Several common spindle transmission modes of CNC machine tools

The spindle refers to the axis used to drive the workpiece or tool to rotate on the machine tool. In the design of spindle components, multiple factors are needed to obtain better processing results.

The machine tool spindle refers to the machine tool used to drive the workpiece or tool rotation axis. The spindle is the most critical part of the machine tool. To say that the CNC system is equivalent to the human brain, then the spindle is equivalent to the human heart. Spindle is an important part of the machine tool. In the design of spindle components, we need to consider rotary accuracy, static stiffness, vibration resistance, temperature rise and thermal deformation, and wear resistance factors, including spindle components of movement accuracy and structural rigidity, to determine the quality of machining and cutting efficiency.

The main drive of CNC machine tools requires a large speed range to ensure that the machining can choose a reasonable amount of cutting so as to obtain the best productivity, machining accuracy, and surface quality. The speed change of CNC machine tools is carried out automatically according to the control instructions, so the main transmission mechanism must be adapted to the requirements of automatic operation.

Common spindle drive mode

There are three common spindle transmission methods, which are gear transmission, synchronous belt transmission, and motor-driven transmission.

Gear drive

Gear transmission, this transmission is more common in large and medium-sized CNC machine tools. It is through several pairs of gear teeth in the completion of the transmission at the same time to achieve the spindle gear step or stepless speed change to ensure that at low speeds it meets the requirements of the spindle output torque characteristics. Most of the slip gear shift is achieved by hydraulic forks or gears driven directly by hydraulic cylinders.

Although this transmission is very effective, it increases the complexity of the hydraulic system of CNC machine tools and must first be sent by the CNC device electrical signals into the mechanical action of the solenoid valve, and then the pressure oil is distributed to the corresponding hydraulic cylinder, thus increasing the intermediate link of the speed change. In addition, this transmission mechanism, caused by vibration and noise, is also larger.

Synchronous belt drive

Synchronous belt drives, mainly used in small CNC machine tools, can avoid the vibration and noise caused by the gear drive, but they can only be applied to low-torque characteristics of the spindle.

Synchronous belt drive is a new type of transmission that combines the advantages of belt and chain transmission. The working surface of the belt and the outer circle of the pulley are made into a tooth type, and through the pulley, the gear teeth are embedded in a non-sliding gear transmission. The internal synchronous belt adopts a strong layer of material without elastic elongation after bearing in order to keep the pitch of the belt unchanged so that the master and slave pulleys can be used for synchronous transmission without relative slip. Compared with general belt drive and gear drive, synchronous belt drive has the following advantages:

1. No sliding, accurate transmission ratio.
2. High transmission efficiency, up to 98% or more.
3. Wide range of use, speed up to 50 m/s, transmission ratio up to 10 or so, transfer power from a few watts to several kilowatts.
4. Smooth transmission, low noise.
5. Easy maintenance, no lubrication.

The shortcomings of the synchronous belt drive are that the installation of the center distance requirements is strict, the belt and pulley manufacturing process is more complex, and the cost is high.

Motor-driven transmission

Motor-driven transmission in the motor is also known as the electric spindle. The motor stator is fixed to the rotor and spindle using an integrated design. This approach greatly simplifies the structure of the spindle box and spindle, effectively improving the stiffness of the spindle components, but the output torque is small because of the motor's heat on the spindle's precision impact.

The advantage is that the spindle parts are compact, have small inertia and mass, can improve the response characteristics of starting and stopping, and are conducive to the control of vibration and noise. The disadvantage is that the heat generated by the motor operation causes thermal deformation of the spindle, so temperature control and cooling when using the spindle with an internal motor are key issues. Japan developed a vertical machining center spindle assembly with a maximum speed of its built-in motor spindle of up to 50,000 r/min.