selected normally without medium pressure, and can also be selected normally, but the rotary seal is subject to a certain oil pressure and relatively high-speed rotation, which requires special selection.
●—≺ Epilogue≻—●
The direction and variation of the thermal elongation of the feed shaft of the precision CNC lathe were analyzed, and the two methods of improving the thermal error through the lead screw pre-stretching and the temperature compensation function in the CNC system were tested and analyzed.
Combined with the practical application results of these two methods, the hollow oil cooling cycle structure is proposed to improve the thermal error. In the lathe feed system, the most calorific part is the heat generated by the high-speed rotation of the lead screw bearing and the heat generated by the rolling friction between the wire mother and the lead screw.
Through the calculation and simulation thermal analysis of the X-axis heating capacity of the lathe, the heat distribution and state of the lead screw under normal working conditions were determined, and the feed shaft structure of the hollow oil cooling cycle was designed accordingly.
In view of the heat generation of lead screw bearings, a circulating oil chamber is designed in the bearing chamber, which not only lubricates the bearing oil but also takes away the heat generated by the bearing through circulation; In view of the frictional heat generated by the wire mother and the lead screw, the hollow lead screw structure is adopted, and the cooling oil circulating inside the screw is used to quickly take away the heat, so as to achieve the temperature stability of the feed system.
Experiments verify that the X-axis accuracy change is less than 2 um during continuous machining, and the X-axis thermal error is eliminated by 50% compared with the original structure of 4 um.
Precision CNC lathes with this structure have been widely used in bearings, robot reducers, new energy vehicles and other industries that require batch processing and high stability, and customers have given a high degree of affirmation to the actual processing effect of products. This method can also provide a reference for improving the thermal error of feed shafts in other types of machine tools.
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