The electric spindle is one of the important core components of high-speed precision machining centers, which determines the reliability of high-speed precision machining centers. The precision requirements for high-speed precision electric spindles are in the micrometer range, such as a radial runout of 0.002 millimeters for the spindle taper hole on the shaft diameter, 0.005 millimeters at a distance of 300 millimeters from the spindle end face, a spindle support shaft diameter roundness of 0.001~0.002, and a spindle diameter accuracy of IT2 level. The precision error and small deformation of the electric spindle itself are the key to control. Therefore, the machining process of the electric spindle should be studied from the aspects of heat treatment, machining process scheme, inspection method, assembly process, and operation test.

1. Heat treatment

The spindle adopts 12CrNi3 alloy carburizing steel, which has good comprehensive mechanical properties after quenching, low-temperature tempering or high-temperature tempering, high low-temperature toughness, low notch sensitivity, good cutting performance, low hardness and good plasticity after annealing.

Using forged blanks, the heat treatment method is as follows: normalizing treatment after forging → high-temperature aging treatment after driving → carburizing treatment of the spindle neck and 7:24 taper hole, with a carburizing layer depth of 1.7mm → quenching treatment of the spindle neck and 7:24 taper hole, with a quenching hardness of HRC59, conducting spindle heat treatment and slicing experiments → sandblasting treatment (spraying fine sand on the left protruding key area and all empty tool slots) → qualitative treatment in oil after rough grinding of the spindle → heating of the electric spindle spindle spindle and hot assembly of the spindle.

2. Machining process

The structural characteristics of the built-in high-speed electric spindle determine the particularity of the spindle machining process. In order to avoid the loss of rotational accuracy caused by thermal deformation during the hot installation of the electric spindle rotor or the imbalance of the spindle rotor assembly caused by assembly errors, the electric spindle rotor must be assembled with the machine tool spindle before precision machining.

Each spindle diameter and inner hole (including taper holes) should reserve a fine grinding amount, and the spindle should be loaded and aligned: the outer circle should be ground according to the measured value of the rotor inner diameter, and the interference fit should be within the range of 0.07~0.09mm.

Heat the balance plate and rotor together with the spindle, and heat the motor rotor: Heat the balance plate and rotor together (heating temperature is 180 ℃~200 ℃, insulation for 1 hour; when the rotor is taken out of the heating furnace, it must be confirmed whether the temperature of the rotor is within the allowable heating temperature range); Place the spindle vertically on the bracket on the platform in advance, and then heat the heated balance sleeve, spindle rotor, and balance plate onto the spindle in sequence (after the workpiece cools down, rotate the machine to add them); Semi precision grinding, precision grinding of rotors (including balance disks), main shaft inner holes, shaft necks, and 7:24 taper holes (7:24 taper holes should be 0.0015 millimeters at the axial A-B reference radial deviation of the main shaft end face and 0.004 millimeters at a distance of 300 millimeters from the main shaft end).

3. Assembly technology of electric spindle components

The built-in high-speed electric spindle can reach a maximum speed of 12000rpm, with the front end completely fixed and the rear end supported by floating bearings. Four rows of front end back-to-back front angular contact ball bearings are used, with the main shaft fixed radially and axially. The rear end adopts cylindrical roller bearings, and the inner ring and rollers of the bearings can move axially along the outer raceway with the main shaft, reducing the impact of thermal elongation on the main shaft and maintaining good accuracy. According to the design characteristics, the key issues that need to be addressed during assembly are bearing preload, temperature rise of front and rear bearings, spindle dynamic balance, hot installation of stator and cooling sleeve, oil and air lubrication device and leakage prevention of cooling sleeve, etc., and the assembly process specifications should be determined.

3.1 Determine the pre tightening force of the bearing

The preload force of the bearing is related to the spindle speed, stiffness, and load of the machine tool. Increasing the preload force of the bearing can improve the stiffness of the spindle, but the friction and loss of the bearing raceway will generate heat. If the heat cannot be dissipated in a timely manner, it will seriously affect the accuracy of the machine tool. Therefore, the preload force of the bearing should be determined through testing or experimentation. Based on repeated experiments and reference to the research results of pre tightening force of electric spindle bearings at home and abroad, the pre tightening amount is determined to be 0.002-0.005 mm. By measuring the height difference between the inner and outer rings of the same set of bearings, the height difference between the inner and outer spacers can be determined. Through precision matching research, the parallelism tolerance of the two end faces of the spacer is 0.002 mm.

3.2 Assembly of the spindle

The specific assembly process of the spindle includes the following eight steps: ① The tolerance interference between the spindle bearing and the spindle journal is 0.002-0.005mm, which can be guaranteed by grinding the measured values of the spindle and spindle bearing; ② Apply SBU15 lithium based grease to the bearing before installation, with a filling amount of 1/3 of the bearing clearance; ③ Before installing the bearing, the main shaft should be vertically fixed on the support of the operating platform. The high point of the axis runout corresponding to the spindle taper hole should be determined according to the markings provided by the machine. During assembly, the high point of the inner ring of the bearing should be aligned with this point and the markings on the outer ring of the bearing; ④ During installation, the bearing heating tool must be used to heat it up, with a heating temperature of 50 ℃; ⑤ The bearings and spacers are installed on the main shaft in order according to the marked positions, and the bearings and spacers are knocked flat in the axial direction with the installation sleeves in order to ensure that all combined end faces of the bearings are tightly fitted; ⑥ When installing the spacer, it is necessary to ensure that the oil and gas holes of the spacer are not blocked, and to ensure that the spindle sleeve is installed on the premise of the correct positioning of the oil and gas lubrication holes and sleeve of the spacer; ⑦ Install the spindle nut; ⑧ Adjust the rotation accuracy of the spindle component.

Therefore, the following requirements should be met: ① The root tolerance for radial runout of the spindle taper hole axis is 0.002 millimeters; ② The allowable deviation at a distance of 300 millimeters from the spindle end face is 0.005 millimeters; ③ The axial displacement tolerance of the spindle is 0.002 millimeters; ④ The radial runout tolerance of the rear journal of the main shaft is 0.02 millimeters

3.3 Dynamic Balance of Spindle Components

In terms of dynamic balance during spindle assembly, the following two points should be noted: ① Assemble the spindle puller and related rotating components, and apply high-pressure resistant grease containing MoS2 on each surface of the butterfly spring to improve the wear resistance between the contact surfaces of the butterfly spring; ② Dynamic balance adjustment of spindle assembly: The spindle assembly is supported by the front support sleeve and the outer ring of the rear bearing. The belt drive can adjust the dynamic balance of the spindle assembly. The balance plates at both ends of the motor rotor are weighted to ensure that the unbalance of the spindle rotating assembly is 0.5g

4. Spindle operation test

The spindle operation test is divided into the following five steps: ① Adjust the parameters of the oil air lubrication device on the spindle box test bench and check the accuracy after the bearing temperature changes. ② The spindle should run at high, medium, and low speeds in sequence from the lowest speed. When running at maximum speed, the time should not exceed 20 minutes, so that the main shaft bearing can reach the standard temperature. The temperature and temperature rise should be measured at the main shaft centering bearing, and the temperature should not exceed 60 ℃ and the temperature rise should not exceed 30 ℃. ③ The spindle should run smoothly without any screaming or impact sounds, and the spindle noise should not exceed 78dB. ④ The spindle loosening and cutting mechanism should be tested. The loosening and pulling force should meet the design requirements. Use a pulling force tester to check the pulling force of the spindle. The pulling force should meet the requirement of 1400-1600kg, and the axial movement of the loose pulling spindle should not exceed 0.15mm. ⑤ Recheck the spindle accuracy to meet the rotation accuracy requirements.

5. Conclusion

The machining technology of high-speed precision machining center electric spindle is an innovative design, and every enterprise is constantly practicing and summarizing experience based on its own machining, inspection, experimental equipment and technical means. The electric spindle assembled according to the above steps has undergone performance testing and user feedback. The radial runout of the spindle taper hole on the spindle diameter is 0.0016 millimeters, 0.004 millimeters at a distance of 300 millimeters from the spindle end face, and the roundness of the spindle supporting spindle diameter is 0.0014. Its accuracy index meets the requirements of high-speed precision machining centers.