0. Introduction

The five axis linkage machining center of SAJO10000 machining center has high machine precision and automation level, with large machine size and worktable size of 1000 × 1000. The stroke of each axis is X=1600, Y=1500, Z=1750, A=0 °~110 °, B=0 °~360 °. This device is easy to use and has strong processing capabilities. Especially its spindle is an electric spindle, which means the rotor of the main motor is the spindle of the machine tool, and it is currently a relatively advanced spindle structure. The mechanical and electrical structure of the electric spindle is complex, with high precision and high technical content in electrical control capability. The spindle can achieve stepless speed regulation from 0 to 6000 r/min, and the rotation control is very precise. It can achieve shaft linkage tapping, high-precision thread machining and other operations.

1. Electric spindle structure of machining center

The structure of the SAJO10000 machining center electric spindle mainly consists of two parts: rotor and stator.

1.1. Rotor

The rotor part includes: fastening components for the tool disc spring, hydraulic cylinder components for loosening the tool, air purification device for the spindle cone, internal cooling and water supply device for the tool, etc.

(1) Tool disc spring tightening component. The tightening part of the tool disc spring is used to firmly fix the tool on the spindle taper hole. The array disc spring drives a retractable four claw tightening hook through a pre tightening nut and a pull rod to hook the tension nail at the rear of the tool tail cone, so that the tool is firmly fixed on the spindle. The tightening force of the tool can be achieved by adjusting the pre tightening nut behind the disc spring pull rod. The tightening force should be adjusted appropriately. If the tool is too loose, it is easy to loosen, and if the disc spring is too tight, it is easy to damage.

(2) Loosen the hydraulic cylinder components of the tool. The tool releases the hydraulic cylinder part to push the disc spring in reverse and separate the tool from the spindle. This type of hydraulic cylinder has a small volume and a working oil pressure of up to 15 megapascals, requiring high quality for the hydraulic cylinder.

(3) Spindle cone air purification device. The function of the spindle cone air cleaning device is to blow compressed air from the inside of the spindle while replacing the spindle tool, cleaning the spindle cone and ensuring accurate tool positioning and reliable fastening. The blowing of compressed air is controlled by a hydraulic pneumatic combination control valve installed on the upper part of the tool release hydraulic cylinder.

(4) Internal cooling water supply device for cutting tools. The internal cooling water supply device for cutting tools is specifically designed for equipment with internal cooling structures in the spindle. The center of the tool is equipped with a cooling liquid channel, through which the cooling liquid in the spindle is directly sprayed onto the contact surface between the tool and the part, thereby achieving good cooling performance. During processing, higher spindle speeds and larger feed rates can be used, greatly improving productivity. The coolant in the central part of the spindle is also controlled by the hydraulic cylinder upper hydraulic pneumatic combination control valve for tool release.

1.2 Stator

The stator part includes: stator coils and stator coil cooling water circulation device, spindle speed detection sensor, tool cooling spray pipeline, bearing oil mist lubrication and circulating water cooling device, etc.

(1) Stator coil. The stator coil of the electric spindle is installed inside a rectangular shell, connected to the A-axis, and rotates with the A-axis. It is equipped with a motor cable and two sets of thermal protection sensors are installed inside the motor cable, which provides good protection against overcurrent and overload of the motor. This type of motor has high power, and the stator coils of the motor generate a large amount of heat. The usual air cooling method cannot meet the requirements. In terms of design, the manufacturer adopted a cooling method of adding circulating water outside the stator coil shell, greatly improving the heat dissipation effect. The heat generated by the motor coil can be quickly carried away, further improving the motor's overcurrent overload capacity. With the addition of water cooling devices, the sealing requirements for the main shaft have also increased.

(2) Spindle speed detection sensor. The speed feedback of the electric spindle is detected by a toothed encoder installed on the spindle rotor and a magnetic sensor located on the motor stator end cover. When each tooth of the encoder passes through the detection surface of the magnetic sensor, the sensor emits a pulse signal. The number of pulses emitted per unit time is processed by the spindle servo controller to generate corresponding control signals to control the spindle speed and angle, and feedback the spindle status to the numerical control system.

(3) Tool cooling spray pipe. The tool cooling spray pipe directly delivers the coolant to the six nozzles on both sides of the spindle tool through the stator shell, forcibly cooling the tool on the spindle. This design has a much larger coolant injection volume than typical machine tools, thereby reducing tool wear and workpiece thermal deformation.

(4) Bearing oil mist lubrication. The motor rotor and stator housing are supported by a set of five precision bearings, bearing 7020ACD/P4A. Three bearings are installed at the front end of the rotor shaft, and two bearings are installed at the rear end. This set of bearings has high speed and high load, especially the three bearings in the front, which require high lubrication. The electric spindle is designed with oil mist lubrication method. The lubricating oil is mixed with compressed air in the oil mist generator and transported to the rolling elements of each bearing through pipelines, so that the bearings are fully lubricated and cooled. Meanwhile, in terms of design, the three front bearing mounting seats are forcibly cooled by a circulating water cooling package, further ensuring the good working condition of the bearings.

2. Repair of typical faults in electric spindles

2.1. Electric spindle water inlet stator coil burnout fault

When the machine tool is processing parts, the worker hears an abnormal sound from the spindle, and the machine automatically stops and opens the work door. It is found that white smoke is emitted from the connection between the spindle and the A-axis, emitting a burnt smell. During this period, it showed that the power output had reached 99%, and there were spindle servo faults and spindle servo measurement circuit current absolute value error alarms.

After shutdown inspection, it was found that the three-phase coil of the main motor had zero resistance to ground, and it was decided to disassemble the main motor. When removing the upper cover of the motor, it was found that there was a mixture of water vapor and cigarette ash sprayed on the upper cover of the motor. The three-phase winding outlet of the motor was burned, and the inner wall of the stator cavity rusted. After the motor rotor was removed with a crane, there was a large amount of water accumulation under the stator coil. When disassembling the cooling water circulation plate at the lower end of the spindle, it was found that the head of the M5 hexagon socket screw with a sealing gasket was broken, and the sealing gasket lost its sealing performance. Cooling water infiltrated the motor, causing damage to the motor. If purchasing original spare parts, it takes 3-4 months for the order cycle to be completed, and the last week will be reworked by a professional domestic manufacturer. After assembly and debugging, the machine was tested and the electric spindle worked normally.

2.2. Replacement of electric spindle bearings

The machine tool experiences an error alarm from the electric spindle encoder during operation. Based on past experience, the clearance between the encoder and the detection gear has been adjusted, but there has been no improvement. Through communication with the staff, it was learned that the spindle vibration has been a bit high recently, and the brightness of the machined holes is not good. Check the static accuracy of the spindle, with a radial runout of 0.02~0.04mm and an axial runout of 0.05~0.08 mm. After excluding the influence of measurement methods and other axes, it was determined that there may be a problem with the spindle bearings, and it was decided to disassemble and replace them.

The electric spindle is relatively heavy, with a motor weighing about 300 kg, and requires the cooperation of multiple people to disassemble. Especially when disassembling the rotor shaft, the entire motor should be stable on the gasket seat of the worktable. When a person climbs to the upper part of the machine tool at a height of more than 4 meters, the Y-axis screw rod of the handwheel will move the motor stator housing upward, so that the motor shaft rotor and the bearing installation housing at the front of the rotor can be removed. After disassembling the motor shaft rotor and rotor front bearing installation housing, all three internal bearings need to be disassembled. However, according to the mechanical diagram, it was found that the fixed bearing front clamping end cover cannot be disassembled using conventional methods. After consulting with the manufacturer, it was found that the interference fit between the inner hole of the pressing end cover at the front end of the bearing and the shaft is 0.1 mm, and disassembly requires an axial force of 2000 kg, which poses a high risk. The machine tool has designed a special hydraulic cavity on the bearing pressure end cover for easy disassembly. High pressure oil can only be removed by injecting it through a special joint with a high-pressure manual pump. After calculation, the high-pressure oil pressure should reach around 50 megapascals. A joint was specially designed and manufactured for this purpose, and finally dismantled with the cooperation of four or five people.

After inspection, the bearings were found to be significantly worn. When replacing a new bearing, the bearing should be heated in advance to avoid affecting the accuracy of the new bearing and facilitate installation. Especially when heating and fixing the front end of the bearing and pressing the end cover, the heating temperature should be carefully calculated. After several days of careful operation and effective control of the bearing preload, the bearing should be replaced.

3. Conclusion

By replacing the SAJO10000 electric spindle bearings this time, the understanding of electric spindle machine tools has been enhanced, and experience has been accumulated for future correction of electric spindles.