Precision Restoration Techniques for Machining Center Spindles
Due to issues such as spindle heating, high noise levels, reduced precision, and unstable operation in the horizontal machining center, we conducted detection, disassembly, and maintenance. This process allowed us to understand the spindle structure better and review the maintenance procedures. We have developed a comprehensive set of methods for the disassembly, installation, detection, and adjustment of the spindle. Our goal is to restore spindle accuracy and provide a reliable reference for the maintenance of similar spindles in the future.
1. Introduction
The spindle of a machining center is experiencing issues: it is hot, has poor precision, and produces high noise levels, which do not meet the production process requirements. Before maintenance, the spindle exhibited an axial runout of 0.02 mm, a radial runout of 0.02 mm at the near end, and a runout of 0.05 mm at 300 mm from the near end. To restore the spindle’s accuracy, it must be disassembled so that each component can be inspected individually. Worn bearings will be replaced, and then the spindle will be reassembled and calibrated to restore its accuracy.
Currently, there are no special tools for disassembly and assembly, nor are there maintenance and inspection platforms for spindle care, which makes it difficult to ensure maintenance quality. Improper disassembly, installation, or adjustment could require a second repair in a short period, significantly impacting equipment utilization and processing efficiency.
To successfully complete the spindle maintenance with high quality, I consulted the machine tool instruction manual, carefully observed the spindle structure diagram, and thoroughly analyzed its internal components. I then developed and implemented a detailed maintenance plan, refined the maintenance process, and created a set of procedures for disassembling, installing, inspecting, and adjusting the spindle.
2. Introduction to machining center components
2.1 Spindle box
The spindle box is positioned in the center of the double-column frame. It features two linear rolling guides and a ball screw assembly to facilitate vertical movement of the spindle (Y-axis). A servo motor, located at the top of the ball screw assembly, is equipped with a power-off brake to ensure braking in the event of a power failure. Behind the spindle box, there is a spindle motor, along with both a spindle tool release device and a spindle taper hole blowing device, which also move up and down with the spindle box.
The precise positioning of the spindle is achieved using an encoder connected to the spindle motor. The vertical movement of the spindle box is supported by a hydraulic balancing system, which includes a balance hydraulic cylinder and an accumulator. This system aids in balancing the spindle box’s movement, enhancing the accuracy of the Y-axis while reducing the load on the Y-axis servo motor. Additionally, the tension of the spindle belt can be adjusted by manipulating the motor mounting plate with screws.
2.2 Spindle Group
The spindle group consists of several components: the spindle, the front spindle bearing group, the rear spindle bearing group, the spindle belt unloading device, the spindle pull rod device (for tool clamping), and the spindle support sleeve.
This spindle group is mounted within the spindle support sleeve and secured to the spindle box using screws that pass through the flange at the end face of the spindle support sleeve. The taper of the spindle taper hole is 7:24. The spindle group is finely processed and assembled with precision. It is important to note that the front and rear spindle bearing groups are marked; be mindful of these marks during disassembly and reassembly to avoid compromising accuracy.
The parallelism of the inner and outer spacers of both the front and rear spindle bearings should be maintained at ≤0.003 mm, while the flatness of the inner and outer spacers must be ≤0.002 mm. These tolerances are crucial to ensure the correct preload of the spindle bearing group and the overall accuracy of the shaft system.
Additionally, both the end face of the spindle support sleeve and the end face of the spindle box installation utilize a scraping process. This ensures that the spindle axis is parallel to the Z axis and perpendicular to the X axis.
2.3 Spindle drawbar tool puller and tool release device
The spindle drawbar tool puller consists of 72 disc spring plates. The tool handle pull pin is secured within the spindle taper hole via the drawbar and the elastic claws at the end of the drawbar. The clamping force is approximately 17800 N. This clamping force can be adjusted using the nut located at the rear end of the spindle drawbar.
To release the tool, a hydraulic cylinder positioned at the rear of the spindle group pushes the spindle drawbar forward. The release position of the spindle drawbar can be fine-tuned using the external thread of the hydraulic cylinder, which is then locked in place with the adjusting nut. The clamping and release positions of the spindle tool are monitored and reported by two proximity switches.
Please note: Before turning on the machine tool spindle, the tool or tool handle must be clamped securely in the spindle taper hole. All front and rear group bearings, as well as the unloading support bearings of the spindle, are lubricated with grease. Additionally, the spindle taper hole is cleaned by blowing air into it from the center hole of the spindle drawbar, using a pneumatic system.
2.4 Spindle tool cooling device
The cooling system directs coolant to the nozzle at the spindle’s front through the 5-way cooling holes in the support sleeve for tool cooling.
2.5 Internally cooled spindle assembly (spindle bearing grease lubrication)
The spindle bearings in the internally cooled spindle assembly are lubricated with grease, and the spindle features external circulation cooling. The tail of the spindle receives coolant and compressed air via a rotary joint.
2.6 Robot
The tool change process between the spindle tool and the tool magazine is carried out by a double-claw robot. During machining, the tool magazine pre-selects the next tool to be used and rotates the tool holder 90 degrees 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 simultaneously. Once the tool change command is given, the tool exchange door opens, and the manipulator extends to grasp the tool. After the tool is securely clamped by the tool claw, the spindle tool release hydraulic cylinder is activated to release the spindle tool pulling mechanism. At this point, the spindle taper hole is cleaned and blown out simultaneously.
Once the tool release is complete, the manipulator’s tool insertion and extraction hydraulic cylinder is activated to remove the tool from both the spindle taper hole and the tool sleeve taper hole. The hydraulic swing cylinder then rotates the manipulator 180 degrees to facilitate the tool change. After the new tool is positioned correctly, the manipulator’s tool insertion and extraction hydraulic cylinder is activated again to reinsert the exchanged tool into either the spindle taper hole or the tool sleeve taper hole of the tool magazine.
Simultaneously, the air blowing from the spindle taper hole stops, the spindle tool release hydraulic cylinder resets, and the spindle tool pulling mechanism reclamps the tool in the spindle taper hole using a disc spring. Finally, the manipulator retracts, the tool exchange door closes, and the tool change action is completed.
3. Methods and measures to solve spindle precision failure 3.1 Spindle maintenance plan
Maintenance Procedure for Spindle Assembly
Preparation:
- Inspect and disassemble the spindle before maintenance.
- Remove the screws from the gland located at the front end of the spindle, and take out the gland.
Disassembly:
- At the rear end of the spindle, remove the loosening piston along with the connected oil and air pipelines. Drain the hydraulic oil from the pipeline, and seal the pipe openings to prevent dust and debris from entering.
- Detach the coupling that connects the spindle to the gearbox, effectively disengaging the spindle.
- Remove the round nut used for adjusting the pull rod at the rear end of the spindle. Make a record of this step for reference during reassembly.
- Knock the pull rod out of the rear end of the spindle, moving it forward. Remove the disc spring afterward.
- Pull out the spindle sleeve and then remove the axial positioning sleeve at the rear end of the spindle.
- Shift the spindle to the left while pulling it. During this process, remove the radial cylindrical roller bearing by knocking it out.
- Remove the adjusting ring, followed by the set screws of the locking nut and the front-end bearing group in sequence.
Cleaning and Storage:
- Thoroughly clean the disassembled spindle, tie rod, and other slender parts. Apply oil for protection and store them vertically to prevent bending or deformation.
- Clean and store the remaining parts properly.
- While disassembling, pay attention to the original assembly benchmarks. Make corresponding marks, and try to identify the fault point.
Reassembly:
- Before reassembly, ensure all CNC components are thoroughly cleaned.
- Apply an appropriate amount of grease to the bearings during assembly.
- Follow the reverse sequence of disassembly to reassemble the spindle.
Final Testing:
- Finally, debug, install, and test the spindle assembly without a load to ensure proper operation.
3.2 Spindle disassembly
1) Check before disassembling the spindle.
Check the spindle operation on-site. When the spindle speed is at 1000 RPM, an obvious abnormal sound is present at the rear end of the spindle. The spindle stops, the magnetic meter holder is attached to the worktable, and the Y-axis is moved downward simultaneously. A dial indicator with an accuracy of 0.01 mm is pressed against the outer surface of the spindle. Using the handwheel gear, the X-axis is moved to find the highest point. The meter contact is pressed at this highest point on the outer surface of the spindle, marking the upper generatrix position. Manually rotate the spindle to complete one full rotation. The meter pointer deflects two grids, indicating a measurement of 0.02 mm.
Next, check the front end cover of the spindle. Everything appears normal, and no bearing issues are found. Move the Y-axis to the lowest position to examine the internal transmission structure of the spindle box. At the upper end of the spindle box, there are three black rubber tubes, arranged from left to right. The two on the left are oil pipes connecting to the spindle cooling device. Upon opening the oil pipe joint, no oil is seen flowing through the pipe. Following the oil pipes, it is confirmed that these two are circulating oil pipes for the spindle refrigerator, while the other rubber pipe is a coolant line.
When assessing the refrigerator’s working condition, the power supply is found to be normal. However, further observation of the display panel shows that the cooling pump is not operational, and the refrigeration device is inactive. Upon opening the rear cover of the refrigerator, it is observed that the cooling pump motor’s fan blades do not rotate, and the cooling fan is also non-functional. Using a multimeter, the glass tube fuses on all three circuits are tested and found to be intact. When a screwdriver is used to manually press the left contactor, the cooling motor starts to rotate. However, once the screwdriver is removed, the contactor resets, and the cooling motor stops. The same method is used on the right contactor, which activates the cooling fan, but it too stops when pressure is released.
Based on these observations, it is concluded that there is an issue with the refrigerator’s control circuit. Further inspection reveals a problem with a control circuit board located in the lower left corner. After replacing this circuit board with one of the same specification, both the refrigerator and spindle cooling device operate normally.
When the spindle runs at a speed of 1000 RPM, it remains idle for 30 minutes, yet the abnormal noise persists. When the speed is reduced to 800 RPM, the noise becomes louder. A comprehensive analysis indicates that it is necessary to disassemble the spindle assembly, replace the bearings, and pre-load the bearings correctly.
2) Remove the peripheral guard plate of the spindle box and the peripheral oil and gas pipelines and lines on the spindle.
To begin the disassembly of the spindle, first stop the machine tool in a position that facilitates access. Press the emergency stop button and cut off the main power supply. Next, remove any peripheral accessories, such as partitions or shields on the spindle box, that may obstruct disassembly.
Before loosening the tool, mark the positions of the oil and gas pipelines as well as the detection switches. After marking, carefully disconnect and remove these components. Then, take out the hydraulic cylinder used for loosening 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 and then loosen the belt to separate the pulley from the belt. Next, loosen the locking screw on the end face of the spindle’s tapered hole. Place an aluminum rod at the rear end of the spindle and gently tap it to knock out the spindle. This completes the spindle disassembly.
3.3 Spindle disassembly
The spindle structure is illustrated in Figure 1. To begin, remove the rear end cover and unscrew the nut. Next, take out the ejector rod from the front end and remove the disc spring from the rear end. Inspect and clean both components, then place them back in order.
Carefully remove the pulley and take off the dust cover. During the disassembly of the spindle bearing, it was discovered that the double-row thrust radial ball bearing cage at the front end of the spindle was broken; this is the point of failure for the spindle. The bearing needs to be replaced and pre-tightened. Clean all disassembled small CNC parts with kerosene and examine them for any damage or wear, particularly focusing on the installation reference surfaces and the end face of the spacer ring.
3.4 Spindle assembly
To begin, securely fix the spindle on the workbench. Apply a uniform layer of lubricating oil to the outer surface of the spindle core. Next, install the lower spacer on the outer surface of the front end of the spindle core, ensuring that the parallelism of the spacer ring is maintained within 0.002 mm.
After this, apply grease evenly in the gap between the bearing cage and the balls, and then install the bearings back-to-back on the outer cylindrical surface of the front end of the spindle core. Proceed to install the inner and outer spacers, ensuring that their parallelism is also within 0.002 mm.
Continue by installing the bearings according to the specified bearing combination. Start with the spindle front support bearing group, followed by the spindle rear support bearing group. After that, install the inner ring spacer and the locking nut (refer to Figure 2). Pre-tighten the bearing assembly and secure the locking nut according to the disassembly dimensions.
After the locking nut is adjusted, 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 flatness of the bearing end face is within 0.002mm, as shown in Figure 3.
To calibrate the radial runout of the free end of the spindle within a tolerance of 0.005 mm, first, fix the micrometer on the bearing. If the runout deviates from this tolerance, adjust it using the anti-loosening screw on the locking nut.
Next, install the rear end support bearing in a back-to-back configuration. Begin by thoroughly cleaning the inner surface of the sleeve, including the cooling circulation groove. Then, apply a layer of grease evenly to the inner surface of the sleeve.
After this, carefully screw the spindle core, along with the bearing assembly, into the sleeve to combine the moving and static parts of the spindle assembly.
Finally, install the spacer ring and then the assembly locking nut in sequence. Pre-tighten the spindle’s rear support bearing group as indicated in Figure 4.
To install the front cover of the spindle, first, place the spindle horizontally on the V-block, as illustrated in Figure 5. Use a feeler gauge to adjust the clearance to within 0.03 mm. Next, utilize a micrometer to measure the radial runout of the spindle, ensuring it is within 0.002 mm. If necessary, make adjustments using a locking nut.
After that, assemble the broaching device, followed by the hydraulic cylinder for loosening the tool. Install the synchronous pulley, and finally, attach the spindle along with its peripheral accessories.
3.5 Spindle test
The spindle test should be conducted gradually, starting from low speed and increasing to high speed. The idling phase must last more than two hours, while high-speed rotation should not exceed 30 minutes. It is important to monitor the temperature of the bearing at all times during the machine’s operation. The maximum temperature for the spindle rolling bearing should not exceed 70°C at the highest speed.
Additionally, listen carefully to the machine’s rotation sounds while it operates. If everything is functioning normally, you should hear a steady whirring sound. However, if there are any issues, you may notice various abnormal noises, such as gear noise, slight knocking, harsh friction sounds, or clanging from metal collisions.
4. Conclusion
After extensive practice and experience, I successfully disassembled and repaired the spindle of the machining center, resolving issues such as spindle heating, poor precision, and high noise levels. As a result, I was able to restore the spindle’s precision. Through this process, I became familiar with the mechanical structure of the machine tool spindle and mastered the techniques for repair, assembly, and adjustment. This experience allowed me to accumulate valuable maintenance knowledge and provide reliable technical data for the future upkeep of the equipment.
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