2021年11月16日火曜日

CNC turntable and its positioning accuracy

Equipment and maintenance Xenon control wheel platform xenon and its fixed image Qinghai ** Machine Tool Factory (Xining 810018) Yue Mingqiang Servo Motor CNC Turntable (1) 1, 3. Reduction gear 2. Servo motor 4. Adjust the loss mother 5. Lock nut 6 .Double-lead tadpole 7. The rotary table of the adjustment sleeve machining center is divided into a numerical control indexing table and a numerical control turntable, which are suitable for different places and each have different characteristics. The numerical control indexing table usually can only complete the indexing of integer degrees, such as integer multiples of 5, integer multiples of T, etc., and cannot achieve continuous indexing, but with the advantages of indexing accuracy and simple production and assembly, it can be rotated in the machining center. A high percentage of the workbench. Because of the low positioning accuracy, poor accuracy retention, complex production, debugging, and use of the CNC turntable, it is rarely used in machining centers. But in the processing of some parts with spatial curved surfaces (such as some tools, spatial roller cams, etc.), CNC turntables must be used. To this end, we must master the typical structure of the CNC turntable and its influence on the indexing accuracy.

In a new product of our factory, the CNC turntable has two structures (A coordinate and B coordinate) as shown in and below.

The structure of the CNC turntable is relatively simple. The servo motor drives the worm through a pair of gears to decelerate, and the worm drives the worm wheel fixed as a whole with the worktable to achieve continuous indexing movement. This structure has fewer parts to participate in the movement, and has less impact on the indexing precision 8 worm gear 9. The axis of the worktable. It can be seen from the structure diagram that the only parts that can affect the indexing accuracy of the turntable are only a worm pair and a gear pair, so that we can consciously improve the manufacturing accuracy of the worm pair and gear pair to ensure the accuracy of the finished product.

The final detection accuracy value of the CNC turntable * is shown in Table 1. Table 1 CNC turntable (1) The measured accuracy value is the first pass 14 ya (continued) the second pass the third pass the i pass the second pass 14 ya the third pass 14 Note: ①The reading value in brackets at position 0 during the first measurement in the table (15’3  ). Both error compensation and reverse compensation are not used.

The value in the first row of the table is the reading value accumulated from 0’by 30 each time to 360, and the value in the second row is the reading value from 360 to 301 each time after the reverse is reversed until (the reading at T.

Due to the structural requirements, the CNC turntable is transmitted from the inside to the outside. The worktable is fixed with a large gear, and the worm gear is driven by the motor after the two-stage gear reduction and the first-stage worm gear reduction, so the transmission chain is longer. After actual measurement, it is found that the indexing accuracy is poor, and the important accuracy is also very poor. For details, see Table 2. Table 2 Numerical Control Turntable (2 The measured accuracy value is the first and the second pass Equipment and Maintenance (continued) 15, 40ff15, 53 “Note: The measurement method and measurement conditions of this table are the same as those in Table 1.

The comparative analysis of the two structures of the CNC turntable shows that the structure (2) has 1 pair of worms at the front end of the transmission chain, and the transmission chain is longer. The *rear-stage transmission chain connected to the worktable is a gear pair, and the worm gear is down 1. Reduction gear 2. Double-lead worm 3. Worm gear speed is relatively small, transmission 4, 5 transition gear 6. Driving gear 7 There are many elastic links in the moving chain of the servo motor. For this reason, combined with the specific structure of the machine tool, the transmission structure has been modified such as.

The measured accuracy values ​​after improvement are shown in Table 3 below.

Table 3 The improved numerical control turntable (2) The real value of the indexing accuracy is the 1st pass 12 Ah, the 2nd pass 12 Ah, the 3rd pass, the 1st pass, the 2nd pass, and the 3rd pass. Note: This table and Table 1 Measurement methods and measurements The conditions are the same.

From the actual measured indexing accuracy values ​​in Table 3, we can see that the indexing accuracy has been improved a lot compared to the unimproved index, and the important positioning accuracy has also been improved a lot, but it has not yet reached the accuracy of the structure (1).

2. Essentials and precautions for designing CNC turntables In the drive structure of CNC turntables, the transmission pair with a large reduction ratio must be used in the final stage of the transmission chain. For example, the middle worm gear pair is directly connected with the worktable, so that the transmission error of the front transmission chain can be transmitted to the worktable after being reduced by a large reduction ratio, and the impact on the worktable is minimal.

The drive chain should be as short as possible. Minimize the number of parts participating in the movement to reduce the manufacturing error of each part. The gap between the parts has an adverse effect on the accuracy of the movement. *Good to be able to directly connect the motor and the reduction worm.

The clearance of each movement pair should be adjusted to the smallest one. After running for a period of time, after the movement pair is worn, the gap produced by it must have a compensation link. If there should be an adjustment pad for adjusting the backlash between the gear pairs, the worm pair must adopt a double-lead worm and worm gear pair with adjustable clearance.

Drive chain * The last stage must use a worm and worm gear pair to improve the rigidity of the drive chain. As far as the structure allows, choose a large-diameter worm gear to achieve a large speed reduction ratio. The commonly used speed reduction ratio is usually 180 or 270 or even greater. In order to reduce the manufacturing error of the worm and worm wheel, the gap between the worm and the worm wheel affects the indexing accuracy.

When the worm rotates one circle, the worktable should rotate a full-degree angle. In this way, the influence of the transmission error on the indexing accuracy is a fixed value, so that the error can be compensated.

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

All parts participating in the movement (such as shafts, bearings, gears, couplings, etc.) should be selected in large sizes and large specifications when the structure allows. In order to improve the rigidity of the entire transmission chain, to avoid instability in accuracy, and at the same time to avoid the vibration of the entire transmission chain caused by intermittent cutting.

After positioning, the worktable clamping device also has a greater impact on the indexing and positioning accuracy of the worktable, so a reasonable clamping method must be adopted. *Easy to use axial clamping, avoid using radial clamping force to change the center position of the worktable and thus destroy the indexing accuracy.

After fully considering the above-mentioned points, we will be able to design a drive device for the CNC turntable that meets various requirements. Of course, a lot of knowledge and practical experience are needed for the CNC turntable to meet various requirements. These problems may be easier to solve.

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