Electric spindle

The high-speed spindle is the core component of high-speed cutting machine tools. With the continuous increase in spindle speed requirements, the traditional gear belt variable speed transmission system can no longer meet the requirements due to its own vibration, noise, and other reasons. Instead, it is a new functional component - the electric spindle, which combines the spindle motor with the machine tool spindle to achieve the integration of the spindle motor and the machine tool spindle. Electronic sensors are used to control the temperature of electric spindles, providing water or oil cooling circulation systems to maintain a constant temperature during high-speed rotation. Generally, a certain set temperature can be controlled within the range of 20 °~25 ° C, with an accuracy of ± 0.7 °. At the same time, new technologies such as oil mist lubrication and hybrid ceramic bearings are used to make the spindle maintenance free, long-life, and high-precision.

Electric spindles were first used for internal grinding machines. In the 1980s, with the development and demand of CNC machine tools and high-speed cutting technology, electric spindle technology gradually applied to high-end CNC machine tools such as machining centers and CNC milling machines, becoming one of the major achievements in machine tool technology in recent years. With the development of machine tool technology, high-speed cutting technology, and the needs of practical applications, people have put forward increasingly high requirements for the performance of machine tool electric spindles. At present, foreign companies engaged in the research and production of high-speed CNC machine tool electric spindles represent a high level of technology in this field. The electric spindles produced by these companies have the following characteristics: high power, high speed; Adopting high-speed and high stiffness bearings, mainly ceramic bearings and liquid hydrostatic bearings, air lubricated bearings and magnetic levitation bearings are used for special occasions; High level precision machining and assembly technology; The supporting control system is of high level, including rotor automatic balancing system, bearing oil and air lubrication and precision control system, fixed rotor cooling temperature precision control system, and spindle deformation temperature compensation precision control system.

Integrated technology of electric spindle

Electric spindle is a set of components, including the electric spindle itself and its accessories: electric spindle, high-frequency frequency converter, oil mist lubricator, cooling device, built-in encoder, and tool changer.

1. High speed bearing technology

Electric spindles usually use dynamic and static pressure bearings, hybrid ceramic bearings, and electromagnetic suspension bearings. Dynamic and static bearings have high stiffness and damping, which can greatly improve machining efficiency, enhance machining quality, extend tool life, and reduce machining costs. Most bearings have an infinite lifespan. At present, composite ceramic bearings are widely used in electric spindle units. The rolling elements of these bearings use hot pressed Si3N4 ceramic balls, and the bearing rings are still steel rings, with a high degree of standardization and minimal changes to the machine tool structure, making them easy to maintain. Electromagnetic suspension bearings have good high-speed performance and high precision, making them easy to diagnose and monitor online. However, due to the complexity of electromagnetic measurement and control systems, this type of bearing is expensive and has maintained a high price for a long time, so it has not been widely used nowadays.

2. High speed motor technology

Electric spindle is a product of the fusion of electric motor and spindle. The rotor of the electric motor is the rotating component of the spindle. In theory, electric spindle can be regarded as a high-speed motor. The key technology is high-speed dynamic balancing;

3. Lubrication

The lubrication of the electric spindle generally adopts timed and quantitative oil air lubrication; You can also use grease lubrication, but the corresponding speed should be reduced. Timing means injecting oil at regular intervals. Quantification refers to the precise control of the amount of lubricating oil each time through a device called a quantitative valve. Oil air lubrication refers to the use of compressed air to blow lubricating oil into ceramic bearings. Oil control is very important, too little cannot lubricate; Too much, when the bearing rotates at high speed, it will heat up due to the resistance of the oil.

4. Cooling device

In order to dissipate heat from the high-speed electric spindle as soon as possible, circulating coolant is usually introduced on the outer wall of the electric spindle, and the function of the cooling device is to maintain the temperature of the coolant.

5. Built in pulse encoder

In order to achieve automatic tool changing and rigid thread tapping, the electric spindle is equipped with a pulse encoder to achieve precise phase angle control and coordination with feed.

6. Automatic tool changing device

In order to be applied to machining centers, electric spindles are equipped with automatic tool changing devices, including disc springs, tool pulling cylinders, etc.

7. Loading of high-speed cutting tools

The well-known BT and ISO cutting tools have been proven to be unsuitable for high-speed machining in practice. In this situation, high-speed tools such as HSK and SKI have emerged.

8. High frequency frequency conversion device

To achieve a speed of tens of thousands or even tens of thousands of revolutions per minute for the electric spindle, a high-frequency frequency converter is needed to drive the built-in high-speed motor of the electric spindle, and the output frequency of the frequency converter needs to reach thousands or thousands of hertz.

Common faults of electric spindle:

1. Decreased machining accuracy.

2. The cylindricity exceeds the tolerance range.

3. The spindle heats up.

4. The noise is loud, but the resistance to manually rotating the spindle is small.

(1) Fault analysis

During the disassembly and inspection of the spindle components, the following fault causes were found:

① The lubricating grease for the spindle bearings of machining centers is mixed with dust and moisture. This is because the compressed air used in the machining center does not have fine filtering and drying devices, so a small amount of dust and water vapor will enter the lubricating grease of the spindle bearings during pneumatic chip blowing, resulting in poor lubrication, heat generation, and loud noise; There are slight scratches on the positioning surface of the taper hole in the spindle, and the taper hole does not fit well with the taper surface of the tool holder, with a slight eccentricity;

② The preload force of the front bearing of the main shaft decreases, and the clearance of the bearing increases;

③ Some of the disc springs in the spindle automatic clamping mechanism have fatigue failure, and the cutting tools have not been fully tightened, resulting in slight clearance.

(2) Fault handling

Replace the spindle bearings and lubricating grease in the machining center, adjust the bearing clearance, make a simple grinder, manually grind the positioning surface of the spindle inner taper hole, and check with the coloring method to ensure that the contact area between the blade handle and the spindle centering taper hole is greater than 85%; Replace the butterfly spring. Place the repaired spindle back into the spindle box and check the radial runout with a dial gauge. The proximal end at 150mm is less than 0.006mm and the distal end is less than 0.010mm. After trial machining, the spindle temperature rise and noise are normal, and the machining accuracy meets the machining process requirements. Then troubleshoot.

Improvement measures:

① Add compressed air fine filtration and drying devices, regularly empty the filter, and regularly clean or replace the filter element;

② Check the cleanliness and fit of the spindle taper hole and blade handle at any time, and check whether the air dryer is working properly;

③ Reasonably arrange processing technology to avoid rapid changes in material cutting rate;

④ Overloading operation is strictly prohibited. If there is a malfunction, it should be reported for repair in a timely manner. It is not allowed to operate with a problem.