Fault phenomenon
In cooperation with an elevator company, our school has processed and produced a stepped shaft used in the elevator. The parts are shown in Figure 1. The equipment used is the CAK4085ni economical CNC lathe produced by Shenyang First Machine Tool Plant. The clamping method adopts a clip and a top, and the center is supported by the center frame, as shown in Figure 2.
figure 1
After the part is first roughed and finished, the outer circle of φ26mm appears to be out of tolerance. It was found by measurement that the end of the φ 26mm outer circle had an inverted cone of about 0.2 mm (right and left small), and the size of the right end of φ26 mm and the outer diameter of the right side of φ21 mm did not change significantly. This phenomenon does not occur when processing each part, sometimes it appears continuously, sometimes occasionally. Because of the contingency of this failure, there are uncertainties in the diagnosis.
figure 2
2. Status of machine tool use
The machine tool has just been used for about one year. The internal lubrication and daily maintenance are normal. There have been several collisions but it is not serious, and the machining accuracy of the machine tool can be guaranteed. In the event of this failure, the daily production of the product can meet the size requirements of the part.
3. Troubleshooting method
Start with the most direct part that affects dimensional changes, and then contact the machine tool for other fault-related links. The case can be resolved according to the external and internal factors. The external factors can be checked for the foot of the bed. The internal factors can be checked in the order of "top, bushing and center frame - tool holder system - middle slide connection system - bed saddle connection system".
4. Fault analysis and elimination
(1) Bed foot If a foot in the foot of the bed is loosened, the vibration of the bed during processing may be tilted due to the instability of the bed, and the phenomenon that occurs in the case may result from the surface of the workpiece. During the troubleshooting, you can check whether the foot of the bed is loose. If it is loose, you need to make the bed foot tight. If necessary, you need to re-adjust the machine level. In this case, the bed foot did not loosen, which can eliminate the impact of the foot of the bed on the fault.
(2) Tip, bushing and center frame According to the long-term accumulated processing experience, the change of the size near the top part may be related to the top point. For example, if the top beat is too large or blocked, it will cause dimensional change near the top. Changes in the dimensions of the portion near the center frame may be related to the center frame or the bushing portion. If the center of the center frame is not tall, the bearing on the bushing may cause dimensional changes near the center frame. The phenomenon of this fault occurs near the center frame. First, check the fixing of the center frame. Then replace the bushing and the bearing and try to cut from the height of the centering frame. The above symptoms will still occur after processing, so the top and center can be excluded. The effect of the frame and sleeve on the size.
(3) Tool holder system The influence of the tool holder on the case may be that the tool holder is not locked, the tool holder and the middle slide plate are loosened, or the tool holder's own anti-lock is not locked.
First check whether the four screws under the tool holder are tightened: attract the magnetic watch holder to the middle slide plate, and align the dial indicator to the bottom of the tool holder. Push the tool holder back and forth to observe whether the dial indicator is beating. If there is a jump, use it. The Allen key tightens the coupling screw.
Then check the locking of the knife holder itself: when the tool holder is locked, the magnetic table holder is sucked on the bottom of the knife holder and cannot be rotated. The dial indicator is aligned with the upper part of the knife holder, and the knife is pressed with the afterburner. The direction of the rotation of the frame is to force the upper part of the tool holder to rotate. Observe whether the dial indicator will jump. If there is a jump, the tool holder cannot be locked during the tool change. The tool holder needs to be cleaned or the internal wear of the tool holder must be replaced. In this case, the tool holder system was checked for no looseness, so the tool holder could be excluded from the case without trial cutting.
(4) Middle slide joint system The joint system of the slide plate mainly includes the screw nut, the coupling, the round nut, the plug iron and the support bearing. The looseness of a certain link in the middle skateboard coupling system may cause the phenomenon in this case, and the locking screws of the screw nut, the coupling and the round nut joint may be checked one by one for looseness. After one year of use of the machine tool, the iron will inevitably wear out, and the iron should be properly adjusted and tightened properly. If the support bearing is damaged, the influence on the size is uncontrollable. In this case, the change of the size is changed by the inverted cone, and there is a certain regularity. In addition, the machine tool has not been subjected to severe impact, and the daily processing load is not very heavy. Therefore, the damage of the bearing can be temporarily excluded. In this case, after the adjustment of the middle slide joint system, the parts were tested and the symptoms were as before.
(5) Bed saddle connection system The bed saddle connection system has some similarities with the middle slide connection system, and can be inspected one by one according to the above method, but the pressure plate under the saddle and the machine guide rail is not on the middle slide. In this case, when checking the pressure plate, it is found that there is a certain gap between the pressure plate and the bottom edge of the guide rail, especially the gap between the left side and the cutting portion of the cutting tool is the largest, about 0.3 mm. The front and rear plates are re-polished and installed, and the gap is controlled at 0.03 to 0.05 mm. After the trial cut, the symptoms did not reappear. It can be seen that the fault in this case is caused by the excessive gap between the pressure plate and the guide rail under the saddle.
5 Conclusion
After the troubleshooting of this case, I discussed it with my colleagues and enterprise engineers. They said that they have not encountered similar phenomena, especially when discussing with enterprise engineers, they said that their machine tools have been used for five or six years. This phenomenon makes me feel very strange. It stands to reason that the platen on their machine tool is unlikely to be worn out. How could it not happen? Later, through comparison, I realized that the equipment used in their factory is a 50 machine tool, and the tool holders are also large squares of 25mm×25mm. The weight of the saddle is large; the machine tool of our school is 40mm small. The machine tool (although the name is 40mm, actually only 36mm bed), the tool holder is only a small knife holder of 20mm × 20mm, the saddle has a small weight.
When the machine is in roughing, the tool will be pressed by the cutting force, and when the roughing is finished, because the machine itself is light, the reaction of the saddle by the machine guide rail may rebound upwards, if it is in front of the finishing car. The rebound gap can be eliminated, and the machine tool is processed normally. When it cannot be eliminated, the phenomenon in this case may occur, so the processed parts are good or bad. If the parts on the saddle have a large weight, the rebound force cannot lift the saddle and there will be no such failure.
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