In view of the spindle heating, high noise, reduced precision and unstable operation in the horizontal machining center, through detection, disassembly and maintenance, we are familiar with the spindle structure, review and sort out the maintenance process, summarize a set of methods for disassembly, installation, detection and adjustment of the spindle, restore the spindle accuracy, and provide a reliable reference for the maintenance of similar spindles in the future.
1. Introduction The spindle of a certain machining center has heating, poor precision, and high noise, which cannot meet the relevant requirements of the production process. Before maintenance, the spindle axial runout is 0.02mm, the spindle radial circle runout is 0.02mm at the near end, and the runout at 300mm from the near end is 0.05mm. To restore the spindle accuracy, the spindle needs to be disassembled, checked one by one, the worn bearings are replaced and reassembled, and the accuracy is debugged and restored. At present, there are no special disassembly and assembly tools and maintenance and inspection platforms for spindle maintenance, and the maintenance quality is difficult to guarantee. If the disassembly, installation and adjustment are improper, the spindle will be repaired twice in a short period of time, which will seriously affect the equipment utilization and processing efficiency.
By consulting the machine tool manual, carefully observing the spindle structure diagram, carefully analyzing and understanding the internal structure of the spindle, formulating and implementing a maintenance plan, refining the maintenance process, forming a set of maintenance ideas for disassembling, installing, testing and adjusting the spindle, and completing the maintenance of the spindle with high quality.
2. Introduction to machining center components 2.1 Spindle box
The spindle box is installed in the middle of the frame of the double-column column. The spindle moves up and down (Y axis) by two linear rolling guides and a ball screw pair. The servo motor installed at the upper end of the ball screw pair has a power-off brake for power-off braking. The spindle motor is installed behind the spindle box, and the spindle tool release device and the spindle taper hole blowing device are also installed behind the spindle box, moving up and down with the spindle box. The spindle accurate stop is achieved by the encoder carried by the spindle motor. The up and down movement of the spindle box adopts a hydraulic balance system. The balance hydraulic cylinder and accumulator are used to complete the up and down balance of the spindle box to improve the movement accuracy of the Y axis and reduce the load of the Y axis servo motor. The tightness of the spindle belt is achieved by adjusting the motor mounting plate with an adjusting screw [1].
2.2 Spindle assembly
The spindle assembly consists of a spindle, a spindle front bearing assembly, a spindle rear bearing assembly, a spindle belt unloading device, a spindle pull rod device (tool clamping) and a spindle support sleeve.
The spindle assembly is installed in the spindle support sleeve and fixed to the spindle box by screws through the flange end face of the spindle support sleeve. The taper of the spindle taper hole is 7:24. The spindle assembly is finely processed and specially assembled. The front and rear bearing assemblies of the spindle are marked. When disassembling and installing, pay attention to the position of the mark to avoid affecting the accuracy. The parallelism of the inner and outer spacers of the front and rear bearings of the spindle assembly should be ≤0.003mm, and the flatness of the inner and outer spacers should be ≤0.002mm to ensure the preload of the spindle bearing assembly and the accuracy of the shaft system. The end face of the spindle support sleeve and the end face of the spindle box installation are scraped and ground to ensure that the spindle axis is parallel to the Z axis and perpendicular to the X axis.
2.3 Spindle drawbar broach and tool release device
The spindle drawbar broach is composed of 72 disc spring plates. The elastic claws at the drawbar end clamp the tool handle pull pin in the spindle taper hole. The clamping force is about 17800N. The clamping force of the tool is adjusted by the nut at the rear end of the spindle drawbar. The hydraulic cylinder installed at the rear end of the spindle group pushes the spindle drawbar forward to release the tool. The position of the spindle drawbar tool release is adjusted by the external thread of the hydraulic cylinder and locked by the adjusting nut. The clamping and release positions of the spindle tool are received and fed back by two proximity switches. Note: When the machine tool spindle is turned on, the tool or tool handle should be clamped in the spindle taper hole first, and then the spindle is turned on. The front and rear group bearings of the spindle and the unloading support bearings are all lubricated with grease. The spindle taper hole cleaning air is blown into the spindle taper hole from the center hole of the spindle tie rod through the air pipe by the pneumatic system [2].
2.4 Spindle tool cooling device
The cooling system sends the cooling air to the cooling nozzle at the front end of the spindle through the 5-way cooling holes on the spindle support sleeve to cool the tool.
2.5 Internally cooled spindle group (spindle bearing grease lubrication)
The spindle bearings of the internally cooled spindle group are grease lubricated, and the spindle has external circulation cooling. The spindle tail is supplied with coolant and compressed air through a rotary joint.
2.6 Robot
The tool change between the spindle tool and the tool magazine is completed by a double-claw robot. During the processing, the tool magazine pre-selects the next tool to be processed and flips the tool holder (tool) 90° to position it, waiting for the tool change command. When a program is completed, each coordinate axis returns to the tool change position and the spindle stops at the same time. When the tool change command is given, the tool exchange door opens and the manipulator extends to grab the tool; after the tool is securely clamped by the claw, the spindle tool release hydraulic cylinder is activated to release the spindle tool puller mechanism, and the spindle taper hole is cleaned and blown; after the tool release is completed, the manipulator inserts and extracts the tool hydraulic cylinder to pull the tool out of the spindle taper hole and the tool sleeve taper hole, and the hydraulic swing cylinder turns the manipulator 180° to change the tool; after the tool is in place, the manipulator inserts and extracts the tool hydraulic cylinder to reinsert the exchanged tool into the spindle taper hole or the tool sleeve taper hole of the tool library, and the spindle taper hole blows the air, the spindle tool release hydraulic cylinder is reset, and the spindle tool puller mechanism re-clamps the tool in the spindle taper hole through the disc spring; finally, the manipulator retracts, the tool exchange door is closed, and the tool change action is completed [3].
III. Methods and measures to solve spindle precision failures 3.1 Spindle maintenance plan
Before maintenance, check and disassemble, remove the spindle front end cover screws, and remove the cover. Remove the rear end of the spindle loosening piston and the oil and air pipelines connected to it, drain the hydraulic oil in the pipeline, and wrap the pipe mouth to prevent dust and debris from entering the pipe. Remove the coupling connecting the spindle and the gearbox to disengage the spindle from the gearbox. Remove the round nut for adjusting the pull rod at the rear end of the spindle, and make a record for reference during assembly. Remove the pull rod from the rear end of the spindle by knocking forward and take out the disc spring. Pull out the spindle sleeve. Remove the axial positioning sleeve at the rear end of the spindle. Pull the spindle to the left and shift it. While pulling the spindle to shift, remove the radial cylindrical roller bearing by knocking. Remove the adjusting ring, and then remove the set screws of the locking nut and the front bearing group in turn.
Clean the disassembled spindle, pull rod and other slender parts, apply oil for protection, and place them vertically to prevent bending and deformation. Then disassemble and clean the remaining parts, and store them properly. Observe more during the disassembly of the spindle, pay attention to some original assembly benchmarks and make corresponding marks, and try to find the fault point.
Before assembly, all parts must be strictly cleaned, and attention should be paid to adding an appropriate amount of grease to the bearings in advance during assembly. The assembly sequence is reversed according to the above disassembly sequence. Finally, the spindle assembly is debugged, installed and tested without load.
3.2 Spindle disassembly
1) Check the spindle before disassembly. Check the spindle operation on site. When the spindle speed is 1000r/min, there is an obvious abnormal sound at the rear end of the spindle. The spindle stops, the magnetic meter holder is adsorbed on the worktable, and the Y axis is moved downward at the same time. The dial indicator contact with an accuracy of 0.01mm is pressed on the outer surface of the spindle. Use the handwheel gear to move the X axis and find the high point. The dial indicator contact is pressed at the highest point of the outer surface of the spindle, which is the upper generatrix position. The spindle is manually rotated to make the spindle rotate one circle. The dial indicator pointer deflects two grids, which is 0.02mm; using the same method, press the dial indicator contact on the end face of the spindle core rod, move the Z axis, press the meter, and after the meter is zeroed, manually rotate the spindle. The dial indicator pointer deflects two grids, that is, the spindle axial movement is 0.02mm. Open the front end cover of the spindle and check. Everything is normal, and no bearings are found. Move the Y axis to the lowest end to see the internal transmission structure of the spindle box. On the upper end of the spindle box, there are 3 black rubber tubes from left to right. The two on the left are oil pipes connecting the spindle cooling device. Open the oil pipe joint, and no oil is seen flowing in the oil pipe. Follow the oil pipe and find that these two oil pipes are circulating oil pipes of the spindle refrigerator, and the other rubber pipe is a coolant pipe.
When checking the working condition of the refrigerator, it was found that the power supply of the refrigerator was normal. Further observation of the display panel showed that the pump of the refrigerator was not working, and the refrigeration device was not working. Open the rear cover of the refrigerator and observe that the fan blades of the cooling pump motor do not rotate, and the cooling fan does not rotate. Use a multimeter to measure the glass tube fuses on the three circuits, all of which are intact. Use a screwdriver to forcefully press the left contactor and find that the cooling motor rotates. However, when the screwdriver is removed, the contactor contacts reset and the cooling motor stops. Use the same method to forcefully press the right contactor and find that the cooling fan of the refrigerator is working. When the pressure stops, the contactor contacts reset and the cooling fan stops working. After the above operations, it was found that there was a problem with the control circuit of the refrigerator. Further inspection revealed that there was a problem with a control circuit board in the lower left corner. After replacing the circuit board with the same specification, the refrigerator worked normally and the spindle refrigeration device circulated normally.
When the spindle speed was 1000r/min, it was idle for 30 minutes and the abnormal noise still existed. When the speed was changed to 800r/min, the noise became louder. Comprehensive analysis showed that it was necessary to disassemble the spindle assembly, replace the bearings, and pre-tighten the bearings [4].
2) Remove the peripheral guard plate of the spindle box and the peripheral oil and gas pipelines and lines on the spindle. Stop the machine tool in a position that is convenient for disassembling the spindle, press the emergency stop button and cut off the main power, disassemble the partition or shield on the spindle box and other peripheral accessories that hinder disassembly, mark the oil and gas pipelines and detection switch positions when grabbing and releasing the tool, and then remove them. Remove the hydraulic cylinder for releasing the tool to expose the rear end cover of the spindle.
3) Loosen the belt on the spindle pulley. Loosen the fixing bolts on the spindle 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 spindle tapered hole, put an aluminum rod on the rear end of the spindle to knock out the spindle, and the spindle is disassembled.
3.3 Spindle disassembly
The spindle structure is shown in Figure 1. Remove the rear cover, unscrew the nut, take out the push rod from the front end, take out the disc spring from the rear end, inspect and clean it, and put it in order.
Figure 1 Spindle structure
1-Axis core 2-Front end cover 3-Axle seat 4, 8-Bearing 5-First locking nut 6-Through hole
7-Disc spring 9-Dust cover 10-First arc toothed pulley 11-Second locking nut
12-Second arc toothed pulley 13-Nut 14-Third locking nut 15-Rear end cover 16-Push rod
17-First spacer 18-Second spacer 19-Pull claw 20-Bolt Gradually remove the pulley and remove the dust cover. When disassembling the spindle bearing, it was found that the retainer of the double-row thrust radial ball bearing at the front end of the spindle was broken. This phenomenon is the fault point of the spindle. The bearing needs to be replaced and pre-tightened. Clean the disassembled parts with kerosene and check for dents and wear, especially the installation reference surface and the end face of the spacer ring [5].
3.4 Spindle assembly
Fix the spindle on the workbench and apply a layer of lubricating oil evenly on the outer surface of the spindle core. Install the lower spacer on the outer surface of the front end of the spindle core. The parallelism of the spacer ring should be controlled within 0.002mm. After evenly applying grease in the gap between the retainer and the ball of the bearing, install the bearing back to back on the outer surface of the front end of the spindle core. Install the inner and outer spacers. The parallelism of the inner and outer spacers is within 0.002mm. Continue to install the bearings according to the bearing combination. Install the spindle front support bearing group first, and then install the spindle rear support bearing group. Install the inner ring spacer and lock nut (see Figure 2), pre-tighten the bearing assembly, and install the lock nut according to the disassembly dimensions.
Figure 2 Install the inner ring spacer and lock nut on the front bearing assembly After adjusting the lock nut, use a micrometer to check whether the inner and outer spacers are within 0.01mm of the spindle center point, and use a micrometer to check whether the bearing end surface flatness is within 0.002mm, as shown in Figure 3.
Figure 3 Use a micrometer to check the inner and outer spacers and the bearing end flatness Fix the micrometer on the bearing to check the radial runout of the spindle free end to within 0.005mm. If it deviates from the tolerance, it needs to be adjusted using the anti-loosening screw on the lock nut. Install the rear end support bearing back to back. First, wipe the inner surface of the sleeve with the cooling circulation groove clean, apply a layer of grease evenly on the inner surface of the sleeve, and then screw the spindle core with the bearing assembly into the sleeve, that is, assemble the moving and static parts of the spindle assembly [6].
Install the spacer ring and the assembly locking nut in sequence, and pre-tighten the rear support bearing group of the spindle (see Figure 4).
Figure 4 Pre-tightening of the rear support bearing group of the spindle is shown in Figure 5. Install the front cover of the spindle, place the spindle horizontally on the V-block, and use a feeler gauge to adjust the gap within 0.03mm. Use a micrometer to check the radial runout of the spindle within 0.002mm, and use the locking nut to adjust. Assemble the broaching device. Assemble the hydraulic cylinder for loosening the tool, install the synchronous pulley, and install the spindle and peripheral accessories.
Figure 5 Installation and adjustment of the front cover of the spindle 3.5 Spindle test
The spindle test should be carried out gradually from low speed to high speed, and the idling should exceed 2h, while the high-speed rotation should not exceed 30min. The temperature of the bearing should be checked at any time during the operation of the machine tool. The temperature of the spindle rolling bearing should not exceed 70℃ at the highest speed. During operation, you should pay attention to the sound of the machine's rotation. If it is running normally, the sound should be a steady whirring sound; if it is abnormal, it will emit various noises, such as gear noise, slight knocking sound, hoarse friction sound or metal collision clanging sound, etc. [7]. IV. Conclusion Through continuous practice and accumulation, by disassembling and repairing the spindle of the machining center, the equipment spindle heating, poor precision and high noise faults were solved, and the spindle accuracy was restored. In this process, I became familiar with the mechanical structure of the machine tool spindle, mastered the repair, assembly and adjustment methods, accumulated rich maintenance experience, and provided reliable technical data for future maintenance of the equipment.
