What should I do if the precision of the imported large-scale machining center changes? Look at the cause analysis and countermeasures
Imported large-scale CNC machining centers are equipment imported by the country at a huge cost. They have the technical characteristics of high precision, reliable performance and large processing range. The number of such machine tools is often a symbol of a company’s precision processing capabilities. Ensuring the normal use of the imported large-scale CNC machining center integrating machine, electricity, hydraulic and gas, grasping the accuracy change range of the machine tool in time, maintaining the accuracy of the machine tool, so that it can process qualified high-precision products is the focus of maintenance work, but also improvement One of the prerequisites for the effective utilization of machine tools. However, in the actual use process, some technical or managerial problems often occur. If these problems cannot be dealt with in a timely and correct manner, they will affect the normal use of the machine tool, and even reduce the service life of the machine tool, which will bring irresponsibility to the enterprise. Loss recovered.
Since 2010, a company has introduced 2 German imported Heckert HEC1600 horizontal machining centers and 4 Czech TOS WHQ13 CNC horizontal boring and milling machining centers, with a maximum axis machining stroke of 3.5m, and Heckert HEC 160 positioning accuracy It is 0.008mm and the positioning accuracy of TOS WHQ13 CNC is 0.01mm, both of which belong to the company’s key precision machine tools. However, during the initial period of more than a year after the first batch of 4 machine tools were put into use, product quality problems occurred frequently, and the accuracy of the machine tools changed abnormally. During the warranty period, foreign manufacturers had a long on-site maintenance period, which put a lot of pressure on production. It is imminent to analyze the factors that affect the accuracy of the machine tool and find an effective solution in time.
According to the contract requirements, the 4 imported machining centers installed in the early stage have passed strict geometric accuracy testing, positioning accuracy and repeat positioning accuracy testing, and NASA test piece processing and testing during installation and acceptance. Why does it happen in a few months? What about the problem of reduced accuracy? Research and analysis believe that the uneven settlement of the machine tool foundation and the deformation of the machine bed after the environmental temperature change are two major factors that cause the geometric accuracy of the machine tool to decrease. In addition to the accuracy change of the machine tool itself, unreasonable processing procedures may also cause poor product quality. Qualified, analyze one by one below.
- Basic production of machine tools
Because the land of the new workshop was originally farmland and the geology is loose, it may be weak if the foundation drawing provided by the machine tool design factory is not piling. To this end, we specially hired Jiangsu Fangjian Engineering Quality Appraisal and Inspection Co., Ltd. to monitor the settlement of the foundations of two of the machine tools. Specific practices such as the selection of monitoring equipment and the marking of benchmark monitoring points are as follows.
(1) Settlement monitoring equipment (see Table 1).
(2) The arrangement of reference points and settlement monitoring points. In accordance with the technical requirements of the relevant regulations and specifications, lay the buried datum points outside the pressure influence range of the building, and ensure its stability, reliability and long-term preservation. According to the actual situation, a total of 1 settlement monitoring reference point is buried around the project, numbered BM1, where BM is the starting point. A total of 9 settlement monitoring points are set up on the foundation of the test bed. The numbered southeast corner is the No. 1 point, and the 9th point is compiled clockwise. The specific point location is shown in Figure 1.
The machine bed of the TOS horizontal machining center is T-shaped connection, and the X-axis bed and the Z-axis bed are fixedly connected with bolts at 7, 8, 3, and 4. The layout in the figure is: the X-axis bed is placed along 1, 9 points, and the Z-axis bed is placed along 5, 7, and 8 points. From the data of the straightness of the guide rail detected by the maintenance personnel, it can be seen that the straightness of the X guide rail changes all For the middle concave. The straightness changes of the Z axis are all upward warping at the 5 points in the figure, and the whole is also concave.
During the period from September 19 to 29, 2011, Jiangsu Fangjian Engineering Quality Appraisal and Inspection Co., Ltd. inspected the settlement of the foundation at 9 points 5 times every 2 days. The cumulative settlement at 9 points is shown in Table 2.
Analyze and judge from the data in Table 2: The settlement of the monitoring point is not related to the actual change curve of the machine bed. In addition, in the foundation construction drawing 1 of the second batch of machine tools, we thickened the concrete layer and thickened the concrete inner steel bars. After observation, it is found that the geometric accuracy of the two Heckert machining centers of the same model is the same after the same period of use. After this series of observations, it is basically possible to rule out the possibility of uneven foundation settlement causing the machine tool guide rail to bend and reduce the accuracy.
What needs to be particularly pointed out here is that the foundation of the precision machine tool must be constructed in strict accordance with the foundation plan provided by the machine tool manufacturer. Water more during the summer construction and maintenance period. Pay attention to the cold during the winter construction and maintenance period. The surface of the foundation is strictly forbidden to have a secondary leveling layer to prevent the surface. Shelling. Before the machine tool is installed, the foundation should also be subjected to a preload test that exceeds the weight of the machine tool itself.
- Ambient temperature change factors
Due to limited conditions, our large-scale machining centers are currently installed and used in a normal temperature environment. Studies have shown that there is a temperature difference between the bed and the bottom of the machine tool during work, forming a temperature gradient from top to bottom. In summer, the temperature of the upper surface of the guide rail is high, and the temperature of the bottom surface is low. The temperature difference (t1-t2) causes the upper surface to extend longer than the lower surface, which is an upper arch shape. In winter, it’s just the opposite. Due to the (t1-t2) temperature difference, the shrinkage of the lower surface is greater than that of the upper surface, and the guide rail assumes a downward deflection shape. This phenomenon has an adverse effect on the accuracy and stability of precision machine tools. The maintenance practice of one of the Heckert machine tools shows that in 2012, our maintenance personnel were in April, May and October when the temperature changed more than 10℃. , In November, the inspection found that the straightness of the machine tool guideway has changed significantly. Among them, the X axis can change up to 0.06mm within a 3m movement range, and the level of accuracy required for debugging during installation is only 0.003mm.
In the statement on the best working conditions for the highest machining accuracy of the machine tool, the Heckert machine tool manufacturer mentioned: The machine tool should be installed in an air-conditioned workshop and a very qualified foundation. The foundation needs to be checked for local soil conditions before construction, and the professional Engineering company construction.
Comparing the following graph of annual average temperature changes in the city where our company is located and the machine tool adjustment record table in 2015, it is found that one of the Czech WHQ13CNC (TOS) with fixed capital number 604699002 had a large temperature change in March, April, and May. The changes in the accuracy of the month and October are shown in Figure 2 and Table 3.
(1) Purchase necessary testing tools: such as 00 grade marble ruler, square ruler, precision electronic level (0.001) and ballbar, etc.
(2) Shorten the machine tool accuracy inspection cycle, inspect the machine tool in advance when the accuracy of the processed parts is high, and restore it in time when the accuracy is found to be lost. For example, in 2012, maintenance personnel conducted more than 50 inspections on 6 large-scale machining centers.
(3) Based on its own maintenance strength, cultivate a highly skilled maintenance team.
(4) Master the measurement technology of precision measuring instruments such as electronic level and ballbar. After training, skilled use of the ballbar can detect whether the geometric accuracy of the machine tool is reduced within 1h. Three or four people can restore the installation level of a Heckert machine tool within 2 days.
When adjusting the accuracy of the Heckert machine tool, please pay attention: The Heckert machine tool is equipped with its own cooling liquid air conditioner. The operator should always set the cooling water temperature according to the environmental temperature change to reduce the temperature difference between the upper and lower guide rails, so as to ensure the high precision machining of the machine tool. When adjusting the geometric accuracy of the machine tool, the maintenance personnel must turn on the cooling water switch beforehand to preheat to ensure that the adjusted accuracy is consistent with the processing.
- The rationality of processing procedures
Take the part shown in Figure 3 as an example: This part requires 3 groups of 6 evenly distributed pin holes of φ147H6, and there is a high degree of position between the common axis of the φ530r6mm bearing block and the two gears of the φ420mm outer circle. Requirements, the process requirement is 0.05mm.
(1) The original processing path (see Figure 4), the tool is quickly positioned to the center 1 of the fitting circle of the three sets of pin holes, and then quickly moved to the A hole, after the A hole is processed, the B hole is quickly positioned, and the B hole is processed again. Quickly move to hole C. This route does not consider eliminating the influence of backlash on hole position processing.
(2) The weight of the external boring tool of φ53 0 mm is about 13 kg, the tool length is about 325mm, the weight of the φ 147mm inner hole boring tool is about 4 kg, the tool length is about 220mm, and the length of the boring bar extending from the spindle is about 4 kg. 30mm. Therefore, the torque generated by the weight of the tool is not equal, which will cause the boring bar to sag and affect the position of the three-pin pin hole.
- Improvement measures
By analyzing the processing route, processing process and test data, the following improvements have been made.
(1) Program optimization: The best processing route is shown in Figure 5. Machine tool accuracy and tool system: Change the extension length of the boring bar to eliminate the inconsistent sagging of the boring bar due to its own weight. The extension length of the φ530mm cylindrical boring tool is reduced by 30mm to 0, and the extension of the boring bar of the φ147mm boring tool is 0. The length is extended by 120mm to 150mm, so that the torque produced by the gravity of the two knives is the same; in addition, the machine tool rail part used for the two knives is reduced within 100mm during processing, which eliminates the machining error caused by the accuracy of the machine tool.
(2) Improvement of the details of the machining process: ① Carefully clean the spindle tool positioning surface to eliminate the difference in positioning of the two tools caused by debris and other debris. ② Loosen the tightening device of the workpiece after rough machining to eliminate rough machining stress and clamping deformation. ③Choose a sharp blade to avoid shape errors in the hole and inner circle due to severe blade wear. ④The processing process is completed continuously to reduce the influence of temperature difference.
(3) Comparison of test inspection data before and after optimization and improvement: The same workpiece is processed by two different processing methods, and the inspection data is obtained by online measurement with Renishaw probes, as shown in Table 4 and Table 5.
The above example illustrates: the machining accuracy of parts is not only related to the manufacturing and installation accuracy of the machine tool itself, but also closely related to the reasonable design of the machining process route.
Large-scale precision machining center type machine tools are used under normal temperature conditions, and the accuracy of the machine tools is kept for a short period of time due to the influence of seasonal temperature changes. Strictly follow the foundation plan and construct correctly to ensure that the quality of the foundation meets the requirements of machine tool installation. This is the prerequisite for determining whether the machine tool can be used normally. Based on its own, cultivate a capable maintenance team, prepare the necessary precision testing equipment, strengthen monitoring, shorten the inspection cycle of machine tool accuracy, and regularly restore machine tool installation accuracy. These measures are the guarantee for normal production. The production process personnel optimize the processing process route, and the operator strives for perfection to ensure that the imported machine tools play the greatest role.