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Countermeasures for Changes in Gantry CNC Milling Center Accuracy

1. Basic production of machine tools

Because the plot of the new workshop was originally farmland and the geology is loose, considering that the foundation map provided by the machine tool design manufacturer has not been piled, it may be weak. To this end, we specially hired Jiangsu Fangjian Engineering Quality Appraisal and Testing Co., Ltd. to monitor the settlement of the foundations of two of the machine tools. The specific practices such as monitoring equipment selection and benchmark monitoring point marking are as follows.

(1) Settlement monitoring equipment (see Table 1).

(2) Arrangement of reference points and settlement monitoring points. In accordance with the technical requirements of the relevant regulations and specifications, the embedded reference points shall be laid out outside the influence range of the building pressure, and the stable, reliable and lasting preservation shall be ensured. According to the actual situation, a total of one subsidence monitoring reference point is buried around the project, numbered BM1, where BM is the starting point. A total of 9 settlement monitoring points are laid out on the foundation of the test bench. The number is the No. 1 point in the southeast corner. The No. 9 point is compiled clockwise.

The bed of the TOS horizontal machining center is a T-shaped connection, and the X-axis bed and the Z-axis bed are connected by bolts at the 7th, 8th, 3rd, and 4th points. The layout in the figure is: the X-axis bed is placed along points 1 and 9, and the Z-axis bed is placed along points 5, 7, and 8. From the data of the maintenance personnel detecting the straightness of the guide rail, it can be seen that the straightness of the X guide rail changes Concave in the middle. The straightness changes of the Z-axis are all upturned at the 5 o’clock position in the figure, and the whole is also concave in the middle.

During the period from September 19 to 29, 2011, Jiangsu Fangjian Engineering Quality Appraisal and Testing Co., Ltd. conducted a total of five times every two days to detect the settlement of 9 points of the foundation. The cumulative settlement of 9 points is shown in Table 2.

Analysis and judgment from the data in Table 2: The settlement of the monitoring point is not related to the actual change curve of the machine bed. Moreover, in the foundation construction drawing 1 of the second batch of machine tools, we thickened the concrete layer and thickened and densified the steel bars in the concrete. After observation, it was found that the geometric accuracy of the two Heckert machining centers of the same model changed in the same way after the same period of use. After this series of observations, it is basically possible to rule out the possibility that the machine tool guide rails are bent and the precision is lowered due to uneven foundation settlement.

It should be pointed out here that the foundation of the precision machine tool must be constructed in strict accordance with the foundation map provided by the machine tool manufacturer, water more during the construction and maintenance period in summer, and pay attention to cold protection during the construction and maintenance period in winter. unshelled. 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.

2. Ambient temperature change factors

Due to limited conditions, our large machining centers are currently installed in normal temperature environments. Studies have shown that there is a temperature difference between the top and bottom of the machine bed during the working process, forming a top-to-bottom temperature gradient. In summer, the upper surface temperature of the guide rail is high, and the lower surface temperature is low. The temperature difference (t1-t2) causes the upper surface to elongate more than the lower surface, showing an upper arch shape. In winter, it’s just the opposite. Due to the temperature difference of (t1-t2), the shrinkage of the lower surface is greater than that of the upper surface, and the guide rail is in a downwardly curved shape. This phenomenon has an adverse effect on the accuracy and stability of precision machine tools. According to the maintenance practice of one of the Heckert machine tools: in 2012, our maintenance personnel were in April, May and October when the temperature changed over 10 °C. . The inspection in November found that the straightness of the machine tool guide rail changed significantly, in which the horizontal change of the X-axis within the moving range of 3m can reach up to 0.06mm, while the debugging level accuracy requirement during installation is only 0.003mm.

The Heckert machine tool manufacturer mentioned in the statement on the best working conditions for the highest machining accuracy of the machine tool: the machine tool should be installed in an air-conditioned workshop and a very qualified foundation. The foundation needs to check the local soil conditions before construction, and needs to be professional construction company.

Comparing the annual average temperature change map of 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 has a large temperature change in March, April, and May. Figure 2 and Table 3 show the changes in the accuracy between months and October.

3. Countermeasures

(1) Purchase necessary testing tools: such as 00-level marble ruler, square ruler, precision electronic level (0.001) and ballbar, etc.

(2) Shorten the inspection cycle of machine tool accuracy, inspect the machine tool in advance when the machining parts accuracy requirements are high, and recover in time when the accuracy is lost. For example, in 2012, maintenance personnel conducted more than 50 inspections on 6 large machining centers.

(3) Based on its own maintenance strength, cultivate a high-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 in 2 days.

Attention should be paid to the precision adjustment of the Haikete machine tool: the Haikete machine tool has its own cooling liquid air conditioner, and the operator should often set the cooling water temperature according to the change of the ambient temperature 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 the maintenance personnel adjust the geometric accuracy of the machine tool, they must turn on the cooling water switch in advance to preheat the machine to ensure that the adjusted accuracy is consistent with the machining time.

4. Rationality of the machining procedure

Take the part shown in Figure 3 as an example: This part needs to fine boring 3 groups of 6 evenly distributed pin holes of φ 147H6 and the common axis of the φ 530r6mm bearing block and the φ 420mm outer circle have a high degree of position. Requirements, the process requirements are 0.05mm.

(1) The original machining 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, and the A hole is quickly positioned to the B hole after machining the B hole. Quickly move to hole C, this solution route does not consider the influence of eliminating backlash on hole position machining.

( 2 ) The weight of the φ530mm cylindrical boring tool is about 13 kg, the tool length is about 325mm, the weight of the φ147mm inner boring tool is about 4 kg, the tool length is about 220mm, and the length of the boring bar extending from the main shaft is 30mm. Therefore, the torque generated by the self-weight of the tool is not equal, which will cause the boring bar to sag in different degrees, thus affecting the position of the three-pin pin hole.

5. 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 inconsistency of the sagging of the boring bar due to its own weight. The extension length of the boring bar for a φ 530mm cylindrical boring tool is shortened by 30 mm to 0, and the extension of the boring bar for a φ 147 mm boring tool is 0. The length is lengthened by 120mm to 150mm, so that the moment generated by the gravity of the two knives is the same; in addition, the guide rail part of the machine tool used by the two knives during processing is reduced within the range of 100mm, which eliminates some machining errors caused by the precision of the machine tool.

(2) Improvement of the details of the machining process: ① Carefully clean the positioning surface of the spindle tool to eliminate the difference in the positioning of the two tools caused by debris such as broken iron filings. ② After rough machining, loosen the clamping device of the workpiece to eliminate rough machining stress and clamping deformation. ③Choose a sharp blade, so as to avoid the shape error of the hole and inner circle caused by the serious wear of the blade. ④The processing process is completed continuously to reduce the influence of temperature difference.

(3) Comparison of test data before and after optimization and improvement: The same workpiece is processed by two different processing methods, and the test data is obtained by online measurement with Renishaw probes, as shown in Table 4 and Table 5.

The above example shows that 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 rational design of the machining process route.

6. Conclusion

Large-scale precision machining center machine tools are used under normal temperature conditions, and are affected by seasonal temperature changes, and the machine tool accuracy retention period is short. Strictly follow the correct construction of the foundation map to ensure that the quality of the foundation meets the requirements of the machine tool installation, which is the premise to determine whether the machine tool can be used normally. Based on oneself, cultivate a capable maintenance team, prepare necessary precision testing equipment, strengthen monitoring, shorten the testing cycle of machine tool accuracy, and regularly restore machine tool installation accuracy. These measures are the guarantees for normal production. The production technicians optimize the processing route, and the operators strive for perfection to ensure that the imported machine tools play the greatest role.

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