Methods and measures for solving spindle accuracy faults

1. Maintenance plan for the main shaft

Before maintenance, conduct an inspection and then disassemble. Remove the screws of the front end cover of the main shaft and take off the cover. Remove the piston at the rear end of the main shaft and the connected oil and gas pipelines, drain the hydraulic oil in the pipelines, and wrap the pipe openings to prevent dust and debris from entering the pipes. Remove the coupling connecting the main shaft to the gearbox to disengage the main shaft from the gearbox. Remove the round nut used for adjusting the rear end of the main shaft's broach rod and make a record for reference during assembly. Remove the pull tool holder from the rear end of the main shaft forward by tapping to take out the disc spring. Pull out the main shaft sleeve. Remove the axial positioning sleeve at the rear end of the main shaft. Pull the main shaft to the left to shift it. While pulling the main shaft to shift it, remove the radial cylindrical roller bearing by tapping. Remove the adjustment ring, and then remove the set screw of the lock nut and the front bearing assembly in sequence.

Clean the elongated parts such as the main shaft and pull rods that have been disassembled, apply oil for protection, and then place them vertically to prevent bending and deformation. Then disassemble and clean the remaining parts and store them properly. During the disassembly of the main shaft, observe more, pay attention to some original assembly references and make corresponding marks, and try your best to find the fault point.

Before assembly, all components must be strictly cleaned, and it should be noted that an appropriate amount of grease should be added to the bearings in advance during assembly. The assembly sequence should be operated in reverse of the aforementioned disassembly sequence. Finally, the main shaft assembly was debugged, installed and tested under no-load conditions.

2.Main shaft disassembly

1) Conduct an inspection before disassembling the main shaft. The operation of the main shaft was inspected on site. When the main shaft speed was 1000r/min, there was a distinct abnormal noise at the rear end of the main shaft. When the main shaft stops rotating, attach the magnetic watch stand to the worktable surface. At the same time, move the Y-axis downward and press the dial indicator contact with an accuracy of 0.01mm onto the outer surface of the main shaft. Use the handwheel position to move the X-axis and find the high point. Press the contact at the highest point on the outer surface of the main shaft, which is the position of the upper generatrix. Manually rotate the main shaft to make it rotate one full circle. The pointer of the dial indicator will deviate by two positions. That is 0.02mm; Using the same method, press the dial indicator contact onto the end face of the main shaft mandrel, move the Z-axis, press the indicator, and after zeroing the indicator, manually rotate the main shaft. The pointer of the dial indicator deviates by two positions, that is, the axial movement of the main shaft is 0.02mm.

The cover at the front end of the main shaft was opened for inspection. Everything was normal and no bearings were found. To see the internal transmission structure of the spindle box, move the Y-axis to the lowest end. On the upper end face of the spindle box, from left to right, there are three black rubber hoses. The two on the left are the oil pipes connecting the spindle refrigeration device. When the oil pipe joints are opened, no oil flow is seen in the oil pipes. Following the oil pipes, it is found that these two oil pipes are the circulation oil pipes of the spindle refrigeration machine, and the other rubber hose is the coolant pipe.

When checking the working condition of the refrigeration machine, it was found that the power display of the refrigeration machine was normal. Further observation of the display panel revealed that the pump of the refrigeration machine was not working and the refrigeration device was also not functioning. When the rear cover of the refrigeration machine was opened for observation, it was found that the fan blades of the cooling pump motor did not rotate, nor did the cooling fan. The glass tube fuses on the three circuits were measured with a multimeter and all were in good condition. When the left contactor was forcibly pressed with a screwdriver, it was found that the cooling motor was rotating. However, when the screwdriver was removed, the contactor contacts returned to their original position and the cooling motor stopped rotating. Using the same method, forcefully press the right contactor. It is found that the cooling fan of the refrigeration machine is working. When the press is stopped, the contactor contacts reset and the cooling fan stops working. After the above operations, it was analyzed that there was a problem with the control circuit of the refrigeration machine. Further inspection revealed that a control circuit board at the lower left corner was faulty. After replacing it with a circuit board of the same specification, the refrigeration machine worked normally and the main shaft refrigeration device cycled normally.

When the spindle speed was 1000r/min, the abnormal noise still existed after idling for 30 minutes. When the speed was changed to 800r/min, the noise became louder. Through comprehensive analysis, it can be concluded that the spindle assembly needs to be disassembled, the bearings replaced, and the bearings pre-tightened [4].

2) Remove the peripheral protective plate of the spindle box and the peripheral oil and gas pipelines and circuits on the spindle. Stop the machine tool at a position conducive to disassembling the spindle, press the emergency stop button and cut off the main power supply. Disassemble the peripheral accessories such as the partition or guard on the spindle box that hinder disassembly. Mark the positions of the grab knife, the oil and gas pipeline when releasing the tool, and the detection switch, then remove them. Disassemble the hydraulic cylinder for releasing the tool to expose the rear end cover of the spindle.

3) Loosen the belt on the main shaft pulley. Loosen the fixing bolts on the main shaft motor, loosen the belt so that the pulley can be separated from the belt, then loosen the locking screws on the end face of the main shaft taper hole, place an aluminum rod at the rear end of the main shaft and knock it out. The main shaft disassembly is complete.

3.Main shaft decomposition

Remove the rear end cover, unscrew the nut, take out the push rod from the front end and the disc spring from the rear end. Inspect, clean and arrange them in sequence.

Gradually remove the pulley and take off the dust cover. When disassembling the main shaft bearings, it was found that the cage of the double-row thrust radial ball bearing at the front end of the main shaft was cracked. This phenomenon is the fault point of this main shaft. The bearings need to be replaced and pre-tightened. Clean the disassembled parts with kerosene and check for any scratches or wear, especially the installation reference surface and the end face of the spacer ring.

4.Main shaft assembly

Fix the main shaft on the worktable and evenly apply a layer of lubricating oil on the outer surface of the main shaft core. Install the lower spacer sleeve on the outer surface of the front end of the main shaft core. The parallelism of the spacer ring should be controlled within 0.002mm. After evenly applying grease to the gap between the cage of the bearing and the balls, install the bearing on the outer surface of the front end of the main shaft core in a back-to-back manner. Install the inner and outer spacers, with the parallelism of the inner and outer spacers within 0.002mm. Continue to install the bearings according to the bearing assembly. First install the front support bearing group of the main shaft, and then install the rear support bearing group of the main shaft. Install the inner ring spacer sleeve and lock nut, and pre-tighten the bearing assembly. The installation of the lock nut can refer to the disassembly size.

After the lock nut is adjusted, use a dial indicator to check whether the inner and outer spacers are within 0.01mm of the center point of the main shaft, and use a dial indicator to check whether the flatness of the bearing end face is within 0.002mm.

Fix the dial indicator on the bearing to correct the radial runout of the free end of the main shaft within 0.005mm. If it deviates from the tolerance, it needs to be adjusted by the anti-loosening screw on the lock nut. Install the rear support bearings back-to-back. First, wipe the inner surface of the shaft sleeve with the cooling circulation groove clean. Then, evenly apply a layer of grease on the inner surface of the shaft sleeve. Next, screw the main shaft core equipped with the bearing assembly into the shaft sleeve, that is, assemble the moving and stationary parts of the main shaft assembly。

Install the spacer ring and the component lock nut in sequence to pre-tighten the rear support bearing group of the main shaft。

Install the front end cover of the main shaft, place the main shaft horizontally on the V-shaped block, and adjust the gap to within 0.03mm with a feeler gauge. Measure the radial circular runout of the main shaft with a dial indicator to be within 0.002mm, and adjust it with a lock nut. Assemble the broach device. Assemble the hydraulic cylinder of the loose knife, install the synchronous pulley, and install the main shaft and peripheral accessories.

5.Spindle test run

The shaft test run should be gradually carried out from low speed to high speed. Idling should exceed 2 hours, while high-speed rotation should not exceed 30 minutes. During the operation of the machine tool, the temperature of the bearings should be checked at any time. At the maximum speed, the temperature of the main shaft rolling bearings must not exceed 70℃. When in operation, pay attention to listening to the sound of the machine's rotation. If it operates normally, the sound it emits should be a smooth hissing sound. If it is abnormal, various noises will be produced, such as gear noise, slight knocking sounds, hoarse friction sounds or the clanging sounds of metal impacts, etc。