CNC rotary table and its positioning accuracy

Equipment and maintenance 瓤 control wheel Taiwan 氙 定 揠 揠 揠 第一 第一 第一 第一 第一 810 810 810 810 810 810 岳 岳 岳 810 810 810 810 810 810 810 810 810 810 810 810 810 810 810 810 810 810 810 810 810 810 810 810 810 810 810 810 810 810 810 Double-guide mast 7. The rotary table of the adjustment sleeve machining center is divided into a numerical control indexing table and a numerical control rotary table. They are applicable to different places and each has different characteristics. CNC indexing table can usually only complete the division of integer degrees, such as integer multiple of 5, integer multiple of T, etc., can not achieve continuous indexing, but with the advantages of indexing accuracy, simple production and assembly, the rotation in the machining center The workbench occupies a high proportion. CNC rotary table is less used in machining centers due to low positioning accuracy, poor precision retention, and unfavorable factors such as production, debugging, and complicated use. However, in some machining with space curved parts (such as some tools, space roller cams, etc.), a CNC rotary table must be used. To this end, we must master the typical structure of the CNC turntable and its impact on the accuracy of indexing.

In a new product of our factory, the CNC rotary table has two structures (the A coordinate B coordinate) as shown in the figure.

The structure of the CNC rotary table is relatively simple. The servo motor drives the worm through a pair of gears to reduce the speed, and the worm wheel is driven by the worm to realize the continuous indexing movement. This type of structure has fewer parts to participate in the movement, and has less influence on the indexing shaft 8 worm wheel 9. It can be seen from the structure diagram that the parts that can affect the indexing accuracy of the turntable have only one pair of worm pairs and one pair of gear pairs, so that we can consciously improve the manufacturing precision of the worm pair and the gear pair to ensure the precision of the finished product.

The final detection accuracy value of the CNC rotary table is shown in Table 1. Table 1 CNC turntable (1) The measured accuracy value bit 1st pass 14丫 (continued) The 2nd pass The 3rd pass The i-th pass The second pass 14丫 The third pass 14 Note: 1 in the first measurement in the table 0 position in the brackets reading value (15'3 The error compensation and reverse compensation are not used in the above values.

The value of the first row in the table is the value of the reading from 0' each time increasing by 30 to 360, and the value of the second row is the reverse from 360 to 301 every time until (T reading).

Due to the structural requirements, the CNC rotary table is transmitted to the outside and the table is fixed with a large gear. After the secondary gear is decelerated and the first stage worm gear is decelerated, the motor drives the worm gear, so the transmission chain is longer. After the actual measurement, it is found that its indexing accuracy is poor, and the important precision is also very poor. See Table 2. Table 2 CNC turntable (2 actual measured accuracy values, 1st, 2nd pass, equipment and maintenance (continued) 15, 40ff15, 53 "Note: This table is the same as the measurement method and measurement conditions of Table 1.

Comparing and analyzing the two kinds of structure CNC rotary table, it can be seen that the worm 1 of the structure (2) is at the front end of the transmission chain, the transmission chain is long, and the last stage transmission chain connected with the worktable is the gear pair, the worm wheel is lowered 1. The reduction gear 2 Double-lead worm 3. The worm gear speed is relatively small, and the 4 and 5 transition gears are transmitted. 6. The active gear 7 servo motor has many elastic links in the moving chain. To this end, combined with the specific structure of the machine tool, the transmission structure has been modified.

The measured values ​​of the improved accuracy are shown in Table 3 below.

Table 3 Improved CNC Turntable (2) Indexing Accuracy Real Value Bit 1st Pass 12丫 2nd Pass 12丫 3rd Pass 1st Pass 2nd 3rd Pass Note: This Table and Table 1 Measurement Method and Measurement The conditions are the same.

In Table 3, we can see that the indexing accuracy is much better than that before the improvement. The important positioning accuracy is also improved a lot, but the accuracy of the structure (1) has not yet been reached.

2. Designing the essentials of CNC turntable and precautions In the drive structure of the CNC turntable, the large speed reduction ratio drive pair must be used at the final stage of the drive chain. For example, the worm gear pair is directly connected to the workbench, so that the transmission error of the front drive chain can be reduced to a large reduction ratio and then transmitted to the workbench, which has little effect on the workbench.

The drive chain is as short as possible. Minimize the number of parts participating in the exercise to reduce the manufacturing error of each part and the adverse effect of the gap between the parts on the motion accuracy. It is best to connect the motor directly to the reduction worm.

The clearance of each movement should be adjusted to the minimum. After running for a period of time, after the movement pair wears, the gap generated must have a compensation link. For example, there should be an adjustment pad for adjusting the backlash between the gear pairs, and the worm pair must use a double-lead worm gear pair with adjustable clearance.

The worm and worm gear pair must be used at the last stage of the transmission chain to improve the rigidity of the transmission chain. The large diameter worm gear is selected by the child to allow for a large speed reduction ratio. Commonly used speed reduction ratios are typically 180 or 270 or even larger. In order to reduce the manufacturing error of the worm and the worm wheel, the gap between the worm and the worm wheel has an influence on the indexing accuracy.

When the worm rotates one turn, the table should turn over the full number of corners. This allows the influence of the transmission error on the indexing accuracy to be a fixed value in order to compensate for the error.

If a high-precision CNC turntable is manufactured, the manufacturing precision of the worm gear pair should also be improved. Under normal circumstances, the manufacturing precision of the last stage worm pair is 2 grades than that of the general dynamic drive, and 4 to 5 grades are commonly used. The manufacturing precision of the reduction gear can be lower than that of the worm gear, and it is usually 5~6.

The components that participate in the movement (such as shafts, bearings, gears, couplings, etc.) should be large and large in size if the structure permits. To improve the rigidity of the entire transmission chain, to avoid the instability of the accuracy, but also to avoid the vibration of the entire transmission chain caused by interrupted cutting.

After positioning, the workbench clamping device has a great influence on the positioning accuracy of the workbench, and a reasonable clamping method must be adopted. It is best to use axial clamping to avoid using the radial clamping force to change the center of the table and destroy the indexing accuracy.

After fully considering the above, we will be able to design a drive unit for the CNC turntable that meets the requirements. Of course, it is necessary to have a lot of knowledge and practical experience in the CNC turntable to meet various requirements. These problems may be easily solved.

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