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Machine tool spindle



Introduction

The machine tool spindle refers to the axis that drives the workpiece or tool to rotate on the machine tool. The main shaft components are usually composed of main shaft, bearings and transmission parts (gears or pulleys). In the machine, it is mainly used to support transmission parts such as gears and pulleys to transmit motion and torque, such as machine tool spindles; some are used to clamp workpieces, such as spindles. Except for machine tools whose main motion is linear motion, such as planers and broaching machines, most machine tools have spindle components. The movement accuracy and structural rigidity of the spindle components are important factors that determine the processing quality and cutting efficiency.

Performance indicators

The main indicators to measure the performance of spindle components are rotation accuracy, stiffness and speed adaptability.

①Rotation accuracy: The radial and axial runout in the direction that affects the machining accuracy when the spindle rotates is mainly determined by the manufacturing and assembly quality of the spindle and the bearing.

②Dynamic and static stiffness: mainly determined by the bending stiffness of the spindle, the stiffness and damping of the bearing.

③Speed ​​adaptability: The maximum allowable speed and speed range are mainly determined by the structure and lubrication of the bearing, as well as the heat dissipation conditions.

Maintenance of machine tool spindle

To reduce the working temperature of the bearing, lubricating oil is often used. There are two types of lubrication: oil-air lubrication and oil circulation lubrication. When using these two methods, pay attention to the following points:

1. When using oil circulation lubrication, ensure that the oil volume of the spindle constant temperature oil tank is sufficient.

2. The oil-air lubrication method is just the opposite of the oil circulation lubrication, it only needs to fill 10% of the bearing space capacity.

The advantage of cyclic lubrication is that it can reduce friction and heat generation when lubrication is satisfied, and it can absorb part of the heat of the spindle assembly.

There are also two release methods for the lubrication of the spindle: oil mist lubrication and injection lubrication.

New technology

1. The technology of the electric spindle:

The electric spindle is the integration of the machine tool spindle and the spindle motor in the field of CNC machine tools The new technology, together with linear motor technology and high-speed tool technology, will push high-speed machining to a new era. 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, and tool change device.

2. The fusion technology of the electric spindle:

High-speed bearing technology: The electric spindle usually uses composite ceramic bearings, which are wear-resistant and heat-resistant, and have a service life several times that of traditional bearings; sometimes Using electromagnetic suspension bearings or hydrostatic bearings, the inner and outer rings are not in contact, and the theoretical life is unlimited;

High-speed motor technology: the electric spindle is the product of the integration of the motor and the spindle, and the rotor of the motor is the rotation of the spindle Partially, in theory, the electric spindle can be regarded as a high-speed motor. The key technology is dynamic balance at high speed;

Lubrication: The lubrication of the electric spindle generally adopts regular and quantitative oil and gas lubrication; grease lubrication can also be used, but the corresponding speed should be discounted. The so-called timing is to inject oil at regular intervals. The so-called quantitative means to precisely control the amount of lubricating oil each time through a device called a quantitative valve. The oil-air lubrication refers to that the lubricating oil is blown into the ceramic bearing with the compressed air. Oil control is very important. Too little will not provide lubrication; too much will cause heat due to oil resistance when the bearing rotates at high speed.

Cooling device: In order to dissipate heat for the high-speed electric spindle as soon as possible, a circulating coolant is usually passed to the outer wall of the electric spindle. The function of the cooling device is to maintain the temperature of the coolant.

Built-in pulse encoder: In order to realize automatic tool change and rigid tapping, a pulse encoder is built in the electric spindle to realize accurate phase angle control and cooperation with feed.

Automatic tool change device: In order to be applied to the machining center, the electric spindle is equipped with an automatic tool change device, including disc springs, broach cylinders, etc.

How to install high-speed tools: The widely-familiar BT and ISO tools have been proved to be unsuitable for high-speed machining. In this case, high-speed tools such as HSK and SKI appeared.

High frequency frequency conversion device: In order to realize 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 Up to thousands or thousands of hertz.

Historical evolution

Machine tool spindles (7 photos)

Before the 1930s, most machine tool spindles used a single oil wedge to slide Bearing. With the improvement of rolling bearing manufacturing technology, a variety of high-precision, high-rigidity rolling bearings for main shafts appeared later. This kind of bearing is convenient to supply, low in price, low in friction coefficient, convenient in lubrication, and can adapt to working conditions with large fluctuations in speed and load, so it is widely used. However, sliding bearings have the advantages of stable operation and good vibration resistance, especially various multi-oil wedge hydrodynamic bearings, which are used a lot in some finishing machine tools such as grinders. The hydrostatic bearing that appeared after the 1950s has high precision, high rigidity, low friction coefficient, good vibration resistance and stability, but it requires a set of complex oil supply equipment, so it is only used in high-precision machine tools and heavy-duty machines. On the machine. Gas bearings have good high-speed performance, but due to their small carrying capacity and complicated gas supply equipment, they are mainly used on high-speed internal grinders and a few ultra-precision machining machines. The electromagnetic bearing that appeared in the early 1970s has the advantages of good high-speed performance and large load-bearing capacity. It can adjust the magnetic field to make a small amount of displacement of the spindle during the cutting process to improve the dimensional accuracy of the processing, but the cost is high, and it can be used for Ultra-precision processing machine tools.

Variable speed mode

Infinitely variable speed

Numerical control machine tool Generally, DC or AC spindle servo motor is used to realize the spindle infinitely variable speed.

AC spindle motor and AC variable frequency drive device (cage induction AC motor is equipped with vector conversion variable frequency speed regulation system), because there is no brush, no sparks, so long service life, and the performance has reached DC drive The level of the system, even in terms of noise, has been reduced. Therefore, it is widely used.

The relationship between the power or torque transmitted by the main shaft and the speed. When the machine tool is in continuous operation, the rotation speed of the spindle is in the range of 437~3500r/min, and the total power of the motor transmitted by the spindle is 11kW, which is the constant power zone II (solid line) of the spindle. In this area, the maximum output torque of the spindle (245N.m) decreases as the spindle speed increases. The spindle speed is within the range of 35~437r/min, and the output torque of the spindle remains unchanged, which is called the constant torque area I (solid line) of the spindle. In this area, the power that the main shaft can transmit decreases as the speed of the main shaft decreases. When the motor is overloaded (overloading is allowed for 30min), constant power area and constant torque area. The overload power of the motor is 15kW, and the maximum output torque of the overload is 334N.m.

Sectional stepless speed change

In actual production, CNC machine tools do not need to have constant power throughout the entire speed range. In general, it is required to drive with constant power in the middle and high speed section, and drive with constant torque in the low speed section. In order to ensure that the spindle of the CNC machine tool has a large torque and the speed range of the spindle is as wide as possible at low speed, some CNC machine tools are equipped with gear speed on the basis of AC or DC motor stepless speed change, making it a stepwise stepless speed change .

Spindle speed adjustment

The spindle is one of the main components of the machining center, because its design directly affects the processing efficiency and the quality of the workpiece. Therefore, the spindle design (static and dynamic stiffness, shaft diameter, bearing, design parameters, etc.) has been studied in depth [5-8]. The performance of the machine tool spindle accelerator mainly depends on the optimal design for the required speed and power transmission ratio. In particular, two factors must be considered because they are very important in the optimization design of the spindle speed control device. These two factors are the smallest volume and the smallest transmission kinetic energy. 

In order to reduce the weight, the volume of the spindle speed control device must be minimized, and the space required for the operation of the machine tool cannot be reduced. But, again, the mechanical spindle accelerator must be designed for long-term production work. Therefore, the transmission of kinetic energy must be at least to ensure the best performance.

The design of the spindle speed control device has led to the use of transmissions based on planetary gear sequences (PGTs), because planetary gear sequences PGTs provide a very compact and efficient solution (reduce the number of ordinary gear sequences) Weight and size), it has high speed and high efficiency. PGTs are also used in many machinery and equipment equipped with automobile gearboxes, thereby extending the constant power range of the low-speed spindle drive motor of the machine tool.

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