Electric spindle is a new technology that integrates machine tool spindle and spindle motor in the field of CNC machine tools in recent years. The electric spindle is a set of components, which includes the electric spindle itself and its accessories: electric spindle, high frequency frequency conversion device, oil mist lubricator, cooling device, built-in encoder, change device, etc. The rotor of the motor is directly used as the main shaft of the machine tool, and the housing of the main shaft unit is the motor base, and other components are used to realize the integration of the motor and the main shaft of the machine tool.
With the rapid development and perfection of electrical transmission technology (frequency conversion speed regulation technology, motor vector control technology, etc.), the mechanical structure of the main drive system of high-speed CNC machine tools has been greatly simplified, and pulley transmission and gear transmission are basically eliminated . The main shaft of the machine tool is directly driven by the built-in motor, thus shortening the length of the main transmission chain of the machine tool to zero, realizing the “zero transmission” of the machine tool. This kind of transmission structure form of spindle motor and machine tool spindle “combined into one” makes the spindle component relatively independent from the transmission system and overall structure of the machine tool, so it can be made into a “spindle unit”, commonly known as “electric spindle”. Since the current electric spindle mainly uses an AC high-frequency motor, it is also called a “high-frequency spindle”. Because there is no intermediate transmission link, it is sometimes called “direct drive spindle”. The characteristics are high speed, high precision, low noise, and the inner ring with lock structure is more suitable for spray lubrication.
The technical structure of the high-frequency spindle
1.high-speed bearing technology
The electric spindle usually uses dynamic and static pressure bearings, composite ceramic bearings or electromagnetic suspension bearings.
Dynamic and static pressure bearings have high rigidity and damping, which can greatly improve processing efficiency, processing quality, extend tool life, and reduce processing costs. This type of bearing has an infinite life.
Composite ceramic bearings are currently widely used in electric spindle units. The rolling elements of this bearing use hot-pressed Si3N4 ceramic balls, and the bearing rings are still steel rings, with a high degree of standardization, small changes to the machine tool structure, and easy maintenance.
Electromagnetic suspension bearings have good high-speed performance, high precision, and are easy to realize diagnosis and online monitoring. However, due to the complex electromagnetic measurement and control system, this kind of bearing is very expensive and has been high for a long time, so it has not been widely used.
2.high-speed motor technology
The electric spindle is the product of the fusion of the motor and the spindle. The rotor of the motor is the rotating part of the spindle. Theoretically, the electric spindle can be regarded as a high-speed motor. The key technology is dynamic balance at high speed;
In order to dissipate heat for the high-speed electric spindle as soon as possible, the outer wall of the electric spindle is usually passed with circulating coolant, and the function of the cooling device is to maintain the temperature of the coolant.
4.built-in pulse encoder
In order to realize automatic change and rigid tapping, a pulse encoder is built in the electric spindle to realize accurate phase angle control and cooperation with feed.
5. Automatic change device
In order to be applied to the machining center, the electric spindle is equipped with an automatic change device, including a disc spring, a pull cylinder, etc.;
6. How to install the high-speed tool
The widely-familiar BT and ISO tools have been proved to be unsuitable for high-speed processing. In this case, high-speed tools such as HSK and SKI appeared.
7.high frequency frequency conversion device
To achieve the speed of tens of thousands or even hundreds of thousands of revolutions per minute of the electric spindle, a high frequency frequency conversion device must be used to drive the built-in high-speed motor of the electric spindle, and the output frequency of the frequency converter must reach thousands or several kilohertz.
High-frequency spindle structure accuracy description and oil-air lubrication:
The electric spindle is composed of a shellless motor, a spindle, a bearing, a spindle unit housing, a drive module, and a cooling device. The rotor of the motor is integrated with the main shaft by a press-fitting method, and the main shaft is supported by front and rear bearings. The stator of the motor is installed in the housing of the spindle unit through a cooling jacket. The speed change of the spindle is controlled by the spindle drive module, and the temperature rise in the spindle unit is limited by the cooling device. The back end of the main shaft is equipped with speed and angular displacement sensors, and the inner cone hole and end face of the front end are used for mounting tools.
The electric spindle is a high-precision actuator, and the main factors that affect the rotation accuracy of the electric spindle are:
① Spindle error
mainly includes the roundness error of the spindle supporting journal, the coaxiality error (deflection of the spindle axis) and the perpendicularity error of the axial bearing surface of the spindle journal and the axis (affecting the axial movement of the spindle)
Bearing errors include the roundness error of the sliding bearing inner hole or rolling bearing raceway, the waviness of the sliding bearing inner hole or rolling bearing raceway, the shape and size error of the rolling bearing roller, the perpendicularity error of the bearing positioning end surface and the axis line, and the bearing end surface The parallelism error between the two, the bearing clearance and the force deformation during cutting, etc.
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